The cardiac action potential (AP) is important for the generation and the propagation of excitation leading to contraction and is the result of ion channels composition in each cardiomyocyte (22–24). AP corresponds to a transient depolarization due to the sequential activation and inactivation of voltage-dependent ion channels, ensuring a flow of ions between the intracellular and extracellular fluid according to the electrochemical gradient. AP is composed of five phases in the non-pacemaker myocytes that determine its duration and amplitude (Figure 1). It is carried by a sequential activation and inactivation of different currents (25, 26). The depolarization phase (Phase 0) is generated by the inward sodium (Na⁺) current (I_Na) encoded by the Na⁺ voltage-gated channel alpha subunit 5 (SCN5A). This phase is followed by a rapid repolarization (Phase 1), carried by the transient outward potassium (K⁺) current (I_to) encoded by the K⁺ voltage-gated channel subfamily D 2 (KCND2). This repolarization is very fast and followed by a plateau phase (Phase 2), which is a balance between the ion calcium (Ca²⁺) inflow through L-type Ca²⁺ current (I_CaL), encoded by the Ca²⁺ voltage-gated channel subunit alpha 1C (CACNA1C) and the outward flow of K⁺ ions. The rapid (I_Kr) and the slow (I_Ks) K⁺ currents, conduct this outward flow of K⁺, and are encoded by human ether-a-go-go–related gene (hERG) and by the K⁺ voltage-gated subfamily Q member1 (KCNQ1), respectively. These two currents terminate the AP resulting in the final repolarization (Phase 3). Finally, Phase 4 corresponds to the resting state around −80 mV in non-pacemaker cells, mainly held by the inwardly rectifying K⁺ currents (I_K1) encoded by the K⁺ inwardly rectifying channel subfamily J 12/14 and 4 (KCNJ12/14/4) (27, 28) and also to the diastolic depolarization in the pacemaker cells carried by the funny current (I_f) encoded by the hyperpolarization activated cyclic nucleotide gated K⁺ channel 4 (HCN4) (29, 30). Under pathological conditions such as HF, these ion channels undergo a remodeling process impacting the AP shape and AP duration (APD), part of which is due to trafficking disorders (31, 32). The APD of both atrial and ventricular cells is prolonged in HFrEF and HFpEF (33). However, Ca²⁺ handling is not impaired in the same way in HFrEF and HFpEF. In HFrEF, the systolic Ca²⁺ is decreased and the diastolic Ca²⁺ is increased, because of a decrease of T-tubule density and an increase of Na⁺/Ca²⁺ exchanger (NCX) expression. In HFpEF, there is a high cytoplasmic Ca²⁺ concentration associated with an unchanged NCX expression and an increased density of T-tubules (34).

I_Na alterations have been implicated in HF. A HF study conducted on mice missing the expression of the Muscle Lim Protein (MLP) demonstrated a decreased of I_Na density by post-translational reduction (35, 36). This I_Na reduction decreased the electrical conduction, favored re-entry, and contributed to inefficient synchronized conduction. Furthermore, studies conducted on dog and human HF led to alterations in the inactivation of I_Na causing a late persistent current (37, 38). Most of I_Na is rapidly inactivated, but a small fraction remains and contributes to generate a late I_Na (I_NaL), thus contributing to increase in the concentration of Na⁺ ions in the cardiomyocyte and consequently increasing the APD (39). However, the molecular mechanisms which explain this I_NaL in HF are still unclear. One study showed that the heterologous expression of Nav1.5 produced a late opening in gating modes (40). Furthermore, another explanation for this late current would be the contribution of the neuronal isoform Nav1.1. One study, on a dog model of HF, demonstrated the participation of this neuronal isoform in inducing the I_NaL (41). Targeting the neuronal type of Na⁺ channel by riluzole was reported to reduce this I_NaL (42). Riluzole could thus offer a therapeutic target to prevent arrhythmia and slow the progression of HF (43). The importance of this I_NaL abnormality has been well demonstrated to contribute to arrhythmias in HF, and medications targeting the I_Na could prevent arrhythmia (44). In fact, some studies used ranolazine, an I_NaL inhibitor, to reduce and prevent arrhythmias in HF (45, 46). To this end, a medication empagliflozin, an SGLT2 inhibitor, was described to interact directly with the Na⁺ channel and reduce the I_NaL in a mouse model (47). Recently, another study used empagliflozin, a SGLT2 inhibitor, to revert the I_NaL upregulation and arrhythmogenic AP in an HFpEF mice model, induced by high-fat diet and nitric oxide synthases inhibition (48). These studies showed that inhibiting I_NaL may be a therapeutic target in the prevention of arrhythmia onset in HF (49). The accumulation of Na⁺ concentration can alter the Ca²⁺ homeostasis because the Ca²⁺ efflux is via the NCX, which will compensate by extruding excessive intracellular Na⁺ resulting in intracellular Ca²⁺ overload and subsequent triggered arrhythmic events (50).

