Unexpected contribution of fibroblasts to muscle lineage as a mechanism for limb muscle patterning

Spatial transcriptomics in the adult Drosophila brain and body

Recently, we have achieved a significant milestone with the creation of the Fly Cell Atlas. This single-nuclei atlas encompasses the entire fly, covering the entire head and body, in addition to all major organs. This atlas catalogs many hundreds of cell types, of which we annotated 250. Thus, a large number of clusters remain to be fully characterized, in particular in the brain. Furthermore, by applying single-nuclei sequencing, all information about the spatial location of the cells in the body and of about possible subcellular localization of the mRNAs within these cells is lost. Spatial transcriptomics promises to tackle these issues. In a proof-of-concept study, we have here applied spatial transcriptomics using a selected gene panel to pinpoint the locations of 150 mRNA species in the adult fly. This enabled us to map unknown clusters identified in the Fly Cell Atlas to their spatial locations in the fly brain. Additionally, spatial transcriptomics discovered interesting principles of mRNA localization and transcriptional diversity within the large and crowded muscle cells that may spark future mechanistic investigations. Furthermore, we present a set of computational tools that will allow for easier integration of spatial transcriptomics and single-cell datasets.

Dynamic interactions between cartilaginous and tendinous/ligamentous primordia during musculoskeletal integration

Proper connections between cartilaginous and muscular primordia through tendinous/ligamentous primordia are essential for musculoskeletal integration. Herein, we report a novel double-reporter mouse model for investigating this process via fluorescently visualising scleraxis (Scx) and SRY-box containing gene 9 (Sox9) expression. We generated ScxTomato transgenic mice and crossed them with Sox9EGFP knock-in mice to obtain ScxTomato;Sox9EGFP mice. Deep imaging of optically cleared double-reporter embryos at E13.5 and E16.5 revealed previously unknown differences in the dynamic interactions between cartilaginous and tendinous/ligamentous primordia in control and Scx-deficient mice. Tendon/ligament maturation was evaluated through simultaneous detection of fluorescence and visualisation of collagen fibre formation using second harmonic generation imaging. Lack of deltoid tuberosity in Scx-deficient mice caused misdirected muscle attachment with morphological changes. Loss of Scx also dysregulated progenitor cell fate determination in the chondrotendinous junction, resulting in the formation of a rounded enthesis rather than the protruding enthesis observed in the control. Hence, our double-reporter mouse system, in combination with loss- or gain-of-function approaches, is a unique and powerful tool that could be used to gain a comprehensive understanding of musculoskeletal integration.

Fibre type differences in the organisation of mononuclear cells and myonuclei at the tips of human myofibres

The myotendinous junction (MTJ) is a weak link in the musculoskeletal system. Here, we isolated the tips of single myofibres from healthy (non-injured) human hamstring muscles for confocal microscopy (n=6) and undertook RNAscope in situ hybridisation (n=6) to gain insight into the profiles of cells and myonuclei in this region, in a fibre type manner. A marked presence of mononuclear cells was observed coating the myofibre tips (confirmed by serial block face scanning electron microscopy and cryosection immunofluorescence), with higher numbers for type I (median 29; range 16-63) than type II (16; 9-23) myofibres (P<0.05). The number of these cells expressing COL22A1 was comparable between fibre types. Myonuclear number and density gradually increased from the myofibre proper towards the tip for both fibre types (P<0.05). COL22A1 was expressed by similar proportions of myonuclei in type I (median 26%; range 13-56) and type II (19%; 3-67) myofibre tips. 70% of the COL22A1-positive nuclei in the MTJ region were myonuclei, and the remaining 30% were MTJ cells. This insight refines our fundamental understanding of the human MTJ at the cell and structural levels.

