Localization-dependent and -independent roles of numb contribute to cell-fate specification in Drosophila

During asymmetric cell division, protein determinants are segregated into one of the two daughter cells. The Numb protein acts as a segregating determinant during both mouse and Drosophila development. In flies, Numb localizes asymmetrically and is required for cell-fate specification in the central and peripheral nervous systems, as well as during muscle and heart development. Whether its asymmetric segregation is important to the performance of these functions is not firmly established. Here, we demonstrate that Numb acts both in a localization-dependent and in a localization-independent manner. We have generated numb mutants that affect only the asymmetric localization of the protein during mitosis. We demonstrate that asymmetric segregation of Numb into one of the two daughter cells is absolutely essential for cell-fate specification in the Drosophila peripheral nervous system. Numb localization is also essential in MP2 neuroblasts in the central nervous system and during muscle development. Surprisingly, in dividing ganglion mother cells or during heart development, Numb function is independent of its ability to segregate asymmetrically in mitosis. Our results suggest that two classes of asymmetric cell division exist, each with different requirements for asymmetric inheritance of cell-fate determinants.     

Asymmetric cell division in the morphogenesis of Drosophila melanogaster macrochaetae

Asymmetric cell division (ACD) is the basic process which creates diversity in the cells of multicellular organisms. As a result of asymmetric cell division, daughter cells acquire the ability to differentiate and specialize in a given direction, which is different from that of their parent cells and from each other. This type of division is observed in a wide range of living organisms from bacteria to vertebrates. It has been shown that the molecular-genetic control mechanism of ACD is evolutionally conservative. The proteins involved in the process of ACD in different kinds of animals have a high degree of homology. Sensory organs--setae (macrochaetae)--of Drosophila are widely used as a model system for studying the genetic control mechanisms of asymmetric division. Setae located in an orderly manner on the head and body of the fly play the role of mechanoreceptors. Each of them consists of four specialized cells--offspring of the only sensory organ precursor cell (SOPC), which differentiates from the imaginal wing disc at the larval stage of the late third age. The basic differentiation and further specialization of the daughter cells of SOPC is an asymmetric division process. In this summary, experimental data on genes and their products controlling asymmetric division of SOPC and daughter cells, and also the specialization of the latter, have been systemized. The basic mechanisms which determine the time cells enter into asymmetric mitosis and which provides the structural characteristics of the asymmetric division process--the polar distribution of protein determinants Numb and Neuralized--the orientation of the mitotic spindle in relation to these determinants, and the uneven segregation of the determinants into the daughter cells that determines the direction of their development have been discussed.     

细胞不对称分裂

细胞不对称分裂是多细胞生物发育的基础。细胞不对称分裂的重要特征是细胞命运决定子在细胞分裂期间的不对称分离。细胞不对称分裂一般要经历4个步骤：在细胞中建立一个极性轴;沿此轴定向并形成纺锤体;细胞命运决定子沿极性轴作极性分布;细胞分裂后,不同的细胞命运决定子指导决定细胞的不同命运。     

Uncovering the link between malfunctions in Drosophila neuroblast asymmetric cell division and tumorigenesis

Asymmetric cell division is a developmental process utilized by several organisms. On the most basic level, an asymmetric division produces two daughter cells, each possessing a different identity or fate. Drosophila melanogaster progenitor cells, referred to as neuroblasts, undergo asymmetric division to produce a daughter neuroblast and another cell known as a ganglion mother cell (GMC). There are several features of asymmetric division in Drosophila that make it a very complex process, and these aspects will be discussed at length. The cell fate determinants that play a role in specifying daughter cell fate, as well as the mechanisms behind setting up cortical polarity within neuroblasts, have proved to be essential to ensuring that neurogenesis occurs properly. The role that mitotic spindle orientation plays in coordinating asymmetric division, as well as how cell cycle regulators influence asymmetric division machinery, will also be addressed. Most significantly, malfunctions during asymmetric cell division have shown to be causally linked with neoplastic growth and tumor formation. Therefore, it is imperative that the developmental repercussions as a result of asymmetric cell division gone awry be understood.     

Monitoring neural progenitor fate through multiple rounds of division in an intact vertebrate brain

The behaviour of neural progenitors in the intact vertebrate brain and spinal cord is poorly understood, chiefly because of the inaccessibility and poor optical qualities inherent in many model systems. To overcome these problems we have studied the optically superior brain of the zebrafish embryo and have monitored the in vivo behaviour of fluorescently labelled neural progenitors and their daughter cells throughout a substantial period of hindbrain development. We find the majority (84%) of hindbrain neurons are born from progenitor divisions that generate two neurons and 68% of reconstructed lineage trees contained no asymmetric stem cell-like divisions. No progenitors divided in the manner expected of a classic stem cell; i.e. one that repeatedly self-renews and generates a differentiated cell type by asymmetric division. We also analysed the orientation of progenitor divisions relative to the plane of the ventricular zone (VZ) and find that this does not correlate with the fate of the daughter cells. Our results suggest that in this vertebrate system the molecular determinants that control whether a cell will become a neuron are usually not linked to a mechanism that generates asymmetric divisions.     

