Patent Publication Number: US-2021163614-A1

Title: Apj antibody, fusion protein thereof with elabela, and pharmaceutical compositions and use thereof

Description:
FIELD 
     Provided herein are an apelin receptor (APJ) antibody and its fusion protein with Elabela. Also provided herein is a pharmaceutical composition of a fusion protein with APJ antibody and Elabela, and use thereof in treating, preventing, or alleviating one or more symptoms of pulmonary arterial hypertension, pulmonary hypertension or one or more symptoms of heart failure. 
     BACKGROUND 
     APJ contains 7 transmembrane units and 308 amino acids and belongs to the family of G protein coupled receptors (GPCRs). The vasoactive peptide Apelin is the earliest discovered endogenous ligand of the APJ receptor. Apelin and APJ are widely distributed in the human central nervous system and various peripheral tissues such as the lung, heart, breast, etc (Kawamata et al., 2001, Biochem. Biophys. Acta. 1538: 162-71; Medhurst et al. 2003,  J. Neurochem.  84: 1162-72), especially in cardiovascular endothelial cells and cardiac tissue (Kleiz et al., 2005.  Regal. Pept.  126: 233-40). A number of studies have shown that the Apelin/APJ signaling system can enhance myocardial contractility, lower blood pressure, promote neovascularization, regulate the immune response and release of pituitary related hormones, regulate insulin secretion, etc, and participate in the pathophysiological events of diabetic vascular disease, cardiac insufficiency, atrial fibrillation and ischemic-reperfusion injury. 
     Elabela is another endogenous ligand of the APJ receptor that has been discovered in recent years and has attracted great attention (Chug et al., 2013,  Lien: Cell  27: 672-680; Pauli et al., 2014,  Science  343: 1248636). Elabela is encoded by three exons on human chromosome 4 and was previously considered to be non-coding RNA. However, studies have found that Elabela contains a conservative ORF that encodes a protein of 54 amino acids, with its mature body consisting of only 32 amino acids. The Elabela/APJ signaling pathway has been shown to play a critical role in the development of the embryo&#39;s heart and vasculature. Studies have confirmed that Elabela and Elabela mutants can activate the Gαil and β-arresting signal transduction pathways of APJ receptors. Further, mutations in the C-terminus cf Elabela polypeptides can trigger a preference for APJ receptor signal transduction pathways (Murza et al., 2016,  J Med. Chem,  59: 2962-72). The preference of receptor signal transduction pathways provides an important direction for new drug development (Bologna et al., 2017,  Biomol. Ther,  25: 12-25). By using the separated perfusion heart and in vivo hemodynamics and echocardiography measurements, it was found that Elabela or Elabela mutants can reduce arterial pressure and exert positive inotropic effects on the heart (Yang et al., 2017,  Circulation  135: 1160-1173; Murza et al., 2016,  J. Med. Chem.  59: 2962-72). Other studies also found that Elabela content in patients with diabetic nephropathy is negatively correlated with urine microalbumin creatinine ratio (ACR) (Zhang et al., 2018,  Cell Physiol Biochem.  48: 1347-1354). 
     The fusion of Elabela and APJ antibody can significantly prolong the half-life of Elabela to retain the biological activity of Elabela molecules. At the same time, the fusion protein of APJ antibody and Elabela has the molecular targeting provided by the antibody, and it helps to improve the druggability of Elabela fusion protein to play a role in the treatment of one or more diseases of pulmonary arterial hypertension (PAH), pulmonary hypertension (PH), type 2 diabetes (T2D) and its related metabolic syndrome, and heart failure. 
     SUMMARY 
     Provided herein is an antibody specifically binding to APJ. 
     Provided herein is an antibody specifically binding to APJ, wherein the antibody comprises 1, 2, 3, 4, 5, or 6 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below: 
     a. light chain CDR1 amino acid sequences: SEQ ID NO: 1, SEQ ID NO: 4, SEQ ID NO: 7, and SEQ ID NO: 10; 
     b. light chain CDR2. amino acid sequences: SEQ ID NO: 2, SEQ ID NO: 5, and SEQ ID NO: 8; 
     c. light chain CDR3 amino acid sequences: SEQ ID NO: 3, SEQ ID NO: 6, SEQ ID NO: 9, and SEQ ID NO: 11; 
     d. heavy chain CDR1 amino acid sequences: SEQ ID NO: 12. SEQ ID NO: 15, and SEQ ID NO: 18; 
     e. heavy chain CDR2 amino acid sequences: SEQ ID NO: 13, SEQ ID NO: 16, SEQ ID NO: 19, and SEQ ID NO: 21; and 
     f. heavy chain CDR3 amino acid sequences: SEQ ID NO: 14, SEQ ID NO: 17, SEQ ID NO: 20, and SEQ ID NO: 22. 
     Provided herein is an Elabela fusion protein, comprising an antibody specifically binding to APJ, and 1, 3, 4, 5, 6, 7 or 8 Elabela fragment peptide linker sequences (Linker); the fusion protein, through a peptide linker sequence (Linker), connects the N-terminus of an Elabela fragment with the C-terminus of a heavy or light chain of APJ antibody, wherein each Elabela fragment is independently a forward Elabela fragment or its mutant; or the fusion protein, through a peptide linker sequence (Linker), connects the C-terminus of an Elabela fragment with the N-terminus of a heavy or light chain of APJ antibody, wherein each Elabela fragment is independently a reverse Elabela fragment or its mutant. 
     Provided herein is an Elabela fusion protein, comprising an APJ antibody and two Elabela fragments and two peptide linker sequences (Linker); the fusion protein, through a peptide linker sequence (Linker), connects the N-terminus of an Elabela. fragment with the C-terminus of a light chain of API antibody: N′-R-Linker-Elabela-C′; or through a peptide linker sequence (Linker), connects the N-terminus of an Elabela. fragment with the C-terminus of a heavy chain of APJ antibody: N′-R-Linker-Elabela-C′, wherein, N′ represents the N-terminus of the fusion protein polypeptide, C′ represents the C-terminus of the fusion protein polypeptide, Elabela represents the forward Elabela fragment or its mutant, R represents the amino acid sequence of the light or heavy chain of APJ antibody, and Linker represents the peptide linker sequence. 
     Provided herein is a polynucleotide encoding an Elabela fusion protein described herein. 
     Provided herein is a vector, comprising a polynucleotide encoding an Elabela fusion protein described herein. 
     Provided herein is a host cell line, comprising one vector described herein. 
     Provided herein is a pharmaceutical composition, comprising an APJ antibody or an Elabela fusion protein, and a pharmaceutically acceptable carrier described herein. 
     Provided herein is the use of an APJ antibody or an Elabela fusion protein described herein in the preparation of a medicament for treating, preventing or ameliorating PAH and PAH related diseases. 
     Provided herein is the use of an APJ antibody or an Elabela fusion protein described herein in the preparation of a medicament for treating, preventing or ameliorating PH and PH related diseases. 
     Provided herein is the use of an APJ antibody or an Elabela fusion protein described herein in the preparation of a medicament for treating, preventing or ameliorating heart failure and hear failure related diseases. 
     Provided herein is the use of an APJ antibody or an Elabela fusion protein described herein in the preparation of a medicament for treating, preventing or ameliorating type 2 diabetes and related metabolic syndrome. 
     Provided herein is the use of an APJ antibody or an Elabela fusion protein described herein in the preparation of a medicament for simultaneously treating, preventing or ameliorating two and more than two diseases of PAH, PH, type 2 diabetes and related metabolic syndrome or heart failure. 
     Provided herein is a method to treat, prevent, or improve one or more symptoms of PAH, comprising giving subjects a therapeutically effective dose of an APJ antibody or an Elabela fusion protein described herein. 
     Provided herein is a method to treat, prevent, or improve one or more symptoms of PH, comprising giving subjects a therapeutically effective dose of an APJ antibody or an Elabela fusion protein described herein. 
     Provided herein is a method to treat, prevent, or improve one or more symptoms of heart failure, comprising giving subjects a therapeutically effective dose of an APJ antibody or an Elabela fusion protein described herein. 
     Provided herein is a method to treat, prevent, or improve one or more symptoms of type 2 diabetes and related metabolic syndrome, comprising giving subjects a therapeutically effective dose of an APJ antibody or an Elabela fusion protein described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1A, 1B, 1C and 1D  show the results of FACS test of the specific binding of recombinant hAPJ antibodies L1H1 (comprising SEQ ID NO: 59 and SEQ ID NO: 64) and L4H4 (comprising SEQ ID NO: 62 and SEQ ID NO: 67) to hAPJ (at antibody concentration of 500 nM or 4 nM). The gray peaks are negative controls, representing the binding curves of or L1H4 to CHO-DHFR-. The solid line peaks represent the binding curves of L1H1 ( FIGS. 1A and 1B ) or L4H4 ( FIGS. 1C and 1D ) to CHO-DHFR-hAPJ. 
         FIG. 2  shows the activation curves of the reporter gene assay test of hAPJ antibody L5H5 (comprising SEQ ID NO: 63 and SEQ ID NO: 68) and Linker2-Elabela-11 (comprising SEQ ID NO: 123 and SEQ ID NO: 93) fusion protein, and L5H5 and Linker2-EA5 (comprising SEQ NO: 123 and SEQ ID NO: 94) fusion protein to activate Elabela/APJ signaling pathway, and EC 50  is: 3.6 nM and 2.6 nM, respectively. 
         FIG. 3  shows the activation curves of the reporter gene assay test of hAPJ antibody L5H5 and Linker2-EA1 (comprising SEQ ID NO: 123 and SEQ ID NO: 103) fusion protein, and L5H5 and Linker2-EA2 (comprising SEQ ID NO: 123 and SEQ ID NO: 107) fusion protein to activate Elabela/APJ signaling pathway, and EC 50  is 5.0 nM and 2.2 nM, respectively. 
         FIG. 4  shows the activation curves of the reporter gene assay test of hAPJ antibody L5H5 and Linker2-EA3 (comprising SEQ ID NO: 123 and SEQ ID NO: 109) fusion protein, and L5H5 and Linker2-EA4 (comprising SEQ ID NO: 123 and SEQ ID NO: 116) fusion protein to activate Elabela/APJ signaling pathway, and EC  50  is: 20.3 nM and 5.4 nM, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     Unless defined otherwise herein, scientific and technical terms described herein shall have the meanings understood by ordinary technicians in the field. Generally, nomenclatures and techniques related to pharmacology, biology, biochemistry, cell and tissue culture, molecular biology, immunology, microbiology, genetics and protein nucleic acid chemistry as well as hybridization are well-known and commonly used in the field. 
     Standard single-letter or three-letter abbreviations are used in this application to indicate polynucleotide and poly peptide sequences. When the polypeptide sequence is written, the first amino acid residue (N′) with the amino group is at the far left and the last amino acid residue (C′) with the carboxyl group is at the far right, for example, the Elabela fragment and its mutant sequence involved in this invention: SEQ ID NO: 91, SEQ ID NO: 92, SEQ ID NO: 93, SEQ ID NO: 94, SEQ ID NO: 95, SEQ ID NO: 96, SEQ ID NO: 97, SEQ ID NO: 98, SEQ ID NO: 99, SEQ ID NO: 100, SEQ ID NO: 101, SEQ ID NO: 102, SEQ ID NO: 103, SEQ ID NO: 104, SEQ ID NO: 105, SEQ ID NO: 106, SEQ ID NO: 107, SEQ ID NO: 108, SEQ ID NO: 109, SEQ ID NO: 110, SEQ ID NO: 111, SEQ ID NO: 112, SEQ ID NO: 113, SEQ ID NO: 114, SEQ ID NO: 115, SEQ ID NO: 116, SEQ ID NO: 117, SEQ ID NO: 118, SEQ ID NO: 119, SEQ ID NO: 120, and SEQ ID NO: 121. Reverse polypeptide sequence refers to a polypeptide sequence wherein amino acids arranged in a reversed order as to the original, for example, the reverse Elabela fragment and its mutant sequences converted from the above Elabela fragment and its mutant sequences: SEQ ID NO: 125, SEQ ID NO: 126, SEQ ID NO: 127, SEQ ID NO: 128, SEQ ID NO: 129, SEQ ID NO: 130, SEQ ID NO: 131, SEQ ID NO: 132, SEQ ID NO: 133, SEQ ID NO: 134, SEQ ID NO: 135, SEQ ID NO: 136, SEQ ID NO: 137, SEQ ID NO: 138, SEQ ID NO: 139, SEQ ID NO: 140, SEQ ID NO: 141, SEQ ID NO: 142, SEQ ID NO: 143, SEQ ID NO: 144, SEQ ID NO: 145, SEQ ID NO: 146. SEQ ID NO: 147, SEQ ID NO: 148. SEQ ID NO: 149, SEQ ID NO: 150, SEQ ID NO: 151, SEQ ID NO: 152, SEQ ID NO: 153, SEQ ID NO: 154, and SEQ ID NO: 155. The 5′ ends of the upstream chains of single-stranded and double-stranded nucleic acid sequences are on the left and their 3′ ends are on the right. The specific portion of a polypeptide can be represented by an amino acid residue number, such as amino acids 67 to 134, or represented by the actual residue of the site, such as Lys67 to Lys134. The specific polypeptide or polynucleotide sequence can also be described by explaining its difference from the reference sequence. 
     The term “individual” refers to animals, including but not limited to primates (e.g., human), cattle, pig, sheep, goal, horse, dog, cat, rabbit, rat, or mouse. The terms “individual” and “patient” are used interchangeably, for example, to refer to a mammalian individual, such as a human individual, and in one embodiment, referring to a human. 
     The meaning of the term “treatment” includes reducing or eliminating the disorder, disease or condition, or one or more symptoms associated with the disorder, disease or condition; or reducing or eliminating the focus of the disorder, disease or condition. 
     The meaning of the term “prevention” includes delaying and/or relieving the onset of the disorder, disease or condition and/or its accompanying symptoms; preventing the individual from acquiring the disorder, disease or condition; or reducing the risk of the individual acquiring the disorder, disease or condition. 
     The term “control” refers to preventing or slowing the progression, spread, or worsening of a disease, disorder or condition, or one or more symptoms (e.g., pain). Sometimes, individuals benefit from the beneficial effects of preventive or therapeutic agents that do not lead to the cure of the condition, disorder or disease, in one embodiment, the term “control” refers to preventing or slowing the progression, spread, or worsening of osteolytic pain. 
     The meaning of the terms “therapeutically effective amount” and “effective amount” refer to an amount of the compound or combination of compounds that when administered is sufficient to prevent the development of or alleviate one or more symptoms of the disorder, disease or condition. The term “therapeutically effective amount” or “effective amount” also refers to an amount of compound that is sufficient to cause biological or medical response in a biomolecule (e.g., protein, enzyme, RNA, or DNA), cell, tissue, system, animal or human, which are sought for by researchers, veterinarians, doctors, or clinicians. 
     The terms “pharmaceutically acceptable carrier,”“pharmaceutically acceptable excipient,” “physiologically acceptable carrier” “physiologically acceptable excipient” refer to pharmaceutically acceptable materials, compositions or carriers, such as liquid or solid fillers, diluents, solvents or packaging materials. In one embodiment, each component of “pharmaceutically acceptable” is compatible with other components of the pharmaceutical formulation and is suitable for contact with human or animal tissues or organs without excessive toxicity, irritation, allergy reactions, immunogenicity or other problems or complications, which is equivalent to a reasonable benefit/risk ratio. 
     The term “about” or “approximately” refers to an acceptable error determined by those skilled in the field, which partially depends on how the value is measured or determined. In certain embodiments, the term “about” or “approximately” means within 1, 3, or 4 standard deviations. In certain embodiments, the term “about” or “approximately” means within 50%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value or range. 
     The terms “peptide,” “polypeptide,” and “protein” refer to a molecule containing two or more amino acids that are interlinked by a peptide bond. These terms cover, for example, natural and artificial proteins and peptide analogues of protein sequences (such as mutant proteins, variants and fusion proteins) and proteins that are post-transcriptional or otherwise covalent or non-covalent modified. A peptide, polypeptide, or protein can be a monomer or a polymer. 
     The term “polypeptide fragment” refers to a polypeptide that has an amino terminus and/or a carboxyl terminus missing from the corresponding full-length protein. For example, the fragment length can be at least 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 50, 70, 80, 90, 100, 150 or 200 amino acids in length. The fragment length can be, for example, up to 1,000, 750, 500, 250, 200, 175, 150, 125, 100, 90, 80, 70, 60, 50, 40, 30, 20, 15, 14, 13, 12, 11, or 10 amino acids. The fragment may further contain one or more additional amino acids at one end or both, such as amino acid sequences from different natural proteins (e.g., Fc or leucine zipper domains) or artificial amino acid sequences (e.g., artificial joint sequences). 
     Polypeptides in this invention include polypeptides modified for any reason or by any means, for example, by: (1) decreasing proteolysis sensitivity, (2) decreasing oxidation sensitivity, (3) altering the affinity for forming protein complexes, (4) altering binding affinity, and (5) conferring or modifying other physicochemical or functional properties. Analogue contains a mutant protein of a polypeptide. For example, single or multiple amino acid substitutions (e.g., conservative amino acid substitutions) can be made in the naturally occurring sequence (e.g., outside the domain of the polypeptide that forms intramolecular contact). The “conservative amino acid substitution” is one that does not significantly change the structural characteristics of the parent sequence (e.g., the substitution of amino acids shall not destroy the helices present in the parent sequence or interfere with other secondary structural types necessary to give the parent sequence its properties or function). 
