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

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

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

Cell fusion is differentially regulated in zebrafish post-embryonic slow and fast muscle

Skeletal muscle fusion occurs during development, growth, and regeneration. To investigate how muscle fusion compares among different muscle cell types and developmental stages, we studied muscle cell fusion over time in wild-type, myomaker (mymk), and jam2a mutant zebrafish. Using live imaging, we show that embryonic myoblast elongation and fusion correlate tightly with slow muscle cell migration. In wild-type embryos, only fast muscle fibers are multinucleate, consistent with previous work showing that the cell fusion regulator gene mymk is specifically expressed throughout the embryonic fast muscle domain. However, by 3 weeks post-fertilization, slow muscle fibers also become multinucleate. At this late-larval stage, mymk is not expressed in muscle fibers, but is expressed in small cells near muscle fibers. Although previous work showed that both mymk and jam2a are required for embryonic fast muscle cell fusion, we observe that muscle force and function is almost normal in mymk and jam2a mutant embryos, despite the lack of fast muscle multinucleation. We show that genetic requirements change post-embryonically, with jam2a becoming much less important by late-larval stages and mymk now required for muscle fusion and growth in both fast and slow muscle cell types. Correspondingly, adult mymk mutants perform poorly in sprint and endurance tests compared to wild-type and jam2a mutants. We show that adult mymk mutant muscle contains small mononucleate myofibers with average myonuclear domain size equivalent to that in wild type adults. The mymk mutant fibers have decreased Laminin expression and increased numbers of Pax7-positive cells, suggesting that impaired fiber growth and active regeneration contribute to the muscle phenotype. Our findings identify several aspects of muscle fusion that change with time in slow and fast fibers as zebrafish develop beyond embryonic stages.

Hypomorphic zebrafish models mimic the musculoskeletal phenotype of β4GalT7-deficient Ehlers-Danlos syndrome

β4GalT7 is a transmembrane Golgi enzyme, encoded by B4GALT7, that plays a pivotal role in the proteoglycan linker region formation during proteoglycan biosynthesis. Defects in this enzyme give rise to a rare autosomal recessive form of Ehlers-Danlos syndrome (EDS), currently known as 'spondylodysplastic EDS (spEDS-B4GALT7)'. This EDS subtype is mainly characterized by short stature, hypotonia and skeletal abnormalities, thereby illustrating its pleiotropic importance during human development. Insights into the pathogenic mechanisms underlying this disabling disease are very limited, in part due to the lack of a relevant in vivo model. As the majority of mutations identified in patients with spEDS-B4GALT7 are hypomorphic, we generated zebrafish models with partial loss of B4galt7 function, including different knockdown (morphant) and mosaic knockout (crispant) b4galt7 zebrafish models and studied the morphologic, functional and molecular aspects in embryonic and larval stages. Morphant and crispant zebrafish show highly similar morphological abnormalities in early development including a small, round head, bowed pectoral fins, short body-axis and mild developmental delay. Several craniofacial cartilage and bone structures are absent or strongly misshapen. In addition, the total amount of sulfated glycosaminoglycans is significantly diminished and particularly heparan and chondroitin sulfate proteoglycan levels are greatly reduced. We also show impaired cartilage patterning and loss of chondrocyte organization in a cartilage-specific Tg(Col2a1aBAC:mcherry) zebrafish reporter line. The occurrence of the same abnormalities in the different models confirms these are specifically caused by B4galt7 deficiency. A disturbed actin pattern, along with a lack of muscle tone, was only noted in morphants in which translation of b4galt7 was blocked. In conclusion, we generated the first viable animal models for spEDS-B4GALT7, and show that in early development the human spEDS-B4GALT7 phenotype is faithfully mimicked in these zebrafish models. Our findings underscore a key role for β4GalT7 in early development of cartilage, bone and muscle. These models will lead to a better understanding of spEDS-B4GALT7 and can be used in future efforts focusing on therapeutic applications.

