Molecular mechanisms of clathrin-independent endocytosis

Cytoskeleton keratin regulation of FasR signaling through modulation of actin/ezrin interplay at lipid rafts in hepatocytes

FasR stimulation by Fas ligand leads to rapid formation of FasR microaggregates, which become signaling protein oligomerization transduction structures (SPOTS), through interactions with actin and ezrin, a structural step that triggers death-inducing signaling complex formation, in association with procaspase-8 activation. In some cells, designated as type I, caspase 8 directly activates effector caspases, whereas in others, known as type II, the caspase-mediated death signaling is amplified through mitochondria. Keratins are the intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatocyte IFs are made solely of keratins 8/18 (K8/K18), the hallmark of all simple epithelia. We have shown recently that in comparison to type II wild-type (WT) mouse hepatocytes, the absence of K8/K18 IFs in K8-null hepatocytes leads to more efficient FasR-mediated apoptosis, in link with a type II/type I-like switch in FasR-death signaling. Here, we demonstrate that the apoptotic process occurring in type I-like K8-null hepatocytes is associated with accelerated SPOTS elaboration at surface membrane, along with manifestation of FasR cap formation and internalization. In addition, the lipid raft organization is altered in K8-null hepatocytes. While lipid raft inhibition impairs SPOTS formation in both WT and K8-null hepatocytes, the absence of K8/K18 IFs in the latter sensitizes SPOTS to actin de-polymerization, and perturbs ezrin compartmentalization. Overall, the results indicate that the K8/K18 IF loss in hepatocytes alters the initial FasR activation steps through perturbation of ezrin/actin interplay and lipid raft organization, which leads to a type II/type I switch in FasR-death signaling.

Rift Valley fever virus strain MP-12 enters mammalian host cells via caveola-mediated endocytosis

Rift Valley fever virus (RVFV) is a zoonotic pathogen capable of causing serious morbidity and mortality in both humans and livestock. The lack of efficient countermeasure strategies, the potential for dispersion into new regions, and the pathogenesis in humans and livestock make RVFV a serious public health concern. The receptors, cellular factors, and entry pathways used by RVFV and other members of the family Bunyaviridae remain largely uncharacterized. Here we provide evidence that RVFV strain MP-12 uses dynamin-dependent caveola-mediated endocytosis for cell entry. Caveolae are lipid raft domains composed of caveolin (the main structural component), cholesterol, and sphingolipids. Caveola-mediated endocytosis is responsible for the uptake of a wide variety of host ligands, as well as bacteria, bacterial toxins, and a number of viruses. To determine the cellular entry mechanism of RVFV, we used small-molecule inhibitors, RNA interference (RNAi), and dominant negative (DN) protein expression to inhibit the major mammalian cell endocytic pathways. Inhibitors and RNAi specific for macropinocytosis and clathrin-mediated endocytosis had no effect on RVFV infection. In contrast, inhibitors of caveola-mediated endocytosis, and RNAi targeted to caveolin-1 and dynamin, drastically reduced RVFV infection in multiple cell lines. Expression of DN caveolin-1 also reduced RVFV infection significantly, while expression of DN EPS15, a protein required for the assembly of clathrin-coated pits, and DN PAK-1, an obligate mediator of macropinocytosis, had no significant impact on RVFV infection. These results together suggest that the primary mechanism of RVFV MP-12 uptake is dynamin-dependent, caveolin-1-mediated endocytosis.

Early endosomal rerouting of major histocompatibility class I conformers

Major histocompatibility class I (MHC-I) molecules are present at the cell surface both as fully conformed trimolecular complexes composed of heavy chain (HC), beta-2-microglobulin (β2m) and peptide, and various open forms, devoid of peptide and/or β2m (open MHC-I conformers). Fully conformed MHC-I complexes and open MHC-I conformers can be distinguished by well characterized monoclonal antibody reagents that recognize their conformational difference in the extracellular domain. In the present study, we used these tools in order to test whether conformational difference in the extracellular domain determines endocytic and endosomal route of plasma membrane (PM) proteins. We analyzed PM localization, internalization, endosomal trafficking, and recycling of human and murine MHC-I proteins on various cell lines. We have shown that fully conformed MHC-I and open MHC-I conformers segregate at the PM and during endosomal trafficking resulting in the exclusion of open MHC-I conformers from the recycling route. This segregation is associated with their partitioning into the membranes of different compositions. As a result, the open MHC-I conformers internalized with higher rate than fully conformed counterparts. Thus, our data suggest the existence of conformation-based protein sorting mechanism in the endosomal system.

Notch ligand endocytosis generates mechanical pulling force dependent on dynamin, epsins, and actin

Notch signaling induced by cell surface ligands is critical to development and maintenance of many eukaryotic organisms. Notch and its ligands are integral membrane proteins that facilitate direct cell-cell interactions to activate Notch proteolysis and release the intracellular domain that directs Notch-specific cellular responses. Genetic studies suggest that Notch ligands require endocytosis, ubiquitylation, and epsin endocytic adaptors to activate signaling, but the exact role of ligand endocytosis remains unresolved. Here we characterize a molecularly distinct mode of clathrin-mediated endocytosis requiring ligand ubiquitylation, epsins, and actin for ligand cells to activate signaling in Notch cells. Using a cell-bead optical tweezers system, we obtained evidence for cell-mediated mechanical force dependent on this distinct mode of ligand endocytosis. We propose that the mechanical pulling force produced by endocytosis of Notch-bound ligand drives conformational changes in Notch that permit activating proteolysis.

Cell-specific intracellular anticancer drug delivery from mesoporous silica nanoparticles with pH sensitivity

A nanoreservoir for efficient intracellular anticancer drug delivery based on mesoporous silica nanoparticles end-capped with lactobionic acid-grafted bovine serum albumin is fabricated. It demonstrates great potential for both cell-specific endocytosis and intracellular pH-responsive controlled release of drugs. A possible endocytosis pathway/mechanism of the smart controlled drug release system is proposed.

