Muscle contracture in children with cerebral palsy: Mechanosensitive pathways and cellular dysfunction

Abstract

Muscle contractures, manifesting as stiffer and shorter muscles, impair muscle growth and function in children with cerebral palsy (CP).¹ The cellular mechanisms underlying these contractures are not well understood, particularly how muscle-generating satellite cells (MuSC) and fibro-adipogenic progenitors (FAP) respond to mechanical stimuli in CP's altered muscle environment. Current treatments for contractures focus on improving the mechanical properties of muscle tissue but overlook cellular dysfunctions. Bridging this gap requires understanding of the interplay between mechanical cues and the intrinsic properties of these cells, which is crucial for developing effective treatments for muscle dysfunction in CP.

Loomis et al.² investigated how substrate stiffness influences MuSC differentiation and FAP activation in contractured CP muscle compared to typically developing muscle. This study examined whether the mechanical properties of extracellular environment modulate the behavior of these cells and whether the contractured muscle environment had a lasting impact on their behavior, thereby providing insights into mechanisms behind muscle contractures in CP.

They found that MuSC isolated from contractured CP muscle consistently showed a lower capacity to fuse and form muscle fibers (myotubes) in vitro. Differentiation on stiffer substrates, which mimic pro-fibrotic muscle tissue, led to increased myonuclear clustering, particularly in CP MuSC. This clustering disrupted the even distribution of nuclei along myotubes, a phenomenon also observed by Corvelyn et al.³ in CP cultures, suggesting nuclear domain disruptions with stiffer environment. Additionally, FAP activation into myofibroblasts was significantly higher in CP cells at all substrate stiffnesses, suggesting a contribution of FAP to alterations in the extracellular matrix characteristics of contractured CP muscle. At the molecular level, lower YAP nuclear localization in CP cells indicated disruptions in mechanosensitive pathways. Attempts to restore regenerative capacity using verteporfin (a YAP/TAZ inhibitor and promoter of differentiation) showed limited effectiveness, highlighting the challenge of pharmacologically correcting these dysfunctions. These findings underscore the need to better understand the mechanobiology of muscle stem cells for potential muscle rehabilitation in CP.

The finding that MuSC fusion was not significantly influenced by hydrogel substrate stiffness contrasts with earlier studies that suggested optimal myotube differentiation on substrates mimicking healthy muscle stiffness (around 10–12 kPa).⁴ Loomis et al. used a broad range of substrate stiffnesses without observing the expected differentiation patterns. Differences in experimental setups (such as using primary MuSC and FAP from genetically heterogeneous human tissue vs more homogeneous murine cells in other studies) and variations in substrate coatings (collagen I vs laminin) could explain some of these discrepancies.

Overall, Loomis et al.'s findings suggest that altering the mechanical environment alone may not address MuSC and FAP dysfunctions in CP. Sibley et al.⁵ demonstrated that MuSC isolated from CP contractured muscle retain intrinsic epigenomic imprints that ‘lock away’ genetic programs for muscle formation and homeostasis. Effective treatments must consider these intrinsic cellular properties and find ways to reactivate genetic programs for improved muscle growth. Multimodal interventions that modulate mechanosensing pathways and address epigenetic modifications may be necessary to restore normal functionality. This approach could lead to better management of muscle contractures by addressing root causes rather than symptoms.

Future research should analyze the interactions and intrinsic properties of mononucleated cells in contractured CP muscle tissue, including MuSC and FAP. Advanced techniques like single-cell RNA sequencing, DNA methylome analysis, and ATAC-seq for chromatin accessibility can provide deeper insights into the regulatory mechanisms affecting MuSC and FAP function in CP. Exploring substrate stiffnesses that mimic pathological conditions and using co-cultures of MuSC and FAP can offer new perspectives on optimal conditions for muscle fiber formation. Addressing these areas could pave the way for novel therapeutic approaches to manage and potentially reverse muscle contractures in CP.