Function of hTim8a in complex IV assembly in neuronal cells provides insight into pathomechanism underlying Mohr-Tranebjærg syndrome.

Diana Stojanovski,Ann E Frazier,Thomas Daniel Jackson,Tegan Stait,David A Stroud,Deidreia Tull,Nicholas J Clemons,Yilin Kang,David R Thorburn,Malcolm J McConville,Catherine S Palmer,David P De Souza,Alexander J Anderson,Kenji M Fujihara,Michael T Ryan

doi:10.7554/elife.48828

Diana Stojanovski, Ann E Frazier + Show 13 more

Open Access

https://doi.org/10.7554/elife.48828

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Abstract

Human Tim8a and Tim8b are members of an intermembrane space chaperone network, known as the small TIM family. Mutations in TIMM8A cause a neurodegenerative disease, Mohr-Tranebjærg syndrome (MTS), which is characterised by sensorineural hearing loss, dystonia and blindness. Nothing is known about the function of hTim8a in neuronal cells or how mutation of this protein leads to a neurodegenerative disease. We show that hTim8a is required for the assembly of Complex IV in neurons, which is mediated through a transient interaction with Complex IV assembly factors, in particular the copper chaperone COX17. Complex IV assembly defects resulting from loss of hTim8a leads to oxidative stress and changes to key apoptotic regulators, including cytochrome c, which primes cells for death. Alleviation of oxidative stress with Vitamin E treatment rescues cells from apoptotic vulnerability. We hypothesise that enhanced sensitivity of neuronal cells to apoptosis is the underlying mechanism of MTS.

Highlights

Mitochondria are fundamental cellular organelles governing many metabolic processes including fatty acid oxidation, the Krebs cycle, oxidative phosphorylation (OXPHOS), and fatty acid ß-oxidation (Kasahara and Scorrano, 2014; McBride et al, 2006)
The parameters measured: (i) mitochondrial membrane potential (Figure 9I); (ii) apoptosis sensitivity following STS treatment (Figure 9J); (iii) oxidative stress (Figure 9K); and (iv) apoptosis sensitivity due to oxidative stress (Figure 9L) were all reverted in hTim8MUT SH cells expressing the COX17 (Figure 9H), suggesting that defective Complex IV and the corresponding oxidative stress is initiating downstream sensitisation of SH-SY5Y cells lacking hTim8a to death. This is the first study to comprehensively analyse the function of TIM chaperones across different human cell models and our findings highlight the functional specialisation of mitochondrial chaperones in different tissues
While loss of hTim9 resulted in profound disruption of the TIM22-biogenesis pathway, removal of hTim8a or hTim8b from both HEK293 or SH-SY5Y cells showed no impact to the biogenesis of the TIM22 complex or associated substrates, in particular hTim23, whose perturbed import has been suggested to be the underlying basis of Mohr-Tranebjærg syndrome (MTS) (Rothbauer et al, 2001)

Summary

Introduction

Mitochondria are fundamental cellular organelles governing many metabolic processes including fatty acid oxidation, the Krebs cycle, oxidative phosphorylation (OXPHOS), and fatty acid ß-oxidation (Kasahara and Scorrano, 2014; McBride et al, 2006). Organelle dysfunction is associated with a broad spectrum of diseases, including mitochondrial diseases that are genetic, often inherited disorders associated with energy generation defects (Nunnari and Suomalainen, 2012; Frazier et al, 2019). The health and functionality of the mitochondrial network relies on the biogenesis of approximately 1500 nuclear encoded proteins, which are imported into mitochondria using sophisticated. The biogenesis of many hydrophobic membrane proteins relies on the small TIM proteins, a chaperone network in the intermembrane space that function to shield hydrophobic proteins as they passage this aqueous space (Curran et al, 2002a; Curran et al, 2002b; Endres et al, 1999; Hoppins and Nargang, 2004; Wiedemann et al, 2004)

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