As the first recipient of both the Women in Cell Biology Junior and Senior Awards, I look back to identify key components that have provided the foundation for my successful research career. In retrospect, the three most important building blocks have been: identifying and pursing important problems; attracting and mentoring talented postdoctoral fellows and students; and establishing and nurturing strong collaborations.
Dynamin (Dyn) is a multidomain and multifunctional GTPase best known for its essential role in clathrin-mediated endocytosis (CME). Dyn2 mutations have been linked to two human diseases, Centronuclear Myopathy (CNM) and Charcot-Marie-Tooth (CMT) disease. Paradoxically, although Dyn2 is ubiquitously expressed and essential for embryonic development, the disease-associated Dyn2 mutants are autosomal dominant, but result in slowly progressing and tissue-specific diseases. Thus, although the cellular defects that cause disease remain unclear, they are expected to be mild. To gain new insight into potential pathogenic mechanisms we utilized mouse Dyn2 conditional knock-out cells combined with retroviral-mediated reconstitution to mimic both heterozygous and homozygous states and characterized cellular phenotypes using quantitative assays for several membrane trafficking events. Surprisingly, none of the four mutants studied exhibited a defect in CME, but all were impaired in their ability to support p75/neurotrophin receptor export from the Golgi, the raft-dependent endocytosis of cholera toxin, and clathrin-independent endocytosis of EGFR. While it will be important to study these mutants in disease-relevant muscle and neuronal cells, given the importance of neurotrophic factors and lipid rafts in muscle physiology, we speculate that these common cellular defects might contribute to the tissue-specific diseases caused by a ubiquitously expressed protein.
Charcot-Marie-Tooth disease; Centronuclear Myopathy; Clathrin-mediated endocytosis; EGF receptor; p75/neurotrophin receptor; lipid rafts
The structurally regular and stable self-assembled capsids derived from viruses can be used as scaffolds for the display of multiple copies of cell- and tissue-targeting molecules and therapeutic agents in a convenient and well-defined manner. The human iron-transfer protein transferrin, a high affinity ligand for receptors upregulated in a variety of cancers, has been arrayed on the exterior surface of the protein capsid of bacteriophage Qβ. Selective oxidation of the sialic acid residues on the glycan chains of transferrin was followed by introduction of a terminal alkyne functionality via an oxime linkage. Attachment of the protein to azide-functionalized Qb capsid particles in an orientation allowing access to the receptor binding site was accomplished by the CuI-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction. Transferrin conjugation to Qβ particles allowed specific recognition by transferrin receptors and cellular internalization via clathrin-mediated endocytosis, as determined by fluorescence microscopy on cells expressing GFP-labeled clathrin light chains. By testing Qβ particles bearing different numbers of transferrin molecules, it was demonstrated that cellular uptake was proportional to ligand density, but that internalization was inhibited by equivalent concentrations of free transferrin. These results suggest that cell targeting with transferrin can be improved by local concentration (avidity) effects.
virus-like particles; transferrin; receptor-mediated endocytosis; polyvalency; click chemistry
Phosphatidylinositol-(4,5)-bisphosphate (PIP2) is the main lipid binding partner of proteins involved in clathrin-mediated endocytosis (CME). Total internal reflection fluorescence microscopy coupled to computational image analysis revealed that the balance of PIP2 synthesis in the bulk plasma membrane and its local turnover within clathrin-coated pits control multiple distinct yet only partly overlapping stages of CME.
Clathrin-mediated endocytosis (CME) is the major mechanism for internalization in mammalian cells. CME initiates by recruitment of adaptors and clathrin to form clathrin-coated pits (CCPs). Nearly half of nascent CCPs abort, whereas others are stabilized by unknown mechanisms and undergo further maturation before pinching off to form clathrin-coated vesicles (CCVs). Phosphatidylinositol-(4,5)-bisphosphate (PIP2), the main lipid binding partner of endocytic proteins, is required for CCP assembly, but little is currently known about its contribution(s) to later events in CCV formation. Using small interfering RNA (siRNA) knockdown and overexpression, we have analyzed the effects of manipulating PIP2 synthesis and turnover on CME by quantitative total internal reflection fluorescence microscopy and computational analysis. Phosphatidylinositol-4-phosphate-5-kinase cannot be detected within CCPs but functions in initiation and controls the rate and extent of CCP growth. In contrast, the 5′-inositol phosphatase synaptojanin 1 localizes to CCPs and controls early stabilization and maturation efficiency. Together these results suggest that the balance of PIP2 synthesis in the bulk plasma membrane and its local turnover within CCPs control multiple stages of CCV formation.
