A developmental analysis of juxtavascular microglia dynamics and interactions with the vasculature

Abstract

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 microglia-vascular interactions, particularly under healthy steady-state conditions. Here, we use the male and female mouse cerebral cortex to show that a higher percentage of microglia associate with the vasculature during the first week of postnatal development compared to older ages and the timing of these associations are dependent on the fractalkine receptor (CX3CR1). Similar developmental microglia-vascular associations were detected in the prenatal human brain. Using live imaging in mice, we found that juxtavascular microglia migrated when microglia are actively colonizing the cortex and became stationary by adulthood to occupy the same vascular space for nearly 2 months. Further, juxtavascular microglia at all ages contact vascular areas void of astrocyte endfeet and the developmental shift in microglial migratory behavior along vessels corresponded to when astrocyte endfeet more fully ensheath vessels. Together, our data provide a comprehensive assessment of microglia-vascular interactions. They support a mechanism by which microglia use the vasculature to migrate within the developing brain parenchyma. This migration becomes restricted upon the arrival of astrocyte endfeet when juxtavascular microglia then establish a long-term, stable contact with the vasculature.SIGNIFICANCE STATEMENTWe report the first extensive analysis of juxtavascular microglia in the healthy, developing and adult brain. Live imaging revealed that juxtavascular microglia within the cortex are highly motile and migrate along vessels as they are colonizing cortical regions. Using confocal, expansion, super-resolution, and electron microscopy, we determined that microglia associate with the vasculature at all ages in areas lacking full coverage astrocyte endfoot coverage and motility of juxtavascular microglia ceases as astrocyte endfeet more fully ensheath the vasculature. Our data lay the fundamental groundwork to investigate microglia-astrocyte crosstalk and juxtavascular microglial function in the healthy and diseased brain. They further provide a potential vascular-dependent mechanism by which microglia colonize the brain to later regulate neural circuit development.

Microglia have a grip on brain microvasculature

“…Surprisingly, about 30% of brain resident microglia observed in adult mice are CAMs, resulting in a capillary-associated soma density that is higher than that of astrocytic cell bodies, but lower than pericyte cell body density. Electron microscopy further reveals that the myeloid cell bodies are closely associated with brain capillaries, localized outside the basement membrane that surrounds capillary endothelium, without any interposing astrocytic processes or endfeet consistent with a previous study 2 .…”

“…In a recent paper published in Nature Communications, Bisht et al identify a signalling mechanism that attracts and maintains microglia at the capillary wall. Moreover, they show that microglia regulate capillary vascular tone, playing a more significant role in blood flow regulation than previously thought.Microglia are the resident immune sentinels of the brain, responsible for the brain immune response as well as a variety of physiological functions including synapse pruning, neuronal regulation, and clearance of cellular and toxic debris [1][2][3][4] . However, much less is known about microglia-vascular interactions, particularly under healthy steady-state conditions.Recent studies using live 2-photon imaging, confocal, expansion, super-resolution, and electron microscopy, suggested that microglia associate with capillary vasculature at all ages and during central nervous system development, notably in areas lacking astrocyte endfoot coverage.…”

“…Microglia are the resident immune sentinels of the brain, responsible for the brain immune response as well as a variety of physiological functions including synapse pruning, neuronal regulation, and clearance of cellular and toxic debris [1][2][3][4] . However, much less is known about microglia-vascular interactions, particularly under healthy steady-state conditions.…”

Impaired developmental microglial pruning of excitatory synapses on CRH-expressing hypothalamic neurons exacerbates stress responses throughout life

“…However, prior analyses did not consider cell-type specificity of the neuronal targets of microglial synapse pruning. Recent data have supported the notion that microglia are a heterogenous population of cells 62,63 , differing across brain regions and even within a brain region 62,64 . Here, we demonstrate that as a result of ELA, microglia engulf fewer synaptic elements of CRH+ neurons in the PVN (Fig.…”

Microglia‐leucocyte axis in cerebral ischaemia and inflammation in the developing brain

“…In the neonatal brain, higher percentage of microglia interact with vasculature in areas lacking astrocyte endfeet coverage, and the high motility of microglia along the vessel ceases when astrocyte endfeet more fully ensheath vessels after development of the brain. The timing of this microglia‐vascular interaction has been recently identified to be regulated by CX3CR1 17 …”