Purification and characterization of a receptor for human parathyroid hormone and parathyroid hormone-related peptide

Highly-Sensitive In Vitro Bioassays for FSH, TSH, PTH, Kp, and OT in Addition to LH in Mouse Leydig Tumor Cell

We demonstrate here that highly sensitive in vitro bioassays for FSH, TSH, and PTH can be set up in mouse Leydig Tumor Cells (mLTC), in addition to the normal LH/CG bioassay, after they were transfected with expression vectors encoding the corresponding Gs Protein-Coupled Receptors (GsPCR), such as FSHR, TSHR, or PTHR. Although the β2 adrenergic receptor is also a GsPCR, its expression in mLTC led to a significant but very low cAMP response compared to those observed with FSH, TSH, or PTH. Similarly, after transfection of the GiPCR MT1 melatonin receptor, we did not observe any inhibitory effect by melatonin of the LH or hCG stimulation. Interestingly, after transfection of mLTC with the human kisspeptin receptor (hKpR), which is a GqPCR, we observed a dose-dependent synergy of 10-12-10-7 M kisspeptin variants with a fixed concentration of 0.3 nM LH or hCG. Without any exogenous receptor transfection, a 2 h preincubation with OT or AVP led to a dose-dependent cAMP response to a fixed dose of LH or hCG. Therefore, highly sensitive in vitro bioassays for various hormones and other GPCR ligands can be set up in mLTC to measure circulating concentrations in only 3-10 µL of blood or other body fluids. Nevertheless, the development of an LHRKO mLTC cell line will be mandatory to obtain strict specificity for these bioassays to eliminate potential cross-reaction with LH or CG.

Parathyroid Hormone Senses Extracellular Calcium To Modulate Endocrine Signaling upon Binding to the Family B GPCR Parathyroid Hormone 1 Receptor

Parathyroid hormone (PTH) binds to a family B G protein coupled receptor, parathyroid hormone 1 receptor (PTH1R). One of its functions is to regulate Ca2+ homeostasis in bone remodeling, during which Ca2+ can reach up to 40 mM. A truncated version of PTH, PTH(1-34), can fully activate PTH1R and has been used for osteoporosis treatments. Here, we used fluorescence anisotropy to examine the binding of PTH(1-34) to PTH1R purified in nanodiscs (PTH1R-ND) and found that the affinity increases 5-fold in the presence of 15 mM Ca2+. However, PTHrP(1-36), another truncated endogenous agonist for PTH1R, does not show this Ca2+ effect. Mutations of Glu19 and Glu22 in PTH(1-34) that are not conserved in PTHrP(1-36) largely abolished the Ca2+ effect. The results support that PTH(1-34) not only activates PTH1R but also uniquely senses Ca2+. This dual function of a peptide hormone is a novel observation that couples changes in extracellular environment with endocrine signaling. Understanding this can potentially reveal the complex role of PTH signaling in bone remodeling and improve the PTH(1-34) treatment for osteoporosis.

Biochemical and immunological characterization of a novel monoclonal antibody against mouse leukotriene B4 receptor 1

Leukotriene B₄ (LTB₄) receptor 1 (BLT1) is a G protein-coupled receptor expressed in various leukocyte subsets; however, the precise expression of mouse BLT1 (mBLT1) has not been reported because a mBLT1 monoclonal antibody (mAb) has not been available. In this study, we present the successful establishment of a hybridoma cell line (clone 7A8) that produces a high-affinity mAb for mBLT1 by direct immunization of BLT1-deficient mice with mBLT1-overexpressing cells. The specificity of clone 7A8 was confirmed using mBLT1-overexpressing cells and mouse peripheral blood leukocytes that endogenously express BLT1. Clone 7A8 did not cross-react with human BLT1 or other G protein-coupled receptors, including human chemokine (C-X-C motif) receptor 4. The 7A8 mAb binds to the second extracellular loop of mBLT1 and did not affect LTB₄ binding or intracellular calcium mobilization by LTB₄. The 7A8 mAb positively stained Gr-1-positive granulocytes, CD11b-positive granulocytes/monocytes, F4/80-positive monocytes, CCR2-high and CCR2-low monocyte subsets in the peripheral blood and a CD4-positive T cell subset, Th1 cells differentiated in vitro from naïve CD4-positive T cells. This mAb was able to detect Gr-1-positive granulocytes and monocytes in the spleens of naïve mice by immunohistochemistry. Finally, intraperitoneal administration of 7A8 mAb depleted granulocytes and monocytes in the peripheral blood. We have therefore succeeded in generating a high-affinity anti-mBLT1 mAb that is useful for analyzing mBLT1 expression in vitro and in vivo.

