Microglia regulate central nervous system myelin growth and integrity.

Myelin is required for the function of neuronal axons in the central nervous system, but the mechanisms that support myelin health are unclear. Although macrophages in the central nervous system have been implicated in myelin health1, it is unknown which macrophage populations are involved and which aspects they influence. Here we show that resident microglia are crucial for the maintenance of myelin health in adulthood in both mice and humans. We demonstrate that microglia are dispensable for developmental myelin ensheathment. However, they are required for subsequent regulation of myelin growth and associated cognitive function, and for preservation of myelin integrity by preventing its degeneration. We show that loss of myelin health due to the absence of microglia is associated with the appearance of a myelinating oligodendrocyte state with altered lipid metabolism. Moreover, this mechanism is regulated through disruption of the TGFß1-TGFßR1 axis. Our findings highlight microglia as promising therapeutic targets for conditions in which myelin growth and integrity are dysregulated, such as in ageing and neurodegenerative disease2,3.

Inhibiting CSF1R alleviates cerebrovascular white matter disease and cognitive impairment.

White matter abnormalities, related to poor cerebral perfusion, are a core feature of small vessel cerebrovascular disease, and critical determinants of vascular cognitive impairment and dementia. Despite this importance there is a lack of treatment options. Proliferation of microglia producing an expanded, reactive population and associated neuroinflammatory alterations have been implicated in the onset and progression of cerebrovascular white matter disease, in patients and in animal models, suggesting that targeting microglial proliferation may exert protection. Colony-stimulating factor-1 receptor (CSF1R) is a key regulator of microglial proliferation. We found that the expression of CSF1R/Csf1r and other markers indicative of increased microglial abundance are significantly elevated in damaged white matter in human cerebrovascular disease and in a clinically relevant mouse model of chronic cerebral hypoperfusion and vascular cognitive impairment. Using the mouse model, we investigated long-term pharmacological CSF1R inhibition, via GW2580, and demonstrated that the expansion of microglial numbers in chronic hypoperfused white matter is prevented. Transcriptomic analysis of hypoperfused white matter tissue showed enrichment of microglial and inflammatory gene sets, including phagocytic genes that were the predominant expression modules modified by CSF1R inhibition. Further, CSF1R inhibition attenuated hypoperfusion-induced white matter pathology and rescued spatial learning impairments and to a lesser extent cognitive flexibility. Overall, this work suggests that inhibition of CSF1R and microglial proliferation mediates protection against chronic cerebrovascular white matter pathology and cognitive deficits. Our study nominates CSF1R as a target for the treatment of vascular cognitive disorders with broader implications for treatment of other chronic white matter diseases.

A story of birth and death: Insights into the formation and dynamics of the microglial population.

Microglia are the main resident immunocompetent cells of the brain with key roles in brain development, homeostasis and function. Here we briefly review our current knowledge of the homeostatic mechanisms regulating the composition and turnover of the microglial population under physiological conditions from development to ageing. A greater understanding of these mechanisms may inform understanding of how dysregulation of microglial dynamics could contribute to the pathogenesis and/or progression of neurological disorders.

Pharmacological targeting of CSF1R inhibits microglial proliferation and prevents the progression of Alzheimer's-like pathology.

The proliferation and activation of microglial cells is a hallmark of several neurodegenerative conditions. This mechanism is regulated by the activation of the colony-stimulating factor 1 receptor (CSF1R), thus providing a target that may prevent the progression of conditions such as Alzheimer's disease. However, the study of microglial proliferation in Alzheimer's disease and validation of the efficacy of CSF1R-inhibiting strategies have not yet been reported. In this study we found increased proliferation of microglial cells in human Alzheimer's disease, in line with an increased upregulation of the CSF1R-dependent pro-mitogenic cascade, correlating with disease severity. Using a transgenic model of Alzheimer's-like pathology (APPswe, PSEN1dE9; APP/PS1 mice) we define a CSF1R-dependent progressive increase in microglial proliferation, in the proximity of amyloid-ß plaques. Prolonged inhibition of CSF1R in APP/PS1 mice by an orally available tyrosine kinase inhibitor (GW2580) resulted in the blockade of microglial proliferation and the shifting of the microglial inflammatory profile to an anti-inflammatory phenotype. Pharmacological targeting of CSF1R in APP/PS1 mice resulted in an improved performance in memory and behavioural tasks and a prevention of synaptic degeneration, although these changes were not correlated with a change in the number of amyloid-ß plaques. Our results provide the first proof of the efficacy of CSF1R inhibition in models of Alzheimer's disease, and validate the application of a therapeutic strategy aimed at modifying CSF1R activation as a promising approach to tackle microglial activation and the progression of Alzheimer's disease.

