Amelioration of Tau and ApoE4-linked glial lipid accumulation and neurodegeneration with an LXR agonist.

Apolipoprotein E (APOE) is a strong genetic risk factor for late-onset Alzheimer's disease (LOAD). APOE4 increases and APOE2 decreases risk relative to APOE3. In the P301S mouse model of tauopathy, ApoE4 increases tau pathology and neurodegeneration when compared with ApoE3 or the absence of ApoE. However, the role of ApoE isoforms and lipid metabolism in contributing to tau-mediated degeneration is unknown. We demonstrate that in P301S tau mice, ApoE4 strongly promotes glial lipid accumulation and perturbations in cholesterol metabolism and lysosomal function. Increasing lipid efflux in glia via an LXR agonist or Abca1 overexpression strongly attenuates tau pathology and neurodegeneration in P301S/ApoE4 mice. We also demonstrate reductions in reactive astrocytes and microglia, as well as changes in cholesterol biosynthesis and metabolism in glia of tauopathy mice in response to LXR activation. These data suggest that promoting efflux of glial lipids may serve as a therapeutic approach to ameliorate tau and ApoE4-linked neurodegeneration.

APOE immunotherapy reduces cerebral amyloid angiopathy and amyloid plaques while improving cerebrovascular function.

The ε4 allele of the apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset Alzheimer's disease (AD) and greatly influences the development of amyloid-ß (Aß) pathology. Our current study investigated the potential therapeutic effects of the anti-human APOE antibody HAE-4, which selectively recognizes human APOE that is co-deposited with Aß in cerebral amyloid angiopathy (CAA) and parenchymal amyloid pathology. In addition, we tested whether HAE-4 provoked brain hemorrhages, a component of amyloid-related imaging abnormalities (ARIA). ARIA is an adverse effect secondary to treatment with anti-Aß antibodies that can occur in blood vessels with CAA. We used 5XFAD mice expressing human APOE4 +/+ (5XE4) that have prominent CAA and parenchymal plaque pathology to assess the efficacy of HAE-4 compared to an Aß antibody that removes parenchymal Aß but increases ARIA in humans. In chronically treated 5XE4 mice, HAE-4 reduced Aß deposition including CAA compared to a control antibody, whereas the anti-Aß antibody had no effect on CAA. Furthermore, the anti-Aß antibody exacerbated microhemorrhage severity, which highly correlated with reactive astrocytes surrounding CAA. In contrast, HAE-4 did not stimulate microhemorrhages and instead rescued CAA-induced cerebrovascular dysfunction in leptomeningeal arteries in vivo. HAE-4 not only reduced amyloid but also dampened reactive microglial, astrocytic, and proinflammatory-associated genes in the cortex. These results suggest that targeting APOE in the core of both CAA and plaques could ameliorate amyloid pathology while protecting cerebrovascular integrity and function.

Targeting of nonlipidated, aggregated apoE with antibodies inhibits amyloid accumulation.

The apolipoprotein E E4 allele of the APOE gene is the strongest genetic factor for late-onset Alzheimer disease (LOAD). There is compelling evidence that apoE influences Alzheimer disease (AD) in large part by affecting amyloid ß (Aß) aggregation and clearance; however, the molecular mechanism underlying these findings remains largely unknown. Herein, we tested whether anti-human apoE antibodies can decrease Aß pathology in mice producing both human Aß and apoE4, and investigated the mechanism underlying these effects. We utilized APPPS1-21 mice crossed to apoE4-knockin mice expressing human apoE4 (APPPS1-21/APOE4). We discovered an anti-human apoE antibody, anti-human apoE 4 (HAE-4), that specifically recognizes human apoE4 and apoE3 and preferentially binds nonlipidated, aggregated apoE over the lipidated apoE found in circulation. HAE-4 also binds to apoE in amyloid plaques in unfixed brain sections and in living APPPS1-21/APOE4 mice. When delivered centrally or by peripheral injection, HAE-4 reduced Aß deposition in APPPS1-21/APOE4 mice. Using adeno-associated virus to express 2 different full-length anti-apoE antibodies in the brain, we found that HAE antibodies decreased amyloid accumulation, which was dependent on Fcγ receptor function. These data support the hypothesis that a primary mechanism for apoE-mediated plaque formation may be a result of apoE aggregation, as preferentially targeting apoE aggregates with therapeutic antibodies reduces Aß pathology and may represent a selective approach to treat AD.

