Studies on the relationship between proteinuria and glomerular ultrastructural change in hyperalbuminaemic female Wistar rats

Podocyte foot process effacement is not correlated with the level of proteinuria in human glomerulopathies

BACKGROUND

Nephrotic syndromes result from increased glomerular permeability to proteins and are structurally believed to be associated with podocyte foot process effacement. Despite increasing knowledge of the molecular composition of the glomerular filtration barrier, the relationship between proteinuria and foot process effacement is unclear.

METHODS

We conducted a morphologic study on the relationship between podocyte foot process effacement and proteinuria. Electron microscope pictures of glomerular capillaries were randomly taken from 27 cases in various stages of minimal change nephrotic syndrome (MCNS), from six cases of IgA nephropathy (IgAN) with high proteinuria and from seven control kidneys. From each picture, the mean width of the foot processes (FPW) was quantitated.

RESULTS

In normal kidney the mean FPW was 580 +/- 40 nm. In biopsies from patients with MCNS without treatment, foot processes were diffusely effaced, reflected by a FPW of 1600 +/- 440 nm. In biopsies from patients with MCNS relapsing under prednisolone treatment, foot processes were significantly less effaced than in untreated MCNS (FPW 920 +/- 200 nm). In biopsies displaying IgAN, effacement was significantly more segmental than in untreated MCNS (FPW 800 +/- 170 nm). Proteinuria did not differ significantly among the groups. Neither in MCNS nor in IgAN was the extent of foot process effacement correlated with the level of proteinuria.

CONCLUSION

Podocyte foot process effacement is not correlated with proteinuria. The differences in podocyte effacement between MCNS, MCNS relapsing under prednisolone treatment, and IgAN may point to different mechanisms of podocyte injury in these diseases.

Peripolar cells and other granulated epithelial cells in renal biopsies

The peripolar cell is a recently described glomerular epithelial cell which is situated within Bowman's capsule at the vascular pole. It contains cytoplasmic granules which contain plasma proteins, although it may also have a secretory function. The relationship between peripolar cells, other granulated glomerular epithelial cells and tubular epithelial cells is unclear. We have studied 242 biopsies from 19 types of renal disease for peripolar cells, other granulated epithelial cells and granulated tubular epithelial cells. Peripolar cells were most numerous in mesangioproliferative glomerulonephritis, IgA nephropathy, focal segmental glomerulosclerosis, membranous glomerulonephritis and lupus nephropathy. Other granulated glomerular epithelial cells were most prominent in diffuse lupus nephropathy, focal glomerulonephritis, acute vascular transplant rejection, crescentic glomerulonephritis and mesangioproliferative glomerulonephritis. Granulation of the tubular epithelium was most prominent in minimal change nephrotic syndrome and amyloidosis. It is likely that the granules in tubular epithelial cells represent lysosomes containing plasma proteins which have been absorbed from the tubular fluid. However, granulation of glomerular cells may represent a more specific response to glomerular damage. In addition, peripolar cells are prominent in only certain diseases, suggesting a specialized function.

The glomerular peripolar cell--an immunohistochemical study

The peripolar cell is a recently described glomerular epithelial cell which may be a new secretory component of the juxtaglomerular apparatus (JGA). The purposes of this study were to identify immunohistochemical markers specific for peripolar cells and to elucidate the composition of their cytoplasmic granules. We examined tissue from normal human and sheep kidneys with a variety of monoclonal antibodies and antisera. A minority of peripolar cells showed immunoreactivity for vimentin but no other intermediate filament. Neuroendocrine markers were negative. The granules of peripolar cells contain a variety of plasma proteins with the exception of IgM. It is likely that peripolar cells absorb plasma proteins from the glomerular filtrate. The reason for this is uncertain; however, their unique position at the vascular pole of normal kidneys suggests a specialized function.

