Distribution of cell membrane-associated proteins along the human nephron

Junctional complexes in epithelial cells: sentinels for extracellular insults and intracellular homeostasis

The cell-cell and cell-ECM junctions within the epithelial tissues are crucial anchoring structures that provide architectural stability, mechanical resistance, and permeability control. Their indispensable role as signaling hubs orchestrating cell shape-related changes such as proliferation, differentiation, migration, and apoptosis has also been well recognized. However, growing amount of evidence now suggests that the multitasking nature of epithelial junctions extends well beyond anchorage-dependent or cell shape change-related biological processes. In this review, we discuss the emerging roles of junctional complexes in regulating innate immune defense, stress resistance, and intracellular proteostasis of the epithelial cells, with emphasis on the upstream regulation of epithelial junctions on various aspects of the epithelial barrier.

A role for tight junction-associated MARVEL proteins in larval sea lamprey (Petromyzon marinus) osmoregulation

This study reports on tight junction-associated MARVEL proteins of larval sea lamprey (Petromyzon marinus) and their potential role in ammocoete osmoregulation. Two Occludin isoforms (designated Ocln and Ocln-a) and a tricellulin (Tric) were identified. Transcripts encoding ocln, ocln-a, and tric were broadly expressed in larval lamprey, with greatest abundance of ocln in gut, liver and kidney, ocln-a in the gill and skin, and tric in the kidney. Ocln and Ocln-a resolved as ∼63 kDa and ∼35 kDa MW proteins respectively while Tric resolved as a ∼50 kDa protein. Ocln immunolocalized to the gill vasculature and in gill mucous cells while Ocln-a localized to the gill pouch and gill epithelium. Both Ocln and Ocln-a localized in the nephron, the epidermis and the luminal side of the gut. In branchial tissue, Tric exhibited punctate localization, consistent with its presence at regions of tricellular contact. Following ion-poor water (IPW) acclimation of ammocoetes, serum [Na+] and [Cl−] reduced, but not [Ca++], and carcass moisture content increased. In association, Ocln abundance increased in skin and kidney, but reduced in gill of IPW-acclimated ammocoetes while Ocln-a abundance reduced in the kidney only. Tric abundance increased in the gill. Region-specific alterations in ocln, ocln-a and tric mRNA abundance was also observed in the gut. Data support a role for Ocln, Ocln-a and Tric in the osmoregulatory strategies of a basal vertebrate.

Curcumin prevents cisplatin-induced decrease in the tight and adherens junctions: relation to oxidative stress

Curcumin is a polyphenol and cisplatin is an antineoplastic agent that induces nephrotoxicity associated with oxidative stress, apoptosis, fibrosis and decrease in renal tight junction (TJ) proteins. The potential effect of curcumin against alterations in TJ structure and function has not been evaluated in cisplatin-induced nephrotoxicity. The present study explored whether curcumin is able to prevent the cisplatin-induced fibrosis and decreased expression of the TJ and adherens junction (AJ) proteins occludin, claudin-2 and E-cadherin in cisplatin-induced nephrotoxicity. Curcumin (200 mg kg(-1)) was administered in three doses, and rats were sacrificed 72 h after cisplatin administration. Curcumin was able to scavenge, in a concentration-dependent way, superoxide anion, hydroxyl radical, peroxyl radical, singlet oxygen, peroxynitrite anion, hypochlorous acid and hydrogen peroxide. Cisplatin-induced renal damage was associated with alterations in plasma creatinine, expression of neutrophil gelatinase-associated lipocalin and of kidney injury molecule-1, histological damage, increase in apoptosis, fibrosis (evaluated by transforming growth factor β1, collagen I and IV and α-smooth muscle actin expressions), increase in oxidative/nitrosative stress (evaluated by Hsp70/72 expression, protein tyrosine nitration, superoxide anion production in isolated glomeruli and proximal tubules, and protein levels of NADPH oxidase subunits p47(phox) and gp91(phox), protein kinase C β2, and Nrf2) as well as by decreased expression of occludin, claudin-2, β-catenin and E-cadherin. Curcumin treatment prevented all the above-described alterations. The protective effect of curcumin against cisplatin-induced fibrosis and decreased proteins of the TJ and AJ was associated with the prevention of glomerular and proximal tubular superoxide anion production induced by NADPH oxidase activity.