     A “mutant” of a polypeptide comprises an amino acid sequence containing the insertion, deletion, and/or replacement of one or more residues in an amino acid sequence relative to another polypeptide sequence. The variants in this invention included fusion proteins. 
     A “derivative” of a polypeptide is a chemically modified poly peptide, for example, by binding to other chemical components such as polyethylene glycol, albumin (such as human serum albumin), phosphorylation, and glycosylation. 
     Unless otherwise indicated, the term “antibody” includes antibodies with two full-length heavy chains and two full-length light chains, as well as their derivatives, variants, fragments, and mutated proteins, instances are listed below. 
     The term “antibody” is a protein that contains the antigen-binding portion and optionally the scaffold or framework portion that allows the antigen-binding portion to adopt a conformation that promotes the binding of the antibody to the antigen. Examples of antibodies include complete antibodies, antibody fragments (such as the antigen-binding portion of an antibody), antibody derivatives, and antibody analogues. For example, the antibody may contain alternative protein scaffolds or artificial scaffolds with transplanted CDRs or derivatives of CDRs. The scaffold includes, but not limited to an antibody-derived scaffold that is introduced, such as one that stabilizes the three-dimensional structure of the antibody, and such as a fully synthetic scaffold for biocompatible polymer. See for example, Korndorfer et al., 2003,  Proteins  53:121-129 Roque et al., 2004,  Biotechnol. Prog.  20:639-654. In addition, the antibody may be either a mock peptide antibody (“PAMs”) or a scaffold containing mock antibodies, therein use of fibrin ligands as scaffolds. 
     Antibodies may have structures such as innate immunoglobulin. “Immunoglobulin” is a tetramer molecule. In natural immunoglobulin, each tetramer consists of two identical polypeptide chain pairs, each pair having a “light” chain (approx. 25 k Da) and a “heavy” chain (approx. 50-70kDa). The amino terminus of each chain includes a variable domain of about 100 to 110 amino acids, which is mainly related to antigen recognition. The carboxyl terminus of each chain determines the constant region mainly associated with the effect of the effectors. The human antibody light chain is divided into κ and λ light chains. The heavy chains were divided into μ, δ, α, or ε, and determined the same type of antigen, such as IgM, IgD, IgG, IgA, and IgE. In light and heavy chains, the variable and constant regions are connected by the “J” region of about 12 or more amino acids, and the heavy chain also includes the “D” region of about 10 more amino acids. Refer to Fundamental Immunology ch.7 (edited by Paul. 2 nd  edition,  Raven Press,  1989). Variable regions of each light/heavy chain pair form antibody binding sites, in this way a complete immunoglobulin has two binding sites. 
     The innate immunoglobulin chains exhibit the same basic structure of a relatively conservative skeletal region (FR) connected by three highly variable regions, also known as the complementary decision region or CDRs. From the N end to the C end, the light and heavy chains contain the structural domains FR1, CDR1, FR2, CDR2, FR3, CDR3, and FR4. The distribution of amino acids in all structural domains was consistent with Kabat et al., in Sequences of Proteins of Immunological Interest, 5 th  edition, U.S. Dept. Of Health and Human Services, PHS, NIH, NIH Publication No. 91-3242, 1991. 
     Unless otherwise specified, “antibody” means either the intact immunoglobulin or the antigen-binding portion of that can compete specifically binding to intact antibody. Antigen-binding portion can be produced by recombinant DNA techniques, and enzymatic or chemical cleavage of intact antibodies. Antigen-binding portion includes, in particular, Fab, Fab′, F(ab) 2 , Fv, structural domain antibodies (dAbs), contain complementary decision area (CDRs), single-chain antibody (scFv), chimeric antibody, double chains antibody (diabodies), three chains antibodies (triabodies), four chains (tetrabodies) and a polypeptide that contains al least a portion of the immunoglobulin that binds to a polypeptide-specific antigen. 
     The Fab fragment is a univalent fragment with V L , V H , C L , and C H1  domains; The F(ab′) 2  fragment is a divalent fragment have two Fab fragments connected by a disulfide bond in the hinge region; Fv fragments have V H  and V L  domains; dAb fragments have V H  domain, V L  domain, or antigen binding fragments of V H  or V L  domain (US patent numbers U.S. Pat. Nos. 6,846,634 and 6,696,245; US patent application public numbers US 2005/0202512, US 2004/0202995, US 2004/0038291, US 2004/0009507, and US 2003/0039958; Ward et al., 1989,  Nature  341:544-546). 
     Single-chain antibody (scFv) is a fusion protein in which the V L  and V H  regions are joined by a connector (for example, a synthetic sequence of amino acid residues) to form a continuous protein antibody, therein the connector is long enough to allow the protein chain to fold back to itself and to form a univalent antigen binding site (See, for example, Bird et al., 1988,  Science  242:423-26; and Huston et al., 1988,  Proc. Natl. Acad. Sci. USA.  85:5879-83). 
     A double-chain antibody is a divalent antibody contain two polypeptide chains, each of which contains the V H  and V L  regions connected by a joint that is so short that it does not allow pairing of the two domains on the same chain. Therefore, each domain is allowed to pair with a complementary domain on another polypeptide chain (See, for example, Holliger et al., 1993,  Proc. Natl. Acad. Sci. USA.  90:6444-48; Poljak et al., 1994,  Structure  2:1121-23). If the two polypeptide chains of the double-stranded antibody are identical, the double-stranded antibody result from their pairing has the same antigen-binding site. Polypeptide chains with different sequences can be used to prepare double-stranded antibodies with different antigen binding sites. Similarly, three-chain and four-chain antibodies are the antibody that contain three and four polypeptide chains and form three and four antigen binding sites, which may be the same or different. 
     This article used the method that Kabat et al., described in Sequences of Proteins of Immunological Interest, 5 th  edition, U.S. Dept. Of Health and Human Services, PHS, NIH, NIH Publication No.91-3242, 1991 to identify the complementary decision region (CDRs) and framework region (FR) of a given antibody. One or more CDRs can be incorporated into a molecule either covalently or noncovalently to make it an antibody. The antibody can incorporate a larger polypeptide chain into the CDR(s). CDR(s) can be covalently attached to another polypeptide chain, or can be non-covalently incorporated. CDRs allows antibodies specifically binding to specific associated antigens. 
     Antibodies can have one or more binding sites. If there is more than one binding site, the binding site can be the same or different from another. For example, natural human immunoglobulin usually has two identical binding sites, while “bi-specific” or “bifunctional” antibodies have two different binding sites, 
     The term “murine antibody” includes antibodies having one or more variable and constant regions derived from mouse immunoglobulin sequences. 
     The term “humanized antibody” is an antibody made by transplanting the sequence of complementary decision regions of mouse antibody molecules into the framework of human anti body variable regions. 
     The terms “antigen-binding domain,” “antigen-binding region,” or “antigen-binding site” are the parts of an antibody that contain amino acid residues that interact with an antigen and contribute to its specificity and affinity for the antigen. For antibodies that bind specifically to their antigens, these terms include at least a part of at least one of its CDR domains. 
     The term “epitope” is the part of a molecule that binds to (for example, by an antibody) the antibody. An epitope may contain a discontinuous part of a molecule (for example, in a polypeptide, the amino acid residues that are discontinuous in the first order of the polypeptide are close enough to each other in the tertiary and quaternary structures of the polypeptide to be bound by an antibody). 
     The “same percentage” of two polynucleotides or two polypeptide sequences is determined using the GAP computer program&#39;s (GCG Wisconsin Package:, part of version 10.3 (Accelrys, San Diego, Calif.)) default parameters comparison sequence. 
     The terms “polynucleotide,” “oligonucleotide” and “nucleic acid” can be used alternatively throughout the full text and include DNA molecules (e.g., cDNA or genomic DNA), RNA molecules (e.g., mRNA), DNA or RNA analogues and their hybrids produced using nucleotide analogues (e.g., peptide nucleic acids and non-natural nucleotide analogues). Nucleic acid molecules can be single or double stranded. In one embodiment, the nucleic acid molecules contained in this invention contain the continuous open reading frames encoding the antibody or its fragments, derivatives, mutant proteins, or variants. 
     If their sequences can be reversed and parallel, two single-stranded nucleotides are “complementary” to each other, so that each nucleotide in one polynucleotide is opposite to the complementary nucleotide in the other, no gaps are introduced and no unpaired nucleotides are found at the 5′ or 3′ ends of each sequence. If two polynucleotides can interbreed under moderately strict conditions, one polynucleotide is “complementary” to the other. Thus, one polynucleotide may be complementary to another polynucleotide, but not its complementary sequence, 
     The term “carrier” is a nucleic acid that can be used to introduce another nucleic acid connected to it into a cell. One type of carrier is a “plasmid” referring to a linear or circular double-stranded DNA molecule that can be attached to an additional nucleic acid segment. Another type of carrier is a viral vector (e.g., replication-detective retroviruses, adenoviruses, and adenoviral companion viruses) in which additional DNA segments can be introduced into the viral genome. Some carriers can replicate autonomously in the host cells into which they are introduced (For example, bacterial carriers containing the origin of bacterial replication and the free-type mammalian carriers). Other carriers (for example, non-free-type mammalian carriers) are integrated into the host cell genome when introduced into the host cell and thus replicate with the host genome. “Expression carrier” is the type of carrier that can guide the expression of selected polynucleotides. 
     If the regulatory sequence affects the expression of a nucleotide sequence (for example, expression level, time, or site), then the nucleotide sequence is “operationally linked” to the regulatory sequence. The “regulatory sequence” is the nucleic acid that affects the expression (for example, expression level, time, or site) of the nucleic acid with which it is operationally linked. Regulatory genes, for example ;  act directly on regulated nucleic acids or through one or more other molecules (e.g., polynucleotides that bind to regulatory sequences and/or nucleic acids). Examples of regulatory sequences include promoters, enhancers, and other expression control elements (e.g., polyadenylation signals). Further examples of regulatory sequences can be described such as Goeddel, 1990,  Gene Expression Technology: Methods in Enzymology , Volume 185, Academic Press, San Diego, Calif.; And Baron et al., 1995,  Nucleic Acids Res.  23:3605-06. 
     The term “host cell” refers to a cell used to express a nucleic acid such as that provided this article. The host cell may be prokaryotes, such as  E. coli , or it can be eukaryotes, such as unicellular eukaryotes (yeast or other fungi, for example), plant cells (such as tobacco or tomato plant cells), animal cells (for example, cells of human, monkey, hamster, rat, mouse or insect) or hybridoma. Usually, the host cell is a culture cell that can be transformed or transfected with a peptide encoding nucleic acid, which can then be expressed in the host cell. The phrase “recombinant host cell” can be used to describe a host cell transformed or transfected with an expected expression of nucleic acid. The host cell may also be a cell that contains the nucleic acid but does not express it at the desired level, unless regulatory sequences are introduced to the host cell so that it is operationally linked to the nucleic acid. It should be understood that the term “host cell” refers to not only the specific subject cell but also to the progeny or possible progeny of that cell. Due to certain modifications occurring in subsequent generations, such as mutations or environmental influences, the progeny may in fact be different from the parent cell but still fall within the scope of the terminology used in this invention. 
     APJ Receptor 
     APJ is a G protein coupled receptor with 7 transmembrane units, consisting of 377 amino acids (O&#39;Dowd, et al., 1993,  Gene.  136:355-60). Up to now, studies show that APJ is widely distributed in the human central nervous system and various peripheral tissues such as the lung, heart, breast, etc. (Kawamata et al., 2001,  Biochem. Biophys. Acta.  1538: 162-71; Medhurst et al., 2003 , J. Neumchem.  84: 1162-72), especially in cardiovascular endothelial cells and cardiac tissue (Kleiz et al., 2005,  Regul. Pept.  126: 233-40). APJ is mainly involved in the regulation of the cardiovascular system, and has also been reported to have important significance in the regulation of insulin and the regulation mechanisms of diabetes and obesity-related diseases (Boucher et al., 2005,  Endocrinology  146:1764-71; Yue et al., 2010,  Am. J. Physiol. Endocrinol. Metab.  298:E59-67). As used herein, “human APJ” and “hAPJ” both refer to human-derived APJ and can be used interchangeably. As used herein, “murine APJ” and “mAPJ” both refer to murine-derived APJ, and can also be used interchangeably. 
     In one embodiment, the antibody presented here is an antibody specifically binding to the human APJ. In one embodiment, the fusion protein presented here is a Elabela fusion protein specifically binding to APJ on cell membrane, where the fusion protein can active Elabela/APJ signal transduction in these cells. In a further embodiment, the fusion protein presented here is an Elabela fusion protein binding to human APJ, where the fusion protein can bind to APJ of other species (such as monkey and mice) and activate Elabela/APJ signal transduction in these species. 
     In one embodiment, the amino acid and polynucleotide sequences of APJ are listed below, with sequence data from the Gene-Bank database of the US national center for biotechnology information and the Uniprot database of the European institute for biological information: 
     Human ( Homo sapiens ) polynucleotide (SEQ ID NO: 55); accession number: X89271. 
     Human ( Homo sapiens ) amino acid (SEQ ID NO: 23); accession number: CAA61546. 
     Monkey ( Rhesus macaque ) polynucleotide (SEQ ID NO: 56); accession number: AF100206. 
     Monkey ( Rhesus macaque ) amino acid (SEQ ID NO: 24); accession number: AAC 72404. 
     Rat ( Rattus norvegicus ) polynucleotide (SEQ ID NO: 57); accession number: BAA033170. 
     Rat ( Rattus norvegicus ) amino acid (SEQ ID NO: 25); accession number: BAA95002. 
     Mouse ( Mus musculus ) polynucleotide (SEQ ID NO: 58); accession number: AJ007612; and mouse ( Mus musculus ) amino acid (SEQ ID NO: 26); accession number: CAB50696. 
     Antibody of Vasoactive Peptide Receptor (APJ Antibody) 
     In one embodiment, provided herein is the APJ antibody. In another embodiment, the APJ antibody provided herein is the complete APJ antibody. In another embodiment, the APJ antibody provided herein is the APJ antibody fragment. In another embodiment, the APJ antibody provided herein is a derivative of APJ antibody. In another embodiment; the APJ antibody provided herein is the APJ antibody mutant protein. In another embodiment, the APJ antibody provided herein is the variant of APJ antibody, 
     In one embodiment, the APJ antibody provided herein comprises 1, 2, 3, 4, 5, or 6 amino acid sequences, each of which is independently selected from the amino acid sequences listed below: 
     a. Light chain CDR 1 amino acid sequences: SEQ ID NO: 1, SEQ ID NO: 4, SEQ ID NO: 7, and SEQ ID NO: 10; 
     b. Light chain CDR2 amino acid sequences: SEQ ID NO: 2, SEQ ID NO: 5, and SEQ ID NO: 8; 
     c. Light chain CDR3 amino acid sequences: SEQ ID NO: 3, SEQ ID NO: 6, SEQ ID NO: 9, and SEQ ID NO: 11; 
     d. Heavy chain CDR1 amino acid sequences: SEQ ID NO: 12, SEQ ID NO: 15, and SEQ ID NO: 18; 
     e. Heavy chain CDR2 amino acid sequences: SEQ ID NO: 13, SEQ ID NO: 16, SEQ ID NO: 19, and SEQ ID NO: 21; and 
     f. Heavy chain CDR3 amino acid sequences: SEQ ID NO: 14, SEQ ID NO: 17, SEQ ID NO: 20, and SEQ ID NO: 22. 
     Table 1 lists the amino acid sequences of the light chain CDRs of the APJ antibody provided herein, as well as the corresponding polynucleotide coding sequences. Table 2 lists the amino acid sequences of the heavy chain CDRs of the APJ antibody provided herein, as well as the corresponding polynucleotide coding sequences. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 light chain CDR amino acid sequences and polvnucleotide coding sequences 
               
