How Stem Cells Decide To Become Either Skeletal Or Smooth Muscle

The newly discovered mechanism also suggests that some current cancer treatments may weaken muscle, and that physician researchers should start watching to see if a previously undetected side effect exists.

Thanks to stem cells, humans develop from a single cell into a complex being with as many as 400 cell types in millions of combinations. The original, single human stem cell, the fertilized embryo, has the potential to develop into every kind of human cell. As we develop in the womb, successive generations of stem cells specialize (differentiate), with each group able to become fewer and fewer cell types.

One set of mostly differentiated stem cells has the ability to become bone, blood, skeletal muscle or smooth muscle. Many human tissues keep a reserve of stem cells on hand in adulthood, ready to differentiate into replacement parts depending on the stimuli they receive. If body signals that skeletal muscle needs replacing, the stem cells take that route. If tissues signal for more blood vessels, the same stem cells may become smooth muscle that supports the lining of blood vessels.

In the current study a team of researchers at the Aab Cardiovascular Research Institute of the University of Rochester School of Medicine & Dentistry and at the University of Texas Southwestern Medical Center found that a transcription factor called myocardin may be the master regulator of whether stem cells become skeletal or smooth muscle. Myocardin is a transcription factor, a protein designed to associate with a section of the DNA code, and to turn the expression of that gene on or off. Until now, Myocardin was only thought of as a protein that turns on genes that make smooth muscle cells. In the PNAS report, Myocardin is shown to also turn off genes that make skeletal muscle.

"These findings could eventually lead to stem-cell based therapies where researchers take control of what the stem cell does once implanted through the action of transcription factors like myocardin, unlike current therapies that "hope" the stem cell will take a correct differentiation path to fight disease," said Joseph M. Miano, Ph.D., senior author of the paper and associate professor within the Aab Cardiovascular Research Institute at the University of Rochester Medical Center "More specifically, many diseases are driven by whether stem cells decide to become skeletal muscle, or instead to become part of new blood vessel formation. These discoveries have created a new wing of medical research that seeks to understand the genetic signals that turn on such stem cell replacement programs."

Atherosclerosis, or hardening of the arteries, for instance, becomes likely to cause heart attack or stroke when cholesterol-driven plaques that build up inside of arteries become fragile. If they rupture, they interact with circulating factors into the blood to cause clots that block arteries and lead to tissue death. Theoretically, injecting stem cells programmed them to become smooth muscle could strengthen the plaques and prevent rupture, Miano said.

Conversely, tumors must be able to grow blood vessels in order to grow. They do so by sending signals for stem cells to form smooth muscle in combination with other signals that turn on vascular endothelial growth factor (VEGF), which together build new blood vessels. Would manipulating myocardin along with VEGF interfere with tumor growth by cutting off its blood supply? Do current VEGF-based treatments kick myocardin into action, creating smooth muscle instead of continually repairing worn out skeletal muscle? Since VEGF is used experimentally to treat peripheral artery disease and coronary artery disease, is this treatment reducing the skeletal muscle strength of these patients?

Miano's team found that myocardin both turns on a set of genes that turns stem cells into smooth muscle, and turns off the genes that turn stem cells into skeletal muscle, making it a bifunctional, developmental switch. The team at Southwestern applied the same idea to the development of the fetus via transgenic mouse studies, providing the biological context that made sense of Miano's finding.

Researchers at many institutions have been studying the somite, a group of cells in the human fetus known to develop into skeletal muscle. The team in Southwestern did cell lineage and tracking studies and found that myocardin is expressed briefly in the somite during development in mice, but then disappears from that region of the fetus. This current data leads to the surprising theory that both skeletal and smooth muscle cells come from the same stem cell region. Myocardin briefly switches on to make the new human's supply of smooth muscle cells, which then migrate to another area where they begin to form blood vessels. Myocardin then quickly shuts off, allowing the somite to continue differentiating into skeletal muscle. If it did not, then skeletal muscle would not develop properly.

Larger Picture

Miano's team is one of many in recent years seeking to define ancient sections of our genetic code that may soon be as important to medical science as genes. A new wave of research is concerned with, not how genes work, but how small regulatory DNA sequences tell genes where, when and to what degree to "turn on" in combination with enzymes that seek them out.

Genes are the chains of deoxyribonucleic acids (DNA) that encode instructions for the building of proteins, the workhorses that make up the body's organs and carry its signals. Growing knowledge of how regulatory sequences control gene behavior has the potential to create new classes of treatment for nerve disorders and heart failure. Regulatory sequences are emerging as an important part of the non-gene majority of human genetic material, once thought of as "junk DNA." A new frontier in genetic research is the defining of the regulome, the complete set of DNA sequences that regulate the precise turning on and off of genes.

In an article by Miano and team published February 2006 in the journal Genome Research, they described one such regulatory sequence: the CArG box. The nucleotide building blocks of DNA chains may contain any one of four nucleobases: adenine (A), thymine (T), guanine (G) and cytosine (C). Any sequence of code starting with 2 Cs, followed by any combination of 6 As or Ts, and ending in 2 Gs is a CArG box.

According to Miano, there are 1,216 variations of CArG box that together occur approximately three million times throughout the human DNA blueprint. CArG boxes exert their influence over genes because they are "shaped" to partner with a nuclear protein called serum response factor (SRF) and several other proteins within a genetic regulatory network, including Myocardin. As many as sixty genes so far have been found to be influenced by the CArG-SRF, including many involved in heart cell and blood vessel function.

Past studies had determined that myocardin is a cofactor with SRF in CArG-Box mediated genetic regulation of stem cells. Up until now, researchers believed myocardin partnered with SRF to turn on smooth muscle genes through CArG box interaction. The current findings suggest, however, that myocardin has a second role, independent of its partnership with CARG-SRF, where it serves as a potent silencer of gene expression for the stem cell to skeletal muscle gene program.

"With its dual action, myocardin is an early example of the efficiency and elegance of the system of genetic controls, where one factor has more than one complementary effect on the development of the body," said Eric Olson, Ph.D., chair of the Department of Molecular Biology at the University of Texas Southwestern Medical Center in Dallas, and also senior author of the study.