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Can we, as adults, grow new nerve cells?

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There's still some confusion
about that question,

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as this is a fairly new field of research.

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For example, I was talking
to one of my colleagues, Robert,

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who is an oncologist,

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and he was telling me,

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"Sandrine, this is puzzling.

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Some of my patients that have been told
they are cured of their cancer

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still develop symptoms of depression."

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And I responded to him,

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"Well, from my point of view
that makes sense.

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The drug you give to your patients
that stops the cancer cells multiplying

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also stops the newborn neurons
being generated in their brain."

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And then Robert looked at me
like I was crazy and said,

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"But Sandrine, these are adult patients --

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adults do not grow new nerve cells."

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And much to his surprise, I said,
"Well actually, we do."

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And this is a phenomenon
that we call neurogenesis.

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[Neurogenesis]

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Now Robert is not a neuroscientist,

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and when he went to medical school
he was not taught what we know now --

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that the adult brain
can generate new nerve cells.

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So Robert, you know,
being the good doctor that he is,

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wanted to come to my lab

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to understand the topic
a little bit better.

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And I took him for a tour

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of one of the most exciting
parts of the brain

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when it comes to neurogenesis --

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and this is the hippocampus.

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So this is this gray structure
in the center of the brain.

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And what we've known already
for very long,

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is that this is important for learning,
memory, mood and emotion.

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However, what we
have learned more recently

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is that this is one of the unique
structures of the adult brain

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where new neurons can be generated.

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And if we slice through the hippocampus

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and zoom in,

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what you actually see here in blue

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is a newborn neuron
in an adult mouse brain.

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So when it comes to the human brain --

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my colleague Jonas Frisén
from the Karolinska Institutet,

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has estimated that we produce
700 new neurons per day

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in the hippocampus.

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You might think this is not much,

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compared to the billions
of neurons we have.

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But by the time we turn 50,

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we will have all exchanged the neurons
we were born with in that structure

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with adult-born neurons.

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So why are these new neurons important
and what are their functions?

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First, we know that they're important
for learning and memory.

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And in the lab we have shown

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that if we block the ability
of the adult brain

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to produce new neurons in the hippocampus,

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then we block certain memory abilities.

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And this is especially new and true
for spatial recognition --

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so like, how you navigate
your way in the city.

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We are still learning a lot,

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and [neurons] are not only important
for memory capacity,

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but also for the quality of the memory.

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And they will have been helpful
to add time to our memory

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and they will help differentiate
very similar memories, like:

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how do you find your bike

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that you park at the station
every day in the same area,

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but in a slightly different position?

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And more interesting
to my colleague Robert,

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is the research we have been doing
on neurogenesis and depression.

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So in an animal model of depression,

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we have seen that we have
a lower level of neurogenesis.

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And if we give antidepressants,

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then we increase the production
of these newborn neurons,

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and we decrease
the symptoms of depression,

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establishing a clear link
between neurogenesis and depression.

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But moreover, if you
just block neurogenesis,

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then you block the efficacy
of the antidepressant.

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So by then, Robert had understood

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that very likely his patients
were suffering from depression

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even after being cured of their cancer,

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because the cancer drug had stopped
newborn neurons from being generated.

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And it will take time
to generate new neurons

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that reach normal functions.

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So, collectively, now we think
we have enough evidence

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to say that neurogenesis
is a target of choice

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if we want to improve
memory formation or mood,

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or even prevent the decline
associated with aging,

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or associated with stress.

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So the next question is:

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can we control neurogenesis?

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The answer is yes.

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And we are now going to do a little quiz.

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I'm going to give you a set
of behaviors and activities,

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and you tell me if you think
they will increase neurogenesis

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or if they will decrease neurogenesis.

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Are we ready?

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OK, let's go.

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So what about learning?

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Increasing?

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Yes.

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Learning will increase the production
of these new neurons.

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How about stress?

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Yes, stress will decrease the production
of new neurons in the hippocampus.

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How about sleep deprivation?

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Indeed, it will decrease neurogenesis.

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How about sex?

