Normal muscle regeneration requires tight control of muscle cell fusion by tetraspanins CD9 and CD81

Distinctive function of Tetraspanins: Implication in viral infections

Harboring four transmembrane domains in their structural hallmark, Tetraspanins (Tspans) are a family of glycoproteins with pivotal functions in a variety of biological and cellular processes. Through interacting laterally with each other or specific membrane proteins, Tspans organize tetraspanin-enriched microdomains (TEMs), modulating cellular signaling, adhesion, fusion, and proliferation. An abundance of evidence has identified the multiple functions in the progression of cancer as well as the underlying molecular mechanisms. Recently, plenty of studies have focused on the utilities of Tspans by pathogens for infection, especially the infection of viruses. The expression of Tspans correlates with the phase of viral infection, the type of virus, and targeted therapies. In particular, perturbations of Tspans in host cells can affect viral attachment, intracellular trafficking, translation, virus assembly, and release. In this review, we summarize and provide a historical overview of the discovery and characterization of various kinds of virus infection and highlight their diversity and complexity, along with the virus life cycle. Furthermore, we examined the current understanding of how various Tspans are involved in the regulatory mechanisms underlying viral infection. This review aims to offer a comprehensive understanding of the targeting of Tspans for therapeutic intervention in infections caused by diverse pathogens.

Targeting Muscle Regeneration with Small Extracellular Vesicles from Adipose Tissue-Derived Stem Cells-A Review

Extracellular vesicles (EVs) are membrane-bound structures released by cells carrying diverse biomolecules involved in intercellular communication. Small EVs are abundant in body fluids, playing a key role in cell signaling. Their natural occurrence and therapeutic potential, especially in the context of muscular disorders, make them a significant area of research. Sarcopenia, characterized by progressive muscle fiber loss, represents a pathological state in which EVs could offer therapeutic benefits, reducing morbidity and mortality. Recent studies have proposed an interplay between adipose tissue (AT) and skeletal muscle regarding sarcopenia pathology. AT dysregulation, as seen in obesity, contributes to skeletal muscle loss in a multifactorial way. While AT-derived stem cell (ATDSC) small EVs have been implicated in musculoskeletal homeostasis, their precise action in muscle regeneration remains incompletely understood. In this context, ATDSC-derived small EVs can stimulate skeletal muscle regeneration through improved proliferation and migration of muscle cells, enhancement of muscular perfusion, improvement of tendon and nerve regeneration, stimulation of angiogenesis, and promotion of myogenic differentiation. However, they can also increase skeletal muscle loss. Notably, this is the first comprehensive review to systematically examine the role of ATDSC-derived small EVs in sarcopenia.

Microbial chaperonin 60 inhibits osteoclast differentiation by interfering with RANK/RANKL binding and overexpression of lipocalin2

Osteoclasts play a crucial role in bone destruction in rheumatoid arthritis (RA). This study aimed to investigate the inhibitory effects of chaperonin 60 (CPN60), identified in the surface proteins of Propionibacterium freudenreichii MJ2, on receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast differentiation, and elucidate the underlying mechanisms. Treatment with CPN60 inhibited RANKL-induced osteoclast differentiation by decreasing the expression of osteoclast differentiation-related genes and proteins. CPN60 interfered with the binding of RANKL to RANK, as elucidated using surface plasmon resonance (SPR) and immunofluorescence. In silico molecular docking analysis further supported the interference of CPN60 with the binding of RANKL and RANK. CPN60 suppressed the expression of molecules linked to the calcium-dependent pathway in RANKL-induced osteoclast differentiation at both mRNA and protein levels. Microarray analysis showed elevated expression of lipocalin 2 (Lcn2), which was closely linked to the inhibition of osteoclast differentiation in CPN60-treated RAW 264.7 cells. Inhibition of Lcn2 decreased the inhibitory effect of CPN60 on osteoclast differentiation, indicating that increased expression of Lcn2 by CPN60 contributes to the inhibition of osteoclastogenesis. In addition, CPN60 treatment alleviated arthritis symptoms in collagen-induced arthritis mice by reducing the generation of collagen-specific antibodies and inhibiting osteoclast differentiation. In conclusion, CPN60 of P. freudenreichii MJ2 interfered with RANKL-RANK binding, reduced the expression of genes and proteins related to osteoclast differentiation and upregulated Lcn2 expression, thereby inhibiting RANKL-induced osteoclast differentiation, which might contribute to ameliorate collagen-induced arthritis.

Combination of transcriptomics and proteomics for analyzing potential biomarker and molecular mechanism underlying skeletal muscle atrophy

BACKGROUND

The skeletal muscle atrophy is prevalently occurred in numerous chronic disease complications. Despite its important clinical significance, there are currently no therapeutic drugs, so new biomarkers and molecular mechanisms need to be discovered urgently.

METHODS

Transcriptome and proteome sequencing data were collected from normal and skeletal muscle atrophic mice. The differentially expressed genes (DEGs) and proteins (DEPs) were analyzed. Applying PPI analysis to obtain overlapping genes and proteins, which were next subjected to GO and KEGG enrichment analysis. Combined analysis of transcriptomics and proteomics was performed to get key genes that were simultaneously found in GO and KEGG enrichment results. Subsequently, RT-qPCR and immunofluorescence were constructed to verify the expression of screened key genes.

RESULTS

By combination of transcriptomics, proteomics and RT-qPCR results, we identified 14 key genes (Cav1, Col3a1, Dnaja1, Postn, Ptges3, Cd44, Clec3b, Igfbp6, Lamc1, Alb, Itga6, Mmp2, Timp2 and Cd9) that were markedly different in atrophic mice. Single-gene GSEA and immunofluorescence suggested Cd9 was probably the biomarker for skeletal muscle atrophy.

CONCLUSIONS

Our study hinted that Cd9 was potential biomarker and may interfere with skeletal muscle atrophy through process of aerobic respiration, oxidative phosphorylation, and metabolism of amino acids and fatty acids.

SIGNIFICANCE

The present study holds the subsequent significance: Frist, we investigated biomarkers for skeletal muscle atrophy using multi-omics approach. A total of 14 genes were markedly different in skeletal muscle atrophic mice. We finally found Cd9 is a potential biomarker for skeletal muscle atrophy. Our work presents novel biomarkers and potential regulatory mechanisms for the early detection and intervention of muscle atrophy.

Suppressing PDGFRβ Signaling Enhances Myocyte Fusion to Promote Skeletal Muscle Regeneration

Muscle cell fusion is critical for forming and maintaining multinucleated myotubes during skeletal muscle development and regeneration. However, the molecular mechanisms directing cell-cell fusion are not fully understood. Here, we identify platelet-derived growth factor receptor beta (PDGFRβ) signaling as a key modulator of myocyte fusion in adult muscle cells. Our findings demonstrate that genetic deletion of Pdgfrβ enhances muscle regeneration and increases myofiber size, whereas PDGFRβ activation impairs muscle repair. Inhibition of PDGFRβ activity promotes myonuclear accretion in both mouse and human myotubes, whereas PDGFRβ activation stalls myotube development by preventing cell spreading to limit fusion potential. Transcriptomics analysis show that PDGFRβ signaling cooperates with TGFβ signaling to direct myocyte size and fusion. Mechanistically, PDGFRβ signaling requires STAT1 activation, and blocking STAT1 phosphorylation enhances myofiber repair and size during regeneration. Collectively, PDGFRβ signaling acts as a regenerative checkpoint and represents a potential clinical target to rapidly boost skeletal muscle repair.

Proteomic landscape of tunneling nanotubes reveals CD9 and CD81 tetraspanins as key regulators

Tunneling nanotubes (TNTs) are open actin- and membrane-based channels, connecting remote cells and allowing direct transfer of cellular material (e.g. vesicles, mRNAs, protein aggregates) from the cytoplasm to the cytoplasm. Although they are important especially, in pathological conditions (e.g. cancers, neurodegenerative diseases), their precise composition and their regulation were still poorly described. Here, using a biochemical approach allowing to separate TNTs from cell bodies and from extracellular vesicles and particles (EVPs), we obtained the full composition of TNTs compared to EVPs. We then focused on two major components of our proteomic data, the CD9 and CD81 tetraspanins, and further investigated their specific roles in TNT formation and function. We show that these two tetraspanins have distinct non-redundant functions: CD9 participates in stabilizing TNTs, whereas CD81 expression is required to allow the functional transfer of vesicles in the newly formed TNTs, possibly by regulating docking to or fusion with the opposing cell.

