Neuropsychology of cognitive aging in rhesus monkeys

A study on the variation of cytokine and electrolyte levels in rhesus macaques, cynomolgus monkeys, and Assamese macaques

BACKGROUND

Non-human primates (NPHs), such as rhesus macaques, cynomolgus monkeys, and Assamese macaques, play a crucial role in biomedical research. However, baseline cytokine and electrolyte data for these three species, particularly data stratified by age and sex, are limited. Therefore, the aim of this study was to establish and analyze age- and sex-specific cytokine and electrolyte profiles in these three species.

METHODS

This study included 40 rhesus macaques (21 males, 19 females), 33 cynomolgus monkeys (17 males, 16 females), and 45 Assamese macaques (25 males, 20 females) classified by age (1-5 years, 6-12 years, >13 years) and sex. The levels of 23 immune function indicators and 5 electrolyte indicators were measured.

RESULTS

Among the three monkey species, the levels of sCD40L, IL-18, MCP-1, MIP-1β, TGFa, K+, Na+, and Cl- exhibited species-, sex-, and age-related differences. Comparison within the same species，sex had no significant impact on cytokine levels in NHPs but did affect electrolyte levels, particularly Cl- and Na+ levels, in cynomolgus monkeys and Assamese macaques. Electrolyte levels in NHPs were not affected by age, whereas the levels of certain cytokines, particularly sCD40L, GM-CSF, and IL-10, varied with age. The remaining 21 cytokines demonstrated no significant age-related changes.

CONCLUSIONS

Significant variations in cytokine and electrolyte levels exist among different monkey species, sexes, and age groups. This research provides valuable resources for NHP researchers and sets the stage for further exploring the impacts of sex and age on NHP physiology and immune function.

Parallel patterns of age-related working memory impairment in marmosets and macaques

As humans age, some experience cognitive impairment while others do not. When impairment does occur, it is not expressed uniformly across cognitive domains and varies in severity across individuals. Translationally relevant model systems are critical for understanding the neurobiological drivers of this variability, which is essential to uncovering the mechanisms underlying the brain's susceptibility to the effects of aging. As such, non-human primates (NHPs) are particularly important due to shared behavioral, neuroanatomical, and age-related neuropathological features with humans. For many decades, macaque monkeys have served as the primary NHP model for studying the neurobiology of cognitive aging. More recently, the common marmoset has emerged as an advantageous model for this work due to its short lifespan that facilitates longitudinal studies. Despite their growing popularity as a model, whether marmosets exhibit patterns of age-related cognitive impairment comparable to those observed in macaques and humans remains unexplored. To address this major limitation for the development and evaluation of the marmoset as a model of cognitive aging, we directly compared working memory ability as a function of age in macaques and marmosets on the identical task. We also implemented varying delays to further tax working memory capacity. Our findings demonstrate that marmosets and macaques exhibit remarkably similar age-related working memory deficits, with macaques performing better than marmosets on longer delays. These results highlight the similarities and differences between the two most commonly used NHP models and support the value of the marmoset as a model for cognitive aging research within the neuroscience community.

The aged female rhesus macaque as a translational model for human menopause and hormone therapy

Progress in understanding the causes of physiological and behavioral changes in post-menopausal women is hampered by the paucity of animal models that accurately recapitulate these age-associated changes. Here we evaluate the translational potential of female rhesus macaques (Macaca mulatta). Like women, these long-lived diurnal primates show marked neuroendocrine changes during aging, as well as perturbed sleep-wake cycles and cognitive decline. Furthermore, the brains of old rhesus macaques show some of the same pathological hallmarks of Alzheimer's disease as do humans, including amyloidosis and tauopathology. Importantly, unlike humans, rhesus macaques can be maintained under tightly controlled environmental conditions, such as photoperiod, temperature and diet, and tissues can be collected with zero postmortem interval; this makes them especially suitable for studies aimed at elucidating underlying molecular mechanisms. Recent findings from female macaques are helping to elucidate how sex-steroids influence gene expression within the brain and contribute to the maintenance of cognitive function and amelioration of age-associated pathologies. Taken together, these findings emphasize the translational value of female rhesus macaques as a model for elucidating causal mechanisms that underlie normative and pathological changes in post-menopausal women. They also provide a pragmatic platform upon which to develop safe and effective therapies.

