Dynamic targeting of the agonist-stimulated m2 muscarinic acetylcholine receptor to caveolae in cardiac myocytes

Dynamic regulation of endothelial nitric oxide synthase: complementary roles of dual acylation and caveolin interactions

N-Terminal myristoylation and thiopalmitoylation of the endothelial isoform of nitric oxide synthase (eNOS) are required for targeting the enzyme to specialized signal-transducing microdomains of plasma membrane termed caveolae. We have previously documented that the subcellular localization of eNOS is dynamically regulated by agonists such as bradykinin, which promotes enzyme depalmitoylation and translocation from caveolae. More recently, we have shown that association of eNOS with caveolin, the principal structural protein in caveolae, leads to enzyme inhibition, in a reversible process modulated by Ca2+-calmodulin (CaM). We now report studies of the respective roles of acylation and caveolin interaction for regulating eNOS activity. Using eNOS truncation and deletion mutants expressed in COS-7 cells, we have identified an obligatory role for the N-terminal half of eNOS in stabilizing its association with caveolin. By exploring the differential effects of detergents (CHAPS vs octyl glucoside), we have shown that this direct interaction between both proteins is facilitated by, but does not require, eNOS acylation, and, importantly, that treatment of intact aortic endothelial cells with the calcium ionophore A23187 leads to the rapid disruption of the eNOS-caveolin complexes. Finally, using transiently transfected COS-7 cells, we have observed that the myristoylation-deficient cytosol-restricted eNOS mutant (myr-) as well as the cytosolic fraction of the palmitoylation-deficient eNOS mutant (palm-) may both interact with caveolin; this association also leads to a marked inhibition of enzyme activity, which is completely reversed by addition of calmodulin. We conclude that the regulatory eNOS-caveolin association is independent of the state of eNOS acylation, indicating that agonist-evoked Ca2+/CaM-dependent disruption of the caveolin-eNOS complex, rather than agonist-promoted depalmitoylation of eNOS, relieves caveolin's tonic inhibition of enzyme activity. We therefore propose that caveolin may serve as an eNOS chaperone regulating NO production independently of the enzyme's residence within caveolae or its state of acylation.

New aspects of G-protein-coupled receptor signalling and regulation

Research on the structure, regulation and signalling properties of the family of seven-transmembrane-helix, heterotrimeric guanine nucleotide-binding protein (G-protein)-coupled receptors (GPCRs) continues at a frantic pace. This reflects their central role in transmission of hormone- and neurotransmitter-encoded information across the plasma membrane of cells. The location of the ligand-binding sites on the extracellular face of the membrane has made them obvious targets for therapeutic intervention in a wide range of conditions resulting from endocrine imbalance. Furthermore, based on the identification of many novel GPCR sequences emerging from expressed sequence tags (ESTs) and other DNA sequencing programmes, it has become clear that the GPCR family is likely to be considerably larger than appreciated in even the recent past. Although neither the natural ligands nor synthetic pharmaceuticals have yet been identified for these so-called ;orphan' GPCRs, they offer the potential for a plethora of new therapeutic targets. Within a short review, it is impossible to cover all the current developments in this field and the topics selected represent a personal view of recent highlights of areas that provide both novel and general insights into the function and regulation of GPCRs.

Endothelial derived vasorelaxation is impaired in human APO A-I transgenic rabbits

Endothelium-derived relaxing factor (nitric oxide: NO) may provide an endogenous defence against atherosclerosis which impairs endothelium-dependent vascular relaxation. Atherosclerosis development is inhibited in cholesterol fed human apo A-I transgenic rabbits (Duverger, N., Circulation, 1996, 94, 713-717). We investigated if endothelium-dependent vascular relaxation is modified in human apo A-I transgenic rabbits by testing in vitro endothelium-dependent receptor-dependent vascular relaxation to acetylcholine and endothelium-dependent receptor-independent vascular relaxation to A23187 of abdominal aorta, precontracted with phenylephrine, in human apo A-I transgenic rabbits (n=4) versus non transgenic littermates (n=4). Endothelium-independent vascular relaxation was investigated with sodium nitroprusside. Vascular precontraction to phenylephrine was significantly increased in human apo A-I transgenic rabbits (p<0.05) while endothelium-independent vascular relaxation to nitroprusside was similar between human apo A-I transgenic rabbits and control rabbits. Endothelium-dependent receptor-dependent and receptor-independent vascular relaxations were reduced in human apo A-I transgenic rabbits (p<0.05). Maximum endothelium-dependent receptor-dependent vascular relaxation was negatively correlated with HDL-cholesterol and total apo A-I (rabbit+ human) plasma levels (r=0.87 and 0.86, p=0.01, respectively) but not with atherogenic plasma lipid (VLDL-cholesterol, LDL-cholesterol, VLDL+LDL cholesterol, triglycerides, apolipoprotein B) levels. These results suggest that the transgenesis of human apo A-I in rabbits impairs signal transduction of endothelial NO synthesis.

Organization of G proteins and adenylyl cyclase at the plasma membrane

There is mounting evidence for the organization and compartmentation of signaling molecules at the plasma membrane. We find that hormone-sensitive adenylyl cyclase activity is enriched in a subset of regulatory G protein-containing fractions of the plasma membrane. These subfractions resemble, in low buoyant density, structures of the plasma membrane termed caveolae. Immunofluorescence experiments revealed a punctate pattern of G protein alpha and beta subunits, consistent with concentration of these proteins at distinct sites on the plasma membrane. Partial coincidence of localization of G protein alpha subunits with caveolin (a marker for caveolae) was observed by double immunofluorescence. Results of immunogold electron microscopy suggest that some G protein is associated with invaginated caveolae, but most of the protein resides in irregular structures of the plasma membrane that could not be identified morphologically. Because regulated adenylyl cyclase activity is present in low-density subfractions of plasma membrane from a cell type (S49 lymphoma) that does not express caveolin, this protein is not required for organization of the adenylyl cyclase system. The data suggest that hormone-sensitive adenylyl cyclase systems are localized in a specialized subdomain of the plasma membrane that may optimize the efficiency and fidelity of signal transduction.

Internalization of the m2 muscarinic acetylcholine receptor. Arrestin-independent and -dependent pathways

Recent studies have identified agonist-dependent phosphorylation as a critical event in the rapid uncoupling of the m2 muscarinic cholinergic receptors (mAChR) from G-proteins and sequestration of the receptors away from the cell surface. However, mutant m2 mAChRs were identified that were phosphorylated but unable to desensitize in adenylyl cyclase assays, while they internalized like wild type (WT) mAChRs. We have tested whether these properties might stem from differences in the abilities of the WT and mutant mAChR to bind arrestins, proteins implicated in both receptor/G-protein uncoupling and internalization. We have determined that arrestin binding requires phosphorylation at a cluster of Ser/Thr residues in amino acids 307-311 in the m2 mAChR. A strong correlation was found between the ability of WT and mutant receptors to bind arrestins in vitro or in vivo and to desensitize in adenylyl cyclase assays. However, the phosphorylation-dependent internalization of the m2 mAChR in HEK-tsA201 cells did not require arrestins and did not proceed via clathrin-mediated endocytosis. While the m2 mAChR was able to enter a clathrin- and arrestin-dependent pathway when arrestin 2 or arrestin 3 was significantly overexpressed, the preferred pathway of internalization of WT and certain mutant m2 mAChR in HEK-tsA201 cells did not involve participation of arrestins. The results suggest that the phosphorylation-mediated regulation of the m2 mAChR may involve arrestin-dependent and -independent events.