Extracellular Matrix Remodeling and the Control of Branching Morphogenetic Programs.

Abstract

Epithelial cells and endothelial cells initiate distinct branching morphogenetic programs during their coordinated invasion and proliferation into the interstitial compartment, a tissue comprised of mesenchymal stromal cells and extracellular matrix (ECM). While mammary gland development occurs in a specialized stromal environment dominated by adipocytes and fibroblasts, endothelial cell branching proceeds throughout all tissues in the absence of a specific stromal cell population. Nevertheless, both epithelial cells and endothelial cells engaged in morphogenetic responses have been posited to mobilize proteolytic enzymes to penetrate ECM barriers. However, transgenic mouse models have failed to identify required proteolytic effectors or uncover the mechanisms whereby proteolytic changes in tissue microenvironments regulate cell behavior. Herein, we characterize functional roles performed by the two dominant transmembrane proteinases expressed during epithelial and endothelial cell branching processes, the membrane-anchored matrix metalloproteinases, MT1-MMP and MT2-MMP. Using a series of transgenic mouse models, we identify new and unanticipated roles for MT1-MMP and MT2-MMP in mammary gland morphogenesis as well as angiogenesis. Tissue-specific targeting of MT1-MMP and MT2-MMP demonstrate that early mammary gland branching, which takes place within an immature ECM, proceeds independently of either proteinase. Instead, both proteinases play important, but diametrically opposed, roles in mammary gland adipocytes, where MT1-MMP stimulates adipogenesis and lipid metabolism, while MT2-MMP represses the development of thermogenic beige/brown adipocytes. In marked contrast, during the major phases of postnatal mammary gland development where a mature ECM is actively deposited, branching requires stromal cell-, rather than epithelial cell-, derived MT1-MMP, where the proteinase regulates branching by remodeling a periductal network of ECM macromolecules dominated by type I collagen. Endothelial cells also rely on MT1-MMP to direct branching, but unexpectedly, the proteinase also controls proliferative responses via a novel regulatory axis wherein pericellular proteolysis of the ECM governs the cytoskeletal-nuclear membrane interactions responsible for regulating transcriptional activity. Together, these data create new paradigms relevant to morphogenesis and tissue remodeling, as well as identify novel roles for membrane-anchored metalloproteinases in governing ECM proteolysis and associated transcriptional programs.