The anionic matrix at the rat glomerular endothelial surface

The unique structural and functional characteristics of glomerular endothelial cell fenestrations and their potential as a therapeutic target in kidney disease

Glomerular endothelial cell (GEnC) fenestrations are a critical component of the glomerular filtration barrier. Their unique nondiaphragmed structure is key to their function in glomerular hydraulic permeability, and their aberration in disease can contribute to loss of glomerular filtration function. This review provides a comprehensive update of current understanding of the regulation and biogenesis of fenestrae. We consider diseases in which GEnC fenestration loss is recognized or may play a role and discuss methods with potential to facilitate the study of these critical structures. Literature is drawn from GEnCs as well as other fenestrated cell types such as liver sinusoidal endothelial cells that most closely parallel GEnCs.

Removal of the endothelial surface layer via hyaluronidase does not modulate monocyte and neutrophil interactions with the glomerular endothelium

OBJECTIVE

The endothelial surface layer (ESL), a layer of macromolecules on the surface of endothelial cells, can both impede and facilitate leukocyte recruitment. However, its role in monocyte and neutrophil recruitment in glomerular capillaries is unknown.

METHODS

We used multiphoton intravital microscopy to examine monocyte and neutrophil behavior in the glomerulus following ESL disruption with hyaluronidase.

RESULTS

Constitutive retention and migration of monocytes and neutrophils within the glomerular microvasculature was unaltered by hyaluronidase. Consistent with this, inhibition of the hyaluronan-binding molecule CD44 also failed to modulate glomerular trafficking of these immune cells. To investigate the contribution of the ESL during acute inflammation, we induced glomerulonephritis via in situ immune complex deposition. This resulted in increases in glomerular retention of monocytes and neutrophils but did not induce marked reduction in the glomerular ESL. Furthermore, hyaluronidase treatment did not modify the prolonged retention of monocytes and neutrophils in the acutely inflamed glomerular microvasculature.

CONCLUSIONS

These observations indicate that, despite evidence that the ESL has the capacity to inhibit leukocyte-endothelial cell interactions while also containing adhesive ligands for immune cells, neither of these functions modulate trafficking of monocytes and neutrophils in steady-state or acutely-inflamed glomeruli.

Early diabetic kidney disease: Focus on the glycocalyx

The incidence of diabetic kidney disease (DKD) is sharply increasing worldwide. Microalbuminuria is the primary clinical marker used to identify DKD, and its initiating step in diabetes is glomerular endothelial cell dysfunction, particularly glycocalyx impairment. The glycocalyx found on the surface of glomerular endothelial cells, is a dynamic hydrated layer structure composed of pro-teoglycans, glycoproteins, and some adsorbed soluble components. It reinforces the negative charge barrier, transduces the shear stress, and mediates the interaction of blood corpuscles and podocytes with endothelial cells. In the high-glucose environment of diabetes, excessive reactive oxygen species and proinflammatory cytokines can damage the endothelial glycocalyx (EG) both directly and indirectly, which induces the production of microalbuminuria. Further research is required to elucidate the role of the podocyte glycocalyx, which may, together with endothelial cells, form a line of defense against albumin filtration. Interestingly, recent research has confirmed that the negative charge barrier function of the glycocalyx found in the glomerular basement membrane and its repulsion effect on albumin is limited. Therefore, to improve the early diagnosis and treatment of DKD, the potential mechanisms of EG degradation must be analyzed and more responsive and controllable targets must be explored. The content of this review will provide insights for future research.

Increased Heparanase Levels in Urine during Acute Puumala Orthohantavirus Infection Are Associated with Disease Severity

Old–world orthohantaviruses cause hemorrhagic fever with renal syndrome (HFRS), characterized by acute kidney injury (AKI) with transient proteinuria. It seems plausible that proteinuria during acute HFRS is mediated by the disruption of the glomerular filtration barrier (GFB) due to vascular leakage, a hallmark of orthohantavirus–caused diseases. However, direct infection of endothelial cells by orthohantaviruses does not result in increased endothelial permeability, and alternative explanations for vascular leakage and diminished GFB function are necessary. Vascular integrity is partly dependent on an intact endothelial glycocalyx, which is susceptible to cleavage by heparanase (HPSE). To understand the role of glycocalyx degradation in HFRS–associated proteinuria, we investigated the levels of HPSE in urine and plasma during acute, convalescent and recovery stages of HFRS caused by Puumala orthohantavirus. HPSE levels in urine during acute HFRS were significantly increased and strongly associated with the severity of AKI and other markers of disease severity. Furthermore, increased expression of HPSE was detected in vitro in orthohantavirus–infected podocytes, which line the outer surfaces of glomerular capillaries. Taken together, these findings suggest the local activation of HPSE in the kidneys of orthohantavirus–infected patients with the potential to disrupt the endothelial glycocalyx, leading to increased protein leakage through the GFB, resulting in high amounts of proteinuria.

