Maintaining The Delicate Balance In The Eyes

Scientists have identified the enzyme that maintains the balance between two key processes that control the size of light-sensitive membranes in the eyes.

AsianScientist (Dec. 16, 2016) – An enzyme known as phospholipase D (PLD) plays an important role in maintaining light-sensitive membranes in fruit flies. These findings, published in eLife, could help scientists understand conditions like retinal degeneration.

The cell membranes of light-sensing nerve cells are packed with rhodopsin, a protein that detects light. Once triggered by light, rhodopsin molecules on the surface membrane must be ‘reset’ in order to sense light again. This requires rhodopsin to be moved into the cell via a process called endocytosis, and for ‘reset’ rhodopsin to be recycled back onto the surfaces. Therefore, constant membrane recycling is essential for vision to function normally.

Scientists from the National Centre for Biological Sciences (NCBS), Bangalore, and the Babraham Institute in UK have recently discovered that PLD is necessary for membrane recycling to sustain normal sight.

“You can think of endocytosis and membrane recycling as two arms of the membrane turnover process,” said Dr. Raghu Padinjat from NCBS. “There needs to be a balance between the two, or else the size of the membrane will shrink—a condition that could lead to retinal degeneration in the eyes. This is actually seen in an inherited genetic disease, a rare disorder called Best’s macular degeneration.”

Using fruit fly photoreceptor cells as a model system, the team found that when these cells are exposed to light, PLD is switched on, and that its activity is essential in coupling endocytosis with recycling of rhodopsin back to the cell surface.

In mutant flies that lack PLD in their photoreceptors, the endocytosis process is decoupled from membrane recycling. When exposed continuously to light, the cell surface of photoreceptors in mutant flies gradually shrinks, reducing rhodopsin levels that make it progressively less sensitive to light. Without PLD activity, the retina gradually degenerates and mutant flies go blind.

Padinjat’s team chose to use the fruit fly eye as a model system because the light-sensing parts of fruit fly photoreceptors are highly expanded, forming a structure called the rhabdomere. When exposed to light, changes in the size of this membrane can be directly visualized through electron microscopy. Furthermore, due to the genetic tractability of the system, the researchers were able to clearly identify PLD as an essential component regulating membrane turnover.

The results from this study also shed light on other cellular processes where membrane turnover plays a critical role, such as maintaining the lining of the lung airways or nutrition-absorbing cells of the gut.

“Therefore, regardless of what the cell type is, there need to be mechanisms to couple endocytosis with recycling of membrane,” said Padinjat. “And that is the importance of our work—we define a mechanism by which cell membrane size is regulated.”

The article can be found at: Thakur et al. (2016) Phospholipase D Activity Couples Plasma Membrane Endocytosis with Retromer Dependent Recycling.


Source: National Center for Biological Sciences.
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