3D-environment and muscle contraction regulate the heterogeneity of myonuclei

Skeletal muscle formation involves tight interactions between muscle cells and associated connective tissue fibroblasts. Every muscle displays the same type of organisation, they are innervated in the middle and attached at both extremities to tendons. Myonuclei are heterogeneous along myotubes and regionalised according to these middle and tip domains. During development, as soon as myotubes are formed, myonuclei at muscle tips facing developing tendons display their own molecular program. In addition to molecular heterogeneity, a subset of tip myonuclei has a fibroblastic origin different to the classical somitic origin, highlighting a cellular heterogeneity of myonuclei in foetal myotubes. To gain insights on the functional relevance of myonucleus heterogeneity during limb development, we used 2D culture and co-culture systems to dissociate autonomous processes (occurring in 2D-cultures) from 3D-environment of tissue development. We also assessed the role of muscle contraction in myonucleus heterogeneity in paralysed limb muscles. The regionalisation of cellular heterogeneity was not observed in 2D cell culture systems and paralyzed muscles. The molecular signature of MTJ myonuclei was lost in a dish and paralysed muscles indicating a requirement of 3D-enviroment and muscle contraction for MTJ formation. Tip genes that maintain a regionalized expression at myotube tips in cultures are linked to sarcomeres. The behaviour of regionalized markers in cultured myotubes and paralyzed muscles allows us to speculate whether the genes intervene in myogenesis, myotube attachment or MTJ formation.

Single-cell transcriptomic analysis of flowering regulation and vernalization in Chinese cabbage shoot apex

In Chinese cabbage development the interplay between shoot apex activity and vernalization is pivotal for flowering timing. The intricate relationship between various cell types in the shoot apex meristem and their roles in regulating flowering gene expression in Chinese cabbage is not yet fully understood. A thorough analysis of single-cell types in the Chinese cabbage shoot apex and their influence on flowering genes and vernalization is essential for deeper insight. Our study first established a single-cell transcriptomic atlas of Chinese cabbage after 25 days of non-vernalization. Analyzing 19 602 single cells, we differentiated them into 15 distinct cell clusters using established marker genes. We found that key genes in shoot apex development and flowering were primarily present in shoot meristematic cells (SMCs), companion cells (CCs), and mesophyll cells (MCs). MADS-box protein FLOWERING LOCUS C 2 (BrFLC2), a gene suppressing flowering, was observed in CCs, mirroring patterns found in Arabidopsis. By mapping developmental trajectories of SMCs, CCs, and MCs, we elucidated the evolutionary pathways of crucial genes in shoot apex development and flowering. The creation of a single-cell transcriptional atlas of the Chinese cabbage shoot apex under vernalization revealed distinct alterations in the expression of known flowering genes, such as VERNALIZATION INSENSITIVE 3 (VIN3), VERNALIZATION 1 (VRN1), VERNALIZATION 2 (VRN2), BrFLC, and FLOWERING LOCUS T (FT), which varied by cell type. Our study underscores the transformative impact of single-cell RNA sequencing (scRNA-seq) for unraveling the complex differentiation and vernalization processes in the Chinese cabbage shoot apex. These insights are pivotal for enhancing breeding strategies and cultivation management of this vital vegetable.

Skeletal Muscle Satellite Cells Co-Opt the Tenogenic Gene Scleraxis to Instruct Regeneration

Skeletal muscles connect bones and tendons for locomotion and posture. Understanding the regenerative processes of muscle, bone and tendon is of importance to basic research and clinical applications. Despite their interconnections, distinct transcription factors have been reported to orchestrate each tissue's developmental and regenerative processes. Here we show that Scx expression is not detectable in adult muscle stem cells (also known as satellite cells, SCs) during quiescence. Scx expression begins in activated SCs and continues throughout regenerative myogenesis after injury. By SC-specific Scx gene inactivation (ScxcKO), we show that Scx function is required for SC expansion/renewal and robust new myofiber formation after injury. We combined single-cell RNA-sequencing and CUT&RUN to identify direct Scx target genes during muscle regeneration. These target genes help explain the muscle regeneration defects of ScxcKO, and are not overlapping with Scx -target genes identified in tendon development. Together with a recent finding of a subpopulation of Scx -expressing connective tissue fibroblasts with myogenic potential during early embryogenesis, we propose that regenerative and developmental myogenesis co-opt the Scx gene via different mechanisms.

