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

Origin of eukaryotic-like Vps23 shapes an ancient functional interplay between ESCRT and ubiquitin system in Asgard archaea

Functional interplay between the endosomal sorting complexes required for transport (ESCRT) and the ubiquitin system underlies the ubiquitin-dependent cargo-sorting pathway of the eukaryotic endomembrane system, yet its evolutionary origin remains unclear. Here, we show that a UEV-Vps23 protein family, which contains UEV and Vps23 domains, mediates an ancient ESCRT and ubiquitin system interplay in Asgard archaea. The UEV binds ubiquitin with high affinity, making the UEV-Vps23 a sensor for sorting ubiquitinated cargo. A steadiness box in the Vps23 domain undergoes ubiquitination through an Asgard E1, E2, and RING E3 cascade. The UEV-Vps23 switches between autoinhibited and active forms, regulating the ESCRT and ubiquitin system interplay. Furthermore, the shared sequence and structural homology among the UEV-Vps23, eukaryotic Vps23, and archaeal CdvA suggest a common evolutionary origin. Together, this work expands our understanding of the ancient ESCRT and ubiquitin system interplay that likely arose antedating divergent evolution between Asgard archaea and eukaryotes.

Ubiquitination in the regulation of autophagy

Autophagy, an efficient and effective approach to clear rapidly damaged organelles, macromolecules, and other harmful cellular components, enables the recycling of nutrient materials and supply of nutrients to maintain cellular homeostasis. Ubiquitination plays an important regulatory role in autophagy. This paper summarizes the most recent progress in ubiquitin modification in various stages of autophagy, including initiation, elongation, and termination. Moreover, this paper shows that ubiquitination is an important way through which selective autophagy achieves substrate specificity. Furthermore, we note the distinction between monoubiquitination and polyubiquitination in the regulation of autophagy. Compared with monoubiquitination, polyubiquitination is a more common strategy to regulate the activity of the autophagy molecular machinery. In addition, the role of ubiquitination in the closure and fusion of autophagosomes warrants further study. This article not only clarifies the regulatory mechanism of autophagy but also contributes to a deeper understanding of the importance of ubiquitination modification.

Hepatitis B virus hijacks TSG101 to facilitate egress via multiple vesicle bodies

Hepatitis B virus (HBV) chronically infects 296 million individuals and there is no cure. As an important step of viral life cycle, the mechanisms of HBV egress remain poorly elucidated. With proteomic approach to identify capsid protein (HBc) associated host factors and siRNA screen, we uncovered tumor susceptibility gene 101 (TSG101). Knockdown of TSG101 in HBV-producing cells, HBV-infected cells and HBV transgenic mice suppressed HBV release. Co-immunoprecipitation and site mutagenesis revealed that VFND motif in TSG101 and Lys-96 ubiquitination in HBc were essential for TSG101-HBc interaction. In vitro ubiquitination experiment demonstrated that UbcH6 and NEDD4 were potential E2 ubiquitin-conjugating enzyme and E3 ligase that catalyzed HBc ubiquitination, respectively. PPAY motif in HBc and Cys-867 in NEDD4 were required for HBc ubiquitination, TSG101-HBc interaction and HBV egress. Transmission electron microscopy confirmed that TSG101 or NEDD4 knockdown reduces HBV particles count in multivesicular bodies (MVBs). Our work indicates that TSG101 recognition for NEDD4 ubiquitylated HBc is critical for MVBs mediated HBV egress.

Multifunctional role of the ubiquitin proteasome pathway in phagocytosis

Phagocytosis is a specialized form of endocytosis where large cells and particles (>0.5μm) are engulfed by the phagocytic cells, and ultimately digested in the phagolysosomes. This process not only eliminates unwanted particles and pathogens from the extracellular sources, but also eliminates apoptotic cells within the body, and is critical for maintenance of tissue homeostasis. It is believed that both endocytosis and phagocytosis share common pathways after particle internalization, but specialized features and differences between these two routes of internalization are also likely. The recruitment and removal of each protein/particle during the maturation of endocytic/phagocytic vesicles has to be tightly regulated to ensure their timely action. Ubiquitin proteasome pathway (UPP), degrades unwanted proteins by post-translational modification of proteins with chains of conserved protein Ubiquitin (Ub), with subsequent recognition of Ub chains by the 26S proteasomes and substrate degradation by this protease. This pathway utilizes different Ub linkages to modify proteins to regulate protein-protein interaction, localization, and activity. Due to its vast number of targets, it is involved in many cellular pathways, including phagocytosis. This chapters describes the basic steps and signaling in phagocytosis and different roles that UPP plays at multiple steps in regulating phagocytosis directly, or through its interaction with other phagosomal proteins. How aberrations in UPP function affect phagocytosis and their association with human diseases, and how pathogens exploit this pathway for their own benefit is also discussed.

The RING finger protein family in health and disease

Ubiquitination is a highly conserved and fundamental posttranslational modification (PTM) in all eukaryotes regulating thousands of proteins. The RING (really interesting new gene) finger (RNF) protein, containing the RING domain, exerts E3 ubiquitin ligase that mediates the covalent attachment of ubiquitin (Ub) to target proteins. Multiple reviews have summarized the critical roles of the tripartite-motif (TRIM) protein family, a subgroup of RNF proteins, in various diseases, including cancer, inflammatory, infectious, and neuropsychiatric disorders. Except for TRIMs, since numerous studies over the past decades have delineated that other RNF proteins also exert widespread involvement in several diseases, their importance should not be underestimated. This review summarizes the potential contribution of dysregulated RNF proteins, except for TRIMs, to the pathogenesis of some diseases, including cancer, autoimmune diseases, and neurodegenerative disorder. Since viral infection is broadly involved in the induction and development of those diseases, this manuscript also highlights the regulatory roles of RNF proteins, excluding TRIMs, in the antiviral immune responses. In addition, we further discuss the potential intervention strategies targeting other RNF proteins for the prevention and therapeutics of those human diseases.

Ubiquitination of the ubiquitin-binding machinery: how early ESCRT components are controlled

To be able to quickly and accurately respond to the environment, cells need to tightly control the amount and localization of plasma membrane proteins. The post-translation modification by the protein modifier ubiquitin is the key signal for guiding membrane-associated cargo to the lysosome/vacuole for their degradation. The machinery responsible for such sorting contains several subunits that function as ubiquitin receptors, many of which are themselves subjected to ubiquitination. This review will focus on what is currently known about the modulation of the machinery itself by ubiquitination and how this might affect its function with a special emphasis on current findings from the plant field.

