Mitochondrial metabolism controls alveolar epithelial cell fate

PROJECT SUMMARY/ABSTRACT Patients with severe pandemic SARS-CoV-2 pneumonia suffered damage of alveolar epithelial cells due to direct viral injury, subsequent immune response, and secondary bacterial pneumonia, which presents clinically as the acute respiratory distress syndrome (ARDS). Despite a similar severity of ARDS, some patients recover their lung function without sequelae, while others develop persistent respiratory symptoms and radiographic abnormalities, or progressive lung fibrosis resulting in death or requiring lung transplantation. The mechanisms driving the heterogeneous outcomes remain elusive. Mitochondrial dysfunction and metabolic changes are commonly observed in patients with severe pneumonia/ARDS and in patients with lung fibrosis but whether this dysfunction is causally related to failed epithelial repair after injury is not known. We focus on an intermediate epithelial cell population expressing genes characteristic of both alveolar epithelial type 2 (AT2) and type 1 (AT1) cells. These "transitional cells" are expanded during postnatal development and in several models of lung injury and fibrosis, and human fibrotic lungs. In our published and preliminary studies, we observed that mitochondrial complex I (MCI)-dependent NAD+ regeneration, independent of ATP synthesis, is necessary for postnatal alveologenesis. Rather than inducing a metabolic crisis and cell death, lung epithelial- specific deletion of NDUFS2, an essential MCI subunit protein, prevented AT2-to-AT1 differentiation resulting in a dramatic expansion of transitional cells and subsequent death of the animal from respiratory failure. Transitional cells lacking MCI function demonstrate activation of the integrated stress response (ISR) and a small molecule inhibitor of the ISR rescued the lethality of the knockout mice. I also observed that loss of NDUFS2 in adult AT2 cells leads to the spontaneous development of lung fibrosis and death of the animal from respiratory failure within several months, highlighting the potential importance of this pathway in lung fibrosis. Collectively, we hypothesize that the loss of MCI function increases the mitochondrial NADH/NAD+ ratio through a pathway that requires OMA1, DELE1, and HRI to activate the ISR and enhance ATF4-mediated transcription, precluding normal alveolar epithelial differentiation. I will test this hypothesis in the following two aims: Aim 1: To determine whether an increased mitochondrial NADH/NAD+ ratio and DELE1 are necessary for ISR activation that precludes AT2 to AT1 differentiation in the absence of mitochondrial complex I. Aim 2: To determine whether epithelial ATF4 activation is necessary and/or sufficient for impaired AT2 to AT1 differentiation. We propose causal experiments using sophisticated genetic murine models to link mitochondrial metabolism, activation of the ISR, and failed epithelial differentiation to the development of fibrosis. We pair our experiments with samples collected from patients with pulmonary fibrosis at the time of lung transplant, with a goal of credentialling mitochondrial metabolism and the ISR as targets for therapy to prevent and treat lung fibrosis.