Age-dependent enhancement of hippocampal long-term potentiation in knock-in mice expressing human apolipoprotein E4 instead of mouse apolipoprotein E

Higher Neuronal Facilitation and Potentiation with APOE4 Suppressed by Angiotensin II

Progressive hippocampal degeneration is a key component of Alzheimer's disease (AD) progression. Therefore, identifying how hippocampal neuronal function is modulated early in AD is an important approach to eventually prevent degeneration. AD-risk factors and signaling molecules likely modulate neuronal function, including APOE genotype and angiotensin II. Compared to APOE3 , APOE4 increases AD risk up to 12-fold, and high levels of angiotensin II are hypothesized to disrupt neuronal function in AD. However, the extent that APOE and angiotensin II modulates the hippocampal neuronal phenotype in AD-relevant models is unknown. To address this issue, we used electrophysiological techniques to assess the impact of APOE genotype and angiotensin II on basal synaptic transmission, presynaptic and post-synaptic activity in mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aβ. We found that compared to E3FAD mice, E4FAD mice had lower basal synaptic activity, but higher levels of paired pulse facilitation (PPF) and Long-Term Potentiation (LTP) in the Schaffer Collateral Commissural Pathway (SCCP) of the hippocampus. We also found that exogenous angiotensin II has a profound inhibitory effect on hippocampal LTP in both E3FAD and E4FAD mice. Collectively, our data suggests that APOE4 and Aβ are associated with a hippocampal phenotype comprised of lower basal activity and higher responses to high frequency stimulation, the latter of which is suppressed by angiotensin II. These novel data suggest a potential mechanistic link between hippocampal activity, APOE4 genotype and angiotensin II in AD.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer's disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell-cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Apolipoprotein E4 Effects a Distinct Transcriptomic Profile and Dendritic Arbor Characteristics in Hippocampal Neurons Cultured in vitro

The APOE gene is diversified by three alleles ε2, ε3, and ε4 encoding corresponding apolipoprotein (apo) E isoforms. Possession of the ε4 allele is signified by increased risks of age-related cognitive decline, Alzheimer's disease (AD), and the rate of AD dementia progression. ApoE is secreted by astrocytes as high-density lipoprotein-like particles and these are internalized by neurons upon binding to neuron-expressed apoE receptors. ApoE isoforms differentially engage neuronal plasticity through poorly understood mechanisms. We examined here the effects of native apoE lipoproteins produced by immortalized astrocytes homozygous for ε2, ε3, and ε4 alleles on the maturation and the transcriptomic profile of primary hippocampal neurons. Control neurons were grown in the presence of conditioned media from Apoe -/- astrocytes. ApoE2 and apoE3 significantly increase the dendritic arbor branching, the combined neurite length, and the total arbor surface of the hippocampal neurons, while apoE4 fails to produce similar effects and even significantly reduces the combined neurite length compared to the control. ApoE lipoproteins show no systemic effect on dendritic spine density, yet apoE2 and apoE3 increase the mature spines fraction, while apoE4 increases the immature spine fraction. This is associated with opposing effects of apoE2 or apoE3 and apoE4 on the expression of NR1 NMDA receptor subunit and PSD95. There are 1,062 genes differentially expressed across neurons cultured in the presence of apoE lipoproteins compared to the control. KEGG enrichment and gene ontology analyses show apoE2 and apoE3 commonly activate expression of genes involved in neurite branching, and synaptic signaling. In contrast, apoE4 cultured neurons show upregulation of genes related to the glycolipid metabolism, which are involved in dendritic spine turnover, and those which are usually silent in neurons and are related to cell cycle and DNA repair. In conclusion, our work reveals that lipoprotein particles comprised of various apoE isoforms differentially regulate various neuronal arbor characteristics through interaction with neuronal transcriptome. ApoE4 produces a functionally distinct transcriptomic profile, which is associated with attenuated neuronal development. Differential regulation of neuronal transcriptome by apoE isoforms is a newly identified biological mechanism, which has both implication in the development and aging of the CNS.

