Late sodium current dysregulation as a causal factor in arrhythmia

Voltage-gated Na⁺ channels (Na_v) are essential for normal cardiac function. Defects in the function of Na_v1.5, the primary Na_v alpha subunit in heart (encoded by SCN5A), have been linked to a host of congenital and acquired cardiac arrhythmia syndromes. Specifically, there has been growing appreciation in the field for the importance of Na_v1.5 defects that produce a unique gating mode characterized by an increase in inappropriate persistent ‘late’ current (I_Na,L). Despite the considerable effort to understand the role of aberrant I_Na,L in disease, there remain fundamental unanswered questions about the pathways responsible for regulation of I_Na,L in health and disease, and, more importantly, whether these pathways may be exploited for therapeutic benefit in human patients.

Despite important advances in anti-arrhythmia therapy over the past half century, cardiac arrhythmia remains a major source of morbidity and mortality [1,2]. Much focus has been given to ion channels as targets for anti-arrhythmia agents, but unintended pro-arrhythmia and off-target effects associated with ion channel blocking drugs motivate the continued pursuit of new and improved anti-arrhythmia agents [3]. Exciting recent work in the field suggests that a novel anti-arrhythmia approach with great potential involves targeting a pathogenic component of voltage-gated Na⁺ current (‘late’ Na⁺ current, I_Na,L) [4–8].

Na_v generate the cardiac action potential (AP) upstroke through a tightly controlled gating process involving rapid channel activation followed immediately by inactivation, which sets the stage for smaller conductance/slower ion channels (mostly voltage-gated Ca²⁺ and K⁺ channels) to repolarize the membrane in preparation for the next heartbeat. While Na_v current (I_Na) is mostly terminated by the rapid voltage-dependent inactivation process within milliseconds, a small percentage of channels (<1%) remain available throughout the AP, giving rise to a persistent (‘late’) current (I_Na,L) even under normal conditions [9]. While the physiological role of I_Na,L is undetermined, it is possible that I_Na,L supports contractility at baseline and in response to acute stress (i.e. ‘fight-or-flight’ response). Regardless of the physiological function, increased I_Na,L has been observed in congenital gain-of-function arrhythmia syndromes (e.g. long QT 3), and in acquired forms of disease (e.g. heart failure, atrial fibrillation (AF)) [9–13]. I_Na,L likely promotes arrhythmogenesis by (1) increasing a depolarizing current that prolongs the AP and increases susceptibility to secondary depolarizations (afterdepolarizations) that serve as potential arrhythmia triggers and (2) increasing intracellular Na⁺ that promotes intracellular Ca²⁺ accumulation via altered activity of Na⁺/Ca²⁺ exchanger [2,6]. Consistent with pro-arrhythmia of I_Na,L, drugs that block I_Na,L have been shown in pre-clinical and clinical trials to reduce arrhythmia incidence, although the precise mode of action is not resolved [4,7,8,14–16]. Despite this important work, questions remain about the mechanisms for dysregulation of I_Na,L in disease and whether a therapeutic strategy focused on I_Na,L is likely the most beneficial in our effort to reduce arrhythmia burden in human patients.

How is I_Na,L regulated in health and disease?

The cardiac cell has evolved elaborate signaling networks for tuning of I_Na,L via post-translational modification of Na_v1.5 (e.g. phosphorylation, nitrosylation, glycosylation) [17]. Among the most well-studied signaling pathways for phosphorylation of Na_v1.5 is the multifunctional Ca^2+/calmodulin-dependent protein kinase II (CaMKII), which is known to be activated in a wide range of cardiac disease states [18]. CaMKII directly phosphorylates Na_v1.5 to alter several I_Na properties, including the magnitude of I_Na,L [19–23]. Although several phosphorylation sites for CaMKII have been identified in the Na_v1.5 Domain I-Domain II (DI-DII) linker [19,20,24,25], mounting data from our group and others point to an important regulatory locus at Ser571 [5,10,19,21,24]. Specifically, increased phosphorylation of Ser571 occurs under stress conditions in vitro and in animal models of disease and in human heart failure [19,21]. We recently developed two knock-in mice models to study the in vivo role of Ser571 phosphorylation: (1) the S571A mouse, which lacks the Ser571 phosphorylation site due to replacement of the serine with an alanine and (2) the S571E mouse, which substitutes a glutamic acid at position 571 to mimic constitutive phosphorylation (phospho-mimetic) [5]. Using these mouse models, we demonstrated that phosphorylation of Ser571 specifically regulates I_Na,L without affecting other I_Na properties (peak current, steady-steady inactivation) Furthermore, ablation of the Ser571 phosphorylation site in S571A mice conferred resistance to maladaptive remodeling and arrhythmias under chronic pressure-overload conditions (transaortic constriction) [5]. While these studies point to an important role for Ser571 in regulating I_Na,L, they do not explain the full range of CaMKII-dependent effects on I_Na. Additional in vivo studies with relevant animal models will be required to assess the role of other phosphorylation sites (e.g. S516) in controlling the final phenotype. We expect that phosphorylation of additional sites act in concert with S571 to mediate CaMKII-dependent control of other channel properties (e.g. channel inactivation). Going forward, systems-based approaches to analyze post-translational modification in an unbiased way may be necessary to fully understand how Na_v1.5 is regulated [26]. Also, it is important to note that while CaMKII-dependent phosphorylation of Na_v1.5 is likely a critical pathway for dysregulation of I_Na,L in disease, there are multiple potential avenues for enhanced I_Na,L downstream of pathogenic stimuli (e.g. adrenergic imbalance, oxidative stress).

