Human skin cells reprogrammed directly into brain cells

Scientists have developed a new method to convert human skin cells directly into a specific type of brain cell affected by Huntington's disease, an ultimately fatal neurodegenerative disorder.
Unlike other techniques that turn one cell type into another, the new process does not pass through a stem cell phase, avoiding the production of multiple cell types, said researchers.
The researchers, at Washington University School of Medicine in St Louis, demonstrated that these converted cells survived at least six months after injection into the brains of mice and behaved similarly to native cells in the brain.
"Not only did these transplanted cells survive in the mouse brain, they showed functional properties similar to those of native cells," said senior author Andrew S Yoo, assistant professor of developmental biology.

"These cells are known to extend projections into certain brain regions. And we found the human transplanted cells also connected to these distant targets in the mouse brain. That's a landmark point about this paper," Yoo said.
The researchers produced a specific type of brain cell called medium spiny neurons, which are important for controlling movement. They are the primary cells affected in Huntington's disease.

The research involved adult human skin cells, rather than more commonly studied mouse cells or even human cells at an earlier stage of development.
To reprogramme these cells, Yoo and his colleagues put the skin cells in an environment that closely mimics the environment of brain cells.

They knew from past work that exposure to two small molecules of RNA, a close chemical cousin of DNA, could turn skin cells into a mix of different types of neurons.
In past research, Yoo and his colleagues showed that exposure to two microRNAs called miR-9 and miR-124 altered the machinery that governs packaging of DNA.
These microRNAs appear to be opening up the tightly packaged sections of DNA important for brain cells, allowing expression of genes governing development and function of neurons.
Knowing exposure to these microRNAs alone could change skin cells into a mix of neurons, the researchers then started to fine tune the chemical signals, exposing the cells to additional molecules called transcription factors that they knew were present in the part of the brain where medium spiny neurons are common.

"We think that the microRNAs are really doing the heavy lifting," said co-first author Matheus B Victor, a graduate student in neuroscience.
"They are priming the skin cells to become neurons. The transcription factors we add then guide the skin cells to become a specific subtype, in this case medium spiny neurons," Victor said.
The study appears in the journal Neuron.