Imaging endocytic clathrin structures in living cells

Identification of a novel function of the clathrin-coated structure at the plasma membrane in facilitating GM-CSF receptor-mediated activation of JAK2

It is well known that ligand binding to the high-affinity GM-CSF receptor (GMR) activates JAK2. However, how and where this event occurs in a cellular environment remains unclear. Here, we demonstrate that clathrin- but not lipid raft-mediated endocytosis is crucial for GMR signaling. Knockdown expression of clathrin heavy chain or intersectin 2 (ITSN2) attenuated GMR-mediated activation of JAK2, whereas inhibiting clathrin-coated pits or plagues to bud off the membrane by the dominant-negative mutant of dynamin enhanced such event. Moreover, unlike the wild-type receptor, an ITSN2-non-binding mutant of GMR defective in targeting to clathrin-coated pits or plagues [collectively referred to as clathrin-coated structures (CCSs) here] failed to activate JAK2 at such locations. Additional experiments demonstrate that ligand treatment not only enhanced JAK2/GMR association at CCSs, but also induced a conformational change of JAK2 which is required for JAK2 to be activated by CCS-localized CK2. Interestingly, ligand-independent activation of the oncogenic mutant of JAK2 (JAK2V617F) also requires the targeting of this mutant to CCSs. But JAK2V617F seems to be constitutively in an open conformation for CK2 activation. Together, this study reveals a novel functional role of CCSs in GMR signaling and the oncogenesis of JAK2V617F.

Dynamic organizing principles of the plasma membrane that regulate signal transduction: commemorating the fortieth anniversary of Singer and Nicolson's fluid-mosaic model

The recent rapid accumulation of knowledge on the dynamics and structure of the plasma membrane has prompted major modifications of the textbook fluid-mosaic model. However, because the new data have been obtained in a variety of research contexts using various biological paradigms, the impact of the critical conceptual modifications on biomedical research and development has been limited. In this review, we try to synthesize our current biological, chemical, and physical knowledge about the plasma membrane to provide new fundamental organizing principles of this structure that underlie every molecular mechanism that realizes its functions. Special attention is paid to signal transduction function and the dynamic aspect of the organizing principles. We propose that the cooperative action of the hierarchical three-tiered mesoscale (2-300 nm) domains--actin-membrane-skeleton induced compartments (40-300 nm), raft domains (2-20 nm), and dynamic protein complex domains (3-10 nm)--is critical for membrane function and distinguishes the plasma membrane from a classical Singer-Nicolson-type model.

Kv2.1 cell surface clusters are insertion platforms for ion channel delivery to the plasma membrane

Voltage-gated K+ (Kv) channels regulate membrane potential in many cell types. Although the channel surface density and location must be well controlled, little is known about Kv channel delivery and retrieval on the cell surface. The Kv2.1 channel localizes to micron-sized clusters in neurons and transfected human embryonic kidney (HEK) cells, where it is nonconducting. Because Kv2.1 is postulated to be involved in soluble N-ethylmaleimide–sensitive factor attachment protein receptor–mediated membrane fusion, we examined the hypothesis that these surface clusters are specialized platforms involved in membrane protein trafficking. Total internal reflection–based fluorescence recovery after photobleaching studies and quantum dot imaging of single Kv2.1 channels revealed that Kv2.1-containing vesicles deliver cargo at the Kv2.1 surface clusters in both transfected HEK cells and hippocampal neurons. More than 85% of cytoplasmic and recycling Kv2.1 channels was delivered to the cell surface at the cluster perimeter in both cell types. At least 85% of recycling Kv1.4, which, unlike Kv2.1, has a homogeneous surface distribution, is also delivered here. Actin depolymerization resulted in Kv2.1 exocytosis at cluster-free surface membrane. These results indicate that one nonconducting function of Kv2.1 is to form microdomains involved in membrane protein trafficking. This study is the first to identify stable cell surface platforms involved in ion channel trafficking.

Diversity of clathrin function: new tricks for an old protein

Clathrin is considered the prototype vesicle coat protein whose self-assembly mediates sorting of membrane cargo and recruitment of lipid modifiers. Detailed knowledge of clathrin biochemistry, structure, and interacting proteins has accumulated since the first observation, almost 50 years ago, of its role in receptor-mediated endocytosis of yolk protein. This review summarizes that knowledge, and focuses on properties of the clathrin heavy and light chain subunits and interaction of the latter with Hip proteins, to address the diversity of clathrin function beyond conventional receptor-mediated endocytosis. The distinct functions of the two human clathrin isoforms (CHC17 and CHC22) are discussed, highlighting CHC22's specialized involvement in traffic of the GLUT4 glucose transporter and consequent role in human glucose metabolism. Analysis of clathrin light chain function and interaction with the actin-binding Hip proteins during bacterial infection defines a novel actin-organizing function for CHC17 clathrin. By considering these diverse clathrin functions, along with intracellular sorting roles and influences on mitosis, further relevance of clathrin function to human health and disease is established.

