Identification and characterization of a new class of trafficking motifs for controlling clathrin-independent internalization and recycling

EFA6A, an Exchange Factor for Arf6, Regulates NGF-Dependent TrkA Recycling From Early Endosomes and Neurite Outgrowth in PC12 Cells

Endosomal trafficking of TrkA is a critical process for nerve growth factor (NGF)-dependent neuronal cell survival and differentiation. The small GTPase ADP-ribosylation factor 6 (Arf6) is implicated in NGF-dependent processes in PC12 cells through endosomal trafficking and actin cytoskeleton reorganization. However, the regulatory mechanism for Arf6 in NGF signaling is largely unknown. In this study, we demonstrated that EFA6A, an Arf6-specific guanine nucleotide exchange factor, was abundantly expressed in PC12 cells and that knockdown of EFA6A significantly inhibited NGF-dependent Arf6 activation, TrkA recycling from early endosomes to the cell surface, prolonged ERK1/2 phosphorylation, and neurite outgrowth. We also demonstrated that EFA6A forms a protein complex with TrkA through its N-terminal region, thereby enhancing its catalytic activity for Arf6. Similarly, we demonstrated that EFA6A forms a protein complex with TrkA in cultured dorsal root ganglion (DRG) neurons. Furthermore, cultured DRG neurons from EFA6A knockout mice exhibited disturbed NGF-dependent TrkA trafficking compared with wild-type neurons. These findings provide the first evidence for EFA6A as a key regulator of NGF-dependent TrkA trafficking and signaling.

Insights of Endocytosis Signaling in Health and Disease

Endocytosis in mammalian cells is a fundamental cellular machinery that regulates vital physiological processes, such as the absorption of metabolites, release of neurotransmitters, uptake of hormone cellular defense, and delivery of biomolecules across the plasma membrane. A remarkable characteristic of the endocytic machinery is the sequential assembly of the complex proteins at the plasma membrane, followed by internalization and fusion of various biomolecules to different cellular compartments. In all eukaryotic cells, functional characterization of endocytic pathways is based on dynamics of the protein complex and signal transduction modules. To coordinate the assembly and functions of the numerous parts of the endocytic machinery, the endocytic proteins interact significantly within and between the modules. Clathrin-dependent and -independent endocytosis, caveolar pathway, and receptor mediated endocytosis have been attributed to a greater variety of physiological and pathophysiological roles such as, autophagy, metabolism, cell division, apoptosis, cellular defense, and intestinal permeabilization. Notably, any defect or alteration in the endocytic machinery results in the development of pathological consequences associated with human diseases such as cancer, cardiovascular diseases, neurological diseases, and inflammatory diseases. In this review, an in-depth endeavor has been made to illustrate the process of endocytosis, and associated mechanisms describing pathological manifestation associated with dysregulated endocytosis machinery.

The Arf6/PIP5K pathway activates IKACh in cigarette smoke mediated atrial fibrillation

Cigarette smoking (CS) is a major cause of cardiovascular diseases. Smokers are at a significantly higher risk for developing atrial fibrillation (AF), a dangerous and abnormal heart rhythm. In the US, 15.5% of adults are current smokers, and it is becoming clear that CS is an independent risk factor for AF, but a detailed mechanistic understanding of how CS contributes to the molecular patho-electrophysiology of AF remains elusive. We investigated if CS related AF is in part mediated through a mechanism that depends on the cardiac acetylcholine activated inward rectifier potassium current (IKACh). We tested the hypothesis that CS increases IKACh via phosphatidylinositol 4-phosphate 5-kinase alpha (PIP5K) and ADP ribosylation factor 6 (Arf6) signaling, leading to AF perpetuation. In vivo inducibility of AF was assessed in mice exposed to CS for 8 weeks. AF duration was increased in CS exposed mice, and TertiapinQ, an IKACh blocker prevented AF development in CS exposed mice. In HEK293 cells stably transfected with Kir3.1 and Kir3.4, the molecular correlates of IKACh, CS exposure increased the expression of the Kir3.1 and Kir3.4 proteins at the cell surface, activated Arf6 and increased the IKACh current. Inhibition of PIP5K, or of Kir3.1/Kir3.4 trafficking via Arf6 abrogated the CS effects on IKACh. Cigarette smoke modifies the atrial electrophysiological substrate, leading to arrhythmogenesis, in part, through IKACh activation via an Arf6/PIP5K dependent pathway.

EFA6A, an exchange factor for Arf6, regulates early steps in ciliogenesis

Ciliogenesis is a coordinated process initiated by the recruitment and fusion of pre-ciliary vesicles at the distal appendages of the mother centriole through mechanisms that remain unclear. Here, we report that EFA6A (also known as PSD), an exchange factor for the small G protein Arf6, is involved in early stage of ciliogenesis by promoting the fusion of distal appendage vesicles forming the ciliary vesicle. EFA6A is present in the vicinity of the mother centriole before primary cilium assembly and prior to the arrival of Arl13B-containing vesicles. During ciliogenesis, EFA6A initially accumulates at the mother centriole and later colocalizes with Arl13B along the ciliary membrane. EFA6A depletion leads to the inhibition of ciliogenesis, the absence of centrosomal Rab8-positive structures and the accumulation of Arl13B-positive vesicles around the distal appendages. Our results uncover a novel fusion machinery, comprising EFA6A, Arf6 and Arl13B, that controls the coordinated fusion of ciliary vesicles docked at the distal appendages of the mother centriole.

Tailoring Iron Oxide Nanoparticles for Efficient Cellular Internalization and Endosomal Escape

Iron oxide nanoparticles (IONs) have been widely explored for biomedical applications due to their high biocompatibility, surface-coating versatility, and superparamagnetic properties. Upon exposure to an external magnetic field, IONs can be precisely directed to a region of interest and serve as exceptional delivery vehicles and cellular markers. However, the design of nanocarriers that achieve an efficient endocytic uptake, escape lysosomal degradation, and perform precise intracellular functions is still a challenge for their application in translational medicine. This review highlights several aspects that mediate the activation of the endosomal pathways, as well as the different properties that govern endosomal escape and nuclear transfection of magnetic IONs. In particular, we review a variety of ION surface modification alternatives that have emerged for facilitating their endocytic uptake and their timely escape from endosomes, with special emphasis on how these can be manipulated for the rational design of cell-penetrating vehicles. Moreover, additional modifications for enhancing nuclear transfection are also included in the design of therapeutic vehicles that must overcome this barrier. Understanding these mechanisms opens new perspectives in the strategic development of vehicles for cell tracking, cell imaging and the targeted intracellular delivery of drugs and gene therapy sequences and vectors.

