Mutant ubiquitin found in Alzheimer's disease causes neuritic beading of mitochondria in association with neuronal degeneration

Modulation of neural gene networks by estradiol in old rhesus macaque females

The postmenopausal decrease in circulating estradiol (E2) levels has been shown to contribute to several adverse physiological and psychiatric effects. To elucidate the molecular effects of E2 on the brain, we examined differential gene expression and DNA methylation (DNAm) patterns in the nonhuman primate brain following ovariectomy (Ov) and subsequent subcutaneous bioidentical E2 chronic treatment. We identified several dysregulated molecular networks, including MAPK signaling and dopaminergic synapse response, that are associated with ovariectomy and shared across two different brain areas, the occipital cortex (OC) and prefrontal cortex (PFC). The finding that hypomethylation (p = 1.6 × 10-51) and upregulation (p = 3.8 × 10-3) of UBE2M across both brain regions provide strong evidence for molecular differences in the brain induced by E2 depletion. Additionally, differential expression (p = 1.9 × 10-4; interaction p = 3.5 × 10-2) of LTBR in the PFC provides further support for the role E2 plays in the brain, by demonstrating that the regulation of some genes that are altered by ovariectomy may also be modulated by Ov followed by hormone replacement therapy (HRT). These results present real opportunities to understand the specific biological mechanisms that are altered with depleted E2. Given E2's potential role in cognitive decline and neuroinflammation, our findings could lead to the discovery of novel therapeutics to slow cognitive decline. Together, this work represents a major step toward understanding molecular changes in the brain that are caused by ovariectomy and how E2 treatment may revert or protect against the negative neuro-related consequences caused by a depletion in estrogen as women approach menopause.

Ubiquitin Is Not a Blood Biomarker of an Early Cognitive Decline in the Polish Elderly

Together with development of new pharmaceutical interventions, as well as the introduction of the concept of initial dementia phase, the demand for early diagnosis has been growing. Research on potential blood biomarkers, amazingly attractive, mainly due to the facility of deriving the material, has provided ambiguous results throughout. The existence of an association between ubiquitin and Alzheimer’s disease pathology suggests that it could be a potential neurodegeneration biomarker. The present study aims to identify and assess the relationship between ubiquitin with regard to the adequacy as a biomarker of an initial dementia and cognitive decline in the elderly. Method: The study sample was composed of 230 participants: 109 women and 121 men aged 65 and older. The relationships of plasma ubiquitin levels with cognitive performance, gender, and age were analyzed. The assessments were performed in three groups of cognitive functioning level: cognitively normal, mild cognitive impairment, and mild dementia, of which the subjects were divided with the Mini-Mental State Examination (MMSE). Results: No significant disparities in plasma ubiquitin levels for various levels of cognitive functioning were identified. Significantly higher plasma ubiquitin levels in women were found in comparison to men. No significant differences were found in ubiquitin concentrations based on age. Results suggest that ubiquitin does not meet the requirements for qualification as a blood biomarker of an early cognitive decline. In order to thoroughly evaluate the potential of research on ubiquitin in connection to an early neurodegenerative process, further studies are needed.

USP7 substrates identified by proteomics analysis reveal the specificity of USP7

Deubiquitylating enzymes (DUBs) remove ubiquitin chains from proteins and regulate protein stability and function. USP7 is one of the most extensively studied DUBs, since USP7 has several well-known substrates important for cancer progression, such as MDM2, N-MYC, and PTEN. Thus, USP7 is a promising drug target. However, systematic identification of USP7 substrates has not yet been performed. In this study, we carried out proteome profiling with label-free quantification in control and single/double-KO cells of USP7and its closest homolog, USP47 Our proteome profiling for the first time revealed the proteome changes caused by USP7 and/or USP47 depletion. Combining protein profiling, transcriptome analysis, and tandem affinity purification of USP7-associated proteins, we compiled a list of 20 high-confidence USP7 substrates that includes known and novel USP7 substrates. We experimentally validated MGA and PHIP as new substrates of USP7. We further showed that MGA deletion reduced cell proliferation, similar to what was observed in cells with USP7 deletion. In conclusion, our proteome-wide analysis uncovered potential USP7 substrates, providing a resource for further functional studies.

Promising protein biomarkers in the early diagnosis of Alzheimer's disease

Alzheimer's disease (AD) is an insidious, multifactorial disease that involves the devastation of neurons leading to cognitive impairments. Alzheimer's have compounded pathologies of diverse nature, including proteins as one important factor along with mutated genes and enzymes. Although various review articles have proposed biomarkers, still, the statistical importance of proteins is missing. Proteins associated with AD include amyloid precursor protein, glial fibrillary acidic protein, calmodulin-like skin protein, hepatocyte growth factor, matrix Metalloproteinase-2. These proteins play a crucial role in the AD hypothesis which includes the tau hypothesis, amyloid-beta (Aβ) hypothesis, cholinergic neuron damage, etc. The present review highlights the role of major proteins and their physiological functions in the early diagnosis of AD. Altered protein expression results in cognitive impairment, synaptic dysfunction, neuronal degradation, and memory loss. On the medicinal ground, efforts of making anti-amyloid, anti-tau, anti-inflammatory treatments are on the peak, having these proteins as putative targets. Few proteins, e.g., Amyloid precursor protein results in the formation of non-soluble sticky Aβ₄₀ and Aβ₄₂ monomers that, over time, aggregate into plaques in the cortical and limbic brain areas and neurogranin is believed to regulate calcium-mediated signaling pathways and thus modulating synaptic plasticity are few putative and potential forthcoming targets for developing effective anti-AD therapies. These proteins may help to diagnose the disease early, bode well for the successful discovery and development of therapeutic and preventative regimens for this devasting public health problem.

Hub Genes, Diagnostic Model, and Predicted Drugs Related to Iron Metabolism in Alzheimer's Disease

Alzheimer's disease (AD), the most common neurodegenerative disease, remains unclear in terms of its underlying causative genes and effective therapeutic approaches. Meanwhile, abnormalities in iron metabolism have been demonstrated in patients and mouse models with AD. Therefore, this study sought to find hub genes based on iron metabolism that can influence the diagnosis and treatment of AD. First, gene expression profiles were downloaded from the GEO database, including non-demented (ND) controls and AD samples. Fourteen iron metabolism-related gene sets were downloaded from the MSigDB database, yielding 520 iron metabolism-related genes. The final nine hub genes associated with iron metabolism and AD were obtained by differential analysis and WGCNA in brain tissue samples from GSE132903. GO analysis revealed that these genes were mainly involved in two major biological processes, autophagy and iron metabolism. Through stepwise regression and logistic regression analyses, we selected four of these genes to construct a diagnostic model of AD. The model was validated in blood samples from GSE63061 and GSE85426, and the AUC values showed that the model had a relatively good diagnostic performance. In addition, the immune cell infiltration of the samples and the correlation of different immune factors with these hub genes were further explored. The results suggested that these genes may also play an important role in immunity to AD. Finally, eight drugs targeting these nine hub genes were retrieved from the DrugBank database, some of which were shown to be useful for the treatment of AD or other concomitant conditions, such as insomnia and agitation. In conclusion, this model is expected to guide the diagnosis of patients with AD by detecting the expression of several genes in the blood. These hub genes may also assist in understanding the development and drug treatment of AD.

