A
new method for generating stem cells from mature cells promises to boost stem
cell production in the laboratory, helping to remove a barrier to regenerative
medicine therapies that would replace damaged or unhealthy body tissues.
The technique, developed by researchers at
the Salk Institute for Biological Studies, allows for the unlimited production
of stem cells and their derivatives as well as reduces production time by more
than half, from nearly two months to two weeks.
"One of the barriers that needs to be
overcome before stem cell therapies can be widely adopted is the difficulty of
producing enough cells quickly for acute clinical application," says
Ignacio Sancho-Martinez, one of the first authors of the paper and a
postdoctoral researcher in the laboratory of Juan Carlos Izpisua Belmonte, the
Roger Guillemin Chair at the Salk Institute.
They and their colleagues, including Fred H.
Gage, professor in Salk's Laboratory of Genetics, have published a new method
for converting cells in Nature Methods.
Stem cells are valued for their pluripotency,
the ability to become nearly any cell in the body. Stem cells for research and
clinical uses are derived in two ways, either directly from cells young enough
to still be pluripotent, or from mature cells that have been reprogrammed to be
pluripotent.
The first kind are called embryonic stem
cells (ESCs), even though the term is a misnomer. They are actually taken from
blastocysts, the hollow bundle of cells approximately the size of a tip of a pin
that is formed by a fertilized egg after five days of cell division. After a
blastocyst implants in the uterus, the embryo stage begins.
Aside from the well-known ethical
controversies, ESCs have a less discussed problem: Tissues grown from ESCs may
trigger immune reactions when they are transplanted into patients.
In order to overcome both ethical and medical
concerns, scientists learned how to coax mature cells (called somatic cells)
that had differentiated into particular types of tissue back to their
pluripotent state. These so-called induced pluripotent stem cells, or iPSCs,
set off whole new rounds of research, including a third way to get desired cell
types.
As it turns out, iPSCs have their own
problems. They take a long time to create in the lab, in a highly inefficient
process that can take up to two months to complete. First, somatic cells must
be reprogrammed to iPSCs, which takes considerable time and effort. Then, the
iPSCs have to be differentiated into specific cell lineages prior to therapeutic
application. Far worse, they can sometimes develop into tumors, called
teratomas, which can be cancerous.
Knowing this, scientists wondered if it might
not be necessary to go all the way back to the blank slate of a pluripotent
stem cell. Key to this idea is that pluripotent stem cells do not immediately
grow into particular cells. They go through intermediate progenitor phases
where they become "multipotent," and can only develop into cell types
within a certain cellular lineage. While a pluripotent cell can become nearly
any cell in the body, a multipotent blood cell, for example, can become red or
white blood cells or platelets, but not distant lineages such as neurons.
Thus, in order to avoid the potential
problems of working with iPSCs, scientists developed the technique of direct
lineage conversion. Unlike the familiar scenario, in which a pluripotent cell
would divide and generate all different cell types of an adult individual, in
direct lineage conversion one somatic cell is turned into just one other cell
type, thus, for example, one skin cell becomes one muscle cell, but nothing
else.
While this technique is effective, the Salk
team and their colleagues wondered if there might be a modification that could
be both more efficient and safer.
"Beyond the obvious issue of safety, the
biggest consideration when thinking about stem cells for clinical use is
productivity," says Salk postdoctoral researcher Leo Kurian, a first
co-author on the paper.
The team developed a new technique, which
they dubbed "indirect lineage conversion" (ILC). In ILC, as explained
in detail in Nature Methods, somatic cells are pushed back to an earlier state
suitable for further specification into progenitor cells.
ILC has the potential to generate multiple
lineages once cells are transferred to the team's specially developed chemical
environment. Most importantly, ILC saves time and reduces the risk of teratomas
by not requiring iPSC generation. Instead, somatic cells are directed to become
the progenitor cells of particular lineages. "We don't push them to zero,
we just push them a bit back," Sancho-Martinez says.
Using ILC, the group reprogrammed human
fibroblasts (skin cells) to become angioblast-like cells, the progenitors of
vascular cells. These new cells could not only proliferate, but also further
differentiate into endothelial and smooth muscle vascular lineages. When
implanted in mice, these cells integrated into the animals' existing
vasculature.
"One of the long-term hopes for stem
cell research is exemplified by this study, where stem cells would
self-assemble into 3D structures and then integrate into existing
tissues," says Juan Carlos Izpisua Belmonte.
While such clinical use may be years away,
this new method has several advantages over current techniques, he explains. It
is safer, since it does not seem to produce tumors or other undesirable genetic
changes, and results in much greater yield than other methods. Most important,
it is faster, and this is part of what makes it not only more productive, but
less risky.
"Generally it can take up to two months
to create iPSCs and their differentiated derivatives, which increases the
chances for mutations to take place," says Emmanuel Nivet, the third of
the first co-authors. "Our method takes only 15 days, so we've
substantially decreased the chances for spontaneous mutations to take
place."
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