Distinct conformations of GPCR-ß-arrestin complexes mediate desensitization, signaling, and endocytosis.

ß-Arrestins (ßarrs) interact with G protein-coupled receptors (GPCRs) to desensitize G protein signaling, to initiate signaling on their own, and to mediate receptor endocytosis. Prior structural studies have revealed two unique conformations of GPCR-ßarr complexes: the "tail" conformation, with ßarr primarily coupled to the phosphorylated GPCR C-terminal tail, and the "core" conformation, where, in addition to the phosphorylated C-terminal tail, ßarr is further engaged with the receptor transmembrane core. However, the relationship of these distinct conformations to the various functions of ßarrs is unknown. Here, we created a mutant form of ßarr lacking the "finger-loop" region, which is unable to form the core conformation but retains the ability to form the tail conformation. We find that the tail conformation preserves the ability to mediate receptor internalization and ßarr signaling but not desensitization of G protein signaling. Thus, the two GPCR-ßarr conformations can carry out distinct functions.

Role of the V2R-ßarrestin-Gßγ complex in promoting G protein translocation to endosomes.

Classically, G protein-coupled receptors (GPCRs) promote signaling at the plasma membrane through activation of heterotrimeric Gαßγ proteins, followed by the recruitment of GPCR kinases and ßarrestin (ßarr) to initiate receptor desensitization and internalization. However, studies demonstrated that some GPCRs continue to signal from internalized compartments, with distinct cellular responses. Both ßarr and Gßγ contribute to such non-canonical endosomal G protein signaling, but their specific roles and contributions remain poorly understood. Here, we demonstrate that the vasopressin V2 receptor (V2R)-ßarr complex scaffolds Gßγ at the plasma membrane through a direct interaction with ßarr, enabling its transport to endosomes. Gßγ subsequently potentiates Gαs endosomal translocation, presumably to regenerate an endosomal pool of heterotrimeric Gs. This work shines light on the mechanism underlying G protein subunits translocation from the plasma membrane to the endosomes and provides a basis for understanding the role of ßarr in mediating sustained G protein signaling.

Uterine Arteriovenous Malformation (AVM) - a Potentially Life-Threatening Cause of Post-Partum Vaginal Bleeding.

BACKGROUND Uterine arteriovenous malformation (AVM) is a rare but potentially life-threatening medical condition. It is a congenital or acquired structural abnormality that may result in potentially life-threatening bleeding. Due to the nonspecific symptoms, this condition may be mistaken for more benign causes of vaginal bleeding, thus potentially leading to adverse outcomes and delay in diagnosis and treatment. Most cases of uterine AVM are acquired, and the post-partum period is an especially vulnerable time. CASE REPORT This is a case of a 26-year-old woman who presented to the Emergency Department with post-partum vaginal bleeding. During her evaluation, a uterine AVM was suspected based on Doppler ultrasound and was confirmed with computed tomography angiography. The patient was admitted to the hospital and treated with catheter embolization with complete resolution of bleeding and return to normal activities shortly after discharge. CONCLUSIONS This report describes a hemodynamically stable patient who presented to the Emergency Department with post-partum vaginal bleeding caused by a large uterine AVM. Despite her benign initial presentation clinically, she had a potentially life-threatening condition that could have resulted in significant morbidity if the diagnosis had been missed or delayed. It is important to maintain a high index of suspicion for even benign-appearing vaginal bleeding in the Emergency Department and to obtain the appropriate diagnostic studies to rule out potentially dangerous causes, especially in the setting of recent pregnancy or gynecologic instrumentation.

Signaling at the endosome: cryo-EM structure of a GPCR-G protein-beta-arrestin megacomplex.

G protein-coupled receptors (GPCRs) are a large class of cell-surface receptor involved in cellular signaling that are currently the target of over one third of all clinically approved therapeutics. Classically, an agonist-bound, active GPCR couples to and activates G proteins through the receptor intracellular core. To attenuate G protein signaling, the GPCR is phosphorylated at its C-terminal tail and/or relevant intracellular loops, allowing for the recruitment of ß-arrestins (ßarrs). ßarrs then couple to the receptor intracellular core in order to mediate receptor desensitization and internalization. However, our laboratory and others have observed that some GPCRs are capable of continuously signaling through G protein even after internalization. This mode of sustained signaling stands in contrast with our previous understanding of GPCR signaling, and its molecular mechanism is still not well understood. Recently, we have solved the structure of a GPCR-G protein-ßarr megacomplex by cryo-electron microscopy. This 'megaplex' structure illustrates the independent and simultaneous coupling of a G protein to the receptor intracellular core, and binding of a ßarr to a phosphorylated receptor C-terminal tail, with all three components maintaining their respective canonically active conformations. The structure provides evidence for the ability of a GPCR to activate G protein even while being bound to and internalized by ßarr. It also reveals that the binding of G protein and ßarr to the same GPCR is not mutually exclusive, and raises a number of future questions to be answered regarding the mechanism of sustained signaling.

Structure of an endosomal signaling GPCR-G protein-ß-arrestin megacomplex.

Classically, G-protein-coupled receptors (GPCRs) are thought to activate G protein from the plasma membrane and are subsequently desensitized by ß-arrestin (ß-arr). However, some GPCRs continue to signal through G protein from internalized compartments, mediated by a GPCR-G protein-ß-arr 'megaplex'. Nevertheless, the molecular architecture of the megaplex remains unknown. Here, we present its cryo-electron microscopy structure, which shows simultaneous engagement of human G protein and bovine ß-arr to the core and phosphorylated tail, respectively, of a single active human chimeric ß2-adrenergic receptor with the C-terminal tail of the arginine vasopressin type 2 receptor (ß2V2R). All three components adopt their canonical active conformations, suggesting that a single megaplex GPCR is capable of simultaneously activating G protein and ß-arr. Our findings provide a structural basis for GPCR-mediated sustained internalized G protein signaling.

HIV/sexually transmitted infections and intimate partner violence: Results from the Togo 2013-2014 Demographic and Health Survey.

Among clinic-based studies, intimate partner violence (IPV) has been shown to contribute to HIV/AIDS among young girls and women. Results from studies among the general population have been less consistent. This study evaluated the associations between HIV infection, any sexually transmitted infections (STIs), and IPV in a population-based sample of Togolese women. Data from the Togo 2013-2014 Demographic and Health Survey were utilized for these analyses. Women aged 15-49, who were currently married, had HIV test results, and answered the Domestic Violence Module were analyzed (n = 2386). Generalized linear mixed-models adjusting for sociodemographic variables, risk behaviors, and cluster effect were used to estimate HIV and STI risks with experience of IPV. HIV prevalence was 2.8%. Prevalence of IPV was 39% among HIV-positive women and 38% among HIV-negative women. Significant associations between IPV and HIV infection were not detected. Adjusted models found significant associations between experience of any IPV and having had STIs (OR 2.05, 95% CI 1.25-3.35). The high rates of violence in this setting warrant community-based interventions that address abuse and gender inequity. These interventions should also discuss the spectrum of STIs in relation to IPV.