V10 Neural stem cells

SS 2009 lecture 10Biological Sequence Analysis

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V10 Neural stem cellsVarious 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

FH Gage, Science 287, 5457 (2000)

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


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FH Gage

Fred H Gage, Salk Institute

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 tree-

like dendrites averaging a thousand synaptic

connections each.

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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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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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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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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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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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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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 system-

wide 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

www.wikipedia.org

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.


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Role of mitogens

www.wikipedia.org

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.


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Neural stem cells

FH Gage, Science 287, 5457 (2000)

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.


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TLX

PNAS 104, 15282 (2007)

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.


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TLX

PNAS 104, 15282 (2007)

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.


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Roles of TLX

Nature 451, 1004 (2008)

Removal of TLX from the adult

mouse brain resulted in a

reduction of stem cell

proliferation and spatial

learning


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Roles of TLX

PNAS 104, 15282 (2007)


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Canonical miRNA biogenesis

www.stembook.org√

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 post-transcriptionally by translational inhibition and/or transcript destablization.


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Non-canonical miRNA biogenesis

www.stembook.org√

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.

V10 Neural stem cells

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