Subcellular localization of moesin in dynamic filopodia, retraction fibers, and other structures involved in substrate exploration, attachment, and cell-cell contacts

Abstract

Moesin, a member of the talin-4.1 superfamily, is a linking protein of the submembraneous cytoskeleton. It is expressed in variable amounts in cells of different phenotypes such as macrophages, lymphocytes, fibroblastic, endothelial, epithelial, and neuronal cell lines. In this report we show that moesin is not randomly distributed throughout the cortical cytoskeleton, but rather that it is concentrated in specialized microdomains. It is localized in the intracellular core of microextensions known as filopodia, microvilli, microspikes, and retraction fibers. This subcellular distribution follows closely the dynamic changes in cell shape that take place when cells attach, spread, and move spontaneously or in response to extracellular signals. This suggests a similar function for moesin in diverse cell types related to the dynamic restructuring of domains of the plasma membrane and underlying membrane skeleton. Support for this comes from studies on PC-12 cells, which respond to NGF by extending neurites and moesin is redistributed from a diffuse localization to growth cone filopodia. In fibroblastic (NIH3T3) or macrophage (RAW264.7) cell lines, moesin is found in filopodia appearing at random on the cell surface soon after the cells are placed in culture, begin to attach, and spread. In polarized epithelial cells (LLC-PK1), moesin is associated with peripheral filopodia and apical microvilli. The cellular microextensions containing moesin are devoid of microtubules, focal contact proteins such as vinculin, and cortical cytoskeletal elements such as protein 4.1, but they do contain varying amounts of actin microfilaments. This localization of moesin in microextensions is not influenced by cytochalasin B. Treatment of cells with phorbolester (PMA) causes rapid cell spreading, disappearance of filopodia and retraction fibers, and moesin does not accumulate in the actin-rich lamellae that form at the cellular edges. After removal of PMA, cells retract and moesin again becomes concentrated in filopodia and retraction fibers. These studies support the hypothesis that filopodia, retraction fibers, and other microextensions of the plasma membrane are unique cellular microdomains with characteristic submembraneous components. Moesin could be involved in the dynamic restructuring of such microdomains by regulating binding interactions between the plasma membrane and the actin cytoskeleton.