Isoproterenol Promotes Rapid Ryanodine Receptor Movement to BIN1 Organized Dyads
Background—The key pathophysiology of human acquired heart failure is impaired calcium transient, which is initiated at dyads consisting ryanodine receptors (RyR) at sarcoplasmic reticulum apposing CaV1.2 channels at t-tubules. Sympathetic tone regulates myocardial calcium transients through β-adrenergic receptor (β-AR) mediated phosphorylation of dyadic proteins. Phosphorylated-RyRs (P-RyR) have increased calcium sensitivity and open probability, amplifying calcium transient at a cost of receptor instability. Given that BIN1 (Bridging Integrator 1) organizes t-tubule microfolds and facilitates CaV1.2 delivery, we explored whether β-AR regulated RyRs are also affected by BIN1.
Methods and Results—Isolated adult mouse hearts or cardiomyocytes were perfused for 5 min with β-AR agonist isoproterenol (1µmol/L) or blockers CGP+ICI (baseline). Using biochemistry and super-resolution fluorescent imaging, we identified that BIN1 clusters P-RyR and CaV1.2. Acute β-AR activation increases coimmunoprecipitation between P-RyR and cardiac spliced BIN1+13+17 (with exons 13 and 17). Isoproterenol redistributes BIN1 to t-tubules, recruiting P-RyRs and improving the calcium transient. In cardiac specific Bin1 heterozygotes (Bin1 HT) mice, isoproterenol fails to concentrate BIN1 to t-tubules, impairing P-RyR recruitment. The resultant accumulation of uncoupled P-RyRs increases the incidence of spontaneous calcium release. In human hearts with end-stage ischemic cardiomyopathy, we find that BIN1 is also 50% reduced, with diminished P-RyR association with BIN1.
Conclusions—Upon β-AR activation, reorganization of BIN1-induced microdomains recruits P-RyR into dyads, increasing the calcium transient while preserving electrical stability. When BIN1 is reduced as in human acquired heart failure, acute stress impairs microdomain formation, limiting contractility and promoting arrhythmias.
- Received July 16, 2015.
- Revision received November 25, 2015.
- Accepted December 21, 2015.