[Show abstract][Hide abstract] ABSTRACT: Glycans cover all cellular surfaces and, not surprisingly, are involved in many facets of stem cell biology and technology. For instance, coaxing stem cells to either proliferate or differentiate into the specific cell types needed for transplantation requires intricate glycan-dependent modulation of signalling molecules such as FGF-2, Wnt, and Notch. Moreover, owing to their prominent cell-surface localization and lineage-specific signatures, glycan epitopes such as the stage-specific embryonic antigens (Lewis X/SSEA-1, SSEA3-4) and tumor-rejection antigens (TRA1-60, 1-81) are ideally suited for identifying and isolating specific cell types from heterogeneous populations. Finally, the non-human sialic acid Neu5Gc has been detected on the surface of human embryonic stem cells because of metabolic incorporation from animal products used for their culture. Transplantation of Neu5Gc-contaminated cells poses immunological risks due to the presence, in humans, of circulating antibodies recognizing this glycan epitope.
Preview · Article · Sep 2007 · Current Opinion in Chemical Biology
[Show abstract][Hide abstract] ABSTRACT: 1. Nitric oxide (NO) is known to affect the properties of various proteins via the S-nitrosylation of cysteine residues. This study evaluated the direct effects of the NO donor sodium nitroprusside (SNP) on the pharmacological properties of the AT1 receptor for angiotensin II expressed in HEK-293 cells. 2. SNP dose-dependently decreased the binding affinity of the AT1 receptor without affecting its total binding capacity. This modulatory effect was reversed within 5 min of removing SNP. 3. The effect of SNP was not modified in the presence of the G protein uncoupling agent GTPgammaS or the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. 4. The binding properties of a mutant AT1 receptor in which all five cysteine residues within the transmembrane domains had been replaced by serine was not affected by SNP. Systematic analysis of mutant AT1 receptors revealed that cysteine 289 conferred the sensitivity to SNP. 5. These results suggest that NO decreased the binding affinity of the AT1 receptor by S-nitrosylation of cysteine 289. This modulatory mechanism may be particularly relevant in pathophysiological situations where the beneficial effects of NO oppose the deleterious effects of angiotensin II.
Full-text · Article · Jul 2006 · British Journal of Pharmacology
[Show abstract][Hide abstract] ABSTRACT: Most G protein-coupled receptors (GPCRs) are N-glycosylated proteins but the role of this post-translational modification in GPCR biosynthesis has not been extensively studied. We previously showed that the non-glycosylated AT(1) receptor is inefficiently expressed at the cell surface. In this study, we addressed whether AT(1) interacts with elements of the ER-based quality control processes. Interestingly, non-glycosylated AT(1) receptors associated with the molecular chaperones calnexin and HSP70, suggesting the importance of protein-based interactions between these partners. We also demonstrate that ER mannosidase I participates in the acquisition of mature glycoforms and in the targeting of the AT(1) receptor to the membrane. Taken together, these results indicate that decreased cell-surface expression of the non-glycosylated receptor cannot be attributed to diminished interactions with molecular chaperones and that mannose trimming of the wild-type AT(1) receptor by ER mannosidase I plays a critical role in its cell-surface expression.
No preview · Article · Mar 2006 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: GPCRs (G-protein-coupled receptors) are preferentially N-glycosylated on ECL2 (extracellular loop 2). We previously showed that N-glycosylation of ECL2 was crucial for cell-surface expression of the hAT1 receptor (human angiotensin II receptor subtype 1). Here, we ask whether positioning of the N-glycosylation sites within the various ECLs of the receptor is a vital determinant in the functional expression of hAT(1) receptor at the cell surface. Artificial N-glycosylation sequons (Asn-Xaa-Ser/Thr) were engineered into ECL1, ECL2 and ECL3. N-glycosylation of ECL1 caused a very significant decrease in affinity and cell surface expression of the resulting receptor. Shifting the position of the ECL2 glycosylation site by two residues led to the synthesis of a misfolded receptor which, nevertheless, was trafficked to the cell surface. The misfolded nature of this receptor is supported by an increased interaction with the chaperone HSP70 (heat-shock protein 70). Introduction of N-glycosylation motifs into ECL3 yielded mutant receptors with normal affinity, but low levels of cell surface expression caused by proteasomal degradation. This behaviour differed from that observed for the aglycosylated receptor, which accumulated in the endoplasmic reticulum. These results show how positioning of the N-glycosylation sites altered many properties of the AT1 receptor, such as targeting, folding, affinity, cell surface expression and quality control.
