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dictyNews Volume 40 Number 05
dictyNews
Electronic Edition
Volume 40, number 5
February 14, 2014
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 dictyNews, the Dicty Reference database and other
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Abstracts
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NNucleocytoplasmic Protein Translocation during Mitosis in the Social
Amoebozoan Dictyostelium discoideum
Danton H. O'Day a,b* and Aldona Budniaka a
a: Department of Biology, University of Toronto Mississauga, 3359
Mississauga Road N., Mississauga, Ontario, Canada, L5L 1C6
b: Department of Cell and Systems Biology, University of Toronto,
25 Harbord St., Toronto, Ontario, Canada, M5S 3G5
Biological Reviews, in press
Mitosis is a fundamental and essential life process. It underlies the
duplication and survival of all cells and, as a result, all eukaryotic
organisms. Since uncontrolled mitosis is a dreaded component of
many cancers, a full understanding of the process is critical. Evolution
has led to the existence of three types of mitosis: closed, open, and
semi-open. The significance of these different mitotic species, how
they can lead to a full understanding of the critical events that underlie
the asexual duplication of all cells, and how they may generate new
insights into controlling unregulated cell division remains to be
determined. The eukaryotic microbe Dictyostelium discoideum has
proven to be a valuable biomedical model organism. While it appears
to utilize closed mitosis, a review of the literature suggests that it
possesses a form of mitosis that lies in the middle between truly open
and fully closed mitosisÑit utilizes a form of semi-open mitosis. Here,
the nucleocytoplasmic translocation patterns of the proteins that have
been studied during mitosis in the social amoebozoan Dictyostelium
are detailed followed by a discussion of how some of them provide
support for the hypothesis of semi-open mitosis.
Submitted by Danton H. O'Day [danton.oday@utoronto.ca]
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Identification of the protein kinases Pyk3 and Phg2 as regulators of
the STATc-mediated response to hyperosmolarity
Linh Hai Vu, Tsuyoshi Araki, Jianbo Na, Christoph S. Clemen,
Jeffrey G.Williams and Ludwig Eichinger
PLoS ONE, accepted
Cellular adaptation to changes in environmental osmolarity is crucial
for cell survival. In Dictyostelium, STATc is a key regulator of the
transcriptional response to hyperosmotic stress. Its phosphorylation
and consequent activation is controlled by two signaling branches,
one cGMP- and the other Ca2+-dependent, of which many signaling
components have yet to be identified. The STATc stress signalling
pathway feeds back on itself by upregulating the expression of STATc
and STATc-regulated genes. Based on microarray studies we chose
two tyrosine-kinase like proteins, Pyk3 and Phg2, as possible modulators
of STATc phosphorylation and generated single and double knock-out
mutants to them. Transcriptional regulation of STATc and STATc
dependent genes was disturbed in pyk3-, phg2-, and pyk3-/phg2- cells.
The absence of Pyk3 and/or Phg2 resulted in diminished or completely
abolished increased transcription of STATc dependent genes in response
to sorbitol, 8-Br-cGMP and the Ca2+ liberator BHQ. Also, phospho-STATc
levels were significantly reduced in pyk3- and phg2- cells and even further
decreased in pyk3-/phg2- cells. The reduced phosphorylation was mirrored
by a significant delay in nuclear translocation of GFP-STATc. The protein
tyrosine phosphatase 3 (PTP3), which dephosphorylates and inhibits
STATc, is inhibited by stress-induced phosphorylation on S448 and S747.
Use of phosphoserine specific antibodies showed that Phg2 but not Pyk3
is involved in the phosphorylation of PTP3 on S747. In pull-down assays
Phg2 and PTP3 interact directly, suggesting that Phg2 phosphorylates
PTP3 on S747 in vivo. Phosphorylation of S448 was unchanged in phg2-
cells. We show that Phg2 and an, as yet unknown, S448 protein kinase
are responsible for PTP3 phosphorylation and hence its inhibition, and
that Pyk3 is involved in the regulation of STATc by either directly or
indirectly activating it. Our results add further complexities to the
regulation of STATc, which presumably ensure its optimal activation
in response to different environmental cues.
