Contrasting patterns of mitochondrial redistribution in the early lineages of Caenorhabditis elegans and Acrobeloides sp. PS1146

Mitochondria-Derived H2O2 Promotes Symmetry Breaking of the C. elegans Zygote

Symmetry breaking is an essential step in cell differentiation and early embryonic development. However, the molecular cues that trigger symmetry breaking remain largely unknown. Here, we show that mitochondrial H₂O₂ acts as a symmetry-breaking cue in the C. elegans zygote. We find that symmetry breaking is marked by a local H₂O₂ increase and coincides with a relocation of mitochondria to the cell cortex. Lowering endogenous H₂O₂ levels delays the onset of symmetry breaking, while artificially targeting mitochondria to the cellular cortex using a light-induced heterodimerization technique is sufficient to initiate symmetry breaking in a H₂O₂-dependent manner. In wild-type development, both sperm and maternal mitochondria contribute to symmetry breaking. Our findings reveal that mitochondrial H₂O₂-signaling promotes the onset of polarization, a fundamental process in development and cell differentiation, and this is achieved by both mitochondrial redistribution and differential H₂O₂-production.

Glutathione reductase gsr-1 is an essential gene required for Caenorhabditis elegans early embryonic development

Glutathione is the most abundant thiol in the vast majority of organisms and is maintained in its reduced form by the flavoenzyme glutathione reductase. In this work, we describe the genetic and functional analysis of the Caenorhabditis elegans gsr-1 gene that encodes the only glutathione reductase protein in this model organism. By using green fluorescent protein reporters we demonstrate that gsr-1 produces two GSR-1 isoforms, one located in the cytoplasm and one in the mitochondria. gsr-1 loss of function mutants display a fully penetrant embryonic lethal phenotype characterized by a progressive and robust cell division delay accompanied by an aberrant distribution of interphasic chromatin in the periphery of the cell nucleus. Maternally expressed GSR-1 is sufficient to support embryonic development but these animals are short-lived, sensitized to chemical stress, have increased mitochondrial fragmentation and lower mitochondrial DNA content. Furthermore, the embryonic lethality of gsr-1 worms is prevented by restoring GSR-1 activity in the cytoplasm but not in mitochondria. Given the fact that the thioredoxin redox systems are dispensable in C. elegans, our data support a prominent role of the glutathione reductase/glutathione pathway in maintaining redox homeostasis in the nematode.

Dual roles for ubiquitination in the processing of sperm organelles after fertilization

BACKGROUND

The process of fertilization involves a cell fusion event between the sperm and oocyte. Although sperm contain mitochondria when they fuse with the oocyte, paternal mitochondrial genomes do not persist in offspring and, thus, mitochondrial inheritance is maternal in most animals. Recent evidence suggests that paternal mitochondria may be eliminated via autophagy after fertilization. In C. elegans, sperm-specific organelles called membraneous organelles (MO) cluster together with paternal mitochondria immediately after fertilization. These MOs but not the mitochondria become polyubiquitinated and associated with proteasomes. The current model for the elimination of paternal mitochondria in C. elegans is that ubiquitination of the MOs induces the formation of autophagosomes which also capture the mitochondria and cause their degradation.

RESULTS

Sperm-derived mitochondria and MOs show a sharp decrease in number during the time between sperm-oocyte fusion and the onset of mitosis. During this time, paternal mitochondria remain closely clustered with the MOs. Two types of polyubiquitin chains are observed on the MOs: K48-linked ubiquitin chains which are known to lead to proteasomal degradation and K63-linked ubiquitin chains which have been linked to autophagy. K48-linked ubiquitin chains and proteasomes show up on MOs very soon after sperm-oocyte fusion. These are present on MOs for only a short period of time. Maternal proteasomes localize to MOs and sperm proteasomes localize to structures that are at the periphery of the MO cluster suggesting that these two proteasome populations may have different roles in degrading paternal material. K63-linked ubiquitin chains appear on MOs early and remain throughout the first several cell divisions.

CONCLUSIONS

Since there are two different types of polyubiquitin chains associated with sperm organelles and their timing differs, it suggests that ubiquitin has two or more roles in the processing of sperm components after fertilization. The K63 chains potentially provide a signal for autophagy of paternal organelles, whereas the K48 chains and proteasomes may be involved in degradation of specific proteins.

How to analyze mitochondrial morphology in healthy cells and apoptotic cells in Caenorhabditis elegans

Mitochondria constantly undergo fusion and fission events. A proper balance of fusion and fission is essential in healthy cells, as disrupting this balance is associated with several neurodegenerative diseases. Mitochondrial fission has also been shown to play an important role during apoptosis. Hence, the machineries that control mitochondrial morphology have both nonapoptotic and apoptotic functions. Seminal work in yeast has identified some of the key components of these machineries. However, the list is certainly not complete and new factors that are specific to metazoans are being identified every year. In this review, we describe methodologies to test whether a particular candidate gene plays a role in the control of mitochondrial morphology in healthy cells and apoptotic cells using Caenorhabditis elegans.

