Patent Publication Number: US-10775375-B2

Title: Methods for identifying modulators of calcium-sensing receptors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/517,409, filed on Apr. 6, 2017, which is a U.S. National Stage Patent Application under 35 U.S.C. § 371 of International Application No. PCT/US2015/055149, filed on Oct. 12, 2015, which claims priority to U.S. Provisional Application Ser. No. 62/062,717, filed on Oct. 10, 2014, the contents of each of which are incorporated in their entirety herein. 
    
    
     SEQUENCE LISTING 
     The instant application contains a Sequence Listing which has been submitted electronically in ASCII format and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Jun. 4, 2019, is named 069269_0328_SL.txt and is 45,583 bytes in size. 
     FIELD 
     The presently disclosed subject matter relates to methods for identifying compounds that modulate the activity and/or expression of a calcium-sensing receptor. 
     BACKGROUND 
     Taste profiles for edible compositions include basic tastes such as sweet, salt, bitter, sour, umami and kokumi. Taste profiles have also been described as including free fatty acid tastes. Chemical compounds that elicit these tastes are often referred to as tastants. Without being bound by theory, it is hypothesized that tastants are sensed by taste receptors in the mouth and throat which transmit signals to the brain where the tastants and resulting taste profiles are registered. Taste receptors include the calcium-sensing receptor (CaSR), which is a G-protein coupled receptor (GPCR) that detects changes in extracellular calcium levels and a close relative to the T1R1, T1R2 and T1R3 receptors, i.e., the sweet and umami receptors. The calcium-sensing receptor has been shown to enhance sweet, salty and umami tastes, and function as a receptor for kokumi taste. 
     Pet food manufacturers have a long-standing desire to provide pet food products that have high nutritional value. In addition, and with particular regard to cat and dog foods, pet food manufacturers desire a high degree of palatability so that pets can receive the full nutritional benefit from their food. Domestic animals, especially cats, are notoriously fickle in their food preferences, and often refuse to eat a pet food product that it has accepted over time or refuse to eat any more than a minimal amount of a pet food product. This phenomenon may be, in part, due to the subtle differences in the sensory profiles of the raw material, which can be perceived by the domestic animals because of their gustatory and olfactory systems. As a result, pet owners frequently change types and brands of pet food in order to maintain their pets in a healthy and contented condition. 
     While there have been recent advances in taste and flavor technologies, there remains a need for compounds that can enhance or modify the palatability of pet food products by enhancing or modifying the taste, texture and/or flavor profiles of the pet food product. The enhancement or modification can be to increase the intensity of a desirable attribute, to replace a desirable attribute not present or somehow lost in the pet food product, or to decrease the intensity of an undesirable attribute. In particular, it is desirable to increase the intensity of a desirable tastant in a pet food product. 
     Therefore, there remains a need in the art for methods to identify compounds that enhance the palatability and/or modulate the kokumi taste of pet food products and for flavor compositions comprising these compounds. 
     SUMMARY OF THE INVENTION 
     The presently disclosed subject matter provides methods for identifying compounds that enhance, increase and/or modulate the activity of a calcium-sensing receptor. Once identified, such compounds can be comprised in a flavor composition that can be added to a variety of pet food products to increase the palatability of the products. For example, in certain embodiments of the present disclosure, such a flavor composition is combined with a pet food product in an amount effective to increase the kokumi taste and/or palatability of the pet food product. 
     In certain embodiments, a method for identifying compounds that enhance, increase and/or modulate the activity and/or expression of a calcium-sensing receptor comprises expressing a calcium-sensing receptor having a nucleotide sequence set forth in SEQ ID NO: 1, 2, 3 or 7, or a fragment or variant thereof, in a cell. The method can further comprise contacting the cell expressing the calcium-sensing receptor with a test compound and determining the activity and/or expression of the calcium-sensing receptor in the presence of the compound as compared to the activity and/or expression of the receptor in the absence of the compound. 
     In certain embodiments, a method for identifying compounds that enhance, increase and/or modulate the activity of a calcium-sensing receptor comprises expressing a calcium-sensing receptor having an amino acid sequence set forth in SEQ ID NO: 4, 5 or 6, or a fragment or variant thereof, in a cell. The method can further comprise contacting the cell expressing the calcium-sensing receptor with a test compound and determining the activity and/or expression of the calcium-sensing receptor in the presence of the compound as compared to the activity and/or expression of the receptor in the absence of the compound. 
     In certain embodiments, the present disclosure provides a method for identifying a composition that modulates the activity of a calcium-sensing receptor (CaSR) comprising (a) contacting a test agent with a CaSR, (b) determining the activity of the CaSR, and (c) selecting as the composition, a test agent that increases the activity of the CaSR. 
     In certain embodiments, the present disclosure provides a method for identifying a composition that modulates the activity of a calcium-sensing receptor (CaSR) comprising (a) contacting a test agent with a CaSR, (b) detecting an interaction between the test agent and one or more amino acids in a Venus Flytrap domain (VFT) or 7 transmembrane domain (7TM) of the CaSR, and (c) selecting as the composition, a test agent that interacts with one or more of the amino acids. 
     In certain embodiments, the present disclosure provides a method for identifying a composition that modulates the activity of a calcium-sensing receptor (CaSR) comprising (a) contacting a CaSR agonist with a CaSR, (b) determining the activity of the CaSR, (c) contacting a test agent with the CaSR, (d) determining the activity of the CaSR, and (e) selecting the test agent as the composition when the activity of (d) is greater than the activity of (b). 
     The foregoing has outlined rather broadly the features and technical advantages of the present application in order that the detailed description that follows may be better understood. Additional features and advantages of the application will be described hereinafter which form the subject of the claims of the application. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present application. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the application as set forth in the appended claims. The novel features which are believed to be characteristic of the application, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a feline CaSR nucleotide sequence (SEQ ID NO: 1). 
         FIG. 2  shows a canine CaSR nucleotide sequence (SEQ ID NO: 2). 
         FIG. 3  shows a human CaSR nucleotide sequence (SEQ ID NO: 3). 
         FIG. 4  shows a feline CaSR amino acid sequence (SEQ ID NO: 4). 
         FIG. 5  shows a canine CaSR amino acid sequence (SEQ ID NO: 5). 
         FIG. 6  shows a human CaSR amino acid sequence (SEQ ID NO: 6). 
         FIG. 7  shows the sequence alignment of a feline CaSR and a human CaSR amino acid sequence. 
         FIG. 8  shows the sequence alignment of a canine CaSR and a human CaSR amino acid sequence. 
         FIG. 9  shows the sequence alignment of a feline CaSR and a canine CaSR amino acid sequence. 
         FIG. 10  shows the sequence alignment of the amino acid sequences of the feline, canine and human CaSRs. 
         FIGS. 11A-11B  show (A) the sequence alignment of the amino acid sequences of the feline, canine and human CaSR VFT domains; and (B) 3D depiction of major structural differences within VFT domains. 
         FIGS. 12A-12B  show (A) the sequence alignment of the amino acid sequences of the feline, canine and human CaSR 7TM domains; and (B) 3D depiction of major structural differences within 7TM domains. 
         FIGS. 13A-13B  show in silico modeling of the interactions between glutathione and a CaSR. A subset of potentially interacting CaSR residues are shown. 
         FIG. 14  shows the snake plot of the feline CaSR. 
         FIG. 15  shows the snake plot of the canine CaSR. 
         FIG. 16  shows the snake plot of the human CaSR. 
         FIGS. 17A-17B  show in silico modeling of the interactions between calindol and a CaSR. A subset of potentially interacting CaSR residues are depicted. 
         FIGS. 18A-18J  show dose response curves for 10 fCaSR ligands determined in an in vitro cellular assay for activation of fCaSR by the 10 ligands in agonist mode. 
         FIGS. 19A-19B  show dose response curves for 2 fCaSR ligands determined in an in vitro cellular assay for activation of fCaSR by the 2 ligands in PAM mode. 
         FIG. 20  shows the pcDNA3.1_fCaSR vector clone comprising the fCaSR coding region. 
         FIG. 21  shows the pcDNA5 TO_fCASR vector clone comprising the fCaSR coding region. 
         FIG. 22  shows a feline CaSR nucleotide sequence with Kozak sequence, and nucleotide substitutions Y987T, M1066C, R1269G and S3131G (SEQ ID NO: 7). 
     
