Activity-driven proprioceptive synaptic refinement in the developing spinal cord by complement signaling mechanisms

Proprioceptive group Ia afferents detect muscle stretch to guide effortless and purposeful movement and make monosynaptic connections with spinal α-motor neurons to mediate reflexes, such as the stretch reflex. It is thought that proprioceptive Ia afferents target motor neurons of the same spinal segment; yet, how this specificity, if any, is established during early development is unknown. Using ex vivo spinal cord electrophysiology preparations from neonatal mice of both sexes, we identified a developmental period during which proprioceptive la afferents evoke both segmental and intersegmental responses at monosynaptic latencies. We provide anatomical evidence that motor neurons in the lumbar segment 4 (L4) receive direct input from proprioceptive Ia afferents in L5 during early postnatal development. Intersegmental responses (L4/L5) were prominent at postnatal days (P) 4-7 but were virtually absent by P11-13. To test the role of proprioceptor activity on segmental specification, we analyzed Na_V1.6 conditional knockout mice (Na_V1.6^cKO), in which proprioceptor signaling is impaired, and found that intersegmental responses persist up to P11-13 but were absent in age-matched floxed controls. We predict this is due to impaired activation of complement signaling pathways, as Na_V1.6^cKO mice showed reduced C1qA expression in the ventral spinal cord at P9. Consistent with this, C1qA knockout mice also retain intersegmental responses at P11-13. Collectively, these findings identify an important postnatal window during which segmental specificity of proprioceptive circuits emerges and suggest that proprioceptor activity induces C1qA-mediated elimination of excessive intersegmental connectivity.