Phosphorylation of the translation initiation factor eIF2alpha increases BACE1 levels and promotes amyloidogenesis

Nicotinic acetylcholine receptor activation induces BACE1 transcription via the phosphorylation and stabilization of nuclear SP1

Epidemiological studies have shown that cigarette smoking increases the risk of Alzheimer disease. However, inconsistent results have been reported regarding the effects of smoking or nicotine on brain amyloid β (Aβ) deposition. In this study, we found that stimulation of the nicotinic acetylcholine receptor (nAChR) increased Aβ production in mouse brains and cultured neuronal cells. nAChR activation triggered the MEK/ERK pathway, which then phosphorylated and stabilized nuclear SP1. Upregulated SP1 acted on two recognition motifs in the BACE1 gene to induce its transcription, resulting in enhanced Aβ production. Mouse brain microdialysis revealed that nAChR agonists increased Aβ levels in the interstitial fluid of the cerebral cortex but caused no delay of Aβ clearance. In vitro assays indicated that nicotine inhibited Aβ aggregation. We also found that nicotine modified the immunoreactivity of anti-Aβ antibodies, possibly through competitive inhibition and Aβ conformation changes. Using anti-Aβ antibody that was carefully selected to avoid these effects, we found that chronic nicotine treatment in Aβ precursor protein knockin mice increased the Aβ content but did not visibly change the aggregated Aβ deposition in the brain. Thus, nicotine influences brain Aβ deposition in the opposite direction, thereby increasing Aβ production and inhibiting Aβ aggregation.

Glucose hypometabolism prompts RAN translation and exacerbates C9orf72-related ALS/FTD phenotypes

The most prevalent genetic cause of both amyotrophic lateral sclerosis and frontotemporal dementia is a (GGGGCC)n nucleotide repeat expansion (NRE) occurring in the first intron of the C9orf72 gene (C9). Brain glucose hypometabolism is consistently observed in C9-NRE carriers, even at pre-symptomatic stages, but its role in disease pathogenesis is unknown. Here, we show alterations in glucose metabolic pathways and ATP levels in the brains of asymptomatic C9-BAC mice. We find that, through activation of the GCN2 kinase, glucose hypometabolism drives the production of dipeptide repeat proteins (DPRs), impairs the survival of C9 patient-derived neurons, and triggers motor dysfunction in C9-BAC mice. We also show that one of the arginine-rich DPRs (PR) could directly contribute to glucose metabolism and metabolic stress by inhibiting glucose uptake in neurons. Our findings provide a potential mechanistic link between energy imbalances and C9-ALS/FTD pathogenesis and suggest a feedforward loop model with potential opportunities for therapeutic intervention.

Amyloid pathology reduces ELP3 expression and tRNA modifications leading to impaired proteostasis

Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by accumulation of β-amyloid aggregates and loss of proteostasis. Transfer RNA (tRNA) modifications play a crucial role in maintaining proteostasis, but their impact in AD remains unclear. Here, we report that expression of the tRNA modifying enzyme ELP3 is reduced in the brain of AD patients and amyloid mouse models and negatively correlates with amyloid plaque mean density. We further show that SH-SY5Y neuronal cells carrying the amyloidogenic Swedish familial AD mutation (SH-SWE) display reduced ELP3 levels, tRNA hypomodifications and proteostasis impairments when compared to cells not carrying the mutation (SH-WT). Additionally, exposing SH-WT cells to the secretome of SH-SWE cells led to reduced ELP3 expression, wobble uridine tRNA hypomodification, and increased protein aggregation. Importantly, correcting tRNA deficits due to ELP3 reduction reverted proteostasis impairments. These findings suggest that amyloid pathology dysregulates proteostasis by reducing ELP3 expression and tRNA modification levels, and that targeting tRNA modifications may be a potential therapeutic avenue to restore neuronal proteostasis in AD and preserve neuronal function.

Neuroprotective Effects and Therapeutic Potential of Dichloroacetate: Targeting Metabolic Disorders in Nervous System Diseases

Dichloroacetate (DCA) is an investigational drug used to treat lactic acidosis and malignant tumours. It works by inhibiting pyruvate dehydrogenase kinase and increasing the rate of glucose oxidation. Some studies have documented the neuroprotective benefits of DCA. By reviewing these studies, this paper shows that DCA has multiple pharmacological activities, including regulating metabolism, ameliorating oxidative stress, attenuating neuroinflammation, inhibiting apoptosis, decreasing autophagy, protecting the blood‒brain barrier, improving the function of endothelial progenitor cells, improving mitochondrial dynamics, and decreasing amyloid β-protein. In addition, DCA inhibits the enzyme that metabolizes it, which leads to peripheral neurotoxicity due to drug accumulation that may be solved by individualized drug delivery and nanovesicle delivery. In summary, in this review, we analyse the mechanisms of neuroprotection by DCA in different diseases and discuss the causes of and solutions to its adverse effects.

The PPP1R15 Family of eIF2-alpha Phosphatase Targeting Subunits (GADD34 and CReP)

The vertebrate PPP1R15 family consists of the proteins GADD34 (growth arrest and DNA damage-inducible protein 34, the product of the PPP1R15A gene) and CReP (constitutive repressor of eIF2α phosphorylation, the product of the PPP1R15B gene), both of which function as targeting/regulatory subunits for protein phosphatase 1 (PP1) by regulating subcellular localization, modulating substrate specificity and assembling complexes with target proteins. The primary cellular function of these proteins is to facilitate the dephosphorylation of eukaryotic initiation factor 2-alpha (eIF2α) by PP1 during cell stress. In this review, we will provide a comprehensive overview of the cellular function, biochemistry and pharmacology of GADD34 and CReP, starting with a brief introduction of eIF2α phosphorylation via the integrated protein response (ISR). We discuss the roles GADD34 and CReP play as feedback inhibitors of the unfolded protein response (UPR) and highlight the critical function they serve as inhibitors of the PERK-dependent branch, which is particularly important since it can mediate cell survival or cell death, depending on how long the stressful stimuli lasts, and GADD34 and CReP play key roles in fine-tuning this cellular decision. We briefly discuss the roles of GADD34 and CReP homologs in model systems and then focus on what we have learned about their function from knockout mice and human patients, followed by a brief review of several diseases in which GADD34 and CReP have been implicated, including cancer, diabetes and especially neurodegenerative disease. Because of the potential importance of GADD34 and CReP in aspects of human health and disease, we will discuss several pharmacological inhibitors of GADD34 and/or CReP that show promise as treatments and the controversies as to their mechanism of action. This review will finish with a discussion of the biochemical properties of GADD34 and CReP, their regulation and the additional interacting partners that may provide insight into the roles these proteins may play in other cellular pathways. We will conclude with a brief outline of critical areas for future study.

Low glucose induced Alzheimer's disease-like biochemical changes in human induced pluripotent stem cell-derived neurons is due to dysregulated O-GlcNAcylation

INTRODUCTION

Sporadic Alzheimer's disease (sAD) is the leading type of dementia. Brain glucose hypometabolism, along with decreased O-GlcNAcylation levels, occurs before the onset of symptoms and correlates with pathogenesis. Heretofore, the mechanisms involved and the roles of O-GlcNAcylation in sAD pathology largely remain unknown due to a lack of human models of sAD.

METHODS

Human cortical neurons were generated from pluripotent stem cells (PSCs) and treated with glucose reduction media.

RESULTS

We found a narrow window of glucose concentration that induces sAD-like phenotypes in PSC-derived neurons. With our model, we reveal that dysregulated O-GlcNAc, in part through mitochondrial dysfunction, causes the onset of sAD-like changes. We demonstrate the therapeutic potential of inhibiting O-GlcNAcase in alleviating AD-like biochemical changes.

DISCUSSION

Our results suggest that dysregulated O-GlcNAc might be a direct molecular link between hypometabolism and sAD-like alternations. Moreover, this model can be exploited to explore molecular processes and for drug development.

HIGHLIGHTS

Lowering glucose to a critical level causes AD-like changes in cortical neurons. Defective neuronal structure and function were also recapitulated in current model. Dysregulated O-GlcNAcylation links impaired glucose metabolism to AD-like changes. Mitochondrial abnormalities correlate with O-GlcNAcylation and precede AD-like phenotype. Our model provides a platform to study sAD as a metabolic disease in human neurons.