An Optimized Method for Protein Extraction from OCT-Embedded Human Kidney Tissue for Protein Quantification by LC-MS/MS Proteomics

The existing biobanks of remnant tissue from clinically indicated kidney biopsies are attractive potential reservoirs for quantification of clinically relevant human tissue proteins by quantitative proteomics. However, a significant caveat of this strategy is that the tissues are often preserved in optimal cutting temperature (OCT) medium. Although OCT is an effective method of preserving the morphologic and immunohistological characteristics of tissues for later study, it significantly impacts efforts to quantify protein expression by liquid chromatography-tandem mass spectrometry methods. We report here a simple, reproducible, and cost-effective procedure to extract proteins from OCT-embedded tissue samples. Briefly, the excess frozen OCT medium was scraped before thawing from the tissue specimens stored at -80°C for ∼3 months. The tissue samples were homogenized and diethyl ether/methanol extraction was performed to remove the remaining OCT medium. The recovered protein was denatured, reduced, and alkylated. The second step of protein extraction and desalting was performed by chloroform/methanol/water extraction of denatured proteins. The resultant protein pellet was trypsin-digested and the marker proteins of various kidney cellular compartments were quantified by targeted selective reaction monitoring proteomics. Upon comparison of peptide signals from OCT-embedded tissue and flash-frozen tissue from the same donors, both individual protein quantities, and their interindividual variabilities, were similar. Therefore, the approach reported here can be applied to clinical reservoirs of OCT-preserved kidney tissue to be used for quantitative proteomics studies of clinically relevant proteins expressed in different parts of the kidney (including drug transporters and metabolizing enzymes).

Biliary tract external drainage alleviates kidney injury in shock

BACKGROUND

Kidney injury is common in hemorrhagic shock (HS). Kidney injury leads to a systemic increase in serum chemokines and cytokines and causes injuries to other vital organs. Our previous studies showed that vitamin C led to organ protection and inflammation inhibitory effects in rat models of HS via induction heme oxygenase-1 (HO-1). We also found that biliary tract external drainage (BTED) increased the expression levels of HO-1 in rat livers. We investigated roles of BTED in kidney injury and its relationship with the HO-1 pathway in HS in this research.

METHODS

Rat models of HS were induced by drawing blood from the femoral artery. BTED was performed by inserting a catheter into the bile duct. Thirty-six Sprague-Dawley rats were randomized to sham group; HS group; zinc protoporphyrin IX (Znpp) group; BTED group; BTED + Znpp group, and BTED + bile infusion group. The expression levels of HO-1 in the kidney were analyzed by Western blotting. The expression levels of occludin messenger RNA in the kidney were analyzed by real-time reverse transcription-polymerase chain reaction. The expression levels of occludin in the kidney were analyzed by immunohistochemistry. Histology of renal was performed by hematoxylin and eosin staining.

RESULTS

Occludin messenger RNA and protein levels in the kidney increased markedly after BTED under HS conditions. Renal histopathologic scores decreased significantly after BTED under HS conditions. Znpp significantly inhibited all mentioned effects.

CONCLUSIONS

BTED alleviates kidney injury in rats of HS via the HO-1 pathway.

Renal tight junction proteins are decreased in cisplatin-induced nephrotoxicity in rats

Cisplatin (CP) is an antineoplastic agent that induces nephrotoxicity and oxidative stress. It is unknown whether renal tight junction (TJ) proteins expression and localization are modified in CP-induced nephrotoxicity.

OBJECTIVE

To study if the expression of the TJ proteins occludin, claudin-2, claudin-5 and zonula occludens-1 (ZO-1) is modified in rats with CP-induced nephrotoxicity.

MATERIALS AND METHODS

Male Wistar rats (n = 5/group) were injected with saline solution (V group), and the other group (CP group) was injected with a single dose of saline solution and CP (7.5 mg/kg i.p.). Rats were sacrificed 72 h after CP injection and blood, and 24-h urine samples were collected. Several plasma and urinary injury biomarkers as well as renal histopathology lesions, oxidative and nitrosative stress markers were evaluated, and protein levels of ocludin, claudin-2, claudin-5, ZO-1 were measured by Western blot. Statistically significant changes noted with different p < 0.05 versus V.

