title: Microphthalmia-associated transcription factor is required for mature myotube formation.
abstract: BACKGROUND
  The roles of microphthalmia-associated transcription factor (Mitf) in the skeletal muscle and during myogenesis are unclear.
  
  
  METHODS
  Expression of Mitf in mouse tissues and during myogenesis was evaluated. Effects of Mitf knockdown on myogenesis and gene expression related to myogenesis were subsequently explored. Furthermore, effects of p21, a cyclin-dependent kinase inhibitor, and integrin α9 (Itga9) were examined.
  
  
  RESULTS
  Mitf was highly expressed in the skeletal muscle; Mitf-A and -J were expressed. Mitf expression increased after differentiation stimulation in C2C12 myogenic cells. Down-regulation of Mitf expression by transfection of siRNA for common Mitf inhibited myotube formation, which was reproduced by Mitf-A knockdown. Morphometric analyses indicated that both multinucleated cell number and the proportion of myotubes with more than 6 nuclei were decreased in Mitf-knockdown cells, suggesting that Mitf is required for not only the formation of nascent myotubes but also their maturation. Searching for genes positively regulated by Mitf revealed p21 and Itga9; decreasing Mitf expression inhibited up-regulation of p21 expression after differentiation stimulation and blocked the induction of Itga9 expression in response to differentiation. Knockdown of p21 decreased the number of multinucleated cells, whereas Itga9 knockdown did not affect the myotube number. Both p21 knockdown and Itga9 knockdown decreased the proportion of myotubes with more than 6 nuclei.
  
  
  CONCLUSIONS
  Mitf positively regulates skeletal muscle formation; Mitf is significantly expressed during myogenesis, and is required for efficient myotube formation through expression of p21 and Itga9.
authors.affiliation: Laboratory of Molecular Biology, Azabu University School of Veterinary Medicine, Sagamihara 252-5201, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Ooishi
authors.forename: Mitsuyuki
authors.initials: M
authors.lastname: Shirai
authors.forename: Masayuki
authors.initials: M
authors.lastname: Funaba
authors.forename: Masaru
authors.initials: M
authors.lastname: Murakami
mesh_terms: D000818:Animals; D002460:Cell Line; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism; D015536:Down-Regulation; D018507:Gene Expression Regulation, Developmental; D055785:Gene Knockdown Techniques; D016023:Integrins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D051739:Microphthalmia-Associated Transcription Factor / Q000235:genetics / Q000378:metabolism*; D024510:Muscle Development / Q000235:genetics; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000378:metabolism*; D032448:Myoblasts, Skeletal / Q000166:cytology / Q000378:metabolism; D009928:Organ Specificity / Q000235:genetics
chemical_list: D050759:Cyclin-Dependent Kinase Inhibitor p21; D016023:Integrins; D051739:Microphthalmia-Associated Transcription Factor