# Patent

## Title
Transgenic mouse defective in WW45 function and use in screening compounds for anti-tunour activity

## Abstract

Mechanisms regulating cell proliferation stop and differentiation initiation during the development stage of mammalian embryo, and the proteins involved therein, are presented. Differentiation regulators, methods of regulating differentiation, transgenic organisms with loss of expression of the differentiation regulator, and methods of preparing the transgenic organisms, are provided.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0056758 filed in the Korean Intellectual Property Office on Jun. 17, 2008, the entire content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
(a) Field of the Invention
Mechanisms regulating cell proliferation stop and differentiation initiation during the development stage of mammalian embryo, and the proteins involved therein, are provided. More specifically, differentiation regulators, methods of regulating differentiation, transgenic organisms with loss of expression of the differentiation regulator, and methods of preparing the transgenic organisms, are provided.
(b) Description of the Related Art
Homeostasis of regenerative epithelial tissues such as skin and intestine is maintained through a tightly balanced process of proliferation and terminal differentiation. During normal epithelial development, proliferating progenitor cells, often referred to as transiently amplifying (TA) cells, actively divide a limited number of times before they undergo cell-cycle exit and terminally differentiate into postmitotic cells. Cancer can develop as a result of inappropriate proliferation of progenitor cells accompanied by a partial or complete loss of differentiation. Therefore, understanding the signaling networks that control cell-cycle exit and terminal differentiation in epithelial tissues will provide insights into the mechanisms underlying tumorigenesis.
A new signaling network, known as the “Hippo pathway” in Drosophila, seems to be a key developmental program in controlling proliferation and apoptosis for proper organ development in Drosophila. The Ste-20 family kinase Hippo, WW adaptor protein Salvador and NDR kinase Warts are key components of the Hippo pathway that restricts cell proliferation and promotes apoptosis in differentiating epithelial cells by regulating expression of cyclin E and Diap1. The Hippo kinase phosphorylates and activates the Warts kinase, and this process is facilitated by the scaffolding protein Salvador or Mats. Warts, together with Mats, then phosphorylates and inhibits the transcription coactivator Yorkie. Expanded, Merlin and Fat, all of which localize to the plasma membrane, function upstream of the Hippo pathway. In flies, mutations of these factors lead to increased cell proliferation and decreased cell death.
The phenotypes of flies with mutations in the Hippo pathway can be rescued with their respective human counterparts, indicating that the Hippo pathway may play an analogous role as a global regulator of epithelial tissue development in mammals. Several reports on each mammalian component of the Hippo pathway have shown that the pathway is involved in cell death and cell-cycle regulation. MST1/2 kinases (Hippo homologs) were originally reported to be involved in apoptosis with caspase-3-mediated proteolytic activation. LATS1/2 (Warts homologs) have been implicated in the regulation of cell-cycle progression, apoptosis, mitotic exit and cytokinesis. YAP (a Yorkie homolog) has been shown to be involved in apoptosis by interacting with p73. Although mutation of WW45 (a Salvador homolog) has been reported in several cancer cell lines, little is known about the functional significance of WW45 in mammals. So far, only limited biochemical interactions have been reported, including the phosphorylation of LATS1/2 by MST1/2, the association of WW45 with MST1/2 and LATS1/2, binding of LATS1 to MOB1 (a MATS homolog) and formation of a complex comprising RASSF1A, MST2, WW45 and LATS1.
The Hippo pathway has also been implicated in mammalian tumorigenesis. Mice lacking LATS1 develop some types of tumor, and hWW45 and Mats are mutated in several cancer cell lines. NF2, the human ortholog of Merlin, is a tumor-suppressor gene, mutations of which lead to neurofibromatosis. YAP is overexpressed in mammalian cancers and transgenic mice overexpressing YAP have an increased liver size and dysplasia with expanded undifferentiated progenitor cells in the intestine. Of the Hippo pathway proteins, only LATS1-, LATS2-, NF2-, and YAP-null mice have been generated; however, these mice are either early embryonic lethal or fail to recapitulate defects seen in the respective Drosophila mutants. Therefore, compared with Drosophila, much less is known about the physiological function of the Hippo pathway in mammalian epithelial development. Furthermore, the molecular mechanisms by which this pathway is regulated during development are not fully understood in mammals.
SUMMARY OF THE INVENTION
An embodiment of the present invention provides a novel use of a WW45 protein and/or a phosphorylated YAP in regulating differentiation in a mammal.
Another embodiment provides a method of regulating differentiation in a mammal by regulating the expression of a WW45 protein and/or the phosphorylation of a Yes-associated protein (YAP).
Another embodiment provides a differentiation regulating agent in mammal including one or more selected from the group consisting of a WW45 protein, a polynucleotide encoding the WW45 protein, a phosphorylated YAP, and a polynucleotide encoding the YAP, as an active ingredient.
Another embodiment provides a non-human transgenic organism whose whole or part of a ww45 protein coding gene is deleted, whereby tumor is induced.
Another embodiment provides a method of preparing a non-human transgenic organism by deleting a whole or part of a ww45 protein coding gene.
Another embodiment provides a method of screening an anti-tumor agent using the non-human transgenic organism.
Still another embodiment provides a method of screening a differentiation regulating agent by measuring the expression level of WW45 protein in a mammalian cell.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a to 1d show targeted disruption of the WW45 gene by homologous recombination, wherein
1a is a schematic representation of the WW45 targeting strategy, showing the mouse WW45 genomic locus, targeting vectors and targeted locus,
1b shows the result of southern-blot analysis of genomic DNA digested with the restriction enzyme Bg1 II using the 5′-probe, and PCR genotype analysis of embryos,
1c shows a macroscopic appearance of wild-type and mutant embryos at E17.5 (top), and growth curves of wild-type (+/+) and mutant (−/−) WW45 embryos (bottom), and
1d shows Hematoxylin and Eosin (H&E)-stained sections of placentas from E17.5 wild-type and mutant embryos. FIG. 2 shows defective angiogenesis in WW45−/− placenta at E17.5 (Scale bar: 100 μm). FIGS. 3a to 3c show hyperproliferation and immature differentiation in WW45−/− epidermis at E17.5, wherein
3a shows the result of histological analysis of Hematoxylin and Eosin (H&E)-stained sections of wild-type (a) and mutant (a′) epidermis,
3b shows the differentiation marker expression in skin of wild-type or mutant embryos, and
3c shows the result of electron-microscopy analysis of the wild-type (upper panel) and mutant (lower panel) epidermis (Scale bar: 100 μm in (A) and (B), 2 μm in (C)). FIGS. 4a to 4c show hyperplasia and immature differentiation of WW45−/− intestinal epithelium at E17.5, wherein
4a shows H&E-stained sections of wild-type (a) and mutant (a′) intestine,
4b shows the differentiation marker expression in the intestine of wild-type or mutant embryos, and
4c shows the result of electron-microscopy analysis of the wild-type (upper panel) and mutant (lower panel) intestines (Scale bar: 100 μm in (A) and (B), 2 μm in (C)).
FIG. 5 shows the result of histological analysis of hematoxylin & eosin (H&E)-stained sections of wild-type (a-d) or mutant (a′-d′) epithelial tissues (Scale bar: 100 μm).
FIG. 6 shows dysmaturation of skin barrier function in mutant embryo at E17.5 measured by skin barrier function assay by X-gal staining.
FIGS. 7a and 7b show hyperproliferation in WW45−/− epithelial tissues at E17.5, wherein
7a shows the result of evaluation of cellular proliferation in wild-type or mutant colonic epithelium by co-immunohistochemistry analysis with anti-Ki67 and anti-β-catenin (Scale bar: 100 μm), and
7b shows the result of quantitative analysis of the percentage of BrdU-positive cells per 1.0-mm2 area of epithelium 2 hours after BrdU injection (Data represent triplicate independent experiments ±SD).
FIGS. 8a to 8d show that increased numbers of cycling cells in the developing WW45−/− epithelium is resulted from inefficient growth arrest, wherein
8a is a graph showing the proportion of progenitor cells (Ki67+) labeled with BrdU after a 1-hour pulse,
8b shows the result of analysis of immunoreactivity for Ki67 in BrdU-positive cells 24 hours after BrdU injection,
8c shows growth curves of primary keratinocytes isolated from wild-type and mutant epidermis show similar proliferation rates, and
8d shows induction of calcium-stimulated differentiation in primary keratinocytes (data of a-d represent triplicate independent experiments (±SD)).
FIGS. 9a and 9b show defective cell-cycle exit in WW45−/− keratinocytes, wherein 9a and 9b show BrdU-labeling index of keratinocytes cultured with or without transforming growth factor (TGF)-β (a) or LiCl (b), respectively.
FIGS. 10a to 10c show activation of MST1 signaling pathway during differentiation, wherein
10a shows protein status of components of the MST1 pathway under differentiation conditions,
10b shows the analysis results for wild-type and mutant primary keratinocytes cultured with or without Ca2+ for 24 hours by immunoprecipitation with antibodies against MST1 and by Western-blot assays, and
10c shows phosphorylation of YAP by activated LATS2 through MST1/WW45.
FIG. 11 shows the result of western-blot analysis with the indicated antibodies wherein the analysis was conducted for precipitates obtained from immunoprecipitation with anti-hemagglutinin (HA) in WW45−/− primary keratinocytes.
FIG. 12a shows partial alignment of the conserved regions of YAP with the Drosophila Yki protein,
FIG. 12b shows the result of identification of the phosphorylation site of YAP,
FIG. 12c shows the result from In-vitro kinase assays using immunoprecipitated HA-tagged LATS1/2 WT or KD and purified His-YAP-WT or His-YAP-SA, where the signals are shown by autoradiography analysis (top two panels); and the input kinase and substrate for Western-blot analysis using anti-LATS1/2 and anti-YAP (bottom two gels), and
FIG. 12d shows the result of western-blot analysis with anti-YAP or an antibody specific for the serine-127-phosphorylated YAP (p-YAP) in wild-type and mutant primary keratinocytes cultured with or without Ca2+ for 24 hours.
FIGS. 13a to 13d show dynamic localization of MST1/YAP during differentiation, wherein
13a to 13c show subcellular localizations of MST1 and YAP in response to differentiation stimuli, and
13d shows the result of immuno peroxidase staining for MST1, YAP and p-YAP in wild-type (a-f) and mutant (a′-f′) epithelium at E17.5 (Scale bar: 10 μm in (c) and 100 μm in (d)).
FIG. 14a shows the result of western-blot analysis performed for lysates of HeLa cells stably transfected with a vector encoding MST1 siRNA or GFP siRNA as a control, and FIG. 14b shows the result from staining Control- and MST1-siRNA-expressing HeLa cells with DAPI and MST1 antibody.
FIG. 15 shows nuclear localization of YAP in proliferative progenitor compartment of the small intestine (Scale bar: 100 μm).
FIGS. 16a to 16f show effects of phosphorylation of YAP serine 127 on epithelium differentiation, wherein
16a shows the percentage of proliferative cells labeled with BrdU under differentiation conditions,
16b shows that YAP-SA-infected cells fail to differentiate in response to differentiation stimuli,
16c shows subcellular localization of YAP, YAP SA and YAP SD in response to differentiation stimuli,
16d shows the percentage of BrdU-positive cells in WW45-deficient keratinocytes infected with the indicated genes under differentiation conditions,
16e shows the result of western-blot analysis for lysates obtained from 16d with the indicated antibodies, and
16f schematically shows a proposed model for the role of WW45 in developing epithelial tissues
FIGS. 17a and 17b expansion of progenitor cells and loss of differentiated cells in WW45−/− epithelium, wherein
17a shows representative H&E and anti-K1 stained sections of epidermis from control and mutant embryos at the indicated developmental stages, and
17b shows representative H&E-stained sections of intestine from control and mutant embryos at the indicated developmental stages (Scale bar: 100 μm).
FIG. 18 shows the number of Ki67-positive cells (average number per 1.0-mm2 area) in wild-type and mutant epithelia at the indicated developmental stages (Data represent triplicate independent experiments ±SD).
FIG. 19 shows the analysis results from quantitative RT-PCR for RNA isolated from wild-type and mutant epidermis (Data represent triplicate independent experiments ±SD).



