Whole-Somite Rotation Generates Muscle Progenitor Cell Compartments in the Developing Zebrafish Embryo

Monocyte-Derived cxcl12 Guides a Directional Migration of Blood Vessels in Zebrafish

BACKGROUND

Sprouting blood vessels, reaching the aimed location, and establishing the proper connections are vital for building vascular networks. Such biological processes are subject to precise molecular regulation. So far, the mechanistic insights into understanding how blood vessels grow to the correct position are limited. In particular, the guide cues and the signaling-originating cells remain elusive.

METHODS

Live imaging analysis was used to observe the vascular developmental process of zebrafish. Whole-mount in situ hybridization and fluorescent in situ hybridization were used to detect the expression profiles of the genes. Single-cell sequencing analysis was conducted to identify the guiding protein and its originating cells.

RESULTS

Taking advantage of live imaging analysis, we described a directional blood vessel migration in the vascularization process of zebrafish pectoral fins. We demonstrated that pectoral fin vessel c migrated over long distances and was anastomosed with the second pair of intersegmental vessels. Furthermore, we found the cxcl12a-cxcr4a axis specifically guided this long-distance extension of pectoral fin vessel c-intersegmental vessel, and either inhibition or overexpression of cxcl12a-cxcr4a signaling both mislead the growth of pectoral fin vessel c to ectopic areas. Finally, based on an analysis of single-cell sequencing data, we revealed that a population of monocytes expresses the Cxcl12a, which guides the migration of the vascular sprout.

CONCLUSIONS

Our study identified Cxcl12a as the signaling molecule for orchestrating the organotypic-specific long-distance migration and anastomosis of the pectoral fin vessel and the intersegmental vessels in zebrafish. We discovered a specific cluster of gata1 (globin transcription factor 1)-positive monocytes responsible for expressing Cxcl12a. The findings offer novel insights into the mechanisms underlying organotypic vascularization in vertebrates.

Twisted cell flow facilitates three-dimensional somite morphogenesis in zebrafish

Tissue elongation is a fundamental morphogenetic process to construct complex embryonic structures. In zebrafish, somites rapidly elongate in both dorsal and ventral directions, transforming from a cuboidal to a V-shape within a few hours of development. Despite its significance, the cellular behaviors that directly lead to somite elongation have not been examined at single-cell resolution. Here, we describe the motion and shapes of all cells composing the dorsal half of the somite in three-dimensional space using lightsheet microscopy. We identified two types of cell movements-in horizontal and dorsal directions-that occur simultaneously within individual cells, creating a complex, twisted flow of cells during somite elongation. Chemical inhibition of Sdf1 signaling disrupted the collective movement in both directions and inhibited somite elongation, suggesting that Sdf1 signaling is crucial for this cell flow. Furthermore, three-dimensional computational modeling suggested that horizontal cell rotation accelerates the perpendicular elongation of the somite along the dorsoventral axis. Together, our study offers novel insights into the role of collective cell migration in tissue morphogenesis, which proceeds dynamically in the three-dimensional space of the embryo.

Experimental models as a tool for research on sarcopenia: A narrative review

Sarcopenia is a musculoskeletal disorder related to muscle mass and function; as the worldwide population ages, its growing prevalence means a decline in quality of life and an increased burden for public health systems. As sarcopenia is a reversible condition, its early diagnosis is of utmost importance. Consensus definitions and diagnosis protocols for sarcopenia have been evolving for a long time, and the identification of molecular pathways subjacent to sarcopenia is a growing research area. The use of liquid biopsies to identify circulating molecules does not provide information about specific regulatory pathways or biomarkers in relevant tissue, and the use of skeletal muscle biopsies from older people has many limitations. Complementary tools are therefore necessary to advance the knowledge of relevant molecular aspects. The development of experimental models, such as animal, cellular, or bioengineered tissue, together with knock-in or knock-out strategies, could therefore be of great interest. This narrative review will explore experimental models of healthy muscle and aged muscle cells as a tool for research on sarcopenia. We will summarize the literature and present relevant experimental models in terms of their advantages and disadvantages. All of the presented approaches could potentially contribute to the accurate and early diagnosis, follow-up, and possible treatment of sarcopenia.

Regulatory Role of Meox1 in Muscle Growth of Sebastes schlegelii

Meox1 is a critical transcription factor that plays a pivotal role in embryogenesis and muscle development. It has been established as a marker gene for growth-specific muscle stem cells in zebrafish. In this study, we identified the SsMeox1 gene in a large teleost fish, Sebastes schlegelii. Through in situ hybridization and histological analysis, we discovered that SsMeox1 can be employed as a specific marker of growth-specific muscle stem cells, which originate from the somite stage and are primarily situated in the external cell layer (ECL) and myosepta, with a minor population distributed among muscle fibers. The knockdown of SsMeox1 resulted in a significant increase in Ccnb1 expression, subsequently promoting cell cycle progression and potentially accelerating the depletion of the stem cell pool, which ultimately led to significant growth retardation. These findings suggest that SsMeox1 arrests the cell cycle of growth-specific muscle stem cells in the G2 phase by suppressing Ccnb1 expression, which is essential for maintaining the stability of the growth-specific muscle stem cell pool. Our study provides significant insights into the molecular mechanisms underlying the indeterminate growth of large teleosts.

Skeletal muscle regeneration after extensive cryoinjury of caudal myomeres in adult zebrafish

Skeletal muscles can regenerate after minor injuries, but severe structural damage often leads to fibrosis in mammals. Whether adult zebrafish possess the capacity to reproduce profoundly destroyed musculature remains unknown. Here, a new cryoinjury model revealed that several myomeres efficiently regenerated within one month after wounding the zebrafish caudal peduncle. Wound clearance involved accumulation of the selective autophagy receptor p62, an immune response and Collagen XII deposition. New muscle formation was associated with proliferation of Pax7 expressing muscle stem cells, which gave rise to MyoD1 positive myogenic precursors, followed by myofiber differentiation. Monitoring of slow and fast muscles revealed their coordinated replacement in the superficial and profound compartments of the myomere. However, the final boundary between the muscular components was imperfectly recapitulated, allowing myofibers of different identities to intermingle. The replacement of connective with sarcomeric tissues required TOR signaling, as rapamycin treatment impaired new muscle formation, leading to persistent fibrosis. The model of zebrafish myomere restoration may provide new medical perspectives for treatment of traumatic injuries.

Review: Understanding fish muscle biology in the indeterminate growth species pacu (Piaractus mesopotamicus)

The muscle phenotype of fish is regulated by numerous factors that, although widely explored, still need to be fully understood. In this context, several studies aimed to unravel how internal and external stimuli affect the muscle growth of these vertebrates. The pacu (Piaractus mesopotamicus) is a species of indeterminate muscular growth that quickly reaches high body weight. For this reason, it adds great importance to the productive sector, along with other round fish. In this context, we aimed to compile studies on fish biology and skeletal muscle growth, focusing on studies by our research group that used pacu as an experimental model along with other species. Based on these studies, new muscle phenotype regulators were identified and explored in vivo, in vitro, and in silico studies, which strongly contribute to advances in understanding muscle growth mechanisms with future applications in the productive sector.