            
           
           
               
               
               
               
            
               
                   
                 CDR1 
                 CDR2 
                 CDR3 
               
               
                   
               
               
                 A-1 
                 catgccagtcagaacattcat 
                 aaggcttccaacttgcacaca 
                 caacagggtcacagttatcctct 
               
               
                 nucleotide 
                 gtttggttaagc 
                 (SEQ ID NO: 28) 
                 gacg 
               
               
                   
                 (SEQ ID NO: 27) 
                   
                 (SEQ ID NO: 29) 
               
               
                   
               
               
                 A-1 
                 HASQNIHVWLS 
                 KASNLHT 
                 QQGHSYPLT 
               
               
                 Amino Acid 
                 (SEQ ID NO: 1) 
                 (SEQ ID NO: 2) 
                 (SEQ ID NO: 3) 
               
               
                   
               
               
                 A-2 
                 agatctagtcagagccttatat 
                 aaagtttccaaccgattttct 
                 tctcaaaatacacatgttcctctc 
               
               
                 nucleotide 
                 acactaatggaaacacctattt 
                 (SEQ ID NO: 31) 
                 acg 
               
               
                   
                 acat 
                   
                 (SEQ ID NO: 32) 
               
               
                   
                 (SEQ ID NO: 30) 
                   
                   
               
               
                   
               
               
                 A-2 
                 RSSQSLIYINGNTY 
                 KVSNRFS 
                 SQNTHVPLT 
               
               
                 Amino Acid 
                 LH 
                 (SEQ ID NO: 5) 
                 (SEQ ID NO: 6) 
               
               
                   
                 (SEQ ID NO: 4) 
                   
                   
               
               
                   
               
               
                 A-3 
                 agatctagtcagaatcttgttca 
                 aaagtttccgaccgactttct 
                 tttcaagcttctcatgttccactca 
               
               
                 nucleotide 
                 tagtagtggaaacacccattta 
                 (SEQ ID NO: 34) 
                 cg 
               
               
                   
                 gat 
                   
                 (SEQ ID NO: 35) 
               
               
                   
                 (SEQ ID NO: 33) 
                   
                   
               
               
                   
               
               
                 A-3 
                 RSSQNLVHSSGNTH 
                 KVSDRLS 
                 FQASHVPLT 
               
               
                 Amino Acid 
                 LD 
                 (SEQ ID NO: 8) 
                 (SEQ ID NO: 9) 
               
               
                   
                 (SEQ ID NO: 7) 
                   
                   
               
               
                   
               
               
                 A-4 
                 agatctgatcagagtcttgtac 
                 aaagtttccaaccgattttct 
                 tttcaagcttcacatattccattca 
               
               
                 nucleotide 
                 atagaactggaaatacccattt 
                 (SEQ ID NO: 31) 
                 ca 
               
               
                   
                 agac 
                   
                 (SEQ ID NO: 37) 
               
               
                   
                 (SEQ ID NO: 36) 
                   
                   
               
               
                   
               
               
                 A-4 
                 RSDQSLVHRTGNT 
                 KVSNRFS 
                 FQASHIPFT 
               
               
                 Amino Acid 
                 HLD 
                 (SEQ ID NO: 5) 
                 (SEQ ID NO: 11) 
               
               
                   
                 (SEQ ID NO: 10) 
                   
                   
               
               
                   
               
               
                 A-5 
                 cggtccgaccagtctctggtg 
                 aaggtgagcaataggttctcc 
                 tttcaggccagccacatcccatt 
               
               
                 nucleotide 
                 cacaggaccggcaacacaca 
                 (SEQ ID NO: 39) 
                 cacc 
               
               
                   
                 cctggat 
                   
                 (SEQ ID NO: 40) 
               
               
                   
                 (SEQ ID NO: 38) 
                   
                   
               
               
                   
               
               
                 A-5 
                 RSDQSLVHRTGNT 
                 KVSNRFS 
                 FQASHIPFT 
               
               
                 Amino Acid 
                 HLD 
                 (SEQ ID NO: 5) 
                 (SEQ ID NO: 11) 
               
               
                   
                 (SEQ ID NO: 10) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 heavy chain CDR amino acid sequences and polynucleotide coding sequences 
               
            
           
           
               
               
               
               
            
               
                   
                 CDR1 
                 CDR2 
                 CDR3 
               
               
                   
               
               
                 A-1 
                 ggattccccttcaatatcaatg 
                 cgcataagaagtaaaagtaataatt 
                 ggcccatatttatatgctatggact 
               
               
                 nucleotide 
                 ccatgaac 
                 acgcaacatattataccgattcagt 
                 ac 
               
               
                   
                 (SEQ ID NO: 41) 
                 gaaagac 
                 (SEQ ID NO: 43) 
               
               
                   
                   
                 (SEQ ID NO: 42) 
                   
               
               
                   
               
               
                 A-1 
                 GFPFNINAMN 
                 RIRSKSNNYATYYADS 
                 GPYLYAMDY 
               
               
                 Amino Acid 
                 (SEQ ID NO: 12) 
                 VKD 
                 (SEQ ID NO: 14) 
               
               
                   
                   
                 (SEQ ID NO: 13) 
                   
               
               
                   
               
               
                 A-2 
                 gggttctcattgaccaactatg 
                 gtggtatggggtgacgggaccac 
                 tccaactgggggtcatttacttat 
               
               
                 nucleotide 
                 gtgtgacc 
                 aagttctcattcaactctcatgtcc 
                 (SEQ ID NO: 46) 
               
               
                   
                 (SEQ ID NO: 44) 
                 (SEQ ID NO: 45) 
                   
               
               
                   
               
               
                 A-2 
                 GFSLTNYGVT 
                 VVWGDGTTSSHSTLM 
                 SNWGSFTY 
               
               
                 Amino Acid 
                 (SEQ ID NO: 15) 
                 S 
                 (SEQ ID NO: 17) 
               
               
                   
                   
                 (SEQ ID NO: 16) 
                   
               
               
                   
               
               
                 A-3 
                 ggattcactttcagtagctatg 
                 tccattagtagtggtggaagtatct 
                 ggccgggggggcagccaggc 
               
               
                 nucleotide 
                 ccatgtct 
                 actatccagagagtgtgaagggc 
                 ctggtttgcttac 
               
               
                   
                 (SEQ ID NO: 47) 
                 (SEQ ID NO: 48) 
                 (SEQ ID NO: 49) 
               
               
                   
               
               
                 A-3 
                 GFTFSSYAMS 
                 SISSGGSIYYPESVKG 
                 GRGGSQAWFAY 
               
               
                 Amino Acid 
                 (SEQ ID NO: 18) 
                 (SEQ ID NO: 19) 
                 (SEQ ID NO: 20) 
               
               
                   
               
               
                 A-4 
                 ggattcactttcagtagctatg 
                 tctattagtagtggtggtagtatcta 
                 ggccggggaaatagctccgcct 
               
               
                 nucleotide 
                 ccatgtct 
                 ccttccagacagtttgaagggc 
                 ggtttgcttac 
               
               
                   
                 (SEQ ID NO: 47) 
                 (SEQ ID NO: 50) 
                 (SEQ ID NO: 51) 
               
               
                   
               
               
                 A-4 
                 GFTFSSYAMS 
                 SISSGGSIYLPDSLKG 
                 GRGNSSAWFAY 
               
               
                 Amino Acid 
                 (SEQ ID NO: 18) 
                 (SEQ ID NO: 21) 
                 (SEQ ID NO: 22) 
               
               
                   
               
               
                 A-5 
                 ggcttcacctttagctcctacg 
                 tctatctctagcggcggcagcatct 
                 ggcagaggcaattcctctgcctg 
               
               
                 nucleotide 
                 ccatgagc 
                 acctgcctgactccctgaagggc 
                 gtttgcctat 
               
               
                   
                 (SEQ ID NO: 52) 
                 (SEQ ID NO: 53) 
                 (SEQ ID NO: 54) 
               
               
                   
               
               
                 A-5 
                 GFTFSSYAMS 
                 SISSGGSIYLPDSLKG 
                 GRGNSSAWFAY 
               
               
                 Amino Acid 
                 (SEQ ID NO: 18) 
                 (SEQ ID NO: 21) 
                 (SEQ ID NO: 22) 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the antibody provided herein comprises a sequence different from one of the CDR amino acid sequences listed in Tables 1 and 2 by five, four, three, two or one single amino acid addition, replacement, and/or deletion. In another embodiment, the antibody provided herein contains a sequence different from one of the CDR amino acid sequences listed in Tables 1 and 2 by four, three, two or one single amino acid addition, replacement, and/or deletion. 
     In another embodiment, the antibody provided herein contains a sequence different from one of the CDR amino acid sequences listed in Tables 1 and 2 by three, two or one single amino acid addition, replacement, and/or deletion. 
     In another embodiment, the antibody provided herein contains a sequence different from one of the CDR amino acid sequences listed in Tables 1 and 2 by two or one single amino acid addition, replacement, and/or deletion. 
     In further embodiments, the antibody provided herein contains a sequence that differs from one of the CDR amino acid sequences listed in Tables 1 and 2 by a single amino acid addition, replacement, and/or deletion. 
     In one embodiment, the APJ antibody provided herein comprises 1 or 2 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below:
         a . Light chain CDR1 amino acid sequences: SEQ ID NO: 1, SEQ ID NO: 4, SEQ ID NO: 7, and SEQ ID NO: 10;   b. Heavy chain CDR1 amino acid sequences: SEQ ID NO: 12, SEQ ID NO: 15, and SEQ ID NO: 18.       

     In another embodiment, the APJ antibody provided herein comprises 1 or 2 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below:
         a. Light chain CDR2 amino acid sequences: SEQ ID NO: 2, SEQ ID NO: 5, and SEQ ID NO: 8;   b. Heavy chain CDR2 amino acid sequences: SEQ ID NO: 13, SEQ ID NO: 16, SEQ ID NO: 19, and SEQ ID NO: 21.       

     In another embodiment, the APJ antibody provided herein comprises 1 or 2 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below:
         a. Light chain CDR3 amino acid sequences: SEQ ID NO: 3. SEQ ID NO: 6, SEQ ID NO: 9. and SEQ ID NO: 11;   b. Heavy chain CDR3 amino acid sequences: SEQ ID NO: 14, SEQ ID NO: 17, SEQ ID NO: 20, and SEQ ID NO: 22.       

     In another embodiment, the APJ antibody provided herein comprises 1, 2, 3 or 4 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below:
         a. Light chain CDR1 amino acid sequences: SEQ ID NO: 1, SEQ ID NO: 4, SEQ ID NO: 7, and SEQ ID NO: 10;   b. a Heavy chain CDR1 amino acid sequences: SEQ ID NO: 12, SEQ ID NO: 15, and SEQ ID NO: 18;   c. Light chain CDR2 amino acid sequences: SEQ ID NO: 2, SEQ ID NO: 5, and SEQ ID NO: 8;   d. Heavy chain CDR2 amino acid sequences: SEQ ID NO: 13, SEQ ID NO: 16, SEQ ID NO: 19, and SEQ ID NO: 21.       

     In another embodiment, the APJ antibody provided herein comprises 1, 2, 3 or 4 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below:
         a. Light chain CDR1 amino acid sequences: SEQ ID NO: 1, SEQ ID NO: 4, SEQ ID NO: 7, and SEQ ID NO: 10;   b. Heavy chain CDR1 amino acid sequences: SEQ ID NO: 12, SEQ ID NO: 15, and SEQ ID NO: 18;   c . Light chain CDR3 amino acid sequences: SEQ ID NO: 3, SEQ ID NO: 6, SEQ ID NO: 9, and SEQ ID NO: 11;   d. Heavy chain CDR3 amino acid sequences: SEQ ID NO: 14, SEQ ID NO: 17, SEQ ID NO: 20, and SEQ ID NO: 22.       

     In further embodiments, the APJ antibody provided herein comprises 1, 2, 3 or 4 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below:
         a. Light chain CDR2 amino acid sequences: SEQ ID NO: 2, SEQ ID NO: 5, and SEQ ID NO: 8;   b. Heavy chain CDR2 amino acid sequences: SEQ ID NO: 13, SEQ ID NO: 16, SEQ ID NO: 19, and SEQ ID NO: 21;   c. Light chain CDR3 amino acid sequences: SEQ ID NO: 3, SEQ ID NO: 6, SEQ ID NO: 9, and SEQ ID NO: 11;   d. Heavy chain CDR3 amino acid sequences: SEQ ID NO: 14, SEQ ID NO: 17, SEQ ID NO: 20, and SEQ ID NO: 22.       

