Identification of cytoskeleton-associated proteins in isolated rat liver endosomes

The temporal association of CapZ with early endosomes regulates endosomal trafficking and viral entry into host cells

BACKGROUND

Many viruses enter host cells by hijacking endosomal trafficking. CapZ, a canonical actin capping protein, participates in endosomal trafficking, yet its precise role in endocytosis and virus infection remains elusive.

RESULTS

Here, we showed that CapZ was transiently associated with early endosomes (EEs) and was subsequently released from the matured EEs after the fusion of two EEs, which was facilitated by PI(3)P to PI(3,5)P2 conversion. Vacuolin-1 (a triazine compound) stabilized CapZ at EEs and thus blocked the transition of EEs to late endosomes (LEs). Likewise, artificially tethering CapZ to EEs via a rapamycin-induced protein-protein interaction system blocked the early-to-late endosome transition. Remarkably, CapZ knockout or artificially tethering CapZ to EEs via rapamycin significantly inhibited flaviviruses, e.g., Zika virus (ZIKV) and dengue virus (DENV), or beta-coronavirus, e.g., murine hepatitis virus (MHV), infection by preventing the escape of RNA genome from endocytic vesicles.

CONCLUSIONS

These results indicate that the temporal association of CapZ with EEs facilitates early-to-late endosome transition (physiologically) and the release of the viral genome from endocytic vesicles (pathologically).

The roles of dynein and myosin VI motor proteins in endocytosis

Endocytosis is indispensable for multiple cellular processes, including signalling, cell adhesion, migration, as well as the turnover of plasma membrane lipids and proteins. The dynamic interplay and regulation of different endocytic entry routes requires multiple cytoskeletal elements, especially motor proteins that bind to membranes and transport vesicles along the actin and microtubule cytoskeletons. Dynein and kinesin motor proteins transport vesicles along microtubules, whereas myosins drive vesicles along actin filaments. Here, we present a brief overview of multiple endocytic pathways and our current understanding of the involvement of these motor proteins in the regulation of the different cellular entry routes. We particularly focus on structural and mechanistic details of the retrograde motor proteins dynein and myosin VI (also known as MYO6), along with their adaptors, which have important roles in the early events of endocytosis. We conclude by highlighting the key challenges in elucidating the involvement of motor proteins in endocytosis and intracellular membrane trafficking.

Capping protein regulates endosomal trafficking by controlling F-actin density around endocytic vesicles and recruiting RAB5 effectors

Actin filaments (F-actin) have been implicated in various steps of endosomal trafficking, and the length of F-actin is controlled by actin capping proteins, such as CapZ, which is a stable heterodimeric protein complex consisting of α and β subunits. However, the role of these capping proteins in endosomal trafficking remains elusive. Here, we found that CapZ docks to endocytic vesicles via its C-terminal actin-binding motif. CapZ knockout significantly increases the F-actin density around immature early endosomes, and this impedes fusion between these vesicles, manifested by the accumulation of small endocytic vesicles in CapZ-knockout cells. CapZ also recruits several RAB5 effectors, such as Rabaptin-5 and Rabex-5, to RAB5-positive early endosomes via its N-terminal domain, and this further activates RAB5. Collectively, our results indicate that CapZ regulates endosomal trafficking by controlling actin density around early endosomes and recruiting RAB5 effectors.

ANXA6 suppresses the tumorigenesis of cervical cancer through autophagy induction

Background

Autophagy is an intracellular degradation pathway conserved in eukaryotes. ANXA6 (annexin A6) belongs to a family of calcium‐dependent membrane and phospholipid‐binding proteins. Here, we identify ANXA6 as a newly synthesized protein in starvation‐induced autophagy and validate it as a novel autophagy modulator that regulates autophagosome formation.

Results

ANXA6 knockdown attenuates starvation‐induced autophagy, while restoration of its expression enhances autophagy. GO (gene ontology) analysis of ANXA6 targets showed that ANXA6 interacts with many RAB GTPases and targets endocytosis and phagocytosis pathways, indicating that ANXA6 exerts its function through protein trafficking. ATG9A (autophagy‐related 9A) is the sole multispanning transmembrane protein and its trafficking through recycling endosomes is an essential step for autophagosome formation. Our results showed that ANXA6 enables appropriate ATG9A⁺ vesicle trafficking from endosomes to autophagosomes through RAB proteins or F‐actin. In addition, restoration of ANXA6 expression suppresses mTOR (mammalian target of rapamycin) activity through the inhibition of the PI3K (phosphoinositide 3‐kinase)‐AKT and ERK (extracellular signal‐regulated kinase) signaling pathways, which is a negative regulator of autophagy. Functionally, ANXA6 expression is correlated with LC3 (microtubule‐associated protein 1 light chain 3) expression in cervical cancer, and ANXA6 inhibits tumorigenesis through autophagy induction.

Conclusions

Our results reveal an important mechanism for ANXA6 in tumor suppression and autophagy regulation.

Transendothelial transport of lipoproteins

The accumulation of low-density lipoproteins (LDL) in the arterial wall plays a pivotal role in the initiation and pathogenesis of atherosclerosis. Conversely, the removal of cholesterol from the intima by cholesterol efflux to high density lipoproteins (HDL) and subsequent reverse cholesterol transport shall confer protection against atherosclerosis. To reach the subendothelial space, both LDL and HDL must cross the intact endothelium. Traditionally, this transit is explained by passive filtration. This dogma has been challenged by the identification of several rate-limiting factors namely scavenger receptor SR-BI, activin like kinase 1, and caveolin-1 for LDL as well as SR-BI, ATP binding cassette transporter G1, and endothelial lipase for HDL. In addition, estradiol, vascular endothelial growth factor, interleukins 6 and 17, purinergic signals, and sphingosine-1-phosphate were found to regulate transendothelial transport of either LDL or HDL. Thorough understanding of transendothelial lipoprotein transport is expected to elucidate new therapeutic targets for the treatment or prevention of atherosclerotic cardiovascular disease and the development of strategies for the local delivery of drugs or diagnostic tracers into diseased tissues including atherosclerotic lesions.

Actin-dependent endosomal receptor recycling

Endosomes constitute major sorting compartments within the cell. There, a myriad of transmembrane proteins (cargoes) are delivered to the lysosome for degradation or retrieved from this fate and recycled through tubulo-vesicular transport carriers to different cellular destinations. Retrieval and recycling are orchestrated by multi-protein assemblies that include retromer and retriever, sorting nexins, and the Arp2/3 activating WASH complex. Fine-tuned control of actin polymerization on endosomes is fundamental for the retrieval and recycling of cargoes. Recent advances in the field have highlighted several roles that actin plays in this process including the binding to cargoes, stabilization of endosomal subdomains, generation of the remodeling forces required for the biogenesis of cargo-enriched transport carriers and short-range motility of the transport carriers.

