The zebrafish dystrophic mutant softy maintains muscle fibre viability despite basement membrane rupture and muscle detachment

Atrogin-1 promotes muscle homeostasis by regulating levels of endoplasmic reticulum chaperone BiP

Skeletal muscle wasting results from numerous pathological conditions affecting both the musculoskeletal and nervous systems. A unifying feature of these pathologies is the upregulation of members of the E3 ubiquitin ligase family, resulting in increased proteolytic degradation of target proteins. Despite the critical role of E3 ubiquitin ligases in regulating muscle mass, the specific proteins they target for degradation and the mechanisms by which they regulate skeletal muscle homeostasis remain ill-defined. Here, using zebrafish loss-of-function models combined with in vivo cell biology and proteomic approaches, we reveal a role of atrogin-1 in regulating the levels of the endoplasmic reticulum chaperone BiP. Loss of atrogin-1 resulted in an accumulation of BiP, leading to impaired mitochondrial dynamics and a subsequent loss in muscle fiber integrity. We further implicated a disruption in atrogin-1-mediated BiP regulation in the pathogenesis of Duchenne muscular dystrophy. We revealed that BiP was not only upregulated in Duchenne muscular dystrophy, but its inhibition using pharmacological strategies, or by upregulating atrogin-1, significantly ameliorated pathology in a zebrafish model of Duchenne muscular dystrophy. Collectively, our data implicate atrogin-1 and BiP in the pathogenesis of Duchenne muscular dystrophy and highlight atrogin-1's essential role in maintaining muscle homeostasis.

Standardization of zebrafish drug testing parameters for muscle diseases

Skeletal muscular diseases predominantly affect skeletal and cardiac muscle, resulting in muscle weakness, impaired respiratory function and decreased lifespan. These harmful outcomes lead to poor health-related quality of life and carry a high healthcare economic burden. The absence of promising treatments and new therapies for muscular disorders requires new methods for candidate drug identification and advancement in animal models. Consequently, the rapid screening of drug compounds in an animal model that mimics features of human muscle disease is warranted. Zebrafish are a versatile model in preclinical studies that support developmental biology and drug discovery programs for novel chemical entities and repurposing of established drugs. Due to several advantages, there is an increasing number of applications of the zebrafish model for high-throughput drug screening for human disorders and developmental studies. Consequently, standardization of key drug screening parameters, such as animal husbandry protocols, drug compound administration and outcome measures, is paramount for the continued advancement of the model and field. Here, we seek to summarize and explore critical drug treatment and drug screening parameters in the zebrafish-based modeling of human muscle diseases. Through improved standardization and harmonization of drug screening parameters and protocols, we aim to promote more effective drug discovery programs.

Optimizing assays of zebrafish larvae swimming performance for drug discovery

INTRODUCTION

Zebrafish larvae are one of the few vertebrates amenable to large-scale drug discovery screens. Larval swimming behavior is often used as an outcome variable and many fields of study have developed assays for evaluating swimming performance. An unintended consequence of this wide interest is that details related to assay methodology and interpretation become scattered across the literature. The aim of this review is to consolidate this information, particularly as it relates to high-throughput approaches.

AREAS COVERED

The authors describe larval swimming behaviors as this forms the basis for understanding their experimentally evoked swimming or spontaneous activity. Next, they detail how swimming activity can serve as an outcome variable, particularly in the multi-well formats used in large-scale screening studies. They also highlight biological and technical factors that can impact the sensitivity and variability of these measurements.

EXPERT OPINION

Careful attention to animal husbandry, experimental design, data acquisition, and interpretation of results can improve screen outcomes by maximizing swimming activity while minimizing intra- and inter-larval variability. The development of more sensitive, quantitative methods of assessing swimming performance that can be incorporated into high-throughput workflows will be important in order to take full advantage of the zebrafish model.

FKRP directed fibronectin glycosylation: A novel mechanism giving insights into muscular dystrophies?

The recently uncovered role of Fukutin-related protein (FKRP) in fibronectin glycosylation has challenged our understanding of the basis of disease pathogenesis in the muscular dystrophies. FKRP is a Golgi-resident glycosyltransferase implicated in a broad spectrum of muscular dystrophy (MD) pathologies that are not fully attributable to the well-described α-Dystroglycan hypoglycosylation. By revealing a new role for FKRP in the glycosylation of fibronectin, a modification critical for the development of the muscle basement membrane (MBM) and its associated muscle linkages, new possibilities for understanding clinical phenotype arise. This modification involves an interaction between FKRP and myosin-10, a protein involved in the Golgi organization and function. These observations suggest a FKRP nexus exists that controls two critical aspects to muscle fibre integrity, both fibre stability at the MBM and its elastic properties. This review explores the new potential disease axis in the context of our current knowledge of muscular dystrophies.

Cavin4 interacts with Bin1 to promote T-tubule formation and stability in developing skeletal muscle

The cavin proteins are essential for caveola biogenesis and function. Here, we identify a role for the muscle-specific component, Cavin4, in skeletal muscle T-tubule development by analyzing two vertebrate systems, mouse and zebrafish. In both models, Cavin4 localized to T-tubules, and loss of Cavin4 resulted in aberrant T-tubule maturation. In zebrafish, which possess duplicated cavin4 paralogs, Cavin4b was shown to directly interact with the T-tubule-associated BAR domain protein Bin1. Loss of both Cavin4a and Cavin4b caused aberrant accumulation of interconnected caveolae within the T-tubules, a fragmented T-tubule network enriched in Caveolin-3, and an impaired Ca2+ response upon mechanical stimulation. We propose a role for Cavin4 in remodeling the T-tubule membrane early in development by recycling caveolar components from the T-tubule to the sarcolemma. This generates a stable T-tubule domain lacking caveolae that is essential for T-tubule function.

