Biogenesis and function of multivesicular bodies

Regulators of Vps4 ATPase activity at endosomes differentially influence the size and rate of formation of intralumenal vesicles

Disassembly of ESCRT-III requires Vps4 ATPase activity under the control of regulatory proteins. Described here are distinct spatiotemporal functions for Vps4 regulators, with Did2 playing a unique role in regulating MVB lumenal vesicle size and Vtal-Vps60 promoting efficient membrane scission and delivery of vesicles into the endosome lumen.

Recruitment of endosomal sorting complexes required for transport (ESCRTs) to the cytosolic face of endosomes regulates selective inclusion of transmembrane proteins into the lumenal vesicles of multivesicular bodies (MVBs). ESCRT-0, -I, and -II bind directly to ubiquitinated transmembrane cargoes of the MVB pathway, whereas polymerization of ESCRT-III at endosomes is thought to bend the membrane and/or provide the energetic force that drives membrane scission and detachment of vesicles into the endosome lumen. Disassembly of the ESCRT-III polymer and dissociation of its subunits from endosomes requires the Vps4 ATPase, the activity of which is controlled in vivo by regulatory proteins. We identify distinct spatiotemporal roles for Vps4-regulating proteins through examinations of subcellular localization and endosome morphology. Did2 plays a unique role in the regulation of MVB lumenal vesicle size, whereas Vtal and Vps60 promote efficient membrane scission and delivery of membrane to the endosome lumen. These morphological effects probably result from Vps4-mediated manipulations of ESCRT-III, because we show dissociation of ESCRT-0, -I, and -II from endosomes is not directly dependent on Vps4 activity.

Mechanism of amyloid plaque formation suggests an intracellular basis of Abeta pathogenicity

The formation of extracellular amyloid plaques is a common patho-biochemical event underlying several debilitating human conditions, including Alzheimer's disease (AD). Considerable evidence implies that AD damage arises primarily from small oligomeric amyloid forms of Abeta peptide, but the precise mechanism of pathogenicity remains to be established. Using a cell culture system that reproducibly leads to the formation of Alzheimer's Abeta amyloid plaques, we show here that the formation of a single amyloid plaque represents a template-dependent process that critically involves the presence of endocytosis- or phagocytosis-competent cells. Internalized Abeta peptide becomes sorted to multivesicular bodies where fibrils grow out, thus penetrating the vesicular membrane. Upon plaque formation, cells undergo cell death and intracellular amyloid structures become released into the extracellular space. These data imply a mechanism where the pathogenic activity of Abeta is attributed, at least in part, to intracellular aggregates.

Internalization and fusion mechanism of vesicular stomatitis virus and related rhabdoviruses

Members of the Rhabdoviridae infect a wide variety of animals and plants, and are the causative agents of many important diseases. Rhabdoviruses enter host cells following internalization into endosomes, with the glycoprotein (G protein) mediating both receptor binding to host cells and fusion with the cellular membrane. The recently solved crystal structure of vesicular stomatitis virus G has allowed considerable insight into the mechanism of rhabdovirus entry, in particular the low pH-dependent conformational changes that lead to fusion activation. Rhabdovirus entry shows several distinct features compared with other enveloped viruses; first, the entry process appears to consist of two distinct fusion events, initial fusion into vesicles within endosomes followed by back-fusion into the cytosol; second, the conformational changes in the G protein that lead to fusion activation are reversible; and third, the G protein is structurally distinct from other viral fusion proteins and is not proteolytically cleaved. The internalization and fusion mechanisms of rhabdoviruses are discussed in this article, with a focus on viral systems where the G protein has been studied extensively: vesicular stomatitis virus and rabies virus, as well as viral hemorrhagic septicemia virus.

Genomic analysis of severe hypersensitivity to hygromycin B reveals linkage to vacuolar defects and new vacuolar gene functions in Saccharomyces cerevisiae

The vacuole of Saccharomyces cerevisiae has been a seminal model for studies of lysosomal trafficking, biogenesis, and function. Several yeast mutants defective in such vacuolar events have been unable to grow at low levels of hygromycin B, an aminoglycoside antibiotic. We hypothesized that such severe hypersensitivity to hygromycin B (hhy) is linked to vacuolar defects and performed a genomic screen for the phenotype using a haploid deletion strain library of non-essential genes. Fourteen HHY genes were initially identified and were subjected to bioinformatics analyses. The uncovered hhy mutants were experimentally characterized with respect to vesicular trafficking, vacuole morphology, and growth under various stress and drug conditions. The combination of bioinformatics analyses and phenotypic characterizations implicate defects in vesicular trafficking, vacuole fusion/fission, or vacuole function in all hhy mutants. The collection was enriched for sensitivity to monensin, indicative of vacuolar trafficking defects. Additionally, all hhy mutants showed severe sensitivities to rapamycin and caffeine, suggestive of TOR kinase pathway defects. Our experimental results also establish a new role in vacuolar and vesicular functions for two genes: PAF1, encoding a RNAP II-associated protein required for expression of cell cycle-regulated genes, and TPD3, encoding the regulatory subunit of protein phosphatase 2A. Thus, our results support linkage between severe hypersensitivity to hygromycin B and vacuolar defects.

The herpes simplex virus-1 encoded glycoprotein B diverts HLA-DR into the exosome pathway

Neutralizing Abs play an important role for immunity against HSV-1 infection. This branch of the immune response is initiated by MHC class II Ag presentation and activation of T cell help. In this study, we show that the HSV-1 encoded glycoprotein B (gB) manipulates the class II processing pathway by perturbing endosomal sorting and trafficking of HLA-DR (DR) molecules. Expression of gB in the human melanoma cell line Mel JuSo results in formation of enlarged DR(+) intracellular vesicles. Costaining of the vesicles revealed the presence of DR, gB, and the late endosomal marker CD63. The lumen of these late endosomal membranes shows a variable content, containing either gB or CD63, or both CD63 and gB. gB targets DR molecules on their biosynthetic route, after the MHC class II invariant chain is released from the DR heterodimer. gB-DR complexes were detected in a post-Golgi compartment and in exosomes, but not on the cell surface. Interestingly, increasing expression of gB strongly elevated the amount of DR and CD63 released into the exosome pathway. In conclusion, this is a previously undescribed mode of viral immune evasion involving hijacking of DR from its normal transport route to the cell surface, followed by viral-mediated release of DR into the exosome pathway.

