Power in the blood
How can we build up the adult stem cells that build
your blood?
Bring up the subject of stem cells, and Stuart Orkin
brings up Martians.
“Imagine,” says the Harvard Medical School
researcher, “if a spaceship landed right in the
middle of Boston and a bunch of extraterrestrials started
listening in on the whole stem cell debate. They’d
probably think that stem cell research is some new and
exotic thing. But nothing could be farther from the
truth. The idea of using stem cells for therapy is old.”
In particular, adult blood stem cells taken from the
bone marrow, also called hematopoietic stem cells (or
HSCs), have been used therapeutically since the 1960s,
saving the lives of tens of thousands of people.
But if our alien visitors are confused, it’s understandable.
Information about the abilities, and liabilities, of
embryonic and adult stem cells has become entirely muddled.
Take, for example, news that came from the University
of Minnesota in 2002. Here, researchers led by biologist
Catherine Verfaillie published findings that suggested
that a certain class of stem cells in the bone marrow,
just like their embryonic counterparts, could create
a variety of different tissues. These cells, she reported,
could form brain, lung, heart, kidney and intestine
tissues.
The implications were huge. Embryonic stem cells exist
for only a few days in a very early stage after an egg
is fertilized. But adult stem cells are sprinkled throughout
our tissues and organs, continuously giving rise to
new cells.
| "There are studies out there showing that you
can take blood stem cells and get them to do X,
Y and Z," says Harvard Medical Researcher Stuart
Orkin. "However, no one can reproduce these
studies. Or some of them just aren't believable.
People cite things to prove their preconceived notions." |
Up until this point, while scientists touted embryonic
stem cells as having endless potential to form nearly
any kind of tissue in the body, adult stem cells were
accorded a single fate on the biological ladder, never
to form any tissue other than that from which they came.
But Verfaillie’s findings indicated that adult stem
cells might be just as therapeutically useful as their
embryonic grandparents. And with no ethical baggage.
“This was probably the biggest thing in adult
stem cell development to come along in years,”
says Whitehead Fellow Fernando
Camargo, then a graduate student at Baylor College
of Medicine. “Still, there were some red flags.”
The Minnesota findings showed that mice and rats who had
received transplants from cultured stem cells derived
from the bone marrow seemed to have cells that genetically
matched the donor’s in other tissues, such as liver
and muscle. It logically followed then that when these
mice received the bone marrow transplant, HSCs made their
way into these tissues, listened to their unique signals,
and then ripened into liver or muscle cells.
But not everything added up. For example, whether a mouse
received a single HSC or 1,000 HSCs from a donor, the
number of donor-matching cells ending up in other tissue
never changed. “That was illogical,” says
Camargo. “If these blood stem cells really were
giving rise to other tissue, we should have seen an exponential
increase based on the size of the transplant.” In
fact, even when blood stem cells were directly injected
into liver or muscle, there was still no increase in new
cells matching the donor’s.
Camargo was one of a handful of scientists to discover
that these HSCs instead were creating mature blood cells
that then fused with cells in the liver. In fact, Camargo
was the lead author on the Nature Medicine
paper that identified macrophages as the fusing culprits.
Today, plasticity of adult stem cells has been largely
discredited. Few scientists seriously pursue it (although
some, like Stanford’s Helen Blau, are investigating
whether fusion itself might have therapeutic value).
The debate roars on, particularly with opponents of embryonic
stem cell research. The notion that HSCs can treat everything
from liver disease to heart disease to Parkinson’s
continues to pop up wherever the debate gets most heated.
While the truth is far more sobering, new findings from
Whitehead researchers may help to render HSCs far more
therapeutically potent than they’ve been thus far,
giving us the ability to treat disease with more precision
while sparing patients many brutal side effects.
Tuning therapies
Many groups opposed to embryonic stem cell research have
made grand claims about adult stem cells, declaring that
they can effectively treat brain cancer, neurodegenerative
diseases, heart disease, and spinal cord injury, to name
a few.
“There are studies out there showing that you can
take blood stem cells and get them to do X, Y and Z,”
says Orkin. “However, no one can reproduce these
studies. Or some of them just aren’t believable. People cite things to prove their preconceived notions.”
The blood system, however, is one area where adult stem
cells can boast an unambiguous track record of success.
“We’ve made a lot of advances over the years
using adult stem cells for treating blood-related disease,”
notes Camargo, who sees plenty of opportunities ahead.
Orkin, for example, has demonstrated that the genes programming
blood cell development are the same ones mutated in leukemias.
His lab is investigating how the basic machinery of blood
cells interfaces with blood cancers. Camargo is interested
in the molecular mechanisms that enable blood stem cells
to maintain their “stemness.”
“If you ask us to draw up a list of every gene that’s
essential to a hematopoietic stem cell, right now it would
be a very short list,” he says. He’s conducting
large-scale screenings of these cells using techniques
such as microarrays and RNA interference in order to find
the key molecular players. His hope is that with such
knowledge, scientists can fine-tune these cells for more
targeted therapies.
It’s likely that this will happen with blood stem
cells long before it happens with any other kind of stem
cell.
“We’ve had deep knowledge of the blood system
for over 100 years,” says John Dick, director of
the University of Toronto’s Program in Stem Cell
Biology. “And we’ve understood the major developmental
lineages of HSCs since the mid-’70s. Compare that
to the liver. Biologists are still arguing over what exactly
a liver stem cell looks like.”
