MDA-9/syntenin interacts with ubiquitin via a novel ubiquitin-binding motif

Prominosomes - a particular class of extracellular vesicles containing prominin-1/CD133?

Extracellular membrane vesicles (EVs) offer promising values in various medical fields, e.g., as biomarkers in liquid biopsies or as native (or bioengineered) biological nanocarriers in tissue engineering, regenerative medicine and cancer therapy. Based on their cellular origin EVs can vary considerably in composition and diameter. Cell biological studies on mammalian prominin-1, a cholesterol-binding membrane glycoprotein, have helped to reveal new donor membranes as sources of EVs. For instance, small EVs can originate from microvilli and primary cilia, while large EVs might be produced by transient structures such as retracting cellular extremities of cancer cells during the mitotic rounding process, and the midbody at the end of cytokinesis. Here, we will highlight the various subcellular origins of prominin-1+ EVs, also called prominosomes, and the potential mechanism(s) regulating their formation. We will further discuss the molecular and cellular characteristics of prominin-1, notably those that have a direct effect on the release of prominin-1+ EVs, a process that might be directly implicated in donor cell reprogramming of stem and cancer stem cells. Prominin-1+ EVs also mediate intercellular communication during embryonic development and adult homeostasis in healthy individuals, while disseminating biological information during diseases.

Biological Role and Aberrant Overexpression of Syntenin-1 in Cancer: Potential Role as a Biomarker and Therapeutic Target

Syntenin-1 is a 298 amino acid protein codified by the melanoma differentiation-associated gene-9 (MDA-9). Structurally, it is composed of four domains: N-terminal, PDZ1, PDZ2, and C-terminal. The PDZ domains of syntenin-1 are involved in the stability and interaction with other molecules such as proteins, glycoproteins, and lipids. Domains are also associated with several biological functions such as the activation of signaling pathways related to cell-to-cell adhesion, signaling translation, and the traffic of intracellular lipids, among others. The overexpression of syntenin-1 has been reported in glioblastoma, colorectal, melanoma, lung, prostate, and breast cancer, which promotes tumorigenesis by regulating cell migration, invasion, proliferation, angiogenesis, apoptosis, and immune response evasion, and metastasis. The overexpression of syntenin-1 in samples has been associated with worst prognostic and recurrence, whereas the use of inhibitors such as shRNA, siRNA, and PDZli showed a diminution of the tumor size and reduction in metastasis and invasion. Syntenin-1 has been suggested as a potential biomarker and therapeutic target in cancer for developing more effective diagnostic/prognostic tests or passive/active immunotherapies.

Radiation induces ESCRT pathway dependent CD44v3+ extracellular vesicle production stimulating pro-tumor fibroblast activity in breast cancer

Despite recent advances in radiotherapeutic strategies, acquired resistance remains a major obstacle, leading to tumor recurrence for many patients. Once thought to be a strictly cancer cell intrinsic property, it is becoming increasingly clear that treatment-resistance is driven in part by complex interactions between cancer cells and non-transformed cells of the tumor microenvironment. Herein, we report that radiotherapy induces the production of extracellular vesicles by breast cancer cells capable of stimulating tumor-supporting fibroblast activity, facilitating tumor survival and promoting cancer stem-like cell expansion. This pro-tumor activity was associated with fibroblast production of the paracrine signaling factor IL-6 and was dependent on the expression of the heparan sulfate proteoglycan CD44v3 on the vesicle surface. Enzymatic removal or pharmaceutical inhibition of its heparan sulfate side chains disrupted this tumor-fibroblast crosstalk. Additionally, we show that the radiation-induced production of CD44v3⁺ vesicles is effectively silenced by blocking the ESCRT pathway using a soluble pharmacological inhibitor of MDA-9/Syntenin/SDCBP PDZ1 domain activity, PDZ1i. This population of vesicles was also detected in the sera of human patients undergoing radiotherapy, therefore representing a potential biomarker for radiation therapy and providing an opportunity for clinical intervention to improve treatment outcomes.

The Ubiquitinated Axon: Local Control of Axon Development and Function by Ubiquitin

Ubiquitin tagging sets protein fate. With a wide range of possible patterns and reversibility, ubiquitination can assume many shapes to meet specific demands of a particular cell across time and space. In neurons, unique cells with functionally distinct axons and dendrites harboring dynamic synapses, the ubiquitin code is exploited at the height of its power. Indeed, wide expression of ubiquitination and proteasome machinery at synapses, a diverse brain ubiquitome, and the existence of ubiquitin-related neurodevelopmental diseases support a fundamental role of ubiquitin signaling in the developing and mature brain. While special attention has been given to dendritic ubiquitin-dependent control, how axonal biology is governed by this small but versatile molecule has been considerably less discussed. Herein, we set out to explore the ubiquitin-mediated spatiotemporal control of an axon's lifetime: from its differentiation and growth through presynaptic formation, function, and pruning.

