Single-cell and spatial transcriptomics identify a macrophage population associated with skeletal muscle fibrosis

Bioactive Nanofiber-Hydrogel Composite Regulates Regenerative Microenvironment for Skeletal Muscle Regeneration after Volumetric Muscle Loss

Volumetric muscle loss (VML) is a severe form of muscle trauma that exceeds the regenerative capacity of skeletal muscle tissue, leading to substantial functional impairment. The abnormal immune response and excessive reactive oxygen species (ROS) accumulation hinder muscle regeneration following VML. Here, an interfacial cross-linked hydrogel-poly(ε-caprolactone) nanofiber composite, that incorporates both biophysical and biochemical cues to modulate the immune and ROS microenvironment for enhanced VML repair, is engineered. The interfacial cross-linking is achieved through a Michael addition between catechol and thiol groups. The resultant composite exhibits enhanced mechanical strength without sacrificing porosity. Moreover, it mitigates oxidative stress and promotes macrophage polarization toward a pro-regenerative phenotype, both in vitro and in a mouse VML model. 4 weeks post-implantation, mice implanted with the composite show improved grip strength and walking performance, along with increased muscle fiber diameter, enhanced angiogenesis, and more nerve innervation compared to control mice. Collectively, these results suggest that the interfacial cross-linked nanofiber-hydrogel composite could serve as a cell-free and drug-free strategy for augmenting muscle regeneration by modulating the oxidative stress and immune microenvironment at the VML site.

Single-nucleus RNA sequencing and lipidomics reveal characteristics of transcriptional and lipid composition in porcine longissimus dorsi muscle

BACKGROUND

Global per capita meat consumption continues to rise, especially pork. Meat quality is influenced by the content of intramuscular fat (IMF) as a key factor. The longissimus dorsi muscle of Dahe pigs (DHM, IMF: 7.98% ± 1.96%) and Dahe black pigs (DHBM, IMF: 3.30% ± 0.64%) was studied to explore cellular heterogeneity and differentially expressed genes (DEGs) associated with IMF deposition using single-nucleus RNA sequencing (snRNA-seq). The lipid composition was then analyzed using non-targeted lipidomics.

RESULTS

A total of seven cell subpopulations were identified, including myocytes, fibroblast/fibro/adipogenic progenitors (FAPs), satellite cells, endothelial cells, macrophages, pericytes, and adipocytes. Among them, FAPs and adipocytes were more focused because they could be associated with lipid deposition. 1623 DEGs in the FAPs subpopulation of DHBM were up-regulated compared with DHM, while 1535 were down-regulated. These DEGs enriched in the glycolysis/gluconeogenesis pathway. 109 DEGs were up-regulated and 806 were down-regulated in the adipocyte subpopulation of DHBM compared with DHM, which were mainly enriched in the PPAR signaling pathway and fatty acid (FA) biosynthesis. The expression level of PPARG, ABP4, LEP, and ACSL1 genes in DHM was higher than that in DHBM. Lipidomics reveals porcine lipid composition characteristics of muscle tissue. A total of 41 lipid classes and 2699 lipid species were identified in DHM and DHBM groups. The top ten relative peak areas of lipid classes in DHM and DHBM were triglyceride (TG), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), diglyceride (DG), cardiolipin (CL), ceramides (Cer), Simple Glc series (Hex1Cer), sphingomyelin (phSM), and phosphatidylinositol (PI). The relative peak areas of 35 lipid species in DHM were lower than DHBM, and 28 lipid species that were higher. There was a significant increase in the TG fatty acyl chains C6:0, C17:0, and C11:4, and a significant decrease in C16:0, C18:1, C18:2, and C22:4 in DHBM (p < 0.05).

CONCLUSIONS

C16:0 FA may downregulate the expression level of PPARG gene, which leads to the downregulation of fat metabolism-related genes such as ACSL, PLIN2, and FABP4 in DHBM compared with DHM. This may be the reason that the lipid deposition ability of Dahe pigs is stronger than that of Dahe black pigs, which need further investigation.

The Multifaceted Nature of Macrophages in Cardiovascular Disease

As the leading cause of mortality worldwide, cardiovascular disease (CVD) represents a variety of heart diseases and vascular disorders, including atherosclerosis, aneurysm, ischemic injury in the heart and brain, arrythmias, and heart failure. Macrophages, a diverse population of immune cells that can promote or suppress inflammation, have been increasingly recognized as a key regulator in various processes in both healthy and disease states. In healthy conditions, these cells promote the proper clearance of cellular debris, dead and dying cells, and provide a strong innate immune barrier to foreign pathogens. However, macrophages can play a detrimental role in the progression of disease as well, particularly those inflammatory in nature. This review will focus on the current knowledge regarding the role of macrophages in cardiovascular diseases.

