Recent progress of in-cell NMR of nucleic acids in living human cells

The inside of living cells is highly crowded with biological macromolecules. It has long been considered that the properties of nucleic acids and proteins, such as their structures, dynamics, interactions, and enzymatic activities, in intracellular environments are different from those under in vitro dilute conditions. In-cell NMR is a robust and powerful method used in the direct measurement of those properties in living cells. However, until 2 years ago, in-cell NMR was limited to Xenopus laevis oocytes due to technical challenges of incorporating exogenous nucleic acids. In the last 2 years, in-cell NMR spectra of nucleic acid introduced into living human cells have been reported. By use of the in-cell NMR spectra of nucleic acids in living human cells, the formation of hairpin structures with Watson–Crick base pairs, and i-motif and G-quadruplex structures with non-Watson–Crick base pairs was demonstrated. Others investigated the mRNA–antisense drug interactions and DNA–small compound interactions. In this article, we review these studies to underscore the potential of in-cell NMR for addressing the structures, dynamics, and interactions of nucleic acids in living human cells.