The diagnostic value and associated molecular mechanism study for fibroblast-related mitochondrial genes on keloid

PURPOSE

This study aims to reveal the mechanism of fibroblast-related mitochondrial genes on keloid formation and explore promising signature genes for keloid diagnosis.

METHOD

The distribution of fibroblasts between the keloid sample and control sample based on three keloid datasets, followed by the differentially expressed genes (DEGs) investigation and associated enrichment analysis. Then, hub genes were explored based on DEGs, mitochondrial genes from an online database, as well as fibroblast-related genes that were revealed by WCGNA. Subsequently, signature genes were screened through machine learning, and their diagnostic value was validated by nomogram. Moreover, the targeted drugs and related transcriptional regulation of these genes were analyzed. Finally, the verification analysis was performed on signature genes using qPCR analysis.

RESULT

A total of totally 329 DEGs were revealed based on three datasets, followed by enrichment analysis. WGCNA revealed a total of 258 fibroblast-related genes, which were primarily assembled in functions like muscle tissue development. By using machine learning, we screened four signature genes (ACSF2, ALDH1B1, OCIAD2, and SIRT4) based on eight hub genes (fibroblast-related mitochondrial genes). Nomogram and validation analyses confirmed the well-diagnostic performance of these four genes in keloid. Immune infiltration and drug correlation analyses showed that SIRT4 was significantly associated with immune cell type 2 T helper cells and molecular drug cyclosporin. All these findings provided new perspectives for the clinical diagnosis and therapy of keloid.

CONCLUSION

The fibroblast-related mitochondrial genes including SIRT4, OCIAD2, ALDH1B1, and ACSF2 were novel signature genes for keloid diagnosis, offering novel targets and strategies for diagnosis and therapy of keloid.

Advances in single-cell transcriptomics in animal research

Understanding biological mechanisms is fundamental for improving animal production and health to meet the growing demand for high-quality protein. As an emerging biotechnology, single-cell transcriptomics has been gradually applied in diverse aspects of animal research, offering an effective method to study the gene expression of high-throughput single cells of different tissues/organs in animals. In an unprecedented manner, researchers have identified cell types/subtypes and their marker genes, inferred cellular fate trajectories, and revealed cell‒cell interactions in animals using single-cell transcriptomics. In this paper, we introduce the development of single-cell technology and review the processes, advancements, and applications of single-cell transcriptomics in animal research. We summarize recent efforts using single-cell transcriptomics to obtain a more profound understanding of animal nutrition and health, reproductive performance, genetics, and disease models in different livestock species. Moreover, the practical experience accumulated based on a large number of cases is highlighted to provide a reference for determining key factors (e.g., sample size, cell clustering, and cell type annotation) in single-cell transcriptomics analysis. We also discuss the limitations and outlook of single-cell transcriptomics in the current stage. This paper describes the comprehensive progress of single-cell transcriptomics in animal research, offering novel insights and sustainable advancements in agricultural productivity and animal health.

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

Limb connective tissue is organized in a continuum of promiscuous fibroblast identities during development

Connective tissue (CT), which includes tendon and muscle CT, plays critical roles in development, in particular as positional cue provider. Nonetheless, our understanding of fibroblast developmental programs is hampered because fibroblasts are highly heterogeneous and poorly characterized. Combining single-cell RNA-sequencing-based strategies including trajectory inference and in situ hybridization analyses, we address the diversity of fibroblasts and their developmental trajectories during chicken limb fetal development. We show that fibroblasts switch from a positional information to a lineage diversification program at the fetal period onset. Muscle CT and tendon are composed of several fibroblast populations that emerge asynchronously. Once the final muscle pattern is set, transcriptionally close populations are found in neighboring locations in limbs, prefiguring the adult fibroblast layers. We propose that the limb CT is organized in a continuum of promiscuous fibroblast identities, allowing for the robust and efficient connection of muscle to bone and skin.