Submitted by Ludwig Eichinger [ludwig.eichinger@uni-koeln.de]
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Excitable Signal Transduction Induces Both Spontaneous and
Directional Cell Asymmetries in the Phosphatidylinositol Lipid
Signaling System for Eukaryotic Chemotaxis
Masatoshi Nishikawa(1), Marcel Hrning(1), Masahiro Ueda(2),
and Tatsuo Shibata(1)
(1)Laboratory for Physical Biology, RIKEN Center for Developmental
Biology, (2) Laboratory of Single Molecule Biology, Graduate School
of Science, Osaka University
Biophysical Journal, in press
106: 723Ð734, http://dx.doi.org/10.1016/j.bpj.2013.12.023
Intracellular asymmetry in the signaling network works as a compass
to navigate eukaryotic chemotaxis in response to guidance cues.
Although the compass variable can be derived from a self-organization
dynamics, such as excit- ability, the responsible mechanism remains
to be clarified. Here, we analyzed the spatiotemporal dynamics of the
phosphatidy- linositol 3,4,5-trisphosphate (PtdInsP3) pathway, which
is crucial for chemotaxis. We show that spontaneous activation of
PtdInsP3-enriched domains is generated by an intrinsic excitable
system. Formation of the same signal domain could be trig- gered by
various perturbations, such as short impulse perturbations that
triggered the activation of intrinsic dynamics to form signal domains.
We also observed the refractory behavior exhibited in typical excitable
systems. We show that the chemotactic response of PtdInsP3 involves
biasing the spontaneous excitation to orient the activation site toward
the chemoattractant. Thus, this biased excitability embodies the
compass variable that is responsible for both random cell migration
and biased random walk. Our finding may explain how cells achieve
high sensitivity to and robust coordination of the downstream activation
that allows chemotactic behavior in the noisy environment outside and
inside the cells.
Submitted by Tatsuo Shibata [tatsuoshibata@cdb.riken.jp]]
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Reversible Membrane Pearling in Live Cells Upon Destruction of
the Actin Cortex
Doris Heinrich, Mary Ecke, Marion Jasnin, Ulrike Engel and
Gnther Gerisch
Biophysical Journal, in press
http://dx.doi.org/10.1016/j.bpj.2013.12.054
Membrane pearling in live cells is observed when the plasma
membrane is depleted of its support, the cortical actin network.
Upon efficient depolymerization of actin, pearls of variable size
are formed, which are connected by nanotubes of about 40 nm
diameter. We show that formation of the membrane tubes and
their transition into chains of pearls do not require external tension,
and that they neither depend on microtubule-based molecular
motors nor pressure generated by myosin-II. Pearling thus differs
from blebbing. The pearling state is stable as long as actin is
prevented from polymerizing. When polymerization is restored,
the pearls are retracted into the cell, indicating continuity of the
membrane. Our data suggest that the alternation of pearls and
strings is an energetically favored state of the unsupported
plasma membrane, and that one of the functions of the actin
cortex is to prevent the membrane from spontaneously assuming
this configuration.
Submitted by Gnther Gerisch [gerisch@biochem.mpg.de]
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Bleb driven chemotaxis of Dictyostelium cells
Evgeny Zatulovskiy1, Richard Tyson2, Till Bretschneider2 and
Robert R. Kay1
J Cell Biol., in press
Blebs and F-actin-driven pseudopods are alternative ways of
extending the leading edge of migrating cells. We show that
Dictyostelium cells switch from using predominantly pseudopods
to blebs when migrating under agarose overlays of increasing
stiffness. Blebs expand faster than pseudopods leaving behind
F-actin scars, but are less persistent. Blebbing cells are strongly
chemotactic to cyclic-AMP, producing nearly all of their blebs up-
gradient. When cells re-orientate to a needle releasing cyclic-AMP,
they stereotypically produce first microspikes, then blebs and
pseudopods only later. Genetically, blebbing requires myosin-II
and increases when actin polymerization or cortical function are
impaired. Cyclic-AMP induces transient blebbing independently
of much of the known chemotactic signal transduction machinery,
but requiring PI3-kinase and downstream PH-domain proteins,
CRAC and PhdA. Impairment of this PI3-kinase pathway results
in slow movement under agarose and cells that produce few blebs,
though actin polymerization appears unaffected. We propose that
mechanical resistance induces bleb-driven movement in
Dictyostelium, which is chemotactic and controlled through
PI3-kinase.
Submitted by Rob Kay [rrk@mrc-lmb.cam.ac.uk]
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[End dictyNews, volume 40, number 5]
