 |
Cell Fate Specification in the C. elegans Embryo
Summary: James Priess uses the nematode C. elegans as a model system for studying early embryonic development and germline specification. His recent research focuses on how regulators of cell fate induce distinct patterns of morphogenesis, and how germline-specific cytoplasmic particles called P granules contribute to germ cell development.
The embryo of the nematode Caenorhabditis elegans provides a convenient system for studying the molecular basis for animal development. C. elegans has an extremely short generation time, allowing the investigation of gene functions through genetic experiments that would be impractical or impossible in other systems. In addition, the complete genome sequence of C. elegans has been determined, making it possible to use powerful reverse genetic techniques to explore gene functions. Within 15 hours of fertilization, the embryo becomes a worm with distinct somatic cell types, such as muscles, neurons, and intestinal cells, that are organized into functional tissues and organs. Our lab is interested in the mechanisms that determine the different fates of the early embryonic cells and shape these cells into distinctive organs.
Previous studies by our lab and others have shown that most of the diversity in cell fates in C. elegans embryos can be attributed to two basic systems. The first system generates, in essence, a binary code. When a cell divides, its daughter cells differ in their levels of POP-1, a transcription factor related to mammalian LEF-1 or TCF: the anterior daughter nucleus has a high level of POP-1, and the posterior nucleus has a low level of POP-1. This pattern repeats at each division, so each cell in the embryo has a unique POP-1 "history"; for example, a cell born in the division pattern anterior/posterior/posterior has a POP-1 history of high/low/low. The anterior/posterior POP-1 asymmetry appears to be initiated by a signaling pathway that shares some components with the related Wnt signaling pathway in vertebrates. Prior to the 48-cell stage of embryogenesis, cells do not show POP-1 asymmetry unless they are in direct contact with a cell that expresses the ligand Wnt. We found, however, that older cells can divide with POP-1 asymmetry even if they have no prior contact with Wnt-expressing cells.
The receptor MOM-5/Frizzled has a role in both the early and late POP-1 asymmetry, and we showed that MOM-5 is localized asymmetrically at most of the embryonic cell divisions after, but not before, the 48-cell stage. Thus it is possible that an asymmetry in MOM-5 activity before the 48-cell stage depends on the asymmetrical presentation of ligand, while after the 48-cell stage the asymmetry in activity is created by the asymmetry in MOM-5 localization.
The early embryonic cells contain different sets of transcription factors that, when coupled with the POP-1 binary code described above, specify several different cell fates. The number of possible cell fates is expanded enormously, however, by a second system—cell interactions mediated by the receptor Notch. The Notch pathway is used in a large number of cell interactions during embryogenesis, and each of these interactions results in a distinct choice in cell fate. To understand the basis for the diverse outcomes, we used genetic and reverse genetic studies to identify several target genes that are regulated directly by Notch signaling. We found that Notch-dependent expression in the intestine involves an enhancer regulated by the combination of Notch plus ELT-2/GATA, encoded by an "organ identity" transcription factor that regulates many aspects of intestinal development. Recently, we found that expression in the pharynx involves the analogous combination of Notch plus PHA-4/Forkhead, encoded by an organ identity transcription factor for pharyngeal development.
Our current projects address how the transcription factors and signaling pathways that regulate cell fate specify organ morphogenesis. In one of these projects, we are studying how two cells form adjacent unicellular tubes. We found that both cells invade surrounding tissue in the organ primordium by moving along a transient tract of laminin. Both cells wrap back onto themselves to form transient C shapes. Then each cell expresses a distinct fusion-promoting protein and self-fuses into a tube; Notch signaling prevents both cells from expressing the same fusogen and cross-fusing. In ongoing work, we are studying how the transient tract of laminin is specified; this involves a novel deposition of laminin between the lateral surfaces of primordial cells and thus is distinct from laminin in the basement membrane that surrounds the primordium. We speculate that similar tracts of laminin might have widespread roles in tissue morphogenesis but have been overlooked because they appear and disappear within only about 20 minutes.
Several projects in our lab are focused on how cells are specified to produce the germline. In previous studies, we identified several proteins that are required for germline development and showed that these proteins are localized to the posterior half of the one-cell embryo. We showed that the posterior localization of the germline proteins requires MEX-5, a protein that localizes in a reciprocal pattern to the anterior half of the embryo. MEX-5 localization requires a complex of proteins called PAR proteins, and the boundary of MEX-5 localization closely matches that of the anterior PAR complex. We showed that the anterior PAR complex is localized by the "capping" of a contractile network of cortical myosin. We recently showed, however, that MEX-5 localization involves a different mechanism: MEX-5 appears to be tethered throughout the cytoplasm before fertilization, but undergoes a phosphorylation-dependent increase in posterior mobility after fertilization.
All of the germline proteins associate transiently with germline-specific granules of unknown function called P granules. P granules are associated with germ cell nuclei during most stages of development, but they detach from the nuclei and become cytoplasmic during oogenesis. We found that depleting the translational regulators GLD-1 and MEX-3 caused germ cells in the adult gonad to lose their P granules; these cells shortly thereafter differentiated inappropriately into somatic cell types, such as neurons and muscles (a germline teratoma). This phenotype is caused, at least in part, by the inappropriate translation of some maternally expressed mRNAs that normally are translated only during embryogenesis. These results suggest that P granules have a role in maintaining the totipotency of germ cells. We discovered that the nuclear-associated P granules are intimately associated with clusters of nuclear pore complexes (NPCs), suggesting that P granules interact with materials moving to or from the nucleus. We showed previously that P granules accumulate large amounts of mRNA, but not rRNA, when germ cells enter quiescence; in recent studies we found that P granule–associated clusters of NPCs are the principal sites of mRNA export in germ cells. We now want to determine how P granules interact with the mRNAs that pass through them.
Last updated December 02, 2008
|
|
|
|
