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Neurodevelopment on Route p63

2005; Cell Press; Volume: 48; Issue: 5 Linguagem: Inglês

10.1016/j.neuron.2005.11.023

1097-4199

Pierluigi Nicotera, Gerry Melino,

Epigenetics and DNA Methylation

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. 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 transcription factor and tumor suppressor p53 and its two recently described homologs, p63 and p73, form a protein family whose members are endowed with reciprocal regulatory functions (Melino et al., 2003Melino G. Lu X. Gasco M. Crook T. Knight R.A. Trends Biochem. Sci. 2003; 28: 663-670Abstract Full Text Full Text PDF PubMed Scopus (253) Google Scholar). They are expressed both as multiple alternatively spliced C-terminal isoforms and as N-terminally deleted isoforms, which—along with posttranslational modifications—accounts for their molecular and regulatory complexity. All three major p53 family members are transcription factors, with a modular structure (Figure 1A), which comprises transactivation (TA), DNA-binding (DBD), and oligomerization (OD) domains, which are essential for their downstream effects, including cell-cycle arrest and apoptosis. In addition to C-terminal splicing, which gives rise to isoforms not discussed here, all three members of the family can also be transcribed starting from a second downstream promoter. This generates N-terminally truncated (ΔN) isoforms that lack the TA domain (full-length proteins are denominated as TA isoforms). The ΔN isoforms can act as dominant-negative inhibitors of the TA isoforms and of p53. Thus, TA and ΔN have pro- and antiapoptotic properties, respectively. The ancestor of this protein family, p63 (Yang et al., 1998Yang A. Kaghad M. Wang Y. Gillett E. Fleming M.D. Dotsch V. Andrews N.C. Caput D. McKeon F. Mol. Cell. 1998; 2: 305-316Abstract Full Text Full Text PDF PubMed Scopus (1769) Google Scholar), has a very high degree of sequence homology with p73, which is known to play an important role in controlling neuronal development. The 86% identity (92% similarity) in the DNA-binding domains suggests that p63 and p73 may target overlapping promoters in those tissues where their degree of expression is similar. Interestingly, although all p53/p63/p73 family members regulate both cell cycle and apoptosis, developmental abnormalities are most prominent in the p73 and p63 null mice. Mice deficient in all p73 isoforms exhibit profound defects, including hippocampal dysgenesis, hydrocephalus, chronic infections, and inflammation, as well as abnormalities in pheromone sensory pathways. Deletion of p63 results in severe limb truncations, craniofacial malformations, and, most important, the absence of skin and most epithelial tissues as well as defective epidermal differentiation (Yang et al., 1999Yang A. Kaghad M. Wang Y. Gillett E. Fleming M.D. Doetsch V. Andrews N.C. Caput D. McKeon F. Nature. 1999; 398: 714-718Crossref PubMed Scopus (1826) Google Scholar, Mills et al., 1999Mills A.A. Zheng B. Wang X.J. Vogel H. Roop D.R. Bradley A. Nature. 1999; 398: 708-713Crossref PubMed Scopus (1633) Google Scholar). In contrast, p53−/− mice do not display major developmental defects. This suggests that while the evolutionarily older family members may primarily function in developmental regulation, the younger member of the family, p53, has a more relevant role in the response to DNA damage (Figure 1B). Neuronal developmental death is required to eliminate redundancies in connecting neuronal networks. In sympathetic neurons, signaling via the TrkA/NGF and p75NTR receptors by target-derived neurotrophins is essential for survival, and lack of this stimulation results in apoptosis. The signaling cascade, involving JNK, c-Jun, BimEL, and Bax, has not yet been completely elucidated. The p53 protein family has been implicated in the death signaling following NGF withdrawal. Initially, a suggestion had been made that the balance between the proapoptotic function of p53 and the prosurvival function of ΔNp73 (attributed to its dominant-negative function on p53) could be a factor deciding the fate of cultured sympathetic neurons deprived of neurotrophin stimulation (Pozniak et al., 2000Pozniak C.D. Radinovic S. Yang A. McKeon F. Kaplan D.R. Miller F.D. Science. 2000; 289: 304-306Crossref PubMed Scopus (397) Google Scholar). However, several other findings have indicated that p53 is not the only proapoptotic effector in sympathetic neurons: p53 null neurons are only partially protected from NGF withdrawal and, in p73−/− mice, the absence of p53 rescues neurons only in part. Finally, p53−/− mice have only a partial deficit in sympathetic neuronal death. In this issue of Neuron, Jacobs and colleagues (Jacobs et al., 2005Jacobs W.B. Govoni G. Ho D. Atwal J.K. Barnabe-Heider F. Keyes W.M. Mills A.A. Miller F.D. Kaplan D.R. Neuron. 