A quantitative analysis of the glomerular charge barrier in the rat

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.

The long journey through renal filtration: new pieces in the puzzle of slit diaphragm architecture

PURPOSE OF REVIEW

The podocyte slit diaphragm is probably the least understood component of the kidney filtration barrier. In this review, we aim to integrate the most recent findings on the molecular make-up and structural architecture of this specialized cell-cell junction into a current concept of glomerular filtration.

RECENT FINDINGS

Analysis of cryopreserved mammalian tissue revealed a bipartite composition of the slit diaphragm. Single NEPH1 molecules span the lower part of the slit close to the glomerular basement membrane whereas NEPHRIN molecules are positioned in the apical part toward Bowman's space. This molecular arrangement could lead to heterogeneous ellipsoidal and circular pores, which are mainly located in the central region of the slit diaphragm.

SUMMARY

Despite having been first identified in the 1970s, the slit diaphragm's structural architecture has not been fully elucidated to date and remains an area of intense research and scientific debate. The slit diaphragm has been initially described as a rigid 'zipper-like' structure in which periodic, rod-like units extend from a podocyte foot processes to a linear central bar, giving rise to homogeneous 4 × 14 nm pores. Several recent findings have challenged these long-held beliefs and instead pointed to an unanticipated complexity of slit diaphragm structure. High-resolution ultrastructural analysis found evidence that the slit diaphragm is a dynamic and adjustable cell-cell junction that forms a nonclogging barrier within the renal filtration system.

The Role of Endothelial Surface Glycocalyx in Mechanosensing and Transduction

The endothelial cells (ECs) forming the inner wall of every blood vessel are constantly exposed to the mechanical forces generated by blood flow. The EC responses to these hemodynamic forces play a critical role in the homeostasis of the circulatory system. A variety of mechanosensors and transducers, locating on the EC surface, intra- and trans-EC membrane, and within the EC cytoskeleton, have thus been identified to ensure proper functions of ECs. Among them, the most recent candidate is the endothelial surface glycocalyx (ESG), which is a matrix-like thin layer covering the luminal surface of the EC. It consists of various proteoglycans, glycosaminoglycans, and plasma proteins and is close to other prominent EC mechanosensors and transducers. This chapter summarizes the ESG composition, thickness, and structure observed by different labeling and visualization techniques and in different types of vessels. It also presents the literature in determining the ESG mechanical properties by atomic force microscopy and optical tweezers. The molecular mechanisms by which the ESG plays the role in EC mechanosensing and transduction are described as well as the ESG remodeling by shear stress, the actin cytoskeleton, the membrane rafts, the angiogenic factors, and the sphingosine-1-phosphate.

Dual Regulation of L-Selectin-Mediated Leukocyte Adhesion by Endothelial Surface Glycocalyx

Endothelial surface glycocalyx (ESG) is a carbohydrate-rich, gel-like layer found on vascular endothelium, serving critical functions in mechanotransduction of blood flows, maintenance of the endothelial permeability, and the control of leukocyte adhesion and inflammation. This study aimed to clarify the role of ESG in the adhesion between leukocytes and Human Umbilical Vein Endothelial Cells (HUVECs) under resting or inflammatory conditions. Using an atomic force microscopy-based single-cell adhesion assay, we directly quantified the detachment force and work perpendicular to the cell membrane. Detachment force and work were measured for every separation event of a leukocyte from a HUVEC with ESG, or with the major ESG glycosaminoglycan components, heparan sulfate (HS) and hyaluronic acid (HA) removed. For the resting HUVECs, when HS and/or HA were removed, the detachment force and work increased dramatically. For the HUVECs activated by inflammatory cytokine tumor necrosis factor alpha, we observed increases in the detachment force and work compared to the resting HUVECs, and removal of HS and/or HA resulted in significant decreases in the detachment force and work. The results demonstrate that the ESG layer serves a dual function: (1) on resting endothelium, it prevents leukocyte adhesion, and (2) under inflammatory conditions, it participates in endothelial-leukocyte interactions with molecules such as selectins.

Progress and controversies in unraveling the glomerular filtration mechanism

PURPOSE OF REVIEW

At first sight, the glomerular filter appears like a problem that should be easily solved. The majority of researchers view the filter like an impermeable wall perforated by specialized and size-selective pores (pore model). However, the fact that this model is in conflict with many of the experimental findings suggests that it may not yet be complete.

RECENT FINDINGS

In the more recent electrokinetic model, we have proposed including electrical effects (streaming potentials). The present review investigates how this can provide a relatively simple mechanistic explanation for the great majority of the so far unexplained characteristics of the filter, for example why the filter never clogs.

