Activation of the epidermal growth factor (EGF) receptor induces formation of EGF receptor- and Grb2-containing clathrin-coated pits

Quantitative modeling of EGF receptor ligand discrimination via internalization proofreading

The epidermal growth factor receptor (EGFR) is a central regulator of cell physiology that is stimulated by multiple distinct ligands. Although ligands bind to EGFR while the receptor is exposed on the plasma membrane, EGFR incorporation into endosomes following receptor internalization is an important aspect of EGFR signaling, with EGFR internalization behavior dependent upon the type of ligand bound. We develop quantitative modeling for EGFR recruitment to and internalization from clathrin domains, focusing on how internalization competes with ligand unbinding from EGFR. We develop two model versions: a kinetic model with EGFR behavior described as transitions between discrete states and a spatial model with EGFR diffusion to circular clathrin domains. We find that a combination of spatial and kinetic proofreading leads to enhanced EGFR internalization ratios in comparison to unbinding differences between ligand types. Various stages of the EGFR internalization process, including recruitment to and internalization from clathrin domains, modulate the internalization differences between receptors bound to different ligands. Our results indicate that following ligand binding, EGFR may encounter multiple clathrin domains before successful recruitment and internalization. The quantitative modeling we have developed describes competition between EGFR internalization and ligand unbinding and the resulting proofreading.

GRB2 dimerization mediated by SH2 domain-swapping is critical for T cell signaling and cytokine production

GRB2 is an adaptor protein required for facilitating cytoplasmic signaling complexes from a wide array of binding partners. GRB2 has been reported to exist in either a monomeric or dimeric state in crystal and solution. GRB2 dimers are formed by the exchange of protein segments between domains, otherwise known as "domain-swapping". Swapping has been described between SH2 and C-terminal SH3 domains in the full-length structure of GRB2 (SH2/C-SH3 domain-swapped dimer), as well as between α-helixes in isolated GRB2 SH2 domains (SH2/SH2 domain-swapped dimer). Interestingly, SH2/SH2 domain-swapping has not been observed within the full-length protein, nor have the functional influences of this novel oligomeric conformation been explored. We herein generated a model of full-length GRB2 dimer with an SH2/SH2 domain-swapped conformation supported by in-line SEC-MALS-SAXS analyses. This conformation is consistent with the previously reported truncated GRB2 SH2/SH2 domain-swapped dimer but different from the previously reported, full-length SH2/C-terminal SH3 (C-SH3) domain-swapped dimer. Our model is also validated by several novel full-length GRB2 mutants that favor either a monomeric or a dimeric state through mutations within the SH2 domain that abrogate or promote SH2/SH2 domain-swapping. GRB2 knockdown and re-expression of selected monomeric and dimeric mutants in a T cell lymphoma cell line led to notable defects in clustering of the adaptor protein LAT and IL-2 release in response to TCR stimulation. These results mirrored similarly-impaired IL-2 release in GRB2-deficient cells. These studies show that a novel dimeric GRB2 conformation with domain-swapping between SH2 domains and monomer/dimer transitions are critical for GRB2 to facilitate early signaling complexes in human T cells.

An overview of receptor endocytosis and signaling

Endocytosis is a cellular process which mediates receptor internalization, nutrient uptake, and the regulation of cell signaling. Microorganisms (many bacteria and viruses) and toxins also use the same process and enter the cells. Generally, endocytosis is considered in the three forms such as phagocytosis (cell eating), pinocytosis (cell drinking), and highly selective receptor-mediated endocytosis (clathrin-dependent and independent). Several endocytic routes exist in an analogous, achieving diverse functions. Most studies on endocytosis have used transformed cells in culture. To visualize the receptor internalization, trafficking, and signaling in subcellular organelles, a green fluorescent protein-tagged receptor has been utilized. It also helps to visualize the endocytosis effects in live-cell imaging. Confocal laser microscopy increases our understanding of receptor endocytosis and signaling. Site-directed mutagenesis studies demonstrated that many short-sequence motifs of the cytoplasmic domain of receptors significantly play a vital role in receptor internalization, subcellular trafficking, and signaling. However, other factors also regulate receptor internalization through clathrin-coated vesicles. Receptor endocytosis can occur through clathrin-dependent and clathrin-independent pathways. This chapter briefly discusses the internalization, trafficking, and signaling of various receptors in normal conditions. In addition, it also highlights the malfunction of the receptor in disease conditions.

Clathrin-coated structures support 3D directed migration through local force transmission

Migrating cells navigate in complex environments through sensing and interpreting biochemical and/or mechanical cues. Here, we report that recently identified tubular clathrin/AP-2 lattices (TCALs), a subset of clathrin-coated structures (CCSs) that pinch collagen fibers, mechanically control directed migration along fibers decorated with ligands of CCS cargoes in three-dimensional (3D) environments. We observed that epidermal growth factor or low-density lipoprotein bound to collagen fibers leads to increased local nucleation and accumulation of TCALs. By using engineered, mixed collagen networks, we demonstrate that this mechanism selectively increases local forces applied on ligand-decorated fibers. We show that these effects depend on the ligand’s receptors but do not rely on their ability to trigger signaling events. We propose that the preferential accumulation of TCALs along ligand-decorated fibers steers migration in 3D environments. We conclude that ligand-regulated, local TCAL accumulation results in asymmetric force distribution that orients cell migration in 3D environments.

Using kICS to Reveal Changed Membrane Diffusion of AQP-9 Treated with Drugs

The formation of nanodomains in the plasma membrane are thought to be part of membrane proteins regulation and signaling. Plasma membrane proteins are often investigated by analyzing the lateral mobility. k-space ICS (kICS) is a powerful image correlation spectroscopy (ICS) technique and a valuable supplement to fluorescence correlation spectroscopy (FCS). Here, we study the diffusion of aquaporin-9 (AQP9) in the plasma membrane, and the effect of different membrane and cytoskeleton affecting drugs, and therefore nanodomain perturbing, using kICS. We measured the diffusion coefficient of AQP9 after addition of these drugs using live cell Total Internal Reflection Fluorescence imaging on HEK-293 cells. The actin polymerization inhibitors Cytochalasin D and Latrunculin A do not affect the diffusion coefficient of AQP9. Methyl-β-Cyclodextrin decreases GFP-AQP9 diffusion coefficient in the plasma membrane. Human epidermal growth factor led to an increase in the diffusion coefficient of AQP9. These findings led to the conclusion that kICS can be used to measure diffusion AQP9, and suggests that the AQP9 is not part of nanodomains.

Rapid whole cell imaging reveals a calcium-APPL1-dynein nexus that regulates cohort trafficking of stimulated EGF receptors

The endosomal system provides rich signal processing capabilities for responses elicited by growth factor receptors and their ligands. At the single cell level, endosomal trafficking becomes a critical component of signal processing, as exemplified by the epidermal growth factor (EGF) receptors. Activated EGFRs are trafficked to the phosphatase-enriched peri-nuclear region (PNR), where they are dephosphorylated and degraded. The details of the mechanisms that govern the movements of stimulated EGFRs towards the PNR, are not completely known. Here, exploiting the advantages of lattice light-sheet microscopy, we show that EGFR activation by EGF triggers a transient calcium increase causing a whole-cell level redistribution of Adaptor Protein, Phosphotyrosine Interacting with PH Domain And Leucine Zipper 1 (APPL1) from pre-existing endosomes within one minute, the rebinding of liberated APPL1 directly to EGFR, and the dynein-dependent translocation of APPL1-EGF-bearing endosomes to the PNR within ten minutes. The cell spanning, fast acting network that we reveal integrates a cascade of events dedicated to the cohort movement of activated EGF receptors. Our findings support the intriguing proposal that certain endosomal pathways have shed some of the stochastic strategies of traditional trafficking and have evolved processes that provide the temporal predictability that typify canonical signaling.

