Renal effects of Tamm-Horsfall protein (uromodulin) deficiency in mice

Role of lipid rafts in membrane delivery of renal epithelial Na+-K+-ATPase, thick ascending limb

Lipid rafts are cholesterol- and shingolipid-enriched membrane microdomains implicated in membrane signaling and trafficking. To assess renal epithelial raft functions through the characterization of their associated membrane proteins, we have isolated lipid rafts from rat kidney by sucrose gradient fractionation after detergent treatment. The low-density fraction was enriched in cholesterol, sphingolipid, and flotillin-1 known as lipid raft markers. Based on proteomic analysis of the low-density fraction, the protein with the highest significance score was the alpha-subunit of Na(+)-K(+)-ATPase (NKA), whose raft association was validated by simultaneous immunoblotting. The beta-subunit of NKA was copurified from the low-density fraction. To test the role of lipid rafts in sorting and membrane delivery of renal-transporting epithelia, we have chosen to study thick ascending limb (TAL) epithelium for its high NKA activity and the property to be stimulated by antidiuretic hormone (ADH). Cultured rabbit TAL cells were studied. Cholesterol depletion and detergent extraction at warmth caused a shift of NKA to the higher-density fractions. Comparative preparations from blood monocytes revealed the absence of NKA from rafts in these nonpolarized cells. Short-term exposure of rabbit TAL cells to ADH (1 h) caused translocation and enhanced raft association of NKA via cAMP activation. Preceding cholesterol depletion prevented this effect. TAL-specific, glycosylphosphatidylinositol-anchored Tamm Horsfall protein was copurified with NKA in the same raft fraction, suggesting functional interference between these products. These results may have functional implications regarding the turnover, trafficking, and regulated surface expression of NKA as the major basolateral ion transporter of TAL.

Alterations of uromodulin biology: a common denominator of the genetically heterogeneous FJHN/MCKD syndrome

Autosomal dominant hyperuricemia, gout, renal cysts, and progressive renal insufficiency are hallmarks of a disease complex comprising familial juvenile hyperuricemic nephropathy and medullary cystic kidney diseases type 1 and type 2. In some families the disease is associated with mutations of the gene coding for uromodulin, but the link between the genetic heterogeneity and mechanism(s) leading to the common phenotype symptoms is not clear. In 19 families, we investigated relevant biochemical parameters, performed linkage analysis to known disease loci, sequenced uromodulin gene, expressed and characterized mutant uromodulin proteins, and performed immunohistochemical and electronoptical investigation in kidney tissues. We proved genetic heterogeneity of the disease. Uromodulin mutations were identified in six families. Expressed, mutant proteins showed distinct glycosylation patterns, impaired intracellular trafficking, and decreased ability to be exposed on the plasma membrane, which corresponded with the observations in the patient's kidney tissue. We found a reduction in urinary uromodulin excretion as a common feature shared by almost all of the families. This was associated with case-specific differences in the uromodulin immunohistochemical staining patterns in kidney. Our results suggest that various genetic defects interfere with uromodulin biology, which could lead to the development of the common disease phenotype. 'Uromodulin-associated kidney diseases' may be thus a more appropriate term for this syndrome.

Does Tamm–Horsfall protein–uric acid binding play a significant role in urate homeostasis?

BACKGROUND

Mutations in Tamm-Horsfall protein (THP), also known as uromodulin, lead to a group of diseases known as the uromodulin storage disorders. Clinically, these diseases present with tubulo-interstitial damage, progressive renal dysfunction, hyperuricaemia, and gout. However, it remains unclear how a mutation in THP, a protein produced in the thick ascending limb, can cause hyperuricaemia when most of the uric acid transport is believed to occur in the proximal tubule. However, one study in humans suggests that uric acid could also be secreted in the distal tubule. Thus, an attractive hypothesis could be that THP would bind to uric acid in the distal tubule, and decrease its subsequent reabsorption in the distal nephron.

METHODS

We screened for uric acid binding to THP using four independent binding assays.

RESULTS

There was no evidence that uric acid could bind to THP.

CONCLUSION

THP-uric acid binding does not seem to play a significant role in the regulation of urate homeostasis.

Renal cortical regulation of COX-1 and functionally related products in early renovascular hypertension (rat)

Renal volume regulation is modulated by the action of cyclooxygenases (COX) and the resulting generation of prostanoids. Epithelial expression of COX isoforms in the cortex directs COX-1 to the distal convolutions and cortical collecting duct, and COX-2 to the thick ascending limb. Partly colocalized are prostaglandin E synthase (PGES), the downstream enzyme for renal prostaglandin E(2) (PGE(2)) generation, and the EP receptors type 1 and 3. COX-1 and related components were studied in two kidney-one clip (2K1C) Goldblatt hypertensive rats with combined chronic ANG II or bradykinin B(2) receptor blockade using candesartan (cand) or the B(2) antagonist Hoechst 140 (Hoe). Rats (untreated sham, 2K1C, sham + cand, 2K1C + cand, sham + Hoe, 2K1C + Hoe) were treated to map expression of parameters controlling PGE(2) synthesis. In 2K1C, cortical COX isoforms did not change uniformly. COX-2 changed in parallel with NO synthase 1 (NOS1) expression with a raise in the clipped, but a decrease in the nonclipped side. By contrast, COX-1 and PGES were uniformly downregulated in both kidneys, along with reduced urinary PGE(2) levels, and showed no clear relations with the NO status. ANG II receptor blockade confirmed negative regulation of COX-2 by ANG II but blunted the decrease in COX-1 selectively in nonclipped kidneys. B(2) receptor blockade reduced COX-2 induction in 2K1C but had no clear effect on COX-1. We suggest that in 2K1C, COX-1 and PGES expression may fail to oppose the effects of renovascular hypertension through reduced prostaglandin signaling in late distal tubule and cortical collecting duct.

