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Jean-Claude Labbé Postdoc, 1999-2002 Currently running his own lab as a PI at the IRIC, University of Montreal Postdoctoral Research: Mechanisms that Position the Mitotic Spindle in Asymmetric Division  We
used a combination of
live imaging, laser-inactivation of centrosomes and genetics to reveal
some of the dynamics, the forces, and the genetic mechanisms underlying
asymmetric spindle positioning.The PAR proteins are known to be
localized asymmetrically in polarized
C. elegans, Drosophila, and human cells and to participate in several
cellular processes, including asymmetric cell division and spindle
orientation. Although astral microtubules are known to play roles in
these processes, their behavior during these events remains poorly
understood. We have developed a method that makes it possible
to examine the residence time of individual astral microtubules at the
cell cortex of developing embryos. Using this method, we found that
microtubules are more dynamic at the posterior cortex of the C. elegans
embryo compared to the anterior cortex during spindle displacement. We
further observed that this asymmetry depends on the PAR-3 protein and
heterotrimeric G protein signaling, and that the PAR-2 protein affects
microtubule dynamics by restricting PAR-3 activity to the anterior of
the embryo. These results indicated that PAR proteins
function to regulate microtubule dynamics at the cortex during
microtubule-dependent cellular processes.
 Regulation of the mitotic spindle's
position is important for cells to
divide asymmetrically. We used laser-mediated inactivation of
centrosomes in C. elegans embryos to analyze the temporal
regulation of forces that
asymmetrically position a mitotic spindle. We find that asymmetric
pulling forces, regulated by cortical PAR proteins, begin to act as
early as prophase and prometaphase, even before the spindle forms and
shifts to a posterior position. The spindle does not shift
asymmetrically during these early phases due to a tethering force,
mediated by astral microtubules that reach the anterior cell cortex. We
have shown that this tether is normally released after spindle assembly
and
independently of anaphase entry. Monitoring microtubule dynamics by
photobleaching segments of microtubules during anaphase revealed that
spindle microtubules do not undergo significant poleward flux in C.
elegans. Together with the known absence of anaphase A, these data
suggest that the major forces contributing to chromosome separation
during anaphase originate outside the spindle. We propose that the
forces positioning the mitotic spindle asymmetrically are tethered
until after the time of spindle assembly and that these same forces are
used later to drive chromosome segregation at anaphase.back |