The relationship between cargo accumulation and clathrin-coated pit initiation and maturation is examined by direct visualization of receptor-engaged clathrin-coated pits.
Clathrin-mediated endocytosis (CME) is the major pathway for concentrative uptake of receptors and receptor–ligand complexes (cargo). Although constitutively internalized cargos are known to accumulate into maturing clathrin-coated pits (CCPs), whether and how cargo recruitment affects the initiation and maturation of CCPs is not fully understood. Previous studies have addressed these issues by analyzing the global effects of receptor overexpression on CME or CCP dynamics. Here, we exploit a refined approach using expression of a biotinylated transferrin receptor (bTfnR) and controlling its local clustering using mono- or multivalent streptavidin. We show that local clustering of bTfnR increased CCP initiation. By tracking cargo loading in individual CCPs, we found that bTfnR clustering preceded clathrin assembly and confirmed that bTfnR-containing CCPs mature more efficiently than bTfnR-free CCPs. Although neither the clustering nor the related changes in cargo loading altered the rate of CCP maturation, bTfnR-containing CCPs exhibited significantly longer lifetimes than other CCPs within the same cell. Together these results demonstrate that cargo composition is a key source of the differential dynamics of CCPs.
We have manipulated the activities of PLD and DGK, enzymes that regulate PA biosynthesis, and directly measured their effects on cellular PA levels and on clathrin-mediated endocytosis (CME). We report a previously unappreciated complexity in PA regulation and show that PA selectively regulates CME of EGF but not transferrin.
Clathrin-mediated endocytosis (CME) is the main route of internalization of receptor-ligand complexes. Relatively little is known about the role of specific lipids in CME, in particular that of phosphatidic acid (PA). We examined the effect of altering cellular PA levels on CME by manipulating the activities and/or levels of either phospholipase D (PLD1 and PLD2) or diacylglycerol kinase (DGK), two enzyme classes involved in PA production. DGK inhibition resulted in a dramatic reduction of cellular PA, measured directly using an enzyme-coupled reaction, which resulted in a decreased rate of EGFR internalization measured biochemically. This corresponded to a decreased rate of clathrin-coated pit (CCP) initiation and increased lifetimes of productive CCPs, as determined by quantitative live-cell total internal reflection fluorescence microscopy. Unexpectedly, PLD inhibition caused an increase in cellular PA, suggesting that PLD activity negatively regulates PA synthesis by other more productive pathways. Consistent with opposite effects on cellular PA levels, PLD inhibition had opposite effects on EGFR internalization and CCP dynamics, compared with DGK inhibition. Importantly, the constitutive internalization of transferrin receptors was unaffected by either treatment. These findings demonstrate that PA plays a regulatory rather than obligatory role in CME and differentially regulates ligand-stimulated CME of EGFR.
Clathrin-coated pit size and dynamic behavior varies with low density lipoprotein receptor (LDLR) expression levels in a manner dependent on the LDLR-specific adaptors, Dab2 and ARH.
Clathrin-mediated endocytosis of surface receptors and their bound ligands (i.e., cargo) is highly regulated, including by the cargo itself. One of the possible sources of the observed heterogeneous dynamics of clathrin-coated pits (CCPs) might be the different cargo content. Consistent with this, we show that CCP size and dynamic behavior varies with low density lipoprotein receptor (LDLR) expression levels in a manner dependent on the LDLR-specific adaptors, Dab2 and ARH. In Dab2-mCherry–expressing cells, varying LDLR expression leads to a progressive increase in CCP size and to the appearance of nonterminal endocytic events. In LDLR and ARH-mCherry–expressing cells in addition to an increase in CCP size, turnover of abortive CCPs increases, and the rate of CCP maturation decreases. Altogether, our results underscore the highly dynamic and cargo-responsive nature of CCP assembly and suggest that the observed heterogeneity is, in part, related to compositional differences (e.g., cargo and adaptors) between CCPs.
