Neurotoxic astrocytic glypican-4 drives APOE4-dependent tau pathology

Apolipoprotein E4 (APOE4) is the crucial genetic risk factor of late-onset Alzheimer disease (AD). Aggregation of tau proteins into insoluble filaments and their spreading across the brain regions are major drivers of neurodegeneration in tauopathies, including in AD. However, the exact mechanisms through which APOE4 induces tau pathology remains unknown. Here, we report that the astrocyte-secreted protein glypican-4 (GPC-4), a novel binding partner of APOE4, drives tau pathology. GPC-4 preferentially interacts with APOE4 in comparison to other APOE isoforms and post-mortem APOE4-carrying AD brains highly express GPC-4 in neurotoxic astrocytes. The astrocyte-secreted GPC-4 induced both tau accumulation and propagation in vitro. CRISPR/dCas9 mediated activation of GPC-4 in a tauopathy animal model robustly induced tau pathology. Further, APOE4-induced tau pathology was greatly diminished in the absence of GPC-4. We found that GPC-4 promoted the stabilization of the APOE receptor low-density lipoprotein receptor-related protein 1 (LRP1) on the cellular surface, which effectively facilitates endocytosis of tau protein. Together, our data comprehensively demonstrate that one of the key APOE4-induced tau pathologies is directly mediated by GPC-4.

Lipid peroxidation and pathological disruption of the ApoE/Reelin-ApoER2-DAB1 axis in sporadic Alzheimer's disease

The strong genetic link between Apolipoprotein E (ApoE) and sporadic Alzheimer's disease (AD) and the marked increase in brain lipid peroxidation observed in early AD suggest that dysfunctional lipid metabolism plays a central role in AD pathogenesis. However, specific mechanisms and targets linking ApoE and lipid peroxidation to AD are not well-defined. Here we used a combination of biochemical experiments, single-marker immunohistochemistry (IHC), and multiplex-IHC to examine the hypothesis that synaptic ApoE receptors and their ligands ApoE and Reelin are susceptible to lipid peroxidation, and that downstream disruptions in ApoE delivery and Reelin-ApoE receptor signaling cascades contribute to the pathogenesis of sporadic AD. We found that (1) Lys and His-enriched sequences within the binding regions of ApoER2, ApoE, VLDLR and Reelin, and recombinant ApoER2, ApoE and Reelin proteins, are vulnerable to attack by aldehydic products of lipid peroxidation, generating lipid-protein adducts and acid-stable ApoE receptor-ligand complexes; (2) ApoER2, lipid peroxidation-modified ApoE, native ApoE, Reelin, and multiple downstream components of Reelin-ApoE receptor signaling cascades that govern synaptic integrity [including DAB1, Tyr232-phosphorylated DAB1, Tyr607-phosphorylated Phosphatidylinositol 3-kinase, Thr508-phosphorylated LIM kinase-1, Ser202/Thr205-phosphorylated Tau and Thr19-phosphorylated-PSD95] accumulate in the immediate vicinity of neuritic plaques and surrounding abnormal neurons, and (3) several of these ApoE/Reelin-ApoE receptor-DAB1 pathway markers positively correlate with Braak stage, Aβ plaque load, and antemortem cognitive impairment. ApoE/Reelin-ApoER2-DAB1 axis pathologies were especially prominent in the dendritic compartments of the molecular layer of the dentate gyrus, cornu ammonis and subiculum, regions that receive synaptic input from the entorhinal-hippocampal projections that underlie memory formation. Taken together, these observations point toward extensive derangements in the ApoE/Reelin-ApoE receptor-DAB1 axis and provide evidence supporting a new, working hypothesis wherein lipid peroxidation-induced adduction and crosslinking of ApoE receptors and ApoE are proximate molecular events that compromise synaptic integrity and contribute to the histopathological hallmarks and cognitive deficits that characterize sporadic AD in humans.

Neuronal ROS-Induced Glial Lipid Droplet Formation is Altered by Loss of Alzheimer’s Disease-associated Genes

SummaryA growing list of Alzheimer’s disease (AD) genetic risk factors is being identified, but the contribution of these genetic mutations to disease remains largely unknown. Accumulating data support a role of lipid dysregulation and excessive ROS in the etiology of AD. Here, we identified cell-specific roles for eight AD risk-associated genes in ROS-induced glial lipid droplet (LD) formation. We demonstrate that ROS-induced glial LD formation requires two ABCA transporters (ABCA1 and ABCA7) in neurons, the APOE receptor (LRP1), endocytic genes (PICALM, CD2AP, and AP2A2) in glia, and retromer genes (VPS26 and VPS35) in both neurons and glia. Moreover, ROS strongly enhances Aβ42-toxicity in flies and Aβ42-plaque formation in mice. Finally, an ABCA1-activating peptide restores glial LD formation in the APOE4-associated loss of LD. This study places AD risk factors in a neuron-to-glia lipid transfer pathway with a critical role in protecting neurons from ROS-induced toxicity.

