Ribonucleotide synthesis by NME6 fuels mitochondrial gene expression

WBSCR16 is essential for mitochondrial 16S rRNA processing in mammals

Mitochondrial rRNAs play important roles in regulating mtDNA-encoded gene expression and energy metabolism subsequently. However, the proteins that regulate mitochondrial 16S rRNA processing remain poorly understood. Herein, we generated adipose-specific Wbscr16-/-mice and cells, both of which exhibited dramatic mitochondrial changes. Subsequently, WBSCR16 was identified as a 16S rRNA-binding protein essential for the cleavage of 16S rRNA-mt-tRNALeu, facilitating 16S rRNA processing and mitochondrial ribosome assembly. Additionally, WBSCR16 recruited RNase P subunit MRPP3 to nascent 16S rRNA and assisted in this specific cleavage. Furthermore, evidence showed that adipose-specific Wbscr16 ablation promotes energy wasting via lipid preference in brown adipose tissue, leading to excess energy expenditure and resistance to obesity. In contrast, overexpression of WBSCR16 upregulated 16S rRNA processing and induced a preference for glucose utilization in both transgenic mouse models and cultured cells. These findings suggest that WBSCR16 plays essential roles in mitochondrial 16S rRNA processing in mammals, and is the key mitochondrial protein to balance glucose and lipid metabolism.

Therapeutic potential of Parkin and its regulation in Parkinson's disease

Parkinson's disease (PD) is a debilitating neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the midbrain substantia nigra, resulting in motor and non-motor symptoms. While the exact etiology of PD remains elusive, a growing body of evidence suggests that dysfunction in the parkin protein plays a pivotal role in the pathogenesis of the disease. Parkin is an E3 ubiquitin ligase that ubiquitinates substrate proteins to control a number of crucial cellular processes including protein catabolism, immune response, and cellular apoptosis.While autosomal recessive mutations in the PARK2 gene, which codes for parkin, are linked to an inherited form of early-onset PD, heterozygous mutations in PARK2 have also been reported in the more commonly occurring sporadic PD cases. Impairment of parkin's E3 ligase activity is believed to play a pathogenic role in both familial and sporadic forms of PD.This article provides an overview of the current understanding of the mechanistic basis of parkin's E3 ligase activity, its major physiological role in controlling cellular functions, and how these are disrupted in familial and sporadic PD. The second half of the manuscript explores the currently available and potential therapeutic strategies targeting parkin structure and/or function in order to slow down or mitigate the progressive neurodegeneration in PD.

Mitochondrial double-stranded RNA homeostasis depends on cell-cycle progression

Mitochondrial gene expression is a compartmentalised process essential for metabolic function. The replication and transcription of mitochondrial DNA (mtDNA) take place at nucleoids, whereas the subsequent processing and maturation of mitochondrial RNA (mtRNA) and mitoribosome assembly are localised to mitochondrial RNA granules. The bidirectional transcription of circular mtDNA can lead to the hybridisation of polycistronic transcripts and the formation of immunogenic mitochondrial double-stranded RNA (mt-dsRNA). However, the mechanisms that regulate mt-dsRNA localisation and homeostasis are largely unknown. With super-resolution microscopy, we show that mt-dsRNA overlaps with the RNA core and associated proteins of mitochondrial RNA granules but not nucleoids. Mt-dsRNA foci accumulate upon the stimulation of cell proliferation and their abundance depends on mitochondrial ribonucleotide supply by the nucleoside diphosphate kinase, NME6. Consequently, mt-dsRNA foci are profuse in cultured cancer cells and malignant cells of human tumour biopsies. Our results establish a new link between cell proliferation and mitochondrial nucleic acid homeostasis.

