Dr. James R. Priess

We are interested in the molecular events of early embryonic development. We use the nematode C. elegans as a model system for our research because its simple anatomy, short life cycle, small genome and complete genome sequence make it ideal for genetic and molecular biological studies. Our work and the work of several other laboratories have demonstrated that the basic pathways that shape the early C. elegans embryo function in most other animals embryos. A signaling pathway called the Notch pathway is used repeatedly in development to create differences between otherwise equivalent cells. This pathway is of additional interest because the ligand-stimulated processing of Notch involves presenillin proteins that are implicated in Alzheimer's disease in humans. Our genetic studies in C. elegans have recently identified two novel components of this pathway, APH-1 and APH-2 (Goutte et al., 2000, 20002). Loss of APH-1 activity prevents the membrane association of APH-2, as do defects in the C. elegans presenillin proteins. A major gap in our understanding of the Notch pathway in nematodes has been the failure to identify direct targets of Notch signaling. We have recently identified two, functionally redundant transcription factors that are very likely to act one step downstream of the direct Notch target. Analysis of these transcription factors has provided insight into the logic of the Notch-mediated cell fate decisions in the early embryo. Two initially equivalent sister cells, called ABa and ABp, both express the receptor Notch, but adopt different fates because ABp is signaled at the 4-cell stage. Two cell cycles later, a subset of ABa descendants are signaled at the 12-cell stage. In essence, the 4-cell Notch-mediated interaction represses mesodermal cell fates, while the 12-cell Notch-mediated interaction induces mesodermal cell fates. How does activation of the same receptor lead to different outcomes? We have found that the novel transcription factors are essential for mesodermal differentiation. Using Green Fluorescent Protein reporters, we have found that Notch signaling at the 4-cell stage represses the expression of these factors in ABp, while these factors are expressed in all ABa descendants independent of Notch signaling; the transcription factors appear to act in combination with direct Notch targets at the 12-cell stage to specify mesoderm. We anticipate that the factor(s) that repress expression in ABp will be a direct target of the Notch pathway, and are currently using genetic and molecular techniques to test this.

Through a complex series of interactions in the early embryo, one cell expresses transcription factors that cause it to form the intestine (Zhu et al., 1997). We recently began an analysis of intestinal morphogenesis with a descriptive study (Leung et al., 1999), then used cell biological and genetic techniques to examine how left-right asymmetry is generated within the intestine (Hermann et al., 2000). We showed that asymmetry involves a novel role for the Notch signaling pathway that more commonly is associated with cell fate decisions. We discovered that left-right asymmetry is closely linked to a system of anterior-posterior asymmetry that we and others described previously (Lin et al., 1998). We are currently using a genetic approach toward understanding how apical-basal polarity is generated in the intestinal cells by isolating mutants defective in intestinal polarity.

Asymmetrically localized transcription factors and cell-cell interactions together cause embryonic cells to adopt specific differentiated fates, such as muscles or skin. However, one embryonic cell does not respond to these signals, and thus remains 'totipotent'; this cell is the precursor to the germline. The special properties of this cell result in part because it alone contains the protein PIE-1, which appears to act as a transient inhibitor of new RNA synthesis (Mello et al., 1996; Seydoux et al., 1996), and the proteins MEX-1 and POS-1 (Guedes et al., 1997; Tabara et al., 1999). We are investigating how these factors are localized asymmetrically to the germline precursor, and recently described a new molecular component of this pathway (Schubert et al., 2000). We interpret factors such as PIE-1, MEX-1 and POS-1 as being 'permissive' for germline development. However, the special cell also contains unique cytoplasmic granules called P granules that may have an 'instructive' role in germline development. We have begun a study of P granules in the gonads of adult animals, when the granules are closely associated with nuclei. We discovered that P granules are localized on clusters of nuclear pores and contain mRNA, suggesting that mRNAs may interact with P granule components as they leave the nucleus (Pitt et al., 2000). In recent work, we have identified specific mRNAs that are in the P granules, and now hope to learn how these mRNAs are effected by the P granules.

Other current projects include an analysis of the POP-1 polarity pathway (see Lin et al.,1998) and a study of the mechanism of gastrulation in C. elegans (Nance and Priess, 2002).