Brain area- and isoform-specific inhibition of synaptic plasticity by apoE4

Environmental enrichment reduces restricted repetitive behavior by altering gray matter microstructure

Restricted, repetitive behaviors are common symptoms in neurodevelopmental disorders including autism spectrum disorder. Despite being associated with poor developmental outcomes, repetitive behaviors remain poorly understood and have limited treatment options. Environmental enrichment attenuates the development of repetitive behaviors, but the exact mechanisms remain obscure. Using the C58 mouse model of repetitive behavior, we performed diffusion tensor imaging to examine microstructural alterations associated with the development of repetitive behavior and its attenuation by environmental enrichment. The C57BL/6 mouse strain, which displays little or no repetitive behavior, was used as a control group. We observed widespread differences in diffusion metrics between C58 mice and C57BL/6 mice. In juvenile C58 mice, repetitive motor behavior displayed strong negative correlations with fractional anisotropy in multiple gray matter regions, whereas in young adult C58 mice, high repetitive motor behavior was most strongly associated with lower fractional anisotropy and higher radial diffusivity in the striatum. Environmental enrichment increased fractional anisotropy and axial diffusivity throughout gray matter regions in the brains of juvenile C58 mice and overlapped predominantly with cerebellar and sensory regions associated with repetitive behavior. Our results suggest environmental enrichment reduces repetitive behavior development by altering gray matter microstructure in the cerebellum, medial entorhinal cortex, and sensory processing regions in juvenile C58 mice. Under standard laboratory conditions, early pathology in these regions appears to contribute to later striatal and white matter dysfunction in adult C58 mice. Future studies should examine the role these regions play in the development of repetitive behavior and the relationship between sensory processing and cerebellar deficits and repetitive behavior.

Lysosomal proteomics reveals mechanisms of neuronal apoE4associated lysosomal dysfunction

ApoE4 is the primary risk factor for Alzheimer's Disease. While apoE is primarily expressed by astrocytes, AD pathology including endosomal abnormalities and mitochondrial dysfunction first occurs in neurons. Lysosomes are poised at the convergence point between these features. We find that apoE4-expressing cells exhibit lysosomal alkalinization, reduced lysosomal proteolysis, and impaired mitophagy. To identify driving factors for this lysosomal dysfunction, we performed quantitative lysosomal proteome profiling. This revealed that apoE4 expression results in lysosomal depletion of Lgals3bp and accumulation of Tmed5 in both Neuro-2a cells and postmitotic human neurons. Modulating the expression of both proteins affected lysosomal function, with Tmed5 knockdown rescuing lysosomal alkalinization in apoE4 cells, and Lgals3bp knockdown causing lysosomal alkalinization and reduced lysosomal density in apoE3 cells. Taken together, our work reveals that apoE4 exerts gain-of-toxicity by alkalinizing the lysosomal lumen, pinpointing lysosomal Tmed5 accumulation and Lgals3bp depletion as apoE4-associated drivers for this phenotype.

The Effect of APOE ɛ4 on the Functional Connectivity in Frontoparietal Network in Hypertensive Patients

Allele 4 of the apolipoprotein E gene (APOE ε4) and hypertension are considered risk factors for Alzheimer’s Disease (AD). The detection of differences in cognitive function and brain networks between hypertensive patients who are APOE ε4 carriers and non-carriers may help in understanding how hypertension and risk genes cumulatively impair brain function, which could provide critical insights into the genetic mechanism by which hypertension serves as a potential risk factor for cognitive decline and even AD. Using behavioral data from 233 elderly hypertensive patients and neuroimaging data from 38 of them from Beijing, China; the study aimed to assess the effects of APOE ε4 on cognition and to explore related changes in functional connectivity. Cognitively, the patients with APOE ε4 showed decreased executive function, memory and language. In the MRI sub-cohort, the frontoparietal networks in the APOE ε4 carrier group exhibited an altered pattern, mainly in the left precentral regions, inferior frontal lobe and angular gyrus. More importantly, the decline of cognitive function was correlated with abnormal FC in the left precentral regions in APOE ε4 carriers. APOE ε4 aggravated the dysfunction in frontal and parietal regions in hypertensive patients. This highlights the importance of brain protection in hypertensive patients, especially those with a genetic risk of AD.

