p63 Is essential for the proliferative potential of stem cells in stratified epithelia

Direct targets of the TRP63 transcription factor revealed by a combination of gene expression profiling and reverse engineering

Genome-wide identification of bona-fide targets of transcription factors in mammalian cells is still a challenge. We present a novel integrated computational and experimental approach to identify direct targets of a transcription factor. This consists of measuring time-course (dynamic) gene expression profiles upon perturbation of the transcription factor under study, and in applying a novel "reverse-engineering" algorithm (TSNI) to rank genes according to their probability of being direct targets. Using primary keratinocytes as a model system, we identified novel transcriptional target genes of TRP63, a crucial regulator of skin development. TSNI-predicted TRP63 target genes were validated by Trp63 knockdown and by ChIP-chip to identify TRP63-bound regions in vivo. Our study revealed that short sampling times, in the order of minutes, are needed to capture the dynamics of gene expression in mammalian cells. We show that TRP63 transiently regulates a subset of its direct targets, thus highlighting the importance of considering temporal dynamics when identifying transcriptional targets. Using this approach, we uncovered a previously unsuspected transient regulation of the AP-1 complex by TRP63 through direct regulation of a subset of AP-1 components. The integrated experimental and computational approach described here is readily applicable to other transcription factors in mammalian systems and is complementary to genome-wide identification of transcription-factor binding sites.

The impact of p53 and p73 on aneuploidy and cancer

Initiation, progression and evasion are sequential steps in cancer formation, with autonomous cell proliferation as a final outcome. Genetic or epigenetic alterations of key regulatory genes of the cell cycle are frequently associated with these phenomena. Recently, chromosomal instability, a long-supposed driving force of tumorigenesis, was associated with dysregulation of mitotic genes, providing advantages to tumor cells. Numerous molecules thus provide a key link in the chain of relationships between chromosomal instability and cancer. Here, we discuss emerging evidence revealing that two p53 family members, p53 and p73, might be key regulatory genes at the heart of the relationship between chromosomal instability and cancer. We argue that the role of members of the p53 family as tumor suppressor proteins, their impact on the control of cellular ploidy, and their newly emerging connection with mitotic checkpoint regulatory genes support the suggestion that p73 and p53 could be two of the missing links among chromosomal instability, the mitotic checkpoint and cancer.

Transforming growth factor-beta regulates mammary carcinoma cell survival and interaction with the adjacent microenvironment

Transforming growth factor (TGF)-beta signaling has been associated with early tumor suppression and late tumor progression; however, many of the mechanisms that mediate these processes are not known. Using Cre/LoxP technology, with the whey acidic protein promoter driving transgenic expression of Cre recombinase (WAP-Cre), we have now ablated the type II TGF-beta receptor (T beta RII) expression specifically within mouse mammary alveolar progenitors. Transgenic expression of the polyoma virus middle T antigen, under control of the mouse mammary tumor virus enhancer/promoter, was used to produce mammary tumors in the absence or presence of Cre (T beta RII((fl/fl);PY) and T beta RII((fl/fl);PY;WC), respectively). The loss of TGF-beta signaling significantly decreased tumor latency and increased the rate of pulmonary metastasis. The loss of TGF-beta signaling was significantly correlated with increased tumor size and enhanced carcinoma cell survival. In addition, we observed significant differences in stromal fibrovascular abundance and composition accompanied by increased recruitment of F4/80(+) cell populations in T beta RII((fl/fl);PY;WC) mice when compared with T beta RII((fl/fl);PY) controls. The recruitment of F4/80(+) cells correlated with increased expression of known inflammatory genes including Cxcl1, Cxcl5, and Ptgs2 (cyclooxygenase-2). Notably, we also identified an enriched K5(+) dNp63(+) cell population in primary T beta RII((fl/fl);PY;WC) tumors and corresponding pulmonary metastases, suggesting that loss of TGF-beta signaling in this subset of carcinoma cells can contribute to metastasis. Together, our current results indicate that loss of TGF-beta signaling in mammary alveolar progenitors may affect tumor initiation, progression, and metastasis through regulation of both intrinsic cell signaling and adjacent stromal-epithelial interactions in vivo.

