Distribution of endogenous albumin in the glomerular wall of streptozotocin-induced diabetic rats as revealed by high-resolution immunocytochemistry

Correlated Endothelial Caveolin Overexpression and Increased Transcytosis in Experimental Diabetes

We investigated the mechanism by which diabetes renders the capillary endothelium more permeable to macromolecules in the lungs of short-term diabetic rats. We used quantitative immunocytochemistry (ICC) to comparatively assess the permeability of alveolar capillaries to serum albumin in diabetic and normoglycemic animals. The effect of diabetes on the population of endothelial caveolae was evaluated by morphometry and by ICC and immunochemical quantification of the amount of caveolin in the whole cell or associated with the purified endothelial plasma membrane. A net increase in the amount of serum albumin taken up by the plasmalemmal vesicles of alveolar endothelial cells and transported to the interstitium was documented in diabetic animals. Interendothelial junctions were not permeated by albumin molecules. The alveolar endothelial cells of hyperglycemic rats contain more caveolae (1.3-fold), accounting for a larger (1.5-fold) fraction of the endothelial volume than those of normal animals. The hypertrophy of the caveolar compartment is accompanied by overexpression of endothelial caveolin 1. Although the aggregated thickness of the endothelial and alveolar epithelium basement membranes increases in diabetes (1.3-fold), the porosity of this structure appears to be unchanged. Capillary hyperpermeability to plasma macromolecules recorded in the early phase of diabetes is explained by an intensification of transendothelial vesicular transport and not by the destabilization of the interendothelial junctions.

Psammomys obesus, a particularly important animal model for the study of the human diabetic nephropathy

The Psammomys obesus lives in natural desert habitat on low energy (LE) diet, however when maintained in laboratory conditions with high energy (HE) diet it exhibits pathological metabolic changes resembling those of type 2 diabetes. We have evaluated and correlated the histopathology, metabolic and functional renal alterations occurring in the diabetic Psammomys. Renal function determined by measuring glomerular filtration rate (GFR), protein excretion, protein/creatinine ratio and morpho-immunocytochemical evaluations were performed on HE diet diabetic animals and compared to LE diet control animals. The diabetic animals present a 54% increase in GFR after one month of hyperglycemic condition and a decrease of 47% from baseline values after 4 months. Protein excretion in diabetic animals was 5 folds increased after 4 months. Light microscopy showed an increase in glomeruli size in the diabetic Psammomys, and electron microscopy and immunocytochemical quantitative evaluations revealed accumulation of basement membrane material as well as frequent splitting of the glomerular basement membrane. In addition, glycogen-filled Armanni-Ebstein clear cells were found in the distal tubules including the thick ascending limbs of the diabetic animals. These renal complications in the Psammomys, including changes in GFR with massive proteinuria sustained by physiological and histopathological changes, are very similar to the diabetic nephropathy in human. The Psamommys obesus represents therefore a reliable animal model of diabetic nephropathy.

Glomerular CD34 expression in short- and long-term diabetes

Aging and diabetes are associated with exacerbated expression of adhesion molecules. Given their importance in endothelial dysfunction and their possible involvement in the alteration of glomerular permeability occurring in diabetes, we have evaluated expression of the sialomucin-type adhesion molecule CD34 in renal glomerular cells of normal and diabetic animals at two different ages by colloidal gold immunocytochemistry and immunoblotting. CD34 labeling was mostly assigned to the plasma membranes of glomerular endothelium and mesangial processes. Podocyte membranes were also labeled, but to a lesser degree. Short- and long-term diabetes triggers a substantial increase in immunogold labeling for CD34 in renal tissues compared with young normoglycemic animals. However, the level of labeling in old diabetic and healthy control rats is similar, suggesting that the effect of diabetes and aging on CD34 expression is similar but not synergistic. Western blotting of isolated glomerular fractions corroborated immunocytochemical results. Increased expression of CD34 may reflect its involvement in the pathogenesis of glomerular alterations related to age and diabetes. Alterations present in early diabetes, resembling those occurring with age, strengthen the concept that diabetes is an accelerated form of aging.