Genetic mutations of APOEε4 carriers in cardiovascular patients lead to the development of insulin resistance and risk of Alzheimer's disease

Type 2 diabetes mellitus and Alzheimer's disease (AD), both are chronic and progressive diseases. Many cardiovascular and genetic risk factors are considered responsible for the development of AD and diabetes mellitus (DM). Genetic risk factor such as apolipoprotein E (APOE) plays a critical role in the progression of AD. Specifically, APOEε4 is genetically the strongest isoform associated with neuronal insulin deficiency, altered lipid homeostasis, and metabolism, decreased glucose uptake, impaired gray matter volume, and cerebrovascular functions. In this article, we have summarized the mechanisms of cardiovascular disturbances associated with AD and DM, impact of amyloid-β aggregation, and neurofibrillary tangles formation in AD. Moreover, cardiovascular risk factors leading to insulin resistance (IR) and amyloid-β aggregation are highlighted along with the effects of APOE risk alleles on cerebral, lipid, and cholesterol metabolism leading to CVD-mediated IR. Correspondingly, the contribution of IR, genetic and cardiovascular risk factors in amyloid-β aggregation, which may lead to the late onset of AD and DM, has been also discussed. In short, IR is related to significantly lower cerebral glucose metabolism, which sequentially forecasts poorer memory performance. Hence, there will be more chances for neural glucose intolerance and impairment of cognitive function in cardiac patients, particularly APOEε4 carriers having IR. Hence, this review provides a better understanding of the corresponding crosstalk among different pathways. This will help to investigate the rational application of preventive measures against IR and cognitive dysfunction, specifically in APOEε4 carriers' cardio-metabolic patients.

Common variants in psychiatric risk genes predict brain structure at birth

Studies in adolescents and adults have demonstrated that polymorphisms in putative psychiatric risk genes are associated with differences in brain structure, but cannot address when in development these relationships arise. To determine if common genetic variants in disrupted-in-schizophrenia-1 (DISC1; rs821616 and rs6675281), catechol-O-methyltransferase (COMT; rs4680), neuregulin 1 (NRG1; rs35753505 and rs6994992), apolipoprotein E (APOE; ε3ε4 vs. ε3ε3), estrogen receptor alpha (ESR1; rs9340799 and rs2234693), brain-derived neurotrophic factor (BDNF; rs6265), and glutamate decarboxylase 1 (GAD1; rs2270335) are associated with individual differences in brain tissue volumes in neonates, we applied both automated region-of-interest volumetry and tensor-based morphometry to a sample of 272 neonates who had received high-resolution magnetic resonance imaging scans. ESR1 (rs9340799) predicted intracranial volume. Local variation in gray matter (GM) volume was significantly associated with polymorphisms in DISC1 (rs821616), COMT, NRG1, APOE, ESR1 (rs9340799), and BDNF. No associations were identified for DISC1 (rs6675281), ESR1 (rs2234693), or GAD1. Of note, neonates homozygous for the DISC1 (rs821616) serine allele exhibited numerous large clusters of reduced GM in the frontal lobes, and neonates homozygous for the COMT valine allele exhibited reduced GM in the temporal cortex and hippocampus, mirroring findings in adults. The results highlight the importance of prenatal brain development in mediating psychiatric risk.