cxcl12a plays an essential role in pharyngeal cartilage development

Background: Neural crest cells constitute a distinct set of multipotent cells that undergo migration along predefined pathways, culmination in the differentiation into a plethora of cell types, including components of the pharyngeal cartilage. The neurocranium is composite structure derived from both cranial neural crest and mesoderm cells, whereas the pharyngeal skeletal elements-including the mandibular and branchial arches-are exclusively formed by craniofacial neural crest cells. Previous studies have elucidated the critical involvement of the chemokine signaling axis Cxcl12b/Cxcr4a in craniofacial development in zebrafish (Danio rerio). Nonetheless, the function contribution of Cxcl12a and Cxcr4b-the homologous counterparts of Cxcl12b and Cxcr4a-remain largely unexplored. Methods: In the present study, mutant lines for cxcl12a and cxcr4b were generated employing CRISPR/Cas9 system. Temporal and spatial expression patterns of specific genes were assessed using in situ hybridization and dual-color fluorescence in situ hybridization techniques. High-resolution confocal microscopy was utilized for in vivo imaging to detect the pharyngeal arch or pouch patterning. Additionally, cartilage formation within the craniofacial region was analyzed via Alcian blue staining, and the proliferation and apoptosis rates of craniofacial neural crest cells were quantified through BrdU incorporation and TUNEL staining. Results: Our data reveals that the deletion of the chemokine gene cxcl12a results in a marked diminution of pharyngeal cartilage elements, attributable to compromised proliferation of post-migratory craniofacial neural crest cells. Subsequent experiments confirmed that Cxcl12a and Cxcl12b exhibit a synergistic influence on pharyngeal arch and pouch formation. Conclusion: Collectively, the present investigation furnishes compelling empirical evidence supporting the indispensable role of Cxcl2a in craniofacial cartilage morphogenesis, albeit cxcr4b mutants exert a minimal impact on this biological process. We delineate that Cxcl12a is essential for chondrogenesis in zebrafish, primarily by promoting the proliferation of craniofacial neural crest cells. Furthermore, we proposed a conceptual framework wherein Cxcl12a and Cxcl12b function synergistically in orchestrating both the pharyngeal arch and pouch morphogenesis.

Migration and differentiation of muscle stem cells are coupled by RhoA signalling during regeneration

Skeletal muscle is highly regenerative and is mediated by a population of migratory adult muscle stem cells (muSCs). Effective muscle regeneration requires a spatio-temporally regulated response of the muSC population to generate sufficient muscle progenitor cells that then differentiate at the appropriate time. The relationship between muSC migration and cell fate is poorly understood and it is not clear how forces experienced by migrating cells affect cell behaviour. We have used zebrafish to understand the relationship between muSC cell adhesion, behaviour and fate in vivo. Imaging of pax7-expressing muSCs as they respond to focal injuries in trunk muscle reveals that they migrate by protrusive-based means. By carefully characterizing their behaviour in response to injury we find that they employ an adhesion-dependent mode of migration that is regulated by the RhoA kinase ROCK. Impaired ROCK activity results in reduced expression of cell cycle genes and increased differentiation in regenerating muscle. This correlates with changes to focal adhesion dynamics and migration, revealing that ROCK inhibition alters the interaction of muSCs to their local environment. We propose that muSC migration and differentiation are coupled processes that respond to changes in force from the environment mediated by RhoA signalling.

Modeling Human Muscular Dystrophies in Zebrafish: Mutant Lines, Transgenic Fluorescent Biosensors, and Phenotyping Assays

Muscular dystrophies (MDs) are a heterogeneous group of myopathies characterized by progressive muscle weakness leading to death from heart or respiratory failure. MDs are caused by mutations in genes involved in both the development and organization of muscle fibers. Several animal models harboring mutations in MD-associated genes have been developed so far. Together with rodents, the zebrafish is one of the most popular animal models used to reproduce MDs because of the high level of sequence homology with the human genome and its genetic manipulability. This review describes the most important zebrafish mutant models of MD and the most advanced tools used to generate and characterize all these valuable transgenic lines. Zebrafish models of MDs have been generated by introducing mutations to muscle-specific genes with different genetic techniques, such as (i) N-ethyl-N-nitrosourea (ENU) treatment, (ii) the injection of specific morpholino, (iii) tol2-based transgenesis, (iv) TALEN, (v) and CRISPR/Cas9 technology. All these models are extensively used either to study muscle development and function or understand the pathogenetic mechanisms of MDs. Several tools have also been developed to characterize these zebrafish models by checking (i) motor behavior, (ii) muscle fiber structure, (iii) oxidative stress, and (iv) mitochondrial function and dynamics. Further, living biosensor models, based on the expression of fluorescent reporter proteins under the control of muscle-specific promoters or responsive elements, have been revealed to be powerful tools to follow molecular dynamics at the level of a single muscle fiber. Thus, zebrafish models of MDs can also be a powerful tool to search for new drugs or gene therapies able to block or slow down disease progression.

Light microscopy-based elastography for the mechanical characterization of zebrafish somitogenesis

We evaluated the elasticity of live tissues of zebrafish embryos using label-free optical elastography. We employed a pair of custom-built elastic microcantilevers to gently compress a zebrafish embryo and used optical-tracking analysis to obtain the induced internal strain. We then built a finite element method (FEM) model and matched the strain with the optical analysis. The elastic moduli were found by minimizing the root-mean-square errors between the optical and FEM analyses. We evaluated the average elastic moduli of a developing somite, the overlying ectoderm, and the underlying yolk of seven zebrafish embryos during the early somitogenesis stages. The estimation results showed that the average elastic modulus of the somite increased from 150 to 700 Pa between 4- and 8-somite stages, while those of the ectoderm and the yolk stayed between 100 and 200 Pa, and they did not show significant changes. The result matches well with the developmental process of somitogenesis reported in the literature. This is among the first attempts to quantify spatially-resolved elasticity of embryonic tissues from optical elastography.

Differentiation and Maturation of Muscle and Fat Cells in Cultivated Seafood: Lessons from Developmental Biology

Cultivated meat, also known as cultured or cell-based meat, is meat produced directly from cultured animal cells rather than from a whole animal. Cultivated meat and seafood have been proposed as a means of mitigating the substantial harms associated with current production methods, including damage to the environment, antibiotic resistance, food security challenges, poor animal welfare, and-in the case of seafood-overfishing and ecological damage associated with fishing and aquaculture. Because biomedical tissue engineering research, from which cultivated meat draws a great deal of inspiration, has thus far been conducted almost exclusively in mammals, cultivated seafood suffers from a lack of established protocols for producing complex tissues in vitro. At the same time, fish such as the zebrafish Danio rerio have been widely used as model organisms in developmental biology. Therefore, many of the mechanisms and signaling pathways involved in the formation of muscle, fat, and other relevant tissue are relatively well understood for this species. The same processes are understood to a lesser degree in aquatic invertebrates. This review discusses the differentiation and maturation of meat-relevant cell types in aquatic species and makes recommendations for future research aimed at recapitulating these processes to produce cultivated fish and shellfish.

Tendon tissue engineering: An overview of biologics to promote tendon healing and repair

Tendons are dense connective tissues with a hierarchical polarized structure that respond to and adapt to the transmission of muscle contraction forces to the skeleton, enabling motion and maintaining posture. Tendon injuries, also known as tendinopathies, are becoming more common as populations age and participation in sports/leisure activities increases. The tendon has a poor ability to self-heal and regenerate given its intrinsic, constrained vascular supply and exposure to frequent, severe loading. There is a lack of understanding of the underlying pathophysiology, and it is not surprising that disorder-targeted medicines have only been partially effective at best. Recent tissue engineering approaches have emerged as a potential tool to drive tendon regeneration and healing. In this review, we investigated the physiochemical factors involved in tendon ontogeny and discussed their potential application in vitro to reproduce functional and self-renewing tendon tissue. We sought to understand whether stem cells are capable of forming tendons, how they can be directed towards the tenogenic lineage, and how their growth is regulated and monitored during the entire differentiation path. Finally, we showed recent developments in tendon tissue engineering, specifically the use of mesenchymal stem cells (MSCs), which can differentiate into tendon cells, as well as the potential role of extracellular vesicles (EVs) in tendon regeneration and their potential for use in accelerating the healing response after injury.

