During larval development of C. elegans hermaphrodites, a pair of distal tip cells (DTCs) lead the extension of two gonadal arms through three sequential phases of migration. These two cells first move centrifugally from the vulva (phase 1), then make a dorsal turn (phase 2) and finally move centripetally back to the vulva (phase 3). We have isolated the mutation
tp3 that caused aberrant DTC migration. A time course analysis showed that
tp3 DTCs had a precocious dorsal turn in phase 2. We have cloned the gene defined by the
tp3 mutation. For simplicity, we will call this gene mig-x in the following experiments. The mig-x gene encodes a protein with a nuclear localization signal and five zinc fingers, suggesting it may function as a transcription factor to regulate DTC migration. Previous studies showed the second dorsal-ventral migration phase requires the UNC-6 netrin guidance cue and its receptors UNC-5 and UNC-40 (Chan et al., 1996; Hedgecock et al., 1990; Ishii et al., 1992; Leung-Hagesteijn et al., 1992)and that the transcriptional upregulation of
unc-5 in the DTCs is required for the initiation of the second migration phase. (Su et al., 2000) Interestingly, we found that
unc-5 is expressed prematurely in the DTC undergoing a precocious dorsal turn in
tp3 mutants. These results altogether suggest that mig-x might negatively regulate
unc-5 expression at a transcriptional level to prevent DTC premature dorsal turns during the first longitudinal migration phase. We are currently testing whether
unc-5 is the direct transcriptionl target of MIG-X. Intriguingly,
unc-5; mig-x,
unc-40;mig-x and
unc-6; mig-x double mutants have a synthetic phenotype of prolonged longitudinal migration without executing any turn, which resembles the Daf-12 DTC defect (Antebi et al., 1998). The
daf-12 encodes a nuclear hormone receptor and is required for
unc-5 expression. We are currently testing if
dpy-24 expression depends on
daf-12. To explore the expression pattern of mig-x, we made a mig-x::gfp translational fusion construct which fully rescued
tp3 mutants. The MIG-X fusion protein was expressed in DTC but not muscle cells, consistent with the model that mig-x acts in DTC to control DTC migration. In addition, MIG-X::GFP is observed in P lineage derived cells, hypodermal V cells, and a few cells in the tail. We are currently generating antibodies against the bacterially expressed MIG-X protein to confirm our GFP expression results. References: Antebi et al.(1998) Development 125, 1191-205; Chan et al.(1996) Cell 87, 187-95; Hedgecock et al. (1990) Neuron 4, 61-85; Ishii et al. (1992) Neuron 9, 873-81; Leung-Hagesteijn et al. (1992) Cell 71, 289-99; Su et al. (2000) Development 127, 585-94.