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内容記述 |
When living organisms encounter inappropriate environment for their survival and reproduction, they either escape from the environment (e.g., bird migration) or adapt to the environment by physiological changes. Dormancy or diapause is a representative example of such adaptation and observed in various organisms. Studies on dormancy have been performed mostly in the fields of physiology, ecology and biochemistry. However, the molecular mechanisms of the regulation of dormancy are not well known.<br /> The nematode C. elegans also has a diapause stage, which is called dauer larva. The dauer larva has common features of dormant animals, i. e., low metabolism, no feeding, accumulation of fat, and resistance to stress. Since many molecular biological and genetic techniques are available, C. elegans is a good model organism for studying the regulation of transition into the diapause stage at molecular and cellular levels. During the life cycle, C. elegans grows up to adults through 4 larval stages (L1-L4) in 2-3 days at 25℃. But under inadequate conditions for growth, that is, under reduced food availability, crowding, and high temperature, animals arrest development and form dauer larvae corresponding to the 3rd larval stage. Dauer larvae can live for several months without feeding, while the life span in normal development is about 2-3 weeks. When the environmental conditions are improved, dauer larvae molt to normal L4 larvae and resume the life cycle. Like insects, the nervous system is involved in the diapause of C. elegans; some neurons in a pair of head sensory organs called amphids have been shown to control dauer larva formation.<br /> Genes that regulate dauer larva formation have been studied by isolation and characterization of mutants that show abnormality in this function. These mutants consist of two groups: dauer-constitutive (daf-c) mutants, which form dauer larvae even under the conditions of abundant food availability and no crowding, and dauer-defective (daf-d) mutants, which do not form dauer larvae even under the conditions of extreme crowding and starvation. The genetic pathways of dauer formation have been revealed by epistasis tests of these mutations and molecular cloning of the mutated genes. At least four signal transduction pathways control dauer larva formation: cGMP related signaling pathway, TGF-β signaling pathway, insulin signaling pathway and recently suggested steroid hormone signaling pathway.<br /> In addition to these mutations that show abnormal phenotypes in dauer larva formation by themselves, mutations that show the dauer-constitutive phenotype only in the background of another mutation have been discovered and called synthetic dauer-constitutive mutations. A great majority of them were isolated by other phenotypes and later found to show this phenotype when double mutants of these mutations were constructed. unc-31 mutation is one of such mutations, while unc-31 gene encodes a homologue of CAPS (calcium activated protein for secretion), which is required for the exocytosis of dense core vesicles, which contain neuropeptides and biogenic amines. Mutants in this gene show many phenotypes: slow locomotion, defective egg-laying, and constitutive pharyngeal pumping, etc., but essentially the wild type phenotype concerning dauer larva formation except at very high temperature (27℃), at which C. elegans cannot reproduce.<br /> To identify new genes regulating dauer formation and to discover new mechanisms, 44 synthetic dauer-constitutive mutants were isolated in the unc-31(e169) background, mapped and named sdf (synthetic abnormality in dauer formation) mutants in our laboratory.<br /> In this study, I cloned one of the mutant genes, sdf-14 gene, by positional cloning, and analyzed its function on dauer larva formation. sdf-14 gene encoded MRP-1 (multidrug resistance-associated protein-1), a member of the ABC transporter superfamily. ABC transporters export or import a wide variety of substrates by directly coupling these functions with the energy of ATP hydrolysis. Human MRP1 has been reported to export unnecessary compounds (conjugates, xenobiotics and detoxification products) from inside cells to outside. Since C. elegans MRP-1/SDF-14 had homology to human MRP1 throughout the amino acid sequence, it was predicted that, like human MRP1, C. elegans MRP-1/SDF-14 consists of 3 membrane spanning domains (MSDs) and 2 nucleotide binding domains (NBDs). The two mutant alleles of sdf-14, ut151 and ut155, had missense mutations in NBD1, while another allele, utl53, had a mutation at the splice acceptor site of the 4th intron. In addition to the four MRP-1 isoforms that have been reported already, I found a new isoform, e-type. These isoforms seemed to differ in their functions, because the b- and c- type isoforms rescued the dauer formation abnormality of the unc-31(e169)；sdf-14(ut153) double mutant, but the e-type isoform did not. These isoforms had variant copies of exon 13, suggesting that exon 13 may code for amino acid sequences that contribute to substrate specificity. The wild type human MRP1 cDNA driven by the sdf-14 promoter, but not the dmL0 mutant MRP1 cDNA, rescued the dauer-constitutive phenotype of the unc-31(e169); sdf-14(ut153) double mutant. Furthermore, the rescue was partially canceled by the addition of a human MRP1 inhibitor. Those results strongly suggested that C. elegans MRP-1/SDF-14 acts as an exporter like human MRP1 in the regulation of dauer larva formation. A functional sdf-14::GFP fusion gene was expressed in many cells, i.e., pharyngeal cells, pharynx-intestinal valve cells, intestinal cells, intestinal-rectum valve cells, vulval epithelial cells, some neurons, and hypodermal seam cells. Moreover, expression in at least two of the three types of cells (neurons, intestinal cells and pharyngeal muscle cells) was needed for the rescue of the dauer formation abnormality of the unc-31(e169)；sdf-14(ut153) mutant. Epistasis analysis revealed that MRP-1/SDF-14 acts neither in the cGMP related signaling pathway nor in the TGF-β signaling pathway. MRP-1/SDF-14 may act in the insulin or steroid hormone signal pathway. Alternatively, it may act in an unknown pathway, or has indirect influence on many pathways. Sodium arsenite, which is a substrate of human MRP1, induced dauer formation of the unc-31(e169) mutant, and sdf-14 mutations enhanced this effect, while sdf-14 single mutants did not form dauer larvae even in the presence of sodium arsenite at 27℃. These results suggest that wild type MRP-1/SDF-14 molecules seemed to suppress dauer larva formation by exporting sodium arsenite or unidentified intrinsic substance of which accumulation may cause of dauer formation due to sdf-14 mutations. |
要旨・審査要旨 / Abstract, Screening Result (343.5 kB)