Stochastic model of clathrin-coated pit assembly

In recent years, fluorescence microscopy has enabled researchers to observe the dynamics of clathrin-coated pit (CCP) assembly in real time. The assembly dynamics of CCPs shows striking heterogeneity. Some CCPs are long-lived (productive CCPs); they bind cargo and grow in size to form clathrin-coated vesicles. In contrast, other CCPs (abortive CCPs) are relatively short-lived and disassemble well before reaching vesicle size. Within both populations there is significant variance in CCP lifetime. We propose a stochastic biophysical model that links these observations with the energetics of CCPs and kinetics of their assembly. We show that without cargo, CCP assembly faces a high energy barrier that is difficult to overcome. As a consequence, CCPs without cargo are almost always abortive. We suggest a mechanism by which cargo binding stabilizes CCPs and facilitates their growth. The lifetime distribution of abortive pits calculated from our model agrees well with published experimental data. We also estimate the lifetimes of productive CCPs and show that the stochastic nature of CCP assembly plays a crucial role in causing their observed wide distribution.

Spatial regulation of receptor tyrosine kinases in development and cancer

During development and tissue homeostasis, patterns of cellular organization, proliferation and movement are highly choreographed. Receptor tyrosine kinases (RTKs) have a crucial role in establishing these patterns. Individual cells and tissues exhibit tight spatial control of the RTKs that they express, enabling tissue morphogenesis and function, while preventing unwarranted cell division and migration that can contribute to tumorigenesis. Indeed, RTKs are deregulated in most human cancers and are a major focus of targeted therapeutics. A growing appreciation of the essential role of spatial RTK regulation during development prompts the realization that spatial deregulation of RTKs is likely to contribute broadly to cancer development and may affect the sensitivity and resistance of cancer to pharmacological RTK inhibitors.

Harnessing the power of the endosome to regulate neural development

Endocytosis and endosomal trafficking play a multitude of roles in cellular function beyond regulating entry of essential nutrients. In this review, we discuss the cell biological principles of endosomal trafficking, the neuronal adaptations to endosomal organization, and the role of endosomal trafficking in neural development. In particular, we consider how cell fate decisions, polarity, migration, and axon outgrowth and guidance are influenced by five endosomal tricks: dynamic modulation of receptor levels by endocytosis and recycling, cargo-specific responses via cargo-specific endocytic regulators, cell-type-specific endocytic regulation, ligand-specific endocytic regulation, and endosomal regulation of ligand processing and trafficking.

A feedback loop between dynamin and actin recruitment during clathrin-mediated endocytosis

Clathrin-mediated endocytosis proceeds by a sequential series of reactions catalyzed by discrete sets of protein machinery. The final reaction in clathrin-mediated endocytosis is membrane scission, which is mediated by the large guanosine triophosphate hydrolase (GTPase) dynamin and which may involve the actin-dependent recruitment of N-terminal containing BIN/Amphiphysin/RVS domain containing (N-BAR) proteins. Optical microscopy has revealed a detailed picture of when and where particular protein types are recruited in the ∼20-30 s preceding scission. Nevertheless, the regulatory mechanisms and functions that underpin protein recruitment are not well understood. Here we used an optical assay to investigate the coordination and interdependencies between the recruitment of dynamin, the actin cytoskeleton, and N-BAR proteins to individual clathrin-mediated endocytic scission events. These measurements revealed that a feedback loop exists between dynamin and actin at sites of membrane scission. The kinetics of dynamin, actin, and N-BAR protein recruitment were modulated by dynamin GTPase activity. Conversely, acute ablation of actin dynamics using latrunculin-B led to a ∼50% decrease in the incidence of scission, an ∼50% decrease in the amplitude of dynamin recruitment, and abolished actin and N-BAR recruitment to scission events. Collectively these data suggest that dynamin, actin, and N-BAR proteins work cooperatively to efficiently catalyze membrane scission. Dynamin controls its own recruitment to scission events by modulating the kinetics of actin and N-BAR recruitment to sites of scission. Conversely actin serves as a dynamic scaffold that concentrates dynamin and N-BAR proteins at sites of scission.

Distinct and separable activities of the endocytic clathrin-coat components Fcho1/2 and AP-2 in developmental patterning

Clathrin-mediated endocytosis occurs at multiple independent import sites on the plasma membrane, but how these positions are selected and how different cargo is simultaneously recognized is obscure. FCHO1 and FCHO2 are early-arriving proteins at surface clathrin assemblies and are speculated to act as compulsory coat nucleators, preceding the core clathrin adaptor AP-2. Here, we show the μ-homology domain (μHD) of FCHO1/2 represents a novel endocytic interaction hub. Translational silencing of fcho1 in zebrafish embryos causes strong dorsoventral patterning defects analogous to Bmp signal failure. The Fcho1 μHD interacts with the Bmp receptor Alk8, uncovering a new endocytic component that positively modulates Bmp signal transmission. Still, the fcho1 morphant phenotype is distinct from severe embryonic defects apparent when AP-2 is depleted. Our data thus contradict the primacy of FCHO1/2 in coat initiation.

