p75NTR and DR6 regulate distinct phases of axon degeneration demarcated by spheroid rupture

The regressive events associated with trophic deprivation are critical for sculpting a functional nervous system. After nerve growth factor withdrawal, sympathetic axons maintain their structural integrity for roughly 18 hours (latent phase) followed by a rapid and near unison disassembly of axons over the next 3 hours (catastrophic phase). Here we examine the molecular basis by which axons transition from latent to catastrophic phases of degeneration following trophic withdrawal. Prior to catastrophic degeneration, we observed an increase in intra-axonal calcium. This calcium flux is accompanied by p75 neurotrophic factor receptor (NTR)-Rho-actin dependent expansion of calcium rich axonal spheroids that eventually rupture, releasing their contents to the extracellular space. Conditioned media derived from degenerating axons is capable of hastening transition into the catastrophic phase of degeneration. We also found that death receptor 6 (DR6) but not p75NTR is required for transition into the catastrophic phase in response to conditioned media but not for the intra-axonal calcium flux, spheroid formation, or rupture that occurs toward the end of latency. Our results support the existence of an inter-axonal degenerative signal that promotes catastrophic degeneration among trophically deprived axons.