Abstract 3153: A Critical Role of the “ER-Mitochondria Unit” and the Reticulon Protein Nogo in Pulmonary Arterial Hypertension (PAH)
The close proximity between the endoplasmic reticulum (ER, a major regulator of [Ca++]i) and mitochondria (a major regulator of redox signaling and apoptosis) facilitates their interaction which is important in regulating respiration, mitochondrial membrane potential (++m) and [Ca++]i. The disruption of this functional unit has been implicated in many diseases associated with the ER stress response. Because mitochondria-driven redox signaling, apoptosis and [Ca++]i are all important for the pro-proliferative and anti-apoptotic environment within the pulmonary artery (PA) wall in PAH, we hypothesized that the ER-mitochondria unit plays a role in the pathogenesis of PAH. We showed that chronic hypoxia induces a classic ER stress response in mouse PA smooth muscle cell (PASMC) associated with over-expression of Nogo, a reticulon protein implicated in the regulation of ER stress and ER shape. This was associated with PASMC ΔΨm hyperpolarization, increased [Ca++]i, activation of the Ca++-sensitive transcription factor NFAT and down-regulation of Kv1.5 (all features of rodent and human PAH). Exogenous over-expression of Nogo in normoxic PASMC mimicked hypoxia and induced this PAH phenotype. In contrast, PASMC lacking Nogo (obtained from Nogo−/−mice, which have a normal phenotype) did not show any of the above changes or evidence of ER stress when exposed to hypoxia. In addition, the minimal distance between the ER and the mitochondria, measured by electron microscopy, increased upon exposure to hypoxia in Nogo+/+ but not Nogo−/− PASMC. In vivo, Nogo+/+ mice developed PAH after 3 weeks of hypoxia (mean PAP 33±1 mmHg, RV/LV+septum 0.37±0.02) and displayed all of the in vitro changes described above; while Nogo−/− mice were completely resistant to PAH (mean PAP 17±2 mmHg, RV/LV±septum 0.23±0.01 (p>0.05 for all, n=9 –10). Lastly, in human PAH tissues (n=3) Nogo was significantly over-expressed in PA media compared to normal lungs (n=3) and similar changes on the ER-mitochondria distance were present. We show for the first time that the ER-mitochondria unit might be important in the vascular remodeling in PAH and we propose that Nogo might be a novel therapeutic target.