Endocytosis: A Turnover Mechanism Controlling Ion Channel Function

Ion channels (IChs) are transmembrane proteins that selectively drive ions across membranes. The function of IChs partially relies on their abundance and proper location in the cell, fine-tuned by the delicate balance between secretory, endocytic, and degradative pathways. The disruption of this balance is associated with several diseases, such as Liddle's and long QT syndromes. Because of the vital role of these proteins in human health and disease, knowledge of ICh turnover is essential. Clathrin-dependent and -independent mechanisms have been the primary mechanisms identified with ICh endocytosis and degradation. Several molecular determinants recognized by the cellular internalization machinery have been discovered. Moreover, specific conditions can trigger the endocytosis of many IChs, such as the activation of certain receptors, hypokalemia, and some drugs. Ligand-dependent receptor activation primarily results in the posttranslational modification of IChs and the recruitment of important mediators, such as β-arrestins and ubiquitin ligases. However, endocytosis is not a final fate. Once internalized into endosomes, IChs are either sorted to lysosomes for degradation or recycled back to the plasma membrane. Rab proteins are crucial participants during these turnover steps. In this review, we describe the major ICh endocytic pathways, the signaling inputs triggering ICh internalization, and the key mediators of this essential cellular process.

Endocytosis in cellular uptake of drug delivery vectors: Molecular aspects in drug development

Drug delivery vectors are widely applied to increase drug efficacy while reducing the side effects and potential toxicity of a drug. They allow for patient-tailored therapy, dose titration, and therapeutic drug monitoring. A major part of drug delivery systems makes use of large nanocarriers: liposomes or virus-like particles (VLPs). These systems allow for a relatively large amount of cargo with good stability of vectors, and they offer multiple options for targeting vectors in vivo. Here we discuss endocytic pathways that are available for drug delivery by large nanocarriers. We focus on molecular aspects of the process, including an overview of potential molecular targets for studies of drug delivery vectors and for future solutions allowing targeted drug delivery.

BRAG2a Mediates mGluR-Dependent AMPA Receptor Internalization at Excitatory Postsynapses through the Interaction with PSD-95 and Endophilin 3

Brefeldin A-resistant ArfGEF 2 (BRAG2) [or Iqsec1 (IQ motif and Sec7 domain-containing protein 1)] is a guanine nucleotide exchange factor for ADP ribosylation factor 6 (Arf6), a small GTPase implicated in the membrane trafficking between the plasma membrane and endosomes. BRAG2 regulates Arf6-dependent endocytosis of AMPA receptors (AMPARs) through the direct interaction during the hippocampal long-term depression. However, the molecular mechanism by which the BRAG2-Arf6 pathway links AMPARs to the endocytic machinery remains elusive. Herein, using mouse brains of both sexes, we demonstrated that BRAG2a, an alternative isoform with a long C-terminal insert containing a proline-rich domain and type I PDZ-binding motif, was selectively localized to the excitatory postsynaptic density (PSD). Using yeast two-hybrid screening, we identified PSD-95 and endophilin 1/3 as BRAG2a-binding partners in the brain. The interaction with PSD-95 was required for synaptic targeting of BRAG2a. In cultured hippocampal neurons, stimulation of group I metabotropic glutamate receptors (mGluRs) increased the interaction of BRAG2a with endophilin 3 and concomitant Arf6 activation in a time-dependent manner. Knockdown of BRAG2 in cultured hippocampal neurons blocked the mGluR-dependent decrease in surface AMPAR levels, which was rescued by introducing wild-type BRAG2a, but not wild-type BRAG2b or BRAG2a mutants lacking the ability to activate Arf6 or to interact with endophilin 3 or PSD-95. Further postembedding immunoelectron microscopic analysis revealed the preorganized lateral distribution of BRAG2a, Arf6, and endophilin 3 for efficient endocytosis at the postsynaptic membrane. Together, the present findings unveiled a novel molecular mechanism by which BRAG2a links AMPARs to the clathrin-dependent endocytic pathway through its interaction with PSD-95 and endophilin 3.SIGNIFICANCE STATEMENT BRAG2/Iqsec1 is a GDP/GTP exchange factor for ADP ribosylation factor 6 (Arf6), a small GTPase implicated in the membrane trafficking between the plasma membrane and endosomes, and regulates Arf6-dependent endocytosis of AMPARs through direct interaction during hippocampal long-term depression, one of the mechanisms of synaptic plasticity related to learning and memory. However, the molecular mechanism by which the BRAG2-Arf6 pathway links AMPARs to the endocytic machinery remains elusive. Here, we identified isoform-specific mechanisms of BRAG2-mediated AMPAR internalization. We demonstrated that the interaction of BRAG2a isoform with PSD-95 and endophilin 3 was required for the mGluR-dependent decrease in surface AMPARs in hippocampal neurons. These results unveiled a novel molecular mechanism by which BRAG2 links AMPARs to the clathrin-mediated endocytic machinery at postsynaptic sites.

Rab10 regulates tubular endosome formation through KIF13A and KIF13B motors

Recycling endosomes are stations that sort endocytic cargoes to their appropriate destinations. Tubular endosomes have been characterized as a recycling endosomal compartment for clathrin-independent cargoes. However, the molecular mechanism by which tubular endosome formation is regulated is poorly understood. In this study, we identified Rab10 as a novel protein localized at tubular endosomes by using a comprehensive localization screen of EGFP-tagged Rab small GTPases. Knockout of Rab10 completely abolished tubular endosomal structures in HeLaM cells. We also identified kinesin motors KIF13A and KIF13B as novel Rab10-interacting proteins by means of in silico screening. The results of this study demonstrated that both the Rab10-binding homology domain and the motor domain of KIF13A are required for Rab10-positive tubular endosome formation. Our findings provide insight into the mechanism by which the Rab10-KIF13A (or KIF13B) complex regulates tubular endosome formation. This article has an associated First Person interview with the first author of the paper.