Effect of arctigenin on neurological diseases: A review

ETHNOPHARMACOLOGICAL RELEVANCE

Arctium lappa L. is a common specie of Asteraceae. Its main active ingredient, Arctigenin (AG), in mature seeds exerts pharmacological effects on the Central Nervous System (CNS).

AIM OF THE STUDY

To review studies on the specific effects of the AG mechanism on various CNS diseases and elucidate signal transduction mechanisms and their pharmacological actions.

MATERIALS AND METHODS

This investigation reviewed the essential role of AG in treating neurological disorders. Basic information on Arctium lappa L. was retrieved from the Pharmacopoeia of the People's Republic of China. The related articles from 1981 to 2022 on the network database (including CNKI, PubMed, and Wan Fang and so on) were reviewed using AG and CNS diseases-related terms such as Arctigenin and Epilepsy.

RESULTS

It was confirmed that AG has a therapeutic effect on Alzheimer's disease, Glioma, infectious CNS diseases (such as Toxoplasma and Japanese Encephalitis Virus), Parkinson's disease, Epilepsy, etc. In these diseases, related experiments such as a Western blot analysis revealed that AG could alter the content of some key factors (such as the reduction of Aβ in Alzheimer's disease). However, in-vivo AG's metabolic process and possible metabolites are still undetermined.

CONCLUSION

Based on this review, the existing pharmacological research has indeed made objective progress to elucidate how AG prevents and treats CNS diseases, especially senile degenerative disease such as Alzheimer's diseases. It was revealed that AG could be used as a potential nervous system drug as it has a wide range of effects in theory with markedly high application value, especially in the elder group. However, the existing studies are limited to in-vitro experiments; therefore, little is known about how AG metabolizes and functions in-vivo, limiting its clinical application and requiring further research.

ABCA7-Associated Clinical Features and Molecular Mechanisms in Alzheimer's Disease

Alzheimer's disease (AD) is the most common type of neurodegenerative disease and its pathogenesis is still unclear. Genetic factors are thought to account for a large proportion of the overall AD phenotypes. ATP-binding cassette transporter A7 (ABCA7) is one of the most important risk gene for AD. Multiple forms of ABCA7 variants significantly increase the risk of AD, such as single-nucleotide polymorphisms, premature termination codon variants, missense variants, variable number tandem repeat, mutations, and alternative splicing. AD patients with ABCA7 variants usually exhibit typical clinical and pathological features of traditional AD with a wide age of onset range. ABCA7 variants can alter ABCA7 protein expression levels and protein structure to affect protein functions such as abnormal lipid metabolism, amyloid precursor protein (APP) processing, and immune cell function. Specifically, ABCA7 deficiency can cause neuronal apoptosis by inducing endoplasmic reticulum stress through the PERK/eIF2α pathway. Second, ABCA7 deficiency can increase Aβ production by upregulating the SREBP2/BACE1 pathway and promoting APP endocytosis. In addition, the ability of microglia to phagocytose and degrade Aβ is destroyed by ABCA7 deficiency, leading to reduced clearance of Aβ. Finally, disturbance of lipid metabolism may also be an important method by which ABCA7 variants influence the incidence rate of AD. In the future, more attention should be given to different ABCA7 variants and ABCA7 targeted therapies for AD.

Evidence of disturbed insulin signaling in animal models of Alzheimer's disease

Since glucose reuptake by neurons is mostly independent of insulin, it has been an intriguing question whether insulin has or not any roles in the brain. Consequently, the identification of insulin receptors in the central nervous system has fueled investigations of insulin functions in the brain. It is also already known that insulin can influence glucose reuptake by neurons, mostly during activities that have the highest energy demand. The identification of high density of insulin receptors in the hippocampus also suggests that insulin may present important roles related to memory. In this context, studies have reported worse performance in cognitive tests among diabetic patients. In addition, alterations in the regulation of central insulin pathways have been observed in the brains of Alzheimer's disease (AD) patients. In fact, some authors have proposed AD as a third type of diabetes and recently, our group proposed insulin resistance as a common link between different AD hypotheses. Therefore, in the present narrative review, we intend to revise and gather the evidence of disturbed insulin signaling in experimental animal models of AD.

Why Is Iron Deficiency/Anemia Linked to Alzheimer's Disease and Its Comorbidities, and How Is It Prevented?

Impaired iron metabolism has been increasingly observed in many diseases, but a deeper, mechanistic understanding of the cellular impact of altered iron metabolism is still lacking. In addition, deficits in neuronal energy metabolism due to reduced glucose import were described for Alzheimer's disease (AD) and its comorbidities like obesity, depression, cardiovascular disease, and type 2 diabetes mellitus. The aim of this review is to present the molecular link between both observations. Insufficient cellular glucose uptake triggers increased ferritin expression, leading to depletion of the cellular free iron pool and stabilization of the hypoxia-induced factor (HIF) 1α. This transcription factor induces the expression of the glucose transporters (Glut) 1 and 3 and shifts the cellular metabolism towards glycolysis. If this first line of defense is not adequate for sufficient glucose supply, further reduction of the intracellular iron pool affects the enzymes of the mitochondrial electron transport chain and activates the AMP-activated kinase (AMPK). This enzyme triggers the translocation of Glut4 to the plasma membrane as well as the autophagic recycling of cell components in order to mobilize energy resources. Moreover, AMPK activates the autophagic process of ferritinophagy, which provides free iron urgently needed as a cofactor for the synthesis of heme- and iron-sulfur proteins. Excessive activation of this pathway ends in ferroptosis, a special iron-dependent form of cell death, while hampered AMPK activation steadily reduces the iron pools, leading to hypoferremia with iron sequestration in the spleen and liver. Long-lasting iron depletion affects erythropoiesis and results in anemia of chronic disease, a common condition in patients with AD and its comorbidities. Instead of iron supplementation, drugs, diet, or phytochemicals that improve energy supply and cellular glucose uptake should be administered to counteract hypoferremia and anemia of chronic disease.

The PERK pathway: beneficial or detrimental for neurodegenerative diseases and tumor growth and cancer

Protein kinase R (PKR)-like endoplasmic reticulum (ER) kinase (PERK) is one of the three major sensors in the unfolded protein response (UPR). The UPR is involved in the modulation of protein synthesis as an adaptive response. Prolonged PERK activity correlates with the development of diseases and the attenuation of disease severity. Thus, the current debate focuses on the role of the PERK signaling pathway either in accelerating or preventing diseases such as neurodegenerative diseases, myelin disorders, and tumor growth and cancer. In this review, we examine the current findings on the PERK signaling pathway and whether it is beneficial or detrimental for the above-mentioned disorders.

The proteomic effects of ketone bodies: implications for proteostasis and brain proteinopathies

A growing body of evidence supports the beneficial effects of the ketone bodies (KBs), acetoacetate and β-hydroxybutyrate (BHB), on diverse physiological processes and diseases. Hence, KBs have been suggested as therapeutic tools for neurodegenerative diseases. KBs are an alternative fuel during fasting and starvation as they can be converted to Ac-CoA to produce ATP. A ketogenic diet (KD), enriched in fats and low in carbohydrates, induces KB production in the liver and favors their use in the brain. BHB is the most abundant KB in the circulation; in addition to its role as energy fuel, it exerts many actions that impact the set of proteins in the cell and tissue. BHB can covalently bind to proteins in lysine residues as a new post-translational modification (PTM) named β-hydroxybutyrylation (Kbhb). Kbhb has been identified in many proteins where Kbhb sites can be critical for binding to other proteins or cofactors. Kbhb is mostly found in proteins involved in chromatin structure, DNA repair, regulation of spliceosome, transcription, and oxidative phosphorylation. Histones are the most studied family of proteins with this PTM, and H3K9bhb is the best studied histone mark. Their target genes are mainly related to cell metabolism, chromatin remodeling and the control of circadian rhythms. The role of Kbhb on physiological processes is poorly known, but it might link KB metabolism to cell signaling and genome regulation. BHB also impacts the proteome by influencing proteostasis. This KB can modulate the Unfolded Protein Response (UPR) and autophagy, two processes involved in the maintenance of protein homeostasis through the clearance of accumulated unfolded and damaged proteins. BHB can support proteostasis and regulate the UPR to promote metabolism adaptation in the liver and prevent cell damage in the brain. Also, BHB stimulates autophagy aiding to the degradation of accumulated proteins. Protein aggregation is common to proteinopathies like Alzheimer's (AD) and Parkinson's (PD) diseases, where the KD and BHB treatment have shown favorable effects. In the present review, the current literature supporting the effects of KBs on proteome conformation and proteostasis is discussed, as well as its possible impact on AD and PD.

