Secretory and endo/exocytic trafficking in invadopodia formation: the MT1-MMP paradigm

Invadopodia are actin-rich, adhesive protrusions that extend into and remodel the extracellular matrix. They are associated with high levels of pericellular proteolysis and correlate with the invasive capacity of a variety of tumour cells. Invadopodia have, thus, been proposed to recapitulate key events of the metastatic process. Although our understanding of the patho-physiology of invadopodia is still in its infancy, the molecular components and signalling pathways leading to their formation have received increasing attention. Recent studies have revealed that diverse membrane polarized secretory and endo/exocytic trafficking pathways converge at these structures for the delivery, in a temporally controlled and spatially confined manner, of key proteolytic enzymes. Here, we will focus our attention on MT1-MMP, a paradigmatic metalloprotease that is primarily responsible for the proteolytic activity of invadopodia. We propose that the biosynthetic/secretory pathway might be critical for the polarized delivery of MT1-MMP to invadopodia that form as "default response" whenever cells have to deal with extracellular matrix (ECM) of variable composition and stiffness. Conversely, "inducible" endo/exocytic trafficking routes might primarily control the delivery of MT1-MMP to invadopodia when cells need to respond in a fast and transient manner to soluble motogenic factors, rather than the insoluble ECM.