Apolipoprotein E epsilon4 genotype and gender: effects on memory

Olfactory Dysfunction Associated with Cognitive Decline in an Elderly Population

BACKGROUND

In many neurological disorders, including Alzheimer disease, early olfactory dysfunction is observed.

OBJECTIVE

In order to determine if deficits in olfactory memory are present in the elderly and if olfactory dysfunction correlates with cognitive impairment in the aging population, olfactory testing has been done on seniors from the NuAge cohort accepting to participate in the Olfactory Response Cognition and Aging (ORCA) secondary sub-study. The t-Mini Mental Statement Examination and the Telephone Interview for Cognitive Status tests were done to assess cognition levels.

RESULTS

Overall, 94% of the ORCA cohort displayed olfactory dysfunction. Deficits in olfactory memory were also present. A correlation was observed between olfactory function and cognitive test scores. Moreover, in women who smoked, there was an association between olfactory memory and cognitive scores.

CONCLUSION

Our results suggest that olfactory dysfunction may predict impending cognitive decline and highlights the need for olfactory training in seniors to improve olfaction and overall well-being.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O).

METHODS

Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus.

RESULTS

The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies.

CONCLUSION

This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Is There a Shared Etiology of Olfactory Impairments in Normal Aging and Neurodegenerative Disease?

As we age, our olfactory function declines. In addition to occurring in normal aging, more rapid decrement of olfactory decline has been associated with several neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). It has been argued that since olfactory deficits occur less frequently or are absent in diseases such as progressive supranuclear palsy, corticobasal degeneration, and multiple system atrophy, olfactory deficits can be used for differential diagnoses of AD and PD. The purpose of this review is to provide a survey of current knowledge about the molecular bases and differential patterns of olfactory deficits present in normal aging, AD, and PD. As substantial research has been conducted in this area, the majority of the content of this review focuses on articles published in the past decade. We hypothesize that olfactory deficits in normal aging, AD, and PD may have different underlying causes, and propose the use of model organisms with small, tractable nervous systems and/or easy to manipulate genomes to further investigate the cellular mechanisms responsible for these deficits.

Characterizing the human APOE epsilon 4 knock-in transgene in female and male rats with multimodal magnetic resonance imaging

The APOE Ɛ4 genotype is the most prevalent genetic risk for Alzheimer's disease (AD). Women carriers of Ɛ4 have higher risk for an early onset of AD than men. Human imaging studies suggest apolipoprotein Ɛ4 may affect brain structures associated with cognitive decline in AD many years before disease onset. It was hypothesized that female APOE Ɛ4 carriers would present with decreased cognitive function and neuroradiological evidence of early changes in brain structure and function as compared to male carriers. Six-month old wild-type (WT) and human APOE Ɛ4 knock-in (TGRA8960), male and female Sprague Dawley rats were studied for changes in brain structure using voxel-based morphometry, alteration in white and gray matter microarchitecture using diffusion weighted imaging with indices of anisotropy, and functional coupling using resting state BOLD functional connectivity. Images from each modality were registered to, and analyzed, using a 3D MRI rat atlas providing site-specific data on over 168 different brain areas. Quantitative volumetric analysis revealed areas involved in memory and arousal were significantly different between Ɛ4 and wild-type (WT) females, with few differences between male genotypes. Diffusion weighted imaging showed few differences between WT and Ɛ4 females, while male genotypes showed significant different measures in fractional anisotropy and apparent diffusion coefficient. Resting state functional connectivity showed Ɛ4 females had greater connectivity between areas involved in cognition, emotion, and arousal compared to WT females, with male Ɛ4 showing few differences from controls. Interestingly, male Ɛ4 showed increased anxiety and decreased performance in spatial and episodic memory tasks compared to WT males, with female genotypes showing little difference across behavioral tests. The sex differences in behavior and diffusion weighted imaging suggest male carriers of the Ɛ4 allele may be more vulnerable to cognitive and emotional complications compared to female carriers early in life. Conversely, the data may also suggest that female carriers are more resilient to cognitive/emotional problems at this stage of life perhaps due to altered brain volumes and enhanced connectivity.

Olfactory dysfunction in Alzheimer's disease Systematic review and meta-analysis

Alzheimer’s disease (AD), a neurodegenerative condition, is one of the most prevalent kinds of dementia, whose frequency doubles for every 5 years of age in elderly.