SUMMARY

Understanding how the glomerular filter functions is a prerequisite to investigate the pathogenesis of proteinuric glomerular diseases and the link between glomerular proteinuria and cardiovascular disease.

Renal albumin filtration: alternative models to the standard physical barriers

Human kidneys produce more than 4 million litres of virtually protein-free primary urine in a lifetime. In healthy individuals, the sieving process is accomplished by the glomerular filter without the smallest sign of clogging, even in old age. How nature accomplishes this extraordinary task is a mystery, but unravelling the functioning of the glomerular filter is important. The basic principles that govern glomerular filtration are probably also true for peripheral filtering by fenestrated capillaries. In addition, understanding the sieving process is a prerequisite to understanding the pathogenesis of proteinuria (that is, the leakage of plasma proteins into the urine). Proteinuria is the hallmark of glomerular disease and a major risk factor for systemic cardiovascular complications, a fact that emphasizes the relationship between the glomerular and peripheral filtering capillaries. In this Review, we briefly summarize the major models that have been proposed for the mechanisms of glomerular filtration and discuss their strengths and limitations. A special emphasis is placed on the 'electrokinetic model' that we have proposed, a model that could potentially resolve many of the seemingly strange characteristics of the glomerular filtration barrier.

Mechano-sensing and transduction by endothelial surface glycocalyx: composition, structure, and function

The endothelial cells (ECs) lining every blood vessel wall are constantly exposed to the mechanical forces generated by blood flow. The EC responses to these hemodynamic forces play a critical role in the homeostasis of the circulatory system. To ensure proper EC mechano-sensing and transduction, there are a variety of mechano-sensors and transducers that have been identified on the EC surface, intra- and trans-EC membrane and within the EC cytoskeleton. Among them, the most recent candidate is the endothelial surface glycocalyx (ESG), which is a matrix-like thin layer covering the luminal surface of the EC. It consists of various proteoglycans, glycosaminoglycans, and plasma proteins, and is close to other prominent EC mechano-sensors and transducers. The ESG thickness was found to be in the order of 0.1-1 µm by different visualization techniques and in different types of vessels. Detailed analysis on the electron microscopy (EM) images of the microvascular ESG revealed a quasi-periodic substructure with the ESG fiber diameter of 10-12 and 20 nm spacing between adjacent fibers. Atomic force microscopy and optical tweezers were applied to investigate the mechanical properties of the ESG on the cultured EC monolayers and in solutions. Enzymatic degradation of specific ESG glycosaminoglycan components was used to directly elucidate the role of the ESG in EC mechano-sensing and transduction by measuring the shear-induced productions of nitric oxide and prostacyclin, two characteristic responses of the ECs to the flow. The unique location, composition, and structure of the ESG determine its role in EC mechano-sensing and transduction.

The glomerular filtration barrier function: new concepts

PURPOSE OF REVIEW

Each day, the human kidneys filter about 140 l of primary urine from plasma. Although this ultrafiltrate is virtually free of plasma protein, the glomerular filter never clogs under physiological conditions. Upto today it is still not entirely resolved as to how the kidney accomplishes this extraordinary task. Most of the proposed models for glomerular filtration have not considered electrical effects.

RECENT FINDINGS

In micropuncture studies, we have directly measured an electrical field across the glomerular filtration barrier. This potential difference is most likely generated by forced passage of the ionic solution of the plasma across the charged glomerular filter ('electrokinetic potential'). As all plasma proteins are negatively charged, the electrical field across the glomerular filtration barrier is predicted to drive plasma proteins from the filter toward the capillary lumen by electrophoresis.

SUMMARY

In this review, we examine our novel model for glomerular filtration in more detail. We outline the physical mechanisms by which electrokinetic effects (streaming potentials) are generated. We investigate the potential impact of the electrical field on the passage of albumin across the glomerular filtration barrier. We review the mathematical heteroporous model including electrical effects and analyse a selection of experimental studies for indications that electrical effects influence glomerular permeability significantly.

The structural stability of the endothelial glycocalyx after enzymatic removal of glycosaminoglycans

Rationale

It is widely believed that glycosaminoglycans (GAGs) and bound plasma proteins form an interconnected gel-like structure on the surface of endothelial cells (the endothelial glycocalyx layer–EGL) that is stabilized by the interaction of its components. However, the structural organization of GAGs and proteins and the contribution of individual components to the stability of the EGL are largely unknown.

Objective

To evaluate the hypothesis that the interconnected gel-like glycocalyx would collapse when individual GAG components were almost completely removed by a specific enzyme.

