MITOL-dependent ubiquitylation negatively regulates the entry of PolγA into mitochondria

Conserved quality control mechanisms of mitochondrial protein import

Mitochondria carry out essential functions for the cell, including energy production, various biosynthesis pathways, formation of co-factors and cellular signalling in apoptosis and inflammation. The functionality of mitochondria requires the import of about 900-1300 proteins from the cytosol in baker's yeast Saccharomyces cerevisiae and human cells, respectively. The vast majority of these proteins pass the outer membrane in a largely unfolded state through the translocase of the outer mitochondrial membrane (TOM) complex. Subsequently, specific protein translocases sort the precursor proteins into the outer and inner membranes, the intermembrane space and matrix. Premature folding of mitochondrial precursor proteins, defects in the mitochondrial protein translocases or a reduction of the membrane potential across the inner mitochondrial membrane can cause stalling of precursors at the protein import apparatus. Consequently, the translocon is clogged and non-imported precursor proteins accumulate in the cell, which in turn leads to proteotoxic stress and eventually cell death. To prevent such stress situations, quality control mechanisms remove non-imported precursor proteins from the TOM channel. The highly conserved ubiquitin-proteasome system of the cytosol plays a critical role in this process. Thus, the surveillance of protein import via the TOM complex involves the coordinated activity of mitochondria-localized and cytosolic proteins to prevent proteotoxic stress in the cell.

Mitochondria in biology and medicine - 2023

Mitochondria are an indispensable part of the cell that plays a crucial role in regulating various signaling pathways, energy metabolism, cell differentiation, proliferation, and cell death. Since mitochondria have their own genetic material, they differ from their nuclear counterparts, and dysregulation is responsible for a broad spectrum of diseases. Mitochondrial dysfunction is associated with several disorders, including neuro-muscular disorders, cancer, and premature aging, among others. The intricacy of the field is due to the cross-talk between nuclear and mitochondrial genes, which has also improved our knowledge of mitochondrial functions and their pathogenesis. Therefore, interdisciplinary research and communication are crucial for mitochondrial biology and medicine due to the challenges they pose for diagnosis and treatment. The ninth annual conference of the Society for Mitochondria Research and Medicine (SMRM)- India, titled "Mitochondria in Biology and Medicine" was organized at the Centre for DNA Fingerprinting and Diagnostics (CDFD), Hyderabad, India, on June 21-23, 2023. The latest advancements in the field of mitochondrial biology and medicine were discussed at the conference. In this article, we summarize the entire event for the benefit of researchers working in the field of mitochondrial biology and medicine.

Hyperubiquitylation of DNA helicase RECQL4 by E3 ligase MITOL prevents mitochondrial entry and potentiates mitophagy

Mutations in the DNA helicase RECQL4 lead to Rothmund-Thomson syndrome (RTS), a disorder characterized by mitochondrial dysfunctions, premature aging, and genomic instability. However, the mechanisms by which these mutations lead to pathology are unclear. Here we report that RECQL4 is ubiquitylated by a mitochondrial E3 ligase, MITOL, at two lysine residues (K1101, K1154) via K6 linkage. This ubiquitylation hampers the interaction of RECQL4 with mitochondrial importer Tom20, thereby restricting its own entry into mitochondria. We show the RECQL4 2K mutant (where both K1101 and K1154 are mutated) has increased entry into mitochondria and demonstrates enhanced mitochondrial DNA (mtDNA) replication. We observed that the three tested RTS patient mutants were unable to enter the mitochondria and showed decreased mtDNA replication. Furthermore, we found that RECQL4 in RTS patient mutants are hyperubiquitylated by MITOL and form insoluble aggregate-like structures on the outer mitochondrial surface. However, depletion of MITOL allows RECQL4 expressed in these RTS mutants to enter mitochondria and rescue mtDNA replication. Finally, we show increased accumulation of hyperubiquitylated RECQL4 outside the mitochondria leads to the cells being potentiated to increased mitophagy. Hence, we conclude regulating the turnover of RECQL4 by MITOL may have a therapeutic effect in patients with RTS.

