Cell cycle-dependent subcellular localization of the TSG101 protein and mitotic and nuclear abnormalities associated with TSG101 deficiency

Peripubertal requirement of Tsg101 in maintaining the integrity of membranous structures in mouse oocytes

Objective

As a component of Endosomal Sorting Complex Required for Transport (ESCRT) complex I, the tumor susceptibility gene 101 (Tsg101) carries out multiple functions. In this work, we report that oocyte‐specific deletion of tumor susceptibility gene 101 (Tsg101) leads to age‐dependent oocyte demise in mice.

Materials and Method

Tsg101 floxed mice (Tsg101^f/f) were bred with Zp3^cre transgenic mice to examine oocyte‐specific roles of Tsg101. Multiple cellular and molecular biological approaches were taken to examine what leads to oocyte demise in the absence of Tsg101.

Results

The death of oocytes from Zp3^cre/Tsg101^f/f (Tsg101^d/d thereafter) mice showed a strong correlation with sexual maturation, as gonadotropin‐releasing hormone antagonist injections improved the survival rate of oocytes from 5‐week‐old Tsg101^d/d mice. Maturation of oocytes from prepubertal Tsg101^d/d mice proceeded normally, but was largely abnormal in oocytes from peripubertal Tsg101^d/d mice, showing shrinkage or rupture. Endolysosomal structures in oocytes from peripubertal Tsg101^d/d mice showed abnormalities, with aberrant patterns of early and late endosomal markers and a high accumulation of lysosomes. Dying oocytes showed plasma membrane blebs and leakage. Blockage of endocytosis in oocytes at 4°C prevented cytoplasmic shrinkage of oocytes from Tsg101^d/d mice until 9 h. The depletion of tsg‐101 in Caenorhabditis elegans increased the permeability of oocytes and embryos, suggesting a conserved role of Tsg101 in maintaining membrane integrity.

Conclusions

Collectively, Tsg101 plays a dual role in maintaining the integrity of membranous structures, which is influenced by age in mouse oocytes.

An ESCRT-dependent step in fatty acid transfer from lipid droplets to mitochondria through VPS13D-TSG101 interactions

Upon starvation, cells rewire their metabolism, switching from glucose-based metabolism to mitochondrial oxidation of fatty acids, which require the transfer of FAs from lipid droplets (LDs) to mitochondria at mitochondria−LD membrane contact sites (MCSs). However, factors responsible for FA transfer at these MCSs remain uncharacterized. Here, we demonstrate that vacuolar protein sorting-associated protein 13D (VPS13D), loss-of-function mutations of which cause spastic ataxia, coordinates FA trafficking in conjunction with the endosomal sorting complex required for transport (ESCRT) protein tumor susceptibility 101 (TSG101). The VPS13 adaptor-binding domain of VPS13D and TSG101 directly remodels LD membranes in a cooperative manner. The lipid transfer domain of human VPS13D binds glycerophospholipids and FAs in vitro. Depletion of VPS13D, TSG101, or ESCRT-III proteins inhibits FA trafficking from LDs to mitochondria. Our findings suggest that VPS13D mediates the ESCRT-dependent remodeling of LD membranes to facilitate FA transfer at mitochondria-LD contacts.

Metabolic rewiring requires the mobilization of fatty acids (FA) from lipid droplets (LDs) at membrane contact sites (MCSs), although the details of FA transfer remain unclear. Here, the authors show that VPS13D and the ESCRT complex remodel LD membranes to promote FA trafficking to mitochondria.

High expression of tumor susceptibility gene 101 (TSG101) is associated with more aggressive behavior in colorectal carcinoma

INTRODUCTION

Identification of genetic determinants such as exosomal content that drives progression and metastasis of colorectal cancer (CRC) has received considerable attention. The present study aims to identify a suitable biomarker in CRC tissues and exosomes based on bioinformatics data to evaluate its expression patterns in CRC tissues as well as its clinicopathological significance.

MATERIALS AND METHODS

Protein-protein interaction (PPI) network and enrichment analysis were applied to identify up-regulated genes that contributed in CRC exosomes to select the marker. The expression patterns and clinical significance of selected exosomal marker were evaluated in tissue microarrays (TMAs) of 445 CRC tumors and 39 adjacent normal tissues using immunohistochemistry method.

RESULTS

Based on bioinformatics data, TSG101 gene was prominent amongst the tumor tissues and exosomes. Expression of TSG101 was significantly up-regulated in tumor cells compared to adjacent normal tissues (p-value = 0.04). Moreover, higher expressions of TSG101 (cytoplasmic and nuclear) were significantly associated with tumor differentiation (p-value = 0.042) and distant metastasis (p-value = 0.027). A significant association was found in the cytoplasmic expression of TSG101 between well and moderate tumor differentiation (p-value = 0.005) as well as moderate and poor differentiation (p-value = 0.050).

CONCLUSION

These findings indicate that the exploration of crosstalk between exosome content and CRC may be valuable for the development of novel exosomal biomarkers. Increased expression of TSG101, as a promising exosome marker, is more associated with more aggressive tumor behaviors, metastasis, and progression of CRC, which paves the way for therapeutic strategies and CRC management. However, further investigations are warranted to clarify the molecular mechanisms of TSG101 in CRC.

Mahogunin Ring Finger 1 Is Required for Genomic Stability and Modulates the Malignant Phenotype of Melanoma Cells

Simple Summary

Melanoma, the most aggressive skin cancer, accounts for the majority of deaths due to this disease. Therefore, identification of genes/proteins involved in melanoma genesis and/or progression is urgent. Mutations abrogating expression of Mahogunin Ring Finger 1 (MGRN1) in mice cause complex phenotypes with hyperpigmentation, and known MGRN1 interactors are important regulators of cell shape and movement. This suggests that MGRN1 may modulate the malignant phenotype of melanoma cells. Analysis of MGRN1-KO mouse melanocytes and melanoma cells showed that lack of MGRN1 leads to cell cycle defects and to a more differentiated, less aggressive phenotype, with increased adhesion to various matrices, decreased motility and high genomic instability. The higher aggressivity of MGRN1-expressing melanoma cells was confirmed in an in vivo mouse melanoma model and is consistent with higher survival of human melanoma patients expressing low levels of MGRN1. Therefore, MGRN1 appears an important determinant of the malignant phenotype of melanoma.

Abstract

The mouse mahoganoid mutation abrogating Mahogunin Ring Finger-1 (MGRN1) E3 ubiquitin ligase expression causes hyperpigmentation, congenital heart defects and neurodegeneration. To study the pathophysiology of MGRN1 loss, we compared Mgrn1-knockout melanocytes with genetically matched controls and melan-md1 (mahoganoid) melanocytes. MGRN1 knockout induced a more differentiated and adherent phenotype, decreased motility, increased the percentage of cells in the S phase of the cell cycle and promoted genomic instability, as shown by stronger γH2AX labelling, increased burden of DNA breaks and higher abundance of aneuploid cells. Lack of MGRN1 expression decreased the ability of melanocytes to cope with DNA breaks generated by oxidizing agents or hydroxyurea-induced replicative stress, suggesting a contribution of genomic instability to the mahoganoid phenotype. MGRN1 knockout in B16-F10 melanoma cells also augmented pigmentation, increased cell adhesion to collagen, impaired 2D and 3D motility and caused genomic instability. Tumors formed by Mgrn1-KO B16-F10 cells had lower mitotic indices, fewer Ki67-positive cells and showed a trend towards smaller size. In short-term lung colonization assays Mgrn1-KO cells showed impaired colonization potential. Moreover, lower expression of MGRN1 is significantly associated with better survival of human melanoma patients. Therefore, MGRN1 might be an important phenotypic determinant of melanoma cells.

