I think most of us know from personal experience that dieting
is really, really hard.
It's very hard to shift the body's,
it's very hard to change body weight.
Most people who diet gain back all the weight they lose, and
then frequently overshoot their starting point.
This is even so for a very highly motivated individual,
such as contestants on The Biggest Loser,
who have tens of millions of people watching their progress,
and its aftermath.
Most of these individuals regain
much of the weight they lose and in some cases, overshoot.
It's almost as if the body has a homeostatic set point
that it returns to.
Regardless of what you put in, regardless of how much
you consume or how you regulate your activity.
Now this set point, however,
has been creeping steadily upwards in recent years.
So this shows you the map you've already seen for Steve.
From the CDC showing, graphing, obesity rates by state.
But the past 30 years or so
have seen a dramatic increase in obesity rates.
In fact, the thinnest state in 2010, which was Colorado,
is substantially fatter than the fattest state,
Mississippi, was in 1990.
So this increase in obesity rates is a worldwide phenomenon.
But as with American states,
this has affected some countries more than others.
And its causes are complex, as you've heard from other
speakers, and still incompletely understood.
Now one factor that is likely to play an important role is
the effects of early life environment, and
specifically the effects of diet.
So again, as with everything, early life experience can have
a major impact on adult body weight and metabolism.
And this can occur as a result
of low caloric intake, through underfeeding,
excess caloric intake, through overfeeding.
Or a change in caloric, a shift towards very high
caloric content food, such as high fat diet.
So if these events occur early in life, in infancy and
early childhood, later on in development, in, say,
late childhood or teenage years, these can lead to
rapid growth, high insulin levels,
often high leptin levels, overeating, and obesity.
And even underfeeding has this effect.
What happens with underfeeding is that once you return,
once an underfed individual is returned to being
able to eat ad libitum, there is a rapid catchup, and essentially
overfeeding to compensate for the earlier low calorie diet.
And these, in adulthood,
lead eventually to metabolic syndrome and obesity, and
this is now fairly well established through experiments.
Through both data from experimental animals and
from human epidemiological study.
So as you've heard, obesity is ultimately a disease not
only of the adipose tissue and the endocrine system,
but ultimately of the brain.
And many different brain regions are involved in control
of food intake, metabolism and activity levels.
But the master regulator, and really the central core
homeostatic modulator is in the hypothalamus, as you've heard.
And this really is the main focus of what my lab does.
We figure out how the hypothalamus is put together
during development.
Now there are various ways you can change,
so ultimately what's going on in regulating the body's
set point is that neural circuitry is being modulated,
is controlling how much an individual eats.
How much activity they expend.
Now various ways that diet could modulate neural circuitry.
So you could modulate the activity of individual neurons.
Make it more or less active.
You can strengthen or
weaken connections among neurons in the circuit.
Or, more radically, you could selectively kill
individual neurons in the circuit.
Take them away.
Or, as I'm going to talk to you about today, you could add new
neurons to the circuit in response to dietary cues.
So, my lab really got into this whole topic more or
less by accident when we were looking at the pattern of
generation of new cells in newborn mice.
Now, it had previously been thought that new neurons were
generated during embryonic development in the hypothalamus,
and not in post-natal animals.
So if we use a chemical label to visualize where cells have
divided, as shown here in green, we were able to identify,
we were able to visualize and area of high levels of cell
proliferation really very deep down in the hypothalamus.
So the hypothalamus is deep down in the brain,
the median eminence, which is this particular point
here in the kind of the middle of the hypothalamus,
is the deepest part of the hypothalamus.
It's relatively cell poor but it contains a number,
it does contain quite a few neurons,
it does contain a number of neurons.
These actually lie outside the blood-brain barrier.
So they're directly sensitive to circulating hormones
and metabolites, and are known to respond to dietary cues.
So we found that in pre-weening
animals, animals that are maybe the equivalent
of about six months to a year in terms of human age.
We see high levels of cell division in the median eminence.
Now this turns out to also correspond to relatively high
levels of neurogenesis.
What we've done here, is that we've taken the label to BrdU,
which labelizes the dividing cells.
And now stained with different markers for
different subtypes of neurons.
And you can see that there are number cells that are labelled
with both the BrdU and
the markers of different neuronal subtypes.
So again, this was done in pre-weaning animals.
So what we then decided to, what we then did was
really look to see when this neurogenesis really ceased.
Did it continue into adulthood, did it slow, or
did it stop relatively early in development?
And, we tested, we figured that because this was a part of
the brain that might be involved in sensing dietary cues, we
looked at the effects of raising animals on both normal chow, and
high fat diet, which is about 60% calories from fat.
So, we found that, if we yeah, animals on high fat diet,
we saw, so we, yeah, animals maintained on normal chow,
the generation of cells in the median
has gradually dropped away, as the animals were weaned.
And in young adult animals, were essentially undetectable.
However, if we maintain the animals on high fat diet,
we found that, during the period,
during the peri-weaning period, when they're drinking milk and
eating solid food, and also in the young adult stage.
