YSM Issue 90.1

A
lizard escaping from a predator can lose its tail as a defense
mechanism—while the detached tail writhes on the ground
and confuses the predator, the lizard scurries away. During
the following months, the lizard grows back a new tail. How does
this regeneration occur? The answer lies in the behavior of stem
cells, specialized cells that can both renew themselves and generate
various other cell types.
Humans have stem cells, too, although they behave differently
from those in the lizard’s tail. Many distinct populations of stem
cells reside within each of us. Neural stem cells in our brains, for
instance, can become new neurons or glial cells. Blood stem cells
in our bone marrow can turn into new white blood cells, red blood
cells, or platelets coursing through our bloodstream. Since stem cells
are extremely versatile and can generate a number of new cell types,
a recent goal of researchers and clinicians has been to harness their
regenerative abilities for replacement therapies—restoring lost organs
and tissues in human patients.
Bo Chen, associate professor of Ophthalmology at Yale, studies
human retinal stem cells. Recently, his team figured out how to
reawaken the stem cell ability of a special group of dormant cells,
called Muller glial cells (MGs). MGs are found in the retina, a tissue
at the back of the eye that detects visual information in the form of
light. While MGs in humans are not true stem cells, they can behave
like stem cells under certain circumstances, such as extreme injury.
MGs are normally asleep, in an inactive state, but injury can reawaken
them and cause them to reenter the cell cycle and develop regenerative
stem cell abilities. Chen and his research team discovered
how to wake up MGs and cause them to proliferate without injuring
them, a strategy that could potentially help to restore retinal cells
damaged by disease.
In invertebrates, such as zebrafish, MGs are permanently active
retinal stem cells capable of regenerating damaged and lost cells. In
mammals, however, MGs typically remain dormant and incapable
of spontaneously re-entering the cell cycle without outside intervention.
While past studies have shown that severe retinal injuries
can stimulate MG proliferation and stem cell activation, Chen found
that injuring MGs was counterproductive to the goal of regeneration.
“We wanted to do something different: activating these cells
without inflicting any damage to the retina. Our goal was to make
neurons without having to kill any neurons in the first place,” Chen
said.
The cell’s decision to remain dormant or become active depends
on a network of signaling pathways that is poorly understood.
Chen’s team suspected that the Wnt signaling pathway was involved,
since this pathway is also involved in embryonic development and
the regulation of stem cell behavior. Thus, the researchers decided
to test if they could reawaken MGs by increasing the activity of the
Wnt pathway.
In the traditional signaling pathway, Wnt proteins bind to cell receptors
to initiate a cascade that eventually results in inhibition of
the GSK3 protein. GSK3 normally degrades β-catenin, a signaling
molecule that causes a number of cellular effects. Therefore, when
the Wnt signaling pathway is activated, GSK3 is inactivated and
β-catenin accumulates inside the cell. The increase in β-catenin then
turns on a set of genes that can change the activity of the cell by
promoting proliferation. To study how Wnt signaling affects MGs,
Chen’s team used viruses to transfer the β-catenin gene into mouse
MG cells, thus mimicking the effects of an activated Wnt pathway.
medicine
FEATURE
After the team transferred the β-catenin gene into the cells, a
significant number of MGs began to reenter the cell cycle and proliferate.
To confirm the role of β-catenin, the researchers next deleted
the GSK3 gene, which encodes for the protein that degrades
β-catenin—again causing a buildup of β-catenin. Once again, they
observed significantly increased proliferation of MGs. They also
discovered that β-catenin directly affected the expression of a gene
called Lin28, which has been shown to regulate stem cell decisions.
Chen’s team had made a remarkable discovery, being the first
to activate the proliferative response of MGs without retinal injury.
Now that they were capable of waking up MGs, they wanted to
know whether their reactivated MGs were behaving like true stem
cells—that is, whether the MGs were capable of generating other
cell types. They transferred the β-catenin gene to a population of
MGs and gave them time to activate and differentiate. When they
analyzed the gene expression of the MGs, they found that the reactivated
MGs expressed similar genetic profiles to several retinal cell
types, suggesting that the MGs were able to differentiate in a stemcell-like
manner.
The impact of Chen’s study lies in its implications for stem cell
regenerative therapy. “Stem cell therapies are promising for treating
diseases of the human retina, where there is a loss of retinal cells,”
Chen explained. These diseases include macular degeneration and
glaucoma, both of which cause vision problems and can eventually
lead to blindness. If stem cells could be used to regenerate lost retinal
cells, then scientists and clinicians would have a novel tool to
treat these types of degenerative retinal diseases. “We can activate
this group of MG cells and potentially direct their differentiation
into any type of retinal neurons, which could replace lost cells. Essentially,
we are asking our retinas to repair themselves,” Chen said.
Although the preliminary results are promising, Chen and the
other scientists still do not fully understand the complex process of
MGs differentiation. For instance, the researchers still do not know
how reactivated MGs choose to become one cell type or another.
“The challenge now is that even when the MGs reenter the cell cycle,
they might not make the exact type of neuron we want,” Chen
said. He would like to better understand the signaling pathways that
guide MG differentiation in order to produce the cell types required
for regenerative therapy. In the future, Chen would like to test different
sets of factors that control MG differentiation. “We hope to use
these factors to guide the differentiation of the reactivated MGs to
make the types of cells that we want.”
Chen and his team are currently focusing on two types of neurons:
photoreceptor cells and ganglion cells. Both cell types are extremely
important in eyesight, detecting light from the environment and
converting it into electrical signals that can be routed to the brain.
If scientists such as Chen’s team successfully produce new photoreceptor
cells and ganglion cells from MGs, then these cell types could
be restored in patients who have lost them due to injury or disease.
Retinal stem cells are an important research topic, and Chen’s findings
will be extremely valuable for other researchers in the field. For
the first time, retinal cells regained stem cell abilities without injury.
As the discovery improves our understanding of the fundamental
signaling pathways behind stem cell differentiation, researchers
could soon harness the abilities of stem cells to heal damaged tissues
in patients. Chen is optimistic that other scientists will continue to
build upon his findings. “This type of research will ultimately greatly
benefit patients if we are successful,” he predicted.
www.yalescientific.org
December 2016
Yale Scientific Magazine
29