     In one embodiment, the APJ antibody provided herein comprises 1, 2 or 3 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below: SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5. SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, SEQ ID NO: 10, and SEQ ID NO: 11. 
     In another embodiment, the APJ antibody provided herein comprises 1, 2 or 3 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below: SEQ ID NO: 12, SEQ ID NO: 13, SEQ NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, and SEQ ID NO: 22. 
     In one embodiment, the APJ antibody provided herein comprises a combination of light chain and heavy chain CDR1 amino acid sequences independently selected from the list below: SEQ ID NO: 1 and SEQ ID NO: 12, SEQ ID NO: 4 and SEQ ID NO: 15, SEQ ID NO: 7 and SEQ ID NO: 18, and SEQ ID NO: 10 and SEQ ID NO: 18. 
     In another embodiment, the APJ antibody provided herein comprises a combination of light chain and heavy chain CDR2 amino acid sequences independently selected from the list below: SEQ ID NO: 2 and SEQ ID NO: 13, SEQ ID NO: 5 and SEQ ID NO: 16, SEQ ID NO: 8 and SEQ ID NO: 19, and SEQ ID NO: 5 and SEQ NO: 21. 
     In further embodiments, the APJ antibody provided herein comprises a combination of light chain and heavy chain CDR3 amino acid sequences independently selected from the list below: SEQ ID NO: 3 and SEQ ID NO: 14, SEQ ID NO: 6 and SEQ ID NO: 17, SEQ ID NO: 9 and SEQ ID NO: 20, and SEQ ID NO: 11 and SEQ ID NO: 22. 
     In one embodiment, the APJ antibody provided herein comprises: 
     a. A combination of light and heavy chain CDR1 amino acid sequences independently selected from the list below: SEQ ID NO: 1 and SEQ ID NO: 12, SEQ ID NO: 4 and SEQ ID NO: 15, SEQ ID NO: 7 and SEQ ID NO: 18, and SEQ ID NO: 10 and SEQ ID NO: 18; and 
     b. A combination of light and heavy chain CDR2 amino acid sequences independently selected from the list below: SEQ ID NO: 2 and SEQ ID NO: 13, SEQ ID NO: 5 and SEQ ID NO: 16, SEQ ID NO: 8 and SEQ ID NO: 19, and SEQ ID NO: 5 and SEQ ID NO: 21. 
     In one embodiment, the APJ antibody provided herein comprises: 
     a. A combination of light and heavy chain CDR1 amino acid sequences independently selected from the list below: SEQ ID NO: 1 and SEQ ID NO: 12, SEQ ID NO: 4 and SEQ ID NO: 15, SEQ ID NO: 7 and SEQ ID NO: 18, and SEQ ID NO: 10 and SEQ ID NO: 18; and 
     b. A combination of light and heavy chain CDR3 amino acid sequences independently selected from the list below: SEQ ID NO: 3 and SEQ ID NO: 14, SEQ ID NO: 6 and SEQ ID NO: 17, SEQ ID NO: 9 and SEQ ID NO: 20, and SEQ ID NO: 11 and SEQ ID NO: 22. 
     In one embodiment, the APJ antibody provided herein comprises: 
     a. A combination of light and heavy chain CDR2 amino acid sequences independently selected from the list below: SEQ ID NO: 2 and SEQ ID NO: 13, SEQ ID NO: 5 and SEQ ID NO: 16, SEQ ID NO: 8 and SEQ ID NO: 19, and SEQ ID NO: 5 a ld SEQ ID NO: 21; 
     b. A combination of light and heavy chain CDR3 amino acid sequences independently selected from the list below: SEQ ID NO: 3 and SEQ ID NO: 14, SEQ ID NO: 6 and SEQ ID NO: 17, SEQ ID NO: 9 and SEQ ID NO: 20. and SEQ ID NO: 11 and SEQ ID NO: 22. 
     In further embodiments, the APJ antibody provided herein comprises: 
     a. A combination of light and heavy chain CDR1 amino acid sequences independently selected from the list below: SEQ ID NO: 1 and SEQ ID NO: 12, SEQ ID NO: 4 and SEQ ID NO: 15, SEQ ID NO: 7 and SEQ ID NO: 18, and SEQ ID NO: 10 and SEQ ID NO: 18; 
     b. A combination of light and heavy chain CDR2 amino acid sequences independently selected from the list below: SEQ ID NO: 2 and SEQ ID NO: 13, SEQ ID NO: 5 and SEQ ID NO: 16, SEQ ID NO: 8 and SEQ ID NO: 19, and SEQ ID NO: 5 and SEQ ID NO: 21; and 
     c. A combination of light and heavy chain CDR3 amino acid sequences independently selected from the list below: SEQ ID NO: 3 and SEQ ID NO: 14, SEQ ID NO: 6 and SEQ ID NO: 17, SEQ ID NO: 9 and SEQ ID NO: 20, and SEQ ID NO: 11 and SEQ ID NO: 22. 
     In one embodiment, the antibody provided herein comprises: 
     (a) Light chain CDR1 amino acid sequence: SEQ ID NO: 1; 
     Light chain CDR2 amino acid sequence: SEQ ID NO: 2; 
     Light chain CDR3 amino acid sequence: SEQ ID NO: 3; 
     Heavy chain CDR1 amino acid sequence: SEQ ID NO: 12; 
     Heavy chain CDR2 amino acid sequence: SEQ ID NO: 13; and 
     Heavy chain CDR3 amino acid sequence: SEQ ID NO: 14; 
     (b) Light chain CDR1 amino acid sequence: SEQ ID NO: 4; 
     Light chain CDR2 amino acid sequence: SEQ ID NO: 5; 
     Light chain CDR3 amino acid sequence: SEQ ID NO: 6; 
     Heavy chain CDR1 amino acid sequence: SEQ ID NO: 15; 
     Heavy chain CDR2. amino acid sequence: SEQ ID NO: 16; and 
     Heavy chain CDR3 amino acid sequence: SEQ ID NO: 17; 
     (c) Light chain CDR1 amino acid sequence: SEQ ID NO: 7; 
     Light chain CDR2 amino acid sequence: SEQ ID NO: 8; 
     Light chain CDR3 amino acid sequence: SEQ ID NO: 9; 
     Heavy chain CDR1 amino acid sequence: SEQ ID NO: 18; 
     Heavy chain CDR2 amino acid sequence: SEQ ID NO: 19; and 
     Heavy chain CDR3 amino acid sequence: SEQ ID NO: 20; 
     (d) Light chain CDR1 amino acid sequences: SEQ ID NO: 10; 
     Light chain CDR2 amino acid sequences: SEQ ID NO: 5; 
     Light chain CDR3 amino acid sequences: SEQ ID NO: 11; 
     Heavy chain CDR1 amino acid sequence: SEQ ID NO: 18; 
     Heavy chain CDR2 amino acid sequence: SEQ ID NO: 21; and 
     Heavy chain CDR3 amino acid sequence: SEQ ID NO: 22; 
     In another embodiment, the antibodies provided herein comprises: 
     Light chain CDR1 amino acid sequence: SEQ ID NO: 10; 
     Light chain CDR2 amino acid sequence: SEQ ID NO: 5; 
     Light chain CDR3 amino acid sequence: SEQ ID NO:  11 ; 
     Heavy chain CDR1 amino acid sequence: SEQ ID NO: 18; 
     Heavy chain CDR2 amino acid sequence: SEQ ID NO: 21; and 
     Heavy chain CDR3 amino acid sequence: SEQ ID NO: 22; 
     In one embodiment, the APJ antibody provided herein comprises one or two amino acid sequences, wherein each amino acid sequence is independently selected from the list below: 
     a. Light chain variable domain amino acid sequences: SEQ ID NO: 59 (L1), SEQ ID NO: 60 (L2), SEQ ID NO: 61 (L3), SEQ ID NO: 62 (L4), and SEQ ID NO: 63 (L5); and an amino acid sequence that is at least 80%, at least 85%, at least 90%, or at least 95% identical to any above sequence; and 
     b. Heavy chain variable domain amino acid sequence: SEQ ID NO: 64 (H1), SEQ ID NO: 65 (H2), SEQ ID NO: 66 (H3), SEQ ID NO: 67 (H4), and SEQ ID NO: 68 (H5); and an amino acid sequence that is at least 80%, at least 85%, at least 90%, or at least 95% identical to any above sequence. 
     In another embodiment, the polynucleotide coding sequence for the APJ antibody provided herein comprises one or two polynucleotide coding sequences, wherein each polynucleotide coding sequence is independently selected from the polynucleotide sequences listed below: 
     a. Light chain variable domain polynucleotide coding sequences: SEQ ID NO: 69, SEQ ID NO: 70, SEQ ID NO: 71, SEQ ID NO: 72, and SEQ ID NO: 73; and a polynucleotide sequence that is at least 80%, at least 85%, at least 90%, or at least 95% identical to an above sequence; and 
     b. Heavy chain variable domain polynucleotide coding sequences: SEQ ID NO: 74, SEQ ID NO: 75, SEQ ID NO: 76, SEQ ID NO: 77, and SEQ ID NO: 78; and a polynucleotide sequence that is at least 80%, at least 85%, at least 90%, or at least 95% identical to any above sequence. 
     In one embodiment, the APJ antibody provided herein comprises an amino acid sequence independently selected from the list below: SEQ ID NO: 59, SEQ ID NO: 60, SEQ ID NO: 61, SEQ ID NO: 62, and SEQ ID NO: 63. 
     In another embodiment, the APJ antibody provided herein comprises an amino acid sequence independently selected from the list below: SEQ ID NO: 64, SEQ ID NO: 65. SEQ ID NO: 66, SEQ ID NO: 67, and SEQ ID NO: 68. 
     In one embodiment, the APJ antibody provided herein comprises a combination of amino acid sequences independently selected from the light chain and heavy chain variable domain amino acid sequences listed below: SEQ ID NO: 59 and SEQ ID NO: 64 (L1H1), SEQ ID NO: 60 and SEQ ID NO: 65 (L2H2), SEQ ID NO: 61 and SEQ ID NO: 66 (L3H3), SEQ ID NO: 62 and SEQ ID NO: 67 (L4H4), and SEQ ID NO: 63 and SEQ ID NO: 68 (L5H5). 
     The symbol “LxHy” can also be used herein to refer to the APJ antibody provided herein, wherein “x” corresponds to the light chain variable region sequence code and “y” corresponds to the heavy chain variable region sequence code. For example, L2H2 is a complete antibody with a light chain variable region comprising the SEQ ID NO: 60 (L2) amino acid sequence and a heavy chain variable region comprising the SEQ ID NO: 65 (H2) amino acid sequence. 
     In one embodiment, the APJ antibody provided herein comprises 1 or 2 amino acid sequences, wherein each amino acid sequence is independently selected from the amino acid sequences listed below: 
     a. Light chain constant domain amino acid sequences: SEQ ID NO: 79, SEQ ID NO: 80 and SEQ ID NO: 81; and 
     b. Heavy chain constant domain amino acid sequences: SEQ ID NO: 82, SEQ ID NO: 83 and SEQ ID NO: 84. 
     In another embodiment, the APJ antibody provided herein comprises a combination of amino acid sequences independently selected from the light chain and heavy chain constant domain amino acid sequences listed below: SEQ ID NO: 79 and SEQ ID NO: 82, SEQ ID NO: 80 and SEQ ID NO: 83, SEQ ID NO: 80 and SEQ ID NO: 84, SEQ ID NO: 81 and SEQ ID NO: 83., and SEQ ID NO: 81 and SEQ ID NO: 84. 
     In one embodiment, the APJ antibody provided herein comprise the light and heavy chain CDRs listed herein, and the amino acid sequences of the FRs (framework). The amino acid sequences of FRs are contained in the light chain or the heavy chain variable domain and are not separately displayed. In one embodiment, the antibody comprises a light chain CDR1 sequence listed herein. In another embodiment, the antibody comprises a light chain CDR2 sequence listed herein. In another embodiment, the antibody comprises a light chain CDR3 sequence listed herein. In another embodiment, the antibody comprises a heavy chain CDR1 sequence listed herein. In another embodiment, the antibody comprises a heavy chain CDR2 sequence listed herein. In another embodiment, the antibody comprises a heavy chain CDR3 sequence listed herein. In another embodiment, the antibody comprises a light chain FR1 sequence herein. In another embodiment, the antibody comprises a light chain FR2 sequence herein. In another embodiment, the antibody comprises a light chain FR3 sequence herein. In another embodiment, the antibody comprises a light chain FR4 sequence herein. In another embodiment, the antibody comprises a heavy chain FR1 sequence herein. In another embodiment, the antibody comprises a heavy chain FR2 sequence herein. In another embodiment, the antibody comprises a heavy chain FR3 sequence herein. In a further embodiment, the antibody comprises a heavy chain FR4 sequence herein. 
     In one embodiment, a light chain CDR3 sequence of the antibody differs from SEQ ID NO: 11 of the light chain CDR3 amino acid sequence illustrated above by no more than 6, 5, 4, 3, 2 or 1 amino acid addition(s), substitution(s), and/or deletion(s). In another embodiment, a heavy chain CDR sequence of the antibody differs from SEQ ID NO: 22 of the heavy chain CDR3 amino acid sequence illustrated above by no more than 6, 5, 4, 3, 2 or 1 amino acid addition(s), substitution(s), and/or deletion(s). In a further embodiment, a light chain CDR3 sequence of the antibody differs from SEQ ID NO: 11 of the light chain CDR3 amino acid sequence illustrated above by no more than 6, 5, 4, 3, 2 or 1 amino acid addition(s), substitution(s), and/or deletion(s), and a heavy chain CDR3 sequence of the antibody differs from SEQ ID NO: 22 of the heavy chain CDR3 amino acid sequence illustrated above by no more than 6, 5, 4, 3, 2 or 1 amino acid addition(s), substitution(s), and/or deletion(s). 
     In one embodiment, the APJ antibody provided herein comprises a light chain variable domain amino acid sequence selected from L4 (SEQ ID NO: 62) or L5 (SEQ ID NO: 63) light chain variable domain sequence listed herein. In one embodiment, the amino acid sequence of the light chain variable domain of the APJ antibody differs from the amino acid sequence of one light chain variable domain of L4 (SEQ ID NO: 62) or L5 (SEQ ID NO: 63) by 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 amino acid difference, wherein the difference in each sequence is independently a deletion, insertion or substitution of an amino acid residue. In another embodiment, the light chain variable domain amino acid sequence of the APJ antibody comprises an amino acid sequence at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least 99% identical to the amino acid sequence of one light chain variable domain of L4 (SEQ ID NO: 62) or L5 (SEQ ID NO: 63). In another embodiment, the polynucleotide coding sequence of the light chain variable domain of the APJ antibody comprises a nucleotide coding sequence at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 97% or at least 99% identical to one polynucleotide coding sequence of L4 (SEQ ID NO: 72) or L5 (SEQ ID NO: 73). In another embodiment, the polynucleotide coding sequence of the light chain variable domain of the APJ antibody comprises a polynucleotide sequence hybridized under moderate conditions with a complementary polynucleotide coding sequence of one light chain variable domain of L4 (SEQ ID NO: 72) or L5 (SEQ ID NO: 73). In a further embodiment, the polynucleotide coding sequence of the light chain variable domain of the APJ antibody comprises a polynucleotide sequence hybridized under stringent conditions with a complementary polynucleotide coding sequence of one light chain variable domain of L4 (SEQ ID NO: 72) or L5 (SEQ ID NO: 73). 
     In one embodiment, the APJ antibody provided herein comprises a heavy chain variable domain amino acid sequence selected from H4 (SEQ ID NO: 67) or H5 (SEQ ID NO: 68) heavy chain variable domain sequences listed herein. In another embodiment, the heavy chain variable domain amino acid sequence of the APJ antibody differs from one heavy chain variable domain amino acid sequence of H4 (SEQ ID NO: 67) or H5 (SEQ ID NO: 68) by 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 amino acid difference, wherein the difference in each sequence is independently a deletion, insertion or substitution of one amino acid residue. In another embodiment, the heavy chain variable domain amino acid sequence of the APJ antibody comprises an amino acid sequence at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least 99% identical to one heavy chain variable domain sequence of H4 (SEQ ID NO: 67) or H5 (SEQ ID NO: 68). In another embodiment, the heavy chain variable domain of the APJ antibody comprises a polynucleotide coding sequence at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least 99% identical to one heavy chain variable domain polynucleotide coding sequence of H4 (SEQ ID NO: 77) or H5 (SEQ ID NO: 78). In another embodiment, the polynucleotide coding sequence of the APJ antibody heavy chain variable domain comprises a polynucleotide sequence hybridized to a complementary polynucleotide coding sequence of one heavy chain variable domain of H4 (SEQ ID NO: 77) or H5 (SEQ ID NO: 78) under moderately strict conditions. In one embodiment, the polynucleotide coding sequence of the APJ antibody heavy chain variable domain comprises a polynucleotide sequence hybridized under stringent conditions with a complementary polynucleotide coding sequence of one heavy chain variable domain of H4 (SEQ ID NO: 77) or H5 (SEQ ID NO: 78). 
     In one embodiment, the antibody provided herein is an antibody comprising a combination of L1H1 (SEQ ID NO: 59 and SEQ ID NO: 64), L2H2 (SEQ ID NO: 60 and SEQ ID NO: 65), L3H3 (SEQ ID NO: 61 and SEQ ID NO: 66), L4H4 (SEQ ID NO: 62 and SEQ ID NO: 67), or L5H5 (SEQ ID NO: 63 and SEQ ID NO: 68), or of a desired phenotype (for example, IgA, IgG1, IgG2a, IgG2b, IgG3, IgE, or IgD), or a Fab or F(ab′)2 fragment thereof. 
     In one embodiment, the antibody provided herein is an antibody comprising a combination of L4H4 (SEQ ID NO: 62 and SEQ ID NO: 67) or L5H5 (SEQ ID NO: 63 and SEQ ID NO: 68), or of an exchanged isotype thereof (for example, IgA, IgG1, IgG2a, IgG2b, IgG3, IgM, IgE, or IgD), or a Fab or F(ab′)2 fragment thereof. 
     The antibodies provided herein can comprise any of the known constant regions of the field. The light chain constant region can be, for example, κ or λ light chain constant region, such as a mouse κ or λ light chain constant region. The heavy chain constant region can be, for example, an α, δ, ε, γ, or μ heavy chain constant region, such as the mouse α, δ, ε, γ, or μ heavy chain constant region. In an embodiment, the light or heavy chain constant region is a fragment, derivative, variant, or mutant of the natural constant region. 
     In one embodiment, the antibody provided herein further comprises a human light chain κ constant domain or fragment thereof. The amino acid sequence of the light chain constant region is as follows: 
     Human light chain κ constant domain amino acid sequence: (SEQ ID NO: 80); and 
     Human light chain λ constant domain amino acid sequence: (SEQ ID NO: 81). 
     In another embodiment, the antibody provided herein further comprises a human heavy chain heavy chain, or a fragment thereof. The heavy chain constant domain amino acid sequences are provided as follows: 
     Human heavy chain constant domain amino acid sequence (h1gG2): (SEQ ID NO: 83), and 
     Human heavy chain constant domain amino acid sequence (h1gG4): (SEQ ID NO: 84). 
     In one embodiment, the heavy and light chain amino acid sequences of the APJ antibody provided herein are as follows: SEQ NO:157 and SEQ ID NO:158. 
     In one embodiment, the APJ antibodies provided herein are selected from mouse-derived antibodies, humanized antibodies, chimeric antibodies, monoclonal antibodies, polyclonal antibodies, recombinant antibodies, antigen-binding antibody fragments, single-chain antibodies, double-chain antibodies, triple-chain antibodies, quadruple-chain antibodies, Fab fragments, F(ab′)x fragments, structural domain antibodies, IgD antibodies, IgE antibodies, IgM antibodies, IgG1 antibodies, IgG2 antibodies, IgG3 antibodies, or IgG4 antibodies. 