RESULTS

Nephrotoxicity was evident by histological alterations, glycosuria, decrease in creatinine clearance, increase in fractional excretion of sodium, serum creatinine and kidney injury molecule-1. These changes were associated with oxidative/nitrosative stress (increased renal abundance of 3-nitrotyrosine and protein kinase Cβ2 and decreased renal expression of nuclear factor-erythroid-2-related factor 2) and decreased activity of antioxidant enzymes. Finally, it was found that CP-induced renal damage was associated with decreased renal expression of occludin and claudin-2.

DISCUSSION AND CONCLUSION

CP altered the TJ proteins expression and localization in the proximal tubule that was associated with oxidative/nitrosative stress.

Oxidative stress induces claudin-2 nitration in experimental type 1 diabetic nephropathy

Renal complications in diabetes are severe and may lead to renal insufficiency. Early alterations in tight junction (TJ) proteins in diabetic nephropathy (DN) have not been explored and the role of oxidative stress in their disassembly has been poorly characterized. We investigated the expression and distribution of TJ proteins: claudin-5 in glomeruli (GL), occludin and claudin-2 in proximal tubules (PTs), and ZO-1 and claudin-1, -4, and -8 in distal tubules (DTs) of rats 21 days after streptozotocin injection. Redox status along the nephron segments was evaluated. Diabetes increased kidney injury molecule-1 expression. Expression of sodium glucose cotransporters (SGLT1 and SGLT2) and facilitative glucose transporter (GLUT2) was induced. Increased oxidative stress was present in GL and PTs and to a lesser extent in DTs (measured by superoxide production and PKCβ2 expression), owing to NADPH oxidase activation and uncoupling of the endothelial nitric oxide synthase-dependent pathway. Claudin-5, occludin, and claudin-2 expression was decreased, whereas claudin-4 and -8 expression increased. ZO-1 was redistributed from membrane to cytosol. Increased nitration of tyrosine residues in claudin-2 was found, which might contribute to decrement of this protein in proximal tubule. In contrast, occludin was not nitrated. We suggest that loss of claudin-2 is associated with increased natriuresis and that loss of glomerular claudin-5 might explain early presence of proteinuria. These findings suggest that oxidative stress is related to alterations in TJ proteins in the kidney that are relevant to the pathogenesis and progression of DN and for altered sodium regulation in diabetes.

Structure and function of the thin limbs of the loop of Henle

The thin limbs of the loop of Henle, which comprise the intermediate segment, connect the proximal tubule to the distal tubule and lie entirely within the renal medulla. The descending thin limb consists of at least two or three morphologically and functionally distinct subsegments and participates in transepithelial transport of NaCl, urea, and water. Only one functionally distinct segment is recognized for the ascending thin limb, which carries out transepithelial transport of NaCl and urea in the reabsorptive and/or secretory directions. Membrane transporters involved with passive transcellular Cl, urea, and water fluxes have been characterized for thin limbs; however, these pathways do not account for all transepithelial fluid and solute fluxes that have been measured in vivo. The paracellular pathway has been proposed to play an important role in transepithelial Na and urea fluxes in defined thin-limb subsegments. As the transport pathways become clearer, the overall function of the thin limbs is becoming better understood. Primary and secondary signaling pathways and protein-protein interactions are increasingly recognized as important modulators of thin-limb cell function and cell metabolism. These functions must be investigated under diverse extracellular conditions, particularly for those cells of the deep inner medulla that function in an environment of wide variation in hyperosmolality. Transgenic mouse models of several key water and solute transport proteins have provided significant insights into thin-limb function. An understanding of the overall architecture of the medulla, including juxtapositions of thin limbs with collecting ducts, thick ascending limbs, and vasa recta, is essential for understanding the role of the kidney in maintaining Na and water homeostasis, and for understanding the urine concentrating mechanism.