Annexins: Ca2+ Effectors Determining Membrane Trafficking in the Late Endocytic Compartment

Despite the discovery of annexins 40 years ago, we are just beginning to understand some of the functions of these still enigmatic proteins. Defined and characterized by their ability to bind anionic membrane lipids in a Ca²⁺-dependent manner, each annexin has to be considered a multifunctional protein, with a multitude of cellular locations and diverse activities. Underlying causes for this considerable functional diversity include their capability to associate with multiple cytosolic and membrane proteins. In recent years, the increasingly recognized establishment of membrane contact sites between subcellular compartments opens a new scenario for annexins as instrumental players to link Ca²⁺ signalling with the integration of membrane trafficking in many facets of cell physiology. In this chapter, we review and discuss current knowledge on the contribution of annexins in the biogenesis and functioning of the late endocytic compartment, affecting endo- and exocytic pathways in a variety of physiological consequences ranging from membrane repair, lysosomal exocytosis, to cell migration.

Endocytosis and Signal Transduction: Basic Science Update

Endocytosis can be separated into the categories of phagocytosis and pinocytosis. Phagocytosis can be distinguished from pinocytosis primarily by the size of particle ingested and by its dependence on actin polymerization as a key step in particle ingestion. Several specific forms of pinocytosis have been identified that can be distinguished based on their dependence on clathrin or caveolin. Both clathrin and caveolin-dependent pinocytosis appear to require the participation of dynamin to internalize the plasma membrane. Other, less well-characterized forms of pinocytosis have also been described. Although endocytosis has long been known to affect receptor density, recent studies have demonstrated that endocytosis through clathrin- and caveolin-dependent processes plays a key role in receptor-mediated signal transduction. In some cases, blockade of these processes attenuates, or even prevents, signal transduction from taking place. This information, coupled with a better understanding of endocytosis mechanisms, will help advance the field of cell biology as well as present new targets for drug development and disease treatment.

Annexin A6 regulates interleukin-2-mediated T-cell proliferation

Annexin A6 (AnxA6) has been implicated in cell signalling by contributing to the organisation of the plasma membrane. Here we examined whether AnxA6 regulates signalling and proliferation in T cells. We used a contact hypersensitivity model to immune challenge wild-type (WT) and AnxA6(-/-) mice and found that the in vivo proliferation of CD4(+) T cells, but not CD8(+) T cells, was impaired in AnxA6(-/-) relative to WT mice. However, T-cell migration and signalling through the T-cell receptor ex vivo was similar between T cells isolated from AnxA6(-/-) and WT mice. In contrast, interleukin-2 (IL-2) signalling was reduced in AnxA6(-/-) compared with WT T cells. Further, AnxA6-deficient T cells had reduced membrane order and cholesterol levels. Taken together, our data suggest that AnxA6 regulates IL-2 homeostasis and sensitivity in T cells by sustaining a lipid raft-like membrane environment.

Subcellular localisation of retromer in post-endocytic pathways of polarised Madin-Darby canine kidney cells

BACKGROUND INFORMATION

Retromer is required for endosome-to-Golgi retrieval of the cation-independent mannose 6-phosphate receptor (CI-MPR), allowing delivery of hydrolases into lysosomes. It is constituted by a conserved heterotrimer formed by vacuolar protein sorting (Vps) gene products Vps26, Vps35 and Vps29, which is in charge of cargo selection, and a dimer of phosphoinositide-binding sorting nexins (SNXs), which has a structural role. Retromer has been implicated in sorting of additional cargo. Thus, retromer also promotes polymeric immunoglobulin A (pIgA) transcytosis by the pIgA receptor (pIgR) in polarised cells, and considerable evidence implicates retromer in controlling epithelial cell polarity. However, the precise localisation of retromer along the endocytic pathway of polarised cells has not been studied in detail.

RESULTS

Our biochemical analysis using rat liver endosome fractions suggests a distinct distribution pattern. Although subunits of the cargo-selective complex were enriched in early endosomes (EEs), levels of SNX2 were greater in sorting endosomes. We then immunolocalised the retromer subunits in polarised Madin-Darby canine kidney (MDCK) cells by confocal microscopy. An estimated 25% of total Vps26 and SNX2 localised to EEs, with negligible portions in recycling endosomes as well as in late endosomes and lysosomes. Although Vps26 was in structures of more heterogeneous size and shape than SNX2, these markedly overlapped. In consequence, the two retromer subcomplexes mostly colocalised. When we analysed retromer overlap with its cargo, we found that structures retromer and pIgA(+) are independent of those structures retromer and CI-MPR(+) . Remarkably, retromer localised preferentially at the transcytotic pathway. Pharmacological inhibition of phosphoinositide 3-kinase affected the co-distribution of retromer with pIgA and the CI-MPR, delaying pIgA progress to the apical surface.

CONCLUSIONS

In polarised MDCK cells, we found retromer associated with certain specialised EE-derived pathways. Our data imply that retromer is largely engaged in pIgA transcytosis in pIgR-expressing MDCK cells, as opposed to endosome-to-Golgi retrieval.

ERK1/2 and p38 MAPKs are complementarily involved in estradiol 17ß-D-glucuronide-induced cholestasis: crosstalk with cPKC and PI3K

Objective

The endogenous, cholestatic metabolite estradiol 17ß-d-glucuronide (E₂17G) induces endocytic internalization of the canalicular transporters relevant to bile formation, Bsep and Mrp2. We evaluated here whether MAPKs are involved in this effect.

Design

ERK1/2, JNK1/2, and p38 MAPK activation was assessed by the increase in their phosphorylation status. Hepatocanalicular function was evaluated in isolated rat hepatocyte couplets (IRHCs) by quantifying the apical secretion of fluorescent Bsep and Mrp2 substrates, and in isolated, perfused rat livers (IPRLs), using taurocholate and 2,4-dinitrophenyl-S-glutathione, respectively. Protein kinase participation in E₂17G-induced secretory failure was assessed by co-administering selective inhibitors. Internalization of Bsep/Mrp2 was assessed by confocal microscopy and image analysis.

Results

E₂17G activated all kinds of MAPKs. The PI3K inhibitor wortmannin prevented ERK1/2 activation, whereas the cPKC inhibitor Gö6976 prevented p38 activation, suggesting that ERK1/2 and p38 are downstream of PI3K and cPKC, respectively. The p38 inhibitor SB203580 and the ERK1/2 inhibitor PD98059, but not the JNK1/2 inhibitor SP600125, partially prevented E₂17G-induced changes in transporter activity and localization in IRHCs. p38 and ERK1/2 co-inhibition resulted in additive protection, suggesting complementary involvement of these MAPKs. In IPRLs, E₂17G induced endocytosis of canalicular transporters and a rapid and sustained decrease in bile flow and biliary excretion of Bsep/Mrp2 substrates. p38 inhibition prevented this initial decay, and the internalization of Bsep/Mrp2. Contrarily, ERK1/2 inhibition accelerated the recovery of biliary secretion and the canalicular reinsertion of Bsep/Mrp2.

Conclusions

cPKC/p38 MAPK and PI3K/ERK1/2 signalling pathways participate complementarily in E₂17G-induced cholestasis, through internalization and sustained intracellular retention of canalicular transporters, respectively.