Ultrastructural analysis of lysosome reactions to inside-out cell membrane vesicles in a cell-free system

Lysosome reactions were ultrastructurally analyzed using a cell-free system with inside-out cell membrane vesicles (IOVs) prepared from rat erythrocyte ghosts in an alkaline buffer and with wheat germ agglutinin-coated colloidal gold particles (WGA-CGs). The submembranous surface coat in the ghosts was depleted from the IOVs' outer surfaces. When lysosomes from rat liver were incubated with these IOVs, some of the trilaminar membranes of the lysosomes and IOVs came into close contact and formed a five-laminar structure without an intermembranous gap. In other reactions, the membranes of both structures formed one continuous trilaminar membrane along the margin of contact and ruffling five-laminar structures in other regions. Several lysosomes exhibited invaginating hollows or projections that entrapped or encircled the IOVs. Similar five-laminar structures were seen at a few points of contact between the IOVs and the hollowing or projecting membranes. In contrast, such reactions were much rarer when IOVs with reconstituted spectrins and actins on their outer surface were used. The formation of tubuliform pits with membrane-bound WGA-CGs was also observed after the incubation of lysosomes with WGA-CGs. These observations suggest that lysosomes fuse with cytoskeleton-depleted IOVs, wrap arm-like projections around them, enclose them by invagination or incorporate their membrane-bound macromolecules through the process of tubuliform invagination. Furthermore, the fusion and wrapping processes are not necessarily independent.

Computational model of membrane fission catalyzed by ESCRT-III

ESCRT-III proteins catalyze membrane fission during multi vesicular body biogenesis, budding of some enveloped viruses and cell division. We suggest and analyze a novel mechanism of membrane fission by the mammalian ESCRT-III subunits CHMP2 and CHMP3. We propose that the CHMP2-CHMP3 complexes self-assemble into hemi-spherical dome-like structures within the necks of the initial membrane buds generated by CHMP4 filaments. The dome formation is accompanied by the membrane attachment to the dome surface, which drives narrowing of the membrane neck and accumulation of the elastic stresses leading, ultimately, to the neck fission. Based on the bending elastic model of lipid bilayers, we determine the degree of the membrane attachment to the dome enabling the neck fission and compute the required values of the protein-membrane binding energy. We estimate the feasible values of this energy and predict a high efficiency for the CHMP2-CHMP3 complexes in mediating membrane fission. We support the computational model by electron tomography imaging of CHMP2-CHMP3 assemblies in vitro. We predict a high efficiency for the CHMP2-CHMP3 complexes in mediating membrane fission.

LRRK2 regulates autophagic activity and localizes to specific membrane microdomains in a novel human genomic reporter cellular model

Leucine rich repeat kinase 2 (LRRK2) mutations are the most common genetic cause of Parkinson's disease (PD) although LRRK2 function remains unclear. We report a new role for LRRK2 in regulating autophagy and describe the recruitment of LRRK2 to the endosomal-autophagic pathway and specific membrane subdomains. Using a novel human genomic reporter cellular model, we found LRRK2 to locate to membrane microdomains such as the neck of caveolae, microvilli/filopodia and intraluminal vesicles of multivesicular bodies (MVBs). In human brain and in cultured human cells LRRK2 was present in cytoplasmic puncta corresponding to MVBs and autophagic vacuoles (AVs). Expression of the common R1441C mutation from a genomic DNA construct caused impaired autophagic balance evident by the accumulation of MVBs and large AVs containing incompletely degraded material and increased levels of p62. Furthermore, the R1441C mutation induced the formation of skein-like abnormal MVBs. Conversely, LRRK2 siRNA knockdown increased autophagic activity and prevented cell death caused by inhibition of autophagy in starvation conditions. The work necessitated developing a new, more efficient recombineering strategy, which we termed Sequential insertion of Target with ovErlapping Primers (STEP) to seamlessly fuse the green fluorescent protein-derivative YPet to the human LRRK2 protein in the LRRK2 genomic locus carried by a bacterial artificial chromosome. Taken together our data demonstrate the functional involvement of LRRK2 in the endosomal-autophagic pathway and the recruitment to specific membrane microdomains in a physiological human gene expression model suggesting a novel function for this important PD-related protein.

Discrete domains of MARCH1 mediate its localization, functional interactions, and posttranscriptional control of expression

Within APCs, ubiquitination regulates the trafficking of immune modulators such as MHC class II and CD86 (B7.2) molecules. MARCH1 (membrane-associated RING-CH), a newly identified ubiquitin E3 ligase expressed in APCs, ubiquitinates MHC class II, thereby reducing its surface expression. Following LPS-induced maturation of dendritic cells, MARCH1 mRNA is down-regulated and MHC class II is redistributed to the cell surface from endosomal compartments. Here, we show that MARCH1 expression is also regulated at the posttranscriptional level. In primary dendritic cell and APC cell lines of murine origin, MARCH1 had a half-life of <30 min. MARCH1 degradation appears to occur partly in lysosomes, since inhibiting lysosomal activity stabilized MARCH1. Similar stabilization was observed when MARCH1-expressing cells were treated with cysteine protease inhibitors. Mutational analyses of MARCH1 defined discrete domains required for destabilization, proper localization, and functional interaction with substrates. Taken together, these data suggest that MARCH1 expression is regulated at a posttranscriptional level by trafficking within the endolysosomal pathway where MARCH1 is proteolyzed. The short half-life of MARCH1 permits very rapid changes in the levels of the protein in response to changes in the mRNA, resulting in efficient induction of Ag presentation once APCs receive maturational signals.

Screening for the drug-phospholipid interaction: correlation to phospholipidosis

Phospholipid bilayers represent a complex, anisotropic environment fundamentally different from bulk oil or octanol, for instance. Even "simple" drug association to phospholipid bilayers can only be fully understood if the slab-of-hydrocarbon approach is abandoned and the complex, anisotropic properties of lipid bilayers reflecting the chemical structures and organization of the constituent phospholipids are considered. The interactions of drugs with phospholipids are important in various processes, such as drug absorption, tissue distribution, and subcellular distribution. In addition, drug-lipid interactions may lead to changes in lipid-dependent protein activities, and further, to functional and morphological changes in cells, a prominent example being the phospholipidosis (PLD) induced by cationic amphiphilic drugs. Herein we briefly review drug-lipid interactions in general and the significance of these interactions in PLD in particular. We also focus on a potential causal connection between drug-induced PLD and steatohepatitis, which is induced by some cationic amphiphilic drugs.