Tetraspanin proteins in membrane remodeling processes

Membrane remodeling is a fundamental cellular process that is crucial for physiological functions such as signaling, membrane fusion and cell migration. Tetraspanins (TSPANs) are transmembrane proteins of central importance to membrane remodeling events. During these events, TSPANs are known to interact with themselves and other proteins and lipids; however, their mechanism of action in controlling membrane dynamics is not fully understood. Since these proteins span the membrane, membrane properties such as rigidity, curvature and tension can influence their behavior. In this Review, we summarize recent studies that explore the roles of TSPANs in membrane remodeling processes and highlight the unique structural features of TSPANs that mediate their interactions and localization. Further, we emphasize the influence of membrane curvature on TSPAN distribution and membrane domain formation and describe how these behaviors affect cellular functions. This Review provides a comprehensive perspective on the multifaceted function of TSPANs in membrane remodeling processes and can help readers to understand the intricate molecular mechanisms that govern cellular membrane dynamics.

Three-dimensional imaging studies in mice identify cellular dynamics of skeletal muscle regeneration

The function of many organs, including skeletal muscle, depends on their three-dimensional structure. Muscle regeneration therefore requires not only reestablishment of myofibers but also restoration of tissue architecture. Resident muscle stem cells (SCs) are essential for regeneration, but how SCs regenerate muscle architecture is largely unknown. We address this problem using genetic labeling of mouse SCs and whole-mount imaging to reconstruct, in three dimensions, muscle regeneration. Unexpectedly, we found that myofibers form via two distinct phases of fusion and the residual basement membrane of necrotic myofibers is critical for promoting fusion and orienting regenerated myofibers. Furthermore, the centralized myonuclei characteristic of regenerated myofibers are associated with myofibrillogenesis and endure months post injury. Finally, we elucidate two cellular mechanisms for the formation of branched myofibers, a pathology characteristic of diseased muscle. We provide a synthesis of the cellular events of regeneration and show that these differ from those used during development.

Smart exosomes enhance PDAC targeted therapy

Exosomes continue to attract interest as a promising nanocarrier drug delivery technology. They are naturally derived nanoscale extracellular vesicles with innate properties well suited to shuttle proteins, lipids, and nucleic acids between cells. Nonetheless, their clinical utility is currently limited by several major challenges, such as their inability to target tumor cells and a high proportion of clearance by the mononuclear phagocyte system (MPS) of the liver and spleen. To overcome these limitations, we developed "Smart Exosomes" that co-display RGD and CD47p110-130 through CD9 engineering (ExoSmart). The resultant ExoSmart demonstrates enhanced binding capacity to αvβ3 on pancreatic ductal adenocarcinoma (PDAC) cells, resulting in amplified cellular uptake in in vitro and in vivo models and increased chemotherapeutic efficacies. Simultaneously, ExoSmart significantly reduced liver and spleen clearance of exosomes by inhibiting macrophage phagocytosis via CD47p110-130 interaction with signal regulatory proteins (SIRPα) on macrophages. These studies demonstrate that an engineered exosome drug delivery system increases PDAC therapeutic efficacy by enhancing active PDAC targeting and prolonging circulation times, and their findings hold tremendous translational potential for cancer therapy while providing a concrete foundation for future work utilizing novel peptide-engineered exosome strategies.

The satellite cell in skeletal muscle: A story of heterogeneity

Skeletal muscle is a highly represented tissue in mammals and is composed of fibers that are extremely adaptable and capable of regeneration. This characteristic of muscle fibers is made possible by a cell type called satellite cells. Adjacent to the fibers, satellite cells are found in a quiescent state and located between the muscle fibers membrane and the basal lamina. These cells are required for the growth and regeneration of skeletal muscle through myogenesis. This process is known to be tightly sequenced from the activation to the differentiation/fusion of myofibers. However, for the past fifteen years, researchers have been interested in examining satellite cell heterogeneity and have identified different subpopulations displaying distinct characteristics based on localization, quiescence state, stemness capacity, cell-cycle progression or gene expression. A small subset of satellite cells appears to represent multipotent long-term self-renewing muscle stem cells (MuSC). All these distinctions led us to the hypothesis that the characteristics of myogenesis might not be linear and therefore may be more permissive based on the evidence that satellite cells are a heterogeneous population. In this review, we discuss the different subpopulations that exist within the satellite cell pool to highlight the heterogeneity and to gain further understanding of the myogenesis progress. Finally, we discuss the long term self-renewing MuSC subpopulation that is capable of dividing asymmetrically and discuss the molecular mechanisms regulating MuSC polarization during health and disease.

Differential proteomics argues against a general role for CD9, CD81 or CD63 in the sorting of proteins into extracellular vesicles

The tetraspanins CD9, CD81 and CD63 are major components of extracellular vesicles (EVs). Yet, their impact on EV composition remains under-investigated. In the MCF7 breast cancer cell line CD63 was as expected predominantly intracellular. In contrast CD9 and CD81 strongly colocalized at the plasma membrane, albeit with different ratios at different sites, which may explain a higher enrichment of CD81 in EVs. Absence of these tetraspanins had little impact on the EV protein composition as analysed by quantitative mass spectrometry. We also analysed the effect of concomitant knock-out of CD9 and CD81 because these two tetraspanins play similar roles in several cellular processes and associate directly with two Ig domain proteins, CD9P-1/EWI-F/PTGFRN and EWI-2/IGSF8. These were the sole proteins significantly decreased in the EVs of double CD9- and CD81-deficient cells. In the case of EWI-2, this is primarily a consequence of a decreased cell expression level. In conclusion, this study shows that CD9, CD81 and CD63, commonly used as EV protein markers, play a marginal role in determining the protein composition of EVs released by MCF7 cells and highlights a regulation of the expression level and/or trafficking of CD9P-1 and EWI-2 by CD9 and CD81.

Targeting of Tetraspanin CD81 with Monoclonal Antibodies and Small Molecules to Combat Cancers and Viral Diseases

Tetraspanin CD81 plays major roles in cell-cell interactions and the regulation of cellular trafficking. This cholesterol-embarking transmembrane protein is a co-receptor for several viruses, including HCV, HIV-1 and Chikungunya virus, which exploits the large extracellular loop EC2 for cell entry. CD81 is also an anticancer target implicated in cancer cell proliferation and mobility, and in tumor metastasis. CD81 signaling contributes to the development of solid tumors (notably colorectal, liver and gastric cancers) and has been implicated in the aggressivity of B-cell lymphomas. A variety of protein partners can interact with CD81, either to regulate attachment and uptake of viruses (HCV E2, claudin-1, IFIM1) or to contribute to tumor growth and dissemination (CD19, CD44, EWI-2). CD81-protein interactions can be modulated with molecules targeting the extracellular domain of CD81, investigated as antiviral and/or anticancer agents. Several monoclonal antibodies anti-CD81 have been developed, notably mAb 5A6 active against invasion and metastasis of triple-negative breast cancer cells. CD81-EC2 can also be targeted with natural products (trachelogenin and harzianoic acids A-B) and synthetic compounds (such as benzothiazole-quinoline derivatives). They are weak CD81 binders but offer templates for the design of new compounds targeting the open EC2 loop. There is no anti-CD81 compound in clinical development at present, but this structurally well-characterized tetraspanin warrants more substantial considerations as a drug target.

Prognostic value and multifaceted roles of tetraspanin CD9 in cancer

CD9 is a crucial regulator of cell adhesion in the immune system and plays important physiological roles in hematopoiesis, blood coagulation or viral and bacterial infections. It is involved in the transendothelial migration of leukocytes which might also be hijacked by cancer cells during their invasion and metastasis. CD9 is found at the cell surface and the membrane of exosomes affecting cancer progression and therapy resistance. High expression of CD9 is mostly associated with good patients outcome, with a few exceptions. Discordant findings have been reported for breast, ovarian, melanoma, pancreatic and esophageal cancer, which might be related to using different antibodies or inherent cancer heterogeneity. According to in vitro and in vivo studies, tetraspanin CD9 is not clearly associated with either tumor suppression or promotion. Further mechanistic experiments will elucidate the role of CD9 in particular cancer types and specific conditions.

Extracellular vesicle distribution and localization in skeletal muscle at rest and following disuse atrophy

BACKGROUND

Skeletal muscle (SkM) is a large, secretory organ that produces and releases myokines that can have autocrine, paracrine, and endocrine effects. Whether extracellular vesicles (EVs) also play a role in the SkM adaptive response and ability to communicate with other tissues is not well understood. The purpose of this study was to investigate EV biogenesis factors, marker expression, and localization across cell types in the skeletal muscle. We also aimed to investigate whether EV concentrations are altered by disuse atrophy.

METHODS

To identify the potential markers of SkM-derived EVs, EVs were isolated from rat serum using density gradient ultracentrifugation, followed by fluorescence correlation spectroscopy measurements or qPCR. Single-cell RNA sequencing (scRNA-seq) data from rat SkM were analyzed to assess the EV biogenesis factor expression, and cellular localization of tetraspanins was investigated by immunohistochemistry. Finally, to assess the effects of mechanical unloading on EV expression in vivo, EV concentrations were measured in the serum by nanoparticle tracking analysis in both a rat and human model of disuse.

