Abstract
Mitochondrial disorders are a group of pathologies characterized by impairment of mitochondrial function mainly due to defects of the respiratory chain and consequent organellar energetics. This affects organs and tissues that require an efficient energy supply, such as brain and skeletal muscle. They are caused by mutations in both nuclear- and mitochondrial DNA (mtDNA)-encoded genes and their clinical manifestations show a great heterogeneity in terms of age of onset and severity, suggesting that patient-specific features are key determinants of the pathogenic process. In order to correlate the genetic defect to the clinical phenotype, we used a cell culture model consisting of fibroblasts derived from patients with different mutations in the mtDNA-encoded ND5 complex I subunit and with different severities of the illness. Interestingly, we found that cells from patients with the 13514A>G mutation, who manifested a relatively late onset and slower progression of the disease, display an increased autophagic flux when compared with fibroblasts from other patients or healthy donors. We characterized their mitochondrial phenotype by investigating organelle turnover, morphology, membrane potential and Ca2+ homeostasis, demonstrating that mitochondrial quality control through mitophagy is upregulated in 13514A>G cells. This is due to a specific downregulation of mitochondrial Ca2+ uptake that causes the stimulation of the autophagic machinery through the AMPK signaling axis. Genetic and pharmacological manipulation of mitochondrial Ca2+ homeostasis can revert this phenotype, but concurrently decreases cell viability. This indicates that the higher mitochondrial turnover in complex I deficient cells with this specific mutation is a pro-survival compensatory mechanism that could contribute to the mild clinical phenotype of this patient.
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Abbreviations
- AEQ:
-
aequorin
- AMPK:
-
AMP-activated protein kinase
- ATP:
-
adenosine triphosphate
- ΔΨmt:
-
mitochondrial membrane potential
- DMEM:
-
Dulbecco’s modified Eagle’s medium
- EMRE:
-
essential MCU regulator
- ETC:
-
electron transport chain
- GFP:
-
green fluorescent protein
- KRB:
-
Krebs–Ringer buffer
- LC3:
-
microtubule-associated protein 1 A/1B light chain 3B
- MCU:
-
mitochondrial Ca2+ uniporter
- MELAS:
-
mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes syndrome
- Mfn2:
-
Mitofusin2
- MICU1:
-
mitochondrial Ca2+ uptake 1
- mtDNA:
-
mitochondrial DNA
- OMM:
-
outer mitochondrial membrane
- PBS:
-
phosphate-buffered saline
- SQSTM1/p62:
-
sequestosome1
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Acknowledgements
We are grateful to Cristina Mammucari and Gyorgy Szabadkai for helpful discussion. This work was supported by grants from Telethon-Italy (GPP10005A), the Italian Ministries of Health (Ricerca Finalizzata) and of Education, University and Research (PRIN, FIRB), the European Union (ERC mitoCalcium, no. 294777), National Institutes of Health (Grant no. 1P01AG025532-01A1), Cariparo and Cariplo Foundations (Padua), the Italian Association for Cancer Research (AIRC). TC is supported by the University of Padova (Progetto Giovani GRIC128SP0, Bando 2012).
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Granatiero, V., Giorgio, V., Calì, T. et al. Reduced mitochondrial Ca2+ transients stimulate autophagy in human fibroblasts carrying the 13514A>G mutation of the ND5 subunit of NADH dehydrogenase. Cell Death Differ 23, 231–241 (2016). https://doi.org/10.1038/cdd.2015.84
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DOI: https://doi.org/10.1038/cdd.2015.84
