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Peter Mundel

Publications and source records attributed to Peter Mundel.

46 records · Page 3Linked to original sources
PubMed2002

Role of nephrin in renal disease including diabetic nephropathy.

Nephrin, a newly described protein, has been localized to the slit membrane between adjacent podocytes of the glomerulus. Its discovery followed the demonstration of the gene NPHS1 and its mutation, resulting in the absence of the protein product, nephrin, in the congenital nephrotic syndrome of the Finnish type. The link between permutations in nephrin expression and proteinuria has been shown in animal models by using neutralizing antibodies or studying mice with inactivation of the nephrin gene. Moreover, the expression of nephrin has been shown to be reduced in various animal models of proteinuric renal disease. The relationship between changes in nephrin expression and proteinuric renal disease in humans is not fully elucidated, with a reduction in expression of this protein reported in a range of renal diseases. Diabetic nephropathy, one of the major causes of end-stage renal disease, is associated with substantial proteinuria and in experimental models with a reduction in slit pore density. In experimental models of diabetes, nephrin expression has been described as being transiently increased in the first 8 weeks of diabetes, followed in longer-term studies with reduced nephrin expression in association with increasing proteinuria. An angiotensin II-receptor blocker has been shown to prevent depletion in glomerular nephrin expression in the diabetic kidney. Human studies in both type 1 and type 2 diabetes suggest down-regulation of nephrin expression in the diabetic kidney and it has been postulated that these changes may play a role in the pathogenesis of diabetic nephropathy, specifically the development of proteinuria in this condition. Although there are other proteins involved in the structure of the epithelial podocyte and specifically the slit pore, nephrin seems to play a pivotal role in preventing passage of protein through the glomerular barrier. Furthermore, it is suggested that the antiproteinuric effects of inhibition of the renin-angiotensin system may partly relate to the effects of these agents on nephrin expression.

Animals↗
PubMed2002

A mutant form of the Wilms' tumor suppressor gene WT1 observed in Denys-Drash syndrome interferes with glomerular capillary development.

The Wilms' tumor suppressor gene WT1 encodes a zinc finger protein that is required for urogenital development. In the kidney, WT1 is most highly expressed in glomerular epithelial cells or podocytes, which are an essential component of the filtering system. Human subjects heterozygous for point mutations in the WT1 gene develop renal failure because of the formation of scar tissue within glomeruli. The relationship between WT1 expression in podocytes during development and glomerular scarring is not well understood. In this study, transgenic mice that expressed a mutant form of WT1 in podocytes were derived. The capillaries within transgenic glomeruli were dilated, indicating that WT1 might regulate the expression of growth factors that affect capillary development. Platelet endothelial cell adhesion molecule-1 expression was greatly reduced on glomerular endothelial cells of transgenic kidneys. These results suggest that WT1 controls the expression of growth factors that regulate glomerular capillary development and that abnormal capillary development might lead to glomerular disease.

Animals↗
PubMed2002

Inhibitory smads and tgf-Beta signaling in glomerular cells.

Smad6 and Smad7 are inhibitory SMADs with putative functional roles at the intersection of major intracellular signaling networks, including TGF-beta, receptor tyrosine kinase (RTK), JAK/STAT, and NF-kappaB pathways. This study reports differential functional roles and regulation of Smad6 and Smad7 in TGF-beta signaling in renal cells, in murine models of renal disease and in human glomerular diseases. Smad7 is upregulated in podocytes in all examined glomerular diseases (focal segmental glomerulosclerosis [FSGS], minimal-change disease [MCD], membranous nephropathy [MNP], lupus nephritis [LN], and diabetic nephropathy [DN]) with a statistically significant upregulation in "classical" podocyte-diseases such as FSGS and MCD. TGF-beta induces Smad7 synthesis in cultured podocytes and Smad6 synthesis in cultured mesangial cells. Although Smad7 expression inhibited both Smad2- and Smad3-mediated TGF-beta signaling in podocytes, it inhibited only Smad3 but not Smad2 signaling in mesangial cells. In contrast, Smad6 had no effect on TGF-beta/Smad signaling in podocytes and enhanced Smad3 signaling in mesangial cells. These data suggest that Smad7 is activated in injured podocytes in vitro and in human glomerular disease and participates in negative control of TGF-beta/Smad signaling in addition to its pro-apoptotic activity, whereas Smad6 has no role in TGF-beta response and injury in podocytes. In contrast, Smad6 is upregulated in the mesangium in human glomerular diseases and may be involved in functions independent of TGF-beta/Smad signaling. These data indicate an important role for Smad6 and Smad7 in glomerular cells in vivo that could be important for the cell homeostasis in physiologic and pathologic conditions.

