Photoaffinity labelling of atrial natriuretic factor (ANF)-R1 receptor by underivatized 125I-ANF. Involvement of lipid peroxidation

Identification of the atrial natriuretic factor-RICreceptor subtype (B-clone) in cultured rat aortic smooth muscle cells

The present report demonstrates the presence in cultured rat aortic smooth muscle cells of a natriuretic factor receptor subtype with a specificity typical of the ANF-R1C (B-clone) receptor subtype. To prove the existence of this receptor subtype in this cell line we show that pCNP-(82-103) is the most potent activator of the intrinsic guanylate cyclase activity, and that [125I]pCNP-(82-103) binds to a specific receptor subtype which is insensitive to the ANF-R2 specific ligand, C-ANF. The investigation of its binding characteristics show the rank potency order of the natriuretic factors in competing for pCNP binding to be pCNP greater than pBNP greater than rANF. Furthermore it was possible to covalently photolabel this receptor subtype with underivatized]125I]pCNP and show that it is composed of a single subunit of 130 kDa with very high specificity for pCNP.

Relationship between age-related increases in rat liver lipid peroxidation and bile canalicular plasma membrane fluidity

Aging is associated with increased cellular levels of lipid peroxides and reactive oxygen species. This increase in the radical species may affect membrane fluidity and consequently membrane functions. We attempted to determine whether age-related increases in lipid peroxide and reactive oxygen species levels affect the annular fluidity of the bile canalicular plasma membrane in young (2-3 weeks old), adult (20 weeks old) and old (100 weeks old) rats. Hepatic levels of lipid peroxides measured with thiobarbituric acid reactive substances (TBARS), reactive oxygen species and reduced glutathione were significantly higher in the old rats than in the young and adult rats. Arachidonic acid (AA) levels increased and docosahexaenoic acid (DHA) levels decreased in the isolated canalicular plasma membrane of the old rats, and the ratio of DHA to AA thus decreased significantly with rat aging. Hepatic TBARS levels correlated negatively with molar ratios of DHA to AA. The annular fluidity of the bile canalicular plasma membrane decreased significantly in the old rats compared with that in the young and adult rats. These results suggest that the age-related decrease in DHA content of the bile canalicular plasma membrane and the molar ratio of DHA to AA may be associated with age-related deterioration of membrane annular fluidity.

Development of p-benzoylbenzoylated [N,C,rANP(1-28)]pBNP32 (pBNP1) derivatives and affinity photolabeling of the bovine NPR-A receptor

Two Nalpha-benzophenone-substituted photoprobes, derived from the high affinity NPR-A chimeric agonist [N, C, rANP(1-28)]pBNP32 (pBNP1) were assembled by solid-phase peptide synthesis. [Nalpha-p-benzoylbenzoyl, Tyr2]pBNP1 (probe A), and [Nalpha-p-benzoylbenzoyl, Tyr18]pBNP1 (probe B) were synthesized and their affinity was tested on bovine zona glomerulosa membrane preparations. Both were found to exert ANP-type high affinities (Kd = 20 pM) with Kd of 10 pM and 30 pM for probe A, and probe B, respectively. Photolabeling of NPR-A with both analogs cross-linked specifically the 130 kDa monomeric NPR-A. The maximal irreversible ligand incorporations were estimated at 18% and 41% for probe A, and probe B, respectively. These results show that the N-terminus of the chimeric compound can be acylated with a large chemical function, such as the benzophenone moiety, without loosing its affinity for the NPR-A receptor. Furthermore, Leu2 or Leu18 can be substituted with tyrosine without disturbing the binding capacity of the ligand. Finally, it appears that the pBNP1 N-terminus is close to the receptor structure as irreversible incorporation is observed after photolabeling.

