Dorsal root ganglia (DRG) contain somatosensory neurons of diverse sensory modalities. Among these different types of sensory neurons, the molecular mechanisms that regulate the development and specification of touch neurons are least well understood. We took a candidate-approach and searched for transcription factors that are expressed in subsets of DRG neurons, and found that the transcription factor Shox2 is expressed in subpopulations of TrkB- and Ret-expressing neurons at neonatal stages. Since TrkB is a known marker selectively expressed in touch sensory neurons, we decided to examine the function of Shox2 in specifying TrkB-positive DRG neurons. Conditional deletion of Shox2 in neural crest cells (which give rise to all DRG neurons) caused a 60~65% reduction in the number of TrkB-expressing neurons. It also resulted in an increase in co-expression of TrkC in Ret-positive sensory neuron. Deletion of Shox2 in differentiating DRG neurons at later time points caused only a moderate reduction in TrkB expression. Overexpression of Shox2 in all neural crest cells resulted in a small increase in the number of TrkB-expressing neurons. Finally, Shox2-deletion also caused reduced touch sensory axonal innervation to layers III/IV of the spinal cord. Taken together, our findings identify Shox2 as an essential but not sufficient component of the transcription programs required in neural progenitor cells for the proper specification of subsets of TrkB-expressing touch/mechanosensory neurons.

Premotor circuits in the brainstem project to pools of orofacial motoneurons to execute essential motor action such as licking, chewing, breathing, and in rodent, whisking. Previous transsynaptic tracing studies only mapped orofacial premotor circuits in neonatal mice, but the adult circuits remain unknown as a consequence of technical difficulties. Here, we developed a three-step monosynaptic transsynaptic tracing strategy to identify premotor neurons controlling vibrissa, tongue protrusion, and jaw-closing muscles in the adult mouse. We registered these different groups of premotor neurons onto the Allen mouse brain common coordinate framework (CCF) and consequently generated a combined 3D orofacial premotor atlas, revealing unique spatial organizations of distinct premotor circuits. We further uncovered premotor neurons that simultaneously innervate multiple motor nuclei and, consequently, are likely to coordinate different muscles involved in the same orofacial motor actions. Our method for tracing adult premotor circuits and registering to Allen CCF is generally applicable and should facilitate the investigations of motor controls of diverse behaviors.

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