Dictyostelium discoideum Perspectives from a Social Amoeba

Many different evolutionary branches seem to have evolved their own ameboid life form. A common aspect of their conditions for thriving is reliance on bacteria as a major food source. On an evolutionary time scale, protozoa-bacteria interactions may have generated a pool of virulence traits, which preadapted some bacterial species as human pathogens. The protozoan mechanisms of phagocytosis use signaling pathways and cytoskeleton proteins closely related to those of macrophages, linking "primitive" organisms to highly organized multicellular species. D. discoideum belongs to a taxonomic entity called the social amoebae, since it is able to transform from a unicellular to a multicellular life stage under adverse living conditions [12]. The entire branch to which this group belongs is thought to constitute a kingdom in the eukaryote tree of life in its own right besides the plant and animal/fungi kingdoms [13]. The haploid genome is 34 Mbp in size. The six chromosomes carry approximately 13 000 genes, of which many are homologous to those in higher eukaryotes (http://dictybase.org) [14]. Since many D. discoideum mutants exist, it is relatively easy to test these mutants for their ability to alter the outcome of an infection [15].

The pathogens predominantly analyzed in D. discoideum are Legionella pneumo-phila, Mycobacterium spp., Pseudomonas aeruginosa, and Cryptococcus neoformans [16-22]. In the case of Legionella, the causative agent of legionnaires' disease, a roadmap of infection-relevant host cell factors is being developed [20]. This road map shows that many general features such as the mode of invasion seem to be the same for macrophages and amoebae. Phagocytosis assays with specific cellular inhibitors and the effects of well-defined host-cell mutants revealed that cyto-plasmic calcium levels, cytoskeleton-associated proteins, and the calcium-binding proteins of the endoplasmic reticulum specifically influence the uptake and intra-cellular growth of L. pneumophila. In contrast, macroautophagy appears to be dis pensable for intracellular replication of L. pneumophila in D.discoideum [15]. For some host factors further analyses were performed by GFP (green fluorescence protein) fusion proteins. On the pathogen side it was shown that Legionella mutants deficient for growth in macrophages are also unable to grow in D. discoi-deum. This also includes the Legionella type IV secretion system Icm/Dot [23].

Dictyostelium wild-type cells were also used as screening system for other muta-genized pathogens. By simple plating assays it was shown that the extracellular pathogen P. aeruginosa utilizes virulence pathways mediated by quorum sensing to infect D. discoideum [24, 25]. An acapsular mutant of the environmental fungus C. neoformans was found to be avirulent in Dictyostelium spp. and mice [22].

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