The rat glomerular filtration barrier does not show negative charge selectivity

New insights into proteinuria/albuminuria

The fractional clearance of proteins as measured in healthy human subjects increases 10,000–100,000- fold when studied in nephrotic patients. This remarkable increase cannot be accounted for by extracellular biophysical mechanisms centered at the glomerular filtration barrier. Rather, it is the nephron and its combination of filtration and cellular uptake that can provide a plausible explanation of these fractional clearance changes. The nephron has two regions that critically determine the level proteinuria/albuminuria. Glomerular filtration of plasma proteins is primarily a size selective event that is basically unchanged in acquired and genetic kidney disease. The glomerular concepts of ‘charge selectivity’ and of ‘large pores’, previously used to explain proteinuria, are now recognized to be flawed and non-existent. Filtered proteins then encounter downstream two protein receptors of the Park and Maack type associated with the proximal tubular cell. The high capacity receptor is thought to retrieve the majority of filtered proteins and return them to the blood supply. Inhibition/saturation of this pathway in kidney disease may create the nephrotic condition and hypoproteinemia/hypoalbuminemia. Inhibitors of this pathway (possibly podocyte derived) are still to be identified. A relatively small proportion of the filtered protein is directed towards a high affinity, low capacity receptor that guides the protein to undergo lysosomal degradation. Proteinuria in normoproteinemic states is derived by inhibition of this pathway, such as in diabetes. The combination of glomerular sieving, and the degradation and retrieval pathways can quantitatively account for the changes in fractional clearance of proteins in the nephrotic condition. Finally, the general retrieval of filtered protein by the proximal tubular cell focuses on the teleological importance of this cell as this retrieval represents the third pillar of retrieval that this cell participates in (it also retrieves water and salt).

Renal Handling of Albumin-From Early Findings to Current Concepts

Albumin is the main protein of blood plasma, lymph, cerebrospinal and interstitial fluid. The protein participates in a variety of important biological functions, such as maintenance of proper colloidal osmotic pressure, transport of important metabolites and antioxidant action. Synthesis of albumin takes place mainly in the liver, and its catabolism occurs mostly in vascular endothelium of muscle, skin and liver, as well as in the kidney tubular epithelium. Long-lasting investigation in this area has delineated the principal route of its catabolism involving glomerular filtration, tubular endocytic uptake via the multiligand scavenger receptor tandem—megalin and cubilin-amnionless complex, as well as lysosomal degradation to amino acids. However, the research of the last few decades indicates that also additional mechanisms may operate in this process to some extent. Direct uptake of albumin in glomerular podocytes via receptor for crystallizable region of immunoglobulins (neonatal FC receptor) was demonstrated. Additionally, luminal recycling of short peptides into the bloodstream and/or back into tubular lumen or transcytosis of whole molecules was suggested. The article discusses the molecular aspects of these processes and presents the major findings and controversies arising in the light of the research concerning the last decade. Their better characterization is essential for further research into pathophysiology of proteinuric renal failure and development of effective therapeutic strategies.

A distributed solute model: an extended two-pore model with application to the glomerular sieving of Ficoll

One of the many unresolved questions regarding the permeability of the glomerular filtration barrier is the reason behind the marked difference in permeability between albumin and polysaccharide probe molecules such as Ficoll and dextran of the same molecular size. Although the differences in permeability have been mainly attributed to charge effects, we have previously shown that this would require a highly charged filtration barrier, having a charge density that is ~10 times more than that on the albumin molecule. In this article, the classic two-pore model was extended by introducing size distributions on the solute molecules, making them conformationally flexible. Experimental sieving data for Ficoll from the rat glomerulus and from precision-made silicon nanopore membranes were analyzed using the model. For the rat glomerulus a small-pore radius of 36.2 Å and a geometric standard deviation (gSD) for the Ficoll size-distribution of 1.16 were obtained. For the nanopore membranes, a gSD of 1.24 and a small-pore radius of 43 Å were found. Interestingly, a variation of only ~16% in the size of the polysaccharide molecule is sufficient to explain the difference in permeability between albumin and Ficoll. Also, in line with previous data, the effects of applying a size distribution on the solute molecule are only evident when the molecular size is close to the pore size. Surely there is at least some variation in the pore radii, and, likely, the gSD obtained in the current study is an overestimation of the "true" variation in the size of the Ficoll molecule.

Are filtered plasma proteins processed in the same way by the kidney?