    
    
     DETAILED DESCRIPTION 
     The presently disclosed subject matter relates to methods for identifying compounds that modulate the activity and/or expression of calcium-sensing receptors, wherein said compounds can be included in a flavor composition that can be used to increase the palatability and/or enhance or modify the taste of various pet food products such as a nutritionally-complete pet food or pet treats. 
     1. Definitions 
     The terms used in this specification generally have their ordinary meanings in the art, within the context of this invention and in the specific context where each term is used. Certain terms are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner in describing the methods and compositions of the invention and how to make and use them. 
     As used herein, the use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” Still further, the terms “having,” “including,” “containing” and “comprising” are interchangeable and one of skill in the art is cognizant that these terms are open ended terms. 
     The term “about” or “approximately” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, “about” can mean within 3 or more than 3 standard deviations, per the practice in the art. Alternatively, “about” can mean a range of up to 20%, preferably up to 10%, more preferably up to 5%, and more preferably still up to 1% of a given value. Alternatively, particularly with respect to biological systems or processes, the term can mean within an order of magnitude, preferably within 5-fold, and more preferably within 2-fold, of a value. 
     As used herein, “taste” refers to a sensation caused by activation of receptor cells in a subject&#39;s taste buds. In certain embodiments, taste can be selected from the group consisting of sweet, sour, salt, bitter, kokumi and umami. In certain embodiments, “taste” can include free fatty acid taste. See, e.g., Cartoni et al., J. of Neuroscience, 30(25): 8376-8382 (2010), the contents of which are incorporated herein by reference. In certain embodiments, a taste is elicited in a subject by a “tastant.” In certain embodiments, a tastant can be a synthetic tastant. In certain embodiments, the tastant is prepared from a natural source. 
     As used herein, “taste profile” refers to a combination of tastes, such as, for example, one or more of a sweet, sour, salt, bitter, umami, kokumi and free fatty acid taste. In certain embodiments, a taste profile is produced by one or more tastant that is present in a composition at the same or different concentrations. In certain embodiments, a taste profile refers to the intensity of a taste or combination of tastes, for example, a sweet, sour, salt, bitter, umami, kokumi and free fatty acid taste, as detected by a subject or any assay known in the art. In certain embodiments, modifying, changing or varying the combination of tastants in a taste profile can change the sensory experience of a subject. 
     As used herein, “flavor” refers to one or more sensory stimuli, such as, for example, one or more of taste (gustatory), smell (olfactory), touch (tactile) and temperature (thermal) stimuli. In certain non-limiting embodiments, the sensory experience of a subject exposed to a flavor can be classified as a characteristic experience for the particular flavor. For example, a flavor can be identified by the subject as being, but not limited to, a floral, citrus, berry, nutty, caramel, chocolate, peppery, smoky, cheesy, meaty, etc., flavor. As used herein, a flavor composition can be selected from a liquid, solution, dry powder, spray, paste, suspension and any combination thereof. The flavor can be a natural composition, an artificial composition, a nature identical, or any combination thereof. 
     As used interchangeably herein, “aroma” and “smell” refer to an olfactory response to a stimulus. For example, and not by way of limitation, an aroma can be produced by aromatic substances that are perceived by the odor receptors of the olfactory system. 
     As used herein, “flavor profile” refers to a combination of sensory stimuli, for example, tastes, such as sweet, sour, bitter, salty, umami, kokumi, free fatty acid tastes, and/or olfactory, tactile and/or thermal stimuli. In certain embodiments, flavor profiles comprise one or more flavors which contribute to the sensory experience of a subject. In certain embodiments, modifying, changing or varying the combination of stimuli in a flavor profile can change the sensory experience of a subject. 
     As used herein, “palatability” can refer to the overall willingness of an animal to eat a certain food product. Increasing the “palatability” of a pet food product can lead to an increase in the enjoyment and acceptance of the pet food by the companion animal to ensure the animal eats a “healthy amount” of the pet food. The term “healthy amount” of a pet food as used herein refers to an amount that enables the companion animal to maintain or achieve an intake contributing to its overall general health in terms of micronutrients, macronutrients and calories, such as set out in the “Mars Petcare Essential Nutrient Standards.” In certain embodiments, “palatability” can mean a relative preference of an animal for one food product over another. For example, when an animal shows a preference for one of two or more food products, the preferred food product is more “palatable,” and has “enhanced palatability.” In certain embodiments, the relative palatability of one food product compared to one or more other food products can be determined, for example, in side-by-side, free-choice comparisons, e.g., by relative consumption of the food products, or other appropriate measures of preference indicative of palatability. Palatability can be determined by a standard testing protocol in which the animal has equal access to both food products such as a test called “two-bowl test” or “versus test.” Such preference can arise from any of the animal&#39;s senses, but can be related to, inter alia, taste, aftertaste, smell, mouth feel and/or texture. 
     The term “pet food” or “pet food product” means a product or composition that is intended for consumption by a companion animal, such as cats, dogs, guinea pigs, mice, rabbits, birds and horses. For example, but not by way of limitation, the companion animal can be a “domestic” cat such as  Felis domesticus . In certain embodiments, the companion animal can be a “domestic” dog, e.g.,  Canis lupus familiaris . A “pet food” or “pet food product” can include any food, feed, snack, food supplement, liquid, beverage, treat, toy (chewable and/or consumable toys), meal substitute or meal replacement. 
     As used herein “nutritionally-complete” refers to pet food products that contain all known required nutrients for the intended recipient of the pet food product, in appropriate amounts and proportions based, for example, on recommendations of recognized or competent authorities in the field of companion animal nutrition. Such foods are therefore capable of serving as a sole source of dietary intake to maintain life, without the addition of supplemental nutritional sources. 
     As used herein “flavor composition” refers to at least one compound or biologically acceptable salt thereof that modulates, including enhancing, multiplying, potentiating, decreasing, suppressing, or inducing, the tastes, smells, flavors and/or textures of a natural or synthetic tastant, flavoring agent, taste profile, flavor profile and/or texture profile in an animal or a human. In certain embodiments, the flavor composition comprises a combination of compounds or biologically acceptable salts thereof. In certain embodiments, flavor composition includes one or more excipients. 
     As used herein, the terms “modulates” or “modifies” refers an increase or decrease in the amount, quality or effect of a particular activity of a receptor and/or an increase or decrease in the expression, activity or function of a receptor. “Modulators,” as used herein, refer to any inhibitory or activating compounds identified using in silico, in vitro and/or in vivo assays for, e.g., agonists, antagonists and their homologs, including fragments, variants and mimetics. 
     “Inhibitors” or “antagonists,” as used herein, refer to modulating compounds that reduce, decrease, block, prevent, delay activation, inactivate, desensitize or downregulate biological activity and/or expression of receptors or pathway of interest. 
     “Inducers,” “activators” or “agonists,” as used herein, refer to modulating compounds that increase, induce, stimulate, open, activate, facilitate, enhance activation, sensitize or upregulate a receptor or pathway of interest. 
     As used herein, the terms “vector” and “expression vector” refer to DNA molecules that are either linear or circular, into which another DNA sequence fragment of appropriate size can be integrated. Such DNA fragment(s) can include additional segments that provide for transcription of a gene encoded by the DNA sequence fragment. The additional segments can include and are not limited to: promoters, transcription terminators, enhancers, internal ribosome entry sites, untranslated regions, polyadenylation signals, selectable markers, origins of replication and such like. Expression vectors are often derived from plasmids, cosmids, viral vectors and yeast artificial chromosomes. Vectors are often recombinant molecules containing DNA sequences from several sources. 
     The term “operably linked,” when applied to DNA sequences, e.g., in an expression vector, indicates that the sequences are arranged so that they function cooperatively in order to achieve their intended purposes, i.e., a promoter sequence allows for initiation of transcription that proceeds through a linked coding sequence as far as the termination signal. 
     The term “nucleic acid molecule” and “nucleotide sequence,” as used herein, refers to a single or double stranded covalently-linked sequence of nucleotides in which the 3′ and 5′ ends on each nucleotide are joined by phosphodiester bonds. The nucleic acid molecule can include deoxyribonucleotide bases or ribonucleotide bases, and can be manufactured synthetically in vitro or isolated from natural sources. 
     The terms “polypeptide,” “peptide,” “amino acid sequence” and “protein,” used interchangeably herein, refer to a molecule formed from the linking of at least two amino acids. The link between one amino acid residue and the next is an amide bond and is sometimes referred to as a peptide bond. A polypeptide can be obtained by a suitable method known in the art, including isolation from natural sources, expression in a recombinant expression system, chemical synthesis or enzymatic synthesis. The terms can apply to amino acid polymers in which one or more amino acid residue is an artificial chemical mimetic of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers and non-naturally occurring amino acid polymers. 
     The term “amino acid,” as used herein, refers to naturally occurring and synthetic amino acids, as well as amino acid analogs and amino acid mimetics that function in a manner similar to the naturally occurring amino acids. Naturally occurring amino acids are those encoded by the genetic code, as well as those amino acids that are later modified, e.g., hydroxyproline, gamma-carboxyglutamate and O-phosphoserine. Amino acid analogs and derivatives can refer to compounds that have the same basic chemical structure as a naturally occurring amino acid, i.e., a carbon that is bound to a hydrogen, a carboxyl group, an amino group and an R group, e.g., homoserine, norleucine, methionine sulfoxide and methionine methyl sulfonium. Such analogs can have modified R groups (e.g., norleucine) or modified peptide backbones, but retain the same basic chemical structure as a naturally occurring amino acid Amino acid mimetics means chemical compounds that have a structure that is different from the general chemical structure of an amino acid, but that functions in a manner similar to a naturally occurring amino acid. 
