Transcript ppt - Chair of Computational Biology

V10 Neural stem cells
Various tissue types contains stem cells to replace dead cells.
E.g. bone marrow.
Typically the number of stem cells is very low.
Stem cells need to migrate to their new destination
Is this also possible for neural cells?
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Neural stem cells
The term "neural stem cell" is used loosely to describe cells that
(i) can generate neural tissue or are derived from the nervous system,
(ii) have some capacity for self-renewal, and
(iii) can give rise to cells other than themselves through asymmetric cell division.
Whether stem cells from neural and other tissues are more defined by their tissue
of origin or by their multipotentiality is at present unclear.
However, neural stem cells can also be derived from more primitive cells that
have the capacity to generate neural stem cells and stem cells of other tissues
FH Gage, Science 287, 5457 (2000)
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FH Gage
Despite the fact that the human brain is composed of
some 100 billion neurons, it’s always been easy to
imagine that this number is somehow fixed at birth—
that we’re born with our full complement of neurons
and then it goes downhill from there.
Certainly these neuronal cells will not divide, as other
cells do.
Not with their enormously extended axons, and treelike dendrites averaging a thousand synaptic
connections each.
Fred H Gage, Salk Institute
So how would an adult brain ever add new neurons,
and how would it possibly wire them successfully
into such an unimaginably complex system?
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FH Gage interviewed by ScienceWatch
Why has it always been so hard for people to people to believe that the
adult brain could give rise to new neurons?
First of all, neurons are very complex cells—long branches, receiving hundreds of
thousands of connections. The idea that confused people is how something as
complex as a neuron could undergo cell division.
This idea was not well integrated with the emerging notion that maybe some
primitive cells remained and that those were doing the dividing.
..
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FH Gage interviewed by ScienceWatch
Why has it always been so hard for people to people to believe that the
adult brain could give rise to new neurons?
...
The other roadblock was that there were several prominent statements in the
literature contending that adult neurogenesis couldn’t happen, because the brain
and structures like the hippocampus need to be stable for memory to be stable.
If new brain cells were added, that would make it hard to store long-term
memories. It was a loose statement, but it resonated with many people ...
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FH Gage interviewed by ScienceWatch
How did you, in fact, convince yourself that neurogenesis was going on in
adult brains?
Among the important elements that helped convince us of this phenomenon were
the application of the molecule BrdU—immunocytochemistry, combined with
confocal microscopy and quantitative stereology to the measurement of
neurogenesis.
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BrdU
Bromodeoxyuridine (5-bromo-2-deoxyuridine, BrdU) is a
synthetic nucleoside that is an analogue of thymidine.
BrdU is commonly used in the detection of proliferating cells in
living tissues.
BrdU can be incorporated into the newly synthesized DNA of
replicating cells (during the S phase of the cell cycle),
substituting for thymidine during DNA replication.
Antibodies specific for BrdU can then be used to detect the
incorporated chemical, thus indicating cells that were actively
replicating their DNA.
www.wikipedia.org
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FH Gage interviewed by ScienceWatch
How did you, in fact, convince yourself that neurogenesis was going on in
adult brains?
In addition, and equally important, was switching the environment of the mice we
studied.
We let these animals grow up in little mouse cages as they normally do, and then,
when they were adults and were matched for sex, age, genetic background, etc,
we took half of them out and put them in a big complex environment and let them
stay there for 45 days.
Then we just asked simply, are there any changes in the numbers of neurons in
the hippocampus? We found this very big effect, and that was the paper we
published in Nature in 1997.
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FH Gage interviewed by ScienceWatch
So does it occur in the cortex also?
So far we haven’t seen it under normal conditions. It’s been claimed in other
areas as well, and we’re not saying that it doesn’t happen at very, very low
frequency or under damaged conditions, but we haven’t seen it.
I’m still open to the idea, however, since we’ve shown that even cells from the
spinal cord can be induced to become neurons after being cultured and
transplanted to the hippocampus, and there’s no neurogenesis going on naturally
in the spinal cord.
So our conclusion is that there are neural stem cells all over the brain and in the
spinal cord, but they don’t give to rise to neurons under normal conditions
because the local environment doesn’t provide them with the appropriate cues.
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FH Gage interviewed by ScienceWatch
So what role does neurogenesis play in the brain, and why in the
hippocampus in particular?
