Fluorescent Visualization of Nucleolar G-Quadruplex RNA and Dynamics of Cytoplasm and Intranuclear Viscosity

The nucleolus,the locus of ribosome biogenesis,was found to be the predominant intracellular target of a new fluorescent probe,V-P1.In solution,the probe demonstrated both a selectivity to RNA G-quadruplexes and a sensitivity to the viscosity,while G-quadruplex binding did not disturb the viscosity sensing.In cells,confocal and fluorescence lifetime imaging,combined with digestion and competition experiments,lent support to the hypothesis of an RNA-based G-quadruplex as the intracellular target,postulated to be nucleolar ribosomal RNA(rRNA).The probe demonstrated a high sensitivity to viscosity in both the cytoplasm and the nuclear compartment and was used to precisely interrogate the viscosity changes resulting from diverse stimuli,such as temperature,monensin treatment,and etoposide-induced apoptosis.Owing to the putative rRNA G-quadruplex binding in vitro and in vivo,and further combined with a relatively low degree of toxicity,the dye enabled the interrogation of cytoplasm and intranuclear viscosity changes under diverse conditions and found applications in studying the influence and significance of cytoplasm and intranuclear viscosity as well as in gaining insight into the native secondary structure of rRNA in nucleoli.