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Characterizing previously unrecognized effects of endogenous molecules on the phagocytic activity of glia, as well as their secretome

University of British Columbia

Neuroinflammation driven by non-neuronal cells, such as microglia and astrocytes, contributes to the progression of neurological disorders, including Alzheimer’s disease. When released from cells, mitochondrial phospholipid cardiolipin (CL), soluble mitochondrial protein cytochrome c (CytC), and gut bacterial metabolites short-chain fatty acids (SCFAs) may interact with microglia and astrocytes, modulating their neuroinflammatory functions. In this thesis, I characterize previously unknown molecular mechanisms and functional consequences of microglia and astrocyte activation by extracellular CL, CytC and SCFAs. In chapter two, I demonstrate that cardiolipin (CL) induces the secretion of monocyte chemoattractant protein (MCP)-1, interferon-γ-induced protein 10 and nitric oxide by microglia, while inhibiting secretion of inflammatory molecules by stimulated microglia-like cells. CL also upregulates microglial phagocytosis. I determine that toll-like receptor (TLR) 4 mediates the effects of CL on microglial phagocytosis and MCP-1 secretion. In chapter three, I demonstrate that microglia and astrocytes release CytC when exposed to cytotoxins. Extracellular CytC induces the secretion of cytotoxins, interleukin (IL)-12 p70, IL-8, IL-1β, and granulocyte-macrophage colony-stimulating factor (GM-CSF) by astrocytic cells, and TLR 4 antagonists inhibit the secretion of the latter three cytokines. In chapter four, I show SCFAs acetate, propionate, butyrate, formate and valerate alone or as a mixture decrease IL-1β, MCP-1, tumor necrosis factor (TNF)-α, and cytotoxin secretions by stimulated microglia-like cells. GLPG 0974, a free fatty acid receptor (FFAR) 2 and 3 antagonist, blocks the inhibitory effect of formate alone, but not the SCFA mixture, on IL-1β secretion by microglia-like cells. Formate and valerate alone decrease the phagocytic activity of stimulated microglia-like cells. Formate, but not valerate, alone inhibits the respiratory burst of microglia-like cells, reducing the production of reactive oxygen species. Extracellularly released CL and CytC regulate immune responses of microglia and astrocytes, respectively, in a TLR 4-dependent manner. Due to its anti-inflammatory properties, cardiolipin could be used to reduce neuroinflammation, while TLR 4 antagonists may reduce the adverse neuroinflammatory effects of extracellular cytochrome c. In addition, SCFAs may regulate the immune responses of microglia. Therefore, therapies that promote the colonization and proliferation of SCFA-producing bacteria may reduce neuroinflammation, and subsequently slow the progression of Alzheimer’s disease.

Text

2020-11-19

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76540

Biochemistry and Molecular Biology

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/