Tae-In Kam

Overexpression of Atg5 in mice activates autophagy and extends lifespan

Autophagy has been implicated in the ageing process, but whether autophagy activation extends lifespan in mammals is unknown. Here we show that ubiquitous overexpression of Atg5, a protein essential for autophagosome formation, extends median lifespan of mice by 17.2%. We demonstrate that moderate overexpression of Atg5 in mice enhances autophagy, and that Atg5 transgenic mice showed anti-ageing phenotypes, including leanness, increased insulin sensitivity and improved motor function. Furthermore, mouse embryonic fibroblasts cultured from Atg5 transgenic mice are more tolerant to oxidative damage and cell death induced by oxidative stress, and this tolerance was reversible by treatment with an autophagy inhibitor. Our observations suggest that the leanness and lifespan extension in Atg5 transgenic mice may be the result of increased autophagic activity.

E2-25K/Hip-2 regulates caspase-12 in ER stress–mediated Aβ neurotoxicity

Amyloid-β (Aβ) neurotoxicity is believed to contribute to the pathogenesis of Alzheimers disease (AD). Previously we found that E2-25K/Hip-2, an E2 ubiquitin-conjugating enzyme, mediates Aβ neurotoxicity. Here, we report that E2-25K/Hip-2 modulates caspase-12 activity via the ubiquitin/proteasome system. Levels of endoplasmic reticulum (ER)–resident caspase-12 are strongly up-regulated in the brains of AD model mice, where the enzyme colocalizes with E2-25K/Hip-2. Aβ increases expression of E2-25K/Hip-2, which then stabilizes caspase-12 protein by inhibiting proteasome activity. This increase in E2-25K/Hip-2 also induces proteolytic activation of caspase-12 through its ability to induce calpainlike activity. Knockdown of E2-25K/Hip-2 expression suppresses neuronal cell death triggered by ER stress, and thus caspase-12 is required for the E2-25K/Hip-2–mediated cell death. Finally, we find that E2-25K/Hip-2–deficient cortical neurons are resistant to Aβ toxicity and to the induction of ER stress and caspase-12 expression by Aβ. E2-25K/Hip-2 is thus an essential upstream regulator of the expression and activation of caspase-12 in ER stress–mediated Aβ neurotoxicity.

Neuropathogenic role of adenylate kinase-1 in Aβ-mediated tau phosphorylation via AMPK and GSK3β

Abnormally hyperphosphorylated tau is often caused by tau kinases, such as GSK3β and Cdk5. Such occurrence leads to neurofibrillary tangle formation and neuronal degeneration in tauopathy, including Alzheimers disease (AD). However, little is known about the signaling cascade underlying the pathologic phosphorylation of tau by Aβ(42). In this study, we show that adenylate kinase 1 (AK1) is a novel regulator of abnormal tau phosphorylation. AK1 expression is markedly increased in the brains of AD patients and AD model mice and is significantly induced by Aβ(42) in the primary neurons. Ectopic expression of AK1 alone augments the pathologic phosphorylation of tau at PHF1, CP13 and AT180 epitopes and enhances the formation of tau aggregates. Inversely, downregulation of AK1 alleviates Aβ(42)-induced hyperphosphorylation of tau. AK1 plays a role in Aβ(42)-induced impairment of AMPK activity and GSK3β activation in the primary neurons. Pharmacologic studies show that treatment with an AMPK inhibitor activates GSK3β, and a GSK3β inhibitor attenuates AK1-mediated tau phosphorylation. In a Drosophila model of human tauopathy, the retinal expression of human AK1 severely exacerbates rough eye phenotype and increases abnormal tau phosphorylation. Further, neural expression of AK1 reduces the lifespan of tau transgenic files. Taken together, these observations indicate that the neuronal expression of AK1 is induced by Aβ(42) to increase abnormal tau phosphorylation via AMPK-GSK3β and contributes to tau-mediated neurodegeneration, providing a new upstream modulator of GSK3β in the pathologic phosphorylation of tau.

Amyloid beta receptors responsible for neurotoxicity and cellular defects in Alzheimer’s disease

Alzheimer’s disease (AD) is the most common neurodegenerative disease. Although a major cause of AD is the accumulation of amyloid-β (Aβ) peptide that induces neuronal loss and cognitive impairments, our understanding of its neurotoxic mechanisms is limited. Recent studies have identified putative Aβ-binding receptors that mediate Aβ neurotoxicity in cells and models of AD. Once Aβ interacts with a receptor, a toxic signal is transduced into neurons, resulting in cellular defects including endoplasmic reticulum stress and mitochondrial dysfunction. In addition, Aβ can also be internalized into neurons through unidentified Aβ receptors and induces malfunction of subcellular organelles, which explains some part of Aβ neurotoxicity. Understanding the neurotoxic signaling initiated by Aβ-receptor binding and cellular defects provide insight into new therapeutic windows for AD. In the present review, we summarize the findings on Aβ-binding receptors and the neurotoxicity of oligomeric Aβ.

