Impacts of increased α-synuclein on clathrin-mediated endocytosis at synapses:implications for neurodegenerative diseases

α-Synuclein causes synaptic pathologies in several neurodegenerative diseases：Parkinson’s disease（PD）is a neurodegenerative disease that impacts the lives of millions of people worldwide.A pathological hallmark of PD,as well as dementia with Lewy bodies（DLB）and several Alzheimer’s disease variants,is the appearanceof intracellular inclusions called Lewy bodies, which contain high levels of aggregated α-synuclein,

Numerical study of clathrin-mediated endocytosis of nanoparticles by cells under tension

In this study, a three-dimensional mathematical model was used to study the contribution of clathrins during the process of cellular uptake of spherical nanoparticles under different membrane tensions. The clathrin-coated pit (CCP) that forms around the inward budding of the cell membrane was modeled as a vesicle with bending rigidity. An optimization algorithm was proposed for minimizing the total energy of the system, which comprises the deforming nanoparticle, receptor-ligand bonds, cell membrane, and CCP, in which way, the profile of the system is acquired. The results showed that the CCP enable full wrapping of the nanoparticles at various membrane tensions. When the cell membrane tension increases, the total deformation energy also increases, but the ratio of CCP bending to the minimum value of the total energy of the system decreases. The results also showed that the diameter of the endocytic vesicles determined by the competition between the stretching of the cell membrane and confinement of the coated pits are much larger than the nanoparticles, which is quit different as the results in passive endocytosis that is not facilitated by the CCPs. The present results indicate that variations of tension on cell membranes constitutes a biophysical marker for understanding the size distribution of CCPs observed in experiments. The present results also suggest that the early abortion of endocytosis is related to that the receptor-ligand bonds cannot generate adequate force to wrap the nanoparticles into the cell membrane before the clathrins respond to support the endocytic vesicles. Correspondingly, late abortion may relate to the inability of CCPs to confine the nanoparticles until the occurrence of the necking stage of endocytosis.

Osmotic Stress Modulates the Balance between Exocytosis and Clathrin-Mediated Endocytosis in Arabidopsis thaliana

The sessile life style of plants creates the need to deal with an often adverse environment, in which water availability can change on a daily basis, challenging the cellular physiology and integrity. Changes in os- motic conditions disrupt the equilibrium of the plasma membrane： hypoosmotic conditions increase and hyperosmotic environment decrease the cell volume. Here, we show that short-term extracellular osmotic treatments are closely followed by a shift in the balance between endocytosis and exocytosis in root mer- istem cells. Acute hyperosmotic treatments （ionic and nonionic） enhance clathrin-mediated endocytosis simultaneously attenuating exocytosis, whereas hypoosmotic treatments have the opposite effects. In addition to clathrin recruitment to the plasma membrane, components of early endocytic trafficking are essential during hyperosmotic stress responses. Consequently, growth of seedlings defective in elements of clathrin or early endocytic machinery is more sensitive to hyperosmotic treatments. We also found that the endocytotic response to a change of osmotic status in the environment is dominant over the presum- ably evolutionary more recent regulatory effect of plant hormones, such as auxin. These results imply that osmotic perturbation influences the balance between endocytosis and exocytosis acting through clathrin- mediated endocytosis. We propose that tension on the plasma membrane determines the addition or removal of membranes at the cell surface, thus preserving cell integrity.

Trafficking pathway between plasma membrane and mitochondria via clathrin-mediated endocytosis

Endocytosis is a basic cellular process that describes a form of active transport across the plasma membrane into the cell.The endocytic pathway consists of distinct membrane compartments;internalized molecules are delivered to early endosomes,and some of them are recycled back to the surface,whereas other molecules are sent to late endosomes and lysosomes for degradation.However,little is known about how mitochondria are involved in the endocytic pathway.Here,we report that FM dyes, membrane-impermeant fluorescent lipid probes,can traffic to mitochondria directly from the plasma membrane by clathrinmediated endocytosis.FM dye entry into mitochondria uses microtubule-dependent active transport,but the mechanism is different from the classical endocytic pathway.Hence,this study reveals a previously unrealized lipid trafficking pathway from the plasma membrane to mitochondria.

