Reduction and the intracellular translocation of EphB2 in Tg2576 mice and the effects of β-amyloid

Co-Expression of Three Wild-Type 3R-Tau Isoforms Induces Memory Deficit via Oxidation-Related DNA Damage and Cell Death: A Promising Model for Tauopathies

The three isoforms of 3R-tau are predominantly deposited in neurons bearing neurofibrillary tangles in Alzheimer's disease (AD), while only 3R-tau accumulation has been detected in Pick's disease (PiD), suggesting the involvement of 3R-tau in neurodegeneration. However, both the role and the molecular mechanism of 3R-tau in neurodegeneration are elusive. Here, we co-expressed three isoforms of human wild-type 3R-tau in adult mouse hippocampal to mimic the pathologic tau accumulating observed in PiD patients. We found that co-expressing three 3R-tau isoforms induced hyperphosphorylation and accumulation of tau proteins; simultaneously, the mice showed remarkable neuron death with synapse and memory deficits. Further in vitro and in vivo studies demonstrated that co-expressing 3R-tau isoforms caused oxidative stress evidenced by an increased malondialdehyde, and the decreased superoxide dismutase and glutathione peroxidase; the 3R-tau accumulation also induced significant glial activation and DNA double-strand breaks (DSBs). Notably, the toxic effects of 3R-tau accumulation were efficiently reversed by administration of antioxidants Vitamin E (VitE) and Vitamin C (VitC), respectively. These data reveal that intracellular accumulation of 3R-tau isoforms in adult brain induces significant neuron death and memory deficits with the mechanism involving oxidation-mediated DSBs; and the antioxidants VitE and VitC can efficiently attenuate the toxicities of 3R-tau. Given that no significant cell death has been detected in the currently available wild-type tau-accumulating models, co-expressing 3R-tau isoforms could be a promising model for drug development of tauopathies, such as PiD.

Kallikrein-related peptidases 6 and 10 are elevated in cerebrospinal fluid of patients with Alzheimer's disease and associated with CSF-TAU and FDG-PET

Background

Alterations in the expression of human kallikrein-related peptidases (KLKs) have been described in patients with Alzheimer’s disease (AD). We elucidated the suitability of KLK6, KLK8 and KLK10 to distinguish AD from NC and explored associations with established AD biomarkers.

Methods

KLK levels in cerebrospinal fluid (CSF), as determined by ELISA, were compared between 32 AD patients stratified to A/T/(N) system with evidence for amyloid pathology and of 23 normal controls with normal AD biomarkers. Associations between KLK levels and clinical severity, CSF and positron emission tomography (PET) based AD biomarkers were tested for.

Results

Levels of KLK6 and KLK10 were significantly increased in AD. KLK6 differed significantly between AD A+/T+/N+ and AD A+/T−/N+ or NC with an AUC of 0.922. CSF pTau and tTau levels were significantly associated with KLK6 in AD.

Conclusions

KLK6 deserves further investigations as a potential biomarker of Tau pathology in AD.

Electronic supplementary material

The online version of this article (10.1186/s40035-019-0168-6) contains supplementary material, which is available to authorized users.

A novel mechanism of synaptic and cognitive impairments mediated via microRNA-30b in Alzheimer's disease

BACKGROUND

It is widely accepted that cognitive and memory deficits in Alzheimer's disease (AD) primarily result from synaptic failure. However, the mechanisms that underlie synaptic and cognitive dysfunction remain unclear.

METHODS

We utilized molecular biology techniques, electrophysiological recordings, fluorescence in situ hybridization (FISH), immuno- and Golgi-staining, chromatin immunoprecipitation (CHIP); lentivirus (LV)-based microRNA overexpression and 'sponging', and behavioral tests to assess upregulated miR-30b causing synaptic and cognitive declines in APP transgenic (TG) mice.

FINDINGS

We provide evidence that expression of miR-30b, which targets molecules important for maintaining synaptic integrity, including ephrin type-B receptor 2 (ephB2), sirtuin1 (sirt1), and glutamate ionotropic receptor AMPA type subunit 2 (GluA2), is robustly upregulated in the brains of both AD patients and APP transgenic (TG) mice, an animal model of AD, while expression of its targets is significantly downregulated. Overexpression of miR-30b in the hippocampus of normal wild-type (WT) mice impairs synaptic and cognitive functions, mimicking those seen in TG mice. Conversely, knockdown of endogenous miR-30b in TG mice prevents synaptic and cognitive decline. We further observed that expression of miR-30b is upregulated by proinflammatory cytokines and Aβ42 through NF-κB signaling.

