Method of the Month — Optogenetics
Our featured method for this month is Optogenetics, which involves controlling the activity of brain cells using light. It is used in the field of biology to study connections between neurons, providing insights into our understanding of psychiatric disorders.
While optogenetics may sound like a new breakthrough in biology, the origins of this method can be traced back to the discovery of proteins known as opsins in the 1970s. These proteins were found in a variety of microorganisms living in high salinity environments such as Halobacterium halobium, and formed channels in the cell membrane that moved ions into the organism when exposed to light. That same decade, Francis Crick (the same scientist who contributed to the discovery of the helical structure of DNA) wrote that light may be breakthrough needed in neuroscience to control only certain cells in the brain, while leaving surrounding neurons unaltered.
However, it would take nearly 30 years for the application of opsins to Crick’s theory to occur, creating the technology that we know today as optogenetics.
The first step of this method is to transfer genes for light reactivity into the neurons of a model organism that normally does not express these genes, such as a mouse. This can be accomplished through the use of a virus which has been specifically engineered to carry genes derived from microorganisms that encode opsins. A region of DNA known as a promoter before the opsin gene will also be present, as this is where proteins will bind to begin transcribing this DNA into RNA.
These viruses will then be injected into genetically engineered mice known as “driver” mice, which can express a protein known as Cre recombinase within their neurons. This is an enzyme that aids in incorporating the viral opsin gene into the mouse’s DNA.
The opsin most commonly used by neuroscientists is called channelrhodopsin-2 (ChR2). It was isolated from Chlamydomonas reinhardtii, a species of green algae, and specifically responds to blue light shining on it. When blue light hits a neuron containing ChR2 channels, positively charged ions such as hydrogen, sodium, and calcium rush into the neuron. This causes an excitatory effect, leading to an electrical impulse known as an action potential to travel down the neuron and allows for communication with other neurons it is connected to.
Another opsin called Halorhodopsin (NpHR) is often used to produce opposite effects of ChR2. When added to neurons, it forms channels in the cell membrane that bring negatively charged ions into the cell, such as chlorine. When exposed to yellow light, these channels open, causing an inhibitory effect. This prevents an action potential from occurring, inhibiting communication to nearby neurons.
Once the mouse has the genes to encode ion channels that are excitatory to blue light and inhibitory to yellow light, the next step involves bringing light into the brain so that we can control the activity of neurons. This is accomplished through surgically implanting a tube known as an optical cannula into the brain region being studied. A fiber-optic cable is passed through this cannula and is attached to an individual neuron. The external end of this cable is attached to a light source, which can be illuminated either blue or yellow to manipulate the activity of the neuron.
Although this technology has not been applied to humans, the rodents that are studied with this method have neural structures that are very similar to ours. This has allowed scientists to gain insight into which regions of the brain are affected by neurological and psychiatric disorders such as Parkinson’s disease, addiction, autism spectrum disorder, schizophrenia, and depression.
