Circadian regulation of brain border immunity across the lifespan

Abstract

Life on Earth is governed by the rhythmic rotation of the planet, dividing time into periods of light and dark to which most life forms are entrained. In mammals, time-keeping is genetically encoded by the circadian clock: a transcription-translation feedback loop lasting twenty-four hours that synchronizes cellular physiology to time-of-day. Core organismal functions are carried out under circadian regulation including even innate immunity, the body’s first line of defense against external pathogens. It is increasingly apparent that circadian disruptions, such as night shift work or aging-related sleep fragmentation, are major risk factors for the development of neurodegenerative diseases including Alzheimer’s Disease (AD). The cellular mechanisms involved in this risk are less clear and may involve perturbation of the timing and function of brain innate immune cells. To further understand how circadian regulation and dysregulation of brain immunity could contribute to disease pathogenesis, we generated an unbiased transcriptional atlas of diurnal rhythms in the brain immune compartment. We identified brain border-associated macrophages (BAMs), a small and long-lived population of perivascular and leptomeningeal scavengers, as highly rhythmic immune cells both in phenotype and function. BAMs are exceptionally efficient at engulfment, by far exceeding other brain myeloid populations in acute uptake of extracellular substrates including amyloid-beta (Aβ), a peptide involved in AD pathogenesis. We found that BAM engulfment capacity is rhythmic and peaks during the murine rest phase. Rhythmicity in BAM engulfment is regulated by the clock gene Bmal1, accompanied by a coordinated wave of upregulation of endocytic genes, and mediated by the fast-recycling scavenger receptor CD206. In the aged brain, BAMs exhibit perturbed clock gene expression, downregulation of CD206, and profoundly impaired uptake of extracellular material including Aβ. Finally, in a mouse model of AD, we found that BAM-specific deletion of Bmal1 worsens perivascular and leptomeningeal Aβ plaque deposition. Together, these results highlight the remarkable endocytic capacity of BAMs and propose one mechanism by which circadian perturbations may precipitate neurodegenerative disease: by disrupting the timing of brain border innate immune functions including the clearance of pathogenic proteins. In parallel, we have sought to generate human models of BAMs using induced pluripotent stem cells (iPSCs). iPS-derived macrophages better recapitulate features of tissue-resident macrophages than circulating monocytes or bone marrow hematopoietic stem cells (HSCs). However, little is known of whether this technology may be used for the generation and study of BAMs. We found that early postnatal xenotransplantation of iPS-derived macrophages to the murine brain leads to robust formation of a human brain border immune compartment. Human xenotransplanted BAMs (xBAMs) develop throughout the leptomeninges and along brain vasculature of chimeric mice. Via transcriptional phenotyping, we identified a conserved signature distinguishing BAMs from microglia, the other long-lived tissue-resident macrophage of the brain. In functional assays, xBAMs exhibit compartment-restricted sampling of draining brain proteins and share the hyper-endocytic phenotype of murine BAMs. Notably, xBAMs are highly enriched for CD206 expression and by far surpass other brain immune cells in acute Aβ uptake. Thus, the “xBAM platform” may be used to model human BAM function in vivo. In addition, we have developed adapted differentiation protocols to generate iPS-derived macrophages in vitro resembling BAMs (the “iBAM platform”). iBAMs share transcriptional and functional features of in vivo BAMs including a hyper-endocytic phenotype and enhanced engulfment capacity. Together, these data indicate that the remarkable endocytic capacity of BAMs is conserved across species and support our ongoing efforts to target BAM scavenging in the context of aging and amyloidosis.