Mechano-regulation by clathrin pit-formation and passive cholesterol-dependent tubules during de-adhesion

Adherent cells ensure membrane homeostasis during de-adhesion by various mechanisms, including endocytosis. Although mechano-chemical feedbacks involved in this process have been studied, the step-by-step build-up and resolution of the mechanical changes by endocytosis are poorly understood. To investigate this, we studied the de-adhesion of HeLa cells using a combination of interference reflection microscopy, optical trapping and fluorescence experiments. We found that de-adhesion enhanced membrane height fluctuations of the basal membrane in the presence of an intact cortex. A reduction in the tether force was also noted at the apical side. However, membrane fluctuations reveal phases of an initial drop in effective tension followed by saturation. The area fractions of early (Rab5-labelled) and recycling (Rab4-labelled) endosomes, as well as transferrin-labelled pits close to the basal plasma membrane, also transiently increased. On blocking dynamin-dependent scission of endocytic pits, the regulation of fluctuations was not blocked, but knocking down AP2-dependent pit formation stopped the tension recovery. Interestingly, the regulation could not be suppressed by ATP or cholesterol depletion individually but was arrested by depleting both. The data strongly supports Clathrin and AP2-dependent pit-formation to be central to the reduction in fluctuations confirmed by super-resolution microscopy. Furthermore, we propose that cholesterol-dependent pits spontaneously regulate tension under ATP-depleted conditions.

Clathrin coated pits as signaling platforms for Akt signaling

Signaling by the activated epidermal growth factor receptor (EGFR) results in diverse cell fates. In this issue, Cabral-Dias et al. (2022. J. Cell Biol.https://doi.org/10.1083/jcb.201808181) demonstrate how plasma membrane clathrin coated pits can act as a signaling platform for one branch of EGFR downstream signaling.

Single molecule localization-based analysis of clathrin-coated pit and caveolar dynamics

Clathrin-coated pits and caveolae are nanoscale invaginations of the plasma membrane of cells, forming through the assembly of membrane coat and accessory proteins in a tightly regulated process. We have analyzed the development of these membrane coat structures with high spatial and temporal resolution and sensitivity using super-resolution single-molecule localization microscopy (SMLM) on live cells. To this end, we developed a sophisticated clustering and data analysis workflow that automatically extracts the relevant information from SMLM image sequences taken on live cells. We quantified lifetime distributions of clathrin-coated and caveolar structures, and analyzed their growth dynamics. Moreover, we observed hotspots in the plasma membrane where coat structures appear repeatedly. The stunningly similar temporal development of clathrin-coated and caveolar structures suggests that key accessory proteins, some of which are shared between the two types of membrane coat structures, orchestrate the temporal evolution of these complex architectures.

Functional characterization of 67 endocytic accessory proteins using multiparametric quantitative analysis of CCP dynamics

Clathrin-mediated endocytosis (CME) begins with the nucleation of clathrin assembly on the plasma membrane, followed by stabilization and growth/maturation of clathrin-coated pits (CCPs) that eventually pinch off and internalize as clathrin-coated vesicles. This highly regulated process involves a myriad of endocytic accessory proteins (EAPs), many of which are multidomain proteins that encode a wide range of biochemical activities. Although domain-specific activities of EAPs have been extensively studied, their precise stage-specific functions have been identified in only a few cases. Using single-guide RNA (sgRNA)/dCas9 and small interfering RNA (siRNA)-mediated protein knockdown, combined with an image-based analysis pipeline, we have determined the phenotypic signature of 67 EAPs throughout the maturation process of CCPs. Based on these data, we show that EAPs can be partitioned into phenotypic clusters, which differentially affect CCP maturation and dynamics. Importantly, these clusters do not correlate with functional modules based on biochemical activities. Furthermore, we discover a critical role for SNARE proteins and their adaptors during early stages of CCP nucleation and stabilization and highlight the importance of GAK throughout CCP maturation that is consistent with GAK's multifunctional domain architecture. Together, these findings provide systematic, mechanistic insights into the plasticity and robustness of CME.

Evolving models for assembling and shaping clathrin-coated pits

Clathrin-mediated endocytosis occurs via the assembly of clathrin-coated pits (CCPs) that invaginate and pinch off to form clathrin-coated vesicles (CCVs). It is well known that adaptor protein 2 (AP2) complexes trigger clathrin assembly on the plasma membrane, and biochemical and structural studies have revealed the nature of these interactions. Numerous endocytic accessory proteins collaborate with clathrin and AP2 to drive CCV formation. However, many questions remain as to the molecular events involved in CCP initiation, stabilization, and curvature generation. Indeed, a plethora of recent evidence derived from cell perturbation, correlative light and EM tomography, live-cell imaging, modeling, and high-resolution structural analyses has revealed more complexity and promiscuity in the protein interactions driving CCP maturation than anticipated. After briefly reviewing the evidence supporting prevailing models, we integrate these new lines of evidence to develop a more dynamic and flexible model for how redundant, dynamic, and competing protein interactions can drive endocytic CCV formation and suggest new approaches to test emerging models.

Dual-Mode Superresolution Imaging Using Charge Transfer Dynamics in Semiconducting Polymer Dots

In a conjugated polymer-based single-particle heterojunction, stochastic fluctuations of the photogenerated hole population lead to spontaneous fluorescence switching. We found that 405 nm irradiation can induce charge recombination and activate the single-particle emission. Based on these phenomena, we developed a novel class of semiconducting polymer dots that can operate in two superresolution imaging modes. The spontaneous switching mode offers efficient imaging of large areas, with <10 nm localization precision, while the photoactivation/deactivation mode offers slower imaging, with further improved localization precision (ca. 1 nm), showing advantages in resolving small structures that require high spatial resolution. Superresolution imaging of microtubules and clathrin-coated pits was demonstrated, under both modes. The excellent localization precision and versatile imaging options provided by these nanoparticles offer clear advantages for imaging of various biological systems.

Clathrin light chain A drives selective myosin VI recruitment to clathrin-coated pits under membrane tension

Clathrin light chains (CLCa and CLCb) are major constituents of clathrin-coated vesicles. Unique functions for these evolutionary conserved paralogs remain elusive, and their role in clathrin-mediated endocytosis in mammalian cells is debated. Here, we find and structurally characterize a direct and selective interaction between CLCa and the long isoform of the actin motor protein myosin VI, which is expressed exclusively in highly polarized tissues. Using genetically-reconstituted Caco-2 cysts as proxy for polarized epithelia, we provide evidence for coordinated action of myosin VI and CLCa at the apical surface where these proteins are essential for fission of clathrin-coated pits. We further find that myosin VI and Huntingtin-interacting protein 1-related protein (Hip1R) are mutually exclusive interactors with CLCa, and suggest a model for the sequential function of myosin VI and Hip1R in actin-mediated clathrin-coated vesicle budding.

Temporal Ordering in Endocytic Clathrin-Coated Vesicle Formation via AP2 Phosphorylation

Clathrin-mediated endocytosis (CME) is key to maintaining the transmembrane protein composition of cells' limiting membranes. During mammalian CME, a reversible phosphorylation event occurs on Thr156 of the μ2 subunit of the main endocytic clathrin adaptor, AP2. We show that this phosphorylation event starts during clathrin-coated pit (CCP) initiation and increases throughout CCP lifetime. μ2Thr156 phosphorylation favors a new, cargo-bound conformation of AP2 and simultaneously creates a binding platform for the endocytic NECAP proteins but without significantly altering AP2's cargo affinity in vitro. We describe the structural bases of both. NECAP arrival at CCPs parallels that of clathrin and increases with μ2Thr156 phosphorylation. In turn, NECAP recruits drivers of late stages of CCP formation, including SNX9, via a site distinct from where NECAP binds AP2. Disruption of the different modules of this phosphorylation-based temporal regulatory system results in CCP maturation being delayed and/or stalled, hence impairing global rates of CME.

