title: Use of phospho-specific antibodies to determine the phosphorylation of endogenous Na+/H+ exchanger NHE3 at PKA consensus sites.
abstract: Transfection studies using mutant constructs have implicated one or both protein kinase A (PKA) consensus phosphorylation sites [serines 552 and 605 in rat Na(+)/H(+) exchanger type 3 (NHE3)] as critical for mediating inhibition of NHE3 in response to several stimuli including dopamine. However, whether one or both of these sites is actually phosphorylated in endogenous NHE3 in proximal tubule cells is unknown. The purpose of this study was to generate phosphospecific antibodies so that the state of phosphorylation of these serine residues in endogenous NHE3 could be assessed in vitro and in vivo. To this end, polyclonal and monoclonal phosphospecific peptide antibodies were generated against each PKA consensus site. Phosphospecificity was established by ELISA and Western blot assays. We then used these antibodies in vitro to evaluate the effect of dopamine on phosphorylation of the corresponding PKA sites (serines 560 and 613) in NHE3 endogenously expressed in opossum kidney cells. Baseline phosphorylation of both sites was detected that was significantly increased by dopamine. Next, we determined the baseline phosphorylation state of each serine in rat kidney NHE3 in vivo. We found that serine 552 of NHE3 is phosphorylated to a much greater extent than serine 605 at baseline in vivo. Moreover, we detected a distinct subcellular localization for NHE3 phosphorylated at serine 552 compared with total NHE3. Specifically, NHE3 phosphorylated at serine 552 localized to the coated pit region of the brush-border membrane, where NHE3 is inactive, while total NHE3 was found throughout the brush-border membrane. These findings strongly suggest that phosphorylation of NHE3 plays a role in its subcellular trafficking in vivo. In conclusion, we successfully generated phosphospecific antibodies that should be useful to assess the phosphorylation of endogenous NHE3 at its two PKA consensus sites under a variety of physiological conditions in vitro and in vivo.
authors.affiliation: Department of Pediatrics, Yale School of Medicine, New Haven, CT 06520-8029, USA.
authors.forename: Hetal S
authors.initials: HS
authors.lastname: Kocinsky
authors.forename: Adriana C C
authors.initials: AC
authors.lastname: Girardi
authors.forename: Daniel
authors.initials: D
authors.lastname: Biemesderfer
authors.forename: Thao
authors.initials: T
authors.lastname: Nguyen
authors.forename: Sueann
authors.initials: S
authors.lastname: Mentone
authors.forename: John
authors.initials: J
authors.lastname: Orlowski
authors.forename: Peter S
authors.initials: PS
authors.lastname: Aronson
mesh_terms: D000818:Animals; D000906:Antibodies / Q000302:isolation & purification; D000911:Antibodies, Monoclonal / Q000096:biosynthesis / Q000276:immunology / Q000302:isolation & purification; D000918:Antibody Specificity; D019556:COS Cells; D002460:Cell Line; D002522:Chlorocebus aethiops; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D004298:Dopamine / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D007150:Immunohistochemistry; D007668:Kidney / Q000378:metabolism; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008856:Microscopy, Fluorescence; D008871:Microvilli / Q000378:metabolism; D009893:Opossums; D010766:Phosphorylation / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D014162:Transfection
chemical_list: D000906:Antibodies; D000911:Antibodies, Monoclonal; C000613386:Slc9a3 protein, mouse; C000613387:Slc9a3 protein, rat; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D012964:Sodium; D017868:Cyclic AMP-Dependent Protein Kinases; D004298:Dopamine