Vascular endothelial growth factor-C, a potential paracrine regulator of glomerular permeability, increases glomerular endothelial cell monolayer integrity and intracellular calcium

Physiological Replication of the Human Glomerulus Using a Triple Culture Microphysiological System

The function of the glomerulus depends on the complex cell-cell/matrix interactions and replication of this in vitro would aid biological understanding in both health and disease. Previous models do not fully reflect all cell types and interactions present as they overlook mesangial cells within their 3D matrix. Herein, the development of a microphysiological system that contains all resident renal cell types in an anatomically relevant manner is presented. A detailed transcriptomic analysis of the contributing biology of each cell type, as well as functionally appropriate albumin retention in the system, is demonstrated. The important role of mesangial cells is shown in promoting the health and maturity of the other cell types. Additionally, a comparison of the incremental advances that each individual cell type brings to the phenotype of the others demonstrates that glomerular cells in simple 2D culture exhibit a state more reflective of the dysfunction observed in human disease than previously recognized. This in vitro model will expand the capability to investigate glomerular biology in a more translatable manner by the inclusion of the important mesangial cell compartment.

Podocyte protease activated receptor 1 stimulation in mice produces focal segmental glomerulosclerosis mirroring human disease signaling events

About 30% of patients who have a kidney transplant with underlying nephrotic syndrome (NS) experience rapid relapse of disease in their new graft. This is speculated to be due to a host-derived circulating factor acting on podocytes, the target cells in the kidney, leading to focal segmental glomerulosclerosis (FSGS). Our previous work suggests that podocyte membrane protease receptor 1 (PAR-1) is activated by a circulating factor in relapsing FSGS. Here, the role of PAR-1 was studied in human podocytes in vitro, and using a mouse model with developmental or inducible expression of podocyte-specific constitutively active PAR-1, and using biopsies from patients with nephrotic syndrome. In vitro podocyte PAR-1 activation caused a pro-migratory phenotype with phosphorylation of the kinase JNK, VASP protein and docking protein Paxillin. This signaling was mirrored in podocytes exposed to patient relapse-derived NS plasma and in patient disease biopsies. Both developmental and inducible activation of transgenic PAR-1 (NPHS2 Cre PAR-1Active+/-) caused early severe nephrotic syndrome, FSGS, kidney failure and, in the developmental model, premature death. We found that the non-selective cation channel protein TRPC6 could be a key modulator of PAR-1 signaling and TRPC6 knockout in our mouse model significantly improved proteinuria and extended lifespan. Thus, our work implicates podocyte PAR-1 activation as a key initiator of human NS circulating factor and that the PAR-1 signaling effects were partly modulated through TRPC6.

Formation of the glomerular microvasculature is regulated by VEGFR-3

Microvascular dysfunction is a key driver of kidney disease. Pathophysiological changes in the kidney vasculature are regulated by vascular endothelial growth factor receptors (VEGFRs), supporting them as potential therapeutic targets. The tyrosine kinase receptor VEGFR-3, encoded by FLT4 and activated by the ligands VEGF-C and VEGF-D, is best known for its role in lymphangiogenesis. Therapeutically targeting VEGFR-3 to modulate lymphangiogenesis has been proposed as a strategy to treat kidney disease. However, outside the lymphatics, VEGFR-3 is also expressed in blood vascular endothelial cells in several tissues including the kidney. Here, we show that Vegfr-3 is expressed in fenestrated microvascular beds within the developing and adult mouse kidney, which include the glomerular capillary loops. We found that expression levels of VEGFR-3 are dynamic during glomerular capillary loop development, with the highest expression observed during endothelial cell migration into the S-shaped glomerular body. We developed a conditional knockout mouse model for Vegfr-3 and found that loss of Vegfr-3 resulted in a striking glomerular phenotype characterized by aneurysmal dilation of capillary loops, absence of mesangial structure, abnormal interendothelial cell junctions, and poor attachment between glomerular endothelial cells and the basement membrane. In addition, we demonstrated that expression of the VEGFR-3 ligand VEGF-C by podocytes and mesangial cells is dispensable for glomerular development. Instead, VEGFR-3 in glomerular endothelial cells attenuates VEGFR-2 phosphorylation. Together, the results of our study support a VEGF-C-independent functional role for VEGFR-3 in the kidney microvasculature outside of lymphatic vessels, which has implications for clinical therapies that target this receptor.NEW & NOTEWORTHY Targeting VEGFR-3 in kidney lymphatics has been proposed as a method to treat kidney disease. However, expression of VEGFR-3 is not lymphatic-specific. We demonstrated developmental expression of VEGFR-3 in glomerular endothelial cells, with loss of Vegfr-3 leading to malformation of glomerular capillary loops. Furthermore, we showed that VEGFR-3 attenuates VEGFR-2 activity in glomerular endothelial cells independent of paracrine VEGF-C signaling. Together, these data provide valuable information for therapeutic development targeting these pathways.

