Fibro-adipogenic Progenitors Research Articles

Differential Acute Adipogenic Signaling Changes Following Neuromusculoskeletal Injuries

Following traumatic injury, neuromusculoskeletal tissue undergoes extensive plastic modification. Changes can be observed at the local and systemic levels and can involve tissue function, signaling, and local composition. Currently there is limited understanding of early adipogenic changes following some traumatic injuries, which is needed to understand the long-term lack of functional muscle recovery. The objective of this project was to examine plasticity after injury, namely systemic and local adipogenic signaling and fat droplet deposition following volumetric muscle loss (VML) and denervation injuries. We hypothesized there would be differential changes in local and systemic adipogenic signaling, specifically changes in fat droplet accumulation following neuromusculoskeletal injuries. Adult C57B1/6J mice (n=36; 6 male, 6 female/group): posterior compartment VML, tibial nerve denervation, and naïve control. Both injury models target plantarflexor muscles. Mice were euthanized 72-hrs post-injury. In vivo muscle function was assessed, and gastrocnemius, liver, and serum were saved for analysis. Whole gastrocnemius muscles from a subset (n=4/group) were saved for histology, and five standardized sections were systematically obtained 1.5mm apart; muscle sections were stained with H&E and Oil-Red-O to evaluate total fiber number and fat droplet accumulation, respectively. Data were analyzed by two- and three-way ANOVA with Tukey’s HSD post hoc. Maximal torque declined ~81 and 98% following VML and denervation injury, respectively (main effect p<0.001), and VML-injured muscle was more fatigable than injury naïve (main effect p=0.001). Following denervation, liver insulin-like growth factor 1 (IGF-1) was increased ~52% compared to other groups, and was most pronounced in males (main effect injury p<0.001; sex p=0.021). VML injury resulted in a 4-fold higher muscle IGF-1 regardless of sex (interaction p=0.021), and greater muscle interleukin-6 (IL-6), which was more pronounced in females (main effect injury p=0.001; sex p=0.049). Platelet-derived factor growth receptor (PDFGRα) expression was diminished following VML injury in both sexes (interaction p=0.016); suggesting a decrease in fibro-adipogenic progenitors (FAPs) and an alternative origin responsible for increases in IGF-1 and IL-6 rather than FAPs. Serum leptin concentrations were blunted following both VML- and denervation-injury (main effect p=0.002), which may illicit alterations in the regenerative process, specifically the maintenance of muscle quality. Serum IGF-1 and IL-6 concentrations were not affected by injury (p≥0.164). Histological analysis revealed a significant decrease in total fiber number across the middle three sections of the VML-injured muscle (i.e., the section initially injured) compared to other groups (p≤0.034). Findings provide a foundation for characterizing adipocyte development in the muscle remaining after injury and identify therapeutic targets for rehabilitative and nutritional approaches to address contractile and metabolic impairments following injury. Future work should elucidate the time course of adipogenic signaling following these injuries, with particular interest in their relationship to changes in metabolic and contractile impairments. R01-AR078903 (JAC & SMG); University of Minnesota UROP (BAC). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Parathyroid hormone promotes induction of beige adipocytes and reversibly ameliorates muscle quality and atrophy following chronic rotator cuff tear in a rat model

