Rab26 modulates the cell surface transport of α2-adrenergic receptors from the Golgi

Protocol to visualize and quantify the COPII concentration and anterograde transport of nascent G protein-coupled receptors

Here, we present a protocol for visualization and quantification of the recruitment of newly synthesized G protein-coupled receptors (GPCRs) to coat protein complex II vesicles and GPCR transport from the endoplasmic reticulum through the Golgi to the cell surface in the retention using the selective hooks assay. We describe steps for plasmid construction, cell transfection, transport synchronization, confocal microscope imaging, and quantification. This protocol is also applicable for studying the transport of non-GPCR cargoes. For complete details on the use and execution of this protocol, please refer to Xu et al.1,2.

The ufmylation cascade controls COPII recruitment, anterograde transport, and sorting of nascent GPCRs at ER

Ufmylation is implicated in multiple cellular processes, but little is known about its functions and regulation in protein trafficking. Here, we demonstrate that the genetic depletion of core components of the ufmylation cascade, including ubiquitin-fold modifier 1 (UFM1), UFM1 activation enzyme 5, UFM1-specific ligase 1 (UFL1), UFM1-specific protease 2, and UFM1-binding protein 1 (UFBP1) each markedly inhibits the endoplasmic reticulum (ER)-Golgi transport, surface delivery, and recruitment to COPII vesicles of a subset of G protein-coupled receptors (GPCRs) and UFBP1's function partially relies on UFM1 conjugation. We also show that UFBP1 and UFL1 interact with GPCRs and UFBP1 localizes at COPII vesicles coated with specific Sec24 isoforms. Furthermore, the UFBP1/UFL1-binding domain identified in the receptors effectively converts non-GPCR protein transport into the ufmylation-dependent pathway. Collectively, these data reveal important functions for the ufmylation system in GPCR recruitment to COPII vesicles, biosynthetic transport, and sorting at ER via UFBP1 ufmylation and interaction directly.

Advances in nanomaterial-targeted treatment of acute lung injury after burns

Acute lung injury (ALI) is a common complication in patients with severe burns and has a complex pathogenesis and high morbidity and mortality rates. A variety of drugs have been identified in the clinic for the treatment of ALI, but they have toxic side effects caused by easy degradation in the body and distribution throughout the body. In recent years, as the understanding of the mechanism underlying ALI has improved, scholars have developed a variety of new nanomaterials that can be safely and effectively targeted for the treatment of ALI. Most of these methods involve nanomaterials such as lipids, organic polymers, peptides, extracellular vesicles or cell membranes, inorganic nanoparticles and other nanomaterials, which are targeted to reach lung tissues to perform their functions through active targeting or passive targeting, a process that involves a variety of cells or organelles. In this review, first, the mechanisms and pathophysiological features of ALI occurrence after burn injury are reviewed, potential therapeutic targets for ALI are summarized, existing nanomaterials for the targeted treatment of ALI are classified, and possible problems and challenges of nanomaterials in the targeted treatment of ALI are discussed to provide a reference for the development of nanomaterials for the targeted treatment of ALI.

Membrane trafficking alterations in breast cancer progression

Breast cancer (BC) is the most common type of cancer in women, and remains one of the major causes of death in women worldwide. It is now well established that alterations in membrane trafficking are implicated in BC progression. Indeed, membrane trafficking pathways regulate BC cell proliferation, migration, invasion, and metastasis. The 22 members of the ADP-ribosylation factor (ARF) and the >60 members of the rat sarcoma (RAS)-related in brain (RAB) families of small GTP-binding proteins (GTPases), which belong to the RAS superfamily, are master regulators of membrane trafficking pathways. ARF-like (ARL) subfamily members are involved in various processes, including vesicle budding and cargo selection. Moreover, ARFs regulate cytoskeleton organization and signal transduction. RABs are key regulators of all steps of membrane trafficking. Interestingly, the activity and/or expression of some of these proteins is found dysregulated in BC. Here, we review how the processes regulated by ARFs and RABs are subverted in BC, including secretion/exocytosis, endocytosis/recycling, autophagy/lysosome trafficking, cytoskeleton dynamics, integrin-mediated signaling, among others. Thus, we provide a comprehensive overview of the roles played by ARF and RAB family members, as well as their regulators in BC progression, aiming to lay the foundation for future research in this field. This research should focus on further dissecting the molecular mechanisms regulated by ARFs and RABs that are subverted in BC, and exploring their use as therapeutic targets or prognostic markers.

Segregation of nascent GPCRs in the ER-to-Golgi transport by CCHCR1 via direct interaction

G protein-coupled receptors (GPCRs) constitute the largest superfamily of cell surface signaling proteins that share a common structural topology. When compared with agonist-induced internalization, how GPCRs are sorted and delivered to functional destinations after synthesis in the endoplasmic reticulum (ER) is much less well understood. Here, we demonstrate that depletion of coiled-coil α-helical rod protein 1 (CCHCR1) by siRNA and CRISPR-Cas9 significantly inhibits surface expression and signaling of α2A-adrenergic receptor (α2A-AR; also known as ADRA2A), without affecting α2B-AR. Further studies show that CCHCR1 depletion specifically impedes α2A-AR export from the ER to the Golgi, but not from the Golgi to the surface. We also demonstrate that CCHCR1 selectively interacts with α2A-AR. The interaction is mediated through multiple domains of both proteins and is ionic in nature. Moreover, mutating CCHCR1-binding motifs significantly attenuates ER-to-Golgi export, surface expression and signaling of α2A-AR. Collectively, these data reveal a novel function for CCHCR1 in intracellular protein trafficking, indicate that closely related GPCRs can be sorted into distinct ER-to-Golgi transport routes by CCHCR1 via direct interaction, and provide important insights into segregation and anterograde delivery of nascent GPCR members.

Sequence-directed concentration of G protein-coupled receptors in COPII vesicles

G protein-coupled receptors (GPCRs) constitute the largest superfamily of plasma membrane signaling proteins. However, virtually nothing is known about their recruitment to COPII vesicles for forward delivery after synthesis in the endoplasmic reticulum (ER). Here, we demonstrate that some GPCRs are highly concentrated at ER exit sites (ERES) before COPII budding. Angiotensin II type 2 receptor (AT2R) and CXCR4 concentration are directed by a di-acidic motif and a 9-residue domain, respectively, and these motifs also control receptor ER-Golgi traffic. We further show that AT2R interacts with Sar1 GTPase and that distinct GPCRs have different ER-Golgi transport rates via COPII which is independent of their concentration at ERES. Collectively, these data demonstrate that GPCRs can be actively captured by COPII via specific motifs and direct interaction with COPII components that in turn affects their export dynamics, and provide important insights into COPII targeting and forward trafficking of nascent GPCRs.