Apo E4 Alleles and Impaired Olfaction as Predictors of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia that affects more than 5 million Americans. It is the only disease among the 10 causes of death that cannot be slowed or cured, thus raising the need for identification of early preclinical markers that could be the focus of preventative efforts. Although evidence is escalating that abnormalities in olfactory structure and function precede AD development and early cognitive impairments by one or more decades, the importance of olfaction is largely overlooked in AD, and such testing is not routinely performed in neurology clinics. Nevertheless, research using the olfactory model, has begun to advance our understanding of the preclinical pathophysiology of AD. Notably, an interesting series of studies is beginning to illuminate the relationship between Apolipoprotein E (ApoE) ε4 polymorphism and olfactory dysfunction and late-onset Alzheimer's disease. In this article, we reviewed present research on the significance of ApoE and olfaction to AD, summarized current studies on the associations and mechanisms of ApoE and olfactory dysfunction, and highlighted important gaps for future work to further advance the translational application of the olfactory paradigm to early, preclinical diagnosis and treatment of AD.

Assessment of Olfactory Memory in Olfactory Dysfunction

To assess all clinically relevant components of olfactory perception, examinations for olfactory sensitivity, discrimination, and identification are performed. Besides the standard perceptual test battery, episodic olfactory memory might offer additional information about olfactory abilities relative to these standard clinical tests. As both olfactory deficits and memory deficits are early symptoms in neurodegenerative disorders, olfactory memory may be of particular interest. However, to date little is known about episodic olfactory memory performance in patients with decreased olfactory function. This study includes the investigation of olfactory memory performance in 14 hyposmic patients (8 female, mean age 52.6 years) completing two episodic odor memory tests (Sniffin' Test of Odor Memory and Odor Memory Test). To control for a general impairment in memory function, a verbal and a figural memory test were carried out. A regression model with multiple predictors was calculated for both odor memory tests separately. Odor identification was identified as the only significant predictor for both odor memory tasks. From our results, we conclude that currently available olfactory memory tests are highly influenced by odor identification abilities, implying the need for the development and validation of additional tests in this field which could serve as additional olfactory perception variables for clinical assessment.

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.

Age and apolipoprotein E ε4 effects on neural correlates of odor memory

Alzheimer's disease (AD) affects 5.4 million Americans. Evidence suggests that individuals who are positive for the apolipoprotein E (ApoE) ε4 allele are at higher risk for developing the disease. Studies have also shown that the ε4 allele is linked to olfactory decline. Olfactory functioning may be investigated using olfactory event-related potentials (OERPs). The high temporal resolution of OERPs enables an understanding of the neural correlates of olfactory processing and functioning. This study investigated the effects of age, ApoE ε4 status, response type, and electrode site on OERP latency and amplitude during encoding and retrieval in an odor recognition memory task. The 60 participants were equally divided into 3 age groups matched on ε4 status: younger, middle, and older. Odors were presented using a computer-controlled olfactometer. Participants were notified during encoding that this was a task of odor memory. Results indicated differences in OERP activity as a function of age, ApoE ε4 status, response type, and electrode site. These findings highlight the potential of OERPs to distinguish ε4- and ε4+ individuals and to contribute to an earlier diagnosis of AD.

Isoform-specific effects of apoE on neurite outgrowth in olfactory epithelium culture

Background

The apolipoprotein E4 (apoE4) genotype is a major risk factor for developing late-onset Alzheimer’s disease (AD). Inheritance of apoE4 is also associated with impairments in olfactory function in early stages of AD. In this project we examined the effects of the three common isoforms of human apoE (apoE2, apoE3, and apoE4) on neuronal differentiation and neurite outgrowth in explant cultures of mouse olfactory epithelium (OE).

Results

The OE cultures derived from apoE-deficient/knockout (KO) mice have significantly fewer neurons with shorter neurite outgrowth than cultures from wild-type (WT) mice. Treatment of the apoE KO culture with either purified human apoE2 or with human apoE3 significantly increased neurite outgrowth. In contrast, treatment with apoE4 did not have an effect on neurite outgrowth. The differential effects of human apoE isoforms on neurite outgrowth were abolished by blocking the low-density lipoprotein receptor-related protein (LRP) with lactoferrin and receptor-associated protein (RAP).

