title: Identification of Map4k4 as a novel suppressor of skeletal muscle differentiation.
abstract: Myoblast differentiation into mature myotubes is a critical step in the development and repair of human skeletal muscle. Here we show that small interfering RNA (siRNA)-based silencing of the Ste20-like mitogen-activated protein 4 kinase 4 (Map4k4) in C2C12 myoblasts markedly enhances expression of myogenic differentiation genes, myoblast fusion, and myotube diameter. In contrast, adenovirus-mediated expression of native Map4k4 in C2C12 cells attenuates each of these processes, indicating that Map4k4 is a negative regulator of myogenic differentiation and hypertrophy. Expression of a Map4k4 kinase-inactive mutant enhances myotube formation, suggesting that the kinase activity of Map4k4 is essential for its inhibition of muscle differentiation. Map4k4 regulation of myogenesis is unlikely to be mediated by classic mitogen-activated protein kinase (MAPK) signaling pathways, because no significant difference in phosphorylation of extracellular signal-regulated kinase (ERK), p38, or c-Jun N-terminal kinase (JNK) is observed in Map4k4-silenced cells. Furthermore, silencing of these other MAPKs does not result in a hypertrophic myotube phenotype like that seen with Map4k4 depletion. Uniquely, Map4k4 silencing upregulates the expression of the myogenic regulatory factor Myf5, whose depletion inhibits myogenesis. Furthermore, Myf5 is required for enhancement of myotube formation in Map4k4-silenced cells, while Myf5 overexpression rescues Map4k4-mediated inhibition of myogenic differentiation. These results demonstrate that Map4k4 is a novel suppressor of skeletal muscle differentiation, acting through a Myf5-dependent mechanism.
authors.affiliation: Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, Massachusetts, USA.
authors.forename: Mengxi
authors.initials: M
authors.lastname: Wang
authors.forename: Shinya U
authors.initials: SU
authors.lastname: Amano
authors.forename: Rachel J Roth
authors.initials: RJ
authors.lastname: Flach
authors.forename: Anil
authors.initials: A
authors.lastname: Chawla
authors.forename: Myriam
authors.initials: M
authors.lastname: Aouadi
authors.forename: Michael P
authors.initials: MP
authors.lastname: Czech
mesh_terms: D000818:Animals; D002454:Cell Differentiation; D002460:Cell Line; D018507:Gene Expression Regulation, Developmental; D020935:MAP Kinase Signaling System; D051379:Mice; D008810:Mice, Inbred C57BL; D024510:Muscle Development; D018485:Muscle Fibers, Skeletal / Q000166:cytology* / Q000378:metabolism; D032446:Myoblasts / Q000166:cytology* / Q000378:metabolism; D051787:Myogenic Regulatory Factor 5 / Q000235:genetics / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000235:genetics* / Q000378:metabolism*; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000235:genetics; D015854:Up-Regulation; D000097782:NF-kappaB-Inducing Kinase
chemical_list: D051787:Myogenic Regulatory Factor 5; D017346:Protein Serine-Threonine Kinases; D034741:RNA, Small Interfering; D000097782:NF-kappaB-Inducing Kinase; C497418:Myf5 protein, mouse