Comparative binding study of rat natriuretic peptide receptor-A

The natriuretic peptide receptor-A (NPR-A) is involved in blood pressure and body fluid regulation in order to help maintain cardiovascular homeostasis. It has been shown that these biological effects are mediated through the natriuretic peptide family of hormones, which bind NPR-A according to the rank order ANP>BNP>>CNP. Previous studies performed with rat kidney papillary tissue suggested the existence of an heterologous NPR-A population since two binding components were obtained for pBNP32, one of high affinity (pK 9.4 +/- 0.1) and the other of lower affinity (pK 7.5 +/- 0.1), while in the same preparation rANP28 binding displayed the expected affinity (pK 10.22 +/- 0.01) and was best fitted with a model involving a single class of binding sites. This apparent heterogeneity of NPR-A in rat kidney papillae could be explained by the presence of two receptor isoforms or of monomeric and oligomeric forms of the same receptor. To investigate the NPR-A binding heterogeneity, we have cloned the rat NPR-A from PC12 cells and compared its pharmacological profile with that of the papillae. Our results with rat NPR-A transfected Cos-P cells show an equivalent pharmacological profile as with the rat tissue, i.e. a high affinity for rANP28 (pK 10.4 +/- 0.1) and two distinctive affinities for pBNP32 (pK 9.74 +/- 0.05 and 7.8 +/- 0.1). Although multiple receptor glycoforms were sometimes detectable by western blotting, only one molecular form was obtained by cross-linking with 125I-rANP28. It thus appears that NPR-A alone can account for the two binding components found in the rat papillae and that a single molecular form of the protein is implicated. We therefore propose that the oligomerization state of the receptors could be responsible for the apparent binding heterogeneity of rat NPR-A.

Glycosylation of asparagine 24 of the natriuretic peptide receptor-B is crucial for the formation of a competent ligand binding domain

UV cross-linking studies of the natriuretic peptide receptor-B (NPR-B) using radiolabeled C-type natriuretic peptide (CNP) indicate that only fully glycosylated receptors are capable of binding ligand. We therefore used site-directed mutagenesis to determine which potential glycosylation sites are occupied by carbohydrate, and the relevant mutants were characterized in order to understand the function of carbohydrate addition at those sites. Our results suggest that five of seven potential N-linked glycosylation sites are modified. In addition, mutation of asparagine 24 results in a loss of approximately 90% of receptor activity. This mutant is expressed at levels comparable to the wild-type receptor, and its activity is not significantly different from that of wild-type NPR-B in terms of EC50 for CNP. Ligand binding studies on this mutant further show that although there is no change in affinity for ligand, approximately 90% of receptor binding is lost. These data suggest that many of the mutant receptors are simply not properly folded. Our results indicate that glycosylation of asparagine 24 of NPR-B receptors may be critical for the formation of a competent ligand binding domain.

Glycosylation is critical for natriuretic peptide receptor-B function

Co-transfection of a truncated natriuretic peptide receptor-B (NPR-B) with the full length receptor results in a decrease of 60-80% in wild-type receptor activity. This reduction correlates with a loss of glycosylation of the full length NPR-B. This effect is dose-dependent, and occurs with no change in the glycosylation of the truncated receptor. Co-transfection of the full length NPR-B with other receptors yields similar results. These data suggest that glycosylation may be crucial for NPR-B function. Cross-linking studies further demonstrate that only fully glycosylated NPR-B receptors are able to bind ligand. Our data therefore argue that carbohydrate modification may be critical for NPR-B receptor ligand binding.