Endocytosis unplugged： multiple ways to enter the cell

Endocytosis occurs at the cell surface and involves internalization of the plasma membrane （PM） along with its constituent membrane proteins and lipids. Endocytosis is involved in sampling of the extracellular milieu and also serves to regulate various processes initiated at the cell surface. These include nutrient uptake, signaling from cell- surface receptors, and many other processes essential for cell and tissue functioning in metazoans. It is also central to the maintenance of PM lipid and protein homeostasis. There are multiple means of internalization that operate concurrently, at the cell surface. With advancement in high-resolution visualization techniques, it is now possible to track multiple endocytic cargo at the same time, revealing a remarkable diversity of endocytic processes in a single cell. A combination of live cell imaging and efficient genetic manipulations has also aided in understanding the functional hierarchy of molecular players in these mechanisms of internalization. Here we provide an account of various endocytic routes, their mechanisms of operation and occurrence across phyla.

The endocytosis and intracellular fate of nanomedicines: Implication for rational design

Nanomedicines employ multiple endocytic pathways to enter cells.Their following fate is interesting,but it is not sufficient understood currently.This review introduces the endocytic pathways,presents new technologies to confirm the specific endocytic pathways and discusses factors for pathway selection.In addition,some intriguing implication about nanomedicine design based on endocytosis will also be discussed at the end.This review may provide new thoughts for the design of novel multifunctional nanomedicines.

Nexus of signaling and endocytosis in oncogenesis driven by non-small cell lung cancer-associated epidermal growth factor receptor mutants

Epidermal growth factor receptor(EGFR) controls a wide range of cellular processes, and aberrant EGFR signaling as a result of receptor overexpression and/or mutation occurs in many types of cancer. Tumor cells in non-small cell lung cancer(NSCLC) patients that harbor EGFR kinase domain mutations exhibit oncogene addiction to mutant EGFR, which confers high sensitivity to tyrosine kinase inhibitors(TKIs). As patients invariably develop resistance to TKIs, it is important to delineate the cell biological basis of mutant EGFR-induced cellular transformation since components of these pathways can serve as alternate therapeutic targets to preempt or overcome resistance. NSCLC-associated EGFR mutants are constitutively-active and induce ligandindependent transformation in nonmalignant cell lines. Emerging data suggest that a number of factors are critical for the mutant EGFR-dependent tumorigenicity, and bypassing the effects of TKIs on these pathways promotes drug resistance. For example, activation of downstream pathways such as Akt, Erk, STAT3 and Src is critical for mutant EGFR-mediated biological processes. It is now well-established that the potency and spatiotemporal features of cellular signaling by receptor tyrosine kinases such as EGFR, as well as the specific pathways activated, is determined by the nature of endocytic traffic pathways through which the active receptors traverse. Recent evidence indicates that NSCLCassociated mutant EGFRs exhibit altered endocytic trafficking and they exhibit reduced Cbl ubiquitin ligasemediated lysosomal downregulation. More recent work has shown that mutant EGFRs undergo ligand-independent traffic into the endocytic recycling compartment, a behavior that plays a key role in Src pathway activation and oncogenesis. These studies are beginning to delineate the close nexus between signaling and endocytic traffic of EGFR mutants as a key driver of oncogenicprocesses. Therefore, in this review, we will discuss the links between mutant EGFR signaling and endocytic properties, and introduce potential mechanisms by which altered endocytic properties of mutant EGFRs may alter signaling and vice versa as well as their implications for NSCLC therapy.

Endocytosis of adiponectin receptor I through a clathrin- and Rab5-dependent pathway

In eukaryotic cells, receptor endocytosis is a key event regulating signaling transduction. Adiponectin receptors belong to a new receptor family that is distinct from G-protein-coupled receptors and has critical roles in the pathogenesis of diabetes and metabolic syndrome. Here, we analyzed the endocytosis of adiponectin and adiponectin receptor 1 （AdipoR1） and found that they are both internalized into transferrin-positive compartments that follow similar traffic routes. Blocking clathrin-mediated endocytosis by expressing Eps15 mutants or depleting K^＋ trapped AdipoR1 at the plasma membrane, and K^＋ depletion abolished adiponectin internalization, indicating that the endocytosis of AdipoR1 and adiponectin is clathrin-dependent. Depletion of K^＋ and overexpression of Eps15 mutants enhance adiponectin- stimulated AMP-activated protein kinase phosphorylation, suggesting that the endocytosis of AdipoR1 might down-regulate adiponectin signaling. In addition, AdipoR1 colocalizes with the small GTPase Rab5, and a dominant negative Rab5 abrogates AdipoR1 endocytosis. These data indicate that AdipoR1 is internalized through a clathrin- and Rab5- dependent pathway and that endocytosis may play a role in the regulation of adiponectin signaling.