The lymphatics in kidney health and disease

The mammalian vascular system consists of two networks: the blood vascular system and the lymphatic vascular system. Throughout the body, the lymphatic system contributes to homeostatic mechanisms by draining extravasated interstitial fluid and facilitating the trafficking and activation of immune cells. In the kidney, lymphatic vessels exist mainly in the kidney cortex. In the medulla, the ascending vasa recta represent a hybrid lymphatic-like vessel that performs lymphatic-like roles in interstitial fluid reabsorption. Although the lymphatic network is mainly derived from the venous system, evidence supports the existence of lymphatic beds that are of non-venous origin. Following their development and maturation, lymphatic vessel density remains relatively stable; however, these vessels undergo dynamic functional changes to meet tissue demands. Additionally, new lymphatic growth, or lymphangiogenesis, can be induced by pathological conditions such as tissue injury, interstitial fluid overload, hyperglycaemia and inflammation. Lymphangiogenesis is also associated with conditions such as polycystic kidney disease, hypertension, ultrafiltration failure and transplant rejection. Although lymphangiogenesis has protective functions in clearing accumulated fluid and immune cells, the kidney lymphatics may also propagate an inflammatory feedback loop, exacerbating inflammation and fibrosis. Greater understanding of lymphatic biology, including the developmental origin and function of the lymphatics and their response to pathogenic stimuli, may aid the development of new therapeutic agents that target the lymphatic system.

Modeling the Glomerular Filtration Barrier and Intercellular Crosstalk

The glomerulus is a compact cluster of capillaries responsible for blood filtration and initiating urine production in the renal nephrons. A trilaminar structure in the capillary wall forms the glomerular filtration barrier (GFB), composed of glycocalyx-enriched and fenestrated endothelial cells adhering to the glomerular basement membrane and specialized visceral epithelial cells, podocytes, forming the outermost layer with a molecular slit diaphragm between their interdigitating foot processes. The unique dynamic and selective nature of blood filtration to produce urine requires the functionality of each of the GFB components, and hence, mimicking the glomerular filter in vitro has been challenging, though critical for various research applications and drug screening. Research efforts in the past few years have transformed our understanding of the structure and multifaceted roles of the cells and their intricate crosstalk in development and disease pathogenesis. In this review, we present a new wave of technologies that include glomerulus-on-a-chip, three-dimensional microfluidic models, and organoids all promising to improve our understanding of glomerular biology and to enable the development of GFB-targeted therapies. Here, we also outline the challenges and the opportunities of these emerging biomimetic systems that aim to recapitulate the complex glomerular filter, and the evolving perspectives on the sophisticated repertoire of cellular signaling that comprise the glomerular milieu.

Endothelial glycocalyx restoration by growth factors in diabetic nephropathy

The endothelial glycocalyx (eGlx) constitutes the first barrier to protein in all blood vessels. This is particularly noteworthy in the renal glomerulus, an ultrafiltration barrier. Leakage of protein, such as albumin, across glomerular capillaries results in albumin in the urine (albuminuria). This is a hall mark of kidney disease and can reflect loss of blood vessel integrity in microvascular beds elsewhere. We discuss evidence demonstrating that targeted damage to the glomerular eGlx results in increased glomerular albumin permeability. EGlx is lost in diabetes and experimental models demonstrate loss from glomerular endothelial cells. Vascular endothelial growth factor (VEGF)A is upregulated in early diabetes, which is associated with albuminuria. Treatment with paracrine growth factors such as VEGFC, VEGF165b and angiopoietin-1 can modify VEGFA signalling, rescue albumin permeability and restore glomerular eGlx in models of diabetes. Manipulation of VEGF receptor 2 signalling, or a common eGlx biosynthesis pathway by these growth factors, may protect and restore the eGlx layer. This would help to direct future therapeutics in diabetic nephropathy.

