Neurobiology of Aging

A decline in declarative or explicit memory has been extensively characterized in cognitive ageing and is a hallmark of cognitive impairments. However, whether and how implicit perceptual memory varies with ageing or cognitive impairment is unclear. Here, we compared implicit perceptual memory and explicit memory measures in three groups of subjects: (1) 59 healthy young volunteers (20-30 years); (2) 238 healthy old volunteers (50-90 years) and (3) 21 patients with mild cognitive impairment MCI (50-90 years). To measure explicit memory, subjects were tested on standard recognition and recall tasks. To measure implicit perceptual memory, we used a classic perceptual priming paradigm. Subjects had to report the shape of a visual search pop-out target. Implicit priming was measured as the speedup in response time for targets with the same vs different color/position on consecutive trials.\n\nOur main findings are as follows: (1) Explicit memory was weaker in old compared to young subjects, and in MCI compared to age-matched controls; (2) Surprisingly, implicit perceptual memory did not always decline with age: color priming was smaller in older subjects but position priming was larger; (3) Position priming was less frequent in the MCI group compared to age-matched controls; (4) Implicit and explicit memory measures were uncorrelated in all three groups. Thus, implicit memory can increase or decrease with age or cognitive impairment, but this decline does not covary with explicit memory. We propose that incorporating explicit and implicit measures can yield a richer characterization of memory.

The APOE {varepsilon}4 gene is associated with increased risk of developing sporadic Alzheimers Disease (AD). Several studies have focused on declarative memory, where episodic memory deficits are reported in {varepsilon}4 carriers, while semantic memory has received much less attention. To clarify whether the impact of APOE {varepsilon}4 on declarative memory is specific to episodic memory, we administered a novel measure of autobiographical memory, the Semantic Autobiographical Interview (SAI). Thirty-eight healthy older adults were recruited, 19 {varepsilon}4 carriers and 19 non-carriers, matched in age, education, and gender. The groups did not significantly differ in any neuropsychological tests except for recognition memory, where {varepsilon}4 carriers showed reduced performance. On the original Autobiographical Interview (AI), results revealed a reduced number of episodic details (internal details and external events) in carriers. Together, these results suggest a reduction of episodic specificity in {varepsilon}4 carriers. In contrast, carriers had very similar semantic production to non-carriers, whether it was for off-task (external) semantic details in the AI, or on-task general and personal semantic details produced in the SAI. These results suggest that older adults retain the gist of their personal experience and that the semanticization of their autobiographical narratives is robust and less sensitive to risk for AD than episodic memory. Public Significance StatementPeople at risk for Alzheimers Disease struggle with recalling memories from specific events (episodic memory), but it remains unclear whether they also have difficulties with personal and general knowledge (semantic memory). We used a new measure, the Semantic Autobiographical Interview, to evaluate these forms of memory. Compared to control participants, people at risk for Alzheimers Disease produced a reduced number of episodic details, but a similar amount of semantic knowledge.

Spatial cognition enables animals to navigate the environment. Impairments in spatial navigation are often preclinical signs of Alzheimers disease (AD) in human. Therefore, evaluating spatial memory deficits is valuable when assessing incipient AD in animal models. The Chinese tree shrew, a close relative of primates, possesses many features that make it suitable for AD research. However, there is a scarcity of reliable behavior paradigms to monitor changes in spatial cognition in this species. To address this, we established reward-based paradigms in the radial-arm maze and the cheeseboard maze for tree shrew, and tested spatial memory of a group of twelve male animals in both tasks, along with a control water maze test, before and after bilateral lesions to the hippocampus, the brain region essential for spatial navigation. Tree shrews memorized target positions during training, and their task performance improved gradually until reaching a plateau in all three mazes. After the lesion, spatial learning was compromised in both newly-developed tasks, whereas memory retrieval was impaired in the water maze. Furthermore, individual task performance in both dry-land paradigms depended heavily on the size of remaining hippocampal tissue. Notably, all lesioned animals displayed spatial memory deficits in the cheeseboard task, but not in the other two paradigms. Our results suggest that the cheeseboard task currently represents the most sensitive paradigm for assessing spatial memory in tree shrew, with the potential to monitor progressive cognitive declines in aged or genetically modified animals developing AD-like symptoms. Significance StatementCognitive tests that monitor impairments in spatial memory play a crucial role in evaluating animal models with early-stage Alzheimers disease (AD). The Chinese tree shrew possesses many features suitable for an AD model, yet behavior tests assessing spatial cognition in this species are lacking. Here we developed novel behavior paradigms tailored to measure spatial memory in tree shrews and evaluated their sensitivity to changes in spatial learning by examining a group of hippocampus-lesioned animals. Our results indicate that the cheeseboard task effectively detects impairments in spatial memory and holds potential for monitoring the progressive cognitive decline in aged or genetically modified tree shrews that develop AD-like symptoms. This research may facilitate the use of tree shrew model in AD research.

