Credit: Gagilas, via Flickr
Credit: Gagilas, via Flickr

Making new neurons in a living brain

Across most of our brain, the neurons that we’re born with are the ones we have throughout our lives. We make new ones in two small areas—the hippocampus, involved in memory and navigation, and (briefly in) the olfactory bulb, responsible for smell. Elsewhere, we’re stuck with the same old supply. That’s partly why diseases like Alzheimer’s and Parkinson’s are so devastating. They destroy neurons that won’t get replaced… at least not without a little help.

Most of our cells are stuck in their ways, so a neuron will always be a neuron and a skin cell will always be a skin cell. But with the right combination of molecules, we can nudge them into a mid-life career change. This research on cellular makeovers had made enormous progress. In just five years, some scientists worked out how to reprogram skin cells into something like a stem cell before coaxing them into neurons, while others managed to convert skin cells directly into neurons, skipping the stemmy middle-men altogether.

Both of these makeovers were done in dishes, using cells that had been extracted from a patient or animal. The resulting neurons would always have to be transplanted back. Now, Olof Torper and Ulrich Pfisterer from Lund University have gone one better: They’ve developed a way of transforming astrocytes—another type of brain cell—directly into neurons, within the brain of a rat. “It is possible to directly reprogram resident cells into neurons in the brain, without a step in a culture dish,” says Malin Parmar, who led the study.

Parmar is reserved about her team’s discovery. They haven’t shown that the new neurons can fit into existing circuits within the brain, let alone that they could replace lost neurons in a diseased brain. Still, it opens the door towards the prospect of reprogramming brain cells without the need for any transplants or surgeries.

Here’s a brief history of the field—brief, because it has done so much in so little time. It took off in 2007, when Shinya Yamanaka used four proteins to shunt a mouse’s skin cells back into a stem-like state, which he called induced pluripotent stem cells (iPSCs). He has since won a Nobel prize for his discovery. In 2008, Marius Wernig used Yamanaka’s cocktail to transform rat skin cells into iPSCs and then into neurons, transplanting them back into the rodents to treat symptoms of Parkinson’s disease. Just months later, Kevin Eggan produced neurons from reprogrammed human skin cells, taken from an 82-year old woman with amyotrophic lateral sclerosis (ALS).

Wernig’s team eventually abandoned the iPSC stage entirely. In 2010, they converted mouse skin cells directly into working neurons, and in 2011, they did the same with human cells taken from discarded foreskins. The new cells slotted seamlessly into existing networks.

In 2012, Benedikt Berninger managed to make neurons from pericytes, a type of cell that embraces around blood vessels in the brain. Berninger’s experiments were done in dishes, but by starting with brain cells rather than skin, he showed that it should theoretically be possible to do all the reprogramming in a living brain. And that’s what Torper and Pfisterer have now done.

First, they worked with human skin cells (fibroblasts). They loaded them with three reprogramming genes known as “BAM factors” – Brn2, Ascl1 and Myt1l—before transplanting them into mouse brains. When the genes were activated, the fibroblasts started producing molecules that are signatures of neurons—a  sign that they had successfully transformed. With a few extra proteins, the team even produced a specific breed of neuron—the dopamine-making variety that’s lost in Parkinson’s disease.

Next, Torper and Pfisterer tried the same trick with cells that were already in the mouse’s head. They focused on astrocytes—the most common type of cell in the brain. These play a supporting role, providing nutrients to neurons, controlling the chemical balance of the fluid around them, and helping to repair any damage. The team used viruses to smuggle the BAM factors directly into astrocytes in the brains of living mice, and successfully converted these into neurons.

It’s a promising start. This direct conversion should be cheaper and quicker than techniques that go through an iPSC stage. It should also minimise the risk of inadvertently forming a tumour, since iPSCs can keep on dividing but neurons never do (and indeed, none of the team’s new neurons went on to form cancers). And astrocytes are in such plentiful supply in the brain that it should be possible to sacrifice a small fraction of them to make new neurons without causing any problems.

Still, there’s still a lot of work to do before this technique could ever be used in the clinic to treat human patients. The team need to make the process much more efficient—for now, only a small fraction of the targeted cells become neurons. They need ways of hitting astrocytes with BAM without using viruses that actually smuggle the genes into the host cell’s DNA—that could increase the risk of cancer if the added genes end up in the wrong place. They need ways of turning the BAM factors off when their work is done.

And of course, they need to show that their new neurons are actually integrating into the brain. That’s what they’re focused on now. If they take mice with symptoms of Parkinson’s disease, can they transform the rodents’ astrocytes into new dopamine neurons and relieving the symptoms of the condition?

These transplant-free transformations aren’t restricted to the brain. Last year, one team converted fibroblasts in the heart into beating muscle cells called cardiomyocytes, providing hope that the scar tissue that accumulates after a heart attack could one day be refreshed into new muscle. Another group performed a similar magic trick in the pancreas, converting cells that release digestive enzymes into those that produce insulin, all within a living mouse. We seem to be on the cusp of an exciting new time when cells have no fate but what we make for them.

Reference: Torper, Pfisterer, Wolf, Pereira, Lau, Jakobsson, Bjorklund, Grealish  & Parmar. 2013. Generation of induced neurons via direct conversion in vivo. PNAS

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