Functional Exhaustion of Type I and II Interferons Production in Severe COVID-19 Patients

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has emerged in Wuhan in December 2019 and has since spread across the world. Even though the majority of patients remain completely asymptomatic, some develop severe systemic complications. In this prospective study we compared the immunological profile of 101 COVID-19 patients with either mild, moderate or severe form of the disease according to the WHO classification, as well as of 50 healthy subjects, in order to identify functional immune factors independently associated with severe forms of COVID-19. Plasma cytokine levels, and cytokine levels upon in vitro non-specific stimulation of innate and adaptive immune cells, were measured at several time points during the course of the disease. As described previously, inflammatory cytokines IL1β, IL6, IL8, and TNFα associated with cytokine storm were significantly increased in the plasma of moderate and severe COVID-19 patients (p < 0.0001 for all cytokines). During follow-up, plasma IL6 levels decreased between the moment of admission to the hospital and at the last observation carried forward for patients with favorable outcome (p = 0.02148). After in vitro stimulation of immune cells from COVID-19 patients, reduced levels of both type I and type II interferons (IFNs) upon in vitro stimulation were correlated with increased disease severity [type I IFN (IFNα): p > 0.0001 mild vs. moderate and severe; type II IFN (IFNγ): p = 0.0002 mild vs. moderate and p < 0.0001 mild vs. severe] suggesting a functional exhaustion of IFNs production. Stimulated IFNα levels lower than 2.1 pg/ml and IFNγ levels lower than 15 IU/mL at admission to the hospital were associated with more complications during hospitalization (p = 0.0098 and p =0.0002, respectively). A low IFNγ level was also confirmed by multivariable analysis [p = 0.0349 OR = 0.98 (0.962; 0.999)] as an independent factor of complications. In vitro treatment with type IFNα restored type IFNγ secretion in COVID-19 patients while the secretion of pro-inflammatory cytokines IL6 and IL1β remained stable or decreased, respectively. These results (a) demonstrate a functional exhaustion of both innate and adaptive immune response in severe forms of COVID-19; (b) identify IFNα and IFNγ as new potential biomarkers of severity; and (c) highlight the importance of targeting IFNs when considering COVID-19 treatment in order to re-establish a normal balance between inflammatory and Th1 effector cytokines.

Specialised endocytic proteins regulate diverse internalisation mechanisms and signalling outputs in physiology and cancer

Although endocytosis was first described as the process mediating macromolecule or nutrient uptake through the plasma membrane, it is now recognised as a critical component of the cellular infrastructure involved in numerous processes, ranging from receptor signalling, proliferation and migration to polarity and stem cell regulation. To realise these varying roles, endocytosis needs to be finely regulated. Accordingly, multiple endocytic mechanisms exist that require specialised molecular machineries and an array of endocytic adaptor proteins with cell-specific functions. This review provides some examples of specialised functions of endocytic adaptors and other components of the endocytic machinery in different cell physiological processes, and how the alteration of these functions is linked to cancer. In particular, we focus on: (i) cargo selection and endocytic mechanisms linked to different adaptors; (ii) specialised functions in clathrin-mediated versus non-clathrin endocytosis; (iii) differential regulation of endocytic mechanisms by post-translational modification of endocytic proteins; (iv) cell context-dependent expression and function of endocytic proteins. As cases in point, we describe two endocytic protein families, dynamins and epsins. Finally, we discuss how dysregulation of the physiological role of these specialised endocytic proteins is exploited by cancer cells to increase cell proliferation, migration and invasion, leading to anti-apoptotic or pro-metastatic behaviours.

Cooperation and Interplay between EGFR Signalling and Extracellular Vesicle Biogenesis in Cancer

Epidermal growth factor receptor (EGFR) takes centre stage in carcinogenesis throughout its entire cellular trafficking odyssey. When loaded in extracellular vesicles (EVs), EGFR is one of the key proteins involved in the transfer of information between parental cancer and bystander cells in the tumour microenvironment. To hijack EVs, EGFR needs to play multiple signalling roles in the life cycle of EVs. The receptor is involved in the biogenesis of specific EV subpopulations, it signals as an active cargo, and it can influence the uptake of EVs by recipient cells. EGFR regulates its own inclusion in EVs through feedback loops during disease progression and in response to challenges such as hypoxia, epithelial-to-mesenchymal transition and drugs. Here, we highlight how the spatiotemporal rules that regulate EGFR intracellular function intersect with and influence different EV biogenesis pathways and discuss key regulatory features and interactions of this interplay. We also elaborate on outstanding questions relating to EGFR-driven EV biogenesis and available methods to explore them. This mechanistic understanding will be key to unravelling the functional consequences of direct anti-EGFR targeted and indirect EGFR-impacting cancer therapies on the secretion of pro-tumoural EVs and on their effects on drug resistance and microenvironment subversion.

Entry of Epidemic Keratoconjunctivitis-Associated Human Adenovirus Type 37 in Human Corneal Epithelial Cells

Purpose

Ocular infection by human adenovirus species D type 37 (HAdV-D37) causes epidemic keratoconjunctivitis, a severe, hyperacute condition. The corneal component of epidemic keratoconjunctivitis begins upon infection of corneal epithelium, and the mechanism of viral entry dictates subsequent proinflammatory gene expression. Therefore, it is important to understand the specific pathways of adenoviral entry in these cells.

Methods

Transmission electron microscopy of primary and tert-immortalized human corneal epithelial cells infected with HAdV-D37 was performed to identify the means of viral entry. Confocal microscopy was used to determine intracellular trafficking. The results of targeted small interfering RNA and specific chemical inhibitors were analyzed by quantitative PCR, and Western blot.

Results

By transmission electron microscopy, HAdV-D37 was seen to enter by both clathrin-coated pits and macropinocytosis; however, entry was both pH and dynamin 2 independent. Small interfering RNA against clathrin, AP2A1, and lysosome-associated membrane protein 1, but not early endosome antigen 1, decreased early viral gene expression. Ethyl-isopropyl amiloride, which blocks micropinocytosis, did not affect HAdV-D37 entry, but IPA, an inhibitor of p21-activated kinase, and important to actin polymerization, decreased viral entry in a dose-dependent manner.

Conclusions

HAdV-D37 enters human corneal epithelial cells by a noncanonical clathrin-mediated pathway involving lysosome-associated membrane protein 1 and PAK1, independent of pH, dynamin, and early endosome antigen 1. We showed earlier that HAdV-D37 enters human keratocytes through caveolae. Therefore, epidemic keratoconjunctivitis-associated viruses enter different corneal cell types via disparate pathways, which could account for a relative paucity of proinflammatory gene expression upon infection of corneal epithelial cells compared with keratocytes, as seen in prior studies.

Molecularly Distinct Clathrin-Coated Pits Differentially Impact EGFR Fate and Signaling

Adaptor protein 2 (AP2) is a major constituent of clathrin-coated pits (CCPs). Whether it is essential for all forms of clathrin-mediated endocytosis (CME) in mammalian cells is an open issue. Here, we demonstrate, by live TIRF microscopy, the existence of a subclass of relatively short-lived CCPs lacking AP2 under physiological, unperturbed conditions. This subclass is retained in AP2-knockout cells and is able to support the internalization of epidermal growth factor receptor (EGFR) but not of transferrin receptor (TfR). The AP2-independent internalization mechanism relies on the endocytic adaptors eps15, eps15L1, and epsin1. The absence of AP2 impairs the recycling of the EGFR to the cell surface, thereby augmenting its degradation. Accordingly, under conditions of AP2 ablation, we detected dampening of EGFR-dependent AKT signaling and cell migration, arguing that distinct classes of CCPs could provide specialized functions in regulating EGFR recycling and signaling.

