Sex differences in cognitive aging: a 4-year longitudinal study in marmosets

Improving preclinical to clinical translation of cognitive function for aging-related disorders: the utility of comprehensive touchscreen testing batteries in common marmosets

Concerns about poor animal to human translation have come increasingly to the fore, in particular with regards to cognitive improvements in rodent models, which have failed to translate to meaningful clinical benefit in humans. This problem has been widely acknowledged, most recently in the field of Alzheimer's disease, although this issue pervades the spectrum of central nervous system (CNS) disorders, including neurodevelopmental, neuropsychiatric, and neurodegenerative diseases. Consequently, recent efforts have focused on improving preclinical to clinical translation by incorporating more clinically analogous outcome measures of cognition, such as touchscreen-based assays, which can be employed across species, and have great potential to minimize the translational gap. For aging-related research, it also is important to incorporate model systems that facilitate the study of the long prodromal phase in which cognitive decline begins to emerge and which is a major limitation of short-lived species, such as laboratory rodents. We posit that to improve translation of cognitive function and dysfunction, nonhuman primate models, which have conserved anatomical and functional organization of the primate brain, are necessary to move the field of translational research forward and to bridge the translational gaps. The present studies describe the establishment of a comprehensive battery of touchscreen-based tasks that capture a spectrum of domains sensitive to detecting aging-related cognitive decline, which will provide the greatest benefit through longitudinal evaluation throughout the prolonged lifespan of the marmoset.

Age- and gender-related dispersion of brain networks across the lifespan

The effects of age and gender on large-scale resting-state networks (RSNs) reflecting within- and between-network connectivity in the healthy brain remain unclear. This study investigated how age and gender influence the brain network roles and topological properties underlying the ageing process. Ten RSNs were constructed based on 998 participants from the REST-meta-MDD cohort. Multivariate linear regression analysis was used to examine the independent and interactive influences of age and gender on large-scale RSNs and their topological properties. A support vector regression model integrating whole-brain network features was used to predict brain age across the lifespan and cognitive decline in an Alzheimer's disease spectrum (ADS) sample. Differential effects of age and gender on brain network roles were demonstrated across the lifespan. Specifically, cingulo-opercular, auditory, and visual (VIS) networks showed more incohesive features reflected by decreased intra-network connectivity with ageing. Further, females displayed distinctive brain network trajectory patterns in middle-early age, showing enhanced network connectivity within the fronto-parietal network (FPN) and salience network (SAN) and weakened network connectivity between the FPN-somatomotor, FPN-VIS, and SAN-VIS networks. Age - but not gender - induced widespread decrease in topological properties of brain networks. Importantly, these differential network features predicted brain age and cognitive impairment in the ADS sample. By showing that age and gender exert specific dispersion of dynamic network roles and trajectories across the lifespan, this study has expanded our understanding of age- and gender-related brain changes with ageing. Moreover, the findings may be useful for detecting early-stage dementia.

Proteomic association with age-dependent sex differences in Wisconsin Card Sorting Test performance in healthy Thai subjects

Sex differences in cognitive function exist, but they are not stable and undergo dynamic change during the lifespan. However, our understanding of how sex-related neural information transmission evolves with age is still in its infancy. This study utilized the Wisconsin Card Sorting Test (WCST) and the label-free proteomics method with bioinformatic analysis to investigate the molecular mechanisms underlying age-related sex differences in cognitive performance in 199 healthy Thai subjects (aged 20-70 years), as well as explore the sex-dependent protein complexes for predicting cognitive aging. The results showed that males outperformed females in two of the five WCST sub-scores: %Corrects and %Errors. Sex differences in these scores were related to aging, becoming noticeable in those over 60. At the molecular level, differently expressed individual proteins and protein complexes between both sexes are associated with the potential N-methyl-D-aspartate type glutamate receptor (NMDAR)-mediated excitotoxicity, with the NMDAR complex being enriched exclusively in elderly female samples. These findings provided a preliminary indication that healthy Thai females might be more susceptible to such neurotoxicity, as evidenced by their cognitive performance. NMDAR protein complex enrichment in serum could be proposed as a potential indication for predicting cognitive aging in healthy Thai females.

