Activatable CRISPR Transcriptional Circuits Generate Functional RNA for mRNA Sensing and Silencing

CRISPR-dCas9 systems that are precisely activated by cell-specific information facilitate the development of smart sensors or therapeutic strategies. We report the development of an activatable dCas9 transcriptional circuit that enables sensing and silencing of mRNA in living cells using hybridization-mediated structure switching for gRNA activation. The gRNA is designed with the spacer sequence blocked by a hairpin structure, and mRNA hybridization induces gRNA structure switching and activates the transcription of reporter RNA. An mRNA sensor developed using a light-up RNA reporter shows high sensitivity and fast-response imaging of survivin mRNA in cells under drug treatments and different cell lines. Furthermore, a feedback circuit is engineered by incorporating a small hairpin RNA in the reporter RNA, demonstrating a smart strategy for dynamic sensing and silencing of survivin with induced tumor cell apoptosis. This circuit illustrates a broadly applicable platform for the development of cell-specific sensing and therapeutic strategies.

Binary (Split) Light-up Aptameric Sensors

This Minireview discusses the design and applications of binary (also known as split) light-up aptameric sensors (BLAS). BLAS consist of two RNA or DNA strands and a fluorogenic organic dye added as a buffer component. When associated, the two strands form a dye-binding site, followed by an increase in fluorescence of the aptamer-bound dye. The design is cost-efficient because it uses short oligonucleotides and does not require conjugation of organic dyes with nucleic acids. In some applications, BLAS design is preferable over monolithic sensors because of simpler assay optimization and improved selectivity. RNA-based BLAS can be expressed in cells and used for the intracellular monitoring of biological molecules. BLAS have been used as reporters of nucleic acid association events in RNA nanotechnology and nucleic-acid-based molecular computation. Other applications of BLAS include the detection of nucleic acids, proteins, and cancer cells, and potentially they can be tailored to report a broad range of biological analytes.

Supramolecular Fluorescence Resonance Energy Transfer in Nucleobase-Modified Fluorogenic RNA Aptamers

RNA aptamers form compact tertiary structures and bind their ligands in specific binding sites. Fluorescence-based strategies reveal information on structure and dynamics of RNA aptamers. Herein, we report the incorporation of the universal emissive nucleobase analog 4-cyanoindole into the fluorogenic RNA aptamer Chili, and its application as a donor for supramolecular FRET to the bound ligands DMHBI+ or DMHBO+ . The photophysical properties of the new nucleobase-ligand-FRET pair revealed structural restraints for the overall RNA aptamer organization and identified nucleotide positions suitable for FRET-based readout of ligand binding. This strategy is generally suitable for binding-site mapping and may also be applied for responsive aptamer devices.