Sex and APOE: A memory advantage in male APOE ε4 carriers in midlife

Dissociable effects of mild COVID-19 on short- and long-term memories

Recent studies have highlighted the presence of cognitive deficits following COVID-19 that persist beyond acute infection, regardless of the initial disease severity. Impairments in short- and long-term memory are among the core deficits reported by patients and observed in objective tests of memory performance. We aimed to extend previous studies by examining performance in a task that allows us to directly compare and contrast memories at different timescales. More specifically, we assessed both short- and long-term memories for contextual-spatial associations encoded during a common session and probed at different durations using an equivalent task in non-hospitalized individuals recovering from mild COVID-19 compared to healthy controls. The approach equated all aspects of memory materials and response demands, isolating performance changes resulting only from memory timescales and thus allowing us to quantify the impact of COVID-19 on cognition. In addition to providing measures of accuracy and response times, the task also provided a sensitive continuous readout of the precision of memory representations, specifically by examining the resolution with which spatial locations were retained in memory. The results demonstrated selective impairment of long-term memory performance in individuals recovering from mild COVID-19 infection. Short-term memory performance remained comparable to healthy controls. Specifically, poor precision of long-term memory representations was demonstrated, which improved with days since diagnosis. No such relationship was observed for short-term memory performance. Our findings reveal a specific impairment to the precision of spatial-contextual long-term memory representations in individuals recovering from mild COVID-19 and demonstrate evidence of recovery in long-term memory over time. Further, the experimental design provides a carefully controlled and sensitive framework to assess memory across different durations with the potential to provide more detailed phenotyping of memory deficits associated with COVID-19 in general.

Dementia risk and thalamic nuclei volumetry in healthy midlife adults: the PREVENT Dementia study

A reduction in the volume of the thalamus and its nuclei has been reported in Alzheimer's disease, mild cognitive impairment and asymptomatic individuals with risk factors for early-onset Alzheimer's disease. Some studies have reported thalamic atrophy to occur prior to hippocampal atrophy, suggesting thalamic pathology may be an early sign of cognitive decline. We aimed to investigate volumetric differences in thalamic nuclei in middle-aged, cognitively unimpaired people with respect to dementia family history and apolipoprotein ε4 allele carriership and the relationship with cognition. Seven hundred participants aged 40-59 years were recruited into the PREVENT Dementia study. Individuals were stratified according to dementia risk (approximately half with and without parental dementia history). The subnuclei of the thalamus of 645 participants were segmented on T1-weighted 3 T MRI scans using FreeSurfer 7.1.0. Thalamic nuclei were grouped into six regions: (i) anterior, (ii) lateral, (iii) ventral, (iv) intralaminar, (v) medial and (vi) posterior. Cognitive performance was evaluated using the computerized assessment of the information-processing battery. Robust linear regression was used to analyse differences in thalamic nuclei volumes and their association with cognitive performance, with age, sex, total intracranial volume and years of education as covariates and false discovery rate correction for multiple comparisons. We did not find significant volumetric differences in the thalamus or its subregions, which survived false discovery rate correction, with respect to first-degree family history of dementia or apolipoprotein ε4 allele status. Greater age was associated with smaller volumes of thalamic subregions, except for the medial thalamus, but only in those without a dementia family history. A larger volume of the mediodorsal medial nucleus (Pfalse discovery rate = 0.019) was associated with a faster processing speed in those without a dementia family history. Larger volumes of the thalamus (P = 0.016) and posterior thalamus (Pfalse discovery rate = 0.022) were associated with significantly worse performance in the immediate recall test in apolipoprotein ε4 allele carriers. We did not find significant volumetric differences in thalamic subregions in relation to dementia risk but did identify an interaction between dementia family history and age. Larger medial thalamic nuclei may exert a protective effect on cognitive performance in individuals without a dementia family history but have little effect on those with a dementia family history. Larger volumes of posterior thalamic nuclei were associated with worse recall in apolipoprotein ε4 carriers. Our results could represent initial dysregulation in the disease process; further study is needed with functional imaging and longitudinal analysis.

APOE ε4-Allele in Middle-Aged and Older Autistic Adults: Associations with Verbal Learning and Memory

Autism spectrum disorder (ASD) is a neurodevelopmental disability and recent evidence suggests that autistic adults are more likely to develop Alzheimer's disease (Alz) and other dementias compared to neurotypical (NT) adults. The ε4-allele of the Apolipoprotein E (APOE) gene is the strongest genetic risk factor for Alz and negatively impacts cognition in middle-aged and older (MA+) adults. This study aimed to determine the impact of the APOE ε4-allele on verbal learning and memory in MA+ autistic adults (ages 40-71 years) compared to matched NT adults. Using the Auditory Verbal Learning Test (AVLT), we found that ε4 carriers performed worse on short-term memory and verbal learning across diagnosis groups, but there was no interaction with diagnosis. In exploratory analyses within sex and diagnosis groups, only autistic men carrying APOE ε4 showed worse verbal learning (p = 0.02), compared to autistic men who were not carriers. Finally, the APOE ε4-allele did not significantly affect long-term memory in this sample. These findings replicate previous work indicating that the APOE ε4-allele negatively impacts short-term memory and verbal learning in MA+ adults and presents new preliminary findings that MA+ autistic men may be vulnerable to the effects of APOE ε4 on verbal learning. Future work with a larger sample is needed to determine if autistic women may also be vulnerable.