Adaptor Proteins, Signal Transducing↗
PubMed2002

Glomerular endothelial fenestrae in vivo are not formed from caveolae.

Previous reports indicate that endothelial fenestrae in vitro can form by fusion of caveolae or caveolae-like vesicles. The principal aim of this study was to determine whether formation of glomerular endothelial cell fenestrae in vivo similarly involves caveolae and caveolin-1. Whereas caveolin-1 immunofluorescence was found around the circumference of human and mouse glomerular capillary loops, it co-localized only partially with the endothelium-specific lectin Ulex Europaeus I in human glomeruli, leaving portions of the endothelium devoid of caveolin-1. Immunogold electron microscopy, used to definitively localize caveolin-1 in glomeruli, showed that caveolin-1 was completely excluded from the fenestrated portion of the endothelium. Moreover, in caveolin-1-deficient mice, which cannot form caveolae, the ultrastructure of glomerular endothelial fenestrae appeared entirely normal. Interestingly, strong caveolin-1 immunogold labeling was observed in podocytes, where some caveolin-1 localized to filtration slits. Caveolin-1 co-immunoprecipitated with the podocyte slit diaphragm proteins nephrin and CD2AP, and dual immunofluorescence confirmed co-localization of caveolin-1 and nephrin. Nevertheless, in caveolin-1-deficient mice, podocyte ultrastructure appeared normal, and the podocyte proteins synaptopodin, nephrin, and podocin were expressed normally. In addition, blood urea nitrogen concentrations and urinary protein excretion in these mice were similar to those in wild-type mice. Thus, unlike caveolae formation, glomerular endothelial cell fenestrae formation in vivo does not require caveolin-1, ruling out the previous hypothesis that endothelial fenestrae represent fused caveolae, at least for glomerular endothelial cells. Localization of caveolin-1 to podocytes and their filtration slits is consistent with the view that the filtration slit plasma membrane represents a type of lipid raft microdomain.

Adaptor Proteins, Signal Transducing↗
PubMed2002

A conditionally immortalized human podocyte cell line demonstrating nephrin and podocin expression.

Recent molecular insights have established the podocyte as a key component of the glomerular filtration barrier, and hence an important common pathway in proteinuric diseases. A conditionally immortalized human podocyte cell line has been developed by transfection with the temperature-sensitive SV40-T gene. These cells proliferate at the "permissive" temperature (33 degrees C). After transfer to the "nonpermissive" temperature (37 degrees C), they entered growth arrest and expressed markers of differentiated in vivo podocytes, including the novel podocyte proteins, nephrin, podocin, CD2AP, and synaptopodin, and known molecules of the slit diaphragm ZO-1, alpha-, beta-, and gamma-catenin and P-cadherin. The differentiation was accompanied by a growth arrest and the upregulation of cyclin-dependent kinase inhibitors, p27 and p57, as well as cyclin D(1), whereas cyclin A was downregulated. These data are consistent with cell cycle protein expression during podocyte maturation in vivo. In conclusion, the development of this cell line provides a new tool in the study of podocyte biology, which will enable accurate assessment of the behavior of these complex cells in health and disease.

Biomarkers↗
PubMed2001

Podocytes respond to mechanical stress in vitro.

