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Emerging evidences suggest that intraneuronal Aβ correlates with the onset of Alzheimer's 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 Aβ1–42or oligomeric Aβ1–40, was blocked byFcgr2bknock-out in neurons and partially in astrocytes. Aβ1–42internalization 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-defectiveFcgr2bmutant. Our findings suggest that FcγRIIb2 is essential for neuropathic uptake of Aβ in AD.SIGNIFICANCE STATEMENTAccumulating 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.

Amyloid-β (Aβ)-containing extracellular plaques and hyperphosphorylated tau-loaded intracellular neurofibrillary tangles are neuropathological hallmarks of Alzheimer's 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, eLife digest In Alzheimer’s disease, damage to neurons in the brain gradually causes memory loss and difficulties with thinking. The main hallmarks of this damage are seen in the accumulation of proteins in and around neurons. First, a protein called amyloid beta forms aggregates outside the cell. This appears to lead to the build up of an abnormal form of a protein called tau inside the cell. These abnormal tau proteins have excessive numbers of phosphate groups attached to them, and so are known as “hyperphosphorylated”. The molecular mechanism underlying amyloid beta’s role in the hyperphosphorylation of tau proteins was not known. Amyloid beta binds to many different receptor proteins – including one called Fc gamma receptor IIb – on the surface of neurons. Kam, Park et al. investigated whether interactions between amyloid beta and Fc gamma receptor IIb might regulate the phosphorylation of tau within neurons. Adding amyloid beta to mouse neurons caused tau proteins to become hyperphosphorylated. However, removing Fc gamma receptor IIb from the neurons, or stopping it from binding to amyloid beta, abolished this effect. When amyloid beta was bound to Fc gamma receptor IIb, the receptor became phosphorylated. This in turn triggered a series of further phosphorylation events, culminating in an increase in the level of a molecule that relays signals from cell receptors – called SHIP2 – inside the neurons. This molecule increases tau phosphorylation when added to neurons. Reducing the activity or amount of SHIP2 in mice that present the symptoms of Alzheimer’s disease reduced the hyperphosphorylation of the tau protein in their neurons and restored their memory to normal levels. Kam, Park et al. also looked at samples taken from the brains of human Alzheimer's disease patients. Unlike samples taken from people without Alzheimer’s disease, neurons in these samples contain both phosphorylated Fc gamma receptor IIb and hyperphosphorylated tau proteins. By uncovering the molecules that link amyloid beta with tau hyperphosphorylation, Kam, Park et al.’s results suggest new targets for therapies to treat the symptoms of Alzheimer’s disease. More research is now needed to investigate whether this could lead to the design of new drugs. DOI: http://dx.doi.org/10.7554/eLife.18691.002

Amyloid-beta (Abeta) neurotoxicity is believed to contribute to the pathogenesis of Alzheimer's disease (AD). Previously we found that E2-25K/Hip-2, an E2 ubiquitin-conjugating enzyme, mediates Abeta 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. Abeta 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 Abeta toxicity and to the induction of ER stress and caspase-12 expression by Abeta. E2-25K/Hip-2 is thus an essential upstream regulator of the expression and activation of caspase-12 in ER stress-mediated Abeta neurotoxicity.

Abnormal Tau protein is known to be closely associated with several neurodegenerative diseases. Previously, we showed that Tau was cleaved by caspase-3 to generate the cleavage product lacking the C-terminus (DeltaTau-1) during neuronal cell death. Here we characterized caspase-8-dependent neurotoxicity of the truncated Tau. Introduction of DeltaTau-1 into primary hippocampal neurons induced loss of neurites in a caspase-dependent manner. Caspase-8 and -6 were proteolytically activated during DeltaTau-1-triggered neuronal cell death, which was suppressed by IETD-fmk, caspase-8 inhibitor. Direct targeting of caspase-8 and its associated FADD with antisense approaches and transient expression of their dominant-negative mutants reduced DeltaTau-1-induced apopotosis. Cells deficient in caspase-8, but not caspase-3, became sensitized to DeltaTau-1-mediated toxicity upon reconstitution with caspase-8. In addition, ectopic expression of mitochondrial antiapoptotic Bcl-2, Bcl-X(L), or inactive caspase-9 short form suppressed DeltaTau-1 toxicity. These results suggest that the truncated Tau protein activates proximal caspase-8 through FADD as a necessary step leading to neuronal cell death and neurite regression, contributing to the progression of abnormal Tau-associated neurodegeneracy.