A marked animal-vegetal polarity in the localization of Na(+),K(+) -ATPase activity and its down-regulation following progesterone-induced maturation

The stage-VI Xenopus oocyte has a very distinct animal-vegetal polarity with structural and functional asymmetry. In this study, we show the expression and distribution pattern of Na(+),K(+) -ATPase in stage-VI oocytes, and its changes following progesterone-induced maturation. Using enzyme-specific electron microscopy phosphatase histochemistry, [(3) H]-ouabain autoradiography, and immunofluorescence cytochemistry at light microscopic level, we find that Na(+),K(+) -ATPase activity is mainly confined to the animal hemisphere. Electron microscopy histochemical results also suggest that polarized distribution of Na(+),K(+) -ATPase activity persists following progesterone-induced maturation, and it becomes gradually more polarized towards the animal pole. The time course following progesterone-induced maturation suggests that there is an initial up-regulation and then gradual down-regulation of Na(+),K(+) -ATPase activity leading to germinal vesicle breakdown (GVBD). By GVBD, the Na(+),K(+) -ATPase activity is completely down-regulated due to endocytotic removal of pump molecules from the plasma membrane into the sub-cortical region of the oocyte. This study provides the first direct evidence for a marked asymmetric localization of Na(+),K(+) -ATPase activity in any vertebrate oocyte. Here, we propose that such asymmetry in Na(+),K(+) -ATPase activity in stage-VI oocytes, and their down-regulation following progesterone-induced maturation, is likely to have a role in the active state of the germinal vesicle in stage-VI oocytes and chromosomal condensation after GVBD.

Epithelial remodeling and claudin mRNA abundance in the gill and kidney of puffer fish (Tetraodon biocellatus) acclimated to altered environmental ion levels

In water of varying ion content, the gills and kidney of fishes contribute significantly to the maintenance of salt and water balance. However, little is known about the molecular architecture of the tight junction (TJ) complex and the regulation of paracellular permeability characteristics in these tissues. In the current studies, puffer fish (Tetraodon biocellatus) were acclimated to freshwater (FW), seawater (SW) or ion-poor freshwater (IPW) conditions. Following acclimation, alterations in systemic endpoints of hydromineral status were examined in conjunction with changes in gill and kidney epithelia morphology/morphometrics, as well as claudin TJ protein mRNA abundance. T. biocellatus were able to maintain endpoints of hydromineral status within relatively tight limits across the broad range of water ion content examined. Both gill and kidney tissue exhibited substantial alterations in morphology as well as claudin TJ protein mRNA abundance. These responses were particularly pronounced when comparing fish acclimated to SW versus those acclimated to IPW. TEM observations of IPW-acclimated fish gills revealed the presence of cells that exhibited the typical characteristics of gill mitochondria-rich cells (e.g. voluminous, Na(+)-K(+)-ATPase-immunoreactive, exposed to the external environment at the apical surface), but were not mitochondria-rich. To our knowledge, this type of cell has not previously been described in hyperosmoregulating fish gills. Furthermore, modifications in the morphometrics and claudin mRNA abundance of kidney tissue support the notion that spatial alterations in claudin TJ proteins along the nephron of fishes will likely play an important role in the regulation of salt and water balance in these organisms.

Spatially resolved quantification of E-cadherin on target hES cells

The local expression and distribution pattern of protein on a cell play essential roles in signal transduction within a cell or between cells. Here we report on the development of a spatially resolved quantification method, which was applied in the study of E-cadherin local expression in identified undifferentiated and differentiated human embryonic stem (hES) cells in their native cellular environment. This was achieved by a novel immunofluorescence assisted affinity mapping (IF-AM) method, in which immunofluorescence provides the guidance to locate a desired type of cell in a cell community for performing affinity mapping to quantify the local protein density. The results unveiled the crucial role of E-cadherin in mediating hES cell proliferation and differentiation: the expression of E-cadherin is markedly higher on undifferentiated cells, and the growth of hES cells in unique colonies is contingent on the homogeneous distribution of E-cadherin. Due to the ability of directly assessing individual proteins of a cell, the IF-AM method is shown to be a sensitive tool for resolving subtle differences in the local expression of membrane proteins even at low abundance.

