Morphogenesis and cell fate determination within the adaxial cell equivalence group of the zebrafish myotome

Mutations in cdon and boc affect trunk neural crest cell migration and slow-twitch muscle development in zebrafish

The transmembrane proteins cdon and boc are implicated in regulating hedgehog signaling during vertebrate development. Recent work showing roles for these genes in axon guidance and neural crest cell migration suggest that cdon and boc may play additional functions in regulating directed cell movements. We use newly generated and existing mutants to investigate a role for cdon and boc in zebrafish neural crest cell migration. We find that single mutant embryos exhibit normal neural crest phenotypes, but that neural crest migration is strikingly disrupted in double cdon;boc mutant embryos. We further show that this migration phenotype is associated with defects in the differentiation of slow-twitch muscle cells, and the loss of a Col1a1a-containing extracellular matrix, suggesting that neural crest defects may be a secondary consequence to defects in mesoderm development. Combined, our data add to a growing literature showing that cdon and boc act synergistically to promote hedgehog signaling during vertebrate development, and suggest that the zebrafish can be used to study the function of hedgehog receptor paralogs.

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.

Slow muscles guide fast myocyte fusion to ensure robust myotome formation despite the high spatiotemporal stochasticity of fusion events

Skeletal myogenesis is dynamic, and it involves cell-shape changes together with cell fusion and rearrangements. However, the final muscle arrangement is highly organized with striated fibers. By combining live imaging with quantitative analyses, we dissected fast-twitch myocyte fusion within the zebrafish myotome in toto. We found a strong mediolateral bias in fusion timing; however, at a cellular scale, there was heterogeneity in cell shape and the relationship between initial position of fast myocytes and resulting fusion partners. We show that the expression of the fusogen myomaker is permissive, but not instructive, in determining the spatiotemporal fusion pattern. Rather, we observed a close coordination between slow muscle rearrangements and fast myocyte fusion. In mutants that lack slow fibers, the spatiotemporal fusion pattern is substantially noisier. We propose a model in which slow muscles guide fast myocytes by funneling them close together, enhancing fusion probability. Thus, despite fusion being highly stochastic, a robust myotome structure emerges at the tissue scale.

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.

NRG1/ErbB signalling controls the dialogue between macrophages and neural crest-derived cells during zebrafish fin regeneration

Fish species, such as zebrafish (Danio rerio), can regenerate their appendages after amputation through the formation of a heterogeneous cellular structure named blastema. Here, by combining live imaging of triple transgenic zebrafish embryos and single-cell RNA sequencing we established a detailed cell atlas of the regenerating caudal fin in zebrafish larvae. We confirmed the presence of macrophage subsets that govern zebrafish fin regeneration, and identified a foxd3-positive cell population within the regenerating fin. Genetic depletion of these foxd3-positive neural crest-derived cells (NCdC) showed that they are involved in blastema formation and caudal fin regeneration. Finally, chemical inhibition and transcriptomic analysis demonstrated that these foxd3-positive cells regulate macrophage recruitment and polarization through the NRG1/ErbB pathway. Here, we show the diversity of the cells required for blastema formation, identify a discrete foxd3-positive NCdC population, and reveal the critical function of the NRG1/ErbB pathway in controlling the dialogue between macrophages and NCdC.

Some fish can regenerate appendages by formation of a structure called the blastema. Here, the authors use single-cell RNA sequencing to characterize the cells required for blastema formation and fin regeneration and identified neural crest cells that orchestrate regeneration via the NRG1/ErbB axis

