Neuronal MHC-I complex is destabilized by amyloid-ß and its implications in Alzheimer's disease.

BACKGROUNDS: The expression of major histocompatibility complex I (MHC-I) in neurons has recently been shown to regulate neurite outgrowth and synaptic plasticity. However, its contribution to neurodegenerative diseases such as Alzheimer's disease (AD) remains largely unknown. METHODS: In this study, we investigated the relationship between impaired MHC-I-ß2M complex and AD in vitro and human AD samples. Interaction between protein was identified by liquid chromatography-tandem mass spectrometry and confirmed by immunoprecipitation. Single-chain trimer of MHC-I-ß2M was generated to study the effect of stabilization of MHC-I-ß2M complex on NCAM1 signaling. RESULTS: MHC-I is destabilized in the brains of AD patients and neuronal cells treated with oligomeric ß-amyloid (Aß). Specifically, Aß oligomers disassemble the MHC-I-ß2-microglobulin (ß2M) complex, leading to reduced interactions with neural cell adhesion molecule 1 (NCAM1), a novel interactor of neuronal MHC-I, and decreased signaling. Inhibition of MHC-I-ß2M complex destabilization by non-dissociable MHC-I-ß2M-peptide complex restored MHC-I-NCAM1 signaling in neuronal cells. CONCLUSIONS: The current study demonstrated that disruption of MHC-1-NCAM1 signaling by Aß induced disassembly of MHC-I-ß2M complex is involved in the pathophysiology of AD. Moreover, our findings suggest modulation of MHC-I stability may be a potential therapeutic target for restoring synaptic function in AD.

Amphiregulin normalizes altered circuit connectivity for social dominance of the CRTC3 knockout mouse.

Social hierarchy has a profound impact on social behavior, reward processing, and mental health. Moreover, lower social rank can lead to chronic stress and often more serious problems such as bullying victims of abuse, suicide, or attack to society. However, its underlying mechanisms, particularly their association with glial factors, are largely unknown. In this study, we report that astrocyte-derived amphiregulin plays a critical role in the determination of hierarchical ranks. We found that astrocytes-secreted amphiregulin is directly regulated by cAMP response element-binding (CREB)-regulated transcription coactivator 3 (CRTC3) and CREB. Mice with systemic and astrocyte-specific CRTC3 deficiency exhibited a lower social rank with reduced functional connectivity between the prefrontal cortex, a major social hierarchy center, and the parietal cortex. However, this effect was reversed by astrocyte-specific induction of amphiregulin expression, and the epidermal growth factor domain was critical for this action of amphiregulin. These results provide evidence of the involvement of novel glial factors in the regulation of social dominance and may shed light on the clinical application of amphiregulin in the treatment of various psychiatric disorders.

Monoclonal antibody Y01 prevents tauopathy progression induced by lysine 280-acetylated tau in cell and mouse models.

The spatiotemporal pattern of the spread of pathologically modified tau through brain regions in Alzheimer's disease (AD) can be explained by prion-like cell-to-cell seeding and propagation of misfolded tau aggregates. Hence, to develop targeted therapeutic antibodies, it is important to identify the seeding- and propagation-competent tau species. The hexapeptide 275VQIINK280 of tau is a critical region for tau aggregation, and K280 is acetylated in various tauopathies, including AD. However, the mechanism that links tau acetylated on lysine 280 (tau-acK280) to subsequent progression to neurodegenerative disease remains unclear. Here, we demonstrate that tau-acK280 is critical for tau propagation processes including secretion, aggregation, and seeding. We developed an antibody, Y01, that specifically targets tau-acK280 and solved the crystal structure of Y01 in complex with an acK280 peptide. The structure confirmed that Y01 directly recognizes acK280 and the surrounding residues. Strikingly, upon interaction with acetylated tau aggregates, Y01 prevented tauopathy progression and increased neuronal viability in neuron cultures and in tau-Tg mice through antibody-mediated neutralization and phagocytosis, respectively. Based on our observations that tau-acK280 is a core species involved in seeding and propagation activities, the Y01 antibody that specifically recognizes acK280 represents a promising therapeutic candidate for AD and other neurodegenerative diseases associated with tauopathy.

