Ca2+ release via InsP3Rs enhances RyR recruitment during Ca2+ transients by increasing dyadic [Ca2+] in cardiomyocytes.

Excitation-contraction coupling (ECC) relies on temporally synchronized sarcoplasmic reticulum (SR) Ca2+ release via ryanodine receptors (RyRs) at dyadic membrane compartments. Neurohormones, such as endothelin-1 (ET-1), that act via Gαq-associated G protein-coupled receptors (GPCRs) modulate Ca2+ dynamics during ECC and induce SR Ca2+ release events involving Ca2+ release via inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs). How the relatively modest Ca2+ release via InsP3Rs elicits this action is not resolved. Here, we investigated whether the actions of InsP3Rs on Ca2+ handling during ECC were mediated by a direct influence on dyadic Ca2+ levels and whether this mechanism contributes to the effects of ET-1. Using a dyad-targeted genetically encoded Ca2+ reporter, we found that InsP3R activation augmented dyadic Ca2+ fluxes during Ca2+ transients and increased Ca2+ sparks. RyRs were required for these effects. These data provide the first direct demonstration of GPCR and InsP3 effects on dyadic Ca2+, and support the notion that Ca2+ release via InsP3Rs influences Ca2+ transients during ECC by facilitating the activation and recruitment of proximal RyRs. We propose that this mechanism contributes to neurohormonal modulation of cardiac function. This article has an associated First Person interview with the first author of the paper.

Demydenko K ，Sipido KR ，Roderick HL 《-》

Functional local crosstalk of inositol 1,4,5-trisphosphate receptor- and ryanodine receptor-dependent Ca2+ release in atrial cardiomyocytes.

Enhanced inositol 1,4,5-trisphosphate receptor (InsP3R2) expression has been associated with a variety of proarrhythmogenic cardiac disorders. The functional interaction between the two major Ca2+ release mechanisms in cardiomyocytes, Ca2+ release mediated by ryanodine receptors (RyR2s) and InsP3-induced intracellular Ca2+ release (IP3ICR) remains enigmatic. We aimed at identifying characterizing local IP3ICR events, and elucidating functional local crosstalk mechanisms between cardiac InsP3R2s and RyR2s under conditions of enhanced cardiac specific InsP3R2 activity. Using confocal imaging and two-dimensional spark analysis, we demonstrate in atrial myocytes (mouse model cardiac specific overexpressing InsP3R2s) that local Ca2+ release through InsP3Rs (Ca2+ puff) directly activates RyRs and triggers elementary Ca2+ release events (Ca2+ sparks). In the presence of increased intracellular InsP3 concentrations IP3ICR can modulate RyRs openings and Ca2+ spark probability. We show as well that IP3ICR remains under local control of Ca2+ release through RyRs. Our results support the concept of bidirectional interaction between RyRs and InsP3Rs (i.e. Ca2+ sparks and Ca2+ puffs) in atrial myocytes. We conclude that highly efficient InsP3 dependent SR-Ca2+ flux constitute the main mechanism of functional crosstalk between InsP3Rs and RyRs resulting in more Ca2+ sensitized RyRs to trigger subsequent Ca2+-induced Ca2+ release activation. In this way, bidirectional local interaction of both SR-Ca2+ release channels may contribute to the shaping of global Ca2+ transients and thereby to contractility in cardiac myocytes.

Wullschleger M ，Blanch J ，Egger M 《-》

InsP(3)R-RyR channel crosstalk augments sarcoplasmic reticulum Ca(2+) release and arrhythmogenic activity in post-MI pig cardiomyocytes.

Ca2+ transients (CaT) underlying cardiomyocyte (CM) contraction require efficient Ca2+ coupling between sarcolemmal Ca2+ channels and sarcoplasmic reticulum (SR) ryanodine receptor Ca2+ channels (RyR) for their generation; reduced coupling in disease contributes to diminished CaT and arrhythmogenic Ca2+ events. SR Ca2+ release also occurs via inositol 1,4,5-trisphosphate receptors (InsP3R) in CM. While this pathway contributes negligeably to Ca2+ handling in healthy CM, rodent studies support a role in altered Ca2+ dynamics and arrhythmogenic Ca2+ release involving InsP3R crosstalk with RyRs in disease. Whether this mechanism persists in larger mammals with lower T-tubular density and coupling of RyRs is not fully resolved. We have recently shown an arrhythmogenic action of InsP3-induced Ca2+ release (IICR) in end stage human heart failure (HF), often associated with underlying ischemic heart disease (IHD). How IICR contributes to early stages of disease is however not determined but highly relevant. To access this stage, we chose a porcine model of IHD, which shows substantial remodelling of the area adjacent to the infarct. In cells from this region, IICR preferentially augmented Ca2+ release from non-coupled RyR clusters that otherwise showed delayed activation during the CaT. IICR in turn synchronised Ca2+ release during the CaT but also induced arrhythmogenic delayed afterdepolarizations and action potentials. Nanoscale imaging identified co-clustering of InsP3Rs and RyRs, thereby allowing Ca2+-mediated channel crosstalk. Mathematical modelling supported and further delineated this mechanism of enhanced InsP3R-RyRs coupling in MI. Our findings highlight the role of InsP3R-RyR channel crosstalk in Ca2+ release and arrhythmia during post-MI remodelling.

Jin X ，Meletiou A ，Chung J ，Tilunaite A ，Demydenko K ，Dries E ，Puertas RD ，Amoni M ，Tomar A ，Gilbert G ，Claus P ，Soeller C ，Rajagopal V ，Sipido K ，Roderick HL ... -  《-》

Inositol 1,4,5-trisphosphate receptors in cardiomyocyte physiology and disease.

Demydenko K ，Ekhteraei-Tousi S ，Roderick HL 《-》

'Eventless' InsP3-dependent SR-Ca2+ release affecting atrial Ca2+ sparks.

Horn T ，Ullrich ND ，Egger M 《-》