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Endocrinology, G.V. (Sonny) Montgomery VA Medical Center (C.E.G.-S., E.P.G.-S.) and University of Mississippi Medical Center (C.E.G.-S., A.F.d.R., D.G.R., J.E., M.P.W., E.P.G.-S.), Jackson, Mississippi 39216; and Research Mississippi, Inc. (M.T.G.-S.), Jackson, Mississippi 39216
Address all correspondence and requests for reprints to: Celso E. Gomez-Sanchez, M.D., Division of Endocrinology, University of Mississippi Medical Center, 2500 North State Street, Jackson, Mississippi 39216. E-mail: cgomez-sanchez{at}medicine.umsmed.edu.
| Abstract |
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| Introduction |
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MR are expressed in many tissues and modulate a variety of functions, the best known being the stimulation of electrolyte and water transfer by transporting epithelia in the kidney and colon. MR are also expressed in select areas of the central nervous system where they modulate multiple cell processes, including trophic effects in hippocampal neurons involved in learning and memory (6) and modulation of sodium appetite control (7) and blood pressure (8) through actions within the amygdala and circumventricular organs.
Several polyclonal antibodies against the MR have been described that were raised against peptides (9, 10, 11), recombinant MR protein fragments (12, 13), or the receptor purified from rat kidney (14). The commercial antibodies currently available have not been well characterized by the vendor; however, their usefulness for the detection of MR in the brain has been published (15). An auto-anti-idiotipic monoclonal antibody generated in animals immunized against an aldosterone-3-carboxymethoxyoxime-BSA conjugate (16) has also been described. This auto-anti-idiotypic antibody presumably recognizes the binding domain of the unbound MR, so aldosterone competes with the antibody for binding to the receptor (17). Because it recognizes the specific conformation of the unbound receptor, the auto-anti-idiotypic antibody has been used for immunohistochemistry (ihc) and does not work for Western blots in which the protein is denatured.
We have produced antibodies to different epitopes of the MR with the anticipation that they would recognize different conformational states of the receptor determined by the steroid ligands and/or cytosolic proteins to which it is bound. The latter presumably depend upon cell type, the location within the cell. and physiological state. In this study, we describe the production of a panel of monoclonal antibodies against various regions of the MR using peptide antigens comprising different portions of the MR molecule conjugated to strong immune-responsive proteins and the patterns of immunoreactivity when these are used for ihc in tissues from normal rats on a standard diet.
| Materials and Methods |
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Preparation of peptide antigens and immunization
Hydropathy plots were used to select peptides (1420 residues) corresponding to various regions of the rat MR (rMR) that have a high probability of being exposed when the molecule is folded normally (18, 19). These peptides were synthesized commercially with a cysteine at the N-terminal, middle, or C-terminal end (Table 1
and Fig. 1
). Peptides were conjugated to keyhole limpet hemocyanin or Blue immunogenic protein, and casein (Pierce Biotechnology, Rockford, IL). For conjugation, 5 mg of protein was dissolved in 1 ml of 0.1 M sodium phosphate buffer (pH 7.4), then 2 mg of 6-(iodoacetamido)caproic acid N-hydroxysuccinimide ester dissolved in 0.1 ml of DMSO was added. After 60 min, the mixture was purified using a 20-ml spin column packed with Sephadex G-25 equilibrated with 0.05 M phosphate, 1 mM EDTA, and 0.5% SDS (pH 7.6), and centrifuged at 800 x g for 10 min. The peptide (34 mg) was dissolved in 1 ml of water, added to the conjugation protein, and stirred for 34 h. After the reaction, the conjugates were dialyzed in 4 liters of PBS overnight. Swiss-Webster female mice were immunized with 50 µg of the keyhole limpet hemocyanin conjugates emulsified in complete Freunds adjuvant, divided into six sc sites, as the primary, then boosted twice, 3 wk apart, with the corresponding conjugates emulsified with incomplete Freunds adjuvant. Two weeks after the last sc inoculation, the mice were injected ip with an aqueous solution of the peptides conjugates, and 3 d later, blood was obtained by cardiac puncture under deep isofluorane anesthesia and the spleens were removed under sterile conditions. Splenocytes were mechanically separated and frozen in two aliquots in freezing media containing 5% DMSO. Sera were screened by ELISA using 96-well plates onto which the peptide conjugated to casein (100 ng) was immobilized. Sera that were positive by ELISA were screened by ihc using rat kidney slices. Hybridomas were produced using splenocytes of mice whose sera were positive by ihc.