Tau acetylation at K280 regulates tau phosphorylation.

PURPOSE/AIM OF THE STUDY: Accumulation of hyperphosphorylated tau is a key pathological finding of Alzheimer's disease. Recently, acetylation of tau is emerging as another key pathogenic modification, especially regarding the acetylation of tau at K280 of the hexapeptide 275VQIINK280, a critical sequence in driving tau aggregation. However, the relationship between these two key post-translational modifications is not well known. In this study, effect of acetylation of tau at K280 on tau phosphorylation profile was investigated. MATERIALS AND METHODS: The human neuroblastoma cell line, SH-SY5Y, was transfected with p300 acetyltransferase and tau to induce acetylation of tau. Phosphorylation profile after acetylation was evaluated on western blot. K280A-mutant tau was transfected to investigate the effect of acetylation of tau at K280 on tau phosphorylation profile. RESULTS: Overexpression of p300 acetyltransferase in tau-transfected SH-SY5Y human neuroblastoma cells increased acetylation of tau. Meanwhile, tau and its phosphorylation also increased at various sites such as S199/202, S202/T205, T231, and S422, but not at S396. However, blocking acetylation only at K280 with K280A-mutant tau reversed the increased phosphorylation of tau at S202/T205, T231, and S422, but not at S199/202 or S396. CONCLUSION: Here we identified tau phosphorylation profile in the context of p300-induced acetylation and K280A-mutant tau, demonstrating that tau acetylation affects phosphorylation differently by residues and that acetylation at K280 is a determinant of phosphorylation at some residues in the context of pathologic acetyltransferase activity. Yet, our results suggest there is a complex interplay yet to be explored between tau acetylation with tau phosphorylation.

TLQP-21 mediated activation of microglial BV2 cells promotes clearance of extracellular fibril amyloid-ß.

ß-Amyloid (Aß) plaque in the brains of patients with Alzheimer's disease (AD) is mainly caused by impaired clearance of Aß by glial cells, including microglia and astrocytes. Because microglia play an important protective role in the central nervous system, many efforts have been made to identify agents that effectively improve microglial Aß phagocytosis. This study found that TLQP-21, which is cleaved from VGF (VGF nerve growth factor inducible) precursor protein, enhanced Aß phagocytosis and degradation by microglial BV2 cells. TLQP-21 also improved microglial phagocytic activity and promoted fibrillar amyloid-ß (fAß) uptake by microglial BV2 cells via a C3AR1-dependent mechanism. Moreover, TLQP-21 stimulated Aß degradation by enhancing lysosome activity, thereby enhancing fAß clearance. These results suggest that treatment with TLQP-21 may be a novel therapeutic strategy to efficiently enhance microglial Aß clearance in AD.

Inhibition of REV-ERBs stimulates microglial amyloid-beta clearance and reduces amyloid plaque deposition in the 5XFAD mouse model of Alzheimer's disease.

A promising new therapeutic target for the treatment of Alzheimer's disease (AD) is the circadian system. Although patients with AD are known to have abnormal circadian rhythms and suffer sleep disturbances, the role of the molecular clock in regulating amyloid-beta (Aß) pathology is still poorly understood. Here, we explored how the circadian repressors REV-ERBα and ß affected Aß clearance in mouse microglia. We discovered that, at Circadian time 4 (CT4), microglia expressed higher levels of the master clock protein BMAL1 and more rapidly phagocytosed fibrillary Aß1-42 (fAß1-42 ) than at CT12. BMAL1 directly drives transcription of REV-ERB proteins, which are implicated in microglial activation. Interestingly, pharmacological inhibition of REV-ERBs with the small molecule antagonist SR8278 or genetic knockdown of REV-ERBs-accelerated microglial uptake of fAß1-42 and increased transcription of BMAL1. SR8278 also promoted microglia polarization toward a phagocytic M2-like phenotype with increased P2Y12 receptor expression. Finally, constitutive deletion of Rev-erbα in the 5XFAD model of AD decreased amyloid plaque number and size and prevented plaque-associated increases in disease-associated microglia markers including TREM2, CD45, and Clec7a. Altogether, our work suggests a novel strategy for controlling Aß clearance and neuroinflammation by targeting REV-ERBs and provides new insights into the role of REV-ERBs in AD.

