Duchenne muscular dystrophy (DMD) is a severe degenerative muscle disease that manifests as progressive muscle weakness, extensive skeletal muscle loss, and cardiomyopathy. DMD is caused by X-linked recessive mutations in the dystrophin gene.Dystrophin is primarily expressed in skeletal and cardiac muscle fibres, where it associates with proteins from the dystroglycan complex to form the dystrophin-associated protein complex. Its interaction with actin ensures that the sarcolemma is anchored to the extracellular matrix as well as the intracellular contractile apparatus, which explains why muscular dystrophies can result from defects in any component of this connecting system. According to protein-protein and protein-lipid interaction studies, dystrophin acts as a shock absorber, stabilising the plasma membrane and preventing damage during muscle contraction, which causes mechanical stress even in normal muscle. Zebrafish have precocious motor locomotor strategies, generating muscle load even before the first 24 hours of development, and mutations that disrupt muscle development are easily identified in large-scale mutagenic screens. Both embryological and genetic studies have used these characteristics to investigate the early stages of muscle development in the zebrafish, with a particular emphasis on the mechanisms used to determine the different fibre types present within the embryonic myotome. However, up until recently, the later stages of muscle development had received little attention.

For several reasons, zebrafish embryos are particularly well suited to the study of muscle development. For starters, they develop externally, are transparent, somitic muscle comprises a large proportion of the body and is accessible, and they begin to move very soon after gastrulation. Both embryological and genetic studies have used these characteristics to investigate early stages of muscle development in the zebrafish, such as the specification of slow-twitch muscle fibres, with great success. However, beyond the initial identification of mutations that affect muscle fibre differentiation, function, and integrity, the later stages of development have received little attention. Axial muscle in fish develops from segmented paraxial mesoderm, which gives rise to somites, which in turn give rise to myotomes.

In the embryonic myotome of zebrafish, the different classes of muscle fibres, slow and fast twitch, are topographically separable. Midline-derived signals direct the most medial cells of the forming myotome to form only slow-twitch fibres. These cells then migrate from their medial origin to traverse the entire length of the myotome, forming a layer of slow-twitch muscle beneath the skin. Behind this migration, the rest of the myotome differentiates as fast-twitch fibres. Muscle fibres initially differentiate to span an entire somite in the anterior–posterior axis, regardless of fate or position within the myotome. By 24 hours post-fertilization (h.p.f.), the somite has taken on its distinctive chevron shape, with the dorsal and ventral halves separated by a sheet of extracellular matrix called the horizontal myoseptum and each pair of adjacent somites separated by the vertical myoseptum, which is similarly constructed. Myosepta serve as attachment points for somitic muscle fibres. These muscle attachment sites have recently come under scrutiny following the discovery that mechanical failure is the pathological mechanism in a zebrafish mutation that provides the first zebrafish model of an inherited skeletal muscle disease.

DMD patients seen in neuromuscular disease centres frequently report that a diet supplemented with bioactive compounds improves their muscle strength in real-world clinical practise. Recent evidence, based on empirical observations in DMD patients and studies in the mdx mouse model of the disease, suggests that proper nutrition can have anti-inflammatory effects and slow the rate of muscle atrophy.For example, resveratrol, a plant-derived natural nutraceutical, appears to improve dystrophic myopathology in human cells and animal models. Although these findings have opened up a new avenue of investigation into chronic muscle diseases, similar studies in larger experimental models of DMD are required to determine the potential efficacy and safety of nutraceuticals in vivo. A validated model of DMD, the sapje strain, carries a recessive nonsense mutation in dystrophin and exhibits muscle disorganisation, motor dysfunction, and early death.The sapje zebrafish phenotype appears 3 days after fertilisation (dpf), and its severity is similar to that of DMD in children. High-throughput screenings with sapje have confirmed the species’ utility in developing new pre-clinical hypotheses. Fluoxetine, sildenafil, and dasatinib are effective compounds that have been validated in sapje and are now being developed for randomised clinical trials. Nonetheless, drugs identified in recent screenings have a number of side effects and, despite being approved for use in humans, cannot be routinely proposed in clinical settings until additional, more expensive preclinical research is completed. It was hypothesised that a nutraceutical screening study targeting early dysfunctional pathways underlying DMD would identify compounds capable of not only improving clinical phenotypes, but also of being rapidly and safely applied in clinical settings via dietary supplements or patient lifestyle changes.

The findings described here gave us the tantalizing prospect of applying the sophisticated embryological and genetic methodologies available in zebrafish to the study of human dystrophic conditions. A particularly promising research direction is the use of second-site enhancer and suppressor screens to identify genes that may act to modulate the dystrophic condition. Furthermore, the molecular defects present in the remaining ‘dystrophic’ mutants have yet to be elucidated, and it is possible that these may represent potentially novel genes that are also mutated in human muscular dystrophies.