RESULTS

In this study, we show that the widely used markers of SkM-derived EVs, α-sarcoglycan and miR-1, are undetectable in serum EVs. We also found that EV biogenesis factors, including the tetraspanins CD63, CD9, and CD81, are expressed by a variety of cell types in SkM. SkM sections showed very low detection of CD63, CD9, and CD81 in myofibers and instead accumulation within the interstitial space. Furthermore, although there were no differences in serum EV concentrations following hindlimb suspension in rats, serum EV concentrations were elevated in human subjects after bed rest.

CONCLUSIONS

Our findings provide insight into the distribution and localization of EVs in SkM and demonstrate the importance of methodological guidelines in SkM EV research.

Juvenile zebrafish (Danio rerio) are able to recover from lordosis

Haemal lordosis, a frequent skeletal deformity in teleost fish, has long been correlated with increased mechanical loads induced by swimming activity. In the present study, we examine whether juvenile zebrafish can recover from haemal lordosis and explore the musculoskeletal mechanisms involved. Juveniles were subjected to a swimming challenge test (SCT) that induced severe haemal lordosis in 49% of the animals and then immediately transferred them to 0.0 total body lengths (TL) per second of water velocity for a week. The recovery from lordosis was examined by means of whole mount staining, histology and gene expression analysis. Results demonstrate that 80% of the lordotic zebrafish are capable of internal and external recovery within a week after the SCT. Recovered individuals presented normal shape of the vertebral centra, maintaining though distorted internal tissue organization. Through the transcriptomic analysis of the affected haemal regions, several processes related to chromosome organization, DNA replication, circadian clock and transcription regulation were enriched within genes significantly regulated behind this musculoskeletal recovery procedure. Genes especially involved in adipogenesis, bone remodeling and muscular regeneration were regulated. A remodeling tissue-repair hypothesis behind haemal lordosis recovery is raised. Limitations and future possibilities for zebrafish as a model organism to clarify mechanically driven musculoskeletal changes are discussed.

Integrative analysis of cell adhesion molecules in glioblastoma identified prostaglandin F2 receptor inhibitor (PTGFRN) as an essential gene

Background

Glioblastoma (GBM) is the most common primary malignant brain tumor in adults exhibiting infiltration into surrounding tissues, recurrence, and resistance to therapy. GBM infiltration is accomplished by many deregulated factors such as cell adhesion molecules (CAMs), which are membrane proteins that participate in cell-cell and cell-ECM interactions to regulate survival, proliferation, migration, and stemness.

Methods

A comprehensive bioinformatics analysis of CAMs (n = 518) in multiple available datasets revealed genetic and epigenetic alterations among CAMs in GBM. Univariate Cox regression analysis using TCGA dataset identified 127 CAMs to be significantly correlated with survival. The poor prognostic indicator PTGFRN was chosen to study its role in glioma. Silencing of PTGFRN in glioma cell lines was achieved by the stable expression of short hairpin RNA (shRNA) against the PTGFRN gene. PTGFRN was silenced and performed cell growth, migration, invasion, cell cycle, and apoptosis assays. Neurosphere and limiting dilution assays were also performed after silencing of PTGFRN in GSCs.

Results

Among the differentially regulated CAMs (n = 181, 34.9%), major proportion of them were found to be regulated by miRNAs (n = 95, 49.7%) followed by DNA methylation (n = 32, 16.7%), and gene copy number variation (n = 12, 6.2%). We found that PTGFRN to be upregulated in GBM tumor samples and cell lines with a significant poor prognostic correlation with patient survival. Silencing PTGFRN diminished cell growth, colony formation, anchorage-independent growth, migration, and invasion and led to cell cycle arrest and induction of apoptosis. At the mechanistic level, silencing of PTGFRN reduced pro-proliferative and promigratory signaling pathways such as ERK, AKT, and mTOR. PTGFRN upregulation was found to be due to the loss of its promoter methylation and downregulation of miR-137 in GBM. PTGFRN was also found to be higher in glioma stem-like cells (GSCs) than the matched differentiated glioma cells (DGCs) and is required for GSC growth and survival. Silencing of PTGFRN in GSCs reduced transcript levels of reprogramming factors (Olig2, Pou3f2, Sall2, and Sox2).

Conclusion

In this study, we provide a comprehensive overview of the differential regulation of CAMs and the probable causes for their deregulation in GBM. We also establish an oncogenic role of PTGFRN and its regulation by miR-137 in GBM, thus signifying it as a potential therapeutic target.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09682-2.

The metalloprotease ADM-4/ADAM17 promotes axonal repair

Axonal fusion is an efficient means of repair following axonal transection, whereby the regenerating axon fuses with its own separated axonal fragment to restore neuronal function. Despite being described over 50 years ago, its molecular mechanisms remain poorly understood. Here, we demonstrate that the Caenorhabditis elegans metalloprotease ADM-4, an ortholog of human ADAM17, is essential for axonal fusion. We reveal that animals lacking ADM-4 cannot repair their axons by fusion, and that ADM-4 has a cell-autonomous function within injured neurons, localizing at the tip of regrowing axon and fusion sites. We demonstrate that ADM-4 overexpression enhances fusion to levels higher than wild type, and that the metalloprotease and phosphatidylserine-binding domains are essential for its function. Last, we show that ADM-4 interacts with and stabilizes the fusogen EFF-1 to allow membranes to merge. Our results uncover a key role for ADM-4 in axonal fusion, exposing a molecular target for axonal repair.

Cell Surface Proteins for Enrichment and In Vitro Characterization of Human Pluripotent Stem Cell-Derived Myogenic Progenitors

Human myogenic progenitors can be derived from pluripotent stem cells (PSCs) for use in modeling natural and pathological myogenesis, as well as treating muscle diseases. Transgene-free methods of deriving myogenic progenitors from different PSC lines often produce mixed populations that are heterogeneous in myogenic differentiation potential, yet detailed and accurate characterization of human PSC-derived myogenic progenitors remains elusive in the field. The isolation and purification of human PSC-derived myogenic progenitors is thus an important methodological consideration when we investigate the properties and behaviors of these cells in culture. We previously reported a transgene-free, serum-free floating sphere culture method for the derivation of myogenic progenitors from human PSCs. In this study, we first performed comprehensive cell surface protein profiling of the sphere culture cells through the screening of 255 antibodies. Next, we used magnetic activated cell sorting and enriched the cells according to the expression of specific surface markers. The ability of muscle differentiation in the resulting cells was characterized by immunofluorescent labeling and quantification of positively stained cells. Our results revealed that myotube-forming cells resided in the differentiated cultures of CD29+, CD56+, CD271+, and CD15– fractions, while thick and multinucleated myotubes were identified in the differentiated cultures from CD9+ and CD146+ fractions. We found that PAX7 localization to the nucleus correlates with myotube-forming ability in these sorted populations. We also demonstrated that cells in unsorted, CD271+, and CD15– fractions responded differently to cryopreservation and prolonged culture expansion. Lastly, we showed that CD271 expression is essential for terminal differentiation of human PSC-derived myogenic progenitors. Taken together, these cell surface proteins are not only useful markers to identify unique cellular populations in human PSC-derived myogenic progenitors but also functionally important molecules that can provide valuable insight into human myogenesis.

Large-scale integration of single-cell transcriptomic data captures transitional progenitor states in mouse skeletal muscle regeneration

Skeletal muscle repair is driven by the coordinated self-renewal and fusion of myogenic stem and progenitor cells. Single-cell gene expression analyses of myogenesis have been hampered by the poor sampling of rare and transient cell states that are critical for muscle repair, and do not inform the spatial context that is important for myogenic differentiation. Here, we demonstrate how large-scale integration of single-cell and spatial transcriptomic data can overcome these limitations. We created a single-cell transcriptomic dataset of mouse skeletal muscle by integration, consensus annotation, and analysis of 23 newly collected scRNAseq datasets and 88 publicly available single-cell (scRNAseq) and single-nucleus (snRNAseq) RNA-sequencing datasets. The resulting dataset includes more than 365,000 cells and spans a wide range of ages, injury, and repair conditions. Together, these data enabled identification of the predominant cell types in skeletal muscle, and resolved cell subtypes, including endothelial subtypes distinguished by vessel-type of origin, fibro-adipogenic progenitors defined by functional roles, and many distinct immune populations. The representation of different experimental conditions and the depth of transcriptome coverage enabled robust profiling of sparsely expressed genes. We built a densely sampled transcriptomic model of myogenesis, from stem cell quiescence to myofiber maturation, and identified rare, transitional states of progenitor commitment and fusion that are poorly represented in individual datasets. We performed spatial RNA sequencing of mouse muscle at three time points after injury and used the integrated dataset as a reference to achieve a high-resolution, local deconvolution of cell subtypes. We also used the integrated dataset to explore ligand-receptor co-expression patterns and identify dynamic cell-cell interactions in muscle injury response. We provide a public web tool to enable interactive exploration and visualization of the data. Our work supports the utility of large-scale integration of single-cell transcriptomic data as a tool for biological discovery.

