Week 2 Topic 2 - Neuron-glial interactions and mental health

Question 1

Section 1 - Embryonic Neural Progenitor Cells To Adult Hippocampal
Neurogenesis, I want to link what you heard from Professor Sarah Guthrie about the generation
of neurons, or neurogenesis, during development, to what you will hear from Dr. Sandrine Thuret
about the generation of new neurons in the adult brain.

Answer 1

In this short section, entitled From Embryonic Neural Progenitor Cells To Adult Hippocampal
Neurogenesis, I want to link what you heard from Professor Sarah Guthrie about the generation
of neurons, or neurogenesis, during development, to what you will hear from Dr. Sandrine Thuret
about the generation of new neurons in the adult brain.

Question 2

Section 2 - adult neurogenesis, then explore the location/environment - also called the
‘niche’ - where adult neurogenesis is occurring. I will also discuss the molecular control of adult
hippocampal neurogenesis, the functionality of adult hippocampal neurogenesis, and finally how
adult hippocampal neurogenesis can be modulated.

Answer 2

So you have just learned about the concept of neural stem cells and the production of neurons
derived from these neural stem cells during development. Now we are going to go through
the concept of adult neurogenesis, then explore the location/environment - also called the
‘niche’ - where adult neurogenesis is occurring. I will also discuss the molecular control of adult
hippocampal neurogenesis, the functionality of adult hippocampal neurogenesis, and finally how
adult hippocampal neurogenesis can be modulated.

Question 3

Section 3 - So, we have learned about the niche. We have learned that astrocytes are important. But what are
the actual molecular controller responsible for adult neurogenesis?

Answer 3

So, we have learned about the niche. We have learned that astrocytes are important. But what are
the actual molecular controller responsible for adult neurogenesis?

Question 4

Section 4 - So can we modulate neurogenesis? And if so, how?

Answer 4

So can we modulate neurogenesis? And if so, how?

Slide 3

Question 5

During development what are neurons are generated by?

Answer 5

Radial glial cells.

[These cells get their name because of their radial morphology.]

Question 6

What type of cell is an astrocyte?

Answer 6

It’s a Glial Cell.

Question 7

What type of cell are radial glial cells generated from?

Answer 7

During development, radial glial cells are generated from new epithelial cells.

Question 8

What are epithelial cells?

Answer 8

These cells are

the cells that form the neural tube and are the characteristics of embryonic neural stem cells,

Question 9

Later in development, what do radial glial cells go

on to generate?

Answer 9

Later in development, these radial glial cells go
on to generate adult neural stem cells as required to generate specific types of neurons in the adult
brain.

Question 10

During adulthood, where do neural stem cells reside?

Answer 10

The stem cells reside in two specific locations in the adult brain, the subventricular zones of the
lateral ventricles and the subgranular zone of the dentate gyrus, which is part of the hippocampal
formation.

Question 11

How do we get from the new epithelial

cells of the neural tube to the adult neural stem cells via radial glial cells?

Answer 11

As I mentioned earlier, the new epithelial cells that form the neural tube are the founder cells of
the central nervous system, and as such, can be thought of as embryonic neural stem cells.

This
means that they have the capability to generate all the different cell types in the developing central
nervous system.

That is, they have the ability to generate all the different types of neurons, and also, two types of
glial cells, astrocytes and oligodendrocytes.

NEW EPITHELIAL CELLS = EMBRYONIC NEURAL STEM CELLS

Question 12

What are two characteristics of new epithelial cells/embryonic neural stem cells?

Answer 12

Like all other stem cells, embryonic neural stem cells are non-specialised cells that have two
specific characteristics.

1. They can self-renew and
2. differentiate. They differentiate into appropriate
   specialised cell types, which for neural stem cells are neurons, astrocytes, and oligodendrocytes.
   Let’s look at each of these characteristics in a little more detail.

Question 13

Can you think of NEW EPITHELIAL CELLS = EMBRYONIC NEURAL STEM CELLS?

Answer 13

yes.

Question 14

What is self renewal?