APOE Polymorphism Is Associated with Changes in the Kynurenine Pathway

BACKGROUND

APOE polymorphism and the Kynurenine pathway (KP) are associated with many disorders, but little is known about associations between APOE polymorphism and the KP. This study explored the associations between the KP and APOE polymorphism in disorders associated with APOE polymorphism and changes in the KP.

METHODS

Subjects with morbid obesity before and after bariatric surgery (numbers 139 and 95, respectively), depression (number 49), and unspecified neurological symptoms (number 39) were included. The following grouping of the APOE genotypes was used: E2 = ɛ2ɛ2 + ɛ2ɛ3, E3 = ɛ3ɛ3 + ɛ2ɛ4, and E4 = ɛ3ɛ4 + ɛ4ɛ4. The KP metabolites Tryptophan, Kynurenine, Kynurenic acid, Quinolinic acid, and Xanthurenic acid were quantified in serum.

RESULTS

The main findings were a significant positive association between E3 and Quinolinic acid (difference between E3 and E2E4: 12.0 (3.5; 18.6) ng/mL); p = 0.005), and a negative association between E4 and Kynurenine (difference between E4 and E2E3: -31.3 (-54.2; -3.2) ng/mL; p = 0.008). Quinolinic acid has been ascribed neurotoxic and inflammatory effects, and Kynurenine is a marker of inflammation.

CONCLUSIONS

The findings indicate that APOE polymorphism might cause changes in the KP that contribute to the disease. Inflammation could be the link between APOE and the KP.

Genetic polymorphism in BIN1 rather than APOE is associated with poor recognition memory among men without dementia

Although several genetic polymorphisms have been linked with the risk of Alzheimer's disease, less is known about their impact on cognitive performance among cognitively healthy individuals. Our aim was to investigate the association of the genetic variant, rs744373 in the bridging integrator 1 gene (BIN1), the strongest genetic risk factor for Alzheimer's disease after the APOE ε4 allele, with different cognitive domains among non-demented older men. Cognitive function was measured using the CogState Brief Battery, which assessed cognitive performance across four domains: psychomotor function, visual attention, recognition memory and working memory. Linear regression analysis revealed that individuals with the BIN1 risk allele performed poorly on the recognition memory task as compared to those without the risk allele. However, this was in contrast with the individuals who harboured the APOE ε4 risk allele as they displayed better performance on the recognition task in comparison to those without the ε4 risk allele. To the best of our knowledge, this is the first study that demonstrates genetic variation in BIN1 to be a better predictor of recognition memory than APOE, which remains the biggest genetic risk factor for Alzheimer's disease.

Crosstalk between high-density lipoproteins and endothelial cells in health and disease: Insights into sex-dependent modulation

Atherosclerotic cardiovascular disease is the leading cause of death worldwide. Intense research in vascular biology has advanced our knowledge of molecular mechanisms of its onset and progression until complications; however, several aspects of the patho-physiology of atherosclerosis remain to be further elucidated. Endothelial cell homeostasis is fundamental to prevent atherosclerosis as the appearance of endothelial cell dysfunction is considered the first pro-atherosclerotic vascular modification. Physiologically, high density lipoproteins (HDLs) exert protective actions for vessels and in particular for ECs. Indeed, HDLs promote endothelial-dependent vasorelaxation, contribute to the regulation of vascular lipid metabolism, and have immune-modulatory, anti-inflammatory and anti-oxidative properties. Sex- and gender-dependent differences are increasingly recognized as important, although not fully elucidated, factors in cardiovascular health and disease patho-physiology. In this review, we highlight the importance of sex hormones and sex-specific gene expression in the regulation of HDL and EC cross-talk and their contribution to cardiovascular disease.

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.

