p63 is an essential proapoptotic protein during neural development

TAp63: The fountain of youth

The mechanisms controlling organismal aging have yet to be clearly defined. In our recent paper [1], we revealed thatTAp63, the p53 family member, is a critical gene in preventing organismal aging by controlling the maintenance of dermal and epidermal precursor and stem cells critical for wound healing and hair growth. In the absence of TAp63, dermal stem cells (skin-derived precursors or SKPs) in young mice are hyperproliferative. As early as one month of age, SKPs and epidermal precursor cells exhibit signs of premature aging including a marked increase in senescence, DNA damage, and genomic instability resulting in an exhaustion of these cells and an overall acceleration in aging. Here, we discuss our findings and its relevance to longevity, regenerative medicine, and tumorigenesis.

TAp63 prevents premature aging by promoting adult stem cell maintenance

The cellular mechanisms that regulate the maintenance of adult tissue stem cells are still largely unknown. We show here that the p53 family member, TAp63, is essential for maintenance of epidermal and dermal precursors and that, in its absence, these precursors senesce and skin ages prematurely. Specifically, we have developed a TAp63 conditional knockout mouse and used it to ablate TAp63 in the germline (TAp63(-/-)) or in K14-expressing cells in the basal layer of the epidermis (TAp63(fl/fl);K14cre+). TAp63(-/-) mice age prematurely and develop blisters, skin ulcerations, senescence of hair follicle-associated dermal and epidermal cells, and decreased hair morphogenesis. These phenotypes are likely due to loss of TAp63 in dermal and epidermal precursors since both cell types show defective proliferation, early senescence, and genomic instability. These data indicate that TAp63 serves to maintain adult skin stem cells by regulating cellular senescence and genomic stability, thereby preventing premature tissue aging.

p63 antagonizes p53 to promote the survival of embryonic neural precursor cells

The molecular mechanisms that regulate survival of embryonic neural precursors are still relatively ill-defined. Here, we have asked whether the p53 family member p63 plays any role during this developmental window, focusing on the embryonic cerebral cortex. We show that genetic knockdown of p63 either in culture or in the embryonic telencephalon causes apoptosis of cortical precursors and newly born cortical neurons, and that this can be rescued by expression of DeltaNp63, but not TAp63 isoforms. This cortical precursor apoptosis is the consequence of deregulated p53 activity, since both basal precursor apoptosis and that induced by loss of p63 are rescued by coincident genetic silencing of p53. Finally, we demonstrate that the third p53 family member, DeltaNp73, does not regulate survival of cortical precursor cells, but that it collaborates with DeltaNp63 to ensure the survival of newly born cortical neurons. Thus, the balance of DeltaNp63 versus p53 determines the life versus death of embryonic cortical precursors, a role that these p53 family members may well play in other populations of developing and/or adult neural precursors.

Stxbp4 regulates DeltaNp63 stability by suppression of RACK1-dependent degradation

p63, a member of the p53 tumor suppressor family, is essential for the development of epidermis as well as other stratified epithelia. Collective evidence indicates that DeltaNp63 proteins, the N-terminally deleted versions of p63, are essential for the proliferation and survival of stratified epithelial cells and squamous cell carcinoma cells. But in response to DNA damage, DeltaNp63 proteins are quickly downregulated in part through protein degradation. To elucidate the mechanisms by which DeltaNp63 proteins are maintained at relatively high levels in proliferating cells but destabilized in response to stress, we sought to identify p63 interactive proteins that regulate p63 stability. We found that Stxbp4 and RACK1, two scaffold proteins, play central roles in balancing DeltaNp63 protein levels. While Stxbp4 functions to stabilize DeltaNp63 proteins, RACK1 targets DeltaNp63 for degradation. Under normal growth conditions, Stxbp4 is indispensable for maintaining high basal levels of DeltaNp63 and preventing RACK1-mediated p63 degradation. Upon genotoxic stress, however, Stxbp4 itself is downregulated, correlating with DeltaNp63 destabilization mediated in part by RACK1. Taken together, we have delineated key mechanisms that regulate DeltaNp63 protein stability in vivo.