Conclusion

ApoE2 and apoE3 stimulate neurite outgrowth in OE cultures by interacting with the lipoprotein receptor, LRP. ApoE4, the isoform associated with AD, failed to promote neurite outgrowth, suggesting a potential mechanism whereby apoE4 may lead to olfactory dysfunction in AD patients.

Individuals at risk for Alzheimer's disease show differential patterns of ERP brain activation during odor identification

BACKGROUND

Studies suggest that older adults at risk of developing Alzheimer's disease may show olfactory processing deficits before other signs of dementia appear.

METHODS

We studied 60 healthy non-demented individuals, half of whom were positive for the genetic risk factor the Apolipoprotein E ε4 allele, in three different age groups. Event-related potentials to visual and olfactory identification tasks were recorded and analyzed for latency and amplitude differences, and plotted via topographical maps.

RESULTS

Varying patterns of brain activation were observed over the post-stimulus epoch for ε4- versus ε4+ individuals on topographical maps. Individuals with the ε4 allele demonstrated different ERP peak latencies during identification of olfactory but not visual stimuli. High correct ApoE classification rates were obtained utilizing the olfactory ERP.

CONCLUSIONS

Olfactory ERPs demonstrate functional decline in individuals at risk for Alzheimer's disease at much earlier ages than previously observed, suggesting the potential for pre-clinical detection of AD at very early stages.

Long-term effects of estradiol replacement in the olfactory system

Olfactory dysfunction often precedes other clinical symptoms in chronic neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. Estrogen deficiency and apoE genotype are known risk factors in these diseases and these factors also affect olfaction. Therefore we examined the effects of estradiol replacement following ovariectomy on expression of apoE and markers of cell proliferation, neuronal maturation, synaptogenesis and reactive gliosis in the primary olfactory pathway of wild-type (WT) and apoE knockout (KO) mice. Estradiol replacement increased apoE staining in the olfactory nerve and glomerular layers. Estradiol increased astrocyte density and olfactory epithelium (OE) thickness regardless of the genotype. In addition estradiol treatment increased the number of mature neurons in the OE and glomerular synaptophysin in both genotypes, but the magnitude of increase was greater in the WT than in the KO mice. These data suggest that estrogen and apoE act synergistically to minimize the loss of mature sensory neurons and synapses following ovariectomy.

Semantic networks for odors and colors in Alzheimer's disease

OBJECTIVE

Impairment in odor-naming ability and in verbal and visual semantic networks raised the hypothesis of a breakdown in the semantic network for odors in Alzheimer's disease (AD). The current study addressed this hypothesis.

METHOD

Twenty-four individuals, half patients with probable AD and half control participants, performed triadic-similarity judgments for odors and colors, separately, which, utilizing the multidimensional scaling (MDS) technique of individual difference scaling analysis (INDSCAL), generated two-dimensional configurations of similarity. The abilities to match odors and colors with written name labels were assessed to investigate disease-related differences in ability to identify and conceptualize the stimuli. In addition, responses on attribute-sorting tasks, requiring the odor and color perceptions to be categorized as one polarity of a certain dimension, were obtained to allow for objective interpretation of the MDS spatial maps.

RESULTS

Whereas comparison subjects generated spatial maps based predominantly on relatively abstract characteristics, patients with AD classified odors on perceptual characteristics. The maps for patients with AD also showed disorganized groupings and loose associations between odors. Their normal configurations for colors imply that the patients were able to comprehend the task per se. The data for label matching and for attribute sorting provide further evidence for a disturbance in semantic odor memory in AD. The patients performed poorer than controls on both these odor tasks, implying that the ability to identify and/or conceptualize odors is impaired in AD.

CONCLUSION

The results provide clear evidence for deterioration of the structure of semantic knowledge for odors in AD.

Effects of hormone therapy on depressive symptoms and cognitive functions in women with Alzheimer disease: a 12 month randomized, double-blind, placebo-controlled study of low-dose estradiol and norethisterone

OBJECTIVE

To elucidate the effects of low-dose 17beta-estradiol and norethisterone (hormone therapy [HT]) versus placebo in women with Alzheimer Disease (AD) on cognition, depressive symptoms, and activities of daily living.

DESIGN

A 12-month randomized, double-blind, placebo-controlled study, stratified by apolipoprotein E (ApoE) genotype (with versus without the epsilon4 allele), duration of education (< or =9 versus >9 years), and age (< or =75 versus >75 years) performed during 2000-2004.