Fatty infiltration (FI) and muscle atrophy (MA) in the rotator cuff muscles following rotator cuff tears (RCT) persist postrepair, increasing the risk of re-tears. Brown adipocyte-like "beige adipocytes" are expected to have a therapeutic effect on intramuscular FI and MA due to their lipolytic activity and the muscle regenerative effects of their secreted factors. However, whether parathyroid hormone (PTH) ameliorates the already advanced FI and MA remains unknown. Therefore, this study aimed to clarify whether PTH promotes the expression of beige adipocytes and ameliorates advanced FI and MA following chronic RCT in rats. Supraspinatus muscles were harvested from rats with chronic RCT after 4 or 8weeks of PTH treatment and compared to those in the control group or to those at the start of treatment. FI was assessed by Oil Red O staining, and the staining area was evaluated as a percentage of the muscle cross-sectional area. MA was evaluated by measuring muscle wet weight and cross-sectional area of muscle fiber. Beige adipocyte expression was evaluated by immunostaining for uncoupling protein 1 (UCP1). Fibro-adipogenic progenitors (FAPs) were separated from muscle-injured mice. We assessed whether PTH could diminish fat droplet accumulation by promoting the differentiation of FAPs into beige adipocytes. After 4 weeks, PTH reduced the area fraction of FI in the rat supraspinatus muscle following chronic RCT compared with that at the beginning of treatment (P=.028). In addition, PTH increased wet muscle mass (P<.001), and muscle fiber cross-sectional area (P=.018) compared with measurements at the start of treatment. PTH administration promoted the expression of UCP1, a beige adipocyte marker, in the supraspinatus muscle (P=.019). PTH increased gene expression of beige adipocyte-related markers and suppressed fat droplet accumulation even after adipogenic differentiation of FAPs (P=.004) but did not reduce fat droplets that had already accumulated in invitro experiments. PTH facilitated beige adipocyte expression and reversibly ameliorated muscle quality and atrophy following chronic RCT by hindering fat droplet accumulation and facilitating muscle regeneration. Therefore, PTH may be a medical treatment for FI and MA following RCT, leading to expanded rotator cuff repair indications.

Exercise in Chronic Kidney Disease Patients Does Not Significantly Alter Fibrosis in Quadriceps Muscle

Background: Chronic kidney disease (CKD) is commonly associated with decreased physical activity and muscle dysfunction. Muscles in CKD become fibrotic, containing excess extracellular matrix (ECM), particularly collagen. Collagen is cross-linked in the ECM, and increased cross-linking is often associated with fibrosis, stiffness, and reduced ECM remodeling. However, it is not known how exercise in patients with CKD may impact the extent of collagen cross-linking or fibrosis. Additionally, muscle fibrosis may impair muscle adaptations to exercise including, fiber size, fiber type, or vasculature. We predicted that collagen content and cross-linking would decrease, and there would be more Type I fibers and larger fiber size in the patients that were in the exercise group. Methods: Participants were randomized to exercise (n=15) or no exercise (n=5) groups in the Exercise Study Testing Enhanced Energetics of Mitochondria (ESTEEM) clinical trial of non-dialysis patients with CKD. The exercise program consisted of 35 min sessions, 3 times a week for 12 weeks consisting of high-intensity interval training, strength training, and power walking. Vastus lateralis biopsies were obtained before and after the 12 weeks of exercise. A bundle of muscle tissue was powdered and separated into a pepsin soluble and insoluble fraction before applying a hydroxyproline assay for collagen content, with the insoluble fraction representing more cross-linked collagen. For histological analysis 10 μm thick cryosections were stained with Picrosirius Red, or immunofluorescent stains to investigate satellite cells, vasculature, fibro-adipogenic progenitors (FAPs), and muscle fiber types. Picrosirius red sections were imaged under polarized light to look at birefringence as a measure of collagen density. Sections that were stained for fiber type and vasculature were analyzed using semi-automatic muscle analysis using segmentation of histology (SMASH). Results: The overall amount of collagen and collagen cross-linking were not altered in either the exercise or no exercise groups. Picrosirius red stained sections did not reveal a change in ECM area or collagen packing density with exercise. Surprisingly minimum ferret diameter and percentage of type I (slow twitch) fibers decreased in most patients that underwent the exercise program. Type I and type IIa (fast oxidative) fiber size also significantly decreased, while there was no change in type IIx (fast glycolytic) fiber size. Conclusion: This study shows that exercise does not dramatically reduce ECM content or muscle fibrosis in patients with CKD. Surprisingly, the response to exercise was opposite of that anticipated in healthy subject with exercise in CKD producing smaller muscle fibers and a shift away from more oxidative type I fibers. This shows that exercise may have limited capacity to enhance function in CKD muscle, potentially due to irreversible fibrosis. Work supported by mitochondrial bioenergetics in persons with chronic kidney disease (NIH R03DK114502). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Parathyroid hormone inhibits fatty infiltration and muscle atrophy after rotator cuff tear by fibro-adipogenic progenitors browning in a rodent model