Sex-modified association between grip strength and mild cognitive impairment: a cross-sectional and follow-up study in rural China

BACKGROUND

The sex difference in the association between grip strength and mild cognitive impairment (MCI) remains controversial and unclear.

METHODS

This is a part of a chronic disease cohort study conducted in rural areas, Fuxin, Liaoning Province, China. At the baseline survey, a total of 2633 participants aged 35- 85 were included in the cross-sectional study. Handgrip strength (HGS, kg) was measured by a dynamometer (Jamar +). MCI were assessed using the Chinese version of the Montreal Cognitive Assessment-Basic (MOCA-BC). Then, a total of 1667 cognitively normal individuals (NCs) were planed to follow up and to assess the incident MCI after two years. We used logistic regression to examine the association between HGS (as a continuous variable and quintiles) and MCI and analyzed the interaction between sex and HGS on MCI. Models stratified by sex were adjusted for demographic information (age, ethnicity, education, marital status, income, physical labor level), modifiable risk factors (body mass index, smoking, drinking) and disease history (hypertension, diabetes, dyslipidemia and coronary heart disease). Baseline MOCA-BC scores were additionally adjusted in the longitudinal study.

RESULTS

In the cross-sectional study, participants were on average 56.6 ± 9.8 years, and 1713 (65.1%) were females. In the cohort study, 743 individuals were followed up with an average age of 55.9 ± 9.6 years, which included 530 (71.3%) females. The cumulative incidence of MCI over a two-year period was 17.1%. In the cross-sectional study, compared to the highest quintile of HGS, the lowest HGS was associated with higher risk of MCI in males (odds ratio [OR]: 2.66; 95% confidence interval [CI]: 1.54, 4.64) and females (OR: 1.70; 95% CI: 1.17, 2.49) with adjustment of potential confounding factors. In the cohort study, compared to the highest quintile of HGS, the lowest HGS was associated with an increased risk of incident MCI in females (OR: 3.93; 95% CI: 1.39, 13.01) but not in males (OR: 0.56; 95% CI: 0.11, 2.94, P for interaction = 0.015).

CONCLUSIONS

Lower grip strength is a risk factor for mild cognitive impairment and predicts a higher risk of MCI in females.

Cognitive development from infancy to young adulthood in common marmosets (Callithrix jacchus): Effect of age, sex, and hormones on learning and affective state

Studies looking at individual variability in cognition have increased in recent years. We followed 43 marmosets (21 males, 22 females) from infancy to young adulthood. At 3-months old, marmosets were trained to touch a rewarded stimulus. At 9-, 15-, and 21-months old, they were given visual discrimination and cognitive bias tests, and urine samples were collected to examine hormone levels. Marmosets were significantly more successful learners at 15 months than 9 months. Individuals who were more successful learners at 9 months were also more successful at 15 months, with more male learners than expected at 15 months. At 9 months, learning success was associated with higher cortisol levels. At 15 months, males with higher estradiol levels were more successful learners, whereas at 21 months, females with higher estradiol and cortisol levels tended to be less successful learners and more pessimistic. Nine months, therefore, appears to be an important developmental timepoint for acquiring cognitive control, which has developed by 15 months. Steroids may have differential effects on each sex, with complex interactions between gonadal and adrenal hormones having an influence on cognitive function over the lifespan. This longitudinal study offers new insight into cognition, including its development and biological underpinnings.

Neuropsychology of cognitive aging in rhesus monkeys

Aged rhesus monkeys, like aged humans, show declines in cognitive function. We present cognitive test data from a large sample of male and female rhesus monkeys, 34 young (aged 3.5-13.6 years) and 71 aged (aged 19.9-32.5 years at the start of cognitive testing). Monkeys were tested on spatiotemporal working memory (delayed response), visual recognition memory (delayed nonmatching to sample), and stimulus-reward association learning (object discrimination), tasks with an extensive evidence base in nonhuman primate neuropsychology. On average, aged monkeys performed worse than young on all 3 tasks. Acquisition of delayed response and delayed nonmatching to sample was more variable in aged monkeys than in young. Performance scores on delayed nonmatching to sample and object discrimination were associated with each other, but neither was associated with performance on delayed response. Sex and chronological age were not reliable predictors of individual differences in cognitive outcome among the aged monkeys. These data establish population norms for multiple cognitive tests in young and aged rhesus monkeys in the largest sample reported to date. They also illustrate independence of cognitive aging in task domains dependent on the prefrontal cortex and medial temporal lobe.