     The terms “isolated” or “purified,” used interchangeably herein, refers to a nucleic acid, a polypeptide, or other biological moiety that is removed from components with which it is naturally associated. The term “isolated” can refer to a polypeptide that is separate and discrete from the whole organism with which the molecule is found in nature or is present in the substantial absence of other biological macromolecules of the same type. The term “isolated” with respect to a polynucleotide can refer to a nucleic acid molecule devoid, in whole or part, of sequences normally associated with it in nature; or a sequence, as it exists in nature, but having heterologous sequences in association therewith; or a molecule disassociated from the chromosome. 
     As used herein, the term “recombinant” can be used to describe a nucleic acid molecule and refers to a polynucleotide of genomic, RNA, DNA, cDNA, viral, semisynthetic or synthetic origin which, by virtue of its origin or manipulation is not associated with all or a portion of polynucleotide with which it is associated in nature. 
     The term “fusion,” as used herein, refers to joining of different peptide or protein segments by genetic or chemical methods wherein the joined ends of peptide or protein segments may be directly adjacent to each other or may be separated by linker or spacer moieties such as amino acid residues or other linking groups. 
     2. Calcium-Sensing Receptors 
     The presently disclosed subject matter provides calcium-sensing receptors (CaSRs) for use in the disclosed methods. The calcium-sensing receptors of the present disclosure can include mammalian calcium-sensing receptors such as, but not limited to, felines, canines and humans. 
     In certain embodiments, a calcium-sensing receptor for use in the presently disclosed subject matter encompasses a feline calcium-sensing receptor having the nucleotide sequence set forth in SEQ ID NO: 1 or 7 and/or the amino acid sequence set forth in SEQ ID NO:4, including fragments thereof (e.g., functional fragments thereof) and variants thereof. 
     In certain embodiments, a calcium-sensing receptor for use in the presently disclosed subject matter encompasses a canine calcium-sensing receptor having the nucleotide sequence set forth in SEQ ID NO: 2 and/or the amino acid sequence set forth in SEQ ID NO:5, including fragments thereof (e.g., functional fragments thereof) and variants thereof. 
     In certain embodiments, a calcium-sensing receptor for use in the presently disclosed subject matter encompasses a human calcium-sensing receptor having the nucleotide sequence set forth in SEQ ID NO: 3 and/or the amino acid sequence set forth in SEQ ID NO:6, including fragments thereof (e.g., functional fragments thereof) and variants thereof. 
     In certain embodiments, the calcium-sensing receptor for use in the presently disclosed subject matter can include a receptor comprising a nucleotide sequence having at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to SEQ ID NO: 1, 2, 3 or 7. 
     In certain embodiments, the calcium-sensing receptor for use in the presently disclosed subject matter can include a receptor comprising an amino acid sequence having at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to SEQ ID NO: 4, 5 or 6. 
     The percent identity of two amino acid sequences or of two nucleotide sequences can be determined by aligning the sequences for optimal comparison purposes (e.g., gaps can be introduced in the first sequence for best alignment with the sequence) and comparing the amino acid residues or nucleotides at corresponding positions. The percent identity can be determined by the number of identical amino acid residues or nucleotides in the sequences being compared (e.g., % identity=number of identical positions/total number of positions×100). 
     The determination of percent identity between two sequences can be determined using a mathematical algorithm known to those of skill in the art. A non-limiting example of a mathematical algorithm for comparing two sequences is the algorithm of Karlin and Altschul (1990) Proc. Natl. Acad. Sci. USA 87:2264-2268, modified as in Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90:5873-5877, the disclosures of which are incorporated herein by reference in their entireties. The NBLAST and XBLAST programs of Altschul, et al. (1990) J. Mol. Biol. 215:403-410 have incorporated such an algorithm. BLAST nucleotide searches can be performed with the NBLAST program, score=100, wordlength=12, to obtain nucleotide sequences homologous to nucleotide sequences of the invention. BLAST protein searches can be performed with the XBLAST program, score=50, wordlength=3 to obtain amino acid sequences homologous to amino acid sequence of the invention. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized as described in Altschul et al. (1997) Nucleic Acids Res. 25:3389-3402, the disclosure of which is incorporated herein by reference in its entirety. Alternatively, PSI-Blast can be used to perform an iterated search, which detects distant relationships between molecules. When utilizing BLAST, Gapped BLAST, and PSI-Blast programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) can be used. See http://www.ncbi.nlm.nih.gov. An additional non-limiting example of a mathematical algorithm utilized for the comparison of sequences is the algorithm of Myers and Miller, CABIOS (1989), the disclosure of which is incorporated herein by reference in its entirety. The ALIGN program (version 2.0), which is part of the CGC sequence alignment software package, has incorporated such an algorithm. Other non-limiting examples of algorithms for sequence analysis known in the art include ADVANCE and ADAM as described in Torellis and Robotti (1994) Comput. Appl. Biosci., 10:3-5; and FASTA described in Pearson and Lipman (1988) Proc. Natl. Acad. Sci. 85:2444-8, the disclosures of which are incorporated herein by reference in their entireties. Within FASTA, ktup is a control option that sets the sensitivity and speed of the search. 
     In certain embodiments, the disclosed subject matter provides for the use of an isolated or purified calcium-sensing receptor and/or variants and fragments thereof. The disclosed subject matter also encompasses the use of sequence variants. In certain embodiments, variation can occur in either or both the coding and non-coding regions of a nucleotide sequence of a calcium-sensing receptor. Variants can include a substantially homologous protein encoded by the same genetic locus in an organism, i.e., an allelic variant. Variants also encompass proteins derived from other genetic loci in an organism, e.g., feline, but having substantial homology to the calcium-sensing receptor, i.e., a homolog. Variants can also include proteins substantially homologous to the calcium-sensing receptor but derived from another organism, i.e., an ortholog. Variants also include proteins that are substantially homologous to the calcium-sensing receptor that are produced by chemical synthesis. Variants also include proteins that are substantially homologous to the calcium-sensing receptor that are produced by recombinant methods. 
     Orthologs, homologs and allelic variants can be identified using methods well known in the art. These variants can include a nucleotide sequence encoding a receptor that is at least about 60-65%, about 65-70%, about 70-75, about 80-85%, about 90-95%, about 95-99% or more homologous to the nucleotide sequence shown in SEQ ID NO: 1, 2, 3 or 7, or fragments thereof. Such nucleic acid molecules can readily be identified as being able to hybridize under stringent conditions, to the nucleotide sequence shown in SEQ ID NO: 1, 2, 3 or 7, or a fragment thereof. In certain embodiments, two polypeptides (or regions thereof) are substantially homologous when the amino acid sequences are at least about 60-65%, about 65-70%, about 70-75, about 80-85%, about 90-95%, about 95-99% or more homologous to the amino acid sequences shown in SEQ ID NO: 4, 5 or 6, or a fragment thereof. A substantially homologous amino acid sequence, according to the disclosed subject matter, will be encoded by a nucleic acid sequence hybridizing to the nucleic acid sequence, or portion thereof, of the nucleotide sequence shown in SEQ ID NOs: 1, 2, 3 or 7 under stringent conditions. 
     The calcium-sensing receptors for use in the methods of the disclosed subject matter include calcium-sensing receptors having additions, deletions or substitutions of amino acid residues (variants) which do not substantially alter the biological activity of the receptor. Those individual sites or regions of the calcium-sensing receptors which may be altered without affecting biological activity can be determined by examination of the structure of the calcium-sensing receptor extracellular domain, for example. Alternatively and/or additionally, one can empirically determine those regions of the receptor which would tolerate amino acid substitutions by alanine scanning mutagenesis (Cunningham et al., Science 244, 1081-1085 (1989), the disclosure of which is hereby incorporated by reference in its entirety). In the alanine scanning mutagenesis method, selected amino acid residues are individually substituted with a neutral amino acid (e.g., alanine) in order to determine the effects on biological activity. 
     It is generally recognized that conservative amino acid changes are least likely to perturb the structure and/or function of a polypeptide. Accordingly, the disclosed subject matter encompasses one or more conservative amino acid changes within a calcium-sensing receptor. Conservative amino acid changes generally involve substitution of one amino acid with another that is similar in structure and/or function (e.g., amino acids with side chains similar in size, charge and shape). Families of amino acid residues having similar side chains have been defined in the art. These families include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine, tryptophan), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). In certain embodiments, one or more amino acid residues within a calcium-sensing receptor can be replaced with other amino acid residues from the same side chain family and the altered protein can be tested for retained function using the functional assays described herein. Modifications can be introduced into a calcium-sensing receptor of the present disclosure by standard techniques known in the art, such as site-directed mutagenesis and PCR-mediated mutagenesis. If such substitutions result in a retention in biological activity, then more substantial changes can be introduced and/or other additions/deletions may be made and the resulting products screened. In certain embodiments, deletions or additions can be from 5-10 residues, alternatively from 2-5 amino acid residues or from 1-2 residues. 
