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Macrophages are innate immune cells that are present in all tissues to maintain immune surveillance. They ingest particles such as bacteria or dead cells, a process called phagocytosis that involves dynamic actin reorganization and generation of forces. In addition to the amoeboid migration used by all leukocytes, we showed in vitro that macrophages use the mesenchymal migration in dense environments involving proteolysis of the extracellular matrix, compaction, and ingestion of degraded matrix to create tunnels.
In most cancers, the density and stiffness of the tissue stroma are enhanced. We found in mouse fibrosarcoma in vivo and in human breast cancer ex vivo, that tumor-associated macrophages (TAM) that help tumor progression, perform the mesenchymal migration using their own matrix metalloproteases (MMPs), and perform the amoeboid migration at the tumor periphery. As a proof of concept that targeting mesenchymal migration would be a novel therapeutic strategy, we showed that MMP inhibition correlates with decreased of both TAM recruitment and tumor growth.
Podosomes are macrophage structures involved in adhesion, proteolytic degradation of the extracellular matrix and 3D mesenchymal migration. They present a submicron-size core of F-actin surrounded by an adhesion ring. Using protrusion force microscopy we showed that podosomes are mechanosensitive and could estimate the forces generated by single podosomes. We are now investigating how podosomes are organised at the nanoscale, and how this organisation regulates protrusion force generation. In particular, we employ in situ cryo-electron tomography to reveal the architecture of podosomes. Quantitative analysis of podosome architecture showed that podosome core filaments are denser, more oblique, and shorter than radial and cortical filaments. Importantly, core filaments are bent and store high elastic energy, supporting that the podosome core consists of a set of highly compressed actin filaments.
Finally, we have recently started to develop a 2.5D model of frustrated phagocytosis consisting of arrays of polyacrylamide micropillars of controlled size and stiffness in order to quantify the forces at play during phagocytosis and address the molecular mechanisms involved. In particular, we focus our attention on the involvement of podosome-like structures, which we also reported to be involved during phagocytosis. |