[Identification of human salivary stem cells from cultured labial minor salivary cells]

Stem cell therapy is expected to be a promising approach for the compensation of lost organs. The various organization cells that compose an animal's body are always being renewed for the maintenance of homeostasis. The cells that become the source of new cells are a body's own stem cells. Cell therapy, using stem cells, has a few of bioethical problems but there is the advantage that it is not necessary to worry about the immunity rejection of the transplant because the body's stem cell is from it's own body. In our present study we identified side population cells (SP cells), which are highly enriched for stem cell activity in human salivary glands. Isolated SP cells expressed high level of DNp63 and PSCA (prostate stem cell antigen), but not nestin, Oct4 and CD34. Real time PCR analysis revealed that the expression of DNp63, detected in immature salivary epithelial cells, gradually decreased through cell differentiation. In contrast, PSCA can be distinguished among early differentiating and later transit-amplifying salivary epithelial cells in tissue culture. Our study suggested that these markers may mark the transition of human salivary epithelial cells.

A skin microRNA promotes differentiation by repressing 'stemness'

In stratified epithelial tissues, homeostasis relies on the self-renewing capacity of stem cells located within the innermost basal layer. As basal cells become suprabasal, they lose proliferative potential and embark on a terminal differentiation programme. Here, we show that microRNA-203 is induced in the skin concomitantly with stratification and differentiation. By altering miR-203's spatiotemporal expression in vivo, we show that miR-203 promotes epidermal differentiation by restricting proliferative potential and inducing cell-cycle exit. We identify p63 as one of the conserved targets of miR-203 across vertebrates. Notably, p63 is an essential regulator of stem-cell maintenance in stratified epithelial tissues. We show that miR-203 directly represses the expression of p63: it fails to switch off suprabasally when either Dicer1 or miR-203 is absent and it becomes repressed basally when miR-203 is prematurely expressed. Our findings suggest that miR-203 defines a molecular boundary between proliferative basal progenitors and terminally differentiating suprabasal cells, ensuring proper identity of neighbouring layers.

Promiscuous gene expression and the developmental dynamics of medullary thymic epithelial cells

Thymic epithelial cells (TEC) form the structural and functional microenvironment necessary for the establishment and quality control of the T cell repertoire. In addition, they provide an ectopic source of numerous tissue-restricted antigens (TRA), a feature called promiscuous gene expression (pGE). How the regulation of pGE is related to the cell biology of TEC subset(s), e.g. their turnover and developmental interrelationship is still poorly understood. The observation that pGE is foremost a property of phenotypically and functionally mature medullary TEC (mTEC) implies that the full implementation of pGE is contingent on mTEC differentiation. Here, we show that the emergence of TEC subsets and pGE is tightly correlated during ontogeny and we provide evidence that mature CD80pos mTEC develop from an immature CD80neg subset. This differentiation step proceeds continuously in the postnatal thymus. While mature mTEC turnover in 2 to 3 weeks, immature mTEC encompass a smaller cycling and a larger non-cycling pool. The latter might serve as a reservoir of committed precursors, which sustain this renewal process. Our data document that mTEC represent a highly dynamic cell population, and they imply that the availability and display of TRA in the thymus undergoes a perpetual temporal and spatial reorganization.

