Insights from Drosophila on mitochondrial complex I.

NADH:ubiquinone oxidoreductase, more commonly referred to as mitochondrial complex I (CI), is the largest discrete enzyme of the oxidative phosphorylation system (OXPHOS). It is localized to the mitochondrial inner membrane. CI oxidizes NADH generated from the tricarboxylic acid cycle to NAD+, in a series of redox reactions that culminates in the reduction of ubiquinone, and the transport of protons from the matrix across the inner membrane to the intermembrane space. The resulting proton-motive force is consumed by ATP synthase to generate ATP, or harnessed to transport ions, metabolites and proteins into the mitochondrion. CI is also a major source of reactive oxygen species. Accordingly, impaired CI function has been associated with a host of chronic metabolic and degenerative disorders such as diabetes, cardiomyopathy, Parkinson's disease (PD) and Leigh syndrome. Studies on Drosophila have contributed to our understanding of the multiple roles of CI in bioenergetics and organismal physiology. Here, we explore and discuss some of the studies on Drosophila that have informed our understanding of this complex and conclude with some of the open questions about CI that can be resolved by studies on Drosophila.

Human mitochondrial MIA40 (CHCHD4) is a component of the Fe-S cluster export machinery.

Mitochondria play an essential role in synthesis and export of iron-sulfur (Fe-S) clusters to other sections of a cell. Although the mechanism of Fe-S cluster synthesis is well elucidated, information on the identity of the proteins involved in the export pathway is limited. The present study identifies hMIA40 (human mitochondrial intermembrane space import and assembly protein 40), also known as CHCHD4 (coiled-coil-helix-coiled-coil-helix domain-containing 4), as a component of the mitochondrial Fe-S cluster export machinery. hMIA40 is an iron-binding protein with the ability to bind iron in vivo and in vitro. hMIA40 harbours CPC (Cys-Pro-Cys) motif-dependent Fe-S clusters that are sensitive to oxidation. Depletion of hMIA40 results in accumulation of iron in mitochondria concomitant with decreases in the activity and stability of Fe-S-containing cytosolic enzymes. Intriguingly, overexpression of either the mitochondrial export component or cytosolic the Fe-S cluster assembly component does not have any effect on the phenotype of hMIA40-depleted cells. Taken together, our results demonstrate an indispensable role for hMIA40 for the export of Fe-S clusters from mitochondria.

Resting mitochondrial complex I from Drosophila melanogaster adopts a helix-locked state.

Respiratory complex I is a proton-pumping oxidoreductase key to bioenergetic metabolism. Biochemical studies have found a divide in the behavior of complex I in metazoans that aligns with the evolutionary split between Protostomia and Deuterostomia. Complex I from Deuterostomia including mammals can adopt a biochemically defined off-pathway 'deactive' state, whereas complex I from Protostomia cannot. The presence of off-pathway states complicates the interpretation of structural results and has led to considerable mechanistic debate. Here, we report the structure of mitochondrial complex I from the thoracic muscles of the model protostome Drosophila melanogaster. We show that although D. melanogaster complex I (Dm-CI) does not have a NEM-sensitive deactive state, it does show slow activation kinetics indicative of an off-pathway resting state. The resting-state structure of Dm-CI from the thoracic muscle reveals multiple conformations. We identify a helix-locked state in which an N-terminal α-helix on the NDUFS4 subunit wedges between the peripheral and membrane arms. Comparison of the Dm-CI structure and conformational states to those observed in bacteria, yeast, and mammals provides insight into the roles of subunits across organisms, explains why the Dm-CI off-pathway resting state is NEM insensitive, and raises questions regarding current mechanistic models of complex I turnover.

Phospholipids can regulate complex I assembly independent of their role in maintaining mitochondrial membrane integrity.

Several phospholipid (PL) molecules are intertwined with some mitochondrial complex I (CI) subunits in the membrane domain of CI, but their function is unclear. We report that when the Drosophila melanogaster ortholog of the intramitochondrial PL transporter, STARD7, is severely disrupted, assembly of the oxidative phosphorylation (OXPHOS) system is impaired, and the biogenesis of several CI subcomplexes is hampered. However, intriguingly, a restrained knockdown of STARD7 impairs the incorporation of NDUFS5 and NDUFA1 into the proximal part of the CI membrane domain without directly affecting the incorporation of subunits in the distal part of the membrane domain, OXPHOS complexes already assembled, or mitochondrial cristae integrity. Importantly, the restrained knockdown of STARD7 appears to induce a modest amount of cardiolipin remodeling, indicating that there could be some alteration in the composition of the mitochondrial phospholipidome. We conclude that PLs can regulate CI biogenesis independent of their role in maintaining mitochondrial membrane integrity.