Answer 14

Self-renewal is the ability of a cell to divide and generate two cells that are identical to the parent
cell. Self-renewal is needed to make sure the cells don’t run out. In other words, that sufficient
numbers of embryonic stem cells are present to enable the generation of all the different brain
cells that we need.

Question 15

What is differentiation?

Answer 15

Differentiation is the ability to divide and generate more specialised cell types. This process is
important for making all the different kinds of cells that are required to generate a proper
functioning brain.
Differentiation can occur in a number of ways. To explain this, I will consider the different ways that
neurons may be generated.

Question 16

How can an embryonic neural stem cell/new epithelial cell differentiate?

Answer 16

1. An embryonic neural stem cell may divide and generate another embryonic neural stem cell and a
   neuron.
2. Or an embryonic neural stem cell may divide, generating a progenitor cell, like a radial glial
   cell, and a neuron. This radial glial cell also has ability to self-renew, but it does this mainly by
   dividing to generate one cell that is like itself and a neuron, but the radial glial cell might also divide
   to generate a dedicated progenitor cell.
   That is, a progenitor cell that has the ability to only generate a single cell type. For instance, a
   neuron. And while doing this, it also generates a neuron.

Question 17

What is asymmetric differentiation?

Answer 17

This process of differentiation, where a parent cell makes two different progeny, is called
asymmetric differentiation.

Question 18

Can radial glial cells make embryonic neural stem cells?

Answer 18

No, radial glial cells cannot make embryonic neural stem cells,

Question 19

Can dedicated progenitor

cells make radial glial cells or embryonic neural stem cells?

Answer 19

No. Dedicated progenitor

cells cannot make radial glial cells or embryonic neural stem cells.

Question 20

Are neurons terminally differentiated? What does this mean?

Answer 20

Yes. Neurons are terminally differentiated, so do not divide at all.

Question 21

Specialization:

Embryonic stem cell (least specialized)
Radial Glial Cell
Dedicated Progenitor Cell
Neuron (most specialized)

Answer 21

So if we think about specialisation, the embryonic neural stem cell is the least specialised cell. Then
we have the radial glial cell, then the dedicated progenitor cell, and then the neuron.

Question 22

What happens after embryonic neural stem cells self-renew during development?

Answer 22

1. Initially during development, embryonic neural stem cells self-renew to expand the progenitor pool.
2. They will then begin to generate neurons, because during development, neurons are generated
   before glial cells.
3. As well as generate neurons, embryonic neural stem cells will also generate radial glial cells. These
   cells, as I told you, also have the ability to self-renew and generate neurons either directly or via a
   dedicated progenitor cell. That is, a cell that will only generate neurons in this case.
4. Just to complicate matters further, embryonic stem cells can also generate neurons via dedicated
   progenitor cells, too.

Question 23

Why are there so many different ways to generate neurons?

Answer 23

Well, we
consider that this is because there are many different types of neurons that need to be made over
a very specific time period during development, and this is especially true when we consider the
complexity of the human brain.

Question 24

Later in development, what do radial glial cells generate?

Answer 24

Later in development, radial glial cells begin to generate oligodendrocytes and astrocytes, again
via dedicated progenitor cells.

Radial glial cells also generate another type of cell, the adult neural stem cell.

As their name
implies, these cells retain the capacity to generate new neurons throughout our lifetime.

Question 25

Who said: Once development has ended…

everything may die, nothing may be regenerated

Answer 25

Cajal.

So the concept of adult neurogenesis, or the birth of a new neuron in the adult brain, is fairly
new. And since the early 1900s, it was generally believed that no new neurons can be generated
in the adult central nervous system. And, as then stated Cajal, ‘Once development has ended…
everything may die, nothing may be regenerated’.

Question 26

When, who, where showed that new neurons were indeed born in the adult hippocampus of rats?

Answer 26

Then, 50 years ago, Altman and collaborator suggested, with autoradiographic and histological
evidence, that some new neurons were indeed born in the adult hippocampus of rats. But it is not
until the early ‘90s that work started again, with the development of new technology, to definitely
prove that neurogenesis was occurring in restricted regions of the adult brain.

Question 27

What are the two adult neurogenic niches that have consistently been found in the rodent?