Isolated Amyloid-β Pathology Is Associated with Preserved Cortical Plasticity in APOE4 Alzheimer's Disease Patients

BACKGROUND

Long-term potentiation (LTP) like-cortical plasticity impairment and cholinergic neurotransmission deficits have been widely demonstrated in Alzheimer's disease (AD) patients.

OBJECTIVE

In this study we aim to investigate the neurophysiological features underlying cognitive decline in AD patients according to the National Institute on Aging-Alzheimer's Association (NIA-AA) classification and APOE genotype.

METHODS

65 newly diagnosed AD patients were enrolled. APOE genotype and lumbar puncture for the analysis of cerebrospinal fluid biomarkers were performed for diagnostic purposes. Patients were subdivided upon NIA-AA criteria, according to the presence of biomarkers of Aβ amyloid deposition (A) and fibrillar tau (T), in four groups: A+/T-E4 (n = 9), A+/T-E3 (n = 18), A+/T+ E4 (n = 21), and A+/T+ E3 (n = 17). We applied intermittent theta burst stimulation over the primary motor cortex to assess LTP-like cortical plasticity and short latency afferent inhibition (SAI) protocol to investigate central cholinergic activity. Patients were followed over 24 months. Cognitive decline was evaluated considering changes in Mini-Mental State Examination scores respect to the baseline.

RESULTS

A+/T-E4 patients showed preserved LTP-like cortical plasticity as compared to A+/T-E3 and to A+/T+ patients independently from genotype (p <  0.001). In addition, A+/T-E4 patients showed a slower cognitive decline with respect to A+/T+ E4 (-0.5±2.12 versus -6.05±4.95; post-hoc p = 0.004) and to A+/T+ E3 patients (-4.12±4.14; post-hoc p = 0.028). No differences were found for SAI protocol (p >  0.05).

CONCLUSION

Our results suggest that APOE4 in patients with isolated amyloid pathology could exert positive effects on LTP-like cortical plasticity with a consequent slower cognitive decline.

Novel Influences of Sex and APOE Genotype on Spinal Plasticity and Recovery of Function after Spinal Cord Injury

Spinal cord injuries can abolish both motor and sensory function throughout the body. Spontaneous recovery after injury is limited and can vary substantially between individuals. Despite an abundance of therapeutic approaches that have shown promise in preclinical models, there is currently a lack of effective treatment strategies that have been translated to restore function after spinal cord injury (SCI) in the human population. We hypothesized that sex and genetic background of injured individuals could impact how they respond to treatment strategies, presenting a barrier to translating therapies that are not tailored to the individual. One gene of particular interest is APOE, which has been extensively studied in the brain because of its allele-specific influences on synaptic plasticity, metabolism, inflammation, and neurodegeneration. Despite its prominence as a therapeutic target in brain injury and disease, little is known about how it influences neural plasticity and repair processes in the spinal cord. Using humanized mice, we examined how the ε3 and ε4 alleles of APOE influence the efficacy of therapeutic intermittent hypoxia (IH) in inducing spinally-mediated plasticity after cervical SCI (cSCI). IH is sufficient to enhance plasticity and restore motor function after experimental SCI in genetically similar rodent populations, but its effect in human subjects is more variable (Golder and Mitchell, 2005; Hayes et al., 2014). Our results demonstrate that both sex and APOE genotype determine the extent of respiratory motor plasticity that is elicited by IH, highlighting the importance of considering these clinically relevant variables when translating therapeutic approaches for the SCI community.