Rab26 promotes macrophage phagocytosis through regulation of MFN2 trafficking to mitochondria

Acute respiratory distress syndrome (ARDS) is an inflammatory disorder of the lungs caused by bacterial or viral infection. Timely phagocytosis and clearance of pathogens by macrophages are important in controlling inflammation and alleviating ARDS. However, the precise mechanism of macrophage phagocytosis remains to be explored. Here, we show that the expression of Rab26 is increased in Escherichia coli- or Pseudomonas aeruginosa-stimulated bone marrow-derived macrophages. Knocking out Rab26 reduced phagocytosis and bacterial clearance by macrophages. Rab26 interacts with mitochondrial fusion protein mitofusin-2 (MFN2) and affects mitochondrial reactive oxygen species generation by regulating MFN2 transport. The levels of MFN2 in mitochondria were reduced in Rab26-deficient bone marrow-derived macrophages, and the levels of mitochondrial reactive oxygen species and ATP were significantly decreased. Knocking down MFN2 using small interfering RNA resulted in decreased phagocytosis and killing ability of macrophages. Rab26 knockout reduced phagocytosis and bacterial clearance by macrophages in vivo, significantly increased inflammatory factors, aggravated lung tissue damage, and increased mortality in mice. Our results demonstrate that Rab26 regulates phagocytosis and clearance of bacteria by mediating the transport of MFN2 to mitochondria in macrophages, thus alleviating ARDS in mice and potentially in humans.

Rab26 restricts insulin secretion via sequestering Synaptotagmin-1

Rab26 is known to regulate multiple membrane trafficking events, but its role in insulin secretion in pancreatic β cells remains unclear despite it was first identified in the pancreas. In this study, we generated Rab26-/- mice through CRISPR/Cas9 technique. Surprisingly, insulin levels in the blood of the Rab26-/- mice do not decrease upon glucose stimulation but conversely increase. Deficiency of Rab26 promotes insulin secretion, which was independently verified by Rab26 knockdown in pancreatic insulinoma cells. Conversely, overexpression of Rab26 suppresses insulin secretion in both insulinoma cell lines and isolated mouse islets. Islets overexpressing Rab26, upon transplantation, also failed to restore glucose homeostasis in type 1 diabetic mice. Immunofluorescence microscopy revealed that overexpression of Rab26 results in clustering of insulin granules. GST-pulldown experiments reveal that Rab26 interacts with synaptotagmin-1 (Syt1) through directly binding to its C2A domain, which interfering with the interaction between Syt1 and SNAP25, and consequently inhibiting the exocytosis of newcomer insulin granules revealed by TIRF microscopy. Our results suggest that Rab26 serves as a negative regulator of insulin secretion, via suppressing insulin granule fusion with plasma membrane through sequestering Syt1.

ROS induced the Rab26 promoter hypermethylation to promote cigarette smoking-induced airway epithelial inflammation of COPD through activation of MAPK signaling

Cigarette smoking (CS) exposure-induced airway inflammatory responses drive the occurrence and development of emphysema and chronic obstructive pulmonary disease (COPD). However, its precise mechanisms have not been fully elucidated. In this study, we explore the role of Rab26 in CS exposure modulating the inflammatory response of airway epithelium and the novel mechanism of CS exposure regulation Rab26. These data showed that CS exposure and H₂O₂ (a type of ROS) suppressed the expression of Rab26 and increased the expression of DNMT3b in vivo and in vitro. GEO data analysis found the level of Rab26 was decreased in the lung tissue of COPD patients. CSE-induced ROS promoted DNA methylation of the Rab26 promoter and inhibited its promoter activity by elevating the DNMT3b level. Antioxidants N-Acetyl-l-cysteine (NAC), 5-Aza-2'-deoxycytidine (5-AZA) (DNA methylation inhibitor) and DNMT3B siRNA alleviated CSE's inhibitory effect on Rab26 expression in vitro. Importantly, NAC alleviated the improved expression of Rab26 and reduced DNMT3B expression, in the airway of smoking exposure as well as attenuated the inflammatory response in vivo. Overexpression of Rab26 attenuated CSE-induced production of inflammatory mediators through part inactivation of p38 and JNK MAPK. On the contrary, silencing Rab26 enhanced p38 and JNK activation and aggravated inflammatory response. These findings suggest that ROS-mediated Rab26 promoter hypermethylation is a critical step in cigarette smoking-induced airway epithelial inflammatory response. Restoring Rab26 in the airway epithelium might be a potential strategy for treating airway inflammation and COPD.

Integrating intracellular nanovesicles into integrin trafficking pathways and beyond

Membrane traffic controls the movement of proteins and lipids from one cellular compartment to another using a system of transport vesicles. Intracellular nanovesicles (INVs) are a newly described class of transport vesicles. These vesicles are small, carry diverse cargo, and are involved in multiple trafficking steps including anterograde traffic and endosomal recycling. An example of a biological process that they control is cell migration and invasion, due to their role in integrin recycling. In this review, we describe what is known so far about these vesicles. We discuss how INVs may integrate into established membrane trafficking pathways using integrin recycling as an example. We speculate where in the cell INVs have the potential to operate and we identify key questions for future investigation.

Human C1orf27 protein interacts with α2A-adrenergic receptor and regulates its anterograde transport

The molecular mechanisms underlying the anterograde surface transport of G protein–coupled receptors (GPCRs) after their synthesis in the endoplasmic reticulum (ER) are not well defined. In C. elegans, odorant response abnormal 4 has been implicated in the delivery of olfactory GPCRs to the cilia of chemosensory neurons. However, the function and regulation of its human homolog, C1orf27, in GPCR transport or in general membrane trafficking remain unknown. Here, we demonstrate that siRNA-mediated knockdown of C1orf27 markedly impedes the ER-to-Golgi export kinetics of newly synthesized α_2A-adrenergic receptor (α_2A-AR), a prototypic GPCR, with the half-time being prolonged by more than 65%, in mammalian cells in retention using the selective hooks assays. Using modified bioluminescence resonance energy transfer assays and ELISAs, we also show that C1orf27 knockdown significantly inhibits the surface transport of α_2A-AR. Similarly, C1orf27 knockout by CRISPR-Cas9 markedly suppresses the ER–Golgi-surface transport of α_2A-AR. In addition, we demonstrate that C1orf27 depletion attenuates the export of β₂-AR and dopamine D2 receptor but not of epidermal growth factor receptor. We further show that C1orf27 physically associates with α_2A-AR, specifically via its third intracellular loop and C terminus. Taken together, these data demonstrate an important role of C1orf27 in the trafficking of nascent GPCRs from the ER to the cell surface through the Golgi and provide novel insights into the regulation of the biosynthesis and anterograde transport of the GPCR family members.

RAB26 contributes to the progression of non-small cell lung cancer after being transcriptionally activated by SMAD3

Non-small cell lung cancer (NSCLC) accounts for 85% of all cases of lung cancer, which constitutes the leading cause of cancer mortality. RAB26, a member of Rab GTPase superfamily, has been suggested to play a role in the tumorigenesis of NSCLC. The present work aimed to explore whether and how RAB26 contributed to the progression of NSCLC. NSCLC cell line A549 was transfection with short hairpin RNA (shRNA) or overexpression (Ov) vector to knockdown RAB26 or overexpress SMAD3, respectively. Then the malignant processes of A549 cells including proliferation, migration, invasion and apoptosis were evaluated by CCK-8, colony formation, wound-healing, transwell and TUNEL assays, respectively. Expression of proteins involved in these processes was measured by western blot. A549 xenograft mice model was established to confirm the effect of RAB26 silence on NSCLC progression in vivo. The relationship between RAB26 and SMAD3 was analyzed by bioinformatics and then verified by dual-luciferase reporter and chromatin immunoprecipitation (ChIP) assays. We found that silence of RAB26 inhibited the proliferation, migration and invasion but promoted apoptosis of A549 cells. In vivo studies revealed that the tumor growth of A549 xenograft was markedly suppressed upon RAB26 silence. Moreover, it was confirmed that SMAD3 bound to the promoter of RAB26 and enhance its expression. Finally, we observed that overexpression of SMAD3 significantly blocked the inhibitory effect of RAB26 silence on NSCLC progression. Collectively, RAB26 may contribute to the progression of NSCLC after being transcriptionally activated by SMAD3.

