The mouse SKD1, a homologue of yeast Vps4p, is required for normal endosomal trafficking and morphology in mammalian cells

Self-Organization of Sinusoidal Vessels in Pluripotent Stem Cell-derived Human Liver Bud Organoids

The induction of tissue-specific vessels in in vitro living tissue systems remains challenging. Here, we directly differentiated human pluripotent stem cells into CD32b+ putative liver sinusoidal progenitors (iLSEP) by dictating developmental pathways. By devising an inverted multilayered air-liquid interface (IMALI) culture, hepatic endoderm, septum mesenchyme, arterial and sinusoidal quadruple progenitors self-organized to generate and sustain hepatocyte-like cells neighbored by divergent endothelial subsets composed of CD32blowCD31high, LYVE1+STAB1+CD32bhighCD31lowTHBD-vWF-, and LYVE1-THBD+vWF+ cells. Wnt2 mediated sinusoidal-to-hepatic intercellular crosstalk potentiates hepatocyte differentiation and branched endothelial network formation. Intravital imaging revealed iLSEP developed fully patent human vessels with functional sinusoid-like features. Organoid-derived hepatocyte- and sinusoid-derived coagulation factors enabled correction of in vitro clotting time with Factor V, VIII, IX, and XI deficient patients' plasma and rescued the severe bleeding phenotype in hemophilia A mice upon transplantation. Advanced organoid vascularization technology allows for interrogating key insights governing organ-specific vessel development, paving the way for coagulation disorder therapeutics.

De Novo VPS4A Mutations Cause Multisystem Disease with Abnormal Neurodevelopment

The endosomal sorting complexes required for transport (ESCRTs) are essential for multiple membrane modeling and membrane-independent cellular processes. Here we describe six unrelated individuals with de novo missense variants affecting the ATPase domain of VPS4A, a critical enzyme regulating ESCRT function. Probands had structural brain abnormalities, severe neurodevelopmental delay, cataracts, growth impairment, and anemia. In cultured cells, overexpression of VPS4A mutants caused enlarged endosomal vacuoles resembling those induced by expression of known dominant-negative ATPase-defective forms of VPS4A. Proband-derived fibroblasts had enlarged endosomal structures with abnormal accumulation of the ESCRT protein IST1 on the limiting membrane. VPS4A function was also required for normal endosomal morphology and IST1 localization in iPSC-derived human neurons. Mutations affected other ESCRT-dependent cellular processes, including regulation of centrosome number, primary cilium morphology, nuclear membrane morphology, chromosome segregation, mitotic spindle formation, and cell cycle progression. We thus characterize a distinct multisystem disorder caused by mutations affecting VPS4A and demonstrate that its normal function is required for multiple human developmental and cellular processes.

Ultrastructure of spinal anterior horn cells in human Niemann-Pick type C (NPC) patient and mouse model of NPC with retroposon insertion in NPC1 genes

Niemann-Pick disease type C (NPC) is a neurovisceral lipid-storage disease. Although NPC patients show lipid storage in anterior horn cells of the spinal cord, little information is available regarding the electron microscopic analyses of the morphologies of intra-endosomal lipid like-materials in the anterior horn cells of NPC patients. In this study, we elucidated the intra-endosomal ultrastructures in spinal anterior horn cells in an NPC patient, as well as in mutant BALB/c NPC1^-/- mice with a retroposon insertion in the NPC1 gene. These morphologies were classified into four types: vesicle, multiple concentric sphere (MCS), membrane, and rose flower. The percentages of the composition in the NPC patient and NPC1^-/- mice were: vesicle (55.5% and 14.9%), MCS (15.7% and 3.4%), membrane (23.6% and 57.1%), and rose flower (5.2% and 24.6%), respectively. Formation of the intra-endosomal structures could proceed as follows: (i) a vesicle or MCS buds off the endosome into the lumen; (ii) when a vesicle breaks down, a membrane is formed; and (iii) after an MCS breaks down, a rose flower structure is formed. Our new finding in this study is that ultrastructural morphology is the same between the NPC patient and NPC1^-/- mice, although there are differences in the composition.

The regulation of Endosomal Sorting Complex Required for Transport and accessory proteins in multivesicular body sorting and enveloped viral budding - An overview

ESCRT (Endosomal Sorting Complex Required for Transport) machinery drives different cellular processes such as endosomal sorting, organelle biogenesis, vesicular trafficking, maintenance of plasma membrane integrity, membrane fission during cytokinesis and enveloped virus budding. The normal cycle of assembly and disassembly of some ESCRT complexes at the membrane requires the AAA-ATPase vacuolar protein sorting 4 (Vps4p). A number of ESCRT proteins are hijacked by clinically significant enveloped viruses including Ebola, and Human Immunodeficiency Virus (HIV) to enable enveloped virus budding and Vps4p provides energy for the disassembly/recycling of these ESCRT proteins. Several years ago, the failure of the terminal budding process of HIV following Vps4 protein inhibition was published; although at that time a detailed understanding of the molecular players was missing. However, later it was acknowledged that the ESCRT machinery has a role in enveloped virus budding from cells due to its role in the multivesicular body (MVB) sorting pathway. The MVB sorting pathway facilitates several cellular activities in uninfected cells, such as the down-regulation of signaling through cell surface receptors as well as the process of viral budding from infected host cells. In this review, we focus on summarising the functional organisation of ESCRT proteins at the membrane and the role of ESCRT machinery and Vps4p during MVB sorting and enveloped viral budding.

Role of SMURF1 ubiquitin ligase in BMP receptor trafficking and signaling

Heterozygous germline mutations in the bone morphogenetic protein type II receptor gene (BMPRII) are associated with hereditary pulmonary arterial hypertension (HPAH). Missense mutations, both in the extracellular ligand-binding and cytoplasmic kinase domains, mostly involve substitution of conserved Cys residues. Singular substitution at any of those Cys residues causes cytoplasmic, perinuclear localization of BMPR with reduced cell surface expression and BMP signaling. The present study examined the effect of Cys residue substitution on BMPR endocytic trafficking and lysosome degradation. We demonstrate that endocytosis/lysosomal degradation of BMPR occurs by two distinct pathways. SMURF1 ubiquitin ligase induces lysosomal degradation of BMPR, while ligase-inactive SMURF1 maintains BMPR protein level and cell surface expression. Substitution of BMPR Cys residues increases lysosomal degradation which is blocked by ligase-inactive SMURF1, elevating protein levels of Cys-substituted BMPRs. Expression of Cys-substituted BMPR suppresses basal BMP signaling activity which is also up-regulated by ligase-inactive SMURF1. Cys-residue substitution thus appears to cause BMPR endocytosis to lysosomes in a SMURF1 ubiquitin ligase-associated pathway. Kinase-activated BMPR undergoes endocytic/lysosomal degradation by a pathway with certain unique properties. Therefore, our results describe a novel mechanism whereby SMURF1 ubiquitin ligase regulates constitutive endocytosis of BMPR which may be mediated by its conserved Cys residues.

