Abstract
Mitochondrial dysfunction is associated with activation of the integrated stress response (ISR) but the underlying triggers remain unclear. We systematically combined acute mitochondrial inhibitors with genetic tools for compartment-specific NADH oxidation to trace mechanisms linking different forms of mitochondrial dysfunction to the ISR in proliferating mouse myoblasts and in differentiated myotubes. In myoblasts, we find that impaired NADH oxidation upon electron transport chain (ETC) inhibition depletes asparagine, activating the ISR via the eIF2α kinase GCN2. In myotubes, however, impaired NADH oxidation following ETC inhibition neither depletes asparagine nor activates the ISR, reflecting an altered metabolic state. ATP synthase inhibition in myotubes triggers the ISR via a distinct mechanism related to mitochondrial inner-membrane hyperpolarization. Our work dispels the notion of a universal path linking mitochondrial dysfunction to the ISR, instead revealing multiple paths that depend both on the nature of the mitochondrial defect and on the metabolic state of the cell.
Highlights
The mitochondrial electron transport chain (ETC) supports myriad, mechanistically linked functions
We show that ETC inhibition elevates the mitochondrial and cytosolic NADH/NAD+ ratios, hindering aspartate synthesis and depleting asparagine, which activates the integrated stress response (ISR) via the eIF2a kinase GCN2
Several recent studies have reported activation of the integrated stress response (ISR) during mitochondrial dysfunction, tracing the underlying mechanisms has proved challenging. We interrogated this response in two cell states, proliferating myoblasts and differentiated myotubes, by employing a battery of small-molecule inhibitors of mitochondria in combination with genetic and chemical tools for selectively buffering their effects
Summary
The mitochondrial electron transport chain (ETC) supports myriad, mechanistically linked functions. In tissues affected by mitochondrial disease, the ISR activates the mitochondrial 1carbon pathway (Kuhl et al, 2017; Tyynismaa et al, 2010; Bao et al, 2016; Nikkanen et al, 2016) and is thought to underlie secretion of the circulating cytokines fibroblast growth factor 21 (FGF21) and growth/differentiation factor 15 (GDF15) that are under consideration as disease biomarkers (Lehtonen et al, 2016; Fujita et al, 2015; Yatsuga et al, 2015; Chung et al, 2017; Miyake et al, 2016; Restelli et al, 2018). Our results reject the notion of a universal trigger of the ISR in mitochondrial dysfunction, and rather, reveal multiple paths to its activation that depend both on the nature of the mitochondrial defect and on the metabolic state of the cell
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