The Multifaceted Roles of the Tumor Susceptibility Gene 101 (TSG101) in Normal Development and Disease

The multidomain protein encoded by the Tumor Susceptibility Gene 101 (TSG101) is ubiquitously expressed and is suggested to function in diverse intracellular processes. In this review, we provide a succinct overview of the main structural features of the protein and their suggested roles in molecular and cellular functions. We then summarize, in more detail, key findings from studies using genetically engineered animal models that demonstrate essential functions of TSG101 in cell proliferation and survival, normal tissue homeostasis, and tumorigenesis. Despite studies on cell lines that provide insight into the molecular underpinnings by which TSG101 might function as a negative growth regulator, a biologically significant role of TSG101 as a tumor suppressor has yet to be confirmed using genuine in vivo cancer models. More recent observations from several cancer research teams suggest that TSG101 might function as an oncoprotein. A potential role of post-translational mechanisms that control the expression of the TSG101 protein in cancer is being discussed. In the final section of the review, we summarize critical issues that need to be addressed to gain a better understanding of biologically significant roles of TSG101 in cancer.

Direct isolation and characterization of circulating exosomes from biological samples using magnetic nanowires

BACKGROUND

Tumor-derived exosomes are gaining attention as important factors that facilitate communication between neighboring cells and manipulate cellular processes associated with cancer development or progression. The conventional techniques for the isolation and detection of exosomes face several limitations, restricting their clinical applications. Hence, a highly efficient technique for the isolation and identification of exosomes from biological samples may provide critical information about exosomes as biomarkers and improve our understanding of their unique role in cancer research. Here, we describe the use of antibody cocktail-conjugated magnetic nanowires to isolate exosomes from plasma of breast and lung cancer patients.

METHODS

The isolated exosomes were characterized based on size and concentration using nanoparticle tracking analysis. Levels of exosomal proteins were measured by bicinchoninic acid assay and enzyme-linked immunosorbent assay. Morphology was visualized by transmission electron microscopy. Immunoblotting (Western blotting) was used to detect the presence of exosomal markers.

RESULTS

The use of antibody cocktail-conjugated magnetic nanowires resulted in approximately threefold greater yield when compared to the conventional methods. The elongated feature of nanowires significantly improved the efficiency of exosome isolation, suggesting its potential to be translated in diverse clinical applications, including cancer diagnosis and treatment.

CONCLUSIONS

The nanowire-based method allows rapid isolation of homogeneous population of exosomes with relatively high yield and purity from even small amounts of sample. These results suggest that this method has the potential for clinical applications requiring highly purified exosomes for the analysis of protein, lipid, mRNA, and miRNA.

All roads lead to the vacuole-autophagic transport as part of the endomembrane trafficking network in plants

Plants regulate their development and response to the changing environment by sensing and interpreting environmental signals. Intracellular trafficking pathways including endocytic-, vacuolar-, and autophagic trafficking are important for the various aspects of responses in plants. Studies in the last decade have shown that the autophagic transport pathway uses common key components of endomembrane trafficking as well as specific regulators. A number of factors previously described for their function in endosomal trafficking have been discovered to be involved in the regulation of autophagy in plants. These include conserved endocytic machineries, such as the endosomal sorting complex required for transport (ESCRT), subunits of the HOPS and exocyst complexes, SNAREs, and RAB GTPases as well as plant-specific proteins. Defects in these factors have been shown to cause impairment of autophagosome formation, transport, fusion, and degradation, suggesting crosstalk between autophagy and other intracellular trafficking processes. In this review, we focus mainly on possible functions of endosomal trafficking components in autophagy.

ESCRT-dependent cargo sorting at multivesicular endosomes

The endosomal sorting complex required for transport (ESCRT) machinery is composed of five multi-subunit protein complexes, which act cooperatively at specialized endosomes to facilitate the movement of specific cargoes from the limiting membrane into vesicles that bud into the endosome lumen. Over the past decade, numerous proteins, lipids, and RNAs have been shown to be incorporated into intralumenal vesicles (ILVs), but the mechanisms by which these unique cargoes are captured are only now becoming better understood. Here, we discuss the potential roles that the ESCRT machinery plays during cargo sorting at multivesicular endosomes (MVEs).

A role of tumor susceptibility gene 101 (TSG101) in innate immune response of crayfish Procambarus clarkii

Tumor susceptibility gene 101 (TSG101) is a multi-functional gene involved in cell growth and proliferation in vertebrates. However, its role in the innate immune response of crustaceans remains unclear. Here, a TSG101 gene was identified in crayfish Procambarus clarkii with an open reading frame of 1320 bp that encoded a predicted 48.3-kDa protein highly homologous to those in other invertebrates. TSG101 mRNA was highly expressed in stomach and hepatopancreas, and its expression was induced significantly in different tissues (hemocytes, gills and intestine) by lipopolysaccharide (LPS) and polyinosinic:polycytidylic acid (poly I: C) with various expression patterns. Recombinant TSG101 protein was expressed in Escherichia coli, and a possible protein-protein interaction between TSG101 and hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) was explored by far-western blotting. RNA interference of TSG101 affected the gene expression of members of the Toll pathway. These results suggest that TSG101 is involved in the innate immune responses of P. clarkii.

TSG101, a tumor susceptibility gene, bidirectionally modulates cell invasion through regulating MMP-9 mRNA expression

Background

Tumor susceptibility gene 101 (TSG101) was initially identified in fibroblasts as a tumor suppressor gene but subsequent studies show that TSG101 also functions as a tumor-enhancing gene in some epithelial tumor cells. Although previous studies have unraveled diverse biological functions of TSG101, the precise mechanism by which TSG101 is involved in carcinogenesis and tumor progression in a bidirectional and multifaceted manner remains unclear.

Methods

To reveal the mechanism underlying bidirectional modulation of cell invasion by TSG101, we used RNA interference to examine whether TSG101 depletion bidirectionally modulated matrix metalloproteinase (MMP)-9 expression in different cell types.

Results

TSG101 depletion promoted cell invasion of HT1080 cells but contrarily reduced cell invasion of HeLaS3 cells. In HT1080 cells, TSG101 depletion increased both baseline and phorbol 12-myristate 13-acetate (PMA)-induced MMP-9 secretion through enhancing MMP-9 mRNA expression, but did not affect the expression or activation of MMP-2. In contrast, TSG101 depletion decreased PMA-induced MMP-9 secretion through reducing MMP-9 mRNA expression in HeLaS3 cells. TSG101 depletion had little impact on the signaling pathways required for the activation of transcription of MMP-9 or MMP-9 mRNA stability in either cell line.

Conclusion

TSG101 bidirectionally modulates cell invasion through regulating MMP-9 mRNA expression in different cell types. Our results provide a mechanistic context for the role of TSG101 in cell invasion as a multifaceted gene.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1942-1) contains supplementary material, which is available to authorized users.