     The disclosed subject matter also provides for fusion proteins that comprise a calcium-sensing receptor, or fragment thereof. In certain embodiments, the disclosed subject matter provides for fusion proteins of a calcium-sensing receptor, or functional fragments thereof, and an immunoglobulin heavy chain constant region. In certain embodiments, a fusion protein of the present disclosure can include a detectable marker, a functional group such as a carrier, a label, a stabilizing sequence or a mechanism by which calcium-sensing receptor agonist binding can be detected. Non-limiting embodiments of a label include a FLAG tag, a His tag, a MYC tag, a maltose binding protein and others known in the art. The presently disclosed subject matter also provides nucleic acids encoding such fusion proteins, vectors containing fusion protein-encoding nucleic acids and host cells comprising such nucleic acids or vectors. In certain embodiments, fusions can be made at the amino terminus (N-terminus) of a calcium-sensing receptor or at the carboxy terminus (C-terminus) of a calcium-sensing receptor. 
     In certain embodiments, the calcium-sensing receptors disclosed herein can contain additional amino acids at the N-terminus and/or at the C-terminus end of the sequences, e.g., when used in the methods of the disclosed subject matter. In certain embodiments, the additional amino acids can assist with immobilizing the polypeptide for screening purposes, or allow the polypeptide to be part of a fusion protein, as disclosed above, for ease of detection of biological activity. 
     3. Methods for Identifying CaSR Modulating Compounds 
     The present disclosure further provides methods for identifying compounds that modulate the activity and/or expression of a calcium-sensing receptor. For example, and not by way of limitation, the modulator can be an agonist or an antagonist. The presently disclosed subject matter provides in silico and in vitro methods for identifying compounds that modulate the activity and/or expression of a calcium-sensing receptor, disclosed above. 
     3.1 In silico Methods 
     The presently disclosed subject matter further provides in silico methods for identifying compounds that can potentially interact with a calcium-sensing receptor and/or modulate the activity and/or expression of a calcium-sensing receptor. 
     In certain embodiments, the method can include predicting the three-dimensional structure (3D) of a calcium-sensing receptor and screening the predicted 3D structure with putative calcium-sensing receptor modulating compounds (i.e., test compounds). The method can further include predicting whether the putative compound would interact with the binding site of the receptor by analyzing the potential interactions with the putative compound and the amino acids of the receptor. The method can further include identifying a test compound that can bind to and/or modulate the biological activity of the calcium-sensing receptor by determining whether the 3D structure of the compound fits within the binding site of the 3D structure of the receptor. 
     In certain embodiments, the calcium-sensing receptor for use in the disclosed method can have the amino acid sequence of SEQ ID NO: 4, 5 or 6, or a fragment or variant thereof. In certain embodiments, the calcium-sensing receptor for use in the presently disclosed subject matter can include a receptor comprising an amino acid sequence having at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to SEQ ID NO: 4, 5 or 6, or a fragment or variant thereof. In certain embodiments, the calcium-sensing receptor for use in the disclosed method can have the nucleotide sequence of SEQ ID NO: 1, 2, 3 or 7, or a fragment or variant thereof. In certain embodiments, the calcium-sensing receptor for use in the presently disclosed subject matter can include a receptor comprising a nucleotide sequence having at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to SEQ ID NO: 1, 2, 3 or 7, or a fragment or variant thereof. 
     Non-limiting examples of compounds (e.g., potential calcium-sensing receptor modulators) that can be tested using the disclosed methods include any small chemical compound, or any biological entity, such as peptides, salts, amino acids and kokumi compounds known in the art, e.g. glutathione. In certain embodiments, the test compound can be a small chemical molecule. 
     In certain embodiments, structural models of a calcium-sensing receptor can be built using crystal structures of other GPCRs as templates for homology modelling. For example, and not by way of limitation, structural models can be generated using the crystal structures of Group C GPCRs. In certain embodiments, a structural model of a calcium-sensing receptor can be based on a known or a combination of known crystal structures of GPCRs. (See, e.g., Lee et al., Eur J Pharmacol. 2015 May 14. pii: S0014-2999(15)30012-1, which is incorporated by reference in its entirety herein). In certain embodiments, a structural model of a calcium-sensing receptor can be generated based on the crystal structure of an mGluR protein. For example, and not by way of limitation, a structural model of the flytrap domain (VFT) of a calcium-sensing receptor can be generated based on the crystal structure having the protein data base (PDB) ID No. 1EWK. In certain embodiments, a structural model of the 7 transmembrane domain (7TM) of a calcium-sensing receptor can be generated based on the crystal structures of mGluR proteins having PDB ID Nos. 4OR2 and 4OO9.  FIG. 13  depict structural models of calcium-sensing receptors that can be used in the disclosed in silico methods. Any suitable modeling software known in the art can be used. In certain embodiments, the Modeller software package can be used to generate the three-dimensional protein structure. 
     In certain embodiments, the in silico methods of identifying a compound that binds to a calcium-sensing receptor comprises determining whether a test compound interacts with one or more amino acids of a calcium-sensing receptor interacting domain, as described herein. 
     Compounds that are identified by the disclosed in silico methods can be further tested using the in vitro methods disclosed herein. 
     3.2 Calcium-Sensing Receptor Binding Site 
     The present application provides for methods of screening for compounds that modulate the activity of a calcium-sensing receptor, for example, a feline, canine or human calcium-sensing receptor, wherein the compounds interact with one or more amino acids of the calcium-sensing receptor. In certain embodiments, the binding site of a calcium-sensing receptor comprises amino acids within the 7 transmembrane domain (7TM) of the receptor, or Venus Flytrap domain (VFT) domain of the receptor, and can be identified by generating an interaction map of the receptor using in silico modeling, as described herein. In one non-limiting example, the presence of an amino acid in the 7TM or VFT interaction map means that the residue is in the vicinity of the ligand binding environment, and interacts with the ligand. 
     In certain embodiments, the interaction between a compound and one or more amino acids of the calcium-sensing receptors described herein can comprises one or more hydrogen bond, covalent bond, non-covalent bond, salt bridge, physical interaction, and combinations thereof. The interactions can also be any interaction characteristic of a ligand receptor interaction known in the art. Such interactions can be determined by, for example, site directed mutagenesis, x-ray crystallography, x-ray or other spectroscopic methods, Nuclear Magnetic Resonance (NMR), cross-linking assessment, mass spectroscopy or electrophoresis, cryo-microscopy, displacement assays based on known agonists, structural determination and combinations thereof. In certain embodiments, the interactions are determined in silico, for example, by theoretical means such as docking a compound into a feline or canine calcium-sensing receptor binding pocket as described herein, for example, using molecular docking, molecular modeling, molecular simulation, or other means known to persons of ordinary skill in the art. 
     In certain embodiments, the interaction is a hydrogen bond interaction. 
     In certain embodiments, the interaction is a hydrophobic interaction. 
     In certain embodiments, the compounds identified according to the methods described herein that modulate the activity of a calcium-sensing receptor interact with one or more amino acids in a VFT domain of the calcium-sensing receptor. In certain embodiments, the amino acids that the compounds interact with comprise 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 or more of ASN64, PHE65, ASN102, THR145, SER169, SER170, ASP190, GLN193, ASP216, TYR218, SER271, SER272, GLY273, GLU297, ALA298, TRP299, ALA300, SER301, SER302, LEU304, ALA321, TYR411, THR412, and HIS413 of a calcium-sensing receptor, for example, a calcium-sensing receptor comprising a feline calcium-sensing receptor described by SEQ ID NO:4, a canine calcium-sensing receptor described by SEQ ID NO:5 or a human calcium-sensing receptor described by SEQ ID NO:6. 
     In certain embodiments, the compounds identified according to the methods described herein that modulate the activity of a calcium-sensing receptor interact with one or more amino acids in a transmembrane domain of the calcium-sensing receptor, for example, a seven transmembrane domain (7TM). In certain embodiments, the amino acids that the compounds interact with comprise 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more of ARG680; PHE684; GLY685; PHE688; VAL689 on Helix 3; GLN735 on Helix 4; ALA772; PHE775; LEU776; THR780; CYS781 on Helix 5; PHE814; VAL817; TRP818; PHE821 on Helix 6; GLU837; ALA840; ILE841; ALA844 on Helix 7; and MET771 and GLU767 on the EC2 loop of a calcium-sensing receptor, for example, a calcium-sensing receptor comprising a feline calcium-sensing receptor described by SEQ ID NO:4, a canine calcium-sensing receptor described by SEQ ID NO:5 or a human calcium-sensing receptor described by SEQ ID NO:6. 
     3.3 In Vitro Methods 
     The presently disclosed subject matter further provides in vitro methods for identifying compounds that can modulate the activity and/or expression of a calcium-sensing receptor. 
     The calcium-sensing receptors for use in the presently disclosed methods can include isolated or recombinant calcium-sensing receptors or cells expressing a calcium-sensing receptor, disclosed herein. In certain embodiments, the calcium-sensing receptor for use in the disclosed methods can have the amino acid sequence of SEQ ID NO: 4, 5 or 6, or a fragment or variant thereof In certain embodiments, the calcium-sensing receptor for use in the disclosed method can have at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to the amino acid sequence of SEQ ID NO: 4, 5 or 6, or a fragment or variant thereof. In certain embodiments, the calcium-sensing receptor for use in the disclosed method can have the nucleotide sequence of SEQ ID NO: 1, 2, 3 or 7, or a fragment or variant thereof. In certain embodiments, the calcium-sensing receptor for use in the presently disclosed subject matter can include a receptor comprising a nucleotide sequence having at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to SEQ ID NO: 1, 2, 3 or 7, or a fragment or variant thereof. 
     In certain embodiments, the method for identifying compounds that modulate the activity and/or expression of a calcium-sensing receptor comprises measuring the biological activity of a calcium-sensing receptor in the absence and/or presence of a test compound. In certain embodiments, the method can include measuring the biological activity of a calcium-sensing receptor in the presence of varying concentrations of the test compound. The method can further include identifying the test compounds that result in a modulation of the activity and/or expression of the calcium-sensing receptor compared to the activity and/or expression of the calcium-sensing receptor in the absence of the test compound. 