C. Tsang I. Exploring stem cell frontiers: definitions, challenges, and perspectives for regenerative medicine

Each year, the European Summer School on Stem Cell Biology and Regenerative Medicine (SCSS) attracts early-career researchers and actively practicing clinicians who specialise in stem cell and regenerative biology. The 16th edition of this influential course took place from 12th to 19th September 2023 on the charming Greek island of Spetses. Focusing on important concepts and recent advances in stem cells, the distinguished faculty included experts spanning the spectrum from fundamental research to clinical trials to market-approved therapies. Alongside an academically intensive programme that bridges the various contexts of stem cell research, delegates were encouraged to critically address relevant questions in stem cell biology and medicine, including broader societal implications. Here, we present a comprehensive overview and key highlights from the SCSS 2023.

Extraocular muscle stem cells exhibit distinct cellular properties associated with non-muscle molecular signatures

Skeletal muscle stem cells (MuSCs) are recognised as functionally heterogeneous. Cranial MuSCs are reported to have greater proliferative and regenerative capacity when compared with those in the limb. A comprehensive understanding of the mechanisms underlying this functional heterogeneity is lacking. Here, we have used clonal analysis, live imaging and single cell transcriptomic analysis to identify crucial features that distinguish extraocular muscle (EOM) from limb muscle stem cell populations. A MyogeninntdTom reporter showed that the increased proliferation capacity of EOM MuSCs correlates with deferred differentiation and lower expression of the myogenic commitment gene Myod. Unexpectedly, EOM MuSCs activated in vitro expressed a large array of extracellular matrix components typical of mesenchymal non-muscle cells. Computational analysis underscored a distinct co-regulatory module, which is absent in limb MuSCs, as driver of these features. The EOM transcription factor network, with Foxc1 as key player, appears to be hardwired to EOM identity as it persists during growth, disease and in vitro after several passages. Our findings shed light on how high-performing MuSCs regulate myogenic commitment by remodelling their local environment and adopting properties not generally associated with myogenic cells.

Regeneration process of myotendinous junction injury induced by collagenase injection between Achilles tendon and soleus muscle in mice

Recently, it has become clear that peri-muscular tissues play a significant role in the deterioration of muscle function. Understanding the function and regeneration of muscle, as well as its surrounding tissues, is crucial to determining the causes of muscular illnesses. However, the regeneration process of the myotendinous junction (MTJ), the most closely related peri-muscular tissue, is still unknown. Therefore, we generated a mouse model of MTJ injury by collagenase injection and searched for the process of regeneration of the MTJ and its adjacent regions. The MTJ region was damaged by collagenase injection, which greatly increased the tendon cross sectional area. Collagenase injections increased the proportion of myofibers with a central nucleus, which is a characteristic of regenerating muscle. The collagenase injection group had myofibers with central nuclei and expressing MTJ markers. Additionally, we measured the length of MTJs using serial cross sections of the soleus muscle and discovered that MTJs at 2 weeks after collagenase injection were shorter compared to the control group, with a propensity to progressively recover their length over time. The results showed that MTJs undergo morphological regeneration even when severely damaged, and that this regeneration occurs in conjunction with muscle regeneration. We anticipate that these findings will be valuable in upcoming research on motor unit regeneration.