Parathyroid hormone inhibits fatty infiltration and muscle atrophy after rotator cuff tear by fibro-adipogenic progenitors browning in a rodent model

Single-Nuclei RNA-Sequencing Uncovers Alterations in the Transcriptional Profile of Skeletal Muscle in Peripheral Artery Disease

BACKGROUND: Lower extremity peripheral artery disease (PAD) is caused by atherosclerotic occlusion of lower limb blood vessels, impacting &gt;200 million people worldwide. PAD causes life-altering symptomatology such as claudication, ischemic rest pain, and gangrene, with limited effective treatment options. Herein, we performed single-nuclei RNA-sequencing (snRNAseq) to facilitate a better understanding of the complex pathobiological environment within the PAD limb and intercellular communication. METHODS: Gastrocnemius muscle specimens were collected from 20 patients with PAD and 12 non-PAD controls. Patients with PAD were classified as Rutherford category 3 (severe claudication). Non-PAD participants were of similar age, body mass index and medication usage as patients with PAD. Nuclei were isolated and snRNAseq was performed. Bioinformatic analysis was conducted to determine differences in gene expression between PAD and non-PAD nuclei, as well as analysis of intercellular signaling networks. Histological analysis of muscle specimens assessed the following: myosin heavy chain fiber distributions, myofiber cross-sectional area and myofiber denervation. RESULTS: 57,364 high-quality nuclei were captured (21,417 PAD; 35,947 non-PAD) and unsupervised clustering revealed 12 distinct nuclei populations based on their transcriptional profile. Quantification of the relative abundance of nuclei populations identified several differences, including PAD muscles having a lower percentage of type I myonuclei, muscle stem cells, and endothelial cells but a higher proportion of type II and regenerating myonuclei. Differentially expressed gene analysis of myonuclei populations uncovered upregulation of genes involved in stress response, autophagy, and atrophy in patients with PAD. Ligand-receptor interactions were used to determine how PAD alters the communication between identified nuclei populations. The inferred strength of interactions was greater in non-PAD compared to PAD patients. Further, intercellular communication network analysis revealed fibro-adipogenic progenitors as a primary signaling hub in PAD muscle. CONCLUSION: snRNAseq uncovered novel transcriptional differences and intercellular communication pathways unique to PAD muscle. This study was supported by National Institutes of Health (NIH) grant R01-HL149704 (T. Ryan). K. Kim supported by the American Heart Association grant POST903198. T. Thome supported by NIH grant F31-DK128920. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

HDACi vorinostat protects muscle from degeneration after acute rotator cuff injury in mice.

Rotator cuff (RC) injuries are characterized by tendon rupture, muscle atrophy, retraction, and fatty infiltration, which increase injury severity and jeopardize adequate tendon repair. Epigenetic drugs, such as histone deacetylase inhibitors (HDACis), possess the capacity to redefine the molecular signature of cells, and they may have the potential to inhibit the transformation of the fibro-adipogenic progenitors (FAPs) within the skeletal muscle into adipocyte-like cells, concurrently enhancing the myogenic potential of the satellite cells. HDACis were added to FAPs and satellite cell cultures isolated from mice. The HDACi vorinostat was additionally administered into a RC injury animal model. Histological analysis was carried out on the isolated supra- and infraspinatus muscles to assess vorinostat anti-muscle degeneration potential. Vorinostat, a HDACi compound, blocked the adipogenic transformation of muscle-associated FAPs in culture, promoting myogenic progression of the satellite cells. Furthermore, it protected muscle from degeneration after acute RC in mice in the earlier muscle degenerative stage after tenotomy. The HDACi vorinostat may be a candidate to prevent early muscular degeneration after RC injury.

Tracking of Nascent Matrix Deposition during Muscle Stem Cell Activation across Lifespan Using Engineered Hydrogels.

