UCSF Identifies Mechanism of Stem Cell Growth

November 13, 2008
News Office: Jennifer O'Brien (415) 502-6397

A class of minuscule molecules called microRNAs has become a major focus of biomedical research. Now, UCSF scientists have identified multiple members of this class that enable embryonic stem cells to divide, and thus proliferate, much more rapidly than the mature, or specialized, cells of the adult body.

The finding offers insight into a critical aspect of normal embryonic development -- the capacity of the early embryo to grow rapidly from a single fertilized cell to an entire embryo. It also suggests, the researcher say, that when microRNAs function inappropriately they likely play a role in cancer.

The findings could improve the efficiency with which embryonic stem cells are produced and maintained in the culture dish, the team says. They also could inform efforts to prevent stem cells transplanted into patients from becoming cancerous. Finally, they could be targets for cancer therapies.

"Our results show that embryonic stem cell-specific microRNAs play a central role in expediting the transition of stem cells through a key step in the cell cycle -- the G1/S transition, which is the check point just before cells duplicate their DNA and prepare to divide," says senior author Dr. Robert Blelloch, of the UCSF Institute for Regeneration Medicine.

"Unraveling the mechanisms that promote the G1/S transition is critical to understanding both normal embryonic development and cancer."

More broadly, he says, "It is clear that microRNAs are central players in development and disease, just like transcription factors. The more we learn about their functions, the greater the opportunities to manipulate them to improve human health."

The study, currently reported in the advanced online issue of Nature Genetics and in the December print edition of the journal, showcases a novel screening technique that the team says offers an alternative and powerful method for identifying the roles of individual microRNAs, whose functions have proven difficult to discern.

The obscure class of molecules debuted in 1993, when scientists reported the discovery of a microRNA in the microscopic roundworm C. elegans. Since then, the field has exploded, says Blelloch, with hundreds of microRNAs discovered in the last eight years across a broad range of species, from plants to animals.

But as the UCSF study demonstrates, he says, microRNAs have a high degree of redundancy -- both in terms of overlapping targets between microRNAs and in the multiple levels within common molecular pathways that each microRNA can target -- making their roles hard to unmask.

MicroRNAs are snippets of single-stranded RNA that prevent a gene's code from being translated from messenger RNA into protein. Produced in the nucleus and released into the cytoplasm, they home in on messenger RNA that share part of their genetic sequence. When they find them, they latch on, preventing the messenger RNA from being processed by the protein-making machines known as ribosomes. As such, microRNAs are able to ratchet down a cell's production of a given protein.

In recent years, microRNAs have been implicated in cancer stem cells, cancer metastases, and even in psychological and metabolic diseases.

In the current study, led by Yangming Wang, a postdoctoral fellow in the Blelloch lab, the scientists focused on microRNAs' role in the embryonic stem cell cycle -- the series of steps a cell takes to grow, duplicate its DNA and divide to form two daughter cells.

Scientists have known that embryonic stem cells divide rapidly, and have suspected that this was due to some special aspects of their cell cycle. However, they have not known what molecular factors would account for this phenomenon. Evidence, including that reported by the UCSF team in 2007 (Nature Genetics, vol. 39), has indicated that microRNAs play a role. But scientists have not known how or which microRNAs might be involved.

In their 2007 study, also led by Wang, the scientists developed a line of mouse embryonic stem cells engineered to lack a gene called Dgcr8. They determined that the deficiency prevented the cells from processing all microRNAs and that, as a result, the cells stalled in the G1/S transition of their cell cycle.

In the current study, using the same line of embryonic stem cells, the scientists added back -- one at a time -- individual microRNA molecules that they suspected played a role in the cell cycle. Of the 300 mouse microRNAs they examined, the researchers found 14 that dramatically increased cell division and proliferation. They then zeroed in on five that also are highly expressed in mouse and human embryonic stem cells.

A computer and experimental analysis showed that the five microRNAs could silence a gene that normally inhibits proteins known as cyclins, which help drive cells through their cell cycle. However, this appears to be just the tip of the iceberg, says Blelloch. As the team went on to discover, these microRNAs inhibit multiple genes that normally would slow the progression of the cell cycle. The list continues to grow as the team's probe deepens.

Blelloch suspects that different families of microRNAs ultimately will be shown to play varying roles in embryonic stem cells. In another revelation of the 2007 study, the team discovered that stem cells lacking microRNAs were less able to differentiate into adult cells. Using their screening technique, the scientists now are investigating whether a set of microRNAs account for this effect.

It's possible, says Blelloch, that the same microRNAs that regulate cell cycle may inhibit differentiation. It would make sense if they did, he says, as rapid progression through the cell cycle may not allow time for the cells to respond to signals telling them how to differentiate.

"I think we're going to find two classes of microRNAs -- those that promote the self-renewal and those that promote differentiation, possibly by having opposing roles on a common set of targets," he says.

The study was funded by the National Institutes of Health, the Stem Cell Research Foundation, the Pew Charitable Trust and the California Institute for Regenerative Medicine.

Co-authors of the study are Archana Shenoy, Joshua E. Babiarz and Laruen Baehner of the UCSF Institute for Regeneration Medicine and Scott Baskerville of Dharmacon Technologies.

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