The Golgi apparatus is a functionally distinct Ca2+ store regulated by the PKA and Epac branches of the ß1-adrenergic signaling pathway.

Ca(2+) release from the Golgi apparatus regulates key functions of the organelle, including vesicle trafficking. We found that the Golgi apparatus was the source of prolonged Ca(2+) release events that originated near the nuclei of primary cardiomyocytes. Golgi Ca(2+) release was unaffected by depletion of sarcoplasmic reticulum Ca(2+), and disruption of the Golgi apparatus abolished Golgi Ca(2+) release without affecting sarcoplasmic reticulum function, suggesting functional and spatial independence of Golgi and sarcoplasmic reticulum Ca(2+) stores. ß1-Adrenoceptor stimulation triggers the production of the second messenger cAMP, which activates the Epac family of Rap guanine nucleotide exchange factors and the kinase PKA (protein kinase A). Phosphodiesterases (PDEs), including those in the PDE3 and PDE4 families, degrade cAMP. Activation of ß1-adrenoceptors stimulated Golgi Ca(2+) release, an effect that required activation of Epac, PKA, and the kinase CaMKII. Inhibition of PDE3s or PDE4s potentiated ß1-adrenergic-induced Golgi Ca(2+) release, which is consistent with compartmentalization of cAMP signaling near the Golgi apparatus. Interventions that stimulated Golgi Ca(2+) release appeared to increase the trafficking of vascular endothelial growth factor receptor-1 (VEGFR-1) from the Golgi apparatus to the surface membrane of cardiomyocytes. In cardiomyocytes from rats with heart failure, decreases in the abundance of PDE3s and PDE4s were associated with increased Golgi Ca(2+) release events. These data suggest that the Golgi apparatus is a focal point for ß1-adrenergic-stimulated Ca(2+) signaling and that the Golgi Ca(2+) store functions independently from the sarcoplasmic reticulum and the global Ca(2+) transients that trigger contraction in cardiomyocytes.

Caveolae compartmentalise ß2-adrenoceptor signals by curtailing cAMP production and maintaining phosphatase activity in the sarcoplasmic reticulum of the adult ventricular myocyte.

Inotropy and lusitropy in the ventricular myocyte can be efficiently induced by activation of ß1-, but not ß2-, adrenoceptors (ARs). Compartmentation of ß2-AR-derived cAMP-dependent signalling underlies this functional discrepancy. Here we investigate the mechanism by which caveolae (specialised sarcolemmal invaginations rich in cholesterol and caveolin-3) contribute to compartmentation in the adult rat ventricular myocyte. Selective activation of ß2-ARs (with zinterol/CGP20712A) produced little contractile response in control cells but pronounced inotropic and lusitropic responses in cells treated with the cholesterol-depleting agent methyl-ß-cyclodextrin (MBCD). This was not linked to modulation of L-type Ca(2+) current, but instead to a discrete PKA-mediated phosphorylation of phospholamban at Ser(16). Application of a cell-permeable inhibitor of caveolin-3 scaffolding interactions mimicked the effect of MBCD on phosphorylated phospholamban (pPLB) during ß2-AR stimulation, consistent with MBCD acting via caveolae. Biosensor experiments revealed ß2-AR mobilisation of cAMP in PKA II signalling domains of intact cells only after MBCD treatment, providing a real-time demonstration of cAMP freed from caveolar constraint. Other proteins have roles in compartmentation, so the effects of phosphodiesterase (PDE), protein phosphatase (PP) and phosphoinositide-3-kinase (PI3K) inhibitors on pPLB and contraction were compared in control and MBCD treated cells. PP inhibition alone was conspicuous in showing robust de-compartmentation of ß2-AR-derived signalling in control cells and a comparatively diminutive effect after cholesterol depletion. Collating all evidence, we promote the novel concept that caveolae limit ß2-AR-cAMP signalling by providing a platform that not only attenuates production of cAMP but also prevents inhibitory modulation of PPs at the sarcoplasmic reticulum. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Effects of cholesterol depletion on compartmentalized cAMP responses in adult cardiac myocytes.

ß(1)-Adrenergic receptors (ß(1)ARs) and E-type prostaglandin receptors (EPRs) both produce compartmentalized cAMP responses in cardiac myocytes. The role of cholesterol-dependent lipid rafts in producing these compartmentalized responses was investigated in adult rat ventricular myocytes. ß(1)ARs were found in lipid raft and non-lipid raft containing membrane fractions, while EPRs were only found in non-lipid raft fractions. Furthermore, ß(1)AR activation enhanced the L-type Ca(2+) current, intracellular Ca(2+) transient, and myocyte shortening, while EPR activation had no effect, consistent with the idea that these functional responses are regulated by cAMP produced by receptors found in lipid raft domains. Using methyl-ß-cyclodextrin to disrupt lipid rafts by depleting membrane cholesterol did not eliminate compartmentalized behavior, but it did selectively alter specific receptor-mediated responses. Cholesterol depletion enhanced the sensitivity of functional responses produced by ß(1)ARs without having any effect on EPR activation. Changes in cAMP activity were also measured in intact cells using two different FRET-based biosensors: a type II PKA-based probe to monitor cAMP in subcellular compartments that include microdomains associated with caveolar lipid rafts and a freely diffusible Epac2-based probe to monitor total cytosolic cAMP. ß(1)AR and EPR activation elicited responses detected by both FRET probes. However, cholesterol depletion only affected ß(1)AR responses detected by the PKA probe. These results indicate that lipid rafts alone are not sufficient to explain the difference between ß(1)AR and EPR responses. They also suggest that ß(1)AR regulation of myocyte contraction involves the local production of cAMP by a subpopulation of receptors associated with caveolar lipid rafts.

Separate elements within a single IQ-like motif in adenylyl cyclase type 8 impart ca2+/calmodulin binding and autoinhibition.

The ubiquitous Ca(2+)-sensing protein calmodulin (CaM) fulfills its numerous signaling functions through a wide range of modular binding and activation mechanisms. By activating adenylyl cyclases (ACs) 1 and 8, Ca(2+) acting via calmodulin impacts on the signaling of the other major cellular second messenger cAMP. In possessing two CaM-binding domains, a 1-5-8-14 motif at the N terminus and an IQ-like motif (IQlm) at the C terminus, AC8 offers particularly sophisticated regulatory possibilities. The IQlm has remained unexplored beyond the suggestion that it bound CaM, and the larger C2b region of which it is part was involved in the relief of autoinhibition of AC8. Here we attempt to distinguish the function of individual residues of the IQlm. From a complementary approach of in vitro and cell population AC activity assays, as well as CaM binding, we propose that the IQlm alone, and not the majority of the C2b, imparts CaM binding and autoinhibitory functions. Moreover, this duality of function is spatially separated and depends on amino acid side-chain character. Accordingly, residues critical for CaM binding are positively charged and clustered toward the C terminus, and those essential for the maintenance of autoinhibition are hydrophobic and more N-terminal. Secondary structure prediction of the IQlm supports this separation, with an ideally placed break in the alpha-helical nature of the sequence. We additionally find that the N and C termini of AC8 interact, which is an association specifically abrogated by fully Ca(2+)-bound, but not Ca(2+)-free, CaM. These data support a sophisticated activation mechanism of AC8 by CaM, in which the duality of the IQlm function is critical.