Outer mitochondrial membrane E3 Ub ligase MARCH5 controls mitochondrial steps in peroxisome biogenesis

Peroxisome de novo biogenesis requires yet unidentified mitochondrial proteins. We report that the outer mitochondrial membrane (OMM)-associated E3 Ub ligase MARCH5 is vital for generating mitochondria-derived pre-peroxisomes. MARCH5 knockout results in accumulation of immature peroxisomes and lower expression of various peroxisomal proteins. Upon fatty acid-induced peroxisomal biogenesis, MARCH5 redistributes to newly formed peroxisomes; the peroxisomal biogenesis under these conditions is inhibited in MARCH5 knockout cells. MARCH5 activity-deficient mutants are stalled on peroxisomes and induce accumulation of peroxisomes containing high levels of the OMM protein Tom20 (mitochondria-derived pre-peroxisomes). Furthermore, depletion of peroxisome biogenesis factor Pex14 leads to the formation of MARCH5- and Tom20-positive peroxisomes, while no peroxisomes are detected in Pex14/MARCH5 dko cells. Reexpression of WT, but not MARCH5 mutants, restores Tom20-positive pre-peroxisomes in Pex14/MARCH5 dko cells. Thus, MARCH5 acts upstream of Pex14 in mitochondrial steps of peroxisome biogenesis. Our data validate the hybrid, mitochondria-dependent model of peroxisome biogenesis and reveal that MARCH5 is an essential mitochondrial protein in this process.

Summary

The authors found that mitochondrial E3 Ub ligase MARCH5 controls the formation of mitochondria-derived pre-peroxisomes. The data support the hybrid, mitochondria-dependent model of peroxisome biogenesis and reveal that MARCH5 is an essential mitochondrial protein in this process.

Mitochondrial OGG1 expression reduces age-associated neuroinflammation by regulating cytosolic mitochondrial DNA

Aging is accompanied by a decline in DNA repair efficiency, which leads to the accumulation of different types of DNA damage. Age-associated chronic inflammation and generation of reactive oxygen species exacerbate the aging process and age-related chronic disorders. These inflammatory processes establish conditions that favor accumulation of DNA base damage, especially 8-oxo-7,8 di-hydroguanine (8-oxoG), which in turn contributes to various age associated diseases. 8-oxoG is repaired by 8-oxoG glycosylase1 (OGG1) through the base excision repair (BER) pathway. OGG1 is present in both the cell nucleus and in mitochondria. Mitochondrial OGG1 has been implicated in mitochondrial DNA repair and increased mitochondrial function. Using transgenic mouse models and cell lines that have been engineered to have enhanced expression of mitochondria-targeted OGG1 (mtOGG1), we show that elevated levels of mtOGG1 in mitochondria can reverse aging-associated inflammation and improve functions. Old male mtOGG1Tg mice show decreased inflammation response, decreased TNFα levels and multiple pro-inflammatory cytokines. Moreover, we observe that male mtOGG1Tg mice show resistance to STING activation. Interestingly, female mtOGG1Tg mice did not respond to mtOGG1 overexpression. Further, HMC3 cells expressing mtOGG1 display decreased release of mtDNA into the cytoplasm after lipopolysacchride induction and regulate inflammation through the pSTING pathway. Also, increased mtOGG1 expression reduced LPS-induced loss of mitochondrial functions. These results suggest that mtOGG1 regulates age-associated inflammation by controlling release of mtDNA into the cytoplasm.

Regulation of PKR-dependent RNA translation inhibition by TRIM21 upon virus infection or other stress

The host always employs various ways to defend against viral infection and spread. However, viruses have evolved their own effective strategies, such as inhibition of RNA translation of the antiviral effectors, to destroy the host's defense barriers. Protein synthesis, commonly controlled by the α-subunit of eukaryotic translation initiation factor 2 (eIF2α), is a basic cellular biological process among all species. In response to viral infection, in addition to inducing the transcription of antiviral cytokines by innate immunity, infected cells also inhibit the RNA translation of antiviral factors by activating the protein kinase R (PKR)-eIF2α signaling pathway. Regulation of innate immunity has been well studied; however, regulation of the PKR-eIF2α signaling pathway remains unclear. In this study, we found that the E3 ligase TRIM21 negatively regulates the PKR-eIF2α signaling pathway. Mechanistically, TRIM21 interacts with the PKR phosphatase PP1α and promotes K6-linked polyubiquitination of PP1α. Ubiquitinated PP1α augments its interaction with PKR, causing PKR dephosphorylation and subsequent translational inhibition release. Furthermore, TRIM21 can constitutively restrict viral infection by reversing PKR-dependent translational inhibition of various previously known and unknown antiviral factors. Our study highlights a previously undiscovered role of TRIM21 in regulating translation, which will provide new insights into the host antiviral response and novel targets for the treatment of translation-associated diseases in the clinic.