     In certain embodiments, the compounds identified according to the methods described herein increase the biological activity of a calcium-sensing receptor by at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or more, compared to the biological activity of the calcium-sensing receptor when the compound is not present. 
     In certain embodiments, the method can further include analyzing two or more, three or more or four or more test compounds in combination. In certain embodiments, the two or more, three or more or four or more test compounds can be from different classes of compounds, e.g., amino acids and small chemical compounds. For example, and not by way of limitation, the method can include analyzing the effect of one or more small chemical test compounds on the biological activity and/or expression of a calcium-sensing receptor in the presence of one or more amino acid test compounds. In certain embodiments, the method for identifying compounds activity and/or expression of a calcium-sensing receptor comprises analyzing the effect of a test compound on the biological activity and/or expression of a calcium-sensing receptor in the presence of a salt, e.g., a calcium salt. 
     In certain embodiments, the method for identifying compounds that modulate the activity and/or expression of a calcium-sensing receptor comprises determining whether a compound modulates the receptor directly, for example, as an agonist or antagonist. In certain embodiments, the method comprises determining whether a compound indirectly modulates the activity of the receptor (e.g., as an allosteric modulator), for example, by enhancing or decreasing the effect of other compounds on activating or inhibiting receptor activity. 
     In certain embodiments, the method for identifying compounds that modulate the activity and/or expression of a calcium-sensing receptor comprises expressing a calcium-sensing receptor in a cell line and measuring the biological activity of the receptor in the presence and/or absence of a test compound. The method can further comprise identifying test compounds that modulate the activity of the receptor by determining if there is a difference in receptor activation in the presence of a test compound compared to the activity of the receptor in the absence of the test compound. In certain embodiments, the selectivity of the putative calcium-sensing receptor modulator can be evaluated by comparing its effects on other GPCRs or taste receptors, e.g., umami, fatty acid, T1R, etc. receptors. 
     Activation of the receptor in the disclosed methods can be detected through the use of a labeling compound and/or agent. In certain embodiments, the activity of the calcium-sensing receptor can be determined by the detection of secondary messengers such as, but not limited to, cAMP, cGMP, IP3, DAG or calcium. In certain embodiments, the activity of the calcium-sensing receptor can be determined by the detection of the intracellular calcium levels. Monitoring can be by way of luminescence or fluorescence detection, such as by a calcium sensitive fluorescent dye. In certain embodiments, the intracellular calcium levels can be determined using a cellular dye, e.g., a fluorescent calcium indicator such as Calcium 4. In certain embodiments, the intracellular calcium levels can be determined by measuring the level of calcium binding to a calcium-binding protein, for example, calmodulin. Alternatively and/or additionally, activity of the calcium-sensing receptor can be determined by detection of the phosphorylation, transcript levels and/or protein levels of one or more downstream protein targets of the calcium-sensing receptor. 
     The cell line used in the disclosed methods can include any cell type that is capable of expressing a calcium-sensing receptor. Non-limiting examples of cells that can be used in the disclosed methods include HeLa cells, Chinese hamster ovary cells (CHO cells), African green monkey kidney cells (COS cells),  Xenopus  oocytes, HEK-293 cells and murine 3T3 fibroblasts. In certain embodiments, the method can include expressing a calcium-sensing receptor in HEK-293 cells. In certain embodiments, the method can include expressing a calcium-sensing receptor in COS cells. In certain embodiments, the cells constitutively express the calcium-sensing receptor. In another embodiment, expression of the CaSR by the cells is inducible. 
     In certain embodiments, the cell expresses a calcium-binding photoprotein, wherein the photoprotein luminesces upon binding calcium. In certain embodiments, the calcium binding photoprotein comprises the protein clytin. In certain embodiments the clytin is a recombinant clytin. In certain embodiments, the clytin comprises an isolated clytin, for example, a clytin isolated from  Clytia gregarium . In certain embodiments, the calcium-binding photoprotein comprises the protein aequorin, for example, a recombinant aequorin or an isolated aequorin, such as an aequorin isolated from  Aequorea victoria . In certain embodiments, the calcium-binding photoprotein comprises the protein obelin, for example, a recombinant obelin or an isolated obelin, such as an obelin isolated from  Obelia longissima.    
     In certain embodiments, expression of a calcium-sensing receptor in a cell can be performed by introducing a nucleic acid encoding a calcium-sensing receptor into the cell. For example, and not by way of limitation, a nucleic acid having the nucleotide sequence set forth in SEQ ID NO: 1, 2, 3 or 7, or a fragment thereof, can be introduced into a cell. In certain embodiments, the introduction of a nucleic acid into a cell can be carried out by any method known in the art, including but not limited to transfection, electroporation, microinjection, infection with a viral or bacteriophage vector containing the nucleic acid sequences, cell fusion, chromosome-mediated gene transfer, microcell-mediated gene transfer, spheroplast fusion, etc. Numerous techniques are known in the art for the introduction of foreign genes into cells (see, e.g., Loeffler and Behr, Meth. Enzymol. 217:599-618 (1993); Cohen et al., Meth. Enzymol. 217:618-644 (1993); Cline, Pharmac. Ther. 29:69-92 (1985), the disclosures of which are hereby incorporated by reference in their entireties) and can be used in accordance with the disclosed subject matter. In certain embodiments, the technique can provide for stable transfer of nucleic acid to the cell, so that the nucleic acid is expressible by the cell and inheritable and expressible by its progeny. In certain embodiments, the technique can provide for a transient transfer of the nucleic acid to the cell, so that the nucleic acid is expressible by the cell, wherein heritability and expressibility decrease in subsequent generations of the cell&#39;s progeny. 
     In certain embodiments, the nucleic acid encoding a calcium-sensing receptor is comprised in a cloning vector, for example, a pcDNA3.1 vector or a pcDNA5 TO vector, that is introduced into the cell. 
     In certain embodiments, the method can include identifying compounds that bind to a calcium-sensing receptor. The method can comprise contacting a calcium-sensing receptor with a test compound and measuring binding between the compound and the calcium-sensing receptor. For example, and not by way of limitation, the methods can include providing an isolated or purified calcium-sensing receptor in a cell-free system, and contacting the receptor with a test compound in the cell-free system to determine if the test compound binds to the calcium-sensing receptor. In certain embodiments, the method can comprise contacting a calcium-sensing receptor expressed on the surface of a cell with a candidate compound and detecting binding of the candidate compound to the calcium-sensing receptor. The binding can be measured directly, e.g., by using a labeled test compound, or can be measured indirectly. In certain embodiments, the detection comprises detecting a physiological event in the cell caused by the binding of the compound to the calcium-sensing receptor, e.g., an increase in the intracellular calcium levels. For example, and not by way of limitation, detection can be performed by way of fluorescence detection, such as a calcium sensitive fluorescent dye, by detection of luminescence, or any other method of detection known in the art. 
     In certain non-limiting embodiments, the in vitro assay comprises cells expressing a calcium-sensing receptor that is native to the cells. Examples of such cells expressing a native calcium-sensing receptor include, for example but not limited to, dog (canine) and/or cat (feline) taste cells (e.g., primary taste receptor cells). In certain embodiments, the dog and/or cat taste cells expressing a calcium-sensing receptor are isolated from a dog and/or cat and cultured in vitro. In certain embodiments, the taste receptor cells can be immortalized, for example, such that the cells isolated from a dog and/or cat can be propagated in culture. 
     In certain embodiments, expression of a calcium-sensing receptor in a cell can be induced through gene editing, for example, through use of the CRISPR gene editing system to incorporate a calcium-sensing receptor gene into the genome of a cell, or to edit or modify a calcium-sensing receptor gene native to the cell. 
     In certain embodiments, the in vitro methods of identifying a compound that binds to a calcium-sensing receptor comprises determining whether a test compound interacts with one or more amino acids of a calcium-sensing receptor interacting domain, as described herein. 
     In certain embodiments, compounds identified as modulators of a calcium-sensing receptor can be further tested in other analytical methods including, but not limited to, in vivo assays, to confirm or quantitate their modulating activity. 
     In certain embodiments, methods described herein can comprise determining whether the calcium-sensing receptor modulator is a kokumi taste enhancing compound, e.g., a calcium-sensing receptor agonist. 
     In certain embodiments, the methods of identifying a calcium-sensing receptor modulator can comprise comparing the effect of a test compound to a calcium-sensing receptor agonist. For example, a test compound that increases the activity of the receptor compared to the activity of the receptor when contacted with a calcium-sensing receptor agonist can be selected as a calcium-sensing receptor modulating compound (e.g., as an agonist). 
     In certain embodiments, the methods of identifying a calcium-sensing receptor modulator can comprise determining whether a test compound modulates the activity of the receptor when the receptor is contacted with an agonist, or whether the test compound can modulate the activity of a positive allosteric modulator (PAM). Test compounds that increase or decrease the effect of said agonist or PAM on the receptor can be selected as a calcium-sensing receptor modulating compound (e.g., as an allosteric modulator). 
     Calcium-sensing receptor agonists and PAMs that can be used according to said methods can comprise one or more compounds described by Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 CaSR agonists and PAMs 
               