The Glomerular Endothelium Restricts Albumin Filtration

Inflammatory activation and/or dysfunction of the glomerular endothelium triggers proteinuria in many systemic and localized vascular disorders. Among them are the thrombotic microangiopathies, many forms of glomerulonephritis, and acute inflammatory episodes like sepsis and COVID-19 illness. Another example is the chronic endothelial dysfunction that develops in cardiovascular disease and in metabolic disorders like diabetes. While the glomerular endothelium is a porous sieve that filters prodigious amounts of water and small solutes, it also bars the bulk of albumin and large plasma proteins from passing into the glomerular filtrate. This endothelial barrier function is ascribed predominantly to the endothelial glycocalyx with its endothelial surface layer, that together form a relatively thick, mucinous coat composed of glycosaminoglycans, proteoglycans, glycolipids, sialomucins and other glycoproteins, as well as secreted and circulating proteins. The glycocalyx/endothelial surface layer not only covers the glomerular endothelium; it extends into the endothelial fenestrae. Some glycocalyx components span or are attached to the apical endothelial cell plasma membrane and form the formal glycocalyx. Other components, including small proteoglycans and circulating proteins like albumin and orosomucoid, form the endothelial surface layer and are bound to the glycocalyx due to weak intermolecular interactions. Indeed, bound plasma albumin is a major constituent of the endothelial surface layer and contributes to its barrier function. A role for glomerular endothelial cells in the barrier of the glomerular capillary wall to protein filtration has been demonstrated by many elegant studies. However, it can only be fully understood in the context of other components, including the glomerular basement membrane, the podocytes and reabsorption of proteins by tubule epithelial cells. Discovery of the precise mechanisms that lead to glycocalyx/endothelial surface layer disruption within glomerular capillaries will hopefully lead to pharmacological interventions that specifically target this important structure.

Novel Insights Into the Role of Glycans in the Pathophysiology of Glomerular Endotheliosis in Preeclampsia

The polysaccharide heparan sulfate is ubiquitously expressed as a proteoglycan in extracellular matrices and on cell surfaces. In the glomerular filtration barrier, the action of the heparan sulfate is directly related to the function of glomerular filtration, mostly attributed to the sulfated domains that occur along the polysaccharide chain, as evidenced by fact that release of fragments of heparan sulfate by heparanase significantly increases the permeability of albumin passage through the glomerular endothelium, event that originates proteinuria. This review aims to show the importance of the structural domains of heparan sulfate in the process of selective permeability and to demonstrate how these domains may be altered during the glomerular inflammation processes that occur in preeclampsia.

Regulation of blood vascular permeability in the skin

Regulation of blood vessel permeability is essential for the homeostasis of peripheral tissues. This regulation controls the trafficking of plasma contents, including water, vitamins, ions, hormones, cytokines, amyloids, lipoproteins, carrier proteins, and immunoglobulins. The properties of blood vessels vary among tissues based on their structural differences: continuous, fenestrated, or sinusoidal. These three types of blood vessels have different charge and size barrier properties. The anionic luminal glycocalyx layer on endothelial cells establishes the "charge barrier" that repels the attachment of negatively charged blood cells and plasma molecules. In contrast, the "size barrier" of blood vessels largely relies on the interendothelial junctions (IEJs) between endothelial cells, which define the paracellular permeability. As in most peripheral tissues, blood capillaries in the skin are composed of continuous and/or fenestrated blood vessels that have relatively tighter IEJs compared to those in the internal organs. Small vesicles in the capillary endothelium were discovered in the 1950s, and studies have since confirmed that blood endothelial cells transport the plasma contents by endocytosis and subsequent transcytosis and exocytosis-this process is called transcellular permeability. The permeability of blood vessels is highly variable as a result of intrinsic and extrinsic factors. It is significantly elevated upon tissue inflammations as a result of disabled IEJs and increased paracellular permeability due to inflammatory mediators. An increase in transcellular permeability during inflammation has also been postulated. Here, we provide an overview of the general properties of vascular permeability based on our recent observations of murine skin inflammation models, and we discuss its physiological significance in peripheral homeostasis.