Purification of family B G protein-coupled receptors using nanodiscs: Application to human glucagon-like peptide-1 receptor

Family B G protein-coupled receptors (GPCRs) play vital roles in hormone-regulated homeostasis. They are drug targets for metabolic diseases, including type 2 diabetes and osteoporosis. Despite their importance, the signaling mechanisms for family B GPCRs at the molecular level remain largely unexplored due to the challenges in purification of functional receptors in sufficient amount for biophysical characterization. Here, we purified the family B GPCR human glucagon-like peptide-1 (GLP-1) receptor (GLP1R), whose agonists, e.g. exendin-4, are used for the treatment of type 2 diabetes mellitus. The receptor was expressed in HEK293S GnTl- cells using our recently developed protocol. The protocol incorporates the receptor into the native-like lipid environment of reconstituted high density lipoprotein (rHDL) particles, also known as nanodiscs, immediately after the membrane solubilization step followed by chromatographic purification, minimizing detergent contact with the target receptor to reduce denaturation and prolonging stabilization of receptor in lipid bilayers without extra steps of reconstitution. This method yielded purified GLP1R in nanodiscs that could bind to GLP-1 and exendin-4 and activate Gs protein. This nanodisc purification method can potentially be a general strategy to routinely obtain purified family B GPCRs in the 10s of microgram amounts useful for spectroscopic analysis of receptor functions and activation mechanisms.

Transmembrane signal transduction by peptide hormones via family B G protein-coupled receptors

Although family B G protein-coupled receptors (GPCRs) contain only 15 members, they play key roles in transmembrane signal transduction of hormones. Family B GPCRs are drug targets for developing therapeutics for diseases ranging from metabolic to neurological disorders. Despite their importance, the molecular mechanism of activation of family B GPCRs remains largely unexplored due to the challenges in expression and purification of functional receptors to the quantity for biophysical characterization. Currently, there is no crystal structure available of a full-length family B GPCR. However, structures of key domains, including the extracellular ligand binding regions and seven-helical transmembrane regions, have been solved by X-ray crystallography and NMR, providing insights into the mechanisms of ligand recognition and selectivity, and helical arrangements within the cell membrane. Moreover, biophysical and biochemical methods have been used to explore functions, key residues for signaling, and the kinetics and dynamics of signaling processes. This review summarizes the current knowledge of the signal transduction mechanism of family B GPCRs at the molecular level and comments on the challenges and outlook for mechanistic studies of family B GPCRs.

Expression, surface immobilization, and characterization of functional recombinant cannabinoid receptor CB2

Human peripheral cannabinoid receptor CB2, a G protein-coupled receptor (GPCR) involved in regulation of immune response has become an important target for pharmaceutical drug development. Structural and functional studies on CB2 may benefit from immobilization of the purified and functional receptor onto a suitable surface at a controlled density and, preferably in a uniform orientation. The goal of this project was to develop a generic strategy for preparation of functional recombinant CB2 and immobilization at solid interfaces. Expression of CB2 as a fusion with Rho-tag (peptide composed of the last nine amino acids of rhodopsin) in E. coli was evaluated in terms of protein levels, accessibility of the tag, and activity of the receptor. The structural integrity of CB2 was tested by ligand binding to the receptor solubilized in detergent micelles, captured on tag-specific monoclonal 1D4 antibody-coated resin. Highly pure and functional CB2 was obtained by sequential chromatography on a 1D4- and Ni-NTA-resin and its affinity to the 1D4 antibody characterized by surface plasmon resonance (SPR). Either the purified receptor or fusion CB2 from the crude cell extract was captured onto a 1D4-coated CM4 chip (Biacore) in a quantitative fashion at uniform orientation as demonstrated by the SPR signal. Furthermore, the accessibility of the extracellular surface of immobilized CB2 and the affinity of interaction with a novel monoclonal antibody NAA-1 was studied by SPR. In summary, we present an integral strategy for purification, surface immobilization, ligand- and antibody binding studies of functional cannabinoid receptor CB2.

