Actin-rich protrusions and nonlocalized GTPase activation in Merlin-deficient schwannomas

Schwannomas lack both alleles for the tumor suppressor Merlin, a cytoskeleton-membrane linker. Previous results showed increased cell spreading of schwannoma cells, but little is known about the underlying mechanisms. Electron microscopy reveals that schwannoma cells not only show more lamellipodia/ruffles but also multiple filopodia. We show that Cdc42, important in filopodia formation, is activated. Both Rac1 and Cdc42 are found all around the cell periphery and in colocalization with their effector phospho-p21 activated kinase in human schwannoma cells. We therefore claim that Rac1 and Cdc42 are activated in a nonlocalized manner, which explains the disperse distribution of lamellipodia/ruffles and filopodia. Using live cell imaging, we further demonstrate continuous remodeling of the many actin-rich protrusions in schwannoma cells. The underlying cytoskeleton of these structures is thin and extensively branched. The actin-related protein 2/3 complex, a major regulator of actin branching, is enriched in the many lamellipodia and ruffles of human primary schwannoma cells. We suggest that the Merlin deficiency in human primary schwannoma cells leads to a random, nonlocalized activation of Rac1 and Cdc42, inducing many actin-rich protrusion zones, not only at the leading edge but also all around the cell periphery. Their nondirectional occurrence together with the continuous and highly dynamic actin remodeling results in the dedifferentiation of these tumor cells.