dictyNews
Electronic Edition
Volume 38, number 22
August 31, 2012

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Abstracts
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The Nramp (Slc11) proteins regulate development, resistance to 
pathogenic bacteria and iron homeostasis in Dictyostelium discoideum

Barbara Peracino, Simona Buracco and Salvatore Bozzaro*
Department of Clinical and Biological Sciences, University of Turin, 
AOU S. Luigi, 10043 Orbassano, Italy


J. Cell Sci,. in press

The Dictyostelium discoideum genome harbours two genes encoding 
members of the Nramp superfamily, which is conserved from bacteria 
(MntH proteins) to humans (Slc11 proteins). Nramps are proton-driven 
metal ion transporters with a preference for iron and manganese. 
Acquisition of these metal cations is vital for all cells, as they act as 
redox cofactors and regulate key cellular processes, such as DNA 
synthesis, electron transport, energy metabolism and oxidative stress. 
Dictyostelium Nramp1 (Slc11a1), like its mammalian ortholog, mediates 
resistance to infection by invasive bacteria. We have extended the 
analysis to the nramp2 gene, by generating single and double 
nramp1/nramp2 knockout mutants and cells expressing GFP fusion 
proteins. In contrast to Nramp1, which is recruited to phagosomes and 
macropinosomes, the Nramp2 protein is localized exclusively in the 
membrane of the contractile vacuole, a vesicular-tubular network 
regulating cellular osmolarity. Both proteins colocalize with the 
V-H+ ATPase, which can provide the electrogenic force for vectorial 
transport. Like nramp1, nramp2 gene disruption affects resistance to 
Legionella pneumophila. Disrupting both genes additionally leads to 
defects in development, with strong delay in cell aggregation, formation 
of large streams and multi-tipped aggregates. Single and double mutants 
display differential sensitivity to cell growth under conditions of iron 
overload or depletion. The data favour the hypothesis that Nramp1 and 
Nramp2, under control of the V-H+ ATPase, synergistically regulate iron 
homeostasis, with the contractile vacuole possibly acting as a store for 
metal cations. 


Submitted by Salvatore Bozzaro [[log in to unmask]]
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Regulation of multiple tip formation by caffeine in cellular 
slime molds

Pundrik Jaiswal, Shashi Prakash Singh, Prasad Aiyar, 
Rakhil Akkali and Ramamurthy Baskar 


BMC Development Biology 2012, 12:26 

Background
The multicellular slug in Dictyostelium has a single tip that acts as 
an organising centre patterning the rest of the slug. High adenosine 
levels at the tip are believed to be responsible for this tip dominance 
and the adenosine antagonist, caffeine overrides this dominance 
promoting multiple tip formation.

Results
Caffeine induced multiple tip effect is conserved in all the Dictyostelids 
tested. Two key components of cAMP relay namely, cAMP 
phosphodiesterase (Pde4) and adenyl cyclase-A (AcaA) levels get 
reduced during secondary tip formation in Dictyostelium discoideum. 
Pharmacological inhibition of cAMP phosphodiesterase also resulted 
in multiple tips. Caffeine reduces cAMP levels by 16.4, 2.34, 4.71 and 
6.30 folds, respectively in D. discoideum, D. aureostipes, D. minutum 
and Polysphondylium pallidum. We propose that altered cAMP levels, 
perturbed cAMP gradient and impaired signalling may be the critical 
factors for the origin of multiple tips in other Dictyostelids as well. In 
the presence of caffeine, slug cell movement gets impaired and 
restricted. The cell type specific markers, ecmA (prestalk) and pspA 
(prespore) cells are not equally contributing during additional tip 
formation. During additional tip emergence, prespore cells 
transdifferentiate to compensate the loss of prestalk cells.

Conclusion
Caffeine decreases adenyl cyclase--A (AcaA) levels and as a 
consequence low cAMP is synthesised altering the gradient. Further 
if cAMP phosphodiesterase (Pde4) levels go down in the presence 
of caffeine, the cAMP gradient breaks down. When there is no cAMP 
gradient, directional movement is inhibited and might favour 
re-differentiation of prespore to prestalk cells. 


Submitted by Pundrik Jaiswal [[log in to unmask]]
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[End dictyNews, volume 38, number 22]