The endothelium as part of the integrative glomerular barrier complex
2013; Elsevier BV; Volume: 85; Issue: 1 Linguagem: Inglês
10.1038/ki.2013.317
ISSN1523-1755
Autores Tópico(s)Acute Kidney Injury Research
ResumoA new study by Xu et al. presents compelling evidence for an important role of the glomerular endothelium in acute kidney injury. They show that lipopolysaccharide reduces the endothelial surface layer, resulting in mild albuminuria, reduced glomerular filtration rate, and fewer endothelial fenestrae. Tumor necrosis factor-α (TNF-α) is identified as instrumental in these lipopolysaccharide effects through the TNF-α type 1 receptor. The study highlights that the glomerular endothelium has a key role in the maintenance of the glomerular filtration barrier. A new study by Xu et al. presents compelling evidence for an important role of the glomerular endothelium in acute kidney injury. They show that lipopolysaccharide reduces the endothelial surface layer, resulting in mild albuminuria, reduced glomerular filtration rate, and fewer endothelial fenestrae. Tumor necrosis factor-α (TNF-α) is identified as instrumental in these lipopolysaccharide effects through the TNF-α type 1 receptor. The study highlights that the glomerular endothelium has a key role in the maintenance of the glomerular filtration barrier. Most patients with kidney disease suffer from injury to their glomerular structures. Xu and co-workers1.Xu C. Chang A. Hack B.K. et al.TNF-mediated damage to glomerular endothelium is an important determinant of acute kidney injury in sepsis.Kidney Int. 2013; 85: 72-81Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar (this issue) show that damage to the glomerular endothelium is crucial for the development of acute kidney injury after injection of lipopolysaccharide (LPS) to mice. Before discussing their data, let us examine our current knowledge of the glomerular barrier. Figure 1 shows the four main elements of the glomerular barrier: the endothelial surface layer (ESL), the endothelium, the glomerular basement membrane (GBM), and the podocyte.2.Haraldsson B. Nyström J. Deen W.M. Properties of the glomerular barrier and mechanisms of proteinuria.Physiol Rev. 2008; 88: 451-487Crossref PubMed Scopus (608) Google Scholar Actually, there is a fifth component, the mesangial cell, which indirectly contributes to the filtration unit by providing the scaffold for the capillary loops, has contractile and phagocytic properties, and communicates with the endothelial cells and with the podocytes.3.Schlondorff D. Banas B. The mesangial cell revisited: no cell is an island.J Am Soc Nephrol. 2009; 20: 1179-1187Crossref PubMed Scopus (302) Google Scholar Indeed, as seen in Figure 1, all the glomerular cells are in close proximity and constantly exchange growth hormones and other signaling molecules. The glomerular filtration area is huge, so that even at the highest filtration rates, the flow velocities are low across the GBM. As a consequence, most larger molecules can easily diffuse back and forth, and against the filtration flow and across the gel-like GBM between glomerular cells.2.Haraldsson B. Nyström J. Deen W.M. Properties of the glomerular barrier and mechanisms of proteinuria.Physiol Rev. 2008; 88: 451-487Crossref PubMed Scopus (608) Google Scholar Theoretically, the first of these four selective filtration permeability barriers, the ESL, needs to be at least as selective as the last, the podocyte, in order to avoid concentration-polarization effects, which would result in a buildup of proteins within the layers, leading to 'clogging' of the filtration unit.2.Haraldsson B. Nyström J. Deen W.M. Properties of the glomerular barrier and mechanisms of proteinuria.Physiol Rev. 2008; 88: 451-487Crossref PubMed Scopus (608) Google Scholar The idea that the endothelium plays an important role in the glomerular barrier was proposed four decades ago.4.Ryan G.B. Karnovsky M.J. Distribution of endogenous albumin in the rat glomerulus: role of hemodynamic factors in glomerular barrier function.Kidney Int. 1976; 9: 36-45Abstract Full Text PDF PubMed Scopus (182) Google Scholar