Preclinical Safety Profile of a Depleting Antibody against CRTh2 for Asthma: Well Tolerated Despite Unexpected CRTh2 Expression on Vascular Pericytes in the Central Nervous System and Gastric Mucosa.

CRTh2 is expressed on immune cells that drive asthma pathophysiology. Current treatment options for severe asthma are inadequate and therapeutic antibody-mediated depletion of CRTh2-expressing cells represents a promising new therapeutic strategy. Here we report for the first time that CRTh2 is not only expressed on immune cells, but also on microvasculature in the central nervous system (CNS) and gastric mucosa in humans. Microvascular expression of CRTh2 raises a safety concern because a therapeutic antiCRTh2 antibody with enhanced depletion capacity could lead to vascular damage. To evaluate this safety risk, we characterized microvascular expression in human and in transgenic mice expressing human CRTh2 protein (hCRTh2.BAC.Tg) and found that CRTh2 is not localized to microvascular endothelium that is directly exposed to circulating therapeutic antibody, but rather, to pericytes that in the CNS are shielded from direct circulatory exposure by the blood-brain barrier. Immunohistochemical visualization of an intravenously administered antiCRTh2 antibody in transgenic mice revealed localization to microvascular pericytes in the gastric mucosa but not in the CNS, suggesting the blood-brain barrier effectively limits pericyte exposure to circulating therapeutic antibody in the CNS. Repeated dosing with a depleting antiCRTh2 antibody in hCRTh2.BAC.Tg mice revealed linear pharmacokinetics and no drug-related adverse findings in any tissues, including the CNS and gastric mucosa, despite complete depletion of CRTh2 expressing circulating eosinophils and basophils. Collectively, these studies demonstrate that the likelihood of drug-related CNS or gastrointestinal toxicity in humans treated with a therapeutic depleting antiCRTh2 antibody is low despite pericyte expression of CRTh2 in these tissues.

Discovery of Novel Blood-Brain Barrier Targets to Enhance Brain Uptake of Therapeutic Antibodies.

The blood-brain barrier (BBB) poses a major challenge for developing effective antibody therapies for neurological diseases. Using transcriptomic and proteomic profiling, we searched for proteins in mouse brain endothelial cells (BECs) that could potentially be exploited to transport antibodies across the BBB. Due to their limited protein abundance, neither antibodies against literature-identified targets nor BBB-enriched proteins identified by microarray facilitated significant antibody brain uptake. Using proteomic analysis of isolated mouse BECs, we identified multiple highly expressed proteins, including basigin, Glut1, and CD98hc. Antibodies to each of these targets were significantly enriched in the brain after administration in vivo. In particular, antibodies against CD98hc showed robust accumulation in brain after systemic dosing, and a significant pharmacodynamic response as measured by brain Aß reduction. The discovery of CD98hc as a robust receptor-mediated transcytosis pathway for antibody delivery to the brain expands the current approaches available for enhancing brain uptake of therapeutic antibodies.

Lack of Widespread BBB Disruption in Alzheimer's Disease Models: Focus on Therapeutic Antibodies.

The blood-brain barrier (BBB) limits brain uptake of therapeutic antibodies. It is believed that the BBB is disrupted in Alzheimer's disease (AD), potentially increasing drug permeability de facto. Here we compared active versus passive brain uptake of systemically dosed antibodies (anti-transferrin receptor [TfR] bispecific versus control antibody) in mouse models of AD. We first confirmed BBB disruption in a mouse model of multiple sclerosis as a positive control. Importantly, we found that BBB permeability was vastly spared in mouse models of AD, including PS2-APP, Tau transgenics, and APOE4 knockin mice. Brain levels of TfR in mouse models or in human cases of AD resembled controls, suggesting target engagement of TfR bispecific is not limited. Furthermore, infarcts from human AD brain showed similar occurrences compared to age-matched controls. These results question the widely held view that the BBB is largely disrupted in AD, raising concern about assumptions of drug permeability in disease.

Therapeutic bispecific antibodies cross the blood-brain barrier in nonhuman primates.

