Background: Myocilin is a 55 to 57 kD secreted glycoprotein and member of the olfactomedin protein family. It is expressed in high amounts in the outflow tissues of the aqueous humor in the eye where it is supposed to contribute to outflow resistance. Myocilin is mutated in some forms of primary open angle glaucoma and affected patients show very high intraocular pressures because of an increase in resistance to aqueous humor outflow. To obtain information, if myocilin may play a comparable role in other tissues with transendothelial fluid flow, we investigated its expression in the rat kidney.
Methods: The expression of myocilin in the normal rat kidney and its changes during mesangioproliferative glomerulonephritis were investigated by immunohistochemistry, one- and two-dimensional gel electrophoresis with Western blotting, and reverse transcription-polymerase chain reaction (RT-PCR).
Results: Myocilin and its mRNA were detected in isolated glomeruli. Immunohistochemistry showed specific labeling of glomerular cells, while tubular and interstitial regions were essentially negative. Double staining with the podocyte-specific markers synaptopodin and ezrin indicated that myocilin-positive cells were predominately podocytes. During mesangioproliferative glomerulonephritis, an induction of myocilin immunoreactivity was observed. Labeling for myocilin was now observed in activated mesangial cells and areas of glomerular sclerosis. In parallel cell culture experiments, mRNA for myocilin was detected in cultured murine podocytes and rat mesangial cells.
Conclusion: Myocilin is expressed in podocytes of the kidney and induced in mesangial cells during experimental mesangioproliferative glomerulonephritis. The specific function of myocilin in the kidney is not clear, but in a parallel to functions of other olfactomedin proteins, it might have a role in cell-cell adhesion and/or signaling processes.