Microglia colonize the developing brain by clonal expansion of highly proliferative progenitors, following allometric scaling.

Microglia arise from the yolk sac and enter the brain during early embryogenesis. Upon entry, microglia undergo in situ proliferation and eventually colonize the entire brain by the third postnatal week in mice. However, the intricacies of their developmental expansion remain unclear. Here, we characterize the proliferative dynamics of microglia during embryonic and postnatal development using complementary fate-mapping techniques. We demonstrate that the developmental colonization of the brain is facilitated by clonal expansion of highly proliferative microglial progenitors that occupy spatial niches throughout the brain. Moreover, the spatial distribution of microglia switches from a clustered to a random pattern between embryonic and late postnatal development. Interestingly, the developmental increase in microglial numbers follows the proportional growth of the brain in an allometric manner until a mosaic distribution has been established. Overall, our findings offer insight into how the competition for space may drive microglial colonization by clonal expansion during development.

Measuring Microglial Turnover in the Adult Brain.

Microglia are the main resident immunocompetent cells of the brain with key roles in brain development, homeostasis, and function. Recent reports have started to shed light on the homeostatic mechanisms regulating the composition and turnover of the microglial population under physiological conditions from development to ageing, but our knowledge of the dynamics of microglia is incomplete. Therefore, it appears relevant to provide a standardized approach to quantify the turnover of microglia, with direct application to create a greater understanding of the dynamics of this cell population, and how it may contribute to the pathogenesis and/or progression of neurological disorders. Here we describe a robust immunohistochemical method to analyze microglial proliferation in mouse brain, aiming at providing a shared and universal approach to analyze microglial dynamics across different laboratories.

Development and maintenance of the brain's immune toolkit: Microglia and non-parenchymal brain macrophages.

Microglia and non-parenchymal macrophages located in the perivascular space, the meninges and the choroid plexus are independent immune populations that play vital roles in brain development, homeostasis, and tissue healing. Resident macrophages account for a significant proportion of cells in the brain and their density remains stable throughout the lifespan thanks to constant turnover. Microglia develop from yolk sac progenitors, later evolving through intermediate progenitors in a fine-tuned process in which intrinsic factors and external stimuli combine to progressively sculpt their cell type-specific transcriptional profiles. Recent evidence demonstrates that non-parenchymal macrophages are also generated during early embryonic development. In recent years, the development of powerful fate mapping approaches combined with novel genomic and transcriptomic methodologies have greatly expanded our understanding of how brain macrophages develop and acquire specialized functions, and how cell population dynamics are regulated. Here, we review the transcription factors, epigenetic remodeling, and signaling pathways orchestrating the embryonic development of microglia and non-parenchymal macrophages. Next, we describe the dynamics of the macrophage populations of the brain and discuss the role of progenitor cells, to gain a better understanding of their functions in the healthy and diseased brain. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 561-579, 2018.

Coupled Proliferation and Apoptosis Maintain the Rapid Turnover of Microglia in the Adult Brain.

Microglia play key roles in brain development, homeostasis, and function, and it is widely assumed that the adult population is long lived and maintained by self-renewal. However, the precise temporal and spatial dynamics of the microglial population are unknown. We show in mice and humans that the turnover of microglia is remarkably fast, allowing the whole population to be renewed several times during a lifetime. The number of microglial cells remains steady from late postnatal stages until aging and is maintained by the spatial and temporal coupling of proliferation and apoptosis, as shown by pulse-chase studies, chronic in vivo imaging of microglia, and the use of mouse models of dysregulated apoptosis. Our results reveal that the microglial population is constantly and rapidly remodeled, expanding our understanding of its role in the maintenance of brain homeostasis.