The Role of Toll‐Like Receptor 4 in Enteric Glia

Abstract

Toll‐like receptors (TLRs) are a class of pattern recognition receptors that play an important role in mediating inflammatory responses to pathogens. TLR4 recognizes lipopolysaccharide (LPS), a membrane component of gram negative bacteria. In the GI tract, neuronal integrity may be dependent on basal activation of TLR4 receptors suggesting a protective role for TLR4. A compromised gut epithelial barrier leads to enhanced entry of bacterial proteins into the lamina propria, leading to neuronal toxicity. Thus pro‐survival vs pro‐toxic effects of LPS may be concentration‐dependent. Enteric glia are important for the maintenance of neuronal integrity. We determined the expression of pro‐inflammatory cytokines in rat enteric glia cells in response to low and high concentrations of LPS. LPS dose‐dependently increased mRNA expression of IL‐6 and TNF‐α with 0.1 μg/ml inducing a 25‐fold and 10‐fold IL‐6 and TNF‐α mRNA expression respectively as compared to untreated glia. A 100‐fold higher concentration of LPS (1‐100 μg/ml) also induced ATP release indicative of cell damage. Whole cell voltage clamp studies showed that this high concentration of LPS increased ATP‐induced inward currents with a corresponding increase in P2X1,2,3,4,and 7 receptor mRNA expression up to 50‐fold as compared to untreated glia. Thus low concentrations of LPS can induce cytokine release whereas high concentrations of LPS induce ATP release which increases P2X receptor activity and further enhances the release of pro‐inflammatory cytokines. Understanding the role of P2X in LPS induced enteric glia activation will point to new targets to control gut bacterial inflammation while still maintaining the beneficial effects of basal TLR4 signaling. Supported by NIH DK046367, DA024009.