Research
PKA Phosphorylation Dissociates Calstabin2 (FKBP12.6) from the Calcium Release Channel (Ryanodine Receptor): Defective Regulation in Failing Hearts.
The ryanodine receptor (RyR)/calcium release channel on the sarcoplasmic reticulum (SR) is the major source of calcium (Ca2+) required for cardiac muscle excitation contraction (EC) coupling. The channel is a tetramer comprised of four identical type 2 RyR polypeptides (RyR2) and four FK506 binding proteins (calstabin2, FKBP12.6). We show that protein kinase A (PKA) phosphorylation of RyR2 transiently dissociates FKBP12.6 and increases the channel open probability (Po). Using co-sedimentation and co-immunoprecipitation we have defined a macromolecular complex comprised of RyR2, calstabin2, PKA, the protein phosphatases PP1 and PP2A, the phosphodiesterase PDE4D3, and an anchoring protein, mAKAP. In failing human hearts, RyR2 is PKA hyperphosphorylated, resulting in defective channel function due to increased sensitivity to Ca2+-induced activation. RyR2 PKA hyperphosphorylation may promote chronic Ca2+ leak from intracellular Ca2+ stores. LINK TO ARTICLE
FKBP12.6 deficiency and defective calcium release channel (ryanodine receptor) function linked to exercise-induced sudden cardiac death.
Arrhythmias, a common cause of sudden cardiac death, can occur in structurally normal hearts, although the mechanism is not known. In cardiac muscle, the ryanodine receptor (RyR2) on the sarcoplasmic reticulum releases the calcium required for muscle contraction. The FK506 binding protein (FKBP12.6) stabilizes RyR2, preventing aberrant activation of the channel during the resting phase of the cardiac cycle. We show that during exercise, RyR2 phosphorylation by cAMP-dependent protein kinase A (PKA) partially dissociates FKBP12.6 from the channel, increasing intracellular Ca(2+) release and cardiac contractility. FKBP12.6(-/-) mice consistently exhibited exercise-induced cardiac ventricular arrhythmias that cause sudden cardiac death. Mutations in RyR2 linked to exercise-induced arrhythmias (in patients with catecholaminergic polymorphic ventricular tachycardia [CPVT]) reduced the affinity of FKBP12.6 for RyR2 and increased single-channel activity under conditions that simulate exercise. These data suggest that "leaky" RyR2 channels can trigger fatal cardiac arrhythmias, providing a possible explanation for CPVT. LINK TO ARTICLE
Phosphodiesterase 4D deficiency in the ryanodine-receptor complex promotes heart failure and arrhythmias.
Phosphodiesterases (PDEs) regulate the local concentration of 3',5' cyclic adenosine monophosphate (cAMP) within cells. cAMP activates the cAMP-dependent protein kinase (PKA). In patients, PDE inhibitors have been linked to heart failure and cardiac arrhythmias, although the mechanisms are not understood. We show that PDE4D gene inactivation in mice results in a progressive cardiomyopathy, accelerated heart failure after myocardial infarction, and cardiac arrhythmias. The phosphodiesterase 4D3 (PDE4D3) was found in the cardiac ryanodine receptor (RyR2)/calcium-release-channel complex (required for excitation-contraction [EC] coupling in heart muscle). PDE4D3 levels in the RyR2 complex were reduced in failing human hearts, contributing to PKA-hyperphosphorylated, "leaky" RyR2 channels that promote cardiac dysfunction and arrhythmias. Cardiac arrhythmias and dysfunction associated with PDE4 inhibition or deficiency were suppressed in mice harboring RyR2 that cannot be PKA phosphorylated. These data suggest that reduced PDE4D activity causes defective RyR2-channel function associated with heart failure and arrhythmias. LINK TO ARTICLE
Sirolimus for the Prevention of In-Stent Restenosis in a Coronary Artery.
Sirolimus (rapamycin), an inhibitor of in-stent restenosis in the coronary arteries, is having a substantial effect on the care of patients with coronary artery disease, was discovered in a soil sample from Easter Island (known locally as Rapa Nui). A naturally occurring product that is isolated from Streptomyces hygroscopicus, sirolimus is an extremely lipophilic (hydrophobic) macrolide that was initially developed as an antifungal agent on the basis of its ability to inhibit the growth of yeast. However, sirolimus was quickly observed to have potent immunosuppressive activity in mammals, which put a halt to its development as an antibiotic. LINK TO ARTICLE
Phosphorylation-dependent regulation of ryanodine receptors: a novel role for leucine/isoleucine zippers.
Ryanodine receptors (RyRs), intracellular calcium release channels required for cardiac and skeletal muscle contraction, are macromolecular complexes that include kinases and phosphatases. Phosphorylation/dephosphorylation plays a key role in regulating the function of many ion channels, including RyRs. However, the mechanism by which kinases and phosphatases are targeted to ion channels is not well understood. We have identified a novel mechanism involved in the formation of ion channel macromolecular complexes: kinase and phosphatase targeting proteins binding to ion channels via leucine/isoleucine zipper (LZ) motifs. Activation of kinases and phosphatases bound to RyR2 via LZs regulates phosphorylation of the channel, and disruption of kinase binding via LZ motifs prevents phosphorylation of RyR2. Elucidation of this new role for LZs in ion channel macromolecular complexes now permits: (a) rapid mapping of kinase and phosphatase targeting protein binding sites on ion channels; (b) predicting which kinases and phosphatases are likely to regulate a given ion channel; (c) rapid identification of novel kinase and phosphatase targeting proteins; and (d) tools for dissecting the role of kinases and phosphatases as modulators of ion channel function.
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