Contributions of sex and genotype to exploratory behavior differences in an aged humanized APOE mouse model of late-onset Alzheimer's disease

Age, genetics, and chromosomal sex have been identified as critical risk factors for late-onset Alzheimer's disease (LOAD). The predominant genetic risk factor for LOAD is the apolipoprotein E ε4 allele (APOE4), and the prevalence of LOAD is higher in females. However, the translational validity of APOE4 mouse models for AD-related cognitive impairment remains to be fully determined. The present study investigated the role of both sex and genotype on learning and memory in aged, humanized APOE knock-in mice. Aged (23.27 mo ± 1.21 mo; 39 male/37 female) APOE3/3, APOE3/4, and APOE4/4 mice performed a novel object recognition (NOR) assay. Task-related metrics were analyzed using two-way sex by genotype ANOVAs. Sex differences were more prominent relative to APOE genotype. Prior to NOR, female mice exhibited thigmotaxic center zone avoidance during the open field task relative to males, regardless of genotype. Within object familiarization and NOR tasks, females had greater object interaction and locomotion. Interestingly, only APOE4/4 females on average recognized the novel object. These results suggest that APOE4, although strongly related to LOAD pathogenesis, does not drive cognitive decline in the absence of other risk factors even in very aged mice. Chromosomal sex is a key driver of behavioral phenotypes and thus is a critical variable for translatability of interventions designed to preserve learning and memory in animal models of LOAD. Last, there was a very high degree of variability in behavioral performance across APOE genotypes. A cluster analysis of the behavioral data revealed a low-activity and a high-activity cluster. APOE4 carriers were overrepresented in the low-activity cluster, while male:female distributions did not differ. Collectively, the behavioral data indicate that chromosomal sex has the greatest impact on behavioral phenotype, and APOE4 carrier status may confer greater risk for cognitive decline in some animals.

Effects of item distinctiveness on the retrieval of objects and object-location bindings from visual working memory

Visual working memory (VWM) is prone to interference from stored items competing for its limited capacity. Distinctiveness or similarity of the items is acknowledged to affect this competition, such that poor item distinctiveness causes a failure to discriminate between items sharing common features. In three experiments, we studied how the distinctiveness of studied real-world objects (i.e., whether the objects belong to the same or different basic categories) affects the retrieval of objects themselves (simple recognition) and object-location conjunctions (information about which object was where in a display, cued recall). In Experiments 1 and 2, we found that distinctiveness did not affect memories for objects or for locations, but low-distinctive objects were more frequently reported at "swapped" locations that originally contained other objects, showing object-location memory swaps. In Experiments 3 we found that observers swapped the location of a tested object with another object from the same category more frequently than with any of the objects from another category. This suggests that more similar studied objects cause more retrieval competition in object-location judgments than in simple recognition. Additionally, we discuss a possible role of categorical labeling of locations that can support object-location retrieval when the studied objects are highly distinct.

Dissociable effects of APOE-ε4 and β-amyloid pathology on visual working memory

Although APOE-ε4 carriers are at significantly higher risk of developing Alzheimer's disease than non-carriers1, controversial evidence suggests that APOE-ε4 might confer some advantages, explaining the survival of this gene (antagonistic pleiotropy)2,3. In a population-based cohort born in one week in 1946 (assessed aged 69-71), we assessed differential effects of APOE-ε4 and β-amyloid pathology (quantified using 18F-Florbetapir-PET) on visual working memory (object-location binding). In 398 cognitively normal participants, APOE-ε4 and β-amyloid had opposing effects on object identification, predicting better and poorer recall respectively. ε4-carriers also recalled locations more precisely, with a greater advantage at higher β-amyloid burden. These results provide evidence of superior visual working memory in ε4-carriers, showing that some benefits of this genotype are demonstrable in older age, even in the preclinical stages of Alzheimer's disease.

Moderators of gene-outcome associations following traumatic brain injury

The field of genomics is the principal avenue in the ongoing development of precision/personalised medicine for a variety of health conditions. However, relating genes to outcomes is notoriously complex, especially when considering that other variables can change, or moderate, gene-outcome associations. Here, we comprehensively discuss moderation of gene-outcome associations in the context of traumatic brain injury (TBI), a common, chronically debilitating, and costly neurological condition that is under complex polygenic influence. We focus our narrative review on single nucleotide polymorphisms (SNPs) of three of the most studied genes (apolipoprotein E, brain-derived neurotrophic factor, and catechol-O-methyltransferase) and on three demographic variables believed to moderate associations between these SNPs and TBI outcomes (age, biological sex, and ethnicity). We speculate on the mechanisms which may underlie these moderating effects, drawing widely from biomolecular and behavioural research (n = 175 scientific reports) within the TBI population (n = 72) and other neurological, healthy, ageing, and psychiatric populations (n = 103). We conclude with methodological recommendations for improved exploration of moderators in future genetics research in TBI and other populations.