Opposing actions of environmental enrichment and Alzheimer's disease on the expression of hippocampal microRNAs in mouse models

Alzheimer's disease (AD) is the most common form of dementia in the elderly. Although there are no drugs that modify the disease process, exposure to an enriched environment (EE) can slow the disease progression. Here, we characterize the effects of AD and EE on the post-transcriptional regulators, microRNAs (miRNAs), which may contribute to the detrimental and beneficial effects of AD and EE, respectively, on synaptic plasticity-related proteins and AD pathology. We found for the first time miRNAs that were inversely regulated in AD and EE, and may affect synaptic proteins and modulators, molecular factors associated with AD pathology, and survival and neuroprotective factors. MiRNAs that were upregulated only in 3xTgAD mice model of AD compared with their control mice were localized to synapses, predicted to downregulate essential synaptic proteins and are highly associated with regulating apoptosis, AD-associated processes and axon guidance. Studying the progressive change in miRNAs modulation during aging of 3xTgAD mice, we identified miRNAs that were regulated in earlier stages of AD, suggesting them as potential AD biomarkers. Last, we characterized AD- and EE-related effects in the mouse hippocampus on tomosyn protein levels, an inhibitor of the synaptic transmission machinery. While EE reduced tomosyn levels, tomosyn levels were increased in old 3xTgAD mice, suggesting a role for tomosyn in the impairment of synaptic transmission in AD. Interestingly, we found that miR-325 regulates the expression levels of tomosyn as demonstrated by a luciferase reporter assay, and that miR-325 was downregulated in AD and upregulated following EE. These findings improve our understanding of the molecular and cellular processes in AD pathology, following EE, and the interplay between the two processes, and open new avenues for the studies of understanding and controlling AD.

Impact of a multi-nutrient diet on cognition, brain metabolism, hemodynamics, and plasticity in apoE4 carrier and apoE knockout mice

Lipid metabolism and genetic background together strongly influence the development of both cardiovascular and neurodegenerative diseases like Alzheimer's disease (AD). A non-pharmacological way to prevent the genotype-induced occurrence of these pathologies is given by dietary behavior. In the present study, we tested the effects of long-term consumption of a specific multi-nutrient diet in two models for atherosclerosis and vascular risk factors in AD: the apolipoprotein ε4 (apoE4) and the apoE knockout (apoE ko) mice. This specific multi-nutrient diet was developed to support neuronal membrane synthesis and was expected to contribute to the maintenance of vascular health. At 12 months of age, both genotypes showed behavioral changes compared to control mice and we found increased neurogenesis in apoE ko mice. The specific multi-nutrient diet decreased anxiety-related behavior in the open field, influenced sterol composition in serum and brain tissue, and increased the concentration of omega-3 fatty acids in the brain. Furthermore, we found that wild-type and apoE ko mice fed with this multi-nutrient diet showed locally increased cerebral blood volume and decreased hippocampal glutamate levels. Taken together, these data suggest that a specific dietary intervention has beneficial effects on early pathological consequences of hypercholesterolemia and vascular risk factors for AD.

Sex Differences in Presynaptic Density and Neurogenesis in Middle-Aged ApoE4 and ApoE Knockout Mice

Atherosclerosis and apolipoprotein E ε4 (APOE4) genotype are risk factors for Alzheimer's disease (AD) and cardiovascular disease (CVD). Sex differences exist in prevalence and manifestation of both diseases. We investigated sex differences respective to aging, focusing on cognitive parameters in apoE4 and apoE knockout (ko) mouse models of AD and CVD. Presynaptic density and neurogenesis were investigated immunohistochemically in male and female apoE4, apoE ko, and wild-type mice. Middle-aged female apoE4 mice showed decreased presynaptic density in the inner molecular layer of the dentate gyrus of the hippocampus. Middle-aged female apoE ko mice showed a trend towards increased neurogenesis in the hippocampus compared with wild-type mice. No differences in these parameters could be observed in middle-aged male mice. Specific harmful interactions between apoE4 and estrogen could be responsible for decreased presynaptic density in female apoE4 mice. The trend of increased neurogenesis found in female apoE ko mice supports previous studies suggesting that temporarily increased amount of synaptic contacts and/or neurogenesis is a compensatory mechanism for synaptic failure. To our knowledge, no other studies investigating presynaptic density in aging female apoE4 or apoE ko mice are available. Sex-specific differences between APOE genotypes could account for some sex differences in AD and CVD.

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.

Selective effect of Apo e4 on CA3 and dentate in normal aging and Alzheimer's disease using high resolution MRI at 4 T

BACKGROUND

Details of the internal hippocampal structure visible at 4 T allow for in vivo volumetry of subfields. The aims of this study were: 1. To determine if Apo e4 has subfield specific effects in controls. 2. To study the influence of Apo e4 on hippocampal subfields in AD.

