Search

Most of the protocols to analyze metabolic features of cell populations from different tissues rely on

Summary Microbiome research has grown significantly in the last decade, highlighting manifold implications of the microbiota to the host’s health. The gut microbiota is connected to the brain through several avenues that allow their interaction. Thus, recent studies have attemtpted to characterize these connections and enhance our understanding of the so called ‘gut-brain-axis’. Microglia, the central nervous system resident macrophages, are crucial for the proper development and maintenance of brain functions. As immune cells, they are in the spotlight for relaying signals between the microbiota and cells of the brain. In this review, we contemplate on interactions between the gut microbiota and microglia, and their influence on brain functions in health and disease.

Microglial function declines during aging. The interaction of microglia with the gut microbiota has been well characterized during development and adulthood but not in aging. Here, we compared microglial transcriptomes from young-adult and aged mice housed under germ-free and specific pathogen-free conditions and found that the microbiota influenced aging associated-changes in microglial gene expression. The absence of gut microbiota diminished oxidative stress and ameliorated mitochondrial dysfunction in microglia from the brains of aged mice. Unbiased metabolomic analyses of serum and brain tissue revealed the accumulation of N

CD33/Sialic acid-binding Ig-like lectin 3 (SIGLEC3) is an innate immune receptor expressed on myeloid cells and mediates inhibitory signaling via tyrosine phosphatases. Variants of CD33 are associated with Alzheimer’s disease (AD) suggesting that modulation of CD33 signaling might be beneficial in AD. Hence, there is an urgent need for reliable cellular CD33 reporter systems. Therefore, we generated a CD33 reporter cell line expressing a fusion protein consisting of the extracellular domain of either human full-length CD33 (CD33M) or the AD-protective variant CD33ΔE2(D2-CD33/CD33m) linked to TYRO protein tyrosine kinase binding protein (TYROBP/DAP12) to investigate possible ligands and antibodies for modulation of CD33 signaling. Application of the CD33-specific antibodies P67.6 and 1c7/1 to the CD33M-DAP12 reporter cells resulted in increased phosphorylation of the kinase SYK, which is downstream of DAP12. CD33M-DAP12 but not CD33ΔE2-DAP12 expressing reporter cells showed increased intracellular calcium levels upon treatment with CD33 antibody P67.6 and partially for 1c7/1. Furthermore, stimulation of human induced pluripotent stem cell-derived microglia with the CD33 antibodies P67.6 or 1c7/1 directly counteracted the triggering receptor expressed on myeloid cells 2 (TREM2)-induced phosphorylation of SYK and decreased the phagocytic uptake of bacterial particles. Thus, the developed reporter system confirmed CD33 pathway activation by CD33 antibody clones P67.6 and 1c7/1. In addition, data showed that phosphorylation of SYK by TREM2 activation and phagocytosis of bacterial particles can be directly antagonized by CD33 signaling.

As tissue macrophages of the central nervous system (CNS), microglia constitute the pivotal immune cells of this organ. Microglial features are strongly dependent on environmental cues such as commensal microbiota. Gut bacteria are known to continuously modulate microglia maturation and function by the production of short-chain fatty acids (SCFAs). However, the precise mechanism of this crosstalk is unknown. Here we determined that the immature phenotype of microglia from germ-free (GF) mice is epigenetically imprinted by H3K4me3 and H3K9ac on metabolic genes associated with substantial functional alterations including increased mitochondrial mass and specific respiratory chain dysfunctions. We identified acetate as the essential microbiome-derived SCFA driving microglia maturation and regulating the homeostatic metabolic state, and further showed that it is able to modulate microglial phagocytosis and disease progression during neurodegeneration. These findings indicate that acetate is an essential bacteria-derived molecule driving metabolic pathways and functions of microglia during health and perturbation.

The authors identify two subsets of peripheral nerve macrophages residing in the endoneurium and the epineurium and displaying a distinct transcriptome and response to injury. These cells lack the main microglia identity and have a distinct origin. While CNS microglia have been extensively studied, relatively little is known about macrophages populating the peripheral nervous system. Here we performed ontogenic, transcriptomic and spatial characterization of sciatic nerve macrophages (snMacs). Using multiple fate-mapping systems, we show that snMacs do not derive from the early embryonic precursors colonizing the CNS, but originate primarily from late embryonic precursors and become replaced by bone-marrow-derived macrophages over time. Using single-cell transcriptomics, we identified a tissue-specific core signature of snMacs and two spatially separated snMacs: Relm alpha(+)Mgl1(+) snMacs in the epineurium and Relm alpha(-)Mgl1(-) snMacs in the endoneurium. Globally, snMacs lack most of the core signature genes of microglia, with only the endoneurial subset expressing a restricted number of these genes. In response to nerve injury, the two resident snMac populations respond differently. Moreover, and unlike in the CNS, monocyte-derived macrophages that develop during injury can engraft efficiently in the pool of resident peripheral nervous system macrophages.