Nrf2 mediates the expression of BAG3 and autophagy cargo adaptor proteins and tau clearance in an age-dependent manner

New aspects of a small GTPase RAB35 in brain development and function

In eukaryotic cells, organelles in the secretory, lysosomal, and endocytic pathways actively exchange biological materials with each other through intracellular membrane trafficking, which is the process of transporting the cargo of proteins, lipids, and other molecules to appropriate compartments via transport vesicles or intermediates. These processes are strictly regulated by various small GTPases such as the RAS-like in rat brain (RAB) protein family, which is the largest subfamily of the RAS superfamily. Dysfunction of membrane trafficking affects tissue homeostasis and leads to a wide range of diseases, including neurological disorders and neurodegenerative diseases. Therefore, it is important to understand the physiological and pathological roles of RAB proteins in brain function. RAB35, a member of the RAB family, is an evolutionarily conserved protein in metazoans. A wide range of studies using cultured mammalian cells and model organisms have revealed that RAB35 mediates various processes such as cytokinesis, endocytic recycling, actin bundling, and cell migration. RAB35 is also involved in neurite outgrowth and turnover of synaptic vesicles. We generated brain-specific Rab35 knockout mice to study the physiological roles of RAB35 in brain development and function. These mice exhibited defects in anxiety-related behaviors and spatial memory. Strikingly, RAB35 is required for the precise positioning of pyramidal neurons during hippocampal development, and thereby for normal hippocampal lamination. In contrast, layer formation in the cerebral cortex occurred superficially, even in the absence of RAB35, suggesting a predominant role for RAB35 in hippocampal development rather than in cerebral cortex development. Recent studies have suggested an association between RAB35 and neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. In this review, we provide an overview of the current understanding of subcellular functions of RAB35. We also provide insights into the physiological role of RAB35 in mammalian brain development and function, and discuss the involvement of RAB35 dysfunction in neurodegenerative diseases.

The interplay between tauopathy and aging through interruption of UPR/Nrf2/autophagy crosstalk in the Alzheimer's disease transgenic experimental models

PURPOSE

Alzheimer's disease (AD) is the most common form of tauopathy that usually occursduring aging and unfolded protein response (UPR), oxidative stress and autophagy play a crucialrole in tauopathy-induced neurotoxicity. The aim of this study was to investigate the effects oftauopathy on normal brain aging in a Drosophila model of AD.

METHOD

We investigated the interplay between aging (10, 20, 30, and 40 days) and human tauR406W (htau)-induced cell stress in transgenic fruit flies.

RESULTS

Tauopathy caused significant defects in eye morphology, a decrease in motor function and olfactory memory performance (after 20 days), and an increase in ethanol sensitivity (after 30 days). Our results showed a significant increase in UPR (GRP78 and ATF4), redox signalling (p-Nrf2, total GSH, total SH, lipid peroxidation, and antioxidant activity), and regulatory associated protein of mTOR complex 1 (p-Raptor) activity in the control group after 40 days, while the tauopathy model flies showed an advanced increase in the above markers at 20 days of age. Interestingly, only the control flies showed reduced autophagy by a significant decrease in the autophagosome formation protein (dATG1)/p-Raptor ratio at 40 days of age. Our results were also confirmed by bioinformatic analysis of microarray data from tauPS19 transgenic mice (3, 6, 9, and 12 months), in which tauopathy increased expression of heme oxygenase 1, and glutamate-cysteine ligase catalytic subunit and promote aging in transgenic animals.

CONCLUSIONS

Overall, we suggest that the neuropathological effects of tau aggregates may be accelerated brain aging, where redox signaling and autophagy efficacy play an important role.

Therapeutic effect of nicotinamide mononucleotide on Alzheimer's disease through activating autophagy and anti-oxidative stress

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the deposition of β-amyloid (Aβ) plaques and neurofibrillary tangles composed of tau protein in the brain. These neuropathological hallmarks contribute to cognitive impairment by inducing neuronal loss in the cerebral cortex and hippocampus. Unfortunately, current therapeutic approaches only target symptomatic relief and do not impede disease progression. Nicotinamide mononucleotide (NMN), a precursor of nicotinamide adenine dinucleotide (NAD+), has emerged as a promising candidate for the treatment of age-related neurodegenerative disorders. NMN supplementation could restore NAD+ levels, thereby alleviating neuronal damage and slowing the progression of AD and other aging-associated diseases. AD is closely associated with autophagic impairment and oxidative stress. Our in vivo experiments demonstrated that NMN could ameliorate pathological and behavioral impairments in AD mice. Specifically, NMN enhanced autophagy and promoted p-tau clearance. Meanwhile, NMN could activate the Nrf2/Keap1/NQO1 pathway, thereby reducing the oxidative stress. Immunofluorescence results demonstrated that NMN could alleviate neuronal damage in AD mice. Furthermore, in vitro results showed that the p-tau clearance and antioxidant stress effects of NMN were suppressed by autophagy inhibitor, chloroquine (CQ) or bafilomycin A1 (BafA1), in Aβ-induced PC12 cells. Lastly, when Nrf2 was knocked down, the antioxidant stress, autophagy enhancement, and p-tau clearance effects of NMN were all inhibited. In conclusion, our research indicates that NMN exerts therapeutic effect against AD by activating autophagy and the Nrf2/Keap1/NQO1 pathway through a mutual regulating mechanism of autophagy and antioxidative stress. These findings highlight the promising potential of NMN for the treatment of AD.

Αnti-prion effects of anthocyanins

Prion diseases, also known as Transmissible Spongiform Encephalopathies (TSEs), are protein-based neurodegenerative disorders (NDs) affecting humans and animals. They are characterized by the conformational conversion of the normal cellular prion protein, PrPC, into the pathogenic isoform, PrPSc. Prion diseases are invariably fatal and despite ongoing research, no effective prophylactic or therapeutic avenues are currently available. Anthocyanins (ACNs) are unique flavonoid compounds and interest in their use as potential neuroprotective and/or therapeutic agents against NDs, has increased significantly in recent years. Therefore, we investigated the potential anti-oxidant and anti-prion effects of Oenin and Myrtillin, two of the most common anthocyanins, using the most accepted in the field overexpressing PrPScin vitro model and a cell free protein aggregation model. Our results, indicate both anthocyanins as strong anti-oxidant compounds, upregulating the expression of genes involved in the anti-oxidant response, and reducing the levels of Reactive Oxygen Species (ROS), produced due to pathogenic prion infection, through the activation of the Keap1-Nrf2 pathway. Importantly, they showcased remarkable anti-prion potential, as they not only caused the clearance of pathogenic PrPSc aggregates, but also completely inhibited the formation of PrPSc fibrils in the Cerebrospinal Fluid (CSF) of patients with Creutzfeldt-Jakob disease (CJD). Therefore, Oenin and Myrtillin possess pleiotropic effects, suggesting their potential use as promising preventive and/or therapeutic agents in prion diseases and possibly in the spectrum of neurodegenerative proteinopathies.