DETAILED DESCRIPTION OF THE EMBODIMENTS
The present inventors generated mice lacking WW45, which is the unique Sav homolog, to examine the role of the Hippo pathway in mammals, and revealed that mutant embryos displayed unchecked proliferation and defects in terminal differentiation of epithelial cells, to complete the present invention. The present inventors also revealed the molecular mechanism by which MST1 signaling is spatiotemporally regulated to allow cell-cycle exit and activation of terminal differentiation in epithelial cells.
Based on the above findings, mechanisms regulating cell proliferation stop and differentiation initiation during the development stage of mammalian embryo, and the proteins involved therein, are presented. More specifically, differentiation regulators, methods of regulating differentiation, transgenic organisms with loss of expression of the differentiation regulator, and methods of preparing the transgenic organisms, are provided.
One object of the present invention is to reveal the processes of cell-cycle exit (cell proliferation stop) and terminal differentiation, and provide a novel use of a WW45 protein involved in the processes.
Therefore, an embodiment provides a method of regulating differentiation in a mammal by regulating the expression of a WW45 protein, the phosphorylation of a Yes-associated protein (YAP), or the movement of phosphorylated YAP from nucleus to cytoplasm, and a differentiation regulating agent in a mammal including one or more selected from the group consisting of a WW45 protein, a polynucleotide encoding the WW45 protein, a phosphorylated YAP, and a polynucleotide encoding the YAP, as an active ingredient.
In embodiments of the present invention, the role of WW45 protein in cell-cycle exit (cell proliferation stop) and differentiation initiation is examined from the finding that in mice lacking WW45, cells proliferation is continued, and differentiation cannot be initiated. Based on the findings of the present invention including the above, the processes of terminating cell proliferation and initiating differentiation may be as follows: The WW45 protein is located in cytoplasm during cell proliferation, and when differentiation conditions are given, the WW45 protein forms a complex with MST1/2 that is present in cytoplasm, to move into nucleus. The MST1/2-WW45 complex that moves into nucleus stimulates LATS1/2 that is present in nucleus, thereby phosphorylating YAP that is present in nucleus. The phosphorylated YAP moves from nucleus to cytoplasm, and binds to 14-3-3 protein, resulting in terminating cell proliferation and initiating differentiation. The above processes are schematically shown in FIG. 16f. 
The WW45 protein may be any WW45 proteins present in mammals. For example, the WW45 protein may be one or more selected from the group consisting of a human WW45 protein (e.g., SEQ ID NO: 1, GenBank Accession No.: NP—068590.1), and rodent WW proteins such as a mouse (Mus musculus) WW45 protein (e.g., SEQ ID NO: 2, GenBank Accession No.: NP—071311), a rat (Rattus norvegicus) WW45 protein (e.g., SEQ ID NO: 3, GenBank Accession No.: NP—001091050), and the like, but not be limited thereto. The WW45 protein may form a complex with MST1/2 in cytoplasm and moves into nucleus.
In embodiments of the present invention, it is revealed that the termination of cell proliferation and initiation of differentiation occur when phosphorylted YAP moves from nucleus to cytoplasm, indicating that the movement of the phosphorylted YAP from nucleus to cytoplasm plays an important role in initiating cell differentiation. The YAP may be any YAP present in mammals. For example, the YAP may be one or more selected from the group consisting of a human YAP (e.g., SEQ ID NO: 4, GenBank Accession No.: NP—006097.1), and rodent YABs such as a mouse (Mus musculus) YAP (e.g., SEQ ID NO: 5, GenBank Accession No.: NP—033560.1), a rat (Rattus norvegicus) YAP (e.g., SEQ ID NO: 6, GenBank Accession No.: NP—001029174), and the like, but not be limited thereto. The phosphorylated site of YAP that is phosphorylated by LATS1/2 which is stimulated by the MST1/2-WW45 protein complex may be 127th amino acid, serine, (hereinafter, ‘serine 127’), among other phosphorylation sites. The YAP where serine 127 is phosphorylated may move to cytoplasm, and binds with 14-3-3 protein, allowing cell proliferation terminated, and cell differentiation initiated.
As used herein, the term ‘differentiation regulating agent’ means any material that is capable of regulating (promoting or inhibiting) any process of a series of processes involved in terminating cell proliferation and initiating cell differentiation.
The method or agent for regulating differentiation according to the present invention may be applied to cells, tissues or organs of any mammal including human. Preferably, the mammal may be human or rodent, and the cells, tissues or organs may be epithelial cells or tissues of skin or internal organs, for example, epithelial cells of intestine, lung, kidney, thymus, pancreas, and the like, and/or epidermal cells of skin, more specifically, keratinocytes. The time to apply the method or agent for regulating differentiation according to the present invention may be the time when the cell proliferation is terminated and the cell differentiation is initiated, which may be determined depending on the cells, tissues or organs to which the method or agent is applied. For example, the time to apply the method or agent may be the period between around E14.5 to P1. The method or agent for regulating differentiation according to the present invention may regulate the differentiation from epithelial progenitor cells to epithelial cells.
Another embodiment of the present invention provides a non-human transgenic organism with inhibited expression or functional loss of the WW5 protein. In a concrete embodiment, the non-human transgenic organism may be a transgenic animal with the receipt number of KCTC11343BP.
Another embodiment of the present invention provides a method of preparing a non-human transgenic organism with inhibited expression or functional loss of the WW5 protein. The method may include the steps of:
1) preparing a recombinant gene where whole or part of WW45 protein coding gene is deleted or substituted, or a recombinant expression vector (gene targeting vector) comprising the recombinant gene and a promoter operably linked the recombinant gene;
2) transfecting an embryo, fertilized ovum, or embryonic stem cell (ES cell) of a non-human mammal with the prepared recombinant gene or recombinant expression vector; and
3) culturing the transfected embryo, fertilized ovum, or ES cell, to obtain a transgenic animal (gene targeting animal), and a tissue or organ of the transgenic animal.
The transfecting step may be performed by any conventionally known method that is used for preparation of a transgenic animal using ES cell. For example, the method of preparing a transgenic mouse may be performed by: selectively injecting the recombinant gene or recombinant expression vector into a ES cell; injecting the resulted ES cell into blastocyst (E3.5), to prepare a chimeric mouse through a surrogate mother; mating the prepared chimeric mouse with a normal mouse, to obtain a heteromouse where only a copy of the WW45 coding gene is deleted; and then, mating the heteromice, to construct a transgenic mouse where two copies of the WW45 coding gene is deleted.
The transgenic animal according to the present invention may be characterized in that the WW45 protein is operated so as not to be expressed or to have functional loss, whereby apoptosis is inhibited, and thus, cell proliferation is not terminated and cell differentiation is not initiated, resulting in inducing tumor. Therefore, the transgenic animal may be useful as a animal model with tumor.
As used herein, the term ‘animal model’ refers to any animal that has a specific disease similar to human disease, thereby being used as an experimental subject for revealing the pathogens, and checking the pathological condition. The animal to be used as an animal model may be one that is easily prepared, has repeatability and reproducibility, and is expected that the similar result to human can be obtainable. In addition, in the animal, it is preferable that pathogen and progress of diseases are similar to human. Therefore, the animal may be preferably one belonging to mammals like human, where the internal structure of the body such as internal organs, immune system, body temperature, and the like, are similar to human, and diseases such as hypertension, cancer, immunodeficiency, and the like, can be induced.
From this point of view, the transgenic animal may be produced from any mammal, for example, a horse, a sheep, a pig, a goat, a camel, an antelope, a dog, or a rodent such as a mouse, a rat, a guinea pig, hamster, and the like, wherein a rodent such as a mouse, a rat, a guinea pig, hamster, and the like may be preferable. In particular, the mouse has been most widely used in studying human diseases because of several advantages such as a small size, a superior fertility, an easy feeding management, a strong resistance to diseases, a genetic uniformity, development of various species, and a possibility to produce an animal that has a similar disease or pathological condition.
The induction of tumor in the transgenic animal may occur at one or more tissues or organs selected from the group consisting of skin epithelium, intestine epithelium, kidney epithelium, lung epithelium, thymus epithelium, pancreas epithelium, and the like. Therefore, the transgenic animal may be useful as a tumor-induced animal model, where the tumor may be one or more selected from a liver cancer, a lung cancer, a blood tumor, a skin cancer, a gastric cancer, a large intestine cancer, a small intestine tumor, a kidney cancer, and the like.
In an embodiment, the inhibition of WW45 protein expression and functional loss may be performed by any conventional transfection technique using a recombinant gene where whole or part of WW45 protein coding gene is deleted or substituted, or a recombinant expression vector prepared by inserting the recombinant gene into a proper expression vector. For example, the inhibition of WW45 protein expression and functional loss may be achieved by deleting or substituting whole or part of WW45 protein coding gene, where the part of WW45 protein coding gene may be a region containing at least SARAH domain.
The WW45 protein coding gene may be a WW45 protein coding gene of mammal including human. For example, the WW45 protein coding gene may be one or more selected from the group consisting of human WW45 protein coding gene (the region from 339th to 1490th positions of SEQ ID NO: 7 (GenBank Accession No. NM—021818.2), mouse (Mus musculus) WW45 protein coding gene (the region from 236th to 1396th positions of SEQ ID NO: 8 (GenBank Accession No. NM—022028.2), rat (Rattus norvegicus) WW45 protein coding gene (the region from 172nd to 1335th positions of SEQ ID NO: 9 (GenBank Accession No. NM—001097581), and the like, but not be limited thereto.
The deletion or substitution of whole or part of the WW45 protein coding gene may include any operations to prevent a complete expression of WW45 protein, or make the expressed WW45 protein incapable of forming a complex with MST1/2 to fail to move into nucleus. Therefore, in embodiment, the deletion of part of WW45 protein coding gene may mean a deletion of at least SARAH domain that plays a role to interact with MST1.
The identical sequences of SARAH domain of WW45 are as follows:
Human: ilkwelfql adldtyqgml kllfmkeleq ivkmyeayrq alltelenr (the region from 321st to 368th positions of SEQ ID NO: 1, SEQ ID NO: 10),
Mus musculus: ilkwelfql adldtyqgml kllfmkeleq ivklyeayrq alltelenr (the region from 322nd to 369th of SEQ ID NO: 2, SEQ ID NO: 11),
Rattus norvegicus: ilkwelfql adldtyqgml kllfmkeleq ivklyeayrq alvtelenr (the region from 323rd to 370th positions of SEQ ID NO: 3, SEQ ID NO: 12).
The transfection method used in the present invention may not be limited, and for example, the method may be one using embryo cells. The method using embryo cells is one using embryonic stem cells (ES cells) isolated from embryos at blastocyst stage. An ES cell is an undifferentiated cell having non-stopped cell division ability and differentiation ability to various tissues. A chimera animal can be produced by introducing a specific gene into ES cell or removing a specific gene from ES cell, and injecting the ES cell into blastocyst cavity of blastocyst stage embryo.
