Identifying the Key Genes for Regeneration | HHMI BioInteractive Video


ALEJANDRO SANCHEZ
ALVARADO: Imagine that somebody came and told
you that there were animals whose heads could
be decapitated, and that, in the span of
a short period of time, these heads would begin to
grow again from the animal that has just lost his head. I would say, yeah, that’s
great science fiction. But then, it turns out that many
animals out there can actually do this. My lab has been studying
planarians, a type of flatworm, since 1998. These warms have
an amazing ability to regenerate lost body parts. My lab is studying
them because we hope to understand
the fundamental rules of this process, which could be
important for our own health. Our lives depend on the
ability of our tissues, like our skin, gut, and
blood, to regenerate. But unlike planarians,
we can’t regenerate a new head or half our body. How do planarians do this? Planarians are everywhere. And the best way to find them is
to go to a clean body of water, fresh water, a stream or a pond,
and get your hand in the water, grab a rock, lift it,
turn it upside down, and expose it to the sunlight. And what you’ll see on that
rock is this little blob beginning to stretch out. They just glide. They’re very beautiful,
very elegant. And it’s going to have these
cockeyes on their head. That’s a flatworm. Planaria, humans, and
every other animal have to make new tissues at
some points in their lives, like during development
or to heal a wound. Understanding how the
process works in one group, such as planaria, can
give us important clues about how it might work in
other species, including humans. So what is the key to
planarian regeneration? They have in their body
plan a large number of these adult stem cells. Human embryos also have
lots of stem cells. They’re needed
during development to produce different types
of differentiated cells, such as skin, brain,
muscle, and liver cells. But stem cells become
more rare as we age. Planaria, on the other hand,
have abundant stem cells throughout their lives. Up to a fifth of their adult
body cells are stem cells. When we cut a
planarian, these stem cells multiply and differentiate
into all the cell types needed to replace the missing parts. But how do stem cells
know what kind of cells they need to produce to
regenerate the correct body part? The stem cells are not
swimming in vacuum. They’re in contact with
differentiated cells, which are already specialized
to perform particular functions. These differentiated
cells can make molecules that signal
to the stem cells to also start differentiating
and become specialized. Inside of cells, the
molecular signals are transmitted
like dominoes, where the action of one
molecule affects the function of the next,
ultimately steering the stem cells into rebuilding
a missing body part. One way to understand
this complicated process is to eliminate one
player at a time and see what happens to
planarian regeneration. Dr. Alicce Accorsi, a
postdoctoral scientist training in my lab, is working
with me on this quest using a technique
called RNA interference, or RNAi for short. DNA is the template for
making messenger RNA, which in turn is a template to
make a particular protein. RNAi use uses a molecule
of double stranded RNA that is complementary to
specific messenger RNA. The double stranded RNA
causes the destruction of the matching
messenger RNA, blocking the production of the protein
and deleting its function in the cell. In humans, a protein
called beta-catenin is important during
embryonic development. It acts like a master switch
that turns on many genes. We wondered whether
the planarian version of beta-catenin might
be important for regeneration. What we did was to take
advantage of RNA interference to test the function of this
protein during regeneration. So we made a double
stranded RNA, complementary to
beta-catenin messenger RNA, to block beta-catenin
protein production and see if regeneration
was affected. But first, we needed to
introduce this double stranded RNA into planarian cells. So what you’re seeing here
is a gourmet top drawer feed for planarians. This is actually made
from organic beef liver. We prepared a double standard
RNA, mixed it with the liver. We take a little bit of that
mixture, put it in a Petri dish where the planarians
are, and they will now congregate towards the food. To eat, planarians stick
out two black structures called pharynges on the
underside of their body. At the end of these
tubes is the mouth. As they ingest the
food, it gets irrigated throughout the
anatomy of the animal, guaranteeing essentially that
every single cell in the animal will receive a dose of
double standard RNA. Normally, when you amputate
a planarian’s head and tail, the animal will regenerate
both head and tail in the right positions. But when we did this procedure
with the animals that were treated by RNAi, we
got an amazing result. So this is a worm that was
treated with beta-catenin RNAi. In the absence of beta-catenin,
what you see essentially is that the head regenerates
where it should regenerate. But if you now look
at the tail, where it should have
grown a tail, you’re actually now growing a head. If removing in beta-catenin
results in two-headed worms, what would happen if there
is too much beta-catenin? We knew that the
levels of bet-catenin are normally turned down
in cells by the action of another protein called APC. So to increase beta-catenin,
we needed to remove APC. To do that, we made
double stranded RNA complementary
to APC messenger RNA to block the production
of APC protein and see if regeneration
was affected. And so if we now introduce
RNAi against APC, now you’re going to get a huge
amount of beta-catenin inside of the cell. We feed them RNAi against APC. We cut the animals. And now we look at
the trunk pieces. And we look at both ends. And to our surprise,
what we found was that instead of
regenerating a head and a tail, it regenerated tail and tail. This research shows that
beta-catenin and APC are part of the line
of molecular dominoes that direct stem
cells to regenerate either a head or a tail. By removing beta-catenin, we
manipulated the normal process of regeneration,
signaling to stem cells on both sides of the body
to regenerate a head, while removing APC caused
beta-catenin to accumulate throughout the
planarian body, we signaled to stem cells
on both sides of the body to regenerate a tail. We’ve repeated these
experiments many times. And together with research
from other scientists, they suggest that heads
normally regenerate where they’re supposed to
because beta-catenin activity is naturally low in tissues
at the front of the animal, while tails regenerate at
the back of the animal, because beta-catenin
activity is naturally high in those tissues. We now know that
beta-catenin and APC are two important
proteins for regeneration. But there are
undoubtedly many more. Using RNAi and
other techniques, we hope to identify all
the major players. Humans have versions of many,
if not all, of these molecules. So these planarian
experiments can guide us to further
understand the biology of human regeneration. And while regeneration
is probably one of the last wild
frontiers of biology, it’s not very difficult
to tell people when we work on
regeneration for them to see why this is important. What would happen if we could
harness the power of planarian regeneration? Wouldn’t that be a
good thing for humans.

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