Emergence of microglia structural and functional heterogeneity between hippocampal subregions during development into early adulthood

Phagocytosis is a key function performed by microglia to maintain tissue homeostasis. The degree of microglial phagocytic activity differs across ages and between gross anatomical brain regions, dictated by the local environment. Here, we asked whether microglial phagocytic phenotype exhibits subregion-scale tuning by circuit-specific features within a brain region. To address this, we took advantage of the highly stereotyped architecture of the hippocampus. We examined three adjacent synaptic subregions, the CA1 stratum radiatum (SR), stratum lacunosum moleculare (SLM) and dentate gyrus molecular layer (DGML). These three subregions provide an ideal system for examining local microglia heterogeneity, as each subregion contains distinct neuropil features, creating three adjacent unique micro-environments to which the microglia are exposed. We measured the phagocytic activity and morphological properties of over 1,000 individual microglia at two developmental points, mid-postnatal (P16) and early adulthood (P60) in the CA1 SR, SLM and DGML. We found that microglial phenotype diversified with development into early adulthood. At the mid-postnatal age, phagocytic activity and morphology were homogeneous across subregions. Conversely, in young adulthood, microglia in the CA1 SR and DGML exhibited a reduction in phagocytic activity, while microglia in the CA1 SLM maintained a highly phagocytic phenotype reminiscent of an immature-like state. These findings uncover a fine-scale tuning of microglia activity that emerges during maturation and is dictated at the sub-region level of the hippocampus, uncovering a distinct population of microglia in the CA1 SLM that exhibit a persistent immature phenotype under physiological conditions. Understanding the target(s) of this phagocytosis and consequences for CA1 SLM function will provide new insight into the role of local tuning of microglia properties for circuit-specific needs in both health and disease.