The GTPase dynamin catalyzes the scission of deeply invaginated clathrin-coated pits at the plasma membrane, but the mechanisms governing dynamin-mediated membrane fission remain poorly understood. Through mutagenesis, we have altered the hydrophobic nature of the membrane-inserting variable loop 1 (VL1) of the pleckstrin homology (PH) domain of dynamin-1 and demonstrate that its stable insertion into the lipid bilayer is critical for high membrane curvature generation and subsequent membrane fission. Dynamin PH domain mutants defective in curvature generation regain function when assayed on precurved membrane templates in vitro, but they remain defective in the scission of clathrin-coated pits in vivo. These results demonstrate that, in concert with dynamin self-assembly, PH domain membrane insertion is essential for fission and vesicle release in vitro and for clathrin-mediated endocytosis in vivo.
The GTPase dynamin assembles at the necks of budded vesicles in vivo and functions in membrane fission. We have developed fluid supported bilayers with excess membrane reservoir, (SUPER) templates, to assay vesicle formation and membrane fission. Consistent with previous studies, in the absence of GTP, dynamin assembles in spirals forming long membrane tubules. GTP addition triggers disassembly, but not membrane fission arguing against models that fission is mediated by concerted and global GTP-driven conformational changes. In contrast, under physiological conditions in the constant presence of GTP, dynamin mediates membrane fission. Under these conditions, fluorescently-labeled dynamin cooperatively organizes into self-limited assemblies that continuously cycle at the membrane and drive vesicle release. When visualized at the necks of emergent vesicles, self-limited dynamin assemblies display intensity fluctuations and persist for variable time periods before fission. Thus, self-limited assemblies of dynamin generated in the constant presence of GTP catalyze membrane fission.
Dynamin; supported bilayers; membrane reservoir; membrane fission; GTPase activity
Dynamin exhibits a high basal rate of GTP hydrolysis that is enhanced by self-assembly on a lipid template. Dynamin's GTPase effector domain (GED) is required for this stimulation, though its mechanism of action is poorly understood. Recent structural work has suggested that GED may physically dock with the GTPase domain to exert its stimulatory effects. To examine how these interactions activate dynamin, we engineered a minimal GTPase-GED fusion protein (GG) that reconstitutes dynamin's basal GTPase activity and utilized it to define the structural framework that mediates GED's association with the GTPase domain. Chemical cross-linking of GG and mutagenesis of full-length dynamin establishes that the GTPase-GED interface is comprised of the N- and C-terminal helices of the GTPase domain and the C-terminus of GED. We further show that this interface is essential for structural stability in full-length dynamin. Finally, we identify mutations in this interface that disrupt assembly-stimulated GTP hydrolysis and dynamin-catalyzed membrane fission in vitro and impair the late stages of clathrin-mediated endocytosis in vivo. These data suggest that the components of the GTPase-GED interface act as an intramolecular signaling module, which we term the bundle signaling element, that can modulate dynamin function in vitro and in vivo.
Diverse cargo molecules (i.e., receptors and ligand/receptor complexes) are taken into the cell by clathrin-mediated endocytosis (CME) utilizing a core machinery consisting of cargo-specific adaptors, clathrin and the GTPase dynamin. Numerous endocytic accessory proteins are also required, but their differential roles and functional hierarchy during CME are not yet understood. Here, we used a combination of quantitative live-cell imaging by total internal reflection fluorescence microscopy (TIR-FM), and decomposition of the lifetime distributions of clathrin-coated pits (CCPs) to measure independent aspects of CCP dynamics, including the turnover of abortive and productive CCP species and their relative contributions. Capitalizing on the sensitivity of this assay, we have examined the effects of specific siRNA-mediated depletion of endocytic accessory proteins on CME progression. Of the 12 endocytic accessory proteins examined, we observed seven qualitatively different phenotypes upon protein depletion. From this data we derive a temporal hierarchy of protein function during early steps of CME. Our results support the idea that a subset of accessory proteins, which mediate coat assembly, membrane curvature, and cargo selection, can provide input into an endocytic restriction point/checkpoint mechanism that monitors CCP maturation.