atoh8 expression pattern in early zebrafish embryonic development

Atonal homologue 8 (atoh8) is a basic helix-loop-helix transcription factor expressed in a variety of embryonic tissues. While several studies have implicated atoh8 in various developmental pathways in other species, its role in zebrafish development remains uncertain. So far, no studies have dealt with an in-depth in situ analysis of the tissue distribution of atoh8 in embryonic zebrafish. We set out to pinpoint the exact location of atoh8 expression in a detailed spatio-temporal analysis in zebrafish during the first 24 h of development (hpf). To our surprise, we observed transcription from pre-segmentation stages in the paraxial mesoderm and during the segmentation stages in the somitic sclerotome and not—as previously reported—in the myotome. With progressing maturation of the somites, the restriction of atoh8 to the sclerotomal compartment became evident. Double in situ hybridisation with atoh8 and myoD revealed that both genes are expressed in the somites at coinciding developmental stages; however, their domains do not spatially overlap. A second domain of atoh8 expression emerged in the embryonic brain in the developing cerebellum and hindbrain. Here, we observed a specific expression pattern which was again in contrast to the previously published suggestion of atoh8 transcription in neural crest cells. Our findings point towards a possible role of atoh8 in sclerotome, cerebellum and hindbrain development. More importantly, the results of this expression analysis provide new insights into early sclerotome development in zebrafish—a field of research in developmental biology which has not received much attention so far.

Pro-resolving mediator protectin D1 promotes epimorphic regeneration by controlling immune cell function in vertebrates

BACKGROUND AND PURPOSE

Specialized pro-resolving mediators (SPMs) are a family of lipids controlling the resolution of inflammation and playing a role in many processes including organ protection and tissue repair. While SPMs are potent bioactive molecules in vivo, their role in epimorphic regeneration of organs in vertebrates has not been tested. Using the zebrafish larva as a robust regenerative vertebrate system, we studied the role of the SPM neuroprotectin/protectin D1 (PD1) during the caudal fin fold regeneration.

EXPERIMENTAL APPROACH

Regeneration of the fin fold was analysed when exposed to a synthetic PD1. The effect of PD1 on immune cell recruitment and activation was further investigated using live imaging combined with fluorescent reporter lines. Using genetic and pharmacological approaches, we dissected the role of neutrophils and macrophages on driving the pro-regenerative effect of PD1.

KEY RESULTS

We showed that PD1 improves fin fold regeneration. Acting in a narrow time window during regeneration, PD1 accelerates the resolution of inflammation without affecting the initial kinetic of neutrophil recruitment but instead, promotes their reverse migration potential. In addition, PD1 induces macrophage polarization switch towards non-inflammatory states in both zebrafish and mammalian system. Finally, macrophages but not neutrophils are essential for PD1-mediated regeneration.

CONCLUSION AND IMPLICATIONS

These results reveal the pro-regenerative action of PD1 and its role in regulating neutrophil and macrophage response in vertebrates. These findings strongly support the development of pro-resolving mediators as natural therapeutic candidates for degenerative disorders and the use of the zebrafish as a tool to investigate pro-regenerative drugs.

Relationship between surrounding tissue morphology and directional collective migration of the posterior lateral line primordium in zebrafish

Collective cell migration, in which cells assemble and move together, is an essential process in embryonic development, wound healing and cancer metastasis. Chemokine signaling guides cell assemblies to their destinations. In zebrafish posterior lateral line primordium (PLLP), a model system for collective cell migration, it has been proposed that the chemokine ligand Cxcl12a secreted from muscle pioneer cells (MPs) and muscle fast fibers (MFFs), which are distributed along with the horizontal midline, binds to the receptor Cxcr4b in PLLP and that Cxcl12a-Cxcr4b signaling guides the anterior-to-posterior migration of PLLP along the horizontal midline. However, how the surrounding tissues affect PLLP migration remains to be elucidated. Here, we investigated the relationship between the PLLP and the surrounding tissues and found that a furrow between the dorsal and ventral myotomes is generated by Sonic hedgehog (Shh) signaling-dependent MP and MFF differentiation and that the PLLP migrates in this furrow. When transient inhibition of Shh signaling impaired both the furrow formation and differentiation of cxcl12a-expressing MPs/MFFs, directional PLLP migration was severely perturbed. Furthermore, when differentiated MPs and MFFs were ablated by femtosecond laser irradiations, the furrow remained and PLLP migration was relatively unaffected. These results suggest that the furrow formation between the dorsal and ventral myotomes is associated with the migratory behavior of PLLP.