Sex differences in Parkinson Disease (PD) risk are well-known. However, it is still unclear the role of sex chromosomes in the development and progression of PD. We performed the first X-chromosome Wide Association Study (XWAS) for PD risk in Latin American individuals. We used data from three admixed cohorts: (i) Latin American Research consortium on the GEnetics of Parkinsons Disease (n=1,504) as discover cohort and (ii) Latino cohort from International Parkinson Disease Genomics Consortium (n = 155) and (iii) Bambui Aging cohort (n= 1,442) as replication cohorts. After developing a X-chromosome framework specifically designed for admixed populations, we identified eight linkage disequilibrium regions associated with PD. We fully replicated one of these regions (top variant rs525496; discovery OR [95%CI]: 0.60 [0.478 - 0.77], p = 3.13 x 10-5 ; replication OR: 0.60 [0.37-0.98], p = 0.04). rs525496 is an expression quantitative trait loci for several genes expressed in brain tissues, including RAB9B, H2BFM, TSMB15B and GLRA4. We also replicated a previous XWAS finding (rs28602900), showing that this variant is associated with PD in non-European populations. Our results reinforce the importance of including X-chromosome and diverse populations in genetic studies.

Age-related deficits in episodic memory and mnemonic discrimination are associated with an increased risk of neurodegenerative diseases, such as Alzheimers disease (AD) (Stark et al., 2013). While much research has focused on hippocampal contributions to these age-related changes (Stark et al., 2019), less is known about the role of posterior cingulate cortex (PCC) especially reduced inhibition in episodic memory deficit. PCC has connections to the medial temporal lobe and is linked to memory declines (Greicius et al., 2004). It is also one of the most vulnerable regions to amyloid deposition in AD (Yokoi et al., 2018). This study hypothesized and found that age-related declines in GABAergic function (brains major inhibitory neurotransmitter) within the PCC contributes to individual differences in memory performance in healthy older adults. Using Magnetic Resonance Spectroscopy, we measured GABA levels in the PCC in 22 healthy younger and 30 older adults. We assessed episodic memory using Rey Delayed Auditory Verbal Learning Test (RAVLT) and Mnemonic Similarity Task (MST). We found that both raw GABA levels and episodic memory performance are lower in older adults compared to young. This reduction in GABA levels is subserved by age-related changes in tissue-composition as evidenced by no age-group differences in corrected GABA levels. More importantly, lower GABA levels (independent of tissue-correction) were associated with poorer episodic performance including delayed recall and mnemonic discrimination. This research suggests that therapeutically targeting posterior cingulate GABA levels might help slow or alleviate memory decline. Significance StatementThis study provides novel insights into the role of posterior cingulate cortex (PCC) GABA+ levels in age-related memory deficits. Our findings demonstrate that lower PCC GABA+ levels in older adults are associated with poorer performance on episodic memory tasks, particularly those involving mnemonic discrimination and word-list learning. This research expands on the growing body of literature linking GABAergic dysfunction to age-related cognitive impairments and suggests that GABAergic changes in the PCC contribute to episodic memory deficits. Importantly, our results highlight the potential of targeting PCC GABA levels as a therapeutic strategy to slow or mitigate memory decline in aging. These findings also offer promising avenues for future research into early biomarkers for Alzheimers disease and other neurodegenerative conditions.

