The functional crosstalk between MT1-MMP and ADAMs in craniofacial & vascular development

Abstract

Membrane TypeI-Matrix Metalloproteinase (MT1-MMP/MMP14), a membrane anchored enzyme, is essential for diverse physiological and pathological processes through extracellular matrix remodeling and pericellular proteolysis. Mutation in MT1-MMP causes Winchester Syndome characterized by the multicentric osteolysis and arthritis. Deficiency in MT1-MMP leads to severe growth retardation, craniofacial deformities, angiogenic defects and premature death in mouse.

Previously, it was found that defective calvarial osteogenesis in Mmp14-/-mice is a consequence of compromised Fibroblast Growth Factor (FGF) signaling. However, the precise molecular mechanism by which MT1-MMP regulates FGF signaling remains unclear. In this study, I have elucidated the regulatory mechanism for MT1-MMP dependent FGF signaling in calvarial development. In calvarial osteoblasts, the loss of MT1-MMP reduces cell surface presentation of FGFR2 in conjunction with formation of truncated FGFR2 fragments. The increased ectodomain shedding of FGFR2, mainly mediated by ADAM9, subsequently compromises FGF signal perception on cell surface and results in defective FGF-induced proliferation and downstream signaling. Furthermore, I found that MT1-MMPcan form complex with FGFR2 and ADAM9, in whichMT1-MMP proteolytically inactivates ADAM9 to protect FGFR2 from shedding mediated by ADAM9. Interestingly, targeted deletion of Adam9completely restores the defective FGF signaling and largely rescues the calvarial bone defects in Mmp14-/- embryos. These results suggest that MT1-MMP inactivates ADAM9 to maintain optimal FGF signaling for calvarial development, revealing a novel regulatory mechanism for FGF signaling. However, this ADAM9/MT1-MMPdependentregulatory loop for FGF signaling is tissue-specific as depletion of ADAM9 fails to rescue defective FGF-2 induced corneal angiogenesis in Mmp14-/-mice. Besides the MT1-MMP and ADAM9 interaction, MT1-MMP also cleaves ADAM15. Indeed, loss of ADAM15 leads to aberrant spontaneous angiogenesis in Mmp14-/-corneas, indicating that the interplay between ADAM15 and MT1-MMP is essential for the maintenance of vascular homeostasis. Although MT1-MMP being a key regulator for angiogenesis is well established, little is known about its role in lymphangiogenesis.

Lymphangiogenesis plays an important role in various pathological conditions including tissue rejection in transplantation, tumorigenesis and metastasis. I here discovered that loss of MT1-MMP in mice results in extensive spontaneous corneal lymphangiogenesis without concomitant changes in angiogenesis, which can be detected as early as P8. The lymphangiogenic phenotype in Mmp14-/-mice is not tissue-specific; as lymphatic hyperplasia is also observed in the diaphragms. This lymphatic defect is likely a consequence of increased secretion of pro-lymphangiogenic factors from macrophages. Loss of MT1-MMP in macrophages causes increased production of VEGF-C/D. Mmp14-/- macrophages are more potent than wild-type macrophages to promote cell proliferation, cell migration and tube formation of lymphatic endothelial cells in vitro, which can be inhibited by blocking VEGF-C/D activities. Blocking VEGFR3 partially rescues the lymphatic defects of MT1-MMP deficient mice. These results reveal a novel functional crosstalk between MT1-MMP and VEGF-C/VEGFR3signaling cascade. These findings also suggest that MT1-MMP likely functions as anon-redundant physiological suppressor of lymphangiogenesis, partly via modulation of lymphangiogenic responses of macrophages.