US scientists have discovered a way to transform living pancreatic cells in mice into another type of cell that produces insulin without having to revert to the stem cell stage, creating what is now a third route for cell reprogramming to add to the existing methods of iPS (induced pluripotent stem cells) and hES (human embryonic stem cells).
The study was the work of Harvard Stem Cell Institute co-director Doug Melton and post doctoral fellow Qiao “Joe” Zhou , at Harvard University, Boston, Massachusetts, and colleagues and was published as an advanced online paper in Nature on 27th August.
Melton and colleagues appear to have attained the “holy grail” of stem cell research, they have turned one type of adult cell directly into another type of adult cell, opening the door to living cell transformation directly in patients.
Using what they called “direct reprogramming,” the researchers turned exocrine cells, which account for around 95 percent of pancreatic tissue, into rare insulin-producing beta cells, which account for a much smaller 1 per cent or less of pancreatic cells and are therefore very precious. Beta cells are the ones that die off in Type I diabetes.
The feat is being heralded as a major step in developing treatments for Type 2 and Type 1 diabetes, bringing closer the day when patients won’t have to be checking their blood sugar all the time, or even take insulin medication. The researchers did however caution that it will be some time before this is a reality, and there are many difficult hurdles to overcome before the method can be tested in humans.
For example, as with iPS, Melton’s team used viruses to introduce various transcription factors (bits of DNA that alter the way that genes express themselves) into the target cells. This is a risky approach in humans because the viruses could behave in other unplanned and unpredictable ways. The researchers are looking for safer alternatives based on chemicals for this reason.
Melton pointed out that direct reprogramming will not remove the need for iPS and hES, and said his lab will continue to use them as well as the new techniques.
“We need to attack problems from multiple angles,” said Melton.
The researchers had a bit of luck when they discovered they could directly reprogram adult cells. Usually, as in iPS, you have to use hundreds of trascription factors to revert a mature cell back into a pluripotent cell that can then be encouraged to develop into any one of a range of cells. If you were lucky to pick the right combination of a much smaller subset of transcription factors, in theory you should be able to short circuit the iPS route and go straight from one type of mature cell to another.
That is what happened with Melton’s team. With a judicious mix of luck and two years of constant trial and error they hit on three transcription factors, Ngn3, Pdx1, and MafA. They just kept asking themselves the question “What genes do you have to turn on for the cell to become a beta cell?”
As Melton commented:
“If you want to do reprogramming it doesn’t take great insight to figure out that the key genes are transcription factors – the proteins that bind DNA and tell cells which genes to turn on and which to turn off.”
He said that a stem cell goes through many steps before it becomes a particular type of adult cell, each step is like passing through a door with a specific lock and each lock is a trascription factor. He and his team asked themselves which “locks” were in the beta cells, and that gave them 1,100 possible transcription factors. Eventually they discovered that only 200 of them are expressed in the cells of the pancreas, so that helped to narrow the field somewhat.
The next step was to find which of the 200 were particular to pancreatic cells that surrounded the beta cells, and that brought the number down to a more manageable 28. The 28 became 9 after the did further “lineage studies”, as Joe explained, “my best guess is it’s these nine”. And he was right. They then mixed all 9 and injected them into the pancreas. Then by removing them one by one they discovered it worked best when only three transcription factors were present, the other six weren’t as important, said Joe.
The luck of fate came when they chose those 9, because if they had not produced results, Melton and his team would have given up, they said.
Through the experiment Melton and colleagues were able to show that the newly induced beta-cells were “indistinguishable from endogenous islet beta-cells in size, shape and ultrastructure”.
They expressed genes essential for beta-cell function and were able to “ameliorate hyperglycaemia by remodelling local vasculature and secreting insulin”.
The researchers concluded that:
“This study provides an example of cellular reprogramming using defined factors in an adult organ and suggests a general paradigm for directing cell reprogramming without reversion to a pluripotent stem cell state.”
“In vivo reprogramming of adult pancreatic exocrine cells to beta-cells.”
Qiao Zhou, Juliana Brown, Andrew Kanarek, Jayaraj Rajagopal & Douglas A. Melton
Nature Advance published online 27 August 2008.
doi:10.1038/nature07314
Sources: Journal Abstract, Harvard Medical School.
Written by: Catharine Paddock, PhD