Answer 27

So two adult neurogenic niches have consistently been found in the rodent, during normal
physiological conditions - the subventricular zones of the lateral ventricle, where the neural stem
cells give rise to newborn neurons that will migrate to the olfactory bulb, and the second niche is
the subgranular zone of the dentate gyrus in the hippocampus. Importantly, the generation of new
neurons throughout adulthood has not only been demonstrated in the hippocampus of rodents but
also in the hippocampus of humans.

Question 28

Who is credited with assessing the
generation of hippocampal cells in postmortem human brains by measuring the concentration of
nuclear-bomb-test-derived C14 in genomic DNA?

Answer 28

Indeed, elegant work from the group of Jonas Frisen, from the Karolinska Institute, assessed the
generation of hippocampal cells in postmortem human brains by measuring the concentration of
nuclear-bomb-test-derived C14 in genomic DNA. In this figure, extracted from the original article,
we can see C14 concentration in the hippocampal neurogenomic DNA correspond to a time after
the date of birth of the individual, demonstrating neurogenesis throughout life.

Question 29

Where do adult neural stem cells proliferate, differentiate, and mature and then extend projection into (rats)?

Answer 29

So when we zoom in to the dentate gyrus of the hippocampus, and then, in the granular cell layer,
we have our neural-stem-cell niche, where they will proliferate, differentiate and mature into
neurons through the granular cell layer, where they will mature and receive input from the
entorhinal cortex and extend projection into the CA3. x

Question 30

How long does it take in rodents to go

from neural stem cells to mature neurons?

Answer 30

And it will take up to four to six weeks to go

from neural stem cells to mature neurons in rodent, as we see here, on the figure on the right.

Question 31

How many new neurons are produced in each

hippocampus per day in humans and in rodents?

Answer 31

So how relevant is the amount of newborn neuron generating during adulthood? In the adult
hippocampus, in human, it is estimated that we produce around 700 new neurons in each
hippocampus per day. It does not seem a lot, among the billions of neurons we have in the brain,
but by the time we turn 50 we will have replaced the entire granular-cell population we were born
with, with adult-born neurons. When investigated adult rodent and a very particular neurogenic
niche, it has been found that 70% of the bulbar neurons are replaced then during a six-week
period.

Question 32

What makes both two niches so special? Why neural stem cells from only in those
privileged areas of the adult brain can generate neurons? What environment makes neurogenesis
possible?

Answer 32

So, now, what makes both two niches so special? Why neural stem cells from only in those
privileged areas of the adult brain can generate neurons? What environment makes neurogenesis
possible?

So there has been some classic transplantation studies providing direct evidence for the regulation
of fate determination by extrinsic signals that are derived from the neurogenic environment.

Such as, if you extract neural stem cells from non-neurogenic regions, like the spinal cord, then
grow them in a dish, expand them, then take these cells you have grown in the dish, and then you
transplant them back into a non-neurogenic region - so, back into the spinal cord - you still do not
get any neurons.

However, if you take them, grow them in the dish, and then transplant them in a
neurogenic region like the dentate gyrus or the subventricular zone, then these neural stem cells
issued from a non-neurogenic region, transplanted in a neurogenic region, give rise to neurons.

Conversely, if you take neural stem cells from a neurogenic region - let’s say, from the dentate
gyrus - then transplant them in a non-neurogenic region, like the spinal cord, you do not get
neurons.

Of course, you transfer them back in a neurogenic region, like the dentate gyrus or the
SVZ– the subventricular zone - then you get a neuron.

So, demonstrating direct evidence for the
regulation of neuronal fate, determination of stem cells by extrinsic signals that are derived from
the neurogenic microenvironment by the niche.

Question 33

So, what constitutes a neurogenic niche?

Answer 33

1. We know that endothelial cells and
2. proximity to blood
   vessels do play a critical role but that also
3. astrocytes within the niche are very important.

Question 34

Who provided evidence that astrocytes are key players in the neurogenic niche, to instruct neural stem
cells to adopt a neuronal fate?

Answer 34

And I would like to highlight here, the first article by Song and collaborator, from the Gage lab,
providing evidence that astrocytes are key players in the neurogenic niche, to instruct neural stem
cells to adopt a neuronal fate.

