Eps15: a multifunctional adaptor protein regulating intracellular trafficking

Redox Signaling in Endosomes Using the Example of EGF Receptors: A Graphical Review

Early endosomes represent first-line sorting compartments or even organelles for internalized molecules. They enable the transport of molecules or ligands to other compartments of the cell, such as lysosomes, for degradation or recycle them back to the membrane by various mechanisms. Moreover, early endosomes function as signaling and scaffolding platforms to initiate or prolong distinct signaling pathways. Accordingly, early endosomes have to be recognized as either part of a degradation or recycling pathway. The physical proximity of many ligand-binding receptors with other membrane-bound proteins or complexes such as NADPH oxidases may result in an interaction of second messengers, like reactive oxygen species (ROS) and early endosomes, that promote the correct recognition of individual early endosomes. In fact, redoxosomes comprise an endosomal subsection of signaling endosomes. One example of such potential interaction is epidermal growth factor receptor (EGFR) signaling. Here we summarize recent findings on EGFR signaling as a well-studied example for receptor trafficking and trans-activation and illustrate the interplay between cellular and endosomal ROS.

The Neglected Sibling: NLRP2 Inflammasome in the Nervous System

While classical NOD-like receptor pyrin domain containing protein 1 (NLRP1) and NLRP3 inflammasomal proteins have been extensively investigated, the contribution of NLRP2 is still ill-defined in the nervous system. Given the putative significance of NLRP2 in orchestrating neuroinflammation, further inquiry is needed to gain a better understanding of its connectome, hence its specific targeting may hold a promising therapeutic implication. Therefore, bioinformatical approach for extracting information, specifically in the context of neuropathologies, is also undoubtedly preferred. To the best of our knowledge, there is no review study selectively targeting only NLRP2. Increasing, but still fragmentary evidence should encourage researchers to thoroughly investigate this inflammasome in various animal- and human models. Taken together, herein we aimed to review the current literature focusing on the role of NLRP2 inflammasome in the nervous system and more importantly, we provide an algorithm-based protein network of human NLRP2 for elucidating potentially valuable molecular partnerships that can be the beginning of a new discourse and future therapeutic considerations.

Apicomplexa micropore: history, function, and formation

The micropore, a mysterious structure found in apicomplexan species, was recently shown to be essential for nutrient acquisition in Plasmodium falciparum and Toxoplasma gondii. However, the differences between the micropores of these two parasites questions the nature of a general apicomplexan micropore structure and whether the formation process model from Plasmodium can be applied to other apicomplexans. We analyzed the literature on different apicomplexan micropores and found that T. gondii probably harbors a more representative micropore type than the more widely studied ones in Plasmodium. Using recent knowledge of the Kelch 13 (K13) protein interactome and gene depletion phenotypes in the T. gondii micropore, we propose a model of micropore formation, thus enriching our wider understanding of micropore protein function.

EGFR-dependent endocytosis of Wnt9a and Fzd9b promotes β-catenin signaling during hematopoietic stem cell development in zebrafish

Cell-to-cell communication through secreted Wnt ligands that bind to members of the Frizzled (Fzd) family of transmembrane receptors is critical for development and homeostasis. Wnt9a signals through Fzd9b, the co-receptor LRP5 or LRP6 (LRP5/6), and the epidermal growth factor receptor (EGFR) to promote early proliferation of zebrafish and human hematopoietic stem cells during development. Here, we developed fluorescently labeled, biologically active Wnt9a and Fzd9b fusion proteins to demonstrate that EGFR-dependent endocytosis of the ligand-receptor complex was required for signaling. In human cells, the Wnt9a-Fzd9b complex was rapidly endocytosed and trafficked through early and late endosomes, lysosomes, and the endoplasmic reticulum. Using small-molecule inhibitors and genetic and knockdown approaches, we found that Wnt9a-Fzd9b endocytosis required EGFR-mediated phosphorylation of the Fzd9b tail, caveolin, and the scaffolding protein EGFR protein substrate 15 (EPS15). LRP5/6 and the downstream signaling component AXIN were required for Wnt9a-Fzd9b signaling but not for endocytosis. Knockdown or loss of EPS15 impaired hematopoietic stem cell development in zebrafish. Other Wnt ligands do not require endocytosis for signaling activity, implying that specific modes of endocytosis and trafficking may represent a method by which Wnt-Fzd specificity is established.

Large-scale phosphoproteomics reveals activation of the MAPK/GADD45β/P38 axis and cell cycle inhibition in response to BMP9 and BMP10 stimulation in endothelial cells

BACKGROUND

BMP9 and BMP10 are two major regulators of vascular homeostasis. These two ligands bind with high affinity to the endothelial type I kinase receptor ALK1, together with a type II receptor, leading to the direct phosphorylation of the SMAD transcription factors. Apart from this canonical pathway, little is known. Interestingly, mutations in this signaling pathway have been identified in two rare cardiovascular diseases, hereditary hemorrhagic telangiectasia and pulmonary arterial hypertension.

METHODS

To get an overview of the signaling pathways modulated by BMP9 and BMP10 stimulation in endothelial cells, we employed an unbiased phosphoproteomic-based strategy. Identified phosphosites were validated by western blot analysis and regulated targets by RT-qPCR. Cell cycle analysis was analyzed by flow cytometry.

RESULTS

Large-scale phosphoproteomics revealed that BMP9 and BMP10 treatment induced a very similar phosphoproteomic profile. These BMPs activated a non-canonical transcriptional SMAD-dependent MAPK pathway (MEKK4/P38). We were able to validate this signaling pathway and demonstrated that this activation required the expression of the protein GADD45β. In turn, activated P38 phosphorylated the heat shock protein HSP27 and the endocytosis protein Eps15 (EGF receptor pathway substrate), and regulated the expression of specific genes (E-selectin, hyaluronan synthase 2 and cyclooxygenase 2). This study also highlighted the modulation in phosphorylation of proteins involved in transcriptional regulation (phosphorylation of the endothelial transcription factor ERG) and cell cycle inhibition (CDK4/6 pathway). Accordingly, we found that BMP10 induced a G1 cell cycle arrest and inhibited the mRNA expression of E2F2, cyclinD1 and cyclinA1.

CONCLUSIONS

Overall, our phosphoproteomic screen identified numerous proteins whose phosphorylation state is impacted by BMP9 and BMP10 treatment, paving the way for a better understanding of the molecular mechanisms regulated by BMP signaling in vascular diseases.

Epsin Endocytic Adaptor Proteins in Angiogenic and Lymphangiogenic Signaling

Circulating vascular endothelial growth factor (VEGF) ligands and receptors are central regulators of vasculogenesis, angiogenesis, and lymphangiogenesis. In response to VEGF ligand binding, VEGF receptor tyrosine kinases initiate the chain of events that transduce extracellular signals into endothelial cell responses such as survival, proliferation, and migration. These events are controlled by intricate cellular processes that include the regulation of gene expression at multiple levels, interactions of numerous proteins, and intracellular trafficking of receptor-ligand complexes. Endocytic uptake and transport of macromolecular complexes through the endosome-lysosome system helps fine-tune endothelial cell responses to VEGF signals. Clathrin-dependent endocytosis remains the best understood means of macromolecular entry into cells, although the importance of non-clathrin-dependent pathways is increasingly recognized. Many of these endocytic events rely on adaptor proteins that coordinate internalization of activated cell-surface receptors. In the endothelium of both blood and lymphatic vessels, epsins 1 and 2 are functionally redundant adaptors involved in receptor endocytosis and intracellular sorting. These proteins are capable of binding both lipids and proteins and are important for promoting curvature of the plasma membrane as well as binding ubiquitinated cargo. Here, we discuss the role of epsin proteins and other endocytic adaptors in governing VEGF signaling in angiogenesis and lymphangiogenesis and discuss their therapeutic potential as molecular targets.

