Condensed desmin and actin cytoskeletal communication in lipid droplets

The interplay between intermediate filaments (IFs) and other cytoskeletal components is important for the integrity and motility of cells. The impact of IF assembly on other components and cell morphology is not yet fully understood. Therefore, we examined the effects of combined desmin and actin assembly on cytoskeletal network arrangement in artificial cell-sized droplets. Fluorescently labeled desmin, with or without actin, was enclosed in droplets prepared with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) using the water-in-oil method. Protein networks were observed using fluorescence microscopy in the presence of 150 mM KCl, 20 mM imidazole-HCl (pH 7.4), 2 mM MgCl₂ , and 1 mM adenosine 5'-triphosphate for both desmin and actin assembly. As desmin alone can assemble into filaments within seconds, desmin networks mainly localizing at the inner margins of the droplets were observed within 10 min after assembly initiation. Subsequently, deformations of droplets appeared. Furthermore, a portion of droplets formed desmin-rich protrusions of several micrometers. Notably, actin alone rarely formed protrusions under the same conditions. When 1,2-dioleoyl-sn-glycero-3-phosphocholine was used instead of DOPE, protrusions became less frequent. The combination of desmin and actin increased the number of deformed droplets in which the proteins were considerably colocalized. The assembly process of desmin facilitated colocalization. Atomic force microscopy failed to reveal interactions between the two filament types. These results suggest that the mechanical properties of desmin networks may influence the behavior of actin networks, as well as membrane morphology, possibly reflecting the mechanical function of desmin filaments in muscle cells.