Effects of Inhibiting VPS4 Support a General Role for ESCRTs in Extracellular Vesicle Biogenesis

Extracellular vesicles (EVs) are proposed to play important roles in intercellular communication. Two classes of EVs can be distinguished based on their intracellular origin. Exosomes are generated within endosomes and released when these fuse with the plasma membrane, whereas ectosomes bud directly from the plasma membrane. Studies of EV function have been hindered by limited understanding of their biogenesis. Components of the endosomal sorting complex required for transport (ESCRT) machinery play essential roles in topologically equivalent processes at both the endosome and the plasma membrane and are consistently recovered in EVs, but whether they are generally required to produce EVs is still debated. Here, we study the effects of inhibiting the ESCRT-associated AAA+ ATPase VPS4 on EV release from cultured cells using two methods for EV recovery, differential centrifugation and polyethylene glycol precipitation followed by lectin affinity chromatography. We find that inhibiting VPS4 in HEK293 cells decreases release of EV-associated proteins and miRNA as well as the overall number of EV particles. The tetraspanins CD63 and CD9 are among the most frequently monitored EV proteins, but they differ in their subcellular localization, with CD63 primarily in endosomes and CD9 on the plasma membrane. We find that CD63 and CD9 are enriched in separable populations of EVs that are both sensitive to VPS4 inhibition. Serum stimulation increases release of both types of EVs and is also reduced by inhibiting VPS4. Taken together, our data indicate that VPS4 activity is important for generating exosomes and ectosomes, thereby generally implicating the ESCRT machinery in EV biogenesis.

Phosphorylated CAV1 activates autophagy through an interaction with BECN1 under oxidative stress

CAV1/Caveolin1, an integral membrane protein, is involved in caveolae function and cellular signaling pathways. Here, we report that CAV1 is a positive regulator of autophagy under oxidative stress and cerebral ischemic injury. Treatment with hydrogen peroxide enhanced autophagy flux and caused the localization of BECN1 to the mitochondria, whereas these changes were impaired in the absence of CAV1. Among many autophagy signals, only LC3 foci formation in response to hydrogen peroxide was abolished by CAV1 deficiency. Under oxidative stress, CAV1 interacted with a complex of BECN1/VPS34 through its scaffolding domain, and this interaction facilitated autophagosome formation. Interestingly, the phosphorylation of CAV1 at tyrosine-14 was essential for the interaction with BECN1 and their localization to the mitochondria, and the activation of autophagy in response to hydrogen peroxide. In addition, the expression of a phosphatase PTPN1 reduced the phosphorylation of CAV1 and inhibited autophagy. Further, compared to that in wild-type mice, autophagy was impaired and cerebral infarct damage was aggravated in the brain of Cav1 knockout mice. These results suggest that the phosphorylated CAV1 functions to activate autophagy through binding to the BECN1/VPS34 complex under oxidative stress and to protect against ischemic damage.

The expression and localization of RNase and RNase inhibitor in blood cells and vascular endothelial cells in homeostasis of the vascular system

RNA may be released from vascular cells including endothelial cells in the event of injury and in vascular disease. Extracellular RNAs have been recognized as novel procoagulant and permeability-increasing factors. Extracellular RNA may function as inflammatory host alarm signals that serve to amplify the defense mechanism, but it may provide important links to thrombus formation. Extracellular RNA is degraded by RNase. We propose that RNase and its inhibitor RNase inhibitor (RI) act as modulators of homeostasis in the vasculature to control the functions of extracellular RNA. We aimed to investigate the expression and localization of RNase 1 and RI in cells that contact blood, such as platelets, mononuclear cells, polymorphonuclear cells, and red blood cells. RNase 1 and RI expression and localization in blood cells were compared with those in the human umbilical vein endothelial cell line, EAhy926. Additionally, we further investigated the effect of thrombin on the expression of RNase 1 and RI in platelets. We used an RNase activity assay, reverse transcription-polymerase chain reaction, western blot, immunocytochemistry, transmission electron microscopy, and immunoelectron microscopy (pre- and post-embedding methods). RNase activity in the supernatant from EAhy926 cells was 50 times than in blood cells (after 60 min). RNase 1 mRNA and protein expression in EAhy926 cells was highest among the cells examined. However, RI mRNA and protein expression was similar in most cell types examined. Furthermore, we observed that RNase 1 and von Willebrand factor were partially colocalized in EAhy926 cells and platelets. In conclusion, we propose that high RNase activity is ordinarily released from endothelial cells to support anticoagulation in the vasculature. On the other hand, platelets and leukocytes within thrombi at sites of vascular injury show very low RNase activity, which may support hemostatic thrombus formation. However, activated platelets and leukocytes may accelerate pathologic thrombus formation.

The ATG conjugation systems are important for degradation of the inner autophagosomal membrane

In macroautophagy, cytoplasmic contents are sequestered into the double-membrane autophagosome, which fuses with the lysosome to become the autolysosome. It has been thought that the autophagy-related (ATG) conjugation systems are required for autophagosome formation. Here, we found that autophagosomal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) syntaxin 17-positive autophagosome-like structures could be generated even in the absence of the ATG conjugation systems, although at a reduced rate. These syntaxin 17-positive structures could further fuse with lysosomes, but degradation of the inner autophagosomal membrane was significantly delayed. Accordingly, autophagic activity in ATG conjugation-deficient cells was strongly suppressed. We suggest that the ATG conjugation systems, which are likely required for the closure (i.e., fission) of the autophagosomal edge, are not absolutely essential for autolysosome formation but are important for efficient degradation of the inner autophagosomal membrane.

TDP-43 loss of function inhibits endosomal trafficking and alters trophic signaling in neurons

Nuclear clearance of TDP-43 into cytoplasmic aggregates is a key driver of neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), but the mechanisms are unclear. Here, we show that TDP-43 knockdown specifically reduces the number and motility of RAB11-positive recycling endosomes in dendrites, while TDP-43 overexpression has the opposite effect. This is associated with delayed transferrin recycling in TDP-43-knockdown neurons and decreased β2-transferrin levels in patient CSF Whole proteome quantification identified the upregulation of the ESCRT component VPS4B upon TDP-43 knockdown in neurons. Luciferase reporter assays and chromatin immunoprecipitation suggest that TDP-43 represses VPS4B transcription. Preventing VPS4B upregulation or expression of its functional antagonist ALIX restores trafficking of recycling endosomes. Proteomic analysis revealed the broad reduction in surface expression of key receptors upon TDP-43 knockdown, including ErbB4, the neuregulin 1 receptor. TDP-43 knockdown delays the surface delivery of ErbB4. ErbB4 overexpression, but not neuregulin 1 stimulation, prevents dendrite loss upon TDP-43 knockdown. Thus, impaired recycling of ErbB4 and other receptors to the cell surface may contribute to TDP-43-induced neurodegeneration by blocking trophic signaling.

Herpes Simplex Virus Capsid Localization to ESCRT-VPS4 Complexes in the Presence and Absence of the Large Tegument Protein UL36p

UL36p (VP1/2) is the largest protein encoded by herpes simplex virus 1 (HSV-1) and resides in the innermost layer of tegument, the complex protein layer between the capsid and envelope. UL36p performs multiple functions in the HSV life cycle, including a critical but unknown role in capsid cytoplasmic envelopment. We tested whether UL36p is essential for envelopment because it is required to engage capsids with the cellular ESCRT/Vps4 apparatus. A green fluorescent protein (GFP)-fused form of the dominant negative ATPase Vps4-EQ was used to irreversibly tag ESCRT envelopment sites during infection by UL36p-expressing and UL36-null HSV strains. Using fluorescence microscopy and scanning electron microscopy, we quantitated capsid/Vps4-EQ colocalization and examined the ultrastructure of the corresponding viral assembly intermediates. We found that loss of UL36p resulted in a two-thirds reduction in the efficiency of capsid/Vps4-EQ association but that the remaining UL36p-null capsids were still able to engage the ESCRT envelopment apparatus. It appears that although UL36p helps to couple HSV capsids to the ESCRT pathway, this is likely not the sole reason for its absolute requirement for envelopment.