Direct Observation of Enhanced Nitric Oxide in a Murine Model of Diabetic Nephropathy

Uncoupling of nitric oxide synthase (NOS) secondary to redox signaling is a central mechanism in endothelial and macrophage activation. To date studies on the production of nitric oxide (NO) during the development of diabetic complications show paradoxical results. We previously showed that recoupling eNOS by increasing the eNOS cofactor tetrahydrobiopterin (BH4) could restore endothelial function and prevent kidney injury in experimental kidney transplantation. Here, we employed a diabetic mouse model to investigate the effects of diabetes on renal tissue NO bioavailability. For this, we used in vivo NO trapping, followed by electron paramagnetic resonance spectroscopy. In addition, we investigated whether coupling of NOS by supplying the cofactor BH4 could restore glomerular endothelial barrier function. Our data show that overall NO availability at the tissue level is not reduced sixteen weeks after the induction of diabetes in apoE knockout mice, despite the presence of factors that cause endothelial dysfunction, and the presence of the endogenous NOS inhibitor ADMA. Targeting uncoupled NOS with the BH4 precursor sepiapterin further increases NO availability, but did not modify renal glomerular injury. Notably, glomerular heparanase activity as a driver for loss of glomerular barrier function was not reduced, pointing towards NOS-independent mechanisms. This was confirmed by unaltered increased glomerular presence of cathepsin L, the protease that activates heparanase.

Atrasentan Reduces Albuminuria by Restoring the Glomerular Endothelial Glycocalyx Barrier in Diabetic Nephropathy

Atrasentan, a selective endothelin A receptor antagonist, has been shown to reduce albuminuria in type 2 diabetes. We previously showed that the structural integrity of a glomerular endothelial glycocalyx is required to prevent albuminuria. Therefore we tested the potential of atrasentan to stabilize the endothelial glycocalyx in diabetic apolipoprotein E (apoE)-deficient mice in relation to its antialbuminuric effects. Treatment with atrasentan (7.5 mg/kg/day) for 4 weeks reduced urinary albumin-to-creatinine ratios by 26.0 ± 6.5% (P < 0.01) in apoE knockout (KO) mice with streptozotocin-induced diabetes consuming an atherogenic diet, without changes in gross glomerular morphology, systemic blood pressure, and blood glucose concentration. Endothelial cationic ferritin surface coverage, investigated using large-scale digital transmission electron microscopy, revealed that atrasentan treatment increases glycocalyx coverage in diabetic apoE KO mice from 40.7 ± 3.2% to 81.0 ± 12.5% (P < 0.05). This restoration is accompanied by increased renal nitric oxide concentrations, reduced expression of glomerular heparanase, and a marked shift in the balance of M1 and M2 glomerular macrophages. In vitro experiments with endothelial cells exposed to laminar flow and cocultured with pericytes confirmed that atrasentan reduced endothelial heparanase expression and increased glycocalyx thickness in the presence of a diabetic milieu. Together these data point toward a role for the restoration of endothelial function and tissue homeostasis through the antialbuminuric effects of atrasentan, and they provide a mechanistic explanation for the clinical observations of reduced albuminuria with atrasentan in diabetic nephropathy.

Dynamic, size-selective effects of protamine sulfate and hyaluronidase on the rat glomerular filtration barrier in vivo

The proteinuric actions of protamine sulfate (PS) have classically been, at least partly, attributed to alterations of the negatively charged glomerular endothelial glycocalyx. To investigate whether the charge-selective properties of the glomerular filtration barrier (GFB) would be altered by PS, we assessed the glomerular sieving of conventional, uncharged, polydispersed Ficoll (n-Ficoll) compared with charge modified, conformationally intact, anionic (carboxymethylated) Ficoll (a-Ficoll) before and after systemic infusions of PS in rats. For comparison, we also investigated the impact of hyaluronidase (hyase), which partially degrades the glycocalyx, on GFB permeability. In anaesthetized Wistar rats, blood access was achieved, and the left ureter was cannulated for urine collection. Rats were infused with either n-Ficoll or a-Ficoll before and during systemic infusions with either PS or hyase. Plasma and urine samples were taken repeatedly and analyzed by high-performance size exclusion chromatography to assess glomerular sieving coefficients (θ) for Ficoll (radius 10-80 Å). The GFB showed a significant glomerular charge selectivity for Ficoll molecules of radius 20-35 Å. PS and hyase infusions reversibly increased θ for large Ficoll molecules (Ficoll molecules of radius 50-80 Å). Thus, for PS, θ for a-Ficoll molecules of radius 70 Å increased from 2.47 × 10(-5) ± 1.1(-5) to 7.25 × 10(-5) ± 1.1(-5) (P < 0.05) at 15 min. For hyase, changes in a-Ficoll molecules of radius 50-80 Å were, however, not statistically significant. Neither PS nor hyase had any effect on θ for n-Ficoll molecules of radius 20-45 Å or a-Ficoll molecules of radius 20-45 Å. It is concluded that systemically administered PS and hyase in moderate doses dynamically decreased the size selectivity of the rat GFB without affecting its charge selective properties.

