Oligodendroglial deletion of ESCRT-I component TSG101 causes spongiform encephalopathy

Distribution and Localization of Mahogunin Ring Finger 1 in the Mouse Central Nervous System

Mahogunin ring finger 1 (MGRN1), an E3 ubiquitin, is involved in several physiological and neuropathological processes. Although mgrn1 mRNA is widely distributed in the central nervous system (CNS), detailed information on its cellular and subcellular localization is lacking and its physiological role remains unclear. In this study, we aimed to determine the distribution of MGRN1 in the mouse CNS using a newly produced antibody against MGRN1. We found that the MGRN1 protein was expressed in most neuronal cell bodies. An intense MGRN1 expression was also observed in the neuropil of the gray matter in different regions of the CNS, including the main olfactory bulb, cerebral cortex, caudate, putamen, thalamic nuclei, hypothalamic nuclei, medial eminence, superior colliculus, hippocampus, dentate gyrus, and spinal cord. Contrastingly, no MGRN1 expression was observed in glial cells. Double fluorescence and immunoelectron microscopic analyses revealed the intracellular distribution of MGRN1 in pre-synapses and near the outer membrane of the mitochondria in neurons. These findings indicate that MGRN1 is more widely expressed throughout the CNS; additionally, the intracellular expression of MGRN1 suggests that it may play an important role in synaptic and mitochondrial functions.

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.

Tumor susceptibility gene 101 is required for the maintenance of uterine epithelial cells during embryo implantation

BACKGROUND

The tumor susceptibility gene 101 (Tsg101), a component of the endosomal sorting complex required for transport (ESCRT) complex I, is involved in multiple biological processes involving endomembranous structures and the plasma membrane. The role of Tsg101 in the uterine epithelium was investigated in Tsg101 floxed mice crossed with Lactoferrin-iCre mice (Tsg101^d/d).

METHODS

Tsg101^d/d mice were bred with stud male mice and the status of pregnancy was examined on days 4 and 6. Histological analyses were performed to examine the uterine architecture. Immunofluorescence staining of several markers was examined by confocal microscopy. Uterine epithelial cells (UECs) were isolated from Tsg101^f/f and Tsg101^d/d mice, and the expression of necroptosis effectors was examined by RT-PCR, western blotting, and immunofluorescence staining. UECs were also subjected to RNA expression profiling.

RESULTS

Tsg101^d/d female mice were subfertile with implantation failure, showing unattached blastocysts on day 6 of pregnancy. Histological and marker analyses revealed that some Tsg101^d/d day 4 pregnant uteri showed a disintegrated uterine epithelial structure. Tsg101^d/d UECs began to degenerate within 18 h of culture. In UECs, expression of necroptosis effectors, such as RIPK1, RIPK3, and MLKL were first confirmed. UECs responded to a stimulus to activate necroptosis and showed increased cell death.

CONCLUSIONS

Tsg101 deficiency in the uterine epithelium causes implantation failure, which may be caused by epithelial defects. This study provides evidence that UECs harbor a necroptotic machinery that responds to death-inducing signals. Thus, Tsg101 expression in the uterine epithelium is required for normal pregnancy in mice.

Tumor Susceptibility Gene 101 facilitates rapamycin-induced autophagic flux in neuron cells

Tumor Susceptibility Gene 101 (TSG101) is a member of endosomal sorting complexes responsible for endocytic pathway, which is associated with autophagic process. However, the role of TSG101 in autophagy remains unclear. To investigate the effect of TSG101 on the membrane-bound MAP1LC3-II, p62 and ubiquitinated protein levels in neuron cells, immunoblotting was used to evaluate the effects in cells silenced with siRNA against TSG101 and treated with autophagy inducer rapamycin. GFP-MAP1LC3 and tandem fluorescent-tagged LC3 (mTagRFP-mWasabi-MAP1LC3) reporter vectors were used to monitor autophagy in cells using confocal microcopy. The autophagic vacuoles were further validated with transmission electron microscopy. Our results showed that TSG101 expression was slightly increased in neuron cells when exposed to rapamycin. Depletion of TSG101 with siRNA lead to accumulation of MAP1LC3-II, GFP-MAP1LC3 puncta and autophagic vacuoles in the cells. Rapamycin-elevated MAP1LC3-II turnover and RFP⁺Wasabi^- puncta were repressed in TSG101 silenced cells, indicating that TSG101 is involved in rapamycin-induced autophagic flux in cells. Moreover, silencing TSG101 reduced colocalization of Rab7, MAP1LC3 and cell viability, increased p62, ubiquitinated proteins in the neuron cells. Taken together, our results suggested that TSG101 might be required for amphisome formation to promote autophagic flux in neuron cells when exposed to rapamycin.

