Acute INDUCTION OF Transcription Factors in Fast-Twitch Muscle DURING CHRONIC SCIATIC NERVE STIMULATION

Abstract

The molecular signals whereby contractile activity induces physiologic and biochemical adaptations in muscles are not fully understood. PURPOSE: To identify transcription factors affected in fast twitch muscle (extensor digitorum longus; EDL) subjected to extrinsic slow motor unit stimulation. METHODS: Adult male Swiss-Webster mice (n=4) were anesthetized and the sciatic nerve of one leg exposed and stimulated electrically at 10 Hz for up to 4 hours through implanted Pt electrodes. The non-stimulated leg served as a paired control. At the end of each experiment, both muscles were flash-frozen for RNA extraction and microarray analysis (GE: CodeLink Mouse 10k Uniset). Paired comparisons corrected for multiple testing were employed to detect differentially expressed genes which were classified into functional groups using NIAID:DAVID. RESULTS: A 50% decrease in peak force and tension time integral occurred within 10 min of stimulation onset and persisted for the entire time course, however time to peak force and half relaxation times remained constant. Stimulation increased gene expression in 78% of the differentially expressed genes detected in the stimulus vs. contralateral control EDL. Significant changes in transcript levels associated with fast to slow phenotype transformation were present including increased troponin C-slow, laminin-beta 3, and tropomyosin 3-γ and decreased myosin heavy chain 2A fast, creatine kinase, myosin light chain-fast, and troponin I fast 2. Transcripts associated with induction and regulation of transcription were statistically overrepresented (vizXlabs: GeneSifter) above other functional classes with 65 of 70 (625 assayed) increased in the stimulated EDL. Specifically, PPARGC1-α (8.4-fold), PPARGC1-β (2.8-fold), PPAR-γ (2.5-fold) and HIF1-α (3.5-fold) were significantly elevated after stimulation although present in the unstimulated leg. Twenty-nine additional transcription factors detected at or below threshold levels in the unstimulated EDL were induced to significant levels in the stimulated EDL. CONCLUSIONS: Our results suggest that 10 Hz short-term stimulation activates both latent and constitutive transcription factors as part of a broad molecular response associated with phenotype modification.