Early neuroinflammation is associated with lower amyloid and tau levels in cognitively normal older adults

CNS inflammation is a key factor in Alzheimer's Disease (AD), but its relation to pathological Aβ, tau, and APOE4 is poorly understood, particularly prior to the onset of cognitive symptoms. To better characterize early relationships between inflammation, APOE4, and AD pathology, we assessed correlations between cerebrospinal fluid (CSF) inflammatory markers and brain levels of Aβ and tau in cognitively normal older adults. Each participant received a lumbar puncture to collect and quantify CSF levels of TNFα, IL-6, IL-8, and IL-10, a T1-weighted MRI, and PET scanning with [¹⁸F]flortaucipir (FTP; n = 57), which binds to tau tangles and/or [¹⁸F]florbetapir (FBP; n = 58), which binds to Aβ. Parallel voxelwise regressions assessed relationships between each CSF inflammatory marker and FTP and FBP SUVR, as well as APOE4*CSF inflammation interactions. Unexpectedly, we detected significant negative associations between regional Aβ and tau PET uptake and CSF inflammatory markers. For Aβ PET, we detected negative associations with CSF IL-6 and IL-8 in regions known to show early accumulation of Aβ (i.e. lateral and medial frontal lobes). For tau PET, negative relationships were observed with CSF TNFα and IL-8, predominantly in regions known to exhibit early tau accumulation (i.e. medial temporal lobe). In subsequent analyses, significant interactions between APOE4 status and IL-8 on Aβ and tau PET levels were observed in spatially distinct regions from those showing CSF-Aβ/tau relationships. Results from the current cross-sectional study support previous findings that neuroinflammation may be protective against AD pathology at a given stage of the disease, and extend these findings to a cognitively normal aging population. This study provides new insight into a dynamic relationship between neuroinflammation and AD pathology and may have implications for whom and when neuroinflammatory therapies may be appropriate.

Memory precision of object-location binding is unimpaired in APOE ε4-carriers with spatial navigation deficits

Research suggests that tests of memory fidelity, feature binding and spatial navigation are promising for early detection of subtle behavioural changes related to Alzheimer's disease. In the absence of longitudinal data, one way of testing the early detection potential of cognitive tasks is through the comparison of individuals at different genetic risk for Alzheimer's dementia. Most studies have done so using samples aged 70 years or older. Here, we tested whether memory fidelity of long-term object-location binding may be a sensitive marker even among cognitively healthy individuals in their mid-60s by comparing participants at low and higher risk based on presence of the ε4-allele of the apolipoprotein gene (n = 26 ε3ε3, n = 20 ε3ε4 carriers). We used a continuous report paradigm in a visual memory task that required participants to recreate the spatial position of objects in a scene. We employed mixture modelling to estimate the two distinct memory processes that underpin the trial-by-trial variation in localization errors: retrieval success which indexes the proportion of trials where participants recalled any information about an object's position and the precision with which participants retrieved this information. Prior work has shown that these memory paradigms that separate retrieval success from precision are capable of detecting subtle differences in mnemonic fidelity even when retrieval success could not. Nonetheless, Bayesian analyses found good evidence that ε3ε4 carriers did not remember fewer object locations [F(1, 42) = 0.450, P = 0.506, BF01 = 3.02], nor was their precision for the spatial position of objects reduced compared to ε3ε3 carriers [F(1, 42) = 0.12, P = 0.726, BF01 = 3.19]. Because the participants in the sample presented here were a subset of a study on apolipoprotein ε4-carrier status and spatial navigation in the Sea Hero Quest game [Coughlan et al., 2019. PNAS, 116(9)], we obtained these data to contrast genetic effects on the two tasks within the same sample (n = 33). Despite the smaller sample size, wayfinding deficits among ε3ε4 carriers could be replicated [F(1, 33) = 5.60, P = 0.024, BF10 = 3.44]. Object-location memory metrics and spatial navigation scores were not correlated (all r < 0.25, P > 0.1, 0 < BF10 < 3). These findings show spared object-location binding in the presence of a detrimental apolipoprotein ε4 effect on spatial navigation. This suggests that the sensitivity of memory fidelity and binding tasks may not extend to individuals with one ε4-allele in their early to mid-60s. The results provide further support to prior proposals that spatial navigation may be a sensitive marker for the earliest cognitive changes in Alzheimer's disease, even before episodic memory.

Different patterns of short-term memory deficit in Alzheimer's disease, Parkinson's disease and subjective cognitive impairment

It has recently been proposed that short-term memory (STM) binding deficits might be an important feature of Alzheimer's disease (AD), providing a potential avenue for earlier detection of this disorder. By contrast, work in Parkinson's disease (PD), using different tasks, has suggested that the STM impairment in this condition is characterised by increased random guessing, possibly due to fluctuating attention. In the present study, to establish whether a misbinding impairment is present in sporadic late-onset AD (LOAD) and increased guessing is a feature of PD, we compared the performance of these patient groups to two control populations: healthy age-matched controls and individuals with subjective cognitive impairment (SCI) with comparable recruitment history as patients. All participants performed a sensitive task of STM that required high resolution retention of object-location bindings. This paradigm also enabled us to explore the underlying sources of error contributing to impaired STM in patients with LOAD and PD using computational modelling of response error. Patients with LOAD performed significantly worse than other groups on this task. Importantly their impaired memory was associated with increased misbinding errors. This was in contrast to patients with PD who made significantly more guessing responses. These findings therefore provide additional support for the presence of two doubly dissociable signatures of STM deficit in AD and PD, with binding impairment in AD and increased random guessing characterising the STM deficit in PD. The task used to measure memory precision here provides an easy-to-administer assessment of STM that is sensitive to the different types of deficit in AD and PD and hence has the potential to inform clinical practice.