Cellular interactions and microenvironment dynamics in skeletal muscle regeneration and disease

Skeletal muscle regeneration relies on the intricate interplay of various cell populations within the muscle niche-an environment crucial for regulating the behavior of muscle stem cells (MuSCs) and ensuring postnatal tissue maintenance and regeneration. This review delves into the dynamic interactions among key players of this process, including MuSCs, macrophages (MPs), fibro-adipogenic progenitors (FAPs), endothelial cells (ECs), and pericytes (PCs), each assuming pivotal roles in orchestrating homeostasis and regeneration. Dysfunctions in these interactions can lead not only to pathological conditions but also exacerbate muscular dystrophies. The exploration of cellular and molecular crosstalk among these populations in both physiological and dystrophic conditions provides insights into the multifaceted communication networks governing muscle regeneration. Furthermore, this review discusses emerging strategies to modulate the muscle-regenerating niche, presenting a comprehensive overview of current understanding and innovative approaches.

A Novel MAO-B/SSAO Inhibitor Improves Multiple Aspects of Dystrophic Phenotype in mdx Mice

Duchenne muscular dystrophy (DMD) is one of the most frequent and severe childhood muscle diseases. Its pathophysiology is multifaceted and still incompletely understood, but we and others have previously shown that oxidative stress plays an important role. In particular, we have demonstrated that inhibition of mitochondrial monoamine oxidases could improve some functional and biohumoral markers of the pathology. In the present study we report the use of dystrophic mdx mice to evaluate the efficacy of a dual monoamine oxidase B (MAO-B)/semicarbazide-sensitive amine oxidase (SSAO) inhibitor, PXS-5131, in reducing inflammation and fibrosis and improving muscle function. We found that a one-month treatment starting at three months of age was able to decrease reactive oxygen species (ROS) production, fibrosis, and inflammatory infiltrate in the tibialis anterior (TA) and diaphragm muscles. Importantly, we also observed a marked improvement in the capacity of the gastrocnemius muscle to maintain its force when challenged with eccentric contractions. Upon performing a bulk RNA-seq analysis, PXS-5131 treatment affected the expression of genes involved in inflammatory processes and tissue remodeling. We also studied the effect of prolonged treatment in older dystrophic mice, and found that a three-month administration of PXS-5131 was able to greatly reduce the progression of fibrosis not only in the diaphragm but also in the heart. Taken together, these results suggest that PXS-5131 is an effective inhibitor of fibrosis and inflammation in dystrophic muscles, a finding that could open a new therapeutic avenue for DMD patients.

Histopathological evaluation of the lungs in experimental autoimmune encephalomyelitis

IMPORTANCE

Experimental autoimmune encephalomyelitis (EAE) is an animal model of multiple sclerosis characterized by inflammation within the central nervous system. However, inflammation in non-neuronal tissues, including the lungs, has not been fully evaluated.

OBJECTIVE

This study evaluated the inflammatory response in lungs of EAE mice by immunohistochemistry and histochemistry.

METHODS

Eight adult C57BL/6 mice were injected with myelin oligodendrocyte glycoprotein35-55 to induce the EAE. Lungs and spinal cords were sampled from the experimental mice at the time of sacrifice and used for the western blotting, histochemistry, and immunohistochemistry.

RESULTS

Histopathological examination revealed inflammatory lesions in the lungs of EAE mice, characterized by infiltration of myeloperoxidase (MPO)- and galectin-3-positive cells, as determined by immunohistochemistry. Increased numbers of collagen fibers in the lungs of EAE mice were confirmed by histopathological analysis. Western blotting revealed significantly elevated level of osteopontin (OPN), cluster of differentiation 44 (CD44), MPO and galectin-3 in the lungs of EAE mice compared with normal controls (p < 0.05). Immunohistochemical analysis revealed both OPN and CD44 in ionized calcium-binding adapter molecule 1-positive macrophages within the lungs of EAE mice.

CONCLUSIONS AND RELEVANCE

Taken together, these findings suggest that the increased OPN level in lungs of EAE mice led to inflammation; concurrent increases in proinflammatory factors (OPN and galectin-3) caused pulmonary impairment.