IMPORTANCE

Envelopment of the HSV capsid is essential for the assembly of an infectious virion and requires the complex interplay of a large number of viral and cellular proteins. Critical to envelope assembly is the virally encoded protein UL36p, whose function is unknown. Here we test the hypothesis that UL36p is essential for the recruitment of cellular ESCRT complexes, which are also known to be required for envelopment.

Loss of functional MYO1C/myosin 1c, a motor protein involved in lipid raft trafficking, disrupts autophagosome-lysosome fusion

MYO1C, a single-headed class I myosin, associates with cholesterol-enriched lipid rafts and facilitates their recycling from intracellular compartments to the cell surface. Absence of functional MYO1C disturbs the cellular distribution of lipid rafts, causes the accumulation of cholesterol-enriched membranes in the perinuclear recycling compartment, and leads to enlargement of endolysosomal membranes. Several feeder pathways, including classical endocytosis but also the autophagy pathway, maintain the health of the cell by selective degradation of cargo through fusion with the lysosome. Here we show that loss of functional MYO1C leads to an increase in total cellular cholesterol and its disrupted subcellular distribution. We observe an accumulation of autophagic structures caused by a block in fusion with the lysosome and a defect in autophagic cargo degradation. Interestingly, the loss of MYO1C has no effect on degradation of endocytic cargo such as EGFR, illustrating that although the endolysosomal compartment is enlarged in size, it is functional, contains active hydrolases, and the correct pH. Our results highlight the importance of correct lipid composition in autophagosomes and lysosomes to enable them to fuse. Ablating MYO1C function causes abnormal cholesterol distribution, which has a major selective impact on the autophagy pathway.

[Analysis of differentially expressed proteome in urine from non-small cell lung cancer patients]

背景与目的

筛查非小细胞肺癌（non-small cell lung cancer, NSCLC）患者尿液中差异表达蛋白，确定可用于NSCLC早期诊断、监测预后和治疗评估的生物标记物。

方法

分别收集40例已病理证实初诊NSCLC患者、8例肺部良性疾病患者和22例健康志愿者的尿液样本。利用0.9%一维十二烷基硫酸钠-聚丙烯酰胺凝胶电泳（sodium dode-cyl sulfate polyacrylamide gel electrophoresis, 1D SDS-PAGE）技术和MS-Thermo-Orbitrap-Velos质谱分析仪对NSCLC组和非肿瘤组尿液中蛋白质进行分离、提取及识别，鉴定出NSCLC患者尿液中的差异表达蛋白。应用SPSS 20.0软件中受试者工作特征曲线（receiver operating characteristic curve, ROC）分别对其敏感性、特异性进行分析，并进行实验验证，从而确定出与NSCLC相关的生物标记物。

结果

NSCLC患者组和非肿瘤组尿液差异性表达蛋白质集中表现在90 kDa、60 kDa和20 kDa-30 kDa凝胶条带中。在NSCLC患者尿液蛋白分析中发现了4种与NSCLC相关的差异表达蛋白，包括上调蛋白LRG1、CA1和下调蛋白VPS4B、YWHAZ。这4种差异表达蛋白作为独立的NSCLC生物标记物其敏感性较低：LRG1蛋白敏感性83.0%（25/30）、特异性90.0%（18/20）；CA1蛋白敏感性60.0%（18/30）、特异性90.0%（18/20)；VPS4B蛋白敏感性73.3%（22/30）、特异性90.0%（18/20）；YWHAZ蛋白敏感性60.0%（18/30）、特异性95.0%（19/20）。而采用蛋白质组合模式对NSCLC进行筛查、诊断，则其敏感性和特异性分别可高达96.7%（29/30）和85%（17/20）。

结论

LRG1、CA1蛋白在NSCLC患者尿液中高表达，而VPS4B、YWHAZ蛋白呈低表达，差异表达蛋白均提示有可能成为用于NSCLC早期筛查、监测预后和治疗评估的生物标记物。LRG1、CA1、VPS4B和YWHAZ尿液蛋白作为单一生物标记物应用于NSCLC筛查和诊断的敏感性较低，而采用蛋白质组合模式明显优于独立模式对NSCLC的筛查和诊断，故蛋白质组合模式在临床诊疗中将更具有良好应用价值和前景。

Mammalian farnesyltransferase α subunit regulates vacuolar protein sorting-associated protein 4A (Vps4A)--dependent intracellular trafficking through recycling endosomes

The protein farnesyltransferase (FTase) mediates posttranslational modification of proteins with isoprenoid lipids. FTase is a heterodimer and although the β subunit harbors the active site, it requires the α subunit for its activity. Here we explore the other functions of the FTase α subunit in addition to its established role in protein prenylation. We found that in the absence of the β subunit, the α subunit of FTase forms a stable autonomous dimeric structure in solution. We identify interactors of FTase α using mass spectrometry, followed by rapid in vitro analysis using the Leishmania tarentolae cell - free system. Vps4A was validated for direct binding to the FTase α subunit both in vitro and in vivo. Analysis of the interaction with Vps4A in Hek 293 cells demonstrated that FTase α controls trafficking of transferrin receptor upstream of this protein. These results point to the existence of previously undetected biological functions of the FTase α subunit that includes control of intracellular membrane trafficking.

Drosophila Vps4 promotes Epidermal growth factor receptor signaling independently of its role in receptor degradation

Endocytic trafficking of signaling receptors is an important mechanism for limiting signal duration. Components of the Endosomal Sorting Complexes Required for Transport (ESCRT), which target ubiquitylated receptors to intra-lumenal vesicles (ILVs) of multivesicular bodies, are thought to terminate signaling by the epidermal growth factor receptor (EGFR) and direct it for lysosomal degradation. In a genetic screen for mutations that affect Drosophila eye development, we identified an allele of Vacuolar protein sorting 4 (Vps4), which encodes an AAA ATPase that interacts with the ESCRT-III complex to drive the final step of ILV formation. Photoreceptors are largely absent from Vps4 mutant clones in the eye disc, and even when cell death is genetically prevented, the mutant R8 photoreceptors that develop fail to recruit surrounding cells to differentiate as R1-R7 photoreceptors. This recruitment requires EGFR signaling, suggesting that loss of Vps4 disrupts the EGFR pathway. In imaginal disc cells mutant for Vps4, EGFR and other receptors accumulate in endosomes and EGFR target genes are not expressed; epistasis experiments place the function of Vps4 at the level of the receptor. Surprisingly, Vps4 is required for EGFR signaling even in the absence of Shibire, the Dynamin that internalizes EGFR from the plasma membrane. In ovarian follicle cells, in contrast, Vps4 does not affect EGFR signaling, although it is still essential for receptor degradation. Taken together, these findings indicate that Vps4 can promote EGFR activity through an endocytosis-independent mechanism.

