The corticospinal tract (CST) is the most important motor system in humans, yet robust regeneration of this projection after spinal cord injury (SCI) has not been accomplished. In rodent models of SCI, we report robust corticospinal axon regeneration, functional synapse formation and improved skilled forelimb function after grafting multipotent neural progenitor cells into sites of spinal cord injury. Corticospinal regeneration requires that grafts are driven toward caudalized (spinal cord), rather than rostralized, fates. Fully mature caudalized neural grafts also support corticospinal regeneration. Moreover, corticospinal axons can emerge from neural grafts and regenerate beyond the lesion, potentially related to attenuation of the glial scar. Rodent corticospinal axons also regenerate into human donor grafts of caudal spinal cord identity. Collectively, these findings indicate that spinal cord “replacement” with homologous neural stem cells enables robust regeneration of the corticospinal projection within and beyond spinal cord lesion sites, achieving a major unmet goal of spinal cord injury research and opening new possibilities for translation.

We grafted human spinal cord-derived neural progenitor cells (NPCs) into sites of cervical spinal cord injury in rhesus monkeys (Macaca mulatta). Under three-drug immunosuppression, grafts survived at least 9 months postinjury and expressed both neuronal and glial markers. Monkey axons regenerated into grafts and formed synapses. Hundreds of thousands of human axons extended out from grafts through monkey white matter and synapsed in distal gray matter. Grafts gradually matured over 9 months and improved forelimb function beginning several months after grafting. These findings in a 'preclinical trial' support translation of NPC graft therapy to humans with the objective of reconstituting both a neuronal and glial milieu in the site of spinal cord injury.

Summary Despite advances in promoting axonal regeneration after acute spinal cord injury (SCI), elicitation of bridging axon regeneration after chronic SCI remains a formidable challenge. We report that combinatorial therapies administered 6 weeks, and as long as 15 months, after SCI promote axonal regeneration into and beyond a mid-cervical lesion site. Provision of peripheral nerve conditioning lesions, grafts of marrow stromal cells and establishment of NT-3 gradients, supports bridging regeneration. Controls receiving partial components of the full combination fail to exhibit bridging. Notably, intraneuronal molecular mechanisms recruited by delayed therapies mirror those of acute injury, including activation of transcriptional activators and regeneration-associated genes. Collectively, these findings provide evidence that regeneration is achievable at unprecedented post-injury time points.

Spinal cord neural stem cells (NSCs) have great potential to reconstitute damaged spinal neural circuitry, but they have yet to be generated in vitro. We now report the derivation of spinal cord NSCs from human pluripotent stem cells (hPSCs). Our observations show that these spinal cord NSCs differentiate into a diverse population of spinal cord neurons occupying multiple positions along the dorso-ventral axis, and can be maintained for prolonged time periods. Grafts into injured spinal cords were rich with excitatory neurons, extended large numbers of axons over long distances, innervated their target structures, and enabled robust corticospinal regeneration. The grafts synaptically integrated into multiple host intraspinal and supraspinal systems, including the corticospinal projection, and improved functional outcomes after injury. hPSC-derived spinal cord NSCs could enable a broad range of biomedical applications for in vitro disease modeling and constitute an improved clinically translatable cell source for 'replacement' strategies in several spinal cord disorders.