Limited transferrin receptor clustering allows rapid diffusion of canine parvovirus into clathrin endocytic structures

Viral pathogens usurp cell surface receptors to access clathrin endocytic structures, yet the mechanisms of virus incorporation into these structures remain incompletely understood. Here we used fluorescence microscopy to directly visualize the association of single canine parvovirus (CPV) capsids with cellular transferrin receptors (TfR) on the surfaces of live feline cells and to monitor how these CPV-TfR complexes access endocytic structures. We found that most capsids associated with fewer than five TfRs and that ∼25% of TfR-bound capsids laterally diffused into assembling clathrin-coated pits less than 30 s after attachment. Capsids that did not encounter a coated pit dissociated from the cell surface with a half-life of ∼30 s. Together, our results show how CPV exploits the natural mechanism of TfR endocytosis to engage the clathrin endocytic pathway and reveal that the low affinity of capsids for feline TfRs limits the residence time of capsids on the cell surface and thus the efficiency of virus internalization.

Membrane mechanisms for signal transduction: the coupling of the meso-scale raft domains to membrane-skeleton-induced compartments and dynamic protein complexes

Virtually all biological membranes on earth share the basic structure of a two-dimensional liquid. Such universality and peculiarity are comparable to those of the double helical structure of DNA, strongly suggesting the possibility that the fundamental mechanisms for the various functions of the plasma membrane could essentially be understood by a set of simple organizing principles, developed during the course of evolution. As an initial effort toward the development of such understanding, in this review, we present the concept of the cooperative action of the hierarchical three-tiered meso-scale (2-300 nm) domains in the plasma membrane: (1) actin membrane-skeleton-induced compartments (40-300 nm), (2) raft domains (2-20 nm), and (3) dynamic protein complex domains (3-10nm). Special attention is paid to the concept of meso-scale domains, where both thermal fluctuations and weak cooperativity play critical roles, and the coupling of the raft domains to the membrane-skeleton-induced compartments as well as dynamic protein complexes. The three-tiered meso-domain architecture of the plasma membrane provides an excellent perspective for understanding the membrane mechanisms of signal transduction.

Getting in touch with the clathrin terminal domain

The N-terminal domain (TD) of the clathrin heavy chain is folded into a seven-bladed β-propeller that projects inward from the polyhedral outer clathrin coat. As the most membrane-proximal portion of assembled clathrin, the TD is a major protein-protein interaction node. Contact with the TD β-propeller occurs through short peptide sequences typically located within intrinsically disordered segments of coat components that usually are elements of the membrane-apposed, inner 'adaptor' coat layer. A huge variation in TD-binding motifs is known and now four spatially discrete interaction surfaces upon the β-propeller have been delineated. An important operational feature of the TD interaction sites in vivo is functional redundancy. The recent discovery that 'pitstop' chemical inhibitors apparently occupy only one of the four TD interaction surfaces, but potently block clathrin-mediated endocytosis, warrants careful consideration of the underlying molecular basis for this inhibition.

Conservation and innovation in Tetrahymena membrane traffic: proteins, lipids, and compartments

The past decade has seen a significant expansion in our understanding of membrane traffic in Tetrahymena thermophila, facilitated by the development of new experimental tools and by the availability of the macronuclear genome sequence. Here we review studies on multiple pathways of uptake and secretion, as well as work on metabolism of membrane lipids. We discuss evidence for conservation versus innovation in the mechanisms used in ciliates compared with those in other eukaryotic lineages, and raise the possibility that existing gene expression databases can be exploited to analyze specific pathways of membrane traffic in these cells.

Cell biology: actin keeps endocytosis on a short leash

High-resolution structural analysis of branched actin networks at the sites of clathrin-mediated endocytosis sheds light on the role of actin in endocytosis and mechanisms controlling actin assembly.

Clathrin-mediated endocytosis: the gateway into plant cells

Endocytosis in plants has an essential role not only for basic cellular functions but also for growth and development, hormonal signaling and communication with the environment including nutrient delivery, toxin avoidance, and pathogen defense. The major endocytic mechanism in plants depends on the coat protein clathrin. It starts by clathrin-coated vesicle formation at the plasma membrane, where specific cargoes are recognized and packaged for internalization. Recently, genetic, biochemical and advanced microscopy studies provided initial insights into mechanisms and roles of clathrin-mediated endocytosis in plants. Here we summarize the present state of knowledge and compare mechanisms of clathrin-mediated endocytosis in plants with animal and yeast paradigms as well as review plant-specific regulations and roles of this process.

Mechanisms of membrane curvature sensing

Bacteria and eukaryotic cells contain geometry-sensing tools in their cytosol: protein motifs or domains that recognize the curvature, concave or convex, deep or shallow, of lipid membranes. These sensors contrast with classical lipid-binding domains by their extended structure and, sometimes, counterintuitive chemistry. Among the sensors are long amphipathic helices, such as the ALPS motif and the N-terminal region of α-synuclein, whose apparent "design defects" translate into a remarkable ability to specifically adsorb to the surface of small vesicles. Fundamental differences in the lipid composition of membranes of the early and late secretory pathways probably explain why some sensors use mostly electrostatics whereas others take advantage of the hydrophobic effect. Membrane curvature sensors help to organize very diverse reactions, such as lipid transfer between membranes, the tethering of vesicles at the Golgi apparatus, and the assembly-disassembly cycle of protein coats.