As early as the late 1800s, thanks in large part to the
Russian biologist Alexander Maximov, scientists knew much
about all the different lineages of blood cells. (There
are about 12 blood cell lineages, compared to only three
cell lineages in brain tissue.) Interest soared in the
mid-20th century, when scientists discovered just how
vulnerable the blood system was to atomic radiation. By
the 1940s the concept of bone marrow transplantation had
worked its way into the biomedical world. Bone marrow
trials began in the 1950s. But nearly all the patients
died.
Canadian researchers James Till and Earnest McCulloch
at the Ontario Cancer Institute in Toronto finally identified
and characterized the first blood stem cell in the early
1960s (work that has dubbed them the “fathers of
stem cell research”). Discovering the proteins that
enable HSCs to differentiate and mature, Till and McCulloch
made it possible to quantitatively analyze a single hematopoietic
stem cell. That revolutionized the success of bone marrow
transplants.
Then, in the mid-1980s, Irving Weissman of Stanford University
developed methods for purifying HSCs using monoclonal
antibodies (antibodies created in mass quantities from
a single immune system cell).
“Even in the last five years we’ve come such
a long way in understanding the molecular mechanisms that
regulate and maintain blood stem cells,” says Camargo.
This fine-grained understanding of the blood system and
its stem cells has created a multi-billion-dollar therapeutic
industry. Blood is one of the most highly characterized
and best-understood tissues in the human body.
That doesn’t mean that we can make it do what
we want. “We’ve gotten very good
at taking blood cells out of one person and transplanting
them directly into another person,” says Dick.
“But if you want to do something with those cells
in culture before transplanting them back, like expand
them, we’re still met largely with failure.”
Stunted growth
If there’s one thing that Whitehead Member Harvey
Lodish has learned over the last few decades, it’s
that hematopoietic stem cells are finicky.
“Our goal is to take these cells out of their
natural environment and get them to do in the lab what
we want them to do,” says Lodish. “The tricky
part is, these stem cells hate being taken
out of their natural environment.”
Quite simply, HSCs are happy in the bone marrow. Normally,
when you place HSCs in a dish that tries to mimic that
environment, they either die, or immediately mature into
red and white blood cells.
Scientists would love to maintain HSCs in their stem
cell state, multiply their number by 10 or 100, and
then transplant them into the patient. “There
just simply aren’t enough of them in the bone
marrow,” explains Lodish. “The more stem
cells you transplant, the more successful the procedure
will be. Even in cord blood, the amount of stem cells
just isn’t adequate for treating an adult.”
Because of these limitations, bone marrow transplants
take a huge toll on patients. For a typical transplant
procedure, a patient is first irradiated, which destroys
all his or her own diseased bone marrow. A sample of donor
marrow is then transplanted into the patient, where it
eventually repopulates him or her with healthy blood stem
cells.
Unfortunately, because families in the U.S. are getting
smaller, only about one-third of the population has related
donors. So physicians painstakingly try to match donors
with patients so as to minimize immune system rejection.
Sometimes after a transplant patients need to take immunosuppressant
drugs for months. Other times, they do so for the rest
of their lives.
One way to ease immune system rejection is to remove all
the T cells from the donor marrow prior to the transplant.
(T cells are a kind of white blood cell and are often
the first to be recognized as foreign.) And although this
effectively deals with the immune rejection, removing
the T cells decreases the therapeutic potency of the transplant.
In order to increase the potency, you need to increase
the number of stem cells coming from the donor. But at
the moment, we can’t.
It’s a catch-22: We can give either an effective
transplant with immune system complications, or a less-effective
transplant without these complications.
“Almost every roadblock we come to with blood stem
cells comes down to our inability to multiply them in
the lab,” says Lodish.
Over the years many labs have reported success in this
area, only to find that these purported advancements have
been false leads. But in 2003, Lodish’s lab stumbled
upon what might just be the answer.
Secrets of expansion
Chengcheng Zhang, a postdoctoral researcher, was studying
fetal tissue in mice when he discovered a new population
of cells that, in the natural environment, appeared to
have a preserving effect on HSCs. When he isolated the
stem cells and placed them in a lab dish by themselves,
they died. When he mixed in these newly discovered cells,
the stem cells thrived. But how did these cells manage
to sustain the stem cells so dramatically?
Zhang reasoned that they might be secreting certain proteins
that sustained the stem cells. Using a series of microarray
platforms, Zhang located a number of such proteins.
In the fall of 2003 and early 2005, Zhang reported
in the journal Blood that one of these proteins
called IGF-2, when added to a solution of HSCs, increased
their number eightfold. Later he discovered that two
more growth factor proteins, angpt12 and angpt13, when
combined with IGF-2 into a cocktail, caused a 30-fold
increase. These results were reported in Nature
Medicine.
Lodish is cautiously optimistic. “If these results,
which occurred in mice, are repeated with human cells,
this will have huge implications for not only bone marrow
transplants but for cord blood transplants, for gene therapy
and especially for basic research,” he says. His
lab now is collaborating with researchers at Lund University
in Sweden to repeat these results with human cells.
As Lodish and colleagues continue exploring ways to multiply
these slippery cells, others are still trying to discover
if HSCs have therapeutic reach beyond blood.
For now, trying to get adult stem cells to behave more
like their embryonic cousins appears doomed to failure.
When a bigger payoff arrives from these highly specialized
cells, it most likely will come from getting them to do
what they already do best.
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