Superior short-term memory in APOE ε2 carriers across the age range

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The Apolipoprotein-E (APOE) ε2 allele is known to be protective against Alzheimer’s disease.

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We tested the effect of this allele on cognitive performance, as measured by a sensitive short-term memory task.

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A large cohort of genotyped participants performed this task remotely.

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ε2 carriers demonstrated superior memory performance in young, middle-aged, and older participants.

The Apolipoprotein-E (APOE) gene is now known to be associated with individual differences in cognitive health in ageing. However, while the APOE ε4 allele confers significantly increased risk of developing Alzheimer’s disease (AD), the APOE ε2 allele is hypothesized to be protective against the development of AD. This is in line with neuroimaging and pathological findings associated with ε2 APOE allele, which go in the opposite direction to those observed in AD-related pathology. However, the precise impact of this allele on cognition remains inconclusive, with some small-cohort studies raising the possibility of an advantageous memory performance in these individuals. Here, we tested short-term memory (STM) performance in a large cohort of individuals, 300 of which were ε2/ε3 carriers. Their performance was compared to 554 ε3/ε3 carriers. We included participants from a wide age range spanning young, middle-aged and elderly adults. All of them performed a STM task that has previously been shown to be sensitive to subtle changes in memory in various patient and at-risk cohorts. Individuals carrying the APOE-ε2 allele exhibited a significant memory advantage, regardless of STM task difficulty and across all ages. The observed memory advantage was present across the age range, suggestive of a phenotypical effect of this allele on cognition, possibly independent of any effects of this genetic allele that occur later life in these individuals.

Putative Survival Advantages in Young Apolipoprotein ɛ4 Carriers are Associated with Increased Neural Stress

Inheritance of a single copy of the apolipoprotein E (APOE) ɛ4 allele increases risk of Alzheimer’s disease (AD) by 3-4-fold, with homozygosity associated with a 12-16-fold increase in risk, relative to ɛ3 allele homozygosity. There is a decreased risk associated with the APOE ɛ2 allele. The pathological consequence of APOE genotype has led to intense efforts to understand the mechanistic basis of the interplay between APOE status and loss of synapses. Numerous ɛ4 allele-related associations have been reported with the potential relevance of these associations to the pathogenesis of AD unknown at this time. In primarily young subjects, we have reviewed a representative body of literature on ɛ4 allele-associations related to the following: cardiovascular responses; impacts on reproduction and fetal development; co-morbidities; resistance to infectious disease; responses to head injury; biochemical differences possibly related to neural stress; and brain structure-function differences. In addition, the literature on the association between the ɛ4 allele and cognitive performance has been reviewed comprehensively. The weight-of-the-evidence supports the hypothesis that possession of the ancestral ɛ4 allele in youth is associated with improved fitness during fetal development, infancy, and youth relative to the more recently appearing ɛ3 allele, at the expense of decreased fitness in old age, which is substantially improved by the ɛ3 allele. However, possession of the ɛ4 allele is also associated with higher levels of synaptic macromolecular turnover, which likely stresses basic cellular neuroplasticity mechanisms. Clinical trials of potential AD therapeutics should consider APOE status as an enrollment criterion.

Short-term memory advantage for brief durations in human APOE ε4 carriers

The Apolipoprotein-E (APOE) ε4 gene allele, the highest known genetic risk factor for Alzheimer's disease, has paradoxically been well preserved in the human population. One possible explanation offered by evolutionary biology for survival of deleterious genes is antagonistic pleiotropy. This theory proposes that such genetic variants might confer an advantage, even earlier in life when humans are also reproductively fit. The results of some small-cohort studies have raised the possibility of such a pleiotropic effect for the ε4 allele in short-term memory (STM) but the findings have been inconsistent. Here, we tested STM performance in a large cohort of individuals (N = 1277); nine hundred and fifty-nine of which included carrier and non-carriers of the APOE ε4 gene, those at highest risk of developing Alzheimer's disease. We first confirm that this task is sensitive to subtle deterioration in memory performance across ageing. Importantly, individuals carrying the APOE ε4 gene actually exhibited a significant memory advantage across all ages, specifically for brief retention periods but crucially not for longer durations. Together, these findings present the strongest evidence to date for a gene having an antagonistic pleiotropy effect on human cognitive function across a wide age range, and hence provide an explanation for the survival of the APOE ε4 allele in the gene pool.