Central and peripheral innervation patterns of defined axial motor units in larval zebrafish

Spinal motor neurons and the peripheral muscle fibers they innervate form discrete motor units that execute movements of varying force and speed. Subsets of spinal motor neurons also exhibit axon collaterals that influence motor output centrally. Here, we have used in vivo imaging to anatomically characterize the central and peripheral innervation patterns of axial motor units in larval zebrafish. Using early born "primary" motor neurons and their division of epaxial and hypaxial muscle into four distinct quadrants as a reference, we define three distinct types of later born "secondary" motor units. The largest is "m-type" units, which innervate deeper fast-twitch muscle fibers via medial nerves. Next in size are "ms-type" secondaries, which innervate superficial fast-twitch and slow fibers via medial and septal nerves, followed by "s-type" units, which exclusively innervate superficial slow muscle fibers via septal nerves. All types of secondaries innervate up to four axial quadrants. Central axon collaterals are found in subsets of primaries based on soma position and predominantly in secondary fast-twitch units (m, ms) with increasing likelihood based on number of quadrants innervated. Collaterals are labeled by synaptophysin-tagged fluorescent proteins, but not PSD95, consistent with their output function. Also, PSD95 dendrite labeling reveals that larger motor units receive more excitatory synaptic input. Collaterals are largely restricted to the neuropil, however, perisomatic connections are observed between motor units. These observations suggest that recurrent interactions are dominated by motor neurons recruited during stronger movements and set the stage for functional investigations of recurrent motor circuitry in larval zebrafish.

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

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

Zebrafish Embryonic Slow Muscle Is a Rapid System for Genetic Analysis of Sarcomere Organization by CRISPR/Cas9, but Not NgAgo

Zebrafish embryonic slow muscle cells, with their superficial localization and clear sarcomere organization, provide a useful model system for genetic analysis of muscle cell differentiation and sarcomere assembly. To develop a quick assay for testing CRISPR-mediated gene editing in slow muscles of zebrafish embryos, we targeted a red fluorescence protein (RFP) reporter gene specifically expressed in slow muscles of myomesin-3-RFP (Myom3-RFP) zebrafish embryos. We demonstrated that microinjection of RFP-sgRNA with Cas9 protein or Cas9 mRNA resulted in a mosaic pattern in loss of RFP expression in slow muscle fibers of the injected zebrafish embryos. To uncover gene functions in sarcomere organization, we targeted two endogenous genes, slow myosin heavy chain-1 (smyhc1) and heat shock protein 90 α1 (hsp90α1), which are specifically expressed in zebrafish muscle cells. We demonstrated that injection of Cas9 protein or mRNA with respective sgRNAs targeted to smyhc1 or hsp90a1 resulted in a mosaic pattern of myosin thick filament disruption in slow myofibers of the injected zebrafish embryos. Moreover, Myom3-RFP expression and M-line localization were also abolished in these defective myofibers. Given that zebrafish embryonic slow muscles are a rapid in vivo system for testing genome editing and uncovering gene functions in muscle cell differentiation, we investigated whether microinjection of Natronobacterium gregoryi Argonaute (NgAgo) system could induce genetic mutations and muscle defects in zebrafish embryos. Single-strand guide DNAs targeted to RFP, Smyhc1, or Hsp90α1 were injected with NgAgo mRNA into Myom3-RFP zebrafish embryos. Myom3-RFP expression was analyzed in the injected embryos. The results showed that, in contrast to the CRISPR/Cas9 system, injection of the NgAgo-gDNA system did not affect Myom3-RFP expression and sarcomere organization in myofibers of the injected embryos. Sequence analysis failed to detect genetic mutations at the target genes. Together, our studies demonstrate that zebrafish embryonic slow muscle is a rapid model for testing gene editing technologies in vivo and uncovering gene functions in muscle cell differentiation.