Adult stem cells occupy a niche that contributes to their function, but how stem cells rebuild their microenvironment after injury remains an open-ended question. Herein, biomaterial-based systems and metabolic labeling are utilized to evaluate how skeletal muscle stem cells deposit extracellular matrix. Muscle stem cells and committed myoblasts are observed to generate less nascent matrix than muscle resident fibro-adipogenic progenitors. When cultured on substrates that matched the stiffness of physiological uninjured and injured muscles, muscle stem cells increased nascent matrix deposition with activation kinetics. Reducing the ability to deposit nascent matrix by an inhibitor of vesicle trafficking (Exo-1) attenuated muscle stem cell function and mimicked impairments observed from muscle stem cells isolated from old muscles. Old muscle stem cells are observed to deposit less nascent matrix than young muscle stem cells, which is rescued with therapeutic supplementation of insulin-like growth factors. These results highlight the role of nascent matrix production with muscle stem cell activation.

The Impact of FBN1-α5β1 Axis in Fibro/Adipogenic Progenitor Cells (FAPCD9-) on Intramuscular Fat Content in Pigs.

Intramuscular fat (IMF) plays a crucial role in enhancing meat quality, enriching meat flavor, and overall improving palatability. In this study, Single-cell RNA sequencing was employed to analyze the longissimus dorsi (LD) obtained from Guangdong small-ear spotted pigs (GDSS, with high IMF) and Yorkshire pigs (YK, with low IMF). GDSS had significantly more Fibro/Adipogenic Progenitor (FAPs), in which the CD9 negative FAPs (FAPCD9-) having adipogenic potential, as demonstrated by in vitro assays using cells originated from mouse muscle. On the other hand, Yorkshire had more fibro-inflammatory progenitors (FIPs, marked with FAPCD9+), presenting higher expression of the FBN1-Integrin α5β1. FBN1-Integrin α5β1 could inhibit insulin signaling in FAPCD9-, suppressing adipogenic differentiation. Our results demonstrated that fat-type pigs possess a greater number of FAPCD9-, which are the exclusive cells in muscle capable of differentiating into adipocytes. Moreover, lean-type pigs exhibit higher expression of FBN1-Integrin α5β1 axis, which inhibits adipocyte differentiation. These results appropriately explain the observed higher IMF content in fat-type pigs.

Metformin mitigates adipogenesis of fibro-adipogenic progenitors after rotator cuff tears via activating mTOR/ULK1-mediated autophagy.

Muscular fatty infiltration is a common issue after rotator cuff tears (RCTs), which impair shoulder function. Females suffer a higher prevalence and a more severe degree of muscular fatty infiltration after RCT when compared with males, with the underlying mechanisms remaining unclear. Fibro-adipogenic progenitors (FAPs) are the primary source of muscular fatty infiltration following RCT. Our findings disclose that gender-specific disparities in muscular fatty infiltration are linked to mTOR/ULK1-mediated autophagy of FAPs. Decreased autophagic activity contributes to adipogenic differentiation in female FAPs after RCT. Furthermore, metformin could enhance mTOR/ULK1-mediated autophagic processes of FAPs, thereby alleviating fatty infiltration and improving shoulder functionality after RCT. Together, our study reveals that gender differences in muscular fatty infiltration arise from distinct autophagic activities. Metformin could be a promising noninvasive intervention to ameliorate muscular fatty infiltration of RCT.NEW & NOTEWORTHY The current study demonstrated that gender-specific disparities in muscular fatty infiltration are attributed to mTOR/ULK1-mediated autophagy of FAPs. Decreased autophagic activity contributes to adipogenic differentiation in female FAPs after RCT. Moreover, metformin could enhance mTOR/ULK1-mediated autophagic processes of FAPs, thereby alleviating fatty infiltration and improving shoulder functionality after RCT. Therefore, metformin could be a promising noninvasive intervention to ameliorate muscular fatty infiltration of RCT.

Cellular and Molecular Mechanisms of Heterotopic Ossification in Fibrodysplasia Ossificans Progressiva.

