Endocytosis and recycling of immune complexes by follicular dendritic cells enhances B cell antigen binding and activation

Stromal-derived follicular dendritic cells (FDCs) are a major reservoir for antigen that are essential for formation of germinal centers, the site where memory and effector B cells differentiate. A long-standing question is how FDCs retain antigen in its native form for extended periods and how they display it to specific B cells. Here we found that FDCs acquired complement-coated immune complexes (ICs) from noncognate B cells via complement receptors 1 and 2 (CD35 and CD21, respectively) and rapidly internalized them by an actin-dependent pathway. ICs were retained intact within a nondegradative cycling compartment and were displayed periodically on the cell surface where they were accessible to antigen-specific B cells. This would explain how antigens are protected from damage and retained over long periods of time, while remaining accessible for B cells.

Similar uptake but different trafficking and escape routes of reovirus virions and infectious subvirion particles imaged in polarized Madin-Darby canine kidney cells

Polarized epithelial cells that line the digestive, respiratory, and genitourinary tracts form a barrier that many viruses must breach to infect their hosts. Current understanding of cell entry by mammalian reovirus (MRV) virions and infectious subvirion particles (ISVPs), generated from MRV virions by extracellular proteolysis in the digestive tract, are mostly derived from in vitro studies with nonpolarized cells. Recent live-cell imaging advances allow us for the first time to visualize events at the apical surface of polarized cells. In this study, we used spinning-disk confocal fluorescence microscopy with high temporal and spatial resolution to follow the uptake and trafficking dynamics of single MRV virions and ISVPs at the apical surface of live polarized Madin-Darby canine kidney cells. Both types of particles were internalized by clathrin-mediated endocytosis, but virions and ISVPs exhibited strikingly different trafficking after uptake. While virions reached early and late endosomes, ISVPs did not and instead escaped the endocytic pathway from an earlier location. This study highlights the broad advantages of using live-cell imaging combined with single-particle tracking for identifying key steps in cell entry by viruses.

Small molecule-mediated TGF-β type II receptor degradation promotes cardiomyogenesis in embryonic stem cells

The cellular signals controlling the formation of cardiomyocytes, vascular smooth muscle, and endothelial cells from stem cell-derived mesoderm are poorly understood. To identify these signals, a mouse embryonic stem cell (ESC)-based differentiation assay was screened against a small molecule library resulting in a 1,4-dihydropyridine inducer of type II TGF-β receptor (TGFBR2) degradation-1 (ITD-1). ITD analogs enhanced proteasomal degradation of TGFBR2, effectively clearing the receptor from the cell surface and selectively inhibiting intracellular signaling (IC(50) ~0.4-0.8 μM). ITD-1 was used to evaluate TGF-β involvement in mesoderm formation and cardiopoietic differentiation, which occur sequentially during early development, revealing an essential role in both processes in ESC cultures. ITD-1 selectively enhanced the differentiation of uncommitted mesoderm to cardiomyocytes, but not to vascular smooth muscle and endothelial cells. ITD-1 is a highly selective TGF-β inhibitor and reveals an unexpected role for TGF-β signaling in controlling cardiomyocyte differentiation from multipotent cardiovascular precursors.

Visualizing the high curvature regions of post-mitotic nascent nuclear envelope membrane

We previously reported that mitotic endoplasmic reticulum (ER) membrane cisternae or sheets directly assemble mammalian nuclear envelope (NE) at the end of mitosis. In this study, we investigated the dynamics of the high curvature regions of partially assembled nuclear envelope membrane using reticulon4a as a probe. We found that, after sorting out reticulon4a from the nascent NE membrane sheets, reticulon4a is specifically localized to the leading edges. Our 3D time lapse images suggested that ER tubules could be incompetent in assembling the NE membrane. Our findings suggest a possible role of reticulons at the leading edges during the NE re-assembly and provide further evidences that the mitotic assembly of NE is by ER cisternae rather than tubules.

Host cell nucleolin is required to maintain the architecture of human cytomegalovirus replication compartments

Drastic reorganization of the nucleus is a hallmark of herpesvirus replication. This reorganization includes the formation of viral replication compartments, the subnuclear structures in which the viral DNA genome is replicated. The architecture of replication compartments is poorly understood. However, recent work with human cytomegalovirus (HCMV) showed that the viral DNA polymerase subunit UL44 concentrates and viral DNA synthesis occurs at the periphery of these compartments. Any cellular factors involved in replication compartment architecture are largely unknown. Previously, we found that nucleolin, a major protein component of nucleoli, associates with HCMV UL44 in infected cells and is required for efficient viral DNA synthesis. Here, we show that nucleolin binds to purified UL44. Confocal immunofluorescence analysis demonstrated colocalization of nucleolin with UL44 at the periphery of replication compartments. Pharmacological inhibition of viral DNA synthesis prevented the formation of replication compartments but did not abrogate association of UL44 and nucleolin. Thus, association of UL44 and nucleolin is unlikely to be a nonspecific effect related to development of replication compartments. No detectable colocalization of 5-ethynyl-2'-deoxyuridine (EdU)-labeled viral DNA with nucleolin was observed, suggesting that nucleolin is not directly involved in viral DNA synthesis. Small interfering RNA (siRNA)-mediated knockdown of nucleolin caused improper localization of UL44 and a defect in EdU incorporation into viral DNA. We propose a model in which nucleolin anchors UL44 at the periphery of replication compartments to maintain their architecture and promote viral DNA synthesis.