FGFR2IIIb signaling regulates thymic epithelial differentiation

Heterogeneous epithelial populations comprising the thymic environment influence early and late stages of T-cell development. The processes that regulate the differentiation of thymic epithelium and that are responsible for this heterogeneity are not well understood, although mesenchymal/epithelial interactions are clearly involved. Here, we show that targeted expression by thymocytes of an fibroblast growth factor receptor-2IIIb (FGFR2IIIb) ligand, FGF10, profoundly alters the differentiation and function of thymic epithelium (TE). Reconstitution of irradiated lckFGF10 mice with normal bone marrow restores normal thymic organization and function, while wild-type mice reconstituted with lckFGF10 bone marrow recapitulates some of the thymic alterations seen in lckFGF10 mice. We also demonstrate that interference with FGFR2IIIb signaling in the thymus with a soluble FGFR2IIIb dominant-negative fusion protein leads to precocious reductions in thymic size and cellularity that resemble age-related thymic involution. These findings indicate that TE compartments are dynamically maintained and that FGF signals are involved in this process.

Reciprocal intraepithelial interactions between TP63 and hedgehog signaling regulate quiescence and activation of progenitor elaboration by mammary stem cells

TP63 is required for preservation of epithelial regenerative stasis and regulates the activity of diverse genetic pathways; however, specific effector pathways are poorly understood. Data presented here indicate that reciprocal regulatory interactions between hedgehog signaling and TP63 mediate stage-specific effects on proliferation and clonigenicity of separable enriched mammary stem and progenitor fractions. Analysis of DeltaN-p63 and TA-p63 indicates segregated expression in mammary stem and progenitor fractions, respectively, demonstrating that differential TP63 promoter selection occurs during elaboration of mammary progenitors by mammary stem cells. This segregation underlies mammary progenitor-specific expression of Indian Hedgehog, identifying it as a binary transcriptional target of TP63. Hedgehog activation in vivo enhances elaboration of mammary progenitors and decreases label retention within mammary stem cell-enriched fractions, suggesting that hedgehog exerts a mitogenic effect on mammary stem cells. Hedgehog signaling promotes differential TP63 promoter usage via disruption of Gli3 or Gli3(R) accumulation, and shRNA-mediated disruption of Gli3 expression was sufficient to alter TP63 promoter usage and enhance clonigenicity of mammary stem cells. Finally, hedgehog signaling is enhanced during pregnancy, where it contributes to expansion of the mammary progenitor compartment. These studies support a model in which hedgehog activates elaboration and differentiation of mammary progenitors via differential TP63 promoter selection and forfeiture of self-renewing capacity.

Skin stem cells: rising to the surface

The skin epidermis and its appendages provide a protective barrier that is impermeable to harmful microbes and also prevents dehydration. To perform their functions while being confronted with the physicochemical traumas of the environment, these tissues undergo continual rejuvenation through homeostasis, and, in addition, they must be primed to undergo wound repair in response to injury. The skin's elixir for maintaining tissue homeostasis, regenerating hair, and repairing the epidermis after injury is its stem cells, which reside in the adult hair follicle, sebaceous gland, and epidermis. Stem cells have the remarkable capacity to both self-perpetuate and also give rise to the differentiating cells that constitute one or more tissues. In recent years, scientists have begun to uncover the properties of skin stem cells and unravel the mysteries underlying their remarkable capacity to perform these feats. In this paper, I outline the basic lineages of the skin epithelia and review some of the major findings about mammalian skin epithelial stem cells that have emerged in the past five years.

P-cadherin is a p63 target gene with a crucial role in the developing human limb bud and hair follicle