Annexins as organizers of cholesterol- and sphingomyelin-enriched membrane microdomains in Niemann-Pick type C disease

Growing evidence suggests that membrane microdomains enriched in cholesterol and sphingomyelin are sites for numerous cellular processes, including signaling, vesicular transport, interaction with pathogens, and viral infection, etc. Recently some members of the annexin family of conserved calcium and membrane-binding proteins have been recognized as cholesterol-interacting molecules and suggested to play a role in the formation, stabilization, and dynamics of membrane microdomains to affect membrane lateral organization and to attract other proteins and signaling molecules onto their territory. Furthermore, annexins were implicated in the interactions between cytosolic and membrane molecules, in the turnover and storage of cholesterol and in various signaling pathways. In this review, we focus on the mechanisms of interaction of annexins with lipid microdomains and the role of annexins in membrane microdomains dynamics including possible participation of the domain-associated forms of annexins in the etiology of human lysosomal storage disease called Niemann-Pick type C disease, related to the abnormal storage of cholesterol in the lysosome-like intracellular compartment. The involvement of annexins and cholesterol/sphingomyelin-enriched membrane microdomains in other pathologies including cardiac dysfunctions, neurodegenerative diseases, obesity, diabetes mellitus, and cancer is likely, but is not supported by substantial experimental observations, and therefore awaits further clarification.

Cholesterol transport from late endosomes to the Golgi regulates t-SNARE trafficking, assembly, and function

This study shows that impaired cholesterol egress from late endosomes in cells with high annexin A6 levels is associated with altered soluble N-ethylmaleimide–sensitive fusion protein 23 (SNAP23) and syntaxin-4 cellular distribution and assembly and accumulation in Golgi membranes. This correlates with reduced secretion of cargo along the constitutive and SNAP23/syntaxin-4–dependent secretory pathway.

Cholesterol regulates plasma membrane (PM) association and functioning of syntaxin-4 and soluble N-ethylmaleimide-sensitive fusion protein 23 (SNAP23) in the secretory pathway. However, the molecular mechanism and cellular cholesterol pools that determine the localization and assembly of these target membrane SNAP receptors (t-SNAREs) are largely unknown. We recently demonstrated that high levels of annexin A6 (AnxA6) induce accumulation of cholesterol in late endosomes, thereby reducing cholesterol in the Golgi and PM. This leads to an impaired supply of cholesterol needed for cytosolic phospholipase A₂ (cPLA₂) to drive Golgi vesiculation and caveolin transport to the cell surface. Using AnxA6-overexpressing cells as a model for cellular cholesterol imbalance, we identify impaired cholesterol egress from late endosomes and diminution of Golgi cholesterol as correlating with the sequestration of SNAP23/syntaxin-4 in Golgi membranes. Pharmacological accumulation of late endosomal cholesterol and cPLA₂ inhibition induces a similar phenotype in control cells with low AnxA6 levels. Ectopic expression of Niemann-Pick C1 (NPC1) or exogenous cholesterol restores the location of SNAP23 and syntaxin-4 within the PM. Importantly, AnxA6-mediated mislocalization of these t-SNAREs correlates with reduced secretion of cargo via the SNAP23/syntaxin-4–dependent constitutive exocytic pathway. We thus conclude that inhibition of late endosomal export and Golgi cholesterol depletion modulate t-SNARE localization and functioning along the exocytic pathway.

Cholesterol transport from late endosomes to the Golgi regulates t-SNARE trafficking, assembly, and function

Cholesterol regulates plasma membrane (PM) association and functioning of syntaxin-4 and soluble N-ethylmaleimide-sensitive fusion protein 23 (SNAP23) in the secretory pathway. However, the molecular mechanism and cellular cholesterol pools that determine the localization and assembly of these target membrane SNAP receptors (t-SNAREs) are largely unknown. We recently demonstrated that high levels of annexin A6 (AnxA6) induce accumulation of cholesterol in late endosomes, thereby reducing cholesterol in the Golgi and PM. This leads to an impaired supply of cholesterol needed for cytosolic phospholipase A2(cPLA2) to drive Golgi vesiculation and caveolin transport to the cell surface. Using AnxA6-overexpressing cells as a model for cellular cholesterol imbalance, we identify impaired cholesterol egress from late endosomes and diminution of Golgi cholesterol as correlating with the sequestration of SNAP23/syntaxin-4 in Golgi membranes. Pharmacological accumulation of late endosomal cholesterol and cPLA2inhibition induces a similar phenotype in control cells with low AnxA6 levels. Ectopic expression of Niemann-Pick C1 (NPC1) or exogenous cholesterol restores the location of SNAP23 and syntaxin-4 within the PM. Importantly, AnxA6-mediated mislocalization of these t-SNAREs correlates with reduced secretion of cargo via the SNAP23/syntaxin-4–dependent constitutive exocytic pathway. We thus conclude that inhibition of late endosomal export and Golgi cholesterol depletion modulate t-SNARE localization and functioning along the exocytic pathway.

Annexin A6 is an organizer of membrane microdomains to regulate receptor localization and signalling

Annexin A6 (AnxA6) belongs to the conserved annexin protein family--a group of Ca(2+) -dependent membrane binding proteins. It is the largest of all annexin proteins and upon activation, binds to negatively charged phospholipids in the plasma membrane and endosomes. In addition, AnxA6 associates with cholesterol-rich membrane microdomains termed lipid rafts. Membrane cholesterol triggers Ca(2+) -independent translocation of AnxA6 to membranes and AnxA6 levels determine the number of caveolae, a form of specialized rafts at the cell surface. AnxA6 also has an F-actin binding domain and interacts with cytoskeleton components. Taken together, this suggests that AnxA6 has a scaffold function to link membrane microdomains with the organization of the cytoskeleton. Such a link facilitates AnxA6 to participate in plasma membrane repair and it would also impact on receptor signalling at the cell surface, growth factor, and lipoprotein receptor trafficking, Ca(2+) -channel activity and T cell activation. Hence, the regulation of cell surface receptors by AnxA6 may be facilitated by its unique structure that allows recruitment of interaction partners and simultaneously bridging specialized membrane domains with cortical actin surrounding activated receptors.

