Abstract Body: Background: The biophysical regulation of HDL function is driven by its lipid composition. While research has transitioned from HDL-cholesterol to assessing HDL functionality, the molecular mechanisms modifying the directionality and stoichiometry of HDL phospholipid and cholesterol transfer in human plasma are poorly understood.
Objectives: To determine how nanoparticle phospholipid acyl-chain composition controls the directionality, stoichiometry, and surface remodeling of HDL during free cholesterol (FC) exchange in human plasma.
Methods and Results: We developed nanoparticles coated with phospholipids (PL) with varying acyl-chain length and saturation (DMPC, SOPC, DOPC, POPC, SAPC, EggPC) with matching molar PL content. Lipid exchange between nanoparticles containing either PL alone (acceptor particle) or equimolar PL:FC ratios (donor particle) was assayed in either whole or apoB-depleted plasma. Total plasma FC removal by acceptor nanoparticles was invariant across PL compositions (30-40%), suggesting a surface-limited exchange process dominated by apoB-containing lipoproteins. In marked contrast, HDL-specific FC exchange was highly sensitive to nanoparticle PL composition (22-66%). When nanoparticles acted as FC donors, unsaturated and polyunsaturated PL selectively transferred FC to HDL, whereas saturated or short-chain PL did so to a lesser extent. This directional FC delivery lowered HDL PL:FC ratios from 5.6 at baseline to 1.7-2.0, demonstrating a highly efficient FC uptake (47-94% increase). Conversely, when nanoparticles acted as acceptors, HDL PL:FC ratios increased markedly (from 5.6 up to 11-18), revealing net PL enrichment of HDL, despite FC loss. These HDL lipid stoichiometric shifts reveal a dynamic adaptive remodeling of both PL and FC on the HDL surface, which may be modulated by acyl-chain-dependent physical constraints.
Conclusions: The directionality of plasma HDL-cholesterol transfer may be regulated by surface lipid dynamics. Phospholipid acyl-chain-dependent membrane lipid disorder selectively alters FC transferability, while HDL PL content dynamically remodels to preserve surface stability. HDL acts as a mechanically adaptive surface rather than a fixed cholesterol reservoir. Thus, the directionality of surface lipid exchange, PL:FC stoichiometry, and acyl-chain-dependent surface plasticity underlies HDL functionality.