Rationale Acute pulmonary oxygen sensing is essential to avoid life-threatening hypoxemia via hypoxic pulmonary vasoconstriction (HPV) which matches perfusion to ventilation. Hypoxia-induced mitochondrial superoxide release has been suggested as critical step in the signaling pathway underlying HPV. However, the identity of the primary oxygen sensor and mechanism of superoxide release in acute hypoxia, as well as its relevance for chronic pulmonary oxygen sensing remains unresolved. Objectives To investigate the role of the pulmonary specific isoform 2 of subunit 4 of mitochondrial complex IV (Cox4i2) and the subsequent mediators superoxide and hydrogen peroxide for pulmonary oxygen sensing and signaling. Methods and Results Isolated ventilated and perfused lungs from Cox4i2−/− mice lacked acute HPV. In parallel, pulmonary arterial smooth muscle cells (PASMCs) from Cox4i2−/− mice showed no hypoxia-induced increase of intracellular calcium. Hypoxia-induced superoxide release which was detected by electron spin resonance spectroscopy in wild type (WT) PASMCs was absent in Cox4i2−/− PASMCs and was dependent on cysteine residues of Cox4i2. HPV could be inhibited by mitochondrial superoxide inhibitors proving functional relevance of superoxide release for HPV. Mitochondrial hyperpolarization, which can promote mitochondrial superoxide release, was detected during acute hypoxia in WT but not Cox4i2−/− PASMCs. Downstream signaling determined by patch clamp measurements showed decreased hypoxia-induced cellular membrane depolarization in Cox4i2−/− PASMCs compared to WT PASMCs, which could be normalized by application of hydrogen peroxide. In contrast, chronic hypoxia-induced pulmonary hypertension and pulmonary vascular remodeling were not or only slightly affected by Cox4i2 deficiency, respectively. Conclusion Cox4i2 is essential for acute but not chronic pulmonary oxygen sensing by triggering mitochondrial hyperpolarization and release of mitochondrial superoxide which, after conversion to hydrogen peroxide, contributes to cellular membrane depolarization and HPV. These findings provide a new model for oxygen sensing processes in the lung and possibly also in other organs.

Our results indicate an important role of TRPC1 in pulmonary vascular remodeling underlying the development of hypoxia-induced PH.

Oxygen (O 2 ) is essential for the viability and function of most metazoan organisms and thus is closely monitored at both the organismal and the cellular levels. However, alveoli often encounter decreased O 2 levels (hypoxia), leading to activation of physiological or pathophysiological responses in the pulmonary arteries. Such changes are achieved by activation of transcription factors. The hypoxia-inducible factors (HIFs) are the most prominent hypoxiaregulated transcription factors in this regard. HIFs bind to hypoxiaresponse elements (HREs) in the Christine Veith studied biology at Justus-Liebig-University Giessen, Germany and joined the department of Internal Medicine there in 2008. After finishing her dissertation in 2012 she became a postdoc. Her research is mainly focused on the dysregulation of cytoskeletal proteins in the pathogenesis of pulmonary hypertension, with special interest in hypoxia/HIF-regulated pathways contributing to pulmonary vascular remodelling in pulmonary hypertension. Norbert Weissmann studied biology at Justus-Liebig-University, Giessen. After his graduation as PhD, he worked as a postdoc researcher and subsequently as research assistant and University Lecturer at the Department of Internal Medicine at Justus-Liebig-University. There he became Extraordinary Professor for Pathophysiology and Experimental Pneumology in 2007, and full Professor for Molecular Mechanisms of Emphysema, Hypoxia and Lung Aging in 2008 at the Excellence Cluster Cardio-Pulmonary System (ECCPS). His research centred on pulmonary oxygen sensing and signalling, including pulmonary hypertension. Moreover, he is interested in mechanisms of chronic obstructive lung disease and accelerated ageing of the lung. Ralf P. Brandes studied medicine at Hannover Medical School, Hannover, Germany and Emory University, Atlanta, GA, USA. promoter region of target genes, whose expression and translation allows the organism, amongst other factors, to cope with decreased environmental O 2 partial pressure (pO 2 ). However, prolonged HIF activation can contribute to major structural alterations, especially in the lung, resulting in the development of pulmonary hypertension (PH). PH is characterized by a rise in pulmonary arterial pressure associated with pulmonary arterial remodelling, concomitant with a reduced intravascular lumen area. Patients with PH develop right heart hypertrophy and eventually die from right heart failure. Thus, understanding the molecular mechanisms of HIF regulation in PH is critical for the identification of novel therapeutic strategies. This review addresses the relationship of hypoxia and the HIF system with pulmonary arterial dysfunction in PH. We particularly focus on the cellular and molecular mechanisms underlying the HIF-driven pathophysiological processes.