EGFR endocytosis: more than meets the eye

Behind the scenes of signaling cascades initiated by activated receptors, endocytosis determines the fate of internalized proteins through degradation in lysosomes or recycling. Over the years, significant progress has been made in understanding the mechanisms of endocytosis and deregulation in disease states. Here we review the role of the EGF-SNX3-EGFR axis in breast cancers with an extended discussion on deregulated EGFR endocytosis in cancer.

Orientia tsutsugamushi OtDUB Is Expressed and Interacts with Adaptor Protein Complexes during Infection

Orientia tsutsugamushi is an etiologic agent of scrub typhus, a globally emerging rickettsiosis that can be fatal. The bacterium's obligate intracellular lifestyle requires its interaction with host eukaryotic cellular pathways. The proteins it employs to do so and their functions during infection are understudied. Recombinant versions of the recently characterized O. tsutsugamushi deubiquitylase (OtDUB) exhibit high-affinity ubiquitin binding, mediate guanine nucleotide exchange to activate Rho GTPases, bind clathrin adaptor protein complexes 1 and 2, and bind the phospholipid phosphatidylserine. Whether OtDUB is expressed and its function during O. tsutsugamushi infection have yet to be explored. Here, OtDUB expression, location, and interactome during infection were examined. O. tsutsugamushi transcriptionally and translationally expresses OtDUB throughout infection of epithelial, monocytic, and endothelial cells. Results from structured illumination microscopy, surface trypsinization of intact bacteria, and acetic acid extraction of non-integral membrane proteins indicate that OtDUB peripherally associates with the O. tsutsugamushi cell wall and is at least partially present on the bacterial surface. Analyses of the proteins with which OtDUB associates during infection revealed several known O. tsutsugamushi cell wall proteins and others. It also forms an interactome with adapter protein complex 2 and other endosomal membrane traffic regulators. This study documents the first interactors of OtDUB during O. tsutsugamushi infection and establishes a strong link between OtDUB and the host endocytic pathway.

UBQLN proteins in health and disease with a focus on UBQLN2 in ALS/FTD

Ubiquilin (UBQLN) proteins are a dynamic and versatile family of proteins found in all eukaryotes that function in the regulation of proteostasis. Besides their canonical function as shuttle factors in delivering misfolded proteins to the proteasome and autophagy systems for degradation, there is emerging evidence that UBQLN proteins play broader roles in proteostasis. New information suggests the proteins function as chaperones in protein folding, protecting proteins prior to membrane insertion, and as guardians for mitochondrial protein import. In this review, we describe the evidence for these different roles, highlighting how different domains of the proteins impart these functions. We also describe how changes in the structure and phase separation properties of UBQLNs may regulate their activity and function. Finally, we discuss the pathogenic mechanisms by which mutations in UBQLN2 cause amyotrophic lateral sclerosis and frontotemporal dementia. We describe the animal model systems made for different UBQLN2 mutations and how lessons learnt from these systems provide fundamental insight into the molecular mechanisms by which UBQLN2 mutations drive disease pathogenesis through disturbances in proteostasis.

FCHO controls AP2's initiating role in endocytosis through a PtdIns(4,5)P2-dependent switch

Clathrin-mediated endocytosis (CME) is the main mechanism by which mammalian cells control their cell surface proteome. Proper operation of the pivotal CME cargo adaptor AP2 requires membrane-localized Fer/Cip4 homology domain-only proteins (FCHO). Here, live-cell enhanced total internal reflection fluorescence-structured illumination microscopy shows that FCHO marks sites of clathrin-coated pit (CCP) initiation, which mature into uniform-sized CCPs comprising a central patch of AP2 and clathrin corralled by an FCHO/Epidermal growth factor potential receptor substrate number 15 (Eps15) ring. We dissect the network of interactions between the FCHO interdomain linker and AP2, which concentrates, orients, tethers, and partially destabilizes closed AP2 at the plasma membrane. AP2's subsequent membrane deposition drives its opening, which triggers FCHO displacement through steric competition with phosphatidylinositol 4,5-bisphosphate, clathrin, cargo, and CME accessory factors. FCHO can now relocate toward a CCP's outer edge to engage and activate further AP2s to drive CCP growth/maturation.

Endocytosis at the Crossroad of Polarity and Signaling Regulation: Learning from Drosophila melanogaster and Beyond

Cellular trafficking through the endosomal–lysosomal system is essential for the transport of cargo proteins, receptors and lipids from the plasma membrane inside the cells and across membranous organelles. By acting as sorting stations, vesicle compartments direct the fate of their content for degradation, recycling to the membrane or transport to the trans-Golgi network. To effectively communicate with their neighbors, cells need to regulate their compartmentation and guide their signaling machineries to cortical membranes underlying these contact sites. Endosomal trafficking is indispensable for the polarized distribution of fate determinants, adaptors and junctional proteins. Conversely, endocytic machineries cooperate with polarity and scaffolding components to internalize receptors and target them to discrete membrane domains. Depending on the cell and tissue context, receptor endocytosis can terminate signaling responses but can also activate them within endosomes that act as signaling platforms. Therefore, cell homeostasis and responses to environmental cues rely on the dynamic cooperation of endosomal–lysosomal machineries with polarity and signaling cues. This review aims to address advances and emerging concepts on the cooperative regulation of endocytosis, polarity and signaling, primarily in Drosophila melanogaster and discuss some of the open questions across the different cell and tissue types that have not yet been fully explored.

Virus interactions with the actin cytoskeleton-what we know and do not know about SARS-CoV-2

The actin cytoskeleton and actin-dependent molecular and cellular events are responsible for the organization of eukaryotic cells and their functions. Viruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), depend on host cell organelles and molecular components for cell entry and propagation. Thus, it is not surprising that they also interact at many levels with the actin cytoskeleton of the host. There have been many studies on how different viruses reconfigure and manipulate the actin cytoskeleton of the host during successive steps of their life cycle. However, we know relatively little about the interactions of SARS-CoV-2 with the actin cytoskeleton. Here, we describe how the actin cytoskeleton is involved in the strategies used by different viruses for entry, assembly, and egress from the host cell. We emphasize what is known and unknown about SARS-CoV-2 in this regard. This review should encourage further investigation of the interactions of SARS-CoV-2 with cellular components, which will eventually be helpful for developing novel antiviral therapies for mitigating the severity of COVID-19.

Placental genomics mediates genetic associations with complex health traits and disease

As the master regulator in utero, the placenta is core to the Developmental Origins of Health and Disease (DOHaD) hypothesis but is historically understudied. To identify placental gene-trait associations (GTAs) across the life course, we perform distal mediator-enriched transcriptome-wide association studies (TWAS) for 40 traits, integrating placental multi-omics from the Extremely Low Gestational Age Newborn Study. At [Formula: see text], we detect 248 GTAs, mostly for neonatal and metabolic traits, across 176 genes, enriched for cell growth and immunological pathways. In aggregate, genetic effects mediated by placental expression significantly explain 4 early-life traits but no later-in-life traits. 89 GTAs show significant mediation through distal genetic variants, identifying hypotheses for distal regulation of GTAs. Investigation of one hypothesis in human placenta-derived choriocarcinoma cells reveal that knockdown of mediator gene EPS15 upregulates predicted targets SPATA13 and FAM214A, both associated with waist-hip ratio in TWAS, and multiple genes involved in metabolic pathways. These results suggest profound health impacts of placental genomic regulation in developmental programming across the life course.

