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dictyNews Volume 42 Number 30
dictyNews
Electronic Edition
Volume 42, number 30
December 16, 2016
Please submit abstracts of your papers as soon as they have been
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Abstracts
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Mycobacterium marinum degrades both triacylglycerols and
phospholipids from its Dictyostelium host to synthesise its
own triacylglycerols and generate lipid inclusions
Caroline Barisch and Thierry Soldati
PLoS Pathogens, in press
During a tuberculosis infection and inside lipid-laden foamy
macrophages, fatty acids (FAs) and sterols are the major energy
and carbon source for Mycobacterium tuberculosis. Mycobacteria
can be found both inside a vacuole and the cytosol, but how this
impacts their access to lipids is not well appreciated. Lipid droplets
(LDs) store FAs in form of triacylglycerols (TAGs) and are energy
reservoirs of prokaryotes and eukaryotes. Using the Dictyostelium
discoideum/Mycobacterium marinum infection model we showed
that M. marinum accesses host LDs to build up its own intracytosolic
lipid inclusions (ILIs). Here, we show that host LDs aggregate at
regions of the bacteria that become exposed to the cytosol, and
appear to coalesce on their hydrophobic surface leading to a transfer
of diacylglycerol O-acyltransferase 2 (Dgat2)-GFP onto the bacteria.
Dictyostelium knockout mutants for both Dgat enzymes are unable to
generate LDs. Instead, the excess of exogenous FAs is esterified
predominantly into phospholipids, inducing uncontrolled proliferation
of the endoplasmic reticulum (ER). Strikingly, in absence of host LDs,
M. marinum alternatively exploits these phospholipids, resulting in
rapid reversal of ER-proliferation. In addition, the bacteria are unable
to restrict their acquisition of lipids from the dgat1 and 2 double
knockout leading to vast accumulation of ILIs. Recent data indicate
that the presence of ILIs is one of the characteristics of dormant
mycobacteria. During Dictyostelium infection, ILI formation in M.
marinum is not accompanied by a significant change in intracellular
growth and a reduction in metabolic activity, thus providing evidence
that storage of neutral lipids does not necessarily induce dormancy.
submitted by: Thierry Soldati [thierry.soldati@unige.ch]
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Adenylate cyclase A acting on PKA mediates induction of stalk
formation by c-di-GMP at the Dictyostelium organizer
Zhi-hui Chen a, Reema Singh a,b, Christian Cole b, Hajara Lawal a,
Christina Schilde a, Melanie Febrer c, Geoffrey J. Barton b and
Pauline Schaap a*
Divisions of Cell and Developmental Biology a, Computational Biology b,
Molecular Medicine c, School of Life Sciences, University of Dundee,
Dundee, UK.
Proc. Natl. Acad. Sci. USA, in press
Coordination of cell movement with cell differentiation is a major feat
of embryonic development. The Dictyostelium stalk always forms at
the organizing tip by a mechanism that is not understood. We previously
reported that c-di-GMP, synthesized by diguanylate cyclase A (DgcA),
induces stalk formation. We here used transcriptional profiling of dgca-
structures to identify target genes for c-di-GMP, and used these genes
to investigate the c-di-GMP signal transduction pathway. We found that
knock-down of PKA activity in prestalk cells reduced stalk gene induction
by c-di-GMP, while PKA activation bypassed the c-di-GMP requirement
for stalk gene expression. c-di-GMP caused a persistent increase in cAMP,
which still occurred in mutants lacking the adenylate cyclases ACG or ACR,
or the cAMP phosphodiesterase RegA. However, both inhibition of
adenylate cyclase A (ACA) with SQ22536, and incubation of a temperature-
sensitive ACA mutant at the restrictive temperature prevented c-di-GMP
induced cAMP synthesis as well as c-di-GMP induced stalk gene
transcription. ACA produces the cAMP signals that coordinate Dictyostelium
morphogenetic cell movement and is highly expressed at the organizing tip.
The stalk-less dgca- mutant regained its stalk by expression of a light-
activated adenylate cyclase from the ACA promoter and exposure to light,
indicating that cAMP is also the intermediate for c-di-GMP in vivo. Our data
show that the more widely expressed DgcA activates tip-expressed ACA,
which then acts on PKA to induce stalk genes. These results explain why
stalk formation in Dictyostelia always initiates at the site of the
morphogenetic organizer.
submitted by: Pauline Schaap [p.schaap@dundee.ac.uk]
———————————————————————————————————————
A polycycstin-type transient receptor potential (Trp) channel that is
activated by ATP
David Traynor & Robert R. Kay
Open Biology, in press
ATP and ADP are ancient extra-cellular signalling molecules that in
Dictyostelium amoebae cause rapid, transient increases in cytosolic
calcium due to an influx through the plasma membrane. This response
is independent of hetero-trimeric G-proteins, the putative IP3 receptor
IplA and all P2X channels. We show, unexpectedly, that it is abolished
in mutants of the polycystin-type transient receptor potential channel,
TrpP. Responses to the chemoattractants cyclic-AMP and folic acid are
unaffected in TrpP mutants. We report that the DIF morphogens,
cyclic-di-GMP, GABA, glutamate and adenosine all induce strong
cytoplasmic calcium responses, likewise independently of TrpP. Thus
TrpP is dedicated to purinergic signalling. ATP treatment causes cell
blebbing within seconds but this does not require TrpP, implicating a
separate purinergic receptor. We could detect no effect of ATP on
chemotaxis and TrpP mutants grow, chemotax and develop almost
normally in standard conditions. No gating ligand is known for the human
homologue of TrpP, polycystin-2, which causes polycystic kidney disease.
Our results now show that TrpP mediates purinergic signalling in
Dictyostelium and is directly or indirectly gated by ATP.
submitted by: Rob Kay [rrk@mrc-lmb.cam.ac.uk]
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[End dictyNews, volume 42, number 30]
