Functional wild-type and carboxyl-terminal-tagged rat substance P receptors expressed in baculovirus-infected insect Sf9 cells

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Recombinant Semliki Forest virus for over-expression and pharmacological characterisation of the histamine H(2) receptor in mammalian cells

We describe the use of recombinant Semliki Forest virus (SFV) vectors for efficient expression of the rat histamine H(2) (rH(2)) receptor in COS-7 (African green monkey kidney cells) cells. Recombinant SFV-infected COS-7 cells express the histamine rH(2) receptor in a time-dependent fashion with a maximum expression level of 50 pmol mg(-1) after 40 h. SFV-mediated histamine rH(2) receptor expression shows similar pharmacological properties as the receptor expressed transiently or stably in mammalian cells. In addition, we demonstrate the pharmacological and functional characterisation of the D(115)N mutated histamine rH(2) receptor. It has been shown that the D(115)N mutation renders the receptor constitutively active and structurally unstable. The rapid onset of and high maximal expression levels obtained from SFV-infected COS-7 cells enabled us to characterise this mutant receptor. We prove that recombinant SFV vectors are powerful tools for heterologous expression of G-protein-coupled receptors and that one can achieve both the high-level gene expression described for baculovirus-infected insect cells and the use of mammalian cells as hosts.

Dopamine D2 receptor dimer formation: evidence from ligand binding

We have examined the binding of two radioligands ([(3)H]spiperone and [(3)H]raclopride) to D(2) dopamine receptors expressed in Chinese hamster ovary cells. In saturation binding experiments in the presence of sodium ions, both radioligands labeled a similar number of sites, whereas in the absence of sodium ions [(3)H]raclopride labeled about half the number of sites labeled by [(3)H]spiperone. In competition experiments in the absence of sodium ions, however, raclopride was able to inhibit [(3)H]spiperone binding fully. In saturation analyses with [(3)H]spiperone in the absence of sodium ions raclopride exerted noncompetitive effects, decreasing the number of sites labeled by the radioligand. These data are interpreted in terms of a model where the receptor exists as a dimer, and in the absence of sodium ions, raclopride exerts negative cooperativity across the dimer both for its own binding and the binding of spiperone. A model of the receptor has been produced that provides a good description of the experimental phenomena described here.

Ig-hepta, a novel member of the G protein-coupled hepta-helical receptor (GPCR) family that has immunoglobulin-like repeats in a long N-terminal extracellular domain and defines a new subfamily of GPCRs

A novel member of the G protein-coupled receptor (GPCR) family was cloned and characterized, which is unique, among the members, in its long extracellular domain comprising Ig-like repeats and in its high expression predominantly in the lung. The clone (Ig-Hepta) was first identified as a polymerase chain reaction product generated with primers designed to amplify secretin receptor family members including the parathyroid hormone-related peptide receptors. Analysis of the open reading frame of cDNAs isolated from a rat lung cDNA library indicated that Ig-Hepta is a protein of 1389 amino acid residues and has two Ig-like repeats in the N-terminal extracellular domain (exodomain) of 1053 amino acid residues and 7 transmembrane spans in the C-terminal region. Northern blot analysis revealed very high expression of its mRNA in the lung and low but detectable levels in the kidney and heart. The mRNA expression in the lung was found to be strongly induced postnatally. Biochemical analysis indicated that Ig-Hepta is a highly glycosylated protein and exists as a disulfide-linked dimer. Immunohistochemistry on rat lung and kidney sections revealed dense localization of Ig-Hepta in alveolar walls and intercalated cells in the collecting duct, respectively, suggesting a role in the regulation of acid-base balance. Ig-Hepta defines a new subfamily of GPCRs.

Dimerization of the calcium-sensing receptor occurs within the extracellular domain and is eliminated by Cys --> Ser mutations at Cys101 and Cys236

