ApoM binds endotoxin contributing to neutralization and clearance by High Density Lipoprotein

Area of Science:

• Biochemistry
• Immunology
• Molecular Biology

Background:

• High-density lipoprotein (HDL) exhibits anti-inflammatory properties, but the precise mechanisms remain unclear.
• Endotoxin, a lipopolysaccharide (LPS), triggers Toll-like receptor 4 (TLR4) signaling and inflammatory mediator release.
• While HDL clears most circulating LPS, the role of apolipoprotein M (ApoM) in this process is under investigation, especially given its reduction during sepsis.

Purpose of the Study:

• To investigate the binding interaction between ApoM and endotoxin (LPS).
• To determine if ApoM binding contributes to the anti-inflammatory effects of HDL.
• To elucidate the role of ApoM in LPS neutralization and clearance by HDL.

Main Methods:

• Isothermal Titration Calorimetry (ITC) to assess ApoM-LPS binding affinity.
• Macrophage-based assays using human and murine cells to evaluate LPS neutralization.
• Computational simulations to predict the binding mode of ApoM and LPS.

Main Results:

• ApoM demonstrated high-affinity binding to E. coli LPS (Kd < 1 μM) in a 1:1 complex, driven by exothermic, enthalpy-driven interactions.
• Computational modeling predicted stable docking of LPS to ApoM, near but not blocking its calyx.
• HDL containing ApoM effectively neutralized E. coli LPS in macrophages, significantly reducing TNFα release.

Conclusions:

• ApoM directly binds to LPS, facilitating its neutralization.
• This interaction is a key mechanism by which HDL contributes to endotoxin clearance and exerts anti-inflammatory effects.