Novel function of MDA-9/Syntenin (SDCBP) as a regulator of survival and stemness in glioma stem cells

Glioblastoma multiforme (GBM) is an aggressive cancer with current therapies only marginally impacting on patient survival. Glioma stem cells (GSCs), a subpopulation of highly tumorigenic cells, are considered major contributors to glioma progression and play seminal roles in therapy resistance, immune evasion and increased invasion. Despite clinical relevance, effective/selective therapeutic targeting strategies for GSCs do not exist, potentially due to the lack of a definitive understanding of key regulators of GSCs. Consequently, there is a pressing need to identify therapeutic targets and novel options to effectively target this therapy-resistant cell population. The precise roles of GSCs in governing GBM development, progression and prognosis are under intense scrutiny, but key upstream regulatory genes remain speculative. MDA-9/Syntenin (SDCBP), a scaffold protein, regulates tumor pathogenesis in multiple cancers. Highly aggressive cancers like GBM express elevated levels of MDA-9 and contain increased populations of GSCs. We now uncover a unique function of MDA-9 as a facilitator and determinant of glioma stemness and survival. Mechanistically, MDA-9 regulates multiple stemness genes (Nanog, Oct4 and Sox2) through activation of STAT3. MDA-9 controls survival of GSCs by activating the NOTCH1 pathway through phospho-Src and DLL1. Once activated, cleaved NOTCH1 regulates C-Myc expression through RBPJK, thereby facilitating GSC growth and proliferation. Knockdown of MDA-9 affects the NOTCH1/C-Myc and p-STAT3/Nanog pathways causing a loss of stemness and initiation of apoptosis in GSCs. Our data uncover a previously unidentified relationship between MDA-9 and GSCs, reinforcing relevance of this gene as a potential therapeutic target in GBM.

Regulation of intestinal homeostasis by the ulcerative colitis-associated gene RNF186

Genome-wide association studies and subsequent deep sequencing analysis have identified susceptible loci for inflammatory bowel diseases (IBDs) including ulcerative colitis (UC). A gene encoding RING finger protein 186 (RNF186) is located within UC-susceptible loci. However, it is unclear whether RNF186 is involved in IBD pathogenesis. Here, we show that RNF186 controls protein homeostasis in colonic epithelia and regulates intestinal inflammation. RNF186, which was highly expressed in colonic epithelia, acted as an E3 ligase mediating polyubiquitination of its substrates. Permeability of small organic molecules was augmented in the intestine of Rnf186^-/- mice. Increased expression of several RNF186 substrates, such as occludin, was found in Rnf186^-/- colonic epithelia. The disturbed protein homeostasis in Rnf186^-/- mice correlated with enhanced endoplasmic reticulum (ER) stress in colonic epithelia and increased sensitivity to intestinal inflammation after dextran sulfate sodium (DSS) treatment. Introduction of an UC-associated Rnf186 mutation led to impaired E3 ligase activity and increased sensitivity to DSS-induced intestinal inflammation in mice. Thus, RNF186 maintains gut homeostasis by controlling ER stress in colonic epithelia.

MDA-9/Syntenin-Slug transcriptional complex promote epithelial-mesenchymal transition and invasion/metastasis in lung adenocarcinoma

Melanoma differentiation-associated gene-9 (MDA-9)/Syntenin is a novel therapeutic target because it plays critical roles in cancer progression and exosome biogenesis. Here we show that Slug, a key epithelial-mesenchymal-transition (EMT) regulator, is a MDA-9/Syntenin downstream target. Mitogen EGF stimulation increases Slug expression and MDA-9/Syntenin nuclear translocation. MDA-9/Syntenin uses its PDZ1 domain to bind with Slug, and this interaction further leads to HDAC1 recruitment, up-regulation of Slug transcriptional repressor activity, enhanced Slug-mediated EMT, and promotion of cancer invasion and metastasis. The PDZ domains and nuclear localization of MDA-9/Syntenin are both required for promoting Slug-mediated cancer invasion. Clinically, patients with high MDA-9/Syntenin and high Slug expressions were associated with poor overall survival compared to those with low expression in lung adenocarcinomas. Our findings provide evidence that MDA-9/Syntenin acts as a pivotal adaptor of Slug and it transcriptionally enhances Slug-mediated EMT to promote cancer invasion and metastasis.

