Ca2+ ions play an important role in many physiological processes, including pacemakering, contraction, release of neurotransmitters, ECC, gene expression, etc. Unsurprisingly, that with a relatively low [Ca(2+)]i, a significant amount of Ca2+ is preserved by cells in intracellular Ca2+ stores. In CM and vascular smooth muscle cells (VSMCs), Ca2+ is stored mainly in SR and mitochondria. Calcium levels in cells are precisely regulated by various transporters and ion channels. Reduction in the intracellular concentration of Ca2+ serves as a signal to refill the Ca2+ stores through store-operated calcium entry mechanism (SOCE).
3.1. Cardiac ryanodine receptor (RyR2)
Ryanodine receptors (RyR2s) are Ca2+-permeable ion channels in the membrane of the SR. These channels are responsible for local Ca2+-induced Ca2+ release from SR. The Ca2+ released (Ca2+ sparks) activates contraction, pacemakering, ECC, and affects other Ca2+-dependent intracellular processes.
RyRs are found in both atrial and ventricular CM, as well as in vascular smooth muscle cells (VSMCs) [
45].
Currently, three isoforms of the RyRs have been characterized, one of which is the type 2 ryanodine receptor (RyR2) has been found in CM. Modern innovations in cryo-electron microscopy have made possible to obtain a number of near-atomic RyRs structures that have contributed to a better understanding of the RyRs architecture [
46]. The RyR2 consists of 4 subunits combined into a homotetramer and the FC-binding protein calstabin 2 in a stoichiometric ratio of 1:4 [
47].
The distinctive feature of the channels formed by RyR2 is that they are activated by an extracellular influx of Ca2+ as a result of the Cav1-RyR2 interaction that triggers local Ca
2+ release [
46].
Coupling between the Ca2+ entry and intracellular Ca2+ store in CM and VSMCs is mainly mediated by stromal interaction molecule STIM1 located in the endoplasmic reticulum and ORAI1 membrane protein. These proteins are the primary components of the calcium-release activated calcium (CRAC) channel (
Figure 4) [
46,
48,
49]. In response to a decrease in the concentration of Ca2+ in SR, STIM1 is homooligomerized and translocated to the SR-PM contact sites, where it colocalizes with Ca2+-ATPase (SERCA), IP3R, and Orai1 forming the Ca2+ selective Orai1 pore [
48]. However, it is still unclear how such a close arrangement of proteins is conducted [
49].
STIM1 domains include the ER luminal domains that have the SAM and EF hands. These domains function as the Ca2+ sensor of SR store content. Cytosolic oriented STIM1 contains regions critical for Orai channel gained within the CC domains. Additional domains include the ERM and lysine rich regions. The first protein, STIM1, is the ER-Ca2+ sensor protein involved in activation of SOCE. The second protein, Orai1, is the pore-forming component of the CRAC channel.
The STIM1 protein contains specific sequences localized in the transmembrane and cytoplasmic domains responsible for modular protein-protein interactions, as well as two Ca2+-binding EF-hand motifs facing the CP lumen. These Ca2+-binding sites are the primary detectors of the Ca2+ level. Over the last years, studies there have been demonstrated evidences of the involvement of STIM1 in the growth of the heart in response to signals of development and pathology [
50].
RyR2s are part of the pacemaker molecular mechanism that ensures the heart automaticity. The contribution of the RyR2-dependent Ca
2+ releases (Ca2+ sparks) to the automaticity is currently interpreted within the framework of a model dubbed "calcium clock". [
51]. According to the coupled-clock pacemaker cell system concept, the “clock” in pacemaker cells forms two competing oscillators: "Ca2+-clock" mechanism based on the spontaneous release of Ca2+ from SR and "membrane clock" ("M-clock") which includes surface membrane cation channels to ignite an AP [
51]. The phenomenon of Ca2+ oscillations, underlying the “Ca2+-clock”, is associated with the local diastolic intracellular Ca2+ releases (LCRs), and it is independent of the membrane potential (MP). Indeed, spontaneous LCRs can be observed in the absence of changes in MP and are a distinctive feature of the "Ca2+-clock" [
52]. RyRs act as a gear in a "Ca2+-clock" inducing rhythmically discharges of LCRs, which, in turn, activate an inward current (I
NCX1) that prompts the "M-clock" to start an AP [
51]. This electrogenic transport mechanism generates an internal ion current, which contributes to the onset of diastolic depolarization. In the final phase of diastolic depolarization, Cav channels cooperate with Na+/Ca2+-exchanger (NCX) to raise MP to the threshold value of the AP [
51]. During the AP phase, the intake of Ca2+ through LTCC channels refills the leakage of Ca2+ from SR allowing new diastolic intracellular Ca2+ release to occur in the next cycle. Consequently, under the normal physiological conditions, the "Ca2+-clock" and the "M-clock" synchronize the pacemaker activity of the heart cells and create a reliable basis for the heart automaticity [
51,
53].
