High cAMP in spares of Dictyostelium discoideum: association with spore dormancy and inhibition of germination

Signalling mechanisms involving cAMP have a well-documented role in the coo rdination of multicellular development and differentiation leading to spore formation in the social amoeba, Dictyostelium discoideum. The involvement of cAMP in the poorly understood developmental stages of spore dormancy and germination have been investigated in this study. Dormant spores contained up to Ii-fold more cAMP than nascent amoebae. The spore cAMP levels were n ot constant, but typically underwent a surge at 14-18 d when spores acquire d the ability to germinate spontaneously. The high cAMP levels decreased on ly during successful spore germination, i.e. emergence of nascent amoebae. The temporal pattern of cAMP decrease was complex and unique to the method of spore activation, supporting our hypothesis that exogenously (e.g. heat) activated and autoactivated spores germinate by different mechanisms. Duri ng heat-induced activation, transcription of acg (a gene encoding adenylyl cyclase associated with germination) correlated well with spore cAMP conten t. Young wild-type spores, incapable of spontaneous germination, maintained a uniformly high cAMP level, and spore cAMP levels also remained high if g ermination was inhibited. When activated spores were deactivated by applyin g increased osmotic pressure, cAMP concentrations rose and ultimately level led off at the high levels typical of dormant spores. The correlation betwe en high cAMP and failure to germinate was also evident when autoactivation was inhibited by the cAMP analogue, 8-bromo-cAMP. Also, spores from a strai n (HTY217) with unrestrained protein kinase A activity were incapable of sp ontaneous germination. Overall, our experiments provide evidence for contin ued cAMP signalling in spores up to 18 d after sporulation and for linkages between elevated cAMP, spore deactivation and inhibition of spontaneous ge rmination.