ESCRT proteins: Double-edged regulators of cellular signaling

ESCRT pathway proteins play a key role in sorting ubiquitinated membrane receptors towards lysosomes providing an important mechanism for attenuating cell surface receptor signaling. However, recent studies point to a positive role of ESCRT proteins in signal transduction in multiple species studied under physiological and pathological conditions. ESCRT components such as Tsg101 and Hrs are overexpressed in human cancers and Tsg101 depletion is detrimental for cell proliferation, survival and transformed phenotype of tumor cells. However, the mechanisms underlying the positive contributions of ESCRT pathway to surface receptor signaling have remained unclear. In a recent study, we showed that Tsg101 and Vps4 are essential for translocation of active Src from endosomes to focal adhesion and invadopodia, thereby revealing a role of ESCRT pathway in promoting Src-mediated migration and invasion. We discuss the implications of these and other recent studies which together suggest a role for the ESCRT pathway in recycling of endocytic cargo proteins, aside from its role in lysosomal targeting, potentially explaining the positive roles of ESCRT proteins in signal transduction.

Blocking ESCRT-mediated envelopment inhibits microtubule-dependent trafficking of alphaherpesviruses in vitro

Herpes simplex virus (HSV) and, as reported here, pseudorabies virus (PRV) utilize the ESCRT apparatus to drive cytoplasmic envelopment of their capsids. Here, we demonstrate that blocking ESCRT-mediated envelopment using the dominant-negative inhibitor Vps4A-EQ (Vps4A in which glutamate [E] at position 228 in the ATPase active site is replaced by a glutamine [Q]) reduced the ability of HSV and PRV particles to subsequently traffic along microtubules in vitro. HSV and PRV capsid-associated particles with bound green fluorescent protein (GFP)-labeled Vps4A-EQ were readily detected by fluorescence microscopy in cytoplasmic extracts of infected cells. These Vps4A-EQ-associated capsid-containing particles bound to microtubules in vitro but were unable to traffic along them. Using a PRV strain expressing a fluorescent capsid and a fluorescently tagged form of the envelope protein gD, we found that similar numbers of gD-positive and gD-negative capsid-associated particles accumulated in cytoplasmic extracts under our conditions. Both classes of PRV particle bound to microtubules in vitro with comparable efficiency, and similar results were obtained for HSV using anti-gD immunostaining. The gD-positive and gD-negative PRV capsids were both capable of trafficking along microtubules in vitro; however, motile gD-positive particles were less numerous and their trafficking was more sensitive to the inhibitory effects of Vps4A-EQ. We discuss our data in the context of microtubule-mediated trafficking of naked and enveloped alphaherpesvirus capsids.

IMPORTANCE

The alphaherpesviruses include several important human pathogens. These viruses utilize microtubule-mediated transport to travel through the cell cytoplasm; however, the molecular mechanisms of trafficking are not well understood. In this study, we have used a cell-free system to examine the requirements for microtubule trafficking and have attempted to distinguish between the movement of so-called "naked" and membrane-associated cytoplasmic alphaherpesvirus capsids.

Pre-embedding immunogold labeling to optimize protein localization at subcellular compartments and membrane microdomains of leukocytes

Precise immunolocalization of proteins within a cell is central to understanding cell processes and functions such as intracellular trafficking and secretion of molecules during immune responses. Here we describe a protocol for ultrastructural detection of proteins in leukocytes. The method uses a pre-embedding approach (immunolabeling before standard processing for transmission electron microscopy (TEM)). This protocol combines several strategies for ultrastructure and antigen preservation, robust blocking of nonspecific binding sites, as well as superior antibody penetration for detecting molecules at subcellular compartments and membrane microdomains. A further advantage of this technique is that electron microscopy (EM) processing is quick. This method has been used to study leukocyte biology, and it has helped demonstrate how activated leukocytes deliver specific cargos. It may also potentially be applied to a variety of different cell types. Excluding the initial time required for sample preparation (15 h) and the final resin polymerization step (16 h), the protocol (immunolabeling and EM procedures) can be completed in 8 h.

The basic amino acids in the coiled-coil domain of CIN85 regulate its interaction with c-Cbl and phosphatidic acid during epidermal growth factor receptor (EGFR) endocytosis

Background

During EGFR internalization CIN85 bridges EGFR-Cbl complex, endocytic machinery and fusible membrane through the interactions of CIN85 with c-Cbl, endophilins and phosphatidic acid. These protein-protein and protein-lipid interactions are mediated or regulated by the positively charged C-terminal coiled-coil domain of CIN85. However, the details of CIN85-lipid interaction remain unknown. The present study suggested a possible electric interaction between the negative charge of phosphatidic acid and the positive charge of basic amino acids in coiled-coil domain.

Results

Mutations of the basic amino acids in the coiled-coil domain, especially K645, K646, R648 and R650, into neutral amino acid alanine completely blocked the interaction of CIN85 with c-Cbl or phosphatidic acid. However, they did not affect CIN85-endophilin interaction. In addition, CIN85 was found to associate with the internalized EGFR endosomes. It interacted with several ESCRT (Endosomal Sorting Complex Required for Transport) component proteins for ESCRT assembly on endosomal membrane. Mutations in the coiled-coil domain (deletion of the coiled-coil domain or point mutations of the basic amino acids) dissociated CIN85 from endosomes. These mutants bound the ESCRT components in cytoplasm to prevent them from assembly on endosomal membrane and inhibited EGFR sorting for degradation.

Conclusions

As an adaptor protein, CIN85 interacts with variety of partners through several domains. The positive charges of basic amino acids in the coiled-coil domain are not only involved in the interaction with phosphatidic acid, but also regulate the interaction of CIN85 with c-Cbl. CIN85 also interacts with ESCRT components for protein sorting in endosomes. These CIN85-protein and CIN85-lipid interactions enable CIN85 to link EGFR-Cbl endocytic complex with fusible membrane during EGFR endocytosis and subsequently to facilitate ESCRT formation on endosomal membrane for EGFR sorting and degradation.

The oligomeric state of the active Vps4 AAA ATPase

The cellular ESCRT (endosomal sorting complexes required for transport) pathway drives membrane constriction toward the cytosol and effects membrane fission during cytokinesis, endosomal sorting, and the release of many enveloped viruses, including the human immunodeficiency virus. A component of this pathway, the AAA ATPase Vps4, provides energy for pathway progression. Although it is established that Vps4 functions as an oligomer, subunit stoichiometry and other fundamental features of the functional enzyme are unclear. Here, we report that although some mutant Vps4 proteins form dodecameric assemblies, active wild-type Saccharomyces cerevisiae and Sulfolobus solfataricus Vps4 enzymes can form hexamers in the presence of ATP and ADP, as assayed by size-exclusion chromatography and equilibrium analytical ultracentrifugation. The Vta1p activator binds hexameric yeast Vps4p without changing the oligomeric state of Vps4p, implying that the active Vta1p-Vps4p complex also contains a single hexameric ring. Additionally, we report crystal structures of two different archaeal Vps4 homologs, whose structures and lattice interactions suggest a conserved mode of oligomerization. Disruption of the proposed hexamerization interface by mutagenesis abolished the ATPase activity of archaeal Vps4 proteins and blocked Vps4p function in S. cerevisiae. These data challenge the prevailing model that active Vps4 is a double-ring dodecamer, and argue that, like other type I AAA ATPases, Vps4 functions as a single ring with six subunits.