Isolation, Culture, and Analysis of Zebrafish Myofibers and Associated Muscle Stem Cells to Explore Adult Skeletal Myogenesis

Adult skeletal musculature experiences continuous physical stress, and hence requires maintenance and repair to ensure its continued efficient functioning. The population of resident muscle stem cells (MuSCs), termed satellite cells, resides beneath the basal lamina of adult myofibers, contributing to both muscle hypertrophy and regeneration. Upon exposure to activating stimuli, MuSCs proliferate to generate new myoblasts that differentiate and fuse to regenerate or grow myofibers. Moreover, many teleost fish undergo continuous growth throughout life, requiring continual nuclear recruitment from MuSCs to initiate and grow new fibers, a process that contrasts with the determinate growth observed in most amniotes. In this chapter, we describe a method for the isolation, culture, and immunolabeling of adult zebrafish myofibers that permits examination of both myofiber characteristics ex vivo and the MuSC myogenic program in vitro. Morphometric analysis of isolated myofibers is suitable to assess differences among slow and fast muscles or to investigate cellular features such as sarcomeres and neuromuscular junctions. Immunostaining for Pax7, a canonical stemness marker, identifies MuSCs on isolated myofibers for study. Furthermore, the plating of viable myofibers allows MuSC activation and expansion and downstream analysis of their proliferative and differentiative dynamics, thus providing a suitable, parallel alternative to amniote models for the study of vertebrate myogenesis.

wnt16 regulates spine and muscle morphogenesis through parallel signals from notochord and dermomyotome

Bone and muscle are coupled through developmental, mechanical, paracrine, and autocrine signals. Genetic variants at the CPED1-WNT16 locus are dually associated with bone- and muscle-related traits. While Wnt16 is necessary for bone mass and strength, this fails to explain pleiotropy at this locus. Here, we show wnt16 is required for spine and muscle morphogenesis in zebrafish. In embryos, wnt16 is expressed in dermomyotome and developing notochord, and contributes to larval myotome morphology and notochord elongation. Later, wnt16 is expressed at the ventral midline of the notochord sheath, and contributes to spine mineralization and osteoblast recruitment. Morphological changes in wnt16 mutant larvae are mirrored in adults, indicating that wnt16 impacts bone and muscle morphology throughout the lifespan. Finally, we show that wnt16 is a gene of major effect on lean mass at the CPED1-WNT16 locus. Our findings indicate that Wnt16 is secreted in structures adjacent to developing bone (notochord) and muscle (dermomyotome) where it affects the morphogenesis of each tissue, thereby rendering wnt16 expression into dual effects on bone and muscle morphology. This work expands our understanding of wnt16 in musculoskeletal development and supports the potential for variants to act through WNT16 to influence bone and muscle via parallel morphogenetic processes.

In humans, genetic variants (DNA sequences that vary amongst individuals) have been identified that appear to influence two tissues, bone and skeletal muscle. However, how single genes and genetic variants exert dual influence on both tissues is not well understood. In this study, we found that the wnt16 gene is necessary for specifying the size and shape of both muscle and bone during development in zebrafish. We also disentangled how wnt16 affects both tissues: distinct cellular populations adjacent to muscle and bone secrete Wnt16, where it acts as a signal guiding the size and shape of each tissue. This is important because in humans, genetic variants near the WNT16 gene have effects on both bone- and muscle-related traits. This study expands our understanding of the role of WNT16 in bone and muscle development, and helps to explain how genetic variants near WNT16 affect traits for both tissues. Moreover, WNT16 is actively being explored as a target for osteoporosis therapies, thus our study could have implications with regard to the potential of targeting WNT16 to treat bone and muscle simultaneously.

Clonal behaviour of myogenic precursor cells throughout the vertebrate lifespan

To address questions of stem cell diversity during skeletal myogenesis, a Brainbow-like genetic cell lineage tracing method, dubbed Musclebow2, was derived by enhancer trapping in zebrafish. It is shown that, after initial formation of the primary myotome, at least 15 muscle precursor cells (mpcs) seed each somite, where they proliferate but contribute little to muscle growth prior to hatching. Thereafter, dermomyotome-derived mpc clones rapidly expand while some progeny undergo terminal differentiation, leading to stochastic clonal drift within the mpc pool. No evidence of cell-lineage-based clonal fate diversity was obtained. Neither fibre nor mpc death was observed in uninjured animals. Individual marked muscle fibres persist across much of the lifespan indicating low rates of nuclear turnover. In adulthood, early-marked mpc clones label stable blocks of tissue comprising a significant fraction of either epaxial or hypaxial somite. Fusion of cells from separate early-marked clones occurs in regions of clone overlap. Wounds are regenerated from several local mpcs; no evidence for specialised stem mpcs was obtained. In conclusion, our data indicate that most mpcs in muscle tissue contribute to local growth and repair and suggest that cellular turnover is low in the absence of trauma.

Unique Features of River Lamprey (Lampetra fluviatilis) Myogenesis

The river lamprey (L. fluviatilis) is a representative of the ancestral jawless vertebrate group. We performed a histological analysis of trunk muscle fiber differentiation during embryonal, larval, and adult musculature development in this previously unstudied species. Investigation using light, transmission electron (TEM), and confocal microscopy revealed that embryonal and larval musculature differs from adult muscle mass. Here, we present the morphological analysis of L. fluviatilis myogenesis, from unsegmented mesoderm through somite formation, and their differentiation into multinucleated muscle lamellae. Our analysis also revealed the presence of myogenic factors LfPax3/7 and Myf5 in the dermomyotome. In the next stages of development, two types of muscle lamellae can be distinguished: central surrounded by parietal. This pattern is maintained until adulthood, when parietal muscle fibers surround the central muscles on both sides. The two types show different morphological characteristics. Although lampreys are phylogenetically distant from jawed vertebrates, somite morphology, especially dermomyotome function, shows similarity. Here we demonstrate that somitogenesis is a conservative process among all vertebrates. We conclude that river lamprey myogenesis shares features with both ancestral and higher vertebrates.

Wnt11 acts on dermomyotome cells to guide epaxial myotome morphogenesis

The dorsal axial muscles, or epaxial muscles, are a fundamental structure covering the spinal cord and vertebrae, as well as mobilizing the vertebrate trunk. To date, mechanisms underlying the morphogenetic process shaping the epaxial myotome are largely unknown. To address this, we used the medaka zic1/zic4-enhancer mutant Double anal fin (Da), which exhibits ventralized dorsal trunk structures resulting in impaired epaxial myotome morphology and incomplete coverage over the neural tube. In wild type, dorsal dermomyotome (DM) cells reduce their proliferative activity after somitogenesis. Subsequently, a subset of DM cells, which does not differentiate into the myotome population, begins to form unique large protrusions extending dorsally to guide the epaxial myotome dorsally. In Da, by contrast, DM cells maintain the high proliferative activity and mainly form small protrusions. By combining RNA- and ChIP-sequencing analyses, we revealed direct targets of Zic1, which are specifically expressed in dorsal somites and involved in various aspects of development, such as cell migration, extracellular matrix organization, and cell-cell communication. Among these, we identified wnt11 as a crucial factor regulating both cell proliferation and protrusive activity of DM cells. We propose that dorsal extension of the epaxial myotome is guided by a non-myogenic subpopulation of DM cells and that wnt11 empowers the DM cells to drive the coverage of the neural tube by the epaxial myotome.

