Molecular heterogeneity of the actin filament cytoskeleton associated with microvilli of photoreceptors, Müller's glial cells and pigment epithelial cells of the retina.

The present study addressed the question as to whether the four different actin-associated proteins that are associated with the actin core bundle in intestinal microvilli (i.e. villin, fimbrin, myosin I and ezrin) are essential components of all microvilli of the body. The retina provides an excellent example of a tissue supplied with three different sets of microvilli, namely those of Müller's glial cells (Müller baskets), photoreceptors (calycal processes), and pigment epithelial cells. The main outcome of this study is that none of these microvilli contain all four actin-associated proteins present in intestinal microvilli. Müller cell microvilli contain villin, ezrin and myosin I (95 kDa isoform) but not fimbrin. Calycal processes of photoreceptors contain fimbrin but not villin, myosin I and ezrin. Finally, microvilli of pigment epithelial cells are positive for ezrin but not for villin, fimbrin and myosin I. Because of limited cross-reactivities of the antibodies to myosin I and ezrin, the myosin I data refer to the chicken retina whereas the findings with anti-ezrin were obtained with the rat retina. A further outcome of this study is that the actin filament core bundles in microvilli of chicken pigment epithelial cells are presumed to contain a crosslinking protein, which is not immunologically related to either villin, fimbrin or myosin I of the intestinal brush border.