Functional Interaction Between the ESCRT-I Component TSG101 and the HSV-1 Tegument Ubiquitin Specific Protease

Similar to phosphorylation, transient conjugation of ubiquitin to target proteins (ubiquitination) mediated by the concerted action of ubiquitin ligases and de-ubiquitinating enzymes (DUBs) can affect substrate function. As obligate intracellular parasites, viruses rely on different cellular pathways for their own replication and the well conserved ubiquitin conjugating/de-conjugating system is not an exception. Viruses not only usurp the host proteins involved in the ubiquitination/de-ubiquitination process, but they also encode their own ubiquitin ligases and DUBs. Here we report that an N-terminal variant of the herpes simplex virus (HSV) type-1 large tegument protein VP1/2 (VP1/2(1-767)), encompassing an active DUB domain (herpesvirus tegument ubiquitin specific protease, htUSP), and TSG101, a component of the endosomal sorting complex required for transport (ESCRT)-I, functionally interact. In particular, VP1/2(1-767) modulates TSG101 ubiquitination and influences its intracellular distribution. Given the role played by the ESCRT machinery in crucial steps of both cellular pathways and viral life cycle, the identification of TSG101 as a cellular target for the HSV-1 specific de-ubiquitinating enzyme contributes to the clarification of the still under debate function of viral encoded DUBs highly conserved throughout the Herpesviridae family.

Expression of TSG101 protein and LSF transcription factor in HPV-positive cervical cancer cells

Our previous study demonstrated a decreased expression of tumor susceptibility gene 101 (TSG101) in cervical cancer cells. To identify the mechanism responsible for TSG101 downregulation during cervical cancer development, we analyzed the TSG101 promoter using cis-element cluster finder software. One of the transcription factors whose binding site was detected in the TSG101 promoter was late SV40 factor (LSF). The aim of this study was to analyze the TSG101 protein and LSF expression levels during cervical cancer development. Immunohistochemical analysis confirmed a previously observed decreased expression of TSG101, whereas quantitative polymerase chain reaction (qPCR) and immunohistochemistry analysis revealed high expression of LSF in cervical, precancer and cancer cells compared with human papillomavirus (HPV)-negative non-cancer samples. High expression of LSF in cervical cancer HPV-positive cells suggests that this protein may be important in the regulation of TSG101 expression, as well as in cervical carcinogenesis. The role of LSF as a mediator in cervical cancer development must be confirmed in future studies.

The ESCRT machinery: from the plasma membrane to endosomes and back again

The manipulation and reorganization of lipid bilayers are required for diverse cellular processes, ranging from organelle biogenesis to cytokinetic abscission, and often involves transient membrane disruption. A set of membrane-associated proteins collectively known as the endosomal sorting complex required for transport (ESCRT) machinery has been implicated in membrane scission steps, which transform a single, continuous bilayer into two distinct bilayers, while simultaneously segregating cargo throughout the process. Components of the ESCRT pathway, which include 5 distinct protein complexes and an array of accessory factors, each serve discrete functions. This review focuses on the molecular mechanisms by which the ESCRT proteins facilitate cargo sequestration and membrane remodeling and highlights their unique roles in cellular homeostasis.

Identification of TSG101 functional domains and p21 loci required for TSG101-mediated p21 gene regulation

TSG101 (tumor susceptibility gene 101) is a multi-domain protein known to act in the cell nucleus, cytoplasm, and periplasmic membrane. Remarkably, TSG101, whose location within cells varies with the stage of the cell cycle, affects biological events as diverse as cell growth and proliferation, gene expression, cytokinesis, and endosomal trafficking. The functions of TSG101 additionally are recruited for viral and microvesicle budding and for intracellular survival of invading bacteria. Here we report that the TSG101 protein also interacts with and down-regulates the promoter of the p21^CIP1/WAF1tumor suppressor gene, and identify a p21 locus and TSG101 domains that mediate this interaction. TSG101 deficiency in Saos-2 human osteosarcoma cells was accompanied by an increased abundance of p21 mRNA and protein and the retardation of cell proliferation. A cis-acting element in the p21 promoter that interacts with TSG101 and is required for promoter repression was located using chromatin immunoprecipitation (ChIP) analysis and p21-driven luciferase reporter gene expression, respectively. Additional analysis of TSG101 deletion mutants lacking specific domains established the role of the central TSG101 domains in binding to the p21 promoter and demonstrated the additional essentiality of the TSG101 C-terminal steadiness box (SB) in the repression of p21 promoter activity. Neither binding of TSG101 to the p21 promoter nor repression of this promoter required the TSG101 N-terminal UEV domain, which mediates the ubiquitin-recognition functions of TSG101 and its actions as a member of ESCRT endocytic trafficking complexes, indicating that regulation of the p21 promoter by TSG101 is independent of its role in such trafficking.

Loss of the E3 ubiquitin ligase LRSAM1 sensitizes peripheral axons to degeneration in a mouse model of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous condition characterized by peripheral axon degeneration with subsequent motor and sensory deficits. Several CMT gene products function in endosomal sorting and trafficking to the lysosome, suggesting that defects in this cellular pathway might present a common pathogenic mechanism for these conditions. LRSAM1 is an E3 ubiquitin ligase that is implicated in this process, and mutations in LRSAM1 have recently been shown to cause CMT. We have generated mouse mutations in Lrsam1 to create an animal model of this form of CMT (CMT2P). Mouse Lrsam1 is abundantly expressed in the motor and sensory neurons of the peripheral nervous system. Both homozygous and heterozygous mice have largely normal neuromuscular performance and only a very mild neuropathy phenotype with age. However, Lrsam1 mutant mice are more sensitive to challenge with acrylamide, a neurotoxic agent that causes axon degeneration, indicating that the axons in the mutant mice are indeed compromised. In transfected cells, LRSAM1 primarily localizes in a perinuclear compartment immediately beyond the Golgi and shows little colocalization with components of the endosome to lysosome trafficking pathway, suggesting that other cellular mechanisms also merit consideration.

Membrane fission reactions of the mammalian ESCRT pathway

The endosomal sorting complexes required for transport (ESCRT) pathway was initially defined in yeast genetic screens that identified the factors necessary to sort membrane proteins into intraluminal endosomal vesicles. Subsequent studies have revealed that the mammalian ESCRT pathway also functions in a series of other key cellular processes, including formation of extracellular microvesicles, enveloped virus budding, and the abscission stage of cytokinesis. The core ESCRT machinery comprises Bro1 family proteins and ESCRT-I, ESCRT-II, ESCRT-III, and VPS4 complexes. Site-specific adaptors recruit these soluble factors to assemble on different cellular membranes, where they carry out membrane fission reactions. ESCRT-III proteins form filaments that draw membranes together from the cytoplasmic face, and mechanistic models have been advanced to explain how ESCRT-III filaments and the VPS4 ATPase can work together to catalyze membrane fission.

