In Vivo Expression of an SCA27A-Linked <i>FGF14</i> Mutation Results in Haploinsufficiency and Impaired Firing of Cerebellar Purkinje Neurons

Autosomal dominant mutations in <i>FGF14</i>, 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 <i>FGF14</i> into the mouse <i>Fgf14</i> locus (<i>Fgf14^F145S</i> ). Current-clamp recordings from Purkinje neurons in acute cerebellar slices from adult male and female <i>Fgf14^F145S/+</i> 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 <i>Fgf14</i> knock-out (<i>Fgf14^+/-</i> ) Purkinje neurons, suggesting that the impaired firing of <i>Fgf14^F145S/+</i> 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 <i>Fgf14^F145S/+</i> (and <i>Fgf14^+/-</i> ) animals and voltage-clamp experiments revealed a hyperpolarizing shift in the voltage dependence of closed-state Nav channel inactivation in <i>Fgf14^F145S/+</i> (and <i>Fgf14^+/-</i> ) Purkinje neurons. Together, these results indicate that <i>Fgf14</i> haploinsufficiency and reduced Nav channel availability underlie impaired firing in <i>Fgf14^F145S/+</i> Purkinje neurons.