Annexin A6-Linking Ca(2+) signaling with cholesterol transport

Annexin A6 (AnxA6) belongs to a conserved family of Ca(2+)-dependent membrane-binding proteins. Like other annexins, the function of AnxA6 is linked to its ability to bind phospholipids in cellular membranes in a dynamic and reversible fashion, in particular during the regulation of endocytic and exocytic pathways. High amounts of AnxA6 sequester cholesterol in late endosomes, thereby lowering the levels of cholesterol in the Golgi and the plasma membrane. These AnxA6-dependent redistributions of cellular cholesterol pools give rise to reduced cytoplasmic phospholipase A2 (cPLA(2)) activity, retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. In addition to regulating cholesterol and caveolin distribution, AnxA6 acts as a scaffold/targeting protein for several signaling proteins, the best characterized being the Ca(2+)-dependent membrane targeting of p120GAP to downregulate Ras activity. AnxA6 also stimulates the Ca(2+)-inducible involvement of PKC in the regulation of HRas and possibly EGFR signal transduction pathways. The ability of AnxA6 to recruit regulators of the EGFR/Ras pathway is likely potentiated by AnxA6-induced actin remodeling. Accordingly, AnxA6 may function as an organizer of membrane domains (i) to modulate intracellular cholesterol homeostasis, (ii) to create a scaffold for the formation of multifactorial signaling complexes, and (iii) to regulate transient membrane-actin interactions during endocytic and exocytic transport. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Plasmon-resonant nanoparticles and nanostars with magnetic cores: synthesis and magnetomotive imaging

Plasmon-resonant gold nanostars (NSTs) with magnetic cores were synthesized by a multistep sequence from superparamagnetic Fe3O4 nanoparticles (NPs) and evaluated as optical contrast agents under magnetomotive (MM) imaging conditions. Core-shell Fe3O4@Au NPs were prepared in nonpolar organic solvents with nanometer control over shell thickness and with good epitaxy to the Fe3O4 surface. Anisotropic growth was performed in micellar solutions of cetyltrimethylammonium bromide (CTAB) under mildly reducing conditions, resulting in NSTs with physical features similar to those produced from colloidal gold seeds. NSTs could be produced below 100 nm from tip to tip, but seed size had a significant impact on growth habit, with larger seed particles producing submicrometer-sized "morning stars". Both NSTs and aggregated core-shell NPs are responsive to in-plane magnetic field gradients and can provide enhanced near-infrared (NIR) contrast under MM conditions, but do so by different mechanisms. NSTs can modulate polarized NIR scattering with minimal translational motion, giving the appearance of a periodic but stationary "blinking", whereas core-shell NP aggregates require lateral displacement for signal modulation. The polarization-sensitive MM imaging modality offers the dual advantage of enhanced signal quality and reduced background signal and can be applied toward the detection of magnetomotive NSTs in heterogeneous biological samples, as illustrated by their detection inside of granular cells such as macrophages.

Annexin A6-induced alterations in cholesterol transport and caveolin export from the Golgi complex

Annexin A6 (AnxA6) belongs to a family of Ca(2+)-dependent membrane-binding proteins and is involved in the regulation of endocytic and exocytic pathways. We previously demonstrated that AnxA6 regulates receptor-mediated endocytosis and lysosomal targeting of low-density lipoproteins and translocates to cholesterol-enriched late endosomes (LE). As cholesterol modulates the membrane binding and the cellular location of AnxA6, but also affects the intracellular distribution of caveolin, we investigated the localization and trafficking of caveolin in AnxA6-expressing cells. Here, we show that cells expressing high levels of AnxA6 are characterized by an accumulation of caveolin-1 (cav-1) in the Golgi complex. This is associated with a sequestration of cholesterol in the LE and lower levels of cholesterol in the Golgi and the plasma membrane, both likely contributing to retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. Further strengthening these findings, knock down of AnxA6 and the ectopic expression of the Niemann-Pick C1 protein in AnxA6-overexpressing cells restore the cellular distribution of cav-1 and cholesterol, respectively. In summary, this study demonstrates that elevated expression levels of AnxA6 perturb the intracellular distribution of cholesterol, which indirectly inhibits the exit of caveolin from the Golgi complex.

Role of exosomes in immune regulation

Exosomes are small vesicles originating from late endosomes, 30–100 nm in diameter with typical cup-shape morphology. They are reported to bear high levels of a narrow spectrum of molecules involved in immune response and signal transduction. Apart from removing obsolete membrane proteins, some surprising biological functions of exosomes were unveiled recently and their applications in immunotherapy of tumors are currently intensively investigated. Dendritic cell- (DC) and tumor-derived exosomes have considerable anti-tumor effects in experimental studies and several clinical trials. Despite their potential applications in eliciting a “positive” immune response, exosomes might induce some “unwanted” immune responses, such as immune tolerance and immune evasion. Therefore further investigations about the physiological functions of exosomes and the optimal way of exosome application in tumor immunotherapy are necessary. This review presents recent developments in the field of exosome research and focuses on its applications to tumor immunotherapy.

Adaptor heat shock protein complex formation regulates trafficking of the asialoglycoprotein receptor

In the asialoglycoprotein receptor (ASGPR) endocytic pathway, internalized receptors pass through early, recycling, and sorting endosomal compartments before returning to the cell surface. Sorting motifs in the cytoplasmic domain (CD) and protein interactions with these sequences presumably direct receptor trafficking. Previous studies have shown that association of a potential sorting heat shock protein (HSP) heterocomplex with the ASGPR-CD was regulated by casein kinase 2 (CK2)-mediated phosphorylation. Mass spectrometry and immunoblot analyses identified five of these ASGPR-CD-associated proteins as the molecular chaperones glycoprotein 96, HSP70, HSP90, cyclophilin A, and FK 506 binding protein. The present study was undertaken to determine whether any of the adaptor protein complexes (AP1, AP2, or AP3) were selectivity associated with the ASGPR-CD. In conjunction with molecular chaperones, AP2 and AP1 were recovered from a CK2 phosphorylated agarose-GSH-GST-ASGPR-CD matrix. Binding of AP3 was independent of the phosphorylation status of the CD matrix. Inhibition of CK2-mediated phosphorylation with tetrabromobenzotriazole prevented AP recovery within an immunoadsorbed ASGPR complex. Rapamycin, which dissociates the HSP heterocomplex from ASGPR-CD, thereby altering receptor trafficking also, inhibited AP association. Similar results were obtained with an inhibitor of HSP90 heterocomplex formation, geldanmycin. The data presented provide evidence that recruitment of AP1 and AP2, which is necessary for appropriate receptor trafficking, is mediated by the interaction of AP with the ASGPR-CD-bound HSP complex.

CART: an Hrs/actinin-4/BERP/myosin V protein complex required for efficient receptor recycling

Altering the number of surface receptors can rapidly modulate cellular responses to extracellular signals. Some receptors, like the transferrin receptor (TfR), are constitutively internalized and recycled to the plasma membrane. Other receptors, like the epidermal growth factor receptor (EGFR), are internalized after ligand binding and then ultimately degraded in the lysosome. Routing internalized receptors to different destinations suggests that distinct molecular mechanisms may direct their movement. Here, we report that the endosome-associated protein hrs is a subunit of a protein complex containing actinin-4, BERP, and myosin V that is necessary for efficient TfR recycling but not for EGFR degradation. The hrs/actinin-4/BERP/myosin V (CART [cytoskeleton-associated recycling or transport]) complex assembles in a linear manner and interrupting binding of any member to its neighbor produces an inhibition of transferrin recycling rate. Disrupting the CART complex results in shunting receptors to a slower recycling pathway that involves the recycling endosome. The novel CART complex may provide a molecular mechanism for the actin-dependence of rapid recycling of constitutively recycled plasma membrane receptors.

