Juxtanuclear Drebrin-Enriched Zone

Abstract

Drebrin E contributes to remodeling of the actin cytoskeleton and formation of cell processes. Therefore, its role in cell migration was studied in prototypes of motile cells with prominent lamellipodia such as murine B16F1 melanoma and Swiss 3T3 cells and in human SV80 fibroblasts. Confocal microscopy revealed absence of drebrin from the tips of lamellipodia but enrichment in the tail of the cells, in retraction zones and in a specific juxtanuclear actin filament compartment, named “drebrin-enriched zone.” A similar subset of juxtanuclear actin filaments is characterized by the actin-binding protein SWAP-70, but drebrin and SWAP-70 localized to different compartments, suggesting the existence of novel distinct subdomains within the actin filament system. In cells overexpressing drebrin-EGFP, numerous long, branched cell processes were formed which slowly retracted and extended. However, in stable transfectants containing lower amounts of the fusion protein, drebrin-EGFP was recruited to the same sites as the endogenous protein during cell migration, i.e., to retracting membrane domains and into the juxtanuclear drebrin-enriched zone. In the leading edges of SV80 cells, characterized by pronounced actin microspikes, drebrin was concentrated along posterior portions of the microspikes, together with tropomyosin, with which it competes for actin binding. Drebrin knockdown by siRNA did not impact forward migration or ruffling. Taken together, these findings suggest that during cell migration drebrin is involved in retraction processes but not in lamellipodia formation. The novel, sizable juxtanuclear drebrin-enriched zone remains to be characterized in detail with respect to its molecular assembly and functions.