Parkin-dependent degradation of the F-box protein Fbw7β promotes neuronal survival in response to oxidative stress by stabilizing Mcl-1

Targeting Parkin-regulated metabolomic change in cartilage in the treatment of osteoarthritis

Articular cartilage degeneration may lead to osteoarthritis (OA) during the aging process, but its underlying mechanism remains unknown. Here, we found that chondrocytes exhibited an energy metabolism shift from glycolysis to oxidative phosphorylation (OXPHOS) during aging. Parkin regulates various cellular metabolic processes. Reprogrammed cartilage metabolism by Parkin ablation decreased OXPHOS and increased glycolysis, with ameliorated aging-related OA. Metabolomics analysis indicated that lauroyl-L-carnitine (LLC) was decreased in aged cartilage, but increased in Parkin-deficient cartilage. In vitro, LLC improved the cartilage matrix synthesis of aged chondrocytes. In vivo, intra-articular injection of LLC in mice with anterior cruciate ligament transaction (ACLT) ameliorated OA progression. These results suggest that metabolic changes are regulated by Parkin-impaired cartilage during aging, and targeting this metabolomic changes by supplementation with LLC is a promising treatment strategy for ameliorating OA.

Molecular insights and clinical implications for the tumor suppressor role of SCFFBXW7 E3 ubiquitin ligase

FBXW7 is one of the most well-characterized F-box proteins, serving as substrate receptor subunit of SKP1-CUL1-F-box (SCF) E3 ligase complexes. SCFFBXW7 is responsible for the degradation of various oncogenic proteins such as cyclin E, c-MYC, c-JUN, NOTCH, and MCL1. Therefore, FBXW7 functions largely as a major tumor suppressor. In keeping with this notion, FBXW7 gene mutations or downregulations have been found and reported in many types of malignant tumors, such as endometrial, colorectal, lung, and breast cancers, which facilitate the proliferation, invasion, migration, and drug resistance of cancer cells. Therefore, it is critical to review newly identified FBXW7 regulation and tumor suppressor function under physiological and pathological conditions to develop effective strategies for the treatment of FBXW7-altered cancers. Since a growing body of evidence has revealed the tumor-suppressive activity and role of FBXW7, here, we updated FBXW7 upstream and downstream signaling including FBXW7 ubiquitin substrates, the multi-level FBXW7 regulatory mechanisms, and dysregulation of FBXW7 in cancer, and discussed promising cancer therapies targeting FBXW7 regulators and downstream effectors, to provide a comprehensive picture of FBXW7 and facilitate the study in this field.

Adult-onset deletion of ATP13A2 in mice induces progressive nigrostriatal pathway dopaminergic degeneration and lysosomal abnormalities

Although most cases of Parkinson's disease (PD) are sporadic, mutations in over 20 genes are known to cause heritable forms of the disease. Recessive loss-of-function mutations in ATP13A2, a lysosomal transmembrane P5B-type ATPase and polyamine exporter, can cause early-onset familial PD. Familial ATP13A2 mutations are also linked to related neurodegenerative diseases, including Kufor-Rakeb syndrome, hereditary spastic paraplegias, neuronal ceroid lipofuscinosis, and amyotrophic lateral sclerosis. Despite the severe effects of ATP13A2 mutations in humans, ATP13A2 knockout (KO) mice fail to exhibit neurodegeneration even at advanced ages, making it challenging to study the neuropathological effects of ATP13A2 loss in vivo. Germline deletion of ATP13A2 in rodents may trigger the upregulation of compensatory pathways during embryonic development that mask the full neurotoxic effects of ATP13A2 loss in the brain. To explore this idea, we selectively deleted ATP13A2 in the adult mouse brain by the unilateral delivery of an AAV-Cre vector into the substantia nigra of young adult mice carrying conditional loxP-flanked ATP13A2 KO alleles. We observe a progressive loss of striatal dopaminergic nerve terminals at 3 and 10 months after AAV-Cre delivery. Cre-injected mice also exhibit robust dopaminergic neuronal degeneration in the substantia nigra at 10 months. Adult-onset ATP13A2 KO also recreates many of the phenotypes observed in aged germline ATP13A2 KO mice, including lysosomal abnormalities, p62-positive inclusions, and neuroinflammation. Our study demonstrates that the adult-onset homozygous deletion of ATP13A2 in the nigrostriatal pathway produces robust and progressive dopaminergic neurodegeneration that serves as a useful in vivo model of ATP13A2-related neurodegenerative diseases.

The SCF-FBW7β E3 ligase mediates ubiquitination and degradation of the serine/threonine protein kinase PINK1

Understanding the mechanisms that govern the stability of functionally crucial proteins is essential for various cellular processes, development, and overall cell viability. Disturbances in protein homeostasis are linked to the pathogenesis of neurodegenerative diseases. PTEN-induced kinase 1 (PINK1), a protein kinase, plays a significant role in mitochondrial quality control and cellular stress response, and its mutated forms lead to early-onset Parkinson's disease. Despite its importance, the specific mechanisms regulating PINK1 protein stability have remained unclear. This study reveals a cytoplasmic interaction between PINK1 and F-box and WD repeat domain-containing 7β (FBW7β) in mammalian cells. FBW7β, a component of the Skp1-Cullin-1-F-box protein complex-type ubiquitin ligase, is instrumental in recognizing substrates. Our findings demonstrate that FBW7β regulates PINK1 stability through the Skp1-Cullin-1-F-box protein complex and the proteasome pathway. It facilitates the K48-linked polyubiquitination of PINK1, marking it for degradation. When FBW7 is absent, PINK1 accumulates, leading to heightened mitophagy triggered by carbonyl cyanide 3-chlorophenylhydrazone treatment. Moreover, exposure to the toxic compound staurosporine accelerates PINK1 degradation via FBW7β, correlating with increased cell death. This study unravels the intricate mechanisms controlling PINK1 protein stability and sheds light on the novel role of FBW7β. These findings deepen our understanding of PINK1-related pathologies and potentially pave the way for therapeutic interventions.

FBXW7 tumor suppressor regulation by dualspecificity tyrosine-regulated kinase 2

FBXW7 is a member of the F-box protein family, which functions as the substrate recognition component of the SCF E3 ubiquitin ligase. FBXW7 is a main tumor suppressor due to its ability to control proteasome-mediated degradation of several oncoproteins such as c-Jun, c-Myc, Cyclin E1, mTOR, and Notch1-IC. FBXW7 inactivation in human cancers results from a somatic mutation or downregulation of its protein levels. This work describes a novel regulatory mechanism for FBXW7 dependent on the serine/threonine protein kinase DYRK2. We show that DYRK2 interacts with and phosphorylates FBXW7 resulting in its proteasome-mediated degradation. DYRK2-dependent FBXW7 destabilization is independent of its ubiquitin ligase activity. The functional analysis demonstrates the existence of DYRK2-dependent regulatory mechanisms for key FBXW7 substrates. Finally, we provide evidence indicating that DYRK2-dependent regulation of FBXW7 protein accumulation contributes to cytotoxic effects in response to chemotherapy agents such as Doxorubicin or Paclitaxel in colorectal cancer cell lines and to BET inhibitors in T-cell acute lymphoblastic leukemia cell lines. Altogether, this work reveals a new regulatory axis, DYRK2/FBXW7, which provides an understanding of the role of these two proteins in tumor progression and DNA damage responses.