Paracetamol perturbs neuronal arborization and disrupts the cytoskeletal proteins SPTBN1 and TUBB3 in both human and chicken in vitro models

Epidemiological studies have linked long-term/high-dose usage of paracetamol (N-acetyl-para-aminophenol, APAP) during pregnancy to adverse neuropsychiatric outcomes, primarily attention-deficit hyperactive disorder (ADHD), in the offspring. Though variable, ADHD has been associated with phenotypic alterations characterized by reductions in grey matter densities and aberrations in structural connectivity, effects which are thought to originate in neurodevelopment. We used embryonic chicken cerebellar granule neurons (CGNs) and neuronally differentiating human NTERA2 cells (NT2Ns) to investigate the in vitro effects of APAP on cell viability, migration, neuritogenesis, and the intracellular levels of various proteins involved in neurodevelopment as well as in the maintenance of the structure and function of neurites. Exposure to APAP ranging from 100 to 1600 μM yielded concentration- and time-dependent reductions in cell viability and levels of neurite arborization, as well as reductions in the levels of the cytoskeletal protein β2-spectrin, with the highest APAP concentration resulting in between 50 and 75% reductions in the aforementioned metrics over the course of 72 h. Exposure to APAP also reduced migration in the NT2Ns but not CGNs. Moreover, we found concentration- and time-dependent increases in punctate aggregation of the cytoskeletal protein β3-tubulin following exposure to APAP in both cell model systems, with the highest APAP concentration approximately doubling the number of aggregates over 72-120 h. Our findings demonstrate that APAP negatively perturbs neurite arborization degree, with concurrent reductions in the protein levels of β2-spectrin and disruption of the integrity of β3-tubulin, both proteins of which play important roles in neuronal structure and function.

Ubiquitin and Ubiquitin-like Proteins in Cancer, Neurodegenerative Disorders, and Heart Diseases

Post-translational modification (PTM) is an essential mechanism for enhancing the functional diversity of proteins and adjusting their signaling networks. The reversible conjugation of ubiquitin (Ub) and ubiquitin-like proteins (Ubls) to cellular proteins is among the most prevalent PTM, which modulates various cellular and physiological processes by altering the activity, stability, localization, trafficking, or interaction networks of its target molecules. The Ub/Ubl modification is tightly regulated as a multi-step enzymatic process by enzymes specific to this family. There is growing evidence that the dysregulation of Ub/Ubl modifications is associated with various diseases, providing new targets for drug development. In this review, we summarize the recent progress in understanding the roles and therapeutic targets of the Ub and Ubl systems in the onset and progression of human diseases, including cancer, neurodegenerative disorders, and heart diseases.

UBB+1 reduces amyloid-β cytotoxicity by activation of autophagy in yeast

UBB+1 is a mutated version of ubiquitin B peptide caused by a transcriptional frameshift due to the RNA polymerase II "slippage". The accumulation of UBB+1 has been linked to ubiquitin-proteasome system (UPS) dysfunction and neurodegeneration. Alzheimer's disease (AD) is defined as a progressive neurodegeneration and aggregation of amyloid-β peptides (Aβ) is a prominent neuropathological feature of AD. In our previous study, we found that yeast cells expressing UBB+1 at lower level display an increased resistance to cellular stresses under conditions of chronological aging. In order to examine the molecular mechanisms behind, here we performed genome-wide transcriptional analyses and molecular/cellular biology assays. We found that low UBB+1 expression activated the autophagy pathway, increased vacuolar activity, and promoted transport of autophagic marker ATG8p into vacuole. Furthermore, we introduced low UBB+1 expression to our humanized yeast AD models, that constitutively express Aβ42 and Aβ40 peptide, respectively. The co-expression of UBB+1 with Aβ42 or Aβ40 peptide led to reduced intracellular Aβ levels, ameliorated viability, and increased chronological life span. In an autophagy deficient background strain (atg1Δ), intracellular Aβ levels were not affected by UBB+1 expression. Our findings offer insights for reducing intracellular Aβ toxicity via autophagy-dependent cellular pathways under low level of UBB+1 expression.

The Dose-Dependent Pleiotropic Effects of the UBB+1 Ubiquitin Mutant

The proteolytic machinery activity diminishes with age, leading to abnormal accumulation of aberrant proteins; furthermore, a decline in protein degradation capacity is associated with multiple age-related proteinopathies. Cellular proteostasis can be maintained via the removal of ubiquitin (Ub)-tagged damaged and redundant proteins by the ubiquitin-proteasome system (UPS). However, during aging, central nervous system (CNS) cells begin to express a frameshift-mutated Ub, UBB⁺¹. Its accumulation is a neuropathological hallmark of tauopathy, including Alzheimer’s disease and polyglutamine diseases. Mechanistically, in cell-free and cell-based systems, an increase in the UBB⁺¹ concentration disrupts proteasome processivity, leading to increased aggregation of toxic proteins. On the other hand, a low level of UBB⁺¹ improves stress resistance and extends lifespan. Here we summarize recent findings regarding the impact of UBB⁺¹ on Ub signaling and neurodegeneration. We also review the molecular basis of how UBB⁺¹ affects UPS components as well as its dose-dependent switch between cytoprotective and cytotoxic roles.

Anatomical and neurochemical organization of the serotonergic system in the mammalian brain and in particular the involvement of the dorsal raphe nucleus in relation to neurological diseases

The brainstem is a neglected brain area in neurodegenerative diseases, including Alzheimer's and Parkinson's disease, frontotemporal lobar degeneration and autonomic dysfunction. In Depression, several observations have been made in relation to changes in one particular the Dorsal Raphe Nucleus (DRN) which also points toward as key area in various age-related and neurodevelopmental diseases. The DRN is further thought to be related to stress regulated processes and cognitive events. It is involved in neurodegeneration, e.g., amyloid plaques, neurofibrillary tangles, and impaired synaptic transmission in Alzheimer's disease as shown in our autopsy findings. The DRN is a phylogenetically old brain area, with projections that reach out to a large number of regions and nuclei of the central nervous system, particularly in the forebrain. These ascending projections contain multiple neurotransmitters. One of the main reasons for the past and current interest in the DRN is its involvement in depression, and its main transmitter serotonin. The DRN also points toward the increased importance and focus of the brainstem as key area in various age-related and neurodevelopmental diseases. This review describes the morphology, ascending projections and the complex neurotransmitter nature of the DRN, stressing its role as a key research target into the neural bases of depression.

