Tubulin autoregulation factors SCAPER and TTC5 recruit γ-tubulin to non-centrosomal MTOCs for neuronal microtubule nucleation and axon regeneration

Neuronal function and survival depend on highly stereotyped non-centrosomal microtubule (MT) arrays. How these arrays form remains poorly understood. Here we identified a role for SCAPER and TTC5, two factors previously implicated in tubulin mRNA autoregulation, in controlling neuronal MT content through {gamma}-tubulin-dependent nucleation. In C. elegans neurons, loss of scpr-1, ttc-5, or both reduced MT numbers to a similar degree as depletion of {gamma}-tubulin, the main MT nucleator. Using conditional single-cell degradation alleles and endogenous tagging, we find that {gamma}-tubulin nucleates MTs in the neuronal cell body from endosomal puncta, and that scpr-1 and ttc-5 are required to recruit {gamma}-tubulin to these structures. SCPR-1 is also instructive, as its overexpression drastically increases {gamma}-tubulin levels and enhances MT density. We propose that these mechanisms are conserved since human SCAPER rescues C. elegans mutants, and SCAPER knockdown in rat hippocampal neurons reduces both {gamma}-tubulin clustering at presynaptic sites and activity-dependent synaptic MT nucleation. Finally, while scpr-1, ttc-5, and{gamma} -tubulin are not required for developmental axon elongation, they are essential for regeneration, where SCPR-1 directs {gamma}-tubulin to the growth cone to facilitate regrowth following injury. These findings reveal mechanisms governing neuronal cytoskeleton assembly and function, and suggest potential crosstalk between tubulin autoregulation and microtubule nucleation.