Lamb1a regulates atrial growth by limiting second heart field addition during zebrafish heart development

During early vertebrate heart development, the heart transitions from a linear tube to a complex asymmetric structure, a morphogenetic process that occurs simultaneously with growth of the heart. Cardiac growth during early heart morphogenesis is driven by deployment of cells from the second heart field (SHF) into both poles of the heart. Laminin is a core component of the extracellular matrix and, although mutations in laminin subunits are linked with cardiac abnormalities, no role for laminin has been identified in early vertebrate heart morphogenesis. We identified tissue-specific expression of laminin genes in the developing zebrafish heart, supporting a role for laminins in heart morphogenesis. Analysis of heart development in lamb1a zebrafish mutant embryos reveals mild morphogenetic defects and progressive cardiomegaly, and that Lamb1a functions to limit heart size during cardiac development by restricting SHF addition. lamb1a mutants exhibit hallmarks of altered haemodynamics, and blocking cardiac contractility in lamb1a mutants rescues heart size and atrial SHF addition. Together, these results suggest that laminin mediates interactions between SHF deployment and cardiac biomechanics during heart morphogenesis and growth in the developing embryo.

An optimized QF-binary expression system for use in zebrafish

The zebrafish model organism has been of exceptional utility for the study of vertebrate development and disease through the application of tissue-specific labelling and overexpression of genes carrying patient-derived mutations. However, there remains a need for a binary expression system that is both non-toxic and not silenced over animal generations by DNA methylation. The Q binary expression system derived from the fungus Neurospora crassa is ideal, because the consensus binding site for the QF transcription factor lacks CpG dinucleotides, precluding silencing by CpG-meditated methylation. To optimize this system for zebrafish, we systematically tested several variants of the QF transcription factor: QF full length; QF2, which lacks the middle domain; QF2^w, which is an attenuated version of QF2; and chimeric QFGal4. We found that full length QF and QF2 were strongly toxic to zebrafish embryos, QF2^w was mildly toxic, and QFGal4 was well tolerated, when injected as RNA or expressed ubiquitously from stable transgenes. In addition, QFGal4 robustly activated a Tg(QUAS:GFP^NLS) reporter transgene. To increase the utility of this system, we also modified the QF effector sequence termed QUAS, which consists of five copies of the QF binding site. Specifically, we decreased both the CpG dinucleotide content, as well as the repetitiveness of QUAS, to reduce the risk of transgene silencing via CpG methylation. Moreover, these modifications to QUAS removed leaky QF-independent neural expression that we detected in the original QUAS sequence. To demonstrate the utility of our QF optimizations, we show how the Q-system can be used for lineage tracing using a Cre-dependent Tg(ubi:QFGal4-switch) transgene. We also demonstrate that QFGal4 can be used in transient injections to tag and label endogenous genes by knocking in QFGal4 into sox2 and ubiquitin C genes.

Zebrafish Models of LAMA2-Related Congenital Muscular Dystrophy (MDC1A)

LAMA2-related congenital muscular dystrophy (CMD; LAMA2-MD), also referred to as merosin deficient CMD (MDC1A), is a severe neonatal onset muscle disease caused by recessive mutations in the LAMA2 gene. LAMA2 encodes laminin α2, a subunit of the extracellular matrix (ECM) oligomer laminin 211. There are currently no treatments for MDC1A, and there is an incomplete understanding of disease pathogenesis. Zebrafish, due to their high degree of genetic conservation with humans, large clutch sizes, rapid development, and optical clarity, have emerged as an excellent model system for studying rare Mendelian diseases. They are particularly suitable as a model for muscular dystrophy because they contain at least one orthologue to all major human MD genes, have muscle that is similar to human muscle in structure and function, and manifest obvious and easily measured MD related phenotypes. In this review article, we present the existing zebrafish models of MDC1A, and discuss their contribution to the understanding of MDC1A pathomechanisms and therapy development.

Effect of Ataluren on dystrophin mutations

Duchenne muscular dystrophy is a severe muscle wasting disease caused by mutations in the dystrophin gene (dmd). Ataluren has been approved by the European Medicines Agency for treatment of Duchenne muscular dystrophy. Ataluren has been reported to promote ribosomal read‐through of premature stop codons, leading to restoration of full‐length dystrophin protein. However, the mechanism of Ataluren action has not been fully described. To evaluate the efficacy of Ataluren on all three premature stop codons featuring different termination strengths (UAA > UAG > UGA), novel dystrophin‐deficient zebrafish were generated. Pathological assessment of the muscle by birefringence quantification, a tool to directly measure muscle integrity, did not reveal a significant effect of Ataluren on any of the analysed dystrophin‐deficient mutants at 3 days after fertilization. Functional analysis of the musculature at 6 days after fertilization by direct measurement of the generated force revealed a significant improvement by Ataluren only for the UAA‐carrying mutant dmd^ta222a. Interestingly however, all other analysed dystrophin‐deficient mutants were not affected by Ataluren, including the dmd^pc3 and dmd^pc2 mutants that harbour weaker premature stop codons UAG and UGA, respectively. These in vivo results contradict reported in vitro data on Ataluren efficacy, suggesting that Ataluren might not promote read‐through of premature stop codons. In addition, Ataluren had no effect on dystrophin transcript levels, but mild adverse effects on wild‐type larvae were identified. Further assessment of N‐terminally truncated dystrophin opened the possibility of Ataluren promoting alternative translation codons within dystrophin, thereby potentially shifting the patient cohort applicable for Ataluren.

Polarization contrast optical diffraction tomography

We demonstrate large scale polarization contrast optical diffraction tomography (ODT). In cross-polarized sample arm detection configuration we determine, from the amplitude of the optical wavefield, a relative measure of the birefringence projection. In parallel-polarized sample arm detection configuration we image the conventional phase projection. For off-axis sample placement we observe for polarization contrast ODT, similar as for phase contrast ODT, a strongly reduced noise contribution. In the limit of small birefringence phase shift δ we demonstrate tomographic reconstruction of polarization contrast images into a full 3D image of an optically cleared zebrafish. The polarization contrast ODT reconstruction shows muscular zebrafish tissue, which cannot be visualized in conventional phase contrast ODT. Polarization contrast ODT images of the zebrafish show a much higher signal to noise ratio (SNR) than the corresponding phase contrast images, SNR=73 and SNR=15, respectively.

