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Proteomic profiling and pathway analysis of the response of rat renal proximal convoluted tubules to metabolic acidosis.

ABSTRACT: Metabolic acidosis is a relatively common pathological condition that is defined as a decrease in blood pH and bicarbonate concentration. The renal proximal convoluted tubule responds to this condition by increasing the extraction of plasma glutamine and activating ammoniagenesis and gluconeogenesis. The combined processes increase the excretion of acid and produce bicarbonate ions that are added to the blood to partially restore acid-base homeostasis. Only a few cytosolic proteins, such as phosphoenolpyruvate carboxykinase, have been determined to play a role in the renal response to metabolic acidosis. Therefore, further analysis was performed to better characterize the response of the cytosolic proteome. Proximal convoluted tubule cells were isolated from rat kidney cortex at various times after onset of acidosis and fractionated to separate the soluble cytosolic proteins from the remainder of the cellular components. The cytosolic proteins were analyzed using two-dimensional liquid chromatography and tandem mass spectrometry (MS/MS). Spectral counting along with average MS/MS total ion current were used to quantify temporal changes in relative protein abundance. In all, 461 proteins were confidently identified, of which 24 exhibited statistically significant changes in abundance. To validate these techniques, several of the observed abundance changes were confirmed by Western blotting. Data from the cytosolic fractions were then combined with previous proteomic data, and pathway analyses were performed to identify the primary pathways that are activated or inhibited in the proximal convoluted tubule during the onset of metabolic acidosis.

SUBMITTER: Schauer KL

PROVIDER: S-EPMC3761203 | biostudies-literature | 2013 Sep

REPOSITORIES: biostudies-literature

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Proteomic profiling and pathway analysis of the response of rat renal proximal convoluted tubules to metabolic acidosis.

Schauer Kevin L KL Freund Dana M DM Prenni Jessica E JE Curthoys Norman P NP

American journal of physiology. Renal physiology 20130626 5

Metabolic acidosis is a relatively common pathological condition that is defined as a decrease in blood pH and bicarbonate concentration. The renal proximal convoluted tubule responds to this condition by increasing the extraction of plasma glutamine and activating ammoniagenesis and gluconeogenesis. The combined processes increase the excretion of acid and produce bicarbonate ions that are added to the blood to partially restore acid-base homeostasis. Only a few cytosolic proteins, such as phos ...[more]

PMID: 23804448

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Project description:The proximal convoluted tubule is the primary site of renal fluid, electrolyte and nutrient reabsorption, processes that consume large amounts of ATP. Previous proteomic studies have profiled the adaptions that occur in this segment of the nephron in response to onset of metabolic acidosis. Proximal convoluted tubule isolation and mitochondrial enrichment procedures were described previously. Briefly, rat renal cortex was digested with collagenase and proximal convoluted tubules were purified by Percoll density gradient centrifugation. Mitochondria were isolated by differential and sucrose density centrifugation, which produced an 8-fold enrichment in specific activity of cytochrome C oxidase . The isolated mitochondria were resuspended in H2O, incubated on ice for 20 min with gentle vortexing, and then centrifuged at 12,000xg for 5 min. The pellet and little remaining supernatant were centrifuged again at 12,000xg for 5 min to tighten the pellet and remove residual supernatant. The combined supernatants contain the mitochondrial outer membrane and intermembrane proteins while the final pellet contains mitoplasts. The mitoplasts were subsequently treated with 0.1 M Na2CO3, pH 11.5, incubated on ice for 20 min, and then centrifuged at 100,000xg for 30 min. The supernatant contains matrix proteins and the pellet is the enriched MIM fraction. Samples containing 50 µg of protein were precipitated with acetone and an in-solution tryptic digestion preformed with ProteaseMax (Promega). The peptides were fractionated on a reverse phase column (1200 nanoHPLC, Zorbax C18, 5 um, 75 um ID x 150mm column, Agilent) using a 90 minute linear gradient from 25%-55% buffer B (buffer B = 90% acetonitrile and 0.1% formic acid) at a flow rate of 300 nl/min. Peptides were eluted directly into the mass spectrometer (LTQ linear ion trap, Thermo Scientific) and spectra were collected over a m/z range of 200-2000 Da using a dynamic exclusion limit of 3 MS/MS spectra of a given peptide mass for 30 sec and an exclusion duration of 90 sec. The compound lists from the resulting spectra were produced using Xcalibar 2.2 software (Thermo Scientific) with an intensity threshold of 5,000 and 1 scan/group. Each biological sample was injected five times for LC-MS/MS analysis. MS/MS spectra were searched against the Uniprot-KB Rattus norvegicus protein sequence database (74,140 sequence entries) concatenated with a reverse database using both the Mascot (version 2.3.02, Matrix Science) and SEQUEST (version v.27, rev. 11, Sorcerer, Sage-N Research) database search engines. The following search parameters were used: average mass, peptide mass tolerance of 2.5 Da, fragment ion mass tolerance of 1.0 Da, complete tryptic digestion allowing four missed cleavages, variable modifications of methionine oxidation and lysine acetylation, and a fixed modification of cysteine carbamidomethylation. Peptide identifications from both of the search engine results were combined using protein identification algorithms in Scaffold 3 (Version 3.6.0, Proteome Software). Peptide and protein probability thresholds of 90% and 99% were applied with Peptide and Protein Prophet algorithms in Scaffold 3 [6] [7]. Proteins containing shared peptides were grouped by Scaffold 3 to satisfy the laws of parsimony. A peptide false discovery rate (FDR) was determined by the target decoy approach of searching the reversed database [8]. Only proteins that were identified by a minimum of 2 unique peptides in at least 1 biological replicate were considered confidently identified. Manual validation of MS/MS spectra was performed for protein identifications that met the initial thresholds that were based on two unique peptides and for all of the acetylated peptides. The analysis confidently identified 207 proteins in the combined samples. Further proteomic analysis identified 14 peptides that contain an N-epsilon-acetyl-lysine, 7 of which are novel sites. This study provides the first proteomic profile of the mitochondrial inner membrane proteome of this segment of the rat renal nephron.