OCIAD2 activates γ-secretase to enhance amyloid β production by interacting with nicastrin

The gamma (γ)-secretase holoenzyme is composed of four core proteins and cleaves APP to generate amyloid beta (Aβ), a key molecule that causes major neurotoxicity during the early stage of Alzheimer’s disease (AD). However, despite its important role in Aβ production, little is known about the regulation of γ-secretase. OCIAD2, a novel modulator of γ-secretase that stimulates Aβ production, and which was isolated from a genome-wide functional screen using cell-based assays and a cDNA library comprising 6,178 genes. Ectopic expression of OCIAD2 enhanced Aβ production, while reduction of OCIAD2 expression suppressed it. OCIAD2 expression facilitated the formation of an active γ-secretase complex and enhanced subcellular localization of the enzyme components to lipid rafts. OCIAD2 interacted with nicastrin to stimulate γ-secretase activity. OCIAD2 also increased the interaction of nicastrin with C99 and stimulated APP processing via γ-secretase activation, but did not affect Notch processing. In addition, a cell-permeable Tat-OCIAD2 peptide that interfered with the interaction of OCIAD2 with nicastrin interrupted the γ-secretase-mediated AICD production. Finally, OCIAD2 expression was significantly elevated in the brain of AD patients and PDAPP mice. This study identifies OCIAD2 as a selective activator of γ-secretase to increase Aβ generation.

Essential role of POLDIP2 in Tau aggregation and neurotoxicity via autophagy/proteasome inhibition

In Alzheimers disease and other tauopathy, abnormal Tau proteins form intracellular aggregates and Tau filaments. However, the mechanisms that regulate Tau aggregation are not fully understood. In this paper, we show that POLDIP2 is a novel regulator of Tau aggregation. From a cell-based screening using cDNA expression library, we isolated POLDIP2 which increased Tau aggregation. Expression of POLDIP2 was increased in neuronal cells by the multiple stresses, including Aβ, TNF-α and H2O2. Accordingly, ectopic expression of POLDIP2 enhanced the formation of Tau aggregates without affecting Tau phosphorylation, while down-regulation of POLDIP2 alleviated ROS-induced Tau aggregation. Interestingly, we found that POLDIP2 overexpression induced impairments of autophagy activity and partially proteasome activity and this activities were retained in DUF525 domain of POLDIP2. In a drosophila model of human tauopathy, knockdown of the drosophila POLDIP2 homolog, CG12162, attenuated rough eye phenotype induced by Tau overexpression. Further, the lifespan of neural-Tau(R406W) transgenic files were recovered by CG12162 knockdown. Together, these observations indicate that POLDIP2 plays a crucial role in Tau aggregation via the impairment of autophagy activity, providing insight into Tau aggregation in Tau pathology.

FcγRIIb-SHIP2 axis links Aβ to tau pathology by disrupting phosphoinositide metabolism in Alzheimer's disease model

Amyloid-β (Aβ)-containing extracellular plaques and hyperphosphorylated tau-loaded intracellular neurofibrillary tangles are neuropathological hallmarks of Alzheimers disease (AD). Although Aβ exerts neuropathogenic activity through tau, the mechanistic link between Aβ and tau pathology remains unknown. Here, we showed that the FcγRIIb-SHIP2 axis is critical in Aβ1-42-induced tau pathology. Fcgr2b knockout or antagonistic FcγRIIb antibody inhibited Aβ1-42-induced tau hyperphosphorylation and rescued memory impairments in AD mouse models. FcγRIIb phosphorylation at Tyr273 was found in AD brains, in neuronal cells exposed to Aβ1-42, and recruited SHIP2 to form a protein complex. Consequently, treatment with Aβ1-42 increased PtdIns(3,4)P2 levels from PtdIns(3,4,5)P3 to mediate tau hyperphosphorylation. Further, we found that targeting SHIP2 expression by lentiviral siRNA in 3xTg-AD mice or pharmacological inhibition of SHIP2 potently rescued tau hyperphosphorylation and memory impairments. Thus, we concluded that the FcγRIIb-SHIP2 axis links Aβ neurotoxicity to tau pathology by dysregulating PtdIns(3,4)P2 metabolism, providing insight into therapeutic potential against AD. DOI: http://dx.doi.org/10.7554/eLife.18691.001

TOM1 Regulates Neuronal Accumulation of Amyloid-β Oligomers by FcγRIIb2 Variant in Alzheimer's Disease