Sex differences in Alzheimers disease (AD) risk and progression are increasingly recognized, with females exhibiting higher global prevalence rates. Yet it remains unclear how genetic and biomarker indicators of Alzheimers risk, such as the apolipoprotein E-{varepsilon}4 (APOE4) allele and amyloid burden, relate to sex differences in brain and cognitive health during the preclinical stage. Using established normative models trained on ~58,000 healthy participants, we computed regional z-scores from T1-weighted MRI scans in 372 cognitively normal participants from the Insight 46 cohort. Scans were acquired at two timepoints, approximately three years apart, beginning at age 70. Regions with z-scores below -1.96 were classified as brain-structure outliers and summarized as total outlier count (tOC). We used linear mixed-effects models to examine how sex, age, and AD risk (APOE4 status and amyloid burden) relate to tOC and cognitive outcomes measured by Preclinical Alzheimer Cognitive Composite (PACC) scores. Both cross-sectional associations and longitudinal changes in tOC and PACC scores were examined, and we tested whether the effects of APOE4 status and amyloid burden on brain and cognitive measures differed by sex. Cross-sectional analyses showed that males had greater tOC than females at younger ages. At timepoint 1, spatial maps showed more regions with outliers in males, though high outlier proportions were limited to occipital areas. By timepoint 2, group differences became more spatially distinct, with males and females showing deviations in different regions. Longitudinally, older males exhibited steeper increases in tOC over time compared to females. Females showed higher PACC scores overall, while no sex differences were observed in cognitive change over time. Greater tOC and amyloid burden were both associated with poorer cognitive outcomes, with the strongest association observed in female APOE4 carriers. However, we found no evidence that AD risk influenced age-related changes in tOC or cognition over time. These findings highlight the complex interplay between sex, age, and AD risk in shaping brain structure and cognition in later life. Some of the observed patterns may reflect emerging vulnerability not yet captured by short-term longitudinal change, underscoring the importance of continued observation. In conclusion, normative modelling provides a valuable approach for detecting subtle variation in brain and cognitive aging across risk groups.

Researchers have identified changes in dopaminergic neuromodulation as playing a key role in adult memory decline. Facilitated by technical advancements, recent research has also implicated noradrenergic neuromodulation in shaping late-life memory performance. However, it is not yet clear whether these two neuromodulators have distinct roles in age-related cognitive changes. Combining longitudinal high-resolution magnetic resonance imaging of the dopaminergic substantia nigra-ventral tegmental area (SN-VTA) and the noradrenergic locus coeruleus (LC) in younger (n = 69) and older adults (n = 251), we found that dopaminergic and noradrenergic integrity are differentially associated with individual differences in memory performance. While LC integrity was related to better episodic memory across several memory tasks, SN-VTA integrity was linked to working memory. Moreover, consistent with their dense interconnection and a largely shared biosynthesis, dopaminergic and noradrenergic brain regions integrity were positively related, and correlated with medial temporal lobe volumes. Longitudinally, we found that older age was associated with more-negative change in SN- VTA and LC integrity (time point 1-time point 2; mean delay [~]1.9 years). Importantly, changes in LC integrity reliably predicted future episodic memory performance (at time point 3). These findings support the feasibility of in-vivo indices for catecholaminergic integrity with potential clinical utility, given the degeneration of both neuromodulatory systems in several age-associated diseases. Moreover, they highlight differential roles of dopaminergic and noradrenergic neuromodulatory nuclei in late-life cognitive decline.

Emerging evidence suggests that some personality traits may link to the vulnerability to or protection for Alzheimers disease (AD). A causal mechanism underlying this relationship, however, remains largely unknown. Using 18F-Florbetaben positron emission tomography (PET) binding to beta-amyloid (A{beta}) plaques, a pathological feature of AD, and functional magnetic resonance imaging (fMRI), we investigated pathological and functional correlates of extraversion and neuroticism in a group of healthy young and older subjects. We quantified a level of brain A{beta} deposition in older individuals. Brain activity was measured in young adults using a task-switching fMRI paradigm. When we correlated personality scores of extraversion and neuroticism with these pathological and functional measures, higher extraversion, but not neuroticism, was significantly associated with lower global A{beta} measures among older adults, accounting for age and sex. This association was present across widespread brain regions. Among young subjects, higher extraversion was associated with lower activity during task switching in anterior cingulate cortex, left anterior insular cortex, left putamen, and middle frontal gyrus bilaterally, while higher neuroticism was associated with increased activity throughout the brain. The present results suggest that possibly via efficient neuronal activity, extraversion, one of lifelong personality traits, may confer the protective mechanism against the development of A{beta} pathology during aging.