Embryonic stem cells isolated from blastocyst stage embryo have high efficiency in producing a chimera and/or transgenic animal compared with EC cells. A transgenic animal lacking a specific genetic character, as well as a transgenic animal obtaining a specific genetic character, can be produced by selecting ES cells in which the introduced extrinsic gene is homologously inserted into a specific site of chromosome. Recently, several studies produce transgenic animals using a primordial germ cell (PGC) that is collected from fetus at early developing stage, in addition to ES cells.
Another embodiment provides a method of screening an anti-tumor agent using the transgenic organism where a tumor is induced. More specifically, the method of screening an anti-tumor agent may include the steps of:
preparing the transgenic organism having lack of WW45 protein expression or functional loss of the WW45 protein, whereby tumor is induced, or a tumor cell isolated from the transgenic organism;
treating the prepared organism or cell with a candidate compound; and
determining the tumor level of the candidate compound-treated organism or cell.
The tumor level may be determined by measuring the number of tumor cells, the volume of tumor, and/or tumor cell viability. When the number of tumor cells and/or the volume of tumor is decreased, and/or apoptosis of tumor cell is induced, by treating the candidate compound, the candidate compound may be determined as an anti-tumor agent. The anti-tumor agent may be used against one or more selected from the group consisting of a liver cancer, a lung cancer, a blood tumor, a skin cancer, a gastric cancer, a large intestine cancer, a small intestine tumor, a kidney cancer, and the like.
Another embodiment provides a method of screening a differentiation regulating agent where the WW45 protein is target. The differentiation regulating agent may include both of a differentiation promoting agent and a differentiation inhibiting agent. More specifically, the method of screening a differentiation regulating agent may include the steps of:
treating a sample cell with a candidate compound; and
measuring the level of WW45 protein expression, MST1/2 activation, LATS1/2 phosphorylation, or YAP phosphorylation, in the cell.
The sample cell may be any cell obtained from any mammal including human, and for example, epithelial cells of skin, intestine, and the like, more specifically, epidermal cells, keratinocytes, crypt cells, and the like. The levels of WW45 protein expression, MST1/2 activation, LATS1/2 phosphorylation, and YAP phosphorylation may be determined by any conventional quantitative or analyzing method of protein known to the relevant art.
The method of screening a differentiation regulating agent may be a method of screening a differentiation promoting agent characterized by determining a candidate compound as a differentiation promoting agent when the level of WW45 protein expression, MST1/2 activation, LATS1/2 phosphorylation, and/or YAP phosphorylation is increased after treating the candidate compound. Alternatively, the method of screening a differentiation regulating agent may be a method of screening a differentiation inhibiting agent characterized by determining a candidate compound as a differentiation inhibiting agent when the level of WW45 protein expression, MST1/2 activation, LATS1/2 phosphorylation, and/or YAP phosphorylation is decreased after treating the candidate compound.
Embodiments of the present invention provide novel insights into the role of WW45 in mammalian epithelial tissue development. WW45 protein is a key regulator of the MST1 signaling pathway, which promotes cell-cycle exit and terminal differentiation in developing epithelial tissues. It is found that ablation of WW45 leads to hyperproliferation accompanied by immature differentiation in epithelial cells of the skin and intestine (FIGS. 3 and 4), and that this phenotype results from re-entry of differentiating cells to the cell cycle rather than intrinsic acceleration of proliferation (FIG. 8).
This conclusion is strongly supported by our observations that WW45-deficient primary keratinocytes cannot be efficiently induced to exit the cell cycle in response to differentiation signals such as Ca2+, TGF- or LiCl, and that increased proliferation rates were detected from E15.5 onwards, which is the onset of terminal differentiation events in the epithelium (FIGS. 8, 17 and 18). The Hippo pathway has been implicated in the restriction of proliferation and promotion of apoptosis in epithelial cells, but there is little evidence that this pathway is involved in terminal differentiation in Drosophila. Interestingly, the mouse MST1 signaling pathway seems to have a role in terminal differentiation in the developing epithelial tissues.
Previous studies with Drosophila have identified several genes and their interactions in the Hippo pathway; however, the intracellular signaling of spatiotemporal regulation during epithelial differentiation remains unclear. The inventors determine the underlying mechanism by which components of the MST1 signaling pathway spatiotemporally regulate cell-cycle exit for epithelial differentiation in mammals.
The inventors show that as-yet-unknown differentiation signals specifically activate the MST1 kinase, which then dynamically localizes to the nucleus and activates LATS1/2, and that wild-type WW45 is required for this process. Activated LATS1/2 then phosphorylates serine 127 of YAP, and this phosphorylated YAP localizes to the cytoplasm where it is inactivated (FIG. 16d). Recently, it has been reported that serine 127 of YAP is the main phosphorylation site in the Hippo pathway and its phosphorylation results in its cytoplasmic translocation; overexpression of YAP in mice induces a severe dysplasia accompanied by expansion of multipotent undifferentiated progenitor cells in the skin and intestine.
These phenotypes are quite similar to those seen in the WW45-null embryos. Here, the inventors have shown that phosphorylation of serine 127 of YAP by LATS1/2 in MST signaling occurs during epithelial differentiation. Based on the observations and the phenotypic similarities between YAP transgenic and WW45-null mice, the inventors propose a hypothesis that, during formation of mature epithelial tissues in mammals, the MST1 pathway is activated by differentiation signals and determines when precursor cells stop dividing and terminally differentiate, and that WW45 may be central to this process (FIG. 16f).
Interestingly, transgenic mice overexpressing YAP S127A in the liver displayed enlarged livers, supporting the hypothesis that Hippo signaling regulates mammalian organ size. However, no organs of increased size were seen in WW45−/− embryos, which could be due to placental defects or another genetic compensation. Thus, tissue-specific or conditional knockout experiments in mice are required for further clarification of the role of the MST1 pathway in regulation of organ size in mammals.
Recent studies in Drosophila have indicated that, together with cyclin E, Diap1 and bantam may be an additional target of Yorkie. However, unlike in Drosophila, no direct target genes were identified in the MST1 pathway. Of interest, quantitative RT-PCR revealed that, although slightly increased, expression levels of cyclin E, cIAP, XIAP and survivin were not significantly upregulated in WW45−/− cells, indicating that other targets of YAP control developmental homeostasis in mammalian epithelial tissues (FIG. 19). Based on the observation that YAP DN is sufficient to rescue the cell-cycle exit and differentiation of WW45−/− cells, identification of the downstream targets or mediators of YAP will provide important insights into how YAP could control both cell-cycle exit and terminal differentiation.
The inventors also found that WW45 deficiency led to disruption of contact inhibition of proliferation, as is also seen with loss of LATS2 and NF2. WW45 is also likely to participate in the contact inhibition signaling pathway. Loss of contact inhibition might be associated with the hyperplasia observed in epithelial tissues of WW45−/− embryos. Recently, inactivation of YAP in the Hippo pathway has been shown to contribute to cell contact inhibition and tissue growth. Thus, further studies should investigate whether defects in contact inhibition in WW45−/− cells are due to a failure of YAP inactivation.
Perturbations in MST1 signaling lead to inappropriate proliferation and expansion of cell compartments, which in turn lead to increased risks of cancer-associated mutations. Several results support the importance of the Hippo pathway in mammalian tumorigenesis. Loss of expression of LATS1 and LATS2, and mutation of hWW45 and Mats have been reported in cancer cell lines. Mice with mutations in LATS1 or NF2 develop tumors, and YAP transcription is increased in the mouse tumor model.
Moreover, YAP can transform immortalized mammary epithelial cells in vitro and transgenic mice overexpressing YAP develop liver cancers or intestinal dysplasia with loss of differentiated cell types. In addition to these reports, the inventors strikingly revealed that dysfunction of a single gene, WW45, affects most developing epithelial tissues, and induces characteristics of a precancerous state: uncontrolled proliferation, partial loss of epithelial polarity and block of terminal differentiation. However, no such phenotypes have been reported in mice lacking LATS1, LATS2 and NF2.
Multiple homologs of the fly Hippo pathway exist in mammals; hence, there may be functional redundancy of these proteins. However, WW45 is the only mammalian homolog of the Sav scaffold protein in the Hippo pathway, and loss of WW45 led to marked phenotypic changes in many epithelial tissues. Importantly, it is found that, during a 14-month observation period, 22% of WW45-heterozygous mice developed some type of tumor including osteosarcoma and hepatoma. Therefore, future studies should investigate how the heterozygous status of WW45 initiates tumorigenesis and the molecular characteristics of developed tumors in these mice. Finally, according to embodiments of the present invention, the WW45-knockout embryo or WW45-heterozygous mice could be a suitable model for tumorigenesis studies of the MST1 pathway.
Here, a novel use of WW45 protein in termination of proliferation (cell cycle exit) and initiation of differentiation, which is expected to be useful in searching a differentiation regulators, and preparing a WW45-knockout or WW45-heterozygous transgenic animal for various studies.
EXAMPLES
The present invention is further explained in more detail with reference to the following examples. These examples, however, should not be interpreted as limiting the scope of the present invention in any manner.
Example 1
Generation of WW45 Null Mice
WW45 gene targeting vector was injected into ES cells by electroporation, and WW45 gene-deficient ES clones were examined by Southern blotting. The WW45 gene-deficient targeting ES clones were injected into blastocyst, to prepare a chimeric mouse.
More specifically, a WW45−/− mouse was generated using standard ES cell (R1) homologous recombination and blastocyst injection techniques (Lim et al., 1996; Lee et al., 2006). A WW45 targeting vector was prepared by subcloning left arm (obtained from 6338 bp containing exon1 using EcoR I restriction enzyme (New England Biolabs, NEB)) and right arm (obtained from 3443 bp using BamH I restriction enzyme (NEB)) into pGK-Puro vector (obtained from pBluscript II KS(+) vector (Stratagene) as a backbone) using 129SvJ mouse BAC clone (obtained from Korea Research Institute of Bioscience & Biotechnology) (FIG. 1a). More specifically, puromycin resistance gene, which is a positive selection marker, was cloned between left arm and right arm, to replace exon2, and diphtheria toxin-A (DT-A), which is a negative selection marker, was cloned into Not I site, to produce a pGK-Puro-WW45 gene targeting vector.
The produced WW45 gene targeting vector was deposited with the Korean Collection for Type Cultures (KCTC) at Yuseong-gu, Deajeon, Korea, on Jun. 12, 2008, and allotted receipt number KCTC11342BP.
WW45−/− mice were generated by injecting the WW45 gene targeting vector into a mouse ES cell (BayGenomics) through electroporation to a targeted ES cell clone, and injecting the ES cell clone into C57BL/6 blastocysts (E3.5, purchased from Jackson Lab) to generate germ-line-transmitting chimeric mice. Mice and ES cells were genotyped by polymerase chain reaction (PCR) assays. The primers and conditions used in the PCR are as follows:








