Copper restriction unmasks axonal degeneration in a mouse model of X-linked hereditary motor neuropathy

Mutations in the copper (Cu) transporter ATP7A cause a spectrum of X-linked diseases, including Menkes Disease, Occipital horn syndrome and distal hereditary motor neuropathy (dHMNX). We previously generated a conditional knock-in mouse model of dHMNX expressing Atp7aT985I, the murine orthologue of the human T994I variant identified in dHMNX patients. Although Atp7aT985I mice did not develop overt motor degeneration, affected males showed a trend toward reduced Cu levels in the peripheral nervous system (PNS). The high-affinity copper transporter Ctr1, encoded by Slc31a1, regulates Cu uptake, and ubiquitous heterozygosity for Slc31a1 (Ctr1+ /-) has been reported to limit Cu availability in the nervous system without impairing motor performance. In this study we genetically restricted Cu availability in Atp7aT985I mice by crossing them with Ctr1+ /- animals. Atp7aT985I/Ctr1+/- males exhibited significantly reduced Cu levels in both the central nervous system (CNS) and PNS compared to wild-type littermates. At six months of age, behavioural testing and histopathological assessment revealed mild motor deficits and axonal loss, preferentially affecting small-caliber fibers exclusively in the Cu-restricted Atp7aT985Imales. Tandem mass tag-based proteomics of sciatic nerve identified significant changes linked to energy metabolism, cytoskeletal integrity and cellular stress responses. Together, these data show that limiting Cu availability unmasks a dHMNX like phenotype in Atp7aT985I /Ctr1+ /- mice, demonstrate the critical role of Cu availability in maintaining peripheral axons and suggest that reduced Cu in motor neurons contributes to axonal degeneration in dHMNX.