Cells were fixed in 4% w/v paraformaldehyde and subjected to immunofluorescence microscopy as previously described [4 (link)]. The antibodies and their dilutions are listed in Supplementary Table 2. Secondary antibodies conjugated to Alexa-488 and Alexa-647 (Molecular Probes) were used for imaging. Nuclei were stained using 4′,6-diamidino-2-phenylindole (DAPI). In the experiments monitoring mitophagy, nucleus counterstaining was performed using Hoescht 33342 and lysossome staining was performed with Lysotracker Red and mitochondria with Mitotracker Green (Molecular Probes), as recommended by the manufacturer. In order to evaluate mito/autophagic flux, cells stimulated to differentiate into chondrocytes for 48h were exposed to 5 mM 3-methyladenine (3-MA, Santa Cruz, USA) or 100 μM chloroquine (CQ, Molecular Probes, USA) for 8 h before imaging (the drug loading control groups were exposed to equal quantities of the solvent DMSO). Imaging was performed using a Zeiss LSM 510-Meta laser scanning confocal microscope. Figures were prepared using ImageJ.