James R. Priess

Identification of genes required for cytoplasmic localization in early C. elegans embryos

We have isolated and analyzed eight strict maternal effect mutations identifying four genes, par-1, par-2, par-3, and par-4, required for cytoplasmic localization in early embryos of the nematode C. elegans. Mutations in these genes lead to defects in cleavage patterns, timing of cleavages, and localization of germ line-specific P granules. Four mutations in par-1 and par-4 are fully expressed maternal effect lethal mutations; all embryos from mothers homozygous for these mutations arrest as amorphous masses of differentiated cells but are specifically lacking intestinal cells. Four mutations in par-2, par-3, and par-4 are incompletely expressed maternal effect lethal mutations and are also grandchildless; some embryos from homozygous mothers survive and grow to become infertile adults due to absence of functional germ cells. We propose that all of these defects result from the failure of a maternally encoded system for intracellular localization in early embryos.

Wnt Signaling and an APC-Related Gene Specify Endoderm in Early C. elegans Embryos

In a 4-cell stage C. elegans embryo, signaling by the P2 blastomere induces anterior-posterior polarity in the adjacent EMS blastomere, leading to endoderm formation. We have taken genetic and reverse genetic approaches toward understanding the molecular basis for this induction. These studies have identified a set of genes with sequence similarity to genes that have been shown to be, or are implicated in, Wnt/Wingless signaling pathways in other systems. The C. elegans genes described here are related to wnt/wingless, porcupine, frizzled, beta-catenin/armadillo, and the human adenomatous polyposis coli gene, APC. We present evidence that there may be partially redundant inputs into endoderm specification and that a subset of these genes appear also to function in determining cytoskeletal polarity in certain early blastomeres.

APH-1 is a multipass membrane protein essential for the Notch signaling pathway in Caenorhabditis elegans embryos

Early embryonic cells in Caenorhabditis elegans embryos interact through a signaling pathway closely related to the Notch signaling pathway in Drosophila and vertebrates.Components of this pathway include a ligand, receptor, the presenilin proteins, and a novel protein, APH-2, that is related to the Nicastrin protein in humans. Here we identify the aph-1 gene as a new component of the Notch pathway in Caenorhabditis elegans. aph-1 is predicted to encode a novel, highly conserved multipass membrane protein. We show that aph-1 and the presenilin genes share a similar function in that they are both required for proper cell-surface localization of APH-2/Nicastrin.

pop-1 Encodes an HMG box protein required for the specification of a mesoderm precursor in Early C. elegans embryos

In C. elegans embryogenesis, the MS blastomere produces predominantly mesodermal cell types, while its sister E generates only endodermal tissue. We show that a maternal gene, pop-1, is essential for the specification of MS fate and that a mutation in pop-1 results in MS adopting an E fate. Previous studies have shown that the maternal gene skn-1 is required for both MS and E development and that skn-1 encodes a transcription factor. We show here that the pop-1 gene encodes a protein with an HMG box similar to the HMG boxes in the vertebrate lymphoid-specific transcriptional regulators TCF-1 and LEF-1. We propose that POP-1 and SKN-1 function together in the early embryo to allow MS-specific differentiation.

WRM-1 Activates the LIT-1 Protein Kinase to Transduce Anterior/Posterior Polarity Signals in C. elegans

During C. elegans development, Wnt/WG signaling is required for differences in cell fate between sister cells born from anterior/posterior divisions. A beta-catenin-related gene, wrm-1, and the lit-1 gene are effectors of this signaling pathway and appear to downregulate the activity of POP-1, a TCF/LEF-related protein, in posterior daughter cells. We show here that lit-1 encodes a serine/threonine protein kinase homolog related to the Drosophila tissue polarity protein Nemo. We demonstrate that the WRM-1 protein binds to LIT-1 in vivo and that WRM-1 can activate the LIT-1 protein kinase when coexpressed in vertebrate tissue culture cells. This activation leads to phosphorylation of POP-1 and to apparent changes in its subcellular localization. Our findings provide evidence for novel regulatory avenues for an evolutionarily conserved Wnt/WG signaling pathway.

