Overview of diet and autoimmune demyelinating optic neuritis: a narrative review

This review summarizes the cellular and molecular underpinnings of autoimmune demyelinating optic neuritis (ADON), a common sequela of multiple sclerosis and other demyelinating diseases. We further present nutritional interventions tested for people with multiple sclerosis focusing on strategies that have shown efficacy or associations with disease course and clinical outcomes. We then close by discuss the potential dietary guidance for preventing and/or ameliorating ADON.

Dietary protection against the visual and motor deficits induced by experimental autoimmune encephalomyelitis

Introduction

Five to eight percent of the world population currently suffers from at least one autoimmune disorder. Despite multiple immune modulatory therapies for autoimmune demyelinating diseases of the central nervous system, these treatments can be limiting for subsets of patients due to adverse effects and expense. To circumvent these barriers, we investigated a nutritional intervention in mice undergoing experimental autoimmune encephalomyelitis (EAE), a model of autoimmune-mediated demyelination that induces visual and motor pathologies similar to those experienced by people with multiple sclerosis (MS).

Methods

EAE was induced in female and male mice and the impact of limiting dietary carbohydrates by feeding a ketogenic diet (KD) enriched in medium chain triglycerides (MCTs), alpha-linolenic acid (an omega-3 fatty acid), and fiber was evaluated in both a preventive regimen (prior to immunization with MOG antigen) and an interventional regimen (following the onset of symptoms). Motor scores were assigned daily and visual acuity was measured using optokinetic tracking. Immunohistochemical analyses of optic nerves were done to assess inflammatory infiltrates and myelination status. Fatty acid and cytokine profiling from blood were performed to evaluate systemic inflammatory status.

Results

The KD was efficacious when fed as a preventive regimen as well as when initiated as an interventional regimen following symptom onset. The KD minimally impacted body weight during the experimental time course, increased circulating ketones, prevented motor and ocular deficits, preserved myelination of the optic nerve, and reduced infiltration of immune cells to optic nerves. The KD also increased anti-inflammatory-associated omega-3 fatty acids in the plasma and reduced select cytokines in the circulation associated with EAE-mediated pathological inflammation.

Discussion

In light of ongoing clinical trials using dietary strategies to treat people with MS, these findings support that a KD enriched in MCTs, omega-3 fatty acids, and fiber promotes a systemic anti-inflammatory milieu and ameliorates autoimmune-induced demyelinating visual and motor deficits.

Sulforaphane recovers cone function in an Nrf2-dependent manner in middle-aged mice undergoing RPE oxidative stress

PURPOSE

Sulforaphane (SFN) is an isothiocyanate derived from cruciferous vegetables that has therapeutic efficacy in numerous animal models of human disease, including mouse models of retinal degeneration. However, despite dozens of clinical trials, the compound remains to be tested as a clinical treatment for ocular disease. Numerous cellular activities of SFN have been identified, including the activation of Nrf2, a transcription factor that induces a battery of target gene products to neutralize oxidative and xenobiotic stresses. As Nrf2 expression and function reportedly decrease with aging, we tested whether the loss of the transcription factor limits the therapeutic efficacy of SFN against retinal degeneration.

METHODS

Six- to 8-month-old wild-type and Nrf2 knockout mice were treated with SFN beginning 1 month after ribozyme-mediated knockdown of superoxide dismutase 2 (SOD2) mRNA in the RPE. The impacts of MnSOD (the protein product of SOD2) knockdown and the efficacy of SFN were evaluated using a combination of electroretinography (ERG), spectral domain optical coherence tomography (SD-OCT), and postmortem histology.

RESULTS

SFN restored the ERG photopic b-wave suppressed by MnSOD loss in wild-type mice, but not in the Nrf2 knockout mice. In contrast, ERG scotopic a- and b-wave loss was not restored for either genotype. SFN significantly improved retinal thickness in the Nrf2 knockout mice with MnSOD knockdown, but this was not observed in the wild-type mice. In both genotypes, SFN treatment reduced morphological markers of RPE atrophy and degeneration, although these improvements did not correlate proportionally with functional recovery.

CONCLUSIONS

These findings highlight the capacity of SFN to preserve cone function, as well as the potential challenges of using the compound as a standalone treatment for age-related retinal degeneration under conditions associated with reduced Nrf2 function.

Dimethyl fumarate mitigates optic neuritis

Purpose

Dimethyl fumarate (DMF) has been approved by the U.S. Food and Drug Administration (FDA) for the treatment of relapsing-remitting multiple sclerosis (RRMS), a demyelinating autoimmune disease characterized by acute episodes of motor, sensory, and cognitive symptoms. Optic neuritis is an episodic sequela experienced by some patients with RRMS that typically presents as acute, monocular vision loss. Episodes of optic neuritis damage and kill retinal ganglion cells (RGCs), and can culminate in permanent vision loss. The purpose of these studies was to evaluate the capacity of DMF to mitigate optic neuritis. The work presented combines studies of a mouse model of MS and a retrospective chart analysis of files of patients with RRMS treated at the MS Center of Excellence within the Oklahoma Medical Research Foundation.

Methods

Experimental autoimmune encephalomyelitis (EAE) is a well-established mouse model that recapitulates cardinal features of somatic and visual MS pathologies. EAE was induced in female C57BL/6J mice by inoculation with myelin oligodendrocyte glycoprotein peptide (residues 35-55; MOG_35-55). DMF or vehicle was administered twice a day by oral gavage. Visual acuity was measured longitudinally with optokinetic tracking. Post-mortem analyses included quantification of RGCs in retinal flatmounts and quantitative PCR (qPCR) of Nrf2 target genes and regulators of myelin. Retrospective chart analyses were performed using data obtained from deidentified files of patients with RRMS.