The membrane domain of respiratory complex I accumulates during muscle aging in Drosophila melanogaster.

The boot-shaped respiratory complex I (CI) consists of a mitochondrial matrix and membrane domain organized into N-, Q- and P-modules. The N-module is the most distal part of the matrix domain, whereas the Q-module is situated between the N-module and the membrane domain. The proton-pumping P-module is situated in the membrane domain. We explored the effect of aging on the disintegration of CI and its constituent subcomplexes and modules in Drosophila flight muscles. We find that the fully-assembled complex remains largely intact in aged flies. And while the effect of aging on the stability of many Q- and N-module subunits in subcomplexes was stochastic, NDUFS3 was consistently down-regulated in subcomplexes with age. This was associated with an accumulation of many P-module subunits in subcomplexes. The potential significance of these studies is that genetic manipulations aimed at boosting, perhaps, a few CI subunits may suffice to restore the whole CI biosynthesis pathway during muscle aging.

IDH2-mediated regulation of the biogenesis of the oxidative phosphorylation system.

Several subunits in the matrix domain of mitochondrial complex I (CI) have been posited to be redox sensors for CI, but how elevated levels of reactive oxygen species (ROS) impinge on CI assembly is unknown. We report that genetic disruption of the mitochondrial NADPH-generating enzyme, isocitrate dehydrogenase 2 (IDH2), in Drosophila flight muscles results in elevated ROS levels and impairment of assembly of the oxidative phosphorylation system (OXPHOS). Mechanistically, this begins with an inhibition of biosynthesis of the matrix domain of CI and progresses to involve multiple OXPHOS complexes. Despite activation of multiple compensatory mechanisms, including enhanced coenzyme Q biosynthesis and the mitochondrial unfolded protein response, ferroptotic cell death ensues. Disruption of enzymes that eliminate hydrogen peroxide, but not those that eliminate the superoxide radical, recapitulates the phenotype, thereby implicating hydrogen peroxide as the signaling molecule involved. Thus, IDH2 modulates the assembly of the matrix domain of CI and ultimately that of the entire OXPHOS.

Analyzing the integrity of oxidative phosphorylation complexes in Drosophila flight muscles.

Drosophila flight muscles are highly enriched with mitochondria and have emerged as a powerful genetic system for studying how oxidative phosphorylation (OXPHOS) complexes are assembled. Here, we describe a series of protocols for analyzing the integrity of OXPHOS complexes in Drosophila via blue native polyacrylamide gel electrophoresis (BN PAGE). We have also included protocols for the additional steps that are typically performed after OXPHOS complexes are separated by BN PAGE, such as Coomassie staining, silver staining, and in-gel OXPHOS activities. For complete details on the use and execution of this protocol, please refer to Murari et al. (2020).

Dissecting the concordant and disparate roles of NDUFAF3 and NDUFAF4 in mitochondrial complex I biogenesis.

Distinct sub-assemblies (modules) of mitochondrial complex I (CI) are assembled with the assistance of CI Assembly Factors (CIAFs) through mechanisms that are incompletely defined. Here, using genetic analyses in Drosophila, we report that when either of the CIAFs - NDUFAF3 or NDUFAF4 - is disrupted, biogenesis of the Q-, N-, and PP-b-modules of CI is impaired. This is due, at least in part, to the compromised integration of NDUFS3 and NDUFS5 into the Q-, and PP-b-modules, respectively, coupled with a destabilization of another CIAF, TIMMDC1, in assembly intermediates. Notably, forced expression of NDUFAF4 rescues the biogenesis defects in the Q-module and some aspects of the defects in the PP-b-module of CI when NDUFAF3 is disrupted. Altogether, our studies furnish new fundamental insights into the mechanism by which NDUFAF3 and NDUFAF4 regulate CI assembly and raises the possibility that certain point mutations in NDUFAF3 may be rescued by overexpression of NDUFAF4.

An antibody toolbox to track complex I assembly defines AIF's mitochondrial function.