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.

Shaping the zebrafish myotome by intertissue friction and active stress

How do tissues self-organize to generate the complex organ shapes observed in vertebrates? Organ formation requires the integration of chemical and mechanical information, yet how this is achieved is poorly understood for most organs. Muscle compartments in zebrafish display a V shape, which is believed to be required for efficient swimming. We investigate how this structure emerges during zebrafish development, combining live imaging and quantitative analysis of cellular movements. We use theoretical modeling to understand how cell differentiation and mechanical interactions between tissues guide the emergence of a specific tissue morphology. Our work reveals how spatially modulating the mechanical environment around and within tissues can lead to complex organ shape formation.

Organ formation is an inherently biophysical process, requiring large-scale tissue deformations. Yet, understanding how complex organ shape emerges during development remains a major challenge. During zebrafish embryogenesis, large muscle segments, called myotomes, acquire a characteristic chevron morphology, which is believed to aid swimming. Myotome shape can be altered by perturbing muscle cell differentiation or the interaction between myotomes and surrounding tissues during morphogenesis. To disentangle the mechanisms contributing to shape formation of the myotome, we combine single-cell resolution live imaging with quantitative image analysis and theoretical modeling. We find that, soon after segmentation from the presomitic mesoderm, the future myotome spreads across the underlying tissues. The mechanical coupling between the future myotome and the surrounding tissues appears to spatially vary, effectively resulting in spatially heterogeneous friction. Using a vertex model combined with experimental validation, we show that the interplay of tissue spreading and friction is sufficient to drive the initial phase of chevron shape formation. However, local anisotropic stresses, generated during muscle cell differentiation, are necessary to reach the acute angle of the chevron in wild-type embryos. Finally, tissue plasticity is required for formation and maintenance of the chevron shape, which is mediated by orientated cellular rearrangements. Our work sheds light on how a spatiotemporal sequence of local cellular events can have a nonlocal and irreversible mechanical impact at the tissue scale, leading to robust organ shaping.

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.

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.

TNF signaling and macrophages govern fin regeneration in zebrafish larvae

Macrophages are essential for appendage regeneration after amputation in regenerative species. The molecular mechanisms through which macrophages orchestrate blastema formation and regeneration are still unclear. Here, we use the genetically tractable and transparent zebrafish larvae to study the functions of polarized macrophage subsets during caudal fin regeneration. After caudal fin amputation, we show an early and transient accumulation of pro-inflammatory macrophages concomitant with the accumulation of non-inflammatory macrophages which, in contrast to pro-inflammatory macrophages, remain associated to the fin until the end of the regeneration. Chemical and genetic depletion of macrophages suggested that early recruited macrophages that express TNFα are critical for blastema formation. Combining parabiosis and morpholino knockdown strategies, we show that TNFα/TNFR1 signaling pathway is required for the fin regeneration. Our study reveals that TNFR1 has a necessary and direct role in blastema cell activation suggesting that macrophage subset balance provides the accurate TNFα signal to prime regeneration in zebrafish.

Slow Muscle Precursors Lay Down a Collagen XV Matrix Fingerprint to Guide Motor Axon Navigation

The extracellular matrix (ECM) provides local positional information to guide motoneuron axons toward their muscle target. Collagen XV is a basement membrane component mainly expressed in skeletal muscle. We have identified two zebrafish paralogs of the human COL15A1 gene, col15a1a and col15a1b, which display distinct expression patterns. Here we show that col15a1b is expressed and deposited in the motor path ECM by slow muscle precursors also called adaxial cells. We further demonstrate that collagen XV-B deposition is both temporally and spatially regulated before motor axon extension from the spinal cord in such a way that it remains in this region after the adaxial cells have migrated toward the periphery of the myotome. Loss- and gain-of-function experiments in zebrafish embryos demonstrate that col15a1b expression and subsequent collagen XV-B deposition and organization in the motor path ECM depend on a previously undescribed two-step mechanism involving Hedgehog/Gli and unplugged/MuSK signaling pathways. In silico analysis predicts a putative Gli binding site in the col15a1b proximal promoter. Using col15a1b promoter-reporter constructs, we demonstrate that col15a1b participates in the slow muscle genetic program as a direct target of Hedgehog/Gli signaling. Loss and gain of col15a1b function provoke pathfinding errors in primary and secondary motoneuron axons both at and beyond the choice point where axon pathway selection takes place. These defects result in muscle atrophy and compromised swimming behavior, a phenotype partially rescued by injection of a smyhc1:col15a1b construct. These reveal an unexpected and novel role for collagen XV in motor axon pathfinding and neuromuscular development.

