Human spinal interneurons repair the injured spinal cord through synaptic integration.

Advances in cell therapy offer promise for some of the most devastating neural injuries, including spinal cord injury (SCI). Endogenous VSX2-expressing spinal V2a interneurons have been implicated as a key component in plasticity and therapeutically driven recovery post-SCI. While transplantation of generic V2a neurons may have therapeutic value, generation of human spinal V2a neurons with rostro-caudal specificity and assessment of their functional synaptic integration with the injured spinal cord has been elusive. Here, we efficiently differentiated optogenetically engineered cervical V2a spinal interneurons (SpINs) from human induced pluripotent stem cells and tested their capacity to form functional synapses with injured diaphragm motor networks in a clinically-relevant sub-acute model of cervical contusion injury. Neuroanatomical tracing and immunohistochemistry demonstrated transplant integration and synaptic connectivity with injured host tissue. Optogenetic activation of transplanted human V2a SpINs revealed functional synaptic connectivity to injured host circuits, culminating in improved diaphragm activity assessed by electromyography. Furthermore, optogenetic activation of host supraspinal pathways revealed functional innervation of transplanted cells by host neurons, which also led to enhanced diaphragm contraction indicative of a functional neuronal relay. Single cell analyses pre- and post-transplantation suggested the in vivo environment resulted in maturation of cervical SpINs that mediate the formation of neuronal relays, as well as differentiation of glial progenitors involved in repair of the damaged spinal cord. This study rigorously demonstrates feasibility of generating human cervical spinal V2a interneurons that develop functional host-transplant and transplant-host connectivity resulting in improved muscle activity post-SCI.

Transneuronal tracing to map connectivity in injured and transplanted spinal networks.

It has become widely appreciated that the spinal cord has significant neuroplastic potential, is not hard-wired, and that with traumatic injury and anatomical plasticity, the networks that we once understood now comprise a new anatomy. Harnessing advances in neuroanatomical tracing to map the neuronal networks of the intact and injured spinal cord has been crucial to elucidating this new spinal cord anatomy. Many new techniques have been developed to identify these networks using a variety of retrograde and anterograde tracers. One method of tracing that has become more widely used to map anatomical changes is transneuronal tracing. Viral tracers are being increasingly used to map spinal networks, leading to an advanced understanding of spinal circuitry and host-donor-host interactions between the injured spinal cord and neural transplants. This review will highlight advances in neuronal tracing, specifically using pseudorabies virus (PRV), and its use in the intact, injured, and transplanted spinal cord.