SERAC1 is a component of the mitochondrial serine transporter complex required for the maintenance of mitochondrial DNA

Mitochondrial membrane synthesis, remodelling and cellular trafficking

Mitochondria are dynamic cellular organelles with complex roles in metabolism and signalling. Primary mitochondrial disorders are a group of approximately 400 monogenic disorders arising from pathogenic genetic variants impacting mitochondrial structure, ultrastructure and/or function. Amongst these disorders, defects of complex lipid biosynthesis, especially of the unique mitochondrial membrane lipid cardiolipin, and membrane biology are an emerging group characterised by clinical heterogeneity, but with recurrent features including cardiomyopathy, encephalopathy, neurodegeneration, neuropathy and 3-methylglutaconic aciduria. This review discusses lipid synthesis in the mitochondrial membrane, the mitochondrial contact site and cristae organising system (MICOS), mitochondrial dynamics and trafficking, and the disorders associated with defects of each of these processes. We highlight overlapping functions of proteins involved in lipid biosynthesis and protein import into the mitochondria, pointing to an overarching coordination and synchronisation of mitochondrial functions. This review also focuses on membrane interactions between mitochondria and other organelles, namely the endoplasmic reticulum, peroxisomes, lysosomes and lipid droplets. We signpost disorders of these membrane interactions that may explain the observation of secondary mitochondrial dysfunction in heterogeneous pathological processes. Disruption of these organellar interactions ultimately impairs cellular homeostasis and organismal health, highlighting the central role of mitochondria in human health and disease.

A novel pathogenic mitochondrial DNA variant m.4344T>C in tRNAGln causes developmental delay

Mitochondrial diseases are a group of genetic diseases caused by mutations in mitochondrial DNA and nuclear DNA. However, the genetic spectrum of this disease is not yet complete. In this study, we identified a novel variant m.4344T>C in mitochondrial tRNAGln from a patient with developmental delay. The mutant loads of m.4344T>C were 95% and 89% in the patient's blood and oral epithelial cells, respectively. Multialignment analysis showed high evolutionary conservation of this nucleotide. TrRosettaRNA predicted that m.4344T>C variant would introduce an additional hydrogen bond and alter the conformation of the T-loop. The transmitochondrial cybrid-based study demonstrated that m.4344T>C variant impaired the steady-state level of mitochondrial tRNAGln and decreased the contents of mitochondrial OXPHOS complexes I, III, and IV, resulting in defective mitochondrial respiration, elevated mitochondrial ROS production, reduced mitochondrial membrane potential and decreased mitochondrial ATP levels. Altogether, this is the first report in patient carrying the m.4344T>C variant. Our data uncover the pathogenesis of the m.4344T>C variant and expand the genetic mutation spectrum of mitochondrial diseases, thus contributing to the clinical diagnosis of mitochondrial tRNAGln gene variants-associated mitochondrial diseases.

Multi-omics analysis identifies sex-specific hepatic protein-metabolite networks in yellow catfish (Pelteobagrus fulvidraco) exposed to chronic hypoxia

Hypoxia disrupts the endocrine system of teleosts. The liver plays important roles in the endocrine system, energy storage, and metabolic processes. The aim of this study was to investigate the sex-specific hepatic response of yellow catfish under chronic hypoxia at the multi-omics level. Common hepatic responses in both sexes included the HIF-1 signaling pathway, glycolysis/gluconeogenesis, and steroid biosynthesis. Hypoxia dysregulated primary bile acid biosynthesis, lipid metabolism, and vitellogenin levels in female fish. Endoplasmic reticulum function in females also tended to be disrupted by hypoxia, as evidenced by significantly enriched pathways, including ribosome, protein processing in the endoplasmic reticulum, and RNA degradation. Other pathways, including the TCA cycle, oxidative phosphorylation, and Parkinson's and Huntington's disease, were highly enriched by hypoxia in male fish, suggesting that mitochondrial function was dysregulated. In both sexes of yellow catfish, the cell cycle was arrested and apoptosis was inhibited under chronic hypoxia. Multi-omics suggested that SLC2A5, CD209, LGMN, and NEDD8 served as sex-specific markers in these fish under chronic hypoxia. Our results provide insights into hepatic adaptation to chronic hypoxia and facilitate our understanding of sex-specific responses in fish.

Comprehensive analysis of the role of SFXN family in breast cancer

The sideroflexin (SFXN) family is a group of mitochondrial membrane proteins. Although the function of the SFXN family in mitochondria has been widely recognized, the expression levels, role, and prognostic value of this family in breast cancer (BC) have not been clearly articulated and systematically analysed. In our research, SFXN1 and SFXN2 were significantly upregulated in BC versus normal samples based on Gene Expression Profiling Interactive Analysis 2 and the Human Protein Atlas databases. We found that high SFXN1 expression was significantly related to poor prognosis in BC patients and that high SFXN2 expression was significantly associated with good prognosis in BC patients. Gene Ontology analysis of the SFXN family was performed based on the STRING database to explore the potential functions of this family, including biological processes, cellular components, and molecular functions. Based on the MethSurv database, we found that two SFXN1 CpG sites (5′-UTR-S_Shelf-cg06573254 and TSS200-Island-cg17647431), two SFXN2 CpG sites (3′-UTR-Open_Sea-cg04774043 and Body-Open_Sea-cg18994254), one SFXN3 CpG site (Body-S_Shelf-cg17858697), and nine SFXN5 CpG sites (1stExon;5′-UTR-Island-cg03856450, Body-Open_Sea-cg04016113, Body-Open_Sea-cg04197631, Body-Open_Sea-cg07558704, Body-Open_Sea-cg08383863, Body-Open_Sea-cg10040131, Body-Open_Sea-cg10588340, Body-Open_Sea-cg17046766, and Body-Open_Sea-cg22830638) were significantly related to the prognosis of BC patients. According to the ENCORI database, four negative regulatory miRNAs for SFXN1 (hsa-miR-22-3p, hsa-miR-140-5p, hsa-miR-532-5p, and hsa-miR-582-3p) and four negative regulatory miRNAs for SFXN2 (hsa-miR-9-5p, hsa-miR-34a-5p, hsa-miR-532-5p, and hsa-miR-885-5p) were related to poor prognosis for BC patients. This study suggests that SFXN1 and SFXN2 are valuable biomarkers and treatment targets for patients with BC.

