Endocytosis of activated receptors and clathrin-coated pit formation: deciphering the chicken or egg relationship

The Battle of LPS Clearance in Host Defense vs. Inflammatory Signaling

Lipopolysaccharide (LPS) in blood circulation causes endotoxemia and is linked to various disease conditions. Current treatments focus on preventing LPS from interacting with its receptor Toll-like receptor 4 (TLR4) and reducing inflammation. However, our body has a natural defense mechanism: reticuloendothelial cells in the liver rapidly degrade and inactivate much of the circulating LPS within minutes. But this LPS clearance mechanism is not perfect. Excessive LPS that escape this clearance mechanism cause systemic inflammatory damage through TLR4. Despite its importance, the role of reticuloendothelial cells in LPS elimination is not well-studied, especially regarding the specific cells, receptors, and mechanisms involved. This gap hampers the development of effective therapies for endotoxemia and related diseases. This review consolidates the current understanding of LPS clearance, narrates known and explores potential mechanisms, and discusses the relationship between LPS clearance and LPS signaling. It also aims to highlight key insights that can guide the development of strategies to reduce circulating LPS by way of bolstering host defense mechanisms. Ultimately, we seek to provide a foundation for future research that could lead to innovative approaches for enhancing the body's natural ability to clear LPS and thereby lower the risk of endotoxin-related inflammatory diseases, including sepsis.

Opposite changes in the expression of clathrin and caveolin-1 in normal and cancerous human prostate tissue: putative clathrin-mediated recycling of EGFR

Endocytosis, an important macromolecule uptake process in cells, is known to be dysregulated in cancer. Clathrin and caveolin-1 proteins play a major role in receptor-mediated endocytosis. We have used a quantitative, unbiased and semi-automated method to measure in situ protein expression of clathrin and caveolin-1 in cancerous and paired normal (cancer adjacent, non-cancerous) human prostate tissue. There was a significant (p < 0.0001) increase in the expression of clathrin in prostate cancer samples (N = 29, n = 91) compared to normal tissue (N = 29, n = 67) (N = number of patients, n = number of cores in tissue arrays). Conversely, there was a significant (p < 0.0001) decrease in expression of caveolin-1 in prostate cancer tissue compared to normal prostate tissue. The opposite change in expression of the two proteins was highly correlated to increasing cancer aggressiveness. There was also a concurrent increase in the expression of epidermal growth factor receptor (EGFR), a key receptor in carcinogenesis, with clathrin in prostate cancer tissue, indicating recycling of EGFR through clathrin-mediated endocytosis (CME). These results indicate that in prostate cancer, caveolin-1-mediated endocytosis (CavME) may be acting as a brake and increase in CME may facilitate tumorigenicity and aggressiveness of prostate cancer through recycling of EGFR. Changes in the expression of these proteins can also potentially be used as a biomarker for prostate cancer to aid in diagnosis and prognosis and clinical decision-making.

Distinct mechanisms enable inward or outward budding from late endosomes/multivesicular bodies

Regulating the residence time of membrane proteins on the cell surface can modify their response to extracellular cues and allow for cellular adaptation in response to changing environmental conditions. The fate of membrane proteins that are internalized from the plasma membrane and arrive at the limiting membrane of the late endosome/multivesicular body (MVB) is dictated by whether they remain on the limiting membrane, bud into internal MVB vesicles, or bud outwardly from the membrane. The molecular details underlying the disposition of membrane proteins that transit this pathway and the mechanisms regulating these trafficking events are unclear. We established a cell-free system that reconstitutes budding of membrane protein cargo into internal MVB vesicles and onto vesicles that bud outwardly from the MVB membrane. Both budding reactions are cytosol-dependent and supported by Saccharomyces cerevisiae (yeast) cytosol. We observed that inward and outward budding from the MVB membrane are mechanistically distinct but may be linked, such that inhibition of inward budding triggers a re-routing of cargo from inward to outward budding vesicles, without affecting the number of vesicles that bud outwardly from MVBs.

Cholesterol modulates CFTR confinement in the plasma membrane of primary epithelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR) is a plasma-membrane anion channel that, when mutated, causes the disease cystic fibrosis. Although CFTR has been detected in a detergent-resistant membrane fraction prepared from airway epithelial cells, suggesting that it may partition into cholesterol-rich membrane microdomains (lipid rafts), its compartmentalization has not been demonstrated in intact cells and the influence of microdomains on CFTR lateral mobility is unknown. We used live-cell imaging, spatial image correlation spectroscopy, and k-space image correlation spectroscopy to examine the aggregation state of CFTR and its dynamics both within and outside microdomains in the plasma membrane of primary human bronchial epithelial cells. These studies were also performed during treatments that augment or deplete membrane cholesterol. We found two populations of CFTR molecules that were distinguishable based on their dynamics at the cell surface. One population showed confinement and had slow dynamics that were highly cholesterol dependent. The other, more abundant population was less confined and diffused more rapidly. Treatments that deplete the membrane of cholesterol caused the confined fraction and average number of CFTR molecules per cluster to decrease. Elevating cholesterol had the opposite effect, increasing channel aggregation and the fraction of channels displaying confinement, consistent with CFTR recruitment into cholesterol-rich microdomains with dimensions below the optical resolution limit. Viral infection caused the nanoscale microdomains to fuse into large platforms and reduced CFTR mobility. To our knowledge, these results provide the first biophysical evidence for multiple CFTR populations and have implications for regulation of their surface expression and channel function.

Regulation of fc receptor endocytic trafficking by ubiquitination

Most immune cells, particularly phagocytes, express various receptors for the Fc portion of the different immunoglobulin isotypes (Fc receptors, FcRs). By binding to the antibody, they provide a link between the adaptive immune system and the powerful effector functions triggered by innate immune cells such as mast cells, neutrophils, macrophages, and NK cells. Upon ligation of the immune complexes, the downstream signaling pathways initiated by the different receptors are quite similar for different FcR classes leading to the secretion of preformed and de novo synthesized pro-inflammatory mediators. FcR engagement also promotes negative signals through the combined action of several molecules that limit the extent and duration of positive signaling. To this regard, ligand-induced ubiquitination of FcRs for IgE (FcεR) and IgG (FcγR) has become recognized as a key modification that generates signals for the internalization and/or delivery of engaged receptor complexes to lysosomes or cytoplasmic proteasomes for degradation, providing negative-feedback regulation of Fc receptor activity. In this review, we discuss recent advances in our understanding of the molecular mechanisms that ensure the clearance of engaged Fcε and Fcγ receptor complexes from the cell surface with an emphasis given to the cooperation between the ubiquitin pathway and endosomal adaptors including the endosomal sorting complex required for transport (ESCRT) in controlling receptor internalization and sorting along the endocytic compartments.

3-Dimensional Tracking of Non-blinking 'Giant' Quantum Dots in Live Cells

While semiconductor quantum dots (QDs) have been used successfully in numerous single particle tracking (SPT) studies due to their high photoluminescence efficiency, photostability, and broad palette of emission colors, conventional QDs exhibit fluorescence intermittency or 'blinking,' which causes ambiguity in particle trajectory analysis and limits tracking duration. Here, non-blinking 'giant' quantum dots (gQDs) are exploited to study IgE-FcεRI receptor dynamics in live cells using a confocal-based 3D SPT microscope. There is a 7-fold increase in the probability of observing IgE-FcεRI for longer than 1 min using the gQDs compared to commercially available QDs. A time-gated photon-pair correlation analysis is implemented to verify that selected SPT trajectories are definitively from individual gQDs and not aggregates. The increase in tracking duration for the gQDs allows the observation of multiple changes in diffusion rates of individual IgE-FcεRI receptors occurring on long (>1 min) time scales, which are quantified using a time-dependent diffusion coefficient and hidden Markov modeling. Non-blinking gQDs should become an important tool in future live cell 2D and 3D SPT studies, especially in cases where changes in cellular dynamics are occurring on the time scale of several minutes.