Mechanical force-driven TNFαendocytosis governs stem cell homeostasis

Mesenchymal stem cells(MSCs)closely interact with the immune system,and they are known to secrete inflammatory cytokines in response to stress stimuli.The biological function of MSC-derived inflammatory cytokines remains elusive.Here,we reveal that even under physiological conditions,MSCs produce and release a low level of tumor necrosis factor alpha(TNFα),which is unexpectedly required for preserving the self-renewal and differentiation of MSCs via autocrine/paracrine signaling.Furthermore,TNFαcritically maintains MSC function in vivo during bone homeostasis.Mechanistically,we unexpectedly discovered that physiological levels of TNFαsafeguard MSC homeostasis in a receptor-independent manner through mechanical force-driven endocytosis and that endocytosed TNFαbinds to mammalian target of rapamycin(mTOR)complex 2 and restricts mTOR signaling.Importantly,inhibition of mTOR signaling by rapamycin serves as an effective osteoanabolic therapeutic strategy to protect against TNFαdeficiency and mechanical unloading.Collectively,these findings unravel the physiological framework of the dynamic TNFαshuttlebased mTOR equilibrium that governs MSC and bone homeostasis.

Effects of grafting cell penetrate peptide and RGD on endocytosis and biological effects of Mg-CaPNPs-CKIP-1 siRNA carrier system in vitro

Calcium phosphate nanoparticles(CaPNPs)have good biocompatibility as gene carriers;however,CaPNPs typically exhibit a low transfection efficiency.Cell penetrate peptide(TAT)can increase the uptake of nanoparticles but is limited by its non-specificity.Grafting adhesion peptide adhesion peptide on carriers can enhance their targeting.The Plekho1 gene encodes casein kinase-2 interacting protein-1(CKIP-1),which can negatively regulate osteogenic differentiation.Based on the above,we produced a Mg-CaPNPs-RGD-TAT-CKIP-1 siRNA carrier system via hydrothermal synthesis,silanization and adsorption.The effects of this carrier system on cell endocytosis and biological effects were evaluated by cell culture in vitro.The results demonstrate that CaPNPs with 7%Mg(60 nm particle size,short rod shape and good dispersion)were suitable for use as gene carriers.The carrier system boosted the endocytosis of MG63 cells and was helpful for promoting the differentiation of osteoblasts,and the dual-ligand system possessed a synergistic effect.The findings of this study show the tremendous potential of the Mg-CaPNPs-RGD-TAT-CKIP-1 siRNA carrier system for efficient delivery into cells and osteogenesis inducement.

Endocytosis of FcαR is clathrin and dynamin dependent, but its cytoplasmic domain is not required

FcαR, the Fc receptor for IgA, is essential for IgA-mediated immune responses. Previous studies have shown that IgA and IgA immune complexes can be rapidly endocytosed by FcαR. However, the underlying mechanism remains unclear. Here, we investigated the endocytic pathway of FcαR in monocytic cell line, U937, that naturally express FcuR and in transfected Chinese hamster ovary （CHO）, COS-7 and Hela cells. By using selective chemical inhibitors of different endocytic pathways, overexpression of dominant-negative mutants of Eps15 and knockdown of clathrin heavy chain （CHC） via RNA interference, we demonstrated that endocytosis of FcaR was through a clathrin-mediated pathway. The endocytosed FcαR went into Rab5- and Rabll-positive endosomes. However, endocytosis of FcaR could not be blocked by a dominant-negative mutant of Rab5. We also demonstrated that endocytosis of FcαR was dynamin-dependent by overexpressing a dominant-negative mutant of dynamin. The potential endocytic motif for FcαR was also examined. Unexpectedly, we found that the entire cytoplasmic domain of FcaR was not required for the endocytic process of FcαR. We conclude that endocytosis of FcaR is clathrin- and dynamin-dependent, but is not regulated by RabS, and the endocytic motif is not located in the cytoplasmic domain of FcαR.

A Numerical Study of Passive Receptor-Mediated Endocytosis of Nanoparticles:The Effect of Mechanical Properties

In this work,a three-dimensional axisymmetric model with nanoparticle,receptor-ligand bonds and cell membrane as a system was used to study the quasi-static receptor-mediated endocytosis process of spherical nanoparticles in drug delivery.The minimization of the system energy function was carried out numerically,and the deformations of nanoparticle,receptor-ligand bonds and cell membrane were predicted.Results show that passive endocytosis may fail due to the rupture of receptor-ligand bonds during the wrapping process,and the size and rigidity of nanoparticles affect the total deformation energy and the terminal wrapping stage.Our results suggest that,in addition to the energy requirement,the success of passive endocytosis also depends on the maximum strength of the receptor-ligand bonds.

