For years scientists have been searching for stem cells in bone marrow to find the precise location of these important but elusive cells, yet with no results. Researchers from Japan’s University of Tsukuba and the University of Michigan Medical School report in the Proceedings of the National Academy of Sciences online Early Edition that they have finally found the location of these cells:
“We took time-lapse movies of sections from mouse leg bone as seen under a fluorescent microscope,” says Douglas Engel, Ph.D., chair and professor of cell and developmental biology at the U-M Medical School, who is the corresponding author for the study. “They clearly show individual, isolated hematopoietic stem cells at the edge of the bone marrow.”
According to Engel, the discovery will make it possible to study hematopoietic stem cells undisturbed and in their natural environment. That’s important, he says, because when stem cells are removed from bone marrow, they either die or start differentiating – changing into different types of specialized blood cells.
“There’s something about the physical location and cellular environment surrounding stem cells in their bone marrow niche that is at least partly responsible for their ability to maintain a primitive, pluripotent state,” Engel says. “Now that we can visualize them in vivo, we are in a better position to find out how they do it.”
In order to visualize hematopoietic stem cells or HSCs in living bone marrow, the researchers had to figure how to get the gene for green fluorescent protein, called GFP, to be expressed only in HSCs. When GFP is expressed inside a cell, it generates a vivid fluorescent green color that is easy to see under a microscope.
Norio Suzuki, Ph.D., the study’s first author, and colleagues at the University of Tsukuba in Tsukuba, Japan solved the problem by splicing the GFP reporter gene into a gene called Gata2, which is known to regulate the activity of hematopoietic stem cells, and into a promoter called IS that specifically controls the expression level of the Gata2 gene in these stem cells. Suzuki and the research team then injected the two modified genes into laboratory mice to create two lines of mutant “knock-in” animals.
“Disrupting Gata2 is embryonic lethal, so mice with two copies of the modified Gata2 gene all died before birth and had few hematopoietic stem cells,” Suzuki says. “But mice that inherited two copies of the modified Gata2 promoter were healthy with levels of hematopoietic stem cells similar to normal mice. By following the Gata2-directed expression of GFP in mouse bone marrow and another marker protein called Sca1, we could easily isolate the hematopoietic stem cells.”
The press release…