Effect of Inositol 1, 4, 5-Trisphosphate Receptor Dependent Ca2+ Release in Atrial Fibrillation
Atrial fibrillation (AF) is a common cardiac arrhythmia characterized by disordered electrical activity in the atria. The pathogenesis of AF is closely associated with myocardial electrical and structural remodeling, which are largely driven by disruptions in calcium (Ca2+) homeostasis. Inositol 1,4,5-trisphosphate receptors (IP3Rs) play a critical role in regulating intracellular Ca2+ signaling, and their dysregulation has been implicated in the development and maintenance of AF. This article explores the mechanisms by which IP3R-dependent Ca2+ release contributes to AF, focusing on atrial remodeling, inflammation, oxidative stress (OS), and the interplay between IP3Rs and other Ca2+ regulatory proteins.
IP3Rs and Ca2+ Homeostasis in Atrial Fibrillation
IP3Rs are intracellular Ca2+ channels located on the sarcoplasmic reticulum (SR) membrane. They are activated by the binding of inositol 1,4,5-trisphosphate (IP3) and Ca2+, which leads to the release of Ca2+ from the SR into the cytosol. In atrial myocytes, IP3Rs are more abundant compared to ventricular myocytes, making them particularly important in atrial Ca2+ signaling. The activation of IP3Rs can influence conduction velocity and rhythm in the sinoatrial nodes, and their dysregulation is closely associated with the pathophysiology of AF.
Clustered IP3Rs exhibit a higher open probability compared to isolated IP3Rs, indicating that they function more cooperatively in the clustered state. The activation of IP3Rs is dependent on appropriate concentrations of IP3 and Ca2+. When IP3R expression is upregulated, Ca2+ influx into the SR is regulated by the sodium-calcium exchanger (NCX). This Ca2+ influx triggers Na+ influx, leading to the prolongation of the action potential duration and the refractory period, which facilitates the maintenance of AF. Ca2+ and Na+ overload also provide a pathological basis for early and delayed after depolarizations, which can further contribute to AF.
Atrial Remodeling and IP3Rs
Atrial structural remodeling is a hallmark of AF and includes fibrosis, enlargement, and fatty infiltration. These changes create an arrhythmogenic substrate by promoting inhomogeneous conduction and re-entry. IP3Rs have been shown to play a role in atrial structural remodeling. For example, the accumulation of collagen I, a key component of fibrosis, is inhibited in IP3R-deficient models. Additionally, IP3R-mediated electrical remodeling can facilitate atrial structural remodeling by enhancing afterload and peripheral resistance.
Clinical studies have demonstrated that structural remodeling in the atrium is detectable in patients with both paroxysmal and permanent AF. The role of IP3Rs in atrial remodeling is further supported by the observation that the incidence of AF is abolished in IP3R2-knockout transgenic mice. These findings highlight the importance of IP3Rs in the development of the arrhythmogenic substrate in AF.
Inflammation and IP3Rs
Inflammation is another key factor in the pathogenesis of AF. Post-operative AF can be predicted based on the detection of circulating inflammatory markers such as C-reactive protein, interleukin (IL)-2, and IL-6. Atrial inflammation and fibrosis are closely interrelated and are associated with similar signaling pathways that promote heterogeneity in conduction. IP3R-mediated signaling can promote the secretion of inflammatory cytokines, including IL-6, IL-8, and macrophage inflammatory protein-1β.
Transforming growth factor-β (TGF-β), a fibrotic protein, can positively support the release of inflammatory cytokines, predisposing individuals to AF. Interestingly, 2-aminoethoxydiphenyl borate, an IP3R inhibitor, has been shown to inhibit the secretion of pro-inflammatory cytokines. These findings suggest that inhibition of IP3Rs may abolish the pro-arrhythmic effects of inflammatory cytokines, providing a potential therapeutic target for AF.