Specific motifs mediate post-synaptic and surface transport of G protein-coupled receptors

G protein-coupled receptors (GPCRs) are key regulators of synaptic functions. However, their targeted trafficking to synapses after synthesis is poorly understood. Here, we demonstrate that multiple motifs mediate α2B-adrenergic receptor transport to the dendritic and post-synaptic compartments in primary hippocampal neurons, with a single leucine residue on the first intracellular loop being specifically involved in synaptic targeting. The N-terminally located tyrosine-serine motif operates differently in neuronal and non-neuronal cells. We further show that the highly conserved dileucine (LL) motif in the C-terminus is required for the dendritic and post-synaptic traffic of all GPCRs studied. The LL motif also directs the export from the endoplasmic reticulum of a chimeric GPCR and confers its transport ability to vesicular stomatitis virus glycoprotein in cell lines. Collectively, these data reveal the intrinsic structural determinants for the synaptic targeting of nascent GPCRs and their cell-type-specific trafficking along the biosynthetic pathways.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Gβγ translocation to the Golgi apparatus activates ARF1 to spatiotemporally regulate G protein-coupled receptor signaling to MAPK

After activation of G protein-coupled receptors, G protein βγ dimers may translocate from the plasma membrane to the Golgi apparatus (GA). We recently report that this translocation activates extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) via PI3Kγ; however, how Gβγ-PI3Kγ activates the ERK1/2 pathway is unclear. Here, we demonstrate that chemokine receptor CXCR4 activates ADP-ribosylation factor 1 (ARF1), a small GTPase important for vesicle-mediated membrane trafficking. This activation is blocked by CRISPR-Cas9-mediated knockout of the GA-translocating Gγ9 subunit. Inducible targeting of different Gβγ dimers to the GA can directly activate ARF1. CXCR4 activation and constitutive Gβγ recruitment to the GA also enhance ARF1 translocation to the GA. We further demonstrate that pharmacological inhibition and CRISPR-Cas9-mediated knockout of PI3Kγ markedly inhibit CXCR4-mediated and Gβγ translocation-mediated ARF1 activation. We also show that depletion of ARF1 by siRNA and CRISPR-Cas9 and inhibition of GA-localized ARF1 activation abolish ERK1/2 activation by CXCR4 and Gβγ translocation to the GA and suppress prostate cancer PC3 cell migration and invasion. Collectively, our data reveal a novel function for Gβγ translocation to the GA to activate ARF1 and identify GA-localized ARF1 as an effector acting downstream of Gβγ-PI3Kγ to spatiotemporally regulate G protein-coupled receptor signaling to mitogen-activated protein kinases.

Rab26 suppresses migration and invasion of breast cancer cells through mediating autophagic degradation of phosphorylated Src

Rab proteins play crucial roles in membrane trafficking. Some Rab proteins are implicated in cancer development through regulating protein sorting or degradation. In this study, we found that the expression of Rab26 is suppressed in the aggressive breast cancer cells as compared to the levels in non-invasive breast cancer cells. Over-expression of Rab26 inhibits cell migration and invasion, while Rab26 knockdown significantly promotes the migration and invasion of breast cancer cells. Rab26 reduces focal adhesion association of Src kinase and induces endosomal translocation of Src. Further experiments revealed that Rab26 mediates the autophagic degradation of phosphorylated Src through interacting with ATG16L1, consequently, resulting in the suppression of the migration and invasion ability of breast cancer cells.

Rab43 GTPase directs postsynaptic trafficking and neuron-specific sorting of G protein-coupled receptors

G protein–coupled receptors (GPCRs) are important modulators of synaptic functions. A fundamental but poorly addressed question in neurobiology is how targeted GPCR trafficking is achieved. Rab GTPases are the master regulators of vesicle-mediated membrane trafficking, but their functions in the synaptic presentation of newly synthesized GPCRs are virtually unknown. Here, we investigate the role of Rab43, via dominant-negative inhibition and CRISPR–Cas9–mediated KO, in the export trafficking of α₂-adrenergic receptor (α₂-AR) and muscarinic acetylcholine receptor (mAChR) in primary neurons and cells. We demonstrate that Rab43 differentially regulates the overall surface expression of endogenous α₂-AR and mAChR, as well as their signaling, in primary neurons. In parallel, Rab43 exerts distinct effects on the dendritic and postsynaptic transport of specific α_2B-AR and M3 mAChR subtypes. More interestingly, the selective actions of Rab43 toward α_2B-AR and M3 mAChR are neuronal cell specific and dictated by direct interaction. These data reveal novel, neuron-specific functions for Rab43 in the dendritic and postsynaptic targeting and sorting of GPCRs and imply multiple forward delivery routes for different GPCRs in neurons. Overall, this study provides important insights into regulatory mechanisms of GPCR anterograde traffic to the functional destination in neurons.

Chemical composition and pharmacological mechanism of ephedra-glycyrrhiza drug pair against coronavirus disease 2019 (COVID-19)

Traditional Chinese medicine (TCM) had demonstrated effectiveness in the prevention and control of COVID-19. Statistics showed that Ephedra and Glycyrrhiza were frequently used in the treatment of COVID-19. We hypothesized that the Ephedra-Glycyrrhiza drug pair is a potential choice for the treatment of COVID-19. Here, 112 active compounds were identified from Ephedra-Glycyrrhiza via network pharmacology approach. Ephedra-Glycyrrhiza pair enrichment analysis demonstrated that these compounds might participate in the cAMP, PI3K-Akt, JAK-STAT and chemokine signaling pathways, which had a high correlation with respiratory, nervous, blood circulation and digestive system-related diseases. Pathway analysis between Ephedra-Glycyrrhiza and COVID-19 showed that the key targets were TNF-α, IL2, FOS, ALB, and PTGS2. They might control PI3K-Akt signaling pathway to exert immune regulation, organ protection and antiviral effects. Molecular docking results showed that the active compounds from the Ephedra-Glycyrrhiza pair bound well to COVID-19 related targets, including the main protease (Mpro, also called 3CLpro), the spike protein (S protein), and the angiotensin-converting enzyme 2 (ACE2). The Molecular dynamics simulation was analyzed for the stability and flexibility of the complex. In conclusion, our study elucidated the potential pharmacological mechanism of Ephedra-Glycyrrhiza in the treatment of COVID-19 through multiple targets and pathways.