Methods and Results

Using confocal microscopy, we observed that the coverage and thickness of heparan sulfate (HS), chondroitin sulfate (CS), hyaluronic acid (HA), and adsorbed albumin were similar, and that the thicknesses of individual GAGs were spatially nonuniform. The individual GAGs were degraded by specific enzymes in a dose-dependent manner, and decreased much more in coverage than in thickness. Removal of HS or HA did not result in cleavage or collapse of any of the remaining components. Simultaneous removal of CS and HA by chondroitinase did not affect HS, but did reduce adsorbed albumin, although the effect was not large.

Conclusion

All GAGs and adsorbed proteins are well inter-mixed within the structure of the EGL, but the GAG components do not interact with one another. The GAG components do provide binding sites for albumin. Our results provide a new view of the organization of the endothelial glycocalyx layer and provide the first demonstration of the interaction between individual GAG components.

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.

Imaging of the porous ultrastructure of the glomerular epithelial filtration slit

The ultrastructure of the glomerular filtration slit is still controversial. In the last 30 years, observations from transmission electron microscopy (TEM) and theoretical analysis of solute clearance produced conflicting results. Here, we used scanning EM with a high-sensitivity detector to image the deepest regions of the filtration slits and report a previously undescribed organization of the slits' ultrastructure. In contrast to previous TEM imaging, we observed circular and ellipsoidal pores in the podocyte junctions mainly located in the central region of the slit diaphragm. The normal mean pore radius estimated by digital morphometric analysis had a log-normal distribution, with an average value of 12.1 nm. In proteinuric pathologic conditions, the mean pore radius values were also log-normally distributed with the presence of some very large pores, exceeding the sizes observed in normal conditions. Our morphologic analysis suggests that the filtration slit is a heteroporous structure instead of the previously proposed zipper-like structure. Selective changes in the ultrastructural organization of the pores may be responsible for the increased filtration of plasma proteins in glomerular disease.

Update on the glomerular filtration barrier

PURPOSE OF REVIEW

The nephrology community lacks a unified view of protein sieving through the glomerular capillary wall. The glomerular capillary wall consists of three distinct but closely interacting layers: the fenestrated endothelium, with its glycocalyx; the podocytes, with their interdigitated foot processes and slit diaphragms; and the intervening glomerular basement membrane. Proteinuria is associated with abnormalities in any one layer, suggesting that each contributes to the glomerular filtration barrier (GFB). Proteinuria can also be induced in the context of a normal glomerular capillary wall. Here, we review some classic studies as well as some newer concepts and present competing hypotheses about the GFB.

RECENT FINDINGS

Two almost forgotten concepts have recently emerged. One group has challenged the exquisite selectivity of the GFB to albumin and suggested that proteinuria is the result of abnormal tubular uptake. There has also been a reemphasis on diffusion through the glomerular basement membrane as the driving force behind macromolecular filtration. New evidence suggests that the endothelial glycocalyx is an important charge-selective barrier.

SUMMARY

We suggest viewing the GFB as a dynamic rather than as a rigid barrier, requiring three healthy layers and a hemodynamic steady state. Multiple challenges to studying the endothelium, the tubular handling of albumin, and the role of hemodynamic forces will require new tools, new hypotheses, and open minds.

Adriamycin alters glomerular endothelium to induce proteinuria

The pathophysiology underlying the nephrotic syndrome is becoming clear for several inherited podocytopathies; the mechanisms of injury that lead to the acquired forms of this disease are not well understood. We explored these mechanisms using the mouse model of adriamycin-induced proteinuria.We estimated the fractional clearances for FITC-Ficolls, albumin, and neutral albumin in cooled, isolated,perfused kidneys (cIPK) in situ. Treatment with adriamycin led to a significant increase in the fractional clearance of albumin and of Ficoll with radii larger than 20 A. Neutral albumin (33.4 A) and similarly sized Ficoll behaved similarly to each other. In addition, adriamycin led to a significant loss of charge density and size selectivity of the glomerular barrier. The thickness of the glomerular endothelial surface layer(i.e., or the glycocalyx) in adriamycin-treated animals was only 20% of that in normal animals. Finally,several proteoglycans were downregulated in isolated glomeruli. In summary, adriamycin thins the glomerular glycocalyx, perhaps by downregulating proteoglycan synthesis, and alters glomerular charge- and size selectivity. These data suggest that the glomerular endothelium may play a role in the pathogenesis of proteinuric renal diseases.

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.

Functional and molecular alterations of the glomerular barrier in long-term diabetes in mice

AIMS/HYPOTHESIS

Despite the fact that diabetic nephropathy is an increasingly common disorder that may lead to uraemia, the underlying mechanisms are still poorly understood and there is no specific therapy. To clarify whether long-term diabetes alters glomerular size- or charge-selectivity or both, we studied non-obese diabetic mice for up to 40 weeks.

