"This is a wonderful model system," Singh said. "We know more about the differentiation of B cells and red cells than other cell types in the blood system."
After a series of experiments that involved manipulating multiple genes encoding regulatory proteins, Singh and colleagues came up with a "hierarchical regulatory network" that orchestrates the differentiation of a stem cell into a committed B-cell precursor.
Five transcriptional regulators guide future B cells along this pathway, activating genes that move the cell to the next stage and enabling the cell to respond to specific chemical signals later on. For example, the transcription factors PU.1 and lkaros are crucial early in the process, nudging a multi-potent progenitor cell -- stage 1, which could become any type of blood cell -- toward becoming a lymphoid progenitor, stage 2. They trigger the expression of certain receptors on the cell surface, such as Flk2 followed by IL-7R, which are necessary for receiving subsequent external signals.
In the next step, the gene for a regulatory protein known as E2A cooperates with PU.1 to activate another regulatory gene called EBF. EBF and E2A act together to push the lymphoid progenitor towards stage 3, a specified pro-B cell. At this stage, many of the genes expressed in B cells have been activated and the genes that encode antibodies have begun the process of recombination.
Finally, EBF and E2A activate a regulator called Pax-5, which pushes the specified pro-B cell to stage 4, a committed pro-B cell. After this point, there is no turning back.
"This is a complicated sequence of events," Singh notes. "There's no denying it." At each stage, different markers or receptors appear on the cell surface, which helps the researchers monitor a cell's progress and enables the cell to reach the next stage.
"To make real use of stem cells we will have to assemble genetic regulatory networks such as this for
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Contact: John Easton
John.Easton@uchospitals.edu
773-702-6241
University of Chicago Medical Center
11-Oct-2004