     In another embodiment, the APJ antibody provided herein is an APJ monoclonal antibody. 
     In another embodiment, the APJ antibody provided herein is a monoclonal antibody comprising a combination of amino acid sequences selected from the list below: SEQ ID NO: 59 and SEQ ID NO: 64, SEQ ID NO: 60 and SEQ ID NO: 65, SEQ ID NO: 61 and SEQ ID NO: 66, SEQ ID NO: 62 and SEQ ID NO: 67, and SEQ ID NO: 63 and SEQ ID NO: 68. 
     In one embodiment, the APJ antibody provided herein is a mouse APJ antibody. In another embodiment, the APJ antibody provided herein is a humanized APJ antibody. 
     In one embodiment, the K d  value of the APJ antibody provided herein is about 1 nM to 200 nM or about 1 nM to 100 nM. 
     Antibodies and Antibody Fragments 
     In one embodiment, the antibody provided herein is a full-length antibody (including polyclonal, monoclonal, chimeric, humanized or human antibody with full length heavy and/or light chains). In another embodiment, the antibody provided herein is an antibody fragment, for example, F(ab′) 2 , Fab, Fab′, Fv, Fc, or Ed fragment, and can be incorporated into single domain antibodies, single-chain antibodies, maxibodies, minibodies, intrabodies, double-chain antibodies, triple-chain antibodies, tetra-chain antibodies, v-NAR, or bis-scFv (see e.g., Hollinger and Hudson, 2005,  Nature Biotechnology,  23, 9, 1126-1136). In another embodiment, the antibody provided herein also includes antibody polypeptides such as those disclosed in U.S. Pat. No. 6,703,199, including fibronectin polypeptide monobodies. In another embodiment, the antibody provided herein also includes other antibody polypeptides disclosed in U.S. Patent Publication 2005/0238646, which are single-chain polypeptides. 
     In one embodiment, the variable regions of the IgG gene expressing a monoclonal antibody of interest in a hybridoma are amplified using nucleotide primers. These primers can be synthesized by one of ordinary skill in the field, or can be purchased from commercially available vendors, which synthesizes primers for mouse and human variable regions including, among others, primers for V Ha , V Hb , V Hc , V Hd , C H1 , V L  and C L  regions. These primers can be used to amplify heavy or light chain variable regions, which can then be inserted into vectors such as IMMUNOZAP™H or IMMUNOZAP™L (Stratagene), respectively. These vectors can then be introduced into  E. coli , yeast, or mammalian-based systems for expression. Large amounts of a single-chain protein containing a fusion of the V H  and V L  regions can be produced using these methods (see Bird el al., 1988,  Science  242:423-426). 
     It should be understood by one skilled in the field that certain proteins, such as antibodies, can undergo a variety of post-translational modifications. The types and extents of these modifications often depend on the host cell lines used to express the protein as well as the culture conditions. Such modifications can include variations in glycosylation, methionine oxidation, diketopiperizine formation, aspartate isomerization and asparagine deamidation. The carboxyl-terminal basic residue (such as lysine or arginine) may be lost due to the frequent modification of carboxypeptidases (as described in Harris, R. J., 1995,  Journal of Chromatography  705:129-134). 
     A common method for production of a murine monoclonal antibody is by hybridoma cells. Monoclonal antibodies can be isolated and purified by a variety of well-established techniques. Such isolation techniques include affinity chromatography with Protein-A Sepharose, size-exclusion chromatography, and ion-exchange chromatography (see, e.g., Coligan at pages 2.7.1-2.7.12 and pages 2.9.1-2.9.3; Baines et al., “Purification of Immunoglobulin G (IgG),”  Methods in Molecular Biology, Vol.  10, pages 79-104 (The Humana Press, Inc. 1992)). A monoclonal antibody can be purified by affinity chromatography using an appropriate ligand selected based on particular properties of the antibody (e.g., heavy or light chain isotype, binding specificity, etc.). Examples of suitable ligands immobilized on a solid support include Protein A, Protein G, an anti-constant region (light chain or heavy chain) antibody, an anti-idiotype antibody, and a TGF-β binding protein, or a fragment or variant thereof. 
     Molecular evolution of the complementarity determining regions (CDRs) in the center of the antibody binding site also has been used to isolate antibodies with increased affinities, for example, antibodies having increased affinities for c-erbB-2, as described by Schier et al., 1996,  J. Mol. Biol.  263:551-567. Accordingly, such techniques are useful in preparing antibodies against human APJ. 
     Antibodies against human APJ can be used, for example, in assays to detect the presence of APJ, either in vitro or in vivo. 
     Antibodies can also be prepared by any of the conventional techniques. For example, they can be purified from cells that naturally express them (e.g., an antibody can be purified from a hybridoma that produces it) or produced in recombinant expression systems using any technique known in the field. For example, Monoclonal Antibodies, Hybridomas: A New Dimension in Biological Analyses, Kennet et al. (eds.), Plenum Press, New York (1980); and Antibodies: A Laboratory Manual, Harlow and Land (eds.), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., (1988). This is discussed in the nucleic acid section below. 
     Antibodies can be prepared and screened for desired properties by any known techniques. Some techniques relate to the isolation of nucleic acids encoding polypeptide chains (or portions thereof) of related antibodies (e.g., anti-APJ antibodies) and manipulation of nucleic acid. Nucleic acids can be fused with another relevant nucleic acid or modified by recombinant DNA techniques (e.g., induced mutations or other conventional techniques) to add, delete or replace one or more amino acid residues. 
     Where it is desired to improve the affinity of antibodies according to the invention containing one or more of the above-mentioned CDRs, such antibodies can be obtained by a number of affinity maturation protocols, including maintaining the CDRs (Yang et al., 1995 , J. Mol. Biol.,  254:392-403), chain shuffling (Marks et al., 1992,  Bio/Technology,  10:779-783), use of mutation strains of  E. coli . (Low et al., 1996,  J. Mol. Biol.,  250:350-368), DNA shuffling (Patten et at., 1997,  Curr: Opin. Biotechnol.,  8:724-733), phage display (Thompson et al., 1996,  J. Mot. Biol.,  256:7-88) and additional PCR techniques (Crameri et al., 1998.  Nature,  391:288-291). All of these methods of affinity maturation are discussed in Vaughan et al., 1998.  Nature Biotechnology,  16:535-539. 
     In one embodiment, fragments of the APJ antibody are provided herein. Such fragments can comprise entirely antibody-derived sequences or additional sequences. Examples of antigen binding fragments include Fab, F(ab′)2, single chain antibodies, diabodies, tribodies, tetrabodies, and domain antibodies. Other examples are provided in Lunde et al., 2002,  Biochem. Soc. Trans.  30:500-06. 
     Single chain antibodies can be formed by linking heavy and light chain variable domain (Fv region) fragments via an amino acid bridge (short peptide linker), resulting in a single polypeptide chain. Such single-chain Fvs (scFvs) have been prepared by fusion DNA encoding a peptide linker between DNAs encoding the two variable domain polypeptides (V L  and V H ). The resulting polypeptides can fold back on themselves to form antigen-binding monomers, or they can form multimers (e.g., dimers, trimers, or tetramers), depending on the length of a flexible linker between the two variable domains (Kortt et at., 1997,  Prot. Eng.  10:423; Kortt et at., 2001,  Biomol. Eng.  18:95-108). By combining different V L  and V H -comprising polypeptides, multimeric scFvs that bind to different epitopes can be formed (Kriangkum et al., 2001,  Biomol. Eng.  18:31-40). Techniques developed for the production of single chain antibodies include those described in U.S. Pat. No. 4946778; Bird, 1988,  Science  242:423; Huston et al., 1988,  Proc. Natl. Acad. Sci. USA  85:5879-83; Ward et al., 1989,  Nature  334:544-6; de Graaf et al., 2002,  Methods Mol. Biol.  178:379-87. Single chain antibodies derived from antibodies provided herein including, but not limited to, scFvs comprising the variable domain combination L1H1, are encompassed by the present invention. 
     Antigen binding fragments derived from an antibody can also be obtained, for example, by proteolytic hydrolysis of the antibody, for example, pepsin or papain digestion of a whole antibody according to conventional methods. By way of example, antibody fragments can be produced by enzymatic cleavage of antibodies with pepsin to provide a SS fragment termed F(ab′)2. This fragment can be further cleaved using a thiol reducing agent to produce 3.5S Fab′ monovalent fragments. Optionally, the cleavage reaction can be performed using a blocking group for the sulfhydryl groups that results in cleavage of disulfide linkages. As an alternative, an enzymatic cleavage using papain produces two monovalent Fab fragments and an Fc fragment directly. These methods are described, for example, by Goldenberg, U.S. Pat. No. 4,331,647, Nisonoffet et al., 1960,  Arch. Biochem. Biophys.  89:230; Porter, 1959,  Biochem. J.  73 : 119 ;  Edelman et at., Methods in Enzymology 1:422 (Academic Press 1967); and by Andrews, S. M. and Titus, J. A. in Current Protocols in Immunology (Cohgan J. E., et al., eds, John Wiley &amp; Sons, New York, 2003), pages 2.8.1-2.8.10 and 2.10A.1-2.10A.5. Other methods for cleaving antibodies, such as separating heavy chains to form monovalent light-heavy chain fragments (Fd), further cleaving of fragments, or other enzymatic, chemical, or genetic techniques can also be used, so long as the fragments bind to the antigen that is recognized by the intact antibody. 
     Another form of an antibody fragment is a peptide comprising one or more complementarity determining regions (CDRs) of an antibody. CDRs can be obtained by constructing polynucleotides that encode the CDRs. Such polynucleotides are prepared, for example, by using the polymerase chain reaction to synthesize the variable region using mRNA of antibody-producing cells as a template (see, for example, Larrick et al., 1991,  Methods: A Companion to Methods in Enzymology  2:106; Courtenay-Luck, “Genetic Manipulation of Monoclonal Antibodies,” Monoclonal Antibodies: Production, Engineering and Clinical Application, Ritter et al. (eds.), page 166 (Cambridge University Press 1995); and Ward el al., “Genetic Manipulation and Expression or Antibodies,” in Monoclonal Antibodies: Principles and Applications, Birch et al., (eds.), page 137 (Wiley-Liss, Inc. 1995)). The antibody fragment further can comprise at least one variable region domain of an antibody described herein. Thus, for example, the V region domain can be monomeric and be a V H  or V L  domain, which can bind to APJ with an affinity of 1×10 −7  M or more as described below. 
     The variable region domain can be any naturally occurring variable domain or an engineered version thereof. By engineered version is meant a variable region domain that has been created using recombinant DNA engineering techniques. Such engineered versions include those created, for example, from a specific antibody variable region by insertions, deletions, or changes in or to the amino acid sequences of the specific antibody. Particular examples include engineered variable region domains containing at least one CDR and optionally one or more framework amino acids from a first antibody and the remainder of the variable region domain from a second antibody. 
     The variable region domain can be covalently attached at a C-terminal amino acid to at least one other antibody domain or a fragment thereof. Thus, for example, a V H  domain that is present in the variable region domain can be linked to an immunoglobulin C H1  domain or a fragment thereof. Similarly, a V L  domain can be linked to a C κ  domain or a fragment thereof. In this way, for example, the antibody can be a Fab fragment, wherein the antigen binding domain contains associated V H  and V L  domains covalently linked at their C-termini to a C H1  and C κ  domain, respectively. The C H1  domain can be extended with further amino acids, for example to provide a hinge region or a portion of a hinge region domain as found in a Fab′ fragment, or to provide further domains, such as antibody C H2  and Ci H3  domains. 
     Derivatives and Variants of Antibodies 
     The nucleotide sequences of L1 and H1 can be altered, for example, by random mutagenesis or by site-directed mutagenesis (e.g., oligonucleotide-directed site-specific mutagenesis) to create an altered polynucleotide comprising one or more particular nucleotide substitutions, deletions, or insertions as compared to the non-mutated polynucleotide. Examples of techniques for making such alterations are described in Walder et al., 1986,  Gene  42:133; Bauer et al., 1985, Gene 37:73; Craik, 1985, BioTechniques, 3:12-19; Smith et al., 1981,  Genetic Engineering: Principles and Methods, Plenum Press ; and U.S. Pat. Nos. 4,518,584 and 4,737,462. These and other methods can be used to make, for example, derivatives of APJ antibodies that have a desired property, for example, an increase in affinity, avidity, or specificity for APJ or in vivo or in vitro stability, or reduced in vivo side-effects as compared to the underivatized antibody. 
     Other derivatives of anti-APJ receptor antibodies within the scope or this invention include covalent or aggregative conjugates of anti-APJ receptor antibodies or fragments thereof, with other proteins or polypeptides, such as by expression of recombinant fusion proteins comprising heterologous polypeptides fused to the N-terminus or C-terminus or an anti-APJ antibody polypeptide. For example, the conjugated peptide can be a heterologous signal (or leader) polypeptide, e.g., the yeast alpha-factor leader or a peptide such as an epitope tag. An antibody containing fusion proteins can comprise peptides added to facilitate purification or identification of antigen binding protein (e.g., poly-His). An antibody also can be linked to the FLAG peptide as described in Hopp et al., 1988,  Bio/Technology  6:1204, and U.S. Pat. No. 5,011,912. The FLAG peptide is highly antigenic and provides an epitope reversibly bound by a specific monoclonal antibody (mAb), enabling rapid assay and facile purification of an expressed recombinant protein. Reagents useful for preparing fusion proteins in which the FLAG peptide is fused to a given polypeptide are commercially available (Sigma, St. Louis, Mo.). In another embodiment, oligomers that contain one or more antibodies can be employed as APJ receptor antagonists. Oligomers can be in the form of covalently-linked or non-covalently-linked dimers, trimers, or higher oligomers. Oligomers comprising two or more antibodies are contemplated for use, with one example being a homodimer. Other oligomers include heterodimers, homotrimers, heterotrimers, homotetramers, heterotetramers, etc. 
     One embodiment is directed to oligomers comprising multiple antibodies joined via covalent or non-covalent interactions between peptide moieties fused to the antibodies. Such peptides can be peptide linkers (spacers), or peptides that have the property of promoting oligomerization. Leucine zippers and certain polypeptides derived from antibodies are among the peptides that can promote oligomerization of antibodies attached thereto, as described in more detail below. 
     In particular embodiments, the oligomers comprise from two to four antibodies. The antibodies of the oligomer can be in any form, such as any of the forms described above, e.g., variants or fragments. Preferably, the oligomers comprise antibodies that have APJ binding activity. 
     In one embodiment, an oligomer is prepared using polypeptides derived from immunoglobulins. Preparation of fusion proteins comprising certain heterologous polypeptides fused to various portions of antibody-derived polypeptides (including the Fc domain) has been described, e.g., by Ashkenazi et al., 1991,  PNAS USA  88:10535; Byrn et al., 1990,  Nature  344:677; and Hollenbaugh et al., 1992 “Construction of Immunoglobulin Fusion Proteins,” in  Current Protocols in Immunology , Suppl. 4, pages 10.19.1-10.19.11. One embodiment provided herein is directed to a dimer comprising two fusion proteins created by fusing an Elabela fragment of an anti-APJ antibody to the Fc region of an antibody. The dimer can be made by, for example, inserting a gene fusion encoding the fusion protein into an appropriate expression vector, expressing the gene fusion in host cells transformed with the recombinant expression vector, and allowing the expressed fusion protein to assemble much like antibody molecules, whereupon inter-chain disulfide bonds form between the Fc moieties to yield the dimer. 
     The term “Fc polypeptide” as used herein includes native and mutein forms of polypeptides derived from the Fc region of an antibody. Truncated forms of such polypeptides containing the hinge region that promotes dimerization also are included. Fusion proteins comprising Fc moieties (and oligomers formed therefrom) offer the advantage of facile purification by affinity chromatography over Protein A or Protein G columns. 
     One suitable Fc poly peptide, described in PCT application WO 93/10151 (hereby incorporated by reference), is a single chain polypeptide extending from the N-terminal hinge region to the native C-terminus of the Fc region of a human IgG1 antibody. Another useful Fc polypeptide is the Fc mutein described in U.S. Pat. No. 5,457,035 and in Baum et at., 1994,  EMBO J.  13:3992-4001. The amino acid sequence of this mutein is identical to that of the native Fc sequence presented in WO 93/10151, except that amino acid 19 has been changed from Leu to Ala, amino acid 20 has been changed from Leu to Glu, and amino acid 22 has been changed from Gly to Ala. The mutein exhibits reduced affinity for Fc receptors. In other embodiments, the variable portion of the heavy and/or light chains of an APJ antibody can be substituted for the variable portion of an antibody heavy and/or light chain. 