Optimal aggregation of FcεRI with a structurally defined trivalent ligand overrides negative regulation driven by phosphatases

To investigate why responses of mast cells to antigen-induced IgE receptor (FcεRI) aggregation depend nonlinearly on antigen dose, we characterized a new artificial ligand, DF3, through complementary modeling and experimentation. This ligand is a stable trimer of peptides derived from bacteriophage T4 fibritin, each conjugated to a hapten (DNP). We found low and high doses of DF3 at which degranulation of mast cells sensitized with DNP-specific IgE is minimal, but ligand-induced receptor aggregation is comparable to aggregation at an intermediate dose, optimal for degranulation. This finding makes DF3 an ideal reagent for studying the balance of negative and positive signaling in the FcεRI pathway. We find that the lipid phosphatase SHIP and the protein tyrosine phosphatase SHP-1 negatively regulate mast cell degranulation over all doses considered. In contrast, SHP-2 promotes degranulation. With high DF3 doses, relatively rapid recruitment of SHIP to the plasma membrane may explain the reduced degranulation response. Our results demonstrate that optimal secretory responses of mast cells depend on the formation of receptor aggregates that promote sufficient positive signaling by Syk to override phosphatase-mediated negative regulatory signals.

Combined mathematical modelling and experimentation to predict polymersome uptake by oral cancer cells

This study is motivated by understanding and controlling the key physical properties underlying internalisation of nano drug delivery. We consider the internalisation of specific nanometre size delivery vehicles, comprised of self-assembling amphiphilic block copolymers, called polymersomes that have the potential to specifically deliver anticancer therapeutics to tumour cells. The possible benefits of targeted polymersome drug delivery include reduced off-target toxic effects in healthy tissue and increased drug uptake by diseased tissue. Through a combination of in vitro experimentation and mathematical modelling, we develop a validated model of nanoparticle uptake by cells via the clathrin-mediated endocytotic pathway, incorporating receptor binding, clustering and recycling. The model predicts how the characteristics of receptor targeting, and the size and concentration of polymersomes alter uptake by tumour cells. The number of receptors per cell was identified as being the dominant mechanism accounting for the difference between cell types in polymersome uptake rate.

FROM THE CLINICAL EDITOR

This article reports on a validated model developed through a combination of in vitro experimentation and mathematical modeling of nanoparticle uptake by cells via the clathrin-mediated endocytotic pathway. The model incorporates receptor binding, clustering, and recycling and predicts how the characteristics of receptor targeting, the size and concentration alter polymersome uptake by cancer cells.

Ubiquitination and endocytosis of the high affinity receptor for IgE

The high affinity receptor for IgE (FcvarepsilonRI) is constitutivelly expressed on the surface of mast cells and basophils as a multimeric complex. Upon antigen ligation to FcvarepsilonRI-bound IgE molecules, the receptor complex transduces intracellular signals leading to the release of preformed and newly synthesised pro-inflammatory mediators. FcvarepsilonRI engagement also generates negative intracellular signals involving the coordinated action of adapters, phosphatases and ubiquitin ligases that limits the intensity and duration of positive signals. Relevant to this, antigen-induced FcvarepsilonRI ubiquitination has become recognized as an important signal for the internalization and delivery of engaged receptor complexes to lysosomes for degradation. In this article, we review recent advances in our understanding of molecular mechanisms that guarantee the clearance of antigen-stimulated FcvarepsilonRI complexes from the cell surface. A particular emphasis will be given on how lipid rafts and the ubiquitin pathway cooperate to ensure receptor internalization and sorting along the endocytic compartments. A brief discussion regarding how ubiquitination regulates the endocytosis of Fc receptors other than FcvarepsilonRI will be included.

Cell-free reconstitution of multivesicular body formation and receptor sorting

The number of surface membrane proteins and their residence time on the plasma membrane are critical determinants of cellular responses to cues that can control plasticity, growth and differentiation. After internalization, the ultimate fate of many plasma membrane proteins is dependent on whether they are sorted for internalization into the lumenal vesicles of multivesicular bodies (MVBs), an obligate step prior to lysosomal degradation. To help to elucidate the mechanisms underlying MVB sorting, we have developed a novel cell-free assay that reconstitutes the sorting of a prototypical membrane protein, the epidermal growth factor receptor, with which we have probed some of its molecular requirements. The sorting event measured is dependent on cytosol, ATP, time, temperature and an intact proton gradient. Depletion of Hrs inhibited biochemical and morphological measures of sorting that were rescued by inclusion of recombinant Hrs in the assay. Moreover, depletion of signal-transducing adaptor molecule (STAM), or addition of mutated ATPase-deficient Vps4, also inhibited sorting. This assay reconstitutes the maturation of late endosomes, including the formation of internal vesicles and the sorting of a membrane protein, and allows biochemical investigation of this process.

Small, mobile FcepsilonRI receptor aggregates are signaling competent

Crosslinking of IgE-bound FcepsilonRI triggers mast cell degranulation. Previous fluorescence recovery after photobleaching (FRAP) and phosphorescent anisotropy studies suggested that FcepsilonRI must immobilize to signal. Here, single quantum dot (QD) tracking and hyperspectral microscopy methods were used for defining the relationship between receptor mobility and signaling. QD-IgE-FcepsilonRI aggregates of at least three receptors remained highly mobile over extended times at low concentrations of antigen that induced Syk kinase activation and near-maximal secretion. Multivalent antigen, presented as DNP-QD, also remained mobile at low doses that supported secretion. FcepsilonRI immobilization was marked at intermediate and high antigen concentrations, correlating with increases in cluster size and rates of receptor internalization. The kinase inhibitor PP2 blocked secretion without affecting immobilization or internalization. We propose that immobility is a feature of highly crosslinked immunoreceptor aggregates and a trigger for receptor internalization, but is not required for tyrosine kinase activation leading to secretion.

Endocytosis and intracellular trafficking of human natural killer cell receptors

Natural killer (NK) cells play a vital role in the defense against viral infections and tumor development. NK cell function is primarily regulated by the sum of signals from a broad array of activation and inhibitory receptors. Key to generating the input level of either activating or inhibitory signals is the maintenance of receptor expression levels on the cell surface. Although the mechanisms of endocytosis and trafficking for some cell surface receptors, such as transferrin receptor and certain immune receptors, are very well known, that is not the situation for receptors expressed by NK cells. Recent studies have uncovered that endocytosis and trafficking routes characteristic for specific activation and inhibitory receptors can regulate the functional responses of NK cells. In this review, we summarize the current knowledge of receptor endocytosis and trafficking, and integrate this with our current understanding of NK cell receptor trafficking.