Exploitation of Autophagy Inducers in the Management of Dementia: A Systematic Review

The social burden of dementia is remarkable since it affects some 57.4 million people all over the world. Impairment of autophagy in age-related diseases, such as dementia, deserves deep investigation for the detection of novel disease-modifying approaches. Several drugs belonging to different classes were suggested to be effective in managing Alzheimer's disease (AD) by means of autophagy induction. Useful autophagy inducers in AD should be endowed with a direct, measurable effect on autophagy, have a safe tolerability profile, and have the capability to cross the blood-brain barrier, at least with poor penetration. According to the PRISMA 2020 recommendations, we propose here a systematic review to appraise the measurable effectiveness of autophagy inducers in the improvement of cognitive decline and neuropsychiatric symptoms in clinical trials and retrospective studies. The systematic search retrieved 3067 records, 10 of which met the eligibility criteria. The outcomes most influenced by the treatment were cognition and executive functioning, pointing at a role for metformin, resveratrol, masitinib and TPI-287, with an overall tolerable safety profile. Differences in sample power, intervention, patients enrolled, assessment, and measure of outcomes prevents generalization of results. Moreover, the domain of behavioral symptoms was found to be less investigated, thus prompting new prospective studies with homogeneous design. PROSPERO registration: CRD42023393456.

Insulin and aging - a disappointing relationship

Experimental studies in animal models of aging such as nematodes, fruit flies or mice have observed that decreased levels of insulin or insulin signaling promotes longevity. In humans, hyperinsulinemia and concomitant insulin resistance are associated with an elevated risk of age-related diseases suggestive of a shortened healthspan. Age-related disorders include neurodegenerative diseases, hypertension, cardiovascular disease, and type 2 diabetes. High ambient insulin concentrations promote increased lipogenesis and fat storage, heightened protein synthesis and accumulation of non-functional polypeptides due to limited turnover capacity. Moreover, there is impaired autophagy activity, and less endothelial NO synthase activity. These changes are associated with mitochondrial dysfunction and oxidative stress. The cellular stress induced by anabolic activity of insulin initiates an adaptive response aiming at maintaining homeostasis, characterized by activation of the transcription factor Nrf2, of AMP activated kinase, and an unfolded protein response. This protective response is more potent in the long-lived human species than in short-lived models of aging research resulting in a stronger pro-aging impact of insulin in nematodes and fruit flies. In humans, resistance to insulin-induced cell stress decreases with age, because of an increase of insulin and insulin resistance levels but less Nrf2 activation. These detrimental changes might be contained by adopting a lifestyle that promotes low insulin/insulin resistance levels and enhances an adaptive response to cellular stress, as observed with dietary restriction or exercise.

Aging, NRF2, and TAU: A Perfect Match for Neurodegeneration?

Although the trigger for the neurodegenerative disease process is unknown, the relevance of aging stands out as a major risk for the development of neurodegeneration. In this review, we highlighted the relationship between the different cellular mechanisms that occur as a consequence of aging and transcription factor nuclear factor erythroid-2-related factor 2 (NRF2) and the connection with the TAU protein. We focused on the relevance of NRF2 in the main processes involved in neurodegeneration and associated with aging, such as genomic instability, protein degradation systems (proteasomes/autophagy), cellular senescence, and stem cell exhaustion, as well as inflammation. We also analyzed the effect of aging on TAU protein levels and its aggregation and spread process. Finally, we investigated the interconnection between NRF2 and TAU and the relevance of alterations in the NRF2 signaling pathway in both primary and secondary tauopathies. All these points highlight NRF2 as a possible therapeutic target for tauopathies.

Commentary: BAG3 as a Mediator of Endosome Function and Tau Clearance

Tauopathies are a group of heterogeneous neurodegenerative conditions characterized by the deposition of abnormal tau protein in the brain. The underlying mechanisms that contribute to the accumulation of tau in these neurodegenerative diseases are multifactorial; nonetheless, there is a growing awareness that dysfunction of endosome-lysosome pathways is a pivotal factor. BCL2 associated athanogene 3 (BAG3) is a multidomain protein that plays a key role in maintaining neuronal proteostasis. Further, recent data indicate that BAG3 plays an important role in mediating vacuolar-dependent degradation of tau. Overexpression of BAG3 in a tauopathy mouse model decreased pathological tau levels and alleviated synapse loss. High throughput screens of BAG3 interactors have identified key players in the vacuolar system; these include clathrin and regulators of small GTPases. These findings suggest that BAG3 is an important regulator of endocytic pathways. In this commentary, we discuss the potential mechanisms by which BAG3 regulates the vacuolar system and tau proteostasis.

The Role of the Transcription Factor Nrf2 in Alzheimer’s Disease: Therapeutic Opportunities

Alzheimer’s disease (AD) is a common neurodegenerative disorder that affects the elderly. One of the key features of AD is the accumulation of reactive oxygen species (ROS), which leads to an overall increase in oxidative damage. The nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a master regulator of the antioxidant response in cells. Under low ROS levels, Nrf2 is kept in the cytoplasm. However, an increase in ROS production leads to a translocation of Nrf2 into the nucleus, where it activates the transcription of several genes involved in the cells’ antioxidant response. Additionally, Nrf2 activation increases autophagy function. However, in AD, the accumulation of Aβ and tau reduces Nrf2 levels, decreasing the antioxidant response. The reduced Nrf2 levels contribute to the further accumulation of Aβ and tau by impairing their autophagy-mediated turnover. In this review, we discuss the overwhelming evidence indicating that genetic or pharmacological activation of Nrf2 is as a potential approach to mitigate AD pathology.

