October 12, 2003
Doubt Cast on Adult Stem-Cell Plasticity Studies
In a study that calls into question the plasticity of adult stem
cells, Howard Hughes Medical Institute (HHMI) researchers and
colleagues at the University of California, San Francisco, have
demonstrated that adult bone marrow cells can fuse with brain, heart
and liver cells in the body.
The phenomenon of fusion would give the appearance that bone marrow
stem cells are altering themselves to become mature cells in other
tissues, when in fact they are not, according to one of the study's
senior authors, HHMI investigator Sean J.
Morrison at the University of Michigan.
“Scientists should exercise caution in using adult bone marrow cells in clinical trials designed to generate new cells in other tissues.”
Sean J. Morrison
The researchers published their findings October 12, 2003, in the
online version of the journal Nature. The studies were carried
out by collaborating scientists, Manuel Alvarez-Dolado and Ricardo
Pardal, in the laboratories of Arturo Alvarez-Buylla of the University
of California, San Francisco and Morrison at the University of
Michigan. Other co-authors are from the University of Valencia in
Spain, the University of Dusseldorf in Germany and MIT.
The ability of bone marrow cells to contribute at very low levels to
other tissues was previously interpreted as indicating that these cells
had the plasticity to make new cells in other tissues. As a result,
clinical trials have been initiated in which bone marrow cells have
been injected into heart muscle in an effort to stimulate the formation
of new heart muscle cells after heart attack.
The new findings indicate that bone marrow contributes to other
tissues by fusing with pre-existing cells rather than by forming new
cells. This finding suggests that scientists should exercise caution in
using adult bone marrow cells in clinical trials designed to generate
new cells in other tissues. It remains uncertain whether the fusion of
blood cells with cells in other tissues can contribute to the survival
or regeneration of cells in those tissues. The researchers emphasize
that their findings underscore the importance of continuing research
with both embryonic and adult stem cells.
Stem cells are immature progenitor cells that have the theoretical
potential to differentiate into adult cells of many types. Scientists
believe that it might be possible to introduce stem cells to regenerate
damaged brain, spinal cord, heart, liver and other tissues. There are
many different types of adult stem cells that were each thought to
generate cells from different tissues - for example, blood-forming stem
cells had been thought to make only blood cells. However, studies over
the past four years have observed a contribution of bone marrow cells
to unrelated tissues, like heart, causing some scientists to believe
that adult stem cells can mature into specialized cells of unrelated
tissue types, in a process known as “transdifferentiation.”
Thus, for example, hematopoietic, or blood stem cells, could give rise
to mature neurons, or vice versa, if they were placed in the
appropriate environment.
“The concept of transdifferentiation has been important
because papers in major journals over the last few years have suggested
that there is widespread potential for transdifferentiation among stem
cells from a number of different tissues,” said Morrison.
“And these findings were the basis for political arguments
against the use of embryonic stem cells. Some critics of this research
argued that if adult stem cells really had developmental plasticity,
there was no need to work on embryonic stem cells,” he said.
Other studies raised the possibility that cell fusion might account
for the seeming “plasticity” observed in stem cells.
Morrison, Alvarez-Buylla and their colleagues set out to develop a
technique that could directly and unequivocally determine whether cell
fusion actually took place in vivo.
Their approach consisted of installing in blood cells a kind of
genetic switch, called Cre, that had the capability of turning on
another “reporter” gene whose activity could be detected by
a characteristic blue staining of tissues. This reporter gene is
normally off in cells of different tissues. Thus, only when cells come
together and fuse, does the Cre protein gain access to the switch and
turn on expression of the telltale blue reporter gene.
In initial experiments in vitro, Alvarez-Buylla and his
colleagues showed clear evidence that the system worked and it signaled
when cell fusion took place.
Next, researchers in both laboratories began experiments to
determine whether cell fusion occurred in live mice. In those
experiments, they transplanted bone marrow cells containing the Cre
protein into mice whose endogenous marrow cells had been eliminated by
irradiation. Those mice had cells throughout their bodies with the
reporter gene that could be switched on by Cre.
“In these mice, we consistently found small numbers of blue
neurons in the brain, hepatocytes in the liver and cardiac muscle cells
in the heart,” said Morrison. Many of these blue cells had two or
more nuclei, which further confirmed that they had been formed by
fusion.
In additional experiments, Morrison and his colleagues inserted bone
marrow cells that contained the genetic trigger under the control of
DNA that caused it to be expressed only in blood cells in mice with the
reporter gene. The results suggested that cell fusion was occurring
between blood cells and cells in other tissues.
Alvarez-Buylla and his colleagues went a step further and performed
indicator experiments that were designed to detect transdifferentiation
in the cells of mice. They used a second reporter gene in the bone
marrow cells that did not depend on fusion, and found no evidence that
the bone marrow cells transdifferentiated into brain, heart or liver
cells.
According to Morrison and Alvarez-Buylla, their findings offer
caution to researchers who have already begun clinical trials in which
they are inserting bone marrow cells into damaged heart tissue, in an
attempt to regenerate healthy muscle.
“Our findings raise a red flag about going too fast to
clinical trials based on the assumption that transdifferentiation is
the mechanism by which stem cells give rise to other cell types,”
said Alvarez-Buylla. “Our paper suggests that previous claims of
transdifferentiation may be explained by cell fusion.” The
scientists said they cannot rule out that transdifferentiation might be
occurring, but that they saw no evidence of it in their experimental
system.
In any case, according to Morrison, the findings emphasize the
importance of using a wide range of studies to determine the properties
of stem cells. “Responsible stem cell researchers have argued all
along that it was important for research to continue with both
embryonic stem cells and adult stem cells,” he said. “And I
think these findings further support that idea by providing evidence
that the plasticity of the adult stem cells was over-estimated.
“In this paper we described a relatively simple method for
looking directly for evidence of fusion. And I hope that future studies
of transdifferentiation will use methods like this to determine whether
the contribution of bone marrow under other conditions could also be
accounted for by fusion.” said Morrison.
According to Alvarez-Buylla, the findings of the two laboratories
are also significant because they might reveal a new biological
mechanism. “Although this remains quite speculative, cell fusion
might be a physiologically relevant phenomenon,” he said.
“While investigators have long used cell fusion as an
experimental tool to explore the relative influence of one cell's
cytoplasm over another's nucleus, they never suspected that fusion was
occurring naturally,” said Alvarez-Buylla.
“It is possible that nature uses cell fusion to enable cells
such as neurons with damaged nuclei to obtain donor nuclei from blood
cells that fuse with them. This may be a rescue mechanism for cells
that are long-lived and that have no other alternative to avoid
death.” Alvarez-Buylla and Morrison will now use their
fusion-detection method to search for fusion in other tissues and to
determine whether it does indeed play a rescue role in damaged
cells.
Nonetheless, additional research will be required in animal models
to determine whether there is any possible therapeutic benefit from
cell fusion. Until the consequences of cell fusion between blood cells
and cells in other tissues are better understood, Alvarez-Buylla and
Morrison urge caution in proceeding with clinical trials that involve
the injection of bone marrow cells into other tissues to promote
repair.
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