2005; 48 (this issue): 743-756Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar) find a more convincing solution to the saga of the p53 gene family and its involvement in developmental neuronal death. They reveal an unexpected and essential role of TAp63γ in naturally occurring sympathetic neuronal death (Figure 1C). Developing sympathetic neurons express TAp63γ at the time of developmental death, and most importantly, the TAp63γ level increases upon NGF withdrawal. Adenoviral expression of p63γ induces apoptosis even in the presence of NGF, and finally, embryonic p63−/− mice display a deficit in naturally occurring developmental neuronal death. Moreover, while both p63 and p53 transactivate Bax to induce cell death, TAp63 can trigger neuronal apoptosis in the absence of p53, whereas p53 requires coincident p63 expression. Altogether these findings strongly support the central role of TAp63γ in sympathetic neuronal development, which can be explained by its proapoptotic function. Bax expression is required for TAp63γ-induced neuronal death in analogy with TAp73 and p53 and with TAp63α. Interestingly, in a recent study by Gressner and colleagues (Gressner et al., 2005Gressner O. Schilling T. Lorenz K. Schleithoff E.S. Koch A. Schultze-Bergkamen H. Lena A.M. Candi E. Terrinoni A. Catani M.V. et al.EMBO J. 2005; 24: 2458-2471Crossref PubMed Scopus (226) Google Scholar), it was found that TAp63 can mediate apoptosis triggered via death receptor complexes (CD95, TRAIL, TNF, FLIP) as well as that involving primarily the mitochondrial (Bax, Noxa, Puma, Apaf-1) death pathway. This places p63 on both apoptotic routes and suggests that TAp63γ may behave in a similar manner in sympathetic neurons. Because Bax is downstream of JNK in sympathetic neuronal death, TAp63γ acts between JNK signaling and the effector Bax (Figure 1C). Neuronal demise triggered by a variety of conditions including brain ischemia and β-amyloid neurotoxicity seems to require the signaling cascade involving JNK, BH3-only Bcl-2 family members, and Bax. Also, a large body of literature has investigated the possible role of p53, and more recently of p73, in neuronal death. In view of the finding presented here, it would be interesting to explore whether p63 may also have a dominant role in postnatal neuronal demise nested in the JNK-Bax route and independently of p53. Along these lines, of particular importance is finding that the proapoptotic role of TAp63γ in sympathetic neurons does not require p53. In fact, p53 requires TAp63 to fulfill its death mission. Thus, the function of the antiapoptotic protein ΔNp73 in sympathetic development reported earlier can be explained by its regulation of TAp63 rather than its ability to neutralize p53 proapoptotic function (see scheme in Figure 1C). Indeed, p63−/− sympathetic neurons are resistant to developmental cell death both in vitro and in vivo. The coincident requirement of p73 and p63 to selectively activate proapoptotic molecules such as Noxa, Perp, and Bax initially shown by Flores and colleagues (Flores et al., 2002Flores E.R. Tsai K.Y. Crowley D. Sengupta S. Yang A. McKeon F. Jacks T. Nature. 2002; 416: 560-564Crossref PubMed Scopus (708) Google Scholar) suggests that a close cooperation between the two molecules is required to trigger apoptosis, at least in some tissues. Surprisingly, cells of mice deficient in both p63 and p73 exhibit a significant resistance to neuronal apoptosis induced by cisplatin or γ-radiation, despite the presence of functionally competent p53. This suggests that p53 may be upstream of TAp73 and TAp63 in the apoptotic pathway, and it is unable to trigger cell death in the absence of its siblings. The literature, which has investigated the involvement of p53 in different forms of neuronal demise, should perhaps be revisited in light of the findings presented here. Clearly, the suggestion that TAp63 could be the effector of the p53 proapoptotic function raises crucial queries. What is the molecular basis of this apparently incestuous relationship? Is the ΔNp63 dominant-negative loop involved? How are different members of the family recruited/upregulated to selectively transactivate target genes? Does the p53 and p63, p73 partnership have fundamentally different biological implications than those caused by selective expression of the individual genes? The function and expression level of p63 and the other family members can obviously be regulated by several factors, including selective induction, but also posttranslational modifications, including phosphorylation, control of protein stability, and regulation by other proteins (Figure 2A). Most of what is currently known about p63 regulation comes from studies in nonneuronal tissues. p63 transcriptional activity is complex and requires a specific spatial localization as well as the interaction with other non-family member proteins, such as the promyelocytic leukemia protein (PML), which physically interacts with the p63 protein and determines the p63 localization into the PML Nuclear-Bodies (PML-NBs) (Bernassola et al., 2005Bernassola F. Oberst A. Melino G. Pandolfi P.P. Oncogene. 