[Show abstract][Hide abstract] ABSTRACT: Asn111, localized in the third transmembrane domain of the AT1 receptor for angiotensin II, plays a critical role in stabilizing the inactive conformation of the receptor. We evaluated the functional and G protein-coupling properties of mutant AT1 receptors in which Asn111 was substituted with smaller (Ala or Gly) or larger residues (Gln or Trp). All four mutants were expressed at high levels in COS-7 cells and, except for N111W-AT1, recognized 125I-Ang II with high affinities comparable to that of the wild-type AT1 receptor. In phospholipase C assays, the four mutants encompassed the entire spectrum of functional states, ranging from constitutive activity (without agonist) for N111A-AT1 and N111G-AT1 to a significant loss of activity (upon maximal stimulation) for N111Q-AT1 and a major loss of activity for N111W-AT1. In Ca2+ mobilization studies, N111W-AT1 produced a weak Ca2+ transient and, unexpectedly, N111G-AT1 also produced a Ca2+ transient that was much weaker than that of the wild-type AT1. The agonist binding affinity of N111W-AT1 was not modified in the presence of GTPgamma S, suggesting that this receptor is not basally coupled to a G protein. GTPgamma S did not modify the high agonist-binding affinity of N111G-AT1 but abolished the coimmunoprecipitation of Gq/11alpha with this constitutively active mutant receptor. These results are a direct demonstration that the N111G-AT1 receptor maintains a high affinity conformation despite being uncoupled from the G protein Gq/11.
[Show abstract][Hide abstract] ABSTRACT: The type 1 receptor for angiotensin II (AT(1)) is a member of the G protein-coupled receptor family. The presence of a caveolin-binding-like motif (phiXphiXXXXphiXXphi where phi is an aromatic residue) within the cytoplasmic tail of the AT(1) receptor suggests an implication for caveolae in the functionality of this receptor. We constructed a mutant AT(1) receptor where each of the aromatic residues in the caveolin-binding-like motif were replaced by alanine (AT(1)-YFFY/A). Mutation of this motif considerably reduced the plasma membrane expression of the receptor that accumulated in a perinuclear compartment. The agonist-induced internalization rate of the AT(1)-YFFY/A receptor was also significantly reduced. Finally, the AT(1)-YFFY/A receptor was poorly activated as indicated by a low agonist-induced production of inositol phosphates. Unexpectedly, the proportion of AT(1) receptor found in caveolae was minor under basal conditions and did not increase under stimulated conditions. Coexpression of the AT(1) receptor with dopamine receptor interacting protein of 78 kDa, a protein implicated in the cellular routing of the dopamine D1 receptor, increased plasma membrane expression of the AT(1) receptor. However, dopamine receptor interacting protein of 78 kDa had no effect on the expression of the AT(1)-YFFY/A receptor. Taken together, these results suggest that the caveolin-binding-like motif of the AT(1) receptor does not promote localization of the receptor to caveolae but rather may act as a docking site for regulatory proteins modulating the routing and the functionality of the receptor.
[Show abstract][Hide abstract] ABSTRACT: The role of N-glycosylation in the pharmacological properties and cell surface expression of AT1 receptor was evaluated. Using site-directed mutagenesis, we substituted both separately and simultaneously the asparagine residues in all three putative N-linked glycosylation consensus sequences (N-X-S/T) of AT1 receptor (positions 4, 176, and 188) with aspartic acid. Expression of these mutant receptors in COS-7 cells followed by photolabeling with [125I]-[p-benzoyl-Phe8]AngII and SDS−PAGE revealed ligand−receptor complexes of four different molecular sizes, indicating that the three N-glycosylation sites are actually occupied by oligosaccharides. Binding studies showed that the affinity of each mutant receptor for [Sar1,Ile8]Ang II was not significantly different from that of wild-type AT1 receptor. Moreover, the functional properties of all mutant receptors were unaffected as evaluated by inositol phosphate production. However, the expression levels of the aglycosylated mutant were 5-fold lower than that of the wild-type AT1 receptor. Use of green fluorescent protein−AT1 receptor fusion proteins in studying the cellular location of the aglycosylated mutant demonstrated that it was distributed at a much higher density to the ER−Golgi complex than to the plasma membrane in HEK 293 cells. Together, these results suggest an important role of N-glycosylation in the proper trafficking of AT1 receptor to the plasma membrane.