Re-splicing of mature mRNA in cancer cells promotes activation of distant weak alternative splice sites

Transcripts of the human tumor susceptibility gene 101 (TSG101) are aberrantly spliced in many cancers. A major aberrant splicing event on the TSG101 pre-mRNA involves joining of distant alternative 5′ and 3′ splice sites within exon 2 and exon 9, respectively, resulting in the extensive elimination of the mRNA. The estimated strengths of the alternative splice sites are much lower than those of authentic splice sites. We observed that the equivalent aberrant mRNA could be generated from an intron-less TSG101 gene expressed ectopically in breast cancer cells. Remarkably, we identified a pathway-specific endogenous lariat RNA consisting solely of exonic sequences, predicted to be generated by a re-splicing between exon 2 and exon 9 on the spliced mRNA. Our results provide evidence for a two-step splicing pathway in which the initial constitutive splicing removes all 14 authentic splice sites, thereby bringing the weak alternative splice sites into close proximity. We also demonstrate that aberrant multiple-exon skipping of the fragile histidine triad (FHIT) pre-mRNA in cancer cells occurs via re-splicing of spliced FHIT mRNA. The re-splicing of mature mRNA can potentially generate mutation-independent diversity in cancer transcriptomes. Conversely, a mechanism may exist in normal cells to prevent potentially deleterious mRNA re-splicing events.

Vesicle formation within endosomes: An ESCRT marks the spot

Vesicle-mediated cargo transport within the endomembrane system requires precise coordination between adaptor molecules, which recognize sorting signals on substrates, and factors that promote changes in membrane architecture. At endosomal compartments, a set of protein complexes collectively known as the ESCRT machinery sequesters transmembrane cargoes that harbor a ubiquitin modification and packages them into vesicles that bud into the endosome lumen. Several models have been postulated to describe this process. However, consensus in the field remains elusive. Here, we discuss recent findings regarding the structure and function of the ESCRT machinery, highlighting specific roles for ESCRT-0 and ESCRT-III in regulating cargo selection and vesicle formation.

Cell cycle regulation and cytoskeletal remodelling are critical processes in the nutritional programming of embryonic development

Many mechanisms purport to explain how nutritional signals during early development are manifested as disease in the adult offspring. While these describe processes leading from nutritional insult to development of the actual pathology, the initial underlying cause of the programming effect remains elusive. To establish the primary drivers of programming, this study aimed to capture embryonic gene and protein changes in the whole embryo at the time of nutritional insult rather than downstream phenotypic effects. By using a cross-over design of two well established models of maternal protein and iron restriction we aimed to identify putative common “gatekeepers” which may drive nutritional programming.

Both protein and iron deficiency in utero reduced the nephron complement in adult male Wistar and Rowett Hooded Lister rats (P<0.05). This occurred in the absence of damage to the glomerular ultrastructure. Microarray, proteomic and pathway analyses identified diet-specific and strain-specific gatekeeper genes, proteins and processes which shared a common association with the regulation of the cell cycle, especially the G1/S and G2/M checkpoints, and cytoskeletal remodelling. A cell cycle-specific PCR array confirmed the down-regulation of cyclins with protein restriction and the up-regulation of apoptotic genes with iron deficiency.

The timing and experimental design of this study have been carefully controlled to isolate the common molecular mechanisms which may initiate the sequelae of events involved in nutritional programming of embryonic development. We propose that despite differences in the individual genes and proteins affected in each strain and with each diet, the general response to nutrient deficiency in utero is perturbation of the cell cycle, at the level of interaction with the cytoskeleton and the mitotic checkpoints, thereby diminishing control over the integrity of DNA which is allowed to replicate. These findings offer novel insight into the primary causes and mechanisms leading to the pathologies which have been identified by previous programming studies.

Association of the endosomal sorting complex ESCRT-II with the Vps20 subunit of ESCRT-III generates a curvature-sensitive complex capable of nucleating ESCRT-III filaments

The scission of membranes necessary for vesicle biogenesis and cytokinesis is mediated by cytoplasmic proteins, which include members of the ESCRT (endosomal sorting complex required for transport) machinery. During the formation of intralumenal vesicles that bud into multivesicular endosomes, the ESCRT-II complex initiates polymerization of ESCRT-III subunits essential for membrane fission. However, mechanisms underlying the spatial and temporal regulation of this process remain unclear. Here, we show that purified ESCRT-II binds to the ESCRT-III subunit Vps20 on chemically defined membranes in a curvature-dependent manner. Using a combination of liposome co-flotation assays, fluorescence-based liposome interaction studies, and high-resolution atomic force microscopy, we found that the interaction between ESCRT-II and Vps20 decreases the affinity of ESCRT-II for flat lipid bilayers. We additionally demonstrate that ESCRT-II and Vps20 nucleate flexible filaments of Vps32 that polymerize specifically along highly curved membranes as a single string of monomers. Strikingly, Vps32 filaments are shown to modulate membrane dynamics in vitro, a prerequisite for membrane scission events in cells. We propose that a curvature-dependent assembly pathway provides the spatial regulation of ESCRT-III to fuse juxtaposed bilayers of elevated curvature.

Urocortin and corticotropin-releasing factor receptor 2 in human renal cell carcinoma: disruption of an endogenous inhibitor of angiogenesis and proliferation

Purpose

Urocortin (Ucn) exerts its actions through activation of two corticotropin-releasing factor receptors (CRFRs), CRFR1 and CRFR2. Involvement of Ucn/CRFR2 system in pathophysiological conditions such as the regulation of angiogenesis and inhibition of proliferation has been already reported. Suppression of neovascularization through reduction of vascular endothelial growth factor and inhibition of tumor cell cycling is modulated mainly through activation of CRFR2. To find out a possible involvement of Ucn/CRFR2 in kidney tumor, we examined the expression of Ucn and CRFR2 in normal and tumoral kidney specimens.

Methods

We applied reverse transcriptase PCR (n = 14), immunofluorescence (IF) on tissue microarrays (n = 25) and confocal microscopy to examine the mRNA expression and peptide/protein localization of Ucn and CRFR2 in normal kidney versus clear cell renal cell carcinoma, respectively.

Results

Ucn and CRFR2 mRNAs are expressed in normal and tumor specimens. In normal tissue, IF showed a cytoplasmic staining of Ucn mainly in proximal tubules, whereas a diffuse nuclear staining with diverse intensity was observed in tumoral tissues. CRFR2 was detected in proximal tubules and vasculature of normal specimens. Intriguingly, an almost complete loss of CRFR2 was observed in epithelial cells and microvessels within tumor tissues.

Conclusions

Here, and for the first time, we show the expression of Ucn and CRFR2 in human kidney and renal cell carcinoma. We propose that the nuclear translocation of Ucn along with the loss of CRFR2 in epithelial cells and microvasculature of tumoral specimens may be involved in the pathobiology of renal cell carcinoma.

The ESCRT complexes

The ESCRT machinery consists of the peripheral membrane protein complexes ESCRT-0, -I, -II, -III, and Vps4-Vta1, and the ALIX homodimer. The ESCRT system is required for degradation of unneeded or dangerous plasma membrane proteins; biogenesis of the lysosome and the yeast vacuole; the budding of most membrane enveloped viruses; the membrane abscission step in cytokinesis; macroautophagy; and several other processes. From their initial discovery in 2001-2002, the literature on ESCRTs has grown exponentially. This review will describe the structure and function of the six complexes noted above and summarize current knowledge of their mechanistic roles in cellular pathways and in disease.