            
           
           
               
               
               
            
               
                 Compound: 
                 CAS number: 
                 Chemical structure: 
               
               
                   
               
               
                 Calcium (Ca 2+ ) 
                 7440-70-2 
                 CaCl 2   
               
               
                 (agonist) 
                   
                   
               
               
                 Magnesium (Mg 2+ ) 
                 7786-30-3 
                 MgCl 2   
               
               
                 (agonist) 
                   
                   
               
               
                   
               
               
                 Spermine (agonist) 
                 71-44-3 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Spermidine (agonist) 
                 124-20-9 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Putrescine (not active) 
                 110-60-1 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 L-Glutathione (agonist) 
                 70-18-8 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Neomycin (agonist) 
                 1404-04-2 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Poly-L-Arginine (agonist) 
                 26982-20-7 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Cinacalcet (PAM) 
                 226256-56-0 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Calindol (PAM) 
                 729610-18-8 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     In certain embodiments, the calcium-sensing receptor modulators of the present disclosure comprise a salt of the calcium-sensing receptor modulator, for example, but not limited to, an acetate salt or a formate salt. In certain embodiments, the calcium-sensing receptor modulator salt comprises an anion (−) (for example, but not limited to, Cl − , O 2− , CO 3   2− , HCO 3   − , OH − , NO 3   − , PO 4   3− , SO 4   2− , CH 3 COO − , HCOO −  and C 2 O 4   2− ) bonded via an ionic bond with a cation (+) (for example, but not limited to, Al 3+ , Ca 2+ , Na + , K + , Cu 2+ , H + , Fe 3+ , Mg 2+ , NH 4   +  and H 3 O + ). In other embodiments, the calcium-sensing receptor agonist salt comprises a cation (+) bonded via an ionic bond with an anion (−). 
     In certain embodiments, the calcium-sensing receptor modulators of the present application are identified through in silico modeling of a calcium-sensing receptor (“Kokumi receptor”), e.g., a feline or a canine calcium-sensing receptor, wherein the calcium-sensing receptor agonists of the present application comprise a structure that fits within a binding site of the calcium-sensing receptor. In certain embodiments, the in silico method comprises the in silico methods described above and in the Examples section of the present application. 
     In certain embodiments, the calcium-sensing receptor modulators of the present application are identified through an in vitro method, wherein the calcium-sensing receptor agonist compounds activate and/or modulate a calcium-sensing receptor, disclosed herein, expressed by cells in vitro. In certain embodiments, the in vitro method comprises the in vitro methods described above and in the Examples section of the present application. 
     EXAMPLES 
     The presently disclosed subject matter will be better understood by reference to the following Examples, which are provided as exemplary of the invention, and not by way of limitation. 
     Example 1—Identification of CaSR Modulators Using In Silico Assays 
     The present example describes the computational modeling of the feline and canine calcium-sensing receptor (CaSR) to identify putative compound modulators. 
     Computational approaches were used to analyze the three-dimensional structure of the calcium-sensing receptor to identify polypeptide regions that can be exploited to selectively modulate the calcium-sensing receptor. A structural homology model of the various domains of the calcium-sensing receptor was generated based on the structures of class C GPCRs (See Binet et al., J. Biol. Chem, 282(16): 12154-63 (2007); Wu et. al., Science, 344(6179):58-64 (2014); and Dore et al., Nature 511:557-562 (2014); each of which are incorporated by reference herein in their entireties). The homology models were built with the Discovery Studio (DS) suite of programs from Accelrys. Specifically, the Modeller program from DS was used (see Eswar et al., Current Protocols in Bioinformatics, Supplement 15:5.6.1-5.6.30 (2006), which is incorporated by reference herein in its entirety). “In silico” screening was used to identify compounds that interact with the structural domains of the calcium-sensing receptor. 
     The GPCR group C family of proteins includes T1R1, T1R2, T1R3, CaSR, GabaB and mGlu proteins. Group C proteins have (1) a large external domain, called a Venus Flytrap domain (VFT), (2) a 7 transmembrane domain (7TM) and (3) a cysteine rich domain that connects the VFT and the 7TM. A sequence alignment of the amino acid sequence of the feline and canine CaSRs was performed and showed an overall 95% sequence identity ( FIG. 9 ). A sequence alignment of the amino acid sequences of the feline, canine and human CaSRs was performed and showed an overall 93.2% sequence identity ( FIG. 10 ). The VFT domain of CaSR consists of amino acids 1-523 and the 7TM domain consists of amino acids 601-865. The active site of the feline, canine and human VFT domain and the allosteric site of their 7TM domains are almost identical ( FIGS. 11 and 12 ). Only two differences were observed near the active site of feline and human CaSR VFTs ( FIG. 11 ).  FIG. 12  shows the differences in amino acids within the 7TM domains are shown on the structure in the Corey-Pauling-Koltun (CPK) representation. No differences were observed in the makeup of the allosteric site of CaSR. 
     A homology model of the VFT domain of the CaSR receptor was generated based on numerous crystal structures of mGlu available from the Protein Data Bank (PDB). There is 27% sequence identity between the VFT domain of mGluR1 (1EWK structure from PDB) and the CaSR alignment of VFT domains. Glutathione, a known CaSR agonist, was docked to the hinge area of CaSR ( FIG. 13 ). Without being bound to a particular theory, it appears that glutathione binds to the hinge region of the VFT domain of CaSR, where the carboxyl end of glutathione forms a hydrogen bond with the backbone of CaSR ( FIG. 13 ). Glutathione was observed to have potential interactions with amino acids ASN64, PHE65, ASN102, THR145, SER169, SER170, ASP190, GLN193, ASP216, TYR218, SER271, SER272, GLY273, GLU297, ALA298, TRP299, ALA300, SER301, SER302, LEU304, ALA321, TYR411, THR412, and HIS413 ( FIG. 13 ). 
     Phenylalanine, a compound that can bind CaSR, was docked in the in silico model along with Calcium (Ca 2+ ), which was docked to the hinge region of CaSR. Ca 2+  formed a salt bridge to ASP216 and GLU297, and the carboxyl group of phenylalanine formed interactions with the hinge region of CaSR and with the calcium. The following amino acids were shown to interact with phenylalanine: TYR218, THR145, SER147, SER170 and SER272. 
     A homology model of the feline and canine CaSR 7M domain was generated based on the crystal structures of 4OR2 and 4OO9 from PDB (see helix plot of  FIG. 14 ; see also,  FIGS. 14, 15 and 16  for helix plots of canine, feline and human CaSRs, respectively). 4OR2 is the crystal structure of the transmembrane domain of mGluR1 from Group C GPCR bound to a negative allosteric modulator (NAM) (see Wu et. al., Science, 344(6179):58-64 (2014), which is incorporated by reference herein in its entirety). 4OO9 is the crystal structure of the transmembrane domain of mGluR5 from Group C GPCR bound to NAM (see Dore et al., Nature 511:557-562 (2014), which is incorporated by reference herein in its entirety). There is approximately 30% sequence identity between the 7TM domain of mGluR1 (4OR2 structure from PDB) and the CaSR 7TM domain. The known potent positive allosteric modulator, calindol, was docked into the allosteric site of the transmembrane domain of human, canine and feline CaSR ( FIG. 17 ). The docking program, BioDock, from BioPredict was used. Calindol was observed to have the following potential interactions with the amino acids of allosteric site of human, canine and feline CaSR: ARG680; PHE684; GLY685; PHE688; VAL689 on Helix 3; GLN735 on Helix 4; ALA772; PHE775; LEU776; THR780; CYS781 on Helix 5; PHE814; VAL817; TRP818; PHE821 on Helix 6; GLU837; ALA840; ILE841; ALA844 on Helix 7; and MET771 and GLU767 on the EC2 loop of the CaSR. 
     Example 2—Identification of Feline CaSR Agonists Using In Vitro Assays 
     The present example describes an in vitro assay for identifying compounds that modulate the activation of the feline calcium-sensing receptor. 
     Compounds identified by in silico modeling with the calcium-sensing receptor, as detailed above in Example 1, as putative calcium-sensing receptor modulators are selected for further testing in vitro. In vitro functional characterization of the selected agonist compounds is used to evaluate the effectiveness of a putative agonist compound in activating the calcium-sensing receptor. 
