Functional overload increases β-MHC promoter activity in rodent fast muscle via the proximal MCAT (βe3) site

Abstract

. Functional overload increases ␤-MHC promoter activity in rodent fast muscle via the proximal MCAT (␤e3) site. Am J Physiol Cell Physiol 282: C518-C527, 2002. First published October 24, 2001 10.1152/ ajpcell.00444.2001 of the rat plantaris muscle by the removal of synergistic muscles induces a shift in the myosin heavy chain (MHC) isoform expression profile from the fast isoforms toward the slow type I, or, ␤-MHC isoform. Different length rat ␤-MHC promoters were linked to a firefly luciferase reporter gene and injected in control and OL plantaris muscles. Reporter activities of Ϫ3,500, Ϫ914, Ϫ408, and Ϫ215 bp promoters increased in response to 1 wk of OL. The smallest Ϫ171 bp promoter was not responsive to OL. Mutation analyses of putative regulatory elements within the Ϫ171 and Ϫ408 bp region were performed. The Ϫ408 bp promoters containing mutations of the ␤e1, distal muscle CAT (MCAT; ␤e2), CACC, or A/T-rich (GATA), were still responsive to OL. Only the proximal MCAT (␤e3) mutation abolished the OL response. Gel mobility shift assays revealed a significantly higher level of complex formation of the ␤e3 probe with nuclear protein from OL plantaris compared with control plantaris. These results suggest that the ␤e3 site functions as a putative OL-responsive element in the rat ␤-MHC gene promoter. gel mobility shift assay; plantaris muscle; direct gene transfer; dual luciferase; ␤-myosin heavy chain SKELETAL MUSCLE FIBERS have the potential to adapt their phenotypic properties to meet different functional demands. The ␤-myosin heavy chain (MHC) isoform is typically expressed in "slow" muscles, which are characterized by a relatively slow speed of shortening but exhibit fatigue resistance and thus are able to maintain contractile activity for long periods of time. They typically function as weight-bearing and postural muscles. In the soleus, a slow muscle in the hindlimb, ϳ90% of fibers express the ␤-form of MHC (13, 16). The relationship between weight-bearing activity and the expression of ␤-MHC in soleus muscle is demonstrated by removing the factor of load in experimental animal models. This is achieved by subjecting the animal to either the microgravity environment of spaceflight (14) or through the ground-based model of hindlimb suspension (4) such that the leg extensor muscles bear little or no weight. When these manipulations are imposed, the expression of ␤-MHC is significantly reduced along with a shift toward increased expression of faster MHC isoforms (4,14).At the other extreme, functional overload induces the expression of ␤-MHC in the plantaris, a fast-twitch ankle extensor muscle that normally expresses the fast MHC isoforms, IIb and IIx, which account for Ͼ80% of the MHC pool (28). In this model, the overload stress is induced by the removal of synergistic muscles along with normal weight-bearing activity, which collectively causes compensatory hypertrophy of muscle fibers (1,26). Adaptation to the overload stimulus also prompts a shift in the fiber-type profile toward a slow fiber-type patte...

IIx myosin heavy chain promoter regulation cannot be characterized in vivo by direct gene transfer

“…Also, to rule out the possibility of interresearcher differences in technique or reagent variances, previous experiments of promoters that gave us high activity and were intervention responsive (e.g., type I MHC, myosin light chain kinase) were repeated in the process of collecting the data reported in this report (data not shown). These experiments resulted in responses similar to those reported previously (11,12,15) and thus confirmed the reliability and validity of the methods utilized here.…”

“…To improve uptake of plasmids in the plantaris muscle, electropermeabilization was induced on the plantaris muscle immediately after plasmid injection. The electroporation procedure was applied with two gold-plated stainless steel electrodes delivered in four pulses of 200 V/cm for 50 ms each at 1 Hz, followed by four additional pulses of opposite polarity, as described previously (12). All experiments were carried out for 7 days, which was shown in prior pilot experiments to be within the time period of maximum promoter-reporter expression.…”

“…In a separate experiment we validated that the luciferase activity of the pRL-null vector is not sensitive to the Den stimulus. Other candidate reference vectors (e.g., cytomegalovirus, SV40, thymidine kinase) have been deemed by us (11,12) and others (17,18) to be invalid choices for the types of experiments carried out here.…”

MCAT Elements and the TEF-1 Family of Transcription Factors in Muscle Development and Disease

“…7 Mutational analyses have revealed that functional overloadinduced activity of the ␤-MHC gene is mediated through the proximal MCAT element in rat plantaris muscle. 48 Moreover, denervation-induced decrease in ␤-MHC expression is also mediated by the proximal MCAT element in rat soleus. 49 Taken together, these results clearly indicate the contribution of the MCAT element to the regulation of muscle-specific genes in skeletal muscle.…”

Transcriptional regulation of IGF-I expression in skeletal muscle

“…The inability of these IGF-I promoter regions to drive more robust expression in vivo was not due to low levels of plasmid uptake, because a coinjected skeletal muscle ␣-actin promoter expressing the RLuc gene showed very high activity (ϳ400-fold higher than background) and increased by ϳ50% in FO plantaris. In addition, Giger et al (15) used the direct gene transfer approach to study regulation of the type I MHC promoter in overloaded skeletal muscle and reported an ϳ100-130% increase in the type I MHC promoter activity in FO plantaris.…”