Squid discovery could revolutionise molecular engineering

The “marvellous molecular machine” that enables a Pacific squid to change its appearance may have implications for pharmaceuticals and energy applications

Cephalopods are fascinating creatures. Possessed of great intelligence of a form entirely alien to humans, octopus and squid have physical abilities that often defy belief. Among these is their ability to change their appearance thanks to structures in their highly complex skin that control colour and texture. Studies of the mechanisms behind these transformations from researchers at the University of California in Santa Barbara (UCSB)  may have profound implications for molecular engineering, and may even point the way towards treatments for Alzheimer’s disease.

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The adaptive iridocytes in the skin of the California market squid are able to tune colour through most of the spectrum. Image: UCSB

Daniel Morse, an emeritus professor of molecular, cellular and developmental biology at UCSB, has been studying Doryteuthisa opalescens, otherwise known as the opalescent inshore squid or the California market squid (because as well as being fascinating, it is also abundant and delicious). These animals take the colour control ability of cephalopods to an extreme level: they can continuously tune their colour and reflectivity, allowing them to both communicate with each other and to hide in the open ocean. They have a variety of mechanisms to do this, but an important contribution is made by proteins in their skin called reflectins, which control reflective pigment cells called iridocytes.

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It has previously been unknown how reflectins worked, but Morse and his colleagues report in the Journal of Biological Chemistry that the mechanism is far more elegant and powerful than they thought, and could be copied to make designer proteins with highly useful properties. Reflectin, they explain, has a structure resembling beads on a string, and normally the links between each of these beads carries a strong positive charge, making the protein strand stretch out. However, nerve signals to the iridocyte cells add negatively charged phosphate groups to the links, neutralising their positive charges and allowing the protein to fold up. This, in turn, exposes sticky surfaces on the beads which allows up to four reflectin molecules to clump together. As the strength of the nerve signals increases, more phosphate groups are added, causing more proteins to clump together.

This process contributes to a change of fluid pressure inside the stacks of membranes that form the cell walls, driving out water and reducing the thickness of the stack, which is around the same as the wavelengths of visible light. This changes the wavelength of the light they reflect from red to yellow, then onto greens and blues. The colour also becomes brighter.

“We had no idea that the mechanism we would discover would turn out to be so remarkably complex yet contained and so elegantly integrated in one multifunctional molecule — the block-copolymeric reflectin — with opposing domains so delicately poised that they act like a metastable machine, continually sensing and responding to neuronal signalling by precisely adjusting the osmotic pressure of an intracellular nanostructure to precisely fine-tune the colour and brightness of its reflected light,” Morse said.

One particularly interesting feature of this process is that it is reversible and cyclable, but some of the processes involved in the transformation are similar to those seen when proteins assemble in the brain during the progress of protein-related diseases like Alzheimer’s and Parkinson’s. These processes, however, are irreversible. Morse believes that further study of this aspect of the reflectin’s properties may hold the key to understanding how brain-damaging pathology might be reversed.

There might also be applications outside medicine, he adds. “Because reflectin works to control osmotic pressure, I can envision applications for novel means of energy storage and conversion, pharmaceutical and industrial applications involving viscosity and other liquid properties, and medical applications,” said Morse.