Lipid raft-dependent FcepsilonRI ubiquitination regulates receptor endocytosis through the action of ubiquitin binding adaptors

The best characterized role for ubiquitination of membrane receptors is to negatively regulate signaling by targeting receptors for lysosomal degradation. The high affinity receptor for IgE (FcεRI) expressed on mast cells and basophils is rapidly ubiquitinated upon antigen stimulation. However, the nature and the role of this covalent modification are still largelly unknown. Here, we show that FcεRI subunits are preferentially ubiquitinated at multiple sites upon stimulation, and provide evidence for a role of ubiquitin as an internalization signal: under conditions of impaired receptor ubiquitination a decrease of receptor entry is observed by FACS analysis and fluorescence microscopy. We also used biochemical approaches combined with fluorescence microscopy, to demonstrate that receptor endocytosis requires the integrity of specific membrane domains, namely lipid rafts. Additionally, by RNA interference we demonstrate the involvement of ubiquitin-binding endocytic adaptors in FcεRI internalization and sorting. Notably, the triple depletion of Eps15, Eps15R and Epsin1 negatively affects the early steps of Ag-induced receptor endocytosis, whereas Hrs depletion retains ubiquitinated receptors into early endosomes and partially prevents their sorting into lysosomes for degradation. Our results are compatible with a scenario in which the accumulation of engaged receptor subunits into lipid rafts is required for receptor ubiquitination, a prerequisite for efficient receptor internalization, sorting and delivery to a lysosomal compartment.

Cross-linking of FcepsilonRI causes Ca2+ mobilization via a sphingosine kinase pathway in a clathrin-dependent manner

Clathrin-coated pits are now recognized to be involved in cell signaling in addition to receptor down-regulation. Here we tried to identify signaling pathways that might be dependent on clathrin. Our initial data with pharmacological inhibitors of formation of clathrin-coated pits or lipid-rafts indicated that Ca(2+) response evoked by cross-linking of the high affinity receptors for IgE (FcepsilonRI) was dependent on clathrin. To confirm this finding, we created clathrin-knockdown cells by transfecting the mast cell line RBL-2H3 with a shRNA-clathrin heavy chain construct. In these cells, the FcepsilonRI-mediated Ca(2+) response was almost completely abolished, which was accompanied by the inhibition of sphingosine 1-phosphate (S1P) production with no changes in inositol 1,4,5-trisphosphate (IP(3)) production. This suggests that the Ca(2+) signaling pathway via a sphingosine kinase (SK) is dependent on clathrin. Furthermore, antigen-induced tyrosine phosphorylation of p85 and p110 subunits of PI3K was almost completely inhibited in clathrin-knockdown cells. In contrast, antigen-induced tyrosine phosphorylation of phospholipase Cgamma was not affected by clathrin-knockdown and tyrosine phosphorylation of Syk and degranulation were partially inhibited in clathrin-knockdown cells. The present study identifies the SK/Ca(2+) pathway to be dependent on clathrin.

Mint3/X11gamma is an ADP-ribosylation factor-dependent adaptor that regulates the traffic of the Alzheimer's Precursor protein from the trans-Golgi network

Beta-amyloid peptides (Abeta) are the major component of plaques in brains of Alzheimer's patients, and are they derived from the proteolytic processing of the beta-amyloid precursor protein (APP). The movement of APP between organelles is highly regulated, and it is tightly connected to its processing by secretases. We proposed previously that transport of APP within the cell is mediated in part through its sorting into Mint/X11-containing carriers. To test our hypothesis, we purified APP-containing vesicles from human neuroblastoma SH-SY5Y cells, and we showed that Mint2/3 are specifically enriched and that Mint3 and APP are present in the same vesicles. Increasing cellular APP levels increased the amounts of both APP and Mint3 in purified vesicles. Additional evidence supporting an obligate role for Mint3 in traffic of APP from the trans-Golgi network to the plasma membrane include the observations that depletion of Mint3 by small interference RNA (siRNA) or mutation of the Mint binding domain of APP changes the export route of APP from the basolateral to the endosomal/lysosomal sorting route. Finally, we show that increased expression of Mint3 decreased and siRNA-mediated knockdowns increased the secretion of the neurotoxic beta-amyloid peptide, Abeta(1-40). Together, our data implicate Mint3 activity as a critical determinant of post-Golgi APP traffic.

Clathrin-Coated Pits: Vive La Différence?

Because of the discovery of coated pits and vesicles more than 40 years ago and the identification of clathrin as a major component of the coat, it has been assumed that clathrin-coated pits (CCPs) are responsible for the uptake of most plasma membrane receptors undergoing internalization. The recent molecular characterization of clathrin-independent routes of endocytosis confirms that several alternative endocytic pathways operate at the plasma membrane of mammalian cells. This heterogeneous view of endocytosis has been expanded still further by recent studies, suggesting that different subpopulations of CCPs responsible for the internalization of specific sets of cargo may coexist. In the present review, we have discussed the experimental evidence in favor or against the existence of distinct parallel clathrin-dependent pathways at the plasma membrane.

Functional and structural requirements for the internalization of distinct BCR-ligand complexes

Antigen (Ag) binding to the BCR rapidly initiates two important events: a phosphorylation cascade that results in the production of secondary signaling intermediaries and the internalization of Ag-BCR complexes. Previous studies using anti-BCR antibodies (Ab) have suggested that BCR signaling is an essential requirement for BCR endocytosis and have further implicated lipid rafts as essential platforms for both BCR functions. However, published data from our laboratory indicate that lipid rafts and consequently raft-mediated signaling are dispensable for BCR-mediated internalization of Ag-specific BCR. Therefore, we investigated the relationship between BCR signaling and endocytosis by defining the role of early kinase signaling in the BCR-mediated internalization of a model Ag (haptenated protein). The results demonstrate that Src kinases and Syk-mediated BCR signaling are not essential for BCR-mediated Ag internalization. Moreover, by comparing Ag and Ab, it was determined that while both localize to clathrin-coated pits, the internalization of Ab-BCR complexes is more susceptible to inhibition of signaling and highly sensitive to disruption of lipid rafts and the actin cytoskeleton compared to Ag-BCR complexes. Thus, these results demonstrate that the nature of the ligand ultimately determines the functional requirements and relative contribution of lipid rafts and other membrane structures to the internalization of BCR-ligand complexes.

Trafficking of the human transferrin receptor in plant cells: effects of tyrphostin A23 and brefeldin A

Plant cells possess much of the molecular machinery necessary for receptor-mediated endocytosis (RME), but this process still awaits detailed characterization. In order to identify a reliable and well-characterized marker to investigate RME in plant cells, we have expressed the human transferrin receptor (hTfR) in Arabidopsis protoplasts. We have found that hTfR is mainly found in endosomal (Ara7- and FM4-64-positive) compartments, but also at the plasma membrane, where it mediates binding and internalization of its natural ligand transferrin (Tfn). Cell surface expression of hTfR increases upon treatment with tyrphostin A23, which inhibits the interaction between the YTRF endocytosis signal in the hTfR cytosolic tail and the mu2-subunit of the AP2 complex. Indeed, tyrphostin A23 inhibits Tfn internalization and redistributes most of hTfR to the plasma membrane, suggesting that the endocytosis signal of hTfR is functional in Arabidopsis protoplasts. Co-immunoprecipitation experiments show that hTfR is able to interact with a mu-adaptin subunit from Arabidopsis cytosol, a process that is blocked by tyrphostin A23. In contrast, treatment with brefeldin A, which inhibits recycling from endosomes back to the plasma membrane in plant cells, leads to the accumulation of Tfn and hTfR in larger patches inside the cell, reminiscent of BFA compartments. Therefore, hTfR has the same trafficking properties in Arabidopsis protoplasts as in animal cells, and cycles between the plasma membrane and endosomal compartments. The specific inhibition of Tfn/hTfR internalization and recycling by tyrphostin A23 and BFA, respectively, thus provide valuable molecular tools to characterize RME and the recycling pathway in plant cells.