SETTING

Ambulatory memory clinic in a general hospital.

PARTICIPANTS

Sixty-five female outpatients aged 65-89 years who met criteria for probable AD according to Diagnostic and Statistical Manual of Mental Disorders, fourth edition and International Classification of Diseases, tenth edition. Ten patients were excluded, resulting in 55 participants who had at least one posttreatment efficacy evaluation.

INTERVENTION

Randomly assigned to receive either 1-mg estradiol and 0.5-mg norethisterone or placebo once daily.

MEASUREMENTS

Cognitive variables were the Dementia Rating Scale, tests from Consortium to Establish a Registry for AD, Global Deterioration Scale (GDS) and Barthel Index.

RESULTS

When only treatment effects were compared by analysis of variance, there were nonsignificant differences between treatment groups for all efficacy variables. A linear model analysis, including stratifying factors in addition to treatment in the model, revealed a significant main effect on mood. The depressive symptoms were lower in the HT group than in the placebo group. Those treated with HT without the ApoE epsilon4 allele had better mood, Word Learning Memory score, and GDS score. Those in the HT group with a higher level of education obtained a better GDS score. Adverse events did not differ between the groups.

CONCLUSION

HT interacts with ApoE genotype in women with AD. Women without an ApoE epsilon4 allele may get better mood and cognition with HT. HT may reduce depressive mood and give less cognitive decline.

Olfaction in aging and Alzheimer's disease: event-related potentials to a cross-modal odor-recognition memory task discriminate ApoE epsilon4+ and ApoE epsilon 4- individuals

Alzheimer's disease (AD) is a devastating neurodegenerative condition that affects more than 5 million Americans. Currently, a definitive and unequivocal diagnosis of AD can only be confirmed histopathogically via postmortem autopsy, demonstrating the need for objective measures of cognitive functioning for those at risk for AD. The single most important genetic risk factor of AD is the apolipoprotein E (ApoE) epsilon4 allele. The present study investigated olfactory and cognitive processing deficits in ApoE epsilon4(+) individuals using a cross-modal recognition memory task and an objective electrophysiological measure, the event-related potential (ERP). Ten epsilon4(+) individuals (5 M, 5 F, mean [M]= 75.1 years) and 10 age- and gender-matched epsilon4(-) individuals (5 M, 5 F, M = 71 years) sequentially encoded a set of 16 olfactory stimuli and were subsequently shown names of odors previously presented (targets) or not (foils). EEG activity was recorded from 19 electrodes as participants distinguished targets from foils using a two-button mouse. P3 latencies were significantly longer in epsilon4(+) individuals, and intraclass correlations demonstrated differential activity between the two groups. These findings are consistent with a compensatory hypothesis, which posits that nondemented epsilon4(+) individuals will expend greater effort in cognitive processing or engage in alternative strategies and therefore require greater activation of neural tissue or recruitment of different neural populations. The findings also suggest that cross-modal ERP studies of recognition memory discriminate early neurocognitive changes in ApoE epsilon4(+) and ApoE epsilon4(-) individuals and may contribute to identifying the phenotype of persons who will develop Alzheimer's disease.

Genetic and vascular modifiers of age-sensitive cognitive skills: effects of COMT, BDNF, ApoE, and hypertension

Several single nucleotide polymorphisms have been linked to neural and cognitive variation in healthy adults. We examined contribution of three polymorphisms frequently associated with individual differences in cognition (Catechol-O-Methyl-Transferase Val158Met, Brain-Derived-Neurotrophic-Factor Val66Met, and Apolipoprotein E epsilon4) and a vascular risk factor (hypertension) in a sample of 189 volunteers (age 18-82). Genotypes were determined from buccal culture samples, and cognitive performance was assessed in 4 age-sensitive domains?fluid intelligence, executive function (inhibition), associative memory, and processing speed. We found that younger age and COMT Met/Met genotype, associated with low COMT activity and higher prefrontal dopamine content, were independently linked to better performance in most of the tested domains. Homozygotes for Val allele of BDNF polymorphism exhibited better associative memory and faster speed of processing than the Met allele carriers, with greater effect for women and persons with hypertension. Carriers of ApoE epsilon4 allele evidenced steeper age-related increase in costs of Stroop color interference, but showed no negative effects on memory. The findings indicate that age-related cognitive performance is differentially affected by distinct genetic factors and their interactions with vascular health status.