Mitochondrial Labeling with Mulberrin-Cy3: A New Fluorescent Probe for Live Cell Visualization

Mitochondria, crucial intracellular organelles, are central to energy metabolism, signal transduction, apoptosis, calcium homeostasis, and a myriad of other biological processes, making them a focal point in diverse research fields. The capacity to fluorescently label and visually track mitochondria is crucial for understanding their biological roles. We present mulberrin-Cy3, a novel small molecule fluorescent probe that selectively labels mitochondria in animal cells, including cancer cells, with relative ease. This protocol details the synthesis of mulberrin-Cy3 and its use for visualizing mitochondria in living cells. The synthesis is straightforward and time-efficient, and the labeling method is more accessible than traditional approaches, providing a cost-effective option for mitochondrial visualization at room temperature. The labeling is rapid, with effective labeling achieved within 5 min of incubation. The fluorescent signal is stable and brighter, offering a significant advantage over existing methods. Mulberrin-Cy3 represents a promising mitochondrial labeling compound, providing researchers with a novel experimental tool to explore the complex biological functions of mitochondria. This innovation has the potential to significantly advance our comprehension of mitochondrial dynamics and their role in cellular health and disease.

Mitochondrial NME6 Influences Basic Cellular Processes in Tumor Cells In Vitro

NME6 belongs to the family of nucleoside diphosphate kinase enzymes, whose major role is to transfer the terminal phosphate from NTPs, mostly ATP, to other (d)NDPs via a high-energy intermediate. Beside this basic enzymatic activity, the family, comprising 10 genes/proteins in humans, executes a number of diverse biochemical/biological functions in the cell. A few previous studies have reported that NME6 resides in the mitochondria and influences oxidative phosphorylation while interacting with RCC1L, a GTPase involved in mitochondrial ribosome assembly and translation. Considering the multifunctional role of NME family members, the goal of the present study was to assess the influence of the overexpression or silencing of NME6 on fundamental cellular events of MDA-MB-231T metastatic breast cancer cells. Using flow cytometry, Western blotting, and a wound-healing assay, we demonstrated that the overexpression of NME6 reduces cell migration and alters the expression of EMT (epithelial-mesenchymal transition) markers. In addition, NME6 overexpression influences cell cycle distribution exclusively upon DNA damage and impacts the MAPK/ERK signaling pathway, while it has no effect on apoptosis. To conclude, our results demonstrate that NME6 is involved in different cellular processes, providing a solid basis for future, more precise investigations of its role.

Mitochondrial NME6: A Paradigm Change within the NME/NDP Kinase Protein Family?

Eukaryotic NMEs/NDP kinases are a family of 10 multifunctional proteins that occur in different cellular compartments and interact with various cellular components (proteins, membranes, and DNA). In contrast to the well-studied Group I NMEs (NME1-4), little is known about the more divergent Group II NMEs (NME5-9). Three recent publications now shed new light on NME6. First, NME6 is a third mitochondrial NME, largely localized in the matrix space, associated with the mitochondrial inner membrane. Second, while its monomeric form is inactive, NME6 gains NDP kinase activity through interaction with mitochondrial RCC1L. This challenges the current notion that mammalian NMEs require the formation of hexamers to become active. The formation of complexes between NME6 and RCC1L, likely heterodimers, seemingly obviates the necessity for hexamer formation, stabilizing a NDP kinase-competent conformation. Third, NME6 is involved in mitochondrial gene maintenance and expression by providing (d)NTPs for replication and transcription (in particular the pyrimidine nucleotides) and by a less characterized mechanism that supports mitoribosome function. This review offers an overview of NME evolution and structure and highlights the new insight into NME6. The new findings position NME6 as the most comprehensively studied protein in NME Group II and may even suggest it as a new paradigm for related family members.

Mitochondria at the crossroads of health and disease

Mitochondria reside at the crossroads of catabolic and anabolic metabolism-the essence of life. How their structure and function are dynamically tuned in response to tissue-specific needs for energy, growth repair, and renewal is being increasingly understood. Mitochondria respond to intrinsic and extrinsic stresses and can alter cell and organismal function by inducing metabolic signaling within cells and to distal cells and tissues. Here, we review how the centrality of mitochondrial functions manifests in health and a broad spectrum of diseases and aging.

