Effects of second intracellular loop mutations on signal transduction and internalization of the gonadotropin-releasing hormone receptor

Inhibition of gonadotropin-releasing hormone receptor signaling by expression of a splice variant of the human receptor

GnRH binds to a specific G protein-coupled receptor in the pituitary to regulate synthesis and secretion of gonadotropins. Using RT-PCR and human pituitary poly(A)+ RNA as a template, the full-length GnRH receptor (wild type) and a second truncated cDNA characterized by a 128-bp deletion between nucleotide positions 522 and 651 were cloned. The deletion causes a frame shift in the open reading frame, thus generating new coding sequence for further 75 amino acids. The truncated cDNA arises from alternative splicing by accepting a cryptic splicing acceptor site in exon 2. Distinct translation products of approximately 45-50 and 42 kDa were immunoprecipitated from COS-7 cells transfected with cDNA coding for wild type GnRH receptor and the truncated splice variant, respectively. Immunocytochemical and enzyme-linked immunosorbent assay studies revealed a membranous expression pattern for both receptor isoforms. Expression of the splice variant, however, occurred at a significantly lower cell surface receptor density. In terms of ligand binding and phospholipase C activation, the wild type receptor showed characteristics of a typical GnRH receptor, whereas the splice variant was incapable of ligand binding and signal transduction. Coexpression of wild type and truncated proteins in transiently or stably transfected cells, however, resulted in impaired signaling via the wild type receptor by reducing maximal agonist-induced inositol phosphate accumulation. The inhibitory effect depended on the amount of splice variant cDNA cotransfected and was specific for the GnRH receptor because signaling via other G(q/11)-coupled receptors, such as the thromboxane A2, M5 muscarinic, and V1 vasopressin receptors, was not affected. Immunological studies revealed that coexpression of the wild type receptor and the truncated splice variant resulted in impaired insertion of the wild type receptor into the plasma membrane. Thus, expression of truncated receptor proteins may highlight a novel principle of specific functional inhibition of G protein-coupled receptors.

Endocytosis and recycling of G protein-coupled receptors

Agonist stimulation of G protein-coupled receptors causes a dramatic reorganization of their intracellular distribution. Activation of receptors triggers receptor endocytosis and, since receptors recycle back to the surface continuously, a new steady state is reached where a significant proportion of receptors is located internally. Although this movement of receptors is remarkable, its role has been enigmatic. Recent developments have provided insight into the compartments through which the receptors move, the nature of the signals that trigger receptor translocation, and the significance of receptor cycling for cell function. In this article, Jennifer Koenig and Michael Edwardson review recent progress in this field and place receptor cycling into a mathematical framework that reveals the extent and rate of intracellular receptor movement.

Ligand-induced internalization of cholecystokinin receptors. Demonstration of the importance of the carboxyl terminus for ligand-induced internalization of the rat cholecystokinin type B receptor but not the type A receptor

Internalization of a variety of different heptahelical G protein-coupled receptors has been shown to be influenced by a number of different structural determinants of the receptors, including the carboxyl terminus. To investigate the role of the carboxyl terminus of cholecystokinin (CCK) receptors in receptor internalization, the rat wild type (WT) CCK-A receptor (WT CCKAR) and the rat WT CCK-B receptor (WT CCKBR) were truncated after amino acid residue 399 (CCKAR Tr399) and 408 (CCKBR Tr408), thereby deleting the carboxyl-terminal 45 and 44 residues, respectively. All WT and mutant CCK receptors were stably expressed in NIH/3T3 cells. Internalization of the CCKAR Tr399 was not significantly different from the WT CCKAR. In contrast, internalization of the CCKBR Tr408 was decreased to 26% compared with the WT CCKBR internalization of 92%. The mutation of all 10 serine and threonine residues (as potential phosphorylation sites) in the carboxyl terminus of the CCKBR to alanines (mutant CCKBR DeltaS/T) could account for the majority of this effect (39% internalization). All mutant receptors displayed similar ligand binding characteristics, G protein coupling, and signal transduction as their respective WT receptors, indicating that the carboxyl termini are not necessary for these processes. Thus, internalization of the CCKBR, unlike that of the CCKAR, depends on the carboxyl terminus of the receptor. These results suggest that, despite the high degree of homology between CCKAR and CCKBR, the structural determinants that mediate the interaction with the endocytic pathway reside in different regions of the receptors.

