Skeletal muscles from chronic stroke patients are fibrotic with decreased myogenic potential

Introduction: Skeletal muscle spasticity after stroke is debilitating often leading to loss of function and joint contracture. While the primary insult is clearly neural in nature, patients with stroke often require orthopedic procedures such as neurotoxin injection, contracture release, muscle lengthenings and nerve transfers. The actual changes to muscle that occur secondary to the neural insult are not known and may have significant therapeutic implications. Thus, the purpose of this study was to define structural and biological alterations in stroke muscles and contrast and compare these changes with those already reported for a different upper motor neuron (UMN) lesion, children with cerebral palsy (CP). Methods: Biopsies were obtained from flexor carpi ulnaris (FCU, n=11) and pronator teres (PT, n=12) muscles in patients with stroke during therapeutic surgery and from FCU (n=10) and PT (n=13) in control patients during standard submuscular ulnar nerve transposition. From fresh biopsies, flow assisted cell sorting (FACS) measured the percentage of mononuclear cells identified as satellite cells (SCs) using positive selection for CD29 (integrin b1 subunit) and CD56 (human NCAM). Then, a small fiber bundle was harvested at its in-situ length, Formalin-fixed and sarcomere length (SL) measured by laser diffraction 2 The remainder of the biopsy was flash frozen for hydroxyproline (HP) 3 and global DNA methylation (GDM) assays. Data were analyzed by two-way ANOVA with muscle (FCU vs. PT) and condition (stroke vs. control) as grouping factors followed by subsequent multiple t-tests to compare stroke and control muscles. Significance level was set to 0.05 for all statistical tests. Results: Both FCU and PT muscles demonstrated dramatic and highly significant increases collagen content with the magnitude being greater in the FCU (significant muscle x condition interaction term, p<0.001). The same differences were seen for SL and GDM. Surprisingly, SL was increased in both muscles reflecting a lossin serial sarcomere number, even though these patients were no longer growing (p<0.05). The magnitude of the SL increase was comparable to that reported using microendoscopy. Global DNA methylation was significantly greater in spastic muscles compared to controls (p<0.05) representing a large epigenetic effect, especially in PT, which apparently resulted in decreased myogenic potential for both muscles as reflected by a significant decrease in their SC’s ability to fuse into myotubes. Taken together, these data reveal dramatic structural and biological alterations in response to UMN even in the absence of a primary myopathy. Discussion: This study revealed quite dramatic changes in muscles from stroke patients due to UMN. Such muscle changes were interesting in light of the fact that the injury occurred to the brain. We propose that the UMN resulting from the stroke epigenetically modifies the peripheral skeletal muscle resulting in these important structural alterations in sarcomere length and collagen content. The functional effects of SL and collagen increases would be increased resting passive mechanical tension in the muscle that may indeed be causally related to the joint deformity and decreased skeletal muscle strength. In CP studies, we demonstrated epigenetic suppression of muscle genes that promote myogenesis and differentiation which ultimately leads to smaller muscle fibers, longer sarcomere lengths and tissue fibrosis. Interestingly, structural changes in stroke muscle parallel the changes observed in CP muscle and may point to a common epigenetic mechanism by which an UMN results in musculoskeletal deformity, even in the absence of growth. This work was supported by Research Career Scientist Award IK6 RX003351 from the United States (U.S.) Department of Veterans Affairs Rehabilitation R&D (Rehab RD) Service. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.