Hic1 identifies a specialized mesenchymal progenitor population in the embryonic limb responsible for bone superstructure formation

The musculoskeletal system relies on the integration of multiple components with diverse physical properties, such as striated muscle, tendon, and bone, that enable locomotion and structural stability. This relies on the emergence of specialized, but poorly characterized, interfaces between these various elements during embryonic development. Within the appendicular skeleton, we show that a subset of mesenchymal progenitors (MPs), identified by Hic1, do not contribute to the primary cartilaginous anlagen but represent the MP population, whose progeny directly contribute to the interfaces that connect bone to tendon (entheses), tendon to muscle (myotendinous junctions), and the associated superstructures. Furthermore, deletion of Hic1 leads to skeletal defects reflective of deficient muscle-bone coupling and, consequently, perturbation of ambulation. Collectively, these findings show that Hic1 identifies a unique MP population that contributes to a secondary wave of bone sculpting critical to skeletal morphogenesis.

Effects of Myostatin on Nuclear Morphology at the Myotendinous Junction

Myostatin (Myo) is known to suppress skeletal muscle growth, and was recently reported to control tendon homeostasis. The purpose of the present study was to investigate the regulatory involvement of Myo in the myotendinous junction (MTJ) in vivo and in vitro. After Achilles tendon injury in mice, we identified unexpected cell accumulation on the tendon side of the MTJ. At postoperative day 7 (POD7), the nuclei had an egg-like profile, whereas at POD28 they were spindle-shaped. The aspect ratio of nuclei on the tendon side of the MTJ differed significantly between POD7 and POD28 (p = 4.67 × 10−34). We then investigated Myo expression in the injured Achilles tendon. At the MTJ, Myo expression was significantly increased at POD28 relative to POD7 (p = 0.0309). To investigate the action of Myo in vitro, we then prepared laminated sheets of myoblasts (C2C12) and fibroblasts (NIH3T3) (a pseudo MTJ model). Myo did not affect the expression of Pax7 and desmin (markers of muscle development), scleraxis and temonodulin (markers of tendon development), or Sox9 (a common marker of muscle and tendon development) in the cell sheets. However, Myo changed the nuclear morphology of scleraxis-positive cells arrayed at the boundary between the myoblast sheet and the fibroblast sheet (aspect ratio of the cell nuclei, myostatin(+) vs. myostatin(-): p = 0.000134). Myo may strengthen the connection at the MTJ in the initial stages of growth and wound healing.

Hox11-expressing interstitial cells contribute to adult skeletal muscle at homeostasis

Interstitial stromal cells play critical roles in muscle development, regeneration and repair and we have previously reported that Hoxa11 and Hoxd11 are expressed in the interstitial cells of muscles attached to the zeugopod, and are crucial for the proper embryonic patterning of these muscles. Hoxa11eGFP expression continues in a subset of muscle interstitial cells through adult stages. The induction of Hoxa11-CreERT2-mediated lineage reporting (Hoxa11iTom) at adult stages in mouse results in lineage induction only in the interstitial cells. However, Hoxa11iTom+ cells progressively contribute to muscle fibers at subsequent stages. The contribution to myofibers exceeds parallel Pax7-CreERT2-mediated lineage labeling. Nuclear-specific lineage labeling demonstrates that Hoxa11-expressing interstitial cells contribute nuclear contents to myofibers. Crucially, at no point after Hoxa11iTom induction are satellite cells lineage labeled. When examined in vitro, isolated Hoxa11iTom+ interstitial cells are not capable of forming myotubes, but Hoxa11iTom+ cells can contribute to differentiating myotubes, supporting Hox-expressing interstitial cells as a new population of muscle progenitors, but not stem cells. This work adds to a small but growing body of evidence that supports a satellite cell-independent source of muscle tissue in vivo.

TWIST1, a gene associated with Saethre-Chotzen syndrome, regulates extraocular muscle organization in mouse