Glomerular epithelial cell lesions in rat renal isografts

Visceral glomerular epithelial cell lesions--microvillus formation, loss of foot processes, osmiophilic inclusion droplets, balloon-like malformation of cell processes, degeneration, necrosis, and loss of cell processes from capillary basement membranes--are found in rat renal isografts 1 mth after transplantation. The lesions, which are most readily recognized in perfusion-fixed material, are essentially focal, affecting neither all glomeruli, nor all cells in any glomerulus, bear no relation to the degree of interstitial nephritis in the graft, and are associated with albuminuria and with focal capillary sclerosis in some glomeruli. They are not restricted to renal isografts but are found in aging rats, in different experimental models of glomerular disease and in clinical glomerular disorders, again in association with proteinuria and glomerulosclerosis. It is therefore proposed that glomerular epithelial cell damage increases capillary permeability and impairs maintenance of the integrity of the capillary wall, leading to proteinuria and focal glomerulosclerosis.

Normal and abnormal aspects of proteinuria. Part I: Mechanisms, characteristics and analyses of urinary protein. Part II: Clinical considerations

Part I highlights the mechanisms of glomerular filtration and tubular reabsorption of plasma proteins, selected characteristics of urinary proteins based upon electrophoretic properties and recent advances in clinical laboratory analysis of proteinuria. Both structural characteristic of the glomerular capillary wall and molecular properties of plasma proteins are important determinants of glomerular filtration. Proteins filtered by the glomerulus subsequently appear in urine only after escaping the efficient mechanisms of tubular reabsorption. Albumin is one such protein and constitutes the major protein in normal urine although trace amounts of alpha, beta, and gamma globulins are also detectable. Several techniques of protein analysis have thus been developed to specifically measure albumin as well as other plasma proteins. Other methods have been adapted to measure total urinary protein content enabling the clinician to readily monitor renal function in health and disease. The second part of this review will consider conditions associated with proteinuria in both asymptomatic individuals and patients with renal disease. Asymptomatic proteinuria encompasses states of excess protein excretion during conditions of orthostasis, exercise, travel to high altitude of fever. Proteinuria during renal disease has received considerable interest as a means to monitor kidney function. It is therefore classified according to the type of damage incurred: (1) glomerular-type where large molecular weight proteins are excreted (2) tubular-type where small molecular weight proteins are excreted and (3) mixed-type characterized by both large and small molecular weight proteinuria.

[Biophysicochemical structures of the glomerular filter]

The ultrastructural and the biophysical and biochemical qualities of glomerular permeability to protein molecules are reviewed. With regard to differently located immune deposition in human glomerulonephritis, description and discussion are addressed in a fixed order of layers: 1. endothelial-subendothelial, i.e. the endothelial cells with fenestrate and the lamina rara interna of the basement membrane (bm), 2. membranous, i.e. the lamina densa of the bm, 3. subepithelial-epithelial, i.e. the lamina rara externa of the bm and the podocytes with food processes and slit diaphragms. It is emphasized that the 3 layers act as gradually (coarse to fine) filter barriers. On the basis of well known structural peculiarities, in the last 10 years experimental studies revealed that the meshwork of type IV collagen and the negatively charged heparan sulfate-proteoglycans - "the glomerular polyanion" - are integrated in sieving of protein molecules. These components are differently located in the stratified cellular and extracellular layers of the glomerular filter and their combined action is the basis of a size, charge and configuration depended filtration of macromolecules. In this way the passage of the mostly negative charge blood proteins, especially albumin, is prevented under normal conditions.

Glomerular podocytic injury in protein overload proteinuria

Injection of rats with large doses of bovine serum albumin causes proteinuria which may persist long after the period of overload has ended. In order to assess in this model of proteinuria the relative importance of podocytic epithelial changes versus alterations in anionic groups in the glomerular capillary wall a morphological study has been made of animals in which the kidneys were fixed by vascular perfusion or by in situ drip fixation. By transmission electron microscopy, podocytes showed protein droplets, cytoplasmic vacuoles, spreading of epithelial cytoplasm with loss of foot processes, and focal separation of epithelium from the glomerular basement membrane, occasionally with cytoplasmic disruption. Staining with colloidal iron showed no reduction in the density of anionic groups per unit area on epithelial cell surfaces or elsewhere in glomeruli. However, the reduced surface area of epithelial cells caused by the changes to their structure accounts adequately for the less intense glomerular colloidal iron staining evident by light microscopy. Changes in podocyte structure, particularly those leading to focal cytoplasmic defects on the outer surface of the glomerular basement membrane, appear to be more important than loss of glomerular anionic groups for the development of proteinuria in protein overload nephropathy.