Expression and localization of MT1-MMP and furin in the glomerular wall of short- and long-term diabetic rats

Diabetic glomerulopathy has been linked to shifts in balance between the synthetic and degradative pathways of the glomerular basement membrane (GBM), a key player in the permselectivity properties of the glomerular wall. The goal of this study was to trace the expression and localization of membrane type-1 metalloprotease (MT1-MMP) and its activating enzyme furin, key proteins involved in basement membrane turnover, in short- and long-term diabetic rat renal tissues. Quantitative immunogold was carried out for MT1-MMP and furin and their expression was evaluated in renal tissues of young and old, control and diabetic rats. To corroborate immunocytochemical findings, Western blots were performed on glomerular lysates. Electron microscopy revealed that the overall expression of MT1-MMP and furin is reduced in plasma membranes of all glomerular cell types of old normoglycemic animals, a phenomenon that is exacerbated in long-term diabetic animals. This observation supports the prevailing theory that diabetes fosters acceleration in the aging process. Interestingly, while biochemical results confirmed a decrease in MT1-MMP expression, an increase in furin was observed. Immunocytochemical studies resolved this discrepancy by tracing the increased furin expression in endoplasmic reticulum and Golgi membranes of podocytes, indicating that furin is retained in the secretory pathway in a diabetic environment. Disturbances at the molecular level of the otherwise tightly regulated MT1-MMP/furin interactions found at the cell surface must account for a lack in extracellular matrix remodeling, increased deposition of GBM material, and loss of glomerular filtration integrity.

Glomerular handling of native albumin in the presence of circulating modified albumins by the normal rat kidney

Persistent hyperglycemia, as occurring in diabetes, induces changes in circulating as well as in structural proteins. These changes involve substitution of lysine residues by glucose adducts resulting in early Amadori products that evolve into toxic and active substances, the advanced glycation end adducts. In previous studies, we demonstrated that early glycated (Amadori) albumin infused into the circulation of normal animals induces transitory alterations of glomerular filtration. Attempting to elucidate the mechanisms underlying these changes, various molecular modifications were introduced in vitro to serum albumin. Glycation, acetylation, carboxymethylation, methylation, and succinylation, involving either a few or a significant number of amino acid residues, produced heavier and more anionic albumin molecules compared with the native one. Native and each of the modified albumin molecules were injected intravenously into normal rats, followed, 30 min later, by hapten-tagged native BSA. Changes in glomerular filtration were evaluated by morphometrical analysis of gold immunolabelings. Compared with native albumin, all the modified forms of albumin induced a deeper penetration of the tracer through the glomerular basement membrane revealing alterations in glomerular permselectivity. This was more evident for severely modified albumin molecules which displayed high labelings in the urinary space and endocytic compartments of proximal tubule epithelial cells. These results indicate that modifications of serum albumin, even minimal, as those occurring in early diabetes, could immediately affect the permselectivity properties of the glomerular wall leading, with time, to severe glomerulopathies.

Morphofunctional studies of the glomerular wall in mice lacking entactin-1

The architecture of the basement membranes is essential for proper function. This architecture is based on interactions among its components, which assemble in a complex network. Entactin-1 appears to be the mastermind of this assembling. In entactin-1-null transgenic mice, immunocytochemistry established the absence of entactin-1 in the glomerular basement membrane, and morphological thickening of this membrane was demonstrated. This prompted us to investigate the organization of other components of the glomerular basement membrane in the transgenic animals. The distribution of type IV collagen and laminin remained unchanged, whereas that of anionic charges was significantly altered. We also evaluated the impact of the absence of entactin-1 on cell relays by studying the alpha(3)- and the alpha(v)-integrins along the endothelial and epithelial glomerular cell plasma membranes. Only the density of alpha(v) was found to be increased. Finally, the filtration properties of the glomerular wall were evaluated by revealing endogenous albumin distribution across the basement membrane. This was altered in transgenic animals, suggesting changes in permselectivity properties. Entactin-1 appears to be an essential component in basement membranes because its absence appears to modify the molecular organization leading to alterations in functional properties.