Receptor control by membrane-tethered ubiquitin ligases in development and tissue homeostasis

Paracrine cell-cell communication is central to all developmental processes, ranging from cell diversification to patterning and morphogenesis. Precise calibration of signaling strength is essential for the fidelity of tissue formation during embryogenesis and tissue maintenance in adults. Membrane-tethered ubiquitin ligases can control the sensitivity of target cells to secreted ligands by regulating the abundance of signaling receptors at the cell surface. We discuss two examples of this emerging concept in signaling: (1) the transmembrane ubiquitin ligases ZNRF3 and RNF43 that regulate WNT and bone morphogenetic protein receptor abundance in response to R-spondin ligands and (2) the membrane-recruited ubiquitin ligase MGRN1 that controls Hedgehog and melanocortin receptor abundance. We focus on the mechanistic logic of these systems, illustrated by structural and protein interaction models enabled by AlphaFold. We suggest that membrane-tethered ubiquitin ligases play a widespread role in remodeling the cell surface proteome to control responses to extracellular ligands in diverse biological processes.

Mahogunin Ring Finger 1 regulates pigmentation by controlling the pH of melanosomes in melanocytes and melanoma cells

Mahogunin Ring Finger 1 (MGRN1) is an E3-ubiquitin ligase absent in dark-furred mahoganoid mice. We investigated the mechanisms of hyperpigmentation in Mgrn1-null melan-md1 melanocytes, Mgrn1-KO cells obtained by CRISPR-Cas9-mediated knockdown of Mgrn1 in melan-a6 melanocytes, and melan-a6 cells depleted of MGRN1 by siRNA treatment. Mgrn1-deficient melanocytes showed higher melanin content associated with increased melanosome abundance and higher fraction of melanosomes in highly melanized maturation stages III-IV. Expression, post-translational processing and enzymatic activity of the rate-limiting melanogenic enzyme tyrosinase measured in cell-free extracts were comparable in control and MGRN1-depleted cells. However, tyrosinase activity measured in situ in live cells and expression of genes associated with regulation of pH increased upon MGRN1 repression. Using pH-sensitive fluorescent probes, we found that downregulation of MGRN1 expression in melanocytes and melanoma cells increased the pH of acidic organelles, including melanosomes, strongly suggesting a previously unknown role of MGRN1 in the regulation of melanosomal pH. Among the pH regulatory genes upregulated by Mgrn1 knockdown, we identified those encoding several subunits of the vacuolar adenosine triphosphatase V-ATPase (mostly Atp6v0d2) and a calcium channel of the transient receptor potential channel family, Mucolipin 3 (Mcoln3). Manipulation of expression of the Mcoln3 gene showed that overexpression of Mcoln3 played a significant role in neutralization of the pH of acidic organelles and activation of tyrosinase in MGRN1-depleted cells. Therefore, lack of MGRN1 led to cell-autonomous stimulation of pigment production in melanocytes mostly by increasing tyrosinase specific activity through neutralization of the melanosomal pH in a MCOLN3-dependent manner.

Intracellular and exosomal microRNAome profiling of human vascular smooth muscle cells during replicative senescence

Vascular aging is highly associated with cardiovascular morbidity and mortality. Although the senescence of vascular smooth muscle cells (VSMCs) has been well established as a major contributor to vascular aging, intracellular and exosomal microRNA (miRNA) signaling pathways in senescent VSMCs have not been fully elucidated. This study aimed to identify the differential expression of intracellular and exosomal miRNA in human VSMCs (hVSMCs) during replicative senescence. To achieve this aim, intracellular and exosomal miRNAs were isolated from hVSMCs and subsequently subjected to whole genome small RNA next-generation sequencing, bioinformatics analyses, and qPCR validation. Three significant findings were obtained. First, senescent hVSMC-derived exosomes tended to cluster together during replicative senescence and the molecular weight of the exosomal protein tumor susceptibility gene 101 (TSG-101) increased relative to the intracellular TSG-101, suggesting potential posttranslational modifications of exosomal TSG-101. Second, there was a significant decrease in both intracellular and exosomal hsa-miR-155-5p expression [n = 3, false discovery rate (FDR) < 0.05], potentially being a cell type-specific biomarker of hVSMCs during replicative senescence. Importantly, hsa-miR-155-5p was found to associate with cell-cycle arrest and elevated oxidative stress. Lastly, miRNAs from the intracellular pool, that is, hsa-miR-664a-3p, hsa-miR-664a-5p, hsa-miR-664b-3p, hsa-miR-4485-3p, hsa-miR-10527-5p, and hsa-miR-12136, and that from the exosomal pool, that is, hsa-miR-7704, were upregulated in hVSMCs during replicative senescence (n = 3, FDR < 0.05). Interestingly, these novel upregulated miRNAs were not functionally well annotated in hVSMCs to date. In conclusion, hVSMC-specific miRNA expression profiles during replicative senescence potentially provide valuable insights into the signaling pathways leading to vascular aging.NEW & NOTEWORTHY This is the first study on intracellular and exosomal miRNA profiling on human vascular smooth muscle cells during replicative senescence. Specific dysregulated sets of miRNAs were identified from human vascular smooth muscle cells. Hsa-miR-155-5p was significantly downregulated in both intracellular and exosomal hVSMCs, suggesting its crucial role in cellular senescence. Hsa-miR-155-5p might be the mediator in linking cellular senescence to vascular aging and atherosclerosis.

Novel Tsg101 Binding Partners Regulate Viral L Domain Trafficking

Two decades ago, Tsg101, a component of the Endosomal Sorting Complexes Required for Transport (ESCRT) complex 1, was identified as a cellular factor recruited by the human immunodeficiency virus type 1 (HIV-1) to facilitate budding of viral particles assembled at the cell periphery. A highly conserved Pro-(Thr/Ser)-Ala-Pro [P(T/S)AP] motif in the HIV-1 structural polyprotein, Gag, engages a P(T/S)AP-binding pocket in the Tsg101 N-terminal domain. Since the same domain in Tsg101 that houses the pocket was found to bind mono-ubiquitin (Ub) non-covalently, Ub binding was speculated to enhance P(T/S)AP interaction. Within the past five years, we found that the Ub-binding site also accommodates di-Ub, with Lys63-linked di-Ub exhibiting the highest affinity. We also identified small molecules capable of disrupting Ub binding and inhibiting budding. The structural similarity of these molecules, prazoles, to nucleosides prompted testing for nucleic acid binding and led to identification of tRNA as a Tsg101 binding partner. Here, we discuss these recently identified interactions and their contribution to the viral assembly process. These new partners may provide additional insight into the control and function of Tsg101 as well as identify opportunities for anti-viral drug design.

Tumor Susceptibility Gene 101 Regulates the Glucocorticoid Receptor through Disorder-Mediated Allostery

Tumor susceptibility gene 101 (TSG101) is involved in endosomal maturation and has been implicated in the transcriptional regulation of several steroid hormone receptors, although a detailed characterization of such regulation has yet to be conducted. Here we directly measure binding of TSG101 to one steroid hormone receptor, the glucocorticoid receptor (GR). Using biophysical and cellular assays, we show that the coiled-coil domain of TSG101 (1) binds and folds the disordered N-terminal domain of the GR, (2) upon binding improves the DNA binding of the GR in vitro, and (3) enhances the transcriptional activity of the GR in vivo. Our findings suggest that TSG101 is a bona fide transcriptional co-regulator of the GR and reveal how the underlying thermodynamics affect the function of the GR.