Effects of intracellular tyrosine residue mutation and carboxyl terminus truncation on signal transduction and internalization of the rat bradykinin B2 receptor

Presently, little is known of the amino acid motif(s) participating in bradykinin B2 receptor-mediated signal transduction processes. In this report we investigate the potential role of the two existing tyrosine (Tyr) residues in the intracellular regions and the carboxyl terminus in the regulatory function of this receptor. Rat-1 cells, which do not contain detectable bradykinin B2 receptor, were transfected with wild type and mutant receptor cDNAs. Tyr-131 and Tyr-321 were each mutated to corresponding alanine-, serine-, and phenylalanine-containing sequences. The last 34 amino acid residues of the carboxyl terminus were truncated. Rat-1 cells transfected with the mutant forms of the receptor cDNA including the truncated COOH-terminal cDNA all bound [3H]bradykinin with essentially the same Kd of approximately 2.2 nM as cells transfected with the wild type bradykinin B2 receptor. However, mutating Tyr-131 resulted in important changes in bradykinin-stimulated phosphoinositide turnover and arachidonate release. For example, exchanging Tyr-131 for alanine led to an 80% decreased arachidonate release (p < 0.005), 90% decrease in inositol phosphate (IP) accumulation (p < 0.001), with receptor uptake at 15 min remaining essentially unchanged. Mutating the same Tyr to phenylalanine resulted in unchanged bradykinin-stimulated IP accumulation, only a slightly lowered arachidonate release, and unchanged receptor uptake at 15 min. Mutating Tyr-321 to alanine resulted in a very different pattern. There was a small but significant reduction in arachidonate release (p < 0.03) and IP accumulation (p < 0.008) with a large, 30%, increase in receptor uptake at 15 min (p < 0.010). Truncation of a portion of the carboxyl tail also proved meaningful, with a 60% decrease in arachidonate release and an 80% decrease in IP accumulation. The truncation also resulted in a large, 130%, decrease in receptor uptake at 15 min (p < 0.023). Taken together, these results point to Tyr-131 as an important element in determining bradykinin-stimulated arachidonate release and IP accumulation. Tyrosine phosphorylation at this site apparently does not play a major role. Tyr-131, Tyr-321, and the carboxyl tail appear to be important in determining receptor uptake.

Evidence for an important functional role of intracellular loop II of the lutropin receptor

The lutropin receptor (LHR) is a G protein-coupled receptor in which high affinity ligand binding occurs to the relatively large extracellular N-terminal domain. Various portions of the receptor have been mapped for their relative importance in localization and in hormone-mediated signaling. There is, however, a paucity of information available on the intracellular loops (ICL), where, along with the C-terminal cytoplasmic tail, G protein coupling is expected to occur. Site-directed mutagenesis was used to investigate the role of several conserved ionizable groups and one tyrosyl residue in ICLs I-III of the rat LHR. The pSVL expression vector, containing the LHR cDNA (wild-type and mutants), was transiently transfected into COS-7 cells, and human choriogonadotropin (hCG) binding and hCG-mediated cAMP production were determined. Several point mutants of amino acid residues in ICL II were prepared and characterized with the following results: replacements of Lys-455 and of His-460 with Glu gave mutant LHRs that failed to localize or fold properly at the cell surface as evidenced by the lack of significant binding to intact cells, although hCG binding could be detected in broken cell preparations, and a neighboring Arg-459 --> Glu replacement had no apparent effect on receptor trafficking, hCG binding or hCG-mediated cAMP-production. A reversal mutant in ICL II in which Glu-441, at the boundary of transmembrane helix III and ICL II, and His-460, at the interface between ICL II and transmembrane helix IV, were interchanged, exhibited hCG binding to intact cells, but the maximal cAMP level at high concentrations of ligand was less than that obtained with COS-7 cells transfected with wild-type LHR. The total number of cell surface receptors determined with the reversal mutant was less than that found with wild-type LHR. This difference, however, is not believed to be responsible for the reduced signaling, since maximal cAMP responses to hCG were obtained with comparable receptor densities of wild-type and various mutant LHRs. Other single replacements in ICL I, Lys-368 --> Glu and to Gln, and in ICL III, Arg-526 --> Glu and Tyr-528 --> Ser, resulted in mutant LHRs with characteristics of wild-type LHR in trafficking, hCG binding and hCG-mediated cAMP production. These findings suggest an important functional role of several amino acid residues in ICL II of LHR.

