C3 and CD47 dependent mechanisms in the refinement of sensory-motor circuits during spinal cord development

Overground movement in mammals requires the assembly and refinement of sensory-motor circuits. Within the spinal cord, sensory neurons, interneurons and motor neurons form intricate neuronal circuits to ensure proper motor control. In the developing brain, supernumerary synapses are initially formed and subsequently pruned making way for the emergence of mature circuits. However, whether this occurs within spinal sensory-motor circuits, it has not been firmly established. Moreover, it is also unknown if a combination of distinct molecules are required to refine spinal cord neuronal circuits. Here, we demonstrate the presence of supernumerary synapses which form inappropriate contacts, resulting in miswired immature spinal neuronal circuits. We determined that inappropriate synapses are of proprioceptive sensory origin and are functional, leading to impaired motor behavior. Using mouse genetics, viral-mediated neuronal map strategies, electrophysiology, and behavioral assessments, we demonstrate that two molecularly distinct mechanisms are responsible for the refinement of spinal circuits. First, we identify C3 as a major contributing factor through classical complement activation. Second, a CD47-dependent mechanism, operating in parallel to classical complement, causing elimination of inappropriate synapses. This finding underlies an unexpected function for CD47 within the spinal cord, in striking contrast to its function in the brain. Our study demonstrates that during early development, the natural course of elimination of inappropriately-generated synapses utilizes a dual fail-safe system to ensure the emergence of normal spinal reflexes and proper behavior in mice.