P-cadherin is a member of the classical cadherin family that forms the transmembrane core of adherens junctions. Recently, mutations in the P-cadherin gene (CDH3) have been shown to cause two inherited diseases in humans: hypotrichosis with juvenile macular dystrophy (HJMD) and ectodermal dysplasia, ectrodactyly, macular dystrophy (EEM syndrome). The common features of both diseases are sparse hair and macular dystrophy of the retina, while only EEM syndrome shows the additional finding of split hand/foot malformation (SHFM). We identified five consanguineous Pakistani families with either HJMD or EEM syndrome, and detected pathogenic mutations in the CDH3 gene of all five families. In order to define the role of P-cadherin in hair follicle and limb development, we performed expression studies on P-cadherin in the mouse embryo, and demonstrated the predominant expression of P-cadherin not only in the hair follicle placode, but also at the apical ectodermal ridge (AER) of the limb bud. Based on the evidence that mutations in the p63 gene also result in hypotrichosis and SHFM, and that the expression patterns of p63 and P-cadherin overlap in the hair follicle placode and AER, we postulated that CDH3 could be a direct transcriptional target gene of p63. We performed promoter assays and ChIP, which revealed that p63 directly interacts with two distinct regions of the CDH3 promoter. We conclude that P-cadherin is a newly defined transcriptional target gene of p63, with a crucial role in hair follicle morphogenesis as well as the AER during limb bud outgrowth in humans, whereas it is not required for either in mice.

DeltaNp63 isoforms regulate CD44 and keratins 4, 6, 14 and 19 in squamous cell carcinoma of head and neck

The human p63 gene codes for multiple protein isoforms and is commonly over-expressed in squamous cell carcinoma of head and neck (SCCHN). This expression is predominantly of the DeltaN- and beta-isoforms, the former lacking the p53-related transactivation domain. p63 can activate or repress transcription of p53 and p73 target genes, but also has unique transcriptional targets and, unlike other p53 family members, is required for normal development and differentiation of squamous epithelia. We have identified novel targets of p63, using microarray analysis of SCCHN cells that stably over-express individual DeltaNp63 isoforms. All three isoforms induced expression of the cancer stem cell marker, CD44, with the DeltaNp63beta isoform showing strongest induction. Using chromatin immunoprecipitation, we were unable to show direct binding of p63 to the CD44 promoter, but found that p63 specifically increased expression of CD44 lacking variant exon 2. Each of the DeltaNp63 isoforms up-regulated expression of keratins 6A and 14 and down-regulated expression of keratins 4 and 19, in keeping with their expression patterns in SCCHN. The data strengthen the idea that p63 has key roles in regulating normal and abnormal differentiation processes through both induction and repression of genes with opposite functions. The identification of up-regulation and differential splicing of CD44 following p63 over-expression indicates roles in the regulation of adhesion, metastasis and the cancer stem cell phenotype.

Identification of Plet-1 as a specific marker of early thymic epithelial progenitor cells

The thymus is essential for a functional immune system, because the thymic stroma uniquely supports T lymphocyte development. We have previously identified the epithelial progenitor population from which the thymus arises and demonstrated its ability to generate an organized functional thymus upon transplantation. These thymic epithelial progenitor cells (TEPC) are defined by surface determinants recognized by the mAbs MTS20 and MTS24, which were also recently shown to identify keratinocyte progenitor cells in the skin. However, the biochemical nature of the MTS20 and MTS24 determinants has remained unknown. Here we show, via expression profiling of fetal mouse TEPC and their differentiated progeny and subsequent analyses, that both MTS20 and MTS24 specifically bind an orphan protein of unknown function, Placenta-expressed transcript (Plet)-1. In the postgastrulation embryo, Plet-1 expression is highly restricted to the developing pharyngeal endoderm and mesonephros until day 11.5 of embryogenesis, consistent with the MTS20 and MTS24 staining pattern; both MTS20 and MTS24 specifically bind cell lines transfected with Plet-1; and antibodies to Plet-1 recapitulate MTS20/24 staining. In adult tissues, we demonstrate expression in a number of sites, including mammary and prostate epithelia and in the pancreas, where Plet-1 is specifically expressed by the major duct epithelium, providing a specific cell surface marker for this putative reservoir of pancreatic progenitor/stem cells. Plet-1 will thus provide an invaluable tool for genetic analysis of the lineage relationships and molecular mechanisms operating in the development, homeostasis, and injury in several organ/tissue systems.