Improved neurocognitive performance in FIV infected cats following treatment with the p75 neurotrophin receptor ligand LM11A-31

HIV rapidly infects the central nervous system (CNS) and establishes a persistent viral reservoir within microglia, perivascular macrophages and astrocytes. Inefficient control of CNS viral replication by antiretroviral therapy results in chronic inflammation and progressive cognitive decline in up to 50% of infected individuals with no effective treatment options. Neurotrophin based therapies have excellent potential to stabilize and repair the nervous system. A novel non-peptide ligand, LM11A-31, that targets the p75 neurotrophin receptor (p75^NTR) has been identified as a small bioavailable molecule capable of strong neuroprotection with minimal side effects. To evaluate the neuroprotective effects of LM11A-31 in a natural infection model, we treated cats chronically infected with feline immunodeficiency virus (FIV) with 13 mg/kg LM11A-31 twice daily over a period of 10 weeks and assessed effects on cognitive functions, open field behaviors, activity, sensory thresholds, plasma FIV, cerebrospinal fluid (CSF) FIV, peripheral blood mononuclear cell provirus, CD4 and CD8 cell counts and general physiology. Between 12 and 18 months post-inoculation, cats began to show signs of neural dysfunction in T maze testing and novel object recognition, which were prevented by LM11A-31 treatment. Anxiety-like behavior was reduced in the open field and no changes were seen in sensory thresholds. Systemic FIV titers were unaffected but treated cats exhibited a log drop in CSF FIV titers. No significant adverse effects were observed under all conditions. The data indicate that LM11A-31 is likely to be a potent adjunctive treatment for the control of neurodegeneration in HIV infected individuals.

Proteome-wide Analysis Reveals Substrates of E3 Ligase RNF146 Targeted for Degradation

Specific E3 ligases target tumor suppressors for degradation. Inhibition of such E3 ligases may be an important approach to cancer treatment. RNF146 is a RING domain and PARylation-dependent E3 ligase that functions as an activator of the β-catenin/Wnt and YAP/Hippo pathways by targeting the degradation of several tumor suppressors. Tankyrases 1 and 2 (TNKS1/2) are the only known poly-ADP-ribosyltransferases that require RNF146 to degrade their substrates. However, systematic identification of RNF146 substrates have not yet been performed. To uncover substrates of RNF146 that are targeted for degradation, we generated RNF146 knockout cells and TNKS1/2-double knockout cells and performed proteome profiling with label-free quantification as well as transcriptome analysis. We identified 160 potential substrates of RNF146, which included many known substrates of RNF146 and TNKS1/2 and 122 potential TNKS-independent substrates of RNF146. In addition, we validated OTU domain-containing protein 5 and Protein mono-ADP-ribosyltransferase PARP10 as TNKS1/2-independent substrates of RNF146 and SARDH as a novel substrate of TNKS1/2 and RNF146. Our study is the first proteome-wide analysis of potential RNF146 substrates. Together, these findings not only demonstrate that proteome profiling can be a useful general approach for the systemic identification of substrates of E3 ligases but also reveal new substrates of RNF146, which provides a resource for further functional studies.

The Ubiquitin System in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, most prevalent in the elderly population and has a significant impact on individuals and their family as well as the health care system and the economy. While the number of patients affected by various forms of dementia including AD is on the increase, there is currently no cure. Although genome-wide association studies have identified genetic markers for familial AD, the molecular mechanisms underlying the initiation and development of both familial and sporadic AD remain poorly understood. Most neurodegenerative diseases and in particular those associated with dementia have been defined as proteinopathies due to the presence of intra- and/or extracellular protein aggregates in the brain of affected individuals. Although loss of proteostasis in AD has been known for decades, it is only in recent years that we have come to appreciate the role of ubiquitin-dependent mechanisms in brain homeostasis and in brain diseases. Ubiquitin is a highly versatile post-translational modification which regulates many aspects of protein fate and function, including protein degradation by the Ubiquitin-Proteasome System (UPS), autophagy-mediated removal of damaged organelles and proteins, lysosomal turnover of membrane proteins and of extracellular molecules brought inside the cell through endocytosis. Amyloid-β (Aβ) fragments as well as hyperphosphorylation of Tau are hallmarks of AD, and these are found in extracellular plaques and intracellular fibrils in the brain of individuals with AD, respectively. Yet, whether it is the oligomeric or the soluble species of Aβ and Tau that mediate toxicity is still unclear. These proteins impact on mitochondrial energy metabolism, inflammation, as well as a number of housekeeping processes including protein degradation through the UPS and autophagy. In this chapter, we will discuss the role of ubiquitin in neuronal homeostasis as well as in AD; summarise crosstalks between the enzymes that regulate protein ubiquitination and the toxic proteins Tau and Aβ; highlight emerging molecular mechanisms in AD as well as future strategies which aim to exploit the ubiquitin system as a source for next-generation therapeutics.

Paradoxical effects of mutant ubiquitin on Aβ plaque formation in an Alzheimer mouse model

Amyloid-β (Aβ) plaques are a prominent pathological hallmark of Alzheimer's disease (AD). They consist of aggregated Aβ peptides, which are generated through sequential proteolytic processing of the transmembrane protein amyloid precursor protein (APP) and several Aβ-associated factors. Efficient clearance of Aβ from the brain is thought to be important to prevent the development and progression of AD. The ubiquitin-proteasome system (UPS) is one of the major pathways for protein breakdown in cells and it has been suggested that impaired UPS-mediated removal of protein aggregates could play an important role in the pathogenesis of AD. To study the effects of an impaired UPS on Aβ pathology in vivo, transgenic APP_Swe/PS1ΔE9 mice (APPPS1) were crossed with transgenic mice expressing mutant ubiquitin (UBB⁺¹), a protein-based inhibitor of the UPS. Surprisingly, the APPPS1/UBB⁺¹ crossbreed showed a remarkable decrease in Aβ plaque load during aging. Further analysis showed that UBB⁺¹ expression transiently restored PS1-NTF expression and γ-secretase activity in APPPS1 mice. Concurrently, UBB⁺¹ decreased levels of β-APP-CTF, which is a γ-secretase substrate. Although UBB⁺¹ reduced Aβ pathology in APPPS1 mice, it did not improve the behavioral deficits in these animals.

Recent Insights on Alzheimer's Disease Originating from Yeast Models

In this review article, yeast model-based research advances regarding the role of Amyloid-β (Aβ), Tau and frameshift Ubiquitin UBB+1 in Alzheimer’s disease (AD) are discussed. Despite having limitations with regard to intercellular and cognitive AD aspects, these models have clearly shown their added value as complementary models for the study of the molecular aspects of these proteins, including their interplay with AD-related cellular processes such as mitochondrial dysfunction and altered proteostasis. Moreover, these yeast models have also shown their importance in translational research, e.g., in compound screenings and for AD diagnostics development. In addition to well-established Saccharomyces cerevisiae models, new upcoming Schizosaccharomyces pombe, Candida glabrata and Kluyveromyces lactis yeast models for Aβ and Tau are briefly described. Finally, traditional and more innovative research methodologies, e.g., for studying protein oligomerization/aggregation, are highlighted.