Results

In the EAE mouse studies, DMF decreased optic neuritis severity, preserved vision and RGCs, and concomitantly reduced motor deficits when administered by two different treatment regimens (prevention or interventional). DMF was more efficacious when administered as an interventional therapy, and the beneficial effects occurred independently of the induction of Nrf2 target genes. A complementary retrospective chart analysis demonstrated that DMF increased the time to a recurrence of optic neuritis, and protected against subsequent bouts of optic neuritis.

Conclusions

This work underscores the potential of DMF to mitigate the severity and recurrence of optic neuritis episodes in patients with RRMS.

Clinically-approved CFTR modulators rescue Nrf2 dysfunction in cystic fibrosis airway epithelia

Cystic Fibrosis (CF) is a multi-organ progressive genetic disease caused by loss of functional cystic fibrosis transmembrane conductance regulator (CFTR) channel. Previously, we identified a significant dysfunction in CF cells and model mice of the transcription factor nuclear-factor-E2-related factor-2 (Nrf2), a major regulator of redox balance and inflammatory signaling. Here we report that approved F508del CFTR correctors VX809/VX661 recover diminished Nrf2 function and colocalization with CFTR in CF human primary bronchial epithelia by proximity ligation assay, immunoprecipitation, and immunofluorescence, concordant with CFTR correction. F508del CFTR correctors induced Nrf2 nuclear translocation, Nrf2-dependent luciferase activity, and transcriptional activation of target genes. Rescue of Nrf2 function by VX809/VX661 was dependent on significant correction of F508del and was blocked by inhibition of corrected channel function, or high-level shRNA knockdown of CFTR or F508del-CFTR. Mechanistically, F508del-CFTR modulation restored Nrf2 phosphorylation and its interaction with the coactivator CBP. Our findings demonstrate that sufficient modulation of F508del CFTR function corrects Nrf2 dysfunction in CF.

Loss of the ubiquitin conjugating enzyme UBE2E3 induces cellular senescence

Cellular senescence plays essential roles in tissue homeostasis as well as a host of diseases ranging from cancers to age-related neurodegeneration. Various molecular pathways can induce senescence and these different pathways dictate the phenotypic and metabolic changes that accompany the transition to, and maintenance of, the senescence state. Here, we describe a novel senescence phenotype induced by depletion of UBE2E3, a highly-conserved, metazoan ubiquitin conjugating enzyme. Cells depleted of UBE2E3 become senescent in the absence of overt DNA damage and have a distinct senescence-associated secretory phenotype, increased mitochondrial and lysosomal mass, an increased sensitivity to mitochondrial and lysosomal poisons, and an increased basal autophagic flux. This senescence phenotype can be partially suppressed by co-depletion of either p53 or its cognate target gene, p21CIP1/WAF1, or by co-depleting the tumor suppressor p16INK4a. Together, these data describe a direct link of a ubiquitin conjugating enzyme to cellular senescence and further underscore the consequences of disrupting the integration between the ubiquitin proteolysis system and the autophagy machinery.

A PGAM5-KEAP1-Nrf2 complex is required for stress-induced mitochondrial retrograde trafficking

The Nrf2 transcription factor is a master regulator of the cellular anti-stress response. A population of the transcription factor associates with the mitochondria through a complex with KEAP1 and the mitochondrial outer membrane histidine phosphatase, PGAM5. To determine the function of this mitochondrial complex, we knocked down each component and assessed mitochondrial morphology and distribution. We discovered that depletion of Nrf2 or PGAM5, but not KEAP1, inhibits mitochondrial retrograde trafficking induced by proteasome inhibition. Mechanistically, this disrupted motility results from aberrant degradation of Miro2, a mitochondrial GTPase that links mitochondria to microtubules. Rescue experiments demonstrate that this Miro2 degradation involves the KEAP1-cullin-3 E3 ubiquitin ligase and the proteasome. These data are consistent with a model in which an intact complex of PGAM5-KEAP1-Nrf2 preserves mitochondrial motility by suppressing dominant-negative KEAP1 activity. These data further provide a mechanistic explanation for how age-dependent declines in Nrf2 expression impact mitochondrial motility and induce functional deficits commonly linked to neurodegeneration.

The nuclear transport receptor Importin-11 is a tumor suppressor that maintains PTEN protein

Phosphatase and tensin homologue (PTEN) protein levels are critical for tumor suppression. However, the search for a recurrent cancer-associated gene alteration that causes PTEN degradation has remained futile. In this study, we show that Importin-11 (Ipo11) is a transport receptor for PTEN that is required to physically separate PTEN from elements of the PTEN degradation machinery. Mechanistically, we find that the E2 ubiquitin-conjugating enzyme and IPO11 cargo, UBE2E1, is a limiting factor for PTEN degradation. Using in vitro and in vivo gene-targeting methods, we show that Ipo11 loss results in degradation of Pten, lung adenocarcinoma, and neoplasia in mouse prostate with aberrantly high levels of Ube2e1 in the cytoplasm. These findings explain the correlation between loss of IPO11 and PTEN protein in human lung tumors. Furthermore, we find that IPO11 status predicts disease recurrence and progression to metastasis in patients choosing radical prostatectomy. Thus, our data introduce the IPO11 gene as a tumor-suppressor locus, which is of special importance in cancers that still retain at least one intact PTEN allele.