However, for the next two decades researchers considered the GBM to be the only important component of the glomerular membrane.5.Kanwar Y.S. Linker A. Farquhar M.G. Increased permeability of the glomerular basement membrane to ferritin after removal of glycosaminoglycans (heparan sulfate) by enzyme digestion.J Cell Biol. 1980; 86: 688-693Crossref PubMed Scopus (492) Google Scholar For the past decade, the field has been, and still is, dominated by the exploration of podocytes, and for good reasons. Since the discovery of nephrin as the culprit for the Finnish-type congenital nephrotic syndrome and as a major component of the slit diaphragm, most genetic forms of proteinuria and focal segmental glomerulosclerosis have been found to be due to mutations in structural proteins of the podocyte slit membrane or the attached podocyte cytoskeleton.6.McCarthy H.J. Bierzynska A. Wherlock M. et al.Simultaneous sequencing of 24 genes associated with steroid-resistant nephrotic syndrome.Clin J Am Soc Nephrol. 2013; 8: 637-648Crossref PubMed Scopus (143) Google Scholar Other hereditary diseases have been shown to be due to mutations of GBM proteins.7.Miner J.H. The glomerular basement membrane.Exp Cell Res. 2012; 318: 973-978Crossref PubMed Scopus (191) Google Scholar However, far less is known about mechanisms that contribute to proteinuria in acquired kidney disease, which is more common and unfortunately still lacks highly effective forms of therapy. With this as background, the glomerular endothelium and cross-talk between glomerular cells and their various communications are gaining new and growing interest. Recently the mild and transient form of proteinuria, which may occur during sepsis and with LPS administration, was mostly attributed to direct LPS effects on podocytes.8.Reiser J. von Gersdorff G. Loos M. et al.Induction of B7-1 in podocytes is associated with nephrotic syndrome.J Clin Invest. 2004; 113: 1390-1397Crossref PubMed Scopus (459) Google Scholar Xu et al. now report that LPS not only lowered the glomerular filtration rate (GFR) and caused albuminuria but also induced important alterations in the glomerular endothelium and its surface layer (ESL).1.Xu C. Chang A. Hack B.K. et al.TNF-mediated damage to glomerular endothelium is an important determinant of acute kidney injury in sepsis.Kidney Int. 2013; 85: 72-81Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar Thus, evaluation by electron microscopy revealed that LPS markedly reduced the density of endothelial fenestrations from 3.6 per micrometer of glomerular capillary in the wild-type control mice to 0.6 per micrometer at 24h after LPS. As many effects of LPS are mediated by tumor necrosis factor-α (TNF-α), they then examined the potential signaling through TNF-α using mice with genetic deletion of the type 1 receptor. LPS injection in TNF receptor 1 knockout (Tnfr1–/–) mice did not reduce GFR or induce proteinuria, and the number of endothelial fenestrae remained similar to that in control, uninjected mice. Regarding possible functional effects of the reduced number of fenestrae after LPS, the authors speculate that it may reflect a reduced hydraulic conductance and hence GFR. Although this is a plausible explanation, one must not forget that in sepsis GFR is reduced mainly because of hemodynamic changes including a low blood pressure. In wild-type mice, LPS injection not only reduced the number of glomerular endothelial fenestrae but also at the same time increased the diameter of the fenestrae from 64 to 195nm, while the 76-nm diameter of fenestrae in Tnfr1–/– mice injected with LPS remained close to that in uninjected controls. Despite the LPS-induced albuminuria, no changes could be detected with electron microscopy in the podocytes' morphology, slit diaphragm, number or size of foot processes, and so on. The mRNA and protein levels of vascular endothelial growth factor (VEGF) were found to be low upon LPS injection, albeit it was measured in total kidney tissue and not specifically in glomeruli. The expression of VEGF receptor 2 was not altered. LPS injection caused a threefold increase in the expression