Variable-angle total internal reflection fluorescence microscopy of intact cells of Arabidopsis thaliana

BACKGROUND

Total internal reflection fluorescence microscopy (TIRFM) is a powerful tool for observing fluorescently labeled molecules on the plasma membrane surface of animal cells. However, the utility of TIRFM in plant cell studies has been limited by the fact that plants have cell walls, thick peripheral layers surrounding the plasma membrane. Recently, a new technique known as variable-angle epifluorescence microscopy (VAEM) was developed to circumvent this problem. However, the lack of a detailed analysis of the optical principles underlying VAEM has limited its applications in plant-cell biology.

RESULTS

Here, we present theoretical and experimental evidence supporting the use of variable-angle TIRFM in observations of intact plant cells. We show that when total internal reflection occurs at the cell wall/cytosol interface with an appropriate angle of incidence, an evanescent wave field of constant depth is produced inside the cytosol. Results of experimental TIRFM observations of the dynamic behaviors of phototropin 1 (a membrane receptor protein) and clathrin light chain (a vesicle coat protein) support our theoretical analysis.

CONCLUSIONS

These findings demonstrate that variable-angle TIRFM is appropriate for quantitative live imaging of cells in intact tissues of Arabidopsis thaliana.

Let's go bananas: revisiting the endocytic BAR code

Against the odds of membrane resistance, members of the BIN/Amphiphysin/Rvs (BAR) domain superfamily shape membranes and their activity is indispensable for a plethora of life functions. While crystal structures of different BAR dimers advanced our understanding of membrane shaping by scaffolding and hydrophobic insertion mechanisms considerably, especially life-imaging techniques and loss-of-function studies of clathrin-mediated endocytosis with its gradually increasing curvature show that the initial idea that solely BAR domain curvatures determine their functions is oversimplified. Diagonal placing, lateral lipid-binding modes, additional lipid-binding modules, tilde shapes and formation of macromolecular lattices with different modes of organisation and arrangement increase versatility. A picture emerges, in which BAR domain proteins create macromolecular platforms, that recruit and connect different binding partners and ensure the connection and coordination of the different events during the endocytic process, such as membrane invagination, coat formation, actin nucleation, vesicle size control, fission, detachment and uncoating, in time and space, and may thereby offer mechanistic explanations for how coordination, directionality and effectiveness of a complex process with several steps and key players can be achieved.

Actin dynamics counteract membrane tension during clathrin-mediated endocytosis

Clathrin-mediated endocytosis is independent of actin dynamics in many circumstances but requires actin polymerization in others. We show that membrane tension determines the actin dependence of clathrin-coat assembly. As found previously, clathrin assembly supports formation of mature coated pits in the absence of actin polymerization on both dorsal and ventral surfaces of non-polarized mammalian cells, and also on basolateral surfaces of polarized cells. Actin engagement is necessary, however, to complete membrane deformation into a coated pit on apical surfaces of polarized cells and, more generally, on the surface of any cell in which the plasma membrane is under tension from osmotic swelling or mechanical stretching. We use these observations to alter actin dependence experimentally and show that resistance of the membrane to propagation of the clathrin lattice determines the distinction between 'actin dependent and 'actin independent'. We also find that light-chain-bound Hip1R mediates actin engagement. These data thus provide a unifying explanation for the role of actin dynamics in coated-pit budding.

Systems biology and physical biology of clathrin-mediated endocytosis

In this review, we describe the application of experimental data and modeling of intracellular endocytic trafficking mechanisms with a focus on the process of clathrin-mediated endocytosis. A detailed parts-list for the protein-protein interactions in clathrin-mediated endocytosis has been available for some time. However, recent experimental, theoretical, and computational tools have proved to be critical in establishing a sequence of events, cooperative dynamics, and energetics of the intracellular process. On the experimental front, total internal reflection fluorescence microscopy, photo-activated localization microscopy, and spinning-disk confocal microscopy have focused on assembly and patterning of endocytic proteins at the membrane, while on the theory front, minimal theoretical models for clathrin nucleation, biophysical models for membrane curvature and bending elasticity, as well as methods from computational structural and systems biology, have proved insightful in describing membrane topologies, curvature mechanisms, and energetics.

The generation of curved clathrin coats from flat plaques

Flat clathrin lattices or 'plaques' are commonly believed to be the precursors to clathrin-coated buds and vesicles. The sequence of steps carrying the flat hexagonal lattice into a highly curved polyhedral cage with exactly 12 pentagons remains elusive, however, and the large numbers of disrupted interclathrin connections in previously proposed conversion pathways make these scenarios rather unlikely. The recent notion that clathrin can make controlled small conformational transitions opens new avenues. Simulations with a self-assembling clathrin model suggest that localized conformational changes in a plaque can create sufficiently strong stresses for a dome-like fragment to break apart. The released fragment, which is strongly curved but still hexagonal, may subsequently grow into a cage by recruiting free triskelia from the cytoplasm, thus building all 12 pentagonal faces without recourse to complex topological changes. The critical assembly concentration in a slightly acidic in vitro solution is used to estimate the binding energy of a cage at 25-40 k(B) T/clathrin.

