Author ORCID Identifier

https://orcid.org/0000-0001-5297-6636

Date of Award

4-2026

Document Type

Thesis (Ph.D.)

Department or Program

Biological Sciences

First Advisor

Robert A. Hill, PhD

Abstract

Microglia continually surveil the brain, allowing for rapid detection of tissue damage or infection. Microglial metabolism is linked to tissue homeostasis, yet how mitochondria are subcellularly partitioned in microglia and dynamically reorganize during surveillance, injury responses, and phagocytic engulfment in the intact brain has remained unclear. In this dissertation, I combine intravital imaging, ultrastructural analyses, and metabolic and cytoskeletal markers to investigate how mitochondrial organization relates to microglial function across health, inflammation, and aging.

I first show that microglial processes diverge in their mitochondrial content, with some containing multiple mitochondria while others are completely void. This heterogeneous partitioning is conserved in the mouse and human neocortex. Microtubules and hexokinase 2 mirror this uneven mitochondrial distribution indicating that these cytoskeletal and metabolic components are linked to mitochondrial organization in microglia. Microglial processes that engage in minute-to-minute surveillance typically do not have mitochondria. Moreover, unlike process surveillance, mitochondrial motility does not change with animal anesthesia. Likewise, the processes that acutely chemoattract to a lesion site or initially engage with a neuron undergoing programmed cell death do not contain mitochondria. Rather, microglia mitochondria have a delayed arrival into the responding cell processes. Thus, there is subcellular heterogeneity of mitochondrial partitioning. Moreover, microglial processes that surveil and acutely respond to damage do not contain mitochondria.

To extend these findings beyond the healthy brain, I examined how inflammatory and aged contexts influence mitochondrial organization. Inflammatory exposure led to distinct mitochondrial remodeling, including localization to the microglial soma and increased circularity. Mitochondrial motility, on the other hand, was unaffected by both inflamed and aged conditions.

Together, these findings reveal that subcellular mitochondrial heterogeneity is a defining feature of microglial biology. I also show that mitochondrial positioning, motility, and remodeling are relatively rigid compared to the dynamic and fluid microglia functions. The data presented in this dissertation provides a foundation for future studies to interrogate how microglial mitochondrial dysfunction arises in aging and neurodegenerative disease.

Original Citation

Pietramale, A.N., Bame, X., Doty, M.E. et al. Mitochondria are absent from microglial processes performing surveillance, chemotaxis, and phagocytic engulfment. Nat Commun 16, 11104 (2025). https://doi.org/10.1038/s41467-025-66708-6

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