Molecular advances shed light on cardiac myosin heavy chain expression in health and disease

Myosin is the most abundant contractile protein, contained in all types of muscle tissue, and myosin heavy chain (MHC) constitutes the molecular motor for driving muscle contraction. Cardiac muscle expresses two MHC isoforms, designated as α and β, exhibiting high and low ATPase activity, respectively, which are encoded by two distinct genes located in close proximity on the same chromosome. The cardiac myosin phenotype is developmentally regulated, and the MHC ATPase activity plays a crucial role in the determination of the rate of cross-bridge cycling and consequently the speed of muscle contraction. Hence, throughout adult life, the fast αMHC expression predominates in a normal rodent heart, while βMHC expression gradually increases with ageing (Haddad et al. 2003). However, the pattern of cardiac MHC isoform expression is a highly dynamic process and can be altered at any time during life, in either direction, in response to hormonal and mechanical stimuli, thus affecting the intrinsic contraction properties of the heart. For instance, constrictive overload, diabetes, caloric restriction and hypothyroidism elicit a shift in expression of MHC isoforms, favouring a significantly higher proportion of βMHC isoform, followed by lower rates of contraction but a greater economy of force production (Giger et al. 2007). Conversely, thyroid hormone has emerged as a major regulator of MHC expression, and it is known that thyroid hormone administration exhibits opposite effects on MHC isoform expression by increasing the αMHC and down-regulating the βMHC isoform. A series of studies aimed at unravelling the precise molecular mechanisms responsible for the tightly co-ordinated switching of cardiac αMHC and βMHC isoforms have shown that the β–αMHC intergenic DNA possesses a bidirectional transcriptional activity, regulating α and β isoform expression in a well-co–ordinated opposite fashion in various mammals (Haddad et al. 2003, 2008; Giger et al. 2007). It has been proposed that this intergenic region plays a fundamental role in controlling the cardiac MHC gene locus. In particular, it has been shown that apart from the normal transcription of the αMHC and βMHC genes, a transcription of the opposite strand occurs as well, giving rise to an antisense RNA that starts in the intergenic region between the β and α genes and extends to overlap with βMHC gene. Giger et al. (2007) recently suggested the existence of an intergenic promoter that may control transcription of both sense α and antisense β genes, with antisense β transcription exerting an inhibitory effect on βMHC gene expression. In a report appearing in this issue of Experimental Physiology, Giger et al. (2009) employed modern technical advancements and nicely addressed an important molecular mechanism of cardiac muscle physiology, providing new insights on the regulation of the βMHC antisense promoter. In order to gain a better understanding of the molecular mechanisms that regulate cardiac MHC gene expression, Giger et al. (2009) examined βMHC antisense promoter regulation using electrophoretic mobility shift analyses, shedding light on the regulatory elements that can influence the activity of this bidirectional promoter and confer responsiveness to diabetes and thyroid hormone manipulations. These authors elegantly show that ventricular proteins from rodent heart interact with a nuclear factor 1 binding site in the antisense promoter sequence, a promoter element which has previously been shown also to regulate rat smooth muscle MHC gene transcription, while a mutation of this binding site abolishes promoter activity in vivo in healthy, diabetic and hypothyroid animals (Giger et al. 2009). Thus, when the intergenic bidirectional promoter is activated, as is the case in adult euthyroid and hyperthyroid rat hearts, the transcription of both α sense and β antisense genes is induced, whereas the transcription of the βMHC gene is inhibited via an as yet not well-defined mechanism. The importance of this finding originates in the fact that a mutation of only two base pairs in the nuclear factor 1 site abolishes promoter activity, indicating that this binding site is critical for gene activation. From the study by Giger et al. (2009) it becomes evident that the characterization of the molecular mechanisms that control cardiac MHC isoform expression is of vital interest and may become the milestone for therapeutic interventions aiming at controlling gene transcription and altering cardiac myosin gene expression. By modifying the expression of βMHC antisense RNA, one will be allowed to co-ordinate and elicit simultaneous shifts in phenotype that take place as the cardiac muscle adapts to altered functional demands or pathological changes. In addition, the ability to slightly increase αMHC expression in failing hearts could be beneficial in terms of improving the cardiac contractility and power output, which should have a significant functional impact. For instance, a direct relation has been established between improvements of left ventricular function in humans with dilated cardiac myopathy and increases in αMHC expression (Abraham et al. 2002). Giger et al. (2009) illustrate the importance of the nuclear factor 1 binding site in the regulation of MHC antisense promoter.