DNA-responsive polymer gels used for releasing drugs, encapsulating cells, and much more now have greater adaptability thanks to the Cas12a nuclease.
Ruth Williams
Dec 1, 2019
DNA is more than just a genetic molecule. Its physical structure and predictable behavior also make it a versatile biological building material. Indeed, DNA has been used to create nanoscale robots, patterns, and 3-D structures for various purposes, and it has been incorporated into hydrophilic polymer gels (hydrogels) for a variety of innovative applications, including biosensing, drug delivery, and more.
But such gels have limited versatility, says Max English, a graduate student in the laboratory of MIT bioengineer Jim Collins. Often, DNA-containing gels are designed with strands that are complementary to the intended DNA activators. This means that “whenever you want to design a material that responds to a different [DNA] cue, you have to redesign the material in its entirety,” English explains.
To avoid such overhauls, English and colleagues created a system for making DNA-containing gels that are capable of responding to nearly any DNA cue simply by providing the CRISPR system’s Cas12a nuclease and a guide RNA (gRNA) that matches the desired DNA trigger. The team exploited a feature of Cas12a called collateral cleavage, in which the enzyme, after cutting its target double-stranded (ds) DNA, nonspecifically chops up surrounding single-stranded (ss) DNAs. The hydrogels are thus fabricated with ssDNAs that are cleaved by Cas12a when, and only when, a given gRNA and dsDNA combination is present.
Using this principle, the team created DNA-containing hydrogels that, in response to a dsDNA cue provided by the researchers, could either release DNA-bound compounds or fully degrade. Such degradation could be used for applications such as liberating encapsulated contents like cells or nanoparticles, initiating flow of a buffer through a microfluidic device, or opening an electrical circuit. These last two examples could potentially be used in diagnostic devices, says Collins, with a change in buffer flow or electrical output signaling the presence of a DNA sequence of interest in a patient sample.
“They showed some really novel applications of responsive hydrogels,” says Rebecca Schulman, a chemical and biomolecular engineer at Johns Hopkins University who did not participate in the study, in an email to The Scientist.
“Their approach is totally customizable . . . [and] is really cleverly designed,” adds bio-engineer Dan Luo of Cornell University who was not involved in the research. “It’s a real integration of molecular biology and materials science.” (Science, 365:780–85, 2019)
Hydrogel Activation | How it works | Sensitivity | Versatility |
Via complementary strands | A gel that contains cross-linked DNA strands can be dissolved or expanded when a complementary DNA strand binds and either displaces or extends the crosslinks. | Low, because each molecule of activating DNA targets just one cross-linked strand | Limited. Each gel must be redesigned to match each input DNA. |
Via CRISPR-based system |
The Cas12a nuclease cuts ssDNAs within hydrogels when given a particular dsDNA cue along with a matching gRNA. |
High, because Cas12 cuts multiple ssDNAs within the gel for each dsDNA molecule. |
High. Each gel can be specifically activated by nearly any gRNA/dsDNA combination. |
Ruth Williams is a freelance journalist based in Connecticut. Email her at ruth@wordsbyruth.com or find her on Twitter @rooph.