(SEQ ID NO: 13)












WW45-L:
5′-TGACCATGTGTCCAGCCTTA-3′,




 








(SEQ ID NO: 14)












WW45-R:
5′-CGAATGGATGCTGCATATTG-3′,




 








(SEQ ID NO: 15)












pGK-3:
5′-GCACGAGACTAGTGAGACGTGCTAC-3′.







Conditions: denature at 94° C. for 30 seconds; annealing at 60° C. for 45 seconds; elongation at 72° C. for 1 minute, 30 cycle.
Using Applied biosystems Geneamp PCR system 2700 PCR apparatus.
Example 2
Histological Analysis
The chimeric mouse produced in Example 1 was mated with a normal mouse (purchased from C57BL/06, Jackson Lab) to produce WW45 hetero mouse where only one copy of WW45 gene is deleted. It was confirmed by PCR that the embryo obtained by mating the resulted WW45 hetero mice is mutants where the WW45 gene is deleted. The resulted embryo of the transgenic animal was deposited to the Korean Collection for Type Cultures (KCTC) at Yuseong-gu, Deajeon, Korea, on Jun. 12, 2008 and allotted receipt number KCTC11343BP.
The embryos were fixed in 4% paraformaldehyde overnight at 4° C., embedded in paraffin and sectioned to the size of 4 μm. The sections (4 μm) were stained with hematoxylin and eosin (H&E) or subjected to immunohistochemical analysis. Immunohistochemical analysis was performed using standard protocols (Koo et al., 2007) with antibodies against PECAM-1 (BD), laminin (Abcam), BrdU (Sigma), E-cadherin (BD), β-catenin (BD), Ki67 (Novocastra), anti-filaggrin (Covance), loricrin (Covance), K10 (Covance), K14 (Covance), K1 (Covance), iFABP (gift from Dr J. I. Gordon), chromogranin A (Immunostar), YAP (Cell Signaling, Santa Cruz), and p-YAP (Cell Signaling). Peroxidase levels were assessed using the EnVision®+ Dual Link System-HRP(DAB+) (DakoCytomation). Intestinal goblet cells were stained with Alcian blue to detect mucin. The TUNEL assay (koo et al., 2005) was performed using a commercial staining kit (Roche). BrdU incorporation experiments were performed by injecting pregnant females intraperitoneally with BrdU (Sigma) at a concentration of 100 μg/g body weight. At 1 hour, 2 hours, or 24 hours after injection, embryos were dissected and fixed for immunohistochemical analysis.
Example 3
Barrier Function Assays
At E17.5, the embryos were rinsed in phosphate-buffered saline (PBS) and immersed in acidic X-gal mix (100 mM phosphate buffer at pH4.3, 3 mM K3Fe(CN)6, 3 mM K4Fe(CN)6, 2 mM MgCl2, 1 mg/mL X-gal), then incubated for 8 hours at 37° C. in the dark (Hardman et al., 1998).
Example 4
Electron Microscopy
Electron-microscopy analysis of the obtained tissue samples was performed according to standard protocols. Briefly, the samples were fixed with 3% glutaraldehyde for 2 hours and then washed with 0.1 M cacodylate buffer containing 0.1% CaCl2. The Samples were then post-fixed with 1% OsO4 in 0.1 M cacodylate buffer (pH 7.2) containing 0.1% CaCl2 for 2 hours at 4° C. After dehydration in graded alcohol concentrations, the cells were embedded in Spurr's epoxy resin. After polymerization of the resin at 70° C. for 36 hours, serial sections were cut and mounted on formvar-coated slot grids. The sections were stained with 4% uranyl acetate for 10 minutes and with lead citrate for 7 minutes. A Tecnai G2 Spirit Twin transmission electron microscope (FEI Company, USA) and a JEM ARM 1300S high-voltage electron microscope (JEOL, Japan) were used.
Example 5
Quantitative Real-Time PCR
Total RNAs from the tissue samples were extracted using Trizol (Gibco). cDNA was generated using SS II RT kit (Invitrogen). Expression of genes indicated in the text was measured by real-time qRT-PCR (Bio-rad iQ5 multicolor real-time PCR detection system), and normalized to GAPDH (glyceraldehyde-3-phosphate dehydrogenase) expression levels.
Example 6
Plasmid Construction
Human cDNAs for WW45, MST1, LATS1/2 and YAP (WW45: GenBank Accession No. NP—068590.1, MST1: GenBank Accession No. NP—006273.1, LATS1: GenBank Accession No. NP—004681.1, LATS2: GenBank Accession No. NP—055387.2, YAP: GenBank Accession No. NP—006097.1) were cloned into pDK-Flag2 or pCMV-HA, which had been modified from pcDNA3.1 or pcDNA3 (Invitrogen), respectively, where the pDK-Flag2 or pCMV-HA was generated by inserting Flag or HA tagging using the pcDNA3.1 or pcDNA3 (Invitrogen) as backbone. Site-directed PCR mutagenesis was used to introduce the missense changes S61A, S109A, S127A, T328A and S347A into the YAP sequence. The S127A-nuclear-localizing form with a deletion of the C-terminal TA-domain of YAP was generated and used for YAP-DN mutant form. The cDNAs for YAP and its serine to alanine (SA) mutant form were cloned into pET-15b (Novagen) to generate recombinant hexahistidine (His6)-tagged YAP and SA proteins, respectively.
Example 7
Generation of Antibodies
Rabbit polyclonal antibodies to MST1 were prepared with purified recombinant hexahistidine (His6)-tagged MST1 (K59R, KAIST) as an antigen and then affinity purified and used for immunostaining. Rabbit polyclonal antibodies to WW45 (Rb Ctr#1) were generated by injecting rabbits with a keyhole-limpet-hemocyanin-conjugated peptide corresponding to the 14 C-terminal amino acids of mouse WW45 (RKQRQQWYAQQHGK; SEQ ID NO: 16). Specific antibodies were affinity purified with the appropriate antigens (RKQRQQWYAQQHGK).
Example 8
Primary Keratinocyte Cultures
Keratinocytes isolated from embryos at E17.5 were cultured in PCT Epidermal Keratinocyte Medium (Chemicon), where the embryos were prepared by removing a womb from a pregnant mouse generated by mating ww45 hetero mice at 17.5th day by caesarean section, then collecting embryos therefrom, removing placenta and amnion from each embryo, conforming genotype of each embryo by the removed amnion, and then selecting embryos that correspond to each genotype. Differentiation was induced by adding CaCl2 to a concentration of 1.2 mM, TGF-β to a concentration of 1 ng/ml (R&D) and LiCl to concentration of 10 mM to the culture medium. BrdU (Sigma) was added to the medium (10 μM) and cells were incubated for 1 hour before being fixed and processed for immunostaining. For growth-curve analysis, 3×105 cells from passage 2 were plated in six-well plates, and the total numbers of cells were counted daily. Transfections were performed with Effectene reagents (Qiagen) or with polyethyleneimine (Sigma). Retroviral infections were performed according to standard protocols (Song et al., 2004).
Example 9
Immunoprecipitation, Subcellular Fractionation, Western-Blot Analysis and Immunofluorescence
Primary keratinocytes were lysed in 25 mM Tris-HCl (pH 7.4), 150 mM NaCl, 1 mM EDTA, 1 mM MgCl2, 0.2% TritonX-100, 0.3% NP-40, protease inhibitors (Roche Protease inhibitor cocktails) and phosphatase inhibitors (NaF, sodium fluoride), where the primary keratinocytes were prepared by extracting epithelium from the E17.5 embryo's skin, treating dispase thereto on the embryo's skin, separating epithelial layer, and then isolating single cell from the epithelial layer. The resulted cell lysates were incubated for 2 hours at 4° C. with antibodies (antibodies against LATS1/2: Bethyl_Laboratories, INC; and antibodies against MST1: prepared in Example 7), and then with protein A/G plus-agarose beads to be subjected to immunoprecipitation. The resulted immunoprecipitates were subjected to Western-blot analysis or the immunoprecipitation (IP)-kinase assay. Separation of nuclear and cytoplasmic extracts was performed using NE-PER® Nuclear (PIERCE) and Cytoplasmic Extraction Reagents (PIERCE). Western-blot analysis was performed using antibodies against YAP (Cell Signaling), p-YAP (Cell Signaling), MST1 (Cell Signaling), p-MST1 (Cell Signaling), LATS1 (Bethyl Laboratories, INC), LATS2 (Bethyl Laboratories, INC), hemagglutinin (HA, Covance), filaggrin (Covance), laminin B (Santa Cruz), α-tubulin (Chemicon), Flag (Sigma) and WW45 (Example 7). For immunostaining analysis, keratinocytes were fixed in absolute methanol at −20° C. for 10 minutes and then exposed consecutively to primary and secondary antibodies. Slides were mounted with 4′,6-diamidino-2-phenylindole (DAPI) and imaged.
Example 10
In-Vitro Kinase Assay
Immunoprecipitated HA-LATS1/2-WT or -KD were incubated for 30 minutes at 30° C. with purified His-YAP-WT or His-YAP-SA in kinase buffer (25 mM HEPES pH 7.4, 50 mM NaCl, 5 mM MgCl2, 5 mM MnCl2, 5 mM β-glycerophosphate and 1 mM dithiothreitol) supplemented with 10 μM adenosine 5′triphosphate (ATP) and 2 μCi [γ-32P] ATP. The reaction mixtures were analyzed by SDS-PAGE and autoradiography to detect 32P-labeled YAP.
Experimental Example 1
Retarded Growth and Perinatal Lethality of the WW45-Deficient Mice
To identify the role of WW45 in vivo, WW45 mutant mice were generated using embryonic-stem (ES)-cell technology (see Example 1). The targeted mutation of the example was characterized by replacing a 2.4-kb genomic region containing WW45 exon 2 (at 330th to 773rd positions of SEQ ID NO: 8 (GenBank Accession No. NM— 022028): atctcatgcc ttcattcatt cggcacggtc caacaattcc cagacggact gacctctgtcttccagattc aagtgctact gctttctcag cttctggaga tggtgtagtt tcaagaaaccagagtttcct gagaactgca attcaaagga cacctcatga agtaatgaga agagaaagccacagactgtc tgccccttct taccttgtca ggagcctagc agatgtccct cgagagtgtggctcatcaca gtcatttttg acagaagtta actttgctgt tgagaatgga gactctggctcccgatactt cttctcagat aacttttttg atggacagag aaggcggcca cttggagatcgtgcacaaga agattacaga tattatgaat acaaccatga tctcttccag aggatgccacagagtcaggg gaggcacact tcag) with a puromycin cassette (Lim et al., 1996; Lee et al., 2006), leading to premature stop in WW45.
Electroporation, puromycin positive selection and diphtheria toxin negative selection were performed, and a 5′ external flanking probe was prepared by PCR on genomic DNA using two primers, SP-l and SP-r.
The nucleotide sequences of the primers are as follows:








