Scientists have figured out how to put human neurons (the "thinking" cells) into mice brains. Does that mean Fievel Mousekewitz from An American Tale met Dr. Frankenstein?
"It's still a mouse brain, not a human brain," Steve Goldman, University of Rochester Medical Center, N.Y., said, according to New Scientist. "But all the non-neuronal cells are human."
Immature glial cells (cells that play a supportive role) were taken from donated human fetuses and injected into mouse pups, according to New Scientist. The glial cells turned into astrocytes, a type of star-shaped glial cell responsible for structure and cell "clean up."
After a year, the mouse glial cells had been taken over by the human cells. Each mouse had received 300,000 human cells that multiplied into 12 million cells that evicted the mouse cells.
"We could see the human cells taking over the whole space," Goldman said, according to New Scientist. "It seemed like the mouse counterparts were fleeing to the margins."
Astrocytes are important to conscious thought, since they fortify connections between neurons. The astrocytes' tendrils coordinate the spread of electrical signals across synapses. Human astrocytes are up to 20 times larger than mouse astrocytes and have 100 times as many tendrils. "It's like ramping up the power of your computer," Goldman said, according to New Scientist.
What did this power up do for the mice? Goldman and his team said it made them smarter.
One standard test created an association between a sound and a mild electric shock. The astrocytes-boosted mice froze four times longer when they heard the sound than their mouse-only brained peers. "These were whopping effects," Goldman said, according to New Scientist. "We can say they were statistically and significantly smarter than control mice."
The first time the experiment was conducted, Goldman and his team injected mature human cells, which just stayed where they were put. In this most recent test, glial progenitor cells (the step before a glial cell), were used. Goldman said that is why the cells were able to take over the mouse brains.
"It would be interesting to find out whether the human astrocytes function the same way in the mice as they do in humans," said Fred Gage, a stem cell researcher at the Salk Institute in California, according to New Scientist. "It would show whether the host modifies the fate of cells, or whether the cells retain the same features in mice as they do in humans."
"That the cells work at all in a different species is amazing, and poses the question of which properties are being driven by the cell itself and which by the new environment," said Wolfgang Enard of Ludwig-Maximilians University Munich in Germany, according to New Scientist. Enard has shown through his experiments that mice learn better if they have a human gene called Foxp2, which is linked to language development in humans.
In a side experiment, Goldman injected glial progenitor cells into mouse pups that were deficient in making myelin, the protein that covers nerves. Inside the mouse brain, the glial cells developed into oligodendrocytes, brain cells that specialize in making myelin.
This suggests that the cells were able to detect and correct the defect, according to New Scientist.
Goldman has applied for permission to treat multiple sclerosis patients, who lack the ability to create the myelin insulation. He hopes the trial will start next year.
"This does not provide the animals with additional capabilities that could in any way be ascribed or perceived as specifically human," Goldman said, according to New Scientist. "Rather, the human cells are simply improving the efficiency of the mouse's own neural networks. It's still a mouse."
Goldman said he and his team decided not to put human cells into monkeys citing "potential ethical issues."
"If you make animals more human-like, where do you stop?" Enard asked, according to New Scientist.