Interfering with Autophagy: The Opposing Strategies Deployed by Legionella pneumophila and Coxiella burnetii Effector Proteins

Autophagy is a fundamental and highly conserved eukaryotic process, responsible for maintaining cellular homeostasis and releasing nutrients during times of starvation. An increasingly important function of autophagy is its role in the cell autonomous immune response; a process known as xenophagy. Intracellular pathogens are engulfed by autophagosomes and targeted to lysosomes to eliminate the threat to the host cell. To counteract this, many intracellular bacterial pathogens have developed unique approaches to overcome, evade, or co-opt host autophagy to facilitate a successful infection. The intracellular bacteria Legionella pneumophila and Coxiella burnetii are able to avoid destruction by the cell, causing Legionnaires' disease and Q fever, respectively. Despite being related and employing homologous Dot/Icm type 4 secretion systems (T4SS) to translocate effector proteins into the host cell, these pathogens have developed their own unique intracellular niches. L. pneumophila evades the host endocytic pathway and instead forms an ER-derived vacuole, while C. burnetii requires delivery to mature, acidified endosomes which it remodels into a large, replicative vacuole. Throughout infection, L. pneumophila effectors act at multiple points to inhibit recognition by xenophagy receptors and disrupt host autophagy, ensuring it avoids fusion with destructive lysosomes. In contrast, C. burnetii employs its effector cohort to control autophagy, hypothesized to facilitate the delivery of nutrients and membrane to support the growing vacuole and replicating bacteria. In this review we explore the effector proteins that these two organisms utilize to modulate the host autophagy pathway in order to survive and replicate. By better understanding how these pathogens manipulate this highly conserved pathway, we can not only develop better treatments for these important human diseases, but also better understand and control autophagy in the context of human health and disease.

Specific TBC Domain-Containing Proteins Control the ER-Golgi-Plasma Membrane Trafficking of GPCRs

G-protein-coupled receptors (GPCRs) constitute the largest superfamily of cell surface signaling proteins. However, the molecular mechanisms underlying their cell surface delivery after synthesis remain poorly understood. Here, we screen the TBC domain-containing proteins, putative Rab GTPase-activating proteins (GAPs), in the intracellular trafficking of GPCRs and identify several TBC proteins that activity-dependently regulate the anterograde transport, en route from the endoplasmic reticulum to the Golgi or from the Golgi to the cell surface, of several prototypic GPCR members without affecting other plasma membrane proteins. We also show that TBC1D6 functions as a GAP for Rab26, physically associates with Rab26, and attenuates Rab26 interaction with GPCRs. Furthermore, both overexpression and depletion of TBC1D6 inhibit the post-Golgi traffic of GPCRs. These data demonstrate important roles of the TBC proteins in forward trafficking of nascent GPCRs and reveal regulatory mechanisms of GPCR targeting to the functional destination.

Wei et al. report that several TBC proteins specifically and activity-dependently regulate ER-Golgi-plasma membrane transport of nascent GPCRs. They also show that TBC1D6 is a GAP for Rab26 and controls GPCR post-Golgi traffic. Their results reveal crucial roles of TBC proteins in and provide regulatory mechanisms of GPCR trafficking.

Quantification of The Surface Expression of G Protein-coupled Receptors Using Intact Live-cell Radioligand Binding Assays

G protein-coupled receptors (GPCRs) are the most structurally diverse family of signaling proteins and regulate a variety of cell function. For most GPCRs, the cell surface is their functional destination where they are able to respond a wide range of extracellular stimuli, leading to the activation of intracellular signal transduction cascades. Thus, the quantity of receptor expression at the cell surface is a crucial factor regulating the functionality of the receptors. Over the past decades, many methods have been developed to measure the cell surface expression of GPCRs. Here, we describe an intact live-cell radioligand binding assay to quantify the surface expression of GPCRs at the endogenous levels or after overexpression. In this assay, cell cultures will be incubated with specific cell-nonpermeable radioligands which selectively and stoichiometrically bind to individual GPCRs and the receptor numbers at the cell surface are quantified by the radioactivity of receptor-bound ligands. This method is highly specific for measuring the functional GPCRs at the surface of intact live cells and is particularly useful for endogenous, low-abundant GPCRs.

Coxiella effector protein CvpF subverts RAB26-dependent autophagy to promote vacuole biogenesis and virulence

Coxiella burnetii, the etiological agent of the zoonosis Q fever, replicates inside host cells within a large vacuole displaying autolysosomal characteristics. The development of this compartment is mediated by bacterial effectors, which interfere with a number of host membrane trafficking pathways. By screening a Coxiella transposon mutant library, we observed that transposon insertions in cbu0626 led to intracellular replication and vacuole biogenesis defects. Here, we demonstrate that CBU0626 is a novel member of the Coxiella vacuolar protein (Cvp) family of effector proteins, which is translocated by the Dot/Icm secretion system and localizes to vesicles with autolysosomal features as well as Coxiella-containing vacuoles (CCVs). We thus renamed this effector CvpF for Coxiella vacuolar protein F. CvpF specifically interacts with the host small GTPase RAB26, leading to the recruitment of the autophagosomal marker MAP1LC3B/LC3B (microtubule associated protein 1 light chain 3 beta) to CCVs. Importantly, cvpF::Tn mutants were highly attenuated compared to wild-type bacteria in the SCID mouse model of infection, highlighting the importance of CvpF for Coxiella virulence. These results suggest that CvpF manipulates endosomal trafficking and macroautophagy/autophagy induction for optimal C. burnetii vacuole biogenesis.

Abbreviations: ACCM: acidified citrate cystein medium; AP: adaptor related protein complex; CCV: Coxiella-containing vacuole; Cvp: Coxiella vacuolar protein; GDI: guanosine nucleotide dissociation inhibitor; GDF: GDI dissociation factor; GEF: guanine exchange factor; LAMP1: lysosomal associated membrane protein 1; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MTORC1: mechanistic target of rapamycin kinase MTOR complex 1; PBS: phosphate-buffered saline; PMA: phorbol myristate acetate; SQSTM1/p62: sequestosome 1; WT: wild-type.

A Naturally Occurring Splice Variant of GGA1 Inhibits the Anterograde Post-Golgi Traffic of α2B-Adrenergic Receptor

The regulatory mechanisms of cell surface targeting of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum through the Golgi remain poorly understood. We have recently demonstrated that three Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding proteins (GGAs) mediate the post-Golgi export of α_2B-adrenergic receptor (α_2B-AR), a prototypic GPCR, and directly interact with the receptor. In particular, GGA1 interaction with α_2B-AR is mediated via its hinge domain. Here we determined the role of a naturally occurring truncated form of GGA1 (GGA1t) which lacks the N-terminal portion of the hinge domain in α_2B-AR trafficking and elucidated the underlying mechanisms. We demonstrated that both GGA1 and GGA1t were colocalized and mainly expressed at the Golgi. In marked contrast to GGA1, the expression of GGA1t significantly attenuated the cell surface export of newly synthesized α_2B-AR from the Golgi and in parallel receptor-mediated signaling. Furthermore, we found that GGA1t formed homodimers and heterodimers with GGA1. More interestingly, GGA1t was unable to bind the cargo α_2B-AR and to recruit clathrin onto the trans-Golgi network. These data provide evidence implicating that the truncated form of GGA1 behaviors as a dominant-negative regulator for the cell surface export of α_2B-AR and this function of GGA1t is attributed to its abilities to dimerize with its wide type counterpart and to inhibit cargo interaction and clathrin recruitment to form specialized transport vesicles.