Loss of Nrf2 exacerbates the visual deficits and optic neuritis elicited by experimental autoimmune encephalomyelitis

PURPOSE

Optic neuritis, inflammation of the optic nerve, is experienced by most patients with multiple sclerosis (MS) and is typically characterized by episodes of acute, monocular vision loss. These episodes of inflammation can lead to damage or degeneration of the retinal ganglion cells (RGCs), the axons of which comprise the optic nerve. Experimental autoimmune encephalomyelitis (EAE) is a well-established model of MS in which mice are immunized to produce a neuroautoimmunity that recapitulates the cardinal hallmarks of human disease, namely, inflammation, demyelination, and neurodegeneration of the brain, spinal cord, and optic nerve. Inflammation-associated oxidative stress plays a key role in promoting spinal cord damage in EAE. However, the role of oxidative stress in optic neuritis and the associated visual deficits has not been studied. To address this gap in research, we sought to determine how a deficiency in the master antioxidant transcription factor (using nuclear factor-E2-related factor [Nrf2]-deficient mice) affects visual pathology in the EAE model.

METHODS

EAE was induced in 8-week-old wild-type (WT) and Nrf2 knockout (KO) mice by immunization against the myelin oligodendrocyte glycoprotein (MOG) peptide antigen. Motor deficits were monitored daily, as was visual acuity using the established functional optokinetic tracking (OKT) assay. Mice were euthanized 21 days post-immunization for histological analyses. The optic nerves were paraffin-embedded and stained with hematoxylin and eosin (H&E) or immune cell type-specific antibodies to analyze inflammatory infiltrates. The retinas were flatmounted and stained with an RGC-specific antibody, and the RGCs were counted to assess neurodegeneration. T-helper (Th) cell-associated cytokines were measured in spleens with enzyme-linked immunosorbent assay (ELISA). Immune analyses of healthy, non-EAE mice were characterized with flow cytometry to assess the baseline immune cell profiles.

RESULTS

Female Nrf2 KO mice exhibited more severe EAE-induced motor deficits compared with female WT mice. In both genders, EAE elicited more severe visual acuity deficits, inflammation of the optic nerve, and RGC degeneration in KO mice compared with their strain- and age-matched WT counterparts. Visual acuity deficits were primarily present in (and only exacerbated in) one eye of each mouse. Excess inflammatory cells within the optic nerves of the KO mice were primarily comprised of T-cells, and greater RGC degeneration in the KO mice was most prevalent in the central retina compared with the peripheral retina. Nrf2 KO spleens exhibited an increased Th1- but not Th17-associated immune response. This enhanced pathology in the KO mice was not due to global differences in immune system development between the two genotypes.

CONCLUSIONS

This is the first study to report that genetic ablation of Nrf2 exacerbates visual deficits, inflammation of the optic nerve, and RGC degeneration in a murine model of MS, suggesting that Nrf2 plays a neuro- and cytoprotective role in EAE-associated optic neuritis.

Sulforaphane is a Nrf2-independent inhibitor of mitochondrial fission

The KEAP1-Nrf2-ARE antioxidant system is a principal means by which cells respond to oxidative and xenobiotic stresses. Sulforaphane (SFN), an electrophilic isothiocyanate derived from cruciferous vegetables, activates the KEAP1-Nrf2-ARE pathway and has become a molecule-of-interest in the treatment of diseases in which chronic oxidative stress plays a major etiological role. We demonstrate here that the mitochondria of cultured, human retinal pigment epithelial (RPE-1) cells treated with SFN undergo hyperfusion that is independent of both Nrf2 and its cytoplasmic inhibitor KEAP1. Mitochondrial fusion has been reported to be cytoprotective by inhibiting pore formation in mitochondria during apoptosis, and consistent with this, we show Nrf2-independent, cytoprotection of SFN-treated cells exposed to the apoptosis-inducer, staurosporine. Mechanistically, SFN mitigates the recruitment and/or retention of the soluble fission factor Drp1 to mitochondria and to peroxisomes but does not affect overall Drp1 abundance. These data demonstrate that the beneficial properties of SFN extend beyond activation of the KEAP1-Nrf2-ARE system and warrant further interrogation given the current use of this agent in multiple clinical trials.

Expression profiling of the ubiquitin conjugating enzyme UbcM2 in murine brain reveals modest age-dependent decreases in specific neurons

BACKGROUND

UbcM2 is a ubiquitin-conjugating enzyme with roles in the turnover of damaged and misfolded proteins, cell cycle progression, development, and regulation of the antioxidant transcription factor, Nrf2. Recent screens have identified binding partners of the enzyme that are associated with various neurodegenerative diseases, and our previous studies have shown that UbcM2 is enriched in retina and brain.

RESULTS

In the current study, we characterized UbcM2 protein expression in various structures and cell types in the murine brain. Immunofluorescence analysis of paraffin-embedded brain sections revealed that UbcM2 is ubiquitously expressed throughout the brain, is enriched in hindbrain and cortex, and is robustly expressed in neurons. In contrast, the enzyme is undetectable in most astrocytes and microglia. As dysfunction of the ubiquitin proteasome system (UPS) has been linked to many age-related neurological diseases, we compared UbcM2 expression levels in young versus aged wild-type mice and found a global decrease in expression in aged brains, with reductions of 10 % or greater in five substructures (cerebellar granule cell layer, primary motor cortex, olfactory nucleus, superior colliculus, and secondary visual cortex).