RNA helicase, DDX27 regulates skeletal muscle growth and regeneration by modulation of translational processes

Gene expression in a tissue-specific context depends on the combined efforts of epigenetic, transcriptional and post-transcriptional processes that lead to the production of specific proteins that are important determinants of cellular identity. Ribosomes are a central component of the protein biosynthesis machinery in cells; however, their regulatory roles in the translational control of gene expression in skeletal muscle remain to be defined. In a genetic screen to identify critical regulators of myogenesis, we identified a DEAD-Box RNA helicase, DDX27, that is required for skeletal muscle growth and regeneration. We demonstrate that DDX27 regulates ribosomal RNA (rRNA) maturation, and thereby the ribosome biogenesis and the translation of specific transcripts during myogenesis. These findings provide insight into the translational regulation of gene expression in myogenesis and suggest novel functions for ribosomes in regulating gene expression in skeletal muscles.

New era in genetics of early-onset muscle disease: Breakthroughs and challenges

Early-onset muscle disease includes three major entities that present generally at or before birth: congenital myopathies, congenital muscular dystrophies and congenital myasthenic syndromes. Almost exclusively there is weakness and hypotonia, although cases manifesting hypertonia are increasingly being recognised. These diseases display a wide phenotypic and genetic heterogeneity, with the uptake of next generation sequencing resulting in an unparalleled extension of the phenotype-genotype correlations and "diagnosis by sequencing" due to unbiased sequencing. Perhaps now more than ever, detailed clinical evaluations are necessary to guide the genetic diagnosis; with arrival at a molecular diagnosis frequently occurring following dialogue between the molecular geneticist, the referring clinician and the pathologist. There is an ever-increasing blurring of the boundaries between the congenital myopathies, dystrophies and myasthenic syndromes. In addition, many novel disease genes have been described and new insights have been gained into skeletal muscle development and function. Despite the advances made, a significant percentage of patients remain without a molecular diagnosis, suggesting that there are many more human disease genes and mechanisms to identify. It is now technically- and clinically-feasible to perform next generation sequencing for severe diseases on a population-wide scale, such that preconception-carrier screening can occur. Newborn screening for selected early-onset muscle diseases is also technically and ethically-achievable, with benefits to the patient and family from early management of these diseases and should also be implemented. The need for world-wide Reference Centres to meticulously curate polymorphisms and mutations within a particular gene is becoming increasingly apparent, particularly for interpretation of variants in the large genes which cause early-onset myopathies: NEB, RYR1 and TTN. Functional validation of candidate disease variants is crucial for accurate interpretation of next generation sequencing and appropriate genetic counseling. Many published "pathogenic" variants are too frequent in control populations and are thus likely rare polymorphisms. Mechanisms need to be put in place to systematically update the classification of variants such that accurate interpretation of variants occurs. In this review, we highlight the recent advances made and the challenges ahead for the molecular diagnosis of early-onset muscle diseases.

Chromatin immunoprecipitation and an open chromatin assay in zebrafish erythrocytes

Zebrafish is an excellent genetic and developmental model for the study of vertebrate development and disease. Its ability to produce an abundance of transparent, externally developed embryos has facilitated large-scale genetic and chemical screens for the identification of critical genes and chemical factors that modulate developmental pathways. These studies can have profound implications for the diagnosis and treatment of a variety of human diseases. Recent advancements in molecular and genomic studies have provided valuable tools and resources for comprehensive and high-resolution analysis of epigenomes during cell specification and lineage differentiation throughout development. In this chapter, we describe two simple methods to evaluate protein-DNA interaction and chromatin architecture in erythrocytes from adult zebrafish. These are chromatin immunoprecipitation coupled with next-generation sequencing (ChIP-seq) and an assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq). These techniques, together with gene expression profiling, are useful for analyzing epigenomic regulation of cell specification, differentiation, and function during zebrafish development in both normal and disease models.