Involvement of adaptor proteins in clathrin-mediated endocytosis of virus entry

The first step in the initiation of effective viral infection is breaking through the cytomembrane to enter the cell. Clathrin-mediated endocytosis is a key vesicular trafficking process in which a variety of cargo molecules are transported from the outside to the inside of the cell. This process is hijacked by numerous families of enveloped or non-enveloped viruses, which use it to enter host cells, followed by trafficking to their replicating sites. Various adaptor proteins that assist in cargo selection, coat assembly, and clathrin-coated bud maturation are important in this process. Research data documented on the involvement of adaptor proteins, such as AP-2, Eps-15, Epsin1, and AP180/CALM, in the invasion of viruses via the clathrin-mediated endocytosis have provided novel insights into understanding the viral life cycle and have led to the development of novel therapeutics. Here, we summarize the latest discoveries on the role of these adaptor proteins in clathrin-mediated endocytosis of virus entry and also discuss the future trends in this field.

The newly discovered role of endocytosis in artemisinin resistance

Artemisinin and its derivatives (ART) are the cornerstone of malaria treatment as part of artemisinin combination therapy (ACT). However, reduced susceptibility to artemisinin as well as its partner drugs threatens the usefulness of ACTs. Single point mutations in the parasite protein Kelch13 (K13) are necessary and sufficient for the reduced sensitivity of malaria parasites to ART but several alternative mechanisms for this resistance have been proposed. Recent work found that K13 is involved in the endocytosis of host cell cytosol and indicated that this is the process responsible for resistance in parasites with mutated K13. These studies also identified a series of further proteins that act together with K13 in the same pathway, including previously suspected resistance proteins such as UBP1 and AP-2μ. Here, we give a brief overview of artemisinin resistance, present the recent evidence of the role of endocytosis in ART resistance and discuss previous hypotheses in light of this new evidence. We also give an outlook on how the new insights might affect future research.

Whole exome sequencing, a hypothesis-free approach to investigate recurrent early miscarriage

RESEARCH QUESTION

Are there genetic determinants shared by unrelated women with unexplained recurrent early miscarriage (REM)?

DESIGN

Thirty REM cases and 30 controls were selected with extreme phenotype among women from Eastern Brittany (France), previously enrolled in an incident case-control study on thrombophilic mutations. Cases and controls were selected based on the number of early miscarriages or live births, respectively. Peripheral blood was collected for DNA extraction at initial visit. The burden of low-frequency variants in the coding part of the genes was compared using whole exome sequencing (WES).

RESULTS

Cases had 3 to 17 early miscarriages (20 cases: ≥5 previous losses). Controls had 1 to 4 live births (20 controls: ≥3 previous live births) and no miscarriages. WES data were available for 29 cases and 30 controls. A total of 209,387 variants were found (mean variant per patient: 59,073.05) with no difference between groups (P = 0.68). The top five most significantly associated genes were ABCA4, NFAM1, TCN2, AL078585.1 and EPS15. Previous studies suggest the involvement of vitamin B12 deficiency in REM. TCN2 encodes for vitamin B12 transporter into cells. Therefore, holotranscobalamin (active vitamin B12) was measured for both cases and controls (81.2 ± 32.1 versus 92.9 ± 34.3 pmol/l, respectively, P = 0.186). Five cases but no controls were below 50 pmol/l (P = 0.052).

CONCLUSIONS

This study highlights four new genes of interest in REM, some of which belong to known networks of genes involved in embryonic development (clathrin-mediated endocytosis and ciliary pathway). The study also confirms the involvement of TCN2 (vitamin B12 pathway) in the early first trimester of pregnancy.

The landscape of long noncoding RNA-involved and tumor-specific fusions across various cancers

The majority of the human genome encodes long noncoding RNA (lncRNA) genes, critical regulators of various cellular processes, which largely outnumber protein-coding genes. However, lncRNA-involved fusions have not been surveyed and characterized yet. Here, we present a systematic study of the lncRNA fusion landscape across cancer types and identify >30 000 high-confidence tumor-specific lncRNA fusions (using 8284 tumor and 6946 normal samples). Fusions positively correlated with DNA damage and cancer stemness and were specifically low in microsatellite instable (MSI)-High or virus-infected tumors. Moreover, fusions distribute differently among cancer molecular subtypes, but with shared enrichment in tumors that are microsatellite stable (MSS), with high somatic copy number alterations (SCNA), and with poor survival. Importantly, we find a potentially new mechanism, mediated by enhancer RNAs (eRNA), which generates secondary fusions that form densely connected fusion networks with many fusion hubs targeted by FDA-approved drugs. Finally, we experimentally validate functions of two tumor-promoting chimeric proteins derived from mRNA-lncRNA fusions, KDM4B-G039927 and EPS15L1-lncOR7C2-1. The EPS15L1 fusion protein may regulate (Gasdermin E) GSDME, critical in pyroptosis and anti-tumor immunity. Our study completes the fusion landscape in cancers, sheds light on fusion mechanisms, and enriches lncRNA functions in tumorigenesis and cancer progression.

Structure, dynamics and functions of UBQLNs: at the crossroads of protein quality control machinery

Cells rely on protein homeostasis to maintain proper biological functions. Dysregulation of protein homeostasis contributes to the pathogenesis of many neurodegenerative diseases and cancers. Ubiquilins (UBQLNs) are versatile proteins that engage with many components of protein quality control (PQC) machinery in cells. Disease-linked mutations of UBQLNs are most commonly associated with amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and other neurodegenerative disorders. UBQLNs play well-established roles in PQC processes, including facilitating degradation of substrates through the ubiquitin-proteasome system (UPS), autophagy, and endoplasmic-reticulum-associated protein degradation (ERAD) pathways. In addition, UBQLNs engage with chaperones to sequester, degrade, or assist repair of misfolded client proteins. Furthermore, UBQLNs regulate DNA damage repair mechanisms, interact with RNA-binding proteins (RBPs), and engage with cytoskeletal elements to regulate cell differentiation and development. Important to the myriad functions of UBQLNs are its multidomain architecture and ability to self-associate. UBQLNs are linked to numerous types of cellular puncta, including stress-induced biomolecular condensates, autophagosomes, aggresomes, and aggregates. In this review, we focus on deciphering how UBQLNs function on a molecular level. We examine the properties of oligomerization-driven interactions among the structured and intrinsically disordered segments of UBQLNs. These interactions, together with the knowledge from studies of disease-linked mutations, provide significant insights to UBQLN structure, dynamics and function.