Different Expression Levels of Human Mutant Ubiquitin B+1 (UBB+1) Can Modify Chronological Lifespan or Stress Resistance of Saccharomyces cerevisiae

The ubiquitin-proteasome system (UPS) is the main pathway responsible for the degradation of misfolded proteins, and its dysregulation has been implicated in several neurodegenerative diseases, including Alzheimer's disease (AD). UBB⁺¹, a mutant variant of ubiquitin B, was found to accumulate in neurons of AD patients and it has been linked to UPS dysfunction and neuronal death. Using the yeast Saccharomyces cerevisiae as a model system, we constitutively expressed UBB⁺¹ to evaluate its effects on proteasome function and cell death, particularly under conditions of chronological aging. We showed that the expression of UBB⁺¹ caused inhibition of the three proteasomal proteolytic activities (caspase-like (β1), trypsin-like (β2) and chymotrypsin-like (β5) activities) in yeast. Interestingly, this inhibition did not alter cell viability of growing cells. Moreover, we showed that cells expressing UBB⁺¹ at lower level displayed an increased capacity to degrade induced misfolded proteins. When we evaluated cells during chronological aging, UBB⁺¹ expression at lower level, prevented cells to accumulate reactive oxygen species (ROS) and avert apoptosis, dramatically increasing yeast life span. Since proteasome inhibition by UBB⁺¹ has previously been shown to induce chaperone expression and thus protect against stress, we evaluated our UBB⁺¹ model under heat shock and oxidative stress. Higher expression of UBB⁺¹ caused thermotolerance in yeast due to induction of chaperones, which occurred to a lesser extent at lower expression level of UBB⁺¹ (where we observed the phenotype of extended life span). Altering UPS capacity by differential expression of UBB⁺¹ protects cells against several stresses during chronological aging. This system can be valuable to study the effects of UBB⁺¹ on misfolded proteins involved in neurodegeneration and aging.

Degradation or aggregation: the ramifications of post-translational modifications on tau

Tau protein is encoded in the microtubule-associated protein tau (MAPT) gene and contributes to the stability of microtubules in axons. Despite of its basic isoelectric point and high solubility, tau is often found in intraneuronal filamentous inclusions such as paired helical filaments (PHFs), which are the primary constituent of neurofibrillary tangles (NFTs). This pathological feature is the nosological entity termed "tauopathies" which notably include Alzheimer's disease (AD). A proteinaceous signature of all tauopathies is hyperphosphorylation of the accumulated tau, which has been extensively studied as a major pharmacological target for AD therapy. However, in addition to phosphorylation events, tau undergoes a number of diverse posttranslational modifications (PTMs) which appear to be controlled by complex crosstalk. It remains to be elucidated which of the PTMs or their combinations have pro-aggregation or anti-aggregation properties. In this review, we outline the consequences of and communications between several key PTMs of tau, such as acetylation, phosphorylation, and ubiquitination, focusing on their roles in aggregation and degradation. We place emphasis on the structure of tau protofilaments from the human AD brain, which may be good targets to modulate etiological PTMs which cause tau aggregation. [BMB Reports 2018; 51(6): 265-273].

The Deacetylase HDAC6 Mediates Endogenous Neuritic Tau Pathology

The initiating events that promote tau mislocalization and pathology in Alzheimer's disease (AD) are not well defined, partly because of the lack of endogenous models that recapitulate tau dysfunction. We exposed wild-type neurons to a neuroinflammatory trigger and examined the effect on endogenous tau. We found that tau re-localized and accumulated within pathological neuritic foci, or beads, comprised of mostly hypo-phosphorylated, acetylated, and oligomeric tau. These structures were detected in aged wild-type mice and were enhanced in response to neuroinflammation in vivo, highlighting a previously undescribed endogenous age-related tau pathology. Strikingly, deletion or inhibition of the cytoplasmic shuttling factor HDAC6 suppressed neuritic tau bead formation in neurons and mice. Using mass spectrometry-based profiling, we identified a single neuroinflammatory factor, the metalloproteinase MMP-9, as a mediator of neuritic tau beading. Thus, our study uncovers a link between neuroinflammation and neuritic tau beading as a potential early-stage pathogenic mechanism in AD.

Dysfunction of Protein Quality Control in Parkinsonism-Dementia Complex of Guam

Guam parkinsonism–dementia complex (G-PDC) is an enigmatic neurodegenerative disease that is endemic to the Pacific island of Guam. G-PDC patients are clinically characterized by progressive cognitive impairment and parkinsonism. Neuropathologically, G-PDC is characterized by abundant neurofibrillary tangles, which are composed of hyperphosphorylated tau, marked deposition of 43-kDa TAR DNA-binding protein, and neuronal loss. Although both genetic and environmental factors have been implicated, the etiology and pathogenesis of G-PDC remain unknown. Recent neuropathological studies have provided new clues about the pathomechanisms involved in G-PDC. For example, deposition of abnormal components of the protein quality control system in brains of G-PDC patients indicates a role for proteostasis imbalance in the disease. This opens up promising avenues for new research on G-PDC and could have important implications for the study of other neurodegenerative disorders.

Amyloid is essential but insufficient for Alzheimer causation: addition of subcellular cofactors is required for dementia

OBJECTIVE

The aim of this study is to examine the hypotheses stating the importance of amyloid or of its oligomers in the pathogenesis of Alzheimer's disease (AD).

METHODS

Published studies were examined.

RESULTS

The importance of amyloid in the pathogenesis of AD is well established, yet accepting it as the main cause for AD is problematic, because amyloid-centric treatments have provided no clinical benefit and about one-third of cognitively normal, older persons have cerebral amyloid plaques. Also problematic is the alternative hypothesis that, instead of amyloid plaques, it is oligomers of amyloid precursor protein that cause AD.Evidence is presented suggesting amyloid/oligomers as necessary but insufficient causes of the dementia and that, for dementia to develop, requires the addition of cofactors known to be associated with AD. Those cofactors include several subcellular processes: mitochondrial impairments; the Wnt signaling system; the unfolded protein response; the ubiquitin proteasome system; the Notch signaling system; and tau, calcium, and oxidative damage.

CONCLUSIONS

A modified amyloid/oligomer hypothesis for the pathogenesis of AD is that activation of one or more of the aforementioned cofactors creates a burden of functional impairments that, in conjunction with amyloid/oligomers, now crosses a threshold of dysfunction that results in clinical dementia. Of considerable importance, several treatments that might reverse the activation of some of the subcellular processes are available, for example, lithium, pioglitazone, erythropoietin, and prazosin; they should be given in combination in a clinical trial to test their safety and efficacy. © 2017 John Wiley & Sons, Ltd.

Systematic approaches to identify E3 ligase substrates

Protein ubiquitylation is a widespread post-translational modification, regulating cellular signalling with many outcomes, such as protein degradation, endocytosis, cell cycle progression, DNA repair and transcription. E3 ligases are a critical component of the ubiquitin proteasome system (UPS), determining the substrate specificity of the cascade by the covalent attachment of ubiquitin to substrate proteins. Currently, there are over 600 putative E3 ligases, but many are poorly characterized, particularly with respect to individual protein substrates. Here, we highlight systematic approaches to identify and validate UPS targets and discuss how they are underpinning rapid advances in our understanding of the biochemistry and biology of the UPS. The integration of novel tools, model systems and methods for target identification is driving significant interest in drug development, targeting various aspects of UPS function and advancing the understanding of a diverse range of disease processes.

Selective Transgenic Expression of Mutant Ubiquitin in Purkinje Cell Stripes in the Cerebellum

The ubiquitin-proteasome system (UPS) is one of the major mechanisms for protein breakdown in cells, targeting proteins for degradation by enzymatically conjugating them to ubiquitin molecules. Intracellular accumulation of ubiquitin-B+1 (UBB+1), a frameshift mutant of ubiquitin-B, is indicative of a dysfunctional UPS and has been implicated in several disorders, including neurodegenerative disease. UBB+1-expressing transgenic mice display widespread labeling for UBB+1 in brain and exhibit behavioral deficits. Here, we show that UBB+1 is specifically expressed in a subset of parasagittal stripes of Purkinje cells in the cerebellar cortex of a UBB+1-expressing mouse model. This expression pattern is reminiscent of that of the constitutively expressed Purkinje cell antigen HSP25, a small heat shock protein with neuroprotective properties.

