Three-dimensional structure of Phospholipase D. Image by Jawahar Swaminathan (public domain)

How do our eyes adjust to changing light?

An enzyme called phospholipase D alters the sensitivity of photoreceptor cells in the eye.

Certain cells in the eye contain a receptor protein known as rhodopsin that enables them to detect light. Rhodopsin is found in distinct patches on the membrane surrounding each of these “photoreceptor” cells and the number of rhodopsin molecules present controls how sensitive the cell is to light. In humans, vitamin A deficiency or genetic defects can decrease the number of rhodopsin molecules on the membrane, leading to difficulty in seeing in dim light.

Fruit fly eyes also contain rhodopsin. Exposure to normal levels of light triggers parts of the membranes of fly photoreceptor cells to detach and move into the interior of the cell. These internalized pieces of membrane have two possible fates: they can either be destroyed or recycled back to the cell surface. This membrane turnover adjusts the size of the membrane surrounding the cell and the number of rhodopsin molecules in it to regulate the cell’s sensitivity to light. It is crucial that turnover is tightly regulated in order to maintain the integrity of the cell membrane. However, it is not clear how the process is regulated during light exposure.

Rajan Thakur and colleagues set out to address this question in fruit flies. The experiments show that an enzyme called phospholipase D is activated when photoreceptors are exposed to light. Active phospholipase D — which generates a molecule called phosphatidic acid — coordinates the internalization of pieces of membrane with the recycling of rhodopsin back to the cell surface. Thakur and colleagues generated fly mutants that lacked phospholipase D and in these animals the internalized rhodopsin was not transported back to the cell membrane. This caused the membrane to shrink in size and decreased the number of rhodopsin molecules in it. As a result, the photoreceptor cells became less sensitive to light.

The findings of Thakur and colleagues show that in response to normal levels of light, phospholipase D balances membrane internalization and recycling to maintain the size and rhodopsin composition of the membrane. Future challenges will be to work out exactly how phospholipase D is activated and how phosphatidic acid tunes membrane internalization and recycling.

To find out more

Read the eLife research paper on which this eLife digest is based: “Phospholipase D activity couples plasma membrane endocytosis with retromer dependent recycling” (November 16, 2016).
eLife is an open-access journal that publishes outstanding research in the life sciences and biomedicine.
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