Inactivation of MTOR promotes autophagy-mediated epithelial injury in particulate matter-induced airway inflammation

Particulate matter (PM) is able to induce airway epithelial injury, while the detailed mechanisms remain unclear. Here we demonstrated that PM exposure inactivated MTOR (mechanistic target of rapamycin kinase), enhanced macroautophagy/autophagy, and impaired lysosomal activity in HBE (human bronchial epithelial) cells and in mouse airway epithelium. Genetic or pharmaceutical inhibition of MTOR significantly enhanced, while inhibition of autophagy attenuated, PM-induced IL6 expression in HBE cells. Consistently, club-cell-specific deletion of Mtor aggravated, whereas loss of Atg5 in bronchial epithelium reduced, PM-induced airway inflammation. Interestingly, the augmented inflammatory responses caused by MTOR deficiency were markedly attenuated by blockage of downstream autophagy both in vitro and in vivo. Mechanistically, the dysregulation of MTOR-autophagy signaling was partially dependent on activation of upstream TSC2, and interacted with the TLR4-MYD88 to orchestrate the downstream NFKB activity and to regulate the production of inflammatory cytokines in airway epithelium. Moreover, inhibition of autophagy reduced the expression of EPS15 and the subsequent endocytosis of PM. Taken together, the present study provides a mechanistic explanation for how airway epithelium localized MTOR-autophagy axis regulates PM-induced airway injury, suggesting that activation of MTOR and/or suppression of autophagy in local airway might be effective therapeutic strategies for PM-related airway disorders.Abbreviations: ACTB: actin beta; AKT: AKT serine/threonine kinase; ALI: air liquid interface; AP2: adaptor related protein complex 2; ATG: autophagy related; BALF: bronchoalveolar lavage fluid; COPD: chronic obstructive pulmonary disease; CXCL: C-X-C motif chemokine ligand; DOX: doxycycline; EGF: epidermal growth factor; EGFR: epidermal growth factor receptor; EPS15: epidermal growth factor receptor pathway substrate 15; HBE: human bronchial epithelial; H&E: hematoxylin & eosin; IKK: IKB kinase; IL: interleukin; LAMP2: lysosomal-associated membrane protein 2; LPS: lipopolysaccharide; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MTEC: mouse tracheal epithelial cells; MTOR: mechanistic target of rapamycin kinase; MYD88: MYD88 innate immune signal transduction adaptor; NFKB: nuclear factor of kappa B; NFKBIA: NFKB inhibitor alpha; PM: particulate matter; PtdIns3K: phosphatidylinositol 3-kinase; Rapa: rapamycin; RELA: RELA proto-oncogene, NFKB subunit; SCGB1A1: secretoglobin family 1A member 1; siRNA: small interfering RNAs; SQSTM1: sequestosome 1; TEM: transmission electronic microscopy; TLR4: toll like receptor 4; TSC2: TSC complex subunit 2.

mGluR5/ERK signaling regulated the phosphorylation and function of glycine receptor α1ins subunit in spinal dorsal horn of mice

Inhibitory glycinergic transmission in adult spinal cord is primarily mediated by glycine receptors (GlyRs) containing the α1 subunit. Here, we found that α1^ins, a longer α1 variant with 8 amino acids inserted into the intracellular large loop (IL) between transmembrane (TM)3 and TM4 domains, was expressed in the dorsal horn of the spinal cord, distributed at inhibitory synapses, and engaged in negative control over nociceptive signal transduction. Activation of metabotropic glutamate receptor 5 (mGluR5) specifically suppressed α1^ins-mediated glycinergic transmission and evoked pain sensitization. Extracellular signal-regulated kinase (ERK) was critical for mGluR5 to inhibit α1^ins. By binding to a D-docking site created by the 8-amino–acid insert within the TM3–TM4 loop of α1^ins, the active ERK catalyzed α1^ins phosphorylation at Ser380, which favored α1^ins ubiquitination at Lys379 and led to α1^ins endocytosis. Disruption of ERK interaction with α1^ins blocked Ser380 phosphorylation, potentiated glycinergic synaptic currents, and alleviated inflammatory and neuropathic pain. These data thus unraveled a novel, to our knowledge, mechanism for the activity-dependent regulation of glycinergic neurotransmission.

Activity-dependent phosphorylation of the glycine receptor α1ins subunit by metabotropic glutamate receptor 5 and ERK kinase signalling causes endocytosis of α1ins and glycinergic disinhibition in the spinal cord dorsal horn, contributing to pain sensitization.

Amyloid-β Induces AMPA Receptor Ubiquitination and Degradation in Primary Neurons and Human Brains of Alzheimer's Disease

As the primary mediator for synaptic transmission, AMPA receptors (AMPARs) are crucial for synaptic plasticity and higher brain functions. A downregulation of AMPAR expression has been indicated as one of the early pathological molecular alterations in Alzheimer's disease (AD), presumably via amyloid-β (Aβ). However, the molecular mechanisms leading to the loss of AMPARs remain less clear. We report that in primary neurons, application of Aβ triggers AMPAR internalization accompanied with a decrease in cell-surface AMPAR expression. Importantly, in both Aβ-treated neurons and human brain tissue from AD patients, we observed a significant decrease in total AMPAR amount and an enhancement in AMPAR ubiquitination. Consistent with facilitated receptor degradation, AMPARs show higher turnover rates in the presence of Aβ. Furthermore, AD brain lysates and Aβ-incubated neurons show increased expression of the AMPAR E3 ligase Nedd4 and decreased expression of AMPAR deubiquitinase USP46. Changes in these enzymes are responsible for the Aβ-dependent AMPAR reduction. These findings indicate that AMPAR ubiquitination acts as the key molecular event leading to the loss of AMPARs and thus suppressed synaptic transmission in AD.

A nanobody-based molecular toolkit provides new mechanistic insight into clathrin-coat initiation

Besides AP-2 and clathrin triskelia, clathrin coat inception depends on a group of early-arriving proteins including Fcho1/2 and Eps15/R. Using genome-edited cells, we described the role of the unstructured Fcho linker in stable AP-2 membrane deposition. Here, expanding this strategy in combination with a new set of llama nanobodies against EPS15 shows an FCHO1/2-EPS15/R partnership plays a decisive role in coat initiation. A nanobody containing an Asn-Pro-Phe peptide within the complementarity-determining region 3 loop is a function-blocking pseudoligand for tandem EPS15/R EH domains. Yet, in living cells, EH domains gathered at clathrin-coated structures are poorly accessible, indicating residence by endogenous NPF-bearing partners. Forcibly sequestering cytosolic EPS15 in genome-edited cells with nanobodies tethered to early endosomes or mitochondria changes the subcellular location and availability of EPS15. This combined approach has strong effects on clathrin coat structure and function by dictating the stability of AP-2 assemblies at the plasma membrane.

Mutation-specific peripheral and ER quality control of hERG channel cell-surface expression

Impaired functional plasma membrane (PM) expression of the hERG K⁺-channel is associated with Long-QT syndrome type-2 (LQT2) and increased risk of cardiac arrhythmia. Reduced PM-expression is primarily attributed to retention and degradation of misfolded channels by endoplasmic reticulum (ER) protein quality control (QC) systems. However, as the molecular pathogenesis of LQT2 was defined using severely-misfolded hERG variants with limited PM-expression, the potential contribution of post-ER (peripheral) QC pathways to the disease phenotype remains poorly established. Here, we investigate the cellular processing of mildly-misfolded Per-Arnt-Sim (PAS)-domain mutant hERGs, which display incomplete ER-retention and PM-expression defects at physiological temperature. We show that the attenuated PM-expression of hERG is dictated by mutation-specific contributions from both the ER and peripheral QC systems. At the ER, PAS-mutants experience inefficient conformational maturation coupled with rapid ubiquitin-dependent proteasomal degradation. In post-ER compartments, they are rapidly endocytosed from the PM via a ubiquitin-independent mechanism and rapidly targeted for lysosomal degradation. Conformational destabilization underlies aberrant cellular processing at both ER- and post-ER compartments, since conformational correction by a hERG-specific pharmacochaperone or low-temperatures can restore WT-like trafficking. Our results demonstrate that the post-ER QC alone or jointly with the ER QC determines the loss-of-PM-expression phenotype of a subset of LQT2 mutations.

Redundant and nonredundant organismal functions of EPS15 and EPS15L1

This study unveils a redundant function for the endocytic proteins Eps15 and Eps15L1 in mouse embryo development and erythropoiesis, and a unique nonredundant role for Eps15L1 in the nervous system.