Clathrin-independent endocytosis contributes to uptake of glucose into BY-2 protoplasts

In eukaryotic cells, several pathways exist for the internalization of plasma membrane proteins and extracellular cargo molecules. These endocytic pathways can be divided into clathrin-dependent and clathrin-independent pathways. While clathrin-dependent pathways are known to be involved in a variety of cellular processes in plants, clathrin-independent pathways have so far only been identified in animal and yeast cells. Here we show that internalization of fluorescent glucose into BY-2 cells leads to accumulation of the sugar in compartments of the endocytic pathway. This endocytic uptake of glucose was not blocked by ikarugamycin, an inhibitor of clathrin-dependent endocytosis, suggesting a role for clathrin-independent endocytosis in glucose uptake. Investigations of fusion and fission of single vesicles by membrane capacitance measurements revealed stimulation of endocytic activity by extracellular glucose. Glucose-stimulated fission of vesicles was not affected by addition of ikarugamycin or blocking of clathrin coat formation by transient over-expression of HUB1 (the C-terminal part of the clathrin heavy chain). These data demonstrate that clathrin-independent endocytosis does occur in plant cells. This pathway may represent a common mechanism for the uptake of external nutrients.

Caveolin-1 and dynamin-2 are essential for removal of the complement C5b-9 complex via endocytosis

The complement system, an important element of both innate and adaptive immunity, is executing complement-dependent cytotoxicity (CDC) with its C5b-9 protein complex that is assembled on cell surfaces and transmits to the cell death signals. In turn, cells, and in particular cancer cells, protect themselves from CDC in various ways. Thus, cells actively remove the C5b-9 complexes from their plasma membrane by endocytosis. Inhibition of clathrin by transfection with shRNA or of EPS-15 with a dominant negative plasmid had no effect on C5b-9 endocytosis and on cell death. In contrast, inhibition of caveolin-1 (Cav-1) by transfection with an shRNA or a dominant negative plasmid sensitized cells to CDC and inhibited C5b-9 endocytosis. Similarly, both inhibition of dynamin-2 by transfection with a dominant negative plasmid or by treatment with Dynasore reduced C5b-9 endocytosis and enhanced CDC. C5b-9 endocytosis was also disrupted by pretreatment of the cells with methyl-β-cyclodextrin or Filipin III, hence implicating membrane cholesterol in the process. Analyses by confocal microscopy demonstrated co-localization of Cav-1-EGFP with C5b-9 at the plasma membrane, in early endosomes, at the endocytic recycling compartment and in secreted vesicles. Further investigation of the process of C5b-9 removal by exo-vesiculation demonstrated that inhibition of Cav-1 and cholesterol depletion abrogated C5b-9 exo-vesiculation, whereas, over-expression of Cav-1 increased C5b-9 exo-vesiculation. Our results show that Cav-1 and dynamin-2 (but not clathrin) support cell resistance to CDC, probably by facilitating purging of the C5b-9 complexes by endocytosis and exo-vesiculation.

The roles of flotillin microdomains--endocytosis and beyond

Flotillins are membrane proteins that form microdomains in the plasma membrane of all mammalian cell types studied to date. They span the evolutionary spectrum, with proteins related to flotillins present in bacteria, fungi, plants and metazoans, which suggests that they perform important, and probably conserved, functions. Flotillins have been implicated in myriad processes that include endocytosis, signal transduction and regulation of the cortical cytoskeleton, yet the molecular mechanisms that underlie flotillin function in these different cases are still poorly understood. In this Commentary, we will provide an introduction to these intriguing proteins, summarise their proposed functions and discuss in greater detail some recent insights into the role of flotillin microdomains in endocytosis that have been provided by several independent studies. Finally, we will focus on the questions that are raised by these new experiments and their implications for future studies.

Entry of human papillomavirus type 16 by actin-dependent, clathrin- and lipid raft-independent endocytosis

Infectious endocytosis of incoming human papillomavirus type 16 (HPV-16), the main etiological agent of cervical cancer, is poorly characterized in terms of cellular requirements and pathways. Conflicting reports attribute HPV-16 entry to clathrin-dependent and -independent mechanisms. To comprehensively describe the cell biological features of HPV-16 entry into human epithelial cells, we compared HPV-16 pseudovirion (PsV) infection in the context of cell perturbations (drug inhibition, siRNA silencing, overexpression of dominant mutants) to five other viruses (influenza A virus, Semliki Forest virus, simian virus 40, vesicular stomatitis virus, and vaccinia virus) with defined endocytic requirements. Our analysis included infection data, i.e. GFP expression after plasmid delivery by HPV-16 PsV, and endocytosis assays in combination with electron, immunofluorescence, and video microscopy. The results indicated that HPV-16 entry into HeLa and HaCaT cells was clathrin-, caveolin-, cholesterol- and dynamin-independent. The virus made use of a potentially novel ligand-induced endocytic pathway related to macropinocytosis. This pathway was distinct from classical macropinocytosis in regards to vesicle size, cholesterol-sensitivity, and GTPase requirements, but similar in respect to the need for tyrosine kinase signaling, actin dynamics, Na⁺/H⁺ exchangers, PAK-1 and PKC. After internalization the virus was transported to late endosomes and/or endolysosomes, and activated through exposure to low pH.

Human papillomavirus type 16 is the main etiological agent of cervical cancer. Despite advances in our understanding of transformation and cancer progression, as well as preventative vaccination strategies, the early events in papillomavirus infections are incompletely understood. Here, we investigated which strategies and cellular mechanisms the virus uses to enter epithelial cells. Entry was slow and asynchronous likely due to several structural alterations, which needed to occur on the cell exterior. Interestingly, the virus hijacked a potentially novel pathway of endocytosis for entry, which was distinct from classical macropinocytosis in regards to vesicle size, cholesterol-sensitivity, and GTPase requirements, but similar in respect to tyrosine kinase signaling, actin dynamics, Na⁺/H⁺ exchangers, PAK-1 and PKC requirements. This cellular mechanism may also be used by other viruses such as influenza A virus, echo virus 1, and choriomeningitis virus.