The chaperone-assisted selective autophagy complex dynamics and dysfunctions

Each protein must be synthesized with the correct amino acid sequence, folded into its native structure, and transported to a relevant subcellular location and protein complex. If any of these steps fail, the cell has the capacity to break down aberrant proteins to maintain protein homeostasis (also called proteostasis). All cells possess a set of well-characterized protein quality control systems to minimize protein misfolding and the damage it might cause. Autophagy, a conserved pathway for the degradation of long-lived proteins, aggregates, and damaged organelles, was initially characterized as a bulk degradation pathway. However, it is now clear that autophagy also contributes to intracellular homeostasis by selectively degrading cargo material. One of the pathways involved in the selective removal of damaged and misfolded proteins is chaperone-assisted selective autophagy (CASA). The CASA complex is composed of three main proteins (HSPA, HSPB8 and BAG3), essential to maintain protein homeostasis in muscle and neuronal cells. A failure in the CASA complex, caused by mutations in the respective coding genes, can lead to (cardio)myopathies and neurodegenerative diseases. Here, we summarize our current understanding of the CASA complex and its dynamics. We also briefly discuss how CASA complex proteins are involved in disease and may represent an interesting therapeutic target.Abbreviation ALP: autophagy lysosomal pathway; ALS: amyotrophic lateral sclerosis; AMOTL1: angiomotin like 1; ARP2/3: actin related protein 2/3; BAG: BAG cochaperone; BAG3: BAG cochaperone 3; CASA: chaperone-assisted selective autophagy; CMA: chaperone-mediated autophagy; DNAJ/HSP40: DnaJ heat shock protein family (Hsp40); DRiPs: defective ribosomal products; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK1/HRI: eukaryotic translation initiation factor 2 alpha kinase 1; GABARAP: GABA type A receptor-associated protein; HDAC6: histone deacetylase 6; HSP: heat shock protein; HSPA/HSP70: heat shock protein family A (Hsp70); HSP90: heat shock protein 90; HSPB8: heat shock protein family B (small) member 8; IPV: isoleucine-proline-valine; ISR: integrated stress response; KEAP1: kelch like ECH associated protein 1; LAMP2A: lysosomal associated membrane protein 2A; LATS1: large tumor suppressor kinase 1; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOC: microtubule organizing center; MTOR: mechanistic target of rapamycin kinase; NFKB/NF-κB: nuclear factor kappa B; NFE2L2: NFE2 like bZIP transcription factor 2; PLCG/PLCγ: phospholipase C gamma; polyQ: polyglutamine; PQC: protein quality control; PxxP: proline-rich; RAN translation: repeat-associated non-AUG translation; SG: stress granule; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; STUB1/CHIP: STIP1 homology and U-box containing protein 1; STK: serine/threonine kinase; SYNPO: synaptopodin; TBP: TATA-box binding protein; TARDBP/TDP-43: TAR DNA binding protein; TFEB: transcription factor EB; TPR: tetratricopeptide repeats; TSC1: TSC complex subunit 1; UBA: ubiquitin associated; UPS: ubiquitin-proteasome system; WW: tryptophan-tryptophan; WWTR1: WW domain containing transcription regulator 1; YAP1: Yes1 associated transcriptional regulator.

A macrocyclic molecule with multiple antioxidative activities protects the lens from oxidative damage

Growing evidence links oxidative stress to the development of a cataract and other diseases of the eye. Treatments for lens-derived diseases are still elusive outside of the standard surgical interventions, which still carry risks today. Therefore, a potential drug molecule OHPy2N2 was explored for the ability to target multiple components of oxidative stress in the lens to prevent cataract formation. Several pathways were identified. Here we show that the OHPy2N2 molecule activates innate catalytic mechanisms in primary lens epithelial cells to prevent damage induced by oxidative stress. This protection was linked to the upregulation of Nuclear factor erythroid-2-related factor 2 and downstream antioxidant enzyme for glutathione-dependent glutaredoxins, based on Western Blot methods. The anti-ferroptotic potential was established by showing that OHPy2N2 increases levels of glutathione peroxidase, decreases lipid peroxidation, and readily binds iron (II) and (III). The bioenergetics pathway, which has been shown to be negatively impacted in many diseases involving oxidative stress, was also enhanced as evidence by increased levels of Adenosine triphosphate product when the lens epithelial cells were co-incubated with OHPy2N2. Lastly, OHPy2N2 was also found to prevent oxidative stress-induced lens opacity in an ex vivo organ culture model. Overall, these results show that there are multiple pathways that the OHPy2N2 has the ability to impact to promote natural mechanisms within cells to protect against chronic oxidative stress in the eye.

AR12 increases BAG3 expression which is essential for Tau and APP degradation via LC3-associated phagocytosis and macroautophagy

We defined the mechanisms by which the chaperone ATPase inhibitor AR12 and the multi-kinase inhibitor neratinib interacted to reduce expression of Tau and amyloid-precursor protein (APP) in microglia and neuronal cells. AR12 and neratinib interacted to increase the phosphorylation of eIF2A S51 and the expression of BAG3, Beclin1 and ATG5, and in parallel, enhanced autophagosome formation and autophagic flux. Knock down of BAG3, Beclin1 or ATG5 abolished autophagosome formation and significantly reduced degradation of p62, LAMP2, Tau, APP, and GRP78 (total and plasma membrane). Knock down of Rubicon, a key component of LC3-associated phagocytosis (LAP), significantly reduced autophagosome formation but not autophagic flux and prevented degradation of Tau, APP, and cell surface GRP78, but not ER-localized GRP78. Knock down of Beclin1, ATG5 or Rubicon or over-expression of GRP78 prevented the significant increase in eIF2A phosphorylation. Knock down of eIF2A prevented the increase in BAG3 expression and significantly reduced autophagosome formation, autophagic flux, and it prevented Tau and APP degradation. We conclude that AR12 has the potential to reduce Tau and APP levels in neurons and microglia via the actions of LAP, endoplasmic reticulum stress signaling and macroautophagy. We hypothesize that the initial inactivation of GRP78 catalytic function by AR12 facilitates an initial increase in eIF2A phosphorylation which in turn is essential for greater levels of eIF2A phosphorylation, greater levels of BAG3 and macroautophagy and eventually leading to significant amounts of APP/Tau degradation.

BAG3 Regulation of RAB35 Mediates the Endosomal Sorting Complexes Required for Transport/Endolysosome Pathway and Tau Clearance

BACKGROUND

Declining proteostasis with aging contributes to increased susceptibility to neurodegenerative diseases, including Alzheimer's disease (AD). Emerging studies implicate impairment of the endosome-lysosome pathway as a significant factor in the pathogenesis of these diseases. Previously, we demonstrated that BAG3 regulates phosphorylated tau clearance. However, we did not fully define how BAG3 regulates endogenous tau proteostasis, especially in the early stages of disease progression.

METHODS

Mass spectrometric analyses were performed to identify neuronal BAG3 interactors. Multiple biochemical assays were used to investigate the BAG3-HSP70-TBC1D10B (EPI64B)-RAB35-HRS regulatory networks. Live-cell imaging was used to study the dynamics of the endosomal pathway. Immunohistochemistry and immunoblotting were performed in human AD brains and in P301S tau transgenic mice with BAG3 overexpressed.

RESULTS

The primary group of neuronal BAG3 interactors identified are involved in the endocytic pathway. Among them were key regulators of small GTPases, such as the RAB35 GTPase-activating protein TBC1D10B. We demonstrated that a BAG3-HSP70-TBC1D10B complex attenuates the ability of TBC1D10B to inactivate RAB35. Thus, BAG3 interacts with TBC1D10B to support the activation of RAB35 and recruitment of HRS, initiating endosomal sorting complex required for transport-mediated endosomal tau clearance. Furthermore, TBC1D10B shows significantly less colocalization with BAG3 in AD brains than in age-matched controls. Overexpression of BAG3 in P301S tau transgenic mice increased the colocalization of phosphorylated tau with the endosomal sorting complex required for transport III protein CHMP2B and reduced the levels of the mutant human tau.