In order to understand the mechanism of albuminuria we have explored how other plasma proteins are processed by the kidney as compared to inert molecules like Ficolls. When fractional clearances are plotted versus protein radius there is a remarkable parallelism between protein (molecular weight range 30-150kDa) clearance in healthy controls, in Dent's disease, in nephrotic states and the clearance of Ficolls. Although there are significant differences in the levels of fractional clearances in these states. Dent's disease results in a 2-fold increase in the fractional clearance of proteins as compared to healthy controls whereas in nephrotic states there is a further 3-fold increase in fractional clearance. Previous thinking that albumin uptake was controlled primarily by the megalin/cubilin receptor does not explain the albumin urinary excretion data and is therefore an incorrect concept. Protein clearance in nephrotic states approach the fractional clearance of inert Ficolls for a given radius. It therefore appears that there are two pathways processing these proteins. A low capacity pathway associated with megalin/cubilin that degrades filtered protein (that is inhibited in Dent's disease) and a high capacity pathway that retrieves the filtered protein and returns it to the blood supply (without retrieval nephrotic protein excretion will occur and this will account for hypoproteinemia). On the other hand low molecular weight proteins (<20kDa) are processed entirely differently by the kidney. They are not retrieved but are comprehensively degraded in the kidney with the degradation products predominantly returned to the blood supply.

Comparative modeling of combined transport of water and graded-size molecules across the glomerular capillary wall

Chronic kidney disease is a common and growing problem worldwide that necessitates recognition of individual risk and appropriate laboratory testing before its progression to end-stage renal failure, requiring dialysis or transplantation for survival. Clearance studies using various graded-size probe molecules established that the passage of molecules/proteins across the glomerular capillary barrier of mammalian kidneys is increasingly restricted as their size increase. Few mathematical models were developed to describe the dynamics of the size-selective functions of macromolecules across membranes and gelatins. In the present study, we compare the behavior of three mathematical descriptions for the Fiber Matrix theory, an Extended Fiber Matrix theory, and an Alternative Statistical Physics analysis to describe the size-selective function of the glomerular capillary barrier; using mainly its hemodynamic, morphometric and hydrodynamic variables; in two experimental rat models. The glomerular basement membrane was represented as a homogeneous three-dimensional network of fibers of uniform length (Lf), radius (Rf), total fractional solid volume of fibers (Vf) and characteristic Darcy permeability. The models were appropriate for simulating in vivo fractional clearance data of neutral Dextran and Ficoll macromolecules from two experimental rat models. We believe that the Lf, Rf and Vf best-fit numerical values may signify new insights for the diagnosis of human nephropathies.

High risks of all-cause and cardiovascular deaths in apparently healthy middle-aged people with preserved glomerular filtration rate and albuminuria: A prospective cohort study

BACKGROUND

The reason why coexistence of preserved estimated glomerular filtration rate (eGFR) and albuminuria contributes to a high risk of death and which cause of death increases all-cause mortality have not been elucidated.

METHODS

A total of 16,759 participants aged 40 to 69 years with normal or mildly reduced eGFR (45-119 ml/min/1.73 m(2)) were enrolled and divided into six groups (group 1, eGFR: 90-119 without albuminuria; group 2, eGFR: 90-119 with albuminuria; group 3, eGFR: 60-89 without albuminuria (reference); group 4, eGFR: 60-89 with albuminuria; group 5, eGFR: 45-59 without albuminuria; group 6, eGFR: 45-59 with albuminuria) based on GFR estimated by using the CKD-EPI study equation modified by a Japanese coefficient and albuminuria (urine albumin-creatinine ratio ≥ 30 mg/g). Outcomes included all-cause death (ACD), cardiovascular death (CVD) and neoplasm-related death (NPD). Multivariable-adjusted mortality rate ratios (RR) and their 95% confidence intervals (CIs) in the groups were estimated by Poisson's regression analysis.

RESULTS

The highest risk of ACD (RR (95% CIs): 3.95 (2.08-7.52)), CVD (7.15 (2.25-22.7)) and NPB (3.25 (1.26-8.38)) was observed in group 2. Subjects in group 2 were relatively young and had the highest levels of body mass index, blood pressure and HbA1c and the highest prevalence of diabetes and metabolic syndrome.

CONCLUSION

Coexistence of preserved eGFR and albuminuria increases risks for ACD, CVD and NPD. Relatively young metabolic persons having both preserved eGFR and albuminuria should be considered as a very high-risk population.

