Functional Characterization of Parallel Fiber-Purkinje Cell Synapses in Two Friedreich's Ataxia Mouse Models

Friedreich ataxia (FRDA) is an autosomal recessive disorder caused by GAA expansions in the FXN gene, which codes for the protein frataxin (FXN). These mutations reduce FXN expression, leading to mitochondrial dysfunction and multisystemic disease. Accumulating evidence suggests that neuronal dysfunction, rather than neuronal death, may drive the neurological phenotypes of FRDA, but the mechanisms underlying such neurological phenotypes remain unclear. To investigate the neural circuit basis of this dysfunction, we employed field recordings to measure Purkinje cell (PC) function and synaptic properties along with western blotting and immunohistochemistry to determine their density and structure in two established FRDA mouse models, the shRNA-frataxin (FRDAkd) and the frataxin knock in-knockout (KIKO) mice. Western blotting demonstrated subtle changes in mitochondrial proteins and only a modest reduction in the density of calbindin positive cells PCs in the cerebellar cortex of the FRDAkd mice, with no change in the density of PCs in the KIKO mice. Though PC density differed slightly in the two models, field recordings of parallel fiber-PC synapses in the molecular layer demonstrated concordant hypo-excitability of basal synaptic transmission and impairments of long-term plasticity using induction protocols associated with both potentiation and depression of synaptic strength. These results indicate that synaptic instability might be a common feature in FRDA mouse models.