The high-affinity immunoglobulin-E receptor (FcepsilonRI) is endocytosed by an AP-2/clathrin-independent, dynamin-dependent mechanism

Aggregation of the high-affinity immunoglobulin E (IgE) receptor (FcepsilonRI), expressed on mast cells and basophils, initiates the immediate hypersensitivity reaction. Aggregated FcepsilonRI has been reported to rapidly migrate to lipid rafts in RBL-2H3 cells. We confirmed that aggregated FcepsilonRI is found in the lipid raft fractions of cellular lysates. Furthermore, we show that the cross-linked FcepsilonRI remains associated with detergent-resistant structures upon internalization. Previous morphological studies have reported that aggregated FepsiloncRI is endocytosed via clathrin-coated pits, which in general are not lipid raft associated. To address this apparent discrepancy, we employed siRNA to suppress expression of components of the clathrin-mediated internalization machinery, namely, clathrin heavy chain, and the AP-2 (alpha-adaptin or mu2-subunit). Transferrin receptor (TfR) is endocytosed by a clathrin-mediated process and, as expected, each transfected siRNA caused a two to threefold elevation of TfR surface expression and almost completely inhibited its endocytosis. In contrast, there was no effect on surface expression levels of FcepsilonRI nor on the endocytosis of the dinitrophenyl-human serum albumin (DNP-HSA)/IgE/FcepsilonRI complex. On the contrary, internalization of DNP-HSA/IgE/FcepsilonRI was inhibited by overexpression of a dominant-negative dynamin mutant. We conclude that internalization of cross-linked FcRI does not require the AP-2/clathrin complex but is dynamin-dependent and may be lipid raft mediated.

Trafficking of a ligand-receptor complex on the growth cones as an essential step for the uptake of nerve growth factor at the distal end of the axon: a single-molecule analysis

The behavior of single molecules of neurotrophins on growth cones was observed by the use of the fluorescent conjugate of nerve growth factor (NGF), Cy3-NGF. After the application of 0.4 nm Cy3-NGF, chick dorsal root ganglion growth cones responded within 1 min of adding the stimulus by expanding their lamellipodia. Only 40 molecules of Cy3-NGF, which occupied <5% of the estimated total binding sites on a single growth cone, were required to initiate the motile responses. After binding to the high-affinity receptor, Cy3-NGF displayed lateral diffusion on the membrane of the growth cones with a diffusion constant of 0.3 microm2 s(-1). The behavior of Cy3-NGF was shifted to a one-directional rearward movement toward the central region of the growth cone. The one-directional movement of Cy3-NGF displayed the same rate as the rearward flow of actin, approximately 4 microm/min. This movement could be stopped by the application of the potent inhibitor of actin polymerization, latrunculin B. Molecules of Cy3-NGF were suggested to be internalized in the vicinity of the central region of the growth cone during this rearward trafficking, because Cy3-NGF remained in the growth cone after the growth cones had been exposed to an acidic surrounding medium: acidic medium causes the complete dissociation of Cy3-NGF from the receptors on the surface of growth cones. These results suggested that actin-driven trafficking of the NGF receptor complex is an essential step for the accumulation and endocytosis of NGF at the growth cone and for the retrograde transport of NGF toward the cell body.

Individual Phosphoinositide 3-Kinase C2α Domain Activities Independently Regulate Clathrin Function

Phosphoinositide 3-kinase C2alpha (PI3K-C2alpha) is a member of the class II PI-3 kinases, defined by the presence of a second C2 domain at their C termini. The cellular functions of the class II enzymes are incompletely understood, though they have been implicated in receptor activation pathways initiated by epidermal growth factor, insulin, and chemokines. PI3K-C2alpha was recently found to be localized to clathrin-coated membranes in the trans-Golgi network and at endocytic sites on the plasma membrane. Further, a specific binding site was identified for clathrin on the N terminus of PI3K-C2alpha, whose occupancy resulted in lipid kinase activation. Expression of PI3K-C2alpha in cells dramatically affected clathrin distribution and function in cells, leading to accumulation of intracellular clathrin-coated structures, which are visualized here at the ultrastructural level, and inhibition of clathrin-mediated transport from both the plasma membrane and the trans-Golgi network. In this study we have demonstrated that the isolated clathrin binding domain of PI3K-C2alpha can drive clathrin lattice assembly and that both it and the lipid kinase activity of the protein can independently modulate clathrin distribution and function when expressed in cells. Together, these results suggest that PI3K-C2alpha employs both protein-protein interaction and localized production of 3-phosphoinositides to affect clathrin dynamics at sites of membrane budding and targeting.

CIN85 regulates the ligand-dependent endocytosis of the IgE receptor: a new molecular mechanism to dampen mast cell function

Ligation of the high-affinity receptor for IgE (Fc epsilonRI), constitutively expressed on mast cells and basophils, promotes cell activation and immediate release of allergic mediators. Furthermore, Fc epsilonRI up-regulation on APC from atopic donors is involved in the pathophysiology of allergic diseases. In consideration of the clinical relevance of the IgE receptor, the down-modulation of Fc epsilonRI expression in mast cells may represent a potential target for handling atopic diseases. In an effort to identify new molecular mechanisms involved in attenuating Fc epsilonRI expression and signaling, we focused our attention on CIN85, a scaffold molecule that regulates, in concert with the ubiquitin ligase Cbl, the clathrin-mediated endocytosis of several receptor tyrosine kinases. In the present study, we show that endogenous CIN85 is recruited in Cbl-containing complexes after engagement of the Fc epsilonRI on a mast cell line and drives ligand-induced receptor internalization. By confocal microscopic analysis, we provide evidence that CIN85 directs a more rapid receptor sorting in early endosomes and delivery to a lysosomal compartment. Furthermore, biochemical studies indicate that CIN85 plays a role in reducing the expression of receptor complex. Finally, we demonstrate that CIN85-overexpressing mast cells are dramatically impaired in their ability to degranulate following Ag stimulation, suggesting that the accelerated internalization of activated receptors by perturbing the propagation of Fc epsilonRI signaling may contribute to dampen the functional response. This role of CIN85 could be extended to include other multimeric immune receptors, such as the T and B cell receptors, providing a more general molecular mechanism for attenuating immune responses.

The quantity and duration of FcRγ signals determine mast cell degranulation and survival

Immunoglobulin E (IgE) bound to multivalent antigen (Ag) elicits mast cell degranulation but not survival; on the contrary, IgE in the absence of Ag (IgE(-Ag)) induces survival only but not degranulation. Although these distinct responses are mediated through the same receptor, FcϵRI, the molecular mechanism generating the divergence is largely unknown. We recently showed that the signals through FcRγ chain are essential for IgE(-Ag)–induced mast cell survival as well as IgE(+Ag)–induced degranulation. To determine whether the cellular output is regulated by the quantity of FcRγ signal, we expressed CD8/FcRγ chimeras (CD8/γ) in bone marrow–derived mast cells (BMMCs) from FcRγ-/- mice to manipulate the strength of FcRγ signals by anti-CD8 cross-linking. Cross-linking of CD8/γ induced mast cell survival and degranulation. Survival was induced by weaker stimulation than needed for degranulation in terms of anti-CD8 concentration and the valency of chimera. However, sustained extracellular signal-regulated kinase (Erk) activation seems to regulate survival even when the activation signal was strong enough to elicit degranulation. Generation of sustained Erk activation by active mitogen-activated protein kinase kinase (MEK) induced BMMC survival. These results suggest that the duration and the magnitude of FcRγ signals may determine mast cell survival and degranulation, respectively. (Blood. 2004;103:3093-3101)

Lipid rafts unite signaling cascades with clathrin to regulate BCR internalization

A major function of the B cell is the internalization of antigen through the BCR for processing and presentation to T cells. While there is evidence suggesting that lipid raft signaling may regulate internalization, the molecular machinery coordinating these two processes remains to be defined. Here we present a link between the B cell signaling and internalization machinery and show that Src-family kinase activity is required for inducible clathrin heavy chain phosphorylation, BCR colocalization with clathrin, and regulated internalization. An analysis of different B cell lines shows that BCR uptake occurs only when clathrin is associated with rafts and is tyrosine phosphorylated following BCR crosslinking. We therefore propose that lipid rafts spatially organize signaling cascades with clathrin to regulate BCR internalization.