Calcium-dependent ligand binding and G-protein signaling of family B GPCR parathyroid hormone 1 receptor purified in nanodiscs

GPCRs mediate intracellular signaling upon external stimuli, making them ideal drug targets. However, little is known about their activation mechanisms due to the difficulty in purification. Here, we introduce a method to purify GPCRs in nanodiscs, which incorporates GPCRs into lipid bilayers immediately after membrane solubilization, followed by single-step purification. Using this approach, we purified a family B GPCR, parathyroid hormone 1 receptor (PTH1R), which regulates calcium and phosphate homeostasis and is a drug target for osteoporosis. We demonstrated that the purified PTH1R in nanodiscs can bind to PTH(1-34) and activate G protein. We also observed that Ca(2+) is a weak agonist of PTH1R, and Ca(2+) in millimolar concentration can switch PTH(1-34) from an inverse agonist to an agonist. Hence, our results show that nanodiscs are a viable vehicle for GPCR purification, enabling studies of GPCRs under precise experimental conditions without interference from other cellular or membrane components.

Purification of functional human Cl(-)/HCO(3)(-) exchanger, AE1, over-expressed in Saccharomyces cerevisiae

There is no high-resolution structure for the membrane domain of the human erythrocyte anion exchanger, AE1 (Band 3). In this report, we have developed an expression and purification strategy for AE1 to be used in crystallization trials. Saccharomyces cerevisiae strain BJ5457 was transformed with an expression vector encoding the AE1 membrane domain (AE1MD, amino acids 388-911), fused C-terminally to an epitope tag, corresponding to the nine C-terminal amino acids of rhodopsin. The fusion protein, AE1MD-Rho, was expressed at a concentration of 0.3 mg/l of culture. Confocal immunofluorescence microscopy and sucrose gradient ultracentrifugation revealed that AE1MD-Rho did not process to the plasma membrane of S. cerevisiae, but was retained in an intracellular membrane fraction. Treatment with the endoglycosidase, PNGase F, showed that AE1MD-Rho is not N-glycosylated. AE1MD-Rho solubilized from yeast membranes, with Fos-choline detergent, was purified to 93% homogeneity in a single-step, using a 1D4 antibody affinity resin, in amounts up to 2.5 mg from 18 l of culture. The ability of purified AE1MD-Rho to transport sulfate was examined in reconstituted vesicles. The rate of sulfate efflux mediated by vesicles reconstituted with AE1MD-Rho was indistinguishable from vesicles with purified erythrocyte-source AE1. Using this purification strategy, sufficient amounts of functional, homogeneous AE1MD-Rho can be purified to enable crystallization trials.

Role of the signal peptide in the synthesis and processing of the glucagon-like peptide-1 receptor

BACKGROUND AND PURPOSE

The glucagon-like peptide-1 receptor (GLP-1R) belongs to Family B of the G protein-coupled receptor superfamily and is a target for treatment of type 2 diabetes. Family B G protein-coupled receptors contain a putative N-terminal signal peptide, but its role in receptor synthesis and trafficking are unclear. Further, the signal peptide is not cleaved in at least one family member.

EXPERIMENTAL APPROACH

We examined receptor glycosylation and the role of the signal peptide in GLP-1R synthesis and trafficking using constructs containing epitope tags at the N- and/or C-terminus and in which the signal peptide sequence was either present or absent.

KEY RESULTS

The signal peptide was absolutely required for GLP-1R synthesis but could be substituted to some extent by increasing positive charge in the N-terminal region of the receptor flanking the signal peptide. The signal peptide is cleaved during synthesis and processing of the receptor. An enhanced GFP-epitope tag at the N-terminus of the receptor permitted synthesis of the receptor but blocked signal peptide cleavage and prevented trafficking to the plasma membrane. Cleavage site mutation allowed synthesis of a full-length receptor, blocked signal peptide cleavage and caused retention within the endoplasmic reticulum.

CONCLUSIONS AND IMPLICATIONS

Signal peptide cleavage was not essential for receptor synthesis but was obligatory for processing and trafficking of receptors to the plasma membrane. Further, the GLP-1R is subject to N-linked glycosylation and only the mature, fully glycosylated form of the receptor is present in the plasma membrane. Inhibition of glycosylation prevents processing and cell surface expression of the GLP-1R.