     Alternatively, the oligomer is a fusion protein comprising multiple antibodies, with or without peptide linkers (spacer peptides). Among the suitable peptide linkers are those described in U.S. Pat. Nos. 4,751,180 and 4,935,233. 
     Another method for preparing oligomeric antibodies involves use of a leucine zipper. Leucine zipper domains are peptides that promote oligomerization of the proteins in which they are found. Leucine zippers were originally identified in several DNA-binding proteins (Landschulz et al., 1988,  Science  240:1759), and have since been found in a variety of different proteins. Among the known leucine zippers are naturally occurring peptides and derivatives thereof that dimerize or trimerize. Examples of leucine zipper domains suitable for producing soluble oligomeric proteins are described in PCT application WO 94/10308, and the leucine zipper derived from lung surfactant protein D (SPD) described in Hoppe et al., 1994,  FEBS Letters  344:191, hereby incorporated by reference. The use of a modified leucine zipper that allows for stable trimerization of a heterologous protein fused thereto is described in Fanslow et at., 1994,  Semin. Immunol.  6:267-78. In one method, recombinant fusion proteins comprising an anti-APJ antibody fragment or derivative fused to a leucine zipper peptide are expressed in suitable host cells, and the soluble oligomeric anti-APJ antibody fragments or derivatives that form are recovered from the culture supernatant. 
     In another embodiment, the antibody derivatives can comprise at least one of the CDRs disclosed herein. For example, one or more CDR can be incorporated into known antibody framework regions (IgG1, IgG2, etc.), or conjugated to a suitable vehicle to enhance the half-life thereof. Suitable vehicles include, but are not limited to Fc, albumin, transferrin, and the like. These and other suitable vehicles are known in the field. Such conjugated CDR peptides can be in monomeric, dimeric, tetrameric, or other form. In one embodiment, one or more water-soluble polymer is bonded at one or more specific position, for example at the amino terminus, of a binding agent. In an example, an antibody derivative comprises one or more water soluble polymer attachments, including, but not limited to, polyethylene glycol, polyoxyethylene glycol, or polypropylene glycol. See, e.g., U.S. Pat. Nos. 4,640,835, 4,496,689, 4,301,144, 4,670,417, 4,791,192 and 4,179,337. In certain embodiments, a derivative comprises one or more of monomethoxy-polyethylene glycol, dextran, cellulose, or other carbohydrate based polymers, poly-(N-vinyl pyrrolidone)-polyethylene glycol, propylene glycol homopolymers, a polypropylene oxide/ethylene oxide co-polymer, polyoxyethylated polyols (e.g., glycerol) and polyvinyl alcohol, as well as mixtures of such polymers. In certain embodiments, one or more water-soluble polymer is randomly attached to one or more side chains. In certain embodiments, PEG can act to improve the therapeutic capacity for a binding agent, such as an antibody. Certain such methods are discussed, for example, in U.S. Pat. No. 6,133,426, which is hereby incorporated by reference for any purpose. 
     Provided herein is an antibody having at least one amino acid substitution, providing that the antibody retains binding specificity. Therefore, modifications to the antibody structures are encompassed within the scope of the invention. These can include amino acid substitutions, which may be conservative or non-conservative, that do not destroy the human APJ binding capability of an antibody. Conservative amino acid substitutions may encompass non-naturally occurring amino acid residues, which are typically incorporated by chemical peptide synthesis rather than by synthesis in biological systems. This include peptidomimetics and other reversed or inverted forms of amino acid moieties. A conservative amino acid substitution can also involve a substitution of a native amino acid residue with a normative residue such that there is little or no effect on the polarity or charge of the amino acid residue at that position. Non-conservative substitutions can involve the exchange of a member of one class of amino acids or amino acid mimetics for a member from another class with different physical properties (e.g., size, polarity, hydrophobicity, charge). 
     Moreover, one skilled in the field may generate variants to be tested, which contain a single amino acid substitution at each desired amino acid residue. The variants can then be screened using activity assays known to those skilled in the field. Such variants could be used to gather information about suitable variants. For example, if one discovered that a change to a particular amino acid residue resulted in destroyed, undesirably reduced, or unsuitable activity, variants with such a change may be avoided. In other words, based on information gathered from such routine experiments, one skilled in the field can readily determine the amino acids where further substitutions should be avoided either alone or in combination with other mutations. 
     In certain embodiments, one skilled in the field is able to determine suitable variants of the polypeptide as set forth herein using well-known techniques. In certain embodiments, one skilled in the field may identify suitable areas of the molecule that may be changed without destroying activity by targeting regions not to be important for activity. In certain embodiments, one can identify residues and portions of the molecules that are conserved among similar polypeptides. In certain embodiments, even areas that may be important for biological activity or for structure may be subject to conservative amino acid substitutions without destroying the biological activity or without adversely affecting the polypeptide structure. Additionally, one skilled in the field can review structure-function studies identifying residues in similar polypeptides that are important for activity or structure. In view of such a comparison, one can predict the importance of amino acid residues in a protein that correspond to amino acid residues which are important for activity or structure in similar proteins. One skilled in the field may opt for chemically similar amino acid substitutions for such predicted important amino acid residues. 
     In certain embodiments, one skilled in the field can also analyze the three-dimensional structure and amino acid sequence in relation to that structure in similar polypeptides. In view of such information, one skilled in the field may predict the alignment of amino acid residues of an antibody with respect to its three-dimensional structure. In certain embodiments, one skilled in the field may choose not to make radical changes to amino acid residues predicted to be on the surface of the protein, since such residues may be involved in important interactions with other molecules. A number of scientific publications have been devoted to the prediction of secondary structure. See Moult, 1996,  Curr Op. Biotech.  7:422-427; Chou et al., 1974,  Biochemistry  13:222-245; Chou et al., 1974,  Biochemistry  113:211-222; Chou e: al., 1978,  Adv Enzymol. Relat. Areas Mol. Biol.  47:45-148 Chou et al., 1979,  Ann. Rev. Biochem.  47:251-276 and Chou et al.,  Biophys.  26:367-384. Moreover, computer programs are currently available to assist with predicting secondary structure. For example, two polypeptides or proteins which have a sequence identity of greater than 30%, or similarity greater than 40% often have similar structural topologies. The recent growth of the protein structural database (PDB) has provided enhanced predictability of secondary structure, including the potential number of folds within the structure of a polypeptide or protein. See Holm et al., 1999 , Nucl. Acid. Res.  27:244-247. It has been suggested (Brenner et al., 1997,  Curt: Op. Struct. Biol.  7:369-376) that there are a limited number of folds in a given polypeptide or protein and that once a critical number of structures have been resolved, structural prediction becomes significantly more accurate. 
     Additional methods of predicting secondary structure include “threading” (Jones, 1997,  Curr Opin. Struct. Biol.  7:377-87; Sippl et al., 1996,  Structure  4:15-19), “profile analysis” (Bowie etal., 1991,  Science  253:164-170; Gribskov et al., 1990,  Meth. Enzym.  183:146-159; Gribskov et al., 1987,  Proc. Nat. Acad. Sci.  84:4355-4358), and “evolutionary linkage” (see Holm, supra (1999), and Brenner, supra (1997)). In certain embodiments, variants of antibodies include glycosylation variants, wherein the number and/or type of glycosylation sites have been altered compared to the amino acid sequences of a parent polypeptide. In certain embodiments, variants comprise a greater or lesser number of N-linked glycosylation sites than the native protein. Alternatively, elimination of such a sequence by substitutions removes an existing N-linked carbohydrate chain. Also provided is a rearrangement of N-linked carbohydrate chains, wherein one or more N-linked glycosylation sites (typically those that are naturally occurring) are eliminated and one or more new N-linked sites are created. Additional preferred antibody variants include cysteine variants, wherein one or more cysteine residues are deleted from or substituted for another amino acid (e.g., serine) as compared to the parent amino acid sequence. Cysteine variants can be useful when antibodies must be refolded into a biologically active conformation such as after the isolation of insoluble inclusion bodies. Cysteine variants generally have fewer cysteine residues than the native protein, and typically have an even number to minimize interactions resulting from unpaired cysteines. 
     Desired amino acid substitutions (whether conservative or non-conservative) can be determined by those skilled in the field at the time such substitutions are desired. In certain embodiments, amino acid substitutions can be used to identify important residues of antibodies to human APJ, or to increase or decrease the affinity of the antibodies to human APJ described herein. 
     According to certain embodiments, preferred amino acid substitutions are those which: (1) reduce susceptibility to proteolysis, (2) reduce susceptibility to oxidation, (3) alter binding affinity for forming protein complexes, (4) alter binding affinities, and/or (4) confer or modify other physiochemical or functional properties on such polypeptides. According to certain embodiments, single or multiple amino acid substitutions (in certain embodiments, conservative amino acid substitutions) can be made in the naturally-occurring sequence (in certain embodiments, in the portion of the polypeptide outside the domain(s) forming intermolecular contacts). In certain embodiments, a conservative amino acid substitution typically cannot substantially change the structural characteristics of the parent sequence (e.g., a replacement amino acid should not break a helix that occurs in the parent sequence, or disrupt other types of secondary structure that characterizes the parent sequence). Examples of field-recognized polypeptide secondary and tertiary structures are described in Proteins, Structures and Molecular Principles (Creighton, Ed., W. H. Freeman and Company, New York (1.984)); Introduction to Protein Structure (Branden and Tooze, Eds., Garland Publishing, New York, N.Y. (1991)); and Thornton et al., 1991,  Nature  354:105, each of which is incorporated herein by reference. 
     In certain embodiments, antibodies of the invention can be chemically bonded with polymers, lipids, or other moieties. 
     The antigen binding agents can comprise at least one of the CDRs described herein incorporated into a biocompatible framework structure. In one embodiment, the biocompatible framework structure comprises a polypeptide or portion thereof that is sufficient to form a conformationally stable structural support, or framework, or scaffold, which is able to present one or more sequences of amino acids that bind to an antigen (e.g., CDRs, a variable region, etc.) in a localized surface region. Such structures can be a naturally occurring polypeptide or polypeptide “fold” (a structural motif), or can have one or more modifications, such as additions, deletions or substitutions of amino acids, relative to a naturally occurring polypeptide or fold. These scaffolds can be derived from a polypeptide of any species (or of more than one species), such as a human, other mammal, other vertebrate, invertebrate, plant, bacteria or virus. 
     Typically, the biocompatible framework structures are based on protein scaffolds or skeletons other than immunoglobulin domains. For example, those based on fibronectin, ankyrin, lipocalin, neocarzinostain, cytochrome b, CPI zinc finger, PST1, coiled coil, LACI-D1, Z domain and tendamistat domains can be used (see, e.g., Nygren and Uhlen, 1997,  Current Opinion in Structural Biology  7:463-469). 
     Additionally, one skilled in the field recognizes that suitable binding agents include portions of these antibodies, such as one or more of heavy chain CDR1 CDR2, CDR3, light chain CDR1, CDR2 and CDR3 as specifically disclosed herein. At least one of the regions of heavy chain CDR1, CDR2, CDR3, CDR1, CDR2 and CDR3 can have at least one amino acid substitution, provided that the antibody retains the binding specificity of the non-substituted CDR. The non-CDR portion of the antibody can be a non-protein molecule, wherein the binding agent cross-blocks the binding of an antibody disclosed herein to human APJ. The non-CDR portion of the antibody can be a non-protein molecule in which the antibody exhibits a similar binding pattern to human Elabela peptides in a competition binding assay as that exhibited by antibody L4H4. The non-CDR portion of the antibody can be composed of amino acids, wherein the antibody is a recombinant binding protein or a synthetic peptide, and the recombinant binding protein cross-blocks the binding of an antibody disclosed herein to human APJ. The non-CDR portion of the antibody can be composed of amino acids, wherein the antibody is a recombinant antibody, and the recombinant antibody exhibits a similar binding pattern to human APJ peptides in a competition binding assay as exhibited by at least one of the antibodies L4H4. 
     Fusion Protein of APJ Antibody and Elabela (Elabela Fusion Protein) 
     In one embodiment, provided herein is an Elebela fusion protein comprising an antibody specifically binding to APJ and an Elabela fragment. 
     In another embodiment, provided herein is an Elabela fusion protein comprising an antibody specifically binding to APJ, as well as 1, 2, 3, 4, 5, 6, 7 or 8 Elabela fragments and peptide linker (Linker); the fusion protein connects the N-terminus of one Elabela fragment to the C-terminus of the heavy or light chain of one APJ antibody through a peptide linker sequence (Linker), wherein each Elabela fragment is independently a forward Elabela fragment or mutant; or the fusion protein connects the C-terminus of one Elabela fragment with the N-terminus of the heavy or light chain of one APJ antibody, wherein each Elabela fragment is independently a reverse Elabela fragment or mutant. 
     In another embodiment, provided herein is an Elabela fusion protein comprising an antibody specifically binding to APJ, as well as 1, 2, 3 or 4 Elabela fragments and peptide linker (Linker); the fusion protein connects the N-terminus of one Elabela fragment with the C-terminus of the heavy or light chain of one APJ antibody through a peptide linker sequence (Linker), wherein each Elabela fragment is independently a forward Elabela fragment or mutant. 
     In another embodiment, provided herein is an Elabela fusion protein comprising an antibody specifically binding to APJ, as well as 1, 2, 3 or 4 Elabela fragments and peptide linker (Linker); the fusion protein connects the C-terminus of one Elabela fragment with the N-terminus of the heavy or light chain of one APJ antibody through a peptide linker sequence (Linker), wherein each Elabela fragment is independently a reverse Elabela fragment or mutant. 
     In another embodiment, provided herein is an Elabela fusion protein comprising an antibody specifically binding to APJ, as well as two Elabela fragments and peptide linker (Linker); the fusion protein connects the N-terminus of one Elabela fragment with the C-terminus of the heavy or light chain of one APJ antibody through a peptide linker sequence (Linker), wherein each Elabela fragment is independently a forward Elabela fragment or mutant. 
     In another embodiment, provided herein is an Elabela fusion protein comprising an antibody specifically binding to APJ, as well as two Elabela fragments and peptide linker (Linker); the fusion protein connects the C-terminus of one Elabela fragment with the N-terminus of the heavy or light chain of one APJ antibody through a peptide linker sequence (Linker), wherein each Elabela fragment is independently a reverse Elabela fragment or mutant. 
     In another embodiment, provided herein is an Elabela fusion protein comprising a APJ antibody and two Elabela fragments and peptide linker (Linker); the fusion protein connects the N-terminus of one Elabela fragment with the C-terminus of one APJ antibody light chain: N′-R-Linker-Elabela-C′; or connects the N-terminus of one Elabela fragment with the C-terminus of one APJ antibody heavy chain: N′-R-Linker-Elebela-C′; wherein: N′ represents the N-terminus of the fusion protein polypeptide chain. C′ represents the C-terminus of the fusion protein polypeptide chain, Elebela represents a forward Elabela fragment or mutant, R represents the amino acid sequence of a light chain or heavy chain of APJ antibody, and Linker represents a peptide linker sequence. 
