High-Resolution Spatial Transcriptomic Atlas of Mouse Soleus Muscle: Unveiling Single Cell and Subcellular Heterogeneity in Health and Denervation.

Skeletal muscle is essential for both movement and metabolic processes, characterized by a complex and ordered structure. Despite its importance, a detailed spatial map of gene expression within muscle tissue has been challenging to achieve due to the limitations of existing technologies, which struggle to provide high-resolution views. In this study, we leverage the Seq-Scope technique, an innovative method that allows for the observation of the entire transcriptome at an unprecedented submicron spatial resolution. By applying this technique to the mouse soleus muscle, we analyze and compare the gene expression profiles in both healthy conditions and following denervation, a process that mimics aspects of muscle aging. Our approach reveals detailed characteristics of muscle fibers, other cell types present within the muscle, and specific subcellular structures such as the postsynaptic nuclei at neuromuscular junctions, hybrid muscle fibers, and areas of localized expression of genes responsive to muscle injury, along with their histological context. The findings of this research significantly enhance our understanding of the diversity within the muscle cell transcriptome and its variation in response to denervation, a key factor in the decline of muscle function with age. This breakthrough in spatial transcriptomics not only deepens our knowledge of muscle biology but also sets the stage for the development of new therapeutic strategies aimed at mitigating the effects of aging on muscle health, thereby offering a more comprehensive insight into the mechanisms of muscle maintenance and degeneration in the context of aging and disease.

Maintenance of subsynaptic myonuclei number is not driven by neural input.

The development and maintenance of neuromuscular junctions (NMJ) are supported by a specialized population of myonuclei that are referred to as the subsynaptic myonuclei (SSM). The relationship between the number of SSM and the integrity of the NMJ as well as the impact of a loss of innervation on SSM remain unclear. This study aimed to clarify these associations by simultaneously analyzing SSM counts and NMJ innervation status in three distinct mouse models of acute and chronic NMJ disruption. SSM were identified using fluorescent immunohistochemistry for Nesprin1 expression, which is highly enriched in SSM, along with anatomical location beneath the muscle fiber motor endplate. Acute denervation, induced by surgical nerve transection, did not affect SSM number after 7 days. Additionally, no significant changes in SSM number were observed during normal aging or in mice with chronic oxidative stress (Sod1 -/-). Both aging WT mice and Sod1 -/- mice accumulated degenerating and denervated NMJ in skeletal muscle, but there was no correlation between innervation status of a given NMJ and SSM number in aged or Sod1 -/- mice. These findings challenge the notion that a loss of SSM is a primary driver of NMJ degradation and leave open questions of the mechanisms that regulate SSM number as well as the physiological significance of the precise SSM number. Further investigations are required to define other properties of the SSM, such as transcriptional profiles and structural integrity, to better understand their role in NMJ maintenance.

Skeletal muscle structure, physiology, and function.

Contractions of skeletal muscles provide the stability and power for all body movements. Consequently, any impairment in skeletal muscle function results in some degree of instability or immobility. Factors that influence skeletal muscle structure and function are therefore of great interest scientifically and clinically. Injury, neuromuscular disease, and old age are among the factors that commonly contribute to impairments in skeletal muscle function. The goal of this chapter is to summarize the fundamentals of skeletal muscle structure and function to provide foundational knowledge for this Handbook volume. We examine the molecular interactions that provide the basis for the generation of force and movement, discuss mechanisms of the regulation of contraction at the level of myofibers, and introduce concepts of the activation and control of muscle function in vivo. Where appropriate, the chapter updates the emerging science that will increase understanding of muscle function.

Decoding muscle-resident Schwann cell dynamics during neuromuscular junction remodeling.

Understanding neuromuscular junction (NMJ) repair mechanisms is essential for addressing degenerative neuromuscular conditions. Here, we focus on the role of muscle-resident Schwann cells in NMJ reinnervation. In young Sod1-/- mice, a model of progressive NMJ degeneration, we identified a clear NMJ 'regenerative window' that allowed us to define regulators of reinnervation and crossing Sod1-/- mice with S100GFP-tg mice permitted visualization and analysis of Schwann cells. High-resolution imaging and single-cell RNA sequencing provide a detailed analysis of Schwann cell number, morphology, and transcriptome revealing multiple subtypes, including a previously unrecognized terminal Schwann cell (tSC) population expressing a synapse promoting signature. We also discovered a novel SPP1-driven cellular interaction between myelin Schwann cells and tSCs and show that it promotes tSC proliferation and reinnervation following nerve injury in wild type mice. Our findings offer important insights into molecular regulators critical in NMJ reinnervation that are mediated through tSCs to maintain NMJ function.