Single-Molecule Microscopy Reveals Dynamic FLNA Interactions Governing SSTR2 Clustering and Internalization

The cytoskeletal protein filamin A (FLNA) has been suggested to play an important role in the responsiveness of GH-secreting pituitary tumors to somatostatin receptor subtype 2 (SSTR2) agonists by regulating SSTR2 expression and signaling. However, the underlying mechanisms are unknown. In this study, we use fast multicolor single-molecule microscopy to image individual SSTR2 and FLNA molecules at the surface of living cells with unprecedented spatiotemporal resolution. We find that SSTR2 and FLNA undergo transient interactions, which occur preferentially along actin fibers and contribute to restraining SSTR2 diffusion. Agonist stimulation increases the localization of SSTR2 along actin fibers and, subsequently, SSTR2 clustering and recruitment to clathrin-coated pits (CCPs). Interfering with FLNA-SSTR2 binding with a dominant-negative FLNA fragment increases SSTR2 mobility, hampers the formation and alignment of SSTR2 clusters along actin fibers, and impairs both SSTR2 recruitment to CCPs and SSTR2 internalization. These findings indicate that dynamic SSTR2-FLNA interactions critically control the nanoscale localization of SSTR2 at the plasma membrane and are required for coupling SSTR2 clustering to internalization. These mechanisms explain the critical role of FLNA in the control of SSTR2 expression and signaling and suggest the possibility of targeting SSTR2-FLNA interactions for the therapy of pharmacologically resistant GH-secreting pituitary tumors.

FerriTag is a new genetically-encoded inducible tag for correlative light-electron microscopy

A current challenge is to develop tags to precisely visualize proteins in cells by light and electron microscopy. Here, we introduce FerriTag, a genetically-encoded chemically-inducible tag for correlative light-electron microscopy. FerriTag is a fluorescent recombinant electron-dense ferritin particle that can be attached to a protein-of-interest using rapamycin-induced heterodimerization. We demonstrate the utility of FerriTag for correlative light-electron microscopy by labeling proteins associated with various intracellular structures including mitochondria, plasma membrane, and clathrin-coated pits and vesicles. FerriTagging has a good signal-to-noise ratio and a labeling resolution of approximately 10 nm. We demonstrate how FerriTagging allows nanoscale mapping of protein location relative to a subcellular structure, and use it to detail the distribution and conformation of huntingtin-interacting protein 1 related (HIP1R) in and around clathrin-coated pits.

New tools for "hot-wiring" clathrin-mediated endocytosis with temporal and spatial precision

Clathrin-mediated endocytosis (CME) is the major route of receptor internalization at the plasma membrane. Analysis of constitutive CME is difficult because the initiation of endocytic events is unpredictable. When and where a clathrin-coated pit will form and what cargo it will contain are difficult to foresee. Here we describe a series of genetically encoded reporters that allow the initiation of CME on demand. A clathrin-binding protein fragment ("hook") is inducibly attached to an "anchor" protein at the plasma membrane, which triggers the formation of new clathrin-coated vesicles. Our design incorporates temporal and spatial control by the use of chemical and optogenetic methods for inducing hook-anchor attachment. Moreover, the cargo is defined. Because several steps in vesicle creation are bypassed, we term it "hot-wiring." We use hot-wired endocytosis to describe the functional interactions between clathrin and AP2. Two distinct sites on the β2 subunit, one on the hinge and the other on the appendage, are necessary and sufficient for functional clathrin engagement.

Covalent Protein Labeling by SpyTag-SpyCatcher in Fixed Cells for Super-Resolution Microscopy

Labeling proteins with high specificity and efficiency is a fundamental prerequisite for microscopic visualization of subcellular protein structures and interactions. Although the comparatively small size of epitope tags makes them less perturbative to fusion proteins, they require the use of large antibodies that often limit probe accessibility and effective resolution. Here we use the covalent SpyTag-SpyCatcher system as an epitope-like tag for fluorescent labeling of intracellular proteins in fixed cells for both conventional and super-resolution microscopy. We also applied this method to endogenous proteins by gene editing, demonstrating its high labeling efficiency and capability for isoform-specific labeling.

The role of the Lowe syndrome protein OCRL in the endocytic pathway

Mutations of the inositol-5-phosphatase OCRL cause Lowe syndrome and Dent-II disease. Both are rare genetic disorders characterized by renal defects. Lowe syndrome is furthermore characterized by defects of the eye (congenital cataracts) and nervous system (mental disabilities, hypotonia). OCRL has been localised to various endocytic compartments suggesting impairments in the endocytic pathway as possible disease mechanism. Recent evidence strongly supports this view and shows essential roles of OCRL at clathrin coated pits, transport of cargo from endosomes to the trans-Golgi network as well as recycling of receptors from endosomes to the plasma membrane. In particular in vitro and in vivo evidence demonstrates an important role of OCRL in recycling of megalin, a multi-ligand receptor crucial for reabsorption of nutrients in the proximal tubulus, a process severely impaired in Lowe syndrome patients. Thus defects in the endocytic pathway are likely to significantly contribute to the kidney phenotype in Lowe syndrome and Dent-II disease.

Activation of ERK and NF-κB during HARE-Mediated Heparin Uptake Require Only One of the Four Endocytic Motifs

Fifteen different ligands, including heparin (Hep), are cleared from lymph and blood by the Hyaluronan (HA) Receptor for Endocytosis (HARE; derived from Stabilin-2 by proteolysis), which contains four endocytic motifs (M1-M4). Endocytosis of HARE•Hep complexes is targeted to coated pits by M1, M2, and M3 (Pandey et al, Int. J. Cell Biol. 2015, article ID 524707), which activates ERK1/2 and NF-κB (Pandey et al J. Biol. Chem. 288, 14068-79, 2013). Here, we used a NF-κB promoter-driven luciferase gene assay and cell lines expressing different HARE cytoplasmic domain mutants to identify motifs needed for Hep-mediated signaling. Deletion of M1, M2 or M4 singly had no effect on Hep-mediated ERK1/2 activation, whereas signaling (but not uptake) was eliminated in HARE(ΔM3) cells lacking NPLY2519. ERK1/2 signaling in cells expressing WT HARE(Y2519A) or HARE(Y2519A) lacking M1, M2 and M4 (containing M3-only) was decreased by 75% or eliminated, respectively. Deletion of M3 (but not M1, M2 or M4) also inhibited the formation of HARE•Hep•ERK1/2 complexes by 67%. NF-κB activation by HARE-mediated uptake of Hep, HA, dermatan sulfate or acetylated LDL was unaffected in single-motif deletion mutants lacking M1, M2 or M4. In contrast, cells expressing HARE(ΔM3) showed loss of HARE-mediated NF-κB activation during uptake of each of these four ligands. NF-κB activation by the four signaling ligands was also eliminated in HARE(Y2519A) or HARE(M3-only;Y2519A) cells. We conclude that the HARE NPLY2519 motif is necessary for both ERK1/2 and NF-κB signaling and that Tyr2519 is critical for these functions.

