Muscle weakness in TPM3-myopathy is due to reduced Ca2+-sensitivity and impaired acto-myosin cross-bridge cycling in slow fibres.
Author(s)
Type
Journal Article
Abstract
Dominant mutations in TPM3, encoding α-tropomyosinslow, cause a congenital myopathy characterised by generalised muscle weakness. Here, we used a multidisciplinary approach to investigate the mechanism of muscle dysfunction in twelve TPM3-myopathy patients.We confirm that slow myofibre hypotrophy is a diagnostic hallmark of TPM3-myopathy, and is commonly accompanied by skewing of fibre-type ratios (either slow or fast fibre predominance). Patient muscle contained normal ratios of the three tropomyosin isoforms and normal fibre-type expression of myosins and troponins. Using 2D-PAGE, we demonstrate that mutant α-tropomyosinslow was expressed, suggesting muscle dysfunction is due to a dominant-negative effect of mutant protein on muscle contraction. Molecular modelling suggested mutant α-tropomyosinslow likely impacts actin-tropomyosin interactions and, indeed, co-sedimentation assays showed reduced binding of mutant α-tropomyosinslow (R168C) to filamentous actin.Single fibre contractility studies of patient myofibres revealed marked slow myofibre specific abnormalities. At saturating [Ca(2+)] (pCa 4.5), patient slow fibres produced only 63% of the contractile force produced in control slow fibres and had reduced acto-myosin cross-bridge cycling kinetics. Importantly, due to reduced Ca(2+)-sensitivity, at sub-saturating [Ca(2+)] (pCa 6, levels typically released during in vivo contraction) patient slow fibres produced only 26% of the force generated by control slow fibres.Thus, weakness in TPM3-myopathy patients can be directly attributed to reduced slow fibre force at physiological [Ca(2+)], and impaired acto-myosin cross-bridge cycling kinetics. Fast myofibres are spared; however, they appear to be unable to compensate for slow fibre dysfunction. Abnormal Ca(2+)-sensitivity in TPM3-myopathy patients suggests Ca(2+)-sensitising drugs may represent a useful treatment for this condition.
Date Issued
2015-08-24
Date Acceptance
2015-08-10
Citation
Human Molecular Genetics, 2015, 24 (22), pp.6278-6292
ISSN
1460-2083
Publisher
Oxford University Press (OUP)
Start Page
6278
End Page
6292
Journal / Book Title
Human Molecular Genetics
Volume
24
Issue
22
Copyright Statement
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Human Molecular Genetics following peer review. The version of record Hum. Mol. Genet. (2015) 24 (22): 6278-6292.
doi: 10.1093/hmg/ddv334 is available online at: https://dx.doi.org/10.1093/hmg/ddv334
doi: 10.1093/hmg/ddv334 is available online at: https://dx.doi.org/10.1093/hmg/ddv334
Identifier
PII: ddv334
Publication Status
Published