Morphological and cytochemical aspects of capillary permeability

Transport of plasma soluble constituents across the capillary wall is of primordial importance in cardiovascular physiology. While physiological experiments have concluded with the existence of two sets of pores, a large one responsible for the transport of proteins and a small one designed for the diffusion of small solutes, the morphological counterparts have yet to get general agreement. In this review, we present the different proposed paths within and between the endothelial cells that do allow passage of plasma constituents and may respond to the definitions established by physiological means. The vesicular system existing in endothelial cells has been the first transendothelial path to be proposed. Several data have demonstrated the involvement of this system in transport, although others have systematically brought controversy. One alternative to the vesicles has been the demonstration of membrane-bound tubules creating, in certain cases, transendothelial channels that would allow diffusion of plasma proteins and other constituents across the capillary wall. Access to this tubulo-vesicular system could be restrained by the stomatal diaphragm and facilitated by specific membrane receptors. Further, we have demonstrated for the first time with morpho-cytochemical tools, that the intercellular clefts are the site of diffusion for small molecules such as peptides having a molecular weight inferior to 3,000. For the fenestrated capillary bed, we have shown that fenestrae are the site through which plasma constituents cross the capillary wall. However, and in spite of the existence of these large open pores, the endothelial cells still display the tubulo-vesicular system involved in transport of large molecules and their intercellular clefts are also the site of diffusion of small molecules. Making consensus on the existence of an intracellular tubulo-vesicular system in non-fenestrated capillaries, responsible for the transport of large molecules by the endothelial cells, and understanding the rational for the fenestrated capillary to have three paths for transport--the fenestrae, the tubulo-vesicular system, and the inter-endothelial clefts--require further investigation.

Temporary effects of circulating Amadori products on glomerular filtration properties in the normal mouse

Previous studies have established a preferential glomerular filtration of glycated BSA (gBSA), as well as a facilitated filtration of BSA in the presence of gBSA. We intend to determine whether these modifications are permanent or transitory. gBSA was intravenously injected into anesthetized normal mice and maintained in circulation for 30 min, 1, 2, 24, and 48 h. Five minutes before death, FITC-BSA was injected. On immunocytochemical evaluations, increased glomerular filtration of FITC-BSA was found at all circulating time points. Changes at 24 and 48 h were less pronounced. Glomerular basement membrane (GBM)-to-lumen gBSA labeling ratios were similar at all time points suggesting no accumulation of gBSA in the GBM. Seventy percent of the gBSA was cleared from the circulation and the GBM after 24 h, and 95% after 48 h. This was confirmed in experiments with radiolabeled tracers. These results suggest that the alteration in GBM permeability to BSA in the normal mouse are due to the presence of gBSA and are gradually overcome along with its clearance from circulation. In early diabetes, increasing concentrations of circulating glycated proteins could be responsible for changes in glomerular permselectivity and probably for the alteration in glomerular filtration properties leading to diabetic nephropathy.

Alterations of the rat mesentery vasculature in experimental diabetes

The alteration induced by diabetes on vascular permeability to serum albumin was investigated in the mesentery of streptozotocin-induced hyperglycemic rats. Double-tagged ((125)I and dinitrophenol-haptenated) heterologous albumin was intravenously administered in normal and hyperglycemic animals, and the extravasation of the tracer was evaluated by radioactivity measurements and by morphometry at the ultrastructural level using quantitative protein A-colloidal gold immunocytochemistry. The results demonstrate that diabetes induces a significant increase in the permeability of the mesentery vessels to albumin. This increase is due to a more efficient transport of macromolecules by endothelial plasmalemmal vesicles and not to leakier interendothelial junctions. Passage across the endothelial basement membranes did not appear to be restricted in either the control or diabetic condition. However, in diabetes, the mesothelial basement membrane appeared to become modified and to restrain the passage of albumin toward the peritoneal cavity. After 3 months of diabetes, the rats presented a net increase in the average diameter of the blood vessels localized in the mesentery arcada (macrovascular hyperplasy) and a notable angiogenesis, manifested at the level of the microvasculature in the mesenteric windows.