Quantification of the electrostatic properties of the glomerular filtration barrier modeled as a charged fiber matrix separating anionic from neutral Ficoll

In the current study we explore the electrostatic interactions on the transport of anionic Ficoll (aFicoll) vs. neutral Ficoll (nFicoll) over the glomerular filtration barrier (GFB) modeled as a charged fiber matrix. We first analyze experimental sieving data for the rat glomerulus, and second, we explore some of the basic implications of a theoretical model for the electrostatic interactions between a charged solute and a charged fiber-matrix barrier. To explain the measured difference in glomerular transport between nFicoll and aFicoll (Axelsson J, Sverrisson K, Rippe A, Fissell W, Rippe B. Am J Physiol 301: F708-F712, 2011), the present simulations demonstrate that the surface charge density needed on a charged fiber matrix must lie between -0.005 C/m(2) and -0.019 C/m(2), depending on the surface charge density of the solute. This is in good agreement with known surface charge densities for many proteins in the body. In conclusion, the current results suggest that electrical charge makes a moderate contribution to glomerular permeability, while molecular size and conformation seem to be more important. Yet, the weak electrical charge obtained in this study can be predicted to nearly totally exclude albumin from permeating through "high-selectivity" pathways in a charged-fiber matrix of the GFB.

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.

Reduced diffusion of charge-modified, conformationally intact anionic Ficoll relative to neutral Ficoll across the rat glomerular filtration barrier in vivo

The glomerular filtration barrier (GFB) is commonly conceived as a negatively charged sieve to proteins. Recent studies, however, indicate that glomerular charge effects are small for anionic, carboxymethylated (CM) dextran vs. neutral dextran. Furthermore, two studies assessing the glomerular sieving coefficients (θ) for negative CM-Ficoll vs. native Ficoll have demonstrated an increased glomerular permeability for CM-Ficoll (Asgeirsson D, Venturoli D, Rippe B, Rippe C. Am J Physiol Renal Physiol 291: F1083-F1089, 2006; Guimarães M, Nikolovski J, Pratt L, Greive K, Comper W. Am Physiol Renal Physiol 285: F1118-F1124, 2003.). The CM-Ficoll used, however, showed a larger Stokes-Einstein radius (a(e)) than neutral Ficoll, and it was proposed that the introduction of negative charges in the Ficoll molecule had made it more flexible and permeable. Recently, a negative FITC-labeled CM-Ficoll (CMI-Ficoll) was produced with a conformation identical to that of neutral FITC-Ficoll. Using these probes, we determined their θ:s in anesthetized Wistar rats (259 ± 2.5 g). After blood access had been achieved, the left ureter was cannulated for urine sampling. Either polysaccharide was infused (iv) together with a filtration marker, and urine and plasma were collected. Assessment of θ FITC-Ficoll was achieved by high-performance size-exclusion chromatography (HPSEC). CMI-Ficoll and native Ficoll had identical elugrams on the HPSEC. Diffusion of anionic Ficoll was significantly reduced compared with that of neutral Ficoll across the GFB for molecules of a(e) ∼20-35 Å, while there were no charge effects for Ficoll of a(e) = 35-80 Å. The data are consistent with a charge effect present in "small pores," but not in "large pores," of the GFB and mimicked those obtained for anionic membranes in vitro for the same probes.

Permeability of Peritoneal and Glomerular Capillaries: What are the Differences According to Pore Theory?

Pore and fiber-matrix theory can both be used to model the peritoneal and glomerular filtration barriers in an attempt to shed light on their differing structure–function relationships. The glomerular filtration barrier (GFB) is structurally more specialized, morphologically complex, and also highly dynamic; but paradoxically, because of its uniformity, it conforms more closely to the predictions of pore theory than does the peritoneum, and it in fact resembles a more simple synthetic membrane. Compared with the peritoneal capillary wall, the GFB has no transcellular “third” pores (aquaporins), and it is far less leaky and more size-selective to proteins, mainly as a result of having far fewer “large” pores. It does have charge-selective properties, although these are considered much less important in excluding albumin than was once thought, and it is also able to select polymers according to their shape and flexibility. Even this property might reflect the relative uniformity of the GFB, which has a high diffusion area and short diffusion distances, compared with the peritoneal barrier, which behaves more like a gel filtration column. Furthermore, the length of the diffusion path across the peritoneal membrane is much greater for small solutes, given the relatively high ultrafiltration coefficient for that membrane compared with the GFB—a situation that reflects both the tortuosity of the interendothelial clefts and the distribution of peritoneal capillaries within the interstitium. These comparisons reveal the peritoneal barrier as a relatively complex structure to model; and yet this model may be more representative of the general microcirculation, and thus shed light on systemic endothelial function in renal failure.