(SEQ ID NO: 17)












Sp-1:
5′-CCT AGA CCC TTT CAA CAA GCA-3′




 








(SEQ ID NO: 18)












Sp-r:
5′-TGC TAT CAC TCA TCG GGA TT-3′







After screening by Southern blotting (Combi H12, FinePCR) using the prepared probe, the targeted clone was identified, and transmitted through the germline after crossing the chimeric mice with C57BL/6 mice (purchased from Jackson Lab) (FIG. 1a).
FIG. 1 shows targeted disruption of the WW45 gene by homologous recombination, wherein FIG. 1a is a schematic representation of the WW45 targeting strategy, showing the mouse WW45 genomic locus, targeting vectors and targeted locus. Exon 2 was replaced with a puro cassette, and diphtheria toxin A was used for negative selection (Lee et al., 2006). Five exons are indicated by black boxes. Also indicated are the 5′-external probe for Southern hybridization of genomic DNA, predicted sizes of hybridizing fragments with restriction sites, and primer pairs for PCR. Arrowheads represent primers (see Example 1) used for genotyping of the WT (1 and 2) or MT (1 and 3) alleles, and Bg indicates Bgl II restriction site.
The absence of WW45 protein was confirmed by fractioning liquid protein obtained from mutant mouse embryonic fibroblasts (MEFs) by electrophoresis and then Western blotting (FIG. 1B), indicating a null allele in mutant mice.
FIG. 1b shows the result of Southern-blot analysis of genomic DNA digested with the restriction enzyme Bgl II using the 5′-probe, and PCR genotype analysis of embryos (see Example 1). Western-blot analysis (WB) shows the presence of WW45 in cultured primary fibroblasts. Arrow indicates bands of WW45.
The Southern blotting shown in FIG. 1b was performed by extracting genomic DNA from embryonic stem cells and transgenic mice, treating a restriction enzyme, and performing electrophoresis on 0.9% agarose gel, to transfer onto nylon membrane. The DNA attached nylon membrane was hybridized with P32-CTP-labeled 5′-external probe, and washed, and the obtained band was imaged.
Heterozygous mice were born healthy and fertile, and developed normally. However, only 3 dwarf homozygotes were found among 954 littermates generated from heterozygous intercrosses, indicating that most of the null mice were embryonic lethal. Viable WW45−/− embryos were found at embryonic days 17.5 (E17.5) and E18.5. The obtained results are shown in Table 1.







