Dark microglia: A new phenotype predominantly associated with pathological states

Abstract

The past decade has witnessed a revolution in our understanding of microglia. These immune cells were shown to actively remodel neuronal circuits, leading to propose new pathogenic mechanisms. To study microglial implication in the loss of synapses, the best pathological correlate of cognitive decline across chronic stress, aging, and diseases, we recently conducted ultrastructural analyses. Our work uncovered the existence of a new microglial phenotype that is rarely present under steady state conditions, in hippocampus, cerebral cortex, amygdala, and hypothalamus, but becomes abundant during chronic stress, aging, fractalkine signaling deficiency (CX3CR1 knockout mice), and Alzheimer's disease pathology (APP‐PS1 mice). Even though these cells display ultrastructural features of microglia, they are strikingly distinct from the other phenotypes described so far at the ultrastructural level. They exhibit several signs of oxidative stress, including a condensed, electron‐dense cytoplasm and nucleoplasm making them as “dark” as mitochondria, accompanied by a pronounced remodeling of their nuclear chromatin. Dark microglia appear to be much more active than the normal microglia, reaching for synaptic clefts, while extensively encircling axon terminals and dendritic spines with their highly ramified and thin processes. They stain for the myeloid cell markers IBA1 and GFP (in CX3CR1‐GFP mice), and strongly express CD11b and microglia‐specific 4D4 in their processes encircling synaptic elements, and TREM2 when they associate with amyloid plaques. Overall, these findings suggest that dark microglia, a new phenotype that we identified based on their unique properties, could play a significant role in the pathological remodeling of neuronal circuits, especially at synapses. GLIA 2016;64:826–839

Phagocytic clearance of presynaptic dystrophies by reactive astrocytes in Alzheimer's disease

“…However, it has been recently reported in another APP/PS1 model, by EM analysis, that activated (dark) microglia is able to encircle synaptic dystrophic elements (Bisht et al, 2016) which might be indicative of their potential phagocytic capacity to eliminate these structures. Therefore, we cannot completely discard a microglial role in the dystrophic clearance but if exists, it must be very infrequent or, on the contrary, a very rapid and highly dynamic process difficult to detect.…”

Microglia in Alzheimer’s disease

“…However, a new phenotype of microglia, referred to as "dark microglia," was found in conditions such as chronic stress, including in the APP/PS1 mouse model of AD. Notably, dark microglia exhibited a highly activated phenotype with strong expression of CD11b and TREM2 and extensive encircling of synaptic clefts when the microglia were associated with amyloid deposits (67). This observation suggests the presence of multiple microglial phenotypes with distinct functions.…”

CNS inflammation and neurodegeneration

“…A phagocytic program plays an important role in clearing aberrant proteins, particularly in AD, PD, and ALS. More recent work identifies a microglia subtype called "dark microglia" with ramified and thin processes and prominent staining for IBA1, CD11b, and microglia-specific 4D4, as well as TREM2 that associates with amyloid plaques (59). Future studies examining the molecular signatures of microglia in health and disease may identify novel targets for diagnosis and modulation of neurodegenerative diseases.…”