EPS15 and its homologous EPS15L1 are endocytic accessory proteins. Studies in mammalian cell lines suggested that EPS15 and EPS15L1 regulate endocytosis in a redundant manner. However, at the organismal level, it is not known to which extent the functions of the two proteins overlap. Here, by exploiting various constitutive and conditional null mice, we report redundant and nonredundant functions of the two proteins. EPS15L1 displays a unique nonredundant role in the nervous system, whereas both proteins are fundamental during embryo development as shown by the embryonic lethality of -Eps15/Eps15L1-double KO mice. At the cellular level, the major process redundantly regulated by EPS15 and EPS15L1 is the endocytosis of the transferrin receptor, a pathway that sustains the development of red blood cells and controls iron homeostasis. Consequently, hematopoietic-specific conditional Eps15/Eps15L1-double KO mice display traits of microcytic hypochromic anemia, due to a cell-autonomous defect in iron internalization.

Anti-enterovirus 71 activities of Melissa officinalis extract and its biologically active constituent rosmarinic acid

Enterovirus 71 (EV71) infection is endemic in the Asia-Pacific region. No specific antiviral drug has been available to treat EV71 infection. Melissa officinalis (MO) is a medicinal plant with long history of usage in the European and Middle East. We investigated whether an aqueous solution of concentrated methanolic extract (MOM) possesses antiviral activity. MOM inhibited plaque formation, cytopathic effect, and viral protein synthesis in EV71-infected cells. Using spectral techniques, we identified rosmarinic acid (RA) as a biologically active constituent of MOM. RA reduced viral attachment and entry; cleavage of eukaryotic translation initiation factor 4 G (eIF4G); reactive oxygen species (ROS) generation; and translocation of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) from nucleus to cytoplasm. It alleviated EV71-induced hyperphosphorylation of p38 kinase and EPS15. RA is likely to suppress ROS-mediated p38 kinase activation, and such downstream molecular events as hnRNP A1 translocation and EPS15-regulated membrane trafficking in EV71-infected cells. These findings suggest that MO and its constituent RA possess anti-EV71 activities, and may serve as a candidate drug for therapeutic and prophylactic uses against EV71 infection.

Antiviral activities of Schizonepeta tenuifolia Briq. against enterovirus 71 in vitro and in vivo

No effective drug is currently available for treatment of enterovirus 71 (EV71) infection. Schizonepeta tenuifolia Briq. (ST) has been used as a herbal constituent of traditional Chinese medicine. We studied whether the aqueous extract of Schizonepeta tenuifolia Briq (STE) has antiviral activity. STE inhibited replication of EV71, as evident by its ability to diminish plaque formation and cytopathic effect induced by EV71, and to inhibit the synthesis of viral RNA and protein. Moreover, daily single-dose STE treatment significantly improved the survival of EV71-infected mice, and ameliorated the symptoms. Mechanistically, STE exerts multiple effects on enteroviral infection. Treatment with STE reduced viral attachment and entry; the cleavage of eukaryotic translation initiation factor 4 G (eIF4G) by EV71 protease, 2A^pro; virus-induced reactive oxygen species (ROS) formation; and relocation of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) from the nucleus to the cytoplasm. It was accompanied by a decline in EV71-associated hyperphosphorylation of p38 kinase and EPS15. It is plausible that STE may inhibit ROS-induced p38 kinase activation, and subsequent hnRNP A1 relocation and EPS15-mediated membrane trafficking in infected cells. These findings suggest that STE possesses anti-EV71 activities, and may serve as health food or candidate antiviral drug for protection against EV71.

Interaction of phosphorylated Rab11-FIP2 with Eps15 regulates apical junction composition

MARK2 regulates the establishment of polarity in Madin-Darby canine kidney (MDCK) cells in part through phosphorylation of serine 227 of Rab11-FIP2. We identified Eps15 as an interacting partner of phospho-S227-Rab11-FIP2 (pS227-FIP2). During recovery from low calcium, Eps15 localized to the lateral membrane before pS227-FIP2 arrival. Later in recovery, Eps15 and pS227-FIP2 colocalized at the lateral membrane. In MDCK cells expressing the pseudophosphorylated FIP2 mutant FIP2(S227E), during recovery from low calcium, Eps15 was trapped and never localized to the lateral membrane. Mutation of any of the three NPF domains within GFP-FIP2(S227E) rescued Eps15 localization at the lateral membrane and reestablished single-lumen cyst formation in GFP-FIP2(S227E)-expressing cells in three-dimensional (3D) culture. Whereas expression of GFP-FIP2(S227E) induced the loss of E-cadherin and occludin, mutation of any of the NPF domains of GFP-FIP2(S227E) reestablished both proteins at the apical junctions. Knockdown of Eps15 altered the spatial and temporal localization of pS227-FIP2 and also elicited formation of multiple lumens in MDCK 3D cysts. Thus an interaction of Eps15 and pS227-FIP2 at the appropriate time and location in polarizing cells is necessary for proper establishment of epithelial polarity.

The temporally controlled expression of Drongo, the fruit fly homolog of AGFG1, is achieved in female germline cells via P-bodies and its localization requires functional Rab11

To achieve proper RNA transport and localization, RNA viruses exploit cellular vesicular trafficking pathways. AGFG1, a host protein essential for HIV-1 and Influenza A replication, has been shown to mediate release of intron-containing viral RNAs from the perinuclear region. It is still unknown what its precise role in this release is, or whether AGFG1 also participates in cytoplasmic transport. We report for the first time the expression patterns during oogenesis for Drongo, the fruit fly homolog of AGFG1. We find that temporally controlled Drongo expression is achieved by translational repression of drongo mRNA within P-bodies. Here we show a first link between the recycling endosome pathway and Drongo, and find that proper Drongo localization at the oocyte's cortex during mid-oogenesis requires functional Rab11.

Over-expression of EPS15 is a favorable prognostic factor in breast cancer

As a crucial player in terminating growth factor signaling, EPS15 plays important roles in many malignancies including breast cancer. To explore the potential association of EPS15 with the clinical outcome of breast cancer, we conducted gene expression survival analysis using six independent datasets, checked its expression quantitative loci and their associated genes, and explored the networking of these genes with EPS15. Our results show that over-expression of EPS15 is significantly associated with a favorable clinical outcome of breast cancer, especially in tumors harbouring a positive estrogen receptor status. 21 unique SNPs were found to be associated with EPS15 expression. Among the neighboring genes of these SNPs, five (MTUS1, DOCK5, MSRA, SLIT3 and SKAP1) are genetically connected with EPS15 and its physical partners. These genes including EPS15 also show significant concurrent expressions, and four exhibit distinct relevance regarding patient survival. High expressions of EPS15 and MSRA show a distinct combinatorial favorable survival, suggesting the clinical relevance of their co-activation. In summary, over-expression of EPS15 is a potential favorable prognostic marker in breast cancer, which can be used clinically alone or together with other genes such as MSRA to avail therapeutic decision-making.

A SPOPL/Cullin-3 ubiquitin ligase complex regulates endocytic trafficking by targeting EPS15 at endosomes

Cullin-3 (CUL3)-based ubiquitin ligases regulate endosome maturation and trafficking of endocytic cargo to lysosomes in mammalian cells. Here, we report that these functions depend on SPOPL, a substrate-specific CUL3 adaptor. We find that SPOPL associates with endosomes and is required for both the formation of multivesicular bodies (MVBs) and the endocytic host cell entry of influenza A virus. In SPOPL-depleted cells, endosomes are enlarged and fail to acquire intraluminal vesicles (ILVs). We identify a critical substrate ubiquitinated by CUL3-SPOPL as EPS15, an endocytic adaptor that also associates with the ESCRT-0 complex members HRS and STAM on endosomes. Indeed, EPS15 is ubiquitinated in a SPOPL-dependent manner, and accumulates with HRS in cells lacking SPOPL. Together, our data indicates that a CUL3-SPOPL E3 ubiquitin ligase complex regulates endocytic trafficking and MVB formation by ubiquitinating and degrading EPS15 at endosomes, thereby influencing influenza A virus infection as well as degradation of EGFR and other EPS15 targets.

