Identifying markers for primitive blood-forming stem cells

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Identifying markers for primitive blood-forming stem cells

01 Jul 2005 Medical News Today

Scientists at the University of Michigan Medical School have

discovered the biological equivalent of a grocery store bar code on

the surface of primitive, blood-forming stem cells in mice. Called

hematopoietic stem cells, they give rise to all the different types

of specialized cells found in blood.

By reading the bar code, scientists can separate stem cells from

their more advanced descendants - progenitor cells that already are

committed to becoming one type of blood cell. The secret, according

to U-M scientists, is to look for the presence or absence of cell

surface receptor proteins expressed by a family of genes called SLAM.

Scientists knew that the 10 or 11 genes in the SLAM family helped

regulate the development and activation of white blood cells called

lymphocytes, but no one knew they also were associated with

hematopoietic stem cells.

" SLAM is the first family of receptors whose patterns of gene

expression can be used to precisely distinguish hematopoietic stem

cells from progenitor cells, and to identify stem cells in tissue

sections, " says on, Ph.D., an associate professor of

internal medicine and of cell and development biology in the U-M

Medical School and a Medical Institute investigator.

Results of the research study will be published in the July 1 issue

of Cell.

The discovery will be valuable to scientists working in the rapidly

advancing field of stem cell science. Currently, scientists must

search for many different markers and use complex procedures to

separate rare hematopoietic stem cells, or HSCs, from other cells in

a blood sample. Using SLAM markers will streamline the process

considerably, says Mark J. Kiel, a student in the U-M Medical

School's M.D./Ph.D. program who is co-first author of the study.

" The classical markers work, but they are complex and it takes a high

level of skill to obtain a pure sample of HSCs, " Kiel says. " We

showed that using just two SLAM markers gives you similar or better

purity than 10 to 12 of the best classical markers combined. It

reduces the skill and time required to pick out a hematopoietic stem

cell and makes the process simpler and less labor intensive. "

" Many of the current markers we use to purify HSCs are expressed as a

gradient from high to low across the hematopoietic cell hierarchy, "

says Omer H. Yilmaz, co-first author and a student in the Medical

School's M.D./Ph.D. program. " What's really exciting about this

finding is that now we have markers that turn on and off abruptly as

stem cells differentiate. "

Kiel and Yilmaz used microarray analysis to measure levels of gene

activity in three types of cells - hematopoietic stem cells,

multipotent progenitor cells and fully developed bone marrow cells.

They selected genes that were expressed at higher levels in the HSC

population, as opposed to the other two types of cells. CD150, the

founding member of the SLAM family of cell surface receptors, was

near the top of the list.

To see if CD150 was expressed on hematopoietic stem cells, Kiel and

Yilmaz transplanted CD150+ and CD150- cells into laboratory mice

whose bone marrow had been destroyed by lethal doses of radiation.

Healthy bone marrow was restored in six of six mice receiving CD150+

cells, but reconstitution was successful in only one of nine mice

receiving CD150- cells.

When they followed the same procedure to test another SLAM marker,

CD244, the results were completely opposite. Cells with CD244

receptors on their surface membranes were unable to reconstitute bone

marrow in irradiated mice for more than a few weeks, while cells

without CD244 receptors were able to reconstitute indefinitely. This

indicated that CD244+ cells were a marker for multipotent progenitor

cells (MPPs), but not for hematopoietic stem cells.

In a third step, the team tested another SLAM marker called CD48, and

found it to be expressed only on more advanced cells. It was not

present on either HSCs or progenitor cells.

" Each marker is expressed at a different stage of the hematopoiesis

hierarchy, " on says. " Selecting for cells with the combination

of CD150+, CD48- and CD244- markers produced a sample that was highly

enriched for HSCs. These cells were extremely rare - fewer than one

out of 10,000 whole bone marrow cells had this combination of surface

markers. "

In the Cell paper, U-M scientists describe how the new markers helped

them see hematopoietic stem cells in stained tissue sections from

mouse bone marrow and spleen, something that was not possible with

existing markers. Knowing where primitive hematopoietic stem cells

hang out in blood-forming organs will give scientists important clues

to how they work, according to on. For this reason, the

identification of markers that allow the localization of HSCs in

tissues has been a long-sought goal in the field of hematopoiesis.

" Until this study, we didn't know exactly where to look for HSCs

within bone marrow and the spleen, " on explains. " Most

scientists believed they stayed in close contact with cells called

osteoblasts, which line the inside of hollow cavities in bone that

contain bone marrow. Using SLAM markers, we discovered hematopoietic

stem cells also congregate in the sinusoidal endothelium - porous

cells lining the inside of blood vessels that carry oxygen throughout

bone marrow and the spleen.

" The presence of HSCs in sinusoidal endothelium could explain how

stem cells are able to enter the bloodstream so quickly, " on

adds. " It also suggests that endothelial cells produce substances

that regulate and nurture the growth of these stem cells. If we can

identify these growth factors, it could help us learn how to

manipulate and grow colonies of HSCs outside the body, which could

lead to new medical treatments for sickle cell anemia, leukemia and

other types of cancer. "

As with most discoveries in stem cell science, U-M scientists say

their work raises new questions for future research. So far they have

identified three SLAM markers on hematopoietic stem cells, but they

have no idea what the other eight genes in the family do. SLAM gene

activity creates receptors on the surface of blood-forming stem

cells, but what do these receptors do? Are SLAM receptors present on

other types of stem cells?

Perhaps the most important unanswered question for future

applications in medicine is whether these same markers are expressed

on human hematopoietic stem cells. Kiel and Yilmaz confirmed the

existence of SLAM markers on HSCs in several strains of laboratory

mice, but whether they will be found on human cells remains " a big

maybe, " according to Kiel.

" If SLAM family receptors are expressed on human HSCs, these new

markers could dramatically enhance the purification of such cells,

potentially making bone marrow transplants safer and more effective, "

says on, who adds that Kiel and Yilmaz have already started

tests to look for them.

Toshide Iwashita, M.D., Ph.D., a former research fellow in on's

laboratory and co-first author on the study, was responsible for the

tissue section analysis. Terhorst, Ph.D, from the Beth Israel

Deaconess Medical Center and Harvard Medical School also collaborated

in the research.

on's research is supported by the Medical

Institute, the National Institutes of Health and the U.S. Army

Research Office. Kiel is supported by a U-M Medical Scientist

Training Program Fellowship and Yilmaz is funded by a predoctoral

fellowship from the U-M's Institute of Gerontology.

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