Cilia and centrosomes: a unifying pathogenic concept for cystic kidney disease?

Cystic kidney diseases are among the most frequent lethal genetic diseases. Positional cloning of novel cystic kidney disease genes revealed that their products (cystoproteins) are expressed in sensory organelles called primary cilia, in basal bodies or in centrosomes. Primary cilia link mechanosensory, visual, osmotic, gustatory and other stimuli to mechanisms of cell-cycle control and epithelial cell polarity. The ciliary expression of cystoproteins explains why many other organs might be also affected in patients with cystic kidney disease. Protein-protein interactions among cystoproteins, and their strong evolutionary conservation, provide a basis for a multidisciplinary approach to unravelling the novel signalling mechanisms that are involved in this disease group.

Genetic factors associated with gout and hyperuricemia

Hyperuricemia and gout are common conditions that have long been known to have a heritable component. Obesity, diabetes, and chronic kidney failure are conditions with multifactorial inheritance that are associated with gout. In addition, social factors such as protein and alcohol intake affect serum uric acid levels. The current review discusses basic uric acid metabolism and the multigenetic inheritance of hyperuricemia. Several monogenic disorders affecting uric acid metabolism are reviewed. The genetics, pathophysiology, diagnosis, and treatment of familial juvenile hyperuricemic nephropathy/medullary cystic kidney disease, autosomal dominant disorders associated with hyperuricemia and progressive kidney failure, are described.

The secretory granule protein syncollin localizes to HL-60 cells and neutrophils

The secretory granule protein syncollin was first identified in the exocrine pancreas where a population of the protein is associated with the luminal surface of the zymogen granule membrane. In this study we provide first morphological and biochemical evidence that, in addition to its pancreatic localization, syncollin is also present in neutrophilic granulocytes of rat and human origin. By immunohistological studies, syncollin was detected in neutrophilic granulocytes of the spleen. Furthermore, syncollin is expressed by the promyelocytic HL-60 cells, where it is stored in azurophilic granules and in a vesicular compartment. These findings were confirmed by fractionation experiments and immunoelectron microscopy. Treatment with a phorbol ester triggered the release of syncollin indicating that in HL-60 cells it is a secretory protein that can be mobilized upon stimulation. A putative role for syncollin in host defense is discussed.

Proteomics and diabetic nephropathy

Diabetes mellitus is acknowledged to be a group of metabolic diseases and heterogeneous in natural history, pathogenesis, response to treatment, and disease progression and remission. Diabetic nephropathy (DN) accounts for approximately 40% of all newly diagnosed cases of end-stage renal disease. The complexity of diabetes and its complications requires a broad-based, unbiased, scientific approach such as proteomics. Recently, proteomics (the systematic analysis of protein identity, quantity, and function) has been applied to the study of DN. Proteomic investigations into diabetic kidney disease have identified new mechanisms of diabetic renal pathology, as well as potential urinary markers of DN. Other current proteomic advances in understanding DN include identifying the role of advanced glycation end products in decreased mitochondrial respiration and also the rapid development of mass spectrometric methods for protein and peptide markers of DN development and markers to pharmacologic therapies. Proteomic analysis has only recently been applied to the study of DN, yet it has shown substantial potential.

Prospects for urinary proteomics: exosomes as a source of urinary biomarkers

Recent progress in biotechnology offers the promise of better medical care at lower costs. Among the techniques that show the greatest promise is mass spectrometry of proteins, which can identify proteins present in body fluids and tissue specimens at a large scale. Because urine can be collected in large amounts in a non-invasive fashion, the potential exists to use mass spectrometry to discover urinary biomarkers that are early predictors of renal disease, or useful in making therapeutic choices. Recently, the authors demonstrated that both membrane proteins and cytosolic proteins from renal epithelia are highly enriched in low-density urinary structures identified as exosomes. Exosomes were found to contain many disease-associated proteins including aquaporin-2, polycystin-1, podocin, non-muscle myosin II, angiotensin-converting enzyme, Na+ K+ 2Cl- cotransporter (NKCC2), thiazide-sensitive Na-Cl cotransporter (NCC), and epithelial sodium channel (ENaC). Potentially, other disease biomarkers could be discovered by mass spectrometry-based proteomic studies in well-defined patient populations. Herein is described the advantages of using urinary exosomes as a starting material for biomarker discovery. In addition, the purpose of this review is to present an overall strategy for biomarker discovery in urine using exosomes and for developing cost-effective clinical assays for these biomarkers, which can potentially be used for early detection of disease, as a means of differential diagnosis, or as a means of guiding therapy. Finally, potential barriers that need to be overcome before urinary proteomics can be applied clinically, are emphasized.

Molecular physiology of urate transport

Humans excrete uric acid as the final breakdown product of unwanted purine nucleotides. Urate scavenges potential harmful radicals in our body. However, in conjunction with genetic or environmental (especially dietary) factors, urate may cause gout, nephrolitiasis, hypertension, and vascular disease. Blood levels of urate are maintained by the balance between generation and excretion. Excretion requires specialized transporters located in renal proximal tubule cells, intestinal epithelial cells, and vascular smooth muscle cells. The recently identified human urate transporters URAT1, MRP4, OAT1, and OAT3 are thought to play central roles in homeostasis and may prove interesting targets for future drug development.