CONCLUSIONS

We identified a novel BAG3-TBC1D10B-RAB35 regulatory axis that modulates endosomal sorting complex required for transport-dependent protein degradation machinery and tau clearance. Dysregulation of BAG3 could contribute to the pathogenesis of AD.

Redox signaling at the crossroads of human health and disease

Redox biology is at the core of life sciences, accompanied by the close correlation of redox processes with biological activities. Redox homeostasis is a prerequisite for human health, in which the physiological levels of nonradical reactive oxygen species (ROS) function as the primary second messengers to modulate physiological redox signaling by orchestrating multiple redox sensors. However, excessive ROS accumulation, termed oxidative stress (OS), leads to biomolecule damage and subsequent occurrence of various diseases such as type 2 diabetes, atherosclerosis, and cancer. Herein, starting with the evolution of redox biology, we reveal the roles of ROS as multifaceted physiological modulators to mediate redox signaling and sustain redox homeostasis. In addition, we also emphasize the detailed OS mechanisms involved in the initiation and development of several important diseases. ROS as a double-edged sword in disease progression suggest two different therapeutic strategies to treat redox-relevant diseases, in which targeting ROS sources and redox-related effectors to manipulate redox homeostasis will largely promote precision medicine. Therefore, a comprehensive understanding of the redox signaling networks under physiological and pathological conditions will facilitate the development of redox medicine and benefit patients with redox-relevant diseases.

Keeping synapses in shape: degradation pathways in the healthy and aging brain

Synapses maintain their molecular composition, plasticity and function through the concerted action of protein synthesis and removal. The complex and polarized neuronal architecture poses specific challenges to the logistics of protein and organelle turnover since protein synthesis and degradation mainly happen in the cell soma. In addition, post-mitotic neurons accumulate damage over a lifetime, challenging neuronal degradative pathways and making them particularly susceptible to the effects of aging. This review will summarize the current knowledge on neuronal protein turnover mechanisms with a particular focus on the presynapse, including the proteasome, autophagy and the endolysosomal route and their roles in regulating presynaptic proteostasis and function. In addition, the author will discuss how physiological brain aging, which entails a progressive decline in cognitive functions, affects synapses and the degradative machinery.

Hepatic lipid accumulation induced by a high-fat diet is regulated by Nrf2 through multiple pathways

Nuclear factor erythroid 2-related factor 2 (Nrf2) is reportedly involved in hepatic lipid metabolism, but the results are contradictory, and the underlying mechanism remains unclear. Here, we focused on elucidating the effects of Nrf2 on hepatic adipogenesis and on determining the possible underlying mechanism. We established a non-alcoholic fatty liver disease (NAFLD) model in a high-fat diet (HFD)-fed Nrf2 knockout (Nrf2 KO) mice; further, a cell model of lipid accumulation was established using mouse primary hepatocytes (MPHs) treated with free fatty acids (FAs). Using these models, we investigated the relationship between Nrf2 and autophagy and its role in the development of NAFLD. We observed that Nrf2 expression levels were upregulated in patients with NAFLD and diet-induced obese mice. Nrf2 deficiency led to hepatic lipid accumulation in vivo and in vitro, in addition to, promoting lipogenesis mainly by increasing SREBP-1c activity. Moreover, Nrf2 deficiency attenuated autophagic flux and inhibited the fusion of autophagosomes and lysosomes in vivo and in vitro. Decreased autophagy caused reduced lipolysis in the liver. Importantly, chromatin immunoprecipitation-qPCR (ChIP-qPCR) and dual-luciferase assay results proved that Nrf2 bound to the LAMP1 promoter and regulated its transcriptional activity. Accordingly, we report that Nrf2-LAMP1 interaction plays an indispensable role in Nrf2-regulated hepatosteatosis. Our data collectively confirm that Nrf2 deficiency promotes hepatosteatosis by enhancing SREBP-1c activity and attenuating autophagy. Our findings provide a novel multi-pathway effect of Nrf2 on lipid metabolism in the liver. We believe that multi-target intervention of Nrf2 is a novel strategy for the treatment of NAFLD.

A Perspective on Nrf2 Signaling Pathway for Neuroinflammation: A Potential Therapeutic Target in Alzheimer's and Parkinson's Diseases

Neuroinflammation plays a pivotal role in Alzheimer's disease (AD) and Parkinson's disease (PD), the leading causes of dementia. These neurological disorders are characterized by the accumulation of misfolded proteins such as amyloid-ß (Aß), tau protein and α-synuclein, contributing to mitochondrial fragmentation, oxidative stress, and neuroinflammation. Misfolded proteins activate microglia, which induces neuroinflammation, expression of pro-inflammatory cytokines and subsequently facilitates synaptic damage and neuronal loss. So far, all the proposed drugs were based on the inhibition of protein aggregation and were failed in clinical trials. Therefore, the treatment options of dementia are still a challenging issue. Thus, it is worthwhile to study alternative therapeutic strategies. In this context, there is increasing data on the pivotal role of transcription factor NF- E2 p45-related factor 2 (Nrf2) on the redox homeostasis and anti-inflammatory functions in neurodegenerative disorders. Interestingly, Nrf2 signaling pathway has shown upregulation of antioxidant genes, inhibition of microglia-mediated inflammation, and improved mitochondrial function in neurodegenerative diseases, suggesting Nrf2 activation could be a novel therapeutic approach to target pathogenesis. The present review will examine the correlation between Nrf2 signaling with neuroinflammation in AD and PD.

The role of BAG3 in health and disease: A "Magic BAG of Tricks"

The multi-domain structure of Bcl-2-associated athanogene 3 (BAG3) facilitates its interaction with many different proteins that participate in regulating a variety of biological pathways. After revisiting the BAG3 literature published over the past ten years with Citespace software, we classified the BAG3 research into several clusters, including cancer, cardiomyopathy, neurodegeneration, and viral propagation. We then highlighted recent key findings in each cluster. To gain greater insight into the roles of BAG3, we analyzed five different published mass spectrometry data sets of proteins that co-immunoprecipitate with BAG3. These data gave us insight into universal, as well as cell-type-specific BAG3 interactors in cancer cells, cardiomyocytes, and neurons. Finally, we mapped variable BAG3 SNPs and also mutation data from previous publications to further explore the link between the domains and function of BAG3. We believe this review will provide a better understanding of BAG3 and direct future studies towards understanding BAG3 function in physiological and pathological conditions.