Human ESCRT-III and VPS4 proteins are required for centrosome and spindle maintenance

The ESCRT pathway helps mediate the final abscission step of cytokinesis in mammals and archaea. In mammals, two early acting proteins of the ESCRT pathway, ALIX and TSG101, are recruited to the midbody through direct interactions with the phosphoprotein CEP55. CEP55 resides at the centrosome through most of the cell cycle but then migrates to the midbody at the start of cytokinesis, suggesting that the ESCRT pathway may also have centrosomal links. Here, we have systematically analyzed the requirements for late-acting mammalian ESCRT-III and VPS4 proteins at different stages of mitosis and cell division. We found that depletion of VPS4A, VPS4B, or any of the 11 different human ESCRT-III (CHMP) proteins inhibited abscission. Remarkably, depletion of individual ESCRT-III and VPS4 proteins also altered centrosome and spindle pole numbers, producing multipolar spindles (most ESCRT-III/VPS4 proteins) or monopolar spindles (CHMP2A or CHMP5) and causing defects in chromosome segregation and nuclear morphology. VPS4 proteins concentrated at spindle poles during mitosis and then at midbodies during cytokinesis, implying that these proteins function directly at both sites. We conclude that ESCRT-III/VPS4 proteins function at centrosomes to help regulate their maintenance or proliferation and then at midbodies during abscission, thereby helping ensure the ordered progression through the different stages of cell division.

ESCRT & Co

Components of the ESCRT (endosomal sorting complex required for transport) machinery mediate endosomal sorting of ubiquitinated membrane proteins. They are key regulators of biological processes important for cell growth and survival, such as growth-factor-mediated signalling and cytokinesis. In addition, enveloped viruses, such as HIV-1, hijack and utilize the ESCRTs for budding during virus release and infection. Obviously, the ESCRT-facilitated pathways require tight regulation, which is partly mediated by a group of interacting proteins, for which our knowledge is growing. In this review we discuss the different ESCRT-modulating proteins and how they influence ESCRT-dependent processes, for example, by acting as positive or negative regulators or by providing temporal and spatial control. A number of the interactors influence the classical ESCRT-mediated process of endosomal cargo sorting, for example, by modulating the interaction between ubiquitinated cargo and the ESCRTs. Certain accessory proteins have been implicated in regulating the activity or steady-state expression levels of the ESCRT components, whereas other interactors control the cellular localization of the ESCRTs, for example, by inducing shuttling between cytosol and nucleus or endosomes. In conclusion, the discovery of novel interactors has and will extend our knowledge of the biological roles of ESCRTs.

Proline-rich sequence recognition: II. Proteomics analysis of Tsg101 ubiquitin-E2-like variant (UEV) interactions

The tumor maintenance protein Tsg101 has recently gained much attention because of its involvement in endosomal sorting, virus release, cytokinesis, and cancerogenesis. The ubiquitin-E2-like variant (UEV) domain of the protein interacts with proline-rich sequences of target proteins that contain P(S/T)AP amino acid motifs and weakly binds to the ubiquitin moiety of proteins committed to sorting or degradation. Here we performed peptide spot analysis and phage display to refine the peptide binding specificity of the Tsg101 UEV domain. A mass spectrometric proteomics approach that combines domain-based pulldown experiments, binding site inactivation, and stable isotope labeling by amino acids in cell culture (SILAC) was then used to delineate the relative importance of the peptide and ubiquitin binding sites. Clearly "PTAP" interactions dominate target recognition, and we identified several novel binders as for example the poly(A)-binding protein 1 (PABP1), Sec24b, NFkappaB2, and eIF4b. For PABP1 and eIF4b the interactions were confirmed in the context of the corresponding full-length proteins in cellular lysates. Therefore, our results strongly suggest additional roles of Tsg101 in cellular regulation of mRNA translation. Regulation of Tsg101 itself by the ubiquitin ligase TAL (Tsg101-associated ligase) is most likely conferred by a single PSAP binding motif that enables the interaction with Tsg101 UEV. Together with the results from the accompanying article (Kofler, M., Schuemann, M., Merz, C., Kosslick, D., Schlundt, A., Tannert, A., Schaefer, M., Lührmann, R., Krause, E., and Freund, C. (2009) Proline-rich sequence recognition: I. Marking GYF and WW domain assembly sites in early spliceosomal complexes. Mol. Cell. Proteomics 8, 2461-2473) on GYF and WW domain pathways our work defines major proline-rich sequence-mediated interaction networks that contribute to the modular assembly of physiologically relevant protein complexes.

Endocytic proteins in the regulation of nuclear signaling, transcription and tumorigenesis

Accumulating evidence argues that many proteins governing membrane sorting during endocytosis participate also in nuclear signaling and transcriptional regulation, mostly by modulating the activity of various nuclear factors. Some adaptors and accessory proteins acting in clathrin-mediated internalization, as well as endosomal sorting proteins can undergo nuclear translocation and affect gene expression directly, while for others the effects may be more indirect. Although it is often unclear to what extent the endocytic and nuclear functions are interrelated, several of such proteins are implicated in the regulation of cell proliferation and tumorigenesis, arguing that their dual-function nature may be of physiological importance.

Cell biology of HIV-1 infection of macrophages

HIV infection of macrophages is a critically important component of viral pathogenesis and progression to AIDS. Although the virus follows the same life cycle in macrophages and T lymphocytes, several aspects of the virus-host relationship are unique to macrophage infection. Examples of these are the long-term persistence of productive infection, sustained by the absence of cell death, and the ability of progeny virus to bud into and accumulate in endocytic compartments designated multivesicular bodies (MVBs). Recently, the hypothesis that viral exploitation of the macrophage endocytic machinery is responsible for perpetuating the chronic state of infection unique to this cell type has been challenged in several independent studies employing a variety of experimental strategies. This review examines the evidence supporting and refuting the canonical hypothesis and highlights recently identified cellular factors that may contribute to the unique aspects of the HIV-macrophage interaction.

A genomic-based approach combining in vivo selection in mice to identify a novel virulence gene in Leishmania

Background

Infection with Leishmania results in a broad spectrum of pathologies where L. infantum and L. donovani cause fatal visceral leishmaniasis and L. major causes destructive cutaneous lesions. The identification and characterization of Leishmania virulence genes may define the genetic basis for these different pathologies.

Methods and Findings

Comparison of the recently completed L. major and L. infantum genomes revealed a relatively small number of genes that are absent or present as pseudogenes in L. major and potentially encode proteins in L. infantum. To investigate the potential role of genetic differences between species in visceral infection, seven genes initially classified as absent in L. major but present in L. infantum were cloned from the closely related L. donovani genome and introduced into L. major. The transgenic L. major expressing the L. donovani genes were then introduced into BALB/c mice to select for parasites with increased virulence in the spleen to determine whether any of the L. donovani genes increased visceral infection levels. During the course of these experiments, one of the selected genes (LinJ32_V3.1040 (Li1040)) was reclassified as also present in the L. major genome. Interestingly, only the Li1040 gene significantly increased visceral infection in the L. major transfectants. The Li1040 gene encodes a protein containing a putative component of an endosomal protein sorting complex involved with protein transport.

Conclusions

These observations demonstrate that the levels of expression and sequence variations in genes ubiquitously shared between Leishmania species have the potential to significantly influence virulence and tissue tropism.