Recruitment of cellular clathrin to viral factories and disruption of clathrin-dependent trafficking

The viral factories of mammalian reovirus (MRV) are cytoplasmic structures that serve as sites of viral genome replication and particle assembly. A 721-aa MRV non-structural protein, µNS, forms the factory matrix and recruits other viral proteins to these structures. In this report, we show that µNS contains a conserved C-proximal sequence (711-LIDFS-715) that is similar to known clathrin-box motifs and is required for recruitment of clathrin to viral factories. Clathrin recruitment by µNS occurs independently of infecting MRV particles or other MRV proteins. Ala substitution for a single Leu residue (mutation L711A) within the putative clathrin-binding motif of µNS inhibits clathrin recruitment, but does not prevent formation or expansion of viral factories. Notably, clathrin-dependent cellular functions, including both endocytosis and secretion, are disrupted in cells infected with MRV expressing wild-type, but not L711A, µNS. These results identify µNS as a novel adaptor-like protein that recruits cellular clathrin to viral factories, disrupting normal functions of clathrin in cellular membrane trafficking. To our knowledge, this is the only viral or bacterial protein yet shown to interfere with clathrin functions in this manner. The results additionally establish a new approach for studies of clathrin functions, based on µNS-mediated sequestration.

ePMV embeds molecular modeling into professional animation software environments

Increasingly complex research has made it more difficult to prepare data for publication, education, and outreach. Many scientists must also wade through black-box code to interface computational algorithms from diverse sources to supplement their bench work. To reduce these barriers we have developed an open-source plug-in, embedded Python Molecular Viewer (ePMV), that runs molecular modeling software directly inside of professional 3D animation applications (hosts) to provide simultaneous access to the capabilities of these newly connected systems. Uniting host and scientific algorithms into a single interface allows users from varied backgrounds to assemble professional quality visuals and to perform computational experiments with relative ease. By enabling easy exchange of algorithms, ePMV can facilitate interdisciplinary research, smooth communication between broadly diverse specialties, and provide a common platform to frame and visualize the increasingly detailed intersection(s) of cellular and molecular biology.

Defects in coatomer protein I (COPI) transport cause blood feeding-induced mortality in Yellow Fever mosquitoes

Blood feeding by vector mosquitoes provides the entry point for disease pathogens and presents an acute metabolic challenge that must be overcome to complete the gonotrophic cycle. Based on recent data showing that coatomer protein I (COPI) vesicle transport is involved in cellular processes beyond Golgi-endoplasmic reticulum retrograde protein trafficking, we disrupted COPI functions in the Yellow Fever mosquito Aedes aegypti to interfere with blood meal digestion. Surprisingly, we found that decreased expression of the γCOPI coatomer protein led to 89% mortality in blood-fed mosquitoes by 72 h postfeeding compared with 0% mortality in control dsRNA-injected blood-fed mosquitoes and 3% mortality in γCOPI dsRNA-injected sugar-fed mosquitoes. Similar results were obtained using dsRNA directed against five other COPI coatomer subunits (α, β, β', δ, and ζ). We also examined midgut tissues by EM, quantitated heme in fecal samples, and characterized feeding-induced protein expression in midgut, fat body, and ovary tissues of COPI-deficient mosquitoes. We found that COPI defects disrupt epithelial cell membrane integrity, stimulate premature blood meal excretion, and block induced expression of several midgut protease genes. To study the role of COPI transport in ovarian development, we injected γCOPI dsRNA after blood feeding and found that, although blood digestion was normal, follicles in these mosquitoes were significantly smaller by 48 h postinjection and lacked eggshell proteins. Together, these data show that COPI functions are critical to mosquito blood digestion and egg maturation, a finding that could also apply to other blood-feeding arthropod vectors.

Exit from the trans-Golgi network: from molecules to mechanisms

The trans-Golgi network is a major sorting platform of the secretory pathway from which proteins and lipids, both newly synthesized and retrieved from endocytic compartments, are targeted to different destinations. These sorting processes occur during the formation of pleomorphic tubular-vesicular carriers. The past years have provided insights into basic mechanisms coordinating the spatial and temporal organization of machineries necessary for the segregation of membrane components into distinct microdomains, for the bending, elongation, and fission of corresponding membranes, thus revealing a complex interplay of protein-protein and protein-lipid interactions.