A mouse model of chemotherapy-related cognitive impairments integrating the risk factors of aging and APOE4 genotype

Some cancer survivors experience marked cognitive impairment, referred to as cancer-related cognitive impairment (CRCI). CRCI has been linked to the genetic factor APOE4, the strongest genetic risk factor for Alzheimer's disease (AD). We used APOE knock-in mice to test whether the relationship between APOE4 and CRCI can be demonstrated in a mouse model, to identify associations of chemotherapy with behavioural and structural correlates of cognition, and to test whether chemotherapy affects markers of AD. Twelve-month old C57BL/6 J female APOE3 (n = 30) and APOE4 (n = 31) knock-in mice were randomized to treatment with either doxorubicin (10 mg/kg) or saline. Behavioural assays at 2-21 weeks-post exposure included open field maze, elevated zero maze, pre-pulse inhibition, Barnes maze, and fear conditioning. Ex-vivo magnetic resonance imaging was used to determine regional volume differences at 31-35 weeks-post exposure, and tissue sections were analyzed for markers of AD pathogenesis. Minimal toxicities were observed in the aged mice after doxorubicin exposure. In the Barnes maze assay, APOE3 mice did not exhibit impairment in spatial learning after doxorubicin treatment, but APOE4 mice demonstrated significant impairments in both the initial identification of the escape hole and the latency to full escape at 6 weeks post-exposure. Both APOE3 and APOE4 mice treated with doxorubicin showed impairment of spatial memory. Grey matter volume in the frontal cortex decreased in APOE4 mice treated with doxorubicin vs. APOE3 mice. This study demonstrates cognitive impairments in aged APOE4 knock-in mice after doxorubicin treatment and establishes this system as a novel and powerful model of CRCI.

THE EXPOSOME IN HUMAN EVOLUTION: FROM DUST TO DIESEL

Global exposures to air pollution and cigarette smoke are novel in human evolutionary history and are associated with about 16 million premature deaths per year. We investigate the history of the human exposome for relationships between novel environmental toxins and genetic changes during human evolution in six phases. Phase I: With increased walking on savannas, early human ancestors inhaled crustal dust, fecal aerosols, and spores; carrion scavenging introduced new infectious pathogens. Phase II: Domestic fire exposed early Homo to novel toxins from smoke and cooking. Phases III and IV: Neolithic to preindustrial Homo sapiens incurred infectious pathogens from domestic animals and dense communities with limited sanitation. Phase V: Industrialization introduced novel toxins from fossil fuels, industrial chemicals, and tobacco at the same time infectious pathogens were diminishing. Thereby, pathogen-driven causes of mortality were replaced by chronic diseases driven by sterile inflammogens, exogenous and endogenous. Phase VI: Considers future health during global warming with increased air pollution and infections. We hypothesize that adaptation to some ancient toxins persists in genetic variations associated with inflammation and longevity.

Opposing Roles of apolipoprotein E in aging and neurodegeneration

Apolipoprotein E (APOE) effects on brain function remain controversial. Removal of APOE not only impairs cognitive functions but also reduces neuritic amyloid plaques in mouse models of Alzheimer's disease (AD). Can APOE simultaneously protect and impair neural circuits? Here, we dissociated the role of APOE in AD versus aging to determine its effects on neuronal function and synaptic integrity. Using two-photon calcium imaging in awake mice to record visually evoked responses, we found that genetic removal of APOE improved neuronal responses in adult APP/PSEN1 mice (8-10 mo). These animals also exhibited fewer neuritic plaques with less surrounding synapse loss, fewer neuritic dystrophies, and reactive glia. Surprisingly, the lack of APOE in aged mice (18-20 mo), even in the absence of amyloid, disrupted visually evoked responses. These results suggest a dissociation in APOE's role in AD versus aging: APOE may be neurotoxic during early stages of amyloid deposition, although being neuroprotective in latter stages of aging.

Apolipoprotein E, Receptors, and Modulation of Alzheimer's Disease

Apolipoprotein E (apoE) is a lipid carrier in both the peripheral and the central nervous systems. Lipid-loaded apoE lipoprotein particles bind to several cell surface receptors to support membrane homeostasis and injury repair in the brain. Considering prevalence and relative risk magnitude, the ε4 allele of the APOE gene is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). ApoE4 contributes to AD pathogenesis by modulating multiple pathways, including but not limited to the metabolism, aggregation, and toxicity of amyloid-β peptide, tauopathy, synaptic plasticity, lipid transport, glucose metabolism, mitochondrial function, vascular integrity, and neuroinflammation. Emerging knowledge on apoE-related pathways in the pathophysiology of AD presents new opportunities for AD therapy. We describe the biochemical and biological features of apoE and apoE receptors in the central nervous system. We also discuss the evidence and mechanisms addressing differential effects of apoE isoforms and the role of apoE receptors in AD pathogenesis, with a particular emphasis on the clinical and preclinical studies related to amyloid-β pathology. Finally, we summarize the current strategies of AD therapy targeting apoE, and postulate that effective strategies require an apoE isoform-specific approach.