Biogenesis, Membrane Trafficking, Functions, and Next Generation Nanotherapeutics Medicine of Extracellular Vesicles

Extracellular vesicles (EVs) are a heterogeneous group of membrane-limited vesicles and multi-signal messengers loaded with biomolecules. Exosomes and ectosomes are two different types of EVs generated by all cell types. Their formation depends on local microdomains assembled in endocytic membranes for exosomes and in the plasma membrane for ectosomes. Further, EV release is a fundamental process required for intercellular communication in both normal physiology and pathological conditions to transmit/exchange bioactive molecules to recipient cells and the extracellular environment. The unique structure and composition of EVs enable them to serve as natural nanocarriers, and their physicochemical properties and biological functions can be used to develop next-generation nano and precision medicine. Knowledge of the cellular processes that govern EVs biology and membrane trafficking is essential for their clinical applications. However, in this rapidly expanding field, much remains unknown regarding EV origin, biogenesis, cargo sorting, and secretion, as well as EV-based theranostic platform generation. Hence, we present a comprehensive overview of the recent advances in biogenesis, membrane trafficking, and functions of EVs, highlighting the impact of nanoparticles and oxidative stress on EVs biogenesis and release and finally emphasizing the role of EVs as nanotherapeutic agents.

DNA methylome profiling identifies novel methylated genes in epithelial ovarian cancer patients with platinum resistance

AIM

Platinum-based chemotherapy is widely used for epithelial ovarian cancer (EOC). As high as 20-25% of EOC patients will not respond to the initial chemotherapy. Accumulated evidences have implied that DNA methylation may serve as a potential bio-marker for chemotherapy-resistant phenotypic screening; however, the pattern underlying primary platinum resistance remains unclear.

METHODS

Reduced representation bisulfite sequencing (RRBS) analysis was performed to identify differences in methylation status between primary platinum-resistant patients Progression free survival (PFS) (PFS < 6 months, n = 8) and extreme sensitive patients (PFS ≥ 24 months, n = 8). The Qubit 3.0 Fluorometer was used for the quantification of RRBS library. The RRBS library was sequenced on Illumina HiSeq2500 sequencer as 50 bp paired-end reads.

RESULTS

After screening, 94 valid hyper-/hypo-methylated regions were identified to be located within 94 gene promoter and exon regions (adjusted q ≤ 0.5), which were primarily associated with cell-cell adhesion, B cell activation and lymphocyte activation according to GO analysis. The 19 differentially methylated regions (DMR) located in the promoter region including TRC-GCA11-1, LOC105370912, ANO7P1, DHX4,MSH2, CDCP2, CCNL1, ARHGAP42P2, PRDM13, LOC101928344, USP29, ZIC5,IL1RAPL1, EVX2, ABR, MGRN1, UBALD1, LINC00261, and ISL2 were identified according to the order of P-values from low to high, of which MSH2, LINC00261, MGRN1, ZIC5, EVX2, CCNL1, and DHX40 were presented to play a variety of roles in cancers process based on the previous studies.

CONCLUSION

DNA methylome profiling based on RRBS assay is an effective method for screening aberrantly methylated genes in primary platinum-resistant patients, which may serve as a potential epigenetic bio-marker for the prediction of primary platinum resistance.

SINAT E3 Ubiquitin Ligases Mediate FREE1 and VPS23A Degradation to Modulate Abscisic Acid Signaling

In plants, the ubiquitin-proteasome system, endosomal sorting, and autophagy are essential for protein degradation; however, their interplay remains poorly understood. Here, we show that four Arabidopsis (Arabidopsis thaliana) E3 ubiquitin ligases, SEVEN IN ABSENTIA OF ARABIDOPSIS THALIANA1 (SINAT1), SINAT2, SINAT3, and SINAT4, regulate the stabilities of FYVE DOMAIN PROTEIN REQUIRED FOR ENDOSOMAL SORTING1 (FREE1) and VACUOLAR PROTEIN SORTING23A (VPS23A), key components of the endosomal sorting complex required for transport-I, to modulate abscisic acid (ABA) signaling. GFP-SINAT1, GFP-SINAT2, and GFP-SINAT4 primarily localized to the endosomal and autophagic vesicles. SINATs controlled FREE1 and VPS23A ubiquitination and proteasomal degradation. SINAT overexpressors showed increased ABA sensitivity, ABA-responsive gene expression, and PYRABACTIN RESISTANCE1-LIKE4 protein levels. Furthermore, the SINAT-FREE1/VPS23A proteins were codegraded by the vacuolar pathway. In particular, during recovery post-ABA exposure, SINATs formed homo- and hetero-oligomers in vivo, which were disrupted by the autophagy machinery. Taken together, our findings reveal a novel mechanism by which the proteasomal and vacuolar turnover systems regulate ABA signaling in plants.

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.

Cellular Prion Protein (PrPc): Putative Interacting Partners and Consequences of the Interaction

Cellular prion protein (PrPc) is a small glycosylphosphatidylinositol (GPI) anchored protein most abundantly found in the outer leaflet of the plasma membrane (PM) in the central nervous system (CNS). PrPc misfolding causes neurodegenerative prion diseases in the CNS. PrPc interacts with a wide range of protein partners because of the intrinsically disordered nature of the protein’s N-terminus. Numerous studies have attempted to decipher the physiological role of the prion protein by searching for proteins which interact with PrPc. Biochemical characteristics and biological functions both appear to be affected by interacting protein partners. The key challenge in identifying a potential interacting partner is to demonstrate that binding to a specific ligand is necessary for cellular physiological function or malfunction. In this review, we have summarized the intracellular and extracellular interacting partners of PrPc and potential consequences of their binding. We also briefly describe prion disease-related mutations at the end of this review.

Post-translational Modification Regulates Formation and Cargo-Loading of Extracellular Vesicles

Accumulating evidence suggests that post-translational modifications (PTMs) regulate the selective encapsulation of non-coding RNA molecules into extracellular vesicles (EVs) and contribute to the downstream functions of EVs or EV-cargo non-coding RNAs. EVs are a newly studied mechanism of intercellular communication that involves the transfer of molecules, including but not limited to proteins, lipids, and non-coding RNAs, to induce functional changes in the recipient cells. In this present mini-review, we focus on the PTM-regulated protein and non-coding RNA selection into eukaryotic EVs.

Loss of tumor susceptibility gene 101 (TSG101) perturbs endoplasmic reticulum structure and function

Tumor susceptibility gene 101 (TSG101), an ESCRT-I protein, is implicated in multiple cellular processes and its functional depletion can lead to blocked lysosomal degradation, cell cycle arrest, demyelination and neurodegeneration. Here, we show that loss of TSG101 results in endoplasmic reticulum (ER) stress and this causes ER membrane remodelling (EMR). This correlates with an expansion of ER, increased vacuolation, altered relative distribution of the rough and smooth ER and disruption of three-way junctions. Blocked lysosomal degradation due to TSG101 depletion leads to ER stress and Ca²⁺ leakage from ER stores, causing destabilization of actin cytoskeleton. Inhibiting Ca²⁺ release from the ER by blocking ryanodine receptors (RYRs) with Dantrolene partially rescues the ER stress phenotypes. Hence, in this study we have identified the involvement of TSG101 in modulating ER stress mediated remodelling by engaging the actin cytoskeleton. This is significant because functional depletion of TSG101 effectuates ER-stress, perturbs the structure, mobility and function of the ER, all aspects closely associated with neurodegenerative diseases. SUMMARY STATEMENT: We show that tumor susceptibility gene (TSG) 101 regulates endoplasmic reticulum (ER) stress and its membrane remodelling. Loss of TSG101 perturbs structure, mobility and function of the ER as a consequence of actin destabilization.