Concordance of Several Subcellular Interactions Initiates Alzheimer's Dementia: Their Reversal Requires Combination Treatment

The pathogenesis of Alzheimer's disease involves multiple pathways that, at the macrolevel, include decreased proliferation plus increased loss affecting neurons, astrocytes, and capillaries and, at the subcellular level, involve several elements: amyloid/amyloid precursor protein, presenilins, the unfolded protein response, the ubiquitin/proteasome system, the Wnt/catenin system, the Notch signaling system, mitochondria, mitophagy, calcium, and tau. Data presented show the intimate, anatomical interactions between neurons, astrocytes, and capillaries; the interactions between the several subcellular factors affecting those cells; and the treatments that are currently available and that might correct dysfunctions in the subcellular factors. Available treatments include lithium, valproate, pioglitazone, erythropoietin, and prazosin. Since the subcellular pathogenesis involves multiple interacting elements, combination treatment would be more effective than administration of a single drug directed at only 1 element. The overall purpose of this presentation is to describe the pathogenesis in detail and to explain the proposed treatments.

The ubiquitin proteasomal system: a potential target for the management of Alzheimer's disease

The cellular quality control system degrades abnormal or misfolded proteins and consists of three different mechanisms: the ubiquitin proteasomal system (UPS), autophagy and molecular chaperones. Any disturbance in this system causes proteins to accumulate, resulting in neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's disease (AD), Parkinson's disease, Huntington's disease and prion or polyglutamine diseases. Alzheimer's disease is currently one of the most common age-related neurodegenerative diseases. However, its exact cause and pathogenesis are unknown. Currently approved medications for AD provide symptomatic relief; however, they fail to influence disease progression. Moreover, the components of the cellular quality control system represent an important focus for the development of targeted and potent therapies for managing AD. This review aims to evaluate whether existing evidence supports the hypothesis that UPS impairment causes the early pathogenesis of neurodegenerative disorders. The first part presents basic information about the UPS and its molecular components. The next part explains how the UPS is involved in neurodegenerative disorders. Finally, we emphasize how the UPS influences the management of AD. This review may help in the design of future UPS-related therapies for AD.

The mechanisms of Mallory-Denk body formation are similar to the formation of aggresomes in Alzheimer's disease and other neurodegenerative disorders

There is a possibility that the aggresomes that form in the brain in neurodegenerative diseases like Alzheimer's disease (AD) and in the liver where aggresomes like Mallory-Denk Bodies (MDB) form, share mechanisms. MDBs can be prevented by feeding mice sadenosylmethionine (SAMe) or betaine. Possibly these proteins could prevent AD. We compared the literature on MDBs and AD pathogenesis, which include roles played by p62, ubiquitin UBB +1, HSPs70, 90, 104, FAT10, NEDD8, VCP/97, and the protein quality control mechanisms including the 26s proteasome, the IPOD and JUNQ and autophagosome pathways.

The Ubiquitin-Proteasome System: Potential Therapeutic Targets for Alzheimer's Disease and Spinal Cord Injury

The ubiquitin-proteasome system (UPS) is a crucial protein degradation system in eukaryotes. Herein, we will review advances in the understanding of the role of several proteins of the UPS in Alzheimer’s disease (AD) and functional recovery after spinal cord injury (SCI). The UPS consists of many factors that include E3 ubiquitin ligases, ubiquitin hydrolases, ubiquitin and ubiquitin-like molecules, and the proteasome itself. An extensive body of work links UPS dysfunction with AD pathogenesis and progression. More recently, the UPS has been shown to have vital roles in recovery of function after SCI. The ubiquitin hydrolase (Uch-L1) has been proposed to increase cellular levels of mono-ubiquitin and hence to increase rates of protein turnover by the UPS. A low Uch-L1 level has been linked with Aβ accumulation in AD and reduced neuroregeneration after SCI. One likely mechanism for these beneficial effects of Uch-L1 is reduced turnover of the PKA regulatory subunit and consequently, reduced signaling via CREB. The neuron-specific F-box protein Fbx2 ubiquitinates β-secretase thus targeting it for proteasomal degradation and reducing generation of Aβ. Both Uch-L1 and Fbx2 improve synaptic plasticity and cognitive function in mouse AD models. The role of Fbx2 after SCI has not been examined, but abolishing ß-secretase reduces neuronal recovery after SCI, associated with reduced myelination. UBB+1, which arises through a frame-shift mutation in the ubiquitin gene that adds 19 amino acids to the C-terminus of ubiquitin, inhibits proteasomal function and is associated with increased neurofibrillary tangles in patients with AD, Pick’s disease and Down’s syndrome. These advances in understanding of the roles of the UPS in AD and SCI raise new questions but, also, identify attractive and exciting targets for potential, future therapeutic interventions.

Misframed ubiquitin and impaired protein quality control: an early event in Alzheimer's disease

Amyloid β (Aβ) plaque formation is a prominent cellular hallmark of Alzheimer's disease (AD). To date, immunization trials in AD patients have not been effective in terms of curing or ameliorating dementia. In addition, γ-secretase inhibitor strategies await clinical improvements in AD. These approaches were based upon the idea that autosomal dominant mutations in amyloid precursor protein (APP) and Presenilin 1 (PS1) genes are predictive for treatment of all AD patients. However most AD patients are of the sporadic form which partly explains the failures to treat this multifactorial disease. The major risk factor for developing sporadic AD (SAD) is aging whereas the Apolipoprotein E polymorphism (ε4 variant) is the most prominent genetic risk factor. Other medium-risk factors such as triggering receptor expressed on myeloid cells 2 (TREM2) and nine low risk factors from Genome Wide Association Studies (GWAS) were associated with AD. Recently, pooled GWAS studies identified protein ubiquitination as one of the key modulators of AD. In addition, a brain site specific strategy was used to compare the proteomes of AD patients by an Ingenuity Pathway Analysis. This strategy revealed numerous proteins that strongly interact with ubiquitin (UBB) signaling, and pointing to a dysfunctional ubiquitin proteasome system (UPS) as a causal factor in AD. We reported that DNA-RNA sequence differences in several genes including ubiquitin do occur in AD, the resulting misframed protein of which accumulates in the neurofibrillary tangles (NFTs). This suggests again a functional link between neurodegeneration of the AD type and loss of protein quality control by the UPS. Progress in this field is discussed and modulating the activity of the UPS opens an attractive avenue of research towards slowing down the development of AD and ameliorating its effects by discovering prime targets for AD therapeutics.

