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MicroRNA (purple) guides cells to become platelets (pictured in green) instead of red blood cells.
Image courtesy of University of Edinburgh/Wellcome Images, Janet Iwasa, and Bang Wong

Scientists have wondered for decades how two very different types of blood cells — platelets and red blood cells — arise from the exact same precursor cell. In work described in the June issue of Developmental Cell, a team of Boston area researchers has unearthed a tiny and unexpected answer: a small snippet of nucleic acid called microRNA-150.

Like all microRNAs, microRNA-150 (miR-150) is only a few handfuls of nucleotides long. Yet despite their diminutive presence, microRNAs have been recognized to play crucial roles in controlling the flow of genetic information. Most genes are made active through a chemical intermediate known as mRNA, which serves as a template for synthesizing proteins. MicroRNAs act as a brake on this process, binding to mRNAs and preventing proteins from being made.

In 2005, Ó³»­´«Ã½ Cancer Program director Todd Golub led a study that showed that microRNAs may also serve a diagnostic purpose. By determining which microRNAs are present in certain tumor cells, the researchers found they could deduce the cells' developmental origins — that is, pinpoint the types of cells from which the tumors first arose.

That pioneering work laid the foundation for the current study and led Golub and his colleagues to ask, do microRNAs play an active role in normal cell development, actually instructing cells what to become? To explore this question, lead author Jun Lu and his colleagues turned their attention to a special subset of developing blood cells. These cells, known as megakaryocyte-erythrocyte progenitors (MEPs), give rise to platelets and to red blood cells, but are extremely rare in circulation. Lu estimates that studying these cells with conventional techniques would have been prohibitively laborious and time consuming. "Instead of doing that," he said, "we decided to develop a new methodology for microRNA profiling that's capable of running with a very small number of cells."

This new methodology, called plate-based capture, allows researchers to analyze microRNAs by capturing and immobilizing them in a small plastic dish. Lu and his colleagues used this technique to monitor how microRNA levels change as MEPs mature. They observed the most dramatic change in the levels of miR-150, a surprising result because miR-150 was previously thought to be unique to immune cells. Nevertheless, the researchers' data pointed to an important role for miR-150 in this stage of blood cell development.

This notion was confirmed in a series of in vitro and in vivo experiments, which showed that miR-150 can shift the balance of cell development towards platelets and away from red blood cells. The clarity of this shift in differentiation impressed co-first author Shangqin Guo, a researcher who works in the laboratory of David Scadden, Ó³»­´«Ã½ associate member and the director of the Harvard Stem Cell Institute and Center for Regenerative Medicine at Massachusetts General Hospital. "It is not unusual to see changes in the characteristics of cells as they are being molecularly engineered," she said. "But excitingly in our experiment, the change induced by miR-150 overexpression translated to the functionality of the cells, evident in the increased production of circulating platelets."

One of the proteins that miR-150 regulates — a transcription factor called MYB — is ubiquitously expressed, meaning it is present in all of the blood's progenitor cellular components and beyond. Mice lacking MYB protein die at an early embryonic stage because no blood lineages form. "MYB is apparently crucial for the maintenance of the whole blood system," Lu said. "It has to be down-regulated for any blood lineage to form."

One of the reasons that researchers had not previously identified this transcription factor's involvement in platelet and red blood cell development is that at an mRNA level, the expression of MYB doesn't appear to change. "If you could look at protein levels, you would find differences, but it's extremely hard to measure protein level differences with such a small number of cells," said Lu. Having the ability to look at an upstream regulator like a microRNA, however, revealed the critical connection between the protein and its function.

Although it is a too early to know what the therapeutic implications of miR-150 and its target transcription factor MYB could be, future investigations of the causes and treatments of blood diseases might be able to capitalize on the researchers' discoveries. For instance, cancer patients can develop bleeding disorders following chemotherapy, which critically harms the bone marrow and can lead to a dangerously low platelet count. If, during that crucial time, scientists could devise a way to increase platelet production, patients might be able to avoid suffering life-threatening bleeding.

"So far, the focus has been largely on the physiological regulation of cells with microRNAs," said Guo. "The next front is the therapeutic challenge and understanding what the role of miR-150 is in disease settings. We now have a basic mechanistic understanding and there will be a lot more work to follow."

Other Ó³»­´«Ã½ researchers contributing to the work include Benjamin Ebert, Hao Zhang, Xiao Peng, Jocelyn Bosco, Jennifer Pretz, Judy Wang, and Raymond Mak. Rita Schlanger, Frederic Preffer and David Dombkowski of MGH and the Harvard Stem Cell Institute also contributed to this work.