LRP1 and SORL1 regulate tau internalization and degradation and enhance tau seeding

ABSTRACTThe identification of the apoE receptor, LRP1, as an endocytic receptor for tau raises several questions about LRP1s’ role in tauopathies. Is internalized tau, like other LRP1 ligands, delivered to lysosomes for degradation? Does LRP1 internalize pathological tau leading to cytosolic seeding? Do other, related receptors participate in these processes? We confirm that LRP1 rapidly internalizes tau, leading to efficient lysosomal degradation. Employing brain homogenates from human Alzheimer brain, we find that LRP1 also mediates cytosolic tau seeding. We additionally found that another apoE receptor, SORL1, a gene implicated in AD risk, also mediates tau endocytosis, degradation, and release into the cytoplasm of seed competent species. These data suggest a role for these apoE receptors in tau uptake, as well as the competing processes of degradation and release to the cytoplasm. The balance of these processes may be fundamental to spread of neuropathology across the brain in Alzheimer disease.

Apolipoprotein E4 disrupts the neuroprotective action of sortilin in neuronal lipid metabolism and endocannabinoid signaling

ABSTRACTINTRODUCTIONApoE is a carrier for brain lipids and the most important genetic risk factor for Alzheimer’s disease (AD). ApoE binds the receptor sortilin which mediates uptake of apoE-bound cargo into neurons. The significance of this uptake route for brain lipid homeostasis and AD risk seen with apoE4, but not apoE3, remains unresolved.METHODSCombining neurolipidomics in patient specimens with functional studies in mouse models, we interrogated apoE isoform-specific functions for sortilin in brain lipid metabolism and AD.RESULTSSortilin directs uptake and conversion of polyunsaturated fatty acids into endocannabinoids, lipid-based neurotransmitters that act through nuclear receptors to sustain neuroprotective gene expression in the brain. This sortilin function requires apoE3, but is disrupted by binding of apoE4, impairing endocannabinoid signaling and increasing amyloidogenic processing.DISCUSSIONWe uncovered the significance of neuronal apoE receptor sortilin in facilitating neuroprotective actions of brain lipids, and its relevance for AD risk seen with apoE4.

Differential Signaling Mediated by ApoE2, ApoE3, and ApoE4 in Human Neurons Parallels Alzheimer’s Disease Risk

Apolipoprotein E (ApoE) mediates clearance of circulating lipoproteins from blood by binding to ApoE receptors. Humans express three genetic variants, ApoE2, ApoE3, and ApoE4, that exhibit distinct ApoE receptor binding properties. In brain, ApoE is abundantly produced by activated astrocytes and microglia, and three variants differentially affect Alzheimer’s disease (AD), such that ApoE2 protects against, and ApoE4 predisposes to the disease. A role for ApoE4 in driving microglial dysregulation and impeding Aβ clearance in AD is well documented, but the direct effects of three variants on neurons are poorly understood. Extending previous studies, we here demonstrate that ApoE variants differentially activate multiple neuronal signaling pathways and regulate synaptogenesis. Specifically, using human neurons cultured in the absence of glia to exclude indirect glial mechanisms, we show that ApoE broadly stimulates signal transduction cascades which among others enhance synapse formation with an ApoE4>ApoE3>ApoE2 potency rank order, paralleling the relative risk for AD conferred by these variants. Unlike the previously described induction of APP transcription, however, ApoE-induced synaptogenesis involves CREB rather than cFos activation. We thus propose that in brain, ApoE acts as a glia-secreted paracrine signal and activates neuronal signaling pathways in what may represent a protective response, with the differential potency of ApoE variants causing distinct levels of chronic signaling that may contribute to AD pathogenesis.

Reversal of ApoE4-induced recycling block as a novel prevention approach for Alzheimer’s disease

ApoE4 genotype is the most prevalent and also clinically most important risk factor for late-onset Alzheimer’s disease (AD). Available evidence suggests that the root cause for this increased risk is a trafficking defect at the level of the early endosome. ApoE4 differs from the most common ApoE3 isoform by a single amino acid that increases its isoelectric point and promotes unfolding of ApoE4 upon endosomal vesicle acidification. We found that pharmacological and genetic inhibition of NHE6, the primary proton leak channel in the early endosome, in rodents completely reverses the ApoE4-induced recycling block of the ApoE receptor Apoer2/Lrp8 and the AMPA- and NMDA-type glutamate receptors that are regulated by, and co-endocytosed in a complex with, Apoer2. Moreover, NHE6 inhibition restores the Reelin-mediated modulation of excitatory synapses that is impaired by ApoE4. Our findings suggest a novel potential approach for the prevention of late-onset AD.