IMPORTANCE

Human cytomegalovirus (HCMV) is an important human pathogen. HCMV infection causes considerable rearrangement of the structure of the nucleus, largely due to the formation of viral replication compartments within the nucleus. Within these compartments, the virus replicates its DNA genome. We previously demonstrated that nucleolin is required for efficient viral DNA synthesis and now find that the nucleolar protein nucleolin interacts with a subunit of the viral DNA polymerase, UL44, specifically at the periphery of replication compartments. Moreover, we find that nucleolin is required to properly localize UL44 at this region. Nucleolin is, therefore, involved in the organization of proteins within replication compartments. This, to our knowledge, is the first report identifying a cellular protein required for maintaining replication compartment architecture.

Redistribution of caveolae during mitosis

Caveolae form a specialized platform within the plasma membrane that is crucial for an array of important biological functions, ranging from signaling to endocytosis. Using total internal reflection fluorescence (TIRF) and 3D fast spinning-disk confocal imaging to follow caveola dynamics for extended periods, and electron microscopy to obtain high resolution snapshots, we found that the vast majority of caveolae are dynamic with lifetimes ranging from a few seconds to several minutes. Use of these methods revealed a change in the dynamics and localization of caveolae during mitosis. During interphase, the equilibrium between the arrival and departure of caveolae from the cell surface maintains the steady-state distribution of caveolin-1 (Cav1) at the plasma membrane. During mitosis, increased dynamics coupled to an imbalance between the arrival and departure of caveolae from the cell surface induces a redistribution of Cav1 from the plasma membrane to intracellular compartments. These changes are reversed during cytokinesis. The observed redistribution of Cav1 was reproduced by treatment of interphase cells with nocodazole, suggesting that microtubule rearrangements during mitosis can mediate caveolin relocalization. This study provides new insights into the dynamics of caveolae and highlights precise regulation of caveola budding and recycling during mitosis.

Formation of the postmitotic nuclear envelope from extended ER cisternae precedes nuclear pore assembly

Live-cell imaging and electron tomography show that nuclear pore complexes only assemble on a previously formed nuclear envelope.

During mitosis, the nuclear envelope merges with the endoplasmic reticulum (ER), and nuclear pore complexes are disassembled. In a current model for reassembly after mitosis, the nuclear envelope forms by a reshaping of ER tubules. For the assembly of pores, two major models have been proposed. In the insertion model, nuclear pore complexes are embedded in the nuclear envelope after their formation. In the prepore model, nucleoporins assemble on the chromatin as an intermediate nuclear pore complex before nuclear envelope formation. Using live-cell imaging and electron microscope tomography, we find that the mitotic assembly of the nuclear envelope primarily originates from ER cisternae. Moreover, the nuclear pore complexes assemble only on the already formed nuclear envelope. Indeed, all the chromatin-associated Nup107–160 complexes are in single units instead of assembled prepores. We therefore propose that the postmitotic nuclear envelope assembles directly from ER cisternae followed by membrane-dependent insertion of nuclear pore complexes.

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.

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.

ARFGAP1 promotes AP-2-dependent endocytosis

COPI (coat protein I) and the clathrin-AP-2 (adaptor protein 2) complex are well-characterized coat proteins, but a component that is common to these two coats has not been identified. The GTPase-activating protein (GAP) for ADP-ribosylation factor 1 (ARF1), ARFGAP1, is a known component of the COPI complex. Here, we show that distinct regions of ARFGAP1 interact with AP-2 and coatomer (components of the COPI complex). Selectively disrupting the interaction of ARFGAP1 with either of these two coat proteins leads to selective inhibition in the corresponding transport pathway. The role of ARFGAP1 in AP-2-regulated endocytosis has mechanistic parallels with its roles in COPI transport, as both its GAP activity and coat function contribute to promoting AP-2 transport.

