Using an engineered sound-sensitive protein, researchers have developed neurons that can be activated by ultrasound. This could be useful in studying, and even developing therapies for, neurological diseases, including Parkinson’s.
The study, “Sonogenetic Modulation of Cellular Activities Using an Engineered Auditory-Sensing Protein,” was published in Nano Letters.
The ability to control neuronal activity in a non-invasive manner is useful for basic research. For example, optogenetics is a technique that uses light-sensitive proteins, such that neurons can be activated by particular wavelengths of light.
However, existing technologies for this purpose have numerous drawbacks. In particular, they often require a fairly high level of invasiveness; in optogenetics with living mice, for example, it is necessary to get a light into the mouse’s brain in order to activate the neurons.
In the new study, researchers set out to create a similar system, but one that used sound, rather than light — a sonogenetic system. Specifically, they wanted to create neurons sensitive to focused ultrasound (FUS), which uses sound waves that can penetrate much deeper into biological tissue than light.
To do this, the researchers turned to a sound-sensitive protein called prestin. A version of this protein is present in ear cells in most mammals, including mice and humans.
The mouse version of prestin is not particularly sensitive to FUS. But some other types of mammals, such as bats, need to be very good at hearing ultrasound for echolocation, and so have ultrasound-sensitive versions of prestin. So, the researchers analyzed the prestin produced by bats and other echolocating mammals, and they introduced two mutations into the gene that provides instructions for making the prestin protein (N7T and N308S, named for the amino acid substitution at the respective position) found in echolocating animals into the mouse version.
When cells were engineered to express this modified version of prestin, they became sensitive to specific frequencies of FUS. More specifically, FUS triggered fluctuations in calcium, which are important for many signaling processes in cells in general, and in neurons in particular.
Of note, the prestin-based FUS activation was not blocked when other types of sensory proteins, such as those that respond to changes in pressure or voltage, were blocked.
Other auditory proteins also were explored, but none were as effective as the modified prestin.
Those experiments were done using cells in dishes. The researchers then tested their modified prestin system in the brains of mice.
The team used an approach, similar to that used for gene therapy, to introduce the modified version of the gene that provides instructions for making the prestin protein, into brain cells. They injected an adeno-associated virus (AAV) containing the modified protein into the brain. AAV-based therapies work by using a virus, engineered not to cause any disease, to infect cells and deliver a working copy of a gene.
They demonstrated that using the modified prestin allowed them to engineer FUS-sensitive neurons in the mices’ brains.
Importantly, the same effect was found in deaf mice, confirming that the observed activation was due to the modified prestin and was not dependent on auditory mechanisms.
“The ultrahigh ultrasound sensitivity of mPrestin(N7T, N308S) [the modified mouse prestin] allows for non-invasively stimulation of target neurons in deep mouse brains by a short pulsed FUS,” the researchers wrote.
They noted that it’s still not completely clear how prestin itself senses FUS. Presumably, there are specific interactions with certain frequencies of sound waves, but more research will be needed to clarify the exact mechanisms, which could allow further refinement of this technology.
This technology could have applications in basic science, where researchers often need to precisely control neuronal activation. It also could have clinical applications — for instance, being modified to allow the activation of certain brain regions in people with neurological conditions such as Parkinson’s or Alzheimer’s diseases.
“Parkinson’s disease and Alzheimer’s disease are caused by the degeneration and death of the cells in the brain. But once the cells with prestin gene fragments have been transplanted into the target area, ultrasound can be applied to awaken atrophied cells so that they can begin to form new neural connections,” study author Chih-kuang Yeh said in a press release. Yeh is a professor at National Tsing Hua University in Taiwan.
“With ongoing development, engineered ultrasound-responsive proteins and sonogenetic systems should become versatile and powerful tools for non-invasively and precisely manipulating activities of genetically modified cells,” the researchers wrote.
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