Paired helical filaments from Alzheimer disease brain induce intracellular accumulation of Tau protein in aggresomes

Abnormal folding of tau protein leads to the generation of paired helical filaments (PHFs) and neurofibrillary tangles, a key neuropathological feature in Alzheimer disease and tauopathies. A specific anatomical pattern of pathological changes developing in the brain suggests that once tau pathology is initiated it propagates between neighboring neuronal cells, possibly spreading along the axonal network. We studied whether PHFs released from degenerating neurons could be taken up by surrounding cells and promote spreading of tau pathology. Neuronal and non-neuronal cells overexpressing green fluorescent protein-tagged tau (GFP-Tau) were treated with isolated fractions of human Alzheimer disease-derived PHFs for 24 h. We found that cells internalized PHFs through an endocytic mechanism and developed intracellular GFP-Tau aggregates with attributes of aggresomes. This was particularly evident by the perinuclear localization of aggregates and redistribution of the vimentin intermediate filament network and retrograde motor protein dynein. Furthermore, the content of Sarkosyl-insoluble tau, a measure of abnormal tau aggregation, increased 3-fold in PHF-treated cells. An exosome-related mechanism did not appear to be involved in the release of GFP-Tau from untreated cells. The evidence that cells can internalize PHFs, leading to formation of aggresome-like bodies, opens new therapeutic avenues to prevent propagation and spreading of tau pathology.

Polyplexes traffic through caveolae to the Golgi and endoplasmic reticulum en route to the nucleus

The cellular machinery involved in the internalization of nonviral gene carriers and their subsequent trafficking to the nucleus directly impacts their therapeutic efficiency. Hence, identifying key endocytic pathways and organelles that contribute to the successful transfer of polyplexes to the nucleus generates new opportunities for improving carrier design. Previously, we showed that histone H3 tail peptides encoding a sequence known to participate in chromatin activation exhibit synergistic gene delivery activity with poly(ethylenimine) (PEI). Polyplexes containing H3 and PEI exhibited a reduced dependence on endocytic pathways that trafficked to lysosomes, and had enhanced sensitivity to an inhibitor associated with retrograde trafficking through the Golgi apparatus. Thus, we sought to determine whether caveolar uptake and transport through the Golgi and/or endoplasmic reticulum (ER) preceded nuclear delivery. By the use of a panel of chemical endocytic inhibitors, we determined that H3 polyplexes utilized caveolar pathways to a greater degree than PEI polyplexes. Caveolae-mediated endocytosis was found to be a productive route for gene expression by the H3/PEI-pDNA polyplexes, consistent with previous studies of polymer-mediated gene delivery. Additionally, the polyplexes substantially colocalized within the ER after only 5 min of incubation, and utilized retrograde Golgi-to-ER pathways at levels similar to pathogens known to traffic by these routes during infection. The results of this study have expanded our understanding of how caveolar polyplexes are trafficked to cell nuclei, and provide new evidence for the role of Golgi-ER pathways in transfection. These findings suggest new design criteria and opportunities to stragetically target nonviral gene delivery vehicles.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Effect of polyethylene glycol modification of TiO₂nanoparticles on cytotoxicity and gene expressions in human cell lines

Nanoparticles (NPs) are tiny materials used in a wide range of industrial and medical applications. Titanium dioxide (TiO(2)) is a type of nanoparticle that is widely used in paints, pigments, and cosmetics; however, little is known about the impact of TiO(2) on human health and the environment. Therefore, considerable research has focused on characterizing the potential toxicity of nanoparticles such as TiO(2) and on understanding the mechanism of TiO(2) NP-induced nanotoxicity through the evaluation of biomarkers. Uncoated TiO(2) NPs tend to aggregate in aqueous media, and these aggregates decrease cell viability and induce expression of stress-related genes, such as those encoding interleukin-6 (IL-6) and heat shock protein 70B' (HSP70B'), indicating that TiO(2) NPs induce inflammatory and heat shock responses. In order to reduce their toxicity, we conjugated TiO(2) NPs with polyethylene glycol (PEG) to eliminate aggregation. Our findings indicate that modifying TiO(2) NPs with PEG reduces their cytotoxicity and reduces the induction of stress-related genes. Our results also suggest that TiO(2) NP-induced effects on cytotoxicity and gene expression vary depending upon the cell type and surface modification.

EPI64 interacts with Slp1/JFC1 to coordinate Rab8a and Arf6 membrane trafficking

ETOC: EPI64 is a TBC domain containing a microvillar protein that binds to Arf6 and induces actin-coated vacuole accumulation when overexpressed. EPI64 does this by stabilizing Arf6-GTP levels to induce clathrin-independent endocytosis and by preventing endosomes from maturing to the tubular endosome by lowering Rab8a GTP levels via association with JFC1.

Cell function requires the integration of cytoskeletal organization and membrane trafficking. Small GTP-binding proteins are key regulators of these processes. We find that EPI64, an apical microvillar protein with a Tre-2/Bub2/Cdc16 (TBC) domain that stabilizes active Arf6 and has RabGAP activity, regulates Arf6-dependent membrane trafficking. Expression of EPI64 in HeLa cells induces the accumulation of actin-coated vacuoles, a distinctive phenotype seen in cells expressing constitutively active Arf6. Expression of EPI64 with defective RabGAP activity does not induce vacuole formation. Coexpression of Rab8a suppresses the vacuole phenotype induced by EPI64, and EPI64 expression lowers the level of Rab8-GTP in cells, strongly suggesting that EPI64 has GAP activity toward Rab8a. JFC1, an effector for Rab8a, colocalizes with and binds directly to a C-terminal region of EPI64. Together this region and the N-terminal TBC domain of EPI64 are required for the accumulation of vacuoles. Through analysis of mutants that uncouple JFC1 from either EPI64 or from Rab8-GTP, our data suggest a model in which EPI64 binds JFC1 to recruit Rab8a-GTP for deactivation by the RabGAP activity of EPI64. We propose that EPI64 regulates membrane trafficking both by stabilizing Arf6-GTP and by inhibiting the recycling of membrane through the tubular endosome by decreasing Rab8a-GTP levels.

