Inaugural Meeting of the European Research Network in Signal Transduction - GPCR Pharmacology: Activation, Signalling and Drug Design (U.K.) - Endosomal G protein signalling by US28 as a new target for glioblastoma multiforme

Glioblastoma Multiforme (GBM) is the most common and aggressive type of brain cancer. The human cytomegalovirus is detected in most GBM patient samples (50-90%) and encodes US28, a viral chemokine G protein-coupled receptor (GPCR). The constitutive activation (agonist-independent) of Gαq by US28 is particularly important as it induces important oncomodulatory properties such as cell inflammation, proliferation, and angiogenesis. Using bioluminescence resonance energy transfer (BRET)-based biosensors, we show that US28 constitutively activates all the Gα protein isoforms tested, but most importantly Gαq. Unlike typical viral GPCRs which are found in the plasma membrane, US28 is mainly localised in endosomes due to chronic internalisation. This poor expression at the plasma membrane is somehow surprising given the high constitutive activity associated to US28. However, since the last decade a growing number of GPCRs have been reported to activate Gα proteins from intracellular compartments such as endosomes upon GPCR internalisation. Using enhanced bystander BRET (EbBRET)-based biosensors, we show a gradual redistribution of Gα proteins, in particular Gαq, from the plasma membrane to early, fast-recycling and slow-recycling endosomes concomitant with a gradual increase of Gαq constitutive activity when US28 is progressively overexpressed. These results suggest that the important endosomal pool of US28 may be actively stimulating Gαq and the associated oncomodulatory signalling. Consequently, directly targeting a US28 blocker (inverse agonist or nanobody) to the endosomes may represent a promising therapeutic approach to successfully block GBM malignancy.