Discovery suggests why stem cells run through stop signs

[Guest guest]

Discovery suggests why stem cells run through stop signs

11 Jun 2005 Medical News Today

Everyone knows that stem cells are controversial. Many people know

that stem cells can grow into virtually any cell type found in the

body, from a red blood cell to a muscle cell to a brain cell. But no

one really knows why stem cells continue to divide and renew

themselves long after the point where other cells stop dividing.

Now scientists at Northwestern University and the University of

Washington offer one of the first clues as to why stem cells ignore

stop signs in the cell cycle: a special molecular mechanism has cut

the brakes. The researchers found that tiny bits of genetic material

called microRNAs are necessary for stem cell division to take place,

suggesting that microRNAs shut off the signals that stop cell

division in most other cells.

The findings were published online this week by the journal Nature.

In the paper, the researchers also speculate that microRNAs may play

a similar role in cancer cells, encouraging their proliferation. This

speculation is supported by three other new papers published this

week in Nature linking microRNAs to cancer.

According to authors Carthew, Owen L. Coon Professor of

Molecular Biology at Northwestern University, and Hannele Ruohola-

Baker, professor of biochemistry at the University of Washington,

microRNAs can regulate gene expression and give stem cells a green

light to pass from the normal stop phase to the stage in which they

begin replicating their DNA for later division.

In their work, Carthew and Ruohola-Baker focused on fruit flies,

which have approximately 80 types of microRNAs. They genetically

modified stem cells from the fruit flies' ovaries and studied how

many egg chambers the mutant stem cells produced as compared to

normal stem cells. The production rate in the mutant cells fell over

the course of 12 days, and the researchers concluded it was because

the mutant stem cells were no longer dividing.

Without the microRNAs at work, the brakes were applied to the cell

division of the mutant stem cells, just like ordinary cells. The

cellular brake (in this case a protein called Dacapo, a fruit fly

homologue of a human tumor suppressor) kept the stem cells from

proliferating.

" Determining which of the 80 microRNAs is responsible for

deactivating the stop signal is the next step of our research, " said

Ruohola-Baker.

" The list of chores that microRNAs do within cells keeps growing in

new and surprising ways, " added Carthew. " This latest discovery with

stem cell division makes us wonder if microRNAs also control division

of other types of cells such as cancer cells. "

Other authors on the Nature paper are Kenji Nakahara of Northwestern

University and Karin Fischer, Steve Hatfield and Halyna Shcherbata of

the University of Washington.

www.northwestern.edu