Researchers map out networks that determine cell fate

A two-step process appears to regulate cell-fate decisions for many types of developing cells, according to researchers from the University.

This finding sheds light on a puzzling behavior. For some differentiating stem cells, the first step leads not to a final decision but to a new choice. In response to the initial chemical signal, these cells take on the genetic signatures of two different cell types. It often requires a second signal for them to commit to a single cellular identity.

The researchers describe their findings in a paper published in the Aug. 25 issue of Cell. Working with hematopoietic stem cells, which give rise to the many types of blood cells, the researchers show how “pioneer transcription factors” trigger the first step, pushing the stem cells toward this mixed lineage, midway between two related cell types—in this case between a macrophage and a neutrophil.

Then one of two rival “secondary factors” activates the genes that lead to one cell type and shuts down the genes that lead to the alternative.

Understanding the circuitry that controls these decisions is central to learning how different kinds of stem cells develop. It provides insights into how to transform stem cells into therapeutically useful cells and suggests possible new treatments for leukemias, in which a persistent mixed lineage seems to drive cancerous proliferation.

Although the researchers worked only with blood-forming stem cells, they suspect that the same basic regulatory principles govern cell type determination in other tissues, such as skin, brain and intestine.

“We see elements of this framework of primary and secondary cell-fate determinants throughout the hematopoietic system,” said study author Harinder Singh, the Louis Block Professor in Molecular Genetics & Cell Biology and a Howard Hughes Medical Institute Investigator at the University, “and we suspect such networks also regulate cell fate in other systems.”

“Understanding the genetic circuitry that orchestrates development of each specialized cell type,” Singh added, “should enable us to manipulate it for our own purposes.”

The researchers focused on how hematopoietic stem cells developed into one of two types of white blood cells: macrophages or neutrophils. Macrophages are the long-lived garbage disposals of the immune system, indiscriminately engulfing and digesting cellular debris and pathogens.

The shorter-lived neutrophils are the immune system’s vultures, flocking to the site of an infection to target and ingest invading organisms.

Although both cell types come from cells known as myeloid progenitors, each type relies on its own set of functionally active genes to carry out its particular role in fighting infection. A major scientific puzzle has been how and why immature hematopoietic stem cells initially express genes that are characteristic of both cell lineages.

In the Cell paper, Singh and colleagues identified the primary signal, a central genetic switch that pushed stem cells toward the first choice. The researchers then identified the antagonistic signals that guided cells toward their second choice. Two of these secondary regulators, Egr-1 and Egr-2, turn on macrophage genes and turn off neutrophil genes.

An opposing signal, Gif-1, is required to turn on neutrophil genes and repress macrophage counterparts.

Such counteracting repression circuitry may be the key to understanding stem cell regulation in general, Singh said.

In collaboration with colleague Aaron Dinner, the research team also formulated a mathematical model that depicts the regulatory network governing progenitor cell development. This model, he said, could have important implications for the therapeutic use of stem cells to rejuvenate damaged tissues.

Insights into this regulatory circuitry may also aid in understanding leukemias, Singh said, since many leukemias exhibit mixed-lineage patterns of gene expression, for example, both macrophage and lymphocyte genes.

“It may be that these cells are stuck in a progenitor-like state,” said Singh. “If you could induce them to resolve that state—to differentiate into one or the other cell type—they would cease to be tumorigenic.”

The Howard Hughes Medical Institute and the National Institutes of Health funded the study, which was spearheaded by Peter Laslo, Research Associate in Molecular Genetics & Cell Biology.

Other members of the study team are Benjamin Gantner, a postdoctoral fellow in Molecular Genetics & Cell Biology; Chauncey Spooner, David Lancki and Roger Sciarmmas, HHMI investigators; Aaron Dinner, Assistant Professor in the Physical Sciences Collegiate Division, the James Franck Institute and Chemistry; and physics student Aryah Warmflash.

The authors are members of the Gordon Center for Integrative Science.