Patent Publication Number: US-2021163554-A1

Title: Method for regulating secretion ability of homeoproteins and their variants having altered secretion ability

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2019-0159210 (filed on Dec. 3, 2019), which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     The present invention relates to a method for regulating the extracellular secretion ability of a homeoprotein and a homeoprotein variant having regulated extracellular secretion ability using the same, and more particularly, to a method for increasing or decreasing the extracellular secretion ability through the increase or decrease in hydrophobicity of a homeodomain external amino acid sequence, or a homeoprotein whose extracellular secretion ability is increased or decreased through the increase or decrease in hydrophobicity of the homeodomain external amino acid sequence. 
     Since cells are surrounded by a hydrophobic cell membrane, hydrophilic biopolymers such as proteins and nucleic acids produced inside the cell cannot pass through the cell membrane and can act while staying only inside the cell. However, proteins which act by being secreted outside the cell, such as growth hormones, are secreted outside the cell via organelles that secrete intracellular proteins, including the endoplasmic reticulum and Golgi apparatus. Proteins which can migrate by the above secretory pathway have a specific secretory label, but most proteins do not have this label, and thus are produced inside the cell and disappear after completing their functions. 
     A transcription factor is present by moving to the nucleus of a cell where target DNA is located to perform an original function of inducing the transcription process which produces RNA by decoding the genetic information included in a DNA sequence, and since RNA produced through transcription induces the formation of proteins responsible for various intracellular reactions through the translation process, the presence or absence of transcription factors which induce the transcription of specific RNA has an important meaning for defining the characteristics of the corresponding cell. Therefore, the characteristics of the corresponding cell may be completely determined only when the transcription factor is produced, acts, and disappears. 
     Among transcription factors, homeoproteins, which are known to be particularly important for determining cell fate during development, are known to have various intracellular functions in addition to typical transcriptional regulatory functions. In particular, the characteristics that regulate the process of translating cytoplasmic RNA into a protein have been found to be important in determining the head and body of a Drosphilia early embryo. 
     Meanwhile, a foreign homeoprotein is introduced through the cell membrane, and such cell membrane-penetrating function was first found by French researchers in 1991. To date, it has been confirmed that intercellular protein transfer is possible among about 10 types of homeoproteins, and although research on the introduction of proteins using the homeodomain of human homeoproteins has been conducted (Korean Patent No. 10-1106262), research has been insufficiently conducted on the effects of amino acid sequences outside the homeodomain on the external secretion of homeoproteins. 
     SUMMARY 
     The present inventors have first elucidated that the presence or absence of a hydrophobic amino acid sequence outside the homeodomain as well as the homeodomain itself are associated with the factors which affect the extracellular secretion ability of homeoproteins, and confirmed that the extracellular secretion ability is decreased in cultured cells and in vivo tissues by substituting the hydrophobic amino acid sequence with a hydrophilic amino acid sequence, and in contrast, homeoproteins with increased extracellular secretion ability in cultured cells and in vivo tissues could be induced by substituting a hydrophilic amino acid sequence at the corresponding position with the hydrophobic amino acid sequence, thereby completing the present invention based on this. 
     Thus, an object of the present invention is to provide a homeoprotein variant whose extracellular secretion ability is increased and decreased, which is selected from the group consisting of the following variants: 
     1) a variant in which amino acid 233 of a HOXD4 protein consisting of an amino acid sequence represented by SEQ ID NO: 1 has been substituted; 