The non-apoptotic role of p53 in neuronal biology: enlightening the dark side of the moon

The transcription factor p53 protects neurons from transformation and DNA damage through the induction of cell-cycle arrest, DNA repair and apoptosis in a range of in vitro and in vivo conditions. Indeed, p53 has a crucial role in eliciting neuronal cell death during development and in adult organisms after exposure to a range of stressors and/or DNA damage. Nevertheless, accumulating evidence challenges this one-sided view of the role of p53 in the nervous system. Here, we discuss how-unexpectedly-p53 can regulate the proliferation and differentiation of neural progenitor cells independently of its role in apoptosis, and p53 post-translational modifications might promote neuronal maturation, as well as axon outgrowth and regeneration, following neuronal injury. We hope to encourage a more comprehensive view of the non-apoptotic functions of p53 during neural development, and to warn against oversimplifications regarding its role in neurons. In addition, we discuss how further insight into the p53-dependent modulation of these mechanisms is necessary to elucidate the decision-making processes between neuronal cell death and differentiation during development, and between neuronal degeneration and axonal regeneration after injury.

The alpha/beta carboxy-terminal domains of p63 are required for skin and limb development. New insights from the Brdm2 mouse which is not a complete p63 knockout but expresses p63 gamma-like proteins

p63, an ancestral transcription factor of the p53 family, has three C-terminal isoforms whose relative in vivo functions are elusive. The p63 gene is essential for skin and limb development, as vividly shown by two independent global knockout mouse models. Both strains, although constructed differently, have identical and severe phenotypes, characterized by absent epidermis and hindlimbs and only rudimentary forelimbs at birth. Here we show that mice from one model, Brdm2, express normal levels of truncated p63 proteins that contain the DNA binding and oligomerization domain but lack the long carboxy-terminal SAM (sterile alpha-motif) and post-SAM domains that are specific for the alpha and beta isoforms. As such, transcriptionally active p63 proteins from Brdm2 mice resemble the naturally occurring p63gamma isoforms, which of all the p63 isoforms most closely resemble p53. Thus, Brdm2 mice are p63alpha/beta isoform-specific knockout mice, gaining unexpected new importance. Our studies identify that p63alpha/beta but not p63gamma are absolutely required for proper skin and limb development.

ZEB1 links p63 and p73 in a novel neuronal survival pathway rapidly induced in response to cortical ischemia

Background

Acute hypoxic/ischemic insults to the forebrain, often resulting in significant cellular loss of the cortical parenchyma, are a major cause of debilitating injury in the industrialized world. A clearer understanding of the pro-death/pro-survival signaling pathways and their downstream targets is critical to the development of therapeutic interventions to mitigate permanent neurological damage.

Methodology/Principal Findings

We demonstrate here that the transcriptional repressor ZEB1, thought to be involved in regulating the timing and spatial boundaries of basic-Helix-Loop-Helix transactivator-mediated neurogenic determination/differentiation programs, functions to link a pro-survival transcriptional cascade rapidly induced in cortical neurons in response to experimentally induced ischemia. Employing histological, tissue culture, and molecular biological read-outs, we show that this novel pro-survival response, initiated through the rapid induction of p63, is mediated ultimately by the transcriptional repression of a pro-apoptotic isoform of p73 by ZEB1. We show further that this phylogenetically conserved pathway is induced as well in the human cortex subjected to episodes of clinically relevant stroke.

Conclusions/Significance

The data presented here provide the first evidence that ZEB1 induction is part of a protective response by neurons to ischemia. The stroke-induced increase in ZEB1 mRNA and protein levels in cortical neurons is both developmentally and phylogenetically conserved and may therefore be part of a fundamental cellular response to this insult. Beyond the context of stroke, the finding that ZEB1 is regulated by a member of the p53 family has implications for cell survival in other tissue and cellular environments subjected to ischemia, such as the myocardium and, in particular, tumor masses.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

p73 regulates neurodegeneration and phospho-tau accumulation during aging and Alzheimer's disease

The genetic mechanisms that regulate neurodegeneration are only poorly understood. We show that the loss of one allele of the p53 family member, p73, makes mice susceptible to neurodegeneration as a consequence of aging or Alzheimer's disease (AD). Behavioral analyses demonstrated that old, but not young, p73+/- mice displayed reduced motor and cognitive function, CNS atrophy, and neuronal degeneration. Unexpectedly, brains of aged p73+/- mice demonstrated dramatic accumulations of phospho-tau (P-tau)-positive filaments. Moreover, when crossed to a mouse model of AD expressing a mutant amyloid precursor protein, brains of these mice showed neuronal degeneration and early and robust formation of tangle-like structures containing P-tau. The increase in P-tau was likely mediated by JNK; in p73+/- neurons, the activity of the p73 target JNK was enhanced, and JNK regulated P-tau levels. Thus, p73 is essential for preventing neurodegeneration, and haploinsufficiency for p73 may be a susceptibility factor for AD and other neurodegenerative disorders.