CONCLUSIONS

These studies represent the first protein expression profiling of a ubiquitin-conjugating enzyme in the brain and support the notion that deficits in protein degradation and proteostasis associated with neurodegenerative diseases may be, in part, attributable to age-dependent reductions in the enzymatic machinery of the UPS.

OKN-007 decreases free radical levels in a preclinical F98 rat glioma model

Free radicals are associated with glioma tumors. Here, we report on the ability of an anticancer nitrone compound, OKN-007 [Oklahoma Nitrone 007; a disulfonyl derivative of α-phenyl-tert-butyl nitrone (PBN)] to decrease free radical levels in F98 rat gliomas using combined molecular magnetic resonance imaging (mMRI) and immunospin-trapping (IST) methodologies. Free radicals are trapped with the spin-trapping agent, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), to form DMPO macromolecule radical adducts, and then further tagged by immunospin trapping by an antibody against DMPO adducts. In this study, we combined mMRI with a biotin-Gd-DTPA-albumin-based contrast agent for signal detection with the specificity of an antibody for DMPO nitrone adducts (anti-DMPO probe), to detect in vivo free radicals in OKN-007-treated rat F98 gliomas. OKN-007 was found to significantly decrease (P < 0.05) free radical levels detected with an anti-DMPO probe in treated animals compared to untreated rats. Immunoelectron microscopy was used with gold-labeled antibiotin to detect the anti-DMPO probe within the plasma membrane of F98 tumor cells from rats administered anti-DMPO in vivo. OKN-007 was also found to decrease nuclear factor erythroid 2-related factor 2, inducible nitric oxide synthase, 3-nitrotyrosine, and malondialdehyde in ex vivo F98 glioma tissues via immunohistochemistry, as well as decrease 3-nitrotyrosine and malondialdehyde adducts in vitro in F98 cells via ELISA. The results indicate that OKN-007 effectively decreases free radicals associated with glioma tumor growth. Furthermore, this method can potentially be applied toward other types of cancers for the in vivo detection of macromolecular free radicals and the assessment of antioxidants.

The ubiquitin-conjugating enzyme UBE2E3 and its import receptor importin-11 regulate the localization and activity of the antioxidant transcription factor NRF2

The transcription factor NF-E2 p45-related factor (Nrf2) induces the expression of cytoprotective proteins that maintain and restore redox homeostasis. Nrf2 levels and activity are tightly regulated, and three subcellular populations of the transcription factor have been identified. During homeostasis, the majority of Nrf2 is degraded in the cytoplasm by ubiquitin (Ub)-mediated degradation. A second population is transcriptionally active in the nucleus, and a third population localizes to the outer mitochondrial membrane. Still unresolved are the mechanisms and factors that govern Nrf2 distribution between its subcellular locales. We show here that the Ub-conjugating enzyme UBE2E3 and its nuclear import receptor importin 11 (Imp-11) regulate Nrf2 distribution and activity. Knockdown of UBE2E3 reduces nuclear Nrf2, decreases Nrf2 target gene expression, and relocalizes the transcription factor to a perinuclear cluster of mitochondria. In a complementary manner, Imp-11 functions to restrict KEAP1, the major suppressor of Nrf2, from prematurely extracting the transcription factor off of a subset of target gene promoters. These findings identify a novel pathway of Nrf2 modulation during homeostasis and support a model in which UBE2E3 and Imp-11 promote Nrf2 transcriptional activity by restricting the transcription factor from partitioning to the mitochondria and limiting the repressive activity of nuclear KEAP1.

The ubiquitin-conjugating enzyme, UbcM2, is restricted to monoubiquitylation by a two-fold mechanism that involves backside residues of E2 and Lys48 of ubiquitin

Proteins can be modified on lysines (K) with a single ubiquitin (Ub) or with polymers of Ub (polyUb). These different configurations and their respective topologies are primary factors for determining whether substrates are targeted to the proteasome for degradation or directed to nonproteolytic outcomes. We report here on the intrinsic ubiquitylation properties of UbcM2 (UBE2E3/UbcH9), a conserved Ub-conjugating enzyme linked to cell proliferation, development, and the cellular antioxidant defense system. Using a fully recombinant ubiquitylation assay, we show that UbcM2 is severely limited in its ability to synthesize polyUb chains with wild-type Ub. Restriction to monoubiquitylation is governed by multiple residues on the backside of the enzyme, far removed from its active site, and by lysine 48 of Ub. UbcM2 with mutated backside residues can synthesize K63-linked polyUb chains and to a lesser extent K6- and K48-linked chains. Additionally, we identified a single residue on the backside of the enzyme that promotes monoubiquitylation. Together, these findings reveal that a combination of noncatalytic residues within the Ubc catalytic core domain of UbcM2 as well as a lysine(s) within Ub can relegate a Ub-conjugating enzyme to monoubiquitylate its cognate targets despite having the latent capacity to construct polyUb chains. The two-fold mechanism for restricting activity to monoubiquitylation provides added insurance that UbcM2 will not build polyUb chains on its substrates, even under conditions of high local Ub concentrations.

Abstract 5258: PTEN levels are controlled by a nuclear transport receptor in lung cancer

The maintenance of PTEN protein levels is critical for tumor suppression. Yet, the ubiquitination system has been shown to affect PTEN levels both adversely through degradation, as well as positively through nuclear import, and it has remained unclear how these two processes are integrated to prevent cancer. Here we show, that a nuclear import receptor is at the heart of a failsafe system that maintains PTEN levels by mediating its nuclear transport. Loss of import receptor function not only leads to cytoplasmic PTEN accumulation but also prompts PTEN degradation through a novel component of the PTEN ubiquitination system.