Rab35 GTPase Triggers Switch-like Recruitment of the Lowe Syndrome Lipid Phosphatase OCRL on Newborn Endosomes

Phosphoinositide (PtdIns) homeostasis requires a tight spatial and temporal regulation during the endocytic process [1]. Indeed, PtdIns(4,5)P2 plays a crucial role in endocytosis by controlling clathrin-coated pit formation, whereas its conversion into PtdIns4P right after scission of clathrin-coated vesicles (CCVs) is essential for successful uncoating and cargo sorting [1-6]. In non-neuronal cells, endosomal PtdIns(4,5)P2 hydrolysis critically relies on the lipid phosphatase OCRL [7-9], the inactivation of which causes the Oculo-Cerebro-Renal syndrome of Lowe [10, 11]. To understand the coupling between PtdIns(4,5)P2 hydrolysis and endosome formation, a key issue is thus to unravel the mechanism by which OCRL is recruited on CCVs precisely after their scission from the plasma membrane. Here we found that the Rab35 GTPase, which plays a fundamental but poorly understood role in endosomal trafficking after cargo internalization [12-21], directly recruits the OCRL phosphatase immediately after scission of the CCVs. Consistent with Rab35 and OCRL acting together, depletion of either Rab35 or OCRL leads to retention of internalized receptors such as the endogenous cation-independent mannose-6-phosphate receptor (CI-MPR) in peripheral clathrin-positive endosomes that display abnormal association with PtdIns(4,5)P2- and actin-binding proteins. Remarkably, Rab35 loading on CCVs rapidly follows the recruitment of the AP2-binding Rab35 GEF/activator DENND1A (connecdenn 1) and the disappearance of the Rab35 GAP/inhibitor EPI64B. We propose that the precise spatial and temporal activation of Rab35 acts as a major switch for OCRL recruitment on newborn endosomes, post-scission PtdIns(4,5)P2 hydrolysis, and subsequent endosomal trafficking.

Single Particle Tracking Reveals that EGFR Signaling Activity Is Amplified in Clathrin-Coated Pits

Signaling from the epidermal growth factor receptor (EGFR) via phosphorylation on its C-terminal tyrosine residues requires self-association, which depends on the diffusional properties of the receptor and its density in the plasma membrane. Dimerization is a key event for EGFR activation, but the role of higher order clustering is unknown. We employed single particle tracking to relate the mobility and aggregation of EGFR to its signaling activity. EGFR mobility alternates between short-lived free, confined and immobile states. In the immobile state, EGFR tends to aggregate in clathrin-coated pits, which is further enhanced in a phosphorylation-dependent manner and does not require ligand binding. EGFR phosphorylation is further amplified by cross-phosphorylation in clathrin-coated pits. Because phosphorylated receptors can escape from the pits, local gradients of signaling active EGFR are formed. These results show that amplification of EGFR phosphorylation by receptor clustering in clathrin-coated pits supports signal activation at the plasma membrane.

Dynamin assembly strategies and adaptor proteins in mitochondrial fission

Mitochondrial fission is mediated by a dynamin-related GTPase that assembles at constricted sites on the organelle. The mechanism of action of this GTPase in fission is related to that of classical dynamin, which severs the necks of clathrin-coated pits at the plasma membrane. The scale of these membrane remodeling events differs by an order of magnitude, however, and structural studies have revealed variations in the assembly properties of classical and mitochondrial dynamins that accommodate these differences. Despite this progress, structural and mechanistic models have not yet incorporated a growing number of adaptor proteins that are required for the membrane recruitment and function of mitochondrial dynamins. Here, we review the structure and assembly properties of the yeast and mammalian mitochondrial dynamins and discuss what is known about the activities of their adaptor proteins.

The function of endocytosis in podocytes

PURPOSE OF REVIEW

In this review, we discuss the role of endocytosis, a fundamental process internalizing molecules from the plasma membrane, and its critical importance in podocyte biology.

RECENT FINDINGS

Endocytic clathrin and nonclathrin-coated pits have been visualized in podocytes using electron microscopy, but the functional biological relevance has not been well defined. Recent evidence suggests that loss of key clathrin endocytic regulatory apparatus, such as dynamin, synaptojanin 1 or endophilin, in genetic mouse models of disease results in severe proteinuria and foot process effacement. In addition, several genes implicated in human nephrotic syndrome directly or indirectly associate with these endocytic proteins, thus creating a protein network that is linked in actin dynamics, signalling and endocytosis.

SUMMARY

This review summarizes our current understanding of membrane trafficking specifically in podocytes, thus giving further novel insights into the molecular mechanisms and pathogenesis of nephrotic syndrome.

Divergent modes for cargo-mediated control of clathrin-coated pit dynamics

Clathrin-mediated endocytosis has long been viewed as a process driven by core endocytic proteins, with internalized cargo proteins being passive. In contrast, an emerging view suggests that signaling receptor cargo may actively control its fate by regulating the dynamics of clathrin-coated pits (CCPs) that mediate their internalization. Despite its physiological implications, very little is known about such "cargo-mediated regulation" of CCPs by signaling receptors. Here, using multicolor total internal reflection fluorescence microscopy imaging and quantitative analysis in live cells, we show that the μ-opioid receptor, a physiologically relevant G protein-coupled signaling receptor, delays the dynamics of CCPs in which it is localized. This delay is mediated by the interactions of two critical leucines on the receptor cytoplasmic tail. Unlike the previously known mechanism of cargo-mediated regulation, these residues regulate the lifetimes of dynamin, a key component of CCP scission. These results identify a novel means for selectively controlling the endocytosis of distinct cargo that share common trafficking components and indicate that CCP regulation by signaling receptors can operate via divergent modes.

Uncoupling the functions of CALM in VAMP sorting and clathrin-coated pit formation

CALM (clathrin assembly lymphoid myeloid leukemia protein) is a cargo-selective adaptor for the post-Golgi R-SNAREs VAMPs 2, 3, and 8, and it also regulates the size of clathrin-coated pits and vesicles at the plasma membrane. The present study has two objectives: to determine whether CALM can sort additional VAMPs, and to investigate whether VAMP sorting contributes to CALM-dependent vesicle size regulation. Using a flow cytometry-based endocytosis efficiency assay, we demonstrate that CALM is also able to sort VAMPs 4 and 7, even though they have sorting signals for other clathrin adaptors. CALM homologues are present in nearly every eukaryote, suggesting that the CALM family may have evolved as adaptors for retrieving all post-Golgi VAMPs from the plasma membrane. Using a knockdown/rescue system, we show that wild-type CALM restores normal VAMP sorting in CALM-depleted cells, but that two non-VAMP-binding mutants do not. However, when we assayed the effect of CALM depletion on coated pit morphology, using a fluorescence microscopy-based assay, we found that the two mutants were as effective as wild-type CALM. Thus, we can uncouple the sorting function of CALM from its structural role.