Resolution of the three dimensional structure of components of the glomerular filtration barrier

Background

The human glomerulus is the primary filtration unit of the kidney, and contains the Glomerular Filtration Barrier (GFB). The GFB had been thought to comprise 3 layers – the endothelium, the basement membrane and the podocyte foot processes. However, recent studies have suggested that at least two additional layers contribute to the function of the GFB, the endothelial glycocalyx on the vascular side, and the sub-podocyte space on the urinary side. To investigate the structure of these additional layers is difficult as it requires three-dimensional reconstruction of delicate sub-microscopic (<1 μm) cellular and extracellular elements.

Methods

Here we have combined three different advanced electron microscopic techniques that cover multiple orders of magnitude of volume sampled, with a novel staining methodology (Lanthanum Dysprosium Glycosaminoglycan adhesion, or LaDy GAGa), to determine the structural basis of these two additional layers. Serial Block Face Scanning Electron Microscopy (SBF-SEM) was used to generate a 3-D image stack with a volume of a 5.3 x 10⁵ μm³ volume of a whole kidney glomerulus (13% of glomerular volume). Secondly, Focused Ion Beam milling Scanning Electron Microscopy (FIB-SEM) was used to image a filtration region (48 μm³ volume). Lastly Transmission Electron Tomography (Tom-TEM) was performed on a 0.3 μm³ volume to identify the fine structure of the glycocalyx.

Results

Tom-TEM clearly showed 20 nm fibre spacing in the glycocalyx, within a limited field of view. FIB-SEM demonstrated, in a far greater field of view, how the glycocalyx structure related to fenestrations and the filtration slits, though without the resolution of TomTEM. SBF-SEM was able to determine the extent of the sub-podocyte space and glycocalyx coverage, without additional heavy metal staining. Neither SBF- nor FIB-SEM suffered the anisotropic shrinkage under the electron beam that is seen with Tom-TEM.

Conclusions

These images demonstrate that the three dimensional structure of the GFB can be imaged, and investigated from the whole glomerulus to the fine structure of the glycocalyx using three dimensional electron microscopy techniques. This should allow the identification of structural features regulating physiology, and their disruption in pathological states, aiding the understanding of kidney disease.

Loss of the endothelial glycocalyx links albuminuria and vascular dysfunction

Patients with albuminuria and CKD frequently have vascular dysfunction but the underlying mechanisms remain unclear. Because the endothelial surface layer, a meshwork of surface-bound and loosely adherent glycosaminoglycans and proteoglycans, modulates vascular function, its loss could contribute to both renal and systemic vascular dysfunction in proteinuric CKD. Using Munich-Wistar-Fromter (MWF) rats as a model of spontaneous albuminuric CKD, multiphoton fluorescence imaging and single-vessel physiology measurements revealed that old MWF rats exhibited widespread loss of the endothelial surface layer in parallel with defects in microvascular permeability to both water and albumin, in both continuous mesenteric microvessels and fenestrated glomerular microvessels. In contrast to young MWF rats, enzymatic disruption of the endothelial surface layer in old MWF rats resulted in neither additional loss of the layer nor additional changes in permeability. Intravenous injection of wheat germ agglutinin lectin and its adsorption onto the endothelial surface layer significantly improved glomerular albumin permeability. Taken together, these results suggest that widespread loss of the endothelial surface layer links albuminuric kidney disease with systemic vascular dysfunction, providing a potential therapeutic target for proteinuric kidney disease.

The gel hypothesis applied to the rat renal capillary membranes: a review

In the gel model for the glomerular (and peritubular) capillary membrane, the integrity of the membrane is supposed to result from the fluid reabsorption induced by the osmotic action of the counter-ions attracted to negative fixed charges, increasing the gel pressure such that it becomes the same as in the capillaries. From this point on, the gel will be unaffected by the high capillary pressure. The same fluid reabsorption will also suspend the fibrils in the matrix such that they form a series of grids composed of, for example, horizontal fibrils spaced similarly from one another. The model thereby explains the well-known phenomenon of a uniform 'pore' size, although slits rather than pores constitute the transport routes. The model also explains the fact that the plasma proteins are free to move in the membrane matrix, which is the consequence of a recent finding that a major restriction to albumin is offered by a unique protein, nephrin, located between the podocytes in Bowman's space cells. A large molecule, which may become trapped in a slit between two fibrils, will thus push out the positive counter-ions whereby the charges become free and hence repel one another, widening the slit such that the molecule is free to move in any direction. It is furthermore concluded that the restriction to proteins is also dependent on the width of the slits closest to plasma.