Glucose Hypometabolism Prompts RAN Translation and Exacerbates C9orf72-related ALS/FTD Phenotypes

The most prevalent genetic cause of both amyotrophic lateral sclerosis and frontotemporal dementia is a (GGGGCC)n nucleotide repeat expansion (NRE) occurring in the first intron of the C9orf72 gene (C9). Brain glucose hypometabolism is consistently observed in C9-NRE carriers, even at pre-symptomatic stages, although its potential role in disease pathogenesis is unknown. Here, we identified alterations in glucose metabolic pathways and ATP levels in the brain of asymptomatic C9-BAC mice. We found that, through activation of the GCN2 kinase, glucose hypometabolism drives the production of dipeptide repeat proteins (DPRs), impairs the survival of C9 patient-derived neurons, and triggers motor dysfunction in C9-BAC mice. We also found that one of the arginine-rich DPRs (PR) can directly contribute to glucose metabolism and metabolic stress. These findings provide a mechanistic link between energy imbalances and C9-ALS/FTD pathogenesis and support a feedforward loop model that opens several opportunities for therapeutic intervention.

Microvascular Contributions to Alzheimer Disease Pathogenesis: Is Alzheimer Disease Primarily an Endotheliopathy?

Alzheimer disease (AD) models are based on the notion that abnormal protein aggregation is the primary event in AD, which begins a decade or longer prior to symptom onset, and culminates in neurodegeneration; however, emerging evidence from animal and clinical studies suggests that reduced blood flow due to capillary loss and endothelial dysfunction are early and primary events in AD pathogenesis, which may precede amyloid and tau aggregation, and contribute to neuronal and synaptic injury via direct and indirect mechanisms. Recent data from clinical studies suggests that endothelial dysfunction is closely associated with cognitive outcomes in AD and that therapeutic strategies which promote endothelial repair in early AD may offer a potential opportunity to prevent or slow disease progression. This review examines evidence from clinical, imaging, neuropathological, and animal studies supporting vascular contributions to the onset and progression of AD pathology. Together, these observations support the notion that the onset of AD may be primarily influenced by vascular, rather than neurodegenerative, mechanisms and emphasize the importance of further investigations into the vascular hypothesis of AD.

GADD34 suppresses eIF2α phosphorylation and improves cognitive function in Alzheimer's disease-model mice

Alzheimer's disease (AD) causes neurodegeneration, leading to cognitive impairment and memory loss. Our previous studies have demonstrated that the induction of growth arrest and DNA damage-inducible gene 34 (GADD34) by quercetin can affect eukaryotic translation initiation factor 2α (eIF2α) phosphorylation-activated transcription factor 4 (ATF4) signaling. However, the relationship between GADD34 expression and cognitive function has not been clarified. In this study, we determined the direct effect of GADD34 on memory. To achieve this, truncated GADD34 (GADD34.5) was injected into the mouse brain to suppress eIF2α phosphorylation and evaluate memory. The injection of GADD34.5 into the hippocampus in AD-model mice did not improve novel object recognition but improved novel object location. The injection of GADD34.5 into the amygdala also resulted in the maintenance of contextual fear memory based on the fear condition test. These results suggest that GADD34 is effective in improving memory for spatial cognition and contextual fear conditioning in AD by inhibiting eIF2α phosphorylation. In summary, GADD34 suppresses eIF2α phosphorylation in the brain and prevents memory loss. As quercetin feeding increases GADD34 expression, it might be used in preventative applications for AD.

Ferroptosis and Neurodegenerative Diseases: Insights into the Regulatory Roles of SLC7A11

Programed cell death plays a key role in promoting human development and maintaining homeostasis. Ferroptosis is a recently identified pattern of programmed cell death that is closely associated with the onset and progression of neurodegenerative diseases. Ferroptosis is mainly caused by the intracellular accumulation of iron-dependent lipid peroxides. The cysteine/glutamate antibody Solute carrier family 7 member 11 (SLC7A11, also known as xCT) functions to import cysteine for glutathione biosynthesis and antioxidant defense. SLC7A11 has a significant impact on ferroptosis, and inhibition of SLC7A11 expression promotes ferroptosis. Moreover, SLC7A11 is also closely associated with neurodegenerative diseases. In this paper, we summarize the relationship between ferroptosis and neurodegenerative diseases and the role of SLC7A11 during this process. The various regulatory mechanisms of SLC7A11 are also discussed. In conclusion, we are looking forward to a theoretical basis for further understanding the occurrence and development of ferroptosis in SLC7A11 and neurodegenerative diseases, and to seek new clues for the treatment of neurodegenerative diseases.

The potential roles of ATF family in the treatment of Alzheimer's disease

Activating transcription factors, ATFs, is a family of transcription factors that activate gene expression and transcription by recognizing and combining the cAMP response element binding proteins (CREB). It is present in various viruses as a cellular gene promoter. ATFs is involved in regulating the mammalian gene expression that is associated with various cell physiological processes. Therefore, ATFs play an important role in maintaining the intracellular homeostasis. ATF2 and ATF3 is mostly involved in mediating stress responses. ATF4 regulates the oxidative metabolism, which is associated with the survival of cells. ATF5 is presumed to regulate apoptosis, and ATF6 is involved in the regulation of endoplasmic reticulum stress (ERS). ATFs is actively studied in oncology. At present, there has been an increasing amount of research on ATFs for the treatment of neurological diseases. Here, we have focused on the different types of ATFs and their association with Alzheimer's disease (AD). The level of expression of different ATFs have a significant difference in AD patients when compared to healthy control. Recent studies have suggested that ATFs are implicated in the pathogenesis of AD, such as neuronal repair, maintenance of synaptic activity, maintenance of cell survival, inhibition of apoptosis, and regulation of stress responses. In this review, the potential role of ATFs for the treatment of AD has been highlighted. In addition, we have systematically reviewed the progress of research on ATFs in AD. This review will provide a basic and innovative understanding on the pathogenesis and treatment of AD.

Mechanistic and therapeutic role of NLRP3 inflammasome in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder, has emerged as the most common form of dementia in the elderly. Several pathological hallmarks have been identified, including neuroinflammation. A comprehensive insight into the underlying mechanisms that can fuel the development of novel therapeutic approaches is necessary because of the alarmingly rapid increase in the frequency of incidence. Recently, NLRP3 inflammasome was identified as a critical mediator of neuroinflammation. Activation of nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) inflammasome by amyloid, neurofibrillary tangles, impaired autophagy and endoplasmic reticulum stress, triggers the release of pro-inflammatory cytokines such as IL-1β and IL-18. Subsequently, these cytokines can promote neurodegeneration and cognitive impairment. It is well established that genetic or pharmacological ablation of NLRP3 alleviates AD-related pathological features in in vitro and in vivo models. Therefore, several synthetic and natural compounds have been identified that exhibit the potential to inhibit NLRP3 inflammasome and alleviate AD-associated pathology. The current review article will highlight the various mechanisms by which activation of NLRP3 inflammation occurs during Alzheimer's disease, and how it influences neuroinflammation, neurodegeneration and cognitive impairment. Moreover, we will summarise the different small molecules that possess the potential to inhibit NLRP3 and can pave the path for developing novel therapeutic interventions for AD.

Type 2 Diabetes and Alzheimer's Disease: The Emerging Role of Cellular Lipotoxicity

Type 2 diabetes (T2D) and Alzheimer's diseases (AD) represent major health issues that have reached alarming levels in the last decades. Although growing evidence demonstrates that AD is a significant comorbidity of T2D, and there is a ~1.4-2-fold increase in the risk of developing AD among T2D patients, the involvement of possible common triggers in the pathogenesis of these two diseases remains largely unknown. Of note, recent mechanistic insights suggest that lipotoxicity could represent the missing ring in the pathogenetic mechanisms linking T2D to AD. Indeed, obesity, which represents the main cause of lipotoxicity, has been recognized as a major risk factor for both pathological conditions. Lipotoxicity can lead to inflammation, insulin resistance, oxidative stress, ceramide and amyloid accumulation, endoplasmic reticulum stress, ferroptosis, and autophagy, which are shared biological events in the pathogenesis of T2D and AD. In the current review, we try to provide a critical and comprehensive view of the common molecular pathways activated by lipotoxicity in T2D and AD, attempting to summarize how these mechanisms can drive future research and open the way to new therapeutic perspectives.

