Modelling of BCS1L-related human mitochondrial disease in Drosophila melanogaster

Uncovering a Novel Pathogenic Mechanism of BCS1L in Mitochondrial Disorders: Insights from Functional Studies on the c.38A>G Variant

The BCS1L gene encodes a mitochondrial chaperone which inserts the Fe2S2 iron-sulfur Rieske protein into the nascent electron transfer complex III. Variants in the BCS1L gene are associated with a spectrum of mitochondrial disorders, ranging from mild to severe phenotypes. Björnstad syndrome, a milder condition, is characterized by sensorineural hearing loss (SNHL) and pili torti. More severe disorders include Complex III Deficiency, which leads to neuromuscular and metabolic dysfunctions with multi-systemic issues and Growth Retardation, Aminoaciduria, Cholestasis, Iron Overload, and Lactic Acidosis syndrome (GRACILE). The severity of these conditions varies depending on the specific BCS1L mutation and its impact on mitochondrial function. This study describes a 27-month-old child with SNHL, proximal renal tubular acidosis, woolly hypopigmented hair, developmental delay, and metabolic alterations. Genetic analysis revealed a homozygous BCS1L variant (c.38A>G, p.Asn13Ser), previously reported in a patient with a more severe phenotype that, however, was not functionally characterized. In this work, functional studies in a yeast model and patient-derived fibroblasts demonstrated that the variant impairs mitochondrial respiration, complex III activity (CIII), and also alters mitochondrial morphology in affected fibroblasts. Interestingly, we unveil a new possible mechanism of pathogenicity for BCS1L mutant protein. Since the interaction between BCS1L and CIII is increased, this suggests the formation of a BCS1L-containing nonfunctional preCIII unable to load RISP protein and complete CIII assembly. These findings support the pathogenicity of the BCS1L c.38A>G variant, suggesting altered interaction between the mutant BCS1L and CIII.

Analysis and identification of mitochondria-related genes associated with age-related hearing loss

BACKGROUND

To explore the mitochondrial genes that play a key role in the occurrence and development of age-related hearing loss (ARHL) and provide a basis for the study of the mechanism of ARHL.

RESULTS

503 differentially expressed genes (DEGs) were detected in the GSE49543 dataset. 233 genes were up-regulated, and 270 genes were down-regulated. There are 1140 genes in the mitochondrial gene bank, and 28 differentially expressed genes related to ARHL. These genes are mainly involved in mitochondrial respiratory chain complex assembly, small molecule catabolism, NADH dehydrogenase complex assembly, organic acid catabolism, precursor metabolites and energy production, and mitochondrial span Membrane transport, metabolic processes of active oxygen species. Then, Cytoscape software identified the three key genes: Aco2, Bcs1l and Ndufs1. Immunofluorescence and Western blot experiments confirmed that the protein content of three key genes in aging cochlear hair cells decreased.

CONCLUSION

We employed bioinformatics analysis to screen 503 differentially expressed genes and identified three key genes associated with ARHL. Subsequently, we conducted in vitro experiments to validate their significance, thereby providing a valuable reference for further elucidating the role of mitochondrial function in the pathogenesis and progression of ARHL.

Structural rather than catalytic role for mitochondrial respiratory chain supercomplexes

Mammalian mitochondrial respiratory chain (MRC) complexes are able to associate into quaternary structures named supercomplexes (SCs), which normally coexist with non-bound individual complexes. The functional significance of SCs has not been fully clarified and the debate has been centered on whether or not they confer catalytic advantages compared with the non-bound individual complexes. Mitochondrial respiratory chain organization does not seem to be conserved in all organisms. In fact, and differently from mammalian species, mitochondria from Drosophila melanogaster tissues are characterized by low amounts of SCs, despite the high metabolic demands and MRC activity shown by these mitochondria. Here, we show that attenuating the biogenesis of individual respiratory chain complexes was accompanied by increased formation of stable SCs, which are missing in Drosophila melanogaster in physiological conditions. This phenomenon was not accompanied by an increase in mitochondrial respiratory activity. Therefore, we conclude that SC formation is necessary to stabilize the complexes in suboptimal biogenesis conditions, but not for the enhancement of respiratory chain catalysis.