Jagged 2b induces intercellular signaling within somites to establish hematopoietic stem cell fate in zebrafish

During development, the somites play a key role in the specification of hematopoietic stem cells (HSCs). In zebrafish, the somitic Notch ligands Delta-c (Dlc) and Dld, both of which are regulated by Wnt16, directly instruct HSC fate in a shared vascular precursor. However, it remains unclear how this signaling cascade is spatially and temporally regulated within somites. Here, we show in zebrafish that an additional somitic Notch ligand, Jagged 2b (Jag2b), induces intercellular signaling to drive wnt16 expression. Jag2b activated Notch signaling in segmented somites at the early stage of somitogenesis. Loss of jag2b led to a reduction in the expression of wnt16 in the somites and an HSC marker, runx1, in the dorsal aorta, whereas overexpression of jag2b increased both. However, Notch-activated cells were adjacent to, but did not overlap with, wnt16-expressing cells within the somites, suggesting that an additional signaling molecule mediates this intercellular signal transduction. We uncover that Jag2b-driven Notch signaling induces efna1b expression, which regulates wnt16 expression in neighboring somitic cells. Collectively, we provide evidence for previously unidentified spatiotemporal regulatory mechanisms of HSC specification by somites.

Somite morphogenesis is required for axial blood vessel formation during zebrafish embryogenesis

Angioblasts that form the major axial blood vessels of the dorsal aorta and cardinal vein migrate toward the embryonic midline from distant lateral positions. Little is known about what controls the precise timing of angioblast migration and their final destination at the midline. Using zebrafish, we found that midline angioblast migration requires neighboring tissue rearrangements generated by somite morphogenesis. The somitic shape changes cause the adjacent notochord to separate from the underlying endoderm, creating a ventral midline cavity that provides a physical space for the angioblasts to migrate into. The anterior to posterior progression of midline angioblast migration is facilitated by retinoic acid-induced anterior to posterior somite maturation and the subsequent progressive opening of the ventral midline cavity. Our work demonstrates a critical role for somite morphogenesis in organizing surrounding tissues to facilitate notochord positioning and angioblast migration, which is ultimately responsible for creating a functional cardiovascular system.

Evolution of Somite Compartmentalization: A View From Xenopus

Somites are transitory metameric structures at the basis of the axial organization of vertebrate musculoskeletal system. During evolution, somites appear in the chordate phylum and compartmentalize mainly into the dermomyotome, the myotome, and the sclerotome in vertebrates. In this review, we summarized the existing literature about somite compartmentalization in Xenopus and compared it with other anamniote and amniote vertebrates. We also present and discuss a model that describes the evolutionary history of somite compartmentalization from ancestral chordates to amniote vertebrates. We propose that the ancestral organization of chordate somite, subdivided into a lateral compartment of multipotent somitic cells (MSCs) and a medial primitive myotome, evolves through two major transitions. From ancestral chordates to vertebrates, the cell potency of MSCs may have evolved and gave rise to all new vertebrate compartments, i.e., the dermomyome, its hypaxial region, and the sclerotome. From anamniote to amniote vertebrates, the lateral MSC territory may expand to the whole somite at the expense of primitive myotome and may probably facilitate sclerotome formation. We propose that successive modifications of the cell potency of some type of embryonic progenitors could be one of major processes of the vertebrate evolution.

Dynamics of muscle growth and regeneration: Lessons from the teleost

The processes of myogenesis during both development and regeneration share a number of similarities across both amniotes and teleosts. In amniotes, the process of muscle formation is considered largely biphasic, with developmental myogenesis occurring through hyperplastic fibre deposition and postnatal muscle growth driven through hypertrophy of existing fibres. In contrast, teleosts continue generating new muscle fibres during adult myogenesis through a process of eternal hyperplasia using a dedicated stem cell system termed the external cell layer. During developmental and regenerative myogenesis alike, muscle progenitors interact with their niche to receive cues guiding their transition into myoblasts and ultimately mature myofibres. During development, muscle precursors receive input from neighbouring embryological tissues; however, during repair, this role is fulfilled by other injury resident cell types, such as those of the innate immune response. Recent work has focused on the role of macrophages as a pro-regenerative cell type which provides input to muscle satellite cells during regenerative myogenesis. As zebrafish harbour a satellite cell system analogous to that of mammals, the processes of regeneration can be interrogated in vivo with the imaging intensive approaches afforded in the zebrafish system. This review discusses the strengths of zebrafish with a focus on both the similarities and differences to amniote myogenesis during both development and repair.

Notch Signaling Regulates Muscle Stem Cell Homeostasis and Regeneration in a Teleost Fish

Muscle regeneration is mediated by the activity of resident muscle satellite cells (muSCs) that express Pax7. In mouse Notch signaling regulates muSCs during quiescence and promotes muSC proliferation in regeneration. It is unclear if these roles of Notch in regulating muSC biology are conserved across vertebrates or are a mammalian specific feature. We have therefore investigated the role of Notch in regulating muSC homeostasis and regeneration in a teleost fish, the zebrafish. We have also tested whether muSCs show differential sensitivity to Notch during myotome development. In an absence of injury Notch is important for preventing muSC proliferation at the vertical myoseptum. In contrast, Notch signaling promotes proliferation and prevents differentiation in the context of injury. Notch is required for the proliferative response to injury at early and later larval stages, suggesting it plays a similar role in regulating muSCs at developing and adult stages. Our results reveal a conserved role for Notch signaling in regulating muSCs under homeostasis and for promoting proliferation during regeneration in teleost fish.

Cellular aspects of somite formation in vertebrates

Vertebrate segmentation, the process that generates a regular arrangement of somites and thereby establishes the pattern of the adult body and of the musculoskeletal and peripheral nervous systems, was noticed many centuries ago. In the last few decades, there has been renewed interest in the process and especially in the molecular mechanisms that might account for its regularity and other spatial-temporal properties. Several models have been proposed but surprisingly, most of these do not provide clear links between the molecular mechanisms and the cell behaviours that generate the segmental pattern. Here we present a short survey of our current knowledge about the cellular aspects of vertebrate segmentation and the similarities and differences between different vertebrate groups in how they achieve their metameric pattern. Taking these variations into account should help to assess each of the models more appropriately.

Pax3 and Pax7 Exhibit Distinct and Overlapping Functions in Marking Muscle Satellite Cells and Muscle Repair in a Marine Teleost, Sebastes schlegelii

Pax3 and Pax7 are members of the Pax gene family which are essential for embryo and organ development. Both genes have been proved to be markers of muscle satellite cells and play key roles in the process of muscle growth and repair. Here, we identified two Pax3 genes (SsPax3a and SsPax3b) and two Pax7 genes (SsPax7a and SsPax7b) in a marine teleost, black rockfish (Sebastes schlegelii). Our results showed SsPax3 and SsPax7 marked distinct populations of muscle satellite cells, which originated from the multi-cell stage and somite stage, respectively. In addition, we constructed a muscle injury model to explore the function of these four genes during muscle repair. Hematoxylin–eosin (H–E) of injured muscle sections showed new-formed myofibers occurred at 16 days post-injury (dpi). ISH (in situ hybridization) analysis demonstrated that the expression level of SsPax3a and two SsPax7 genes increased gradually during 0–16 dpi and peaked at 16 dpi. Interestingly, SsPax3b showed no significant differences during the injury repair process, indicating that the satellite cells labeled by SsPax3b were not involved in muscle repair. These results imply that the muscle stem cell populations in teleosts are more complicated than in mammals. This lays the foundation for future studies on the molecular mechanism of indeterminant growth and muscle repair of large fish species.

Molecular Mechanisms Regulating Muscle Plasticity in Fish

Growth rates in fish are largely dependent on genetic and environmental factors, of which the latter can be highly variable throughout development. For this reason, muscle growth in fish is particularly dynamic as muscle structure and function can be altered by environmental conditions, a concept referred to as muscle plasticity. Myogenic regulatory factors (MRFs) like Myogenin, MyoD, and Pax7 control the myogenic mechanisms regulating quiescent muscle cell maintenance, proliferation, and differentiation, critical processes central for muscle plasticity. This review focuses on recent advancements in molecular mechanisms involving microRNAs (miRNAs) and DNA methylation that regulate the expression and activity of MRFs in fish. Findings provide overwhelming support that these mechanisms are significant regulators of muscle plasticity, particularly in response to environmental factors like temperature and nutritional challenges. Genetic variation in DNA methylation and miRNA expression also correlate with variation in body weight and growth, suggesting that genetic markers related to these mechanisms may be useful for genomic selection strategies. Collectively, this knowledge improves the understanding of mechanisms regulating muscle plasticity and can contribute to the development of husbandry and breeding strategies that improve growth performance and the ability of the fish to respond to environmental challenges.