Structural analysis of the interaction between Dishevelled2 and clathrin AP-2 adaptor, a critical step in noncanonical Wnt signaling

Wnt association with its receptor, Frizzled (Fz), and recruitment by the latter of an adaptor, Dishevelled (Dvl), initiates signaling through at least two distinct pathways ("canonical" and "noncanonical"). Endocytosis and compartmentalization help determine the signaling outcome. Our previous work has shown that Dvl2 links at least one Frizzled family member (Fz4) to clathrin-mediated endocytosis by interacting with the μ2 subunit of the AP-2 clathrin adaptor, through both a classical endocytic tyrosine motif and a so-called "DEP domain." We report here the crystal structure of a chimeric protein that mimics the Dvl2-μ2 complex. The DEP domain binds at one end of the elongated, C-terminal domain of μ2. This domain:domain interface shows that parts of the μ2 surface distinct from the tyrosine-motif site can help recruit specific receptors or adaptors into a clathrin coated pit. Mutation of residues at the DEP-μ2 contact or in the tyrosine motif reduce affinity of Dvl2 for μ2 and block efficient internalization of Fz4 in response to ligation by Wnt5a. The crystal structure has thus allowed us to identify the specific interaction that leads to Frizzled uptake and to downstream, noncanonical signaling events.

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.

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.

Structure of clathrin coat with bound Hsc70 and auxilin: mechanism of Hsc70-facilitated disassembly

The chaperone Hsc70 drives the clathrin assembly–disassembly cycle forward by stimulating dissociation of a clathrin lattice. A J-domain containing co-chaperone, auxilin, associates with a freshly budded clathrin-coated vesicle, or with an in vitro assembled clathrin coat, and recruits Hsc70 to its specific heavy-chain-binding site. We have determined by electron cryomicroscopy (cryoEM), at about 11 Å resolution, the structure of a clathrin coat (in the D6-barrel form) with specifically bound Hsc70 and auxilin. The Hsc70 binds a previously analysed site near the C-terminus of the heavy chain, with a stoichiometry of about one per three-fold vertex. Its binding is accompanied by a distortion of the clathrin lattice, detected by a change in the axial ratio of the D6 barrel. We propose that when Hsc70, recruited to a position close to its target by the auxilin J-domain, splits ATP, it clamps firmly onto its heavy-chain site and locks in place a transient fluctuation. Accumulation of the local strain thus imposed at multiple vertices can then lead to disassembly.

Distinct dynamics of endocytic clathrin-coated pits and coated plaques

Here we classify endocytic structures at the adherent (bottom) surface of many cells in culture into shorter-lived coated pits and longer-lived coated plaques which internalize by different mechanisms.

Clathrin is the scaffold of a conserved molecular machinery that has evolved to capture membrane patches, which then pinch off to become traffic carriers. These carriers are the principal vehicles of receptor-mediated endocytosis and are the major route of traffic from plasma membrane to endosomes. We report here the use of in vivo imaging data, obtained from spinning disk confocal and total internal reflection fluorescence microscopy, to distinguish between two modes of endocytic clathrin coat formation, which we designate as “coated pits” and “coated plaques.” Coated pits are small, rapidly forming structures that deform the underlying membrane by progressive recruitment of clathrin, adaptors, and other regulatory proteins. They ultimately close off and bud inward to form coated vesicles. Coated plaques are longer-lived structures with larger and less sharply curved coats; their clathrin lattices do not close off, but instead move inward from the cell surface shortly before membrane fission. Local remodeling of actin filaments is essential for the formation, inward movement, and dissolution of plaques, but it is not required for normal formation and budding of coated pits in the cells we have studied. We conclude that there are at least two distinct modes of clathrin coat formation at the plasma membrane—classical coated pits and coated plaques—and that these two assemblies interact quite differently with other intracellular structures.

Here, we identify and characterize two distinct modes of clathrin-mediated uptake at the plasma membrane. The “canonical” coated pit is where assembly of a curved clathrin lattice, linked to deformation of the underlying membrane, gives rise to coated vesicles. Clathrin coated “plaques” are extended clathrin lattices of low curvature (enriched in hexagonal arrays) that have been observed by electron microscopy at the bottom of cells, but their relationship to the canonical, curved pit assembly has been obscure. Recognition of the difference between two distinguishable classes of events detected by fluorescence microscopy has resolved a number of conflicts and misconceptions in the literature. In particular, a large fraction of the clathrin endocytic processes studied at the adherent surface of HeLa, Swiss 3T3, and astrocyte cells are the long-lived coated plaques, not canonical coated pits, whereas most, if not all, of the clathrin endocytic processes at the free surface of these cells are coated pits. Conflation of data from the two distinct processes has previously led to misleading mechanistic conclusions, which are now resolved.