Human Alzheimer's Disease ATN/ABC Staging Applied to Aging Rhesus Macaque Brains: Association With Cognition and MRI-Based Regional Gray Matter Volume

The brain changes of Alzheimer's disease (AD) include Abeta (Aβ) amyloid plaques ("A"), abnormally phosphorylated tau tangles ("T"), and neurodegeneration ("N"). These have been used to construct in vivo and postmortem diagnostic and staging classifications for evaluating the spectrum of AD in the "ATN" and "ABC" ("B" for Braak tau stage, "C" for Consortium to Establish a Registry for Alzheimer's Disease [CERAD] neuritic plaque density) systems. Another common AD feature involves cerebral amyloid angiopathy (CAA). We report the first experiment to examine relationships among cognition, brain distribution of amyloid plaques, CAA, tau/tangles, and magnetic resonance imaging (MRI)-determined volume changes (as a measure of "N") in the same group of behaviorally characterized nonhuman primates. Both ATN and ABC systems were applied to a group of 32 rhesus macaques aged between 7 and 33 years. When an immunohistochemical method for "T" and "B" was used, some monkeys were "triple positive" on ATN, with a maximum ABC status of A1B2C3. With silver or thioflavin S methods, however, all monkeys were classified as T-negative and B0, indicating the absence of mature neurofibrillary tangles (NFTs) and hence neuropathologically defined AD. Although monkeys at extremes of the ATN and ABC classifications, or with frequent CAA, had significantly lower scores on some cognitive tests, the lack of fully mature NFTs or dementia-consistent cognitive impairment indicates that fully developed AD may not occur in rhesus macaques. There were sex differences noted in the types of histopathology present, and only CAA was significantly related to gray matter volume.

The Common Marmoset as a Translational Model for Longitudinal Studies of Cognitive Aging and Individual Vulnerability to Decline

In humans, cognitive aging is highly variable, with some individuals experiencing decline while others remain stable, and different cognitive domains exhibiting uneven vulnerability to aging. The neural mechanisms driving this intra- and inter-individual variability are not fully understood, making longitudinal studies in translational models essential for elucidating the timelines and processes involved. The common marmoset (Callithrix jacchus), a short-lived nonhuman primate, offers an unprecedented opportunity to conduct longitudinal investigations of aging and age-related disease over a condensed time frame, in a highly translatable animal model. The potential of the marmoset as a model for cognitive aging is indisputable, but a comprehensive cognitive battery tailored for longitudinal aging studies has not yet been developed, applied, or validated. This represents a critical missing piece for evaluating the marmoset as a model and understanding the extent to which marmoset cognitive aging mirrors the patterns found in humans, including whether marmosets have individual variability in their vulnerability to age-related cognitive decline. To address this, we developed a comprehensive touchscreen-based neuropsychological test battery for marmosets (MarmoCog), targeting five cognitive domains: working memory, stimulus-reward association learning, cognitive flexibility, motor speed, and motivation. We tested a large cohort of marmosets, ranging from young adults to geriatrics, over several years. We found significant variability in cognitive aging, with the greatest decline occurring in domains dependent on the prefrontal cortex and hippocampus. Additionally, we observed significant inter-individual variability in vulnerability to age-related cognitive decline: some marmosets declined across multiple domains, others in just one, and some showed no decline at all. This pattern mirrors human cognitive aging, solidifies the marmoset as an advantageous model for age-related cognitive decline, and provides a strong foundation for identifying the neural mechanisms involved.

