Introduction: 
Triggered by passive congestion of blood from the left heart into the pulmonary vasculature, PH-LHD is further aggravated by pulmonary arterial (PA) remodeling, the pathomechanism of which is little understood. Here, we addressed the role of the endothelial cell (EC)-derived basement membrane (BM) in regulating smooth muscle cell (SMC) dynamics leading to neointimal hyperplasia (NH) in PH-LHD.

Methods and Results: By histological analysis of human PA samples collected during heart transplantation, we identified SMC-formed NH as a hallmark of PH-LHD. NH increased PA wall thickness up to 10% and correlated with mean pulmonary arterial pressure indicating a potential role in PH pathogenesis. In vitro evaluation of SMCs isolated from PAs of PH-LHD patients revealed a significant increase in cell migration and proliferation, as compared to SMCs from control PAs. Further transcriptome analysis and immunohistological detection of increased levels of BM markers collagen IV and laminin identified BM remodeling as an early event in PA remodeling in LHD patients prior to clinical pulmonary hypertension (PH) and NH. Based on these findings we hypothesized that the remodeled endothelial BM may trigger NH by enhancing SMC migration and proliferation. Indeed EC-produced de-cellularized BM from PAs of LHD patients without PH increased proliferation and migration in control SMCs, while control BM inhibited these responses in PH-LHD SMCs. In accordance with YAP-1 being a mechanosensitive transcriptional co-activator of the Hippo signaling pathway potentially mediating effects of the BM on SMCs, nuclear (active) YAP-1 protein was detected in SMCs within the neointimal region in PH-LHD PAs. Concomitantly, in vitro experiments showed that de-cellularized BM produced by ECs of LHD patients with or without PH, could increase nuclear YAP-1 in control SMCs, while control BM decreased it in PH-LHD SMCs. YAP-1 inhibition by Verteporfin decreased YAP-1 nuclear translocation in isolated primary SMCs, and attenuated the BM-induced increase in SMC proliferation and migration.

Conclusion: Our findings identify remodeling of the endothelial BM as an early event in PH-LHD development that enhances SMC proliferation and migration via YAP-1 activation, potentially leading to NH.  BM remodeling and downstream YAP-1 activation present promising therapeutic targets for preventing NH in PH-LHD.

Acknowledgements: This work was supported by the DZHK, the BMBF, the DSHF, the DGK, and the DFG.