And so over the years, just like with counting and choice, there have been a
very number, very large number of theories about how this might, might
occur. But I think now the prevailing model, and
the one everybody seems to accept, is that there is a progressive recruitment
of the X chromosome into a silent Nuclear compartment.
So what I mean by silent nuclear compartment is a region where RNA
polymerese two is excluded so the RNA polymerase that actually transcribes
genes is removed from this region. and the X chromosome is drawn into this
region. So what I'm showing you in the picture
along the bottom of the slide is first of all a nucleus that's stained with DNA.
A DNA stain DAPI so we can see the DNA and you can see two regions that are more
densely stained than the others in the top right hand side.
These are two inactive X chromosomes. These particular cells are cancer cells
that happen to have two inactive X chromosomes.
The second oh, sorry, in the third image you can see Xist expression in green, and
you can see these two DAPI dense or these heterochromatic regions.
These Barr bodies are also where Xist is found, but the, this image in
between, which is called h/m Cot-1 is a stain for repetitive RNA, so the
expression of repetitive elements, and what you can see indicated by the yellow
arrows. Is that these two regions where these
Barr bodies are found, where Xist is found, are devoid of this repetitive RNA.
So, then when you look at the pixel profile, so this is when a line has been
drawn through the image showing the merge, the line is shown in yellow.
You can see that throughout this region, you have enrichment of DAPI.
So that the DNA stain in blue, and that strain by the mound in the graph.
And the mound in the Xist graph, so that the green line.
But you can see that the red goes down. So you see much less, repetitive RNA
being found in that region. And this is because we know that one of
the earliest events after Xist expression is exclusion of RNA polymerase two from
this region where, in which the X chromosome is found.
And silencing of the repeats, and so we find that this, the repetitive, the
repeats that we sometimes be expressed, the ones that are found in this region,
so the ones in the X chromosome are not being expressed at this time, and it's a
very early event in X inactivation. So this happens, this silent nuclear
compartment and the silencing in the repeat actually happens before the genes
on the X chromosome are silenced. And we know that the X inactivation
spreading, correlates with how the genes are actually drawn into
this silent nuclear compartment. And I'll show you this in more detail in
a diagrammatic form in the next few slides.
But here, I'd like to mention about some of those factors that are differentially
expressed or that these competent factors, competence factors which need to
be present in the nucleus for Xist to be able to bring
about silencing. So these factors, some of these factors
are SATB1 and SATB2. So, SATB1 and SATB2 were found because
they were expressed in both embryonic stem cells, early post differentiation
but also in the limited number of other cell types.
that are actually competent to perform this initiation of X inactivation.
And SATB1 and SATB2 are factors that are involved in nuclear reorganisation.
So in other words, they move the chromosomes around.