Location bias contributes to functionally selective responses of biased CXCR3 agonists

Some G protein-coupled receptor (GPCR) ligands act as biased agonists which preferentially activate specific signaling transducers over others. Although GPCRs are primarily found at the plasma membrane, GPCRs can traffic to and signal from many subcellular compartments. Here, we determine that differential subcellular signaling contributes to the biased signaling generated by three endogenous ligands of the chemokine GPCR CXCR3. The signaling profile of CXCR3 changed as it trafficked from the plasma membrane to endosomes in a ligand-specific manner. Endosomal signaling was critical for biased activation of G proteins, β-arrestins, and ERK1/2. In CD8+ T cells, the chemokines promoted unique transcriptional responses predicted to regulate inflammatory pathways. In a mouse model of contact hypersensitivity, β-arrestin-biased CXCR3-mediated inflammation was dependent on receptor internalization. Our work demonstrates that differential subcellular signaling is critical to the overall biased response observed at CXCR3, which has important implications for drugs targeting chemokine receptors and other GPCRs.

Zika Virus Enters Soma of Neuron through NGF/TrkA-Like Endosomal Signaling Pathway

Infection with the Zika virus results in severe neurological disease in adults or congenital Zika syndrome in newborns. We employed the domain search strategy to study the Zika virus glycoprotein E in this work. The results revealed that immature E contains a NGF domain (“MNKCYIQIMDLGHMCDATMSYECPMLDEGVEPDDVDCWCNTTSTWVVYGTCHH”) and is capable of interacting with TrkA. The E/TrkA complex increased E's interaction with receptors such as Axl and facilitated Zika virus endocytosis via clathrin. Rab5 retrograded transmission of Zika virus-containing E/TrkA endosomal signals to neuronal soma. Rab7 helped dissociation of E/TrkA in late acidic endosomes, and then E became mature after the NGF domain was cut. After membrane fusion with the endosome, the Zika virus was released into the neuron cell body. It showed only the immature E protein of Zika had NGF activity. The retrograde trafficking of endosomal signals (E/TrkA) similar to NGF/TrkA enabled Zika virus to infect neuronal cells. E's interference with the TrkA signal impaired neuronal cell growth and results in neuronal cell apoptosis.

Glucagon Receptor-mediated Regulation of Gluconeogenic Gene Transcription is Endocytosis-dependent in Primary Hepatocytes

A number of G protein-coupled receptors (GPCRs) are now thought to use endocytosis to promote cellular cAMP signaling that drives downstream transcription of cAMP-dependent genes. We tested if this is true for the Glucagon Receptor (GCGR), which mediates physiological regulation of hepatic glucose metabolism via cAMP signaling. We show that epitope-tagged GCGRs undergo clathrin and dynamin-dependent endocytosis in HEK293 cells after activation by glucagon, and transit via EEA1-marked endosomes shown previously to be sites of GPCR/Gs-stimulated production of cAMP. We further show that endocytosis potentiates cytoplasmic cAMP elevation produced by GCGR activation and promotes transcription of PCK1, the gene which encodes the enzyme catalyzing the rate-limiting step in gluconeogenesis. We verify endocytosis-dependent induction of PCK1 expression by endogenous GCGRs in primary hepatocytes, and show similar control of two other gluconeogenic genes (PGC1α and G6PC). Together, these results implicate the endosomal signaling paradigm in metabolic regulation by glucagon.

High affinity FcγR activating function depends on IRAP+ endosomal-signaling platforms

Although endocytosis of cell surface receptors is generally thought to terminate the signaling, for some receptors, endocytosis sustains signaling. We wondered if endosomal signaling participates to the function of the receptors for Fc immunoglobulin fragments (FcRs) that are highly internalized after their activation. We demonstrate here that four different FcRs follow distinct endocytic pathways after activation. While FcαRI is internalized into lysosomes, FcγRIIA is internalized and partially retained in early endosomes, whereas the inhibitory receptor FcγRIIB is internalized in endosomes decorated by the autophagy marker LC3. Interestingly, the high affinity FcγRI is internalized in specialized endosomal compartments described by the Insulin Responsive AminoPeptidase (IRAP), where it remains associated with the active form of the signaling kinase Syk. Our results show that FcγRI has the ability to build endosomal-signaling platforms, which depend on the presence of IRAP and Rab14. Destabilization of the endosomal signaling platforms compromised the ability of peritoneal macrophages to kill tumor cells by antibody-dependent cell mediated cytotoxicity, indicating that FcγRI endosomal signaling is required for the therapeutic action of anti-tumor monoclonal antibodies.

Class II phosphatidylinositol 3-kinase isoforms in vesicular trafficking

Phosphatidylinositol 3-kinases (PI3Ks) are critical regulators of many cellular processes including cell survival, proliferation, migration, cytoskeletal reorganization, and intracellular vesicular trafficking. They are a family of lipid kinases that phosphorylate membrane phosphoinositide lipids at the 3′ position of their inositol rings, and in mammals they are divided into three classes. The role of the class III PI3K Vps34 is well-established, but recent evidence suggests the physiological significance of class II PI3K isoforms in vesicular trafficking. This review focuses on the recently discovered functions of the distinct PI3K-C2α and PI3K-C2β class II PI3K isoforms in clathrin-mediated endocytosis and consequent endosomal signaling, and discusses recently reported data on class II PI3K isoforms in different physiological contexts in comparison with class I and III isoforms.

