Activated cAMP receptors switch encystation into sporulation

Abstract

Metazoan embryogenesis is controlled by a limited number of signaling modules that are used repetitively at successive developmental stages. The development of social amoebas shows similar reiterated use of cAMP-mediated signaling. In the model Dictyostelium discoideum, secreted cAMP acting on 4 cAMP receptors (cARs1-4) coordinates cell movement during aggregation and fruiting body formation, and induces the expression of aggregation and sporulation genes at consecutive developmental stages. To identify hierarchy in the multiple roles of cAMP, we investigated cAR heterogeneity and function across the social amoeba phylogeny. The gene duplications that yielded cARs 2-4 occurred late in evolution. Many species have only a cAR1 ortholog that duplicated independently in the Polysphondylids and Acytostelids. Disruption of both cAR genes of Polysphondylium pallidum (Ppal) did not affect aggregation, but caused complete collapse of fruiting body morphogenesis. The stunted structures contained disorganized stalk cells, which supported a mass of cysts instead of spores; cAMP triggered spore gene expression in Ppal, but not in the cAR null mutant, explaining its sporulation defect. Encystation is the survival strategy of solitary amoebas, and lower taxa, like Ppal, can still encyst as single cells. Recent findings showed that intracellular cAMP accumulation suffices to trigger encystation, whereas it is a complementary requirement for sporulation. Combined, the data suggest that cAMP signaling in social amoebas evolved from cAMP-mediated encystation in solitary amoebas; cAMP secretion in aggregates prompted the starving cells to form spores and not cysts, and additionally organized fruiting body morphogenesis. cAMP-mediated aggregation was the most recent innovation.developmental signaling ͉ evolution of multicellularity ͉ Dictyostelia ͉ Amoebozoa

Evolutionary crossroads in developmental biology: Dictyostelium discoideum

“…This role of oscillatory cAMP signalling therefore evolved first and its additional role in coordinating aggregation appeared more recently in Dictyostelia evolution. cAR gene disruption in the group 2 species P. pallidum not only disorganizes fruiting body morphogenesis but also results in stunted structures that form cysts instead of spores in the spore head (Kawabe et al, 2009). This is because the cAR-null mutant no longer expresses prespore genes in response to cAMP stimulation (Kawabe et al, 2009).…”

“…cAR gene disruption in the group 2 species P. pallidum not only disorganizes fruiting body morphogenesis but also results in stunted structures that form cysts instead of spores in the spore head (Kawabe et al, 2009). This is because the cAR-null mutant no longer expresses prespore genes in response to cAMP stimulation (Kawabe et al, 2009). As discussed above, sporulation and encystation both require intracellular cAMP acting on PKA, but sporulation additionally requires extracellular cAMP acting on cARs.…”

Dictyostelium Finds New Roles to Model

“…This is being addressed by whole-genome sequence analysis of at least one representative from each of the four groups and by parallel studies of genes known to be key to the development of D. discoideum (Kawabe et al 2002(Kawabe et al , 2009). The new sequences, from A. subglobosum (http:/ /acyto.sequence.info/), D. purpurem (http:/ / dictybase.org/), P. pallidum (http:/ /sacgb.fli-leibniz.de/ ), Dictyostelium fasciculatum (http:/ /sacgb.fli-leibniz.de/), and D. lacteum (P. Schaap, personal communication), will also be extremely valuable in other, more general ways.…”

Evolution of self-organisation in Dictyostelia by adaptation of a non-selective phosphodiesterase and a matrix component for regulated cAMP degradation

“…Both PCR products were cloned into vector pLoxP-NeoIII, yielding pPpal PdsA-KO. To construct pLoxP-NeoIII, the XbaI site in pLoxP-NeoI (Kawabe et al, 2009) was destroyed by digestion with XbaI, fill-in with Klenow and self-ligation with T4-ligase. The A6Neo cassette with flanking LoxP sites was next excised with BamHI/HindIII and cloned into pBluescript SK -, creating pLoxP-NeoIII.…”

“…Ppal cells were transformed with either of the vectors. -Galactosidase activity was visualised with X-gal in developing Ppal structures as described previously (Dingermann et al, 1989;Kawabe et al, 2009). …”