Using therapeutic antibodies that need to cross the blood-brain barrier (BBB) to treat neurological disease is a difficult challenge. We have shown that bispecific antibodies with optimized binding to the transferrin receptor (TfR) that target ß-secretase (BACE1) can cross the BBB and reduce brain amyloid-ß (Aß) in mice. Can TfR enhance antibody uptake in the primate brain? We describe two humanized TfR/BACE1 bispecific antibody variants. Using a human TfR knock-in mouse, we observed that anti-TfR/BACE1 antibodies could cross the BBB and reduce brain Aß in a TfR affinity-dependent fashion. Intravenous dosing of monkeys with anti-TfR/BACE1 antibodies also reduced Aß both in cerebral spinal fluid and in brain tissue, and the degree of reduction correlated with the brain concentration of anti-TfR/BACE1 antibody. These results demonstrate that the TfR bispecific antibody platform can robustly and safely deliver therapeutic antibody across the BBB in the primate brain.

Dissecting gene expression at the blood-brain barrier.

The availability of genome-wide expression data for the blood-brain barrier is an invaluable resource that has recently enabled the discovery of several genes and pathways involved in the development and maintenance of the blood-brain barrier, particularly in rodent models. The broad distribution of published data sets represents a viable starting point for the molecular dissection of the blood-brain barrier and will further direct the discovery of novel mechanisms of blood-brain barrier formation and function. Technical advances in purifying brain endothelial cells, the key cell that forms the critical barrier, have allowed for greater specificity in gene expression comparisons with other central nervous system cell types, and more systematic characterizations of the molecular composition of the blood-brain barrier. Nevertheless, our understanding of how the blood-brain barrier changes during aging and disease is underrepresented. Blood-brain barrier data sets from a wider range of experimental paradigms and species, including invertebrates and primates, would be invaluable for investigating the function and evolution of the blood-brain barrier. Newer technologies in gene expression profiling, such as RNA-sequencing, now allow for finer resolution of transcriptomic changes, including isoform specificity and RNA-editing. As our field continues to utilize more advanced expression profiling in its ongoing efforts to elucidate the blood-brain barrier, including in disease and drug delivery, we will continue to see rapid advances in our understanding of the molecular mediators of barrier biology. We predict that the recently published data sets, combined with forthcoming genomic and proteomic blood-brain barrier data sets, will continue to fuel the molecular genetic revolution of.

Apolipoprotein E4 produced in GABAergic interneurons causes learning and memory deficits in mice.

Apolipoprotein (apo) E4 is expressed in many types of brain cells, is associated with age-dependent decline of learning and memory in humans, and is the major genetic risk factor for AD. To determine whether the detrimental effects of apoE4 depend on its cellular sources, we generated human apoE knock-in mouse models in which the human APOE gene is conditionally deleted in astrocytes, neurons, or GABAergic interneurons. Here we report that deletion of apoE4 in astrocytes does not protect aged mice from apoE4-induced GABAergic interneuron loss and learning and memory deficits. In contrast, deletion of apoE4 in neurons does protect aged mice from both deficits. Furthermore, deletion of apoE4 in GABAergic interneurons is sufficient to gain similar protection. This study demonstrates a detrimental effect of endogenously produced apoE4 on GABAergic interneurons that leads to learning and memory deficits in mice and provides a novel target for drug development for AD related to apoE4.

Transferrin receptor (TfR) trafficking determines brain uptake of TfR antibody affinity variants.

Antibodies to transferrin receptor (TfR) have potential use for therapeutic entry into the brain. We have shown that bispecific antibodies against TfR and ß-secretase (BACE1 [ß-amyloid cleaving enzyme-1]) traverse the blood-brain barrier (BBB) and effectively reduce brain amyloid ß levels. We found that optimizing anti-TfR affinity improves brain exposure and BACE1 inhibition. Here we probe the cellular basis of this improvement and explore whether TfR antibody affinity alters the intracellular trafficking of TfR. Comparing high- and low-affinity TfR bispecific antibodies in vivo, we found that high-affinity binding to TfR caused a dose-dependent reduction of brain TfR levels. In vitro live imaging and colocalization experiments revealed that high-affinity TfR bispecific antibodies facilitated the trafficking of TfR to lysosomes and thus induced the degradation of TfR, an observation which was further confirmed in vivo. Importantly, high-affinity anti-TfR dosing induced reductions in brain TfR levels, which significantly decreased brain exposure to a second dose of low-affinity anti-TfR bispecific. Thus, high-affinity anti-TfR alters TfR trafficking, which dramatically impacts the capacity for TfR to mediate BBB transcytosis.