By testing the consequences of importin loss in vivo, we found that hypomorphic mice developed lung adenocarcinoma, which presented with aberrant cytoplasmic PTEN localization and degradation, as predicted by our in vitro findings. Since the corresponding human locus suffers frequent deletion as well as inactivating mutations in lung cancer, we propose that this import receptor is a novel tumor suppressor that antagonizes PI 3-Kinase signaling in settings with at least one intact PTEN gene.

Citation Format: Muhan Chen, Tali M. Herzka, Martha E. Zeeman, Kendra S. Plafker, Matthias Minderer, Mireia Castillo-Martin, Carlos Cordon-Cardo, Scott M. Plafker, Lloyd C. Trotman. PTEN levels are controlled by a nuclear transport receptor in lung cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5258. doi:10.1158/1538-7445.AM2013-5258

High dietary fat selectively increases catalase expression within cardiac mitochondria

Obesity is a predictor of diabetes and cardiovascular disease. One consequence of obesity is dyslipidemia characterized by high blood triglycerides. It has been proposed that oxidative stress, driven by utilization of lipids for energy, contributes to these diseases. The effects of oxidative stress are mitigated by an endogenous antioxidant enzyme network, but little is known about its response to high fat utilization. Our experiments used a multiplexed quantitative proteomics method to measure antioxidant enzyme expression in heart tissue in a mouse model of diet-induced obesity. This experiment showed a rapid and specific up-regulation of catalase protein, with subsequent assays showing increases in activity and mRNA. Catalase, traditionally considered a peroxisomal protein, was found to be present in cardiac mitochondria and significantly increased in content and activity during high fat feeding. These data, coupled with the fact that fatty acid oxidation enhances mitochondrial H(2)O(2) production, suggest that a localized catalase increase is needed to consume excessive mitochondrial H(2)O(2) produced by increased fat metabolism. To determine whether the catalase-specific response is a common feature of physiological conditions that increase blood triglycerides and fatty acid oxidation, we measured changes in antioxidant expression in fasted versus fed mice. Indeed, a similar specific catalase increase was observed in mice fasted for 24 h. Our findings suggest a fundamental metabolic process in which catalase expression is regulated to prevent damage while preserving an H(2)O(2)-mediated sensing of diet composition that appropriately adjusts insulin sensitivity in the short term as needed to prioritize lipid metabolism for complete utilization.

Mechanisms for countering oxidative stress and damage in retinal pigment epithelium

Clinical and experimental evidence supports that chronic oxidative stress is a primary contributing factor to numerous retinal degenerative diseases, such as age-related macular degeneration (AMD). Eyes obtained postmortem from AMD patients have extensive free radical damage to the proteins, lipids, DNA, and mitochondria of their retinal pigment epithelial (RPE) cells. In addition, several mouse models of chronic oxidative stress develop many of the pathological hallmarks of AMD. However, the extent to which oxidative stress is an etiologic component versus its involvement in disease progression remains a major unanswered question. Further, whether the primary target of oxidative stress and damage is photoreceptors or RPE cells, or both, is still unclear. In this review, we discuss the major functions of RPE cells with an emphasis on the oxidative challenges these cells encounter and the endogenous antioxidant mechanisms employed to neutralize the deleterious effects that such stresses can elicit if left unchecked.

The ubiquitin-conjugating enzyme UbcM2 can regulate the stability and activity of the antioxidant transcription factor Nrf2

The transcription factor nuclear factor E2-related factor 2 (Nrf2) induces the expression of antioxidant gene products that neutralize reactive oxygen species and restore redox homeostasis. Nrf2 is constitutively degraded by the ubiquitin proteolytic system in unperturbed cells, but this turnover is arrested in response to oxidative stress, thereby leading to Nrf2 accumulation. Yet, a mechanistic understanding of how Nrf2 stabilization and transcriptional activation are coupled remains to be determined. We have discovered that the ubiquitin-conjugating enzyme UbcM2 is a novel regulator of Nrf2. Recombinant Nrf2 and UbcM2 form a complex upon alkylation of a non-catalytic cysteine in UbcM2, Cys-136. Substitution of this cysteine with a phenylalanine (C136F) to mimic cysteine oxidation/alkylation results in constitutive binding of UbcM2 to Nrf2 and an increased half-life of the transcription factor in vivo. We provide evidence that UbcM2 and Nrf2 form a nuclear complex utilizing the DNA binding, Neh1 domain, of Nrf2. Finally, we demonstrate that UbcM2 can enhance the transcriptional activity of endogenous Nrf2 and that Cys-136 and the active-site cysteine, Cys-145, jointly contribute to this regulation. Collectively, these data identify UbcM2 as a novel component of the Nrf2 regulatory circuit and position cysteine 136 as a putative redox sensor in this signaling pathway. This work implicates UbcM2 in the restoration of redox homeostasis following oxidative stress.

