Unique insights into maternal mitochondrial inheritance in mice

Uniparental mitochondrial DNA inheritance is not affected in Ustilago maydis Δatg11 mutants blocked in mitophagy

Background

Maternal or uniparental inheritance (UPI) of mitochondria is generally observed in sexual eukaryotes, however, the underlying mechanisms are diverse and largely unknown. Recently, based on the use of mutants blocked in autophagy, it has been demonstrated that autophagy is required for strict maternal inheritance in the nematode Caenorhabditis elegans. Uniparental mitochondrial DNA (mtDNA) inheritance has been well documented for numerous fungal species, and in particular, has been shown to be genetically governed by the mating-type loci in the isogamous species Cryptococcus neoformans, Phycomyces blakesleeanus and Ustilago maydis. Previously, we have shown that the a2 mating-type locus gene lga2 is decisive for UPI during sexual development of U. maydis. In axenic culture, conditional overexpression of lga2 triggers efficient loss of mtDNA as well as mitophagy. To assess a functional relationship, we have investigated UPI in U. maydis Δatg11 mutants, which are blocked in mitophagy.

Results

This study has revealed that Δatg11 mutants are not affected in pathogenic development and this has allowed us to analyse UPI under comparable developmental conditions between mating-compatible wild-type and mutant strain combinations. Explicitly, we have examined two independent strain combinations that gave rise to different efficiencies of UPI. We demonstrate that in both cases UPI is atg11-independent, providing evidence that mitophagy is not critical for UPI in U. maydis, even under conditions of strict UPI.

Conclusions

Until now, analysis of a role of mitophagy in UPI has not been reported for microbial species. Our study suggests that selective autophagy does not contribute to UPI in U. maydis, but is rather a consequence of selective mtDNA elimination in response to mitochondrial damage.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0358-z) contains supplementary material, which is available to authorized users.

Autophagy and ubiquitin-mediated proteolysis may not be involved in the degradation of spermatozoon mitochondria in mouse and porcine early embryos

The mitochondrial genome is maternally inherited in animals, despite the fact that paternal mitochondria enter oocytes during fertilization. Autophagy and ubiquitin-mediated degradation are responsible for the elimination of paternal mitochondria in Caenorhabditis elegans; however, the involvement of these two processes in the degradation of paternal mitochondria in mammals is not well understood. We investigated the localization patterns of light chain 3 (LC3) and ubiquitin in mouse and porcine embryos during preimplantation development. We found that LC3 and ubiquitin localized to the spermatozoon midpiece at 3 h post-fertilization, and that both proteins were colocalized with paternal mitochondria and removed upon fertilization during the 4-cell stage in mouse and the zygote stage in porcine embryos. Sporadic paternal mitochondria were present beyond the morula stage in the mouse, and paternal mitochondria were restricted to one blastomere of 4-cell embryos. An autophagy inhibitor, 3-methyladenine (3-MA), did not affect the distribution of paternal mitochondria compared with the positive control, while an autophagy inducer, rapamycin, accelerated the removal of paternal mitochondria compared with the control. After the intracytoplasmic injection of intact spermatozoon into mouse oocytes, LC3 and ubiquitin localized to the spermatozoon midpiece, but remnants of undegraded paternal mitochondria were retained until the blastocyst stage. Our results show that paternal mitochondria colocalize with autophagy receptors and ubiquitin and are removed after in vitro fertilization, but some remnants of sperm mitochondrial sheath may persist up to morula stage after intracytoplasmic spermatozoon injection (ICSI).

Manipulating mitochondrial genomes in the clinic: playing by different rules

Recently, several publications have surfaced describing methods to manipulate mitochondrial genomes in tissues and embryos. With them, a somewhat sensationalistic uproar about the generation of children with 'three parents' has dominated the discussion in the lay media. It is important that society understands the singularities of mitochondrial genetics to judge these procedures in a rational light, so that this ongoing discussion does not preclude the helping of patients and families harboring mutated mitochondrial genomes.