The glomerular endothelial cell coat is essential for glomerular filtration

The endothelial cell surface layer (ESL) is believed to contribute to the glomerular barrier, and the nature of its molecular structure is still largely unknown. The ESL consists of the membrane-bound glycocalyx and the loosely attached endothelial cell coat (ECC). A brief injection of hypertonic sodium chloride into the left renal artery was used to displace, elute, and collect non-covalently bound components of the renal ESL in rats. This procedure increased the fractional clearance of albumin 12-fold without detectable morphological changes as assessed by electron microscopy compared with the control group injected with isotonic saline. Mathematical modeling suggested a reduced glomerular charge density. Mass spectrometry of the renal eluate identified 17 non-covalently bound proteins normally present in the ECC. One of these proteins, orosomucoid, has previously been shown to be important for capillary permselectivity. Another protein, lumican, is expressed by glomerular endothelial cells and likely contributes to maintaining an intact barrier. Thus, the absence of one or more of these proteins causes proteinuria and illustrates the importance of the ECC in glomerular permselectivity.

Physiologic upper limits of pore size of different blood capillary types and another perspective on the dual pore theory of microvascular permeability

BACKGROUND

Much of our current understanding of microvascular permeability is based on the findings of classic experimental studies of blood capillary permeability to various-sized lipid-insoluble endogenous and non-endogenous macromolecules. According to the classic small pore theory of microvascular permeability, which was formulated on the basis of the findings of studies on the transcapillary flow rates of various-sized systemically or regionally perfused endogenous macromolecules, transcapillary exchange across the capillary wall takes place through a single population of small pores that are approximately 6 nm in diameter; whereas, according to the dual pore theory of microvascular permeability, which was formulated on the basis of the findings of studies on the accumulation of various-sized systemically or regionally perfused non-endogenous macromolecules in the locoregional tissue lymphatic drainages, transcapillary exchange across the capillary wall also takes place through a separate population of large pores, or capillary leaks, that are between 24 and 60 nm in diameter. The classification of blood capillary types on the basis of differences in the physiologic upper limits of pore size to transvascular flow highlights the differences in the transcapillary exchange routes for the transvascular transport of endogenous and non-endogenous macromolecules across the capillary walls of different blood capillary types.

METHODS

The findings and published data of studies on capillary wall ultrastructure and capillary microvascular permeability to lipid-insoluble endogenous and non-endogenous molecules from the 1950s to date were reviewed. In this study, the blood capillary types in different tissues and organs were classified on the basis of the physiologic upper limits of pore size to the transvascular flow of lipid-insoluble molecules. Blood capillaries were classified as non-sinusoidal or sinusoidal on the basis of capillary wall basement membrane layer continuity or lack thereof. Non-sinusoidal blood capillaries were further sub-classified as non-fenestrated or fenestrated based on the absence or presence of endothelial cells with fenestrations. The sinusoidal blood capillaries of the liver, myeloid (red) bone marrow, and spleen were sub-classified as reticuloendothelial or non-reticuloendothelial based on the phago-endocytic capacity of the endothelial cells.

RESULTS

The physiologic upper limit of pore size for transvascular flow across capillary walls of non-sinusoidal non-fenestrated blood capillaries is less than 1 nm for those with interendothelial cell clefts lined with zona occludens junctions (i.e. brain and spinal cord), and approximately 5 nm for those with clefts lined with macula occludens junctions (i.e. skeletal muscle). The physiologic upper limit of pore size for transvascular flow across the capillary walls of non-sinusoidal fenestrated blood capillaries with diaphragmed fenestrae ranges between 6 and 12 nm (i.e. exocrine and endocrine glands); whereas, the physiologic upper limit of pore size for transvascular flow across the capillary walls of non-sinusoidal fenestrated capillaries with open 'non-diaphragmed' fenestrae is approximately 15 nm (kidney glomerulus). In the case of the sinusoidal reticuloendothelial blood capillaries of myeloid bone marrow, the transvascular transport of non-endogenous macromolecules larger than 5 nm into the bone marrow interstitial space takes place via reticuloendothelial cell-mediated phago-endocytosis and transvascular release, which is the case for systemic bone marrow imaging agents as large as 60 nm in diameter.