Aside from common forms of acquired arrhythmia, increased I_Na,L has been shown to underlie dozens of gain-of-function mutations in SCN5A linked to long-QT syndrome type 3 [12]. Most of these mutations are missense mutations that destabilize or slow the rapid inactivation process. For example, the first inherited LQT3 mutation to be identified, ΔKPQ, results in a 3-amino acid deletion in the Domain III-Domain IV (DIII-DIV) linker of Na_v1.5, which promotes channel reopenings at depolarized potentials [27,28]. Interestingly, in recent studies, we found that inherited arrhythmia mutations involving phospho-mimetic mutations at sites adjacent to the Ser571 site (A572D and Q573E) conferred arrhythmia susceptibility, in part, by mimicking CaMKII-dependent phosphorylation of Na_v1.5 [21]. These studies support the importance of Ser571 in modulation of I_Na,L and suggest a novel mechanism for dysregulation of I_Na,L in inherited disease.

Is I_Na,L an optimal target for preventing arrhythmia?

Based on the strong link between increased I_Na,L and inherited and acquired forms of arrhythmia, it is not surprising that I_Na,L has emerged as a potential target for reducing risk of arrhythmia. A wealth of pre-clinical data support the utility of I_Na,L blockers in preventing a wide range of arrhythmias [2,4,8,14–16]. Moreover, recent clinical trials demonstrate efficacy of the I_Na,L blocker ranolazine for reducing incidence of supraventricular arrhythmias and new onset AF in myocardial infarction patients, as well as termination of paroxysmal AF and prevention of post-operative AF [6,7]. These studies all point to the anti-arrhythmia efficacy of I_Na,L blocking drugs. While there is clearly a basis for optimism, it is important to note limitations, including the nonspecific nature of these compounds with effects on other I_Na properties (i.e. peak current) and other channels, and effects that may vary across regions of the heart [29]. Next-generation compounds such as GS-458967 attempt to address some of these limitations by increasing selectivity for I_Na,L with promising anti-arrhythmic results in pre-clinical studies [30]. Furthermore, neuronal Na_v isoforms (e.g. Na_v1.1, Na_v1.2), in addition to Na_v1.5, likely make an important contribution to I_Na,L and may complicate therapeutic targeting of late current [31]. At the same time, while studies from our group and others point to I_Na,L as an important gateway for arrhythmogenic imbalance of intracellular ion homeostasis, it is clearly not the only source. Pathogenic signals upstream of I_Na,L (e.g. CaMKII) alter the function of other key substrates important for normal cell excitability and ion homeostasis. For example, sarcoplasmic reticulum Ca²⁺ release channels are also phosphorylated by CaMKII in stress conditions and promote arrhythmia by increasing likelihood of inappropriate spontaneous Ca²⁺ release from the sarcoplasmic reticulum [32]. Thus, while the data support the potential of an anti-arrhythmic strategy involving suppression of I_Na,L, it remains to be determined whether I_Na,L block alone will be necessary/sufficient or whether it should be considered in the context of an adjuvant therapy.

Financial and competing interests disclosure

The authors have received support from the National Institutes of Health (grant number HL114893 to TJ Hund) and from the James S. McDonnell Foundation (to TJ Hund). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.