MATERIALS AND METHODS

During the study period, spot urine was collected and blood pressure measured. At weeks 10 and 40, the right kidney was isolated and perfused at 8 degrees C to inhibit tubular function, allowing for analysis of glomerular selectivity with albumin and Ficoll clearance. The left kidney was removed for further investigation using electron microscopy and molecular biology. Real-time PCR with low-density arrays was done to evaluate renal cortex mRNA expression of proteoglycans and other components in the glomerular barrier. After 40 weeks of diabetes, kidneys showed morphological changes typical of diabetic complications.

RESULTS

At 40 weeks, the fractional clearance for negatively charged albumin was three times higher in the diabetic animals (0.0160) than in controls (0.0051, p<0.001), while fractional clearance for neutral Ficoll 35.5 A with a Stokes Einstein radius similar to that of albumin was unaffected. In addition, protein and mRNA levels for versican and decorin were downregulated after 40 weeks of diabetes.

CONCLUSIONS/INTERPRETATION

We conclude that glomerular charge- but not size-selectivity was impaired in the diabetic animals with proteinuria. Also, glomerular components such as versican, decorin and fibromodulin were found to be downregulated after 40 weeks of diabetes.

Morphological and functional evidence for an important role of the endothelial cell glycocalyx in the glomerular barrier

In this study, we pursued the somewhat controversial issue whether the glycosaminoglycans (GAG) in the endothelial cell glycocalyx are important for glomerular size and charge selectivity. In isoflurane-anesthetized mice, Intralipid droplets were used as indirect markers of the glomerular endothelial cell-surface layer, i.e., the glycocalyx. The mice were given intravenous injections of GAG-degrading enzymes, which due to their high molecular weight remained and acted intravascularly. Flow-arrested kidneys were fixed and prepared for electron microscopy, and the distance between glomerular endothelial cells and the luminal Intralipid droplets was measured. The relative frequency of Intralipid droplets was calculated for each 50-nm increment zone up to 500 nm from the endothelial cell membrane surface as were the mean distances. Glomerular size and charge selectivity were estimated from the clearance data for neutral Ficolls (molecular radii of 12–72 Å), and albumin in isolated kidneys was perfused at 8°C. In enzyme-treated animals (hyaluronidase, heparinase, and chondroitinase), the relative Intralipid droplet frequency in the zone closest to the endothelial cells, i.e., 0–50 nm, was increased ∼2.5 times compared with controls. Also, the mean distance between the Intralipid droplets and the endothelium was decreased from 176 to 115–122 nm by enzyme treatment. These changes were accompanied by an increase in the fractional clearance for albumin. In conclusion, both morphological and functional measurements suggest the endothelial cell glycocalyx to be an important component of the glomerular barrier.

Charge modification of the endothelial surface layer modulates the permeability barrier of isolated rat mesenteric small arteries

We hypothesized that modulation of the effective charge density of the endothelial surface layer (ESL) results in altered arterial barrier properties to transport of anionic solutes. Rat mesenteric small arteries (diameter approximately 190 microm) were isolated, cannulated, perfused, and superfused with MOPS-buffered physiological salt solutions. MOPS-solutions were of normal ionic strength (162 mM, MOPS), low ionic strength (81 mM, LO-MOPS), or high ionic strength (323 mM, HI-MOPS), to modulate ESL charge density (normal, high, or low ESL charge, respectively). Osmolarity of MOPS, LO-MOPS, and HI-MOPS was kept constant at 297 mosmol/l, using additional glucose when necessary. Perfusate solutions were supplemented with 1% BSA. Arteries were cannulated with a double-barreled theta-pipet on the inlet side and a regular pipet on the outlet side. After infusion of FITC-labeled dextran of 50 kDa (FITC-Delta50) and the endothelial membrane dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate, the dynamics of arterial dye filling were determined with confocal microscopy. ESL thickness, as determined from the initial exclusion zone for FITC-Delta50 on the luminal endothelial surface, was 6.3 +/- 1.4 microm for LO-MOPS, 2.7 +/- 1.0 microm for MOPS, and 1.1 +/- 1.3 microm for HI-MOPS. At low ionic strength, FITC-Delta50 permeated into the ESL with a total ESL permeation time (tauESL) of 26 min, and at normal ionic strength with a tauESL of 20 min. No apparent exclusion of FITC-Delta50 from the ESL could be observed at high ionic strength. In conclusion, we demonstrate that the modulation of solvent ionic strength influences the thickness and barrier properties of the ESL.