Small-molecule drug screening identifies drug Ro 31-8220 that reduces toxic phosphorylated tau in Drosophila melanogaster.

The intraneuronal aggregates of hyperphosphorylated and misfolded tau (neurofibrillary tangles, NFTs) cause a stereotypical spatiotemporal Alzheimer's disease (AD) progression that correlates with the severity of the associated cognitive decline. Kinase activity contributes to the balance between neuron survival and cell death. Hyperactivation of kinases including the conventional protein kinase C (PKC) is a defective molecular event accompanying associative memory loss, tau phosphorylation, and progression of AD or related neurodegenerative diseases. Here, we investigated the ability of small therapeutic compounds (a custom library) to improve tau-induced rough-eye phenotype in a Drosophila melanogaster model of frontotemporal dementia. We also assessed the tau phosphorylation in vivo and selected hit compounds. Among the potential hits, we investigated Ro 31-8220, described earlier as a potent PKCα inhibitor. Ro 31-8220 robustly improved the rough-eye phenotype, reduced phosphorylated tau species in vitro and in vivo, reversed tau-induced memory impairment, and improved the fly motor functions. In a human neuroblastoma cell line, Ro 31-8220 reduced the PKC activity and the tau phosphorylation pattern, but we also have to acknowledge the compound's wide range of biological activity. Nevertheless, Ro 31-8220 is a novel therapeutic mitigator of tau-induced neurotoxocity.

V232M substitution restricts a distinct O-glycosylation of PLD3 and its neuroprotective function.

The link between Val232Met variant of phospholipase D3 (PLD3) and late-onset Alzheimer's disease (AD) is still obscure. While it may not affect directly the amyloid precursor protein function, PLD3 could be regulating multiple cellular compartments. Here, we investigated the function of wild-type human PLD3 (PLD3WT) and the Val232Met variant (PLD3VM) in the presence of ß-amyloid (Aß) in a Drosophila melanogaster model of AD. We expressed PLD3WT in CNS of the Aß-model flies and monitored its effect on the ER stress, cell apoptosis and recovery the Aß-induced cognitive impairment. The expression reduced ER stress and neuronal apoptosis, which resulted in normalized antioxidative phospholipids levels and brain protection. A specific O-glycosylation at pT271 in PLD3 is essential for its normal trafficking and cellular localization. The V232 M substitution impairs this O-glycosylation, leading to enlarged lysosomes and plausibly aberrant protein recycling. PLD3VM was less neuroprotective, and while, PLD3WT expression enhances the lysosomal functions, V232 M attenuated PLD3's trafficking to the lysosomes. Thus, the V232 M mutation may affect AD pathogenesis. Further understanding of the mechanistic role of PLD3 in AD could lead to developing novel therapeutic agents.

Ouabain activates transcription factor EB and exerts neuroprotection in models of Alzheimer's disease.

The number of neurofibrillary tangles containing abnormal hyperphosphorylated tau protein correlates with the degree of dementia in Alzheimer's disease (AD). In addition, autophagosome accumulation and disturbance of autophagy, the process by which toxic aggregate proteins are degraded in the cytosol, are also found in AD models. These indicate that regulation of the autophagy-lysosome system may be a potential therapeutic target for AD. Activation of transcription factor EB (TFEB), a master regulator of autophagy-lysosome system gene transcription, reduces the amount of tau in APP mice. Here, to identify potential therapeutic compounds for AD, we performed two types of screening to determine pharmacologically active compounds that increase 1) neuronal viability in okadaic acid-induced tau hyperphosphorylation-related neurodegeneration models and 2) nuclear localization of TFEB in high-contents screening. Ouabain, a cardiac glycoside, was discovered as a common hit compound in both screenings. It also exhibited a significant protective effect in tau transgenic fly and mouse models in vivo. This work demonstrates that ouabain enhances activation of TFEB through inhibition of the mTOR pathway and induces downstream autophagy-lysosomal gene expression and cellular restorative properties. Therefore, therapeutic approaches using ouabain reduce the accumulation of abnormal toxic tau in vitro and in vivo.

Orexin Impairs the Phagocytosis and Degradation of Amyloid-ß Fibrils by Microglial Cells.