Monoubiquitination of syntaxin 3 leads to retrieval from the basolateral plasma membrane and facilitates cargo recruitment to exosomes

Monoubiquitination of Stx3 leads to efficient endocytosis from the basolateral plasma membrane and trafficking into the multivesicular body/exosomal pathway. Stx3 plays a role in cargo recruitment into exosomes. This pathway is exploited by HCMV for virion excretion.

Syntaxin 3 (Stx3), a SNARE protein located and functioning at the apical plasma membrane of epithelial cells, is required for epithelial polarity. A fraction of Stx3 is localized to late endosomes/lysosomes, although how it traffics there and its function in these organelles is unknown. Here we report that Stx3 undergoes monoubiquitination in a conserved polybasic domain. Stx3 present at the basolateral—but not the apical—plasma membrane is rapidly endocytosed, targeted to endosomes, internalized into intraluminal vesicles (ILVs), and excreted in exosomes. A nonubiquitinatable mutant of Stx3 (Stx3-5R) fails to enter this pathway and leads to the inability of the apical exosomal cargo protein GPRC5B to enter the ILV/exosomal pathway. This suggests that ubiquitination of Stx3 leads to removal from the basolateral membrane to achieve apical polarity, that Stx3 plays a role in the recruitment of cargo to exosomes, and that the Stx3-5R mutant acts as a dominant-negative inhibitor. Human cytomegalovirus (HCMV) acquires its membrane in an intracellular compartment and we show that Stx3-5R strongly reduces the number of excreted infectious viral particles. Altogether these results suggest that Stx3 functions in the transport of specific proteins to apical exosomes and that HCMV exploits this pathway for virion excretion.

EFA6 regulates lumen formation through alpha-actinin 1

A key step of epithelial morphogenesis is the creation of the lumen. Luminogenesis by hollowing proceeds through the fusion of apical vesicles at cell-cell contact. The small nascent lumens grow through extension, coalescence and enlargement coordinated with cell division to give rise to a single central lumen. Here, using MDCK cells grown in 3D-culture, we show that EFA6A participates in luminogenesis. EFA6A recruits α-actinin 1 (ACTN1) through direct binding. In polarized cells, ACTN1 was found to be enriched at the tight junction where it acts as a primary effector of EFA6A for normal luminogenesis. Both proteins are essential for the lumen extension and enlargement, where they mediate their effect by regulating the cortical acto-myosin contractility. Finally, ACTN1 was also found to act as an effector for the isoform EFA6B in the human mammary tumoral MCF7 cell line. EFA6B restored the glandular morphology of this tumoral cell line in an ACTN1-dependent manner. Thus, we identified new regulators of cyst luminogenesis essential for the proper maturation of a newly-formed lumen into a single central lumen.

Synaptic activity regulates AMPA receptor trafficking through different recycling pathways

Changes in glutamatergic synaptic strength in brain are dependent on AMPA-type glutamate receptor (AMPAR) recycling, which is assumed to occur through a single local pathway. In this study, we present evidence that AMPAR recycling occurs through different pathways regulated by synaptic activity. Without synaptic stimulation, most AMPARs recycled in dynamin-independent endosomes containing the GTPase, Arf6. Few AMPARs recycled in dynamin-dependent endosomes labeled by transferrin receptors (TfRs). AMPAR recycling was blocked by alterations in the GTPase, TC10, which co-localized with Arf6 endosomes. TC10 mutants that reduced AMPAR recycling had no effect on increased AMPAR levels with long-term potentiation (LTP) and little effect on decreased AMPAR levels with long-term depression. However, internalized AMPAR levels in TfR-containing recycling endosomes increased after LTP, indicating increased AMPAR recycling through the dynamin-dependent pathway with synaptic plasticity. LTP-induced AMPAR endocytosis is inconsistent with local recycling as a source of increased surface receptors, suggesting AMPARs are trafficked from other sites.

DOI:http://dx.doi.org/10.7554/eLife.06878.001

Cells called neurons transmit information around the brain in the form of electrical signals. At a junction between two neurons—called a synapse—an electrical signal triggers the release of small molecules called neurotransmitters. These molecules travel across the gap between the two neurons and trigger a new electrical signal in the second neuron. Memories can be stored in synapses: high levels of activity can ‘strengthen’ the synapse, which increases the transfer of information between the neurons.

In many synapses, a molecule called glutamate is the neurotransmitter. Proteins called AMPARs, which are found on the surface of the neuron, can detect glutamate and transmit the signal along the second neuron. The strength of synapses is controlled by changes in AMPAR levels through ‘recycling’, where AMPAR proteins are removed from synapses, internalized and later returned to synapses.

It was thought that AMPARs are recycled via just one pathway at synapses. However, the amount of recycling is much higher when the synapses are active and it is not clear how this works. Now, Zheng et al. have used fluorescent tags to track the recycling of AMPARs in synapses from rats under a microscope. The experiments show that when the synapses are not active, most AMPARs are recycled via a pathway marked by a protein called Arf6. However, when the synapses are active, most AMPAR is recycled via a different route marked by so-called ‘transferrin receptor’ proteins.

The experiments also reveal that a protein called TC10 is involved in recycling AMPARs alongside Arf6, but is not required for recycling when the synapses are active and being strengthened. Unexpectedly, AMPAR internalization—via the process involving transferrin receptors—increases during synapse strengthening. This suggests that some of the extra AMPAR proteins sent to the membrane have come from other parts of the neuron away from the synapse.

Zheng et al.'s findings provide evidence that AMPARs are recycled through different routes depending on the activity of the synapse. The next challenge will be to directly test whether AMPARs are transported from other parts of the neuron to the strengthened synapse and to understand how this works.