The impact of mitochondrial function/dysfunction on IVF and new treatment possibilities for infertility

Mitochondria play vital roles in oocyte functions and they are critical indicators of oocyte quality which is important for fertilization and development into viable offspring. Quality-compromised oocytes are correlated with infertility, developmental disorders, reduced blastocyst cell number and embryo loss in which mitochondrial dysfunctions play a significant role. Increasingly, women affected by metabolic disorders such as diabetes or obesity and oocyte aging are seeking treatment in IVF clinics to overcome the effects of adverse metabolic conditions on mitochondrial functions and new treatments have become available to restore oocyte quality. The past decade has seen enormous advances in potential therapies to restore oocyte quality and includes dietary components and transfer of mitochondria from cells with mitochondrial integrity into mitochondria-impaired oocytes. New technologies have opened up new possibilities for therapeutic advances which will increase the success rates for IVF of oocytes from women with compromised oocyte quality.

Mitochondrial DNA disease and developmental implications for reproductive strategies

Mitochondrial diseases are potentially severe, incurable diseases resulting from dysfunctional mitochondria. Several important mitochondrial diseases are caused by mutations in mitochondrial DNA (mtDNA), the genetic material contained within mitochondria, which is maternally inherited. Classical and modern therapeutic approaches exist to address the inheritance of mtDNA disease, but are potentially complicated by the fact that cellular mtDNA populations evolve according to poorly-understood dynamics during development and organismal lifetimes. We review these therapeutic approaches and models of mtDNA dynamics during development, and discuss the implications of recent results from these models for modern mtDNA therapies. We particularly highlight mtDNA segregation—differences in proliferative rates between different mtDNA haplotypes—as a potential and underexplored issue in such therapies. However, straightforward strategies exist to combat this and other potential therapeutic problems. In particular, we describe haplotype matching as an approach with the power to potentially ameliorate any expected issues from mtDNA incompatibility.

Versatile roles of k63-linked ubiquitin chains in trafficking

Modification by Lys63-linked ubiquitin (UbK63) chains is the second most abundant form of ubiquitylation. In addition to their role in DNA repair or kinase activation, UbK63 chains interfere with multiple steps of intracellular trafficking. UbK63 chains decorate many plasma membrane proteins, providing a signal that is often, but not always, required for their internalization. In yeast, plants, worms and mammals, this same modification appears to be critical for efficient sorting to multivesicular bodies and subsequent lysosomal degradation. UbK63 chains are also one of the modifications involved in various forms of autophagy (mitophagy, xenophagy, or aggrephagy). Here, in the context of trafficking, we report recent structural studies investigating UbK63 chains assembly by various E2/E3 pairs, disassembly by deubiquitylases, and specifically recognition as sorting signals by receptors carrying Ub-binding domains, often acting in tandem. In addition, we address emerging and unanticipated roles of UbK63 chains in various recycling pathways that function by activating nucleators required for actin polymerization, as well as in the transient recruitment of signaling molecules at the plasma or ER membrane. In this review, we describe recent advances that converge to elucidate the mechanisms underlying the wealth of trafficking functions of UbK63 chains.

Mitochondrial dynamics and inheritance during cell division, development and disease

During cell division, it is critical to properly partition functional sets of organelles to each daughter cell. The partitioning of mitochondria shares some common features with that of other organelles, particularly in the use of interactions with cytoskeletal elements to facilitate delivery to the daughter cells. However, mitochondria have unique features - including their own genome and a maternal mode of germline transmission - that place additional demands on this process. Consequently, mechanisms have evolved to regulate mitochondrial segregation during cell division, oogenesis, fertilization and tissue development, as well as to ensure the integrity of these organelles and their DNA, including fusion-fission dynamics, organelle transport, mitophagy and genetic selection of functional genomes. Defects in these processes can lead to cell and tissue pathologies.