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.

LRSAM1 E3 ubiquitin ligase: molecular neurobiological perspectives linked with brain diseases

Cellular protein quality control (PQC) plays a significant role in the maintenance of cellular homeostasis. Failure of PQC mechanism may lead to various neurodegenerative diseases due to accumulation of aberrant proteins. To avoid such fatal neuronal conditions PQC employs autophagy and ubiquitin proteasome system (UPS) to degrade misfolded proteins. Few quality control (QC) E3 ubiquitin ligases interplay an important role to specifically recognize misfolded proteins for their intracellular degradation. Leucine-rich repeat and sterile alpha motif-containing 1 (LRSAM1) is a really interesting new gene (RING) class protein that possesses E3 ubiquitin ligase activity with promising applications in PQC. LRSAM1 is also known as RING finger leucine repeat rich (RIFLE) or TSG 101-associated ligase (TAL). LRSAM1 has various cellular functions as it modulates the protein aggregation, endosomal sorting machinery and virus egress from the cells. Thus, this makes LRSAM1 interesting to study not only in protein conformational disorders such as neurodegeneration but also in immunological and other cancerous disorders. Furthermore, LRSAM1 interacts with both cellular protein degradation machineries and hence it can participate in maintenance of overall cellular proteostasis. Still, more research work on the quality control molecular functions of LRSAM1 is needed to comprehend its roles in various protein aggregatory diseases. Earlier findings suggest that in a mouse model of Charcot-Marie-Tooth (CMT) disease, lack of LRSAM1 functions sensitizes peripheral axons to degeneration. It has been observed that in CMT the patients retain dominant and recessive mutations of LRSAM1 gene, which encodes most likely a defective protein. However, still the comprehensive molecular pathomechanism of LRSAM1 in neuronal functions and neurodegenerative diseases is not known. The current article systematically represents the molecular functions, nature and detailed characterization of LRSAM1 E3 ubiquitin ligase. Here, we review emerging molecular mechanisms of LRSAM1 linked with neurobiological functions, with a clear focus on the mechanism of neurodegeneration and also on other diseases. Better understanding of LRSAM1 neurobiological and intracellular functions may contribute to develop promising novel therapeutic approaches, which can also propose new lines of molecular beneficial targets for various neurodegenerative diseases.

Chronic and age-dependent effects of the spongiform neurodegeneration-associated MGRN1 E3 ubiquitin ligase on mitochondrial homeostasis

Spongiform encephalopathy is an intriguing yet poorly understood neuropathology characterized by vacuoles, demyelination, and gliosis. It is observed in patients with prion disease, primary mitochondrial disease, HIV-1 infection of the brain, and some inherited disorders, but the underlying mechanism of disease remains unclear. The brains of mice lacking the MGRN1 E3 ubiquitin ligase develop vacuoles by 9 months of age. MGRN1-dependent ubiquitination has been reported to regulate mitofusin 1 and GP78, suggesting MGRN1 may have a direct effect on mitochondrial homeostasis. Here, we demonstrate that some MGRN1 localizes to mitochondria, most likely due to N-myristoylation, and mitochondria in cells from Mgrn1 null mutant mice display fragmentation and depolarization without recruitment of the parkin E3 ubiquitin ligase. The late onset of pathology in the brains of Mgrn1 null mutant mice suggests that a further, age-dependent effect on mitochondrial homeostasis may be required to trigger vacuolation. Parkin protein and mRNA levels showed a significant decline in the brains of Mgrn1 null mutant mice by 12 months of age. To test whether loss of parkin triggers vacuolation through a synergistic effect, we generated Mgrn1; parkin double mutant mice. By 1 month of age, their brains demonstrated more severe mitochondrial dysfunction than Mgrn1 null mutants, but there was no effect on the age-of-onset of spongiform neurodegeneration. Expression of the ATF4 transcription factor, a key regulator of the mitochondrial stress response, also declined in the brains of aged Mgrn1 null mutant mice. Together, the data presented here indicate that loss of MGRN1 has early, direct effects on mitochondrial homeostasis and late, indirect effects on the ability of cells to respond to mitochondrial stress.

Ticket to a bubble ride: Cargo sorting into exosomes and extracellular vesicles

Extracellular vesicles (EVs) are released by cells into the extracellular milieu to facilitate intercellular communication in both physiological and pathological condition. EVs contain selective repertoires of proteins, RNAs, lipids and metabolites that moderate signalling pathways in the recipient cells. The enrichment of a particular set of proteins or RNAs within the EVs highlights the existence of specific sorting mechanisms that orchestrate the selective packaging of the cargo. The molecular machinery of cargo sorting has remained obscure over the years and functional studies are required to understand this complex mechanism. In this article, we offer a brief overview of the molecular mechanisms that are known to regulate sorting of various molecules into EVs. We also discuss how different pathways of biogenesis alter the exosomal cargo as well and the implications of the cellular state on the content of the EVs. Understanding the sorting of exosomal cargo could further be exploited in clinical settings for targeted drug delivery and to block disease progression.

Calmodulin regulates MGRN1-GP78 interaction mediated ubiquitin proteasomal degradation system

The mechanism by which the endoplasmic reticulum (ER) ubiquitin ligases sense stress to potentiate their activity is poorly understood. GP78, an ER E3 ligase, is best known for its role in ER-associated protein degradation, although its activity is also linked to mitophagy, ER-mitochondria junctions, and MAPK signaling, thus highlighting the importance of understanding its regulation. In healthy cells, Mahogunin really interesting new gene (RING) finger 1 (MGRN1) interacts with GP78 and proteasomally degrades it to alleviate mitophagy. Here, we identify calmodulin (CaM) as the adapter protein that senses fluctuating cytosolic Ca²⁺ levels and modulates the Ca²⁺-dependent MGRN1-GP78 interactions. When stress elevates cytosolic Ca²⁺ levels in cultured and primary neuronal cells, CaM binds to both E3 ligases and inhibits their interaction. Molecular docking, simulation, and biophysical studies show that CaM interacts with both proteins with different affinities and binding modes. The physiological impact of this interaction switch manifests in the regulation of ER-associated protein degradation, ER-mitochondria junctions, and relative distribution of smooth ER and rough ER.-Mukherjee, R., Bhattacharya, A., Sau, A., Basu, S., Chakrabarti, S., Chakrabarti, O. Calmodulin regulates MGRN1-GP78 interaction mediated ubiquitin proteasomal degradation system.