Cell Tracking Profiler - a user-driven analysis framework for evaluating 4D live-cell imaging data

Accurate measurements of cell morphology and behaviour are fundamentally important for understanding how disease, molecules and drugs affect cell function in vivo Here, by using muscle stem cell (muSC) responses to injury in zebrafish as our biological paradigm, we established a 'ground truth' for muSC behaviour. This revealed that segmentation and tracking algorithms from commonly used programs are error-prone, leading us to develop a fast semi-automated image analysis pipeline that allows user-defined parameters for segmentation and correction of cell tracking. Cell Tracking Profiler (CTP) is a package that runs two existing programs, HK Means and Phagosight within the Icy image analysis suite, to enable user-managed cell tracking from 3D time-lapse datasets to provide measures of cell shape and movement. We demonstrate how CTP can be used to reveal changes to cell behaviour of muSCs in response to manipulation of the cell cytoskeleton by small-molecule inhibitors. CTP and the associated tools we have developed for analysis of outputs thus provide a powerful framework for analysing complex cell behaviour in vivo from 4D datasets that are not amenable to straightforward analysis.

Animal models of sarcopenia

Sarcopenia is the age-related decline in muscle mass and function without any underlying disease. The exact molecular mechanisms responsible for this pathology remain unknown. The use of model organisms, such as mice, rats, flies, and worms, has advanced the field of sarcopenia research by identifying therapeutic strategies and genetic mutations that result in improved muscle mass and function of elderly animals. This review discusses molecular and therapeutic discoveries made using these model organisms and how these animals can be further utilized to better understand sarcopenia pathogenesis. In rodents, flies, and worms, dietary restriction improves muscle performance in old animals. In rodents and worms, exercise and a number of naturally occurring compounds alleviate sarcopenia. Reduction in the insulin/IGF1 receptor pathway, well known to promote longevity, improves sarcopenia in worms and flies. Mitochondrial dysfunction may contribute to the pathogenesis of sarcopenia: In rodents, there is age-dependent reduction in mitochondrial mass and a change in morphology; in nematodes, there is age-dependent fragmentation of mitochondria that precedes sarcomeric disorganization. In Drosophila and rats, components of the 26S proteasome are elevated in aged muscle. An advantage of the worm and fly models is that these organisms lack muscle stem cells, and thus processes that promote the maintenance of already assembled muscle, can be identified without the confounding influence of muscle regeneration. Zebrafish are an up and coming model of sarcopenia for future consideration. A better understanding of the molecular changes behind sarcopenia will help researchers develop better therapies to improve the muscle health of elderly individuals.

Myogenin is an essential regulator of adult myofibre growth and muscle stem cell homeostasis

Growth and maintenance of skeletal muscle fibres depend on coordinated activation and return to quiescence of resident muscle stem cells (MuSCs). The transcription factor Myogenin (Myog) regulates myocyte fusion during development, but its role in adult myogenesis remains unclear. In contrast to mice, myog-/-zebrafish are viable, but have hypotrophic muscles. By isolating adult myofibres with associated MuSCs, we found that myog-/- myofibres have severely reduced nuclear number, but increased myonuclear domain size. Expression of fusogenic genes is decreased, Pax7 upregulated, MuSCs are fivefold more numerous and mis-positioned throughout the length of myog-/-myofibres instead of localising at myofibre ends as in wild-type. Loss of Myog dysregulates mTORC1 signalling, resulting in an 'alerted' state of MuSCs, which display precocious activation and faster cell cycle entry ex vivo, concomitant with myod upregulation. Thus, beyond controlling myocyte fusion, Myog influences the MuSC:niche relationship, demonstrating a multi-level contribution to muscle homeostasis throughout life.

Horizontal Boundary Cells, a Special Group of Somitic Cells, Play Crucial Roles in the Formation of Dorsoventral Compartments in Teleost Somite

Establishment of robust gene expression boundary is crucial for creating elaborate morphology during development. However, mechanisms underlying boundary formation have been extensively studied only in a few model systems. We examined the establishment of zic1/zic4-expression boundary demarcating dorsoventral boundary of the entire trunk of medaka fish (Oryzias latipes) and identified a subgroup of dermomyotomal cells called horizontal boundary cells (HBCs) as crucial players for the boundary formation. Embryological and genetic analyses demonstrated that HBCs play crucial roles in the two major events of the process, i.e., refinement and maintenance. In the refinement, HBCs could serve as a chemical barrier against Wnts from the neural tube by expressing Hhip. At later stages, HBCs participate in the maintenance of the boundary by differentiating into the horizontal myoseptum physically inhibiting cell mixing across the boundary. These findings reveal the mechanisms underlying the dorsoventral boundary in the teleost trunk by specialized boundary cells.

Zebrafish models of sarcopenia

Sarcopenia - the accelerated age-related loss of muscle mass and function - is an under-diagnosed condition, and is central to deteriorating mobility, disability and frailty in older age. There is a lack of treatment options for older adults at risk of sarcopenia. Although sarcopenia's pathogenesis is multifactorial, its major phenotypes - muscle mass and muscle strength - are highly heritable. Several genome-wide association studies of muscle-related traits were published recently, providing dozens of candidate genes, many with unknown function. Therefore, animal models are required not only to identify causal mechanisms, but also to clarify the underlying biology and translate this knowledge into new interventions. Over the past several decades, small teleost fishes had emerged as powerful systems for modeling the genetics of human diseases. Owing to their amenability to rapid genetic intervention and the large number of conserved genetic and physiological features, small teleosts - such as zebrafish, medaka and killifish - have become indispensable for skeletal muscle genomic studies. The goal of this Review is to summarize evidence supporting the utility of small fish models for accelerating our understanding of human skeletal muscle in health and disease. We do this by providing a basic foundation of the (zebra)fish skeletal muscle morphology and physiology, and evidence of muscle-related gene homology. We also outline challenges in interpreting zebrafish mutant phenotypes and in translating them to human disease. Finally, we conclude with recommendations on future directions to leverage the large body of tools developed in small fish for the needs of genomic exploration in sarcopenia.

Rainbow trout slow myoblast cell culture as a model to study slow skeletal muscle, and the characterization of mir-133 and mir-499 families as a case study

Muscle fibres are classified as fast, intermediate and slow. In vitro myoblast cell culture model from fast muscle is a very useful tool to study muscle growth and development; however, similar models for slow muscle do not exist. Owing to the compartmentalization of fish muscle fibres, we have developed a slow myoblast cell culture for rainbow trout (Oncorhynchus mykiss). Slow and fast muscle-derived myoblasts have similar morphology, but with differential expression of slow muscle markers such as slow myhc, sox6 and pgc-1α We also characterized the mir-133 and mir-499 microRNA families in trout slow and fast myoblasts as a case study during myogenesis and in response to electrostimulation. Three mir-133 (a-1a, a-1b and a-2) and four mir-499 (aa, ab, ba and bb) paralogues were identified for rainbow trout and named base on their phylogenetic relationship to zebrafish and Atlantic salmon orthologues. Omy-mir-499ab and omy-mir-499bb had 0.6 and 0.5-fold higher expression in slow myoblasts compared with fast myoblasts, whereas mir-133 duplicates had similar levels in both phenotypes and little variation during development. Slow myoblasts also showed increased expression for omy-mir-499b paralogues in response to chronic electrostimulation (7-fold increase for omy-mir-499ba and 2.5-fold increase for omy-mir-499bb). The higher expression of mir-499 paralogues in slow myoblasts suggests a role in phenotype determination, while the lack of significant differences of mir-133 copies during culture development might indicate a different role in fish compared with mammals. We have also found signs of sub-functionalization of mir-499 paralogues after electrostimulation, with omy-mir-499b copies more responsive to electrical signals.