Structure of human annexin a6 at the air-water interface and in a membrane-bound state

We postulate the existence of a pH-sensitive domain in annexin A6 (AnxA6), on the basis of our observation of pH-dependent conformational and orientation changes of this protein and its N- (AnxA6a) and C-terminal (AnxA6b) halves in the presence of lipids. Brewster angle microscopy shows that AnxA6, AnxA6a, and AnxA6b in the absence of lipids accumulate at the air-water interface and form a stable, homogeneous layer at pH below 6.0. Under these conditions polarization modulation IR absorption spectroscopy reveals significant conformational changes of AnxA6a whereas AnxA6b preserves its alpha-helical structure. The orientation of protein alpha-helices is parallel with respect to the interface. In the presence of lipids, polarization modulation IR reflection absorption spectroscopy experiments suggest that AnxA6a incorporates into the lipid/air interface, whereas AnxA6b is adsorbed under the lipid monolayer. In this case AnxA6a regains its alpha-helical structures. At a higher pressure of the lipid monolayer the average orientation of the alpha-helices of AnxA6a changes from flat to tilted by 45 degrees with respect to normal to the membrane interface. For AnxA6b no such changes are detected, even at a high pressure of the lipid monolayer-suggesting that the putative pH-sensitive domain of AnxA6 is localized in the N-terminal half of the protein.

Microtubule-dependent movement of late endocytic vesicles in vitro: requirements for Dynein and Kinesin

Our previous studies demonstrated that fluorescent early endocytic vesicles prepared from rat liver after injection of Texas red asialoorosomucoid contain asialoglycoprotein and its receptor and move and undergo fission along microtubules using kinesin I and KIFC2, with Rab4 regulating KIFC2 activity (J. Cell Sci. 116, 2749, 2003). In the current study, procedures to prepare fluorescent late endocytic vesicles were devised. In addition, flow cytometry was utilized to prepare highly purified fluorescent endocytic vesicles, permitting validation of microscopy-based experiments as well as direct biochemical analysis. These studies revealed that late vesicles bound to and moved along microtubules, but in contrast to early vesicles, did not undergo fission. As compared with early vesicles, late vesicles had reduced association with receptor, Rab4, and kinesin I but were highly associated with dynein, Rab7, dynactin, and KIF3A. Dynein and KIF3A antibodies inhibited late vesicle motility, whereas kinesin I and KIFC2 antibodies had no effect. Dynamitin antibodies prevented the association of late vesicles with microtubules. These results indicate that acquisition and exchange of specific motor and regulatory proteins characterizes and may regulate the transition of early to late endocytic vesicles. Flow cytometric purification should ultimately facilitate detailed proteomic analysis and mapping of endocytic vesicle-associated proteins.

The FAM deubiquitylating enzyme localizes to multiple points of protein trafficking in epithelia, where it associates with E-cadherin and beta-catenin

Ubiquitylation is a necessary step in the endocytosis and lysosomal trafficking of many plasma membrane proteins and can also influence protein trafficking in the biosynthetic pathway. Although a molecular understanding of ubiquitylation in these processes is beginning to emerge, very little is known about the role deubiquitylation may play. Fat Facets in mouse (FAM) is substrate-specific deubiquitylating enzyme highly expressed in epithelia where it interacts with its substrate, beta-catenin. Here we show, in the polarized intestinal epithelial cell line T84, FAM localized to multiple points of protein trafficking. FAM interacted with beta-catenin and E-cadherin in T84 cells but only in subconfluent cultures. FAM extensively colocalized with beta-catenin in cytoplasmic puncta but not at sites of cell-cell contact as well as immunoprecipitating with beta-catenin and E-cadherin from a higher molecular weight complex ( approximately 500 kDa). At confluence FAM neither colocalized with, nor immunoprecipitated, beta-catenin or E-cadherin, which were predominantly in a larger molecular weight complex ( approximately 2 MDa) at the cell surface. Overexpression of FAM in MCF-7 epithelial cells resulted in increased beta-catenin levels, which localized to the plasma membrane. Expression of E-cadherin in L-cell fibroblasts resulted in the relocalization of FAM from the Golgi to cytoplasmic puncta. These data strongly suggest that FAM associates with E-cadherin and beta-catenin during trafficking to the plasma membrane.

Receptor-mediated endocytosis by the asialoglycoprotein receptor: effect of ethanol administration on endosomal distribution of receptor and ligand

Using the asialoglycoprotein receptor (ASGP-R) and a representative ligand, asialoorosomucoid (ASOR), we have previously shown ethanol-induced impairment of endosomal acidification, receptor recycling and ligand binding, internalization, and degradation. In the current study, we further investigated ethanol-induced alterations in receptor/ligand trafficking by labeling endosomes in vivo with either Texas-Red-ASOR or 125I-ASOR, and then assessing the receptor/ligand content of endosomes. We assessed two fractions after both 5 and 25 min of labeling: 'early endosomes' (EEs; endosomes from the cell periphery) and 'late endosomes' (LEs; endosomes farther into the cell interior). At both time points, significantly more ligand was found in EE fractions isolated from chow- and pair-fed controls (3:1, EE to LE, respectively). However, endosomes isolated from ethanol-fed animals showed a shift over time toward a more equal ligand distribution between endosome fractions (P < or = 0.05). Analysis of the ASGP-R content revealed a distribution pattern between the endosome fractions similar to that observed for ligand distribution. Impairment of receptor-ligand dissociation was assessed in endosome fractions by determining bound/free ligand ratios. Analysis showed that most of the ligand present in both endosome fractions was free (56-99%), although more was bound to receptor in EE vs LE of both control and ethanol animals (P < or = 0.05). At 5 min, more ligand remained bound in endosomes from ethanol-fed animals compared with control endosomes (P < or = 0.05), and the same pattern was observed at the latter time point. These results suggest that delayed dissociation may cause the receptor ligand complexes to travel farther into the cell interior, which may impair proper trafficking of the ligand to lysosomes and alter the receptor recycling.

Roles of the cytoskeleton and motor proteins in endocytic sorting

After internalization, endocytic material is actively transported through the cytoplasm, predominantly by microtubule motor proteins. Microtubule-based endocytic transport facilitates sorting of endocytic contents, vesicle fusion and fission, delivery to lysosomes, cytosolic dispersal, as well as nuclear uptake and cytosolic egress of pathogens. Endosomes, like most organelles, move bidirectionally through the cytosol and regulate their cellular location by controlling the activity of motor proteins, and potentially by controlling microtubule and actin polymerization. Control of motor protein activity is manifest by increased microtubule "run lengths", and the binding of motor proteins to organelles can be regulated by motor protein receptors. A mechanistic understanding of how organelles control motor protein activity to allow for endocytic sorting presents an exciting avenue for future research.

Nuclear annexin II negatively regulates growth of LNCaP cells and substitution of ser 11 and 25 to glu prevents nucleo-cytoplasmic shuttling of annexin II

BACKGROUND

Annexin II heavy chain (also called p36, calpactin I) is lost in prostate cancers and in a majority of prostate intraepithelial neoplasia (PIN). Loss of annexin II heavy chain appears to be specific for prostate cancer since overexpression of annexin II is observed in a majority of human cancers, including pancreatic cancer, breast cancer and brain tumors. Annexin II exists as a heterotetramer in complex with a protein ligand p11 (S100A10), and as a monomer. Diverse cellular functions are proposed for the two forms of annexin II. The monomer is involved in DNA synthesis. A leucine-rich nuclear export signal (NES) in the N-terminus of annexin II regulates its nuclear export by the CRM1-mediated nuclear export pathway. Mutation of the NES sequence results in nuclear retention of annexin II.