Cis and trans regulatory mechanisms control AP2-mediated B cell receptor endocytosis via select tyrosine-based motifs

Following antigen recognition, B cell receptor (BCR)-mediated endocytosis is the first step of antigen processing and presentation to CD4+ T cells, a crucial component of the initiation and control of the humoral immune response. Despite this, the molecular mechanism of BCR internalization is poorly understood. Recently, studies of activated B cell-like diffuse large B cell lymphoma (ABC DLBCL) have shown that mutations within the BCR subunit CD79b leads to increased BCR surface expression, suggesting that CD79b may control BCR internalization. Adaptor protein 2 (AP2) is the major mediator of receptor endocytosis via clathrin-coated pits. The BCR contains five putative AP2-binding YxxØ motifs, including four that are present within two immunoreceptor tyrosine-based activation motifs (ITAMs). Using a combination of in vitro and in situ approaches, we establish that the sole mediator of AP2-dependent BCR internalization is the membrane proximal ITAM YxxØ motif in CD79b, which is a major target of mutation in ABC DLBCL. In addition, we establish that BCR internalization can be regulated at a minimum of two different levels: regulation of YxxØ AP2 binding in cis by downstream ITAM-embedded DCSM and QTAT regulatory elements and regulation in trans by the partner cytoplasmic domain of the CD79 heterodimer. Beyond establishing the basic rules governing BCR internalization, these results illustrate an underappreciated role for ITAM residues in controlling clathrin-dependent endocytosis and highlight the complex mechanisms that control the activity of AP2 binding motifs in this receptor system.

Regulation of endocytic clathrin dynamics by cargo ubiquitination

VIDEO ABSTRACT

Some endocytic cargoes control clathrin-coated pit (CCP) maturation, but it is not known how such regulation is communicated. We found that μ-opioid neuropeptide receptors signal to their enclosing CCPs by ubiquitination. Nonubiquitinated receptors delay CCPs at an intermediate stage of maturation, after clathrin lattice assembly is complete but before membrane scission. Receptor ubiquitination relieves this inhibition, effectively triggering CCP scission and producing a receptor-containing endocytic vesicle. The ubiquitin modification that conveys this endocytosis-promoting signal is added to the receptor's first cytoplasmic loop, catalyzed by the Smurf2 ubiquitin ligase, and coordinated with activation-dependent receptor phosphorylation and clustering through Smurf2 recruitment by the endocytic adaptor beta-arrestin. Epsin1 detects the signal at the CCP and is required for ubiquitin-promoted scission. This cargo-to-coat communication system mediates a biochemical checkpoint that ensures appropriate receptor ubiquitination for later trafficking, and it controls specific receptor loading into CCPs by sensing when a sufficient quorum is reached.

A high precision survey of the molecular dynamics of mammalian clathrin-mediated endocytosis

The molecular dynamics of clathrin-mediated endocytosis in living cells has been mapped with an approximately ten-fold improvement in temporal accuracy, yielding new insights into the molecular mechanism.

Dual colour total internal reflection fluorescence microscopy is a powerful tool for decoding the molecular dynamics of clathrin-mediated endocytosis (CME). Typically, the recruitment of a fluorescent protein–tagged endocytic protein was referenced to the disappearance of spot-like clathrin-coated structure (CCS), but the precision of spot-like CCS disappearance as a marker for canonical CME remained unknown. Here we have used an imaging assay based on total internal reflection fluorescence microscopy to detect scission events with a resolution of ∼2 s. We found that scission events engulfed comparable amounts of transferrin receptor cargo at CCSs of different sizes and CCS did not always disappear following scission. We measured the recruitment dynamics of 34 types of endocytic protein to scission events: Abp1, ACK1, amphiphysin1, APPL1, Arp3, BIN1, CALM, CIP4, clathrin light chain (Clc), cofilin, coronin1B, cortactin, dynamin1/2, endophilin2, Eps15, Eps8, epsin2, FBP17, FCHo1/2, GAK, Hip1R, lifeAct, mu2 subunit of the AP2 complex, myosin1E, myosin6, NECAP, N-WASP, OCRL1, Rab5, SNX9, synaptojanin2β1, and syndapin2. For each protein we aligned ∼1,000 recruitment profiles to their respective scission events and constructed characteristic “recruitment signatures” that were grouped, as for yeast, to reveal the modular organization of mammalian CME. A detailed analysis revealed the unanticipated recruitment dynamics of SNX9, FBP17, and CIP4 and showed that the same set of proteins was recruited, in the same order, to scission events at CCSs of different sizes and lifetimes. Collectively these data reveal the fine-grained temporal structure of CME and suggest a simplified canonical model of mammalian CME in which the same core mechanism of CME, involving actin, operates at CCSs of diverse sizes and lifetimes.

The molecular machinery of clathrin-mediated endocytosis concentrates receptors at the cell surface in a patch of membrane that curves into a vesicle, pinches off, and internalizes membrane cargo and a tiny volume of extracellular fluid. We know that dozens of proteins are involved in this process, but precisely when and where they act remains poorly understood. Here we used a fluorescence imaging assay to detect the moment of scission in living cells and used this as a reference point from which to measure the characteristic recruitment signatures of 34 fluorescently tagged endocytic proteins. Pair-wise comparison of these recruitment signatures allowed us to identify seven modules of proteins that were recruited with similar kinetics. For the most part the recruitment signatures were consistent with what was previously known about the proteins' structure and their binding affinities; however, the recruitment signatures for some components (such as some BAR and F-BAR domain proteins) could not have been predicted from existing structural or biochemical data. This study provides a paradigm for mapping molecular dynamics in living cells and provides new insights into the mechanism of clathrin-mediated endocytosis.

An expectation maximization based method for subcellular particle tracking using multi-angle TIRF microscopy

Multi-angle total internal reflection fluorescence microscopy (MA-TIRFM) is a new generation of TIRF microscopy to study cellular processes near dorsal cell membrane in 4 dimensions (3D+t). To perform quantitative analysis using MA-TIRFM, it is necessary to track subcellular particles in these processes. In this paper, we propose a method based on a MAP framework for automatic particle tracking and apply it to track clathrin coated pits (CCPs). The expectation maximization (EM) algorithm is employed to solve the MAP problem. To provide the initial estimations for the EM algorithm, we develop a forward filter based on the most probable trajectory (MPT) filter. Multiple linear models are used to model particle dynamics. For CCP tracking, we use two linear models to describe constrained Brownian motion and fluorophore variation according to CCP properties. The tracking method is evaluated on synthetic data and results show that it has high accuracy. The result on real data confirmed by human expert cell biologists is also presented.

Surface aggregation patterns of LDL receptors near coated pits II. The retrograde membrane flow-diffusion and generalized plaque-form insertion mechanism

This study presents a theoretical exploration of the effects of mechanisms that, in addition to diffusion, may influence the surface dynamics and display of unbound receptors in the low-density lipoprotein (LDL) endocytic cycle in human fibroblasts. The factors considered here are a transverse membrane flow and a generalized plaque-form insertion mode. The proposed model permits estimations of aggregation rates of unbound receptors in coated pits as well as pictorial representations of their expected steady-state display on the cell surface. Our findings show that this display is determined in a fundamental way by the ratio of the strength of the flow to the diffusion coefficient. For measured values of the diffusion coefficient and the estimated value of the flow rate strength (and independent of the receptor insertion mode), the display predicted by our model is consistent with the capping phenomenon, i.e., a gradated clustering in the direction of flow streamlines. There could be suitable characterizations of the receptor reinsertion mode that would produce a substantial reduction in the mean capture time of LDL receptors by coated pits. In any event, our results show that the existence of a transverse membrane flow precludes the display of steady-state plaque-form surface clusters.

