The Caenorhabditis elegans spe-39 gene is required for intracellular membrane reorganization during spermatogenesis

Caenorhabditis elegans spermatid formation involves asymmetric partitioning of cytoplasm during the second meiotic division. This process is mediated by specialized ER/Golgi-derived fibrous body-membranous organelles (FB-MOs), which have a fibrous body (FB) composed of bundled major sperm protein filaments and a vesicular membranous organelle (MO). spe-39 mutant spermatocytes complete meiosis but do not usually form spermatids. Ultrastructural examination of spe-39 spermatocytes reveals that MOs are absent, while FBs are disorganized and not surrounded by the membrane envelope usually observed in wild type. Instead, spe-39 spermatocytes contain many small vesicles with internal membranes, suggesting they are related to MOs. The spe-39 gene was identified and it encodes a novel hydrophilic protein. Immunofluorescence with a specific SPE-39 antiserum reveals that it is distributed through much of the cytoplasm and not specifically associated with FB-MOs in spermatocytes and spermatids. The spe-39 gene has orthologs in Drosophila melanogaster and humans but no homolog was identified in the yeast genome. This suggests that the specialized membrane biogenesis steps that occur during C. elegans spermatogenesis are part of a conserved process that requires SPE-39 homologs in other metazoan cell types.     

The nonmuscle myosin regulatory light chain gene mlc-4 is required for cytokinesis, anterior-posterior polarity, and body morphology during Caenorhabditis elegans embryogenesis.

Using RNA-mediated genetic interference in a phenotypic screen, we identified a conserved nonmuscle myosin II regulatory light chain gene in Caenorhabditis elegans, which we name mlc-4. Maternally supplied mlc-4 function is required for cytokinesis during both meiosis and mitosis and for establishment of anterior-posterior (a-p) asymmetries after fertilization. Reducing the function of mlc-4 or nmy-2, a nonmuscle myosin II gene, also leads to a loss of polarized cytoplasmic flow in the C. elegans zygote, supporting models in which cytoplasmic flow may be required to establish a-p differences. Germline P granule localization at the time of cytoplasmic flow is also lost in these embryos, although P granules do become localized to the posterior pole after the first mitosis. This result suggests that a mechanism other than cytoplasmic flow or mlc-4/nmy-2 activity can generate some a-p asymmetries in the C. elegans zygote. By isolating a deletion allele, we show that removing zygotic mlc-4 function results in an elongation phenotype during embryogenesis. An mlc-4/green fluorescent protein transgene is expressed in lateral rows of hypodermal cells and these cells fail to properly change shape in mlc-4 mutant animals during elongation.     

The conserved oligomeric Golgi complex is required for fucosylation of N-glycans in Caenorhabditis elegans

The conserved oligomeric Golgi complex (COG) is a hetero-octomeric peripheral membrane protein required for retrograde vesicular transport and glycoconjugate biosynthesis within the Golgi. Mutations in subunits 1, 4, 5, 6, 7 and 8 are the basis for a rare inheritable human disease termed congenital disorders of glycosylation type-II. Defects to COG complex function result in aberrant glycosylation, protein trafficking and Golgi structure. The cellular function of the COG complex and its role in protein glycosylation are not completely understood. In this study, we report the first detailed structural analysis of N-glycans from a COG complex-deficient organism. We employed sequential ion trap mass spectrometry of permethylated N-glycans to demonstrate that the COG complex is essential for the formation of fucose-rich N-glycans, specifically antennae fucosylated structures in Caenorhabditis elegans. Our results support the supposition that disruption to the COG complex interferes with normal protein glycosylation in the medial and/or trans-Golgi.     

Novel and conserved protein macoilin is required for diverse neuronal functions in Caenorhabditis elegans

Neural signals are processed in nervous systems of animals responding to variable environmental stimuli. This study shows that a novel and highly conserved protein, macoilin (MACO-1), plays an essential role in diverse neural functions in Caenorhabditis elegans. maco-1 mutants showed abnormal behaviors, including defective locomotion, thermotaxis, and chemotaxis. Expression of human macoilin in the C. elegans nervous system weakly rescued the abnormal thermotactic phenotype of the maco-1 mutants, suggesting that macoilin is functionally conserved across species. Abnormal thermotaxis may have been caused by impaired locomotion of maco-1 mutants. However, calcium imaging of AFD thermosensory neurons and AIY postsynaptic interneurons of maco-1 mutants suggest that macoilin is required for appropriate responses of AFD and AIY neurons to thermal stimuli. Studies on localization of MACO-1 showed that C. elegans and human macoilins are localized mainly to the rough endoplasmic reticulum. Our results suggest that macoilin is required for various neural events, such as the regulation of neuronal activity.     