Endothelial Cell Inflammation and Barriers Are Regulated by the Rab26-Mediated Balance between β2-AR and TLR4 in Pulmonary Microvessel Endothelial Cells

Rab26 GTPase modulates the trafficking of cell surface receptors, such as G protein-coupled receptors including α2-adrenergic receptors in some cell types. However, the effect of Rab26 on β2-adrenergic receptor (β2-AR) trafficking or/and Toll-like receptor 4 (TLR4) expression in human pulmonary microvascular endothelial cells (HPMECs) is still unclear. Here, we investigated the role of Rab26 in regulating the expression of β2-ARs and TLR4 in HPMECs and the effect of these receptors' imbalance on endothelial cell barrier function. The results showed that there was unbalance expression in these receptors, where β2-AR expression was remarkably reduced, and TLR4 was increased on the cell membrane after lipopolysaccharide (LPS) treatment. Furthermore, we found that Rab26 overexpression not only upregulated β2-ARs but also downregulated TLR4 expression on the cell membrane. Subsequently, the TLR4-related inflammatory response was greatly attenuated, and the hyperpermeability of HPMECs also was partially relived. Taken together, these data suggest that basal Rab26 maintains the balance between β2-ARs and TLR4 on the cell surface, and it might be a potential therapeutic target for diseases involving endothelial barrier dysfunction.

Network Pharmacology Deciphering Mechanisms of Volatiles of Wendan Granule for the Treatment of Alzheimer's Disease

Objective

To explore the mechanisms of the volatiles of Wendan granule (WDG) for the treatment of Alzheimer's disease, network pharmacology method integrating absorption, distribution, metabolism, and excretion (ADME) screening, target fishing, network constructing, pathway analysing, and correlated diseases prediction was applied.

Methods

Twelve small molecular compounds of WDG were selected as the objects from 74 volatiles with the relative abundances above 2 %, and their ADME parameters were collected from Traditional Chinese Medicine Systems Pharmacology platform (TCMSP), and the corresponding targets, genes, pathways, and diseases were predicted according to the data provided by TCMSP, DrugBank, Uniport, and the Database for Annotation, Visualization, and Integrated Discovery (DAVID). Then the related pathways and correlation analysis were explored by the Kyoto Encyclopedia and Genomes (KEGG) database. Finally, the networks of compound target, target pathway, and pathway disease of WDG were constructed by Cytoscape software.

Results

Twelve compounds interacted with 49 targets, of which top three targets were gamma-aminobutyric acid receptor subunit alpha-1 (GABRA1), prostaglandin G/H synthase 2 (PGHS-2), and sodium-dependent noradrenaline transporter. Interestingly, these targets were highly associated with depression, insomnia, and Alzheimer's disease that mainly corresponded to mental and emotional illnesses.

Conclusion

The integrated network pharmacology method provides precise probe to illuminate the molecular mechanisms of the main volatiles of WDG for relieving senile dementia related syndromes, which will also facilitate the application of traditional Chinese medicine as an alternative or supplementary to conventional treatments of AD, as well as follow-up studies such as upgrading the quality standard of clinically applied herbal medicine and novel drug development.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

Regulation of protein-coding gene and long noncoding RNA pairs in liver of conventional and germ-free mice following oral PBDE exposure

Gut microbiome communicates with the host liver to modify hepatic xenobiotic biotransformation and nutrient homeostasis. Polybrominated diphenyl ethers (PBDEs) are persistent environmental contaminants that are detected in fatty food, household dust, and human breast milk at worrisome levels. Recently, long noncoding RNAs (lncRNAs) have been recognized as novel biomarkers for toxicological responses and may regulate the transcriptional/translational output of protein-coding genes (PCGs). However, very little is known regarding to what extent the interactions between PBDEs and gut microbiome modulate hepatic lncRNAs and PCGs, and what critical signaling pathways are impacted at the transcriptomic scale. In this study, we performed RNA-Seq in livers of nine-week-old male conventional (CV) and germ-free (GF) mice orally exposed to the most prevalent PBDE congeners BDE-47 and BDE-99 (100 μmol/kg once daily for 4-days; vehicle: corn oil, 10 ml/kg), and unveiled key molecular pathways and PCG-lncRNA pairs targeted by PBDE-gut microbiome interactions. Lack of gut microbiome profoundly altered the PBDE-mediated transcriptomic response in liver, with the most prominent effect observed in BDE-99-exposed GF mice. The top pathways up-regulated by PBDEs were related to xenobiotic metabolism, whereas the top pathways down-regulated by PBDEs were in lipid metabolism and protein synthesis in both enterotypes. Genomic annotation of the differentially regulated lncRNAs revealed that majority of these lncRNAs overlapped with introns and 3'-UTRs of PCGs. Lack of gut microbiome profoundly increased the percentage of PBDE-regulated lncRNAs mapped to the 3'-UTRs of PCGs, suggesting the potential involvement of lncRNAs in increasing the translational efficiency of PCGs by preventing miRNA-3'-UTR binding, as a compensatory mechanism following toxic exposure to PBDEs. Pathway analysis of PCGs paired with lncRNAs revealed that in CV mice, BDE-47 regulated nucleic acid and retinol metabolism, as well as circadian rhythm; whereas BDE-99 regulated fatty acid metabolism. In GF mice, BDE-47 differentially regulated 19 lncRNA-PCG pairs that were associated with glutathione conjugation and transcriptional regulation. In contrast, BDE-99 up-regulated the xenobiotic-metabolizing Cyp3a genes, but down-regulated the fatty acid-metabolizing Cyp4 genes. Taken together, the present study reveals common and unique lncRNAs and PCG targets of PBDEs in mouse liver, and is among the first to show that lack of gut microbiome sensitizes the liver to toxic exposure of BDE-99 but not BDE-47. Therefore, lncRNAs may serve as specific biomarkers that differentiate various PBDE congeners as well as environmental chemical-mediated dysbiosis. Coordinate regulation of PCG-lncRNA pairs may serve as a more efficient molecular mechanism to combat against xenobiotic insult, and especially during dysbiosis-induced increase in the internal dose of toxicants.

RAB26-dependent autophagy protects adherens junctional integrity in acute lung injury

Microvascular barrier dysfunction is the central pathophysiological feature of acute lung injury (ALI). RAB26 is a newly identified small GTPase involved in the regulation of endothelial cell (EC) permeability. However, the mechanism behind this protection has not been clearly elucidated. Here we found that RAB26 promoted the integrity of adherens junctions (AJs) in a macroautophagy/autophagy-dependent manner in ALI. RAB26 is frequently downregulated in mouse lungs after LPS treatment. Mice lacking Rab26 exhibited phosphorylated SRC expression and increased CDH5/VE-cadherin phosphorylation, leading to AJ destruction. rab26-null mice showed further aggravation of the effects of endotoxin insult on lung vascular permeability and water content. Depletion of RAB26 resulted in upregulation of phosphorylated SRC, enhancement of CDH5 phosphorylation, and aggravation of CDH5 internalization, thereby weakening AJ integrity and endothelial barrier function in human pulmonary microvascular endothelial cells (HPMECs). RAB26 overexpression caused active interaction between SRC and the autophagy marker LC3-II and promoted degradation of phosphorylated SRC. Furthermore, RAB26 was involved in a direct and activation-dependent manner in autophagy induction through interaction with ATG16L1 in its GTP-bound form. These findings demonstrate that RAB26 exerts a protective effect on endothelial cell (EC) permeability, which is in part dependent on autophagic targeting of active SRC, and the resultant CDH5 dephosphorylation maintains AJ stabilization. Thus, RAB26-mediated autophagic targeting of phosphorylated SRC can maintain barrier integrity when flux through the RAB26-SRC pathway is protected. These findings suggest that activation of RAB26-SRC signaling provides a new therapeutic opportunity to prevent vascular leakage in ALI.

