Adaptive iridocytes in the skin of the California squid market can adjust their color across most of the spectrum. Credit: University of California ̵
1; Santa Barbara
Squid, octopus and sepia are the undisputed masters of fraud and camouflage. Their exceptional ability to change color, texture and shape is unmatched even by modern technology.
Researchers at the laboratory of Santa Barbara professor Daniel Morse have long been interested in the optical properties of changing animal colors and are particularly intrigued by the opalescent coastal squid. Also known as California squid, these animals have developed the ability to fine-tune and continually adjust their color and shine to a degree unmatched by other creatures. This gives them the opportunity to communicate as well as to hide in sight in the bright and often useless upper ocean.
In previous work, researchers discovered that specialized proteins, called reflexes, control reflex pigment cells – iridocytes – which in turn contribute to altering the overall appearance and appearance of a being. But it's still a mystery how the reflexes actually worked.
"Now we wanted to understand how this remarkable molecular machine works," says Morse, an excellent professor of emergencies in the Department of Molecular, Cellular and Biological Development and lead author of a book that appeared in Journal of Biological Chemistry . Understanding this mechanism, he said, will give a glimpse into the adjustable control of emergent properties, which could open the door for the next generation of bio-inspired synthetic materials.
Like most cephalopods, opalescent coastal squids, practice your sorcery through what may be the most complex skin found anywhere in nature. The small muscles manipulate the texture of the skin, while the pigments and striations affect its appearance. One group of cells controls their color by expanding and contracting cells in their skin that contain pigment sacks.
Behind these pigment cells lies a layer of iridescent cells – those iridocytes – that reflect light and contribute to the color of animals. the entire visible spectrum. Squids also have leucophores that control the reflection of white light. Together, these layers of pigments containing and reflecting cells give squid the ability to control the brightness, color and hue of their skin through a remarkably wide palette.
Unlike pigment color, the highly dynamic shades of opalescent coastal squid are the result of a change in the structure of the iridocyte itself. Light bounces between nanometer size features about the same size as the wavelengths in the visible spectrum, producing colors. As these structures change in size, the colors change. Reflexin proteins are behind the ability of these features to displace shapes, and it is the researchers' job to understand how they handle the job.
Thanks to a combination of genetic engineering and biophysical analysis, scientists have found the answer, and this has proven to be a mechanism far more elegant and powerful than previously thought.
"The results were very surprising," said first author Robert Levenson, a doctoral researcher at Morse Laboratory. The group expected to find one or two spots on the protein that controls its activity, he said. "Instead, our evidence has shown that the reflexin characteristics that control its signal recognition and the resulting assembly are spread throughout the protein chain."
Reflectin contained in tightly packed layers of membrane in iridocytes, it looks a bit like a string of string beads, the researchers found. Usually, the links between the beads are very positively charged, so they repel each other, straightening the proteins like unheated spaghetti.
Morse and his team find that nerve signals to reflecting cells cause the addition of phosphate groups to the bonds, These negatively charged phosphate groups neutralize the repulsion of the bonds, allowing the proteins to fold. The team was particularly excited to find that this fold reveals new, sticky surfaces of the parts that resemble reflex beads, allowing them to adhere. Up to four phosphates can bind to each reflex protein, providing the squid a fine-tuning process: The more phosphates added, the more proteins fold, progressively exposing more emergent hydrophobic surfaces and the larger lumps grow.  As these lumps grow, many, single, small proteins in solution become smaller, larger groups of multiple proteins. This changes the fluid pressure inside the membrane stacks, expelling the water, a kind of "osmotic engine" that responds to the smallest changes in charge generated by the neurons to which patches of thousands of leucophores and iridocytes are attached. The resulting dehydration reduces the thickness and distance of the membrane stacks, which shifts the wavelength of reflected light progressively from red to yellow, then to green and finally blue. The more concentrated the solution, the higher the refractive index, which increases the brightness of the cells.
"We had no idea that the mechanism we would find would be so remarkably complex, yet containing so elegantly integrated into a multifunctional molecule – the block copolymer reflexin – with opposing domains so delicately tuned that they act as metastases machine, constantly sensing and responding to neural signaling by precisely adjusting the osmotic pressure of the intracellular nanostructure to precisely refine the color and brightness of reflected light, "said Morse.
already, the researchers found, the whole process is reversible and cyclical, allowing squids to continuously refine whatever optical properties the situation needs.
New design principles
have successfully manipulated reflexin in previous experiments, but this study marks the first demonstration of the underlying mechanism and could now provide new ideas to scientists and engineers designing materials with adjustable properties. "Our findings reveal a fundamental link between the properties of biomolecular materials produced in living systems and the highly engineered synthetic polymers that are now being developed across industry and technology," Morse says.
"Because reflexin works to control osmotic pressure, I can anticipate applications for new energy storage and conversion tools, pharmaceutical and industrial applications including viscosity and other liquid properties, and medical applications," he added.
It is remarkable that some of the processes of operation in these proteins of reflexin are shared by proteins that collect pathologically in Alzheimer's disease and other degenerative conditions, observed Morse. He plans to investigate why this mechanism is reversible, cyclic, harmless and useful in the case of reflexin, but irreversible and pathological to other proteins. Perhaps finely structured differences in their sequences may explain the discrepancy and even indicate new pathways for disease prevention and treatment.
Marine biologists clarify how specialized cells in squid skin are able to control the coloration of the animal
Robert Levenson et al. Trigger-to-color calibration: Neutralizing a genetically encoded cubic switch and dynamic braking, fine-tuning reflex summation, Journal of Biological Chemistry (2019). Doi: 10.1074 / jbc.RA119.010339
University of California – Santa Barbara
Great Molecular Machine (2019, November 15)
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