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 derived from male and female neonatal mice 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. SIGNIFICANCE STATEMENT Developmental pruning shares several morphological similarities to both disease- and injury-induced degeneration including spheroid formation. The function and underlying mechanisms governing axonal spheroid formation, however, remain unclear. In this study we report that axons coordinate each other9s degeneration during development via axonal spheroid rupture. Prior to irreversible breakdown of the axon in response to trophic withdrawal, p75NTR-RhoA signaling governs the formation and growth of spheroids. These spheroids then rupture allowing exchange of contents under 10 kDa between the intracellular and extracellular space to drive DR6 and calpain dependent catastrophic degeneration. This finding informs not only our understanding of regressive events during development but may also provide a rationale for designing new treatments toward myriad neurodegenerative disorders.