Mutations in the second cytoplasmic loop of the rat parathyroid hormone (PTH)/PTH-related protein receptor result in selective loss of PTH-stimulated phospholipase C activity

To define the structural requirements of the parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor necessary for activation of phospholipase C (PLC), receptors with random mutations in their second cytoplasmic loop were synthesized, and their properties were assessed. A mutant in which the wild type (WT) rat PTH/PTHrP receptor sequence EKKY (amino acids 317-320) was replaced with DSEL had little or no PTH-stimulated PLC activity when expressed transiently in COS-7 cells, but it retained full capacity to bind ligand and to generate cAMP. This phenotype was confirmed in LLC-PK1 cells stably expressing the DSEL mutant receptor, where both PTH-stimulated PLC activity and sodium-dependent phosphate co-transport were essentially abolished. Individual mutations of these four residues point to a critical role for Lys-319 in receptor-G protein coupling. PTH-generated IPs were reduced to 27 +/- 13% when K319E, compared with the WT receptor, and PLC activation was fully recovered in a receptor revertant in which Glu-319 in the DSEL mutant cassette was restored to the WT residue, Lys. Moreover, the WT receptor and a mutant receptor in which K319R had indistinguishable properties, thus suggesting that a basic amino acid at this position may be important for PLC activation. All of these receptors had unimpaired capacity to bind ligand and to generate cAMP. To ensure adequacy of Galphaq-subunits for transducing the receptor signal, Galphaq was expressed in HEK293 and in LLC-PK1 cells together with either WT receptors or receptors with the DSEL mutant cassette. PTH generated no inositol phosphates (IPs) in either HEK293 or LLC-PK1 cells, when they expressed DSEL mutant receptors together with Galphaq. In contrast, PTH generated 2- and 2. 5-fold increases in IPs, respectively, when these cells co-expressed both the WT receptor and Galphaq. Thus, generation of IPs by the activated PTH/PTHrP receptor can be selectively abolished without affecting its capacity to generate cAMP, and Lys-319 in the second intracellular loop is critical for activating the PLC pathway. Moreover, alpha-subunits of the Gq family, rather than betagamma-subunits, transduce the signal from the activated receptor to PLC, and the PLC, rather than the adenylyl cyclase, pathway mediates sodium-dependent phosphate co-transport in LLC-PK1 cells.

The role of the aspartate-arginine-tyrosine triad in the m1 muscarinic receptor: mutations of aspartate 122 and tyrosine 124 decrease receptor expression but do not abolish signaling