METHODS

81 subjects (66 controls, mean age 60.8+/-13.6, range: 28-85 years), and 15 AD (mean age 67.5+/-9.3) were studied. Entorhinal cortex, subiculum, CA1, CA1-CA2 transition zone, CA3-4 and dentate gyrus (CA3&DG) and total hippocampal volume were determined using a manual marking strategy.

RESULTS

Significant effects for Apo e4 on the CA3&DG were found in the total control population (p=0.042) and in older controls (61-85 years) (p=0.036) but not in younger (28-60 years) controls. Significant effects for Apo e4 (p=0.0035) on CA3&DG were also found in a subgroup of older subjects and AD subjects. AD with Apo e4 had smaller CA3&DG than AD without Apo e4 (p=0.027).

CONCLUSIONS

These findings suggest that Apo e4 exerts a regionally selective effect on CA3&DG in normal aging and AD.

Rosiglitazone increases dendritic spine density and rescues spine loss caused by apolipoprotein E4 in primary cortical neurons

Convergent evidence has revealed an association between insulin resistance and Alzheimer's disease (AD), and the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonist, rosiglitazone, an insulin sensitizer and mitochondrial activator, improves cognition in patients with early or mild-to-moderate AD. Apolipoprotein (apo) E4, a major genetic risk factor for AD, exerts neuropathological effects through multiple pathways, including impairment of dendritic spine structure and mitochondrial function. Here we show that rosiglitazone significantly increased dendritic spine density in a dose-dependent manner in cultured primary cortical rat neurons. This effect was abolished by the PPAR-gamma-specific antagonist, GW9662, suggesting that rosiglitazone exerts this effect by activating the PPAR-gamma pathway. Furthermore, the C-terminal-truncated fragment of apoE4 significantly decreased dendritic spine density. Rosiglitazone rescued this detrimental effect. Thus, rosiglitazone might improve cognition in AD patients by increasing dendritic spine density.

Intraneuronal amyloid-β plays a role in mediating the synergistic pathological effects of apoE4 and environmental stimulation

The allele E4 of apolipoprotein E4 (apoE4), which is the most prevalent genetic risk factor of Alzheimer's disease (AD), inhibits synaptogenesis and neurogenesis and stimulates apoptosis in brains of apoE4 transgenic mice that have been exposed to an enriched environment. In the present study, we investigated the hypothesis that the brain activity-dependent impairments in neuronal plasticity, induced by apoE4, are mediated via the amyloid cascade. Importantly, we found that exposure of mice transgenic for either apoE4, or the Alzheimer's disease benign allele apoE3, to an enriched environment elevates similarly the hippocampal levels of amyloid-beta peptide (Abeta) and apoE of these mice, but that the degree of aggregation and spatial distribution of Abeta in these mice are markedly affected by the apoE genotype. Accordingly, environmental stimulation triggered the formation of extracellular plaque-like Abeta deposits and the accumulation of intra-neuronal oligomerized Abeta specifically in brains of apoE4 mice. Further experiments revealed that hippocampal dentate gyrus neurons are particularly susceptible to apoE4 and environmental stimulation and that these neurons are specifically enriched in both oligomerized Abeta and apoE. These findings show that the impairments in neuroplasticity which are induced by apoE4 following environmental stimulation are associated with the accumulation of intraneuronal Abeta and suggest that oligomerized Abeta mediates the synergistic pathological effects of apoE4 and environmental stimulation.

