InsP3 regulated calcium signaling microdomains in the physiology and pathophysiology of ventricular cardiomyocytes

Heart disease accounts for more than 40% of all death in Europe. It affects every third person in a world and remains the leading cause of mortality and morbidity. The current therapy based on antagonism of beta-adrenergic receptors, angiotensin converting enzyme and/or angiotensin type 2 receptors improves the outcome for people with heart failure. Nevertheless, the prevalence of heart failure continues to grow rapidly, highlighting the need for novel therapies to target pathways that drive heart failure progression. One such pathway is activated by endothelin-1 (ET-1) that was seen to enhance contractile efficiency of the heart. However, an excess of circulating levels of ET-1 is of prognostic significance, predicting worsening of heart failure. Therapies, involving blockade of ET-1 signaling, have shown promise in animal heart failure models. In clinical trials in humans however, these agents failed to reduce morbidity and mortality. Despite, the growing knowledge about the contribution of ET-1-induced signaling to disease progression, the underlying mechanisms on a cellular level remain to be fully resolved. In the last decades, Ca2+ cycling defects and altered gene expression in cardiomyocytes were shown to be common processes associated with the progression of heart failure. In this regard, ET-1 has pleiotropic effects on cardiomyocytes ranging from positive inotropy, diastolic Ca2+ leak, induction of pro-arrhythmic spontaneous Ca2+ releases and promotion of gene expression and hypertrophic growth. ET-1 elicits its effects on cardiomyocytes in part via inositol 1,4,5-trisphospahte Ca2+ release channel (InsP3R). Although the contribution of InsP3R-mediated Ca2+ signals to Ca2+ handling and hypertrophic remodeling associated transcriptional changes have been shown for decades, the underlying molecular mechanisms remain poorly understood. First, we examined expression of InsP3Rs relative to the ryanodine receptor calcium release channels (RyRs) and transverse-axial tubular invaginations of sarcolemma (TATS), which together form the dyad structures central to the generation of Ca2+ signals responsible for rhythmic contraction of the heart. We showed the presence of ~30% and ~50% of InsP3Rs in the dyad and overlapping with RyRs, thereby enabling Ca2+ signals through InsP3R to influence Ca2+ release via RyR clusters (Chapter 4). We then investigated how the spatial distribution of InsP3Rs is changed during hypertrophic remodeling. In post-MI cardiomyocytes, we did not find alterations in the fraction of InsP3R associated with RyRs and TATS, although InsP3R clusters were significantly larger when compared to those in healthy cells. Next, we investigated the contribution of crosstalk between InsP3R and RyR to dyadic Ca2+ signaling and whole-cell Ca2+ transients. In this thesis, we showed that Ca2+ released via InsP3Rs signals to RyRs enhancing their activation. In this way, InsP3-induced Ca2+ release may both promote greater Ca2+ flux underlying enhanced contraction as well as stimulate spontaneous release, which may in turn contribute to arrhythmia generation (Chapter 5). These data suggest that targeting Ca2+ release via InsP3Rs, represents a strategy to suppress the activity of ET-1 on Ca2+ release and may in turn prove more efficacious than previous strategies to inhibit the ET receptor. Finally, we investigated the contribution of InsP3 signals to induction of hypertrophic responses in neonatal rat ventricular cardiomyocytes (NRVMs). In in vitro experiments in NRVMs, we showed that buffering of InsP3 signals in either the cytosol or nucleus prevented transcription of hypertrophy associated genes following exposure to either a physiological (insulin-like growth factor; IGF-1) or pathological (endothelin-1; ET-1) mediator of hypertrophy (Chapter 6). However, hypertrophy-induced cardiomyocyte growth was shown to be regulated selectively by InsP3Rs located in the nucleus, whereas expression of several proto-oncogenes (Fos, Jun, Egr1, etc.) was primarily under the control of InsP3 signals originated in the cytosol. Future work is required to uncover whether different transcription regulators are activated by Ca2+ release via InsP3Rs during pathological and physiological hypertrophy. Together, these data show that improving our understanding of ET-1-induced signaling pathways could provide efficient targets and strategies to curb the rising trend in heart failure.

Jaar van publicatie:2022

Toegankelijkheid:Embargoed