     2) a variant in which amino acid 305 of a SHOX2 protein consisting of an amino acid sequence represented by SEQ ID NO: 2 has been substituted; 
     3) a variant in which amino acids 143 and 191 of an EN2 protein consisting of an amino acid sequence represented by SEQ ID NO: 3 have been substituted; 
     4) a variant in which amino acid 220 or 258 of an OTX2 protein consisting of an amino acid sequence represented by SEQ ID NO: 4 has been substituted; 
     5) a variant in which amino acid 130 of a PAX6 protein consisting of an amino acid sequence represented by SEQ ID NO: 5 has been substituted; and 
     6) a variant in which amino acid 305 of a VAX1 protein consisting of an amino acid sequence represented by SEQ ID NO: 6 has been substituted. 
     Further, another object of the present invention is to provide a method for increasing or decreasing the extracellular secretion ability of a homeoprotein or a method for producing a homeoprotein whose extracellular secretion ability is increased or decreased, the method including substituting one amino acid selected from the group consisting of the following groups. 
     In addition, still another object of the present invention is to provide a method for predicting the extracellular secretion ability of a homeoprotein, the method including confirming one amino acid from the group consisting of the following amino acids: 
     1) amino acid 233 of a HOXD4 protein, consisting of an amino acid sequence represented by SEQ ID NO: 1; 
     2) amino acid 305 of a SHOX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 2; 
     3) amino acids 143 and 191 of an EN2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 3; 
     4) amino acid 220 or 258 of an OTX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 4; 
     5) amino acid 130 of a PAX6 protein, consisting of an amino acid sequence represented by SEQ ID NO: 5; and 
     6) amino acid 305 of a VAX1 protein, consisting of an amino acid sequence represented by SEQ ID NO: 6. 
     However, a technical problem to be achieved by the present invention is not limited to the aforementioned problem, and the other problems that are not mentioned may be clearly understood by a person skilled in the art from the following description. 
     To achieve the objects of the present invention as described above, the present invention provides a homeoprotein variant whose extracellular secretion ability is increased or decreased, which is selected from the group consisting of the following variants: 
     1) a variant in which amino acid 233 of a HOXD4 protein consisting of an amino acid sequence represented by SEQ ID NO: 1 has been substituted; 
     2) a variant in which amino acid 305 of a SHOX2 protein consisting of an amino acid sequence represented by SEQ ID NO: 2 has been substituted; 
     3) a variant in which amino acids 143 and 191 of an EN2 protein consisting of an amino acid sequence represented by SEQ ID NO: 3 have been substituted; 
     4) a variant in which amino acid 220 or 258 of an OTX2 protein consisting of an amino acid sequence represented by SEQ ID NO: 4 has been substituted; 
     5) a variant in which amino acid 130 of a PAX6 protein consisting of an amino acid sequence represented by SEQ ID NO: 5 has been substituted; and 6) a variant in which amino acid 305 of a VAX1 protein consisting of an amino acid sequence represented by SEQ ID NO: 6 has been substituted. 
     As an exemplary embodiment of the present invention, the amino acids may be an amino acid within an external motif of a homeodomain. 
     As another exemplary embodiment of the present invention, when the amino acid is substituted with a hydrophobic amino acid, the extracellular secretion ability may be increased. 
     As still another exemplary embodiment of the present invention, the hydrophobic amino acid may be leucine (Leu)(L). 
     As yet another exemplary embodiment of the present invention, when the amino acid is substituted with a hydrophilic amino acid, the extracellular secretion ability may be decreased. 
     As yet another object of the present invention, the hydrophilic amino acid may be glutamate (Glu)(E). 
     In addition, the present invention provides a method for increasing or decreasing the extracellular secretion ability of a homeoprotein or a method for producing a homeoprotein whose extracellular secretion ability is increased or decreased, the method including substituting one amino acid selected from the group consisting of the following groups. 
