Quantitative Assessment of Receptor Internalization

Receptor internalization was quantitatively assessed using HaloTag technology (Promega) as described previously (4 (link)). HEK293T cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM, 5919, Nissui, Tokyo, Japan) supplemented with 10% fetal bovine serum. The cells were transfected with Halo-expressing vector and labeled with the cell-impermeable Alexa Fluor 488 ligand (Promega) in Opti-MEM for 15 min at 37°C. Each inhibitor or antagonist pretreatment was for 30 min. The cells were then treated with 1 µM PACAP, 5-HT or saline, washed with phosphate-buffered saline and fixed in 4% paraformaldehyde. Cells were imaged using an FV1000D confocal microscope (Olympus, Tokyo, Japan) in sequential mode and membrane protein internalization was quantified using ImageJ software (NIH, MD, USA). To assess the internalization ratio, we defined the shape of a whole-cell (region of interest, ROI, A) and its cytoplasmic region (ROI B) by reducing the size by 5–10 pixels and then determining the fluorescence in both ROIs. The internalization ratio (%) was defined by dividing the amount of luminescence in ROI B by that in ROI A.