Microglia are the primary immune cells of the central nervous system (CNS) parenchyma, and have evolved conceptually from silent sentinels awaiting pathogenic or injurious disturbance to active and central regulators of brain homeostasis and disease (Prinz et al., 2019). Their functional repertoire equips microglia to perform immune as well as non-immune roles, but less is known about the latter. As exacerbated or, on the other hand, deficient microglial function may contribute to brain dyshomeostasis, shedding light on such functions may provide insight into mechanisms underlying disease etiology and pathogenesis. The development of microglial depletion paradigms via inhibition of colony-stimulating factor 1 receptor (CSF1R), expressed in the brain by microglia and required for their survival (Elmore et al., 2014), provides unprecedented capacity for functional investigation by allowing researchers to observe and draw conclusions from the consequences of microglial absence (up to 99% depletion) on CNS processes for virtually any duration of time (Green et al., 2020). Such methods have suggested that microglia dynamically regulate the dendritic spines of neuronal circuits in the adult homeostatic brain (Rice et al., 2017; Rice et al., 2015), for instance, as well as amyloid plaque compaction and deposition in the context of Alzheimer’s disease (AD) (Casali et al., 2020; Huang et al., 2021; Spangenberg et al., 2019).
The aim of my thesis is to identify and elucidate the role of microglia in the regulation of the brain extracellular matrix (ECM) – the perisynaptic and synaptic manifestations of which serve as the fourth and most recently addended component of the contemporary model of the quad- or tetra-partite synapse, also consisting of pre- and post-synaptic elements and associated glia (Dityatev and Schachner, 2003; Dityatev et al., 2010). While the ECM is divided into several separate compartments in the brain, one particularly salient instance is the perineuronal net (PNN), a specialized reticular formation that surrounds neuronal subsets and proximal synapses to provide synaptic stability, physical and chemical protection, and other unique physiological properties (Fawcett et al., 2019; Reichelt et al., 2019). Not only are PNNs associated with long-term memory storage (Shi et al., 2019a; Thompson et al., 2018; Tsien, 2013), they protect against oxidative stress and amyloid-β (Cabungcal et al., 2013; Miyata et al., 2007), underscoring their particular relevance to disorders like AD. Among other changes, I found that these ECM structures are reduced in Huntington’s disease (HD) and AD, two brain disorders with disparate etiologies, pathologies, and microglial activation phenotypes. Importantly, I found that early microglial depletion with CSF1R inhibitors prevented PNN loss in both cases. Surprisingly, elimination of microglia also induced dramatic upregulation of PNN density throughout the brains of healthy adult mice. These results define a novel role of microglia in the regulation of PNNs in the homeostatic CNS, which may in turn go awry in neurodegenerative diseases where microglia adopt dyshomeostatic phenotypes.