     Furthermore, the present invention provides a method for predicting the extracellular secretion ability of a homeoprotein, the method including confirming one amino acid selected from the group consisting of the following amino acids: 
     1) amino acid 233 of a HOXD4 protein, consisting of an amino acid sequence represented by SEQ ID NO: 1; 
     2) amino acid 305 of a SHOX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 2; 
     3) amino acids 143 and 191 of an EN2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 3; 
     4) amino acid 220 or 258 of an OTX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 4; 
     5) amino acid 130 of a PAX6 protein, consisting of an amino acid sequence represented by SEQ ID NO: 5; and 
     6) amino acid 305 of a VAX1 protein, consisting of an amino acid sequence represented by SEQ ID NO: 6. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates images of V5-HP secretion of 293T, GT1-7 and MDCK cells in a medium (dot color indicates secretion observed in the corresponding cell line); 
         FIG. 2  illustrates the confirmation of the correlation between the homeodomain whose expression is observed in 293T cells and secretion by isolating the homeodomain from the homeoprotein; 
         FIG. 3  illustrates the results of confirming the correlation between mutations of homeoproteins from which homeodomains have been deleted and secretion; 
         FIG. 4  illustrates the results of confirming shared motifs in sHPs using the multiple sequence alignment described in the STAR method; 
         FIG. 5  illustrates the amino acid sequences of sHP and nsHP at the points indicated by the arrows in  FIG. 4 ; 
         FIG. 6  illustrates the positions of amino acids which are expected to be shared by sHP and nsHP, respectively (homeodomains are colored in red); 
         FIG. 7  illustrates a comparison of the extracellular secretion abilities of mutant homeoproteins (the scores below the image show the average relative secretion index (RSI) value compared with the secretion index of the wild-type homeoproteins); 
         FIG. 8  illustrates images comparing the abilities of wild-type homeoproteins and mutant homeoproteins to be transferred between HeLa cells and relative transfer index (RTI) values (*p&lt;0.01; **p&lt;0.005; ANOVA., scale bars: 25 μm); 
         FIG. 9  illustrates images comparing the abilities of wild-type homeoproteins and mutant homeoproteins to be transferred between mouse embryonic brain cells and RTI values (*p&lt;0.01; **p&lt;0.005; ANOVA., scale bars: 25 μm); 
         FIG. 10  illustrates the transcriptional activity of secretion-defective OTX2 (F258E) by measuring the activity of luciferase induced by the OTX2 target CRX promotor in transfected cells (*p&lt;0.01; **p&lt;0.005; ***p&lt;0.001; ANOVA.); 
         FIG. 11  illustrates the transcriptional activity of a PAX6 variant by measuring the activity of luciferase induced by a tandem PAX6 target DNA sequence (*p&lt;0.01; **p&lt;0.005; ***p&lt;0.001; ANOVA.); 
         FIG. 12  illustrates the transcriptional activity of a VAX1 variant by measuring the activity of luciferase induced by a TCF7L2 promotor (*p&lt;0.01; **p&lt;0.005; ***p&lt;0.001; ANOVA.); 
         FIG. 13  illustrates the results of co-culturing 293T cells overexpressing V5-VAX1, V5-VAX1 (L305E) or V5-VAX1 (WFSR) with E13.5 mouse retinal explants for 48 hours (*p&lt;0.01; **p&lt;0.005; ***p&lt;0.001; ANOVA.); and 
         FIG. 14  illustrates the results of immunostaining of retinal explants treated with recombinant VS-tagged wild-type and mutant VAX1 proteins for 48 hours (*p&lt;0.01; **p&lt;0.005; ***p&lt;0.001; ANOVA.). 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the present invention will be described in detail. 
     The present inventors have first elucidated that the presence or absence of a hydrophobic amino acid sequence outside the homeodomain as well as the homeodomain itself are associated with the factors which affect the extracellular secretion ability of homeoproteins, and confirmed that a homeoprotein whose extracellular secretion ability is increased or decreased could be induced through an increase or decrease in hydrophobicity of the amino acid sequence, thereby completing the present invention based on this. 