Structure and function of the ESCRT-II-III interface in multivesicular body biogenesis

The ESCRT-II-ESCRT-III interaction coordinates the sorting of ubiquitinated cargo with the budding and scission of intralumenal vesicles into multivesicular bodies. The interacting regions of these complexes were mapped to the second winged helix domain of human ESCRT-II subunit VPS25 and the first helix of ESCRT-III subunit VPS20. The crystal structure of this complex was determined at 2.0 A resolution. Residues involved in structural interactions explain the specificity of ESCRT-II for Vps20, and are critical for cargo sorting in vivo. ESCRT-II directly activates ESCRT-III-driven vesicle budding and scission in vitro via these structural interactions. VPS20 and ESCRT-II bind membranes with nanomolar affinity, explaining why binding to ESCRT-II is dispensable for the recruitment of Vps20 to membranes. Docking of the ESCRT-II-VPS20(2) supercomplex reveals a convex membrane-binding surface, suggesting a hypothesis for negative membrane curvature induction in the nascent intralumenal vesicle.

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.

Ubiquitylation of the gap junction protein connexin-43 signals its trafficking from early endosomes to lysosomes in a process mediated by Hrs and Tsg101

Gap junctions are dynamic plasma membrane domains, and their protein constituents, the connexins, have a high turnover rate in most tissue types. However, the molecular mechanisms involved in degradation of gap junctions have remained largely unknown. Here, we show that ubiquitin is strongly relocalized to connexin-43 (Cx43; also known as Gja1) gap junction plaques in response to activation of protein kinase C. Cx43 remained ubiquitylated during its transition to a Triton X-100-soluble state and along its trafficking to early endosomes. Following internalization, Cx43 partly colocalized with the ubiquitin-binding proteins Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate; also known as Hgs) and Tsg101 (tumor susceptibility gene 101). Depletion of Hrs or Tsg101 by small interfering RNA abrogated trafficking of Cx43 from early endosomes to lysosomes. Under these conditions, Cx43 was able to undergo dephosphorylation and deubiquitylation, locate to the plasma membrane and form functional gap junctions. Simultaneous depletion of Hrs and Tsg101 caused accumulation of a phosphorylated and ubiquitylated subpopulation of Cx43 in early endosomes and in hybrid organelles between partly degraded annular gap junctions and endosomes. Collectively, these data reveal a central role of early endosomes in sorting of ubiquitylated Cx43, and identify Hrs and Tsg101 as crucial regulators of trafficking of Cx43 to lysosomes.

Abnormal regulation of TSG101 in mice with spongiform neurodegeneration

Spongiform neurodegeneration is characterized by the appearance of vacuoles throughout the central nervous system. It has many potential causes, but the underlying cellular mechanisms are not well understood. Mice lacking the E3 ubiquitin ligase Mahogunin Ring Finger-1 (MGRN1) develop age-dependent spongiform encephalopathy. We identified an interaction between a "PSAP" motif in MGRN1 and the ubiquitin E2 variant (UEV) domain of TSG101, a component of the endosomal sorting complex required for transport I (ESCRT-I), and demonstrate that MGRN1 multimonoubiquitinates TSG101. We examined the in vivo consequences of loss of MGRN1 on TSG101 expression and function in the mouse brain. The pattern of TSG101 ubiquitination differed in the brains of wild-type mice and Mgrn1 null mutant mice: at 1 month of age, null mutant mice had less ubiquitinated TSG101, while in adults, mutant mice had more ubiquitinated, insoluble TSG101 than wild-type mice. There was an associated increase in epidermal growth factor receptor (EGFR) levels in mutant brains. These results suggest that loss of MGRN1 promotes ubiquitination of TSG101 by other E3s and may prevent its disassociation from endosomal membranes or cause it to form insoluble aggregates. Our data implicate loss of normal TSG101 function in endo-lysosomal trafficking in the pathogenesis of spongiform neurodegeneration in Mgrn1 null mutant mice.

MHC II and the endocytic pathway: regulation by invariant chain

The major histocompatibility complex (MHC) class I and II molecules perform vital functions in innate and adaptive immune responses towards invading pathogens. MHC class I molecules load peptides in the endoplasmatic reticulum (ER) and display them to the T cell receptors (TcR) on CD8(+) T lymphocytes. MHC class II molecules (MHC II) acquire their peptides in endosomes and present these to the TcR on CD4+ T lymphocytes. They are vital for the generation of humoral immune responses. MHC II assembly in the ER and trafficking to endosomes is guided by a specialized MHC II chaperone termed the invariant chain (Ii). Ii self-associates into a trimer in the ER, this provides a scaffold for the assembly of three MHC II heterodimers and blocks their peptide binding grooves, thereby avoiding premature peptide binding. Ii then transports the nascent MHC II to more or less specialized compartment where they can load peptides derived from internalized pathogens.

Intracellular assembly and trafficking of MHC class I molecules

The presentation of antigenic peptides by class I molecules of the major histocompatibility complex begins in the endoplasmic reticulum (ER) where the co-ordinated action of molecular chaperones, folding enzymes and class I-specific factors ensures that class I molecules are loaded with high-affinity peptide ligands that will survive prolonged display at the cell surface. Once assembled, class I molecules are released from the quality-control machinery of the ER for export to the plasma membrane where they undergo dynamic endocytic cycling and turnover. We review recent progress in our understanding of class I assembly, anterograde transport and endocytosis and highlight some of the events targeted by viruses as a means to evade detection by cytotoxic T cells and natural killer cells.

