Dynamic aspects of the supramolecular organization of intermediate filament networks in cultured epidermal cells

Abstract

We have shown, by indirect immunofluorescence microscopy using an antiserum against the mouse keratin subunit K2 and by electron microscopy, that transformed (PAM) and primary (PME) mouse epidermal cells possess extensive net works of IF bundles. Following trypsinization and replating of PAM cells, IF bundles are seen to move as a continuous net work from a perinuclear zone into the peripheral cytoplasmic regions. In PAM cells lysed in high‐ionic‐strength solutions containing Triton ×‐100 and DNAase‐1, IF bundles appear to be closely associated with nuclear envelope remnants and, in some cases, appear to be attached to nuclear pore complexes. PME cells cultivated in low Ca²⁺‐containing medium possess perinuclear birefringent arrays of IF bundles. Within 2 hours of switching the cells to normal Ca²⁺ levels, the PME IF bundle network moves towards and establishes contact with the cell surface as desmosomes form. Live cells observed by phase contrast and fixed cells observed by immunofluorescence microscopy demonstrate that desmosomes can be distinguished as dark bands separating neighboring cells. There is little difference between the major proteins seen in SDS‐polyacrylamide gel profiles of isolated IF bundle net works from PME cells before and after the Ca²⁺ switch. Therefore, a reorganization of relatively insoluble membrane‐associated protein following the Ca²⁺ switch may be involved in desmosome formation. The isolated IF networks from PAM cells differ in protein composition compared to the PME IF networks. This may be related to the greatly reduced number of desmosomes in PAM cells. The IF bundle system in epidermal cells appears to be involved in shape formation, shape maintenance, the establishment of desmosomes, nuclear centration, and cell‐cell contact.