The role of p53 as a tumor-suppressor protein is well known and is based on its ability to transcriptionally activate effector proteins that cause cell cycle arrest in G1 that allows time for repair of DNA damage. An alternate cellular choice is apoptosis to eliminate cells with damaged genomes. Loss of p53 activity by mutation is a major predisposing factor to transformation and cancer. The consequences of p53 and loss of function are appreciated in the cancer field but the functional significance of p53 in other diseases is not well studied. A major question in the p53 field continues to be how cell signaling decides between cell cycle arrest versus apoptosis. This question is a particularly important for possible p53 contributions towards programmed cell death in neurodegenerative processes and diseases are less recognized but are likely to be very influential in neuronal stress and injury. In particular, we found that p53 contributes to neuronal differentiation during the process of cell cycle arrest during neuronal differentiation. We asked the question what genes might be transcriptionally regulated by p53 in a neuronal cell culture model of differentiation using rat PC12 cells under the influence of nerve growth factor (NGF). We and others have found that NGF treatment greatly induces p53 during the growth arrest accompanying neuronal differentiation leading us to hypothesize that p53 might influence genes governing the differentiation process. A genome-wide chromatin immunoprecipitation (ChIP) cloning technique was used in which we reported 14 new genes involved in making new connections between nerves, or dendrites, that are important important in nerve cell development. These genes including members of the wnt and grainyhead families are under the control of NGF. Possible roles for these new genes are in maintenance of normal neural health, repair of damaged nerves, tissue regeneration through stem cells and neuronal development.? ? Control over the cell death process is critical in tissues throughout the body, particularly within the nervous system in which the underlying pathologies of many neurodegenerative diseases such as Parkinsons, Alzheimers, amyotrophic lateral sclerosis are not well understood. Nerve growth factor is one of several endogenous growth factors that maintains health and stability of differentiated neuronal tissues by controlling expression of several genes including the transcription factor, p53, an important determinant in programmed cell death. We conducted experiments to determine the influence of nerve growth factor upon apoptosis induced by reactive oxygen species that involved p53 activation. Our hypothesis was that NGF would protect neuronal cells against p53-induced apoptosis. We reported data that showed nerve growth factor protects differentiated neuronal cells from programmed cell death by p53 after exposure to the endogenous chemical, nitric oxide, but such nerve cells eventually die by necrosis after accumulation of direct injury from nitric oxide. While neuronal cultures imperfectly model what happens to the nervous system, the current study suggests that endogenous growth factors exert powerful protective inhibitory effects against nerve cell self-destruction and that combating neuronal necrosis might be key to treating some long-term neurodegenerative diseases.? ? In our recently published work, we identified novel NGF-dependent transcriptional targets of p53 during neuronal differentiation, including expression of the morphogen, wnt7b. We further exploited this finding by testing and validating the hypothesis that wnt7b expression occurs in a p53-dependent manner and that wnt7b expression is a critical factor in NGF-mediated neurite outgrowth of differentiating PC12 cells. We showed that NGF treatment causes a sustained elevation of p53 protein related to growth arrest, neurite outgrowth and wnt7b expression. P53 overexpression induced wnt7b mRNA and protein levels in neuronal PC12 cells. Ectopic wnt7b overexpression was able to rescue neurite outgrowth in NGF-treated p53-silenced PC12 cells and could be blocked by JNK inhibitors. Interestingly, we also found a unique subset of wnt receptors, Fzd9 and Fzd7, induced by NGF exposure. Based on gene expression studies, we propose a model by which wnt7b expression cooperates with NGF-inducible Fzd receptor expression to increase both the number and length of neurite extensions. Our data suggest wnt7b is a novel p53-regulated neuritogenic factor in PC12 cells that in conjunction with NGF-regulated Fzd expression is involved in p53-dependent neurite outgrowth through non-canonical JNK signaling. This most recent work is currently under editiorial review and has been accepted with some minor revisions.? ? Further investigation of wnt7b expression and other p53-regulated genes such as tfcp2l4/grhl3 grainyhead homolog in neuronal tissues could provide valuable knowledge on neurite outgrowth, neuronal repair processes and have implications for stem cell research in differentiation of neuronal progenitors and also in neuronal development. In addition, wnt7b may also influence plasma membrane processes in cell growth and motility important in non-neuronal tissues as an important area for future investigation.