An Asp-Arg-Tyr triad occurs in a majority of rhodopsin-like G protein-coupled receptors. The fully conserved Arg is critical for G protein activation, but the function of the flanking residues is not well understood. We expressed in COS-7 cells m1 muscarinic receptors that were mutated at Asp122 and Tyr124. Most mutations at either position strongly attenuated or prevented the expression of binding sites for the antagonist [3H]N-methylscopolamine. However, sites that were expressed displayed unaltered affinity for the antagonist. Receptor protein, visualized with a carboxyl-terminally directed antibody, was reduced but never completely abolished. The effects of these mutations were partially reversed by the deletion of 129 amino acids from the third intracellular loop of the receptor. In several cases, comparison of immunocytochemistry with binding measurements suggested the presence of substantial amounts of inactive, presumably misfolded, receptor protein. Some of the variants that bound [3H]N-methylscopolamine underwent small changes in their affinities for acetylcholine. All retained nearly normal abilities to mediate an acetylcholine-induced phosphoinositide response. We propose that Asp122 and Tyr124 make intramolecular contacts whose integrity is important for efficient receptor folding but that they do not participate directly in signaling. The role of these residues is completely distinct from that of Arg123, whose mutation abolishes signaling without diminishing receptor expression.

Dependence of fibroblast migration on actin severing activity of gelsolin

Gelsolin nucleates actin filament assembly, blocks the fast-exchanging ends of actin filaments, and severs filaments, processes that may play an important role in cell motility. We studied the relationship between cell migration, gelsolin content, and actin severing activity in human gingival fibroblasts. These cells were keratin negative and desmin negative but expressed vimentin and myosin II. Cells were separated by their ability to migrate in response to a chemoattractant stimulus. Northern analysis of mRNA, [35S]methionine incorporation into immunoprecipitated gelsolin, immunoblots of cell lysates, and quantitative confocal microscopy showed 1.4-2-fold higher levels of gelsolin in nonmigrant compared with migrant cells. Because the concentration of intracellular gelsolin did not appear to be a central determinant of cell migration, we assessed its requirement for motility. Cells that were electroinjected with a gelsolin antibody that inhibits actin severing by gelsolin in vitro showed a 72% reduction of the number of migrant cells compared with controls treated with an irrelevant antibody. Cells that were electroinjected with free gelsolin exhibited a 33% increase in migration compared with cells electroinjected with bovine serum albumin. Compared with nonmigrant cells, migrant cells contained abundant free gelsolin and exhibited gelsolin-dependent F-actin severing activity, which required Ca2+. Serum stimulation of cell migration required increases in [Ca2+]i because incubation with 3 microM 1,2-bis-(o-aminophenoxy)-ethane-N,N, N',N'-tetraacetic acid tetra(acetoxymethyl)-ester blocked calcium flux and cell migration. Serum also stimulated the recruitment of gelsolin into the supernatants of lysates from migrant but not from nonmigrant cells. In fibroblasts, gelsolin concentration alone does not apparently determine migratory capacity. Instead, the Ca2+-dependent actin severing activity of free gelsolin appears to be a major determinant of cell migration.

Reactive oxygen species mediate cytokine activation of c-Jun NH2-terminal kinases

Interleukin 1 (IL-1) and tumor necrosis factor alpha (TNFalpha) are known to induce production of reactive oxygen species (ROS), which have been suggested to act as second messengers. Here we demonstrate that ROS production by bovine chondrocytes upon cytokine stimulation induces c-jun expression. Since c-jun expression is regulated by its own gene product via phosphorylation by c-Jun NH2-terminal kinases (JNKs), we investigated if cytokines and ROS could modulate JNK activity in chondrocyte monolayer cultures. Treatment of bovine chondrocytes with both IL-1 and TNFalpha leads to rapid induction of JNK activity, stimulating JNK activity 7- and 20-fold, respectively. Importantly, the observation that antioxidant treatment antagonizes IL-1 and TNFalpha activation of JNK provides strong evidence that ROS can act as mediators of JNK activity. Moreover, potent activation of JNK is also observed by direct addition of the ROS hydrogen peroxide (H2O2) to the chondrocyte cultures. Nitric oxide (NO), a multifunctional ROS, also appears to simulate JNK, albeit to a lesser extent. These findings identify JNK as another molecular target for the actions of NO and H2O2. In addition, the inhibitory effect of diphenyleneiodonium on JNK activation implicates the involvement of flavonoid-containing enzymes in the ROS-mediated signaling process. Overstimulation of JNK activity by excessive production of ROS may, therefore, underlie pathological conditions such as arthritis and cancer.