SIGNIFICANCE STATEMENT

In addition to the archetypal axon guidance cues, the extracellular matrix provides local information that guides motor axons from the spinal cord to their muscle targets. Many of the proteins involved are unknown. Using the zebrafish model, we identified an unexpected role of the extracellular matrix collagen XV in motor axon pathfinding. We show that the synthesis of collagen XV-B by slow muscle precursors and its deposition in the common motor path are dependent on a novel two-step mechanism that determines axon decisions at a choice point during motor axonogenesis. Zebrafish and humans use common molecular cues and regulatory mechanisms for the neuromuscular system development. And as such, our study reveals COL15A1 as a candidate gene for orphan neuromuscular disorders.

Quadruple zebrafish mutant reveals different roles of Mesp genes in somite segmentation between mouse and zebrafish

The segmental pattern of somites is generated by sequential conversion of the temporal periodicity provided by the molecular clock. Whereas the basic structure of this clock is conserved among different species, diversity also exists, especially in terms of the molecular network. The temporal periodicity is subsequently converted into the spatial pattern of somites, and Mesp2 plays crucial roles in this conversion in the mouse. However, it remains unclear whether Mesp genes play similar roles in other vertebrates. In this study, we generated zebrafish mutants lacking all four zebrafish Mesp genes by using TALEN-mediated genome editing. Contrary to the situation in the mouse Mesp2 mutant, in the zebrafish Mesp quadruple mutant embryos the positions of somite boundaries were clearly determined and morphological boundaries were formed, although their formation was not completely normal. However, each somite was caudalized in a similar manner to the mouse Mesp2 mutant, and the superficial horizontal myoseptum and lateral line primordia were not properly formed in the quadruple mutants. These results clarify the conserved and species-specific roles of Mesp in the link between the molecular clock and somite morphogenesis.

Fgf regulates dedifferentiation during skeletal muscle regeneration in adult zebrafish

Fibroblast growth factors (Fgfs) regulate critical biological processes such as embryonic development, tissue homeostasis, wound healing, and tissue regeneration. In zebrafish, Fgf signaling plays an important role in the regeneration of the spinal cord, liver, heart, fin, and photoreceptors, although its exact mechanism of action is not fully understood. Utilizing an adult zebrafish extraocular muscle (EOM) regeneration model, we demonstrate that blocking Fgf receptor function using either a chemical inhibitor (SU5402) or a dominant-negative transgenic construct (dnFGFR1a:EGFP) impairs muscle regeneration. Adult zebrafish EOMs regenerate through a myocyte dedifferentiation process, which involves a muscle-to-mesenchyme transition and cell cycle reentry by differentiated myocytes. Blocking Fgf signaling reduced cell proliferation and active caspase 3 levels in the regenerating muscle with no detectable levels of apoptosis, supporting the hypothesis that Fgf signaling is involved in the early steps of dedifferentiation. Fgf signaling in regenerating myocytes involves the MAPK/ERK pathway: inhibition of MEK activity with U0126 mimicked the phenotype of the Fgf receptor inhibition on both muscle regeneration and cell proliferation, and activated ERK (p-ERK) was detected in injured muscles by immunofluorescence and western blot. Interestingly, following injury, ERK2 expression is specifically induced and activated by phosphorylation, suggesting a key role in muscle regeneration. We conclude that the critical early steps of myocyte dedifferentiation in EOM regeneration are dependent on Fgf signaling.

