It turns out that the genes that make muscle stem cells in the embryo are surprisingly not needed in adult muscle stem cells to regenerate muscles after injury, a result that seems to relate to a parallel finding in salamander limb regeneration.
“The paired-box genes, Pax3 and Pax7 are involved in the development of the skeletal muscles," explained lead researcher Christoph Lepper, a predoctoral fellow at Carnegie Institution for Science’s Department of Embryology. "It is well established that both genes are needed to produce muscle stem cells in the embryo. A previous student, Alice Chen, studied how these genes are turned on in embryonic muscle stem cells. I thought that if they are so important in the embryo, they must be important for adult muscle stem cells. Using genetic tricks, I was able to suppress both genes in adult muscle stem cells. I was totally surprised to find that the muscle stem cells are normal without them.”
Perhaps coincidentally, perhaps not, the Carnegie finding brings to mind a recent research result from the Max Planck Institute regarding salamanders. Salamanders regrow lost limbs and Elly Tanaka at Planck has been studying how they do so. Tanaka and colleagues found that salamander regeneration begins when a clump of cells called a blastema forms at the tip of a lost limb. From the blastema come skin, muscle, bone, blood vessels and neurons, ultimately growing into a limb virtually identical to the old one.
But here's the surprise: Rather than having their cellular clocks fully reset and reverting to an embryonic state, cells in the salamanders’ stumps became slightly less mature versions of the cells they’d been before. In other words they don't revert to pluripotency.
Researchers, many of whom hoped their findings could someday be used to heal people, hypothesized that as cells joined blastemas, they “de-differentiated” and became pluripotent — able to become any type of tissue. Embryonic stem cells are also pluripotent, as are cells that have been genetically reprogrammed through the process of induced pluripotency.
“People working on stem cells are trying to de-differentiate cells in an artificial fashion,” said Alejandro Sánchez Alvarado, a Howard Hughes Medical Institute stem cell biologist who was not involved in the Planck study. “It will be very important for the regenerative-medicine community to take stock of what’s going on in the salamander, because they’ve been doing it for 360 million years, and found a natural way to de-differentiate their tissues.”
So what does this have to do with the Carnegie findings? Carnegie's researchers found that muscle stem cells are normal without the genes necessary to form muscle in the embryonic state. They then looked at whether this is true upon injury in an adult, after which the repair process requires muscle stem cells to make new muscles. For this, they injured the leg muscles between the knee and ankle. They were again surprised that these muscle stem cells, without the two key embryonic muscle stem cell genes, could generate muscles as well as normal muscle stem cells. They even performed a second round of injury and found that the stem cells were still active.
The Carnegie scientists then wondered when these genes become unnecessary for muscle stem cells to regenerate muscles. It turned out that these embryonic genes are important to muscle stem cell creation up to the first three weeks after birth. What makes the muscle stem cells different after three weeks? The scientist believe that these two embryonic muscle stem cell genes also tell the stem cells to become quiet as the organism matures. After that time is reached, they “hand over” their jobs to a different set of genes. The researchers suggest that since the adult muscle stem cells are only activated when injury occurs (by trauma or exercise), they use a new set of genes from those used during embryonic development, which proceeds without injury. They are eager to find these adult muscle stem cell genes.
Back to Planck and Tanaka for a moment, in his experiment with Salamanders he first added a gene that makes a fluorescent protein into the genomes of axolotl salamanders, The team removed from the salamander eggs the cells that would eventually become legs. They fused the cells into new eggs; when these matured into adult salamanders, cells in their legs glowed under a microscope.
After the researchers amputated their salamanders’ legs, the legs regrew. Cells in the new legs also contained the fluorescent protein and glowed under a microscope, so the scientists could watch blastemas form and legs regrow in cell-by-cell detail.
Contrary to expectation, salamander skin cells that joined the blastema later divided into skin cells. Muscle became muscle. Cartilage became cartilage. Only cells from just beneath the skin could become more than one cell type.
“People didn’t know if the salamanders were special because they forced adult tissues to become pluripotent, and whether we should look for factors that did that — or if, as we find now, we actually shouldn’t try to force cells back to a pluripotent state,” said Tanaka.
Whether this striking absence of pluripotency is universal is still unknown but the two very different areas of research would seem to combine to suggest fascinating possibilities.
If Tanaka’s findings hold, they suggest a relatively new avenue for stem cell research. Bodies might find it easier to accept cells that have been only partially reprogrammed, like those in the axolotl’s blastema, than embryonic or fully reprogrammed cells.
This partial reversion approach has shown promise in the lab of Harvard Stem Cell Institute co-director Douglas Melton, who last year used partial reprogramming on pancreas cells that subsequently formed other pancreas cell types.
“This represents a parallel approach for how to make cells in regenerative medicine,” said Melton at the time. “If you’ve got extra cells of one type and need another, why go all the way back to a stem cell?”
Adapted from Wired Magazine and the Carnegie Institute for Science announcement.
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