Casting a Wider Net: Whole-Cell Assays to Capture Varied and Biased Signaling

The observation of complex receptor behaviors has shown how ligands can have multiple efficacies and can also differentially stimulate certain cellular signaling pathways over others (i.e., biased signaling). Conventional pharmacological assays (usually proximal to the receptor) will detect ligands that produce the signal defined by the assay (i.e., Ca2+, cAMP, and others) but otherwise may miss biased ligands that produce little activation of pathways not measured by the assay. In theory, this is less of a hazard for generic whole-cell assays, which may be sensitive to multiple signaling inputs. Whole-cell assays have the advantage of detecting effects induced by a variety of receptor interactions with cytosolic proteins, including those that may be previously unknown. These ideas are discussed within the context of the high-throughput flow cytometry measurement of receptor internalization described by Wu et al. in the current issue of the journal (p. 645). The internalization of receptors can be a useful therapeutic modality and the article by Wu et al. illustrates how this new assay, targeted to downstream cellular effects, can uncover unique ligand efficacies linked to receptor internalization.