Targeting angiogenesis and lymphangiogenesis in kidney disease

The kidney is permeated by a highly complex vascular system with glomerular and peritubular capillary networks that are essential for maintaining the normal functions of glomerular and tubular epithelial cells. The integrity of the renal vascular network depends on a balance of proangiogenic and antiangiogenic factors, and disruption of this balance has been identified in various kidney diseases. Decreased levels of the predominant proangiogenic factor, vascular endothelial growth factor A (VEGFA), can result in glomerular microangiopathy and contribute to the onset of preeclampsia, whereas upregulation of VEGFA has roles in diabetic kidney disease (DKD) and polycystic kidney disease (PKD). Other factors that regulate angiogenesis, such as angiopoietin 1 and vasohibin 1, have been shown to be protective in animal models of DKD and renal fibrosis. The renal lymphatic system is important for fluid homeostasis in the kidney, as well as the transport of immune cells and antigens. Experimental studies suggest that the lymphangiogenic factor VEGFC might have protective effects in PKD, DKD and renal fibrosis. Understanding the physiological and pathological roles of factors that regulate angiogenesis and lymphangiogenesis in the kidney has led to the development of novel therapeutic strategies for kidney diseases.

VEGFC Reduces Glomerular Albumin Permeability and Protects Against Alterations in VEGF Receptor Expression in Diabetic Nephropathy

Elevated levels of vascular endothelial growth factor (VEGF) A are thought to cause glomerular endothelial cell (GEnC) dysfunction and albuminuria in diabetic nephropathy. We hypothesized that VEGFC could counteract these effects of VEGFA to protect the glomerular filtration barrier and reduce albuminuria. Isolated glomeruli were stimulated ex vivo with VEGFC, which reduced VEGFA- and type 2 diabetes-induced glomerular albumin solute permeability (Ps'alb). VEGFC had no detrimental effect on glomerular function in vivo when overexpression was induced locally in podocytes (podVEGFC) in otherwise healthy mice. Further, these mice had reduced glomerular VEGFA mRNA expression, yet increased glomerular VEGF receptor heterodimerization, indicating differential signaling by VEGFC. In a model of type 1 diabetes, the induction of podVEGFC overexpression reduced the development of hypertrophy, albuminuria, loss of GEnC fenestrations and protected against altered VEGF receptor expression. In addition, VEGFC protected against raised Ps'alb by endothelial glycocalyx disruption in glomeruli. In summary, VEGFC reduced the development of diabetic nephropathy, prevented VEGF receptor alterations in the diabetic glomerulus, and promoted both glomerular protection and endothelial barrier function. These important findings highlight a novel pathway for future investigation in the treatment of diabetic nephropathy.

Physiological Role of Vascular Endothelial Growth Factors as Homeostatic Regulators

The vascular endothelial growth factor (VEGF) family of proteins are key regulators of physiological systems. Originally linked with endothelial function, they have since become understood to be principal regulators of multiple tissues, both through their actions on vascular cells, but also through direct actions on other tissue types, including epithelial cells, neurons, and the immune system. The complexity of the five members of the gene family in terms of their different splice isoforms, differential translation, and specific localizations have enabled tissues to use these potent signaling molecules to control how they function to maintain their environment. This homeostatic function of VEGFs has been less intensely studied than their involvement in disease processes, development, and reproduction, but they still play a substantial and significant role in healthy control of blood volume and pressure, interstitial volume and drainage, renal and lung function, immunity, and signal processing in the peripheral and central nervous system. The widespread expression of VEGFs in healthy adult tissues, and the disturbances seen when VEGF signaling is inhibited support this view of the proteins as endogenous regulators of normal physiological function. This review summarizes the evidence and recent breakthroughs in understanding of the physiology that is regulated by VEGF, with emphasis on the role they play in maintaining homeostasis. © 2017 American Physiological Society. Compr Physiol 8:955-979, 2018.

Glomerular endothelial cell maturation depends on ADAM10, a key regulator of Notch signaling

The principal function of glomeruli is to filter blood through a highly specialized filtration barrier consisting of a fenestrated endothelium, the glomerular basement membrane and podocyte foot processes. Previous studies have uncovered a crucial role of endothelial a disintegrin and metalloprotease 10 (ADAM10) and Notch signaling in the development of glomeruli, yet the resulting defects have not been further characterized nor understood in the context of kidney development. Here, we used several different experimental approaches to analyze the kidneys and glomeruli from mice lacking ADAM10 in endothelial cells (A10ΔEC mice). Scanning electron microscopy of glomerular casts demonstrated enlarged vascular diameter and increased intussusceptive events in A10ΔEC glomeruli compared to controls. Consistent with these findings, genes known to regulate vessel caliber (Apln, AplnR and Vegfr3) are significantly upregulated in A10ΔEC glomeruli. Moreover, transmission electron microscopy revealed the persistence of diaphragms in the fenestrae of A10ΔEC glomerular endothelial cells, which was corroborated by the elevated expression of the protein PLVAP/PV-1, an integral component of fenestral diaphragms. Analysis of gross renal vasculature by light sheet microscopy showed no major alteration of the branching pattern, indicating a localized importance of ADAM10 in the glomerular endothelium. Since intussusceptions and fenestrae with diaphragms are normally found in developing, but not mature glomeruli, our results provide the first evidence for a crucial role of endothelial ADAM10, a key regulator of Notch signaling, in promoting the development and maturation of the glomerular vasculature.

