Moshe Levi

Lipid rafts reconstituted in model membranes

One key tenet of the raft hypothesis is that the formation of glycosphingolipid- and cholesterol-rich lipid domains can be driven solely by characteristic lipid-lipid interactions, suggesting that rafts ought to form in model membranes composed of appropriate lipids. In fact, domains with raft-like properties were found to coexist with fluid lipid regions in both planar supported lipid layers and in giant unilamellar vesicles (GUVs) formed from 1) equimolar mixtures of phospholipid-cholesterol-sphingomyelin or 2) natural lipids extracted from brush border membranes that are rich in sphingomyelin and cholesterol. Employing headgroup-labeled fluorescent phospholipid analogs in planar supported lipid layers, domains typically several microns in diameter were observed by fluorescence microscopy at room temperature (24 degrees C) whereas non-raft mixtures (PC-cholesterol) appeared homogeneous. Both raft and non-raft domains were fluid-like, although diffusion was slower in raft domains, and the probe could exchange between the two phases. Consistent with the raft hypothesis, GM1, a glycosphingolipid (GSL), was highly enriched in the more ordered domains and resistant to detergent extraction, which disrupted the GSL-depleted phase. To exclude the possibility that the domain structure was an artifact caused by the lipid layer support, GUVs were formed from the synthetic and natural lipid mixtures, in which the probe, LAURDAN, was incorporated. The emission spectrum of LAURDAN was examined by two-photon fluorescence microscopy, which allowed identification of regions with high or low order of lipid acyl chain alignment. In GUVs formed from the raft lipid mixture or from brush border membrane lipids an array of more ordered and less ordered domains that were in register in both monolayers could reversibly be formed and disrupted upon cooling and heating. Overall, the notion that in biomembranes selected lipids could laterally aggregate to form more ordered, detergent-resistant lipid rafts into which glycosphingolipids partition is strongly supported by this study.

Mouse Models of Diabetic Nephropathy

Diabetic nephropathy is a major cause of ESRD worldwide. Despite its prevalence, a lack of reliable animal models that mimic human disease has delayed the identification of specific factors that cause or predict diabetic nephropathy. The Animal Models of Diabetic Complications Consortium (AMDCC) was created in 2001 by the National Institutes of Health to develop and characterize models of diabetic nephropathy and other complications. This interim report and our online supplement detail the progress made toward that goal, specifically in the development and testing of murine models. Updates are provided on validation criteria for early and advanced diabetic nephropathy, phenotyping methods, the effect of background strain on nephropathy, current best models of diabetic nephropathy, negative models, and views of future directions. AMDCC investigators and other investigators in the field have yet to validate a complete murine model of human diabetic kidney disease. Nonetheless, the critical analysis of existing murine models substantially enhances our understanding of this disease process.

Partitioning of Thy-1, GM1, and cross-linked phospholipid analogs into lipid rafts reconstituted in supported model membrane monolayers

As shown earlier, raft-like domains resembling those thought to be present in natural cell membranes can be formed in supported planar lipid monolayers. These liquid-ordered domains coexist with a liquid-disordered phase and form in monolayers prepared both from synthetic lipid mixtures and lipid extracts of the brush border membrane of mouse kidney cells. The domains are detergent-resistant and are highly enriched in the glycosphingolipid GM1. In this work, the properties of these raft-like domains are further explored and compared with properties thought to be central to raft function in plasma membranes. First, it is shown that domain formation and disruption critically depends on the cholesterol density and can be controlled reversibly by treating the monolayers with the cholesterol-sequestering reagent methyl-β-cyclodextrin. Second, the glycosylphosphatidylinositol-anchored cell-surface protein Thy-1 significantly partitions into the raft-like domains. The extent of this partitioning is reduced when the monolayers contain GM1, indicating that different molecules can compete for domain occupation. Third, the partitioning of a saturated phospholipid analog into the raft phase is dramatically increased (15% to 65%) after cross-linking with antibodies, whereas the distribution of a doubly unsaturated phospholipid analog is not significantly affected by cross-linking (≈10%). This result demonstrates that cross-linking, a process known to be important for certain cell-signaling processes, can selectively translocate molecules to liquid-ordered domains.

Advanced glycation end products: a Nephrologist's perspective.

