Electrical remodeling in human atrial fibrillation

Atrial fibrillation is the most common clinically important cardiac arrhythmia accounting for 20% to 25% of strokes and is a common cause of congestive heart failure.1,2 With the aging population and changing demographics, atrial fibrillation has become an epidemic affecting 2.66 million people in the United States. The prevalence of atrial fibrillation is estimated to increase by 5 fold to 12 million by 2050.3 The diagnosis and treatment of atrial fibrillation represent a significant health care burden of $15.7 billion per year.4 Treatment of atrial fibrillation using antiarrhythmic drugs has been disappointing, while radiofrequency ablation approaches have limitations, including unclear long-term efficacy. Innovation in treatment is needed and pursuit of novel modalities of therapy requires fundamental knowledge in the molecular mechanisms that lead to atrial fibrillation, including electrical remodeling in atrial fibrillation. Electrical remodeling in atrial fibrillation refers to the changes in electrophysiological properties as a result of atrial fibrillation. It is believed to occur as a compensatory mechanism to avoid intracellular Ca2+ overload, which is detrimental to the viability of cardiac cells. However, this maladaptation renders the heart susceptible to the perpetuation of atrial fibrillation.5 It is known that the longer a patient remains in atrial fibrillation, the less likely that spontaneous cardioversion will occur, and the less likely the patient can be successfully cardioverted electrically. Moreover, the incidence for relapse after cardioversion is increased, suggesting changes in the properties of the heart occur with increased duration in atrial fibrillation. Recent studies have shown that the duration of atrial fibrillation is inversely correlated with the success of cardioversion and is the most powerful predictor of outcome, more so than left atrial size or age.6 The concept of electrical remodeling in atrial fibrillation was firmly established in the mid 1990's when some very elegant and insightful experiments were performed. ATRIAL FIBRILLATION BEGETS ATRIAL FIBRILATION Repetitive rapid atrial pacing in animals has been shown to induce atrial fibrillation. The duration of induced atrial fibrillation increased progressively over time from 5 seconds in control animals to sustained atrial fibrillation of over 24 hours after 2 weeks of rapid atrial pacing. These experiments popularized the concept of “atrial fibrillation begets atrial fibrillation” and brought electrical remodeling to the forefront of arrhythmia research.7–9 These studies also showed that artificial maintenance of atrial fibrillation resulted in an increased propensity of atrial fibrillation to become more and more sustained. This was associated with a progressive shortening in atrial fibrillation intervals (the R-R intervals during atrial fibrillation) and in the atrial effective refractory period, while atrial conduction velocity remained unchanged. Two key electrophysiological features of atrial fibrillation were noted. First, as the duration of tachycardia-induced atrial fibrillation was lengthened, atrial refractoriness was progressively shortened at any given pacing rate. Second, the normal shortening of atrial refractoriness with increase in pacing rate seen in control was lost if atrial fibrillation lasted for 24 hours or longer. This loss of rate adaptation in refractoriness is a key feature associated with the development of atrial fibrillation. Similar findings were confirmed in human studies.8 ION CHANNEL REMODELING IN HUMAN ATRIAL FIBRILLATION A considerable body of work exists in delineating the ionic mechanisms that underlie the electrical remodeling in atrial fibrillation in humans. Here, the ion channel remodeling in human atrial fibrillation was reviewed according to the different phases of the cardiac action potential. The results are summarized in Figure 1.Figure 1.: Ion channel remodeling in human atrial fibrillation. The figure on the left depicts an atrial action potential in sinus rhythm with atrial fibrillation-induced remodeling of ionic currents at different phases of the action potential. The result is represented by the figure on the right with significant shortening of the action potential in atrial fibrillation.Phase 0 The major determinant of phase 0 or the upstroke of action potential is the voltage-gated sodium current. Neither the current densities nor the voltage dependence of the current is altered by chronic atrial fibrillation. While the voltage-dependent activation of the sodium current is not changed, the voltage-dependent inactivation of the sodium current is shifted in the depolarizing direction by about 10 mV in atrial fibrillation, suggesting that the biophysical properties of the channel are changed.8,9 Another major determinant of electrical impulse conduction in cardiac tissue is the gap junction. The major gap junction subunits in the atria are connexin 40 and connexin 43. The most consistent finding regarding connexin remodeling in atrial fibrillation is that there is increased lateralization of connexins 40 and 43. In human atria with chronic atrial fibrillation, the pattern of connexin distribution is greatly disturbed with extensive lateral distribution (more than 40%).5,9,10 In addition, there is marked heterogeneity in the distribution of connexins in different parts of the heart. In the fibrillating atrium, these changes may