TABLE 1







Genotypes of progeny from WW45 heterozygous intercrosses










No. per genotype












Age (dpc)
+/+
+/−
−/−
Total














17.5
168
352
85
605


18.0
17
34
6
57


18.5
22
46
6
72


19.0
4
2
0
6


Neonates
350
601
3
954









WW45−/− embryos up to E11.5 were morphologically indistinguishable from their control littermates. However, from about E13.5 onwards, WW45−/− embryos were slightly smaller than the controls, indicating a slower gain in body weight (FIG. 1c). The upper part of FIG. 3c shows a macroscopic appearance of wild-type and mutant embryos at E17.5, and the bottom part shows growth curves of wild-type (+/+) and mutant (−/−) WW45 embryos, indicating growth retardation of mutant (−/−) WW45 embryos. Despite the growth retardation, no consistent overt defects that would cause the embryonic lethality of mutants were observed.
This result prompted the inventors to examine placentas from mutants and their littermates. Vascular invasion normally proceeded with increasing development of the labyrinth layer in wild-type placentas. By contrast, mutant placentas displayed immature development with poor growth and vascularization of the labyrinth layers (FIG. 1d). FIG. 1d shows hematoxylin and eosin (H&E)-stained sections of placentas from E17.5 wild-type and mutant embryos. It was found that the major layers had defective maturations with reduced and disordered vasculature in mutant placentas (de, decidua; sp, spongiotrophoblast layer; 1a, labyrinth layer. Scale bar: 500 μm). Boxed regions are shown at high magnification.
Defective intermingling of fetal and maternal vessels was confirmed by staining with anti-PECAM1 and anti-Laminin according to Example 2, and the obtained result was shown in FIG. 2. FIG. 2 is picture showing defective vascularization of WW45−/− placenta at E17.5. As shown in FIG. 2, Extra-embryonic vascular defects were confirmed by immunohistochemistry analysis with antibodies against PECAM-1 and laminin. Scale bar: 100 μm.
In conclusion, the malfunctional labyrinth layer may affect the growth and viability of WW45−/− embryos.
Experimental Example 2
Hyperplasia and Immature Differentiation of Epithelial Cells in the WW45-Deficient Embryos
Previous Drosophila studies have proposed important roles for SAV1 in regulation of proliferation and apoptosis in epithelial tissues. Thus, histological analyses of WW45−/− embryos were performed at various embryonic stages. Interestingly, hyperproliferation of epithelial cells was clearly observed in the skin and intestine (FIGS. 2 and 3) and other organs (FIG. 5) of the WW45−/− embryos at E17.5. The experiment was performed according to the method of Example 2.
The inventors first characterized skin development in these WW45−/− mice. FIG. 3 shows hyperproliferation and immature differentiation in WW45−/− epidermis at E17.5. Dividing keratinocytes are normally restricted to the basal layer of wild-type epidermis, and as cells exit from the cell cycle, these keratinocytes move outward and differentiate to form the spinous layers, the granular layers, and the dead enucleated stratum corneum layers at the skin surface (FIG. 3a-a).
By contrast, the epidermis of null embryos had a more dense basal layer and the expanded suprabasal layers were less differentiated, with reduced enucleation and compaction of the developing granular cells (FIG. 3a-a′). Development of hair follicles was rarely seen, and only small premature hair follicles were seen in null embryos at this stage. Co-immunostaining for E-cadherin and Ki67 revealed that the skin of mutant embryos contained increased numbers of proliferative epithelial cells, compared with wild-type embryos (FIG. 3a-b, b′). Almost all the basal cells and several suprabasal cells expressed Ki67 in the mutant epidermis, whereas proliferation was restricted to the basal layer in the wild-type epidermis.
FIG. 3a shows the result of histological analysis of Hematoxylin and Eosin (H&E)-stained sections of wild-type (a) and mutant (a′) epidermis. Evaluation of cellular proliferation was conducted by co-immunohistochemistry analysis with anti-Ki67 and anti-E-cadherin (b, b′). The increased numbers of Ki67-positive cells, including in multiple cell layers, were found in the mutant epidermis. Quantitation of the percentage of proliferating cells per 1-mm2 area in mutant versus control epithelium from three independent experiments (±SD). Boxed regions are shown at high magnification in the right panel of each genotype. wt, wild-type; mt, mutant.
TUNEL (terminal deoxynucleotidyl transferase biotin-dUTP nick-end labeling)-positive cells were also shown in epidermis from wild-type embryos, but not in epidermis from null embryos (FIG. 3b-e, e′). FIG. 3b shows differentiation marker expression in skin of wild-type or mutant embryos. Immunohistochemistry analysis was performed with antibodies against K14, K10, loricrin and filaggrin, in addition to TUNEL staining for analysis of apoptosis. 4′,6-diamidino-2-phenylindole (DAPI)-stained nuclei are shown in blue. The lack of stratification and differentiation and the increased numbers of progenitor cells in the mutant epidermis were found. Thus, increased proliferation in the suprabasal layer and repressed apoptosis of terminally differentiated keratinocytes contribute to hyperplasia in the epidermis of mutant embryos.
Epithelia of WW45−/− embryos were hyperproliferative but did not seem to undergo normal differentiation; therefore, the inventors investigated whether epithelial differentiation was delayed and/or defective in mutant epithelia using a panel of antibodies against proteins that are expressed at defined stages of differentiation. Keratin 14 was normally expressed in one or two layers of basal cells in wild-type embryos, whereas it was strongly expressed in the multilayered basal cells in WW45−/− embryos (FIG. 3b-a, a′).
FIG. 4 shows hyperplasia and immature differentiation of WW45−/− intestinal epithelium at E17.5, wherein FIG. 4a shows H&E-stained sections of wild-type (a) and mutant (a′) intestine. The results from immunohistochemistry analysis with anti-Ki67 and anti-E-cadherin were indicated (b, b′). The increased numbers of Ki67-positive cells, including in multiple cell layers, were observed in the mutant epithelium. Quantitation of the percentage of proliferating cells per 1-mm2 area in mutant versus control epithelium from three independent experiments (±SD). Boxed regions are shown at high magnification in the right panel of each genotype. wt, wild-type; mt, mutant. FIG. 4b shows differentiation marker expression in the intestine of wild-type or mutant embryos. Immunohistochemistry analysis was performed with antibodies against iFABP and chromogranin A, in addition to Alcian-blue staining. The significant reductions in levels of terminal differentiating cells, goblet cells and entero-endocrine cells were observed in mutants, compared with wild-type littermates. FIG. 4c shows the result of electron-microscopy analysis of the wild-type (upper panel) and mutant (lower panel) intestines. The density of brush-border microvilli of mutant enterocytes is reduced compared with the densely compacted, uniformly distributed microvilli of wild-type enterocytes. Scale bar: 100 μm in (a) and (b), 2 μm in (c).
FIG. 5 shows hyperplasia in WW45−/− epithelial tissues at E17.5. FIG. 5 shows the result of histological analysis of hematoxylin & eosin (H&E)-stained sections of wild-type (a-d) or mutant (a′-d′) epithelial tissues, wherein (a, a′) indicates large intestine, (b, b′) lung, (c, c′) retina, and (d, d′) tongue. Right panels are enlarged images of each black boxed part. The dense and disorganized epithelia were observed in mutant embryos. Scale bar: 100 μm.
There were increased numbers of keratin-10-expressing cells in the suprabasal layers of the WW45−/− epidermis compared with wild-type embryos (FIG. 4b-b′). Expression levels of loricrin and filaggrin, which are markers of late keratinocyte differentiation, were significantly downregulated in mutant epidermis, indicating defects in late differentiation (FIG. 4b-c′-d′). The absence of terminally differentiated layers was also confirmed by assaying skin-barrier development with X-gal staining (Hardman et al., 1998). Dye exclusion in control embryos was established at E17.5, whereas WW45−/− embryos turned an intense blue color when immersed in solution (FIG. 6). FIG. 6 shows dysmaturation of skin barrier function in mutant embryo at E17.5. Skin barrier function was assayed by X-gal staining. Blue-dye incorporation represents disruption to formation of the epidermal barrier in mutant embryos compared with their wild-type littermates.
Electron-microscopy analysis clearly showed that the epidermis of WW45-null embryos was thicker than the wild-type epidermis. Moreover, the granular and cornified layers present dysmaturation in the mutant epidermis, with nucleated cells reaching the epidermal surface (FIG. 3c). FIG. 3c shows the result of electron-microscopy analysis of the wild-type (upper panel) and mutant (lower panel) epidermis. The loss of columnar morphology in basal cells and the disorganized suprabasal cells were observed as well as the loss of flattened granular and cornified layers in the mutant skin.
These data indicate that the suprabasal mutant keratinocytes fail to stop proliferating and terminally differentiate.
Intestinal development in mutant embryos was also examined. Wild-type intestinal epithelium consists of a monolayer of polarized epithelial cells organized into crypts. By contrast, the WW45−/− mutant epithelium was multilayered and displayed hypercellularity with pseudostratified and enlarged nuclei, perturbed differentiation with loss of goblet cells, and increased numbers of mitotic cells (FIG. 4a-a′ and FIG. 5-a, a′). Furthermore, Ki67 staining revealed extensive proliferation throughout the villus epithelium in the small intestine, whereas proliferative cells were restricted to the crypt bases in the control epithelium (FIG. 3a-b-b′).
Indeed, all epithelial cells in the mutant colons were Ki67-positive, indicating dysplasia (FIG. 7a). FIG. 7 shows hyperproliferation in WW45−/− epithelial tissues at E17.5, wherein FIG. 7a shows the result of evaluation of cellular proliferation in wild-type or mutant colonic epithelium by co-immunohistochemistry analysis with anti-Ki67 and anti-β-catenin. The numbers of Ki67-positive cells, including those located in multiple cell layers, in the mutant epithelium were increased compared with the control epithelium; in the control epithelium, Ki67-positive cells were present mainly in the restricted proliferation zone. Scale bar: 100 μm. In agreement with these results, bromodeoxyuridine (BrdU) pulse experiments further confirmed significantly increased numbers of dividing cells in mutant epithelia of many organs (FIG. 7b). FIG. 7b shows the result of quantitative analysis of the percentage of BrdU-positive cells per 1.0-mm2 area of epithelium 2 hours after BrdU injection. Data represent triplicate independent experiments ±SD.
Then, the differentiation of the various intestinal epithelial cell lineages was examined. During differentiation of enterocytes, the main epithelial cell type, the FABP protein was detected at normal levels in the villi of wild-type embryos but at markedly reduced levels in the mutant embryos (FIG. 4b-a, a′). Similarly, chromogranin labeling, which detects differentiation along the entero-endocrine lineage, was rarely detected in mutant embryos (FIG. 4b-b, b′). Staining with Alcian blue, a marker for goblet cells, revealed a complete absence of muco-secreting goblet cells in all mutant intestinal tracts (FIG. 4b-c, c′). Ultrastructural analysis also revealed poorly developed microvillus brush borders on the apical surfaces of the villous enterocytes, indicating defective enterocyte differentiation in mutant epithelium of the small intestine (FIG. 4c). Interestingly, these mutant cells had enlarged nuclei located close to the apical region, indicating a loss of apical-basal polarity.
In addition to immature differentiation of the mutant skin and intestine, immature differentiation was also detected in the lungs of mutant embryos (Lee and Lim, personal observation). Taken together, these results indicate that WW45 deficiency induces hyperplasia and immature differentiation in epithelial tissues.
Experimental Example 3
WW45 Regulates Cell-Cycle Exit in Epithelial Progenitor Cells During Differentiation
It was tested whether excessive proliferation of mutant cells was due to increased proliferation rates or failure of cell-cycle exit for terminal differentiation (according to Example 2). First, the cell-cycle duration in epithelial progenitor cells was examined by analyzing the proportion of cycling cells (Ki67+) in S-phase 1 hour after injection of BrdU (Schmahl, 1983). Although the percentage of BrdU-labeled cells was increased in mutant embryos, the BrdU+Ki67+/Ki67+ labeling index was approximately the same in wild-type and mutant embryos, indicating similar proliferation rates in wild-type and mutant embryos (FIG. 8a). FIG. 8 shows increased numbers of cycling cells in the developing WW45−/− epithelium resulted from inefficient growth arrest, wherein FIG. 8a is a graph showing the proportion of progenitor cells (Ki67+) labeled with BrdU after a 1-hour pulse. There is no difference in cell-cycle length between wild-type and mutant embryos.
Second, the frequency of cell-cycle re-entry was determined by assessing the proportion of dividing cells (BrdU+) 24 hours after the BrdU injection (Chenn and Walsh, 2002). During the time interval between BrdU application and analysis, cells can leave (Ki67−) or re-enter the cell cycle (Ki67+). The mean ratio of BrdU+Ki67+/BrdU+ cells was significantly increased by 49% in the mutant small intestine and by 58% in the mutant colon compared with wild-type controls (FIG. 8b). FIG. 8b shows the result of analysis of immunoreactivity for Ki67 in BrdU-positive cells 24 hours after BrdU injection. The fraction of cells re-entering the cell cycle (BrdU+Ki67+) is significantly increased in mutant embryos.
The above results indicate that WW45 promotes to exit from the cell cycle in epithelial progenitors during embryonic development.
Experimental Example 4
Failure of Cell-Cycle Exit of WW45−/− Keratinocytes During In-Vitro Differentiation
To further analyze the rates of proliferation and differentiation of epithelial cells, primary keratinocytes from the skin of embryos were isolated. Consistent with in-vivo data (FIG. 8a), WW45−/− keratinocytes had normal cell-cycle distribution and the rate of proliferation was not significantly increased compared with control cells (FIG. 8c). FIG. 8c shows growth curves of primary keratinocytes isolated from wild-type and mutant epidermis, which shows similar proliferation rates.
In addition, the results suggest that WW45 is unlikely to regulate the rate of proliferation. The ability of WW45 to regulate proliferation arrest and differentiation of developing epidermal cells was examined by adding calcium, transforming growth factor-β (TGF-β) or lithium chloride (LiCl), which has been shown to induce proliferation exit, and possibly terminal differentiation, of keratinocytes (Hennings et al., 1980; Shipley et al., 1986; Olmeda et al., 2003). With Ca2+, TGF-β or LiCl treatment (see Example 8), the wild-type keratinocytes showed efficient growth arrest and the numbers of BrdU-labeled cells were reduced. By contrast, mutant cells continued to proliferate and were BrdU-positive 24 hours after Ca2+, TGF-β or LiCl treatment (FIGS. 8d and 9).
FIG. 8d is a graph showing induction of calcium-stimulated differentiation in primary keratinocytes. Level of BrdU incorporation in keratinocytes cultured in the absence or presence of Ca2+ for the times was indicated. WW45-deficient embryos show inefficient growth arrest in the mutant keratinocytes under differentiation conditions. FIG. 9 shows defective cell-cycle exit in WW45−/− keratinocytes, wherein 9a and 9b show BrdU-labeling index of keratinocytes cultured with or without transforming growth factor (TGF)-β (a) or LiCl (b), respectively. The failure to stop cycling in WW45-deficient keratinocytes in response to TGF-β or LiCl treatment was observed.
WW45-deficient embryos display failure of cell-cycle exit of mutant keratinocytes under differentiation conditions. Consistent with these results, the expression of filaggrin was evident in wild-type but not mutant keratinocytes after induction of differentiation (FIG. 10a). FIG. 10 shows activation of MST1 signaling pathway during differentiation, 10a shows protein status of components of the MST1 pathway under differentiation conditions. Primary keratinocytes were incubated with or without Ca2+ for 24 hours and analyzed by Western-blot analysis with the indicated antibodies. The activation of MST1, as shown by pMST1 blotting and the mobility shift of YAP and LATS1/2 was observed in the control keratinocytes, but not in mutant keratinocytes, under differentiation conditions. Keratinocyte differentiation was confirmed by Western-blot analysis of filaggrin expression.