RESULTS

Annexin II localized in the nucleus is phosphorylated, and the appearance of nuclear phosphorylated annexin II is cell cycle dependent, indicating that phosphorylation may play a role in nuclear entry, retention or export of annexin II. By exogenous expression of annexin II in the annexin II-null LNCaP cells, we show that wild-type annexin II is excluded from the nucleus, whereas the NES mutant annexin II localizes in both the nucleus and cytoplasm. Nuclear retention of annexin II results in reduced cell proliferation and increased doubling time of cells. Expression of annexin II, both wild type and NES mutant, causes morphological changes of the cells. By site-specific substitution of glutamic acid in the place of serines 11 and 25 in the N-terminus, we show that simultaneous phosphorylation of both serines 11 and 25, but not either one alone, prevents nuclear localization of annexin II.

CONCLUSION

Our data show that nuclear annexin II is phosphorylated in a cell cycle-dependent manner and that substitution of serines 11 and 25 inhibit nuclear entry of annexin II. Aberrant accumulation of nuclear annexin II retards proliferation of LNCaP cells.

Annexin II expression is reduced or lost in prostate cancer cells and its re-expression inhibits prostate cancer cell migration

While studying Bim, a BH3-only proapoptotic protein, we identified an approximately 36 kDa protein, which was abundantly expressed in all five strains of primary normal human prostate (NHP) epithelial cells but significantly reduced or lost in seven prostate cancer cell lines. The approximately 36 kDa protein was subsequently identified as annexin II by proteomic approach and confirmed by Western blotting using an annexin II-specific antibody. Conventional and 2D SDS-PAGE, together with Western blotting, also revealed reduced or lost expression of annexin I in prostate cancer cells. Subcellular localization studies revealed that in NHP cells, annexin II was distributed both in the cytosol and underneath the plasma membrane, but not on the cell surface. Prostate cancer cells showed reduced levels as well as altered expression patterns of annexin II. Since annexins play important roles in maintaining Ca(2+) homeostasis and regulating the cytoskeleton and cell motility, we hypothesized that the reduced or lost expression of annexin I/II might promote certain aggressive phenotypes of prostate cancer cells. In subsequent experiments, we indeed observed that restoration of annexin II expression inhibited the migration of the transfected prostate cancer cells without affecting cell proliferation or apoptosis. Hence, our results suggest that annexin II, and, likely, annexin I, may be endogenous suppressors of prostate cancer cell migration and their reduced or lost expression may contribute to prostate cancer development and progression.

Nongenomic estrogen action regulates tyrosine phosphatase activity and tuberin stability

Estrogen action and tuberin function has been suggested to play a crucial role in the proliferation of lung smooth muscle-like cells and/or myofibroblasts in pulmonary lymphangioleiomyomatosis (LAM). Tuberin is a tumor suppressor phosphoprotein, which also regulates fluid phase endocytosis. Its activity, turnover and complex association with hamartin depends on its phosphorylation status. We have recently reported that nongenomic estrogen action regulates the phosphorylation status of several cytoplasmic proteins. Herein, we demonstrate that estrogen increases tyrosine phosphatase activity, which can be abrogated by antiestrogen ICI 182780 and tyrosine phosphatase inhibitor bpV(phen), but not by the protein synthesis inhibitor cyclohexamide. Furthermore, we show that estrogen transiently enhances the turnover of tuberin, which follows an inverse pattern to that observed for tyrosine phosphatase and endocytosis activity. We showed that tuberin phosphorylation protects it from degradation and induces its accumulation in female human lung fibroblasts and myofibroblasts. Our results suggest that nongenomic estrogen action induces tyrosine phosphatase activity that regulates stability of tyrosine phosphorylated proteins, including tuberin, which may play a crucial role in cellular specific functions such as endocytosis.

A balance of KIF1A-like kinesin and dynein organizes early endosomes in the fungus Ustilago maydis

In Ustilago maydis, bidirectional transport of early endosomes is microtubule dependent and supports growth and cell separation. During early budding, endosomes accumulate at putative microtubule organizers within the bud, whereas in medium-budded cells, endosome clusters appear at the growing ends of microtubules at the distal cell pole. This suggests that motors of opposing transport direction organize endosomes in budding cells. Here we set out to identify these motors and elucidate the molecular mechanism of endosome reorganization. By PCR we isolated kin3, which encodes an UNC-104/KIF1-like kinesin from U.maydis. Recombinant Kin3 binds microtubules and has ATPase activity. Kin3-green fluorescent protein moves along microtubules in vivo, accumulates at sites of growth and localizes to endosomes. Deletion of kin3 reduces endosome motility to approximately 33%, and abolishes endosome clustering at the distal cell pole and at septa. This results in a transition from bipolar to monopolar budding and cell separation defects. Double mutant analysis indicates that the remaining motility in Deltakin3-mutants depends on dynein, and that dynein and Kin3 counteract on the endosomes to arrange them at opposing cell poles.

Prolonged unloading of rat soleus muscle causes distinct adaptations of the gene profile

Using commercially available microarray technology, we investigated a series of transcriptional adaptations caused by atrophy of rat m. soleus due to 35 days of hindlimb suspension. We detected 395 out of 1,200 tested transcripts, which reflected 1%-5% of totally expressed genes. From various cellular functional pathways, we detected multiple genes that spanned a 200-fold range of gene expression levels. Statistical analysis combining L1 regression with the sign test based on the conservative Bonferroni correction identified 105 genes that underwent transcriptional adaptations with atrophy. Generally, expressional changes were discrete (<50%) and pointed in the same direction for genes belonging to the same cellular functional units. In particular, a distinct expressional adaptation of genes involved in fiber transformation; that is, metabolism, protein turnover, and cell regulation were noted and matched to corresponding transcriptional changes in nutrient trafficking. Expressional changes of extracellular proteases, and of genes involved in nerve-muscle interaction and excitation-contraction coupling identify previously not recognized adaptations that occur in atrophic m. soleus. Considerations related to technical and statistical aspects of the array approach for profiling the skeletal muscle genome and the impact of observed novel adaptations of the m. soleus transcriptome are put into perspective of the physiological adaptations occurring with muscular atrophy.

Integrin clustering induces kinectin accumulation

Integrin receptors mediate the formation of adhesion complexes and play important roles in signal transduction from the extracellular matrix. Integrin-based adhesion complexes (IAC) contain proteins that link integrins to the cytoskeleton and recruit signaling molecules, including vinculin,paxillin, focal adhesion kinase, talin and α-actinin. In this study, we describe a ∼160 kDa protein that is markedly enriched at IAC induced by clustering integrins with fibronectin-coated beads. Protein sequence analysis reveals that this ∼160 kDa protein is kinectin. Kinectin is an integral membrane protein found in endoplasmic reticulum, and it serves as a receptor for the motor protein kinesin. Fibronectin-induced IAC sequestered over half of the total cellular content of kinectin within 20 minutes. In addition, two other ER-resident proteins, RAP [low-density lipoprotein receptor-related protein (LRP) receptor-associated protein] and calreticulin, were found to be clustered at IAC, whereas kinesin was not. Our results identify a novel class of constituents of IAC.