So what they did is co-culture experiments.

So they extracted neural stem cells from the adult
hippocampus - so, here, labelled in green– and then co-cultured them with astrocytes, either
extracted from the adult hippocampus or with astrocytes extracted from a non-neurogenic region,
like the spinal cord.

So, leaving those neural stem cells to differentiate, together with these
astrocytes in cultures, so they did produce more neurons - so, then expressing this neuronal
marker mapped to in red - when co-cultured with hippocampal astrocytes.

So it is nicely quantified on the graph, where the neural stem cells produced the most neurons
when co-cultured with neonatal hippocampal astrocytes - so, young hippocampal astrocytes.

Then
the next-best were adult hippocampal astrocytes, and then with spinal-cord-derived astrocytes
leading to the lower number of neurons - comparable, actually, to control condition - without any
astrocytes.

Question 35

What do we know about molecular control of adult neurogenesis?

EGF and FGF2 - primary mitogens used to propagate neural stem cells in vitro and are believed to be very important for the control of in vivo proliferation of neural stem cells or progenitor cells

BMP - instructs adult neural stem cells to adopt a glial cell fate

Answer 35

1. At the time, we understood very well how adult neural stem
   cells proliferate - with, for example, epidermal and fibroblast growth factors, EGF and FGF2, which are the primary mitogens used to propagate neural stem cells in vitro and are believed to be very important for the control of in vivo proliferation of neural stem cells or progenitor cells.

Next, the molecular mechanism underlying fate specification of adult neural stem cells over a decision to actually become neuron at this time had just began to be revealed.

2. So we knew, back in 2004, that adult neural stem cells express member of the bone morphogenic protein, BMP, family that instruct them to adopt a glial cell fate.
3. However, in the neurogenics niche, the BMP inhibitor Noggin is secreted by the ependymal cells and presumably serves to block the gliogenic effect of BMPs, so driving the fate of the neural stem cells towards a neuronal fate in the niche.
4. When we think about factor control in later step, in neurogenesis, such as functional maturation, synapse formation and integration into the neuronal circuit and the survival, were then, at the time, unknown.
5. We now know a lot more on the molecular mechanism controlling all the steps of adult neurogenesis.

And I invite you to have a look at this updated review and find your favourite molecule involved in proliferation, differentiation, migration, all the way down to integration.

Question 36

What does the bone morphogenic protein, BMP, do to a cell?

Answer 36

When expressed, it instructs a cell to adopt a glial cell fate.

Question 37

What does Wnt signalling do?

Answer 37

Wnt signalling regulates

adult hippocampal neurogenesis.

Question 38

What is wnt?

Answer 38

it is a protein, that regulates cell growth, function, differentiation, and cell death. Wnt proteins play a central role in bone development, modeling, and remodeling and is secreted by astrocytes, one of our
niche key player.

Question 39

When was it first shown that Wnt signalling regulates adult hippocampal neurogenesis?

Answer 39

So in this 2005 Nature article, the author showed for the first time that Wnt signalling regulates
adult hippocampal neurogenesis.

The first clue they got was via in situ hybridisation, identifying cells expressing Wnt in the
subgranular zone, where neural stem cells reside within the hippocampal niche.

They then
extracted adult hippocampal stem cells and cultured them in vitro, with or without Wnt factors.
T

hey show here that Wnt-3 pushed them towards a neuronal fate, as indicated by the increased
number of neuroblasts, or young neuron, labelled here with doublecortin, or DCX, in red.

It is
nicely quantified on the graph, with a fourfold increase of neurons produced when neural stem
cells are cultured in the presence of Wnt.

For their next experiment, they moved in vivo and injected the hippocampus with a controlled
antivirus only expressing a green fluorescent marker, or they injected the hippocampus with
a dominant negative Wnt antivirus blocking Wnt signalling.

WHEN WNT WAS BLOCKED, LESS NEUROGENESIS.

They show here that the number
of newborn neurons has decreased of eightfold when Wnt signalling was blocked, their data
demonstrating that Wnt signalling was an important regulator of adult hippocampal neurogenesis.
Slide 7
So we have gone through the importance of the niche. We have looked at some of the molecular
control.