Everybody wants to move-Evolutionary implications of trunk muscle differentiation in vertebrate species

In our review we have completed current knowledge on myotomal myogenesis in model and non-model vertebrate species (fishes, amphibians, reptiles, birds and mammals) at morphological and molecular levels. Data obtained from these studies reveal distinct similarities and differences between amniote and anamniote species. Based on the available data, we decided to present evolutionary implications in vertebrate trunk muscle development. Despite the fact that in all vertebrates muscle fibres are multinucleated, the pathways leading to them vary between vertebrate taxa. In fishes during early myogenesis myoblasts differentiate into multinucleated lamellae or multinucleate myotubes. In amphibians, myoblasts fuse to form multinucleated myotubes or, bypassing fusion, directly differentiate into mononucleated myotubes. Furthermore, mononucleated myotubes were also observed during primary myogenesis in amniotes. The mononucleated state of myogenic cells could be considered as an old phylogenetic, plesiomorphic feature, whereas direct multinuclearity of myotubes has a synapomorphic character. On the other hand, the explanation of this phenomenon could also be linked to the environmental conditions in which animals develop. The similarities observed in vertebrate myogenesis might result from a conservative myogenic programme governed by the Pax3/Pax7 and myogenic regulatory factor (MRF) network, whereas differences in anamniotes and amniotes are established by spatiotemporal pattern expression of MRFs during muscle differentiation and/or environmental conditions.

Stem cells in skeletal muscle growth and regeneration in amniotes and teleosts: Emerging themes

Skeletal muscle is a contractile, postmitotic tissue that retains the capacity to grow and regenerate throughout life in amniotes and teleost. Both muscle growth and regeneration are regulated by obligate tissue resident muscle stem cells. Given that considerable knowledge exists on the myogenic process, recent studies have focused on examining the molecular markers of muscle stem cells, and on the intrinsic and extrinsic signals regulating their function. From this, two themes emerge: firstly, muscle stem cells display remarkable heterogeneity not only with regards to their gene expression profile, but also with respect to their behavior and function; and secondly, the stem cell niche is a critical regulator of muscle stem cell function during growth and regeneration. Here, we will address the current understanding of these emerging themes with emphasis on the distinct processes used by amniotes and teleost, and discuss the challenges and opportunities in the muscle growth and regeneration fields. This article is characterized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Early Embryonic Development > Development to the Basic Body Plan Vertebrate Organogenesis > Musculoskeletal and Vascular.

Single cell dynamics of embryonic muscle progenitor cells in zebrafish

Muscle stem cells hold a great therapeutic potential in regenerating damaged muscles. However, the in vivo behavior of muscle stem cells during muscle growth and regeneration is still poorly understood. Using zebrafish as a model, we describe the in vivo dynamics and function of embryonic muscle progenitor cells (MPCs) in the dermomyotome. These cells are located in a superficial layer external to muscle fibers and express many extracellular matrix (ECM) genes, including collagen type 1 α2 (col1a2). Utilizing a new col1a2 transgenic line, we show that col1a2⁺ MPCs display a ramified morphology with dynamic cellular processes. Cell lineage tracing demonstrates that col1a2⁺ MPCs contribute to new myofibers in normal muscle growth and also during muscle regeneration. A combination of live imaging and single cell clonal analysis reveals a highly choreographed process of muscle regeneration. Activated col1a2⁺ MPCs change from the quiescent ramified morphology to a polarized and elongated morphology, generating daughter cells that fuse with existing myofibers. Partial depletion of col1a2⁺ MPCs severely compromises muscle regeneration. Our work provides a dynamic view of embryonic muscle progenitor cells during zebrafish muscle growth and regeneration.

The Developmental Phases of Zebrafish Myogenesis

The development and growth of vertebrate axial muscle have been studied for decades at both the descriptive and molecular level. The zebrafish has provided an attractive model system for investigating both muscle patterning and growth due to its simple axial musculature with spatially separated fibre types, which contrasts to complex muscle groups often deployed in amniotes. In recent years, new findings have reshaped previous concepts that define how final teleost muscle form is established and maintained. Here, we summarise recent findings in zebrafish embryonic myogenesis with a focus on fibre type specification, followed by an examination of the molecular mechanisms that control muscle growth with emphasis on the role of the dermomyotome-like external cell layer. We also consider these data sets in a comparative context to gain insight into the evolution of axial myogenic patterning systems within the vertebrate lineage.

Cdc42 Effector Protein 3 Interacts With Cdc42 in Regulating Xenopus Somite Segmentation

Somitogenesis is a critical process during vertebrate development that establishes the segmented body plan and gives rise to the vertebra, skeletal muscles, and dermis. While segmentation clock and wave front mechanisms have been elucidated to control the size and time of somite formation, regulation of the segmentation process that physically separates somites is not understood in detail. Here, we identified a cytoskeletal player, Cdc42 effector protein 3 (Cdc42ep3, CEP3) that is required for somite segmentation in Xenopus embryos. CEP3 is specifically expressed in somite tissue during somite segmentation. Loss-of-function experiments showed that CEP3 is not required for the specification of paraxial mesoderm, nor the differentiation of muscle cells, but is required for the segmentation process. Live imaging analysis further revealed that CEP3 is required for cell shape changes and alignment during somitogenesis. When CEP3 was knocked down, somitic cells did not elongate efficiently along the mediolateral axis and failed to undertake the 90° rotation. As a result, cells remained in a continuous sheet without an apparent segmentation cleft. CEP3 likely interacts with Cdc42 during this process, and both increased and decreased Cdc42 activity led to defective somite segmentation. Segmentation defects caused by Cdc42 knockdown can be partially rescued by the overexpression of CEP3. Conversely, loss of CEP3 resulted in the maintenance of high levels of Cdc42 activity at the cell membrane, which is normally reduced during and after somite segmentation. These results suggest that there is a feedback regulation between Cdc42 and CEP3 during somite segmentation and the activity of Cdc42 needs to be fine-tuned to control the coordinated cell shape changes and movement required for somite segmentation.

Lineage tracing of sclerotome cells in amphibian reveals that multipotent somitic cells originate from lateral somitic frontier

The two somite compartments, dorso-lateral dermomyotome and medio-ventral sclerotome are major vertebrate novelties, but little is known about their evolutionary origin. We determined that sclerotome cells in Xenopus come from lateral somitic frontier (LSF) by lineage tracing, ablation experiments and histological analysis. We identified Twist1 as marker of migrating sclerotome progenitors in two amphibians, Xenopus and axolotl. From these results, three conclusions can be drawn. First, LSF is made up of multipotent somitic cells (MSCs) since LSF gives rise to sclerotome but also to dermomytome as already shown in Xenopus. Second, the basic scheme of somite compartmentalization is conserved from cephalochordates to anamniotes since in both cases, lateral cells envelop dorsally and ventrally the ancestral myotome, suggesting that lateral MSCs should already exist in cephalochordates. Third, the transition from anamniote to amniote vertebrates is characterized by extension of the MSCs domain to the entire somite at the expense of ancestral myotome since amniote somite is a naive tissue that subdivides into sclerotome and dermomyotome. Like neural crest pluripotent cells, MSCs are at the origin of major vertebrate novelties, namely hypaxial region of the somite, dermomyotome and sclerotome compartments. Hence, change in MSCs properties and location is involved in somite evolution.