CSF tau is associated with impaired cortical plasticity, cognitive decline and astrocyte survival only in APOE4-positive Alzheimer's disease

In Alzheimer’s disease (AD) patients, apopoliprotein (APOE) polymorphism is the main genetic factor associated with more aggressive clinical course. However, the interaction between cerebrospinal fluid (CSF) tau protein levels and APOE genotype has been scarcely investigated. A possible key mechanism invokes the dysfunction of synaptic plasticity. We investigated how CSF tau interacts with APOE genotype in AD patients. We firstly explored whether CSF tau levels and APOE genotype influence disease progression and long-term potentiation (LTP)-like cortical plasticity as measured by transcranial magnetic stimulation (TMS) in AD patients. Then, we incubated normal human astrocytes (NHAs) with CSF collected from sub-groups of AD patients to determine whether APOE genotype and CSF biomarkers influence astrocytes survival. LTP-like cortical plasticity differed between AD patients with apolipoprotein E4 (APOE4) and apolipoprotein E3 (APOE3) genotype. Higher CSF tau levels were associated with more impaired LTP-like cortical plasticity and faster disease progression in AD patients with APOE4 but not APOE3 genotype. Apoptotic activity was higher when cells were incubated with CSF from AD patients with APOE4 and high tau levels. CSF tau is detrimental on cortical plasticity, disease progression and astrocyte survival only when associated with APOE4 genotype. This is relevant for new therapeutic approaches targeting tau.

Hippocampal synaptic and neural network deficits in young mice carrying the human APOE4 gene

INTRODUCTION

Apolipoprotein E4 (APOE4) is a major genetic risk factor for late-onset sporadic Alzheimer disease. Emerging evidence demonstrates a hippocampus-associated learning and memory deficit in aged APOE4 human carriers and also in aged mice carrying human APOE4 gene. This suggests that either exogenous APOE4 or endogenous APOE4 alters the cognitive profile and hippocampal structure and function. However, little is known regarding how Apoe4 modulates hippocampal dendritic morphology, synaptic function, and neural network activity in young mice.

AIM

In this study, we compared hippocampal dendritic and spine morphology and synaptic function of young (4 months) mice with transgenic expression of the human APOE4 and APOE3 genes.

METHODS

Hippocampal dendritic and spine morphology and synaptic function were assessed by neuronal imaging and electrophysiological approaches.

RESULTS

Morphology results showed that shortened dendritic length and reduced spine density occurred at hippocampal CA1 neurons in Apoe4 mice compared to Apoe3 mice. Electrophysiological results demonstrated that in the hippocampal CA3-CA1 synapses of young Apoe4 mice, basic synaptic transmission, and paired-pulse facilitation were enhanced but long-term potentiation and carbachol-induced hippocampal theta oscillations were impaired compared to young Apoe3 mice. However, both Apoe genotypes responded similarly to persistent stimulations (4, 10, and 40 Hz for 4 seconds).

CONCLUSION

Our results suggest significant alterations in hippocampal dendritic structure and synaptic function in Apoe4 mice, even at an early age.