The NRF2-Dependent Transcriptional Regulation of Antioxidant Defense Pathways: Relevance for Cell Type-Specific Vulnerability to Neurodegeneration and Therapeutic Intervention

Oxidative stress has been implicated in the etiology and pathobiology of various neurodegenerative diseases. At baseline, the cells of the nervous system have the capability to regulate the genes for antioxidant defenses by engaging nuclear factor erythroid 2 (NFE2/NRF)-dependent transcriptional mechanisms, and a number of strategies have been proposed to activate these pathways to promote neuroprotection. Here, we briefly review the biology of the transcription factors of the NFE2/NRF family in the brain and provide evidence for the differential cellular localization of NFE2/NRF family members in the cells of the nervous system. We then discuss these findings in the context of the oxidative stress observed in two neurodegenerative diseases, Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), and present current strategies for activating NFE2/NRF-dependent transcription. Based on the expression of the NFE2/NRF family members in restricted populations of neurons and glia, we propose that, when designing strategies to engage these pathways for neuroprotection, the relative contributions of neuronal and non-neuronal cell types to the overall oxidative state of tissue should be considered, as well as the cell types which have the greatest intrinsic capacity for producing antioxidant enzymes.

Operationally defining cognitive reserve genes

Variability in cognitive decline is related to the environment, lifestyle factors, and individual differences in biological aging, including cognitive reserve, plastic properties of the brain, which account for better-than-expected cognition for a given level of brain aging or pathology. Cognitive reserve has not been thoroughly investigated in aged rodents. To address this gap, cognitive reserve was examined using Gene Expression Omnibus data for the CA1 region of the hippocampus of young and aged behaviorally characterized male rats. Statistical filtering identified brain aging and potential cognitive reserve genes, and multiple regression was employed to confirm cognitive reserve genes as genes that predicted better-than-expected cognition for a given level of brain aging. In general, cognitive reserve genes, in which increased expression was associated with better cognition, were not different with age or directly correlated with measures of cognition and appear to act as negative regulators of aging processes, including neuroinflammation and oxidative stress. The results suggest that, for some animals, resilience mechanisms are activated to counteract aging stressors that impair cognition. In contrast, cognitive reserve genes, in which decreased expression was associated with better cognition, were linked to nervous system development and cation transport, suggesting adaptive changes in the circuit to preserve cognition.

Postnatal development of BAG3 expression in mouse cerebral cortex and hippocampus

The decreased efficiency of autophagic processing in the central nervous system during aging may be a contributing factor in neurodegenerative diseases. BAG3 (Bcl2 associated athanogene 3) is a major member of the BAG family of co-molecular chaperones that mediate selective macroautophagy. Therefore, we analyzed the expression and distribution of BAG3 in the brain at postnatal 0 day (P0), P15, 1-, 2-, 9-, 12-, and 18 month-old C57BL/6 mice, thus covering almost all ages. Except for a significant steep drop in mRNA and protein levels in the cortex and hippocampus soon after birth, there were minimal differences in the expression and distribution of BAG3 among P15, M1, M2, M9, and M12 mice; however, at 18 months, BAG3 expression was significantly higher. Immunohistochemical analyses showed that BAG3 is mainly located in the neuronal cytoplasm and processes in C57BL/6 the cerebral cortex and hippocampus from P0 to M18 postnatal development. These findings indicate that BAG3 might be stable in young and middle-aged mice, but unstable in aged mice.

Roles of Phase Separation for Cellular Redox Maintenance

The oxidation reaction greatly alters characteristics of various cellular components. In exchange for efficient energy production, mitochondrial aerobic respiration substantially increases the risk of excess oxidation of cellular biomolecules such as lipids, proteins, nucleic acids, and numerous small molecules. To maintain a physiologically balanced cellular reduction-oxidation (redox) state, cells utilize a variety of molecular machineries including cellular antioxidants and protein degradation complexes such as the ubiquitin-proteasome system or autophagy. In the past decade, biomolecular liquid-liquid phase separation (LLPS) has emerged as a subject of great interest in the biomedical field, as it plays versatile roles in the maintenance of cellular homeostasis. With regard to redox homeostasis, LLPS arose as a major player in both well-characterized and newly emerging redox pathways. LLPS is involved in direct redox imbalance sensing, signal transduction, and transcriptional regulation. Also, LLPS is at play when cells resist redox imbalance through metabolic switching, translational remodeling, activating the DNA damage response, and segregation of vulnerable lipids and proteins. On the other hand, chronic accumulation of phase-separated molecular condensates such as lipid droplets and amyloid causes neurotoxic outcomes. In this review we enumerate recent progress on understanding how cells utilize LLPS to deal with oxidative stress, especially related to cell survival or pathogenesis, and we discuss future research directions for understanding biological phase separation in cellular redox regulation.

H2S probe CPC inhibits autophagy and promotes apoptosis by inhibiting glutathionylation of Keap1 at Cys434

H2S is actual an endogenous signaling gas molecule and involved in a range of cell physiological processes. However, the mechanism of endogenous H2S regulating autophagy and apoptosis has not been thoroughly investigated. Here, we try to address this issue by using a H2S probe, (E)-2-(4-(4-(7-(diethylamino)-2-oxo-2H-chromene-3-carbonyl)-piperazin-1-yl)-styryl)-1, 3, 3-trimethyl-3H-indol-1-ium iodide (CPC), which could react with endogenous H2S. Herein, we reported that CPC inhibited autophagy and decreased the expression and activity of NF-E2-related factor 2 (Nrf2), then induced cell apoptosis. CPC inhibited autophagy and promoted apoptosis by inhibiting Nrf2 activation, which was H2S dependent. Furthermore, we found that CPC inhibited Nrf2 nucleus translocation by inhibiting glutathionylation of Kelch-like ECH-associated protein 1 (Keap1) at the Cys434 residue. CPC also inhibited various cancer cell growth, but had no effect on normal cell growth in vitro, and inhibited A549 cancer growth, but did not affect normal angiogenesis in vivo. Therefore, we not only found a new inhibitor of autophagy and Nrf2, but also suggested a novel mechanism that endogenous H2S could regulate autophagy, apoptosis and Nrf2 activity through regulating glutathionylation of Keap1 at the Cys434 residue.

Nrf2: a dark horse in Alzheimer's disease treatment

Alzheimer's disease (AD), an age-dependent neurodegenerative disorder, is the main cause of dementia. Common hallmarks of AD include the amyloid β-peptide (Aβ) aggregation, high levels of hyperphosphorylated tau protein (p-tau) and failure in redox homeostasis. To date, all proposed drugs affecting Aβ and/or p-tau have been failed in clinical trials. A decline in the expression of the transcription factor Nrf2 (nuclear factor-erythroid 2-p45 derived factor 2) and its driven genes (NQO1, HO-1, and GCLC), and alteration of the Nrf2-related pathways have been observed in AD brains. Nrf2 plays a critical role in maintaining cellular redox homeostasis and regulating inflammation response. Nrf2 activation also provides cytoprotection against increasing pathologies including neurodegenerative diseases. These lines of evidence imply that Nrf2 activation may be a novel AD treatment option. Interestingly, recent studies have also demonstrated that Nrf2 interferes with several key pathogenic processes in AD including Aβ and p-tau pathways. The current review aims to provide insights into the role of Nrf2 in AD. Also, we discuss the progress and challenges regarding the Nrf2 activators for AD treatment.

