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Too much of a good thing? Establishing a role of excessive RyR2 dephosphorylation in heart disease

The rhythmic contraction of the heart is due to the exquisitely controlled movement of calcium into each cardiac contractile cell. The bulk of this calcium is released from intracellular stores within individual cells through a calcium channel, the ryanodine receptor (RyR2). Accordingly, increasing the magnitude of calcium flux through RyR2 is a critical element in increasing the force of contraction and consequently the amount of blood ejected from the heart per beat. This makes RyR2 a key target of the sympathetic nervous system during the fight or flight response, the physiological response to acute stress and exercise. However, paradoxically, the very pathway that results in increased cardiac output during exercise can also lead to severely reduced output in multiple forms of heart disease. Consistent with other protein targets, an increase in sympathetic activity results in the phosphorylation of RyR2, which increases its opening, thus augmenting calcium release. Acutely, this is beneficial as more calcium enters the cell per beat, increasing contractility. However, in many forms of heart disease, this phosphorylation becomes excessive (Dobrev & Wehrens, 2014). Under these circumstances, the probability of RyR2 opening becomes so great that calcium is continuously released at low levels. This persistent calcium leak depletes the intracellular stores, resulting in a diminished release per beat, ultimately reducing contractile force and the amount of blood ejected – as observed in heart disease and failure. The persistent leak of calcium between beats is also an important driver of arrhythmias (Jones et al. 2017). Consequently, the level of RyR2 phosphorylation must be carefully controlled to prevent cardiac dysfunction. In terms of RyR2 phosphorylation, protein kinase A and calmodulin-dependent kinase II dominate, with the specificity of each enzyme largely defined for each of the three major phosphorylation sites. While the phosphorylation of RyR2 has been extensively researched, with studies dating back over two decades, it is only more recently that a greater emphasis has been placed on the proteins responsible for restoring RyR2 to its dephosphorylated form (Huke & Bers, 2008). As the study of the balance between phosphorylation and dephosphorylation has progressed, it has become apparent that the effect on calcium release though RyR2 is non-linear. It is now generally accepted that the function of RyR2 has a Vor U-shaped relationship where RyR2 activity is highest at very low or very high phosphorylation levels (Bovo et al. 2017). While the primary kinases implicated in phosphorylation of RyR2 have been well established, the nature of the specific phosphatase(s) responsible for dephosphorylating each site in vivo, until now, was less clear. In this issue of the Journal of Physiology, Potenza et al. (2020) establish protein phosphatase 1 (PP-1) as an endogenous regulator of RyR2 phosphorylation at one of these key sites. They additionally demonstrate that excessive dephosphorylation of RyR2 by PP-1, much like excessive phosphorylation, results in enhanced calcium leak, further supporting the idea that RyR2 requires an intermediate level of phosphorylation to function correctly. Despite the authors demonstrating a clear role of PP-1 in modulating the function of RyR2, it remains unclear as to the overall role of the enzyme in disease. Although this current work adds to the evidence that too little phosphorylation of RyR2 results in calcium leak, making targeting PP-1 activity a compelling treatment option, whether hypo-phosphorylation of RyR2 occurs in disease remains to be determined. As is well discussed by the authors, although an increase in PP-1 expression and activity is frequently observed in the failing heart, there is almost a universal finding of hyper-phosphorylation of RyR2 at all three major sites (Dobrev & Wehrens, 2014). This suggests an uncoupling of PP-1 expression and activity toward RyR2 and leaves the clinical relevance of increased PP-1 expression unclear. However, a current lack of implication of RyR2 hypo-phosphorylation in a specific disease phenotype does not detract from the data indicating a specific role of PP-1 in modulating RyR2 function. It is widely acknowledged that the exact mechanism underlying the roles of specific kinases and phosphorylation sites on RyR2 function in cardiac disease remains controversial (Dobrev & Wehrens, 2014). This theme appears to also be maintained in the context of RyR2 dephosphorylation and the role of protein phosphatases. As described above, Potenza et al. (2020) convincingly demonstrate that excessive RyR2 dephosphorylation by PP-1 can acutely enhance calcium leak in wild-type mice, which is in contrast to previous reports of a protective role of increased PP-1 activity by reducing RyR2-mediated calcium leak and arrhythmias in a setting of heart failure (Fischer et al. 2018). Rather than lessening the impact of these opposing findings, this highlights the need to further understand the intricacy of RyR2 dephosphorylation as a mechanism contributing to calcium dysfunction in cardiac disease. Looking forward, it will be important to explore the interplay between PP-1 and the kinases throughout the progression of disease and determine when PP-1 may have beneficial or detrimental effects on RyR2 function. No doubt the work of Potenza et al. (2020) will further fuel the efforts of researchers to consolidate the complexities of the relationship between RyR2 function and its phosphorylation state in cardiac pathophysiology.