     In another embodiment, provided herein is an Elabela fusion protein comprising a APJ antibody and two Elabela fragments and peptide linker (Linker); the fusion protein connects the C-terminus of one Elabela fragment with the N-terminus of one APJ antibody light chain through a peptide linker (Linker): N′-Elabela-Linker-R-C′; or connects the C-terminus of one Elabela fragment with the N-terminus of one APJ antibody heavy chain: N′-Elabela-Linker-R-C′; wherein: N′ represents the N-terminus of the fusion protein polypeptide chain, C′ represents the C-terminus of the fusion protein polypeptide chain, Elebela represents a reverse Elabela fragment or mutant, R represents the amino acid sequence of a light chain or heavy chain of APJ antibody, and Linker represents the peptide linker sequence. 
     In another embodiment, provided herein is an Elabela fusion protein comprising a APJ antibody and two Elabela fragments and peptide linker (Linker); the fusion protein connects the N-terminus of one Elabela fragment with the C-terminus of one APJ antibody light chain through a peptide linker (Linker): N′-R-Linker-Elabela-C′; wherein: N′ represents the N-terminus of the fusion protein polypeptide chain, C′ represents the C-terminus of the fusion protein polypeptide chain, Elebela represents a forward Elabela fragment or mutant, R represents an APJ antibody light chain amino acid sequence, and Linker represents a peptide linker sequence. 
     In another embodiment, provided herein is an Elabela fusion protein comprising a APJ antibody and two Elabela fragments and peptide linker (Linker); the fusion protein connects the N-terminus of one Elabela fragment with the C-terminus of one APJ antibody heavy chain through a peptide linker (Linker): N′-R-Linker-Elabela-C′; wherein: N′ represents the N-terminus of the fusion protein polypeptide chain, C′ represents the C-terminus of the fusion protein polypeptide chain, Elebela represents a forward Elabela fragment or mutant, R represents an APJ antibody heavy chain amino acid sequence, and Linker represents a peptide linker sequence. 
     In one embodiment, in the Elebela fusion proteins provided herein, the forward Elebela fragment or variant thereof is independently selected from one of the following amino acid sequences: SEQM NO: 91, SEQ ID NO: 92, SEQ ID NO: 93, SEQ ID NO: 94, SEQ ID NO: 95, SEQ ID NO: 96, SEQ ID NO: 97, SEQ ID NO: 98, SEQ ID NO: 99, SEQ ID NO: 100, SEQ ID NO: 101, SEQ ID NO: 102, SEQ ID NO: 103, SEQ ID NO: 104, SEQ ID NO: 105, SEQ ID NO: 106, SEQ ID NO: 107, SEQ ID NO: 108, SEQ ID NO: 109, SEQ ID NO: 110, SEQ ID NO: 111, SEQ ID NO: 112, SEQ ID NO: 113, SEQ ID NO: 114, SEQ ID NO: 115, SEQ ID NO: 116, SEQ ID NO: 117, SEQ ID NO: 118, SEQ ID NO: 119, SEQ ID NO: 120, SEQ ID NO: 121, SEQ ID NO: 166, and SEQ ID NO: 167. 
     In one embodiment, in the Elebela fusion proteins provided herein, the forward Elebela fragment is independently selected from one of the following amino acid sequences: SEQ ID NO: 92, SEQ ID NO: 93, SEQ ID NO: 103, SEQ ID NO: 107, SEQ ID NO: 115, SEQ ID NO: 116, SEQ ID NO: 166, and SEQ ID NO: 167. 
     In one embodiment, in the Elebela fusion proteins provided herein, the reversed Elebela fragment or variant thereof is independently selected from one of the following amino acid sequences: SEQ ID NO: 125, SEQ ID NO: 126, SEQ ID NO: 127, SEQ ID NO: 128, SEQ ID NO: 129, SEQ ID NO: 130, SEQ ID NO: 131, SEQ ID NO: 132, SEQ ID NO: 133, SEQ ID NO: 134, SEQ ID NO: 135, SEQ ID NO: 136, SEQ ID NO: 137, SEQ ID NO: 138, SEQ ID NO: 139, SEQ ID NO: 140, SEQ ID NO: 141, SEQ ID NO: 142, SEQ ID NO: 143, SEQ ID NO: 144, SEQ ID NO: 145, SEQ ID NO: 146, SEQ ID NO: 147, SEQ ID NO: 148, SEQ ID NO: 149, SEQ ID NO: 150, SEQ ID NO: 151, SEQ ID NO: 152, SEQ ID NO: 153, SEQ ID NO: 154, and SEQ ID NO: 155. 
     In one embodiment, in the Elebela fusion proteins provided herein, wherein the peptide linker (Linker) sequence independently comprises from 1 to 200 amino acid residues, from 2 to 100 amino acid residues, from 5 to 50 amino acid residues, from 6 to 25 amino acid residues, or from 10 to 20 amino acid residues. 
     In another embodiment, in the Elebela fusion proteins provided herein, wherein the linker (Linker) sequence independently comprises the full length, a part of, or repeated amino acid sequence of one of the following amino acid sequences: SEQ ID NO: 122, SEQ ID NO: 123, and SEQ ID NO: 124. 
     In another embodiment, in the Elebela fusion proteins provided herein, the Linker sequence is selected independently from amino acid sequences listed below: SEQ ID NO: 122, SEQ ID NO: 123, and SEQ ID NO: 124. 
     In one embodiment, the light chain amino acid sequence of the Elebela fusion protein provided herein is as follows: SEQ ID NO: 158, and the heavy chain amino acid sequence is one of the following sequences: SEQ ID NO: 156, SEQ ID NO: 159, SEQ ID NO: 160, SEQ ID NO: 161, SEQ ID NO: 162, SEQ ID NO: 163, SEQ ID NO: 164, and SEQ ID NO: 165. In one embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 156, respectively. In another embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 159, respectively. In another embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 160, respectively. In another embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 161, respectively. In another embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 162, respectively. In another embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 163, respectively. In another embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 164, respectively. In further embodiment, the light and heavy chain amino acid sequence of the Elebela fusion protein provided herein are SEQ ID NO: 158 and SEQ ID NO: 165, respectively. 
     Nucleic Acids 
     In one aspect, the present invention provides isolated nucleic acid molecules. The nucleic acids comprise, for example, polynucleotides that encode all or part of the Elebela fusion protein, for example, one or both chains of the Elebela fusion protein of the invention, or a fragment, derivative, mutein, or variant thereof; polynucleotides sufficient for use as hybridization probes; PCR primers or sequencing primers for identifying, analyzing, mutating or amplifying a polynucleotide encoding a polypeptide; anti-sense nucleic acids for inhibiting expression of a polynucleotide, and complementary sequences of the foregoing. The nucleic acids can be any length. They can be, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400, 450, 500, 750, 1,000, 1,500, 3,000, 5,000 or more nucleotides in length, and/or can comprise one or more additional sequences, for example, regulatory sequences, and/or be part of a larger nucleic acid, for example, a vector. The nucleic acids can be single-stranded or double-stranded and can comprise RNA and/or DNA nucleotides, and artificial variants thereof (e.g., peptide nucleic acids). 
     Nucleic acids encoding antibody polypeptides (e.g., heavy or light chain, variable domain only, or full length) can be isolated from B-cells of mice that have been immunized with APJ antigen. The nucleic acid of the antibody of Elabela fusion protein can be isolated by conventional procedures such as polymerase chain reaction (PCR). 
     Nucleic acid sequences encoding the variable regions of the heavy and light chain variable regions are shown above. The skilled in the field appreciates that, due to the degeneracy of the genetic code, each of the polypeptide sequences disclosed herein is encoded by a large number of other nucleic acid sequences. The present invention provides each degenerate nucleotide sequence encoding each antibody or Elabela fusion protein of the invention. 
     The invention further provides nucleic acids that hybridize to other nucleic acids (e.g., nucleic acids comprising a nucleotide sequence of any of Elebela fusion proteins) under particular hybridization conditions. Methods for hybridizing nucleic acids are well-known in the field. See, e.g., Current Protocols in Molecular Biology, John Wiley &amp; Sons, N.Y. (1989), 6.3.1-6.3.6. As defined herein, for example, a moderately stringent hybridization condition uses a prewashing solution containing 5× sodium chloride/sodium citrate (SSC), 0.5% SDS, 1.0 mM EDTA (pH 8.0), hybridization buffer of about 50% formamide, 6× SSC, and a hybridization temperature of 55° C. (or other similar hybridization solutions, such as one containing about 50% formamide, with a hybridization temperature of 42° C.), and washing conditions of 60° C., in 0.5× SSC. 0.1% SDS. A stringent hybridization condition hybridizes in 6×SSC at 45° C., followed by one or more washes in 0.1×SSC, SDS at 68° C. Furthermore, one of skill in the field can manipulate the hybridization and/or washing conditions to increase or decrease the stringency of hybridization such that nucleic acids comprising nucleotide sequences that are at least 65, 70, 75, 80, 85, 90, 95, 98 or 99% identical to each other typically remain hybridized to each other. The basic parameters affecting the choice of hybridization conditions and guidance for devising suitable conditions are set forth by, for example, Sambrook, Fritsch, and Maniatis (1989, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., chapters 9 and 11; and Current Protocols in Molecular Biology, 1995. Ausubel et al., Eds., John Wiley Sons, Inc., sections 2.10 and 6.3-6.4), and can be readily determined by those having ordinary skill in the field based on, for example, the length and/or base composition of the DNA. Changes can be introduced by mutation into a nucleic acid, thereby leading to changes in the amino acid sequence of a polypeptide (e.g., an antibody) that it encodes. Mutations can be introduced using any technique known in the field. In one embodiment, one or more particular amino acid residues are changed using, for example, a site-directed mutagenesis protocol. In another embodiment, one or more randomly selected residues are changed using, for example, a random mutagenesis protocol. No matter how it is made, a mutant polypeptide can be expressed and screened for a desired property. 
     Mutations can be introduced into a nucleic acid without significantly altering the biological activity of a polypeptide that it encodes. For example, one can make nucleotide substitutions leading to amino acid substitutions at non-essential amino acid residues. In one embodiment, nucleotide sequences provided herein for Elebela fusion proteins, or fragments, variants, or derivatives thereof, are mutated such that they encode amino acid sequences provided herein for Elebela fusion proteins, comprising one or more deletions or substitutions of amino acid residues to result in sequences bearing two or more different amino acid residues. In another embodiment, the mutagenesis inserts an amino acid adjacent to one or more amino acid residues shown herein for Elebela fusion proteins to result in sequences with two or more different amino acid residues. Alternatively, one or more mutations can be introduced into a nucleic acid that selectively change the biological activity (e.g., binding to APJ) of a polypeptide that it encodes. For example, the mutation can quantitatively or qualitatively change the biological activity. Examples of quantitative changes include increasing, reducing or eliminating the activity. Examples of qualitative changes include changing the antigen specificity of Elebela fusion proteins. 
     In another aspect, the present invention provides nucleic acid molecules that are suitable for use as primers or hybridization probes for the detection of nucleic acid sequences of the invention. A nucleic acid molecule of the invention can comprise only a portion of a nucleic acid sequence encoding a full-length polypeptide of the invention, for example, a fragment that can be used as a probe or primer or a fragment encoding an active portion (e.g., a APJ binding portion) of a polypeptide of the invention. 
     Probes based on the sequence of a nucleic acid of the invention can be used to detect the nucleic acid or similar nucleic acids, for example, transcripts encoding a polypeptide of the invention. The probe can comprise a label group, e.g., a radioisotope, a fluorescent compound, an enzyme, or an enzyme co-factor. Such probes can be used to identify a cell that expresses the polypeptide. 
     In another aspect, the vectors provided herein comprise a nucleic acid encoding a polypeptide of the invention or a portion thereof. Examples of vectors include, but are not limited to, plasmids, viral vectors, non-episomal mammalian vectors and expression vectors, for example, recombinant expression vectors. 
     The recombinant expression vectors provided herein can comprise a nucleic acid of the invention in a form suitable for expression of the nucleic acid in a host cell. The recombinant expression vectors include one or more regulatory sequences, selected on the basis of the host cells to be used for expression, which is operably linked to the nucleic acid sequence to be expressed. Regulatory sequences include those that direct constitutive expression of a nucleotide sequence in many types of host cells (e.g., SV40 early gene enhancer, Rous sarcoma virus promoter and cytomegalovirus promoter), those that direct expression of the nucleotide sequence only in certain host cells (e.g., tissue-specific regulatory sequences, see Voss et al., 1986,  Trends Biochem. Sci.  11:287, Maniatis et al., 1987,  Science  236:1237, the disclosure of each of which is incorporated by reference herein in its entirety), and those that direct inducible expression of a nucleotide sequence in response to particular treatment or condition (e.g., the metallothionin promoter in mammalian cells and the tet-responsive and/or streptomycin responsive promoter in both prokaryotic and eukaryotic systems (see Id.). Those skilled in the field recognize that the design of the expression vector can depend on such factors as the choice of the host cell to be transformed, the level of expression of protein desired, etc. The expression vectors of the invention can be introduced into host cells to thereby produce proteins or peptides, including fusion proteins or peptides, encoded by nucleic acids as described herein. 
     In another aspect, the present invention provides host cells into which a recombinant expression vector of the invention has been introduced. A host cell can be any prokaryotic cell or eukaryotic cell. Prokaryotic host cells include gram-negative or gram-positive organisms, for example,  E. coli  or  bacilli . Higher eukaryotic cells include insect cells, yeast cells, and established cell lines of mammalian origin. Examples of suitable mammalian host cell lines include Chinese hamster ovary (CHO) cells or their derivatives such as Veggie CHO and related cell lines which grow in serum-free media (see Rasmussen et al., 1998,  Cylotechnology  28:31) or CHO strain DXB-11, which is deficient in DHFR (see Urlaub et al., 1980,  Proc. Natl. Acad. Sci. USA  77:4216-20). Additional CHO cell lines include CHO-K1 (ATCC#CCL-61), EM9 (ATCC# CRL-1861), and W20 (ATCC# CRL-1862). Additional host cells include the COS-7 line of monkey kidney cells (ATCC# CRL-1651) (see Gluzman et al., 1981,  Cell  23:175), L cells, C127 cells, 3T3 cells (ATCC CCL-163), AM-1/D cells (described in U.S. Pat. No. 6,210,924), HeLa cells, BHK (ATCC CRL-10) cell lines, the CV1/EBNA cell line derived from the African green monkey kidney cell line CV1 (ATCC CCL-70) (see McMahan et 1991,  EMBO J.  10:2821), human embryonic kidney cells such as 293, 293 EBNA or MSR 293, human epidermal A431 cells, human Colo205 cells, other transformed primate cell lines, normal diploid cells, cell strains derived from in vitro culture of primary tissue, primary explants, HL-60, U937, HaK or Jurkat cells. Appropriate cloning and expression vectors for use with bacterial, fungal, yeast, and mammalian cellular hosts are described by Pouwels et al. (Cloning Vectors: A Laboratory Manual, Elsevier, N.Y., 1985). 
     Vector DNA can be introduced into prokaryotic or eukaryotic cells via conventional transformation or transfection techniques. For stable transfection of mammalian cells, it is known that, depending upon the expression vector and transfection technique used, only a small fraction of cells can integrate the foreign DNA into their genome. In order to identify and select these integrants, a gene that encodes a selectable marker (e.g., for resistance to antibiotics) is generally introduced into the host cells along with the gene of interest. Preferred selectable markers include those which confer resistance to drugs, such as G418, hygromycin and methotrexate. Cells stably transfected with the introduced nucleic acid can be identified by drug selection (e.g., cells that have incorporated the selectable marker gene survive, while the other cells die), among other methods. 
     The transformed cells can be cultured under conditions that promote expression of a polypeptide, and the polypeptide recovered by conventional protein purification procedures. One such purification procedure is described in the Examples below. Polypeptides contemplated for use herein include substantially homogeneous recombinant mammalian Elebela fusion protein polypeptides substantially free of contaminating endogenous materials. 
     Activity of APJ Antibody 
     The activity of the APJ antibody refers to the effect of the antibody provided herein in binding specifically to APJ. In one embodiment, a mouse or humanized antibody provided herein specifically binds to a human APJ receptor. 
     In one embodiment, the K d  of the antibody provided herein binding to a human APJ receptor is ranging approximately from 0.01 nM to 1000 nM, from 0.1 nM to 500 nM, from 0.5 nM to 200 nM, from 1 nM to 200 nM, or from 10 nM to 100 nM. In another embodiment, the K d  of the antibody provided herein binding to a human APJ receptor is ranging approximately from 1 nM to 200 nM. In yet another embodiment, the K d  of the antibody provided herein binding to a human APJ receptor is ranging approximately from 1 nM to 100 nM. In yet another embodiment, the K d  of the antibody provided herein binding to a human APJ receptor is approximately 1 nM, 2 nM, 5 nM, 10 nM, 20 nM, 30 nM, 40 nM, 50 nM, 60 nM, 70 nM, 80 nM, 90 nM, or 100 nM. 
     In one embodiment, the antibody provided herein specifically binds to a human APJ receptor with one or more following properties:
         a. providing the substantially similar or better K d  as comparing to a reference antibody in binding to a human APJ receptor; and   b. cross-competing binding with a reference antibody to a human APJ receptor,       

