Sympathetic neuropathology is revealed in muscles affected by amyotrophic lateral sclerosis

Abstract

Rationale: The anatomical substrate of skeletal muscle autonomic innervation has remained underappreciated since it was described many decades ago. As such, the structural and functional features of muscle sympathetic innervation are largely undetermined in both physiology and pathology, mainly due to methodological limitations in the histopathological analysis of small neuronal fibers in tissue samples. Amyotrophic lateral sclerosis (ALS) is a fatal neuromuscular disease which mainly targets motor neurons, and despite autonomic symptoms occurring in a significant fraction of patients, peripheral sympathetic neurons (SNs) are generally considered unaffected and, as such, poorly studied.Purpose: In this research, we compared sympathetic innervation of normal and ALS muscles, through structural analysis of the sympathetic network in human and murine tissue samples.Methods and Results: We first refined tissue processing to circumvent methodological limitations interfering with the detection of muscle sympathetic innervation. The optimized “Neuro Detection Protocol” (NDP) was validated in human muscle biopsies, demonstrating that SNs innervate, at high density, both blood vessels and skeletal myofibers, independent of the fiber metabolic type. Subsequently, NDP was exploited to analyze sympathetic innervation in muscles of SOD1G93A mice, a preclinical ALS model. Our data show that ALS murine muscles display SN denervation, which has already initiated at the early disease stage and worsened during aging. SN degeneration was also observed in muscles of MLC/SOD1G93A mice, with muscle specific expression of the SOD1G93A mutant gene. Notably, similar alterations in SNs were observed in muscle biopsies from an ALS patient, carrying the SOD1G93A mutation.Conclusion: We set up a protocol for the analysis of murine and, more importantly, human muscle sympathetic innervation. Our results indicate that SNs are additional cell types compromised in ALS and suggest that dysfunctional SOD1G93A muscles affect their sympathetic innervation.

Sympathetic innervation in skeletal muscle and its role at the neuromuscular junction

“…Potential roles of sympathetic co-innervation at the NMJ in the etiology of neuromuscular disorders and in their therapy are manifold and have been discussed with respect to congenital myasthenic syndromes, myasthenia gravis, amyotrophic lateral sclerosis, spinal muscular atrophy, and sarcopenia in recent reviews (Cao et al 2020 ; Rodríguez Cruz et al 2020 ; Bukharaeva et al 2021 ; Delbono et al 2021 ; Mazzaro et al 2023 ; Ohno et al 2023 ). In many of these disorders, sympathomimetic drugs have been used for treatment, with variable success.…”