ApoE and TDP-43 neuropathology in two siblings with familial FTLD-motor neuron disease.

Frontotemporal lobar degeneration with motor neuron disease (FTLD-MND) is characterized by neuronal cytoplasmic inclusions containing TDP-43. Apolipoprotein E4 (apoE4), derived from the apoE ϵ4 allele, enhances brain atrophy in FTLD through unknown mechanisms. Here, we studied two siblings with C9ORF72-linked familial FTLD-MND, an apoE ϵ4 homozygote and an apoE ϵ3 homozygote. The apoE ϵ4 homozygote had more cognitive-behavioral symptoms, fronto-insulo-temporal atrophy, and apoE fragments and aggregates in the anterior cingulate cortex. ApoE formed complexes with TDP-43 that were more abundant in the apoE ϵ4 homozygote. Although differences seen in a sibling pair could arise due to chance, these findings raise the possibility that apoE4 exacerbates brain pathology in FTLD through formation of neurotoxic apoE fragments and interactions with TDP-43.

Hilar GABAergic interneuron activity controls spatial learning and memory retrieval.

BACKGROUND: Although extensive research has demonstrated the importance of excitatory granule neurons in the dentate gyrus of the hippocampus in normal learning and memory and in the pathogenesis of amnesia in Alzheimer's disease (AD), the role of hilar GABAergic inhibitory interneurons, which control the granule neuron activity, remains unclear. METHODOLOGY AND PRINCIPAL FINDINGS: We explored the function of hilar GABAergic interneurons in spatial learning and memory by inhibiting their activity through Cre-dependent viral expression of enhanced halorhodopsin (eNpHR3.0)--a light-driven chloride pump. Hilar GABAergic interneuron-specific expression of eNpHR3.0 was achieved by bilaterally injecting adeno-associated virus containing a double-floxed inverted open-reading frame encoding eNpHR3.0 into the hilus of the dentate gyrus of mice expressing Cre recombinase under the control of an enhancer specific for GABAergic interneurons. In vitro and in vivo illumination with a yellow laser elicited inhibition of hilar GABAergic interneurons and consequent activation of dentate granule neurons, without affecting pyramidal neurons in the CA3 and CA1 regions of the hippocampus. We found that optogenetic inhibition of hilar GABAergic interneuron activity impaired spatial learning and memory retrieval, without affecting memory retention, as determined in the Morris water maze test. Importantly, optogenetic inhibition of hilar GABAergic interneuron activity did not alter short-term working memory, motor coordination, or exploratory activity. CONCLUSIONS AND SIGNIFICANCE: Our findings establish a critical role for hilar GABAergic interneuron activity in controlling spatial learning and memory retrieval and provide evidence for the potential contribution of GABAergic interneuron impairment to the pathogenesis of amnesia in AD.

Reducing human apolipoprotein E levels attenuates age-dependent Aß accumulation in mutant human amyloid precursor protein transgenic mice.

Apolipoprotein E4 (apoE4) plays a major role in the pathogenesis of Alzheimer's disease. Brain amyloid-ß (Aß) accumulation depends on age and apoE isoforms (apoE4 > apoE3) both in humans and in transgenic mouse models. Brain apoE levels are also isoform dependent, but in the opposite direction (apoE4 < apoE3). Thus, one prevailing hypothesis is to increase brain apoE expression to reduce Aß levels. To test this hypothesis, we generated mutant human amyloid precursor protein transgenic mice expressing one or two copies of the human APOE3 or APOE4 gene that was knocked in and flanked by LoxP sites. We report that reducing apoE3 or apoE4 expression by 50% in 6-month-old mice results in efficient Aß clearance and does not increase Aß accumulation. However, 12-month-old mice with one copy of the human APOE gene had significantly reduced Aß levels and plaque loads compared with mice with two copies, regardless of which human apoE isoform was expressed, suggesting a gene dose-dependent effect of apoE on Aß accumulation in aged mice. Additionally, 12-month-old mice expressing one or two copies of the human APOE4 gene had significantly higher levels of Aß accumulation and plaque loads than age-matched mice expressing one or two copies of the human APOE3 gene, suggesting an isoform-dependent effect of apoE on Aß accumulation in aged mice. Moreover, Cre-mediated APOE4 gene excision in hippocampal astrocytes significantly reduced insoluble Aß in adult mice. Thus, reducing, rather than increasing, apoE expression is an attractive approach to lowering brain Aß levels.

