Preeclamptic sera induce nephrin shedding from podocytes through endothelin-1 release by endothelial glomerular cells

Human liver stem cell-derived microvesicles accelerate hepatic regeneration in hepatectomized rats

Several studies indicate that adult stem cells may improve the recovery from acute tissue injury. It has been suggested that they may contribute to tissue regeneration by the release of paracrine factors promoting proliferation of tissue resident cells. However, the factors involved remain unknown. In the present study we found that microvesicles (MVs) derived from human liver stem cells (HLSC) induced in vitro proliferation and apoptosis resistance of human and rat hepatocytes. These effects required internalization of MVs in the hepatocytes by an alpha(4)-integrin-dependent mechanism. However, MVs pre-treated with RNase, even if internalized, were unable to induce hepatocyte proliferation and apoptosis resistance, suggesting an RNA-dependent effect. Microarray analysis and quantitative RT-PCR demonstrated that MVs were shuttling a specific subset of cellular mRNA, such as mRNA associated in the control of transcription, translation, proliferation and apoptosis. When administered in vivo, MVs accelerated the morphological and functional recovery of liver in a model of 70% hepatectomy in rats. This effect was associated with increase in hepatocyte proliferation and was abolished by RNase pre-treatment of MVs. Using human AGO2, as a reporter gene present in MVs, we found the expression of human AGO2 mRNA and protein in the liver of hepatectomized rats treated with MVs. These data suggested a translation of the MV shuttled mRNA into hepatocytes of treated rats. In conclusion, these results suggest that MVs derived from HLSC may activate a proliferative program in remnant hepatocytes after hepatectomy by a horizontal transfer of specific mRNA subsets.

Endothelin receptor selectivity in chronic kidney disease: rationale and review of recent evidence

Endothelin (ET) is a potent vasoconstrictory peptide with proinflammatory and profibrotic properties that exerts its biological effects through two pharmacologically distinct receptor subtypes, namely ET(A) and ET(B). In addition to its substantial contribution to normal renal function, a large body of evidence suggests that derangement of the renal ET system is involved in the initiation and progression of chronic kidney disease (CKD) in diabetes, hypertension and glomerulonephritis. Thus, the use of ET receptor antagonists (ERAs) may offer potential novel treatment strategies in CKD. Recent literature on the role of the renal ET system in the healthy kidney was reviewed. In addition, an unbiased PubMed search was performed for studies published during the last 5 years that addressed the effects of ERAs in CKD. A particular objective was to extract information regarding whether selective or nonselective ERAs may have therapeutic potential in humans. ET-1 acts primarily as an autocrine or paracrine factor in the kidney. In normal physiology, ET-1 promotes diuresis and natriuresis by local production and action through ET(B) receptors in the renal medulla. In pathology, ET-1 mediates vasoconstriction, mesangial-cell proliferation, extracellular matrix production and inflammation, effects that are primarily conveyed by ET(A) receptors. Results obtained in animal models and in humans with the use of ERAs in CKD are encouraging; nevertheless, it is still under debate which receptor subtype should be targeted. According to most studies, selective inhibition of ET(A) receptors appears superior compared with nonselective ERAs because this approach does not interfere with the natriuretic, antihypertensive and ET clearance effects of ET(B) receptors. Although preliminary data in humans are promising, the potential role of ERAs in patients with CKD and the question of which receptor subtype should be targeted can only be clarified in randomized clinical trials.

Mesenchymal stem cell-derived microvesicles protect against acute tubular injury

Administration of mesenchymal stem cells (MSCs) improves the recovery from acute kidney injury (AKI). The mechanism may involve paracrine factors promoting proliferation of surviving intrinsic epithelial cells, but these factors remain unknown. In the current study, we found that microvesicles derived from human bone marrow MSCs stimulated proliferation in vitro and conferred resistance of tubular epithelial cells to apoptosis. The biologic action of microvesicles required their CD44- and beta1-integrin-dependent incorporation into tubular cells. In vivo, microvesicles accelerated the morphologic and functional recovery of glycerol-induced AKI in SCID mice by inducing proliferation of tubular cells. The effect of microvesicles on the recovery of AKI was similar to the effect of human MSCs. RNase abolished the aforementioned effects of microvesicles in vitro and in vivo, suggesting RNA-dependent biologic effects. Microarray analysis and quantitative real time PCR of microvesicle-RNA extracts indicate that microvesicles shuttle a specific subset of cellular mRNA, such as mRNAs associated with the mesenchymal phenotype and with control of transcription, proliferation, and immunoregulation. These results suggest that microvesicles derived from MSCs may activate a proliferative program in surviving tubular cells after injury via a horizontal transfer of mRNA.

Nephrin and endothelial injury

PURPOSE OF REVIEW

Nephrin, the main structural protein of the slit diaphragm, is expressed on the surface of glomerular podocytes and is critical in maintaining permselectivity and preventing proteinuria. This review focuses on the fate of nephrin in the context of endothelial injury and gives an update on the recent progress in understanding the pathomechanisms that lead to proteinuria.

