Arnaud Chevrollier

Mitochondria are central hubs for cellular fitness, empowered by plastic remodeling of their shape, proteome composition, and/or metabolic state. MFN2 (mitofusin 2) mediates mitochondrial fusion and ensures adaptations in response to metabolic changes and stresses. Besides this canonical role, MFN2 serves as a communication hub with other organelles. It tethers mitochondria to the endoplasmic reticulum (ER), lipid droplets, and peroxisomes, regulating calcium buffering, apoptosis, lipid biosynthesis, and lipolysis. Dysfunctional MFN2 causes the hereditary neuropathy Charcot–Marie–Tooth type 2A (CMT2A) and is linked to several metabolic diseases. In a recent publication, we described another fusion-independent role of MFN2 in proteostasis and mitophagy. MFN2 binds the chaperone HSPA8/HSC70 (heat shock protein family A [Hsp70] member 8) and the proteasome, a key function in maintaining mitochondrial and cellular protein quality control, which appears to be lost in the context of CMT2A-associated MFN2 variants.

The bioenergetic machinery of the cell is protected and structured within two layers of mitochondrial membranes. The mitochondrial inner membrane is extremely rich in proteins, including respiratory chain complexes, substrate transport proteins, ion exchangers, and structural fusion proteins. These proteins participate directly or indirectly in shaping the membrane’s curvature and facilitating its folding, as well as promoting the formation of nanotubes, and proton-rich pockets known as cristae. Recent fluorescent super-resolution images have demonstrated the strong dynamics of these events, with constant remodeling processes. The mitochondrial outer membrane itself is also highly dynamic, interacting with the endoplasmic reticulum and its environment to ensure a rapid diffusion of surface components throughout the mitochondrial networks. All these movements occur besides migration, fusion, and fission of the mitochondria themselves. These dynamic events at the level of mitochondrial membranes are primarily dependent on their unique lipid composition. In this review, we discuss the latest advances in phospholipid research, focusing on their metabolism and role in mitochondrial dynamics. This process emphasizes the importance of interactions with the endoplasmic reticulum and mitochondrial matrix enzymes, extending its relevance to lipid sources, in particular, cardiolipins and phosphatidylethanolamines at the cellular, tissue and even whole-organism level. Given the expanding array of characterized mitochondrial functions, ranging from calcium homeostasis to inflammation and cellular senescence, research in the field of mitochondrial lipids is particularly significant. As mitochondria play a central role in various pathological processes, including cancer and neurodegenerative disorders, lipid metabolism may offer promising therapeutic approaches.

The centrosome, as the main microtubule organizing centre, plays key roles in cell polarity, genome stability and ciliogenesis. The recent identification of ribosomes, RNA-binding proteins and transcripts at the centrosome suggests local protein synthesis. In this context, we hypothesized that TDP-43, a highly conserved RNA binding protein involved in the pathophysiology of amyotrophic lateral sclerosis and frontotemporal lobar degeneration, could be enriched at this organelle. Using dedicated high magnification sub-diffraction microscopy on human cells, we discovered a novel localization of TDP-43 at the centrosome during all phases of the cell cycle.These results were confirmed on purified centrosomes by western blot and immunofluorescence microscopy. In addition, the co-localization of TDP-43 and pericentrin suggested a pericentriolar enrichment of the protein, leading us to hypothesize that TDP-43 might interact with local mRNAs and proteins. Supporting this hypothesis, we found four conserved centrosomal mRNAs and 16 centrosomal proteins identified as direct TDP-43 interactors. More strikingly, all the 16 proteins are implicated in the pathophysiology of TDP-43 proteinopathies, suggesting that TDP-43 dysfunction in this organelle contributes to neurodegeneration.This first description of TDP-43 centrosomal enrichment paves the way for a more comprehensive understanding of TDP-43 physiology and pathology.

Mutations in DNM1L (DRP1), which encode a key player of mitochondrial and peroxisomal fission, have been reported in patients with the variable phenotypic spectrum, ranging from non-syndromic optic atrophy to lethal infantile encephalopathy. Here, we report a case of an adult female patient presenting with a complex neurological phenotype that associates axonal sensory neuropathy, spasticity, optic atrophy, dysarthria, dysphasia, dystonia, and ataxia, worsening with aging. Whole-exome sequencing revealed a heterozygous de novo variant in the GTPase domain of DNM1L [NM_001278464.1: c.176C>A p.(Thr59Asn)] making her the oldest patient suffering from encephalopathy due to defective mitochondrial and peroxisomal fission-1. In silico analysis suggested a protein destabilization effect of the variant Thr59Asn. Unexpectedly, Western blotting disclosed profound decrease of DNM1L expression, probably related to the degradation of DNM1L complexes. A detailed description of mitochondrial and peroxisomal anomalies in transmission electron and 3D fluorescence microscopy studies confirmed the exceptional phenotype of this patient.

(1) Background: The development of mitochondrial medicine has been severely impeded by a lack of effective therapies. (2) Methods: To better understand Mitochondrial Encephalopathy Lactic Acidosis Syndrome Stroke-like episodes (MELAS) syndrome, neuronal cybrid cells carrying different mutation loads of the m.3243A > G mitochondrial DNA variant were analysed using a multi-omic approach. (3) Results: Specific metabolomic signatures revealed that the glutamate pathway was significantly increased in MELAS cells with a direct correlation between glutamate concentration and the m.3243A > G heteroplasmy level. Transcriptomic analysis in mutant cells further revealed alterations in specific gene clusters, including those of the glutamate, gamma-aminobutyric acid pathways, and tricarboxylic acid (TCA) cycle. These results were supported by post-mortem brain tissue analysis from a MELAS patient, confirming the glutamate dysregulation. Exposure of MELAS cells to ketone bodies significantly reduced the glutamate level and improved mitochondrial functions, reducing the accumulation of several intermediate metabolites of the TCA cycle and alleviating the NADH-redox imbalance. (4) Conclusions: Thus, a multi-omic integrated approach to MELAS cells revealed glutamate as a promising disease biomarker, while also indicating that a ketogenic diet should be tested in MELAS patients.

Dominant Optic Atrophy (DOA) is one of the most common inherited mitochondrial diseases, leading to blindness. It is caused by the chronic degeneration of the retinal ganglion cells (RGCs) and their axons forming the optic nerve. Until now, DOA has been mainly associated with genes encoding proteins involved in mitochondrial network dynamics. Using next-generation and exome sequencing, we identified for the first time heterozygous PMPCA variants having a causative role in the pathology of late-onset primary DOA in five patients. PMPCA encodes an α subunit of the mitochondrial peptidase (MPP), responsible for the cleavage and maturation of the mitochondrial precursor proteins imported from the cytoplasm into mitochondria. Recently, PMPCA has been identified as the gene responsible for Autosomal Recessive Cerebellar Ataxia type 2 (SCAR2) and another severe recessive mitochondrial disease. In this study, four PMPCA variants were identified, two are frameshifts (c.309delA and c.820delG) classified as pathogenic and two are missenses (c.1363G>A and c.1547G>A) classified with uncertain pathological significance. Functional assays on patients’ fibroblasts show a hyperconnection of the mitochondrial network and revealed that frameshift variants reduced α-MPP levels, while not significantly affecting the respiratory machinery. These results suggest that alterations in mitochondrial peptidase function can affect the fusion-fission balance, a key element in maintaining the physiology of retinal ganglion cells, and consequently lead to their progressive degeneration.