“Our interest is in understanding,
at the level of gene expression, how muscle cells develop,” said Dr.
Da-Zhi Wang, the senior and corresponding author of the study. Wang is
an assistant professor of cell and developmental biology in the School
of Medicine and a member of both the Carolina Cardiovascular Biology
Center and the UNC Lineberger Comprehensive Cancer Center.
“As microRNAs are gaining acceptance as global regulators of gene
expression, we questioned whether they could be involved in the
development of muscle,” he said.
Muscle tissue is generated when myoblasts, or pre-muscle cells, stop
proliferating and instead undergo irreversible changes (differentiation)
that cause them to become myotubes, or mature muscle cells.
Wang’s group studied two miRNAs – miR-1 and miR-133 – found exclusively
in muscle cells. Because their genes are located so close to one
another, miR-1 and miR-133 are always expressed together, yet they carry
out opposing tasks.
“This is the first case that two miRNAs are co-expressed together but
perform different functions,” Wang said.
The two miRNAs described in the UNC study are instrumental in
determining if myoblasts proliferate or differentiate. The research
showed that increasing the amount of miR-1 caused myoblasts to
differentiate into mature muscle cells, but prevented their
proliferation. To the contrary, increasing the amount of miR-133 caused
the myoblasts to proliferate even more, but prevented them from
undergoing differentiation.
Similar experiments carried out in developing frog embryos confirmed
their finding. Increasing miR-1 caused more muscle tissue overall and
fewer myoblasts, while increasing miR-133 led to more myoblasts but less
muscle overall in the frog embryo.
“That was quite a surprise to many people because those two miRNAs are
both equally expressed when muscles are differentiating, so we assumed
that they are probably pushing muscle cells in the same direction. But
after analyzing them, we found they have contradictory roles,” Wang
said.
Exactly how far-reaching their effect is on diverse biological processes
remains unclear.
As with all RNA molecules, miRNAs are copied from genes contained in the
DNA of a cell. Whereas typical RNA molecules contain thousands of
ribonucleotide bases – abbreviated “A’s,” “U’s,” “G’s” and “C’s” –
miRNAs are only 22 bases in length.
Unlike the main class of RNA molecule called messenger RNAs (mRNAs),
miRNAs are not a blueprint for making protein. Rather, miRNAs bind to
and actually prevent specific mRNA molecules from making their protein.
In this way, miRNAs inhibit gene expression.
In the miRNA field there are two types of thinking, Wang said. One is
that miRNAs are master regulators of a variety of biological processes,
and the second is that miRNAs provide a way to fine-tune the master
regulators.
“Because these two miRNAs are expressed together, but act oppositely in
muscle cells, our data suggest that miR-1 and miR-133 act as fine tuners
of the muscle development process,” Wang said.
Indeed, the researchers found that miR-1 and miR-133 lower the amounts
of two proteins, HDAC4 and SRF, which play important roles in,
respectively, muscle proliferation and differentiation.
Ongoing studies by the authors are exploring the possible roles that
these two miRNAs may play in muscle pathology, such as skeletal muscle
damage or cardiac hypertrophy.
Wang’s collaborators at UNC were Dr. Scott M. Hammond, assistant
professor of cell and developmental biology and Dr. Frank L. Conlon,
assistant professor of biology in the College of Arts and Sciences and
of genetics in the School of Medicine.
“Three junior faculty members put their strengths together in this
research. It reflects the cooperation in the scientific community here
at UNC,” Wang said.
Contributing authors also include graduate students Jian-Fu Chen, the
lead author, and Thomas E. Callis, as well as technician Qiulian Wu,
from Wang’s lab. Other collaborators were graduate student Elizabeth M.
Mandel of Conlon’s lab and postdoctoral scientist Dr. J. Michael
Thompson of Hammond’s lab.
The research was supported by grants from the National Institutes of
Health, the American Heart Association, the March of Dimes Birth Defect
Foundation and the Muscular Dystrophy Association.
Source: University
of North Carolina School of Medicine
Published on 6th
January 2006