Clostridial glucosylating toxins enter cells via clathrin-mediated endocytosis

Clostridium difficile toxin A (TcdA) and toxin B (TcdB), C. sordellii lethal toxin (TcsL) and C. novyi α-toxin (TcnA) are important pathogenicity factors, which represent the family of the clostridial glucosylating toxins (CGTs). Toxin A and B are associated with antibiotic-associated diarrhea and pseudomembraneous colitis. Lethal toxin is involved in toxic shock syndrome after abortion and α-toxin in gas gangrene development. CGTs enter cells via receptor-mediated endocytosis and require an acidified endosome for translocation of the catalytic domain into the cytosol. Here we studied the endocytic processes that mediate cell internalization of the CGTs. Intoxication of cells was monitored by analyzing cell morphology, status of Rac glucosylation in cell lysates and transepithelial resistance of cell monolayers. We found that the intoxication of cultured cells by CGTs was strongly delayed when cells were preincubated with dynasore, a cell-permeable inhibitor of dynamin, or chlorpromazine, an inhibitor of the clathrin-dependent endocytic pathway. Additional evidence about the role of clathrin in the uptake of the prototypical CGT family member toxin B was achieved by expression of a dominant-negative inhibitor of the clathrin-mediated endocytosis (Eps15 DN) or by siRNA against the clathrin heavy chain. Accordingly, cells that expressed dominant-negative caveolin-1 were not protected from toxin B-induced cell rounding. In addition, lipid rafts impairment by exogenous depletion of sphingomyelin did not decelerate intoxication of HeLa cells by CGTs. Taken together, our data indicate that the endocytic uptake of the CGTs involves a dynamin-dependent process that is mainly governed by clathrin.

Mechanisms of antibody-mediated insulin-like growth factor I receptor (IGF-IR) down-regulation in MCF-7 breast cancer cells

The insulin-like growth factor I receptor (IGF-IR) plays a critical role in cell proliferation and survival. We previously reported that a recombinant anti-IGF-IR antibody, scFv-Fc, consisting of 1H7 monoclonal antibody (mAb)-derived single chain antibody (scFv) and human IgG1 Fc, significantly suppressed breast tumor growth, and we proposed IGFIR down-regulation as a mechanism for tumor growth inhibition (Horm Metab Res. 2003; 35:836, Cancer Res. 2003; 63:627). This study used MCF-7 breast cancer cells to investigate the effects of anti-IGF-IR mAbs with various epitope specificities on IGF-IR downregulation and signaling pathways. Despite their differing effects on IGF-IR signaling, all five mAbs used down-regulated IGF-IR. Inhibitor experiments indicated that anti-IGF-IR mAbs induced internalization of IGF-IR from clathrin coated-pits. Pretreatment of MCF-7 cells with methylamine substantially reduced the antibody-mediated IGF-IR down-regulation while MG115 did not. Ubiquitination of IGF-IR did not occur in MCF-7 cells after mAb treatment. These results suggest that anti-IGF-IR antibodies with different epitope-specificities can cause internalization of IGF-IR from clathrin-coated pits and down-regulation via a lysosome-dependent pathway in an IGF-IR activation-independent manner.

HIP1 exhibits an early recruitment and a late stage function in the maturation of coated pits

Huntingtin interacting protein 1 (HIP1) is an accessory protein of the clathrin-mediated endocytosis (CME) pathway, yet its precise role and the step at which it becomes involved are unclear. We employed live-cell imaging techniques to focus on the early steps of CME and characterize HIP1 dynamics. We show that HIP1 is highly colocalized with clathrin at the plasma membrane and shares similar dynamics with a subpopulation of clathrin assemblies. Employing transferrin receptor fused to pHluorin, we distinguished between open pits to which HIP1 localizes and newly internalized vesicles that are devoid of HIP1. Moreover, shRNA knockdown of clathrin compromised HIP1 membranal localization, unlike the reported behavior of Sla2p. HIP1 fragment, lacking its ANTH and Talin-like domains, inhibits internalization of transferrin, but retains colocalization with membranal clathrin assemblies. These data demonstrate HIP1's role in pits maturation and formation of the coated vesicle, and its strong dependence on clathrin for membranal localization.

The preprophase band is a localized center of clathrin-mediated endocytosis in late prophase cells of the onion cotyledon epidermis

The preprophase band (PPB) marks the site on the plant cell cortex where the cell plate will fuse during the final stage of cytokinesis. Recent studies have shown that several cytoskeletal proteins are depleted at the PPB site, but the processes that bring about these changes are still unknown. We have investigated the membrane systems associated with the PPB regions of epidermal cells of onion cotyledons by means of serial thin sections and electron tomograms. In contrast with specimens preserved by chemical fixatives, our high-pressure frozen cells demonstrated the presence of large numbers of clathrin-coated pits and vesicles in the PPB regions. The vesicles were of two types: clathrin-coated and structurally related, non-coated vesicles. Quantitative analysis of the data revealed that the number of clathrin-coated pits and vesicles is higher in the PPB regions than outside of these regions. Immunofluorescent microscopy using anti-plant clathrin-antibody confirmed this result. In contrast, no differences in secretory activities were observed. We postulate that the removal of membrane proteins by endocytosis plays a role in the formation of PPB 'memory' structures.

Dissecting the cell entry pathway of dengue virus by single-particle tracking in living cells

Dengue virus (DENV) is an enveloped RNA virus that causes the most common arthropod-borne infection worldwide. The mechanism by which DENV infects the host cell remains unclear. In this work, we used live-cell imaging and single-virus tracking to investigate the cell entry, endocytic trafficking, and fusion behavior of DENV. Simultaneous tracking of DENV particles and various endocytic markers revealed that DENV enters cells exclusively via clathrin-mediated endocytosis. The virus particles move along the cell surface in a diffusive manner before being captured by a pre-existing clathrin-coated pit. Upon clathrin-mediated entry, DENV particles are transported to Rab5-positive endosomes, which subsequently mature into late endosomes through acquisition of Rab7 and loss of Rab5. Fusion of the viral membrane with the endosomal membrane was primarily detected in late endosomal compartments.

AAK1 regulates Numb function at an early step in clathrin-mediated endocytosis

Numb is an endocytic protein that is proposed to influence clathrin-coated pit assembly, although its mode of action and the mechanisms that regulate its activity are unknown. In this study, we show that Numb binds to and is phosphorylated by adaptor-associated kinase 1 (AAK1), a key endocytic kinase. We find that AAK1 redistributes Numb to perinuclear endosomes when overexpressed, while kinase depletion causes Numb to accumulate at the plasma membrane. Overexpression of a Numb point mutant (T102A) that lacks the AAK1 phosphorylation site potently disrupts transferrin and low-density lipoprotein internalization but does not impact EGF uptake. Consistent with Numb redistribution results, we find that T102A Numb no longer localizes to perinuclear endosomes. Instead, it is enriched at the plasma membrane where it shows elevated levels of colocalization with coated pit markers. Collectively, these observations demonstrate that Numb endocytic activity is regulated by AAK1 and that phosphorylation may be a critical step in promoting coated pit maturation.

Regulation of endocytic recycling by C. elegans Rab35 and its regulator RME-4, a coated-pit protein

Using Caenorhabditis elegans genetic screens, we identified receptor-mediated endocytosis (RME)-4 and RME-5/RAB-35 as important regulators of yolk endocytosis in vivo. In rme-4 and rab-35 mutants, yolk receptors do not accumulate on the plasma membrane as would be expected in an internalization mutant, rather the receptors are lost from cortical endosomes and accumulate in dispersed small vesicles, suggesting a defect in receptor recycling. Consistent with this, genetic tests indicate the RME-4 and RAB-35 function downstream of clathrin, upstream of RAB-7, and act synergistically with recycling regulators RAB-11 and RME-1. We find that RME-4 is a conserved DENN domain protein that binds to RAB-35 in its GDP-loaded conformation. GFP-RME-4 also physically interacts with AP-2, is enriched on clathrin-coated pits, and requires clathrin but not RAB-5 for cortical association. GFP-RAB-35 localizes to the plasma membrane and early endocytic compartments but is lost from endosomes in rme-4 mutants. We propose that RME-4 functions on coated pits and/or vesicles to recruit RAB-35, which in turn functions in the endosome to promote receptor recycling.

