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Resumen:
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Chronic mountain sickness (CMS) is a disease that potentially threatens a large segment of the high altitude population (more than 140 million highlanders) during extended living over 2500m. Patients with CMS suffer from severe hypoxemia, erythrocytosis and neurologic manifestations including migraine, headache, mental fatigue, confusion, and memory loss. The only therapy, which is inadequate, is periodic phlebotomy. By understanding the mechanisms of CMS neuropathology, we wish to be able to develop better therapeutic strategies, not only for CMS but also for those diseases that are relevant at the sea level as well. In the current study, we used induced pluripotent stem cells (iPSCs)-derived neurons from subjects with CMS and others without any evidence of CMS (non-CMS) living at the same altitude, as a model to study CMS in vitro. We noticed that CMS neurons are more sensitive to the culture environment than non-CMS, with more blebbing or beadings of dendrites after 3 weeks in culture. Although intracellular ATP level is significantly decreased in the CMS group, there was no difference in LDH release between the two groups. Transmission electron microscopic images illustrated that CMS neurons have a decreased mitochondrial volume density, cristae and mitochondrial length. Real-time PCR analysis confirmed that CMS neurons have 1) decreased mitochondrial biogenesis gene such as PGC1 and increased mitophagy genes including PARKIN and PINK; 2) decreased fission and fusion genes including DRP1, FIS, OPA, and MFN2, as well as 3) many altered nuclear and mitochondrial gene including UCP2, ANT1, VDAC, BCL2-XL, NMNAT1, and VEGF, suggesting the present of mitochondrial dysfunction in CMS neurons. Within these genes, NMNAT1, a central enzyme of NAD biogenesis, attracts attention because overexpression of NMNAT1 is believed to be neuroprotective in neuronal morphogenesis. Decreased NAD level, due to a down-regulation of NMNAT1, has been shown to disrupt the nuclear-mitochondrial communication and lead to mitochondrial dysfunction and render cells in a pseudohypoxic state under normoxia. Thus, we speculate that a similar phenomenon takes place in CMS neurons; indeed we have confirmed that both NMNAT1 gene expression and NAD level are lower in CMS neurons as compared to non-CMS neurons. We conclude that a decreased NAD level might be one of reasons for mitochondrial dysfunction and results in a vulnerable survival phenotype in CMS neurons.
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