Mitochondrial Neurodegeneration: Lessons from Drosophila melanogaster Models

The fruit fly-i.e., Drosophila melanogaster-has proven to be a very useful model for the understanding of basic physiological processes, such as development or ageing. The availability of straightforward genetic tools that can be used to produce engineered individuals makes this model extremely interesting for the understanding of the mechanisms underlying genetic diseases in physiological models. Mitochondrial diseases are a group of yet-incurable genetic disorders characterized by the malfunction of the oxidative phosphorylation system (OXPHOS), which is the highly conserved energy transformation system present in mitochondria. The generation of D. melanogaster models of mitochondrial disease started relatively recently but has already provided relevant information about the molecular mechanisms and pathological consequences of mitochondrial dysfunction. Here, we provide an overview of such models and highlight the relevance of D. melanogaster as a model to study mitochondrial disorders.

A Drosophila model of the neurological symptoms in Mpv17-related diseases

Mutations in the Mpv17 gene are responsible for MPV17-related hepatocerebral mitochondrial DNA depletion syndrome and Charcot-Marie-Tooth (CMT) disease. Although several models including mouse, zebrafish, and cultured human cells, have been developed, the models do not show any neurological defects, which are often observed in patients. Therefore, we knocked down CG11077 (Drosophila Mpv17; dMpv17), an ortholog of human MPV17, in the nervous system in Drosophila melanogaster and investigated the behavioral and cellular phenotypes. The resulting dMpv17 knockdown larvae showed impaired locomotor activity and learning ability consistent with mitochondrial defects suggested by the reductions in mitochondrial DNA and ATP production and the increases in the levels of lactate and reactive oxygen species. Furthermore, an abnormal morphology of the neuromuscular junction, at the presynaptic terminal, was observed in dMpv17 knockdown larvae. These results reproduce well the symptoms of human diseases and partially reproduce the phenotypes of Mpv17-deficient model organisms. Therefore, we suggest that neuron-specific dMpv17 knockdown in Drosophila is a useful model for investigation of MPV17-related hepatocerebral mitochondrial DNA depletion syndrome and CMT caused by Mpv17 dysfunction.

CG7630 is the Drosophila melanogaster homolog of the cytochrome c oxidase subunit COX7B

The mitochondrial respiratory chain (MRC) is composed of four multiheteromeric enzyme complexes. According to the endosymbiotic origin of mitochondria, eukaryotic MRC derives from ancestral proteobacterial respiratory structures consisting of a minimal set of complexes formed by a few subunits associated with redox prosthetic groups. These enzymes, which are the “core” redox centers of respiration, acquired additional subunits, and increased their complexity throughout evolution. Cytochrome c oxidase (COX), the terminal component of MRC, has a highly interspecific heterogeneous composition. Mammalian COX consists of 14 different polypeptides, of which COX7B is considered the evolutionarily youngest subunit. We applied proteomic, biochemical, and genetic approaches to investigate the COX composition in the invertebrate model Drosophila melanogaster. We identified and characterized a novel subunit which is widely different in amino acid sequence, but similar in secondary and tertiary structures to COX7B, and provided evidence that this object is in fact replacing the latter subunit in virtually all protostome invertebrates. These results demonstrate that although individual structures may differ the composition of COX is functionally conserved between vertebrate and invertebrate species.

Measurement of mitochondrial respiratory chain enzymatic activities in Drosophila melanogaster samples

Mitochondrial respiratory chain (MRC) dysfunction is linked to mitochondrial disease as well as other common conditions such as diabetes, neurodegeneration, cancer, and aging. Thus, the evaluation of MRC enzymatic activities is fundamental for diagnostics and research purposes on experimental models. Here, we provide a verified and reliable protocol for mitochondria isolation from various D. melanogaster samples and subsequent measurement of the activity of MRC complexes I–V plus citrate synthase (CS) through UV-VIS spectrophotometry.

For complete details on the use and execution of this protocol, please refer to Brischigliaro et al. (2021).

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A detailed and quick protocol to isolate mitochondria from D. melanogaster samples

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A step-by-step procedure to measure MRC enzymatic activities in isolated mitochondria

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A comprehensive guide for data analysis, with examples of validated systems