CONCLUSIONS

The physiologic upper limit of pore size in the capillary walls of most non-sinusoidal blood capillaries to the transcapillary passage of lipid-insoluble endogenous and non-endogenous macromolecules ranges between 5 and 12 nm. Therefore, macromolecules larger than the physiologic upper limits of pore size in the non-sinusoidal blood capillary types generally do not accumulate within the respective tissue interstitial spaces and their lymphatic drainages. In the case of reticuloendothelial sinusoidal blood capillaries of myeloid bone marrow, however, non-endogenous macromolecules as large as 60 nm in diameter can distribute into the bone marrow interstitial space via the phago-endocytic route, and then subsequently accumulate in the locoregional lymphatic drainages of tissues following absorption into the lymphatic drainage of periosteal fibrous tissues, which is the lymphatic drainage of myeloid bone marrow. When the ultrastructural basis for transcapillary exchange across the capillary walls of different capillary types is viewed in this light, it becomes evident that the physiologic evidence for the existence of aqueous large pores ranging between 24 and 60 nm in diameter in the capillary walls of blood capillaries, is circumstantial, at best.

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.

Proteinuria in diabetic kidney disease: a mechanistic viewpoint

Proteinuria is the hallmark of diabetic kidney disease (DKD) and is an independent risk factor for both renal disease progression, and cardiovascular disease. Although the characteristic pathological changes in DKD include thickening of the glomerular basement membrane and mesangial expansion, these changes per se do not readily explain how patients develop proteinuria. Recent advances in podocyte and glomerular endothelial cell biology have shifted our focus to also include these cells of the glomerular filtration barrier in the development of proteinuria in DKD. This review describes the pathophysiological mechanisms at a cellular level which explain why patients with DKD develop proteinuria.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

Resolved: capillary endothelium is a major contributor to the glomerular filtration barrier

There is growing debate as to the primal role of capillary endothelial cells in the barrier function of the glomerular filter. Two experts argue the points for and against. Clearly unresolved is agreement whether glomerular capillary endothelium has fenestrae subtended by diaphragms.

Glomerular endothelial glycocalyx constitutes a barrier to protein permeability

Glycocalyx, composed of glycoproteins including proteoglycans, coats the luminal surface of the glomerular capillaries. Human heparanase degrades heparan sulphate glycosaminoglycans and is up-regulated in proteinuric states. In this study, we analyze the structure of the human glomerular endothelial cell glycocalyx in vitro and examine its functional relevance, especially after treatment with human heparanase. Electron microscopy of conditionally immortalized glomerular endothelial cells revealed a 200-nm thick glycocalyx over the plasma membrane, which was also demonstrated by confocal microscopy. Neuraminidase treatment removed the majority of glycocalyx, reduced trans-endothelial electrical resistance by 59%, and increased albumin flux by 207%. Heparinase III and human heparanase specifically cleaved heparan sulphate: this caused no change in trans-endothelial electrical resistance, but increased the albumin passage across the monolayers by 40% and 39%, respectively. Therefore, we have characterized the glomerular endothelial cell glycocalyx and have shown that it contributes to the barrier to flux of albumin across the cell layer. These results suggest an important role for this glycocalyx in the restriction of glomerular protein passage in vivo and suggest ways in which human heparanase levels may be linked to proteinuria in clinical disease.

In Vivo Degradation of Heparan Sulfates in the Glomerular Basement Membrane Does Not Result in Proteinuria

Heparan sulfates (HS) are long, unbranched, negatively charged polysaccharides that are bound to core proteins. HS in the glomerular basement membrane (GBM) is reported to be important for charge-selective permeability. Aberrant GBM HS expression has been observed in several glomerular diseases, such as diabetic nephropathy and membranous glomerulopathy, and a decrease in HS generally is associated with proteinuria. This study, with the use of a controlled in vivo approach, evaluated whether degradation of HS in rat GBM resulted in acute proteinuria. Rats received two intravenous injections of either heparinase III to digest HS or neuraminidase to remove neuraminic acids (positive control). Urine samples were taken at various time points, and at the end of the experiment, kidneys were removed and analyzed. Injection with heparinase III resulted in a complete loss of glomerular HS as demonstrated by immunofluorescence staining using anti-HS antibodies and by electron microscopy using cupromeronic blue in a critical electrolyte concentration mode. In the urine, a strong increase in HS was found within 2 h after the first injection. Staining for agrin, the major HS proteoglycan core protein in the GBM, was unaltered. No urinary albumin or other proteins were detected at any time point, and no changes in glomerular morphology were noticed. Injection of rats with neuraminidase, however, resulted in a major increase of urinary albumin and was associated with an increase in urinary free neuraminic acid. An increased glomerular staining with Peanut agglutinin lectin, indicative of removal of neuraminic acid, was noted. In conclusion, removal of HS from the GBM does not result in acute albuminuria, whereas removal of neuraminic acid does.