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Distinct classes of CCPs exist, molecularly defined by the presence or lack of AP2

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The AP2-negative CCPs support the internalization of EGFR but not of TfR

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The AP2-negative CCPs rely on the endocytic adaptors eps15/eps15L1 and epsin1

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The two classes of CCPs determine distinct EGFR fates and signaling outputs

Clathrin regulates Wnt/β-catenin signaling by affecting Golgi to plasma membrane transport of transmembrane proteins

The canonical Wnt/β-catenin signaling pathway regulates cell proliferation in development and adult tissue homeostasis. Dysregulated signaling contributes to human diseases, in particular cancer. Growing evidence suggests a role for clathrin and/or endocytosis in the regulation of this pathway, but conflicting results exist and demand a deeper mechanistic understanding. We investigated the consequences of clathrin depletion on Wnt/β-catenin signaling in cell lines and found a pronounced reduction in β-catenin protein levels, which affects the amount of nuclear β-catenin and β-catenin target gene expression. Although we found no evidence that clathrin affects β-catenin levels via endocytosis or multivesicular endosome formation, an inhibition of protein transport through the biosynthetic pathway led to reduced levels of a Wnt co-receptor, low-density lipoprotein receptor-related protein 6 (LRP6), and cell adhesion molecules of the cadherin family, thereby affecting steady-state levels of β-catenin. We conclude that clathrin impacts on Wnt/β-catenin signaling by controlling exocytosis of transmembrane proteins, including cadherins and Wnt co-receptors that together control the membrane-bound and soluble pools of β-catenin.

Regulation of the EGFR/ErbB signalling by clathrin in response to various ligands in hepatocellular carcinoma cell lines

Membrane receptor intracellular trafficking and signalling are frequently altered in cancers. Our aim was to investigate whether clathrin‐dependent trafficking modulates signalling of the ErbB receptor family in response to amphiregulin (AR), EGF, heparin‐binding EGF‐like growth factor (HB‐EGF) and heregulin‐1β (HRG). Experiments were performed using three hepatocellular carcinoma (HCC) cell lines, Hep3B, HepG2 and PLC/PRF/5, expressing various levels of EGFR, ErbB2 and ErbB3. Inhibition of clathrin‐mediated endocytosis (CME), by down‐regulating clathrin heavy chain expression, resulted in a cell‐ and ligand‐specific pattern of phosphorylation of the ErbB receptors and their downstream effectors. Clathrin down‐regulation significantly decreased the ratio between phosphorylated EGFR (pEGFR) and total EGFR in all cell lines when stimulated with AR, EGF, HB‐EGF or HRG, except in HRG‐stimulated Hep3B cells in which pEGFR was not detectable. The ratio between phosphorylated ErbB2 and total ErbB2 was significantly decreased in clathrin down‐regulated Hep3B cells stimulated with any of the ligands, and in HRG‐stimulated PLC/PRF/5 cells. The ratio between phosphorylated ErbB3 and total ErbB3 significantly decreased in clathrin down‐regulated cell lines upon stimulation with EGF or HB‐EGF. STAT3 phosphorylation levels significantly increased in all cell lines irrespective of stimulation, while that of AKT remained unchanged, except in AR‐stimulated Hep3B and HepG2 cells in which pAKT was significantly decreased. Finally, ERK phosphorylation was insensitive to clathrin inhibition. Altogether, our observations indicate that clathrin regulation of ErbB signalling in HCC is a complex process that likely depends on the expression of ErbB family members and on the autocrine/paracrine secretion of their ligands in the tumour environment.

Rab25-Mediated EGFR Recycling Causes Tumor Acquired Radioresistance

Tumor acquired radioresistance remains as the major limit in cancer radiotherapy (RT). Rab25, a receptor recycling protein, has been reported to be enhanced in tumors with aggressive phenotype and chemotherapy resistance. In this study, elevated Rab25 expression was identified in an array of radioresistant human cancer cell lines, in vivo radioresistant xenograft tumors. Clinical investigation confirmed that Rab25 expression was also associated with a worse prognosis in patients with lung adenocarcinoma (LUAD) and nasopharyngeal carcinoma (NPC). Enhanced activities of EGFR were observed in both NPC and LUAD radioresistant cells. Rab25 interacts with EGFR to enhance EGFR recycling to cell surface and to decrease degradation in cytoplasm. Inhibition of Rab25 showed synergized radiosensitivity with reduced aggressive phenotype. This study provides the clinical and experimental evidence that Rab25 is a potential therapeutic target to alleviate the hyperactive EGFR signaling and to prevent RT-acquired tumor resistance in patients with LUAD and NPC.

Transport of the Thyroid Hormone Carrier Protein Transthyretin into Human Epidermoid Cells

Purpose: Transthyretin (TTR) is a protein with a growing number of biological functions in addition to its well-established binding and circulatory transport of thyroxine, and indirect retinoid transport through interaction with retinol-binding protein. Misfolded and aggregated wild-type and mutant TTRs are involved in amyloid diseases. Several aspects of TTR pathology and physiology remain poorly understood. Receptor-mediated cellular transport of TTR has been described in a few cell types; and such studies suggest the possibility of different TTR receptors and endocytic pathways. Our main objective was to further understand the endocytic pathways for TTR.Methods: In the current study, analyses of TTR endocytic transport were performed in the human A431 cell line. The results of TTR uptake were compared with those of the iron-carrier protein transferrin (Tf, a common stardard for endocytosis studies) in the same cell types.Results: A comparison of TTR and Tf endocytosis suggested similar early, 5-10 min, accumulation kinetics. But at a later time, 30 min, TTR accumulation was 20-30% lower than that of Tf (p < .05), a result that suggests different post-endocytic fates for these two ligands. Through the use of multiple endocytosis inhibitors, biochemical evidence is provided for an internalization pathway that differs from the clathrin-mediated endocytosis of Tf.Conclusions: These results for A431 cells are compared with others reported for different cell types; and it is suggested that this same hormone carrier protein can transit into cells through multiple endocytic pathways.

Emerging concepts of receptor endocytosis and concurrent intracellular signaling: Mechanisms of guanylyl cyclase/natriuretic peptide receptor-A activation and trafficking

Endocytosis is a prominent clathrin-mediated mechanism for concentrated uptake and internalization of ligand-receptor complexes, also known as cargo. Internalization of cargo is the fundamental mechanism for receptor-dependent regulation of cell membrane function, intracellular signal transduction, and neurotransmission, as well as other biological and physiological activities. However, the intrinsic mechanisms of receptor endocytosis and contemporaneous intracellular signaling are not well understood. We review emerging concepts of receptor endocytosis with concurrent intracellular signaling, using a typical example of guanylyl cyclase/natriuretic peptide receptor-A (NPRA) internalization, subcellular trafficking, and simultaneous generation of second-messenger cGMP and signaling in intact cells. We highlight the role of short-signal motifs located in the carboxyl-terminal regions of membrane receptors during their internalization and subsequent receptor trafficking in organelles that are not traditionally studied in this context, including nuclei and mitochondria. This review sheds light on the importance of future investigations of receptor endocytosis and trafficking in live cells and intact animals in vivo in physiological context.

Clathrin-dependent internalization, signaling, and metabolic processing of guanylyl cyclase/natriuretic peptide receptor-A

Cardiac hormones, atrial and brain natriuretic peptides (ANP and BNP), have pivotal roles in renal hemodynamics, neuroendocrine signaling, blood pressure regulation, and cardiovascular homeostasis. Binding of ANP and BNP to the guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) induces rapid internalization and trafficking of the receptor via endolysosomal compartments, with concurrent generation of cGMP. However, the mechanisms of the endocytotic processes of NPRA are not well understood. The present study, using ¹²⁵I-ANP binding assay and confocal microscopy, examined the function of dynamin in the internalization of NPRA in stably transfected human embryonic kidney-293 (HEK-293) cells. Treatment of recombinant HEK-293 cells with ANP time-dependently accelerated the internalization of receptor from the cell surface to the cell interior. However, the internalization of ligand-receptor complexes of NPRA was drastically decreased by the specific inhibitors of clathrin- and dynamin-dependent receptor internalization, almost 85% by monodansylcadaverine, 80% by chlorpromazine, and 90% by mutant dynamin, which are specific blockers of endocytic vesicle formation. Visualizing the internalization of NPRA and enhanced GFP-tagged NPRA in HEK-293 cells by confocal microscopy demonstrated the formation of endocytic vesicles after 5 min of ANP treatment; this effect was blocked by the inhibitors of clathrin and by mutant dynamin construct. Our results suggest that NPRA undergoes internalization via clathrin-mediated endocytosis as part of its normal itinerary, including trafficking, signaling, and metabolic degradation.