     As used herein, a homeodomain refers to a protein having a structure showing gene-binding activity and consisting of about 60 amino acids. The homeodomain is expressed from a gene having a size of 180 bp, which is known to be a homeobox. It is known that the homeodomain binds specifically to a target gene promotor site to promote or inhibit gene expression, the homeodomain is mainly identified in a protein group that regulates early Drosophila development, and is named as the homeodomain related to Drosophila development, and it is possible to find a group of segmental genes, genes which control the morphogenesis of vertebrate Hox genes and the like, a conjugation determinant gene MAT of yeast, transcription factor groups which control tissue-specific gene expression, and the like. Each of these groups of homeodomains has a characteristic amino acid alignment and exhibits evolutionary conservation in the animal kingdom. Proteins having a homeodomain are collectively referred to as homeoproteins, and in the present invention, the homeoprotein may be any one selected from the group consisting of HOXD4, SHOX2, EN2, OTX2, PAX6, and VAX1, but is not limited thereto. 
     As used herein, the external motif of the homeodomain refers to an amino acid sequence in a specific region out of the region of the homeodomain rather than the inside of the homeodomain, and the amino acid sequence fragment in the specific region may be located upstream or downstream of the homeodomain. 
     The present inventors specifically confirmed the correlation between the extracellular secretion ability of homeoproteins and the hydrophobic amino acids of the external motifs of homeodomains, and more specifically, an exemplary embodiment of the present invention confirmed that when leucine was substituted with glutamate at amino acid positions 143 and 191 of EN2 by performing an experiment of replacing a hydrophobic amino acid at each corresponding position of secretory homeodomain-containing proteins (sHP) with a hydrophilic amino acid glutamate (Glu)(E) in order to confirm the importance of hydrophobic residues in external motifs of homeodomains, the amount of mutant EN2 to be measured in growth media was reduced, and confirmed that the extracellular secretion ability of mutants in which leucine 220 of OTX, phenylalanine 258 of OTX, leucine 130 of PAX6, or leucine 305 of VAX1 had been changed to glutamate was reduced (see Example 3). 
     Further, another exemplary embodiment of the present invention confirmed that when a hydrophilic or hydrophobic amino acid sequence in the external motifs of homeodomains was substituted with a hydrophobic amino acid or another hydrophobic amino acid, and more specifically, the extracellular secretion ability of a mutant in which serine 233 of HOXD4 had been substituted with leucine or proline 305 of SHOX2 had been substituted with leucine was enhanced (see Example 3). 
     In addition, through still another exemplary embodiment of the present invention, it was confirmed that OTX2 (F258E) and PAX6 (L130E) still exhibited transcriptional activities, and luciferase reporter expression downstream of OTX2 target CRX promotor and tandem PAX6 binding sequences, respectively, as strong as wild-type OTX2 and PAX6 was induced, and it was confirmed that the hydrophobic residues were involved in the secretion of homeoproteins outside the cell without affecting the transcriptional activities of the sHPs by confirming that VAX1 (L305E) also activated the expression of luciferase, whose transcription was regulated by a VAX1 target transcription factor 7-like 2 (TCF7L2) gene upstream sequence, as significantly as wild-type VAX1 (see Example 5). 
     Through the experimental results of the exemplary embodiments, the present inventors suggest that when a specific hydrophobic amino acid of the external motifs of homeodomains of a homeoprotein is substituted with a hydrophilic amino acid, only the extracellular secretion ability of the homeoprotein can be reduced without reducing the transcriptional activities of the homeoprotein. 
     Thus, the present invention provides a homeoprotein variant whose extracellular secretion ability is increased or decreased, which is selected from the group consisting of the following variants: 
     1) a variant in which amino acid 233 of a HOXD4 protein consisting of an amino acid sequence represented by SEQ ID NO: 1 has been substituted; 
     2) a variant in which amino acid 305 of a SHOX2 protein consisting of an amino acid sequence represented by SEQ ID NO: 2 has been substituted; 
     3) a variant in which amino acids 143 and 191 of an EN2 protein consisting of an amino acid sequence represented by SEQ ID NO: 3 have been substituted; 
     4) a variant in which amino acid 220 or 258 of an OTX2 protein consisting of an amino acid sequence represented by SEQ ID NO: 4 has been substituted; 
