Abstract

Lymphatic valves are essential for maintaining tissue fluid homeostasis and their dysfunction leads to lymphedema, a morbid and disfiguring disease without a cure. Mechanical forces due to lymph flow are required for proper lymphatic valve development, yet it remains unclear how lymphatic endothelial cells (LECs) sense and decode mechanical signals. In this study, we identify the cell guidance semaphorin receptor Plexin D1 (PLXND1) as a lymphatic mechanosensor required for lymphatic valve morphogenesis. Conditional genetic ablation of plxnd1 in LECs causes major defects in lymphatic valve development in two different lymphatic vascular beds. Mechanistically, PLXND1 acts as a mechanosensor within a lymphatic mechanocomplex, initiating distinct mechanical signals and activating the lymphatic valve transcriptional program through an unconventional pathway. Screening of primary lymphoedema patients identified PLXND1 missense variants and functional analysis establishes two pathogenic variants that selectively disrupt the ligand vs mechanosensing functions of this receptor. Variants associated with lymphoedema in members of the mechanocomplex disrupt its formation, underscoring the central role of this complex in lymphatic valve biology. Our work uncovers a novel mechanosensing mechanism guiding lymphatic valve development, with profound implications for the understanding and treatment of primary lymphoedema in humans.