G protein-coupled receptor/arrestin3 modulation of the endocytic machinery

Nonvisual arrestins (arr) modulate G protein-coupled receptor (GPCR) desensitization and internalization and bind to both clathrin (CL) and AP-2 components of the endocytic coated pit (CP). This raises the possibility that endocytosis of some GPCRs may be a consequence of arr-induced de novo CP formation. To directly test this hypothesis, we examined the behavior of green fluorescent protein (GFP)-arr3 in live cells expressing beta2-adrenergic receptors and fluorescent CL. After agonist stimulation, the diffuse GFP-arr3 signal rapidly became punctate and colocalized virtually completely with preexisting CP spots, demonstrating that activated complexes accumulate in previously formed CPs rather than nucleating new CP formation. After arr3 recruitment, CP appeared larger: electron microscopy analysis revealed an increase in both CP number and in the occurrence of clustered CPs. Mutant arr3 proteins with impaired binding to CL or AP-2 displayed reduced recruitment to CPs, but were still capable of inducing CP clustering. In contrast, though constitutively present in CPs, the COOH-terminal moiety of arr3, which contains CP binding sites but lacks receptor binding, did not induce CP clustering. Together, these results indicate that recruitment of functional arr3-GPCR complexes to CP is necessary to induce clustering. Latrunculin B or 16 degrees C blocked CP rearrangements without affecting arr3 recruitment to CP. These results and earlier studies suggest that discrete CP zones exist on cell surfaces, each capable of supporting adjacent CPs, and that the cortical actin membrane skeleton is intimately involved with both the maintenance of existing CPs and the generation of new structures.

Recruitment of activated G protein-coupled receptors to pre-existing clathrin-coated pits in living cells

The process of clathrin-mediated endocytosis tightly regulates signaling of the superfamily of seven-transmembrane G protein-coupled receptors (GPCRs). A fundamental question in the cell biology of membrane receptor endocytosis is whether activated receptors can initiate the formation of clathrin-coated pits (CPs) or whether they are simply mobilized to pre-existing CPs. Here, using various approaches, including a dynamic assay to monitor the distribution of CPs and GPCR-beta-arrestin complexes in live HeLa cells, we demonstrate for the first time that activated GPCRs do not initiate the de novo formation of CPs but instead are targeted to pre-existing CPs.

Myosin VI, a new force in clathrin mediated endocytosis

The integrity of the actin cytoskeleton and associated motor proteins are essential for the efficient functioning of clathrin mediated endocytosis at least in polarised cells. Myosin VI, the only motor protein so far identified that moves towards the minus end of actin filaments, is the first motor protein to be shown to associate with clathrin coated pits/vesicles at the plasma membrane and to modulate clathrin mediated endocytosis. Recent kinetic studies suggest that myosin VI may move processively along actin filaments providing clues about its functions in the cell. The possible role(s) of myosin VI in the sequential steps involved in receptor mediated endocytosis are discussed.

Selective recruitment of arrestin-3 to clathrin coated pits upon stimulation of G protein-coupled receptors

Non-visual arrestins (arrestin-2 and arrestin-3) play critical roles in the desensitization and internalization of many G protein-coupled receptors. In vitro experiments have shown that both non-visual arrestins bind with high and approximately comparable affinities to activated, phosphorylated forms of receptors. They also exhibit high affinity binding, again of comparable magnitude, to clathrin. Further, agonist-promoted internalization of many receptors has been found to be stimulated by exogenous over-expression of either arrestin2 or arrestin3. The existence of multiple arrestins raises the question whether stimulated receptors are selective for a specific endogenous arrestin under more physiological conditions. Here we address this question in RBL-2H3 cells, a cell line that expresses comparable levels of endogenous arrestin-2 and arrestin-3. When (beta)(2)-adrenergic receptors are stably expressed in these cells the receptors internalize efficiently following agonist stimulation. However, by immunofluorescence microscopy we determine that only arrestin-3, but not arrestin-2, is rapidly recruited to clathrin coated pits upon receptor stimulation. Similarly, in RBL-2H3 cells that stably express physiological levels of m1AChR, the addition of carbachol selectively induces the localization of arrestin-3, but not arrestin-2, to coated pits. Thus, this work demonstrates coupling of G protein-coupled receptors to a specific non-visual arrestin in an in vivo setting.

Observing FcepsilonRI signaling from the inside of the mast cell membrane

We have determined the membrane topography of the high-affinity IgE receptor, FcstraightepsilonRI, and its associated tyrosine kinases, Lyn and Syk, by immunogold labeling and transmission electron microscopic (TEM) analysis of membrane sheets prepared from RBL-2H3 mast cells. The method of Sanan and Anderson (Sanan, D.A., and R.G.W. Anderson. 1991. J. Histochem. Cytochem. 39:1017-1024) was modified to generate membrane sheets from the dorsal surface of RBL-2H3 cells. Signaling molecules were localized on the cytoplasmic face of these native membranes by immunogold labeling and high-resolution TEM analysis. In unstimulated cells, the majority of gold particles marking both FcepsilonRI and Lyn are distributed as small clusters (2-9 gold particles) that do not associate with clathrin-coated membrane. Approximately 25% of FcepsilonRI clusters contain Lyn. In contrast, there is essentially no FcepsilonRI-Syk colocalization in resting cells. 2 min after FcepsilonRI cross-linking, approximately 10% of Lyn colocalizes with small and medium-sized FcepsilonRI clusters (up to 20 gold particles), whereas approximately 16% of Lyn is found in distinctive strings and clusters at the periphery of large receptor clusters (20-100 gold particles) that form on characteristically osmiophilic membrane patches. While Lyn is excluded, Syk is dramatically recruited into these larger aggregates. The clathrin-coated pits that internalize cross-linked receptors bud from membrane adjacent to the Syk-containing receptor complexes. The sequential association of FcstraightepsilonRI with Lyn, Syk, and coated pits in topographically distinct membrane domains implicates membrane segregation in the regulation of FcstraightepsilonRI signaling.

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Phosphoinositide-AP-2 interactions required for targeting to plasma membrane clathrin-coated pits

The clathrin-associated AP-2 adaptor protein is a major polyphosphoinositide-binding protein in mammalian cells. A high affinity binding site has previously been localized to the NH(2)-terminal region of the AP-2 alpha subunit (Gaidarov et al. 1996. J. Biol. Chem. 271:20922-20929). Here we used deletion and site- directed mutagenesis to determine that alpha residues 21-80 comprise a discrete folding and inositide-binding domain. Further, positively charged residues located within this region are involved in binding, with a lysine triad at positions 55-57 particularly critical. Mutant peptides and protein in which these residues were changed to glutamine retained wild-type structural and functional characteristics by several criteria including circular dichroism spectra, resistance to limited proteolysis, and clathrin binding activity. When expressed in intact cells, mutated alpha subunit showed defective localization to clathrin-coated pits; at high expression levels, the appearance of endogenous AP-2 in coated pits was also blocked consistent with a dominant-negative phenotype. These results, together with recent work indicating that phosphoinositides are also critical to ligand-dependent recruitment of arrestin-receptor complexes to coated pits (Gaidarov et al. 1999. EMBO (Eur. Mol. Biol. Organ.) J. 18:871-881), suggest that phosphoinositides play a critical and general role in adaptor incorporation into plasma membrane clathrin-coated pits.