Overexpression of Toll-like receptor 4 contributes to the internalization and elimination of Escherichia coli in sheep by enhancing caveolae-dependent endocytosis

Background:Gram-negative bacterial infections have a major economic impact on both the livestock industry and public health.Toll-like receptor 4(TLR4)plays a crucial role in host defence against Gram-negative bacteria.Exploring the defence mechanism regulated by TLR4 may provide new targets for treatment of inflammation and control of bacterial infections.In a previous study,we generated transgenic sheep overexpressing TLR4 by microinjection to improve disease resistance.The defence mechanism through which TLR4 overexpression protected these sheep against pathogens is still not fully understood.Results:In the present study,we used Escherichia coli to infect monocytes isolated from peripheral blood of the animal model.The overexpression of TLR4 strongly enhanced the percentage of endocytosis and capacity of elimination in monocytes during the early stages of infection.This phenomenon was mainly due to overexpression of TLR4 promoting caveolae-mediated endocytosis.Pretreatment of the transgenic sheep monocytes with inhibitors of TLR4,Src signalling,or the caveolae-mediated endocytosis pathway reduced the internalization of bacteria,weakened the ability of the monocytes to eliminate the bacteria,and increased the pH of the endosomes.Conclusion:Together,our results reveal the effects of TLR4 on the control of E.coli infection in the innate immunity of sheep and provide crucial evidence of the caveolae-mediated endocytosis pathway required for host resistance to invading bacteria in a large animal model,providing theoretical support for breeding disease resistance in the future.Furthermore,Src and caveolin 1(CAV1)could be potentially valuable targets for the control of infectious diseases.

The relationship between endocytosis of peritoneal macrophages induced by concanavalin A and intracellular free calcium in mouse

In this experiment the morphological changes of mouse peritoneal macrophages in the course of their conjugation with colloidal gold-labelled concanavalin A(ConA-Au) i by the surface receptor and then the endocytosis and transport of the ConA were observed

Localization of proteins involved in endocytosis at tubulobulbar complexes in rat testes

Tubulobulbar Complexes (TBCs) are actin-rich structures formed between Sertoli-cells and spermatids at the time of sperm release. The main functions of the TBCs are to remove excess spermatid cytoplasm and acrosomal contents, internalize and recycle junctional complexes by endocytosis prior to spermiation. However, in addition to recycling some of the molecules undergo lysosomal degradation. The molecular machinery involved in endocytosis at the TBCs is not well understood. To bridge this gap localization of various proteins, involved at various steps of endocytosis studied in other systems, was demonstrated in TBCs using testicular fragmented material or sections by immunoblotting and immunofluroscence. The presence of key molecules like Vamp-2, syntaxin and Lamp-2 indicates occurrence of lysosomal degradation in addition to junctional recycling at the TBCs present at the time of sperm release. TBCs are endocytic devices functioning to recycle junctional molecules or remove spermatid cytoplasm that were present between spermatids and Sertoli-cells all through the process of spermatid maturation and in turn regulate male fertility.

Endocytosis and Vesicular Transport of Plasmid DNA in Cells During Electric Field-Mediated Gene Delivery

