Title: Regulation of HLH-2/E2A during Caenorhabditis elegans gonadogenesis
Description: Organisms are comprised of many cells with multiple distinct cell types, each of which must be decided precisely to ensure proper formation of a functional organism. In C. elegans, the basic helix-loop helix transcription factor HLH-2 is required for the specification of the anchor cell, or AC. The AC arises from a group of four somatic gonad cells, all of which initially express HLH-2. Two of the four cells, which we call β cells, lose AC competence early and instead become ventral uterine precursor cells, or VUs. We call the remaining two cells α cells. One α cell becomes the AC, while the other becomes a VU. Which α cell becomes the AC is random—50% of the time one α cell becomes the AC, while the other 50% of the time the other α cell becomes the AC. The choice of which cell becomes the AC and which becomes the VU is called the AC/VU decision, and occurs through reciprocal signaling by LIN-12/Notch and its ligand LAG-2/DSL. At first, both α cells express similar levels of lin-12 and lag-2.

As the AC/VU decision progresses, the AC expresses higher levels of lag-2, and the VU expresses higher levels of lin-12. By this time, HLH-2 is only present in the specified AC, while it is post-translationally degraded in VUs. The mechanism by which HLH-2 is degraded and the consequences of disrupting its degradation on AC specification are unknown. In this work, we studied the function and regulation of HLH-2 during two stages of somatic gonad development. First, we used long-term fluorescence microscopy to visualize HLH-2 over the course of somatic gonad development. We found that HLH-2 expression begins in the parents of the α and β cells a consistent amount of time after their birth, and that the parent cell that first expresses HLH-2 almost always gives rise to the α cell that becomes the VU, while the second cell to express HLH-2 gives rise to the AC. This led us to study the effect of a loss of hlh-2 activity in the α and β cells. We generated an α and β cell-specific hlh-2(0) allele using genome editing tools and found that LIN-12 protein is not present in the absence of hlh-2 activity.

Based on this discovery, we conceived a model where HLH-2 expression biases the first-expressing cell towards the VU fate by endowing it with an edge in lin-12 activity. Next, we focused on restriction of HLH-2 to the AC. Typically, HLH-2 protein is degraded in VUs, which we hypothesized was a crucial step in restriction of the AC fate to a single cell. We found that in a lin-12(0) background, HLH-2 is stabilized in VUs even when the resulting cell does not become an AC, indicating that lin-12 directly promotes HLH-2 degradation. This led us to search for a lin-12-regulated factor that targets HLH-2 for degradation in VUs. We identified seven ubiquitin-related genes whose depletion resulted in stabilized HLH-2 in VUs, but surprisingly did not cause an AC/VU defect. We suspect that HLH-2 degradation in VUs is one of multiple negative regulatory mechanisms that ensure the robustness of the AC/VU decision. The following research contributes new insights into how stochastic cell fate decisions amplify noise to ensure a consistent and reproducible outcome.