Transplanted brain cells restore circuitry

What if we could transplant brain cells like bone marrow? German researchers have shown that it’s certainly possible – at least in mice.

Magdalena Götz from Ludwig Maximilians University of Munich and colleagues transplanted healthy brain cells from a mouse embryo into a damaged adult mouse brain. Imaging showed the new cells grew into and restored function in the injured areas.

The work, published in Nature, adds to a growing bulk of research on how brain cell transplants might one day treat neurodegenerative diseases such as Parkinson’s and Alzheimer’s.

Unlike most other cells in the body, brain cells – called neurons – are generally built to last. When they die – from a stroke, for instance, or a knock on the head – they rarely replenish.

There are exceptions, of course: new cells grow in brain regions such as the hippocampus, the structure responsible for memory and learning. It’s a process called neurogenesis.

But could parts of the brain that don’t usually undergo neurogenesis be able to welcome and nurture replacement cells?

Previous studies have shown transplanted neurons fill a damaged visual cortex – the part of the brain that processes vision – in mice to restore sight. But no one knew how much like the native brain cell population the newcomers became.

Götz and her colleagues wanted to find out. They took healthy brain cells from mouse embryos, tagged them with a fluorescent protein so they could be traced, and popped them into adult mouse brains with a damaged visual cortex.

They then followed the new neurons’ growth.

And after four weeks of settling in, the transplanted neurons formed connections with other parts of the brain and fired in response to visual stimuli. In essence, the transplanted cells looked like and acted like the missing cortical cells.

But, Götz says, “the most exciting part is that indeed all the information to wire up new neurons properly can still be activated in brain regions that normally never incorporate new neurons”. 

The researchers suggest the signposts that guide a neuron’s growth in the brain remain even after the cell has died. In other words, new neurons are able to follow the signs that guided the growth of the now-lost cells.

But this new research has been a long time coming.

Götz says while performing neuron reprogramming experiments for brain damage in 2002, “I started to think about integration of these reprogrammed neurons and then found to my amazement that the key question of functional activity had not even been solved for transplant neurons – which we then embarked upon”.

Clinical neurologist Matthew Kiernan from the University of Sydney in Australia says one problem such studies run into is that the transplanted neurons may become over-active and can form benign tumours.

“This is a critical area of ongoing research, but it’s still relatively underdeveloped,” he says.

Still, he says the authors are “trying to either give more function or maintain function, and I think that’s incredibly exciting”.