Regulation of muscle hypertrophy through granulin: Relayed communication among mesenchymal progenitors, macrophages, and satellite cells

Skeletal muscles exert remarkable regenerative or adaptive capacities in response to injuries or mechanical loads. However, the cellular networks underlying muscle adaptation are poorly understood compared to those underlying muscle regeneration. We employed single-cell RNA sequencing to investigate the gene expression patterns and cellular networks activated in overloaded muscles and compared these results with those observed in regenerating muscles. The cellular composition of the 4-day overloaded muscle, when macrophage infiltration peaked, closely resembled that of the 10-day regenerating muscle. In addition to the mesenchymal progenitor-muscle satellite cell (MuSC) axis, interactome analyses or targeted depletion experiments revealed communications between mesenchymal progenitors-macrophages and macrophages-MuSCs. Furthermore, granulin, a macrophage-derived factor, inhibited MuSC differentiation, and Granulin-knockout mice exhibited blunted muscle hypertrophy due to the premature differentiation of overloaded MuSCs. These findings reveal the critical role of granulin through the relayed communications of mesenchymal progenitors, macrophages, and MuSCs in facilitating efficient muscle hypertrophy.

Dietary pyruvate targets cytosolic phospholipase A2 to mitigate inflammation and obesity in mice

Obesity has a multifactorial etiology and is known to be a state of chronic low-grade inflammation, known as meta-inflammation. This state is associated with the development of metabolic disorders such as glucose intolerance and nonalcoholic fatty liver disease. Pyruvate is a glycolytic metabolite and a crucial node in various metabolic pathways. However, its role and molecular mechanism in obesity and associated complications are obscure. In this study, we reported that pyruvate substantially inhibited adipogenic differentiation in vitro and its administration significantly prevented HFD-induced weight gain, white adipose tissue inflammation, and metabolic dysregulation. To identify the target proteins of pyruvate, drug affinity responsive target stability was employed with proteomics, cellular thermal shift assay, and isothermal drug response to detect the interactions between pyruvate and its molecular targets. Consequently, we identified cytosolic phospholipase A2 (cPLA2) as a novel molecular target of pyruvate and demonstrated that pyruvate restrained diet-induced obesity, white adipose tissue inflammation, and hepatic steatosis in a cPLA2-dependent manner. Studies with global ablation of cPLA2 in mice showed that the protective effects of pyruvate were largely abrogated, confirming the importance of pyruvate/cPLA2 interaction in pyruvate attenuation of inflammation and obesity. Overall, our study not only establishes pyruvate as an antagonist of cPLA2 signaling and a potential therapeutic option for obesity, but it also sheds light on the mechanism of its action. Pyruvate's prior clinical use indicates that it can be considered a safe and viable alternative for obesity, whether consumed as a dietary supplement or as part of a regular diet.

Mapping Cell Atlases at the Single-Cell Level

Recent advancements in single-cell technologies have led to rapid developments in the construction of cell atlases. These atlases have the potential to provide detailed information about every cell type in different organisms, enabling the characterization of cellular diversity at the single-cell level. Global efforts in developing comprehensive cell atlases have profound implications for both basic research and clinical applications. This review provides a broad overview of the cellular diversity and dynamics across various biological systems. In addition, the incorporation of machine learning techniques into cell atlas analyses opens up exciting prospects for the field of integrative biology.

Deciphering Immune Landscape Remodeling Unravels the Underlying Mechanism for Synchronized Muscle and Bone Aging

Evidence from numerous studies has revealed the synchronous progression of aging in bone and muscle; however, little is known about the underlying mechanisms. To this end, human muscles and bones are harvested and the aging-associated transcriptional dynamics of two tissues in parallel using single-cell RNA sequencing are surveyed. A subset of lipid-associated macrophages (triggering receptor expressed on myeloid cells 2, TREM2+ Macs) is identified in both aged muscle and bone. Genes responsible for muscle dystrophy and bone loss, such as secreted phosphoprotein 1 (SPP1), are also highly expressed in TREM2+ Macs, suggesting its conserved role in aging-related features. A common transition toward pro-inflammatory phenotypes in aged CD4+ T cells across tissues is also observed, activated by the nuclear factor kappa B subunit 1 (NFKB1). CD4+ T cells in aged muscle experience Th1-like differentiation, whereas, in bone, a skewing toward Th17 cells is observed. Furthermore, these results highlight that degenerated myocytes produce BAG6-containing exosomes that can communicate with Th17 cells in the bone through its receptor natural cytotoxicity triggering receptor 3 (NCR3). This communication upregulates CD6 expression in Th17 cells, which then interact with TREM2+ Macs through CD6-ALCAM signaling, ultimately stimulating the transcription of SPP1 in TREM2+ Macs. The negative correlation between serum exosomal BCL2-associated athanogene 6 (BAG6) levels and bone mineral density further supports its role in mediating muscle and bone synchronization with aging.

