Maintaining cooperation among cardiac myofilament proteins through thick and thin

One of the potentially important but often unappreciated mechanisms in the control of cardiac function is that diastolic filling, the triggering and sustaining of developed pressure, and the dynamics of relaxation are likely to involve cooperative interactions among the sarcomeric proteins (Moss et al. 2004; Kobayashi & Solaro, 2005). Measures of the steady-state relation between Ca²⁺ and force generation by membrane-free (skinned) myocytes provide strong evidence for this cooperativity; Ca²⁺–force relations are steep with Hill coefficients commonly in the range of 3–5. This steep relation occurs despite the presence of a single regulatory Ca²⁺-binding site on troponin C (cTnC), the thin filament receptor triggering contraction. Theories for the mechanism of this steep dependence of force on Ca²⁺ are couched in terms of interactions among regulatory units (RU) of myofilament proteins (Tobacman & Sawyer, 1990; Moss et al. 2004; Kobayashi & Solaro, 2005). The RU consists of seven actins in a module with one each of cTnI, cTnT, cTnI, tropomyosin (Tm) and myosin. One theory is rooted in detailed balance, which dictates that the protein–protein interactions by which Ca²⁺ binds to cTnC not only promote the cross-bridge reaction with actin but, in turn, the reaction of the cross-bridge with actin promotes Ca²⁺ binding to cTnC. In this theory the affinity of the single regulatory Ca²⁺-binding site on cTnC is not a constant, but increases as cross-bridges bind thus giving rise to a steep Ca²⁺–force relation. A second theory is that RU–RU interactions are responsible for cooperative spread of activation. The idea is that there is a longitudinal propagation of activation along the thin filament from one RU to a near neighbour by end to end interactions between contiguous Tm strands or by actin–actin interactions. These RU–RU interactions have been hypothesized to be promoted by the following mechanisms: (1) an effect dependent on strongly bound cross-bridges that modify actin structure and move Tm further away from its blocking position than occurs by Ca²⁺ binding alone; and (2) an effect intrinsic to the thin filament that induces RU–RU interactions independent of the actin–myosin reaction (Tobacman & Sawyer, 1990). In this issue of The Journal of Physiology, Sun et al. (2009) provide evidence in support of the idea that the cooperative regulation of cardiac function under physiological conditions is at the level of processes intrinsic to the thin filaments, and that during pathological conditions in which there is a loss of MgATP there is a different cooperative mechanism involving the activation by strongly bound rigor cross-bridges. They provide an excellent discussion relating their findings to earlier studies. Their data support earlier studies demonstrating cooperative Ca²⁺ binding to thin filaments in reconstituted preparations (Tobacman & Sawyer, 1990) in the absence of myosin heads. They also report an effect, albeit relatively weak, of cycling, force-generating cross-bridges on cTnC structure.

There are important implications of the report by Sun et al. (2009) that relate to our understanding of physiological control of contractility as well as to our understanding of important disorders of the heart. There is a substantial body of evidence associating activation of the thin filaments by strongly bound cross-bridges with fundamental control mechanisms in the heart such as the Frank–Starling relation, rates of rise and fall of force, and stretch activation (Moss et al. 2004; Kobayashi & Solaro, 2005). These interpretations will now have to take into account the findings of Sun et al. with regard to the relative significance of activation of the thin filaments by force-generating cross-bridges.

Understanding the control of the relation between Ca²⁺ binding to thin filaments and function has emerged as a key element in the understanding of mechanisms by which familial cardiomyopathies lead to hypertrophy and sudden arrhythmic death. In the case of hypertrophic cardiomyopathies (HCM) there is substantial evidence that sensitization of the myofilaments to Ca²⁺ may be the trigger for the massive hypertrophy and eventual sudden death (Baudenbacher et al. 2008). Moreover, there is evidence that specific reversal of the enhanced Ca²⁺ sensitivity in animal models reverses the HCM phenotype (Jagatheesan et al. 2007). Detailed understanding of the cooperative processes described by Sun et al. and further follow-up studies may define an approach to reverse the enhanced sensitivity to Ca²⁺.

The findings by Sun et al. with regard to the effect of rigor cross-bridges on activation of the thin filaments are also important. Their data demonstrate a cooperative activation of thin filaments by strongly bound rigor cross-bridges that is different from that with cycling cross-bridges. Understanding the molecular mechanism of this mode of cooperative activation of the thin filaments is of particular importance to pathologies associated with ischaemia/reperfusion injury. When hearts are reperfused following a period of ischaemia and loss of ATP, rigor cross-bridge-induced activation may contribute significantly to the size of infarcts (Piper et al. 2004). Interventions that reduce this myosin-induced activation are likely to reduce the extent of post-ischaemic necrosis. The unique molecular mechanisms demonstrated by Sun et al. to occur with rigor cross-bridges may provide clues for specific interventions.