These data indicate that increased proliferation in WW45−/− epithelial tissues results from impaired growth arrest of progenitor cells during differentiation rather than from an increased rate of proliferation.
Experimental Examine 5
Activation of the MST Signaling Pathway During Keratinocyte Differentiation In Vitro
To further characterize the molecular mechanisms by which WW45 regulates cell-cycle exit during differentiation, the phosphorylation and localization of MST, LATS and YAP in keratinocytes during differentiation were investigated. Interestingly, autophosphorylation of MST1 was induced upon differentiation, as shown by phospho-MST1 immunoblotting, indicating MST1 activation (FIG. 10a). By contrast, this autophosphorylation of MST1 was not detected in the WW45−/− keratinocytes. Therefore, WW45 is required for MST1 activation after induction of differentiation with calcium treatment. In addition, the mobility patterns of LATS1/2 and YAP differ after induction of differentiation with calcium treatment (FIG. 10a). Phosphorylation of LATS1/2 and YAP was detected in differentiated wild-type keratinocytes, but not differentiated mutant keratinocytes. Thus, phosphorylation of YAP seems to be dependent on the MST1 signaling pathway, in particular on LATS1/2, during epithelial differentiation in mammals.
It was also investigated whether the formation of components of the MST1 signaling pathway might be associated with and affect differentiation of keratinocytes. First, WW45−/− keratinocytes were transfected with LATS1/2, MST1 and YAP with or without WW45, and then induced differentiation prior to immunoprecipitation. In WW45-deficient keratinocytes, LATS1 and LATS2 co-precipitated with YAP, but not with MST1. By contrast, these proteins form a stable complex in the presence of WW45 (FIG. 11). FIG. 11 shows requirement of WW45 for interaction between MST1/2 and LATS1/2 in primary keratinocytes. Physical associations between MST1, LATS1/2, WW45 and YAP under differentiation conditions were observed. WW45−/− primary keratinocytes were co-transfected with the plasmids indicated at the top of each panel. After 24 hours of transfection, cells were maintained in Ca2+-containing medium for a further 24 hours before harvesting for immunoprecipitation with anti-hemagglutinin (HA). The resulting precipitates were subjected to Western-blot analysis with the indicated antibodies. Note that complex formation was only detected in the presence of WW45.
Moreover, endogenous MST1 and LATS1/2 formed a complex in keratinocytes under differentiated conditions and in the presence of WW45 (FIG. 10b). FIG. 10b shows the analysis results for wild-type and mutant primary keratinocytes cultured with or without Ca2+ for 24 hours by immunoprecipitation with antibodies against MST1 and by Western-blot assays. Complex formation is only detected with Ca2+ treatment in the control samples. In addition, YAP was fully phosphorylated in keratinocytes complemented with wild-type WW45 and expressing LATS2 wild-type (WT), but not in cells with wild-type WW45 and expressing LATS2 kinase dead (KD) and MST1 or in cells complemented with WW45 lacking the SARAH-domain, which is responsible for interaction with MST1 (FIG. 10c). FIG. 10c shows phosphorylation of YAP by activated LATS2 through MST1/WW45. WW45−/− primary keratinocytes were co-transfected with the indicated plasmids and probed with the indicated antibodies. The mobility shift of YAP and LATS2 in the presence of MST1 and intact WW45 was observed.
These results indicate that, in mammals, WW45 is required for MST1 activation and promotes LATS1/2 phosphorylation by recruiting MST1 into the complex, and that activated LATS1/2 then phosphorylates YAP. Surprisingly, activation of the MST1 signaling pathway in mammals seems to be specific for differentiation signals, at least for keratinocyte differentiation.
Then, the YAP residue that is phosphorylated by LATS1/2 was identified (FIG. 12). Key conserved residues between human YAP and Drosophila Yki were mutated to alanine (S61A, S109A, S127A, S328A, and S347A). Among these, only the S127A mutant was not phosphorylated by activated LATS2 in cells (FIG. 12b). Indeed, in-vitro kinase assays further showed markedly reduced levels of phosphorylation of YAP S127A mutant by LATS1/2, indicating that serine 127 is a major phosphorylation site (FIG. 12c). This result is consistent with the recent finding that serine 127 of YAP is the primary Hippo-responsive phosphorylation site (Dong et al., 2007). Finally, it was confirmed that this is a phosphorylation site of endogenous YAP with a phospho-serine 127 antibody during keratinocyte differentiation (FIG. 12d).
FIG. 12 shows that phosphorylation of YAP serine 127 by LATS1/2 in the MST1 pathway is induced by differentiation signals in primary keratinocytes. FIG. 12a shows partial alignment of the conserved regions of YAP with the Drosophila Yki protein. The consensus motif of the site phosphorylated by LATS1/2 is underlined. Asterisk indicates the YAP serine 127 residue, which is the phosphorylation site for LATS1/2. FIG. 12b shows the result of identification of the phosphorylation site of YAP. WW45-deficient primary keratinocytes were co-transfected with the indicated plasmids and probed with the indicated antibodies. FIG. 12c shows the result of In-vitro kinase assays that are performed using immunoprecipitated HA-tagged LATS1/2 WT or KD and purified His-YAP-WT or His-YAP-SA, wherein the signals are shown by autoradiography analysis (top two panels). The input kinase and substrate for Western-blot analysis using anti-LATS1/2 and anti-YAP are also shown (bottom two gels). FIG. 12d shows the result of Western-blot analysis with anti-YAP or an antibody specific for the serine-127-phosphorylated YAP (p-YAP) in wild-type and mutant primary keratinocytes cultured with or without Ca2+ for 24 hours. Differentiation of keratinocytes and the presence of WW45 were assessed by Western-blot analysis using anti-filaggrin and anti-WW45.
Experimental Example 6
Dynamic Cellular Localizations of MST1 and YAP During Epithelial Differentiation
YAP associates with the Src family kinase at the plasma membrane, with 14-3-3 family proteins in the cytoplasm and with transcription factors in the nucleus, suggesting a dynamic localization of YAP in cells (Sudol, 1994; Yagi et al., 1999; Vassilev et al., 2001; Mattallansa et al., 2007). However, the localization of other components of MST1 pathway has not been determined. Thus, the subcellular localization of MST1 pathway components during epithelial cell differentiation by nuclear-cytoplasmic fractionation experiments was examined. MST1 was mainly detected in the cytoplasmic fraction under undifferentiated conditions, but a significant amount was detected in the nuclear fraction after differentiation induction in wild-type cells, suggesting that differentiation triggered MST1 translocation to the nucleus.
However, this nuclear localization of MST1 was severely compromised in mutant cells (FIG. 13a). In addition to this change in MST1 localization, it was also found that YAP was found mainly in the nucleus under undifferentiated conditions, but it was phosphorylated and mainly detected in the cytoplasm of differentiated wild-type cells. However, cytoplasmic localization and phosphorylation of YAP were compromised in mutant cells under differentiated conditions (FIG. 13a). These results indicate that induction of keratinocyte differentiation triggers translocation of MST1 into the nucleus and then activates LATS1/2, which in turn phosphorylates YAP, and that WW45 is necessary for this event.
FIG. 13 shows dynamic localization of MST1/YAP during differentiation, 13a to 13c show subcellular localizations of MST1 and YAP in response to differentiation stimuli. In FIG. 13a, fractionation experiments were conducted with primary keratinocytes from wild-type or mutant embryos with or without Ca2+ for 24 hours. Fractionated lysates were subjected to Western-blot analysis with the indicated antibodies. N, nuclear; C, cytoplasmic.
The ability of MST1/WW45/LATS2 to modulate the localization of YAP was further examined by ectopically overexpressing these proteins in WW45-null keratinocytes. Consistent with previous results, YAP was phosphorylated by LATS2 and this phosphorylated form was found in the cytoplasm, probably owing to nuclear translocation of MST1 by WW45 (FIG. 13b).
FIG. 13b shows the result obtained by co-transfecting WW45-deficient keratinocytes with the indicated plasmids. After 24 hours of transfection, cells were maintained in Ca2+ medium for a further 24 hours before harvesting for fractionation experiments. Western-blot analysis was performed with the indicated antibodies. It is revealed that the major cytoplasm translocation of YAP and nuclear translocation of MST1 were only detected in the presence of intact WW45.
By contrast, these dynamic localizations of MST1 and YAP were not seen in cells expressing WW45 lacking the SARAH domain, which indicates that the WW45 SARAH domain is required for this process. Moreover, the YAP S127A mutant was mainly localized to the nucleus regardless of MST1 signaling activation, indicating that phosphorylation of serine 127 of YAP by LATS2 in the nucleus promotes translocation of YAP into the cytoplasm. These results are consistent with the presence of a MST1/WW45/LATS/YAP complex in differentiated keratinocytes (FIG. 10b).
The distinct localization of MST1 and YAP was also examined by performing an immunostaining assay in differentiated keratinocytes as well as embryo tissue sections (FIG. 13c and 13d). FIG. 13c shows the result obtained by culturing primary keratinocytes from wild-type or mutant embryos with or without Ca2+ for 24 hours, and then, being subjected to immunostaining with anti-MST1, anti-YAP and anti-p-YAP. The phospho-dependent translocation of YAP and MST1 was observed in Ca2+-induced differentiated control cells, but not in mutant cells. FIG. 13d shows the result of immunoperoxidase staining for MST1, YAP and p-YAP in wild-type (a-f) and mutant (a′-f′) epithelium at E17.5. The intense nuclear detection of YAP and cytoplasmic detection of MST1 in most mutant epithelia cells were observed, which is in contrast to the dynamic distributions according to differentiation stage in control epithelial cells. Serine 127 phospho-specific immunoreactivity was cytoplasm-specific in the differentiating zones in the control, whereas the signal was absent from the mutant cells. Counterstaining was performed with hematoxylin. (a-c, a′-c′) Skin. (d-f, d′-f′) Small intestine. Scale bar: 10 μm in (C) and 100 μm in (D).
The specificity of MST1 antibody was verified by immunostaining and immunoblot using MST1-deleted cells (FIG. 14). FIG. 14 shows the verification of the MST1 antibody, wherein FIG. 14a shows the result from Western-blot analysis that was performed with lysates of HeLa cells stably transfected with a vector encoding MST1 siRNA or GFP siRNA as a control. α-tubulin was used as a loading control. FIG. 14b shows the results obtained by staining control- and MST1-siRNA-expressing HeLa cells with DAPI and with MST1 antibody.
Consistent with the fractionation results, staining for MST1 and YAP revealed dynamic translocation of endogenous YAP and MST1 during differentiation in the wild-type keratinocytes but such translocation was not apparent in the mutant keratinocytes during differentiation (FIG. 13c). Moreover, although phosphorylated YAP was evident and mainly found in the cytoplasm in differentiated wild-type cells, it was barely detectable in mutant cells. In wild-type tissue sections, the inventors detected nuclear-localized YAP in proliferative basal cells, which were Ki67-positive, and cytoplasm-localized YAP in differentiated cells (FIG. 13d and FIG. 15). FIG. 15 shows nuclear localization of YAP in proliferative progenitor compartment of the small intestine. The serial sections of the wild-type embryos were stained with anti-YAP and anti Ki67. The nuclear-localized YAP in the Ki67-positive cycling cells of the crypt compartment of the small intestine was detected. Scale bar: 100 μm. These results are consistent with the recent finding that YAP is expressed and localized in the nuclei of the crypt compartment of the small intestine (Camargo et al., 2007).
However, MST1 was detected in the cytoplasm of proliferating cells, but in the nucleus of differentiated cells in the wild-type skin and intestine. By contrast, many proliferating cells of mutant embryos showed cytoplasm-localized MST1 and nuclear-localized YAP (FIG. 13d). In addition to changes in MST1 and YAP localizations, the cytoplasm of the differentiated regions in control embryos stained positive for phosphor-YAP, but staining levels were significantly reduced in mutant embryos (FIG. 13d). Therefore, consistent with in-vitro epithelial differentiation results, the changes in dynamic localization of MST1 and YAP are also likely to occur as epithelial cells undergo differentiation in vivo, and WW45 is likely to be a key protein in this process.
Experimental Example 7
Failure of Proliferation Arrest and Differentiation of Normal Keratinocytes Expressing YAP S127A
Based on our observations that phosphorylation and translocation of YAP into the cytoplasm are dependent on LATS1/2, which acts via MST1/WW45, it was assessed whether the phosphorylation of serine 127 of YAP is required for inducing keratinocyte growth arrest and differentiation. Thus, keratinocytes were infected with a retrovirus expressing YAP WT, YAP S127A, which is a non-phospho form, or YAP S127D, which is a phospho-mimic form. Calcium-induced differentiation signals suppressed proliferation of growing keratinocytes expressing YAP WT or YAP S127D. By contrast, YAP S127A was mainly localized to the nucleus and its overexpression in wild-type keratinocytes failed to cause proliferation arrest and differentiation (FIG. 16a to 16c).
These results indicate that the non-phosphorylated form of YAP is localized to and acts in the nucleus to allow cells to proliferate even in the presence of differentiation signals. It was also tested whether inactivation of YAP suppresses the differentiation defect in the WW45-deficient keratinocytes. To do this, the dominant-negative form of YAP (DN), which dominantly inhibits the function of endogenous YAP protein, was generated (Zhao et al., 2007). Importantly, YAP DN was sufficient to rescue the differentiation of the mutant cells (FIGS. 16d and 16e). Taken together, these results show that inactivation and subcellular targeting to the cytoplasm of phosphorylated YAP is required for cell-cycle exit and differentiation initiation.
FIG. 16 shows the effects of phosphorylation of YAP serine 127 on epithelium differentiation. FIG. 16a shows the percentage of proliferative cells labeled with BrdU under differentiation conditions. Levels of BrdU incorporation in primary keratinocytes were determined at the indicated times after retroviral infection with the indicated genes and incubation with or without Ca+. WT, wild-type; SA, S127A mutant, SD, S127D mutant. The failure of cell-cycle exit in response to Ca2+-induced differentiation in YAP-SA-infected cells was observed.
FIG. 16b shows the failure of YAP-SA-infected cells to differentiate in response to differentiation stimuli. Lysates from 16a were subjected to Western-blot analysis with the indicated antibodies. The suppression of YAP mobility shift with the absence of processed forms of filaggrin, which is a late-terminal differentiation marker, in YAP-SA-infected cells, was observed.
FIG. 16c shows subcellular localization of YAP, YAP SA and YAP SD in response to differentiation stimuli. Fractionations from 16a were subjected to immunoblot analysis with the indicated antibodies. The lack of cytoplasmic translocation of YAP SA under the Ca2+-induced differentiation conditions was observed.
FIG. 16d shows the percentage of BrdU-positive cells in WW45-deficient keratinocytes infected with the indicated genes under differentiation conditions. WT, wild-type; DN, dominant negative form. The suppression of cell-cycle re-entry in response to Ca2+ treatment in WW45-deficient keratinocytes infected with YAP-DN was observed.
FIG. 16e shows the result of Western-blot analysis on lysates from 16d with the indicated antibodies. The rescue of differentiation in response to Ca2+ treatment in WW45-deficient keratinocytes infected with YAP-DN was observed.
FIG. 16f shows a proposed model for the role of WW45 in developing epithelial tissues. Under differentiation conditions, WW45 promotes LATS1/2 phosphorylation by recruiting MST1/2 into the nuclear-localized complex, and this activated LATS1/2 then phosphorylates YAP, resulting in translocation of YAP into the cytoplasm.
This phosphorylation and subcellular targeting of YAP to the cytoplasm are required for proliferation stop and differentiation initiation in developing epithelial cells. Ultimately, during development, the MST1 pathway regulates proper epithelial tissue development by controlling the balance between proliferation and terminal differentiation.