Dysregulated proteostasis network in neuronal diseases

Long-term maintenance of synaptic connections is important for brain function, which depends on varying proteostatic regulations to govern the functional integrity of neuronal proteomes. Proteostasis supports an interconnection of pathways that regulates the fate of proteins from synthesis to degradation. Defects in proteostatic signaling are associated with age-related functional decline and neurodegenerative diseases. Recent studies have advanced our knowledge of how cells have evolved distinct mechanisms to safely control protein homeostasis during synthesis, folding and degradation, and in different subcellular organelles and compartments. Neurodegeneration occurs when these protein quality controls are compromised by accumulated pathogenic proteins or aging to an irreversible state. Consequently, several therapeutic strategies, such as targeting the unfolded protein response and autophagy pathways, have been developed to reduce the burden of misfolded proteins and proved useful in animal models. Here, we present a brief overview of the molecular mechanisms involved in maintaining proteostatic networks, along with some examples linking dysregulated proteostasis to neuronal diseases.

Exploring links between 2-oxoglutarate-dependent oxygenases and Alzheimer's disease

Hypoxia, that is, an inadequate oxygen supply, is linked to neurodegeneration and patients with cardiovascular disease are prone to Alzheimer's disease (AD). 2-Oxoglutarate and ferrous iron-dependent oxygenases (2OGDD) play a key role in the regulation of oxygen homeostasis by acting as hypoxia sensors. 2OGDD also have roles in collagen biosynthesis, lipid metabolism, nucleic acid repair, and the regulation of transcription and translation. Many biological processes in which the >60 human 2OGDD are involved are altered in AD patient brains, raising the question as to whether 2OGDD are involved in the transition from normal aging to AD. Here we give an overview of human 2OGDD and critically discuss their potential roles in AD, highlighting possible relationships with synapse dysfunction/loss. 2OGDD may regulate neuronal/glial differentiation through enzyme activity-dependent mechanisms and modulation of their activity has potential to protect against synapse loss. Work linking 2OGDD and AD is at an early stage, especially from a therapeutic perspective; we suggest integrated pathology and in vitro discovery research to explore their roles in AD is merited. We hope to help enable long-term research on the roles of 2OGDD and, more generally, oxygen/hypoxia in AD. We also suggest shorter term empirically guided clinical studies concerning the exploration of 2OGDD/oxygen modulators to help maintain synaptic viability are of interest for AD treatment.

Hypoxia Inducible Factor-1α binds and activates γ-secretase for Aβ production under hypoxia and cerebral hypoperfusion

Hypoxic-ischemic injury has been linked with increased risk for developing Alzheimer's disease (AD). The underlying mechanism of this association is poorly understood. Here, we report distinct roles for hypoxia-inducible factor-1α (Hif-1α) in the regulation of BACE1 and γ-secretase activity, two proteases involved in the production of amyloid-beta (Aβ). We have demonstrated that Hif-1α upregulates both BACE1 and γ-secretase activity for Aβ production in brain hypoxia-induced either by cerebral hypoperfusion or breathing 10% O2. Hif-1α binds to γ-secretase, which elevates the amount of active γ-secretase complex without affecting the level of individual subunits in hypoxic-ischemic mouse brains. Additionally, the expression of full length Hif-1α increases BACE1 and γ-secretase activity in primary neuronal culture, whereas a transcriptionally incompetent Hif-1α variant only activates γ-secretase. These findings indicate that Hif-1α transcriptionally upregulates BACE1 and nontranscriptionally activates γ-secretase for Aβ production in hypoxic-ischemic conditions. Consequently, Hif-1α-mediated Aβ production may be an adaptive response to hypoxic-ischemic injury, subsequently leading to increased risk for AD. Preventing the interaction of Hif-1α with γ-secretase may therefore be a promising therapeutic strategy for AD treatment.

Ser/Thr phosphatases: One of the key regulators of insulin signaling

Protein phosphorylation is an important post-translational modification that regulates several cellular processes including insulin signaling. The evidences so far have already portrayed the importance of balanced actions of kinases and phosphatases in regulating the insulin signaling cascade. Therefore, elucidating the role of both kinases and phosphatases are equally important. Unfortunately, the role of phosphatases is less studied as compared to kinases. Since brain responds to insulin and insulin signaling is reported to be crucial for many neuronal processes, it is important to understand the role of neuronal insulin signaling regulators. Ser/Thr phosphatases seem to play significant roles in regulating neuronal insulin signaling. Therefore, in this review, we discussed the involvement of Ser/Thr phosphatases in regulating insulin signaling and insulin resistance in neuronal system at the backdrop of the same phosphatases in peripheral insulin sensitive tissues.

Error-prone protein synthesis recapitulates early symptoms of Alzheimer disease in aging mice

Age-related neurodegenerative diseases (NDDs) are associated with the aggregation and propagation of specific pathogenic protein species (e.g., Aβ, α-synuclein). However, whether disruption of synaptic homeostasis results from protein misfolding per se rather than accumulation of a specific rogue protein is an unexplored question. Here, we show that error-prone translation, with its frequent outcome of random protein misfolding, is sufficient to recapitulate many early features of NDDs, including perturbed Ca²⁺ signaling, neuronal hyperexcitability, and mitochondrial dysfunction. Mice expressing the ribosomal ambiguity mutation Rps9 D95N exhibited disrupted synaptic homeostasis resulting in behavioral changes reminiscent of early Alzheimer disease (AD), such as learning and memory deficits, maladaptive emotional responses, epileptiform discharges, suppressed circadian rhythmicity, and sleep fragmentation, accompanied by hippocampal NPY expression and cerebral glucose hypometabolism. Collectively, our findings suggest that random protein misfolding may contribute to the pathogenesis of age-related NDDs, providing an alternative framework for understanding the initiation of AD.

AdipoRon induces AMPK activation and ameliorates Alzheimer's like pathologies and associated cognitive impairment in APP/PS1 mice

Alzheimer's disease (AD) is a progressive devastating neurodegenerative disorder characterized by extracellular amyloid beta (Aβ₄₂) plaque formation, hyperphosphorylation of tau protein leading to intracellular neurofibrillary tangle formation. Recently discovered hallmark features responsible for AD pathogenesis are neuronal insulin resistance, dysregulation in adiponectin and AMPK signaling. The presence of adiponectin and its receptor in the brain with its unique anti-diabetic effects and association with neurodegenerative diseases has raised our interest in exploring orally active small molecule adiponectin receptor agonist, AdipoRon. To date, all the available drugs for the treatment of AD provides symptomatic relief and do not stall the progression of the disease. Indeed, it is becoming increasingly apparent to find appropriate targets. Here, we attempt to shed lights on adiponectin receptor agonist, AdipoRon and its downstream molecular targets in reducing disease pathogenesis and insulin resistance. In brain, AdipoRon induced AMPK activation, increased insulin sensitivity, reduced amyloid beta plaque deposition and improved cognitive impairment. Levels of BACE were also downregulated while LDLR, APOE and neprilysin were upregulated promoting amyloid beta clearance from brain. AdipoRon further reduced the chronic inflammatory marker, GFAP and improved synaptic markers PSD-95 and synaptophysin in APP/PS1 mice. Our in-vitro studies further confirmed the potential role of AdipoRon in improving insulin sensitivity by increasing GLUT 4 translocation, glucose uptake and insulin signaling under hyperinsulinemic condition. Our findings suggest that AdipoRon could be a promising lead in the future treatment strategies in the development of effective AD treatment.

Behavioral and Molecular Effects of Thapsigargin-Induced Brain ER- Stress: Encompassing Inflammation, MAPK, and Insulin Signaling Pathway

Accumulation of misfolded proteins, known as endoplasmic reticulum (ER) stress, is known to participate in Alzheimer’s disease (AD). AD is also correlated with impaired central insulin signaling. However, few studies have probed the relationship between memory, central ER stress, inflammation, hippocampal mitogen-activated protein kinase (MAPK) activity and insulin resistance. The present study aimed to investigate the causative role and underlying mechanisms of brain ER stress in memory impairment and develop a reliable animal model for ER-mediated memory loss. Thapsigargin (TG), a known ER stress activator, was centrally administered. The cognitive function of animals was evaluated by the Morris Water Maze (MWM). To verify the induction of central ER stress, we investigated the mRNA expression of UPR markers in the hippocampus. In addition, the activation of ER stress markers, including Bip, CHOP, and some related apoptosis and pro-inflammatory proteins, such as caspase-3, Bax, Bcl-2, TNF-α, MAPK, and insulin signaling markers, were assessed by Western-blots. The results demonstrated that TG impairs spatial cognition and hippocampal insulin signaling. Meanwhile, molecular results showed a concurrent increment of hippocampal UPR markers, apoptosis, P38 activity, and TNF-α. This study introduced TG-induced ER stress as a pharmacological model for memory impairment in rats and revealed some underlying mechanisms.