Protein p0071, an armadillo plaque protein of adherens junctions, is predominantly expressed in distal renal tubules

Protein p0071 is a member of the p120-subfamily of armadillo proteins and is well known as a junctional plaque component involved in cell-cell adhesion, especially in adherens junctions. By systematic immunohistochemical analysis of mouse and human kidney tissues, p0071 was prominently detected in distinct kidney tubules. Upon double-labeling immunolocalization experiments with segment-specific markers, p0071 was predominantly localized in distal straight and convoluted tubules and to a lesser extent in proximal tubules, in the ascending thin limb of loop of Henle and in the collecting ducts. In capillaries of the kidney, p0071 co-localized with VE-cadherin an endothelium-specific cadherin. Protein p0071 was also detected in both, renal cell carcinomas derived from distal tubules and in maturing nephrons of early mouse developmental stages. Immunoblotting of total extracts of cultured cells of renal origin showed that p0071 was detected in all human and murine cells analyzed. Upon immunolocalization, p0071 was observed in adherens junctions but also in distinct cytoplasmic structures at the cell periphery of cultured cells. Possible structural and functional roles of p0071 are suggested by its preferential occurrence in distinct tubule segments, and its potential use as a cytodiagnostic cell type marker in renal pathology is discussed.

Occludin and hydromineral balance in Xenopus laevis

To investigate the response of the tight junction (TJ) protein occludin to environmental change in an anuran amphibian, we examined occludin tissue distribution, immunolocalization and alterations in mRNA expression in African clawed frogs (Xenopus laevis) acclimated to brackish water (BW) conditions (from freshwater to 2 per thousand, 5 per thousand or 10 per thousand salt water). Occludin mRNA is widely expressed in Xenopus and is abundant in tissues involved in regulating salt and water balance, such as the gastrointestinal (GI) tract, kidney and urinary bladder. Immunohistochemical analyses revealed strong occludin immunolabelling in the apicolateral region of epithelia lining the GI tract and mRNA expression increased along the longitudinal axis of the gut. In kidney tissue, occludin was differentially expressed on the luminal side of the nephron tubule, appearing in the distal tubules and collecting ducts only. In response to BW acclimation, Xenopus exhibited a significant loss of tissue water as well as salinity-dependent elevations in serum osmolality as a result of increased urea levels followed by elevated serum Na(+) and Cl(-) levels. Tissue-specific alterations in the ionomotive enzyme Na(+),K(+)-ATPase were also observed in Xenopus in response to BW acclimation. Most notably, Na(+),K(+)-ATPase activity in the rectum increased in response to elevated environmental salt concentrations while renal activity decreased. Furthermore, acclimation to BW caused tissue-specific and salinity-dependent alterations in occludin mRNA expression within select Xenopus osmoregulatory organs. Taken together, these studies suggest that alterations in occludin, in conjunction with active transport processes, may contribute to amphibian hydromineral homeostasis during environmental change.

Differential expression pattern of protein ARVCF in nephron segments of human and mouse kidney

The protein ARVCF is a member of the p120 subfamily of armadillo proteins whose members have been described to occur in junction-bound and non-junction-bound forms. Studies on ARVCF were constrained because the endogenous protein was difficult to detect with the available reagents. We have generated novel monoclonal and polyclonal antibodies usable for biochemical and localization studies. By systematic immunohistochemical analysis of various tissues protein ARVCF is prominently detected in mouse, bovine and human kidney. Using antibodies against specific markers of nephron segments protein ARVCF is localized in proximal tubules according to double label immunofluorescence. Besides its occurrence in proximal tubules of adult kidney and in renal cell carcinoma derived from proximal tubules ARVCF is also detected in maturing nephrons in early mouse developmental stages such as, for example, 15 days of gestation (E15). Immunoblotting of total extracts of cultured cells of renal origin showed that ARVCF is detected in all human and murine cultured cells analyzed. Upon immunolocalization ARVCF is mostly detected in the cytoplasm occurring in a fine granular form. This prominent cytoplasmic localization of ARVCF in cultured cells and its occurrence in proximal tubules implies an involvement of ARVCF in specific functional processes of proximal tubules of kidney.

