ATP transduces signals from ASGM1, a glycolipid that functions as a bacterial receptor

Abstract

The flagella of the Gram-negative bacterium Pseudomonas aeruginosa serve not only for motility but also to bind bacteria to the host cell glycolipid asialoGM1 (ASGM1) through the protein flagellin. This interaction triggers defensive responses in host cells. How this response occurs is unclear because ASGM1 lacks transmembrane and cytoplasmic domains and there is little information about the downstream effectors that connect ASGM1 ligation to the initiation of host defense responses. Here, we show that ASGM1 ligation promotes ATP release from the host cell, followed by autocrine activation of a nucleotide receptor. This response links ASGM1 to cytoplasmic signaling molecules and results in activation of phospholipase C, Ca(2+) mobilization, phosphorylation of a mitogen-activated protein kinase (Erk 1/2), and activation of mucin transcription. These results indicate that bacterial interaction with host cells can trigger autocrine nucleotide signaling and suggest that agents affecting nucleotide receptors may modulate host responses to bacteria.

Regulation of Airway Mucin Gene Expression

“…However, TLR signaling can activate NF-κB through two parallel routes: the MyD88-dependent and -independent pathways (36). In the above studies (32)(33)(34), MUC2 activation by LPS, LTA, and flagellin was suspected not to involve the TLRs at all. LPS, for example, is usually bound to a serum protein called LPS binding protein (LBP) that is then brought to the TLR4 by either a membrane-bound or a soluble receptor called CD14 (37).…”

“…These EGF ligands then bind to EGFR, which then activates RAS-MEK1/2-ERK1/2, as does LPS, to cause NF-κB activation of MUC2 transcription (33). P. aeruginosa also has a protein called flagellin that can activate airway epithelial cells and MUC2 expression (34). Although flagellin normally binds to TLR5 and activates NF-κB in airway cells (35), the induction of MUC2 appears to work through the binding of flagella by ASGM1, a membrane ganglioside.…”

“…After binding ASGM1, ATP is released and binds to an extracellular nucleotide receptor that signals through phospholipase C activation and the generation of inositol triphosphate and Ca 2+ . As do LPS and LTA, the signal then converges on the mitogen-activated protein kinase (MAP kinase) ERK and leads to its activation and subsequent downstream transcriptional activation of MUC2 (34) (Figure 1). …”

“…LPS, for example, is usually bound to a serum protein called LPS binding protein (LBP) that is then brought to the TLR4 by either a membrane-bound or a soluble receptor called CD14 (37). In the studies above (32)(33)(34), however, neutralizing anti-CD14 antibodies could not inhibit Pseudomonas's LPS stimulation of MUC2, suggesting that another receptor besides TLR4 is used (32). Likewise, LTA was believed to bind to surface platelet-activating factor receptors (PAFRs), and the flagellin was thought to involve the ASGM1 instead of TLRs (33,34).…”

Airway epithelial cell signaling in response to bacterial pathogens

“…58,92 Several other Ca 2þ dependent transcription factors are also activated by bacterial ligands leading to local cytokine expression and mucin production. 81 Peptidoglycan activates the leucine zipper containing transcription factors cAMP-responsive element binding protein (CREB)/ATF and AP-1. 93 CREB senses changes in cyclic nucleotides released at the surface of the airway in response to Ca 2þ fluxes.…”