Electron microscopic evaluation of the endothelial surface layer of glomerular capillaries

Recent data from various vascular beds suggest that a layer of mucopolysaccharides covering the endothelial cells play an important role in transport processes, among others. In this study, electron microscopy (EM) was used to explore the presence of an endothelial surface layer (ESL) in rat glomerular capillaries. We adopted various fixation and labeling techniques, as follows: (1) negatively charged lipid particles were used as a tracer that was expected to be excluded from the ESL. The density of intravascular lipid particles in flow-arrested capillaries was 89% lower in a 200-nm periendothelial area than in the rest of the luminal space (n = 6 rats, P < 0.001); (2) podocytes of cryofixed fresh tissue had a 20-nm extramembranous coat, interpreted as the true glycocalyx; the coat was less expressed on the endothelium; (3) on unfixed endothelial cells, colloidal lanthanum labeled a 60-nm-thick layer, occasionally forming lumps; (4) perfusion with a fluorocarbon-based oxygen-carrying fixative, followed by tannic acid contrast enhancement, revealed an extensive (> 200 nm) ESL not previously described; however, this finding was restricted to superficial glomerular capillaries; (5) Cupromeronic Blue cytochemistry displayed a loose proteoglycan network in fenestral openings and, occasionally, a semiordered ESL; (6) ferricyanide-reduced osmication resulted in increased numbers of fenestral diaphragms. In conclusion, this study provides novel morphological evidence to support the presence of a significant glomerular ESL.

Ultrastructural model for size selectivity in glomerular filtration

A theoretical model was developed to relate the size selectivity of the glomerular barrier to the structural characteristics of the individual layers of the capillary wall. Thicknesses and other linear dimensions were evaluated, where possible, from previous electron microscopic studies. The glomerular basement membrane (GBM) was represented as a homogeneous material characterized by a Darcy permeability and by size-dependent hindrance coefficients for diffusion and convection, respectively; those coefficients were estimated from recent data obtained with isolated rat GBM. The filtration slit diaphragm was modeled as a single row of cylindrical fibers of equal radius but nonuniform spacing. The resistances of the remainder of the slit channel, and of the endothelial fenestrae, to macromolecule movement were calculated to be negligible. The slit diaphragm was found to be the most restrictive part of the barrier. Because of that, macromolecule concentrations in the GBM increased, rather than decreased, in the direction of flow. Thus the overall sieving coefficient (ratio of Bowman's space concentration to that in plasma) was predicted to be larger for the intact capillary wall than for a hypothetical structure with no GBM. In other words, because the slit diaphragm and GBM do not act independently, the overall sieving coefficient is not simply the product of those for GBM alone and the slit diaphragm alone. Whereas the calculated sieving coefficients were sensitive to the structural features of the slit diaphragm and to the GBM hindrance coefficients, variations in GBM thickness or filtration slit frequency were predicted to have little effect. The ability of the ultrastructural model to represent fractional clearance data in vivo was at least equal to that of conventional pore models with the same number of adjustable parameters. The main strength of the present approach, however, is that it provides a framework for relating structural findings to the size selectivity of the glomerular barrier.

Incorporation of [35S]sulfate into glomerular membranes of rats chronically exposed to cadmium and its relation with urinary glycosaminoglycans and proteinuria

The aim of the present work was to assess the effects of long-term exposure to Cd on the sulfatation of glomerular membranes and their relation with proteinuria and urinary glycosaminoglycans (GAG). For this purpose the in vitro incorporation of [35S]sulfate was investigated in female Sprague-Dawley rats given 100 ppm of Cd in drinking water for 7 months. When compared with their controls, glomeruli from Cd-treated rats showed a 12.8% decrease in the incorporation of the label into glomerular membranes. This effect, which was not explained by differences in viability or in sulfate uptake by the glomeruli, suggests that sulfatation of glomerular membranes is impaired in Cd-treated rats. In support of this, in another independent experiment, a decrease, 17.4% on average, of the sulfate content of glomerular membranes was observed in long-term Cd-treated rats (100 ppm in drinking water for 4 months). This effect was significantly correlated with albuminuria and transferrinuria but not with beta 2-microglobinuria, suggesting that a loss of heparan sulfate of the glomerular capillary wall could be involved in the Cd-induced glomerular proteinuria. On the other hand an enhanced urinary excretion of GAG, negatively correlated with the sulfate content of glomerular membranes, was also observed in Cd-treated rats. Moreover GAG excretion was associated with tubular and glomerular proteinuria, which suggests that GAG might be a useful marker of Cd-induced nephrotoxicity.

