Safer Nanotech

Although so tiny they are invisible, it’s easy to see that nanomaterials are becoming a big thing. There are odor-fighting socks and antibacterial dishrags impregnated with silver nanoparticles. Nano-sized titanium dioxide can be found in a long list of food and consumer products, including salad dressing, cake frosting, toothpaste and sunscreen. The vibrantly colored screen of the Kindle Fire can be attributed to quantum dots, a.k.a. nano-scale crystals of semiconductors such as cadmium selenide. And the list goes on.

Nanomaterials are tiny by definition, measuring between 1 and 100 nanometers along one or more dimension. (By comparison, a human hair is approximately 100,000 nanometers in width.) At this scale, they possess unique physical and chemical properties that make them useful for a wide array of applications, including consumer products, environmental remediation and medicine. Yet there are many unanswered questions about their safety.

“We don’t know a lot about the toxicity of nanomaterials, and we have much to learn about the potential risks associated with the release of these materials into the environment,” says Joel Pedersen, Rothermel Bascom Professor of Soil Science at CALS.

Pedersen is part of a collaborative, multidisciplinary research team exploring these unknowns as part of the UW–Madison-based Center for Sustainable Nanotechnology, which was founded in 2012 with support from the National Science Foundation. Center scientists are working to understand how nanomaterials interact with living systems and the environment, with the practical goal of developing the insights needed to start creating nanomaterials that are designed to be more environmentally benign. This includes re-engineering them to make them safer, if needed.

With expertise in chemistry, biology and engineering, Pedersen is in charge of the Center’s efforts to develop laboratory models to assess the biological impacts of nanomaterials. While he has done some experiments in zebrafish, Pedersen’s work for the Center focuses on innovative, non-biological approaches, including creating “artificial cell surfaces” in the lab.

“Our intent is to get down to the molecular level,” Pedersen explains. “What are the rules that govern how these materials interact with biological systems? In particular, how do these particles interact with cell membranes?”

One way Pedersen’s group makes artificial cell surfaces is by depositing lipid vesicles on a special quartz crystal sensor until the vesicles spontaneously rupture and then fuse to form a lipid bilayer—the basic structure of a cell membrane—on the sensor’s surface.

When electricity is applied to the sensor, it causes the system to vibrate at a particular frequency. Next, Pedersen’s team applies nanomaterials to the artificial cell surface. The sensor can detect subtle changes in the frequency of the vibration, yielding clues about the interaction between the material and the membrane.

By combining the results of this approach with others, Pedersen is finding that some nanoparticles, by virtue of their unique physical and chemical properties, seem to be able to extract lipids from the cell surface.

“Our results are consistent with the idea that these nanoparticles are grabbing lipids out of the membrane and acquiring a lipid coating when they come in contact with a cell,” explains Pedersen.

This cell membrane-disrupting behavior is a concern for the health of humans and animals. And while Pedersen’s team hasn’t observed this behavior in models of bacterial cell surfaces, there are other, broader concerns about the impacts of nanomaterials on microbial communities in the environment.

“Eukaryotes are our main focus, but there is some concern that nanomaterials in the environment can alter microbial community compositions. At present, we don’t know to what extent such changes could be problematic,” says Pedersen.

The information gained from Pedersen’s research will help inform the work of other scientists in the Center for Sustainable Nanotechnology who focus on tweaking nanoparticles to make them safer.

“Ultimately, the goal is to redesign nanomaterials to minimize their adverse effects, or find better ways to embed them in materials so they aren’t released into the environment,” Pedersen says.

The Shocking Truth

It is well known that a certain kind of fish swims the world’s waters protected, as it were, by its very own stun gun.

Unknown, until now, is how electric fish evolved such a defense. A team of researchers led by CALS biochemistry professor Michael Sussman has established the genetic basis for the electric organ, an anatomical feature found only in fish. It evolved independently half a dozen times in environments ranging from the flooded forests of the Amazon to murky marine environments.

“These fish have converted a muscle to an electric organ,” says UW Biotechnology Center director Sussman, who began this research almost a decade ago. The study, recently published in the journal Science, provides evidence to support the idea that the six electric fish lineages used essentially the same genes and developmental and cellular pathways to make the electric organ, which fish use to communi- cate with mates, navigate, stun prey, and as a shock- ing defense. The jolt from an electric organ can be several times more powerful than the current from a standard household electrical outlet.

Worldwide, there are hundreds of electric fish in six broad lineages. Their taxonomic diversity is so great that Darwin himself cited electric fishes as critical examples of convergent evolution, where unrelated animals independently evolve similar traits to adapt to a particular environment or ecological niche.

The new work includes the first draft assembly of the complete genome of the South American electric eel. “A six-foot eel is a top predator in the water and is in essence a frog with a built-in five-and-a- half-foot cattle prod,” says Sussman. “Since all of the visceral organs are near the face, the remaining 90 percent of the fish is almost all electric organ.”

Electric fish have long fascinated humans. The ancient Egyptians used the torpedo, an electric marine ray, in an early form of electrotherapy to treat epilepsy. Much of what Benjamin Franklin and other pioneering scientists learned about electric- ity came from studies of electric fish. In Victorian times, parties were organized where guests would form a chain to experi- ence the shock of an electric fish.

All muscle cells have electrical potential. Simple contraction of a muscle will release a small

amount of voltage. But at least 100 million years ago some fish began to amplify that potential by evolv- ing from muscle cells another type of cell called an electrocyte—larger cells, organized in sequence and capable of generating much higher voltages than those used to make muscles work.

The “in-series alignment” of the electrocytes and the unique polarity of each cell allows for the “sum- mation of voltages, much like batteries stacked in series in a flashlight,” says Sussman.

In addition to sequencing and assembling DNA from the electric eel genome, the team produced protein sequences from the cells of the electric organs and skeletal muscles of three other electric fish lin- eages using RNA sequencing and analysis.

“I consider exotic organisms such as the electric fish to be one of nature’s wonders and an important gift to humanity,” says Sussman. “Our study dem- onstrates nature’s creative powers and its parsimony, using the same genetic and developmental tools to invent an adaptive trait time and again in widely disparate environments.”

And the findings may be useful to humans. “By learning how nature does this, we may be able to manipulate the process with muscle in other organ- isms and, in the near future, perhaps use the tools of synthetic biology to create electrocytes for generating electrical power in bionic devices within the human body or for uses we have not thought of yet,” says Sussman.

Sussman’s collaborators include Harold Zakon of the University of Texas at Austin and Manoj Samanta of the Systemix Institute in Redmond, Washington. The study was funded by the National Science Foundation, the W. M. Keck Foundation and the National Institutes of Health.