A high-throughput Galectin-9 imaging assay for quantifying nanoparticle uptake, endosomal escape and functional RNA delivery

Abstract RNA-based therapies have great potential to treat many undruggable human diseases. However, their efficacy, in particular for mRNA, remains hampered by poor cellular delivery and limited endosomal escape. Advances in the development and rational optimisation of delivery vectors, such as lipid nanoparticles (LNPs), are impeded by the limited availability of screening methods that probe the intracellular processing of LNPs in sufficient detail. We have developed a high-throughput imaging-based endosomal escape assay utilising a Galectin-9 reporter and fluorescently labelled mRNA to probe correlations between nanoparticle-mediated uptake, endosomal escape frequency, and mRNA translation. This assay has, furthermore, been integrated with a screening platform for nanoparticle formulation to optimise LNPs. We show that Galectin-9 recruitment is a robust, quantitative reporter of endosomal escape events induced by different mRNA delivery nanoparticles and small molecules. We also identify nanoparticles with superior escape properties and demonstrate significant cell line variances in endosomal escape response, highlighting the need for fine-tuning of delivery formulations for specific applications.