An RNAi screen for mitochondrial proteins required to maintain the morphology of the organelle in Caenorhabditis elegans

Mitochondria are dynamic organelles that frequently divide and fuse together, resulting in the formation of intracellular tubular networks. In yeast and mammals, several factors including Drp1/Dnm1 and Mfn/Fzo1 are known to regulate mitochondrial morphology by controlling membrane fission or fusion. Here, we report the systematic screening of Caenorhabditis elegans mitochondrial proteins required to maintain the morphology of the organelle using an RNA interference feeding library. In C. elegans body wall muscle cells, mitochondria usually formed tubular structures and were severely fragmented by the mutation in fzo-1 gene, indicating that the body wall muscle cells are suitable for monitoring changes in mitochondrial morphology due to gene silencing. Of 719 genes predicted to code for most of mitochondrial proteins, knockdown of >80% of them caused abnormal mitochondrial morphology, including fragmentation and elongation. These findings indicate that most fundamental mitochondrial functions, including metabolism and oxidative phosphorylation, are necessary for maintenance of the tubular networks as well as membrane fission and fusion. This is the first evidence that known mitochondrial activities are prerequisite for regulating the morphology of the organelle. Furthermore, 88 uncharacterized or poorly characterized genes were found in the screening to be implicated in mitochondrial morphology.

Arrest of cytoplasmic streaming induces algal proliferation in green paramecia

A green ciliate Paramecium bursaria, bearing several hundreds of endosymbiotic algae, demonstrates rotational microtubule-based cytoplasmic streaming, in which cytoplasmic granules and endosymbiotic algae flow in a constant direction. However, its physiological significance is still unknown. We investigated physiological roles of cytoplasmic streaming in P. bursaria through host cell cycle using video-microscopy. Here, we found that cytoplasmic streaming was arrested in dividing green paramecia and the endosymbiotic algae proliferated only during the arrest of cytoplasmic streaming. Interestingly, arrest of cytoplasmic streaming with pressure or a microtubule drug also induced proliferation of endosymbiotic algae independently of host cell cycle. Thus, cytoplasmic streaming may control the algal proliferation in P. bursaria. Furthermore, confocal microscopic observation revealed that a division septum was formed in the constricted area of a dividing paramecium, producing arrest of cytoplasmic streaming. This is a first report to suggest that cytoplasmic streaming controls proliferation of eukaryotic cells.

SPD-3 is required for spindle alignment in Caenorhabditis elegans embryos and localizes to mitochondria

During the development of multicellular organisms, cellular diversity is often achieved through asymmetric cell divisions that produce two daughter cells having different developmental potentials. Prior to an asymmetric cell division, cellular components segregate to opposite ends of the cell defining an axis of polarity. The mitotic spindle rotationally aligns along this axis of polarity, thereby ensuring that the cleavage plane is positioned such that segregated components end up in individual daughter cells. Here we report our characterization of a novel gene required for spindle alignment in Caenorhabditis elegans. During the first mitosis in spd-3(oj35) embryos the spindle failed to align along the anterior/posterior axis, leading to abnormal cleavage configurations. spd-3(oj35) embryos had additional defects reminiscent of dynein/dynactin loss-of-function possibly caused by the mislocalization of dynactin. Surprisingly, we found that SPD-3GFP localized to mitochondria. Consistent with this localization, spd-3(oj35) worms exhibited slow growth and increased ATP concentrations, which are phenotypes similar to those described for other mitochondrial mutants in C. elegans. To our knowledge, SPD-3 is the first example of a link between mitochondria and spindle alignment in C. elegans.

CAR-1, a protein that localizes with the mRNA decapping component DCAP-1, is required for cytokinesis and ER organization in Caenorhabditis elegans embryos

The division of one cell into two requires the coordination of multiple components. We describe a gene, car-1, whose product may provide a link between disparate cellular processes. Inhibition of car-1 expression in Caenorhabditis elegans embryos causes late cytokinesis failures: cleavage furrows ingress but subsequently regress and the spindle midzone fails to form, even though midzone components are present. The localized accumulation of membrane that normally develops at the apex of the cleavage furrow during the final phase of cytokinesis does not occur and organization of the endoplasmic reticulum is aberrant, indicative of a disruption in membrane trafficking. The car-1 gene has homologues in a number of species, including proteins that associate with RNA binding proteins. CAR-1 localizes to P-granules (germ-line specific ribonucleoprotein particles) and discrete, developmentally regulated cytoplasmic foci. These foci also contain DCAP-1, a protein involved in decapping mRNAs. Thus, CAR-1, a protein likely to be associated with RNA metabolism, plays an essential role in the late stage of cytokinesis, suggesting a novel link between RNA, membrane trafficking and cytokinesis in the C. elegans embryo.

Asymmetric cell division in C. elegans: cortical polarity and spindle positioning

The one-cell Caenorhabditis elegans embryo divides asymmetrically into a larger and smaller blastomere, each with a different fate. How does such asymmetry arise? The sperm-supplied centrosome establishes an axis of polarity in the embryo that is transduced into the establishment of anterior and posterior cortical domains. These cortical domains define the polarity of the embryo, acting upstream of the PAR proteins. The PAR proteins, in turn, determine the subsequent segregation of fate determinants and the plane of cell division. We address how cortical asymmetry could be established, relying on data from C. elegans and other polarized cells, as well as from applicable models. We discuss how cortical polarity influences spindle position to accomplish an asymmetric division, presenting the current models of spindle orientation and anaphase spindle displacement. We focus on asymmetric cell division as a function of the actin and microtubule cytoskeletons, emphasizing the cell biology of polarity.