title: Activity-dependent repression of muscle genes by NFAT.
abstract: Adult skeletal muscles retain an adaptive capacity to switch between slow- and fast-twitch properties that largely depend on motoneuron activity. The NFAT (nuclear factor of activated T cells) family of calcium-dependent transcription factors has been implicated in the up-regulation of genes encoding slow contractile proteins in response to slow-patterned motoneuron depolarization. Here, we demonstrate an unexpected, novel function of NFATc1 in slow-twitch muscles. Using the troponin I fast (TnIf) intronic regulatory element (FIRE), we identified sequences that down-regulate its function selectively in response to patterns of electrical activity that mimic slow motoneuron firing. A bona fide NFAT binding site in the TnIf FIRE was identified by site-directed mutations and by electrophoretic mobility and supershift assays. The activity-dependent transcriptional repression of FIRE is mediated through this NFAT site and, importantly, its mutation did not alter the up-regulation of TnIf transcription by fast-patterned activity. siRNA-mediated knockdown of NFATc1 in adult muscles resulted in ectopic activation of the FIRE in the slow soleus, without affecting enhancer activity in the fast extensor digitorum longus muscle. These findings demonstrate that NFAT can function as a repressor of fast contractile genes in slow muscles and they exemplify how an activity pattern can increase or decrease the expression of distinct contractile genes in a use-dependent manner as to enhance phenotypic differences among fiber types or induce adaptive changes in adult muscles.
authors.affiliation: Section on Molecular Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892.
authors.forename: Zaheer A
authors.initials: ZA
authors.lastname: Rana
authors.forename: Kristian
authors.initials: K
authors.lastname: Gundersen
authors.forename: Andres
authors.initials: A
authors.lastname: Buonanno
mesh_terms: D000818:Animals; D001665:Binding Sites; D005786:Gene Expression Regulation / Q000502:physiology*; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism*; D050778:NFATC Transcription Factors / Q000235:genetics* / Q000502:physiology*; D034741:RNA, Small Interfering / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D014158:Transcription, Genetic
chemical_list: D050778:NFATC Transcription Factors; D034741:RNA, Small Interfering