Cell surface dynamics – how Rho GTPases orchestrate the interplay between the plasma membrane and the cortical cytoskeleton

Small GTPases are known to regulate hundreds of cell functions. In particular, Rho family GTPases are master regulators of the cytoskeleton. By regulating actin nucleation complexes, Rho GTPases control changes in cell shape, including the extension and/or retraction of surface protrusions and invaginations. Protrusion and invagination of the plasma membrane also involves the interaction between the plasma membrane and the cortical cytoskeleton. This interplay between membranes and the cytoskeleton can lead to an increase or decrease in the plasma membrane surface area and its tension as a result of the fusion (exocytosis) or internalization (endocytosis) of membranous compartments, respectively. For a long time, the cytoskeleton and plasma membrane dynamics were investigated separately. However, studies from many laboratories have now revealed that Rho GTPases, their modulation of the cytoskeleton, and membrane traffic are closely connected during the dynamic remodeling of the cell surface. Arf- and Rab-dependent exocytosis of specific vesicles contributes to the targeting of Rho GTPases and their regulatory factors to discrete sites of the plasma membrane. Rho GTPases regulate the tethering of exocytic vesicles and modulate their subsequent fusion. They also have crucial roles in the different forms of endocytosis, where they participate in the sorting of membrane domains as well as the sculpting and sealing of membrane flasks and cups. Here, we discuss how cell surface dynamics depend on the orchestration of the cytoskeleton and the plasma membrane by Rho GTPases.

Pacsin 2 is recruited to caveolae and functions in caveolar biogenesis

The pacsin (also termed syndapin) protein family is well characterised structurally. They contain F-BAR domains associated with the generation or maintenance of membrane curvature. The cell biology of these proteins remains less understood. Here, we initially confirm that EHD2, a protein previously shown biochemically to be present in caveolar fractions and to bind to pacsins, is a caveolar protein. We go on to report that GFP-pacsin 2 can be recruited to caveolae, and that endogenous pacsin 2 partially colocalises with caveolin 1 at the plasma membrane. Analysis of the role of pacsin 2 in caveolar biogenesis using small interfering RNA (siRNA) reveals that loss of pacsin 2 function results in loss of morphologically defined caveolae and accumulation of caveolin proteins within the plasma membrane. Overexpression of the F-BAR domain of pacsin 2 (but not the related F-BAR domains of CIP4 and FBP17) disrupts caveolar morphogenesis or trafficking, implying that pacsin 2 interacts with components required for these processes. We propose that pacsin 2 has an important role in the formation of plasma membrane caveolae.

ANGUSTIFOLIA, a plant homolog of CtBP/BARS, functions outside the nucleus

CtBP/BARS is a unique protein family in having quite diversified cellular functions, intercellular localizations, and developmental roles. ANGUSTIFOLIA (AN) is the sole homolog of CtBP/BARS from Arabidopsis thaliana, although it has plant AN-specific motifs and a long C-terminus. Previous studies suggested that AN would function in the nucleus as a transcriptional co-repressor, as CtBPs function in animals; however, precise verification has been lacking. In this paper, we isolated a homologous gene (MAN) of AN from liverwort, Marchantia polymorpha. Transformation of the Arabidopsis an-1 mutant with 35S-driven MAN completely complemented the an-1 phenotype, although it lacks the putative nuclear localization signal (NLS) that exists in AN proteins isolated from other plant species. We constructed several plasmids for expressing modified ANs with amino acid substitutions in known motifs. The results clearly indicated that modified AN with mutations in the putative NLS-like domain could complement the an-1 phenotype. Therefore, we re-examined localization of AN using several techniques. Our results demonstrated that AN localizes on punctuate structures around the Golgi, partially overlapping with a trans-Golgi network resident, which highlighted an unexpected link between leaf development and membrane trafficking. We should reconsider the roles and evolutionary traits of AN based on these findings.

Endocytic adaptors--social networking at the plasma membrane

Receptor-mediated endocytosis is a dynamic process that is crucial for maintaining plasma membrane composition and controlling cell-signaling pathways. A variety of entry routes have evolved to ensure that the vast array of molecules on the cell surface can be differentially internalized by endocytosis. This diversity has extended to include a growing list of endocytic adaptor proteins, which are thought to initiate the internalization process. The key function of adaptors is to select the proteins that should be removed from the cell surface. Thus, they have a central role in defining the physiology of a cell. This has made the study of adaptor proteins a very active area of research that is ripe for exciting future discoveries. Here, we review recent work on how adaptors mediate endocytosis and address the following questions: what characteristics define an endocytic adaptor protein? What roles do these proteins fulfill in addition to selecting cargo and how might adaptors function in clathrin-independent endocytic pathways? Through the findings discussed in this Commentary, we hope to stimulate further characterization of known adaptors and expansion of the known repertoire by identification of new adaptors.

Redistribution of caveolae during mitosis

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

Coordinated regulation of caveolin-1 and Rab11a in apical recycling compartments of polarized epithelial cells

Recent studies have identified caveolin-1, a protein best known for its functions in caveolae, in apical endocytic recycling compartments in polarized epithelial cells. However, very little is known about the regulation of caveolin-1 in the endocytic recycling pathway. To address this question, in the current study we compared the relationship between compartments enriched in sub-apical caveolin-1 and Rab11a, a well-defined marker of apical recycling endosomes, using polarized MDCK cells as a model. We show that caveolin-1-containing vesicles define a compartment that partially overlaps with Rab11a, and that the distribution of subapical caveolin-1 and Rab11a shows a similar dependence on microtubule disruption. Mutants of the Rab11a effector, Rab11-FIP2 also altered the localization of caveolin-1. These findings indicate that caveolin-1 is coordinately regulated with Rab11a within the apical recycling system of polarized epithelial cells, suggesting that the two proteins are components of the same pathway.

