Neuromuscular Junction Remodeling: the Role of Sub-Synaptic Nuclei, Terminal Schwann Cells, and Trophic Signaling Following Neuromusculoskeletal Injuries

Abstract

Peripheral nerve crush injury is a well-established model of neuromuscular junction (NMJ) denervation and subsequent re-innervation. Functionally, the muscle tissue follows a similar pattern as neural recovery, with immediate loss of force production that steadily improves in parallel with rates of re-innervation. On the other hand, traumatic injury to the muscle tissue itself, specifically volumetric muscle loss (VML), results in an irrecoverable loss of muscle function. Recent work has indicated significant impairments to the NMJ following VML that appear chronic in nature, alongside functional recovery. Thus, the goal of this study was to compare the effects of nerve and muscle injury on NMJ remodeling, with the hypothesis that VML will result in chronic alterations to the NMJ, including a higher degree of cellular localization to the NMJ. Even numbers of adult male and female mice were used with three experimental groups: injury naive, nerve crush, and VML-injury; and three terminal timepoints: 3-, 48-, and 112-days post injury. Data were analyzed by three-way ANOVA with Tukey’s HSD post-hoc. Confirming the assumed recoverability of the two injury models, we found in vivo maximal torque was fully restored following nerve crush injury (Tukey’s HSD post hoc p=0.730), but remained at a significant deficit following VML (~41% lower than injury naïve; Tukey’s HSD post hoc p<0.001). Compared to nerve crush and injury naïve, we found VML results in aberrantly high trophic signaling and numbers of supporting cells. Protein expression of neuregulin-1 and connective tissue growth factor were both highest in the VML group (~1.2-fold and 12-fold increase compared to injury naïve, respectively; Main effect of group, p<0.001). While nerve crush injury elicited a transient increase in terminal Schwann cell numbers (~38% higher than injury naïve at 48dpi; Tukey’s HSD post hoc p<0.001), VML resulted in a chronic increase (~53% and 27% higher than injury naïve at 48 and 112dpi, respectively; Tukey’s HSD post hoc p<0.001). Moreover, nerve crush injury did not alter the number of sub-synaptic nuclei per NMJ, while VML resulted in significant increases at both 48- and 112-days post injury (~27% and 24% higher than injury naïve, respectively; Tukey’s HSD post hoc p=0.002). In some cases, sex differences were detected, including higher rates of innervation in females than males (Main effect of sex p=0.003), particularly following VML-injury (Tukey’s HSD post hoc p=0.004). The findings herein bear a striking resemblance to NMJs found in dystrophic muscle, which are thought to be the result of continuous cycles of muscle fiber degeneration and regeneration. Thus, the NMJ changes observed following VML may be indicative of a larger issue, such as chronic muscle fiber remodeling and pervasive fibrotic accumulation. R01-AR078903 (JAC & SMG); K02-AG081488 (SMG). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.