The neuropeptide Y Y1 receptor is a somatic receptor on dorsal root ganglion neurons and a postsynaptic receptor on somatostatin dorsal horn neurons

Using indirect immunofluorescence, neuropeptide Y Y1 receptor (Y1 receptor)-like immunoreactivity (LI) was localized close to the plasmalemma of small neurons in lumbar dorsal root ganglia (DRGs) and neurons in the inner lamina II of the lumbar spinal cord of the rat. Using confocal microscopy, colocalization of Y1 receptor-LI and transferrin receptor-LI, a marker for endosomes and coated vesicles, was observed in dot-like structures along the plasmalemma. Under the electron microscope, Y1 receptor-LI was localized in coated vesicles and endosomes, in the membrane of tubular cisternae, sometimes connected to multivesicular bodies, and in the plasmalemma. These complex distribution patterns may reflect receptor turnover and internalization processes. In the lamina II of the spinal dorsal horn, Y1 receptor-LI was localized in the plasmalemma of neurons without any apparent association with paramembrane structures, as described above for the DRG neurons. Many dendrites were Y1 receptor-positive, and some of them made synaptic contacts with unstained axonal terminals. In general, Y1 receptor-LI was localized in the membrane outside the postsynaptic density. Double-immunofluorescence staining showed that most Y1 receptor-immunoreactive neurons in lamina II contained somatostatin-LI. Both in DRG and dorsal horn neurons, the Y1 receptor thus seems to represent a postjunctional/postsynaptic receptor.

Coupled inositide phosphorylation and phospholipase D activation initiates clathrin-coat assembly on lysosomes

Adaptors appear to control clathrin-coat assembly by determining the site of lattice polymerization but the nucleating events that target soluble adaptors to an appropriate membrane are poorly understood. Using an in vitro model system that allows AP-2-containing clathrin coats to assemble on lysosomes, we show that adaptor recruitment and coat initiation requires phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) synthesis. PtdIns(4,5)P2 is generated on lysosomes by the sequential action of a lysosome-associated type II phosphatidylinositol 4-kinase and a soluble type I phosphatidylinositol 4-phosphate 5-kinase. Phosphatidic acid, which potently stimulates type I phosphatidylinositol 4-phosphate 5-kinase activity, is generated on the bilayer by a phospholipase D1-like enzyme located on the lysosomal surface. Quenching phosphatidic acid function with primary alcohols prevents the synthesis of PtdIns(4, 5)P2 and blocks coat assembly. Generating phosphatidic acid directly on lysosomes with exogenous bacterial phospholipase D in the absence of ATP still drives adaptor recruitment and limited coat assembly, indicating that PtdIns(4,5)P2 functions, at least in part, to activate the PtdIns(4,5)P2-dependent phospholipase D1. These results provide the first direct evidence for the involvement of anionic phospholipids in clathrin-coat assembly on membranes and define the enzymes responsible for the production of these important lipid mediators.

Acute cholesterol depletion inhibits clathrin-coated pit budding

Many biologically important macromolecules are internalized into cells by clathrin-coated pit endocytosis. The mechanism of clathrin-coated pit budding has been investigated intensively, and considerable progress has been made in characterizing the proteins involved in internalization. Membrane lipid composition and the lateral organization of lipids and proteins within membranes are believed to play an important role in the regulation of membrane-trafficking processes. Here we report that membrane cholesterol plays a critical role in clathrin-coated pit internalization. We show that acute cholesterol depletion, using beta-methyl-cyclodextrin, specifically reduces the rate of internalization of transferrin receptor by more than 85%, without affecting intracellular receptor trafficking back to the cell surface. The effect on endocytosis is attributable to a failure of coated pits to detach from the plasma membrane, as visualized by using a green fluorescent protein-clathrin conjugate in living cells. Ultrastructural studies indicate that acute cholesterol depletion causes accumulation of flat-coated membranes and a corresponding decrease in deep-coated pits, consistent with the possibility that flat clathrin lattices are direct precursors of indented pits and endocytic vesicles in intact cells. We conclude that clathrin is unable to induce curvature in the membrane depleted of cholesterol.

Spatial control of coated-pit dynamics in living cells

Here we visualize new aspects of the dynamics of endocytotic clathrin-coated pits and vesicles in mammalian cells by using a fusion protein consisting of green fluorescent protein and clathrin light chain a. Clathrin-coated pits invaginating from the plasma membrane show definite, but highly limited, mobility within the membrane that is relaxed upon treatment with latrunculin B, an inhibitor of actin assembly, indicating that an actin-based framework may be involved in the mobility of these pits. Transient, motile coated vesicles that originate from coated pits can be detected, with multiple vesicles occasionally appearing to emanate from a single pit. Despite their seemingly random distribution, coated pits tend to form repeatedly at defined sites while excluding other regions. This spatial regulation of coated-pit assembly and function is attributable to the attachment of the coated pits to the membrane skeleton.

EGF receptor signaling stimulates SRC kinase phosphorylation of clathrin, influencing clathrin redistribution and EGF uptake

Epidermal growth factor (EGF) binding to its receptor causes rapid phosphorylation of the clathrin heavy chain at tyrosine 1477, which lies in a domain controlling clathrin assembly. EGF-mediated clathrin phosphorylation is followed by clathrin redistribution to the cell periphery and is the product of downstream activation of SRC kinase by EGF receptor (EGFR) signaling. In cells lacking SRC kinase, or cells treated with a specific SRC family kinase inhibitor, EGF stimulation of clathrin phosphorylation and redistribution does not occur, and EGF endocytosis is delayed. These observations demonstrate a role for SRC kinase in modification and recruitment of clathrin during ligand-induced EGFR endocytosis and thereby define a novel effector mechanism for regulation of endocytosis by receptor signaling.

High-affinity binding of the AP-1 adaptor complex to trans-golgi network membranes devoid of mannose 6-phosphate receptors

The GTP-binding protein ADP-ribosylation factor (ARF) initiates clathrin-coat assembly at the trans-Goli network (TGN) by generating high-affinity membrane-binding sites for the AP-1 adaptor complex. Both transmembrane proteins, which are sorted into the assembling coated bud, and novel docking proteins have been suggested to be partners with GTP-bound ARF in generating the AP-1-docking sites. The best characterized, and probably the major transmembrane molecules sorted into the clathrin-coated vesicles that form on the TGN, are the mannose 6-phosphate receptors (MPRs). Here, we have examined the role of the MPRs in the AP-1 recruitment process by comparing fibroblasts derived from embryos of either normal or MPR-negative animals. Despite major alterations to the lysosome compartment in the MPR-deficient cells, the steady-state distribution of AP-1 at the TGN is comparable to that of normal cells. Golgi-enriched membranes prepared from the receptor-negative cells also display an apparently normal capacity to recruit AP-1 in vitro in the presence of ARF and either GTP or GTPgammaS. The AP-1 adaptor is recruited specifically onto the TGN and not onto the numerous abnormal membrane elements that accumulate within the MPR-negative fibroblasts. AP-1 bound to TGN membranes from either normal or MPR-negative fibroblasts is fully resistant to chemical extraction with 1 M Tris-HCl, pH 7, indicating that the adaptor binds to both membrane types with high affinity. The only difference we do note between the Golgi prepared from the MPR-deficient cells and the normal cells is that AP-1 recruited onto the receptor-lacking membranes in the presence of ARF1.GTP is consistently more resistant to extraction with Tris. Because sensitivity to Tris extraction correlates well with nucleotide hydrolysis, this finding might suggest a possible link between MPR sorting and ARF GAP regulation. We conclude that the MPRs are not essential determinants in the initial steps of AP-1 binding to the TGN but, instead, they may play a regulatory role in clathrin-coated vesicle formation by affecting ARF.GTP hydrolysis.