POP-1 and Anterior–Posterior Fate Decisions in C. elegans Embryos

Blastomeres in C. elegans embryos execute lineage programs wherein the fate of a cell is correlated reproducibly with the division sequence by which that cell is born. We provide evidence that the pop-1 gene functions to link anterior-posterior cell divisions with cell fate decisions. Each anterior cell resulting from an anterior-posterior division appears to have a higher level of nuclear POP-1 protein than does its posterior sister. Genes in the C. elegans Wnt pathway are required for this inequality in POP-1 levels. We show that loss of pop-1(+) activity leads to several types of anterior cells adopting the fates of their posterior sisters. These results suggest a mechanism for the invariance of blastomere lineages.

The maternal gene skn-1 encodes a protein that is distributed unequally in early C. elegans embryos

The autonomous or cell-intrinsic developmental properties of early embryonic blastomeres in nematodes are thought to result from the action of maternally provided determinants. After the first cleavage of the C. elegans embryo, only the posterior blastomere, P1, has a cell-intrinsic ability to produce pharyngeal cells. The product of the maternal gene skn-1 is required for P1 to produce pharyngeal cells. We show here that the Skn-1 protein is nuclear localized and that P1 appears to accumulate markedly higher levels of Skn-1 protein than its sister, the AB blastomere. We have examined the distribution of Skn-1 protein in embryos from mothers with maternal-effect mutations in the genes mex-1, par-1, and pie-1. These results suggest that mex-1(+) and par-1(+) activities are required for the unequal distribution of the Skn-1 protein and that pie-1(+) activity may function to regulate the activity of Skn-1 protein in the descendants of the posterior blastomere P1.

MEX-3 Is a KH Domain Protein That Regulates Blastomere Identity in Early C. elegans Embryos

After the first division of the C. elegans embryo, the posterior blastomere can produce numerous muscles while the anterior blastomere cannot. We show here that maternal-effect lethal mutations in the gene mex-3 cause descendants of the anterior blastomere to produce muscles by a pattern of development similar to that of a descendant of the wild-type posterior blastomere. mex-3 encodes a probable RNA-binding protein that is distributed unequally in early embryos and that is a component of germline-specific granules called P granules. We propose that MEX-3 contributes to anterior-posterior asymmetry by regulating one or more mRNAs involved in specifying the fate of the posterior blastomere.

The pie-1 and mex-1 genes and maternal control of blastomere identity in early C. elegans embryos

During C. elegans embryogenesis an 8-cell stage blastomere, called MS, undergoes a reproducible cleavage pattern, producing pharyngeal cells, body wall muscles, and cell deaths. We show here that maternal-effect mutations in the pie-1 and mex-1 genes cause additional 8-cell stage blastomeres to adopt a fate very similar to that of the wild-type MS blastomere. In pie-1 mutants one additional posterior blastomere adopts an MS-like fate, and in mex-1 mutants four additional anterior blastomeres adopt an MS-like fate. We propose that maternally provided pie-1(+) and mex-1(+) gene products may function in the early embryo to localize or regulate factors that determine the fate of the MS blastomere.

MEX-5 and MEX-6 Function to Establish Soma/Germline Asymmetry in Early C. elegans Embryos

An asymmetrical network of cortically localized PAR proteins forms shortly after fertilization of the C. elegans egg. This network is required for subsequent asymmetries in the expression patterns of several proteins that are encoded by nonlocalized, maternally expressed mRNAs. We provide evidence that two nearly identical genes, mex-5 and mex-6, link PAR asymmetry to those subsequent protein asymmetries. MEX-5 is a novel, cytoplasmic protein that is localized through PAR activities to the anterior pole of the 1-cell stage embryo. MEX-5 localization is reciprocal to that of a group of posterior-localized proteins called germline proteins. Ectopic expression of MEX-5 is sufficient to inhibit the expression of germline proteins, suggesting that MEX-5 functions to inhibit anterior expression of the germline proteins.