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Western blots
Western blot analysis was used to screen the media from the clones positive by ELISA. MR protein sources were whole cell extracts of Chinese hamster ovary (CHO)-K1 cells transfected with enhanced green fluorescent protein (EGFP)-rMR [kindly provided by Drs. A. Naray-Fejes-Toth (Dartmouth University, Lebanon, NH) and D. Pearce (University of California San Francisco, San Francisco, CA)] (21) and CHO cells infected with a lentivirus expressing the complete sequence of the rMR, as well as cytosol from hippocampus of adrenalectomized rats. The 7.5% PAGE gels were transferred to polyvinylidene difluoride membranes using a tank technique, blocked with 1% Carnation dry skim milk in Tris-buffered saline with 0.05% Tween 20. The membranes were incubated with the primary antibody overnight, washed, and developed using a screened affinity purified goat antimouse IgG peroxidase-labeled antibody (Jackson ImmunoResearch Laboratories, West Grove, PA). The membranes were developed using a chemiluminescence substrate (Pierce WestPico Super Signal; Pierce Biotechnology) and recorded using auto-radiographic film (Fujifilm; Fisher Scientific).
Immunohistochemistry
Tissues were collected from intact rats, anesthetized, then perfused with saline, followed by Streck Tissue Fixative (Streck Tissue Fixative, Omaha, NE). The kidneys, hearts, and brains were further fixed in Streck Tissue Fixative overnight, then embedded in paraffin. Six-micrometer sections were cut, deparaffinized, treated with 0.1% phenylhydrazine for 30 min to inhibit peroxidases, then blocked with 0.05 M Tris, 5% dry milk, 2% normal donkey serum, and 0.2% SDS for 1 h. The slides were then incubated overnight with the antibody in a similar buffer containing 0.05% Tween 20. After washing, the sections were incubated for 1 h with an affinity purified donkey antimouse IgG biotin-labeled antibody, washed, and incubated with peroxidase-labeled streptavidin (Zymed, Invitrogen, Carlsbad, CA) in Superblocker (Pierce Biotechnology) for 30 min. The slides were developed using diaminobenzidine and counterstained with hematoxylin (22). Negative controls included incubation with no primary antibody and with primary antibody preincubated with an excess of immunizing peptide.
| Results |
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Five monoclonal antibodies against the rMR 118 peptide were studied; 6G1 and 1D5 were evaluated fully and seemed to be very similar. Figure 2
shows the ihc using the rMR 118 6G1 antibody in kidney, heart, colon, and central nervous system of intact rats. rMR 118 6G1 staining was primarily nuclear with lighter cytoplasmic staining in all tissues. Immunoreactivity in kidneys of intact rats was seen in connecting tubules, distal convoluted tubules, and cortical collecting tubules (Fig. 2
, B and C), hippocampus (Fig. 2E
), choroid plexus (Fig. 2G
), cerebellum (Fig. 2H
), and colon (Fig. 2I
). Staining of the heart was also primarily nuclear with cytosolic staining (Fig. 2K
) and strong staining of cytosol and nuclei of the vascular smooth muscle layer of the coronary arteries (Fig. 2L
). Western blot analysis of hippocampal cytosol from adrenalectomized animals using rMR 118 antibodies produces a single band at a molecular mass of approximately 107 kDa, the expected molecular mass of the rMR (Fig. 3
). The sequence of the rMR 118 peptide is identical in the rat, human, and mouse, so this antibody should be useful for ihc and Western blots in these species.
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Antibodies from the rMR 365381 (4D6) recognized nuclei of distal and cortical collecting tubular cells of the kidney (Fig. 4J
), hippocampus (Fig. 4K
), cerebellum (Fig. 4L
), in the heart (Fig. 4
, M and N) and vascular smooth muscle of coronary vessels (Fig. 4O
). They produced a Western blot band at approximately 107 kDa, as did the other antibodies, but were less sensitive (Fig. 3
). The sequence of this region is identical between the rat and mouse, but differs from the human at amino acids 366, a histidine vs. glutamine, and amino acids 371372, histidine-aspartic acid vs. glutamine-glutamic acid. Because the conjugation was done through a cysteine added to the C-terminal portion, one expects that the antibody might not cross-react with the human MR (not tested).