## Claims

1. A transgenic mouse whose genome comprises a heterozygous, null allele of the gene encoding WW45 protein, wherein at least the region encoding the SARAH domain of the WW45 protein is deleted, and wherein said mouse exhibits formation of tumors.

2. The transgenic mouse according to claim 1, wherein the gene encoding WW45 protein is the gene encoding mouse WW45 protein (SEQ ID NO: 8).

3. The transgenic mouse according to claim 1, wherein the SARAH domain has the amino acid sequence of SEQ ID NO: 11.

4. A method of screening a compound for anti-tumor activity, the method comprising the steps of:
preparing a transgenic mouse whose genome comprises a heterozygous, null allele of the gene encoding WW45 protein, wherein at least the region encoding the SARAH domain of the WW45 protein is deleted, and wherein said mouse exhibits formation of tumors;
treating the prepared transgenic mouse with a candidate compound;
determining a level of the tumor in the transgenic mouse treated with the candidate compound by measuring the number of tumor cells, volume of the tumor, or tumor cell viability; and
identifying the candidate compound as a compound having anti-tumor activity if the number of the tumor cells or the volume of the tumor has been decreased relative to the number of tumor cells or volume of the tumor in the transgenic mouse prior to the treatment with the candidate compound, or if apoptosis of the tumor cells has been induced after the treatment with the candidate compound.

5. The method according to claim 4, wherein the compound has anti-tumor activity against osteosarcoma or hepatoma.