C-terminal-truncated apolipoprotein (apo) E4 inefficiently clears amyloid-beta (Abeta) and acts in concert with Abeta to elicit neuronal and behavioral deficits in mice.

Apolipoprotein (apo) E4 is the major known genetic risk factor for Alzheimer's disease (AD). We have shown in vitro and in vivo that apoE4 preferentially undergoes aberrant cleavage in neurons, yielding neurotoxic C-terminal-truncated fragments. To study the effect of these fragments on amyloid-ß (Aß) clearance/deposition and their potential synergy with Aß in eliciting neuronal and behavioral deficits, we cross-bred transgenic mice expressing apoE3, apoE4, or apoE4(Δ272-299) with mice expressing human amyloid protein precursor (APP) harboring familial AD mutations (hAPP(FAD)). At 6-8 mo of age, hAPP(FAD) mice expressing apoE3 or apoE4 had lower levels of hippocampal Aß (94% and 89%, respectively) and less Aß deposition (89% and 87%) than hAPP(FAD) mice without apoE, whereas hAPP(FAD) mice expressing mouse apoE had higher Aß levels. Thus, human apoE stimulates Aß clearance, but mouse apoE does not. Expression of apoE4(Δ272-299) reduced total Aß levels by only 63% and Aß deposition by 46% compared with hAPP(FAD) mice without apoE. Unlike apoE3 and apoE4, the C-terminal-truncated apoE4 bound poorly with Aß peptides, leading to decreased Aß clearance and increased Aß deposition. Despite their lower levels of Aß and Aß deposition, hAPP(FAD)/apoE4(Δ272-299) mice accumulated pathogenic Aß oligomers and displayed neuronal and behavioral deficits similar to or more severe than those in hAPP(FAD) mice. Thus, the C-terminal-truncated apoE4 fragment inefficiently clears Aß peptides and acts in concert with low levels of Aß to elicit neuronal and behavioral deficits in mice.

Apolipoprotein E4 causes age- and Tau-dependent impairment of GABAergic interneurons, leading to learning and memory deficits in mice.

Apolipoprotein E4 (apoE4) is the major genetic risk factor for Alzheimer's disease. However, the underlying mechanisms are unclear. We found that female apoE4 knock-in (KI) mice had an age-dependent decrease in hilar GABAergic interneurons that correlated with the extent of learning and memory deficits, as determined in the Morris water maze, in aged mice. Treating apoE4-KI mice with daily peritoneal injections of the GABA(A) receptor potentiator pentobarbital at 20 mg/kg for 4 weeks rescued the learning and memory deficits. In neurotoxic apoE4 fragment transgenic mice, hilar GABAergic interneuron loss was even more pronounced and also correlated with the extent of learning and memory deficits. Neurodegeneration and tauopathy occurred earliest in hilar interneurons in apoE4 fragment transgenic mice; eliminating endogenous Tau prevented hilar GABAergic interneuron loss and the learning and memory deficits. The GABA(A) receptor antagonist picrotoxin abolished this rescue, while pentobarbital rescued learning deficits in the presence of endogenous Tau. Thus, apoE4 causes age- and Tau-dependent impairment of hilar GABAergic interneurons, leading to learning and memory deficits in mice. Consequently, reducing Tau and enhancing GABA signaling are potential strategies to treat or prevent apoE4-related Alzheimer's disease.

GABAergic interneuron dysfunction impairs hippocampal neurogenesis in adult apolipoprotein E4 knockin mice.

Apolipoprotein (apo) E, a polymorphic protein with three isoforms (apoE2, apoE3, and apoE4), is essential for lipid homeostasis. Carriers of apoE4 are at higher risk for developing Alzheimer's disease. We have investigated adult neurogenesis in mice with knockout (KO) for apoE or with knockin (KI) alleles for human apoE3 or apoE4, and we report that neurogenesis is reduced in both apoE-KO and apoE4-KI mice. In apoE-KO mice, increased BMP signaling promoted glial differentiation at the expense of neurogenesis. In contrast, in apoE4-KI mice, presynaptic GABAergic input-mediated maturation of newborn neurons was diminished. Tau phosphorylation, an Alzheimer's disease characteristic, and levels of neurotoxic apoE fragments were both elevated in apoE4-KI hippocampal neurons concomitant with decreased GABAergic interneuron survival. Potentiating GABAergic signaling restored neuronal maturation and neurogenesis in apoE4-KI mice to normal levels. These findings suggest that GABAergic signaling can be targeted to mitigate the deleterious effects of apoE4 on neurogenesis.

Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease.

Neural network dysfunction may play an important role in Alzheimer's disease (AD). Neuronal circuits vulnerable to AD are also affected in human amyloid precursor protein (hAPP) transgenic mice. hAPP mice with high levels of amyloid-beta peptides in the brain develop AD-like abnormalities, including cognitive deficits and depletions of calcium-related proteins in the dentate gyrus, a region critically involved in learning and memory. Here, we report that hAPP mice have spontaneous nonconvulsive seizure activity in cortical and hippocampal networks, which is associated with GABAergic sprouting, enhanced synaptic inhibition, and synaptic plasticity deficits in the dentate gyrus. Many Abeta-induced neuronal alterations could be simulated in nontransgenic mice by excitotoxin challenge and prevented in hAPP mice by blocking overexcitation. Aberrant increases in network excitability and compensatory inhibitory mechanisms in the hippocampus may contribute to Abeta-induced neurological deficits in hAPP mice and, possibly, also in humans with AD.

Accelerating amyloid-beta fibrillization reduces oligomer levels and functional deficits in Alzheimer disease mouse models.

Many proteins suspected of causing neurodegenerative diseases exist in diverse assembly states. For most, it is unclear whether shifts from one state to another would be helpful or harmful. We used mutagenesis to change the assembly state of Alzheimer disease (AD)-associated amyloid-beta (Abeta) peptides. In vitro, the "Arctic" mutation (AbetaE22G) accelerated Abeta fibrillization but decreased the abundance of nonfibrillar Abeta assemblies, compared with wild-type Abeta. In human amyloid precursor protein (hAPP) transgenic mice carrying mutations adjacent to Abeta that increase Abeta production, addition of the Arctic mutation markedly enhanced the formation of neuritic amyloid plaques but reduced the relative abundance of a specific nonfibrillar Abeta assembly (Abeta*56). Mice overexpressing Arctic mutant or wild-type Abeta had similar behavioral and neuronal deficits when they were matched for Abeta*56 levels but had vastly different plaque loads. Thus, Abeta*56 is a likelier determinant of functional deficits in hAPP mice than fibrillar Abeta deposits. Therapeutic interventions that reduce Abeta fibrils at the cost of augmenting nonfibrillar Abeta assemblies could be harmful.

Reelin depletion in the entorhinal cortex of human amyloid precursor protein transgenic mice and humans with Alzheimer's disease.

Reelin regulates nervous system development and modulates synaptic plasticity in the adult brain. Several findings suggest that alterations in Reelin signaling may contribute to neuronal dysfunction associated with Alzheimer's disease (AD). Cell surface receptors for Reelin, including integrins and very-low-density lipoprotein receptor/apolipoprotein E2 receptor, may be targets of amyloid-beta (Abeta) peptides presumed to play key roles in the pathogenesis of AD. Reelin also regulates the extent of tau phosphorylation. Finally, increased amounts of Reelin fragments have been found in CSF from AD patients, suggesting altered processing of Reelin. We therefore hypothesized that Reelin levels might be altered in the brains of human amyloid precursor protein (hAPP) transgenic mice, particularly in brain regions vulnerable to AD such as hippocampus and entorhinal cortex. Compared with nontransgenic controls, hAPP mice had significantly fewer Reelin-expressing pyramidal cells in the entorhinal cortex, the major population of glutamatergic neurons expressing Reelin in the brain. Western blot analysis of the hippocampus, which receives projections from the entorhinal cortex, revealed significant reductions in Reelin levels. In contrast, the number of Reelin-expressing GABAergic interneurons was not altered in either the entorhinal cortex or the hippocampus. Thus, neuronal expression of hAPP/Abeta is sufficient to reduce Reelin expression in a specific population of entorhinal cortical pyramidal neurons in vivo. Underscoring the relevance of these findings, we found qualitatively similar reductions of Reelin-expressing pyramidal neurons in the entorhinal cortex of AD brains. We conclude that alterations in Reelin processing or signaling may be involved in AD-related neuronal dysfunction.