Aging, Alzheimer's, and APOE genotype influence the expression and neuronal distribution patterns of microtubule motor protein dynactin-P50

Reports from neural cell cultures and experimental animal studies provide evidence of age- and disease-related changes in retrograde transport of spent or misfolded proteins destined for degradation or recycling. However, few studies address these issues in human brain from those who either age without dementia and overt neuropathology, or succumb to Alzheimer's; especially as such propensity may be influenced by APOE genotype. We studied the expression and distribution of the dynein subunit dynactin-P50, the β amyloid precursor protein (βAPP), and hyperphosphorylated tau (P-tau) in tissues and tissue sections of brains from non-demented, neuropathology-free patients and from Alzheimer patients, with either APOE ε3,3 or APOE ε4,4. We found that advanced age in patients without dementia or neuropathological change was associated with coordinated increases in dynactin-P50 and βAPP in neurons in pyramidal layers of the hippocampus. In contrast, in Alzheimer's, βAPP and dynactin were significantly reduced. Furthermore, the dynactin-P50 and βAPP that was present was located primarily in dystrophic neurites in Aβ plaques. Tissues from Alzheimer patients with APOE ε3,3 had less P-tau, more βAPP, dynactin-P50, and synaptophysin than did tissues from Alzheimer patients carrying APOE ε4,4. It is logical to conclude, then, that as neurons age successfully, there is coordination between retrograde delivery and maintenance and repair, as well as between retrograde delivery and degradation and/or recycling of spent proteins. The buildup of proteins slated for repair, synaptic viability, transport, and re-cycling in neuron soma and dystrophic neurites suggest a loss of this coordination in Alzheimer neurons. Inheritance of APOE ε3,3 rather than APOE ε4,4, is associated with neuronal resilience, suggestive of better repair capabilities, more synapses, more efficient transport, and less hyperphosphorylation of tau. We conclude that even in disease the ε3 allele is neuroprotective.

Ubiquitin-dependent proteolysis in yeast cells expressing neurotoxic proteins

Critically impaired protein degradation is discussed to contribute to neurodegenerative disorders, including Parkinson's, Huntington's, Alzheimer's, and motor neuron diseases. Misfolded, aggregated, or surplus proteins are efficiently degraded via distinct protein degradation pathways, including the ubiquitin-proteasome system, autophagy, and vesicular trafficking. These pathways are regulated by covalent modification of target proteins with the small protein ubiquitin and are evolutionary highly conserved from humans to yeast. The yeast Saccharomyces cerevisiae is an established model for deciphering mechanisms of protein degradation, and for the elucidation of pathways underlying programmed cell death. The expression of human neurotoxic proteins triggers cell death in yeast, with neurotoxic protein-specific differences. Therefore, yeast cell death models are suitable for analyzing the role of protein degradation pathways in modulating cell death upon expression of disease-causing proteins. This review summarizes which protein degradation pathways are affected in these yeast models, and how they are involved in the execution of cell death. I will discuss to which extent this mimics the situation in other neurotoxic models, and how this may contribute to a better understanding of human disorders.

Accumulation of Basic Amino Acids at Mitochondria Dictates the Cytotoxicity of Aberrant Ubiquitin

Neuronal accumulation of UBB⁺¹, a frameshift variant of ubiquitin B, is a hallmark of Alzheimer’s disease (AD). How UBB⁺¹ contributes to neuronal dysfunction remains elusive. Here, we show that in brain regions of AD patients with neurofibrillary tangles UBB⁺¹ co-exists with VMS1, the mitochondrion-specific component of the ubiquitin-proteasome system (UPS). Expression of UBB⁺¹ in yeast disturbs the UPS, leading to mitochondrial stress and apoptosis. Inhibiting UPS activity exacerbates while stimulating UPS by the transcription activator Rpn4 reduces UBB⁺¹-triggered cytotoxicity. High levels of the Rpn4 target protein Cdc48 and its cofactor Vms1 are sufficient to relieve programmed cell death. We identified the UBB⁺¹-induced enhancement of the basic amino acids arginine, ornithine, and lysine at mitochondria as a decisive toxic event, which can be reversed by Cdc48/Vms1-mediated proteolysis. The fact that AD-induced cellular dysfunctions can be avoided by UPS activity at mitochondria has potentially far-reaching pathophysiological implications.

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UBB⁺¹ co-exists with the UPS component VMS1 in neurofibrillary tangles

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UBB⁺¹ accumulation impairs the UPS and mitochondria, triggering cell death

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UBB⁺¹ causes accumulation of basic amino acids at mitochondria

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Vms1 reverts UBB⁺¹-triggered basic amino acid accumulation and cell death

The ubiquitin-proteasome system in neurodegeneration

SIGNIFICANCE

Impairment of the ubiquitin-proteasome system (UPS) has been implicated in the pathogenesis of a wide variety of neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. The most significant risk factor for the development of these disorders is aging, which is associated with a progressive decline in UPS activity and the accumulation of oxidatively modified proteins. To date, no therapies have been developed that can specifically up-regulate this system.

RECENT ADVANCES

In the neurodegenerative brain, dysfunction of the UPS has been associated with the deposition of ubiquitinated protein aggregates and widespread disruption of the proteostasis network. Recent research has identified further evidence of impairment in substrate ubiquitination and proteasomal degradation, which could contribute to the loss of cellular proteostasis in neurodegenerative disease. Novel strategies for activation of the UPS by genetic manipulation and treatment with synthetic compounds have also recently been identified.

CRITICAL ISSUES

Here, we discuss the specific roles of the UPS in the healthy central nervous system and establish how dysfunctional components can contribute to neurotoxicity in the context of disease.

FUTURE DIRECTIONS

Knowledge of the UPS components that are specifically or preferentially involved in neurodegenerative disease will be critical in the development of targeted therapies which aim at limiting the accumulation of misfolded proteins without gross disturbance of this major proteolytic pathway.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

Dietary methanol regulates human gene activity

Methanol (MeOH) is considered to be a poison in humans because of the alcohol dehydrogenase (ADH)-mediated conversion of MeOH to formaldehyde (FA), which is toxic. Our recent genome-wide analysis of the mouse brain demonstrated that an increase in endogenous MeOH after ADH inhibition led to a significant increase in the plasma MeOH concentration and a modification of mRNA synthesis. These findings suggest endogenous MeOH involvement in homeostasis regulation by controlling mRNA levels. Here, we demonstrate directly that study volunteers displayed increasing concentrations of MeOH and FA in their blood plasma when consuming citrus pectin, ethanol and red wine. A microarray analysis of white blood cells (WBC) from volunteers after pectin intake showed various responses for 30 significantly differentially regulated mRNAs, most of which were somehow involved in the pathogenesis of Alzheimer's disease (AD). There was also a decreased synthesis of hemoglobin mRNA, HBA and HBB, the presence of which in WBC RNA was not a result of red blood cells contamination because erythrocyte-specific marker genes were not significantly expressed. A qRT-PCR analysis of volunteer WBCs after pectin and red wine intake confirmed the complicated relationship between the plasma MeOH content and the mRNA accumulation of both genes that were previously identified, namely, GAPDH and SNX27, and genes revealed in this study, including MME, SORL1, DDIT4, HBA and HBB. We hypothesized that human plasma MeOH has an impact on the WBC mRNA levels of genes involved in cell signaling.

