Investigating Microglia-Vascular Interactions in the Developing and Adult Central Nervous System

Microglia, the resident macrophages of the central nervous system (CNS), are dynamic cells, constantly extending and retracting their processes as they contact and functionally regulate neurons and other glial cells. There is far less known about how microglia interact with the CNS vasculature, particularly under healthy steady-state conditions. Here, I provide the first extensive characterization of juxtavascular microglia in the healthy, postnatal brain and identify a molecular mechanism regulating the timing of these interactions during development. Using the mouse cerebral cortex, I show that microglia are intimately associated with the vasculature in the CNS, directly contacting the basal lamina in vascular sites that are devoid of astrocyte endfeet. I demonstrate a high percentage of microglia are associated with the vasculature during the first week of postnatal development, which is concomitant with a peak in microglial colonization of the cortex and recruitment to synapses. I find that as microglia colonize the cortex, juxtavascular microglia are highly motile along vessels and become largely stationary as the brain matures. 2-photon live imaging in adult mice reveals that these vascular-associated microglia in the mature brain are stable and stationary for several weeks. Further, a decrease in microglia motility along the vasculature is tightly correlated with the expansion of astrocyte endfeet along the vasculature. Finally, I provide evidence that the timing of these microglia-vascular interactions during development is regulated by the microglial fractalkine receptor (CX3CR1). Together, these data support a model by which microglia use the vasculature as a scaffold to migrate and colonize the developing brain and the timing of these associations is modulated by CX3CR1. This migration along the vasculature becomes restricted as astrocyte vascular endfoot territory expands and, upon maturation, vascular-associated microglia become largely stationary