Parasites of the genus Leishmania cause a variety of human diseases that range from destructive skin lesions caused by L. major to visceral infections of the liver and spleen caused by L. donovani that result in death. The Leishmania genes responsible for these different pathologies are not known. In the present study, we used a comparative genome-based approach to introduce and over-express L. donovani genes in L. major to determine whether this results in increased virulence of L. major in visceral organs of infected mice. Through this approach, a novel gene termed Li1040 was identified that is potentially involved in protein transport and was shown to increase pathogenesis in the visceral organs in mice. The Li1040 gene may therefore represent a Leishmania virulence gene that has the potential to regulate the pathology of infection in the mammalian host. These observations help to define how Leishmania causes fatal infections in humans and therefore provide a parasite-specific target for therapy.

Human ESCRT and ALIX proteins interact with proteins of the midbody and function in cytokinesis

TSG101 and ALIX both function in HIV budding and in vesicle formation at the multivesicular body (MVB), where they interact with other Endosomal Sorting Complex Required for Transport (ESCRT) pathway factors required for release of viruses and vesicles. Proteomic analyses revealed that ALIX and TSG101/ESCRT-I also bind a series of proteins involved in cytokinesis, including CEP55, CD2AP, ROCK1, and IQGAP1. ALIX and TSG101 concentrate at centrosomes and are then recruited to the midbodies of dividing cells through direct interactions between the central CEP55 'hinge' region and GPP-based motifs within TSG101 and ALIX. ESCRT-III and VPS4 proteins are also recruited, indicating that much of the ESCRT pathway localizes to the midbody. Depletion of ALIX and TSG101/ESCRT-I inhibits the abscission step of HeLa cell cytokinesis, as does VPS4 overexpression, confirming a requirement for these proteins in cell division. Furthermore, ALIX point mutants that block CEP55 and CHMP4/ESCRT-III binding also inhibit abscission, indicating that both interactions are essential. These experiments suggest that the ESCRT pathway may be recruited to facilitate analogous membrane fission events during HIV budding, MVB vesicle formation, and the abscission stage of cytokinesis.

Nuclear functions of endocytic proteins

An increasing number of proteins appear to perform multiple, sometimes unrelated functions in the cell. Such moonlighting properties have been recently demonstrated for proteins involved in clathrin-mediated endocytosis. Some clathrin adaptors and endosomal proteins can undergo nucleocytoplasmic shuttling, which is often based on intrinsic sequence motifs and requires active transport mechanisms. Endocytic proteins can associate with nuclear molecules, changing their localization and/or activity and may modulate the levels and specificity of gene transcription. It is not clear how the nuclear and cytoplasmic pools of endocytic proteins are interconnected, or whether these molecules act as nuclear second messengers upon extracellular stimuli, but alike in endocytosis, they seem to form multi-component scaffolding platforms in the nucleus. Added to their endocytic functions, the nuclear roles of Eps15, Epsin1, CALM, HIP1, Dab1/2, beta-arrestins, APPL1/2 and the components of ESCRTs clearly increase the complexity of signaling networks affecting cellular growth, proliferation and homeostasis.

Parallels between cytokinesis and retroviral budding: a role for the ESCRT machinery

During cytokinesis, as dividing animal cells pull apart into two daughter cells, the final stage, termed abscission, requires breakage of the midbody, a thin membranous stalk connecting the daughter cells. This membrane fission event topologically resembles the budding of viruses, such as HIV-1, from infected cells. We found that two proteins involved in HIV-1 budding-tumor susceptibility gene 101 (Tsg101), a subunit of the endosomal sorting complex required for transport I (ESCRT-I), and Alix, an ESCRT-associated protein-were recruited to the midbody during cytokinesis by interaction with centrosome protein 55 (Cep55), a centrosome and midbody protein essential for abscission. Tsg101, Alix, and possibly other components of ESCRT-I were required for the completion of cytokinesis. Thus, HIV-1 budding and cytokinesis use a similar subset of cellular components to carry out topologically similar membrane fission events.

The Arabidopsis elch mutant reveals functions of an ESCRT component in cytokinesis

Recently, an alternative route to the proteasomal protein-degradation pathway was discovered that specifically targets transmembrane proteins marked with a single ubiquitin to the endosomal multivesicular body (MVB) and, subsequently, to the vacuole (yeast) or lysosome (animals), where they are degraded by proteases. Vps23p/TSG101 is a key component of the ESCRT I-III machinery in yeast and animals that recognizes mono-ubiquitylated proteins and sorts them into the MVB. Here, we report that the Arabidopsis ELCH (ELC) gene encodes a Vps23p/TSG101 homolog, and that homologs of all known ESCRT I-III components are present in the Arabidopsis genome. As with its animal and yeast counterparts, ELC binds ubiquitin and localizes to endosomes. Gel-filtration experiments indicate that ELC is a component of a high-molecular-weight complex. Yeast two-hybrid and immunoprecipitation assays showed that ELC interacts with Arabidopsis homologs of the ESCRT I complex. The elc mutant shows multiple nuclei in various cell types, indicating a role in cytokinesis. Double-mutant analysis with kaktus shows that increased ploidy levels do not influence the cytokinesis effect of elc mutants, suggesting that ELC is only important during the first endoreduplication cycle. Double mutants with tubulin folding cofactor a mutants show a synergistic phenotype, suggesting that ELC regulates cytokinesis through the microtubule cytoskeleton.

Spongiform neurodegeneration-associated E3 ligase Mahogunin ubiquitylates TSG101 and regulates endosomal trafficking

A null mutation in the gene encoding the putative E3 ubiquitin-protein ligase Mahogunin causes spongiform neurodegeneration, a recessively transmitted prion-like disease in mice. However, no substrates of Mahogunin have been identified, and the cellular role of Mahogunin is unknown. Here, we report the identification of TSG101, a key component of the endosomal sorting complex required for transport (ESCRT)-I, as a specific Mahogunin substrate. We find that Mahogunin interacts with the ubiquitin E2 variant (UEV) domain of TSG101 via its PSAP motif and that it catalyzes monoubiquitylation of TSG101 both in vivo and in vitro. Depletion of Mahogunin by small interfering RNAs in mammalian cells disrupts endosome-to-lysosome trafficking of epidermal growth factor receptor, resulting in prolonged activation of a downstream signaling cascade. Our findings support a role for Mahogunin in a proteasome-independent ubiquitylation pathway and suggest a link between dysregulation of endosomal trafficking and spongiform neurodegeneration.

Human MDM2 isoforms translated differentially on constitutive versus p53-regulated transcripts have distinct functions in the p53/MDM2 and TSG101/MDM2 feedback control loops

Proteins encoded by the mdm2 gene, which has a pivotal role in the regulation of growth and differentiation, exist principally in human and murine cells as two isoforms that migrate in gels as 75-kDa and 90-kDa proteins. There is limited understanding of the respective biological roles of these isoforms, their molecular nature, and their mechanism of formation. We report here that human p75(MDM2) is an N-terminally truncated mixture of protein isoforms produced by the initiation of translation at two distinct internal AUG codons. The p75(MDM2) doublets and p90(MDM2), which is the full-length MDM2 protein, are expressed in approximately equal amounts from transcripts initiated at the constitutive P1 promoter of mdm2. Unlike murine transcripts initiated at the p53-activated P2 promoter, human cell transcripts initiated at the P2 promoter preferentially produce p90(MDM2). The ubiquitin enzyme variant protein TSG101, which interacts functionally with MDM2 in an autoregulatory loop that parallels the p53/MDM2 feedback control loop, interferes with degradation of both isoforms; however, only p90(MDM2) promotes proteolysis of TSG101 and p53. Our results reveal the mechanism of formation of the principal MDM2 isoforms, the differential effects of p53 on the production of these isoforms, and the differential abilities of human MDM2 isoforms as regulators of the MDM2/TSG101 and p53/MDM2 feedback control loops.

