Dynamics of Myosin Heavy Chain Gene Regulation in Slow Skeletal Muscle

Abstract

The evolutionarily conserved order of the skeletal muscle myosin heavy chain (MHC) genes and their close tandem proximity on the same chromosome are intriguing and may be important for their coordinated regulation. We investigated type II MHC gene regulation in slow-type muscle fibers undergoing a slow to fast MHC transformation in response to inactivity, 7 days after spinal cord isolation (SI) in rats. We examined the transcriptional products of both the sense and antisense strands across the IIa-IIx-IIb MHC gene locus. A strand-specific reverse transcription (RT)-PCR approach was utilized to study the expression of the mRNA, the primary transcript (pre-mRNA), the antisense RNA overlapping the MHC genes, and both the intergenic sense and antisense RNAs. Results showed that the mRNA and pre-mRNA of each MHC had a similar response to SI, suggesting regulation of these genes at the transcriptional level. In addition, we detected previously unknown antisense strand transcription that produced natural antisense transcripts (NATs). RT-PCR mapping of the RNA products revealed that the antisense activity resulted in the formation of three major products: aII, xII, and bII NATs (antisense products of the IIa, IIx, and IIb genes, respectively). The aII NAT begins in the IIa-IIx intergenic region in close proximity to the IIx promoter, extends across the 27-kb IIa MHC gene, and continues to the IIa MHC gene promoter. The expression of the aII NAT was significantly up-regulated in muscles after SI, was negatively correlated with IIa MHC gene expression, and was positively correlated with IIx MHC gene expression. The exact role of the aII NAT is not clear; however, it is consistent with the inhibition of IIa MHC gene transcription. In conclusion, NATs may mediate cross-talk between adjacent genes, which may be essential to the coordinated regulation of the skeletal muscle MHC genes during dynamic phenotype shifts.Skeletal muscle is highly adaptable when subjected to altered loading and hormone states. Its size, metabolic makeup, and contractile properties can all be altered to optimize function (1). Variability in contractile properties is achieved mainly by diversification in the motor protein myosin heavy chain (MHC), 2 where different isoforms are encoded by distinct genes (1, 2). Of this family of eight MHC genes, six are tandemly linked and span ϳ420 kb in the rat on chromosome 10, with embryonic MHC situated at the most 5Ј end, sequentially followed by IIa, IIx, IIb, neonatal, and extraocular MHC. ␤ (or type I) and ␣ MHCs are located tandemly on separate chromosomes (chromosome 14 in the rat); they span ϳ50 kb and are separated by 4.5 kb. Interestingly, the genomic order and orientation on the chromosomes of the MHC genes are conserved in all mammalian species, leading researchers to suspect that this organization might be an important feature in the strategy for the coordinated regulation of these genes (2-5).Types I, IIa, IIx, and IIb, in respective order of increasing ATPase activity, are the four predominately exp...

Regeneration of Reinnervated Rat Soleus Muscle Is Accompanied by Fiber Transition Toward a Faster Phenotype

“…In the present study, however, we performed a more detailed analysis of the slow, hybrid, and fast fibers at both immunohistochemical and mRNA levels, examining also the size (atrophy state) of the different fiber populations, because these data are necessary for judging the reinnervation level of fibers before the induction of regeneration. Our new results, namely the significantly high number of I/II hybrids, as well as the increase in the number of both MHCIIA-or MHCIIX-expressing fibers, accompanied by respective changes in their mRNA levels, suggest a gradual slow-to-fast fiber switch in reinnervated soleus, which is regulated predominantly at the mRNA level (Schiaffino and Reggiani 1996;Baldwin and Haddad 2001;Pandorf et al 2006).…”

Factors controlling cardiac myosin-isoform shift during hypertrophy and heart failure

“…Although the exact role of this anti-sense transcript is not yet understood, it has been found to be associated with MHC isoform switching in the heart in response to pathologic stimuli, such as hemodynamic overload and diabetes [35][36][37]. A similar anti-sense mediated regulation of the MHC genes has also been suggested for isoform switching in slow skeletal muscle fibers [38].…”

Regulation of an antisense RNA with the transition of neonatal to IIb myosin heavy chain during postnatal development and hypothyroidism in rat skeletal muscle

“…The genomic order, relative spacing, and the coding sequence are highly conserved across a broad evolutionary span. We have previously shown that functional significance for this gene order and intergenic spacing is apparent by the transcriptional coordination of MHC genes with long noncoding RNA transcripts that are transcribed in the antisense orientation to the IIa, IIx, and IIb MHC genes during alterations to muscle loading state (42,47). For example, muscle inactivity and unloading induce a coordinated IIa to IIx MHC shift in the soleus muscle whereby IIa decreases while IIx increases concurrently.…”

“…For example, muscle inactivity and unloading induce a coordinated IIa to IIx MHC shift in the soleus muscle whereby IIa decreases while IIx increases concurrently. Along with these shifts, an antisense transcript is also induced, the so-called aII natural antisense transcript (NAT), which is transcribed in reverse orientation across the entire length of the IIa MHC gene (42). The aII NAT appears to repress IIa MHC gene transcription (42).…”

“…Along with these shifts, an antisense transcript is also induced, the so-called aII natural antisense transcript (NAT), which is transcribed in reverse orientation across the entire length of the IIa MHC gene (42). The aII NAT appears to repress IIa MHC gene transcription (42). Sense-antisense pairs were also observed to regulate IIx and IIb MHC in both fast and slow skeletal muscle and in cardiac muscle in coordination of ␣-and ␤-MHC expression (18,20,42,47).…”