βIV-Spectrin regulates STAT3 targeting to tune cardiac response to pressure overload

Heart failure (HF) remains a major source of morbidity and mortality in the US. The multifunctional Ca2+/calmodulin-dependent kinase II (CaMKII) has emerged as a critical regulator of cardiac hypertrophy and failure, although the mechanisms remain unclear. Previous studies have established that the cytoskeletal protein βIV-spectrin coordinates local CaMKII signaling. Here, we sought to determine the role of a spectrin-CaMKII complex in maladaptive remodeling in HF. Chronic pressure overload (6 weeks of transaortic constriction [TAC]) induced a decrease in cardiac function in WT mice but not in animals expressing truncated βIV-spectrin lacking spectrin-CaMKII interaction (qv3J mice). Underlying the observed differences in function was an unexpected differential regulation of STAT3-related genes in qv3J TAC hearts. In vitro experiments demonstrated that βIV-spectrin serves as a target for CaMKII phosphorylation, which regulates its stability. Cardiac-specific βIV-spectrin-KO (βIV-cKO) mice showed STAT3 dysregulation, fibrosis, and decreased cardiac function at baseline, similar to what was observed with TAC in WT mice. STAT3 inhibition restored normal cardiac structure and function in βIV-cKO and WT TAC hearts. Our studies identify a spectrin-based complex essential for regulation of the cardiac response to chronic pressure overload. We anticipate that strategies targeting the new spectrin-based "statosome" will be effective at suppressing maladaptive remodeling in response to chronic stress.

Abstract 394: The Role Of Camkii-dependent Late Sodium Current In Ischemia/reperfusion-associated Arrhythmogenesis

Ischemic heart disease is the leading cause of death in the world. Importantly, CaMKII activity is upregulated during ischemia/reperfusion (I/R). However, the mechanisms by which CaMKII induces arrhythmias remains incompletely understood. We tested the hypothesis that CaMKII-dependent upregulation of the late Na+ current (INa,L) via direct phosphorylation of Nav1.5 at Ser571 is responsible for increased arrhythmia susceptibility during I/R. The role of CaMKII-dependent regulation of INa,L was explored using mice harboring Nav channel variants Nav1.5-Ser571E (S571E, phosphomimetic) and Nav1.5-Ser571A (S571A, phosphorylation incompetent). Isolated, perfused WT, S571E and S571A hearts were subjected to global ischemia (15 min) followed by reperfusion (15 min). Immunoblots demonstrated a significant increase in phosphorylated Nav1.5 at Ser571 in WT hearts during I/R compared to baseline (2.2±0.3 vs 1.0±0.3, respectively; P<0.05, n=5). Optical mapping using the voltage-sensitive dye di-4-ANEPPs (4 μM) revealed a significant increase in action potential duration in WT and S571E mice compared to S571A during I/R (WT: 55.2±0.6 ms*, S571E: 70.0±2.3 ms*, S571A: 46.0±2.9 ms; *P<0.05 vs. S571A, n=5/genotype). Furthermore, spontaneous arrhythmia incidence was increased in WT and S571E hearts (WT: 0.60±0.06 events/min*, S571E: 0.81±0.14 events/min*, S571A : 0.23±0.03 events/min; *P<0.05 vs. S571A, n=5/genotype). Interestingly, arrhythmia inducibility using an S1-S2 protocol was also increased in WT and S571E hearts (WT: 80%*, S571E: 100%*, S571A: 0%; *P<0.05 vs. S571A, n=5/genotype). The Na+ channel blocker mexiletine (10 μM) normalized APD and decreased arrhythmia inducibility in WT hearts during I/R (0% arrhythmia incidence in WT hearts treated with mexiletine). These data indicate a central role for CaMKII-dependent phosphorylation of Nav1.5 and augmentation of INa,L in the development of arrhythmogenesis during I/R.

CaMKII-dependent late Na+ current increases electrical dispersion and arrhythmia in ischemia-reperfusion

The mechanisms underlying Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-induced arrhythmias in ischemia-reperfusion (I/R) are not fully understood. We tested the hypothesis that CaMKII increases late Na⁺ current ( I_Na,L) via phosphorylation of Na_v1.5 at Ser⁵⁷¹ during I/R, thereby increasing arrhythmia susceptibility. To test our hypothesis, we studied isolated, Langendorff-perfused hearts from wild-type (WT) mice and mice expressing Na_v channel variants Na_v1.5-Ser571E (S571E) and Na_v1.5-Ser571A (S571A). WT hearts showed a significant increase in the levels of phosphorylated CaMKII and Na_v1.5 at Ser⁵⁷¹ [p-Na_v1.5(S571)] after 15 min of global ischemia (just before the onset of reperfusion). Optical mapping experiments revealed an increase in action potential duration (APD) and APD dispersion without changes in conduction velocity during I/R in WT and S571E compared with S571A hearts. At the same time, WT and S571E hearts showed an increase in spontaneous arrhythmia events (e.g., premature ventricular contractions) and an increase in the inducibility of reentrant arrhythmias during reperfusion. Pretreatment of WT hearts with the Na⁺ channel blocker mexiletine (10 μM) normalized APD dispersion and reduced arrhythmia susceptibility during I/R. We conclude that CaMKII-dependent phosphorylation of Na_v1.5 is a crucial driver for increased I_Na,L, arrhythmia triggers, and substrate during I/R. Selective targeting of this CaMKII-dependent pathway may have therapeutic potential for reducing arrhythmias in the setting of I/R. NEW & NOTEWORTHY Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of Na_v1.5 at Ser⁵⁷¹ leads to a prolongation of action potential duration (APD), increased APD dispersion, and increased arrhythmia susceptibility after ischemia-reperfusion in isolated mouse hearts. Genetic ablation of the CaMKII-dependent phosphorylation site Ser⁵⁷¹ on Na_v1.5 or low-dose mexiletine (to inhibit late Na⁺ current) reduced APD dispersion, arrhythmia triggers, and ventricular tachycardia inducibility.