     HEK293 cells that stably express a calcium-sensing receptor are exposed to putative compounds to modulate the activity and/or expression of the calcium-sensing receptor. Activation of the calcium-sensing receptor is detected by a change in intracellular calcium levels using a calcium sensitive fluorescent dye. Cells that do not express the calcium-sensing receptor or that are not contacted with the compound are used as a control. A FLIPR® Tetra or a FlexStation® 3 is used for data capture. 
     For each putative calcium-sensing receptor modulator, dose response curves are generated and the EC 50  value of the putative calcium-sensing receptor modulator is determined. The term half maximal effective concentration (EC 50 ) refers to the concentration of a compound which induces a response halfway between the baseline and the maximum after a specified exposure time. 
     Example 3—Identification of Canine CaSR Agonists Using In Vitro Assays 
     The present example describes an in vitro assay for identifying compounds that modulate the activation of the canine calcium-sensing receptor. 
     Compounds identified by in silico modeling with a calcium-sensing receptor, as detailed above in Example 1, as putative calcium-sensing receptor modulators are selected for further testing in vitro. In vitro functional characterization of the selected modulators is used to evaluate the effectiveness of a putative agonist compound in activating the calcium-sensing receptor. 
     HEK293 cells that stably express the canine CaSR are exposed to putative compounds to modulate the activity and/or expression of the calcium-sensing receptor. Activation of the calcium-sensing receptor is detected by a change in intracellular calcium levels using a calcium sensitive fluorescent dye. Cells that do not express the calcium-sensing receptor or that are not contacted with the compound are used as a control. A FLIPR® Tetra or a FlexStation® 3 is used for data capture. 
     For each putative calcium-sensing receptor modulator, dose response curves are generated and the EC 50  value of the putative calcium-sensing receptor modulator is determined. The term half maximal effective concentration (EC 50 ) refers to the concentration of a compound which induces a response halfway between the baseline and the maximum after a specified exposure time. 
     Example 4—In Vitro Assay for Identification of Feline CaSR Modulators 
     The present example describes an in vitro assay for identifying compounds that modulate the activation of the feline calcium-sensing receptor. 
     Summary. The full length coding sequence of the calcium-sensing receptor (CaSR, a GPCR (3, C or Glutamate family), naturally coupled to Gαq/11 and Gαi (ligand-directed signaling)) from  Felis catus  (fCaSR) having the sequence described by SEQ ID NO: 7 was synthesized and sub-cloned into suitable expression vectors. The cat CaSR expression constructs were stably transfected in two mammalian cell lines, the HEK-natClytin and the HEK T-Rex/natClytin, two cell lines that allow for detection of changes in intracellular calcium levels through luminescence. A pure selected clone (K 5.1) was used to analyze 12 CaSR ligands (including calcium as a reference agonist control) in a high-throughput screening (HTS) multi-plate test for agonist and positive allosteric modulator (PAM) effects. 7 of the ligands tested (including calcium control) were identified as agonists, and 2 were identified as PAM of CaSR, demonstrating that the in vitro cellular assay described by the present study is a reliable, reproducible and robust functional cell based assay for the cat CaSR receptor suitable for HTS. 
     Results. HEK-natClytin and HEK T-Rex/natClytin cells were transfected with vectors comprising fCaSR. Mock transfections were carried out in parallel as negative controls. After antibiotic selections, a 1º limiting dilution of the two transfected target and mock pools were performed and then analyzed with calcium as a reference agonist and luminescence photoprotein as read out of activated fCaSR using a FLIPR® Tetra screening system. Based on clone pool analysis results, the HEK T-Rex/natClytin responding clones were selected for further evaluation. A 2º limiting dilution of selected clones was performed. Four of the 2º limiting dilution clones were characterized, and two responding clones, (K1.5,K10.3) were completely optimized for cell growth, cell density and cell seeding time conditions, for DMSO sensitivity, for signal stability over time and for frozen cells use. 
     A final pure selected clone (K 5.1) was analyzed in a HTS multi-plate test for agonist or positive allosteric modulator (PAM) effects of 12 CaSR ligands (including calcium as a reference agonist control). 10.000 cw of K 5.1 were seeded on poly-D-lys coated 384 well MTPs in medium containing 1 μg/mL of Doxycycline. Screening with the 12 ligands (including calcium control) was conducted 24 hours later. Dose response curves for the activation of CaSR by the ligands in agonist mode testing are shown in  FIG. 18 . Dose response curves for the activation of CaSR by the ligands cinacalcet and calindol in PAM mode testing are shown in  FIG. 19 . As described by Table 2, 7 of the ligands tested (including calcium control) activated CaSR as agonists, and 2 (cinacalcet and calindol) activated CaSR as PAMs. For each ligand that activated CaSR as an agonist or PAM, the EC 50  value of the ligand was determined. The term half maximal effective concentration (EC 50 ) refers to the concentration of a compound which induces a response halfway between the baseline and the maximum after a specified exposure time. 
                     TABLE 2                  The Effect of CaSR ligands on CaSR activation as an agonist or PAM                                         Enhancer       COMPOUND   MAX dose   Agonist EC 50     (PAM) EC 50                                           Calcium (Ca 2+ )   15   mM   1.5 mM   not active       Magnesium (Mg 2+ )   30   mM   6.1 mM   not active       Spermine   3   mM   0.5 mM   Not active       Spermidine   3   mM   1.3 mM   Not active       Putrescine   30   mM   Not active   Not active       L-Glutathione   30   mM   8.2 mM   Not active       Neomycin   30   mM    10 mM   Not active       Poly-L-Arginine   60   μg/mL   4.6 μg/mL   Not active       Cinacalcet   100   μM   Not active   1.0 μM       Calindol   100   μM   Not active   1.9 μM                    
Methods
 
     Cloning of the feline calcium-sensitive receptor into a constitutive expression vector._The Felis catus CaSR gene (fCaSR) is encoded by a gene described by GenBank accession no. NM_001164654.1, and expresses a protein described by GenBank accession no. NP_001158126. The fCaSR coding sequence (SEQ ID NO:7) was synthesized by GeneArt (construct 15AAQ7BP_fCaSR_pMK), with Kozak sequence, Y987T, M1066C, R1269G and S3131G. The coding sequence was excised from the construct with 5′NotI-3′ApaI, and inserted into a pcDNA3.1 vector opened with the same restriction enzymes. After restriction analysis with BamHI enzyme for the screening of positive constructs, clone K4, with correct DNA fragmentation (6217+2781 pb), was selected. The presence of the entire fCaSR coding region was confirmed by sequencing. The pcDNA3.1_fCaSR vector clone comprising the fCaSR coding region is shown in  FIG. 20 . 
     Cloning of the feline calcium-sensitive receptor into an inducible expression vector for the T-REx™ System._The fCaSR coding sequence (SEQ ID NO:7) described above was inserted into a pcDNA5 TO vector opened with the same restriction enzymes. After restriction analysis with BamHI enzyme for the screening of positive constructs, clone K14, with correct DNA fragmentation (5888+2781 pb), was selected. The presence of the entire fCaSR coding region was confirmed by sequencing. The pcDNA5 TO_fCASR vector clone comprising the fCaSR coding region is shown in  FIG. 21 . 
     Transfection of HEK-natClytin and HEK-293 T-REx cell lines. Transfection was performed by electroporation. 2×10 6  cells detached from 60-70% confluent flasks were transfected with a total amount of 10 μg of DNA construct by using the 
     Gene Pulser II electroporator (Biorad) (parameters: 300 Volts, 950 μF). Transfected cells were immediately diluted in wild-type complete medium and seeded in T75 flasks. After 48 h, the proper antibiotic concentration was added to the medium and cells were cultured at 37° C. 5% CO 2  for about 3 weeks to generate stable pools. 
     Detection of CaSR receptor activation. Transfected cells were analyzed for a response to a reference agonist (calcium) or various CaSR ligands by detecting Ca 2+  sensitive photoprotein luminescence. The experiments were performed in a 384 well format according to one of the following procedures:
         7.500 cw were plated on poly-D-lys coated 384 well microtiter plates (MTPs). 24 h later, medium was manually discarded and replaced with fresh medium containing 1 μg/mL of Doxycycline. Experiments were then carried out 48 h after seeding.   10.000 cw were plated on poly-D-lys coated 384 well MTPs in medium containing 1 μg/mL of Doxycycline. Experiments were carried out 24 hrs later.       