BACKGROUND: Intracranial accumulation of amyloid-ß (Aß) is a characteristic finding of Alzheimer's disease (AD). It is thought to be the result of Aß overproduction by neurons and impaired clearance by several systems, including degradation by microglia. Sleep disturbance is now considered a risk factor for AD, but studies focusing on how sleep modulates microglial handling of Aß have been scarce. OBJECTIVE: To determine whether phagocytosis and degradation of extracellular Aß fibrils by BV2 microglial cells were impaired by treatment with orexin-A/B, a major modulator of the sleep-wake cycle, which may mimic sleep deprivation conditions. METHODS: BV2 cells were treated with orexin and Aß for various durations and phagocytic and autophagic processes for degradation of extracellular Aß were examined. RESULTS: After treatment with orexin, the formation of actin filaments around Aß fibrils, which is needed for phagocytosis, was impaired, and phagocytosis regulating molecules such as PI3K, Akt, and p38-MAPK were downregulated in BV2 cells. Orexin also suppressed autophagic flux, through disruption of the autophagosome-lysosome fusion process, resulting in impaired Aß degradation in BV2 cells. CONCLUSIONS: Our results demonstrate that orexin can hinder clearance of Aß through the suppression of phagocytosis and autophagic flux in microglia. This is a novel mechanism linking AD and sleep, and suggests that attenuated microglial function, due to sleep deprivation, may increase Aß accumulation in the brain.

Sorting nexin-4 regulates ß-amyloid production by modulating ß-site-activating cleavage enzyme-1.

BACKGROUND: Amyloid precursor protein (APP) is cleaved by ß-site amyloid precursor protein-cleaving enzyme 1 (BACE1) to produce ß-amyloid (Aß), a critical pathogenic peptide in Alzheimer's disease (AD). Aß generation can be affected by the intracellular trafficking of APP or its related secretases, which is thus important to understanding its pathological alterations. Although sorting nexin (SNX) family proteins regulate this trafficking, the relevance and role of sorting nexin-4 (SNX4) regarding AD has not been studied yet. METHODS: In this study, human brain tissue and APP/PS1 mouse brain tissue were used to check the disease relevance of SNX4. To investigate the role of SNX4 in AD pathogenesis, several experiments were done, such as coimmunoprecipitation, Western blotting, immunohistochemistry, and gradient fractionation. RESULTS: We found that SNX4 protein levels changed in the brains of patients with AD and of AD model mice. Overexpression of SNX4 significantly increased the levels of BACE1 and Aß. Downregulation of SNX4 had the opposite effect. SNX4 interacts with BACE1 and prevents BACE1 trafficking to the lysosomal degradation system, resulting in an increased half-life of BACE1 and increased production of Aß. CONCLUSIONS: We show that SNX4 regulates BACE1 trafficking. Our findings suggest novel therapeutic implications of modulating SNX4 to regulate BACE1-mediated ß-processing of APP and subsequent Aß generation.

Alzheimer's disease genes and autophagy.

Autophagy is a process to degrade and recycle cellular constituents via the lysosome for regulating cellular homeostasis. Its dysfunction is now considered to be involved in many diseases, including neurodegenerative diseases. Many features reflecting autophagy impairment, such as autophagosome accumulation and lysosomal dysfunction, have been also revealed to be involved in Alzheimer's disease (AD). Recent genetic studies such as genome-wide association studies in AD have identified a number of novel genes associated with AD. Some of the identified genes have demonstrated dysfunction in autophagic processes in AD, while others remain under investigation. Since autophagy is strongly regarded to be one of the major pathogenic mechanisms of AD, it is necessary to review how the AD-associated genes are related to autophagy. We anticipate our current review to be a starting point for future studies regarding AD-associated genes and autophagy. This article is part of a Special Issue entitled SI:Autophagy.

c-Jun N-terminal Kinase (JNK) induces phosphorylation of amyloid precursor protein (APP) at Thr668, in okadaic acid-induced neurodegeneration.