Vascular Growth Factors and Glomerular Disease

The glomerulus is a highly specialized microvascular bed that filters blood to form primary urinary filtrate. It contains four cell types: fenestrated endothelial cells, specialized vascular support cells termed podocytes, perivascular mesangial cells, and parietal epithelial cells. Glomerular cell-cell communication is critical for the development and maintenance of the glomerular filtration barrier. VEGF, ANGPT, EGF, SEMA3A, TGF-β, and CXCL12 signal in paracrine fashions between the podocytes, endothelium, and mesangium associated with the glomerular capillary bed to maintain filtration barrier function. In this review, we summarize the current understanding of these signaling pathways in the development and maintenance of the glomerulus and the progression of disease.

The endoderm indirectly influences morphogenetic movements of the zebrafish head kidney through the posterior cardinal vein and VegfC

Integration of blood vessels and organ primordia determines organ shape and function. The head kidney in the zebrafish interacts with the dorsal aorta (DA) and the posterior cardinal vein (PCV) to achieve glomerular filtration and definitive hematopoiesis, respectively. How the head kidney co-develops with both the axial artery and vein remains unclear. We found that in endodermless sox32-deficient embryos, the head kidney associated with the PCV but not the DA. Disrupted convergent migration of the PCV and the head kidney in sox32-deficient embryos was rescued in a highly coordinated fashion through the restoration of endodermal cells. Moreover, grafted endodermal cells abutted the host PCV endothelium in the transplantation assay. Interestingly, the severely-disrupted head kidney convergence in the sox32-deficient embryo was suppressed by both the cloche mutation and the knockdown of endothelial genes, indicating that an interaction between the endoderm and the PCV restricts the migration of the head kidney. Furthermore, knockdown of either vegfC or its receptor vegfr3 suppressed the head kidney convergence defect in endodermless embryos and perturbed the head kidney-PCV association in wild-type embryos. Our findings thus underscore a role for PCV and VegfC in patterning the head kidney prior to organ assembly and function.

Vascular growth factors play critical roles in kidney glomeruli

Kidney glomeruli ultrafilter blood to generate urine and they are dysfunctional in a variety of kidney diseases. There are two key vascular growth factor families implicated in glomerular biology and function, namely the vascular endothelial growth factors (VEGFs) and the angiopoietins (Angpt). We present examples showing not only how these molecules help generate and maintain healthy glomeruli but also how they drive disease when their expression is dysregulated. Finally, we review how manipulating VEGF and Angpt signalling may be used to treat glomerular disease.

Role of connexin 43 in vascular hyperpermeability and relationship to Rock1-MLC20 pathway in septic rats

Connexin (Cx)43 has been shown to participate in several cardiovascular diseases. Increased vascular permeability is a common and severe complication in sepsis or septic shock. Whether or not Cx43 takes part in the regulation of vascular permeability in severe sepsis is not known, and the underlying mechanism has not been described. With cecal ligation and puncture-induced sepsis in rats and lipopolysaccharide (LPS)-treated vascular endothelial cells (VECs) from pulmonary veins, the role of Cx43 in increased vascular permeability and its relationship to the RhoA/Rock1 pathway were studied. It was shown that vascular permeability in the lungs, kidneys, and mesentery in sepsis rats and LPS-stimulated monolayer pulmonary vein VECs was significantly increased and positively correlated with the increased expression of Cx43 and Rock1 in these organs and cultured pulmonary vein VECs. The connexin inhibitor carbenoxolone (10 mg/kg iv) and the Rock1 inhibitor Y-27632 (2 mg/kg iv) alleviated the vascular leakage of lung, mesentery, and kidney in sepsis rats. Overexpressed Cx43 increased the phosphorylation of 20-kDa myosin light chain (MLC20) and the expression of Rock1 and increased the vascular permeability and decreased the transendothelial electrical resistance of pulmonary vein VECs. Cx43 RNA interference decreased the phosphorylation of MLC20 and the expression of Rock1 and decreased LPS-stimulated hyperpermeability of cultured pulmonary vein VECs. The Rock1 inhibitor Y-27632 alleviated LPS- and overexpressed Cx43-induced hyperpermeability of monolayer pulmonary vein VECs. This report shows that Cx43 participates in the regulation of vascular permeability in sepsis and that the mechanism is related to the Rock1-MLC20 phosphorylation pathway.