Advanced glycation end products (AGEs) are a heterogeneous group of molecules that accumulate in plasma and tissues with advancing age, diabetes, and renal failure. There is emerging evidence that AGEs are potential uremic toxins and may have a role in the pathogenesis of vascular and renal complications associated with diabetes and aging. AGEs are formed when a carbonyl of a reducing sugar condenses with a reactive amino group in target protein. These toxic molecules interact with specific receptors and elicit pleiotropic responses. AGEs accelerate atherosclerosis through cross-linking of proteins, modification of matrix components, platelet aggregation, defective vascular relaxation, and abnormal lipoprotein metabolism. In vivo and in vitro studies indicate that AGEs have a vital role in the pathogenesis of diabetic nephropathy and the progression of renal failure. The complications of normal aging, such as loss of renal function, Alzheimers disease, skin changes, and cataracts, may also be mediated by progressive glycation of long-lived proteins. AGEs accumulate in renal failure as a result of decreased excretion and increased generation resulting from oxidative and carbonyl stress of uremia. AGE-modified beta(2)-microglobulin is the principal pathogenic component of dialysis-related amyloidosis in patients undergoing dialysis. Available dialytic modalities are not capable of normalizing AGE levels in patients with end-stage renal disease. A number of reports indicated that restoration of euglycemia with islet-cell transplantation normalized and prevented further glycosylation of proteins. Aminoguanidine (AGN), a nucleophilic compound, not only decreases the formation of AGEs but also inhibits their action. A number of studies have shown that treatment with AGN improves neuropathy and delays the onset of retinopathy and nephropathy. N-Phenacylthiazolium bromide is a prototype AGE cross-link breaker that reacts with and can cleave covalent AGE-derived protein cross-links. Thus, there is an exciting possibility that the complications of diabetes, uremia, and aging may be prevented with these novel agents.

Two-photon fluorescence microscopy of laurdan generalized polarization domains in model and natural membranes.

Two-photon excitation microscopy shows coexisting regions of different generalized polarization (GP) in phospholipid vesicles, in red blood cells, in a renal tubular cell line, and in purified renal brushborder and basolateral membranes labeled with the fluorescent probe laurdan. The GP function measures the relative water content of the membrane. In the present study we discuss images obtained with polarized laser excitation, which selects different molecular orientations of the lipid bilayer corresponding to different spatial regions. The GP distribution in the gel-phase vesicles is relatively narrow, whereas the GP distribution in the liquid-crystalline phase vesicles (DOPC and DLPC) is broad. Analysis of images obtained with polarized excitation of the liquid-crystalline phase vesicles leads to the conclusion that coexisting regions of different GP must have dimensions smaller than the microscope resolution (approximately 200 nm radially and 600 nm axially). Vesicles of an equimolar mixture of DOPC and DPPC show coexisting rigid and fluid domains (high GP and low GP), but the rigid domains, which are preferentially excited by polarized light, have GP values lower than the pure gel-phase domains. Cholesterol strongly modifies the domain morphology. In the presence of 30 mol% cholesterol, the broad GP distribution of the DOPC/DPPC equimolar sample becomes narrower. The sample is still very heterogeneous, as demonstrated by the separations of GP disjoined regions, which are the result of photoselection of regions of different lipid orientation. In intact red blood cells, microscopic regions of different GP can be resolved, whereas in the renal cells GP domains have dimensions smaller than the microscope resolution. Preparations of renal apical brush border membranes and basolateral membranes show well-resolved GP domains, which may result from a different local orientation, or the domains may reflect a real heterogeneity of these membranes.

Regulation of Renal Fatty Acid and Cholesterol Metabolism, Inflammation, and Fibrosis in Akita and OVE26 Mice With Type 1 Diabetes

In Akita and OVE26 mice, two genetic models of type 1 diabetes, diabetic nephropathy is characterized by mesangial expansion and loss of podocytes, resulting in glomerulosclerosis and proteinuria, and is associated with increased expression of profibrotic growth factors, proinflammatory cytokines, and increased oxidative stress. We have also found significant increases in renal triglyceride and cholesterol content. The increase in renal triglyceride content is associated with 1) increased expression of sterol regulatory element–binding protein (SREBP)-1c and carbohydrate response element–binding protein (ChREBP), which collectively results in increased fatty acid synthesis, 2) decreased expression of peroxisome proliferator–activated receptor (PPAR)-α and -δ, which results in decreased fatty acid oxidation, and 3) decreased expression of farnesoid X receptor (FXR) and small heterodimer partner (SHP). The increase in cholesterol content is associated with 1) increased expression of SREBP-2 and 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase, which results in increased cholesterol synthesis, and 2) decreased expression of liver X receptor (LXR)-α, LXR-β, and ATP-binding cassette transporter-1, which results in decreased cholesterol efflux. Our results indicate that in type 1 diabetes, there is altered renal lipid metabolism favoring net accumulation of triglycerides and cholesterol, which are driven by increases in SREBP-1, ChREBP, and SREBP-2 and decreases in FXR, LXR-α, and LXR-β, which may also play a role in the increased expression of profibrotic growth hormones, proinflammatory cytokines, and oxidative stress.

Renal Control of Calcium, Phosphate, and Magnesium Homeostasis

Calcium, phosphate, and magnesium are multivalent cations that are important for many biologic and cellular functions. The kidneys play a central role in the homeostasis of these ions. Gastrointestinal absorption is balanced by renal excretion. When body stores of these ions decline significantly, gastrointestinal absorption, bone resorption, and renal tubular reabsorption increase to normalize their levels. Renal regulation of these ions occurs through glomerular filtration and tubular reabsorption and/or secretion and is therefore an important determinant of plasma ion concentration. Under physiologic conditions, the whole body balance of calcium, phosphate, and magnesium is maintained by fine adjustments of urinary excretion to equal the net intake. This review discusses how calcium, phosphate, and magnesium are handled by the kidneys.

