Adenylyl cyclase and the control of cell differentiation in Dictyostelium dicoideum

Adenylyl cyclase is part of a biochemical network that controls cell differentiation in Dictyostelium discoideum. At a certain stage of development the enzyme is rhythmically activated, with periods of about 8 min. These oscillations are superimposed upon an increase of the basal activity extending over a period of hours. The basal activity remains low in a mutant blocked at an early stage of development. In strain Ax-2 two periods of strongly increasing basal activity have been found: the first from 2 to 4 h after the end of the growth phase, the other beginning at about 8 h. Based on the periodic regulation of adenylyl cyclase, cyclic AMP is released into the extracellular space in the form of pulses. Application of cyclic-AMP pulses, but not its continuous influx, stimulates the increase of basal adenylyl cyclase activity. Two other constituents of the cyclic-AMP signal system cyclic-AMP receptors and cell-surface phosphodiesterase, are similarly controlled. The principal importance of positive feedback loops in a network controlling cell differentiation is discussed.