BAG3 Proteomic Signature under Proteostasis Stress

The multifunctional HSP70 co-chaperone BAG3 (BCL-2-associated athanogene 3) represents a key player in the quality control of the cellular proteostasis network. In response to stress, BAG3 specifically targets aggregation-prone proteins to the perinuclear aggresome and promotes their degradation via BAG3-mediated selective macroautophagy. To adapt cellular homeostasis to stress, BAG3 modulates and functions in various cellular processes and signaling pathways. Noteworthy, dysfunction and deregulation of BAG3 and its pathway are pathophysiologically linked to myopathies, cancer, and neurodegenerative disorders. Here, we report a BAG3 proteomic signature under proteostasis stress. To elucidate the dynamic and multifunctional action of BAG3 in response to stress, we established BAG3 interactomes under basal and proteostasis stress conditions by employing affinity purification combined with quantitative mass spectrometry. In addition to the identification of novel potential BAG3 interactors, we defined proteins whose interaction with BAG3 was altered upon stress. By functional annotation and protein-protein interaction enrichment analysis of the identified potential BAG3 interactors, we confirmed the multifunctionality of BAG3 and highlighted its crucial role in diverse cellular signaling pathways and processes, ensuring cellular proteostasis and cell viability. These include protein folding and degradation, gene expression, cytoskeleton dynamics (including cell cycle and transport), as well as granulostasis, in particular.

Network Simulations Reveal Molecular Signatures of Vulnerability to Age-Dependent Stress and Tau Accumulation

Alzheimer’s disease (AD) is the leading cause of dementia and one of the most common causes of death worldwide. As an age-dependent multifactorial disease, the causative triggers of AD are rooted in spontaneous declines in cellular function and metabolic capacity with increases in protein stressors such as the tau protein. This multitude of age-related processes that cause neurons to change from healthy states to ones vulnerable to the damage seen in AD are difficult to simultaneously investigate and even more difficult to quantify. Here we aimed to diminish these gaps in our understanding of neuronal vulnerability in AD development by using simulation methods to theoretically quantify an array of cellular stress responses and signaling molecules. This temporally-descriptive molecular signature was produced using a novel multimethod simulation approach pioneered by our laboratory for biological research; this methodology combines hierarchical agent-based processes and continuous equation-based modeling in the same interface, all while maintaining intrinsic distributions that emulate natural biological stochasticity. The molecular signature was validated for a normal organismal aging trajectory using experimental longitudinal data from Caenorhabditis elegans and rodent studies. In addition, we have further predicted this aging molecular signature for cells impacted by the pathogenic tau protein, giving rise to distinct stress response conditions needed for cytoprotective aging. Interestingly, our simulation experiments showed that oxidative stress signaling (via daf-16 and skn-1 activities) does not substantially protect cells from all the early stressors of aging, but that it is essential in preventing a late-life degenerative cellular phenotype. Together, our simulation experiments aid in elucidating neurodegenerative triggers in the onset of AD for different genetic conditions. The long-term goal of this work is to provide more detailed diagnostic and prognostic tools for AD development and progression, and to provide more comprehensive preventative measures for this disease.

Diabetes Promotes Development of Alzheimer's Disease Through Suppression of Autophagy

Recent studies suggest that diabetes predisposes patients to develop neurodegenerative Alzheimer's disease (AD), although the underlying mechanisms have yet to be determined. Compromised autophagy of neuronal cells, which occurs in the early stages of AD, has been shown to enhance disease progression. However, autophagic regulation as a mechanism connecting diabetes and AD has not been shown before. Here, we found that streptozotocin (STZ)-induced diabetic rats exhibited poorer performance on the social recognition test, Morris water maze, and plus-maze discriminative avoidance task, compared to PBS-treated normoglycemic control rats, likely resulting from increased brain deposition of amyloid-β peptide aggregates (Aβ) and increased phosphorylation of tau protein, two pathological features of AD. Moreover, diabetes-induced AD-like behavioral and pathological changes were associated with a decrease in neuronal cell autophagy. Furthermore, compromised cell autophagy was recapitulated in vitro in neuronal cells cultured in high glucose conditions. Thus, our data suggest that hyperglycemia in diabetes may directly inhibit neuronal cell autophagy, which subsequently enhances AD-associated pathological progression.

Protein–protein interaction network with machine learning models and multiomics data reveal potential neurodegenerative disease-related proteins

Research of protein–protein interaction in several model organisms is accumulating since the development of high-throughput experimental technologies and computational methods. The protein–protein interaction network (PPIN) is able to examine biological processes in a systematic manner and has already been used to predict potential disease-related proteins or drug targets. Based on the topological characteristics of the PPIN, we investigated the application of the random forest classification algorithm to predict proteins that may cause neurodegenerative disease, a set of pathological changes featured by protein malfunction. By integrating multiomics data, we further showed the validity of our machine learning model and narrowed down the prediction results to several hub proteins that play essential roles in the PPIN. The novel insights into neurodegeneration pathogenesis brought by this computational study can indicate promising directions for future experimental research.

Nrf2 deficiency promotes the increasing trend of autophagy during aging in skeletal muscle: a potential mechanism for the development of sarcopenia

This study aims to explore the impact of nuclear factor erythroid 2-related factor 2 (Nrf2) deficiency on skeletal muscle autophagy and the development of sarcopenia. LC3b, P62, Bnip3, Lamp-1, and AMPK protein levels were measured in muscle from young, middle-aged, old Nrf2-/- (knockout, KO) mice and age-matched wild-type (WT) C57/BL6 mice. Autophagy flux was measured in young WT, young KO, old WT, old KO mice, using colchicine as autophagy inhibitor. There was a trend of higher accumulation of LC3b-II, P62, Bnip3, Lamp-1 induced by colchicine in old WT mice compared with young WT mice. Colchicine induced a significantly higher accumulation of LC3b-II, P62, Bnip3, Lamp-1 in KO mice compared with WT mice, both in the young and old groups. AMPK and reactive oxygen species (ROS) were unregulated following Nrf2 KO and increasing age, which was consistent with the increasing trend of autophagy flux following Nrf2 KO and increasing age. Nrf2 KO and increasing age caused decreased cross-sectional area of extensor digitorum longus and soleus muscles. We concluded that Nrf2 deficiency and increasing age may activate AMPK and ROS signals to cause excessive autophagy activation in skeletal muscle, which can be a potential mechanism for the development of sarcopenia.