Hypoxia preconditioned bone marrow-derived mesenchymal stromal/stem cells enhance myoblast fusion and skeletal muscle regeneration

Background

The skeletal muscle reconstruction occurs thanks to unipotent stem cells, i.e., satellite cells. The satellite cells remain quiescent and localized between myofiber sarcolemma and basal lamina. They are activated in response to muscle injury, proliferate, differentiate into myoblasts, and recreate myofibers. The stem and progenitor cells support skeletal muscle regeneration, which could be disturbed by extensive damage, sarcopenia, cachexia, or genetic diseases like dystrophy. Many lines of evidence showed that the level of oxygen regulates the course of cell proliferation and differentiation.

Methods

In the present study, we analyzed hypoxia impact on human and pig bone marrow-derived mesenchymal stromal cell (MSC) and mouse myoblast proliferation, differentiation, and fusion. Moreover, the influence of the transplantation of human bone marrow-derived MSCs cultured under hypoxic conditions on skeletal muscle regeneration was studied.

Results

We showed that bone marrow-derived MSCs increased VEGF expression and improved myogenesis under hypoxic conditions in vitro. Transplantation of hypoxia preconditioned bone marrow-derived MSCs into injured muscles resulted in the improved cell engraftment and formation of new vessels.

Conclusions

We suggested that SDF-1 and VEGF secreted by hypoxia preconditioned bone marrow-derived MSCs played an essential role in cell engraftment and angiogenesis. Importantly, hypoxia preconditioned bone marrow-derived MSCs more efficiently engrafted injured muscles; however, they did not undergo myogenic differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02530-3.

Dynamic changes of miRNAs in skeletal muscle development at New Zealand rabbits

BACKGROUND

miRNA is one of the crucial roles in the complex and dynamic network that regulates the development of skeletal muscle. The landscape of skeletal muscle miRNAs from fetus to adult in New Zealand rabbits has not been revealed yet.

RESULTS

In this study, nine RNA-seq libraries of fetus, child and adult rabbits' leg muscles were constructed. A total of 278 differentially expressed miRNAs (DEmiRNAs) were identified. In the fetus vs. child group, the main functional enrichments were involved in membrane and transport. Pathway enriched terms of up-regulated DEmiRNAs were connected with the differentiation and hypertrophy of skeletal muscle, and down-regulated ones were related to muscle structure and metabolic capacity. In the child vs. adult group, functions were associated to positioning and transportation, and pathways were relevant to ECM, muscle structure and hypertrophy. Finally, ocu-miR-185-3p and ocu-miR-370-3p, which had the most target genes, were identified as hub-miRNAs in these two groups.

CONCLUSIONS

In short, we summarized the highly expressed and uniquely expressed DEmiRNAs of fetus, child and adult rabbits' leg muscles. Besides, the potential functional changes of miRNAs in two consecutive stages have been explored. Among them, the ocu-miR-185-3p and ocu-miR-370-3p with the most target genes were selected as hub-miRNAs. These data improved the understanding of the regulatory molecules of meat rabbit development, and provided a novel perspective for molecular breeding of meat rabbits.

Roles of tetraspanins during trophoblast development: bioinformatics and new perspectives

Tetraspanins are a superfamily of membrane proteins found in all eukaryotic organisms. They act as scaffold molecules that regulate the traffic and function of other membrane/signaling proteins, resulting in important downstream cellular consequences. The aim of this work was to use transcriptomes and bioinformatics analysis to identify the tetraspanins (and their partners) involved in trophoblast differentiation. We built a protein-protein interaction network around tetraspanins which revealed that tetraspanins CD9, CD81, and CD82 show a specific expression during trophoblast differentiation. These proteins appeared to be interconnected and to recruit several membrane partners which include integrins, immune-related molecules, and a variety of receptors. During weeks 8 to 24, a CD9 expression trajectory was identified in extravillous trophoblasts, and a website was developed: ( https://extravillous.shinyapps.io/CD9humanEVT/ ). In conclusion, CD81 may, together with CD9 and CD82, be interconnected in controlling trophoblast invasion in the endometrium. CD9 expression trajectory in extravillous trophoblast between weeks 8 and 24 shows the involvement of CD9 in cell adhesion and migration.

Tetraspanins: Novel Molecular Regulators of Gastric Cancer

Gastric cancer is the fourth and fifth most common cancer worldwide in men and women, respectively. However, patients with an advanced stage of gastric cancer still have a poor prognosis and low overall survival rate. The tetraspanins belong to a protein superfamily with four hydrophobic transmembrane domains and 33 mammalian tetraspanins are ubiquitously distributed in various cells and tissues. They interact with other membrane proteins to form tetraspanin-enriched microdomains and serve a variety of functions including cell adhesion, invasion, motility, cell fusion, virus infection, and signal transduction. In this review, we summarize multiple utilities of tetraspanins in the progression of gastric cancer and the underlying molecular mechanisms. In general, the expression of TSPAN8, CD151, TSPAN1, and TSPAN4 is increased in gastric cancer tissues and enhance the proliferation and invasion of gastric cancer cells, while CD81, CD82, TSPAN5, TSPAN9, and TSPAN21 are downregulated and suppress gastric cancer cell growth. In terms of cell motility regulation, CD9, CD63 and CD82 are metastasis suppressors and the expression level is inversely associated with lymph node metastasis. We also review the clinicopathological significance of tetraspanins in gastric cancer including therapeutic targets, the development of drug resistance and prognosis prediction. Finally, we discuss the potential clinical value and current limitations of tetraspanins in gastric cancer treatments, and provide some guidance for future research.

Myogenic progenitor cell transplantation for muscle regeneration following hindlimb ischemia and reperfusion

Background

Muscle is severely affected by ischemia/reperfusion injury (IRI). Quiescent satellite cells differentiating into myogenic progenitor cells (MPC) possess a remarkable regenerative potential. We herein established a model of local application of MPC in murine hindlimb ischemia/reperfusion to study cell engraftment and differentiation required for muscle regeneration.

Methods

A clamping model of murine (C57b/6 J) hindlimb ischemia was established to induce IRI in skeletal muscle. After 2 h (h) warm ischemic time (WIT) and reperfusion, reporter protein expressing MPC (TdTomato or Luci-GFP, 1 × 10⁶ cells) obtained from isolated satellite cells were injected intramuscularly. Surface marker expression and differentiation potential of MPC were analyzed in vitro by flow cytometry and differentiation assay. In vivo bioluminescence imaging and histopathologic evaluation of biopsies were performed to quantify cell fate, engraftment and regeneration.

Results

2h WIT induced severe IRI on muscle, and muscle fiber regeneration as per histopathology within 14 days after injury. Bioluminescence in vivo imaging demonstrated reporter protein signals of MPC in 2h WIT animals and controls over the study period (75 days). Bioluminescence signals were detected at the injection site and increased over time. TdTomato expressing MPC and myofibers were visible in host tissue on postoperative days 2 and 14, respectively, suggesting that injected MPC differentiated into muscle fibers. Higher reporter protein signals were found after 2h WIT compared to controls without ischemia, indicative for enhanced growth and/or engraftment of MPC injected into IRI-affected muscle antagonizing muscle damage caused by IRI.

Conclusion

WIT-induced IRI in muscle requests increased numbers of injected MPC to engraft and persist, suggesting a possible rational for cell therapy to antagonize IRI. Further investigations are needed to evaluate the regenerative capacity and therapeutic advantage of MPC in the setting of ischemic limb injury.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02208-w.

CD9, a tetraspanin target for cancer therapy?

In the present minireview, we intend to provide a brief history of the field of CD9 involvement in oncogenesis and in the metastatic process of cancer, considering its potential value as a tumor-associated antigenic target. Over the years, CD9 has been identified as a favorable prognostic marker or predictor of metastatic potential depending on the cancer type. To understand its implications in cancer beside its use as an antigenic biomarker, it is essential to know its physiological functions, including its molecular partners in a given cell system. Moreover, the discovery that CD9 is one of the most specific and broadly expressed markers of extracellular membrane vesicles, nanometer-sized entities that are released into extracellular space and various physiological body fluids and play a role in intercellular communication under physiological and pathological conditions, notably the establishment of cancer metastases, has added a new dimension to our knowledge of CD9 function in cancer. Here, we will discuss these issues as well as the possible cancer therapeutic implications of CD9, their limitations, and pitfalls.

