Muscle weakness in TPM3-myopathy is due to reduced Ca2+-sensitivity and impaired acto-myosin cross-bridge cycling in slow fibres

M. Yuen, S.T. Cooper, S.B. Marston, Kristen Nowak, Elyshia Mcnamara, N. Mokbe, B. Ilkovski, Gina Ravenscroft, J. Rendu, J.M. Dewinter, L. Klinge, A.H. Beggs, K.N. North, C.A.C. Ottenheijm, N.F. Clarke

Research output: Contribution to journalArticle

19 Citations (Scopus)

Abstract

© The author 2015. Published by oxford university press. All rights reserved. Dominant mutations in TPM3, encoding a-tropomyosinslow, cause a congenital myopathy characterized by generalized muscle weakness. Here, we used a multidisciplinary approach to investigate the mechanism of muscle dysfunction in 12 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 a-tropomyosinslow was expressed, suggesting muscle dysfunction is due to a dominant-negative effect of mutant protein on muscle contraction. Molecular modelling suggested mutant a-tropomyosinslow likely impacts actin-tropomyosin interactions and, indeed, co-sedimentation assays showed reduced binding of mutant a-tropomyosinslow (R168C) to filamentous actin. Single fibre contractility studies of patient myofibres revealed marked slow myofibre specific abnormalities. At saturating [Ca2+] (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 Ca2+-sensitivity, at sub-saturating [Ca2+] (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 [Ca2+], and impaired acto-myosin cross-bridge cycling kinetics. Fastmyofibres are spared; however, they appear to be unable to compensate for slow fibre dysfunction. Abnormal Ca2+-sensitivity in TPM3-myopathy patients suggests Ca2+-sensitizing drugs may represent a useful treatment for this condition.
Original languageEnglish
Pages (from-to)6278-6292
Number of pages15
JournalHuman Molecular Genetics
Volume24
Issue number22
Early online date24 Aug 2015
DOIs
Publication statusPublished - 22 Nov 2015

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