The endocytic protein GRAF1 is directed to cell-matrix adhesion sites and regulates cell spreading

GTPase regulator associated with focal adhesion kinase-1 (GRAF1) interacted with endocytic and adhesion proteins, and GRAF1 endocytic activity was up-regulated in spreading cells and concentrated at the leading edge of migrating cells. Depletion of GRAF1 resulted in profound defects in cell spreading. GRAF1 remodeled membrane microdomains at adhesions, aiding membrane turnover during cell morphological changes.

The rho GTPase-activating protein GTPase regulator associated with focal adhesion kinase-1 (GRAF1) remodels membranes into tubulovesicular clathrin-independent carriers (CLICs) mediating lipid-anchored receptor endocytosis. However, the cell biological functions of this highly prevalent endocytic pathway are unclear. In this article, we present biochemical and cell biological evidence that GRAF1 interacted with a network of endocytic and adhesion proteins and was found enriched at podosome-like adhesions and src-induced podosomes. We further demonstrate that these sites comprise microdomains of highly ordered lipid enriched in GRAF1 endocytic cargo. GRAF1 activity was upregulated in spreading cells and uptake via CLICs was concentrated at the leading edge of migrating cells. Depletion of GRAF1, which inhibits CLIC generation, resulted in profound defects in cell spreading and migration. We propose that GRAF1 remodels membrane microdomains at adhesion sites into endocytic carriers, facilitating membrane turnover during cell morphological changes.

The role of electrically stimulated endocytosis in gene electrotransfer

Gene electrotransfer is an established method for transfer of genes into cells, however, the mechanism of transfer of DNA across the cell membrane is still not known. Some studies suggest that DNA is translocated through membrane pores while others propose that DNA enters the cell via electro-endocytosis, but no direct observation was performed. In this paper we investigated the second hypothesis. Cells were stained with membrane dye FM 1-43FX, which is used for observation of endocytosis, and then exposed to electric pulses. We analyzed if endocytosis was stimulated by applying electric pulses with intensities below and above the threshold value for gene electrotransfer. No increase in endocytosis from 20 min or even up to 2h after the pulse delivery was observed, regardless of the electric field strength. These observations do not correlate with electrotransfer efficiency, which increases with field strength and is observed only above the threshold value. Our results suggest that electro-endocytosis is not a crucial mechanism for gene electrotransfer and that the hypothesis of DNA entry by translocation through permeabilized membrane is more plausible. The presented results are important for better understanding of the mechanisms of gene electrotransfer and for its optimization for clinical applications.

Increased expression of the large GTPase dynamin 2 potentiates metastatic migration and invasion of pancreatic ductal carcinoma

Pancreatic ductal tumors invade local parenchyma and metastasize to distant organs. Src-mediated tyrosine kinase signaling pathways promote pancreatic ductal adenocarcinoma (PDAC) metastasis, though the molecular mechanisms supporting this invasive process are poorly understood and represent important and novel therapeutic targets. The large GTPase Dynamin2 (Dyn2), a Src-kinase substrate, regulates membrane-cytoskeletal dynamics although it is yet to be defined if this mechanoenzyme contributes to tumor cell migration and invasion. Therefore the goal of this study was to test if Dyn2 is upregulated in human pancreatic tumors and to define its role in cell migration and metastatic invasion using in vitro assays and nude mouse models. Histological analysis showed that 81% of the 85 patients tested had elevated Dyn2 in PDAC tissue. To test if Dyn2 overexpression alters metastatic properties of human pancreatic tumor cells, stable clones of BxPC-3 cells overexpressing either wild-type Dyn2 or a phosphorylation-deficient mutant Dyn2Y(231/597)F known to attenuate Dyn2 function, were generated and analyzed for migratory capacity. Importantly, tumor cells expressing 2-3 fold levels of Dyn2 protruded lamellipodia at twice the rate, migrated faster (180%) and farther (2.5-fold greater net distance) on glass and through transwell chambers (2-3 fold more cells through the filter) compared to cells expressing Dyn2Y(231/597)F or vector alone. Further, siRNA-mediated depletion of Dyn2 and dynamin inhibitors MiTMAB and Dynasore significantly reduced cell migration (>66%), wound healing (>75%) and invasion in transwell assays (>95%) compared to DMSO treated cells. To test the metastatic potential conferred by increased Dyn2 expression, the BxPC-3 clonal cell lines were implanted orthotopically into the pancreas of nude mice. Cells expressing Dyn2-GFP exhibited a 3-fold increase in large distal tumors compared to cells expressing Dyn2Y(231/597)F or vector alone. Finally, histological analysis of pancreatic metastases from human patients revealed that Dyn2 is upregulated in 60% of metastatic tumors examined. These findings are the first to implicate dynamin in any neoplastic condition and to directly demonstrate a role for this mechanoenzyme in invasive cell migration.

Clathrin- and dynamin-independent endocytosis of FGFR3--implications for signalling

Endocytosis of tyrosine kinase receptors can influence both the duration and the specificity of the signal emitted. We have investigated the mechanisms of internalization of fibroblast growth factor receptor 3 (FGFR3) and compared it to that of FGFR1 which is internalized predominantly through clathrin-mediated endocytosis. Interestingly, we observed that FGFR3 was internalized at a slower rate than FGFR1 indicating that it may use a different endocytic mechanism than FGFR1. Indeed, after depletion of cells for clathrin, internalization of FGFR3 was only partly inhibited while endocytosis of FGFR1 was almost completely abolished. Similarly, expression of dominant negative mutants of dynamin resulted in partial inhibition of the endocytosis of FGFR3 whereas internalization of FGFR1 was blocked. Interfering with proposed regulators of clathrin-independent endocytosis such as Arf6, flotillin 1 and 2 and Cdc42 did not affect the endocytosis of FGFR1 or FGFR3. Furthermore, depletion of clathrin decreased the degradation of FGFR1 resulting in sustained signalling. In the case of FGFR3, both the degradation and the signalling were only slightly affected by clathrin depletion. The data indicate that clathrin-mediated endocytosis is required for efficient internalization and downregulation of FGFR1 while FGFR3, however, is internalized by both clathrin-dependent and clathrin-independent mechanisms.