Parkinsons Disease (PD) is an age-related neurodegenerative disorder that has been associated with increased DNA damage. To test if PD is associated with increased somatic mutations, we analyzed RNA-seq data in whole blood from 5 visits of the Parkinsons Progression Markers Initiative for clonally amplified somatic variants. Comprehensive analysis of RNA-sequencing data revealed a total of 5,927 somatic variants (2.4 variants per sample on average). Mutation frequencies were significantly elevated in PD subjects as compared to age-matched controls at the time of the last visit. This was confirmed by RNA analysis of substantia nigra. By contrast, the fraction of carriers with clonal hematopoiesis, was significantly reduced in old PD patients as compared to old healthy controls. These results indicate that while the overall mutation rate is higher in PD, specific clonally amplified mutations are protective against PD, as has been found for Alzheimers Disease.

INTRODUCTIONBoth Alzheimers disease (AD) and ageing have a strong genetic component. In each case, many associated variants have been discovered, but how much missing heritability remains to be discovered is debated. Variability in the estimation of SNP-based heritability could explain the differences in reported heritability. METHODSWe compute heritability in five large independent cohorts (N=7,396, 1,566, 803, 12,528 and 3,963) to determine whether a consensus for the AD heritability estimate can be reached. These cohorts vary by sample size, age of cases and controls and phenotype definition. We compute heritability a) for all SNPs, b) excluding APOE region, c) excluding both APOE and genome-wide association study hit regions, and d) SNPs overlapping a microglia gene-set. RESULTSSNP-based heritability of Alzheimers disease is between 38 and 66% when age and genetic disease architecture are correctly accounted for. The heritability estimates decrease by 12% [SD=8%] on average when the APOE region is excluded and an additional 1% [SD=3%] when genome-wide significant regions were removed. A microglia gene-set explains 69-84% of our estimates of SNP-based heritability using only 3% of total SNPs in all cohorts. CONCLUSIONThe heritability of neurodegenerative disorders cannot be represented as a single number, because it is dependent on the ages of cases and controls. Genome-wide association studies pick up a large proportion of total AD heritability when age and genetic architecture are correctly accounted for. Around 13% of SNP-based heritability can be explained by known genetic loci and the remaining heritability likely resides around microglial related genes. Author SummaryEstimates of heritability in Alzheimers disease, the proportion of phenotypic variance explained by genetics, are very varied across different studies, therefore, the amount of missing heritability not yet captured by current genome-wide association studies is debated. We investigate this in five independent cohorts, provide estimates based on these cohorts and detail necessary suggestions to accurately calculate heritability in age-related disorders. We also confirm the importance of microglia relevant genetic markers in Alzheimers disease. This manuscript provides suggestions for other researchers computing heritability in late-onset disorders and the microglia gene-set used in this study will be published alongside this manuscript and made available to other researchers. The correct assessment of disease heritability will aid in better understanding the amount of missing heritability in Alzheimers disease.

Substantia nigra pars compacta (SNc) dopamine neurons are required for voluntary movement and reward learning, and advanced age is associated with motor and cognitive decline. In the midbrain, D2-type autoreceptors located on dendrodendritic synapses between dopamine neurons control cell firing through G protein-activated potassium (GIRK) channels. We previously showed that aging disrupts dopamine neuron pacemaker firing in mice, but only in males. Here we show that D2-receptor inhibitory postsynaptic currents (D2-IPSCs) in aged male mice are moderately smaller compared to young males as well as females, regardless of age. Local application of dopamine revealed a reduction in the amplitude of the D2-receptor currents in old males compared to young, pointing to a postsynaptic mechanism that could not be explained by impairment of the GIRK channels or degeneration of the dendritic arbor. Kinetic analysis showed no differences in D2-IPSCs in old versus young mice or between sexes. Potentiation of D2-IPSCs by corticotropin releasing factor (CRF) is also conserved in aging, indicating preservation of plasticity mechanisms. These findings have implications for understanding dopamine transmission in aging in both sexes and could explain in part the increased susceptibility of males to SNc degeneration of dopamine neurons in neurodegenerative disorders such as Parkinsons disease (PD).

Robust evidence points to mnemonic deficits in older adults related to dedifferentiated, i.e., less distinct, neural responses during memory encoding. However, less is known about retrieval-related dedifferentiation and its role in age-related memory decline. In this study, younger and older adults were scanned both while incidentally learning face and house stimuli and while completing a surprise recognition memory test. Using pattern similarity searchlight analyses, we looked for indicators of neural dedifferentiation during retrieval and asked whether this might explain interindividual differences in memory performance. Our findings revealed age-related reductions in neural distinctiveness during memory retrieval as well as in encoding-retrieval reinstatement in visual processing regions. We further demonstrated that the degree to which patterns elicited during encoding were reinstated during retrieval tracked variability in memory performance better than retrieval-related distinctiveness only. All in all, we contribute to meager existing evidence for age-related neural dedifferentiation during memory retrieval. We propose that the recognition task (as opposed to a cued recall task) may have revealed impairment in perceptual processing in older adults, leading to particularly widespread age differences in neural distinctiveness. We additionally provide support for the idea that well-defined reactivation of encoding patterns plays a major role in successful memory retrieval.

