dictyNews
Electronic Edition
Volume 39, number 14
May 10, 2013
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Abstracts
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Hydrolysis of aromatic b-glucosides by non-pathogenic bacteria
confers a chemical weapon against predators
Authors: Robert Sonowal1, Krithi Nandimath1, Sucheta S. Kulkarni2,
Sandhya P. Koushika2,3, Vidyanand Nanjundiah1 and S. Mahadevan1
Affiliations: 1 Department of Molecular Reproduction, Development
and Genetics, Indian Institute of Science, Bangalore 560 012, India
2 National Centre for Biological Sciences, Bangalore 560065, India
3 Department of Biological Sciences, Tata Institute of Fundamental
Research, Mumbai, India
Proceedings of The Royal Society B, in press
Bacteria present in natural environments such as soil have evolved
multiple strategies to escape from predation.We report that natural
isolates of Enterobacteriaceae that actively hydrolyze plant-derived
aromatic beta-glucosides such as salicin, arbutin and esculin, are
able to avoid predation by the bacteriovorous amoeba Dictyostelium
discoideum and nematodes of multiple genera belonging to the family
Rhabditidae. This advantage can be observed under laboratory culture
conditions as well as in the soil environment. The aglycone moiety
released by the hydrolysis of beta-glucosides is toxic to predators and
acts via the dopaminergic receptor Dop-1 in the case of Caenorhabditis
elegans. While soil isolates of nematodes belonging to the family
Rhabditidae are repelled by the aglycone, laboratory strains and natural
isolates of Caenorhabditis sp. are attracted to the compound, mediated
by receptors that are independent of Dop-1, leading to their death. The
beta-glucosides–positive (Bgl+) bacteria that are otherwise
non-pathogenic can obtain additional nutrients from the dead predators,
thereby switching their role from prey to predator. This study also offers
an evolutionary explanation for the retention by bacteria of ‘cryptic’ or
‘silent’ genetic systems such as the bgl operon.
Submitted by Robert Sonowal [[log in to unmask]]
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Bacterial discrimination by Dictyostelid amoebae reveals the complexity
of ancient interspecies interactions
Waleed Nasser1, Balaji Santhanam2,3, Edward Roshan Miranda2,4,
Anup Parikh 2,3, Kavina Juneja5, Gregor Rot6, Chris Dinh1,
Rui Chen2,3,4, Blaz Zupan2,6, Gad Shaulsky2,3,4 and Adam Kuspa1,2,4*
1 - Verna and Marrs McLean Department of Biochemistry and Molecular
Biology,
2 – Department of Molecular and Human Genetics,
3 – Structural and Computational Biology and Molecular Biophysics Program,
4 – Developmental Biology Program, Baylor College of Medicine, Houston,
TX 77030;
5 – Department of Biochemistry and Cell Biology, Rice University, Houston,
TX 77005;
6 – Faculty of Computer and Information Science, University of Ljubljana,
Slovenia.
Current Biology, in press
Background: Amoebae and bacteria interact within predator/prey and
host/pathogen relationships, but the general response of amoeba to
bacteria is not well understood. The amoeba Dictyostelium discoideum
feeds on, and is colonized by diverse bacterial species including Gram-
positive [Gram(+)] and Gram-negative [Gram(–)] bacteria, two major
groups of bacteria that differ in structure and macromolecular composition.
Results: Transcriptional profiling of D. discoideum revealed sets of genes
whose expression is enriched in amoebae interacting with different species
of bacteria, including sets that appear specific to amoebae interacting with
Gram(+), or with Gram(–) bacteria. In a genetic screen utilizing the growth
of mutant amoebae on a variety of bacteria as a phenotypic readout, we
identified amoebal genes that are only required for growth on Gram(+)
bacteria, including one that encodes the cell surface protein gp130, as well
as several genes that are only required for growth on Gram(–) bacteria
including one that encodes a putative lysozyme, AlyL. These genes are
required for parts of the transcriptional response of wild-type amoebae,
and this allowed their classification into potential response pathways.
Conclusions: We have defined genes that are critical for amoebal survival
during feeding on Gram(+), or Gram(–), bacteria which we propose form
part of a regulatory network that allows D. discoideum to elicit specific
cellular responses to different species of bacteria in order to optimize
survival.
Submitted by Adam Kuspa [[log in to unmask]]
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Systematic analysis of gamma-aminobutyric acid (GABA) metabolism and
function in the social amoeba Dictyostelium discoideum.
Wu Y, Janetopoulos C.
Department of Biological Sciences, Vanderbilt University, United States.
Journal of Biological Chemistry, in press
While GABA has been suggested to regulate spore encapsulation in
the social amoeba Dictyostellium discoideum, the metabolic profile and
other potential functions of GABA during development remain unclear.
In this study, we investigated the homeostasis of GABA metabolism by
disrupting genes related to GABA metabolism and signaling. Extracellular
levels of GABA are tightly regulated during early development, and GABA
is generated by the glutamate decarboxylase, GadB, during growth and
in early development. However, overexpression of the prespore-specific
homologue, GadA, in the presence of GadB reduces production of
extracellular GABA. Perturbation of extracellular GABA levels delays the
process of aggregation. Cytosolic GABA is degraded by the GABA
transaminase, GabT, in the mitochondria. Disruption of a putative vesicular
GABA transporter (vGAT) homologue DdvGAT reduces secreted GABA.
We identified the GABAB receptor-like family member GrlB as the major
GABA receptor during early development, and either disruption or
overexpression of GrlB delays aggregation. This delay is likely the result
of an abolished pre-starvation response and late expression of several
early developmental genes. Distinct genes are employed for GABA
generation during sporulation. During sporulation, GadA alone is required
for generating GABA and DdvGAT is likely responsible for GABA
secretion. GrlE but not GrlB is the GABA receptor during late development.
Submitted by Chris Janetopoulos [[log in to unmask]]
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The G alpha subunit Galpha8 inhibits proliferation, promotes adhesion
and regulates cell differentiation
Wu Y, Janetopoulos C.
Department of Biological Sciences, Vanderbilt University, United States.
Developmental Biology, in press
Heterotrimeric G protein-mediated signal transduction plays a pivotal
role in both vegetative and developmental stages in the eukaryote
Dictyostelium discoideum. Here we describe novel functions of the
G protein alpha subunit Galpha8 during vegetative and development
stages. Galpha8 is expressed at low levels during vegetative growth.
Loss of Galpha8 promotes cell proliferation, whereas excess Galpha8
expression dramatically inhibits growth and induces aberrant cytokinesis
on substrates in a Gbeta-dependent manner. Overexpression of Galpha8
also leads to increased cell-cell cohesion and cell-substrate adhesion.
We demonstrate that the increased cell-cell cohesion is mainly caused
by induced CadA expression, and the induced cell-substrate adhesion
is responsible for the cytokinesis defects. However, the expression of
several putative constitutively active mutants of Galpha8 does not
augment the phenotypes caused by intact Galpha8. Galpha8 is strongly
induced after starvation, and loss of Galpha8 results in decreased
expression of certain adhesion molecules including CsA and tgrC1.
Interestingly, Galpha8 is preferentially distributed in the upper and
lower cup of the fruiting body. Lack of Galpha8 decreases the
expression of the specific marker of the anterior-like cells, suggesting
that Galpha8 is required for anterior-like cell differentiation.
Submitted by Chris Janetopoulos [[log in to unmask]]
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[End dictyNews, volume 39, number 14]
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