dictyNews
Electronic Edition
Volume 33, number 2
July 17, 2009
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Abstracts
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The trishanku gene and terminal morphogenesis in Dictyostelium
discoideum
Nameeta Mujumdar1, Kei Inouye2 and Vidyanand Nanjundiah1
1Indian Institute of Science, Bangalore, India and 2Kyoto University,
Kyoto,
Japan
Evolution & Development, in press
The positioning of spores relative to the stalk is a characteristic
feature
of morphogenesis in the cellular slime moulds. In Dictyostelium
discoideum
the rising non-motile spore mass is cradled above and below by tissues
derived from both prestalk cells and anterior-like cells, that are known
as the upper and lower cup respectively. Trishanku (triA) is a novel
gene expressed in posterior prespore cells of the D. discoideum slug;
in the absence of triA function prespore cells halt their ascent midway
on the stalk (Jaiswal et al., Differentiation 74:596–607, 2006).
Intra- and inter-strain grafting experiments were carried out with
anterior and posterior fragments of Ax2 and triA- slugs. Intra-triA-
grafts show that (a) cells freely cross the prestalk-prespore boundary
and change their fates accordingly and (b) the lower cup is normal but
the upper cup is not. The aberrant terminal morphogenesis seen in
triA- can be traced to the improper functioning of the upper cup.
When wild-type upper cup function is supplied via other cells, trishanku
spores can reach the tip of the stalk. Conversely, Ax2 spores fail to
do so in chimeras in which the upper cup is largely made up of mutant
cells. A lowered level of expression of the gene encoding the cell
adhesion molecule lagC during culmination could be responsible for
the poor attachment of the upper cup to the spore mass in triA- .
These observations reinforce Sternfeld’s finding (Dev Genes Evol
208:487–494, 1998) that the wild-type phenotype of the upper cup is
important for the elevation of the spores in D. discoideum. Further,
they show that a gene that is expressed in one cell type elicits
behaviour in a second cell type that can ‘feed back’ and enhance the
fitness of the first cell type.
Submitted by Vidyanand Nanjundiah ([log in to unmask])
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Yersinia outer protein YopE affects the actin cytoskeleton in
Dictyostelium
discoideum through targeting of multiple Rho family GTPases
Georgia Vlahou, Oxana Schmidt, Bettina Wagner, Handan Uenlue,
Petra Dersch, Francisco Rivero and Barbara A. Weissenmayer
BMC Microbiology, in press
Background
All human pathogenic Yersinia species share a virulence-associated
type III secretion system that translocates Yersinia effector proteins
into host cells to counteract infection-induced signaling responses and
prevent phagocytosis. Dictyostelium discoideum has been recently used to
study the effects of bacterial virulence factors produced by
internalized
pathogens. In this study we explored the potential of Dictyostelium as
model organism for analyzing the effects of ectopically expressed
Yersinia
outer proteins (Yops).
Results
TheYersinia pseudotuberculosis virulence factors YopE, YopH, YopM and
YopJ were expressed de novo within Dictyostelium and their effects on
growth in axenic medium and on bacterial lawns were analyzed. No severe
effect was observed for YopH, YopJ and YopM, but expression of YopE,
which is a GTPase activating protein for Rho GTPases, was found to be
highly detrimental. GFP-tagged YopE expressing cells had less
conspicuous
cortical actin accumulation and decreased amounts of F-actin. The actin
polymerization response upon cAMP stimulation was impaired, although
chemotaxis was unaffected. YopE also caused reduced uptake of yeast
particles. These alterations are probably due to impaired Rac1
activation.
We also found that YopE predominantly associates with intracellular
membranes including the Golgi apparatus and inhibits the function of
moderately overexpressed RacH.
Conclusion
The phenotype elicited by YopE in Dictyostelium can be explained, at
least in part, by inactivation of one or more Rho family GTPases. It
further demonstrates that the social amoeba Dictyostelium discoideum
can be used as an efficient and easy-to-handle model organism in order
to analyze the function of a translocated GAP protein of a human
pathogen.
Submitted by: Barbara A. Weissenmayer [[log in to unmask]]
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[End dictyNews, volume 32, number 2]
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