of heparanase in wild-type mice but not in Tnfr1–/– mice. Heparanase specifically removes heparan sulfate from heparan sulfate glycoproteins, which contribute to the glomerular filtration barrier.2.Haraldsson B. Nyström J. Deen W.M. Properties of the glomerular barrier and mechanisms of proteinuria.Physiol Rev. 2008; 88: 451-487Crossref PubMed Scopus (608) Google Scholar In addition, LPS was found to reduce the amount of sialic acid-containing proteoglycans in the ESL as analyzed by use of wheat germ agglutinin, a lectin that specifically binds sialic acid. All of these changes were diminished in Tnfr1–/– mice, and the effects were mimicked by infusions of TNF-α. Thus, the endothelium and its surface layer seem to play important roles in the development of the glomerular functional changes of LPS-induced acute kidney injury and involve predominantly the type 1 TNF-α receptor.1.Xu C. Chang A. Hack B.K. et al.TNF-mediated damage to glomerular endothelium is an important determinant of acute kidney injury in sepsis.Kidney Int. 2013; 85: 72-81Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar How then do the findings of the study by Xu et al.1.Xu C. Chang A. Hack B.K. et al.TNF-mediated damage to glomerular endothelium is an important determinant of acute kidney injury in sepsis.Kidney Int. 2013; 85: 72-81Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar fit into the context of previous work in the field? It has been known for two decades that in the endothelium the TNFR1 mediates most of the response to TNF-α. Moreover, the endothelial cells in most organs seem to be affected by TNF-α during sepsis, resulting in capillary leak.9.von Drygalski A. Furlan-Freguia C. Ruf W. et al.Organ-specific protection against lipopolysaccharide-induced vascular leak is dependent on the endothelial protein C receptor.Arterioscler Thromb Vasc Biol. 2013; 33: 769-776Crossref PubMed Scopus (32) Google Scholar However, lately the mild and transient proteinuria induced by LPS has been attributed predominantly to direct and LPS-mediated effects on podocytes and, as such, has been used as a 'podocyte-specific' tool to elicit albuminuria.8.Reiser J. von Gersdorff G. Loos M. et al.Induction of B7-1 in podocytes is associated with nephrotic syndrome.J Clin Invest. 2004; 113: 1390-1397Crossref PubMed Scopus (459) Google Scholar On the basis of the present data of Xu et al.,1.Xu C. Chang A. Hack B.K. et al.TNF-mediated damage to glomerular endothelium is an important determinant of acute kidney injury in sepsis.Kidney Int. 2013; 85: 72-81Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar this exclusively podocyte-based explanation for LPS-induced glomerular function has to be modified to also include changes in the glomerular endothelial cells (GECs) as contributing to the altered permselectivity that occurs in response to LPS or TNF-α in the glomerulus. Primary GEC studies have provided considerable knowledge about the properties of these cells.10.Ballermann B.J. Regulation of bovine glomerular endothelial cell growth in vitro.Am J Physiol. 1989; 256: C182-C189PubMed Google Scholar In terms of permeability of these fenestrated cells, the focus has been on their surface layer.2.Haraldsson B. Nyström J. Deen W.M. Properties of the glomerular barrier and mechanisms of proteinuria.Physiol Rev. 2008; 88: 451-487Crossref PubMed Scopus (608) Google Scholar Thus, Sörensson et al. studied the proteoglycan synthesis of GECs,11.Sörensson J. Björnson A. Ohlson M. et al.Synthesis of sulfated proteoglycans by bovine glomerular endothelial cells in culture.Am J Physiol Renal Physiol. 2003; 284: F373-F380Crossref PubMed Scopus (41) Google Scholar and Singh et al. could reproduce the results in immortalized human GECs.12.Singh A. Satchell S.C. Neal C.R. et al.Glomerular endothelial glycocalyx constitutes a barrier to protein permeability.J Am Soc Nephrol. 2007; 18: 2885-2893Crossref PubMed Scopus (207) Google Scholar However, these studies mostly describe the inner glycocalyx component of the ESL. To study the intact ESL, in vivo studies are required, and these have shown that enzymatic removal of the ESL induces albuminuria.13.Jeansson M. Haraldsson B. Morphological and functional evidence for an important role of the endothelial cell glycocalyx in the glomerular barrier.Am J Physiol Renal Physiol. 