Involvement of vps33a in the fusion of uroplakin-degrading multivesicular bodies with lysosomes

The apical surface of the terminally differentiated mouse bladder urothelium is largely covered by urothelial plaques, consisting of hexagonally packed 16-nm uroplakin particles. These plaques are delivered to the cell surface by fusiform vesicles (FVs) that are the most abundant cytoplasmic organelles. We have analyzed the functional involvement of several proteins in the apical delivery and endocytic degradation of uroplakin proteins. Although FVs have an acidified lumen and Rab27b, which localizes to these organelles, is known to be involved in the targeting of lysosome-related organelles (LROs), FVs are CD63 negative and are therefore not typical LROs. Vps33a is a Sec1-related protein that plays a role in vesicular transport to the lysosomal compartment. A point mutation in mouse Vps33a (Buff mouse) causes albinism and bleeding (Hermansky-Pudlak syndrome) because of abnormalities in the trafficking of melanosomes and platelets. These Buff mice showed a novel phenotype observed in urothelial umbrella cells, where the uroplakin-delivering FVs were almost completely replaced by Rab27b-negative multivesicular bodies (MVBs) involved in uroplakin degradation. MVB accumulation leads to an increase in the amounts of uroplakins, Lysosomal-associated membrane protein (LAMP)-1/2, and the activities of beta-hexosaminidase and beta-glucocerebrosidase. These results suggest that FVs can be regarded as specialized secretory granules that deliver crystalline arrays of uroplakins to the cell surface, and that the Vps33a mutation interferes with the fusion of MVBs with mature lysosomes thus blocking uroplakin degradation.

Silencing by small RNAs is linked to endosomal trafficking

Small RNAs direct RNA-induced silencing complexes (RISCs) to regulate stability and translation of mRNAs. RISCs associated with target mRNAs often accumulate in discrete cytoplasmic foci known as GW-bodies. However, RISC proteins can associate with membrane compartments such as the Golgi and endoplasmic reticulum. Here, we show that GW-bodies are associated with late endosomes (multivesicular bodies, MVBs). Blocking the maturation of MVBs into lysosomes by loss of the tethering factor HPS4 (ref. 5) enhances short interfering RNA (siRNA)- and micro RNA (miRNA)-mediated silencing in Drosophila melanogaster and humans. It also triggers over-accumulation of GW-bodies. Blocking MVB formation by ESCRT (endosomal sorting complex required for transport) depletion results in impaired miRNA silencing and loss of GW-bodies. These results indicate that active RISCs are physically and functionally coupled to MVBs. We further show that MVBs promote the competence of RISCs in loading small RNAs. We suggest that the recycling of RISCs is promoted by MVBs, resulting in RISCs more effectively engaging with small RNA effectors and possibly target RNAs. It may provide a means to enhance the dynamics of RNA silencing in the cytoplasm.

Membrane-bending proteins

Cellular membranes can assume a number of highly dynamic shapes. Many cellular processes also require transient membrane deformations. Membrane shape is determined by the complex interactions of proteins and lipids. A number of families of proteins that directly bend membranes have been identified. Most associate transiently with membranes and deform them. These proteins work by one or more of three types of mechanisms. First, some bend membranes by inserting amphipathic domains into one of the leaflets of the bilayer; increasing the area of only one leaflet causes the membrane to bend. Second, some proteins form a rigid scaffold that deforms the underlying membrane or stabilizes an already bent membrane. Third, some proteins may deform membranes by clustering lipids or by affecting lipid ordering in membranes. Still other proteins may use novel but poorly understood mechanisms. In this review, we summarize what is known about how different families of proteins bend membranes.

Extracellular K+ concentration controls cell surface density of IKr in rabbit hearts and of the HERG channel in human cell lines

Although the modulation of ion channel gating by hormones and drugs has been extensively studied, much less is known about how cell surface ion channel expression levels are regulated. Here, we demonstrate that the cell surface density of both the heterologously expressed K+ channel encoded by the human ether-a-go-go-related gene (HERG) and its native counterpart, the rapidly activating delayed rectifier K+ channel (IKr), in rabbit hearts in vivo is precisely controlled by extracellular K+ concentration ([K+]o) within a physiologically relevant range. Reduction of [K+]o led to accelerated internalization and degradation of HERG channels within hours. Confocal analysis revealed colocalization between HERG and ubiquitin during the process of HERG internalization, and overexpression of ubiquitin facilitated HERG degradation under low [K+]o. The HERG channels colocalized with a marker of multivesicular bodies during internalization, and the internalized HERG channels were targeted to lysosomes. Our results provide the first evidence to our knowledge that the cell surface density of a voltage-gated K+ channel, HERG, is regulated by a biological factor, extracellular K+. Because hypokalemia is known to exacerbate long QT syndrome (LQTS) and Torsades de pointes tachyarrhythmias, our findings provide a potential mechanistic link between hypokalemia and LQTS.

Herpes simplex virus type 1 production requires a functional ESCRT-III complex but is independent of TSG101 and ALIX expression

Herpes simplex virus type 1 (HSV-1) acquires its mature virus envelope by budding into the lumen of cytoplasmic membranous compartments carrying the viral glycoproteins. In a cellular context, a budding process with identical topology occurs during the formation of intraluminal vesicles in multivesicular bodies. The cellular machinery that mediates this budding process is composed of four protein complexes termed endosomal sorting complexes required for transport (ESCRTs) and several associated proteins, including the ATPase VPS4. We have recently shown that functional VPS4 is specifically required for the cytoplasmic envelopment of HSV-1. We now demonstrate that, consistent with a role of VPS4 in virus envelopment, dominant-negative ESCRT-III proteins potently block HSV-1 production. Retroviruses are known to recruit the ESCRT machinery by small peptide motifs termed late domains. These late domains interact with various ESCRT components and thereby promote ESCRT recruitment. The best-characterized late-domain interacting ESCRT proteins are ALIX and TSG101. The presence of potential ALIX and TSG101 binding sequence motifs in various structural HSV-1 proteins suggested a functional role of these proteins in HSV-1 envelopment. We therefore used a set of dominant-negative proteins, as well as RNA interference, to characterize the contribution of ALIX and TSG101 to HSV-1 production. Interestingly, despite the strict requirement for a functional ESCRT-III complex, our data suggest that HSV-1 production is independent of ALIX and TSG101 expression. In line with these data, we also find that ESCRT-III proteins and VPS4A/B are specifically incorporated into mature HSV-1 virions.