Dynamin (Dyn) is a multifunctional GTPase implicated in several cellular events, including endocytosis, intracellular trafficking, cell signaling, and cytokinesis. The mammalian genome encodes three isoforms, Dyn1, Dyn2, and Dyn3, and several splice variants of each, leading to the suggestion that distinct isoforms and/or distinct splice variants might mediate distinct cellular functions. We generated a conditional Dyn2 KO cell line and performed knockout and reconstitution experiments to explore the isoform- and splice variant specific cellular functions of ubiquitously expressed Dyn2. We find that Dyn2 is required for clathrin-mediated endocytosis (CME), p75 export from the Golgi, and PDGF-stimulated macropinocytosis and cytokinesis, but not for other endocytic pathways. Surprisingly, CME and p75 exocytosis were efficiently rescued by reintroduction of Dyn2, but not Dyn1, suggesting that these two isoforms function differentially in vesicular trafficking in nonneuronal cells. Both isoforms rescued macropinocytosis and cytokinesis, suggesting that dynamin function in these processes might be mechanistically distinct from its role in CME. Although all four Dyn2 splice variants could equally restore CME, Dyn2ba and -bb were more effective at restoring p75 exocytosis. This splice variant specificity correlated with their differential targeting to the Golgi. These studies reveal isoform and splice-variant specific functions for Dyn2.
Biological membrane fission is conducted by protein-driven stress. To create such membrane stress the GTPase dynamin-1, protein orchestrating membrane fission in endocytosis, assembles into helical scaffolds that constrict the necks of endocytic vesicles. We found that under constant GTP turnover two-rung dynamin scaffold is sufficient to produce fission of lipid nanotubes. Analyzing membrane fission by short dynamin scaffolds, we reveal a catalytic cycle which translates constriction stresses into fission. Upon constriction, coordinated membrane wedging by the scaffold facilitates reversible merger of the inner leaflet of the nanotube, the hemifission. Modeling of this reversible step identifies a low-energy path based on geometric coupling of the scaffold and the membrane. The final translation of the metastable hemifission into complete fission is stochastically linked to disassembly of the scaffold. This catalytic conversion of localized stresses into membrane remodeling suggests a novel paradigm for fission and fusion of cellular membranes.
Numerous endocytic accessory proteins (EAPs) mediate assembly and maturation of clathrin-coated pits (CCPs) into cargo-containing vesicles. Analysis of EAP function through bulk measurement of cargo uptake has been hampered due to potential redundancy among EAPs and, as we show here, the plasticity and resilience of clathrin-mediated endocytosis (CME). Instead, EAP function is best studied by uncovering the correlation between variations in EAP association to individual CCPs and the resulting variations in maturation. However, most EAPs bind to CCPs in low numbers, making the measurement of EAP association via fused fluorescent reporters highly susceptible to detection errors. Here, we present a framework for unbiased measurement of EAP recruitment to CCPs and their direct effects on CCP dynamics. We identify dynamin and the EAP-binding α-adaptin appendage domain of the AP2 adaptor as switches in a regulated, multistep maturation process and provide direct evidence for a molecular checkpoint in CME.
The mechanism by which the self-assembling GTPase dynamin functions in vesicle formation remains controversial. Point mutations in shibire, the Drosophila dynamin, cause temperature-sensitive (ts) defects in endocytosis. We show that the ts2 mutation, which occurs in the switch 2 region of dynamin's GTPase domain, compromises GTP binding affinity. Three second-site suppressor mutations, one in the switch 1 region of the GTPase domain and two in the GTPase effector domain (GED), dynamin's putative GAP, fully rescue the shits2 defects in synaptic vesicle recycling. The functional rescue in vivo correlates with a reduction in both the basal and assembly-stimulated GTPase activity in vitro. These findings demonstrate that GED is indeed an internal dynamin GAP and establish that, as for other GTPase superfamily members, dynamin's function in vivo is negatively regulated by its GAP activity. Based on these and other observations, we propose a two-step model for dynamin during vesicle formation in which an early regulatory GTPase-like function precedes late, assembly-dependent steps during which GTP hydrolysis is required for vesicle release.
