Lymphatic dysfunctions are associated with several human diseases, including lymphedema and metastatic spread of cancer. Although it is well recognized that lymphatic capillaries attach directly to interstitial matrix mainly composed of fibrillar type I collagen, the interactions occurring between lymphatics and their surrounding matrix have been overlooked. In this study, we demonstrate how matrix metalloproteinase (MMP)-2 drives lymphatic morphogenesis through Mmp2-gene ablation in mice, mmp2 knockdown in zebrafish and in 3D-culture systems, and through MMP2 inhibition. In all models used in vivo (3 murine models and thoracic duct development in zebrafish) and in vitro (lymphatic ring and spheroid assays), MMP2 blockage or down-regulation leads to reduced lymphangiogenesis or altered vessel branching. Our data show that lymphatic endothelial cell (LEC) migration through collagen fibers is affected by physical matrix constraints (matrix composition, density, and crosslinking). Transmission electron microscopy and confocal reflection microscopy using DQ-collagen highlight the contribution of MMP2 to mesenchymal-like migration of LECs associated with collagen fiber remodeling. Our findings provide new mechanistic insight into how LECs negotiate an interstitial type I collagen barrier and reveal an unexpected MMP2-driven collagenolytic pathway for lymphatic vessel formation and morphogenesis. (Blood. 2012;119(21):5048-5056)
IntroductionLymphatic vessels are essential for body fluid balance and immunologic surveillance. Several pathologies manifest abnormal lymphatic vessel growth (lymphangiogenesis), either excessive (inflammatory diseases, tumor metastases, cornea-graft rejection) or defective (lymphedema). [1][2][3] Our understanding of the molecular mechanisms underlying lymphangiogenesis is still in its infancy. Although several key molecular determinants (vascular endothelial growth factor [VEGF]-C/D, VEGF receptor-3, angiopoietin, ephrins, integrins, etc) have been identified, 2,4-7 the importance of cell-matrix interactions during the lymphangiogenic process is not well documented.One of the features that discriminates lymphatic capillaries from blood capillaries is the kind of extracellular matrix (ECM) to which each vessel type is exposed in its natural environment. Blood vascular endothelium is in direct contact with basement membrane components (laminin, type IV collagen), 8 whereas the basal lamina is largely absent in lymphatic capillaries. Lymphatic endothelial cells (LECs) interact intimately with the adjacent interstitial matrix and attach to it through anchorage filaments. 9,10 Therefore, LECs sprouting from preexisting vessels and their migration through the ECM require that they be able to negotiate an interstitial collagen barrier mainly composed of fibrillar type I collagen. Recently, evidence has accumulated demonstrating that integrins, by promoting cell-matrix interaction, influence normal lymphatic function and lymphangiogenesis. 5,10,11 How LECs deal with fibrillar collagens to migrate an...