Statin-sensitive endocytosis of albumin by glomerular podocytes

Abstract

Whether podocytes are able to endocytose proteins is uncertain. We studied protein endocytosis in conditionally immortalized mouse and human podocytes using FITC-albumin by direct quantitative assay and by fluorescence microscopy and electron microscopy in mouse podocytes. Furthermore, in vivo uptake was studied in human, rat, and mouse podocytes. Both mouse and human podocytes displayed specific one-site binding for FITC-albumin with K d of 0.91 or 0.44 mg/ml and Bmax of 3.15 or 0.81 g/mg cell protein, respectively. In addition, they showed avid endocytosis of FITC-albumin with Km of 9.48 or 4.5 mg/ml and Vmax of 474.3 or 97.4 g⅐mg cell protein Ϫ1 ⅐h Ϫ1 , respectively. Immunoglobulin and transferrin were inefficient competitors of this process, indicating some specificity for albumin. Accumulation of endocytosed albumin could be demonstrated in intracellular vesicles by fluorescence confocal microscopy and electron microscopy. Endocytosis was sensitive to pretreatment with simvastatin.

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

“…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.…”

“…In line with our hypothesis, cultured immortalized human podocytes were shown to perform caveolae-dependent endocytosis of albumin, 7 and albumin-containing vesicles could be observed by electron microscopy in mouse, rat, and human tissue. 8 Using intravital multiphoton microscopy (MPM) and electron microscopy, we found that Ang II infusion promotes the endocytosis of albumin by podocytes. The podocyte albumin endocytosis during Ang II infusion was mediated by the AT1 receptor, megalin dependent, and facilitated by an increase in the albumin concentration of the SP.…”

Functional Human Podocytes Generated in Organoids from Amniotic Fluid Stem Cells

“…Both BSA and nuclear HNA ( Figure 7A, right inset, arrowhead and arrows, respectively) were found within the same cell, which is clear evidence of active endocytosis by viable and functional human podocytes, as happens in normal podocytes in vivo. 18 BSA was also detected in the tubular lumen and in subapical vesicles of proximal tubular cells ( Figure 7, B and B9, arrowheads), indicating tubular reabsorption in the chimeric tissue. Unlike differentiated podocytes, in vitro cultured AFSCs were not able to take up FITC-BSA (Supplemental Figure 7), demonstrating that this property was acquired upon differentiation within the glomerular structures.…”

Distinct Requirements for Vacuolar Protein Sorting 34 Downstream Effector Phosphatidylinositol 3-Phosphate 5-Kinase in Podocytes Versus Proximal Tubular Cells

“…9 More recently, podocyte uptake of Evans Blue-labeled albumin was observed in a puromycin aminonucleoside injury model by transcytosis, a well characterized process for moving materials from one cell membrane domain to another. 10 However, many of the studies showing the ability of podocytes to endocytose labeled albumin or IgG have been performed in vitro [11][12][13][14] (i.e., in isolated cultured podocytes) and may not truly mirror the events occurring in vivo. Human diseases and mouse models with disruption of the filtration barrier resulting in large amounts of protein leak are not ideally suited to elucidate the role of endocytosis in podocyte homeostasis, because the behavior of podocytes is likely altered.…”