A new chemical that restores vision for a short while in blind mice has been discovered. This is generating great excitement because of its potential to treat human blindness.
Age-related macular degeneration and retinitis pigmentosa (RP) are two forms of blindness. In these conditions, the cells on the retina – the surface in the posterior of the eye on which nerve cells detect light– are unable to respond to light, while the rest of the visual system remains intact. Current treatments include implanting optical ‘chips’, gene therapy, which inserts genes that allow the retina to respond to light and the use of embryonic stem cells which form light-sensitive cells that integrate into the defective retina. All of these are highly invasive procedures.
An easier alternative may be in the reckoning. A small molecule called AAQ (it has the unfortunate name acrylamide-azobenzene-quaternary ammonium) has been found to act as a ‘photoswitch’ which can bestow light sensitivity on nerve cells. Researchers injected small amounts of this chemical into the eyes of blind mice, and found that the mice showed contraction of their pupils and light avoidance, both of which are clear indicators of restoration of vision. This light sensitivity lasted for a few hours.
How Do We Respond to Light?
How do nerve cells respond to signals anyway? They are ‘activated’ by signals by the flow of ions. Nerve cells have numerous ion channels on their surfaces which open and close to control the ‘chemical current’ of the nerve cell. These ion channels can open or close, and thus control the flow of this current through the nerve cell, which is what we mean when we say that nerve cells are activated. AAQ binds to potassium channels in nerve cells in the eye, and makes them open up in the presence of ultraviolet light, and close when hit by green light. By offering this optical control, they thus bypass the defective retinal cells which do not pass on the light signal to nerve cells.
Though the effects of AAQ last for a few hours, researchers are looking to produce versions that produce longer-lasting effects. They are also trying to modify it so it can automatically deactivate in the dark instead of requiring a specific colour to deactivate. “The advantage of this approach is that it is a simple chemical, which means that you can change the dosage, you can use it in combination with other therapies, or you can discontinue the therapy if you don’t like the results. As improved chemicals become available, you could offer them to patients. You can’t do that when you surgically implant a chip or after you genetically modify somebody,” said lead researcher Richard Kramer.
This research was conducted by a team from the University of California, Berkeley, the University of Washington and the University of Munich. It was published in the journal ‘Neuron’. You can read about the research here.