INTERPRETATION

Our results provide a previously undefined mechanism by which unregulated miR-30b causes synaptic and cognitive dysfunction in AD, suggesting that reversal of dysregulated miR-30b in the brain may prevent or slow cognitive declines in AD. FUND: This work was supported by National Institutes of Health grants R01NS076815, R01MH113535, R01AG058621, P30GM103340 Pilot Project, and by the LSUHSC School of Medicine Research Enhancement Program grant (to C.C.).

Epigenetic Control of the Notch and Eph Signaling Pathways by the Prion Protein: Implications for Prion Diseases

Among the ever-growing number of self-replicating proteins involved in neurodegenerative diseases, the prion protein PrP remains the most infamous for its central role in transmissible spongiform encephalopathies (TSEs). In these diseases, pathogenic prions propagate through a seeding mechanism, where normal PrP^C molecules are converted into abnormally folded scrapie isoforms termed PrP^Sc. Since its discovery over 30 years ago, much advance has contributed to define the host-encoded cellular prion protein PrP^C as a critical relay of prion-induced neuronal cell demise. A current consensual view is that the conversion of PrP^C into PrP^Sc in neuronal cells diverts the former from its normal function with subsequent molecular alterations affecting synaptic plasticity. Here, we report that prion infection is associated with reduced expression of key effectors of the Notch pathway in vitro and in vivo, recapitulating changes fostered by the absence of PrP^C. We further show that both prion infection and PrP^C depletion promote drastic alterations in the expression of a defined set of Eph receptors and their ephrin ligands, which represent important players in synaptic function. Our data indicate that defects in the Notch and Eph axes can be mitigated in response to histone deacetylase inhibition in PrP^C-depleted as well as prion-infected cells. We thus conclude that infectious prions cause a loss-of-function phenotype with respect to Notch and Eph signaling and that these alterations are sustained by epigenetic mechanisms.

Kallikrein-8 inhibition attenuates Alzheimer's disease pathology in mice

INTRODUCTION

Memory loss and increased anxiety are clinical hallmarks of Alzheimer's disease (AD). Kallikrein-8 is a protease implicated in memory acquisition and anxiety, and its mRNA is known to be up-regulated in AD-affected human hippocampus. Therefore, an involvement of Kallikrein-8 in Alzheimer's pathogenesis is conceivable but remains to be proved.

METHODS

We determined the cerebral expression of Kallikrein-8 mRNA and protein during the course of AD in patients and in transgenic mice and tested the impact of Kallikrein-8 inhibition on AD-related pathology in mice and in primary glial cells.

RESULTS

Kallikrein-8 mRNA and protein were up-regulated in both species at incipient stages of AD. Kallikrein-8 inhibition impeded amyloidogenic amyloid-precursor-protein processing, facilitated amyloid β (Aβ) clearance across the blood-brain-barrier, boosted autophagy, reduced Aβ load and tau pathology, enhanced neuroplasticity, reversed molecular signatures of anxiety, and ultimately improved memory and reduced fear.

DISCUSSION

Kallikrein-8 is a promising new therapeutic target against AD.

Identification of approved drugs that inhibit the binding of amyloid β oligomers to ephrin type-B receptor 2

Ephrin type‐B receptor 2 (EphB2) is a member of the receptor tyrosine kinase family and plays an important role in learning and memory functions. In patients with Alzheimer's disease (AD) and in mouse models of AD, a reduction in the hippocampal EphB2 level is observed. It was recently reported that normalization of the EphB2 level in the dentate gyrus rescues memory function in a mouse model of AD, suggesting that drugs that restore EphB2 levels may be beneficial in the treatment of AD. Amyloid β (Aβ) oligomers, which are believed to be key molecules involved in the pathogenesis of AD, induce EphB2 degradation through their direct binding to EphB2. Thus, compounds that inhibit the binding of Aβ oligomers to EphB2 may be beneficial. Here, we screened for such compounds from drugs already approved for clinical use in humans. Utilizing a cell‐free screening assay, we determined that dihydroergotamine mesilate, bromocriptine mesilate, cepharanthine, and levonorgestrel inhibited the binding of Aβ oligomers to EphB2 but not to cellular prion protein, another endogenous receptor for Aβ oligomers. Additionally, these four compounds did not affect the binding between EphB2 and ephrinB2, an endogenous ligand for EphB2, suggesting that the compounds selectively inhibited the binding of Aβ oligomers to EphB2. This is the first identification of compounds that selectively inhibit the binding of Aβ oligomers to EphB2. These results suggest that these four compounds may be safe and effective drugs for treatment of AD.