2005; 24: 6982-6986Crossref PubMed Scopus (38) Google Scholar). This interaction between p63 and PML increases the steady-state protein levels of p63 in cultured cells and p63 transcriptional activation of Bax and cell cycle-related genes such as gadd45 and p21. PML bodies are found in healthy neurons but also in pathological settings. Their assembly is particularly evident in disease states (i.e., in the reactive sensory ganglion neurons of the Guillain-Barre syndrome, in amyotrophic lateral sclerosis, and possibly in intranuclear inclusion disease). The function of p63 is also modulated by phosphorylation and prolyl-isomerization by Pin-1 (G.M., unpublished data). PML regulation and posttranslational modifications are essentially similar for all p53 family members and indicate that p63 follows the strict sequence of activation control described for its siblings (see Figure 2B). From biblical times, our culture is populated by the antagonism of life and death, good and evil, and often these dichotomies run in close families. Like Cain and Abel, Romulus and Remus, and countless others, the members of the p53 family play dangerously one against the other in a game of death and survival. But is it as simple as "live and let die"? Or is there a possibility that the members of this family may entertain more subtle interactions? According to Jacobs and colleagues (Jacobs et al., 2005Jacobs W.B. Govoni G. Ho D. Atwal J.K. Barnabe-Heider F. Keyes W.M. Mills A.A. Miller F.D. Kaplan D.R. Neuron. 2005; 48 (this issue): 743-756Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar), TAp63 regulates neuronal development primarily by acting through the cell death machinery. This is consistent with the evidence that NGF withdrawal primarily promotes cell death. However, in nonneuronal settings (i.e., epidermal biology) there is substantial evidence that p63 controls differentiation (according to Dennis Roop and colleagues; Mills et al., 1999Mills A.A. Zheng B. Wang X.J. Vogel H. Roop D.R. Bradley A. Nature. 1999; 398: 708-713Crossref PubMed Scopus (1633) Google Scholar) or the stem cell compartment (according to Frank McKeon and colleagues; Yang et al., 1999Yang A. Kaghad M. Wang Y. Gillett E. Fleming M.D. Doetsch V. Andrews N.C. Caput D. McKeon F. Nature. 1999; 398: 714-718Crossref PubMed Scopus (1826) Google Scholar) (see Figure 2C). Although the epidermal p63−/− phenotype reported by the two independent laboratories was identical, the interpretation of the underlying mechanism responsible for the p63 null phenotype remains in sharp contrast. While Roop and Bradley conclude that p63 controls epidermal differentiation, McKeon suggests that differentiation of the epidermis remains normal, whereas the p63 defect influences the proliferation potential of the stem cell compartment (see Figure 2C). It is clear that both TAp63 and ΔNp63 control the transcriptional activation or derepression of many genes involved in signaling and development, in addition to apoptosis. For example, the phosphoinoside-3-kinase pathway can regulate p63 downstream of the epidermal growth factor receptor (Barbieri et al., 2003Barbieri C.E. Barton C.E. Pietenpol J.A. J. Biol. Chem. 2003; 278: 51408-51414Crossref PubMed Scopus (65) Google Scholar), which links p63 to the PTEN pathway. Furthermore, p63 can physically interact with its own family members, can induce β-catenin accumulation with subsequent gene regulation (Patturajan et al., 2002Patturajan M. Nomoto S. Sommer M. Fomenkov A. Hibi K. Zangen R. Poliak N. Califano J. Trink B. Ratovitski E. Sidransky D. Cancer Cell. 2002; 1: 369-379Abstract Full Text Full Text PDF PubMed Scopus (168) Google Scholar), and can regulate the redox balance through REDD1 (Ellisen et al., 2002Ellisen L.W. Ramsayer K.D. Johannessen C.M. Yang A. Beppu H. Minda K. Oliner J.D. McKeon F. Haber D.A. Mol. Cell. 2002; 10: 995-1005Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar). Finally and most relevant in neurons, p63 can regulate the c-fos proto-oncogene, two ligands of Notch, JAG1 and JAG2, and the Wnt pathway gene hDKK1 (Wu et al., 2003Wu G. Nomoto S. Hoque M.O. Dracheva T. Osada M. Lee C.C. Dong S.M. Guo Z. Benoit N. Cohen Y. et al.Cancer Res. 2003; 63: 2351-2357PubMed Google Scholar). These are obviously all crucial for neural function and development. In addition, p73 has direct effects on neuronal development, unrelated to cell death (De Laurenzi et al., 2000De Laurenzi V. Raschella G. Barcaroli D. Annicchiarico-Petruzzelli M. Ranalli M. Catani M.V. Tanno B. Costanzo A. Levrero M. Melino G. J. Biol. Chem. 2000; 275: 15226-15231Crossref PubMed Scopus (161) Google Scholar, Billion et al., 2004Billion N. Terrinoni A. Jolicoer C. McCarthy A. Richardson W.D. Melino G. Raff M. Development. 2004; 131: 1211-1220Crossref PubMed Scopus (90) Google Scholar). The work by Jacobs and colleagues highlights a new dimension in the complicated network of the p53 gene family and its role in neuronal development. Clearly, as new horizons are opened by these observations, many exciting questions still lie ahead. The understanding of the role of these proteins in responding to external cues (or lack thereof), versus internal cell-autonomous signals to differentiation, will likely be one of the most challenging. P63 Is an Essential Proapoptotic Protein during Neural DevelopmentJacobs et al.NeuronDecember 08, 2005In BriefThe p53 family member p63 is required for nonneural development, but has no known role in the nervous system. Here, we define an essential proapoptotic role for p63 during naturally occurring neuronal death. Sympathetic neurons express full-length TAp63 during the developmental death period, and TAp63 levels increase following NGF withdrawal. Overexpression of TAp63 causes neuronal apoptosis in the presence of NGF, while cultured p63−/− neurons are resistant to apoptosis following NGF withdrawal. Full-Text PDF Open Archive