Clathrin mediates endocytosis and polar distribution of PIN auxin transporters in Arabidopsis

Endocytosis is a crucial mechanism by which eukaryotic cells internalize extracellular and plasma membrane material, and it is required for a multitude of cellular and developmental processes in unicellular and multicellular organisms. In animals and yeast, the best characterized pathway for endocytosis depends on the function of the vesicle coat protein clathrin. Clathrin-mediated endocytosis has recently been demonstrated also in plant cells, but its physiological and developmental roles remain unclear. Here, we assessed the roles of the clathrin-mediated mechanism of endocytosis in plants by genetic means. We interfered with clathrin heavy chain (CHC) function through mutants and dominant-negative approaches in Arabidopsis thaliana and established tools to manipulate clathrin function in a cell type-specific manner. The chc2 single mutants and dominant-negative CHC1 (HUB) transgenic lines were defective in bulk endocytosis as well as in internalization of prominent plasma membrane proteins. Interference with clathrin-mediated endocytosis led to defects in constitutive endocytic recycling of PIN auxin transporters and their polar distribution in embryos and roots. Consistent with this, these lines had altered auxin distribution patterns and associated auxin transport-related phenotypes, such as aberrant embryo patterning, imperfect cotyledon specification, agravitropic growth, and impaired lateral root organogenesis. Together, these data demonstrate a fundamental role for clathrin function in cell polarity, growth, patterning, and organogenesis in plants.

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.

Rapid and efficient clathrin-mediated endocytosis revealed in genome-edited mammalian cells

Clathrin-mediated endocytosis (CME) is the best-studied pathway by which cells selectively internalize molecules from the plasma membrane and surrounding environment. Previous live-cell imaging studies using ectopically overexpressed fluorescent fusions of endocytic proteins indicated that mammalian CME is a highly dynamic but inefficient and heterogeneous process. In contrast, studies of endocytosis in budding yeast using fluorescent protein fusions expressed at physiological levels from native genomic loci have revealed a process that is very regular and efficient. To analyse endocytic dynamics in mammalian cells in which endogenous protein stoichiometry is preserved, we targeted zinc finger nucleases (ZFNs) to the clathrin light chain A and dynamin-2 genomic loci and generated cell lines expressing fluorescent protein fusions from each locus. The genome-edited cells exhibited enhanced endocytic function, dynamics and efficiency when compared with previously studied cells, indicating that CME is highly sensitive to the levels of its protein components. Our study establishes that ZFN-mediated genome editing is a robust tool for expressing protein fusions at endogenous levels to faithfully report subcellular localization and dynamics.

Structure of the PTEN-like region of auxilin, a detector of clathrin-coated vesicle budding

Auxilin, a J-domain containing protein, recruits the Hsc70 uncoating ATPase to newly budded clathrin-coated vesicles. The timing of auxilin arrival determines that uncoating will commence only after the clathrin lattice has fully assembled and after membrane fission is complete. Auxilin has a region resembling PTEN, a PI3P phosphatase. We have determined the crystal structure of this region of bovine auxilin 1; it indeed resembles PTEN closely. A change in the structure of the P loop accounts for the lack of phosphatase activity. Inclusion of phosphatidylinositol phosphates substantially enhances liposome binding by wild-type auxilin, but not by various mutants bearing changes in loops of the C2 domain. Nearly all these mutations also prevent recruitment of auxilin to newly budded coated vesicles. We propose a specific geometry for auxilin association with a membrane bilayer and discuss implications of this model for the mechanism by which auxilin detects separation of a vesicle from its parent membrane.

Asymmetry as the key to clathrin cage assembly

The self-assembly of clathrin proteins into polyhedral cages is simulated for the first time (to our knowledge) by introducing a coarse-grain triskelion particle modeled after clathrin's characteristic shape. The simulations indicate that neither this shape, nor the antiparallel binding of four legs along the lattice edges, is sufficient to induce cage formation from a random solution. Asymmetric intersegmental interactions, which probably result from a patchy distribution of interactions along the legs' surfaces, prove to be crucial for the efficient self-assembly of cages.

Gateways for the intracellular access of nanocarriers: a review of receptor-mediated endocytosis mechanisms and of strategies in receptor targeting

IMPORTANCE OF THE FIELD

The last 10 years have seen a dramatic growth in understanding and controlling how complex, drug-loaded (nano)structures, as well as pathogens, or biopharmaceuticals can gather access to the cytoplasm, which is a key step to increasing the effectiveness of their action.

AREAS COVERED IN THIS REVIEW

The review offers an updated overview of the current knowledge of endocytic processes; furthermore, the cell surface receptors most commonly used in drug delivery are here discussed on the basis of their reported internalization mechanisms, with examples of their use as nanocarrier targets taken from the most recent scientific literature.

WHAT THE READER WILL GAIN

Knowledge of molecular biology details is increasingly necessary for a rational design of drug delivery systems. Here, the aim is to provide the reader with an attempt to link a mechanistic knowledge of endocytic mechanisms with the identification of appropriate targets (internalization receptors) for nanocarriers.

TAKE HOME MESSAGE

Much advance is still needed to create a complete and coherent biological picture of endocytosis, but current knowledge already allows individuation of a good number of targetable groups for a predetermined intracellular fate of nanocarriers.