Glomerular size and charge selectivity in the rat as revealed by FITC-ficoll and albumin

The fractional clearances (theta) for FITC-Ficoll and albumin were estimated in isolated perfused rat kidneys in which the tubular activity was inhibited by low temperature (8 degrees C) and/or 10 mM NH(4)Cl. The Ficoll data were analyzed according to a two-pore model giving small and large pore radii of 46 A and 80-87 A, respectively. The estimated negative charge density was 35-45 meq/l at 8 degrees C. Perfusion with erythrocyte-free solutions of kidneys at 37 degrees C reduced glomerular size and charge permselectivity. Thus the large pore fraction of the glomerular filtrate (f(L)) was 1.64% at 37 degrees C compared with 0.94% at 8 degrees C. The theta for albumin was four times higher at 37 degrees C than at 8 degrees C (0.86% vs. 0.19%, respectively). NH(4)Cl caused further irreversible damage to the glomerular barrier. We conclude that there are no deleterious effects on the glomerular barrier of a reduction in temperature from 37 degrees C to 8 degrees C. Therefore our data seem to disprove the hypothesis of low glomerular permselectivity and transtubular uptake of intact albumin and support the classic concept of a highly selective glomerular barrier.

Immunocytochemical detection of advanced glycated end products in rat renal tissue as a function of age and diabetes

BACKGROUND

High blood glucose levels play major roles in the pathogenesis of renal diabetic complications through non-enzymatic glycation. For long-lived molecules this leads to formation of advanced glycation end products (AGE), and the renal extracellular matrix appears to be one of the targets for such processes. Using immunocytochemistry, we studied the appearance and deposition of AGE products in renal tissues from normal and diabetic rats at different ages, to evaluate the effects of aging and hyperglycemia.

METHODS

The streptozotocin-injected rat represented our model of hyperglycaemic condition. The immunogold techniques were applied at the light and electron microscope levels using specific monoclonal and polyclonal antibodies against AGE adducts. The results were analyzed by morphometry.

RESULTS

In normoglycemic animals, significant increases in labeling were detected in tubular basement membranes and mesangial matrix at 12 to 15 months of age. In contrast, in diabetic animals, significant increases in labeling were found for all extracellular matrices as soon as after two months of hyperglycemia. Labelings were also detected in cellular compartments, particularly in nuclei that showed increases in diabetic conditions. The labeling was particularly intense in proximal convoluted tubules and their endosomal compartment, due to the reabsorption of urinary AGE products.

CONCLUSION

The presence of AGE products in the renal extracellular matrix of old normoglycemic animals and their rapid appearance in hyperglycemia, indicate that AGE products may participate in the pathogenesis of renal complications. Furthermore, the non-enzymatic glycation is not restricted to extracellular matrices but also affects cellular proteins.

Increased NADH oxidase activity in the retina of the BBZ/Wor diabetic rat

This morphological study demonstrates a role for endothelial cells in generating reactive oxygen species in early stages of retinopathy in the BBZ/Wor rat, an obese, noninsulin dependent model of diabetes. Hyperglycemia induced pseudohypoxia results in an imbalance in cytosolic NADH/NAD+. In the oxygen-rich environment of the retina, NADH oxidase generates superoxide radical which is dismutated to hydrogen peroxide. Localization of hydrogen peroxide by the cerium NADH oxidase enzyme activity cytochemical localization technique shows a statistically significant increase of peroxide localization in the central retina of diabetic rats as compared to age-matched, nondiabetic controls. Endothelial cell dysfunction, indicated by leakage of endogenous serum albumin, coincided with areas of NADH oxidase activity localization. In diabetic rats there are increased levels of fibronectin in areas of hydrogen peroxide localization. This in vivo, morphological study is the first demonstration of oxidative injury and endothelial cell dysfunction in the retina of a spontaneous, noninsulin dependent model of diabetes.

Renal fate of circulating advanced glycated end products (AGE): evidence for reabsorption and catabolism of AGE-peptides by renal proximal tubular cells