Tumor suppressor p53 regulates heat shock factor 1 protein degradation in Huntington's disease

p53 and HSF1 are two major transcription factors involved in cell proliferation and apoptosis, whose dysregulation contributes to cancer and neurodegeneration. Contrary to most cancers, p53 is increased in Huntington's disease (HD) and other neurodegenerative diseases, while HSF1 is decreased. p53 and HSF1 reciprocal regulation has been shown in different contexts, but their connection in neurodegeneration remains understudied. Using cellular and animal models of HD, we show that mutant HTT stabilized p53 by abrogating the interaction between p53 and E3 ligase MDM2. Stabilized p53 promotes protein kinase CK2 alpha prime and E3 ligase FBXW7 transcription, both of which are responsible for HSF1 degradation. Consequently, p53 deletion in striatal neurons of zQ175 HD mice restores HSF1 abundance and decrease HTT aggregation and striatal pathology. Our work shows the mechanism connecting p53 stabilization with HSF1 degradation and pathophysiology in HD and sheds light on the broader molecular differences and commonalities between cancer and neurodegeneration.

The Role of Membrane-Associated E3 Ubiquitin Ligases in Cancer

The cell membrane system comprises the plasma membrane, endoplasmic reticulum, Golgi apparatus, lysosome, mitochondria, and nuclear membrane, which are essential for maintaining normal physiological functions of cells. The proteins associated with these membrane-organelles are frequently modified to regulate their functions, the most common of which is ubiquitin modification. So far, many ubiquitin E3 ligases anchored in the membrane system have been identified as critical players facilitating intracellular biofunctions whose dysfunction is highly related to cancer. In this review, we summarized membrane-associated E3 ligases and revealed their relationship with cancer, which is of great significance for discovering novel drug targets of cancer and may open up new avenues for inducing ubiquitination-mediated degradation of cancer-associated membrane proteins via small chemicals such as PROTAC and molecular glue.

FKBP51 modulates hippocampal size and function in post-translational regulation of Parkin

FK506-binding protein 51 (encoded by Fkpb51, also known as Fkbp5) has been associated with stress-related mental illness. To investigate its function, we studied the morphological consequences of Fkbp51 deletion. Artificial Intelligence-assisted morphological analysis revealed that male Fkbp51 knock-out (KO) mice possess more elongated dentate gyrus (DG) but shorter hippocampal height in coronal sections when compared to WT. Primary cultured Fkbp51 KO hippocampal neurons were shown to exhibit larger dendritic outgrowth than wild-type (WT) controls and pharmacological manipulation experiments suggest that this may occur through the regulation of microtubule-associated protein. Both in vitro primary culture and in vivo labeling support a role for FKBP51 in the regulation of microtubule-associated protein expression. Furthermore, Fkbp51 KO hippocampi exhibited decreases in βIII-tubulin, MAP2, and Tau protein levels, but a greater than 2.5-fold increase in Parkin protein. Overexpression and knock-down FKBP51 demonstrated that FKBP51 negatively regulates Parkin in a dose-dependent and ubiquitin-mediated manner. These results indicate a potential novel post-translational regulatory mechanism of Parkin by FKBP51 and the significance of their interaction on disease onset. KO has more flattened hippocampus using AI-assisted measurement Both pyramidal cell layer (PCL) of CA and granular cell layer (GCL) of DG distinguishable as two layers: deep cell layer and superficial layer. Distinct MAP2 expression between deep and superficial layer between KO and WT, Higher Parkin expression in KO brain Mechanism of FKBP51 inhibition resulting in Parkin, MAP2, Tau, and Tubulin expression differences between KO and WT mice, and resulting neurite outgrowth differences.

The regulation of neuronal autophagy and cell survival by MCL1 in Alzheimer's disease

Maintaining neuronal integrity and functions requires precise mechanisms controlling organelle and protein quality. Alzheimer's disease (AD) is characterized by functional defects in the clearance and recycling of intracellular components. As such, neuronal homeostasis involves autophagy, mitophagy, and apoptosis. Compromised activity in these cellular processes may cause pathological phenotypes of AD. Dysfunction of mitochondria is one of the hallmarks of AD. Mitophagy is a critical mitochondria quality control system, and the impaired mitophagy is observed in AD. Myeloid cell leukemia 1 (MCL1), a member of the pro-survival B-cell lymphoma protein 2 (BCL2) family, is a mitochondria-targeted protein that contributes to maintaining mitochondrial integrity. Mcl1 knockout mice display peri-implantation lethality. The studies on conditional Mcl1 knockout mice demonstrate that MCL1 plays a central role in neurogenesis and neuronal survival during brain development. Accumulating evidence reveals the critical role of MCL1 as a regulator of neuronal autophagy, mitophagy, and survival. In this review, we discuss the emerging neuroprotective function of MCL1 and how dysregulation of MCL1 signaling is involved in the pathogenesis of AD. As the pro-survival BCL2 family of proteins are promising targets of pharmacological intervention with BH3 mimetic drugs, we also discuss the promise of MCL1-targeting therapy in AD.

Myeloid Fbxw7 Prevents Pulmonary Fibrosis by Suppressing TGF-β Production

Idiopathic pulmonary fibrosis (IPF) is a group of chronic interstitial pulmonary diseases characterized by an inexorable decline in lung function with limited treatment options. The abnormal expression of transforming growth factor-β (TGF-β) in profibrotic macrophages is linked to severe pulmonary fibrosis, but the regulation mechanisms of TGF-β expression are incompletely understood. We found that decreased expression of E3 ubiquitin ligase Fbxw7 in peripheral blood mononuclear cells (PBMCs) was significantly related to the severity of pulmonary fibrosis in IPF patients. Fbxw7 is identified to be a crucial suppressing factor for pulmonary fibrosis development and progression in a mouse model induced by intratracheal bleomycin treatment. Myeloid cell-specific Fbxw7 deletion increases pulmonary monocyte-macrophages accumulation in lung tissue, and eventually promotes bleomycin-induced collagen deposition and progressive pulmonary fibrosis. Notably, the expression of TGF-β in profibrotic macrophages was significantly upregulated in myeloid cell-specific Fbxw7 deletion mice after bleomycin treatment. C-Jun has long been regarded as a critical transcription factor of Tgfb1, we clarified that Fbxw7 inhibits the expression of TGF-β in profibrotic macrophages by interacting with c-Jun and mediating its K48-linked ubiquitination and degradation. These findings provide insight into the role of Fbxw7 in the regulation of macrophages during the pathogenesis of pulmonary fibrosis.

Implications of FBXW7 in Neurodevelopment and Neurodegeneration: Molecular Mechanisms and Therapeutic Potential

The ubiquitin-proteasome system (UPS) mediated protein degradation is crucial to maintain quantitive and functional homeostasis of diverse proteins. Balanced cellular protein homeostasis controlled by UPS is fundamental to normal neurological functions while impairment of UPS can also lead to some neurodevelopmental and neurodegenerative disorders. Functioning as the substrate recognition component of the SCF-type E3 ubiquitin ligase, FBXW7 is essential to multiple aspects of cellular processes via targeting a wide range of substrates for proteasome-mediated degradation. Accumulated evidence shows that FBXW7 is fundamental to neurological functions and especially implicated in neurodevelopment and the nosogenesis of neurodegeneration. In this review, we describe general features of FBXW7 gene and proteins, and mainly present recent findings that highlight the vital roles and molecular mechanisms of FBXW7 in neurodevelopment such as neurogenesis, myelination and cerebral vasculogenesis and in the pathogenesis of some typical neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease and Huntington’s disease. Additionally, we also provide a prospect on focusing FBXW7 as a potential therapeutic target to rescue neurodevelopmental and neurodegenerative impairment.