Dynamic reorganization of flotillins in chemokine-stimulated human T-lymphocytes

Background

Different types of membrane microdomains (rafts) have been postulated to be present in the rear and front of polarized migrating T-lymphocytes. Disruption of rafts by cholesterol sequestration prevents T-cell polarization and migration. Reggie/flotillin-1 and -2 are two highly homologous proteins that are thought to shape membrane microdomains. We have previously demonstrated the enrichment of flotillins in the uropod of human neutrophils. We have now investigated mechanisms involved in chemokine-induced flotillin reorganization in human T-lymphocytes, and possible roles of flotillins in lymphocyte polarization.

Results

We studied flotillin reorganization and lateral mobility at the plasma membrane using immunofluorescence staining and FRAP (fluorescence recovery after photobleaching). We show that flotillins redistribute early upon chemokine stimulation, and form very stable caps in the uropods of human peripheral blood T-lymphocytes, colocalizing with the adhesion molecule PSGL-1 and activated ezrin/radixin/moesin (ERM) proteins. Chemokine-induced formation of stable flotillin caps requires integrity and dynamics of the actin cytoskeleton, but is not abolished by inhibitors suppressing Rho-kinase or myosin II activity. Tagged flotillin-2 and flotillin-1 coexpressed in T-lymphocytes, but not singly expressed proteins, colocalize in stable caps at the tips of uropods. Lateral mobility of coexpressed flotillins at the plasma membrane is already partially restricted in the absence of chemokine. Incubation with chemokine results in almost complete immobilization of flotillins. Capping is abolished when wild-type flotillin-1 is coexpressed with a mutant of flotillin-2 (G2A) that is unable to interact with the plasma membrane, or with a deletion mutant of flotillin-2 that lacks a putative actin-binding domain. Wild-type flotillin-2 in contrast forms caps when coexpressed with a mutant of flotillin-1 unable to interact with membranes. Transfection of T-lymphocytes with flotillin-2-G2A reduces cell polarization and uropod recruitment of endogenous flotillin-1 and PSGL-1.

Conclusions

Our data suggest that stable flotillin cap formation in the rear of polarized T-lymphocytes requires flotillin heterooligomer formation, as well as direct F-actin interactions of flotillin-2 and raft/membrane association of flotillin-2, but not -1. Our data also implicate flotillin-rich actin-dependent membrane microdomains in T-lymphocyte uropod formation.

Structural remodeling, trafficking and functions of glycosylphosphatidylinositol-anchored proteins

Glycosylphosphatidylinositol (GPI) is a glycolipid that is covalently attached to proteins as a post-translational modification. Such modification leads to the anchoring of the protein to the outer leaflet of the plasma membrane. Proteins that are decorated with GPIs have unique properties in terms of their physical nature. In particular, these proteins tend to accumulate in lipid rafts, which are critical for the functions and trafficking of GPI-anchored proteins (GPI-APs). Recent studies mainly using mutant cells revealed that various structural remodeling reactions occur to GPIs present in GPI-APs as they are transported from the endoplasmic reticulum to the cell surface. This review examines the recent progress describing the mechanisms of structural remodeling of mammalian GPI-anchors, such as inositol deacylation, glycan remodeling and fatty acid remodeling, with particular focus on their trafficking and functions, as well as the pathogenesis involving GPI-APs and their deficiency.

Molecular profiling of the human nasal epithelium: A proteomics approach

A comprehensive proteomic profiling of nasal epithelium (NE) is described. This study relies on simple subcellular fractionation used to obtain soluble- and membrane-enriched fractions followed by 2-dimensional liquid chromatography (2D-LC) separation and tandem mass spectrometry (MS/MS). The cells were collected using a brushing technique applied on NE of clinically evaluated volunteers. Subsequently, the soluble- and the membrane-protein enriched fractions were prepared and analyzed in parallel using 2D-LC-MS/MS. In a set of 1482 identified proteins, 947 (63.9%) proteins were found to be associated to membrane fraction. Grand average hydropathy value index (GRAVY) analysis, the transmembrane protein mapping and annotations of primary location deposited in the Human Protein Reference Database (HPRD) confirmed an enrichment of hydrophobic proteins on this dataset. Ingenuity Pathway Analysis (IPA) of soluble fraction revealed an enrichment of molecular and cellular functions associated with cell death, protein folding and drug metabolism while in membrane fraction showed an enrichment of functions associated with molecular transport, protein trafficking and cell-to-cell signaling and interaction. The IPA showed similar enrichment of functions associated with cellular growth and proliferation in both soluble and membrane subproteomes. This finding was in agreement with protein content analysis using exponentially modified protein abundance index (emPAI). A comparison of our data with previously published studies focusing on respiratory tract epithelium revealed similarities related to identification of proteins associated with physical barrier function and immunological defence. In summary, we extended the NE molecular profile by identifying and characterizing proteins associated to pivotal functions of a respiratory epithelium, including the control of fluid volume and ionic composition at the airways' surface, physical barrier maintenance, detoxification and immunological defence. The extent of similarities supports the applicability of a less invasive analysis of NE to assess prognosis and treatment response of lung diseases such as asthma, cystic fibrosis and chronic obstructive pulmonary disease.

