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 ~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.