Tbx16 and Msgn1 are required to establish directional cell migration of zebrafish mesodermal progenitors

The epithelial to mesenchymal transition (EMT) is an essential process that occurs repeatedly during embryogenesis whereby stably adherent cells convert to an actively migrating state. While much is known about the factors and events that initiate the EMT, the steps that cells undergo to become directionally migratory are far less well understood. Zebrafish embryos lacking the transcription factors Tbx16/Spadetail and Mesogenin1 (Msgn1) are a valuable system for investigating the EMT. Mesodermal cells in these embryos are unable to perform the EMT necessary to leave the most posterior end of the body (the tailbud) and join the pre-somitic mesoderm, a process that is conserved in all vertebrates. It has previously been very difficult to study this EMT in vertebrates because of the multiple cell types in the tailbud and the morphogenetic changes the whole embryo undergoes. Here, we describe a novel tissue explant system for imaging the mesodermal cell EMT in vivo that allows us to investigate the requirements for cells to acquire migratory properties during the EMT with high spatio-temporal resolution. This method revealed that, despite the inability of tbx16;msgn1-deficient cells to leave the tailbud, actin-based protrusions form surprisingly normally in these cells and they become highly motile. However, tbx16;msgn1-deficient cells have specific cell-autonomous defects in the persistence and anterior direction of migration because the lamellipodia they form are not productive in driving anteriorward migration. Additionally, we show that mesoderm morphogenesis and differentiation are separable and that there is a migratory cue that directs mesodermal cell migration that is independent of Tbx16 and Msgn1. This work defines changes that cells undergo as they complete the EMT and provides new insight into the mechanisms required in vivo for cells to become mesenchymal.

Identification of polarized macrophage subsets in zebrafish

While the mammalian macrophage phenotypes have been intensively studied in vitro, the dynamic of their phenotypic polarization has never been investigated in live vertebrates. We used the zebrafish as a live model to identify and trail macrophage subtypes. We generated a transgenic line whose macrophages expressing tumour necrosis factor alpha (tnfa), a key feature of classically activated (M1) macrophages, express fluorescent proteins Tg(mpeg1:mCherryF/tnfa:eGFP-F). Using 4D-confocal microscopy, we showed that both aseptic wounding and Escherichia coli inoculation triggered macrophage recruitment, some of which started to express tnfa. RT-qPCR on Fluorescence Activated Cell Sorting (FACS)-sorted tnfa⁺ and tnfa⁻ macrophages showed that they, respectively, expressed M1 and alternatively activated (M2) mammalian markers. Fate tracing of tnfa⁺ macrophages during the time-course of inflammation demonstrated that pro-inflammatory macrophages converted into M2-like phenotype during the resolution step. Our results reveal the diversity and plasticity of zebrafish macrophage subsets and underline the similarities with mammalian macrophages proposing a new system to study macrophage functional dynamic.

DOI:http://dx.doi.org/10.7554/eLife.07288.001

Inflammation plays an important role in helping the body to heal wounds and fight off certain diseases. Immune cells called macrophages—which are perhaps best known for their ability to engulf and digest microbes and cell debris—help to control inflammation. In mammals, different types of macrophage exist; the most functionally extreme of which are the M1 macrophages that stimulate inflammation and M2 macrophages that reduce the inflammatory response.

Macrophages acquire different abilities through a process called polarization, which is controlled by signals produced by a macrophage's environment. Polarization has been well investigated in human and mouse cells grown in the laboratory, but less is understood about how this process occurs in live animals.

Nguyen Chi, Laplace-Builhe et al. investigated whether zebrafish larvae (which are naturally transparent) could form an experimental model in which to investigate macrophage polarization in living animals. Zebrafish were first genetically engineered to produce two fluorescent proteins: one that marks macrophages and one that marks M1 macrophages. These fluorescent proteins allow the movement and polarization of macrophages to be tracked in real time in living larvae using a technique called confocal microscopy. Nguyen Chi, Laplace-Builhe et al. also isolated macrophage cells from these zebrafish at different times during the inflammatory process to identify which macrophage subtypes form and when.