     5) a variant in which amino acid 130 of a PAX6 protein consisting of an amino acid sequence represented by SEQ ID NO: 5 has been substituted; and 6) a variant in which amino acid 305 of a VAX1 protein consisting of an amino acid sequence represented by SEQ ID NO: 6 has been substituted. 
     The amino acid according to the present invention may be an amino acid in the external motifs of homeodomains, and when the amino acid according to the present invention is substituted with a hydrophobic amino acid, the extracellular secretion ability may be enhanced, and the hydrophobic amino acid may be leucine (Leu)(L), but is not limited thereto. 
     In the present invention, a hydrophobic amino acid is a general term for amino acids which strongly tend to be present inside proteins in an aqueous solution, and more specifically, examples of the hydrophobic amino acid include phenylalanine, tryptophan, isoleucine, leucine, proline, methionine, valine, glycine, alanine, cysteine, and the like. The conformation of the protein is supported by hydrophobic interactions between hydrophobic residues. In addition, there is a hydrophobic amino acid which forms a part of the substrate binding site at the enzyme active center site. Hydrophobic amino acids other than proline also have a great ability to form β structures. A hydrophobic region of the protein surface is considered to be a place where the protein surface binds to other proteins, lipid parts of biological membranes, or hydrophobic ligands. 
     The amino acid according to the present invention may be an amino acid in the external motifs of homeodomains, and when the amino acid according to the present invention is substituted with a hydrophilic amino acid, the extracellular secretion ability may be reduced, and the hydrophilic amino acid may be glutamate (Glu)(E), but is not limited thereto. 
     In addition, as another aspect of the present invention, the present invention provides a method for increasing or decreasing the extracellular secretion ability of a homeoprotein or a method for producing a homeoprotein whose extracellular secretion ability is increased or decreased, the method including substituting one amino acid selected from the group consisting of the following amino acids: 
     1) amino acid 233 of a HOXD4 protein, consisting of an amino acid sequence represented by SEQ ID NO: 1; 
     2) amino acid 305 of a SHOX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 2; 
     3) amino acids 143 and 191 of an EN2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 3; 
     4) amino acid 220 or 258 of an OTX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 4; 
     5) amino acid 130 of a PAX6 protein, consisting of an amino acid sequence represented by SEQ ID NO: 5; and 
     6) amino acid 305 of a VAX1 protein, consisting of an amino acid sequence represented by SEQ ID NO: 6. 
     Furthermore, as still another aspect of the present invention, the present invention provides a method for predicting the extracellular secretion ability of a homeoprotein, the method including confirming one amino acid selected from the group consisting of the following amino acids: 
     1) amino acid 233 of a HOXD4 protein, consisting of an amino acid sequence represented by SEQ ID NO: 1; 
     2) amino acid 305 of a SHOX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 2; 
     3) amino acids 143 and 191 of an EN2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 3; 
     4) amino acid 220 or 258 of an OTX2 protein, consisting of an amino acid sequence represented by SEQ ID NO: 4; 
     5) amino acid 130 of a PAX6 protein, consisting of an amino acid sequence represented by SEQ ID NO: 5; and 6) amino acid 305 of a VAX1 protein, consisting of an amino acid sequence represented by SEQ ID NO: 6. 
     Hereinafter, preferred examples for helping the understanding of the present invention will be suggested. However, the following examples are provided only to more easily understand the present invention, and the contents of the present invention are not limited by the following examples. 
     EXAMPLES 
     Example 1 
     Experimental Preparation and Experimental Methods 
     1-1. Animal 
     All experiments done with the mice were performed according to approved Institutional Animal Care and Use Committee (IACUC) protocols (KAIST IACUC 13-130) of Korea Advanced Institute of Science and Technology (KAIST). 
     1-2. Gateway Cloning of Human HP ORFs 