Ultrastructural analysis of autophagosome organization using mammalian autophagy-deficient cells

Autophagy is mediated by a unique organelle, the autophagosome. Autophagosome formation involves a number of autophagy-related (ATG) proteins and complicated membrane dynamics. Although the hierarchical relationships of ATG proteins have been investigated, how individual ATG proteins or their complexes contribute to the organization of the autophagic membrane remains largely unknown. Here, systematic ultrastructural analysis of mouse embryonic fibroblasts and HeLa cells deficient in various ATG proteins revealed that the emergence of the isolation membrane (phagophore) requires FIP200/RB1CC1, ATG9A, and PtdIns 3-kinase activity. By contrast, small premature isolation membrane- and autophagosome-like structures were generated in cells lacking VMP1 and ATG2A/B, respectively. The isolation membranes could elongate in cells lacking ATG5, but these did not mature into autophagosomes. We also found that ferritin clusters accumulated at the autophagosome formation site together with p62/SQSTM1 in autophagy-deficient cells. These results reveal the specific functions of these representative ATG proteins in autophagic membrane organization and ATG-independent recruitment of ferritin to the autophagosome formation site.

The maize AAA-type protein SKD1 confers enhanced salt and drought stress tolerance in transgenic tobacco by interacting with Lyst-interacting protein 5

ATPase associated with various cellular activities (AAA) proteins are important regulators involved in diverse cellular functions. To date, the molecular mechanisms of AAA proteins involved in response to salt and drought stresses in plants are largely unknown. In this study, a putative SKD1 (suppressor of K(+) transport growth defect 1) ortholog from Zea mays (ZmSKD1), which encodes a putative AAA protein, was isolated. The transcript levels of ZmSKD1 were higher in aerial tissues and were markedly up-regulated by salt or drought stress. Over-expression of ZmSKD1 in tobacco plants enhanced their tolerances not only to salt but to drought. Moreover, reactive oxygen species accumulations in ZmSKD1 transgenic lines were relative less than those in wild-type plants during salt or PEG-induced water stress. The interaction between ZmSKD1 and NtLIP5 (Lyst-Interacting Protein 5 homolog from Nicotiana tabacum) was confirmed by both yeast two-hybrid and immuno-precipitation assays; moreover, the α-helix-rich domain in the C-terminus of ZmSKD1 was identified to be required for its interaction with NtLIP5 using truncation mutations. Collectively, these data demonstrate that ZmSKD1could be involved in salt and drought stress responses and its over-expression enhances salt or drought stress tolerance possibly through interacting with LIP5 in tobacco. This study may facilitate our understandings of the biological roles of SKD1-mediated ESCRT pathway under stress conditions in higher plants and accelerate genetic improvement of crop plants tolerant to environmental stresses.

Relief of autoinhibition enhances Vta1 activation of Vps4 via the Vps4 stimulatory element

The endosomal sorting complexes required for transport (ESCRTs) impact multiple cellular processes including multivesicular body sorting, abscission, and viral budding. The AAA-ATPase Vps4 is required for ESCRT function, and its full activity is dependent upon the co-factor Vta1. The Vta1 carboxyl-terminal Vta1 SBP1 Lip5 (VSL) domain stimulates Vps4 function by facilitating oligomerization of Vps4 into its active state. Here we report the identification of the Vps4 stimulatory element (VSE) within Vta1 that is required for additional stimulation of Vps4 activity in vitro and in vivo. VSE activity is autoinhibited in a manner dependent upon the unstructured linker region joining the amino-terminal microtubule interacting and trafficking domains and the carboxyl-terminal VSL domain. The VSE is also required for Vta1-mediated Vps4 stimulation by ESCRT-III subunits Vps60 and Did2. These results suggest that ESCRT-III binding to the Vta1 microtubule interacting and trafficking domains relieves linker region autoinhibition of the VSE to produce maximal activation of Vps4 during ESCRT function.

MIT domain of Vps4 is a Ca2+-dependent phosphoinositide-binding domain

The microtubule interacting and trafficking (MIT) domain is a small protein module that is conserved in proteins of diverged function, such as Vps4, spastin and sorting nexin 15 (SNX15). The molecular function of the MIT domain is protein-protein interaction, in which the domain recognizes peptides containing MIT-interacting motifs. Recently, we identified an evolutionarily related domain, 'variant' MIT domain at the N-terminal region of the microtubule severing enzyme katanin p60. We found that the domain was responsible for binding to microtubules and Ca(2+). Here, we have examined whether the authentic MIT domains also bind Ca(2+). We found that the loop between the first and second α-helices of the MIT domain binds a Ca(2+) ion. Furthermore, the MIT domains derived from Vps4b and SNX15a showed phosphoinositide-binding activities in a Ca(2+)-dependent manner. We propose that the MIT domain is a novel membrane-associating domain involved in endosomal trafficking.

The hairpin-type tail-anchored SNARE syntaxin 17 targets to autophagosomes for fusion with endosomes/lysosomes

The lysosome is a degradative organelle, and its fusion with other organelles is strictly regulated. In contrast to fusion with the late endosome, the mechanisms underlying autophagosome-lysosome fusion remain unknown. Here, we identify syntaxin 17 (Stx17) as the autophagosomal SNARE required for fusion with the endosome/lysosome. Stx17 localizes to the outer membrane of completed autophagosomes but not to the isolation membrane (unclosed intermediate structures); for this reason, the lysosome does not fuse with the isolation membrane. Stx17 interacts with SNAP-29 and the endosomal/lysosomal SNARE VAMP8. Depletion of Stx17 causes accumulation of autophagosomes without degradation. Stx17 has a unique C-terminal hairpin structure mediated by two tandem transmembrane domains containing glycine zipper-like motifs, which is essential for its association with the autophagosomal membrane. These findings reveal a mechanism by which the SNARE protein is available to the completed autophagosome.

An Internal Standard-Assisted Synthesis and Degradation Proteomic Approach Reveals the Potential Linkage between VPS4B Depletion and Activation of Fatty Acid β-Oxidation in Breast Cancer Cells

The endosomal/lysosomal system, in particular the endosomal sorting complexes required for transport (ESCRTs), plays an essential role in regulating the trafficking and destination of endocytosed receptors and their associated signaling molecules. Recently, we have shown that dysfunction and down-regulation of vacuolar protein sorting 4B (VPS4B), an ESCRT-III associated protein, under hypoxic conditions can lead to the abnormal accumulation of epidermal growth factor receptor (EGFR) and aberrant EGFR signaling in breast cancer. However, the pathophysiological consequences of VPS4B dysfunction remain largely elusive. In this study, we used an internal standard-assisted synthesis and degradation mass spectrometry (iSDMS) method, which permits the direct measurement of protein synthesis, degradation and protein dynamic expression, to address the effects of VPS4B dysfunction in altering EGF-mediated protein expression. Our initial results indicate that VPS4B down-regulation decreases the expression of many proteins involved in glycolytic pathways, while increased the expression of proteins with roles in mitochondrial fatty acid β-oxidation were up-regulated in VPS4B-depleted cells. This observation is also consistent with our previous finding that hypoxia can induce VPS4B down-regulated, suggesting that the adoption of fatty acid β-oxidation could potentially serve as an alternative energy source and survival mechanism for breast cancer cells in response to hypoxia-mediated VPS4B dysfunction.

