Production and Multi-Parameter Live Cell Fluorescence Lifetime Imaging Microscopy (FLIM) of Multicellular Spheroids

In this work, we present and compare spheroid formation methods that can be used for analysis of cell metabolism and oxygen distribution using live cell microscopy. In recent years, fluorescence lifetime imaging microscopy has been extensively used to investigate metabolic biomarkers like NADPH and FAD in live cells, and several fluorescent nanoparticles have been developed to multiplex such measurements with imaging of cell and tissue oxygenation. Multicellular spheroids, organoids, and organ-on-a-chip can replicate a complex, in-vivo-like microenvironment, minimizing the need in animal research.

For spheroid production, we show different formation methods, can span from low to high throughput. They also highlight their optical accessibility, compatibility with fluorescence lifetime imaging microscope, and the possibility of including extracellular matrix components. So, even though 3D in vitro models provide better context in comparison to 2D cultures, their high variability, low reproducibility, and incomplete experimental reporting remain a problem.

Parameters such as the spheroid size, the nutrient composition, the extracellular viscosity, and even spheroid formation methods can all lead to increased cellular heterogeneity. With this protocol, we aim to harmonize and standardize the spheroid production methods, highlighting key aspects important for life-continuous and multiparametric analysis of spheroids using FLIM microscopy.