Paternal mitochondrial destruction after fertilization is mediated by a common endocytic and autophagic pathway in Drosophila

Almost all animals contain mitochondria of maternal origin only, but the exact mechanisms underlying this phenomenon are still vague. We investigated the fate of Drosophila paternal mitochondria after fertilization. We demonstrate that the sperm mitochondrial derivative (MD) is rapidly eliminated in a stereotypical process dubbed paternal mitochondrial destruction (PMD). PMD is initiated by a network of vesicles resembling multivesicular bodies and displaying common features of the endocytic and autophagic pathways. These vesicles associate with the sperm tail and mediate the disintegration of its plasma membrane. Subsequently, the MD separates from the axoneme and breaks into smaller fragments, which are then sequestered by autophagosomes for degradation in lysosomes. We further provide evidence for the involvement of the ubiquitin pathway and the autophagy receptor p62 in this process. Finally, we show that the ubiquitin ligase Parkin is not involved in PMD, implying a divergence from the autophagic pathway of damaged mitochondria.

Regulation of mitochondrial genome inheritance by autophagy and ubiquitin-proteasome system: implications for health, fitness, and fertility

Mitochondria, the energy-generating organelles, play a role in numerous cellular functions including adenosine triphosphate (ATP) production, cellular homeostasis, and apoptosis. Maternal inheritance of mitochondria and mitochondrial DNA (mtDNA) is universally observed in humans and most animals. In general, high levels of mitochondrial heteroplasmy might contribute to a detrimental effect on fitness and disease resistance. Therefore, a disposal of the sperm-derived mitochondria inside fertilized oocytes assures normal preimplantation embryo development. Here we summarize the current research and knowledge concerning the role of autophagic pathway and ubiquitin-proteasome-dependent proteolysis in sperm mitophagy in mammals, including humans. Current data indicate that sperm mitophagy inside the fertilized oocyte could occur along multiple degradation routes converging on autophagic clearance of paternal mitochondria. The influence of assisted reproductive therapies (ART) such as intracytoplasmic sperm injection (ICSI), mitochondrial replacement (MR), and assisted fertilization of oocytes from patients of advanced reproductive age on mitochondrial function, inheritance, and fitness and for the development and health of ART babies will be of particular interest to clinical audiences. Altogether, the study of sperm mitophagy after fertilization has implications in the timing of evolution and developmental and reproductive biology and in human health, fitness, and management of mitochondrial disease.

Role of autophagy in embryogenesis

Eukaryotes have evolved multiple mechanisms for inactivating macromolecules in order to maintain their functionality. Autophagy-the process of self-eating-leads to the degradation of cytoplasmic components for the dynamic remodeling of subcellular compartments, turnover and recycling of macromolecules, and regulation of cellular activity through the control of specific intracellular signaling pathways. This fundamental process is also implicated in systemic response to starvation and immune challenges, as well as anti-tumorigenesis and anti-senescence. Recent studies have also highlighted an important role for autophagy in embryonic development. In this review, we discuss the emerging evidence for the varied functions of autophagy at different stages of development, with an emphasis on the early events of embryogenesis.

To be or not to be? How selective autophagy and cell death govern cell fate

The health of metazoan organisms requires an effective response to organellar and cellular damage either by repair of such damage and/or by elimination of the damaged parts of the cells or the damaged cell in its entirety. Here, we consider the progress that has been made in the last few decades in determining the fates of damaged organelles and damaged cells through discrete, but genetically overlapping, pathways involving the selective autophagy and cell death machinery. We further discuss the ways in which the autophagy machinery may impact the clearance and consequences of dying cells for host physiology. Failure in the proper removal of damaged organelles and/or damaged cells by selective autophagy and cell death processes is likely to contribute to developmental abnormalities, cancer, aging, inflammation, and other diseases.

Dynamic survey of mitochondria by ubiquitin

Ubiquitin is a post-translational modifier with proteolytic and non-proteolytic roles in many biological processes. At mitochondria, it performs regulatory homeostatic functions and contributes to mitochondrial quality control. Ubiquitin is essential for mitochondrial fusion, regulates mitochondria-ER contacts, and participates in maternal mtDNA inheritance. Under stress, mitochondrial dysfunction induces ubiquitin-dependent responses that involve mitochondrial proteome remodeling and culminate in organelle removal by mitophagy. In addition, many ubiquitin-dependent mechanisms have been shown to regulate innate immune responses and xenophagy. Here, we review the emerging roles of ubiquitin at mitochondria.

