Programmable, target-induced fluorogenic CRISPR-tDeg platform for live-cell RNA visualization

RNA molecules display remarkable heterogeneity in structure, dynamics, and function, yet methods for their precise visualization in living cells remain limited. While CRISPR-based RNA imaging holds great potential, existing systems often suffer from high background fluorescence due to constitutive signal emission or non-specific binding. To overcome these challenges, we developed CtDeg (CRISPR-dCas13-tDeg), a modular RNA imaging platform that links fluorescence activation directly to target RNA recognition while leveraging degron-mediated degradation to suppress background signals. By engineering the crRNA scaffold to embed the Pepper RNA motif, CtDeg ensures that fluorescence is present only upon binding to the native RNA target. We systematically optimized C-terminal tDeg variants to maximize the signal-to-noise ratio and demonstrated that CtDeg achieves substantially lower background and higher specificity than conventional fluorescent protein-CRISPR-based RNA imaging approaches. Using CtDeg, we captured real-time paraspeckle assembly dynamics and visualized early-stage SARS-CoV-2 genomic RNA transport. Remarkably, CtDeg provided the first direct imaging evidence of virus-induced NEAT1_2 lncRNA accumulation, revealing a host-virus regulatory interaction. Beyond these applications, CtDeg is compatible with multiple Cas13 orthologs and fluorescent proteins, establishing a versatile, target-induced platform for probing RNA localization, dynamics, and function in living cells, with broad applications in synthetic biology and RNA biology.