Metabolism at the crossroads of microglial function and inflammation

Abstract

The brain relies on a specialized population of immune cells called microglia to maintain its internal balance. Unlike most immune cells which circulate throughout the body, microglia reside permanently in the central nervous system. There, they monitor tissue, clear away cellular waste, and coordinate responses to injury, infection, or other stressors. Beyond these immune functions, they help shape neural circuits and maintain the brains’ protective barriers. To carry out such diverse roles, microglia are highly adaptable, constantly adjusting their behavior to the needs of the surrounding tissue. Recent advances in genomic technologies have revealed that microglia adopt a wide range of functional states, each defined by distinct patterns of gene activity and cellular behavior. These states are particularly relevant in ageing and neurodegenerative conditions like Alzheimer’s disease, where microglial responses can either protect the brain or contribute to disease progression. Understanding the biological processes that drive these shifts is a central challenge in modern neuroscience. Lipid metabolism - the way cells process, store, and use fats - has emerged as a key influence on microglial behavior. While lipids are essential for energy and cell structure, they also act as signals that control immune activity. In ageing and neurodegeneration, microglia often accumulate lipid-filled droplets. Whether these droplets actively shape cellular behavior or simply reflect underlying stress is still unclear. This thesis investigates how lipid metabolism and intracellular pathways shape microglial immune responses. Using microglia derived from human stem cells as a model, this work examines how microglial metabolic state and cellular organization influences inflammation. Chapter 2 examines how the methods used to generate microglia influence both metabolism and immune properties. We found that different growth methods led to significant differences in microglial lipid profiles, particularly in triglycerides, a storage form of fatty acids found in lipid droplets. These differences were due to the presence or absence of a specific nutrient in these cultures. Adding this nutrient, or its key component L-carnitine, not only reduced lipid droplet accumulation but also reduced inflammatory markers and improved responsiveness to immune signals. These results demonstrate that experimental conditions can strongly influence both metabolism and immune behavior, highlighting the importance of carefully defined culture systems when modeling disease-relevant processes. Chapter 3 explores the functional consequences of lipid droplet accumulation. By exposing microglia to excess fatty acids, we induced triglyceride-rich lipid droplet formation and immune signaling. This metabolic change notably increased the expression of IL32, an inflammatory molecule primarily found in humans and higher mammals, while other inflammatory signals were influenced in a context-dependent fashion. These findings suggest that lipid droplets do not simply cause general inflammation but modulate specific pathways, shaping microglial immune behavior in a nuanced manner. Chapter 4 investigates the cell’s internal transport and recycling systems, known as the endolysosomal and autophagic pathways, respectively. These systems move materials within the cell and break down waste to prevent buildup and subsequent damage. By testing a selected group of 34 chemical compounds, we identified the enzyme VPS34 as a key regulator of inflammation. Inhibiting this enzyme disrupted the cell’s recycling process and exacerbated the inflammatory response to immune triggers. These results suggest that intact internal trafficking serves as a crucial brake on excessive inflammation. Together, the findings of this thesis reveal that microglial immune behavior is a product of its metabolism, lipid storage, and internal organization. In ageing and neurodegeneration, persistent metabolic stress, lipid accumulation, and impaired trafficking can shift this balance toward maladaptive chronic inflammation. By using human stem cell models, this work provides a blueprint for understanding microglial diversity and suggests avenues for restoring balance in ageing and neurodegenerative contexts.