p53 family in development

The p53 family network is a unique cellular processor that integrates information from various pathways and determines cellular choices between proliferation, replication arrest/repair, differentiation, senescence, or apoptosis. The most studied role of the p53 family is the regulation of stress response and tumor suppression. By removing damaged cells from the proliferating pool, p53 family members preserve the integrity of the genome. In addition to this well recognized role, recent data implicate the p53 protein family in a broader role of controlling cell proliferation, differentiation and death. Members of the p53 protein family with opposing activity perform coordination of these processes. Imbalance of p53 protein family may contribute to a significant proportion of congenital developmental abnormalities in humans.

ATM kinase is a master switch for the Delta Np63 alpha phosphorylation/degradation in human head and neck squamous cell carcinoma cells upon DNA damage

We previously found that the pro-apoptotic DNA damaging agent, cisplatin, mediated the proteasome-dependent degradation of Delta Np63 alpha associated with its increased phosphorylated status. Since Delta Np63 alpha usually plays an opposite role to p53 and TAp63 in human cancers, we tested the notion that phosphorylation events induced by DNA damage would affect the protein degradation of Delta Np63 alpha in HNSCC cells upon cisplatin exposure. We found that Delta Np63 alpha is phosphorylated in the time-dependent fashion at the following positions: S385, T397 and S466, which were surrounded by recognition motifs for ATM, CDK2 and p70s6K kinases, respectively. We showed that chemical agents or siRNA inhibiting the activity of ATM, CDK2 and p70s6K kinases blocked degradation of Delta Np63 alpha in HNSCC cells after cisplatin exposure. Site-specific mutagenesis of Delta Np63 alpha residues targeted for phosphorylation by ATM, CDK2 or p70s6k led to dramatic modulation of Delta Np63 alpha degradation. Finally, we demonstrated that the Delta Np63 alpha protein is a target for direct in vitro phosphorylation by ATM, CDK2 or p70s6K. Our results implicate specific kinases, and target phosphorylation sites in the degradation of Delta Np63 alpha following DNA damage.

Pathological apoptosis in the developing brain

More than half of the initially-formed neurons are deleted in certain brain regions during normal development. This process, whereby cells are discretely removed without interfering with the further development of remaining cells, is called programmed cell death (PCD). The term apoptosis is used to describe certain morphological manifestations of PCD. Many of the effectors of this developmental cell death program are highly expressed in the developing brain, making it more susceptible to accidental activation of the death machinery, e.g. following hypoxia-ischemia or irradiation. Recent evidence suggests, however, that activation and regulation of cell death mechanisms under pathological conditions do not exactly mirror physiological, developmentally regulated PCD. It may be argued that the conditions after e.g. ischemia are not even compatible with the execution of PCD as we know it. Under pathological conditions cells are exposed to various stressors, including energy failure, oxidative stress and unbalanced ion fluxes. This results in parallel triggering and potential overshooting of several different cell death pathways, which then interact with one another and result in complex patterns of biochemical manifestations and cellular morphological features. These types of cell death are here called "pathological apoptosis," where classical hallmarks of PCD, like pyknosis, nuclear condensation and caspase-3 activation, are combined with non-PCD features of cell death. Here we review our current knowledge of the mechanisms involved, with special focus on the potential for therapeutic intervention tailored to the needs of the developing brain.

SuPr-1-mediated desumoylation regulates the repressor activity of DeltaNp63alpha

DeltaNp63alpha is exclusively expressed in stem cells and progenitor cells of the stratified epithelia. It promotes cell proliferation by antagonizing p53 and related TAp63/TAp73. Here, we report that specific desumoylation by SUMO protease SuPr-1 provides a fine-tuning mechanism for DeltaNp63alpha repressor activity. We found that disrupting the sumoylation site compromised DeltaNp63alpha repressor activity profoundly against TAp63gamma and TAp73beta-mediated transcription activation, but not to p53-mediated transcription. We further found that SuPr-1 specifically bound to sumoylated DeltaNp63alpha and hydrolyzed SUMO. Consequently, SuPr-1 expression reduced DeltaNp63alpha repressor activity to TAp63gamma and TAp73beta, whereas p53-mediated transactivation was unaffected. Collectively, these data suggest that SuPr-1-mediated DeltaNp63alpha desumoylation elaborately regulates epithelial growth.

