Real-Time Investigation of Intracellular Polynucleotide Kinase Using a Cascaded Amplification Circuit

Polynucleotide kinase (PNK) shows an in-depth correlationship with DNA repair and metabolism processes. The in situ visualization of intracellular PNK revealed an extremely biological significance in supplementing reliable and quantitative information on its spatiotemporal distribution in live cells. Herein, we developed a versatile cascaded DNA amplification circuit through the integration of catalytic DNA assembly and hybridization chain reaction circuits and realized the accurate evaluation of intracellular PNK activity via the Förster resonance energy transfer (FRET) principle. Initially, without PNK, trigger T was firmly caged in the PNK-recognizing hairpin H, resulting in no disturbance of the concatenated circuit. However, with the introduction of PNK, the 5'-OH terminal of PNK-addressing H was phosphorylated, then the phosphorylated H could be subsequently digested by λ exonuclease (λ Exo) to produce trigger T of the cascaded DNA circuit. As a result, the integrated circuit was stimulated to produce an amplified FRET signal for quantitatively monitoring the activity of PNK. Due to the λ Exo-specific digestion of 5'-phosphate DNA and the high signal gain of the cascade circuit, our proposed strategy enables the sensitive analysis of PNK activity in vitro and in complex biological samples. Furthermore, our PNK-sensing platform was extensively explored in HeLa cells for realizing reliable intracellular PNK imaging and thus showed high potential in the future diagnosis and treatment of kinase-related diseases.