     170 human HP cDNAs were obtained from human ORFeome v7.0 developed by the Dana-Farber Cancer Research Institute (DFCRI) and human ORFeome developed by Johns Hopkins University, and the HP ORFs were cloned into a pCAGIG-V5 vector using Gateway cloning technology (Invitrogen) for expression in cultured cell-lines and mouse embryonic brains. 
     1-3. Cell Culture, Transfection, and Dot Blot Analysis 
     293T, HeLa, GT1-7, and MDCK cells were maintained in Dulbecco&#39;s modified Eagle medium (DMEM) containing 10% fetal bovine serum (FBS). 293T cells were then transfected with polyethylenimine (PEI), and HeLa, GT1-7, and MDCK cells were transfected using the GenJet plus DNA in vitro transfection reagent (Signagen) following the manufacturer&#39;s manual. The growth media of the transfected cells were replaced with FreeStyle serum-free media (GIBCO BRL) 6 hours after transfection, and were collected. 500 ml of the media were added into each well of the GE Whatman Dot-Blot 96 Well Plate System, which was applied under vacuum to aspirate the media across the PVDF membrane. The membranes, which captured proteins and nucleic acids in the growth media, were then blotted with anti-V5 and anti-GFP antibodies, simultaneously, the transfected cells were lysed in RIPA buffer (PBS with 0.1% SDS), and supernatants were collected for western blot analyses. 
     1-4. Immunostaining 
     Mouse embryonic brains and eyes were isolated for subsequent fixation in PBS containing 4% paraformaldehyde (PFA) for 1 hour. The samples were then moved into 20% sucrose/PBS solution for subsequent incubation at 4° C. for 16 hours prior to cryopreservation in the TissueTek O.C.T. compound for freezing. 
     Frozen samples having a thickness between 12 mm and 20 mm were then cryosectioned onto a slide glass. Alternatively, HeLa cells cultured on the coverslips were fixed in 4% PFA/PBS for 20 minutes. The tissues on the slides and HeLa cells on the coverslips were then incubated in a blocking solution (PBS including 10% normal donkey serum and 0.1% Triton X-100) at room temperature for 1 hour. The cells were further incubated in a blocking solution including primary antibodies without Triton X-100 at 4° C. for 16 hours, and subsequently with fluorophore-conjugated secondary antibodies recognizing the primary antibodies. Fluorescent images of the IHC signals were then obtained by the Olympus FV1000 confocal microscope. 
     1-5. Recombinant V5-HP Affinity Purification 
     293T cells expressing V5-tagged HP were lysed in a buffer consisting of 20 mM Tris-HCl (pH 7.9), 500 mM NaCl, 20% glycerol, 4 mM MgCl 2 , 0.4 mM EDTA, and a protease inhibitor cocktail (Millipore). Supernatant fractions of the cells obtained after centrifugation at 13,200 rpm for 10 minutes were incubated with anti-V5 antibodies at 4° C. for 16 hours, and then with protein-G Sepharose (GE Healthcare) beads for 2 hours. The protein-G Sepharose immune complexes were washed five times with a wash buffer (20 mM Tris-HCl (pH 7.9), 150 mM NaCl, 2 mM MgCl 2 , 0.2 mM EDTA, 0.1% NP40, and a protease inhibitor) before V5-tagged HPs. The protein-G Sepharose immune complexes were eluted from the protein-G Sepharose beads in the wash buffer containing 0.25 mg/ml V5 peptide. The V5 peptide was then removed by the Amicon Ultracentrifugal filter device (Millipore). 
     1-6. Retinal Explants and Axon Growth Analysis 
     Retinal explants were prepared as it was described in a previous report (Kim et al., 2014). Briefly, retinas were isolated from E13.5 mouse embryos and explants were added to a collagen mixture and positioned on plates coated with poly-L-lysine (10 μg/ml) and laminin (10 μg/ml). The explants were then incubated at 37° C. for 1 hour to allow gelling before adding a Neurobasal medium containing the B27 supplement (Invitrogen). The explants were cultured for 48 hours before being treated with proteins or co-cultured with 293T cell aggregates for 48 hours. 
     1-7. Quantification &amp; Statistical Analysis 
     The multiple sequence alignment was then built by using Multiple Sequence Comparison by Log-Expectation (MUSCLE) software with the default option, which is designed to give the most accurate gapped alignment (Edgar, 2004). The multiple sequence alignment (MSA) results were split into two sub-MSAs for sHP and nsHP sequences with different RSI cut-offs, respectively, and the sequence profiles, which represent the amino acid frequencies in each MSA column, were calculated. To identify the distinctive MSA columns discriminating sHP and nsHP sequences, the difference between two sub-MSAs in MSA column i was calculated by using the JS-divergence defined as. 
     
