A programmable encapsulation system improves delivery of therapeutic bacteria in mice.

Living bacteria therapies have been proposed as an alternative approach to treating a broad array of cancers. In this study, we developed a genetically encoded microbial encapsulation system with tunable and dynamic expression of surface capsular polysaccharides that enhances systemic delivery. Based on a small RNA screen of capsular biosynthesis pathways, we constructed inducible synthetic gene circuits that regulate bacterial encapsulation in Escherichia coli Nissle 1917. These bacteria are capable of temporarily evading immune attack, whereas subsequent loss of encapsulation results in effective clearance in vivo. This dynamic delivery strategy enabled a ten-fold increase in maximum tolerated dose of bacteria and improved anti-tumor efficacy in murine models of cancer. Furthermore, in situ encapsulation increased the fraction of microbial translocation among mouse tumors, leading to efficacy in distal tumors. The programmable encapsulation system promises to enhance the therapeutic utility of living engineered bacteria for cancer.

Complex multicomponent patterns rendered on a 3D DNA-barrel pegboard.

DNA origami, in which a long scaffold strand is assembled with a many short staple strands into parallel arrays of double helices, has proven a powerful method for custom nanofabrication. However, currently the design and optimization of custom 3D DNA-origami shapes is a barrier to rapid application to new areas. Here we introduce a modular barrel architecture, and demonstrate hierarchical assembly of a 100 megadalton DNA-origami barrel of ~90 nm diameter and ~250 nm height, that provides a rhombic-lattice canvas of a thousand pixels each, with pitch of ~8 nm, on its inner and outer surfaces. Complex patterns rendered on these surfaces were resolved using up to twelve rounds of Exchange-PAINT super-resolution microscopy. We envision these structures as versatile nanoscale pegboards for applications requiring complex 3D arrangements of matter, which will serve to promote rapid uptake of this technology in diverse fields beyond specialist groups working in DNA nanotechnology.

Extrusion of RNA from a DNA-Origami-Based Nanofactory.

Cells often spatially organize biomolecules to regulate biological interactions. Synthetic mimicry of complex spatial organization may provide a route to similar levels of control for artificial systems. As a proof-of-principle, we constructed an RNA-extruding nanofactory using a DNA-origami barrel with an outer diameter of 60 nm as a chassis for integrated rolling-circle transcription and processing of RNA through spatial organization of DNA templates, RNA polymerases, and RNA endonucleases. The incorporation efficiency of molecular components was quantified to be roughly 50% on designed sites within the DNA-origami chassis. Each integrated nanofactory with RNA-producing units, composed of DNA templates and RNA polymerases, produced 100 copies of target RNA in 30 min on average. Further integration of RNA endonucleases that cleave rolling-circle transcripts from concatemers into monomers resulted in 30% processing efficiency. Disabling spatial organization of molecular components on DNA origami resulted in suppression of RNA production as well as processing.

Thermal cycling of DNA devices via associative strand displacement.

DNA-based devices often operate through a series of toehold-mediated strand-displacement reactions. To achieve cycling, fluidic mixing can be used to introduce 'recovery' strands to reset the system. However, such mixing can be cumbersome, non-robust, and wasteful of materials. Here we demonstrate mixing-free thermal cycling of DNA devices that operate through associative strand-displacement cascades. These cascades are favored at low temperatures due to the primacy of a net increase in base pairing, whereas rebinding of 'recovery' strands is favored at higher temperatures due to the primacy of a net release of strands. The temperature responses of the devices could be modulated by adjustment of design parameters such as the net increase of base pairs and the concentrations of strands. Degradation of function was not observable even after 500 thermal cycles. We experimentally demonstrated simple digital-logic circuits that evaluate at 35°C and reset after transient heating to 65°C. Thus associative strand displacement enables robust thermal cycling of DNA-based devices in a closed system.

Addressing the instability of DNA nanostructures in tissue culture.

DNA nanotechnology is an advanced technique that could contribute diagnostic, therapeutic, and biomedical research devices to nanomedicine. Although such devices are often developed and demonstrated using in vitro tissue culture models, these conditions may not be compatible with DNA nanostructure integrity and function. The purpose of this study was to characterize the sensitivity of 3D DNA nanostructures produced via the origami method to the in vitro tissue culture environment and identify solutions to prevent loss of nanostructure integrity. We examined whether the physiological cation concentrations of cell culture medium and the nucleases present in fetal bovine serum (FBS) used as a medium supplement result in denaturation and digestion, respectively. DNA nanostructure denaturation due to cation depletion was design- and time-dependent, with one of four tested designs remaining intact after 24 h at 37 °C. Adjustment of medium by addition of MgSO4 prevented denaturation. Digestion of nanostructures by FBS nucleases in Mg(2+)-adjusted medium did not appear design-dependent and became significant within 24 h and when medium was supplemented with greater than 5% FBS. We estimated that medium supplemented with 10% FBS contains greater than 256 U/L equivalent of DNase I activity in digestion of DNA nanostructures. Heat inactivation at 75 °C and inclusion of actin protein in medium inactivated and inhibited nuclease activity, respectively. We examined the impact of medium adjustments on cell growth, viability, and phenotype. Adjustment of Mg(2+) to 6 mM did not appear to have a detrimental impact on cells. Heat inactivation was found to be incompatible with in vitro tissue culture, whereas inclusion of actin had no observable effect on growth and viability. In two in vitro assays, immune cell activation and nanoparticle endocytosis, we show that using conditions compatible with cell phenotype and nanostructure integrity is critical for obtaining reliable experimental data. Our study thus describes considerations that are vital for researchers undertaking in vitro tissue culture studies with DNA nanostructures and some potential solutions for ensuring that nanostructure integrity and functions are maintained during experiments.

Cerenkov imaging - a new modality for molecular imaging.

Cerenkov luminescence imaging (CLI) is an emerging hybrid modality that utilizes the light emission from many commonly used medical isotopes. Cerenkov radiation (CR) is produced when charged particles travel through a dielectric medium faster than the speed of light in that medium. First described in detail nearly 100 years ago, CR has only recently applied for biomedical imaging purposes. The modality is of considerable interest as it enables the use of widespread luminescence imaging equipment to visualize clinical diagnostic (all PET radioisotopes) and many therapeutic radionuclides. The amount of light detected in CLI applications is significantly lower than other that in other optical imaging techniques such as bioluminescence and fluorescence. However, significant advantages include the use of approved radiotracers and lack of an incident light source, resulting in high signal to background ratios. As well, multiple subjects may be imaged concurrently (up to 5 in common bioluminescent equipment), conferring both cost and time benefits. This review summarizes the field of Cerenkov luminescence imaging to date. Applications of CLI discussed include intraoperative radionuclide-guided surgery, monitoring of therapeutic efficacy, tomographic optical imaging capabilities, and the ability to perform multiplexed imaging using fluorophores excited by the Cerenkov radiation. While technical challenges still exist, Cerenkov imaging has materialized as an important molecular imaging modality.