Genes Implicated in Familial Parkinson's Disease Provide a Dual Picture of Nigral Dopaminergic Neurodegeneration with Mitochondria Taking Center Stage

The mechanism of nigral dopaminergic neuronal degeneration in Parkinson's disease (PD) is unknown. One of the pathological characteristics of the disease is the deposition of α-synuclein (α-syn) that occurs in the brain from both familial and sporadic PD patients. This paper constitutes a narrative review that takes advantage of information related to genes (SNCA, LRRK2, GBA, UCHL1, VPS35, PRKN, PINK1, ATP13A2, PLA2G6, DNAJC6, SYNJ1, DJ-1/PARK7 and FBXO7) involved in familial cases of Parkinson's disease (PD) to explore their usefulness in deciphering the origin of dopaminergic denervation in many types of PD. Direct or functional interactions between genes or gene products are evaluated using the Search Tool for the Retrieval of Interacting Genes/Proteins (STRING) database. The rationale is to propose a map of the interactions between SNCA, the gene encoding for α-syn that aggregates in PD, and other genes, the mutations of which lead to early-onset PD. The map contrasts with the findings obtained using animal models that are the knockout of one of those genes or that express the mutated human gene. From combining in silico data from STRING-based assays with in vitro and in vivo data in transgenic animals, two likely mechanisms appeared: (i) the processing of native α-syn is altered due to the mutation of genes involved in vesicular trafficking and protein processing, or (ii) α-syn mutants alter the mechanisms necessary for the correct vesicular trafficking and protein processing. Mitochondria are a common denominator since both mechanisms require extra energy production, and the energy for the survival of neurons is obtained mainly from the complete oxidation of glucose. Dopamine itself can result in an additional burden to the mitochondria of dopaminergic neurons because its handling produces free radicals. Drugs acting on G protein-coupled receptors (GPCRs) in the mitochondria of neurons may hopefully end up targeting those receptors to reduce oxidative burden and increase mitochondrial performance. In summary, the analysis of the data of genes related to familial PD provides relevant information on the etiology of sporadic cases and might suggest new therapeutic approaches.

The tumor suppressor archipelago E3 ligase is required for spermatid differentiation in Drosophila testis

The human orthologue of the tumor suppressor protein FBW7 is encoded by the Drosophila archipelago (ago) gene. Ago is an F-box protein that gives substrate specificity to its SCF ubiquitin ligase complex. It has a central role in multiple biological processes in a tissue-specific manner such as cell proliferation, cellular differentiation, hypoxia-induced gene expression. Here we present a previously unknown tissue-specific role of Ago in spermatid differentiation. We identified a classical mutant of ago which is semi-lethal and male-sterile. During the characterization of ago function in testis, we found that ago plays role in spermatid development, following meiosis. We confirmed spermatogenesis defects by silencing ago by RNAi in testes. The ago mutants show multiple abnormalities in elongating and elongated spermatids, including aberration of the cyst morphology, malformed mitochondrial structures, and individualization defects. Additionally, we determined the subcellular localization of Ago protein with mCherry-Ago transgene in spermatids. Our findings highlight the potential roles of Ago in different cellular processes of spermatogenesis, like spermatid individualization, and regulation of mitochondrial morphology.

Parkinson's Disease Master Regulators on Substantia Nigra and Frontal Cortex and Their Use for Drug Repositioning

Parkinson's disease (PD) is among the most prevalent neurodegenerative diseases. Available evidences support the view of PD as a complex disease, being the outcome of interactions between genetic and environmental factors. In face of diagnosis and therapy challenges, and the elusive PD etiology, the use of alternative methodological approaches for the elucidation of the disease pathophysiological mechanisms and proposal of novel potential therapeutic interventions has become increasingly necessary. In the present study, we first reconstructed the transcriptional regulatory networks (TN), centered on transcription factors (TF), of two brain regions affected in PD, the substantia nigra pars compacta (SNc) and the frontal cortex (FCtx). Then, we used case-control studies data from these regions to identify TFs working as master regulators (MR) of the disease, based on region-specific TNs. Twenty-nine regulatory units enriched with differentially expressed genes were identified for the SNc, and twenty for the FCtx, all of which were considered MR candidates for PD. Three consensus MR candidates were found for SNc and FCtx, namely ATF2, SLC30A9, and ZFP69B. In order to search for novel potential therapeutic interventions, we used these consensus MR candidate signatures as input to the Connectivity Map (CMap), a computational drug repositioning webtool. This analysis resulted in the identification of four drugs that reverse the expression pattern of all three MR consensus simultaneously, benperidol, harmaline, tubocurarine chloride, and vorinostat, thus suggested as novel potential PD therapeutic interventions.

Parkin, an E3 ubiquitin ligase, enhances airway mitochondrial DNA release and inflammation

INTRODUCTION

Parkin (Park2), an E3 ubiquitin ligase, is critical to maintain mitochondrial function by regulating mitochondrial biogenesis and degradation (mitophagy), but recent evidence suggests the involvement of Parkin in promoting inflammation. In the present study, we determined if Parkin regulates airway mitochondrial DNA (mtDNA) release and inflammatory responses to type 2 cytokine interleukin (IL)-13 and allergens.

METHODS

We measured Parkin mRNA expression in brushed bronchial epithelial cells and mtDNA release in the paired bronchoalveolar lavage fluid (BALF) from normal subjects and asthmatics. Parkin-deficient primary human tracheobronchial epithelial (HTBE) cells generated using the CRISPR-Cas9 system were stimulated with IL-13. To determine the in vivo function of Parkin, Parkin knockout (PKO) and wild-type (WT) mice were treated with IL-13 or allergen (house dust mite, HDM) in the presence or absence of mtDNA isolated from normal mouse lungs.

RESULTS

Parkin mRNA expression in asthmatic airway epithelium was upregulated, which positively correlated with the levels of released mtDNA in BALF. IL-13-stimulated HTBE cells increased Parkin expression. Moreover, IL-13 induced mtDNA release in Parkin-sufficient, but not in Parkin-deficient HTBE cells. PKO (vs WT) mice attenuated airway mtDNA release and inflammation following IL-13 or HDM treatments. mtDNA amplified airway inflammation in mice treated with IL-13 or HDM. Notably, Parkin also mediated mtDNA-induced exacerbation of airway inflammation.

CONCLUSION

Our research findings suggest that Parkin promotes mtDNA release and inflammation in airways, thus improving our understanding of the complex role of Parkin and mitochondrial dysfunction in asthma pathogenesis.