Structural neuroimaging in preclinical dementia: From microstructural deficits and grey matter atrophy to macroscale connectomic changes

The last decade has witnessed a proliferation of neuroimaging studies characterising brain changes associated with Alzheimer's disease (AD), where both widespread atrophy and 'signature' brain regions have been implicated. In parallel, a prolonged latency period has been established in AD, with abnormal cerebral changes beginning many years before symptom onset. This raises the possibility of early therapeutic intervention, even before symptoms, when treatments could have the greatest effect on disease-course modification. Two important prerequisites of this endeavour are (1) accurate characterisation or risk stratification and (2) monitoring of progression using neuroimaging outcomes as a surrogate biomarker in those without symptoms but who will develop AD, here referred to as preclinical AD. Structural neuroimaging modalities have been used to identify brain changes related to risk factors for AD, such as familial genetic mutations, risk genes (for example apolipoprotein epsilon-4 allele), and/or family history. In this review, we summarise structural imaging findings in preclinical AD. Overall, the literature suggests early vulnerability in characteristic regions, such as the medial temporal lobe structures and the precuneus, as well as white matter tracts in the fornix, cingulum and corpus callosum. We conclude that while structural markers are promising, more research and validation studies are needed before future secondary prevention trials can adopt structural imaging biomarkers as either stratification or surrogate biomarkers.

DHA brain uptake and APOE4 status: a PET study with [1-11C]-DHA

BACKGROUND

The apolipoprotein E ɛ4 (APOE4) allele is the strongest genetic risk factor identified for developing Alzheimer's disease (AD). Among brain lipids, alteration in the ω-3 polyunsaturated fatty acid docosahexaenoic acid (DHA) homeostasis is implicated in AD pathogenesis. APOE4 may influence both brain DHA metabolism and cognitive outcomes.

METHODS

Using positron emission tomography, regional incorporation coefficients (k*), rates of DHA incorporation from plasma into the brain using [1-¹¹C]-DHA (J _in), and regional cerebral blood flow using [¹⁵O]-water were measured in 22 middle-aged healthy adults (mean age 35 years, range 19-65 years). Data were partially volume error-corrected for brain atrophy. APOE4 phenotype was determined by protein expression, and unesterified DHA concentrations were quantified in plasma. An exploratory post hoc analysis of the effect of APOE4 on DHA brain kinetics was performed.

RESULTS

The mean global gray matter DHA incorporation coefficient, k*, was significantly higher (16%) among APOE4 carriers (n = 9) than among noncarriers (n = 13, p = 0.046). Higher DHA incorporation coefficients were observed in several brain regions, particularly in the entorhinal subregion, an area affected early in AD pathogenesis. Cerebral blood flow, unesterified plasma DHA, and whole brain DHA incorporation rate (J _in) did not differ significantly between the APOE groups.

CONCLUSIONS

Our findings suggest an increase in the DHA incorporation coefficient in several brain regions in APOE4 carriers. These findings may contribute to understanding how APOE4 genotypes affect AD risk.

Apolipoprotein E ϵ4 is positively related to spatial performance but unrelated to hippocampal volume in healthy young adults

The apolipoprotein E (APOE) ϵ4 allele is known to be a major genetic risk factor for Alzheimer's disease (AD). It has been linked to especially episodic memory decline and hippocampal atrophy in both healthy and demented elderly populations. In young adults, ϵ4 carriers have shown better performance in episodic memory compared to non-carriers. Spatial memory, however, has not been thoroughly assessed in relation to APOE in spite of its dependence on the hippocampus. In this study, we assessed the effect of APOE genotype on a variety of spatial and episodic memory tasks as well as hippocampal volume assessed through manual tracing in a sample of young adults (N=123). We also assessed whether potential effects were modulated by sex. The presence of one or more ϵ4 alleles had positive effects on spatial function and memory and object location memory, but no effect on word recognition. Men were superior to women in spatial function and memory but there were no sex differences in the other tasks. In spite of APOE ϵ4 carriers having superior performance in several memory tasks, no difference was found as a function of APOE genotype in hippocampal volume. To our knowledge, this study is the first to show that APOE ϵ4 has a positive effect on spatial ability in young adults.

