dictyNews
Electronic Edition
Volume 37, number 4
August 12, 2011

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Abstracts
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Salmonella typhimurium is pathogenic for Dictyostelium cells and subverts
the starvation response

Alessio Sillo*1, Jan Matthias#1, Roman Konertz#, Salvatore Bozzaro*2 and
Ludwig Eichinger#2§

*Department of Clinical and Biological Sciences, University of Turin, AOU
S. Luigi, 10043 Orbassano (Torino), Italy 
#Center for Biochemistry, Medical Faculty, University of Cologne,
Joseph-Stelzmann-Str. 52, D-50931 Cologne (Germany)


Cell. Microbiol., in press

In unicellular amoebae, such as Dictyostelium discoideum, bacterial
phagocytosis is a food hunting device, while in higher organisms it is the
first defence barrier against microbial infection. In both cases,
pathogenic bacteria exploit phagocytosis to enter the cell and multiply
intracellularly. Salmonella typhimurium, the agent of food-borne
gastroenteritis, is phagocytosed by both macrophages and Dictyostelium
cells. By using cell biological assays and global transcriptional analysis
with DNA microarrays covering the Dictyostelium genome, we show here that
S. typhimurium is pathogenic for Dictyostelium cells. Depending on the
degree of virulence, which in turn depended on bacterial growth conditions,
Salmonella could kill Dictyostelium cells or inhibit their growth and
development. In the early phase of infection in non-nutrient buffer, the
ingested bacteria escaped degradation, induced a starvation-like
transcriptional response but inhibited selectively genes required for
chemotaxis and aggregation. This way differentiation of the host cells into
spore and stalk cells was blocked or delayed, which in turn is likely to be
favourable for the establishment of a replicative niche for Salmonella.
Inhibition of the aggregation competence and chemotactic streaming of
aggregation-competent cells in the presence of Salmonella suggests
interference with cAMP signalling.


Submitted by: Eichinger Ludwig [[log in to unmask]]
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A dual role for Rac1 GTPases in the regulation of cell motility

Vedrana Filic, Maja Marinovic, Jan Faix and Igor Weber


Journal of Cell Science, in press

Rac proteins are the only canonical Rho family GTPases in Dictyostelium, 
where they act as key regulators of the actin cytoskeleton. In order to monitor 
the dynamics of activated Rac1 in Dictyostelium cells, a fluorescent probe 
was developed that specifically binds to GTP-bound form of Rac1. The 
probe is based on the GTPase-binding domain (GBD) from PAK1 kinase, 
and was selected on the basis of yeast two-hybrid, GST pull-down and 
fluorescence resonance energy transfer assays. PAK1_GBD localizes to 
leading edges of migrating cells and to endocytotic cups. As in vertebrates, 
activated Rac1 therefore appears to control de novo actin polymerization 
at protruding regions of the cell. Additionally, we found that the IQGAP-
related protein DGAP1, which sequesters active Rac1 into a quaternary 
complex with actin-binding proteins cortexillin I and II, localizes to the 
trailing regions of migrating cells. Notably, PAK1_GBD and DGAP1, 
which both bind to Rac1-GTP, display mutually exclusive localizations in 
cell migration, phagocytosis and cytokinesis, and opposite dynamics of 
recruitment to the cell cortex upon stimulation with chemoattractants. 
Moreover, cortical localization of PAK1_GBD depends on integrity of the 
actin cytoskeleton, whereas cortical localization of DGAP1 does not. 
Taken together, these results imply that Rac1 GTPases play a dual role 
in regulation of cell motility and polarity in Dictyostelium.


Submitted by Igor Weber [[log in to unmask]]
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[End dictyNews, volume 37, number 4]