Global connectivity and function of descending spinal input revealed by 3D microscopy and retrograde transduction

The brain communicates with the spinal cord through numerous axon tracts that arise from discrete nuclei, transmit distinct functions, and often collateralize to facilitate the coordination of descending commands. In efforts to restore supraspinal connectivity after injury or disease, this complexity presents a major challenge to interpreting functional outcomes, while the wide distribution of supraspinal nuclei complicates the delivery of therapeutics. Here we harness retrograde viral vectors to overcome these challenges. We demonstrate highly efficient retrograde transduction by AAV2-Retro in adult female mice, and employ tissue clearing to visualize descending tracts and create a mesoscopic projectome for the spinal cord. Moreover, chemogenetic silencing of supraspinal neurons with retrograde vectors results in complete and reversible forelimb paralysis, illustrating effective modulation of supraspinal function. Retrograde efficacy persists even after spinal injury, highlighting therapeutic potential. These data provide a global view of supraspinal connectivity and illustrate the potential of retrograde vectors to parse the functional contributions of supraspinal inputs.