In HF, alterations in I_Kr, I_Ks, and I_to can be present. I_K plays a crucial role in the cardiac AP shape, and its remodeling is an important contributor to repolarization abnormalities in HF (51, 52). First, the I_to is known to be reduced in HF and to participate in APD prolongation (53, 54). The reduction of I_to participates in increasing the Ca²⁺ influx and Ca²⁺ concentration in the cells, which triggered the activation of hypertrophic pathway in the mice model (55, 56). Hypertrophy is a compensatory response to HF, but sustained cardiac hypertrophy contributes to the development of HF. Remodeling in I_Kr and I_Ks in HF have also been reported (57–59). A study conducted in rabbits with atrioventricular block showed a reduction of both I_Kr and I_Ks, using the patch-clamp technique (60, 61). I_K1 was also shown to be downregulated in HF (59). The small-conductance Ca²⁺-activated K⁺ currents (SK) are upregulated in HF (62). This current participates in the repolarization of healthy atria but not in the ventricle (63). Other studies have shown an upregulation in this channel in ventricular myocytes, which prolongs the APD and promotes the arrhythmogenesis in HF (64, 65). Finally, a downregulation of the TREK channel was observed in patients with atrial fibrillation complicated by HF (66). In all, the remodeling of I_K in HF underlies, in part, the electrical abnormality and associated arrhythmogenesis (67).

During the development of HF, important changes in Ca²⁺ handling occur (68). Key players in Ca²⁺ homeostasis include L-type Ca²⁺ channels, NCX, ryanodine receptor type 2 (RYR2), sarcoplasmic reticulum Ca²⁺-ATPase (SERCA), and Na⁺/H⁺ exchangers (NHE).

Clinical and experimental studies have demonstrated that the sinus node function is impaired in HF (118, 119). These observations are correlated with a downregulation of the I_f in the sinus node pacemaker in a rabbit HF model (120). Another study in dogs with HF reported a decrease of the transcript and protein level of both HCN4 and HCN2, which explains the decrease of I_f and the impairment of the sinus node function indicating that the pacemaker current is remodeled in HF (119). Unlike these studies, the study conducted on human HF demonstrated an upregulation of both atrial and ventricular HCN2 and HCN4, accounts for the observed increase in I_f,  which could contribute to atrial and ventricular arrhythmias (121). A healthy adult ventricle normally has low levels of HCN2 and HCN4. However, the expressions of these genes increases as a result of HF (122). In support of this study, clinical trials have demonstrated the beneficial effects of ivabradine, an I_f inhibitor, in HF (123–125). Additional studies are warranted to establish the exact role of I_f and its genes HCN2/HCN4 in HF.

The electrical propagation in cardiomyocytes occurs through the connexins and forms an electrical cell-to-cell coupling (126). The connexin-43 (Cx43) protein is highly expressed in the ventricles (127). In diseased hearts such as in the case of hypertrophy and HF, Cx43 is localized at the lateral side of the ventricular myocytes (128). While Cx43 remodeling has been associated with spontaneous ventricular arrhythmia, the exact mechanisms for arrhythmogenesis are just emerging. Interestingly, the expression of Cx43 has been described to be downregulated in HF condition (129, 130). In addition, a decreased phosphorylation of Cx43 was detected in HF cardiomyocytes, which impairs the function and the localization from the intercalated disc to the lateral membrane (131, 132). Abnormalities in function and expression of Cx43 result in slow conduction in HF, which increases the propensity for arrhythmic events. Lahnwong et al. showed that pretreatment with dapagliflozin, a SGLT2 inhibitor, in a rat model of HF had lower arrhythmia score (133). The anti-arrhythmic effects of SGLT2 inhibitor are likely explained by its effect on the phosphorylation level of Cx43.

These ion channels, exchangers, and connexins remodeling in HF increase the propensity of arrhythmias. EADs and DADs represent triggers of arrhythmias. EADs are due to AP prolongation caused by a reduction of repolarizing currents, such as I_to, I_Kr, and I_Ks, and an increase the I_CaL and I_NaL (Figure 1A). The DADs are caused by Ca²⁺ handling abnormalities during the diastole, due to an increased activity of NCX, decreased SERCA function, and Ca²⁺ leak from the RYR (Figure 1B). Furthermore, the re-entry mechanisms are the result of the remodeling process that leads to slower conduction velocities and the heterogeneity of APD, which can create conduction blocks. A reduction, observed in the HF model, of I_Na, I_f, and Cx43 favors this phenomenon and contributes to sustained arrhythmic events (Figure 1C). These ion channel changes are summarized in Table 1.