Sex modulates the apolipoprotein E ε4 effect on white matter and cortical functional connectivity in individuals with amnestic mild cognitive impairment

BACKGROUND AND PURPOSE

Recent studies from the Alzheimer's Disease Neuroimaging Initiative show that, in the USA, 75% of patients with Alzheimer's disease are female. To date, there have rarely been any attempts to analyze data by sex or gender, which limits the potential for discovering the effects of sex or gender on disease. Little evidence is available regarding the effect of gender and apolipoprotein E (APOE) ε4 on white matter (WM) connection from the functional perspective due to the lack of appropriate techniques for detecting blood-oxygen-level-dependent signals in WM.

METHODS

We took advantage of a new framework known as functional tensor imaging to investigate the effect of sex and APOEε4 on WM cortical functional connectivity throughout the brain.

RESULTS

In a group of female patients with amnestic mild cognitive impairment, we found a significantly reduced functional connectivity in the left posterior limb of the internal capsule, left superior fronto-occipital fasciculus, bilateral temporopolar area and right somatosensory association cortex in APOEε4 carriers in contrast to non-carriers. We also found a significant APOEε4 by sex interaction effect on the right somatosensory association cortex, left temporopolar area and left superior temporal gyrus. The clinical Montreal Cognitive Assessment score was significantly negatively associated with the right somatosensory association cortex with APOEε4 by sex interaction in males.

CONCLUSIONS

These results indicate that increased APOE-related risk in women may be associated with decreased activity in both gray matter and WM in patients with amnestic mild cognitive impairment compared with men. The finding suggests accounting for sex differences in neuroimaging biomarkers, diagnostics and treatment strategy.

Impact of Apolipoprotein E gene polymorphism during normal and pathological conditions of the brain across the lifespan

The central nervous system (CNS) is the cellular substrate for the integration of complex, dynamic, constant, and simultaneous interactions among endogenous and exogenous stimuli across the entire human lifespan. Numerous studies on aging-related brain diseases show that some genes identified as risk factors for some of the most common neurodegenerative diseases - such as the allele 4 of APOE gene (APOE4) for Alzheimer's disease (AD) - have a much earlier neuro-anatomical and neuro-physiological impact. The impact of APOE polymorphism appears in fact to start as early as youth and early-adult life. Intriguingly, though, those same genes associated with aging-related brain diseases seem to influence different aspects of the brain functioning much earlier actually, that is, even from the neonatal periods and earlier. The APOE4, an allele classically associated with later-life neurodegenerative disorders as AD, seems in fact to exert a series of very early effects on phenomena of neuroplasticity and synaptogenesis that begin from the earliest periods of life such as the fetal ones.We reviewed some of the findings supporting the hypothesis that APOE polymorphism is an early modifier of various neurobiological aspects across the entire human lifespan - from the in-utero to the centenarian life - during both normal and pathological conditions of the brain.

The Role of Sex and Sex Hormones in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a wide class of disorders of the central nervous system (CNS) with unknown etiology. Several factors were hypothesized to be involved in the pathogenesis of these diseases, including genetic and environmental factors. Many of these diseases show a sex prevalence and sex steroids were shown to have a role in the progression of specific forms of neurodegeneration. Estrogens were reported to be neuroprotective through their action on cognate nuclear and membrane receptors, while adverse effects of male hormones have been described on neuronal cells, although some data also suggest neuroprotective activities. The response of the CNS to sex steroids is a complex and integrated process that depends on (i) the type and amount of the cognate steroid receptor and (ii) the target cell type-either neurons, glia, or microglia. Moreover, the levels of sex steroids in the CNS fluctuate due to gonadal activities and to local metabolism and synthesis. Importantly, biochemical processes involved in the pathogenesis of NDs are increasingly being recognized as different between the two sexes and as influenced by sex steroids. The aim of this review is to present current state-of-the-art understanding on the potential role of sex steroids and their receptors on the onset and progression of major neurodegenerative disorders, namely, Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, and the peculiar motoneuron disease spinal and bulbar muscular atrophy, in which hormonal therapy is potentially useful as disease modifier.

When time's arrow doesn't bend: APOE-ε4 influences episodic memory before old age

Episodic memory impairment is the hallmark symptom of Alzheimer's Disease (AD). However, episodic memory has also been shown to decline across the lifespan. Here, we investigated whether episodic memory is differentially affected relative to other cognitive abilities before old age, and whether being an Apolipoprotein E (APOE) ε4 carrier -a genetic risk factor for developing AD-exacerbates any such impairments. We used general linear models to test for performance differences within 4 composite measures of cognition - episodic memory, semantic memory, speed of processing, and fluid reasoning-- as a function of age group (young, Mage = 30.21 vs. middle-aged, Mage = 50.84) and APOE-ε4 genotype status (ε4+ vs. ε4-). We replicated findings of age-related reductions in episodic memory, speed of processing, and fluid reasoning, and age-related increases in semantic memory. However, we also found that APOE genotype status moderated the age-related declines in episodic memory: APOE-ε4+ middle-aged adults exhibited impairments relative to both APOE-ε4- middle-aged participants, and APOE-ε4+ younger adults. These results suggest specific and dynamic alterations to episodic memory as a function of APOE allelic variation and age.

