Energy Pathway of Lipid Monolayer Fusion: From Droplet Contact to Coalescence

Neutral fats in living organisms are stored in lipid droplets, intracellular organelles enveloped by a phospholipid monolayer. The fusion of these lipid droplets is vital for numerous physiological functions and is regulated by specific proteins and lipids. Dysregulation of this process, leading to excessive droplet growth, is associated with various pathological conditions. Notably, changes in the lipid composition of the boundary monolayers can significantly influence the fusion rate, mirroring fusion dynamics of membranous compartments surrounded by lipid bilayers. In this study, we conducted a theoretical and computational analysis of monolayer fusion, extending the established bilayer fusion model to this context. We characterize the energy trajectory associated with monolayer fusion, tracing the process from the initial unperturbed state to the formation of physical contact between monolayers, and subsequently to the expansion of this structure, which we refer to as the monolayer stalk, analogous to bilayer fusion. Unlike bilayer fusion, monolayer fusion features a single energy barrier, determining the process efficiency. Once this barrier is overcome, further droplet merging occurs spontaneously, highlighting the dynamic nature of lipid droplet interactions. We analyze how lipid composition influences this energy barrier and explore the effects of factors such as Gaussian curvature and hydration-induced repulsion on the energy landscape. Our calculations reveal that Gaussian curvature energy significantly contributes to barrier height. An increase in the proportion of lipids exhibiting large negative spontaneous curvature, which enhances fusion likelihood, can substantially decrease this barrier. Our findings are consistent with existing experimental data and allow us to quantify the barrier height as a function of lipid composition. Specifically, we demonstrate that incorporating 50 mol % of dioleoylphosphatidylethanolamine (DOPE) into pure dioleoylphosphatidylcholine (DOPC) monolayers reduces the energy barrier height by approximately 16 k_BT - half of this reduction attributed to changes in spontaneous curvature, with the other half due to modification in hydration repulsion parameters. These findings provide quantitative insights into lipid droplet fusion mechanisms, advancing our understanding of lipid metabolism and its physiological regulation.