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dictyNews Volume 23 Number 16
Dicty News
Electronic Edition
Volume 23, number 16
November 12, 2004
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu
or by using the form at
http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit.
Back issues of Dicty-News, the Dicty Reference database and other
useful information is available at dictyBase - http://dictybase.org.
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Abstracts
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Gamete fusion and cytokinesis preceding zygote establishment in the sexual
process of Dictyostelium discoideum
Kentaro Ishida, Toshihiro Hata, and Hideko Urushihara
Develop.Growth & Differ. in press
Cells of Dictyostelium discoideum become sexually mature under submerged and
dark conditions, and fuse with opposite mating-type cells to form zygote
giant cells, which gather surrounding cells and finally develop into dormant
structures called macrocysts. In the present study, we found that the
multinuclear fused cells formed during this process frequently underwent
cytokinesis driven by random local movements. The split cells were capable
of re-fusion, and repeated fission. These radical behaviors continued until
the extensive cell aggregation started around the giant cells. Thus, gamete
fusion and initiation of zygote development do not coincide in the mating of
D. discoideum. Analyses by confocal microscopy and flow cytometry indicated
that the cessation of the random movement followed pronuclear fusion, and
that microtubule organizing centers (MTOCs), abundant in the fused cells at
the beginning, gradually decreased and only one of them remained within the
developed macrocyst. Some of the genes known to control cell movement, such
as rasGEFB and rasS, increased shortly before the cessation of repeated
fusion-fission and initiation of phagocytosis. These results suggest that
the sequential molecular events are necessary in D. discoideum after gamete
fusion to establish a new individuality of zygotes.
Submitted by: Hideko Urushihara [hideko@biol.tsukuba.ac.jp]
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Quantitative imaging of single live cells reveals spatiotemporal dynamics of
multi-step signaling events of chemoattractant gradient sensing in
Dictyostelium
Xuehua Xu1, Martin Meier-Schellersheim2, Xuanmao Jiao1, Lauren E. Nelson1,
and Tian Jin1*
Laboratory of Immunogenetics1, Laboratory of Immunology2, National Institute
of Allergy and Infectious Diseases, NIH
Twinbrook II Facility, 12441 Parklawn Drive, Rockville, Maryland 20852, USA
*Corresponding author
Phone: 301-480-1430
Email: tjin@niaid.nih.gov
Molecular biology of the cell, in press.
Activation of G-protein-coupled chemoattractant receptors triggers
dissociation of G alpha and G betagamma subunits. These subunits induce
intracellular responses that can be highly polarized when a cell experiences
a gradient of chemoattractant. Exactly how a cell achieves this amplified
signal polarization is still not well understood. Here, we quantitatively
measure temporal and spatial changes of receptor occupancy, G-protein
activation by FRET imaging, and PIP3 levels by monitoring the dynamics of
PHCrac-GFP translocation in single living cells in response to different
chemoattractant fields. Our results provided the first direct evidence that
G-proteins are activated to different extents on the cell surface in
response to asymmetrical stimulations. A stronger, uniformly applied
stimulation triggers not only a stronger G-protein activation but also a
faster adaptation of downstream responses. When nave cells (which have
not experienced chemoattractant) were abruptly exposed to stable cAMP
gradients, G-proteins were persistently activated throughout the entire
cell surface, whereas the response of PHCrac-GFP translocation surprisingly
consisted of two phases, an initial transient and asymmetrical
translocation around the cell membrane, followed by a second phase
producing a highly polarized distribution of PHCrac-GFP. We propose a
revised model of gradient sensing, suggesting an important role for locally
controlled components that inhibit PI3Kinase activity.
Submitted by: [Xuehua Xu <xxu@niaid.nih.gov]
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[End Dicty News, volume 23, number 16] 
