*4.3. Immunofluorescence Microscopy*

Immunocytofluorescence staining was performed after 48 h of incubation with LPV or control DMSO, which corresponds to the time required for ST formation, as previously described [11]. Briefly, cells were fixed and permeabilized in methanol and blocked in Phosphate Buffered Saline (PBS) 1× Bovine Serum Albumine (BSA) 1%-Tween 0.1%. Cells were first incubated overnight at 4 ◦C with primary polyclonal rabbit antibody to desmoplakin (5 µg/mL, ab16434 abcam®, Cambridge, UK). They were then incubated for 1 h at room temperature and protected from light, with secondary goat anti-rabbit antibody conjugated with Alexa Fluor 488 (1/500, A-11008 LifeTechnologies, Carlsbad, CA, USA). Nuclei were stained with 40 ,6-diamidino-2-phenylindole (DAPI) and samples were conserved in mounted-medium. Pictures were taken using a BX60 epifluorescence microscope (Olympus) equipped with a 40× oil objective (Olympus 1.00), an ultrahigh-vacuum mercury lamp and a Hamamatsu camera (C4742-95) and analyzed with VisionStage Orca software (v 1.6). The "control immunoglobulin without specific epitope" conditions were used to evaluate the background signal and to set up the acquisition and colorization of pictures. Resulting pictures allowed us to calculate fusion index i.e., (nuclei number in ST-ST number)/total nuclei number. ST was considered when at least two nuclei were not separated by plasmic membrane, observed thanks to desmoplakin staining in green.