Fibrodysplasia ossificans progressiva (FOP) is a debilitating genetic disorder characterized by recurrent episodes of heterotopic ossification (HO) formation in muscles, tendons, and ligaments. FOP is caused by a missense mutation in the ACVR1 gene (activin A receptor type I), an important signaling receptor involved in endochondral ossification. The ACVR1R206H mutation induces increased downstream canonical SMAD-signaling and drives tissue-resident progenitor cells with osteogenic potential to participate in endochondral HO formation. In this article, we review aberrant ACVR1R206H signaling and the cells that give rise to HO in FOP. FOP mouse models and lineage tracing analyses have been used to provide strong evidence for tissue-resident mesenchymal cells as cellular contributors to HO. We assess how the underlying mutation in FOP disrupts muscle-specific dynamics during homeostasis and repair, with a focus on muscle-resident mesenchymal cells known as fibro-adipogenic progenitors (FAPs). Accumulating research points to FAPs as a prominent HO progenitor population, with ACVR1R206H FAPs not only aberrantly differentiating into chondro-osteogenic lineages but creating a permissive environment for bone formation at the expense of muscle regeneration. We will further discuss the emerging role of ACVR1R206H FAPs in muscle regeneration and therapeutic targeting of these cells to reduce HO formation in FOP.

Dispase/collagenase cocktail allows for coisolation of satellite cells and fibroadipogenic progenitors from human skeletal muscle.

Satellite cells (SCs) and fibroadipogenic progenitors (FAPs) are progenitor populations found in muscle that form new myofibers postinjury. Muscle development, regeneration, and tissue-engineering experiments require robust progenitor populations, yet their isolation and expansion are difficult given their scarcity in muscle, limited muscle biopsy sizes in humans, and lack of methodological detail in the literature. Here, we investigated whether a dispase and collagenase type 1 and 2 cocktail could allow dual isolation of SCs and FAPs, enabling significantly increased yield from human skeletal muscle. Postdissociation, we found that single cells could be sorted into CD56 + CD31-CD45- (SC) and CD56-CD31-CD45- (FAP) cell populations, expanded in culture, and characterized for lineage-specific marker expression and differentiation capacity; we obtained ∼10% SCs and ∼40% FAPs, with yields twofold better than what is reported in current literature. SCs were PAX7+ and retained CD56 expression and myogenic fusion potential after multiple passages, expanding up to 1012 cells. Conversely, FAPs expressed CD140a and differentiated into either fibroblasts or adipocytes upon induction. This study demonstrates robust isolation of both SCs and FAPs from the same muscle sample with SC recovery more than two times higher than previously reported, which could enable translational studies for muscle injuries.NEW & NOTEWORTHY We demonstrated that a dispase/collagenase cocktail allows for simultaneous isolation of SCs and FAPs with 2× higher SC yield compared with other studies. We provide a thorough characterization of SC and FAP in vitro expansion that other studies have not reported. Following our dissociation, SCs and FAPs were able to expand by up to 1012 cells before reaching senescence and maintained differentiation capacity in vitro demonstrating their efficacy for clinical translation for muscle injury.

Abstract 1601: The role of complement in pancreatic cancer cachexia

Abstract Cancer cachexia is a multifactorial condition characterized by skeletal muscle atrophy and dysfunction that impairs longevity and quality of life for the vast majority of cancer patients. Recently, we discovered through unbiased proteomic analyses that the complement system is activated in the skeletal muscle of cachectic pancreatic cancer patients. Therefore, the purpose of this study was to investigate the role of complement in pancreatic cancer cachexia. To accomplish this, we obtained genetically modified, C57Bl6/J congenic mice lacking the central regulator of complement, C3 (C3−/− mice). Wild-type (C57Bl6/J, WT, n=12) and C3−/− mice (n=11) (male, 12 weeks old) received orthotopic injections of mouse pancreatic cancer cells (KPC) into the pancreas, and as non-cancer controls, WT (n=8) and C3−/− (n=4) mice received injections of PBS into the pancreas. Mice were monitored daily and euthanized at IACUC-mandated tumor endpoint (body condition score ≤ 2; range: 14-15 days). In-vivo diaphragm function was assessed immediately prior to euthanasia via M-mode ultrasound. Skeletal muscles (soleus, SOL; tibialis anterior, TA; diaphragm, DIA) were harvested and ex-vivo contractile function was assessed on strips of costal diaphragm. Diaphragmatic excursion and respiratory rate were reduced with KPC tumor burden in WT mice, but preserved in C3−/− mice. Deletion of C3 did not influence the KPC-induced reduction of body mass or KPC tumor growth. Atrophy of limb muscle mass (SOL, TA) and fiber size (TA) during KPC tumor burden was attenuated in C3−/− mice. Maximal specific force of DIA was greater in C3−/− versus WT KPC-bearing mice. KPC tumor burden increased collagen content in the DIA of WT mice, but not C3−/− mice. Diaphragm fiber size was reduced in WT KPC-bearing mice, but preserved in C3−/− mice. KPC tumor burden induced significant increases in immune cell (CD45+) infiltration and fibroadipogenic progenitor cell (PDGFRa+) abundance in the DIA of WT tumor-bearing mice, but to a lesser extent in C3−/− mice. These findings suggest complement activation is causative in pancreatic cancer-induced skeletal muscle wasting and dysfunction. Inhibition of complement during cancer cachexia may possess therapeutic potential for the preservation of skeletal muscle mass and function. Citation Format: Andrew C. D'Lugos, Chandler Callaway, Sarah M. Judge, Andrew R. Judge. The role of complement in pancreatic cancer cachexia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1601.

