PTPσ mediates the integration of grafted neuronal tissueoids with host neural pathways after complete spinal cord...

Cell transplantation-based regenerative medicine offers a promising strategy for repairing damaged neural pathways following spinal cord injury. Nonetheless, the integration of transplanted cells-particularly neurons-into host tissue remains insufficiently characterized. Notably, the molecular mechanisms underlying graft-host interaction are still poorly defined. In this study, we investigated how directly transplanted mature neurons contribute to the structural repair of fully transected spinal circuits in a rat xenotransplantation model and sought to identify candidate molecules involved in this process. To this end, we engineered human iPSC-derived neuronal tissueoids (Ntoids) in vitro using tissue engineering approaches. These Ntoids primarily consisted of mature, post-mitotic neurons interconnected into functional neural networks. Dependent on excitatory neurotransmission, they displayed electrophysiological signatures characteristic of excitatory neural networks. Importantly, their transplantable properties enabled them to fill tissue defects resulting from complete spinal cord injury. Histological analyses demonstrated that Ntoids survived for at least 8 weeks after spinal cord transplantation, with grafted cells retaining neuronal phenotypic characteristics. Furthermore, Ntoid transplantation significantly promoted reinnervation, synaptogenesis, and motor function recovery at the injury/graft site. Analysis of single-cell sequencing data from the developing rodent spinal cord suggested that the PTPσ-TrkC complex is involved in excitatory synaptogenesis. This was corroborated in human brain-spinal cord assembloid models, where PTPσ⁺ neurites extended from brain organoids into spinal cord regions and established connections with TrkC⁺ spinal neurons. Similarly, co-culture of rat organotypic brain slices with Ntoids showed that PTPσ and TrkC co-localized at developing synaptic junctions. Additionally, transplanted TrkC-expressing Ntoids established synaptic connections with host PTPσ⁺ supraspinal motor fibers (5-HT⁺) and sensory fibers (CGRP⁺). Thus, our findings revealed that the PTPσ-TrkC complex may participate in synaptic integration between transplanted neurons and host circuits, constituting a structural foundation for functional neural relay restoration in spinal cord injury repair.