Molecular conformation and filtration properties of anionic Ficoll

The physiology of glomerular permselectivity remains mechanistically obscure, despite its importance in human disease. Although electrical contributions to glomerular permselectivity have long been considered important, two recent reports demonstrated enhanced glomerular permeability to anionic versus neutral polysaccharides. The interpretation of these observations is complicated by confounding of the effects of chemical modification on charge with effects on size and shape. In this report, neutral and anionic Ficoll are characterized by size-exclusion chromatography with online light scattering and viscometry and filtration through a highly defined anionic filtration membrane. Neutral and carboxymethylated Ficoll are nearly identical in size and conformation, yet carboxymethylated Ficoll is retained by an anionic membrane in excess of neutral Ficoll. This suggests that comparisons between clearances of neutral and carboxymethylated Ficoll may be a sensitive probe of electrostatic interactions independent of size and conformation.

Renal FcRn reclaims albumin but facilitates elimination of IgG

The widely distributed neonatal Fc receptor (FcRn) contributes to maintaining serum levels of albumin and IgG in adults. In the kidney, FcRn is expressed on the podocytes and the brush border of the proximal tubular epithelium. Here, we evaluated the role of renal FcRn in albumin and IgG metabolism. Compared with wild-type controls, FcRn(-/-) mice had a lower t((1/2)) for albumin (28.7 versus 39.9 h) and IgG (29.5 versus 66.1 h). Renal loss of albumin could account for the former, suggested by the progressive development of hypoalbuminemia in wild-type mice transplanted with FcRn-deficient kidneys. Furthermore, serum albumin levels returned to normal in FcRn(-/-) recipients of wild-type kidneys after removing the native FcRn-deficient kidneys. In contrast, renal loss could not account for the enhanced elimination of IgG in FcRn(-/-) mice. These mice had minimal urinary excretion of native and labeled IgG, which increased to wild-type levels in FcRn(-/-) recipients of a single FcRn-sufficient kidney (t((1/2)) of IgG was 21.7 h). Taken together, these data suggest that renal FcRn reclaims albumin, thereby maintaining the serum concentration of albumin, but facilitates the loss of IgG from plasma protein pools.

Glomerular permeability to macromolecules in the Necturus kidney

Many aspects of the glomerular filtration of macromolecules remain controversial, including the location of the major filtration barrier, the effects of electrical charge, and the reason the filtration barrier does not clog. We examined these issues in anesthetized Necturus maculosus, using fluorescently labeled probes and a two-photon microscope. With the high resolution of this system and the extraordinary width ( approximately 3.5 mum) of the glomerular basement membrane (GBM) in this salamander, we were able to visualize fluorescent molecules in the GBM in vivo. GBM/plasma concentration ratios for myoglobin, ovalbumin, and serum albumin did not differ from that of inulin, indicating that the GBM does not discriminate among these molecules. The GBM/plasma concentration ratios for fluoresceinated dextran 500 and 2,000 kDa were significantly below that of inulin. Glomerular sieving coefficients (GSCs) for various macromolecules decreased as molecular mass increased, and the GSCs for bovine or human serum albumin were extremely low. The effect of electrical charge on filterability of a macromolecule was also examined. The GSCs for native (anionic) and neutral human serum albumin were not significantly different, nor did GSCs for anionic and neutral dextran 40 kDa differ, indicating that charge has no detectable effect on filterability of these macromolecules. These studies indicate that the main filtration barrier to albumin is the podocyte slit diaphragm. Electron microscopic studies revealed many cell processes within the GBM. Macromolecules that penetrated the GBM were taken up by mesangial cells and endothelial cells, suggesting that these cells help to prevent clogging of the filter.

Where does albuminuria come from in diabetic kidney disease?

The classic mechanism to explain albumin excretion in diabetes has been permeability defects in the glomerular filter. However, a new concept has emerged that albuminuria can be explained by the two major pathways the proximal tubular cell uses to process filtered albumin. Specifically, albumin permeability through the glomerular filter is only governed by size selectivity. Most of the filtered albumin is retrieved by the proximal tubular cell and returned to the peritubular blood supply. Albuminuria in the nephrotic range would arise from retrieval pathway dysfunction. The small quantities of filtered albumin that are not retrieved undergo obligatory lysosomal degradation before urinary excretion as small peptide fragments. This pathway is sensitive to metabolic factors responsible for hypertrophy and fibrosis, particularly molecules such as angiotensin II and transforming growth factor-beta1, whose production is stimulated by hyperglycemic environments. Dysfunction in this degradation pathway may lead to albuminuria below the nephrotic range.

