Specific cell-cell contact serves as the developmental signal to deactivate discoidin I gene expression in Dictyostelium discoideum

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Abstract
Specific cell-cell contact is a major regulatory signal controlling cell differentiation in D. discoideum, causing dramatic changes in the developmental program of gene expression. This report focuses on the relationships between specific cell-cell contact and the activity of the genes for discoidin I, an endogenous lectin that has been implicated in the cell-cell cohesion process. By performing quantitative RNA dot-hybridization assays and RNA gel blot-hybridization analyses, using as a probe a recombinant plasmid containing a discoidin I c[complementary]DNA insert, changes in discoidin I mRNA levels are measured during normal development and in response to specific manipulations of the state of cellular aggregation. During normal development on filters, there is a close temporal correspondence between the establishment of specific cell-cell contacts and the decline in discoidin I mRNA levels. By the tight-aggregate stage, discoidin I mRNA is barely detectable. When tight aggregates are disaggregated and the cells are maintained in the disaggregated state, there is a dramatic rise in discoidin I mRNA content. When cells are developed in suspension (conditions that interfere with the establishment of tight cell-cell contacts), discoidin I mRNA accumulates to abnormally high levels, and these persist well after the levels in filter-developed cells have declined. These results strongly suggest that cell-cell contact is the normal developmental signal to deactivate discoidin I gene expression; thus, a contact-deactivated gene for which a recombinant DNA probe is available has now been identified. Exogenous cAMP almost completely blocks the disaggregation-induced reactivation of discoidin I gene expression. Possible mechanistic relationships between specific cell-cell contact, intracellular cAMP levels, and developmental gene expression are discussed.