Regulation of totipotency in the "Caenorhabditis elegans" germ line

The fertilization of an oocyte with sperm leads to the formation of a zygote, which has the unique ability to differentiate into any cell type. This specific ability is defined as totipotency. Germ cells differentiate into highly specialized cells, oocytes and sperm, but germ cells also have an underlying totipotency, as totipotent cells can be derived from germ cells. However the mechanisms that allow germ cells to establish/maintain germline identity and to specialize, while maintaining an underlying totipotency, are not understood.
In C.elegans, germ cells in the gld-1, and gld-1, mex-3 mutants fail to progress through meiosis and instead form a germline tumor. Recently Dr. Rafal Ciosk found that germ cells in the gld-1, and gld-1, mex-3 germline tumor lose their germline identity and instead acquired a somatic fate, a phenotype that is reminiscent to a special human germline tumor, called teratoma. This finding provided us with a genetic model system that allowed us to investigate the mechanisms that are required to maintain germline identity, and totipotency.
To address these questions, we first needed to understand how teratoma formation occurs in C.elegans. What is the etiology of the cells undergoing teratoma formation? To address this question we used a compound mutant background in which the major mitotic and meiotic pathways were deleted and the gonad was lacking a distal to proximal orientation. As cells within this gonad showed a synchronized development we could follow the different cell cycle stages preceding teratoma formation. After an initial phase of proliferation germ cells enter meiosis, however fail to progress through meiosis, re-enter proliferation and undergo germ line to soma transition. This knowledge allowed us to reveal the cells that lead to teratoma formation in the simplest genetic background, the gld-1 mutant. This analysis showed us that the germline tumor in the gld-1 mutant is formed by two major populations of cells, a central and proximal tumor. As already the loss of GLD-1 alone leads to teratoma formation we sought to identify GLD-1 targets. In this analysis we could define core cell cycle factors as new GLD-1 targets, namely cyclin E and Cyclin Bs. Genetic experiments showed that ectopic expression of Cyclin E together with CDK-2 promotes the re-entry into mitosis and tumor initiation in the central region of the gld-1 gonad. This re-entry into mitosis leads to loss of germ line identity and unexpectedly to a change in the transcriptional program of the cells, preceding expression of markers of terminally differentiated cells. Furthermore we found that ectopic expression of a known GLD-1 target, GLP-1, promotes proximal tumor formation and suppresses germ line to soma transition in these cells.
Taken together this study revealed that different cell populations lead to the formation of the heterogeneous germline tumor in the gld-1, or gld-1, mex-3 mutant, and identified its major regulators. Further this study provides a first mechanism promoting germline to soma transition. We propose that the loss of GLD-1 leads to ectopic expression of its targets, such as Cyclin E and the somatic determinant PAL-1/Caudal. Ectopic expression of Cyclin E promotes re-entry into mitosis and a change in the transcriptional profile of the cell, which creates an environment that allows a somatic determinant to promote germ line to soma transition. The importance of this finding is that it is not only the loss of translational control that leads to teratoma formation, but also a change in the transcriptional competence of the cells, and it emphasizes the importance of cell cycle control during meiosis as a fundamental mechanism to maintain germline identity.