In 2006, Andre Nel proposed four generations of nanostructures: simple passive, simple active, multi-componented, and self-evolving. The present state of nanotechnology lies mostly within the second generation of active particles that can perform conceptually simple tasks such as releasing drugs in acidic environments (i.e. in a cancer microenvironment), walking along tracks of DNA, or detecting disease. Researchers at the University of Toronto now report in Science on a new shape-shifting nanostructure that can be toggled between two states to turn cell targeting on and off. This pushes the field towards the third generation of multi-componented systems that can be intentionally adjusted to do certain tasks.
The shape shifters are assembled from gold nanoparticles linked by DNA. DNA is often used in building nanoscale systems because of its inherent high-specificity in binding: two complementary strands bind each other only when their sequences align – you know, the whole A-T, G-C business you learned from high school biology. On top of that, the binding is reversible, allowing for release of a strand if another strand with the same sequence can outcompete it. In this new shape-shifting system, two gold nanoparticles (a large and a medium-sized sphere) were connected to each other, and a cloud of smaller particles surrounded either the large or medium particle. After adding in specific DNA strands, the cloud of small particles could be made to switch from being attached to the large particle, to being attached to the medium particle, and back, with yields of nearly 90%.
Now here’s the beauty. A targeting molecule, folic acid, was also attached to the big particle and initially hidden underneath the cloud of small particles. When the team added DNA to move the cloud off the big particle and onto the medium-sized particle, the folic became exposed and toggled the targeting function on. As such, more of these targeting-on particles were found inside cells that overexpressed the folate receptor. Inhibition studies with additional free folic acid in the cell culture reduced the uptake, suggesting that the receptor could be saturated.
“Biological molecules are really complicated, but made of simple parts,” said Dr. Warren Chan, principal investigator for the study. “There are only 20 essential amino acids, but they make up all of our proteins, which undergo shape changes to create function. Here, we also assembled simple components and showed that we can specifically control their conformations to change their behaviour.”
The Chan Lab, also known as the “Integrated Nanobiotechnology and Biomedical Sciences Laboratory” (INBS), has been working on nanoassembly with gold and DNA for many years. Medgadget reported on their previous study in Nature Nanotechnology, where the assembly was still a first-generation passive nanostructure.
“This [DNA assembly] field essentially came about in 1996. I was a grad student then and had been curious about shape-shifting structures, but we didn’t understand how to make nor test them at the time.”
Almost 20 years later, the time was ripe. Dr. Seiitchi Ohta and Dylan Glancy joined the lab and led the project. “They were smart and very organized and completed the project very quickly,” said Chan.
While this proof-of-concept has made a leap to show a modifiable DNA nanostructure, there’s still plenty to characterize and adapt to give it the targeting capabilities that it promises. For example, will it avoid liver sequestration in the body, a universal challenge in nanoparticle therapeutics? Strategies to deploy the particles in “stealth mode” and subsequent shape-change upon arrival at their target could be explored. The dream of having nanoscale robots doing automated tasks in the body is still many years away, but the field inches ever closer.