       
         
           
             
               
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     where p i  (k) and q i  (k) represent the frequencies of the amino acid k in MSA column i in the sub-MSAs for sHP and nsHP sequences, respectively, and l is 0.5, and r i  (k) is defined as (p i  (k)+q i  (k))=2. 
     1-8. Statistical Analysis 
     Statistical analysis was performed using Prism Software (GraphPad; v5.0) measurement tools. All statistical analysis data is expressed as the mean±STE. Comparison between two groups was done by an unpaired Student&#39;s t test, and the differences among multiple groups were determined by analysis of variance (ANOVA) with a Tukey&#39;s post-test used to determine the significant differences among multiple groups. P values&lt;0.01 were considered as statistically significant results. 
     Example 2 
     Confirmation of Domains Affecting Extracellular Secretion of Homeoproteins 
     It has been suggested that the Sec motif, which is a linker region between the second and third helices of the homeodomain, is responsible for the secretion of En2, and as illustrated in  FIG. 1 , given that the Sec motif is shared by all HDs, it was confirmed there is no chance of or very little chance of secretion of HPX/HOPX, which has only a PRD class homeodomain without any other functional domain (RSI (293T): 0.00, RSI (GT1-7): 3.97, RSI (MDCK): 0.96). 
     Further, as illustrated in  FIG. 2 , as a result of further examining the secretabilities of various isolated homeodomains, no evidence of secretion was found in any of the three cell lines. 
     In contrast, as illustrated in  FIG. 1 , it was confirmed that the BARX1, EVX1, PAX3, and ZFHX3 isoforms lacking their homeodomains had an extracellular secretion ability even though there was no homeodomain in certain cell situations. 
     Thus, as a result of testing the secretion of secretory homeodomain-containing proteins (sHPs), as illustrated in  FIG. 3 , it was confirmed that OTX2 and PAX6 were still secreted without the homeodomain, whereas VAX1 should have its homeodomain for secretion. 
     Together, it can be confirmed that although the presence or absence of the homeodomain is an important requirement for the secretion of homeoproteins, it is not a sufficient requirement for the secretion of homeoproteins. In addition, it could be speculated that other domain motifs outside the homeodomain were required for extracellular secretion of homeoproteins. 
     Example 3 
     Confirmation of Hydrophobic Amino Acid Sequence Affecting Extracellular Secretion of Homeoproteins 
     Referring to the experimental results of Example 2, as illustrated in  FIG. 4 , it was tried to identify the motifs, which are located outside the homeodomain and are shared among various secretory homeodomain-containing proteins (sHPs), by multiple sequence alignment. 
     As a result, as illustrated in  FIG. 5 , it was confirmed that hydrophobic amino acids, including phenylalanine (Phe)(F), leucine (Leu)(L), and methionine (Met)(M), were commonly detectable at the same locations of the sHPs outside the homeodomain. 
     To confirm the importance of the residues in extracellular secretion of homeoproteins, an experiment of replacing amino acids at corresponding positions of secretory homeodomain-containing proteins (sHPs) with an acidic amino acid glutamate (Glu) was performed. 
     As a result, as illustrated in  FIGS. 6 and 7 , it was confirmed that changing leucine (Leu)(L) residues at positions 143 and 191 of EN2 to glutamate (Glu)(E) reduced the amount of the secreted mutant EN2 measured in the growth media. 
     Together with the results, as illustrated in  FIGS. 6 and 7 , it was confirmed that the extracellular secretion ability of a mutant in which leucine (or phenylalanine) of other secretory homeodomain-containing proteins (sHPs) such as leucine 220 of OTX2, phenylalanine 258 of OTX2, leucine 130 of PAX6, and leucine 305 of VAX1 had been substituted with glutamate was reduced, and thus it could be seen that it is a general rule that the extracellular secretion ability of a homeoprotein in which leucine (or phenylalanine) had been substituted with glutamate was reduced. Here, each mutant is indicated as EN2 (L143E), EN2 (L191E), OTX2 (L220E), OTX2 (F258E), PAX6 (L130E), and VAX1 (L305E), respectively. 
     Conversely, as illustrated in  FIGS. 6 and 7 , it was confirmed that the extracellular secretion ability of a mutant in which serine 233 of HOXD4 was substituted with leucine or proline 305 of SHOX2 was substituted with leucine was enhanced in 293T cells. Here, each mutant is indicated as HOXD4 (S233L) and SHOX2 (P305L). The following Table 1 shows the sequence number of each mutant. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 SEQ ID NO 
                 Mutant 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 7 
                 HOXD4 (S233L) 
               
