Balancing act in cell membranes to sustain vision

Shedding light on the balancing act necessary to maintain size of cell membranes, researchers have found a critical link -- an enzyme named Phospholipase D or PLD -- that is essential for proper recycling of membranes to sustain normal sight.

Using the light-sensitive membranes in fruit fly eyes as a model system, scientists from the National Centre for Biological Sciences (NCBS), Bengaluru, and the Babraham Institute, Cambridge, UK, have recently discovered the link, details on which are published in the journal eLife in November.

Their results could help in understanding membrane turnover, a process that occurs in almost every cell in our bodies, said a statement from NCBS.

There are millions of photoreceptors within our eyes. They are nerve cells that capture light to form images of the world around us. The surface membranes of these nerve cells are packed with rhodopsin, a protein that detects light.

These are the light-sensing membranes of the eyes that absorb packets of light to trigger nerves causing the sensation of sight.

Once triggered by light, however, rhodopsin molecules must be 'reset' in order to sense light again.

This begins with a process called endocytosis, where the cell pinches off parts of its surface membranes into structures called endosomes.

The rhodopsin in endosomes is eventually recycled back onto the cell surface for further events of light detection.

Since a photoreceptor's sensitivity depends on how many rhodopsin molecules it has on its surface, membrane turnover in these cells is critical in preserving normal eyesight.

"You can think of endocytosis and membrane recycling as two arms of the membrane turnover process," said 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," he added.

The team has 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.

But the results from this study are not limited only to membrane turnover in the light-sensitive membranes of the eyes.

Every cell in our body undergoes membrane turnover for a variety of reasons - from resetting detectors on their surfaces to changing the molecules on their membranes in response to a signal.

"Therefore regardless of what the cell type it is, there need to be mechanisms to couple endocytosis with recycling of membrane. And that is the importance of our work - we define a mechanism by which cell membrane size is regulated," the researchers added.

--IANS

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