Abnormalities in the axonal cytoskeleton induced by a connexin32 mutation in nerve xenografts

The X-linked form of Charcot-Marie-Tooth neuropathy is associated with mutations in the connexin32 (Cx32) gene. The functional role of Cx32 in Schwann cells and the relationship of these mutations to the progressive axonal loss and distal limb weakness seen in this disease have not been elucidated. To investigate the local influence of Schwann cells bearing the Cx32 gene defect on axonal cytoskeleton and the myelination process, the nerve xenograft model was used to transfer a Cx32 missense mutation (Glu102Gly) from human to an in vivo myelination system in nude mice. Twelve nerve grafts from two family members with Cx32 mutations and 17 grafts from three healthy individuals were generated by end-to-end anastomosis of approximately 6-mm sural nerve fascicles into the cut ends of the sciatic nerve in nude mice. Specimens were examined at 2, 4, 8, 12, and 16 weeks. Ultrastructural morphometric analysis showed Schwann cells with Cx32 mutation have a profound effect on the nude mice axons, resulting in an increase in neurofilament density, a depletion of microtubules associated with fragmentation of smooth axonal reticulum, and increased vesicles and mitochondria. At 16 weeks, axonal enlargement was evident within the proximal part of the graft; axonal atrophy, degeneration, and fiber loss were seen in distal-graft and host segments. The myelination process was not affected. We conclude that Cx32 mutation impairs a modulatory function of Schwann cells on axons, resulting in profound cytoskeletal alterations leading to distal axonal degeneration. These observations emphasize the role of impaired Schwann cell-axon interactions in the pathogenesis of hereditary neuropathies.