The parathyroid hormone 1 receptor directly binds to the FERM domain of ezrin, an interaction that supports apical receptor localization and signaling in LLC-PK1 cells

PTH 1 receptor (PTH1R) regulates mineral ion homeostasis. Both apical and basolateral PTH1R subpopulations exist within the renal proximal tubule. The purpose of this research was to examine determinants within the PTH1R that direct apical localization. When expressed in LLC-PK1 cells, a proximal tubule cell model, the PTH1R localizes to both apical and basolateral membranes. The C terminus of the PTH1R contains a psd-95, discs large, ZO-1 domain interaction motif that binds the sodium-hydrogen exchanger regulatory factor 1 (NHERF-1), a renal tubule scaffold protein. Receptors lacking the psd-95, discs large, ZO-1 domain interaction motif (PTH1R-CDelta4) partly localize to apical membranes, suggesting that additional factors may be involved. Ezrin, a membrane-cytoskeleton linking protein, directly binds NHERF-1 and thus links assembled complexes to actin. In vitro, subdomain C of the ezrin band 4.1, ezrin, radixin, domain interacts with the C-terminal tail of the PTH1R on a site that is mutually exclusive from the NHERF-1 interaction domain, suggesting the presence of a ternary complex. Mutating the lysine-arginine-lysine motif within the juxtamembrane region of the PTH1R C-terminal tail to alanines markedly disrupts interactions with the band 4.1, ezrin, radixin, domain of ezrin both in vitro and within cells. Inclusion of these mutations in the context of the full-length PTH1R disrupts apical localization with no effect on basolateral expression. Expression of a dominant-negative ezrin selectively disrupts apical expression and signaling of the PTH1R. However, dominant-negative ezrin does not affect expression or signaling of the basolateral PTH1R subpopulation. These findings reveal that direct ezrin interactions promote PTH1R apical localization and signaling in LLC-PK1 cells.

Novel affinity tag system using structurally defined antibody-tag interaction: application to single-step protein purification

Biologically important human proteins often require mammalian cell expression for structural studies, presenting technical and economical problems in the production/purification processes. We introduce a novel affinity peptide tagging system that uses a low affinity anti-peptide monoclonal antibody. Concatenation of the short recognition sequence enabled the successful engineering of an 18-residue affinity tag with ideal solution binding kinetics, providing a low-cost purification means when combined with nondenaturing elution by water-miscible organic solvents. Three-dimensional information provides a firm structural basis for the antibody-peptide interaction, opening opportunities for further improvements/modifications.

Study of a synthetic human olfactory receptor 17-4: expression and purification from an inducible mammalian cell line

In order to begin to study the structural and functional mechanisms of olfactory receptors, methods for milligram-scale purification are required. Here we demonstrate the production and expression of a synthetically engineered human olfactory receptor hOR17-4 gene in a stable tetracycline-inducible mammalian cell line (HEK293S). The olfactory receptor gene was fabricated from scratch using PCR-based gene-assembly, which facilitated codon optimization and attachment of a 9-residue bovine rhodopsin affinity tag for detection and purification. Induction of adherent cultures with tetracycline together with sodium butyrate led to hOR17-4 expression levels of ∼30 µg per 150 mm tissue culture plate. Fos-choline-based detergents proved highly capable of extracting the receptors, and fos-choline-14 (N-tetradecylphosphocholine) was selected for optimal solubilization and subsequent purification. Analysis by SDS-PAGE revealed both monomeric and dimeric receptor forms, as well as higher MW oligomeric species. A two-step purification method of immunoaffinity and size exclusion chromatography was optimized which enabled 0.13 milligrams of hOR17-4 monomer to be obtained at >90% purity. This high purity of hOR17-4 is not only suitable for secondary structural and functional analyses but also for subsequent crystallization trials. Thus, this system demonstrates the feasibility of purifying milligram quantities of the GPCR membrane protein hOR17-4 for fabrication of olfactory receptor-based bionic sensing device.