Dynamin, a central player in clathrin-mediated endocytosis, interacts with several functionally diverse SH3 domain-containing proteins. However, the role of these interactions with regard to dynamin function is poorly defined. We have investigated a recently identified protein partner of dynamin, SNX9, sorting nexin 9. SNX9 binds directly to both dynamin-1 and dynamin-2. Moreover by stimulating dynamin assembly, SNX9 stimulates dynamin's basal GTPase activity and potentiates assembly-stimulated GTPase activity on liposomes. In fixed cells, we observe that SNX9 partially localizes to clathrin-coated pits. Using total internal reflection fluorescence microscopy in living cells, we detect a transient burst of EGFP-SNX9 recruitment to clathrin-coated pits that occurs during the late stages of vesicle formation and coincides spatially and temporally with a burst of dynamin-mRFP fluorescence. Transferrin internalization is inhibited in HeLa cells after siRNA-mediated knockdown of SNX9. Thus, our results establish that SNX9 is required for efficient clathrin-mediated endocytosis and suggest that it functions to regulate dynamin activity.
Clathrin-mediated endocytosis in mammalian cells is critical for a variety of cellular processes including nutrient uptake and cell surface receptor down-regulation. Despite the findings that numerous endocytic accessory proteins directly or indirectly regulate actin dynamics and that actin assembly is spatially and temporally coordinated with endocytosis, direct functional evidence for a role of actin during clathrin-coated vesicle formation is lacking. Here, we take parallel biochemical and microscopic approaches to address the contribution of actin polymerization/depolymerization dynamics to clathrin-mediated endocytosis. When measured using live-cell fluorescence microscopy, disruption of the F-actin assembly and disassembly cycle with latrunculin A or jasplakinolide results in near complete cessation of all aspects of clathrin-coated structure (CCS) dynamics. Stage-specific biochemical assays and quantitative fluorescence and electron microscopic analyses establish that F-actin dynamics are required for multiple distinct stages of clathrin-coated vesicle formation, including coated pit formation, constriction, and internalization. In addition, F-actin dynamics are required for observed diverse CCS behaviors, including splitting of CCSs from larger CCSs, merging of CCSs, and lateral mobility on the cell surface. Our results demonstrate a key role for actin during clathrin-mediated endocytosis in mammalian cells.
The μ2 subunit of the AP2 complex is known to be phosphorylated in vitro by a copurifying kinase, and it has been demonstrated recently that μ2 phosphorylation is required for transferrin endocytosis (Olusanya, O., P.D. Andrews, J.R. Swedlow, and E. Smythe. 2001. Curr. Biol. 11:896–900). However, the identity of the endogenous kinase responsible for this phosphorylation is unknown. Here we identify and characterize a novel member of the Prk/Ark family of serine/threonine kinases, adaptor-associated kinase (AAK)1. We find that AAK1 copurifies with adaptor protein (AP)2 and that it directly binds the ear domain of α-adaptin in vivo and in vitro. In neuronal cells, AAK1 is enriched at presynaptic terminals, whereas in nonneuronal cells it colocalizes with clathrin and AP2 in clathrin-coated pits and at the leading edge of migrating cells. AAK1 specifically phosphorylates the μ subunit in vitro, and stage-specific assays for endocytosis show that μ phosphorylation by AAK1 results in a decrease in AP2-stimulated transferrin internalization. Together, these results provide strong evidence that AAK1 is the endogenous μ2 kinase and plays a regulatory role in clathrin-mediated endocytosis. These results also lend support to the idea that clathrin-mediated endocytosis is controlled by cycles of phosphorylation/desphosphorylation.
