Asymmetric segregation of PIE-1 in C. elegans is mediated by two complementary mechanisms that act through separate PIE-1 protein domains

Asymmetric cell division during animal development

Although most cells produce two equal daughters during mitosis, some can divide asymmetrically by segregating protein determinants into one of their two daughter cells. Interesting parallels exist between such asymmetric divisions and the polarity established in epithelial cells, and heterotrimeric G proteins might connect these aspects of cell polarity. The discovery of asymmetrically segregating proteins in vertebrates indicates that the results obtained in invertebrate model organisms might also apply to mammalian stem cells.

DNA replication defects delay cell division and disrupt cell polarity in early Caenorhabditis elegans embryos

In early Caenorhabditis elegans embryos, asymmetric cell divisions produce descendants with asynchronous cell cycle times. To investigate the relationship between cell cycle regulation and pattern formation, we have identified a collection of embryonic-lethal mutants in which cell divisions are delayed and cell fate patterns are abnormal. In div (for division delayed) mutant embryos, embryonic cell divisions are delayed but remain asynchronous. Some div mutants produce well-differentiated cell types, but they frequently lack the endodermal and mesodermal cell fates normally specified by a transcriptional activator called SKN-1. We show that mislocalization of PIE-1, a negative regulator of SKN-1, prevents the specification of endoderm and mesoderm in div-1 mutant embryos. In addition to defects in the normally asymmetric distribution of PIE-1, div mutants also exhibit other losses of asymmetry during early embryonic cleavages. The daughters of normally asymmetric divisions are nearly equal in size, and cytoplasmic P-granules are not properly localized to germline precursors in div mutant embryos. Thus the proper timing of cell division appears to be important for multiple aspects of asymmetric cell division. One div gene, div-1, encodes the B subunit of the DNA polymerase alpha-primase complex. Reducing the function of other DNA replication genes also results in a delayed division phenotype and embryonic lethality. Thus the other div genes we have identified are likely to encode additional components of the DNA replication machinery in C. elegans.

Embryonic polarity: a role for microtubules

Researchers have suspected that initial polarization of the Caenorhabditis elegans embryo might be directed by microtubules, but demonstrating this has faced obstacles. A new study has cleverly bypassed these obstacles.

Polarization of the anterior-posterior axis of C. elegans is a microtubule-directed process

In Caenorhabditis elegans, polarity along the anterior-posterior (A/P) axis is established shortly after fertilization and is determined by the sperm, whose position specifies the posterior end of the embryo'. Although many factors required for the establishment of A/P polarity have been described, the nature of the spatial cue provided by the sperm remains unknown. Here we show that a microtubule-organizing centre is necessary and sufficient to establish several aspects of A/P polarity. In wildtype embryos, appearance of the first molecular asymmetries along the A/P axis correlates with and requires nucleation of microtubules by the sperm-derived centrosomes (sperm asters). In mutant embryos arrested in meiosis, sperm asters fail to form, and posterior is defined by the position of the persistent meiotic spindle rather than by the position of the sperm. Together, our data indicate that the primary spatial cue for A/P polarity in C. elegans derives from microtubules emanating from the sperm asters. Our findings support a parallel between C. elegans zygotes and other cells, such as Drosophila oocytes, which rely on microtubules to regulate polarity.

Embryonic polarity: protein stability in asymmetric cell division

Recent work on pattern formation in Caenorhabditis elegans has uncovered a new mechanism of asymmetric cell division: the cytoplasm is polarized by cortical proteins, and this polarization then influences the stability of other maternally expressed proteins that in turn determine early embryonic cell fates.