Endosomal cAMP production broadly impacts the cellular phosphoproteome

Endosomal signaling from G protein-coupled receptors (GPCRs) has emerged as a novel paradigm with important pharmacological and physiological implications. Yet, our knowledge of the functional consequences of activating intracellular GPCRs is incomplete. To address this gap, we combined an optogenetic approach for site-specific generation of the prototypical second messenger cyclic AMP (cAMP) with unbiased mass spectrometry-based analysis of phosphoproteomic effects. We identified 218 unique, high-confidence sites whose phosphorylation is either increased or decreased in response to cAMP production. We next determined that cAMP produced from endosomes led to more robust changes in phosphorylation than cAMP produced from the plasma membrane. Remarkably, this was true for the entire repertoire of identified targets, and irrespective of their annotated sub-cellular localization. Furthermore, we identified a particularly strong endosome bias for a subset of proteins that are dephosphorylated in response to cAMP. Through bioinformatics analysis, we established these targets as putative substrates for protein phosphatase 2A (PP2A), and we propose compartmentalized activation of PP2A-B56δ as the likely underlying mechanism. Altogether, our study extends the concept that endosomal signaling is a significant functional contributor to cellular responsiveness by establishing a unique role for localized cAMP production in defining categorically distinct phosphoresponses.

Pharmacological Programming of Endosomal Signaling Activated by Small Molecule Ligands of the Follicle Stimulating Hormone Receptor

Follicle-stimulating hormone receptor (FSHR) is a G protein-coupled receptor (GPCR) with pivotal roles in reproduction. One key mechanism dictating the signal activity of GPCRs is membrane trafficking. After binding its hormone FSH, FSHR undergoes internalization to very early endosomes (VEEs) for its acute signaling and sorting to a rapid recycling pathway. The VEE is a heterogeneous compartment containing the Adaptor Protein Phosphotyrosine Interacting with Pleckstrin homology Domain and Leucine Zipper 1 (APPL1) with distinct functions in regulating endosomal Gαs/cAMP signaling and rapid recycling. Low molecular weight (LMW) allosteric FSHR ligands were developed for use in assisted reproductive technology yet could also provide novel pharmacological tools to study FSHR. Given the critical nature of receptor internalization and endosomal signaling for FSHR activity, we assessed whether these compounds exhibit differential abilities to alter receptor endosomal trafficking and signaling within the VEE. Two chemically distinct LMW agonists (benzamide, termed B3 and thiazolidinone, termed T1) were employed. T1 was able to induce a greater level of cAMP than FSH and B3. As cAMP signaling drives gonadotrophin hormone receptor recycling, rapid exocytic events were evaluated at single event resolution. Strikingly, T1 was able to induce a 3-fold increase in recycling events compared to FSH and two-fold more compared to B3. As T1-induced internalization was only marginally greater, the dramatic increase in recycling and cAMP signaling may be due to additional mechanisms. All compounds exhibited a similar requirement for receptor internalization to increase cAMP and proportion of FSHR endosomes with active Gαs, suggesting regulation of cAMP signaling induced by T1 may be altered. APPL1 plays a central role for GPCRs targeted to the VEE, and indeed, loss of APPL1 inhibited FSH-induced recycling and increased endosomal cAMP signaling. While T1-induced FSHR recycling was APPL1-dependent, its elevated cAMP signaling was only partially increased following APPL1 knockdown. Unexpectedly, B3 altered the dependence of FSHR to APPL1 in an opposing manner, whereby its endosomal signaling was negatively regulated by APPL1, while B3-induced FSHR recycling was APPL1-independent. Overall, FSHR allosteric compounds have the potential to re-program FSHR activity via altering engagement with VEE machinery and also suggests that these two distinct functions of APPL1 can potentially be selected pharmacologically.

Endosomal signaling of delta opioid receptors is an endogenous mechanism and therapeutic target for relief from inflammatory pain

Whether G protein-coupled receptors signal from endosomes to control important pathophysiological processes and are therapeutic targets is uncertain. We report that opioids from the inflamed colon activate δ-opioid receptors (DOPr) in endosomes of nociceptors. Biopsy samples of inflamed colonic mucosa from patients and mice with colitis released opioids that activated DOPr on nociceptors to cause a sustained decrease in excitability. DOPr agonists inhibited mechanically sensitive colonic nociceptors. DOPr endocytosis and endosomal signaling by protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) pathways mediated the sustained inhibitory actions of endogenous opioids and DOPr agonists. DOPr agonists stimulated the recruitment of Gαi/oand β-arrestin1/2 to endosomes. Analysis of compartmentalized signaling revealed a requirement of DOPr endocytosis for activation of PKC at the plasma membrane and in the cytosol and ERK in the nucleus. We explored a nanoparticle delivery strategy to evaluate whether endosomal DOPr might be a therapeutic target for pain. The DOPr agonist DADLE was coupled to a liposome shell for targeting DOPr-positive nociceptors and incorporated into a mesoporous silica core for release in the acidic and reducing endosomal environment. Nanoparticles activated DOPr at the plasma membrane, were preferentially endocytosed by DOPr-expressing cells, and were delivered to DOPr-positive early endosomes. Nanoparticles caused a long-lasting activation of DOPr in endosomes, which provided sustained inhibition of nociceptor excitability and relief from inflammatory pain. Conversely, nanoparticles containing a DOPr antagonist abolished the sustained inhibitory effects of DADLE. Thus, DOPr in endosomes is an endogenous mechanism and a therapeutic target for relief from chronic inflammatory pain.