Integrative cross-species analysis reveals conserved and unique signatures in fatty skeletal muscles

Fat infiltration in skeletal muscle is now recognized as a standard feature of aging and is directly related to the decline in muscle function. However, there is still a limited systematic integration and exploration of the mechanisms underlying the occurrence of myosteatosis in aging across species. Here, we re-analyzed bulk RNA-seq datasets to investigate the association between fat infiltration in skeletal muscle and aging. Our integrated analysis of single-nucleus transcriptomics in aged humans and Laiwu pigs with high intramuscular fat content, identified species-preference subclusters and revealed core gene programs associated with myosteatosis. Furthermore, we found that fibro/adipogenic progenitors (FAPs) had potential capacity of differentiating into PDE4D+/PDE7B+ preadipocytes across species. Additionally, cell-cell communication analysis revealed that FAPs may be associated with other adipogenic potential clusters via the COL4A2 and COL6A3 pathways. Our study elucidates the correlation mechanism between aging and fat infiltration in skeletal muscle, and these consensus signatures in both humans and pigs may contribute to increasing reproducibility and reliability in future studies involving in the field of muscle research.

Reciprocal communication between FAPs and muscle cells via distinct extracellular vesicle miRNAs in muscle regeneration.

Muscle regeneration is a complex process relying on precise teamwork between multiple cell types, including muscle stem cells (MuSCs) and fibroadipogenic progenitors (FAPs). FAPs are also the main source of intramuscular adipose tissue (IMAT). Muscles without FAPs exhibit decreased IMAT infiltration but also deficient muscle regeneration, indicating the importance of FAPs in the repair process. Here, we demonstrate the presence of bidirectional crosstalk between FAPs and MuSCs via their secretion of extracellular vesicles (EVs) containing distinct clusters of miRNAs that is crucial for normal muscle regeneration. Thus, after acute muscle injury, there is activation of FAPs leading to a transient rise in IMAT. These FAPs also release EVs enriched with a selected group of miRNAs, a number of which come from an imprinted region on chromosome 12. The most abundant of these is miR-127-3p, which targets the sphingosine-1-phosphate receptor S1pr3 and activates myogenesis. Indeed, intramuscular injection of EVs from immortalized FAPs speeds regeneration of injured muscle. In late stages of muscle repair, in a feedback loop, MuSCs and their derived myoblasts/myotubes secrete EVs enriched in miR-206-3p and miR-27a/b-3p. The miRNAs repress FAP adipogenesis, allowing full muscle regeneration. Together, the reciprocal communication between FAPs and muscle cells via miRNAs in their secreted EVs plays a critical role in limiting IMAT infiltration while stimulating muscle regeneration, hence providing an important mechanism for skeletal muscle repair and homeostasis.

The reciprocity of skeletal muscle and bone: an evolving view from mechanical coupling, secretory crosstalk to stem cell exchange.