Clathrin, adaptors and eps15 in endosomes containing activated epidermal growth factor receptors

Activation of the epidermal growth factor receptor (EGFR) by EGF results in binding of clathrin adaptor protein complex AP-2 to the receptor cytoplasmic tail. The transient interaction with AP-2 is thought to be responsible for the selective recruitment of the EGFR into coated pits during endocytosis. In this study we found that EGF-induced EGFR/AP-2 association, measured by co-immunoprecipitation, persists after receptor internalization. Double-label immunofluorescence of EGF-treated A-431 and COS-1 cells revealed the presence of AP-2, clathrin and eps15, another component of the plasma membrane coated pits, in the large perinuclear endosomes loaded with EGFRs. By optical sectioning and image deconvolution, the immunoreactivities were seen to be distributed within vesicular and tubular elements of these endosomes. In addition, these compartments contained the transferrin receptors and a EEA.1 protein, markers of early endosomes. Furthermore, Golgi clathrin adaptor complex AP-1 was found in EGFR-containing endosomes and EGFR immunoprecipitates in A-431 cells. The direct interaction of the EGFR with micro1 as well as micro2 subunits of AP-1 and AP-2, correspondingly, was shown using the yeast two-hybrid assay. Brefeldin A, a drug that releases AP-1 from the trans-Golgi membranes, had no effect on AP-1 association with endosomes and its co-precipitation with EGFR. Taken together, the data suggest that endosomal EGFR-AP complexes make up a significant portion of the total amount of these complexes detectable by co-immunoprecipitation. It can be proposed that APs are capable of binding to the endosomal membrane via a mechanism that requires AP interaction with the intracellular tails of multimeric receptors like activated EGFR, which in turn allows recruitment of clathrin and eps15. The hypothesis that the competition between adaptor complexes for binding to the receptor tails in endosomes may regulate of the sorting of receptors is discussed.

Protein transport from the secretory to the endocytic pathway in mammalian cells

The trans-Golgi network (TGN) is the last station of the secretory pathway where soluble and membrane proteins are sorted for subsequent transport to endocytic compartments. This pathway is primarily followed by two distinct but related mannose 6-phosphate receptors which exhibit complementary functions in soluble lysosomal enzyme targeting. These transmembrane proteins and their bound ligands are packaged in transport intermediates coated with clathrin and the AP-1 assembly complex. Their segregation is determined by the interaction of tyrosine- and di-leucine-based sorting determinants present in their cytoplasmic domains with AP-1. Other membrane proteins such as the lysosomal membrane glycoproteins or envelope glycoproteins of herpes viruses, which contain similar sorting signals, may also follow the same pathway. In this review, we will summarize our current understanding of the molecular mechanisms leading to membrane protein sorting in the TGN and the formation of AP-1-coated transport intermediates.

Endocytic clathrin-coated pit formation is independent of receptor internalization signal levels

The mechanisms responsible for coated pit formation in cells remain unknown, but indirect evidence has argued both for and against a critical role of receptor cytoplasmic domains in the process. If the endocytic motifs of receptors are responsible for recruiting AP2 to the plasma membrane, thereby driving coated pit formation, then the level of constitutively internalized receptors at the membrane would be expected to govern the steady-state level of coated pits in cells. Here we directly test this hypothesis for broad classes of receptors containing three distinct constitutive internalization signals. Chimeric proteins consisting of an integral membrane reporter protein (Tac) coupled to cytoplasmic domains bearing tyrosine-, di-leucine-, or acidic cluster/casein kinase II-based internalization signals were overexpressed to levels that saturated the internalization pathway. Quantitative confocal immunofluorescence microscopy indicated that the number of plasma membrane clathrin-coated pits and the concentration of their structural components were invariant when comparing cells expressing saturating levels of the chimeric receptors to nonexpressing cells or to cells expressing only the Tac reporter lacking cytoplasmic internalization signals. Biochemical analysis showed that the distribution of coat proteins between assembled coated pits and soluble pools was also not altered by receptor overexpression. Finally, the cellular localizations of AP2 and AP1 were similarly unaffected. These results provide a clear indication that receptor endocytic signals do not determine coated pit levels by directly recruiting AP2 molecules. Rather, the findings support a model in which coated pit formation proceeds through recruitment and activation of AP2, likely through a limited number of regulated docking sites that act independently of endocytic signals.

Dileucine-based sorting signals bind to the beta chain of AP-1 at a site distinct and regulated differently from the tyrosine-based motif-binding site

In previous work, we showed that peptides from endocytosed proteins containing the tyrosine YXXphi sorting motif are recognized by the mu 2 subunit of AP-2, the plasma membrane clathrin adaptor protein complex. This interaction is activated by phosphoinositide lipids that are phosphorylated at the D-3 position of the inositol ring, and is also enhanced by the formation of clathrin-AP-2 coats. Here, we describe the detection of a specific interaction between peptides containing a second sorting motif, the dileucine motif, and AP-1, the clathrin adaptor complex responsible for sorting proteins at the trans-Golgi network (TGN). Surprisingly, the site of dileucine binding is the beta1 subunit, not mu 1. A YXXphi-containing peptide from a protein trafficked within the TGN does bind to mu 1, however. Phosphatidylinositol 3,4-diphosphate and 3,4, 5-triphosphate did not activate the interaction between dileucine-containing peptides and AP-1 but instead inhibited it, and clathrin-AP-1 coat formation did not alter the interaction. Thus, there are at least two physically separate binding sites for sorting signals on APs, which are also regulated independently.

Modulation of intracellular transport by transported proteins: insight from regulation of COPI-mediated transport

Intracellular transport is best understood for how proteins are shuttled among different compartments of the secretory pathway by membrane-bound transport carriers. However, it remains unclear whether regulation of this transport is modulated by the transported (cargo) proteins in the lumen of transport pathways. In the early secretory pathways that connect the endoplasmic reticulum (ER) and the Golgi complex, the small GTPase ADP-ribosylation factor 1 (ARF1) recruits a cytosolic coat protein complex named COPI onto membranes as a key step in the formation of transport vesicles. Transport of newly synthesized proteins that leave the ER includes a class of cargo proteins with a sequence motif of KDEL. When these KDEL proteins leave the ER to reach the Golgi complex, they are recognized by their receptor and transported retrograde in COPI-coated vesicles back to the ER. We now demonstrate that stimulation of the KDEL receptor by a KDEL protein enhances an interaction between the KDEL receptor and a GTPase-activating protein for ARF1. As a result, more cytosolic GTPase-activating protein is recruited to membranes to inactivate ARF1. Thus, the KDEL proteins are examples of luminal cargo proteins that regulate transport by activating their receptor. Most likely, this regulation affects retrograde transport from the Golgi complex to the ER, as activated KDEL receptor appears to reside only in retrograde COPI-coated vesicles.