Properties of the six isoforms of p63: p53-like regulation in response to genotoxic stress and cross talk with DeltaNp73

TP63, a member of the TP53 gene family, encodes two groups of three isoforms (alpha, beta and gamma). The TAp63 isoforms act as transcription factors. The DeltaNp63 isoforms lack the main transcription activation domain and act as dominant-negative inhibitors of transactivation (TA) isoforms. To clarify the role of these isoforms and to better understand their functional overlap with p53, we ectopically expressed each p63 isoform in the p53-null hepatocellular carcinoma cell line Hep3B. All TA isoforms, as well as DeltaNp63alpha, had a half-life of <1 h when transiently expressed and were degraded by the proteasome pathway. The most stable form was DeltaNp63gamma, with a half-life of >8 h. As expected, TA isoforms differed in their transcriptional activities toward genes regulated by p53, TAp63gamma being the most active form. In contrast, DeltaNp63 isoforms were transcriptionally inactive on genes studied and inhibited TA isoforms in a dose-dependent manner. When stably expressed in polyclonal cell populations, TAp63beta and gamma isoforms were undetectable. However, when treated with doxorubicin (DOX), p63 proteins rapidly accumulated in the cells. This stabilization was associated with an increase in phosphorylation. Strikingly, in DOX-treated polyclonal populations, increase in TAp63 levels was accompanied by overexpression of DeltaNp73. This observation suggests complex regulatory cross talks between the different isoforms of the p53 family. In conclusion, p63 exhibits several transcriptional and stress-response properties similar to those of p53, suggesting that p63 activities should be taken into consideration in approaches to improve cancer therapies based on genotoxic agents.

The p53 tumor suppressor-like protein nvp63 mediates selective germ cell death in the sea anemone Nematostella vectensis

Here we report the identification and molecular function of the p53 tumor suppressor-like protein nvp63 in a non-bilaterian animal, the starlet sea anemone Nematostella vectensis. So far, p53-like proteins had been found in bilaterians only. The evolutionary origin of p53-like proteins is highly disputed and primordial p53-like proteins are variably thought to protect somatic cells from genotoxic stress. Here we show that ultraviolet (UV) irradiation at low levels selectively induces programmed cell death in early gametes but not somatic cells of adult N. vectensis polyps. We demonstrate with RNA interference that nvp63 mediates this cell death in vivo. Nvp63 is the most archaic member of three p53-like proteins found in N. vectensis and in congruence with all known p53-like proteins, nvp63 binds to the vertebrate p53 DNA recognition sequence and activates target gene transcription in vitro. A transactivation inhibitory domain at its C-terminus with high homology to the vertebrate p63 may regulate nvp63 on a molecular level. The genotoxic stress induced and nvp63 mediated apoptosis in N. vectensis gametes reveals an evolutionary ancient germ cell protective pathway which relies on p63-like proteins and is conserved from cnidarians to vertebrates.

CD74 induces TAp63 expression leading to B-cell survival

Most mature follicular B cells circulate within the periphery in a quiescent state, without actively contributing to an acute immune response. Lasting B-cell persistence in the periphery is dependent on survival signals that are transduced by cell surface receptors. We recently demonstrated that cell surface CD74 controls mature B-cell survival. Stimulation of cell surface CD74 leads to NF-kappaB activation, which enables entry of the stimulated B cells into the S phase, induction of DNA synthesis, and cell division, and augments the expression of survival genes. In the present study, we investigated CD74 target genes to determine the identities of the molecules whose expression is modulated by CD74, thereby regulating B-cell survival. We report that CD74 activates the p65 member of the NF-kappaB family, which in turn up-regulates the expression of p53-related TAp63 proteins. TAp63 then binds and transactivates the Bcl-2gene and induces the production of Bcl-2 protein, thereby providing the cells with increased survival capacity. Thus, the CD74/NF-kappaB/TAp63 axis defines a novel antiapoptotic pathway in mature B cells, resulting in the shaping of both the B-cell repertoire and the immune response.

