Activation of β-catenin in mesenchymal progenitors leads to muscle mass loss

Integrating postsynaptic morphology and dynamics to evaluate neuromuscular synapse status: Insights from α-bungarotoxin

The neuromuscular junction (NMJ) is a crucial peripheral synapse that controls muscle contraction. It consists of a presynaptic motor terminal, a postsynaptic muscle domain, and associated cells, such as terminal Schwann cells and kranocytes. Its larger size compared to central synapses has allowed detailed analyses of NMJ morphology that have been widely used as a reliable parameter of synaptic formation, maturation, function, and decline. Due to its high affinity for postsynaptic acetylcholine receptors (AChRs), the snake venom-derived α-bungarotoxin (BTX) has been pivotal in advancing our understanding of NMJ organization, enabling a detailed mapping of postsynaptic morphologies associated to distinct functional outcomes. Although certain morphological features are often associated with NMJ worsening, some of these cellular changes also occur in biological contexts where synaptic function remains intact. In this review, we draw on previous studies and our recent findings using BTX-based pulse-chase assays to suggest that combining morphological analyses with assessments of postsynaptic stability offers a more comprehensive understanding of NMJ function and regenerative potential. We propose that integrating diverse BTX-based tools into studies of NMJ morphology and stability will provide particularly valuable insights in contexts such as aging, injury, and neuromuscular diseases, where these combined parameters may serve as robust predictors of functional outcomes.

Exploring the therapeutic potential of fibroadipogenic progenitors in muscle disease

Skeletal muscle relies on its inherent self-repair ability to withstand continuous mechanical damage. Myofiber-intrinsic processes facilitate the repair of damage to sarcolemma and sarcomeres, but it is the coordinated interaction between muscle-resident satellite and stromal cells that are crucial in the regeneration of muscles to replace the lost muscle fibers. Fibroadipogenic progenitors (FAPs), are muscle-resident mesenchymal cells that are notable for their role in creating the dynamic stromal niche required to support long-term muscle homeostasis and regeneration. While FAP-mediated extracellular matrix formation and the establishment of a homeostatic muscle niche are essential for maintaining muscle health, excessive accumulation of FAPs and their aberrant differentiation leads to the fibrofatty degeneration that is a hallmark of myopathies and muscular dystrophies. Recent advancements, including single-cell RNA sequencing and in vivo analysis of FAPs, are providing deeper insights into the functions and specialization of FAPs, shedding light on their roles in both health and disease. This review will explore the above insights, discussing how FAP dysregulation contributes to muscle diseases. It will offer a concise overview of potential therapeutic interventions targeting FAPs to restore disrupted interactions among FAPs and muscle-resident cells, ultimately addressing degenerative muscle loss in neuromuscular diseases.

Ubiquitination Insight from Spinal Muscular Atrophy-From Pathogenesis to Therapy: A Muscle Perspective

Spinal muscular atrophy (SMA) is one of the most frequent causes of death in childhood. The disease's molecular basis is deletion or mutations in the SMN1 gene, which produces reduced survival motor neuron protein (SMN) levels. As a result, there is spinal motor neuron degeneration and a large increase in muscle atrophy, in which the ubiquitin-proteasome system (UPS) plays a significant role. In humans, a paralogue of SMN1, SMN2 encodes the truncated protein SMNΔ7. Structural differences between SMN and SMNΔ7 affect the interaction of the proteins with UPS and decrease the stability of the truncated protein. SMN loss affects the general ubiquitination process by lowering the levels of UBA1, one of the main enzymes in the ubiquitination process. We discuss how SMN loss affects both SMN stability and the general ubiquitination process, and how the proteins involved in ubiquitination could be used as future targets for SMA treatment.

Fibro-adipogenic progenitors in physiological adipogenesis and intermuscular adipose tissue remodeling

Excessive accumulation of intermuscular adipose tissue (IMAT) is a common pathological feature in various metabolic and health conditions and can cause muscle atrophy, reduced function, inflammation, insulin resistance, cardiovascular issues, and unhealthy aging. Although IMAT results from fat accumulation in muscle, the mechanisms underlying its onset, development, cellular components, and functions remain unclear. IMAT levels are influenced by several factors, such as changes in the tissue environment, muscle type and origin, extent and duration of trauma, and persistent activation of fibro-adipogenic progenitors (FAPs). FAPs are a diverse and transcriptionally heterogeneous population of stromal cells essential for tissue maintenance, neuromuscular stability, and tissue regeneration. However, in cases of chronic inflammation and pathological conditions, FAPs expand and differentiate into adipocytes, resulting in the development of abnormal and ectopic IMAT. This review discusses the role of FAPs in adipogenesis and how they remodel IMAT. It highlights evidence supporting FAPs and FAP-derived adipocytes as constituents of IMAT, emphasizing their significance in adipose tissue maintenance and development, as well as their involvement in metabolic disorders, chronic pathologies and diseases. We also investigated the intricate molecular pathways and cell interactions governing FAP behavior, adipogenesis, and IMAT accumulation in chronic diseases and muscle deconditioning. Finally, we hypothesize that impaired cellular metabolic flexibility in dysfunctional muscles impacts FAPs, leading to IMAT. A deeper understanding of the biology of IMAT accumulation and the mechanisms regulating FAP behavior and fate are essential for the development of new therapeutic strategies for several debilitating conditions.

