dictyNews
Electronic Edition
Volume 32, number 8
March 20, 2009

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Abstracts
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Cortical Factor Feedback Model for Cellular Locomotion and Cytofission

Shin I. Nishimura, Masahiro Ueda and Sasai Masaki


PLoS Computational Biology 5(3):e1000310 (doi:10.1371)

Eukaryotic cells can move spontaneously without being guided by external
cues. For such spontaneous movements, a variety of different modes have
been observed, including the amoeboid-like locomotion with protrusion of
multiple pseudopods, the keratocyte-like locomotion with a widely spread
lamellipodium, cell division with two daughter cells crawling in  
opposite
directions, and fragmentations of a cell to multiple pieces. Mutagenesis
studies have revealed that cells exhibit these modes depending on which
genes are deficient, suggesting that seemingly different modes are the
manifestation of a common mechanism to regulate cell motion. In this  
paper,
we propose a hypothesis that the positive feedback mechanism working
through the inhomogeneous distribution of regulatory proteins underlies
this variety of cell locomotion and cytofission. In this hypothesis, a
set of regulatory proteins, which we call cortical factors, suppress  
actin
polymerization. These suppressing factors are diluted at the extending
front and accumulated at the retracting rear of cell, which establishes
a cellular polarity and enhances the cell motility, leading to the  
further
accumulation of cortical factors at the rear. Stochastic simulation of
cell movement shows that the positive feedback mechanism of cortical
factors stabilizes or destabilizes modes of movement and determines
the cell migration pattern. The model predicts that the pattern is  
selected
by changing the rate of formation of the actin-filament network or the
threshold to initiate the network formation.


Submitted by: Shin Nishimura [[log in to unmask]]
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The Effects of Extracellular Calcium on Motility, Pseudopod and Uropod  
Formation,
Chemotaxis and the Cortical Localization of Myosin II in Dictyostelium  
discoideum

Daniel F. Lusche, Deborah Wessels and David R. Soll

The W.M. Keck Dynamic Image Analysis Facility
Department of Biology The University of Iowa Iowa City, IA 52242


Cell Motility and the Cytoskeleton, in press

Extracellular Ca++, a ubiquitous cation in the soluble environment of  
cells both
free living and within the human body, regulates most aspects of  
amoeboid cell
motility, including shape, uropod formation, pseudopod formation,  
velocity and
turning in Dictyostelium discoideum. Hence it affects the efficiency  
of both basic
motile behavior and chemotaxis. Extracellular Ca++ is optimal at 10  
mM. A gradient
of the chemoattractant cAMP generated in the absence of added Ca++  
only affects
turning, butin combination with extracellular Ca++, enhances the  
effects of
extracellular Ca++.

Potassium, at 40 mM, can substitute for Ca++. Mg++, Mn++, Zn++ and Na+  
cannot.
Extracellular Ca++, or K+, also induce the cortical localization of  
myosin II in a polar
fashion. The effects of Ca++, K+ or a cAMP gradient do not appear to  
be similarly
mediated by an increase in the general pool of free cytosolic Ca++.  
These results
suggest a model, in which each agent functioning through different  
signaling
systems, converge toaffect the cortical localization of myosin II,  
which in turn
effects the behavioral changes leading to efficient cell motility and  
chemotaxis.


Submitted by: Deborah Wessels [[log in to unmask]]
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Acidic Ca2+ stores, excitability and cell patterning in Dictyostelium  
discoideum

Julian D. Gross

Dept of Biochemistry, University of Oxford, Oxford OX13QU,
United Kingdom


Eukaryotic Cell, in press

In this minireview I argue that the properties of the anterior and  
posterior
cells of aggregates (slugs) can be accounted for by the following  
assumptions:
1) Cytosolic Ca2+ is sequestered into a specific type of internal  
store by
an ATP-dependent Ca2+/H+ exchanger acting in conjunction with a vacuolar
H+-ATPase that transfers protons into the compartment interior. 2)  
Cyclic AMP
relay by the adenylyl cyclase, ACA, is dependent inter alia on cytosolic
Ca2+ transients resulting from release of this stored Ca2+ in response  
to
binding of cyclic AMP to its cell surface receptors 3) The vacuolar H+- 
ATPase
is active in the anterior cells of aggregates but inactive in the  
posterior cells.
The former can therefore fill these stores and experience Ca2+  
transients,
whereas the latter cannot. 4) The Ca2+ transients are responsible for  
driving
prestalk cell-specific (PST) gene expression and inhibiting prespore  
cell specific
(PSP) gene expression. Hence anterior cells express PST genes but not  
PSP
genes, and posterior cells do not express PST genes.  5) Posterior  
cells express
PSP genes as a result of activation of cAMP-dependent protein kinase A  
by
cAMP generated by a separate, constitutively active, adenylyl cyclase  
(ACG),
present only in the posterior cells.


Submitted by: Julian Gross [[log in to unmask]]
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[End dictyNews, volume 32, number 8]