Tubulin autoregulation factors recruit γ-tubulin to non-centrosomal MTOCs for neuronal microtubule nucleation

Neuronal function and survival depend on stereotyped non-centrosomal microtubule (MT) arrays. How these arrays form remains poorly understood. Here, we found that two factors previously implicated in tubulin mRNA turnover control neuronal MT content through γ-tubulin-dependent nucleation. In C. elegans neurons, loss of scaper, ttc5, or both reduced MT numbers to a similar degree as depletion of γ-tubulin, the main MT nucleator. Using conditional single-cell degradation alleles and endogenous tagging, we find that scaper and ttc5 are required to recruit γ-tubulin to non-centrosomal MT organizing centers. SCAPER is also instructive, as its overexpression drastically raises γ-tubulin levels at nucleation sites and increases 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 γ-tubulin clustering at presynaptic sites and activity-dependent synaptic MT nucleation. SCAPER, TTC5 and γ-tubulin are dispensable for developmental axon growth, but play essential roles during regeneration, where SCAPER directs γ-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.