A New Take on Prion Protein Dynamics in Cellular Trafficking

The mobility of cellular prion protein (PrP^C) in specific cell membrane domains and among distinct cell compartments dictates its molecular interactions and directs its cell function. PrP^C works in concert with several partners to organize signaling platforms implicated in various cellular processes. The scaffold property of PrP^C is able to gather a molecular repertoire to create heterogeneous membrane domains that favor endocytic events. Dynamic trafficking of PrP^C through multiple pathways, in a well-orchestrated mechanism of intra and extracellular vesicular transport, defines its functional plasticity, and also assists the conversion and spreading of its infectious isoform associated with neurodegenerative diseases. In this review, we highlight how PrP^C traffics across intra- and extracellular compartments and the consequences of this dynamic transport in governing cell functions and contributing to prion disease pathogenesis.

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.

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.

The Role of Vesicle Trafficking and Release in Oligodendrocyte Biology

Oligodendrocytes are a subtype of glial cells found within the central nervous system (CNS), responsible for the formation and maintenance of specialized myelin membranes which wrap neuronal axons. The development of myelin requires tight coordination for the cell to deliver lipid and protein building blocks to specific myelin segments at the right time. Both internal and external cues control myelination, thus the reception of these signals also requires precise regulation. In late years, a growing body of evidence indicates that oligodendrocytes, like many other cell types, may use extracellular vesicles (EVs) as a medium for transferring information. The field of EV research has expanded rapidly over the past decade, with new contributions that suggest EVs might have direct involvement in communications with neurons and other glial cells to fine tune oligodendroglial function. This functional role of EVs might also be maladaptive, as it has likewise been implicated in the spreading of toxic molecules within the brain during disease. In this review we will discuss the field's current understanding of extracellular vesicle biology within oligodendrocytes, and their contribution to physiologic and pathologic conditions.

UBAP1 mutations cause juvenile-onset hereditary spastic paraplegias (SPG80) and impair UBAP1 targeting to endosomes

We aimed to find a new causative gene and elucidate the molecular mechanisms underlying a new type of hereditary spastic paraplegia (HSP). Patients with HSP were recruited from the Japan Spastic Paraplegia Research Consortium (JASPAC). Exome sequencing of genomic DNA from patients in four families was carried out, followed by Sanger sequencing of the UBAP1 gene. A mouse homolog of one UBAP1 frameshift mutation carried by one of the patients was created as a disease model. Functional properties of the UBAP1 wild type and UBAP1-mutant in mouse hippocampus neurons were examined. We identified three novel heterozygous loss of function mutations (c.425_426delAG, c.312delC, and c.535G>T) in the UBAP1 gene as the genetic cause of a new type of HSP (SPG80). All the patients presented identical clinical features of a pure type of juvenile-onset HSP. Functional studies on mouse hippocampal neurons revealed that the C-terminal deletion UBAP1-mutant of our disease model had lost its ability to bind ubiquitin in vitro. Overexpression of the UBAP1 wild type interacts directly with ubiquitin on enlarged endosomes, while the UBAP1-mutant cannot be recruited to endosome membranes. Our study demonstrated that mutations in the UBAP1 gene cause a new type of HSP and elucidated its pathogenesis. The full-length UBAP1 protein is involved in endosomal dynamics in neurons, while loss of UBAP1 function may perturb endosomal fusion and sorting of ubiquitinated cargos. These effects could be more prominent in neurons, thereby giving rise to the phenotype of a neurodegenerative disease such as HSP.

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.

Live Imaging of ESCRT Proteins in Microfluidically Isolated Hippocampal Axons

Live imaging of microfluidically isolated axons permits study of the dynamic behavior of fluorescently tagged proteins and vesicles in these neuronal processes. We use this technique to study the motility and transport of ESCRT proteins in axons of primary hippocampal neurons. This chapter details the preparation of microfluidic chambers, as well as the seeding, fluidic isolation, and lentiviral transduction of hippocampal neurons in these chambers, optimized for the study of ESCRT protein dynamics.

Lysosomal Quality Control in Prion Diseases

Prion diseases are transmissible, familial or sporadic. The prion protein (PrP), a normal cell surface glycoprotein, is ubiquitously expressed throughout the body. While loss of function of PrP does not elicit apparent phenotypes, generation of misfolded forms of the protein or its aberrant metabolic isoforms has been implicated in a number of neurodegenerative disorders such as scrapie, kuru, Creutzfeldt-Jakob disease, fatal familial insomnia, Gerstmann-Sträussler-Scheinker and bovine spongiform encephalopathy. These diseases are all phenotypically characterised by spongiform vacuolation of the adult brain, hence collectively termed as late-onset spongiform neurodegeneration. Misfolded form of PrP (PrP^Sc) and one of its abnormal metabolic isoforms (the transmembrane ^CtmPrP) are known to be disease-causing agents that lead to progressive loss of structure or function of neurons culminating in neuronal death. The aberrant forms of PrP utilise and manipulate the various intracellular quality control mechanisms during pathogenesis of these diseases. Amongst these, the lysosomal quality control machinery emerges as one of the primary targets exploited by the disease-causing isoforms of PrP. The autophagosomal-lysosomal degradation pathway is adversely affected in multiple ways in prion diseases and may hence be regarded as an important modulator of neurodegeneration. Some of the ESCRT pathway proteins have also been shown to be involved in the manifestation of disease phenotype. This review discusses the significance of the lysosomal quality control pathway in affecting transmissible and familial types of prion diseases.