Some muscle cells are multi-talented.
http://www.nature.com/nsu/011213/011213-9.html
Stem cells' fates are a multiple
choice.
Stem cells found in mashed up muscle
can migrate into the bone marrow and make blood cells1. Muscle contains
many different types of cell, however, and the exact identity of the one
responsible remains unclear.
Johnny Huard, of the University of
Pittsburgh in Pennsylvania, and his colleagues selected a group of adult
mouse muscle cells that kept dividing for days and were marked by a distinct
array of proteins.
When injected into mice whose bone
marrow had been killed by radiation, the stem cells replaced it. Mice that
would otherwise have died after 2 weeks survived for 6 months. And, when
new blood stem cells were recovered and injected into a third mouse, they
reverted to producing more muscle. This backtracking to their original
job is the "most amazing thing", says Huard.
"It shows that cells can go in many
different directions given the right environment," says stem-cell researcher
Helen Blau of Stanford University in California. The traditional view -
that stem cells progressively and permanently lose their initial ability
to produce many cell types - is changing, she argues.
Rather than a one-way road of cell
destiny, "It looks like a San Francisco highway", says Blau. Stem cells
can go off at one exit to make nerve cells and rejoin to make liver cells
when the need arises.
Embryonic stem (ES) cells may still
have properties that adult stem cells lack, cautions Ron McKay of the Memorial
Sloan-Kettering Cancer Center in New York. Adult nerve stem cells are more
likely to stop producing new nerve cells than are ES cells, he says, arguing
for continued experimentation with the controversial human cells.
"I'll say it because we're in Washington:
they [ES cells] grow without changing their developmental potential," he
says.
Muscling in
"We weren't looking for stem cells,"
explains Huard. He and his team were trying to find muscle cells that could
restore the missing protein dystrophin in patients suffering from the wasting
muscle disease Duchenne muscular dystrophy (DMD). They wanted cells that
were tough enough to survive transplantation into a patient.
They injected their selected cell
group, labelled so that they could be tracked, into mice with a form of
DMD. But the cells rarely turned up in muscle. Instead, Huard found them
in heart, liver, lung, spleen - but mainly bone marrow. "I got sidetracked,"
he says.
Huard is now trying to coax his stem
cells back to muscle by searching for the molecules that lure them there.
Working muscle cells would bump up dystrophin levels. "It will be very
exciting," he predicts.
To finally identify the elusive muscle
stem cell, researchers must start from a single cell, warns Blau: even
Huard's group of purified cells could contain outliers with unknown effects.
Such a technique identified an 'ultimate' stem cell from bone marrow earlier
this year2.
© Nature News Service
11 December 2001
HELEN PEARSON
A single stem cell from adult mouse
muscle can form enough blood cells to save another animal's life - and
still switch back to making brawn, researchers announced at the Washington
meeting of the American Society for Cell Biology this week.
References