Toll-like receptor 2 is required for LPS-induced toll-like receptor 4 signaling and inhibition of ion transport in renal thick ascending limb

Previously we demonstrated that basolateral LPS inhibits HCO ₃^- absorption in the renal medullary thick ascending limb (MTAL) through TLR4-dependent ERK activation. Here we report that the response of the MTAL to basolateral LPS requires TLR2 in addition to TLR4. The basolateral addition of LPS (ultrapure Escherichia coli K12) decreased HCO ₃^- absorption in isolated, perfused MTALs from wild-type mice but had no effect in MTALs from TLR2^-/- mice. In contrast, inhibition of HCO₃^- absorption by lumen LPS was preserved in TLR2^-/- MTALs, indicating that TLR2 is involved specifically in mediating the basolateral LPS response. LPS also did not increase ERK phosphorylation in MTALs from TLR2^-/- mice. TLR2 deficiency had no effect on expression of TLR4, MD-2, or MyD88. However, LPS-induced recruitment of MyD88 to the basolateral membrane was impaired in TLR2^-/- MTALs. Inhibition of HCO₃^- absorption by LPS did not require CD14. Co-immunoprecipitation studies demonstrated an association between TLR4 and TLR2. Inhibition of HCO₃^- absorption by TLR2-specific ligands was preserved in MTALs from TLR4^-/- mice. These results indicate that the effect of basolateral LPS to inhibit HCO₃ ^- absorption in the MTAL through MyD88-dependent ERK activation depends on a novel interaction between TLR4 and TLR2. TLR2 plays a dual role in the induction of intracellular signals that impair MTAL function, both through cooperation with TLR4 to mediate ERK signaling by LPS and through a TLR4-independent signaling pathway activated by Gram-positive bacterial ligands. Regulation of TLR2 expression and its interaction with TLR4 may provide new mechanisms for controlling and therapeutic targeting of TLR4-mediated LPS responses.