An unequal divide

Before cell division, an axis of polarity is established, which causes differing concentrations of determinants that induce different cell fates in the daughter cells.

A fundamental dilemma in development is unravelling how a single cell can divide asymmetrically to generate two daughter cells with distinct fates. Thanks to pioneering studies that began in the 1980s, we now know that an important mechanism for generating differences between daughter cells is the unequal distribution of developmentally important molecules or determinants.

Our knowledge of the machinery that directs asymmetric cell division has come largely from experiments using the nematode worm Caenorhabditis elegans and the fruitfly Drosophila melanogaster. The worm has proved to be particularly useful in this respect, because its early development is characterized by a cascade of asymmetric divisions. Indeed, the very first division of the single-cell C. elegans embryo is asymmetric — the cell (P0) divides along its anterior–posterior axis to produce a larger AB cell and a smaller posterior P1 cell.

A landmark paper, published in 1988 by Kemphues et al., identified genes that are involved in cell-fate specification in C. elegans. A study of maternal-effect lethal mutations led to the identification of the par (partitioning defective) genes par1–4. Wild-type C. elegans oocytes contain a uniform distribution of P granules — large ribonuclear particles that normally segregate into P1. Mutations in the four par genes led to abnormal positioning of the mitotic spindle and aberrant localization of the P granules, leading the authors to propose that the par genes encode products that are required for spindle placement and cytoplasmic localization.

In the mid 1990s, Kemphues and colleagues cloned the C. elegans par-1 and par-3 genes. PAR-1 is a conserved serine/threonine kinase, whereas PAR-3 turned out to be a novel protein. Both of these proteins are asymmetrically distributed in the zygote: PAR-1 is enriched at the posterior periphery, whereas PAR-3 is found at the anterior periphery. In early germ-line precursor cells, PAR-1 localization correlates strikingly with P granule distribution and requires the kinase activity of PAR-1. Meanwhile, the distribution of PAR-3 controls spindle orientation. PAR-3 localization depends on the par-2 gene, and PAR-3 is required for PAR-1 localization.

Experiments in Drosophila uncovered additional asymmetrically segregating cell-fate determinants. In the developing peripheral nervous system, sensory organ precursors (SOPs) divide symmetrically to give the secondary precursors IIa and IIb. These precursors then divide asymmetrically — IIa produces cells that form the hair and socket, whereas IIb generates cells that differentiate into the neuron and sheath. A breakthrough in elucidating the underlying molecular mechanism came from the Jan laboratory. In a 1994 paper, they identified a protein in Drosophila sensory organs called Numb, which is localized within the SOP before cell division and segregates to only one of the daughter cells during division. So, is Numb necessary to give IIa and IIb distinct fates? Overexpression of Numb during SOP division results in the formation of two IIb cells rather than the normal IIa/IIb pair. Furthermore, in the absence of Numb, IIb cells differentiate inappropriately, forming cells that normally derive from IIa. These results demonstrated that the asymmetric segregation of a determinant during cell division could induce a specific fate in one of the two daughter cells.

In the developing fly central nervous system, neuroblasts divide asymmetrically to produce an apical daughter that remains a neuroblast and a smaller basal daughter called a ganglion mother cell. Knoblich and colleagues showed that expression of the inscuteable gene is required for asymmetric segregation of Numb, as well as correct spindle orientation, in fly neuroblasts and epithelial cells. The Inscuteable protein localizes to the apical-cell cortex before mitosis and precedes the basal localization of Numb. These results indicated that asymmetric localization of Inscuteable establishes positional information for both spindle orientation and asymmetric localization of Numb. The Par proteins have also been shown to be involved in asymmetric Numb localization in neuroblasts.

Together, these pioneering papers demonstrated that asymmetric distribution of cellular determinants underlies the generation of cellular diversity, and researchers are now trying to establish whether similar principles determine asymmetric cell division in vertebrates.