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dictyNews Volume 24 Number 08
Dicty News
Electronic Edition
Volume 24, number 8
March 25, 2005
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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Temporal and Spatial Regulation of Phosphoinositide Signaling Mediates
Cytokinesis
Chris Janetopoulos, Jane Borleis, Francisca Vazquez, Miho Iijima, and
Peter Devreotes
Department of Cell Biology, Johns Hopkins University School of Medicine
Dev Cell, in press
Polarity is a prominent feature of both cell division and migration. While
a key role for PI(3,4,5)P3 has been established for migration,
phosphoinositide signaling during cytokinesis has not been reported. In
chemotaxis, local accumulation of PI(3,4,5)P3 at the cellâs leading edge,
achieved through temporal and spatial regulation of PI3-kinases and the
tumor suppressor, PTEN, biases the actin cytoskeleton and thereby controls
directional sensing and polarity. We find that as migrating D. discoideum
cells round up to enter cytokinesis, PI(3,4,5)P3 signaling is uniformly
suppressed . Then as the spindle and cell elongate, PI3Ks and PTEN move to
and function at the poles and furrow, respectively. Cell lines lacking both
these enzymatic activities fail to modulate PI(3,4,5)P3 levels, are
defective in cytokinesis, and cannot divide in suspension. The cells
continue to grow and duplicate their nuclei at a normal rate, generating
large multinucleate cells. Furrows that fail to ingress between nuclei
are unable to stably accumulate myosin filaments or suppress actin-filled
ruffles. We propose that phosphoinositide-linked circuits, similar to those
that bring about asymmetry during cell migration, also regulate polarity in
cytokinesis.
Submitted by: Chris Janetopoulos [cjanetop@jhmi.edu]
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New prestalk and prespore inducing signals in Dictyostelium
Ioannis Serafimidis and Robert R. Kay
MRC Laboratory of Molecular Biology,
Hills Road,
Cambridge, CB2 2QH
UK
Developmental Biology, in press
The differentiation inducing signals (DIFs) currently known in Dictyostelium
appear unable to account for the full diversity of cell types produced in
development. To search for new signals, we analysed the differentiation in
monolayers of cells expressing prestalk (ecmAO, ecmA, ecmO, ecmB and cAR2)
and prespore (psA) markers. Expression of each marker drops off as the cell
density is reduced, suggesting that cell interaction is required. Expression
of each marker is inhibited by cerulenin, an inhibitor of polyketide
synthesis, and can be restored by conditioned medium. However, the known
stalk-inducing polyketide, DIF-1, could not replace conditioned medium and
induce the ecmA or cAR2 prestalk markers, suggesting that they require
different polyketide inducers. Polyketide production by fungi is stimulated
by cadmium ions, which also dramatically stimulates differentiation in
Dictyostelium cell cultures and the accumulation of medium factors.
Factors produced with cadmium present were extracted from conditioned medium
and fractionated by HPLC. A new factor inducing prespore cell
differentiation, called PSI-2, and two inducing stalk cell differentiation
(DIFs 6 and 7) were resolved. All are distinct from currently identified
factors. DIF-6, but not DIF-7 or PSI-2, appears to have an essential
carbonyl group. Thus Dictyostelium may use extensive polyketide signalling
in its development.
Submitted by: Rob Kay [rrk@mrc-lmb.cam.ac.uk]
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Dictyostelium LIS1 is a centrosomal protein required for microtubule/cell
cortex interactions, nucleus/centrosome linkage and actin dynamics
Markus Rehberg, Julia Kleylein-Sohn, Jan Faix, Thi-Hieu Ho, Irene Schulz
and Ralph Grf*
A.-Butenandt-Institut/Zellbiologie, Ludwig-Maximilians-Universitt Mnchen,
Schillerstrasse 42, D-80336 Mnchen, Germany
*corresponding author
Mol. Biol. Cell in press
The widespread LIS1-proteins were originally identified as the target for
sporadic mutations causing lissencephaly in humans. Dictyostelium LIS1
(DdLIS1) is a microtubule-associated protein exhibiting 53 % identity to
human LIS1. It colocalizes with dynein at isolated, microtubule-free
centrosomes, suggesting that both are integral centrosomal components.
Replacement of the DdLIS1 gene by the hypomorphic D327H allele or
overexpression of an MBP-DdLIS1 fusion disrupted various dynein-associated
functions. Microtubules lost contact with the cell cortex and were dragged
behind an unusually motile centrosome. Previously, this phenotype was
observed in cells overexpressing fragments of dynein or the
XMAP215-homologue DdCP224. DdLIS1 was coprecipitated with DdCP224,
suggesting that both act together in dynein-mediated cortical attachment
of microtubules. Furthermore, DdLIS1-D327H mutants showed Golgi dispersal
and reduced centrosome/nucleus association. Defects in DdLIS1 function also
altered actin dynamics characterized by traveling waves of actin
polymerization correlated with a reduced F-actin content. DdLIS1 could be
involved in actin dynamics through Rho-GTPases, since DdLIS1 interacted
directly with Rac1A in vitro. Our results show that DdLIS1 is required for
maintenance of the microtubule cytoskeleton, Golgi apparatus and
nucleus/centrosome association, and they suggest that LIS1-dependent
alterations of actin dynamics could also contribute to defects in neuronal
migration in lissencephaly patients.
Submitted by: Ralph Graef [rgraef@lrz.uni-muenchen.de]
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[End Dicty News, volume 24, number 8] 