Affinity purification and characterization of a G-protein coupled receptor, Saccharomyces cerevisiae Ste2p

We present an example of expression and purification of a biologically active G-protein coupled receptor (GPCR) from yeast. An expression vector was constructed to encode the Saccharomyces cerevisiae GPCR alpha-factor receptor (Ste2p, the STE2 gene product) containing a 9-amino acid sequence of rhodopsin that served as an epitope/affinity tag. In the construct, two glycosylation sites and two cysteine residues were removed to aid future structural and functional studies. The receptor was expressed in yeast cells and was detected as a single band in a western blot indicating the absence of glycosylation. Ligand binding and signaling assays of the epitope-tagged, mutated receptor showed it maintained the full wild-type biological activity. For extraction of Ste2p, yeast membranes were solubilized with 0.5% n-dodecyl maltoside (DM). Approximately 120 microg of purified alpha-factor receptor was obtained per liter of culture by single-step affinity chromatography using a monoclonal antibody to the rhodopsin epitope. The binding affinity (K(d)) of the purified alpha-factor receptor in DM micelles was 28 nM as compared to K(d)=12.7 nM for Ste2p in cell membranes, and approximately 40% of the purified receptor was correctly folded as judged by ligand saturation binding. About 50% of the receptor sequence was retrieved from MALDI-TOF and nanospray mass spectrometry after CNBr digestion of the purified receptor. The methods described will enable structural studies of the alpha-factor receptor and may provide an efficient technique to purify other GPCRs that have been functionally expressed in yeast.

Large-scale purification and characterization of human parathyroid hormone-1 receptor stably expressed in HEK293S GnTI− cells

Human parathyroid hormone-1 receptor (hPTHR1) belongs to class II of the G protein-coupled receptor (GPCR) family, whose members all contain a seven-transmembrane helix domain. The receptor regulates bone metabolism through interactions with its ligand, human parathyroid hormone (hPTH). For structural studies of the hPTHR1/hPTH complex, we constructed a mammalian cell line to stably express recombinant hPTHR1 in large-scale. The receptor was solubilized with dodecyl maltoside and purified with affinity chromatography. The purified receptor displayed restricted N-glycosylation as expected. Functionality was demonstrated: the hPTHR1 retained affinity for bPTH-(1-34) and specifically cross-linked to a radioiodinated bPTH-(1-34) analog. This work describes an approach for preparing milligram-scale quantities of receptor for elucidation of the structural biology of this seven-transmembrane GPCR.

Calcitonin gene-related peptide receptor expression in healthy and inflamed human pulp tissue

AIM

To use radioreceptor analysis for comparing calcitonin gene-related peptide (CGRP) receptor expression in human pulp tissue samples collected from teeth having a clinical diagnosis of acute irreversible pulpitis, healthy pulps and teeth with induced inflammation.

METHODOLOGY

Six pulp samples were obtained from teeth having a clinical diagnosis of acute irreversible pulpitis. Another eight pulp samples were obtained from healthy premolars where extraction was indicated for orthodontic purposes. In four of these premolars, inflammation was induced prior to pulp collection. All the samples were processed and labelled with 125I-CGRP. Binding sites were identified by 125I-CGRP and standard CGRP competition assays.

RESULTS

CGRP receptor expression was found in all human pulp tissue samples. Most receptors were found in the group of pulps from teeth having a clinical diagnosis of acute irreversible pulpitis, followed by the group of pulps having induced inflammation. The least number of receptors was expressed in the group of healthy pulps. The Kruskal-Wallis and Mann-Whitney (post-hoc) tests showed statistically significant differences between the groups (P < 0.05).

CONCLUSION

CGRP receptor expression in human pulp tissue is significantly increased during inflammatory phenomena such as acute irreversible pulpitis.

