title: Muscle-specific overexpression of IGF-I improves E-C coupling in skeletal muscle fibers from dystrophic mdx mice.
abstract: Duchenne muscular dystrophy (DMD) is a lethal X-linked disease caused by the absence of functional dystrophin. Abnormal excitation-contraction (E-C) coupling has been reported in dystrophic muscle fibers from mdx mice, and alterations in E-C coupling components may occur as a direct result of dystrophin deficiency. We hypothesized that muscle-specific overexpression of insulin-growth factor-1 (IGF-I) would reduce E-C coupling failure in mdx muscle. Mechanically skinned extensor digitorum longus muscle fibers from mdx mice displayed a faster decline in depolarization-induced force responses (DIFR); however, there were no differences in sarcoplasmic reticulum (SR)-mediated Ca(2+) resequestration or in the properties of the contractile apparatus when compared with nondystrophic controls. The rate of DIFR decline was restored to control levels in fibers from transgenic mdx mice that overexpressed IGF-I in skeletal muscle (mdx/IGF-I mice). Dystrophic muscles have a lower transcript level of a specific dihydropyridine receptor (DHPR) isoform, and IGF-I-mediated changes in E-C coupling were associated with increased transcript levels of specific DHPR isoforms involved in Ca(2+) regulation. Importantly, IGF-I overexpression also increased the sensitivity of the contractile apparatus to Ca(2+). The results demonstrate that IGF-I can ameliorate fundamental aspects of E-C coupling failure in dystrophic muscle fibers and that these effects are important for the improvements in cellular function induced by this growth factor.
authors.affiliation: Basic and Clinical Myology Laboratory, Department of Physiology, The University of Melbourne, Victoria, Australia.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Schertzer
authors.forename: Chris
authors.initials: C
authors.lastname: van der Poel
authors.forename: Thea
authors.initials: T
authors.lastname: Shavlakadze
authors.forename: Miranda D
authors.initials: MD
authors.lastname: Grounds
authors.forename: Gordon S
authors.initials: GS
authors.lastname: Lynch
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000235:genetics / Q000378:metabolism; D004195:Disease Models, Animal; D007334:Insulin-Like Growth Factor I / Q000235:genetics / Q000378:metabolism*; D008297:Male; D051379:Mice; D018101:Mice, Inbred mdx; D008822:Mice, Transgenic; D009119:Muscle Contraction; D018485:Muscle Fibers, Skeletal / Q000378:metabolism*; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000503:physiopathology; D020388:Muscular Dystrophy, Duchenne / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology; D020033:Protein Isoforms / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013997:Time Factors
chemical_list: D020746:Calcium Channels, L-Type; D020033:Protein Isoforms; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; D002118:Calcium