Negative Cross-Talk between TLR2/4-Independent AMPKa1 and TLR2/4-Dependent JNK Regulates S. pneumoniae-Induced Mucosal Innate Immune Response

Streptococcus pneumoniae is major cause of otitis media (OM) and life-threatening pneumonia. Overproduction of mucin, the major component of mucus, plays a critical role in the pathogenesis of both OM and pneumonia. However, the molecular mechanisms underlying the tight regulation of mucin upregulation in the mucosal epithelium by S. pneumoniae infection remain largely unknown. In this study, we show that S. pneumoniae pneumolysin (PLY) activates AMP-activated protein kinase a1 (AMPKa1), the master regulator of energy homeostasis, which is required for S. pneumoniae-induced mucin MUC5AC upregulation in vitro and in vivo. Moreover, we found that PLY activates AMPKa1 via cholesterol-dependent membrane binding of PLY and subsequent activation of the Ca2+- Ca2+/calmodulin- dependentkinase kinase b (CaMKKb) and Cdc42-mixed-lineage protein kinase 3 (MLK3) signaling axis in a TLR2/4-independent manner. AMPKa1 positively regulates PLY-induced MUC5AC expression via negative cross-talk with TLR2/4-dependent activation of MAPK JNK, the negative regulator of MUC5AC expression. Moreover, pharmacological inhibition of AMPKa1 suppressed MUC5AC induction in the S. pneumoniae-induced OM mouse model, thereby demonstrating its therapeutic potential in suppressing mucus overproduction in OM. Taken together, our data unveil a novel mechanism by which negative cross-talk between TLR2/4-independent activation of AMPKa1 and TLR2/4-dependent activation of JNK tightly regulates the S. pneumoniae PLY-induced host mucosal innate immune response. The Journal of Immunology, 2022, 209: 1532-1544.

Automated summary

Streptococcus pneumoniae is a major cause of otitis media and life-threatening pneumonia. Overproduction of mucin, the major component of mucus, plays a critical role in the pathogenesis of both conditions. This study reveals a novel mechanism by which pneumolysin from S. pneumoniae activates AMP-activated protein kinase a1 (AMPKa1), leading to upregulation of mucin MUC5AC. The study also demonstrates the negative cross-talk between TLR2/4-independent activation of AMPKa1 and TLR2/4-dependent activation of JNK in regulating the host mucosal innate immune response to S. pneumoniae infection.