Mechanisms of ligand binding to the parathyroid hormone (PTH)/PTH-related protein receptor: selectivity of a modified PTH(1-15) radioligand for GalphaS-coupled receptor conformations

Mechanisms of ligand binding to the PTH/PTHrP receptor (PTHR) were explored using PTH fragment analogs as radioligands in binding assays. In particular, the modified amino-terminal fragment analog, (125)I-[Aib(1,3),Nle8,Gln10,homoarginine11,Ala12,Trp14,Tyr15]rPTH(1-15)NH2, (125)I-[Aib(1,3),M]PTH(1-15), was used as a radioligand that we hypothesized to bind solely to the juxtamembrane (J) portion of the PTHR containing the extracellular loops and transmembrane helices. We also employed (125)I-PTH(1-34) as a radioligand that binds to both the amino-terminal extracellular (N) and J domains of the PTHR. Binding was examined in membranes derived from cells expressing either wild-type or mutant PTHRs. We found that the binding of (125)I-[Aib(1,3),M]PTH(1-15) to the wild-type PTHR was strongly (approximately 90%) inhibited by guanosine 5'-O-(3-thio)triphosphate (GTPgammaS), whereas the binding of (125)I-PTH(1-34) was only mildly (approximately 25%) inhibited by GTPgammaS. Of these two radioligands, only (125)I-[Aib(1,3),M]PTH(1-15) bound to PTHR-delNt, which lacks most of the receptor's N domain, and again this binding was strongly inhibited by GTPgammaS. Binding of (125)I-[Aib(1,3),M]PTH(1-15) to the constitutively active receptor, PTHR-H223R, was only mildly (approximately 20%) inhibited by GTPgammaS, as was the binding of (125)I-PTH(1-34). In membranes prepared from cells lacking Galpha(S) via knockout mutation of Gnas, no binding of (125)I-[Aib(1,3),M]PTH(1-15) was observed, but binding of (125)I-[Aib(1,3),M]PTH(1-15) was recovered by virally transducing the cells to heterologously express Galpha(S). (125)I-PTH(1-34) bound to the membranes with or without Galpha(S). The overall findings confirm the hypothesis that (125)I-[Aib(1,3),M]PTH(1-15) binds solely to the J domain of the PTHR. They further show that this binding is strongly dependent on coupling of the receptor to Galpha(S)-containing heterotrimeric G proteins, whereas the binding of (125)I-PTH(1-34) can occur in the absence of such coupling. Thus, (125)I-[Aib(1,3),M]PTH(1-15) appears to function as a selective probe of Galpha(S)-coupled, active-state PTHR conformations.

The receptor for parathyroid hormone and parathyroid hormone-related peptide is hydrolyzed and its signaling properties are altered by directly binding the calpain small subunit

We show calcium-dependent, direct binding between the N-terminal portion of the PTH/PTHrP receptor (PTH1R) C-terminal intracellular tail and the calpain small subunit. Binding requires, but may not be limited to, amino acids W474, S475, and W477. The wild-type, full-length rat (r) PTH1R, but not rPTH1R with W474A/W477A substitutions, copurifies with the endogenous calpain small subunit in HEK293 cells. Calpain hydrolyzes delta Nt-rPTH1R, a receptor with a 156-amino acid N-terminal deletion, in a calcium-dependent manner in vitro and in intact cells. Most importantly, PTH stimulation increases the cleavage of delta Nt-rPTH1R and rPTH1R-yellow fluorescent protein in HEK293 cells, and of talin in HEK293 cells expressing rPTH1R-yellow fluorescent protein and in ROS17/2.8 osteoblast-like cells that express rPTH1R endogenously. The absence of calpain in Capn4-null embryonic fibroblasts and the lowered calpain activity in MC3T3-E1 osteoblastic cells due to stable expression of the calpain inhibitor, calpastatin, reduce PTH-stimulated cAMP accumulation. The calpain small subunit is the second protein, in addition to the sodium-hydrogen exchanger regulatory factor, and the first enzyme that binds the PTH1R; PTH1R bound to both of these proteins results in altered PTH signaling.

Presence of a N-terminal signal peptide in class II G protein-coupled receptors: crucial role for expression of the human VPAC1 receptor

The hVPAC1 receptor for vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase activating peptide (PACAP) has an N-terminal signal peptide like all other class II G protein-coupled receptors (GPCRs). We determined the role of the signal peptide in expression of human VPAC1 receptor in transfected CHO cells. Three constructs were transfected: Flag30-hVPAC1, a receptor containing an inserted FLAG sequence between Ala30 and Ala31 and fused in the C-terminal position to GFP; Flag30-[delta1-30]-hVPAC1, the same construct as Flag30-hVPAC1 but lacking the 1-30 putative signal peptide (SP) sequence; Flag0-hVPAC1, a receptor containing an N-terminal FLAG sequence and fused in the C-terminal position to GFP. For each construct, we determined 125I-VIP binding, VIP-induced cAMP production, GFP fluorescence and indirect immunofluorescence on nonpermeabilized cells incubated with mouse monoclonal anti-Flag antibodies. The data were consistent with a crucial role of the signal peptide for expression of functional VPAC1 receptors at the cell surface and suggested that the signal peptide is cleaved during the translocation of the receptor to the plasma membrane, probably in the endoplasmic reticulum.