Sex Differences in Neuropsychological Test Performance in Alzheimer's Disease and the Influence of the ApoE Genotype

BACKGROUND

Only few studies exist reporting sex differences in the Alzheimer disease (AD) patients regarding cognitive profile, brain damage, and risk factors. The present study investigated the influence of sex in combination with the Alzheimer risk allele, ε4-allele of apolipoprotein E, on cognitive performance.

MATERIALS AND METHODS

We examined the effect of sex and ApoE genotype on a range of neuropsychological markers from the German version of the Consortium to Establish a Registry in Alzheimer's Disease Neuropsychological Battery in a monocentric study of 399 AD patients.

RESULTS

Male patients had significantly more years of school and occupational education compared with women. Male AD patients outperformed female patients in tasks of object naming, constructional praxis, and constructional praxis recall. There was no statistically significant interaction effect between sex and ε4-allele of apolipoprotein E for any of the examined variables.

CONCLUSIONS

The superiority of healthy men compared with women in tasks of object naming, constructional praxis, and visual memory seems to remain stable when people develop AD, indicating larger cognitive reserves in men. In contrast, findings that cognitively healthy women outperform men in tests of verbal memory and verbal fluency are not stable in AD. Further studies are needed to gain insight in the reasons for sex differences.

Dopamine affects short-term memory corruption over time in Parkinson's disease

Cognitive deficits are a recognised component of Parkinson's disease (PD). However, particularly within the domain of short-term memory, it is unclear whether these impairments are masked, or caused, by patients' dopaminergic medication. The effect of medication on pure maintenance in PD patients has rarely been explored, with most assessments examining maintenance intercalated between other executive tasks. Moreover, few studies have utilised methods that can measure the quality of mental representations, which can enable the decomposition of recall errors into their underlying neurocognitive components. Here, we fill this gap by examining pure maintenance in PD patients in high and low dopaminergic states. Participants had to encode the orientation of two stimuli and reproduce these orientations after a short (2 s) or long (8 s) delay. In addition, we also examined the performance of healthy, age-matched older adults to contextualise these effects and determine whether PD represents an exacerbation of the normal ageing process. Patients showed improved recall OFF compared to ON their dopaminergic medication, but only for long-duration trials. Moreover, PD patients OFF their medication actually performed at a level superior to age-matched controls, indicative of a paradoxical enhancement of memory in the low dopaminergic state. The application of a probabilistic model of response selection suggested that PD patients made fewer misbinding errors in the low, compared with high, dopaminergic state for longer-delay trials. Thus, unexpectedly, the mechanisms that prevent memoranda from being corrupted by misbinding over time appear to be enhanced in PD patients OFF dopaminergic medication. Possible explanations for this paradoxical effect are discussed.

Subjective cognitive decline, APOE ε4, and incident mild cognitive impairment in men and women

Introduction

Possible joint effects of subjective cognitive decline (SCD) and apolipoprotein E (APOE) ε4 genotype on incident mild cognitive impairment (MCI) were examined for men and women separately.

Methods

Cognitively normal participants with and without SCD were included from the first follow-up examination of the population-based Heinz Nixdorf Recall study. Sex-stratified logistic regression models estimated main effects and interactions (additive, multiplicative) of SCD at the first follow-up (yes+/no−) and APOE ε4 (positive+/negative−) groups for MCI 5 years later.

Results

Odds for MCI 5 years later were higher in SCD/APOE ε4 group +/+ than the sum of groups +/− and −/+ in women, with a trend for positive interaction. Odds for incident MCI in men was highest in group +/−, with no interaction effect.

Discussion

Our findings indicate that APOE ε4 may play an important role in the association of SCD and incident MCI, especially considering sex. Further studies need to examine these associations with larger sample sizes.

Task-irrelevant financial losses inhibit the removal of information from working memory

The receipt of financial rewards or penalties - though task-irrelevant - may exert an obligatory effect on manipulating items in working memory (WM) by constraining a forthcoming shift in attention or reinforcing attentional shifts that have previously occurred. Here, we adjudicate between these two hypotheses by varying – after encoding- the order in which task-irrelevant financial outcomes and cues indicating which items need to be retained in memory are presented (so called retrocues). We employed a “what-is-where” design that allowed for the fractionation of WM recall into separate components: identification, precision and binding (between location and identity). Principally, valence-dependent effects were observed only for precision and binding, but only when outcomes were presented before, rather than after, the retrocue. Specifically, task-irrelevant financial losses presented before the retrocue caused a systematic breakdown in binding (misbinding), whereby the features of cued and non-cued memoranda became confused, i.e., the features that made up relevant memoranda were displaced by those of non-cued (irrelevant) items. A control experiment, in which outcomes but no cues were presented, failed to produce the same effects, indicating that the inclusion of retrocues were necessary for generating this effect. These results show that the receipt of financial penalties – even when uncoupled to performance – can prevent irrelevant information from being effectively pruned from WM. These results illustrate the importance of reward-related processing to controlling the contents of WM.

