Chemogenetic Manipulation of Microglia: Effects on neuronal structure and function

Abstract

Microglia, the resident immune cells of the central nervous system (CNS), play a critical role in maintaining homeostasis, immune surveillance, and modulating neuronal function. They are highly dynamic cells capable of responding to environmental changes by altering their morphology and function. Recent research has highlighted that microglia are not only involved in immune responses but also play a pivotal role in synaptic plasticity, synaptic pruning, and overall cognitive processes. Activation of microglia, whether by inflammatory processes or chronic diseases, has profound consequences on neurons and synapses, leading to alterations in synaptic strength, connectivity, and ultimately cognitive functions. However, the detailed mechanisms underlying microglia-neuron communication remain under active investigation. Traditional methods to activate microglia, such as pharmacological agents like lipopolysaccharide (LPS) or genetic approaches including global gene knockout models, can be non-specific and affect multiple cell types, making it difficult to isolate the direct role of microglia in neuronal modulation. These methods also typically lack precise timing, further complicating our understanding of microglia's direct influence. To address these challenges, I took a more targeted approach by directly activating microglia using a chemogenetic tool to target the Gq signaling pathway. In this dissertation, I employed a chemogenetic strategy with the tamoxifen-inducible Cre-ER loxP system to selectively express Gq-DREADD (Designer Receptors Exclusively Activated by Designer Drugs) in microglia. This approach allowed us to activate microglia in a controlled and targeted way, avoiding unwanted effects on other brain cells. The activation of the Gq pathway led to changes in microglial morphology, specifically in reduced branching complexity and increased intracellular calcium levels suggesting a shift in their functional state. Additionally, the chronic Gq-DREADD activation induced a decrease in the number of excitatory synapses on CA1 pyramidal neurons., which was found to be dependent on brain-derived neurotrophic factor (BDNF) signaling. As well as a reduction in long-term potentiation (LTP) in the hippocampus, which is a key mechanism for synaptic plasticity and memory. Despite these significant changes at the synaptic level, the mice with activated microglia initially performed well in learning and memory tasks, such as locating a hidden platform in the Morris water maze. However, in the following days, their ability to retain spatial information declined compared to controls, indicating that while learning was unaffected, the stability of the memories formed was compromised. Our findings show that selective activation of microglia can negatively impact synaptic plasticity and memory retention without inducing other overt symptoms of sickness. This method offers a valuable tool for studying the effects of microglial activation on neuronal function with high specificity and precise timing.