As a model for investigating gene regulation in relation to cell and tissue morphogenesis, we studied the expression of the adherens junction proteins, vinculin, α-actinin and actin, and that of desmosomal junctions containing the desmoplakin–cytokeratin complex, in response to changes in cell contacts and configuration. In monolayer or suspension cultures of kidney epithelial cells we found high levels of synthesis of cytokeratin and desmoplakin where extensive cell–cell contacts were established. In contrast, cells in sparse monolayers had high levels of the vimentin-type intermediate filaments, but very low levels of cytokeratins and desmoplakin I. Whereas in kidney epithelial cells all cytokeratins were coordinately regulated in response to changes in culture conditions, in mammary epithelial cells a new 45×103Mr. cytokeratin was induced in dense monolayer and suspension cultures. By treating cells with TPA, intercellular junctions were rapidly disrupted and expression of cytokeratin and desmoplakin was dramatically reduced; however, vimentin expression was not affected. In mammary epithelial cells only synthesis of the 45×103Mr cytokeratin was reduced in TPA-treated cells. Thus the synthesis of the cytokeratin–desmoplakin complex was coordinately regulated in response to changes in cell–cell contact and cell shape in a way that is compatible with the organization of these cells in vivo.
The relationship between the organization and expression of adherens junction proteins and their role in the acquisition of the differentiated phenotype was studied in fibroblasts and in differentiating ovarian granulosa cells. The synthesis of vinculin in cultured fibroblasts increased dramatically when the cell culture density was high, concomitant with the establishment of extensive cell–substratum and cell–cell contacts of the adherens type. When fibroblasts were plated on substrata of varying adhesiveness, to modulate cell shape from a flat and well-spread to a poorly adherent spherical shape, there was a relationship between vinculin organization and expression: vinculin synthesis decreased dramatically in round cells.
The differentiation of freshly isolated ovarian granulosa cells (as measured by production of high levels of progesterone) in response to gonadotropic hormones was followed by dramatic changes in cell shape and organization and expression of adherens junction proteins. Cell shape changed from a flat fibroblastic type to a spherical one, with a reduction in vinculin-containing plaques and the disappearance of actin-containing stress fibres. Synthesis of vinculin, α-actinin and actin was significantly reduced but that of tubulin and vimentin was unchanged. Interestingly, when granulosa cells were plated on an extracellular matrix derived from endothelial cells, they underwent differentiation, even in the absence of gonadotropins, producing high levels of progesterone with similar changes in adherens junction protein synthesis and organization. The involvement of changes in organization and expression of adherens junctions in granulosa cell differentiation was further suggested by the observation that treatment with cytochalasin B alone was sufficient to induce simultaneous changes in adherens junction protein expression and progesterone production. Thus the modulation of expression and organization of these junctional proteins may be a central part of the programme of granulosa cell differentiation.
The experimental systems described in this overview demonstrate a link between changes in cell contacts, cell configuration and the expression of differentiated tissue functions. They also provide us with a model with which to study the regulation of the organization and expression of junctional components in response to changes in cellular and tissue morphogenesis.