FUNCTIONAL PROMISCUITY OF GENE-REGULATION BY SERPENTINE RECEPTORS IN DICTYOSTELIUM-DISCOIDEUM

Serpentine receptors such as smoothened and frizzled play important ro les in cell fate determination during animal development. In Dictyaste lium discoideum, four serpentine cyclic AMP (cAMP) receptors (cARs) re gulate expression of multiple classes of developmental genes. To under stand their function, it is essential to know whether each cAR is coup led to a specific gene regulatory pathway or whether specificity resul ts from the different developmental regulation of individual cARs. To distinguish between these possibilities, we measured gene induction in car1 car3 double mutant cell lines that express equal levels of eithe r cAR1, cAR2, or cAR3 under a constitutive promoter. We found that all cARs efficiently mediate both aggregative gene induction by cAMP puls es and induction of postaggregative and prespore genes by persistent c AMP stimulation. Two exceptions to this functional promiscuity were ob served. (i) Only cAR1 can mediate adenosine inhibition of cAMP-induced prespore gene expression, a phenomenon that was found earlier in wild -type cells. cAR1's mediation of adenosine inhibition suggests that cA R1 normally mediates prespore gene induction. (ii) Only cAR2 allows en try into the prestalk pathway. Prestalk gene expression is induced by differentiation-inducing factor (DIF) but only after cells have been p restimulated with cAMP. We found that DIF-induced prestalk gene expres sion is 10 times higher in constitutive cAR2 expressors than in consti tutive cAR1 or cAR3 expressors (which still have endogenous cAR2), sug gesting that cAR2 mediates induction of DEF competence. Since in wild- type slugs cAR2 is expressed only in anterior cells, this could explai n the so far puzzling observations that prestalk cells differentiate a t the anterior region but that DIF levels are actually higher at the p osterior region. After the initial induction of DIF competence, cAMP b ecomes a repressor of prestalk gene expression. This function can agai n be mediated by cAR1, cAR2, and cAR3.