Investigating Real‐Time Internalization of Membrane Proteins Using a Novel Technology Based on Bioluminescence

The trafficking of membrane proteins including G‐protein coupled receptors (GPCRs) is an exciting area in pharmacology. Proteins, acting in a diverse array of physiological systems, can have differential signaling consequences depending on their subcellular localization. The objective of this work is the development of a universal method to study, under physiological conditions and in real‐time, the different membrane protein internalization processes. This novel method to measure membrane protein internalization will accelerate the drug discovery process for a wide variety of diseases. Our hypothesis is that receptor trafficking can represent a universal feature of membrane proteins and can be exploited to study receptor pharmacology as well as drug development. Methods A short domain of the human endofin was synthesized and subcloned into the four NanoBiT vectors. Cells were seeded in 96‐well plates at a density of 6×104 cells per well. The following day, cells were transfected using four different endofin‐receptor plasmid combinations. The combination with the highest signal was chosen for further experimentation. At 24 hr post‐transfection, the medium was aspirated and replaced with 100μl OPTIMEM (Life Technologies, Grand Island, NY, USA), which was then followed by the addition of 25μl substrate (furimazine). Luminescence measurements were taken once every minute for 10 min for signal stabilization. Finally, 10μl of ligand solution was added to each well, and luminescence measurements were immediately recorded. Three independent experiments were performed in triplicate, and each triplicate was averaged before calculating the s.e.m. Results In the quantitative pharmacological analysis of GPCRs, HEK293 cells were treated with increasing concentrations of ligand. The time course graph displays an increase in normalized luminescence over time with increasing ligand concentrations. The area under the curve analysis of this response demonstrates a clear concentration‐dependent response to b2AR stimulation. We also explored the pharmacological analysis of a non‐GPCR membrane protein, termed FAM19A5 Isoform II, and monitored its internalization by two monoclonal antibodies. Cells were treated with the antibodies A and B, with an EC50 value of 72.5±5nM for antibody A and 33.1±3nM for antibody B. Finally, we were also able to monitor virus entry in living cells via ACE2 internalization by binding with the SARS‐CoV2 Spike protein in HEK293 cells treated with lentiviruses expressing the SARS‐CoV2 Spike protein at their surface. Conclusions By using this novel approach we were able to characterize the trafficking of a wide variety of membrane proteins ranging from GPCRs to even antibody internalization and SARS‐CoV2 virus entry in real‐time living cells. We were also able to observe that internalization rates vary dramatically across different families of receptors, with the antibody‐mediated internalization having the slowest kinetics. Compared to other technologies where specialized microscopes and fluorophores are required, our approach represents a simple low‐cost universal assay to monitor the internalization of membrane proteins under physiological conditions.