VISUAL CYCLE

An imperfect recycling system

Plant 1 The retina is the thin membrane located at the back of the eye. It contains millions of photoreceptors used for vision, and plays a similar role as the light sensitive film at the back of a camera. These specialized receptors cover the retina and are responsible for black and white (rods) and for colored vision (cones). Vitamin A, which can be found on the tip of the photoreceptors, is the key molecule to vision. Its exact role was explicited by George Wald and resulted in a Nobel Prize of 1967. In other words, vitamin A is the fuel of vision.

Vitamin A and its derivatives (such as beta-carotene), originate from certain types of food: for example, eggs, milk and other dairy products all contain vitamin A while vegetables, fruits, carrots, etc. contain beta-carotene. After vitamin A is transported into the retina, it is struck by light forcing it to change its molecular shape, and acting like an electrical switch which enables the delivery of an electric signal to the brain. This signals the presence of light to the brain.

After vitamin A has changed its shape, it becomes insensitive to light and needs to be reactivated by specialized cells, the retinal pigmented epithelium (RPE). The geometric switch of vitamin A as well as its recycling by the RPE is called the "visual cycle".

While we would wish vitamin A to be a clean burning fuel, unfortunately, the visual cycle is imperfect and some vitamin A molecules are able to escape the recycling system: these vitamin A can then bind to other vitamin A molecules and create toxic aggregates of vitamin A called vitamin A dimers (or A2E). A2E is then absorbed and stored in the RPE cells where they are considered to be responsible for the formation of other toxic granules named lipofuscin.

With age, accumulation of lipofuscin reduces the proper function of the RPE cells and is thought to be partly responsible for inducing macular degeneration.

Stargardt disease patients present a defective gene which prevents proper transport of vitamin A back into the RPE, which results in even faster formation and accumulation of A2E and lipofuscin. This is why Stargardt disease is also called "juvenile macular degeneration" as the clinical presentations may be somehow similar.

Visual cycle imperfections lead to A2E formation and to macular degeneration. Alkeus Pharmaceuticals is developing compounds that can help perfect this cycle, prevent the formation of toxic A2E and lipofuscin pigments and potentially slow down vision loss in dry-AMD and Stargardt disease.