Germline NF1, c-RET, SDH, and VHL mutations cause familial pheochromocytoma. Pheochromocytomas derive from sympathetic neuronal precursor cells. Many of these cells undergo c-Jun-dependent apoptosis during normal development as NGF becomes limiting. NF1 encodes a GAP for the NGF receptor TrkA, and NF1 mutations promote survival after NGF withdrawal. We found that pheochromocytoma-associated c-RET and VHL mutations lead to increased JunB, which blunts neuronal apoptosis after NGF withdrawal. We also found that the prolyl hydroxylase EglN3 acts downstream of c-Jun and is specifically required among the three EglN family members for apoptosis in this setting. Moreover, EglN3 proapoptotic activity requires SDH activity because EglN3 is feedback inhibited by succinate. These studies suggest that failure of developmental apoptosis plays a role in pheochromocytoma pathogenesis.

Activation of the p75 neurotrophin receptor leads to a variety of effects within the nervous system, including neuronal apoptosis. Both c-Jun N-terminal kinase (JNK) and the tumor suppressor p53 have been reported to be critical for this receptor to induce cell death; however, the mechanisms by which p75 activates these pathways is undetermined. Here we report that the neurotrophin receptor interacting factor (NRIF) is necessary for p75-dependent JNK activation and apoptosis. Upon nerve growth factor withdrawal, nrif؊/؊ sympathetic neurons underwent apoptosis, whereas p75-mediated death was completely abrogated. The lack of cell death correlated with a lack of JNK activation in the nrif؊/؊ neurons, suggesting that NRIF is a selective mediator for p75-dependent JNK activation and apoptosis. Moreover, we document that NRIF expression is sufficient to induce cell death through a mechanism that requires p53. Taken together, these results establish NRIF as an essential component of the p75 apoptotic pathway.The p75 neurotrophin receptor is a pleiotropic signaling molecule that regulates cellular survival, neurite outgrowth, and myelin formation (1). This founding member of the tumor necrosis factor receptor superfamily can directly bind all of the neurotrophins, including nerve growth factor (NGF), 1 brainderived neurotrophic factor (BDNF), and neurotrophin-3 and -4 (NT-3, NT-4), but it also functions as a co-receptor in a variety of protein complexes. p75 can interact with TrkA to form a high affinity neurotrophin binding site (2) and enhance survival signaling (3). It can also associate with the Nogo receptor and Lingo-1 and, upon interaction with myelin proteins, block neurite outgrowth (4, 5), and, together with Sortilin, the neurotensin 3 receptor, it binds the proform of NGF and initiates apoptosis (6). The divergent cellular responses depend on the receptor complex as well as the cellular context. For example, sympathetic neurons of superior cervical ganglia undergo a period of programmed cell death during ontogenesis, and NGF, supplied by the tissues innervated, prevents the loss of these neurons through binding to a p75-TrkA complex (7). In contrast, specific activation of p75 (8) or a p75-sortilin complex (6) induces apoptosis in the neurons. Genetic deletion of p75 prevents the normal period of cell death in the developing superior cervical ganglia (8, 9), thus demonstrating the key role of this receptor in regulating the survival of this neuronal population.How p75 initiates this variety of biological effects is not well understood; however, the stress kinase c-Jun N-terminal kinase, JNK, has been suggested to play a role in mediating this apoptotic signal. Neurotrophin activation of JNK through p75 correlates with the induction of cell death (43) and inhibition of the kinase prevents the receptor from killing (23, 10). Interestingly, c-Jun, the downstream target of the kinase, is not required for the receptor to activate apoptosis (11); however, other JNK substrates have been implicated in the p75 death ...