Human nerve cells transplanted into a mouse brain

Understanding the mechanisms underlying brain circuit formation is important, for example, if we want to treat brain disease. Now, Belgian researchers have developed a novel strategy to transplant human neurons as individual cells into a mouse brain and to follow their development over time.

As explained by Professor Pierre Vanderhaeghen, from the VIB-KU Leuven Center for Brain & Disease Research and Université libre de Bruxelles, “One remarkable feature of human neurons is their unusually long development. Neural circuits take years to reach full maturity in humans, but only a few weeks in mice or some months in monkeys.

“This long period of maturation allows much more time for the modulation of brain cells and circuits, which allows us to learn efficiently for an extended period up until late adolescence. It’s a very important and unique feature for our species, but what lies at its origin remains a mystery.”

Dr Daniele Linaro, also from VIB-KU Leuven and Université libre de Bruxelles, revealed, “We differentiated human embryonic stem cells into neurons and injected them into the brains of young mouse pups. This allows us to investigate human neurons in a living brain over many months. We can also apply a whole range of biological tools in these cells to study human neural circuit formation and human brain diseases.”

The researchers discovered that the transplanted human cells follow the same developmental plan as they would in a human brain, with a months-long period of maturation typical for human neurons. This means that our nerve cells may follow an ‘internal clock’ of development that is surprisingly independent of the surrounding environment. Moreover, the human cells were able to function in the mouse neural circuits.

“After months of maturation, the human neurons began to process information, for example, responding to visual inputs from the environment,” said Dr Ben Vermaercke, from VIB-KU Leuven and NeuroElectronics Research Flanders (NERF). “The human cells even showed different responses depending on the type of stimulus, indicating a surprisingly high degree of precision in the connections between the transplanted cells and the host mouse’s brain circuits.”

Published in the journal Neuron, the study is said to constitute the first demonstration of genuine circuit integration of neurons derived from human pluripotent stem cells. According to NERF’s Professor Vincent Bonin, “It’s a technological milestone that opens up exciting possibilities to study how genetic information, environmental cues and behaviour together shape how the brain wires itself up.”

On the one hand, this model could be applied to study a whole range of diseases that are thought to impact the development of human neurons into neural circuits. The researchers plan to use neurons with genetic mutations linked to diseases such as intellectual disability to try and understand what goes wrong during maturation and circuit formation.

“Disturbances of circuit development have been linked to intellectual disability, for example, and to psychiatric diseases such as schizophrenia,” Prof Bonin said. “However, it has remained impossible to study human neural circuits in action in great detail — until now.”

“Our findings also imply that nerve cells retain their ‘juvenile’ properties even in an adult (mouse) brain,” Prof Vanderhaeghen added. “This could have potentially important implications for neural repair. The fact that transplanted young human neurons can integrate into adult circuits is promising news in terms of treatment development for neurodegeneration or stroke, where lost neurons could potentially be replaced by transplanting new neurons.”

Image credit: ©stock.adobe.com/au/mgkuijpers

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