Mutant ubiquitin UBB+1 induces mitochondrial fusion by destabilizing mitochondrial fission-specific proteins and confers resistance to oxidative stress-induced cell death in astrocytic cells

Mutant ubiquitin UBB+1 is observed in a variety of aging-related neurodegenerative diseases and acts as a potent inhibitor of the ubiquitin proteasome system (UPS). In the present study, we investigated the relationship between impaired UPS (using ectopic expression of UBB+1) and mitochondrial dynamics in astrocytes, which are the most abundant glial cells in the central nervous system. Immunocytochemistry and fluorescence recovery after photobleaching revealed that ectopic expression of UBB+1 induced mitochondrial elongation. We further demonstrated that overexpression of UBB+1 destabilized mitochondrial fission-specific proteins including Drp1, Fis1, and OPA3, but not the mitochondrial fusion-specific proteins Mfn1, Mfn2, and OPA1. The reduction in mitochondrial fission-specific proteins by UBB+1 was prevented by inhibiting the 26 S proteasome using chemical inhibitors, including MG132, lactacystin and epoxomicin. We then assessed the involvement of proteases that target mitochondrial proteins by using various protease inhibitors. Finally, we confirmed that either overexpression of UBB+1 or inhibiting the proteasome can protect astrocytic cells from H₂O₂-induced cell death compared with control cells. Our results suggest that UBB+1 destabilizes mitochondrial fission-specific proteins, leading to mitochondrial fusion and the subsequent resistance to oxidative stress. We therefore propose a protective role of UBB+1 overexpression or the proteasome inhibition in astrocytes in degenerative brains.

AD-linked, toxic NH2 human tau affects the quality control of mitochondria in neurons

Functional as well as structural alterations in mitochondria size, shape and distribution are precipitating, early events in progression of Alzheimer's Disease (AD). We reported that a 20-22kDa NH2-tau fragment (aka NH2htau), mapping between 26 and 230 amino acids of the longest human tau isoform, is detected in cellular and animal AD models and is neurotoxic in hippocampal neurons. The NH2htau -but not the physiological full-length protein- interacts with Aβ at human AD synapses and cooperates with it in inhibiting the mitochondrial ANT-1-dependent ADP/ATP exchange. Here we show that the NH2htau also adversely affects the interplay between the mitochondria dynamics and their selective autophagic clearance. Fragmentation and perinuclear mislocalization of mitochondria with smaller size and density are early found in dying NH2htau-expressing neurons. The specific effect of NH2htau on quality control of mitochondria is accompanied by (i) net reduction in their mass in correlation with a general Parkin-mediated remodeling of membrane proteome; (ii) their extensive association with LC3 and LAMP1 autophagic markers; (iii) bioenergetic deficits and (iv) in vitro synaptic pathology. These results suggest that NH2htau can compromise the mitochondrial biology thereby contributing to AD synaptic deficits not only by ANT-1 inactivation but also, indirectly, by impairing the quality control mechanism of these organelles.

Mutant ubiquitin attenuates interleukin-1β- and tumor necrosis factor-α-induced pro-inflammatory signaling in human astrocytic cells

A frameshift mutation of ubiquitin called ubiquitin⁺¹ (UBB⁺¹) was found in the aging and Alzheimer’s disease brains and thought to be associated with neuronal dysfuction and degeneration. Even though ubiquitylation has been known to regulate vital cellular functions mainly through proteasome-dependent degradation of polyubiquitinated substrates, proteolysis-independent roles of ubiquitylation have emerged as key mechanisms in various signaling cascades. In this study, we have investigated the effect of UBB⁺¹ on proinflammatory signaling such as interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in human astrocytes. Treatment with TNF-α and IL-1β induced expression of CCL2 and CXCL8 by human astrocytic cells; while ectopic expression of UBB⁺¹ significantly abrogated the proinflammatory cytokine-induced expression of chemokines. Ectopic expression of UBB⁺¹ suppressed TNF-α- and IL-1β-induced activation of NF-κB and JNK signaling pathway. Furthermore, we have demonstrated that polyubiquitylation of TRAFs and subsequent phosphorylation of TAK1 were significantly inhibited by stable expression of UBB⁺¹. Collectively, these results suggest that UBB⁺¹ may affect proinflammatory signaling in the central nervous system via inhibitory mechanisms of ubiquitin-dependent signaling in human astrocytes.

Autophagy in axonal and dendritic degeneration

Degeneration of axons and dendrites is a common and early pathological feature of many neurodegenerative disorders, and is thought to be regulated by mechanisms distinct from those determining death of the cell body. The unique structures of axons and dendrites (collectively neurites) may cause them to be particularly vulnerable to the accumulation of protein aggregates and damaged organelles. Autophagy is a catabolic mechanism in which cells clear protein aggregates and damaged organelles. Basal autophagy occurs continuously as a housekeeping function, and can be acutely expanded in response to stress or injury. Emerging evidence shows that insufficient or excessive autophagy contributes to neuritic degeneration. Here, we review the recent progress that has begun to reveal the role of autophagy in neurite function and degeneration.

Tau degradation: the ubiquitin-proteasome system versus the autophagy-lysosome system

The ubiquitin-proteasome system (UPS) and the autophagy-lysosome system are two major protein quality control mechanisms in eukaryotic cells. While the UPS has been considered for decades as the critical regulator in the degradation of various aggregate-prone proteins, autophagy has more recently been shown to be an important pathway implicated in neuronal health and disease. The two hallmark lesions of Alzheimer's disease (AD) are extracellular β-amyloid plaques and intracellular tau tangles. It has been suggested that tau accumulation is pathologically more relevant to the development of neurodegeneration and cognitive decline in AD patients than β-amyloid plaques. Here, we review the UPS and autophagy-mediated tau clearance mechanisms and outline the biochemical connections between these two processes. In addition, we discuss pharmacological methods that target these degradation systems for the treatment and prevention of AD.

Role of the ubiquitin-proteasome system in nervous system function and disease: using C. elegans as a dissecting tool

In addition to its central roles in protein quality control, regulation of cell cycle, intracellular signaling, DNA damage response and transcription regulation, the ubiquitin-proteasome system (UPS) plays specific roles in the nervous system, where it contributes to precise connectivity through development, and later assures functionality by regulating a wide spectrum of neuron-specific cellular processes. Aberrations in this system have been implicated in the etiology of neurodevelopmental and neurodegenerative diseases. In this review, we provide an updated view on the UPS and highlight recent findings concerning its role in normal and diseased nervous systems. We discuss the advantages of the model organism Caenorhabditis elegans as a tool to unravel the major unsolved questions concerning this biochemical pathway and its involvement in nervous system function and dysfunction, and expose the new possibilities, using state-of-the-art techniques, to assess UPS function using this model system.

Review: unchained maladie - a reassessment of the role of Ubb(+1) -capped polyubiquitin chains in Alzheimer's disease

Molecular misreading allows the formation of mutant proteins in the absence of gene mutations. A mechanism has been proposed by which a frameshift mutant of the ubiquitin protein, Ubb(+1) , which accumulates in an age-dependent manner as a result of molecular misreading, contributes to neuropathology in Alzheimer's disease (Lam et al. 2000). Specifically, in the Ubb(+1) -mediated proteasome inhibition hypothesis Ubb(+1) 'caps' unanchored (that is, nonsubstrate linked) polyubiquitin chains, which then act as dominant inhibitors of the 26S proteasome. A review of subsequent literature indicates that this original hypothesis is broadly supported, and offers new insights into the mechanisms accounting for the age-dependent accumulation of Ubb(+1) , and how Ubb(+1) -mediated proteasome inhibition may contribute to Alzheimer's disease. Further, recent studies have highlighted a physiological role for free endogenous unanchored polyubiquitin chains in the direct activation of certain protein kinases. This raises the possibility that Ubb(+1) -capped unanchored polyubiquitin chains could also exert harmful effects through the aberrant activation of tau or other ubiquitin-dependent kinases, neuronal NF-κB activity or NF-κB-mediated neuroinflammatory processes.