Selective regulation of clathrin-mediated epidermal growth factor receptor signaling and endocytosis by phospholipase C and calcium

Clathrin-mediated endocytosis is a major regulator of cell-surface protein internalization. Clathrin and other proteins assemble into small invaginating structures at the plasma membrane termed clathrin-coated pits (CCPs) that mediate vesicle formation. In addition, epidermal growth factor receptor (EGFR) signaling is regulated by its accumulation within CCPs. Given the diversity of proteins regulated by clathrin-mediated endocytosis, how this process may distinctly regulate specific receptors is a key question. We examined the selective regulation of clathrin-dependent EGFR signaling and endocytosis. We find that perturbations of phospholipase Cγ1 (PLCγ1), Ca²⁺, or protein kinase C (PKC) impair clathrin-mediated endocytosis of EGFR, the formation of CCPs harboring EGFR, and EGFR signaling. Each of these manipulations was without effect on the clathrin-mediated endocytosis of transferrin receptor (TfR). EGFR and TfR were recruited to largely distinct clathrin structures. In addition to control of initiation and assembly of CCPs, EGF stimulation also elicited a Ca²⁺- and PKC-dependent reduction in synaptojanin1 recruitment to clathrin structures, indicating broad control of CCP assembly by Ca²⁺ signals. Hence EGFR elicits PLCγ1-calcium signals to facilitate formation of a subset of CCPs, thus modulating its own signaling and endocytosis. This provides evidence for the versatility of CCPs to control diverse cellular processes.

Segmentation of 3D Trajectories Acquired by TSUNAMI Microscope: An Application to EGFR Trafficking

Whereas important discoveries made by single-particle tracking have changed our view of the plasma membrane organization and motor protein dynamics in the past three decades, experimental studies of intracellular processes using single-particle tracking are rather scarce because of the lack of three-dimensional (3D) tracking capacity. In this study we use a newly developed 3D single-particle tracking method termed TSUNAMI (Tracking of Single particles Using Nonlinear And Multiplexed Illumination) to investigate epidermal growth factor receptor (EGFR) trafficking dynamics in live cells at 16/43 nm (xy/z) spatial resolution, with track duration ranging from 2 to 10 min and vertical tracking depth up to tens of microns. To analyze the long 3D trajectories generated by the TSUNAMI microscope, we developed a trajectory analysis algorithm, which reaches 81% segment classification accuracy in control experiments (termed simulated movement experiments). When analyzing 95 EGF-stimulated EGFR trajectories acquired in live skin cancer cells, we find that these trajectories can be separated into three groups-immobilization (24.2%), membrane diffusion only (51.6%), and transport from membrane to cytoplasm (24.2%). When EGFRs are membrane-bound, they show an interchange of Brownian diffusion and confined diffusion. When EGFRs are internalized, transitions from confined diffusion to directed diffusion and from directed diffusion back to confined diffusion are clearly seen. This observation agrees well with the model of clathrin-mediated endocytosis.

A Christianson syndrome-linked deletion mutation (∆(287)ES(288)) in SLC9A6 disrupts recycling endosomal function and elicits neurodegeneration and cell death

Background

Christianson Syndrome, a recently identified X-linked neurodevelopmental disorder, is caused by mutations in the human gene SLC9A6 encoding the recycling endosomal alkali cation/proton exchanger NHE6. The patients have pronounced limitations in cognitive ability, motor skills and adaptive behaviour. However, the mechanistic basis for this disorder is poorly understood as few of the more than 20 mutations identified thus far have been studied in detail.

Methods

Here, we examined the molecular and cellular consequences of a 6 base-pair deletion of amino acids Glu²⁸⁷ and Ser²⁸⁸ (∆ES) in the predicted seventh transmembrane helix of human NHE6 expressed in established cell lines (CHO/AP-1, HeLa and neuroblastoma SH-SY5Y) and primary cultures of mouse hippocampal neurons by measuring levels of protein expression, stability, membrane trafficking, endosomal function and cell viability.

Results

In the cell lines, immunoblot analyses showed that the nascent mutant protein was properly synthesized and assembled as a homodimer, but its oligosaccharide maturation and half-life were markedly reduced compared to wild-type (WT) and correlated with enhanced ubiquitination leading to both proteasomal and lysosomal degradation. Despite this instability, a measurable fraction of the transporter was correctly sorted to the plasma membrane. However, the rates of clathrin-mediated endocytosis of the ∆ES mutant as well as uptake of companion vesicular cargo, such as the ligand-bound transferrin receptor, were significantly reduced and correlated with excessive endosomal acidification. Notably, ectopic expression of ∆ES but not WT induced apoptosis when examined in AP-1 cells. Similarly, in transfected primary cultures of mouse hippocampal neurons, membrane trafficking of the ∆ES mutant was impaired and elicited marked reductions in total dendritic length, area and arborization, and triggered apoptotic cell death.

Conclusions

These results suggest that loss-of-function mutations in NHE6 disrupt recycling endosomal function and trafficking of cargo which ultimately leads to neuronal degeneration and cell death in Christianson Syndrome.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0129-9) contains supplementary material, which is available to authorized users.

Interaction with epsin 1 regulates the constitutive clathrin-dependent internalization of ErbB3

BACKGROUND

In contrast to other members of the EGF receptor family, ErbB3 is constitutively internalized in a clathrin-dependent manner. Previous studies have shown that ErbB3 does not interact with the coated pit localized adaptor complex 2 (AP-2), and that ErbB3 lacks two AP-2 interacting internalization signals identified in the EGF receptor. Several other clathrin-associated sorting proteins which may recruit cargo into coated pits have, however, been identified, and the study was performed to identify adaptors needed for constitutive internalization of ErbB3.

METHODS

A high-throughput siRNA screen was used to identify adaptor proteins needed for internalization of ErbB3. Upon knock-down of candidate proteins internalization of ErbB3 was identified using an antibody-based internalization assay combined with automatic fluorescence microscopy.

RESULTS

Among 29 candidates only knock-down of epsin 1 turned out to inhibit ErbB3. Epsin 1 has ubiquitin interacting motifs (UIMs) and we show that ErbB3 interacts with an epsin 1 deletion mutant containing these UIMs. In support of an ErbB3-epsin 1 UIM dependent interaction, we show that ErbB3 is constitutively ubiquitinated, but that both ubiquitination and the ErbB3-epsin 1 interaction increase upon ligand binding.

CONCLUSION

Altogether the results are consistent with a model whereby both constitutive and ligand-induced internalization of ErbB3 are regulated through interaction with epsin 1.

GENERAL SIGNIFICANCE

Internalization is an important regulator of growth factor receptor mediated signaling and the current study identify mechanisms regulating plasma membrane turnover of ErbB3.