Ficoll and dextran vs. globular proteins as probes for testing glomerular permselectivity: effects of molecular size, shape, charge, and deformability

Polydisperse mixtures of dextran or Ficoll have been frequently used as molecular probes for studies of glomerular permselectivity because they are largely inert and not processed (reabsorbed) by the proximal tubules. However, dextrans are linear, flexible molecules, which apparently are hyperpermeable across the glomerular barrier. By contrast, the Ficoll molecule is almost spherical. Still, there is ample evidence that Ficoll fractional clearances (sieving coefficients) across the glomerular capillary wall (GCW) are markedly higher than those for neutral globular proteins of an equivalent in vitro Stokes-Einstein (SE) radius. Physical data, obtained by "crowding" experiments or measurements of intrinsic viscosity, suggest that the Ficoll molecule exhibits a rather open, deformable structure and thus deviates from an ideally hard sphere. This is also indicated from the relationship between (log) in vitro SE radius and (log) molecular weight (MW). Whereas globular proteins seem to behave in a way similar to hydrated hard spheres, polydisperse dextran and Ficoll exhibit in vitro SE radii that are much larger than those for compact spherical molecules of equivalent MW. For dextran, this can be partially explained by a high-molecular-size asymmetry. However, for Ficoll the explanation may be that the Ficoll molecule is more flexible (deformable) than are globular proteins. An increased compressibility of Ficoll and an increased deformability and size asymmetry for dextran may be the explanation for the fact that the permeability of the GCW is significantly higher when assessed using polysaccharides such as Ficoll or dextran compared with that obtained using globular proteins as molecular size probes. We suggest that molecular deformability, besides molecular size, shape, and charge, plays a crucial role in determining the glomerular permeability to molecules of different species.

Primary human glomerular endothelial cells produce proteoglycans, and puromycin affects their posttranslational modification

This article describes the possible role of the endothelial cell-surface coat, containing proteoglycans (PGs) with connected glycosaminoglycans (GAGs), in maintaining glomerular permselectivity. Primary human glomerular endothelial cells (HGEC) in culture were treated with the nephrosis-inducing agent puromycin aminonucleoside (PAN). Analysis was made by TaqMan real-time PCR, Western blot analysis, and by metabolic labeling with [35S]sulfate. The HGECs express several PGs: syndecan, versican, glypican, perlecan, decorin, and biglycan, which may contribute to the glomerular charge barrier. PAN treatment downregulated both the protein expression (by 25%) and the mRNA expression (by 37 ± 6%, P < 0.001, n = 8) of versican compared with control. Transferases important for chondroitin and heparan sulfate biosynthesis were also significantly downregulated by PAN, resulting in less sulfate groups, shorter GAG chains, and reduced PG net-negative charge. Moreover, analysis of the cell media after PAN treatment revealed a reduced content of [35S]sulfate-labeled PGs (40% of control). We conclude that PAN may cause proteinuria by affecting the endothelial cell-surface layer and not only by disrupting the foot process arrangement of the podocytes. Thus the endothelium may be a more important component of the glomerular barrier than hitherto acknowledged.

Why do we not all have proteinuria? An update of our current understanding of the glomerular barrier

The key question is not why some patients have proteinuria but rather why not all people have it. In the present review, we will present an update on the glomerular barrier after the recent breakthroughs in podocyte biology. In particular, we will discuss the role of the endothelium, which seems to be a neglected part of the glomerular membrane.

Glomerular charge selectivity is impaired in hypertensive nephropathy

BACKGROUND

In hypertensive nephropathy the morphological changes and albuminuria seem to start in the inner cortex but the mechanism of proteinuria is unknown. We tested the hypothesis of reduced glomerular charge selectivity in the juxtamedullary cortex of old spontaneously hypertensive rats (SHR) as a cause of proteinuria in rats with hypertensive nephropathy.

METHODS

The glomerular charge barrier was evaluated in 80-week-old SHR and age-matched normotensive Wistar-Kyoto rats (WKY) by measuring the glomerular clearance of radiolabelled cationic and anionic chymotrypsinogen (Chym and aChym, MW 25,000) accumulated by the proximal tubular cells in the outer, middle and inner cortex following intravenous injection. The glomerular filtration rates (GFR) in the cortical zones were obtained by aprotinin (MW 6500) and the sieving coefficient (theta) of the protein tracers calculated as their glomerular clearance/GFR.