Purification and biochemical characterization of the D6 chemokine receptor

There is much interest in chemokine receptors as therapeutic targets in diseases such as AIDS, autoimmune and inflammatory disorders, and cancer. Hampering such studies is the lack of accurate three-dimensional structural models of these molecules. The CC-chemokine receptor D6 is expressed at exceptionally high levels in heterologous transfectants. Here we report the purification and biochemical characterization of milligram quantities of D6 protein from relatively small cultures of transfected mammalian cells. Importantly, purified D6 retains full functional activity, shown by displaceable binding of 125I-labelled MIP-1beta (macrophage inflammatory protein-1beta) and by complete binding of the receptor to a MIP-1alpha affinity column. In addition, we show that D6 is decorated on the N-terminus by N-linked glycosylation. Mutational analysis reveals that this glycosylation is dispensable for ligand binding and high expression in transfected cells. Metabolic labelling has revealed the receptor to also be sulphated and phosphorylated. Phosphorylation is ligand independent and is not enhanced by ligand binding and internalization, suggesting similarities with the viral chemokine receptor homologue US28. Like US28, an analysis of the full cellular complement of D6 in transfected cells indicates that >80% is found associated with intracellular vesicular structures. This may account for the high quantities of D6 that can be synthesized in these cells. These unusual properties of D6, and the biochemical characterization described here, leads the way towards work aimed at generating the three-dimensional structure of this seven-transmembrane-spanning receptor.

Characterization of PTH/PTHrP receptor in rat duodenum: effects of ageing

In rat enterocytes, signaling through the parathyroid hormone (PTH)/PTH-related peptide receptor type 1(PTHR1) includes stimulation of adenylyl cyclase, increases of intracellular calcium, activation of phospholipase C, and the MAP kinase pathway, mechanisms that suffer alterations with ageing. The purpose of this study was to evaluate whether an alteration at the level of the PTH receptor (PTHR1) is the basis for impaired PTH signaling in aged rat enterocytes. Western Blot analysis with a specific monoclonal anti-PTHR1 antibody revealed that a 85 kDa PTH binding component, the size expected for the mature PTH/PTHrP receptor, localizes in the basolateral (BLM) and brush border (BBM) membranes of the enterocyte, being the protein expression about 7-fold higher in the BLM. Two other bands of 105 kDa (corresponding to highly glycosylated, incompletely processed receptor form) and 65 kDa (proteolytic fragment) were also seen. BLM PTHR1 protein expression significantly decreases with ageing, while no substantial decrease was observed in the BBM from old rats. PTHR1 immunoreactivity was also present in the nucleus where PTHR1 protein levels were similar in enterocytes from young and aged rats. Immunohistochemical analysis of rat duodenal sections showed localization of PTHR1 in epithelial cells all along the villus with intense staining of BBM, BLM, and cytoplasm. The nuclei of these cells were reactive to the PTHR1 antiserum, but not all cells showed the same nuclear staining. The receptor was also detected in the mucosae lamina propria cells, but was absent in globets cells from epithelia. In aged rats, PTHR1 immunoreactivity was diffused in both membranes and cytoplasm and again, PTH receptor expression was lower than in young animals, while the cell nuclei showed a similar staining pattern than in young rats. Ligand binding to PTHR1 was performed in purified BLM. rPTH(1-34) displaced [I(125)]PTH(1-34) binding to PTHR1 in a concentration-dependent fashion. In both, aged (24 months) and young (3 months) rats, binding of [I(125)]PTH was characterized by a single class of high-affinity binding sites. The affinity of the receptor for PTH was not affected by age. The maximum number of specific PTHR1 binding sites was decreased by 30% in old animals. The results of this study suggest that age-related declines in PTH regulation of signal transduction pathways in rat enterocytes may be due, in part, to the loss of hormone receptors.