Apolipoprotein E in synaptic plasticity and Alzheimer's disease: potential cellular and molecular mechanisms

Alzheimer's disease (AD) is clinically characterized with progressive memory loss and cognitive decline. Synaptic dysfunction is an early pathological feature that occurs prior to neurodegeneration and memory dysfunction. Mounting evidence suggests that aggregation of amyloid-β (Aβ) and hyperphosphorylated tau leads to synaptic deficits and neurodegeneration, thereby to memory loss. Among the established genetic risk factors for AD, the ɛ4 allele of apolipoprotein E (APOE) is the strongest genetic risk factor. We and others previously demonstrated that apoE regulates Aβ aggregation and clearance in an isoform-dependent manner. While the effect of apoE on Aβ may explain how apoE isoforms differentially affect AD pathogenesis, there are also other underexplored pathogenic mechanisms. They include differential effects of apoE on cerebral energy metabolism, neuroinflammation, neurovascular function, neurogenesis, and synaptic plasticity. ApoE is a major carrier of cholesterols that are required for neuronal activity and injury repair in the brain. Although there are a few conflicting findings and the underlying mechanism is still unclear, several lines of studies demonstrated that apoE4 leads to synaptic deficits and impairment in long-term potentiation, memory and cognition. In this review, we summarize current understanding of apoE function in the brain, with a particular emphasis on its role in synaptic plasticity and the underlying cellular and molecular mechanisms, involving low-density lipoprotein receptor-related protein 1 (LRP1), syndecan, and LRP8/ApoER2.

Young APOE4 targeted replacement mice exhibit poor spatial learning and memory, with reduced dendritic spine density in the medial entorhinal cortex

The apolipoprotein E4 (APOE-ε4) allele is the strongest genetic risk factor for developing late-onset Alzheimer's disease, and may predispose individuals to Alzheimer's-related cognitive decline by affecting normal brain function early in life. To investigate the impact of human APOE alleles on cognitive performance in mice, we trained 3-mo-old APOE targeted replacement mice (E2, E3, and E4) in the Barnes maze to locate and enter a target hole along the perimeter of the maze. Long-term spatial memory was probed 24 h and 72 h after training. We found that young E4 mice exhibited significantly impaired spatial learning and memory in the Barnes maze compared to E3 mice. Deficits in spatial cognition were also present in a second independent cohort of E4 mice tested at 18 mo of age. In contrast, cognitive performance in the hidden platform water maze was not as strongly affected by APOE genotype. We also examined the dendritic morphology of neurons in the medial entorhinal cortex of 3-mo-old TR mice, neurons important to spatial learning functions. We found significantly shorter dendrites and lower spine densities in basal shaft dendrites of E4 mice compared to E3 mice, consistent with spatial learning and memory deficits in E4 animals. These findings suggest that human APOE-ε4 may affect cognitive function and neuronal morphology early in life.

ApoE4 confers better spatial memory than apoE3 in young adult hAPP-Yac/apoE-TR mice

The APOE-ɛ4 allele is associated with increased cognitive decline during normal aging and Alzheimer's disease. However, several studies intriguingly found a beneficial effect on cognition in young adult human APOE-ɛ4 carriers. Here, we show that 3-month old bigenic hAPP-Yac/apoE4-TR mice outperformed their hAPP-Yac/apoE3-TR counterparts on learning and memory performances in the highly hippocampus-dependent, hidden-platform version of the Morris water maze task. The two mouse lines did not differ in a non-spatial visible-platform version of the task. This hAPP-Yac/apoE-TR model may thus provide a useful tool to study the mechanisms involved in the antagonistic pleiotropic effects of APOE-ɛ4 on cognitive functions.

White matter differences between healthy young ApoE4 carriers and non-carriers identified with tractography and support vector machines

The apolipoprotein E4 (ApoE4) is an established risk factor for Alzheimer's disease (AD). Previous work has shown that this allele is associated with functional (fMRI) changes as well structural grey matter (GM) changes in healthy young, middle-aged and older subjects. Here, we assess the diffusion characteristics and the white matter (WM) tracts of healthy young (20-38 years) ApoE4 carriers and non-carriers. No significant differences in diffusion indices were found between young carriers (ApoE4+) and non-carriers (ApoE4-). There were also no significant differences between the groups in terms of normalised GM or WM volume. A feature selection algorithm (ReliefF) was used to select the most salient voxels from the diffusion data for subsequent classification with support vector machines (SVMs). SVMs were capable of classifying ApoE4 carrier and non-carrier groups with an extremely high level of accuracy. The top 500 voxels selected by ReliefF were then used as seeds for tractography which identified a WM network that included regions of the parietal lobe, the cingulum bundle and the dorsolateral frontal lobe. There was a non-significant decrease in volume of this WM network in the ApoE4 carrier group. Our results indicate that there are subtle WM differences between healthy young ApoE4 carriers and non-carriers and that the WM network identified may be particularly vulnerable to further degeneration in ApoE4 carriers as they enter middle and old age.

