Fibroadipogenic Progenitors Regulate the Basal Proliferation of Satellite Cells and Homeostasis of Pharyngeal Muscles via HGF Secretion

Fibro-Adipogenic Progenitors require autocrine IGF-I in homeostatic and regenerating skeletal muscle

Fibro-Adipogenic Progenitors (FAPs) are mesenchymal stem cells that are vital for muscle homeostasis and regeneration but produce fibrosis and intramuscular fat under pathological conditions. Insulin-like Growth Factor-I (IGF-I) is a key regulator of muscle repair, satellite cell activity, macrophage polarization, and extracellular matrix (ECM) remodeling. We generated inducible FAP-specific Igf1 deficient (FID) mice to determine the necessity of FAP IGF-I. After BaCl 2 injury, FID mice exhibited impaired muscle regeneration, with fewer Pax7+ cells, increased macrophage accumulation, smaller fibers, reduced ECM, and depressed FAP proliferation. Following glycerol injury, FID muscles exhibited reduced adipocyte accumulation. Primary FAPs isolated from injured FID muscles had blunted growth, upregulation of immune-regulatory genes and downregulation of ECM and cell proliferation genes, with delayed responses to fibrogenic and to adipogenic media. FAP property alterations were already present in homeostatic muscle, indicated by scRNASeq, with decreased indices of protein translation and ECM production as well as increased markers of senescence, confirmed in vivo and in vitro . Overall, FAP IGF-I is a critical autocrine factor, with further paracrine consequences for muscle regenerative capacity.

Optimizing the Host Niche to Fuel Cleft Lip Muscle Regeneration

OBJECTIVE

Lip muscle reconstruction is a key strategy for cleft lip repair, but the outcome of muscle regeneration is suboptimal. The lack of relevant models restricts studies on cleft lip muscle regeneration.

DESIGN

In this study, we developed an animal xenograft model by transplanting muscle samples from patients with cleft lip into different host sites in immunodeficient mice. The grafted muscles were harvested after 1, 2, and 5 months to investigate the temporal dynamics of myofiber growth and maturation. Comparisons were made among muscle biopsies, xenografts in the masseter (MAS) muscle of the mice, and xenografts in the tibialis anterior (TA) muscle of the mice to determine the optimal host niche.

Histological analysis of myofiber number and size, fiber type switching, innervation, and blood supply was performed to evaluate the cleft lip muscle regeneration process.

RESULTS

The muscles from cleft lip patients underwent an active degeneration-regeneration cycle. The fiber diameter in the MAS niche was significantly larger than that in the TA niche and was comparable to the fiber diameter of the donor biopsy from which it originated. The innervation and blood supply of the muscle grafts at the MAS host site were also superior to those of the grafts at the TA host site.

CONCLUSIONS

The MAS muscles of mice provided the most favorable microenvironment for cleft lip muscle regeneration. This patient-centered xenograft model offers a platform for cleft lip muscle regeneration studies.

FAPs orchestrate homeostasis of muscle physiology and pathophysiology

As a common clinical manifestation, muscle weakness is prevalent in people with mobility disorders. Further studies of muscle weakness have found that patients with muscle weakness present with persistent muscle inflammation, loss of muscle fibers, fat infiltration, and interstitial fibrosis. Therefore, we propose the concept of muscle microenvironment homeostasis, which explains the abnormal pathological changes in muscles through the imbalance of muscle microenvironment homeostasis. And we identified an interstitial progenitor cell FAP during the transition from normal muscle microenvironment homeostasis to muscle microenvironment imbalance caused by muscle damage diseases. As a kind of pluripotent stem cell, FAPs do not participate in myogenic differentiation, but can differentiate into fibroblasts, adipocytes, osteoblasts, and chondrocytes. As a kind of mesenchymal progenitor cell, it is involved in the generation of extracellular matrix, regulate muscle regeneration, and maintain neuromuscular junction. However, the muscle microenvironment is disrupted by the causative factors, and the abnormal activities of FAPs eventually contribute to the complex pathological changes in muscles. Targeting the mechanisms of these muscle pathological changes, we have identified appropriate signaling targets for FAPs to improve and even treat muscle damage diseases. In this review, we propose the construction of muscle microenvironmental homeostasis and find the key cells that cause pathological changes in muscle after homeostasis is broken. By studying the mechanism of abnormal differentiation and apoptosis of FAPs, we found a strategy to inhibit the abnormal pathological changes in muscle damage diseases and improve muscle regeneration.