The ubiquitin/proteasome system has been proposed to play an important role in Alzheimer's disease (AD) pathogenesis. However, the critical factor(s) modulating both amyloid-beta peptide (Abeta) neurotoxicity and ubiquitin/proteasome system in AD are not known. We report the isolation of an unusual ubiquitin-conjugating enzyme, E2-25K/Hip-2, as a mediator of Abeta toxicity. The expression of E2-25K/Hip-2 was upregulated in the neurons exposed to Abeta(1-42) in vivo and in culture. Enzymatic activity of E2-25K/Hip-2 was required for both Abeta(1-42) neurotoxicity and inhibition of proteasome activity. E2-25K/Hip-2 functioned upstream of apoptosis signal-regulating kinase 1 (ASK1) and c-Jun N-terminal kinase (JNK) in Abeta(1-42) toxicity. Further, the ubiquitin mutant, UBB+1, a potent inhibitor of the proteasome which is found in Alzheimer's brains, was colocalized and functionally interacted with E2-25K/Hip-2 in mediating neurotoxicity. These results suggest that E2-25K/Hip-2 is a crucial factor in regulating Abeta neurotoxicity and could play a role in the pathogenesis of Alzheimer's disease.

Amyloid-β (Aβ) induces neuronal loss and cognitive deficits and is believed to be a prominent cause of Alzheimer’s disease (AD); however, the cellular pathology of the disease is not fully understood. Here, we report that IgG Fcγ receptor II-b (FcγRIIb) mediates Aβ neurotoxicity and neurodegeneration. We found that FcγRIIb is significantly upregulated in the hippocampus of AD brains and neuronal cells exposed to synthetic Aβ. Neuronal FcγRIIb activated ER stress and caspase-12, and Fcgr2b KO primary neurons were resistant to synthetic Aβ-induced cell death in vitro. Fcgr2b deficiency ameliorated Aβ-induced inhibition of long-term potentiation and inhibited the reduction of synaptic density by naturally secreted Aβ. Moreover, genetic depletion of Fcgr2b rescued memory impairments in an AD mouse model. To determine the mechanism of action of FcγRIIb in Aβ neurotoxicity, we demonstrated that soluble Aβ oligomers interact with FcγRIIb in vitro and in AD brains, and that inhibition of their interaction blocks synthetic Aβ neurotoxicity. We conclude that FcγRIIb has an aberrant, but essential, role in Aβ-mediated neuronal dysfunction.

Ubiquitin-positive deposits are histopathologically found in patients with Alzheimer's disease (AD). It is not understood why ubiquitin is accumulated in intra- and extra-cellular deposits or how it is involved in AD pathogenesis. Interestingly, recent evidence, including studies of E2-25K/Hip-2, has elucidated the molecular mechanism of the ubiquitin-proteasome system (UPS) malfunction in AD. The neurotoxicity and proteasome inhibition by Abeta, a main cause of AD pathogenesis, are mediated by increased E2-25K/Hip-2 in the brains of patients with AD. Furthermore, E2-25K/Hip-2 is required for the neurotoxicity that is mediated by a ubiquitin B mutant (UBB+1), which is a potent inhibitor of proteasomes that is found in patients with AD. Intensive research is required to identify the components of the UPS that are involved in AD pathogenesis.

Calsenilin/DREAM/KChIP3 was identified as a calcium-binding protein that interacts with presenilins, serves as a transcription repressor, and binds to the A-type potassium channel. In this study, we hypothesized that calsenilin might be involved in the neurodegeneration of Alzheimer's disease and examined calsenilin expression in Alzheimer's disease. Calsenilin levels were elevated in the cortex region of Alzheimer's patient brains and in the neocortex and the hippocampus of Swedish mutant beta-amyloid precursor protein transgenic mice brains. Induction of calsenilin was also observed in the activated astroglia as well as in the neurons surrounding beta-amyloid (Abeta)- and Congo red-positive plaques. Exposing cultured cortical and hippocampal neurons to Abeta42, an amyloid-beta peptide whose deposition in the brain is a characteristic of Alzheimer's disease, induced both calsenilin protein and mRNA expression, and cell death. Moreover, blocking the calsenilin expression protected the neuronal cells from Abeta toxicity. These findings suggest that chronic up-regulation of calsenilin may be a risk factor for developing Alzheimer's disease, perhaps by facilitating calsenilin-mediated neurodegeneration.

The ubiquitin-proteasome system (UPS) is involved in many biological pathways via the degradation of short-lived and regulatory proteins important in cellular processes. Moreover, in the most of the chronic human neurodegenerative diseases, including Alzheimer’s disease (AD) and Parkinson’s disease (PD), protein aggregation co-localized with ubiquitin is a common feature and may be caused by the abrogation of UPS. Parkin gene isolated from autosomal recessive-juvenile parkinsonism (AR-JP) is ubiquitin-protein ligase (E3) and many mutations of parkin in familial PD disrupted the ubiquitin-protein ligase activity to eventually accumulate its substrates in intracellular aggresome (Lewy body). Pathogenic proteins of AD, including presenilin and Amyloid-b Precursor Protein (APP), are the substrates of UPS and the mutations related with AD pathogenesis give resistance to their degradation by proteasome. Therefore, UPS is the important target of therapeutic trial for neurodegenerative disorders.