Worm Breeder's Gazette 14(4): 56 (October 1, 1996)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Department of Genetics and Cell Biology University of Minnesota St. Paul MN 55108
When a cell divides to generate daughters of different cell fates, the division is said to be asymmetric. In C. elegans, 807 of the 949 nongonadal cell divisions that occur during development are asymmetric. An important feature of most asymmetric cell divisions in C. elegans is that they occur with a defined orientation with respect to the body axes. The relative orientation of the different daughter cells to each other and to the body axes gives an asymmetric cell division a polarity. Mutations in lin-44 cause the polarities of certain asymmetric cell divisions in the tail to be frequently reversed, while maintaining the asymmetries and the orientations of division planes. Genetic analysis indicated that the polarity reversal defect is the result of a complete loss of lin-44 function (Herman and Horvitz, 1994, Development, 120: 1035-1047). Molecular analysis has revealed that lin-44 encodes a member of the Wnt family of secretory glycoproteins, which have been shown to function as short range signaling molecules. Examination of animals containing reporter constructs designed to mimic patterns of lin-44 expression and in situ hybridization experiments have demonstrated that lin-44 is expressed in the developing tail hypodermis, which is posterior to the cells affected by lin-44 mutation. Mosaic analysis has shown that lin-44 function is not required in the cells affected by lin-44 mutation, but in cells in which we observe expression. Based upon these results we propose that lin-44 protein is secreted by the tail hypodermal cells and affects the polarity of asymmetric cell divisions that occur more anteriorly in the tail (Herman et al., 1995, Cell, 83: 101-110).
lin-44 animals have defective phasmids. The phasmids are sensory structures in the tail that consist of two neurons, a sheath cell and two socket cells which provide an opening to the environment through which fluorescent dyes can enter and fill the neurons. As a result of the reversal of T cell polarity in lin-44 mutants, the phasmid socket cells are misplaced, which blocks dye filling by the phasmid neurons. To identify other genes that interact with lin-44 to specify cell polarity, we are screening for additional mutations that reduce or abolish the dye filling of phasmid neurons without affecting amphid neuron dye-filling in the head. We have isolated 28 such mutations. Each of these mutations has been scored for the presence of the phasmid socket cells in their normal positions. Six mutations lacked phasmid socket cells in the normal positions. Two of these are new alleles of known genes: one is in lin-44 and the other is in lin-17. Mutations in lin-17 cause a loss of T cell polarity and hence block phasmid dye-filling. lin-17 has been shown by Sawa and Horvitz (1996, Genes & Dev., in press) to encode a homolog of the Drosophila frizzled (fz) gene. A homolog of Fz, DFz2, has recently been shown to function as a receptor for the Drosophila Wnt gene, Wg (Bhanot et al., 1996, Nature, 382: 225-230). These data suggest that LIN-17 may function as a receptor for LIN-44 signal.
The four other mutations define new genes involved in the control of cell polarity, which we have named lop for loss of cell polarity. We have begun analysis of two of these new mutations, defining lop-1 and lop-2; the remaining two were isolated very recently and have not been characterized. Most lop-1 mutant animals arrest during larval development, and all arrest when the mutation is heterozygous to a deficiency of the locus. lop-1 mutants that do not arrest display a loss of T cell polarity (Figure 1), male tail defects, and variable egg-laying defects. Interval and deficiency mapping places lop-1 between dpy-10 and tra-2 on LGII, a region that is covered by sequenced cosmid clones. We have recently rescued lop-1 by microinjection of one of these clones, C04A2. lop-2 also maps to LGII, and mutants appear to display a loss of T cell polarity, male tail defects, and variable egg-laying defects. Some lop-2 hermaphrodites have abnormal body and tail morphologies in addition to the loss of T cell polarity. Since mutations in both lop-1 and lop-2 cause defects in addition to the loss of T cell polarity, these genes may be involved in the reception or transduction of LIN-44 as well as one or more of the other four known C. elegans Wnts. Alternatively, mutations in these genes could affect the polarity of other cells in the animal in ways which we cannot detect. Further genetic and molecular analysis of these and other new genes we identify should distinguish between these possibilities.
Diagrams of T cell lineages in wild-type and two representative lop-1 hermaphrodites. Cell fates of the T cell descendants are indicated in the wild-type lineage. In the lop-1 lineages, exact cell fates cannot be determined, and cells with the nuclear morphology of hypodermal cell are designated hyp. Anterior is to the left. Division planes are all anterior-posterior.