Development of myofibres and associated connective tissues in fish axial muscle: Recent insights and future perspectives

Fish axial muscle consists of a series of W-shaped muscle blocks, called myomeres, that are composed primarily of multinucleated contractile muscle cells (myofibres) gathered together by an intricate network of connective tissue that transmits forces generated by myofibre contraction to the axial skeleton. This review summarises current knowledge on the successive and overlapping myogenic waves contributing to axial musculature formation and growth in fish. Additionally, this review presents recent insights into muscle connective tissue development in fish, focusing on the early formation of collagenous myosepta separating adjacent myomeres and the late formation of intramuscular connective sheaths (i.e. endomysium and perimysium) that is completed only at the fry stage when connective fibroblasts expressing collagens arise inside myomeres. Finally, this review considers the possibility that somites produce not only myogenic, chondrogenic and myoseptal progenitor cells as previously reported, but also mesenchymal cells giving rise to muscle resident fibroblasts.

Spatiotemporal Coordination of FGF and Shh Signaling Underlies the Specification of Myoblasts in the Zebrafish Embryo

Somitic cells give rise to a variety of cell types in response to Hh, BMP, and FGF signaling. Cell position within the developing zebrafish somite is highly dynamic: how, when, and where these signals specify cell fate is largely unknown. Combining four-dimensional imaging with pathway perturbations, we characterize the spatiotemporal specification and localization of somitic cells. Muscle formation is guided by highly orchestrated waves of cell specification. We find that FGF directly and indirectly controls the differentiation of fast and slow-twitch muscle lineages, respectively. FGF signaling imposes tight temporal control on Shh induction of slow muscles by regulating the time at which fast-twitch progenitors displace slow-twitch progenitors from contacting the Shh-secreting notochord. Further, we find a reciprocal regulation of fast and slow muscle differentiation, morphogenesis, and migration. In conclusion, robust cell fate determination in the developing somite requires precise spatiotemporal coordination between distinct cell lineages and signaling pathways.

Myogenin promotes myocyte fusion to balance fibre number and size

Each skeletal muscle acquires its unique size before birth, when terminally differentiating myocytes fuse to form a defined number of multinucleated myofibres. Although mice in which the transcription factor Myogenin is mutated lack most myogenesis and die perinatally, a specific cell biological role for Myogenin has remained elusive. Here we report that loss of function of zebrafish myog prevents formation of almost all multinucleated muscle fibres. A second, Myogenin-independent, fusion pathway in the deep myotome requires Hedgehog signalling. Lack of Myogenin does not prevent terminal differentiation; the smaller myotome has a normal number of myocytes forming more mononuclear, thin, albeit functional, fast muscle fibres. Mechanistically, Myogenin binds to the myomaker promoter and is required for expression of myomaker and other genes essential for myocyte fusion. Adult myog mutants display reduced muscle mass, decreased fibre size and nucleation. Adult-derived myog mutant myocytes show persistent defective fusion ex vivo. Myogenin is therefore essential for muscle homeostasis, regulating myocyte fusion to determine both muscle fibre number and size.

Loss of the transcription factor Myogenin in mice reduces skeletal myogenesis and leads to perinatal death but how Myogenin regulates muscle formation is unclear. Here, the authors show that zebrafish Myogenin enhances Myomaker expression, muscle cell fusion and myotome size, yet decreases fast muscle fibre number.

Xenopus SOX5 enhances myogenic transcription indirectly through transrepression

In anamniotes, somite compartimentalization in the lateral somitic domain leads simultaneously to myotome and dermomyotome formation. In the myotome, Xenopus Sox5 is co-expressed with Myod1 in the course of myogenic differentiation. Here, we studied the function of Sox5 using a Myod1-induced myogenic transcription assay in pluripotent cells of animal caps. We found that Sox5 enhances myogenic transcription of muscle markers Des, Actc1, Ckm and MyhE3. The use of chimeric transactivating or transrepressive Sox5 proteins indicates that Sox5 acts as a transrepressor and indirectly stimulates myogenic transcription except for the slow muscle-specific genes Myh7L, Myh7S, Myl2 and Tnnc1. We showed that this role is shared by Sox6, which is structurally similar to Sox5, both belonging to the SoxD subfamily of transcription factors. Moreover, Sox5 can antagonize the inhibitory function of Meox2 on myogenic differentiation. Meox2 which is a dermomyotome marker, represses myogenic transcription in Myod-induced myogenic transcription assay and in Nodal5-induced mesoderm from animal cap assay. The inhibitory function of Meox2 and the pro-myogenic function of Sox5 were confirmed during Xenopus normal development by the use of translation-blocking oligomorpholinos and dexamethasone inducible chimeric Sox5 and Meox2 proteins. We have therefore identified a new function for SoxD proteins in muscle cells, which can indirectly enhance myogenic transcription through transrepression, in addition to the previously identified function as a direct repressor of slow muscle-specific genes.

microRNA-206 modulates an Rtn4a/Cxcr4a/Thbs3a axis in newly forming somites to maintain and stabilize the somite boundary formation of zebrafish embryos

Although microRNA-206 (miR-206) is known to regulate proliferation and differentiation of muscle fibroblasts, the role of miR-206 in early-stage somite development is still unknown. During somitogenesis of zebrafish embryos, reticulon4a (rtn4a) is specifically repressed by miR-206. The somite boundary was defective, and actin filaments were crossing over the boundary in either miR-206-knockdown or rtn4a-overexpressed embryos. In these treated embryos, C-X-C motif chemokine receptor 4a (cxcr4a) was reduced, while thrombospondin 3a (thbs3a) was increased. The defective boundary was phenocopied in either cxcr4a-knockdown or thbs3a-overexpressed embryos. Repression of thbs3a expression by cxcr4a reduced the occurrence of the boundary defect. We demonstrated that cxcr4a is an upstream regulator of thbs3a and that defective boundary cells could not process epithelialization in the absence of intracellular accumulation of the phosphorylated focal adhesion kinase (p-FAK) in boundary cells. Therefore, in the newly forming somites, miR-206-mediated downregulation of rtn4a increases cxcr4a. This activity largely decreases thbs3a expression in the epithelial cells of the somite boundary, which causes epithelialization of boundary cells through mesenchymal-epithelial transition (MET) and eventually leads to somite boundary formation. Collectively, we suggest that miR-206 mediates a novel pathway, the Rtn4a/Cxcr4a/Thbs3a axis, that allows boundary cells to undergo MET and form somite boundaries in the newly forming somites of zebrafish embryos.

Ethanol Exposure Causes Muscle Degeneration in Zebrafish

Alcoholic myopathies are characterized by neuromusculoskeletal symptoms such as compromised movement and weakness. Although these symptoms have been attributed to neurological damage, EtOH may also target skeletal muscle. EtOH exposure during zebrafish primary muscle development or adulthood results in smaller muscle fibers. However, the effects of EtOH exposure on skeletal muscle during the growth period that follows primary muscle development are not well understood. We determined the effects of EtOH exposure on muscle during this phase of development. Strikingly, muscle fibers at this stage are acutely sensitive to EtOH treatment: EtOH induces muscle degeneration. The severity of EtOH-induced muscle damage varies but muscle becomes more refractory to EtOH as muscle develops. NF-kB induction in muscle indicates that EtOH triggers a pro-inflammatory response. EtOH-induced muscle damage is p53-independent. Uptake of Evans blue dye shows that EtOH treatment causes sarcolemmal instability before muscle fiber detachment. Dystrophin-null sapje mutant zebrafish also exhibit sarcolemmal instability. We tested whether Trichostatin A (TSA), which reduces muscle degeneration in sapje mutants, would affect EtOH-treated zebrafish. We found that TSA and EtOH are a lethal combination. EtOH does, however, exacerbate muscle degeneration in sapje mutants. EtOH also disrupts adhesion of muscle fibers to their extracellular matrix at the myotendinous junction: some detached muscle fibers retain beta-Dystroglycan indicating failure of muscle end attachments. Overexpression of Paxillin, which reduces muscle degeneration in zebrafish deficient for beta-Dystroglycan, is not sufficient to rescue degeneration. Taken together, our results suggest that EtOH exposure has pleiotropic deleterious effects on skeletal muscle.