More than 20 years of the discovery of Park2

Discovery of Park2 is our finding of a family of young onset parkinsonism, in which this family was thought to be associated with a polymorphism of the manganese superoxide gene. The gene locus of the manganese superoxide dismutase has been known. We were able to pick up a gene for this family and related families in the close approximate position at the long arm of chromosome 6. The gene for this disease has a ubiquitin-like motif in the N-terminus and two RING finger structures. It was shown that this gene had a ubiquitin-protein ligase activity. But it is not elucidated the substrate of this enzyme. Meanwhile, it has become clear that PINK1 and Parkin work together to remove the mitochondria of the lowered membrane potential in the autophagosomes (mitophagy). Now that the molecular mechanisms of mitophagy is under investigation. In addition, many hot topics are going on such as Lewy body in Park2, single heterozygotes, rare clinical manifestations, and so on.

Ehrlichia chaffeensis TRP120-mediated ubiquitination and proteasomal degradation of tumor suppressor FBW7 increases oncoprotein stability and promotes infection

Ehrlichia chaffeensis (E. chaffeensis) exploits evolutionarily conserved Notch and Wnt host cell signaling pathways to downregulate innate immune host defenses and promote infection. The multifunctional E. chaffeensis TRP120 effector which has HECT E3 ubiquitin ligase activity, interacts with the host nuclear tumor suppressor F-BOX and WD domain repeating-containing 7 (FBW7). FBW7 is the substrate recognition subunit of the Skp1-cullin-1-FBOX E3 ubiquitin (Ub) ligase complex (SCF) known to negatively regulate a network of oncoproteins (Notch, cyclin E, c-Jun, MCL1 and cMYC). In this study, we demonstrate that TRP120 and FBW7 colocalize strongly in the nucleus by confocal immunofluorescent microscopy and interactions between TRP120 and FBW7 FBOX and WD40 domains were demonstrated by ectopic expression and co-immunoprecipitation. Although FBW7 gene expression increased during E. chaffeensis infection, FBW7 levels significantly decreased (>70%) by 72 h post infection. Moreover, an iRNA knockdown of FBW7 coincided with increased E. chaffeensis infection and levels of Notch intracellular domain (NICD), phosphorylated c-Jun, MCL-1 and cMYC, which are negatively regulated by FBW7. An increase in FBW7 K48 ubiquitination was detected during infection by co-IP, and FBW7 degradation was inhibited in infected cells treated with the proteasomal inhibitor bortezomib. Direct TRP120 ubiquitination of native and recombinant FBW7 was demonstrated in vitro and confirmed by ectopic expression of TRP120 HECT Ub ligase catalytic site mutant. This study identifies the tumor suppressor, FBW7, as a TRP120 HECT E3 Ub ligase substrate, and demonstrates that TRP120 ligase activity promotes ehrlichial infection by degrading FBW7 to maintain stability of Notch and other oncoproteins involved in cell survival and apoptosis.

MCL1 as a Therapeutic Target in Parkinson's Disease?

Dopamine neurons in the substantia nigra (SN) pars compacta are selectively lost during the progression of Parkinson's disease (PD). Recent work performed on the role of the Bcl2 family (highly specialized proteins which control cellular survival and death) in midbrain dopamine neurons has led to the identification of the Bcl2 factor Mcl1 as a weak link in the survival of these neurons. We hypothesize that the regulation of BCL2 proteins may explain this selective vulnerability, and may even provide a novel therapeutic opportunity - strengthening weak links such as MCL1 could result in a delay or complete abrogation of cell death during PD.

Carbon monoxide ameliorates acetaminophen-induced liver injury by increasing hepatic HO-1 and Parkin expression

Acetaminophen (APAP) is widely used as an antifebrile and analgesic drug at recommended doses, whereas an overdose of APAP can cause severe liver damage. The molecular mechanisms underlying APAP-induced liver damage remain incompletely understood. Carbon monoxide (CO), an end-product of heme oxygenase (HO)-1 activity, can confer anti-inflammatory and antiapoptotic properties in cellular models of toxicity via regulation of mitochondrial function. The objective of this study was to evaluate the effects of CO on APAP-induced hepatotoxicity and CO's relationship to regulation of endoplasmic reticulum (ER) stress and mitochondrial signaling using CO-releasing molecules or low concentrations of CO applied as pretreatment or posttreatment. Using genetic deletion or knockdown approaches in alpha mouse liver cells or primary hepatocytes, respectively, we investigated the role of HO-1 and the mitophagy regulator protein Parkin on APAP-induced expression of the ER stress-associated apoptosis regulator cytosine-cytosine-adenosine-adenosine-thymidine (CCAAT)/enhancer-binding protein homologous protein (CHOP). We found that CO induced Parkin expression in hepatocytes via the protein kinase RNA-like ER kinase/eukaryotic translation initiation factor 2-α/activating transcription factor-4 signaling pathway. Additionally, CO gas inhalation significantly alleviated APAP-induced liver damage in vivo and correspondingly reduced serum alanine aminotransferase and aspartate aminotransferase levels as well as proinflammatory cytokines and reduced the expression of CHOP in liver tissues while dramatically increasing hepatic HO-1 and Parkin expression. We found that the protective effects of CO on APAP-induced liver damage were mediated by down-regulation of CHOP at a transcriptional and post-translational level via induction of HO-1 and Parkin, respectively, and associated with decreases in reactive oxygen species production and JNK phosphorylation. We conclude that CO may represent a promising therapeutic agent for APAP-induced liver injury.-Chen, Y., Park, H.-J., Park, J., Song, H.-C., Ryter, S. W., Surh, Y.-J., Kim, U.-H., Joe, Y., Chung, H. T. Carbon monoxide ameliorates acetaminophen-induced liver injury by increasing hepatic HO-1 and Parkin expression.

Stress-induced phospho-ubiquitin formation causes parkin degradation

Mutations in the E3 ubiquitin ligase parkin are the most common known cause of autosomal recessive Parkinson’s disease (PD), and parkin depletion may play a role in sporadic PD. Here, we sought to elucidate the mechanisms by which stress decreases parkin protein levels using cultured neuronal cells and the PD-relevant stressor, L-DOPA. We find that L-DOPA causes parkin loss through both oxidative stress-independent and oxidative stress-dependent pathways. Characterization of the latter reveals that it requires both the kinase PINK1 and parkin’s interaction with phosphorylated ubiquitin (phospho-Ub) and is mediated by proteasomal degradation. Surprisingly, autoubiquitination and mitophagy do not appear to be required for such loss. In response to stress induced by hydrogen peroxide or CCCP, parkin degradation also requires its association with phospho-Ub, indicating that this mechanism is broadly generalizable. As oxidative stress, metabolic dysfunction and phospho-Ub levels are all elevated in PD, we suggest that these changes may contribute to a loss of parkin expression.

FBXO45-MYCBP2 regulates mitotic cell fate by targeting FBXW7 for degradation

Cell fate decision upon prolonged mitotic arrest induced by microtubule-targeting agents depends on the activity of the tumor suppressor and F-box protein FBXW7. FBXW7 promotes mitotic cell death and prevents premature escape from mitosis through mitotic slippage. Mitotic slippage is a process that can cause chemoresistance and tumor relapse. Therefore, understanding the mechanisms that regulate the balance between mitotic cell death and mitotic slippage is an important task. Here we report that FBXW7 protein levels markedly decline during extended mitotic arrest. FBXO45 binds to a conserved acidic N-terminal motif of FBXW7 specifically under a prolonged delay in mitosis, leading to ubiquitylation and subsequent proteasomal degradation of FBXW7 by the FBXO45-MYCBP2 E3 ubiquitin ligase. Moreover, we find that FBXO45-MYCBP2 counteracts FBXW7 in that it promotes mitotic slippage and prevents cell death in mitosis. Targeting this interaction represents a promising strategy to prevent chemotherapy resistance.