Importance of low-grade albuminuria

The well-described association between chronic kidney disease and cardiovascular disease is typically thought to originate from loss of renal function, as estimated by the glomerular filtration rate. However, recent data suggest that urinary albumin excretion has an important role in this association. Albuminuria is a marker of underlying vascular dysfunction and has been correlated with structural and functional integrity of the vasculature. Although the traditional upper limit of normal daily albumin excretion has been 30 mg/d, recent epidemiologic data suggest that levels in the general population are actually much lower. Further, within this range of low-grade albuminuria (LGA), increasing excretion rates are associated with increasing risk of cardiovascular disease. This association is independent of renal function, and in the earliest stages of chronic kidney disease, LGA seems to be a more important determinant than the glomerular filtration rate. This emerging association underscores the complexity of albumin excretion, in which subtle changes in albumin excretion reflect widespread vascular processes. Using the key words albuminuria, low-grade albuminuria, and microalbuminuria in a PubMed search of literature from January 1, 1995, to February 29, 2008, this review summarizes the most recent data on LGA and its association with cardiovascular and renal disease.

Disease-dependent mechanisms of albuminuria

The mechanism of albuminuria is perhaps one of the most complex yet important questions in renal physiology today. Recent studies have directly demonstrated that the normal glomerulus filters substantial amounts of albumin and that charge selectivity plays little or no role in preventing this process. This filtered albumin is then processed by proximal tubular cells by two distinct pathways; dysfunction in either one of these pathways gives rise to discrete forms of albuminuria. Most of the filtered albumin is returned to the peritubular blood supply by a retrieval pathway. Albuminuria in the nephrotic range would arise from retrieval pathway dysfunction. The small quantities of filtered albumin that are not retrieved undergo obligatory lysosomal degradation before urinary excretion as small peptide fragments. This degradation pathway is sensitive to metabolic factors responsible for hypertrophy and fibrosis, particularly molecules such as angiotensin II and transforming growth factor-beta1, whose production is stimulated by hyperglycemic and hypertensive environments. Dysfunction in this degradation pathway leads to albuminuria below the nephrotic range. These new insights into albumin filtration and processing argue for a reassessment of the role of podocytes and the slit diaphragm as major direct determinants governing albuminuria, provide information on how glomerular morphology and "tubular" albuminuria may be interrelated, and offer a new rationale for drug development.

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.

Controversies in nephrology: Response to ‘renal albumin handling, facts, and artifacts’

For 40 years indirect measurements of the glomerular sieving coefficient of albumin yielded very low values. The first direct measurement by 2-photon microscopy by Russo et al (Kidney Int (2007) 71, 504-513) gives values 50-times higher. This demonstrated that relatively large quantities of albumin are normally filtered based on size selectivity alone. Most of this albumin is retrieved and returned to the blood supply. These new discoveries represent a paradigm shift in our understanding of albumin processing by the kidney. They also serve to explain several anomalous aspects of previous studies on glomerular filtration and mechanism of albuminuria and support the fact that glomerular charge selectivity is not a major factor controlling glomerular permselectivity.

Evidence for a role of transforming growth factor (TGF)-beta1 in the induction of postglomerular albuminuria in diabetic nephropathy: amelioration by soluble TGF-beta type II receptor

Transforming growth factor-beta (TGF-beta) has previously been implicated in the progression of diabetic nephropathy, including the onset of fibrosis and albuminuria. Here we report for the first time the use of a high-affinity TGF-beta1 binding molecule, the soluble human TGF-beta type II receptor (sTbetaRII.Fc), in the treatment of diabetic nephropathy in 12-week streptozotocin-induced diabetic Sprague-Dawley rats. In vitro studies using immortalized rat proximal tubule cells revealed that 50 pmol/l TGF-beta1 disrupted albumin uptake (P < 0.001 vs. control), an inhibition significantly reversed by the use of the sTbetaRII.Fc (1,200 pmol/l). In vivo studies demonstrated that treatment with sTbetaRII.Fc reduced urinary albumin excretion by 36% at 4 weeks, 59% at 8 weeks (P < 0.001), and 45% at 12 weeks (P < 0.01 for diabetic vs. treated). This was correlated with an increase in megalin expression (P < 0.05 for diabetic vs. treated) and a reduction in collagen IV expression following sTbetaRII.Fc treatment (P < 0.001 for diabetic vs. treated). These changes occurred independently of changes in blood glucose levels. This study demonstrates that the sTbetaRII.Fc is a potential new agent for the treatment of fibrosis and albuminuria in diabetic nephropathy and may reduce albuminuria by reducing TGF-beta1-induced disruptions of renal proximal tubule cell uptake of albumin.

