The Dictyostelium MAP kinase ERK2 regulates multiple, independent developmental pathways

We showed previously that the MAP kinase ERK2 is essential for aggregation. erk2 null cells lack cAMP stimulation of adenylyl cyclase and thus cannot relay the cAMP chemotactic signal, although the cells chemotax to cAMP (Segall et al. 1995). In this paper we have examined the role of ERK2 in controlling developmental gene expression and morphogenesis during the multicellular stages, making use of a temperature-sensitive ERK2 mutation. Using suspension assays, we show that ERK2 is not essential for aggregation- stage, cAMP pulse-induced gene expression, or for the expression of postaggregative genes, which are induced at the onset of mound formation in response to cAMP in wild-type cells. In contrast, the prespore-specific gene SP60 is not induced and the prestalk-specific gene ecmA is induced but at a significantly reduced level. Chimeric organisms, comprised of wild-type and erk2 null cells expressing the prestalk-specific ecmA/lacZ reporter, show an abnormal spatial patterning, in which Erk2(ts)/erk2 cells are excluded from the very anterior prestalk A region. To further examine the function of ERK2 during the multicellular stages, we bypassed the requirement of ERK2 for aggregation by creating an ERK2 temperature-sensitive mutant. erk2 null cells expressing the ERK2(ts) mutant develop normally at 20°C and express cell- type-specific genes but do not aggregate at temperatures above 25°C. Using temperature shift experiments, we showed that ERK2 is essential for proper morphogenesis and for the induction and maintenance of prespore but not prestalk gene expression. Our results indicate that ERK2 functions at independent stages during Dictyostelium development to control distinct developmental programs: during aggregation, ERK2 is required for the activation of adenylyl cyclase and during multicellular development, ERK2 is essential for morphogenesis and cell-type-specific gene expression. Analysis of these results and others supports the conclusion that the requirement of ERK2 for cell-type differentiation is independent of its role in the activation of adenylyl cyclase.