Fbxw7 increases CCL2/7 in CX3CR1hi macrophages to promote intestinal inflammation

Resident and inflammatory mononuclear phagocytes (MPh) with functional plasticity in the intestine are critically involved in the pathology of Inflammatory Bowel Diseases (IBD), in which the mechanism remains incompletely understood. In the present study, we found that increased expression of E3 ligase FBXW7 in the inflamed intestine was significantly correlated to IBD severity in both human diseases and mice model. Myeloid-Fbxw7 deficiency protected mice from dextran sodium sulfate (DSS) and 2,6,4-trinitrobenzene sulfonic acid (TNBS) induced colitis. Fbxw7 deficiency resulted in decreased production of chemokines CCL2 and CCL7 by colonic CX3CR1hi resident macrophages and reduced accumulation of CX3CR1int pro-inflammatory MPh in colitis colon tissue. Mice received AAV-shFbxw7 administration showed significantly improved survival rate and alleviated colitis. Mechanisms screening demonstrated that FBXW7 suppresses H3K27me3 modification and promotes Ccl2 and Ccl7 expression via degradation of histone-lysine N-methyltransferase EZH2 in macrophages. Taken together, our results indicate that FBXW7 degrades EZH2 and increases Ccl2/Ccl7 in CX3CR1hi macrophages, which promotes the recruiting CX3CR1int pro-inflammatory MPh into local colon tissues with colitis. Targeting FBXW7 might represent a potential therapeutic approach for intestine inflammation intervention.

E3 ligase Fbw7 participates in oxidative stress‑induced myocardial cell injury via interacting with Mcl‑1

Oxidative stress participates in several heart diseases and is an important mechanism contributing to the pathological alterations of myocardial cell injury. In recent years, ubiquitylation has been demonstrated to be an important biochemical reaction associated with apoptosis. To investigate the effects and interactions of the E3 ligase F-box and WD repeat domain containing 7 (Fbw7) and MCL1 apoptosis regulator, BCL2 family member (Mcl-1) in myocardial cells during oxidative stress, Cell Counting Kit-8, flow cytometry, western blot, reactive oxygen species and co-immunoprecipitation assays were conducted. The current study revealed that Fbw7 may facilitate apoptosis via the Mcl-1-Bax pathway in oxidative stress-induced myocardial H9c2 cell injury. Mcl-1 inhibits the functions of Bcl-2 family members, including the mitochondrial apoptosis factor Bax, to maintain cell viability; however, the present study suggested that Fbw7 may degrade Mcl-1 and impaired this process. Therefore, it may be hypothesized that Fbw-7 promotes myocardial cell injury via interacting with Mcl-1.

Oncostatin M, an Underestimated Player in the Central Nervous System

For a long time, the central nervous system (CNS) was believed to be an immune privileged organ. In the last decades, it became apparent that the immune system interacts with the CNS not only in pathological, but also in homeostatic situations. It is now clear that immune cells infiltrate the healthy CNS as part of immune surveillance and that immune cells communicate through cytokines with CNS resident cells. In pathological conditions, an enhanced infiltration of immune cells takes place to fight the pathogen. A well-known family of cytokines is the interleukin (IL)-6 cytokine family. All members are important in cell communication and cell signaling in the immune system. One of these members is oncostatin M (OSM), for which the receptor is expressed on several cells of the CNS. However, the biological function of OSM in the CNS is not studied in detail. Here, we briefly describe the general aspects related to OSM biology, including signaling and receptor binding. Thereafter, the current understanding of OSM during CNS homeostasis and pathology is summarized.

Reducing Mcl-1 gene dosage induces dopaminergic neuronal loss and motor impairments in Park2 knockout mice

Mutations in the PARK2 gene are associated with early onset Parkinsonism. The Park2 -/- mouse, however, does not exhibit neurodegeneration or other Parkinson's disease (PD) phenotypes. Previously, we discovered that translation of Mcl-1, a pro-survival factor, is upregulated in the Park2 -/- mouse, suggesting a compensatory mechanism during development. Here we generated the Park2 -/- Mcl-1 +/- mouse and show that by reducing Mcl-1 gene dosage by 50%, the Park2 -/- genotype is sensitized, conferring both dopaminergic neuron loss and motor impairments. We propose that this murine model could be a useful tool for dissecting PD etiology and developing treatment strategies against this neurodegenerative disease.

F-box/WD-40 repeat-containing protein 7: A potential target in the progression and treatment of gastrointestinal malignancy

Cancer is a principal cause of human morbidity and mortality, with gastrointestinal malignancies, in particular, resulting in a marked number of tumor-associated mortalities. The progression of gastrointestinal malignancy is regulated by a variety of aberrantly expressed proteins, a number of which facilitate tumor progression, whereas, others function as tumor suppressors. The detection of such proteins not only contributes to the early diagnosis of cancer, they may additionally serve as potential therapeutic targets. In normal tissues, numerous proteins encoded by proto-oncoproteins are degraded by ubiquitylation enzymes, consisting of F-box/WD-40 repeat-containing protein 7 (Fbw7) and other proteins, thus avoiding tumorigenesis and maintaining homeostasis. In tumor tissues, the downregulation of Fbw7, caused by various factors, leads to disorders in ubiquitinase synthesis, which may induce tumor progression and chemoresistance, particularly in gastrointestinal malignancy. Therefore, an in-depth study of the regulatory mechanisms involved in disorders of Fbw7 expression and the role of Fbw7 in chemoresistance of gastrointestinal tumors may suggest improvements for early diagnostic screening and targeted therapy.

Molecular control of chaperone-mediated autophagy

Chaperone-mediated autophagy (CMA) is a selective form of autophagy in which cytosolic proteins bearing a pentapeptide motif biochemically related to the KFERQ sequence, are recognized by the heat shock protein family A member 8 (HSPA8) chaperone, delivered to the lysomal membrane, and directly translocated across the lysosomal membrane by a protein complex containing lysosomal associated membrane protein 2a (Lamp2a). Since its discovery over two decades ago, the importance of this pathway in cell proteostasis has been made increasingly apparent. Deregulation of this pathway has been implicated in a variety of diseases and conditions, including lysosomal storage diseases, cancer, neurodegeneration and even aging. Here, we describe the main molecular features of the pathway, its regulation, cross-talk with other degradation pathways and importance in disease.

CKS Proteins Promote Checkpoint Recovery by Stimulating Phosphorylation of Treslin

CKS proteins are small (9-kDa) polypeptides that bind to a subset of the cyclin-dependent kinases. The two paralogs expressed in mammals, Cks1 and Cks2, share an overlapping function that is essential for early development. However, both proteins are frequently overexpressed in human malignancy. It has been shown that CKS protein overexpression overrides the replication stress checkpoint, promoting continued origin firing. This finding has led to the proposal that CKS protein-dependent checkpoint override allows premalignant cells to evade oncogene stress barriers, providing a causal link to oncogenesis. Here, we provide mechanistic insight into how overexpression of CKS proteins promotes override of the replication stress checkpoint. We show that CKS proteins greatly enhance the ability of Cdk2 to phosphorylate the key replication initiation protein treslin in vitro Furthermore, stimulation of treslin phosphorylation does not occur by the canonical adapter mechanism demonstrated for other substrates, as cyclin-dependent kinase (CDK) binding-defective mutants are capable of stimulating treslin phosphorylation. This effect is recapitulated in vivo, where silencing of Cks1 and Cks2 decreases treslin phosphorylation, and overexpression of wild-type or CDK binding-defective Cks2 prevents checkpoint-dependent dephosphorylation of treslin. Finally, we provide evidence that the role of CKS protein-dependent checkpoint override involves recovery from checkpoint-mediated arrest of DNA replication.