Internalization of hydroxyapatite nanoparticles in liver cancer cells

Hydroxyapatite (HAP) is the main inorganic component of hard tissues and shows excellent biocompatibility and osteoconductivity properties. Nanoparticles of HAP can be synthesised by the precipitation method in distilled water. The needle shaped particles are below 100 nm in size with low-crystallinity and high-surfacial activation. Recent studies showed toxic effects of HAP nanoparticles on cancer cells. Other studies focus on the application of HAP nanoparticles as drug and gene delivery system or cell marker. However, to date, the exact internalization pathway of HAP nanoparticles into cells has not been determined. When HAP nanoparticles were added to cell culture medium, the particles immediately became instable and formed agglomerates with a size of about 500-700 nm. Hence, cells seldom encounter single HAP nanoparticles in the environment of cell culture or body fluid. The TEM showed internalized HAP captured by vacuoles in the cytoplasm of the hepatocellular carcinoma cells. The invaginations in the cell membrane before nanoparticle uptake suggested endocytic pathways as internalization mechanism. This study revealed that agglomerated HAP nanoparticles were internalized by cells through the energy-dependent process of clathrin-mediated endocytosis. Depletion of intracellular potassium arrested the formation of coated pit, which inhibited the uptake of HAP.

The alpha1a-adrenergic receptor occupies membrane rafts with its G protein effectors but internalizes via clathrin-coated pits

The alpha(1a)-adrenergic receptor (alpha(1a)AR) occupies intracellular and plasma membranes in both native and heterologous expression systems. Based on multiple independent lines of evidence, we demonstrate the alpha(1a)AR at the cell surface occupies membrane rafts but exits from rafts following stimulation. In non-detergent raft preparations, basal alpha(1a)AR is present in low density membrane rafts and colocalizes with its G protein effectors on density gradients. Raft disruption by cholesterol depletion with methyl-beta-cyclodextrin eliminates these light rafts. To confirm the presence of the alpha(1a)AR in plasma membrane rafts, fluorescence resonance energy transfer measurements were used to demonstrate colocalization of surface receptor and the raft marker, cholera toxin B. This colocalization was largely lost following alpha(1a)AR stimulation with phenylephrine. Similarly, receptor stimulation causes exit of the alpha(1a)AR from light rafts within 3-10 min in contrast to the G proteins, which largely remain in light rafts. Importantly, this delayed exit of the alpha(1a)AR suggests acute receptor signaling and desensitization occur entirely within rafts. Interestingly, both confocal analysis and measurement of surface alpha(1a)AR levels indicate modest receptor internalization during the 10 min following stimulation, suggesting most of the receptor has entered non-raft plasma membrane. Nevertheless, activation does increase the rate of receptor internalization as does disruption of rafts with methyl-beta-cyclodextrin, suggesting raft exit enables internalization. Confocal analysis of surface-labeled hemagglutinin-alpha(1a)AR reveals that basal and stimulated receptor occupies clathrin pits in fixed cells consistent with previous indirect evidence. The evidence presented here strongly suggests the alpha(1a)AR is a lipid raft protein under basal conditions and implies agonist-mediated signaling occurs from rafts.

Visualizing clathrin-mediated IgE receptor internalization by electron and atomic force microscopy

A significant step in the immunoglobulin E (IgE) receptor signaling pathway in mast cell membranes is receptor internalization by clathrin-coated vesicles. Visualization in native membrane sheets of the emerging clathrin lattice structures containing the IgE receptor and associated signaling partners has been accomplished with high-resolution transmission electron microscopy (TEM). More recently, membrane sheets with labeled clathrin have also been characterized with atomic force microscopy (AFM) in combination with fluorescence imaging. We discuss here the procedure for creating fixed, native cell membrane sheets, labeling with immunogold or fluorescent labels, and utilization for TEM or AFM/fluorescence imaging of clathrin-mediated IgE internalization.

Endocytosis of neurotransmitter receptors: location matters

Endocytosis of excitatory glutamate receptors from the postsynaptic plasma membrane plays a fundamental role in synaptic function and plasticity. In a recent study published in Neuron, Lu et al. (2007) describe protein interactions that link zones of receptor endocytosis directly to the postsynaptic scaffold and propose that local trafficking of receptors facilitated by these endocytic zones is required to maintain synaptic responsiveness.

"Alternative" endocytic mechanisms exploited by pathogens: new avenues for therapeutic delivery?

Some pathogens utilize unique routes to enter cells that may evade the intracellular barriers encountered by the typical clathrin-mediated endocytic pathway. Retrograde transport and caveolar uptake are among the better characterized pathways, as alternatives to clathrin-mediated endocytosis, that are known to facilitate entry of pathogens and potential delivery agents. Recent characterization of the trafficking mechanisms of prion proteins and certain bacteria may present new paradigms for strategizing improvements in therapeutic spread and retention of therapy. This review will provide an overview of such endocytic pathways, and discuss current and future possibilities in using these routes as a means to improve therapeutic delivery.

Curved EFC/F-BAR-domain dimers are joined end to end into a filament for membrane invagination in endocytosis

Pombe Cdc15 homology (PCH) proteins play an important role in a variety of actin-based processes, including clathrin-mediated endocytosis (CME). The defining feature of the PCH proteins is an evolutionarily conserved EFC/F-BAR domain for membrane association and tubulation. In the present study, we solved the crystal structures of the EFC domains of human FBP17 and CIP4. The structures revealed a gently curved helical-bundle dimer of approximately 220 A in length, which forms filaments through end-to-end interactions in the crystals. The curved EFC dimer fits a tubular membrane with an approximately 600 A diameter. We subsequently proposed a model in which the curved EFC filament drives tubulation. In fact, striation of tubular membranes was observed by phase-contrast cryo-transmission electron microscopy, and mutations that impaired filament formation also impaired membrane tubulation and cell membrane invagination. Furthermore, FBP17 is recruited to clathrin-coated pits in the late stage of CME, indicating its physiological role.

"Wunder" F-BAR domains: going from pits to vesicles

Clathrin-mediated endocytosis is a key mechanism by which cells take up extracellular cargo. In this issue, Shimada et al. (2007) reveal the mode of action of the F-BAR domain, which deepens the initial membrane pit that forms during clathrin-mediated endocytosis.

Membranous structures transfer cell surface proteins across NK cell immune synapses

Intercellular transfer of cell surface proteins is widespread and facilitates several recently discovered means for immune cell communication. Here, we examined the molecular mechanism for intercellular exchange of the natural killer (NK) cell receptor KIR2DL1 and HLA-C, prototypical proteins that swap between NK cells and target cells. Transfer was contact dependent and enhanced for cells expressing cognate receptor/ligand pairs but did not depend on KIR2DL1 signaling. To a lesser extent, proteins transferred independent from specific recognition. Intracellular domains of transferred proteins were not exposed to the extracellular environment and transferred proteins were removed by brief exposure to low pH. By fluorescence microscopy, transferred proteins localized to discrete regions on the recipient cell surface. Higher resolution scanning electron micrographs revealed that transferred proteins were located within specific membranous structures. Transmission electron microscopy of the immune synapse revealed that membrane protrusions from one cell interacted with the apposing cell surface within the synaptic cleft. These data, coupled with previous observations, lead us to propose that intercellular protein transfer is mediated by membrane protrusions within and surrounding the immunological synapse.