The length of vesicular stomatitis virus particles dictates a need for actin assembly during clathrin-dependent endocytosis

Microbial pathogens exploit the clathrin endocytic machinery to enter host cells. Vesicular stomatitis virus (VSV), an enveloped virus with bullet-shaped virions that measure 70 x 200 nm, enters cells by clathrin-dependent endocytosis. We showed previously that VSV particles exceed the capacity of typical clathrin-coated vesicles and instead enter through endocytic carriers that acquire a partial clathrin coat and require local actin filament assembly to complete vesicle budding and internalization. To understand why the actin system is required for VSV uptake, we compared the internalization mechanisms of VSV and its shorter (75 nm long) defective interfering particle, DI-T. By imaging the uptake of individual particles into live cells, we found that, as with parental virions, DI-T enters via the clathrin endocytic pathway. Unlike VSV, DI-T internalization occurs through complete clathrin-coated vesicles and does not require actin polymerization. Since VSV and DI-T particles display similar surface densities of the same attachment glycoprotein, we conclude that the physical properties of the particle dictate whether a virus-containing clathrin pit engages the actin system. We suggest that the elongated shape of a VSV particle prevents full enclosure by the clathrin coat and that stalling of coat assembly triggers recruitment of the actin machinery to finish the internalization process. Since some enveloped viruses have pleomorphic particle shapes and sizes, our work suggests that they may use altered modes of endocytic uptake. More generally, our findings show the importance of cargo geometry for specifying cellular entry modes, even when the receptor recognition properties of a ligand are maintained.

Animating the model figure

In all branches of scientific inquiry, researchers build models that enable them to visualize, formulate and communicate their hypotheses to others. In cell biology, our conceptual understanding of a process is typically embodied in a model figure. These visual models should ideally represent pre-existing knowledge of molecular interactions, movement, structure and localization but, in reality, they often fall short. Cell biologists have begun to look to the use of three-dimensional animation to visualize and describe complex molecular and cellular events. In addition to aiding teaching and communication, animation is emerging as a powerful tool for providing researchers with insight into the processes that they study. Two case studies focusing on the structure/function of the motor protein dynein and the structure of the centriole are discussed.

Toxic effects of iron oxide nanoparticles on human umbilical vein endothelial cells

Iron oxide nanoparticles (IONPs) have been employed for hyperthermia treatments, stem cell therapies, cell labeling, and imaging modalities. The biocompatibility and cytotoxic effects of iron oxide nanoparticles when used in biomedical applications, however, are an ongoing concern. Endothelial cells have a critical role in this research dealing with tumors, cardiovascular disease and inflammation. However, there is little information dealing with the biologic effects of IONPs on the endothelial cell. This paper deals with the influence of dextran and citric acid coated IONPs on the behavior and function of human umbilical vein endothelial cells (HUVECs). After exposing endothelial cells to IONPs, dose-dependent effects on HUVECs viability, cytoskeleton and function were determined. Both citric acid and dextran coated particles appeared to be largely internalized by HUVECs through endocytosis and contribute to eventual cell death possibly by apoptosis. Cytoskeletal structures were greatly disrupted, as evidenced by diminished vinculin spots, and disorganized actin fiber and tubulin networks. The capacity of HUVECs to form a vascular network on Matrigel™ diminished after exposure to IONPs. Cell migration/invasion were inhibited significantly even at very low iron concentrations (0.1 mM). The results of this study indicate the great importance of thoroughly understanding nanoparticle-cell interactions, and the potential to exploit this understanding in tumor therapy applications involving IONPs as thermo/chemoembolization agents.

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

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

Internalization of the opioid growth factor, [Met5]-enkephalin, is dependent on clathrin-mediated endocytosis for downregulation of cell proliferation

The opioid growth factor (OGF; [Met(5)]-enkephalin), a constitutively expressed and tonically active inhibitory peptide, interacts with the OGF receptor (OGFr) to form an endogenous growth-regulating pathway in homeostasis. Amplification of OGF-OGFr interfacing in animal and clinical studies depresses development, neoplasia, angiogenesis, and immunity. Disruption of the OGF-OGFr axis accelerates cell proliferation and has been particularly important in wound repair. To investigate how OGF enters cells, OGF was labeled with 5,6-tetramethylrhodamine OGF (RhoOGF) to study its uptake in live cells. African green monkey kidney cells (COS-7) incubated with RhoOGF exhibited a temperature-dependent course of entry, being internalized at 37 degrees C but not at 4 degrees C. RhoOGF was detected in the cytoplasm 15 min after initial exposure, observed in both cytoplasm and nucleus within 30 min, and remained in the cells for as long as 5 h. A 100-fold excess of OGF or the opioid antagonist naltrexone, but not other opioid ligands (some selective for classic opioid receptors), markedly reduced entry of RhoOGF into cells. RhoOGF was functional because DNA synthesis in cells incubated with RhoOGF (10(-5) to 10(-8) M) was decreased 24-36%, and was comparable to cells treated with unlabeled OGF (reductions of 26-39%). OGF internalization was dependent on clathrin-mediated endocytosis, with addition of clathrin siRNA diminishing the uptake of RhoOGF and upregulating DNA synthesis. RhoOGF clathrin-mediated endocytosis was unrelated to endosomal or Golgi pathways. Taken together, these results suggest that OGF enters cells by active transport in a saturable manner that requires clathrin-mediated endocytosis.

