In Vivo Expression of an SCA27A-Linked FGF14 Mutation Results in Haploinsufficiency and Impaired Firing of Cerebellar Purkinje Neurons.

Autosomal dominant mutations in FGF14, which encodes intracellular fibroblast growth factor 14 (iFGF14), underlie spinocerebellar ataxia type 27A (SCA27A), a devastating multisystem disorder resulting in progressive deficits in motor coordination and cognitive function. Mice lacking iFGF14 exhibit similar phenotypes, which have been linked to iFGF14-mediated modulation of the voltage-gated sodium (Nav) channels that regulate high-frequency repetitive firing of cerebellar Purkinje neurons, the main output neurons of the cerebellar cortex. To investigate the in vivo mechanisms underlying SCA27A, we developed a targeted knock-in strategy to introduce the first point mutation identified in FGF14 into the mouse Fgf14 locus (Fgf14F145S ). Current-clamp recordings from Purkinje neurons in acute cerebellar slices from adult male and female Fgf14F145S/+ mice revealed that high-frequency repetitive firing, which is characteristic of wild-type Purkinje neurons, was replaced by prolonged bursts of action potentials. A shift from tonic to burst firing was mimicked in wild-type Purkinje neurons by bath application of the Nav channel toxin, tetrodotoxin. Burst firing was also measured in heterozygous Fgf14 knock-out (Fgf14+/- ) Purkinje neurons, suggesting that the impaired firing of Fgf14F145S/+ Purkinje neurons reflects reduced Nav channel availability, owing to the loss of the iFGF14 protein. Western blot analyses confirmed reduced iFGF14 protein expression in cerebellar lysates prepared from Fgf14F145S/+ (and Fgf14+/- ) animals and voltage-clamp experiments revealed a hyperpolarizing shift in the voltage dependence of closed-state Nav channel inactivation in Fgf14F145S/+ (and Fgf14+/- ) Purkinje neurons. Together, these results indicate that Fgf14 haploinsufficiency and reduced Nav channel availability underlie impaired firing in Fgf14F145S/+ Purkinje neurons.