Submitted on July 18, 2006
Accepted on October 5, 2006
STATIN-SENSITIVE ENDOCYTOSIS OF ALBUMIN BY GLOMERULAR PODOCYTES
Jeanette Eyre1, Kyriakos Iannou2, Blair D Grubb3, Moin Saleem4, Peter W Mathieson4, Nigel John Brunskill5*, Erik Ilso Christensen6, and Peter S Topham7
1 Department of Infection, Immunity and Inflammation, University of Leicester, Leicester, Leics, United Kingdom
2 Department of Nephrology, Leicester General Hospital, Leicester, Leics, United Kingdom
3 Department of Cell Physiology and Pharmacology, University of Leicester, Leicester, Leics, United Kingdom
4 Academic and Childrens Renal Unit, University of Bristol, Bristol, United Kingdom
5 Infection, Immunity and Inflammation, University of Leicester, Medical Sciences Building, Leicester, Leicestershire, LE1 9HN, United Kingdom; Department of Nephrology, Leicester General Hospital, Leicester, Leics, United Kingdom
6 Department of Cell Biology, Institute of Anatomy, University of Aarhus, DK-8000 Aarhus C, Denmark
7 Department of Infection, Immunity and Inflammation, University of Leicester, Leicester, Leics, United Kingdom; Department of Nephrology, Leicester General Hospital, Leicester, Leics, United Kingdom
* To whom correspondence should be addressed. E-mail: njb18{at}le.ac.uk.
Glomerular podocytes are critical regulators of glomerular permeability via the slit diaphragm and may play a role in cleaning the glomerular filter. Whether podocytes are able to endocytose proteins is uncertain. We studied protein endocytosis in conditionally immortalised 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 Kd 0.91mg/ml or 0.44mg/ml, and Bmax 3.15µg/mg cell protein or 0.81µg/mg cell protein respectively. In addition they showed avid endocytosis of FITC-albumin with Km 9.48mg/ml or 4.5mg/ml, and Vmax 474.3 µg/mg cell protein/hr or 97.4µg/mg cell protein/hr 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 pre-treatment with simvastatin. In vivo accumulation of albumin was found in all three species but most pronounced in rat.
We conclude that podocytes are able to endocytose protein in a statin sensitive manner. This function is likely to be highly significant in health and disease. In addition protein endocytosis by podocytes may represent a useful, measurable phenotypic characteristic against which potentially injurious or beneficial interventions can be assessed.