Binding deficits in visual short-term memory in patients with temporal lobe lobectomy

Classical views of the medial temporal lobe (MTL) have established that it plays a crucial role in long-term memory (LTM). Here we demonstrate, in a sample of patients who have undergone anterior temporal lobectomy for the treatment of pharmacoresistant epilepsy, that the MTL additionally plays a specific, causal role in short-term memory (STM). Patients (n=22) and age-matched healthy control participants (n=26) performed a STM task with a sensitive continuous report measure. This paradigm allowed us to examine recall memory for object identity, location and object-location binding, independently on a trial-by-trial basis. Our findings point to a specific involvement of MTL in object-location binding, but, crucially, not retention of either object identity or location. Therefore the MTL appears to perform a specific computation: binding disparate features that belong to a memory. These results echo findings from previous studies, which have identified a role for the MTL in relational binding for LTM, and support the proposal that MTL regions perform such a function for both STM and LTM, independent of the retention duration. Furthermore, these findings and the methodology employed here may provide a simple, sensitive and clinically valuable means to test memory dysfunuction in MTL disorders.

Dissociable effects of the apolipoprotein-E (APOE) gene on short- and long-term memories

Short- and long-term memory performance as a function of apolipoprotein-E (APOE) genotype was examined in older, healthy individuals using sensitive and comparable tasks to provide a more detailed description of influences of the ε4 allele (highest genetic risk factor for Alzheimer's disease) on memory. Older heterozygous and homozygous ε4 carriers and noncarriers performed 2 tasks of memory. Both tasks allowed us to measure memory for item identity and locations, using a sensitive, continuous measure of report. Long-term memory for object locations was impaired in ε4/ε4 carriers, whereas, paradoxically, this group demonstrated superior short-term memory for locations. The dissociable effects of the gene on short- and long-term memory suggest that the effect of genotype on these two types of memories, and their neural underpinnings, might not be co-extensive. Whereas the long-term memory impairment might be linked to preclinical Alzheimer's disease, the short-term memory advantage may reflect an independent, phenotypical effect of this allele on cognition.

Working Memory in Alzheimer's Disease and Parkinson's Disease

Working memory impairments are frequently observed in patients with Alzheimer's disease (AD) and Parkinson's disease (PD). Recent research suggests that the mechanisms underlying these deficits might be dissociable using sensitive tasks, specifically those that rely on the reproduction of the exact quality of features held in memory.In patients with AD, working memory impairments are mainly due to an increase in misbinding errors. They arise when patients misremember which features (e.g., color, orientation, shape, and location) belong to different objects held in memory. Hence, they erroneously report features that belong to items in memory other than the one they are probed on. This misbinding of features that belong to different objects in memory can be considered a form of interference between stored items. Such binding errors are evident even in presymptomatic individuals with familial AD (due to gene mutations) who do not have AD yet. Overall, these findings are in line with the role of the medial temporal lobes, and specifically the hippocampus, in retention of feature bindings, regardless of retention duration, i.e., in both short- or long-term memory.Patients with PD, on the other hand, do not show increased misbinding. Their working memory deficits are associated with making more random errors or guesses. These random responses are not modulated by manipulations of their dopaminergic medication and hence may reflect involvement of non-dopaminergic neurotransmitters in this deficit. In addition, patients with PD demonstrate impairments in gating of information into relevant vs. irrelevant items in memory, a cognitive operation that is modulated by dopaminergic manipulation in line with a frontal executive effect of this neurotransmitter. Thus, although AD and PD are both associated with working memory impairments, these surface manifestations appear to be underpinned by very different mechanisms.

Harnessing Genetic Complexity to Enhance Translatability of Alzheimer's Disease Mouse Models: A Path toward Precision Medicine

An individual's genetic makeup plays a large role in determining susceptibility to Alzheimer's disease (AD) but has largely been ignored in preclinical studies. To test the hypothesis that incorporating genetic diversity into mouse models of AD would improve translational potential, we combined a well-established mouse model of AD with a genetically diverse reference panel to generate mice that harbor identical high-risk human mutations but differ across the remainder of their genome. We first show that genetic variation profoundly modifies the impact of human AD mutations on both cognitive and pathological phenotypes. We then validate this complex AD model by demonstrating high degrees of genetic, transcriptomic, and phenotypic overlap with human AD. Overall, work here both introduces a novel AD mouse population as an innovative and reproducible resource for the study of mechanisms underlying AD and provides evidence that preclinical models incorporating genetic diversity may better translate to human disease.