The stability of Fbw7α in M-phase requires its phosphorylation by PKC

Fbw7 is a tumor suppressor often deleted or mutated in human cancers. It serves as the substrate-recruiting subunit of a SCF ubiquitin ligase that targets numerous critical proteins for degradation, including oncoproteins and master transcription factors. Cyclin E was the first identified substrate of the SCF^Fbw7 ubiquitin ligase. In human cancers bearing FBXW7-gene mutations, deregulation of cyclin E turnover leads to its aberrant expression in mitosis. We investigated Fbw7 regulation in Xenopus eggs, which, although arrested in a mitotic-like phase, naturally express high levels of cyclin E. Here, we report that Fbw7α, the only Fbw7 isoform detected in eggs, is phosphorylated by PKC (protein kinase C) at a key residue (S18) in a manner coincident with Fbw7α inactivation. We show that this PKC-dependent phosphorylation and inactivation of Fbw7α also occurs in mitosis during human somatic cell cycles, and importantly is critical for Fbw7α stabilization itself upon nuclear envelope breakdown. Finally, we provide evidence that S18 phosphorylation, which lies within the intrinsically disordered N-terminal region specific to the α-isoform reduces the capacity of Fbw7α to dimerize and to bind cyclin E. Together, these findings implicate PKC in an evolutionarily-conserved pathway that aims to protect Fbw7α from degradation by keeping it transiently in a resting, inactive state.

Twenty years since the discovery of the parkin gene

Nearly 20 years have passed since we identified the causative gene for a familial Parkinson's disease, parkin (now known as PARK2), in 1998. PARK2 is the most common gene responsible for young-onset Parkinson's disease. It codes for the protein Parkin RBR E3 ubiquitin-protein ligase (PARK2), which directly links to the ubiquitin-proteasome as a ubiquitin ligase. PARK2 is involved in mitophagy, which is a type of autophagy, in collaboration with PTEN-induced putative kinase 1 (PINK1). The PINK1 gene (previously known as PARK6) is also a causative gene for young-onset Parkinson's disease. Both gene products may be involved in regulating quality control within the mitochondria. The discovery of PARK2 as a cause of young-onset Parkinson's disease has had a major impact on other neurodegenerative diseases. The involvement of protein degradation systems has been implicated as a common mechanism for neurodegenerative diseases in which inclusion body formation is observed. The discovery of the involvement of PARK2 in Parkinson's disease focused attention on the involvement of protein degradation systems in neurodegenerative diseases. In this review, we focus on the history of the discovery of PARK2, the clinical phenotypes of patients with PARK2 mutations, and its functional roles.

Cell cycle-dependent degradation of the methyltransferase SETD3 attenuates cell proliferation and liver tumorigenesis

Histone modifications, including lysine methylation, are epigenetic marks that influence many biological pathways. Accordingly, many methyltransferases have critical roles in various biological processes, and their dysregulation is often associated with cancer. However, the biological functions and regulation of many methyltransferases are unclear. Here, we report that a human homolog of the methyltransferase SET (SU(var), enhancer of zeste, and trithorax) domain containing 3 (SETD3) is cell cycle-regulated; SETD3 protein levels peaked in S phase and were lowest in M phase. We found that the β-isoform of the tumor suppressor F-box and WD repeat domain containing 7 (FBXW7β) specifically mediates SETD3 degradation. Aligning the SETD3 sequence with those of well known FBXW7 substrates, we identified six potential non-canonical Cdc4 phosphodegrons (CPDs), and one of them, CPD1, is primarily phosphorylated by the kinase glycogen synthase kinase 3 (GSK3β), which is required for FBXW7β-mediated recognition and degradation. Moreover, depletion or inhibition of GSK3β or FBXW7β resulted in elevated SETD3 levels. Mutations of the phosphorylated residues in CPD1 of SETD3 abolished the interaction between FBXW7β and SETD3 and prevented SETD3 degradation. Our data further indicated that SETD3 levels positively correlated with cell proliferation of liver cancer cells and liver tumorigenesis in a xenograft mouse model, and that overexpression of FBXW7β counteracts the SETD3's tumorigenic role. We also show that SETD3 levels correlate with cancer malignancy, indicated by SETD3 levels that the 54 liver tumors are 2-fold higher than those in the relevant adjacent tissues. Collectively, these data elucidated that a GSK3β-FBXW7β-dependent mechanism controls SETD3 protein levels during the cell cycle and attenuates its oncogenic role in liver tumorigenesis.

Parkin mediates neuroprotection through activation of Notch1 signaling

Parkin, an E3 ubiquitin ligase, is the most frequently mutated gene in hereditary Parkinson's disease. Inactivation of Parkin leads to impairment of the ubiquitin-proteasome system, resulting in the accumulation of misfolded or aggregated proteins and ensuing neurodegeneration. In this study, we show that Parkin positively regulates the Notch1 signaling pathway. Overexpression of Parkin stabilized Notch1-IC protein levels, whereas knockdown of Parkin decreased Notch1-IC protein stability. Notably, overexpression of Parkin disrupted oxidative stress-induced apoptosis in neuronal cells. However, knockdown of Notch1 inhibited Parkin-induced neuronal cell survival. Together, these results indicate that Parkin is a novel regulator of the Notch1 signaling pathway, which promotes neuronal cell survival.

In Situ Peroxidase Labeling and Mass-Spectrometry Connects Alpha-Synuclein Directly to Endocytic Trafficking and mRNA Metabolism in Neurons

Synucleinopathies, including Parkinson's disease (PD), are associated with the misfolding and mistrafficking of alpha-synuclein (α-syn). Here, using an ascorbate peroxidase (APEX)-based labeling method combined with mass spectrometry, we defined a network of proteins in the immediate vicinity of α-syn in living neurons to shed light on α-syn function. This approach identified 225 proteins, including synaptic proteins, proteins involved in endocytic vesicle trafficking, the retromer complex, phosphatases and mRNA binding proteins. Many were in complexes with α-syn, and some were encoded by genes known to be risk factors for PD and other neurodegenerative diseases. Endocytic trafficking and mRNA translation proteins within this spatial α-syn map overlapped with genetic modifiers of α-syn toxicity, developed in an accompanying study (Khurana et al., this issue of Cell Systems). Our data suggest that perturbation of these particular pathways is directly related to the spatial localization of α-syn within the cell. These approaches provide new avenues to systematically examine protein function and pathology in living cells.

GSK3B-mediated phosphorylation of MCL1 regulates axonal autophagy to promote Wallerian degeneration

The pathophysiological function and induction mechanism of autophagy in neuronal axons have remained unclear. Wakatsuki et al. show that the GSK3B-mediated phosphorylation of MCL1 leads to its UPS-dependent degradation, which induces axonal autophagy and promotes axonal degeneration.