Oxidative Stress and IP3Rs
Oxidative stress (OS) occurs due to an imbalance between oxidants and antioxidants, leading to the production of reactive oxygen species (ROS). ROS can activate IP3R-mediated Ca2+ signaling, particularly in atrial myocytes, where IP3Rs are more sensitive to ROS compared to ventricular myocytes. The opening of mitochondrial permeability transition pores (mPTPs), which is controlled by IP3R-mediated Ca2+ release, triggers electrical remodeling in the atrium and promotes the development of AF.
Pre-treatment with N-acetylcysteine, an IP3R inhibitor, can abolish the effects of IP3R1-mediated Ca2+ overload. ROS also triggers the activation of protein kinase A, C, and G (PKA/PKC/PKG), leading to the phosphorylation of IP3Rs. For example, PKA promotes Ca2+ influx into the SR by mediating the phosphorylation of IP3R1 and IP3R2. PKC, on the other hand, increases intracellular Ca2+ concentration through the phospholipase C (PLC)-PKC-IP3R pathway. PKG selectively phosphorylates IP3R1 and prevents Ca2+ release, thereby reducing the amplitude and frequency of Ca2+ oscillations.
IP3Rs and Apoptosis
Atrial remodeling, inflammation, and OS are closely associated with cell apoptosis, which can lead to abnormal conduction velocity and rhythm in atrial tissues. IP3Rs play a pivotal role in the regulation of cell apoptosis by modulating Ca2+ elevation and ATP metabolism. For example, Bax and Bak, members of the anti-apoptotic Bcl-2 family, decrease Ca2+ leakage by regulating the phosphorylation of IP3R1. Bcl-2 and Bax/Bak can interact with IP3Rs, forming a macromolecular complex that stimulates mitochondrial Ca2+ uptake and controls cell apoptosis.
Mutations in IP3Rs can also affect their function. The P1059L mutation in the IP3R regulatory domain increases the binding affinity to IP3, contributing to enhanced Ca2+ signaling. IP3R1/IP3R2 double-knockout models exhibit severe structural abnormalities in cardiac tissues, such as thin myocardial walls and poor trabeculation, leading to embryonic lethality. Mutations in sodium channels (Nav1.5) can also affect IP3R1 function via co-localization with calcium/calmodulin-dependent protein kinase II, resulting in Na+ and Ca2+ overload and arrhythmic disease.
Cross-Talk Between IP3Rs and Other Ca2+ Regulatory Proteins
IP3Rs interact with several other Ca2+ regulatory proteins, including ryanodine receptor 2 (RyR2), transient receptor potential canonical 3 (TRPC3), stromal interaction molecule (STIM), and Orai calcium release-activated calcium modulator 1 (ORAI1). Functional cross-talk between IP3Rs and RyRs has been observed in human atrial myocytes. Although RyR2 is more frequently expressed in ventricular myocytes, IP3Rs and RyRs co-localize in the microspace of atrial myocytes, providing a substrate for the modulation of channel gating.
TRPC3 plays a significant role in mediating cardiac fibrosis, which serves as the etiological basis for AF. In TRPC3 knockout mice, the effect of angiotensin II-induced AF is inhibited. A complex involving TRPC3, NCX, and IP3R1 contributes to the modulation of Ca2+ homeostasis during the inflammatory response. IP3Rs can also interact with STIM and ORAI1, leading to Ca2+ leakage from the SR. The activity of store-operated calcium entry (SOCE) is reversely controlled by STIM/ORAI1 signaling cascades, highlighting the complex interplay between IP3Rs and other Ca2+ regulatory proteins.
Conclusion
In summary, IP3R-mediated Ca2+ release plays a critical role in the pathogenesis of AF by contributing to atrial remodeling, inflammation, oxidative stress, and cell apoptosis. The dysregulation of IP3Rs leads to Ca2+ and Na+ overload, which promotes the development and maintenance of AF. IP3Rs also interact with other Ca2+ regulatory proteins, such as RyRs, TRPC3, STIM, and ORAI1, to modulate Ca2+ homeostasis. Understanding the mechanisms underlying IP3R-mediated Ca2+ signaling provides valuable insights into the pathophysiology of AF and may inform the development of targeted therapeutic strategies to alleviate the morbidity and mortality associated with this condition.
doi.org/10.1097/CM9.0000000000000898
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