Cholinergic tone and microglial dynamics in aging​

Microglia are the resident immune cells of the CNS involved in the regulation of synaptic development and function through neuroimmune signaling. Microglia express receptors for the neurotransmitter acetylcholine (ACh), which is known to directly impact their inflammatory activity. Changes in ACh levels occurring during aging may contribute to neurodegenerative disease. We hypothesize age-related changes in ACh transmission and microglia number in the primate brain. Specifically, we predict interaction between ACh and microglia within the dentate gyrus (DG), a region critical for learning and neurogenesis. Using high-resolution images of brain sections of the DG from Macaca mulatta (Macaques), we quantified changes in ACh and microglia expression based on acetylcholinesterase (AChE) and ionized-calcium binding adaptor molecule 1 (IBA1) immunolabeling, respectively. We conducted analysis across DG subregions: granule cell layer, GCL; Hilus, Hil; inner molecular layer, IML; outer molecular layer, OML. Our results indicate an effect of age on AChE and microglial marker expression suggesting a role for cholinergic neuroimmune modulation in memory decline.

Hi, my name is Sajud Aladhab and I’m a senior at George Mason University majoring in neuroscience and my projects on cholinergic tone and microglia dynamics in aging. So some background about my project. My project focuses primarily on the hippocampus which is a region in the brain located in this area and the hippocampus is essential for learning and memory and is one of the first regions affected by aging and neurodegenerative diseases such as Alzheimer’s. As humans age, acetylcholine signaling declines which causes a weakening of synaptic plasticity as well as connection between neurons. As this occurs, it has been seen that microglia the brain immune cells become more active which causes an increase in neuroinflammation. This may be due to microglia containing acetylcholine receptors such as alpha 7 which allow acetylcholine to directly influence their activation and inflammatory state. So some context about my project. So my project uses two key markers in the hippocampus. The first being IBA1 which is this lighter stain here and that labels microglia, the brain immune cells and acetylcholinesterase which is cholinergic marker which is this dark stain here and it determines where acetylcholinesterase which is an enzyme breaks down acetylcholine. The region that I chose to parcellate and quantify in the hippocampus was the dentate gyrus. The reason as to why I chose this region in particular is because it’s crucial for memory encoding as well as being extremely sensitive to neurodegenerative diseases such as Alzheimer’s. So my hypothesis and goals. So my goals are to compare the dentate gyrus across infancy, adulthood, and aging to see how cholinergic and microglia signalings change over the lifespan and to track cholinergic changes across the macaque ages and to determine whether or not there’s a correlation with microgilla density. And my hypothesis is as aging occurs, it should produce a significant change in acetylcholinesterase and microgilla density and the patterns should suggest an interaction between cholinergic tone and microglia activity in the dentate gyrus.

So methods and how my project was done. So my project included the use of macaque hippocampal sections which were stained for AChE and IBA1. The dentate gyrus was then parcellated into its sub regions which included the granule cellular layer, the hilus, the inner molecular layer and the outer molecular layer. ImageJ and QuPath paths were then used for thresholding, normalization and quantification of the marker intensities for AChE and IBA1 across their ages respectively.

So my results so in the IBA1 stain across ages there seemed to be fluctuations which suggested microglia remodeling in early development and then a drop in density in aging in the aged groups. In the hilus there seemed to be variable microglial levels throughout the whole lifespan which reflected how dynamic this region was and in the molecular layer it seemed to be stable throughout which suggested that microgilla had low structural or immunity demands here. In AChE across aging the granule cellular layer, the hilus and the outer molecular layer had similar results where cholinergic tone slowly rose during development and peaked in adulthood and then dropped to lower levels in the aged groups. In the inner molecular layer it was a bit different where there wasn’t any early developmental significance but it did peak in adulthood like right here and once again it dropped in cholinergic tone when it when it reached the aged groups. So implications and what this means so in development AChE increased across the entire dentate gyrus while microglia changes were subregional specific like in the granule cellular layer and in the hilus. In aging AChE declines were in aged groups and correlated to microgilla declines mainly in the granule cellular layer. Overall, these different aging patterns suggested interacting changes that may contribute to dentate gyrus vulnerability and memory decline. And I would like to acknowledge the OSCAR undergraduate research program at George Mason University for funding this project.