Introduction: Muscle and bone constitute the two main parts of the musculoskeletal system and generate an intricately coordinated motion system. The crosstalk between muscle and bone has been under investigation, leading to revolutionary perspectives in recent years. Method and results: In this review, the evolving concept of muscle-bone interaction from mechanical coupling, secretory crosstalk to stem cell exchange was explained in sequence. The theory of mechanical coupling stems from the observation that the development and maintenance of bone mass are largely dependent on muscle-derived mechanical loads, which was later proved by Wolff's law, Utah paradigm and Mechanostat hypothesis. Then bone and muscle are gradually recognized as endocrine organs, which can secrete various cytokines to modulate the tissue homeostasis and remodeling to each other. The latest view presented muscle-bone interaction in a more direct way: the resident mesenchymal stromal cell in the skeletal muscle, i.e., fibro-adipogenic progenitors (FAPs), could migrate to the bone injury site and contribute to bone regeneration. Emerging evidence even reveals the ectopic source of FAPs from tissue outside the musculoskeletal system, highlighting its dynamic property. Conclusion: FAPs have been established as the critical cell connecting muscle and bone, which provides a new modality to study inter-tissue communication. A comprehensive and integrated perspective of muscle and bone will facilitate in-depth research in the musculoskeletal system and promote novel therapeutic avenues in treating musculoskeletal disorders.

Adipose Stromal Cell-Derived Cancer-Associated Fibroblasts Suppress FGFR Inhibitor Efficacy.

Cancer aggressiveness has been linked with obesity, and studies have shown that adipose tissue can enhance cancer progression. In this issue of Cancer Research, Hosni and colleagues discover a paracrine mechanism mediated by adipocyte precursor cells through which urothelial carcinomas become resistant to erdafitinib, a recently approved therapy inhibiting fibroblast growth factor receptors (FGFR). They identified neuregulin 1 (NRG1) secreted by adipocyte precursor cells as an activator of HER3 signaling that enables resistance. The NRG1-mediated FGFR inhibitor resistance was amenable to intervention with pertuzumab, an antibody blocking the NRG1/HER3 axis. To investigate the nature of the resistance-associated NRG1-expressing cells in human patients, the authors analyzed published single-cell RNA sequencing data and observed that such cells appear in a cluster assigned as inflammatory cancer-associated fibroblasts (iCAF). Notably, the gene signature corresponding to these CAFs is highly similar to that shared by adipose stromal cells (ASC) in fat tissue and fibro-adipogenic progenitors (FAP) in skeletal muscle of cancer-free individuals. Because fibroblasts with the ASC/FAP signature are enriched in various carcinomas, it is possible that the paracrine signaling conferred by NRG1 is a pan-cancer mechanism of FGFR inhibitor resistance and tumor aggressiveness. See related article by Hosni et al., p. 725.

Integrated meta-omics reveals the regulatory landscape involved in lipid metabolism between pig breeds

BackgroundDomesticated pigs serve as an ideal animal model for biomedical research and also provide the majority of meat for human consumption in China. Porcine intramuscular fat content associates with human health and diseases and is essential in pork quality. The molecular mechanisms controlling lipid metabolism and intramuscular fat accretion across tissues in pigs, and how these changes in response to pig breeds, remain largely unknown.ResultsWe surveyed the tissue-resident cell types of the porcine jejunum, colon, liver, and longissimus dorsi muscle between Lantang and Landrace breeds by single-cell RNA sequencing. Combining lipidomics and metagenomics approaches, we also characterized gene signatures and determined key discriminating markers of lipid digestibility, absorption, conversion, and deposition across tissues in two pig breeds. In Landrace, lean-meat swine mainly exhibited breed-specific advantages in lipid absorption and oxidation for energy supply in small and large intestinal epitheliums, nascent high-density lipoprotein synthesis for reverse cholesterol transport in enterocytes and hepatocytes, bile acid formation, and secretion for fat emulsification in hepatocytes, as well as intestinal-microbiota gene expression involved in lipid accumulation product. In Lantang, obese-meat swine showed a higher synthesis capacity of chylomicrons responsible for high serum triacylglycerol levels in small intestinal epitheliums, the predominant characteristics of lipid absorption in muscle tissue, and greater intramuscular adipcytogenesis potentials from muscular fibro-adipogenic progenitor subpopulation.ConclusionsThe findings enhanced our understanding of the cellular biology of lipid metabolism and opened new avenues to improve animal production and human diseases.D7xkEMpXE5aNHjWuhXhcDpVideo

Depletion of SMN protein in mesenchymal progenitors impairs the development of bone and neuromuscular junction in spinal muscular atrophy.

