The anti‐arrhythmic drIP3 from a leaky SR

Cardiac excitation–contraction coupling (ECC) is initiated by depolarization-triggered Ca2+ influx through voltage-dependent L-type Ca2+ channels on the sarcolemma of ventricular myocytes (VMs). This triggers larger magnitude Ca2+-induced Ca2+ release (CICR) from clusters of ryanodine receptors (RyR2) on the nearby junctional sarcoplasmic reticulum (jSR). The resultant large, synchronous Ca2+ release leads to a global elevation in cytoplasmic Ca2+ concentration ([Ca2+]cyt), activating the myofilaments, and culminating in contraction. RyR2s are thus critical players in ECC. In the current issue of The Journal of Physiology, Blanch and Egger focus their attention on another, less abundant (∼10% the number of RyR2) intracellular Ca2+-release protein with a more controversial role in ECC, the inositol 1,4,5-trisphosphate receptor type 2 (IP3R2) (Blanch & Egger, 2018). In healthy VMs, IP3R2s are only weakly expressed on the jSR, and predominantly occupy peri-nuclear sites quite distant from the contractile machinery of the cell. However, enhanced SR-localized expression of IP3R2 has been reported in humans with heart failure, VMs isolated from mammals with pathophysiological conditions such as cardiac hypertrophy and hypertension (Harzheim et al. 2009), and cardiac-specific IP3R2 overexpressing transgenic (TG) mice (Nakayama et al. 2010). Thus upregulation of IP3R2 has been implicated as pro-arrhythmic. However, using the TG mouse model of IP3R2 overexpression, the provocative study of Blanch & Egger (2018) suggests quite the opposite. A combination of biochemistry, Ca2+ imaging and pharmacology was used to investigate the role of IP3R2 in ECC and in the generation of Ca2+ waves, the cellular trigger for cardiac arrhythmias. In the control, unstimulated state, the authors report that Ca2+ waves were 8 times more likely to occur in TG myocytes than WT (Fig. 1A and C). Transient amplitude was higher in TG myocytes despite lower expression of RyR2 and an SR load that was comparable to WT. This was rationalized by a higher fractional release in the TG myocytes. But why would fractional release be higher in TG myocytes when SR load was seen to be normal? One possibility is that the higher expression of jSR-localized IP3R2 in TG cells could position the receptors in close proximity to RyR2, enabling functional interplay between them, and boosting CICR as previously reported in atrial cells (Wullschleger et al. 2017). If this were the case then blocking IP3R2s would be predicted to decrease transient amplitude in TG myocytes. Surprisingly, a more complicated picture emerged as pharmacological blockade of the receptors actually increased transient amplitude and Ca2+ wave occurrence, and led to a significant elevation in SR load. It may still be the case that functional cross-talk between RyR2 and IP3R2 does occur in TG myocytes and this explains the higher Ca2+ transient amplitude and wave propensity in these cells but a dual role of the receptors masks this conclusion. It appears that in TG myocytes, intrinsically active IP3R2s play an essential role in regulating SR Ca2+ content. Once this regulation is lost (with receptor blockade), SR Ca2+ overload ensues with Ca2+ content soaring to 1.4 times that of WT cells. This high luminal Ca2+ concentration would be expected to sensitize RyR2s and lead to asynchronous Ca2+ release and waves as observed. These results contrast with those seen in spontaneously hypertensive rat VMs where 2-aminoethyl diphenylborinate (2-APB) reduced Ca2+ transient amplitudes (Harzheim et al. 2009), adding a note of caution about the conservation of this dual role of IP3R2 across species. The authors also evaluated the role of the receptors during physiological IP3 production by applying endothelin to initiate a Gq protein coupled receptor (GqPCR) signalling cascade in which activated phospholipase C hydrolyses phosphatidylinositol 4,5-bisphosphate and liberates IP3. Again, this may be predicted to increase SR Ca2+ release and boost CICR from nearby RyR2, elevating Ca2+ transient amplitudes, and enhancing the probability of asynchronous Ca2+ release and Ca2+ waves. Indeed this prediction manifested as such in WT myocytes where SR Ca2+ load increased 1.4-fold, transient amplitude increased 1.7-fold and Ca2+ waves were 5 times more likely (Fig. 1B). But in another interesting and unexpected finding, the authors report that this treatment actually halved the likelihood of Ca2+ wave occurrence in TG myocytes (Fig. 1D). Examination of the relative contributions of IP3R2 and RyR2 to SR Ca2+ leak revealed the answer to this conundrum. In TG myocytes, application of endothelin induces an even greater IP3R2-mediated leak of Ca2+ from the SR than observed basally, while the role of RyR2 in SR leak was reduced. This regulates SR Ca2+ content and maintains the luminal [Ca2+]SR at a level below the threshold for spark activation. Thus, IP3R2 overexpression represents an efficient means to control SR Ca2+ content and protects against Ca2+ overload and arrhythmia. In summary, Blanch and Egger provide evidence that in a pathological state (mimicked by the TG mouse model), IP3 induced Ca2+ release leads to: (1) reduced SR Ca2+ content, (2) reduced asynchronous Ca2+ release and, (3) a consequent reduction in Ca2+ wave likelihood. The additional IP3R2s observed in cellular remodelling during cardiac pathologies thus may actually serve a protective role, providing fine control of SR Ca2+ content through leak. This maintains [Ca2+]SR at a level below the threshold for spontaneous spark activation, preventing arrhythmogenic Ca2+ release. The findings of this study paint IP3R2 overexpression during cardiac pathology in a new light and acquits these receptors as the perpetrators of arrhythmia substrate formation (at least in this model). While in healthy hearts IP3R2 can initiate arrhythmogenic Ca2+ release, when a critical level of expression is passed, they instead become anti-arrhythmic regulators of SR Ca2+ content. It would be an interesting quantitative question to determine what that critical level is. Also, can the expression go beyond that critical level and reduce SR Ca2+ content too much as cardiac hypertrophy progresses from a healthy adaptive response (such as that seen during pregnancy or exercise) to a pathological one during heart failure? It is striking that IP3R2 blockade was once posited as an anti-arrhythmic strategy in patients suffering from various cardiac pathologies but, in light of this new evidence, that strategy could have proven catastrophic. It would be quite poetic to conclude this perspective by suggesting that the legacy of this work could prove to be its shifting of the emphasis of attention away from IP3R2s as pro-arrhythmic proteins and instead ignite a new focus on IP3R2 agonists to decrease SR Ca2+ load during cardiac pathologies. However, a salient dichotomy exists between these results in VMs and those previously reported in atrial myocytes (Hohendanner et al. 2015). There, where IP3R2 expression levels are around 6 times higher than in VMs, basal IP3 production during heart failure can produce inotropic effects while acute receptor stimulated production may be arrhythmogenic. Therefore, at the whole heart level, it remains to be seen whether IP3R2s will prove to be friends or foes. None Sole author. This work was supported by an American Heart Association Scientist Development Grant to the author (15SDG25560035).