Cytoskeletal Tropomyosin Tm5NM1 Is Required for Normal Excitation-contraction Coupling in Skeletal Muscle

Nicole Vlahovich,* ${dagger}$ ${ddagger}$ ${sect}$ Anthony J. Kee,* ${ddagger}$ || Chris Van der Poel,¶ Emma Kettle,* Delia Hernandez-Deviez,# Christine Lucas,*@ Gordon S. Lynch,¶ Robert G. Parton,# Peter W. Gunning,||@ ${dagger}$ ${dagger}$ and Edna C. Hardeman* ${ddagger}$ ${ddagger}$

^*Muscle Development Unit, Children’s Medical Research Institute, Westmead, NSW 2145, Australia; University of Western Sydney, Australia; ^||Faculty of Medicine, University of Sydney, Sydney, NSW 2006, Australia; ^¶Department of Physiology, University of Melbourne, Australia; ^#Institute for Molecular Biosciences, University of Queensland and Centre for Microscopy and Microanalysis, Brisbane, QLD, Australia; ^@Oncology Research Unit, The Children’s Hospital at Westmead, Westmead, NSW 2145, Australia; Department of Pharmacology, School of Medical Sciences, University of New South Wales, Sydney, NSW 2052, Australia; Department of Anatomy, School of Medical Sciences, University of New South Wales, Sydney, NSW 2052, Australia
^*Muscle Development Unit, Children’s Medical Research Institute, Westmead, NSW 2145, Australia; University of Western Sydney, Australia; ^||Faculty of Medicine, University of Sydney, Sydney, NSW 2006 Australia; ^¶Department of Physiology, University of Melbourne, Australia; ^#Institute for Molecular Biosciences, University of Queensland and Centre for Microscopy and Microanalysis, Brisbane, QLD, Australia; ^@Oncology Research Unit, The Children’s Hospital at Westmead, Westmead, NSW 2145, Australia; Department of Pharmacology, School of Medical Sciences, University of New South Wales, Sydney, NSW 2052, Australia; Department of Anatomy, School of Medical Sciences, University of New South Wales, Sydney, NSW 2052, Australia

Monitoring Editor: Thomas D. Pollard

The functional diversityof the actin microfilaments relies in part on the actin bindingprotein, tropomyosin (Tm). The muscle-specific Tms regulateactin-myosin interactions and hence contraction. However, thereis less known about the roles of the numerous cytoskeletal isoforms.We have shown that a cytoskeletal Tm, Tm5NM1, defines a Z-lineadjacent cytoskeleton in skeletal muscle (Kee et al., 2004).Recently we identified a second cytoskeletal Tm in this region,Tm4 (Vlahovich et al., 2008). Here we show that Tm4 and Tm5NM1define separate actin filaments; the former associated withthe terminal sarcoplasmic reticulum (SR) and other tubulovesicularstructures. In skeletal muscles of Tm5NM1 knockout (KO) mice,Tm4 localization was unchanged demonstrating the specificityof the membrane association. Tm5NM1 KO muscles exhibit potentiationof T-system depolarization and decreased force rundown withrepeated T-tubule depolarizations consistent with altered T-tubulefunction. These results indicate that a Tm5NM1-defined actincytoskeleton is required for the normal excitation-contractioncoupling in skeletal muscle.

These authors contributed equally to this work.

Present address: The Burnham Institute for Medical Research, 10901 North Torrey Pines Road, La Jolla, CA 92037.

Address correspondence to: Edna C. Hardeman (e.hardeman{at}unsw.edu.au)