Consistent effects of TSG101 genetic variability on multiple outcomes of exposure to human immunodeficiency virus type 1

Tumor susceptibility gene 101 (TSG101) encodes a host cellular protein that is appropriated by human immunodeficiency virus type 1 (HIV-1) in the budding process of viral particles from infected cells. Variation in the coding or noncoding regions of the gene could potentially affect the degree of TSG101-mediated release of viral particles. While the coding regions of the gene were found to lack nonsynonymous variants, two polymorphic sites in the TSG101 5' area were identified that were associated with the rate of AIDS progression among Caucasians. These single-nucleotide polymorphisms (SNPs), located at positions -183 and +181 relative to the translation start, specify three haplotypes termed A, B, and C, which occur at frequencies of 67%, 21%, and 12%, respectively. Haplotype C is associated with relatively rapid AIDS progression, while haplotype B is associated with slower disease progression. Both effects were dominant over the intermediate haplotype A. The haplotypes also demonstrated parallel effects on the rate of CD4 T-cell depletion and viral load increase over time, as well as a possible influence on HIV-1 infection. The data raise the hypothesis that noncoding variation in TSG101 affects the efficiency of TSG101-mediated release of viral particles from infected cells, thereby altering levels of plasma viral load and subsequent disease progression.

Bcr interacts with components of the endosomal sorting complex required for transport-I and is required for epidermal growth factor receptor turnover

Virtually all patients with chronic myelogenous leukemia (CML) express an aberrant protein (p210 Bcr-Abl) that contains NH2-terminal sequences from Bcr fused to COOH-terminal sequences from Abl. In a yeast two-hybrid screen, we have identified TSG101 as a binding partner for Bcr. Because TSG101 is a subunit of the mammalian endosomal sorting complex required for transport (ESCRT), which regulates protein sorting during endosomal trafficking, this association suggests that Bcr may have a related cellular function. The docking site for TSG101 has been mapped to the COOH terminus of Bcr, indicating that this interaction may be disrupted in CML. Overexpression studies with full-length TSG101 and Bcr reveal that this interaction can be recapitulated in mammalian cells. The association can also be observed between natively expressed proteins in a panel of hematopoietic and nonhematopoietic cell lines, where a second subunit of the ESCRT complex, vacuolar sorting protein 28 (Vps28), was also found to interact with Bcr. Both Bcr and TSG101 exhibit a punctate cytoplasmic distribution and seem to colocalize in HeLa cells, which would be consistent with an in vivo association. Bacterially purified Bcr and TSG101 also bind, suggesting that the interaction is direct and is not dependent on ubiquitination. Disruption of the endosomal pathway with an ATPase-defective Vps4 mutant results in the cellular redistribution of Bcr, and suppression of Bcr in HeLa cells by small interfering RNA impairs epidermal growth factor receptor turnover. Taken together, these observations suggest that Bcr is a component of the mammalian ESCRT complexes and plays an important role in cellular trafficking of growth factor receptors.

Genetic structure and evolution of the Vps25 family, a yeast ESCRT-II component

Background

Vps25p is the product of yeast gene VPS25 and is found in an endosomal sorting complex required for transport (ESCRT)-II, along with Vps22p and Vps36p. This complex is essential for sorting of ubiquitinated biosynthetic and endosomal cargoes into endosomes.

Results

We found that VPS25 is a highly conserved and widely expressed eukaryotic gene, with single orthologs in chromalveolate, excavate, amoebozoan, plant, fungal and metazoan species. Two paralogs were found in Trichomonas vaginalis. An ortholog was strikingly absent from the Encephalitozoon cuniculi genome. Intron positions were analyzed in VPS25 from 36 species. We found evidence for five ancestral VPS25 introns, intron loss, and single instances of intron gain (a Paramecium species) and intron slippage (Theileria species). Processed pseudogenes were identified in four mammalian genomes, with a notable absence in the mouse genome. Two retropseudogenes were found in the chimpanzee genome, one more recently inserted, and one evolving from a common primate ancestor. The amino acid sequences of 119 Vps25 orthologs are aligned, compared with the known secondary structure of yeast Vps25p, and used to carry out phylogenetic analysis. Residues in two amino-terminal PPXY motifs (motif I and II), involved in dimerization of Vps25p and interaction with Vps22p and Vps36p, were closely, but not absolutely conserved. Specifically, motif I was absent in Vps25 homologs of chromalveolates, euglenozoa, and diplomonads. A highly conserved carboxy-terminal lysine was identified, which suggests Vps25 is ubiquitinated. Arginine-83 of yeast Vps25p involved in Vps22p interaction was highly, but not absolutely, conserved. Human tissue expression analysis showed universal expression.

Conclusion

We have identified 119 orthologs of yeast Vps25p. Expression of mammalian VPS25 in a wide range of tissues, and the presence in a broad range of eukaryotic species, indicates a basic role in eukaryotic cell function. Intron splice site positions were highly conserved across all major eukaryotic species, suggesting an ancestral origin. Amino acid sequence analysis showed the consensus for the amino-terminal proline-rich motifs is P- [WP]-X-[YF] for motif I (when present) and P-P-[FYL]-[FY] for motif II, and that Vps25 may be ubiquitinated.

Txr1: a transcriptional regulator of thrombospondin-1 that modulates cellular sensitivity to taxanes

Using transcripts initiated at a chromosomally integrated retrovirus-based promoter to perturb gene expression randomly in human prostate cancer cells, we isolated cell clones resistant to taxane lethality and discovered the role of a previously uncharacterized gene, txr1, in this phenotype. We show that txr1 impedes taxane-induced apoptosis in tumor cells by transcriptionally down-regulating the production of thrombospondin-1 (TSP-1)--known earlier for both its anti-angiogenic and proapoptotic actions. Decrease of Txr1 or treatment with TSP-1 or TSP-1 mimetic peptide sensitized cells to taxane cytotoxicity by activating signaling through the CD47 receptor (also known as the integrin-associated protein), whereas interference with CD47 function reduced taxane-induced cell death. Cellular abundance of Txr1 and TSP-1 varied inversely, and alteration of the level of both proteins correlated highly with taxol resistance in 13 of 19 NCI-60 cancer cell lines. Our results reveal a hitherto unsuspected mechanism of taxane resistance, elucidate the role of txr1 in this resistance, and identify txr1 as a regulator of TSP-1 production and an agent for its chemotherapeutic modulation.

Endosomal and non-endosomal functions of ESCRT proteins

The three endosomal sorting complexes required for transport (ESCRTs) are integral to the degradation of endocytosed membrane proteins and multivesicular body (MVB) biogenesis. Here, we review evidence that ESCRTs have evolved as a specialized machinery for the degradative sorting of ubiquitinated membrane proteins and we highlight recent studies that have shed light on the mechanisms by which these complexes mediate protein sorting, MVB biogenesis, tumour suppression and viral budding. We also discuss evidence that some ESCRT subunits have evolved additional functions that are unrelated to membrane trafficking.