Spatial omics: An innovative frontier in aging research

Disentangling the impact of aging on health and disease has become critical as population aging progresses rapidly. Studying aging at the molecular level is complicated by the diverse aging profiles and dynamics. However, the examination of cellular states within aging tissues in situ is hampered by the lack of high-resolution spatial data. Emerging spatial omics technologies facilitate molecular and spatial analysis of tissues, providing direct access to precise information on various functional regions and serving as a favorable tool for unraveling the heterogeneity of aging. In this review, we summarize the recent advances in spatial omics application in multi-organ aging research, which has enhanced the understanding of aging mechanisms from multiple standpoints. We also discuss the main challenges in spatial omics research to date, the opportunities for further developing the technology, and the potential applications of spatial omics in aging and aging-related diseases.

Macrophage phenotypes and functions: resolving inflammation and restoring homeostasis

Inflammation must be tightly regulated to both defend against pathogens and restore tissue homeostasis. The resolution of inflammatory responses is a dynamic process orchestrated by cells of the immune system. Macrophages, tissue-resident innate immune cells, are key players in modulating inflammation. Here, we review recent work highlighting the importance of macrophages in tissue resolution and the return to homeostasis. We propose that enhancing macrophage pro-resolution functions represents a novel and widely applicable therapeutic strategy to dampen inflammation, promote repair, and restore tissue integrity and function.

Influences of Aged Bone Marrow Macrophages on Skeletal Health and Senescence

PURPOSE OF REVIEW

The purpose of this review is to discuss the role of macrophages in the regulation of skeletal health with age, particularly in regard to both established and unexplored mechanisms in driving inflammation and senescence.

RECENT FINDINGS

A multitude of research has uncovered mechanisms of intrinsic aging in macrophages, detrimental factors released by these immune cells, and crosstalk from senescent mesenchymal cell types, which altogether drive age-related bone loss. Furthermore, bone marrow macrophages were recently proposed to be responsible for the megakaryocytic shift during aging and overall maintenance of the hematopoietic niche. Studies on extra-skeletal macrophages have shed light on possible conserved mechanisms within bone and highlight the importance of these cells in systemic aging. Macrophages are a critically important cell type in maintaining skeletal homeostasis with age. New discoveries in this area are of utmost importance in fully understanding the pathogenesis of osteoporosis in aged individuals.

Spatial Transcriptomics Reveals Signatures of Histopathological Changes in Muscular Sarcoidosis

Sarcoidosis is a multisystemic disease characterized by non-caseating granuloma infiltrating various organs. The form with symptomatic muscular involvement is called muscular sarcoidosis. The impact of immune cells composing the granuloma on the skeletal muscle is misunderstood. Here, we investigated the granuloma-skeletal muscle interactions through spatial transcriptomics on two patients affected by muscular sarcoidosis. Five major transcriptomic clusters corresponding to perigranuloma, granuloma, and three successive muscle tissue areas (proximal, intermediate, and distal) around the granuloma were identified. Analyses revealed upregulated pathways in the granuloma corresponding to the activation of T-lymphocytes and monocytes/macrophages cytokines, the upregulation of extracellular matrix signatures, and the induction of the TGF-β signaling in the perigranuloma. A comparison between the proximal and distal muscles to the granuloma revealed an inverse correlation between the distance to the granuloma and the upregulation of cellular response to interferon-γ/α, TNF-α, IL-1,4,6, fibroblast proliferation, epithelial to mesenchymal cell transition, and the downregulation of muscle gene expression. These data shed light on the intercommunications between granulomas and the muscle tissue and provide pathophysiological mechanisms by showing that granuloma immune cells have a direct impact on proximal muscle tissue by promoting its progressive replacement by fibrosis via the expression of pro-inflammatory and profibrosing signatures. These data could possibly explain the evolution towards a state of disability for some patients.

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.