Localization by photoaffinity labeling of natriuretic peptide receptor-A binding domain

A portion of the ligand binding domain for atrial natriuretic peptide (ANP) was identified as an affinity cross-linked proteolytic fragment of bovine adrenal natriuretic peptide receptor type-A (NPR-A). Affinity purified NPR-A was UV-cross-linked to the amino terminus of 125I-[Tyr2] rat ANP-(2-27). A chymotryptic fragment of the affinity labeled NPR-A was isolated by chromatography and electrophoresis. This fragment yielded a major microsequence corresponding to a region from Met173 to Phe188 of the receptor extracellular domain and containing one N-glycosylation site at Asn180. Bovine NPR-A receptor was then cross-linked to the carboxy terminus of the highly efficient photoaffinity derivative 125I-[Tyr18,Bpa27] rat ANP(1-27). Proteolysis of the affinity labeled NPR-A with cyanogen bromide and trypsin produced radiolabeled and glycosylated fragments of size 15 and 9 kDa, respectively, which contained the epitope Ile181-Phe188 (CS328) and which were detectable by immunoprecipitation with a monospecific polyclonal antibody against CS328. Proteolysis with cyanogen bromide followed by Glu-C produced a shorter photolabeled 6 kDa fragment which was not immunoprecipitable by anti-CS328 antibody and which was not glycosylated. The results lead to the identification of the short segment Asp191-Arg198 as the site of covalent binding of [Tyr18,Bpa27] rat ANP(1-27). This hydrophilic region is adjacent to the epitope Ile181-Phe188 and to the glycosylation site Asn180. It displays the species variability and the high surface probability expected for a portion of the binding domain of NPR-A in contact with ANP.

Molecular weight determination of the hepatic vasopressin receptor with a high-affinity photoprobe

We report here a study of photoaffinity labeling of the V1a-vasopressin receptor with high-affinity, V1-specific radioiodinated antagonist ligands: one containing an azidophenylalanine residue ([beta,beta-dimethyl-beta-mercaptopropionyl(1), p-azido-Phe2,Val4,Lys8,D-Tyr9] vasopressin), two others containing nitrophenylalanine, and one, highly similar but without a photosensitive function, as control. All analogues competed in the dark for the same binding site with vasopressin. Long-wavelength UV irradiation of rat liver membranes incubated in presence of the radio-iodinated azido photolabel produced a specifically labeled protein band at 53 kDa in SDS-PAGE. Identical experiments with the nitrophenylalanyl peptides produced only non-specific labeling and control experiments with the non-photosensitive analogue produced no labeling at all. Chemical crosslinking of 3H-VP to the same membrane preparation produced a result identical to that of the azido photolabel, confirming the receptor nature of the labeled protein. Deglycosylation of the labeled receptor with endoglycosidase F reduced the observed molecular weight of 53 kDa to 43 kDa. The molecular parameters reported herein of the presumed hepatic vasopressin receptor confirm the values deduced from the molecular cloning of the rat V1a receptor.

Production of polyclonal antibody to the bovine adrenal atrial natriuretic factor-R1 receptor

A polyclonal antibody monospecific for an intracellular epitope of the atrial natriuretic factor (ANF)-R1 receptor was produced. The receptor protein (200 pmoles) was purified to homogeneity from bovine adrenal zona glomerulosa (BAZG), reduced, alkylated and digested with trypsin. The tryptic fragments were purified by reverse-phase h.p.l.c. on a C18 column. Based on the sequence of one of these fragments, a peptide was chemically synthesized, coupled to thyroglobulin and injected into rabbits. The antibody obtained was shown to be specific for the R1-type as no receptor was detected in bovine red blood cells (RBC) (which are devoid of ANF receptors) and in NIH-3T3 cell membranes (where only the R2-type is expressed). Several other tissues were screened and comparison of the immunoreactive receptor density estimates with those obtained by ANF binding yielded a correlation coefficient (r2) of 0.965. The minimal detectable dose was typically 3 fmoles/tube and the ED50 of the RIA was 30 fmoles/tube. Cyanogen bromide digestion of the receptor was essential for antigenic detection, indicating that the epitope is probably hindered due to the tertiary structure of the native protein. Moreover, location of the epitope in the kinase homology domain of the receptor, combined with partial tryptic digestion, suggests that the proteolysis-sensitive region of the receptor is located between the transmembrane-spanning domain and the amino acid 586. This method of production of antibodies should be useful to precisely map the amino acids involved in various functions of the receptor.

