Heart failure treatment has historically undergone several paradigm shifts. Early pharmacological approaches included cardiac glycosides, which may be effective in relieving patients' symptoms, but carry a notoriously high risk of toxicity. In the 1980s, adrenergic agonists and phosphodiesterase inhibitors were introduced with great expectations, as it seemed intuitive to treat impaired contractility with positive inotropes.
However, these agents were observed to increase mortality as they induced maladaptive cardiac responses and exhausted the failing heart. Consequently, the mainstay of today’s treatment is based on inhibitors of the chronic neurohumoral activation in heart failure, such as β-adrenergic blockers and angiotensin converting enzyme (ACE) inhibitors. Unfortunately, these treatments often only provide heart failure patients with symptomatic relief and temporarily impede disease progression. Therefore, new treatment strategies are needed that target the basal pathogenic mechanisms of this disease.
SERCA2a contributes to Ca2+ removal from the cytosol by recycling Ca2+ into the SR. SERCA2a is thus an important regulator of diastolic function since, together with NCX, it sets the rate of Ca2+ transient decline and resting Ca2+ levels, and thereby the rate and extent of cardiomyocyte relaxation . SERCA2a function also contributes to control of systolic function, by regulating the SR Ca2+ load available for release. SERCA2a activity is primarily determined by cytosolic Ca2+ levels and its endogenous inhibitor phospholamban (PLB). Since Ca2+ is usually the rate limiting factor for SERCA2a activity, increased [Ca2+]i directly enhances SR Ca2+ pumping. PLB is a reversible inhibitor of SERCA2a, which acts by decreasing the Ca2+ affinity, but not the maximum pumping capacity (Vmax) of SERCA2a. Phosphorylation of PLB by protein kinase A (PKA), a cAMP- dependent kinase, relieves this inhibition.
SERCA2a in Heart Failure
In late 1980s, SERCA2a levels were reported to be reduced in animal models of hypertrophy and in 1990 Mercadier et al were first to report reduced mRNA levels of SERCA2a in human heart failure. The relation between SERCA2a levels and loss of contractile force in heart failure was later demonstrated by Hasenfuss et al . Numerous subsequent studies have confirmed the significance of reduced SERCA2a levels in heart failure pathogenesis. While some studies have reported reduced mRNA levels but unaltered protein levels different disease etiologies, stages and animal models employed may contribute to this discrepancy. Regional differences in SERCA expression have also recently been suggested to underlie these conflicting results.
While SERCA2a recycles Ca2+ into the SR, NCX is the main route for Ca2+ extrusion from the cardiomyocyte. NCX plays an important role in setting resting Ca2+ levels, and thus the extent of cardiomyocyte relaxation. However, since resting [Ca2+]i regulates SERCA2a activity, SR content, and Ca2+ transient magnitude, the net effect of altered NCX activity on the rate of Ca2+ decline can be difficult to predict, and may vary between species. Another mechanism of NCX regulation is the small inhibitory protein phospholemman. This is a member of the FXYD family of ion transport regulators and is found to co-localize with both NKA and NCX in heart. PKA and PKC phosphorylation modulate its inhibitory effect, but in a different manner in NCX compared to NKA; phospholemman phosphorylation relieves NKA inhibition but increases NCX inhibition. During phosphorylation, phospholemman therefore increases contractility, by inhibiting NCX and increasing [Ca2+]i, but also protects against Na+ overload by relieved inhibition of NKA.
As described above, increased NCX activity and reduced SERCA2a activity are considered to be important contributors to reduced SR Ca2+ content in heart failure. The third pathway by which SR Ca2+ content may be reduced is via increased RyR Ca2+ leakage. As with SERCA2a and NCX, RyR is also recognized as a therapeutic target. In order to understand its potential in therapy, we will first outline its function in normal and diseased hearts.
RyR is a large tetrameric protein localized to the SR membrane. RyR2, which is the major cardiac RyR isoform, acts as a scaffolding protein that associates with a number of proteins to form a macromolecular complex [127, 128]. This complex, which is important for RyR regulation and integrity, includes regulatory proteins such as protein kinase A, protein phosphatase 1 and 2a, calmodulin, calmodulin kinase II and phosphodiesterase 4D3 (PDE4D3) (Fig. ?4B4B). This structure allows tight control of RyR function via several phosphorylation sites, as well as Ca2+ activation and inactivation sites. The RyR2 binding protein FKBP12.6 (calstabin 2) stabilizes the tetrameric conformation.
As described in the introduction, the transverse tubules (t-tubules) are invaginations of the cellular membrane which form an organized, primarily transverse, network that enables the formation of dyadic junctions between the cellular membrane and the SR . T-tubules have been identified in ventricular cardiomyocytes in a wide variety of mammalian species (for a thorough review see ) and recent studies on larger mammals such as pig, sheep, cow, horse, and human [167-170] also found t-tubules in atrial myocytes (for review see ).
The majority of t-tubules lie in close proximity to the Z-lines of healthy ventricular cells [172-174]. However, in addition to the transverse elements of the t-tubule system, a smaller proportion of longitudinal elements extend between Z-lines [6, 175, 176]. T-tubule geometry and distribution vary between species, and it has been suggested that ventricular cells from smaller species (with higher heart rates) have thinner t-tubules but higher overall t-tubule densities
The past several decades of investigation have revealed that altered Ca2+ handling is as an important pathophysiological mechanism in heart failure, and that SERCA2a, NCX, RyR, and t-tubule structure are key players. Such insight has enabled the recent therapeutic targeting of these systems, resulting in attenuation or reversal of important facets of the heart failure phenotype, including mechanical dysfunction, arrhythmogenesis, and pathological signaling.
Although the majority of these data are from experimental studies, recent and ongoing clinical trials have shown promise. Since heart failure is a diverse disease entity, with variations in etiology, phenotype, stage and molecular basis, it is anticipated that future therapies based on cardiomyocyte mechanisms will have the potential to individualize treatment, and improve patient outcomes.