That’s an open question.
Why has this part of the brain reserved the capacity to generate neurons?
It’s not a ubiquitous phenomenon.
So why does it happen in this brain structure?
We don’t know yet, although I think it will be resolved in the next couple of years.
...
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FH Gage interviewed by ScienceWatch
So what role does neurogenesis play in the brain, and why in the
hippocampus in particular?
...
In order to know what role neurogenesis plays in hippocampal function or systemwide function, we have to know what role the hippocampus is playing.
We’re not able to understand neurogenesis itself, without understanding this
structure in which it occurs.
So this is a very exciting time for developing model systems—knockout
technologies, for instance.
Every day in the literature, there’s another neurogenesis article published.
There are some really smart people getting into this field, and they’re discovering
some wonderful things.
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Neural stem cells
Neural stem cells (NSCs) are the self-renewing, multipotent cells that generate
the main phenotypes of the nervous system.
In 1992, Reynolds and Weiss were the first to isolate neural progenitor and stem
cells from adult mice brain tissue.
Since then, neural progenitor and stem cells have been isolated from various
areas of the adult brain and from various species including human.
www.wikipedia.org
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Role of mitogens
Epidermal growth factor (EGF) and fibroblast growth factor (FGF) are mitogens
for neural progenitor and stem cells in vitro, though other factors synthesized by
the neural progenitor and stem cells in culture are required for their growth.
It is hypothesized that neurogenesis in the adult brain originates from NSCs.
The origin and identity of NSCs in the adult brain remain to be defined.
www.wikipedia.org
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Neural stem cells
An illustration proposing the classes of
mammalian stem cells that can give rise to
neurons, presented as a hierarchy
beginning with the most primitive and
multipotent stem cell and progressing to the
most restricted.
The restrictions of fate at each step and
examples of sites in the body where they
can be obtained are also presented.
As our understanding of the true potential
and nature of stem cells is still unfolding,
modifications will clearly be added.
E.g., the small arrows pointing up suggest
the potential, although not well
documented, dedifferentiation of the more
restricted cell below.
FH Gage, Science 287, 5457 (2000)
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TLX
Nuclear receptors are ligand-dependent transcription factors that regulate the
expression of genes critical for a variety of biological processes, including
development,
growth, and
differentiation.
TLX is an orphan nuclear receptor that plays an important role in vertebrate
brain functions.
TLX is an essential regulator of neural stem cell proliferation and self-renewal in
the adult brain.
PNAS 104, 15282 (2007)
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TLX
Expression of TLX in the mouse
- starts at embryonic day 8 (E8),
- peaks at E13.5 and
- decreases by E16,
with barely detectable levels at birth.
TLX expression increases again after birth and is high in the adult brain.
Although TLX-null mice appear grossly normal at birth, mature mice manifest a
rapid retinopathy with reduced cerebral hemispheres.
TLX could act by controlling the expression of a network of downstream target
genets to establish the undifferentiated and self-renewable state of neural stem
cells.
PNAS 104, 15282 (2007)
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Roles of TLX
Removal of TLX from the
adult mouse brain resulted in
a reduction of stem cell
proliferation and spatial
learning
Nature 451, 1004 (2008)
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Roles of TLX
PNAS 104, 15282 (2007)
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Canonical miRNA biogenesis
The Canonical miRNA biogenesis pathway. pri-miRNAs are transcribed primarily as
RNA pol II transcripts. The pri-miRNAs are processed co-transcriptionally by the
Microprocessor (Drosha/Dgcr8). Following Microprocessor cleavage, Exportin-5
transports the pre-miRNA hairpin into the cytoplasm.
There, the pre-miRNA is cleaved by Dicer, resulting in a miRNA/miRNA* complex.
With the help of TRBP, the mature miRNA is loaded into the Argonaute subunit of
the silencing complex. This complex then goes onto silence target mRNAs posttranscriptionally by translational inhibition and/or transcript destablization.
www.stembook.org√
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Non-canonical miRNA biogenesis
The Non-canonical miRNA biogenesis pathway. Mirtrons are short introns which
form pre-miRNA hairpins following splicing and debranching of the transcript. The
endogenous shRNAs are directly transcribed as pre-miRNA hairpins.
www.stembook.org√
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