Sensorimotor peripheral innervation in Duchenne muscular Dystrophy: spotlights on the sciatic nerve of dystrophic mdx mice

Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease, characterized by the lack of dystrophin (Dp427), a cytoskeletal protein expressed in skeletal muscles and several other cell types. Along or in substitution of Dp427, different tissues other than muscles may also express shorter dystrophin isoforms, as in the peripheral nervous system (PNS), where myelinating Schwann cells (SCs) contain the Dp116 isoform. By binding to a large multiprotein complex (DGC), described as similar in muscle and organized around the alpha and beta-dystroglycan (DG) dymer, Dp116 contributes to myelin integrity and nerve fiber conduction. Structural and functional integrity of nerve fibers rely on axon-SC intense cross-talk, a well-known mediator of which is the neurotransmitter GABA. To unveil the effects that DMD has on peripheral sensory-motor nerves, we analyzed in the sciatic nerve of mdx mice, a model of DMD, expression and localization of: myelin proteins (e.g. MBP, P0, PMP22, MAG), Dp116, proteins of the DGC (dystrobrevin, beta-DG), as well as inotropic GABAA (α4, β3, δ) and metabotropicGABAB-1b receptor subunits. Compared to wild-type, in mdx mice, level, distribution and intensity of immunolabeling of all proteins analyzed, except for Dp116, were significantly reduced, indicating alteration of myelin integrity and SCs-axons crosstalk. This was also confirmed by parallel electrophysiological studies showing a significant reduction in Abeta- mechanoreceptor fiber excitability. Furthermore, gel zymography and Western immunoblot demonstrated activation of the pro-inflammatory extracellular matrix metalloproteinase (MMP) 2, which targets both alpha and beta-DG, suggesting that cleavage of DG within the myelin sheet, induced by retrograde inflammation from muscle to nerve, could be an upstream determinant for Dp116-DGC degradation and consequent deregulation of SC-axon signaling. Altogether, these findings support the hypothesis of significant alterations of the neuron-SC cross-talk in peripheral nerves of dystrophic subject, an aspect to be considered for supporting DMD therapeutic strategies