Pharmacodynamics and pharmacogenomics of diverse receptor-mediated effects of methylprednisolone in rats using microarray analysis

Corticosteroids such as methylprednisolone (MPL) produce many of their anti-inflammatory, immunosuppressive, and exaggerated physiological effects by receptor and gene-mediated mechanisms. The temporal pattern of change in four genes in rat tissues was measured by quantitative Northern hybridization and rtPCR after a single dose of MPL. Two profiles were observed: two genes with enhanced expression showed a slow onset and moderate rate of decline within a 24 hr time frame while two genes with reduced expression exhibited a rapid onset and prolonged suppression over a > or = 72 hr time span. These patterns are consistent with and rationalized by pharmacodynamic expectations based on earlier models. cDNA microarrays used to assess the expression levels of 5200 genes at one optimal time-point showed marked variation in baseline values. Of these, 20 genes showed statistically significant enhanced expression with increases ranging from 130 to 1690%, 31 genes exhibited reduced expression ranging from 31 to 72% of control. Many genes could be categorized as affecting acute phase/immune response, energy metabolism, microsomal metabolism, and hepatic function. These studies provide the first simultaneous assessment of the diversity in pharmacogenomic effects of corticosteroids. They also provide some insight into the advantages and limitations of microarray measurements in regard to the pharmacodynamics of drugs having complex, multi-faceted, and integrated mechanisms of action.

Legumain from bovine kidney: its purification, molecular cloning, immunohistochemical localization and degradation of annexin II and vitamin D-binding protein

Legumain (asparaginyl endopeptidase) was purified to homogeneity from bovine kidneys. The molecular mass of the purified enzyme was calculated to be 34000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence of beta-mercaptoethanol. The enzyme rapidly hydrolyzed the substrate Z-Ala-Ala-Asn-MCA and was strongly inhibited by N-ethylmaleimide, p-chloromercuribenzene-sulfonic acid, Hg(2+) and Cu(2+). The amino acid sequence of the first 26 residues of the enzyme was Gly-Gly-Lys-His-Trp-Val-Val-Ile-Val-Ala-Gly-Ser-Asn-Gly-Gln-Tyr-Asn-Tyr-Arg-His-Gln-Ala-Phe-Ala-Asp-His-. This sequence is highly homologous to the sequences in the N-terminal of pig kidney legumain. We screened a bovine kidney cortex cDNA library using a DNA probe that originated from rat legumain, and we determined the bovine kidney cDNA structure and deduced the amino acid sequence. The cDNA is composed 1934 bp and encodes 433 amino acids in the coding region. The enzyme was strongly stained in the proximal tubules of the rat kidney in an immunohistochemical study. Vitamin D-binding protein which is known to be a ligand to megalin existing in the proximal tubules, was cleaved in a limited proteolytic manner by bovine kidney legumain. These results suggested that legumain contributes to the processing of macromolecules absorbed by proximal tubule cells. The enzyme also cleaved an N-terminal synthetic peptide of bovine annexin II (Gly(24)-Ser-Val-Lys-Ala-Tyr-Thr(30)-Asn-Phe-Asp-Ala-Glu(35)-Arg-Asp(37)) at a position between Asn(31) and Phe(32). The amino-terminal domain of annexin II has p11 subunit binding sites and phosphorylation sites for both pp60(src) and protein kinase C. This suggests that legumain plays an important role in inactivation and degradation of annexin II, which is abundant in the receptor-recycling compartments of endosomes/lysosomes.

VP40, the matrix protein of Marburg virus, is associated with membranes of the late endosomal compartment

Localization of VP40 in Marburg virus (MBGV)-infected cells was studied by using immunofluorescence and immunoelectron microscopic analysis. VP40 was detected in association with nucleocapsid structures, present in viral inclusions and at sites of virus budding. Additionally, VP40 was identified in the foci of virus-induced membrane proliferation and in intracellular membrane clusters which had the appearance of multivesicular bodies (MVBs). VP40-containing MVBs were free of nucleocapsids. When analyzed by immunogold labeling, the concentration of VP40 in MVBs was six times higher than in nucleocapsid structures. Biochemical studies showed that recombinant VP40 represented a peripheral membrane protein that was stably associated with membranes by hydrophobic interaction. Recombinant VP40 was also found in association with membranes of MVBs and in filopodia- or lamellipodia-like protrusions at the cell surface. Antibodies against marker proteins of various cellular compartments showed that VP40-positive membranes contained Lamp-1 and the transferrin receptor, confirming that they belong to the late endosomal compartment. VP40-positive membranes were also associated with actin. Western blot analysis of purified MBGV structural proteins demonstrated trace amounts of actin, Lamp-1, and Rab11 (markers of recycling endosomes), while markers for other cellular compartments were absent. Our data indicate that MBGV VP40 was able to interact with membranes of late endosomes in the course of viral infection. This capability was independent of other MBGV proteins.

Evidence for the Involvement of annexin 6 in the trafficking between the endocytic compartment and lysosomes

Annexins are a family of calcium-dependent phospholipid-binding proteins, which have been implicated in a variety of biological processes including membrane trafficking. The annexin 6/lgp120 prelysosomal compartment of NRK cells was loaded with low-density lipoprotein (LDL) and then its transport from this endocytic compartment and its degradation in lysosomes were studied. NRK cells were microinjected with the mutated annexin 6 (anx6(1-175)), to assess the possible involvement of annexin 6 in the transport of LDL from the prelysosomal compartment. The results indicated that microinjection of mutated annexin 6, in NRK cells, showed the accumulation of LDL in larger endocytic structures, denoting retention of LDL in the prelysosomal compartment. To confirm the involvement of annexin 6 in the trafficking and the degradation of LDL we used CHO cells transfected with mutated annexin 6(1-175). Thus, in agreement with NRK cells the results obtained in CHO cells demonstrated a significant inhibition of LDL degradation in CHO cells expressing the mutated form of annexin 6 compared to controls overexpressing wild-type annexin 6. Therefore, we conclude that annexin 6 is involved in the trafficking events leading to LDL degradation.

Activation of Raf-1 is defective in annexin 6 overexpressing Chinese hamster ovary cells

Annexin 6 is a Ca2+-dependent phospholipid-binding protein involved in membrane trafficking. In this study we demonstrate the association of Raf-1 with recombinant rat annexin 6. Raf-annexin 6 interaction was shown to be independent of cell activation by epidermal growth factor (EGF) or phorbol esters (12-O-tetradecanoyl-phorbol-13-acetate (TPA)). A stable Chinese hamster ovary (CHO)-anx6 cell line overexpressing annexin 6 was established to examine the function of annexin 6. In these cells, no increase of Ras-GTP levels, induced by EGF or TPA, was detected. In addition, the activity of Raf was completely inhibited, whereas the mitogen-activated protein kinase-P was unaffected.

