Evidence that ΔNp73 promotes neuronal survival by p53-dependent and p53-independent mechanisms

The p53 family member, p73, is essential for the survival of sympathetic neurons during the developmental period of naturally occurring neuronal death. Here, we have asked whether DeltaNp73, which is the only p73 isoform expressed in sympathetic neurons, mediates this survival by p53- dependent and/ or p53-independent mechanisms. Initially, we used a genetic approach and crossed p53(+/-) and p73(+/-) mice. Quantitation of neurons in the sympathetic superior cervical ganglion during the period of naturally occurring cell death revealed that the loss of p53 partially rescued the death of neurons seen in p73(-/-) animals. Moreover, exogenous expression of DeltaNp73 in cultured p53(-/-) sympathetic neurons rescued these neurons from apoptosis after NGF withdrawal. Biochemical studies asking how DeltaNp73 inhibited NGF withdrawal-induced apoptosis in wild-type neurons demonstrated that it prevented the upregulation of the direct p53 targets p21 and Apaf-1 as well as cleavage of caspase-3. It also inhibited events at the mitochondrial apoptotic checkpoint, suppressing the induction of BimEL and the release of mitochondrial cytochrome c. Interestingly, DeltaNp73 expression also inhibited one very early event in the apoptotic cascade, the activation of c-Jun N-terminal protein kinase (JNK), likely by binding directly to JNK. Finally, we show that neuronal cell size is decreased in p73(-/-) mice, and that this decrease is not rescued by the lack of p53, suggesting a role for p73 in regulating cell size that does not involve interactions with p53. Thus, DeltaNp73 promotes neuronal survival via p53- dependent and - independent mechanisms, and it does so at multiple points, including some of the most proximal events that occur after NGF withdrawal.