The presence of excessive amounts of advanced glycation end products (AGE) in tissues or in the circulation may critically affect the progression of diabetic nephropathy. Circulating AGE levels, mainly in the form of small peptides, increase in diabetic patients or in patients with end-stage renal disease. This rise correlates with the severity of the nephropathy. However, so far little is known about the fate of AGE-proteins and AGE-peptides in renal tissue, and in order to elucidate this issue we undertook the present study. AGE-bovine serum albumin (AGE-BSA) and AGE-peptides were prepared, characterized by spectrophotometry, spectrofluorometry, chromatography and SDS-PAGE. AGE-peptides reacted in vitro with LDL producing biochemical and ultrastructural modifications. Using colloidal gold post-embedding immunoelectron microscopy with an anti-AGE antibody generated in our laboratory, we followed, in a short-term kinetic study, the cellular and sub-cellular localisation of circulating AGE-products throughout the nephron. AGE-peptides or AGE-BSA were injected into otherwise normal rats and detected by protein A-gold immuno-cytochemistry after 15, 30 or 45 min of circulation. Most of the AGE-BSA was found in the lumen of capillary vessels and distributed along the endothelial side of the glomerular basement membrane. Presence on mesangial matrix was also apparent. AGE-peptides were easily filtered and actively reabsorbed by the proximal convoluted tubule. At 15 min, little labelling was found in the glomerular wall. Instead, the labelling was present in the urinary space and microvilli of epithelial cells. Early endosomes displayed intense labelling as well. At 45 min, late endosomes and lysosomes added to the pattern of labelling. The distal tubule epithelial cells were devoid of labelling for any of the intervals studied. AGE-peptides but not AGE-BSA could be detected in the urine of injected rats. These observations point to participation of the endo-lysosomal apparatus of the proximal convoluted tubule to the disposal of AGE-peptides, while giving an ultrastructural support for a key role of the kidney in AGE catabolism.

Immunocytochemical investigation of the in vivo endocytosis by renal tubular epithelial cells

The internalization and degradation of glomerular filtered serum proteins by the proximal tubular epithelium has been extensively studied by microperfusion methods. By using a cationic probe that easily traverses the glomerular wall into the urinary space, we have performed a morpho-cytochemical and quantitative study of the in vivo endocytotic activity of the proximal tubular epithelial cell. Bovine serum albumin (BSA) was tagged with dinitrophenol (DNP) and cationized to pI over 8. It was introduced into the circulation of normal mice for 5, 10, and 30 minutes and the distribution of the labeling was determined by protein A-gold immunocytochemistry, using specific antiDNP antibodies on tissue sections of routinely aldehyde-fixed, osmiumpostfixed, and Epon-embedded kidneys. Cationic BSA-DNP was detected at the endothelial and epithelial sides of the glomerular basement membrane, and over capillary and tubular basement membranes. In the proximal tubular epithelial cell, labeling was present over microvilli as well as over endosomal and lysosomal compartments, with labeling intensities varying from one compartment to the other. Morphometric evaluations of the labeling demonstrated a progressive incorporation of the probe from microvilli and endocytic compartments at 5 minutes to endocytic and lysosomal compartments at 10 and then 30 minutes. When considering labeling densities, no significant differences were found on microvilli and basolateral membranes between times of circulation; however, the labeling density over endosomal and lysosomal compartments was very intense at 10 minutes compared with 5 minutes, decreasing at 30 minutes. Results from this study validate the cationic albumin tagged with DNP as a tool in the study of the quantitative aspects of protein endocytosis at the ultrastructural level, in the kidney tubular epithelium.

Intra-GBM site of the functional filtration barrier for endogenous proteins in rats

The passage of various endogenous proteins [such as albumin, transferrin, immunoglobulin G (IgG), and immunoglobulin M (IgM)] across GBM was studied in vivo in normal Munich-Wistar rats. Glomeruli were fixed by three different methods: in situ drip-fixation, perfusion- and immersion-fixation; then they were processed for immunogold electron microscopy. The most reproducible results were obtained with in situ drip-fixation. Albumin, transferrin and IgG penetrated into GBM, but IgM did not. Morphometry revealed that density of albumin increased towards the inner 1/5 to 1/3 of GBM (junction of lamina rara interna and lamina densa) and decreased towards the subepithelial region of GBM, whereas density of IgG and transferrin was the highest at the subendothelial site and declined towards the subepithelial side of GBM. These findings suggest that central and/or outer zone of GBM constitute the main filtration barrier for albumin, and that subendothelial zone may contribute also to the charge-selective barrier. It is also suggested that the subendothelial zone acts more effectively as a filtration barrier for IgG and transferrin than for albumin. In the outer zone of GBM, which roughly corresponds to lamina rara externa visualized by conventional electron microscopy, the relative density of IgG and transferrin was higher than that of albumin. Since the pI of albumin was lower than that of IgG and transferrin, this finding suggests that subepithelial zone of GBM also acts as a charge-selective barrier. In conclusion, the main GBM filtration barrier for albumin might be the central and outer zones of GBM, and that for transferrin and IgG might be the entire width of GBM.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased vascular permeability in pancreas of diabetic rats: detection with high resolution protein A-gold cytochemistry