Proteomic analysis of dendritic cell-derived exosomes: a secreted subcellular compartment distinct from apoptotic vesicles

Dendritic cells constitutively secrete a population of small (50-90 nm diameter) Ag-presenting vesicles called exosomes. When sensitized with tumor antigenic peptides, dendritic cells produce exosomes, which stimulate anti-tumor immune responses and the rejection of established tumors in mice. Using a systematic proteomic approach, we establish the first extensive protein map of a particular exosome population; 21 new exosomal proteins were thus identified. Most proteins present in exosomes are related to endocytic compartments. New exosomal residents include cytosolic proteins most likely involved in exosome biogenesis and function, mainly cytoskeleton-related (cofilin, profilin I, and elongation factor 1alpha) and intracellular membrane transport and signaling factors (such as several annexins, rab 7 and 11, rap1B, and syntenin). Importantly, we also identified a novel category of exosomal proteins related to apoptosis: thioredoxin peroxidase II, Alix, 14-3-3, and galectin-3. These findings led us to analyze possible structural relationships between exosomes and microvesicles released by apoptotic cells. We show that although they both represent secreted populations of membrane vesicles relevant to immune responses, exosomes and apoptotic vesicles are biochemically and morphologically distinct. Therefore, in addition to cytokines, dendritic cells produce a specific population of membrane vesicles, exosomes, with unique molecular composition and strong immunostimulating properties.

The small GTPase Rab4A interacts with the central region of cytoplasmic dynein light intermediate chain-1

Rab4 belongs to the Rab family of small GTPases involved in the regulation of intracellular transport, and has been localized to early endosomes. We have employed the yeast two-hybrid system to identify proteins that specifically interact with Rab4AQ67L, a GTPase-deficient mutant form of Rab4A. Screening a mouse embryo cDNA library identified a clone (M449) that interacted with Rab4A in a nucleotide-dependent fashion. Data base searches identified this clone as the mouse cytoplasmic dynein light intermediate chain-1 (LIC-1). Based on this finding, the full-length equivalent human cytoplasmic dynein LIC-1 was isolated by PCR. When Rab4A was overexpressed together with either M449 or dynein LIC-1 in HeLa cells, the proteins were found to colocalize in the perinuclear region. We characterize the localization of both overexpressed human dynein LIC-1 and the endogenous protein with respect to microtubules and show that it concentrates to the microtubule-organizing center and mitotic spindle. Additionally, GFPRab4A endosomes localize to microtubules and are redistributed by nocodazole treatment. This is the first described interaction between cytoplasmic dynein, a retrograde motor protein, and a Rab protein.

Endocytic traffic in polarized epithelial cells: role of the actin and microtubule cytoskeleton

The cytoskeleton is required for multiple cellular events including endocytosis and the transfer of cargo within the endocytic system. Polarized epithelial cells are capable of endocytosis at either of their distinct apical or basolateral plasma membrane domains. Actin plays a role in internalization at both cell surfaces. Microtubules and actin are required for efficient transcytosis and delivery of proteins to late endosomes and lysosomes. Microtubules are also important in apical recycling pathways and, in some polarized cell types, basolateral recycling requires actin. The microtubule motor proteins dynein and kinesin and the class I unconventional myosin motors play a role in many of these trafficking steps. This review examines the endocytic pathways of polarized epithelial cells and focuses on the emerging roles of the actin cytoskeleton in these processes.

Identification of receptor ligands by screening phage-display peptide libraries ex vivo on microdissected kidney tubules

A novel method to identify receptor ligands for defined renal tubular segments has been developed. Ex vivo screening of phage-display peptide libraries on isolated intact rat proximal convoluted tubules (PCT) and cortical collecting ducts (CCD) allowed the direct access of phage to the basolateral surface of tubular epithelial cells. Two distinct peptide motifs were selected for CCD and PCT, indicating differential expression of some membrane receptors on the basolateral surface of defined kidney tubule segments. Using the linear peptide motif ELRGD(R/M)AX(W/L), recovered from freshly isolated rat CCD, mediated 16-fold selectivity of phage binding to CCD compared with PCT. Binding to CCD was 39-fold higher than that of a random control phage. Binding and subsequent internalization of phage, most likely by an integrin-mediated endocytosis pathway, was abolished by the addition of the corresponding synthetic peptide. Furthermore, the results demonstrate that presentation and flanking amino acids determine the specific binding properties of RGD ligands to their putative integrin receptors. The results emphasize the need of a native cell system for the identification of renal epithelial cell surface ligands. Such ligands are of potential relevance for the analysis of interactions between extracellular matrix and kidney tubules or for the development of improved vectors for kidney-specific drug delivery or gene transfer.

Annexins as nucleotide-binding proteins: facts and speculations

Annexins are ubiquitous multifunctional Ca2+ and phospholipid-binding proteins whose mechanism of function remains largely unknown. The accumulated in vitro experimental evidence indicates that ATP and GTP are functional ligands for nucleotide-sensitive annexin isoforms. Such nucleotide binding could modulate Ca2+ homeostasis, vesicular transport and/or signal transduction pathways and link them to cellular energy metabolism. Alternatively, since annexins are able to interact with other nucleotide-utilizing proteins, such as various kinases, GTPases and structural proteins, these proteins could influence the guanine nucleotide exchange metabolism and/or control the activity of various G proteins. The nucleotide-binding properties of annexins may affect the development or maintenance of some pathologies and diseases in which changes in physiological concentrations of purine nucleotides or disruption of Ca2+ homeostasis are crucial targets.

Analysis of gap junction formation in rat hepatocytes by intravenous injection of an anti-connexin32 monoclonal antibody (HAM8)

HAM8 monoclonal antibody was used to examine the mechanism of connexin32 (Cx32) formation in the rat model in vivo by immunofluorescence and immunoelectron microscopy. After a single intravenous injection of HAM8 IgG, a number of HAM8 signals were observed at the sinusoidal face, between adjacent hepatocytes as well as in the cytoplasm stained with only 2nd fluorescent antibody. Moreover, the in vivo localization of the HAM8 antigen appeared to change with time. At 5 min after the antibody injection, Cx32 signals from liver sections were clearly detected at the sinusoidal face. Fifteen minutes later, numerous linear and dotted fluorescent signals were observed between hepatocytes; in addition, much punctate staining was found at the sinusoidal face and in hepatocytes. These findings were identified by immunoelectron microscopy. Interestingly, 1 hr later, much punctate staining considered to be similar to those seen in the normal rat liver tissues was observed between adjacent hepatocytes, suggesting that a great deal of Cx32 combined with HAM8 have been assembled into identifiable gap junction plaques. Five hours later, intercellular and intracellular Cx32 signals were infrequently detected. When staining was performed with HAM8 and 2nd antibody, however, numerous Cx32 signals were again observed between neighboring hepatocytes, as punctate staining appearing in a pattern approximately the same as that seen in the normal liver tissue. Based on these results, we assumed that a precursor gap was present during Cx32 formation, and discussed the pathways of Cx32 formation and the degradation of Cx32 as well as that of HAM8.