ABBREVIATIONS

AJs: adherens junctions; ALI: acute lung injury; ARDS: acute respiratory distress syndrome; ATG5: autophagy related 5; ATG12: autophagy related 12; ATG 16L1: autophagy related 16 like; 1 BALF: bronchoalveolar lavage fluidCQ: chloroquine; Ctrl: control; EC: endothelial cell; GFP: green fluorescent protein; HA-tagged; RAB26^WT: HA-tagged wild-type; RAB26  HA-tagged; RAB26^QL: HA-tagged; RAB26^Q123LHA-tagged; RAB26^NI: HA-tagged; RAB26^N177IHPMECs: human pulmonary microvascular endothelial cells; H&E: hematoxylin & eosin; IgG: immunoglobulin; GIF: immunofluorescence; IP: immunoprecipitationi;. p.: intraperitoneal; LPS: lipopolysaccharide; PBS: phosphate-buffered salinesi; RNA: small interfering;RNASQSTM1/p62, sequestosome; 1TBS: Tris-buffered saline; VEGF: vascular endothelial growth factor; WB: western blot; WT: wild-type.

Role of Rab GTPases in the export trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) constitute a superfamily of cell surface receptors that regulate a variety of cell functions. As the cell surface is the functional destination for most GPCRs, the cell surface targeting process represents a crucial checkpoint in controlling the functionality of the receptors. However, the molecular mechanisms underlying the cell surface delivery of newly synthesized GPCRs remain poorly understood. In this review, we will highlight the role of Rab GTPases in GPCR cell surface transport, particularly post-Golgi traffic, and discuss the underlying molecular mechanisms.

ARF1 promotes prostate tumorigenesis via targeting oncogenic MAPK signaling

ADP-ribosylation factor 1 (ARF1) is a crucial regulator in vesicle-mediated membrane trafficking and involved in the activation of signaling molecules. However, virtually nothing is known about its function in prostate cancer. Here we have demonstrated that ARF1 expression is significantly elevated in prostate cancer cells and human tissues and that the expression levels of ARF1 correlate with the activation of mitogen-activated protein kinases (MAPK) ERK1/2. Furthermore, we have shown that overexpression and knockdown of ARF1 produce opposing effects on prostate cancer cell proliferation, anchorage-independent growth and tumor growth in mouse xenograft models and that ARF1-mediated cell proliferation can be abolished by the Raf1 inhibitor GW5074 and the MEK inhibitors U0126 and PD98059. Moreover, inhibition of ARF1 activation achieved by mutating Thr48 abolishes ARF1's abilities to activate the ERK1/2 and to promote cell proliferation. These data demonstrate that the aberrant MAPK signaling in prostate cancer is, at least in part, under the control of ARF1 and that, similar to Ras, ARF1 is a critical regulator in prostate cancer progression. These data also suggest that ARF1 may represent a key molecular target for prostate cancer therapeutics and diagnosis.

The GTPase Rab43 Controls the Anterograde ER-Golgi Trafficking and Sorting of GPCRs

G-protein-coupled receptors (GPCRs) constitute the largest superfamily of cell-surface signaling proteins. However, mechanisms underlying their surface targeting and sorting are poorly understood. Here, we screen the Rab family of small GTPases in the surface transport of multiple GPCRs. We find that manipulation of Rab43 function significantly alters the surface presentation and signaling of all GPCRs studied without affecting non-GPCR membrane proteins. Rab43 specifically regulates the transport of nascent GPCRs from the endoplasmic reticulum (ER) to the Golgi. More interestingly, Rab43 directly interacts with GPCRs in an activation-dependent fashion. The Rab43-binding domain identified in the receptors effectively converts non-GPCR membrane protein transport into a Rab43-dependent pathway. These data reveal a crucial role for Rab43 in anterograde ER-Golgi transport of nascent GPCRs, as well as the ER sorting of GPCR members by virtue of its ability to interact directly.

Regulation on Toll-like Receptor 4 and Cell Barrier Function by Rab26 siRNA-loaded DNA Nanovector in Pulmonary Microvascular Endothelial Cells

The small GTPase Rab26 is involved in multiple processes, such as vesicle-mediated secretion and autophagy. However, the mechanisms and functions of Rab26 in the human pulmonary microvascular endothelial cells (HPMVECs) are not clear. In this study, we thoroughly investigated the role and novel mechanism of Rab26 in permeability and apoptosis of HPMVECs using a self-assembled Rab26 siRNA loaded DNA Y-motif nanoparticle (siRab26-DYM) and Rab26 adenovirus. We found that siRab26-DYM could be efficiently transfected into HPMVECs in a time- and dose-dependent manner. Importantly, the siRab26-DYM nanovector markedly aggravated the LPS-induced apoptosis and hyper-permeability of HPMVECs by promoting the nuclear translocation of Foxo1, and subsequent activation of Toll-like receptor 4 (TLR4) signal pathway. Overexpression of Rab26 by Rab26 adenoviruses partially inactivated LPS-induced TLR4 signaling pathway, suppressed the cell apoptosis and attenuated the hyperpermeability of HPMVECs. These results suggest that the permeability and apoptosis of HPMVECs can be modulated by manipulating Rab26 derived TLR4 signaling pathway, and that Rab26 can be potential therapeutic target for the treatment of vascular diseases related to endothelial barrier functions.

Protein Kinase C Activation Promotes α1B-Adrenoceptor Internalization and Late Endosome Trafficking through Rab9 Interaction. Role in Heterologous Desensitization

Upon agonist stimulation, α1B-adrenergic receptors couple to Gq proteins, calcium signaling and protein kinase C activation; subsequently, the receptors are phosphorylated, desensitized, and internalized. Internalization seems to involve scaffolding proteins, such as β-arrestin and clathrin. However, the fine mechanisms that participate remain unsolved. The roles of protein kinase C and the small GTPase, Rab9, in α1B-AR vesicular traffic were investigated by studying α1B-adrenergic receptor-Rab protein interactions, using Förster resonance energy transfer (FRET), confocal microscopy, and intracellular calcium quantitation. In human embryonic kidney 293 cells overexpressing Discosoma spp. red fluorescent protein (DsRed)-tagged α1B-ARs and enhanced green fluorescent protein--tagged Rab proteins, pharmacological protein kinase C activation mimicked α1B-AR traffic elicited by nonrelated agents, such as sphingosine 1-phosphate (i.e., transient α1B-AR-Rab5 FRET signal followed by a sustained α1B-AR-Rab9 interaction), suggesting brief receptor localization in early endosomes and transfer to late endosomes. This latter interaction was abrogated by blocking protein kinase C activity, resulting in receptor retention at the plasma membrane. Similar effects were observed when a dominant-negative Rab9 mutant (Rab9-GDP) was employed. When α1B-adrenergic receptors that had been mutated at protein kinase C phosphorylation sites (S396A, S402A) were used, phorbol ester-induced desensitization of the calcium response was markedly decreased; however, interaction with Rab9 was only partially decreased and internalization was observed in response to phorbol esters and sphingosine 1-phosphate. Finally, Rab9-GDP expression did not affect adrenergic-mediated calcium response but abolished receptor traffic and altered desensitization. Data suggest that protein kinase C modulates α1B-adrenergic receptor transfer to late endosomes and that Rab9 regulates this process and participates in G protein-mediated signaling turn-off.