We see that neurogenesis in the median eminence is substantially
higher, as you can see in this third bar graph here.
Now, the majority of studies
with experimental mice use only males.
We were lucky in that we chose to be a little different and
use only females.
And we were very lucky in that respect, because this
effect is really seems to be only confined to females.
So only females show elevated median eminence neurogenesis in
the median eminence following administration of high fat diet.
Okay so it looks like there are cells that are dividing,
there are neurons being made.
Where are these neurons coming from?
So that brings me to the title of our talk.
And the hypothalamic tanycytes.
So the tanycytes are a peculiar type of glial, or support cell.
These line the walls of the hypothalamic ventricles.
They're found in the hypothalamus and
only in a few other regions in the adult brain.
Now, these cells closely resemble neural stem cells or
stem or progenitor cells.
So called radial glia, that are found throughout the nervous
system, when neurogenesis is occurring.
But as development proceeds, are pretty much
are lost from almost all parts of the adult nervous system.
They express many genes that are characteristic of stem and
progenitor cells as well.
And as you can see, again,
at the bottom of this graph, at the bottom here.
This is the median eminence,
very highly enriched with tanycytes.
So we wanted to see whether tanycytes were giving rise to
neurons.
And the way we did that was essentially to use
a genetic tagging approach.
We used a molecular marker, so
we used a combination of a site-specific recombinase and
a recombinase-dependant molecular marker.
Here shown in green that is only activated in tanycytes,
but not in other cell types in the median eminence,
such as astrocytes or neurons.
Now this is immediately after we've switched on this label.
Now, if we then wait, we start to see this label coming up
in cells that express markers of immature neurons.
And also, if we wait for a month,
now starts to come on in markers of mature neurons.
So, it looks like tanycytes in the median eminence are dividing
and giving rise to neurons.
Okay, so that's all well and good, but what does it mean?
So it looks like generation of these cells is activated by
high fat diet.
What are the functional consequences, if anything?
So at the time,
we didn't have a way to selectively kill these cells.
But what we could do was block cell division,
specifically in the median eminence.
And the way we do that is essentially use
a system that's very much like a gamma knife system for mice.
So we use focal irradiation that
selectively targeted the median eminence.
And that inhibits the division of cells just as does
radiotherapy for a tumor.
Now, the way we do this is shown here.
So we take the mouse,
anesthetize it, and put it on a rotating platform.
And we aim a bean about one millimeter in diameter
that delivers soft X-rays.
As you rotate the stage, this beam now forms a cone.
And the apex of that cone gets enough radiation to
efficiently block cell division just in that region,
and nowhere else in the brain.
This was developed by collaborators in the radiation
oncology department of Hopkins.
So what we found was, in many ways, somewhat surprising.
So, first we found that animals that had been kept
maintained on normal chow,
where there was no indetectable cell division, showed
no affect on body weight when maintained on high fat diet.
So they didn't gain weight,
they didn't lose weight relative to irradiated animals.
We found that males did not show a significant difference in
weight gain relative to females, and there's a small trend.
This essentially matched exactly what we saw
the sex specific differences in generation of the neurons.
However, what we saw, we saw a dramatic effect in females.
And so you can see here, irradiated females,
shown here in red, actually gained substantially less weight
than animals that have just gone though the sham surgery,
when fed high fat diet.
So it's almost as if these new cells that are being generated
are acting to make the mice fatter,
and how are they doing that?
It turns out this effect does not occur through changing food,
food intake, or feeding.
But it does so entirely through regulation of activity, so
if we look at measures of oxygen consumption or
energy expenditure and total activity, these go substantially
hot, these go up in the irradiated animals.
So I just want to sum up what we've found here.
We've first shown that new neurons are generated
in the juvenile hypothalamic median eminence, and
that neurogenesis can be triggered by high-fat diet
in young adult females, but not appreciably in males.
These neurons appear to be generated from tanycytes, and
selectively inhibiting cell proliferation in the median
eminence in females, reduces weight gain and increases
activity when animals are maintained on a high fat diet.
Now what we don't yet
know is if the effects of focal irradiation in median eminence
actually occur from blocking neurogenesis.
There are other cells there, they're glia or support cells,
there are cells that make up the blood vessels.
We are now using a number of genetic tools to selectively
kill the neurons derived from tanycytes or to silence
their activity, which will allow us to test this directly.
Secondly, what molecular signals actually induce tanycyte derived
neurogenesis?
We've heard about a lot of hormones and
secreted factors whose levels are modulated by diet.
Insulin, lectin,
cytokines that are associated with inflammation.
We know that diet is important, but we don't yet
know what the molecular signals are.
And do signals other than diet regulate hypothalamic
neurogenesis, psychogenic stress, sex hormones?
We, again, don't know, these are areas of active investigation.
And then finally, and perhaps most importantly, we do not yet
know if what we see in mice applies to humans.
But again, this is ongoing research.
I just wanna close and thank the people who did the work,
particularly Daniel Lee, a graduate student in the lab who
really started this work off,
and Sue Younu, a post doc who's now carrying it on.
And thank you very much.