A biochemical and immunological investigation into the physiological basis of the increased albumin filtration induced in hyperalbuminaemic female Wistar rats

Albuminuria increases cystatin C excretion: implications for urinary biomarkers

BACKGROUND

Low-molecular weight (LMW) proteins, including albumin and novel urinary biomarkers of acute kidney injury (AKI) such as cystatin C and neutrophil gelatinase-associated lipocalin (NGAL), are normally absorbed from the glomerular filtrate by receptor-mediated transport. We evaluated the effect of albuminuria on urinary excretion of novel biomarkers.

METHODS

Sprague-Dawley rats given four injections over 2 days of 5 mg/g body wt/day bovine serum albumin (BSA) in saline were compared with controls given saline alone. Urinary cystatin C, albumin and protein excretion rates were compared prior to treatment (Day -1), after treatment (Day 2) and 4 days later (Day 6). A preliminary assessment of the clinical effect of proteinuria on the filtered urinary biomarkers cystatin C and NGAL was made by comparison with the effect on urinary interleukin-18 (IL-18) that is not absorbed from the glomerular filtrate, in a cohort of intensive care unit patients.

RESULTS

BSA induced transient increases in albuminuria, proteinuria and cystatinuria (P < 0.01, P < 0.001 and P < 0.001, respectively). Beyond a threshold 6-fold increase in albuminuria, cystatin C absorption was reduced by competitive inhibition. The excretion rates of all analytes returned to preinjection levels by Day 6. Clinical proteinuria was associated with increasing cystatin C and NGAL concentrations (n = 90, P < 0.0001) but not IL-18 (P = 0.12).

CONCLUSIONS

Proteinuria may increase the threshold for detection of AKI by increasing the excretion of LMW protein biomarkers.

Inducible rodent models of acquired podocyte diseases

Glomerular diseases remain the leading cause of chronic and end-stage kidney disease. Significant advances in our understanding of human glomerular diseases have been enabled by the development and better characterization of animal models. Diseases of the glomerular epithelial cells (podocytes) account for the majority of proteinuric diseases. Rodents have been extensively used experimentally to better define mechanisms of disease induction and progression, as well as to identify potential targets and therapies. The development of podocyte-specific genetically modified mice has energized the research field to better understand which animal models are appropriate to study acquired podocyte diseases. In this review we discuss inducible experimental models of acquired nondiabetic podocyte diseases in rodents, namely, passive Heymann nephritis, puromycin aminonucleoside nephrosis, adriamycin nephrosis, liopolysaccharide, crescentic glomerulonephritis, and protein overload nephropathy models. Details are given on the model backgrounds, how to induce each model, the interpretations of the data, and the benefits and shortcomings of each. Genetic rodent models of podocyte injury are excluded.

A morphometric, biochemical and histochemical comparison of puromycin aminonucleoside and hyperalbuminaemic induced proteinurias in the female Wistar rat

Biochemical, immunological, histochemical and electron microscope morphometric techniques were used to monitor the changes in urinary protein composition, albumin clearance and glomerular ultrastructure induced in female Wistar rats following i.p. injection of puromycin aminonucleoside (PAN) or bovine albumin (BSA). BSA injected rats maintained a high degree of selectivity with albumin constituting 90 per cent. of the total protein excreted even when mean protein excretion was in the order of 500 mg/24 hr. A similar degree of selectivity was only evident in PAN nephrotic rats at low levels of proteinuria. Levels of 500 mg/24 hr only 57 per cent. of the total protein was albumin. These differences correlated well with the increased number of glomeruli from PAN nephrotics compared with hyperalbuminaemic rats which, at these high levels of proteinuria, had bare areas of glomerular basement membrane caused by epithelial cell detachment (88 and 7 per cent. respectively). Detailed electron microscope morphometric and immunohistochemical studies showed that there were also important quantitative differences in a number of superficially similar glomerular structural alterations. In PAN nephrotic rats all glomeruli showed very marked epithelial cell foot process loss and reduced staining with colloidal iron. In glomeruli from hyperalbuminaemic rats there was a wide variation in the extent of epithelial cell foot process loss and reduced colloidal iron staining was only demonstrable in those glomeruli which had swollen epithelial containing large numbers of vacuoles and protein droplets. Similarly, while protein droplets were smaller and less numerous in glomeruli from PAN-injected rats, they were present in most glomeruli whereas their distribution was much more variable in glomeruli from BSA-injected rats. All the data collected therefore indicated that there were important differences in the types of proteinuria and glomerular ultrastructural damage present in PAN nephrotic and hyperalbuminaemic rats and suggested that their induction may have involved entirely different mechanisms. Evidence gathered from one experimental model should thus only be used with extreme caution to aid in interpretation of data obtained from the other.

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

Increased albumin filtration was shown to cause glomerular epithelial cell foot process loss in female Wistar rats made hyperalbuminaemic by means of a series of intraperitoneal bovine albumin injections. It appeared to act by stimulating glomerular epithelial cell endocytosis with the result that protein droplets and vacuoles accumulated within the epithelium causing marked cytoplasmic swelling which forced the foot processes to spread out and fuse. The severity of this glomerular ultrastructural damage rose with increasing bovine albumin dose and correlated well with induced proteinuria.