Considering that synaptic loss is the best pathological correlate of cognitive decline across the significance of this newly defined cellular mechanism –microglial phagocytosis of synaptic elements– in the pathogenesis of brain disorders. The Neuroimmune Plasticity lab investigates the role of microglia as vectors for effecting targeted changes in neuronal circuits in order to spare memory, learning and other cognitive functions that can be compromised during chronic stress, aging, and neurodegenerative disease. Since chronic stress accelerates cellular aging, potentiates oxidative stress, neuroinflammation, and predisposes to various diseases across the lifespan, therapeutic interventions promoting resilience –of the brain immune cells– to stress could be crucial for healthy aging (in vulnerable individuals).

Microglia are extremely sensitive to environmental challenges, including chronic stress, which is associated with a substantial remodeling of neuronal circuits and elimination of synapses. Our recent work revealed that neuron-microglia communication underlies adaptation of the brain and behavior to chronic stress, and that neuronal signaling between the chemokine fractalkine (CX3CL1) and its unique receptor CX3CR1, mainly expressed by microglia, plays a crucial role in this process. Fractalkine receptor deficiency (in CX3CR1 knockout mice) prevents the effects of chronic unpredictable stress on the phagocytosis of synaptic elements by microglia, short- and long-term neuronal plasticity, as well as depressive-like behavior. In CX3CR1 knockout mice, microglial phagocytosis of terminals and spines is basally elevated, targeting synaptic elements in a non-specific manner, and is not modulated by chronic stress. These findings indicate that microglia-regulated mechanisms could underlie the differential susceptibility to chronic stress and consequently the vulnerability to disorders, including major depression, triggered by stressful events.

In parallel, we also uncovered the existence of an ultrastructurally distinct microglial phenotype that is predominantly associated with pathological states. These cells are rare in steady state conditions, but become prevalent upon chronic stress, aging, or Alzheimer’s disease (AD) pathology, and account for almost 50% of the microglial population in CX3CR1 knockout mice. They exhibit several signs of oxidative stress including a condensed, electron-dense cytoplasm and nucleoplasm giving them a ‘dark’ appearance similar to mitochondria, accompanied by endoplasmic reticulum dilation (the best characterized sign of oxidative stress at the ultrastructural level), mitochondrial disruption, and heterochromatin remodeling associated with epigenetic alteration. The physiological significance of these dark microglia has yet to be elucidated but they appear extremely active, frequently reaching for synaptic clefts, while extensively encircling terminals, spines, and entire synapses with their highly ramified and thin processes. They strongly express CD11b, which forms complement receptor 3 involved in synaptic pruning, specifically in their processes encircling synaptic elements, and myeloid-cell specific TREM2 when associated with amyloid-β plaques. In AD pathology, TREM2 positive cells were recently shown to express the phagocytic effectors MERTK and AXL. These findings indicate that dark microglia could represent a subset of cells that become stressed as a result of their hyperactivity under adaptive pressure, leading to abnormal (or perhaps specialized) interactions with synapses.

We are now testing the hypothesis that stressed microglia are specifically implicated in the loss of synapses that impairs learning, memory, and other cognitive functions along the aging trajectory. These experiments are conducted during chronic stress and aging, the greatest risk factors for AD, in preclinical mouse models that we have developed. We use an integrative approach that combines non-invasive imaging (two-photon, super-resolution, and 3D-scanning electron microscopy) with whole-transcriptomics and behavioral assessments. We also validate our findings in human brain samples from healthy individuals, young vs aged, and AD patients.