Alteration in the intracellular Na+ i/K+ i ratio triggers gene expression and G-quadruplex dynamics in the nuclei of living cells

Monovalent metal cations are crucial for the formation of DNA secondary structures G-quadruplexes (G4s), which are abundant in gene promoters. Various physiological and pathological conditions are associated with the Na + i /K + i ratio imbalance and gene expression alterations. However, it remains unclear whether G4s are involved in Na + i /K + i -dependent gene expression regulation. We altered the intracellular Na + i /K + i ratio in HeLa cells by inhibiting Na,K-ATPase with specific inhibitor ouabain or by loading the cells with Li + . The intracellular content of monovalent metal cations was assessed using atomic absorption spectrometry. The effect of monovalent metal cation imbalance on the mRNA levels of the FOS , JUN , ZFP36 , EGR1 , PTGS2 , KIT , MYC , and NFAT5 genes was evaluated using real-time polymerase chain reaction. The influence of varying Na + , K + , and Li + concentrations on the topology of G4 of the FOS , KIT , and MYC gene promoters was measured using circular dichroism spectroscopy. G4 dynamics in the nuclei of living cells was studied using fluorescence lifetime imaging microscopy. In this study, we demonstrate that both the replacement of Na + i and K + i with Li + i and the elevation of the Na + i /K + i ratio affect several early response genes expression. We observed that the topology of G4s of the FOS , KIT , and MYС promoters is affected by the ratio of Na + , K + , Li + in vitro, which correlates with the monovalent metal cation-dependent expression pattern of these genes. Besides that, monovalent metal cations perturbations were accompanied with alterations in intranuclear G4 dynamics in cellulo. Our results suggest that G4s may function as intracellular sensors of the Na + i /K + i ratio and switch their topology, thereby regulating gene expression.