Differential roles of p63 isoforms in epidermal development: selective genetic complementation in p63 null mice

Epidermal development requires the transcription factor p63, as p63-/- mice are born dead, without skin. The gene expresses two proteins, one with an amino-terminal transactivation domain (TAp63) and one without (deltaNp63), although their relative contribution to epidermal development is unknown. To address this issue, we reintroduced TAp63alpha and/or deltaNp63alpha under the K5 promoter into p63-/- mice by in vivo genetic complementation. Whereas p63-/- and p63-/-;TA mice showed extremely rare patches of poorly differentiated keratinocytes, p63-/-;deltaN mice showed significant epidermal basal layer formation. Double TAp63alpha/deltaNp63alpha complementation showed greater patches of differentiated skin; at the ultrastructural level, there was clear reformation of a distinct basal membrane and hemidesmosomes. At the molecular level, deltaNp63 regulated expression of genes characteristic of the basal layer (K14), interacting (by Chip, luc assay) with the third p53 consensus site. Conversely, TAp63 transcribed the upper layer's genes (Ets-1, K1, transglutaminases, involucrin). Therefore, the two p63 isoforms appear to play distinct cooperative roles in epidermal formation.

Cleavage of the transactivation-inhibitory domain of p63 by caspases enhances apoptosis

p63 is a p53-related transcription factor. Utilization of two different promoters and alternative splicing at the C terminus lead to generation of six isoforms. The alpha isoforms of TAp63 and DeltaNp63 contain a transactivation-inhibitory (TI) domain at the C termini, which can bind to the transactivation (TA) domain and inhibit its transcriptional activity. Consequently, TAp63alpha can directly inhibit its activity through an intramolecular interaction; similarly, DeltaNp63alpha can inhibit the activity of the active TAp63 isoforms through an intermolecular interaction. In this work, we demonstrate that after induction of apoptosis, the TI domain of the p63alpha isoforms is cleaved by activated caspases. Cleavage of DeltaNp63alpha relieves its inhibitory effect on the transcriptionally active p63 proteins, and the cleavage of TAp63alpha results in production of a TAp63 protein with enhanced transcriptional activity. In agreement with these data, generation of the N-terminal TAp63 fragment has a role in apoptosis because stable cell lines expressing wild-type TAp63 are more sensitive to apoptosis compared with cells expressing the noncleavable mutant. We also used a model system in which TAp63 expression was induced by trichostatin-A treatment in HCT116 cells. Trichostatin-A sensitized these cells to apoptosis, and this sensitization was associated with cleavage of up-regulated p63.

p53 and Delta Np63 alpha differentially bind and regulate target genes involved in cell cycle arrest, DNA repair and apoptosis

The mechanism by which the p53 family of proteins coordinately regulates select target genes after various types of cell stress is not well understood. To further define factors that dictate regulation of target genes, we examined the binding of p53, DeltaNp63alpha and RNA polymerase II (pol II) to the regulatory regions of select target genes in primary human epidermal keratinocytes (HEKs) using chromatin immunoprecipitation. In rapidly proliferating cells, we observed constitutive binding of DeltaNp63alpha and varying levels of p53 binding, to consensus sites in target genes involved in cell cycle arrest, DNA repair and apoptosis. Following genotoxic stress, p53 occupancy increased whereas DeltaNp63alpha occupancy decreased at the majority of binding sites examined. Microarray analysis of transcripts isolated from HEKs ectopically expressing p53 and DeltaNp63alpha revealed an inverse regulation of select target genes by the two family members. Collectively, our results suggest that DeltaNp63alpha can function as a repressor of select p53 target genes involved in growth arrest, DNA repair and apoptosis, and that the location of the p53 consensus binding site(s) in a target gene may dictate whether pol II is constitutively bound in proliferating cells.

A new technique for real-time analysis of caspase-3 dependent neuronal cell death

Several markers are available to identify cells undergoing programmed cell death, but so far they are only applicable on fixed material. Therefore, no information on the kinetics of apoptosis can be obtained, although apoptosis is a dynamic cell process. Here, we describe a new technique that allows the real-time observation of the onset of apoptosis in primary neurons. Neurons are transfected with a plasmid that codes for a fluorescent protein localized in the soma. Upon activation of caspase-3, which represents the point-of-no-return in the apoptosis process, the fusion protein is cleaved and as a consequence translocates into the nucleus. The onset of apoptosis is thus visualized by translocation of the fluorescent signal from the soma to the nucleus. The translocation process was found to be specific for the apoptosis process as it correlates with the activation of caspase-3 and TUNEL staining. This tool does not require complex detection systems and allows for the first time the analysis of the kinetics of apoptosis in a simple and efficient manner.

