title: Muscle weakness in TPM3-myopathy is due to reduced Ca2+-sensitivity and impaired acto-myosin cross-bridge cycling in slow fibres.
abstract: Dominant mutations in TPM3, encoding α-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 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+)-sensitizing drugs may represent a useful treatment for this condition.
authors: Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Westmead, Australia, Discipline of Paediatrics and Child Health, University of Sydney, Sydney, Australia, michaela.kreissl@sydney.edu.au.
authors: Michaela
authors: M
authors: Yuen
authors: Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Westmead, Australia, Discipline of Paediatrics and Child Health, University of Sydney, Sydney, Australia.
authors: Sandra T
authors: ST
authors: Cooper
authors: National Heart and Lung Institute, Imperial College London, London, UK.
authors: Steve B
authors: SB
authors: Marston
authors: Harry Perkins Institute of Medical Research and the Centre for Medical Research, University of Western Australia, Nedlands, Australia.
authors: Kristen J
authors: KJ
authors: Nowak
authors: Harry Perkins Institute of Medical Research and the Centre for Medical Research, University of Western Australia, Nedlands, Australia.
authors: Elyshia
authors: E
authors: McNamara
authors: Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Westmead, Australia, Faculty of Health Sciences, St. George Health Complex, The University of Balamand, Beirut, Lebanon.
authors: Nancy
authors: N
authors: Mokbel
authors: Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Westmead, Australia.
authors: Biljana
authors: B
authors: Ilkovski
authors: Harry Perkins Institute of Medical Research and the Centre for Medical Research, University of Western Australia, Nedlands, Australia.
authors: Gianina
authors: G
authors: Ravenscroft
authors: Département de Biochimie Toxicologie et Pharmacologie, Département de Biochimie Génétique et Moléculaire, Centre Hospitalier Universitaire de Grenoble, Grenoble, France.
authors: John
authors: J
authors: Rendu
authors: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center, Amsterdam, The Netherlands.
authors: Josine M
authors: JM
authors: de Winter
authors: Department of Pediatrics and Adolescent Medicine, Division of Pediatric Neurology, Faculty of Medicine, Georg August University, Göttingen, Germany.
authors: Lars
authors: L
authors: Klinge
authors: Division of Genetics and Genomics, The Manton Center for Orphan Disease Research, Boston Children's Hospital, Harvard Medical School, Boston, MA, USA.
authors: Alan H
authors: AH
authors: Beggs
authors: Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Westmead, Australia, Discipline of Paediatrics and Child Health, University of Sydney, Sydney, Australia, Murdoch Children's Research Institute, the Royal Children's Hospital, Parkville, Australia and Department of Paediatrics, University of Melbourne, Melbourne, Australia.
authors: Kathryn N
authors: KN
authors: North
authors: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center, Amsterdam, The Netherlands.
authors: Coen A C
authors: CA
authors: Ottenheijm
authors: Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Westmead, Australia, Discipline of Paediatrics and Child Health, University of Sydney, Sydney, Australia.
authors: Nigel F
authors: NF
authors: Clarke
mesh_terms: D000199:Actins / Q000235:genetics / Q000378:metabolism; D000293:Adolescent; D000328:Adult; D002118:Calcium / Q000378:metabolism; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism*; D018908:Muscle Weakness / Q000235:genetics / Q000378:metabolism; D009133:Muscular Atrophy / Q000235:genetics / Q000378:metabolism*; D009135:Muscular Diseases / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D009218:Myosins / Q000235:genetics / Q000378:metabolism*; D020033:Protein Isoforms; D014335:Tropomyosin / Q000235:genetics* / Q000378:metabolism
chemical_list: D000199:Actins; D020033:Protein Isoforms; C498783:TPM3 protein, human; D014335:Tropomyosin; D009218:Myosins; D002118:Calcium