MADD-4 is a secreted cue required for midline-oriented guidance in Caenorhabditis elegans

The netrins and slits are two families of widely conserved cues that guide axons and cells along the dorsal-ventral (D-V) axis of animals. These cues typically emanate from the dorsal or ventral midlines and provide spatial information to migrating cells by?forming gradients along the D-V axis. Some cell types, however, extend processes to both the dorsal and ventral midlines, suggesting the existence of additional guidance cues that are secreted from both midlines. Here, we report that a previously uncharacterized protein called MADD-4 is secreted by the dorsal and ventral nerve cords of the nematode C.?elegans to attract sensory axons and muscle?membrane extensions called muscle arms. MADD-4's activity is dependent on UNC-40/DCC, a netrin receptor, which functions cell-autonomously to direct membrane extension. The biological role of MADD-4 orthologs, including ADAMTSL1 and 3 in mammals, is unknown. MADD-4 may therefore represent the founding member of a family of guidance proteins.     

The gon-1 gene is required for gonadal morphogenesis in Caenorhabditis elegans

In wild-type Caenorhabditis elegans, the gonad is a complex epithelial tube that consists of long arms composed predominantly of germline tissue as well as somatic structures specialized for particular reproductive functions. In gon-1 mutants, the adult gonad is severely disorganized with essentially no arm extension and no recognizable somatic structure. The developmental defects in gon-1 mutants are limited to the gonad; other cells, tissues, and organs appear to develop normally. Previous work defined the regulatory "leader" cells as crucial for extension of the gonadal arms (J. E. Kimble and J. G. White, 1981, Dev. Biol. 81, 208-219). In gon-1 mutants, the leader cells are specified correctly, but they fail to migrate and gonadal arms are not generated. In addition, gon-1 is required for morphogenesis of the gonadal somatic structures. This second role appears to be independent of that required for leader migration. Parallel studies have shown that gon-1 encodes a secreted metalloprotease (R. Blelloch and J. Kimble, 1999, Nature 399, 586-590). We discuss how a metalloprotease may control two aspects of gonadal morphogenesis.     

Expression of the homeotic gene mab-5 during Caenorhabditis elegans embryogenesis.

mab-5 is a member of a complex of homeobox-containing genes evolutionarily related to the Antennapedia and bithorax complexes of Drosophila melanogaster. Like the homeotic genes in Drosophila, mab-5 is required in a particular region along the anterior-posterior body axis, and acts during postembryonic development to give cells in this region their characteristic identities. We have used a mab-5-lacZ fusion integrated into the C. elegans genome to study the posterior-specific expression of mab-5 during embryogenesis. The mab-5-lacZ fusion was expressed in the posterior of the embryo by 180 minutes after the first cleavage, indicating that the mechanisms responsible for the position-specific expression of mab-5-lacZ act at a relatively early stage of embryogenesis. In embryos homozygous for mutations in the par genes, which disrupt segregation of factors during early cleavages, expression of mab-5-lacZ was no longer localized to the posterior. This suggests that posterior-specific expression of mab-5 depends on the appropriate segregation of developmental factors during early embryogenesis. After extrusion of any blastomere of the four-cell embryo, descendants of the remaining three cells could still express the mab-5-lacZ fusion. In these partial embryos, however, the fusion was often expressed in cells scattered throughout the embryo, suggesting that cell-cell interactions and/or proper positioning of early blastomeres are required for mab-5 expression to be localized to the posterior.     

glo-3, a novel Caenorhabditis elegans gene, is required for lysosome-related organelle biogenesis

