Giving sight to the blind - a clinical application of channelrhodopsins
I say this in my classes, and I stand by it: Karl Deisseroth or Ed Boyden or one of those folks who described optogenetics in their 2005 Nature Neuroscience paper are going to earn a Nobel Prize.
The technique of optogenetics is a brilliantly creative feat of genetic engineering. Generally speaking, optogenetics refers to a group of special transmembrane proteins that respond to light. These proteins change their structure when struck by a photon of light.
In a protein such as channelrhodopsin-2, the one described by Boyden et al in 2005,1 blue light causes an ion channel to open, which allows sodium to move into the cell. The movement of sodium causes depolarization, or cellular excitation. These channels are amazingly fast, able to respond and change shape on the order of milliseconds. This light-sensitivity allows researchers to change neuronal activity using light, and when put into a rodent, allows us to modulate their behavior in real time.
For many years, optogenetic control of behaviors was only performed on experimental animals, like rats or mice.
A study published in 2021 changed everything. A channelrhodopsin protein was used in a 58-year-old man to restore some aspects of his vision. 2
When he was 18, the patient was first diagnosed with a genetic condition called retinitis pigmentosa (RP). RP is a degenerative disease affecting about 1 in 4,000 people, resulting in severe vision loss and eventual blindness. Although it can be easily detected during an eye exam, there is no effective cure for RP, and therapeutic strategies are aimed at dealing with vision loss or slowing the progress of the disease. 3
Enter channelrhodopsins.
These proteins are made up of two major structural elements. One is the channel, the portion that spans the cell membrane and allows for ions to move across the cell membrane by facilitated diffusion. The other part is the rhodopsin, the photon sensor, that triggers the channel to open or close in response to photons of light.
Rhodopsins are part of a healthy visual system, found in the rod photoreceptors that make up the majority of the visual sensory cells of the retina. These cells are mainly responsible for detecting light in our peripheral vision and in low-light conditions.
In order to counteract the progressing loss of retinal neurons in RP, the research team delivered channelrhodopsin to the man’s retina. Since these proteins are actually only effective when created by the neurons themselves, the research team couldn’t simply put the channelrhodopsin into the man’s eyeball. Instead they had to use a virus as a tool for tricking the cell into making the channelrhodopsin proteins on their own.
Viruses get a bad rap for all the diseases and premature loss of life they can cause. But in actuality, they are not sentient malicious creatures, and are barely considered “living” - depending on who you ask. Many viruses are just protein shells enclosing some genetic material. Despite (or maybe because of?) their simplicity, they are extremely effective at doing one thing: penetrating the surface of cells and delivering their genetic material to that cell.
And if you were a researcher who wants to put the channelrhodopsin protein into the retinal neurons, you could exploit this function.
In this study, the research team developed an adeno-associated viral vector containing the genetic material that codes for a specific channelrhodopsin called ChrimsonR. Then, in the most unpleasant part of the experiment, they delivered this virus via an intravitreal injection - a shot to the eyeball. Because it takes some time for the virus to deliver its payload and for the cells to take the genetic instructions, the team and the patient waited for four months.
Because patients with RP experience a loss of the photoreceptor cells, the target for the treatment were the downstream cells in the “chain of vision signaling”, specifically the retinal ganglion cells of the fovea. Normally, only a tiny fraction of these cells are light sensitive. However, once they start producing the ChrimsonR protein, they will gain the ability to change their activity in response to light. Since these retinal ganglion cells make up the optic nerve, which communicates with the brain, a successful restoration of vision would essentially bypass the lost photoreceptor cells.
The photoreceptors that we have in the fovea, mainly the cones, come in three flavors, each of them maximally responsive to a specific wavelength of light. This particular ChrimsonR construct is most strongly activated by 590 nm wavelength light, which corresponds to an amber color. This color was preferable since this color causes less pupil constriction compared to blue light, the wavelength described in the early channelrhodopsin papers. Less constriction means that more light can enter into the eye and signal through the retinal neurons.
Unfortunately, the viral delivery of ChrimsonR alone was not sufficient to restore vision. The patient had to be fitted with specialized light stimulating goggles, which look a little bit like a VR headset. These goggles detect the light signals of the surroundings, and deliver a similar pattern of amber light directly to the man’s eyes.
Occipital lobe shown in pink at the posterior end of the brain.
In behavioral tests, the patient demonstrated an improvement in his ability to identify the presence or absence of objects in front of him, accurately doing so in the majority of trials. Ultimately, this restored ability to distinguish things in front of him will serve him well in life outside the lab settings.
The expermenters also wanted to demonstrate that those visual signals are making their way into the brain. To accomplish this, they equipped the patient with an electroencephalogram (EEG) to search for changes in brain activity at the moments when he was getting light stimulation into his goggles. As expected, the changes in cortical activity were most robust in the occipital lobe, the part of the brain that corresponds to the visual cortex.
One of the tricky issues concerning RP is the heterogeneity of the disease. It is likely that each case of RP differs from the others. For example, there are at least 50 genes that contribute to the onset of RP, ranging in functions such as ion channels and enzymes. Many of the genes have still unknown functions. 4
This publication demonstrated that optogenetic strategies may have some benefit for patients in the clinic, beyond restoring sight to the blind.
1 https://pubmed.ncbi.nlm.nih.gov/16116447/
2 https://www.nature.com/articles/s41591-021-01351-4
3 https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/retinitis-pigmentosa