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内容記述 |
Many sensory neurons such as vertebrate olfactory receptor neurons have cilia at the tip. These sensory cilia have a special arrangement of microtubules, like motile cilia. Because the cilia contain transduction components including various chemo-, mechano-, photo-receptors, G proteins, adenylate cyclase, Jon channels, etc., it is generally assumed that they are the primary sites of transduction where environmental stimuli are converted into neuronal signals known as receptor potentials. While signal transduction in sensory neurons has been studied to some extent, the way sensory cilia are formed during development and the spatial organization of signal components in the cilia remain to be studied.<br /> C. elegans is a good model organism for studying the structure, function, and development of the nervous system. Of the 118 classes of neurons in C elegans hermaphrodites, 24 classes (60 neurons) have sensory cilia. Mutants that are defective in the morphology of cilia have been isolated and mapped to 29 genes. The results of cloning of some of these genes suggested that transport on microtubules and signal transduction involving an appropriate level of some Ga proteins are necessary for correct cilium formation. However, how they act in cilium morphogenesis remains to be studied.<br /> Mutants that map in che-2 gene of C. elegans have defects in sensory cilia. Almost all the sensory cilia in che-2 mutants are much shorter than those of wild type animals and lack their middle and distal segment. Microtubules assemble ectopically in an abnormal posterior projection adjacent to the defective cilium. It was speculated that the abnormal posterior projection is formed either by the degeneration of a cilium once extended correctly or by the accumulation of the components of a cilium that cannot extend correctly. In either case, the che-2 product may be required for the stability or assembly of microtubules in the middle and distal segment. che-2 mutants also show defects in various functions mediated by ciliated sensory neurons, such as chemotaxis, osmotic avoidance, dauer formation and male mating.<br /> In this study, I first characterized four alleles of che-2, since only the reference allele, e1033, had been analyzed in detail. I found that the allele mn395 shows weaker defects in various phenotypes than the other alleles.<br /> Second, I visualized cilia using a GFP fusion gene under a promoter specific to a few sensory neurons. This technique enabled me to trace the development of cilia. The results showed that the abnormality of cilia in che-2 mutants is due to a defect in extension and not due to degeneration.<br /> Third, che-2 gene was cloned by the cosmid rescue method. It encodes a novel protein of 760 amino acids with four W040 repeats in the N-terminal region. WD40 repeat is known to be involved in protein-protein interaction and formation of multiprotein complexes. Three alleles of che-2 seem to produce truncated forms of CHE-2 protein lacking about 160 amino acids in the C-terminal region. As they show relatively severe phenotypes, the C-terminal region, which has no similarity to any known protein, must be essential for the CHI-2 function. On the other hand, the weaker allele, mn395, has a missense mutation in the first WD40 repeat. It may means that the protein-protein interaction through WD40 motif is also necessary for the CHE-2 function.<br /> Fourth, I determined the expression pattern of CHE-2 using GFP reporter gene. CHE-2 is expressed in almost all the ciliated sensory neurons of C. elegans. The expression starts at the late embryonic stage when cilia are formed and continues to the adult stage. Detailed analysis of CHE-2 expression revealed its localization at cilia, suggesting CHI-2 as a component of cilia.<br /> Fifth, rescue of functions by expressing che-2 in a subset of sensory neurons demonstrated that che-2 acts cell-autonomously. Furthermore, this experiment provides the bits of a method for determining the functional identity of sensory neurons, provided that the function is impaired in che-2 mutants.<br /> Sixth, I expressed che-2 cDNA under the control of a C. elegans heat shock promoter, to know the developmental stage of che-2 expression sufficient for cilium formation. Surprisingly, the animals could extend cilia, even if they were heat-shocked at adults, although cilia are normally formed at the embryonic stage in normal development. The cilia that extended at the adult stage by heat-shock induction were often bent. Thus, there may be a mechanism that enables cilia to extend straight in embryos, and such a mechanism may be lost at the adult stage. Moreover, the data indicates that the cilium which once extend by heat-shock induction seems to degenerate again, implying that CHE-2 is necessary for both the formation and the maintenance of sensory cilia.<br /> These experiments suggest that CHE-2 is a novel component of sensory cilia that is necessary for their formation and maintenance. It acts cell-autonomously and probably interacts with other components with the WD40 repeats. The existence of mammalian homologue indicates a general role of CHE-2. However, the exact mode of interaction of CHE-2 with other cilium components such as those in a microtubular transport system and a signal transduction system remains to be elucidated. |
要旨・審査要旨 / Abstract, Screening Result (376.4 kB)