underlie the localized conduction abnormalities and contribute to the initiation and self-perpetuation of reentry in atrial fibrillation. Phase 1 The major ionic determinant of phase 1, which is the early repolarization phase of the action potential, is the transient outward potassium current, Ito. Ito is a very dynamic current and is significantly modulated in various disease states, including cardiac hypertrophy and congestive heart failure. In atrial fibrillation, Ito is dramatically downregulated, by 50 to 60%. Myocytes from both left and right atria in atrial fibrillation showed similar reductions in Ito density compared to controls.5,8,9 Expression of Kv4.3 mRNA and protein, which encodes the Ito, was likewise reduced by about 75% in atrial fibrillation, suggesting that the molecular mechanism of ionic current remodeling is due to changes in gene expression and downregulation of Kv4.3 expression. Phase 2 Phase 2 is determined by the activities of the L-type Ca2+ current and the repolarizing K+ currents. Reduction of the L-type Ca2+ currents has been found consistently in all models of atrial fibrillation. The Ca2+ currents in atrial myocytes from patients with atrial fibrillation showed a 60% to 70% reduction in current density, but the biophysical properties of the channel including voltage-dependent activation and inactivation were not changed in atrial fibrillation.8,9 Downregulation of mRNA encoding Cav1.2, the ICa,L α-subunit, as well as downregulation of the β subunit of the Ca2+ channel have been reported in atrial fibrillation.5,8,9 Interestingly, the decrease in L-type Ca2+ current attenuates the rate-dependent adaptation of human atrial effective refractory period. Atrial myocytes from patients with sinus rhythm shorten their action potential duration appropriately as stimulation frequency is increased. However, cells from patients with atrial fibrillation showed markedly reduced action potential durations which fail to shorten as stimulation frequency increases. This can be mimicked in the normal atrial myocyte by blocking the calcium currents with nifedipine, suggesting that modulation of the L-type Ca2+ current is a major determinant in rate adaptation of atrial refractoriness.8,9 An atrial-specific repolarizing current that is active during phase 2 of the action potential is the ultrarapid delayed rectifier K+ current, IKur. In patients with atrial fibrillation, IKur has been reported to be significantly reduced in association with decreased expression of Kv1.5, which underlies IKur.8,9 However, other studies found no significant changes in IKur in patients with atrial fibrillation. The clinical relevance of IKur in the pathogenesis of atrial fibrillation is underscored by the finding that human familial atrial fibrillation can be caused by a loss-of-function mutation in KCNA5, which encodes Kv1.5.11 The significance of IKur remodeling in human atrial fibrillation warrants further investigation. Phase 3 The major phase 3 currents are the delayed rectifier potassium currents, IKr and IKs. These currents are technically difficult to study and there are no human data on IKr or IKs, in human atrial myocytes with fibrillation.8,9 An important repolarizing current in the human atrium is the acetylcholine-activated K+ current, IKACh, which is a weak inward rectifier potassium current. Activation of IKACh has been shown to strongly promote atrial fibrillation, and IKACh is significantly increased in atrial fibrillation. Moreover, IKACh becomes constitutively active in atrial fibrillation, resulting in a large basal current without activation by agonists.8,9 The major ionic determinant of terminal portion of phase 3 is the strong inward rectifier potassium current, IK1, which is encoded by Kir2.1. In patients with atrial fibrillation, IK1 is uniformly upregulated.8,9 In addition, Kir2.1 mRNA and protein expression are also increased. The upregulation of IK1 is an important contributor to the development of atrial fibrillation as gain-of-function mutations of Kir2.1 are associated with familial atrial fibrillation.12 Phase 4 There are no human data on phase 4 pacemaker currents and no information on If or the T-type Ca2+ currents in atrial myocytes from patients with atrial fibrillation. MECHANISMS CONTRIBUTING TO THE CARDIAC ELECTRICAL REMODELING IN ATRIAL FIBRILLATION Atrial fibrillation is a multifaceted disease with many factors contributing to the cardiac electrical remodeling. The short-term factors that act within minutes to hours include: (1) intracellular Ca2+ increases inactivate the L-type Ca2+ channel and activates proteases like calpain, Ca2+ dependent kinases such as calcium-calmodulin kinase II, and phosphatases, including calcineurin; (2) post-translational modification of ion channels and channel turnover by proteolysis; (3) neurohumoral factors that modulate ion channel and cellular function; (4) increase in oxidative stress alters ion channel function through protein oxidation and nitration. The long-term factors contributing to cardiac electrical remodeling that act over days to weeks include: (1) changes in ion channel gene transcription and protein expression; (2) ion channel redistribution as exemplified by connexin; (3) altered Ca2+ handling and homeostasis; (4) development of fibrosis and structural remodeling such as increase in atrial size; (5) increase in oxidative stress; (6) the role of microRNAs. INCREASED INTRACELLULARC CA2+ OVERLOAD Atrial fibrillation increases the rate of atrial