The normal kidney filters nephrotic levels of albumin retrieved by proximal tubule cells: retrieval is disrupted in nephrotic states

The origin of albuminuria remains controversial owing to difficulties in quantifying the actual amount of albumin filtered by the kidney. Here we use fluorescently labeled albumin, together with the powerful technique of intravital 2-photon microscopy to show that renal albumin filtration in non-proteinuric rats is approximately 50 times greater than previously measured and is followed by rapid endocytosis into proximal tubule cells (PTCs). The endocytosed albumin appears to undergo transcytosis in large vesicles (500 nm in diameter), identified by immunogold staining of endogenous albumin by electron microscopy, to the basolateral membrane where the albumin is disgorged back to the peritubular blood supply. In nephrotic rats, the rate of uptake of albumin by the proximal tubule (PT) is decreased. This is consistent with reduced expression of clathrin, megalin, and vacuolar H(+)-ATPase A subunit, proteins that are critical components of the PT endocytotic machinery. These findings strongly support the paradigm-shifting concept that the glomerular filter normally leaks albumin at nephrotic levels. Albuminuria does not occur as this filtered albumin load is avidly bound and retrieved by PTCs. Dysfunction of this retrieval pathway leads to albuminuria. Thus, restoration of the defective endocytotic and processing function of PT epithelial cells might represent an effective strategy to limit urinary albumin loss, at least in some types of nephrotic syndrome.

Increased glomerular permeability to negatively charged Ficoll relative to neutral Ficoll in rats

It is established that the glomerular filter sieves macromolecules based on their size, shape, and charge. Anionic proteins are thus retarded compared with their neutral or cationic counterparts. However, recent studies have indicated that charge effects are small, or even “anomalous,” for polysaccharides. We therefore investigated the impact of charge on the glomerular permeability to polysaccharides by comparing sieving coefficients (θ; primary urine-to-plasma concentration ratio) for negatively charged, carboxymethylated (CM) FITC-Ficoll and FITC-dextran with their neutral counterparts. For these probes, θ were determined in anesthetized Wistar rats [269 ± 2.7 g (±SE; n = 36)], whose ureters were cannulated for urine sampling. The glomerular filtration rate was assessed using FITC-inulin. Polysaccharides were constantly infused, and after equilibration, urine was collected and a midpoint plasma sample was taken. Size and concentration determinations of the FITC-labeled polysaccharides were achieved by size-exclusion HPLC (HPSEC). For CM-Ficoll, θ was significantly increased (32 times at 55 Å) compared with that of uncharged Ficoll. A small increase in θ for CM-dextran compared with neutral dextran was also observed (1.8 times at 55 Å). In conclusion, negatively charged Ficoll relative to neutral Ficoll was found to be markedly hyperpermeable across the glomerular filter. Furthermore, negatively charged Ficoll was observed to be larger on HPSEC compared with its neutral counterpart of the same molecular weight. It is proposed that the introduction of negative charges in the “dendrimeric,” cross-linked Ficoll molecule may alter its configuration, so as to make it more extended, and conceivably, more flexible, thereby increasing its glomerular permeability.

Nature of glomerular capillary permeability changes following acute renal ischemia-reperfusion injury in rats

This study was performed to evaluate the alterations of glomerular filtration barrier characteristics following acute renal ischemia-reperfusion (I/R). Ischemia was induced in anesthetized rats by unilateral renal artery occlusion for either 20 or 60 min, followed by reperfusion during 20 or 60 min, respectively, with the contralateral kidney serving as control. Sieving coefficients (theta) were obtained by analyzing Ficoll [mol.radius (a(e)) 13-85 A] in urine and plasma after 20 and 60 min I/R. Furthermore, theta for human serum albumin (HSA) was estimated using a tissue uptake technique after 20 and 60 min of I/R, while clearance of HSA compared with that for neutralized HSA (nHSA) was assessed after 20 min of I/R only. Glomerular filtration rate (GFR) was measured by [51Cr]EDTA and inulin. I/R reduced GFR and increased theta for Ficoll molecules of a(e) >55 A and theta for albumin. theta for Ficoll vs. a(e), analysed using a two-pore model, demonstrated that, despite increases in theta, the large-pore fractional ultrafiltration coefficient (alpha(L)) was unchanged after 20 min of I/R, owing to the decline in GFR, but increased after 60 min of I/R. However, the apparent alpha(L) for albumin increased already after 20 min of I/R (P < 0.005) and the nHSA/HSA clearance ratio was slightly reduced, possibly reflecting a diminished negative charge barrier. In conclusion, after 20 min of I/R, indications of a reduced charge selectivity were noted, while after 60 min of I/R, there was mainly a reduction in size selectivity, compatible with an increased formation of large pores.

