Protocol for labeling and fixation of intact lysosomes with esterified amino acid analogs to assess lysosomal expansion in living eukaryotic cells

The lysosomal compartment is a key hub for cell metabolism, and morphological alterations have been described in several pathological conditions. Here, we describe the use of amino acid analogs modified by the presence of a methyl ester group that accumulates within lysosomes. This generates an intraluminal osmotic effect able to trigger a rapid and selective expansion of the lysosomal compartment within 2 h of treatment. We also present protocols to preserve lysosomal morphology, which yields a more accurate size measurement. For complete details on the use and execution of this protocol, please refer to Scerra et al. (2021). [Display omitted] •Methyl-group-modified amino acid analogs (LEU-ME) can localize to lysosomes•LEU-ME triggers a rapid expansion of the lysosomal compartment•Fixation procedure is a key step for preserving lysosomal morphology•Quantification and analysis of lysosomal dimensions The lysosomal compartment is a key hub for cell metabolism, and morphological alterations have been described in several pathological conditions. Here, we describe the use of amino acid analogs modified by the presence of a methyl ester group that accumulates within lysosomes. This generates an intraluminal osmotic effect able to trigger a rapid and selective expansion of the lysosomal compartment within 2 h of treatment. We also present protocols to preserve lysosomal morphology, which yields a more accurate size measurement.