Synaptic and Functional Connectivity Between Neural Progenitor Cell Grafts and Spinal Locomotor Circuits After Spinal Cord Injury

Abstract

Spinal cord injury (SCI) is a traumatic and life-altering event that frequently results in the loss of voluntary motor function. Currently, there are no clinically effective treatments that can improve locomotor function after SCI. Neural progenitor cells (NPCs) are a promising potential therapy for SCI, because of their ability to mature into spinal cord neuronal subtypes and act as a neuronal relay between surviving host neurons rostral and caudal to the site of the injury. In order to use NPCs to repair locomotor circuitry following traumatic spinal cord injury, it is first critical to determine the number and phenotypes of transplanted NPC-derived neurons that establish direct and indirect synaptic connections onto spinal cord motor neurons (MNs) after spinal cord injury and to ascertain their function in modulating locomotor recovery. We first utilized the monosynaptic rabies virus and pseudorabies virus to characterize direct and indirect graft inputs onto spinal cord motor neurons in adult mice following SCI and NPC transplantation. We found that less than 1% of graft neurons established synaptic connections onto host lumbar MNs; these graft-derived neurons were identified to be cholinergic and Chx10+ . We next visualized cell type-specific graft projections using AAV-SynTag to label anatomical projections from all graft neurons, cholinergic graft neurons, or V2a graft neurons. We found that each of these subtypes exhibited a distinct termination pattern in the lumbar spinal cord, suggesting that graft neuron phenotype influences patterns of synaptic connectivity. Finally, we examined the effects of modulating graft activity on locomotor behavior. We performed an SCI on adult C57BL/6 mice and transplanted NPCs expressing excitatory DREADDs (hM3Dq) in all graft neurons. Ten weeks later, we observed that DREADD-mediated activation of graft neurons significantly alters muscle activity This appears to be the first study to show direct and indirect synaptic connections of NPCs onto spinal locomotor circuitry and demonstrate that graft activity directly modulates muscle function. Ultimately, these findings serves as an important foundation to inform future work on development of cell-based therapies to restore locomotor function after spinal cord injury.