The effect of ASK1 on vascular permeability and edema formation in cerebral ischemia

Apoptosis signal-regulating kinase-1 (ASK1) is the mitogen-activated protein kinase kinase kinase (MAPKKK) and participates in the various central nervous system (CNS) signaling pathways. In cerebral ischemia, vascular permeability in the brain is an important issue because regulation failure of it results in edema formation and blood-brain barrier (BBB) disruption. To determine the role of ASK1 on vascular permeability and edema formation following cerebral ischemia, we first investigated ASK1-related gene expression using microarray analyses of ischemic brain tissue. We then measured protein levels of ASK1 and vascular endothelial growth factor (VEGF) in brain endothelial cells after hypoxia injury. We also examined protein expression of ASK1 and VEGF, edema formation, and morphological alteration through cresyl violet staining in ischemic brain tissue using ASK1-small interference RNA (ASK1-siRNA). Finally, immunohistochemistry was performed to examine VEGF and aquaporin-1 (AQP-1) expression in ischemic brain injury. Based on our findings, we propose that ASK1 is a regulating factor of vascular permeability and edema formation in cerebral ischemia.

Podocytes regulate neutrophil recruitment by glomerular endothelial cells via IL-6-mediated crosstalk

Stromal cells actively modulate the inflammatory process, in part by influencing the ability of neighboring endothelial cells to support the recruitment of circulating leukocytes. We hypothesized that podocytes influence the ability of glomerular endothelial cells (GEnCs) to recruit neutrophils during inflammation. To address this, human podocytes and human GEnCs were cultured on opposite sides of porous inserts and then treated with or without increasing concentrations of TNF-α prior to addition of neutrophils. The presence of podocytes significantly reduced neutrophil recruitment to GEnCs by up to 50% when cultures were treated with high-dose TNF-α (100 U/ml), when compared with GEnC monocultures. Importantly, this phenomenon was dependent on paracrine actions of soluble IL-6, predominantly released by podocytes. A similar response was absent when HUVECs were cocultured with podocytes, indicating a tissue-specific phenomenon. Suppressor of cytokine signaling 3 elicited the immunosuppressive actions of IL-6 in a process that disrupted the presentation of chemokines on GEnCs by altering the expression of the duffy Ag receptor for chemokines. Interestingly, suppressor of cytokine signaling 3 knockdown in GEnCs upregulated duffy Ag receptor for chemokines and CXCL5 expression, thereby restoring the neutrophil recruitment. In summary, these studies reveal that podocytes can negatively regulate neutrophil recruitment to inflamed GEnCs by modulating IL-6 signaling, identifying a potential novel anti-inflammatory role of IL-6 in renal glomeruli.

The role of the glomerular endothelium in albumin handling

The unique permeability characteristics of the glomerular capillary wall depend on its three-layer structure, consisting of endothelial cells, the basement membrane and podocytes. These components form the glomerular filtration barrier (GFB). That albuminuria may occur in the absence of changes in podocyte foot processes suggests that GFB components other than podocytes have essential roles in albumin handling. The endothelium forms the first part of the GFB and is characterized by fenestrations-transcellular holes that are filled with endothelial glycocalyx, a hydrated mesh principally comprised of proteoglycans. The glycocalyx and adsorbed plasma constituents form the endothelial surface layer (ESL). Human and animal studies have shown that the glomerular ESL restricts macromolecule passage and ensures that plasma albumin is largely excluded from the GFB. The glomerular endothelium is also likely to indirectly influence glomerular albumin handling by modifying podocyte behaviour. These modifications may occur physiologically through soluble mediators and/or pathologically through increased exposure of podocytes to plasma components as a consequence of ESL dysfunction. The importance of the glomerular endothelium and ESL in albumin handling also sheds light on the relationship between albuminuria and vascular disease. The therapeutic potential that this relationship offers will become evident with better understanding of the structure, composition and regulation of the glycocalyx.

Glycosaminoglycan regulation by VEGFA and VEGFC of the glomerular microvascular endothelial cell glycocalyx in vitro

