Synapsin 2 deficiency impairs NCX1 transport and increases susceptibility to ventricular arrhythmias and heart failure mortality

Abstract Background Synapsin 2 (Syn2) modulate vesicles in the post-synaptic terminal and is connected to sudden unexplained death in epilepsy. Given the unknown role of Syn2 in the heart, and the association of vesicle transport in neuronal cells and cardiomyocytes, we have investigated the role of Syn2 in the myocardium and during heart failure (HF) development. Purpose To characterize the localization and functional role of Syn2 in cardiomyocytes, and to compare mortality and the prevalence of ventricular arrhythmia in Syn2 KO mice and WT littermates. Methods We explored left ventricular (LV) Syn2 levels in two different experimental HF models and assessed mortality in Syn2 KO mice vs. WT littermates after pressure-overload induced by aortic banding. We used confocal imaging and virus transduction to characterize Syn2 localization and co-localization with NCX1 and Rab proteins. We verified Syn2-NCX1 interaction by immunoprecipitation, mass spectrometry, and surface resonance experiments. We investigated calcium handling in isolated cardiomyocytes. Susceptibility for ventricular arrhythmias was tested during ISO stress testing in the Langendorff-model with explanted hearts. Results We found Syn2, but not synapsin 1, to be expressed in the myocardium. LV Syn2 levels were markedly downregulated in the failing myocardium, both after myocardial infarction and pressure-overload. Syn2 KO mice had increased mortality compared to WT littermates following aortic banding, but demonstrated no clear clinical or echocardiographic phenotype, except reduced fractional shortening. Given no clear etiology for increased mortality, we next explored the association between Syn2 and ion-channel vesicle transport, calcium handling and ventricular arrhythmias. By confocal imaging and viral transduction, we found Syn2 to localize in vesicles in HL-1 cells, where Syn2 co-localized with Rab2, Rab3, Rab7 and NCX1. Syn2 was also found to directly interact with NCX1 by other methods. NCX1 levels were downregulated in the membrane fraction in the LV of Syn2 KO mice compared to WT littermates following aortic banding. We observed increased frequency of calcium sparks and waves in isolated Syn2 KO cardiomyocytes compared to controls, and Syn2 KO mice had increased frequency of ventricular arrhythmias compared to WT littermate mice. Explanted hearts from Syn2 KO mice demonstrated more severe ventricular arrhythmias compared to hearts from WT littermate controls. Conclusions We report for the first time that Syn2 is expressed in the myocardium and that Syn2 is involved in NCX1 transport and localization. We also found markedly reduced left ventricle Syn2 levels in HF individuals and mice that lacked Syn2 more frequent displayed severe ventricular arrhythmias and had increased mortality. Hence, our data suggest that reduced Syn2 in the failing myocardium may lead to increased mortality, possibly linked to altered NCX trafficking and subsequent ventricular arrhythmias.