title: Genetic loss of calcineurin blocks mechanical overload-induced skeletal muscle fiber type switching but not hypertrophy.
abstract: The serine/threonine phosphatase calcineurin is an important regulator of calcium-activated intracellular responses in eukaryotic cells. In higher eukaryotes, calcium/calmodulin-mediated activation of calcineurin facilitates direct dephosphorylation and nuclear translocation of the transcription factor nuclear factor of activated T-cells (NFAT). Recently, controversy has surrounded the role of calcineurin in mediating skeletal muscle cell hypertrophy. Here we examined the ability of calcineurin-deficient mice to undergo skeletal muscle hypertrophic growth following mechanical overload (MOV) stimulation or insulin-like growth factor-1 (IGF-1) stimulation. Two distinct models of calcineurin deficiency were employed: calcineurin Abeta gene-targeted mice, which show a approximately 50% reduction in total calcineurin, and calcineurin B1-LoxP-targeted mice crossed with a myosin light chain 1f cre knock-in allele, which show a greater than 80% loss of total calcineurin only in skeletal muscle. Calcineurin Abeta-/- and calcineurin B1-LoxP(fl/fl)-MLC-cre mice show essentially no defects in muscle growth in response to IGF-1 treatment or MOV stimulation, although calcineurin Abeta-/- mice show a basal defect in total fiber number in the plantaris and a mild secondary reduction in growth, consistent with a developmental defect in myogenesis. Both groups of gene-targeted mice show normal increases in Akt activation following MOV or IGF-1 stimulation. However, overload-mediated fiber-type switching was dramatically impaired in calcineurin B1-LoxP(fl/fl)-MLC-cre mice. NFAT-luciferase reporter transgenic mice failed to show a correlation between IGF-1- or MOV-induced hypertrophy and calcineurin-NFAT-dependent signaling in vivo. We conclude that calcineurin expression is important during myogenesis and fiber-type switching, but not for muscle growth in response to hypertrophic stimuli.
authors.affiliation: Department of Pediatrics, University of Cincinnati, Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio 45229-3039, USA.
authors.forename: Stephanie A
authors.initials: SA
authors.lastname: Parsons
authors.forename: Douglas P
authors.initials: DP
authors.lastname: Millay
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Wilkins
authors.forename: Orlando F
authors.initials: OF
authors.lastname: Bueno
authors.forename: Gretchen L
authors.initials: GL
authors.lastname: Tsika
authors.forename: Joel R
authors.initials: JR
authors.lastname: Neilson
authors.forename: Christine M
authors.initials: CM
authors.lastname: Liberatore
authors.forename: Katherine E
authors.initials: KE
authors.lastname: Yutzey
authors.forename: Gerald R
authors.initials: GR
authors.lastname: Crabtree
authors.forename: Richard W
authors.initials: RW
authors.lastname: Tsika
authors.forename: Jeffery D
authors.initials: JD
authors.lastname: Molkentin
mesh_terms: D000483:Alleles; D000818:Animals; D015153:Blotting, Western; D019703:Calcineurin / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D002455:Cell Division; D003433:Crosses, Genetic; D017930:Genes, Reporter; D013006:Growth Hormone / Q000378:metabolism; D006984:Hypertrophy; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D008156:Luciferases / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D019076:Transgenes
chemical_list: D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D008156:Luciferases; D019703:Calcineurin; D010100:Oxygen