RESULTS

The theta aChym was similar in SHR and WKY except in the inner cortex where it was 35% higher in SHR (0.65+/-0.05) than in WKY (0.48+/-0.03) (P = 0.01). The ratio of theta aChym to theta Chym was increased from 0.55+/-0.06 to 0.77+/-0.05 (P<0.02) in the inner cortex of SHR, whereas theta Chym remained the same in all cortical zones. Finally, the percentage of glomeruli with adsorption droplets in podocytes quantified by light microscopy was higher in the inner than the outer cortex of SHR (P<0.05).

CONCLUSIONS

The study supports the theory of a functioning glomerular charge barrier. An increased relative clearance of aChym in the inner cortex of SHR indicates impairment of the charge barrier, which, at least in part, could explain the increased protein excretion in SHR with hypertensive nephropathy.

Characterization and analysis of thermal denaturation of antibodies by size exclusion high-performance liquid chromatography with quadruple detection

Size exclusion chromatography (SEC) coupled with online light scattering, viscometry, refractometry, and UV-visible spectroscopy provides a very powerful tool for studying protein size, shape, and aggregation. This technique can be used to determine the molecular weight of the component peaks independent of the retention times in the SEC column and simultaneously measure the hydrodynamic radius and polydispersity of the protein. We applied this technology by coupling an Agilent Chemstation high-performance liquid chromatography system with a diode array UV-visible detector and a Viscotek 300 EZ Pro triple detector (combination of a light scattering detector, refractometer, and differential pressure viscometer) to characterize and compare the molecular properties of a number of monoclonal antibodies. Our studies reveal that different monoclonal immunoglobulin Gs (IgGs) and chimeric IgGs show slightly different retention times and therefore different molecular weights in gel filtration analysis. However, when they are analyzed by light scattering, refractometry, and viscometry, different IgGs have comparable molecular weight, molecular homogeneity (polydispersity), and size. Gel filtration coupled with UV or refractive index detection suggests that antibodies purified and formulated for preclinical and clinical development are more than 95% monomer with little or no detectable soluble aggregates. Light scattering measurements showed the presence of trace amounts of soluble aggregate in all the IgG preparations. The different IgG molecules showed different susceptibility to heat and pH. One of the murine antibodies was considerably less stable than the others at 55 degrees C. The application of this powerful technology for the characterization of monoclonal antibodies of therapeutic potential is discussed.

Anomalous fractional clearance of negatively charged Ficoll relative to uncharged Ficoll

Recent studies, using low-temperature perfusion of rat kidneys, have claimed the existence of renal charge selectivity simply on the basis of the differential excretion rates of uncharged Ficoll and charged proteins. To test for the existence of charge selectivity in vivo, we examined the clearance of negatively charged Ficoll compared with uncharged Ficoll. A short-term approach to steady state was used to study the fractional clearances. Relative clearances were also examined using an osmotic pump technique where the tracers reach a steady-state value in conscious rats after 7 days. Carboxymethyl Ficoll was stable during filtration and renal passage, was not taken up by the kidneys, and did not bind to plasma proteins. There was no significant difference in the fractional clearance of molecules with radius of 36 A for Ficoll (fractional clearance = 0.048 +/- 0.038, n = 5) and negatively charged carboxymethyl Ficoll (fractional clearance = 0.028 +/- 0.019, n = 5). For molecules with radii greater than 36 A, carboxymethyl Ficoll had facilitated clearance with respect to uncharged Ficoll [for example, at a radius of 60 A fractional clearance for Ficoll = 0.0012 +/- 0.0005 (n = 5), whereas that for carboxymethyl Ficoll = 0.015 +/- 0.005 (n = 5)]. Renal function was not compromised by carboxymethyl Ficoll as uncharged Ficoll in urine exhibited similar hydrodynamic size profiles when studied in the presence of excess unlabeled carboxymethyl Ficoll. The facilitated clearance of negatively charged Ficoll with respect to uncharged Ficoll reveals a property of the capillary wall, which has been previously observed with other nonproteinaceous polyanions. This study demonstrates that the glomerular capillary wall is not charge selective in the form of excluding negatively charged Ficoll. However, the charge properties of the capillary wall may influence the facilitated transport of charged Ficoll compared with uncharged Ficoll.