The results show that unpolarized macrophages move to the sites of inflammation (caused by wounds or bacterial infection), where they become polarized into M1 cells. Over time, these M1 macrophages progressively convert into an M2-like macrophage subtype, presumably to help clear up the inflammation.

Furthermore, Nguyen Chi, Laplace-Builhe et al. show that the M1 and M2 macrophage subtypes in zebrafish are similar to those found in mammals. Therefore, genetically engineered zebrafish larvae are likely to prove useful for studying macrophage activity and polarization in living animals.

DOI:http://dx.doi.org/10.7554/eLife.07288.002

Hanging on for the ride: adhesion to the extracellular matrix mediates cellular responses in skeletal muscle morphogenesis and disease

Skeletal muscle specification and morphogenesis during early development are critical for normal physiology. In addition to mediating locomotion, skeletal muscle is a secretory organ that contributes to metabolic homeostasis. Muscle is a highly adaptable tissue, as evidenced by the ability to increase muscle cell size and/or number in response to weight bearing exercise. Conversely, muscle wasting can occur during aging (sarcopenia), cancer (cancer cachexia), extended hospital stays (disuse atrophy), and in many genetic diseases collectively known as the muscular dystrophies and myopathies. It is therefore of great interest to understand the cellular and molecular mechanisms that mediate skeletal muscle development and adaptation. Muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction. This process requires carefully orchestrated interactions between cells and their extracellular matrix microenvironment. These interactions are dynamic, allowing muscle cells to sense biophysical, structural, organizational, and/or signaling changes within their microenvironment and respond appropriately. In many musculoskeletal diseases, these cell adhesion interactions are disrupted to such a degree that normal cellular adaptive responses are not sufficient to compensate for accumulating damage. Thus, one major focus of current research is to identify the cell adhesion mechanisms that drive muscle morphogenesis, with the hope that understanding how muscle cell adhesion promotes the intrinsic adaptability of muscle tissue during development may provide insight into potential therapeutic approaches for muscle diseases. Our objectives in this review are to highlight recent studies suggesting conserved roles for cell-extracellular matrix adhesion in vertebrate muscle morphogenesis and cellular adaptive responses in animal models of muscle diseases.

Skeletal myogenesis in the zebrafish and its implications for muscle disease modelling

Current evidence indicates that post-embryonic muscle growth and regeneration in amniotes is mediated almost entirely by stem cells derived from muscle progenitor cells (MPCs), known as satellite cells. Exhaustion and impairment of satellite cell activity is involved in the severe muscle loss associated with degenerative muscle diseases such as Muscular Dystrophies and is the main cause of age-associated muscle wasting. Understanding the molecular and cellular basis of satellite cell function in muscle generation and regeneration (myogenesis) is critical to the broader goal of developing treatments that may ameliorate such conditions. Considerable knowledge exists regarding the embryonic stages of amniote myogenesis. Much less is known about how post-embryonic amniote myogenesis proceeds, how adult myogenesis relates to embryonic myogenesis on a cellular or genetic level. Of the studies focusing on post-embryonic amniote myogenesis, most are post-mortem and in vitro analyses, precluding the understanding of cellular behaviours and genetic mechanisms in an undisturbed in vivo setting. Zebrafish are optically clear throughout much of their post-embryonic development, facilitating their use in live imaging of cellular processes. Zebrafish also possess a compartment of MPCs, which appear similar to satellite cells and persist throughout the post-embryonic development of the fish, permitting their use in examining the contribution of these cells to muscle tissue growth and regeneration.