The ubiquitin conjugating enzyme, UbcM2, engages in novel interactions with components of cullin-3 based E3 ligases

The class III ubiquitin conjugating enzymes (E2s) are distinguished from other E2s by the presence of unique N-terminal domains, and the utilization of importin-11 for transport into the nucleus in an activation dependent fashion. To begin determining the physiological roles of these enzymes, we carried out a yeast two-hybrid screen with the class III E2, UbcM2. This screen retrieved RCBTB1, a putative substrate adaptor for a cullin3 (CUL3) E3 ligase. We initially established through biochemical studies that RCBTB1 has the properties of a CUL3 substrate adaptor. Further analysis of the UbcM2-RCBTB1 complex led to the discovery and characterization of the following novel interactions: (i) UbcM2 binds an N-terminal domain of CUL3 requiring the first 57 amino acids, the same domain that binds to RCBTB1 and other substrate adaptors; (ii) UbcM2 does not bind mutants of CUL3 that are deficient in substrate adaptor recruitment; (iii) UbcM2 interacts with CUL3 independent of a bridging RING-finger protein; and (iv) can engage the neddylated (i.e., activated) form of CUL3. We also present evidence that UbcM2 can bind to the N-terminal halves of multiple cullins, implying that this E2 is a general cofactor for this class of ligases. Together, these studies represent the first evidence that UbcM2, in concert with substrate adaptors, engages activated CUL3 ligases, thus suggesting that class III E2s are novel regulators of cullin ligases.

Human papillomavirus 16 E5 induces bi-nucleated cell formation by cell-cell fusion

Human papillomaviruses (HPV) 16 is a DNA virus encoding three oncogenes--E5, E6, and E7. The E6 and E7 proteins have well-established roles as inhibitors of tumor suppression, but the contribution of E5 to malignant transformation is controversial. Using spontaneously immortalized human keratinocytes (HaCaT cells), we demonstrate that expression of HPV16 E5 is necessary and sufficient for the formation of bi-nucleated cells, a common characteristic of precancerous cervical lesions. Expression of E5 from non-carcinogenic HPV6b does not produce bi-nucleate cells. Video microscopy and biochemical analyses reveal that bi-nucleates arise through cell-cell fusion. Although most E5-induced bi-nucleates fail to propagate, co-expression of HPV16 E6/E7 enhances the proliferation of these cells. Expression of HPV16 E6/E7 also increases bi-nucleated cell colony formation. These findings identify a new role for HPV16 E5 and support a model in which complementary roles of the HPV16 oncogenes lead to the induction of carcinogenesis.

The human ubiquitin conjugating enzyme, UBE2E3, is required for proliferation of retinal pigment epithelial cells

PURPOSE

Cell cycle progression is governed by the coordinated activities of kinases, phosphatases, and the ubiquitin system. The entire complement of ubiquitin pathway components that mediate this process in retinal pigment epithelial (RPE) cells remains to be identified. This study was undertaken to determine whether the human ubiquitin-conjugating enzyme, UBE2E3, is essential for RPE cell proliferation.

METHODS

UBE2E3 expression and localization in telomerase-immortalized, human RPE cells was determined with a UBE2E3-specific antibody. The necessity for UBE2E3 in RPE proliferation was determined using small interfering (si)RNA to target the expression of the enzyme. Cell counts and immunolabeling for the proliferation marker Ki-67 and the cyclin-dependent kinase inhibitor p27(Kip1) were performed to assess the consequences of UBE2E3 depletion. A mouse strain harboring a disrupted allele of UbcM2 (the mouse counterpart of UBE2E3) with the coding sequence for beta-galactosidase was used to track the developmental expression of the enzyme in murine RPE cells.

RESULTS

UBE2E3 localized in the nucleus of the immortalized RPE cells. Depletion of the enzyme by siRNA resulted in a cell-cycle exit accompanied by a loss of Ki-67, an increase in p27(Kip1), and a doubling in cell area. Rescue experiments confirmed the specificity of the RNA interference. In vivo, UbcM2 was transcriptionally downregulated during RPE development in the mouse.

CONCLUSIONS

UBE2E3 is essential for the proliferation of RPE-1 cells and is downregulated during RPE layer maturation in the developing mouse eye. These findings indicate that UBE2E3 is a major enzyme in modulating the balance between RPE cell proliferation and differentiation.

Mammalian DET1 regulates Cul4A activity and forms stable complexes with E2 ubiquitin-conjugating enzymes

DET1 (de-etiolated 1) is an essential negative regulator of plant light responses, and it is a component of the Arabidopsis thaliana CDD complex containing DDB1 and COP10 ubiquitin E2 variant. Human DET1 has recently been isolated as one of the DDB1- and Cul4A-associated factors, along with an array of WD40-containing substrate receptors of the Cul4A-DDB1 ubiquitin ligase. However, DET1 differs from conventional substrate receptors of cullin E3 ligases in both biochemical behavior and activity. Here we report that mammalian DET1 forms stable DDD-E2 complexes, consisting of DDB1, DDA1 (DET1, DDB1 associated 1), and a member of the UBE2E group of canonical ubiquitin-conjugating enzymes. DDD-E2 complexes interact with multiple ubiquitin E3 ligases. We show that the E2 component cannot maintain the ubiquitin thioester linkage once bound to the DDD core, rendering mammalian DDD-E2 equivalent to the Arabidopsis CDD complex. While free UBE2E-3 is active and able to enhance UbcH5/Cul4A activity, the DDD core specifically inhibits Cul4A-dependent polyubiquitin chain assembly in vitro. Overexpression of DET1 inhibits UV-induced CDT1 degradation in cultured cells. These findings demonstrate that the conserved DET1 complex modulates Cul4A functions by a novel mechanism.