DeltaNp63alpha promotes apoptosis of human epidermal keratinocytes

In this study we show that deltaNp63alpha overexpression in primary human epidermal keratinocytes causes decreased cell proliferation and increased apoptosis. These changes are associated with increased levels of p21 and p27, decreased cyclin D1 and cyclin E levels, reduced mitochondrial membrane potential, and enhanced procaspase and poly(ADP-ribose) polymerase cleavage. Bcl-xS and Bax levels are increased and Bcl-xL level is reduced. p53 levels are increased in the deltaNp63alpha-expressing cells and p53 overexpression reproduces features of the deltaNp63alpha phenotype. Increased p53 expression results in reduced deltaNp63alpha, suggesting that p53 may negatively regulate deltaNp63alpha level. DeltaNp63alpha also induces apoptosis in HaCaT and SCC-13 cells, which encode inactive p53 genes, suggesting that the response is p53 independent in these cell lines. Both deltaNp63alpha and TAp63alpha reduce SCC-13 cell survival. These studies indicate that both deltaNp63alpha and TAp63alpha can negatively regulate keratinocyte survival.

p63 and p73 in human cancer: defining the network

The p53-related genes p63 and p73 exhibit significant structural homology to p53; however, they do not function as classical tumor suppressors and are rarely mutated in human cancers. Both p63 and p73 exhibit tissue-specific roles in normal development and a complex contribution to tumorigenesis that is due to their expression as multiple protein isoforms. The predominant p63/p73 isoforms expressed both in normal development and in many tumors lack the conserved transactivation (TA) domain; these isoforms instead exhibit a truncated N-terminus (DeltaN) and function at least in part as transcriptional repressors. p63 and p73 isoforms are regulated through both transcriptional and post-translational mechanisms, and they in turn regulate diverse cellular functions including proliferation, survival and differentiation. The net effect of p63/p73 expression in a given context depends on the ratio of TA/DeltaN isoforms expressed, on physical interaction between p63 and p73 isoforms, and on functional interactions with p53 at the promoters of specific downstream target genes. These multifaceted interactions occur in diverse ways in tumor-specific contexts, demonstrating a functional 'p53 family network' in human tumorigenesis. Understanding the regulation and mechanistic contributions of p63 and p73 in human cancers may ultimately provide new therapeutic opportunities for a variety of these diseases.

p63 overexpression induces the expression of Sonic Hedgehog

p63 and p73 are members of the p53 protein family and have been shown to play an important role in cell death, development, and tumorigenesis. In particular, p63 has been shown to be involved in the maintenance of epidermal stem cells and in the stratification of the epidermis. Sonic Hedgehog (Shh) is a morphogen that has also been implicated to play a role in epithelial stem cell proliferation and in the development of organs. Recently, Shh has also been shown to play an important role in the progression of a variety of cancers. In this report, we show that p63 and p73 but not p53 overexpression induces Shh expression. In particular, p63gamma and p63beta (both TA and DeltaN isoforms) and TAp73beta isoform induce Shh. Expression of Shh was found to be significantly reduced in mouse embryo fibroblasts obtained from p63-/- mice. The naturally occurring p63 mutant TAp63gamma(R279H) and the tumor suppressor protein p14(ARF) inhibited the TAp63gamma-mediated transactivation of Shh. The region -228 to -102 bp of Shh promoter was found to be responsive to TAp63gamma-induced transactivation and TAp63gamma binds to regions within the Shh promoter in vivo. The results presented in this study implicate p63 in the regulation of the Shh signaling pathway.

DeltaNp63 plays an anti-apoptotic role in ventral bladder development

The bladder, the largest smooth-muscle organ in the human body, is responsible for urine storage and micturition. P63, a homolog of the p53 tumor-suppressor gene, is essential for the development of all stratified epithelia, including the bladder urothelium. The N-terminal truncated isoform of p63, DeltaNp63, is known to have anti-apoptotic characteristics. We have established that DeltaNp63 is not only the predominant isoform expressed throughout the bladder, but is also preferentially expressed in the ventral bladder urothelium during early development. We observed a host of ventral defects in p63-/- embryos, including the absence of the abdominal and ventral bladder walls. This number of ventral defects is identical to bladder exstrophy, a congenital anomaly exhibited in human neonates. In the absence of p63, the ventral urothelium was neither committed nor differentiated, whereas the dorsal urothelium was both committed and differentiated. Furthermore, in p63-/- bladders, apoptosis in the ventral urothelium was significantly increased. This was accompanied by the upregulation of mitochondrial apoptotic mediators Bax and Apaf1, and concurrent upregulation of p53. Overexpression of DeltaNp63gamma and DeltaNp63beta in p63-/- bladder primary cell cultures resulted in a rescue, evidenced by significantly reduced expressions of Bax and Apaf1. We conclude that DeltaNp63 plays a crucial anti-apoptotic role in normal bladder development.