You are what you eat: multifaceted functions of autophagy during C. elegans development

Autophagy involves the sequestration of a portion of the cytosolic contents in an enclosed double-membrane autophagosomal structure and its subsequent delivery to lysosomes for degradation. Autophagy activity functions in multiple biological processes during Caenorhabditis elegans development. The basal level of autophagy in embryos removes aggregate-prone proteins, paternal mitochondria and spermatid-specific membranous organelles (MOs). Autophagy also contributes to the efficient removal of embryonic apoptotic cell corpses by promoting phagosome maturation. During larval development, autophagy modulates miRNA-mediated gene silencing by selectively degrading AIN-1, a component of miRNA-induced silencing complex, and thus participates in the specification of multiple cell fates controlled by miRNAs. During development of the hermaphrodite germline, autophagy acts coordinately with the core apoptotic machinery to execute genotoxic stress-induced germline cell death and also cell death when caspase activity is partially compromised. Autophagy is also involved in the utilization of lipid droplets in the aging process in adult animals. Studies in C. elegans provide valuable insights into the physiological functions of autophagy in the development of multicellular organisms.

Mitochondrial DNMT3A and DNA methylation in skeletal muscle and CNS of transgenic mouse models of ALS

Cytosine methylation is an epigenetic modification of DNA catalyzed by DNA methyltransferases. Cytosine methylation of mitochondrial DNA (mtDNA) is believed to have relative underrepresentation; however, possible tissue and cell differences in mtDNA methylation and relationships to neurodegenerative disease have not been examined. We show by immunoblotting that DNA methyltransferase 3A (Dnmt3a) isoform is present in pure mitochondria of adult mouse CNS, skeletal muscle, and testes, and adult human cerebral cortex. Dnmt1 was not detected in adult mouse CNS or skeletal muscle mitochondria but appeared bound to the outer mitochondrial membrane. Immunofluorescence confirmed the mitochondrial localization of Dnmt3a and showed 5-methylcytosine (5mC) immunoreactivity in mitochondria of neurons and skeletal muscle myofibers. DNA pyrosequencing of two loci (D-loop and 16S rRNA gene) and twelve cytosine-phosphate-guanine (CpG) sites in mtDNA directly showed a tissue differential presence of 5mC. Because mitochondria have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS), but the disease mechanisms are uncertain, we evaluated mitochondrial Dnmt3a and 5mC levels in human superoxide dismutase-1 (SOD1) transgenic mouse models of ALS. Mitochondrial Dnmt3a protein levels were reduced significantly in skeletal muscle and spinal cord at presymptomatic or early disease. Immunofluorescence showed that 5mC immunoreactivity was present in mitochondria of neurons and skeletal myofibers, and 5mC immunoreactivity became aggregated in motor neurons of ALS mice. DNA pyrosequencing revealed significant abnormalities in 16S rRNA gene methylation in ALS mice. Immunofluorescence showed that 5mC immunoreactivity can be sequestered into autophagosomes and that mitophagy was increased and mitochondrial content was decreased in skeletal muscle in ALS mice. This study reveals a tissue-preferential mitochondrial localization of Dnmt3a and presence of cytosine methylation in mtDNA of nervous tissue and skeletal muscle and demonstrates that mtDNA methylation patterns and mitochondrial Dnmt3a levels are abnormal in skeletal muscle and spinal cord of presymptomatic ALS mice, and these abnormalities occur in parallel with loss of myofiber mitochondria.

Ubiquitination regulates the morphogenesis and function of sperm organelles

It is now understood that protein ubiquitination has diverse cellular functions in eukaryotes. The molecular mechanism and physiological significance of ubiquitin-mediated processes have been extensively studied in yeast, Drosophila and mammalian somatic cells. Moreover, an increasing number of studies have emphasized the importance of ubiquitination in spermatogenesis and fertilization. The dysfunction of various ubiquitin systems results in impaired sperm development with abnormal organelle morphology and function, which in turn is highly associated with male infertility. This review will focus on the emerging roles of ubiquitination in biogenesis, function and stability of sperm organelles in mammals.