FBL2 regulates amyloid precursor protein (APP) metabolism by promoting ubiquitination-dependent APP degradation and inhibition of APP endocytosis

The ubiquitin-proteasome pathway is a major protein degradation pathway whose dysfunction is now widely accepted as a cause of neurodegenerative diseases, including Alzheimer's disease. Here we demonstrate that the F-box and leucine rich repeat protein2 (FBL2), a component of the E3 ubiquitin ligase complex, regulates amyloid precursor protein (APP) metabolism through APP ubiquitination. FBL2 overexpression decreased the amount of secreted amyloid β (Aβ) peptides and sAPPβ, whereas FBL2 mRNA knockdown by siRNA increased these levels. FBL2 overexpression also decreased the amount of intracellular Aβ in Neuro2a cells stably expressing APP with Swedish mutation. FBL2 bound with APP specifically at its C-terminal fragment (CTF), which promoted APP/CTF ubiquitination. FBL2 overexpression also accelerated APP proteasome-dependent degradation and decreased APP protein localization in lipid rafts by inhibiting endocytosis. These effects were not observed in an F-box-deleted FBL2 mutant that does not participate in the E3 ubiquitin ligase complex. Furthermore, a reduced insoluble Aβ and Aβ plaque burden was observed in the hippocampus of 7-month-old FBL2 transgenic mice crossed with double-transgenic mice harboring APPswe and PS1(M146V) transgenes. These findings indicate that FBL2 is a novel and dual regulator of APP metabolism through FBL2-dependent ubiquitination of APP.

Protein homeostasis, aging and Alzheimer's disease

Alzheimer’s disease (AD) is one key medical challenge of the aging society and despite a great amount of effort and a huge collection of acquired data on molecular mechanisms that are associated with the onset and progression of this devastating disorder, no causal therapy is in sight. The two main hypotheses of AD, the amyloid cascade hypothesis and the Tau hypothesis, are still in the focus of AD research. With aging as the accepted main risk factor of the most important non familial and late onset sporadic forms of AD, it is now mandatory to discuss more intensively aspects of cellular aging and aging biochemistry and its impact on neurodegeneration. Since aging is accompanied by changes in cellular protein homeostasis and an increasing demand for protein degradation, aspects of protein folding, misfolding, refolding and, importantly, protein degradation need to be linked to AD pathogenesis. This is the purpose of this short review.

Nucleolar stress characterized by downregulation of nucleophosmin: a novel cause of neuronal degeneration

Nucleophosmin (NPM) is a multifunctional nucleolar protein that has been linked with nucleolar stress. In non-neuronal cell lines, NPM may enhance or inhibit the activity of tumor suppressor p53, a major apoptotic protein. The relationship between NPM and p53 in the central nervous system (CNS) remains unknown. Here, we assessed the role of NPM in the CNS using a model of seizure-induced neurodegeneration. We show that NPM overexpression is neuroprotective against kainic acid-induced excitotoxicity, and that downregulation of NPM is pro-apoptotic in a p53-independent manner. These results suggest a key role for NPM in promoting neuronal survival and a novel mechanism of neuronal degeneration triggered by nucleolar stress.

Magnetically enhanced nucleic acid delivery. Ten years of magnetofection-progress and prospects

Nucleic acids carry the building plans of living systems. As such, they can be exploited to make cells produce a desired protein, or to shut down the expression of endogenous genes or even to repair defective genes. Hence, nucleic acids are unique substances for research and therapy. To exploit their potential, they need to be delivered into cells which can be a challenging task in many respects. During the last decade, nanomagnetic methods for delivering and targeting nucleic acids have been developed, methods which are often referred to as magnetofection. In this review we summarize the progress and achievements in this field of research. We discuss magnetic formulations of vectors for nucleic acid delivery and their characterization, mechanisms of magnetofection, and the application of magnetofection in viral and nonviral nucleic acid delivery in cell culture and in animal models. We summarize results that have been obtained with using magnetofection in basic research and in preclinical animal models. Finally, we describe some of our recent work and end with some conclusions and perspectives.

Ubiquitin over-expression phenotypes and ubiquitin gene molecular misreading during aging in Drosophila melanogaster

Molecular Misreading (MM) is the inaccurate conversion of genomic information into aberrant proteins. For example, when RNA polymerase II transcribes a GAGAG motif it synthesizes at low frequency RNA with a two-base deletion. If the deletion occurs in a coding region, translation will result in production of misframed proteins. During mammalian aging, misframed versions of human amyloid precursor protein (hApp) and ubiquitin (hUbb) accumulate in the aggregates characteristic of neurodegenerative diseases, suggesting dysfunctional degradation or clearance. Here cDNA clones encoding wild-type hUbb and the frame-shifted version hUbb⁺¹ were expressed in transgenic Drosophila using the doxycycline-regulated system. Misframed proteins were abundantly produced, both from the transgenes and from endogenous Drosophila ubiquitin-encoding genes, and their abundance increased during aging in whole-fly extracts. Over-expression of wild-type hUbb, but not hUbb⁺¹, was toxic during fly development. In contrast, when over-expressed specifically in adult flies, hUbb⁺¹ caused small decreases in life span, whereas hUbb was associated with small increases, preferentially in males. The data suggest that MM occurs in Drosophila and that the resultant misframed proteins accumulate with age. MM of the ubiquitin gene can produce alternative ubiquitin gene products with different and sometimes opposing phenotypic effects.

The regulation of mitochondrial morphology: intricate mechanisms and dynamic machinery

Mitochondria typically form a reticular network radiating from the nucleus, creating an interconnected system that supplies the cell with essential energy and metabolites. These mitochondrial networks are regulated through the complex coordination of fission, fusion and distribution events. While a number of key mitochondrial morphology proteins have been identified, the precise mechanisms which govern their activity remain elusive. Moreover, post translational modifications including ubiquitination, phosphorylation and sumoylation of the core machinery are thought to regulate both fusion and division of the network. These proteins can undergo several different modifications depending on cellular signals, environment and energetic demands of the cell. Proteins involved in mitochondrial morphology may also have dual roles in both dynamics and apoptosis, with regulation of these proteins under tight control of the cell to ensure correct function. The absolute reliance of the cell on a functional mitochondrial network is highlighted in neurons, which are particularly vulnerable to any changes in organelle dynamics due to their unique biochemical requirements. Recent evidence suggests that defects in the shape or distribution of mitochondria correlate with the progression of neurodegenerative diseases such as Alzheimer's, Huntington's and Parkinson's disease. This review focuses on our current understanding of the mitochondrial morphology machinery in cell homeostasis, apoptosis and neurodegeneration, and the post translational modifications that regulate these processes.