MHC II in dendritic cells is targeted to lysosomes or T cell-induced exosomes via distinct multivesicular body pathways

Dendritic cells (DCs) express major histocompatibility complex class II (MHC II) to present peptide antigens to T cells. In immature DCs, which bear low cell surface levels of MHC II, peptide-loaded MHC II is ubiquitinated. Ubiquitination drives the endocytosis and sorting of MHC II to the luminal vesicles of multivesicular bodies (MVBs) for lysosomal degradation. Ubiquitination of MHC II is abrogated in activated DCs, resulting in an increased cell surface expression. We here provide evidence for an alternative MVB sorting mechanism for MHC II in antigen-loaded DCs, which is triggered by cognately interacting antigen-specific CD4+ T cells. At these conditions, DCs generate MVBs with MHC II and CD9 carrying luminal vesicles that are secreted as exosomes and transferred to the interacting T cells. Sorting of MHC II into exosomes was, in contrast to lysosomal targeting, independent of MHC II ubiquitination but rather correlated with its incorporation into CD9 containing detergent-resistant membranes. Together, these data indicate two distinct MVB pathways: one for lysosomal targeting and the other for exosome secretion.

Cell proliferation and epidermal growth factor signaling in non-small cell lung adenocarcinoma cell lines are dependent on Rin1

Rin1 is a Rab5 guanine nucleotide exchange factor that plays an important role in Ras-activated endocytosis and growth factor receptor trafficking in fibroblasts. In this study, we show that Rin1 is expressed at high levels in a large number of non-small cell lung adenocarcinoma cell lines, including Hop62, H650, HCC4006, HCC827, EKVX, HCC2935, and A549. Rin1 depletion from A549 cells resulted in a decrease in cell proliferation that was correlated to a decrease in epidermal growth factor receptor (EGFR) signaling. Expression of wild type Rin1 but not the Rab5 guanine nucleotide exchange factor-deficient Rin1 (Rin1Delta) complemented the Rin1 depletion effects, and overexpression of Rin1Delta had a dominant negative effect on cell proliferation. Rin1 depletion stabilized the cell surface levels of EGFR, suggesting that internalization was necessary for robust signaling in A549 cells. In support of this conclusion, introduction of either dominant negative Rab5 or dominant negative dynamin decreased A549 proliferation and EGFR signaling. These data demonstrate that proper internalization and endocytic trafficking are critical for EGFR-mediated signaling in A549 cells and suggest that up-regulation of Rin1 in A549 cell lines may contribute to their proliferative nature.

Platelet alpha-granules: basic biology and clinical correlates

alpha-Granules are essential to normal platelet activity. These unusual secretory granules derive their cargo from both regulated secretory and endocytotic pathways in megakaryocytes. Rare, inheritable defects of alpha-granule formation in mice and man have enabled identification of proteins that mediate cargo trafficking and alpha-granule formation. In platelets, alpha-granules fuse with the plasma membrane upon activation, releasing their cargo and increasing platelet surface area. The mechanisms that control alpha-granule membrane fusion have begun to be elucidated at the molecular level. SNAREs and SNARE accessory proteins that control alpha-granule secretion have been identified. Proteomic studies demonstrate that hundreds of bioactive proteins are released from alpha-granules. This breadth of proteins implies a versatile functionality. While initially known primarily for their participation in thrombosis and hemostasis, the role of alpha-granules in inflammation, atherosclerosis, antimicrobial host defense, wound healing, angiogenesis, and malignancy has become increasingly appreciated as the function of platelets in the pathophysiology of these processes has been defined. This review will consider the formation, release, and physiologic roles of alpha-granules with special emphasis on work performed over the last decade.

Cell proliferation and epidermal growth factor signaling in non-small cell lung adenocarcinoma cell lines are dependent on Rin1

Rin1 is a Rab5 guanine nucleotide exchange factor that plays an important role in Ras-activated endocytosis and growth factor receptor trafficking in fibroblasts. In this study, we show that Rin1 is expressed at high levels in a large number of non-small cell lung adenocarcinoma cell lines, including Hop62, H650, HCC4006, HCC827, EKVX, HCC2935, and A549. Rin1 depletion from A549 cells resulted in a decrease in cell proliferation that was correlated to a decrease in epidermal growth factor receptor (EGFR) signaling. Expression of wild type Rin1 but not the Rab5 guanine nucleotide exchange factor-deficient Rin1 (Rin1Delta) complemented the Rin1 depletion effects, and overexpression of Rin1Delta had a dominant negative effect on cell proliferation. Rin1 depletion stabilized the cell surface levels of EGFR, suggesting that internalization was necessary for robust signaling in A549 cells. In support of this conclusion, introduction of either dominant negative Rab5 or dominant negative dynamin decreased A549 proliferation and EGFR signaling. These data demonstrate that proper internalization and endocytic trafficking are critical for EGFR-mediated signaling in A549 cells and suggest that up-regulation of Rin1 in A549 cell lines may contribute to their proliferative nature.

Structural basis for ESCRT-III protein autoinhibition

Endosomal sorting complexes required for transport-III (ESCRT-III) subunits cycle between two states: soluble monomers and higher-order assemblies that bind and remodel membranes during endosomal vesicle formation, midbody abscission and enveloped virus budding. Here we show that the N-terminal core domains of increased sodium tolerance-1 (IST1) and charged multivesicular body protein-3 (CHMP3) form equivalent four-helix bundles, revealing that IST1 is a previously unrecognized ESCRT-III family member. IST1 and its ESCRT-III binding partner, CHMP1B, both form higher-order helical structures in vitro, and IST1-CHMP1 interactions are required for abscission. The IST1 and CHMP3 structures also reveal that equivalent downstream alpha5 helices can fold back against the core domains. Mutations within the CHMP3 core-alpha5 interface stimulate the protein's in vitro assembly and HIV-inhibition activities, indicating that dissociation of the autoinhibitory alpha5 helix from the core activates ESCRT-III proteins for assembly at membranes.

Cell biology of the ESCRT machinery

The ESCRT (endosomal sorting complex required for transport) machinery comprises a set of protein complexes that regulate sorting and trafficking into multivesicular bodies en route to the lysosome. The physical mechanism responsible for generating lumenal vesicles in this pathway is unknown. Here we review recent studies suggesting that components of the ESCRT-III complex drive lumenal vesicle formation and consider possible mechanisms for this reaction.