Auto-activation of the apoptosis protein Bax increases mitochondrial membrane permeability and is inhibited by Bcl-2

Interactions among Bcl-2 family proteins mediated by Bcl-2 homology (BH) regions transform apoptosis signals into actions. The interactions between BH3 region-only proteins and multi-BH region proteins such as Bax and Bcl-2 have been proposed to be the dominant interactions required for initiating apoptosis. Experimental evidence also suggests that both homo- and hetero-interactions are mediated primarily by the BH3 regions in all Bcl-2 family proteins and contribute to commitment to or inhibition of apoptosis. We found that a peptide containing the BH3 helix of Bax was not sufficient to activate recombinant Bax to permeabilize mitochondria. However, an extended peptide containing the BH3 helix and additional downstream sequences activated Bax to permeabilize mitochondria and liposomes. Bcl-2 inhibited the membrane-permeabilizing activity of peptide-activated Bax. This activity of Bcl-2 was inhibited by the extended but not the BH3-only peptide despite both peptides binding to Bcl-2 with similar affinity. Further, membrane-bound Bax activation intermediates directly activated soluble Bax further permeabilizing the membrane. Bcl-2 inhibited Bax auto-activation. We therefore propose that Bax auto-activation amplifies the initial death signal produced by BH3-only proteins and that Bcl-2 functions as an inhibitor of Bax auto-activation.

Ubiquitin charging of human class III ubiquitin-conjugating enzymes triggers their nuclear import

Ubiquitin is a small polypeptide that is conjugated to proteins and commonly serves as a degradation signal. The attachment of ubiquitin (Ub) to a substrate proceeds through a multi-enzyme cascade involving an activating enzyme (E1), a conjugating enzyme (E2), and a protein ligase (E3). We previously demonstrated that a murine E2, UbcM2, is imported into nuclei by the transport receptor importin-11. We now show that the import mechanism for UbcM2 and two other human class III E2s (UbcH6 and UBE2E2) uniquely requires the covalent attachment of Ub to the active site cysteine of these enzymes. This coupling of E2 activation and transport arises from the selective interaction of importin-11 with the Ub-loaded forms of these enzymes. Together, these findings reveal that Ub charging can function as a nuclear import trigger, and identify a novel link between E2 regulation and karyopherin-mediated transport.

Ribosomal protein L12 uses a distinct nuclear import pathway mediated by importin 11

Ribosome biogenesis requires the nuclear translocation of ribosomal proteins from their site of synthesis in the cytoplasm to the nucleus. Analyses of the import mechanisms have revealed that most ribosomal proteins can be delivered to the nucleus by multiple transport receptors (karyopherins or importins). We now provide evidence that ribosomal protein L12 (rpL12) is distinguished from the bulk of ribosomal proteins because it accesses the importin 11 pathway as a major route into the nucleus. rpL12 specifically and directly interacted with importin 11 in vitro and in vivo. Both rpL12 binding to and import by importin 11 were inhibited by another importin 11 substrate, UbcM2, indicating that these two cargoes may bind overlapping sites on the transport receptor. In contrast, the import of rpL23a, a ribosomal protein that uses the general ribosomal protein import system, was not competed by UbcM2, and in an in vitro binding assay, importin 11 did not bind to the nuclear localization signal of rpL23a. Furthermore, in a transient transfection assay, the nuclear accumulation of rpL12 was increased by coexpressed importin 11, but not by other importins. These data are consistent with importin 11 being a mediator of rpL12 nuclear import. Taken together, these results indicate that rpL12 uses a distinct nuclear import pathway that may contribute to a mechanism for regulating ribosome synthesis and/or maturation.

Importin-11, a nuclear import receptor for the ubiquitin-conjugating enzyme, UbcM2

Importins are members of a family of transport receptors (karyopherins) that mediate the nucleocytoplasmic transport of protein and RNA cargoes. We identified importin-11 as a potential new human member of this family, on the basis of limited similarity to the Saccharomyces cerevisiae protein, Lph2p, and cloned the complete open reading frame. Importin-11 interacts with the Ran GTPase, and constitutively shuttles between the nuclear and cytoplasmic compartments. A yeast dihybrid screen identified UbcM2, an E2-type ubiquitin-conjugating enzyme, as a binding partner and potential transport cargo for importin-11. Importin-11 and UbcM2 interact directly, and the complex is disassembled by Ran:GTP but not by Ran:GDP. UbcM2 is constitutively nuclear and shuttles between the nuclear and cytoplasmic compartments. Nuclear import of UbcM2 requires Ran and importin-11, and is inhibited by wheatgerm agglutinin, energy depletion or dominant interfering mutants of Ran and importin-beta. These data establish importin-11 as a new member of the karyopherin family of transport receptors, and identify UbcM2 as a nuclear member of the E2 ubiquitin-conjugating enzyme family.

Phosphorylation of simian cytomegalovirus assembly protein precursor (pAPNG.5) and proteinase precursor (pAPNG1): multiple attachment sites identified, including two adjacent serines in a casein kinase II consensus sequence

The assembly protein precursor (pAP) of cytomegalovirus (CMV), and its homologs in other herpesviruses, functions at several key steps during the process of capsid formation. This protein, and the genetically related maturational proteinase, is distinguished from the other capsid proteins by posttranslational modifications, including phosphorylation. The objective of this study was to identify sites at which pAP is phosphorylated so that the functional significance of this modification and the enzyme(s) responsible for it can be determined. In the work reported here, we used peptide mapping, mass spectrometry, and site-directed mutagenesis to identify two sets of pAP phosphorylation sites. One is a casein kinase II (CKII) consensus sequence that contains two adjacent serines, both of which are phosphorylated. The other site(s) is in a different domain of the protein, is phosphorylated less frequently than the CKII site, does not require preceding CKII-site phosphorylation, and causes an electrophoretic mobility shift when phosphorylated. Transfection/expression assays for proteolytic activity showed no gross effect of CKII-site phosphorylation on the enzymatic activity of the proteinase or on the substrate behavior of pAP. Evidence is presented that both the CKII sites and the secondary sites are phosphorylated in virus-infected cells and plasmid-transfected cells, indicating that these modifications can be made by a cellular enzyme(s). Apparent compartmental differences in phosphorylation of the CKII-site (cytoplasmic) and secondary-site (nuclear) serines suggest the involvement of more that one enzyme in these modifications.