Sample drift correction following 4D confocal time-lapse imaging

The generation of four-dimensional (4D) confocal datasets; consisting of 3D image sequences over time; provides an excellent methodology to capture cellular behaviors involved in developmental processes.  The ability to track and follow cell movements is limited by sample movements that occur due to drift of the sample or, in some cases, growth during image acquisition. Tracking cells in datasets affected by drift and/or growth will incorporate these movements into any analysis of cell position. This may result in the apparent movement of static structures within the sample. Therefore prior to cell tracking, any sample drift should be corrected. Using the open source Fiji distribution (1)  of ImageJ (2,3) and the incorporated LOCI tools (4), we developed the Correct 3D drift plug-in to remove erroneous sample movement in confocal datasets. This protocol effectively compensates for sample translation or alterations in focal position by utilizing phase correlation to register each time-point of a four-dimensional confocal datasets while maintaining the ability to visualize and measure cell movements over extended time-lapse experiments.

In vivo zebrafish assays for analyzing drug toxicity

INTRODUCTION

Off-target effects represent one of the major concerns in the development of new pharmaceuticals, requiring large-scale animal toxicity testing. Faster, cheaper and more reliable assays based on zebrafish embryos (ZE) are being developed as major tools for assessing toxicity of chemicals during the drug-discovery process.

AREAS COVERED

This paper reviews techniques aimed to the analysis of in vivo sublethal toxic effects of drugs on major physiological functions, including the cardiovascular, nervous, neuromuscular, gastrointestinal and thyroid systems among others. Particular emphasis is placed on high-throughput screening techniques (HTS), including robotics, imaging technologies and image-analysis software.

EXPERT OPINION

The analysis of off-target effects of candidate drugs requires systemic analyses, as they often involve the complete organism rather than specific, tissue- or cell-specific targets. The unique physical and physiological characteristics of ZE make this system an essential tool for drug discovery and toxicity assessment. Different HTS methodologies applicable to ZE allow the screening of large numbers of different chemicals for many diverse and relevant toxic endpoints.

Control of muscle fibre-type diversity during embryonic development: the zebrafish paradigm

Vertebrate skeletal muscle is composed of distinct types of fibre that are functionally adapted through differences in their physiological and metabolic properties. An understanding of the molecular basis of fibre-type specification is of relevance to human health and fitness. The zebrafish provides an attractive model for investigating fibre type specification; not only are their rapidly developing embryos optically transparent, but in contrast to amniotes, the embryonic myotome shows a discrete temporal and spatial separation of fibre type ontogeny that simplifies its analysis. Here we review the current state of understanding of muscle fibre type specification and differentiation during embryonic development of the zebrafish, with a particular focus on the roles of the Prdm1a and Sox6 transcription factors, and consider the relevance of these findings to higher vertebrate muscle biology.

The ventral to dorsal BMP activity gradient in the early zebrafish embryo is determined by graded expression of BMP ligands

In the early zebrafish embryo, a ventral to dorsal gradient of bone morphogenetic protein (BMP) activity is established, which is essential for the specification of cell fates along this axis. To visualise and mechanistically determine how this BMP activity gradient forms, we have used a transgenic zebrafish line that expresses monomeric red fluorescent protein (mRFP) under the control of well-characterised BMP responsive elements. We demonstrate that mRFP expression in this line faithfully reports BMP and GDF signalling at both early and late stages of development. Taking advantage of the unstable nature of mRFP transcripts, we use in situ hybridisation to reveal the dynamic spatio-temporal pattern of BMP activity and establish the timing and sequence of events that lead to the formation of the BMP activity gradient. We show that the BMP transcriptional activity gradient is established between 30% and 40% epiboly stages and that it is preceded by graded mRNA expression of the BMP ligands. Both Dharma and FGF signalling contribute to graded bmp transcription during these early stages and it is subsequently maintained through autocrine BMP signalling. We show that BMP2B protein is also expressed in a gradient as early as blastula stages, but do not find any evidence of diffusion of this BMP to generate the BMP transcriptional activity gradient. Thus, in contrast to diffusion/transport-based models of BMP gradient formation in Drosophila, our results indicate that the establishment of the BMP activity gradient in early zebrafish embryos is determined by graded expression of the BMP ligands.