Gut granules are specialized lysosome-related organelles that act as sites of fat storage in Caenorhabditis elegans intestinal cells. We identified mutations in a gene, glo-3, that functions in the formation of embryonic gut granules. Some glo-3(−) alleles displayed a complete loss of embryonic gut granules, while other glo-3(−) alleles had reduced numbers of gut granules. A subset of glo-3 alleles led to mislocalization of gut granule contents into the intestinal lumen, consistent with a defect in intracellular trafficking. glo-3(−) embryos lacking gut granules developed into adults containing gut granules, indicating that glo-3(+) function may be differentially required during development. We find that glo-3(+) acts in parallel with or downstream of the AP-3 complex and the PGP-2 ABC transporter in gut granule biogenesis. glo-3 encodes a predicted membrane-associated protein that lacks obvious sequence homologs outside of nematodes. glo-3 expression initiates in embryonic intestinal precursors and persists almost exclusively in intestinal cells through adulthood. GLO-3GFP localizes to the gut granule membrane, suggesting it could play a direct role in the trafficking events at the gut granule. smg-1(−) suppression of glo-3(−) nonsense alleles indicates that the C-terminal half of GLO-3, predicted to be present in the cytoplasm, is not necessary for gut granule formation. Our studies identify GLO-3 as a novel player in the formation of lysosome-related organelles.     

The Caenorhabditis elegans NR4A nuclear receptor is required for spermatheca morphogenesis

The gene nhr-6 encodes the Caenorhabditis elegans ortholog of the NR4A nuclear receptor. We determined the biological functions of NHR-6 through the isolation and characterization of a deletion allele of nhr-6, lg6001. We demonstrate that nhr-6 has an essential role in the development of the C. elegans somatic gonad. Specifically, nhr-6 is required for the development of the hermaphrodite spermatheca, a somatic gonad organ that serves as the site of sperm storage and oocyte fertilization. Using a variety of spermatheca cell markers, we have determined that loss of nhr-6 function causes severe morphological defects in the spermatheca and associated spermathecal valves. This appears to be due to specific requirements for nhr-6 in regulating cell proliferation and cell differentiation during development of these structures. The improper development of these structures in nhr-6(lg6001) mutants leads to defects in ovulation and significantly reduced fecundity of C. elegans hermaphrodites. The phenotypes of nhr-6(lg6001) mutants are consistent with a role for nhr-6 in organogenesis, similar to the functions of its mammalian homologs.     

Separase is required for chromosome segregation during meiosis I in Caenorhabditis elegans.

BACKGROUND: Chromosome segregation during mitosis and meiosis is triggered by dissolution of sister chromatid cohesion, which is mediated by the cohesin complex. Mitotic sister chromatid disjunction requires that cohesion be lost along the entire length of chromosomes, whereas homolog segregation at meiosis I only requires loss of cohesion along chromosome arms. During animal cell mitosis, cohesin is lost in two steps. A nonproteolytic mechanism removes cohesin along chromosome arms during prophase, while the proteolytic cleavage of cohesin's Scc1 subunit by separase removes centromeric cohesin at anaphase. In Saccharomyces cerevisiae and Caenorhabditis elegans, meiotic sister chromatid cohesion is mediated by Rec8, a meiosis-specific variant of cohesin's Scc1 subunit. Homolog segregation in S. cerevisiae is triggered by separase-mediated cleavage of Rec8 along chromosome arms. In principle, chiasmata could be resolved proteolytically by separase or nonproteolytically using a mechanism similar to the mitotic "prophase pathway." RESULTS: Inactivation of separase in C. elegans has little or no effect on homolog alignment on the meiosis I spindle but prevents their timely disjunction. It also interferes with chromatid separation during subsequent embryonic mitotic divisions but does not directly affect cytokinesis. Surprisingly, separase inactivation also causes osmosensitive embryos, possibly due to a defect in the extraembryonic structures, referred to as the "eggshell." CONCLUSIONS: Separase is essential for homologous chromosome disjunction during meiosis I. Proteolytic cleavage, presumably of Rec8, might be a common trigger for the first meiotic division in eukaryotic cells. Cleavage of proteins other than REC-8 might be necessary to render the eggshell impermeable to solutes.     