Impaired autophagy by soluble endoglin, under physiological hypoxia in early pregnant period, is involved in poor placentation in preeclampsia

In early pregnancy, trophoblasts and the fetus experience hypoxic and low-nutrient conditions; nevertheless, trophoblasts invade the uterine myometrium up to one third of its depth and migrate along the lumina of spiral arterioles, replacing the maternal endothelial lining. Here, we showed that autophagy, an intracellular bulk degradation system, occurred in extravillous trophoblast (EVT) cells under hypoxia in vitro and in vivo. An enhancement of autophagy was observed in EVTs in early placental tissues, which suffer from physiological hypoxia. The invasion and vascular remodeling under hypoxia were significantly reduced in autophagy-deficient EVT cells compared with wild-type EVT cells. Interestingly, soluble endoglin (sENG), which increased in sera in preeclamptic cases, suppressed EVT invasion by inhibiting autophagy. The sENG-inhibited EVT invasion was recovered by TGFB1 treatment in a dose-dependent manner. A high dose of sENG inhibited the vascular construction by EVT cells and human umbilical vein endothelial cells (HUVECs), meanwhile a low dose of sENG inhibited the replacement of HUVECs by EVT cells. A protein selectively degraded by autophagy, SQSTM1, accumulated in EVT cells in preeclamptic placental biopsy samples showing impaired autophagy. This is the first report showing that impaired autophagy in EVT contributes to the pathophysiology of preeclampsia.

Continuous endocytic recycling of tight junction proteins: how and why?

Tight junctions consist of many proteins, including transmembrane and associated cytoplasmic proteins, which act to provide a barrier regulating transport across epithelial and endothelial tissues. These junctions are dynamic structures that are able to maintain barrier function during tissue remodelling and rapidly alter it in response to extracellular signals. Individual components of tight junctions also show dynamic behaviour, including migration within the junction and exchange in and out of the junctions. In addition, it is becoming clear that some tight junction proteins undergo continuous endocytosis and recycling back to the plasma membrane. Regulation of endocytic trafficking of junctional proteins may provide a way of rapidly remodelling junctions and will be the focus of this chapter.

Microtubule-severing enzymes at the cutting edge

ATP-dependent severing of microtubules was first reported in Xenopus laevis egg extracts in 1991. Two years later this observation led to the purification of the first known microtubule-severing enzyme, katanin. Katanin homologs have now been identified throughout the animal kingdom and in plants. Moreover, members of two closely related enzyme subfamilies, spastin and fidgetin, have been found to sever microtubules and might act alongside katanins in some contexts (Roll-Mecak and McNally, 2010; Yu et al., 2008; Zhang et al., 2007). Over the past few years, it has become clear that microtubule-severing enzymes contribute to a wide range of cellular activities including mitosis and meiosis, morphogenesis, cilia biogenesis and disassembly, and migration. Thus, this group of enzymes is revealing itself to be among the most important of the microtubule regulators. This Commentary focuses on our growing understanding of how microtubule-severing enzymes contribute to the organization and dynamics of diverse microtubule arrays, as well as the structural and biophysical characteristics that afford them the unique capacity to catalyze the removal of tubulin from the interior microtubule lattice. Our goal is to provide a broader perspective, focusing on a limited number of particularly informative, representative and/or timely findings.

Identification of an AAA ATPase VPS4B-dependent pathway that modulates epidermal growth factor receptor abundance and signaling during hypoxia

VPS4B, an AAA ATPase (ATPase associated with various cellular activities), participates in vesicular trafficking and autophagosome maturation in mammalian cells. In solid tumors, hypoxia is a common feature and an indicator of poor treatment outcome. Our studies demonstrate that exogenous or endogenous (assessed with anchorage-independent three-dimensional multicellular spheroid culture) hypoxia induces VPS4B downregulation by the ubiquitin-proteasome system. Inhibition of VPS4B function by short hairpin VPS4B (sh-VPS4B) or expression of dominant negative VPS4B(E235Q) promotes anchorage-independent breast cancer cell growth and resistance to gefitinib, U0126, and genotoxicity. Biochemically, hyperactivation of epidermal growth factor receptor (EGFR), a receptor tyrosine kinase essential for cell proliferation and survival, accompanied by increased EGFR accumulation and altered intracellular compartmentalization, is observed in cells with compromised VPS4B. Furthermore, enhanced FOS/JUN induction and AP-1 promoter activation are noted in EGF-treated cells with VPS4B knockdown. However, VPS4B depletion does not affect EGFRvIII stability or its associated signaling. An inverse correlation between VPS4B expression and EGFR abundance is observed in breast tumors, and high-grade or recurrent breast carcinomas exhibit lower VPS4B expression. Together, our findings highlight a potentially critical role of VPS4B downregulation or chronic-hypoxia-induced VPS4B degradation in promoting tumor progression, unveiling a nongenomic mechanism for EGFR overproduction in human breast cancer.

Structures containing Atg9A and the ULK1 complex independently target depolarized mitochondria at initial stages of Parkin-mediated mitophagy

Mitochondria can be degraded by autophagy; this process is termed mitophagy. The Parkinson disease-associated ubiquitin ligase Parkin can trigger mitophagy of depolarized mitochondria. However, how the autophagy machinery is involved in this specific type of autophagy remains to be determined. It has been speculated that adaptor proteins such as p62 may mediate interaction between the autophagosomal LC3 family of proteins and ubiquitinated protein on mitochondria. Here, we describe our systematic analysis of the recruitment of Atg proteins in Parkin-dependent mitophagy. Structures containing upstream Atg proteins, including ULK1, Atg14, DFCP1, WIPI-1, and Atg16L1, can associate with depolarized mitochondria even in the absence of membrane-bound LC3. Atg9A structures are also recruited to these damaged mitochondria as well as the autophagosome formation site during starvation-induced canonical autophagy. At initial steps of Parkin-mediated mitophagy, the structures containing the ULK1 complex and Atg9A are independently recruited to depolarized mitochondria and both are required for further recruitment of downstream Atg proteins except LC3. Autophagosomal LC3 is important for efficient incorporation of damaged mitochondria into the autophagosome at a later stage. These findings suggest a process whereby the isolation membrane is generated de novo on damaged mitochondria as opposed to one where a preformed isolation membrane recognizes mitochondria.

Structure and function of the membrane deformation AAA ATPase Vps4

The ATPase Vps4 belongs to the type-I AAA family of proteins. Vps4 functions together with a group of proteins referred to as ESCRTs in membrane deformation and fission events. These cellular functions include vesicle formation at the endosome, cytokinesis and viral budding. The highly dynamic quaternary structure of Vps4 and its interactions with a network of regulators and co-factors has made the analysis of this ATPase challenging. Nevertheless, recent advances in the understanding of the cell biology of Vps4 together with structural information and in vitro studies are guiding mechanistic models of this ATPase.

Multivesicular bodies mature from the trans-Golgi network/early endosome in Arabidopsis

The plant trans-Golgi network/early endosome (TGN/EE) is a major hub for secretory and endocytic trafficking with complex molecular mechanisms controlling sorting and transport of cargo. Vacuolar transport from the TGN/EE to multivesicular bodies/late endosomes (MVBs/LEs) is assumed to occur via clathrin-coated vesicles, although direct proof for their participation is missing. Here, we present evidence that post-TGN transport toward lytic vacuoles occurs independently of clathrin and that MVBs/LEs are derived from the TGN/EE through maturation. We show that the V-ATPase inhibitor concanamycin A significantly reduces the number of MVBs and causes TGN and MVB markers to colocalize in Arabidopsis thaliana roots. Ultrastructural analysis reveals the formation of MVBs from the TGN/EE and their fusion with the vacuole. The localization of the ESCRT components VPS28, VPS22, and VPS2 at the TGN/EE and MVBs/LEs indicates that the formation of intraluminal vesicles starts already at the TGN/EE. Accordingly, a dominant-negative mutant of VPS2 causes TGN and MVB markers to colocalize and blocks vacuolar transport. RNA interference-mediated knockdown of the annexin ANNAT3 also yields the same phenotype. Together, these data indicate that MVBs originate from the TGN/EE in a process that requires the action of ESCRT for the formation of intraluminal vesicles and annexins for the final step of releasing MVBs as a transport carrier to the vacuole.