ErbB4 tyrosine kinase inhibition impairs neuromuscular development in zebrafish embryos

Tyrosine kinase inhibitors are widely used in the clinic, but limited information is available about their toxicity in developing organisms. Here, we tested the effect of tyrosine kinase inhibitors targeting the ErbB receptors for their effects on developing zebrafish (Danio rerio) embryos. Embryos treated with wide-spectrum pan-ErbB inhibitors or erbb4a-targeting antisense oligonucleotides demonstrated reduced locomotion, reduced diameter of skeletal muscle fibers, and reduced expression of muscle-specific genes, as well as reduced motoneuron length. The phenotypes in the skeletal muscle, as well as the defect in motility, were rescued both by microinjection of human ERBB4 mRNA and by transposon-mediated muscle-specific ERBB4 overexpression. The role of ErbB4 in regulating motility was further controlled by targeted mutation of the endogenous erbb4a locus in the zebrafish genome by CRISPR/Cas9. These observations demonstrate a potential for the ErbB tyrosine kinase inhibitors to induce neuromuscular toxicity in a developing organism via a mechanism involving inhibition of ErbB4 function.

Tmem2 regulates cell-matrix interactions that are essential for muscle fiber attachment

Skeletal muscle morphogenesis depends upon interactions between developing muscle fibers and the extracellular matrix (ECM) that anchors fibers to the myotendinous junction (MTJ). The pathways that organize the ECM and regulate its engagement by cell-matrix adhesion complexes (CMACs) are therefore essential for muscle integrity. Here, we demonstrate the impact of transmembrane protein 2 (tmem2) on cell-matrix interactions during muscle morphogenesis in zebrafish. Maternal-zygotic tmem2 mutants (MZtmem2) exhibit muscle fiber detachment, in association with impaired laminin organization and ineffective fibronectin degradation at the MTJ. Similarly, disorganized laminin and fibronectin surround MZtmem2 cardiomyocytes, which could account for their hindered movement during cardiac morphogenesis. In addition to ECM defects, MZtmem2 mutants display hypoglycosylation of α-dystroglycan within the CMAC, which could contribute to the observed fiber detachment. Expression of the Tmem2 ectodomain can rescue aspects of the MZtmem2 phenotype, consistent with a possible extracellular function of Tmem2. Together, our results suggest that Tmem2 regulates cell-matrix interactions by affecting both ECM organization and CMAC activity. These findings evoke possible connections between the functions of Tmem2 and the etiologies of congenital muscular dystrophies, particularly dystroglycanopathies.

Cavin4b/Murcb Is Required for Skeletal Muscle Development and Function in Zebrafish

Skeletal muscles provide metazoans with the ability to feed, reproduce and avoid predators. In humans, a heterogeneous group of genetic diseases, termed muscular dystrophies (MD), lead to skeletal muscle dysfunction. Mutations in the gene encoding Caveolin-3, a principal component of the membrane micro-domains known as caveolae, cause defects in muscle maintenance and function; however it remains unclear how caveolae dysfunction underlies MD pathology. The Cavin family of caveolar proteins can form membrane remodeling oligomers and thus may also impact skeletal muscle function. Changes in the distribution and function of Cavin4/Murc, which is predominantly expressed in striated muscles, have been reported to alter caveolae structure through interaction with Caveolin-3. Here, we report the generation and phenotypic analysis of murcb mutant zebrafish, which display impaired swimming capacity, skeletal muscle fibrosis and T-tubule abnormalities during development. To understand the mechanistic importance of Murc loss of function, we assessed Caveolin-1 and 3 localization and found it to be abnormal. We further identified an in vivo function for Murc in Erk signaling. These data link Murc with developmental defects in T-tubule formation and progressive muscle dysfunction, thereby providing a new candidate for the etiology of muscular dystrophy.

Zebrafish models for nemaline myopathy reveal a spectrum of nemaline bodies contributing to reduced muscle function

Nemaline myopathy is characterized by muscle weakness and the presence of rod-like (nemaline) bodies. The genetic etiology of nemaline myopathy is becoming increasingly understood with mutations in ten genes now known to cause the disease. Despite this, the mechanism by which skeletal muscle weakness occurs remains elusive, with previous studies showing no correlation between the frequency of nemaline bodies and disease severity. To investigate the formation of nemaline bodies and their role in pathogenesis, we generated overexpression and loss-of-function zebrafish models for skeletal muscle α-actin (ACTA1) and nebulin (NEB). We identify three distinct types of nemaline bodies and visualize their formation in vivo, demonstrating these nemaline bodies not only exhibit different subcellular origins, but also have distinct pathological consequences within the skeletal muscle. One subtype is highly dynamic and upon breakdown leads to the accumulation of cytoplasmic actin contributing to muscle weakness. Examination of a Neb-deficient model suggests this mechanism may be common in nemaline myopathy. Another subtype results from a reduction of actin and forms a more stable cytoplasmic body. In contrast, the final type originates at the Z-disk and is associated with myofibrillar disorganization. Analysis of zebrafish and muscle biopsies from ACTA1 nemaline myopathy patients demonstrates that nemaline bodies also possess a different protein signature. In addition, we show that the ACTA1^D286G mutation causes impaired actin incorporation and localization in the sarcomere. Together these data provide a novel examination of nemaline body origins and dynamics in vivo and identifies pathological changes that correlate with muscle weakness.

Spatio-Temporal Differences in Dystrophin Dynamics at mRNA and Protein Levels Revealed by a Novel FlipTrap Line

Dystrophin (Dmd) is a structural protein that links the extracellular matrix to actin filaments in muscle fibers and is required for the maintenance of muscles integrity. Mutations in Dmd lead to muscular dystrophies in humans and other vertebrates. Here, we report the characterization of a zebrafish gene trap line that fluorescently labels the endogenous Dmd protein (Dmd-citrine, Gt(dmd-citrine) ^ct90a). We show that the Dmd-citrine line recapitulates endogenous dmd transcript expression and Dmd protein localization. Using this Dmd-citrine line, we follow Dmd localization to the myosepta in real-time using time-lapse microscopy, and find that the accumulation of Dmd protein at the transverse myosepta coincides with the onset of myotome formation, a critical stage in muscle maturation. We observed that Dmd protein localizes specifically to the myosepta prior to dmd mRNA localization. Additionally, we demonstrate that the Dmd-citrine line can be used to assess muscular dystrophy following both genetic and physical disruptions of the muscle.