The role of the pancreatic microcirculation in the pathogenesis of Type 1 (insulin-dependent) diabetes mellitus remains poorly understood. Herein, a method is described for the ultrastructural investigation of the integrity of the pancreatic microvasculature. The method consists of histochemical detection and isolation of the islets followed by albumin and protein A-gold immunocytochemistry, whereby the distribution of endogenous albumin is used as a marker of endothelial integrity. This technique, applied to the study of spontaneously diabetic rats, reveals a selective increase in permeability of islet capillaries and post-capillary venules at the onset of diabetes, while acinar capillaries and arterioles remain intact. At 50 days of age, before the onset of diabetes, the microvasculature of diabetes-prone rats shows no alterations in permeability to albumin. When used in conjunction with morphometric analyses, this methodological approach may be useful for further studies in pathologic or experimental conditions involving the pancreatic microvasculature.

Hapten-tagged plasma proteins as immunocytochemical probes for the study of vascular permeability

Bovine serum albumin and transferrin were covalently coupled with fluorescein isothiocyanate and digoxigenin, respectively, and intravenously co-injected in equal amounts in mouse. The derivation of the two proteins induces minor alterations of their physicochemical properties as well as of their physiological functions. The two tracers were revealed within vascular and extravascular compartments of diaphragm by quantitative postembedding immunocytochemistry, using antibodies against each of the haptens in conjunction with the protein AG-gold complexes. The influence of different fixatives and embedding protocols on the immunodetectability of the hapten-tagged proteins was assessed. Both resist reasonably well to osmication and embedding in Epon. None of the haptens reacted with the heterologous antibody. At 30 minutes after injection, the tracers were detected in blood plasma, interstitium, and endothelial plasmalemmal vesicles. The presence of both proteins within the interendothelial clefts was inconspicuous. The ratios between the labeling densities found over endothelium, interstitial space, and vascular lumen were similar for both tracers. This suggests that the endothelium of mouse diaphragm capillaries might exhibit comparable permeabilities towards serum albumin and transferrin which are similar in size and charge. The study shows that hapten-tagged polypeptides are close to the corresponding native macromolecules, and represent interesting tools for the morphological study of dynamic processes such as transcytosis.

Immunocytochemical study of glomerular permeability to anionic, neutral and cationic albumins

The renal handling of albumin of various isoelectric points (pI) was studied in mice by high resolution quantitative immunocytochemistry. Bovine serum albumin (BSA) was tagged with dinitrophenol (DNP) and cationized to pI 6.5 to 7.5 and to pI over 8.5. These tracers, including the anionic BSA-DNP (pI 4.8) were injected in the iliac vein of mice and the post-embedding protein A-gold technique was used with antibodies against DNP to localize the different tracers in renal tissue. Morphometric analysis of the labeling over the glomerular wall has demonstrated variations in its distribution according to the pI of the markers. Anionic and neutral BSA molecules were detected on the endothelial side of the basement membrane, while labeling for cationic BSA was present mainly in the laminae rarae interna and externa known to display fixed polyanionic sites. Neutral and cationic BSA-DNP reached the urinary space and were detected within the endocytotic apparatus of the proximal tubule epithelium. Neutralization of the anionic sites of the basement membrane with cationic BSA resulted in an increase in the permeability towards anionic BSA-DNP. In addition to the demonstration of glomerular permeability properties towards various probes, the present study has demonstrated that dinitrophenylated albumin represents an excellent versatile tool for the quantitative morphological investigation of glomerular permeability.

