The effects of mutant Schwann cells on the axonal cytoskeleton and regeneration-associated myelination in hereditary neuropathies: Studies on nerve xenografts

Differential distal axonal loss is a common feature of Charcot-Marie-Tooth (CMT) hereditary neuropathies and primary Schwann cell genetic defects, mutations affecting the peripheral myelin protein 22 (PMP22) and Connexin 32 (Cx32) genes, are now known to be involved in the molecular pathogenesis of these disorders. The general hypothesis tested here, is that the axonal loss in these disorders is due to perturbed Schwann cell-axonal interactions resulting from primary Schwann genetic defects. The nerve xenograft paradigm used in these studies creates an easy access model to study the local influence of mutant Schwann cells derived from patients with known mutations upon axonal properties and the regeneration-associated myelination process. In this study, sural nerve segments from individuals with PMP22 duplications or deletions and point mutations, as well as Cx32 point mutations were grafted into the cut ends of the sciatic nerve of nude mice. The xenografts and host segments were studied at different time intervals after grafting and compared to controls obtained from healthy volunteers. Xenografts from CMT1A (PMP22 duplication) and x-linked form of CMT neuropathy (CMTX resulting from Cx32 point mutation) examined at 16 weeks show that the nude mice axons within the proximal part of grafts demonstrate a significant increase in axonal area with an increase in the neurofilament and membranous organelle density compared to distal graft and distal host segments. A preferential distal axonal loss associated with a perpetual axonal atrophy, degeneration, and axonal sprouting was observed over time with increasing frequency at 8 to 16 weeks. The distal summation of the pathology, as evidenced by a greater amount of fiber loss in the distal graft segments is in accordance with the concept of dying back degeneration, similar to that observed in patients. These alterations were seen to a lesser extent in PMP22 deletion or point mutation xenografts and were not observed in controls. The quantitative morphometric data indicate a segmental impairment of both slow and fast transport components in nude mice axons where they pass through the grafted segments, which probably is the underlying cause of preferential distal axonal degeneration. In addition, similar nodal and cleft alterations (increases in lengths of constricted axon segment and numbers of Schmidt-Lanterman incisures) were seen in PMP22 duplication and Cx32 point mutation grafts, suggesting that they may represent common compensatory changes in response to impaired Schwann cell-axon interactions. Collectively, these studies show that Schwann cells bearing the PMP22 or Cx32 genetic defects cause major perturbations in Schwann cell-axon interactions emphasizing the role of axonal component in the pathogenesis of hereditary neuropathies.