Ateam of scientists has used fiber optics to turn on human embryonic stem cells in a mouse model to show that it is possible to establish a neural network.
The hope is that the technique could one day be used to deliver neurons derived from human embryonic stem cells (hESC) or induced pluripotent stem cells (iPSC) to target tissue and use light signals to enhance a neural network damaged by disease.
The study, led by Jason P. Weick, PhD, assistant scientist at the Waisman Center at the University of Wisconsin-Madison, offers proof of principle that transplanted neurons can actually integrate into the network to send information to and from the synapse and modulate neural network activity both in a lab dish and in a mouse.
Dr. Weick collaborated with Su-Chun Zhang, PhD, and Yan Liu, PhD, to show that hESC-derived neurons co-cultured with mouse neurons and optically stimulated could talk to the mouse cells by eliciting inhibitory and excitatory post-synaptic currents that trigger mouse neurons into action.
The investigators transplanted the hESC-derived neurons into the hippocampus in adult mice and used light to stimulate the cells. The mice were observed to receive the information from the hESC-derived neurons in its own pyramidal neuronal network.
The experiments, published in the Nov. 21 online edition of the Proceedings of the National Academy of Sciences, suggest that transplanted populations of iPSCs or hESCs can be used to control network behavior in an intact brain,” said Dr. Weick. “These cells may recreate the circuitry that has been lost or damaged in a range of neurological diseases such as Parkinson and Huntington disease.”
“Scientists have been able to show functional integration of hESC-derived neurons, labeling and recording the neurons to show they receive signals from endogenous tissue,” explained Dr. Weick. “But previous techniques have not allowed us to specifically stimulate transplanted cells in an attempt to show that they are sending appropriate signals to the host network.”
The advent of optogenetics provided the opportunity to test the power of the cells to take up residence and function as if they naturally belonged to the neural network. The technique was created by Stanford University's Karl Deisseroth, MD, PhD. [See “References” for past articles on Dr. Deisseroth's work in Neurology Today.] Scientists insert a channel that is responsive to light into hESCs. When they shine light, the cells excite and they can follow the electrical charge as it makes its way through the network.
The technique allowed the Wisconsin scientists to incorporate the light sensitive channel into hESC-derived neurons before transplantation. The cells are also tagged with a red fluorescent protein that labels the cells.
In the first experiment, they exposed the hESC-derived neurons to cortical mouse cells that are normally at work in the cortex. The mouse neurons fire simultaneously and then go quiet. This bursting pattern is not seen in hESC-derived neurons. The pairing of the human and mouse cells offered an opportunity to measure the output of the human cells.
According to Dr. Weick, in a matter of weeks the human cells were showing this bursting pattern. “It means that the human neurons are receiving information from the mouse cells and the information is incorporated into the human neurons,” he said.
Also, shining a light on the human cells worked like a light switch to induce mouse-bursting activity. They can actually control the electrical bursting pattern like a conductor orchestrates the timing and timber of instruments working together. When the light goes on, the channel is activated and the neurons send signals to the mouse cells to simultaneously fire.
They then transplanted the hESC-derived neurons into the hippocampus in mice, excited the cells with light and recorded the currents. They reported that they were able to follow the activity of the light-induced activity of the human cells.
The next step is to transplant the cells and stimulate neuronal networks to see if it influences behavior. The vision for human diseases, said Dr. Weick, is to transplant specific cell types and place a fiber optic cable that can be controlled externally to turn on light and activate the channel to excite the neurons.
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“The biggest significance is using the light channel technology to activate cells,” said Clive Svendsen, PhD, director of the Regenerative Medicine Institute at Cedars Sinai Medical Center. “It's very cool. They activated the human cells in a mouse brain.”
Dr. Svendsen said there is a lot more work to be done to figure out whether such a technology could be used to deliver cell therapies into humans. The problem, as he sees it, is that “light doesn't penetrate very far and reaching deep targets will be a challenge.”
Allison D. Ebert, an assistant professor in the department of cell biology, neurobiology and anatomy at the Medical College of Wisconsin, said that hundreds of papers have shown that cells can be transplanted into the brains of rodent and primate models of disease, but most of the papers demonstrate how the animals function rather than how the cells function.
“Although not in a disease-specific situation, “this paper is really the first to look at circuit integration of transplanted human cells,” she said.
“This is a step in the right direction by using slice cultures following transplantation, but checking the cells' functions in a live animal will be the most crucial experiment to determine how the cells are behaving, particularly over the long term,” she added.
But, Dr. Ebert said, there are other challenges. “One of the big ones is circuit integration. Jason's paper is a great proof of concept. The next steps would be to examine transplanted cells in models of disease to see how the diseased environment impacts the cells' functions.”
The optogenetics tool to control cells' behavior is advantageous,” she added. “The idea that cells can be modified to be precisely controlled by light is a great option. Finding non-invasive methods will be difficult, but I think science and technology are advancing to that point.”