A novel kinase activity associated with Nef derived from neurovirulent simian immunodeficiency virus

The Nef proteins of Simian immunodeficiency virus (SIV) and human immunodeficiency virus (HIV) have been shown to associate with several cellular kinases. Further, the ability of SIVmac239 Nef to associate with a p21-activated kinase (PAK)-related kinase has been correlated with pathogenic progression to AIDS in rhesus macaques. Because the ability of Nef to associate with the PAK-related kinase is viral isolate dependent, we reasoned that viral isolates derived from distinct physiological locations may encode Nef proteins that exhibit distinct kinase association profiles. In this study, we compared kinase activities associated with Nef proteins derived from the prototypic lymphocyte-tropic SIVmac239 and a macrophage-tropic, neurovirulent clone, SIV/17E-Fr. Our findings not only support previous studies that have documented the association of SIVmac239 Nef with a PAK-related kinase and a Nef-associated kinase complex (NAKC) but describe a novel serine kinase activity detectable only in conjunction with the Nef protein derived from the neurovirulent clone, SIV/17E-Fr. The latter Nef protein does not associate with PAK, and unlike PAK or NAKC, this novel kinase activity is enhanced in association with nonmyristoylated forms of Nef and can utilize both ATP and GTP as phosphodonors. We also show that at least one substrate for the kinase is Nef itself and demonstrate that the SIV/17E-Fr Nef protein is phosphorylated in SIV-infected cells. These results suggest that the ability to associate with cellular kinases in general may be a conserved feature of Nef, but particular kinase/Nef associations may evolve with changes in the host environment concomitant with viral spread.

Cytomegalovirus assembly protein precursor and proteinase precursor contain two nuclear localization signals that mediate their own nuclear translocation and that of the major capsid protein

The cytomegalovirus (CMV) assembly protein precursor (pAP) interacts with the major capsid protein (MCP), and this interaction is required for nuclear translocation of the MCP, which otherwise remains in the cytoplasm of transfected cells (L. J. Wood et al., J. Virol. 71:179-190, 1997). We have interpreted this finding to indicate that the CMV MCP lacks its own nuclear localization signal (NLS) and utilizes the pAP as an NLS-bearing escort into the nucleus. The CMV pAP amino acid sequence has two clusters of basic residues (e.g., KRRRER [NLS1] and KARKRLK [NLS2], for simian CMV) that resemble the simian virus 40 large-T-antigen NLS (D. Kalderon et al., Cell 39:499-509, 1984) and one of these (NLS1) has a counterpart in the pAP homologs of other herpesviruses. The work described here establishes that NLS1 and NLS2 are mutually independent NLS that can act (i) in cis to translocate pAP and the related proteinase precursor (pNP1) into the nucleus and (ii) in trans to transport MCP into the nucleus. By using combinations of NLS mutants and carboxy-terminal deletion constructs, we demonstrated a self-interaction of pAP and cytoplasmic interactions of pAP with pNP1 and of pNP1 with itself. The relevance of these findings to early steps in capsid assembly, the mechanism of MCP nuclear transport, and the possible cytoplasmic formation of protocapsomeric substructures is discussed.

Human cytomegalovirus capsid assembly protein precursor (pUL80.5) interacts with itself and with the major capsid protein (pUL86) through two different domains

We have used the yeast GAL4 two-hybrid system to examine interactions between the human cytomegalovirus (HCMV) major capsid protein (MCP, encoded by UL86) and the precursor assembly protein (pAP, encoded by UL80.5 and cleaved at its carboxyl end to yield AP) and found that (i) the pAP interacts with the MCP through residues located within the carboxy-terminal 21 amino acids of the pAP, called the carboxyl conserved domain (CCD); (ii) the pAP interacts with itself through a separate region, called the amino conserved domain (ACD), located between amino acids His34 and Arg52 near the amino end of the molecule; (iii) the simian CMV (SCMV) pAP and AP can interact with or replace their HCMV counterparts in these interactions, whereas the herpes simplex virus pAP and AP homologs cannot; and (iv) the HCMV and SCMV maturational proteinase precursors (ACpra, encoded by UL80a and APNG1, respectively) can interact with the pAP and MCP. The ACD and CCD amino acid sequences are highly conserved among members of the betaherpesvirus group and appear to have counterparts in the alpha- and gammaherpesvirus pAP homologs. Deleting the ACD from the HCMV pAP, or substituting Ala for a conserved Leu in the ACD, eliminated detectable pAP self-interaction and also substantially reduced MCP binding in the two-hybrid assay. This finding indicates that the pAP self-interaction influences the pAP-MCP interaction. Immunofluorescence studies corroborated the pAP-MCP interaction detected in the GAL4 two-hybrid experiments and showed that nuclear transport of the MCP was mediated by pAP but not AP. We conclude that the pAP interacts with the MCP, that this interaction is mediated by the CCD and is influenced by pAP self-interaction, and that one function of the pAP-MCP interaction may be to provide a controlled mechanism for transporting the MCP into the nucleus.