Ceramide as a TLR4 agonist; a putative signalling intermediate between sphingolipid receptors for microbial ligands and TLR4

Abstract

SummaryMucosal Toll-like receptors (TLRs) respond to pathogens, but remain inert to the indigenous flora, suggesting that the TLRs can receive pathogen-specific signals. For example, TLR4 signalling is activated in CD14-negative epithelial cells by P-fimbriated, uropathogenic Escherichia coli, but not by lipopolysaccharide. The fimbriae use glycosphingolipids as recognition receptors and there is release of ceramide, which is the membrane-anchoring domain of the receptors. In this study, ceramide was identified as a TLR4 agonist and as a putative signalling intermediate between the glycosphingolipid recognition receptors and TLR4. Exogenous ceramide activated a TLR4-dependent epithelial cell response, as shown by exposing stably transfected TLR4-positive or -negative human embryonal kidney cells to C2 and C6 ceramide. A similar, TLR4-dependent response occurred after deliberate release of endogenous long-chained ceramide with sphingomyelinase. Microbial ligands with glycosphingolipid specificity (P fimbriae or the B subunit of Shiga toxin) were shown to increase the levels of ceramide and to trigger a TLR4-dependent response in epithelial cells. The results show that ceramide activates TLR4 signalling and suggest that this mechanism might allow pathogens to elicit mucosal TLR4 responses by perturbing sphingolipid receptors for virulence ligands like P fimbriae.

Mechanism of pathogen‐specific TLR4 activation in the mucosa: Fimbriae, recognition receptors and adaptor protein selection

“…Ceramide is the membrane anchor of the receptor oligosaccharide in the outer leaflet of the lipid bilayer, and the intracellular levels of free ceramide were shown to increase in response to P fimbriated E. coli [37]. More recently, other agonists that release ceramide were shown to trigger a TLR4-dependent response, and exogenous free ceramide was shown to have a similar effect [38]. Thus, structural modifications of the recognition receptor may participate in the recruitment of TLR4.…”

Lipidomic Analysis of Toxoplasma gondii Reveals Unusual Polar Lipids

“…PE-cer and other ceramide derivatives have structural resemblance to lipopolysaccharide and represent alternative ligands for host toll receptors (40). If PE-cer does bind to toll receptors, PE-cer may be made by the parasite to modulate host cell function.…”

Antimicrobial Peptides, Innate Immunity, and the Normally Sterile Urinary Tract

“…CD14 is present in low levels in healthy kidney and uroepithelia are only weakly responsive to LPS. 32 However, an endogenous lipid from uroepithelial cells, ceramide, also acts as a TLR4 stimulus, and microbial attachment to the epithelia activates sphingomyelinase, liberating endogenous ceramides that trigger TLR4-dependent responses, such as chemokine production. 32 This scenario has not been described in other human organ systems as yet.…”