Dynamic alterations of glomerular charge density in fixed rat kidneys suggest involvement of endothelial cell coat

In a previous paper, we found that low ionic strength (I) reversibly reduced the glomerular charge density, suggesting increased volume of the charge-selective barrier. Because glutaraldehyde makes most structures rigid, we considered the isolated, perfusion-fixed rat kidney to be an ideal model for further analysis. The fixed kidneys were perfused with albumin solutions containing FITC-Ficoll at two different Is (I = 151 and 34 mM). At normal I, the fractional clearance () for albumin was 0.0049 (SE -0.0017, +0.0027, n = 6), whereas for neutral Ficoll35.5A of similar size was significantly higher 0.104 (SE 0.010, n = 5, P < 0.001). At low I, for albumin was 0.0030 (SE -0.0011, +0.0018, n = 6, not significant from albumin at normal I) and for Ficoll35.5A was identical to that at normal I, 0.104 (SE 0.015, n = 6, P < 0.01 compared with albumin at low I). According to a heterogeneous charged fiber model, low I reduced the fiber density from 0.056 to 0.0315, suggesting a 78% gel volume expansion. We conclude that 1) there is a significant glomerular charge barrier. 2) Solutions with low I increase the volume of the charge barrier even in kidneys fixed with glutaraldehyde. Our findings suggest that polysaccharide-rich structures, such as the endothelial cell coat, are key components in the glomerular barrier.

Glomerular size and charge selectivity in the mouse after exposure to glucosaminoglycan-degrading enzymes

This is the first functional study of glomerular size and charge selectivity in mice. The aim was to investigate the controversial issue of glomerular permselectivity in animals exposed to glucosaminoglycan-degrading enzymes, hyaluronidase, and heparinase. Fractional clearances (theta) for FITC-Ficoll and albumin were estimated in isoflurane anesthetized mice in vivo and in cooled isolated perfused kidneys (cIPK). In cIPK, a significant increase of theta(albumin) from 0.0023 (95% confidence interval, 0.0014 to 0.0033) in controls to 0.0130 (95% confidence interval, 0.0055 to 0.0206) was seen after hyaluronidase treatment. The theta for neutral Ficoll of similar size as albumin was 0.063 to 0.093 in all groups. According to a heterogeneous charged fiber model, the fiber volume fraction of negatively charged fibers decreased by 10% after enzyme treatments. It is concluded that glomerular size and charge selectivity in mice is similar to that previously shown for rats. Moreover, hyaluronic acid, chondroitin sulfate, and heparan sulfate are of importance for charge selectivity.

Glomerular filtration rate dependence of sieving of albumin and some neutral proteins in rat kidneys

The size and charge-selective properties of the glomerular barrier are partly controversial. Glomerular sieving coefficients (theta) for proteins have rarely been determined noninvasively before in vivo. Therefore, theta was assessed vs. glomerular filtration rate (GFR; (51)Cr-EDTA clearance) in intact rats for radiolabeled myoglobin, kappa-dimer, neutral horseradish peroxidase (nHRP), neutral human serum albumin (nHSA), and native albumin (HSA). To obtain theta, glomerular tracer clearance, assessed from the 7- to 8-min kidney uptake of protein, was divided by the GFR. The data were fitted with a two-pore model of glomerular permeability, where the small-pore radius was 37.35 +/- 1.11 (SE) A, and the "unrestricted pore area over diffusion path length" (A(0)/DeltaX) 1.84 +/- 0.43 x 10(6) cm. Although seemingly horizontal for nHRP and nHSA, the log theta vs. GFR curves showed slightly negative slopes for the proteins investigated in the GFR interval of 2-4.5 ml/min. Strong negative (linear) correlations between (log) theta and GFR were obtained for myoglobin (P = 0.002) and HSA (P = 0.006), whereas they were relatively weak for nHRP and nHSA and nonsignificant for kappa-dimer. Theta for nHSA was markedly higher than that for HSA. In conclusion, there were no indications of increases in theta vs. GFR, as indicative of concentration polarization, for the proteins investigated at high GFRs. Furthermore, the glomerular small-pore radius assessed from endogenous (neutral) protein sieving data was found to be smaller than previously determined using dextran or Ficoll as test molecules.