Evolutionarily conserved nuclear migration genes required for early embryonic development in Caenorhabditis elegans

The nudF and nudC genes of the fungus Aspergillus nidulans encode proteins that are members of two evolutionarily conserved families. In A. nidulans these proteins mediate nuclear migration along the hyphae. The human ortholog of nudF is Lis1, a gene essential for neuronal migration in the developing cerebral cortex. The mammalian ortholog of nudC encodes a protein that interacts with Lis1. We have identified orthologs of nudC and Lis1 from the nematode Caenorhabditis elegans. Heterologous expression of the C. elegans nudC ortholog, nud-1, complements the A. nidulans nudC3 mutant, demonstrating evolutionary conservation of function. A C. elegans nud-1::GFP fusion produces sustained fluorescence in sensory neurons and embryos, and transient fluorescence in the gonad, gut, vulva, ventral cord, and hypodermal seam cells. Fusion of GFP to C. elegans lis-1 revealed expression in all major neuronal processes of the animal as well as the multinucleate spermathecal valves and adult seam cells. Phenotypic analysis of either nud-1 and lis-1 by RNA interference yielded similar phenotypes, including embryonic lethality, sterility, altered vulval morphology, and uncoordinated movement. Digital time-lapse video microscopy was used to determine that RNAi-treated embryos exhibited nuclear positioning defects in early embryonic cell division similar to those reported for dynein/dynactin depletion. These results demonstrate that the LIS-1/NUDC-like proteins of C. elegans represent a link between nuclear positioning, cell division, and neuronal function.     

Caenorhabditis elegans reticulon interacts with RME-1 during embryogenesis

Reticulon (RTN) family proteins are localized in the endoplasmic reticulum (ER). At least four different RTN genes have been identified in mammals, but in most cases, the functions of the encoded proteins except mammalian RTN4-A and RTN4-B are unknown. Each RTN gene produces 1-3 proteins by different promoters and alternative splicing. In Caenorhabditis elegans, there is a single gene (rtn gene) encoding three reticulon proteins, nRTN-A, B, and C. mRNA of nRTN-C is expressed in germ cells and embryos. However, nRTN-C protein is only expressed during embryogenesis and rapidly disappears after hatch. By yeast two-hybrid screening, two clones encoding the same C-terminal region of RME-1, a protein functioning in the endocytic recycling, were isolated. These findings suggest that nRTN-C functions in the endocytic pathway during embryogenesis.     

Autophagy is required for necrotic cell death in Caenorhabditis elegans

Autophagy is the main process for bulk protein and organelle recycling in cells under extracellular or intracellular stress. Deregulation of autophagy has been associated with pathological conditions such as cancer, muscular disorders and neurodegeneration. Necrotic cell death underlies extensive neuronal loss in acute neurodegenerative episodes such as ischemic stroke. We find that excessive autophagosome formation is induced early during necrotic cell death in C. elegans. In addition, autophagy is required for necrotic cell death. Impairment of autophagy by genetic inactivation of autophagy genes or by pharmacological treatment suppresses necrosis. Autophagy synergizes with lysosomal catabolic mechanisms to facilitate cell death. Our findings demonstrate that autophagy contributes to cellular destruction during necrosis. Thus, interfering with the autophagic process may protect neurons against necrotic damage in humans.     

The NED-8 conjugating system in Caenorhabditis elegans is required for embryogenesis and terminal differentiation of the hypodermis

This work has identified the enzymes involved in the activation and conjugation of the ubiquitin-like protein NED-8 in Caenorhabditis elegans. A C. elegans conjugating enzyme, UBC-12, is highly specific in its ability to utilize NED-8 as a substrate. Immunostaining shows that NED-8 is conjugated in vivo to a major target protein with a conjugate size of 90 kDa. While the amount of this conjugate is developmentally regulated with reduced levels in the larval stages, the mRNA encoding C. elegans UBC-12 is constitutively produced throughout development, as is NED-8 itself. The importance of the NED-8 conjugating system in C. elegans was determined by RNA interference (RNAi) assays using double-stranded RNA encoding NED-8, UBC-12, or the NED-8 activating enzyme component ULA-1. The progeny of both ned-8 and ubc-12 RNAi-treated hermaphrodites either arrested during embryonic development or underwent abnormal postembryonic development. The effect on postembryonic development was pleiotropic, the most frequent gross abnormality being vulval eversion during the L4 stage. Individuals with an everted vulva either burst at the L4 to adult molt or gave rise to adults incapable of egg laying. Additionally, both ned-8 and ubc-12 RNAi induced a striking abnormality in the alae, structures produced by the lateral hypodermal seam cells in the adult nematode. Affected alae were patchy and frequently diverged around a central space. Vulval defects were also produced by RNAi directed at C. elegans ula-1. This is the first demonstration of a requirement for NED-8 conjugation in metazoan development.