Ubiquitin and ubiquitin-like modifications of the p53 family

Regulation of p53 by the ubiquitin-proteasomal pathway has been studied considerably. Studies have also demonstrated that the ubiquitin-like proteins SUMO-1 and NEDD8 modify p53. Similarly, p63 and p73 are subject to regulation by ubiquitin and ubiquitin-like modifications, and perturbations of these pathways in the regulation of the p53 family have been implicated in tumorigenesis and developmental abnormalities. Here, we provide an overview of the current understanding of the regulation of the p53 family by covalent modification by ubiquitin, SUMO-1, and NEDD8.

The p53 family in nervous system development and disease

The p53 family, consisting of the tumor suppressors p53, p63 and p73, play a vital role as regulators of survival and apoptosis in the developing, adult and injured nervous system. These proteins function as key survival and apoptosis checkpoints in neurons, acting as either rheostats or sensors responsible for integrating multiple pro-apoptotic and survival cues. A dramatic example of this checkpoint function is observed in developing sympathetic neurons, where a pro-survival and truncated form of p73 antagonizes the apoptotic functions of p53 and p63. Thus the levels and activities of the different p53 family members may ultimately determine whether neurons either live or die during nervous system development and disease.

A dominant negative form of p63 inhibits apoptosis in a p53-independent manner

Stem cells are a source of differentiated cells in multiple tissues. If genetic alterations occur in stem cells, the problem persists and malignant cancers may arise. DeltaNp63alpha-a homologue of the tumor suppressor p53-is exclusively expressed in proliferating undifferentiated epithelial cells and cancer cells of epidermal origin. Here, we show that DeltaNp63alpha antagonizes DNA damage-induced apoptosis in a p53-independent manner. We found that upon cellular injury, DeltaNp63alpha must be downregulated before apoptotic program can be activated. The 5637 cell line has abundant levels of DeltaNp63alpha and mutant p53, and it is resistant to DNA damage-induced apoptosis. The knockdown of DeltaNp63alpha by RNA interference sensitized these cells to apoptosis upon genotoxic insult. This suggests that DeltaNp63alpha plays an anti-apoptotic role regardless of the p53 status. Considering the frequent mutations of p53 in tumor cells, our results provide important implications for the treatment of cancers in which p63 is amplified.

Neurodevelopment on route p63

All known members of the p53 gene family, including the two homologs p73 and p63, have multiple biological functions. In neurons, p53 and p73 are known to regulate cell death in the developing and adult nervous system. A report by Jacobs et al. in this issue of Neuron shows that the more ancestral member of this gene family, p63, is an essential proapoptotic protein during neuronal development.

The role of p63 in germ cell apoptosis in the developing testis

The fetal and neonatal development of male germ cells (gonocytes) is a poorly understood but crucial process for establishing fertility. In rodents, gonocytes go through two phases of proliferation accompanied by apoptosis and separated by a quiescent period during the end of fetal development. P63 is a member of the P53 gene family that yields six isoforms. We detected only the p63 protein and no p53 and p73 in the nucleus of the gonocytes of mouse testes. We report for the first time the ontogeny of each p63 mRNA isoform during testis development. We observed a strong expression of p63gamma mRNA and protein when gonocytes are in the quiescent period. In vitro treatment with retinoic acid prevented gonocytes from entering the quiescent period and was correlated with a reduced production of p63gamma isoform mRNA. We investigated the function of p63 by studying the testicular phenotype of P63-null mice. P63 invalidation slightly, but significantly increased the number of gonocytes counted during the quiescent period. As P63-null animals die at birth we used an original organ culture that mimicked neonatal in vivo development to study further the testicular development. P63 invalidation resulted in a sharply increased number of gonocytes during the culture period due to a decrease in spontaneous apoptosis with no change in proliferation. P63 invalidation also caused abnormal morphologies in the germ cells that were also found in P63(+/-) adult male mice. Thus, p63 appears as an important regulator of germ cell development.