Endocytosis and trafficking of BMP receptors: Regulatory mechanisms for fine-tuning the signaling response in different cellular contexts

Signaling by bone morphogenetic protein (BMP) receptors is regulated at multiple levels in order to ensure proper interpretation of BMP stimuli in different cellular settings. As with other signaling receptors, regulation of the amount of exposed and signaling-competent BMP receptors at the plasma-membrane is predicted to be a key mechanism in governing their signaling output. Currently, the endocytosis of BMP receptors is thought to resemble that of the structurally related transforming growth factor-β (TGF-β) receptors, as BMP receptors are constitutively internalized (independently of ligand binding), with moderate kinetics, and mostly via clathrin-mediated endocytosis. Also similar to TGF-β receptors, BMP receptors are able to signal from the plasma membrane, while internalization to endosomes may have a signal modulating effect. When at the plasma membrane, BMP receptors localize to different membrane domains including cholesterol rich domains and caveolae, suggesting a complex interplay between membrane distribution and internalization. An additional layer of complexity stems from the putative regulatory influence on the signaling and trafficking of BMP receptors exerted by ligand traps and/or co-receptors. Furthermore, the trafficking and signaling of BMP receptors are subject to alterations in cellular context. For example, genetic diseases involving changes in the expression of auxiliary factors of endocytic pathways hamper retrograde BMP signals in neurons, and perturb the regulation of synapse formation. This review summarizes current understanding of the trafficking of BMP receptors and discusses the role of trafficking in regulation of BMP signals.

Polarised clathrin-mediated endocytosis of EGFR during chemotactic invasion

Directed cell migration is critical for numerous physiological processes including development and wound healing. However chemotaxis is also exploited during cancer progression. Recent reports have suggested links between vesicle trafficking pathways and directed cell migration. Very little is known about the potential roles of endocytosis pathways during metastasis. Therefore we performed a series of studies employing a previously characterised model for chemotactic invasion of cancer cells to assess specific hypotheses potentially linking endocytosis to directed cell migration. Our results demonstrate that clathrin-mediated endocytosis is indispensable for epidermal growth factor (EGF) directed chemotactic invasion of MDA-MB-231 cells. Conversely, caveolar endocytosis is not required in this mode of migration. We further found that chemoattractant receptor (EGFR) trafficking occurs by clathrin-mediated endocytosis and is polarised towards the front of migrating cells. However, we found no role for clathrin-mediated endocytosis in focal adhesion disassembly in this migration model. Thus, this study has characterised the role of endocytosis during chemotactic invasion and has identified functions mechanistically linking clathrin-mediated endocytosis to directed cell motility.

Change your TPLATE, change your fate: plant CME and beyond

Clathrin-mediated endocytosis (CME) is the predominant and evolutionarily conserved pathway by which eukaryotes internalize cargoes (i.e., plasma membrane proteins, lipids, and extracellular material) that are engaged in a variety of processes. Initiation of CME relies on adaptor proteins, which precisely select the cargoes for internalization, recruit the clathrin cage, and start membrane curvature. The recently identified CME early adaptor complex, the TPLATE complex (TPC), is essential for CME in plants. Phylogenetic analyses suggest that the TPC evolved from an ancient protein complex involved in vesicle trafficking in early eukaryotes, which raises questions about CME evolution and adaptation within the eukaryotic Kingdoms. In this review, we focus on the early events of plant CME and explore evolutionary aspects related to CME in other eukaryotes.

The Mysterious Ways of ErbB2/HER2 Trafficking

The EGFR- or ErbB-family of receptor tyrosine kinases consists of EGFR/ErbB1, ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4. Receptor activation and downstream signaling are generally initiated upon ligand-induced receptor homo- or heterodimerization at the plasma membrane, and endocytosis and intracellular membrane transport are crucial for regulation of the signaling outcome. Among the receptors, ErbB2 is special in several ways. Unlike the others, ErbB2 has no known ligand, but is still the favored dimerization partner. Furthermore, while the other receptors are down-regulated either constitutively or upon ligand-binding, ErbB2 is resistant to down-regulation, and also inhibits down-regulation of its partner upon heterodimerization. The reason(s) why ErbB2 is resistant to down-regulation are the subject of debate. Contrary to other ErbB-proteins, mature ErbB2 needs Hsp90 as chaperone. Several data suggest that Hsp90 is an important regulator of factors like ErbB2 stability, dimerization and/or signaling. Hsp90 inhibitors induce degradation of ErbB2, but whether Hsp90 directly makes ErbB2 endocytosis resistant is unclear. Exposure to anti-ErbB2 antibodies can also induce down-regulation of ErbB2. Down-regulation induced by Hsp90 inhibitors or antibodies does at least partly involve internalization and endosomal sorting to lysosomes for degradation, but also retrograde trafficking to the nucleus has been reported. In this review, we will discuss different molecular mechanisms suggested to be important for making ErbB2 resistant to down-regulation, and review how membrane trafficking is involved when down-regulation and/or relocalization of ErbB2 is induced.

Proteomic analysis of the epidermal growth factor receptor (EGFR) interactome and post-translational modifications associated with receptor endocytosis in response to EGF and stress

Aberrant expression, activation, and stabilization of epidermal growth factor receptor (EGFR) are causally associated with several human cancers. Post-translational modifications and protein-protein interactions directly modulate the signaling and trafficking of the EGFR. Activated EGFR is internalized by endocytosis and then either recycled back to the cell surface or degraded in the lysosome. EGFR internalization and recycling also occur in response to stresses that activate p38 MAP kinase. Mass spectrometry was applied to comprehensively analyze the phosphorylation, ubiquitination, and protein-protein interactions of wild type and endocytosis-defective EGFR variants before and after internalization in response to EGF ligand and stress. Prior to internalization, EGF-stimulated EGFR accumulated ubiquitin at 7 K residues and phosphorylation at 7 Y sites and at S(1104). Following internalization, these modifications diminished and there was an accumulation of S/T phosphorylations. EGFR internalization and many but not all of the EGF-induced S/T phosphorylations were also stimulated by anisomycin-induced cell stress, which was not associated with receptor ubiquitination or elevated Y phosphorylation. EGFR protein interactions were dramatically modulated by ligand, internalization, and stress. In response to EGF, different E3 ubiquitin ligases became maximally associated with EGFR before (CBL, HUWE1, and UBR4) or after (ITCH) internalization, whereas CBLB was distinctively most highly EGFR associated following anisomycin treatment. Adaptin subunits of AP-1 and AP-2 clathrin adaptor complexes also became EGFR associated in response to EGF and anisomycin stress. Mutations preventing EGFR phosphorylation at Y(998) or in the S(1039) region abolished or greatly reduced EGFR interactions with AP-2 and AP-1, and impaired receptor trafficking. These results provide new insight into spatial, temporal, and mechanistic aspects of EGFR regulation.

Review series: From uncertain beginnings: initiation mechanisms of clathrin-mediated endocytosis

Clathrin-mediated endocytosis is a central and well-studied trafficking process in eukaryotic cells. How this process is initiated is likely to be a critical point in regulating endocytic activity spatially and temporally, but the underlying mechanisms are poorly understood. During the early stages of endocytosis three components—adaptor and accessory proteins, cargo, and lipids—come together at the plasma membrane to begin the formation of clathrin-coated vesicles. Although different models have been proposed, there is still no clear picture of how these three components cooperate to initiate endocytosis, which may indicate that there is some flexibility underlying this important event.

SARA and RNF11 at the crossroads of EGFR signaling and trafficking

The classical view that endocytosis serves only for growth factor receptor degradation and signaling termination has recently been challenged by an increasing number of reports showing that various growth factor receptors such as epidermal growth factor receptor (EGFR) continue to activate downstream signaling molecules en route to lysosomes prior to their degradation. Moreover, the trafficking route that the ligand-receptor complexes follow to enter the cell is mutually interconnected with the final signaling output. Endosomal resident effector proteins are compartmentalized and regulate the signaling and trafficking of the ligand-bound receptor complexes. Smad anchor for receptor activation (SARA) is an early endosomal protein facilitating TGF-β signaling cascade. Even though SARA was identified as an adaptor protein that regulates SMAD2 activation and TGF-β signal propagation, an increasing number of reports in various systems describe SARA as a trafficking regulator. Recently, SARA has been shown to interact with the E3 ubiquitin ligase RNF11 (RING finger protein 11) and members of the ESCRT-0 (endosomal sorting complex required for transport) complex functionally participating in the degradation of EGFR.