Functional ESCRT machinery is required for constitutive recycling of claudin-1 and maintenance of polarity in vertebrate epithelial cells

Drosophila ESCRT mutants lose epithelial polarity and show increased proliferation, suggesting that ESCRT proteins act as tumor suppressors. In this study, we show for the first time to our knowledge that ESCRT proteins are required to maintain polarity in mammalian epithelial cells, supporting the idea that ESCRT proteins are tumor suppressors.

Genetic screens in Drosophila have identified regulators of endocytic trafficking as neoplastic tumor suppressor genes. For example, Drosophila endosomal sorting complex required for transport (ESCRT) mutants lose epithelial polarity and show increased cell proliferation, suggesting that ESCRT proteins could function as tumor suppressors. In this study, we show for the for the first time to our knowledge that ESCRT proteins are required to maintain polarity in mammalian epithelial cells. Inhibition of ESCRT function caused the tight junction protein claudin-1 to accumulate in intracellular vesicles. In contrast E-cadherin and occludin localization was unaffected. We investigated the cause of this accumulation and show that claudin-1 is constitutively recycled in kidney, colon, and lung epithelial cells, identifying claudin-1 recycling as a newly described feature of diverse epithelial cell types. This recycling requires ESCRT function, explaining the accumulation of intracellular claudin-1 when ESCRT function is inhibited. We further demonstrate that small interfering RNA knockdown of the ESCRT protein Tsg101 causes epithelial monolayers to lose their polarized organization and interferes with the establishment of a normal epithelial permeability barrier. ESCRT knockdown also reduces the formation of correctly polarized three-dimensional cysts. Thus, in mammalian epithelial cells, ESCRT function is required for claudin-1 trafficking and for epithelial cell polarity, supporting the hypothesis that ESCRT proteins function as tumor suppressors.

Small GTPase Rab12 regulates transferrin receptor degradation: Implications for a novel membrane trafficking pathway from recycling endosomes to lysosomes

Plasma membrane receptor proteins play a key role in signal transduction and nutrient uptake, thereby controlling quality of receptor proteins is one of the most important issues in cellular logistics. After endocytosis, receptor proteins are generally delivered to lysosomes for degradation or recycled back to the plasma membrane for recycling. Transferrin receptor (TfR) is a well-known representative of recycling receptor proteins, which are traveled between plasma membrane and recycling endosomes. Although the molecular mechanism of the TfR recycling pathway has been extensively investigated in the literature, almost nothing is known about its degradation mechanism. We have recently shown that small GTPase Rab12 and its upstream activator Dennd3 regulate the constitutive degradation of TfR without modulating a conventional endocytic degradation pathway or TfR recycling pathway. Our findings suggest that Rab12 regulates membrane trafficking of TfR from recycling endosomes to lysosomes. In this addendum, we discuss the physiological significance of TfR degradation and the fate of determination of TfR (recycling or degradation).

Parkin mediates proteasome-dependent protein degradation and rupture of the outer mitochondrial membrane

Upon mitochondrial depolarization, Parkin, a Parkinson disease-related E3 ubiquitin ligase, translocates from the cytosol to mitochondria and promotes their degradation by mitophagy, a selective type of autophagy. Here, we report that in addition to mitophagy, Parkin mediates proteasome-dependent degradation of outer membrane proteins such as Tom20, Tom40, Tom70, and Omp25 of depolarized mitochondria. By contrast, degradation of the inner membrane and matrix proteins largely depends on mitophagy. Furthermore, Parkin induces rupture of the outer membrane of depolarized mitochondria, which also depends on proteasomal activity. Upon induction of mitochondrial depolarization, proteasomes are recruited to mitochondria in the perinuclear region. Neither proteasome-dependent degradation of outer membrane proteins nor outer membrane rupture is required for mitophagy. These results suggest that Parkin regulates degradation of outer and inner mitochondrial membrane proteins differently through proteasome- and mitophagy-dependent pathways.

The Arabidopsis ESCRT protein-protein interaction network

In yeast, endosomal sorting of monoubiquitylated transmembrane proteins is performed by a subset of the 19 "class E vacuolar protein sorting" proteins. The core machinery consists of 11 proteins that are organised in three complexes termed ESCRT I-III (endosomal sorting complex required for transport I-III) and is conserved in eukaryotic cells. While the pathway is well understood in yeast and animals, the plant ESCRT system is largely unexplored. At least one sequence homolog for each ESCRT component can be found in the Arabidopsis genome. Generally, sequence conservation between yeast/animals and the Arabidopsis proteins is low. To understand details about participating proteins and complex organization we have performed a systematic pairwise yeast two hybrid analysis of all Arabidopsis proteins showing homology to the ESCRT core machinery. Positive interactions were validated using bimolecular fluorescence complementation. In our experiments, most putative ESCRT components exhibited interactions with other ESCRT components that could be shown to occur on endosomes suggesting that despite their low homology to their yeast and animal counterparts they represent functional components of the plant ESCRT pathway.

The Arabidopsis ESCRT protein-protein interaction network

In yeast, endosomal sorting of monoubiquitylated transmembrane proteins is performed by a subset of the 19 "class E vacuolar protein sorting" proteins. The core machinery consists of 11 proteins that are organised in three complexes termed ESCRT I-III (endosomal sorting complex required for transport I-III) and is conserved in eukaryotic cells. While the pathway is well understood in yeast and animals, the plant ESCRT system is largely unexplored. At least one sequence homolog for each ESCRT component can be found in the Arabidopsis genome. Generally, sequence conservation between yeast/animals and the Arabidopsis proteins is low. To understand details about participating proteins and complex organization we have performed a systematic pairwise yeast two hybrid analysis of all Arabidopsis proteins showing homology to the ESCRT core machinery. Positive interactions were validated using bimolecular fluorescence complementation. In our experiments, most putative ESCRT components exhibited interactions with other ESCRT components that could be shown to occur on endosomes suggesting that despite their low homology to their yeast and animal counterparts they represent functional components of the plant ESCRT pathway.

Proteome of ubiquitin/MVB pathway: possible involvement of iron-induced ubiquitylation of transferrin receptor in lysosomal degradation

Ubiquitylation of membrane proteins triggers their endocytosis at the plasma membrane and subsequent lysosomal degradation through multivesicular bodies (MVBs). A dominant-negative mutant SKD1/Vps4B caused an accumulation of ubiquitylated membrane proteins in MVBs. We have identified 22 membrane proteins whose trafficking is potentially regulated by ubiquitylation. Nine of them, including transferrin receptor (TfR), are indeed ubiquitylated and/or accumulated in MVBs in the cells expressing mutant Vps4. While the recycling route and iron-regulated expression of TfR are well characterized, the mechanism by which the degradation of TfR is regulated is largely unknown. We show that an excess of iron enhances both TfR's ubiquitylation and degradation in lysosomes. Probably, the up-regulated expression of ferritin, an endogenous iron-chelating molecule, attenuated the iron-induced degradation of TfR. Exogenously introduced lysine-less TfR, compared to the wild-type one, showed resistance to the iron-induced ubiquitylation and degradation, when endogenous TfR, which most certainly heterodimerizes with exogenous ones, was depleted with siRNA. These data suggest that the iron-induced ubiquitylation and degradation of TfR along with MVB pathway physiologically plays an important role in iron homeostasis.

