G-protein-coupled receptors (GPCRs) comprise the largest and most pharmacologically important family of cellsurface receptors encoded by the human genome. In many instances, the distinct signaling behavior of certain GPCRs has been explained in terms of the formation of heteromers with, for example, distinct signaling properties and allosteric crossregulation. Confirmation of this has, however, been limited by the paucity of reliable methods for probing heteromeric GPCR interactions in situ. The most widely used assays for GPCR stoichiometry, based on resonance energy transfer, are unsuited to reporting heteromeric interactions. Here, we describe a targeted bioluminescence resonance energy transfer (BRET) assay, called type-4 BRET, which detects both homo-and heteromeric interactions using induced multimerization of protomers within such complexes, at constant expression. Using type-4 BRET assays, we investigate heterodimerization among known GPCR homodimers: the CXC chemokine receptor 4 and sphingosine-1-phosphate receptors. We observe that CXC chemokine receptor 4 and sphingosine-1-phosphate receptors can form heterodimers with GPCRs from their immediate subfamilies but not with more distantly related receptors. We also show that heterodimerization appears to disrupt homodimeric interactions, suggesting the sharing of interfaces. Broadly, these observations indicate that heterodimerization results from the divergence of homodimeric receptors and will therefore likely be restricted to closely related homodimeric GPCRs.

A case note audit was undertaken of HIV-positive men who have sex with men (MSM) to ascertain whether national guidelines for taking sexual histories, including recreational drug use and sexually transmitted infection (STI) screening were being met. The notes of 142 HIV-positive men seen in 2015 were available, of whom 85 were MSM. Information was collected regarding sexual history, recreational drug use documentation, sexually transmitted infection screen offer and test results. Seventy-seven (91%) of the MSM had a sexual history documented, of whom 60 (78%) were sexually active. STI screens were offered to 58/60 (97%) of those who were sexually active and accepted by 53 (91%). Twelve (23%) of these had an STI. A recreational drug history was taken in 63 (74%) with 17 (27%) reporting use and 3 (5%) chemsex. The high rate of STIs highlights that regular screening in this group is essential. Additionally, the fact that over a quarter reported recreational drug use and given the increasing concern around chemsex, questions about this should be incorporated into the sexual history proforma.

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