Reactive oxygen species mediate modification of glycocalyx during ischemia-reperfusion injury

The glycocalyx (Gcx) is a complex and poorly understood structure covering the luminal surface of endothelial cells. It is known to be a determinant of vascular rheology and permeability and may be a key control site for the vascular injuries caused by ischemia-reperfusion (I/R). We used intravital-microscopy to evaluate the effects of I/R injury on two properties of Gcx in mouse cremasteric microvessels: exclusion of macromolecules (anionic-dextrans) and intracapillary distribution of red blood cells (RBC). In this model, the Gcx is rapidly modified by I/R injury with an increase in 70-kDa anionic-dextran penetration without measurable effect on the penetration of 580-kDa anionic-dextran or on RBC exclusion. The effects of I/R injury appear to be mediated by the rapid production of reactive oxygen species (ROS) because they are ameliorated by the addition of exogenous superoxide dismutase-catalase. Intravenous application of allopurinol or heparin also inhibited the effects of I/R injury, and we interpret efficacy of allopurinol as evidence for a role for xanthine-oxidoreductase (XOR) in the response to I/R injury. Heparin, which is hypothesized to displace XOR from a heparin-binding domain in the Gcx, reduced the effects of I/R. The effects of I/R injury were also partially prevented or fully reversed by the intravascular infusion of exogenous hyaluronan. These data demonstrate: 1) the liability of Gcx during I/R injury; 2) the importance of locally produced ROS in the injury to Gcx; and 3) the potential importance of heparin-binding sites in modulating the ROS production. Our findings further highlight the relations between glycosaminoglycans and the pathophysiology of Gcx in vivo.

Quantitative microscopic approaches for studying kidney functions

Intravital ratiometric microscopy is a powerful method for quantitative study of kidney functions. As demonstrated in the examples, both the generalized polarity and direct ratio imaging approaches allow investigators to address basic and important questions such as those related to filtration, permeability and reabsorption. Both approaches have similar advantageous for in vivo imaging. However, they have different sensitivities and responses to the component intensity changes. Therefore, investigators must design their experiments based on the characteristics of the generalized polarity function and direct ratio approach.

Microalbuminuria is associated with impaired glomerular permselectivity in lymphoma patients

Slightly increased urinary albumin excretion is frequently found in patients with lymphoma and other malignancies but the pathophysiological mechanisms have yet to be clarified. In this study, parameters of renal function in lymphoma patients with microalbuminuria were evaluated. Sixty-seven patients with histologically proven diffuse large B-cell lymphoma were included in the study at diagnosis. Urinary albumin excretion was measured by immunoturbidimetry and microalbuminuria was defined as an excretion rate between 20 and 200 microg/min. Glomerular function was further estimated by renal clearance of creatinine and IgG, and the IgG/IgG4 charge selectivity index. Tubular function was evaluated by renal clearance of beta(2)-microglobulin. The median value of IgG clearance was increased in the microalbuminuric patients (0.22 versus 0.18 microl/min; p = 0.03). The median selectivity index was significantly lower in patients with microalbuminuria (1.0 versus 2.2; p<0.0001). Urinary albumin excretion was correlated with both the renal clearance of IgG (p<0.0001) and the selectivity index (p<0.0001). These data suggest that a slightly elevated level of urinary albumin excretion in a population of patients with aggressive lymphoma reflects altered glomerular permselectivity probably due to a defect in charge selectivity. The glomerular sieving dysfunction may be associated with an inflammatory response to the malignancy. Further studies are needed to validate the clinical impact of the renal parameters in lymphoma patients.

Quantitative intravital microscopy using a Generalized Polarity concept for kidney studies

In this article, we describe a ratiometric intravital two-photon microscopy technique for studying glomerular permeability and differences in proximal tubule cell reabsorption. This quantitative approach is based on the Generalized Polarity (GP) concept, in which the intensity difference between two fluorescent molecules is normalized to the total intensity produced by the two dyes. After an initial intravenous injection of a mixture of 3-, 40-, and 70-kDa fluorescently labeled dextrans into live Munich-Wistar-Frömter (MWF) rats, we were able to monitor changes in the GP values between any two dyes within local regions of the kidney, including the glomerulus, Bowman's capsule, proximal tubule lumens and proximal tubule cells, and individual capillary vessels. We were able to quantify accumulations of different dextrans in the Bowman's space and in tubular lumens as well as reabsorption by proximal tubular cells at different time points in the same rat. We found that for 6- to 8-wk-old MWF rats that developed spontaneous albuminuria, the 40- and 70-kDa dextrans, with hydrodynamic radii larger than albumin, were differentially filtered, but both were able to pass the glomerular filtration barrier and enter into the urinary space of the Bowman's capsule within a few seconds after intravenous infusion. Using GP image analysis, we found that negatively charged dextrans of both 40 and 70 kDa were better reabsorbed by the proximal tubule cells than the neutrally charged 40-kDa dextran. These results demonstrate the potential power of the GP imaging technique for quantitative studies of glomerular filtration and tubular reabsorption.

