A tool for imaging targeted RNAs in living human cells is reported online this week in Nature Methods. This improved version of an RNA-based probe, known as Spinach2, broadens the range of RNAs that can be labelled to reveal the dynamic localization of disease-associated ‘toxic RNAs’.

Spinach is a designer RNA sequence that can bind to a small synthetic molecule and cause it to fluoresce green. The sequence for Spinach can, in principle, be attached to any normally occurring cellular RNA in order to label it with fluorescence and track its location in the cell. But in practice, attaching it to an RNA leads to very weak fluorescence due to poor folding and instability, limiting its usefulness as a probe.

Samie Jaffrey and colleagues engineered Spinach2 to improve on the original sequence’s folding and stability, allowing a broader range of RNAs to be imaged. As a demonstration, the authors use Spinach2 to label GCC RNA sequence repeats, which form toxic aggregates linked to fragile X?associated tremor and ataxia syndrome, a disease that affects muscle movement, coordination and cognition. The ability to live-image these labelled RNA molecules allowed the team to identify tautomycin as the first drug that can disrupt existing aggregates in the nucleus.