Quantitative analysis of the mechanism of endocytic actin patch assembly and disassembly in fission yeast

We report time courses of the accumulation and loss of 16 fluorescent fusion proteins at sites of clathrin-mediated endocytosis in fission yeast. Mathematical modeling shows that dendritic nucleation hypothesis can account for the kinetics of actin assembly in vivo and disassembly requires actin filament severing along with depolymerization.

We used quantitative confocal microscopy to measure the numbers of 16 proteins tagged with fluorescent proteins during assembly and disassembly of endocytic actin patches in fission yeast. The peak numbers of each molecule that accumulate in patches varied <30–50% between individual patches. The pathway begins with accumulation of 30–40 clathrin molecules, sufficient to build a hemisphere at the tip of a plasma membrane invagination. Thereafter precisely timed waves of proteins reach characteristic peak numbers: endocytic adaptor proteins (∼120 End4p and ∼230 Pan1p), activators of Arp2/3 complex (∼200 Wsp1p and ∼340 Myo1p) and ∼300 Arp2/3 complexes just ahead of a burst of actin assembly into short, capped and highly cross-linked filaments (∼7000 actins, ∼200 capping proteins, and ∼900 fimbrins). Coronin arrives last as all other components disperse upon patch internalization and movement over ∼10 s. Patch internalization occurs without recruitment of dynamins. Mathematical modeling, described in the accompanying paper (Berro et al., 2010, MBoC 21: 2803–2813), shows that the dendritic nucleation hypothesis can account for the time course of actin assembly into a branched network of several hundred filaments 100–200 nm long and that patch disassembly requires actin filament fragmentation in addition to depolymerization from the ends.

A lipid matrix model of membrane raft structure

Domains in cell membranes are created by lipid-lipid interactions and are referred to as membrane rafts. Reliable isolation methods have been developed which have shown that rafts from the same membranes have different proteins and can be sub-fractionated by immunoaffinity methods. Analysis of these raft subfractions shows that they are also comprised of different molecular species of lipids. The major lipid classes present are phospholipids, glycosphingolipids and cholesterol. Model studies show that mixtures of phospholipids, particularly sphingomyelin, and cholesterol form liquid-ordered phase with properties intermediate between a gel and fluid phase. This type of liquid-ordered phase dominates theories of domain formation and raft structure in biological membranes. Recently it has been shown that sphingolipids with long (22-26C) N-acyl fatty acids form quasi-crystalline bilayer structures with diacylphospholipids that have well-defined stoichiometries. A two tier heuristic model of membrane raft structure is proposed in which liquid-ordered phase created by a molecular complex between sphingolipids with hydrocarbon chains of approximately equal length and cholesterol acts as a primary staging area for selecting raft proteins. Tailoring of the lipid anchors of raft proteins takes place at this site. Assembly of lipid-anchored proteins on a scaffold of sphingolipids with asymmetric hydrocarbon chains and phospholipids arranged in a quasi-crystalline bilayer structure serves to concentrate and orient the proteins in a manner that couples them functionally within the membrane. Specificity is inherent in the quasi-crystalline lipid structure of liquid-ordered matrices formed by both types of complex into which protein lipid anchors are interpolated. An interaction between the sugar residues of the glycolipids and the raft proteins provides an additional level of specificity that distinguishes one raft from another.

Roles of AP-2 in clathrin-mediated endocytosis

Background

The notion that AP-2 clathrin adaptor is an essential component of an endocytic clathrin coat appears to conflict with recent observations that substantial AP-2 depletion, using RNA interference with synthesis of AP-2 subunits, fails to block uptake of certain ligands known to internalize through a clathrin-based pathway.

Methodology/Principal Findings

We report here the use of in vivo imaging data obtained by spinning-disk confocal microscopy to study the formation of clathrin-coated structures at the plasma membranes of BSC1 and HeLa cells depleted by RNAi of the clathrin adaptor, AP-2. Very few clathrin coats continue to assemble after AP-2 knockdown. Moreover, there is a total absence of clathrin-containing structures completely lacking AP-2 while all the remaining coats still contain a small amount of AP-2. These observations suggest that AP-2 is essential for endocytic coated-pit and coated-vesicle formation. We also find that AP-2 knockdown strongly inhibits light-density lipoprotein (LDL) receptor-mediated endocytosis, as long as cells are maintained in complete serum and at 37°C. If cells are first incubated with LDL at 4°C, followed by warming, there is little or no decrease in LDL uptake with respect to control cells. LDL uptake at 37°C is also not affected in AP-2 depleted cells first deprived of LDL by incubation with either serum-starved or LDL-starved cells for 24 hr. The LDL-deprived cells display a significant increase in endocytic structures enriched on deeply invaginated tubes that contain LDL and we suggest that under this condition of stress, LDL might enter through this alternative pathway.

Conclusions/Significance

These results suggest that AP-2 is essential for endocytic clathrin coated-pit and coated-vesicle formation. They also indicate that under normal conditions, functional endocytic clathrin coated pits are required for LDL internalization. We also show that under certain conditions of stress, cells can upregulate alternative endocytic structures with the potential to provide compensatory trafficking pathways.