Novel protein interaction network of human calcitonin receptor-like receptor revealed by label-free quantitative proteomics

Background: G protein-coupled receptor (GPCR) calcitonin receptor-like receptor (CLR) signalling is implicated in skin-related and cardiovascular diseases, migraine and cancer. However, beyond its agonists and receptor activity-modifying proteins (RAMPs), proteins which bind to CLR and define its properties in primary human cells remain insufficiently understood. Aim: We aimed to profile the CLR interactome in primary human dermal lymphatic endothelial cells (HDLEC), where this GPCR is expressed. Materials and methods: Immunoprecipitation (IP) of core- and terminally-glycosylated CLR from primary in vitro cultured HDLEC was conducted using rabbit polyclonal anti-human CLR serum (with pre-immune serum serving as a control) and confirmed by immunoblotting. Total HDLEC and co-immunoprecipitated CLR proteomes were analysed by label-free quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS). Quantitative in-situ proximity ligation assay (PLA) using ZEISS LSM 710 confocal microscope and ZEN Blue 3.0 and Image J software was performed to confirm LC-MS/MS findings. All experiments were repeated at least three times (biological replicates). For statistical analysis of PLA data, distribution was analysed using Shapiro-Wilk normality test followed by an unpaired t-test or Mann-Whitney test with a p-value of ≤0.05 interpreted as significant. For MS data of CLR IP samples, statistical analysis was performed using t-test with a permutation-based false discovery rate (FDR)-adjusted p-value of ≤0.006 interpreted as significant. Results: A total of 4,902 proteins were identified and quantified by LC-MS/MS in primary HDLEC and 46 were co-immunoprecipitated with CLR (p<0.006). Direct interaction with the GPCR was confirmed for five of these by PLA (p<0.01). Conclusions: This is the first study of its kind to identify novel binding partners of CLR expressed in primary human cells. Our integrative quantitative approach, combining immunoprecipitation of core- and terminally-glycosylated CLR, LC-MS/MS, and PLA, could be applied in a similar fashion to study its interactome in a variety of human cells and tissues, and its contribution to a range of diseases, where the role of this GPCR is implicated. ### Competing Interest Statement The authors have declared no competing interest.