Most trafficking defects come from a direct variant on ion channels, which induces a misfolded protein and the retention in ER. Variants in the Na⁺ channel were discovered to be linked with a trafficking defect, leading to HF (212). A recent study demonstrated that two different variants of Nav1.5 can interact with each other to rescue the I_Na (181). Specifically, this study showed that one variant of Na⁺ channel, trafficking-deficient but gating-competent, was able to restore a fraction of the I_Na by interacting with another Na⁺ channel variant that was trafficking-competent but gating-deficient. This study supports the notion that Na⁺ channel α-subunits interact and cooperate with each other. The interaction between Na⁺ channels will allow to rescue the trafficking and could lead to a new approach to treat patients.

The post-translational modifications are important for the trafficking of Na⁺ channels (213). Specifically, the phosphorylation of Na⁺ channel protein process is well known to be a key factor in the trafficking (214, 215). The PKA, protein kinase C (PKC), and CAMKII are among the most abundant kinases expressed in the heart (216, 217). More recently, a study conducted on rabbit cardiomyocyte showed that the ubiquitination of Na⁺ channel by the ubiquitin ligase NEDD4 could regulate the endosomal trafficking (218). Targeting these enzymes could lead to enhancing the ion channel trafficking of Na⁺ channel and rescue the I_Na in HF. Interestingly, empagliflozin was recently described to reduce the I_NaL by inhibition of CAMKII activity (219). This medication could be used to restore a normal I_Na in patients with HF.

Changes in microtubule dynamics are known to alter ion channel trafficking. In fact, an upregulation of the detyrosination of microtubules can induce HF (Figure 3D) (187). The detyrosination is a post-translational modification of tubulin, the protein building block of microtubules, which are essential components of the cytoskeleton. In this process, the C-terminal tyrosine residue of tubulin is removed enzymatically, resulting in a stable and long-lived form of the microtubules. It was previously described that the detyrosination reduces the density and the peak I_Na in mice cardiomyocytes (220). It will be interesting to suppress the detyrosination to restore normal ion channel trafficking of the Na⁺ channel. Chen et al. showed that a pharmacological suppression of detyrosinated microtubules restores 50% of the lost contractile function in the left ventricular myocardium of a failing human heart (221).

Furthermore, Marchal et al. demonstrated in human-induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CM) that the plus-end binding protein (EB1) improves the trafficking and the function of Na⁺ channel trafficking (190). In mammals, the EB protein family consists of three proteins: EB1, EB2, and EB3, which bind directly to the end of microtubules and facilitate the anchoring of proteins at the plasma membrane (222, 223). In addition, other microtubule plus-end tracking proteins, also called +TIP, have been found to be associated with the Na⁺ channel and potentially involved in its trafficking. For example, alterations in EB1, EB2, CLASP, CLIP1 CENP-F, MACF1, or iASPP are linked to a decrease in conduction or abnormal contraction, thereby emphasizing the impact of +TIP function on cardiac electrophysiology and function (Figure 3E) (189, 192, 224). One of the consequences of HF is the dysregulation of microtubule homeostasis, which is necessary for guiding ion channels to the membrane and this process explains the reduced I_Na reported in patients with HF.

Variants in K⁺ channels were the first described and represent the most important factor in the development of HF (52, 225). Indeed, a study described that 90% of hERG variants inhibited the trafficking from the ER to the Golgi (226). These variants cause hERG transport disorders by stimulating the UPR and leading the degradation of the ion channel protein (182). Variants on KCNQ1 were also revealed to play a role in the abnormal KCNQ1 trafficking, leading to HF and long QT syndrome (227, 228). Furthermore, another study described that a variant of KCNQ1 impaired the hERG trafficking (229). Indeed, this variant of KCNQ1 interacts in the cytosol with hERG and impairs the trafficking. The alteration of hERG trafficking, caused by the KCNQ1 variant, leads to HF and long QT syndrome.

Hypokalemia or hyperkaliemia disturbs the trafficking of hERG (198). Guo et al. were the first to reveal how the density of hERG channels is regulated under hypokalemic conditions (230). This situation triggered an ubiquitination of hERG, internalization, and finally degradation (231) (Figure 3F).