The PSAP gene encodes prosaposin, which is later cleaved into four active saposins: saposin A, B, C and D. Mutations in these enzymes have been linked to specific lysosomal storage disorders. Recently, a genetic association between mutations in saposin D and Parkinsons disease (PD) has been reported. To further examine whether variants in saposin D or the other saposins could be associated with Parkinsons disease, we performed Optimized Sequence Kernel Association Test (SKAT-O) in 4,132 Parkinsons disease patients and 4,470 controls. Furthermore, we analyzed data from a PD Genome Wide Association Study (GWAS) to examine the association of common variants in the PSAP locus with Parkinsons disease risk (analysis on 56,308 patients) and age at onset (analysis on 28,568 patients). We did not find any statistically significant associations between neither rare nor common variants in saposin D, nor any of the other saposins, and PD risk or onset. These results suggest that PSAP variants play either a very minor role, or more likely, no role, in PD.

Relational memory, the ability to flexibly encode and retrieve associations among distinct elements, is critically dependent on the hippocampus and declines with age in humans. The Transverse Patterning (TP) task is designed to probe relational memory by requiring learning of hierarchical, circular stimulus relationships (e.g., A+ B-, B+ C-, C+ A-), a structure akin to rock-paper-scissors. In humans, TP performance is reliably impaired by hippocampal damage and aging. In non-human primates, however, findings have been inconsistent with some studies demonstrating clear hippocampal dependence, while others report no impairment, or even improvements, following hippocampal lesions. This raises the possibility that species differences in cognitive strategy use may underlie these divergent outcomes. We hypothesized that non-human primates rely on an elemental learning strategy, supported by corticostriatal systems, even when relational memory is required. To test this, we trained young and aged common marmosets (Callithrix jacchus) on the TP task and several control tasks designed to isolate elemental versus configural learning. Marmosets successfully acquired reward contingencies for individual stimulus pairs but failed when success required integrating all three stimulus relationships. In contrast, all animals readily acquired control tasks solvable via simple stimulus-response associations. Notably, there was no evidence of age-related impairment on TP or control task performance. Given the early vulnerability of the hippocampus to aging and the relative preservation of striatal systems, this pattern further supports the conclusion that marmosets rely on a habit-based learning strategy that is poorly suited to relational demands. These findings suggest that humans and non-human primates may approach the same tasks using different cognitive strategies. This has critical implications for interpreting cross-species differences in memory performance and highlights the need to validate which neural systems a task engages in each species before using it as a translational model of hippocampal function or cognitive aging.

The current study investigates whether electroencephalographic (EEG) activity reflects age-related memory changes during encoding. We recorded scalp EEG in 151 young adults (aged 18-30) and 37 older adults (aged 60-85) as they memorized lists of words. Subjects studied the word lists either under full attention or while performing a secondary task that required them to make semantic judgments about each word. Although the secondary task reduced recall among all subjects, differences in recall performance between the age groups were smaller when participants performed a secondary task at encoding. Older adults also exhibited distinct neural subsequent memory effects, characterized by less negativity in the alpha frequencies compared to young adults. Multivariate classifiers trained on neural features successfully predicted subsequent memory at the trial level in both young and older adults, and captured the differential effects of task demands on memory performance between young and older adults. The findings indicate that neural biomarkers of successful memory vary with both cognitive aging and task demands. Public significanceThe current study investigates the EEG spectral biomarkers of memory encoding in young and older adults, and identifies key features of neural activity associated with age-related memory change. We further find that age-related memory differences are smaller when participants perform a semantic judgment task during encoding, and that multivariate classifiers trained on the EEG data predict these effects.

