Chemical Turns Stem Cells into Neurons
By Jason Socrates
Bardi
A group of researchers from The Scripps Research Institute
(TSRI) and the Genomics Institute of the Novartis Research
Foundation (GNF) have identified a small chemical molecule
that controls the fate of embryonic stem cells.
"We found molecules that can direct the embryonic stem cells
to [become] neurons," says Sheng Ding, who recently completed
his Ph.D. work at TSRI and is becoming an assistant professor
in the Department of Chemistry. Ding is the lead author on
the study, which is described in an upcoming issue of the
journal Proceedings of the National Academy of Sciences.
Peter Schultz, TSRI professor of chemistry and Scripps Family
Chair of TSRI's Skaggs Institute for Chemical Biology, adds:
"This is an important step in our efforts to understand how
to modulate stem cell proliferation and fate."
The Promise of Stem Cell Therapy
Stem cells have huge potential in medicine because they
have the ability to differentiate into many different cell
types—potentially providing doctors with the ability
to regenerate cells that have been permanently lost by a patient.
For instance, the damage of neurodegenerative diseases like
Parkinson's, in which dopaminergic neurons in the brain are
lost, may be ameliorated by regenerating neurons. Another
example is Type 1 diabetes, an autoimmune condition in which
pancreatic islet cells are destroyed by the body's immune
system. Because stem cells have the power to differentiate
into islet cells, stem cell therapy could potentially cure
this chronic condition.
However bright this promise, many barriers must be overcome
before stem cells can be used in medicine. Scientists have
yet to understand the natural signaling mechanisms that control
stem cell fate and to develop ways to manipulate these controls.
"We still have much to learn about how to direct stem cells
to specific lineages," says Ding.
In order to address this problem, Schultz and Ding sought
to find small chemical molecules that could permit precise
control over the fate of pluripotent mouse embryonic stem
cells—which, like human embryonic stem cells, have the
ability to differentiate into all cell types.
The scientists screened some 50,000 small molecules from
a combinatorial small molecule library that they synthesized
at GNF. Just as a common library is filled with different
books, this combinatorial library is filled with different
small organic compounds.
From this assortment, Schultz and Ding designed a method
to identify molecules able to differentiate the cells into
neurons. They engineered embryonal carcinoma (EC) cells with
a reporter gene encoding a protein called luciferase, and
they inserted this luciferase gene downstream of the promoter
sequence of a gene that is only expressed in neuronal cells.
Then they placed these EC cells into separate wells and added
different chemicals from the library to each. If the engineered
EC cells in any particular well were induced to become neurons,
the neurons would express luciferase—which can convert
a non-luminescent substrate to a luminescent product. This
product makes that well easy to detect from tens of thousands
of other wells with GNF's state-of-the-art high-throughput
screening equipment.
Once they found some cells they believed to be neurons by
treatment with certain small molecules, the scientists used
more rigorous assays to confirm this, including staining the
cells for characteristic markers and examining the shape of
individual cells under the microscope. Neurons have a characteristic
round soma body and asymmetric multiple processes.
In the end, Schultz and Ding found a number of molecules
that were able to induce neuronal differentiation, and they
chose one, called TWS119, for further studies.
When they examined the mechanism of TWS119 in detail, they
found that it binds to a cellular kinase enzyme called glycogen
synthase kinase-3beta (GSK-3beta). This is a multifunctional
"signaling" enzyme involved in a number of physiological signaling
processes whereby it modulates other enzymes by attaching
a phosphate group to them.
The fact that modulating GSK-3beta leads the cells to become
neurons reveals basic information on the complicated signaling
cascade that turns a stem cell into a neuron. And the fact
that TWS119 modulates the activity of GSK- 3beta suggests
that TWS119 is likely to provide new insights into the molecular
mechanism that controls stem cell fate, and may ultimately
be useful to in vivo stem cell therapy.
Schultz and Ding are still working on describing the exact
mechanism whereby this binding directs the cell to become
a neuron.
The article, "Synthetic Small Molecules that Control Stem
Cell Fate" is authored by Sheng Ding, Tom Y.H. Wu, Achim Brinker,
Eric C. Peters, Wooyoung Hur, Nathanael S. Gray, and Peter
G. Schultz and will be available online next week at: http://www.pnas.org/cgi/10.1073/pn
as.0732087100. The article will also be published in an
upcoming issue of the journal Proceedings of the National
Academy of Sciences.
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