The trafficking of K⁺ channels, especially hERG protein, involves a chaperone protein from the family of heat shock protein (HSP) (232, 233). HSP90 and HSP70 interact with hERG and play a crucial role for the maturation. In HF, the role of these chaperone proteins was not well described, but it was suggested as playing a potential role for therapeutic modulation of HSP in HF (196). Indeed, an inhibition of HSP90 in mice model of HF demonstrated a preserved left ventricular pressure (234). More recently, this team showed that simvastatin also inhibits HSP90 and reduces cardiac remodeling in HF rat model (235). For hERG protein trafficking, a study showed that the reduced expression level of hERG protein is linked with impaired trafficking because of the diminished interaction between hERG and HSP90 (Figure 3G) (197). This observation could explain the decrease of I_K observed in HF, and showed the potential therapeutic role of HSP90 in HF (236). There is limited knowledge regarding the trafficking defects for the I_to and I_K1 in HF. However, the K_v channel-interacting protein (KChIP) is downregulated in HF and this contributes to the inhibition of the trafficking of I_to (237). Furthermore, one study mentioned the N-glycosylation defect of TREK channel, which contributes to reducing the I_K in HF (238).

Ca²⁺ channel trafficking and function are regulated by auxiliary protein subunits, including the β-subunits, which help in their trafficking (239, 240). These β-subunits are important in facilitating the Ca²⁺ channel exiting from the ER (241), and different β-subunit variants have been involved in HF (180, 241). ER stress also induced a repression of Ca²⁺ channel trafficking. Indeed, a study conducted on hiPSC-CM showed that the activation IRE1, a pathway activated by the UPR response, decreases the protein level of the Ca²⁺ channel (Figure 3B) (186). In failing human hearts a decrease of Myc box-dependent-interacting protein 1 (BIN1) protein has been demonstrated. In myocytes, BIN1 facilitates microtubule-based delivery of Cav1.2 channels directly to T-tubules for normal Ca²⁺ transient development (242). BIN1 acts like a membrane scaffolding protein to anchor the Ca²⁺ channel at the plasma membrane and could explain the decrease of I_CaL observed in HF (193) (Figure 3E). In case of this channel, a recent study conducted in mice knock-out for CASK demonstrated that the loss of I_CaL accelerates HF development (194). This latest study demonstrated the importance of the actors involved in the trafficking of ion channels and that their destabilization can lead to aggravating the development of HF.

Furthermore, studies have shown that the post-translational modifications could affect the other components of the Ca²⁺ handling like the RYR2. PKA hyperphosphorylation, CAMKII, oxidation, and nitrosylation trigger the destabilization and dissociation of FKBP12.6 from the channel and contribute to SR Ca²⁺ leak that is thought to induce arrhythmias and decrease myocardium contractile strength in HF (243). These post-translational modifications can cause defects of RYR2 trafficking and contribute to development of HF.

Few studies attempted to understand the mechanisms of NCX trafficking in HF. In fact, the NCX activity in HF occurs more in response to a persistent I_Na that increases its reverse mode or by a Ca²⁺ leak coming from the SR, thus increasing its activity and its arrhythmic role. However, a recent study highlighted the role of the synapsin-2 on the NCX regulation of trafficking in HF (195). The synapsin-2 is a protein associated with the vesicle transport during protein trafficking. First, the authors showed a decrease of synapsin-2 expression in the left ventricle in mice model of HF and demonstrated a co-localization of synapsin-2 and NCX. These data suggest that reduced synapsin-2 in HF alters the NCX trafficking and leads to ventricular arrhythmias (Figure 3H) (195).

For the Cx43, the localization of this hemichannel is at the intercalated discs. The trafficking of Cx43 involves a major plus-end EB1 protein (244, 245). EB1 allows anterograde movement to the intercalated discs by linking the microtubules to the desmosomes via the interaction with another plus-end protein (p150) (246). In case of HF, oxidative stress causes the microtubule plus-end EB1 dissociation from the end of the microtubules (Figure 3E). Consequently, it impairs the microtubule attachment to adherent junction structures and the delivery of Cx43 to the plasma membrane at the intercalated discs (191, 247). This observation could explain the decrease of Cx43 observed in patients with HF (248).

Collectively, ion channel trafficking disturbances lead to HF. A better understanding of these mechanisms allows to develop new therapeutic strategies or to better understand certain therapies used. First, targeting HSP chaperone proteins, suppressing microtubule detyrosination, or even targeting Rab recycling proteins could be interesting new strategies to restore ion channels trafficking. Therapy using an adeno-associated virus (AAV), as in the CUPID2 study, does not seem to be the best solution as of yet in the treatment of HF (111). However, therapies carried by SGLT2 inhibitors seem the most promising for trafficking restoration, specifically empagliflozin, which makes it possible to restore normal ion channels trafficking through its chaperone-like effect by increasing the forward trafficking of ion channels (249). Finally, Entresto® is a new therapeutic option to reduce the occurrence of arrhythmias in patients with HF. Its precise role in the treatment of these arrhythmias is still being explored, but it appears that its role affects the three pathways of B-type natriuretic peptide, angiotensin II, and bradykinin (250). Its role in ion channel trafficking is yet to be discovered.