Neuroligin 3 promotes functional recovery of spinal cord injury through coordinating with Sar1a and Hspa8 at synapses
The recovery of locomotor function following incomplete spinal cord injury (SCI) primarily relies on the precise reconstruction of synaptic communications between spared corticospinal tract (CST) axons and propriospinal neurons. However, components capable of rebuilding synaptic machinery after injury remain elusive.  Here, using RNA-Seq of human umbilical cord-derived MSCs (hUC-MSCs) before and after transplantation in SCI rats, along with in vitro and in vivo functional screenings, we identified Neuroligin 3 (NLGN3) as a spinal cord microenvironment (SCE)-activated cell adhesion molecule (CAM) in hUC-MSCs, promoting MSC-mediated functional recovery of SCI. Importantly, spinal neuron-specific activation of Nlgn3 led to significant functional improvement in SCI rats, resembling the effects of hUC-MSC transplantation. We elucidated the protein interactome of Nlgn3, enriched with proteins involved in synaptic transmission. Specifically, Nlgn3 interacted with synaptic vesicle (SV)-associated proteins, Sar1a and Hspa8, modulating their recruitment at synapses. Remarkably, restoring either of these two proteins in injured spinal neurons improved locomotor recovery of SCI, with further enhancement when combined with Nlgn3. Thus, our results not only reveal the previously unknown therapeutic effect of Nlgn3 in SCI repair but also propose a novel approach for treating SCI by targeting protein complexes centered around Nlgn3. This study also suggests that MSCs can serve as a stem cell platform for identifying innovative therapeutic targets and mechanisms to reestablish corticospinal-spinal relay circuits after SCI. Overall design: In this study, 10-mm spinal cord segments centered at the epicenter were collected from rats subjected to different treatments for total RNA extraction. A total of 20 sequencing libraries were prepared, encompassing three biological replicates for MSC-in vitro, four replicates for the MSC-SCI-1d, MSC-sham-1d and MSC-sham-3d groups, and five replicates for the MSC-SCI-3d groups