Loss of the Atp2c1 secretory pathway Ca(2+)-ATPase (SPCA1) in mice causes Golgi stress, apoptosis, and midgestational death in homozygous embryos and squamous cell tumors in adult heterozygotes

Loss of one copy of the human ATP2C1 gene, encoding SPCA1 (secretory pathway Ca(2+)-ATPase isoform 1), causes Hailey-Hailey disease, a skin disorder. We performed targeted mutagenesis of the Atp2c1 gene in mice to analyze the functions of this Golgi membrane Ca(2+) pump. Breeding of heterozygous mutants yielded a normal Mendelian ratio among embryos on gestation day 9.5; however, null mutant (Spca1(-/-)) embryos exhibited growth retardation and did not survive beyond gestation day 10.5. Spca1(-/-) embryos had an open rostral neural tube, but hematopoiesis and cardiovascular development were ostensibly normal. Golgi membranes of Spca1(-/-) embryos were dilated, had fewer stacked leaflets, and were expanded in amount, consistent with increased Golgi biogenesis. The number of Golgi-associated vesicles was also increased, and rough endoplasmic reticulum had fewer ribosomes. Coated pits, junctional complexes, desmosomes, and basement membranes appeared normal in mutant embryos, indicating that processing and trafficking of proteins in the secretory pathway was not massively impaired. However, apoptosis was increased, possibly the result of secretory pathway stress, and a large increase in cytoplasmic lipid was observed in mutant embryos, consistent with impaired handling of lipid by the Golgi. Adult heterozygous mice appeared normal and exhibited no evidence of Hailey-Hailey disease; however, aged heterozygotes had an increased incidence of squamous cell tumors of keratinized epithelial cells of the skin and esophagus. These data show that loss of the Golgi Ca(2+) pump causes Golgi stress, expansion of the Golgi, increased apoptosis, and embryonic lethality and demonstrates that SPCA1 haploinsufficiency causes a genetic predisposition to cancer.

Differentiation restricted endocytosis of cell penetrating peptides in MDCK cells corresponds with activities of Rho-GTPases

PURPOSE

Cellular entry of biomacromolecules is restricted by the barrier function of cell membranes. Tethering such molecules to cell penetrating peptides (CPPs) that can translocate cell membranes has opened new horizons in biomedical research. Here, we investigate the cellular internalization of hCT(9-32)-br, a human calcitonin derived branched CPP, and SAP, a gamma-zein related sequence.

METHODS

Internalization of fluorescence labelled CPPs was performed with both proliferating and confluent MDCK cells by means of confocal laser scanning microscopy (CLSM) and fluorescence activated cell sorting (FACS) using appropriate controls. Internalization was further elaborated in an inflammatory, IFN-gamma/TNF-alphaa induced confluent MDCK model mimicking inflammatory epithelial pathologies. Activities of active form Rho-GTPases (Rho-A and Rac-1) in proliferating and confluent MDCK cells were monitored by pull-down assay and Western blot analysis.

RESULTS

We observed marked endocytic uptake of the peptides into proliferating MDCK by a process suggesting both lipid rafts and clathrin-coated pits. In confluent MDCK, however, we noted a massive but compound-unspecific slow-down of endocytosis. This corresponded with a down-regulation of endocytosis by Rho-GTPases, previously identified to be intimately involved in endocytic traffic. In fact, we found endocytic internalization to relate with active Rho-A; vice versa, MDCK cell density, degree of cellular differentiation and endocytic slow-down were found to relate with active Rac-1. To our knowledge, this is the first study to cast light on the previously observed differentiation restricted internalization of CPPs into epithelial cell models. In the inflammatory IFN-gamma/TNF-alphaa induced confluent MDCK model mimicking inflammatory epithelial pathologies, CPP internalization was enhanced in a cytokine concentration-dependent way resulting in maximum enhancement rates of up to 90%. We suggest a cytokine induced redistribution of lipid rafts in confluent MDCK to cause this enhancement.

CONCLUSION

Our findings emphasize the significance of differentiated cell models in the study of CPP internalization and point towards inflammatory epithelial pathologies as potential niche for the application of CPPs for cellular delivery.

Role of the AP2 beta-appendage hub in recruiting partners for clathrin-coated vesicle assembly

Adaptor protein complex 2 α and β-appendage domains act as hubs for the assembly of accessory protein networks involved in clathrin-coated vesicle formation. We identify a large repertoire of β-appendage interactors by mass spectrometry. These interact with two distinct ligand interaction sites on the β-appendage (the “top” and “side” sites) that bind motifs distinct from those previously identified on the α-appendage. We solved the structure of the β-appendage with a peptide from the accessory protein Eps15 bound to the side site and with a peptide from the accessory cargo adaptor β-arrestin bound to the top site. We show that accessory proteins can bind simultaneously to multiple appendages, allowing these to cooperate in enhancing ligand avidities that appear to be irreversible in vitro. We now propose that clathrin, which interacts with the β-appendage, achieves ligand displacement in vivo by self-polymerisation as the coated pit matures. This changes the interaction environment from liquid-phase, affinity-driven interactions, to interactions driven by solid-phase stability (“matricity”). Accessory proteins that interact solely with the appendages are thereby displaced to areas of the coated pit where clathrin has not yet polymerised. However, proteins such as β-arrestin (non-visual arrestin) and autosomal recessive hypercholesterolemia protein, which have direct clathrin interactions, will remain in the coated pits with their interacting receptors.

Formation of clathrin-coated vesicles, important in endocytosis, relies on accessory proteins assembled by adaptor protein complex 2 (AP2). Here, mass spectrometry and crystallization identifies proteins recruited by AP2's β-appendage for this purpose.

Epsin: inducing membrane curvature

Epsin was originally discovered by virtue of its binding to another accessory protein, Eps15. Members of the epsin family play an important role as accessory proteins in clathrin-mediated endocytosis. Epsin isoforms have been described that differ in intracellular site of action and/or in tissue distribution, although all epsins essentially contribute to membrane deformation. Besides inducing membrane curvature, epsin also plays a key function as adaptor protein, coupling various components of the clathrin-assisted uptake and fulfils an important role in selecting and recognizing cargo. Furthermore, epsin possesses the ability to block vesicle formation during mitosis. To perform all these functions, epsin, apart from interacting with PtdIns(4,5)P2 via its ENTH domain, also engages in several protein interactions with different components of the clathrin-mediated endocytic system. Recently, RNA interference has successfully been exploited to generate a cell line constitutively silencing epsin expression, which can be used to study internalization of multiple ligands.

New Insights into Membrane Trafficking and Protein Sorting

Protein transport in the secretory and endocytic pathways is a multistep process involving the generation of transport carriers loaded with defined sets of cargo, the shipment of the cargo-loaded transport carriers between compartments, and the specific fusion of these transport carriers with a target membrane. The regulation of these membrane-mediated processes involves a complex array of protein and lipid interactions. As the machinery and regulatory processes of membrane trafficking have been defined, it is increasingly apparent that membrane transport is intimately connected with a number of other cellular processes, such as quality control in the endoplasmic reticulum (ER), cytoskeletal dynamics, receptor signaling, and mitosis. The fidelity of membrane trafficking relies on the correct assembly of components on organelles. Recruitment of peripheral proteins plays a critical role in defining organelle identity and the establishment of membrane subdomains, essential for the regulation of vesicle transport. The molecular mechanisms for the biogenesis of membrane subdomains are also central to understanding how cargo is sorted and segregated and how different populations of transport carriers are generated. In this review we will focus on the emerging themes of organelle identity, membrane subdomains, regulation of Golgi trafficking, and advances in dissecting pathways in physiological systems.