Down-regulation of epidermal growth factor receptor by curcumin-induced UBE1L in human bronchial epithelial cells

UBE1L, ubiquitin-activating enzyme E1-like, is the activating enzyme of ISG15ylation (ISG15, interferon stimulated gene 15). Loss of UBE1L and activation of epidermal growth factor receptor (EGFR) signaling are common events in lung carcinogenesis. Curcumin, a well-studied chemopreventive agent, is known to down-regulate EGFR. The present study demonstrated that curcumin decreased EGFR expression in human bronchial epithelial (HBE) Beas-2B cells and lung cancer A549 cells. For the first time, UBE1L was found to be induced by curcumin in HBE cells. Interestingly, overexpression of UBE1L reduced EGFR at posttranslational level in HBE cells. UBE1L triggered EGFR membrane internalization and promoted complex formation between ISG15 and EGFR. Curcumin decreased EGFR downstream signaling pAKT and nuclear factor κB (NF-κB). Overexpression or knockdown of UBE1L also resulted in down-regulation or up-regulation of phosphoinositide 3-kinase/AKT/NF-κB correspondently. In human samples, there was an inverse relationship between UBE1L and EGFR/AKT/NF-κB in non-small cell lung cancer tissues and adjacent tissues. These results uncover a novel chemopreventive mechanism of curcumin in inducing UBE1L and down-regulating EGFR signaling in HBE cells.

Endophilin, Lamellipodin, and Mena cooperate to regulate F-actin-dependent EGF-receptor endocytosis

The epidermal growth factor receptor (EGFR) plays an essential role during development and diseases including cancer. Lamellipodin (Lpd) is known to control lamellipodia protrusion by regulating actin filament elongation via Ena/VASP proteins. However, it is unknown whether this mechanism supports endocytosis of the EGFR. Here, we have identified a novel role for Lpd and Mena in clathrin-mediated endocytosis (CME) of the EGFR. We have discovered that endogenous Lpd is in a complex with the EGFR and Lpd and Mena knockdown impairs EGFR endocytosis. Conversely, overexpressing Lpd substantially increases the EGFR uptake in an F-actin-dependent manner, suggesting that F-actin polymerization is limiting for EGFR uptake. Furthermore, we found that Lpd directly interacts with endophilin, a BAR domain containing protein implicated in vesicle fission. We identified a role for endophilin in EGFR endocytosis, which is mediated by Lpd. Consistently, Lpd localizes to clathrin-coated pits (CCPs) just before vesicle scission and regulates vesicle scission. Our findings suggest a novel mechanism in which Lpd mediates EGFR endocytosis via Mena downstream of endophilin.

Cooperation between a BAR domain protein and a regulator of actin filament elongation during lamellipodia protrusion reveals actin cytoskeleton roles in endocytic vesicle scission in mammalian cells.

Endocytosis of receptor tyrosine kinases

Endocytosis is the major regulator of signaling from receptor tyrosine kinases (RTKs). The canonical model of RTK endocytosis involves rapid internalization of an RTK activated by ligand binding at the cell surface and subsequent sorting of internalized ligand-RTK complexes to lysosomes for degradation. Activation of the intrinsic tyrosine kinase activity of RTKs results in autophosphorylation, which is mechanistically coupled to the recruitment of adaptor proteins and conjugation of ubiquitin to RTKs. Ubiquitination serves to mediate interactions of RTKs with sorting machineries both at the cell surface and on endosomes. The pathways and kinetics of RTK endocytic trafficking, molecular mechanisms underlying sorting processes, and examples of deviations from the standard trafficking itinerary in the RTK family are discussed in this work.

The role of endosomal signaling triggered by metastatic growth factors in tumor progression

Within tumor microenvironment, a lot of growth factors such as hepatocyte growth factor and epidermal growth factor may induce similar signal cascade downstream of receptor tyrosine kinase (RTK) and trigger tumor metastasis synergistically. In the past decades, the intimate relationship of RTK-mediated receptor endocytosis with signal transduction was well established. In general, most RTK undergoes clathrin-dependent endocytosis and/or clathrin-independent endocytosis. The internalized receptors may sustain the signaling within early endosome, recycling to plasma membrane for subsequent ligand engagement or sorting to late endosomes/lysosome for receptor degradation. Moreover, receptor endocytosis influences signal transduction in a temporal and spatial manner for periodical and polarized cellular processes such as cell migration. The endosomal signalings triggered by various metastatic factors are quite similar in some critical points, which are essential for triggering cell migration and tumor progression. There are common regulators for receptor endocytosis including dynamin, Rab4, Rab5, Rab11 and Cbl. Moreover, many critical regulators within the RTK signal pathway such as Grb2, p38, PKC and Src were also modulators of endocytosis. In the future, these may constitute a new category of targets for prevention of tumor metastasis.

Internalization mechanisms of the epidermal growth factor receptor after activation with different ligands

The epidermal growth factor receptor (EGFR) regulates normal growth and differentiation, but dysregulation of the receptor or one of the EGFR ligands is involved in the pathogenesis of many cancers. There are eight ligands for EGFR, however most of the research into trafficking of the receptor after ligand activation focuses on the effect of epidermal growth factor (EGF) and transforming growth factor-α (TGF-α). For a long time it was believed that clathrin-mediated endocytosis was the major pathway for internalization of the receptor, but recent work suggests that different pathways exist. Here we show that clathrin ablation completely inhibits internalization of EGF- and TGF-α-stimulated receptor, however the inhibition of receptor internalization in cells treated with heparin-binding EGF-like growth factor (HB-EGF) or betacellulin (BTC) was only partial. In contrast, clathrin knockdown fully inhibits EGFR degradation after all ligands tested. Furthermore, inhibition of dynamin function blocked EGFR internalization after stimulation with all ligands. Knocking out a number of clathrin-independent dynamin-dependent pathways of internalization had no effect on the ligand-induced endocytosis of the EGFR. We suggest that EGF and TGF-α lead to EGFR endocytosis mainly via the clathrin-mediated pathway. Furthermore, we suggest that HB-EGF and BTC also lead to EGFR endocytosis via a clathrin-mediated pathway, but can additionally use an unidentified internalization pathway or better recruit the small amount of clathrin remaining after clathrin knockdown.

SARA and RNF11 interact with each other and ESCRT-0 core proteins and regulate degradative EGFR trafficking

Smad anchor for receptor activation (SARA) is highly enriched on endocytic membranes via binding to phosphatidylinositol 3-phosphates through its FYVE (Fab1p-YOTB-Vps27p-EEA1) domain. SARA was originally identified as a protein that recruits non-phosphorylated SMAD2/3 to the activated TGFβ receptors for phosphorylation, but later reports suggested a regulatory role in endocytic trafficking. Here we demonstrate that the ubiquitin ligase RNF11 is a SARA-interacting protein residing on early and late endosomes, as well as the fast recycling compartment. RNF11 and SARA interact with the ESCRT-0 subunits STAM2 and Eps15b, but only RNF11 associates with the core subunit Hrs. Both gain- and loss-of-function perturbation of RNF11 and SARA levels result in delayed degradation of epidermal growth factor (EGF)-activated EGF receptor (EGFR), while loss-of-function sustained/enhanced EGF-induced ERK1/2 phosphorylation. These findings suggest that RNF11 and SARA are functional components of the ESCRT-0 complexes. Moreover, SARA interacts with clathrin, the ESCRT-I subunit Tsg101 and ubiquitinated cargo exhibiting all the properties of Hrs concerning ESCRT-0 function, indicating that it could substitute Hrs in some ESCRT-0 complexes. These results suggest that RNF11 and SARA participate structurally and functionally in the ESCRT-dependent lysosomal degradation of receptors. As a consequence, the negative influence that perturbation of RNF11 and SARA levels exerts on the lysosomal degradation of EGFRs could underscore the reported overexpression of RNF11 in several cancers. In these cancers, deficient termination of the oncogenic signaling of mutated receptors, such as the EGFRs, through suboptimal lysosomal degradation could contribute to the process of malignant transformation.

