Title: Function and regulation of the transcription factor HLH-2/E2A during gonadogenesis in Caenorhabditis elegans
Description: During animal development, cells are born with the potential to adopt different cell fates. Many of the mechanisms that control cell fate choice are highly conserved. Here I study how the fates of two important cell types, the Anchor Cell (AC) and the Distal Tip Cell (DTC), are specified during C. elegans gonadogenesis. The AC is an important signaling hub that directs uterine and vulval development. Its specification and function require the E protein HLH-2/Da/E2A, an essential basic Helix-Loop-Helix transcription factor. In the developing gonad, two cells Z1.ppp and Z4.aaa have naturally variable fates. They require hlh-2 for the potential to adopt AC fate, for the AC/VU decision to specify one AC and one ventral uterine precursor cell (VU), and for execution of AC fate. hlh-2 has been shown to be post-transcriptionally regulated during the AC/VU decision: HLH-2 protein is observed in the AC and not in the VU, though hlh-2 is transcribed in both cells. Until this work, little was known about how hlh-2 is post-transcriptionally regulated in C. elegans.

Understanding how E proteins are regulated is also important in other contexts, because increased E protein activity has been associated with developmental defects and lymphoma. Here, a novel dimerization-dependent mechanism for HLH-2 down-regulation is identified in the VU. I provide evidence that HLH-2 homodimers promote AC competence, the AC/VU decision, and AC function, and that HLH-2 homodimers are recognized in the VU and targeted for degradation. The human ortholog E2A is found to be regulated similarly, raising the possibility that the mechanism for negative regulation of HLH-2 and E2A is conserved. A simple model could explain the difference in stability of HLH-2 homodimers: lin-12/Notch activity is low in the AC but high in the VU, where it promotes the turnover of HLH-2 homodimers. The C. elegans gonad also contains two distal tip cells (DTC), which direct the shape of the developing gonad and promote germline proliferation. In addition to AC fate, HLH-2 is required for the specification and function of the DTCs.

However, it was not known what dimerizes with HLH-2 to promote DTC specification. Here, LIN-32/Atonal and HLH-12 are identified as two functionally redundant partners for HLH-2 in promoting DTC fate and function. Loss of both lin-32 and hlh-12 causes a complete failure of DTC migration, which likely reflects a highly penetrant failure of DTC specification. lin-32 and hlh-12 are both expressed in the DTC around the time of specification, consistent with a cell-autonomous role. These results suggest that LIN-32 and HLH-12 can heterodimerize with HLH-2 in the DTC to specify fate and promote migration. This work advances our understanding of how HLH-2 is regulated and how it functions with different dimerization partners to specify different cell fates during gonad development.