     On the day of the experiment, medium was removed by plate overthrow and tapping on a paper towel. Cells were then incubated for 3-4 hrs at 37° C. with 10 μM coelenterazine dissolved in 2 mM Ca 2+  or Ca 2+ -Free Tyrode&#39;s Buffer (20 μL). Cells were exposed to calcium control and/or ligand and the luminescence changes over a period of 1 minute (at 35.000 gain of sensitivity) were monitored using a FLIPR® Tetra screening system. For agonist evaluation, a single exposure protocol was applied (ligands 1º injection, 20 μL-2×), whereas for PAM evaluation a double injection protocol was applied as following:
         1° injection: ligands dose-response (20 μL-2×)   2° injection: Reference agonist EC20 (20 μL-3×)   Data Analysis. FLIPR® Tetra measurements were analysed with Screenworks© software (Molecular Devices, Version 3.0.1.4) and data were exported as Maximum statistics of Absolute Response (RLU) calculated after compound injection. RLU is obtained applying “Subtract Bias on Sample n” (where n=Time point of compound injection). Mean and standard deviation values were calculated on the exported data with Excel software, and then values were used to create sigmoidal dose-response curves (variable slope) or histograms with GraphPad PRISM® software (Version 6). The obtained EC 50 /IC 50  values were used to calculate EC 50 /IC 100  values according to the following formula:
 
 EC   X =[( X/ 100− X )1/HillSlope]* EC   50  
       

     As the calculated IC 100  value would be infinite, the IC 99  value was calculated as an approximation. The robust Z prime (RZ′), the Intraplate Variability and the Interplate Variability were calculated onto minimum (Control Reference, CR) well signals and maximum (Signal Reference, SR) well signals, according to the following formulae: 
     
       
         
           
             
                 
             
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     Cell lines, Medium and Culture Conditions. The experiments were done using HEK-natClytin or HEK T-Rex/natClytin reporter cell lines that were generated by stable transfection of the photoprotein natClytin into HEK-293 or HEK-293 T-Rex (HEK T-Rex, Invitrogen) cell lines respectively. 
     Culture medium: HEK-natClytin cell line. Minimum Essential Medium Eagle with Earle&#39;s Salts (EMEM, BioWhittaker cat. BE12-125F); 10% FBS (Fetal Bovine Serum, Sigma cat. F7524); 1% Penicillin-Streptomycin (BioWhittaker cat. DE17-602E); 2 mM Ultraglutamine 1 (BioWhittaker cat. BE17-605E/U1), 0.2 μg/mL of Puromicin (InvivoGen cat. ant-pr-1). HEK-natClytin/fCaSR cell medium was supplemented with 0.8 mg/mL G418 (InvivoGen cat. ant-gn-5). 
     Culture medium: HEK-293 T-REx cell line. DMEM High Glucose (Lonza BioWhittaker cat. BE12-604F/U1; 500 mL) supplemented with Fetal Bovine Serum TET-FREE (Euroclone cat. EC 50182L; 50 mL), Penicillin-Streptomycin (BioWhittaker, cat. DE17-602E; 5 mL of 100× Solution), Blasticidin 5 μg/ml (InvivoGen, cat. ant-bl-1). HEK T-Rex/natClytin cell medium was supplemented with 1 μg/mL of Puromicin (InvivoGen cat. ant-pr-1). HEK T-Rex/natClytin-fCaSR cell medium was supplemented with 0.5 μg/mL of Puromicin and 50 μg/mL of Hygromycin (InvivoGen cat. ant-hg-5) during selection and then maintained in medium supplemented with 0.25 μg/mL of Puromicin and 25 μg/mL of Hygromycin. 
     Culture and seeding conditions. Cells were split every 3-4 days by gentle wash with PBS, followed by 5 min. incubation at 37° C. with Trypsin. Detached cells were diluted with complete medium and counted using the Becman Coulter Z1TM Particle Counter. The desired number of cell was plated into a new flask or used for adherent experiments. 1.5-2.0×10 6  cells were seeded in a T75 flask twice a week, recovering about 10-12×10 6  cells at ˜80% confluence after 3-4 days. As an alternative, an 80% confluent flask could be diluted 1:5-1:10 twice a week. 
     For experimental conditions, cells were plated on poly-D-lys coated 384 well MTPs according to one of the following conditions:
         7.500 cw. 24 h later, medium was manually discarded and replaced with fresh medium containing 1 μg/mL of Doxycycline. Experiments were then carried out 48 h after seeding.   10.000 cw in medium containing 1 μg/mL of Doxycycline. 24 h later experiments were then carried out.   Buffers and Ligands. The following buffers and ligands were used in the present study:   PBS: D-PBS without calcium and magnesium; EuroClone, cat. ECB4004L.   Trypsin: Trypsin—0.05% EDTA 0.02% in PBS: EuroClone, cat. EC B3052D.   Doxycycline: Sigma, cat. D9891; 20 mg/mL stock solution is prepared in water and stored in the dark at −20° C.   2 mM Ca 2+  Tyrode&#39;s Buffer: 130 mM NaCl, 5 mM KCl, 2 mM CaCl 2 , 5 mM NaHCO 3 , 1 mM MgCl 2 , 20 mM HEPES, pH 7.4 sterile filtered and autoclaved.   Ca 2+ -Free Tyrode&#39;s Buffer: 130 mM NaCl, 5 mM KCl, 1 mM MgCl 2 , 5 mM NaHCO 3 , 20 mM HEPES; pH 7.4 sterile filtered and autoclaved.   Dimethyl sulfoxide (DMSO): Sigma, cat. 34869.   Coelenterazine: (Biosynth, cat. C-7001), 5 mg/mL prepared in DMSO-Glutathione 30 μM (Sigma G-6529), stored in aliquots at −20° C.   Calcium chloride (dihydrate): Sigma, cat. 223506, 1 M stock solution, prepared in water and stored at RT.   Spermine: Sigma, cat. 53256, 100 mM stock solution, prepared in 100% DMSO, stored in aliquots at −20° C.   Spermidine: Sigma, cat. 52626, 100 mM stock solution, prepared in 100% DMSO, stored in aliquots at −20° C.   Putrescine: Sigma, cat. P7505, 100 mM stock solution, prepared in water and stored in aliquots at −20° C.   L-Glutathione (γ-Glu-Cys-Gly): Sigma, cat. G6013, 100 mM stock solution, prepared in water and stored in aliquots at −20° C.   Neomycin: InvivoGen, cat. ant-gn-1, 50 mg/mL G418 sol, stored at 4° C.   Poly-L-Arginine (hydrochloride): Sigma, cat. P7762, 100 ug/mL stock solution, prepared in 100% DMSO, stored in aliquots at −20° C.   Cinacalcet (hydrochloride): Cayman Chemical, cat. 16042, 100 mM stock solution, prepared in 100% DMSO, stored in aliquots at −20° C.   Calindol (hydrochloride): SantaCruzBiotech (DBA), cat. sc-211006, 100 mM stock solution, prepared in 100% DMSO, stored in aliquots at −20° C.   Magnesium chloride (hexahydrate): Sigma, cat. M2393, 1 M stock solution, prepared in water and stored at RT.   Instrumentation and Disposables. Experiments were performed using the ICCD camera FLIPR® Tetra (MDC). The analysis was performed in 384-well polystyrene assay plates.
 
Test plates: Poly-D-lysine coated 384 Well Assay Plates (MTP), Black/clear bottom, MATRIX, cat. CPL-4332.
 
Compound plates: 384 Well Polypropylene Assay Plates, V bottom, MATRIX, cat. 4312.
       

     Although the presently disclosed subject matter and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the presently disclosed subject matter, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the presently disclosed subject matter. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 
     Patents, patent applications, publications, product descriptions and protocols are cited throughout this application the disclosures of which are incorporated herein by reference in their entireties for all purposes.