Project description:Crosstalk of renal epithelial cells with interstitial fibroblasts plays an important role in kidney pathophysiology. A previous study showed that crosstalk between renal epithelial cells and renal fibroblasts protects against acidosis-induced damage. In order to gain further mechanistic insight into this crosstalk, we investigated the effect of acidosis on the transcriptome of renal epithelial cells (NRK-52E) and renal fibroblasts (NRK-49F) in co-culture by RNASeq, bioinformatics analysis and experimental validation. Cells were exposed to acidic media or control media for 48 h. RNA and protein from whole cell lysate were isolated. In addition, cells were fractionated into cytosol, nucleus and chromatin. RNASeq data were analyzed for differential expression and pathway enrichment (ingenuity pathway analysis, IPA, QIAGEN). Total and phosphorylated protein expression was assessed by Western blot (WB). Transcription factor activity was assessed by luciferase reporter assay. Bioinformatic analysis using differentially expressed genes according to RNASeq (7834 for NRK-52E and 3197 for NRK-49F) predicted the antioxidant and cell-protective Nrf2 pathway as acidosis-induced in NRK-52E and NRK-49F cells. Activation of Nrf2 comprises enhanced Nrf2 phosphorylation, nuclear translocation, DNA binding and initiation of a cell protective transcriptional program. Our data show that acidosis enhances chromatin-associated Nrf2 expression and the abundance of phosphorylated Nrf2 in the chromatin fraction of NRK-52E cells in co-culture but not in monoculture. Furthermore, acidosis enhances the activity of a reporter for Nrf2 (ARE-luciferase). Despite the bioinformatics prediction, NRK-49F cells did not respond with Nrf2 activation. Transketolase (TKT) is an important regulator of antioxidant and homeostatic responses in the kidney and a canonical Nrf2 target gene. We show that protein and mRNA expression of TKT is increased in NRK-52E cells under co-culture but not under monoculture conditions. In conclusion, our data show that extracellular acidosis activates the cytoprotective transcription factor Nrf2 in renal epithelial cells co-cultivated with renal fibroblasts, thereby enhancing the expression of cytoprotective TKT. This protective response is not observed in monoculture. Activation of the Nrf2 pathway represents a co-operative cellular strategy of protection against acidosis.

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Proteomic profiling and pathway analysis of the response of rat renal proximal convoluted tubules to metabolic acidosis.

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Proteomic profiling and pathway analysis of the response of rat renal proximal convoluted tubules to metabolic acidosis.

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