Quantitative analysis of multivesicular bodies (MVBs) in the hypoglossal nerve: evidence that neurotrophic factors do not use MVBs for retrograde axonal transport

Multivesicular bodies (MVBs) are defined by multiple internal vesicles enclosed within an outer, limiting membrane. MVBs have previously been quantified in neuronal cell bodies and in dendrites, but their frequencies and significance in axons are controversial. Despite lack of conclusive evidence, it is widely believed that MVBs are the primary organelle that carries neurotrophic factors in axons. Reliable information about axonal MVBs under physiological and pathological conditions is needed for a realistic assessment of their functional roles in neurons. We provide a quantitative ultrastructural analysis of MVBs in the normal postnatal rat hypoglossal nerve and under a variety of experimental conditions. MVBs were about 50 times less frequent in axons than in neuronal cell bodies or dendrites. Five distinct types of MVBs were distinguished in axons, based on MVB size, electron density, and size of internal vesicles. Although target manipulations did not significantly change MVBs in axons, dystrophic conditions such as delayed fixation substantially increased the number of axonal MVBs. Radiolabeled brain- and glial-cell derived neurotrophic factors (BDNF and GDNF) injected into the tongue did not accumulate during retrograde axonal transport in MVBs, as determined by quantitative ultrastructural autoradiography, and confirmed by analysis of quantum dot-labeled BDNF. We conclude that for axonal transport, neurotrophic factors utilize small vesicles or endosomes that can be inconspicuous at transmission electron microscopic resolution, rather than MVBs. Previous reports of axonal MVBs may be based, in part, on artificial generation of such organelles in axons due to dystrophic conditions.

Perturbation of vacuolar maturation promotes listeriolysin O-independent vacuolar escape during Listeria monocytogenes infection of human cells

Listeria monocytogenes is a bacterial pathogen that replicates within the cytosol of infected host cells. The ability to rapidly escape the phagocytic vacuole is essential for efficient intracellular replication. In the murine model of infection, the pore-forming cytolysin listeriolysin O (LLO) is absolutely required for vacuolar dissolution, as LLO-deficient (DeltaLLO) mutants remain trapped within vacuoles. In contrast, in many human cell types DeltaLLO L. monocytogenes are capable of vacuolar escape at moderate to high frequencies. To better characterize the mechanism of LLO-independent vacuolar escape in human cells, we conducted an RNA interference screen to identify vesicular trafficking factors that play a role in altering vacuolar escape efficiency of DeltaLLO L. monocytogenes. RNA interference knockdown of 18 vesicular trafficking factors resulted in increased LLO-independent vacuolar escape. Our results suggest that knockdown of one factor, RABEP1 (rabaptin-5), decreased the maturation of vacuoles containing DeltaLLO L. monocytogenes. Thus, we provide evidence that increased vacuolar escape of DeltaLLO L. monocytogenes in human cells correlates with slower vacuolar maturation. We also determined that increased LLO-independent dissolution of vacuoles during RABEP1 knockdown required the bacterial broad-range phospholipase C (PC-PLC). We hypothesize that slowing the kinetics of vacuolar maturation generates an environment conducive for vacuolar escape mediated by the bacterial phospholipases.

Structural basis of Ist1 function and Ist1-Did2 interaction in the multivesicular body pathway and cytokinesis

The ESCRT machinery functions in several important eukaryotic cellular processes. The AAA-ATPase Vps4 catalyzes disassembly of the ESCRT-III complex and may regulate membrane deformation and vesicle scission as well. Ist1 was proposed to be a regulator of Vps4, but its mechanism of action was unclear. The crystal structure of the N-terminal domain of Ist1 (Ist1NTD) reveals an ESCRT-III subunit-like fold, implicating Ist1 as a divergent ESCRT-III family member. Ist1NTD specifically binds to the ESCRT-III subunit Did2, and cocrystallization of Ist1NTD with a Did2 fragment shows that Ist1 interacts with the Did2 C-terminal MIM1 (MIT-interacting motif 1) via a novel MIM-binding structural motif. This arrangement indicates a mechanism for intermolecular ESCRT-III subunit association and may also suggest one form of ESCRT-III subunit autoinhibition via intramolecular interaction.

Deficiency of niemann-pick type C-1 protein impairs release of human immunodeficiency virus type 1 and results in Gag accumulation in late endosomal/lysosomal compartments

Human immunodeficiency virus type 1 (HIV-1) relies on cholesterol-laden lipid raft membrane microdomains for entry into and egress out of susceptible cells. In the present study, we examine the need for intracellular cholesterol trafficking pathways with respect to HIV-1 biogenesis using Niemann-Pick type C-1 (NPC1)-deficient (NPCD) cells, wherein these pathways are severely compromised, causing massive accumulation of cholesterol in late endosomal/lysosomal (LE/L) compartments. We have found that induction of an NPC disease-like phenotype through treatment of various cell types with the commonly used hydrophobic amine drug U18666A resulted in profound suppression of HIV-1 release. Further, NPCD Epstein-Barr virus-transformed B lymphocytes and fibroblasts from patients with NPC disease infected with a CD4-independent strain of HIV-1 or transfected with an HIV-1 proviral clone, respectively, replicated HIV-1 poorly compared to normal cells. Infection of the NPCD fibroblasts with a vesicular stomatitis virus G-pseudotyped strain of HIV-1 produced similar results, suggesting a postentry block to HIV-1 replication in these cells. Examination of these cells using confocal microscopy showed an accumulation and stabilization of Gag in LE/L compartments. Additionally, normal HIV-1 production could be restored in NPCD cells upon expression of a functional NPC1 protein, and overexpression of NPC1 increased HIV-1 release. Taken together, our findings demonstrate that intact intracellular cholesterol trafficking pathways mediated by NPC1 are needed for efficient HIV-1 production.

ESCRT ubiquitin-binding domains function cooperatively during MVB cargo sorting

Ubiquitin (Ub) sorting receptors facilitate the targeting of ubiquitinated membrane proteins into multivesicular bodies (MVBs). Ub-binding domains (UBDs) have been described in several endosomal sorting complexes required for transport (ESCRT). Using available structural information, we have investigated the role of the multiple UBDs within ESCRTs during MVB cargo selection. We found a novel UBD within ESCRT-I and show that it contributes to MVB sorting in concert with the known UBDs within the ESCRT complexes. These experiments reveal an unexpected level of coordination among the ESCRT UBDs, suggesting that they collectively recognize a diverse set of cargo rather than act sequentially at discrete steps.