Pulsed electric field has been used widely as a nonviral approach to improving gene delivery in basic and translational research[1-2].The technique has been called electrotransfection(ET),electroporation,electrogene transfer,and gene electroinjection in the literature [1,3].It has a great potential to improve clinical treatment of diseases through delivery of vaccines and therapeutic genes,genome and epigenome editing,and generation of human induced pluripotent stem cells for tissue engineering[1-3].During ET,extracellular transport of plasmid DNA(pDNA)relies on electrophoresis,which is critical for applications in vivo.However,mechanisms of intracellular transport remain to be understood.The lack of understanding has hindered the translation of ET technology to the clinic.It is well known that pulsed electric field can generate transient hydrophilic pores in the plasma membrane(i.e.,electroporation)that permit membrane-impermeant molecules to enter cells.Although the pores have yet to be visualized directly under a microscope,the electric field-induced membrane permeabilization has been demonstrated through experimental measurements of electrical conductance of synthetic lipid membranes and plasma membranes,direct observation of fluorescent markers crossing the membranes facing both cathode and anode,and numerical simulations of the membrane permeabilization[1,3].Results from the simulations have predicted that the cutoff size of the pores is on the order of a few hundred nanometers,and the lifetime of the pores that are larger than 100 nm is on the order of 10 msec.Although these data provide a solid evidence of the membrane permeabilization,recent studies have demonstrated that the generation of the pores is insufficient for ET[1,4].The reasons are as follows.First,the lifetime of the pores is several orders of magnitude shorter than the time scale for pDNA uptake,which is on the order of 10 min.Second,complex formation between pDNA and plasma membrane is a necessary condition for successful gene transfer.Third,inhibition of clathrin mediated endocytosis or Rac-1 dependent micropinocytosis can reduce the amount of pDNA internalized by cells [1].Finally,we demonstrate that few pDNA molecules can be observed in the cytosol that are not associated with the intracellular vesicles[5],suggesting that pDNA uptake is mediated by endocytosis.In addition to the internalization,ET requires the pDNA in the cytoplasm to reach the nucleus.To understand mechanisms of intracellular trafficking of pDNA,we have examined time-dependent pDNA distributions in cells,quantitatively determined percentages of pDNA molecules associated with different endocytic compartments using transmission electron microscopy(TEM),and investigated different approaches to facilitate cytoplasmic transport and nuclear entry of pDNA.Our data have shown that electrotransfected pDNA is located in different vesicular ultrastructures at or near the plasma membrane at10 min post application of electric pulses[5].In the hard-to-transfect cells(e.g.,4T1),pDNA penetration from the cell surface is less active,and the total number of vesicular structures associated with pDNA is low,compared to those in the easyto-transfect cells(e.g.,COS7).Our data have also shown that macropinocytosis is the most common pathway shared by all types of cells.To investigate how improve pDNA transport in cells,we have photochemically treated cells to non-specifically induce pDNA escape from intracellular vesicles,or blocked endosome and autophagic vacuole maturation through treatment of cells with Bafilomycin Al,an inhibitor of vacuolar H+ATPase.Our data demonstrate that both treatments can lead to reduction of ET efficiency although the treatment for inducing endosomal escape can enhance poly-L-lysine mediated gene delivery.These data suggest that the vesicles play an important role in protecting the naked pDNA during intracellular trafficking.The nuclear envelope is another major barrier to ET.To facilitate the nuclear entry,we have examined three different approaches.One is to synchronize the nuclear envelope breakdown(NEBD)prior to ET;the second approach is to pre-treat cells with a nuclear pore dilating agent(i.e.,trans-1,2-cyclohexanediol);and the third one is to incorporate a nuclear targeting sequence(NTS)(i.e.,SV40)into the pDNA.Our data have shown that the synchronization of the NEBD can significantly improve the ET efficiency without compromising the cell viability.The nuclear pore dilation can improve the ET as well but the dilating agent is cytotoxic.The incorporation of NTS into pDNA can improve the gene delivery efficiency but the improvement is cell-type dependent,suggesting that the NTS has to be screened and optimized for the cells of interest.In summary,the transient pores in the plasma membrane induced by the electric pulses will enable cellular uptake of membrane-impermeant molecules up to the size of small proteins.Larger molecules(e.g.,pDNA)have to be internalized via endocytic processes triggered by the pulsed electric field.Within the cells,pDNA transport is mediated by vesicles and can be blocked by non-specific escape from vesicles or inhibition of vesicle maturation.The nuclear entry of pDNA can be enhanced,without compromising cell viability,through the use of the NTS or the synchronization of the NEBD.

Study progress of cell endocytosis

Endocytosis is a process through which extracellular materials are transported into cell through membrane deformation. This process is not a simple step-by-step process in which a series of proteins function according to the chronological order, but rather a complex process comprising many members which are regulated precisely. The role of endocytosis is broadly divided into two categories, phagocytosis and pinocytosis, the latter is divided into four species in accordance with the size of endocytosis substances： clathrin dependent endocytosis, the diameter of clathrin-coated vesicle is 100-150 nm; caveolin dependent endocytosis, the diameter of caveolin protein-coated vesicle is 50-100 nm; macropinocytosis, the diam- eter of macropinocytosis is generally 0.5-2 μm, sometimes up to 5 μm; clathrin and caveolin independent endocytosis. Many proteins including endophilin A1, A2, A3, and endocytotic proteins B, B1a, and Blb as well as dynamin, actin and Rab protein families are involved in endocytosis and play an important role in different stages. The abnormal endocytosis may be involved in the development of certain diseases.