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Oh, wow!

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(Laughter)

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Yes, you are right, it will increase
the production of new neurons.

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However, it's all about balance here.

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We don't want to fall in a situation --

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(Laughter)

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about too much sex
leading to sleep deprivation.

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(Laughter)

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How about getting older?

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So the neurogenesis rate
will decrease as we get older,

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but it is still occurring.

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And then finally, how about running?

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I will let you judge that one by yourself.

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So this is one of the first studies

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that was carried out by one of my mentors,
Rusty Gage from the Salk Institute,

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showing that the environment
can have an impact

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on the production of new neurons.

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And here you see a section
of the hippocampus of a mouse

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that had no running wheel in its cage.

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And the little black dots you see
are actually newborn neurons-to-be.

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And now, you see a section
of the hippocampus of a mouse

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that had a running wheel in its cage.

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So you see the massive increase

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of the black dots representing
the new neurons-to-be.

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So activity impacts neurogenesis,
but that's not all.

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What you eat will have an effect

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on the production of new neurons
in the hippocampus.

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So here we have a sample of diet --

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of nutrients that have been shown
to have efficacy.

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And I'm just going
to point a few out to you:

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Calorie restriction of 20 to 30 percent
will increase neurogenesis.

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Intermittent fasting --
spacing the time between your meals --

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will increase neurogenesis.

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Intake of flavonoids,

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which are contained
in dark chocolate or blueberries,

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will increase neurogenesis.

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Omega-3 fatty acids,

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present in fatty fish, like salmon,

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will increase the production
of these new neurons.

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Conversely, a diet rich
in high saturated fat

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will have a negative impact
on neurogenesis.

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Ethanol -- intake of alcohol --
will decrease neurogenesis.

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However, not everything is lost;

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resveratrol, which is
contained in red wine,

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has been shown to promote
the survival of these new neurons.

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So next time you are at a dinner party,

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you might want to reach for this possibly
"neurogenesis-neutral" drink.

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(Laughter)

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And then finally,
let me point out the last one --

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a quirky one.

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So Japanese groups are fascinated
with food textures,

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and they have shown that actually
soft diet impairs neurogenesis,

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as opposed to food that requires
mastication -- chewing -- or crunchy food.

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So all of this data,

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where we need to look
at the cellular level,

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has been generated using animal models.

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But this diet has also been given
to human participants,

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and what we could see is that
the diet modulates memory and mood

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in the same direction
as it modulates neurogenesis,

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such as: calorie restriction
will improve memory capacity,

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whereas a high-fat diet will exacerbate
symptoms of depression --

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as opposed to omega-3 fatty acids,
which increase neurogenesis,

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and also help to decrease
the symptoms of depression.

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So we think that the effect of diet

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on mental health, on memory and mood,

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is actually mediated by the production
of the new neurons in the hippocampus.

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And it's not only what you eat,

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but it's also the texture
of the food, when you eat it,

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and how much of it you eat.

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On our side -- neuroscientists
interested in neurogenesis --

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we need to understand better
the function of these new neurons,

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and how we can control their survival
and their production.

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We also need to find a way to protect
the neurogenesis of Robert's patients.

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And on your side --

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I leave you in charge
of your neurogenesis.

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Thank you.

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(Applause)

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Margaret Heffernan:
Fantastic research, Sandrine.

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Now, I told you you changed my life --

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I now eat a lot of blueberries.

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Sandrine Thuret: Very good.

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MH: I'm really interested
in the running thing.

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Do I have to run?

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Or is it really just
about aerobic exercise,

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getting oxygen to the brain?

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Could it be any kind of vigorous exercise?

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ST: So for the moment,

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we can't really say
if it's just the running itself,

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but we think that anything that indeed
will increase the production --

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or moving the blood flow to the brain,

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should be beneficial.

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MH: So I don't have to get
a running wheel in my office?

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ST: No, you don't!

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MH: Oh, what a relief! That's wonderful.

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Sandrine Thuret, thank you so much.

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ST: Thank you, Margaret.

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(Applause)