Differential expression of TrkB isoforms switches climbing fiber-Purkinje cell synaptogenesis to selective synapse elimination.

Correct neural function depends on precisely organized connectivity, which is refined from broader projections through synaptic/collateral elimination. In the rat, olivocerebellar topography is refined by regression of multiple climbing fiber (CF) innervation of Purkinje cells (PC) during the first two postnatal weeks. The molecules that initiate this regression are not fully understood. We assessed the role of cerebellar neurotrophins by examining tropomycin receptor kinase (Trk) receptor expression in the inferior olive and cerebellum between postnatal days (P)3-7, when CF-PC innervation changes from synapse formation to selective synapse elimination, and in a denervation-reinnervation model when synaptogenesis is delayed. Trks A, B, and C are expressed in olivary neurons; although TrkA was not transported to the cerebellum and TrkC was unchanged during innervation and reinnervation, suggesting that neither receptor is involved in CF-PC synaptogenesis. In contrast, both total and truncated TrkB (TrkB.T) increased in the olive and cerebellum from P4, whereas full-length and activated phosphorylated TrkB (phospho-TrkB) decreased from P4-5. This reveals less TrkB signaling at the onset of CF regression. This expression pattern was reproduced during CF-PC reinnervation: in the denervated hemicerebellum phospho-TrkB decreased as CF terminals degenerated, then increased in parallel with the delayed neosynaptogenesis as new CFs reinnervated the denervated hemicerebellum. In the absence of this signaling, CF reinnervation did not develop. Our data reveal that olivocerebellar TrkB activity parallels CF-PC synaptic formation and stabilization and is required for neosynaptogenesis. Furthermore, TrkB.T expression rises to reduce TrkB signaling and permit synapse elimination.