The neuroprotective effects of glucagon-like peptide 1 in Alzheimer’s and Parkinson’s disease: An in-depth review

Currently, there is no disease-modifying treatment available for Alzheimer’s and Parkinson’s disease (AD and PD) and that includes the highly controversial approval of the Aβ-targeting antibody aducanumab for the treatment of AD. Hence, there is still an unmet need for a neuroprotective drug treatment in both AD and PD. Type 2 diabetes is a risk factor for both AD and PD. Glucagon-like peptide 1 (GLP-1) is a peptide hormone and growth factor that has shown neuroprotective effects in preclinical studies, and the success of GLP-1 mimetics in phase II clinical trials in AD and PD has raised new hope. GLP-1 mimetics are currently on the market as treatments for type 2 diabetes. GLP-1 analogs are safe, well tolerated, resistant to desensitization and well characterized in the clinic. Herein, we review the existing evidence and illustrate the neuroprotective pathways that are induced following GLP-1R activation in neurons, microglia and astrocytes. The latter include synaptic protection, improvements in cognition, learning and motor function, amyloid pathology-ameliorating properties (Aβ, Tau, and α-synuclein), the suppression of Ca2+ deregulation and ER stress, potent anti-inflammatory effects, the blockage of oxidative stress, mitochondrial dysfunction and apoptosis pathways, enhancements in the neuronal insulin sensitivity and energy metabolism, functional improvements in autophagy and mitophagy, elevated BDNF and glial cell line-derived neurotrophic factor (GDNF) synthesis as well as neurogenesis. The many beneficial features of GLP-1R and GLP-1/GIPR dual agonists encourage the development of novel drug treatments for AD and PD.

APP Genetic Deficiency Alters Intracellular Ca2+ Homeostasis and Delays Axonal Degeneration in Dorsal Root Ganglion Sensory Neurons

The activation of self-destructive cellular programs helps sculpt the nervous system during development, but the molecular mechanisms used are not fully understood. Prior studies have investigated the role of the APP in the developmental degeneration of sensory neurons with contradictory results. In this work, we sought to elucidate the impact of APP deletion in the development of the sensory nervous system in vivo and in vitro. Our in vivo data show an increase in the number of sciatic nerve axons in adult male and female APP-null mice, consistent with the hypothesis that APP plays a pro-degenerative role in the development of peripheral axons. In vitro, we show that genetic deletion of APP delays axonal degeneration triggered by nerve growth factor deprivation, indicating that APP does play a pro-degenerative role. Interestingly, APP depletion does not affect caspase-3 levels but significantly attenuates the rise of axoplasmic Ca2+ that occurs during degeneration. We examined intracellular Ca2+ mechanisms that could be involved and found that APP-null DRG neurons had increased Ca2+ levels within the endoplasmic reticulum and enhanced store-operated Ca2+ entry. We also observed that DRG axons lacking APP have more mitochondria than their WT counterparts, but these display a lower mitochondrial membrane potential. Finally, we present evidence that APP deficiency causes an increase in mitochondrial Ca2+ buffering capacity. Our results support the hypothesis that APP plays a pro-degenerative role in the developmental degeneration of DRG sensory neurons, and unveil the importance of APP in the regulation of calcium signaling in sensory neurons.SIGNIFICANCE STATEMENT The nervous system goes through a phase of pruning and programmed neuronal cell death during development to reach maturity. In such context, the role played by the APP in the peripheral nervous system has been controversial, ranging from pro-survival to pro-degenerative. Here we present evidence in vivo and in vitro supporting the pro-degenerative role of APP, demonstrating the ability of APP to alter intracellular Ca2+ homeostasis and mitochondria, critical players of programmed cell death. This work provides a better understanding of the physiological function of APP and its implication in developmental neuronal death in the nervous system.

A Pipeline for Natural Small Molecule Inhibitors of Endoplasmic Reticulum Stress

The homeostasis of eukaryotic cells is inseverable of that of the endoplasmic reticulum (ER). The main function of this organelle is the synthesis and folding of a significant portion of cellular proteins, while it is also the major calcium reservoir of the cell. Upon unresolved ER stress, a set of stress response signaling pathways that are collectively labeled as the unfolded protein response (UPR) is activated. Prolonged or intense activation of this molecular machinery may be deleterious. It is known that compromised ER homeostasis, and consequent UPR activation, characterizes the pathogenesis of neurodegenerative diseases. In an effort to discover new small molecules capable of countering ER stress, we subjected a panel of over 100 natural molecules to a battery of assays designed to evaluate several hallmarks of ER stress. The protective potential of these compounds against ER stress was evaluated at the levels of calcium homeostasis, key gene and protein expression, and levels of protein aggregation in fibroblasts. The most promising compounds were subsequently tested in neuronal cells. This framework resulted in the identification of several bioactive molecules capable of countering ER stress and deleterious events associated to it. Delphinidin stands out as the most promising candidate against neurodegeneration. This compound significantly inhibited the expression of UPR biomarkers, and displayed a strong potential to inhibit protein aggregation in the two aforementioned cell models. Our results indicate that natural products may be a valuable resource in the development of an effective therapeutic strategy against ER stress-related diseases.

Long-Term Depression-Inducing Low Frequency Stimulation Enhances p-Tau181 and p-Tau217 in an Age-Dependent Manner in Live Rats

Background: Cognitive decline in Alzheimer’s disease (AD) correlates with the extent of tau pathology, in particular tau hyperphosphorylation, which is strongly age-associated. Although elevation of cerebrospinal fluid or blood levels of phosphorylated tau (p-Tau) at residues Thr181 (p-Tau181), Thr217 (p-Tau217), and Thr231 (p-Tau231) are proposed to be particularly sensitive markers of preclinical AD, the generation of p-Tau during brain activity is poorly understood. Objective: To study whether the expression levels of p-Tau181, p-Tau217, and p-Tau231 can be enhanced by physiological synaptic long-term depression (LTD) which has been linked to the enhancement of p-Tau in hippocampus. Methods: In vivo electrophysiology was performed in urethane anesthetized young adult and aged male rats. Low frequency electrical stimulation (LFS) was used to induce LTD at CA3 to CA1 synapses. The expression level of p-Tau and total tau was measured in dorsal hippocampus using immunofluorescent staining and/or western blotting. Results: We found that LFS enhanced p-Tau181 and p-Tau217 in an age-dependent manner in the hippocampus of live rats. In contrast, phosphorylation at residues Thr231, Ser202/Thr205, and Ser396 appeared less sensitive to LFS. Pharmacological antagonism of either N-methyl-D-aspartate or metabotropic glutamate 5 receptors inhibited the elevation of both p-Tau181 and p-Tau217. Targeting the integrated stress response, which increases with aging, using a small molecule inhibitor ISRIB, prevented the enhancement of p-Tau by LFS in aged rats. Conclusion: Together, our data provide a novel in vivo means to uncover brain plasticity-related cellular and molecular processes of tau phosphorylation at key sites in health and aging.

The role of eIF2 phosphorylation in cell and organismal physiology: new roles for well-known actors

Control of protein synthesis (mRNA translation) plays key roles in shaping the proteome and in many physiological, including homeostatic, responses. One long-known translational control mechanism involves phosphorylation of initiation factor, eIF2, which is catalysed by any one of four protein kinases, which are generally activated in response to stresses. They form a key arm of the integrated stress response (ISR). Phosphorylated eIF2 inhibits eIF2B (the protein that promotes exchange of eIF2-bound GDP for GTP) and thus impairs general protein synthesis. However, this mechanism actually promotes translation of certain mRNAs by virtue of specific features they possess. Recent work has uncovered many previously unknown features of this regulatory system. Several studies have yielded crucial insights into the structure and control of eIF2, including that eIF2B is regulated by several metabolites. Recent studies also reveal that control of eIF2 and the ISR helps determine organismal lifespan and surprising roles in sensing mitochondrial stresses and in controlling the mammalian target of rapamycin (mTOR). The latter effect involves an unexpected role for one of the eIF2 kinases, HRI. Phosphoproteomic analysis identified new substrates for another eIF2 kinase, Gcn2, which senses the availability of amino acids. Several genetic disorders arise from mutations in genes for eIF2α kinases or eIF2B (i.e. vanishing white matter disease, VWM and microcephaly, epileptic seizures, microcephaly, hypogenitalism, diabetes and obesity, MEHMO). Furthermore, the eIF2-mediated ISR plays roles in cognitive decline associated with Alzheimer's disease. New findings suggest potential therapeutic value in interfering with the ISR in certain settings, including VWM, for example by using compounds that promote eIF2B activity.