Monoclonal antibodies against the peptide 832846 (n = 4) produced weak ihc staining that was similar to that of the other antibodies, but the monoclonal antibodies were negative by Western blots. Therefore these were not studied further.
| Discussion |
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Our monoclonal antibodies were generated against epitopes located at both the N and C termini of the molecule. Several of the N-terminal antibodies are useful for both ihc and Western blots (rMR 118, rMR 6482, rMR 365381), although another produced excellent ihc staining, but did not work for Western blot analyses (rMR 7987). No useful C-terminal monoclonal antibodies were obtained.
The antibody rMR 6482 (2D6) primarily stained nuclei of cells in the distal, connecting, and cortical collecting tubules (Fig. 2
, J and K). It also stained nuclei and, to a lesser extent, cytoplasm of cardiomyocytes and specific neurons in the brain in a pattern like that produced by tritiated aldosterone autoradiography, including the hippocampus where the highest density of MR occurs (24). Antibody rMR 6482 (2D6) and several other clones from the same fusion also stained the luminal membrane of proximal tubular cells of nephrons in the inner cortex (Fig. 2
, M and O). This luminal membrane staining is probably due to cross-reactivity with an epitope of an unknown protein and is not related to the MR, because no other antibody exhibited this pattern of staining. A more remote possibility is that this represents a membrane-attached configuration of the MR for which there is evidence but little information (25). This antibody was the most sensitive in Western blots and produced a single band with protein from hippocampal cytosol unless the gel was overloaded (Fig. 3
).
The rMR 118 (6G1) antibody stained both nuclei and the cytosol, but not brush borders in the kidneys. Western blots of hippocampal cytosolic protein-using rMR 118 (6G1) also showed a single band at the predicted molecular mass (Fig. 3
). A functional alternatively translated MR starting at methionine 15 has been demonstrated by in vitro transcription/translation studies of the human MR (26). This MR would not be detected by the rMR 118 (6G1) antibody if it exists in nature, but it would be detected by the other MR antibodies. Western blots using the rMR 118 (6G1) produced similar results as those using rMR 365381 (4D6) and rMR 6482, suggesting that if the alternatively spliced MR starting at amino acid 15 exists, the full-length MR predominates.
The rMR 7997 (3F10) antibody stained both cytoplasm and nuclei of rat kidneys. It also gave excellent labeling in the heart and brain, including the choroid plexus (Fig. 4G
). This antibody did not work for Western blots, producing only several faint bands when large amounts of protein were used.
It was originally thought that the unoccupied MR resided in the cytoplasm and moved into the nucleus upon occupation by an agonist (27); however, others using immunofluorescence staining have found three different patterns of subcellular distribution including diffuse, nuclear, and cytoplasmic (28). Similar cytoplasmic and nuclear staining has been found using the monoclonal auto-anti-idiotypic antibody (17, 29). In the absence of ligand, MR is located in both the cytosol and nucleus bound by a variety of chaperone proteins, including hsp90. Upon exposure to either aldosterone or corticosterone, most MR are found in the nucleus, where they form prominent clusters that are not seen in the absence of hormone (21, 30). It appears that several of our monoclonal antibodies have a strong preference for the MR in the nucleus. This could be due to masking of MR epitopes by binding to cytoplasmic proteins that dissociate before the receptor enters the nuclei. In vitro transcription translation experiments have demonstrated that the transcriptional activity of the MR is altered by the modification of the receptor by sumoylation (31); however, we found no evidence by Western blots of an alteration of molecular weight of the MR in hypothalamic cytosol.
In conclusion, monoclonal antibodies against several distinct regions of the rMR have been produced to facilitate its study. Although most of the conjugated peptides elicited the formation of specific antibodies that recognize the peptide in an ELISA, some of these antibodies do not recognize the native MR. Several of the best MR antibodies, while recognizing the protein in the same cells as the others, differ in their affinity for the receptor depending on its location within the cell, presumably due to conformational changes wrought by association with other proteins. These should be useful tools for the study of the MR biology.
| Footnotes |
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First Published Online November 17, 2005
Abbreviations: CHO, Chinese hamster ovary; EGFP, enhanced green fluorescent protein; ihc, immunohistochemistry; MR, mineralocorticoid receptor; rMR, rat MR.
Received July 11, 2005.
Accepted for publication November 10, 2005.
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