Occludin immunolocalization and protein expression in goldfish

Tight junctions (TJs) are an integral component of models illustrating ion transport mechanisms across fish epithelia; however, little is known about TJ proteins in fishes. Using immunohistochemical methods and Western blot analysis, we examined the localization and expression of occludin, a transmembrane TJ protein, in goldfish tissues. In goldfish gills,discontinuous occludin immunostaining was detected along the edges of secondary gill lamellae and within parts of the interlamellar region that line the lateral walls of the central venous sinus. In the goldfish intestine,occludin immunolocalized in a TJ-specific distribution pattern to apical regions of columnar epithelial cells lining the intestinal lumen. In the goldfish kidney, occludin was differentially expressed in discrete regions of the nephron. Occludin immunostaining was strongest in the distal segment of the nephron, moderate in the collecting duct and absent in the proximal segment. To investigate a potential role for occludin in the maintenance of the hydromineral balance of fishes, we subjected goldfish to 1, 2 and 4 weeks of food deprivation, and then examined the endpoints of hydromineral status,Na+,K+-ATPase activity and occludin protein expression in the gills, intestine and kidney. Occludin expression altered in response to hydromineral imbalance in a tissue-specific manner suggesting a dynamic role for this TJ protein in the regulation of epithelial permeability in fishes.

A p38 mitogen-activated protein kinase inhibitor protects against renal damage in a non-heart-beating donor model

Ischemia-reperfusion injury is one of the central nonimmunologic processes involved in renal allograft dysfunction. Kidneys from non-heart beating donors (NHBD) exhibit higher rates of delayed graft function (DGF) than those from other donors. Primary nonfunction and DGF are the main barriers to the use of kidneys from NHBD. Using a pig model of NHBD transplantation, we studied the effect of FR167653 (a p38 MAP kinase inhibitor) on the recovery and reparation of kidneys exposed to both warm (WI: 1 h) and cold ischemia (24 h). Our results demonstrate that the addition of FR167653 increases the kinetics of proximal tubule cell regeneration after 60 min of WI. Hypoxia-inducible factor and vascular endothelial growth factor expression was also more important in FR167653-treated kidneys compared with those in nontreated groups. Also, expression of peripheral-type benzodiazepine receptor, involved in tissue repair, was increased in the FR167653-treated groups. At 3 mo, the protective effects of FR167653 were accompanied by a reduction of long-term inflammation process and tubulointerstitial fibrosis development associated with a limitation of ischemia-induced remodeling. This study suggests that such treatment may be useful in protocols aimed at improving the quality of renal transplants from NHBD. In addition, the beneficial role of FR167653 in limiting early injury is associated with secondary reduction in development of tubular atrophy and interstitial fibrosis which are together the hallmark of failing renal transplants. The more efficient effect was observed when FR167653 was added in combination before WI, during cold storage and reperfusion.

Na,K-adenosine triphosphatase alpha1-subunit predicts survival of renal clear cell carcinoma

PURPOSE

Na,K-adenosine triphosphatase, which is composed of a catalytic alpha-subunit and a regulatory beta-subunit, generates an electrochemical gradient across the plasma membrane. Previous studies demonstrated altered Na,K-adenosine triphosphatase subunit expression in renal clear cell carcinoma and an association of subunit levels with the prediction of recurrent bladder cancer. We determined the clinical association of protein expression patterns of the Na,K-adenosine triphosphatase alpha1 and beta1-subunits in renal clear cell carcinoma using tissue microarrays with linked clinicopathological data.

MATERIALS AND METHODS

The UCLA kidney cancer tissue microarray was used to investigate the protein expression of Na,K-adenosine triphosphatase alpha1 and beta1-subunits by immunohistochemistry in 342 patients with renal clear cell carcinoma who were treated with radical nephrectomy. Of these patients clinical outcomes studies were performed in 317. The resultant expression reactivity was correlated with clinicopathological variables.

RESULTS

We found that the alpha1-subunit was a significant and independent predictor of disease specific death from renal clear cell carcinoma on multivariate Cox proportional hazards analysis that included established prognostic factors Eastern Cooperative Oncology Group performance status, pT status, metastasis status and tumor grade. Significance was found when examining all patients with clear cell renal cell carcinoma as well as patient substrata with low or high grade tumors and localized or metastatic disease, suggesting that the Na,K-adenosine triphosphatase alpha1-subunit could be used as a new prognosticator for disease specific death from renal clear cell carcinoma.