myocyte activation, resulting in an increase in Ca2+ influx into the cell. This increase in intracellular Ca2+ enhances Ca2+ current inactivation, while activating Ca2+ dependent proteases, kinases, and phosphatases. Also, transcription factors such as NFAT (nuclear factor of activated T-cells) are activated.13 To minimize intracellular Ca2+ overload, which threatens the viability of the cell, the action potential duration is abbreviated as a compensatory mechanism to avoid Ca2+ overload. However, the reduced atrial refractoriness promotes the development and maintenance of atrial fibrillation. The increase in intracellular Ca2+ activates phosphatases PP1 and PP2A, dephosphorylates the Ca2+ channel, and reduces the L-type Ca2+ currents. CAMKII is activated and phosphorylates the ryanodine receptor, causing dissociation of calstabin (FKBP12.6) while promoting diastolic leakage of the ryanodine receptor, hence increasing cytosolic Ca2+ overload.14 IP3 receptor expression is increased in AF, and may lead to increased Ca2+-induced Ca2+ release. At the same time, phospholamban and sacrolipin are phosphorylated, leading to increased sarcoplasmic reticulum reuptake of Ca2+. The increase in Ca2+ enhances Na+/Ca2+ exchange activity and promotes the development of delayed afterdepolarizations and triggered activity.13,15 In addition, the increase in cellular Ca2+ activates calcineurin, a Ca2+-activated phosphatase, which dephosphorylates NFAT, causing it to translocate to the nucleus and downregulate the transcription of the gene encoding Cav1.2, the α-subunit of the L-type Ca2+ channel.16 This further reduces the L-type Ca2+ current during the cardiac action potential plateau, leading to shortening of the action potential duration, and thereby promoting atrial fibrillation. OXIDATIVE STRESS AND INFLAMMATION Another major mechanism of electrical remodeling in atrial fibrillation is oxidative stress and inflammation. The evidence that atrial fibrillation is associated with enhanced oxidative stress and inflammation includes the presence of increased lymphomononuclear cell infiltrates in biopsies of atria with fibrillation, an increase in C-reactive protein, a decrease in reducing equivalents like glutathione, increased cellular protein oxidation and tyrosine nitration, and the effects of statins on post-op atrial fibrillation.5,17–20 There are several sources of reactive oxygen species, but the major contributors in atrial fibrillation are thought to be enhanced angiotensin and TGFβ signaling. Activation of the angiotensin receptor and the TGFβ receptor activates NADPH oxidase, which produces reactive oxygen species including superoxide. These reactive oxygen species in turn activate mitogen kinases and trigger a host of cellular events including activation of transcription factors that result in cardiomyocyte hypertrophy, apoptosis, and necrosis, as well as fibroblast proliferation and collagen synthesis leading to fibrosis.5,17 Extracellular superoxide production is also elevated in the left atrial appendage of fibrillating atria due to elevated NADPH oxidase and xanthine oxidase.21 A study by Kim et al22 showed that atrial NADPH oxidase activity is independently associated with an increased risk of post-operative atrial fibrillation. Those at the lowest quartile of NADPH oxidase activity in the right atrial appendage biopsy had a 12% incidence of post-operative atrial fibrillation, whereas those in the top quartile had a 67% percent incidence and the trend had a P value of <0.0001. ROLE OF MICRORNAS MicroRNAs have emerged as important regulators of gene expression, and are predicted to regulate at least a third of all human genes.23 Recent reports have shown that microRNAs are involved with the regulation of cardiac electrophysiology.24 Various microRNAs have been shown to participate in the electrical and structural remodeling of atrial fibrillation. The greatly reduced miR-1 expression in human atrial fibrillation contributes to the upregulation of Kir2.1 channel and leads to increased in IK1.25 MiR-328 was found to be elevated by 3.5-fold in atrial fibrillation patients with rheumatic heart disease and was responsible for the downregulation of the L-type Ca2+ channel genes CACNA1C and CACNB1.26 We have recently found that microRNA-499 is upregulated in fibrillating human atria and is responsible for the downregulation of SK3 channels.27 The clinical relevance of this finding is underscored by the recent finding in genome-wide association studies that identified the KCNN3 gene as closely associated with atrial fibrillation.28 CONCLUSION In summary, the electrical remodeling in atrial fibrillation involves the downregulation of the Ca2+ current and upregualtion of IK1 and IKACh. These changes cause shortening of the action potential and atrial refractoriness, thus rendering the atrial fibrillation wavelength smaller and providing substrates for multiple wavelets. These changes are accompanied by the redistribution of gap junctions, leading to abnormal electrical impulse conduction. In addition, the intracellular Ca2+ overload and abnormal Ca2+ handling may enhance the development of triggered activities, hence promoting the development and maintenance of atrial fibrillation. The underlying mechanisms that produce these changes are multi-factorial and may include enhanced oxidative stress and inflammatory changes, altered Ca2+ homeostasis, and abnormal ion channel expression due to altered microRNA profiles in atrial fibrillation.