Spinal muscular atrophy (SMA) is a neuromuscular disorder characterized by the deficiency of the survival motor neuron (SMN) protein, which leads to motor neuron dysfunction and muscle atrophy. In addition to the requirement for SMN in motor neurons, recent studies suggest that SMN deficiency in peripheral tissues plays a key role in the pathogenesis of SMA. Using limb mesenchymal progenitor cell (MPC)-specific SMN-depleted mouse models, we reveal that SMN reduction in limb MPCs causes defects in the development of bone and neuromuscular junction (NMJ). Specifically, these mice exhibited impaired growth plate homeostasis and reduced insulin-like growth factor (IGF) signaling from chondrocytes, rather than from the liver. Furthermore, the reduction of SMN in fibro-adipogenic progenitors (FAPs) resulted in abnormal NMJ maturation, altered release of neurotransmitters, and NMJ morphological defects. Transplantation of healthy FAPs rescued the morphological deterioration. Our findings highlight the significance of mesenchymal SMN in neuromusculoskeletal pathogenesis of SMA and provide insights into potential therapeutic strategies targeting mesenchymal cells for the treatment of SMA.

Depletion of SMN protein in mesenchymal progenitors impairs the development of bone and neuromuscular junction in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disorder characterized by the deficiency of the survival motor neuron (SMN) protein, which leads to motor neuron dysfunction and muscle atrophy. In addition to the requirement for SMN in motor neurons, recent studies suggest that SMN deficiency in peripheral tissues plays a key role in the pathogenesis of SMA. Using limb mesenchymal progenitor cell (MPC)-specific SMN-depleted mouse models, we reveal that SMN reduction in limb MPCs causes defects in the development of bone and neuromuscular junction (NMJ). Specifically, these mice exhibited impaired growth plate homeostasis and reduced insulin-like growth factor (IGF) signaling from chondrocytes, rather than from the liver. Furthermore, the reduction of SMN in fibro-adipogenic progenitors (FAPs) resulted in abnormal NMJ maturation, altered release of neurotransmitters, and NMJ morphological defects. Transplantation of healthy FAPs rescued the morphological deterioration. Our findings highlight the significance of mesenchymal SMN in neuromusculoskeletal pathogenesis of SMA and provide insights into potential therapeutic strategies targeting mesenchymal cells for the treatment of SMA.

Utilization of the Rat Tibial Nerve Transection Model to Evaluate Cellular and Molecular Mechanisms Underpinning Denervation-Mediated Muscle Injury.

Peripheral nerve injury denervates muscle, resulting in muscle paralysis and atrophy. This is reversible if timely muscle reinnervation occurs. With delayed reinnervation, the muscle's reparative ability declines, and muscle-resident fibro-adipogenic progenitor cells (FAPs) proliferate and differentiate, inducing fibro-fatty muscle degradation and thereby physical disability. The mechanisms by which the peripheral nerve regulates FAPs expansion and differentiation are incompletely understood. Using the rat tibial neve transection model, we demonstrated an increased FAPs content and a changing FAPs phenotype, with an increased capacity for adipocyte and fibroblast differentiation, in gastrocnemius muscle post-denervation. The FAPs response was inhibited by immediate tibial nerve repair with muscle reinnervation via neuromuscular junctions (NMJs) and sensory organs (e.g., muscle spindles) or the sensory protection of muscle (where a pure sensory nerve is sutured to the distal tibial nerve stump) with reinnervation by muscle spindles alone. We found that both procedures reduced denervation-mediated increases in glial-cell-line-derived neurotrophic factor (GDNF) in muscle and that GDNF promoted FAPs adipogenic and fibrogenic differentiation in vitro. These results suggest that the peripheral nerve controls FAPs recruitment and differentiation via the modulation of muscle GDNF expression through NMJs and muscle spindles. GDNF can serve as a therapeutic target in the management of denervation-induced muscle injury.