Dissection of the influenza A virus endocytic routes reveals macropinocytosis as an alternative entry pathway

Influenza A virus (IAV) enters host cells upon binding of its hemagglutinin glycoprotein to sialylated host cell receptors. Whereas dynamin-dependent, clathrin-mediated endocytosis (CME) is generally considered as the IAV infection pathway, some observations suggest the occurrence of an as yet uncharacterized alternative entry route. By manipulating entry parameters we established experimental conditions that allow the separate analysis of dynamin-dependent and -independent entry of IAV. Whereas entry of IAV in phosphate-buffered saline could be completely inhibited by dynasore, a specific inhibitor of dynamin, a dynasore-insensitive entry pathway became functional in the presence of fetal calf serum. This finding was confirmed with the use of small interfering RNAs targeting dynamin-2. In the presence of serum, both IAV entry pathways were operational. Under these conditions entry could be fully blocked by combined treatment with dynasore and the amiloride derivative EIPA, the hallmark inhibitor of macropinocytosis, whereas either drug alone had no effect. The sensitivity of the dynamin-independent entry pathway to inhibitors or dominant-negative mutants affecting actomyosin dynamics as well as to a number of specific inhibitors of growth factor receptor tyrosine kinases and downstream effectors thereof all point to the involvement of macropinocytosis in IAV entry. Consistently, IAV particles and soluble FITC-dextran were shown to co-localize in cells in the same vesicles. Thus, in addition to the classical dynamin-dependent, clathrin-mediated endocytosis pathway, IAV enters host cells by a dynamin-independent route that has all the characteristics of macropinocytosis.

Attachment to and entry into a host cell are the first crucial steps in establishing a successful virus infection and critical factors in determining host cell and species tropism. Influenza A virus (IAV) attaches to host cells by binding of its major surface protein, hemagglutinin, to sialic acids that are omnipresent on the glycolipids and glycoproteins exposed on the surfaces of cells. IAV subsequently enters cells of birds and a wide variety of mammals via receptor-mediated endocytosis using clathrin as well as via (an) alternative uncharacterized route(s). The elucidation of the endocytic pathways taken by IAV has been hampered by their apparent redundancy in establishing a productive infection. By manipulating the entry conditions we have established experimental settings that allow the separate analysis of dynamin-dependent (including clathrin-mediated endocytosis) and independent entry of IAV. Collectively, our results indicate macropinocytosis, the main route for the non-selective uptake of extracellular fluid by cells, as an alternative IAV entry route. As the dynamin-dependent and -independent IAV entry routes are redundant and independent, their separate manipulation was crucial for the identification and characterization of the alternative IAV entry route. A similar strategy might be applicable to the study of endocytic pathways taken by other viruses.

Polystyrene nanoparticle trafficking across MDCK-II

Polystyrene nanoparticles (PNP) cross rat alveolar epithelial cell monolayers via non-endocytic transcellular pathways. To evaluate epithelial cell type-specificity of PNP trafficking, we studied PNP flux across Madin Darby canine kidney cell II monolayers (MDCK-II). The effects of calcium chelation (EGTA), energy depletion (sodium azide (NaN(3)) or decreased temperature), and endocytosis inhibitors methyl-β-cyclodextrin (MBC), monodansylcadaverine and dynasore were determined. Amidine-modified PNP cross MDCK-II 500 times faster than carboxylate-modified PNP. PNP flux did not increase in the presence of EGTA. PNP flux at 4 °C and after treatment with NaN(3) decreased 75% and 80%, respectively. MBC exposure did not decrease PNP flux, whereas dansylcadaverine- or dynasore-treated MDCK-II exhibited ∼80% decreases in PNP flux. Confocal laser scanning microscopy revealed intracellular colocalization of PNP with clathrin heavy chain. These data indicate that PNP translocation across MDCK-II (1) occurs via clathrin-mediated endocytosis and (2) is dependent on PNP physicochemical properties. We conclude that uptake/trafficking of nanoparticles (NPs) into/across epithelia depends both on properties of the NPs and on the specific epithelial cell type.

Endocytic response of type I alveolar epithelial cells to hypertonic stress

We present plasma membrane (PM) internalization responses of type I alveolar epithelial cells to a 50 mosmol/l increase in tonicity. Our research is motivated by interest in ATI repair, for which endocytic retrieval of PM appears to be critical. We validated pharmacological and molecular tools to dissect the endocytic machinery of these cells and used these tools to test the hypothesis that osmotic stress triggers a pathway-specific internalization of PM domains. Validation experiments confirmed the fluorescent analogs of lactosyl-ceramide, transferrin, and dextran as pathway-specific cargo of caveolar, clathrin, and fluid-phase uptake, respectively. Pulse-chase experiments indicate that hypertonic exposure causes a downregulation of clathrin and fluid-phase endocytosis while stimulating caveolar endocytosis. The tonicity-mediated increase in caveolar endocytosis was associated with the translocation of caveolin-1 from the PM and was absent in cells that had been transfected with dominant-negative dynamin constructs. In separate experiments we show that hypertonic exposure increases the probability of PM wound repair following micropuncture from 82 ± 4 to 94 ± 2% (P < 0.01) and that this effect depends on Src pathway activation-mediated caveolar endocytosis. The therapeutic and biological implications of our findings are discussed.

Come in and take your coat off - how host cells provide endocytosis for virus entry

Viruses are intracellular parasites that rely upon the host cell machinery for their life cycle. Newly generated virus particles have to transmit their genomic information to uninfected cells/organisms. Viral entry is the process to gain access to viral replication sites within uninfected cells, a multistep course of events that starts with binding to target cells. Since viruses are simple in structure and composition and lack any locomotive capacity, viruses depend on hundreds of host cell proteins during entry. Most animal viruses take advantage of endocytosis to enter cells. Cell biological, morphological and biochemical studies, live cell imaging and systematic approaches have identified various new endocytic mechanisms besides clathrin-mediated endocytosis, macropinocytosis and caveolar/lipid raft-mediated endocytosis. Hence, studying virus entry has become ever more complex. This review provides a cell biological overview of the existing endocytic mechanisms and strategies used or potentially used by viruses to enter cells.