Survival rates and mortality risks of Plecturocebus cupreus at the California National Primate Research Center

This article describes survivorship and explores factors affecting mortality risks in a captive colony of coppery titi monkeys (Plecturocebus cupreus) housed at the California National Primate Research Center (CNPRC), at UC Davis, in Davis, CA. We analyzed data collected on individuals since the colony's creation in the 1960s, with a sample of 600 animals with partially complete information (date of birth, age at death, body mass, parental lineage). We used three methods: (1) Kaplan-Meier regressions followed by a log-rank test to compare survival in male and female titi monkeys, (2) a breakpoint analysis to identify shifts in the survival curves, and (3) Cox regressions to test the effect of body mass change, parental pair tenure, and parental age on mortality risk. We found that males tend to have a longer median lifespan than females (14.9 and 11.4 years; p = 0.094) and that survival decreases earlier in males than in females during adulthood (9.8 and 16.2 years). A body mass loss of 10% from adulthood to the time of death led to a 26% higher risk of dying (p < 0.001) as compared to an individual with stable body mass. We found no evidence of sociobiological factors on mortality risks (parental age, parental pair tenure), but an exploratory analysis suggested that a higher rate of offspring conceptions increases mortality risks. This description of factors influencing survival and mortality in titi monkeys is a first step toward understanding aging in this species to consider titi monkeys as a primate model for socioemotional aging.

Domain-Specific Cognitive Impairment Reflects Prefrontal Dysfunction in Aged Common Marmosets

Age-related cognitive impairment is not expressed uniformly across cognitive domains. Cognitive functions that rely on brain areas that undergo substantial neuroanatomical changes with age often show age-related impairment, whereas those that rely on brain areas with minimal age-related change typically do not. The common marmoset has grown in popularity as a model for neuroscience research, but robust cognitive phenotyping, particularly as a function of age and across multiple cognitive domains, is lacking. This presents a major limitation for the development and evaluation of the marmoset as a model of cognitive aging and leaves open the question of whether they exhibit age-related cognitive impairment that is restricted to some cognitive domains, as in humans. In this study, we characterized stimulus-reward association learning and cognitive flexibility in young adults to geriatric marmosets using a Simple Discrimination task and a Serial Reversal task, respectively. We found that aged marmosets show transient impairment in learning-to-learn but have conserved ability to form stimulus-reward associations. Furthermore, aged marmosets have impaired cognitive flexibility driven by susceptibility to proactive interference. As these impairments are in domains critically dependent on the prefrontal cortex, our findings support prefrontal cortical dysfunction as a prominent feature of neurocognitive aging. This work positions the marmoset as a key model for understanding the neural underpinnings of cognitive aging.

Bridging the rodent to human translational gap: Marmosets as model systems for the study of Alzheimer's disease

Introduction

Our limited understanding of the mechanisms that trigger the emergence of Alzheimer's disease (AD) has contributed to the lack of interventions that stop, prevent, or fully treat this disease. We believe that the development of a non-human primate model of AD will be an essential step toward overcoming limitations of other model systems and is crucial for investigating primate-specific mechanisms underlying the cellular and molecular root causes of the pathogenesis and progression of AD.

Methods

A new consortium has been established with funding support from the National Institute on Aging aimed at the generation, characterization, and validation of Marmosets As Research Models of AD (MARMO-AD). This consortium will study gene-edited marmoset models carrying genetic risk for AD and wild-type genetically diverse aging marmosets from birth throughout their lifespan, using non-invasive longitudinal assessments. These include characterizing the genetic, molecular, functional, behavioral, cognitive, and pathological features of aging and AD.

