Mitochondrial Integrated Stress Response in Neurological Diseases

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Mitochondria play essential roles in cell biology because are central hubs of most metabolic pathways. They are not only essential for energy conversion, but also for the biosynthesis and catabolism of virtually all cell constituents. Mitochondrial dysfunction causes havoc in all cells, but especially in those cell types that are highly dependent on mitochondrial energetic and metabolic functions, such as neurons and glia. Genetic alterations of the mitochondrial proteome, which includes more than 1000 proteins, encoded by both the nuclear and the mitochondrial genomes, result in primary mitochondrial disorders. These diseases, for which there is currently no effective treatment, result in severe and often fatal neurodegeneration. Mitochondrial dysfunction also plays a role in the pathogenesis of many age-related neurodegenerative disorders, such as Alzheimer and Parkinson disease and ALS. Therefore, addressing therapeutically the consequences of mitochondrial dysfunction could have a profound impact on the treatment of many human disorders. A major challenge in devising effective treatments for mitochondrial encephalopathies is our limited understanding of the ramifications of the effects of mitochondrial dysfunction. The conventional view that these disorders are caused simply by energy failure is inadequate, as it is becoming increasingly clear that mitochondrial dysfunction affects much more than just ATP generation and leads to an extensive rewiring of cell metabolism. An exciting new development in the field is the observation that various types of mitochondrial dysfunction activate transcriptional and metabolic responses that involve multiple stress signaling pathways. We and others have identified a “mitochondrial integrated stress response” (mtISR) in diverse genetic forms of mitochondrial disorders, suggesting that mtISR is strongly associated with mitochondrial diseases and a potential pathogenic common denominator. We postulate that, while in the short term these responses may be compensatory, if sustained and unresolved, they become maladaptive and causes imbalances of key metabolites, which may be more detrimental than the energy defect itself. While we now fully appreciate these maladaptive mechanisms in peripheral tissues, such as muscle and heart, very little is known about them in the CNS affected by mitochondrial encephalopathies. A deeper knowledge of the characteristics and the consequences of the mtISR in the CNS is needed to understand its pathogenic significance and develop targets therapeutic strategies. Our research group has a long-standing commitment to investigating the pathogenic mechanisms of mitochondrial diseases and we have accumulated over two decades of expertise in studying the mechanisms of mitochondrial encephalopathies and mitochondrial dysfunction in neurodegeneration. In this R35 application, we focus on fundamental gaps in knowledge on the mtISR in mitochondrial encephalopathies by studying disease models that recapitulate human diseases. We will use a series of approaches, both established and technologically innovative, to generate a blueprint of the metabolic rewiring in the diseased CNS and identify targets potentially responsive to therapeutic modulation.