Muscle Stem Cell Identity Confirmed By Researchers

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Muscle Stem Cell Identity Confirmed By Researchers

http://medicalnewscenter.com/out/out.cgi?

http://www.sciencedaily.com/releases/2008/09/080917145135.htm

A single cell can repopulate damaged skeletal muscle in mice, say

scientists at the Stanford University School of Medicine, who devised

a way to track the cell's fate in living animals. The research is the

first to confirm that so-called satellite cells encircling muscle

fibers harbor an elusive muscle stem cell.

Identifying and isolating such a cell in humans would have profound

therapeutic implications for disorders such as muscular dystrophy,

injury and muscle wasting due to aging, disuse or disease.

" We were able to show at the single-cell level that these cells are

true, multipotent stem cells, " said Helen Blau, PhD, the E.

and Delia B. Baxter Professor of Pharmacology. " They fit the classic

definition: they can both self-renew and give rise to specialized

progeny. " Blau is the senior author of the research, which will be

published Sept. 17 in the online issue of Nature.

" We are thrilled with the results, " said Alessandra Sacco, PhD,

senior research scientist in Blau's laboratory and first author of

the research. " It's been known that these satellite cells are crucial

for the regeneration of muscle tissue, but this is the first

demonstration of self-renewal of a single cell. "

One-tenth of the body's mass is skeletal muscle. Satellite cells hang

out between a muscle fiber and its thin, membrane-like sheath,

waiting to spring into action when the fiber is damaged by exercise

or trauma. When necessary, they begin to divide to make more

specialized muscle cells. This property alone, however, doesn't

qualify them as stem cells. That designation requires them to be able

to also make copies of themselves for future use.

Although many researchers suspected that the satellite cell

population included muscle stem cells, it was difficult to prove

because not all satellite cells are identical. It was possible that

one subpopulation was responsible for making lots of specialized

muscle cells, while another replenished the supply of satellite cells.

This divide-and-conquer approach might be efficient, but doesn't have

the same exciting clinical applications as identifying a true stem

cell. However, analyzing the specific properties of a single cell is

technically difficult, and usually requires hundreds of hours of

painstaking microscopic analysis of tissue slices from many

laboratory animals.

Sacco used a trick to overcome these hurdles. She isolated satellite

cells from a mouse genetically engineered to express a glowing

protein, luciferase, first identified in fireflies. She then used a

novel imaging technique developed at Stanford to follow their fate

after transplantation into living animals that did not express the

protein. Because this non-invasive method allows repeated imaging of

the same animal, fewer mice are needed for the research.

" To be able to detect the presence of the cells by bioluminescence

was really a breakthrough, " said Blau, the director of the Baxter

Laboratory of Genetic Pharmacology. " It taught us so much more. We

could see how the cells were responding, and really monitor their

dynamics. "

Sacco transplanted a single satellite cell expressing the glowing

protein into the hind leg muscles of each of 144 mice; in six of the

mice, these cells went on to proliferate and self-renew in the

recipient's existing muscle. The relatively low success rate is most

likely due in part to the fact that not all of the satellite cells

are stem cells and also to the difficulty of keeping a lone cell

alive and happy during isolation and transplantation.

The leg muscles of these six mice were repopulated with between

20,000 to 80,000 glowing progeny of the original satellite cell. Many

cells made new muscle fibers or contributed to the recipient's muscle

fibers. Most exciting, several of the glowing cells expressed cell

markers specific only to satellite cells, indicating the original

cell was also making more copies of itself and confirming that it was

a stem cell.

In another set of experiments, Sacco and her colleagues transplanted

between 10 and 500 satellite cells expressing the glowing protein

into each mouse leg muscle. These cells also engrafted and

proliferated extensively, increasing approximately a hundredfold in

number after transplantation and a hundredfold more in response to

muscle damage. They contributed extensively to the recipient's

muscle, both by forming new fibers and by fusing with injured fibers.

Furthermore, once the need for reinforcements had been met, the

satellite stem cells stopped proliferating; that is, unlike tumor

cells, the transplanted cells were responsive to local cues.

Finally, the researchers were able to induce a second and third wave

of proliferation of the glowing satellite cells with repeated

incidences of damage, showing that the stem cell function persisted

over time.

" Now we can monitor the same mouse over time, and see how various

treatments affect muscle regeneration, " said Sacco. She and her

collaborators are now turning their attention to isolating similar

muscle stem cells from humans.

In addition to visually following the fate of the glowing cells,

researchers can also use the intensity of the signal to assess the

speed and strength of the stem cells' rescue response under a variety

of conditions - an important feature that will allow researchers to

directly compare the function of putative stem cells in a variety of

injury and disease models.

" This technique provides the first quantitative way to compare stem

cells in solid tissues, " said Blau. " By providing a means of

assessing the efficacy of a range of stem cell therapies in a variety

of tissues, I think it will greatly impact not only the study of

muscle stem cells in regenerative medicine, but also the stem cell

field in general. "

Sacco and Blau's Stanford collaborators included Regis Doyonnas, PhD,

senior scientist; Peggy Kraft, research assistant; and Stefan

Vitorovic, a research assistant and Stanford undergraduate student.

The research was funded by the National Institutes of Health and by

the Baxter Foundation.