Temporal emergence of age-associated changes in cognitive and physical function in vervets (Chlorocebus aethiops sabaeus)

Using non-invasive behavioral and physiological data to measure biological age in wild baboons

Biological aging is near-ubiquitous in the animal kingdom, but its timing and pace vary between individuals and over lifespans. Prospective, individual-based studies of wild animals-especially non-human primates-help identify the social and environmental drivers of this variation by indicating the conditions and exposure windows that affect aging processes. However, measuring individual biological age in wild primates is challenging because several of the most promising methods require invasive sampling. Here, we leverage observational data on behavior and physiology, collected non-invasively from 319 wild female baboons across 2402 female-years of study, to develop a composite predictor of age: the non-invasive physiology and behavior (NPB) clock. We found that age predictions from the NPB clock explained 51% of the variation in females' known ages. Further, deviations from the clock's age predictions predicted female survival: females predicted to be older than their known ages had higher adult mortality. Finally, females who experienced harsh early-life conditions were predicted to be about 6 months older than those who grew up in more benign conditions. While the relationship between early adversity and NPB age is noisy, this estimate translates to a predicted 2-3 year reduction in mean adult lifespan in our model. A constraint of our clock is that it is tailored to data collection approaches implemented in our study population. However, many of the clock's components have analogs in other populations, suggesting that non-invasive data can provide broadly applicable insight into heterogeneity in biological age in natural populations.

Development and assessment of a stair ascension challenge as a measure of aging and physical function in nonhuman primates

Nonhuman primates (NHPs) are valuable models for studying healthspan, including frailty development. Frailty metrics in people centers on functional measures, including usual gait speed which can be predictive of all-cause mortality. This concept that physical competencies are able to prognosticate an individual's health trajectory over chronologic aging is well-accepted and has led to refinements in how physical function is evaluated, and include measures of strength and power along with walking speed. NHP studies of aging require evaluation of physical function, which can be difficult in field and research settings. We compared stair climb velocity to usual walking speed in 28 peri-geriatric to geriatric NHPs, as incorporating a climbing obstacle integrates multiple components of physical function: isolated leg and back strength, proprioception, balance, and range of motion. We find that stair climbing speed was reliable between observers, and whether timing was in-person take from video capture. The stair climb rates were 50% more associated with chronological age than walking speed (R = -0.68 vs. -0.45) and only stair climbing speeds were retained as predictive of age when walking speed and bodyweight were included in multivariate models (overall R2  = 0.44; p < 0.0001). When comparing young (10-16 years) versus geriatric (16-29 years) stair climbing speed was significantly different (p < 0.001), while walking speeds only tended to be slower (p = 0.12) suggesting that the additional challenge of a stair climb better unmasks subclinical frailty development that usual walking speed.

Executive function mediates age-related variation in social integration in female vervet monkeys (Chlorocebus sabaeus)

In humans, social participation and integration wane with advanced age, a pattern hypothesized to stem from cognitive or physical decrements. Similar age-related decreases in social participation have been observed in several nonhuman primate species. Here, we investigated cross-sectional age-related associations between social interactions, activity patterns, and cognitive function in 25 group-living female vervets (a.k.a. African green monkeys, Chlorocebus sabaeus) aged 8-29 years. Time spent in affiliative behavior decreased with age, and time spent alone correspondingly increased. Furthermore, time spent grooming others decreased with age, but the amount of grooming received did not. The number of social partners to whom individuals directed grooming also decreased with age. Grooming patterns mirrored physical activity levels, which also decreased with age. The relationship between age and grooming time was mediated, in part, by cognitive performance. Specifically, executive function significantly mediated age's effect on time spent in grooming interactions. In contrast, we did not find evidence that physical performance mediated age-related variation in social participation. Taken together, our results suggest that aging female vervets were not socially excluded but decreasingly engaged in social behavior, and that cognitive deficits may underlie this relationship.

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.

A morphological study and the variability in the number of infraorbital foramina in the African green monkey (Grivet) (Chlorocebus aethiops) using microcomputed tomography