               
                 8 
                 SHOX2 (P305L) 
               
               
                 9 
                 EN2 (L143E) EN2 (L191E) 
               
               
                 10 
                 OTX2 (L220E) 
               
               
                 11 
                 OTX2 (F258E) 
               
               
                 12 
                 PAX6 (L130E) 
               
               
                 13 
                 VAX1 (L305E) 
               
               
                   
               
            
           
         
       
     
     Example 4 
     Confirmation of Correlation Between Extracellular Secretion Ability of Homeoproteins and Transfer Ability to Neighboring Cells 
     As illustrated in  FIG. 8 , it was confirmed that in a mutant in which leucine (Leu)(L) residues at positions 143 and 191 of EN2 had been changed to glutamate (Glu)(E), a mutant in which leucine 220 of OTX2, phenylalanine 258 of OTX2, leucine 130 of PAX6, or leucine 305 of VAX1 had been substituted with glutamate, or secretory homeodomain-containing proteins (sHPs) defective in the cellular secretion ability, the efficiency of transfer between cells is reduced compared to wild-type homeoproteins. 
     Conversely, as illustrated in  FIG. 8 , it was confirmed that the transfer ability between HeLa cells of a mutant in which serine 233 of HOXD4 was substituted with leucine or proline 305 position of SHOX2 was substituted with leucine had been increased. 
     Further, similar to the experimental results in the cultured cells, as illustrated in  FIG. 9 , it was confirmed that hydrophobic residues outside the homeodomain also played an important role for intercellular homeoprotein transfer in mouse brain cells. 
     When the results are combined, it is confirmed that rather than depending solely on the homeodomain, homeoprotein secretion is determined by each homeoprotein&#39;s three-dimensional structure, and the three-dimensional structure is determined by residues outside the homeodomain, such as the hydrophobic amino acids outside the homeodomain. 
     Furthermore, it is possible to predict that mutations or post-translational modifications at those sites can change the extracellular secretion ability of the homeoproteins. 
     Example 5 
     Confirmation of Correlation Between Extracellular Secretion Ability and Transcriptional Activity of Homeoproteins 
     Through the above-described examples, it was confirmed that mutations in hydrophobic amino acid residues outside the homeodomain have a significant effect on homeoprotein secretion. Thus, the following experiments were conducted to confirm how the mutations affect the transcriptional activities of the homeoproteins. 
     As a result, as illustrated in  FIGS. 8 and 9 , it was confirmed that the mutation in which leucine or phenylalanine was substituted with glutamate among the amino acid residues outside the homeodomain did not change intracellular distribution of the sHPs, and all the mutant homeoproteins were detected in the nucleus. 
     Further, as illustrated in  FIGS. 10 and 11 , OTX2 (F258E) and PAX6 (L130E) still showed transcriptional activities and induced luciferase reporter expression downstream of OTX2 target CRX promotor and tandem PAX6 binding sequences, respectively as strong as wild-type OTX2 and PAX6. 
     In addition, as illustrated in  FIG. 12 , it was confirmed that VAX1 (L305E) also activated the expression of luciferase, whose transcription was regulated by a VAX1 target transcription factor 7-like 2 (TCF7L2) gene upstream sequence (Vacik et al., 2011), as significantly as wild-type VAX1. 
     The results suggest that the hydrophobic residues enhance extracellular secretion ability without affecting the transcriptional activities of the sHPs. 
     Next, the abilities of VAX1 (L305E) and wild-type VAX1 to affect retinal axon growth were compared, which is dependent on intercellular transfer of VAX1. 
     V5-VAX1 overexpressed in 293T cells was detectable in the axons projecting from co-cultured mouse retinal explants as illustrated in  FIG. 13 , but V5-VAX1(L305E) proteins expressed in 293T cells were neither detectable in the retinal axons nor promoted the axonal growth. However, as illustrated in  FIG. 14 , recombinant V5-VAX1(L305E) added to the growth media of retinal explants was detectable in the retinal axons and induced axonal growth as efficiently as wild-type VAX1. On the contrary, it was confirmed that recombinant V5-VAX1(WF/SR), which can be secreted but cannot cross the cell membrane, was not detectable in the retinal axons, and it failed to induce axonal growth. 
     Collectively, the results suggest that VAX1(L305E) performs a normal transcription factor function in the nucleus, but cannot be secreted to the outside of the cells. 
     The present inventors have first elucidated that the presence or absence of a hydrophobic amino acid sequence outside the homeodomain as well as the homeodomain itself are associated with the factors which affect the extracellular secretion ability of homeoproteins, and confirmed that a homeoprotein whose extracellular secretion ability is increased or decreased could be induced through an increase or decrease in hydrophobicity of the amino acid sequence, and through research on the secretory mechanism of homeoproteins, the homeoprotein is expected to be usefully used to enhance the understanding of the secretory pathway by which various intracellular proteins which do not include secretory labels are secreted. 
     The above-described description of the present invention is provided for illustrative purposes, and those skilled in the art to which the present invention pertains will understand that the present invention can be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. Therefore, it should be understood that the above-described embodiments are illustrative in all aspects and are not restrictive.