Unexpected solution to fly eye mystery

A new study shows that a flies rapid vision may be a result of their photoreceptors - specialised cells found in the retina - physically contracting in response to light.

The mechanical force then generates electrical responses that are sent to the brain much faster than, for example, in our own eyes, where responses are generated using traditional chemical messengers.

It had been thought that the ion channels responsible for generating the photoreceptors’ electrical response were activated by chemical messengers as is usually the case in cell signalling pathways.

However, these results suggest that the light-sensitive ion channels responsible for the photoreceptor’s electrical response may be physically activated by the contractions - a surprising solution to the mystery of light perception in the fly’s eye and a new concept in cellular signalling.

Professor Roger Hardie, lead author of the study from the University of Cambridge’s Department of Physiology, Development and Neuroscience, said: “The ion channel in question is the so-called ’transient receptor potential’ (TRP) channel, which we originally identified as the light-sensitive channel in the fly in the 1990’s.

“It is now recognised as the founding member of one of the largest ion channel families in the genome, with closely related channels playing vital roles throughout our own bodies. As such, TRP channels are increasingly regarded as potential therapeutic targets for numerous pathological conditions, including pain, hypertension, cardiac and pulmonary disease, cancer, rheumatoid arthritis, and cerebral ischaemia. We are therefore hopeful that these new results may have significance well beyond the humble eye of the fly.”

A fly’s vision is so fast that it is capable of tracking movements up to five times faster than our own eyes. This performance is achieved using microvillar photoreceptor cells, in which the photo-receptive membrane is made up of tiny tubular membranous protrusions known as microvilli.

In each photoreceptor cell, tens of thousands of these are packed together to form a long rod-like structure, which acts as a light-guide to absorb the incident light. Each microvillus also houses the biochemical machinery, which converts the energy of the absorbed light into the electrical responses that are sent to the brain - a process known as phototransduction.

As in all photoreceptors, phototransduction starts with absorption of light by a visual pigment molecule (rhodopsin). In microvillar photoreceptors this leads to activation of a specific enzyme known as phospholipase C (PLC). PLC is a ubiquitous and very well-studied enzyme, which cleaves a large piece from a specific lipid component of the cell membrane (“PIP2”), leaving a smaller membrane lipid (DAG) in its place.

Somehow this enzymatic reaction leads to the opening of “ion channels” in the microvillus membrane; once opened, these allow positively charged ions such as Ca2+ and Na+ to flow into the cell thus generating the electrical response. This basic sequence of events has been established for over 20 years; but exactly how PLC’s enzymatic activity causes the opening of the channels has long remained a mystery and one of the major outstanding questions in sensory biology.

Professor Hardie added: “The conventional wisdom would be that one of the products of this enzyme’s activity is a chemical ’second messenger’ that binds to and activates the channel. However, years of research had previously failed to find compelling evidence for such a straightforward mechanism.”