Modulation of the arrestin-clathrin interaction in cells. Characterization of beta-arrestin dominant-negative mutants

We recently demonstrated that nonvisual arrestins interact via a C-terminal binding domain with clathrin and function as adaptor proteins to promote beta2-adrenergic receptor (beta2AR) internalization. Here, we investigated the potential utility of a mini-gene expressing the clathrin-binding domain of beta-arrestin (beta-arrestin (319-418)) to function as a dominant-negative with respect to beta2AR internalization and compared its properties with those of beta-arrestin and beta-arrestin-V53D, a previously reported dominant-negative mutant. In vitro studies demonstrated that beta-arrestin-V53D bound better to clathrin than beta-arrestin but was significantly impaired in its interaction with phosphorylated G protein-coupled receptors. In contrast, whereas beta-arrestin (319-418) also bound well to clathrin it completely lacked receptor binding activity. When coexpressed with the beta2AR in HEK293 cells, beta-arrestin (319-418) effectively inhibited agonist-promoted receptor internalization, whereas beta-arrestin-V53D was only modestly effective. However, both constructs significantly inhibited the stimulation of beta2AR internalization by beta-arrestin in COS-1 cells. Interestingly, immunofluorescence microscopy analysis reveals that both beta-arrestin (319-418) and beta-arrestin-V53D are constitutively localized in clathrin-coated pits in COS-1 cells. These results indicate the potential usefulness of beta-arrestin (319-418) to effectively block arrestin-clathrin interaction in cells and suggest that this construct may prove useful in further defining the mechanisms involved in G protein-coupled receptor trafficking.

Nef-mediated clathrin-coated pit formation

The sequence of events leading to clathrin-coated pit (CCP) nucleation on the cell surface and to the incorporation of receptors into these endocytic structures is still imperfectly understood. In particular, the question remains as to whether receptor tails initiate the assembly of the coat proteins or whether receptors migrate into preformed CCP. This question was approached through a dissection of the mechanisms implemented by Nef, an early protein of human and simian immunodeficiency virus (HIV and SIV, respectively), to accelerate the endocytosis of cluster of differentiation antigen type 4 (CD4), the major receptor for these viruses. Results collected showed that: (a) Nef promotes CD4 internalization via an increased association of CD4 with CCP; (b) the Nef-mediated increase of CD4 association with CCP is related to a doubling of the plasma membrane area occupied by clathrin-coated structures; (c) this increased CCP number at the plasma membrane has functional consequences preferentially on CD4 uptake and does not significantly affect transferrin receptor internalization or fluid-phase endocytosis; (d) the presence of a CD4 cytoplasmic tail including a critical dileucine motif is required to induce CCP formation via Nef; and (e) when directly anchored to the cytoplasmic side of the plasma membrane, Nef itself can promote CCP formation. Taken together, these observations lead us to propose that CD4 can promote CCP generation via the connector molecule Nef. In this model, Nef interacts on one side with CD4 through a dileucine-based motif present on CD4 cytoplasmic tail and on the other side with components of clathrin-coated surface domain (i.e., adaptins). These Nef-generated complexes would then initiate the nucleation of CCP.

Arrestin/clathrin interaction. Localization of the arrestin binding locus to the clathrin terminal domain

Previously we demonstrated that nonvisual arrestins exhibit a high affinity interaction with clathrin, consistent with an adaptor function in the internalization of G protein-coupled receptors (Goodman, O. B., Jr., Krupnick, J. G., Santini, F., Gurevich, V. V., Penn, R. B., Gagnon, A. W., Keen, J. H., and Benovic, J. L. (1996) Nature 383, 447-450). In this report we show that a short sequence of highly conserved residues within the globular clathrin terminal domain is responsible for arrestin binding. Limited proteolysis of clathrin cages results in the release of terminal domains and concomitant abrogation of arrestin binding. The nonvisual arrestins, beta-arrestin and arrestin3, but not visual arrestin, bind specifically to a glutathione S-transferase-clathrin terminal domain fusion protein. Deletion analysis and alanine scanning mutagenesis localize the binding site to residues 89-100 of the clathrin heavy chain and indicate that residues 1-100 can function as an independent arrestin binding domain. Site-directed mutagenesis identifies an invariant glutamine (Glu-89) and two highly conserved lysines (Lys-96 and Lys-98) as residues critical for arrestin binding, complementing hydrophobic and acidic residues in arrestin3 which have been implicated in clathrin binding (Krupnick, J. G., Goodman, O. B., Jr., Keen, J. H., and Benovic, J. L. (1997) J. Biol. Chem. 272, 15011-15016). Despite exhibiting high affinity clathrin binding, arrestins do not induce coat assembly. The terminal domain is oriented toward the plasma membrane in coated pits, and its binding of both arrestins and AP-2 suggests that this domain is the anchor responsible for adaptor-receptor recruitment to the coated pit.

Regulatory interactions in the recognition of endocytic sorting signals by AP-2 complexes

Many plasma membrane proteins destined for endocytosis are concentrated into clathrin-coated pits through the recognition of a tyrosine-based motif in their cytosolic domains by an adaptor (AP-2) complex. The mu2 subunit of isolated AP-2 complexes binds specifically, but rather weakly, to proteins bearing the tyrosine-based signal. We now demonstrate, using peptides with a photoreactive probe, that this binding is strengthened significantly when the AP-2 complex is present in clathrin coats, indicating that there is cooperativity between receptor-AP-2 interactions and coat formation. Phosphoinositides with a phosphate at the D-3 position of the inositol ring, but not other isomers, also increase the affinity of the AP-2 complex for the tyrosine-based motif. AP-2 is the first protein known (in any context) to interact with phosphatidylinositol 3-phosphate. Our findings indicate that receptor recruitment can be coupled to clathrin coat assembly and suggest a mechanism for regulation of membrane traffic by lipid products of phosphoinositide 3-kinases.

Mannose 6-phosphate receptors regulate the formation of clathrin-coated vesicles in the TGN

The transport of the two mannose 6-phosphate receptors (MPRs) from the secretory pathway to the endocytic pathway is mediated by carrier vesicles coated with the AP-1 Golgi-specific assembly protein and clathrin. Using an in vitro assay that reconstitutes the ARF-1-dependent translocation of cytosolic AP-1 onto membranes of the TGN, we have previously reported that the MPRs are key components for the efficient recruitment of AP-1 (Le Borgne, R., G. Griffiths, and B. Hoflack. 1996. J. Biol. Chem. 271:2162-2170). Using a polyclonal antibody against the mouse gamma-adaptin, we have now examined the steady state distribution of AP-1 after subcellular fractionation of mouse fibroblasts lacking both MPRs or reexpressing physiological levels of either MPR. We report that the amount of AP-1 bound to membranes and associated with clathrin-coated vesicles depends on the expression level of the MPRs and on the integrity of their cytoplasmic domains. Thus, these results indicate that the concentration of the MPRs, i.e., the major transmembrane proteins sorted toward the endosomes, determines the number of clathrin-coated vesicles formed in the TGN.

Saturation of the endocytic pathway for the transferrin receptor does not affect the endocytosis of the epidermal growth factor receptor

Cell-surface receptors that undergo clathrin-mediated endocytosis contain short amino acid sequences in their cytoplasmic domain that serve as internalization signals. Interactions between these sequences and components of the endocytic machinery should become limiting upon overexpression of the constitutively recycling transferrin receptor (TfR). A tetracycline-responsive system was used to induce overexpression of the TfR up to 20-fold in HeLa cells. Internalization assays indicate the rate of 125I-transferrin uptake per surface TfR is reduced by a factor of 4 in induced cells. Consistent with endocytosis being the rate-limiting step, TfRs shift from an endosomal to more of a plasma membrane distribution with TfR overexpression. The clathrin-associated protein AP-2 has been proposed to interact directly with the cytoplasmic domain of many receptors, yet no changes in the amount or distribution of AP-2 were detected in induced cells. The internalization rate for the epidermal growth factor receptor was also measured, with or without induction of TfR expression. Even though endocytosis of the TfR is saturated in induced cells, 125I-labeled epidermal growth factor continues to be internalized at a rate identical to that seen in uninduced cells. We propose that there are different limiting steps for the endocytosis of these two receptors.