Isoproterenol Promotes Rapid Ryanodine Receptor Movement to Bridging Integrator 1 (BIN1)–Organized Dyads

Abstract

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.

Cardiac BIN1 (cBIN1) is a regulator of cardiac contractile function and an emerging biomarker of heart muscle health

“…This particular cardiac isoform cBIN1 is the only BIN1 isoform that is capable of rescuing t-tubule microfolds in transgenic mice with conditional Bin1 knockout in cardiomyocytes (Hong et al, 2014). Furthermore, cBIN1 is the isoform organizing t-tubule LTCC-RyR dyads (Fu et al, 2016). Detailed functions of other BIN1 isoforms in the heart await further studies.…”

“…BIN1 has been implicated in many cellular processes, such as inducing membrane invagination (Frost et al, 2009) and initiating tubulogenesis in skeletal muscle cells (Lee et al, 2002). In human and mouse heart cells, BIN1 localizes to cardiac t-tubules (Hong et al, 2010), forms t-tubule membrane microdomains (Hong et al, 2014;Fu et al, 2016), facilitates cytoskeleton-based calcium channel trafficking to t-tubule membrane (Hong et al, 2010), and causes RyR movement along the SR membrane (Fu et al, 2016).…”

“…Second, BIN1 also facilitates the clustering of calcium channels that are already at the t-tubule membrane. Super-resolution (10-20 nm) fluorescent imaging revealed that LTCCs form large clusters at BIN1-membrnae (Fu et al, 2016), which become smaller in cardiomyocytes with heterozygous Bin1 deletion (Hong et al, 2014;Fu and Hong, 2015). Channel clustering at cBIN1-microdomains may also affect coupled-gating of LTCCs therefore normal channel function.…”

“…Channel clustering at cBIN1-microdomains may also affect coupled-gating of LTCCs therefore normal channel function. Third, in addition to trafficking and clustering LTCCs at the t-tubule membrane, cBIN1 plays a critical role in recruiting the other dyadic protein RyRs to the jSR membrane (Fu et al, 2016). As a result, cBIN1 serves as a protein bridge important for the formation and maintenance of LTCC-RyR couplons, which are essential for normal E-C coupling.…”

“…As a result, cBIN1 serves as a protein bridge important for the formation and maintenance of LTCC-RyR couplons, which are essential for normal E-C coupling. Recruitment of RyRs to cBIN1 is phosphorylation dependent (Fu et al, 2016). By recruiting S2808-phosphorylated RyRs with increased calcium sensitivity and open probability (Marx et al, 2000) to couple with LTCCs, cBIN1 increases CICR gain necessary during an acute stress response in normal hearts.…”