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.

Analysing regenerative potential in zebrafish models of congenital muscular dystrophy

The congenital muscular dystrophies (CMDs) are a clinically and genetically heterogeneous group of muscle disorders. Clinically hypotonia is present from birth, with progressive muscle weakness and wasting through development. For the most part, CMDs can mechanistically be attributed to failure of basement membrane protein laminin-α2 sufficiently binding with correctly glycosylated α-dystroglycan. The majority of CMDs therefore arise as the result of either a deficiency of laminin-α2 (MDC1A) or hypoglycosylation of α-dystroglycan (dystroglycanopathy). Here we consider whether by filling a regenerative medicine niche, the zebrafish model can address the present challenge of delivering novel therapeutic solutions for CMD. In the first instance the readiness and appropriateness of the zebrafish as a model organism for pioneering regenerative medicine therapies in CMD is analysed, in particular for MDC1A and the dystroglycanopathies. Despite the recent rapid progress made in gene editing technology, these approaches have yet to yield any novel zebrafish models of CMD. Currently the most genetically relevant zebrafish models to the field of CMD, have all been created by N-ethyl-N-nitrosourea (ENU) mutagenesis. Once genetically relevant models have been established the zebrafish has several important facets for investigating the mechanistic cause of CMD, including rapid ex vivo development, optical transparency up to the larval stages of development and relative ease in creating transgenic reporter lines. Together, these tools are well suited for use in live-imaging studies such as in vivo modelling of muscle fibre detachment. Secondly, the zebrafish's contribution to progress in effective treatment of CMD was analysed. Two approaches were identified in which zebrafish could potentially contribute to effective therapies. The first hinges on the augmentation of functional redundancy within the system, such as upregulating alternative laminin chains in the candyfloss fish, a model of MDC1A. Secondly high-throughput small molecule screens not only provide effective therapies, but also an alternative strategy for investigating CMD in zebrafish. In this instance insight into disease mechanism is derived in reverse. Zebrafish models are therefore clearly of critical importance in the advancement of regenerative medicine strategies in CMD. This article is part of a Directed Issue entitled: Regenerative Medicine: The challenge of translation.

Loss of Tropomodulin4 in the zebrafish mutant träge causes cytoplasmic rod formation and muscle weakness reminiscent of nemaline myopathy

Nemaline myopathy is an inherited muscle disease that is mainly diagnosed by the presence of nemaline rods in muscle biopsies. Of the nine genes associated with the disease, five encode components of striated muscle sarcomeres. In a genetic zebrafish screen, the mutant träge (trg) was isolated based on its reduction in muscle birefringence, indicating muscle damage. Myofibres in trg appeared disorganised and showed inhomogeneous cytoplasmic eosin staining alongside malformed nuclei. Linkage analysis of trg combined with sequencing identified a nonsense mutation in tropomodulin4 (tmod4), a regulator of thin filament length and stability. Accordingly, although actin monomers polymerize to form thin filaments in the skeletal muscle of tmod4^trg mutants, thin filaments often appeared to be dispersed throughout myofibres. Organised myofibrils with the typical striation rarely assemble, leading to severe muscle weakness, impaired locomotion and early death. Myofibrils of tmod4^trg mutants often featured thin filaments of various lengths, widened Z-disks, undefined H-zones and electron-dense aggregations of various shapes and sizes. Importantly, Gomori trichrome staining and the lattice pattern of the detected cytoplasmic rods, together with the reactivity of rods with phalloidin and an antibody against actinin, is reminiscent of nemaline rods found in nemaline myopathy, suggesting that misregulation of thin filament length causes cytoplasmic rod formation in tmod4^trg mutants. Although Tropomodulin4 has not been associated with myopathy, the results presented here implicateTMOD4 as a novel candidate for unresolved nemaline myopathies and suggest that the tmod4^trg mutant will be a valuable tool to study human muscle disorders.

Thrombospondin-4 controls matrix assembly during development and repair of myotendinous junctions

Tendons are extracellular matrix (ECM)-rich structures that mediate muscle attachments with the skeleton, but surprisingly little is known about molecular mechanisms of attachment. Individual myofibers and tenocytes in Drosophila interact through integrin (Itg) ligands such as Thrombospondin (Tsp), while vertebrate muscles attach to complex ECM fibrils embedded with tenocytes. We show for the first time that a vertebrate thrombospondin, Tsp4b, is essential for muscle attachment and ECM assembly at myotendinous junctions (MTJs). Tsp4b depletion in zebrafish causes muscle detachment upon contraction due to defects in laminin localization and reduced Itg signaling at MTJs. Mutation of its oligomerization domain renders Tsp4b unable to rescue these defects, demonstrating that pentamerization is required for ECM assembly. Furthermore, injected human TSP4 localizes to zebrafish MTJs and rescues muscle detachment and ECM assembly in Tsp4b-deficient embryos. Thus Tsp4 functions as an ECM scaffold at MTJs, with potential therapeutic uses in tendon strengthening and repair.

DOI:http://dx.doi.org/10.7554/eLife.02372.001

Tendons, the tough connective tissues that link muscles to bones, are essential for lifting, running and other movements in animals. A matrix of proteins, called the extracellular matrix, connects the cells in a tendon, giving it the strength it needs to prevent muscles from detaching from bones during strenuous activities.

To achieve this strength, extracellular matrix proteins bind to one another and to receptors on the muscle cell surface that are linked to its internal scaffolding, thereby organizing other proteins into a structure called a myotendinous junction. However, despite the essential roles of tendons, scientists do not fully understand how this organization occurs, or how it can go awry.

Subramanian and Schilling screened zebrafish for genes that are essential for proper muscle attachment, and zeroed in on a gene encoding a protein called Thrombospondin-4b (Tsp4b). A similar protein helps to connect muscle and tendon cells in fruit flies. Without Tsp4b, zebrafish are able to form connections between muscles and tendons, but the muscles detach easily during movement. This weakened connection is caused by disorganization of the proteins in the extracellular matrix, which results in reduced signaling from the muscle cell receptors.

