Extracellular cardiolipin modulates glial phagocytosis and cytokine secretion in a toll‐like receptor 4‐dependent manner

Abstract

AbstractBackgroundCardiolipin (CL) is a mitochondrial membrane phospholipid that acts as a signaling molecule when released into the extracellular space from degenerating and dying brain cells. Previous research has shown that extracellular CL inhibits secretion of pro‐inflammatory mediators by immune‐activated microglia. CL alone also increases the phagocytic activity of microglia and upregulates their expression of brain‐derived neurotrophic factor (BDNF) and glial cell‐derived neurotrophic factor (GDNF). The mechanisms by which CL modulates microglia functions, in a manner that is potentially beneficial in Alzheimer’s disease, are unknown. Furthermore, the effects of CL on astrocytes have not been explored. Since microglia and astrocytes express toll‐like receptor (TLR) 4, we hypothesized that extracellular CL interacts with this receptor, modulating immune functions of glia.MethodWe studied the effects of CL applied to the culture media on select functions of primary murine microglia and astrocytes, BV‐2 murine microglia, U118 MG human astrocytoma cells, and THP‐1 human microglia‐like cells. Secretion of inflammatory cytokines was monitored by enzyme‐linked immunosorbent assay (ELISA) or immunoblotting. The phagocytic activity of primary microglia and astrocytes was measured with fluorescent latex microspheres. The internalization of the microspheres was verified using confocal microscopy.ResultExtracellular CL (0.4‐14 μM) alone upregulates the phagocytic activity and release of monocyte chemoattractant protein (MCP)‐1 by murine microglia and human astrocytic cells. TLR 4‐specific antagonist TAK‐242 and blocking antibodies inhibit the upregulated phagocytic activity and MCP‐1 secretion by CL‐treated microglia and human THP‐1 microglia‐like cells. CL increases the secretion of interferon (IFN) γ‐induced protein (IP)‐10 and IFN‐β by THP‐1 microglia‐like cells and human U118 MG astrocytoma cells, respectively. CL inhibits the secretion of cytotoxins by several types of activated microglia and astrocytes. Amyloid β (Aβ42) peptide‐ and lipopolysaccharide‐induced secretion of cytokines by primary murine microglia and U118 MG astrocytoma cells, respectively, is reduced by extracellular CL.ConclusionExtracellular CL can interact with TLR 4 to modulate select functions of microglia and astrocytes. Importantly, we demonstrate that CL inhibits amyloid‐β peptide‐induced secretion of inflammatory cytokines by microglia and reduces glia‐mediated neuron death. Therefore, CL or liposomes containing CL could be developed as potential therapeutic agents for Alzheimer’s disease.