Abstract
The excitation-contraction (E-C) coupling in heart cells governed by L-type Ca 2+ channels (LCCs) in the cell membrane/T-tubules (TTs) and ryanodine receptors (RyRs) in the junctional sarcoplasmic reticulum (JSR). During heart failure, LCC-RyR signaling became defective, leading to degraded Ca 2+ transients and compromised contractile strength. In the present study, we investigated the structural and molecular mechanisms underlying the compromised LCC-RyR signaling in rat heart failure models produced by transverse aortic constriction surgery. Stereological analysis of transmission electron microscopic images showed that the volume density and the surface area of JSRs and those of JSR-coupled TTs were both decreased in failing heart cells. The TT-JSR junctional structures, known as couplons, were displaced or missing from the Z-line areas. Moreover, the spatial span of individual couplons was markedly reduced in failing heart cells. Knockdown of junctophilin-2 (JP2), a TT-SR linker protein down-regulated in heart failure, fully reproduced the structural deformation and decreased/desynchronized Ca 2+ release, indicating that JP2 down-regulation is at least partially responsible for the structural and functional defects of E-C coupling in heart failure. In searching for upstream regulator of JP2, we found that microRNA-mediated suppression of JP2 expression underlied both TT-SR structural uncoupling and LCC-RyR functional uncoupling. These results revealed molecular and structural mechanisms underlying the defective E-C coupling in failing heart cells, and suggested new therapeutic targets against heart failure.
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