Alix regulates egress of hepatitis B virus naked capsid particles in an ESCRT-independent manner

Hepatitis B virus (HBV) is an enveloped DNA virus that exploits the endosomal sorting complexes required for transport (ESCRT) pathway for budding. In addition to infectious particles, HBV‐replicating cells release non‐enveloped (nucleo)capsids, but their functional implication and pathways of release are unclear. Here, we focused on the molecular mechanisms and found that the sole expression of the HBV core protein is sufficient for capsid release. Unexpectedly, released capsids are devoid of a detectable membrane bilayer, implicating a non‐vesicular exocytosis process. Unlike virions, naked capsid budding does not require the ESCRT machinery. Rather, we identified Alix, a multifunctional protein with key roles in membrane biology, as a regulator of capsid budding. Ectopic overexpression of Alix enhanced capsid egress, while its depletion inhibited capsid release. Notably, the loss of Alix did not impair HBV production, furthermore indicating that virions and capsids use diverse export routes. By mapping of Alix domains responsible for its capsid release‐mediating activity, its Bro1 domain was found to be required and sufficient. Alix binds to core via its Bro1 domain and retained its activity even if its ESCRT‐III binding site is disrupted. Together, the boomerang‐shaped Bro1 domain of Alix appears to escort capsids without ESCRT.

The AAA-type ATPase AtSKD1 contributes to vacuolar maintenance of Arabidopsis thaliana

The vacuole is the most prominent organelle of plant cells. Despite its importance for many physiological and developmental aspects of plant life, little is known about its biogenesis and maintenance. Here we show that Arabidopsis plants expressing a dominant-negative version of the AAA (ATPase associated with various cellular activities) ATPase AtSKD1 (SUPPRESSOR OF K+ TRANSPORT GROWTH DEFECT1) under the control of the trichome-specific GLABRA2 (GL2) promoter exhibit normal vacuolar development in early stages of trichome development. Shortly after its formation, however, the large central vacuole is fragmented and finally disappears completely. Secretion assays with amylase fused to the vacuolar sorting signal of Sporamin show that dominant-negative AtSKD1 inhibits vacuolar trafficking of the reporter that is instead secreted. In addition, trichomes expressing dominant-negative AtSKD1 frequently contain multiple nuclei. Our results suggest that AtSKD1 contributes to vacuolar protein trafficking and thereby to the maintenance of the large central vacuole of plant cells, and might play a role in cell-cycle regulation.

Seeds of Arabidopsis plants expressing dominant-negative AtSKD1 under control of the GL2 promoter show a transparent testa phenotype and a mucilage defect

We have recently shown that overexpression of dominant-negative AtSKD1 versions under control of the trichome and non-root-hair-cell specific GL2 promoter (GL2pro) blocks trafficking of soluble cargo to the vacuole, resulting in its fragmentation and ultimately cell death. GL2pro is also active in the Arabidopsis seeds. When we inspected seeds of the dominant-negative AtSKD1 variants we found two phenotypes. The seeds display a transparent testa phenotype caused by a lack of proanthocyanidin (PA) and do not possess seed coat mucilage. Both phenotypes could be connected by cell death induced by the overexpression of dominant-negative AtSKD1.

Downregulation of autophagy by Bcl-2 promotes MCF7 breast cancer cell growth independent of its inhibition of apoptosis

The anti-apoptotic Bcl-2 protein, which confers oncogenic transformation and drug resistance in most human cancers, including breast cancer, has recently been shown to effectively counteract autophagy by directly targeting Beclin1, an essential autophagy mediator and tumor suppressor. However, it remains unknown whether autophagy inhibition contributes to Bcl-2-mediated oncogenesis. Here, by using a loss-of-function mutagenesis study, we show that Bcl-2-mediated antagonism of autophagy has a critical role in enhancing the tumorigenic properties of MCF7 breast cancer cells independent of its anti-apoptosis activity. A Bcl-2 mutant defective in apoptosis inhibition but competent for autophagy suppression promotes MCF7 breast cancer cell growth in vitro and in vivo as efficiently as wild-type Bcl-2. The growth-promoting activity of this Bcl-2 mutant is strongly correlated with its suppression of Beclin1-dependent autophagy, leading to sustained p62 expression and increased DNA damage in xenograft tumors, which may directly contribute to tumorigenesis. Thus, the anti-autophagic property of Bcl-2 is a key feature of Bcl-2-mediated oncogenesis and may in some contexts, serve as an attractive target for breast and other cancer therapies.

Endosomal-sorting complexes required for transport (ESCRT) pathway-dependent endosomal traffic regulates the localization of active Src at focal adhesions

Active Src localization at focal adhesions (FAs) is essential for cell migration. How this pool is linked mechanistically to the large pool of Src at late endosomes (LEs)/lysosomes (LY) is not well understood. Here, we used inducible Tsg101 gene deletion, TSG101 knockdown, and dominant-negative VPS4 expression to demonstrate that the localization of activated cellular Src and viral Src at FAs requires the endosomal-sorting complexes required for transport (ESCRT) pathway. Tsg101 deletion also led to impaired Src-dependent activation of STAT3 and focal adhesion kinase and reduced cell migration. Impairment of the ESCRT pathway or Rab7 function led to the accumulation of active Src at aberrant LE/LY compartments followed by its loss. Analyses using fluorescence recovery after photo-bleaching show that dynamic mobility of Src in endosomes is ESCRT pathway-dependent. These results reveal a critical role for an ESCRT pathway-dependent LE/LY trafficking step in Src function by promoting localization of active Src to FAs.

Specific behavior of intracellular Streptococcus pyogenes that has undergone autophagic degradation is associated with bacterial streptolysin O and host small G proteins Rab5 and Rab7

Streptococcus pyogenes (group A streptococcus (GAS)) is a pathogen that invades non-phagocytic host cells, and causes a variety of acute infections such as pharyngitis. Our group previously reported that intracellular GAS is effectively degraded by the host-cell autophagic machinery, and that a cholesterol-dependent cytolysin, streptolysin O (SLO), is associated with bacterial escape from endosomes in epithelial cells. However, the details of both the intracellular behavior of GAS and the process leading to its autophagic degradation remain unknown. In this study, we found that two host small G proteins, Rab5 and Rab7, were associated with the pathway of autophagosome formation and the fate of intracellular GAS. Rab5 was involved in bacterial invasion and endosome fusion. Rab7 was clearly multifunctional, with roles in bacterial invasion, endosome maturation, and autophagosome formation. In addition, this study showed that the bacterial cytolysin SLO supported the escape of GAS into the cytoplasm from endosomes, and surprisingly, a SLO-deficient mutant of GAS was viable longer than the wild-type strain although it failed to escape the endosomes. This intracellular behavior of GAS is unique and distinct from that of other types of bacterial invaders. Our results provide a new picture of GAS infection and host-cell responses in epithelial cells.