When a human form of this protein was injected into zebrafish embryos lacking Tsp4b, it settled into the junctions between muscle and tendon cells. The human protein repaired the detached muscles and restored the proper organization of the matrix. This improved the strength of the muscle-tendon attachment in the treated fish embryos, suggesting that similar injections could also help to strengthen and repair muscles and tendons in people.

DOI:http://dx.doi.org/10.7554/eLife.02372.002

Gne depletion during zebrafish development impairs skeletal muscle structure and function

GNE Myopathy is a rare recessively inherited neuromuscular disorder caused by mutations in the GNE gene, which codes for the key enzyme in the metabolic pathway of sialic acid synthesis. The process by which GNE mutations lead to myopathy is not well understood. By in situ hybridization and gne promoter-driven fluorescent transgenic fish generation, we have characterized the spatiotemporal expression pattern of the zebrafish gne gene and have shown that it is highly conserved compared with the human ortholog. We also show the deposition of maternal gne mRNA and maternal GNE protein at the earliest embryonic stage, emphasizing the critical role of gne in embryonic development. Injection of morpholino (MO)-modified antisense oligonucleotides specifically designed to knockdown gne, into one-cell embryos lead to a variety of phenotypic severity. Characterization of the gne knockdown morphants showed a significantly reduced locomotor activity as well as distorted muscle integrity, including a reduction in the number of muscle myofibers, even in mild or intermediate phenotype morphants. These findings were further confirmed by electron microscopy studies, where large gaps between sarcolemmas were visualized, although normal sarcomeric structures were maintained. These results demonstrate a critical novel role for gne in embryonic development and particularly in myofiber development, muscle integrity and activity.

Membrane-myofibril cross-talk in myofibrillogenesis and in muscular dystrophy pathogenesis: lessons from the zebrafish

Striated muscle has a highly ordered structure in which specialized domains of the cell membrane involved in force transmission (costameres) and excitation-contraction coupling (T tubules) as well as the internal membranes of the sarcoplasmic reticulum are organized over specific regions of the sarcomere. Optimal muscle function is dependent on this high level of organization but how it established and maintained is not well understood. Due to its ex utero development and transparency, the zebrafish embryo is an excellent vertebrate model for the study of dynamic relationships both within and between cells during development. Transgenic models have allowed the delineation of cellular migration and complex morphogenic rearrangements during the differentiation of skeletal myocytes and the assembly and organization of new myofibrils. Molecular targeting of genes and transcripts has allowed the identification of key requirements for myofibril assembly and organization. With the recent advances in gene editing approaches, the zebrafish will become an increasingly important model for the study of human myopathies and muscular dystrophies. Its high fecundity and small size make it well suited to high-throughput screenings to identify novel pharmacologic and molecular therapies for the treatment of a broad range of neuromuscular conditions. In this review, we examine the lessons learned from the zebrafish model regarding the complex interactions between the sarcomere and the sarcolemma that pattern the developing myocyte and discuss the potential for zebrafish as a model system to examine the pathophysiology of, and identify new treatments for, human myopathies and muscular dystrophies.

zebraflash transgenic lines for in vivo bioluminescence imaging of stem cells and regeneration in adult zebrafish

The zebrafish has become a standard model system for stem cell and tissue regeneration research, based on powerful genetics, high tissue regenerative capacity and low maintenance costs. Yet, these studies can be challenged by current limitations of tissue visualization techniques in adult animals. Here we describe new imaging methodology and present several ubiquitous and tissue-specific luciferase-based transgenic lines, which we have termed zebraflash, that facilitate the assessment of regeneration and engraftment in freely moving adult zebrafish. We show that luciferase-based live imaging reliably estimates muscle quantity in an internal organ, the heart, and can longitudinally follow cardiac regeneration in individual animals after major injury. Furthermore, luciferase-based detection enables visualization and quantification of engraftment in live recipients of transplanted hematopoietic stem cell progeny, with advantages in sensitivity and gross spatial resolution over fluorescence detection. Our findings present a versatile resource for monitoring and dissecting vertebrate stem cell and regeneration biology.

Quantitative assessment of muscle damage in the mdx mouse model of Duchenne muscular dystrophy using polarization-sensitive optical coherence tomography

Minimally invasive, high-resolution imaging of muscle necrosis has the potential to aid in the assessment of diseases such as Duchenne muscular dystrophy. Undamaged muscle tissue possesses high levels of optical birefringence due to its anisotropic ultrastructure, and this birefringence decreases when the tissue undergoes necrosis. In this study, we present a novel technique to image muscle necrosis using polarization-sensitive optical coherence tomography (PS-OCT). From PS-OCT scans, our technique is able to quantify the birefringence in muscle tissue, generating an image indicative of the tissue ultrastructure, with areas of abnormally low birefringence indicating necrosis. The technique is demonstrated on excised skeletal muscles from exercised dystrophic mdx mice and control C57BL/10ScSn mice with the resulting images validated against colocated histological sections. The technique additionally gives a measure of the proportion (volume fraction) of necrotic tissue within the three-dimensional imaging field of view. The percentage necrosis assessed by this technique is compared against the percentage necrosis obtained from manual assessment of histological sections, and the difference between the two methods is found to be comparable to the interobserver variability of the histological assessment. This is the first published demonstration of PS-OCT to provide automated assessment of muscle necrosis.

