FORMATION AND TURNOVER OF THE KERATIN NETWORK IN MIGRATING EPITHELIAL CELLS.

Keratins are wide used markers for malignant epithelial cells, but their primary function is stabilization of epithelial tissues. Nevertheless, during metastasis, epithelia cells change from sessile cells into migrating cells. Consequently the stable keratin filament network connected to desmosomes has to undergo significant alterations which are required in a moving cell. Here we describe the involvement of keratins in the migration processes, which is typical for malignant cells. We want to answer the question how an altered keratin network of a migrating cell is built up, where this process takes place and how it is regulated. Using time lapse video microscopy of cells expressing fluorescently labelled keratins, we could show the formation of the new keratin network in the emerging lamellipodia of migrating cells. Close to the leading edge of lamellipodia, in direct proximity to focal contacts, the birth of keratin filament precursors (KFPs) could be observed as newly appearing fluorescent granules that subsequently elongate. During this elongation period, the KFPs moved retrograde along actin fibers and merged with the existing keratin network which was located further backwards in the lamellipodium. To further examine the origin of keratin protein forming KFPs, we fused photoactivable GFP (paGFP) to keratin. By examination of living cells expressing this protein, we could follow the dissociation of soluble keratin-paGFP from existing perinuclear filaments and the subsequent incorporation of keratin-paGFP in newly forming GFPs. We propose that these processes are part of a continuous turnover cycle of the keratin network, which enables the fast organization of the keratin network in the lamellipodia of migrating epithelial cells. [ABSTRACT FROM AUTHOR]