With the help of a tiny flatworm best known for its extraordinary ability to regenerate lost tissue, researchers have identified a gene that controls the ability of stem cells to differentiate into specialized cells. The gene encodes a protein that is most similar to the protein PIWI which is an important regulator of stem cells in organisms ranging from plants to humans. The significance of this study is enormous. It sheds light on the most crucial aspect of stem cells - how they can transform into any cell of the body.

The research, performed at the University of Utah School of Medicine, was carried out by Helen Hay Whitney postdoctoral fellow Peter W. Reddien (Associate Member at the Whitehead Institute for Biomedical Research), and led by Howard Hughes Medical Institute investigator Alejandro Sánchez Alvarado.

Freshwater flatworm planaria have amazing ability to regenerate. Cut one of these animals in half, and a week later, two fully functional worms will have developed from the pieces. Cut a piece that is 1/279th the size of the animal, and it too will regrow into a complete worm.

Scientists got their first clues when they individually turned off 1,065 of the worm’s genes, and found 240 that were involved in some aspect of regeneration. Importantly, Sánchez Alvarado noted, 85 percent of these genes are found in the genomes of other organisms, including humans. In the current study, the scientists zeroed in on one of these genes, called smedwi-2, that was active in dividing neoblasts.

Smedwi-2 belongs to the Argonaute/PIWI protein family and is most similar to PIWI proteins found in fruit flies. According to Sánchez Alvarado, PIWI proteins have been shown to play a role in regulating stem cells in plants and fruit flies, as well as humans. “This encompasses millions of years of evolution,” he said. “Still, we don’t know exactly how this particular gene is doing its function in any of these organisms.”

To find out, the scientists used RNA interference to specifically turn off the piwi gene in planaria. When they did this, they found that worms had the same defects as those whose neoblasts have been destroyed by radiation — head regression, curling, and the inability to regenerate — suggesting that the gene was needed for normal neoblast function.

The researchers examined piwi’s role more closely, and found that when they amputated part of a worm where the gene had been turned off, the stem cells were still able to detect the wound. Amputation triggered the stem cells to divide, as in normal worms, and the daughter cells traveled proficiently to the part of the body where they were needed. However, once they arrived, they failed to replace the lost tissue.

When neoblasts divide, they produce at least two cell types – one copy of the original, and one cell that can develop into a specialized cell to replace a lost cell elsewhere in the body. The researchers found that without piwi, the daughter cells from this division failed to differentiate into a specialized cell once they’d reached their destination. Based on their findings, Sánchez Alvarado said, “We think that piwi is actually involved in producing daughter cells that are competent to restore aged differentiated cells during homeostasis as well as missing tissues after amputation. Unlike what’s been thought about piwi for some time, which is that it was required to maintain the stem cell, we think that’s not happening here. The stem cells are being maintained by another mechanism, and it’s the division progeny, instead, that is being affected.”

There’s some evidence, Sánchez Alvarado said, that piwi plays a similar role in regulating the progeny of adult stem cells in humans. He cautioned that more work is needed to determine just how functionally similar the factors regulating stem cells in planaria are to those in higher organisms. But so far there’s good evidence that many of the important genes are the same, he said.

The current study begins the detailed analysis that will be needed to establish whether this humble worm can illuminate the mechanisms underlying the unique biology of stem cells.

Link