DOI:http://dx.doi.org/10.7554/eLife.06878.002

Channel properties of Nax expressed in neurons

Nax is a sodium-concentration ([Na+])-sensitive Na channel with a gating threshold of ~150 mM for extracellular [Na+] ([Na+]o) in vitro. We previously reported that Nax was preferentially expressed in the glial cells of sensory circumventricular organs including the subfornical organ, and was involved in [Na+] sensing for the control of salt-intake behavior. Although Nax was also suggested to be expressed in the neurons of some brain regions including the amygdala and cerebral cortex, the channel properties of Nax have not yet been adequately characterized in neurons. We herein verified that Nax was expressed in neurons in the lateral amygdala of mice using an antibody that was newly generated against mouse Nax. To investigate the channel properties of Nax expressed in neurons, we established an inducible cell line of Nax using the mouse neuroblastoma cell line, Neuro-2a, which is endogenously devoid of the expression of Nax. Functional analyses of this cell line revealed that the [Na+]-sensitivity of Nax in neuronal cells was similar to that expressed in glial cells. The cation selectivity sequence of the Nax channel in cations was revealed to be Na+ ≈ Li+ > Rb+ > Cs+ for the first time. Furthermore, we demonstrated that Nax bound to postsynaptic density protein 95 (PSD95) through its PSD95/Disc-large/ZO-1 (PDZ)-binding motif at the C-terminus in neurons. The interaction between Nax and PSD95 may be involved in promoting the surface expression of Nax channels because the depletion of endogenous PSD95 resulted in a decrease in Nax at the plasma membrane. These results indicated, for the first time, that Nax functions as a [Na+]-sensitive Na channel in neurons as well as in glial cells.

Endocytosis as a mode to regulate functional expression of two-pore domain potassium (K₂p) channels

Two-pore domain potassium (K_2P) channels are implicated in an array of physiological and pathophysiological roles. As a result of their biophysical properties, these channels produce a background leak K⁺ current which has a direct effect on cellular membrane potential and activity. The regulation of potassium leak from cells through K_2P channels is of critical importance to cell function, development and survival. Controlling the cell surface expression of these channels is one mode to regulate their function and is achieved through a balance between regulated channel delivery to and retrieval from the cell surface. Here, we explore the modes of retrieval of K_2P channels from the plasma membrane and observe that K_2P channels are endocytosed in both a clathrin-mediated and clathrin-independent manner. K_2P channels use a variety of pathways and show altered internalisation and sorting in response to external cues. These pathways working in concert, equip the cell with a range of approaches to maintain steady state levels of channels and to respond rapidly should changes in channel density be required.

Clathrin-independent pathways of endocytosis

There are many pathways of endocytosis at the cell surface that apparently operate at the same time. With the advent of new molecular genetic and imaging tools, an understanding of the different ways by which a cell may endocytose cargo is increasing by leaps and bounds. In this review we explore pathways of endocytosis that occur in the absence of clathrin. These are referred to as clathrin-independent endocytosis (CIE). Here we primarily focus on those pathways that function at the small scale in which some have distinct coats (caveolae) and others function in the absence of specific coated intermediates. We follow the trafficking itineraries of the material endocytosed by these pathways and finally discuss the functional roles that these pathways play in cell and tissue physiology. It is likely that these pathways will play key roles in the regulation of plasma membrane area and tension and also control the availability of membrane during cell migration.

Endocytosis of HERG is clathrin-independent and involves arf6

The hERG potassium channel is critical for repolarisation of the cardiac action potential. Reduced expression of hERG at the plasma membrane, whether caused by hereditary mutations or drugs, results in long QT syndrome and increases the risk of ventricular arrhythmias. Thus, it is of fundamental importance to understand how the density of this channel at the plasma membrane is regulated. We used antibodies to an extracellular native or engineered epitope, in conjunction with immunofluorescence and ELISA, to investigate the mechanism of hERG endocytosis in recombinant cells and validated the findings in rat neonatal cardiac myocytes. The data reveal that this channel undergoes rapid internalisation, which is inhibited by neither dynasore, an inhibitor of dynamin, nor a dominant negative construct of Rab5a, into endosomes that are largely devoid of the transferrin receptor. These results support a clathrin-independent mechanism of endocytosis and exclude involvement of dynamin-dependent caveolin and RhoA mechanisms. In agreement, internalised hERG displayed marked overlap with glycosylphosphatidylinositol-anchored GFP, a clathrin-independent cargo. Endocytosis was significantly affected by cholesterol extraction with methyl-β-cyclodextrin and inhibition of Arf6 function with dominant negative Arf6-T27N-eGFP. Taken together, we conclude that hERG undergoes clathrin-independent endocytosis via a mechanism involving Arf6.

Endosomes derived from clathrin-independent endocytosis serve as precursors for endothelial lumen formation

Clathrin-independent endocytosis (CIE) is a form of bulk plasma membrane (PM) endocytosis that allows cells to sample and evaluate PM composition. Once in endosomes, the internalized proteins and lipids can be recycled back to the PM or delivered to lysosomes for degradation. Endosomes arising from CIE contain lipid and signaling molecules suggesting that they might be involved in important biological processes. During vasculogenesis, new blood vessels are formed from precursor cells in a process involving internalization and accumulation of endocytic vesicles. Here, we found that CIE has a role in endothelial lumen formation. Specifically, we found that human vascular endothelial cells (HUVECs) utilize CIE for internalization of distinct cargo molecules and that in three-dimensional cultures CIE membranes are delivered to the newly formed lumen.

EFA6A, a guanine nucleotide exchange factor for Arf6, interacts with sorting nexin-1 and regulates neurite outgrowth

The membrane trafficking and actin cytoskeleton remodeling mediated by ADP ribosylation factor 6 (Arf6) are functionally linked to various neuronal processes including neurite formation and maintenance, neurotransmitter release, and receptor internalization. EFA6A is an Arf6-specific guanine nucleotide exchange factor that is abundantly expressed in the brain. In this study, we identified sorting nexin-1 (SNX1), a retromer component that is implicated in endosomal sorting and trafficking, as a novel interacting partner for EFA6A by yeast two-hybrid screening. The interaction was mediated by the C-terminal region of EFA6A and a BAR domain of SNX1, and further confirmed by pull-down assay and immunoprecipitation from mouse brain lysates. In situ hybridization analysis demonstrated the widespread expression of SNX1 in the mouse brain, which overlapped with the expression of EFA6A in the forebrain. Immunofluorescent analysis revealed the partial colocalization of EFA6A and SNX1 in the dendritic fields of the hippocampus. Immunoelectron microscopic analysis revealed the overlapping subcellular localization of EFA6A and SNX1 at the post-synaptic density and endosomes in dendritic spines. In Neuro-2a neuroblastoma cells, expression of either EFA6A or SNX1 induced neurite outgrowth, which was further enhanced by co-expression of EFA6A and SNX1. The present findings suggest a novel mechanism by which EFA6A regulates Arf6-mediated neurite formation through the interaction with SNX1.