Regulation of α2B-Adrenergic Receptor Cell Surface Transport by GGA1 and GGA2

The molecular mechanisms that control the targeting of newly synthesized G protein-coupled receptors (GPCRs) to the functional destinations remain poorly elucidated. Here, we have determined the role of Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding proteins 1 and 2 (GGA1 and GGA2) in the cell surface transport of α_2B-adrenergic receptor (α_2B-AR), a prototypic GPCR, and studied the underlying mechanisms. We demonstrated that knockdown of GGA1 and GGA2 by shRNA and siRNA significantly reduced the cell surface expression of inducibly expressed α_2B-AR and arrested the receptor in the perinuclear region. Knockdown of each GGA markedly inhibited the dendritic expression of α_2B-AR in primary cortical neurons. Consistently, depleting GGA1 and GGA2 attenuated receptor-mediated signal transduction measured as ERK1/2 activation and cAMP inhibition. Although full length α_2B-AR associated with GGA2 but not GGA1, its third intracellular loop was found to directly interact with both GGA1 and GGA2. More interestingly, further mapping of interaction domains showed that the GGA1 hinge region and the GGA2 GAE domain bound to multiple subdomains of the loop. These studies have identified an important function and revealed novel mechanisms of the GGA family proteins in the forward trafficking of a cell surface GPCR.

GGA3 Interacts with a G Protein-Coupled Receptor and Modulates Its Cell Surface Export

Molecular mechanisms governing the anterograde trafficking of nascent G protein-coupled receptors (GPCRs) are poorly understood. Here, we have studied the regulation of cell surface transport of α2-adrenergic receptors (α2-ARs) by GGA3 (Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding protein 3), a multidomain clathrin adaptor protein that sorts cargo proteins at the trans-Golgi network (TGN) to the endosome/lysosome pathway. By using an inducible system, we demonstrated that GGA3 knockdown significantly inhibited the cell surface expression of newly synthesized α2B-AR without altering overall receptor synthesis and internalization. The receptors were arrested in the TGN. Furthermore, GGA3 knockdown attenuated α2B-AR-mediated signaling, including extracellular signal-regulated kinase 1/2 (ERK1/2) activation and cyclic AMP (cAMP) inhibition. More interestingly, GGA3 physically interacted with α2B-AR, and the interaction sites were identified as the triple Arg motif in the third intracellular loop of the receptor and the acidic motif EDWE in the VHS domain of GGA3. In contrast, α2A-AR did not interact with GGA3 and its cell surface export and signaling were not affected by GGA3 knockdown. These data reveal a novel function of GGA3 in export trafficking of a GPCR that is mediated via a specific interaction with the receptor.

A single mutation in helix 8 enhances the angiotensin II type 1a receptor transport and signaling

The amphipathic helix 8 in the membrane-proximal C-terminus is a structurally conserved feature of class A seven transmembrane-spanning G protein-coupled receptors (GPCRs). Mutations of this helical motif often cause receptor misfolding, defective cell surface transport and dysfunction. Surprisingly, we demonstrated here that a single point mutation at Lys308 in helix 8 markedly enhanced the steady-state surface density of the angiotensin II type 1a receptor (AT1aR). Consistent with the enhanced cell surface expression, Lys308 mutation significantly augmented AT1aR-mediated mitogen-activated protein kinase ERK1/2 activation, inositol phosphate production, and vascular smooth muscle cell migration. This mutation also increased the overall expression of AT1aR without altering receptor degradation. More interestingly, Lys308 mutation abolished AT1aR interaction with β-COP, a component of COPI transport vesicles, and impaired AT1aR responsiveness to the inhibition of Rab6 GTPase involved in the Golgi-to-ER retrograde pathway. Furthermore, these functions of Lys308 were largely dependent on its positively charged property. These data reveal previously unappreciated functions of helix 8 and novel mechanisms governing the cell surface transport and function of AT1aR.

The mechanism and function of mitogen-activated protein kinase activation by ARF1

Mitogen-activated protein kinases (MAPK) can be activated by a number of biochemical pathways through distinct signaling molecules. We have recently revealed a novel function for the Ras-like small GTPase ADP-ribosylation factor 1 (ARF1) in mediating the activation of Raf1-MEK-ERK1/2 pathway by G protein-coupled receptors [Dong C, Li C and Wu G (2011) J Biol Chem 286, 43,361-43,369]. Here, we have further defined the underlying mechanism and the possible function of ARF1-mediated MAPK pathway. We demonstrated that the blockage of ARF1 activation and the disruption of ARF1 localization to the Golgi by mutating Thr48, a highly conserved residue involved in the exchange of GDP for GTP, and the myristoylation site Gly2 abolished ARF1's ability to activate ERK1/2. In addition, treatment with Golgi structure disrupting agents markedly attenuated ARF1-mediated ERK1/2 activation. Furthermore, ARF1 significantly promoted cell proliferation. More interestingly, ARF1 activated 90kDa ribosomal S6 kinase 1 (RSK1) without influencing Elk-1 activation and ERK2 translocation to the nuclei. These data demonstrate that, once activated, ARF1 activates the MAPK pathway likely using the Golgi as a main platform, which in turn activates the cytoplasmic RSK1, leading to cell proliferation.

Regulation of β-adrenergic receptor trafficking and lung microvascular endothelial cell permeability by Rab5 GTPase

Rab5 GTPase modulates the trafficking of the cell surface receptors, including G protein-coupled β-adrenergic receptors (β-ARs). Here, we have determined the role of Rab5 in regulating the internalization of β-ARs in lung microvascular endothelial cells (LMECs) and in maintaining the integrity and permeability of endothelial cell barrier. Our data demonstrate that lipopolysaccharide (LPS) treatment disrupts LMEC barrier function and reduces the cell surface expression of β-ARs. Furthermore, the activation of β-ARs, particularly β2-AR, is able to protect the LMEC permeability from LPS injury. Moreover, siRNA-mediated knockdown of Rab5 inhibits both the basal and agonist-provoked internalization of β-ARs, therefore, enhancing the cell surface expression of the receptors and receptor-mediated ERK1/2 activation. Importantly, knockdown of Rab5 not only inhibits the LPS-induced effects on β-ARs but also protects the LMEC monolayer permeability. All together, these data provide strong evidence indicating a crucial role of Rab5-mediated internalization of β-ARs in functional regulation of LMECs.

