Optimizing miRNA transfection for screening in precision cut lung slices

Precision cut lung slices (PCLS) are complex three-dimensional (3-D) lung tissue models, which preserve the native microenvironment, including cell diversity and cell-matrix interactions. They are an innovative ex vivo platform that allows studying disease as well as the effects of therapeutic agents or regulatory molecules [e.g., microRNA (miRNA)]. The aim of our study was to develop a protocol to transfect PCLS with miRNA using lipid nanoparticles (LNPs) to enable higher throughput screening of miRNA, obviating the need for custom stabilization and internalization approaches. PCLS of 4 mm diameter were generated using agarose-filled rodent lungs and a vibratome. TYE665-labeled scrambled miRNA was used to evaluate transfection efficacy of six different commercially available LNPs. Transfection efficacy was visualized using live high-content fluorescence microscopy, followed by higher-resolution confocal fluorescence microscopy in fixed PCLS. Metabolic activity and cellular damage were assessed using water-soluble tetrazolium salt (WST-1) and lactate dehydrogenase (LDH) release. Using a live staining kit containing a cell membrane impermeant nuclear dye, RedDot2, we established that cellular membranes in PCLS are permeable in the initial 24 h of slicing but diminished thereafter. Therefore, all transfection experiments occurred at least 24 h after slicing. All six commercially available LNPs enabled transfection without inducing significant cytotoxicity or impaired metabolic function. However, RNAiMAX and INTERFERin led to increases in transfection efficacy as compared with other LNPs, with detection possible as low as 25 nM. Therefore, LNP-based transfection of miRNA is possible and can be visualized in live or fixed PCLS, enabling future higher throughput studies using diverse miRNAs.