Mitochondrial abnormalities contribute to muscle weakness in a Dnajb6 deficient zebrafish model

Mutations in DNAJB6 are a well-established cause of limb girdle muscular dystrophy type D1 (LGMD D1). Patients with LGMD D1 develop progressive muscle weakness with histology showing fibre damage, autophagic vacuoles, and aggregates. Whilst there are many reports of LGMD D1 patients, the role of DNAJB6 in the muscle is still unclear. In this study, we developed a loss of function zebrafish model in order to investigate the role of Dnajb6. Using a double dnajb6a and dnajb6b mutant model, we show that loss of Dnajb6 leads to a late onset muscle weakness. Interestingly, we find that adult fish lacking Dnajb6 do not have autophagy or myofibril defects, however, they do show mitochondrial changes and damage. This study demonstrates that loss of Dnajb6 causes mitochondrial defects and suggests that this contributes to muscle weakness in LGMD D1. These findings expand our knowledge of the role of Dnajb6 in the muscle and provides a model to screen novel therapies for LGMD D1.

RGD inhibition of itgb1 ameliorates laminin-α2-deficient zebrafish fibre pathology

Deficiency of muscle basement membrane (MBM) component laminin-α2 leads to muscular dystrophy congenital type 1A (MDC1A), a currently untreatable myopathy. Laminin--α2 has two main binding partners within the MBM, dystroglycan and integrin. Integrins coordinate both cell adhesion and signalling; however, there is little mechanistic insight into integrin's function at the MBM. In order to study integrin's role in basement membrane development and how this relates to the MBM's capacity to handle force, an itgβ1.b-/- zebrafish line was created. Histological examination revealed increased extracellular matrix (ECM) deposition at the MBM in the itgβ1.b-/- fish when compared with controls. Surprisingly, both laminin and collagen proteins were found to be increased in expression at the MBM of the itgβ1.b-/- larvae when compared with controls. This increase in ECM components resulted in a decrease in myotomal elasticity as determined by novel passive force analyses. To determine if it was possible to control ECM deposition at the MBM by manipulating integrin activity, RGD peptide, a potent inhibitor of integrin-β1, was injected into a zebrafish model of MDC1A. As postulated an increase in laminin and collagen was observed in the lama2-/- mutant MBM. Importantly, there was also an improvement in fibre stability at the MBM, judged by a reduction in fibre pathology. These results therefore show that blocking ITGβ1 signalling increases ECM deposition at the MBM, a process that could be potentially exploited for treatment of MDC1A.

Testing of therapies in a novel nebulin nemaline myopathy model demonstrate a lack of efficacy

Nemaline myopathies are heterogeneous congenital muscle disorders causing skeletal muscle weakness and, in some cases, death soon after birth. Mutations in nebulin, encoding a large sarcomeric protein required for thin filament function, are responsible for approximately 50% of nemaline myopathy cases. Despite the severity of the disease there is no effective treatment for nemaline myopathy with limited research to develop potential therapies. Several supplements, including L-tyrosine, have been suggested to be beneficial and consequently self-administered by nemaline myopathy patients without any knowledge of their efficacy. We have characterized a zebrafish model for nemaline myopathy caused by a mutation in nebulin. These fish form electron-dense nemaline bodies and display reduced muscle function akin to the phenotypes observed in nemaline myopathy patients. We have utilized our zebrafish model to test and evaluate four treatments currently self-administered by nemaline myopathy patients to determine their ability to increase skeletal muscle function. Analysis of muscle pathology and locomotion following treatment with L-tyrosine, L-carnitine, taurine, or creatine revealed no significant improvement in skeletal muscle function emphasizing the urgency to develop effective therapies for nemaline myopathy.