cAMP-dependent protein kinase regulates Polysphondylium pallidum development

Emerging roles for diguanylate cyclase during the evolution of soma in dictyostelia

BACKGROUND

Cyclic di-guanylate (c-di-GMP), synthesized by diguanylate cyclase, is a major second messenger in prokaryotes, where it triggers biofilm formation. The dictyostelid social amoebas acquired diguanylate cyclase (dgcA) by horizontal gene transfer. Dictyostelium discoideum (Ddis) in taxon group 4 uses c-di-GMP as a secreted signal to induce differentiation of stalk cells, the ancestral somatic cell type that supports the propagating spores. We here investigated how this role for c-di-GMP evolved in Dictyostelia by exploring dgcA function in the group 2 species Polysphondylium pallidum (Ppal) and in Polysphondylium violaceum (Pvio), which resides in a small sister clade to group 4.

RESULTS

Similar to Ddis, dgcA is upregulated after aggregation in Ppal and Pvio and predominantly expressed in the anterior region and stalks of emerging fruiting bodies. DgcA null mutants in Ppal and Pvio made fruiting bodies with very long and thin stalks and only few spores and showed delayed aggregation and larger aggregates, respectively. Ddis dgcA- cells cannot form stalks at all, but showed no aggregation defects. The long, thin stalks of Ppal and Pvio dgcA- mutants were also observed in acaA- mutants in these species. AcaA encodes adenylate cyclase A, which mediates the effects of c-di-GMP on stalk induction in Ddis. Other factors that promote stalk formation in Ddis are DIF-1, produced by the polyketide synthase StlB, low ammonia, facilitated by the ammonia transporter AmtC, and high oxygen, detected by the oxygen sensor PhyA (prolyl 4-hydroxylase). We deleted the single stlB, amtC and phyA genes in Pvio wild-type and dgcA- cells. Neither of these interventions affected stalk formation in Pvio wild-type and not or very mildly exacerbated the long thin stalk phenotype of Pvio dgcA- cells.

CONCLUSIONS

The study reveals a novel role for c-di-GMP in aggregation, while the reduced spore number in Pvio and Ppal dgcA- is likely an indirect effect, due to depletion of the cell pool by the extended stalk formation. The results indicate that in addition to c-di-GMP, Dictyostelia ancestrally used an as yet unknown factor for induction of stalk formation. The activation of AcaA by c-di-GMP is likely conserved throughout Dictyostelia.

Development of the dictyostelid Polysphondylium violaceum does not require secreted cAMP

Group 4 Dictyostelia, like Dictyostelium discoideum, self-organize into aggregates and fruiting bodies using propagating waves of the chemoattractant cAMP, which are produced by a network containing the adenylate cyclase AcaA, cAMP receptors (Cars) and the extracellular cAMP phosphodiesterase PdsA. Additionally, AcaA and the adenylate cyclases AcrA and AcgA produce secreted cAMP for induction of aggregative and prespore gene expression and intracellular cAMP for PKA activation, with PKA triggering initiation of development and spore and stalk maturation. Non-group 4 species also use secreted cAMP to coordinate post-aggregative morphogenesis and prespore induction but use other attractants to aggregate. To understand how cAMP's role in aggregation evolved, we deleted the acaA, carA and pdsA genes of Polysphondylium violaceum, a sister species to group 4. acaA- fruiting bodies had thinner stalks but otherwise developed normally. Deletion of acrA, which was similarly expressed as acaA, reduced aggregation centre initiation and, as also occurred after D. discoideum acrA deletion, caused spore instability. Double acaA-acrA- mutants failed to form stable aggregates, a defect that was overcome by exposure to the PKA agonist 8Br-cAMP, and therefore likely due to reduced intracellular cAMP. The carA- and pdsA- mutants showed normal aggregation and fruiting body development. Together, the data showed that P. violaceum development does not critically require secreted cAMP, while roles of intracellular cAMP in initiation of development and spore maturation are conserved. Apparently, cell-cell communication underwent major taxon-group specific innovation in Dictyostelia.

From environmental sensing to developmental control: cognitive evolution in dictyostelid social amoebas

Dictyostelid social amoebas respond to starvation by self-organizing into multicellular slugs that migrate towards light to construct spore-bearing structures. These behaviours depend on excitable networks that enable amoebas to produce propagating waves of the chemoattractant cAMP, and to respond by directional movement. cAMP additionally regulates cell differentiation throughout development, with differentiation and cell movement being coordinated by interaction of the stalk inducer c-di-GMP with the adenylate cyclase that generates cAMP oscillations. Evolutionary studies indicate how the manifold roles of cAMP in multicellular development evolved from a role as intermediate for starvation-induced encystation in the unicellular ancestor. A merger of this stress response with the chemotaxis excitable networks yielded the developmental complexity and cognitive capabilities of extant Dictyostelia.

