Sox14 Is Required for a Specific Subset of Cerebello-Olivary Projections

We identify Sox14 as an exclusive marker of inhibitory projection neurons in the lateral and interposed, but not the medial, cerebellar nuclei. Sox14(+) neurons make up similar to 80% of Gad1(+) neurons in these nuclei and are indistinguishable by soma size from other inhibitory neurons. All Sox14(+) neurons of the lateral and interposed cerebellar nuclei are generated at approximately E10/10.5 and extend long-range, predominantly contralateral projections to the inferior olive. A small Sox14(+) population in the adjacent vestibular nucleus "Y" sends an ipsilateral projection to the oculomotor nucleus. Cerebellar Sox14(+) and glutamatergic projection neurons assemble in non-overlapping populations at the nuclear transition zone, and their integration into a coherent nucleus depends on Sox14 function. Targeted ablation of Sox14(+) cells by conditional viral expression of diphtheria toxin leads to significantly impaired motor learning. Contrary to expectations, associative learning is unaffected by unilateral Sox14(+) neuron elimination in the interposed and lateral nuclei.