Calcium-sensing receptors are present in membranes as dimers that can be reduced to monomers with sufhydryl reagents. All studies were carried out on the human calcium-sensing receptor tagged at the carboxyl terminus with green fluorescent protein (hCaR-GFP) to permit identification and localization of expressed proteins. Truncations containing either the extracellular agonist binding domain plus transmembrane helix 1 (ECD/TMH1-GFP) or the transmembrane domain plus the intracellular carboxyl terminus (TMD/carboxyl terminus-GFP) were used to identify the dimerization domain. ECD/TMH1-GFP was a dimer in the absence of reducing reagents, whereas TMD/carboxyl-terminal GFP was a monomer in the absence or presence of reducing agents, suggesting that dimerization occurs via the ECD. To identify the residue(s) involved in dimerization within the ECD, cysteine --> serine point mutations were made in residues that are conserved between hCaR and metabotropic glutamate receptors. Mutations at positions 60 and 131 were expressed at levels comparable to wild type in HEK 293 cells, had minimal effects on hCaR function, and did not eliminate dimerization, whereas mutations at positions 101 and 236 greatly decreased receptor expression and resulted in significant amounts of monomer in the absence of reducing agents. The double point mutant hCaR(C101S/C236S)-GFP was expressed more robustly than either C101S or C236S and covalent dimerization was eliminated. hCaR(C101S/C236S)-GFP had a decreased affinity for extracellular Ca2+ and slower response kinetics upon increases or decreases in agonist concentration. These results suggest that covalent, disulfide bond-mediated dimerization of the calcium-sensing receptor contributes to stabilization of the ECD and to acceleration of the transitions between inactive and active receptor conformations.

Expression of dopamine D3 receptor dimers and tetramers in brain and in transfected cells

The expression and characteristics of the dopamine D3 receptor protein were studied in brain and in stably transfected GH3 cells. Monoclonal antibodies were used for immunoprecipitation and immunoblot experiments. Immunoprecipitates obtained from primate and rodent brain tissues contain a low molecular weight D3 protein and one or two larger protein species whose molecular mass are integral multiples of the low molecular weight protein and thus appear to have resulted from dimerization and tetramerization of a D3 monomer. Whereas D3 receptor multimers were found to be abundantly expressed in brain, the major D3 immunoreactivity expressed in stable D3-expressing rat GH3 cells was found to be a monomer. However, multimeric D3 receptor species with electrophoretic mobilities similar to those expressed in brain were also seen in D3-expressing GH3 cells when a truncated D3-like protein (named D3nf) was co-expressed in these cells. Furthermore, results from immunoprecipitation experiments with D3- and D3nf-specific antibodies show that the higher-order D3 proteins extracted from brain and D3/D3nf double transfectants also contain D3nf immunoreactivity, and immunocytochemical studies show that the expression of D3 and D3nf immunoreactivities overlaps substantially in monkey and rat cortical neurons. Altogether, these data show oligomeric D3 receptor protein expression in vivo and they suggest that at least some of these oligomers are heteroligomeric protein complexes containing D3 and the truncated D3nf protein.

The neostriatal mosaic: basis for the changing distribution of neurokinin-1 receptor immunoreactivity during development

The pattern of neurokinin-1 receptor-like immunoreactivity (NK-1Rir) was mapped in perinatal and adult mouse striatum by using a new polyclonal antiserum. NK-1Rir was detected in the differentiating regions of the ganglionic eminences on embryonic day 12.5 (E12.5). NK-1Rir structures were enriched in the striatal patch compartment between E16.5 and approximately postnatal day 3 (P3); distributed more uniformly, within portions of both the patch and matrix compartments on P7; and enriched in the matrix compartment in the adult. Analysis of the phenotype of NK-1Rir cells on P2, P7, and in the adult suggested that cholinergic cells accounted for the majority of NK-1Rir cells early postnatally, with increasing contributions from somatostatinergic cells later postnatally. In the adult, approximately half of NK-1Rir cells were cholinergic and half were somatostatinergic. The transient enrichment of NK-1R-bearing cells and processes in the patch compartment which contains cells that express substance P (SP), a putative ligand for the NK-1R, may be a consequence of compartment formation or may be functionally important for compartment development.

A peptide derived from a beta2-adrenergic receptor transmembrane domain inhibits both receptor dimerization and activation

One of the assumptions of the mobile receptor hypothesis as it relates to G protein-coupled receptors is that the stoichiometry of receptor, G protein, and effector is 1:1:1 (Bourne, H. R., Sanders, D. A., and McCormick, F.(1990) Nature 348, 125-132). Many studies on the cooperativity of agonist binding are incompatible with this notion and have suggested that both G proteins and their associated receptors can be oligomeric. However, a clear physical demonstration that G protein-coupled receptors can indeed interact as dimers and that such interactions may have functional consequences was lacking. Here, using differential epitope tagging we demonstrate that beta2-adrenergic receptors do form SDS-resistant homodimers and that transmembrane domain VI of the receptor may represent part of an interface for receptor dimerization. The functional importance of dimerization is supported by the observation that a peptide derived from this domain that inhibits dimerization also inhibits beta-adrenergic agonist-promoted stimulation of adenylyl cyclase activity. Moreover, agonist stimulation was found to stabilize the dimeric state of the receptor, while inverse agonists favored the monomeric species, which suggests that interconversion between monomeric and dimeric forms may be important for biological activity.