Endothelial cell apoptosis during glomerular capillary lumen formation in vivo

Transforming growth factor-beta (TGF-beta) stimulates endothelial cell apoptosis in vitro, and inhibition of TGF-beta1 leads to retention of undifferentiated endothelial cells in developing glomerular capillaries and reduced lumen formation in vivo. This study explored the question whether glomerular capillary lumen formation in vivo may involve TGF-beta1-dependent endothelial cell apoptosis. Neutralizing anti-TGF-beta1 or non-immune IgY were infused into the renal arteries of 3-d-old rats, and the kidneys were examined 2 d later. By transmission electron microscopy, endocapillary apoptotic cells were observed at a frequency of 0.10/loop in immature glomeruli of 3-d-old rat pups. In 5-d-old rat pups given neutralizing TGF-beta1 antibody or control IgY, the frequency of endocapillary apoptotic cells was 0.03 and 0.09/loop, respectively (P < 0.001, chi(2)). Dual labeling with TUNEL and anti-von Willebrand factor (vWF) antibody showed that apoptotic cells in immature glomeruli of 5-d-old rat pups are endothelial cells. Quantitative analysis showed significantly fewer TUNEL/vWF-labeled cells in glomeruli after anti-TGF-beta1 antibody infusion than in controls. No endocapillary apoptotic cells were observed in any group in C-shaped or S-shaped bodies, and the TUNEL assay revealed no glomerular apoptotic cells in kidneys from mature rats. These findings suggest that superfluous endothelial cells are cleared from immature glomerular capillaries by apoptosis, a process regulated by TGF-beta1. Taken together with the previous finding, that TGF-beta1 blockade blunts glomerular capillary lumen formation in vivo, it is proposed that TGF-beta1-dependent apoptosis serves to open capillary lumens in this vascular bed during glomerular development.

Synthesis of sulfated proteoglycans by bovine glomerular endothelial cells in culture

It has been suggested that proteinuria is caused by alterations of the charge selectivity of the basement membrane and/or the epithelial cell layer (podocytes). However, recent findings suggest that the endothelial luminal surface coat, consisting of proteoglycans with their connected glycosaminoglycan (GAG) branches and glycoproteins, may contribute to the permselectivity. Therefore, we wanted to investigate the effects on endothelial GAG synthesis during normal and pathological conditions. We treated glomerular endothelial cell cultures with puromycin aminonucleoside (PAN, a nephrosis-inducing agent) or interleukin-1beta (IL-1beta) for a total of 72 h and compared the metabolic turnover and incorporation of [(35)S]sulfate during the last 2 days. In control cultures, the GAG content in the media supernatants increased 66 +/- 6% (mean +/- SE) between 12 and 42 h of incubation with radioactivity (P < 0.01, n = 8). The content of (35)S-labeled GAGs in the media was reduced by 31 +/- 1% by PAN (P < 0.001, n = 8) and increased by 141 +/- 15% by 10 U/ml IL-1beta (P < 0.01, n = 8). Treatment with enzymes revealed a dominance of heparan, chondroitin, and dermatan sulfate GAGs. Thus the glomerular endothelial cell production of GAGs was increased by IL-1beta and reduced by PAN. Therefore, it is conceivable that certain nephrotic conditions may be due to endothelial dysfunction, rather than other renal causes.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Enhanced glomerular permeability to macromolecules in the Nagase analbuminemic rat

Plasma albumin restricts capillary water filtration. Accordingly, the glomerular ultrafiltration coefficient is higher in Nagase analbuminemic rats (NAR) than in Sprague-Dawley controls. We investigated whether the glomerular permeability to macromolecules is also enhanced in NAR. SDS-PAGE fractionation of urine proteins showed several bands with molecular masses between 60 and 90 kDa in NAR only. Acute administration of BSA to NAR led to nearly complete disappearance of these proteins from urine, an effect partially reversed when most of the exogenous albumin was cleared from circulation. The fractional clearance of 70-kDa dextran was increased in NAR, indicating a size defect. Binding of cationized ferritin to the glomerular basement membrane was decreased in NAR, suggesting associated depletion of fixed anions. The magnitude of cationic ferritin binding correlated negatively with the fractional clearance of 70-kDa dextran, suggesting that the two abnormalities may share a common pathogenic mechanism. Collectively, these results suggest enhanced glomerular permeability to macromolecules in NAR. Albumin may be necessary to maintain the normal glomerular permselectivity properties.

Puromycin aminonucleoside damages the glomerular size barrier with minimal effects on charge density

Puromycin aminonucleoside (PAN) has been suggested to reduce glomerular charge density, to create large glomerular "leaks," or not to affect the glomerular barrier. Therefore, we analyzed glomerular charge and size selectivity in vivo and in isolated kidneys perfused at 8 degrees C (cIPK) in control and PAN-treated rats. The fractional clearances (theta) for albumin and Ficoll of similar hydrodynamic size were 0.0017 +/- 0.0004 and 0.15 +/- 0.02, respectively, in control cIPKs. Two-pore analysis gave similar results in vivo and in vitro, with small- and large-pore radii of 47-52 and 85-105 A, respectively, in controls. Puromycin increased the number of large pores 40-50 times, the total pore area over diffusion distance decreased by a factor of 25-30, and the small-pore radius increased by 33% (P < 0.001 for all comparisons of size selectivity and theta). The effect of PAN was less dramatic on the estimated wall charge density, which was 73% of that of controls. We conclude that puromycin effectively destroys the glomerular size barrier with minimal effects on charge density.