In humans, severe cognitive impairment with neuron loss occurs in Alzheimer's Disease (AD) but cognitive decline also occurs in normal healthy aging without neurodegeneration. The rhesus monkey is a valuable model of normal healthy aging as it does not develop the AD pathology but does develop age-related cognitive impairments (Herndon et al, 1997; Moore et al, 2006) which parallel human impairments (Fjell & Walhovd, 2010; Moss et al, 2007). In searching for the cause of cognitive aging, we and others have shown that neurons are not lost with age (Peters et al, 1996; Morrison & Hof 1997) but instead myelin pathology develops, white matter is lost and both correlate with cognitive decline ?(Makris 2007; Wisco et al, 2008; Bowley et al., 2010; Peters & Kemper, 2012). Importantly, normal aging in humans without AD also show neuron preservation with white matter damage (Freeman et al, 2008; Guttmann et al., 1998; Marner et al., 2003; Raz 2003). Possible causes of myelin pathology are inflammation and oxidative stress that damage myelin or dysfunction of oligodendrocytes that maintain it. Phagocytosis may also be a factor as improper clearance of myelin by microglia inhibits normal remyelination (Lampron et al. 2015). Indeed, we found that microglial phagocytic activation increases with age and correlates with cognitive decline (Shobin et al., 2017). Here we propose three aims to study behaviorally tested aging monkeys of both sexes from 5 to over 30 years of age from three sources: Normal aging (NA-1) monkeys acquired and tested for this project (N=25), data and archived tissue samples from normal aging (NA- 2) monkeys studied here over the past decade (N=50), monkeys from the NIA intramural calorie restriction (CR) study (n=30, 17 brains on hand plus12 - 15 expected over 5 years).
Aim 1 will use fresh frozen NA-1&2 tissue to assess gene expression in white matter and fresh NA-1 samples to isolate oligodendroglia and microglia for RNA expression. It will also use fixed tissue from NA-1&2 to verify the cellular correlates of these effects.
Aim 2 will use fresh NA-1 samples of cingulate cortex as a model system to investigate with in vitro slice neurophysiology how myelin pathology affects neuronal signaling and follow-up with immunohistochemistry (IHC) and high resolution confocal microscopy on fixed slices to quantify effects on spines and synapses as well as synaptic pruning by microglia.
Aim 3 will compare CR monkeys to NA-1&2 to determine if CR decreases inflammation and protects against both myelin pathology and cognitive decline. To this end, monkeys at NIA are currently being tested on our cognitive testing battery, and upon their death we receive one fixed hemisphere and fresh frozen frontal slabs from the other. In fresh frozen tissue, we will evaluate gene expression with microarray and targeted qPCR. In fixed tissue we will use IHC to assess myelin damage and markers of oligodendroglia and microglia function. These studies will provide new insights into mechanisms of age-related myelin pathology and determine if CR reduces myelin pathology and cognitive decline of normal aging.
In normal healthy aging humans who escape Alzheimer's disease or other neurodegenerative conditions, there is a progressive decline in cognitive functions from memory to problem solving. In the rhesus monkey model of normal human aging we have found that while neurons do not die, the myelinated axons of the white matter develop significant pathology that appears to lead to cognitive impairment. Here we propose a series of studies in the normal aging rhesus monkey to determine: 1) how dysfunction in the oligodendroglia that make myelin and in the microglia that clean up myelin debris may lead to myelin pathology; 2) how this myelin pathology affects neural circuitry; and 3) whether, and by what process, long term calorie restriction may reduce myelin pathology and slow age-related cognitive decline.