Reduced urinary excretion of sulfated polysaccharides in diabetic rats

The aim of the present study was to further understand the changes in renal filtration that occur in the early stages of diabetes mellitus. Diabetes was induced in male Wistar rats by a single injection of streptozotocin. Glycemia, body weight, 24-h urine volume and urinary excretion of creatinine, protein and glycosaminoglycans were measured 10 and 30 days after diabetes induction. All the diabetic animals used in the present study were hyperglycemic, did not gain weight, and presented proteinuria and creatinine hyperfiltration. In contrast, the glycosaminoglycan excretion decreased. Dextran sulfates of different molecular weights (6.0 to 11.5 kDa) were administered to the diabetic rats, and to age-matched, sham-treated controls. Most of the dextran sulfate was excreted during the first 24 h, and the amounts excreted in the urine were inversely proportional to the dextran sulfate molecular weight for all groups. Nevertheless, diabetic rats excreted less and accumulated more dextran sulfate in kidney and liver, as compared to controls. These differences, which were observed only for the dextran sulfates of higher molecular weights (>7 kDa), increased with the duration of diabetes. Our findings suggest differential renal processing mechanisms for proteins and sulfated polysaccharides, with the possible involvement of kidney cells.

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.

Mechanism of hypoalbuminemia in rodents

Normal albumin loss from the plasma is thought to be minimized by a number of mechanisms, including charge repulsion with the capillary wall and an intracellular rescue pathway involving the major histocompatibility complex-related Fc receptor (FcRn)-mediated mechanism. This study investigates how these factors may influence the mechanism of hypoalbuminemia. Hypoalbuminemia in rats was induced by treatment with puromycin aminonucleoside (PA). To test the effects of PA on capillary wall permeability, plasma elimination rates were determined for tritium-labeled tracers of different-sized Ficolls, negatively charged Ficolls, and (14)C-labeled tracer of albumin in control and PA-treated Sprague-Dawley rats. Urinary excretion and tissue uptake were also measured. Hypoalbuminemia was also examined in two strains of FcRn-deficient mice: beta(2)-microglobulin (beta(2)M) knockout (KO) mice and FcRn alpha-chain KO mice. The excretion rates of albumin and albumin-derived fragments were measured. PA-induced hypoalbuminemia was associated with a 2.5-fold increase in the plasma elimination rate of albumin. This increase could be completely accounted for by the increase in urinary albumin excretion. Changes in the permeability of the capillary wall were not apparent, inasmuch as there was no comparable increase in the plasma elimination rate of 36- to 85-A Ficoll or negatively charged 50- to 80-A Ficoll. In contrast, hypoalbuminemic states in beta(2)M and FcRn KO mice were associated with decreases in excretion of albumin and albumin-derived fragments. This demonstrates that the mechanism of hypoalbuminemia consists of at least two distinct forms: one specifically associated with the renal handling of albumin and the other mediated by systemic processes.

Urinary clearance of albumin is critically determined by its tertiary structure

The excretion of serum albumin in the urine is considered the net result of renal glomerular filtration and tubular uptake. During routine experiments, we observed that a batch of tritium-labeled albumin yielded anomalous results, being excreted in the urine of isolated perfused kidneys at 10 times the rate of normal tritiated albumin. This anomalous albumin, when simultaneously studied with normal carbon 14-labeled albumin, exhibited 10 times greater excretion than normal [(14)C]albumin. Anomalous albumin could not be reversed to normal albumin by means of conditioning with blood. In vivo clearances of anomalous albumin could not be quantitated because anomalous albumin is degraded during circulation. Anomalous albumin appeared to have the same molecular size (as determined with sodium dodecyl sulfate-polyacrylamide gel electrophoresis, capillary electrophoresis, and gel chromatography) and isoelectric-point profile (2-dimensional electrophresis) as normal albumin. Normal albumin could be transformed to anomalous albumin with alkali/heat treatment. Reverse-phase high-pressure liquid chromatography analysis of fragments from tryptic digests of anomalous albumin, alkali/heat-treated albumin, and normal albumin suggest that anomalous albumin and alkali/heat-treated albumin have altered tertiary structure, possibly as a result of denaturation and disulfide exchange. These studies show that the tertiary structure of albumin, beyond simple size and charge, is a critical determinant for albumin processing by the kidney and suggest that a specific albumin-recognition event by the kidneys is critical to normal renal handling of albumin.