Several lines of evidence have revealed that phosphorylation of amyloid precursor protein (APP) at Thr668 is involved in the pathogenesis of Alzheimer's disease (AD). Okadaic acid (OA), a protein phosphatase-2A inhibitor, has been used in AD research models to increase tau phosphorylation and induce neuronal death. We previously showed that OA increased levels of APP and induced accumulation of APP in axonal swellings. In this study, we found that in OA-treated neurons, phosphorylation of APP at Thr668 increased and accumulated in axonal swellings by c-jun N-terminal kinase (JNK), and not by Cdk5 or ERK/MAPK. These results suggest that JNK may be one of therapeutic targets for the treatment of AD. [BMB Reports 2016; 49(7): 376-381].

Regulation of amyloid precursor protein processing by its KFERQ motif.

Understanding of trafficking, processing, and degradation mechanisms of amyloid precursor protein (APP) is important because APP can be processed to produce ß-amyloid (Aß), a key pathogenic molecule in Alzheimer's disease (AD). Here, we found that APP contains KFERQ motif at its C-terminus, a consensus sequence for chaperone-mediated autophagy (CMA) or microautophagy which are another types of autophagy for degradation of pathogenic molecules in neurodegenerative diseases. Deletion of KFERQ in APP increased C-terminal fragments (CTFs) and secreted N-terminal fragments of APP and kept it away from lysosomes. KFERQ deletion did not abolish the interaction of APP or its cleaved products with heat shock cognate protein 70 (Hsc70), a protein necessary for CMA or microautophagy. These findings suggest that KFERQ motif is important for normal processing and degradation of APP to preclude the accumulation of APP-CTFs although it may not be important for CMA or microautophagy. [BMB Reports 2016; 49(6): 337-342].

The Alzheimer's Disease-Associated R47H Variant of TREM2 Has an Altered Glycosylation Pattern and Protein Stability.

The R47H coding variant of the triggering receptor expressed on myeloid cells-2 (TREM2) increases the risk of Alzheimer's disease (AD) similar to apolipoprotein E4. TREM2 R47H has recently been shown to have impaired binding to damage-associated lipid or apolipoprotein ligands. However, it is not known how this R47H variant affects the biochemical characteristics of TREM2 and alters the pathogenesis of AD. We previously reported that TREM2-R47H has a slightly different glycosylation pattern from wild-type. A more detailed characterization in our present study confirms that TREM2 R47H has an altered glycosylation pattern and reduced stability. TREM2 R47H shows different glycosylation profiles from analysis using monensin or kifunensine treatment which were confirmed by mass spectrometry. The solubility of TREM2 R47H and its cleaved products such as intracellular domain (ICD) is also decreased, increasing its proteasomal and lysosomal degradation. The different biochemical characteristics of TREM2 R47H, including glycosylation, solubility and processing, may offer insights into a future therapeutic strategy for AD.

Calpain-mediated cleavage of DARPP-32 in Alzheimer's disease.

Toxicity induced by aberrant protein aggregates in Alzheimer's disease (AD) causes synaptic disconnection and concomitant progressive neurodegeneration that eventually impair cognitive function. cAMP-response element-binding protein (CREB) is a transcription factor involved in the molecular switch that converts short-term to long-term memory. Although disturbances in CREB function have been suggested to cause memory deficits in both AD and AD animal models, the mechanism of CREB dysfunction is still unclear. Here, we show that the dopamine- and cAMP-regulated phosphoprotein 32 kDa (DARPP-32), a key inhibitor of protein phosphate-1 (PP-1) that regulates CREB phosphorylation, is cleaved by activated calpain in both AD brains and neuronal cells treated with amyloid-ß or okadaic acid, a protein phosphatase-2A inhibitor that induces tau hyperphosphorylation and neuronal death. We found that DARPP-32 is mainly cleaved at Thr(153) by calpain and that this cleavage of DARPP-32 reduces CREB phosphorylation via loss of its inhibitory function on PP1. Our results suggest a novel mechanism of DARPP-32-CREB signalling dysregulation in AD.

Disease-Associated Mutations of TREM2 Alter the Processing of N-Linked Oligosaccharides in the Golgi Apparatus.

