In situ analysis of single-stranded and duplex siRNA integrity in living cells.

To attain the full therapeutic promise of short interfering RNA (siRNA), it is believed that improvements such as increased biostability are critical. Regrettably, thus far, insufficient in situ data are on hand regarding the intracellular stability of siRNAs. We report on the use of an advanced fluorescence-based method to probe the nucleolytic decay of double labeled siRNAs, which are subject to fluorescence resonance energy transfer (FRET). In vitro measurements with RNAse A and cellular extracts demonstrate that the ratio of acceptor (5'-Cy5) to donor (3'-rhodamine green) fluorescence can be used to study the degradation of the labeled siRNA substrates upon donor excitation. Intracellular FRET analysis showed substantial degradation of single-stranded siRNA, whereas duplex siRNA stayed intact during the measured time period. These data underline the high intrinsic nuclease resistance of unmodified duplex siRNA and prove that cellular persistence is much more critical for the single-stranded structure. For the first time, the stability of siRNA is investigated in real-time inside living cells. The fluorescence-based method presented here is a straightforward technique to gain direct information on siRNA integrity inside living cells and provides a bright outlook to learn more about the intracellular fate of siRNA therapeutics.