Thbs1 regulates skeletal muscle mass in a TGFβ-Smad2/3-ATF4-dependent manner

Loss of muscle mass is a feature of chronic illness and aging. Here, we report that skeletal muscle-specific thrombospondin-1 transgenic mice (Thbs1 Tg) have profound muscle atrophy with age-dependent decreases in exercise capacity and premature lethality. Mechanistically, Thbs1 activates transforming growth factor β (TGFβ)-Smad2/3 signaling, which also induces activating transcription factor 4 (ATF4) expression that together modulates the autophagy-lysosomal pathway (ALP) and ubiquitin-proteasome system (UPS) to facilitate muscle atrophy. Indeed, myofiber-specific inhibition of TGFβ-receptor signaling represses the induction of ATF4, normalizes ALP and UPS, and partially restores muscle mass in Thbs1 Tg mice. Similarly, myofiber-specific deletion of Smad2 and Smad3 or the Atf4 gene antagonizes Thbs1-induced muscle atrophy. More importantly, Thbs1-/- mice show significantly reduced levels of denervation- and caloric restriction-mediated muscle atrophy, along with blunted TGFβ-Smad3-ATF4 signaling. Thus, Thbs1-mediated TGFβ-Smad3-ATF4 signaling in skeletal muscle regulates tissue rarefaction, suggesting a target for atrophy-based muscle diseases and sarcopenia with aging.

Skeletal muscle niche, at the crossroad of cell/cell communications

Skeletal muscle is composed of a variety of tissue and non-tissue resident cells that participate in homeostasis. In particular, the muscle stem cell niche is a dynamic system, requiring direct and indirect communications between cells, involving local and remote cues. Interactions within the niche must happen in a timely manner for the maintenance or recovery of the homeostatic niche. For instance, after an injury, pro-myogenic cues delivered too early will impact on muscle stem cell proliferation, delaying the repair process. Within the niche, myofibers, endothelial cells, perivascular cells (pericytes, smooth muscle cells), fibro-adipogenic progenitors, fibroblasts, and immune cells are in close proximity with each other. Each cell behavior, membrane profile, and secretome can interfere with muscle stem cell fate and skeletal muscle regeneration. On top of that, the muscle stem cell niche can also be modified by extra-muscle (remote) cues, as other tissues may act on muscle regeneration via the production of circulating factors or the delivery of cells. In this review, we highlight recent publications evidencing both local and remote effectors of the muscle stem cell niche.

Pro-regenerative biomaterials recruit immunoregulatory dendritic cells after traumatic injury

During wound healing and surgical implantation, the body establishes a delicate balance between immune activation to fight off infection and clear debris and immune tolerance to control reactivity against self-tissue. Nonetheless, how such a balance is achieved is not well understood. Here we describe that pro-regenerative biomaterials for muscle injury treatment promote the proliferation of a BATF3-dependent CD103+XCR1+CD206+CD301b+ dendritic cell population associated with cross-presentation and self-tolerance. Upregulation of E-cadherin, the ligand for CD103, and XCL-1 in injured tissue suggests a mechanism for cell recruitment to trauma. Muscle injury recruited natural killer cells that produced Xcl1 when stimulated with fragmented extracellular matrix. Without cross-presenting cells, T-cell activation increases, pro-regenerative macrophage polarization decreases and there are alterations in myogenesis, adipogenesis, fibrosis and increased muscle calcification. These results, previously observed in cancer progression, suggest a fundamental mechanism of immune regulation in trauma and material implantation with implications for both short- and long-term injury recovery.

DPPIV+ fibro-adipogenic progenitors form the niche of adult skeletal muscle self-renewing resident macrophages

Adult tissue-resident macrophages (RMs) are either maintained by blood monocytes or through self-renewal. While the presence of a nurturing niche is likely crucial to support the survival and function of self-renewing RMs, evidence regarding its nature is limited. Here, we identify fibro-adipogenic progenitors (FAPs) as the main source of colony-stimulating factor 1 (CSF1) in resting skeletal muscle. Using parabiosis in combination with FAP-deficient transgenic mice (PdgfrαCreERT2 × DTA) or mice lacking FAP-derived CSF1 (PdgfrαCreERT2 × Csf1flox/null), we show that local CSF1 from FAPs is required for the survival of both TIM4- monocyte-derived and TIM4+ self-renewing RMs in adult skeletal muscle. The spatial distribution and number of TIM4+ RMs coincide with those of dipeptidyl peptidase IV (DPPIV)+ FAPs, suggesting their role as CSF1-producing niche cells for self-renewing RMs. This finding identifies opportunities to precisely manipulate the function of self-renewing RMs in situ to further unravel their role in health and disease.