AAK1; AP2; clathrin; kinase; endocytosis
A role for dynamin in clathrin-mediated endocytosis is now well established. However, mammals express three closely related, tissue-specific dynamin isoforms, each with multiple splice variants. Thus, an important question is whether these isoforms and splice variants function in vesicle formation from distinct intracellular organelles. There are conflicting data as to a role for dynamin-2 in vesicle budding from the TGN. To resolve this issue, we compared the effects of overexpression of dominant-negative mutants of dynamin-1 (the neuronal isoform) and dynamin-2 (the ubiquitously expressed isoform) on endocytic and biosynthetic membrane trafficking in HeLa cells and polarized MDCK cells. Both dyn1(K44A) and dyn2(K44A) were potent inhibitors of receptor-mediated endocytosis; however neither mutant directly affected other membrane trafficking events, including transport mediated by four distinct classes of vesicles budding from the TGN. Dyn2(K44A) more potently inhibited receptor-mediated endocytosis than dyn1(K44A) in HeLa cells and at the basolateral surface of MDCK cells. In contrast, dyn1(K44A) more potently inhibited endocytosis at the apical surface of MDCK cells. The two dynamin isoforms have redundant functions in endocytic vesicle formation, but can be targeted to and function differentially at subdomains of the plasma membrane.
receptor-mediated endocytosis; dynamin; polarized MDCK cells; trans-Golgi network; adenovirus expression
New developments in electron microscopy used in combination reveal the elegant architecture of cellular structures at very high resolution.
Since its inception, electron microscopy (EM) has revealed that cellular membranes are organized into structurally distinct subdomains, created by localized protein and lipid assemblies to perform specific complex cellular functions. Caveolae are membrane subdomains that function as signaling platforms, endocytic carriers, sensors of membrane tension, and mechanical stress, as well as in lipid homeostasis. They were first discovered almost 60 years ago by pioneering electron microscopists. While new and exciting developments in SUPER-resolution fluorescent light microscopy facilitate studies of the spatial organization of fluorescently labeled protein components, these techniques cannot reveal the underlying cellular structures. Thus, equally exciting are developments in EM: genetically encoded probes for protein localization at sub-10 nm resolution, more powerful instruments that allow imaging of larger cell volumes, and computational methods for reconstructing three-dimensional images. Used in combination, as done by Ludwig et al. in the current issue of PLOS Biology, these tools reveal high-resolution insights into the composition and organization of the caveolae coat and the formation of these specialized structures. Together, these advances are contributing to a resurgence in EM.
Bone Morphogenetic Proteins (BMPs) have diverse functions during development in vertebrates. We have recently shown that BMP2 signaling promotes venous specific angiogenesis in zebrafish embryos. However, factors that confer a context dependent pro-angiogenic function of BMP2 signaling within endothelial cells need to be identified. Here, we report that Disabled homolog 2 (Dab2), a cargo specific adaptor protein for Clathrin, is essential to mediate the pro-angiogenic function of BMP2 signaling. We find that inhibition of Dab2 attenuates internalization of BMP receptors and abrogates the pro-angiogenic effects of BMP signaling in endothelial cells. Moreover, inhibition of Dab2 decreases phosphorylation of SMAD-1, 5, and 8, indicating that Dab2 plays an essential role in determining the outcome of BMP signaling within endothelial cells, and may provide a molecular basis for a context dependent pro-angiogenic function of BMP2 signaling.
Dynamin is the most-studied membrane fission machinery and has served as a paradigm for studies of other fission GTPases; however, several critical questions regarding its function remain unresolved. In particular, because most dynamin GTPase domain mutants studied to date equally impair both basal and assembly-stimulated GTPase activities, it has been difficult to distinguish their respective roles in clathrin-mediated endocytosis (CME) or in dynamin catalyzed membrane fission. Here we compared a new dynamin mutant, Q40E, which is selectively impaired in assembly-stimulated GTPase activity with S45N, a GTP-binding mutant equally defective in both basal and assembly-stimulated GTPase activities. Both mutants potently inhibit CME and effectively recruit other endocytic accessory proteins to stalled coated pits. However, the Q40E mutant blocks at a later step than S45N, providing additional evidence that GTP binding and/or basal GTPase activities of dynamin are required throughout clathrin coated pit maturation. Importantly, using in vitro assays for assembly-stimulated GTPase activity and membrane fission, we find that the latter is much more potently inhibited by both dominant-negative mutants than the former. These studies establish that efficient fission from supported bilayers with excess membrane reservoir (SUPER) templates requires coordinated GTP hydrolysis across two rungs of an assembled dynamin collar.