Urinary exosomes have been proposed as starting material for discovery of protein biomarkers of kidney disease. Current protocols for their isolation use a two-step differential centrifugation process. Due to their low density, exosomes are expected to remain in the low-speed (17,000 × g) supernatant and to sediment only when the sample is spun at high speed (200,000 × g). Analysis using western blots and electron microscopy found that urinary exosomes are also present in the low-speed pellet entrapped by polymeric Tamm-Horsfall protein thus diminishing the procedures reproducibility. Here we show that addition of dithiothreitol to the low-speed pellet disrupted the polymeric network presumably by reduction of disulfide bonds linking the monomers. This modification shifted the exosomal proteins from the low-to the high-speed pellet. Also, by shifting the Tamm-Horsfall protein to the high-speed pellet, the use of dithiothreitol makes it feasible to use Tamm-Horsfall protein to normalize excretion rates of exosomal proteins in spot urines. We tested this by western blot, and found that there was a high degree of correlation between exosomal proteins and Tamm-Horsfall protein in the high-speed pellet. Since the yield of exosomes by differential centrifugation can be increased by chemical reduction, Tamm-Horsfall protein may be a suitable normalizing variable for urinary exosome studies when quantitative urine collections are not practical.

We used a systems biology-based approach to investigate the basis of cell-specific expression of the water channel aquaporin-2 (AQP2) in the renal collecting duct. Computational analysis of the 5-flanking region of the AQP2 gene (Genomatix) revealed 2 conserved clusters of putative transcriptional regulator (TR) binding elements (BEs) centered at ؊513 bp (corresponding to the SF1, NFAT, and FKHD TR families) and ؊224 bp (corresponding to the AP2, SRF, CREB, GATA, and HOX TR families). Three other conserved motifs corresponded to the ETS, EBOX, and RXR TR families. To identify TRs that potentially bind to these BEs, we carried out mRNA profiling (Affymetrix) in mouse mpkCCDc14 collecting duct cells, revealing expression of 25 TRs that are also expressed in native inner medullary collecting duct. One showed a significant positive correlation with AQP2 mRNA abundance among mpkCCD subclones (Ets1), and 2 showed a significant negative correlation (Elf1 and an orphan nuclear receptor Nr1h2). Transcriptomic profiling in native proximal tubules (PT), medullary thick ascending limbs (MTAL), and IMCDs from kidney identified 14 TRs (including Ets1 and HoxD3) expressed in the IMCD but not PT or MTAL (candidate AQP2 enhancer roles), and 5 TRs (including HoxA5, HoxA9 and HoxA10) expressed in PT and MTAL but not in IMCD (candidate AQP2 repressor roles). In luciferase reporter assays, overexpression of 3 ETS family TRs transactivated the mouse proximal AQP2 promoter. The results implicate ETS family TRs in cell-specific expression of AQP2 and point to HOX, RXR, CREB and GATA family TRs as playing likely additional roles.aquaporin 2 ͉ kidney ͉ microarrays ͉ transcription ͉ vasopressin R enal water excretion is tightly regulated chiefly through effects of vasopressin on the molecular water channel, aquaporin-2 (AQP2) (1). AQP2 gene expression in the kidney is restricted to collecting duct principal cells and connecting tubule cells (2, 3). Aside from control of trafficking of AQP2-containing vesicles (1), AQP2 is regulated through changes in the total abundance of the AQP2 protein in collecting duct cells. Vasopressin increases the renal abundance of the AQP2 protein (4) via changes in AQP2 mRNA levels (5), in part by transcriptional regulation. Studies in transgenic mice in which 14-15 kb of the 5Ј-flanking region of the AQP2 gene was coupled to reporters established that cell-specific expression of the AQP2 gene in the collecting duct is dependent on cis-elements in this region (6, 7). Altered AQP2 protein abundance in the renal collecting duct is largely responsible for water balance abnormalities associated with diverse clinical states including lithium-induced diabetes insipidus, congestive heart failure, and the syndrome of inappropriate antidiuresis (1). Understanding the roles of AQP2 in these clinical states hinges largely on understanding the mechanism of cell-specific expression of the AQP2 gene.Sequencing of the 5Ј-flanking region of the AQP2 gene revealed several putative cis-binding element (BE) motifs including a c...

Hypercalcemia can cause renal dysfunction such as nephrogenic diabetes insipidus (NDI), but the mechanisms underlying hypercalcemia-induced NDI are not well understood. To elucidate the early molecular changes responsible for this disorder, we employed mass spectrometry-based proteomic analysis of inner medullary collecting ducts (IMCD) isolated from parathyroid hormone-treated rats at onset of hypercalcemia-induced NDI. Forty-one proteins, including the water channel aquaporin-2, exhibited significant changes in abundance, most of which were decreased. Bioinformatic analysis revealed that many of the downregulated proteins were associated with cytoskeletal protein binding, regulation of actin filament polymerization, and cell-cell junctions. Targeted LC-MS/MS and immunoblot studies confirmed the downregulation of 16 proteins identified in the initial proteomic analysis and in additional experiments using a vitamin D treatment model of hypercalcemia-induced NDI. Evaluation of transcript levels and estimated half-life of the downregulated proteins suggested enhanced protein degradation as the possible regulatory mechanism. Electron microscopy showed defective intercellular junctions and autophagy in the IMCD cells from both vitamin D- and parathyroid hormone-treated rats. A significant increase in the number of autophagosomes was confirmed by immunofluorescence labeling of LC3. Colocalization of LC3 and Lamp1 with aquaporin-2 and other downregulated proteins was found in both models. Immunogold electron microscopy revealed aquaporin-2 in autophagosomes in IMCD cells from both hypercalcemia models. Finally, parathyroid hormone withdrawal reversed the NDI phenotype, accompanied by termination of aquaporin-2 autophagic degradation and normalization of both nonphoshorylated and S256-phosphorylated aquaporin-2 levels. Thus, enhanced autophagic degradation of proteins plays an important role in the initial mechanism of hypercalcemic-induced NDI.