Elucidation of cytolinker function: periplakin-associated proteins in epithelial cells.

Periplakin is a cytolinker protein which participates in the barrier formation of the skin by playing part in the cornified envelope assembly of epidermal cells. Previous studies had identified peripłakin, envoplakin and involucrin as constituents of the cornified envelope, but gene-targeting studies had demonstrated that lack of the proteins individually or in any "double knock-out" combination did not disturb the epidermal differentiation or skin barrier function. In order to gain more information about periplakin, which is also expressed in simple epithelial cells, a stably transfected MCF-7 subclone overexpressing the HA-tagged periplakin N-terminus was generated. Co-immunoprecipitation was used for screening of protein complexes associated with the HA-tagged periplakin N-terminus to identify previously uncharacterised periplakin partners. Co-IP with anti-HA antibody and mass-spectrometry revealed a 500 kDa periplakin interacting protein, plectin and another protein around the size of 34 kDa identified as annexin A9. Endogenous periplakin co-localised with annexin A9 in the plasma membrane of MCF-7 cells and showed a similar staining pattern in newborn and adult mouse skin. Transient transfection of periplakin deletion constructs indicated that the first 133 amino acid residues are essential for the co- localisation with plectin at cell borders. Immunofluorescence analysis demonstrated that periplakin N-terminus and different plectin isoforms, such as plectin-1,-1 f and 1k, are co- localise at cell borders of MCF-7 epithelia and also co-localise with endogenous periplakin at suprabasal layers of the skin. Ablation of the plectin by siRNA transfection in HaCaT keratinocytes resulted in aggregation of periplakin into small clusters in the cytoplasm. Scratch wounded MCF-7 epithelia expressing the N-terminal half of periplakin showed accelerated keratin re-organisation which was hampered by plectin downregulation. The role of periplakin and keratin IPs in the migration of simple epithelial cells was also investigated. Stable expression of periplakin C-terminus increased keratin bundling and Ser-431 phosphorylation of keratin 8 at the free wound edge, delaying wound closure. Depletion of periplakin or plectin by SİRNA transfection impaired wound closure, while simultaneous ablation of both proteins reduced the speed of wound healing even further. Knockdown of keratin 8 IPs with SİRNA resulted in the loss of desmoplakin at cell borders and the failure of different simple epithelial sheets migrating as a collective unit

Amyloid-β in mitochondrial disease: mutation in a human metallopeptidase links amyloidotic neurodegeneration with mitochondrial processing.

There is increasing evidence that common molecular pathways in neurons are closely linked with mitochondrial function and that mitochondrial dysfunction is connected to various forms of neurodegenerative diseases. For instance, mitochondria are involved in amyloid‐β (Aβ) deposition in Alzheimer's disease, although the exact molecular pathways remain largely unknown. Brunetti et al (2015) in this issue of EMBO Molecular Medicine provide a novel link between Aβ accumulation and mitochondria. A pathogenic mutation in a Norwegian family in the mitochondrial metallopeptidase PITRM1 is found to underlie a novel mitochondrial neurodegenerative phenotype associated with Aβ accumulation

Mitochondrial aminoacyl-tRNA synthetases trigger unique compensatory mechanisms in neurons

\ua9 The Author(s) 2023. Published by Oxford University Press. Mitochondrial aminoacyl-tRNA synthetase (mt-ARS) mutations cause severe, progressive, and often lethal diseases with highly heterogeneous and tissue-specific clinical manifestations. This study investigates the molecular mechanisms triggered by three different mt-ARS defects caused by biallelic mutations in AARS2, EARS2, and RARS2, using an in vitro model of human neuronal cells. We report distinct molecular mechanisms of mitochondrial dysfunction among the mt-ARS defects studied. Our findings highlight the ability of proliferating neuronal progenitor cells (iNPCs) to compensate for mitochondrial translation defects and maintain balanced levels of oxidative phosphorylation (OXPHOS) components, which becomes more challenging in mature neurons. Mutant iNPCs exhibit unique compensatory mechanisms, involving specific branches of the integrated stress response, which may be gene-specific or related to the severity of the mitochondrial translation defect. RNA sequencing revealed distinct transcriptomic profiles showing dysregulation of neuronal differentiation and protein translation. This study provides valuable insights into the tissue-specific compensatory mechanisms potentially underlying the phenotypes of patients with mt-ARS defects. Our novel in vitro model may more accurately represent the neurological presentation of patients and offer an improved platform for future investigations and therapeutic development

Altered 2-thiouridylation impairs mitochondrial translation in reversible infantile respiratory chain deficiency.

Childhood-onset mitochondrial encephalomyopathies are severe, relentlessly progressive conditions. However, reversible infantile respiratory chain deficiency (RIRCD), due to a homoplasmic mt-tRNA(Glu) mutation, and reversible infantile hepatopathy, due to tRNA 5-methylaminomethyl-2-thiouridylate methyltransferase (TRMU) deficiency, stand out by showing spontaneous recovery, and provide the key to treatments of potential broader relevance. Modification of mt-tRNA(Glu) is a possible functional link between these two conditions, since TRMU is responsible for 2-thiouridylation of mt-tRNA(Glu), mt-tRNA(Lys) and mt-tRNA(Gln). Here we show that down-regulation of TRMU in RIRCD impairs 2-thiouridylation and exacerbates the effect of the mt-tRNA(Glu) mutation by triggering a mitochondrial translation defect in vitro. Skeletal muscle of RIRCD patients in the symptomatic phase showed significantly reduced 2-thiouridylation. Supplementation with l-cysteine, which is required for optimal TRMU function, rescued respiratory chain enzyme activities in human cell lines of patients with RIRCD as well as deficient TRMU. Our results show that l-cysteine supplementation is a potential treatment for RIRCD and for TRMU deficiency, and is likely to have broader application for the growing group of intra-mitochondrial translation disorders

Altered RNA metabolism due to a homozygous RBM7 mutation in a patient with spinal motor neuropathy.