Heterozygous loss of function mutations in TWIST1 cause Saethre-Chotzen syndrome, which is characterized by craniosynostosis, facial asymmetry, ptosis, strabismus, and distinctive ear appearance. Individuals with syndromic craniosynostosis have high rates of strabismus and ptosis, but the underlying pathology is unknown. Some individuals with syndromic craniosynostosis have been noted to have absence of individual extraocular muscles or abnormal insertions of the extraocular muscles on the globe. Using conditional knock-out alleles for Twist1 in cranial mesenchyme, we test the hypothesis that Twist1 is required for extraocular muscle organization and position, attachment to the globe, and/or innervation by the cranial nerves. We examined the extraocular muscles in conditional Twist1 knock-out animals using Twist2-cre and Pdgfrb-cre drivers. Both are expressed in cranial mesoderm and neural crest. Conditional inactivation of Twist1 using these drivers leads to disorganized extraocular muscles that cannot be reliably identified as specific muscles. Tendons do not form normally at the insertion and origin of these dysplastic muscles. Knock-out of Twist1 expression in tendon precursors, using scleraxis-cre, however, does not alter EOM organization. Furthermore, developing motor neurons, which do not express Twist1, display abnormal axonal trajectories in the orbit in the presence of dysplastic extraocular muscles. Strabismus in individuals with TWIST1 mutations may therefore be caused by abnormalities in extraocular muscle development and secondary abnormalities in innervation and tendon formation.

Exploring the Potential Mechanisms of Melilotus officinalis (L.) Pall. in Chronic Muscle Repair Patterns Using Single Cell Receptor-Ligand Marker Analysis and Molecular Dynamics Simulations

Information regarding the function of Melilotus officinalis (L.) Pall. in skeletal muscles is still unknown. In this study, we explored the possible regulatory targets of M. (L.) Pall. that affects the repair patterns in chronic muscle injury. We analyzed the potential target genes and chemical composition of M. (L.) Pall. and constructed a "drug-component-disease target genes" network analysis. Five active ingredients and 87 corresponding targets were obtained. Muscle-tendon junction (MTJ) cells were used to perform receptor-ligand marker analysis using the CellphoneDB algorithm. Targets of M. (L.) Pall. were screened further for the cellular ligand-receptor protein action on MTJs. Enrichment analysis suggests that those protein-associated ligand receptors may be associated with a range of intercellular signaling pathways. Molecular docking validation was then performed. Five proteins (CCL2, VEGFA, MMP2, MET, and EGFR) may be regulated by the active ingredient luteolin and scoparone. Finally, molecular dynamics simulations revealed that luteolin can stably target binding to MMP2. M. (L.) Pall. influences skeletal muscle repair patterns by affecting the fibroblast interactions in the muscle-tendon junctions through the active ingredients luteolin and scoparone.

Microfluidic Tissue Engineering and Bio-Actuation

Bio-hybrid technologies aim to replicate the unique capabilities of biological systems that could surpass advanced artificial technologies. Soft bio-hybrid robots consist of synthetic and living materials and have the potential to self-assemble, regenerate, work autonomously, and interact safely with other species and the environment. Cells require a sufficient exchange of nutrients and gases, which is guaranteed by convection and diffusive transport through liquid media. The functional development and long-term survival of biological tissues in vitro can be improved by dynamic flow culture, but only microfluidic flow control can develop tissue with fine structuring and regulation at the microscale. Full control of tissue growth at the microscale will eventually lead to functional macroscale constructs, which are needed as the biological component of soft bio-hybrid technologies. This review summarizes recent progress in microfluidic techniques to engineer biological tissues, focusing on the use of muscle cells for robotic bio-actuation. Moreover, the instances in which bio-actuation technologies greatly benefit from fusion with microfluidics are highlighted, which include: the microfabrication of matrices, biomimicry of cell microenvironments, tissue maturation, perfusion, and vascularization.