Considering the importance of the RYR2 in providing of the heart automaticity, it seems quite obvious that mutations of genes encoding these receptors or proteins interacting with RYR2 in the CICR, would lead to catastrophic consequences for the organism [
54]. Indeed, studies using a mouse model have shown that a RYR2 mutation with a locus in the CaM-binding site (reducing receptor inactivation) causes cardiac hypertrophy, heart failure, and early sudden death [
55,
56].
Clinical studies suggest that enhancing the interaction of CаM-RyR2 may represent an effective therapeutic strategy for the treatment of cardiac arrhythmias and heart failure. In support of this idea, the mutation of GOF CаM-M37Q and reinforcement of the CаM-RyR2 interaction have been demonstrated to be able to suppress the spontaneous release of Ca2+ from SR and catecholaminergic polymorphic ventricular tachycardia.
In chronic heart diseases accompanied by cardiac arrhythmias, there is an increase in the activity of Cav1.2 channels, which leads to an increase in their permeability to Ca2+ and, as a result, to CM calcium overload. At the same time, spontaneous Ca2+ releases induced by more frequent RYR2 openings form "pathological" calcium waves that are abolished removed under physiological conditions with the participation of NCX1 and other molecular determinants of Ca
2+ homeostasis (CASQ2, FKBP12, SERCA2a, ect.) [
46]. It was found that NCX1 dysfunction and changes in its expression profile during arrhythmia lead to changes in atrial cell morphology and calcium handling together with dramatic alterations in the function of SAN [
58,
59].
Recent studies have shown that spontaneous arrhythmogenic "calcium waves" can result from genetic mutations of RyR2, but more often due to an increase in the time of its open state [
60,
61,
62,
63,
64,
65]. Defects in its modular coupling with regulatory proteins of the cytosol, such as CaM, Epacl, PDE, FKBP12.6, PKA, PP1, calstabin, etc., or with Ca2+-binding proteins localized in the lumen of the SR (junctin, triadin and calsequestrin) and forming temporary macromolecular complexes with it, can lead to a disruption of the RyR2 gate function [
61,
66,
67,
68,
69]. Mechanisms of interaction of the partner molecules with RyR2 are built on a structural basis, while regulatory proteins (predominantly kinases) use RyR2 as a scaffold protein to form functional signal complexes that can modify a large number of other Ca2+-dependent molecules involved in the cascade signal transmission. The structure of these RyR2-multi-domain complexes and mechanisms of regulation of their activity are still far from being fully understood [
65,
69].
It is noted that point mutations of RyR2-associated proteins or changes in their expression can dramatically affect the development of cardiac arrhythmias [
70]. In particular, the expression level of serine/threonine protein phosphatases plays an important role in the pathogenesis of arrhythmias [
71]. Serine/threonine protein phosphatases (PP1, PP2A, and PP2B) control dephosphorylation of numerous cardiac proteins, including a variety of ion channels (Cav1.2, NKA, NCX, ect.), calcium-handling proteins (SERCA, junctin and PLB), contractile proteins MLC2, TnI and MyBP-C [
71,
72], thereby providing post-translational regulation of ECC and other heart functions. Accordingly, dysfunction of this regulation can contribute to the development of cardiac arrhythmias. Atrial fibrillation (AF) is the most common heart rhythm disorder, and it is characterized by electrical and structural cardiac remodeling that among other factors includes changes in the phosphorylation status of a wide range of proteins, such as the RyR2 [
71]. It was found that a decrease in the concentration of PP1 caused by an increase in the level of PP1-regulatory proteins like inhibitor-1 (I-1), inhibitor-2 or heat-shock protein -20 in the sarcoplasm of ventricular CM lead to the rapid development of tachycardia and could cause sudden death [
63,
72], although an experimental increase in the concentration of PP1 in sarcoplasm prevented the development of arrhythmia, which was proven in experiments on mice overexpressing Ang II [
73]. Mice characterized with a highly phosphorylated RyR2-S2808 site (S2808A+/+) demonstrated an increased sensitivity of RyR2 to Ca2+ during dephosphorylation of PP-1 [
74].
