Cytokeratins in mesenchymal cells: impact on functional concepts of the diversity of intermediate filament proteins.

Shape, motility, viscosity and organelle distribution of animal cells is regulated, or at least influenced, by fibrous proteins constituting a 'cytoskeleton' that involves the F-actin-based microfilaments (5-7 nm in diameter) and the microtubules (20-25 nm). In addition, most vertebrate cells contain a third type of highly organized protein bundle, the intermediate filaments (IF; 8-12 nm). Their specific role is less clear, but they certainly contribute to tensile strength of cells and, via their anchorages at desmosomes, to cell-cell interactions (for reviews, see Franke et al. 1982; Traub, 1985; Krohne and Benavente, 1986; Franke, 1987; Steinert and Roop, 1988; Nagle, 1988; Bershadsky and Vasiliev, 1988; Robson, 1989; CarmoFonseca and David-Ferreira, 1990). Ultrastructurally, the IFs of different cell types are very similar, but biochemical and immunological data have revealed in mammals at least seven distinct classes comprising, for example in humans, a total of approximately 40 different polypeptides: vimentin (predominantly, but not exclusively in mesenchymally derived cells), desmin (typical of most types of muscle cells), glial filament protein (GFP; predominantly in astrocytes), several different neurofilament proteins (in most neurons), peripherin (in certain neuronal cells), at least four different lamins (structural components of the nuclear lamina), and a whole panopticum of cytokeratins (present in almost all epithelial cells). A given tissue is characterized by a specific pattern of IF polypeptides, which is extensively used for cell typing in tumor diagnosis (see Kartenbeck, 1989), and as a differentiation marker in embryogenesis (e.g. see LaFlamme and Dawid, 1990).