Developmental molecular signatures define de novo cortico-brainstem circuit for skilled forelimb movement

Skilled motor control requires precise connections between subcerebral projection neurons (SCPN) in the cerebral cortex and their appropriate subcerebral targets in the brainstem or spinal cord. The brainstem has been widely recognized as a key integration and processing hub between “upper” motor centers and spinal circuits involved in execution of movements. Cortical input into the brainstem may fine-tune movement execution and support motor planning and is established by two SCPN supopulations: 1) cortico-brainstem neurons (CBN) that limit their axon extension to supraspinal levels, directly innervating brainstem nuclei, and 2) corticospinal neurons (CSN) that extend their axons into the spinal cord but additionally collateralize within the brainstem. Classical anatomical tracing approaches are not able to distinguish CBN from CSN, since these distinct subpopulations reside interdigitated in cortex, thus precluding the investigation of the function of CBN in skilled motor movement execution and planning until now. We used a combination of transcriptomics, tracing, and imaging techniques to identify the molecular program that CBN vs. CSN at the time point of initial axon extension. We found that Neuropeptide Y (Npy) is specifically enriched in CBN in lateral cortex, while CART prepropeptide (Cartpt) delineates cervical-projecting CSN, which now provides, for the first time in the field, molecular access to specifically investigate direct cortical input into the brainstem. By using established Cre mouse lines for CBN / CSN specific markers, we now aim to investigate the function and connectivity of these CBN vs. CSN within the brainstem. Our preliminary results suggest a topographic organization of these subpopulations within the brainstem. Selectively silencing of Npy+ CBN within the lateral cortex will allow us to investigate the contribution of CBN to skilled motor movements in the single pellet reaching task and other selected motor tasks. Our results provide a foundation for future studies aiming to unravel the complexity of cortico-brainstem and cortico-spinal circuits, and their contribution to movement control in health and disease.