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Wednesday June 2/Evening
SESSION 1: INTRODUCTION
Ira Hall
covers presentations by Davor Solter, Barbara Meyer, David Allis
The opening session of this year’s symposium was started off by
Davor Solter, who delivered an excellent and entertaining review of the
early days of research into nuclear reprogramming and presented some
recent results out of his laboratory. Solter’s
talk focused on the mechanisms by which the developing oocyte can
restore totipotency to differentiated somatic cell nuclei introduced by
nuclear transfer. He first presented the seminal experiments whereby nuclear
transfer was used to create embryos derived exclusively from a paternal
or maternal complement. These
seminal experiments demonstrated the fundamental difference between the
genomes transmitted by the two sexes and gave rise to the field of
genomic imprinting. Solter went
on to discuss the mechanisms by which the oocyte functions as a
“reprogramming milieu”, focusing on the stage-specific control of
translation of maternally-derived transcripts and the role of specific
motifs within the 3’UTRs of such messages.
The results from one of these experiments was summarized with the
statement: “some things that seem too good to be true actually turn
out to be true”. He presented
some recent data showing that transposable elements are differentially
expressed during the oocyte to embryo transition, and put forward the
tantalizing hypothesis that transposable elements function in epigenetic
reprogramming through their ability to recruit chromatin-modifying
activities. Dr. Solter
then spent some time discussing the issue of how axes are determined in
mammalian development. His findings largely contradict the idea that the
axis for the first cleavage of the developing mouse embryo is determined
by the positions of the polar body and the sperm entry point - rather,
Solter's work demonstrates that the plane of the first cleavage
coincides with the plane defined by the apposition of two pronuclei.
Prompted by questions, he also offered some intriguing
perspectives on the stochastic yet robust nature of the developmental
program, asserting that development is surprisingly non-linear and
unpredictable, and marveling at the ability of embryos to take different
paths but still arrive at the same endpoint.
In her talk, Barbara Meyer presented an enlightening overview of the
different mechanisms by which dosage compensation can occur, followed by
a more specific discussion of her laboratory’s work on the strategy
utilized by the nematode C. elegans.
She began with the three known schemes that organisms utilize to
ensure equal levels of gene expression between males and females: a
complete silencing of one of the two female X chromosomes, as in
mammals; a doubling of the expression of the single X, as occurs in
Drosophila males; and a two-fold decrease in the expression of the two X
chromosomes, as in C. elegans hermaphrodites).
Bringing together the diverse fields represented by attendees of
the symposium, Meyer highlighted the mechanistic parallels between
dosage compensation and other forms of silencing including mammalian
imprinting, transposon silencing, centromeric heterochromatin formation
in yeast, and heterochromatic gene silencing in flies.
She noted that a common theme appears to be RNA-directed changes
in histone modification patterns and chromatin structure followed by
spreading of silencing complexes in cis.
Dr. Meyer went on to discuss extensive work from her laboratory
which has focused the identification of cis-acting sequences and
trans-acting factors required for dosage compensation in C. elegans.
She described the cloning of the large dosage compensation
complex (DCC) and laid out four hypotheses for how this repressive
complex could be recruited to the X chromosome during dosage
compensation: single site recruitment followed by spreading, multiple
site recruitment followed by spreading, a limited number of recruitment
sites in the absence of spreading, or simply a high density of binding
sites. She then presented
some elegant experiments in which FISH and immuno-staining of the DCC
was performed in strains containing detached portions of X chromosome to
show that the endogenous X chromosome is composed of interspersed
regions that can or cannot autonomously recruit the DCC.
She went on the describe the isolation of a specific sequence
contained on the X that is sufficient to recruit the DCC, and showed
that when this “X recognition site” is inserted into an autosome the
DCC is recruited and spreading occurs.
The data presented clearly indicated that there are multiple
initiation sites on the X chromosome and that spreading from those sites
does occur. Finally, Meyer
speculated on the role of non-coding RNA in X chromosome recognition and
presented some preliminary genetic data suggesting that RNAi-directed
silencing could be involved in C. elegans dosage compensation.
David
Allis gave an energized, data-packed seminar detailing recent results
from his laboratory’s attempts to decipher the combinatorial code of
histone modifications. His talk
largely described efforts to identify the mechanism by which histone
methylation could be reversed. Histone
methylation is known to play a major role in determining chromatin
structure through its ability to recruit downstream proteins, and is
considered a stable epigenetic mark in that it persists without active
removal. As such, the
failure to identify a demethylase has been a matter of much debate in
the field. The first idea
put forward by Dr. Allis, termed the methyl-phos switch, is that
demethylation of lysine residues may not be required in order to reverse
the downstream effects of the methylation.
As lysines are found next to serine or threonine, which are
phosphor-acceptors, phosphorylation of these adjacent residues may
negate the effects of lysine methylation and have the same functional
consequence of demethylation. Supporting
this hypthesis, he presented evidence that binding of the HP1
chromodomain to H3 lysine9 is inhibited by phosphorylation of the
adjacent serine. For the reversal
of argenine methylation, Allis presented evidence that a demethylase may
indeed exist. This enzyme is a
previously characterized peptidyl argentine deaminase (PAD) whose
biochemical activity suggested to Allis and colleagues that it could
function as a demethylase. He
then showed a series of meticulous biochemical experiments directly
implicating PAD in the demethylation of Argenine residues, resulting in
an irregular amino acid citrulline. This
entertaining talk shed new light on the dynamic regulation of histone
tail modifications and gave rise to a lively discussion afterwards about
the complexities of using antibodies to characterize histone
modification.
Other
Dispatches
Symposium
69 Live
Symposia
Past (a bit of history and photographs from previous Symposia)
Online
Symposium Volumes (searchable database of past Symposia volumes and
currently received manuscripts) |
Ira Hall
(Wigler lab)
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