[HIV budding and Tsg101]

HIV, as well as many enveloped viruses, exits the cells by budding directly from the plasma membrane. HIV budding is dependent on a PTAP motif, which is located within the p6 domain of Gag. Recent studies have shown that the cellular protein Tsg101 binds to the PTAP L-domain motif of HIV p6 and facilitates the final stages of virus release. Tsg101 function in the cellular vacuolar protein sorting pathway, where they play central roles in selecting cargo for incorporation into vesicles that bud into the maturing endosome to create multivesicular bodies (MVBs). Vesicle budding into the MVB and viral budding at the plasma membrane are topologically equivalent, and the same machinery could catalyze both processes. It will be important to understand the mechanism of virus budding in detail, since virus budding may be a potential target for interference with HIV propagation.

Elevated expression of C10orf3 (chromosome 10 open reading frame 3) is involved in the growth of human colon tumor

After analysing gene-expression profiles of colon cancers on a cDNA microarray containing cDNAs corresponding to 23 040 human genes, we focused on a gene annotated as C10orf3 (chromosome 10 open reading frame 3), whose expression was elevated in colorectal cancers (CRC) as well as in tumors arising in the stomach, lung, pancreas, and breast. The gene encodes a putative 464-amino-acid protein containing a domain known as AAA (ATPases associated with a variety of cellular activities). Western blot analysis using an antibody to the gene product confirmed that the protein was overexpressed in nine of the 15 clinical cancer tissues examined, compared to corresponding noncancerous epithelial cells. A subsequent proteomics analysis revealed that C10orf3 product associated with the product of tumor susceptibility gene 101 (TSG101), and that C10orf3 downregulated TSG101 in a post-transcriptional manner. Expression of short interfering RNA in cells derived from CRC caused significant decreases in C10orf3 expression and inhibited growth of the transfected cells, which was associated with increased apoptotic cells. These data suggest that elevated C10orf3 expression might play an essential role in the growth of cancer cells, and that suppression of C10orf3-mediated signal transduction may be a novel therapeutic strategy to a wide range of human tumors.

SIMPLE interacts with NEDD4 and TSG101: evidence for a role in lysosomal sorting and implications for Charcot-Marie-Tooth disease

Mutation of the SIMPLE gene (small integral membrane protein of the lysosome/late endosome) is the molecular basis of Charcot-Marie-Tooth disease type 1C (CMT1C), a demyelinating peripheral neuropathy. Although the precise function of SIMPLE is unknown, prior reports suggest it localizes to the lysosome/late endosome. Furthermore, murine Simple interacts with Nedd4 (neural precursor cell expressed, developmentally downregulated 4), an E3 ubiquitin ligase that is important for regulating lysosomal degradation of plasma membrane proteins. To bring insights into the biochemical function of human SIMPLE, we confirmed that human SIMPLE interacts with NEDD4 and also report a novel interaction with tumor susceptibility gene 101 (TSG101), a class E vacuolar sorting protein. TSG101 is known to function downstream of NEDD4, sorting ubiquitinated substrates into multivesicular bodies (MVBs), which then deliver their cargo into the lysosomal lumen for degradation. Given the interaction with NEDD4 and TSG101, and the localization of SIMPLE along the lysosomal degradation pathway, we hypothesize that SIMPLE plays a role in the lysosomal sorting of plasma membrane proteins. We examine three CMT1C-associated SIMPLE mutations and show that they do not affect the interaction with NEDD4 or TSG101, nor do they lead to altered subcellular localization.

Stabilization of the unliganded glucocorticoid receptor by TSG101

The glucocorticoid receptor (GR) has been shown to undergo hormone-dependent down-regulation via transcriptional, post-transcriptional, and posttranslational mechanisms. However, the mechanisms involved in modulating GR levels in the absence of hormone remain enigmatic. Here we demonstrate that TSG101, a previously identified GR-interacting protein, stabilizes the hypophosphorylated form of GR in the absence of ligand. We found that a non-phosphorylated version of GR (S203A/S211A) showed enhanced interaction with TSG101 as compared with the wild type GR, suggesting that TSG101 interacts more favorably with GR when it is not phosphorylated. A significant accumulation of GR S203A/S211A protein is detected in the absence of ligand when TSG101 is overexpressed, whereas no increase in the wild type phosphorylated GR or phosphomimetic GR S203E/S211E was observed in mammalian cells. In contrast, down-regulation of TSG101 expression by siRNA renders the hypophosphorylated form of GR unstable. We further show that TSG101 stabilizes GR by impeding its degradation by the proteasome and extending receptor half-life. Thus, in absence of a ligand, TSG101 binds GR and protects the non-phosphorylated receptor from degradation.

Crystal structure of subunit VPS25 of the endosomal trafficking complex ESCRT-II

Background

Down-regulation of plasma membrane receptors via the endocytic pathway involves their monoubiquitylation, transport to endosomal membranes and eventual sorting into multi vesicular bodies (MVB) destined for lysosomal degradation. Successive assemblies of Endosomal Sorting Complexes Required for Transport (ESCRT-I, -II and III) largely mediate sorting of plasma membrane receptors at endosomal membranes, the formation of multivesicular bodies and their release into the endosomal lumen. In addition, the human ESCRT-II has been shown to form a complex with RNA polymerase II elongation factor ELL in order to exert transcriptional control activity.

Results

Here we report the crystal structure of Vps25 at 3.1 Å resolution. Vps25 crystallizes in a dimeric form and each monomer is composed of two winged helix domains arranged in tandem. Structural comparisons detect no conformational changes between unliganded Vps25 and Vps25 within the ESCRT-II complex composed of two Vps25 copies and one copy each of Vps22 and Vps36 [1,2].

Conclusions

Our structural analyses present a framework for studying Vps25 interactions with ESCRT-I and ESCRT-III partners. Winged helix domain containing proteins have been implicated in nucleic acid binding and it remains to be determined whether Vps25 has a similar activity which might play a role in the proposed transcriptional control exerted by Vps25 and/or the whole ESCRT-II complex.

Embryos Derived from Porcine Skin-Derived Stem Cells Exhibit Enhanced Preimplantation Development1

Ongoing research to identify the most suitable type of donor cell for nuclear transfer (NT) has suggested that less differentiated stem cells may be better donors than other somatic cell types. Recently, we have reported the isolation and characterization of porcine skin-originated sphere (PSOS) stem cells from fetal skin, making it possible to test this hypothesis in a nonrodent animal model. In the present study, we have investigated and compared the feasibility and preimplantation developmental efficiency of using fetal PSOS cells and fibroblasts as nuclear-transfer donors. The majority of fetal PSOS cells are in the G1/ G0 stage of the cell cycle, which is desirable for NT. During long-term in vitro culture, fetal PSOS cells had greater genome stability, with a lower frequency of abnormal karyotypes than fetal fibroblast cells. Embryos cloned from PSOS cells showed enhanced preimplantation development compared with fibroblast cloned embryos, which is indicated by an increased rate of blastocyst development and a higher total cell number in Day 7 blastocysts. The gene expression profile of genes critical for early development from eight-cell-stage PSOS NT embryos more closely resembled the pattern observed from in vivo-produced embryos compared with that of fibroblast-cloned embryos. Cumulatively, our data suggest that fetal PSOS cells may be better donor cells for NT in the pig.