Glomerular capillary pressure is thought to affect the structure and function of glomerular cells. However, it is unknown whether podocytes are intrinsically sensitive to mechanical forces. In the present study, differentiated mouse podocytes were cultured on flexible silicone membranes. Biaxial cyclic stress (0.5 Hz and 5% linear strain) was applied to the membranes for up to 3 d. Mechanical stress reduced the size of podocyte cell bodies, and processes became thin and elongated. Podocytes did not align in the inhomogeneous force field. Whereas the network of microtubules and that of the intermediate filament vimentin exhibited no major changes, mechanical stress induced a reversible reorganization of the actin cytoskeleton: transversal stress fibers (SF) disappeared and radial SF that were connected to an actin-rich center (ARC) formed. Epithelial and fibroblast cell lines did not exhibit a comparable stress-induced reorganization of the F-actin. Confocal and electron microscopy revealed an ellipsoidal and dense filamentous structure of the ARC. Myosin II, alpha-actinin, and the podocyte-specific protein synaptopodin were present in radial SF, but, opposite to F-actin, they were not enriched in the ARC. The formation of the ARC and of radial SF in response to mechanical stress was inhibited by nonspecific blockade of Ca(2+) influx with Ni(2+) (1 mM), by Rho kinase inhibition with Y-27632 (10 microM), but not by inhibition of stretch-activated cation channels with Gd(3+) (50 microM). In summary, mechanical stress induces a unique reorganization of the actin cytoskeleton in podocytes, featuring radial SF and an ARC, which differ in protein composition. The F-actin reorganization in response to mechanical stress depends on Ca(2+) influx and Rho kinase. The present study provides the first direct evidence that podocytes are mechanosensitive.

Actins↗
PubMed2001

Human immunodeficiency virus-1 induces loss of contact inhibition in podocytes.

Human immunodeficiency virus-associated nephropathy (HIVAN) affects up to 10% of HIV-positive black adults and children and is the leading cause of renal disease in infected individuals. The disease is characterized by proliferation of renal epithelial cells, both glomerular and tubular. Diseased kidneys are enlarged, and glomerular visceral epithelial cells (podocytes) express proliferation markers. In a transgenic murine model of HIVAN expressing a deletion construct of HIV-1, the identical pathologic features are observed. It was demonstrated that HIV-1 mRNA is expressed in renal epithelium of the transgenic mouse and in patients with HIVAN, suggesting a direct role for HIV-1 in disease pathogenesis in both humans and the murine model. For investigating the mechanisms responsible for proliferative changes in podocytes, the HIV-1 transgenic mouse was bred onto the immortomouse background, and conditionally immortalized transgenic and nontransgenic podocyte cell lines were established. Transgenic podocytes demonstrated increased spontaneous proliferation, compared with nontransgenic podocytes at confluence, and they were found to have a greater percentage of cells in the proliferative phase of the cell cycle. It is striking that transgenic podocytes were not contact inhibited and formed aggregates in soft agar. Aggregates also formed when nontransgenic podocytes were infected with the identical HIV-1 construct used to generate the transgenic model. This demonstrates that the loss of contact inhibition is due to a direct effect of HIV-1. Therefore, proliferation induced by HIV-1 gene expression is likely to play a key role in the pathogenesis of HIVAN.

Agar↗
PubMed2000

The glomerular slit diaphragm is a modified adherens junction.

The glomerular slit diaphragm between podocyte foot processes shares typical morphologic features with an adherens junction. Differentiated cultured podocytes form cellular structures comparable to filtration slits in vivo. At those sites, zonula occludens-1 (ZO-1) was coexpressed with P-cadherin as well as with alpha-, beta-, and gamma-catenin. In situ, P-cadherin was detected at the slit diaphragm in association with ZO-1 as shown by confocal microscopy and immunogold double labeling electron microscopy. P-cadherin expression in vivo and in vitro was confirmed by reverse transcription-PCR. These findings led to the concept that the slit diaphragm represents an adherens junction composed of P-cadherin, alpha-, beta-, and gamma-catenin, and ZO-1. In contrast to an adherens junction of a similar composition recently described in cultured fibroblasts, the slit diaphragm complex does not contain vinculin, which was found in nearby focal contacts. A P-cadherin-based adherens junction is well-suited to explain the zipper-like structure of the slit diaphragm. The present study should allow new avenues leading to the identification of additional slit diaphragm-associated proteins conferring specificity to this unique cell junction.

Animals↗