-Sayanti Pal, Amity University Kolkata
Retinal Degeneration is a condition in which there is a slow degeneration of the retinal cells, eventually leading to blindness. People who suffer from these retinal degenerative diseases for years are said to suffer from permanent loss of vision. But thanks to recent advances in science and technology for giving us new hope of vision in the future though bionic vision. Research says that in the case of Retinal Degeneration only the external neuron i.e. the photoreceptor cells gets damaged. The second neuron i.e. the bipolar cells, and the third or internal neuron i.e. the ganglion cells remain intact.
With this knowledge, scientists have developed an artificial retina named the Bionic Eye that mimics the natural environment of our human eye. It contains a mini-camera that captures the image, an electronic microchip that acts as photoreceptor cells. And an electrode that converts the captured light into an electrical signal. These electrical impulses stimulate the remaining retinal cells to pass the information along the optic nerve to the brain. In clinical trials, it was found that patients can regain a partial vision that enables them to recognize certain patterns or to identify the shape of a person. This influences the person to relearn how to see instead of getting back the existing vision. To overcome this difficulty, the artificial retina is then connected with a computing device that scans the whole image captured by the camera. The information is encoded into a light signal that passes through the optic nerve to the brain for creating a high-resolution vision.
We know that our optic nerve sends electrochemical signals to the brain, which implies that our optic nerves are not sensitive towards the light. Here comes the approach of optogenetics, a branch of biotechnology that genetically modifies the optic nerves to convert them into light-sensitive cells. They can take up the light impulses, and transfer it to the brain for vision restoration. The most appropriate way to achieve this is generally done via a viral expression system. Special genes from single-celled organisms (i.e. algae and bacteria) known as microbial opsin genes are cloned with stereotypes of lenti or adeno-associated viral vector to target specific cells by subretinal injection to express particular combinations of opsins and promoters to improve cellular specificity. When these genes are incorporated in the targeted cells, it starts producing opsins that function as light-sensitive ion channels or pumps and activates or inhibits the production of electrical signals in cells by directing the movement of charged ions (e.g., protons or chloride ions) across the cell membrane.
Mostly Channelrhodopsin-2 (ChR2), obtained from Chlamydomonas reinhardtii, is a light-activated cation channel that is used to induce depolarization. Halorhodospin (HR), obtained from halobacteria, is a light-gated chloride ion pump that is used to induce hyperpolarization in the targeted cells. The choice of excitatory or inhibitory opsin largely depends on what type of activity one wishes to induce. Even multiple opsins can be expressed in a particular cell population to evoke two different responses at a time. Once the transfection is completed, the optogenetically modified cells require a high irradiance to be activated. This requirement can cause irreversible photochemical damage to the nerves, and also limits the optoelectronic stimulators from providing a sufficient light intensity. Moreover, these opsins may face an immune rejection from the recipient. Melanopsin-a light sensitive signalling transduction cascade might offer an alternative to this. As most of the research work on this area has used wild-type opsins, we need to see whether and how novel opsins can lead to improving artificial retinal strategies.
Some experiments and demonstrations on animal models (mice) have been achieved until date. It is expected that within a few years we will be able to see the first human optogenetic trials for vision restoration. Two major ethical issues may arise surrounding human optogenetic trials. First, human genetic engineering, and second, the toxicity of viral vector. Despite these challenges, we can say that optogenetics is a fast-developing field, with several optoelectronic devices moving towards human trials. Nevertheless, optogenetics, along with Artificial Retina holds the potential to bring back significantly improved vision in diseases affecting the photoreceptors and promises a future for Bionic vision restoration.
Source
- Barrett, J. M., Berlinguer, R., Degenaar, P.; Optogenetic approaches to retinal prosthesis; 2014; Visual Neuroscience;31(4-5): 345-354; DOI: 10.1017/S0952523814000212
URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4161214/
- The Bionic Eye; Fighting Blindness Canada.
URL: https://www.fightingblindness.ca/resources/the-bionic-eye/
Amazing article, informative and useful.