DOI:http://dx.doi.org/10.7554/eLife.13841.001

Individual cells can move material, collectively referred to as cargo, from the outside environment into the cell interior via a process known as endocytosis. The cell then has different routes to transport the packages of cargo, called endocytic vesicles, to specific locations within the cell. Protein-based molecular machines move the cargo and control how it is selected and targeted to different destinations. For example, a molecular machine that contains a protein called CUL3 labels other components of the system with a chemical tag to regulate the route cargo takes in mammalian cells. However, it was not clear how CUL3 can selectively attach the chemical labels.

Gschweitl, Ulbricht et al. have now found that another protein called SPOPL provides selectivity for the CUL3-based machine during endocytosis in human cells. The experiments show that SPOPL attaches to endocytic vesicles, and that CUL3 and SPOPL work together to label a specific component of these vesicles called EPS15. The label changes how EPS15 interacts with other proteins. When SPOPL is not present in a cell, EPS15 is unnaturally stable and occupies many of the routes used by endocytic cargos. The cargo directly interacting with EPS15 is then routed on the fast lane to its destination, while other cargo accumulate in a kind of molecular traffic jam.

Other proteins like SPOPL are specific for the endocytic system. Exchange of SPOPL with these similar proteins in the CUL3 machine is likely to chemically label a different set of endocytic proteins. Gschweitl, Ulbricht et al.’s next challenge is to identify the selectivity, targeting and coordination of these exchangeable components in the endocytic system.

DOI:http://dx.doi.org/10.7554/eLife.13841.002

Structural basis for the recognition of two consecutive mutually interacting DPF motifs by the SGIP1 μ homology domain

FCHo1, FCHo2, and SGIP1 are key regulators of clathrin-mediated endocytosis. Their μ homology domains (μHDs) interact with the C-terminal region of an endocytic scaffold protein, Eps15, containing fifteen Asp-Pro-Phe (DPF) motifs. Here, we show that the high-affinity μHD-binding site in Eps15 is a region encompassing six consecutive DPF motifs, while the minimal μHD-binding unit is two consecutive DPF motifs. We present the crystal structures of the SGIP1 μHD in complex with peptides containing two DPF motifs. The peptides bind to a novel ligand-binding site of the μHD, which is distinct from those of other distantly related μHD-containing proteins. The two DPF motifs, which adopt three-dimensional structures stabilized by sequence-specific intramotif and intermotif interactions, are extensively recognized by the μHD and are both required for binding. Thus, consecutive and singly scattered DPF motifs play distinct roles in μHD binding.

The human immunodeficiency virus (HIV) Rev-binding protein (HRB) is a co-factor for HIV-1 Nef-mediated CD4 downregulation

Human immunodeficiency virus type 1 (HIV-1)-mediated CD4 downregulation is an important determinant of viral replication in vivo. Research on cellular co-factors involved in this process could lead to the identification of potential therapeutic targets. We found that CD4 surface levels were significantly higher in HIV-1-infected cells knocked-down for the HIV Rev-binding protein (HRB) compared with control cells. HRB knock-down affected CD4 downregulation induced by Nef but not by HIV-1 Vpu. Interestingly, the knock-down of the related protein HRBL (HRB-like), but not of the HRB interaction partner EPS15 (epidermal growth factor receptor pathway substrate 15), increased CD4 levels in Vpu-expressing cells significantly. Both of these proteins are known to be involved in HIV-1-mediated CD4 downregulation as co-factors of HIV-1 Nef. These results identify HRB as a previously unknown co-factor for HIV-1 Nef-mediated CD4 downregulation and highlight differences with the related protein HRBL, which affects the CD4 downregulation in a dual role as co-factor of both HIV-1 Nef and Vpu.

Eps15 homology domain containing protein of Plasmodium falciparum (PfEHD) associates with endocytosis and vesicular trafficking towards neutral lipid storage site

The human malaria parasite, Plasmodium falciparum, takes up numerous host cytosolic components and exogenous nutrients through endocytosis during the intra-erythrocytic stages. Eps15 homology domain-containing proteins (EHDs) are conserved NTPases, which are implicated in membrane remodeling and regulation of specific endocytic transport steps in eukaryotic cells. In the present study, we have characterized the dynamin-like C-terminal Eps15 homology domain containing protein of P. falciparum (PfEHD). Using a GFP-targeting approach, we studied localization and trafficking of PfEHD in the parasite. The PfEHD-GFP fusion protein was found to be a membrane bound protein that associates with vesicular network in the parasite. Time-lapse microscopy studies showed that these vesicles originate at parasite plasma membrane, migrate through the parasite cytosol and culminate into a large multi-vesicular like structure near the food-vacuole. Co-staining of food vacuole membrane showed that the multi-vesicular structure is juxtaposed but outside the food vacuole. Labeling of parasites with neutral lipid specific dye, Nile Red, showed that this large structure is neutral lipid storage site in the parasites. Proteomic analysis identified endocytosis modulators as PfEHD associated proteins in the parasites. Treatment of parasites with endocytosis inhibitors obstructed the development of PfEHD-labeled vesicles and blocked their targeting to the lipid storage site. Overall, our data suggests that the PfEHD is involved in endocytosis and plays a role in the generation of endocytic vesicles at the parasite plasma membrane, that are subsequently targeted to the neutral lipid generation/storage site localized near the food vacuole.

AP-2-associated protein kinase 1 and cyclin G-associated kinase regulate hepatitis C virus entry and are potential drug targets

Hepatitis C virus (HCV) enters its target cell via clathrin-mediated endocytosis. AP-2-associated protein kinase 1 (AAK1) and cyclin G-associated kinase (GAK) are host kinases that regulate clathrin adaptor protein (AP)-mediated trafficking in the endocytic and secretory pathways. We previously reported that AAK1 and GAK regulate HCV assembly by stimulating binding of the μ subunit of AP-2, AP2M1, to HCV core protein. We also discovered that AAK1 and GAK inhibitors, including the approved anticancer drugs sunitinib and erlotinib, could block HCV assembly. Here, we hypothesized that AAK1 and GAK regulate HCV entry independently of their effect on HCV assembly. Indeed, silencing AAK1 and GAK expression inhibited entry of pseudoparticles and cell culture grown-HCV and internalization of Dil-labeled HCV particles with no effect on HCV attachment or RNA replication. AAK1 or GAK depletion impaired epidermal growth factor (EGF)-mediated enhanced HCV entry and endocytosis of EGF receptor (EGFR), an HCV entry cofactor and erlotinib's cancer target. Moreover, either RNA interference-mediated depletion of AP2M1 or NUMB, each a substrate of AAK1 and/or GAK, or overexpression of either an AP2M1 or NUMB phosphorylation site mutant inhibited HCV entry. Last, in addition to affecting assembly, sunitinib and erlotinib inhibited HCV entry at a postbinding step, their combination was synergistic, and their antiviral effect was reversed by either AAK1 or GAK overexpression. Together, these results validate AAK1 and GAK as critical regulators of HCV entry that function in part by activating EGFR, AP2M1, and NUMB and as the molecular targets underlying the antiviral effect of sunitinib and erlotinib (in addition to EGFR), respectively.