The Fate of Oxidative Strand Breaks in Mitochondrial DNA

Mitochondrial DNA (mtDNA) is particularly vulnerable to somatic mutagenesis. Potential mechanisms include DNA polymerase γ (POLG) errors and the effects of mutagens, such as reactive oxygen species. Here, we studied the effects of transient hydrogen peroxide (H₂O₂ pulse) on mtDNA integrity in cultured HEK 293 cells, applying Southern blotting, ultra-deep short-read and long-read sequencing. In wild-type cells, 30 min after the H₂O₂ pulse, linear mtDNA fragments appear, representing double-strand breaks (DSB) with ends characterized by short GC stretches. Intact supercoiled mtDNA species reappear within 2-6 h after treatment and are almost completely recovered after 24 h. BrdU incorporation is lower in H₂O₂-treated cells compared to non-treated cells, suggesting that fast recovery is not associated with mtDNA replication, but is driven by rapid repair of single-strand breaks (SSBs) and degradation of DSB-generated linear fragments. Genetic inactivation of mtDNA degradation in exonuclease deficient POLG p.D274A mutant cells results in the persistence of linear mtDNA fragments with no impact on the repair of SSBs. In conclusion, our data highlight the interplay between the rapid processes of SSB repair and DSB degradation and the much slower mtDNA re-synthesis after oxidative damage, which has important implications for mtDNA quality control and the potential generation of somatic mtDNA deletions.

Mitochondrial PARP1 regulates NAD+-dependent poly ADP-ribosylation of mitochondrial nucleoids

PARPs play fundamental roles in multiple DNA damage recognition and repair pathways. Persistent nuclear PARP activation causes cellular NAD+ depletion and exacerbates cellular aging. However, very little is known about mitochondrial PARP (mtPARP) and poly ADP-ribosylation (PARylation). The existence of mtPARP is controversial, and the biological roles of mtPARP-induced mitochondrial PARylation are unclear. Here, we demonstrate the presence of PARP1 and PARylation in purified mitochondria. The addition of the PARP1 substrate NAD+ to isolated mitochondria induced PARylation, which was suppressed by treatment with the inhibitor olaparib. Mitochondrial PARylation was also evaluated by enzymatic labeling of terminal ADP-ribose (ELTA). To further confirm the presence of mtPARP1, we evaluated mitochondrial nucleoid PARylation by ADP ribose-chromatin affinity purification (ADPr-ChAP) and PARP1 chromatin immunoprecipitation (ChIP). We observed that NAD+ stimulated PARylation and TFAM occupancy on the mtDNA regulatory region D-loop, inducing mtDNA transcription. These findings suggest that PARP1 is integrally involved in mitochondrial PARylation and that NAD+-dependent mtPARP1 activity contributes to mtDNA transcriptional regulation.

Membrane compartmentalisation of the ubiquitin system

We now have a comprehensive inventory of ubiquitin system components. Understanding of any system also needs an appreciation of how components are organised together. Quantitative proteomics has provided us with a census of their relative populations in several model cell types. Here, by examining large scale unbiased data sets, we seek to identify and map those components, which principally reside on the major organelles of the endomembrane system. We present the consensus distribution of > 50 ubiquitin modifying enzymes, E2s, E3s and DUBs, that possess transmembrane domains. This analysis reveals that the ER and endosomal compartments have a diverse cast of resident E3s, whilst the Golgi and mitochondria operate with a more restricted palette. We describe key functions of ubiquitylation that are specific to each compartment and relate this to their signature complement of ubiquitin modifying components.

Protocol to detect in vitro and in cell ubiquitylation of mitochondrial DNA polymerase gamma by mitochondrial E3 ligase MITOL

Mitochondrial polymerase gamma (PolγA) is the only replicative polymerase in mitochondria. To determine PolγA ubiquitylation in cells, Flag-PolγA and MITOL are overexpressed, and subsequently the immunoprecipitated Flag-PolγA is checked for ubiquitylation. Alternately, in vitro synthesized PolγA and MITOL are used to determine whether PolγA is ubiquitylated. Either anti-ubiquitin or anti-Flag antibody is used to detect the ubiquitylated product. Thus, we provide a detailed, reliable, highly reproducible protocol for detecting ubiquitylation of PolγA by MITOL, both in cells and in vitro. For complete details on the use and execution of this protocol, please refer to Hussain et al. (2021).

Understanding the DNA double-strand break repair and its therapeutic implications

Repair of DNA double-strand breaks (DSBs) and its regulation are tightly integrated inside cells. Homologous recombination, nonhomologous end joining and microhomology mediated end joining are three major DSB repair pathways in mammalian cells. Targeting proteins associated with these repair pathways using small molecule inhibitors can prove effective in tumors, especially those with deregulated repair. Sensitization of cancer to current age therapy including radio and chemotherapy, using small molecule inhibitors is promising and warrant further development. Although several are under clinical trial, till date no repair inhibitor is approved for commercial use in cancer patients, with the exception of PARP inhibitors targeting single-strand break repair. Based on molecular profiling of repair proteins, better prognostic and therapeutic output can be achieved in patients. In the present review, we highlight the different mechanisms of DSB repair, chromatin dynamics to provide repair accessibility and modulation of inhibitors in association with molecular profiling and current gold standard treatment modalities for cancer.