Apolipoprotein E (ApoE) genotype is a powerful genetic modifier of Alzheimer's disease (AD). The ApoE4 isoform significantly reduces the mean age-of-onset of dementia through unknown mechanisms. Here, we show that ApoE4 selectively impairs synaptic plasticity and NMDA receptor phosphorylation by Reelin, a regulator of brain development and modulator of synaptic strength. ApoE4 reduces neuronal surface expression of Apoer2, a dual function receptor for ApoE and for Reelin, as well as NMDA and AMPA receptors by sequestration in intracellular compartments, thereby critically reducing the ability of Reelin to enhance synaptic glutamate receptor activity. As a result, the ability of Reelin to prevent LTP suppression by extracts from AD-afflicted human brains in hippocampal slices from knockin mice expressing the human ApoE4 isoform is severely impaired. These findings show an isoform-specific role of ApoE in the localization and intracellular trafficking of lipoprotein and glutamate receptors and thereby reveal an alternative mechanism by which ApoE4 may accelerate onset of dementia and neuronal degeneration by differentially impairing the maintenance of synaptic stability.Alzheimer's disease | lipoprotein receptor | neurodegeneration | NMDA receptor | Reelin

Abnormal processing of the amyloid precursor protein (APP) and ␤-amyloid (A␤) plaque accumulation are defining features of Alzheimer disease (AD), a genetically complex neurodegenerative disease that is characterized by progressive synapse loss and neuronal cell death. A␤ induces synaptic dysfunction in part by altering the endocytosis and trafficking of AMPA and NMDA receptors. Reelin is a neuromodulator that increases glutamatergic neurotransmission by signaling through the postsynaptic ApoE receptors Apoer2 and Vldlr and thereby potently enhances synaptic plasticity. Here we show that Reelin can prevent the suppression of long-term potentiation and NMDA receptors, which is induced by levels of A␤ comparable to those present in an AD-afflicted brain. This reversal is dependent upon the activation of Src family tyrosine kinases. At high concentrations of A␤ peptides, Reelin can no longer overcome the A␤ induced functional suppression and this coincides with a complete blockade of the Reelin-dependent phosphorylation of NR2 subunits. We propose a model in which A␤, Reelin, and ApoE receptors modulate neurotransmission and thus synaptic stability as opposing regulators of synaptic gain control.Alzheimer disease ͉ apolipoprotein E ͉ long-term potentiation ͉ NMDA receptor ͉ A␤ oligomer A lzheimer disease (AD) is a complex genetic neurodegenerative disease that afflicts an increasing fraction of our aging population (1). A characteristic feature of AD is the accumulation of oligomeric and higher-order aggregates of the A␤ 1-42 form of the amyloid-␤ (A␤) peptide, which is physiologically released by sequential proteolytic cleavage from the amyloid-␤ precursor protein (APP) (2, 3). Abnormal, amyloidogenic A␤ processing and plaque formation progressively lead to synaptic dysfunction, synapse loss, and ultimately neuronal death.Recent insight into the pathophysiological functions of A␤ came from studies that demonstrated a potent negative impact of A␤ oligomers on synaptic functions that underlie long-term synaptic plasticity (4-6). Incubation of hippocampal neurons and slices with A␤ oligomers leads to intracellular trapping or functional impairment of AMPA and NMDA-type glutamate receptors (7-9), thereby decreasing long-term potentiation (LTP), an enhancement of synaptic strength that is correlated with memory (4, 5, 10-13). Consistent with these in vitro findings, Palop et al. (14,15) showed that overexpression of mutant APP forms in mice resulted in a global dysregulation of neuronal network activity in vivo.Reelin is a signaling protein that is produced by interneurons in the brain and that has an effect on synaptic functions that is opposite to that of A␤ oligomers. Reelin addition to hippocampal slices results in enhanced LTP (16) as a result of the activation of Src family tyrosine kinases (SFKs) (17-19), which increase NMDA receptor activity by tyrosine phosphorylation of their NR2 subunits (20,21). These effects of Reelin are mediated by a pair of homologous cell surface receptors designated apolipoprotein E r...

Summary Reelin is a glycoprotein that is critical for proper layering of neocortex during development as well as dynamic regulation of glutamatergic postsynaptic signaling in mature synapses. Here we show that Reelin also acts presynaptically, resulting in robust rapid enhancement of spontaneous neurotransmitter release without affecting properties of evoked neurotransmission. This effect of Reelin requires a modest but significant increase in presynaptic Ca2+ initiated via ApoER2 signaling. The specificity of Reelin action on spontaneous neurotransmitter release is encoded at the level of vesicular SNARE machinery as it requires VAMP7 and SNAP-25 but not synaptobrevin2, VAMP4 or vti1a. These results uncover a novel presynaptic regulatory pathway that utilizes the heterogeneity of synaptic vesicle associated SNAREs and selectively augments action potential-independent neurotransmission.