Generating intrathymic microenvironments to establish T-cell tolerance

Alphabeta T cells pass through a series of lymphoid tissue stromal microenvironments to acquire self tolerance and functional competence. In the thymus, positive selection of the developing T-cell receptor repertoire occurs in the cortex, whereas events in the medulla purge the system of self reactivity. T cells that survive are exported to secondary lymphoid organs where they direct first primary and then memory immune responses. This Review focuses on the microenvironments that nurture T-cell development rather than on T cells themselves. We summarize current knowledge on the formation of thymic epithelial-cell microenvironments, and highlight similarities between the environments that produce T cells and those that select and maintain them during immune responses.

Crossing boundaries: stem cells, holoclones, and the fundamentals of squamous epithelial renewal

Renewal of stratified squamous epithelia, eg, the epidermis, the esophagus, or the epithelia lining the oral cavity, normally depends on the presence of keratinocyte stem cells that are thoroughly distributed in the epithelial basal layer. It is commonly thought that stem cells divide asymmetrically and generate transient amplifying cells. In turn, the latter generate postmitotic cells, which will replace the terminally differentiated cells that constantly slough off the epithelial surface. In this model, each stem cell only renews a tiny epithelial column, even if it has the capacity to generate a large amount of epithelium, a property important during wound healing. Interestingly, the cornea is an exception among stratified epithelia, because it does not contain stem cells but rather relies on the centripetal migration of transient amplifying cells generated by stem cells dividing occasionally and located at the limbus, the transition zone of the cornea with the conjunctiva. It is unclear which evolutionary advantage an epithelium with a rapid turn over has gained through the development of such a mechanism. Understanding why and how the cornea has evolved differently from all other stratified epithelia is certainly a major challenge in epithelial stem cell biology.

Impaired repair of cyclobutane pyrimidine dimers in human keratinocytes deficient in p53 and p63

While many p53-deficient cell types are impaired in global genomic nucleotide excision repair of cyclobutane pyrimidine dimers (CPDs), human epidermal keratinocytes expressing human papillomavirus type 16 E6 and E7 are p53 deficient and yet maintain repair of CPD. We hypothesized that the p53 homolog, p63, may participate in governing global repair instead of p53 in keratinocytes. Following ultraviolet radiation (UVR) of E6/E7 keratinocytes, depletion of p63 but not of p73 impaired global genomic repair of CPD relative to control cells. In all cases, repair of pyrimidine(6-4)pyrimidone photoproducts, the other major UVR-induced DNA lesions, was unaffected. In E6/E7 keratinocytes treated with p63 small interfering RNA, reduced global repair of CPD was associated not with reduced levels of messenger RNA-encoding DNA damage recognition proteins but rather with decreased levels of DDB2 and XPC proteins, suggesting that p63 posttranscriptionally regulates levels of these proteins. These results indicate that global repair may be regulated at multiple levels and suggest a novel role for p63 in modulating repair of DNA damage in human keratinocytes. The results may provide insight into mechanisms of genomic stability in epithelia infected with oncogenic human papilloma viruses and may further explain the lack of increased skin cancer incidence in Li-Fraumeni syndrome.

Neuregulin3 alters cell fate in the epidermis and mammary gland

BACKGROUND

The Neuregulin family of ligands and their receptors, the Erbb tyrosine kinases, have important roles in epidermal and mammary gland development as well as during carcinogenesis. Previously, we demonstrated that Neuregulin3 (Nrg3) is a specification signal for mammary placode formation in mice. Nrg3 is a growth factor, which binds and activates Erbb4, a receptor tyrosine kinase that regulates cell proliferation and differentiation. To understand the role of Neuregulin3 in epidermal morphogenesis, we have developed a transgenic mouse model that expresses Nrg3 throughout the basal layer (progenitor/stem cell compartment) of mouse epidermis and the outer root sheath of developing hair follicles.