PL201, a Reported Rhamnoside Against Alzheimer's Disease Pathology, Alleviates Neuroinflammation and Stimulates Nrf2 Signaling

Neuroinflammation induced by overactivated glia cells is believed to be a major hallmark of Alzheimer's disease (AD) and a hopeful target against AD. A rhamnoside PL201 was previously reported to promote neurogenesis and ameliorate AD, and in this study, we revealed that PL201 also significantly reduced accumulation of the activated microglia and proinflammatory cytokines in APP/PS1 mice. In vitro, PL201 consistently suppressed the microglia induction of proinflammatory cytokines after stimulation with lipopolysaccharides and Aβ42. Further mechanistic studies demonstrated that PL201 considerably enhanced the expression level and the nuclear translocation of Nrf2, a key regulator of neuroinflammation. Moreover, PL201 effectively stimulated Nrf2 signaling cascade, including upregulation of HO-1 and downregulation of NF-κB pathway. Thus, our findings indicated the anti-neuroinflammatory effect by PL201 in vivo and suggested that PL201 or the like, with multiple functions such as neurogenesis, mitochondria maintenance, and anti-neuroinflammation, could be a promising candidate in AD treatment.

Targeting Aggrephagy for the Treatment of Alzheimer's Disease

Alzheimer’s disease (AD) is one of the most common neurodegenerative diseases in older individuals with specific neuropsychiatric symptoms. It is a proteinopathy, pathologically characterized by the presence of misfolded protein (Aβ and Tau) aggregates in the brain, causing progressive dementia. Increasing studies have provided evidence that the defect in protein-degrading systems, especially the autophagy-lysosome pathway (ALP), plays an important role in the pathogenesis of AD. Recent studies have demonstrated that AD-associated protein aggregates can be selectively recognized by some receptors and then be degraded by ALP, a process termed aggrephagy. In this study, we reviewed the role of aggrephagy in AD development and discussed the strategy of promoting aggrephagy using small molecules for the treatment of AD.

NBR1-mediated p62-liquid droplets enhance the Keap1-Nrf2 system

p62/SQSTM1 is a multivalent protein that has the ability to cause liquid-liquid phase separation and serves as a receptor protein that participates in cargo isolation during selective autophagy. This protein is also involved in the non-canonical activation of the Keap1-Nrf2 system, a major oxidative stress response pathway. Here, we show a role of neighbor of BRCA1 gene 1 (NBR1), an autophagy receptor structurally similar to p62/SQSTM1, in p62-liquid droplet formation and Keap1-Nrf2 pathway activation. Overexpression of NBR1 blocks selective degradation of p62/SQSTM1 through autophagy and promotes the accumulation and phosphorylation of p62/SQSTM1 in liquid-like bodies, which is required for the activation of Nrf2. NBR1 is induced in response to oxidative stress, which triggers p62-mediated Nrf2 activation. Conversely, loss of Nbr1 suppresses not only the formation of p62/SQSTM1-liquid droplets, but also of p62-dependent Nrf2 activation during oxidative stress. Taken together, our results show that NBR1 mediates p62/SQSTM1-liquid droplet formation to activate the Keap1-Nrf2 pathway.

Perturbation of synapsins homeostasis through HIV-1 Tat-mediated suppression of BAG3 in primary neuronal cells

HIV-1 Tat is known to be released by HIV infected non-neuronal cells in the brain, and after entering neurons, compromises brain homeostasis by impairing pro-survival pathways, thus contributing to the development of HIV-associated CNS disorders commonly observed in individuals living with HIV. Here, we demonstrate that synapsins, phosphoproteins that are predominantly expressed in neuronal cells and play a vital role in modulating neurotransmitter release at the pre-synaptic terminal, and neuronal differentiation become targets for Tat through autophagy and protein quality control pathways. We demonstrate that the presence of Tat in neurons results in downregulation of BAG3, a co-chaperone for heat shock proteins (Hsp70/Hsc70) that is implicated in protein quality control (PQC) processes by eliminating mis-folded and damaged proteins, and selective macroautophagy. Our results show that treatment of cells with Tat or suppression of BAG3 expression by siRNA in neuronal cells disturbs subcellular distribution of synapsins and synaptotagmin 1 (Syt1) leading to their accumulation in the neuronal soma and along axons in a punctate pattern, rather than being properly distributed at axon-terminals. Further, our results revealed that synapsins partially lost their stability and their removal via lysosomal autophagy was noticeably impaired in cells with low levels of BAG3. The observed impairment of lysosomal autophagy, under this condition, is likely caused by cells losing their ability to process LC3-I to LC3-II, in part due to a decrease in the ATG5 levels upon BAG3 knockdown. These observations ascribe a new function for BAG3 in controlling synaptic communications and illuminate a new downstream target for Tat to elicit its pathogenic effect in impacting neuronal cell function and behavior.

BAG3 promotes autophagy and glutaminolysis via stabilizing glutaminase

Bcl-2 associated athanogene 3 (BAG3) is an important molecule that maintains oncogenic features of cancer cells via diverse mechanisms. One of the important functions assigned to BAG3 is implicated in selective macroautophagy/autophagy, which attracts much attention recently. However, the mechanism underlying regulation of autophagy by BAG3 has not been well defined. Here, we describe that BAG3 enhances autophagy via promotion of glutamine consumption and glutaminolysis. Glutaminolysis initiates with deamination of glutamine by glutaminase (GLS), by which yields glutamate and ammonia in mitochondria. The current study demonstrates that BAG3 stabilizes GLS via prohibition its interaction with SIRT5, thereby hindering its desuccinylation at Lys158 and Lys164 sites. As an underlying molecular mechanism, we demonstrate that BAG3 interacts with GLS and decreases SIRT5 expression. The current study also demonstrates that occupation by succinyl at Lys158 and Lys164 sites prohibits its Lys48-linked ubiquitination, thereby preventing its subsequent proteasomal degradation. Collectively, the current study demonstrates that BAG3 enhances autophagy via stabilizing GLS and promoting glutaminolysis. For the first time, this study reports that succinylation competes with ubiquitination to regulate proteasomal GLS degradation.

BAG3 and SYNPO (synaptopodin) facilitate phospho-MAPT/Tau degradation via autophagy in neuronal processes