This article is part of the theme issue ‘Basal cognition: conceptual tools and the view from the single cell’.

Evolution of multicellularity in Dictyostelia

The well-orchestrated multicellular life cycle of Dictyostelium discoideum has fascinated biologists for over a century. Self-organisation of its amoebas into aggregates, migrating slugs and fruiting structures by pulsatile cAMP signalling and their ability to follow separate differentiation pathways in well-regulated proportions continue to be topics under investigation. A striking aspect of D. discoideum development is the recurrent use of cAMP as chemoattractant, differentiation inducing signal and second messenger for other signals that control the developmental programme. D. discoideum is one of >150 species of Dictyostelia and aggregative life styles similar to those of Dictyostelia evolved many times in eukaryotes. Here we review experimental studies investigating how phenotypic complexity and cAMP signalling co-evolved in Dictyostelia. In addition, we summarize comparative genomic studies of multicellular Dictyostelia and unicellular Amoebozoa aimed to identify evolutionary conservation and change in all genes known to be essential for D. discoideum development.

Two potential evolutionary origins of the fruiting bodies of the dictyostelid slime moulds

Dictyostelium discoideum and the other dictyostelid slime moulds ('social amoebae') are popular model organisms best known for their demonstration of sorocarpic development. In this process, many cells aggregate to form a multicellular unit that ultimately becomes a fruiting body bearing asexual spores. Several other unrelated microorganisms undergo comparable processes, and in some it is evident that their multicellular development evolved from the differentiation process of encystation. While it has been argued that the dictyostelid fruiting body had similar origins, it has also been proposed that dictyostelid sorocarpy evolved from the unicellular fruiting process found in other amoebozoan slime moulds. This paper reviews the developmental biology of the dictyostelids and other relevant organisms and reassesses the two hypotheses on the evolutionary origins of dictyostelid development. Recent advances in phylogeny, genetics, and genomics and transcriptomics indicate that further research is necessary to determine whether or not the fruiting bodies of the dictyostelids and their closest relatives, the myxomycetes and protosporangids, are homologous.

The polyketide MPBD initiates the SDF-1 signaling cascade that coordinates terminal differentiation in Dictyostelium

Dictyostelium uses a wide array of chemical signals to coordinate differentiation as it switches from a unicellular to a multicellular organism. MPBD, the product of the polyketide synthase encoded by stlA, regulates stalk and spore differentiation by rapidly stimulating the release of the phosphopeptide SDF-1. By analyzing specific mutants affected in MPBD or SDF-1 production, we delineated a signal transduction cascade through the membrane receptor CrlA coupled to Gα1, leading to the inhibition of GskA so that the precursor of SDF-1 is released. It is then processed by the extracellular protease of TagB on prestalk cells. SDF-1 apparently acts through the adenylyl cyclase ACG to activate the cyclic AMP (cAMP)-dependent protein kinase A (PKA) and trigger the production of more SDF-1. This signaling cascade shows similarities to the SDF-2 signaling pathway, which acts later to induce rapid spore encapsulation.

From drought sensing to developmental control: evolution of cyclic AMP signaling in social amoebas

Amoebas and other protists commonly encyst when faced with environmental stress. Although little is known of the signaling pathways that mediate encystation, the analogous process of spore formation in dictyostelid social amoebas is better understood. In Dictyostelium discoideum, secreted cyclic AMP (cAMP) mediates the aggregation of starving amoebas and induces the differentiation of prespore cells. Intracellular cAMP acting on cAMP-dependent protein kinase (PKA) triggers the maturation of spores and prevents their germination under the prevalent conditions of high osmolality in the spore head. The osmolyte-activated adenylate cyclase, ACG, produces cAMP for prespore differentiation and inhibition of spore germination. To retrace the origin of ACG function, we investigated ACG gene conservation and function in species that span the dictyostelid phylogeny. ACG genes, osmolyte-activated ACG activity, and osmoregulation of spore germination were detected in species that represent the 4 major groups of Dictyostelia. Unlike the derived species D. discoideum, many basal Dictyostelia have retained the ancestral mechanism of encystation from solitary amoebas. In these species and in solitary amoebas, encystation is independently triggered by starvation or by high osmolality. Osmolyte-induced encystation was accompanied by an increase in cAMP and prevented by inhibition of PKA, indicating that ACG and PKA activation mediate this response. We propose that high osmolality signals drought in soil amoebas and that developmental cAMP signaling in the Dictyostelia has evolved from this stress response.