Knockdown of col22a1 gene in zebrafish induces a muscular dystrophy by disruption of the myotendinous junction

The myotendinous junction (MTJ) is the major site of force transfer in skeletal muscle, and defects in its structure correlate with a subset of muscular dystrophies. Col22a1 encodes the MTJ component collagen XXII, the function of which remains unknown. Here, we have cloned and characterized the zebrafish col22a1 gene and conducted morpholino-based loss-of-function studies in developing embryos. We showed that col22a1 transcripts localize at muscle ends when the MTJ forms and that COLXXII protein integrates the junctional extracellular matrix. Knockdown of COLXXII expression resulted in muscular dystrophy-like phenotype, including swimming impairment, curvature of embryo trunk/tail, strong reduction of twitch-contraction amplitude and contraction-induced muscle fiber detachment, and provoked significant activation of the survival factor Akt. Electron microscopy and immunofluorescence studies revealed that absence of COLXXII caused a strong reduction of MTJ folds and defects in myoseptal structure. These defects resulted in reduced contractile force and susceptibility of junctional extracellular matrix to rupture when subjected to repeated mechanical stress. Co-injection of sub-phenotypic doses of morpholinos against col22a1 and genes of the major muscle linkage systems showed a synergistic gene interaction between col22a1 and itga7 (α7β1 integrin) that was not observed with dag1 (dystroglycan). Finally, pertinent to a conserved role in humans, the dystrophic phenotype was rescued by microinjection of recombinant human COLXXII. Our findings indicate that COLXXII contributes to the stabilization of myotendinous junctions and strengthens skeletal muscle attachments during contractile activity.

Zebrafish models flex their muscles to shed light on muscular dystrophies

Muscular dystrophies are a group of genetic disorders that specifically affect skeletal muscle and are characterized by progressive muscle degeneration and weakening. To develop therapies and treatments for these diseases, a better understanding of the molecular basis of muscular dystrophies is required. Thus, identification of causative genes mutated in specific disorders and the study of relevant animal models are imperative. Zebrafish genetic models of human muscle disorders often closely resemble disease pathogenesis, and the optical clarity of zebrafish embryos and larvae enables visualization of dynamic molecular processes in vivo. As an adjunct tool, morpholino studies provide insight into the molecular function of genes and allow rapid assessment of candidate genes for human muscular dystrophies. This unique set of attributes makes the zebrafish model system particularly valuable for the study of muscle diseases. This review discusses how recent research using zebrafish has shed light on the pathological basis of muscular dystrophies, with particular focus on the muscle cell membrane and the linkage between the myofibre cytoskeleton and the extracellular matrix.

NAD+ biosynthesis ameliorates a zebrafish model of muscular dystrophy

NAD+ improves muscle tissue structure and function in dystrophic zebrafish by increasing basement membrane organization.

Muscular dystrophies are common, currently incurable diseases. A subset of dystrophies result from genetic disruptions in complexes that attach muscle fibers to their surrounding extracellular matrix microenvironment. Cell-matrix adhesions are exquisite sensors of physiological conditions and mediate responses that allow cells to adapt to changing conditions. Thus, one approach towards finding targets for future therapeutic applications is to identify cell adhesion pathways that mediate these dynamic, adaptive responses in vivo. We find that nicotinamide riboside kinase 2b-mediated NAD+ biosynthesis, which functions as a small molecule agonist of muscle fiber-extracellular matrix adhesion, corrects dystrophic phenotypes in zebrafish lacking either a primary component of the dystrophin-glycoprotein complex or integrin alpha7. Exogenous NAD+ or a vitamin precursor to NAD+ reduces muscle fiber degeneration and results in significantly faster escape responses in dystrophic embryos. Overexpression of paxillin, a cell adhesion protein downstream of NAD+ in this novel cell adhesion pathway, reduces muscle degeneration in zebrafish with intact integrin receptors but does not improve motility. Activation of this pathway significantly increases organization of laminin, a major component of the extracellular matrix basement membrane. Our results indicate that the primary protective effects of NAD+ result from changes to the basement membrane, as a wild-type basement membrane is sufficient to increase resilience of dystrophic muscle fibers to damage. The surprising result that NAD+ supplementation ameliorates dystrophy in dystrophin-glycoprotein complex– or integrin alpha7–deficient zebrafish suggests the existence of an additional laminin receptor complex that anchors muscle fibers to the basement membrane. We find that integrin alpha6 participates in this pathway, but either integrin alpha7 or the dystrophin-glycoprotein complex is required in conjunction with integrin alpha6 to reduce muscle degeneration. Taken together, these results define a novel cell adhesion pathway that may have future therapeutic relevance for a broad spectrum of muscular dystrophies.

A variety of diseases, both inherited and acquired, affect muscle tissues in humans. Critical to muscle homeostasis is the anchoring of muscle fibers to their surrounding microenvironment through cell adhesion complexes that help to resist the repeated stress experienced during muscle contraction. Genetic mutations in these complexes weaken this mechanical attachment, making fibers more susceptible to damage and death. The resulting increased fiber degeneration can eventually lead to progressive muscle-wasting diseases, known collectively as muscular dystrophies. Although clinical trials are ongoing, there is presently no way to cure the loss of muscle structure and function associated with these diseases. We identified a novel cell adhesion pathway involving integrin alpha6 that promotes adhesion of muscle cells to their microenvironment. Here, we show that activation of this pathway not only significantly reduces muscle degeneration but also improves the swimming ability of dystrophic zebrafish. We explore the likely benefits and limitations of this pathway in treating symptoms of congenital muscular dystrophies. Our findings suggest that activation of this pathway (for example, by boosting levels of NAD+) has the potential to ameliorate loss of muscle structure and function in multiple muscular dystrophies.

Epistatic dissection of laminin-receptor interactions in dystrophic zebrafish muscle

Laminins form essential components of the basement membrane and are integral to forming and maintaining muscle integrity. Mutations in the human Laminin-alpha2 (LAMA2) gene result in the most common form of congenital muscular dystrophy, MDC1A. We have previously identified a zebrafish model of MDC1A called candyfloss (caf), carrying a loss-of-function mutation in the zebrafish lama2 gene. In the skeletal muscle, laminins connect the muscle cell to the extracellular matrix (ECM) by binding either dystroglycan or integrins at the cell membrane. Through epistasis experiments, we have established that both adhesion systems individually contribute to the maintenance of fibre adhesions and exhibit muscle detachment phenotypes. However, larval zebrafish in which both adhesion systems are simultaneously genetically inactivated possess a catastrophic failure of muscle attachment that is far greater than a simple addition of individual phenotypes would predict. We provide evidence that this is due to other crucial laminins present in addition to Lama2, which aid muscle cell attachments and integrity. We have found that lama1 is important for maintaining attachments, whereas lama4 is localized and up-regulated in damaged fibres, which appears to contribute to fibre survival. Importantly, our results show that endogenous secretion of laminins from the surrounding tissues has the potential to reinforce fibre attachments and strengthen laminin-ECM attachments. Collectively these findings provide a better understanding of the cellular pathology of MDC1A and help in designing effective therapies.

