Climbing Fiber Development Is Impaired in Postnatal Car8 Mice.

The cerebellum is critical for an array of motor functions. During postnatal development, the Purkinje cells (PCs) guide afferent topography to establish the final circuit. Perturbing PC morphogenesis or activity during development can result in climbing fiber (CF) multi-innervation or mis-patterning. Structural defects during circuit formation typically have long-term effects on behavior as they contribute to the phenotype of movement disorders such as cerebellar ataxia. The Car8 mouse is one model in which early circuit destruction influences movement. However, although the loss of Car8 leads to the mis-wiring of afferent maps and abnormal PC firing, adult PC morphology is largely intact and there is no neurodegeneration. Here, we sought to uncover how defects in afferent connectivity arise in Car8 mutants to resolve how functional deficits persist in motor diseases with subtle neuropathology. To address this problem, we analyzed CF development during the first 3 weeks of life. By immunolabeling CF terminals with VGLUT2, we found evidence of premature CF synapse elimination and delayed translocation from PC somata at postnatal day (P) 10 in Car8 mice. Surprisingly, by P15, the wiring normalized, suggesting that CAR8 regulates the early but not the late stages of CF development. The data support the hypothesis of a defined sequence of events for cerebellar circuits to establish function.