Study finds genes that 'fine-tune' muscle development process
The genes are among a recently discovered group of genes known as microRNAs (miRNAs), which were first discovered in worms 12 years ago. Only in the past few years have they become recognized as essential gene regulators in many multicellular organisms, including humans.
"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 animal 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 assistant professor 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.
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