title: Effect of levosimendan on the contractility of muscle fibers from nemaline myopathy patients with mutations in the nebulin gene.
abstract: 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.
authors.affiliation: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center Amsterdam, De Boelelaan 1118, 1081, BT Amsterdam, The Netherlands.
authors.forename: Josine M
authors.initials: JM
authors.lastname: de Winter
authors.affiliation: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center Amsterdam, De Boelelaan 1118, 1081, BT Amsterdam, The Netherlands.
authors.forename: Barbara
authors.initials: B
authors.lastname: Joureau
authors.affiliation: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center Amsterdam, De Boelelaan 1118, 1081, BT Amsterdam, The Netherlands.
authors.forename: Vasco
authors.initials: V
authors.lastname: Sequeira
authors.affiliation: INMR, The Children's Hospital at Westmead and Discipline of Paediatrics & Child Health, University of Sydney, Cnr Hawkesbury Road & Hainsworth Street, Sydney, Australia.
authors.forename: Nigel F
authors.initials: NF
authors.lastname: Clarke
authors.affiliation: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center Amsterdam, De Boelelaan 1118, 1081, BT Amsterdam, The Netherlands.
authors.forename: Jolanda
authors.initials: J
authors.lastname: van der Velden
authors.affiliation: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center Amsterdam, De Boelelaan 1118, 1081, BT Amsterdam, The Netherlands ; Department of Physics and Astronomy, Faculty of Science, VU University, De Boelelaan 1105, Amsterdam, The Netherlands.
authors.forename: Ger Jm
authors.initials: GJ
authors.lastname: Stienen
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, 1333 N. Martin Avenue, Tucson, USA.
authors.forename: Henk
authors.initials: H
authors.lastname: Granzier
authors.affiliation: Division of Genetics and Genomics, The Manton Center for Orphan Disease Research, Boston Children's Hospital, Harvard Medical School, 25 Shattuck Street, Boston, USA.
authors.forename: Alan H
authors.initials: AH
authors.lastname: Beggs
authors.affiliation: Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center Amsterdam, De Boelelaan 1118, 1081, BT Amsterdam, The Netherlands ; Department of Cellular and Molecular Medicine, University of Arizona, 1333 N. Martin Avenue, Tucson, USA.
authors.forename: Coen Ac
authors.initials: CA
authors.lastname: Ottenheijm
keywords: Calcium-sensitizer; Levosimendan; Muscle force; Muscle mechanics; Nebulin; Nemaline myopathy