Interaction of endogenous nephrin and CD2-associated protein in mouse epithelial M-1 cell line.

The interpodocyte slit diaphragm is an essential structure for maintaining the functional glomerular filtration barrier. The slit diaphragm is proposed to consist of an interacting meshwork of nephrin molecules. Earlier studies with tagged proteins have suggested that the intracellular part of nephrin interacts with CD2-associated protein (CD2AP). This study was addressed to show by coimmunoprecipitation and pulldown assays an interaction of endogenously expressed nephrin and CD2AP in the kidney-derived mouse epithelial M-1 cell line, to provide evidence of the domain(s) of CD2AP involved in the interaction, and to show the localization of the respective proteins by immunoelectron microscopy in kidney cortex. In addition, the localization of CD2AP, podocin, alpha-actinin 4, and nephrin was studied in human kidney glomeruli and in M-1 cells by immunofluorescence microscopy. The results indicate an endogenous interaction between nephrin and CD2AP in M-1 cells and suggest that this interaction is mediated by the third Src homology 3 (SH3) domain of CD2AP. We also show by immunoelectron microscopy that nephrin and CD2AP are detected at the slit diaphragm area, supporting their interaction in the glomeruli in vivo. In addition, nephrin was found to partially colocalize with CD2AP and podocin in double immunofluorescence microscopy, confirming the close proximity of these proteins and proposing that these proteins may belong to nephrin-associated protein complex in glomeruli. The existence of nephrin, CD2AP, podocin, and alpha-actinin 4 enables further characterization of their relationship in M-1 cells.