Epidermal growth factor cytoplasmic domain affects ErbB protein degradation by the lysosomal and ubiquitin-proteasome pathway in human cancer cells

The cytoplasmic domains of EGF-like ligands, including EGF cytoplasmic domain (EGFcyt), have important biological functions. Using specific constructs and peptides of human EGF cytoplasmic domain, we demonstrate that EGFcyt facilitates lysosomal and proteasomal protein degradation, and this coincided with growth inhibition of human thyroid and glioma carcinoma cells. EGFcyt and exon 22-23-encoded peptide (EGF22.23) enhanced procathepsin B (procathB) expression and procathB-mediated lysosomal degradation of EGFR/ErbB1 as determined by inhibitors for procathB and the lysosomal ATPase inhibitor BafA1. Presence of mbEGFctF, EGFcyt, EGF22.23, and exon 23-encoded peptides suppressed the expression of the deubiqitinating enzyme ubiquitin C-terminal hydrolase-L1 (UCH-L1). This coincided with hyperubiquitination of total cellular proteins and ErbB1/2 and reduced proteasome activity. Upon small interfering RNA-mediated silencing of endogenously expressed UCH-L1, a similar hyperubiquitinylation phenotype, reduced ErbB1/2 content, and attenuated growth was observed. The exon 23-encoded peptide region of EGFcyt was important for these biologic actions. Structural homology modeling of human EGFcyt showed that this molecular region formed an exposed surface loop. Peptides derived from this EGFcyt loop structure may aid in the design of novel peptide therapeutics aimed at inhibiting growth of cancer cells.

Cetuximab in combination with anti-human IgG antibodies efficiently down-regulates the EGF receptor by macropinocytosis

The monoclonal antibody C225 (Cetuximab) blocks binding of ligand to the epidermal growth factor receptor (EGFR). In addition, it is known that incubation with C225 induces endocytosis of the EGFR. This endocytosis has previously been shown to be increased when C225 is combined with an additional monoclonal anti-EGFR antibody. However, the effects of antibody combinations on EGFR activation, endocytosis, trafficking and degradation have been unclear. By binding a secondary antibody to the C225-EGFR complex, we here demonstrate that a combination of antibodies can efficiently internalize and degrade the EGFR. Although the combination of antibodies activated the EGFR kinase and induced ubiquitination of the EGFR, the kinase activity was not required for internalization of the EGFR. In contrast to EGF-induced EGFR down-regulation, the antibody combination efficiently degraded the EGFR without initiating downstream proliferative signaling. The antibody-induced internalization of EGFR was found not to depend on clathrin and/or dynamin, but depended on actin polymerization, suggesting induction of macropinocytosis. Macropinocytosis may cause internalization of large membrane areas, and this could explain the highly efficient internalization of the EGFR induced by combination of antibodies.

The PtdIns3P-binding protein Phafin 2 mediates epidermal growth factor receptor degradation by promoting endosome fusion

Phosphatidylinositol 3-phosphate (PtdIns3P) orchestrates endosomal cargo transport, fusion and motility by recruiting FYVE or PX domain-containing effector proteins to endosomal membranes. In an attempt to discover novel PtdIns3P effectors involved in the termination of growth factor receptor signalling, we performed an siRNA screen for epidermal growth factor (EGF) degradation, targeting FYVE and PX domain proteins in the human proteome. This screen identified several potential regulators of EGF degradation, including HRS (used as positive control), PX kinase, MTMR4 and Phafin2/PLEKHF2. As Phafin2 has not previously been shown to be required for EGF receptor (EGFR) degradation, we performed further functional studies on this protein. Loss of Phafin2 was found to decrease early endosome size, whereas overexpression of Phafin2 resulted in enlarged endosomes. Moreover, both the EGFR and the fluid-phase marker dextran were retained in abnormally small endosomes in Phafin2-depleted cells. In yeast two-hybrid analysis we identified Phafin2 as a novel interactor of the endosomal-tethering protein EEA1, and Phafin2 colocalized strongly with EEA1 in microdomains of the endosome membrane. Our results suggest that Phafin2 controls receptor trafficking and fluid-phase transport through early endosomes by facilitating endosome fusion in concert with EEA1.

Nucleic acid delivery: the missing pieces of the puzzle?

The delivery of genes or RNA interference (RNAi) agents can increase or decrease the expression of virtually any protein in a cell, and this process opens the path for cures to most diseases that afflict humans. However, the high molecular weight, anionic nature, and instability of nucleic acids in the presence of enzymes pose major obstacles to their delivery and frustrates their use as human therapies. This Account describes current ideas about the mechanisms in nonviral nucleic acid delivery and how lipidic and polymeric carriers can overcome some of the critical barriers to delivery. Over the last 20 years, researchers have developed a multitude of polymeric and lipidic vectors, but only a small fraction of these have progressed into clinical trials. None of these vectors has received FDA approval, which indicates that the current vectors do not yet have suitable properties for effective in vivo nucleic acid delivery. Nucleic acid delivery is a multistep process and inefficiencies at any stage result in a dramatic decrease in gene delivery or gene silencing. However, the majority of studies investigating synthetic vectors focus solely on optimization of endosomal escape. A small number of studies address how to improve uptake via targeted delivery, and an even smaller fraction examine the intracellular fate of the delivery systems and nucleic acid cargo. The internalization of genes into the cell nucleus remains an inefficient and mysterious process. In the case of DNA delivery, strategies are needed to increase and accelerate the migration of DNA through the cytoplasm and transport it through the nuclear membrane. siRNA delivery involves fewer barriers. siRNA is more readily released from the carrier and more resistant to enzymatic degradation, and its target is in the cytoplasm; hence, siRNA delivery systems are becoming a clinical reality. With regard to siRNA therapy, the exact cytoplasmic location of RNA-induced silencing complex (RISC) formation and activity is unknown, which makes specific targeting of the RISC for more efficient delivery difficult. Furthermore, we would like to identify the factors that favor the binding of siRNA to Ago-2. If we could understand how the half-life of siRNA and Ago-2/siRNA complex in the cytoplasm can be modulated without interfering with RISC functions that are essential for normal cell activity, we could increase siRNA delivery efficiency. In this Account, we review the current synthetic vectors and propose alternative strategies in a few cases. We also suggest how certain cellular mechanisms might be exploited to improve gene transfection and silencing. Finally, we discuss whether some carriers that deliver the siRNA to cells could also repackage the siRNA into exosomes. The exosomes would then transport the siRNA into a subsequent population of cells that manifest the siRNA effect. This piggy-back mechanism may be responsible for reported deep tissue siRNA effects using certain carriers.

REST is up-regulated by epidermal growth factor in HeLa cells and inhibits apoptosis by influencing histone H3 acetylation

REST (repressor element 1-silencing transcription factor) is a transcription factor that recruits histone deacetylases to silence gene transcription. REST appears to play a paradoxical role in cancer cells: it exhibits tumor suppressor activity or promotes tumorigenesis, depending upon the setting. The extracellular signaling molecules that control REST gene expression in cancer cells remain poorly understood. In this study, we report that REST expression in HeLa cells is elevated in cells exposed to epidermal growth factor or serum, whereas the rate of cell apoptosis is low. Apoptosis induced by serum withdrawal is significantly increased in HeLa cells treated with an antisense phosphorothioate oligodeoxynucleotide (AS ODN) capable of down-regulating REST expression, whereas in HeLa cells transfected with a REST expressing plasmid, REST overexpression reduces the marked apoptosis caused, in absence of serum, by exposure to an anti-Fas receptor antibody imitating the Fas ligand activity plus PD 98059, a blocker of extracellular signal-regulated kinase 1/2 activation. REST knockdown also reduces mRNA levels of the antiapoptotic protein Bcl-X(L) whereas in HeLa cells overexpressing REST, the reduction of Bcl-X(L) mRNA caused by the anti-Fas receptor antibody plus PD 98059 is significantly decreased. Finally, we report that acetylation of histone H3 is increased in HeLa cells exposed to AS ODN or anti-Fas receptor antibody, whereas it is reduced in cells transfected with the REST expressing plasmid. Our findings indicate that REST is a novel gene regulated by EGF in HeLa cells that potentially contributes to the modulation of apoptosis via epigenetic mechanisms.