Alzheimers disease (AD) is characterized by the prion-like propagation of amyloid-{beta} (A{beta}). However, the role of A{beta} in cognitive impairment is still unclear. To determine the causal role of A{beta} in AD, we intracerebrally seeded the entorhinal cortex of two-month-old AppNL-G-F mouse model with an A{beta} peptide derived from patients who died from rapidly progressing AD. When the mice were three and six months of age, or one- and four-months following seeding, respectively, spatial learning and memory were tested using the Morris water task. Immunohistochemical labeling showed seeding with the A{beta} seed increased plaque size one month following seeding, but reduced plaque counts four months following injection compared to the control seeded mice. A significant increase in microgliosis was found. However, we found no correlation between pathology and spatial performance. The results of the present study show that seeding human tissue with or without A{beta} alters learning and memory ability, A{beta} plaque deposition, plaque size, and microgliosis in the AppNL-G-F knock-in model, and these effects are dependent on the presence of a humanized App gene and the presence of A{beta} in the seed. But these pathological changes were not initially causal in memory impairment.

Attention deficits emerge early in Alzheimers disease (AD), where cholinergic dysfunction compromises goal-directed behavior and cognitive control. Therefore, attentional impairments may serve as early indicators of cognitive decline, and also as meaningful targets for therapeutic intervention. Despite their clinical importance, attention deficits remain under- targeted by current treatments, which offer only modest benefit. To support development of more effective therapies, preclinical models that closely mirror human neurobiology and behavior are essential. Non-human primates (NHPs), with their high degree of cortical and functional similarity to humans, particularly in prefrontal regions, offer a uniquely translational platform for evaluating cognitive enhancers. We assessed pharmacological interventions targeting sustained attention using the Continuous Performance Test (CPT) in adult male cynomolgus macaques. Monkeys were trained to detect target stimuli while ignoring distractors, achieving individualized stable performance. To simulate cholinergic dysfunction, we administered scopolamine, a muscarinic acetylcholine receptor antagonist, which produced dose-dependent declines in accuracy and reaction time. Mild and severe impairment levels were identified within each animal. We then tested three compounds: nicotine, guanfacine, and donepezil. Nicotine, a nicotinic receptor agonist, fully restored performance across both impairment levels, suggesting potential benefit in both early and advanced AD. Guanfacine, an 2A adrenergic agonist, improved accuracy only under mild impairment, while donepezil, an acetylcholinesterase inhibitor, showed inconsistent effects. None of the compounds reversed scopolamine-induced slowing of reaction time, indicating specificity for attentional control. These findings highlight the utility of the NHP CPT as a pharmacologically sensitive model for detecting attentional dysfunction and evaluating pro-cognitive therapeutics in aging and neurodegeneration.

BackgroundThe circadian system influences many different biological processes across the lifespan, including memory performance and daily activity patterns. The biological process of aging causes decreased control of the circadian system that is accompanied by a decline in memory performance, suggesting that these two processes may be linked. Indeed, our previous work has shown that in male mice, the clock gene Per1 functions within the dorsal hippocampus to exert diurnal control over memory and repression of Per1 in the old hippocampus contributes to age-related impairments in spatial memory. Although it is clear that Per1 may be a key molecular link between memory and the circadian rhythm, next to nothing is known about how sex impacts this role in the young or old brain. Here, we are interested in understanding how the factors of sex and age impact memory performance, circadian activity patterns, sleep behavior, and hippocampal Per1 expression. MethodsWe used a combination of spatial memory (Object Location Memory (OLM)) and circadian activity monitoring to determine how male and female mice change across the lifespan. In addition, we used RT-qPCR to quantify the change in Per1 levels in response to learning in young and old, male and female mice. ResultsYoung female mice resist diurnal oscillations in memory, showing robust spatial memory across the diurnal cycle. In contrast, old female mice show an emergence of diurnal memory oscillations, with better memory during the day than at night (similar to what we observed previously in young male mice). In contrast, old male mice showed better memory performance during the night than the day, suggesting that their peak memory performance is drastically shifted compared to young males. We also measured activity patterns and sleep behavior across the diurnal cycle and found that sex was more of an influence than age in multiple analyses, but age did have an impact, with old male mice showing stronger circadian rhythm disruptions than any other cohort. Finally, we investigated whether the circadian clock gene Per1 plays a role in these sex- and age-dependent effects in diurnal memory performance. We found that, in general, learning- induced Per1 and memory performance peaked at similar times of day in each group, consistent with our hypothesis that Per1 exerts diurnal control over memory performance. ConclusionsThis work supports a role for Per1 in exerting diurnal control over memory and suggests that Per1 may be an appealing therapeutic target to improve memory and circadian dysfunction in old age. HighlightsO_LIDiurnal oscillations in spatial memory are sex- and age-dependent in mice C_LIO_LIPer1 learning-induced expression matches diurnal memory patterns C_LIO_LICircadian rhythm patterns are sex- and age-dependent in mice C_LIO_LIYoung females show good memory across the diurnal cycle C_LIO_LIDiurnal memory oscillations reemerge in old female mice C_LI Plain language summaryMemory is an integral part of everyday functioning, and one that is known to decline with aging. Our lab has previously shown that the clock gene Period1 (Per1) regulates spatial memory performance in young males, establishing a molecular link between circadian rhythms and memory. Young adult male mice show diurnal oscillations in memory consolidation, with the best memory occurring at midday, and the worst memory occurring at midnight. In the current study, we wanted to expand our work to young adult females, as well as an aged population of male and female mice. Using a simple spatial memory task, we measured diurnal changes in both memory performance and Per1 gene expression within the dorsal hippocampus (a brain region necessary for spatial memory). We found that old mice (both male and female) showed a correlation between high Per1 levels and better memory, as we have previously seen. Conversely, young female mice performed well on the memory task at every timepoint but didnt have a significant change in Per1, indicating that they may be using some different mechanism to modulate memory performance. Finally, we used infrared activity monitoring to investigate several circadian rhythm related measures in young and old, male and female mice. We found that sex influenced the circadian rhythm more than age, and the group with the largest circadian disruption was aged males. Overall, this research provides new information about how both sex and age impact diurnal oscillations in both memory and activity, fundamental knowledge that has been lacking in the field.