The exosome complex is the most important RNA processing machinery within the cell. Mutations in its subunits EXOSC8 and EXOSC3 cause pontocerebellar hypoplasia, spinal muscular atrophy (SMA) and central nervous system demyelination. We present a patient with SMA-like phenotype carrying a homozygous mutation in RBM7-a subunit of the nuclear exosome targeting (NEXT) complex-which is known to bind and carry specific subtypes of coding and non-coding RNAs to the exosome. The NEXT complex with other protein complexes is responsible for the substrate specificity of the exosome. We performed RNA-sequencing (RNA-seq) analysis on primary fibroblasts of patients with mutations in EXOSC8 and RBM7 and gene knock-down experiments using zebrafish as a model system. RNA-seq analysis identified significantly altered expression of 62 transcripts shared by the two patient cell lines. Knock-down of rbm7, exosc8 and exosc3 in zebrafish showed a common pattern of defects in motor neurons and cerebellum. Our data indicate that impaired RNA metabolism may underlie the clinical phenotype by fine tuning gene expression which is essential for correct neuronal differentiation

Clinical and functional characterisation of the combined respiratory chain defect in two sisters due to autosomal recessive mutations in MTFMT.

Exome sequencing identified compound heterozygous mutations in the recently discovered mitochondrial methionyl-tRNA formyltransferase (MTFMT) gene in two sisters with mild Leigh syndrome and combined respiratory chain deficiency. The mutations lead to undetectable levels of the MTFMT protein. Blue native polyacrylamide gel electrophoresis showed decreased complexes I and IV, and additional products stained with complex V antibodies, however the overall steady state level of mt-tRNA(Met) was normal. Our data illustrate that exome sequencing is an excellent diagnostic tool, and its value in clinical medicine is enormous, however it can only be optimally exploited if combined with detailed phenotyping and functional studies

Mitochondrial dysfunction in liver failure requiring transplantation.

Liver failure is a heterogeneous condition which may be fatal and the primary cause is frequently unknown. We investigated mitochondrial oxidative phosphorylation in patients undergoing liver transplantation. We studied 45 patients who had liver transplantation due to a variety of clinical presentations. Blue native polyacrylamide gel electrophoresis with immunodetection of respiratory chain complexes I-V, biochemical activity of respiratory chain complexes II and IV and quantification of mitochondrial DNA (mtDNA) copy number were investigated in liver tissue collected from the explanted liver during transplantation. Abnormal mitochondrial function was frequently present in this cohort: ten of 40 patients (25 %) had a defect of one or more respiratory chain enzyme complexes on blue native gels, 20 patients (44 %) had low activity of complex II and/or IV and ten (22 %) had a reduced mtDNA copy number. Combined respiratory chain deficiency and reduced numbers of mitochondria were detected in all three patients with acute liver failure. Low complex IV activity in biliary atresia and complex II defects in cirrhosis were common findings. All six patients diagnosed with liver tumours showed variable alterations in mitochondrial function, probably due to the heterogeneity of the presenting tumour. In conclusion, mitochondrial dysfunction is common in severe liver failure in non-mitochondrial conditions. Therefore, in contrast to the common practice detection of respiratory chain abnormalities in liver should not restrict the inclusion of patients for liver transplantation. Furthermore, improving mitochondrial function may be targeted as part of a complex therapy approach in different forms of liver diseases

ANO10 mutations cause ataxia and coenzyme Q₁₀ deficiency.

Inherited ataxias are heterogeneous disorders affecting both children and adults, with over 40 different causative genes, making molecular genetic diagnosis challenging. Although recent advances in next-generation sequencing have significantly improved mutation detection, few treatments exist for patients with inherited ataxia. In two patients with adult-onset cerebellar ataxia and coenzyme Q10 (CoQ10) deficiency in muscle, whole exome sequencing revealed mutations in ANO10, which encodes anoctamin 10, a member of a family of putative calcium-activated chloride channels, and the causative gene for autosomal recessive spinocerebellar ataxia-10 (SCAR10). Both patients presented with slowly progressive ataxia and dysarthria leading to severe disability in the sixth decade. Epilepsy and learning difficulties were also present in one patient, while retinal degeneration and cataract were present in the other. The detection of mutations in ANO10 in our patients indicate that ANO10 defects cause secondary low CoQ10 and SCAR10 patients may benefit from CoQ10 supplementation

ANO10 mutations cause ataxia and coenzyme Q(10) deficiency

Inherited ataxias are heterogeneous disorders affecting both children and adults, with over 40 different causative genes, making molecular genetic diagnosis challenging. Although recent advances in next-generation sequencing have significantly improved mutation detection, few treatments exist for patients with inherited ataxia. In two patients with adult-onset cerebellar ataxia and coenzyme Q10 (CoQ10) deficiency in muscle, whole exome sequencing revealed mutations in ANO10, which encodes anoctamin 10, a member of a family of putative calcium-activated chloride channels, and the causative gene for autosomal recessive spinocerebellar ataxia-10 (SCAR10). Both patients presented with slowly progressive ataxia and dysarthria leading to severe disability in the sixth decade. Epilepsy and learning difficulties were also present in one patient, while retinal degeneration and cataract were present in the other. The detection of mutations in ANO10 in our patients indicate that ANO10 defects cause secondary low CoQ10 and SCAR10 patients may benefit from CoQ10 supplementation