Genes encoding the tailless family of nuclear receptors are highly conserved among animals where they play various roles in regulating development. The C. elegans tailless ortholog,
nhr-67, is expressed in a dynamic pattern in pre-uterine cells: initially in the 4 pre-VU cells during the L2 stage, then at high levels in the anchor cell (AC) and lower levels in the VU cells, apparently in response to the
lin-12/lag-2 reciprocal signaling system. During the L3 stage,
nhr-67 expression is maintained at high levels in the AC and briefly at low levels in the six p cells, and later only in the UTSE syncytium.
We have found that
nhr-67 is required for the AC-VU decision. A loss of
nhr-67 results in a two AC phenotype, indicating that
nhr-67 is required for execution of the VU fate in response to
lag-2/lin-12 signaling between AC and VU. But
nhr-67 is also required for differentiation of the AC, including expression of
lag-2 and
zmp-1, a phenotype that is very similar to that of
egl-43 mutants.
nhr-67 is also required for execution of the p cell lineages. Epistasis analysis indicates that this requirement depends on
nhr-67 function in the VU lineages rather than the AC. Finally,
nhr-67 is required for expression of
lin-12 in the pre-VU cells and later VU lineages, suggesting that a major function of
nhr-67 may be differentiation of the pre-VU cells, one of which becomes the AC. The loss of
lin-12 expression may be sufficient to account for both the AC-VU defect and the p lineage defect. The presence of a conserved canonical NHR-67 binding site in the
lin-12 promoter raises the possibility that
nhr-67 may directly control the transcription of
lin-12.
We are currently evaluating a 250 bp fragment of the
nhr-67 promoter that contains six evolutionarily-conserved candidate cis-acting sites. Deletions of this region of the promoter cause a strong loss-of-function for
nhr-67 function in the AC and VU lineages. The 250 bp region is sufficient to drive expression of GFP in the AC, indicating that it contains promoter elements for cell-specific expression. Among these six sites are two predicted binding sites for HLH-2, suggesting that HLH-2 may be a direct regulator of
nhr-67 expression. We are also exploring the functions of other candidate upstream regulators of
nhr-67, such as
egl-43. This project is supported by a grant from the NSF.