Muscle diseases in the zebrafish

Animal models in biomedical research are important for understanding the pathological mechanisms of human diseases at a molecular and cellular level. Several aspects of mammalian animals, however, may limit their use in modelling neuromuscular disorders. Many attributes of zebrafish (Danio rerio) are complementary to mammalian experimental systems, establishing the zebrafish as a powerful model organism in disease biology. This review focuses on a number of key studies using the zebrafish to model hereditary muscle diseases with additional emphasis on recent advances in zebrafish functional genomics and drug discovery. Increasing research in zebrafish disease models, combined with knowledge from mammalian models, will bring novel insights into the disease pathogenesis of neuromuscular disorders, as well as facilitate the development of effective therapeutic strategies.

Macondo crude oil from the Deepwater Horizon oil spill disrupts specific developmental processes during zebrafish embryogenesis

Background

The Deepwater Horizon disaster was the largest marine oil spill in history, and total vertical exposure of oil to the water column suggests it could impact an enormous diversity of ecosystems. The most vulnerable organisms are those encountering these pollutants during their early life stages. Water-soluble components of crude oil and specific polycyclic aromatic hydrocarbons have been shown to cause defects in cardiovascular and craniofacial development in a variety of teleost species, but the developmental origins of these defects have yet to be determined. We have adopted zebrafish, Danio rerio, as a model to test whether water accumulated fractions (WAF) of the Deepwater Horizon oil could impact specific embryonic developmental processes. While not a native species to the Gulf waters, the developmental biology of zebrafish has been well characterized and makes it a powerful model system to reveal the cellular and molecular mechanisms behind Macondo crude toxicity.

Results

WAF of Macondo crude oil sampled during the oil spill was used to treat zebrafish throughout embryonic and larval development. Our results indicate that the Macondo crude oil causes a variety of significant defects in zebrafish embryogenesis, but these defects have specific developmental origins. WAF treatments caused defects in craniofacial development and circulatory function similar to previous reports, but we extend these results to show they are likely derived from an earlier defect in neural crest cell development. Moreover, we demonstrate that exposure to WAFs causes a variety of novel deformations in specific developmental processes, including programmed cell death, locomotor behavior, sensory and motor axon pathfinding, somitogenesis and muscle patterning. Interestingly, the severity of cell death and muscle phenotypes decreased over several months of repeated analysis, which was correlated with a rapid drop-off in the aromatic and alkane hydrocarbon components of the oil.

Conclusions

Whether these teratogenic effects are unique to the oil from the Deepwater Horizon oil spill or generalizable for most crude oil types remains to be determined. This work establishes a model for further investigation into the molecular mechanisms behind crude oil mediated deformations. In addition, due to the high conservation of genetic and cellular processes between zebrafish and other vertebrates, our work also provides a platform for more focused assessment of the impact that the Deepwater Horizon oil spill has had on the early life stages of native fish species in the Gulf of Mexico and the Atlantic Ocean.

Development of the zebrafish myoseptum with emphasis on the myotendinous junction

Zebrafish myosepta connect two adjacent muscle cells and transmit muscular forces to axial structures during swimming via the myotendinous junction (MTJ). The MTJ establishes transmembrane linkages system consisting of extracellular matrix molecules (ECM) surrounding the basement membrane, cytoskeletal elements anchored to sarcolema, and all intermediate proteins that link ECM to actin filaments. Using a series of zebrafish specimens aged between 24 h post-fertilization and 2 years old, the present paper describes at the transmission electron microscope level the development of extracellular and intracellular elements of the MTJ. The transverse myoseptum development starts during the segmentation period by deposition of sparse and loosely organized collagen fibrils. During the hatching period, a link between actin filaments and sarcolemma is established. The basal lamina underlining sarcolemma is well differentiated. Later, collagen fibrils display an orthogonal orientation and fibroblast-like cells invade the myoseptal stroma. A dense network of collagen fibrils is progressively formed that both anchor myoseptal fibroblasts and sarcolemmal basement membrane. The differentiation of a functional MTJ is achieved when sarcolemma interacts with both cytoskeletal filaments and extracellular components. This solid structural link between contractile apparatus and ECM leads to sarcolemma deformations resulting in the formation of regular invaginations, and allows force transmission during muscle contraction. This paper presents the first ultrastructural atlas of the zebrafish MTJ development, which represents an useful tool to analyse the mechanisms of the myotendinous system formation and their disruption in muscle disorders.

Development and evolution of the muscles of the pelvic fin

Locomotor strategies in terrestrial tetrapods have evolved from the utilisation of sinusoidal contractions of axial musculature, evident in ancestral fish species, to the reliance on powerful and complex limb muscles to provide propulsive force. Within tetrapods, a hindlimb-dominant locomotor strategy predominates, and its evolution is considered critical for the evident success of the tetrapod transition onto land. Here, we determine the developmental mechanisms of pelvic fin muscle formation in living fish species at critical points within the vertebrate phylogeny and reveal a stepwise modification from a primitive to a more derived mode of pelvic fin muscle formation. A distinct process generates pelvic fin muscle in bony fishes that incorporates both primitive and derived characteristics of vertebrate appendicular muscle formation. We propose that the adoption of the fully derived mode of hindlimb muscle formation from this bimodal character state is an evolutionary innovation that was critical to the success of the tetrapod transition.