Damage to endothelial glycocalyx impairs vascular barrier function and may contribute to progression of chronic vascular disease. An early indicator is microalbuminuria resulting from glomerular filtration barrier damage. We investigated the contributions of hyaluronic acid (HA) and chondroitin sulfate (CS) to glomerular microvascular endothelial cell (GEnC) glycocalyx and examined whether these are modified by vascular endothelial growth factors A and C (VEGFA and VEGFC). HA and CS were imaged on GEnCs and their resynthesis was examined. The effect of HA and CS on transendothelial electrical resistance (TEER) and labeled albumin flux across monolayers was assessed. Effects of VEGFA and VEGFC on production and charge characteristics of glycosaminoglycan (GAG) were examined via metabolic labeling and liquid chromatography. GAG shedding was quantified using Alcian Blue. NDST2 expression was examined using real-time PCR. GEnCs expressed HA and CS in the glycocalyx. CS contributed to the barrier to both ion (TEER) and protein flux across the monolayer; HA had only a limited effect. VEGFC promoted HA synthesis and increased the charge density of synthesized GAGs. In contrast, VEGFA induced shedding of charged GAGs. CS plays a role in restriction of macromolecular flux across GEnC monolayers, and VEGFA and VEGFC differentially regulate synthesis, charge, and shedding of GAGs in GEnCs. These observations have important implications for endothelial barrier regulation in glomerular and other microvascular beds.

Acute laminar shear stress reversibly increases human glomerular endothelial cell permeability via activation of endothelial nitric oxide synthase

Laminar shear stress is a key determinant of systemic vascular behavior, including through activation of endothelial nitric oxide synthase (eNOS), but little is known of its role in the glomerulus. We confirmed eNOS expression by glomerular endothelial cells (GEnC) in tissue sections and examined effects of acute exposure (up to 24 h) to physiologically relevant levels of laminar shear stress (10-20 dyn/cm(2)) in conditionally immortalized human GEnC. Laminar shear stress caused an orientation of GEnC and stress fibers parallel to the direction of flow and induced Akt and eNOS phosphorylation along with NO production. Inhibition of the phophatidylinositol (PI)3-kinase/Akt pathway attenuated laminar shear stress-induced eNOS phosphorylation and NO production. Laminar shear stress of 10 dyn/cm(2) had a dramatic effect on GEnC permeability, reversibly decreasing the electrical resistance across GEnC monolayers. Finally, the laminar shear stress-induced reduction in electrical resistance was attenuated by the NOS inhibitors l-N(G)-monomethyl arginine (l-NMMA) and l-N(G)-nitroarginine methyl ester (l-NAME) and also by inhibition of the PI3-kinase/Akt pathway. Hence we have shown for GEnC in vitro that acute permeability responses to laminar shear stress are dependent on NO, produced via activation of the PI3-kinase/Akt pathway and increased eNOS phosphorylation. These results suggest the importance of laminar shear stress and NO in regulating the contribution of GEnC to the permeability properties of the glomerular capillary wall.

An in vitro model of the glomerular capillary wall using electrospun collagen nanofibres in a bioartificial composite basement membrane

The filtering unit of the kidney, the glomerulus, contains capillaries whose walls function as a biological sieve, the glomerular filtration barrier. This comprises layers of two specialised cells, glomerular endothelial cells (GEnC) and podocytes, separated by a basement membrane. Glomerular filtration barrier function, and dysfunction in disease, remains incompletely understood, partly due to difficulties in studying the relevant cell types in vitro. We have addressed this by generation of unique conditionally immortalised human GEnC and podocytes. However, because the glomerular filtration barrier functions as a whole, it is necessary to develop three dimensional co-culture models to maximise the benefit of the availability of these cells. Here we have developed the first two tri-layer models of the glomerular capillary wall. The first is based on tissue culture inserts and provides evidence of cell-cell interaction via soluble mediators. In the second model the synthetic support of the tissue culture insert is replaced with a novel composite bioartificial membrane. This consists of a nanofibre membrane containing collagen I, electrospun directly onto a micro-photoelectroformed fine nickel supporting mesh. GEnC and podocytes grew in monolayers on either side of the insert support or the novel membrane to form a tri-layer model recapitulating the human glomerular capillary in vitro. These models will advance the study of both the physiology of normal glomerular filtration and of its disruption in glomerular disease.

Quantitating glomerular endothelial fenestration: an unbiased stereological approach

BACKGROUND/AIMS

Glomerular endothelial cells are fenestrated, allowing for especially high transcellular hydraulic conductivity. Current knowledge about endothelial fenestration structural changes in disease conditions is limited, partly due to the absence of robust methodologies to quantitate these structures. Herein, we propose a novel method for estimating the percentage of endothelial fenestration.

METHODS

An unbiased stereological method based on contiguity of two phases and surface area density estimation using isotropic uniform random line probes was developed. A line grid for intercept counting and classifying endothelial coverage of fenestrated versus non-fenestrated areas was designed. The method was applied to renal biopsies from 15 patients with Fabry disease and 9 normal living kidney donor controls.

RESULTS

The percentage of glomerular capillary endothelial coverage which was fenestrated was lower in Fabry patients (43 ± 12%) versus controls (53 ± 9%; p = 0.047). The fraction of endothelial surface which was fenestrated was greater on the peripheral versus mesangial zones of the capillary walls in both Fabry patients (p = 0.00002) and controls (p = 0.0005).