СaM kinase II is another enzyme that plays an important pathogenetic role in diseases accompanied by cardiac arrhythmia [
52,
54,
75]. Effect of CaM kinase II on RyR2 is controversial. In pharmacological experiments using the method of embedding proteins in an artificial bilayer, some authors revealed the activating effect of this kinase on RyR2. On the other hand, others demonstrated its’ inhibitory effect [
76]. Using molecular genetic methods (transgenic overexpression), more recent studies have found that СаМ kinase II by binds RyR2 and causes its phosphorylation at serine 2814 (RYR2-S2814). This, in turn, increases the frequency of Ca2+ spikes and the spontaneous local diastolic subsarcolemmal Ca2+ releases in the process of ECC [
52]. It was noted that an increase in the level of phosphorylation of RYR-S2814 in SAN pacemaker cells led to the alteration of the "Ca2+-clock" regulation and the development of heart failure (HF) [
77].
Inhibition of CaM kinase II by a specific blocker KN93 reduced the release of Ca2+ from SR and a slowed in heart rate [
52].
In vitro studies have shown the possibility of using this blocker to relieve ventricular tachycardia caused by oxidative stress, which opens up prospects for the therapeutic use of KN-93 and its’ functional analogues in the treatment of arrhythmias [
52].
In addition, an increase in the basal level of CaMKII through phosphorylation of histone deacetylases (HDACs) activates myocyte-enhancer factor 2 (MEF2), which initiates the CaMKII/MEF-2 signaling pathway and hypertrophic remodeling of ventricular myocytes [
78]. Among endogenous biologically active molecules that have arrhythmogenic effects, the most studied are neurohormones, such as endothelin-1 (ET-1), aldosterone, epinephrine, which act through Gαq-associated GPCRs and cAMP-dependent protein kinase A (PKA) [
79,
80,
81]. Increased hormonal stimulation of these receptors leads to hyperphosphorylation of RyR2 and dissociation of calstabin 2 (FKBP12.6) from it. RyR2, deprived of this protein, loses its locking function, which leads to an increase in the time of its open state, leakage of Ca2+, an increase in [(Ca2+)]i, to afterdepolarization and can cause "fatal arrhythmia", heart attack, and SCD [
82]. To date, two genetic diseases associated with mutations in ventricular RyR2 have been described: catecholaminergic polymorphic ventricular tachycardia (CPVT), or familial polymorphic ventricular tachycardia and arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) type 2 [
83]. In patients with CPVT, the affinity of calstabin 2 to RyR2 is reduced due to a defect in RyR2 at its binding site to calstabin [
82]. The use of molecular approaches in the strategy of the targeted therapy of CVD emerged new drugs that suppress hyperphosphorylation of p-RyR2 (Ser2808) and p-RyR2 (Ser2814), thereby, stabilizing RyR2 and normalizing heart rate and contractility of ventricular myocytes [
84,
85]. The ClinVar database describes 165 variants of pathogenic mutations of the
RYR2 gene. The most common mutations associated with CPVT are: Ser2246Leu, Arg2474Ser, Asn4104Lys, Arg4497Cys, Pro2328Ser, 1.1-KB DEL, EX3. Most of these mutations lead to amino acid substitution or to the appearance of a premature stop codon and disruption of the formation of a functional protein [
86].
In addition to the RYR2 gene, encoding ryanodine receptor calcium release channel, mutations in 5 genes encoding proteins from the SR calcium-release complex are involved in pathogenesis of CPVT: CASQ2 (encoding cardiac calsequestrin), TRDN (encoding triadin), CALM1, CALM2 and CALM3 (encoding identical protein calmodulin) [
87].
The development of arrhythmogenic dysplasia of the right ventricle is also associated with mutations in the RYR2 gene. The ARVD2 locus was mapped to chromosome 1q42--q43 [
88]. This disease is characterized by partial fatty or fibrous degeneration of the myocardium of the right ventricle, electrical instability, and sudden death [
88]. The detection of RyR2 mutations causing CPVT and ARVD2 opens the way to pre-symptomatic detection of carriers of the disease in childhood, thus enabling early monitoring and treatment [
87,
88].
