rod-cells

New function for rods in daylight

Vision – so crucial to human health and well-being – depends on job-sharing by just a few cell types, the rod cells and cone cells, in our retina. Botond Roska and his group have identified a novel function for rod photoreceptor cells in the retina in daylight. Driven by cones and mediated by horizontal cells, rods help to increase contrast information at times when they are not directly sensing light. The retina thus repurposes its cells in different light conditions to increase the amount of visual information about the environment.

(Caption: Horizontal cells in the retina)

Task sharing in the retina seemed clear: Two different kinds of photoreceptor cells take on two different visual tasks. Rods allow us to see at night, cones operate during the day and enable color vision. However, the question as to why there are about 20 times more rods than cones in a human retina, when daytime vision is much more relevant for us, has usually led to a shrug of shoulders. It seemed a waste of resources.

Botond Roska and his group at the Friedrich Miescher Institute for Biomedical Research, could now show in a study published recently in Nature Neuroscience that the rods in mouse take on an important function during daytime vision as well.

The scientists showed that in bright light, the rods mediate a so called surround inhibition. Surround inhibition is an important feature in the retina because it allows not only to transmit information about whether a photoreceptor is exposed to light, but also about contrast. While the cone cells hyperpolarize in bright light and thus send a visual signal to the inner retina, the rods depolarize, inversely matching the activity pattern of the cone cells. The response in the rods is driven by cone cells and mediated through horizontal cells. These horizontal cells connect rods and cones through their dendrites and long axons, and at the same time form a mesh of connections among each other. The hyperpolarization of one cone thus leads to the depolarization of many surrounding rods.

During bright light conditions, the cells of the inner retina receive therefore information through two pathways: First through the well-established cone pathway, and second through this newly identified rod pathway. “We think that the surround information relayed to the inner retina through the rod pathway has different functional properties than the information obtained through the cone pathway,” comments Roska. “In any case it is fascinating to see how the retina repurposes the rod cells during bright light conditions to increase contrast information, at times when they are not directly sensing light.”

After all, these large numbers of rods don’t seem to be present in the retina in vain.

The red eye shine seen in alligators arises when light enters the eye and hits a layer of cells called the tapetum lucidum. This membrane is located beneath the photoreceptor cells (rods and cones) in the retina and reflects light back into these cells to increase the amount of light detected, which improves an alligator’s vision in low light conditions.

Several species exhibit this phenomenon, with different colour ‘shines’ observed. Most species with eyeshine are night hunters who must make use of limited light.

-Jean

Photos by Larry Lynch (http://www.lynchphotos.com/) and David Moynahan (http://www.davidmoynahan.com/)

Figuring out the rules of bacterial cell division

Bacteria “know” where to divide into daughter cells using a concentration gradient formed by the protein MinD, which oscillates back and forth in a rod-shaped cell with maxima at the ends and minima in the middle, where the cell divides. Here’s a figure of it in action with green-fluorescent protein-tagged MinD in an E.coli cell over time: 

Researchers from the Netherlands have pushed the limits of how MinD is able to define the cell division boundary by custom-growing E.coli cells into different shapes and sizes. This required chemically suppressing the E.coli’s ability to maintain its rod shape and form a new cell wall between the dividing cells, and then injecting a single cell into bacterium-scale (micrometer, or um) moulds of the desired shape, which they would then expand to fill:

Again, using GFP-tagged MinD its oscillations were tracked over time in the artificially shaped cells. Definite patterns could be seen, with the MinD preferring to travel along symmetry axes:

The cell dimensions of rectangular cells were systematically altered and different patterns were observed, with one common characteristic -for cell lengths of ~3-6um, about the length of normal dividing E.coli cells,  MinD forms 2 poles. Multiple poles appeared at 7um or greater, and lack of poles occurred at <2.5um.

The protein dynamics underlying this can be modeled by Turing’s reaction-diffusion equations

Although Turing is mostly known for his role in deciphering the Enigma coding machine and the Turing Test, the impact of his ‘reaction-diffusion theory’ on biology might be even more spectacular. He predicted how patterns in space and time emerge as the result of only two molecular interactions – explaining for instance how a zebra gets its stripes, or how an embryo hand develops five fingers. Such a Turing process also acts with proteins within a single cell, to regulate cell division.

Read more at: http://phys.org/news/2015-06-squares-triangles-bacteria-figure-alan.html#jCp

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2015.126.html

25 June 2015

Switching Eyes On

Deep in the back layer of our eyeballs, light triggers cells called photoreceptors – the long, rod-shaped cells stained green in this microscope image. Once activated, the photoreceptors send messages via nerve cells into the brain that enable us to see. Sometimes these photoreceptors break down and stop working properly, causing sight loss. But because the nerves wiring them to the brain are still intact, researchers are testing whether new genetic engineering techniques – known as optogenetics – can switch light-sensitivity back on and restore sight. Using a modified virus, they’re adding a specially-designed light-activated protein molecule into cells at the back of the eye in blind mice. These molecular ‘light switches’ work amazingly well, turning on in response to light and bringing back the animals’ vision. Although it’s still early days, the exciting results bring hope that this technique could one day lead to new therapies for sight loss.

Written by Kat Arney

Image by Michiel van Wyk and colleagues
University of Bern, Switzerland
Originally published under a Creative Commons Licence (BY 4.0)
Research published in PLOS Biology, May 2015

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Microglia May Be a Potential Therapeutic Target for Blinding Eye Disease

A new study by researchers at the National Eye Institute (NEI) shows that they also accelerate damage wrought by blinding eye disorders, such as retinitis pigmentosa. NEI is part of the National Institutes of Health.

The research is in EMBO Molecular Medicine. (full open access)

Research: “Microglial phagocytosis of living photoreceptors contributes to inherited retinal degeneration” by Lian Zhao, Matthew K Zabel, Xu Wang, Wenxin Ma, Parth Shah, Robert N Fariss, Haohua Qian, Christopher N Parkhurst, Wen‐Biao Gan, and Wai T Wong in EMBO Molecular Medicine doi:10.15252/emmm.201505298

Image: A microglial cell (green) extends spider-like arms to capture and consume rod photoreceptor cells (blue). Image credit: Dr. Wai T. Wong, National Eye Institute.

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The red eye shine seen in alligators arises when light enters the eye and hits a layer of cells called the tapetum lucidum. This membrane is located beneath the photoreceptor cells (rods and cones) in the retina and reflects light back into these cells to increase the amount of light detected, which improves an alligator’s vision in low light conditions.

Several species exhibit this phenomenon, with different colour ‘shines’ observed. Most species with eyeshine are night hunters who must make use of limited light.

Photos by Larry Lynch and David Moynahan