Mast cell-specific gangliosides and FcepsilonRI follow the same endocytic pathway from lipid rafts in RBL-2H3 cells

Recent studies have shown that, in mast cells, membrane microdomains rich in cholesterol and glycosphingolipids called lipid rafts play an important role in FcepsilonRI signaling. The present study demonstrates that, in RBL-2H3 cells following stimulation, the mast cell-specific gangliosides associated with FcepsilonRI are internalized from lipid rafts along with the receptor. When the cells are labeled with iodinated antibodies against the gangliosides or against FcepsilonRI and the cell components are then fractionated on Percoll density gradients, in stimulated cells the gangliosides are internalized with the same kinetics as FcepsilonRI and at 3 hr are present in the dense lysosome fraction. Using transmission electron microscopy, with antibody against the gangliosides conjugated to horseradish peroxidase and antibody against FcepsilonRI conjugated to colloidal gold, it was possible to demonstrate that the gangliosides and FcepsilonRI are internalized in the same coated vesicles. At 5 min, the gangliosides and FcepsilonRI can be identified in early endosomes and at 3 hr are found together in acid phosphatase-positive lysosomes. This study demonstrates that the mast cell-specific gangliosides are internalized from lipid rafts in the same vesicles and traffic intracellularly with the same kinetics as FcepsilonRI. This study contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Syntaxin 1A promotes the endocytic sorting of EAAC1 leading to inhibition of glutamate transport

The neuronal glutamate transporter, excitatory amino-acid carrier 1 (EAAC1), plays an important role in the modulation of neurotransmission and contributes to synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and to epileptogenesis. However, the mechanisms that regulate EAAC1 endocytic sorting and function remain largely unknown. Here, we first demonstrate that EAAC1 undergoes internalization through the clathrin-mediated pathway and further show that syntaxin 1A, a key molecule in synaptic exocytosis, potentiates EAAC1 internalization, thus leading to the functional inhibition of EAAC1. In the presence of the transmembrane domain of syntaxin 1A, its H3 coiled-coil domain of syntaxin 1A is necessary and sufficient for the inhibition of EAAC1. Furthermore, specific suppression of endogenous syntaxin 1A significantly blocked EAAC1 endocytic sorting and lysosomal degradation promoted by kainic acid, a drug for kindling the animal model of human temporal lobe epilepsy in rat, indicating a potential role of syntaxin 1A in epileptogenesis. These findings provide new evidence that syntaxin 1A serves as an intrinsic enhancer to EAAC1 endocytic sorting and further suggest that syntaxin 1A is conversant with both ;ins' and ;outs' of synaptic neurotransmission.

Clathrin-independent internalization of the human histamine H1-receptor in CHO-K1 cells

The aim of the present study was to investigate the cellular pathway involved in histamine-stimulated internalization of the human H1-receptor in CHO-K1 cells expressing N-terminal myc-tagged H1-receptor (Myc-H1) or N-terminal myc-C-terminal green fluorescent protein (Myc-GFP H1) versions of the receptor. Studies of 3H-mepyramine binding and histamine-stimulated 3H-inositol phosphate accumulation in these cells showed that the Myc-H1 and Myc-GFP H1-receptors had identical pharmacology to the wild-type H1-receptor. The Myc-H1-receptor was rapidly internalized in CHO-K1 cells following stimulation with histamine (0.1 mM). This response occurred within 15 min, and could be prevented by the quaternary H1-receptor antagonist alpha-pirdonium. Similar data were obtained with the Myc-GFP H1-receptors. Internalization of the Myc-GFP H1-receptor was maintained in the absence of extracellular calcium and was not inhibited by the CAM kinase II inhibitor KN-62 (10 microM). Phorbol dibutyrate, an activator of protein kinase C, was also able to stimulate internalization of the H1-receptor. However, inhibition or downregulation of protein kinase C (which significantly modified histamine-stimulated inositol phosphate responses) was without effect on the internalization of the H1-receptor stimulated by histamine. Hypertonic sucrose did not prevent histamine-induced internalization of the Myc-GFP H1-receptor, but was able to attenuate internalization of transferrin via clathrin-mediated endocytosis in the same cells. In contrast, preincubation of cells with filipin or nystatin, which disrupts caveolae and lipid rafts, completely inhibited the histamine-induced internalization of the Myc-GFP H1-receptor, but was without effect on the sequestration of transferrin. The H1-receptor and cholera toxin subunit B were colocalized under resting conditions at the cell surface. Immunohistochemical studies with an antibody to caveolin-1 confirmed that this protein was also localized predominantly to the plasma membrane. However, following stimulation of CHO-Myc-GFP H1 cells with histamine, there was no evidence for internalization of caveolin-1 in parallel with the H1-receptor. These data provide strong evidence that the H1-receptor is internalized via a clathrin-independent mechanism and most likely involves lipid rafts.

Dynamics of clathrin and adaptor proteins during endocytosis

The endocytic adaptor complex AP-2 colocalizes with the majority of clathrin-positive spots at the cell surface. However, we previously observed that AP-2 is excluded from internalizing clathrin-coated vesicles (CCVs). The present studies quantitatively demonstrate that AP-2 disengages from sites of endocytosis seconds before internalization of the nascent CCV. In contrast, epsin, an alternate adaptor for clathrin at the plasma membrane, disappeared, along with clathrin. This suggests that epsin remains an integral part of the CCV throughout endocytosis. Clathrin spots at the cell surface represent a heterogeneous population: a majority (70%) of the spots disappeared with a time course of 4 min, whereas a minority (22%) remained static for > or =30 min. The static clathrin spots undergo constant subunit exchange, suggesting that although they are static structures, these spots comprise functional clathrin molecules, rather than dead-end aggregates. These results support a model where AP-2 serves a cargo-sorting function before endocytosis, whereas alternate adaptors, such as epsin, actually link cargo to the clathrin coat surrounding nascent endocytic vesicles. These data also support a role for static clathrin, providing a nucleation site for endocytosis.

Dynamic Interaction of HIV-1 Nef with the Clathrin-Mediated Endocytic Pathway at the Plasma Membrane

The HIV-1 Nef protein perturbs the trafficking of membrane proteins such as CD4 by interacting with clathrin-adaptor complexes. We previously reported that Nef alters early/recycling endosomes, but its role at the plasma membrane is poorly documented. Here, we used total internal reflection fluorescence microscopy, which restricts the analysis to a approximately 100 nm region of the adherent surface of the cells, to focus on the dynamic of Nef at the plasma membrane relative to that of clathrin. Nef colocalized both with clathrin spots (CS) that remained static at the cell surface, corresponding to clathrin-coated pits (CCPs), and with approximately 50% of CS that disappeared from the cell surface, corresponding to forming clathrin-coated vesicles (CCVs). The colocalization of Nef with clathrin required the di-leucine motif essential for Nef binding to AP complexes and was independent of CD4 expression. Furthermore, analysis of Nef mutants showed that the capacity of Nef to induce internalization and downregulation of CD4 in T lymphocytes correlated with its localization into CCPs. In conclusion, this analysis shows that Nef is recruited into CCPs and into forming CCVs at the plasma membrane, in agreement with a model in which Nef uses the clathrin-mediated endocytic pathway to induce internalization of some membrane proteins from the surface of HIV-1-infected T cells.