BACKGROUND:Nemaline myopathy (NM) is a congenital muscle disease associated with weakness and the presence of nemaline bodies (rods) in muscle fibers. Mutations in seven genes have been associated with NM, but the most commonly mutated gene is nebulin (NEB), which is thought to account for roughly 50% of cases.RESULTS:We describe two siblings with severe NM, arthrogryposis and neonatal death caused by two novel NEB mutations: a point mutation in intron 13 and a frameshift mutation in exon 81. Levels of detectable nebulin protein were significantly lower than those in normal control muscle biopsies or those from patients with less severe NM due to deletion of NEB exon 55. Mechanical studies of skinned myofibers revealed marked impairment of force development, with an increase in tension cost.CONCLUSIONS:Our findings demonstrate that the mechanical phenotype of severe NM is the consequence of mutations that severely reduce nebulin protein levels and suggest that the level of nebulin expression may correlate with the severity of disease.

BACKGROUND: Nemaline myopathy (NM), the most common non-dystrophic congenital myopathy, is characterized by generalized skeletal muscle weakness, often from birth. To date, no therapy exists that enhances the contractile strength of muscles of NM patients. Mutations in NEB, encoding the giant protein nebulin, are the most common cause of NM. The pathophysiology of muscle weakness in NM patients with NEB mutations (NEB-NM) includes a lower calcium-sensitivity of force generation. We propose that the lower calcium-sensitivity of force generation in NEB-NM offers a therapeutic target. Levosimendan is a calcium sensitizer that is approved for use in humans and has been developed to target cardiac muscle fibers. It exerts its effect through binding to slow skeletal/cardiac troponin C. As slow skeletal/cardiac troponin C is also the dominant troponin C isoform in slow-twitch skeletal muscle fibers, we hypothesized that levosimendan improves slow-twitch muscle fiber strength at submaximal levels of activation in patients with NEB-NM. METHODS: To test whether levosimendan affects force production, permeabilized slow-twitch muscle fibers isolated from biopsies of NEB-NM patients and controls were exposed to levosimendan and the force response was measured. RESULTS: No effect of levosimendan on muscle fiber force in NEB-NM and control skeletal muscle fibers was found, both at a submaximal calcium level using incremental levosimendan concentrations, and at incremental calcium concentrations in the presence of levosimendan. In contrast, levosimendan did significantly increase the calcium-sensitivity of force in human single cardiomyocytes. Protein analysis confirmed that the slow skeletal/cardiac troponin C isoform was present in the skeletal muscle fibers tested. CONCLUSIONS: These findings indicate that levosimendan does not improve the contractility in human skeletal muscle fibers, and do not provide rationale for using levosimendan as a therapeutic to restore muscle weakness in NEB-NM patients. We stress the importance of searching for compounds that improve the calcium-sensitivity of force generation of slow-twitch muscle fibers. Such compounds provide an appealing approach to restore muscle force in patients with NEB-NM, and also in patients with other neuromuscular disorders.

Skeletal muscle consists of an extremely regular organization of myofibers that are specialized in contraction. Development and maintenance of skeletal muscle function depends on the precise organization of sarcomeric contractile proteins that consist the myofibrils. Impaired or delayed myofibrillogenesis has been identified as the primary pathological mechanism of many skeletal muscle myopathies. Several members of the Kelch family of proteins have been implicated in skeletal muscle development and diseases, and mutations in these proteins have resulted in perturbations in the ubiquitin proteasome system (UPS), which is the primary means of proteasomal degradation in eukaryotes. In particular, KLHL41 of the BTB-BACK Kelch family is primarily expressed in skeletal muscle and has been identified as a regulator of the skeletal muscle differentiation process that results in the normal development and functioning of mature skeletal muscles. KLHL41 acts as a substrate-specific adaptor for Cullin 3 (Cul3) E3 ubiquitin ligase, implicating the role/s of KLHL41 in proteasomal ubiquitination processes in skeletal muscle. Recent studies have determined that the degradation of nebulin-related anchoring protein (NRAP), which was found to interact with KLHL41, is a critical process in skeletal myofibril maturation that is caused by KLHL41-mediated ubiquitination of the NRAP protein. Through this study, it was further confirmed that KLHL41 changes in localization as maturation occurs, which may provide insight into the mechanism of its functions in myofibril maturation. In addition, the study found that KLHL41 promotes the critical process of nebulin-related anchoring protein (NRAP) degradation. Lastly, mutations in the KLHL41, which are known to cause Nemaline Myopathy (NM) in patients, were modeled in murine C2C12 myoblasts to gain a greater understanding of how KLHL41 mutations may affect protein stability and Cul3 E3 ubiquitin ligase activity. Overall, the findings of this thesis support the critical role of KLHL41 in the formation of mature myofibrils, and provides insight into how deficiency of KLHL41 contributes to a disease state through regulation of the CUL3 protein complex. / 2022-06-30T00:00:00Z

Congenital myopathies are a rare and heterogeneous group of diseases. They are primarily characterised by skeletal muscle weakness and disease-specific pathological features. They harshly limit ordinary life and in severe cases, these myopathies are associated with early death of the affected individuals. The congenital myopathies investigated in this thesis are nemaline myopathy and myofibrillar myopathy. These diseases are usually caused by missense mutations in genes encoding myofibrillar proteins, but the exact mechanisms by which the point mutations in these proteins cause the overall weakness remain mysterious. Hence, in this thesis two different nemaline myopathy-causing actin mutations and one myofibrillar myopathy-causing myosin-mutation found in both human patients and mouse models were used to investigate the cascades of molecular and cellular events leading to weakness. I performed a broad range of functional and structural experiments including skinned muscle fibre mechanics, small-angle X-ray scattering as well as immunoblotting and histochemical techniques. Interestingly, according to my results, point mutations in myosin and actin differently modify myosin binding to actin, cross-bridge formation and muscle fibre force production revealing divergent mechanisms, that is, gain versus loss of function (papers I, II and IV). In addition, one point mutation in actin appears to have muscle-specific effects.  The presence of that mutant protein in respiratory muscles, i.e. diaphragm, has indeed more damaging consequences on myofibrillar structure than in limb muscles complexifying the pathophysiological mechanisms (paper II). As numerous atrophic muscle fibres can be seen in congenital myopathies, I also considered this phenomenon as a contributing factor to weakness and characterised the underlying causes in presence of one actin mutation. My results highlighted a direct muscle-specific up-regulation of the ubiquitin-proteasome system (paper III). All together, my research work demonstrates that mutation- and muscle-specific mechanisms trigger the muscle weakness in congenital myopathies. This gives important insights into the pathophysiology of congenital myopathies and will undoubtedly help in designing future therapies.