Skeletal muscle differentiation and fusion are regulated by the BAR-containing Rho-GTPase-activating protein (Rho-GAP), GRAF1

Although RhoA activity is necessary for promoting myogenic mesenchymal stem cell fates, recent studies in cultured cells suggest that down-regulation of RhoA activity in specified myoblasts is required for subsequent differentiation and myotube formation. However, whether this phenomenon occurs in vivo and which Rho modifiers control these later events remain unclear. We found that expression of the Rho-GTPase-activating protein, GRAF1, was transiently up-regulated during myogenesis, and studies in C2C12 cells revealed that GRAF1 is necessary and sufficient for mediating RhoA down-regulation and inducing muscle differentiation. Moreover, forced expression of GRAF1 in pre-differentiated myoblasts drives robust muscle fusion by a process that requires GTPase-activating protein-dependent actin remodeling and BAR-dependent membrane binding or sculpting. Moreover, morpholino-based knockdown studies in Xenopus laevis determined that GRAF1 expression is critical for muscle development. GRAF1-depleted embryos exhibited elevated RhoA activity and defective myofibrillogenesis that resulted in progressive muscle degeneration, defective motility, and embryonic lethality. Our results are the first to identify a GTPase-activating protein that regulates muscle maturation and to highlight the functional importance of BAR domains in myotube formation.

Zebrafish as a model to understand autophagy and its role in neurological disease

In the past decade, the zebrafish (Danio rerio) has become a popular model system for the study of vertebrate development, since the embryos and larvae of this species are small, transparent and undergo rapid development ex utero, allowing in vivo analysis of embryogenesis and organogenesis. These characteristics can also be exploited by researchers interested in signaling pathways and disease processes and, accordingly, there is a growing literature on the use of zebrafish to model human disease. This model holds great potential for exploring how autophagy, an evolutionarily conserved mechanism for protein degradation, influences the pathogeneses of a range of different human diseases and for the evaluation of this pathway as a potential therapeutic strategy. Here we summarize what is known about the regulation of autophagy in eukaryotic cells and its role in neurodegenerative disease and highlight how research using zebrafish has helped further our understanding of these processes.

Isolation and transcriptome analysis of adult zebrafish cells enriched for skeletal muscle progenitors

INTRODUCTION

Over the past 10 years, the use of zebrafish for scientific research in the area of muscle development has increased dramatically. Although several protocols exist for the isolation of adult myoblast progenitors from larger fish, no standardized protocol exists for the isolation of myogenic progenitors from adult zebrafish muscle.

METHODS

Using a variant of a mammalian myoblast isolation protocol, zebrafish muscle progenitors have been isolated from the total dorsal myotome. These zebrafish myoblast progenitors can be cultured for several passages and then differentiated into multinucleated, mature myotubes.

RESULTS

Transcriptome analysis of these cells during myogenic differentiation revealed a strong downregulation of pluripotency genes, while, conversely, showing an upregulation of myogenic signaling and structural genes.

CONCLUSIONS

Together these studies provide a simple, yet detailed method for the isolation and culture of myogenic progenitors from adult zebrafish, while further promoting their therapeutic potential for the study of muscle disease and drug screening.

The zebrafish dag1 mutant: a novel genetic model for dystroglycanopathies

In a forward genetic approach to identify novel genes for congenital muscle diseases, a zebrafish mutant, designated patchytail, was identified that exhibits degenerating muscle fibers with impaired motility behavior. Genetic mapping identified a genomic locus containing the zebrafish ortholog of the dystroglycan gene (DAG1). Patchytail fish contain a point mutation (c.1700T>A) in dag1, resulting in a missense change p.V567D. This change is associated with reduced transcripts and a complete absence of protein. The absence of α-dystroglycan and β-dystroglycan caused destabilization of dystroglycan complex, resulting in membrane damages. Membrane damage was localized on the extracellular matrix at myosepta as well as basement membrane between adjacent myofibers. These studies also identified structural abnormalities in triads at 3 days post fertilization (dpf) of dystroglycan-deficient muscles, significantly preceding sarcolemmal damage that becomes evident at 7 dpf. Immunofluorescence studies identified a subpopulation of dystroglycan that is expressed at t-tubules in normal skeletal muscles. In dag1-mutated fish, smaller and irregular-shaped t-tubule vesicles, as well as highly disorganized terminal cisternae of sarcoplasmic reticulum, were common. In addition to skeletal muscle defects, dag1-mutated fish have brain abnormalities and ocular defects in posterior as well as anterior chambers. These phenotypes of dystroglycan-deficient fish are highly reminiscent of the phenotypes observed in the human conditions muscle-eye-brain disease and Walker-Warburg syndrome. This animal model will provide unique opportunities in the understanding of biological functions of dystroglycan in a wide range of dystroglycanopathies, as disruption of this gene in higher vertebrates results in early embryonic lethality.

Characterization of the laminin gene family and evolution in zebrafish

Laminins are essential components of all basement membranes and are fundamental to tissue development and homeostasis. Humans possess at least 16 different heterotrimeric laminin complexes formed through different combinations of alpha, beta, and gamma chains. Individual chains appear to exhibit unique expression patterns, leading to the notion that overlap between expression domains governs the constitution of complexes found within particular tissues. However, the spatial and temporal expression of laminin genes has not been comprehensively analyzed in any vertebrate model to date. Here, we describe the tissue-specific expression patterns of all laminin genes in the zebrafish, throughout embryonic development and into the "post-juvenile" period, which is representative of the adult body form. In addition, we present phylogenetic and microsynteny analyses, which demonstrate that the majority of our zebrafish sequences are orthologous to human laminin genes. Together, these data represent a fundamental resource for the study of vertebrate laminins.

Basement membrane diseases in zebrafish

Basement membranes (BMs) are a complex, sheet-like network of specialized extracellular matrix that underlies epithelial cells and surrounds muscle cells. They provide adherence between neighboring tissues, permit some flexibility of these adherent structures, and can act as a store for growth factors and as a guide for cell migration. The BM is not just a static structure; its deposition and remodeling are important for many processes including embryonic development, immune response, and wound healing. To date, dysfunction in BM deposition or remodeling has been linked to many human congenital disorders and diseases, affecting many different tissues in the body, including malformations, dystrophies, and cancer. However, many questions remain to be answered on the role BM proteins, and their mutations, play in the pathogenesis of human disease. In recent years, the zebrafish (Danio rerio) has emerged as a powerful animal model for human development and disease. In the first part of this chapter, we provide an overview of described defects caused by BM dysfunction in zebrafish, including development and function of notochord, muscle, central nervous system, skin, cardiovascular system, and kidney. In the second part, we will describe details of methods used to visualize and assess the structure of the BM in zebrafish, and to functionally analyze its different components.