Pharmacological analysis demonstrates dramatic alteration of D1 dopamine receptor neuronal distribution in the rat analog of L-DOPA-induced dyskinesia

We have associated behavioral, pharmacological, and quantitative immunohistochemical study in a rat analog of l-DOPA-induced dyskinesia to understand whether alterations in dopamine receptor fate in striatal neurons may be involved in mechanisms leading to movement abnormalities. Detailed analysis at the ultrastructural level demonstrates specific alterations of dopamine D(1) receptor (D(1)R) subcellular localization in striatal medium spiny neurons in l-DOPA-treated 6-hydroxydopamine-lesioned rats with abnormal involuntary movements (AIMs). This includes exaggerated D(1)R expression at the plasma membrane. However, D(1)R retains ability of internalization, as a challenge with the potent D(1)R agonist SKF-82958 induces a strong decrease of labeling at membrane in animals with AIMs. Since a functional cross talk between D(1)R and D(3)R has been suggested, we hypothesized that their coactivation by dopamine derived from l-DOPA might anchor D(1)R at the membrane. Accordingly, cotreatment with l-DOPA and the D(3)R antagonist ST 198 restores normal level of membrane-bound D(1)R. Together, these results demonstrate that AIMs are related to abnormal D(1)R localization at the membrane and intraneuronal trafficking dysregulation, and suggest that strategies aiming at disrupting the D(1)R-D(3)R cross talk might reduce l-DOPA-induced dyskinesia by reducing D(1)R availability at the membrane.

High-resolution fractionation of signaling endosomes containing different receptors

Receptor endocytosis is regulated by ligand binding, and receptors may signal after endocytosis in signaling endosomes. We hypothesized that signaling endosomes containing different types of receptors may be distinct from one another and have different physical characteristics. To test this hypothesis, we developed a high-resolution organelle fractionation method based on mass and density, optimized to resolve endosomes from other organelles. Three different types of receptors undergoing ligand-induced endocytosis were localized predominately in endosomes that were resolved from one another using this method. Endosomes containing activated receptor tyrosine kinases (RTKs), TrkA and EGFR, were similar to one another. Endosomes containing p75(NTR) (in the tumor necrosis receptor superfamily) and PAC1 (a G-protein-coupled receptor) were distinct from each other and from RTK endosomes. Receptor-specific endosomes may direct the intracellular location and duration of signal transduction pathways to dictate response to signals and determine cell fate.

The ESCRT-related CHMP1A and B proteins mediate multivesicular body sorting of auxin carriers in Arabidopsis and are required for plant development

Plasma membrane proteins internalized by endocytosis and targeted for degradation are sorted into lumenal vesicles of multivesicular bodies (MVBs) by the endosomal sorting complexes required for transport (ESCRT) machinery. Here, we show that the Arabidopsis thaliana ESCRT-related CHARGED MULTIVESICULAR BODY PROTEIN/CHROMATIN MODIFYING PROTEIN1A (CHMP1A) and CHMP1B proteins are essential for embryo and seedling development. Double homozygous chmp1a chmp1b mutant embryos showed limited polar differentiation and failed to establish bilateral symmetry. Mutant seedlings show disorganized apical meristems and rudimentary true leaves with clustered stomata and abnormal vein patterns. Mutant embryos failed to establish normal auxin gradients. Three proteins involved in auxin transport, PINFORMED1 (PIN1), PIN2, and AUXIN-RESISTANT1 (AUX1) mislocalized to the vacuolar membrane of the mutant. PIN1 was detected in MVB lumenal vesicles of control cells but remained in the limiting membrane of chmp1a chmp1b MVBs. The chmp1a chmp1b mutant forms significantly fewer MVB lumenal vesicles than the wild type. Furthermore, CHMP1A interacts in vitro with the ESCRT-related proteins At SKD1 and At LIP5. Thus, Arabidopsis CHMP1A and B are ESCRT-related proteins with conserved endosomal functions, and the auxin carriers PIN1, PIN2, and AUX1 are ESCRT cargo proteins in the MVB sorting pathway.

Membrane scission by the ESCRT-III complex

The endosomal sorting complex required for transport (ESCRT) system is essential for multivesicular body biogenesis, in which cargo sorting is coupled to the invagination and scission of intralumenal vesicles. The ESCRTs are also needed for budding of enveloped viruses including human immunodeficiency virus 1, and for membrane abscission in cytokinesis. In Saccharomyces cerevisiae, ESCRT-III consists of Vps20, Snf7, Vps24 and Vps2 (also known as Did4), which assemble in that order and require the ATPase Vps4 for their disassembly. In this study, the ESCRT-III-dependent budding and scission of intralumenal vesicles into giant unilamellar vesicles was reconstituted and visualized by fluorescence microscopy. Here we show that three subunits of ESCRT-III, Vps20, Snf7 and Vps24, are sufficient to detach intralumenal vesicles. Vps2, the ESCRT-III subunit responsible for recruiting Vps4, and the ATPase activity of Vps4 were required for ESCRT-III recycling and supported additional rounds of budding. The minimum set of ESCRT-III and Vps4 proteins capable of multiple cycles of vesicle detachment corresponds to the ancient set of ESCRT proteins conserved from archaea to animals.

ESCRT proteins and the regulation of endocytic delivery to lysosomes

In mammalian cells, there is evidence of cargo specificity in the requirement for particular ESCRT (endosomal sorting complex required for transport) proteins to sort cargo into the luminal vesicles of MVBs (multivesicular bodies). We have focussed on studying the ESCRT requirements for delivery of MHC class I to lysosomes following polyubiquitination by the Kaposi's sarcoma-associated herpesvirus protein K3. Down-regulation of polyubiquitinated cell-surface MHC class I in HeLa cells stably expressing K3 is achieved via clathrin-mediated endocytosis, followed by sorting into the luminal vesicles of MVBs and eventual delivery to lysosomes. Depletion of ESCRT-I and some ESCRT-III components interferes with this sorting and allows recycling of MHC class I to the cell surface. Depletion of ESCRT-II components has no effect on K3-mediated down-regulation of MHC class I and no gross morphological effect on endocytic compartments. Thus virally polyubiquitinated MHC class I does not require all of the ESCRT proteins in order to be sorted into the luminal vesicles of MVBs. However, there may be a further requirement for ESCRT-III proteins to ensure the efficient fusion of MVBs with lysosomes.

