Three matrix metalloproteinases are required in vivo for macrophage migration during embryonic development

Macrophages are essential in development, repair and pathology of a variety of tissues via their roles in tissue remodelling, wound healing and inflammation. These biological functions are also associated with a number of human diseases, for example tumour associated macrophages have well defined functions in cancer progression. Xenopus embryonic macrophages arise from a haematopoietic stem cell population by direct differentiation and act as the main mechanism of host defence, before lymphoid cells and a circulatory system have developed. This function is conserved in mouse and human development. Macrophages express a number of matrix metalloproteinases (MMPs), which are central to their function. MMPs are a large family of zinc-dependent endoproteases with multiple roles in extracellular matrix remodelling and the modulation of signalling pathways. We have previously shown MMP-7 to be expressed by Xenopus embryonic macrophages. Here we investigate the role of MMP-7 and two other MMPs (MMP-18 and MMP-9) that are also expressed in the migrating macrophages. Using morpholino (MO) mediated knockdown of each of the MMPs we demonstrate that they are necessary for normal macrophage migration in vivo. The loss-of-function effect can be rescued using the specific MMPs, altered to be resistant to morpholinos but not by overexpression of the other MMPs. Double and triple morpholino knockdowns further suggest that these MMPs act combinatorily to promote embryonic macrophage migration. Thus, our results imply that these three MMPs have distinct functions, which together are crucial to mediate macrophage migration in the developing embryo. This demonstrates conclusively that MMPs are required for normal macrophage cell migration in the whole organism.