MDA-9/Syntenin Control

MDA-9/Syntenin is a small PDZ domain containing scaffolding protein with diverse array of functions regulating membrane trafficking, cell adhesion, neural, and synaptic development, ubiquitination, and exosome biogenesis. An appreciable number of studies also established a pivotal role of MDA-9/Syntenin in cancer development and progression. In this review, we will discuss the dynamic role of MDA-9/Syntenin in regulating normal and abnormal fate of various cellular processes.

Syntenin and syndecan in the biogenesis of exosomes

Cells communicate with their environment in various ways, including by secreting vesicles. Secreted vesicles are loaded with proteins, lipids and RNAs that compose 'a signature' of the cell of origin and potentially can reprogram recipient cells. Secreted vesicles recently gained in interest for medicine. They represent potential sources of biomarkers that can be collected from body fluids and, by disseminating pathogenic proteins, might also participate in systemic diseases like cancer, atherosclerosis and neurodegeneration. The mechanisms controlling the biogenesis and the uptake of secreted vesicles are poorly understood. Some of these vesicles originate from endosomes and are called 'exosomes'. In this review, we recapitulate recent insight on the role of the syndecan (SDC) heparan sulphate proteoglycans, the small intracellular adaptor syntenin and associated regulators in the biogenesis and loading of exosomes with cargo. SDC-syntenin-associated regulators include the endosomal sorting complex required for transport accessory component ALG-2-interacting protein X, the small GTPase adenosine 5'-diphosphate-ribosylation factor 6, the lipid-modifying enzyme phospholipase D2 and the endoglycosidase heparanase. All these molecules appear to support the budding of SDC-syntenin and associated cargo into the lumen of endosomes. This highlights a major mechanism for the formation of intraluminal vesicles that will be released as exosomes.

Syntenin is involved in the bacteria clearance response of kuruma shrimp (Marsupenaeus japonicus)

Syntenin is a multifunctional cytosolic adaptor protein that contributes to cell migration, proliferation, attachment, and apoptosis, as well as immune response to virus, in vertebrates. However, the functions of syntenin in the antibacterial response of invertebrates remain unclear. In this study, we identified a syntenin-like gene (MjSyn) from the kuruma shrimp (Marsupenaeus japonicus) and detected its function in the antibacterial immunity of shrimp. The full-length MjSyn was 1223 bp with a 963 bp open reading frame that encodes 320 amino acids. The deduced MjSyn proteins contained two atypical PDZ domains (sequence repeat that was first reported in the postsynaptic density protein or PSD-95, DlgA, and ZO-1 protein), an N-terminal domain, and a C-terminal domain. Reverse transcription (RT)-PCR results showed that MjSyn was expressed in all tested tissues. Quantitative real-time PCR analysis revealed that MjSyn transcripts in the hemocyte, gill, and intestine were significantly induced at various time points after infection with Staphylococcus aureus and Vibrio anguillarum. The knockdown of the expression of MjSyn by RNA interference resulted in a significant decrease in the phagocytic ability and increased bacteria number in vivo of shrimp. Moreover, the expression of MjCnx, a cytoplasma and membrane location lectin chaperone protein, was inhibited in the MjSyn-knocked down shrimp, which indicated a possible calnexin-related way. Thus, the MjSyn participates in the bacterial clearance response of kuruma shrimp, thereby providing new insight into the function of this kind of important adaptor protein.

Mutation of the highly conserved Ser-40 of the HIV-1 p6 gag protein to Phe causes the formation of a hydrophobic patch, enhances membrane association, and polyubiquitination of Gag

The HIV-1 p6 Gag protein contains two late assembly (l-) domains that recruit proteins of the endosomal sorting complex required for transport (ESCRT) pathway to mediate membrane fission between the nascent virion and the cell membrane. It was recently demonstrated that mutation of the highly conserved Ser-40 to Phe (S40F) disturbs CA-SP1 processing, virus morphogenesis, and infectivity. It also causes the formation of filopodia-like structures, while virus release remains unaffected. Here, we show that the mutation S40F, but not the conservative mutation to Asp (S40D) or Asn (S40N), augments membrane association, K48-linked polyubiquitination, entry into the 26S proteasome, and, consequently, enhances MHC-I antigen presentation of Gag derived epitopes. Nuclear magnetic resonance (NMR) structure analyses revealed that the newly introduced Phe-40, together with Tyr-36, causes the formation of a hydrophobic patch at the C-terminal α-helix of p6, providing a molecular rationale for the enhanced membrane association of Gag observed in vitro and in HIV-1 expressing cells. The extended exposure of the S40F mutant to unidentified membrane-resident ubiquitin E3-ligases might trigger the polyubiquitination of Gag. The cumulative data support a previous model of a so far undefined property of p6, which, in addition to MA, acts as membrane targeting domain of Gag.

