Study uncovers new and basic subtleties of how photoreceptors work

Moving around in the half-light is difficult but not impossible. To help us in this undertaking we have the rods, a type of light-sensitive cells (photoreceptors) present in the retina of vertebrates, capable of detecting very low lights that allow to move about even in poorly lit cellars or caves. They are biological wonders capable of detecting even a single quantum of light, but they need continuous maintenance.

They are the protagonists of this new study published in PNAS by a group of investigators of SISSA – Scuola Internazionale Superiore di Studi Avanzati and the Istituto officina dei materiali of the National research council CNR-Iom which reveals new and essential details of how the retina works and in particular photoreceptors.

With new optical probes we measured the concentration and the distribution of calcium in the OS. Using advanced optical microscopy instruments, we were able to study the distribution of this metal with unprecedented resolution and accuracy. What has emerged from the analyses is that there is greater concentration of calcium at the base of the outer segment with respect to the tip, which helps to understand the structure of the rod showing its non-homogeneity, as was thought until now.”-Dan Cojoc of Cnr

The OS of the sticks is the one where the biological system capable of capturing the light is located, while the IS is responsible for the information to be transferred to the mind. “We have understood that the outer section is more fragile than what was thought”, comments Vincent Torre, neuroscientist of SISSA directing the group that conducted the study, adding”The OS consists of a stack of lipid discs comprising the proteins responsible for phototransduction. New discs are generated at the base of the OS while employed discs are removed at the tip of the OS.

Traditionally, it was believed that in a stack of approximately 1000 disks there was almost perfect uniformity. However, our work shows that only the first 200 or 300 discs at the bottom of the OS are those efficiently capable of detecting the single photon of light, feature from which comes the great sensitivity of the rods. The other discs positioned close to the tip gradually get rid of effectiveness and sensitivity and for this reason they must be disposed of and replaced with fresh discs in excellent condition”.

It was the Calcium, an ion present in large numbers in biological processes that allowed the understanding of the mechanism. Its concentration in the OS is an excellent indicator of the integrity and functionality of phototransduction, the procedure with which the photoreceptors convert light into nerve signals.

These flares are not evenly distributed but found in the tips of the OS, which shows the existence of a functional gradient along the OS, a fundamental property for photoreceptor transduction of vertebrates.” Cojoc concludes. Like a warning light, the Calcium flares imply that the disks start to stop working in their finest and need turnover.

The article was also advised to Faculty Opinions by the editor of PNAS -something reserved only for the main contributions – for these reasons:”This interesting article uses a new Calcium measurement method to show that light-dependent changes of Calcium in the outer section of the sticks are greater at the bottom than at the tip”.

Neuroscientist Gordon Fain of the University of California continues,”These differences can reflect an energy gradient that originates from the mitochondria of the inner segment. The authors of this study also make the amazing observation that Calcium increases spontaneously both in the tip and at the bottom (but more often in the tip), as well as more rarely in the inner segment. These increases produce sudden flares, i.e. peaks of Calcium concentration, which decrease slowly for several seconds and which stay local without propagating within the outer section or between the outer and inner segment.”

The research, funded by SISSA and by the FVG region, offers an important contribution to the understanding of the eye and opens up interesting perspectives in the biomedical field.