Fixing ryanodine receptor Ca leak - a novel therapeutic strategy for contractile failure in heart and skeletal muscle

Abstract

A critical component in regulating cardiac and skeletal muscle contractility is the release of Ca(2+) via ryanodine receptor (RyR) Ca(2+) release channels in the sarcoplasmic reticulum (SR). In heart failure and myopathy, the RyR has been found to be excessively phosphorylated or nitrosylated and depleted of the RyR-stabilizing protein calstabin (FK506 binding protein 12/12.6). This remodeling of the RyR channel complex results in an intracellular SR Ca(2+) leak and impaired contractility. Despite recent advances in heart failure treatment, there are still devastatingly high mortality rates with this disease. Moreover, pharmacological treatment for muscle weakness and myopathy is nearly nonexistent. A novel class of RyR-stabilizing drugs, rycals, which reduce Ca(2+) leak by stabilizing the RyR channels due to preservation of the RyR-calstabin interaction, have recently been shown to improve contractile function in both heart and skeletal muscle. This opens up a novel therapeutic strategy for the treatment of contractile failure in the cardiac and skeletal muscle.

Figure 1. Excitation-contraction coupling in cardiac and skeletal muscle

A) Figure showing the principals of Ca²⁺ cycling in excitation-contraction (E-C) coupling of the cardiomyocyte. Upon depolarization of the sarcolemma, L-type Ca²⁺ channels open to allow Ca²⁺ to enter the cell. This Ca²⁺ will in turn activate the juxtaposed RyR and more Ca²⁺ will be released. The bulk of cytoplasmic [Ca²⁺] increase is due to RyR-mediated release of Ca²⁺ from the SR. Cytoplasmic Ca²⁺ will trigger myofilament activity so that contraction can occur. To relax, cytoplasmic Ca²⁺ is pumped back into the SR via the SR Ca²⁺ ATPase (SERCA2a) and extruded out of the cell through the sarcolemmal Na+-Ca²⁺ exchanger (NCX). B) Depicts Ca²⁺ handling in skeletal muscle E-C coupling. L-type Ca²⁺ channels at the sarcolemma and RyR1 in the SR membrane are in close vicinity with each other. Depolarization of the sarcolemma causes the L-type Ca²⁺ channel to interact with and activate the RyR independent of entry of extracellular Ca²⁺. Virtually all of the increase in cytoplasmic [Ca²⁺] that is needed for myofilament activation and force generation comes from SR Ca²⁺ release. The Ca²⁺ is cleared from the cytoplasm via the SERCA. The absence of transsarcolemmal Ca²⁺ fluxes underscores the importance of RyR1 Ca²⁺ release function in skeletal muscle ECC.

Figure 1. Excitation-contraction coupling in cardiac and skeletal muscle

A) Figure showing the principals of Ca²⁺ cycling in excitation-contraction (E-C) coupling of the cardiomyocyte. Upon depolarization of the sarcolemma, L-type Ca²⁺ channels open to allow Ca²⁺ to enter the cell. This Ca²⁺ will in turn activate the juxtaposed RyR and more Ca²⁺ will be released. The bulk of cytoplasmic [Ca²⁺] increase is due to RyR-mediated release of Ca²⁺ from the SR. Cytoplasmic Ca²⁺ will trigger myofilament activity so that contraction can occur. To relax, cytoplasmic Ca²⁺ is pumped back into the SR via the SR Ca²⁺ ATPase (SERCA2a) and extruded out of the cell through the sarcolemmal Na+-Ca²⁺ exchanger (NCX). B) Depicts Ca²⁺ handling in skeletal muscle E-C coupling. L-type Ca²⁺ channels at the sarcolemma and RyR1 in the SR membrane are in close vicinity with each other. Depolarization of the sarcolemma causes the L-type Ca²⁺ channel to interact with and activate the RyR independent of entry of extracellular Ca²⁺. Virtually all of the increase in cytoplasmic [Ca²⁺] that is needed for myofilament activation and force generation comes from SR Ca²⁺ release. The Ca²⁺ is cleared from the cytoplasm via the SERCA. The absence of transsarcolemmal Ca²⁺ fluxes underscores the importance of RyR1 Ca²⁺ release function in skeletal muscle ECC.

Figure 2. Dissociation of calstabin (FKBP12/FKBP12.6) from the RyR macromolecular complex underlies SR Ca²⁺ leak. A–B)

Under normal conditions, the interaction between calstabin and the RyR subunits stabilize the channel function and minimizes SR Ca²⁺ leak when the cell is in resting condition(A). B) Upon activation of the RyR (e.g. by increased [Ca²⁺]or via interaction with the DHPR), the channel opens and Ca²⁺ is released from the sarcoplasmic reticulum (SR) into the cytosol, driven by the large SR-cytosolic concentration gradient. C–D) The RyR is a target for stress signals (e.g. β-adrenergic receptor-mediated PKA-dependent phosphorylation of serine residues, nitrosylation of cysteines and oxidative modifications of the RyR). Chronic stress signaling can lead to dissociation of calstabin from the Ca²⁺ release channel that then becomes leaky, Ca²⁺ oozes out through the channel and the SR Ca²⁺ load is diminished (C). Under such conditions, the driving force for Ca²⁺ across the SR membrane is reduced and upon activation of the RyR less Ca²⁺ will be released to the cytosol.