The proportion of older adults in Western populations is increasing and there is, therefore, a need to define factors affecting maintenance of physical and cognitive health in old age. Variations in the Klotho (KL) gene, and specifically the KL-VS haplotype, have been identified by several authors as potentially influencing cognitive function and decline. We have attempted to verify the reported associations between KL variants, including the KL-VS haplotype, and cognitive function in up to 335,074 British Caucasian participants aged 40-79 years from the UK Biobank. We do not find evidence that KL-VS affects cognitive function or its decline with increasing age. We examined a further 244 KL variants and found that rs117650866 was associated with Prospective Memory, but could not replicate this in follow-up samples. In conclusion, there is insufficient evidence in the UK Biobank to support the concept that KL variants affect cognitive function or its rate of decline.

Brain cholinergic denervation is among the earliest manifestations of Alzheimers disease, and rapid eye movement (REM) sleep alterations have also been described early in the course of the disease. While cholinergic activity supports cortical activation during REM sleep, direct evidence for a link between cholinergic degeneration and early REM sleep alterations in humans is still lacking. Here, we tested the long-standing hypothesis that early cholinergic denervation may be associated with REM sleep EEG slowing in older adults with and without Mild Cognitive Impairment. Twenty-four older participants (mean age: 71.29 {+/-} 4.85 years; 58.33% women; 25% participants with amnestic Mild Cognitive Impairment) without dementia or moderate-to-severe obstructive sleep apnea underwent a night of in-laboratory polysomnography, comprehensive neuropsychological evaluation, structural MRI and molecular PET imaging with [18F]-Fluoroethoxybenzovesamicol (FEOBV), known for its sensitivity to quantify brain cholinergic innervation. Voxel-wise multiple regressions assessed the associations between REM sleep characteristics (i.e., REM sleep percentage, relative theta power and EEG slowing ratios, defined as [delta + theta]/[alpha + beta] power) and FEOBV-PET standard uptake value ratio maps corrected for partial volume effects, controlling for sex. Given that FEOBV uptake was higher in women compared to men, we also performed exploratory sex-stratified analyses adjusted for age. Higher REM sleep EEG slowing over frontal and parietal derivations was significantly associated with cortical cholinergic denervation, notably in fronto-parietal areas and the medial temporal lobe (P<0.005 level, combined with a cluster-level family-wise error correction). Exploratory sex-stratified analyses revealed that REM sleep EEG slowing was associated with cholinergic denervation in medial temporal regions in women, and neocortical regions in men. These findings provide the first direct in vivo evidence that REM sleep EEG slowing could represent a sensitive marker of cortical cholinergic denervation in older adults, prior to dementia onset. Thus, quantitative REM sleep EEG may constitute a promising marker for early diagnosis and disease-modifying interventions in Alzheimers disease.