Hook1, microtubules, and Rab22: mediators of selective sorting of clathrin-independent endocytic cargo proteins on endosomes

Clathrin-independent endocytosis (CIE) mediates the internalization of many plasma membrane (PM) proteins involved in homeostasis, immune response, and signaling. CIE cargo molecules are internalized independent of clathrin, and dynamin, and modulated by the small G protein Arf6. After internalization the CIE cargo proteins either follow a default pathway of trafficking to lysosomes for degradation or follow a pathway where they are routed directly to the recycling endosomes for return to the PM. The selective endosomal sorting of molecules like CD44, CD98, and CD147, which are involved in cell-cell and cell-extracellular interactions, indicates that sorting mechanisms dictate the post-endocytic fate of CIE cargo proteins. In a recent study, we identified sorting signals that specify the endosomal trafficking of CIE cargo proteins and uncover a role for Hook1 as an endosomal cargo adaptor that routes CIE cargo to the recycling endosomes. Furthermore, we found that Hook1, microtubules, and Rab22a work in coordination to directly recycle the cargo and facilitate cell spreading. Here, we discuss our current view on the endosomal sorting of CIE cargo proteins and their molecular regulators.

Clathrin-independent endocytosis: a cargo-centric view

Clathrin-independent endocytosis occurs in all cells and interest in this mode of cellular entry has grown. Although this form of endocytosis was first described for entry of bacterial toxins, here we focus our attention on the endogenous cell surface "cargo" proteins that enter cells by this mechanism. The cargo proteins entering by this mechanism are varied and include nutrient transporters, ion channels, cell adhesion molecules and proteins associated with the immune system. Despite the apparent lack of selection at the cell surface, we provide some examples of specific sorting of these cargo proteins after entry, leading to distinct itineraries and cellular fates.

Microtubule-dependent endosomal sorting of clathrin-independent cargo by Hook1

Hook1, a microtubule and cargo tethering protein, is important for the sorting of clathrin-independent cargoes away from EEA1+ endosomes and promotes their recycling.

Many plasma membrane (PM) proteins enter cells nonselectively through clathrin-independent endocytosis (CIE). Here, we present evidence that cytoplasmic sequences in three CIE cargo proteins—CD44, CD98, and CD147—were responsible for the rapid sorting of these proteins into endosomal tubules away from endosomes associated with early endosomal antigen 1 (EEA1). We found that Hook1, a microtubule- and cargo-tethering protein, recognized the cytoplasmic tail of CD147 to help sort it and CD98 into Rab22a-dependent tubules associated with recycling. Depletion of Hook1 from cells altered trafficking of CD44, CD98, and CD147 toward EEA1 compartments and impaired the recycling of CD98 back to the PM. In contrast, another CIE cargo protein, major histocompatibility complex class I, which normally traffics to EEA1 compartments, was not affected by depletion of Hook1. Loss of Hook1 also led to an inhibition of cell spreading, implicating a role for Hook1 sorting of specific CIE cargo proteins away from bulk membrane and back to the PM.

ARF6 directs axon transport and traffic of integrins and regulates axon growth in adult DRG neurons

Integrins are involved in axon growth and regeneration. Manipulation of integrins is a route to promoting axon regeneration and understanding regeneration failure in the CNS. Expression of α9 integrin promotes axon regeneration, so we have investigated α9β1 trafficking and transport in axons and at the growth cone. We have previously found that α9 and β1 integrins traffic via Rab11-positive recycling endosomes in peripheral axons and growth cones. However, transport via Rab11 is slow, while rapid transport occurs in vesicles lacking Rab11. We have further studied α9 and β1 integrin transport and traffic in adult rat dorsal root ganglion axons and PC12 cells. Integrins are in ARF6 vesicles during rapid axonal transport and during trafficking in the growth cone. We report that rapid axonal transport of these integrins and their trafficking at the cell surface is regulated by ARF6. ARF6 inactivation by expression of ACAP1 leads to increased recycling of β1 integrins to the neuronal surface and to increased anterograde axonal transport. ARF6 activation by expression of the neuronal guanine nucleotide exchange factors, ARNO or EFA6, increases retrograde integrin transport in axons and increases integrin internalization. ARF6 inactivation increases integrin-mediated outgrowth, while activation decreases it. The coordinated changes in integrin transport and recycling resulting from ARF6 activation or inactivation are the probable mechanism behind this regulation of axon growth. Our data suggest a novel mechanism of integrin traffic and transport in peripheral axons, regulated by the activation state of ARF6, and suggest that ARF6 might be targeted to enhance integrin-dependent axon regeneration after injury.

ARF6-mediated endosomal transport of Telencephalin affects dendritic filopodia-to-spine maturation

The GTPase Arf6 and its exchange factor EFA6A promote internalization of the Ig-like molecule Telencephalin in hippocampal neurons leading to the maturation of filopodia into dendritic spines, important for synapse formation.

Dendritic filopodia are dynamic structures thought to be the precursors of spines during synapse development. Morphological maturation to spines is associated with the stabilization and strengthening of synapses, and can be altered in various neurological disorders. Telencephalin (TLN/intercellular adhesion molecule-5 (ICAM5)) localizes to dendritic filopodia, where it facilitates their formation/maintenance, thereby slowing spine morphogenesis. As spines are largely devoid of TLN, its exclusion from the filopodia surface appears to be required in this maturation process. Using HeLa cells and primary hippocampal neurons, we demonstrate that surface removal of TLN involves internalization events mediated by the small GTPase ADP-ribosylation factor 6 (ARF6), and its activator EFA6A. This endocytosis of TLN affects filopodia-to-spine transition, and requires Rac1-mediated dephosphorylation/release of actin-binding ERM proteins from TLN. At the somato-dendritic surface, TLN and EFA6A are confined to distinct, flotillin-positive membrane subdomains. The co-distribution of TLN with this lipid raft marker also persists during its endosomal targeting to CD63-positive late endosomes. This suggests a specific microenvironment facilitating ARF6-mediated mobilization of TLN that contributes to promotion of dendritic spine development.