Altered renal tubular expression of the complement inhibitor Crry permits complement activation after ischemia/reperfusion

Ischemia/reperfusion (I/R) of several organs results in complement activation, but the kidney is unique in that activation after I/R occurs only via the alternative pathway. We hypothesized that selective activation of this pathway after renal I/R could occur either because of a loss of complement inhibition or from increased local synthesis of complement factors. We examined the relationship between renal complement activation after I/R and the levels and localization of intrinsic membrane complement inhibitors. We found that loss of polarity of complement receptor 1-related protein y (Crry) in the tubular epithelium preceded activation of the alternative pathway along the basolateral aspect of the tubular cells. Heterozygous gene-targeted mice that expressed lower amounts of Crry were more sensitive to ischemic injury. Furthermore, inhibition of Crry expressed by proximal tubular epithelial cells in vitro resulted in alternative pathway-mediated injury to the cells. Thus, altered expression of a complement inhibitor within the tubular epithelium appears to be a critical factor permitting activation of the alternative pathway of complement after I/R. Increased C3 mRNA and decreased factor H mRNA were also detected in the outer medulla after I/R, suggesting that altered synthesis of these factors might further contribute to complement activation in this location.

Role of microtubules in the rapid regulation of renal phosphate transport in response to acute alterations in dietary phosphate content.

Renal proximal tubular response to acute administration of a low Pi diet is characterized by a rapid adaptive increase in apical brush border membrane (BBM) Na-Pi cotransport activity and Na-Pi cotransporter protein abundance, independent of a change in Na-Pi cotransporter mRNA levels (Levi, M., M. Lötscher, V. Sorribas, M. Custer, M. Arar, B. Kaissling, H. Murer, and J. Biber. 1994. Am. J. Physiol. 267: F900-F908). The purposes of the present study were to determine if the acute adaptive response occurs independent of de novo protein synthesis, and if microtubules play a role in the rapid upregulation of the Na-Pi cotransporters at the apical BBM. We found that inhibition of transcription by actinomycin D and translation by cycloheximide did not prevent the rapid adaptive response. In addition, in spite of a 3.3-fold increase in apical BBM Na-Pi cotransporter protein abundance, there was no change in cortical homogenate Na-Pi cotransporter protein abundance. Pretreatment with colchicine, which resulted in almost complete disruption of the microtubular network, abolished the adaptive increases in BBM Na-Pi cotransport activity and Na-Pi cotransporter protein abundance. In contrast, colchicine had no effect on the rapid downregulation of Na-Pi cotransport in response to acute administration of a high Pi diet. We conclude that the rapid adaptive increase in renal proximal tubular apical BBM Na-Pi cotransport activity and Na-Pi cotransporter abundance is independent of de novo protein synthesis, and is mediated by microtubule-dependent translocation of presynthesized Na-Pi cotransporter protein to the apical BBM.

The Na+-Pi cotransporter PiT-2 (SLC20A2) is expressed in the apical membrane of rat renal proximal tubules and regulated by dietary Pi

The principal mediators of renal phosphate (P(i)) reabsorption are the SLC34 family proteins NaPi-IIa and NaPi-IIc, localized to the proximal tubule (PT) apical membrane. Their abundance is regulated by circulatory factors and dietary P(i). Although their physiological importance has been confirmed in knockout animal studies, significant P(i) reabsorptive capacity remains, which suggests the involvement of other secondary-active P(i) transporters along the nephron. Here we show that a member of the SLC20 gene family (PiT-2) is localized to the brush-border membrane (BBM) of the PT epithelia and that its abundance, confirmed by Western blot and immunohistochemistry of rat kidney slices, is regulated by dietary P(i). In rats treated chronically on a high-P(i) (1.2%) diet, there was a marked decrease in the apparent abundance of PiT-2 protein in kidney slices compared with those from rats kept on a chronic low-P(i) (0.1%) diet. In Western blots of BBM from rats that were switched from a chronic low- to high-P(i) diet, NaPi-IIa showed rapid downregulation after 2 h; PiT-2 was also significantly downregulated at 24 h and NaPi-IIc after 48 h. For the converse dietary regime, NaPi-IIa showed adaptation within 8 h, whereas PiT-2 and NaPi-IIc showed a slower adaptive trend. Our findings suggest that PiT-2, until now considered as a ubiquitously expressed P(i) housekeeping transporter, is a novel mediator of P(i) reabsorption in the PT under conditions of acute P(i) deprivation, but with a different adaptive time course from NaPi-IIa and NaPi-IIc.