The Role of Ephs and Ephrins in Memory Formation

The ability to efficiently store memories in the brain is a fundamental process and its impairment is associated with multiple human mental disorders. Evidence indicates that long-term memory formation involves alterations of synaptic efficacy produced by modifications in neural transmission and morphology. The Eph receptors and their cognate ephrin ligands have been shown to be involved in these key neuronal processes by regulating events such as presynaptic transmitter release, postsynaptic glutamate receptor conductance and trafficking, synaptic glutamate reuptake, and dendritic spine morphogenesis. Recent findings show that Ephs and ephrins are needed for memory formation in different organisms. These proteins participate in the formation of various types of memories that are subserved by different neurons and brain regions. Ephs and ephrins are involved in brain disorders and diseases with memory impairment symptoms, including Alzheimer's disease and anxiety. Drugs that agonize or antagonize Ephs/ephrins signaling have been developed and could serve as therapeutic agents to treat such diseases. Ephs and ephrins may therefore induce cellular alterations mandatory for memory formation and serve as a target for pharmacological intervention for treatment of memory-related brain diseases.

Stimulation of EphB2 attenuates tau phosphorylation through PI3K/Akt-mediated inactivation of glycogen synthase kinase-3β

Abnormal tau hyperphosphorylation is an early pathological marker of Alzheimer’s disease (AD), however, the upstream factors that regulate tau phosphorylation are not illustrated and there is no efficient strategy to arrest tau hyperphosphorylation. Here, we find that activation of endogenous EphB2 receptor by ligand stimulation (ephrinB1/Fc) or by ectopic expression of EphB2 plus the ligand stimulation induces a remarkable tau dephosphorylation at multiple AD-associated sites in SK-N-SH cells and human embryonic kidney cells that stably express human tau (HEK293-tau). In cultured hippocampal neurons and the hippocampus of human tau transgenic mice, dephosphorylation of tau proteins was also detected by stimulation of EphB2 receptor. EphB2 activation inhibits glycogen synthase kinase-3β (GSK-3β), a crucial tau kinase, and activates phosphatidylinositol-3-kinase (PI3K)/Akt both in vitro and in vivo, whereas simultaneous inhibition of PI3K or upregulation of GSK-3β abolishes the EphB2 stimulation-induced tau dephosphorylation. Finally, we confirm that ephrinB1/Fc treatment induces tyrosine phosphorylation (activation) of EphB2, while deletion of the tyrosine kinase domain (VM) of EphB2 eliminates the receptor stimulation-induced GSK-3β inhibition and tau dephosphorylation. We conclude that activation of EphB2 receptor kinase arrests tau hyperphosphorylation through PI3K-/Akt-mediated GSK-3β inhibition. Our data provide a novel membranous target to antagonize AD-like tau pathology.

Eph receptors: new players in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is devastating and leads to permanent losses of memory and other cognitive functions. Although recent genetic evidences strongly argue for a causative role of Aβ in AD onset and progression (Jonsson et al., 2012), its role in AD etiology remains a matter of debate. However, even if not the sole culprit or pathological trigger, genetic and anatomical evidences in conjunction with numerous pharmacological studies, suggest that Aβ peptides, at least contribute to the disease. How Aβ contributes to memory loss remains largely unknown. Soluble Aβ species referred to as Aβ oligomers have been shown to be neurotoxic and induce network failure and cognitive deficits in animal models of the disease. In recent years, several proteins were described as potential Aβ oligomers receptors, amongst which are the receptor tyrosine kinases of Eph family. These receptors together with their natural ligands referred to as ephrins have been involved in a plethora of physiological and pathological processes, including embryonic neurogenesis, learning and memory, diabetes, cancers and anxiety. Here we review recent discoveries on Eph receptors-mediated protection against Aβ oligomers neurotoxicity as well as their potential as therapeutic targets in AD pathogenesis.