Post-translational add-ons mark the path in exosomal protein sorting

Extracellular vesicles (EVs) are released by cells to the extracellular environment to mediate inter-cellular communication. Proteins, lipids, nucleic acids and metabolites shuttled in these vesicles modulate specific functions in recipient cells. The enrichment of selected sets of proteins in EVs compared with global cellular levels suggests the existence of specific sorting mechanisms to specify EV loading. Diverse post-translational modifications (PTMs) of proteins participate in the loading of specific elements into EVs. In this review, we offer a perspective on PTMs found in EVs and discuss the specific role of some PTMs, specifically Ubiquitin and Ubiquitin-like modifiers, in exosomal sorting of protein components. The understanding of these mechanisms will provide new strategies for biomedical applications. Examples include the presence of defined PTM marks on EVs as novel biomarkers for the diagnosis and prognosis of certain diseases, or the specific import of immunogenic components into EVs for vaccine generation.

Structural Stability of the Coiled-Coil Domain of Tumor Susceptibility Gene (TSG)-101

The tumor susceptibility gene-101 coiled coil domain (TSG101cc) is an integral component of the endosomal maturation machinery and cytokinesis, and also interacts with several transcription factors. The TSG101cc has been crystallized as a homotetramer but is known to interact with two of its binding partners as a heterotrimer. To investigate this apparent discrepancy, we examined the solution thermodynamics of the TSG101cc. Here, we use circular dichroism, differential scanning calorimetry, analytical ultracentrifugation, fluorescence, and structural thermodynamic analysis to investigate the structural stability and the unfolding of the TSG101cc. We demonstrate that TSG101cc exists in solution primarily as a tetramer, which unfolds in a two-state manner. Surprisingly, no homodimeric or homotrimeric species were detected. Structural thermodynamic analysis of the homotetrameric structure and comparison with known oligomeric coiled-coils suggests that the TSG101cc homotetramer is comparatively unstable on a per residue basis. Furthermore, the homotrimeric coiled-coil is predicted to be much less stable than the functional heterotrimeric coiled-coil in the endosomal sorting complex required for transport 1 (ESCRT1). These results support a model whereby the tetramer–monomer equilibrium of TSG101 serves as the cellular reservoir of TSG101, which is effectively outcompeted when its binding partners are present and the heteroternary complex can form.

Inhibitors of Deubiquitinating Enzymes Block HIV-1 Replication and Augment the Presentation of Gag-Derived MHC-I Epitopes

In recent years it has been well established that two major constituent parts of the ubiquitin proteasome system (UPS)—the proteasome holoenzymes and a number of ubiquitin ligases—play a crucial role, not only in virus replication but also in the regulation of the immunogenicity of human immunodeficiency virus type 1 (HIV-1). However, the role in HIV-1 replication of the third major component, the deubiquitinating enzymes (DUBs), has remained largely unknown. In this study, we show that the DUB-inhibitors (DIs) P22077 and PR-619, specific for the DUBs USP7 and USP47, impair Gag processing and thereby reduce the infectivity of released virions without affecting viral protease activity. Furthermore, the replication capacity of X4- and R5-tropic HIV-1_NL4-3 in human lymphatic tissue is decreased upon treatment with these inhibitors without affecting cell viability. Most strikingly, combinatory treatment with DIs and proteasome inhibitors synergistically blocks virus replication at concentrations where mono-treatment was ineffective, indicating that DIs can boost the therapeutic effect of proteasome inhibitors. In addition, P22077 and PR-619 increase the polyubiquitination of Gag and thus its entry into the UPS and the major histocompatibility complex (MHC)-I pathway. In summary, our data point towards a model in which specific inhibitors of DUBs not only interfere with virus spread but also increase the immune recognition of HIV-1 expressing cells.

E3 ligase mahogunin (MGRN1) influences amyloid precursor protein maturation and secretion

Altered processing of the Amyloid Precursor Protein (APP) is a well-recognized central pathogenic mechanism in Alzheimer's Disease (AD), and regulation of APP processing is a major focus of research in the AD field. However, how age-associated cellular and molecular changes contribute to changes in the amyloidogenic processing of APP have not been extensively clarified so far. We here provide evidence that the processing of APP is influenced by the e3 ubiquitin ligase Mahogunin (MGRN1), a neuroprotective molecule whose levels decrease with aging. Specifically, the expression of MGRN1 inhibits the maturation of APP by sequestering it in the secretory pathway. This sequestration significantly delayed the proteolytic processing of APP, resulting in a reduced β-amyloid (Aβ) peptide release into the extracellular environment. Accordingly, a reduction of MGRN1 levels in hippocampal neurons, as it occurs during physiological aging, leads to an increased Aβ40 and Aβ42 release. We therefore propose that age contributes to the amyloidogenic processing of APP by altering its intracellular trafficking along the secretory pathway due in part to the down-regulation of MGRN1.

Tumor susceptibility gene 101 regulates predisposition to apoptosis via ESCRT machinery accessory proteins

ESCRT proteins are implicated in myriad cellular processes, including endosome formation, fusion of autophagosomes/amphisomes with lysosomes, and apoptosis. The role played by these proteins in either facilitating or protecting against apoptosis is unclear. In this study, while trying to understand how deficiency of Mahogunin RING finger 1 (MGRN1) affects cell viability, we uncovered a novel role for its interactor, the ESCRT-I protein TSG101: it directly participates in mitigating ER stress-mediated apoptosis. The association of TSG101 with ALIX prevents predisposition to apoptosis, whereas ALIX-ALG-2 interaction favors a death phenotype. Altered Ca2+ homeostasis in cells and a simultaneous increase in the protein levels of ALIX and ALG-2 are required to elicit apoptosis by activating ER stress-associated caspase 4/12. We further demonstrate that in the presence of membrane-associated, disease-causing prion protein CtmPrP, increased ALIX and ALG-2 levels are detected along with ER stress markers and associated caspases in transgenic brain lysates and cells. These effects were rescued by overexpression of TSG101. This is significant because MGRN1 deficiency is closely associated with neurodegeneration and prenatal and neonatal mortality, which could be due to excess cell death in selected brain regions or myocardial apoptosis during embryonic development.