Results

The consortium successfully generated viable founders carrying PSEN1 mutations in C410Y and A426P using CRISPR/Cas9 approaches, with germline transmission demonstrated in the C410Y line. Longitudinal characterization of these models, their germline offspring, and normal aging outbred marmosets is ongoing. All data and resources from this consortium will be shared with the greater AD research community.

Discussion

By establishing marmoset models of AD, we will be able to investigate primate-specific cellular and molecular root causes that underlie the pathogenesis and progression of AD, overcome limitations of other model organisms, and support future translational studies to accelerate the pace of bringing therapies to patients.

Domain-specific cognitive impairment reflects prefrontal dysfunction in aged common marmosets

Age-related cognitive impairment is not expressed uniformly across cognitive domains. Cognitive functions that rely on brain areas that undergo substantial neuroanatomical changes with age often show age-related impairment, while those that rely on brain areas with minimal age-related change typically do not. The common marmoset has grown in popularity as a model for neuroscience research, but robust cognitive phenotyping, particularly as a function of age and across multiple cognitive domains, is lacking. This presents a major limitation for the development and evaluation of the marmoset as a model of cognitive aging, and leaves open the question of whether they exhibit age-related cognitive impairment that is restricted to some cognitive domains, as in humans. In this study, we characterized stimulus-reward association learning and cognitive flexibility in young adults to geriatric marmosets using a Simple Discrimination and a Serial Reversal task, respectively. We found that aged marmosets show transient impairment in "learning-to-learn" but have conserved ability to form stimulus-reward associations. Furthermore, aged marmosets have impaired cognitive flexibility driven by susceptibility to proactive interference. Since these impairments are in domains critically dependent on the prefrontal cortex, our findings support prefrontal cortical dysfunction as a prominent feature of neurocognitive aging. This work positions the marmoset as a key model for understanding the neural underpinnings of cognitive aging.

Significance Statement

Aging is the greatest risk factor for neurodegenerative disease development, and understanding why is critical for the development of effective therapeutics. The common marmoset, a short-lived non-human primate with neuroanatomical similarity to humans, has gained traction for neuroscientific investigations. However, the lack of robust cognitive phenotyping, particularly as a function of age and across multiple cognitive domains limits their validity as a model for age-related cognitive impairment. We demonstrate that aging marmosets, like humans, have impairment that is specific to cognitive domains reliant on brain areas that undergo substantial neuroanatomical changes with age. This work validates the marmoset as a key model for understanding region-specific vulnerability to the aging process.

Atrophic astrocytes in aged marmosets present tau hyperphosphorylation, RNA oxidation, and DNA fragmentation

Astrocytes perform multiple essential functions in the brain showing morphological changes. Hypertrophic astrocytes are commonly observed in cognitively healthy aged animals, implying a functional defense mechanism without losing neuronal support. In neurodegenerative diseases, astrocytes show morphological alterations, such as decreased process length and reduced number of branch points, known as astroglial atrophy, with detrimental effects on neuronal cells. The common marmoset (Callithrix jacchus) is a non-human primate that, with age, develops several features that resemble neurodegeneration. In this study, we characterize the morphological alterations in astrocytes of adolescent (mean 1.75 y), adult (mean 5.33 y), old (mean 11.25 y), and aged (mean 16.83 y) male marmosets. We observed a significantly reduced arborization in astrocytes of aged marmosets compared to younger animals in the hippocampus and entorhinal cortex. These astrocytes also show oxidative damage to RNA and increased nuclear plaques in the cortex and tau hyperphosphorylation (AT100). Astrocytes lacking S100A10 protein show a more severe atrophy and DNA fragmentation. Our results demonstrate the presence of atrophic astrocytes in the brains of aged marmosets.