Regulation of VEGF signaling by membrane traffic

Recent findings have drawn attention to the role of membrane traffic in the signaling of vascular endothelial growth factor (VEGF). The significance of this development stems from the pivotal function of VEGF in vasculogenesis and angiogenesis. The outline of the regulation of VEGF receptor (VEGFR) signaling by membrane traffic is similar to that of the epidermal growth factor receptor (EGFR), a prototype of the intertwining between membrane traffic and signaling. There are, however, unique features in VEGFR signaling that are conferred in part by the involvement of the co-receptor neuropilin (Nrp). Nrp1 and VEGFR2 are integrated into membrane traffic through the adaptor protein synectin, which recruits myosin VI, a molecular motor that drives inward trafficking [17,21,64]. The recent detection of only mild vascular defects in a knockin mouse model that expresses Nrp1 lacking a cytoplasmic domain [104], questions the co-receptor's role in VEGF signaling and membrane traffic. The regulation of endocytosis by ephrin-B2 is another feature unique to VEGR2/3 [18,19], but it awaits a mechanistic explanation. Current models do not fully explain how membrane traffic bridges between VEGFR and the downstream effectors that produce its functional outcome, such as cell migration. VEGF-A appears to accomplish this task in part by recruiting endocytic vesicles carrying RhoA to internalized active VEGFR2 [58].

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Differential roles of Grb2 and AP-2 in p38 MAPK- and EGF-induced EGFR internalization

The epidermal growth factor receptor (EGFR) is an important regulator of normal growth and differentiation, and it is involved in the pathogenesis of many cancers. Endocytic downregulation is central in terminating EGFR signaling after ligand stimulation. It has been shown that p38 MAPK activation also can induce EGFR endocytosis. This endocytosis lacks many of the characteristics of ligand-induced EGFR endocytosis. We compared the two types of endocytosis with regard to the requirements for proteins in the internalization machinery. Both types of endocytosis require clathrin, but while epidermal growth factor (EGF)-induced EGFR internalization also required Grb2, p38 MAPK-induced internalization did not. Interestingly, AP-2 knock down blocked p38 MAPK-induced EGFR internalization, but only mildly affected EGF-induced internalization. In line with this, simultaneously mutating two AP-2 interaction sites in EGFR affected p38 MAPK-induced internalization much more than EGF-induced EGFR internalization. Thus, it seems that EGFR in the two situations uses different sets of internalization mechanisms.

CIN85 regulates ubiquitination and degradative endosomal sorting of the EGF receptor

CIN85 has been demonstrated to interact with a number of proteins involved in endocytosis and intracellular sorting. However, the exact functional role of CIN85 in endocytosis remains unclear. We have investigated whether CIN85 plays a role in EGF-induced EGF receptor (EGFR) internalization, as previously suggested, or whether CIN85 is rather involved in endosomal sorting of the EGFR. When over-expressing a dominant negative interfering CIN85 mutant consisting of three SH3 domains only, we found that internalization of EGF was inhibited. However, when knocking down CIN85 by RNAi, the EGF-EGFR uptake appeared similar to in control cells. Furthermore, in CIN85 depleted cells, EGF-induced ubiquitination of the EGFR was decreased, and degradation of EGF-EGFR complexes was delayed. Our data further demonstrated that depletion of CIN85 increased the recycling of EGF, suggesting that CIN85 plays a role in endosomal sorting of the ubiquitinated EGFR. Our data also demonstrated that CIN85 was constitutively associated with Hrs, and this strengthens the hypothesis of a functional role of CIN85 in endosomal EGFR sorting.

Sorting it out in endosomes: an emerging concept in renal epithelial cell transport regulation

Ion and water transport by the kidney is continually adjusted in response to physiological cues. Selective endocytosis and endosomal trafficking of ion transporters are increasingly appreciated as mechanisms to acutely modulate renal function. Here, we discuss emerging paradigms in this new area of investigation.

The epidermal growth factor receptor (EGFR) promotes uptake of influenza A viruses (IAV) into host cells

Influenza A viruses (IAV) bind to sialic-acids at cellular surfaces and enter cells by using endocytotic routes. There is evidence that this process does not occur constitutively but requires induction of specific cellular signals, including activation of PI3K that promotes virus internalization. This implies engagement of cellular signaling receptors during viral entry. Here, we present first indications for an interplay of IAV with receptor tyrosine kinases (RTKs). As representative RTK family-members the epidermal growth factor receptor (EGFR) and the c-Met receptor were studied. Modulation of expression or activity of both RTKs resulted in altered uptake of IAV, showing that these receptors transmit entry relevant signals upon virus binding. More detailed studies on EGFR function revealed that virus binding lead to clustering of lipid-rafts, suggesting that multivalent binding of IAV to cells induces a signaling platform leading to activation of EGFR and other RTKs that in turn facilitates IAV uptake.

Shiga toxin increases formation of clathrin-coated pits through Syk kinase

Clathrin-dependent endocytosis is a main entry mechanism for the glycolipid-binding Shiga toxin (Stx), although clathrin-independent pathways are also involved. Binding of Stx to its receptor Gb3 not only is essential for Stx retrograde transport to the endoplasmic reticulum and toxicity but also activates signaling through the tyrosine kinase Syk. We previously described that Syk activity is important for Stx entry, but it remained unclear how this kinase modulates endocytosis of Stx. Here we characterized the effects of Stx and Syk on clathrin-coated pit formation. We found that acute treatment with Stx results in an increase in the number of clathrin-coated profiles as determined by electron microscopy and on the number of structures containing the endocytic AP-2 adaptor at the plasma membrane determined by live-cell spinning disk confocal imaging. These responses to Stx require functional Syk activity. We propose that a signaling pathway mediated by Syk and modulated by Stx leads to an increased number of endocytic clathrin-coated structures, thus providing a possible mechanism by which Stx enhances its own endocytosis.

Multiple mechanisms collectively regulate clathrin-mediated endocytosis of the epidermal growth factor receptor

Four independent mechanisms for uptake of activated EGFR are identified by a combination of receptor mutagenesis and RNA interference approaches.

Endocytosis of the epidermal growth factor receptor (EGFR) is important for the regulation of EGFR signaling. However, EGFR endocytosis mechanisms are poorly understood, which precludes development of approaches to specifically inhibit EGFR endocytosis and analyze its impact on signaling. Using a combination of receptor mutagenesis and RNA interference, we demonstrate that clathrin-dependent internalization of activated EGFR is regulated by four mechanisms, which function in a redundant and cooperative fashion. These mechanisms involve ubiquitination of the receptor kinase domain, the clathrin adaptor complex AP-2, the Grb2 adaptor protein, and three C-terminal lysine residues (K1155, K1158, and K1164), which are acetylated, a novel posttranslational modification for the EGFR. Based on these findings, the first internalization-defective EGFR mutant with functional kinase and normal tyrosine phosphorylation was generated. Analysis of the signaling kinetics of this mutant revealed that EGFR internalization is required for the sustained activation of protein kinase B/AKT but not for the activation of mitogen-activated protein kinase.