Parallel patterns of cognitive aging in marmosets and macaques

As humans age, some experience cognitive impairment while others do not. When impairment does occur, it is not expressed uniformly across cognitive domains and varies in severity across individuals. Translationally relevant model systems are critical for understanding the neurobiological drivers of this variability, which is essential to uncovering the mechanisms underlying the brain's susceptibility to the effects of aging. As such, non-human primates are particularly important due to shared behavioral, neuroanatomical, and age-related neuropathological features with humans. For many decades, macaque monkeys have served as the primary non-human primate model for studying the neurobiology of cognitive aging. More recently, the common marmoset has emerged as an advantageous model for this work due to its short lifespan that facilitates longitudinal studies. Despite their growing popularity as a model, whether marmosets exhibit patterns of age-related cognitive impairment comparable to those observed in macaques and humans remains unexplored. To address this major limitation for the development and evaluation of the marmoset as a model of cognitive aging, we directly compared working memory ability as a function of age in macaques and marmosets on the identical working memory task. Our results demonstrate that marmosets and macaques exhibit remarkably similar age-related working memory deficits, highlighting the value of the marmoset as a model for cognitive aging research within the neuroscience community.

Cognitive Aging and the Primate Basal Forebrain Revisited: Disproportionate GABAergic Vulnerability Revealed

Basal forebrain (BF) projections to the hippocampus and cortex are anatomically positioned to influence a broad range of cognitive capacities that are known to decline in normal aging, including executive function and memory. Although a long history of research on neurocognitive aging has focused on the role of the cholinergic basal forebrain system, intermingled GABAergic cells are numerically as prominent and well positioned to regulate the activity of their cortical projection targets, including the hippocampus and prefrontal cortex. The effects of aging on noncholinergic BF neurons in primates, however, are largely unknown. In this study, we conducted quantitative morphometric analyses in brains from young adult (6 females, 2 males) and aged (11 females, 5 males) rhesus monkeys (Macaca mulatta) that displayed significant impairment on standard tests that require the prefrontal cortex and hippocampus. Cholinergic (ChAT+) and GABAergic (GAD67+) neurons were quantified through the full rostrocaudal extent of the BF. Total BF immunopositive neuron number (ChAT+ plus GAD67+) was significantly lower in aged monkeys compared with young, largely because of fewer GAD67+ cells. Additionally, GAD67+ neuron volume was greater selectively in aged monkeys without cognitive impairment compared with young monkeys. These findings indicate that the GABAergic component of the primate BF is disproportionally vulnerable to aging, implying a loss of inhibitory drive to cortical circuitry. Moreover, adaptive reorganization of the GABAergic circuitry may contribute to successful neurocognitive outcomes.SIGNIFICANCE STATEMENT A long history of research has confirmed the role of the basal forebrain in cognitive aging. The majority of that work has focused on BF cholinergic neurons that innervate the cortical mantle. Codistributed BF GABAergic populations are also well positioned to influence cognitive function, yet little is known about this prominent neuronal population in the aged brain. In this unprecedented quantitative comparison of both cholinergic and GABAergic BF neurons in young and aged rhesus macaques, we found that neuron number is significantly reduced in the aged BF compared with young, and that this reduction is disproportionately because of a loss of GABAergic neurons. Together, our findings encourage a new perspective on the functional organization of the primate BF in neurocognitive aging.

Integrated multi-omics analysis of brain aging in female nonhuman primates reveals altered signaling pathways relevant to age-related disorders

The prefrontal cortex (PFC) has been implicated as a key brain region responsible for age-related cognitive decline. Little is known about aging-related molecular changes in PFC that may mediate these effects. To date, no studies have used untargeted discovery methods with integrated analyses to determine PFC molecular changes in healthy female primates. We quantified PFC changes associated with healthy aging in female baboons by integrating multiple omics data types (transcriptomics, proteomics, metabolomics) from samples across the adult age span. Our integrated omics approach using unbiased weighted gene co-expression network analysis to integrate data and treat age as a continuous variable, revealed highly interconnected known and novel pathways associated with PFC aging. We found Gamma-aminobutyric acid (GABA) tissue content associated with these signaling pathways, providing 1 potential biomarker to assess PFC changes with age. These highly coordinated pathway changes during aging may represent early steps for aging-related decline in PFC functions, such as learning and memory, and provide potential biomarkers to assess cognitive status in humans.