Detecting Material State Changes in the Nucleolus by Label-free Digital Holographic Microscopy

Abstract

Ribosome biogenesis is initiated in the nucleolus, a multiphase biomolecular condensate formed by liquid-liquid phase separation. The nucleolus is a powerful disease biomarker and stress biosensor whose morphology reflects its function. Here we have used digital holographic microscopy (DHM), a label-free quantitative phase contrast microscopy technique, to detect nucleoli in adherent and suspension cells. Subtle nucleolar alterations induced by drug treatment or by depletion of ribosomal proteins were efficiently detected by DHM. We trained convolutional neural networks to detect and quantify nucleoli automatically on DHM images of cultured human cells (HeLa). Holograms containing cell optical thickness information allowed us to define a novel nucleolar index which we used to distinguish nucleoli whose material state had been optogenetically modulated. We conclude that DHM is a powerful tool for quantitatively characterizing nucleoli, including material state, without any staining.