TGFβ signaling curbs cell fusion and muscle regeneration

Muscle cell fusion is a multistep process involving cell migration, adhesion, membrane remodeling and actin-nucleation pathways to generate multinucleated myotubes. However, molecular brakes restraining cell-cell fusion events have remained elusive. Here we show that transforming growth factor beta (TGFβ) pathway is active in adult muscle cells throughout fusion. We find TGFβ signaling reduces cell fusion, regardless of the cells' ability to move and establish cell-cell contacts. In contrast, inhibition of TGFβ signaling enhances cell fusion and promotes branching between myotubes in mouse and human. Exogenous addition of TGFβ protein in vivo during muscle regeneration results in a loss of muscle function while inhibition of TGFβR2 induces the formation of giant myofibers. Transcriptome analyses and functional assays reveal that TGFβ controls the expression of actin-related genes to reduce cell spreading. TGFβ signaling is therefore requisite to limit mammalian myoblast fusion, determining myonuclei numbers and myofiber size.

A versatile platform for generating engineered extracellular vesicles with defined therapeutic properties

Extracellular vesicles (EVs) are an important intercellular communication system facilitating the transfer of macromolecules between cells. Delivery of exogenous cargo tethered to the EV surface or packaged inside the lumen are key strategies for generating therapeutic EVs. We identified two “scaffold” proteins, PTGFRN and BASP1, that are preferentially sorted into EVs and enable high-density surface display and luminal loading of a wide range of molecules, including cytokines, antibody fragments, RNA binding proteins, vaccine antigens, Cas9, and members of the TNF superfamily. Molecules were loaded into EVs at high density and exhibited potent in vitro activity when fused to full-length or truncated forms of PTGFRN or BASP1. Furthermore, these engineered EVs retained pharmacodynamic activity in a variety of animal models. This engineering platform provides a simple approach to functionalize EVs with topologically diverse macromolecules and represents a significant advance toward unlocking the therapeutic potential of EVs.

Tetraspanin CD82 is necessary for muscle stem cell activation and supports dystrophic muscle function

Background

Tetraspanins are a family of proteins known to assemble protein complexes at the cell membrane. They are thought to play diverse cellular functions in tissues by modifying protein-binding partners, thus bringing complexity and diversity in their regulatory networks. Previously, we identified the tetraspanin KAI/CD82 as a prospective marker for human muscle stem cells. CD82 expression appeared decreased in human Duchenne muscular dystrophy (DMD) muscle, suggesting a functional link to muscular dystrophy, yet whether this decrease is a consequence of dystrophic pathology or a compensatory mechanism in an attempt to rescue muscle from degeneration is currently unknown.

Methods

We studied the consequences of loss of CD82 expression in normal and dystrophic skeletal muscle and examined the dysregulation of downstream functions in mice aged up to 1 year.

Results

Expression of CD82 is important to sustain satellite cell activation, as in its absence there is decreased cell proliferation and less efficient repair of injured muscle. Loss of CD82 in dystrophic muscle leads to a worsened phenotype compared to control dystrophic mice, with decreased pulmonary function, myofiber size, and muscle strength. Mechanistically, decreased myofiber size in CD82^−/− dystrophic mice is not due to altered PTEN/AKT signaling, although increased phosphorylation of mTOR at Ser2448 was observed.

Conclusion

Basal CD82 expression is important to dystrophic muscle, as its loss leads to significantly weakened myofibers and impaired muscle function, accompanied by decreased satellite cell activity that is unable to protect and repair myofiber damage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-020-00252-3.

Implications for tetraspanin-enriched microdomain assembly based on structures of CD9 with EWI-F

Crystal and single-particle cryo-EM structures reveal how the tetraspanin CD9 interacts with its prototypical partner EWI-F and provide a new concatenation model for the assembly of tetraspanin-enriched microdomains.

Tetraspanins are eukaryotic membrane proteins that contribute to a variety of signaling processes by organizing partner-receptor molecules in the plasma membrane. How tetraspanins bind and cluster partner receptors into tetraspanin-enriched microdomains is unknown. Here, we present crystal structures of the large extracellular loop of CD9 bound to nanobodies 4C8 and 4E8 and, the cryo-EM structure of 4C8-bound CD9 in complex with its partner EWI-F. CD9–EWI-F displays a tetrameric arrangement with two central EWI-F molecules, dimerized through their ectodomains, and two CD9 molecules, one bound to each EWI-F transmembrane helix through CD9-helices h3 and h4. In the crystal structures, nanobodies 4C8 and 4E8 bind CD9 at loops C and D, which is in agreement with the 4C8 conformation in the CD9–EWI-F complex. The complex varies from nearly twofold symmetric (with the two CD9 copies nearly anti-parallel) to ca. 50° bent arrangements. This flexible arrangement of CD9–EWI-F with potential CD9 homo-dimerization at either end provides a “concatenation model” for forming short linear or circular assemblies, which may explain the occurrence of tetraspanin-enriched microdomains.

Rbm24 modulates adult skeletal muscle regeneration via regulation of alternative splicing

Rationale: The adult skeletal muscle can self-repair efficiently following mechanical or pathological damage due to its remarkable regenerative capacity. However, regulatory mechanisms underlying muscle regeneration are complicated and have not been fully elucidated. Alternative splicing (AS) is a major mechanism responsible for post-transcriptional regulation. Many aberrant AS events have been identified in patients with muscular dystrophy which is accompanied by abnormal muscle regeneration. However, little is known about the correlation between AS and muscle regeneration. It has been reported that RNA binding motif protein 24 (Rbm24), a tissue-specific splicing factor, is involved in embryo myogenesis while the role of Rbm24 in adult myogenesis (also called muscle regeneration) is poorly understood. Methods: To investigate the role of Rbm24 in adult skeletal muscle, we generated Rbm24 conditional knockout mice and satellite cell-specific knockout mice. Furthermore, a cardiotoxin (CTX)-induced injury model was utilized to assess the effects of Rbm24 on skeletal muscle regeneration. Genome-wide RNA-Seq was performed to identify the changes in AS following loss of Rbm24. Results: Rbm24 knockout mice displayed abnormal regeneration 4 months after tamoxifen treatment. Using RNA-Seq, we found that Rbm24 regulated a complex network of AS events involved in multiple biological processes, including myogenesis, muscle regeneration and muscle hypertrophy. Moreover, using a CTX-induced injury model, we showed that loss of Rbm24 in skeletal muscle resulted in myogenic fusion and differentiation defects and significantly delayed muscle regeneration. Furthermore, satellite cell-specific Rbm24 knockout mice recapitulated the defects in regeneration seen in the global Rbm24 knockout mice. Importantly, we demonstrated that Rbm24 regulated AS of Mef2d, Naca, Rock2 and Lrrfip1 which are essential for myogenic differentiation and muscle regeneration. Conclusions: The present study demonstrated that Rbm24 regulates dynamic changes in AS and is essential for adult skeletal muscle regeneration.

A role for tetraspanin proteins in regulating fusion induced by Burkholderia thailandensis

Burkholderia pseudomallei is the causative agent of melioidosis, a disease with high morbidity that is endemic in South East Asia and northern Australia. An unusual feature of the bacterium is its ability to induce multinucleated giant cell formation (MNGC), which appears to be related to bacterial pathogenicity. The mechanism of MNGC formation is not fully understood, but host cell factors as well as known bacterial virulence determinants are likely to contribute. Since members of the tetraspanin family of membrane proteins are involved in various types of cell:cell fusion, their role in MNGC formation induced by Burkholderia thailandensis, a mildly pathogenic species closely related to B. pseudomallei, was investigated. The effect of antibodies to tetraspanins CD9, CD81, and CD63 in MNGC formation induced by B. thailandensis in infected mouse J774.2 and RAW macrophage cell lines was assessed along with that of recombinant proteins corresponding to the large extracellular domain (EC2) of the tetraspanins. B. thailandensis-induced fusion was also examined in macrophages derived from CD9 null and corresponding WT mice, and in J774.2 macrophages over-expressing CD9. Antibodies to CD9 and CD81 promoted MNGC formation induced by B. thailandensis, whereas EC2 proteins of CD9, CD81, and CD63 inhibited MNGC formation. Enhanced MNGC formation was observed in CD9 null macrophages, whereas a decrease in MNGC formation was associated with overexpression of CD9. Overall our findings show that tetraspanins are involved in MNGC formation induced by B. thailandensis and by implication, B. pseudomallei, with CD9 and CD81 acting as negative regulators of this process.