Inhibitory Effects of Macelignan on Tau Phosphorylation and Aβ Aggregation in the Cell Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder mainly affecting old population. In this study, two Tau overexpressing cell lines (SH-SY5Y/Tau and HEK293/Tau), N2a/SweAPP cell line, and 3× Transgene (APPswe/PS1M146V/TauP301L) mouse primary nerve cell lines were used as AD models to study the activity and molecular mechanism of macelignan, a natural compound extracted from Myristica fragrans, against AD. Our study showed that macelignan could reduce the phosphorylation of Tau at Thr 231 site, Ser 396 site, and Ser 404 site in two overexpressing Tau cell lines. It also could decrease the phosphorylation of Tau at Ser 404 site in mouse primary neural cells. Further investigation of its mechanism found that macelignan could reduce the phosphorylation of Tau by increasing the level of autophagy and enhancing PP2A activity in Tau overexpressing cells. Additionally, macelignan could activate the PERK/eIF2α signaling pathway to reduce BACE1 translation, which further inhibits the cleavage of APP and ultimately suppresses Aβ deposition in N2a/SweAPP cells. Taken together, our results indicate that macelignan has the potential to be developed as a treatment for AD.

Exogenous Ketones and Lactate as a Potential Therapeutic Intervention for Brain Injury and Neurodegenerative Conditions

Metabolic dysfunction is a ubiquitous underlying feature of many neurological conditions including acute traumatic brain injuries and chronic neurodegenerative conditions. A central problem in neurological patients, in particular those with traumatic brain injuries, is an impairment in the utilization of glucose, which is the predominant metabolic substrate in a normally functioning brain. In such patients, alternative substrates including ketone bodies and lactate become important metabolic candidates for maintaining brain function. While the potential neuroprotective benefits of ketosis have been recognized for up to almost a century, the majority of work has focused on the use of ketogenic diets to induce such a state, which is inappropriate in cases of acute disease due to the prolonged periods of time (i.e., weeks to months) required for the effects of a ketogenic diet to be seen. The following review seeks to explore the neuroprotective effects of exogenous ketone and lactate preparations, which have more recently become commercially available and are able to induce a deep ketogenic response in a fraction of the time. The rapid response of exogenous preparations makes their use as a therapeutic adjunct more feasible from a clinical perspective in both acute and chronic neurological conditions. Potentially, their ability to globally moderate long-term, occult brain dysfunction may also be relevant in reducing lifetime risks of certain neurodegenerative conditions. In particular, this review explores the association between traumatic brain injury and contusion-related dementia, assessing metabolic parallels and highlighting the potential role of exogenous ketone and lactate therapies.

Physical Exercise, a Potential Non-Pharmacological Intervention for Attenuating Neuroinflammation and Cognitive Decline in Alzheimer's Disease Patients

This narrative review summarises the evidence for considering physical exercise (PE) as a non-pharmacological intervention for delaying cognitive decline in patients with Alzheimer’s disease (AD) not only by improving cardiovascular fitness but also by attenuating neuroinflammation. Ageing is the most important risk factor for AD. A hallmark of the ageing process is a systemic low-grade chronic inflammation that also contributes to neuroinflammation. Neuroinflammation is associated with AD, Parkinson’s disease, late-onset epilepsy, amyotrophic lateral sclerosis and anxiety disorders. Pharmacological treatment of AD is currently limited to mitigating the symptoms and attenuating progression of the disease. AD animal model studies and human studies on patients with a clinical diagnosis of different stages of AD have concluded that PE attenuates cognitive decline not only by improving cardiovascular fitness but possibly also by attenuating neuroinflammation. Therefore, low-grade chronic inflammation and neuroinflammation should be considered potential modifiable risk factors for AD that can be attenuated by PE. This opens the possibility for personalised attenuation of neuroinflammation that could also have important health benefits for patients with other inflammation associated brain disorders (i.e., Parkinson’s disease, late-onset epilepsy, amyotrophic lateral sclerosis and anxiety disorders). In summary, life-long, regular, structured PE should be considered as a supplemental intervention for attenuating the progression of AD in human. Further studies in human are necessary to develop optimal, personalised protocols, adapted to the progression of AD and the individual’s mental and physical limitations, to take full advantage of the beneficial effects of PE that include improved cardiovascular fitness, attenuated systemic inflammation and neuroinflammation, stimulated brain Aβ peptides brain catabolism and brain clearance.

Inhibition of the ISR abrogates mGluR5-dependent long-term depression and spatial memory deficits in a rat model of Alzheimer's disease

Soluble amyloid-β-protein (Aβ) oligomers, a major hallmark of AD, trigger the integrated stress response (ISR) via multiple pathologies including neuronal hyperactivation, microvascular hypoxia, and neuroinflammation. Increasing eIF2α phosphorylation, the core event of ISR, facilitates metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), and suppressing its phosphorylation has the opposite effect. Having found the facilitation of mGluR5-LTD by Aβ in live rats, we wondered if suppressing eIF2α phosphorylation cascade would protect against the synaptic plasticity and cognitive disrupting effects of Aβ. We demonstrate here that the facilitation of mGluR5-LTD in a delayed rat model by single i.c.v. injection of synthetic Aβ_1-42. Systemic administration of the small-molecule inhibitor of the ISR called ISRIB (trans-isomer) prevents Aβ-facilitated LTD and abrogates spatial learning and memory deficits in the hippocampus in exogenous synthetic Aβ-injected rats. Moreover, ex vivo evidence indicates that ISRIB normalizes protein synthesis in the hippocampus. Targeting the ISR by suppressing the eIF2α phosphorylation cascade with the eIF2B activator ISRIB may provide protective effects against the synaptic and cognitive disruptive effects of Aβ which likely mediate the early stage of sporadic AD.

Endoplasmic Reticulum Stress-Associated Neuronal Death and Innate Immune Response in Neurological Diseases

Neuronal death and inflammatory response are two common pathological hallmarks of acute central nervous system injury and chronic degenerative disorders, both of which are closely related to cognitive and motor dysfunction associated with various neurological diseases. Neurological diseases are highly heterogeneous; however, they share a common pathogenesis, that is, the aberrant accumulation of misfolded/unfolded proteins within the endoplasmic reticulum (ER). Fortunately, the cell has intrinsic quality control mechanisms to maintain the proteostasis network, such as chaperone-mediated folding and ER-associated degradation. However, when these control mechanisms fail, misfolded/unfolded proteins accumulate in the ER lumen and contribute to ER stress. ER stress has been implicated in nearly all neurological diseases. ER stress initiates the unfolded protein response to restore proteostasis, and if the damage is irreversible, it elicits intracellular cascades of death and inflammation. With the growing appreciation of a functional association between ER stress and neurological diseases and with the improved understanding of the multiple underlying molecular mechanisms, pharmacological and genetic targeting of ER stress are beginning to emerge as therapeutic approaches for neurological diseases.

USP25 inhibition ameliorates Alzheimer's pathology through the regulation of APP processing and Aβ generation

Down syndrome (DS), or trisomy 21, is one of the critical risk factors for early-onset Alzheimer’s disease (AD), implicating key roles for chromosome 21–encoded genes in the pathogenesis of AD. We previously identified a role for the deubiquitinase USP25, encoded on chromosome 21, in regulating microglial homeostasis in the AD brain; however, whether USP25 affects amyloid pathology remains unknown. Here, by crossing 5×FAD AD and Dp16 DS mice, we observed that trisomy 21 exacerbated amyloid pathology in the 5×FAD brain. Moreover, bacterial artificial chromosome (BAC) transgene–mediated USP25 overexpression increased amyloid deposition in the 5×FAD mouse brain, whereas genetic deletion of Usp25 reduced amyloid deposition. Furthermore, our results demonstrate that USP25 promoted β cleavage of APP and Aβ generation by reducing the ubiquitination and lysosomal degradation of both APP and BACE1. Importantly, pharmacological inhibition of USP25 ameliorated amyloid pathology in the 5×FAD mouse brain. In summary, we identified the DS-related gene USP25 as a critical regulator of AD pathology, and our data suggest that USP25 serves as a potential pharmacological target for AD drug development.