Regulation of Vps4 ATPase activity by ESCRT-III

MVB (multivesicular body) formation occurs when the limiting membrane of an endosome invaginates into the intraluminal space and buds into the lumen, bringing with it a subset of transmembrane cargoes. Exvagination of the endosomal membrane from the cytosol is topologically similar to the budding of retroviral particles and cytokinesis, wherein membranes bud away from the cytoplasm, and the machinery responsible for MVB sorting has been implicated in these phenomena. The AAA (ATPase associated with various cellular activities) Vps4 (vacuolar protein sorting 4) performs a critical function in the MVB sorting pathway. Vps4 appears to dissociate the ESCRTs (endosomal sorting complexes required for transport) from endosomal membranes during the course of MVB sorting, but it is unclear how Vps4 ATPase activity is synchronized with ESCRT release. We have investigated the mechanisms by which ESCRT components stimulate the ATPase activity of Vps4. These studies support a model wherein Vps4 activity is subject to spatial and temporal regulation via distinct mechanisms during MVB sorting.

Analyses of the recycling receptor, FcRn, in live cells reveal novel pathways for lysosomal delivery

Lysosomes play a central role in the degradation of proteins and other macromolecules. The mechanisms by which receptors are transferred to lysosomes for constitutive degradation are poorly understood. We have analyzed the processes that lead to the lysosomal delivery of the Fc receptor, FcRn. These studies provide support for a novel pathway for receptor delivery. Specifically, unlike other receptors that enter intraluminal vesicles in late endosomes, FcRn is transferred from the limiting membrane of such endosomes to lysosomes, and is rapidly internalized into the lysosomal lumen. By contrast, LAMP-1 persists on the limiting membrane. Receptor transfer is mediated by tubular extensions from late endosomes to lysosomes, or by interactions of the two participating organelles in kiss-and-linger-like processes, whereas full fusion is rarely observed. The persistence of FcRn on the late endosomal limiting membrane, together with selective transfer to lysosomes, allows this receptor to undergo recycling or degradation. Consequently, late endosomes have functional plasticity, consistent with the presence of the Rab5 GTPase in discrete domains on these compartments.

The fat controller: roles of palmitoylation in intracellular protein trafficking and targeting to membrane microdomains (Review)

The attachment of palmitic acid to the amino acid cysteine via thioester linkage (S-palmitoylation) is a common post-translational modification of eukaryotic proteins. In this review, we discuss the role of palmitoylation as a versatile protein sorting signal, regulating protein trafficking between distinct intracellular compartments and the micro-localization of proteins within membranes.

Molecular assemblies and membrane domains in multivesicular endosome dynamics

Along the degradation pathway, endosomes exhibit a characteristic multivesicular organization, resulting from the budding of vesicles into the endosomal lumen. After endocytosis and transport to early endosomes, activated signaling receptors are incorporated into these intralumenal vesicles through the action of the ESCRT machinery, a process that contributes to terminate signaling. Then, the vesicles and their protein cargo are further transported towards lysosomes for degradation. Evidence also shows that intralumenal vesicles can undergo "back-fusion" with the late endosome limiting membrane, a route exploited by some pathogens and presumably followed by proteins and lipids that need to be recycled from within the endosomal lumen. This process depends on the late endosomal lipid lysobisphosphatidic acid and its putative effector Alix/AIP1, and is presumably coupled to the invagination of the endosomal limiting membrane at the molecular level via ESCRT proteins. In this review, we discuss the intra-endosomal transport routes in mammalian cells, and in particular the different mechanisms involved in membrane invagination, vesicle formation and fusion in a space inaccessible to proteins known to control intracellular membrane traffic.

Mechanisms of regulated unconventional protein secretion

Most eukaryotic proteins are secreted through the conventional endoplasmic reticulum (ER)-Golgi secretory pathway. However, cytoplasmic, nuclear and signal-peptide-containing proteins have been shown to reach the cell surface by non-conventional transport pathways. The mechanisms and molecular components of unconventional protein secretion are beginning to emerge, including a role for caspase 1 and for the peripheral Golgi protein GRASP, which could function as a plasma membrane tether for membrane compartments during specific stages of development.

A role for the ESCRT system in cell division in archaea

Archaea are prokaryotic organisms that lack endomembrane structures. However, a number of hyperthermophilic members of the Kingdom Crenarchaea, including members of the Sulfolobus genus, encode homologs of the eukaryotic endosomal sorting system components Vps4 and ESCRT-III (endosomal sorting complex required for transport-III). We found that Sulfolobus ESCRT-III and Vps4 homologs underwent regulation of their expression during the cell cycle. The proteins interacted and we established the structural basis of this interaction. Furthermore, these proteins specifically localized to the mid-cell during cell division. Overexpression of a catalytically inactive mutant Vps4 in Sulfolobus resulted in the accumulation of enlarged cells, indicative of failed cell division. Thus, the archaeal ESCRT system plays a key role in cell division.

Role of the feline immunodeficiency virus L-domain in the presence or absence of Gag processing: involvement of ubiquitin and Nedd4-2s ligase in viral egress

RNA-enveloped viruses bud from infected cells by exploiting the multivesicular body (MVB) pathway. In this context, ubiquitination of structural viral proteins and their direct interaction with cellular factors involved in the MVB biogenesis through short proline rich regions, named late domains (L-domains), are crucial mechanisms. Here we report that, in contrast with the human immunodeficiency virus (HIV), the feline immunodeficiency virus (FIV), a non-primate lentivirus, is strictly dependent for its budding on a "PSAP"-type L-domain, mapping in the carboxy-terminal region of Gag, irrespective of a functional viral protease. Moreover, we provide evidence that FIV egress is related to Gag ubiquitination, that is, linked to the presence of an active L-domain. Finally, although FIV Gag does not contain a PPxY motif, we show that the Nedd4-2s ubiquitin ligase enhances FIV Gag ubiquitination and it is capable to rescue viral mutants lacking a functional L-domain. In conclusion, our data bring to light peculiar aspects of FIV egress, but we also demonstrate that a non-primate lentivirus shares with HIV-1 a novel mechanism of connection to the cellular budding machinery.