Exocytosis,endocytosis and recycling of secretory vesicles in neuroendocrine cells,and its regulation by cortical actin

The cortical actin network is a mesh of filaments distributed beneath the plasmalemma that dynamically reacts in response to stimuli.This dynamic network of cortical filaments,together with motor myosin partners,adjusts the plasmalemma tension,organizes membrane protein microdomains,remodels the cell surface and drives vesicle motion in order to fine-tune exocytosis,endocytosis and recycling of secretory vesicles.In this review,we discuss how these mechanisms work in secretory cells.

Monoclonal antibody targeting mu-opioid receptor attenuates morphine tolerance via enhancing morphine-induced receptor endocytosis

Morphine is a frequently used analgesic that activates the mu-opioid receptor(MOR),which has prominent side effects of tolerance.Although the inefficiency of morphine in inducing the endocytosis of MOR underlies the development of morphine tolerance,currently,there is no effective therapy to treat morphine tolerance.In the current study,we aimed to develop a monoclonal antibody(mAb)precisely targeting MOR and to determine its therapeutic efficacy on morphine tolerance and the underlying molecular mechanisms.We successfully prepared a mAb targeting MOR,named 3A5C7,by hybridoma technique using a strategy of deoxyribonucleic acid immunization combined with cell immunization,and identified it as an immunoglobulin G mAb with high specificity and affinity for MOR and binding ability to antigens with spatial conformation.Treatment of two cell lines,HEK293T and SH-SY5Y,with 3A5C7 enhanced morphine-induced MOR endocytosis via a G protein-coupled receptor kinase 2(GRK2)/b-arrestin2-dependent mechanism,as demonstrated by immunofluorescence staining,flow cytometry,Western blotting,coimmunoprecipitation,and small interfering ribonucleic acid(siRNA)-based knockdown.This mAb also allowed MOR recycling from cytoplasm to plasma membrane and attenuated morphine-induced phosphorylation of MOR.We established an in vitro morphine tolerance model using differentiated SH-SY5Y cells induced by retinoic acid.Western blot,enzyme-linked immunosorbent assays,and siRNA-based knockdown revealed that 3A5C7 mAb diminished hyperactivation of adenylate cyclase,the in vitro biomarker of morphine tolerance,via the GRK2/b-arrestin2 pathway.Furthermore,in vivo hotplate test demonstrated that chronic intrathecal administration of 3A5C7 significantly alleviated morphine tolerance in mice,and withdrawal jumping test revealed that both chronic and acute 3A5C7 intrathecal administration attenuated morphine dependence.Finally,intrathecal electroporation of silencing short hairpin RNA illustrated that the in vivo anti-tolerance and anti-dependence efficacy of 3A5C7 was mediated by enhanced morphine-induced MOR endocytosis via GRK2/b-arrestin2 pathway.Collectively,our study provided a therapeutic mAb,3A5C7,targeting MOR to treat morphine tolerance,mediated by enhancing morphine-induced MOR endocytosis.The mAb 3A5C7 demonstrates promising translational value to treat clinical morphine tolerance.

High-density lipoprotein endocytosis in endothelial cells

AIM: To describe the way stations of high-density lipoprotein(HDL) uptake and its lipid exchange in endothelial cells in vitro and in vivo. METHODS: A combination of fluorescence microscopy using novel fluorescent cholesterol surrogates and electron microscopy was used to analyze HDL endocytosis in great detail in primary human endothelial cells. Further, HDL uptake was quantified using radio-labeled HDL particles. To validate the in vitro findings mice were injected with fluorescently labeled HDL and particle uptake in the liver was analyzed using fluorescencemicroscopy. RESULTS: HDL uptake occurred via clathrin-coated pits, tubular endosomes and multivesicular bodies in human umbilical vein endothelial cells. During uptake and resecretion, HDL-derived cholesterol was exchanged at a faster rate than cholesteryl oleate, resembling the HDL particle pathway seen in hepatic cells. In addition, lysosomes were not involved in this process and thus HDL degradation was not detectable. In vivo, we found HDL mainly localized in mouse hepatic endothelial cells. HDL was not detected in parenchymal liver cells, indicating that lipid transfer from HDL to hepatocytes occurs primarily via scavenger receptor, class B, type Ⅰ mediated selective uptake without concomitant HDL endocytosis. CONCLUSION: HDL endocytosis occurs via clathrincoated pits, tubular endosomes and multivesicular bodies in human endothelial cells. Mouse endothelial cells showed a similar HDL uptake pattern in vivo indicating that the endothelium is one major site of HDL endocytosis and transcytosis.