A team of researchers at the Emory University School of Medicine (Atlanta, GA) has modified optogenetics (the application of light to control cell activity via encoded genes) to yield an approach they call "optochemogenetics," which enables noninvasive, selective brain cell stimulation. The work could lead to a way to noninvasively control transplanted neural progenitor cells with light to restore lost functions in stroke recovery.
In a mouse model of stroke, lead researchers Shan Ping Yu, MD, Ph.D., and Ling Wei, MD of the Emory University School of Medicine's Department of Anesthesiology worked to figure out how to selectively stimulate brain cells noninvasively. To do so, they teamed up with Jack Tung, Ph.D., Ken Berglund, Ph.D., and Robert Gross, MD of the Department of Neurosurgery and the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory, who had created "luminopsins," engineered proteins that are both light-sensitive and generate their own light when provided with a chemical called coelenterazine (CTZ). The protein components come from Volvox algae and from Gaussia princeps, a fingernail-sized crustacean that lives in the deep ocean.
Related: Glowing protein could serve as light source for optogenetics
Yu and Wei were looking for ways to coax neural progenitor cells—capable of multiplying and differentiating into mature neurons—to survive in the brain after the destruction of a stroke. They were working with a mouse model, in which the sensory and motor regions on one side of the brain are damaged.
"It is not sufficient to put the cells into the damaged brain and then not take care of them," Yu says. "If we expect progenitor cells to differentiate and become functional neurons, the cells have to receive stimulation that mimics the kind of activity they will see in the brain. They also need growth factors and a supportive environment."
In experiments described in their published paper, the researchers introduced genes encoding luminopsins into induced pluripotent stem cells, which were cultured to form neural progenitor cells. The neural progenitor cells were delivered into the brains of mice a week after stroke. CTZ, which emits light when acted upon by luminopsins, was then provided intranasally twice a day for two weeks. Intranasal delivery bypasses the blood-brain barrier and repeated administrations are clinically feasible, Yu explains. Bioluminescence could be detected in the cell graft area and was visible for around one hour after CTZ administration.
CTZ promoted an array of positive effects in the progenitor cells: more survival and intact axons, more connections within the brain, and better responses to electrical stimulation. It also promoted recovery of function in the affected limb in the mice. The mice were tested in activities such as reaching and grasping food pellets, or removing adhesive dots from their paws. In young mice, CTZ and progenitor cells together could restore use of the stroke-affected limb back to normal levels, and even in older mice, they produced partial recovery of function.
When Yu was asked about clinical prospects, he said that optochemogenetics represents a significant advance, compared with its constituent technologies: "Optogenetics is a fantastic technical tool, but it presents some barriers to clinical implementation," he says. "You have the invasive fiber-optic light delivery, and the limited distance of light diffusion, especially on the larger scale of the human brain."
Delivery of cells into the brain and making them glow are complex, but they offer scientists some flexibility when designing experiments: direct light application, which can be turned on and off quickly, or the steady support of CTZ stimulation. Luminopsins can provide "the capabilities for the cells to be activated either in a way mimicking neuronal activities of fast activation or manipulations of the channel/cell in clinical treatments," the authors write.
Yu and his colleagues are also testing their approach for the delivery of neural progenitor cells in the context of traumatic brain injury.
Full details of the work appear in the Journal of Neuroscience.