Amiloride enhances antigen specific CTL by faciliting HBV DNA vaccine entry into cells

The induction of relatively weak immunity by DNA vaccines in humans can be largely attributed to the low efficiency of transduction of somatic cells. Although formulation with liposomes has been shown to enhance DNA transduction of cultured cells, little, if any, effect is observed on the transduction of somatic tissues and cells. To improve the rate of transduction, DNA vaccine delivery by gene gun and the recently developed electroporation techniques have been employed. We report here that to circumvent requirement for such equipment, amiloride, a drug that is prescribed for hypertension treatment, can accelerate plasmid entry into antigen presenting cells (APCs) both in vitro and in vivo. The combination induced APCs more dramatically in both maturation and cytokine secretion. Amiloride enhanced development of full CD8 cytolytic function including induction of high levels of antigen specific CTL and expression of IFN-γ+perforin+granzymeB+ in CD8+ T cells. Thus, amiloride is a facilitator for DNA transduction into host cells which in turn enhances the efficiency of the immune responses.

HAX1 deficiency: impact on lymphopoiesis and B-cell development

HAX1 was originally described as HS1-associated protein with a suggested function in receptor-mediated apoptotic and proliferative responses of lymphoid cells. Recent publications refer to a complex and multifunctional role of this protein. To investigate the in vivo function of HAX1 (HS1-associated protein X1) in B cells, we generated a Hax1-deficient mouse strain. Targeted deletion of Hax1 resulted in premature death around the age of 12 wk accompanied by a severe reduction of lymphocytes in spleen, thymus and bone marrow. In the bone marrow, all B-cell populations were lost comparably. In the spleen, B220(+) cells were reduced by almost 70%. However, as investigated by adoptive transfer experiments, this impairment is not exclusively B-cell intrinsic and we hypothesize that a HAX1-deficient environment cannot sufficiently provide the essential factors for proper lymphocyte development, trafficking and survival. Hax1(-/-) B cells show a significantly reduced expression of CXCR4, which might have an influence on the observed defects in B-cell development.

The role of G proteins in assembly and function of Kir3 inwardly rectifying potassium channels

Kir3 channels (also known as GIRK channels) are important regulators of electrical excitability in both cardiomyocytes and neurons. Much is known regarding the assembly and function of these channels and the roles that their interacting proteins play in controlling these events. Further, they are one of the best studied effectors of heterotrimeric G proteins in general and Gβγ subunits in particular. However, our understanding of the roles of multiple Gβγ binding sites on Kir3 channels is still rudimentary. We discuss potential roles for Gβγ in channel assembly and trafficking in addition to their known role in cellular signaling.

Discovery of new cargo proteins that enter cells through clathrin-independent endocytosis

Clathrin-independent endocytosis (CIE) allows internalization of plasma membrane proteins lacking clathrin-targeting sequences, such as the major histocompatibility complex class I protein (MHCI), into cells. After internalization, vesicles containing MHCI fuse with transferrin-containing endosomes generated from clathrin-dependent endocytosis. In HeLa cells, MHCI is subsequently routed to late endosomes or recycled back out to the plasma membrane (PM) in distinctive tubular carriers. Arf6 is associated with endosomal membranes carrying CIE cargo and expression of an active form of Arf6 leads to the generation of vacuolar structures that trap CIE cargo immediately after endocytosis, blocking the convergence with transferrin-containing endosomes. We isolated these trapped vacuolar structures and analyzed their protein composition by mass spectrometry. Here we identify and validate six new endogenous cargo proteins (CD44, CD55, CD98, CD147, Glut1, and ICAM1) that use CIE to enter cells. CD55 and Glut1 appear to closely parallel the trafficking of MHCI, merging with transferrin endosomes before entering the recycling tubules. In contrast, CD44, CD98, and CD147 appear to directly enter the recycling tubules and by-pass the merge with EEA1-positive, transferrin-containing endosomes. This divergent itinerary suggests that sorting may occur along this CIE pathway. Furthermore, the identification of new cargo proteins will assist others studying CIE in different cell types and tissues.

Constitutive endocytic recycling and protein kinase C-mediated lysosomal degradation control K(ATP) channel surface density

Pancreatic ATP-sensitive potassium (K(ATP)) channels control insulin secretion by coupling the excitability of the pancreatic beta-cell to glucose metabolism. Little is currently known about how the plasma membrane density of these channels is regulated. We therefore set out to examine in detail the endocytosis and recycling of these channels and how these processes are regulated. To achieve this goal, we expressed K(ATP) channels bearing an extracellular hemagglutinin epitope in human embryonic kidney cells and followed their fate along the endocytic pathway. Our results show that K(ATP) channels undergo multiple rounds of endocytosis and recycling. Further, activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate significantly decreases K(ATP) channel surface density by reducing channel recycling and diverting the channel to lysosomal degradation. These findings were recapitulated in the model pancreatic beta-cell line INS1e, where activation of PKC leads to a decrease in the surface density of native K(ATP) channels. Because sorting of internalized channels between lysosomal and recycling pathways could have opposite effects on the excitability of pancreatic beta-cells, we propose that PKC-regulated K(ATP) channel trafficking may play a role in the regulation of insulin secretion.

Pathways and mechanisms of endocytic recycling

Endocytic recycling is coordinated with endocytic uptake to control the composition of the plasma membrane. Although much of our understanding of endocytic recycling has come from studies on the transferrin receptor, a protein internalized through clathrin-dependent endocytosis, increased interest in clathrin-independent endocytosis has led to the discovery of new endocytic recycling systems. Recent insights into the regulatory mechanisms that control endocytic recycling have focused on recycling through tubular carriers and the return to the cell surface of cargoes that enter cells through clathrin-independent mechanisms. Recent work emphasizes the importance of regulated recycling in processes as diverse as cytokinesis, cell adhesion, morphogenesis, cell fusion, learning and memory.