An ability to comprehensively track the assembly intermediates (AIs) of complex I (CI) biogenesis in Drosophila will enable the characterization of the precise mechanism(s) by which various CI regulators modulate CI assembly. Accordingly, we generated 21 novel antibodies to various mitochondrial proteins and used this resource to characterize the mechanism by which apoptosis-inducing factor (AIF) regulates CI biogenesis by tracking the AI profile observed when AIF expression is impaired. We find that when the AIF-Mia40 translocation complex is disrupted, the part of CI that transfers electrons to ubiquinone is synthesized but fails to progress in the CI biosynthetic pathway. This is associated with a reduction in intramitochondrial accumulation of the Mia40 substrate, MIC19. Importantly, knockdown of either MIC19 or MIC60, components of the mitochondrial contact site and cristae organizing system (MICOS), fully recapitulates the AI profile observed when AIF is inhibited. Thus, AIF's effect on CI assembly is principally due to compromised intramitochondrial transport of the MICOS complex.

In vitro characterization of a novel Isu homologue from Drosophila melanogaster for de novo FeS-cluster formation.

FeS-clusters are utilized by numerous proteins within several biological pathways that are essential for life. In eukaryotes, the primary FeS-cluster production pathway is the mitochondrial iron-sulfur cluster (ISC) pathway. In Saccharomyces cerevisiae, de novo FeS-cluster formation is accomplished through coordinated assembly with the substrates iron and sulfur by the scaffold assembly protein "Isu1". Sulfur for cluster assembly is provided by cysteine desulfurase "Nfs1", a protein that works in union with its accessory protein "Isd11". Frataxin "Yfh1" helps direct cluster assembly by serving as a modulator of Nfs1 activity, by assisting in the delivery of sulfur and Fe(ii) to Isu1, or more likely through a combination of these and other possible roles. In vitro studies on the yeast ISC machinery have been limited, however, due to the inherent instability of recombinant Isu1. Isu1 is a molecule prone to degradation and aggregation. To circumvent Isu1 instability, we have replaced yeast Isu1 with the fly ortholog to stabilize our in vitro ISC assembly system and assist us in elucidating molecular details of the yeast ISC pathway. Our laboratory previously observed that recombinant frataxin from Drosophila melanogaster has remarkable stability compared to the yeast ortholog. Here we provide the first characterization of D. melanogaster Isu1 (fIscU) and demonstrate its ability to function within the yeast ISC machinery both in vivo and in vitro. Recombinant fIscU has physical properties similar to that of yeast Isu1. It functions as a stable dimer with similar Fe(ii) affinity and ability to form two 2Fe-2S clusters as the yeast dimer. The fIscU and yeast ISC proteins are compatible in vitro; addition of Yfh1 to Nfs1-Isd11 increases the rate of FeS-cluster formation on fIscU to a similar extent observed with Isu1. Finally, fIscU expressed in mitochondria of a yeast strain lacking Isu1 (and its paralog Isu2) is able to completely reverse the deletion phenotypes. These results demonstrate fIscU can functionally replace yeast Isu1 and it can serve as a powerful tool for exploring molecular details within the yeast ISC pathway.

Mge1, a nucleotide exchange factor of Hsp70, acts as an oxidative sensor to regulate mitochondrial Hsp70 function.

Despite the growing evidence of the role of oxidative stress in disease, its molecular mechanism of action remains poorly understood. The yeast Saccharomyces cerevisiae provides a valuable model system in which to elucidate the effects of oxidative stress on mitochondria in higher eukaryotes. Dimeric yeast Mge1, the cochaperone of heat shock protein 70 (Hsp70), is essential for exchanging ATP for ADP on Hsp70 and thus for recycling of Hsp70 for mitochondrial protein import and folding. Here we show an oxidative stress-dependent decrease in Mge1 dimer formation accompanied by a concomitant decrease in Mge1-Hsp70 complex formation in vitro. The Mge1-M155L substitution mutant stabilizes both Mge1 dimer and Mge1-Hsp70 complex formation. Most important, the Mge1-M155L mutant rescues the slow-growth phenomenon associated with the wild-type Mge1 strain in the presence of H2O2 in vivo, stimulation of the ATPase activity of Hsp70, and the protein import defect during oxidative stress in vitro. Furthermore, cross-linking studies reveal that Mge1-Hsp70 complex formation in mitochondria isolated from wild-type Mge1 cells is more susceptible to reactive oxygen species compared with mitochondria from Mge1-M155L cells. This novel oxidative sensor capability of yeast Mge1 might represent an evolutionarily conserved function, given that human recombinant dimeric Mge1 is also sensitive to H2O2.