2006; 290: F111-F116Crossref PubMed Scopus (164) Google Scholar, 14.Meuwese M.C. Broekhuizen L.N. Kuikhoven M. et al.Endothelial surface layer degradation by chronic hyaluronidase infusion induces proteinuria in apolipoprotein E-deficient mice.PLoS One [online]. 2010; 5: e14262Crossref PubMed Scopus (41) Google Scholar, 15.Salmon A.H. Ferguson J.K. Burford J.L. et al.Loss of the endothelial glycocalyx links albuminuria and vascular dysfunction.J Am Soc Nephrol. 2012; 23: 1339-1350Crossref PubMed Scopus (173) Google Scholar, 16.Dane M.J. van den Berg B.M. Avramut M.C. et al.Glomerular endothelial surface layer acts as a barrier against albumin filtration.Am J Pathol. 2013; 182: 1532-1540Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar Recently, the composition of the delicate and elusive 'cell coat' layer of the ESL was revealed by an in vivo ion exchange approach and mass spectrometry.17.Friden V. Oveland E. Tenstad O. et al.The glomerular endothelial cell coat is essential for glomerular filtration.Kidney Int. 2011; 79: 1322-1330Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar It is now evident that the endothelium is also affected early in experimental kidney disease resulting from podocyte damage, such as the adriamycin-induced nephrosis in mice.18.Jeansson M. Björck K. Tenstad O. et al.Adriamycin alters glomerular endothelium to induce proteinuria.J Am Soc Nephrol. 2009; 20: 114-122Crossref PubMed Scopus (131) Google Scholar,19.Sun Y.B. Qu X. Zhang X. et al.Glomerular endothelial cell injury and damage precedes that of podocytes in adriamycin-induced nephropathy.PLoS One [online]. 2013; 8: e55027Crossref PubMed Scopus (82) Google Scholar The paper by Xu et al.1.Xu C. Chang A. Hack B.K. et al.TNF-mediated damage to glomerular endothelium is an important determinant of acute kidney injury in sepsis.Kidney Int. 2013; 85: 72-81Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar further provides support for the importance of the endothelium and its surface layer for the glomerular barrier. As fascinating as the expanding story of the glomerular endothelium may be, it may just be part of an even greater mystery—that of glomerular cell–cell communication. The exciting era started with the discovery by Susan Quaggin's group of VEGF production by the podocyte and its essential role in the adjacent endothelial cell's function and survival.20.Eremina V. Sood M. Haigh J. et al.Glomerular-specific alterations of VEGF-A expression lead to distinct congenital and acquired renal diseases.J Clin Invest. 2003; 111: 707-716Crossref PubMed Scopus (1076) Google Scholar These studies led to the identification of this cross-talk as playing an essential role in eclampsia21.Eremina V. Jefferson J.A. Kowalewska J. et al.VEGF inhibition and renal thrombotic microangiopathy.N Engl J Med. 2008; 358: 1129-1136Crossref PubMed Scopus (1159) Google Scholar and the further evolution of a deeper understanding of the soluble VEGF receptor22.Jin J. Sison K. Li C. et al.Soluble FLT1 binds lipid microdomains in podocytes to control cell morphology and glomerular barrier function.Cell. 2012; 151: 384-399Abstract Full Text Full Text PDF PubMed Scopus (117) Google Scholar and the mechanism for proteinuria as a serious side effect of anti-VEGF therapy for carcinomas.21.Eremina V. Jefferson J.A. Kowalewska J. et al.VEGF inhibition and renal thrombotic microangiopathy.N Engl J Med. 2008; 358: 1129-1136Crossref PubMed Scopus (1159) Google Scholar Furthermore, recent work indicates that this exciting VEGF story may represent only the tip of the iceberg of glomerular cell cross-talk and communication. For example, podocytes can affect the adjacent endothelium via several secreted molecules such as angiopoietins, angiopoietin-like protein 3 (Angptl3), Angptl4,23.Clement L.C. Avila-Casado C. Mace C. et al.Podocyte-secreted angiopoietin-like-4 mediates proteinuria in glucocorticoid-sensitive nephrotic syndrome.Nat Med. 2011; 17: 117-122Crossref PubMed Scopus (243) Google Scholar insulin-like growth factor-binding protein-related protein 1,24.Matsumoto T. Hess S. Kajiyama H. et al.Proteomic analysis identifies insulin-like growth factor-binding protein-related protein-1 as a podocyte product.Am J Physiol Renal Physiol. 