Mechanisms of endocytosis

Endocytic mechanisms control the lipid and protein composition of the plasma membrane, thereby regulating how cells interact with their environments. Here, we review what is known about mammalian endocytic mechanisms, with focus on the cellular proteins that control these events. We discuss the well-studied clathrin-mediated endocytic mechanisms and dissect endocytic pathways that proceed independently of clathrin. These clathrin-independent pathways include the CLIC/GEEC endocytic pathway, arf6-dependent endocytosis, flotillin-dependent endocytosis, macropinocytosis, circular doral ruffles, phagocytosis, and trans-endocytosis. We also critically review the role of caveolae and caveolin1 in endocytosis. We highlight the roles of lipids, membrane curvature-modulating proteins, small G proteins, actin, and dynamin in endocytic pathways. We discuss the functional relevance of distinct endocytic pathways and emphasize the importance of studying these pathways to understand human disease processes.

Clathrin-independent endocytosis: a unique platform for cell signaling and PM remodeling

There is increasing interest in endocytosis that occurs independently of clathrin coats and the fates of membrane proteins internalized by this mechanism. The appearance of clathrin-independent endocytic and membrane recycling pathways seems to vary with different cell types and cargo molecules. In this review we focus on studies that have been performed using HeLa and COS cells as model systems for understanding this membrane trafficking system. These endosomal membranes contain signaling molecules including H-Ras, Rac1, Arf6 and Rab proteins, and a lipid environment rich in cholesterol and PIP(2) providing a unique platform for cell signaling. Furthermore, activation of some of these signaling molecules (H-Ras, Rac and Arf6) can switch the constitutive form of clathrin-independent endocytosis into a stimulated one, associated with PM ruffling and macropinocytosis.

Oxidised LDL internalisation by the LOX-1 scavenger receptor is dependent on a novel cytoplasmic motif and is regulated by dynamin-2

The LOX-1 scavenger receptor recognises pro-atherogenic oxidised low-density lipoprotein (OxLDL) particles and is implicated in atherosclerotic plaque formation, but this mechanism is not well understood. Here we show evidence for a novel clathrin-independent and cytosolic-signal-dependent pathway that regulates LOX-1-mediated OxLDL internalisation. Cell surface labelling in the absence or presence of OxLDL ligand showed that LOX-1 is constitutively internalised from the plasma membrane and its half-life is not altered upon ligand binding and trafficking. We show that LOX-1-mediated OxLDL uptake is disrupted by overexpression of dominant-negative dynamin-2 but unaffected by CHC17 or mu2 (AP2) depletion. Site-directed mutagenesis revealed a conserved and novel cytoplasmic tripeptide motif (DDL) that regulates LOX-1-mediated endocytosis of OxLDL. Taken together, these findings indicate that LOX-1 is internalised by a clathrin-independent and dynamin-2-dependent pathway and is thus likely to mediate OxLDL trafficking in vascular tissues.

Existence of multiple isoforms of HS1-associated protein X-1 in murine and human tissues

To date, the literature concerning the HS1 (haematopoietic cell-specific protein 1)-associated protein X-1 (HAX1) protein has reported considerable variation regarding its function in mammalian cells, subcellular localisation and binding partners. We show here that HAX1 comprises a family of proteins. Murine tissues express three mRNA variants, encoded by two genes on chromosomes 2 and 3. The chromosome 2 gene is intronless and would encode a protein 100% identical with that encoded by chromosome 3. In humans, alternative splice variants, encoded by the chromosome 1 gene, produce a family of transcripts composed of up to eight members. Based on the sequences published in GenBank and Ensembl, we designed specific primers and detected by PCR three mRNA species in murine tissues and eight variants in human cells. We screened a panel of 19 human cell lines as well as primary fibroblasts, oral keratinocytes and freshly isolated peripheral blood mononuclear cells. All human cells studied expressed at least six of the possible HAX1 mRNA variants. In silico analysis of the variants revealed an open reading frame in all of them, suggesting that murine and human tissues can express two and eight HAX1 proteins, respectively. Analysis of human protein lysates by Western blotting with the use of a monoclonal anti-HAX1 antibody revealed multiple bands. These bands were decreased after treatment of cells with a single small interfering RNA duplex targeting a region common to six of the variants, confirming their identity as HAX1 proteins. Comparison of the human variants with the six HAX1 homologues described to date in the chimpanzee (Pan troglodytes) and the four homologues described in macaque (Macaca mulatta) revealed very high conservation with only one amino acid substitution between human and chimpanzee homologues. Moreover, a number of additional products were amplified and sequenced, which indicated that further human isoforms are likely to exist. These findings are likely to explain the current confusion concerning putative HAX1 function.

The EFA6 family: guanine nucleotide exchange factors for ADP ribosylation factor 6 at neuronal synapses

ADP ribosylation factor 6 (ARF6) is a member of the ARF family of small GTPases, which mediates a variety of neuronal functions accompanying the structural changes of developing and mature neurons through its regulation of actin cytoskeleton reorganization and membrane traffic. The activation of ARF6 is strictly regulated by guanine nucleotide exchange factors (GEFs). The EFA6 family is the first member that was identified to be a specific GEF for ARF6 and comprises four structurally related polypeptides (EFA6A, EFA6B, EFA6C and EFA6D). Since the cellular and subcelllular localization of GEFs is a critical determinant for the spatiotemporal activation of ARF6 in neurons, I have focused on the EFA6 family from the anatomical point of view to understand the neuronal functions of ARF6. Three members of the EFA6 family (EFA6A, EFA6C and EFA6D) are abundantly expressed in the mouse brain with distinct spatiotemporal patterns. Interestingly, they are enriched particularly in the postsynaptic density fraction, shedding light on the importance of the EFA-ARF6 pathway in neuronal synapses. Here, I will review the recent advances in the expression and functions of the EFA6 family in the nervous system.