Light-strand bias and enriched zones of embedded ribonucleotides are associated with DNA replication and transcription in the human-mitochondrial genome

Abundant ribonucleoside-triphosphate (rNTP) incorporation into DNA by DNA polymerases in the form of ribonucleoside monophosphates (rNMPs) is a widespread phenomenon in nature, resulting in DNA-structural change and genome instability. The rNMP distribution, characteristics, hotspots and association with DNA metabolic processes in human mitochondrial DNA (hmtDNA) remain mostly unknown. Here, we utilize the ribose-seq technique to capture embedded rNMPs in hmtDNA of six different cell types. In most cell types, the rNMPs are preferentially embedded on the light strand of hmtDNA with a strong bias towards rCMPs; while in the liver-tissue cells, the rNMPs are predominately found on the heavy strand. We uncover common rNMP hotspots and conserved rNMP-enriched zones across the entire hmtDNA, including in the control region, which links the rNMP presence to the frequent hmtDNA replication-failure events. We show a strong correlation between coding-sequence size and rNMP-embedment frequency per nucleotide on the non-template, light strand in all cell types, supporting the presence of transient RNA-DNA hybrids preceding light-strand replication. Moreover, we detect rNMP-embedment patterns that are only partly conserved across the different cell types and are distinct from those found in yeast mtDNA. The study opens new research directions to understand the biology of hmtDNA and genomic rNMPs.

Regulators of mitonuclear balance link mitochondrial metabolism to mtDNA expression

Mitochondrial oxidative phosphorylation (OXPHOS) complexes are assembled from proteins encoded by both nuclear and mitochondrial DNA. These dual-origin enzymes pose a complex gene regulatory challenge for cells requiring coordinated gene expression across organelles. To identify genes involved in dual-origin protein complex synthesis, we performed fluorescence-activated cell-sorting-based genome-wide screens analysing mutant cells with unbalanced levels of mitochondrial- and nuclear-encoded subunits of Complex IV. We identified genes involved in OXPHOS biogenesis, including two uncharacterized genes: PREPL and NME6. We found that PREPL specifically impacts Complex IV biogenesis by acting at the intersection of mitochondrial lipid metabolism and protein synthesis, whereas NME6, an uncharacterized nucleoside diphosphate kinase, controls OXPHOS biogenesis through multiple mechanisms reliant on its NDPK domain. Firstly, NME6 forms a complex with RCC1L, which together perform nucleoside diphosphate kinase activity to maintain local mitochondrial pyrimidine triphosphate levels essential for mitochondrial RNA abundance. Secondly, NME6 modulates the activity of mitoribosome regulatory complexes, altering mitoribosome assembly and mitochondrial RNA pseudouridylation. Taken together, we propose that NME6 acts as a link between compartmentalized mitochondrial metabolites and mitochondrial gene expression.

NME6: ribonucleotide salvage sustains mitochondrial transcription

The building blocks for RNA and DNA are made in the cytosol, meaning mitochondria depend on the import and salvage of ribonucleoside triphosphates (rNTPs) and deoxyribonucleoside triphosphates (dNTPs) for the synthesis of their own genetic material. While extensive research has focused on mitochondrial dNTP homeostasis due to its defects being associated with various mitochondrial DNA (mtDNA) depletion and deletion syndromes, the investigation of mitochondrial rNTP homeostasis has received relatively little attention. In this issue of the EMBO Journal, Grotehans et al provide compelling evidence of a major role for NME6, a mitochondrial nucleoside diphosphate kinase, in the conversion of pyrimidine ribonucleoside diphosphates into the corresponding triphosphates. These data also suggest a significant physiological role for NME6, as its absence results in the depletion of mitochondrial transcripts and destabilization of the electron transport chain (Grotehans et al, 2023).