Role of the E3 ubiquitin ligase RNF157 as a novel downstream effector linking PI3K and MAPK signaling pathways to the cell cycle

The interconnected PI3K and MAPK signaling pathways are commonly perturbed in cancer. Dual inhibition of these pathways by the small-molecule PI3K inhibitor pictilisib (GDC-0941) and the MEK inhibitor cobimetinib (GDC-0973) suppresses cell proliferation and induces cell death better than either single agent in several preclinical models. Using mass spectrometry-based phosphoproteomics, we have identified the RING finger E3 ubiquitin ligase RNF157 as a target at the intersection of PI3K and MAPK signaling. We demonstrate that RNF157 phosphorylation downstream of the PI3K and MAPK pathways influences the ubiquitination and stability of RNF157 during the cell cycle in an anaphase-promoting complex/cyclosome–CDH1-dependent manner. Deletion of these phosphorylation-targeted residues on RNF157 disrupts binding to CDH1 and protects RNF157 from ubiquitination and degradation. Expression of the cyclin-dependent kinase 2 (CDK2), itself a downstream target of PI3K/MAPK signaling, leads to increased phosphorylation of RNF157 on the same residues modulated by PI3K and MAPK signaling. Inhibition of PI3K and MEK in combination or of CDK2 by their respective small-molecule inhibitors reduces RNF157 phosphorylation at these residues and attenuates RNF157 interaction with CDH1 and its subsequent degradation. Knockdown of endogenous RNF157 in melanoma cells leads to late S phase and G₂/M arrest and induces apoptosis, the latter further potentiated by concurrent PI3K/MEK inhibition, consistent with a role for RNF157 in the cell cycle. We propose that RNF157 serves as a novel node integrating oncogenic signaling pathways with the cell cycle machinery and promoting optimal cell cycle progression in transformed cells.

Calcium dependent regulation of protein ubiquitination - Interplay between E3 ligases and calcium binding proteins

The ubiquitination status of proteins and intracellular calcium levels are two factors which keep changing inside any living cell. These two events appear to be independent of each other but recent experimental evidences show that ubiquitination of cellular proteins are influenced by calcium, Calmodulin, Calmodulin-dependent kinase II and other proteins of calcium dependent pathways. E3 ligases like Nedd4, SCF complex, APC, GP78 and ITCH are important regulators of calcium mediated processes. A bioinformatics analysis to inspect sequences and interacting partners of 242 candidate E3 ligases show the presence of calcium and/or Calmodulin binding motifs/domains within their sequences. Building a protein-protein interaction (PPI) network of human E3 ligase proteins identifies Ca²⁺ related proteins as direct interacting partners of E3 ligases. Review of literature, analysis of E3 ligase sequences and their interactome suggests an interconnectivity between calcium signaling and the overall UPS system, especially emphasizing that a subset of E3 ligases have importance in physiological pathways modulated by calcium.

MC1R signaling. Intracellular partners and pathophysiological implications

The melanocortin-1 receptor (MC1R) preferentially expressed in melanocytes is best known as a key regulator of the synthesis of epidermal melanin pigments. Its paracrine stimulation by keratinocyte-derived melanocortins also activates DNA repair pathways and antioxidant defenses to build a complex, multifaceted photoprotective response. Many MC1R actions rely on cAMP-dependent activation of two transcription factors, MITF and PGC1α, but pleiotropic MC1R signaling also involves activation of mitogen-activated kinases and AKT. MC1R partners such as β-arrestins, PTEN and the E3 ubiquitin ligase MGRN1 differentially regulate these pathways. The MC1R gene is complex and polymorphic, with frequent variants associated with skin phenotypes and increased cancer risk. We review current knowledge of signaling from canonical MC1R, its splice isoforms and natural polymorphic variants. Recently discovered intracellular targets and partners are also discussed, to highlight the diversity of mechanisms that may contribute to normal and pathological variation of pigmentation and sensitivity to solar radiation-induced damage. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

ESCRTs and associated proteins in lysosomal fusion with endosomes and autophagosomes

Endolysosomal and autophagosomal degradation pathways are highly connected at various levels, sharing multiple molecular effectors that modulate them individually or simultaneously. These two lysosomal degradative pathways are primarily involved in the disposal of cargo internalized from the cell surface or long-lived proteins or aggregates and aged organelles present in the cytosol. Both of these pathways involve a number of carefully regulated vesicular fusion events that are dependent on ESCRT proteins. The ESCRT proteins especially ESCRT-I and III participate in the regulation of fusion events between autophagosome/amphisome and lysosome. Along with these, a number of functionally diverse ESCRT associated and regulatory proteins such as, endosomal PtdIns (3) P 5-kinase Fab1, ALIX, mahogunin ring finger 1, atrogin 1, syntaxin 17, ATG12-ATG3 complex, and protein kinase CK2α are involved in fusion events in either or both the lysosomal degradative pathways.

Oligodendroglial deletion of ESCRT-I component TSG101 causes spongiform encephalopathy

BACKGROUND INFORMATION

Vacuolation of the central nervous system (CNS) is observed in patients with transmissible spongiform encephalopathy, HIV-related encephalopathy and some inherited diseases, but the underlying cellular mechanisms remain poorly understood. Mice lacking the mahogunin ring finger-1 (MGRN1) E3 ubiquitin ligase develop progressive, widespread spongiform degeneration of the CNS. MGRN1 ubiquitinates and regulates tumour susceptibility gene 101 (TSG101), a central component of the endosomal trafficking machinery. As loss of MGRN1 is predicted to cause partial TSG101 loss-of-function, we hypothesised that CNS vacuolation in Mgrn1 null mice may be caused by the accumulation of multi-cisternal endosome-like 'class E' vacuolar protein sorting (vps) compartments similar to those observed in Tsg101-depleted cells in culture.

RESULTS

To test this hypothesis, Tsg101 was deleted from mature oligodendroglia in vivo. This resulted in severe spongiform encephalopathy, histopathologically similar to that observed in Mgrn1 null mutant mice but with a more rapid onset. Vacuoles in the brains of Tsg101-deleted and Mgrn1 mutant mice labelled with endosomal markers, consistent with an endosomal origin. Vacuoles in the brains of mice inoculated with Rocky Mountain Laboratory (RML) prions did not label with these markers, indicating a different origin, consistent with previously published studies that indicate RML prions have a primary effect on neurons and cause vacuolation in an MGRN1-independent manner. Oligodendroglial deletion of Rab7, which mediates late endosome-to-lysosome trafficking and autophagosome-lysosome fusion, did not cause spongiform change.

CONCLUSIONS

Our data suggest that the formation of multi-cisternal 'class E' vps endosomal structures in oligodendroglia leads to vacuolation.

SIGNIFICANCE

This work provides the first evidence that disrupting multi-vesicular body formation in oligodendroglia can cause white matter vacuolation and demyelination. HIV is known to hijack the endosomal sorting machinery, suggesting that HIV infection of the CNS may also act through this pathway to cause encephalopathy.

The ubiquitin system: a critical regulator of innate immunity and pathogen-host interactions

The ubiquitin system comprises enzymes that are responsible for ubiquitination and deubiquitination, as well as ubiquitin receptors that are capable of recognizing and deciphering the ubiquitin code, which act in coordination to regulate almost all host cellular processes, including host-pathogen interactions. In response to pathogen infection, the host innate immune system launches an array of distinct antimicrobial activities encompassing inflammatory signaling, phagosomal maturation, autophagy and apoptosis, all of which are fine-tuned by the ubiquitin system to eradicate the invading pathogens and to reduce concomitant host damage. By contrast, pathogens have evolved a cohort of exquisite strategies to evade host innate immunity by usurping the ubiquitin system for their own benefits. Here, we present recent advances regarding the ubiquitin system-mediated modulation of host-pathogen interplay, with a specific focus on host innate immune defenses and bacterial pathogen immune evasion.

