Selective albuminuria via podocyte albumin transport in puromycin nephrotic rats is attenuated by an inhibitor of NADPH oxidase

Abstract

The mechanism of selective albuminuria in minimal change nephrotic syndrome, in which glomerular capillaries are diffusely covered by effaced podocyte foot processes with reduced slit diaphragms, is unknown. Podocyte injury is due, in part, to NADPH-induced oxidative stress. Here we studied mechanism of selective albuminuria in puromycin aminonucleoside (PAN) nephrotic rats, a model of minimal change nephrotic syndrome. In these rats, Evans Blue-labeled human albumin was taken up by podocytes and its urinary excretion markedly increased, with retained selectivity for albumin. Immunogold scanning electron micrographic images found increased human albumin in podocyte vesicles and on the apical membrane in nephrotic compared with control rats. Apocynin, an inhibitor of NADPH oxidase, decreased superoxide production in podocytes, and inhibited endocytosis and urinary albumin excretion. Real-time confocal microscopy found an initial delay in the appearance of Evans Blue-labeled human albumin in the tubular lumen, reflecting the time needed for transcellular transport. Immunoprecipitation analysis indicated that FcRn, a receptor for albumin transport, mediated podocyte albumin transport, and treatment with anti-FcRn antibody reduced proteinuria in these nephrotic rats. Thus, podocyte albumin transport was enhanced in PAN nephrotic rats by means of FcRn, which may explain the mechanism of selective proteinuria. This was blocked by apocynin, suggesting a new therapeutic approach.

Intravital Imaging Reveals Angiotensin II–Induced Transcytosis of Albumin by Podocytes

“…42 In accordance with a clearance function of podocytes, the blockade of the fetal Fc fragment receptor, which is relevant for intracellular albumin vesicle sorting, caused a reduction in proteinuria in nephrotic rats. 25 Furthermore, fetal Fc fragment receptor-deficient mice accumulated IgG in the glomerular basement membrane, further supporting the assumption that podocytes have a clearance function for the glomerular filtration barrier. 16,17 We found that endocytosed albumin is partly localized in acid cell compartments, which colocalized with the lysosomal marker LysoTracker in vivo.…”

“…24 Data obtained from human, rat, and mouse kidneys suggested that plasma proteins, such as IgG, ferritin, and albumin, may be localized in podocyte vacuoles. 8,17,[25][26][27][28][29][30] Commensurate with these findings, podocytes were suggested to clear the glomerular filtration barrier from proteins. 17 Our results are in agreement with these studies and further reveal that an Ang II-mediated increase in albumin flux across the glomerular membrane enhances the endocytosis of albumin by podocytes in the normal glomerulus.…”

“…43 In addition to lysosomal degradation, we showed that albumin is subject to a transcellular transport through the podocyte into the urinary space. Transcellular transport of proteins by podocytes has been suggested as a novel pathway of protein filtration during states of disease 25 and a potential mechanism of protein clearance from the SP. 17 Nevertheless, the phenomenon itself has never been visualized in vivo.…”

Albumin-associated free fatty acids induce macropinocytosis in podocytes

“…Numerous studies using electron microscopy have shown increased numbers of protein-containing vesicles in podocytes under nephrotic conditions in which there is significant leakage of proteins across the GBM (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Previously, we showed that expression of the neonatal Fc receptor, a transcytosis receptor for Ig and albumin, allows podocytes to handle these proteins by internalization and transcytosis (19).…”

Nephropathy and Elevated BP in Mice with Podocyte-Specific NADPH Oxidase 5 Expression

“…In animal models of minimal change disease, membranous nephropathy, and FSGS, inhibition of Nox activity is associated with decreased podocyte effacement and amelioration of albuminuria. [11][12][13][14] In models of diabetic nephropathy, treatment with the Nox inhibitor apocynin, as well the antioxidant vitamin E, reduces oxidative stress, podocyte effacement and loss, and albuminuria. 6,15,16 Noxs are regulated by many factors, including the renin angiotensin aldosterone system.…”