Human prominin-1 (CD133) is detected in both neoplastic and non-neoplastic salivary gland diseases and released into saliva in a ubiquitinated form

Prominin-1 (CD133) is physiologically expressed at the apical membranes of secretory (serous and mucous) and duct cells of major salivary glands. We investigated its expression in various human salivary gland lesions using two distinct anti-prominin-1 monoclonal antibodies (80B258 and AC133) applied on paraffin-embedded sections and characterized its occurrence in saliva. The 80B258 epitope was extensively expressed in adenoid cystic carcinoma, in lesser extent in acinic cell carcinoma and pleomorphic adenoma, and rarely in mucoepidermoid carcinoma. The 80B258 immunoreactivity was predominately detected at the apical membrane of tumor cells showing acinar or intercalated duct cell differentiation, which lined duct- or cyst-like structures, and in luminal secretions. It was observed on the whole cell membrane in non-luminal structures present in the vicinity of thin-walled blood vessels and hemorrhagic areas in adenoid cystic carcinoma. Of note, AC133 labeled only a subset of 80B258–positive structures. In peritumoral salivary gland tissues as well as in obstructive sialadenitis, an up-regulation of prominin-1 (both 80B258 and AC133 immunoreactivities) was observed in intercalated duct cells. In most tissues, prominin-1 was partially co-expressed with two cancer markers: carcinoembryonic antigen (CEA) and mucin-1 (MUC1). Differential centrifugation of saliva followed by immunoblotting indicated that all three markers were released in association with small membrane vesicles. Immuno-isolated prominin-1–positive vesicles contained CEA and MUC1, but also exosome-related proteins CD63, flotillin-1, flotillin-2 and the adaptor protein syntenin-1. The latter protein was shown to interact with prominin-1 as demonstrated by its co-immunoisolation. A fraction of saliva-associated prominin-1 appeared to be ubiquitinated. Collectively, our findings bring new insights into the biochemistry and trafficking of prominin-1 as well as its immunohistochemical profile in certain types of salivary gland tumors and inflammatory diseases.

The S40 residue in HIV-1 Gag p6 impacts local and distal budding determinants, revealing additional late domain activities

Background

HIV-1 budding is directed primarily by two motifs in Gag p6 designated as late domain-1 and −2 that recruit ESCRT machinery by binding Tsg101 and Alix, respectively, and by poorly characterized determinants in the capsid (CA) domain. Here, we report that a conserved Gag p6 residue, S40, impacts budding mediated by all of these determinants.

Results

Whereas budding normally results in formation of single spherical particles ~100 nm in diameter and containing a characteristic electron-dense conical core, the substitution of Phe for S40, a change that does not alter the amino acids encoded in the overlapping pol reading frame, resulted in defective CA-SP1 cleavage, formation of strings of tethered particles or filopodia-like membrane protrusions containing Gag, and diminished infectious particle formation. The S40F-mediated release defects were exacerbated when the viral-encoded protease (PR) was inactivated or when L domain-1 function was disrupted or when budding was almost completely obliterated by the disruption of both L domain-1 and −2. S40F mutation also resulted in stronger Gag-Alix interaction, as detected by yeast 2-hybrid assay. Reducing Alix binding by mutational disruption of contact residues restored single particle release, implicating the perturbed Gag-Alix interaction in the aberrant budding events. Interestingly, introduction of S40F partially rescued the negative effects on budding of CA NTD mutations EE75,76AA and P99A, which both prevent membrane curvature and therefore block budding at an early stage.

Conclusions

The results indicate that the S40 residue is a novel determinant of HIV-1 egress that is most likely involved in regulation of a critical assembly event required for budding in the Tsg101-, Alix-, Nedd4- and CA N-terminal domain affected pathways.

Syntenin-a promotes spinal cord regeneration following injury in adult zebrafish

In contrast to mammals, adult zebrafish recover locomotor function after spinal cord injury, in part due to the capacity of the central nervous system to repair severed connections. To identify molecular cues that underlie regeneration, we conducted mRNA expression profiling and found that syntenin-a expression is upregulated in the adult zebrafish spinal cord caudal to the lesion site after injury. Syntenin is a scaffolding protein involved in mammalian cell adhesion and movement, axonal outgrowth, establishment of cell polarity, and protein trafficking. It could thus be expected to be involved in supporting regeneration in fish. Syntenin-a mRNA and protein are expressed in neurons, glia and newly generated neural cells, and upregulated caudal to the lesion site on days 6 and 11 following spinal cord injury. Treatment of spinal cord-injured fish with two different antisense morpholinos to knock down syntenin-a expression resulted in significant inhibition of locomotor recovery at 5 and 6 weeks after injury, when compared to control morpholino-treated fish. Knock-down of syntenin-a reduced regrowth of descending axons from brainstem neurons into the spinal cord caudal to the lesion site. These observations indicate that syntenin-a is involved in regeneration after traumatic insult to the central nervous system of adult zebrafish, potentially leading to novel insights into the cellular and molecular mechanisms that require activation in the regeneration-deficient mammalian central nervous system.

