Discovered forty years ago but underappreciated for decades, the paradigm of biased agonism has now become standard in G protein-coupled receptor (GPCR) pharmacology and drug discovery. Biased agonism refers to the phenomenon where different agonists binding to the same receptor results in differential activation of signaling pathways downstream of the receptor. Biased agonists may have distinct physiological effects from the endogenous agonist or antagonists of a receptor, leading to unique pharmacological profiles that could serve as more effective drugs. This phenomenon is in part due to the stabilization of different ensembles of agonist:GPCR complexes leading to distinct transducer coupling. The delay in the appreciation of biased agonism likely reflects our prior inability to monitor different transducers and signaling pathways downstream of the receptor, as well as a lack of chemical diversity amongst tested agonists. Now, with a wealth of agonist:receptor:transducer structures, biased agonism is not the exception to GPCR pharmacology, but the rule. GPCR biased signaling is encoded throughout a diversity of chemical space, from structurally distinct orthosteric agonists as well as allosteric modulators, which can engage receptors in distinct binding modes. Here, we review our current understanding of biased agonism, from pharmacological, mechanistic and structural perspectives, and the current and potential impact of biased agonism on the development of new therapeutics.
G protein–coupled receptors (GPCRs) are best known for initiating signaling at the plasma membrane, yet a substantial and functionally distinct subset of these receptors localize to the primary cilium, a highly specialized cell-surface organelle. Over the past decade, the primary cilium has emerged as a privileged signaling compartment that concentrates GPCRs, signaling effectors, and second messengers within a physically constrained and biochemically unique environment. In this review, we synthesize current knowledge of GPCR signaling from a primary cilium perspective, with emphasis on receptor repertoire, trafficking mechanisms, and compartment-specific signal transduction. We discuss how ciliary entry and exit are regulated by dedicated transport systems, how canonical GPCR signaling components - G proteins, GRKs, β-arrestins, RGS proteins, and AKAPs - are organized within cilia, and how the ciliary geometry, membrane composition, voltage, pH, and cytoskeletal architecture shape signaling outcomes. We further highlight how cilia support autonomous and highly sensitive microdomains of cAMP and Ca2+ signaling, enabling integration and hierarchical control of multiple GPCR inputs. Finally, we examine the physiological consequences of ciliary GPCR signaling in neural, renal, adipose, and pancreatic, and developmental tissues, where defects in ciliary signaling contribute to metabolic disease, impaired morphogenesis and tissue homeostasis, and ciliopathies – a range of human disorders arising from primary cilium dysfunction. Together, these studies establish the primary cilium as a dynamic and modular GPCR signaling hub and underscore the importance of subcellular localization in determining receptor function and signaling specificity.
The critical involvement of hepatic transporters in both physiological and pathological processes, including metabolic diseases, drug metabolism and toxicity, represent key issues in pharmacology. The expression and localization of transporters involved in hepatic functions are especially important in new drug development and toxicology studies. The value of animal experiments, because of major specific differences in transporter expression, localization and substrate recognition, is limited in this regard, and human liver cell preparations are already available to allow relevant in vitro studies. Currently the gold standard for in vitro studies is the primary human liver cell preparation. However, the inter-individual heterogeneity, low reproducibility and high price of such preparations are prohibiting in their wide-spread use. Hepatocytes derived from liver tumors are widely used in toxicology studies, while each of these cell lines has specific advantages and shortcomings. Stem cell-based liver cell preparations, especially spheroids and organoids derived from human induced pluripotent stem cells, may serve as a new toolbox for studying a variety of membrane transporters and complex metabolic interactions in human hepatocytes. These advanced preparations, while currently somewhat premature, should provide an excellent basis for comprehensive in vitro functional assays. The present paper provides a comparative review of our current understanding of the pharmacological relevance of hepatic transporters, their interactions with the key players of general drug metabolism, their complex regulation, as well as the recent methodologies to apply human-relevant in vitro systems in drug development and toxicology studies.
