dictyNews
Electronic Edition
Volume 32, number 9
April 3, 2009

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Abstracts
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Dictyostelium discoideum Paxillin Regulates Actin-Based Processes

M. Berenice Duran, Asif Rahman, Max Colten and Derrick Brazill

Department of Biological Sciences, Center for the Study of Gene
Structure and Function
Hunter College of the City University of New York, New York, NY 10021


Protist, in press

Paxillin is a key player in integrating the actin cytoskeleton with  
adhesion,
and thus is essential to numerous cellular processes including  
proliferation,
differentiation and migration in animal cells.  PaxB, the Dictyostelium
discoideum orthologue of paxillin, has been shown to be important for
adhesion and development, much like its mammalian counterpart.  Here,
we use the overproduction of PaxB to gain better insight into its role  
in
  regulating the actin cytoskeleton and adhesion.  We find that PaxB
overexpressing (PaxBOE) cells can aggregate and form mounds normally,
but are blocked in subsequent development.  This arrest can be rescued
by addition of wild-type cells, indicating a non-cell autonomous role  
for
PaxB.  PaxBOE cells also have  alterations in several actin-based
processes, including adhesion, endocytosis, motility and chemotaxis.
PaxBOE cells exhibit an EDTA-sensitive increase in cell-cell cohesion,
suggesting that PaxB-mediated adhesion is Ca2+ or Mg2+ dependent.
Interestingly, cells overexpressing paxB are less adhesive to the  
substratum.
In addition, PaxBOE cells display decreased motility under starved  
conditions,
decreased endocytosis, and are unable to efficiently chemotax up a  
folate
gradient.  Taken together, the data suggest that proper expression of  
PaxB
is vital for the regulation of development and actin-dependent  
processes.



Submitted by: Derrick Brazill [[log in to unmask]]
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Activated cAMP receptors switch encystation into sporulation

Yoshinori Kawabe1, Takahiro Morio2, John L. James1, Alan R. Prescott1,
Yoshimasa Tanaka2 and Pauline Schaap1*

1College of Life Sciences, University of Dundee, Dundee, Angus,
DD15EH, UK.
2Graduate School of Life and Environmental Sciences, University of
Tsukuba, Ibaraki, 305-8572, Japan.


PNAS, in press

Metazoan embryogenesis is controlled by a limited number of signalling
modules that are used repetitively at succesive developmental stages.
The development of social amoebas shows similar reiterated use of
cAMP-mediated signalling. In the model Dictyostelium discoideum,
secreted cAMP acting on 4 cAMP receptors (cARs1-4) coordinates cell
movement during aggregation and fruiting body formation, and induces
the expression of aggregation and sporulation genes at consecutive
developmental stages. To identify hierarchy in the multiple roles of  
cAMP,
we investigated cAR heterogeneity and function across the social amoeba
phylogeny. The gene duplications that yielded cARs 2-4 occurred late in
evolution. Many species have only a cAR1 ortholog, that duplicated
independently in the Polysphondylids and Acytostelids. Disruption of  
both
cAR genes of Polysphondylium pallidum did not affect aggregation, but
caused complete collapse of fruiting body morphogenesis. The stunted
structures contained disorganized stalk cells, which supported a mass of
cysts instead of spores. cAMP triggered spore gene expression in
P.pallidum, but not in the cAR null mutant, explaining its sporulation  
defect.
Encystation is the survival strategy of solitary amoebas, and basal  
taxa,
like P.pallidum, can still encyst as single cells. Recent findings  
showed
that intracellular cAMP accumulation suffices to trigger encystation,
whereas it is a complementary requirement for sporulation. Combined,
the data suggest that cAMP signalling in social amoebas evolved from
cAMP-mediated encystation in solitary amoebas. cAMP secretion in
aggregates prompted the starving cells to form spores and not cysts,
and additionally organized fruiting body morphogenesis. cAMP-mediated
aggregation was the most recent innovation.



Submitted by: Pauline Schaap [[log in to unmask]]
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[End dictyNews, volume 32, number 9]