Evidence for atrial natriuretic peptide (ANP) synthesis and the presence of ANP-transducing receptors in the rat olfactory bulb

This study demonstrates the presence of both atrial natriuretic peptide (ANP) precursor and ANP transcripts in the rat olfactory bulb (OB), a key brain structure involved in the generation of olfaction-dependent behavior. In addition to synthesizing ANP, the OB contains ANP-transducing receptors coupled to the guanylate cyclase system but it is devoid of ANP "clearance receptors." The characterization of biologically active ANP receptors and the evidence for in situ ANP synthesis in this region of the CNS adds credence to the hypothesis that the peptide plays a putative role in olfaction.

Allosteric modulation by ATP of the bovine adrenal natriuretic factor R1 receptor functions

Atrial natriuretic factor (ANF-R1) receptor is a 130-kDa protein that contains a cytoplasmic guanylate cyclase domain. We report that ATP interacts in an allosteric manner with the ANF-R1 receptor, resulting in reduced ANF binding and enhanced ANF-stimulated guanylate cyclase activity. The modulatory properties of various nucleotides indicate a preference for the adenine family with a rank order of potency of ATP greater than App(NH)p greater than or equal to ADP greater than or equal to AMP while cyclic and guanine nucleotides except GTP are inactive. The negative modulation by ATP of ANF binding is specific for the ANF-R1 receptor subtype since the amount of ANF bound by the guanylate cyclase uncoupled ANF-R2 subtype is increased in the presence of ATP. Furthermore, the effects of ATP on ANF-R1 receptor binding function are still observed with the affinity-purified ANF-R1 receptor, suggesting an allosteric binding site for ATP on the ANF-R1 receptor. In intact membranes, limited proteolysis of the ANF-R1 receptor with trypsin dose-dependently prevents the ATP-induced decrease in ANF binding concomitantly with the formation of a membrane-associated ANF-binding fragment of 70 kDa. These results confirm the direct modulatory role of ATP on hormone binding activity of ANF-R1 receptor and suggest that the nucleotide regulatory binding site is located in the intracellular domain vicinal to the protease-sensitive region.

Cross-reactivity of amylin with calcitonin-gene-related peptide binding sites in rat liver and skeletal muscle membranes

This study examines whether the high degree of sequence identity between amylin and calcitonin-gene-related peptide (CGRP) is reflected in their cross-reactivity at the level of membrane receptor binding. Rat liver plasma membranes contain a specific saturable binding site for 125I-labelled human CGRP-1. Binding reached equilibrium within 30 min and was rapidly reversed by re-incubating membranes in the presence of 1 microM human CGRP. In addition, the presence of 50 mM- or 500 mM-NaCl lowered specific binding by 30% and 77% respectively. Scatchard analysis was consistent with a single high-affinity site with a dissociation constant (Kd) of 0.125 nM and binding capacity (Bmax.) of 580 fmol/mg of membrane protein. Specific binding of 125I-labelled human CGRP-1 to both liver and skeletal muscle membranes was inhibited by human CGRP-1 [IC50 (concn. causing half-maximal inhibition of binding) 0.1-0.3 nM], and rat amylin (IC50 10 nM), but not by human calcitonin. Covalent cross-linking of 125I-CGRP to its binding site in rat skeletal muscle and liver membranes resulted in labelling of a major species of about 70 kDa under reducing conditions and about 55 kDa under alkylating conditions, as visualized on SDS/PAGE. These radiolabelled species were absent in the presence of CGRP or amylin at 1 microM. These results are indicative of a common binding site for both CGRP and amylin in liver and skeletal muscle, and it is suggested that both peptides mediate their actions through the same effector system. The normal physiological importance and the relevance to the pathology of type 2 diabetes of these data are discussed.