Inducible and tissue-specific cell labeling in Cre-ERT2 transgenic Xenopus lines

Investigating cell lineage requires genetic tools that label cells in a temporal and tissue‐specific manner. The bacteriophage‐derived Cre‐ER^T2/loxP system has been developed as a genetic tool for lineage tracing in many organisms. We recently reported a stable transgenic Xenopus line with a Cre‐ER^T2/loxP system driven by the mouse Prrx1 (mPrrx1) enhancer to trace limb fibroblasts during the regeneration process (Prrx1:CreER line). Here we describe the detailed technological development and characterization of such line. Transgenic lines carrying a CAG promoter‐driven Cre‐ER^T2/loxP system showed conditional labeling of muscle, epidermal, and interstitial cells in both the tadpole tail and the froglet leg upon 4‐hydroxytamoxifen (4OHT) treatment. We further improved the labeling efficiency in the Prrx1:CreER lines from 12.0% to 32.9% using the optimized 4OHT treatment regime. Careful histological examination showed that Prrx1:CreER lines also sparsely labeled cells in the brain, spinal cord, head dermis, and fibroblasts in the tail. This work provides the first demonstration of conditional, tissue‐specific cell labeling with the Cre‐ER^T2/loxP system in stable transgenic Xenopus lines.

Identification of distinct non-myogenic skeletal-muscle-resident mesenchymal cell populations

Mesenchymal progenitors of the lateral plate mesoderm give rise to various cell fates within limbs, including a heterogeneous group of muscle-resident mesenchymal cells. Often described as fibro-adipogenic progenitors, these cells are key players in muscle development, disease, and regeneration. To further define this cell population(s), we perform lineage/reporter analysis, flow cytometry, single-cell RNA sequencing, immunofluorescent staining, and differentiation assays on normal and injured murine muscles. Here we identify six distinct Pdgfra⁺ non-myogenic muscle-resident mesenchymal cell populations that fit within a bipartite differentiation trajectory from a common progenitor. One branch of the trajectory gives rise to two populations of immune-responsive mesenchymal cells with strong adipogenic potential and the capability to respond to acute and chronic muscle injury, whereas the alternative branch contains two cell populations with limited adipogenic capacity and inherent mineralizing capabilities; one of the populations displays a unique neuromuscular junction association and an ability to respond to nerve injury.

Leinroth et al. explore the heterogeneity of Pdgfra⁺ muscle-resident mesenchymal cells, demonstrating that Pdgfra⁺ subpopulations have unique gene expression profiles, exhibit two distinct cell trajectories from a common progenitor, differ in their abilities to respond to muscle injuries, and show variable adipogenic and mineralizing capacities.

Development and Regeneration of Muscle, Tendon, and Myotendinous Junctions in Striated Skeletal Muscle

Owing to a rapid increase in aging population in recent years, the deterioration of motor function in older adults has become an important social problem, and several studies have aimed to investigate the mechanisms underlying muscle function decline. Furthermore, structural maintenance of the muscle–tendon–bone complexes in the muscle attachment sites is important for motor function, particularly for joints; however, the development and regeneration of these complexes have not been studied thoroughly and require further elucidation. Recent studies have provided insights into the roles of mesenchymal progenitors in the development and regeneration of muscles and myotendinous junctions. In particular, studies on muscles and myotendinous junctions have—through the use of the recently developed scRNA-seq—reported the presence of syncytia, thereby suggesting that fibroblasts may be transformed into myoblasts in a BMP-dependent manner. In addition, the high mobility group box 1—a DNA-binding protein found in nuclei—is reportedly involved in muscle regeneration. Furthermore, studies have identified several factors required for the formation of locomotor apparatuses, e.g., tenomodulin (Tnmd) and mohawk (Mkx), which are essential for tendon maturation.

Identification of bipotent progenitors that give rise to myogenic and connective tissues in mouse

How distinct cell fates are manifested by direct lineage ancestry from bipotent progenitors, or by specification of individual cell types is a key question for understanding the emergence of tissues. The interplay between skeletal muscle progenitors and associated connective tissue cells provides a model for examining how muscle functional units are established. Most craniofacial structures originate from the vertebrate-specific neural crest cells except in the dorsal portion of the head, where they arise from cranial mesoderm. Here, using multiple lineage-tracing strategies combined with single cell RNAseq and in situ analyses, we identify bipotent progenitors expressing Myf5 (an upstream regulator of myogenic fate) that give rise to both muscle and juxtaposed connective tissue. Following this bifurcation, muscle and connective tissue cells retain complementary signalling features and maintain spatial proximity. Disrupting myogenic identity shifts muscle progenitors to a connective tissue fate. The emergence of Myf5-derived connective tissue is associated with the activity of several transcription factors, including Foxp2. Interestingly, this unexpected bifurcation in cell fate was not observed in craniofacial regions that are colonised by neural crest cells. Therefore, we propose that an ancestral bi-fated program gives rise to muscle and connective tissue cells in skeletal muscles that are deprived of neural crest cells.