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune-modulatory receptor involved in phagocytosis and inflammation. Mutations of Q33X, Y38C and T66M cause Nasu-Hakola disease (NHD) which is characterized by early onset of dementia and bone cysts. A recent, genome-wide association study also revealed that single nucleotide polymorphism of TREM2, such as R47H, increased the risk of Alzheimer's disease (AD) similar to ApoE4. However, how these mutations affect the trafficking of TREM2, which may affect the normal functions of TREM2, was not known. In this study, we show that TREM2 with NHD mutations are impaired in the glycosylation with complex oligosaccharides in the Golgi apparatus, in the trafficking to plasma membrane and further processing by γ-secretase. Although R47H mutation in AD affected the glycosylation and normal trafficking of TREM2 less, the detailed pattern of glycosylated TREM2 differs from that of the wild type, thus suggesting that precise regulation of TREM2 glycosylation is impaired when arginine at 47 is mutated to histidine. Our results suggest that the impaired glycosylation and trafficking of TREM2 from endoplasmic reticulum/Golgi to plasma membrane by mutations may inhibit its normal functions in the plasma membrane, which may contribute to the disease.

Vancomycin blocks autophagy and induces interleukin-1ß release in macrophages.

Systemic inflammatory response syndrome (SIRS) is a serious condition that can cause organ failure as an exaggerated immunoresponse to the infection or other causes. Recently, autophagy was reported as a key process that regulates inflammatory responses in macrophages. Vancomycin is one of the most commonly prescribed antibiotics for sepsis treatment or following surgery. However, there are no studies on how vancomycin affects autophagy or inflammation. Here, we treated macrophage cell lines with vancomycin and lipopolysaccharides and found that vancomycin blocks autophagy and increases inflammatory responses. This finding suggests that vancomycin should be more cautiously administered in order to prevent unwanted SIRS during sepsis.

Autophagy in microglia degrades extracellular ß-amyloid fibrils and regulates the NLRP3 inflammasome.

Accumulation of ß-amyloid (Aß) and resultant inflammation are critical pathological features of Alzheimer disease (AD). Microglia, a primary immune cell in brain, ingests and degrades extracellular Aß fibrils via the lysosomal system. Autophagy is a catabolic process that degrades native cellular components, however, the role of autophagy in Aß degradation by microglia and its effects on AD are unknown. Here we demonstrate a novel role for autophagy in the clearance of extracellular Aß fibrils by microglia and in the regulation of the Aß-induced NLRP3 (NLR family, pyrin domain containing 3) inflammasome using microglia specific atg7 knockout mice and cell cultures. We found in microglial cultures that Aß interacts with MAP1LC3B-II via OPTN/optineurin and is degraded by an autophagic process mediated by the PRKAA1 pathway. We anticipate that enhancing microglial autophagy may be a promising new therapeutic strategy for AD.

ß-Amyloid is transmitted via neuronal connections along axonal membranes.

OBJECTIVE: ß-amyloid plaque is a critical pathological feature of Alzheimer disease. Pathologic studies suggest that neurodegeneration may occur in a retrograde fashion from axon terminals near ß-amyloid plaques, and that plaque may spread through brain regions. However, there is no direct experimental evidence to show transmission of ß-amyloid. METHODS: Microscopic imaging data of ß-amyloid transmission was acquired in cortical neuron cultures from Sprague-Dawley rat embryos using polydimethylsiloxane (PDMS) microfluidic culture chambers and in brain sections from in vivo ß-amyloid injection. RESULTS: We present direct imaging evidence in cultured cortical neurons, using PDMS microfluidic culture chambers, that ß-amyloid is readily absorbed by axonal processes and retrogradely transported to neuronal cell bodies. Transmission of ß-amyloid via neuronal connections was also confirmed in mouse brain. ß-Amyloid absorbed by distal axons accumulates in axonal swellings, mitochondria, and lysosomes of the cell bodies. Interestingly, dynasore, an inhibitor of dynamin, which is a protein indispensable for endocytosis, did not prevent retrograde transport of ß-amyloid, indicating that ß-amyloid is absorbed onto axonal membranes and transmitted via them to the cell body. Dynasore did decrease the transneuronal transmission of ß-amyloid, suggesting that this requires the internalization and secretion of ß-amyloid. INTERPRETATION: Our findings provide direct in vitro and in vivo evidence for spreading of ß-amyloid through neuronal connections, and suggest possible therapeutic approaches to blocking this spread.