During the larval development of a C. elegans hermaphrodite, a pair of distal tip cells (DTCs) lead the extension of two gonadal arms through three sequential phases of migration. They first move along the ventral side to the ends of the animal (phase 1), then make a dorsal turn (phase 2) and finally move along the dorsal side back to the center (phase 3). We have isolated the mutation
tp3 with aberrant DTC migration. A time-course analysis showed that the
tp3 DTC had an early dorsal turn. We have cloned the gene, tentatively named as mig-x here, defined by the
tp3 mutation. It encodes a protein with a nuclear localization signal and five zinc fingers, suggesting that it may function as a transcription factor to regulate DTC migration. Previous studies showed the dorsal turn of DTC requires 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 DTCs is required and sufficient for the onset of the dorsal turn (Su et al., 2000). Interestingly, premature
unc-5 expression was observed in the DTC undergoing a precocious dorsal turn in
tp3 mutants. Therefore, mig-x may negatively regulate
unc-5 expression to control the proper timing of the dorsal turn. We are testing if
unc-5 may be the direct transcriptional target of MIG-X. Intriguingly,
unc-5;
tp3,
unc-6;
tp3 and
unc-40;
tp3 double mutants exhibited a synthetic phenotype of prolonged longitudinal migration without phase-2 or -3. This defect is similar to that of
daf-12(
rh84) mutants (Antebi et al., 1998).
daf-12 encodes a nuclear hormone receptor and is required for
unc-5 expression (Antebi et al., 2000; Snow et al., 2000; Su et al., 2000). We are testing if mig-x expression may depend on
daf-12. To explore the expression pattern of mig-x, we made a mig-x::gfp translational fusion construct, which fully rescued the
tp3 phenotype. The MIG-X fusion protein was present 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 the P-lineage derived cells, hypodermal cells, and a few cells in the tail. We also generated MIG-X antibodies and are investigating the expression pattern of the endogenous mig-x gene. References: Antebi et al. (1998) Development 125, 1191-205; Antebi et al. (2000) Genes & Development 14, 1512-1527; 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; Snow et al. (2000) Biochimica et Biophysica Acta - Gene Structure & Expression 1494, 104-116; Su et al. (2000) Development 127, 585-94.