IMPORTANCE

Understanding the host pathways hijacked by HCV is critical for developing host-centered anti-HCV approaches. Entry represents a potential target for antiviral strategies; however, no FDA-approved HCV entry inhibitors are currently available. We reported that two host kinases, AAK1 and GAK, regulate HCV assembly. Here, we provide evidence that AAK1 and GAK regulate HCV entry independently of their role in HCV assembly and define the mechanisms underlying AAK1- and GAK-mediated HCV entry. By regulating temporally distinct steps in the HCV life cycle, AAK1 and GAK represent "master regulators" of HCV infection and potential targets for antiviral strategies. Indeed, approved anticancer drugs that potently inhibit AAK1 or GAK inhibit HCV entry in addition to assembly. These results contribute to an understanding of the mechanisms of HCV entry and reveal attractive host targets for antiviral strategies as well as approved candidate inhibitors of these targets, with potential implications for other viruses that hijack clathrin-mediated pathways.

PHD3 regulates EGFR internalization and signalling in tumours

Tumours exploit their hypoxic microenvironment to induce a more aggressive phenotype, while curtailing the growth-inhibitory effects of hypoxia through mechanisms that are poorly understood. The prolyl hydroxylase PHD3 is regulated by hypoxia and plays an important role in tumour progression. Here we identify PHD3 as a central regulator of epidermal growth factor receptor (EGFR) activity through the control of EGFR internalization to restrain tumour growth. PHD3 controls EGFR activity by acting as a scaffolding protein that associates with the endocytic adaptor Eps15 and promotes the internalization of EGFR. In consequence, loss of PHD3 in tumour cells suppresses EGFR internalization and hyperactivates EGFR signalling to enhance cell proliferation and survival. Our findings reveal that PHD3 inactivation provides a novel route of EGFR activation to sustain proliferative signalling in the hypoxic microenvironment.

UIM domain-dependent recruitment of the endocytic adaptor protein Eps15 to ubiquitin-enriched endosomes

BACKGROUND

Eps15 is an endocytic adaptor protein that stimulates clathrin-mediated endocytosis. Among other interactions, Eps15 binds ubiquitin via UIM domains, recruiting ubiquitinated cargo into clathrin-coated vesicles. In EGF-treated cells, Eps15 also localizes to endosomes. The basis of this localization is not known.

RESULTS

We show that accumulation of ubiquitinated cargo can recruit Eps15 to endosomes via UIM domain interactions. First, treatment of SK-Br-3 breast cancer cells, which overexpress the EGFR family member ErbB2, with geldanamycin to promote receptor ubiquitination and endosomal transport, recruited FLAG-Eps15 to endosomes. Two in-frame ubiquitin constructs, PM-GFP-Ub (retained in endosomes after endocytosis), and GFP-FYVE-UbΔGG (targeted directly to endosomes) also recruited Eps15 to endosomes, as did slowing endosome maturation with constitutively-active Rab5-Q79L. Endosomal recruitment required the UIM domains, but not the N-terminal EH domains or central coiled-coil domains, of Eps15. Silencing of the endosomal Eps15 binding partner Hrs did not affect recruitment of Eps15 to ubiquitin-enriched endosomes. In fact, Hrs silencing itself modestly recruited Eps15 to endosomes, probably by accumulating endogenous ubiquitinated cargo. Eps15 silencing did not affect lysosomal degradation of ubiquitinated ErbB2; however, GFP-FYVE-UbΔGG overexpression inhibited internalization of EGFR and transferrin receptor.

CONCLUSIONS

We show for the first time that ubiquitin is sufficient for Eps15 recruitment to endosomes. We speculate that Eps15 recruitment to ubiquitin-rich endosomes may reduce the level of Eps15 at the plasma membrane, slowing endocytosis to allow time for processing of ubiquitinated cargo in endosomes.

Endocytic adaptor epidermal growth factor receptor substrate 15 (Eps15) is involved in the trafficking of ubiquitinated α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

AMPA-type glutamate receptors (AMPARs) play a critical role in mediating fast excitatory synaptic transmission in the brain. Alterations in receptor expression, distribution, and trafficking have been shown to underlie synaptic plasticity and higher brain functions, including learning and memory, as well as brain dysfunctions such as drug addiction and psychological disorders. Therefore, it is essential to elucidate the molecular mechanisms that regulate AMPAR dynamics. We have shown previously that mammalian AMPARs are subject to posttranslational modification by ubiquitin, with AMPAR ubiquitination enhancing receptor internalization and reducing AMPAR cell surface expression. Here we report a crucial role for epidermal growth factor receptor substrate 15 (Eps15), an endocytic adaptor, in ubiquitination-dependent AMPAR internalization. We find that suppression or overexpression of Eps15 results in changes in AMPAR surface expression. Eps15 interacts with AMPARs, which requires Nedd4-mediated GluA1 ubiquitination and the ubiquitin-interacting motif of Eps15. Importantly, we find that Eps15 plays an important role in AMPAR internalization. Knockdown of Eps15 suppresses the internalization of GluA1 but not the mutant GluA1 that lacks ubiquitination sites, indicating a role of Eps15 for the internalization of ubiquitinated AMPARs. These results reveal a novel molecular mechanism employed specifically for the trafficking of the ubiquitin-modified AMPARs.

Ubiquitin pathways in neurodegenerative disease

Control of proper protein synthesis, function, and turnover is essential for the health of all cells. In neurons these demands take on the additional importance of supporting and regulating the highly dynamic connections between neurons that are necessary for cognitive function, learning, and memory. Regulating multiple unique synaptic protein environments within a single neuron while maintaining cell health requires the highly regulated processes of ubiquitination and degradation of ubiquitinated proteins through the proteasome. In this review, we examine the effects of dysregulated ubiquitination and protein clearance on the handling of disease-associated proteins and neuronal health in the most common neurodegenerative diseases.

STRIPAK complexes: structure, biological function, and involvement in human diseases

The mammalian striatin family consists of three proteins, striatin, S/G2 nuclear autoantigen, and zinedin. Striatin family members have no intrinsic catalytic activity, but rather function as scaffolding proteins. Remarkably, they organize multiple diverse, large signaling complexes that participate in a variety of cellular processes. Moreover, they appear to be regulatory/targeting subunits for the major eukaryotic serine/threonine protein phosphatase 2A. In addition, striatin family members associate with germinal center kinase III kinases as well as other novel components, earning these assemblies the name striatin-interacting phosphatase and kinase (STRIPAK) complexes. Recently, there has been a great increase in functional and mechanistic studies aimed at identifying and understanding the roles of STRIPAK and STRIPAK-like complexes in cellular processes of multiple organisms. These studies have identified novel STRIPAK and STRIPAK-like complexes and have explored their roles in specific signaling pathways. Together, the results of these studies have sparked increased interest in striatin family complexes because they have revealed roles in signaling, cell cycle control, apoptosis, vesicular trafficking, Golgi assembly, cell polarity, cell migration, neural and vascular development, and cardiac function. Moreover, STRIPAK complexes have been connected to clinical conditions, including cardiac disease, diabetes, autism, and cerebral cavernous malformation. In this review, we discuss the expression, localization, and protein domain structure of striatin family members. Then we consider the diverse complexes these proteins and their homologs form in various organisms, emphasizing what is known regarding function and regulation. Finally, we explore possible roles of striatin family complexes in disease, especially cerebral cavernous malformation.