Clathrin-independent endocytosis: from nonexisting to an extreme degree of complexity

Today it is generally accepted that there are several endocytic mechanisms, both the clathrin-dependent one and mechanisms which operate without clathrin and with different requirements when it comes to dynamin, small GTP-binding proteins of the Rho family and specific lipids. It should be noted that clathrin-independent endocytosis can occur even when the cholesterol level in the membrane has been reduced to so low levels that caveolae are gone and clathrin-coated membrane areas are flat. Although new investigators in the field take it for granted that there is a multitude of entry mechanisms, it has taken a long time for this to become accepted. However, more work needs to be done, because one can still ask the question: How many endocytic mechanisms does a cell have, what are their function, and how are they regulated? This article describes some of the history of endocytosis research and attempts to give an overview of the complexity of the mechanisms and their regulation.

Clathrin-independent internalization and recycling

The functionality of receptor and channel proteins depends directly upon their expression level on the plasma membrane. Therefore, the ability to selectively adjust the surface level of a particular receptor or channel protein is pivotal to many cellular signalling events. The internalization and recycling pathway plays a major role in the regulation of protein surface level, and thus has been a focus of research for many years. Although several endocytic pathways have been identified, most of our knowledge has come from the clathrin-dependent pathway, while the other pathways remain much less well defined. Considering that clathrin-independent internalization may account for as much as 50% of the total endocytic activity in the cell, the lack of such knowledge constitutes a major gap in our efforts to understand how different internalization pathways are utilized and co-ordinated. Recent studies have provided valuable insights into this area, yet many more questions still remain. In this review, we will give a panoramic introduction to the current knowledge of various internalization and recycling pathways, with an emphasis on the latest findings that have broadened our view of the clathrin-independent pathways. We will also dedicate one section to the emerging studies of the clathrin-independent internalization pathways in neuronal cells.

A specific role of AGS3 in the surface expression of plasma membrane proteins

Activator of G protein signaling 3 (AGS3), originally identified in a functional screen for mammalian proteins that activate heterotrimeric G protein signaling, is known to be involved in drug-seeking behavior and is up-regulated during cocaine withdrawal in animal models. These observations indicate a potential role for AGS3 in the formation or maintenance of neural plasticity. We have found that the overexpression of AGS3 alters the surface-to-total ratios of a subset of heterologously expressed plasma membrane receptors and channels. Further analysis of the endocytic trafficking of one such protein by a biotin-based internalization assay suggests that overexpression of AGS3 moderately affects the internalization or recycling of surface proteins. Moreover, AGS3 overexpression and siRNA-mediated knockdown of AGS3 both result in the dispersal of two endogenously expressed trans-Golgi network (TGN)-associated cargo proteins without influencing those in the cis- or medial-Golgi compartments. Finally, adding a TGN-localization signal to a CD4-derived reporter renders the trafficking of fusion protein sensitive to AGS3. Taken together, our data support a model wherein AGS3 modulates the protein trafficking along the TGN/plasma membrane/endosome loop.

Internalization and degradation of the glutamate transporter GLT-1 in response to phorbol ester

Activation of protein kinase C (PKC) decreases the activity and cell surface expression of the predominant forebrain glutamate transporter, GLT-1. In the present study, C6 glioma were used as a model system to define the mechanisms that contribute to this decrease in cell surface expression and to determine the fate of internalized transporter. As was previously observed, phorbol 12-myristate 13-acetate (PMA) caused a decrease in biotinylated GLT-1. This effect was blocked by sucrose or by co-expression with a dominant-negative variant of dynamin 1, and it was attenuated by co-expression with a dominant-negative variant of the clathrin heavy chain. Depletion of cholesterol with methyl-beta-cyclodextrin, co-expression with a dominant-negative caveolin-1 mutant (Cav1/S80E), co-expression with dominant-negative variants of Eps15 (epidermal-growth-factor receptor pathway substrate clone 15), or co-expression with dominant-negative Arf6 (T27N) had no effect on the PMA-induced loss of biotinylated GLT-1. Long-term treatment with PMA caused a time-dependent loss of biotinylated GLT-1 and decreased the levels of GLT-1 protein. Inhibitors of lysosomal degradation (chloroquine or ammonium chloride) or co-expression with a dominant-negative variant of a small GTPase implicated in trafficking to lysosomes (Rab7) prevented the PMA-induced decrease in protein and caused an intracellular accumulation of GLT-1. These results suggest that the PKC-induced redistribution of GLT-1 is dependent upon clathrin-mediated endocytosis. These studies identify a novel mechanism by which the levels of GLT-1 could be rapidly down-regulated via lysosomal degradation. The possibility that this mechanism may contribute to the loss of GLT-1 observed after acute insults to the CNS is discussed.

Rab-GTPase-dependent endocytic recycling of Kv1.5 in atrial myocytes

The number of ion channels expressed on the cell surface shapes the complex electrical response of excitable cells. Maintaining a balance between anterograde and retrograde trafficking of channel proteins is vital in regulating steady-state cell surface expression. Kv1.5 is an important voltage-gated K(+) channel in the cardiovascular system underlying the ultra-rapid rectifying potassium current (Ik(ur)), a major repolarizing current in atrial myocytes, and regulating the resting membrane potential and excitability of smooth muscle cells. Defects in the expression of Kv1.5 are associated with pathological states such as chronic atrial fibrillation and hypoxic pulmonary hypertension. There is, thus, substantial interest in understanding the mechanisms regulating cell surface channel levels. Here, we investigated the internalization and recycling of Kv1.5 in the HL-1 immortalized mouse atrial myocytes. Kinetic studies indicate that Kv1.5 is rapidly internalized to a perinuclear region where it co-localizes with the early endosomal marker, EEA1. Importantly, we identified that a population of Kv1.5, originating on the cell surface, internalized and recycled back to the plasma membrane. Notably, Kv1.5 recycling processes are driven by specific Rab-dependent endosomal compartments. Thus, co-expression of GDP-locked Rab4S22N and Rab11S25N dominant-negative mutants decreased the steady-state Kv1.5 surface levels, whereas GTPase-deficient Rab4Q67L and Rab11Q70L mutants increased steady-state Kv1.5 surface levels. These data reveal an unexpected dynamic trafficking of Kv1.5 at the myocyte plasma membrane and demonstrate a role for recycling in the maintenance of steady-state ion channel surface levels.