3.2. Ion channels with transient receptor potential (TRPC, TRPM7, TRPA1)
Transient receptor potential channels are nonselective cation channels of the TRP channel superfamily, uniting the related receptor proteins capable of being activated by the potential originating from binding of the ligand to the receptor. This superfamily is divided into a family of canonical TRP channels (TRPC) and several families whose names come from the name of the receptor, binding to which initiates the potential. Most TRPs are polymodal channels, so-called «coincidence detectors» that are activated by both physical and chemical stimuli [
89]. TRP channels vary degrees of selectivity and permeability to ions. TRPV1 - TRPA1 and TRPM6/7 channels are more selective for Ca2+ ions. In CM, they are localized in the sarcolemma adjacent mainly to intercalated disks and are activated by phospholipase C (PLC) via Gaq-associated G protein-coupled receptors [
90].
The common structural features of these channels are: four N-terminal ankyrin repeats, six short TM domains and pore-forming region localized between transmembrane domains 5 and 6. Like the other previously described channels, TRP channels with partner proteins and kinases form signaling complexes that can be involved in the pathogenesis of various cardiovascular diseases [
91,
92,
93,
94].
The existing data suggest that several types of TRP channel (TRPC3, TRPC6, TRPV1, TRPV3, TRPV4, TRPA1, TRPM6 and TRPM7) may play a central role in the progression of fibroproliferative disorders in the heart and blood vessels and contribute to both acute and chronic inflammatory processes involved in them [
95].
The family of canonical TRP channels consists of proteins closely related to the
Drosophila channel proteins of the same name involved in photoreception [
96]. This family includes seven subfamilies (TRPC1 – TRPC7), of which proteins of the TRPC1, -3, -4, -6 and -7 subfamilies were found in CM [
97]. To date, the greatest interest is focused on mechanosensitive TRPC channels. In case of their molecular «breakdown», these channels begin to pass an increased flow of Ca2+ ions, and, thereby, activate processes involving pathological remodeling of CM [
91,
93,
98]. In addition, TRPC7 also mediate apoptosis, thereby contributing to the process of heart failure [
99].
All TRPC channels are dependent on receptors associated with PLC, since they are directly or indirectly activated by phospholipid products formed due to activation of this enzyme and induction of hydrolysis of membrane phospholipids. TRPV1, -2 and -5 channels are activated by binding IP3 to the receptors and responsible for SOCE. In this case, the interaction of TRPC with Orai protein and stromal interacting molecule 1 (STIM1) is noted [
100]. The mechanism of Ca2+ store filling with the participation of these proteins is as follows. After Ca2+ store depletion, the STIM1 protein located in the ER undergoes a complex conformational rearrangement which results in STIM1 translocation into discrete ER-plasma membrane junctions, where it directly interacts with the plasma membrane protein Orai1. Orai 1 in its turn triggers recruitment of TRPC1 into the plasma membrane where it is activated by STIM1 then. TRPC1 and Orai1 form discrete STIM1-gated channel for the entry of Ca2+ into the lumen of the ER [
101,
102]. In addition, STIM1 can also activate TRPC1 through its C-terminal polybasic domain, a distinct from its Orai1-activating domain, SOAR [
101].
TRPM2 is the second member of the TRPM subfamily that includes eight members, specifically TRPM1-8. TRPM2 is widely expressed in СM, where it forms a Ca2+-permeable cation channel and serves as a cellular sensor for oxidative stress or inflammatory response [
103,
104,
105].
The N-terminus is composed of four melastatin homology regions and homology region pre-S1 (melastatin homology regions (MHR) and homology regions (HR). The channel domain contains six TM (S1–S6), corresponding to a voltage-sensor-like domain; the pore is formed by the loop between the S5 and S6. The C-terminus is composed of TRP and the coiled-coil domain (CC) (
Figure 5).
Trpm2 monomer is depicted as having 6 TM (S1 to S6) with the putative pore-forming loop situated between S5 and S6. Four monomers associate to form a Trpm2 channel. Both N- and C-termini are in the cytosol. The N-terminus contains 4 modules of Trpm subfamily melastatin homology domain (MHD). In the second MHD, there is an IQ-like motif which binds Ca2+-calmodulin. The C-terminus contains a Trp box (TRP), a coiled-coil domain (CC), and the adenosine diphosphate ribose (ADPR) pyrophosphatase homolog domain. Trpm2 is a non-specific cation-permeable channel which allows entry of Ca2+, Na+, and K+.
