Mechanisms regulating Ca2+ release in cardiomyocytes

This editorial refers to ‘Sensitized signalling between L-type Ca2+ channels and ryanodine receptors in the absence or inhibition of FKBP12.6 in cardiomyocytes’ by Y.-T. Zhao et al., pp. 332–342. A schematic showing mechanisms regulating Ca2+ release in cardiac cells. (A) Common pool regeneration. A single LCC gates a single RyR, which in turn gates adjacent RyRs. This leads to propagated release of all CRUs. (B) Cluster bomb model that leads to local control. RyRs and LCCs are arranged in geometrically separated clusters (CRUs). Ca2+ entering the cell through an LCC gates one or a number of RyRs in the CRU so that all RyRs in the CRU are activated and release Ca2+. The CRUs are sufficiently separated that they do not activate one another. These CRUs can be recruited stochastically which can lead to graded responses. (C) Cluster bomb models with and without FKBP12.6 which reduces the sensitivity of RyRs to Ca2+. Left side of schematic: in the presence of FKBP12.6 β-adrenergic stimulation does not necessarily lead to regenerative activation of all CRUs. Local control is maintained. Right side of panel: in the absence of FKBP12.6, which results in increased sensitivity of RyRs to Ca2+, β-adrenergic stimulation leads to Ca2+ release from CRUs which is transmitted to adjacent CRUS so that regenerative release from all CRUs can occur. Local control of clusters is lost. Blue arrows indicate Ca2+ movement. Note in (C) that size of arrows indicates magnitude of Ca2+ flux. It has been suggested that additional controls exist within CRUs to limit the sensitivity of the RyRs to triggering Ca2+ so that activation between adjacent CRUs and common pool activation of all CRUs is less likely to occur providing an additional level of local control. It has been proposed for example that the 12.6-kDa-FK506 binding protein (FKP12.6) reduces the sensitivity of RyRs to Ca2+ stabilizing the CICR system and preventing, or at least limiting, common pool regeneration of multiple CRUs (Figure 1C). However, this has proved controversial.11,12 In this edition of Cardiovascular Research, Zhao et al.1 report elegant studies that strongly suggest that indeed FKBP12.6 can reduce the sensitivity of RyRs to Ca2+ by modifying RyR open probability (PRyR). Presumably this effect is mediated by FKBP12.6 binding with recently studied structural domains present in RyR2.13 This effectively stabilizes the system against undesirable Ca2+ release produced by common pool regeneration during for example β-adrenergic stimulation. These authors used a combination of patch clamping and confocal imaging to detect both ‘sparklets’ produced by unitary Ca2+ currents and local release events or sparks in both WT and FKBP12.6 KO mice. This allowed them to establish the temporal relationship between isolated triggers (sparklets) and sparks in situ. Near threshold depolarization, which activated a small fraction of the LCC pool, produced a stochastic activation of sparks by what was very likely a single LCC opening. Spark frequency was found to be higher in the FKBP12.6 KO than in WT mice. In addition, the delay between trigger and sparks was shortened. These observations, among others, led the authors to conclude that FKBP12.6 stabilizes RyRs by desensitizing them to Ca2+. An important finding that helps to resolve controversy surrounding this subject is that neither FKBP12.6 KO nor β-adrenergic stimulation acting independently produce destabilization of RyRs outside the stability margin of the CICR system. Clearly a finding of this nature was only possible in in situ experiments. However, FKBP12.6 KO and β-adrenergic stimulation sum their effects to produce additive sensitization of RyRs causing chaotic Ca2+ waves in most cells which means that the stability margin of the CICR system has been breached. Therefore, it seems likely than under physiological conditions sympathetic stimulation is less likely to produce chaotic release events in the presence of FKBP12.6. The author’s experimental approach may prove particularly useful in the study of disease where the effect of FKBP12.6 is compromised and the CICR system can be destabilized by sympathetic stimulation. For example, it has been reported that a significant number of patients suffering from catecholaminergic polymorphic ventricular tachycardia type 1 (CPVT1) present mutations that affect the RyR2 FKPB12.6 binding domain.14 This is a type of stress-induced inherited arrhythmia in which patients without any structural heart disease develop ventricular arrhythmias, syncope or sudden death under physical or emotional stress that is usually presented with an increased adrenergic response. Presumably these mutations reduce the ability of FKPB12.6 to stabilize CICR. Additionally, the FKPB12.6 presence may help to mitigate the increased sensitivity to luminal SR Ca2+ that is observed in other RyR mutations associated with CPVTs. Along these lines, transverse aortic constriction-associated mortality was reduced in transgenic mice overexpressing FKBP12.6.15 Stimulation after isoproterenol treatment, which induced ventricular tachycardia in 50% of WT mice, did not cause tachycardia in the transgenic mice. This clearly points to the stabilizing effect of FKPB12.6 in this model of overexpression. However, the overexpression of FKBP12.6 appears to have effects that are model dependent. Thus, Bito et al.16 using a mouse model with conditional FBPK12.6 overexpression found a reduced PRyR that nevertheless did not protect against post-myocardial infarction (MI) remodelling. This draws attention to the complexity of FKPB12.6 interaction with RyRs in different disease models, suggesting that numerous pathways need to be targeted to prevent post-MI remodelling. The approach to studies reported by these authors may clearly lead to understanding how these mutations may destabilize the CICR system and help to develop insightful therapeutic strategies. Conflict of interest: none declared.