Sleep deficiency as a driver of cellular stress and damage in neurological disorders

Neurological disorders encompass an extremely broad range of conditions, including those that present early in development and those that progress slowly or manifest with advanced age. Although these disorders have distinct underlying etiologies, the activation of shared pathways, e.g., integrated stress response (ISR) and the development of shared phenotypes (sleep deficits) may offer clues toward understanding some of the mechanistic underpinnings of neurologic dysfunction. While it is incontrovertibly complex, the relationship between sleep and persistent stress in the brain has broad implications in understanding neurological disorders from development to degeneration. The convergent nature of the ISR could be a common thread linking genetically distinct neurological disorders through the dysregulation of a core cellular homeostasis pathway.

Oligomannose-Type Glycan Processing in the Endoplasmic Reticulum and Its Importance in Misfolding Diseases

Simple Summary

Glycans play many roles in biological processes. For instance, they mediate cell–cell interaction, viral infection, and protein folding of glycoproteins. Glycoprotein folding in the endoplasmic reticulum (ER) is closely related to the onset of diseases such as misfolding diseases caused by accumulation of misfolded proteins in the ER. In this review, we focused on oligomannose-type glycan processing in the ER, which has central roles in glycoprotein folding in the ER, and we summarise relationship between oligomannose-type glycan processing and misfolding diseases arising from the disruption of ER homeostasis.

Abstract

Glycoprotein folding plays a critical role in sorting glycoprotein secretion and degradation in the endoplasmic reticulum (ER). Furthermore, relationships between glycoprotein folding and several diseases, such as type 2 diabetes and various neurodegenerative disorders, are indicated. Patients’ cells with type 2 diabetes, and various neurodegenerative disorders induce ER stress, against which the cells utilize the unfolded protein response for protection. However, in some cases, chronic and/or massive ER stress causes critical damage to cells, leading to the onset of ER stress-related diseases, which are categorized into misfolding diseases. Accumulation of misfolded proteins may be a cause of ER stress, in this respect, perturbation of oligomannose-type glycan processing in the ER may occur. A great number of studies indicate the relationships between ER stress and misfolding diseases, while little evidence has been reported on the connection between oligomannose-type glycan processing and misfolding diseases. In this review, we summarize alteration of oligomannose-type glycan processing in several ER stress-related diseases, especially misfolding diseases and show the possibility of these alteration of oligomannose-type glycan processing as indicators of diseases.

Integrated Stress Response in Neuronal Pathology and in Health

Neurodegeneration involves progressive pathological loss of a specific population of neurons, glial activation, and dysfunction of myelinating oligodendrocytes leading to cognitive impairment and altered movement, breathing, and senses. Neuronal degeneration is a hallmark of aging, stroke, drug abuse, toxic chemical exposure, viral infection, chronic inflammation, and a variety of neurological diseases. Accumulation of intra- and extracellular protein aggregates is a common characteristic of cell pathologies. Excessive production of reactive oxygen species and nitric oxide, induction of endoplasmic reticulum stress, and accumulation of misfolded protein aggregates have been shown to trigger a defensive mechanism called integrated stress response (ISR). Activation of ISR is important for synaptic plasticity in learning and memory formation. However, sustaining of ISR may lead to the development of neuronal pathologies and altered patterns in behavior and perception.

ABCA7, a Genetic Risk Factor Associated with Alzheimer's Disease Risk in African Americans

African American/Black adults are twice as likely to have Alzheimer's disease (AD) compared to non-Hispanic White adults. Genetics partially contributes to this disparity in AD risk, among other factors, as there are several genetic variants associated with AD that are more prevalent in individuals of African or European ancestry. The phospholipid-transporting ATPase ABCA7 (ABCA7) gene has stronger associations with AD risk in individuals with African ancestry than in individuals with European ancestry. In fact, ABCA7 has been shown to have a stronger effect size than the apolipoprotein E (APOE) ɛ4 allele in African American/Black adults. ABCA7 is a transmembrane protein involved in lipid homeostasis and phagocytosis. ABCA7 dysfunction is associated with increased amyloid-beta production, reduced amyloid-beta clearance, impaired microglial response to inflammation, and endoplasmic reticulum stress. This review explores the impact of ABCA7 mutations that increase AD risk in African American/Black adults on ABCA7 structure and function and their contributions to AD pathogenesis. The combination of biochemical/biophysical and 'omics-based studies of these variants needed to elucidate their downstream impact and molecular contributions to AD pathogenesis is highlighted.

GADD34-mediated dephosphorylation of eIF2α facilitates pseudorabies virus replication by maintaining de novo protein synthesis

Viruses have evolved multiple strategies to manipulate their host's translational machinery for the synthesis of viral proteins. A common viral target is the alpha subunit of eukaryotic initiation factor 2 (eIF2α). In this study, we show that global protein synthesis was increased but the eIF2α phosphorylation level was markedly decreased in porcine kidney 15 (PK15) cells infected with pseudorabies virus (PRV), a swine herpesvirus. An increase in the eIF2α phosphorylation level by salubrinal treatment or transfection of constructs expressing wild-type eIF2α or an eIF2α phosphomimetic [eIF2α(S51D)] attenuated global protein synthesis and suppressed PRV replication. To explore the mechanism involved in the inhibition of eIF2α phosphorylation during PRV infection, we examined the phosphorylation status of protein kinase R-like endoplasmic reticulum kinase (PERK) and double-stranded RNA-dependent protein kinase R (PKR), two kinases that regulate eIF2α phosphorylation during infection with numerous viruses. We found that the level of neither phosphorylated (p)-PERK nor p-PKR was altered in PRV-infected cells or the lungs of infected mice. However, the expression of growth arrest and DNA damage-inducible protein 34 (GADD34), which promotes eIF2α dephosphorylation by recruiting protein phosphatase 1 (PP1), was significantly induced both in vivo and in vitro. Knockdown of GADD34 and inhibition of PP1 activity by okadaic acid treatment led to increased eIF2α phosphorylation but significantly suppressed global protein synthesis and inhibited PRV replication. Collectively, these results demonstrated that PRV induces GADD34 expression to promote eIF2α dephosphorylation, thereby maintaining de novo protein synthesis and facilitating viral replication.

Therapeutic Effects of Natural Compounds and Small Molecule Inhibitors Targeting Endoplasmic Reticulum Stress in Alzheimer's Disease

Alzheimer’s disease (AD) is the most common neurodegenerative disease, characterized by progressive cognitive impairment and memory loss. So far, the pathogenesis of AD has not been fully understood. Research have shown that endoplasmic reticulum (ER) stress and unfolded protein response (UPR) participate in the occurrence and development of AD. Furthermore, various studies, both in vivo and in vitro, have shown that targeting ER stress and ER stress-mediated apoptosis contribute to the recovery of AD. Thus, targeting ER stress and ER stress-mediated apoptosis may be effective for treating AD. In this review, the molecular mechanism of ER stress and ER stress-mediated apoptosis, as well as the therapeutic effects of some natural compounds and small molecule inhibitors targeting ER stress and ER stress-mediated apoptosis in AD will be introduced.

Aβ initiates brain hypometabolism, network dysfunction and behavioral abnormalities via NOX2-induced oxidative stress in mice

A predominant trigger and driver of sporadic Alzheimer’s disease (AD) is the synergy of brain oxidative stress and glucose hypometabolism starting at early preclinical stages. Oxidative stress damages macromolecules, while glucose hypometabolism impairs cellular energy supply and antioxidant defense. However, the exact cause of AD-associated glucose hypometabolism and its network consequences have remained unknown. Here we report NADPH oxidase 2 (NOX2) activation as the main initiating mechanism behind Aβ_1-42-related glucose hypometabolism and network dysfunction. We utilize a combination of electrophysiology with real-time recordings of metabolic transients both ex- and in-vivo to show that Aβ_1-42 induces oxidative stress and acutely reduces cellular glucose consumption followed by long-lasting network hyperactivity and abnormalities in the animal behavioral profile. Critically, all of these pathological changes were prevented by the novel bioavailable NOX2 antagonist GSK2795039. Our data provide direct experimental evidence for causes and consequences of AD-related brain glucose hypometabolism, and suggest that targeting NOX2-mediated oxidative stress is a promising approach to both the prevention and treatment of AD.