Myotome adaptability confers developmental robustness to somitic myogenesis in response to fibre number alteration

Balancing the number of stem cells and their progeny is crucial for tissue development and repair. Here we examine how cell numbers and overall muscle size are tightly regulated during zebrafish somitic muscle development. Muscle stem/precursor cell (MPCs) expressing Pax7 are initially located in the dermomyotome (DM) external cell layer, adopt a highly stereotypical distribution and thereafter a proportion of MPCs migrate into the myotome. Regional variations in the proliferation and terminal differentiation of MPCs contribute to growth of the myotome. To probe the robustness of muscle size control and spatiotemporal regulation of MPCs, we compared the behaviour of wild type (wt) MPCs with those in mutant zebrafish that lack the muscle regulatory factor Myod. Myod^fh261 mutants form one third fewer multinucleate fast muscle fibres than wt and show a significant expansion of the Pax7⁺ MPC population in the DM. Subsequently, myod^fh261 mutant fibres generate more cytoplasm per nucleus, leading to recovery of muscle bulk. In addition, relative to wt siblings, there is an increased number of MPCs in myod^fh261 mutants and these migrate prematurely into the myotome, differentiate and contribute to the hypertrophy of existing fibres. Thus, homeostatic reduction of the excess MPCs returns their number to normal levels, but fibre numbers remain low. The GSK3 antagonist BIO prevents MPC migration into the deep myotome, suggesting that canonical Wnt pathway activation maintains the DM in zebrafish, as in amniotes. BIO does not, however, block recovery of the myod^fh261 mutant myotome, indicating that homeostasis acts on fibre intrinsic growth to maintain muscle bulk. The findings suggest the existence of a critical window for early fast fibre formation followed by a period in which homeostatic mechanisms regulate myotome growth by controlling fibre size. The feedback controls we reveal in muscle help explain the extremely precise grading of myotome size along the body axis irrespective of fish size, nutrition and genetic variation and may form a paradigm for wider matching of organ size.

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A critical window for early muscle fibre formation is proposed.

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Fish lacking MyoD1 form fewer muscle fibres, but have more myogenic stem cells.

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Stem cell numbers rapidly return to normal during subsequent development.

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GSK3 activity promotes and MyoD1 delays myoblast migration into the myotome.

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Compensatory fibre size increase ensures robustness of overall muscle size.

Global gene expression in muscle from fasted/refed trout reveals up-regulation of genes promoting myofibre hypertrophy but not myofibre production

BACKGROUND

Compensatory growth is a phase of rapid growth, greater than the growth rate of control animals, that occurs after a period of growth-stunting conditions. Fish show a capacity for compensatory growth after alleviation of dietary restriction, but the underlying cellular mechanisms are unknown. To learn more about the contribution of genes regulating hypertrophy (an increase in muscle fibre size) and hyperplasia (the generation of new muscle fibres) in the compensatory muscle growth response in fish, we used high-density microarray analysis to investigate the global gene expression in muscle of trout during a fasting-refeeding schedule and in muscle of control-fed trout displaying normal growth.

RESULTS

The compensatory muscle growth signature, as defined by genes up-regulated in muscles of refed trout compared with control-fed trout, showed enrichment in functional categories related to protein biosynthesis and maturation, such as RNA processing, ribonucleoprotein complex biogenesis, ribosome biogenesis, translation and protein folding. This signature was also enriched in chromatin-remodelling factors of the protein arginine N-methyl transferase family. Unexpectedly, functional categories related to cell division and DNA replication were not inferred from the molecular signature of compensatory muscle growth, and this signature contained virtually none of the genes previously reported to be up-regulated in hyperplastic growth zones of the late trout embryo myotome and to potentially be involved in production of new myofibres, notably genes encoding myogenic regulatory factors, transmembrane receptors essential for myoblast fusion or myofibrillar proteins predominant in nascent myofibres.

CONCLUSION

Genes promoting myofibre growth, but not myofibre formation, were up-regulated in muscles of refed trout compared with continually fed trout. This suggests that a compensatory muscle growth response, resulting from the stimulation of hypertrophy but not the stimulation of hyperplasia, occurs in trout after refeeding. The generation of a large set of genes up-regulated in muscle of refed trout may yield insights into the molecular and cellular mechanisms controlling skeletal muscle mass in teleost and serve as a useful list of potential molecular markers of muscle growth in fish.

RARβ2 is required for vertebrate somitogenesis

During vertebrate somitogenesis, retinoic acid is known to establish the position of the determination wavefront, controlling where new somites are permitted to form along the anteroposterior body axis. Less is understood about how RAR regulates somite patterning, rostral-caudal boundary setting, specialization of myotome subdivisions or the specific RAR subtype that is required for somite patterning. Characterizing the function of RARβ has been challenging due to the absence of embryonic phenotypes in murine loss-of-function studies. Using the Xenopus system, we show that RARβ2 plays a specific role in somite number and size, restriction of the presomitic mesoderm anterior border, somite chevron morphology and hypaxial myoblast migration. Rarβ2 is the RAR subtype whose expression is most upregulated in response to ligand and its localization in the trunk somites positions it at the right time and place to respond to embryonic retinoid levels during somitogenesis. RARβ2 positively regulates Tbx3 a marker of hypaxial muscle, and negatively regulates Tbx6 via Ripply2 to restrict the anterior boundaries of the presomitic mesoderm and caudal progenitor pool. These results demonstrate for the first time an early and essential role for RARβ2 in vertebrate somitogenesis.

A role for planar cell polarity during early endoderm morphogenesis

The zebrafish endoderm begins to develop at gastrulation stages as a monolayer of cells. The behaviour of the endoderm during gastrulation stages is well understood. However, knowledge of the morphogenic movements of the endoderm during somitogenesis stages, as it forms a mesenchymal rod, is lacking. Here we characterise endodermal development during somitogenesis stages, and describe the morphogenic movements as the endoderm transitions from a monolayer of cells into a mesenchymal endodermal rod. We demonstrate that, unlike the overlying mesoderm, endodermal cells are not polarised during their migration to the midline at early somitogenesis stages. Specifically, we describe the stage at which endodermal cells begin to leave the monolayer, a process we have termed 'midline aggregation'. The planar cell polarity (PCP) signalling pathway is known to regulate mesodermal and ectodermal cell convergence towards the dorsal midline. However, a role for PCP signalling in endoderm migration to the midline during somitogenesis stages has not been established. In this report, we investigate the role for PCP signalling in multiple phases of endoderm development during somitogenesis stages. Our data exclude involvement of PCP signalling in endodermal cells as they leave the monolayer.

Stem cell dynamics in muscle regeneration: Insights from live imaging in different animal models

In recent years, live imaging has been adopted to study stem cells in their native environment at cellular resolution. In the skeletal muscle field, this has led to visualising the initial events of muscle repair in mouse, and the entire regenerative response in zebrafish. Here, we review recent discoveries in this field obtained from live imaging studies. Tracking of tissue resident stem cells, the satellite cells, following injury has captured the morphogenetic dynamics of stem/progenitor cells as they facilitate repair. Asymmetric satellite cell division generated a clonogenic progenitor pool, providing in vivo validation for this mechanism. Furthermore, there is an emerging role of stem/progenitor cell guidance at the injury site by cellular protrusions. This review concludes that live imaging is a critical tool for discovering the distinct processes that occur during regeneration, emphasising the importance of imaging in diverse animal models to capture the entire scope of stem cell functions. Also see the Video Abstract. Link to: https://youtube/tgUHSBD1N0g.