RESULTS

Transgenic females formed supernumerary nipples and mammary glands along and adjacent to the mammary line providing strong evidence that Nrg3 has a role in the initiation of mammary placodes along the body axis. In addition, alterations in morphogenesis and differentiation of other epidermal appendages were observed, including the hair follicles. The transgenic epidermis is hyperplastic with excessive sebaceous differentiation and shows striking similarities to mouse models in which c-Myc is activated in the basal layer including decreased expression levels of the adhesion receptors, alpha6-integrin and beta1-integrin.

CONCLUSION

These results indicate that the epidermis is sensitive to Nrg3 signaling, and that this growth factor can regulate cell fate of pluripotent epidermal cell populations including that of the mammary gland. Nrg3 appears to act, in part, by inducing c-Myc, altering the proliferation and adhesion properties of the basal epidermis, and may promote exit from the stem cell compartment. The results we describe provide significant insight into how growth factors, such as Nrg3, regulate epidermal homeostasis by influencing the balance between stem cell renewal, lineage selection and differentiation.

Visualizing the dynamics of p21(Waf1/Cip1) cyclin-dependent kinase inhibitor expression in living animals

Although the role of p21(Waf1/Cip1) gene expression is well documented in various cell culture studies, its in vivo roles are poorly understood. To gain further insight into the role of p21(Waf1/Cip1) gene expression in vivo, we attempted to visualize the dynamics of p21(Waf1/Cip1) gene expression in living animals. In this study, we established a transgenic mice line (p21-p-luc) expressing the firefly luciferase under the control of the p21(Waf1/Cip1) gene promoter. In conjunction with a noninvasive bioluminescent imaging technique, p21-p-luc mice enabled us to monitor the endogenous p21(Waf1/Cip1) gene expression in vivo. By monitoring and quantifying the p21(Waf1/Cip1) gene expression repeatedly in the same mouse throughout its entire lifespan, we were able to unveil the dynamics of p21(Waf1/Cip1) gene expression in the aging process. We also applied this system to chemically induced skin carcinogenesis and found that the levels of p21(Waf1/Cip1) gene expression rise dramatically in benign skin papillomas, suggesting that p21(Waf1/Cip1) plays a preventative role(s) in skin tumor formation. Surprisingly, moreover, we found that the level of p21(Waf1/Cip1) expression strikingly increased in the hair bulb and oscillated with a 3-week period correlating with hair follicle cycle progression. Notably, this was accompanied by the expression of p63 but not p53. This approach, together with the analysis of p21(Waf1/Cip1) knockout mice, has uncovered a novel role for the p21(Waf1/Cip1) gene in hair development. These data illustrate the unique utility of bioluminescence imaging in advancing our understanding of the timing and, hence, likely roles of specific gene expression in higher eukaryotes.

Epidermal stem cell fate: what can we learn from embryonic stem cells?

Because of its constant renewal and high propensity for repair, the epidermis is, together with the gut and the hematopoietic system, a tissue of choice to explore stem cell biology. Previous research over many years has revealed the complexity of the epidermis: the heterogeneity of the stem cell compartment, with its rare, slowly cycling, multipotent, hair-follicle, "bulge" stem cells and the more restricted interfollicular, follicle-matrix, and sebaceous-gland stem cells, which in turn generate the large pool of transit-amplifying progeny. Stem cell activity has been used for some considerable time to repair skin injuries, but ex-vivo keratinocyte amplification has its limitations, and grafted skin homeostasis is not totally satisfactory. Human embryonic stem cells raise the hope that the understanding of the developmental steps leading to the generation of epidermal stem cells and the characterization of the key signaling pathways involved in skin morphogenesis (such as p63) will be translated into therapeutic benefit. Our recent results suggest the feasibility not only of identifying but also of amplifying human ES cells, early ectodermal progenitors with an intact multipotent potential that might improve the quality and functionality of grafts, provided that preclinical in vivo studies confirm our expectations from in vitro analysis.

