REVIEWPodosomes as smart regulators of cellular adhesion
Introduction
Since their early description (Zallone et al., 1983; Marchisio et al., 1984) as cell–matrix adhesion structures podosomes were only found in mesenchymal cells and were particularly abundant in hematopoietic stem cell-derived lineages including osteoclasts and some human leukemias (Marchisio et al., 1984, Marchisio et al., 1988; Gaidano et al., 1990). Podosomes were also abundantly assembled in several cell types transformed by src (Marchisio et al., 1984) where they could cluster in complex rosette-like structures (Fig. 1). Indeed, the podosome denomination was chosen to indicate that these adhesion structures were presumably involved in the regulation of cell locomotion (Tarone et al., 1985). Fine electron microscopic analysis showed that individual podosomes corresponded to columnar arrays of actin filaments surrounding a narrow tubular invagination of the plasmalemma roughly perpendicular to the substratum (Nitsch et al., 1989) that may convey membrane-bound or secreted molecules onto the extracellular matrix as also suggested by independent studies (Chen, 1989). Podosomes have cytoskeletal components similar to those found in focal contacts but also contain some unique markers, such as the actin-binding proteins gelsolin, dynamin, and cortactin. Moreover, they are much more dynamic and their assembly takes place also when protein synthesis is temporarily blocked.
Section snippets
Epithelial podosomes as hemidesmosome-organizing centers
Our studies (Spinardi et al., 2004) showed a new podosome-like structure in adhering epithelial cells. Each podosome structure appears to be surrounded by a rosette of hemidesmosomes that suggests an organized unit of cytoskeletal structures related to the firm epithelial adhesion to the basement membrane (Fig. 2). The epithelial podosome-like structures are remarkably similar to those described in several cells types of mesenchymal origin. Both structures are organized around a small tubular
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Present address: ConStem, Department of Biotechnology and Biosciences, University of Milano Bicocca, Piazza della Scienza 2, I-20126 Milano, Italy.