Dystrophin and the dystrophin associated glycoprotein complex in the zebrafish, Danio rerio

Abstract

Mutations affecting the dystrophin gene and the dystrophin associated glycoprotein complex (DGC) genes in humans cause chronic muscle wasting diseases collectively termed muscular dystrophies (MDs), often with associated cardiomyopathy. The dystrophin gene is associated with the allelic, X-linked diseases Duchenne and Becker muscular dystrophy (D/BMD), of which DMD is fatal in early adulthood. The DGC is associated with the autosomal recessive limb girdle muscular dystrophies (LGMDs) which result from mutations of the human α-, β-, δ- and γ- sarcoglycan genes. Dystrophin and the DGC genes encode a sarcolemmal complex of proteins that forms the structural linkage between the muscle cytoskeleton and extracellular matrix, which is crucial for the maintenance of muscle integrity during normal activity. Dystrophin and the DGC genes are evolutionarily conserved, underpinning the important role of this complex. In terms of DMD modeling, the mouse has served as a suitable vertebrate species but the pathophysiology of the disease in the mouse does not entirely mimic human DMD. LGMD modeling is served by a the BIO 14.6 hamster which is a traditional model of cardiomyopathy, and mouse sarcoglycan gene null strains. The zebrafish is an established model of vertebrate development, and is receiving increasing attention in terms of human disease modeling. This thesis sought to examine the potential of the zebrafish as a further comparative model for these MDs. In order to provide experimental support to realise this modeling potential, this thesis describes the identification of apparent orthologues of many critical members of the DGC in the zebrafish. An apparent zebrafish orthologue of the human dystrophin gene was identified that expresses a 400kDa protein that is localised to the myofibre membrane surface of adult zebrafish muscle. Epitope mapping confirmed that zebrafish dystrophin retains all the conserved functional domains reported in the human protein. Dystrophin expression in the zebrafish embryo was observed at six hours post fertilisation (hpf), although maternal transcript was present in the zygote. Dystrophin protein was localised at the myosepta of zebrafish embryos at 24 hours post fertilisation (hpf). Apparent zebrafish orthologues of human β-dystroglycan and β-, δ-, ε-, γ- and ζ-sarcoglycan were also identified, and in the case of β-dystroglycan and β-, and γ-sarcoglycan were shown to express proteins localised to the myofibre membrane surface in adult zebrafish. Like dystrophin, zebrafish β-dystroglycan and β-sarcoglycan genes express proteins which localised to the myosepta of 24hpf zebrafish embryos. Co-localisation of dystrophin with β-dystroglycan and F-actin confirmed that the zebrafish carries an intact DGC. The DGC in the zebrafish retina was also studied to provide a basis for understanding the diversity of DGC expression and function in respect of the pathology of the human diseases modelled. A DGC which incorporates zebrafish utrophin or dystrophin with dystroglycan, was localised to the glial cell layer (GCL) and outer plexiform layer (OPL) of the adult zebrafish retina. During zebrafish retinal development, dystrophin and dystroglycan were first localised in the GCL at 48hpf and the OPL at72hpf. In terms of targeting gene expression to achieve disease modeling outcomes, two strategies were used targeting the carboxyl-terminus of zebrafish dystrophin transcript. Peptide nucleic acid (PNA) masking of the splice donor site of zebrafish dystrophin exon 7l resulted in mis-splicing of the transcript which conserved the reading frame of the transcript. Gene silencing by short interfering RNAs targeted against exon 53 and exon 68 of zebrafish dystrophin caused temporary loss of dystrophin transcript, and delayed dystrophin and β-sarcoglycan localisation at the myosepta of affected embryos. Together, these data suggest that dystrophin and the DGC in zebrafish may play a highly conserved functional role in muscle architecture that, when disrupted, could offer insight into human neuromuscular disease processes.