The anionic charge barrier in the renal corpuscle of the pronephros in the lamprey, Petromyzon marinus L

The charge barrier within the renal corpuscle of the pronephric kidney of the lamprey, Petromyzon marinus, was investigated at two life cycle intervals using cationized ferritin and polyethyleneimine. In the larval renal corpuscle the endothelium of the glomerular capillaries and the laminae rarae externa and interna of the glomerular basement membrane show regularly-spaced deposits of the tracers. The lamina densa remains unstained. Concomitant with a loss of major processes of the visceral epithelial podocytes and development of an extensive mesangial matrix in late adult life are alterations in the distribution of the anionic sites. The lamina rara interna is no longer a distinct entity and the mesangium contains irregularly-distributed anionic sites surrounding electron-dense deposits. The results indicate that the distribution of the anionic sites during adult life most likely affects the ability of the renal corpuscle to act as an efficient filtration device. This charge distribution is consistent with that seen during some renal pathologies of higher vertebrates.

Effects of maternal bilateral ureteral ligation on the differentiation of glomerular anionic sites in fetal rat kidney

Effects of maternal bilateral ureteral ligation on the differentiation of glomerular anionic sites in fetal kidney were studied with the colloidal iron method. The ligation was performed on days 17, 19, and 21 of pregnancy, and the animal were euthanized 1 day after each ligation. On fetal day 20, in the fetuses from ligated mothers, colloidal iron was concentrated in laminae rarae interna and externa of the glomerular basement membrane. In the fetuses from sham-ligated mothers, however, a different pattern of iron distribution was noted. In most areas of the glomerular basement membrane, colloidal iron was randomly distributed as in both fetuses of fetal day 18. In the remaining areas, the same pattern of colloidal iron distribution as was noted in the fetuses from ligated mothers of fetal day 20 was observed. On fetal day 22, colloidal iron was densely accumulated in laminae rarae interna and externa of the glomerular basement membrane in both groups. However, the iron-free zone was slightly thicker in the fetuses from the ligated mothers. These findings suggest that maternal bilateral ureteral ligation accelerates the differentiation of glomerular anionic sites in the kidney of fetal rats.

Experimental glomerulonephritis induced in rats by a lectin and its antibodies

The aim of the present study was to investigate the events following interaction of antibody with an antigen planted on the surface of glomerular endothelial cell (GEN). A lectin, Helix pomatia agglutinin (HPA), was planted at the surface of rat GEN by the perfusion of the isolated left kidney with neuraminidase (NRD) and HPA. A subsequent perfusion with IgG fraction, but not with Fab fragments, of rabbit anti-HPA serum induced formation of granular immune deposits (IDs) on the surface of GEN. When the left kidney was revascularized after initial formation of IDs, acute glomerulonephritis ensued. Fifteen minutes after revascularization, granular IDs were observed at the subendothelial space. Two days later, there was decrease of subendothelial IDs with concomitant appearance of subepithelial IDs. At the seventh day, IDs were mainly localized in the subepithelial space. The results suggest that this model of experimental glomerulonephritis is characterized by: (1) initial formation of HPA immune complexes (ICs) at the surface of GEN; and by (2) subsequent movement of ICs from luminal side to subepithelial area with local formation of IDs. The results are relevant to the understanding of the kinetics of ICs resulting from interaction of antibodies with exogenous antigens "planted" in the glomerular capillary walls, and of the local formation of IDs.

Distribution of electric charges and concanavalin A binding sites on autophagic vacuoles and lysosomes in mouse hepatocytes

The distributions of electric charges and Concanavalin A binding sites in autophagic vacuoles and lysosomes in mouse hepatocytes were studied by utilizing a frozen ultrathin section labeling method with cationized ferritin (CF) or anionized ferritin and ferritin-conjugated Concanavalin A (Con A-F) as visual probes. Our observations revealed that the inner surface of the autophagic vacuole membrane has more anionic sites (CF binding) than other organelle membranes. This suggests that if the limiting membranes of autophagic vacuoles originate from preexisting membranes, such membranes must undergo structural and compositional alternation during the formation of the autophagic vacuoles. In contrast to CF, Con A-F showed no distinct binding to the membranes of autophagic vacuoles, but the contents of vacuoles displayed varying Con A-F binding, depending on the stage of the autophagic process. Increased binding was seen in more mature autophagic vacuoles. Since lysosomes showed a preferential accumulation of Con A-F particles, molecules with Con A-F binding sites in autophagic vacuoles may be of lysosomal origin. Con A-F distribution varied from lysosome to lysosome in the same cell, indicating heterogeneity of lysosomal contents. These results suggest that ferritin-conjugated lectin labeling methods applied to frozen, ultrathin section are a useful new approach in analyzing the natural history of autophagic vacuoles and the heterogeneity of lysosomes.