Anton Malkov, Irina Popova et al. demonstrate that beta-amyloid application induces oxidative stress and reduces glucose consumption in the mouse brain, leading to network hyperactivity and behavioral changes—pathologies similar to those observed early on in Alzheimer’s disease patients. Inhibition of NADPH oxidase 2 (NOX2) rescued these phenotypes, suggesting that NOX2 may represent an important therapeutic target for Alzheimer’s disease.

Casein Kinase 2 dependent phosphorylation of eIF4B regulates BACE1 expression in Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder. Increased Aβ production plays a fundamental role in the pathogenesis of the disease and BACE1, the protease that triggers the amyloidogenic processing of APP, is a key protein and a pharmacological target in AD. Changes in neuronal activity have been linked to BACE1 expression and Aβ generation, but the underlying mechanisms are still unclear. We provide clear evidence for the role of Casein Kinase 2 in the control of activity-driven BACE1 expression in cultured primary neurons, organotypic brain slices, and murine AD models. More specifically, we demonstrate that neuronal activity promotes Casein Kinase 2 dependent phosphorylation of the translation initiation factor eIF4B and this, in turn, controls BACE1 expression and APP processing. Finally, we show that eIF4B expression and phosphorylation are increased in the brain of APPPS1 and APP-KI mice, as well as in AD patients. Overall, we provide a definition of a mechanism linking brain activity with amyloid production and deposition, opening new perspectives from the therapeutic standpoint.

Subtle genomic DNA damage induces intraneuronal production of amyloid-β (1-42) by increasing β-secretase activity

Aberrant accumulation of amyloid-β (Aβ) in brain is the major trigger for pathogenesis in Alzheimer's disease (AD). It is imperative to understand how Aβ attains such toxic levels in the brain parenchyma. We detected that a subtle and tolerable amount of DNA damage, related to aging, increased intraneuronal Aβ_1-42 production both in cultured neuron and in cortex of rodent brain. Strikingly, we also observed elevated levels of mitochondrial fusion and of its major driver protein, MFN2. Hyperfusion of mitochondria may be seen as an adaptive stress response resulting from the induction of ER stress since we detected the activation of both PERK and IRE1α arms of unfolded protein response of ER stress. We found increased phosphorylation of PERK substrate eukaryotic initiation factor 2 α (eIF2α), and upregulation of the downstream effector proteins, ATF4 and CHOP. Concomitantly, increased XBP1 level, the direct effecter protein of IRE-1α, was observed. Reports suggest that eIF2α phosphorylation can increase BACE1 activity, the rate limiting enzyme in Aβ production. Here, we show that inhibiting PERK, decreased Aβ_1-42 level while direct BACE1 inhibition, reduced the mitochondrial fusion. We found increased MFN2 expression in young 5xFAD mice when Aβ plaques and neurodegeneration were absent. Thus, our study indicates that mild DNA damage leads to increased Aβ_1-42 production almost as a consequence of an initial ER stress-directed protective mitochondrial fusion in brain. We propose that an age-related subtle genomic DNA damage may trigger enhanced intraneuronal Aβ_1-42 production in an apparently healthy neuron way before the appearance of clinical symptoms in AD.

Does oxidatively damaged DNA drive amyloid-β generation in Alzheimer's disease? A hypothesis

In Alzheimer's disease (AD), amyloid-β (Aβ) generation and upstream β-secretase 1 (BACE1) expression appear to be driven by oxidative stress via c-Jun N-terminal kinase (JNK), p38, and Interferon-Induced, Double-Stranded RNA-Activated Protein Kinase (PKR). In addition, inflammatory molecules, including lipopolysaccharide (LPS), induce genes central to Aβ genesis, such as BACE1, via nuclear factor-κB (NFκB). However, additional triggers of Aβ generation remain poorly understood and might represent novel opportunities for therapeutic intervention. Based on mechanistic studies and elevated ectopic oxidatively damaged DNA (oxoDNA) levels in preclinical AD, mild cognitive impairment, and AD patients, we hypothesize oxoDNA contributes to β-amyloidosis starting from the earliest stages of AD through multiple pathways. OxoDNA induces mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4), thereby sensitizing the brain to oxidative stress-induced JNK activation and BACE1 transcription. It also induces myeloid differentiation primary response 88 (MyD88) and activates protein kinase CK2, thereby increasing NFκB activation and BACE1 induction. OxoDNA increases oxidative stress via nuclear factor erythroid 2-related factor 2 (Nrf2) ectopic localization, likely augmenting JNK-mediated BACE1 induction. OxoDNA likely also promotes β-amyloidosis via absent in melanoma 2 (AIM2) induction. Falsifiable predictions of this hypothesis include that deoxyribonuclease treatment should decrease Aβ and possibly slow cognitive decline in AD patients. While formal testing of this hypothesis remains to be performed, a case report has found deoxyribonuclease I treatment improved a severely demented AD patient's Mini-Mental Status Exam score from 3 to 18 at 2 months. There is preliminary preclinical and clinical evidence suggesting that ectopic oxidatively damaged DNA may act as an inflammatory damage-associated molecular pattern contributing to Aβ generation in AD, and deoxyribonuclease I should be formally evaluated to test whether it can decrease Aβ levels and slow cognitive decline in AD patients.

Berberine Reduces Aβ42 Deposition and Tau Hyperphosphorylation via Ameliorating Endoplasmic Reticulum Stress

Alzheimer’s disease (AD) is tightly related to endoplasmic reticulum stress (ER stress), which aggravates two dominant pathological manifestations of AD: senile plaques and neurofibrillary tangles. Berberine is widely applied in the clinical treatment of many diseases and is reported to have anti-AD effects. In the present study, berberine was shown to ameliorate ER stress and cognitive impairment in APP/PS1 mice. We found ER stress plays a role as a central hub for signal transduction, which was evidenced by the hyperactivation of glycogen synthase kinase 3β (GSK3β) to phosphorylate tau and the activation of PRKR-like endoplasmic reticulum kinase (PERK) subsequently to phosphorylate eukaryotic translation initiation factor-2 α (eIF2α). Also, eIF2α has regulated the expression of beta-site APP cleaving enzyme-1 (BACE1), which cleaves APP into pro-oligomerized amyloid beta 42 (Aβ₄₂), the main component of senile plaques, proven by using siRNA targeting at eIF2α. Mechanically, berberine can reduce GSK3β activity, contributing to the downregulation of tau phosphorylation. Berberine also suppressed Aβ₄₂ production via inhibiting the PERK/eIF2α/BACE1 signaling pathway. Taken together, these findings indicated that berberine had the potential to ameliorate two major pathological manifestations of AD mainly by suppressing ER stress. Our work provided knowledge on the pharmacological intervention of AD and the possible targets for future drug development.

Post-translational modifications: Regulators of neurodegenerative proteinopathies

One of the hallmark features in the neurodegenerative disorders (NDDs) is the accumulation of aggregated and/or non-functional protein in the cellular milieu. Post-translational modifications (PTMs) are an essential regulator of non-functional protein aggregation in the pathogenesis of NDDs. Any alteration in the post-translational mechanism and the protein quality control system, for instance, molecular chaperone, ubiquitin-proteasome system, autophagy-lysosomal degradation pathway, enhances the accumulation of misfolded protein, which causes neuronal dysfunction. Post-translational modification plays many roles in protein turnover rate, accumulation of aggregate and can also help in the degradation of disease-causing toxic metabolites. PTMs such as acetylation, glycosylation, phosphorylation, ubiquitination, palmitoylation, SUMOylation, nitration, oxidation, and many others regulate protein homeostasis, which includes protein structure, functions and aggregation propensity. Different studies demonstrated the involvement of PTMs in the regulation of signaling cascades such as PI3K/Akt/GSK3β, MAPK cascade, AMPK pathway, and Wnt signaling pathway in the pathogenesis of NDDs. Further, mounting evidence suggests that targeting different PTMs with small chemical molecules, which acts as an inhibitor or activator, reverse misfolded protein accumulation and thus enhances the neuroprotection. Herein, we briefly discuss the protein aggregation and various domain structures of different proteins involved in the NDDs, indicating critical amino acid residues where PTMs occur. We also describe the implementation and involvement of various PTMs on signaling cascade and cellular processes in NDDs. Lastly, we implement our current understanding of the therapeutic importance of PTMs in neurodegeneration, along with emerging techniques targeting various PTMs.