TMEM8C-mediated fusion is regionalized and regulated by NOTCH signalling during foetal myogenesis

The location and regulation of fusion events within skeletal muscles during development remain unknown. Using the fusion marker myomaker (Mymk), named TMEM8C in chicken, as a readout of fusion, we identified a co-segregation of TMEM8C-positive cells and MYOG-positive cells in single-cell RNA-sequencing datasets of limbs from chicken embryos. We found that TMEM8C transcripts, MYOG transcripts and the fusion-competent MYOG-positive cells were preferentially regionalized in central regions of foetal muscles. We also identified a similar regionalization for the gene encoding the NOTCH ligand JAG2 along with an absence of NOTCH activity in TMEM8C+ fusion-competent myocytes. NOTCH function in myoblast fusion had not been addressed so far. We analysed the consequences of NOTCH inhibition for TMEM8C expression and myoblast fusion during foetal myogenesis in chicken embryos. NOTCH inhibition increased myoblast fusion and TMEM8C expression and released the transcriptional repressor HEYL from the TMEM8C regulatory regions. These results identify a regionalization of TMEM8C-dependent fusion and a molecular mechanism underlying the fusion-inhibiting effect of NOTCH in foetal myogenesis. The modulation of NOTCH activity in the fusion zone could regulate the flux of fusion events.

Towards Tabula Gallus

The Tabula Gallus is a proposed project that aims to create a map of every cell type in the chicken body and chick embryos. Chickens (Gallus gallus) are one of the most recognized model animals that recapitulate the development and physiology of mammals. The Tabula Gallus will generate a compendium of single-cell transcriptome data from Gallus gallus, characterize each cell type, and provide tools for the study of the biology of this species, similar to other ongoing cell atlas projects (Tabula Muris and Tabula Sapiens/Human Cell Atlas for mice and humans, respectively). The Tabula Gallus will potentially become an international collaboration between many researchers. This project will be useful for the basic scientific study of Gallus gallus and other birds (e.g., cell biology, molecular biology, developmental biology, neuroscience, physiology, oncology, virology, behavior, ecology, and evolution). It will eventually be beneficial for a better understanding of human health and diseases.

Master regulators of skeletal muscle lineage development and pluripotent stem cells differentiation

In vertebrates, the skeletal muscles of the body and their associated stem cells originate from muscle progenitor cells, during development. The specification of the muscles of the trunk, head and limbs, relies on the activity of distinct genetic hierarchies. The major regulators of trunk and limb muscle specification are the paired-homeobox transcription factors PAX3 and PAX7. Distinct gene regulatory networks drive the formation of the different muscles of the head. Despite the redeployment of diverse upstream regulators of muscle progenitor differentiation, the commitment towards the myogenic fate requires the expression of the early myogenic regulatory factors MYF5, MRF4, MYOD and the late differentiation marker MYOG. The expression of these genes is activated by muscle progenitors throughout development, in several waves of myogenic differentiation, constituting the embryonic, fetal and postnatal phases of muscle growth. In order to achieve myogenic cell commitment while maintaining an undifferentiated pool of muscle progenitors, several signaling pathways regulate the switch between proliferation and differentiation of myoblasts. The identification of the gene regulatory networks operating during myogenesis is crucial for the development of in vitro protocols to differentiate pluripotent stem cells into myoblasts required for regenerative medicine.