Obesity, an established risk factor of atrial fibrillation (AF), is frequently associated with enhanced inflammatory response. However, whether inflammatory signaling is causally linked to AF pathogenesis in obesity remains elusive. We recently demonstrated that the constitutive activation of the ‘NACHT, LRR, and PYD Domains-containing Protein 3’ (NLRP3) inflammasome promotes AF susceptibility. In this study, we hypothesized that the NLRP3 inflammasome is a key driver of obesity-induced AF. Western blotting was performed to determine the level of NLRP3 inflammasome activation in atrial tissues of obese patients, sheep, and diet-induced obese (DIO) mice. The increased body weight in patients, sheep, and mice was associated with enhanced NLRP3-inflammasome activation. To determine whether NLRP3 contributes to the obesity-induced atrial arrhythmogenesis, wild-type (WT) and NLRP3 homozygous knockout (NLRP3^−/−) mice were subjected to high-fat-diet (HFD) or normal chow (NC) for 10 weeks. Relative to NC-fed WT mice, HFD-fed WT mice were more susceptible to pacing-induced AF with longer AF duration. In contrast, HFD-fed NLRP3^−/− mice were resistant to pacing-induced AF. Optical mapping in DIO mice revealed an arrhythmogenic substrate characterized by abbreviated refractoriness and action potential duration (APD), two key determinants of reentry-promoting electrical remodeling. Upregulation of ultra-rapid delayed-rectifier K⁺-channel (Kv1.5) contributed to the shortening of atrial refractoriness. Increased profibrotic signaling and fibrosis along with abnormal Ca²⁺ release from sarcoplasmic reticulum (SR) accompanied atrial arrhythmogenesis in DIO mice. Conversely, genetic ablation of Nlrp3 (NLRP3^−/−) in HFD-fed mice prevented the increases in Kv1.5 and the evolution of electrical remodeling, the upregulation of profibrotic genes, and abnormal SR Ca²⁺ release in DIO mice. These results demonstrate that the atrial NLRP3 inflammasome is a key driver of obesity-induced atrial arrhythmogenesis and establishes a mechanistic link between obesity-induced AF and NLRP3-inflammasome activation.

The acid sphingomyelinase/ceramide system has been shown to be important for cellular infection with at least some viruses, for instance rhinovirus or SARS-CoV-2. Functional inhibition of the acid sphingomyelinase using tricyclic antidepressants prevented infection of epithelial cells, for instance with SARS-CoV-2. The structure of ambroxol, i.e. trans-4-[(2,4-dibromanilin-6-yl)-methyamino]-cyclohexanol, a mucolytic drug applied by inhalation, suggests that the drug might inhibit the acid sphingomyelinase, and thereby infection with SARS-CoV-2. To test this, we used spike protein pseudotyped viral particles (pp-VSV-SARS-CoV-2 spike), a bona fide system for mimicking SARS-CoV-2 entry into cells. Viral uptake and formation of ceramide localization were determined by fluorescence microscopy, activity of the acid sphingomyelinase by consumption of [14C]sphingomyelin and ceramide was quantified by a kinase method. We found that entry of pp-VSV-SARS-CoV-2 spike required activation of acid sphingomyelinase and release of ceramide, events that were all prevented by pretreatment with ambroxol. We also obtained nasal epithelial cells from human volunteers prior to and after inhalation of ambroxol. Inhalation of ambroxol reduced acid sphingomyelinase activity in nasal epithelial cells, and prevented pp-VSV-SARS-CoV-2 spike-induced acid sphingomyelinase activation, ceramide release, and entry of pp-VSV-SARS-CoV-2 spike ex vivo. The addition of purified acid sphingomyelinase or C16 ceramide restored entry of pp-VSV-SARS-CoV-2 spike into ambroxol-treated epithelial cells. We propose that ambroxol might be suitable for clinical studies to prevent COVID-19.

Chronic kidney disease (CKD) is associated with a higher risk of atrial fibrillation (AF). The mechanistic link between CKD and AF remains elusive. IL-1β, a main effector of NLR family pyrin domain–containing 3 (NLRP3) inflammasome activation, is a key modulator of conditions associated with inflammation, such as AF and CKD. Circulating IL-1β levels were elevated in patients with CKD who had AF (versus patients with CKD in sinus rhythm). Moreover, NLRP3 activity was enhanced in atria of patients with CKD. To elucidate the role of NLRP3/IL-1β signaling in the pathogenesis of CKD-induced AF, Nlrp3^–/– and WT mice were subjected to a 2-stage subtotal nephrectomy protocol to induce CKD. Four weeks after surgery, IL-1β levels in serum and atrial tissue were increased in WT CKD (WT-CKD) mice versus sham-operated WT (WT-sham) mice. The increased susceptibility to pacing-induced AF and the longer AF duration in WT-CKD mice were associated with an abbreviated atrial effective refractory period, enlarged atria, and atrial fibrosis. Genetic inhibition of NLRP3 in Nlrp3^–/– mice or neutralizing anti–IL-1β antibodies effectively reduced IL-1β levels, normalized left atrial dimensions, and reduced fibrosis and the incidence of AF. These data suggest that CKD creates a substrate for AF development by activating the NLRP3 inflammasome in atria, which is associated with structural and electrical remodeling. Neutralizing IL-1β antibodies may be beneficial in preventing CKD-induced AF.