TRPM2 channels are activated by ADPR, Ca2+, H2O2 and other reactive oxygen species (ROS). They serve as a cellular sensor for oxidative stress, mediating oxidative stress-induced [Ca(2+)]i increase and contributing to pathological processes in many cell types, including CM. Overexpression of Trpm2 induces cell injury and death by Ca
2+ overload or enhanced inflammatory response [
103,
105].
Mutations in genes encoding closely related TRPM4 channels lead to impaired automatism, conduction, and the appearance of hereditary progressive familial heart block type I (PFHBI) [
106,
107]. It is also assumed that some forms of provoked cardiac arrhythmia may occur due to a single gain-of-function mutation in TRPM4. To date, 47 mutations of TRPM4 channel have been registered in the Human Gene Mutation Database [
108,
109].
TRPM7 channel mutations are especially dangerous in the prenatal period, as they can lead to intrauterine fatal arrhythmia and fetal death or to a change in the myocardial transcription profile in adulthood, deterioration of ventricular contractile function, conduction and repolarization [
110,
111].
There is evidence of a wide involvement of TRP channels in the pathogenesis of CVD caused by hypoxia and oxidative stress, as well as ischemia-reperfusion (I/R) [
90,
104,
112,
113,
114].
It is known that during hypoxia and I/R, there is formation of ROS and accumulation of lipid peroxidation products, including unsaturated aldehydes such as acrolein and 4-hydroxynonenal, which are TRPA1 agonists, in cardiac tissue [
115]. The mechanism of toxic action of unsaturated aldehydes on CM is associated with their high electrophilicity and the ability to covalently bind to cysteine residues in the TRPA1 molecule, leads to the opening of these channels and an increase sarcoplasmic reticulum Ca2+ release flux into the cytosol [
116]. Overexpression of TRPС1 channels also contributes to Ca2+ leakage from SR [
117]. As a result, Ca2+ overload of СM, leading to impaired contractility, heart failure and myocardial infarction occurs [
117].
The pathogenic significance of TRP channels in the development of heart failure, coronary artery disease, arterial and pulmonary hypertension, as well as coronary microvascular dysfunction and atherosclerosis is no less [
6,
90,
118,
119,
120].
TRPA1, TRPV1-4 and TRPC1-6 are expressed on the surface of endothelial cells and ensure the pass of Ca2+ ions into cells, regulating endothelial-dependent vasodilation in response to a number of signaling molecules, such as endothelial derived hyperpolarizing factor (EDHF), NO, and prostacyclin [
90,
113,
119,
123]. In mice with TRPV1 and TRPC3 channel knockout, there was a decrease in aortic vasodilation in response to carbachol, which proves the involvement of these channels in endothelium-dependent vasodilation [
119,
124]. In experiments on three animal models (dog, rat and mouse), it was demonstrated that i.v. administration of the TRPV4 agonist, GSK1016790A, stimulated endothelial derived EDHF-dependent vasodilation, and led to a subsequent decrease in blood pressure [
125]. The TRPV4 channels have been noted to participate in the coupling of endothelial-dependent vasodilation and relaxation of VSMCs through interaction with RyR2 and the big conduction KCa channel [
126]. TRPV4/7, TRPC1/5/6/7 and TRPV1/2/4 are expressed in VSMCs and participate in myogenic regulation of vascular tone [
119,
120,
127]. Electrophysiological experiments demonstrated that the TRPM4 knockout mice had increased vascular tone and developed hypertension [
90,
128], while inhibition of TRPC6 reduced VSMCs contraction [
127]. TRPV1 channels are involved in the progression of the atherosclerotic process in the apolipoprotein E gene knockout mice [
129]. Further studies have shown that other mechanosensitive TRP channels may also play a role in the development of atherosclerosis and coronary heart disease (CHD) [
6,
130,
131]. In addition, disruption of the TRP channel expression or function may explain the observed increased cardiovascular risk in patients with metabolic syndrome [
132].
Thus, TRP channels have broad cardiovascular plasticity. TRPC3, TRPC5, TRPC6, TRPV1, and TRPM7 are involved in vasoconstriction and regulation of blood pressure and can be considered as potential therapeutic targets for the treatment of chronic CVD, including cardiometabolic diseases and myocardial atrophy [
6,
119,
132,
133,
134].