Article

Changes in cross-bridge cycling underlie muscle weakness in patients with tropomyosin 3-based myopathy

Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center, Amsterdam 1081 BT, The Netherlands.
Human Molecular Genetics (Impact Factor: 6.68). 02/2011; 20(10):2015-25. DOI: 10.1093/hmg/ddr084
Source: PubMed

ABSTRACT Nemaline myopathy, the most common non-dystrophic congenital myopathy, is caused by mutations in six genes, all of which encode thin-filament proteins, including NEB (nebulin) and TPM3 (α tropomyosin). In contrast to the mechanisms underlying weakness in NEB-based myopathy, which are related to loss of thin-filament functions normally exerted by nebulin, the pathogenesis of muscle weakness in patients with TPM3 mutations remains largely unknown. Here, we tested the hypothesis that the contractile phenotype of TPM3-based myopathy is different from that of NEB-based myopathy and that this phenotype is a direct consequence of the loss of the specific functions normally exerted by tropomyosin. To test this hypothesis, we used a multidisciplinary approach, including muscle fiber mechanics and confocal and electron microscopy to characterize the structural and functional phenotype of muscle fibers from five patients with TPM3-based myopathy and compared this with that of unaffected control subjects. Our findings demonstrate that patients with TPM3-based myopathy display a contractile phenotype that is very distinct from that of patients with NEB-based myopathy. Whereas both show severe myofilament-based muscle weakness, the contractile dysfunction in TPM3-based myopathy is largely explained by changes in cross-bridge cycling kinetics, but not by the dysregulation of sarcomeric thin-filament length that plays a prominent role in NEB-based myopathy. Interestingly, the loss of force-generating capacity in TPM3-based myopathy appears to be compensated by enhanced thin-filament activation. These findings provide a scientific basis for differential therapeutics aimed at restoring contractile performance in patients with TPM3-based versus NEB-based myopathy.

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    • "Myofibres from all five TPM3 patients examined in this study, showed a significantly lower rate of force redevelopment and higher tensions costs (as measured by ATP consumption rates per force production) suggesting that the proportion of force-generating cross-bridges is diminished in patient myofibres. A striking feature of these TPM3 myofibres was a greatly increased Ca 2+ sensitivity, resulting in significantly higher pCa 50 values (Ottenheijm et al., 2011). Notably, this increased sensitivity does not lead to muscle stiffness but is rather thought to represent a compensatory mechanism counteracting some of the effect of the diminished force generated by the crossbridges . "
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    • "Other factors such as myofibrillar concentration (i.e. myosin and actin), myosin light chains and Ca 2+ sensitivity could affect the performance of single fibres (Bottinelli, 2001; Ottenheijm et al., 2011). Limitations of the present study are: (i) small sample size for the control BB muscle (n 5 3) and overall power of the statistical analyses, (ii) unknown biopsy location and depth for certain individuals, (iii) no histology available when analyses were performed. "
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    • "The majority of the protein is composed of repeating modules, which interact with thin filament components along the length of the thin filament, whereas its N-and C-terminal regions contain unique sequences that bind to proteins at the actin filament pointed end and the Z-line, respectively. Based on in vivo and in vitro studies, nebulin has recently been implicated in various important processes, including stabilization of thin filaments (Bang et al., 2006; Castillo et al., 2009; Gokhin and Fowler, 2013; Littlefield and Fowler, 2008; Pappas et al., 2010; Witt et al., 2006), myofibrillar force generation (Bang et al., 2006; Gokhin et al., 2009; Ochala et al., 2011; Ottenheijm et al., 2010; Ottenheijm et al., 2009), regulation of the actomyosin interaction (Bang et al., 2009; Castillo et al., 2009; Ochala et al., 2011; Ottenheijm et al., 2011), sarcoplasmic Ca 2+ handling (Ottenheijm et al., 2008; Witt et al., 2006), maintenance of sarcomeric integrity during muscle contraction (Bang et al., 2006), as well as Z-line alignment, width and integrity (Bang et al., 2006; Pappas et al., 2010; Tonino et al., 2010; Witt et al., 2006). However, although recent studies have provided new insights into the role of nebulin in skeletal muscle, its function in the Z-line remains poorly understood. "
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