Atrial fibrillation (AF) is associated with altered cAMP/PKA signaling and an AF-promoting reduction of L-type Ca²⁺-current (I_Ca,L), the mechanisms of which are poorly understood. Cyclic-nucleotide phosphodiesterases (PDEs) degrade cAMP and regulate PKA-dependent phosphorylation of key calcium-handling proteins, including the I_Ca,L-carrying Cav1.2_α1C subunit. The aim was to assess whether altered function of PDE type-8 (PDE8) isoforms contributes to the reduction of I_Ca,L in persistent (chronic) AF (cAF) patients. mRNA, protein levels, and localization of PDE8A and PDE8B isoforms were measured by RT-qPCR, western blot, co-immunoprecipitation and immunofluorescence. PDE8 function was assessed by FRET, patch-clamp and sharp-electrode recordings. PDE8A gene and protein levels were higher in paroxysmal AF (pAF) vs. sinus rhythm (SR) patients, whereas PDE8B was upregulated in cAF only. Cytosolic abundance of PDE8A was higher in atrial pAF myocytes, whereas PDE8B tended to be more abundant at the plasmalemma in cAF myocytes. In co-immunoprecipitation, only PDE8B2 showed binding to Cav1.2_α1C subunit which was strongly increased in cAF. Accordingly, Cav1.2_α1C showed a lower phosphorylation at Ser1928 in association with decreased I_Ca,L in cAF. Selective PDE8 inhibition increased Ser1928 phosphorylation of Cav1.2_α1C, enhanced cAMP at the subsarcolemma and rescued the lower I_Ca,L in cAF, which was accompanied by a prolongation of action potential duration at 50% of repolarization. Both PDE8A and PDE8B are expressed in human heart. Upregulation of PDE8B isoforms in cAF reduces I_Ca,L via direct interaction of PDE8B2 with the Cav1.2_α1C subunit. Thus, upregulated PDE8B2 might serve as a novel molecular mechanism of the proarrhythmic reduction of I_Ca,L in cAF. The reduction of L-type Ca²⁺-current in atrial myocytes promotes atrial fibrillation. Phosphodiesterase type 8B binds L-type Ca²⁺ channels and reduces local cAMP levels and PKA-dependent channel phosphorylation, thereby decreasing L-type Ca²⁺-current and abbreviating atrial action potential that promotes atrial fibrillation persistence. ADP, adenosine diphosphate; cAF, persistent (chronic) atrial fibrillation; cAMP, cyclic adenosine monophosphate; DAD, delayed afterdepolarization; I_Ca,L, L-type Ca²⁺ current; LTCC, L-type Ca²⁺ channel; PDE, phosphodiesterase; PLB, phospholamban; RyR2, ryanodine receptor type 2; SERCA2a, sarcoplasmic reticulum Ca^{2 +} ATPase type 2a; SLN, sarcolipin; SR, sinus rhythm.

Postoperative atrial fibrillation (poAF) is AF occurring days after surgery, with a prevalence of 33% among patients undergoing open-heart surgery. The degree of postoperative inflammation correlates with poAF risk, but less is known about the cellular and molecular mechanisms driving postoperative atrial arrhythmogenesis. We performed single-cell RNA-seq comparing atrial nonmyocytes from mice with and without poAF, which revealed infiltrating CCR2 ⁺ macrophages to be the most altered cell type. Pseudotime trajectory analyses identified Il-6 as a gene of interest driving in macrophages, which we confirmed in pericardial fluid collected from human patients after cardiac surgery. Indeed, macrophage depletion and macrophage-specific Il6ra conditional knockout (cKO) prevented poAF in mice. Downstream STAT3 inhibition with TTI-101 and cardiomyocyte-specific Stat3 cKO rescued poAF, indicating a proarrhythmogenic role of STAT3 in poAF development. Confocal imaging in isolated atrial cardiomyocytes (ACMs) uncovered what we believe to be a novel link between STAT3 and CaMKII-mediated ryanodine receptor–2 (RyR2)-Ser(S)2814 phosphorylation. Indeed, nonphosphorylatable RyR2 ^S2814A mice were protected from poAF, and CaMKII inhibition prevented arrhythmogenic Ca ²⁺ mishandling in ACMs from mice with poAF. Altogether, we provide multiomic, biochemical, and functional evidence from mice and humans that IL-6-STAT3-CaMKII signaling driven by infiltrating atrial macrophages is a pivotal driver of poAF, which portends therapeutic utility for poAF prevention.

Objectives: Lung organ scarcity has led to more generous acceptance of organs under the idea of extended-donor criteria. However, long-term effects have to be monitored to redefine present practice. In this study, we investigated the impact of donor age over 55 years in lung transplantation. Methods: In this retrospective study, 186 consecutive double-lung transplantation procedures from January 2000 to December 2008 were evaluated. A total of 19 recipients received lungs from donors aged 55 years or older (range 55–69 years) (group A) and 167 received lungs from younger donors (range 8–54) (group B). In-hospital mortality, intensive care unit (ICU) stay, rejection episodes, lung function and survival up to 5 years were evaluated. Results: In-hospital mortality was similar in both groups (group A: 10.5%; group B: 13.7%). Postoperative ICU stay was 19 ± 33 days versus 17 ± 34 days (A vs B). Rejection episodes as well as postoperative lung function up to 5 years, and overall cumulative 5-year survival (group A: 52.4%; group B: 50.9%) did not reach statistical significance. However, a trend of increased bronchiolitis obliterans syndrome (BOS) prevalence and reduced lung function was noted. Cause of death showed no differences in both groups. Conclusions: Donor age ≥55 years does not compromise immediate and long-term results after lung transplantation, although long-term observation of patients receiving such an organ suggests earlier lung dysfunction. Due to the rising need of organs, lungs from donors aged 55 or older have to be considered for transplantation. However, the acceptance should be based on donor lung evaluation and individual recipient needs. Long-term outcomes over 5 years need to be further investigated.