     In one embodiment, the reference antibody comprises a combination of light chain variable domain amino acid sequence SEQ ID NO: 62 and heavy chain variable domain amino acid sequence SEQ ID NO: 67. 
     As used herein, the term “substantially similar” means comparable to, or approximately 200%, 180%, 160%, 150%, 140%, 120%, 110%, 100%, 99%, 98%, 97%, 95%, 90%, 85%, 80%, 75%, 70%, 65%, or 50% of the K d  of a reference antibody. In one embodiment, the reference antibody comprises a combination of light chain SEQ ID NO: 62 and heavy chain SEQ ID NO: 67. 
     Biological Activity of the Fusion Protein of APJ Antibody and Elabela 
     The biological activity of the fusion protein of APJ antibody and Elabela comprises the biological activity of Elabela and activity of APJ antibody. The activity of the ARJ antibody is as described above. “Elabela biological activity” refers to the biological activity of the Elabela fusion protein that binds in vivo and activates Elabela receptor (e.g., APJ) and causes cellular stress response. The cellular stress response comprises, but not limited to, enhancing myocardial contractility, relaxing blood vessels, lowering blood pressure, diuresis (reducing the release of antidiuretic hormone), regulating immune response and pituitary related hormone release, etc. Combining the biological activity of Elabela and APJ antibodies, the Elabela fusion protein provided herein can be used to treat various diseases and symptoms associated with Elabela and APJ. The fusion protein exerts its biological effect by acting on Elabela and/or APJ, so the Elabela fusion protein treatment provided herein can be used to treat subjects whose disease or symptom benefit from “increasing Elabela stimulation.” These subjects are referred to as subjects who “need Elabela stimulation therapy.” These subjects include acute heart failure, chronic heart failure, pulmonary hypertension and pulmonary arterial hypertension, and also includes subjects with diabetic vascular disease, cardiac insufficiency, atrial fibrillation, and ischemia-reperfusion injury. 
     In one embodiment, the biological activity of the Elabela fusion protein is detected using report gene assay measurement approach, quantifying the function of Elabela fusion protein in activating APJ in vitro. 
     In one embodiment, the EC 50  of the antibody and Elabela fusion protein provided herein in activating the Elabela/APJ signaling pathway is ranging approximately from 0.1 nM to 100 nM, from 0.5 nM to 20 nM, from 1 nM to 10 nM, or from 1 nM to 5 nM. 
     Pharmaceutical Compositions 
     In one embodiment, a pharmaceutical composition provided herein comprises an Elabela fusion protein provided herein and one or more pharmaceutically acceptable carriers. 
     In one embodiment, a pharmaceutical composition comprises an Elabela fusion protein provided herein and one or more substances selected from the following: buffer solution with pH suitable for Elabela fusion protein, antioxidant (e.g., ascorbic acid), low molecular weight polypeptide (e.g., polypeptide containing less than 10 amino acids), protein, amino acid, sugar example (such as dextrin), complex (e.g., EDTA), glutathione, stabilizer and excipients. In one embodiment, a pharmaceutical composition provided herein can also comprise preservative. In one embodiment, the pharmaceutical composition provided herein can be prepared into a lyophilized powder using an appropriate excipient solution as a diluent. Further examples of components that can be used in drug prescription are given in Remington&#39;s Pharmaceutical Sciences, 16 th  Edition (1980) and 20 th  edition (2000), Mack publishing company. 
     Treatment Method 
     In one embodiment, provided herein is a method of treating, preventing, or ameliorating PAH, comprising administration to a subject a therapeutically effective dosage of the Elabela fusion protein provided herein or a pharmaceutical composition thereof. 
     In another embodiment, provided herein is a method of treating, preventing, or ameliorating PH, comprising administration to a subject a therapeutically effective dosage of the Elabela fusion protein provided herein or a pharmaceutical composition thereof. 
     In another embodiment, provided herein is a method of treating, preventing, or ameliorating heart failure, comprising administration to a subject a therapeutically effective dosage of the Elabela fusion protein provided herein or a pharmaceutical composition thereof. 
     In another embodiment, provided herein is a method of treating, preventing, or ameliorating T2D and its related metabolic syndrome, comprising administration to a subject a therapeutically effective dosage of the Elabela fusion protein provided herein or a pharmaceutical composition thereof. 
     In another embodiment, provided herein is a method of treating, preventing, or ameliorating two or more diseases of pulmonary hypertension, pulmonary arterial hypertension, type 2 diabetes mellitus and its related metabolic syndrome, or heart failure, comprising administration to a subject a therapeutically effective dosage of the Elabela fusion protein provided herein or a pharmaceutical composition thereof. 
     In any one of the uses provided herein, the pharmaceutical composition provided herein is for intravenous or subcutaneous injection. 
     A pharmaceutical composition of an Elabela fusion protein can be administered by any suitable technique, including, but not limited to, parenterally, topically, or by inhalation. If injected, the pharmaceutical composition can be administered by rapid injection or continuous infusion via, for example, intra-articular, intravenous, intramuscular, intralesional, intra-abdominal, or subcutaneous route. It is considered, for example, localized administration at the disease or injury site, such as transdermal administration and sustained release of an implant. Delivery by inhalation includes, for example, nasal or oral inhalation, use of a nebulizer, inhalation of an Elabela fusion protein in aerosol form, and the like. Other alternatives include oral preparations, including pills, syrups, or lozenges. 
     Dosages and the frequency of administration can vary according to such factors as the route of administration, the particular Elabela fusion protein employed, the nature and severity of the disease to be treated, whether the condition is acute or chronic, and the size and general condition of the subject. Appropriate dosages can be determined by procedures known in the pertinent field, e.g. in clinical trials that can involve dose escalation studies. 
     The Elabela fusion protein provided herein can be administered, for example, once or more than once, e.g., at regular intervals, over a period of time. In particular embodiments, the Elabela fusion protein is administered once over a period of at least a month or longer, e.g., for one, two, or three months or even indefinitely. For treating chronic conditions, long-term treatment is generally most effective. However, for treating acute conditions, administration for shorter periods, e.g., from one to six weeks, can be sufficient. In general, the Elabela fusion protein is administered until the patient manifests a medically relevant degree of improvement over baseline for the chosen indicator or indicators. 
     An example of the treatment regimen presented here includes subcutaneous injection of Elabela fusion protein at an appropriate dose once a week or longer to treat symptoms caused by type 2 diabetes and its related metabolic syndrome, acute heart failure, chronic heart failure, pulmonary hypertension or pulmonary arterial hypertension. The Elabela fusion protein can be administered weekly or monthly until desired result is achieved, for example, the patient&#39;s symptoms subside. Treatment can be renewed as needed, or, alternatively, a maintenance dose can be given. 
     The patient&#39;s blood BNP or pro-BNP concentration and body weight can be monitored before, during and/or after treatment with Elabela fusion protein to detect any changes in their pressure. For certain conditions, changes in BNP or pro-BNP can vary with factors such as disease progression. The BNP or pro-BNP concentration can be determined using known techniques. 
     Specific embodiments of the methods and compositions herein involve the use of, for example, an Elabela fusion protein and one or more Elabela agonists, two or more Elabela fusion proteins provided herein, or the Elabela fusion protein provided herein and one or more other Elabela agonists. In a further embodiment, Elabela fusion protein is administered alone or in combination with other agents used to treat symptoms that are painful for the patient. Examples of these agents include protein and non-protein drugs. When multiple drugs are administered in combination, the dosage should be adjusted accordingly as is well known in the field. “Combined administration” combination therapy is not limited to simultaneous administration, but also includes treatment regimens in which the antigen and protein are administered at least once during the course of administration involving the administration of at least one other therapeutic agent to the patient. 
     On the other hand, provided herein is a method for preparing a medicament for treating heart failure and pulmonary arterial hypertension and related disorders, which comprises a mixture of the Elabela fusion protein provided herein and a pharmaceutically acceptable excipient for the treatment of the related diseases of the above diseases. The pharmaceutical preparation method is as described above. 
     Further provided herein are compositions, kits, and methods related to Elabela fusion protein that can specifically bind to human APJ. Nucleic acid molecules and derivatives and fragments thereof are also provided, wherein comprising polynucleotides encoding all or part of a polypeptide that binds to APJ, for example, nucleic acid encoding all or part of Elabela fusion proteins or Elabela fusion protein derivatives. Further provided herein are vectors and plasmids containing such nucleic acids and cells and cell lines containing such nucleic acids and/or vectors and plasmids. Methods provided herein comprise, for example, methods for preparing, identifying, or isolating Elabela fusion proteins that hind to human APJ, a method to determine whether the Elabela fusion protein binds to APJ, and a method of administering the Elabela fusion protein that binds to APJ into an animal model. 
     EXAMPLES 
     The technical solutions described herein are further illustrated by the following examples. 
     If not specified, the stating materials and equipment described herein are commercially available or commonly used in the art. The methods in the following examples, unless otherwise specified, are all conventional methods in the art. 
     1. Preparation of Antigen for Immunization 
     CHO-DHFR-cells were seeded into a 6-well plate. After 24 h culture, the cells were transfected with pTM15 plasmids which were modified to carry hAPJ gene (see SEQ ID NO: 55 for the nucleotide sequence, and SEQ ID NO: 23 for the amino acid sequence). The transfection was carried out by following the transfection conditions recommended by Invitrogen for Lipofectamine 2000. Forty-eight hours after transfection, the medium was replaced with a complete medium containing 300 μg/mL hygromycin, then the medium was changed every 3 days. The stable clones appeared after about two-weeks culture. After digesting and dispersing the cell colonies, the cells were passaged and further cultured until the cells grew to 100% healing degree. The stable cell lines were detected by FACS using V5 labeled antibody (Life Technologies), and the cell populations after pressurization were identified according to the results of FACS detection. The selected CHO-DHFR-hAPJ cells expressed a large number of hAPJ on cell membrane. Finally, after subcloning and further identification, three APJ cell lines were selected as high expression stable cell lines. These cell lines with high expression of hAPJ can be used as immunogens for antibody preparation (refer to example 2). 
     in addition, the fusion protein of hAPJ extracellular domain and hIgG FC can also be used as immunogen for antibody preparation. The preparation method is as following: The fusion protein gene sequence of the extracellular domain of hAPJ, hIgG 2  FC and linker was subcloned into pTM5 plasmid. The APJ extracellular domain fusion protein was highly and transiently expressed in suspended HEK293 cells. Then the cell supernatant was collected and the APJ extracellular domain fusion protein was purified by affinity chromatography. 
     2. Preparation of APJ Antibodies 
     The antigen and aluminum hydroxide adjuvant were mixed and injected subcutaneously into BALB/c mice (6-8 weeks). The mice were boosted once every week. After immunization for 6 times in total, blood samples were collected through clipping tails. Then the serum was collected by centrifugation and serum titer was analyzed by FACS. Once the acceptable antibody titers were achieved, the mice were sacrificed by cervical dislocation, and their spleen cells were harvested under aseptic conditions. Moreover, SP2/0 cells were collected at the logarithmic phase of growth and centrifuged, the cell pellets were resuspended with serum-free culture medium, then centrifuged and resuspended for a second time and counted. Similar number of spleen cells and SP2/0 cells were mixed, followed by 3 rounds of washing-centrifugation. After the cell pellets from the last centrifugation were gently detached, pre-warmed PEG-1500 was added dropwise followed by pipette-mixing, then 30 mL of the pre-warmed serum-free medium was added slowly to terminate the PEG fusion. After centrifugation, the cell pellets were detached followed by addition of fusion culture medium. Spleen cells and feeder layer cells were plated in 96-well plates, and 100 μL culture medium was added into each well. The hybridoma cells after fusion and layer feeder cells were cultured together in 96-well plates followed by HAT (sarcine, amethopterin and thymidine) selection to get rid of the non-fused cells. Ten days later, the supernatants of the hybridoma cells in the culture plates were collected for ELISA analysis. 
     3. ELISA Screening of APJ Antibodies 
     CHO-DHER-hAPJ cells with hAPJ over expression and CHO-DHFR-cells without hAPJ expression were seeded into 96-well plates, respectively. When cells grew to 90% confluence, the supernatant of the culture medium was removed and attached cells were washed twice with PBS, and 100% methanol was added to fix the cells at 4° C., then 100 μL of H 2 O 2 -PBS was added followed by incubation at room temperature for 20 min. Then the cells were washed twice with PBS. After blocking with BSA (dissolved in PBS), the hybridoma supernatant was added and incubated for 90 min at 4° C. After several washes, 100 μL of the diluted secondary antibody goat-anti-mouse Fc-HRP was added into each well and incubated at 37° C. for 30 min. After washing for five times, 100 μL of TMB chromogenic substrate was added into each well and incubated at 37° C. for 15 min, then 50 μL 2M H 2 SO 4  was added to terminate the reaction, and OD 450  values were read. After blocking with 1% BSA (dissolved in PBS), the hybridoma supernatant was added and incubated for 90 min at 4° C. Then the ELISA method described above was performed to screen anti-hAPJ monoclonal antibodies. The positive control was the mouse serum after immunization; the negative control was the cell culture supernatant. After initial analysis by ELISA, several positive hybridoma clones secreting anti-hAPJ antibodies were identified. Those hybridoma clones secreting anti-hAPJ antibodies were selected and went through cloning to get the stable cell lines secreting anti-hAPJ antibodies. Lastly, the affinity of ascites antibodies secreted by positive hybridoma was verified and ranked by FACS analysis (refer to example 9). 
     4. Cloning and Subcloning of APJ Antibody Genes 
     Hybridoma cells secreting antibodies were collected. Hybridoma mRNA was extracted according to the manufacturer protocol of QIAGEN mRNA extraction kit. Then the extracted mRNA was transcribed reversely into cDNA. The reverse transcription primers were specific primers for the light and heavy chain constant regions of a mouse, with the heavy chain reverse transcription primer being (5′-TTTGGRGGGAAGATGAAGAC-3′), and the light chain reverse transcription primers being (5′-TTAACACTCTCCCCTGTTGAA-3′) and (5′-TTAACACTCATTCCTGRTGAA-3′), RT-PCR reaction conditions were as following: 25° C. for 5 min, 50° C. for 60 min, and 70° C. for15 min. Reversely transcribed cDNA was diluted with 0.1 mM TE to 500 then added into the ultrafiltration centrifuge tube (Amicon Ultra-0.5) and centrifuged at 2,000 g for 10 min. The filtrate was removed, 500 μL of 0.1 mM TE was added and centrifuged at 2,000 g for 10 min. The filtrate was removed and the preparation tube was placed in inversion to the new centrifugal tube, and centrifuged at 2,000 g for 10 min to obtain the purified cDNA. Purified cDNA (10 μL) was taken as a template, followed by addition of 4 μL 5× tailing buffer (Promega), 4 μL dATP (1 mM) and 10 U terminal transferase (Promega), mixing uniformly, incubation at 37° C. for 5 min and incubation again at 65° C. for 5 min. Then the PolyA tail cDNA was used as a template and PCR was performed to amplify genes of the light and heavy chain variable region of antibodies. Upstream primers were all oligodT, heavy chain downstream primers were (5′-TGGACAGGGATCCAGAGTTCC-3′) and (5′-TGGACAGGGCTCCATAGTTCC-3′), and light chain downstream primer was (5′-ACTCGTCCTTGGTCAACGTG-3′). The PCR reaction conditions were as following: 95° C. for 5 min; 95° C. for 30 s, 56° C. for 30 s, 72° C. for 1 min, 40 cycles; and 72° C. for 7 min. The PCR products were connected to the PMD 18-T vector (Takata Bio) and then sequenced. Then the PCR primers were designed based on the DNA sequences of the antibodies to ligate the complete light chain, heavy chain signal peptides and variable domains and mouse IgG1 constant region into expression vector pTM5. 
     5. APJ Antibody Humanization, Optimization and Subcloning 
     The sequences of light and heavy chain variable regions of the screened mouse antibodies were aligned with the homologous antibodies, using NCBI database to search the germline gene sequences of a human antibody (Ig Germline Gene sequence) homologous to variable region sequence of the selected antibodies for humanization, and the human gene sequence with highest homology except CDR sequences was used as a template for CDR grafting to obtain humanized antibody variable region sequences. The genes of humanized antibody light and heavy chains were synthesized and then combined with the human IgG2 or IgG4 constant region sequence to obtain whole recombinant humanized antibody sequences. The expression of the recombinant antibodies was achieved according to example 8, and their affinities to APJ was analyzed by FACS as described in step 10. The humanized antibody candidate retaining best affinity to APJ was selected from the group, and its variable region sequence was further modified by site-specific mutagenesis for improved affinity to APJ. 
     The optimized heavy chain and light chain variable region sequences of humanized antibody were synthesized by a CRO, and the complete heavy chain variable region sequence was further connected with the expression vector pTM5 which had been loaded with the heavy chain constant region; similarly, the complete light chain variable region sequence was connected with the expression vector pTM5 which had been loaded with the light chain constant region. 
     6. Construction of the Humanized APJ Antibody and Elabela Fusion Protein 
     The optimized humanized antibody was fused with the Elabela fragment at the C-terminal of the heavy chain to form the Elabela fusion protein. The two sequences were linked by linker as a bridge. The heavy chain nucleic acid sequence of the humanized APJ antibody was partially linked to the “linker Elabela fragment” by overlapping PCR, and the restriction sites of Nhe1 and Not1 were added at both ends of the primers, so as to connect the complete fusion protein sequence with the expression vector pTM5. After the Elabela fusion protein sequence was connected to the expression vector pTM5, sequencing analysis was carried out to confirm the correctness of the construction. 
     7. Transient Expression of the APJ Antibody and Elabela Fusion Protein 
     5×10 5 /mL suspension cells of HEK293 or CHO expression cell line was seeded in a shaker flask. After 24 h rotation at 37° C. and 5% CO 2 , the cell density reached 1×10 6 /mL and was then used for transfection. Polyethylenimine (PEI) was used as a transfection reagent and mixed with DNA during transfection. A mixture of both was incubated quiescently for 15 min then was added into the cell culture. The cells after treated with the PEI/DNA mixture were rotated for 24 h at 37° C. and 5% CO 2 , then tryptone was added into the cell culture as a supplement required by expression. After the completion of expression (more than 96 h), the cell supernatant was collected for the antibody purification and separation. 
     8. Purification and Separation of the APJ Antibody and Elabela Fusion Protein 
     The cell supernatant harvested from example 8 was centrifuged at 8000 rpm to remove cells and cellular debris, then filtered by 0.22 μm filter to get clear supernatant for purification. The purification process was done through chromatograph. Firstly, the supernatant was passed through A/G protein coupled affinity chromatography column, and antibodies or fusion proteins were remained in the column after binding to the ligands of the A/G protein coupled affinity chromatography column. The antibodies or fusion proteins were eluted from the chromatography column using an eluent with pH of 3.0 or less. The collected eluent was neutralized immediately with 1M Tris-HCl. The antibody or fusion protein eluent was then dialyzed into a PBS or other buffer. 
     9. Binding Activity Verification of Functional APJ Antibodies through FACS Analysis 
     PBS containing EDTA was used to detach and collect 10 5  CHO-DHFR-hAPJ cells into 1.5 mL EP tubes. The supernatant was removed after centrifugation. The negative control sample was resuspended with a FACS loading buffer (PBS, 2% FBS). For the positive control, 200 μL antibody supernatant with certain concentration was added to resuspend cells and then incubated at room temperature; the cells were then centrifuged at 1500 rpm to remove the supernatant, washed with a FACS loading buffer and centrifuged again. The cells were resuspended followed by addition (200 μL/well) of a FITC labeled goat anti-mouse fluorescent secondary antibody at 1:50 dilution and incubated at room temperature for 30 min in the dark. Supernatant was removed after centrifugation, cells were washed with FACS loading buffer, centrifuged again and resuspended with the loading buffer for analysis. In the experimental results shown in  FIG. 1A  to  FIG. 1D , the gray peak on the left side was the negative control of 500 nM mouse ascites antibody L1H1 binding with blank cell CHO-DHFR-, and the solid line peaks were the binding curves of mouse ascites antibody L1H1 at 500 nM ( FIG. 1A ) and 4 nM ( FIG. 1B ), respectively, binding with CHO-DHFR-hAPJ, which shifted to the right obviously compared with the negative control of gray peak, and proved the specific binding of L1H1 and hAPJ. The right gray peak was the negative control of 500 nM mouse ascites antibody L4H4 binding with blank cell CHO-DHFR-, and the solid line peaks were the binding curves of mouse ascites antibody L4H4 at 500 nM ( FIG. 1C ) and 4 nM ( FIG. 1D ), respectively, binding with CHO-DIFR-hAPJ, which shifted to the right obviously compared with the negative control of gray peak, and proved the specific binding of L4H4 and hAPJ. 
     10. Reporter Gene Assay Measurement of the in Vitro Activation of Elabela/APJ Signaling Pathway by Elabela and Elabela Fusion Protein. 
     35000 hAN-CRE-Luciferase expressing CHO-DHFR-cells per well were seeded into 96 well plate, and cultured overnight at 37° C. The culture supernatant was removed the second day, cells were washed twice with serum-free medium, and the residue liquid was aspirated. 50 μL of 0.6 μM forskolin was added in 96 well plate in advance, then 50 μL of Elabela fusion protein or Elabela-11 polypeptide diluted in serum-free medium was added, and incubated at 37° C. for 6 hours. After stimulation, 100 μL Bright Glo chemical luminescence substrate (Promega) was added, then the cell lysate was transferred to white 96 well plate, and the relative intensity of fluorescence was read in the SpectraMax L (Molecular Devices). The data were fitted with prism5.0 and EC50 was calculated. The antibodies of L1H1 and L4H4 did not block or activate APJ receptor.  FIG. 2  shows the activation curves of Elabela/APJ signaling pathway activated by the fusion protein of L5H5 and linker2-Elabela-11, and the fusion protein of L5H5 and linker2-EA5 detected by reporter gene experiment, with EC 50  of 3.61 and 2.55 nM, respectively.  FIG. 3  shows the activation curves of Elabela/APJ signaling pathway activated by the fusion protein of L5H5 and linker2-EA1, and fusion protein of L5H5 and linker2-EA2 detected by reporter gene experiment, with EC 50  of 4.95 and 2.16 nM, respectively.  FIG. 4  shows the activation curves of Elabela/APJ signaling pathway activated by the fusion protein of L5H5 and linker2-EA3, and fusion protein of L5H5 and linker2-EA4 detected by reporter gene experiment, with EC 50  of 20.27 and 5.4 nM, respectively. 
     The above examples are provided to fully disclose and explain to those of ordinary skill in the art how to manufacture and use the embodiments requiring protection, without limiting the scope of the disclosure herein. The obvious modifications to those skilled in the art are within the scope of the claims herein. All publications, patents, and patent applications cited in this specification are incorporated herein by reference as if each publication, patent or patent application were specifically and individually incorporated herein by reference.