A major cellular catabolic pathway in neurons is macroautophagy/autophagy, through which misfolded or aggregation-prone proteins are sequestered into autophagosomes that fuse with lysosomes, and are degraded. MAPT (microtubule-associated protein tau) is one of the protein clients of autophagy. Given that accumulation of hyperphosphorylated MAPT contributes to the pathogenesis of Alzheimer disease and other tauopathies, decreasing endogenous MAPT levels has been shown to be beneficial to neuronal health in models of these diseases. A previous study demonstrated that the HSPA/HSP70 co-chaperone BAG3 (BCL2-associated athanogene 3) facilitates endogenous MAPT clearance through autophagy. These findings prompted us to further investigate the mechanisms underlying BAG3-mediated autophagy in the degradation of endogenous MAPT. Here we demonstrate for the first time that BAG3 plays an important role in autophagic flux in the neurites of mature neurons (20-24 days in vitro [DIV]) through interaction with the post-synaptic cytoskeleton protein SYNPO (synaptopodin). Loss of either BAG3 or SYNPO impeded the fusion of autophagosomes and lysosomes predominantly in the post-synaptic compartment. A block of autophagy leads to accumulation of the autophagic receptor protein SQSTM1/p62 (sequestosome 1) as well as MAPT phosphorylated at Ser262 (p-Ser262). Furthermore, p-Ser262 appears to accumulate in autophagosomes at post-synaptic densities. Overall these data provide evidence of a novel role for the co-chaperone BAG3 in synapses. In cooperation with SYNPO, it functions as part of a surveillance complex that facilitates the autophagic clearance of MAPT p-Ser262, and possibly other MAPT species at the post-synapse. This appears to be crucial for the maintenance of a healthy, functional synapse.Abbreviations: aa: amino acids; ACTB: actin beta; BafA1: bafilomycin A_1; BAG3: BCL2 associated athanogene 3; CQ chloroquine; CTSL: cathepsin L; DIV: days in vitro; DLG4/PSD95: discs large MAGUK scaffold protein 4; HSPA/HSP70: heat shock protein family A (Hsp70); MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAP2: microtubule associated protein 2; MAPT: microtubule associated protein tau; p-Ser262: MAPT phosphorylated at serine 262; p-Ser396/404: MAPT phosphorylated at serines 396 and 404; p-Thr231: MAPT phosphorylated at threonine 231; PBS: phosphate buffered saline; PK: proteinase K; scr: scrambled; shRNA: short hairpin RNA; SQSTM1/p62 sequestosome 1; SYN1: synapsin I; SYNPO synaptopodin; SYNPO2/myopodin: synaptopodin 2; VPS: vacuolar protein sorting.

Nrf2 deficiency exacerbates frailty and sarcopenia by impairing skeletal muscle mitochondrial biogenesis and dynamics in an age-dependent manner

AIM

Mitochondrial dysfunction during aging is a key factor that contributes to sarcopenia. Nuclear factor erythroid 2-related factor 2 (Nrf2) has been increasingly recognized to regulate mitochondrial function. The present study aimed to investigate the role of Nrf2 in the development of frailty and sarcopenia during aging, and to demonstrate whether Nrf2 contributes to the maintenance of muscle mass and function by regulation of mitochondrial biogenesis and dynamics during the aging process.

METHODS

Young (5-6 months), middle-aged (11-13 months), old (20-24 months) Nrf2-/- (knockout, KO) mice and age-matched wild-type (WT) C57/BL6 mice were used in this study. Physical function of the mice in the 6 groups was assessed by grip strength test, four paw inverted hanging test, rotarod analysis, open field analysis, and treadmill endurance test. Muscle mass was measured by cross-sectional area (CSA) of tibialis anterior muscles and gastrocnemius muscle weight. The frailty status of the 25 old WT mice and 23 old KO mice were assessed based on the mouse frailty phenotype assessment. Expression levels of genes involved in mitochondrial biogenesis (nuclear respiratory factor 1 (Nrf1), peroxisome proliferative activated receptor, gamma, coactivator 1 alpha (PGC-1α), mitochondrial transcription factor A (TFAM)) and mitochondrial dynamics (optic atrophy protein 1 (Opa1), mitofusin 1 (Mfn1), mitofusin 2 (Mfn2), and dynamin-related protein 1 (Drp1)) were measured in the skeletal muscle. SDH staining was performed and mitochondrial DNA (mtDNA) copy number was measured. Transmission electron microscopy was used to measure the mitochondria number and morphology.

RESULTS

Physical function and muscle mass decreased during aging. The mRNA expression levels of Nrf2 decreased with increasing frailty phenotype scores in the old WT mice. There were minimal differences in the physical function and muscle mass between the WT and KO mice in the young groups, whereas Nrf2 deficiency caused a declined physical function and muscle mass in the middle-aged and old mice, and exacerbated frailty in the old mice. The decreases of the physical function and muscle mass were accompanied by the reduced expression levels of genes involved in mitochondrial biogenesis and dynamics, as well as a reduction of mitochondrial number, mitochondrial content, mtDNA copy number, and an impaired mitochondria morphology in the skeletal muscle.

CONCLUSION

Nrf2 deficiency exacerbated frailty and sarcopenia during aging, at least partially by impairing skeletal muscle mitochondrial biogenesis and dynamics in an age-dependent manner.

Tau Clearance Mechanisms

Efficient quality control mechanisms are essential for a healthy, functional neuron. Recognition and degradation of misfolded, damaged, or potentially toxic proteins, is a crucial aspect of protein quality control. Tau is a protein that is highly expressed in neurons, and plays an important role in modulating a number of physiological processes. Maintaining appropriate levels of tau is key for neuronal health; hence perturbations in tau clearance mechanisms are likely significant contributors to neurodegenerative diseases such as Alzheimer's disease and frontotemporal lobar degeneration. In this chapter we will first briefly review the two primary degradative mechanisms that mediate tau clearance: the proteasome system and the autophagy-lysosome pathway. This will be followed by a discussion about what is known about the contribution of each of these pathways to tau clearance. We will also present recent findings on tau degradation through the endolysosomal system. Further, how deficits in these degradative systems may contribute to the accumulation of dysfunctional or toxic forms of tau in neurodegenerative conditions is considered.

It's all about tau

Tau is a protein that is highly enriched in neurons and was originally defined by its ability to bind and stabilize microtubules. However, it is now becoming evident that the functions of tau extend beyond its ability to modulate microtubule dynamics. Tau plays a role in mediating axonal transport, synaptic structure and function, and neuronal signaling pathways. Although tau plays important physiological roles in neurons, its involvement in neurodegenerative diseases, and most prominently in the pathogenesis of Alzheimer disease (AD), has directed the majority of tau studies. However, a thorough knowledge of the physiological functions of tau and its post-translational modifications under normal conditions are necessary to provide the foundation for understanding its role in pathological settings. In this review, we will focus on human tau, summarizing tau structure and organization, as well as its posttranslational modifications associated with physiological processes. We will highlight possible mechanisms involved in mediating the turnover of tau and finally discuss newly elucidated tau functions in a physiological context.

p62/SQSTM1 - steering the cell through health and disease

SQSTM1 (also known as p62) is a multifunctional stress-inducible scaffold protein involved in diverse cellular processes. Its functions are tightly regulated through an extensive pattern of post-translational modifications, and include the isolation of cargos degraded by autophagy, induction of the antioxidant response by the Keap1-Nrf2 system, as well as the regulation of endosomal trafficking, apoptosis and inflammation. Accordingly, malfunction of SQSTM1 is associated with a wide range of diseases, including bone and muscle disorders, neurodegenerative and metabolic diseases, and multiple forms of cancer. In this Review, we summarize current knowledge regarding regulation, post-translational modifications and functions of SQSTM1, as well as how they are dysregulated in various pathogenic contexts.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual's brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual’s brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.