Short-chain fatty acids (SCFAs) alone or in combination regulate select immune functions of microglia-like cells.

Neuroinflammation contributes to neurodegenerative disorders, including Alzheimer's disease (AD). Gut microbes are involved in regulating systemic inflammation. Short-chain fatty acids (SCFAs), which are among the many metabolites released by gut microbes, can cross the blood-brain barrier (BBB) and interact with microglia. High concentrations of individual SCFAs decrease the inflammatory responses of peripheral monocytes; therefore, we hypothesized that SCFAs act on their own or in combinations to reduce the inflammatory response of microglia. Cultured human THP-1 monocytic cells and differentiated human HL-60 myelomonocytic cells were used to model select immune functions of human microglia. Acetate, propionate, butyrate, formate, and valerate were added to cells alone or as a mixture containing SCFAs at an approximate physiological concentration ratio. The SCFA mixture, as well as several individual SCFAs at the highest concentrations used in the mixture (15-236 µM), decreased the secretion of interleukin (IL)-1ß, monocyte chemoattractant protein (MCP)-1, tumor necrosis factor (TNF)-α, and cytotoxins by immune-stimulated THP-1 cells. GLPG 0974, a free fatty acid receptor (FFAR) 2/3 antagonist, did not block the inhibitory effect of the SCFA mixture on IL-1ß secretion by THP-1 cells while blocking the inhibitory effect of formate alone. We demonstrated that formate and valerate alone reduced the phagocytic activity of immune-stimulated THP-1 cells. Formate, but not valerate, alone also inhibited the N-formylmethionine-leucyl-phenylalanine (fMLP)-induced respiratory burst of HL-60 cells, reducing the production of reactive oxygen species (ROS). Our data indicate that SCFAs could regulate select microglial functions that are disrupted in AD.

Extracellular histones as damage-associated molecular patterns in neuroinflammatory responses.

The four core histones H2A, H2B, H3, H4, and the linker histone H1 primarily bind DNA and regulate gene expression within the nucleus. Evidence collected mainly from the peripheral tissues illustrates that histones can be released into the extracellular space by activated or damaged cells. In this article, we first summarize the innate immune-modulatory properties of extracellular histones and histone-containing complexes, such as nucleosomes, and neutrophil extracellular traps (NETs), described in peripheral tissues. There, histones act as damage-associated molecular patterns (DAMPs), which are a class of endogenous molecules that trigger immune responses by interacting directly with the cellular membranes and activating pattern recognition receptors (PRRs), such as toll-like receptors (TLR) 2, 4, 9 and the receptor for advanced glycation end-products (RAGE). We then focus on the available evidence implicating extracellular histones as DAMPs of the central nervous system (CNS). It is becoming evident that histones are present in the brain parenchyma after crossing the blood-brain barrier (BBB) or being released by several types of brain cells, including neurons, microglia, and astrocytes. However, studies on the DAMP-like effects of histones on CNS cells are limited. For example, TLR4 is the only known molecular target of CNS extracellular histones and their interactions with other PRRs expressed by brain cells have not been observed. Nevertheless, extracellular histones are implicated in the pathogenesis of a variety of neurological disorders characterized by sterile neuroinflammation; therefore, detailed studies on the role these proteins and their complexes play in these pathologies could identify novel therapeutic targets.

Extracellular cardiolipin modulates microglial phagocytosis and cytokine secretion in a toll-like receptor (TLR) 4-dependent manner.

Microglia-driven neuroinflammation contributes to neurodegenerative diseases. Mitochondrial phospholipid cardiolipin acts as a signaling molecule when released from damaged cells. We demonstrate that extracellular cardiolipin induces the secretion of monocyte chemoattractant protein-1 and interferon gamma-induced protein 10 by resting microglia while inhibiting secretion of cytokines by microglia stimulated with lipopolysaccharide, amyloid Aß42 peptides, or α-synuclein. Extracellular cardiolipin also induces nitric oxide secretion by microglia-like cells and upregulates microglial phagocytosis. By using blocking antibodies, we determine that toll-like receptor 4 mediates the latter effect. Under physiological and pathological conditions characterized by cell death, extracellularly released cardiolipin may regulate immune responses of microglia.