A reference single-cell transcriptomic atlas of human skeletal muscle tissue reveals bifurcated muscle stem cell populations

Single-cell RNA-sequencing (scRNA-seq) facilitates the unbiased reconstruction of multicellular tissue systems in health and disease. Here, we present a curated scRNA-seq dataset of human muscle samples from 10 adult donors with diverse anatomical locations. We integrated ~ 22,000 single-cell transcriptomes using Scanorama to account for technical and biological variation and resolved 16 distinct populations of muscle-resident cells using unsupervised clustering of the data compendium. These cell populations included muscle stem/progenitor cells (MuSCs), which bifurcated into discrete "quiescent" and "early-activated" MuSC subpopulations. Differential expression analysis identified transcriptional profiles altered in the activated MuSCs including genes associated with aging, obesity, diabetes, and impaired muscle regeneration, as well as long non-coding RNAs previously undescribed in human myogenic cells. Further, we modeled ligand-receptor cell-communication interactions and observed enrichment of the TWEAK-FN14 pathway in activated MuSCs, a characteristic signature of muscle wasting diseases. In contrast, the quiescent MuSCs have enhanced expression of the EGFR receptor, a recognized human MuSC marker. This work provides a new benchmark reference resource to examine human muscle tissue heterogeneity and identify potential targets in MuSC diversity and dysregulation in disease contexts.

Cd9 Protects Photoreceptors from Injury and Potentiates Edn2 Expression

Purpose

Cd9 is a tetraspanin membrane protein that plays various roles in tissue development and disease pathogenesis, especially in cancer, but the expression patterns and function of Cd9 in retinal development and disease are not well understood. We asked its roles during retinal photoreceptor degeneration by using CD9-knockout mice.

Methods

Cd9 knockout mice and rd1 mice were used to examine roles of Cd9 for progression of photoreceptor degeneration. Reverse transcription-polymerase chain reaction and immunohistochemistry were mainly used as analytical methods.

Results

Cd9 transcripts were only weakly expressed in retina at embryonic day 14, but its expression level subsequently increased and peaked at around postnatal day 12. In 6-week-old female mice derived retina, mRNA expression decreased slightly but was maintained at a significant level. Published RNA-sequencing data and immunohistochemistry indicated that Cd9 was expressed abundantly in Müller glia and weakly in other retinal neurons. Notably, when photoreceptors were damaged, Cd9 expression was increased in rod photoreceptors and decreased in Müller glia. Cd9 knockout mice retinas developed normally; however, once the retina suffered damage, degeneration of photoreceptors was more severe in Cd9 knockout retinas than control retinas. Induction of Edn2, which is known to protect against photoreceptor damage, was severely hampered. In addition, induction of Socs3, which is downstream of gp130 (Il6st), was weaker in Cd9 knockout retinas.

Conclusions

Taken together, these findings indicate that, although Cd9 was dispensable for normal gross morphological development, it protected rod photoreceptors and enhanced Edn2 expression, possibly through modulation of gp130 signaling.

Site-specific functionality and tryptophan mimicry of lipidation in tetraspanin CD9

Lipidation of transmembrane proteins regulates many cellular activities, including signal transduction, cell–cell communication, and membrane trafficking. However, how lipidation at different sites in a membrane protein affects structure and function remains elusive. Here, using native mass spectrometry we determined that wild‐type human tetraspanins CD9 and CD81 exhibit nonstochastic distributions of bound acyl chains. We revealed CD9 lipidation at its three most frequent lipidated sites suffices for EWI‐F binding, while cysteine‐to‐alanine CD9 mutations markedly reduced binding of EWI‐F. EWI‐F binding by CD9 was rescued by mutating all or, albeit to a lesser extent, only the three most frequently lipidated sites into tryptophans. These mutations did not affect the nanoscale distribution of CD9 in cell membranes, as shown by super‐resolution microscopy using a CD9‐specific nanobody. Thus, these data demonstrate site‐specific, possibly conformation‐dependent, functionality of lipidation in tetraspanin CD9 and identify tryptophan mimicry as a possible biochemical approach to study site‐specific transmembrane‐protein lipidation.

EWI-2 Inhibits Cell-Cell Fusion at the HIV-1 Virological Presynapse

Cell-to-cell transfer of virus particles at the Env-dependent virological synapse (VS) is a highly efficient mode of HIV-1 transmission. While cell–cell fusion could be triggered at the VS, leading to the formation of syncytia and preventing exponential growth of the infected cell population, this is strongly inhibited by both viral (Gag) and host (ezrin and tetraspanins) proteins. Here, we identify EWI-2, a protein that was previously shown to associate with ezrin and tetraspanins, as a host factor that contributes to the inhibition of Env-mediated cell–cell fusion. Using quantitative fluorescence microscopy, shRNA knockdowns, and cell–cell fusion assays, we show that EWI-2 accumulates at the presynaptic terminal (i.e., the producer cell side of the VS), where it contributes to the fusion-preventing activities of the other viral and cellular components. We also find that EWI-2, like tetraspanins, is downregulated upon HIV-1 infection, most likely by Vpu. Despite the strong inhibition of fusion at the VS, T cell-based syncytia do form in vivo and in physiologically relevant culture systems, but they remain small. In regard to that, we demonstrate that EWI-2 and CD81 levels are restored on the surface of syncytia, where they (presumably) continue to act as fusion inhibitors. This study documents a new role for EWI-2 as an inhibitor of HIV-1-induced cell–cell fusion and provides novel insight into how syncytia are prevented from fusing indefinitely.

Stromal cell-derived factor-1 (CXCL12) and its role in bone and muscle biology

Musculoskeletal disorders are the leading cause of disability worldwide; two of the most prevalent of which are osteoporosis and sarcopenia. Each affect millions in the aging population across the world and the associated morbidity and mortality contributes to billions of dollars in annual healthcare cost. Thus, it is important to better understand the underlying pathologic mechanisms of the disease process. Regulatory chemokine, CXCL12, and its receptor, CXCR4, are recognized to be essential in the recruitment, localization, maintenance, development and differentiation of progenitor stem cells of the musculoskeletal system. CXCL12 signaling results in the development and functional ability of osteoblasts, osteoclasts, satellite cells and myoblasts critical to maintaining musculoskeletal homeostasis. Interestingly, one suggested pathologic mechanism of osteoporosis and sarcopenia is a decline in the regenerative capacity of musculoskeletal progenitor stem cells. Thus, because CXCL12 is critical to progenitor function, a disruption in the CXCL12 signaling axis might play a distinct role in these pathological processes. Therefore, in this article, we perform a review of CXCL12, its physiologic and pathologic function in bone and muscle, and potential targets for therapeutic development.

Phosphatidylserine on viable sperm and phagocytic machinery in oocytes regulate mammalian fertilization

Fertilization is essential for species survival. Although Izumo1 and Juno are critical for initial interaction between gametes, additional molecules necessary for sperm:egg fusion on both the sperm and the oocyte remain to be defined. Here, we show that phosphatidylserine (PtdSer) is exposed on the head region of viable and motile sperm, with PtdSer exposure progressively increasing during sperm transit through the epididymis. Functionally, masking phosphatidylserine on sperm via three different approaches inhibits fertilization. On the oocyte, phosphatidylserine recognition receptors BAI1, CD36, Tim-4, and Mer-TK contribute to fertilization. Further, oocytes lacking the cytoplasmic ELMO1, or functional disruption of RAC1 (both of which signal downstream of BAI1/BAI3), also affect sperm entry into oocytes. Intriguingly, mammalian sperm could fuse with skeletal myoblasts, requiring PtdSer on sperm and BAI1/3, ELMO2, RAC1 in myoblasts. Collectively, these data identify phosphatidylserine on viable sperm and PtdSer recognition receptors on oocytes as key players in sperm:egg fusion.

Tspan15 Is a New Stemness-Related Marker in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the second cause of cancer-related deaths worldwide. A clearer understanding of the molecular mechanisms underlying tumor growth and invasiveness remains crucial for developing new therapies. Here, the expression of tetraspanins, a family of plasma membrane organizers involved in tumor progression, has been addressed. Integrative approaches combining transcriptomics and bioinformatics allow demonstrating the induced and heterogeneous expression of Tspan15 in HCC. Tspan15 positive tumors exhibit signatures related to hepatic progenitor cells as well as recurrence of cancer. Immunohistochemistry experiments confirm Tspan15 expression in the subset of HCC expressing stemness-related markers such as EpCAM and Cytokeratin-19. Functional networks reveal that most of these genes expressed in correlation to Tspan15 support cell proliferation. Furthermore, Tspan15 overexpression in the hepatoma cell line HepG2 significantly increases cell proliferation. A quantitative proteomic analysis of the secretome reveals a higher abundance of the protein connective tissue growth factor (CTGF), a pleiotropic matricellular signaling protein. Proteomic profiling of Tspan15 complexes allows identifying numerous membrane proteins including several growth factor receptors. Finally, Tspan15 increases ERK1/2 phosphorylation that directly controls CTGF expression and secretion. In conclusion, Tspan15 is a new stemness-related marker in HCC which exhibits high potential of tumor growth and recurrence.