RECENT FINDINGS

The following conditions of endothelial injury were found to induce loss of nephrin.(1) Preeclampsia, in which the associated proteinuria is induced by the soluble variant of vascular endothelial growth factor, which stimulates production of endothelin 1 (ET1) in endothelial cells. ET1 in turn triggers nephrin shedding from podocytes.(2) Hypertension, in which increased levels of angiotensin II induce podocyte apoptosis and reduce nephrin expression, leading to proteinuria in rats.(3) Diabetes and high fat diet, which lead to a significant increase in inflammatory molecules and cytokines, including MCP-1, which induces changes in podocyte cytoskeleton and nephrin loss.

SUMMARY

Recent results showed that damage to endothelial cells may alter endothelial-podocyte interaction and induces nephrin loss, a main cause of proteinuria.

Endothelin: 20 years from discovery to therapy

Since its identification as an endothelial cell-derived vasoconstrictor peptide in 1988, endothelin-1, the predominant member of the endothelin peptide family, has received considerable interest in basic medical science and in clinical medicine, which is reflected by more than 20 000 scientific publications on endothelin research in the past 20 years. The story of endothelin is unique as the gene sequences of endothelin receptors and the first receptor antagonists became available within only 4 years of the identification of the peptide sequence. The first clinical study in patients with congestive heart failure was published only 3 years thereafter. Yet, despite convincing experimental evidence of a pathogenetic role for endothelin in development, cell function, and disease, many initial clinical studies on endothelin antagonism were negative. In many of these studies, study designs or patient selection were inadequate. Today, for diseases such as pulmonary hypertension, endothelin antagonist treatment has become reality in clinical medicine, and ongoing clinical studies are evaluating additional indications, such as renal disease and cancer. Twenty years after the discovery of endothelin, its inhibitors have finally arrived in the clinical arena and are now providing us with new options to treat disease and prolong the lives of patients. Possible future indications include resistant arterial hypertension, proteinuric renal disease, cancer, and connective tissue diseases.

Proteinuria in mice expressing PKB/SGK-resistant GSK3

SGK1 is critically important for mineralocorticoid/salt-induced glomerular injury. SGK1 inactivates GSK3, which downregulates Snail, a DNA-binding molecule repressing the transcription of nephrin, a protein critically important for the integrity of the glomerular slit membrane. PKB/SGK-dependent GSK regulation is disrupted in mice carrying a mutation, in which the serine in the SGK/PKB-phosphorylation consensus sequence is replaced by alanine. The present study explored whether PKB/SGK-dependent GSK3 regulation influences glomerular proteinuria. Gene-targeted knockin mice with mutated and thus PKB/SGK-resistant GSK3alpha,beta (gsk3(KI)) were compared with their wild-type littermates (gsk3(WT)). gsk3(KI) and gsk3(WT) mice were implanted with DOCA release pellets and offered 1% saline as drinking water for 21 days. Under standard diet, tap water intake and absence of DOCA, urinary flow rate, glomerular filtration rate, and urinary albumin excretion were significantly larger and blood pressure was significantly higher in gsk3(KI) than in gsk3(WT) mice. Within 18 days, DOCA/salt treatment significantly increased fluid intake and urinary flow rate, urinary protein and albumin excretion, and blood pressure in both genotypes but the respective values were significantly higher in gsk3(KI) than in gsk3(WT) mice. Plasma albumin concentration was significantly lower in gsk3(KI) than in gsk3(WT) mice. Proteinuria was abrogated by lowering of blood pressure with alpha(1)-blocker prazosin (1 microg/g body wt) in 8-mo-old mice. According to immunofluorescence, nephrin at 3 and 8 mo and podocin expression at 3 mo were significantly lower in gsk3(KI) than in gsk3(WT) mice. After 18 days, DOCA/salt treatment renal glomerular sclerosis and tubulointerstitial damage were significantly more pronounced in gsk3(KI) than in gsk3(WT) mice. The observations reveal that disruption of PKB/SGK-dependent regulation of GSK3 leads to glomerular injury with proteinuria, which may at least partially be secondary to enhanced blood pressure.

Reversal of proteinuric renal disease and the emerging role of endothelin

Proteinuria is a major long-term clinical consequence of diabetes and hypertension, conditions that lead to progressive loss of functional renal tissue and, ultimately, end-stage renal disease. Proteinuria is also a strong predictor of cardiovascular events. Convincing preclinical and clinical evidence exists that proteinuria and the underlying glomerulosclerosis are reversible processes. This Review outlines the mechanisms involved in the development of glomerulosclerosis--particularly those responsible for podocyte injury--with an emphasis on the potential capacity of endothelin receptor blockade to reverse this process. There is strong evidence that endothelin-1, a peptide with growth-promoting and vasoconstricting properties, has a central role in the pathogenesis of proteinuria and glomerulosclerosis, which is mediated via activation of the ET(A) receptor. Several antiproteinuric drugs, including angiotensin-converting-enzyme inhibitors, angiotensin receptor antagonists, statins and certain calcium channel blockers, inhibit the formation of endothelin-1. Preclinical studies have demonstrated that endothelin receptor antagonists can reverse proteinuric renal disease and glomerulosclerosis, and preliminary studies in humans with renal disease have shown that these drugs have remarkable antiproteinuric effects that are additive to those of standard antiproteinuric therapy. Additional clinical studies are needed.