Network Models Predict That Pyramidal Neuron Hyperexcitability and Synapse Loss in the dlPFC Lead to Age-Related Spatial Working Memory Impairment in Rhesus Monkeys

Immune proteins C1q and CD47 may contribute to aberrant microglia-mediated synapse loss in the aging monkey brain that is associated with cognitive impairment

Cognitive impairment in learning, memory, and executive function occurs in normal aging even in the absence of Alzheimer's disease (AD). While neurons do not degenerate in humans or monkeys free of AD, there are structural changes including synapse loss and dendritic atrophy, especially in the dorsolateral prefrontal cortex (dlPFC), and these correlate with cognitive age-related impairment. Developmental studies revealed activity-dependent neuronal properties that lead to synapse remodeling by microglia. Microglia-mediated phagocytosis that may eliminate synapses is regulated by immune "eat me" and "don't eat me" signaling proteins in an activity-dependent manner, so that less active synapses are eliminated. Whether this process contributes to age-related synapse loss remains unknown. The present study used a rhesus monkey model of normal aging to investigate the balance between the "eat me" signal, complement component C1q, and the "don't eat me" signal, transmembrane glycoprotein CD47, relative to age-related synapse loss in dlPFC Area 46. Results showed an age-related elevation of C1q and reduction of CD47 at PSD95+ synapses that is associated with cognitive impairment. Additionally, reduced neuronal CD47 RNA expression was found, indicating that aged neurons were less able to produce the protective signal CD47. Interestingly, microglia do not show the hypertrophic morphology indicative of phagocytic activity. These findings suggest that in the aging brain, changes in the balance of immunologic proteins give microglia instructions favoring synapse elimination of less active synapses, but this may occur by a process other than classic phagocytosis such as trogocytosis.

Myelin dystrophy in the aging prefrontal cortex leads to impaired signal transmission and working memory decline: a multiscale computational study

Normal aging leads to myelin alternations in the rhesus monkey dorsolateral prefrontal cortex (dlPFC), which are often correlated with cognitive impairment. It is hypothesized that remyelination with shorter and thinner myelin sheaths partially compensates for myelin degradation, but computational modeling has not yet explored these two phenomena together systematically. Here, we used a two-pronged modeling approach to determine how age-related myelin changes affect a core cognitive function: spatial working memory. First we built a multicompartment pyramidal neuron model fit to monkey dlPFC data, with axon including myelinated segments having paranodes, juxtaparanodes, internodes, and tight junctions, to quantify conduction velocity (CV) changes and action potential (AP) failures after demyelination and subsequent remyelination in a population of neurons. Lasso regression identified distinctive parameter sets likely to modulate an axon's susceptibility to CV changes following demyelination versus remyelination. Next we incorporated the single neuron results into a spiking neural network model of working memory. While complete remyelination nearly recovered axonal transmission and network function to unperturbed levels, our models predict that biologically plausible levels of myelin dystrophy, if uncompensated by other factors, can account for substantial working memory impairment with aging. The present computational study unites empirical data from electron microscopy up to behavior on aging, and has broader implications for many demyelinating conditions, such as multiple sclerosis or schizophrenia.

Neuronal properties of pyramidal cells in lateral prefrontal cortex of the aging rhesus monkey brain are associated with performance deficits on spatial working memory but not executive function

Age-related declines in cognitive abilities occur as early as middle-age in humans and rhesus monkeys. Specifically, performance by aged individuals on tasks of executive function (EF) and working memory (WM) is characterized by greater frequency of errors, shorter memory spans, increased frequency of perseverative responses, impaired use of feedback and reduced speed of processing. However, how aging precisely differentially impacts specific aspects of these cognitive functions and the distinct brain areas mediating cognition are not well understood. The prefrontal cortex (PFC) is known to mediate EF and WM and is an area that shows a vulnerability to age-related alterations in neuronal morphology. In the current study, we show that performance on EF and WM tasks exhibited significant changes with age, and these impairments correlate with changes in biophysical properties of layer 3 (L3) pyramidal neurons in lateral LPFC (LPFC). Specifically, there was a significant age-related increase in excitability of L3 LPFC pyramidal neurons, consistent with previous studies. Further, this age-related hyperexcitability of LPFC neurons was significantly correlated with age-related decline on a task of WM, but not an EF task. The current study characterizes age-related performance on tasks of WM and EF and provides insight into the neural substrates that may underlie changes in both WM and EF with age.

Age-related alterations to working memory and to pyramidal neurons in the prefrontal cortex of rhesus monkeys begin in early middle-age and are partially ameliorated by dietary curcumin

Layer 3 (L3) pyramidal neurons in aged rhesus monkey lateral prefrontal cortex (LPFC) exhibit significantly elevated excitability in vitro and reduced spine density compared to neurons in young subjects. The time-course of these alterations, and whether they can be ameliorated in middle age by the powerful anti-oxidant curcumin is unknown. We compared the properties of L3 pyramidal neurons from the LPFC of behaviorally characterized rhesus monkeys over the adult lifespan using whole-cell patch clamp recordings and neuronal reconstructions. Working memory (WM) impairment, neuronal hyperexcitability, and spine loss began in middle age. There was no significant relationship between neuronal properties and WM performance. Middle-aged subjects given curcumin exhibited better WM performance and less neuronal excitability compared to control subjects. These findings suggest that the appropriate time frame for intervention for age-related cognitive changes is early middle age, and points to the efficacy of curcumin in delaying WM decline. Because there was no relationship between excitability and behavior, the effects of curcumin on these measures appear to be independent.

Comparative neuropathology in aging primates: A perspective

While humans exhibit a significant degree of neuropathological changes associated with deficits in cognitive and memory functions during aging, non-human primates (NHP) present with more variable expressions of pathological alterations among individuals and species. As such, NHP with long life expectancy in captivity offer an opportunity to study brain senescence in the absence of the typical cellular pathology caused by age-related neurodegenerative illnesses commonly seen in humans. Age-related changes at neuronal population, single cell, and synaptic levels have been well documented in macaques and marmosets, while age-related and Alzheimer's disease-like neuropathology has been characterized in additional species including lemurs as well as great apes. We present a comparative overview of existing neuropathologic observations across the primate order, including classic age-related changes such as cell loss, amyloid deposition, amyloid angiopathy, and tau accumulation. We also review existing cellular and ultrastructural data on neuronal changes, such as dendritic attrition and spine alterations, synaptic loss and pathology, and axonal and myelin pathology, and discuss their repercussions on cellular and systems function and cognition.

Cognitive Reserve in Model Systems for Mechanistic Discovery: The Importance of Longitudinal Studies

The goal of this review article is to provide a resource for longitudinal studies, using animal models, directed at understanding and modifying the relationship between cognition and brain structure and function throughout life. We propose that forthcoming longitudinal studies will build upon a wealth of knowledge gleaned from prior cross-sectional designs to identify early predictors of variability in cognitive function during aging, and characterize fundamental neurobiological mechanisms that underlie the vulnerability to, and the trajectory of, cognitive decline. Finally, we present examples of biological measures that may differentiate mechanisms of the cognitive reserve at the molecular, cellular, and network level.