Mitochondrial transcription factor A (TFAM) as a novel intercellular signaling molecule of the brain : its role in glial cell activation and neuroinflammation

Microglia, a subtype of non-neuronal glial cells, represent the innate immune system of the brain. In Alzheimer’s disease, chronic neuroinflammation caused by dysregulated activation of microglia (termed microgliosis) contributes to neuronal cell death. Endogenous molecules including damage-associated molecular patterns (DAMPs) have been implicated as triggers of microgliosis. High mobility group box 1 (HMGB1), a well-characterized DAMP, induces inflammatory responses from several cell types when released into the extracellular space. Mitochondrial transcription factor A (TFAM), a structural and functional homolog of HMGB1, has been implicated as a possible DAMP. However, the effects of TFAM on glial and neuronal cells of the central nervous system remain unknown. This thesis demonstrates that extracellular TFAM acts as a DAMP capable of inducing pro-inflammatory and cytotoxic responses in glial cells by engaging receptors similar to those activated by HMGB1. Stimulation of THP-1 cells (microglia model) with TFAM in combination with interferon (IFN)-γ, resulted in activation of these cells, as demonstrated by a significant decrease in SH-SY5Y neuronal cell viability after their exposure to the supernatants from stimulated THP-1 cells. Similar results were obtained using the U-118 MG astrocytic cell line and primary human astrocytes. The induced neurotoxicity was accompanied by the release of monocyte chemotactic protein-1 (MCP-1), reactive oxygen species (ROS), and novel intercellular signaling agents called microparticles (MPs). These MPs, in turn, induced THP-1 cell activation in an autocrine manner. Furthermore, my data demonstrate that the receptor for advanced glycation endproducts (RAGE) and the macrophage antigen complex-1 (Mac-1) receptor are engaged by TFAM, as blocking these receptors with specific inhibitors (heparin and soluble RAGE) or antibodies attenuated glial toxicity towards neuronal cells, and decreased the secretion of MCP-1 and ROS. Lastly, TFAM expression in U-373 MG astrocytic cells and primary human astrocytes was upregulated in response to treatment with hydrogen peroxide. This research provides insight into glial activation by TFAM, and the role of endogenous intercellular signaling molecules in communication between different central nervous system cell types. Moreover, identification of the glial receptors targeted by TFAM may provide novel therapeutic targets for pathologies involving sterile neuroinflammatory processes including Alzheimer’s disease.Arts and Sciences, Irving K. Barber School of (Okanagan)Biology, Department of (Okanagan)Graduat