Neuronal vulnerability to brain aging and neurodegeneration in cognitively impaired marmoset monkeys (Callithrix jacchus)

The investigation of neurobiological and neuropathological changes that affect synaptic integrity and function with aging is key to understanding why the aging brain is vulnerable to Alzheimer's disease. We investigated the cellular characteristics in the cerebral cortex of behaviorally characterized marmosets, based on their trajectories of cognitive learning as they transitioned to old age. We found increased astrogliosis, increased phagocytic activity of microglial cells and differences in resting and reactive microglial cell phenotypes in cognitively impaired compared to nonimpaired marmosets. Differences in amyloid beta deposition were not related to cognitive trajectory. However, we found age-related changes in density and morphology of dendritic spines in pyramidal neurons of layer 3 in the dorsolateral prefrontal cortex and the CA1 field of the hippocampus between cohorts. Overall, our data suggest that an accelerated aging process, accompanied by neurodegeneration, that takes place in cognitively impaired aged marmosets and affects the plasticity of dendritic spines in cortical areas involved in cognition and points to mechanisms of neuronal vulnerability to aging.

General cognitive performance declines with female age and is negatively related to fledging success in a wild bird

Identifying the causes and fitness consequences of intraspecific variation in cognitive performance is fundamental to understand how cognition evolves. Selection may act on different cognitive traits separately or jointly as part of the general cognitive performance (GCP) of the individual. To date, few studies have examined simultaneously whether individual cognitive performance covaries across different cognitive tasks, the relative importance of individual and social attributes in determining cognitive variation, and its fitness consequences in the wild. Here, we tested 38 wild southern pied babblers (Turdoides bicolor) on a cognitive test battery targeting associative learning, reversal learning and inhibitory control. We found that a single factor explained 59.5% of the variation in individual cognitive performance across tasks, suggestive of a general cognitive factor. GCP varied by age and sex; declining with age in females but not males. Older females also tended to produce a higher average number of fledglings per year compared to younger females. Analysing over 10 years of breeding data, we found that individuals with lower general cognitive performance produced more fledglings per year. Collectively, our findings support the existence of a trade-off between cognitive performance and reproductive success in a wild bird.

Age-Related Learning and Working Memory Impairment in the Common Marmoset

Aging is the greatest risk factor for the development of neurodegenerative diseases, yet we still do not understand how the aging process leads to pathologic vulnerability. The research community has relied heavily on mouse models, but the considerable anatomic, physiological, and cognitive differences between mice and humans limit their translational relevance. Ultimately, these barriers necessitate the development of novel aging models. As a nonhuman primate (NHP), the common marmoset (Callithrix jacchus) shares many features in common with humans and yet has a significantly shorter lifespan (10 years) than other primates, making it ideally suited to longitudinal studies of aging. Our objective was to evaluate the marmoset as a model of age-related cognitive impairment. To do this, we used the Delayed Recognition Span Task (DRST) to characterize age-related changes in working memory capacity in a cohort of sixteen marmosets, of both sexes, varying in age from young adult to geriatric. These monkeys performed thousands of trials over periods of time ranging up to 50% of their adult lifespan. To our knowledge, this represents the most thorough cognitive profiling of any marmoset aging study conducted to date. By analyzing individual learning curves, we found that aged animals exhibited delayed onset of learning, slowed learning rate after onset, and decreased asymptotic working memory performance. These findings are not accounted for by age-related impairments in motor speed and motivation. This work firmly establishes the marmoset as a model of age-related cognitive impairment.SIGNIFICANCE STATEMENT Understanding the normal aging process is fundamental to identifying therapeutics for neurodegenerative diseases for which aging is the biggest risk factor. Historically, the aging field has relied on animal models that differ markedly from humans, constraining translatability. Here, we firmly establish a short-lived nonhuman primate (NHP), the common marmoset, as a key model of age-related cognitive impairment. We demonstrate, through continuous testing over a substantial portion of the adult marmoset lifespan, that aging is associated with both impaired learning and working memory capacity, unaccounted for by age-related changes in motor speed and motivation. Characterizing individual cognitive aging trajectories reveals inherent heterogeneity, which could lead to earlier identification of the onset of impairment, and extended timelines during which therapeutics are effective.