CONCLUSION

The proposed method provides an unbiased tool to quantitate endothelial fenestration changes in glomeruli. The practical example introduced showed reduced glomerular endothelial fenestration in Fabry nephropathy.

The relevance and role of vascular endothelial growth factor C, matrix metalloproteinase-2 and E-cadherin in epithelial ovarian cancer

We investigated the correlation of vascular endothelial growth factor C, matrix metalloproteinase-2, E-cadherin to explore mechanisms of vascular endothelial growth factor C in the metastasis of ovarian cancer and the relationship of prognosis. We applied immunohistochemistry to investigate the expression of vascular endothelial growth factor C, matrix metalloproteinase-2 and E-cadherin in ovarian tissues of 227 patients. We adopted Pearson chi-square test, Spearman correlation coefficient, univariate analysis, multivariate analysis, and Kaplan-Meier method. The positive rate of vascular endothelial growth factor C, matrix metalloproteinase-2 and E-cadherin in ovarian cancer was higher than that in borderline and benign tumor (P < 0.01). Vascular endothelial growth factor C was positively correlated with matrix metalloproteinase-2 (r = 0.665, P < 0.01) while negatively with E-cadherin(r = -0.185, P < 0.05). Univariate analysis showed clinical stage, pathologic grade, lymphatic metastasis, residual disease, chemotherapy, ascites, vascular endothelial growth factor C, and matrix metalloproteinase-2-influenced survival time (P < 0.05). In Cox multivariate analysis, all the aforementioned factors were found to be independent prognostic factors except pathologic grade. Vascular endothelial growth factor C was a new target to assess the prognosis of ovarian cancer. The expression of vascular endothelial growth factor C in ovarian cancer was related to matrix metalloproteinase-2 and E-cadherin.

High glucose causes dysfunction of the human glomerular endothelial glycocalyx

The endothelial glycocalyx is a gel-like layer which covers the luminal side of blood vessels. The glomerular endothelial cell (GEnC) glycocalyx is composed of proteoglycan core proteins, glycosaminoglycan (GAG) chains, and sialoglycoproteins and has been shown to contribute to the selective sieving action of the glomerular capillary wall. Damage to the systemic endothelial glycocalyx has recently been associated with the onset of albuminuria in diabetics. In this study, we analyze the effects of high glucose on the biochemical structure of the GEnC glycocalyx and quantify functional changes in its protein-restrictive action. We used conditionally immortalized human GEnC. Proteoglycans were analyzed by Western blotting and indirect immunofluorescence. Biosynthesis of GAG was analyzed by radiolabeling and quantified by anion exchange chromatography. FITC-albumin was used to analyze macromolecular passage across GEnC monolayers using an established in vitro model. We observed a marked reduction in the biosynthesis of GAG by the GEnC under high-glucose conditions. Further analysis confirmed specific reduction in heparan sulfate GAG. Expression of proteoglycan core proteins remained unchanged. There was also a significant increase in the passage of albumin across GEnC monolayers under high-glucose conditions without affecting interendothelial junctions. These results reproduce changes in GEnC barrier properties caused by enzymatic removal of heparan sulfate from the GEnC glycocalyx. They provide direct evidence of high glucose-induced alterations in the GEnC glycocalyx and demonstrate changes to its function as a protein-restrictive layer, thus implicating glycocalyx damage in the pathogenesis of proteinuria in diabetes.

Functional distinctions in cytosolic calcium regulation between cells of the glomerular filtration barrier

The importance of intracellular calcium ([Ca(2+)]i) regulation in the glomerular filtration barrier (GFB) has recently been highlighted by mutations in the cation channel TRPC6, resulting in a renal-specific phenotype. We examined the effects of FFA, a tool that can activate TRPC6, on [Ca(2+)]i in human conditionally immortalised glomerular endothelial cells (ciGEnC) and human podocytes (ciPod) that form the GFB. Changes in [Ca(2+)]i stimulated by FFA were measured in Fura 2-AM loaded cells. In GEnC, cell activation by FFA was dependent on external Ca(2+), yet in ciPod it was not. Depletion of internal Ca(2+) stores with thapsigargin did not affect cell activation by FFA in ciGEnC, but inhibited it in ciPod in a nephrin-dependent manner, demonstrated using nephrin deficient (ND) ciPod in conjunction with nephrin rescue experiments. FFA induced [Ca(2+)]i store release in ciPod, but not in ciGEnC or ND ciPod. In parallel, there were differences in the localisation of overexpressed TRPC6 between ciGEnC and ciPod. Furthermore, co-transfection of nephrin with TRPC6 in HEK293 cells reduced the FFA-induced increase in [Ca(2+)]i and nephrin clustering altered TRPC6 distribution. In conclusion, cell activation by FFA in podocytes stimulates the opening of a Ca(2+) channel, probably TRPC6, in a nephrin-dependent manner with a different activation profile to GEnC.