Mahogunin-mediated regulation of Gαi localisation during mitosis and its effect on spindle positioning

Mahogunin RING Finger 1 (MGRN1) is a ubiquitin E3 ligase known to affect spindle tilt in mitotic cells by regulating α-tubulin ubiquitination and polymerization. In cell culture systems we have found that expressing truncated mutants of MGRN1 leads to various other mitotic anomalies, such as lateral and angular spindle displacements. This seems to be independent of the MGRN1 ligase activity. Our experiments suggest that MGRN1 regulates the balance between the lower molecular weight monomeric Gαi and larger trimeric G-protein complex, along with its abundance in the ternary complex that regulates spindle positioning. The cytosolic isoforms of MGRN1 lead to the enrichment of monomeric Gαi in the cytosol and its subsequent recruitment at the plasma membrane. Excess Gαi at the cell cortex results in an imbalance in the assembly of the ternary complex regulating spindle positioning during mitosis. These observations seem independent of the ligase activity of MGRN1, although we cannot exclude the involvement of an intermediate player that acts as a substrate for MGRN1, and in turn, regulates Gαi.

60 YEARS OF POMC: Regulation of feeding and energy homeostasis by α-MSH

The melanocortin peptides derived from pro-opiomelanocortin (POMC) were originally understood in terms of the biological actions of α-melanocyte-stimulating hormone (α-MSH) on pigmentation and adrenocorticotrophic hormone on adrenocortical glucocorticoid production. However, the discovery of POMC mRNA and melanocortin peptides in the CNS generated activities directed at understanding the direct biological actions of melanocortins in the brain. Ultimately, discovery of unique melanocortin receptors expressed in the CNS, the melanocortin-3 (MC3R) and melanocortin-4 (MC4R) receptors, led to the development of pharmacological tools and genetic models leading to the demonstration that the central melanocortin system plays a critical role in the regulation of energy homeostasis. Indeed, mutations in MC4R are now known to be the most common cause of early onset syndromic obesity, accounting for 2-5% of all cases. This review discusses the history of these discoveries, as well as the latest work attempting to understand the molecular and cellular basis of regulation of feeding and energy homeostasis by the predominant melanocortin peptide in the CNS, α-MSH.

Mahogunin ring finger 1 confers cytoprotection against mutant SOD1 aggresomes and is defective in an ALS mouse model

Proteotoxicity of misfolded, disease-causing proteins is deeply implicated in the pathomechanisms for neurodegenerative diseases including copper-zinc superoxide dismutase (SOD1)-linked amyotrophic lateral sclerosis (ALS). However, the precise cellular quality control (QC) mechanisms against aggregation of misfolded mutant SOD1 proteins remain elusive. Here, we found that the Mahogunin ring finger-1 (MGRN1) E3 ubiquitin ligase, which catalyzes mono-ubiquitination to the substrate, was dysregulated in the cellular and mouse models of ALS and that it preferentially interacted with various mutant forms of SOD1. Intriguingly, the motor neurons of presymptomatic ALS mice have diminished MGRN1 cytoplasmic distribution. MGRN1 was partially recruited to mutant SOD1 inclusions where they were positive for p62 and Lamp2. Moreover, overexpression of MGRN1 reduced mutant SOD1 aggregation and alleviated its proteotoxic effects on cells. Taken together, our findings suggest that MGRN1 contributes to the clearance of toxic mutant SOD1 inclusions likely through autophagic pathway, and, most likely, the sequestration of MGRN1 sensitizes motor neurons to degeneration in the ALS mouse model. Furthermore, the present study identifies the MGRN1-mediated protein QC mechanism as a novel therapeutic target in neurodegenerative diseases.

Parkin Protects Against Misfolded SOD1 Toxicity by Promoting Its Aggresome Formation and Autophagic Clearance

Mutations in Cu/Zn superoxide dismutase (SOD1) cause autosomal dominant amyotrophic lateral sclerosis (ALS), a devastating neurodegenerative disease with no effective treatment. Despite ample evidence indicating involvement of mutation-induced SOD1 protein misfolding and aggregation in ALS pathogenesis, the molecular mechanisms that control cellular management of misfolded, aggregation-prone SOD1 mutant proteins remain unclear. Here, we report that parkin, an E3 ubiquitin-protein ligase which is linked to Parkinson's disease, is a novel regulator of cellular defense against toxicity induced by ALS-associated SOD1 mutant proteins. We find that parkin mediates K63-linked polyubiquitination of SOD1 mutants in cooperation with the UbcH13/Uev1a E2 enzyme and promotes degradation of these misfolded SOD1 proteins by the autophagy-lysosome system. In response to strong proteotoxic stress associated with proteasome impairment, parkin promotes sequestration of misfolded and aggregated SOD1 proteins to form perinuclear aggresomes, regulates positioning of lysosomes around misfolded SOD1 aggresomes, and facilitates aggresome clearance by autophagy. Our findings reveal parkin-mediated cytoprotective mechanisms against misfolded SOD1 toxicity and suggest that enhancing parkin-mediated cytoprotection may provide a novel therapeutic strategy for treating ALS.

Mahogunin regulates fusion between amphisomes/MVBs and lysosomes via ubiquitination of TSG101

Aberrant metabolic forms of the prion protein (PrP), membrane-associated (Ctm)PrP and cytosolic (cyPrP) interact with the cytosolic ubiquitin E3 ligase, Mahogunin Ring Finger-1 (MGRN1) and affect lysosomes. MGRN1 also interacts with and ubiquitinates TSG101, an ESCRT-I protein, involved in endocytosis. We report that MGRN1 modulates macroautophagy. In cultured cells, functional depletion of MGRN1 or overexpression of (Ctm)PrP and cyPrP blocks autophagosome-lysosome fusion, alleviates the autophagic flux and its degradative competence. Concurrently, the degradation of cargo from the endo-lysosomal pathway is also affected. This is significant because catalytic inactivation of MGRN1 alleviates fusion of lysosomes with either autophagosomes (via amphisomes) or late endosomes (either direct or mediated through amphisomes), without drastically perturbing maturation of late endosomes, generation of amphisomes or lysosomal proteolytic activity. The compromised lysosomal fusion events are rescued by overexpression of TSG101 and/or its monoubiquitination in the presence of MGRN1. Thus, for the first time we elucidate that MGRN1 simultaneously modulates both autophagy and heterophagy via ubiquitin-mediated post-translational modification of TSG101.

