Age-related Purkinje cell death is steroid dependent: RORα haplo-insufficiency impairs plasma and cerebellar steroids and Purkinje cell survival

A major problem of ageing is progressive impairment of neuronal function and ultimately cell death. Since sex steroids are neuroprotective, their decrease with age may underlie age-related neuronal degeneration. To test this, we examined Purkinje cell numbers, plasma sex steroids and cerebellar neurosteroid concentrations during normal ageing (wild-type mice, WT), in our model of precocious ageing (Rora+/sg, heterozygous staggerer mice in which expression of the neuroprotective factor RORα is disrupted) and after long-term hormone insufficiency (WT post-gonadectomy). During normal ageing (WT), circulating sex steroids declined prior to or in parallel with Purkinje cell loss, which began at 18 months of age. Although Purkinje cell death was advanced in WT long-term steroid deficiency, this premature neuronal loss did not begin until 9 months, indicating that vulnerability to sex steroid deficiency is a phenomenon of ageing Purkinje neurons. In precocious ageing (Rora+/sg), circulating sex steroids decreased prematurely, in conjunction with marked Purkinje cell death from 9 months. Although Rora+/sg Purkinje cells are vulnerable through their RORα haplo-insufficiency, it is only as they age (after 9 months) that sex steroid failure becomes critical. Finally, cerebellar neurosteroids did not decrease with age in either genotype or gender; but were profoundly reduced by 3 months in male Rora+/sg cerebella, which may contribute to the fragility of their Purkinje neurons. These data suggest that ageing Purkinje cells are maintained by circulating sex steroids, rather than local neurosteroids, and that in Rora+/sg their age-related death is advanced by premature sex steroid loss induced by RORα haplo-insufficiency

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 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 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