Transendothelial transport of serum albumin: a quantitative immunocytochemical study

The steady-state distribution of endogenous albumin in mouse diaphragm was determined by quantitative postembedding protein A-gold immunocytochemistry using a specific anti-mouse albumin antibody. Labeling density was recorded over vascular lumen, endothelium, junctions, and subendothelial space. At equilibrium, the volume density of interstitial albumin was 18% of that in circulation. Despite this large difference in albumin concentration between capillary lumen and interstitium, plasmalemmal vesicles labeling was uniformly distributed across the endothelial profile. 68% of the junctions displayed labeling for albumin, which was however low and confined to the luminal and abluminal sides. The scarce labeling of the endothelial cell surface did not confirm the fiber matrix theory. The kinetics of albumin transcytosis was evaluated by injecting radioiodinated and DNP-tagged BSA. At 3, 10, 30, and 60 min, and 3, 5, and 24 h circulation time, blood radioactivity was measured and diaphragms were fixed and embedded. Anti-DNP antibodies were used to map the tracer in aforementioned compartments. A linear relationship between blood radioactivity and vascular labeling density was found, with a detection sensitivity approaching 1 gold particle per DNP-BSA molecule. Tracer presence over endothelial vesicles reached rapidly (10 min) a saturation value; initially localized near the luminal front, it evolved towards a uniform distribution across endothelium during the first hour. An hour was also needed to reach the saturation limit within the subendothelial space. Labeling of the junctions increased slowly, out of phase with the inferred transendothelial albumin fluxes. This suggests that they play little, if any, role in albumin transcytosis, which rather seems to proceed through the vesicular way.

Ultrastructural and immunohistochemical similarities between glomerular arteriolar and mesangial lesions in experimental diabetes

It has thus far been speculated that plasma proteins have leaked and accumulated in diabetic expanded mesangium. We have previously shown that glomerular arteriolar hyalinosis has frequently occurred and has consisted of basement membrane-like extracellular matrix in alloxan diabetic rats. In the present study, the two diabetic glomerular lesions, arteriolar hyaline nodules and increased mesangial matrix, were comparatively evaluated by means of electron microscopy and immunohistochemistry. It was revealed that both of these lesions contained, in common, finely particulate extracellular matrix mingled with cytoplasmic fragments, and exhibited intense immunoreactions for IgG and IgM. It is indicated that both types of diabetic glomerular lesions are essentially of the same nature.

Contractile proteins in podocytes: immunocytochemical localization of actin and alpha-actinin in normal and nephrotic rat kidneys

Actin and alpha-actinin immunoreactive sites have been localized at the electron microscope level by the protein A-gold immunocytochemical technique in podocytes of normal and nephrotic rat renal tissues. In normal renal glomeruli, fibrillar networks located in the core of foot processes or bundles of microfilaments interconnecting them were found to be labelled for these two cytoskeletal proteins. On the other hand, in nephrotic renal glomeruli, concomitant with the loss of podocytic foot processes a reorganization of the podocytic cytoskeleton and a concentration of some of its elements into thick uniform bands was observed. Actin and alpha-actinin were revealed in these bands. Control experiments confirmed the specificity of the labelling obtained. Our results suggest that normal podocytes contain an actin-based contractile system that might contribute to the maintenance of the particular cell shape of these cells and that the rearrangement of the podocytic cytoskeleton occurring in the nephrotic syndrome might account for the changes in the foot processes and contribute to the alteration in glomerular function.

Ontogenesis of glomerular basement membrane: structural and functional properties

Protein A-gold immunocytochemistry was applied in combination with morphometrical approaches to reveal the alpha 1(IV), alpha 2(IV), and alpha 3(IV) chains of type IV collagen as well as entactin on renal basement membranes, particularly on the glomerular one, during maturation. The results have indicated that a heterogeneity between renal basement membranes appears during the maturation process. In the glomerulus at the capillary loop stage, both the epithelial and endothelial cell basement membranes were labeled for the alpha 1(IV) and alpha 2(IV) chains of type IV collagen and entactin. After fusion, both proteins were present on the entire thickness of the typical glomerular basement membrane. At later stages, the labeling for alpha 1(IV) and alpha 2(IV) chains of type IV collagen decreased and drifted towards the endothelial side, whereas the labeling for the alpha 3(IV) chain increased and remained centrally located. Entactin remained on the entire thickness of the basement membrane during maturation and in adult stage. The distribution of endogenous serum albumin in the glomerular wall was studied during maturation, as a reference for the functional properties of the glomerular basement membrane. This distribution, dispersed through the entire thickness of the basement membrane at early stages, shifted towards the endothelial side of the lamina densa with maturation, demonstrating a progressive acquisition of the permselectivity. These results demonstrate that modifications in the content and organization of the different constituents of basement membranes occur with maturation and are required for the establishment of the filtration properties of the glomerular basement membrane.