Elevated expression of syntenin in breast cancer is correlated with lymph node metastasis and poor patient survival

INTRODUCTION

Syntenin is a scaffolding-PDZ domain-containing protein. Although it is reported that syntenin is associated with melanoma growth and metastasis, the possible role of syntenin in breast cancer has not been well elucidated. The present study investigated the expression and function of syntenin in breast cancer.

METHODS

Real-time polymerase chain reaction (PCR) and Western blots were used to determine the mRNA and protein expression of syntenin. With a combination of overexpression and RNA interference, the effect of syntenin on migration, invasion, and ERK1/2 activation was examined in breast cancer cell lines. The effect of syntenin in vivo was assessed with an orthotropic xenograft tumor model in BALB/c nu/nu mice. In addition, the expression level of syntenin in clinical breast cancer tissues was evaluated with immunohistochemistry. The Kaplan-Meier survival curve was used to evaluate patient survival, and the Cox proportional hazards model was used for multivariate analysis.

RESULTS

Our study showed that syntenin expression was upregulated in high-metastasis breast cancer cell lines and breast cancer tissues. Overexpression of syntenin in breast cancer cells promoted cell migration and invasion in vitro. Moreover, overexpression of syntenin promoted breast tumor growth and lung metastasis in vivo. We further showed that activation of integrin β1 and ERK1/2 was required for syntenin-mediated migration and invasion of breast cancer cells. The correlation between syntenin expression and tumor size (P = 0.011), lymph node status (P = 0.001), and recurrence (P = 0.002) was statistically significant. More important, syntenin expression in primary tumors was significantly related to patients' overall survival (OS; P = 0.023) and disease-free survival (DFS; P = 0.001). Its status was an independent prognostic factor of OS (P = 0.049) and DFS (P = 0.002) in our cohort of patients.

CONCLUSIONS

These results suggest that syntenin plays a significant role in breast cancer progression, and it warrants further investigation as a candidate molecular marker of breast cancer metastasis and a potential therapeutic target.

Ubiquitin and its binding domains

Post-translational modification by ubiquitin (ubiquitination, ubiquitylation, ubiquitinylation) is used as a robust signaling mechanism in a variety of processes that are essential for cell homeostasis. Its signaling specificity is conferred by the inherent dynamics of ubiquitin, the multivalency of ubiquitin chains, and its subcellular context, often defined by ubiquitin receptors and the substrate. Greater than 150 ubiquitin receptors have been found and their ubiquitin-binding domains (UBDs) are structurally diverse and include alpha-helical motifs, zinc fingers (ZnF), pleckstrin-homology (PH) domains, ubiquitin conjugating (Ubc)-related structures and src homology 3 (SH3) domains. New UBD structural motifs continue to be identified expanding the ubiquitin-signaling map to proteins and structural families not previously associated with ubiquitin trafficking. In this manuscript, we highlight several ubiquitin receptors from the multiple UBD folds with a focus on the structural characteristics of their interaction with ubiquitin.

Binding to syntenin-1 protein defines a new mode of ubiquitin-based interactions regulated by phosphorylation

Syntenin-1 is a PDZ domain-containing adaptor that controls trafficking of transmembrane proteins including those associated with tetraspanin-enriched microdomains. We describe the interaction of syntenin-1 with ubiquitin through a novel binding site spanning the C terminus of ubiquitin, centered on Arg(72), Leu(73), and Arg(74). A conserved LYPSL sequence in the N terminus, as well as the C-terminal region of syntenin-1, are essential for binding to ubiquitin. We present evidence for the regulation of this interaction through syntenin-1 dimerization. We have also established that syntenin-1 is phosphorylated downstream of Ulk1, a serine/threonine kinase that plays a critical role in autophagy and regulates endocytic trafficking. Importantly, Ulk1-dependent phosphorylation of Ser(6) in the LYPSL prevents the interaction of syntenin-1 with ubiquitin. These results define an unprecedented ubiquitin-dependent pathway involving syntenin-1 that is regulated by Ulk1.