Emerging evidences suggest that intraneuronal Aβ correlates with the onset of Alzheimers disease (AD) and highly contributes to neurodegeneration. However, critical mediator responsible for Aβ uptake in AD pathology needs to be clarified. Here, we report that FcγRIIb2, a variant of Fcγ-receptor IIb (FcγRIIb), functions in neuronal uptake of pathogenic Aβ. Cellular accumulation of oligomeric Aβ1–42, not monomeric Aβ1–42 or oligomeric Aβ1–40, was blocked by Fcgr2b knock-out in neurons and partially in astrocytes. Aβ1–42 internalization was FcγRIIb2 di-leucine motif-dependent and attenuated by TOM1, a FcγRIIb2-binding protein that repressed the receptor recycling. TOM1 expression was downregulated in the hippocampus of male 3xTg-AD mice and AD patients, and regulated by miR-126-3p in neuronal cells after exposure to Aβ1–42. In addition, memory impairments in male 3xTg-AD mice were rescued by the lentiviral administration of TOM1 gene. Augmented Aβ uptake into lysosome caused its accumulation in cytoplasm and mitochondria. Moreover, neuronal accumulation of Aβ in both sexes of 3xTg-AD mice and memory deficits in male 3xTg-AD mice were ameliorated by forebrain-specific expression of Aβ-uptake-defective Fcgr2b mutant. Our findings suggest that FcγRIIb2 is essential for neuropathic uptake of Aβ in AD. SIGNIFICANCE STATEMENT Accumulating evidences suggest that intraneuronal Aβ is found in the early step of AD brain and is implicated in the pathogenesis of AD. However, the critical mediator involved in these processes is uncertain. Here, we describe that the FcγRIIb2 variant is responsible for both neuronal uptake and intraneuronal distribution of pathogenic Aβ linked to memory deficits in AD mice, showing a pathologic significance of the internalized Aβ. Further, Aβ internalization is attenuated by TOM1, a novel FcγRIIb2-binding protein. Together, we provide a molecular mechanism responsible for neuronal uptake of pathogenic Aβ found in AD.

REVERSAL OF LYSOSOMAL DYSFUNCTION RESCUES BETA-AMYLOID PATHOGENESIS IN ALZHEIMER’S DISEASE

(AD-DS). Three copies of the Hsa21 gene APP are sufficient to cause early-onset AD but how trisomy of other Hsa21 genes influences disease development is unclear. Methods:We analysed cathepsin activity in patient fibroblasts, post-mortem brain material from people who have DS and AD pathology and a novel mouse model of AD-DS (progeny of the cross of the Tc1 model of Hsa21 trisomy with the J20 APP transgenic model). To understand how changes in cathepsin activity may affect APP and Ab processing in vitro and in vivo in our AD-DS animal model, we used a combination of pulse-chase analysis, western blotting, immunohistochemistry, ELISA, mass-spectrometry, enzymatic activity assays and in vivo Ab clearance studies. Additionally we undertook behavioural phenotyping and an aging study of our mouse model of AD-DS to understand how changes in APP/Ab processing due to trisomy of Hsa21 effect learning and aging. Results:Here we show that triplication of Hsa21 sequences, other than APP, cause cysteine cathepsin deficits that result in failure to activate these proteases in DS. We successfully modelled these enzymatic changes in a novel AD-DS mouse model system, and found that they occur independently of gross-enlargement of the endo-lysosomes. Using our AD-DS mouse model, we show that trisomy of Hsa21 sequences, other than APP, also alter the metabolism of APP/Ab. These changes decrease the soluble Ab38/42 ratio and are associated with an increase in Ab aggregation and deposition, and result in exacerbation of APP/Ab-associated hyper-activity and specific deficits in two tests of short-term memory. We also show that the trisomyassociated changes in APP/Ab metabolism we observe occur independently of alterations in a-, bor g-secretase activity or changes in the rate of extracellular Ab-clearance in vivo. Conclusions:We propose that trisomy Hsa21-associated cathepsin deficits are a novel AD-DS pathomechanism that alter APP/Ab processing and may contribute to the development of AD in people who have DS.

Role of AK1 in tau phosphorylation and toxicity

Background: Tau is abnormally hyperphosphorylated and becomes the major protein subunit of paired helical filaments (PHF)/neurofibrillary tangles (NFT) found in neurodegenerative diseases, including Alzheimer’s disease (AD). These changes lead to the cognitive decline, loss of synapse, and cell death. However, little is known about mechanisms that regulate tauinduced neurodegeneration. Methods: we have established a cell-based functional assay and screened thousands of human cDNAs and isolated novel tau-aggregation regulating genes. Results: Finally, we found several tau regulating genes. One of them is Adenylate kinase1(AK1). The expression of AK1 was significantly increased by the treatment of amyloid-b1-42 (Ab1-42). And ectopic expression of AK1 increased tau phosphorylation in primary neurons. Tau phosphorylation triggered by AK1 was suppressed by GSK3b inhibitors and AK1 expression decreased the inhibitory phosphorylation(pS9) of GSK3b. In addition, in a drosophila model of human tauopathy that displays a rough eye phenotype, the increased expression of AK1 severely exacerbated rough eye phenotype and increased tau phosphorylation. Conclusions: These results indicate that AK1 may play an essential role in tau phosphorylation and toxicity.