Expression of GATA-3 in epidermis and hair follicle: Relationship to p63

The epidermis is a multi-layered stratified epithelium continuously renewed by differentiating keratinocytes that develops by the action of p63, a member of the p53 family. The TP63 contains two promoters, resulting in the expression of different proteins, containing (TAp63) or not (DeltaNp63) an amino-terminal transactivation domain, which contribution in skin formation is not fully understood. We found that p63 binds and transactivate GATA-3 promoter, which in turn transactivate IKKalpha, two pivotal regulators of epithelial development. Indeed, GATA-3 is a regulator of cell lineage in skin and hair follicles formation. To further study the relationship between GATA-3 and p63 isoforms here we investigated their expression during keratinocyte differentiation, in human epidermis and hair follicle.

Self-renewal and multilineage differentiation in vitro from murine prostate stem cells

Murine prostate stem cells express integrin alpha 6, which modulates survival, proliferation, and differentiation signaling through its interaction with the extracellular protein laminin. When plated in vitro in laminin containing Matrigel medium, 1 of 500-1,000 murine prostate cells can grow and form clonogenic spheroid structures that we term prostate spheres. Prostate spheres can be serially passaged individually or in bulk to generate daughter spheres with similar composition, demonstrating that sphere-forming cells are capable of self-renewal. Spheres spontaneously undergo lineage specification for basal and transit-amplifying cell types. P63-expressing cells localized to the outer layers of prostate spheres possess higher self-renewal capacity, whereas cells toward the center display a more differentiated transit-amplifying phenotype, as demonstrated by the expression of the prostate stem cell antigen. When dihydrotestosterone is added to the medium, the androgen receptor is stabilized, is imported to the nucleus, and drives differentiation to a luminal cell-like phenotype. A fraction of sphere cells returned to an in vivo environment can undergo differentiation and morphogenesis to form prostate tubular structures with defined basal and luminal layers accompanied by prostatic secretions. This study demonstrates self-renewal and multilineage differentiation from single adult prostate stem/progenitor cells in a specific in vitro microenvironment.

DeltaNp63 regulates thymic development through enhanced expression of FgfR2 and Jag2

p63, a homologue of the tumor suppressor p53, is pivotal for epithelial development, because its loss causes severe epithelial dysgenesis, although no information is so far available on the role of p63 in the thymus. We identified the expression of all p63 isoforms in the developing thymus. The p63(-/-) thymi show severe abnormalities in size and cellularity, even though the organ expresses normal levels of keratins 5 and 8, indicating a p63-independent differentiation of thymic epithelial cells (TEC). TEC were sufficiently developed to allow a significant degree of education to produce CD4/CD8 single- and double-positive T cells. To study the selective contribution of transactivation-active p63 (TAp63) and amino-deleted p63 (DeltaNp63) isoforms to the function of the TEC, we genetically complemented p63(-/-) mice by crossing p63(+/-) mice with transgenic mice expressing either TAp63alpha or DeltaNp63alpha under the control of the keratin 5 promoter. Thymic morphology and cellularity were partially restored by complementation with DeltaNp63, but not TAp63, one downstream effector being fibroblast growth factor receptor 2-IIIb (FgfR2-IIIb). Indeed, FgfR2-IIIb is regulated directly by p63, via its interaction with apobec-1-binding protein-1, and its knockout shows thymic defects similar to those observed in p63(-/-) thymi. In addition, expression of Jag2, a component of the Notch signaling pathway known to be required for thymic development, was enhanced by p63 in vivo genetic complementation. Like Jag2(-/-) thymi, p63(-/-) thymi also show reduced gammadelta cell formation. Therefore, p63, and particularly the DeltaNp63 isoform, is essential for thymic development via enhanced expression of FgfR2 and Jag2. The action of DeltaNp63 is not due to a direct regulation of TEC differentiation, but it is compatible with maintenance of their "stemness," the thymic abnormalities resulting from epithelial failure due to loss of stem cells.