Question 40

So now, what is the functionality of adult neurogenesis? What are these newborn neurons
for?

Answer 40

So we know that adult hippocampal neurogenesis is important for learning and memory. The level
of neurogenesis in the dentate gyrus is positively correlated with hippocampal-dependent learning
tasks. And there is plenty of paper out there showing that type of evidence.

Question 41

What happens if we block neurogenesis?

Answer 41

And in many of the studies, if we actually block neurogenesis, then we block hippocampal-dependent learning abilities.

Question 42

Can hippocampal-dependent learning also modulate neurogenesis? What does this mean?

Answer 42

And the dotted line I placed there is to illustrate that actually
hippocampal-dependent learning can also modulate neurogenesis - so, showing a bi-directional link
between learning and neurogenesis.

Question 43

What has been suggested about the relationship between increased neurogenesis and improved
cognition?

Answer 43

So multiple mechanisms for the relationship between increased neurogenesis and improved
cognition have been suggested, including computational theories to demonstrate that new
neurons increase memory capacity, reducing difference between memories - what we call ‘pattern
separation’ - or add information about time to memories.

Question 44

What has been suggested about post-natal new hippocampal

neurons and forgetting during infancy?

Answer 44

Of these, post-natal new hippocampal

neurons could be also involved in forgetting during infancy.

Question 45

What has been suggested about the connection between adult hippocampal neurogenesis and mood regulation and depression?

Answer 45

Adult hippocampal neurogenesis is also implicated in mood regulation and depression.

So, adult
hippocampal neurogenesis is reduced in many animal models of depression, and many treatments
for depression actually promote adult hippocampal neurogenesis.

So, although more evidence suggests that neurogenesis alone cannot mediate the effect of
anti-depressant, it is a key player.

Such as, if you give anti-depressant to an animal model of
depression, you will alleviate the symptom of depression.

But if you block neurogenesis in the same
animal model of depression, then you will prevent the efficacy of the anti-depressant - so, showing
a link between neurogenesis and depressive behaviour.

And research is still ongoing to understand
more precisely the role of this new neuron in mood and depression.

Question 46

So what do we think about learning? Will it increase neurogenesis or decrease neurogenesis?

Answer 46

Yes,
learning will increase neurogenesis.
h.

Question 47

How about stress on neurogenesis?

Answer 47

Stress will decrease the level of adult hippocampal neurogenesis, especially
chronic stress.

Question 48

How about social interaction and neurogenesis?

Answer 48

Social interaction will be associated with a higher level of adult
hippocampal neurogenesis. And conversely, in rodents, social isolation is going to lead to
decreased neurogenesis in the hippocampus.

Question 49

How about chronic sleep deprivation on neurogenesis?

Answer 49

Yes, chronic sleep deprivation is going to decrease the level
of neurogenesis in the hippocampus.

Question 50

What do we think about running and exercise?

Answer 50

Running will increase the level of adult hippocampal

neurogenesis, and I will highlight that in a few slides.

Question 51

What about getting older?

Answer 51

And finally, what about getting older? As we get older, neurogenesis is still occurring. However, the
rate of neurogenesis is going to decrease as we age.

Question 52

Who illustrated how ageing
influences adult hippocampal neurogenesis? And who carried out what experiment to compare neurogenesis in an old rat and a new rat? Yucky.

Answer 52

And this is beautifully illustrated by the work from Villeda, et al. So this is an example of how ageing
influences adult hippocampal neurogenesis. And more precisely, how the ageing systemic milieu, or
blood and serum, modulate adult hippocampal neurogenesis.

So this is a 2011 Nature article by the lab of Wyss-Coray using a parabiosis approach, which means
using the circulatory system of an old and a young mice.

So they have isochronic control - young
and young in yellow.

We have an old and old isochronic pair in grey. And then they have their
heterochronic young, old pair in the middle.

And they let them attach for three months and then
look at their brain and look more precisely at the adult hippocampal neurogenesis.

So what we see when we look closely, and they label the newborn neurons or neuroblasts with
doublecortin, and we see that in the isochronic mice - young and young - we have a nice level of
neurogenesis.

