LSC - 2024 - The lung microvasculature promotes alveolar type 2 cell differentiation via secreted SPARCL1

<bold>Background:</bold> The mammalian lung is highly vascularized to allow for efficient oxygenation of blood. Despite the prevailing image of lung capillaries as passive conduits for blood, they associate with the respiratory epithelium before birth, suggesting an instructive role on neighboring cells. <bold>Aims:</bold> We hypothesized that lung endothelial cells (ECs) secrete factors instructing alveolar epithelial cell differentiation. In this work, we aimed to identify novel molecular mechanisms of vascular-epithelial crosstalk in alveologenesis. <bold>Methods:</bold> Focusing on factors secreted by ECs, we identified candidate genes with increasing expression in late embryonic stages in the mouse lung. Next, we screened recombinant candidate proteins using fetal lung alveolar organoids. Transcriptomic and pharmacological modulation experiments were used to determine the signal transduction pathways activated in epithelial cells in response to EC-secreted factors. <bold>Results:</bold> We identified SPARCL1 as a vascular extracellular matrix molecule capable of promoting alveolar type 2 (AT2) cell differentiation <italic>in vitro</italic>. SPARCL1 induced a dose-dependent increase in <italic>lysozyme</italic> mRNA levels in organoids and is downregulated in hypovascular lungs. Recombinant SPARCL1 treatment of lung organoids induced the upregulation of NFkB targets, and repression of NFkB activation blocked SPARCL1 effects. Ongoing experiments aim to test whether SPARCL1 overexpression <italic>in vivo</italic> leads to increased AT2 cell numbers. <bold>Conclusions:</bold> Our data indicate that extracellular SPARCL1 promotes AT2 cell maturation through epithelial NFkB activation. We thus propose a potential value in targeting the epithelial SPARCL1-NFkB differentiation axis to promote lung maturation.