Environmental enrichment stimulates neurogenesis in apolipoprotein E3 and neuronal apoptosis in apolipoprotein E4 transgenic mice

Neurodegeneration in Alzheimer's disease (AD) is associated with the activation of neurogenesis. The mechanisms underlying this crosstalk between neuronal death and birth and the extent to which it is affected by genetic risk factors of AD are not known. We employed transgenic mice expressing human apolipoprotein E4 (apoE4), the most prevalent genetic risk factor for AD, or expressing human apoE3 (an AD-benign allele), in order to examine the hypothesis that apoE4 tilts the balance between neurogenesis and neuronal cell death in favor of the latter. The results showed an isoform-specific increase in neurogenesis in the hippocampal dentate gyrus (DG) under standard conditions in apoE4-transgenic mice. Environmental stimulation, which increases neurogenesis in the DG of apoE3-transgenic and wild-type mice, had the opposite effect on the apoE4 mice, where it triggered apoptosis while decreasing hippocampal neurogenesis. These effects were specific to the DG and were not observed in the subventricular zone, where neurogenesis was unaffected by either the apoE genotype or the environmental conditions. These in vivo findings demonstrate a linkage between neuronal apoptosis and the impaired neuronal plasticity and cognition of apoE4-transgenic mice, and suggest that similar interactions between apoE4 and environmental factors might occur in AD.

Apolipoprotein E, cholesterol metabolism, diabetes, and the convergence of risk factors for Alzheimer's disease and cardiovascular disease

High fat diets and sedentary lifestyles are becoming major concerns for Western countries. They have led to a growing incidence of obesity, dyslipidemia, high blood pressure, and a condition known as the insulin-resistance syndrome or metabolic syndrome. These health conditions are well known to develop along with, or be precursors to atherosclerosis, cardiovascular disease, and diabetes. Recent studies have found that most of these disorders can also be linked to an increased risk of Alzheimer's disease (AD). To complicate matters, possession of one or more apolipoprotein E epsilon4 (APOE epsilon4) alleles further increases the risk or severity of many of these conditions, including AD. ApoE has roles in cholesterol metabolism and Abeta clearance, both of which are thought to be significant in AD pathogenesis. The apparent inadequacies of ApoE epsilon4 in these roles may explain the increased risk of AD in subjects carrying one or more APOE epsilon4 alleles. This review describes some of the physiological and biochemical changes that the above conditions cause, and how they are related to the risk of AD. A diversity of topics is covered, including cholesterol metabolism, glucose regulation, diabetes, insulin, ApoE function, amyloid precursor protein metabolism, and in particular their relevance to AD. It can be seen that abnormal lipid, cholesterol and glucose metabolism are consistently indicated as central in the pathophysiology, and possibly the pathogenesis of AD. As diagnosis of mild cognitive impairment and early AD are becoming more reliable, and as evidence is accumulating that health conditions such as diabetes, obesity, and coronary artery disease are risk factors for AD, appropriate changes to diets and lifestyles will likely reduce AD risk, and also improve the prognosis for people already suffering from such conditions.

Regulation of hippocampal cholesterol metabolism by apoE and environmental stimulation

Alzheimer's disease is associated with genetic risk factors, of which the allele E4 of apolipoprotein E (apoE4) is the most prevalent, and it is also affected by environmental factors such as early life education. We have recently shown, utilizing apoE-deficient and apoE transgenic mice, that synaptogenesis in the hippocampus following environmental stimulation is affected by apoE. In view of the pivotal role of cholesterol in synaptic plasticity, and of its suggested role in synaptogenesis, we presently examined the effects of apoE and environmental stimulation on brain cholesterol homeostasis. The hippocampal levels of cholesterol and its precursors and metabolites in control mice were not affected by exposure to environmental stimulation. In contrast, the hippocampal levels of cholesterol and its precursors lathosterol and desmosterol and metabolite 24S-hydroxycholesterol were lower in apoE-deficient mice that were maintained in a regular environmental than those of corresponding control mice, whereas they were markedly elevated following environmental stimulation. Histological and immunohistochemical experiments revealed that the combined stimulatory effects of apoE deficiency and environmental stimulation on cholesterol metabolism were associated with marked activation of hippocampal astrocytes and with the abnormal accumulation of cholesterol in neurons and astrocytes. These effects were rescued similarly in apoE3 and apoE4 transgenic mice. These findings suggest that apoE plays an important role in the translocation of cholesterol from astrocytes to neurons in vivo and in the regulation and homeostasis of this process.