Invadopodia are actin-rich cell membrane projections used by invasive cells to penetrate the basement membrane. Control of invadopodia stability is critical for efficient degradation of the extracellular matrix (ECM); however, the underlying molecular mechanisms remain poorly understood. Here, we uncover a new role for podoplanin, a transmembrane glycoprotein closely associated with malignant progression of squamous cell carcinomas (SCCs), in the regulation of invadopodia-mediated matrix degradation. Podoplanin downregulation in SCC cells impairs invadopodia stability, thereby reducing the efficiency of ECM degradation. We report podoplanin as a novel component of invadopodia-associated adhesion rings, where it clusters prior to matrix degradation. Early podoplanin recruitment to invadopodia is dependent on lipid rafts, whereas ezrin/moesin proteins mediate podoplanin ring assembly. Finally, we demonstrate that podoplanin regulates invadopodia maturation by acting upstream of the ROCK-LIMK-Cofilin pathway through the control of RhoC GTPase activity. Thus, podoplanin plays a key role in the regulation of invadopodia function in SCC cells, controlling the initial steps of cancer cell invasion.

Of the 20 Rho GTP-binding proteins in humans, 8 have atypical properties, which are also unusual within the Ras superfamily. These atypical proteins fall into four subfamilies: RhoU/RhoV, Rnd1/Rnd2/Rnd3, RhoH and RhoBTB1/ RhoBTB2. These proteins are known or predicted to be predominantly GTP-bound in cells, because of changes in their ability to exchange GDP for GTP or to hydrolyse GTP. Apart from RhoH, they also have N-terminal and C-terminal extensions that give them unique interacting partners and functions. For example, RhoU can bind SH3 domain-containing proteins, Rnd proteins can bind to 14-3-3 proteins, and RhoBTB proteins can interact via their BTB domains with cullin-3, which is involved in proteasomal degradation. The proteins have been implicated in diverse functions, including cell adhesion and migration, vesicle trafficking and cell proliferation.