Olfactory ERPs in an odor/visual congruency task differentiate ApoE ε4 carriers from non-carriers

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder that impairs memory and semantic processing. AD patients and MCI patients at risk for AD show altered N400 ERP responses to incongruent visual and verbal stimuli. AD patients exhibit neuropathology in olfactory brain areas before cognitive symptoms, suggesting the potential for olfactory processing to reflect early pathology. Despite this, odor congruency has not been examined. We investigated odor-image congruency in older adults at genetic risk for AD. ApoE ε4 carriers and non-carriers were screened for anosmia, severe hyposmia, and dementia. Olfactory ERPs were measured 600-1300 ms following odor-image pairs. Odors were each presented once congruently and once incongruently via an olfactometer. Pz amplitude significantly decreased on incongruent trials in e4 carriers. Pz amplitude differences on congruous and incongruous trials were larger in non carriers. Regression indicated that congruency showed very high sensitivity and specificity for correctly classifying ε4 carriers from non-carriers.

Insights into synaptic function from mouse models of human cognitive disorders

Modern approaches to the investigation of the molecular mechanisms underlying human cognitive disease often include multidisciplinary examination of animal models engineered with specific mutations that spatially and temporally restrict expression of a gene of interest. This approach not only makes possible the development of animal models that demonstrate phenotypic similarities to their respective human disorders, but has also allowed for significant progress towards understanding the processes that mediate synaptic function and memory formation in the nondiseased state. Examples of successful mouse models where genetic manipulation of the mouse resulted in recapitulation of the symptomatology of the human disorder and was used to significantly expand our understanding of the molecular mechanisms underlying normal synaptic plasticity and memory formation are discussed in this article. These studies have broadened our knowledge of several signal transduction cascades that function throughout life to mediate synaptic physiology. Defining these events is key for developing therapies to address disorders of cognitive ability.

Reelin and apoE actions on signal transduction, synaptic function and memory formation

Low-density-lipoprotein receptors (LDLRs) are an evolutionarily ancient surface protein family with the ability to activate a diversity of extracellular signals across the cellular membrane in the adult central nervous system (CNS). Their intimate roles in modulating synaptic plasticity and their necessity in hippocampal-dependent learning and memory have only recently come to light. Two known LDLR ligands, specifically apolipoprotein E (apoE) and reelin, have been the most widely investigated in this regard. Most of our understanding of synaptic plasticity comes from investigation of both pre- and postsynaptic alterations. Therefore, it is interesting to note that neurons and glia that do not contribute to the synaptic junction in question can secrete signaling molecules that affect synaptic plasticity. Notably, reelin and apoE have been shown to modulate hippocampal long-term potentiation in general, and affect NMDA receptor and AMPA receptor regulation specifically. Furthermore, these receptors and signaling molecules have significant roles in neuronal degenerative diseases such as Alzheimer's disease. The recent production of recombinant proteins, knockout and transgenic mice for receptors and ligands and the development of human ApoE targeted replacement mice have significantly expanded our understanding of the roles LDLRs and their ligands have in certain disease states and the accompanying initiation of specific signaling pathways. This review describes the role LDLRs, apoE and reelin have in the regulation of hippocampal synaptic plasticity.