The architectural relationship of components controlling mast cell endocytosis

Eukaryotic cells use multiple routes for receptor internalization. Here, we examine the topographical relationships of clathrin-dependent and clathrin-independent endocytic structures on the plasma membranes of leukemia-derived mast cells. The high affinity IgE receptor (FcεRI) utilizes both pathways, whereas transferrin receptor serves as a marker for the classical clathrin-mediated endocytosis pathway. Both receptors were tracked by live-cell imaging in the presence or absence of inhibitors that established their differential dependence on specific endocytic adaptor proteins. The topology of antigen-bound FcεRI, clathrin, dynamin, Arf6 and Eps15-positive structures were analyzed by 2D and 3D immunoelectron microscopy techniques, revealing their remarkable spatial relationships and unique geometry. We conclude that the mast cell plasma membrane has multiple specialized domains for endocytosis. Their close proximity might reflect shared components, such as lipids and adaptor proteins, that facilitate inward membrane curvature. Intersections between these specialized domains might represent sorting stations that direct cargo to specific endocytic pathways.

A snapshot of the physical and functional wiring of the Eps15 homology domain network in the nematode

Protein interaction modules coordinate the connections within and the activity of intracellular signaling networks. The Eps15 Homology (EH) module, a protein-protein interaction domain that is a key feature of the EH-network, was originally identified in a few proteins involved in endocytosis and vesicle trafficking, and has subsequently also been implicated in actin reorganization, nuclear shuttling, and DNA repair. Here we report an extensive characterization of the physical connections and of the functional wirings of the EH-network in the nematode. Our data show that one of the major physiological roles of the EH-network is in neurotransmission. In addition, we found that the proteins of the network intersect, and possibly coordinate, a number of “territories” of cellular activity including endocytosis/recycling/vesicle transport, actin dynamics, general metabolism and signal transduction, ubiquitination/degradation of proteins, DNA replication/repair, and miRNA biogenesis and processing.

Identification of a novel motif that affects the conformation and activity of the MARCH1 E3 ubiquitin ligase

MARCH1, a member of the membrane-associated RING-CH family of E3 ubiquitin ligases, regulates antigen presentation by downregulating the cell surface expression of Major Histocompatibility Complex class II and CD86 molecules. MARCH1 is a transmembrane protein that exposes both its N- and C-terminus to the cytoplasm. We have conducted a structure-function analysis of its two cytoplasmic tails to gain insights into the trafficking of MARCH1 in the endocytic pathway. Fusion of the N-terminal portion of MARCH1 to a type II transmembrane reporter molecule revealed that this cytoplasmic tail contains endosomal sorting motifs. The C-terminal domain also appears to contain intracellular sorting signals because it reduced surface expression of a type I transmembrane reporter molecule. Mutation of the two putative C-terminal tyrosine-based sorting signals did not affect the activity of human MARCH1; however, it did reduce its incorporation into exosomes. Moreover, site-directed mutagenesis pointed to a functional C-terminal 221VQNC224 sequence that affects the spatial organization of the two cytoplasmic regions. This motif is also found in other RING-type E3 ubiquitin ligases, such as parkin. Altogether, these findings highlight the complex regulation of MARCH1 trafficking in the endocytic pathway as well as the intricate interactions between its cytoplasmic tails.

The clathrin adaptor Dab2 recruits EH domain scaffold proteins to regulate integrin β1 endocytosis

Endocytic adaptor proteins facilitate cargo recruitment and clathrin-coated pit nucleation. The prototypical clathrin adaptor AP2 mediates cargo recruitment, maturation, and scission of the pit by binding cargo, clathrin, and accessory proteins, including the Eps-homology (EH) domain proteins Eps15 and intersectin. However, clathrin-mediated endocytosis of some cargoes proceeds efficiently in AP2-depleted cells. We found that Dab2, another endocytic adaptor, also binds to Eps15 and intersectin. Depletion of EH domain proteins altered the number and size of clathrin structures and impaired the endocytosis of the Dab2- and AP2-dependent cargoes, integrin β1 and transferrin receptor, respectively. To test the importance of Dab2 binding to EH domain proteins for endocytosis, we mutated the EH domain-binding sites. This mutant localized to clathrin structures with integrin β1, AP2, and reduced amounts of Eps15. Of interest, although integrin β1 endocytosis was impaired, transferrin receptor internalization was unaffected. Surprisingly, whereas clathrin structures contain both Dab2 and AP2, integrin β1 and transferrin localize in separate pits. These data suggest that Dab2-mediated recruitment of EH domain proteins selectively drives the internalization of the Dab2 cargo, integrin β1. We propose that adaptors may need to be bound to their cargo to regulate EH domain proteins and internalize efficiently.

Eps15R is required for bone morphogenetic protein signalling and differentially compartmentalizes with Smad proteins

Transforming growth factor β superfamily members signal through Smad transcription factors. Bone morphogenetic proteins (BMPs) act via Smads 1, 5 and 8 and TGF-βs signal through Smads 2 and 3. The endocytic adaptor protein Eps15R, or 'epidermal growth factor (EGF) receptor pathway substrate 15-related protein' is a component of EGF signal transduction, mediating internalization of the EGF receptor. We show that it interacts with Smad proteins, is required for BMP signalling in animal caps and stimulates Smad1 transcriptional activity. This function resides in the Asp-Pro-Phe motif-enriched 'DPF domain' of Eps15R, which activates transcription and antagonizes Smad2 signalling. In living cells, Eps15R segregates into spatially distinct regions with different Smads, indicating an unrecognized level of Smad compartmentalization.

Baculovirus GP64-mediated entry into mammalian cells

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) serves as an efficient viral vector, not only for abundant gene expression in insect cells, but also for gene delivery into mammalian cells. Lentivirus vectors pseudotyped with the baculovirus envelope glycoprotein GP64 have been shown to acquire more potent gene transduction than those with vesicular stomatitis virus (VSV) envelope glycoprotein G. However, there are conflicting hypotheses about the molecular mechanisms of the entry of AcMNPV. Moreover, the mechanisms of the entry of pseudotyped viruses bearing GP64 into mammalian cells are not well characterized. Determination of the entry mechanisms of AcMNPV and the pseudotyped viruses bearing GP64 is important for future development of viral vectors that can deliver genes into mammalian cells with greater efficiency and specificity. In this study, we generated three pseudotyped VSVs, NPVpv, VSVpv, and MLVpv, bearing envelope proteins of AcMNPV, VSV, and murine leukemia virus, respectively. Depletion of membrane cholesterol by treatment with methyl-β-cyclodextrin, which removes cholesterol from cellular membranes, inhibited GP64-mediated internalization in a dose-dependent manner but did not inhibit attachment to the cell surface. Treatment of cells with inhibitors or the expression of dominant-negative mutants for dynamin- and clathrin-mediated endocytosis abrogated the internalization of AcMNPV and NPVpv into mammalian cells, whereas inhibition of caveolin-mediated endocytosis did not. Furthermore, inhibition of macropinocytosis reduced GP64-mediated internalization. These results suggest that cholesterol in the plasma membrane, dynamin- and clathrin-dependent endocytosis, and macropinocytosis play crucial roles in the entry of viruses bearing baculovirus GP64 into mammalian cells.

Mosquito-borne flaviviruses: overview of viral life-cycle and host–virus interactions

Mosquito-borne flaviviruses such as dengue virus, Japanese encephalitis virus and West Nile virus pose a threat to half of the world population and are a serious public health challenge in many developing countries. There are no effective vaccines or antivirals for most of these viruses. Viruses, being obligate parasites, hijack host pathways for efficient replication and therefore each step of viral life-cycle, namely entry into the host cell, genome replication, assembly and exit, requires the participation of host factors. Investigating the biology of mosquito-borne flaviviruses and the complex interplay of virus with its host will help in identifying drug targets and also in developing safer vaccines and antivirals. This article provides insights into the recent developments in our understanding of the virus–host interactions at various steps in the life-cycle of these viruses.