Protocol for mitochondrial isolation and sub-cellular localization assay for mitochondrial proteins

Here, we provide a protocol to isolate mitochondria from cultured cells and extract differently located mitochondrial proteins. We detail steps to separate both integral and peripheral membrane proteins from soluble proteins using sonication. We describe the separation of integral membrane proteins from the peripheral membrane and soluble proteins using sodium carbonate extraction. Furthermore, we detail the use of proteinase K and Triton X-100 to distinguish outer membrane proteins from mitochondrial proteins.

Mitochondrial biology and dysfunction in secondary mitochondrial disease

Mitochondrial diseases are a broad, genetically heterogeneous class of metabolic disorders characterized by deficits in oxidative phosphorylation (OXPHOS). Primary mitochondrial disease (PMD) defines pathologies resulting from mutation of mitochondrial DNA (mtDNA) or nuclear genes affecting either mtDNA expression or the biogenesis and function of the respiratory chain. Secondary mitochondrial disease (SMD) arises due to mutation of nuclear-encoded genes independent of, or indirectly influencing OXPHOS assembly and operation. Despite instances of novel SMD increasing year-on-year, PMD is much more widely discussed in the literature. Indeed, since the implementation of next generation sequencing (NGS) techniques in 2010, many novel mitochondrial disease genes have been identified, approximately half of which are linked to SMD. This review will consolidate existing knowledge of SMDs and outline discrete categories within which to better understand the diversity of SMD phenotypes. By providing context to the biochemical and molecular pathways perturbed in SMD, we hope to further demonstrate the intricacies of SMD pathologies outside of their indirect contribution to mitochondrial energy generation.

Serine active site containing protein 1 depletion alters lipid metabolism and protects against high fat diet-induced obesity in mice

OBJECTIVE

Although the serine active site containing 1 (SERAC1) protein is essential for cardiolipin remodeling and cholesterol transfer, its physiological role in whole-body energy metabolism remains unclear. Thus, we investigated the role of SERAC1 in lipid distribution and metabolism in mice.

METHODS

CRISPR/Cas9 was used to create homozygous Serac1 knockout mice. A range of methods, including electron microscopy, histological analysis, DNA sequencing, glucose and insulin tolerance tests, and biochemical analysis of serum lipid levels, were used to assess lipid distribution and rates of lipid synthesis in mice.

RESULTS

We found that Serac1 depletion in mice prevented high-fat diet-induced obesity but did not affect energy expenditure. The liver was affected by Serac1 depletion, but adipose tissues were not. Serac1 depletion was shown to impair cholesterol transfer from the liver to the serum and led to an imbalance in cholesterol distribution. The livers from mice with Serac1 depletion showed increased cholesterol synthesis because the levels of cholesterol synthesis enzymes were upregulated. Moreover, the accumulation of hepatic lipid droplets in mice with Serac1 depletion were decreased, suggesting that SERAC1 depletion may decrease the risk for hepatic steatosis in high fat diet-induced mice.

CONCLUSION

Our findings demonstrate that SERAC1 can serve as a potential target for the treatment or prevention of diet-induced hepatic lipid metabolic disorders.

A High-Throughput Search for SFXN1 Physical Partners Led to the Identification of ATAD3, HSD10 and TIM50

Simple Summary

Mitochondria are central players in cell fate and cell death. Indeed, mitochondrial dysfunction has been observed in many diseases, including neurodegenerative diseases. The activity of these organelles relies on numerous mitochondrial transporters, among which the sideroflexins have received little attention to date despite their emerging importance in human health. To better understand the cellular functions of these transporters and their associations with diseases, we herein investigated the molecular partners of one human sideroflexin, SFXN1. Several proteins capable of interacting with SFXN1 were identified, including ATAD3 and HSD10, two mitochondrial proteins linked to neuronal disorders.

Abstract

Sideroflexins (SFXN, SLC56) are a family of evolutionarily conserved mitochondrial carriers potentially involved in iron homeostasis. One member of the SFXN family is SFXN1, recently identified as a human mitochondrial serine transporter. However, little is known about the SFXN1 interactome, necessitating a high-throughput search to better characterize SFXN1 mitochondrial functions. Via co-immunoprecipitation followed by shotgun mass spectrometry (coIP-MS), we identified 96 putative SFXN1 interactors in the MCF7 human cell line. Our in silico analysis of the SFXN1 interactome highlights biological processes linked to mitochondrial organization, electron transport chains and transmembrane transport. Among the potential physical partners, ATAD3A and 17β-HSD10, two proteins associated with neurological disorders, were confirmed using different human cell lines. Nevertheless, further work will be needed to investigate the significance of these interactions.