“Exploring the Role of the Gut Microbiome in Mediating Sex-Specific Phenotypes
in Mouse Models of Alzheimer’s Disease”
​
Sangram S. Sisodia, Ph.D.
Thomas Reynolds Sr. Family Professor of Neurosciences
Director, Center for Molecular Neurobiology
Departments of Neurobiology and Neurology
The University of Chicago
947 E. 58th St. MC0928
Chicago, Il 60637
​
Summary of the research:
Since 2016, research in our laboratory has focused on understanding the role of the gut microbiome in modulating the deposition of pathological amyloid (Aβ) peptides in plaques and associated neuroinflammation that is driven by innate immune cells, termed microglia. Our lab published the first study showing that antibiotic (ABX)-mediated perturbations of gut microbiota significantly reduces amyloid burden and neuroinflammation in the brains of independent transgenic mouse strains that express mutant genes that cause familial Alzheimer’s disease (AD). Surprisingly, these alterations only occur in male, but not female mice. Our recent studies have documented that estrogen plays a dominant role in driving amyloidosis and neuroinflammation in female mice. We now seek to assess changes in levels of metabolites and chemokines in peripheral and central compartments, perform deep sequencing and metagenomics to identify specific bacterial species, and transcriptomic approaches to understand the mechanism(s) by which microglial cells in brains of ABX- treated mice exert the observed sex-specific effects that impact amyloid deposition and neuroinflammation.
​
A more detailed lay abstract
Alzheimer’s disease (AD) is a progressive neurodegenerative disease characterized by the extracellular deposition of amyloid beta (Aβ) plaques, the formation of hyperphosphorylated tau-based intracellular neurofibrillary tangles (NFT), and neuroinflammation in the central nervous system (CNS). Studies have shown that microglia cells, the innate immune cells of the CNS are activated in brains of patients with AD and the activation occurs before neuronal loss and cognitive deficits become evident. Microglia cells constantly monitor their milieu through a set of genes that regulate principal functions of these cells related to synaptic remodeling, phagocytosis and clearance. Several studies have demonstrated that a population of activated microglia, termed disease-associated microglia (DAM) cells are present in transgenic AD models and other models of neurodegenerative disease. These DAM microglial cells are found in close proximity to Aβ plaques and exhibit a distinctive transcriptomic signature in mouse models. With AD progression, these cells show reduced expression of certain “homeostatic” genes, and increased expression of “disease-associated” genes and hence, it has been argued that the transition from a homeostatic status to an activated status implicates these cells as principal drivers of AD onset and progression.
​
It has been reported that AD patients have altered gut microbiota and gut microbiota are known to have a profound impact on microglial cell function. In 2016, our lab first showed that broad-spectrum antibiotic (ABX)-mediated perturbations of gut microbiota significantly reduces amyloid burden in the brains of two independent transgenic mouse models of AD. Remarkably, only male, but not female mice showed a dramatic reduction in Aβ plaques, alterations in microglial morphology and the microglial transcriptome.
​
It has been established that the incidence of AD is higher in women than in men, suggesting that there are sex-specific pathogenic mechanism(s) that leads to AD. Interestingly, sex related differences in the gut microbiome are observed in humans, and recent studies have posited a role for the gut microbiome in AD. Late-onset AD risk genes are highly expressed in microglia, the innate immune cells in the brain, and it has been reported that the gut microbiome influences microglial development in a sex-specific manner. While the mechanism(s) underlying these sex specific differences are unknown, it is clear that sex hormones can modulate gut microbiota composition. Indeed, we have recently reported that female mice treated with ABX exhibit significantly elevated levels of circulating estrogen and that ovariectomy, a procedure that lowers circulating estrogen, also leads to alterations in gut microbiome composition that iscoincident with a reduction in Aβ deposition and neuroinflammation. The goal of this proposal is to test our central hypothesis that sex-specific regulation of microglia via the gut microbiome is a mechanism that drives AD pathogenesis in females.
​
Our published studies have revealed that ABX-treatment and OVX leads to changes in the ratios of dominant bacterial phyla, termed Firmicutes and Bacteroidetes. Within these phyla are a group of bacteria that constitute the “estrabolome”…bacteria that directly influence the levels of circulating estrogen. Microbes in the estrobolome express a gene encoding β-glucuronidase, an enzyme that modifies estrogens into their active forms, that in turn bind to estrogen receptors on many cell types, including microglia, that influence estrogen-dependent physiological processes. In addition to variable levels of β-glucuronidase activity, overall intestinal microbial richness also influences the balance of estrogens that circulate in the body. This dysbiosis can lead to either a deficiency, or an excess of, free estrogen and imbalances between the various forms of estrogen and other hormones, which might promote the development of estrogen-related pathologies and chronic diseases, including AD.
​
Hence, our FIRST AIM is to carry out metagenomic studies to identify the bacterial species that contribute to alterations in F/B ratios and ultimately to the disease progression. Our SECOND AIM is to identify metabolites and cytokines in the plasma and brain tissue in female animals subject to ABX treatment that might influence the biology of peripheral immune cells and microglia. The THIRD AIM is to expand our knowledge of the transcriptome of microglia from ABX- and OVX-treated animals using single-cell RNA sequencing and spatial transcriptomics.
​
In summary, our proposed project aims to understand the mechanism(s) by which the gut microbiome metabolizes hormones, and particularly estrogen metabolites, to regulate the circulating levels of this hormone that in turn, influences AD-type pathology and neuroinflammation.
Brief summary as to how the funds will be used.
Funds will be used to cover a small portion of salary and fringe for both Dr. Sisodia and a technician. Funds will also be used to cover general lab supplies, animal expenses and bacterial DNA sequencing (metagenomics) and microglial transcriptomic studies.