Mucin-lipid interactions at a model intestinal interface: lipid-dependent organization and mechanical properties in...

The intestinal mucus barrier is a dynamic mucin-rich interfacial system exposed to lipids originating from the diet, bile, microbial membranes, and epithelial cell membranes. Although the biological role of mucin (MUC2) is well established, the physicochemical mechanisms by which individual lipid species influence mucin-lipid coupling and interfacial mechanics remain poorly understood. Here, we examine whether the interfacial response to MUC2 depends on lipid chemistry and monolayer assembly pathway. Langmuir monolayers were used as controlled model interfaces to investigate MUC2 interactions with sphingomyelin, cholesterol, and phospholipids differing in headgroup chemistry, charge, saturation, and packing behavior: 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoyl-sn-glycero-3-phosphoglycerol (DOPG), and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). Two complementary assembly pathways were compared: MUC2 injection beneath preformed lipid monolayers (final concentration 0.001 mg/mL) and lipid spreading onto a MUC2-containing subphase (0.5 mg/mL). Surface pressure-area isotherms, surface compressional modulus analysis, surface potential measurements, and isobaric relaxation experiments revealed pronounced lipid-dependent and assembly-history-dependent effects. For DPPC, the collapse pressure decreased from 58.5 mN/m on PBS to 43.7 mN/m after MUC2 injection and 41.1 mN/m on the MUC2-containing subphase, while Cₛ^-1_max decreased from 230.1 to 199.0 and 135.5 mN/m, respectively. Unsaturated phospholipid films, particularly DOPE and DOPG, exhibited reduced isobaric area relaxation during dynamic MUC2 exposure, whereas cholesterol showed pronounced area relaxation. DPPC and sphingomyelin displayed distinct interfacial responses consistent with their more ordered packing and limited lateral compressibility. These results indicate that mucin-lipid interactions are determined by the interplay between headgroup chemistry, sterol rigidity, hydration, electrostatic interactions, and monolayer mechanical response. The study provides a physicochemical framework for understanding lipid-specific responses at simplified mucus-conditioned interfaces and establishes a basis for future studies in more complex intestinal mucus and membrane models.