Sympathetic neurons depend on NGF binding to TrkA for their survival during vertebrate development. NGF deprivation initiates a transcription-dependent apoptotic response, which is suggested to require activation of the transcription factor c-Jun. Similarly, apoptosis can also be induced by selective activation of the p75 neurotrophin receptor. The transcriptional dependency of p75-mediated cell death has not been determined; however, c-Jun NH2-terminal kinase has been implicated as an essential component. Because the c-jun–null mutation is early embryonic lethal, thereby hindering a genetic analysis, we used the Cre-lox system to conditionally delete this gene. Sympathetic neurons isolated from postnatal day 1 c-jun–floxed mice were infected with an adenovirus expressing Cre recombinase or GFP and analyzed for their dependence on NGF for survival. Cre immunopositive neurons survived NGF withdrawal, whereas those expressing GFP or those uninfected underwent apoptosis within 48 h, as determined by DAPI staining. In contrast, brain-derived neurotrophic factor (BDNF) binding to p75 resulted in an equivalent level of apoptosis in neurons expressing Cre, GFP, and uninfected cells. Nevertheless, cycloheximide treatment prevented BDNF-mediated apoptosis. These results indicate that whereas c-jun is required for apoptosis in sympathetic neurons on NGF withdrawal, an alternate signaling pathway must be induced on p75 activation.

Activation of the neurotrophin receptor p75 has been shown to elicit opposing cellular signals. Depending on the context of the cell, p75 will either promote survival or induce apoptosis after neurotrophin stimulation. p75-induced apoptosis occurs through activation of c-Jun N-terminal kinase (JNK), whereas the survival signal is mediated by nuclear factor B (NFB). The receptor proximal signals that produce these responses are unknown, although several molecules have been identified that associate with the intracellular domain of p75. One such interactor, TRAF6, a member of the tumor necrosis factor receptor-associated factor family, has been implicated in p75 signaling. To assess the role of TRAF6 in p75 signaling, we analyzed mice with this gene deleted. In Schwann cells isolated from traf6؉/؉ animals, NGF elicited an 80% increase in transcription of an NFB reporter; however, in traf6؊/؊ cells, the NGF response was abrogated. Similarly, NGF activation of JNK was not apparent in Schwann cells from mice lacking traf6. Deficiencies in p75 signaling in traf6؊/؊ animals resulted in a loss of p75-mediated apoptosis. In sympathetic neurons cultured from traf6؉/؉ superior cervical ganglia (SCGs), there was an increase in JNK activation and apoptosis after BDNF binding to p75; however, traf6؊/؊ neurons did not respond. In vivo during naturally occurring cell death, there was a 55.6% reduction in TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling)-positive cells in the SCG of postnatal day 4 traf6؊/؊ animals relative to traf6؉/؉ littermates. These results indicate that TRAF6 plays an essential role in mediating p75 signal transduction and induction of apoptosis.

Neurotrophin signaling through the p75 receptor regulates apoptosis within the nervous system both during development and in response to injury. Whereas a number of p75 interacting factors have been identified, how these upstream factors function in a coordinated manner to mediate receptor signaling is still unclear. Here, we report a functional interaction between TRAF6 and the neurotrophin receptor interacting factor (NRIF), two proteins known to associate with the intracellular domain of the p75 neurotrophin receptor. The association between NRIF and TRAF6 was direct and occurred with both endogenous and ectopically expressed proteins. A KRAB repressor domain of NRIF and the carboxyl-terminal, receptor-binding region of TRAF6 were required for the interaction. Co-expression of TRAF6 increased the levels of NRIF protein and induced its nuclear translocation. Reciprocally, NRIF enhanced TRAF6-mediated activation of the c-Jun NH 2 -terminal kinase (JNK) by 3-fold, while only modestly increasing the stimulation of NF-B. The expression of both NRIF and TRAF6 was required for reconstituting p75 activation of JNK in HEK293 cells, whereas NRIF mutants lacking the TRAF6 interaction domain were unable to substitute for the full-length protein in facilitating activation of the kinase. These results suggest that NRIF and TRAF6 functionally interact to facilitate neurotrophin signaling through the p75 receptor.

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