Development of a NanoBRET-Based Sensitive Screening Method for CXCR4 Ligands.

A critical part of the development of CXCR4 modulators is to have a simple and sensitive assay system to complement the search by screening and evaluating the binding affinity. Herein, a NanoBRET assay system was developed, and its feasibility as a high-throughput screening tool for potent CXCR4 ligands was ascertained. TAMRA-Ac-TZ14011, a fluorescent-labeled CXCR4 antagonist, was adopted as a fluorescent acceptor of bioluminescent energy from N-terminally fused NanoLuc-CXCR4 stably expressed in CHO cells. The ratio of fluorescence at 620 nm to the luminescence at 460 nm represents the interaction between test compounds and CXCR4. We have demonstrated in the present study that the NanoBRET assay system is applicable for the evaluation of CXCR4 ligands using the combination of TAMRA-Ac-TZ14011 as an acceptor and NanoLuc tagged to CXCR4 as a bioluminescent donor expressed in living cells. IC50 values of known CXCR4 ligands were determined and found to be compatible with the values obtained by other existing and sensitive methods, such as SDF-1:3.2 nM, Ac-TZ14011:15.3 nM, and FC131:4.5 nM, which confirmed the feasibility of our system ( Z' values ≥0.5). The introduction of an IL-6 secretory signaling peptide (secNluc-CXCR4) further enhanced the expression and trafficking of the tagged receptor, which, in turn, increased the dynamic range of the NanoBRET assay system. Thus, we have successfully developed a NanoBRET system in living cells, which is simple, homogeneous, and useful in multiwell plate screening of potent CXCR4 ligands.