PI3KC2α, a class II PI3K, is required for dynamin-independent internalization pathways

Increasing evidence indicates that cellular uptake of several molecules can occur independently of functional dynamin, but the molecular players that regulate dynamin-independent endocytosis and the subsequent trafficking steps are still largely unknown. A survival-based short-hairpin (sh) RNA screen using a cell line expressing a diphtheria toxin receptor (DTR, officially known as HBEGF) anchored to GPI (DTR–GPI), which internalizes diphtheria toxin (DT, officially known as DTX) in a dynamin-independent manner, identified PI3KC2α, a class II phosphoinositide 3-kinase (PI3K), as a specific regulator of dynamin-independent DT internalization. We found that the internalization of several proteins that enter the cell through dynamin-independent pathways led to a relocalization of PI3KC2α to cargo-positive vesicles. Furthermore, downregulation of PI3KC2α impaired internalization of CD59 as well as fluid-phase endocytosis. Our data suggest a general role for PI3KC2α in regulating physiologically relevant dynamin-independent internalization pathways by recruiting early endosome antigen 1 (EEA1) to vesicular compartments, a step required for the intracellular trafficking of vesicles generated by dynamin-independent endocytic pathways.

Internalization of p53(14-29) peptide amphiphiles and subsequent endosomal disruption results in SJSA-1 cell death

In vivo peptide inhibition of tumor suppressor p53 binding to the protein MDM2 is hampered by inefficient delivery of the peptide. Our approach to couple a hydrophobic lipid-like tail on the inhibitory peptide p53(14-29) allowed its intracellular delivery in vitro, in a panel of different cell lines. The constructed chimeric molecules, termed peptide amphiphiles, further self-assembled into supramolecular structures, identified as elongated wormlike micelles. Internalization of peptides occurred following micelle disassembly, partly via clathrin-mediated endocytosis of monomers. Incubation of SJSA-1 cells in hypertonic culture media, aimed to disrupt endocytic vesicles, resulted in peptide amphiphile-mediated cell death. Our results provide the basis for the construction of novel therapeutic supramolecular nanoparticles and suggest hydrophobic modification of peptides as a promising strategy for enhancing delivery of impermeable peptides.

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

IMPORTANCE OF THE FIELD

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

AREAS COVERED IN THIS REVIEW

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

WHAT THE READER WILL GAIN

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

TAKE HOME MESSAGE

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

Reticulon short hairpin transmembrane domains are used to shape ER tubules

Reticulons are integral membrane proteins that partition into and shape the tubular endoplasmic reticulum (ER). We propose that reticulons use a membrane insertion mechanism to generate regions of high membrane curvature in the ER. A reticulon contains two short hairpin transmembrane domains (TMDs), which could generate membrane curvature by increasing the area of the cytoplasmic leaflet. Here, we test whether the short length of these hairpin TMDs is required for reticulon membrane-shaping functions in mammalian cells. We lengthened the TMDs of reticulon 4 to resemble a typical bi-pass TMD that spans both leaflets. We find that TMD mutants oligomerize like wild type (wt), however, they are not immobilized, do not partition into tubules, do not constrict tubules and no longer suppress peripheral ER cisternae. Therefore, short hairpin TMD length is required for reticulon protein partitioning and membrane-shaping functions. Another membrane protein with a short hairpin TMD is caveolin. We show that an ER-retained caveolin construct also partitions within the ER in a manner that is dependent on it containing a short hairpin TMD. These data suggest that a short hairpin TMD may be a general feature used by membrane-shaping proteins to partition into and shape regions of high membrane curvature.

Flotillin microdomains interact with the cortical cytoskeleton to control uropod formation and neutrophil recruitment

The association between flotillin microdomains and the cortical cytoskeleton controls myosin IIa activation and neutrophil chemotaxis.

We studied the function of plasma membrane microdomains defined by the proteins flotillin 1 and flotillin 2 in uropod formation and neutrophil chemotaxis. Flotillins become concentrated in the uropod of neutrophils after exposure to chemoattractants such as N-formyl-Met-Leu-Phe (fMLP). Here, we show that mice lacking flotillin 1 do not have flotillin microdomains, and that recruitment of neutrophils toward fMLP in vivo is reduced in these mice. Ex vivo, migration of neutrophils through a resistive matrix is reduced in the absence of flotillin microdomains, but the machinery required for sensing chemoattractant functions normally. Flotillin microdomains specifically associate with myosin IIa, and spectrins. Both uropod formation and myosin IIa activity are compromised in flotillin 1 knockout neutrophils. We conclude that the association between flotillin microdomains and cortical cytoskeleton has important functions during neutrophil migration, in uropod formation, and in the regulation of myosin IIa.

Clathrin-independent carriers form a high capacity endocytic sorting system at the leading edge of migrating cells

Although the importance of clathrin- and caveolin-independent endocytic pathways has recently emerged, key aspects of these routes remain unknown. Using quantitative ultrastructural approaches, we show that clathrin-independent carriers (CLICs) account for approximately three times the volume internalized by the clathrin-mediated endocytic pathway, forming the major pathway involved in uptake of fluid and bulk membrane in fibroblasts. Electron tomographic analysis of the 3D morphology of the earliest carriers shows that they are multidomain organelles that form a complex sorting station as they mature. Proteomic analysis provides direct links between CLICs, cellular adhesion turnover, and migration. Consistent with this, CLIC-mediated endocytosis of key cargo proteins, CD44 and Thy-1, is polarized at the leading edge of migrating fibroblasts, while transient ablation of CLICs impairs their ability to migrate. These studies provide the first quantitative ultrastructural analysis and molecular characterization of the major endocytic pathway in fibroblasts, a pathway that provides rapid membrane turnover at the leading edge of migrating cells.

Hijacking the endocytic machinery by microbial pathogens

Understanding the mechanisms that microbes exploit to invade host cells and cause disease is crucial if we are to eliminate their threat. Although pathogens use a variety of microbial factors to trigger entry into non-phagocytic cells, their targeting of the host cell process of endocytosis has emerged as a common theme. To accomplish this, microbes often rewire the normal course of particle internalization, frequently usurping theoretical maximal sizes to permit entry and reconfiguring molecular components that were once thought to be required for vesicle formation. Here, we discuss recent advances in our understanding of how toxins, viruses, bacteria, and fungi manipulate the host cell endocytic machinery to generate diseases. Additionally, we will reveal the advantages of using these organisms to expand our general knowledge of endocytic mechanisms in eukaryotic cells.