Nanoscale organization of tetraspanins during HIV-1 budding by correlative dSTORM/AFM

Membrane partition and remodeling play a key role in numerous cell mechanisms, especially in viral replication cycles where viruses subvert the plasma membrane to enter and escape from the host cell. Specifically assembly and release of HIV-1 particles require specific cellular components, which are recruited to the egress site by the viral protein Gag. We previously demonstrated that HIV-1 assembly alters both partitioning and dynamics of the tetraspanins CD9 and CD81, which are key players in many infectious processes, forming enriched areas where the virus buds. In this study we correlated super resolution microscopy mapping of tetraspanins with membrane topography delineated by atomic force microscopy (AFM) in Gag-expressing cells. We revealed that CD9 is specifically trapped within the nascent viral particles, especially at buds tips, suggesting that Gag mediates CD9 and CD81 depletion from the plasma membrane. In addition, we showed that CD9 is organized as small membrane assemblies of few tens of nanometers that can coalesce upon Gag expression.

Regulation of cytoskeleton and adhesion signaling in osteoclasts by tetraspanin CD82

We used a myeloid-specific Cre to conditionally delete CD82 in mouse osteoclasts and their precursors. In contrast to global loss of CD82 (gKO), conditional loss of CD82 (cKO) in osteoclasts does not affect cortical bone, osteoblasts, or adipocytes. CD82 loss results in greater trabecular volume and trabecular number but reduced trabecular space in 6-month old male mice. Though this trend is present in females it did not reach significance; whereas there was an increase in osteoclast numbers and eroded surface area only in female cKO mice. In vitro, there is an increase in osteoclast fusion and defects in actin assembly in both gKO and cKO mice, irrespective of sex. This is accompanied by altered osteoclast morphology and decreased release of CTX in vitro. Integrin αvβ3 expression is reduced, while integrin β1 is increased. Signaling to Src, Syk, and Vav are also compromised. We further discovered that expression of Clec2 and its ligand, Podoplanin, molecules that also signal to Syk and Vav, are increased in differentiated osteoclasts. Loss of CD82 reduces their expression. Thus, CD82 is required for correct assembly of the cytoskeleton and to limit osteoclast fusion, both needed for normal osteoclast function.

Palmitoylation is a post-translational modification of Alix regulating the membrane organization of exosome-like small extracellular vesicles

BACKGROUND

Virtually all cell types have the capacity to secrete nanometer-sized extracellular vesicles, which have emerged in recent years as potent signal transducers and cell-cell communicators. The multifunctional protein Alix is a bona fide exosomal regulator and skeletal muscle cells can release Alix-positive nano-sized extracellular vesicles, offering a new paradigm for understanding how myofibers communicate within skeletal muscle and with other organs. S-palmitoylation is a reversible lipid post-translational modification, involved in different biological processes, such as the trafficking of membrane proteins, achievement of stable protein conformations, and stabilization of protein interactions.

METHODS

Here, we have used an integrated biochemical-biophysical approach to determine whether S-palmitoylation contributes to the regulation of extracellular vesicle production in skeletal muscle cells.

RESULTS

We ascertained that Alix is S-palmitoylated and that this post-translational modification influences its protein-protein interaction with CD9, a member of the tetraspanin protein family. Furthermore, we showed that the structural organization of the lipid bilayer of the small (nano-sized) extracellular vesicle membrane with altered palmitoylation is qualitatively different compared to mock control vesicles.

CONCLUSIONS

We propose that S-palmitoylation regulates the function of Alix in facilitating the interactions among extracellular vesicle-specific regulators and maintains the proper structural organization of exosome-like extracellular vesicle membranes.

GENERAL SIGNIFICANCE

Beyond its biological relevance, our study also provides the means for a comprehensive structural characterization of EVs.

Biochemical and pathological changes result from mutated Caveolin-3 in muscle

Background

Caveolin-3 (CAV3) is a muscle-specific protein localized to the sarcolemma. It was suggested that CAV3 is involved in the connection between the extracellular matrix (ECM) and the cytoskeleton. Caveolinopathies often go along with increased CK levels indicative of sarcolemmal damage. So far, more than 40 dominant pathogenic mutations have been described leading to several phenotypes many of which are associated with a mis-localization of the mutant protein to the Golgi. Golgi retention and endoplasmic reticulum (ER) stress has been demonstrated for the CAV3 p.P104L mutation, but further downstream pathophysiological consequences remained elusive so far.

Methods

We utilized a transgenic (p.P104L mutant) mouse model and performed proteomic profiling along with immunoprecipitation, immunofluorescence and immunoblot examinations (including examination of α-dystroglycan glycosylation), and morphological studies (electron and coherent anti-Stokes Raman scattering (CARS) microscopy) in a systematic investigation of molecular and subcellular events in p.P104L caveolinopathy.

Results

Our electron and CARS microscopic as well as immunological studies revealed Golgi and ER proliferations along with a build-up of protein aggregates further characterized by immunoprecipitation and subsequent mass spectrometry. Molecular characterization these aggregates showed affection of mitochondrial and cytoskeletal proteins which accords with our ultra-structural findings. Additional global proteomic profiling revealed vulnerability of 120 proteins in diseased quadriceps muscle supporting our previous findings and providing more general insights into the underlying pathophysiology. Moreover, our data suggested that further DGC components are altered by the perturbed protein processing machinery but are not prone to form aggregates whereas other sarcolemmal proteins are ubiquitinated or bind to p62. Although the architecture of the ER and Golgi as organelles of protein glycosylation are altered, the glycosylation of α-dystroglycan presented unchanged.

Conclusions

Our combined data classify the p.P104 caveolinopathy as an ER-Golgi disorder impairing proper protein processing and leading to aggregate formation pertaining proteins important for mitochondrial function, cytoskeleton, ECM remodeling and sarcolemmal integrity. Glycosylation of sarcolemmal proteins seems to be normal. The new pathophysiological insights might be of relevance for the development of therapeutic strategies for caveolinopathy patients targeting improved protein folding capacity.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0173-y) contains supplementary material, which is available to authorized users.

Monocyte Subsets Have Distinct Patterns of Tetraspanin Expression and Different Capacities to Form Multinucleate Giant Cells

Monocytes are able to undergo homotypic fusion to produce different types of multinucleated giant cells, such as Langhans giant cells in response to M. tuberculosis infection or foreign body giant cells in response to implanted biomaterials. Monocyte fusion is highly coordinated and complex, with various soluble, intracellular, and cell-surface components mediating different stages of the process. Tetraspanins, such as CD9, CD63, and CD81, are known to be involved in cell:cell fusion and have been suggested to play a role in regulating homotypic monocyte fusion. However, peripheral human monocytes are not homogenous: they exist as a heterogeneous population consisting of three subsets, classical (CD14⁺⁺CD16^-), intermediate (CD14⁺⁺CD16⁺), and non-classical (CD14⁺CD16⁺), at steady state. During infection with mycobacteria, the circulating populations of intermediate and non-classical monocytes increase, suggesting they may play a role in the disease outcome. Human monocytes were separated into subsets and then induced to fuse using concanavalin A. The intermediate monocytes were able to fuse faster and form significantly larger giant cells than the other subsets. When antibodies targeting tetraspanins were added, the intermediate monocytes responded to anti-CD63 by forming smaller giant cells, suggesting an involvement of tetraspanins in fusion for at least this subset. However, the expression of fusion-associated tetraspanins on monocyte subsets did not correlate with the extent of fusion or with the inhibition by tetraspanin antibody. We also identified a CD9^High and a CD9^Low monocyte population within the classical subset. The CD9^High classical monocytes expressed higher levels of tetraspanin CD151 compared to CD9^Low classical monocytes but the CD9^High classical subset did not exhibit greater potential to fuse and the role of these cells in immunity remains unknown. With the exception of dendrocyte-expressed seven transmembrane protein, which was expressed at higher levels on the intermediate monocyte subset, the expression of fusion-related proteins between the subsets did not clearly correlate with their ability to fuse. We also did not observe any clear correlation between giant cell formation and the expression of pro-inflammatory or fusogenic cytokines. Although tetraspanin expression appears to be important for the fusion of intermediate monocytes, the control of multinucleate giant cell formation remains obscure.

Regulation of skeletal myotube formation and alignment by nanotopographically controlled cell-secreted extracellular matrix

Skeletal muscle has a well-organized tissue structure comprised of aligned myofibers and an encasing extracellular matrix (ECM) sheath or lamina, within which reside satellite cells. We hypothesize that the organization of skeletal muscle tissues in culture can affect both the structure of the deposited ECM and the differentiation potential of developing myotubes. Furthermore, we posit that cellular and ECM cues can be a strong determinant of myoblast fusion and morphology in 3D tissue culture environments. To test these, we utilized a thermoresponsive nanofabricated substratum to engineer anisotropic sheets of myoblasts which could then be transferred and stacked into multilayered tissues. Within such engineered tissues, we found that myoblasts rapidly sense topography and deposit structurally organized ECM proteins. Furthermore, the initial tissue structure was found to exert significant control over myoblast fusion and eventual myotube organization. These results highlight the importance of ECM structure on myoblast fusion and organization, and provide insights into substrate-mediated control of myotube formation in the development of novel, more effective, engineered skeletal muscle tissues. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1543-1551, 2018.

