Segregation Of Cell Fate Determinants Research Articles

Drosophila Nonmuscle Myosin II Promotes the Asymmetric Segregation of Cell Fate Determinants by Cortical Exclusion Rather Than Active Transport

Cell fate diversity can be achieved through the asymmetric segregation of cell fate determinants. In the Drosophila embryo, neuroblasts divide asymmetrically and in a stem cell fashion. The determinants Prospero and Numb localize in a basal crescent and are partitioned from neuroblasts to their daughters (GMCs). Here we show that nonmuscle myosin II regulates asymmetric cell division by an unexpected mechanism, excluding determinants from the apical cortex. Myosin II is activated by Rho kinase and restricted to the apical cortex by the tumor suppressor Lethal (2) giant larvae. During prophase and metaphase, myosin II prevents determinants from localizing apically. At anaphase and telophase, myosin II moves to the cleavage furrow and appears to “push” rather than carry determinants into the GMC. Therefore, the movement of myosin II to the contractile ring not only initiates cytokinesis but also completes the partitioning of cell fate determinants from the neuroblast to its daughter.

A mouse homologue of Drosophila pins can asymmetrically localize and substitute for pins function in Drosophila neuroblasts.

Asymmetric cell division is a fundamental mechanism used to generate cellular diversity in invertebrates and vertebrates. In Drosophila, asymmetric division of neuroblasts is achieved by the asymmetric segregation of cell fate determinants Prospero and Numb into the basal daughter cell. Asymmetric segregation of cell fate determinants requires an apically localized protein complex that includes Inscuteable, Pins, Bazooka, DmPar-6, DaPKC and Galphai. Pins acts to stabilize the apical complex during neuroblast divisions. Pins interacts and colocalizes with Inscuteable, as well as maintaining its apical localization. We have isolated a mouse homologue of pins (Pins) and characterized its expression profile. Mouse PINS shares high similarity in sequence and structure with Pins and other Pins-like proteins from mammals. Pins is expressed in many mouse tissues but its expression is enriched in the ventricular zone of the developing central nervous systems. PINS localizes asymmetrically to the apical cortex of mitotic neuroblasts when ectopically expressed in Drosophila embryos. Like Pins, its N-terminal tetratricopeptide repeats can directly interact with the asymmetric localization domain of Insc, and its C-terminal GoLoco-containing region can direct localization to the neuroblast cortex. We further show that Pins can fulfill all aspects of pins function in Drosophila neuroblast asymmetric cell divisions. Our results suggest a conservation of function between the fly and mammalian Pins homologues.

Molecular cloning and developmental expression of Par-1/MARK homologues XPar-1A and XPar-1B from Xenopuslaevis

Par-1 encodes a serine/threonine kinase that is involved in asymmetric segregation of cell fate determinants in Caenorhabditis elegans and Drosophila embryos. Recent biochemical studies indicate an association of PAR-1 with the Dishevelled protein and suggest a role in so-called canonical Wnt signaling (Nat. Cell Biol. 3 (2001) 628). Here we describe two Xenopus laevis cDNAs, which encode PAR-1 homologues designated XPar-1A and XPar-1B. Structurally, XPar-1A and XPar-1B are closely related to rat MARK proteins and human Par-1A and Par-1Bα, respectively. XPar-1A and XPar-1B are expressed both maternally and zygotically in an indistinguishable pattern. In the egg and cleavage stage embryos their transcripts are enriched in the animal pole of the embryo. During blastula and gastrula stages, cells in the animal and marginal regions continue to express both genes uniformly. Expression progresses vegetally towards and then through the blastopore lip concomitantly with the movements of epiboly and gastrulation. With the onset of neurulation, XPar-1A and XPar-1B transcripts are restricted to the neurectoderm. At tailbud and tadpole stages they are detected in the head region, including brain, eyes, otic vesicles, cement gland, branchial arches as well as spinal cord and somites. Therefore, this analysis suggests that the Xenopus par-1 homologues XPar-1A and XPar-1B are expressed in frog embryos both maternally and zygotically in a restricted pattern and may play a role in establishing polarity in early embryos as well as in organogenesis during later stages of development.

Steel factor regulates cell cycle asymmetry.

Asymmetric segregation of cell-fate determinants during mitosis (spatial asymmetry) is an essential mechanism by which stem cells are maintained while simultaneously giving rise to differentiated progenitors that ultimately produce all the specialized cells in the hematopoietic system. Temporal cell cycle asymmetry and heterogeneity are attributes of cell proliferation that are also essential for maintaining tissue organization. Hematopoietic stem cells (HSCs) are regulated by a complex network of cytokines, some of which have very specific effects, while others have very broad ranging effects on HSCs. Some cytokines, like steel factor (SLF), are known to synergize with other cytokines to produce rapid expansion of progenitor cells. Using the human growth factor-dependent MO7e cell line as a model for synergistic proliferation, we present evidence that links proliferation asymmetry to SLF synergy with GM-CSF, and suggests that temporal asymmetry and cell cycle heterogeneity can be regulated by SLF in vitro. We also show that CDK-inhibitor and cell cycle regulator, p27kip-1, may be involved in this temporal asymmetry regulation. We propose that SLF/GM-CSF synergy is, in part, due to a shift in proliferation pattern from a heterogeneous and asymmetric one to a more synchronous and symmetric pattern, thus contributing dramatically to the rapid expansion that accompanies SLF synergy observed in MO7e cells. This kinetic model of asymmetry is consistent with recent evidence showing that even though SLF synergy results in a strong proliferative signal, it does not increase primary HSC self-renewal, which is believed to be highly dependent on asymmetric divisions. The factor-dependent MO7e/SCF- synergy/asymmetry model described here may therefore be useful for studies of the effects of various cytokines on cell cycle asymmetry.

Asymmetric cell division: from A to Z.

Cell divisions producing two daughter cells that adopt distinct fates are defined as asymmetric. In all organisms, ranging from bacteria to mammals, in which development has been studied extensively, asymmetric cell divisions generate cell diversity. Asymmetric cell divisions can be achieved by either intrinsic or extrinsic mechanisms (Fig. 1). Intrinsic mechanisms involve the preferential segregation of cell fate determinants to one of two daughter cells during mitosis. Asymmetrically segregated factors that bind cell fate determinants and orient the mitotic spindle may also be necessary to ensure the faithful segregation of determinants into only one daughter cell. Extrinsic mechanisms involve cell‐cell communication. In metazoans, a dividing’s cell’s social contex provides a wealth of positional information and opportunity for cell‐cell interactions. Interactions between daughter cells or between a daughter cell and other nearby cells could specify daughter cell fate. Interaction between a progenitor cell and its environment can also influence cell polarity by directing spindle orientation and the asymmetric distribution of developmental potential to daughter cells. Recent studies have indicated that a combination of intrinsic and extrinsic mechanisms specify distinct daughter cell fates during asymmetric cell divisions. We focus on asymmetric cell divisions that occur during the development of the Caenorhabditis elegans and Drosophila nervous systems. Although we touch upon asymmetric cell divisions in the early C. elegans embryo, more extensive reviews on this subject are available (Guo and Kemphues 1996a; Han 1997; Kemphues and Strome 1997; Bowerman 1998).