dictyNews
Electronic Edition
Volume 40, number 17
July 18, 2014
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Abstracts
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How blebs and pseudopods cooperate during chemotaxis
Richard A. Tyson, Evgeny Zatulovskiy, Robert R. Kay, Till Bretschneider
PNAS, accepted
Two motors can drive extension of the leading edge of motile cells: actin
polymerization, and myosin-driven contraction of the cortex, producing
fluid pressure and the formation of blebs. Dictyostelium cells can move
with both blebs and actin-driven pseudopods at the same time, and blebs,
like pseudopods, can be orientated by chemotactic gradients. Here we ask
how bleb sites are selected and how the two forms of projection cooperate.
We show that membrane curvature is an important, yet overlooked, factor.
Dictyostelium cells were observed moving under agarose, which efficiently
induces blebbing, and the dynamics of membrane deformations analysed.
Blebs preferentially originate from negatively curved regions, generated
on the flanks of either extending pseudopods or blebs themselves. This is
true of cells at different developmental stages, chemotaxing to either folate
or cyclic-AMP, and moving with both blebs and pseudopods, or blebs only.
A physical model of blebbing suggests that detachment of the cell
membrane is facilitated in concave areas of the cell, where membrane
tension produces an outward directed force, as opposed to pulling inwards
in convex regions. Our findings assign a new role to membrane tension
which can spatially couple blebs and pseudopods, thus contributing to
clustering protrusions to the cell front.
Submitted by Till Bretschneider [[log in to unmask]]
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The social amoeba Polysphondylium pallidum loses encystation and
sporulation, but can still erect fruiting bodies in the absence of cellulose.
Qingyou Du and Pauline Schaap1
1corresponding author
Phone: 44 1382 388078; Fax 44 1382 345386.
College of Life Sciences, University of Dundee, MSI/WTB/JBC complex,
Dow Street, Dundee, DD15EH, UK.
E-mail: [log in to unmask]; [log in to unmask]
Protist, accepted
Amoebas and other freely moving protists differentiate into walled cysts
when exposed to stress. As cysts, amoeba pathogens are resistant to
biocides, preventing treatment and eradication. Lack of gene modification
procedures has left the mechanisms of encystation largely unexplored.
Genetically tractable Dictyostelium discoideum amoebas require cellulose
synthase for formation of multicellular fructifications with cellulose-rich stalk
and spore cells. Amoebas of its distant relative Polysphondylium pallidum
(Ppal), can additionally encyst individually in response to stress. Ppal has
two cellulose synthase genes, DcsA and DcsB, which we deleted individually
and in combination. Dcsa- mutants formed fruiting bodies with normal stalks,
but their spores and cyst walls lacked cellulose, which obliterated stress-
resistance of spores and rendered cysts entirely non-viable. A dcsa-/dcsb-
mutant made no walled spores, stalk cells or cysts, although simple fruiting
structures were formed with a droplet of amoeboid cells resting on an
sheathed column of decaying cells. DcsB is expressed in prestalk and stalk
cells, while DcsA is additionally expressed in spores and cysts. We conclude
that cellulose is essential for encystation and that cellulose synthase may be
a suitable target for drugs to prevent encystation and render amoeba
pathogens susceptible to conventional antibiotics.
Submitted by Pauline Schaap [[log in to unmask]]
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[End dictyNews, volume 40, number 17]
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