Thalamic ventrobasal (VB) relay neurones express multiple GABA A receptor subtypes mediating phasic and tonic inhibition. During postnatal development, marked changes in subunit expression occur, presumably reflecting changes in functional properties of neuronal networks. The aims of this study were to characterize the properties of synaptic and extrasynaptic GABA A receptors of developing VB neurones and investigate the role of the α 1 subunit during maturation of GABA-ergic transmission, using electrophysiology and immunohistochemistry in wild type (WT) and α 1 0/0 mice and mice engineered to express diazepam-insensitive receptors (α 1H101R , α 2H101R ). In immature brain, rapid (P8/9-P10/11) developmental change to mIPSC kinetics and increased expression of extrasynaptic receptors (P8-27) formed by the α 4 and δ subunit occurred independently of the α 1 subunit. Subsequently (≥ P15), synaptic α 2 subunit/gephyrin clusters of WT VB neurones were replaced by those containing the α 1 subunit. Surprisingly, in α 1 0/0 VB neurones the frequency of mIPSCs decreased between P12 and P27, because the α 2 subunit also disappeared from these cells. The loss of synaptic GABA A receptors led to a delayed disruption of gephyrin clusters. Despite these alterations, GABA-ergic terminals were preserved, perhaps maintaining tonic inhibition. These results demonstrate that maturation of synaptic and extrasynaptic GABA A receptors in VB follows a developmental programme independent of the α 1 subunit. Changes to synaptic GABA A receptor function and the increased expression of extrasynaptic GABA A receptors represent two distinct mechanisms for fine-tuning GABA-ergic control of thalamic relay neurone activity during development.

It is becoming apparent that the hormone leptin plays an important role in modulating hippocampal function. Indeed, leptin enhances NMDA receptor activation and promotes hippocampal long-term potentiation (LTP). Furthermore, obese rodents with dysfunctional leptin receptors display impairments in hippocampal synaptic plasticity. Here we demonstrate that under conditions of enhanced excitability (evoked in Mg 2+ -free medium or following blockade of GABA A receptors), leptin induces a novel form of long-term depression (LTD) in area CA1 of the hippocampus. Leptin-induced LTD was markedly attenuated in the presence of D-(-)-2-Amino-5-Phosphonopentanoic acid (D-AP5), suggesting that it is dependent on the synaptic activation of NMDA receptors. In addition, low-frequency stimulus-evoked LTD occluded the effects of leptin. In contrast, metabotropic glutamate receptors (mGluRs) did not contribute to leptin-induced LTD as mGluR antagonists failed to either prevent or reverse this process. The signalling mechanisms underlying leptin-induced LTD were independent of the Ras-Raf-mitogen-activated protein kinase signalling pathway, but were markedly enhanced following inhibition of either phosphoinositide 3-kinase or protein phosphatases 1 and 2A. These data indicate that under conditions of enhanced excitability, leptin induces a novel form of homosynaptic LTD, which further underscores the proposed key role for this hormone in modulating NMDA receptor-dependent hippocampal synaptic plasticity.

Alzheimer's disease (AD) is a currently incurable neurodegenerative disorder and the most common form of dementia in people over the age of 65. The predominant genetic risk factor for AD is the ε4 allele encoding apolipoprotein E (ApoE4). The secreted glycoprotein Reelin, which is a physiological ligand for the multifunctional ApoE receptors Apolipoprotein E receptor 2 (Apoer2) and very low-density lipoprotein receptor (Vldlr), enhances synaptic plasticity. We have previously shown that the presence of ApoE4 renders neurons unresponsive to Reelin by impairing the recycling of the receptors, thereby decreasing its protective effects against amyloid β (Aβ) oligomer-induced synaptic toxicity in vitro. Here, we show that when Reelin was knocked out in adult mice, these mice behaved normally without overt learning or memory deficits. However, they were strikingly sensitive to amyloid-induced synaptic suppression, and had profound memory and learning disabilities at very low amounts of amyloid deposition. Our findings highlight the physiological importance of Reelin in protecting the brain against Aβ-induced synaptic dysfunction and memory impairment.

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.

Alzheimer's disease (AD) is the most common form of dementia in individuals over the age of 65 years. The most prevalent genetic risk factor for AD is the 4 allele of apolipoprotein E (ApoE4), and novel AD treatments that target ApoE are being considered. One unresolved question in ApoE biology is whether ApoE is necessary for healthy brain function. ApoE knock-out (KO) mice have synaptic loss and cognitive dysfunction; however, these findings are complicated by the fact that ApoE knock-out mice have highly elevated plasma lipid levels, which may independently affect brain function. To bypass the effect of ApoE loss on plasma lipids, we generated a novel mouse model that expresses ApoE normally in peripheral tissues, but has severely reduced ApoE in the brain, allowing us to study brain ApoE loss in the context of a normal plasma lipid profile. We found that these brain ApoE knock-out (bEKO) mice had synaptic loss and dysfunction similar to that of ApoE KO mice; however, the bEKO mice did not have the learning and memory impairment observed in ApoE KO mice. Moreover, we found that the memory deficit in the ApoE KO mice was specific to female mice and was fully rescued in female bEKO mice. Furthermore, while the AMPA/NMDA ratio was reduced in ApoE KO mice, it was unchanged in bEKO mice compared with controls. These findings suggest that plasma lipid levels can influence cognition and synaptic function independent of ApoE expression in the brain.