Researchers find mechanism that may determine early blood cell fate

Remain a hematopoetic stem cell or become a specialized blood cell?

Hematopoietic stem cells, the mother of all blood cells, face a fundamental dilemma in their lives.

Each must either remain a hematopoietic stem cell (HSC) by renewing itself or it must transform into one of eight specialized types of blood cells, such as a red blood cell, a white blood cell or a platelet.

Until recently, scientists didn’t know how the essential cells, which exist in limited amounts in the body, decide which direction to go. Now, researchers in the University of Wisconsin Medical School Department of Pharmacology have found a mechanism that might determine what each HSC will be. The mechanism involves an unexpected interaction between two related proteins.

Appearing in the July 11, 2003 issue of the Proceedings of the National Academy of Sciences (online), the study should be of special interest to hematologists treating patients with severe cancer, which can deplete blood cells, or blood disorders such as sickle cell disease, beta thalassemia or alpha thalassemia. One common treatment has been to introduce HSCs into the body, where they produce new blood cells to counteract abnormal blood cells or replace absent blood cells.

In the recent past, most blood specialists extracted HSC from bone marrow, where the cells occur in relatively large numbers; however, the procedure can be difficult on some patients. Today, a growing number of hematologists are extracting HSC much more easily from peripheral blood or umbilical cord blood. But since fewer cells are present in these sources, it would be advantageous to expand the cells to increase their numbers, explained Emery Bresnick, UW Medical School professor of pharmacology and senior author on the study.

“Manipulations to expand stem cell numbers can cause the cells to lose their ability to remain stem cells,” Bresnick said. “Any mechanism that tells you how to maintain stem cell status and prevent differentiation is a good target for modulating and improving this whole process. We discovered a mechanism that is an excellent candidate for controlling the decision of whether the HSC should remain undifferentiated or form blood.”

Bresnick and researchers Jeffrey Grass, Meghan Boyer, Soumen Paul and Jing Wu focused on two members of the GATA family of proteins, which are known to play a central role in the development of blood cells. GATA proteins work by attaching to portions of target genes, which either starts or stops the gene activity.

The researchers zeroed in first on GATA-2, which is required in significant levels for HSC to differentiate into intermediate blood cells called multi-potent progenitor cells. But the protein levels must go down before the progenitor cells can undergo further differentiation into distinct blood cell types.

“A central unsolved question was what signals tell the GATA-2 gene to stop producing its proteins so the progenitor cells can form blood,” Bresnick said.

To find the answer, he and his team also examined GATA-1. Significant amounts of this protein are critical for the formation of several types of blood cells: red cells, which carry hemoglobin to the lungs; mast cells, which mediate important aspects of immunity and asthma; and platelets, which are needed for blood clotting. However, GATA-1 levels must be lowered if HSCs are to be sustained in an undifferentiated state.

“We reasoned that there was a reciprocal, opposing relationship between GATA-1 and GATA-2,” Bresnick said.

By observing the way the proteins interact with sequences of genes in living cells, Bresnick and his colleagues found a master region on the GATA-2 gene that regulates the relationship. They found that when the GATA-2 gene is active, GATA-2 proteins attach to the region; when the GATA-2 gene is inactive, GATA-1 proteins attach to the region. As GATA-1 levels increase, the protein attaches to the region, signaling the GATA-2 gene to stop making GATA-2 protein.

The net effect, said Bresnick, is that if GATA-1 is occupying the master region, the gene activity stops, and the cell will differentiate. On the other hand, if GATA-2 binds to the region, gene activity begins, preventing differentiation and supporting stem cell function.

“It’s a delicate balance between two highly related proteins, but if we can shift the balance by modulating this relationship, we can chose to increase the number of these limited HSC or stimulate hematopoisis,” Bresnick said.