Atrial electrical remodeling is a cause and consequence of AF. At the tissue level, electrical remodeling comprises effective refractory period shortening, conduction velocity slowing, wavelength reduction, and frequent atrial ectopy caused by calcium (Ca
2+)-dependent triggered activity [
48‐
50]. While Ca
2+-mediated triggers serve as the critical initiators of AF, an appropriate substrate formed through progressive electrical and structural remodeling of the atria is required for the long-term perpetuation of the arrhythmia and its conversion from paroxysmal to chronic forms.
At the molecular level, electrical remodeling arises from altered expression and function of a host of ion channel, Ca2+ cycling, and gap junction proteins. While a comprehensive discussion of mechanisms by which these proteins are regulated by redox signaling is outside the scope of this review article, we focus on three key players, namely the ryanodine receptors (RYR2), sodium (Na) channels, and gap junction proteins owing to their importance to AF initiation and maintenance. In what follows, we highlight key studies linking oxidative stress to the malfunction of these critical targets.
Oxidative stress in diabetes promotes Ca2+-mediated triggers and the initiation of AF
Triggered activity caused by delayed afterdepolarizations (DADs) is typically required for the initiation of AF. DADs arise from increased Ca
2+ leak from the sarcoplasmic reticulum (SR) via RYR2. This, in turn, causes diastolic SR Ca
2+ release events, which activate the Na
+/Ca
2+ exchanger (NCX), producing a transient-inward current that causes membrane depolarization. If large enough, these depolarizations trigger propagated wavefronts forming premature atrial beats that can initiate AF. Numerous studies have implicated hyperphosphorylation of RYR2 via calmodulin-dependent protein kinase II (CaMKII) in arrhythmogenic SR Ca
2+ leak in ventricular and atrial myocytes of diseased hearts. CAMKII, which typically requires the binding of Ca
2+ to calmodulin for its activation, is also triggered by oxidation. Multiple groups have highlighted the role of mitochondria-derived ROS in the oxidation and therefore activation of CAMKII leading to the arrhythmogenic hyperphosphorylation of RYR2. Indeed, mitochondria-derived ROS have been shown to cause AF in multiple transgenic mouse models harboring leaky RYR2 channels [
51]. With regards to diabetes, Joseph et al. [
52] demonstrated that cardiac lipid overload secondary to peroxisome proliferator-activated receptor gamma (PPAR-γ) overexpression was associated with mitochondrial oxidative stress and increased SR Ca
2+ leak via oxidized RyR2 channels. These defects likely resulted in frequent ventricular ectopy, which was reversed by treatment with a mitochondria targeted anti-oxidant [
52]. Whether a similar mechanism promotes atrial ectopy and AF in diabetes awaits further study.
Abnormal atrial conduction and the maintenance of AF
In large animals and humans, AF is maintained by reentrant excitation forming stable or meandering rotors, leading circle reentry, or multiple circulating wavelets. Unidirectional conduction block is a prerequisite for reentrant excitation and conduction slowing is a key predisposing factor for conduction block. Conduction slowing causes wavelength shortening, which in turn, promotes the stability of AF circuits. Studies by multiple groups have demonstrated substantial conduction slowing and reduced conduction reserve in the atria of diabetic animals. In general, these changes arise as a consequence of structural remodeling (covered in the previous section), decreased Na channel activity, or altered expression, phosphorylation, and localization of gap junction proteins.
Atrial gap junctions are formed by the assembly of connexin (Cx) proteins, namely Cx40 and Cx43. Downregulation of Cx40 as well as hyperphosphorylation and downregulation of Cx43 have been implicated in the electrical remodeling of the diabetic heart that culminates in conduction slowing and AF [
53]. However, these molecular changes have not been corroborated in all studies. For example, Mitasikova et al. [
54] demonstrated paradoxical upregulation (not downregulation) of Cx43 with a decrease (not increase) in Cx43 phosphorylation. These seemingly conflicting results may be attributed to the specific sites of phosphorylation on Cx43 or the stage of diabetes. As case in point, we reported stage-dependent discrepancies in Cx43 phosphorylation in ventricular myocardium of rats with pressure overload hypertrophy [
55]. Specifically, we found hyperphosphorylation and increased expression of Cx43 at the early (compensated) stage of hypertrophy that were followed by marked downregulation and dephosphorylation of the protein at late stages of remodeling [
55]. Notably, conduction slowing was observed at both early and late stages of remodeling but was more severe during the latter [
55]. While the molecular mechanisms underlying gap junction remodeling leading to AF are largely unknown, there is substantial evidence that oxidative stress plays a major role. For one, oxidative stress alters the atrial expression of Cx40 and Cx43 as well as the size of atrial gap junctions in a model of intermittent hypoxia mimicking obstructive sleep apnea [
56]. Moreover, oxidative modification of tyrosine-mediated signaling plays a key role in Cx43 remodeling during the progression of streptozotocin-induced diabetes [
57]. Finally, oxidative stress disrupts Cx43 forward trafficking to the intercalated disk resulting in abnormal gap junction coupling [
58].
Na channel activity plays a major role in mediating proper action potential conduction across the heart. In alloxan-induced diabetic rabbits that are prone to AF, Liu et al. [
44] demonstrated decreased I
Na density that was likely caused by the pro-inflammatory rise in NF-κB levels. In addition to its regulation by inflammatory cytokines, I
Na in ventricular myocytes is highly sensitive to oxidative stress, elevated NADH levels and protein kinase C activation [
59]. Remarkably, treatment with a mitochondria-targeted antioxidant reversed this defect in murine models and in ventricular samples from patients with non-ischemic heart failure [
59]. Although the role of NADPH-ROS signaling in the modulation of atrial Na channel expression and gating will require direct investigation in models of diabetes, elegant findings by the Dudley group highlight the importance of metabolic pathways in the regulation of impulse formation and conduction via direct effects on I
Na activity [
60].
A master metabolic pathway which is highly relevant in the setting of diabetes is mediated by liver kinase B1 (LKB1), an upstream kinase with multiple downstream effectors. One of those effectors is 5′ adenosine monophosphate-activated protein kinase (AMPK), a critical component of the metabolic stress response of the heart to injury and a target of the anti-diabetic agent Metformin. The relevance of this metabolic pathway in diabetes is underscored by studies demonstrating the cardioprotective efficacy of AMPK activation in Goto Kakizaki type-2 diabetic rats [
61] and streptozotocin-induced type-1 diabetic mice [
62]. While LKB1 knockdown was shown by multiple groups to cause adverse ventricular remodeling, hypertrophy and AF, the underlying mechanisms by which defective LKB1 signaling promotes atrial arrhythmias remained unclear. Specifically, whether loss of LKB1
per se causes primary atrial electrical remodeling and AF independently of ventricular dysfunction and heart failure remained unknown. Using LKB1 knockout mice, we showed early remodeling of atrial gap junctions and ion channels that preceded ventricular remodeling or the spontaneous onset of permanent AF [
63]. Specifically, knockdown of LKB1 led to significant downregulation of atrial Cx40 and I
Na peak density, causing prolonged intra-atrial depolarization and inter-atrial conduction block [
63]. Future studies aimed at investigating the role of this metabolic pathway to diabetes-related AF are needed.