Gcn2 Regulates Bmp Signaling: Consequence For Pvod Pathobiology And Therapeutic Management

Introduction Pulmonary veno-occlusive disease (PVOD) occurs in humans as heritable form due to biallelic inactivating mutations of EIF2AK4 (encoding GCN2), or as a sporadic form. The pathobiology of this orphan fatal disease is currently unknown. However, the identification of EIF2AK4 as the major genetic cause of PVOD has clarified the nosology of the disease. On the other hand, BMP/BMPR2 axis is a major axis of pulmonary arterial hypertension pathobiology, a close but better understood form of severe pulmonary hypertension (PH). Objective To investigate the functional relationship between GCN2 and the BMP/BMPR2 axis. Methods We investigated GCN2 and the BMP/BMPR2 axis in human and animal models of PVOD, and in cultured human pulmonary microvascular endothelial cells (hPMEC) with interventional experiments (siRNA and pharmacological inhibitors/activators). Results Pulmonary GCN2 protein expression was decreased in all forms of PVOD. We showed that GCN2 loss-of-function negatively regulates BMP-dependent SMAD1/5/9 signaling in hPMEC. This molecular relationship was confirmed in vivo, in the lungs of a newly created transgenic rat model knock out for Eif2ak4 (Δ152Ex1/Δ152Ex1). We found a 4 fold decrease in SMAD1/5/9 phosphorylation in KO rats. Since those rats don’t have spontaneous PH, the decrease in SMAD1/5/9 phosphorylation is not the mere consequence of high pulmonary artery pressures. We showed this regulation may be mediated through GCN2-dependent repression of chordin, an extracellular antagonist of BMP signaling. GCN2 inhibition induced a dramatic increase in hPMEC proliferation, which is highly relevant of PVOD genesis. BMP9 treatment was able to block this exuberant proliferation. Conclusion GCN2 loss-of-function negatively regulates SMAD1/5/9 phosphorylation. Despite this dampened BMP signaling, exogenous BMP9 was still able to reverse GCN2 inhibition-induced hPMEC proliferation. BMP9 may hence be considered as potential therapeutic options for PVOD.