2010; 299: F776-F784Crossref PubMed Scopus (14) Google Scholar and endothelin-1.25.Daehn I.S. Zhang T. Casalena G. et al.Role of endothelin-1 in podocyte-to-endothelial cross-talk in podocytopathies.Kidney Week 2011 (annual meeting), 8–13 November 2011. American Society of Nephrology, Philadelphia, PA2011: FR-OR297Google Scholar Mesangial cells can also affect podocytes by a number of signaling molecules, such as midkine, transforming growth factor-β, nitric oxide, cytokines, and chemokines.3.Schlondorff D. Banas B. The mesangial cell revisited: no cell is an island.J Am Soc Nephrol. 2009; 20: 1179-1187Crossref PubMed Scopus (302) Google Scholar Interestingly, the endothelial cells also produce a number of growth hormones, such as epidermal,26.Miyazawa T. Zeng F. Wang S. et al.Low nitric oxide bioavailability upregulates renal heparin binding EGF-like growth factor expression.Kidney Int. 2013; 84: 1176-1188Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar insulin-like, hepatocyte, and platelet-derived growth factors,27.Lindahl P. Hellström M. Kalén M. et al.Paracrine PDGF-B/PDGF-Rβ signaling controls mesangial cell development in kidney glomeruli.Development. 1998; 125: 3313-3322Crossref PubMed Google Scholar that may affect both mesangial cells and podocytes.28.Obeidat M. Obeidat M. Ballermann B.J. Glomerular endothelium: a porous sieve and formidable barrier.Exp Cell Res. 2012; 318: 964-972Crossref PubMed Scopus (60) Google Scholar Although the in vivo significance of the cross-talk of these various mediators remains to be fully established, it clearly points to the increasing recognition of intraglomerular cell–cell communication as maintaining the intricate glomerular balance of power required for the maintenance of the structural and functional integrity of this intricate filtration unit. Are we witnessing a paradigm shift in our understanding of the glomerular barrier? This may well be so. Consider the ingenious design of the glomerular filter, with cells on each side of the 'membrane' that communicate with each other to maintain an almost perfect size-selective filtration membrane during a century of human life. So in spite of the strong support for a podocentric view provided by the genetic forms of proteinuric renal diseases, support is also accumulating for contributing roles of the endothelial and mesangial cells in the maintenance of the filtration unit, especially in acquired proteinuric glomerular diseases such as diabetes.29.Weil E.J. Lemley K.V. Mason C.C. et al.Podocyte detachment and reduced glomerular capillary endothelial fenestration promote kidney disease in type 2 diabetic nephropathy.Kidney Int. 2012; 82: 1010-1017Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar It may be time to modify the concept of 'the single most important component' of the glomerular filter and to embrace a more all-encompassing concept of an integrative glomerular barrier complex. At present, we are just beginning to explore the details of the cross-talk between different cell types in the glomerulus. Fortunately, as is so often the case in science, novel tools may allow us to study the in vivo cell–cell communications within the glomerulus by use of different fluorescent probes expressed by different glomerular cells and their visualization in real time by multiphoton laser microscopy. I believe that a better understanding of the various glomerular cell–cell communications will not only enlighten us about the intricacies of glomerular function in health and disease, but will thereby also point toward new therapeutic avenues, which are so desperately needed for our patients with glomerular diseases. The Swedish Research Council (project no. 09898) and the Wenner-Gren Foundation supported this work.
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