Endocrine Abstracts

The free fatty acid 4 receptor (FFA4R) is highly expressed in adipose tissue and other tissues that are involved in metabolic homeostasis, where its pharmacological stimulation improves glucose uptake and insulin sensitivity. Work by our and other groups have revealed that, contrary to textbook knowledge, GPCRs are not only active at the plasma membrane, as previously believed, but also at intracellular sites such as early endosomes or the Golgi complex/trans-Golgi network. However, the relevance of this phenomenon for signalling by FFA4R and other metabolically relevant GPCRs is presently largely unknown. Here, we used highly inclined and laminated optical sheet (HILO) microscopy and bioluminescence resonance energy transfer (BRET) to elucidate the relationship between FFA4R short isoform internalization, trafficking and signalling. Our results indicate that the FFA4R rapidly internalizes to early endosomes upon stimulation with the full agonist TUG-891. Subsequently, the FFA4R was found to recruit mini Gα_i/o protein probes to membranes of early endosomes, indicating that the FFA4R remains active in this compartment after internalization. Further experiments are under way to investigate the consequences of FFA4R endosomal signalling downstream of G protein activation and its implications in adipocyte metabolism. Understanding the mechanisms and relevance of endosomal signalling by FFA4R and other metabolic GPCRs could ultimately pave the way to novel therapeutic strategies for diabetes and other metabolic diseases.