The GTPase Rab26 links synaptic vesicles to the autophagy pathway

Small GTPases of the Rab family not only regulate target recognition in membrane traffic but also control other cellular functions such as cytoskeletal transport and autophagy. Here we show that Rab26 is specifically associated with clusters of synaptic vesicles in neurites. Overexpression of active but not of GDP-preferring Rab26 enhances vesicle clustering, which is particularly conspicuous for the EGFP-tagged variant, resulting in a massive accumulation of synaptic vesicles in neuronal somata without altering the distribution of other organelles. Both endogenous and induced clusters co-localize with autophagy-related proteins such as Atg16L1, LC3B and Rab33B but not with other organelles. Furthermore, Atg16L1 appears to be a direct effector of Rab26 and binds Rab26 in its GTP-bound form, albeit only with low affinity. We propose that Rab26 selectively directs synaptic and secretory vesicles into preautophagosomal structures, suggesting the presence of a novel pathway for degradation of synaptic vesicles.

DOI:http://dx.doi.org/10.7554/eLife.05597.001

Our brain contains billions of cells called neurons that form an extensive network through which information is readily exchanged. These cells connect to each other via junctions called synapses. Our developing brain starts off with far more synapses than it needs, but the excess synapses are destroyed as the brain matures. Even in adults, synapses are continuously made and destroyed in response to experiences and learning.

Inside neurons there are tiny bubble-like compartments called vesicles that supply the synapses with many of the proteins and other components that they need. There is a growing body of evidence that suggests these vesicles are rapidly destroyed once a synapse is earmarked for destruction, but it is not clear how this may occur.

Here, Binotti, Pavlos et al. found that a protein called Rab26 sits on the surface of the vesicles near synapses. This protein promotes the formation of clusters of vesicles, and a membrane sometimes surrounds these clusters. Further experiments indicate that several proteins involved in a process called autophagy—where unwanted proteins and debris are destroyed—may also be found around the clusters of vesicles.

Autophagy starts with the formation of a membrane around the material that needs to be destroyed. This seals the material off from rest of the cell so that enzymes can safely break it down. Binotti, Pavlos et al. found that one of the autophagy proteins—called Atg16L—can bind directly to Rab26, but only when Rab26 is in an ‘active’ state.

These findings suggest that excess vesicles at synapses may be destroyed by autophagy. Further work will be required to establish how this process is controlled and how it is involved in the loss of synapses.

DOI:http://dx.doi.org/10.7554/eLife.05597.002

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.

Role of lipid rafts and GM1 in the segregation and processing of prion protein

The prion protein (PrP^C) is highly expressed within the nervous system. Similar to other GPI-anchored proteins, PrP^C is found in lipid rafts, membrane domains enriched in cholesterol and sphingolipids. PrP^C raft association, together with raft lipid composition, appears essential for the conversion of PrP^C into the scrapie isoform PrP^Sc, and the development of prion disease. Controversial findings were reported on the nature of PrP^C-containing rafts, as well as on the distribution of PrP^C between rafts and non-raft membranes. We investigated PrP^C/ganglioside relationships and their influence on PrP^C localization in a neuronal cellular model, cerebellar granule cells. Our findings argue that in these cells at least two PrP^C conformations coexist: in lipid rafts PrP^C is present in the native folding (α-helical), stabilized by chemico-physical condition, while it is mainly present in other membrane compartments in a PrP^Sc-like conformation. We verified, by means of antibody reactivity and circular dichroism spectroscopy, that changes in lipid raft-ganglioside content alters PrP^C conformation and interaction with lipid bilayers, without modifying PrP^C distribution or cleavage. Our data provide new insights into the cellular mechanism of prion conversion and suggest that GM1-prion protein interaction at the cell surface could play a significant role in the mechanism predisposing to pathology.

RAB26 coordinates lysosome traffic and mitochondrial localization

As they mature, professional secretory cells like pancreatic acinar and gastric chief cells induce the transcription factor MIST1 (also known as BHLHA15) to substantially scale up production of large secretory granules in a process that involves expansion of apical cytoplasm and redistribution of lysosomes and mitochondria. How a scaling factor like MIST1 rearranges cellular architecture simply by regulating expression levels of its transcriptional targets is unknown. RAB26 is a MIST1 target whose role in MIST1-mediated secretory cell maturation is also unknown. Here, we confirm that RAB26 expression, unlike most Rabs which are ubiquitously expressed, is tissue specific and largely confined to MIST1-expressing secretory tissues. Surprisingly, functional studies showed that RAB26 predominantly associated with LAMP1/cathepsin D lysosomes and not directly with secretory granules. Moreover, increasing RAB26 expression - by inducing differentiation of zymogen-secreting cells or by direct transfection - caused lysosomes to coalesce in a central, perinuclear region. Lysosome clustering in turn caused redistribution of mitochondria into distinct subcellular neighborhoods. The data elucidate a novel function for RAB26 and suggest a mechanism for how cells could increase transcription of key effectors to reorganize subcellular compartments during differentiation.

RAB GTPases and RAB-interacting proteins and their role in the control of cognitive functions

A RAS-related class of small monomeric G proteins, the RAB GTPases, is emerging as of key biological importance in compartment specific directional control of vesicles formation, transport and fusion. Thanks to human genetic observation and to the consequent dedicated biochemical work, substantial progress has been made on the understanding of the role played by RAB GTPases and their effector proteins on neuronal development and the shaping of cognitive functions. This review is highlighting these initial elements to broaden the current scope of research on developmental cognitive deficits and take the point of view of RAB GTPases control on membrane transport in neurons and astrocytes.

Rare Genomic Variants Link Bipolar Disorder with Anxiety Disorders to CREB-Regulated Intracellular Signaling Pathways

Bipolar disorder is a common, complex, and severe psychiatric disorder with cyclical disturbances of mood and a high suicide rate. Here, we describe a family with four siblings, three affected females and one unaffected male. The disease course was characterized by early-onset bipolar disorder and co-morbid anxiety spectrum disorders that followed the onset of bipolar disorder. Genetic risk factors were suggested by the early onset of the disease, the severe disease course, including multiple suicide attempts, and lack of adverse prenatal or early life events. In particular, drug and alcohol abuse did not contribute to the disease onset. Exome sequencing identified very rare, heterozygous, and likely protein-damaging variants in eight brain-expressed genes: IQUB, JMJD1C, GADD45A, GOLGB1, PLSCR5, VRK2, MESDC2, and FGGY. The variants were shared among all three affected family members but absent in the unaffected sibling and in more than 200 controls. The genes encode proteins with significant regulatory roles in the ERK/MAPK and CREB-regulated intracellular signaling pathways. These pathways are central to neuronal and synaptic plasticity, cognition, affect regulation and response to chronic stress. In addition, proteins in these pathways are the target of commonly used mood-stabilizing drugs, such as tricyclic antidepressants, lithium, and valproic acid. The combination of multiple rare, damaging mutations in these central pathways could lead to reduced resilience and increased vulnerability to stressful life events. Our results support a new model for psychiatric disorders, in which multiple rare, damaging mutations in genes functionally related to a common signaling pathway contribute to the manifestation of bipolar disorder.