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Although, supernumerary synapses are formed and subsequently pruned in the brain, whether this occurs within spinal sensory-motor circuits remains unclear. Moreover, it is unknown what molecules are involved. Here, we demonstrate the presence of proprioceptive supernumerary synapses forming inappropriate contacts with motor neurons, resulting in miswired immature circuits. Using mouse genetics, neuronal circuit mapping, electrophysiology, and behavioral studies, we demonstrate that the inappropriate synapses are functional, leading to impaired behaviors. We further identify two complementary mechanisms responsible for their elimination: first, C3 through the classical complement pathway and second, CD47 that operates independently of classical complement signaling. This finding underlies an unexpected function for CD47 within the spinal cord, in contrast to its function in the brain. Thus, during early development, the course of elimination of inappropriately generated synapses utilizes a dual fail\u2013safe system to ensure emergence of mature spinal motor circuits.<\/jats:p>","DOI":"10.1084\/jem.20251329","type":"journal-article","created":{"date-parts":[[2026,8,8]],"date-time":"2026-08-08T06:20:46Z","timestamp":1786170046000},"update-policy":"https:\/\/doi.org\/10.1084\/jem.crossmarkpolicy","source":"Crossref","is-referenced-by-count":0,"title":["C3 and CD47 mediate sensory-motor circuit refinement during spinal cord development"],"prefix":"10.1084","volume":"223","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5832-089X","authenticated-orcid":true,"given":"Danny","family":"Florez-Paz","sequence":"first","affiliation":[{"id":[{"id":"https:\/\/ror.org\/00hj8s172","id-type":"ROR","asserted-by":"publisher"}],"name":"Center for Motor Neuron Biology and Disease, Columbia University 1 , New York, NY,","place":["USA"]},{"id":[{"id":"https:\/\/ror.org\/00hj8s172","id-type":"ROR","asserted-by":"publisher"}],"name":"Columbia University 2 Department of Neurology, , New York, NY,","place":["USA"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4610-2016","authenticated-orcid":true,"given":"George Z.","family":"Mentis","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/00hj8s172","id-type":"ROR","asserted-by":"publisher"}],"name":"Center for Motor Neuron Biology and Disease, Columbia University 1 , New York, NY,","place":["USA"]},{"id":[{"id":"https:\/\/ror.org\/00hj8s172","id-type":"ROR","asserted-by":"publisher"}],"name":"Columbia University 2 Department of Neurology, , New York, NY,","place":["USA"]},{"id":[{"id":"https:\/\/ror.org\/00hj8s172","id-type":"ROR","asserted-by":"publisher"}],"name":"Columbia University 3 Department of Pathology and Cell Biology, , New York, NY,","place":["USA"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"291","published-online":{"date-parts":[[2026,9,16]]},"reference":[{"key":"2026091610494818400_bib1","doi-asserted-by":"publisher","first-page":"16877","DOI":"10.1073\/pnas.1419045111","article-title":"Degradation of mouse locomotor pattern in the absence of proprioceptive sensory feedback","volume":"111","author":"Akay","year":"2014","journal-title":"Proc. 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