Atrial Myocyte NLRP3/CaMKII Nexus Forms a Substrate for Postoperative Atrial Fibrillation

Abstract

Rationale: Post-operative atrial fibrillation (POAF) is a common and troublesome complication of cardiac surgery. POAF is generally believed to occur when post-operative triggers act on a pre-existing vulnerable substrate, but the underlying cellular and molecular mechanisms are largely unknown. Objective: To identify cellular POAF-mechanisms in right-atrial samples from patients without a history of atrial fibrillation undergoing open-heart surgery. Methods and Results: Multicellular action potentials, membrane ion-currents (perforated patch-clamp) or simultaneous membrane-current (ruptured patch-clamp) and [Ca 2+ ]i-recordings in atrial cardiomyocytes, along with protein-expression levels in tissue homogenates or cardiomyocytes, were assessed in 265 atrial samples from patients without or with POAF. No indices of electrical, profibrotic, or connexin remodeling were noted in POAF, but Ca 2+ -transient amplitude was smaller while spontaneous sarcoplasmic-reticulum (SR) Ca 2+ -release events and L-type Ca 2+ -current alternans occurred more frequently. Ca 2+ /calmodulin-dependent protein kinase-II (CaMKII) protein-expression, CaMKII-dependent phosphorylation of the cardiac ryanodine-receptor channel type-2 (RyR2) and RyR2 single-channel open-probability were significantly increased in POAF. SR Ca 2+ -content was unchanged in POAF despite greater SR Ca 2+ -leak, with a trend towards increased SR Ca 2+ -ATPase activity. POAF patients also showed stronger expression of activated components of the NLRP3-inflammasome system in atrial whole-tissue homogenates and cardiomyocytes. Acute application of interleukin-1beta caused NLRP3-signaling activation and CaMKII-dependent RyR2/phospholamban hyperphosphorylation in HL-1-cardiomyocytes and enhanced spontaneous SR Ca 2+ -release events in both POAF-cardiomyocytes and HL-1-cardiomyocytes. Computational modeling showed that RyR2-dysfunction and increased SR Ca 2+ -uptake are sufficient to reproduce the Ca 2+ -handling phenotype and indicated an increased risk of proarrhythmic delayed afterdepolarizations in POAF-subjects in response to interleukin-1beta. Conclusions: Pre-existing Ca 2+ -handling abnormalities and activation of NLRP3-inflammasome/CaMKII signaling are evident in atrial cardiomyocytes from patients who subsequently develop POAF. These molecular substrates sensitize cardiomyocytes to spontaneous Ca 2+ -releases and arrhythmogenic afterdepolarizations, particularly upon exposure to inflammatory mediators. Our data reveal a potential cellular and molecular substrate for this important clinical problem.

Surgery-related cardiac stress: A susceptibility test of late atrial fibrillation recurrence?

“…Therefore, the surgery-induced stress likely identifies a subset of patients with propensity to develop clinical AF providing some sort of “susceptibility test” for long-term recurrence of AF [15] . Interestingly, in our previous study [13] , no differences between patients with POAF and postoperative sinus rhythm were noted for sex, body-mass index, comorbidities and echocardiographic parameters including left atrial diameter, degree of valvular regurgitation, and left-ventricular ejection fraction. The only difference was older age and renal function, which suggests that these two factors could play an important role in the evolution of the pro-arrhythmic atrial substrate.…”

“…In particular, post-operative inflammation can exacerbate pre-existing atrial abnormalities, promoting the formation of delayed afterdepolarizations that can lead to POAF. Combined these findings provide a novel unifying paradigm that POAF patients share a common atrial cardiomyopathy with patients with common forms of AF, predisposing POAF-patients to both POAF caused by transient surgery-induced inflammation (explaining the usually self-limited nature of POAF) and to long-term AF (explaining the predilection for recurrence) [13] . Clearly, much more work is needed to detect and precisely define the nature of this silent atrial cardiomyopathy and its causal and mechanistic contribution to new-onset POAF and its late recurrences.…”

“…In agreement, we recently identified molecular evidence for the presence of a pre-existing subclinical atrial cardiomyopathy that predisposes POAF-patients to AF development. Pre-existing Ca 2+ -handling abnormalities and activation of NLRP3-inflammasome/CaMKII signaling are evident in atrial cardiomyocytes from patients who subsequently develop POAF [13] . Despite the presence of abnormalities in patients destined to manifest POAF, none of these patients had manifested clinical AF, indicating that these abnormalities were insufficient in themselves to generate AF.…”

Stress-driven cardiac calcium mishandling via a kinase-to-kinase crosstalk

“…Although protein kinase A (PKA) has also been shown to phosphorylate RyR2 channels, inconsistent findings regarding the arrhythmic effect of PKA hyperphosphorylation of RyR2 at Ser2809 in HF were reported [ 3 , 7 , 16 , 85 , 90 , 110 , 120 , 135 , 138 , 140 , 154 ], further suggesting the key role of CaMKIIδ in HF-evoked arrhythmias. In recent years, accumulating evidence suggests that CaMKIIδ-dependent RyR2 channel dysfunction also leads to SR Ca 2+ mishandling and triggered activities (delayed afterdepolarizations (DADs)) in the atria of chronic AF patients and post-operative AF patients [ 55 , 131 ]. For instance, activated CaMKII was found to increase both arrhythmic Ca 2+ activities and profibrotic activity-caused structural remodeling in chronic AF patients associated with HF [ 28 , 96 ].…”

Systematic review of pre-clinical therapies for post-operative atrial fibrillation

“…Emerging evidence has shown that POAF arises from a combination of pre-existing cardiomyopathic changes in the atria, surgical-induced changes in atrial substrate and post-operative insults (such as inflammation, altered neural regulation and oxidative stress) [ 6 – 8 ]. These electrical and structural changes increase AF vulnerability by creating a pro-fibrillatory substrate while altered calcium handling increases the risk for delayed afterdepolarizations and the stimulation of ectopic atrial beats that trigger the arrhythmia [ 9 – 11 ].…”