Lymphocyte crawling and transendothelial migration require chemokine triggering of high-affinity LFA-1 integrin

Endothelial chemokines are instrumental for integrin-mediated lymphocyte adhesion and transendothelial migration (TEM). By dissecting how chemokines trigger lymphocyte integrins to support shear-resistant motility on and across cytokine-stimulated endothelial barriers, we found a critical role for high-affinity (HA) LFA-1 integrin in lymphocyte crawling on activated endothelium. Endothelial-presented chemokines triggered HA-LFA-1 and adhesive filopodia at numerous submicron dots scattered underneath crawling lymphocytes. Shear forces applied to endothelial-bound lymphocytes dramatically enhanced filopodia density underneath crawling lymphocytes. A fraction of the adhesive filopodia invaded the endothelial cells prior to and during TEM and extended large subluminal leading edge containing dots of HA-LFA-1 occupied by subluminal ICAM-1. Memory T cells generated more frequent invasive filopodia and transmigrated more rapidly than their naive counterparts. We propose that shear forces exerted on HA-LFA-1 trigger adhesive and invasive filopodia at apical endothelial surfaces and thereby promote lymphocyte crawling and probing for TEM sites.

The anaplastic lymphoma kinase controls cell shape and growth of anaplastic large cell lymphoma through Cdc42 activation

Anaplastic large cell lymphoma (ALCL) is a non-Hodgkin's lymphoma that originates from T cells and frequently expresses oncogenic fusion proteins derived from chromosomal translocations or inversions of the anaplastic lymphoma kinase (ALK) gene. The proliferation and survival of ALCL cells are determined by the ALK activity. Here we show that the kinase activity of the nucleophosmin (NPM)-ALK fusion regulated the shape of ALCL cells and F-actin filament assembly in a pattern similar to T-cell receptor-stimulated cells. NPM-ALK formed a complex with the guanine exchange factor VAV1, enhancing its activation through phosphorylation. VAV1 increased Cdc42 activity, and in turn, Cdc42 regulated the shape and migration of ALCL cells. In vitro knockdown of VAV1 or Cdc42 by short hairpin RNA, as well as pharmacologic inhibition of Cdc42 activity by secramine, resulted in a cell cycle arrest and apoptosis of ALCL cells. Importantly, the concomitant inhibition of Cdc42 and NPM-ALK kinase acted synergistically to induce apoptosis of ALCL cells. Finally, Cdc42 was necessary for the growth as well as for the maintenance of already established lymphomas in vivo. Thus, our data open perspectives for new therapeutic strategies by revealing a mechanism of regulation of ALCL cell growth through Cdc42.

Differential evanescence nanometry: live-cell fluorescence measurements with 10-nm axial resolution on the plasma membrane

We present a method to resolve components within a diffraction-limited object by tracking simultaneously the average axial positions of two different sets of fluorescent molecules within it. The axial positions are then subtracted from each other to determine the separation of the two sets of fluorophores. This method follows the dynamic changes in the separation of the two sets of fluorophores with freely rotating dipoles using sequential acquisitions with total internal reflection and wide-field illumination, and it can be used to measure the formation of small structures on living cells. We have verified that we can achieve a resolution of 10 nm, and we have used the method to follow the location of clathrin and its adaptor AP-2 as they are recruited to a diffraction-limited coated pit during its assembly at the plasma membrane. We find a gradually increasing axial separation between the centroids of clathrin and AP-2 distribution, up to a final value of 30 nm just before coated-pit pinching and formation of the coated vesicle.

Mammalian cells change volume during mitosis

Using single cell-imaging methods we have found that the volume of adherent cells grown in culture decreases as the cells rounds when it enters mitosis. A minimal volume is reached at metaphase. Rapid volume recovery initiates before abscission as cells make the transition from metaphase to cytokinesis. These volume changes are simultaneous with the rapid surface area decrease and recovery observed in mitotic cells [1].

Invasive and adherent bacterial pathogens co-Opt host clathrin for infection

Infection by the bacterium Listeria monocytogenes depends on host cell clathrin. To determine whether this requirement is widespread, we analyzed infection models using diverse bacteria. We demonstrated that bacteria that enter cells following binding to cellular receptors (termed "zippering" bacteria) invade in a clathrin-dependent manner. In contrast, bacteria that inject effector proteins into host cells in order to gain entry (termed "triggering" bacteria) invade in a clathrin-independent manner. Strikingly, enteropathogenic Escherichia coli (EPEC) required clathrin to form actin-rich pedestals in host cells beneath adhering bacteria, even though this pathogen remains extracellular. Furthermore, clathrin accumulation preceded the actin rearrangements necessary for Listeria entry. These data provide evidence for a clathrin-based entry pathway allowing internalization of large objects (bacteria and ligand-coated beads) and used by "zippering" bacteria as part of a general mechanism to invade host mammalian cells. We also revealed a nonendocytic role for clathrin required for extracellular EPEC infections.