ApoE isoform-dependent changes in hippocampal synaptic function

The lipoprotein receptor system in the hippocampus is intimately involved in the modulation of synaptic transmission and plasticity. The association of specific apoE isoform expression with human neurodegenerative disorders has focused attention on the role of these apoE isoforms in lipoprotein receptor-dependent synaptic modulation. In the present study, we used the apoE2, apoE3 and apoE4 targeted replacement (TR) mice along with recombinant human apoE isoforms to determine the role of apoE isoforms in hippocampus area CA1 synaptic function. While synaptic transmission is unaffected by apoE isoform, long-term potentiation (LTP) is significantly enhanced in apoE4 TR mice versus apoE2 TR mice. ApoE isoform-dependent differences in LTP induction require NMDA-receptor function, and apoE isoform expression alters activation of both ERK and JNK signal transduction. Acute application of specific apoE isoforms also alters LTP induction while decreasing NMDA-receptor mediated field potentials. Furthermore, acute apoE isoform application does not have the same effects on ERK and JNK activation. These findings demonstrate specific, isoform-dependent effects of human apoE isoforms on adult hippocampus synaptic plasticity and highlight mechanistic differences between chronic apoE isoform expression and acute apoE isoform exposure.

Reelin, lipoprotein receptors and synaptic plasticity

Apolipoprotein E (APOE) is a cholesterol transport protein and an isoform-specific major risk factor for neurodegenerative diseases. The lipoprotein receptors that bind APOE have recently been recognized as pivotal components of the neuronal signalling machinery. The interaction between APOE receptors and one of their ligands, reelin, allows them to function directly as signal transduction receptors at the plasma membrane to control not only neuronal positioning during brain development, but also synaptic plasticity in the adult brain. Here, we review the molecular mechanisms through which APOE, cholesterol, reelin and APOE receptors control synaptic functions that are essential for cognition, learning, memory, behaviour and neuronal survival.

Better memory and neural efficiency in young apolipoprotein E epsilon4 carriers

The apolipoprotein E (APOE) epsilon4 allele is the major genetic risk factor for Alzheimer's disease, but an APOE effect on memory performance and memory-related neurophysiology in young, healthy subjects is unknown. We found an association of APOE epsilon4 with better episodic memory compared with APOE epsilon2 and epsilon3 in 340 young, healthy persons. Neuroimaging was performed in a subset of 34 memory-matched individuals to study genetic effects on memory-related brain activity independently of differential performance. E4 carriers decreased brain activity over 3 learning runs, whereas epsilon2 and epsilon3 carriers increased activity. This smaller neural investment of epsilon4 carriers into learning reappeared during retrieval: epsilon4 carriers exhibited reduced retrieval-related activity with equal retrieval performance. APOE isoforms had no differential effects on cognitive measures other than memory, brain volumes, and brain activity related to working memory. We suggest that APOE epsilon4 is associated with good episodic memory and an economic use of memory-related neural resources in young, healthy humans.

Synaptic dysfunction and oxidative stress in Alzheimer's disease: emerging mechanisms

In this paper, we review experimental advances in molecular neurobiology of Alzheimer's disease (AD), with special emphasis on analysis of neural function of proteins involved in AD pathogenesis, their relation with several signaling pathways and with oxidative stress in neurons. Molecular genetic studies have found that mutations in APP, PS1 and PS2 genes and polymorphisms in APOE gene are implicated in AD pathogenesis. Recent studies show that these proteins, in addition to its role in beta-amyloid processing, are involved in several neuroplasticity-signaling pathways (NMDA-PKA-CREB-BDNF, reelin, wingless, notch, among others). Genomic and proteomic studies show early synaptic protein alterations in AD brains and animal models. DNA damage caused by oxidative stress is not completely repaired in neurons and is accumulated in the genes of synaptic proteins. Several functional SNPs in synaptic genes may be interesting candidates to explore in AD as genetic correlates of this synaptopathy in a "synaptogenomics" approach. Thus, experimental evidence shows that proteins implicated in AD pathogenesis have differential roles in several signaling pathways related to neuromodulation and neurotransmission in adult and developing brain. Genomic and proteomic studies support these results. We suggest that oxidative stress effects on DNA and inherited variations in synaptic genes may explain in part the synaptic dysfunction seen in AD.