Novel insights from human induced pluripotent stem cells on origins and roles of fibro/adipogenic progenitors as heterotopic ossification precursors

Fibro/adipogenic progenitors (FAPs) that reside in muscle tissue are crucial for muscular homeostasis and regeneration as they secrete signaling molecules and components of the extracellular matrix. During injury or disease, FAPs differentiate into different cell types and significantly modulate muscular function. Recent advances in lineage tracing and single-cell transcriptomics have proven that FAPs are heterogeneous both in resting and post-injury or disease states. Their heterogeneity may be owing to the varied tissue microenvironments and their diverse developmental origins. Therefore, understanding FAPs' developmental origins can help predict their characteristics and behaviors under different conditions. FAPs are thought to be the major cell populations in the muscle connective tissue (MCT). During embryogenesis, the MCT directs muscular development throughout the body and serves as a prepattern for muscular morphogenesis. The developmental origins of FAPs as stromal cells in the MCT were studied previously. In adult tissues, FAPs are important precursors for heterotopic ossification, especially in the context of the rare genetic disorder fibrodysplasia ossificans progressiva. A new developmental origin for FAPs have been suggested that differs from conventional developmental perspectives. In this review, we summarize the developmental origins and functions of FAPs as stromal cells of the MCT and present novel insights obtained by using patient-derived induced pluripotent stem cells and mouse models of heterotopic ossification. This review broadens the current understanding of FAPs and suggests potential avenues for further investigation.

Pharyngeal pathology in a mouse model of oculopharyngeal muscular dystrophy is associated with impaired basal autophagy in myoblasts

Oculopharyngeal muscular dystrophy (OPMD) is a late-onset dominant disease that primarily affects craniofacial muscles. Despite the fact that the genetic cause of OPMD is known to be expansion mutations in the gene encoding the nuclear polyadenosine RNA binding protein PABPN1, the molecular mechanisms of pathology are unknown and no pharmacologic treatments are available. Due to the limited availability of patient tissues, several animal models have been employed to study the pathology of OPMD. However, none of these models have demonstrated functional deficits in the muscles of the pharynx, which are predominantly affected by OPMD. Here, we used a knock-in mouse model of OPMD, Pabpn1 +/A17 , that closely genocopies patients. In Pabpn1 +/A17 mice, we detected impaired pharyngeal muscle function, and impaired pharyngeal satellite cell proliferation and fusion. Molecular studies revealed that basal autophagy, which is required for normal satellite cell function, is higher in pharynx-derived myoblasts than in myoblasts derived from limb muscles. Interestingly, basal autophagy is impaired in cells derived from Pabpn1 +/A17 mice. Pabpn1 knockdown in pharyngeal myoblasts failed to recapitulate the autophagy defect detected in Pabpn1 +/A17 myoblasts suggesting that loss of PABPN1 function does not contribute to the basal autophagy defect. Taken together, these studies provide the first evidence for pharyngeal muscle and satellite cell pathology in a mouse model of OPMD and suggest that aberrant gain of PABPN1 function contributes to the craniofacial pathology in OPMD.

Muscle fibro-adipogenic progenitors from a single-cell perspective: Focus on their “virtual” secretome

Skeletal muscle is a highly plastic tissue composed of a number of heterogeneous cell populations that, by interacting and communicating with each other, participate to the muscle homeostasis, and orchestrate regeneration and repair in healthy and diseased conditions. Although muscle regeneration relies on the activity of muscle stem cells (MuSCs), many other cellular players such as inflammatory, vascular and tissue-resident mesenchymal cells participate and communicate with MuSCs to sustain the regenerative process. Among them, Fibro-Adipogenic Progenitors (FAPs), a muscle interstitial stromal population, are crucial actors during muscle homeostasis and regeneration, interacting with MuSCs and other cellular players and dynamically producing and remodelling the extra-cellular matrix. Recent emerging single-cell omics technologies have resulted in the dissection of the heterogeneity of each cell populations within skeletal muscle. In this perspective we have reviewed the recent single-cell omics studies with a specific focus on FAPs in mouse and human muscle. More precisely, using the OutCyte prediction tool, we analysed the “virtual” secretome of FAPs, in resting and regenerating conditions, to highlight the potential of RNAseq data for the study of cellular communication.