Cell surface flip-flop of phosphatidylserine is critical for PIEZO1-mediated myotube formation

Myotube formation by fusion of myoblasts and subsequent elongation of the syncytia is essential for skeletal muscle formation. However, molecules that regulate myotube formation remain elusive. Here we identify PIEZO1, a mechanosensitive Ca²⁺ channel, as a key regulator of myotube formation. During myotube formation, phosphatidylserine, a phospholipid that resides in the inner leaflet of the plasma membrane, is transiently exposed to cell surface and promotes myoblast fusion. We show that cell surface phosphatidylserine inhibits PIEZO1 and that the inward translocation of phosphatidylserine, which is driven by the phospholipid flippase complex of ATP11A and CDC50A, is required for PIEZO1 activation. PIEZO1-mediated Ca²⁺ influx promotes RhoA/ROCK-mediated actomyosin assemblies at the lateral cortex of myotubes, thus preventing uncontrolled fusion of myotubes and leading to polarized elongation during myotube formation. These results suggest that cell surface flip-flop of phosphatidylserine acts as a molecular switch for PIEZO1 activation that governs proper morphogenesis during myotube formation.

Myotube formation by fusion of myoblasts is essential for skeletal muscle formation, but which molecules regulate this process remains elusive. Here authors identify the mechanosensitive PIEZO1 channel as a key element, whose activity is regulated by phosphatidylserine during myotube formation.

Double deletion of tetraspanins CD9 and CD81 in mice leads to a syndrome resembling accelerated aging

Chronic obstructive pulmonary disease (COPD) has been recently characterized as a disease of accelerated lung aging, but the mechanism remains unclear. Tetraspanins have emerged as key players in malignancy and inflammatory diseases. Here, we found that CD9/CD81 double knockout (DKO) mice with a COPD-like phenotype progressively developed a syndrome resembling human aging, including cataracts, hair loss, and atrophy of various organs, including thymus, muscle, and testis, resulting in shorter survival than wild-type (WT) mice. Consistent with this, DNA microarray analysis of DKO mouse lungs revealed differential expression of genes involved in cell death, inflammation, and the sirtuin-1 (SIRT1) pathway. Accordingly, expression of SIRT1 was reduced in DKO mouse lungs. Importantly, siRNA knockdown of CD9 and CD81 in lung epithelial cells additively decreased SIRT1 and Foxo3a expression, but reciprocally upregulated the expression of p21 and p53, leading to reduced cell proliferation and elevated apoptosis. Furthermore, deletion of these tetraspanins increased the expression of pro-inflammatory genes and IL-8. Hence, CD9 and CD81 might coordinately prevent senescence and inflammation, partly by maintaining SIRT1 expression. Altogether, CD9/CD81 DKO mice represent a novel model for both COPD and accelerated senescence.

Preparation of decellularized biphasic hierarchical myotendinous junction extracellular matrix for muscle regeneration

Muscle injury and defect affect people's quality of life, and effective treatment is lacking. Herein, we generated a scaffold to obtain decellularized porcine Achilles tendon myotendinous junction (D-MTJ) extracellular matrix (ECM) with well-preserved native biphasic hierarchical structure, biological composition, and excellent mechanical properties for muscle regeneration. The combined use of potassium chloride, potassium iodide, Triton-X 100, and sodium-dodecyl sulfate (SDS) can completely remove the main immunogenicity, while maintaining the major biological components and microstructure. The specific biomechanics of D-MTJ is comparable to the native muscle-tendon physiological conditions. Additionally, the D-MTJ ECM scaffold induced minimal immunological reaction (histology analysis) through rat subcutaneous implantation. Moreover, in vitro, muscle satellite cells adhered, proliferated, and infiltrated into the D-MTJ scaffold, and myofiber-like cell differentiation was observed as shown by increased expression of myogenesis-related genes during culture. In vivo, newly formed myofibers were observed in a muscle defect model with D-MTJ orthotopic transplantation, while the control group presented mostly with fibrous tissue deposition. Additionally, the number of Myod and MyHC-positive cells in the ECM scaffold group was higher at day 30. We preliminary explored the mechanisms underlying D-MTJ-mediated muscle regeneration, which may be attributed to its specific biphasic hierarchical structure, bio-components, and attractiveness for myogenesis cells. In conclusion, our findings suggest the D-MTJ ECM scaffold prepared in this study is a promising choice for muscle regeneration.

STATEMENT OF SIGNIFICANCE

This study is the first to use decellularization technology obtaining the specifically decellularized myotendinous junction (D-MTJ) with well-preserved biphasic hierarchical structure and constituents, excellent mechanical properties and good biocompatibility. The D-MTJ was further proved to be efficient for muscle regeneration in vitro and in vivo, and the underlying mechanisms may be attributed to its specifically structure and constituents, improved myogenesis and good preservation of repair-related factors. Our study may provide basis for the decellularization of other biphasic hierarchical tissues and a platform for further studies on muscle fiber and tendon integrations in vitro.

R-spondin1 Controls Muscle Cell Fusion through Dual Regulation of Antagonistic Wnt Signaling Pathways

Wnt-mediated signals are involved in many important steps in mammalian regeneration. In multiple cell types, the R-spondin (Rspo) family of secreted proteins potently activates the canonical Wnt/β-catenin pathway. Here, we identify Rspo1 as a mediator of skeletal muscle tissue repair. First, we show that deletion of Rspo1 results in global alteration of muscle regeneration kinetics following acute injury. We find that muscle progenitor cells lacking Rspo1 show delayed differentiation due to reduced activation of Wnt/β-catenin target genes. Furthermore, muscle cells lacking Rspo1 have a fusion phenotype leading to larger myotubes containing supernumerary nuclei both in vitro and in vivo. The increase in muscle fusion was dependent on downregulation of Wnt/β-catenin and upregulation of non-canonical Wnt7a/Fzd7/Rac1 signaling. We conclude that reciprocal control of antagonistic Wnt signaling pathways by Rspo1 in muscle stem cell progeny is a key step ensuring normal tissue architecture restoration following acute damage.

Transiently expressed pattern during myogenesis and candidate miRNAs of Tmem8C in goose

Transmembrane protein 8C (Tmem8C) is a muscle-specific membrane protein that controls myoblast fusion, which is essential for the formation of multinucleated muscle fibres. As most of the birds can fly, they have enormous requirement for the muscle, but there are only a few studies of Tmem8C in birds. In this study, we obtained the coding sequence (CDS) of Tmem8C in goose, predicted miRNAs that can act on the 3'UTR, analysed expression profiles of this gene in breast and leg muscles (BM and LM) during the embryonic period and neonatal stages, and identified miRNAs that might affect the targeted gene. The results revealed a high homology between Tmem8C in goose and other animals (indicated by sequence comparisons and phylogenetic trees), some conservative characteristics (e.g., six transmembrane domains and two E-boxes in the 5'UTR might be the potential binding sites of muscle regulatory factors (MRFs)), and the d_N/d_S ratio indicated purifying selection acting on this gene, facilitating conservatism in vertebrates. Q-PCR indicated Tmem8C had a peak expression pattern, reaching its highest expression levels in stage E15 in LM and E19 in BM, and then dropping transiently in E23 (P < 0.05). We examined 13 candidate miRNAs, and negative relationships were detected both in BM and LM (mir-125b-5p, mir-15a, mir-16-1 and mir-n23). Notably, mir-16-1 significantly decreased luciferase activity in dual luciferase reporter gene (LRG) assay, suggesting that it can be identified as potential factors affecting Tmem8C. This study investigated Tmem8C in water bird for the first time, and provided useful information about this gene and its candidate miRNAs in goose.

Altered expression of ganglioside GM3 molecular species and a potential regulatory role during myoblast differentiation

Gangliosides (sialic acid-containing glycosphingolipids) help regulate many important biological processes, including cell proliferation, signal transduction, and differentiation, via formation of functional microdomains in plasma membranes. The structural diversity of gangliosides arises from both the ceramide moiety and glycan portion. Recently, differing molecular species of a given ganglioside are suggested to have distinct biological properties and regulate specific and distinct biological events. Elucidation of the function of each molecular species is important and will provide new insights into ganglioside biology. Gangliosides are also suggested to be involved in skeletal muscle differentiation; however, the differential roles of ganglioside molecular species remain unclear. Here we describe striking changes in quantity and quality of gangliosides (particularly GM3) during differentiation of mouse C2C12 myoblast cells and key roles played by distinct GM3 molecular species at each step of the process.