CONCLUSIONS

These results suggest that Na,K-adenosine triphosphatase alpha1-subunit expression patterns may be a useful clinical prognosticator for renal clear cell carcinoma. The Na,K-adenosine triphosphatase beta1-subunit was not found to be a useful prognosticator in this setting.

Distinct distribution of specific members of protein 4.1 gene family in the mouse nephron

BACKGROUND

Protein 4.1 is an adapter protein that links the actin cytoskeleton to various transmembrane proteins. These 4.1 proteins are encoded by four homologous genes, 4.1R, 4.1G, 4.1N, and 4.1B, which undergo complex alternative splicing. Here we performed a detailed characterization of the expression of specific 4.1 proteins in the mouse nephron.

METHODS

Distribution of renal 4.1 proteins was investigated by staining of paraformaldehyde-fixed mouse kidney sections with antibodies highly specific for each 4.1 protein. Major 4.1 splice forms, amplified from mouse kidney marathon cDNA, were expressed in transfected COS-7 cells in order to assign species of known exon composition to proteins detected in kidney.

RESULTS

A 105 kD 4.1R splice form, initiating at ATG-2 translation initiation site and lacking exon 16, but including exon 17B, was restricted to thick ascending limb of Henle's loop. A 95 kD 4.1N splice form, lacking exons 15 and 17D, was expressed in either descending or ascending thin limb of Henle's loop, distal convoluted tubule, and all regions of the collecting duct system. A major 108 kD 4.1B splice form, initiating at a newly characterized ATG translation initiation site, and lacking exons 15, 17B, and 21, was present only in Bowman's capsule and proximal convoluted tubule (PCT). There was no expression of 4.1G in kidney.

CONCLUSION

Distinct distribution of 4.1 proteins along the nephron suggests their involvement in targeting of selected transmembrane proteins in kidney epithelium and, therefore, in regulation of specific kidney functions.

Localization of intercellular adherens junction protein p120 catenin during podocyte differentiation

To reveal the role of cadherin complex in podocyte differentiation, the present study describes the localization of the cadherin complex, including p120 catenin (p120ctn), in the developing and in the aminonucleoside nephrosis (PAN nephrosis) rat kidney, by immunofluorescence microscopy and immunogold electron microscopy. p120ctn and beta-catenin were co-localized at the apical part of lateral cell membranes in presumptive podocytes of the S-shaped body, and their localization shifted to the basal margin of lateral cell membranes in the capillary loop stage. There was no expression of the cadherin complex at the slit diaphragm, the intercellular junction of mature podocytes. After the regression of the podocyte junctional structure in PAN nephrosis, the cadherin complex was not re-expressed. The dynamic changes in the localization of the cadherin complex suggest that the it plays an important role during podocyte differentiation, including the rearrangement of the intercellular junction and the formation of the slit diaphragm.

Immunocytochemical localization of Na-K-ATPase alpha- and gamma-subunits in rat kidney

The gamma-subunit of the Na-K-ATPase is a single-span membrane protein that alters the kinetic properties of the enzyme. It is expressed in the kidney, but our initial observations indicated that it is not present in all nephron segments (Arystarkhova E, Wetzel RK, Asinovski NK, and Sweadner KJ. J Biol Chem 274: 33183-33185, 1999). Here we used triple-label confocal immunofluorescence microscopy in rat kidney with antibodies to Na-K-ATPase alpha1- and gamma-subunits and nephron segment-specific markers. Na-K-ATPase alpha1-subunit stain was low but unambiguous in proximal segments, moderate in macula densa, connecting tubules, and cortical collecting ducts, high in thick ascending limb and distal convoluted tubules, and nearly undetectable in glomeruli, descending and ascending thin limb, and medullary collecting ducts. The gamma-subunit colocalized at staining levels similar to alpha1-subunit in basolateral membranes in all segments except cortical thick ascending limb and cortical collecting ducts, which had alpha1-subunit but no detectable gamma-subunit stain. Selective gamma-subunit expression may contribute to the variations in Na-K-ATPase properties in different renal segments.