The glomerular ultrastructural distribution of immunoglobulin G in hyperalbuminaemic (protein-overload) proteinuria

Female Munich-Wistar rats were given intraperitoneal injections either of bovine serum albumin to induce proteinuria or of water as a control. Their kidneys were fixed in situ. An ultrastructural technique was used to demonstrate IgG antiperoxidase antibodies either injected from a heterologous species or autologous, induced by immunization with horseradish peroxidase. Photometry of electron micrographic negative was used to determine the distribution of antiperoxidase antibodies. In glomeruli of control animals IgG was present in the basement membrane. There were three sites at which the passage of IgG across the basement membrane was hindered: between blood plasma and the lamina rara interna, between the lamina densa and the lamina rara externa, and between the lamina rara externa and the urinary space. Glomeruli of proteinuric animals were variable in appearance, some showing little structural damage and others showing marked changes with loss of epithelial foot processes and accumulation of vacuoles and protein droplets in epithelial cells. Both types of glomeruli contained IgG in the urinary space. The distribution of IgG in the basement membrane of both types was similar. Compared with control animals there was less IgG in the basement membrane and IgG was distributed uniformly across the basement membrane. The proteinuria in hyperalbuminaemia (protein-overload) is associated with a diffuse change in the barrier function of the glomerular basement membrane to IgG which is, at least in the initial stages, not related to structural changes in glomerular epithelial cells.

Glomerular lysozyme binding in protein-overload proteinuria

Binding of the cationic molecule lysozyme to the glomerular basement membrane and to the glomerular epithelial cell coat was investigated in the glomerulus of normal female Wistar rats and in rats in which heavy proteinuria was induced by the daily administration of 1 g of bovine serum albumin. In normal rats the binding of lysozyme to the anionic groups in the glomerular basement membrane and the cell coat had no effect on the ultrastructure of the glomerular epithelial cell, in particular the foot processes were unchanged. In the proteinuric rats the lysozyme-binding to the glomerular basement membrane and the epithelial cell coat was completely lost in the damaged glomeruli. In the apparently normal glomeruli present in these proteinuric animals binding was similar to that seen in normal rats. These results suggest that in protein-overload proteinuria there is a loss of glomerular anion and hence a reduction in the glomerular charge barrier. This may account, at least in part, for the increased glomerular leak of negatively charged serum albumin in this experimental model of proteinuria.

Ultrastructural localization of anionic and cationic ferritin in the rat glomerular basement membrane in protein-overload proteinuria

The penetration into the glomerular basement membrane of anionic and cationic ferritin has been studied in rats made proteinuric by intraperitoneal administration of bovine serum albumin. In comparison with control animals anionic ferritin penetrated the glomerular basement membrane to a much greater extent in proteinuric rats. Some ferritin particles were observed in small invaginations of the epithelial cell membrane adjacent to the glomerular basement membrane and incorporated in pinocytotic vesicles within the epithelial cell cytoplasm. This was not seen in control animals. Cationic ferritin distribution in the glomerular basement membrane was similar in control and proteinuric rats suggesting that the increased anionic ferritin penetration observed occurs without any reduction in fixed anionic charge.

Effects of isolation and food restriction begun at 50 days on the development of age-associated renal disease in the male Wistar rat

The development of proteinuria with increasing age was studied in three groups of male Wistar rats: ad libitum fed and isolated, ad libitum fed and group housed 6 to 8 rats per cage, and food restricted (one-third of the isolated ad libitum food intake) and isolated. Studies were begun at age 50 days and continued throughout life. Ad libitum fed rats when isolated ate more food, grew faster, had larger maximum body weights and developed proteinuria at a faster rate than those that were group housed. There was a small increase in the severity of glomerular pathology in old age. However, systolic blood pressure was not affected significantly by isolation, nor was life duration. Food restriction of isolated rats inhibited body growth, prevented the development of proteinuria, reduced the incidence of glomerular and tubular pathology in old age and prolonged life. Electron microscopic examination of the kidneys of old food-restricted rats revealed a much lower incidence of foot process retraction and spreading on the basement membrane of the glomerulus than in ad libitum fed rats. Cardiac enlargement was also prevented by long-term food restriction.