Macroautophagy is a catabolic process, in which portions of cytoplasm or organelles are delivered to lysosomes for degradation. Emerging evidence has indicated a pathological connection between axonal degeneration and autophagy. However, the physiological function and induction mechanism of autophagy in axons remain elusive. We herein show that, through activation of BECLIN1, glycogen synthase kinase 3B (GSK3B)–mediated phosphorylation of BCL2 family member MCL1 induces axonal autophagy and axonal degeneration. Phosphorylated MCL1 is ubiquitinated by the FBXW7 ubiquitin ligase and degraded by the proteasome, thereby releasing BECLIN1 to induce axonal autophagy. Axonal autophagy contributes to local adenosine triphosphate production in degenerating axons and the exposure of phosphatidylserine—an “eat-me” signal for phagocytes—on transected axons and is required for normal recruitment of phagocytes to axonal debris in vivo. These results suggest that GSK3B–MCL1 signaling to regulate autophagy might be important for the successful completion of Wallerian degeneration.

Parkin promotes proteasomal degradation of misregulated BAX

The pro-apoptotic BCL-2 protein BAX commits human cells to apoptosis by permeabilizing the outer mitochondrial membrane. BAX activation has been suggested to require the separation of helix α5 from α6 and thus the ‘latch’ from the ‘core’ domain among other conformational changes. Here we show that conformational changes in this region impair BAX translocation to the mitochondria and retrotranslocation back into the cytosol and therefore BAX inhibition, but not activation. Redirecting misregulated BAX to the mitochondria revealed an alternative mechanism of BAX inhibition. The E3 ligase Parkin, known to trigger mitochondria-specific autophagy, ubiquitinates BAX K128 and targets the pro-apoptotic BCL-2 protein for proteasomal degradation. Retrotranslocation-deficient BAX is completely degraded in a Parkin-dependent manner. Although only a minor pool of endogenous BAX escapes retrotranslocation into the cytosol, Parkin-dependent targeting of misregulated BAX on the mitochondria provides substantial protection against BAX apoptotic activity.

Parkin Overexpression Ameliorates PrP106-126-Induced Neurotoxicity via Enhanced Autophagy in N2a Cells

Transmissible spongiform encephalopathies (TSEs) are caused by the accumulation of the abnormal prion protein scrapie (PrP^Sc). Prion protein aggregation, misfolding, and cytotoxicity in the brain are the major causes of neuronal dysfunction and ultimate neurodegeneration in all TSEs. Parkin, an E3 ubiquitin ligase, has been studied extensively in all major protein misfolding aggregating diseases, especially Parkinson's disease and Alzheimer's disease, but the role of parkin in TSEs remains unknown. Here we investigated the role of parkin in a prion disease cell model in which neuroblastoma2a (N2a) cells were treated with prion peptide PrP106-126. We observed a gradual decrease in the soluble parkin level upon treatment with PrP106-126 in a time-dependent manner. Furthermore, endogenous parkin colocalized with FITC-tagged prion fragment106-126. Overexpression of parkin in N2a cells via transfection repressed apoptosis by enhancing autophagy. Parkin-overexpressing cells also showed reductions in apoptotic BAX translocation to the mitochondria and cytochrome c release to the cytosol, which ultimately inhibited activation of proapoptotic caspases. Taken together, our findings reveal a parkin-mediated cytoprotective mechanism against PrP106-126 toxicity, which is a novel potential therapeutic target for treating prion diseases.

Constitutive Activation of PINK1 Protein Leads to Proteasome-mediated and Non-apoptotic Cell Death Independently of Mitochondrial Autophagy

Phosphatase and tensin homolog-induced putative kinase 1 (PINK1), a Ser/Thr kinase, and PARKIN, a ubiquitin ligase, are causal genes for autosomal recessive early-onset parkinsonism. Multiple lines of evidence indicate that PINK1 and PARKIN cooperatively control the quality of the mitochondrial population via selective degradation of damaged mitochondria by autophagy. Here, we report that PINK1 and PARKIN induce cell death with a 12-h delay after mitochondrial depolarization, which differs from the time profile of selective autophagy of mitochondria. This type of cell death exhibited definite morphologic features such as plasma membrane rupture, was insensitive to a pan-caspase inhibitor, and did not involve mitochondrial permeability transition. Expression of a constitutively active form of PINK1 caused cell death in the presence of a pan-caspase inhibitor, irrespective of the mitochondrial membrane potential. PINK1-mediated cell death depended on the activities of PARKIN and proteasomes, but it was not affected by disruption of the genes required for autophagy. Furthermore, fluorescence and electron microscopic analyses revealed that mitochondria were still retained in the dead cells, indicating that PINK1-mediated cell death is not caused by mitochondrial loss. Our findings suggest that PINK1 and PARKIN play critical roles in selective cell death in which damaged mitochondria are retained, independent of mitochondrial autophagy.

Parkin Regulation and Neurodegenerative Disorders

Parkin is a unique, multifunctional ubiquitin ligase whose various roles in the cell, particularly in neurons, are widely thought to be protective. The pivotal role that Parkin plays in maintaining neuronal survival is underscored by our current recognition that Parkin dysfunction represents not only a predominant cause of familial parkinsonism but also a formal risk factor for the more common, sporadic form of Parkinson’s disease (PD). Accordingly, keen research on Parkin over the past decade has led to an explosion of knowledge regarding its physiological roles and its relevance to PD. However, our understanding of Parkin is far from being complete. Indeed, surprises emerge from time to time that compel us to constantly update the paradigm of Parkin function. For example, we now know that Parkin’s function is not confined to mere housekeeping protein quality control (QC) roles but also includes mitochondrial homeostasis and stress-related signaling. Furthermore, emerging evidence also suggest a role for Parkin in several other major neurodegenerative diseases including Alzheimer’s disease (AD) and Amyotrophic Lateral Sclerosis (ALS). Yet, it remains truly amazing to note that a single enzyme could serve such multitude of functions and cellular roles. Clearly, its activity has to be tightly regulated. In this review, we shall discuss this and how dysregulated Parkin function may precipitate neuronal demise in various neurodegenerative disorders.

Regulating Fbw7 on the road to cancer

The F-box protein Fbw7 targets for degradation critical cellular regulators, thereby controlling essential processes in cellular homeostasis, including cell cycle, differentiation and apoptosis. Most Fbw7 substrates are strongly associated with tumorigenesis and Fbw7 can either suppress or promote tumor development in mouse models. Fbw7 activity is controlled at different levels, resulting in specific and tunable regulation of the abundance and activity of its substrates. Here we highlight recent studies on the role of Fbw7 in controlling tumorigenesis and on the mechanisms that modulate Fbw7 function.

Fbw7 and its counteracting forces in stem cells and cancer: Oncoproteins in the balance

Fbw7 is well characterised as a stem cell regulator and tumour suppressor, powerfully positioned to control proliferation, differentiation and apoptosis by targeting key transcription factors for ubiquitination and destruction. Evidence in support of these roles continues to accumulate from in vitro studies, mouse models and human patient data. Here we summarise the latest of these findings, highlighting the tumour-suppressive role of Fbw7 in multiple tissues, and the rare circumstances where Fbw7 activity can be oncogenic. We discuss mechanisms that regulate ubiquitination by Fbw7, including ubiquitin-specific proteases such as USP28 that counteract Fbw7 activity and thereby stabilise oncoproteins. Deubiquitination of key Fbw7 substrates to prevent their destruction is beginning to be appreciated as an important pro-tumourigenic mechanism. As the ubiquitin-proteasome system represents a largely untapped field for drug development, the interplay between Fbw7 and its counterpart deubiquitinating enzymes in tumours is likely to attract increasing interest and influence future treatment strategies.

