The colony stimulating factor 1 receptor (CSF1R) is a key regulator of myeloid lineage cells. Genetic loss of the CSF1R blocks the normal population of resident microgliain the brain that originates from the yolk sac during early development. However, the role of CSF1R signaling in microglial homeostasis in the adult brain is largely unknown. To this end, we tested the effects of selective CSF1R inhibitors on microglia in adult mice. Surprisingly, extensive treatment results in elimination of ~99% of all microglia brain-wide, showing that microglia in the adult brain are physiologically dependent upon CSF1R signaling. Mice depleted of microglia show no behavioral or cognitive abnormalities, revealing that microglia are not necessary for these tasks. Finally, we discovered that the microglia-depleted brain completely repopulates with new microglia within one week of inhibitor cessation. Microglial repopulation throughout the CNS occurs through proliferation of nestin positive cells that then differentiate into microglia.

In addition to amyloid-β plaque and tau neurofibrillary tangle deposition, neuroinflammation is considered a key feature of Alzheimer's disease pathology. Inflammation in Alzheimer's disease is characterized by the presence of reactive astrocytes and activated microglia surrounding amyloid plaques, implicating their role in disease pathogenesis. Microglia in the healthy adult mouse depend on colony-stimulating factor 1 receptor (CSF1R) signalling for survival, and pharmacological inhibition of this receptor results in rapid elimination of nearly all of the microglia in the central nervous system. In this study, we set out to determine if chronically activated microglia in the Alzheimer's disease brain are also dependent on CSF1R signalling, and if so, how these cells contribute to disease pathogenesis. Ten-month-old 5xfAD mice were treated with a selective CSF1R inhibitor for 1 month, resulting in the elimination of ∼80% of microglia. Chronic microglial elimination does not alter amyloid-β levels or plaque load; however, it does rescue dendritic spine loss and prevent neuronal loss in 5xfAD mice, as well as reduce overall neuroinflammation. Importantly, behavioural testing revealed improvements in contextual memory. Collectively, these results demonstrate that microglia contribute to neuronal loss, as well as memory impairments in 5xfAD mice, but do not mediate or protect from amyloid pathology.

With severe injury or disease, microglia become chronically activated and damage the local brain environment, likely contributing to cognitive decline. We previously discovered that microglia are dependent on colony-stimulating factor 1 receptor (CSF1R) signaling for survival in the healthy adult brain, and we have exploited this dependence to determine whether such activated microglia contribute deleteriously to functional recovery following a neuronal lesion. Here, we induced a hippocampal lesion in mice for 25 d via neuronal expression of diphtheria toxin A-chain, producing both a neuroinflammatory reaction and behavioral alterations. Following the 25 d lesion, we administered PLX3397, a CSF1R inhibitor, for 30 d to eliminate microglia. This post-lesion treatment paradigm improved functional recovery on elevated plus maze and Morris water maze, concomitant with reductions in elevated proinflammatory molecules, as well as normalization of lesion-induced alterations in synaptophysin and PSD-95. Further exploration of the effects of microglia on synapses in a second cohort of mice revealed that dendritic spine densities are increased with long-term microglial elimination, providing evidence that microglia shape the synaptic landscape in the adult mouse brain. Furthermore, in these same animals, we determined that microglia play a protective role during lesioning, whereby neuronal loss was potentiated in the absence of these cells. Collectively, we demonstrate that microglia exert beneficial effects during a diphtheria toxin-induced neuronal lesion, but impede recovery following insult.

Microglia mediate chronic neuroinflammation following central nervous system (CNS) disease or injury, and in doing so, damage the local brain environment by impairing recovery and contributing to disease processes. Microglia are critically dependent on signaling through the colony-stimulating factor 1 receptor (CSF1R) and can be eliminated via administration of CSF1R inhibitors. Resolving chronic neuroinflammation represents a universal goal for CNS disorders, but long-term microglial elimination may not be amenable to clinical use. Notably, withdrawal of CSF1R inhibitors stimulates new microglia to fully repopulate the CNS, affording an opportunity to renew this cellular compartment. To that end, we have explored the effects of acute microglial elimination, followed by microglial repopulation, in a mouse model of extensive neuronal loss. Neuronal loss leads to a prolonged neuroinflammatory response, characterized by the presence of swollen microglia expressing CD68 and CD45, as well as elevated levels of cytokines, chemokines, complement, and other inflammatory signals. These collective responses are largely resolved by microglial repopulation. Furthermore, microglial repopulation promotes functional recovery in mice, with elevated plus maze performance matching that of uninjured mice, despite the loss of 80% of hippocampal neurons. Analyses of synaptic surrogates revealed increases in PSD95 and synaptophysin puncta with microglial repopulation, suggesting that these cells sculpt and regulate the synaptic landscape. Thus, our results show that short-term microglial elimination followed by repopulation may represent a clinically feasible and novel approach to resolve neuroinflammatory events and promote brain recovery.

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