Novel glycylated sugar alcohols protect ESC-specific microRNAs from degradation in iPS cells

Excessive accumulation of embryonic stem cell (ESC)-specific microRNAs occurs in both ESCs and induced pluripotent stem cells (iPSC); yet, the mechanism involved is unknown. In iPSCs, we for the first time found that novel glycylated sugar alcohols, particularly glycylglycerins, are tightly bound with ESC-specific microRNA precursors (pre-miRNA), such as pre-miR-302. Among these isolated glycylglycerins, we further identified that 1,3-diglycylglycerin and 1,2,3-triglycylglycerin are two major compounds bonded with negatively charged nucleic acids via electro-affinity and subsequently forming sugar-like coats in the hairpin-like double helix structures of pre-miRNAs. As a result, such glycylglycerin-formed coating serves as a protection layer against miRNA degradation. Moreover, we found that the pH value of iPSC cytosol determines the charges of these glycylglycerins. During iPSC differentiation, the cytosol pH is increased and hence neutralizes the charges of glycylglycerins, consequently leading to fast miRNA degradation. Therefore, the current findings not only explain how ESC-specific miRNAs are preserved and accumulated in iPSCs and ESCs but also demonstrate an important function of glycylglycerins in protecting the structural integrity of highly degradable miRNAs, providing a useful means for maintaining miRNA/siRNA function as well as developing the related RNA interference (RNAi) applications.