Microscopy

Podosomes are highly dynamic actin-rich adhesive structures formed predominantly by cells of the monocytic lineage, which degrade the extracellular matrix. Normal podosome formation and dissociation accompanies the migration of myeloid cells. Failure to make podosomes, as in the immunodeficiency disorder Wiscott-Aldrich syndrome, severely compromises the migration and chemotaxis of macrophages and dendritic cells. Podosomes consist of a core of F-actin and actinregulating proteins, surrounded by a ring of adhesion-associated proteins such as vinculin. We have characterised the structure of podosomes, particularly the structure of the vinculin ring, using two high resolution fluorescence microscopy techniques: stimulated emission depletion microscopy and structured illumination microscopy. Rather than being round, as previously assumed, we have found the vinculin ring to be created from relatively straight strands of vinculin, resulting in a distinctly polygonal shape. The strands bind preferentially at angles close to 120 degrees. Furthermore, vinculin strands are observed nucleating at the corners of the podosomes, suggesting a mechanism for podosome growth.