Age, adrenal steroids, and cognitive functioning in captive chimpanzees (Pan troglodytes)

Background

Dehydroepiandrosterone-sulfate is the most abundant circulating androgen in humans and other catarrhines. It is involved in several biological functions, such as testosterone production, glucocorticoid antagonist actions, neurogenesis and neuroplasticty. Although the role of dehydroepiandrosterone-sulfate (DHEAS) in cognition remains elusive, the DHEAS/cortisol ratio has been positively associated with a slower cognitive age-decline and improved mood in humans. Whether this relationship is found in nonhuman primates remains unknown.

Methods

We measured DHEAS and cortisol levels in serum of 107 adult chimpanzees to investigate the relationship between DHEAS levels and age. A subset of 21 chimpanzees was used to test the potential associations between DHEAS, cortisol, and DHEAS/cortisol ratio in cognitive function, taking into account age, sex, and their interactions. We tested for cognitive function using the primate cognitive test battery (PCTB) and principal component analyses to categorize cognition into three components: spatial relationship tasks, tool use and social communication tasks, and auditory-visual sensory perception tasks.

Results

DHEAS levels, but not the DHEAS/cortisol ratio, declined with age in chimpanzees. Our analyses for spatial relationships tasks revealed a significant, positive correlation with the DHEAS/cortisol ratio. Tool use and social communication had a negative relationship with age. Our data show that the DHEAS/cortisol ratio, but not DHEAS individually, is a promising predictor of spatial cognition in chimpanzees.

Potential trade-off between olfactory and visual discrimination learning in common marmosets (Callithrix jacchus): Implications for the assessment of age-related cognitive decline

Olfactory dysfunction has been identified as an early biomarker for dementia risk but has rarely been assessed in nonhuman primate models of human aging. To better characterize common marmosets as such models, we assessed olfactory discrimination performance in a sample of 10 animals (5 females), aged 2.5-8.9 years old. The monkeys were proficient in the discrimination and reversal of visual stimuli but naïve to odor stimuli. For olfactory discrimination, the monkeys performed a series of six discriminations of increasing difficulty between two odor stimuli. We found no evidence for an age-related decline as both young and older individuals were able to perform the discriminations in roughly the same number of trials. In addition, the older monkeys had faster responses than the younger animals. However, we noted that when adjusted for age, the speed of acquisition of the first discrimination in the olfactory modality was inversely correlated to the speed of acquisition of their first discrimination of two visual stimuli months earlier. These results suggest that marmosets may compensate for sensory deficits in one modality with higher sensory performance in another. These data have broad implications for the assessment of age-related cognitive decline and the categorization of animals as impaired or nonimpaired.

The marmoset as a model for investigating the neural basis of social cognition in health and disease

Social-cognitive processes facilitate the use of environmental cues to understand others, and to be understood by others. Animal models provide vital insights into the neural underpinning of social behaviours. To understand social cognition at even deeper behavioural, cognitive, neural, and molecular levels, we need to develop more representative study models, which allow testing of novel hypotheses using human-relevant cognitive tasks. Due to their cooperative breeding system and relatively small size, common marmosets (Callithrix jacchus) offer a promising translational model for such endeavours. In addition to having social behavioural patterns and group dynamics analogous to those of humans, marmosets have cortical brain areas relevant for the mechanistic analysis of human social cognition, albeit in simplified form. Thus, they are likely suitable animal models for deciphering the physiological processes, connectivity and molecular mechanisms supporting advanced cognitive functions. Here, we review findings emerging from marmoset social and behavioural studies, which have already provided significant insights into executive, motivational, social, and emotional dysfunction associated with neurological and psychiatric disorders.

Differential Ageing of the Brain Hemispheres: Evidence from a Longitudinal Study of Hand Preferences in Common Marmosets

This paper is concerned with decreasing asymmetry of motor control in ageing. It discusses age-related changes in humans and reports a longitudinal study of hand preferences in common marmosets. An annual assessment of hand preference for holding food was recorded throughout the lifespan of 19 marmosets that lived for at least 9 years, and half of those lived for at least 11 years. Those with a left-hand preference showed a gradual reduction in the strength of their hand preference throughout adult life. No significant change in the strength of hand preference was found in right-handed marmosets. Hence, ageing has a specific effect on motor control by the right hemisphere.