Immunogold studies of monomeric elements from the globular domain (NC1) of type IV collagen in renal basement membranes during experimental diabetes in the rat

The protein A-gold immunocytochemical technique was applied to reveal the monomeric elements M1, M2* and M3 from the non-collagenous globular domain (NC1) of type IV collagen over various renal basement membranes from control and long-term streptozotocin-induced diabetic rats. This study includes the basement membranes of the proximal tubule, the Bowman's capsule and the glomerulus as well as the extracellular matrix of the mesangium. The labellings obtained were confined to basement membrane material. The quantitative analysis demonstrated changes in labelling intensities and distribution between tissues from normal and diabetic animals. Increased labelling intensities were observed for M1 and M2* monomers in all the basement membranes studied except for the mesangial matrix which remained unchanged. In addition, the labelling for M1 monomers, present on the endothelial side of the glomerular basement membrane of control animals, was found to be distributed throughout the entire thickness of the basement membrane of diabetic animals. In contrast, neither the intensity of the labelling, nor the distribution of M3 monomers were altered in diabetic animals. Since M1 monomers are markers of the alpha 1(IV) and alpha 2(IV) chains of type IV collagen while M2* and M3 mark alpha 3(IV) and alpha 4(IV) chains respectively, the present results demonstrate changes in the nature of the collagenous elements of basement membranes during diabetes. Furthermore, the results indicate that the alpha 3(IV) and the alpha 4(IV) chains are not necessarily present in the same molecule. The modifications of the collagenous elements of the basement membranes during diabetes must alter the structural characteristics of these matrices which in turn might influence their functional properties.

Ultrastructural distribution of endogenous IgGs in the glomerular wall of control and diabetic rats

Endogenous IgG molecules were revealed with high resolution EM over the glomerular wall in renal tissues sampled from short and longterm control and streptozotocin induced diabetic rats by applying the protein A-gold immunocytochemical approach. In tissues from control animals, IgG antigenic sites were revealed on the subendothelial side of the basement membrane, the epithelial side being only weakly labelled. In contrast, in longterm diabetic animals IgG antigenic sites were present throughout the entire thickness of the basement membrane, and in patches closely associated with the plasma membrane of the epithelial cells. Deposits of basement membrane-like material present in the mesangial area were also highly labelled for IgG. Numerous intensely labelled lysosome-like structures were present in the epithelial cells. Morphometrical evaluation of the distribution of the labelling over the basement membrane confirmed these observations. In control animals a peak of labelling was found at 30 nm from the endothelial cell region corresponding to the subendothelial side of the lamina densa. In longterm diabetic animals the labelling was more uniformly distributed throughout the entire thickness of the basement membrane. These data were correlated to biochemical determinations of proteinuria and IgG excretion in urine samples from the control and the diabetic animals. These results suggest that in normal conditions the lamina densa may represent the main barrier for the restriction of the passage of IgGs through the glomerular wall. Modifications at that level occur during diabetes leading to or participating in the loss of the selective permeability of the basement membrane.

Distribution of endogenous albumin across the rat aortic wall as revealed by quantitative immunocytochemistry

Endogenous albumin was revealed over thin sections of rat aortic wall, with high resolution and specificity, by applying the protein A-gold immunocytochemical technique. Gold particles, revealing albumin antigenic sites, were observed over plasmalemmal vesicles in endothelial cells and over the interstitial space throughout the thickness of the aortic wall. The distribution of the labeling in the interstitial space varied from region to region and was associated with the collagen fibers, following the orientation of the bundles. The morphometric evaluation of this labeling demonstrated a first peak in labeling intensity in the intima followed by a steep decrease with low levels in the media, and an increasing gradient towards the adventitia. In the subendothelium, a moderate labeling was observed at the base of the endothelial cells of both aortic and capillary endothelia, followed by a decreasing gradient. Ratios between the labeling density in the intima as well as in the adventitia and that in the capillary lumen (plasma albumin) revealed different concentrations of albumin in these compartments. Endogenous albumin, under steady-state conditions, is thus unevenly distributed over the interstitial spaces across the rat aortic wall, and appears associated along the collagen fibers.