Squid and cuttlefish are masters of camouflage, blending in with their surroundings to evade predators or surprise prey. Some aspects of how these cephalopods become reversibly transparent remain unclear, largely because researchers cannot grow cephalopod skin cells in the lab. Now, however, the researchers report that they have mimicked the tunable transparency of certain squid skin cells in cultureable mammalian cells. The work could not only shed light on the basic biology of squid, but also lead to better ways to image many cell types.
The researchers will present their findings at the American Chemical Society (ACS) spring meeting.
Alon Gorodetsky, Ph.D., and his research group have been working on squid-inspired materials for many years. In previous work, they developed “invisibility stickers” consisting of squid reflecting proteins produced by bacteria and stuck to sticky tape. “So we had this crazy idea to see if we could capture some aspects of the ability of squid skin tissue to change transparency in human cell cultures,” says Gorodetsky, the project’s principal investigator.
The University of California, Irvine team focused their efforts on cephalopod cells called leucophores, which have particle-like nanostructures made up of reflective proteins that scatter light. Typically, the reflectins clump together to form the nanoparticles so that light is not absorbed or transmitted directly; Instead, light scatters or bounces off them, making the leucophores appear bright white.
“We wanted to manipulate mammalian cells in such a way that they form stably rather than transiently reflective nanostructures for which we can better control light scattering,” says Gorodetsky. In fact, when the cells transmit light with low scattering, they appear more transparent. Alternatively, scattering much more light makes the cells opaque and more visible. “Then we thought we could predictably change the transparency of the cells relative to the environment or background at the cellular level or even at the culture level,” he says.
To change the way light interacts with cultured cells, Georgii Bogdanov, a graduate student in Gorodetsky’s lab who is presenting the results, introduced squid-derived genes that coded for reflecting into human cells, which then read the DNA used to produce the protein. “A key advance in our experiments was getting cells to stably produce reflections and form light-scattering nanostructures with relatively high refractive indices, which also allowed us to better image cells in three dimensions,” explains Bogdanov.
In experiments, the team added salt to cell culture media and watched reflective proteins clump together into nanostructures. By systematically increasing the salt concentration, Bogdanov obtained detailed 3D time-lapse images of the properties of the nanostructures. As the nanoparticles got larger, more light was reflected off the cells, adjusting their opacity.
Then the COVID-19 pandemic hit, and researchers wondered what they could do to advance their investigation without being physically in the lab. So Bogdanov spent his time at home developing computer models that could predict the expected light scattering and transparency of a cell even before an experiment was started. “It’s a nice loop between theory and experiment, where you introduce design parameters for reflective nanostructures, get specific predicted optical properties, and then make the cells more efficient – for any light scattering properties that might interest you,” says Gorodetsky.
Basically, Gorodetsky suggests that these results will help scientists better understand squid skin cells that have not been successfully cultured in the laboratory. For example, previous researchers have postulated that reflective nanoparticles disassemble and reassemble to alter the transparency of tunable squid leucophores. And now Gorodetsky’s team has shown that similar rearrangements occur in their stable mammalian cells modified by simple changes in salt concentration, a mechanism that appears to be analogous to what has been observed in tunable squid cells.
The researchers are now optimizing their technique to develop better cell imaging strategies based on the cells’ intrinsic optical properties. Gorodetsky envisions that reflectin proteins could act as genetically encoded markers that don’t turn white in human cells. “Reflectin as a molecular probe offers many opportunities to trace structures in cells with advanced microscopy techniques,” adds Bogdanov. For example, the scientists suggest that imaging approaches based on their work could also have implications for better understanding cell growth and development.
Researchers acknowledge funding from the Defense Advanced Research Projects Agency and the US Air Force Office of Scientific Research.