Oncostatin M-dependent Mcl-1 induction mediated by JAK1/2-STAT1/3 and CREB contributes to bioenergetic improvements and protective effects against mitochondrial dysfunction in cortical neurons

Oncostatin M (OSM), a cytokine in the interleukin-6 (IL-6) family, has been proposed to play a protective role in the central nervous system, such as attenuation of excitotoxicity induced by N-methyl-D-aspartate (NMDA) and glutamate. However, the potential neuroprotective effects of OSM against mitochondrial dysfunction have never been reported. In the present study, we tested the hypothesis that OSM may confer neuronal resistance against 3-nitropropionic acid (3-NP), a plant toxin that irreversibly inhibits the complex II of the mitochondrial electron transport chain, and characterized the underlying molecular mechanisms. We found that OSM preconditioning dose- and time-dependently protected cortical neurons against 3-NP toxicity. OSM stimulated expression of myeloid cell leukemia-1 (Mcl-1), an anti-apoptotic Bcl-2 family member expressed in differentiating myeloid cells, that required prior phosphorylation of Janus kinase-1 (JAK1), JAK2, extracellular signal-regulated kinase-1/2 (ERK1/2), signal transducer and activator of transcription-3 (STAT3), STAT1, and cAMP-response element-binding protein (CREB). Pharmacological inhibitors of JAK1, JAK2, ERK1/2, STAT3, STAT1, and CREB as well as the siRNA targeting at STAT3 and Mcl-1 all abolished OSM-dependent 3-NP resistance. Finally, OSM-dependent Mcl-1 induction contributed to the enhancements of mitochondrial bioenergetics including increases in spare respiratory capacity and ATP production. In conclusion, our findings indicated that OSM induces Mcl-1 expression via activation of ERK1/2, JAK1/2, STAT1/3, and CREB; furthermore, OSM-mediated Mcl-1 induction contributes to bioenergetic improvements and neuroprotective effects against 3-NP toxicity in cortical neurons. OSM may thus serve as a novel neuroprotective agent against mitochondrial dysfunction commonly associated with pathogenic mechanisms underlying neurodegeneration.

Molecular mechanisms of ageing and related diseases

Human and other multicellular life species age, and ageing processes become dominant during the late phase of life. Recent studies challenge this dogma, suggesting that ageing does not occur in some animal species. In mammals, cell replicative senescence occurs as early as before birth (i.e. in embryos) under physiological conditions. How the molecular machinery operates and why ageing cells dominate under some circumstances are intriguing questions. Recent studies show that cell ageing involves extensive cellular remodelling, including telomere attrition, heterochromatin formation, endoplasmic reticulum stress, mitochondrial disorders and lysosome processing organelles and chromatins. This article provides an update on the molecular mechanisms underlying the ageing of various cell types, the newly described developmental and programmed replicative senescence and the critical roles of cellular organelles and effectors in Parkinson's disease, diabetes, hypertension and dyskeratosis congenita.

Both ubiquitin ligases FBXW8 and PARK2 are sequestrated into insolubility by ATXN2 PolyQ expansions, but only FBXW8 expression is dysregulated

The involvement of the ubiquitin-proteasome system (UPS) in the course of various age-associated neurodegenerative diseases is well established. The single RING finger type E3 ubiquitin-protein ligase PARK2 is mutated in a Parkinson’s disease (PD) variant and was found to interact with ATXN2, a protein where polyglutamine expansions cause Spinocerebellar ataxia type 2 (SCA2) or increase the risk for Levodopa-responsive PD and for the motor neuron disease Amyotrophic lateral sclerosis (ALS). We previously reported evidence for a transcriptional induction of the multi-subunit RING finger Skp1/Cul/F-box (SCF) type E3 ubiquitin-protein ligase complex component FBXW8 in global microarray profiling of ATXN2-expansion mouse cerebellum and demonstrated its role for ATXN2 degradation in vitro. Now, we documented co-localization in vitro and co-immunoprecipitations both in vitro and in vivo, which indicate associations of FBXW8 with ATXN2 and PARK2. Both FBXW8 and PARK2 proteins are driven into insolubility by expanded ATXN2. Whereas the FBXW8 transcript upregulation by ATXN2- expansion was confirmed also in qPCR of skin fibroblasts and blood samples of SCA2 patients, a FBXW8 expression dysregulation was not observed in ATXN2-deficient mice, nor was a PARK2 transcript dysregulation observed in any samples. Jointly, all available data suggest that the degradation of wildtype and mutant ATXN2 is dependent on FBXW8, and that ATXN2 accumulation selectively modulates FBXW8 levels, while PARK2 might act indirectly through FBXW8. The effects of ATXN2-expansions on FBXW8 expression in peripheral tissues like blood may become useful for clinical diagnostics.

Parkin structure and function

Mutations in the parkin or PINK1 genes are the leading cause of the autosomal recessive form of Parkinson’s disease. The gene products, the E3 ubiquitin ligase parkin and the serine/threonine kinase PINK1, are neuroprotective proteins, which act together in a mitochondrial quality control pathway. Here, we review the structure of parkin and mechanisms of its autoinhibition and function as a ubiquitin ligase. We present a model for the recruitment and activation of parkin as a key regulatory step in the clearance of depolarized or damaged mitochondria by autophagy (mitophagy). We conclude with a brief overview of other functions of parkin and considerations for drug discovery in the mitochondrial quality control pathway.

Chronic Deletion and Acute Knockdown of Parkin Have Differential Responses to Acetaminophen-induced Mitophagy and Liver Injury in Mice

We previously demonstrated that pharmacological induction of autophagy protected against acetaminophen (APAP)-induced liver injury in mice by clearing damaged mitochondria. However, the mechanism for removal of mitochondria by autophagy is unknown. Parkin, an E3 ubiquitin ligase, has been shown to be required for mitophagy induction in cultured mammalian cells following mitochondrial depolarization, but its role in vivo is not clear. The purpose of this study was to investigate the role of Parkin-mediated mitophagy in protection against APAP-induced liver injury. We found that Parkin translocated to mitochondria in mouse livers after APAP treatment followed by mitochondrial protein ubiquitination and mitophagy induction. To our surprise, we found that mitophagy still occurred in Parkin knock-out (KO) mice after APAP treatment based on electron microscopy analysis and Western blot analysis for some mitochondrial proteins, and Parkin KO mice were protected against APAP-induced liver injury compared with wild type mice. Mechanistically, we found that Parkin KO mice had decreased activated c-Jun N-terminal kinase (JNK), increased induction of myeloid leukemia cell differentiation protein (Mcl-1) expression, and increased hepatocyte proliferation after APAP treatment in their livers compared with WT mice. In contrast to chronic deletion of Parkin, acute knockdown of Parkin in mouse livers using adenovirus shRNA reduced mitophagy and Mcl-1 expression but increased JNK activation after APAP administration, which exacerbated APAP-induced liver injury. Therefore, chronic deletion (KO) and acute knockdown of Parkin have differential responses to APAP-induced mitophagy and liver injury in mice.