Intermolecular Failure of L-type Ca ²⁺ Channel and Ryanodine Receptor Signaling in Hypertrophy

Abstract

Pressure overload–induced hypertrophy is a key step leading to heart failure. The Ca ²⁺-induced Ca ²⁺ release (CICR) process that governs cardiac contractility is defective in hypertrophy/heart failure, but the molecular mechanisms remain elusive. To examine the intermolecular aspects of CICR during hypertrophy, we utilized loose-patch confocal imaging to visualize the signaling between a single L-type Ca ²⁺ channel (LCC) and ryanodine receptors (RyRs) in aortic stenosis rat models of compensated (CHT) and decompensated (DHT) hypertrophy. We found that the LCC-RyR intermolecular coupling showed a 49% prolongation in coupling latency, a 47% decrease in chance of hit, and a 72% increase in chance of miss in DHT, demonstrating a state of “intermolecular failure.” Unexpectedly, these modifications also occurred robustly in CHT due at least partially to decreased expression of junctophilin, indicating that intermolecular failure occurs prior to cellular manifestations. As a result, cell-wide Ca ²⁺ release, visualized as “Ca ²⁺ spikes,” became desynchronized, which contrasted sharply with unaltered spike integrals and whole-cell Ca ²⁺ transients in CHT. These data suggested that, within a certain limit, termed the “stability margin,” mild intermolecular failure does not damage the cellular integrity of excitation-contraction coupling. Only when the modification steps beyond the stability margin does global failure occur. The discovery of “hidden” intermolecular failure in CHT has important clinical implications.

Author Summary

High blood pressure induces hypertrophy, a thickening of the cardiac muscle that eventually leads to heart failure, a leading cause of morbidity and mortality. The contractile power of the heart depends in part on signaling between calcium channels on the cell membrane (L-type Ca ²⁺ channels) and calcium release channels on a specialized calcium-regulating organelle called the sarcoplasmic reticulum. This signaling process is defective in heart failure. We have found that the signaling efficiency between a single L-type channel and its controlled Ca ²⁺ release channels decreases during the transition from hypertrophy to heart failure. Moreover, we find unexpectedly that the signaling failure between channels occurs even before any obvious defect in the cardiac cell's ability to contract is seen. In normal cells, the timing between calcium influx and release is rapid; but in hypertrophy before heart failure manifests, there is a delay in this signaling process. In seeking the underlying mechanisms of this intermolecular failure, we find that a protein known as junctophilin, which anchors the sarcoplasmic reticulum to the cell membrane system, is expressed at a lower level. These results reveal early molecular events associated with the progression of hypertrophy, and may provide new insights for developing methods of early diagnosis and treatment to prevent heart failure.

Abstract

The authors show that although whole-cell coupling of L-type calcium channels and ryanodine receptor current activation remains intact during compensated hypertrophy (before heart failure manifests), intermolecular coupling at a molecular level is already slipping.