Mahogunin Ring Finger-1 (MGRN1), a Multifaceted Ubiquitin Ligase: Recent Unraveling of Neurobiological Mechanisms

In healthy cell, inappropriate accumulation of poor or damaged proteins is prevented by cellular quality control system. Autophagy and ubiquitin proteasome system (UPS) provides regular cytoprotection against proteotoxicity induced by abnormal or disruptive proteins. E3 ubiquitin ligases are crucial components in this defense mechanism. Mahogunin Ring Finger-1 (MGRN1), an E3 ubiquitin ligase of the Really Interesting New Gene (RING) finger family, plays a pivotal role in many biological and cellular mechanisms. Previous findings indicate that lack of functions of MGRN1 can cause spongiform neurodegeneration, congenital heart defects, abnormal left-right patterning, and mitochondrial dysfunctions in mice brains. However, the detailed molecular pathomechanism of MGRN1 in cellular functions and diseases is not well known. This article comprehensively represents the molecular nature, characterization, and functions of MGRN1; we also summarize possible beneficiary aspects of this novel E3 ubiquitin ligase. Here, we review recent literature on the role of MGRN1 in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic intervention.

Ubiquitin in regulation of spindle apparatus and its positioning: implications in development and disease

Emerging data implicates ubiquitination, a post-translational modification, in regulating essential cellular events, one of them being mitosis. In this review we discuss how various E3 ligases modulate the cortical proteins such as dynein, LGN, NuMa, Gα, along with polymerization, stability, and integrity of spindles. These are responsible for regulating symmetric cell division. Some of the ubiquitin ligases regulating these proteins include PARK2, BRCA1/BARD1, MGRN1, SMURF2, and SIAH1; these play a pivotal role in the correct positioning of the spindle apparatus. A direct connection between developmental or various pathological disorders and the ubiquitination mediated cortical regulation is rather speculative, though deletions or mutations in them lead to developmental disorders and disease conditions.

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.

ATP13A2/PARK9 regulates secretion of exosomes and α-synuclein

Kufor-Rakeb syndrome (KRS) is caused by loss-of-function mutations in ATP13A2 (PARK9) and characterized by juvenile-onset parkinsonism, pyramidal signs, and cognitive decline. Previous studies suggested that PARK9 deficiency causes lysosomal dysfunction and α-synuclein (α-syn) accumulation, whereas PARK9 overexpression suppresses toxicity of α-syn. However, the precise mechanism of PARK9 effect on lysosomes and α-syn has been unknown. Here, we found that overexpressed PARK9 localized to multivesicular bodies (MVBs) in the human H4 cell line. The results from patient fibroblasts showed that loss of PARK9 function leads to decreased number of the intraluminal vesicles in MVBs and diminished release of exosomes into culture media. By contrast, overexpression of PARK9 results in increased release of exosomes in H4 cells and mouse primary cortical neurons. Moreover, loss of PARK9 function resulted in decreased secretion of α-syn into extracellular space, whereas overexpressed PARK9 promotes secretion of α-syn, at least in part via exosomes. Finally, we found that PARK9 regulates exosome biogenesis through functional interaction with the endosomal sorting complex required for transport machinery. Together, these data suggest the involvement of PARK9 in the biogenesis of exosomes and α-syn secretion and raise a possibility that disruption of these pathways in patients with KRS contributes to the disease pathogenesis.

Cullin-3 and the endocytic system: New functions of ubiquitination for endosome maturation

Cullin-RING-ligases (CRLs) comprise the largest class of multisubunit E3 ubiquitin ligases, which regulate a broad range of cellular processes. Cullin3 (Cul3) recently emerged as an important regulator of intracellular trafficking, in particular secretion and endosome maturation. Here we summarize and discuss possible functions and substrates of Cul3 in the endocytic system.

Interaction with Tsg101 is necessary for the efficient transport and release of nucleocapsids in marburg virus-infected cells

Endosomal sorting complex required for transport (ESCRT) machinery supports the efficient budding of Marburg virus (MARV) and many other enveloped viruses. Interaction between components of the ESCRT machinery and viral proteins is predominantly mediated by short tetrapeptide motifs, known as late domains. MARV contains late domain motifs in the matrix protein VP40 and in the genome-encapsidating nucleoprotein (NP). The PSAP late domain motif of NP recruits the ESCRT-I protein tumor susceptibility gene 101 (Tsg101). Here, we generated a recombinant MARV encoding NP with a mutated PSAP late domain (rMARV(PSAPmut)). rMARV(PSAPmut) was attenuated by up to one log compared with recombinant wild-type MARV (rMARV(wt)), formed smaller plaques and exhibited delayed virus release. Nucleocapsids in rMARV(PSAPmut)-infected cells were more densely packed inside viral inclusions and more abundant in the cytoplasm than in rMARV(wt)-infected cells. A similar phenotype was detected when MARV-infected cells were depleted of Tsg101. Live-cell imaging analyses revealed that Tsg101 accumulated in inclusions of rMARV(wt)-infected cells and was co-transported together with nucleocapsids. In contrast, rMARV(PSAPmut) nucleocapsids did not display co-localization with Tsg101, had significantly shorter transport trajectories, and migration close to the plasma membrane was severely impaired, resulting in reduced recruitment into filopodia, the major budding sites of MARV. We further show that the Tsg101 interacting protein IQGAP1, an actin cytoskeleton regulator, was recruited into inclusions and to individual nucleocapsids together with Tsg101. Moreover, IQGAP1 was detected in a contrail-like structure at the rear end of migrating nucleocapsids. Down regulation of IQGAP1 impaired release of MARV. These results indicate that the PSAP motif in NP, which enables binding to Tsg101, is important for the efficient actin-dependent transport of nucleocapsids to the sites of budding. Thus, the interaction between NP and Tsg101 supports several steps of MARV assembly before virus fission.

Post-translational modifications of exosomal proteins

Exosomes mediate intercellular communication and participate in many cell processes such as cancer progression, immune activation or evasion, and the spread of infection. Exosomes are small vesicles secreted to the extracellular environment through the release of intraluminal vesicles contained in multivesicular bodies (MVBs) upon the fusion of these MVBs with the plasma membrane. The composition of exosomes is not random, suggesting that the incorporation of cargo into them is a regulated process. However, the mechanisms that control the sorting of protein cargo into exosomes are currently elusive. Here, we review the post-translational modifications detected in exosomal proteins, and discuss their possible role in their specific sorting into exosomes.

Regulation of proteasomes in prion disease

The hallmark of prion disease is the accumulation of misfolded protein PrP(Sc), which is toxic to neuronal cells. The proteasome system is responsible for the rapid, precise, and timely degradation of proteins and plays an important role in cellular protein quality control. Increasing evidence indicates impaired activity of proteasomes in prion diseases. Accumulated PrP(Sc) can directly or indirectly affect proteasome activity. Misfolded protein may influence the assembly and activity of 19S regulatory particle, or post-translational modification of 20S proteasome, which may adversely affect the protein degradation activity of proteasomes. In this review, we summarized the recent findings concerning the possible regulation of proteasomes in prion and other neurodegenerative diseases. The proteasome system may enhance its degradation activity by changing its structure, and this activity can also be increased by related chaperones when neuronal cells are subject to stress. When the proteasome system is inhibited, degradation of protein aggregates via autophagy may increase as a compensatory system. It is possible that a balance exists between the proteasome and autophagy in vivo; when one is impaired, the activity of the other may increase to maintain homeostasis. However, more studies are needed to elucidate the relationship between the proteasome system and autophagy.