Characterization of novel VEGF (vascular endothelial growth factor)-C splicing isoforms from mouse

VEGF (vascular endothelial growth factor)-C is a major growth factor implicated in various physiological processes, such as angiogenesis and lymphangiogenesis. In the present paper, we report the identification of three short VEGF-C splicing isoforms (VEGF-C62, VEGF-C129 and VEGF-C184) from immortalized mouse kidney PTECs (proximal tubular epithelial cells). Semi-quantitative RT (reverse transcription)-PCR analysis showed these isoforms were universally expressed to varying degrees in different tissues with high expression levels in the kidney. In immortalized PTECs and podocytes, VEGF-C62 can activate phosphorylation of FAK (focal adhesion kinase) and promote cell adhesion to substratum. Cell survival was also increased by VEGF-C62 treatment in the absence of serum. VEGF-C62 can also reduce cell proliferation in PTECs and podocytes. Nucleolin was one of the proteins that associated with VEGF-C62 in pull-down assays using GST (glutathione transferase) fusion proteins as bait, indicating different protein binding requirements for VEGF-C62 compared with VEGF-C. In conclusion, these newly identified VEGF-C isoforms represent a new class of proteins, which are potentially involved in epithelial cell adhesion and proliferation through novel receptor pathways.

New aspects of glomerular filtration barrier structure and function: five layers (at least) not three

PURPOSE OF REVIEW

Three structures (glomerular endothelial fenestrae, glomerular basement membrane and podocyte interfoot process/slit diaphragms) have traditionally been considered as the major determinants of glomerular permeability. We review recent work demonstrating the functional importance of two additional layers: the endothelial surface layer (ESL) and the subpodocyte space (SPS).

RECENT FINDINGS

Removing glomerular endothelial cell monolayer ESL in vitro significantly alters monolayer permeability, supporting previous in-vivo demonstrations of the importance of the ESL in determining glomerular permeability. Whether fenestral diaphragms are present to support the ESL in healthy adult glomeruli has been examined in a recent report. On the downstream side of the glomerular filtration barrier, the SPS is a recently described structure that covers approximately two-thirds of the barrier, has highly restrictive dimensions and contributes to the hydraulic resistance and ultrafiltration characteristics of the glomerulus. Different layers of the barrier have also been shown to influence the permeability characteristics of one another, either through biophysical interactions, or through the activities of ligand-receptor axes that cross the various layers of the barrier.

SUMMARY

The structure and function of the glomerular filtration barrier remains an area of significant new discovery, and recent work continues to highlight the complexity of this dynamic multilayered watershed.

Glomerular endothelial cell fenestrations: an integral component of the glomerular filtration barrier

Glomerular endothelial cell (GEnC) fenestrations are analogous to podocyte filtration slits, but their important contribution to the glomerular filtration barrier has not received corresponding attention. GEnC fenestrations are transcytoplasmic holes, specialized for their unique role as a prerequisite for filtration across the glomerular capillary wall. Glomerular filtration rate is dependent on the fractional area of the fenestrations and, through the glycocalyx they contain, GEnC fenestrations are important in restriction of protein passage. Hence, dysregulation of GEnC fenestrations may be associated with both renal failure and proteinuria, and the pathophysiological importance of GEnC fenestrations is well characterized in conditions such as preeclampsia. Recent evidence suggests a wider significance in repair of glomerular injury and in common, yet serious, conditions, including diabetic nephropathy. Study of endothelial cell fenestrations is challenging because of limited availability of suitable in vitro models and by the requirement for electron microscopy to image these sub-100-nm structures. However, extensive evidence, from glomerular development in rodents to in vitro studies in human GEnC, points to vascular endothelial growth factor (VEGF) as a key inducer of fenestrations. In systemic endothelial fenestrations, the intracellular pathways through which VEGF acts to induce fenestrations include a key role for the fenestral diaphragm protein plasmalemmal vesicle-associated protein-1 (PV-1). The role of PV-1 in GEnC is less clear, not least because of controversy over existence of GEnC fenestral diaphragms. In this article, the structure-function relationships of GEnC fenestrations will be evaluated in depth, their role in health and disease explored, and the outlook for future study and therapeutic implications of these peculiar structures will be approached.