Then when we look at our heterochronic young brain that was fused with an old
brain, we see that there is a decrease of the level of neurogenesis.

When we look at how old
isochronic pair - both brains - we see that there’s a dramatic reduced level of neurogenesis.

But
then if we look at the heterochronic old brain that was fused with the young animal, we see that
compared to the old animal it has an increased level of neurogenesis.

And it is nicely quantified in
the C section of the figure and D section of the figure, where we see that our young mice that was
fused with old mice has a decreased level of neurogenesis.

Whereas the old mice that was fused
with a young animal actually has an increased level of neurogenesis.

And they could recapitulate that experiment by simply injecting the plasma.

So if we look on the
right side of the figure, and they take a young mice as a control and then inject young mice with the
young plasma, and then they take an old blood plasma that they inject into young mice.

Then we
look at the level of neurogenesis.

And you see that the young mice injected with the young plasma
have a normal level of doublecortin of neuroblast.

But then if you look at the brain of the young mice injected with the old plasma, you see this
dramatic decreased level of neurogenesis.

And we could correlate that with, actually, their learning
and memory abilities.

Question 53

Why does neurogenesis emerge as a target of choice to enhance
learning and memory and mood, or prevent their decline?

Answer 53

So because of the functional role of adult hippocampal neurogenesis on learning and memory,
and mood and depression, and the fact that we can modulate adult hippocampal neurogenesis,
we think that neurogenesis in the adult hippocampus emerges as a target of choice to enhance
learning and memory and mood, or prevent their decline.

Question 54

Who showed

for the first time that neurogenesis can be modified by an intervention such as running?

Answer 54

So now, I want to highlight this article published in the Gage Lab - from the Salk Institute showing
for the first time that neurogenesis can be modified by an intervention such as running.

So here we see the hippocampus of a control mouse not having access to a running wheel. The
cells labelled in black are neural stem cells proliferating that will lead next to neurogenesis. And
compare this to the amount of black cells in mice who did run in D. We have nearly an increase
of 30%, so showing that running is probably a very efficient intervention to increase the level of
neurogenesis in the adult hippocampus.

Question 55

What are the 7 interventions to modulate adult hippocampal neurogenesis?

Answer 55

So let’s summarise what we have seen so far.

1. So we know that learning, exercise is going to
   increase neurogenesis.
2. We have not discussed particularly enriched environment. But be aware
   that by enriched environment, I mean putting toys, for example, in cages of rodent will increase
   the level neurogenesis, and in line then will increase their learning and memory abilities and
   improve their mood.
3. Diet also can have a positive impact, but equally another type of diet can have a negative impact.
4. We talked about ageing can decrease the level of neurogenesis.
5. Chronic stress will decrease
6. Chronic sleep deprivation will decrease the level of neurogenesis.
7. Diet - what we eat can modulate the production of new
   neurons.

Question 56

How can diets modulate neurogenesis?

Answer 56

1. So limiting calorie intake of 30% or doing intermittent fasting - so eating every
   other day - increased neurogenesis.

2.Flavonoids contained in cocoa and fruits with dark skins like
blueberry will increase neurogenesis.

3. Omega-3 fatty acid contained in oily fish like salmon will
   increase the production of new neurons.
4. Conversely, diets rich in saturated fat will decrease neurogenesis.
5. Alcohol will also be detrimental
   to the production of new neurons. However, resveratrol contained in red wine has a positive effect.
   So now for a quirky one.
6. There are entire groups of Japanese scientists fascinated about the
   role of food texture, and they have shown that soft food will decrease neurogenesis.

So these
data are derived from animal work, but actually the same diets that have been shown to impact
memory and mood in human studies, and food modulates behaviour in the same direction as foodmodulated neurogenesis, such as decreasing calorie intake, intake of flavonoids, Omega-3 fatty
acid, has increased the production of new neurons, will improve cognition and mood. Conversely,
diets rich in saturated fat will decrease learning and memory abilities and exacerbate symptoms of
depression. And some food seems to be linked to poor learning and memory abilities.
Slide 11

Therefore, we have more and more evidence that suggests that neurogenesis mediates the effect
of diet on mental health