Knowledge of the nonhuman primate morphology and anatomy related to craniofacial mechanoreception is essential for a fundamental understanding of the incidents that have occurred during the evolution of craniofacial features. The present study focuses on the variability in the number of infraorbital foramina and associated anatomical structures such as the infraorbital canal (IOC) and the infraorbital groove (IOG), as they are considered to play an important role in the behavioral ecology of these animals. A total of 19 skulls of Chlorocebus aethiops were analyzed. The number of infraorbital foramina was assessed macroscopically using a magnifying glass and a small diameter probe. Three dimensional (3D) projections and morphometric analysis of the infraorbital foramina, IOCs, and IOGs were performed using microcomputed tomography (micro-CT) for two skulls that represent one of the most common morphological types. Regardless of sex and body side, the most common morphological type observed in the studied species is the presence of three infraorbital foramina. The IOC takes a funnel or pinched shape. 3D projections were made to assess the course of the infraorbital vascular and nerve bundles in selected individuals. The results indicate a high morphological diversity within the species, although there appears to be a consistent distribution pattern of infraorbital neurovascular bundles in species of the Cercopithecidae family. The use of X-ray micro-CT allowed 3D visualization of the maxillary region to determine the variability of the infraorbital foramina and to track the division of the infraorbital neurovascular bundle in the case of the most common macroscopic expression of the number of the infraorbital foramen in C. aethiops, as well as the morphometric of the IOCs and IOGs which are related to mechanoreception of the primate's snout.

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 (3.5-13.6 years) and 71 aged (19.9-32.5 years of age 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 three 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 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. (181 words).

Transcriptional profiles in olfactory pathway-associated brain regions of African green monkeys: Associations with age and Alzheimer's disease neuropathology

Introduction

Olfactory impairment in older individuals is associated with an increased risk of Alzheimer's disease (AD). Characterization of age versus neuropathology‐associated changes in the brain olfactory pathway may elucidate processes underlying early AD pathogenesis. Here, we report age versus AD neuropathology–associated differential transcription in four brain regions in the olfactory pathway of 10 female African green monkeys (vervet, Chlorocebus aethiops sabaeus), a well‐described model of early AD‐like neuropathology.

Methods

Transcriptional profiles were determined by microarray in the olfactory bulb (OB), piriform cortex (PC), temporal lobe white matter (WM), and inferior temporal cortex (ITC). Amyloid beta (Aβ) plaque load in parietal and temporal cortex was determined by immunohistochemistry, and concentrations of Aβ42, Aβ40, and norepinephrine in ITC were determined by enzyme‐linked immuosorbent assay (ELISA). Transcriptional profiles were compared between middle‐aged and old animals, and associations with AD‐relevant neuropathological measures were determined.

Results

Transcriptional profiles varied by brain region and age group. Expression levels of TRO and RNU4‐1 were significantly lower in all four regions in the older group. An additional 29 genes were differentially expressed by age in three of four regions. Analyses of a combined expression data set of all four regions identified 77 differentially expressed genes (DEGs) by age group. Among these DEGs, older subjects had elevated levels of CTSB, EBAG9, LAMTOR3, and MRPL17, and lower levels of COMMD10 and TYW1B. A subset of these DEGs was associated with neuropathology biomarkers. Notably, CTSB was positively correlated with Aβ plaque counts, Aβ42:Aβ40 ratios, and norepinephrine levels in all brain regions.

Discussion

These data demonstrate age differences in gene expression in olfaction‐associated brain regions. Biological processes exhibiting age‐related enrichment included the regulation of cell death, vascular function, mitochondrial function, and proteostasis. A subset of DEGs was specifically associated with AD phenotypes. These may represent promising targets for future mechanistic investigations and perhaps therapeutic intervention.

Aging-related Alzheimer's disease-like neuropathology and functional decline in captive vervet monkeys (Chlorocebus aethiops sabaeus)

Age-related neurodegeneration characteristic of late-onset Alzheimer's disease (LOAD) begins in middle age, well before symptoms. Translational models to identify modifiable risk factors are needed to understand etiology and identify therapeutic targets. Here, we outline the evidence supporting the vervet monkey (Chlorocebus aethiops sabaeus) as a model of aging-related AD-like neuropathology and associated phenotypes including cognitive function, physical function, glucose handling, intestinal physiology, and CSF, blood, and neuroimaging biomarkers. This review provides the most comprehensive multisystem description of aging in vervets to date. This review synthesizes a large body of evidence that suggests that aging vervets exhibit a coordinated suite of traits consistent with early AD and provide a powerful, naturally occurring model for LOAD. Notably, relationships are identified between AD-like neuropathology and modifiable risk factors. Gaps in knowledge and key limitations are provided to shape future studies to illuminate mechanisms underlying divergent neurocognitive aging trajectories and to develop interventions that increase resilience to aging-associated chronic disease, particularly, LOAD.

Walking speed declines with age in male and female baboons (Papio sp.): Confirmation of findings with sex as a biological variable

We measured walking speed in baboons (67 female, 36 male; 5-22 years) to develop regression formulas to predict biological age. The final model strongly predicted age from just speed and sex. Walking speed is a valuable baboon aging biomarker. We present the first male speed data in a nonhuman primate.