More insight into how vision works

Date:
January 3, 2022
Source:
Paul Scherrer Institute
Summary:
Scientists have shed light on an important component of the eye:
a protein in the rod cells of the retina which helps us see in
dim light.

Acting as an ion channel in the cell membrane, the protein is
responsible for relaying the optical signal from the eye to the
brain. If a genetic disorder disrupts the molecular function in
a person, they will go blind.

Scientists have deciphered the protein's three-dimensional
structure, preparing the way for innovative medical treatments.



FULL STORY ==========================================================================
PSI scientists have shed light on an important component of the eye:
a protein in the rod cells of the retina which helps us see in dim
light. Acting as an ion channel in the cell membrane, the protein is responsible for relaying the optical signal from the eye to the brain. If
a genetic disorder disrupts the molecular function in a person, they will
go blind. Scientists have deciphered the protein's three-dimensional
structure, preparing the way for innovative medical treatments. The
study is published in the scientific journal Nature Structural &
Molecular Biology.


========================================================================== "It's thanks to the rod cells in our eye that we can observe the stars in
the night sky," explains Jacopo Marino, a biologist with PSI's Laboratory
of Biomolecular Research. "These photo cells are so sensitive to light
that they can detect even a single photon reaching us from a very remote
part of the universe -- a truly incredible feat." The ability of our
brain to eventually translate these light beams into a visual impression
is partly down to the cyclic nucleotide-gated (CNG) ion channels whose three-dimensional structure has now been illuminated by a PSI research
group led by Jacopo Marino.

The ion channel acts as a gatekeeper controlling whether specific
particles are allowed through to the interior of the receptor cell. It
is embedded in the protein-rich shell -- the cell membrane -- of the
rod cells. In darkness, the ion channel, and thus the gate to the cell,
is completely open. But when light hits the eye, it triggers a cascade
of processes in the rod cells. This ultimately causes the gate to close,
with the result that positively charged particles, such as calcium ions,
can no longer enter into the cell.

This electrochemical signal continues via the nerve cells into the
brain's visual cortex, where a visual impression -- such as a flash of
light -- is created. "The idea of solving the structure of this channel
dates back to nearly 20 years ago, when Gebhard Schertler and Benjamin
Kaupp already collaborated on this topic," says Jacopo Marino. Both are co-authors of the new study.

Endurance paid off PhD student Diane Barret first had to extract the
channel protein from cows' eyes supplied by an abattoir -- a complicated
and arduous process. "This was a very challenging task, as the protein
is extremely sensitive and decomposes very quickly. In addition, it
is only available in tiny quantities in the source material," Barret
explains. It took a whole two years to obtain enough protein to work
with. "We were both too stubborn to simply give up," says Jacopo Marino, laughing. "But in the end that stubbornness paid off." The scientists
then used cryo-electron microscopy to reveal the three- dimensional
structure of the ion channel. "In contrast to previous studies on the
structure of the ion channel, we investigated the native protein as it
exists in the eye. We are therefore much closer to the real conditions
that exist in living creatures," Diane Barret says.

One of the reasons why a clearer understanding of the channel protein's
natural structure is important is to advance the development of treatments
for genetic disorders for which there is no known cure, such as retinitis pigmentosa. With this disease, photoreceptors gradually die off, leaving
people blind. One possible cause is that the body is unable to correctly produce the CNG channel protein due to a genetic defect. As a result, the
ion channel does not close completely when light hits the eye, disrupting
the cell's electrochemical balance and causing the cells to die.

"If we could find molecules that affect the protein in such a way that
the channel would completely close, we could prevent the cells from dying
-- and thus stop people going blind," explains Jacopo Marino. Now that researchers have identified the precise structure of the protein they
are able to search specifically for such molecules.

Additional barrier The protein comprises four parts: three lots of subunit
A, and one lot of subunit B. A correctly functioning ion channel is
only possible in this combination. In their study, PSI scientists show
why the B subunit seems to play such an important role: a side arm of
the protein -- a single amino acid - - protrudes from the rest of the
protein, like a barrier across a gateway. This narrows the passage in
the channel to the point where no ions can pass through.

"No one expected that -- it came as a total surprise," says Diane
Barret. Other narrow places already exist in the A subunit -- like
main gateways -- which were previously thought to be the only ones. It
is interesting to note that the additional barrier is found not only
in the protein from the cow's eye, but seems to apply to all types of
animal, as the scientists showed. Whether crocodiles, eagles or humans
-- all living creatures with an ion channel in their eye have the same protruding amino acid at this position in the protein.

As it has been preserved so consistently during evolution, it must be
essential for the functioning of the channel.

special promotion Explore the latest scientific research on sleep and
dreams in this free online course from New Scientist -- Sign_up_now_>>> academy.newscientist.com/courses/science-of-sleep-and-dreams ========================================================================== Story Source: Materials provided by Paul_Scherrer_Institute. Original
written by Brigitte Osterath. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Diane C. A. Barret, Gebhard F. X. Schertler, U. Benjamin Kaupp,
Jacopo
Marino. The structure of the native CNGA1/CNGB1 CNG channel from
bovine retinal rods. Nature Structural & Molecular Biology, 2021;
DOI: 10.1038/ s41594-021-00700-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220103121742.htm
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