S100A9 interacts with a dynamic region on CD14 to activate Toll-like receptor 4

Area of Science:

• Immunology
• Molecular Biology

Background:

• S100A9, a Damage Associated Molecular Pattern (DAMP), triggers inflammatory responses through Toll-like receptor 4 (TLR4).
• While crucial for tissue repair, S100A9's role in inflammatory diseases necessitates understanding its activation mechanism.
• CD14 has been identified as a co-receptor essential for S100A9-mediated TLR4 activation.

Approach:

• Investigated the role of membrane-bound CD14 in S100A9/TLR4 signaling using cell-based assays, mutations, and pharmacological inhibitors.
• Utilized mutagenesis, structural modeling, and in vitro binding experiments to characterize the S100A9-CD14 interaction at a molecular level.
• Employed molecular dynamics simulations to analyze the dynamic nature of the CD14 binding site.

Key Points:

• Membrane-bound CD14 is critical for potentiating TLR4 activation by S100A9.
• S100A9-induced TLR4 activation is sensitive to inhibitors of pathways downstream of TLR4 internalization, suggesting a role for endocytosis.
• S100A9 binds to the N-terminus of CD14 in a region distinct from, but overlapping with, the lipopolysaccharide (LPS) binding site.

Conclusions:

• The findings suggest that S100A9 activates TLR4 through CD14-dependent internalization of the receptor complex.
• Molecular characterization reveals S100A9 binds to a dynamic N-terminal region of CD14, accommodating both soluble proteins and small molecules like LPS.
• This study provides the first molecular insights into the S100A9/CD14 interaction, advancing the understanding of S100A9-mediated TLR4 activation.