Obesity Induced Microglial Activation and its Role in Hippocampal Dysfunction

Abstract

The prevalence of global obesity has rapidly risen over the past several decades. Management of obesity and its co-morbidities place an immense burden on patients and healthcare systems. Clinical studies show an association between mid-life obesity and late-life cognitive impairment and dementia, and childhood obesity is associated with cognitive deficits as well. A better understanding of mechanisms mediating cognitive dysfunction in obesity across the lifespan will be critical for developing appropriate therapeutic interventions.

Rodent models of high-diet diet (HFD)-induced obesity demonstrate HFD induced inflammation in the hippocampus, a brain region involved in declarative memory. The immune response contributes to complications of obesity and may play a role in obesity-associated hippocampal-dependent cognitive impairment. Hippocampal microglia, the resident innate immune cells of the central nervous system, are activated in obese mice, and are proposed to contribute to cognitive deficits by excessive pruning of neuronal dendritic spines. Mechanisms responsible for this microglial activation are unclear. The goal of this dissertation was to determine the temporal progression and distinct activation phenotype of hippocampal microglia and to identify potential mechanisms mediating activation, focusing on young, adolescent mice.

To address this goal, we proposed four aims. For Aim 1, we fed adolescent mice control or HFD longitudinally for 2 wk, 1 mo, or 3 mo and used three-dimensional morphology measures to quantify microglial morphology as an indicator of activation state. HFD feeding induced obesity and metabolic dysfunction, which both increased in severity over time. Unexpectedly, in the hippocampal CA1 region, HFD did not alter microglia morphology metrics at any duration of diet, suggesting that effects on activation state may be subtle. Given potentially subtle effects, and because microglia manifest a heterogenous population of cells, we moved beyond morphology measures for Aim 2 and performed single-cell RNA-sequencing to investigate the activation phenotype of hippocampal microglia after 1 mo and 3 mo of control or HFD. Our work, to our knowledge, is the first scRNA-seq study of microglia in obesity, and transcriptomics revealed that HFD feeding alters intercellular immune signaling among microglia and dysregulates endoplasmic reticulum homeostasis.

To investigate the effect of age, in Aim 3 we determined the impact of HFD on cognitive function and hippocampal inflammation in young and middle-aged mice. We found that obese mice are cognitively impaired using the fear conditioning task, which measures associative learning, and that older age exacerbates performance deficits in this task. Our final set of ongoing experiments in Aim 4 seek to determine the role of endoplasmic reticulum stress in microglial activation in obesity.

In summary, our data demonstrate that diet-induced obesity in young mice alters the hippocampal microglial transcriptome but does not change microglial morphology in the CA1 region. We identified endoplasmic reticulum dysregulation as a potential mechanism mediating the microglia response. Future work is required to determine whether a potential ER stress response plays a protective or injurious role in microglial activation and subsequent cognitive impairment. Further, we found that older obese mice display more severe cognitive deficits, suggesting that young age may protect against pathology. This dissertation builds a foundation to better understand critical periods across the lifespan for tackling cognitive deficits in obesity.