The Drosophila NuMA Homolog Mud regulates spindle orientation in asymmetric cell division

During asymmetric cell division, the mitotic spindle must be properly oriented to ensure the asymmetric segregation of cell fate determinants into only one of the two daughter cells. In Drosophila neuroblasts, spindle orientation requires heterotrimeric G proteins and the G alpha binding partner Pins, but how the Pins-G alphai complex interacts with the mitotic spindle is unclear. Here, we show that Pins binds directly to the microtubule binding protein Mud, the Drosophila homolog of NuMA. Like NuMA, Mud can bind to microtubules and enhance microtubule polymerization. In the absence of Mud, mitotic spindles in Drosophila neuroblasts fail to align with the polarity axis. This can lead to symmetric segregation of the cell fate determinants Brat and Prospero, resulting in the mis-specification of daughter cell fates and tumor-like over proliferation in the Drosophila nervous system. Our data suggest a model in which asymmetrically localized Pins-G alphai complexes regulate spindle orientation by directly binding to Mud.     

Two types of asymmetric divisions in the Drosophila sensory organ precursor cell lineage

Asymmetric partitioning of cell-fate determinants during development requires coordinating the positioning of these determinants with orientation of the mitotic spindle. In the Drosophila peripheral nervous system, sensory organ progenitor cells (SOPs) undergo several rounds of division to produce five cells that give rise to a complete sensory organ. Here we have observed the asymmetric divisions that give rise to these cells in the developing pupae using green fluorescent protein fusion proteins. We find that spindle orientation and determinant localization are tightly coordinated at each division. Furthermore, we find that two types of asymmetric divisions exist within the sensory organ precursor cell lineage: the anterior-posterior pI cell-type division, where the spindle remains symmetric throughout mitosis, and the strikingly neuroblast-like apical-basal division of the pIIb cell, where the spindle exhibits a strong asymmetry at anaphase. In both these divisions, the spindle reorientates to position itself perpendicular to the region of the cortex containing the determinant. On the basis of these observations, we propose that two distinct mechanisms for controlling asymmetric cell divisions occur within the same lineage in the developing peripheral nervous system in Drosophila.     

Roles for polarity and nuclear determinants in specifying daughter cell fates after an asymmetric cell division in the maize leaf

Asymmetric cell divisions occur repeatedly during plant development, but the mechanisms by which daughter cells are directed to adopt different fates are not well understood [1,2]. Previous studies have demonstrated roles for positional information in specification of daughter cell fates following asymmetric divisions in the embryo [3] and root [4]. Unequally inherited cytoplasmic determinants have also been proposed to specify daughter cell fates after some asymmetric cell divisions in plants [1,2,5], but direct evidence is lacking. Here we investigate the requirements for specification of stomatal subsidiary cell fate in the maize leaf by analyzing four mutants disrupting the asymmetric divisions of subsidiary mother cells (SMCs). We show that subsidiary cell fate does not depend on proper localization of the new cell wall during the SMC division, and is not specified by positional information acting on daughter cells after completion of the division. Instead, our data suggest that specification of subsidiary cell fate depends on polarization of SMCs and on inheritance of the appropriate daughter nucleus. We thus provide evidence of a role for unequal inheritance of an intracellular determinant in specification of cell fate after an asymmetric plant cell division.     

Rotation and asymmetry of the mitotic spindle direct asymmetric cell division in the developing central nervous system

The asymmetric segregation of cell-fate determinants and the generation of daughter cells of different sizes rely on the correct orientation and position of the mitotic spindle. In the Drosophila embryo, the determinant Prospero is localized basally and is segregated equally to daughters of similar cell size during epidermal cell division. In contrast, during neuroblast division Prospero is segregated asymmetrically to the smaller daughter cell. This simple switch between symmetric and asymmetric segregation is achieved by changing the orientation of cell division: neural cells divide in a plane perpendicular to that of epidermoblast division. Here, by labelling mitotic spindles in living Drosophila embryos, we show that neuroblast spindles are initially formed in the same axis as epidermal cells, but rotate before cell division. We find that daughter cells of different sizes arise because the spindle itself becomes asymmetric at anaphase: apical microtubules elongate, basal microtubules shorten, and the midbody moves basally until it is positioned asymmetrically between the two spindle poles. This observation contradicts the widely held hypothesis that the cleavage furrow is always placed midway between the two centrosomes.     

Drosophila mechanoreceptors as a model for studying asymmetric cell division

Asymmetric cell division (ACD) is one of the processes creating the overall diversity of cell types in multicellular organisms. The essence of this process is that the daughter cells exit from it being different from both the parental cell and one another in their ability to further differentiation and specialization. The large bristles (macrochaetae) that are regularly arranged on the surface of the Drosophila adult function as mechanoreceptors, and since their development requires ACD, they have been extensively used as a model system for studying the genetic control of this process. Each macrochaete is composed of four specialized cells, the progeny resulting from several ACDs from a single sensory organ precursor (SOP) cell, which differentiates from the ectodermal cells of the wing imaginal disc in the third-instar larva and pupa. In this paper we review the experimental data on the genes and their products controlling the ACDs of the SOP cell and its daughter cells, and their further specialization. We discuss the main mechanisms determining the time when the cell enters ACD, as well as the mechanisms providing for the structural characteristics of asymmetric division, namely, polar distribution of protein determinants (Numb and Neuralized), orientation of the division spindle relative to these determinants, and unequal segregation of the determinants specifying the direction of daughter cell development.     

Mechanisms of asymmetric stem cell division

Stem cells self-renew but also give rise to daughter cells that are committed to lineage-specific differentiation. To achieve this remarkable task, they can undergo an intrinsically asymmetric cell division whereby they segregate cell fate determinants into only one of the two daughter cells. Alternatively, they can orient their division plane so that only one of the two daughter cells maintains contact with the niche and stem cell identity. These distinct pathways have been elucidated mostly in Drosophila. Although the molecules involved are highly conserved in vertebrates, the way they act is tissue specific and sometimes very different from invertebrates.     

When timing is everything: role of cell cycle regulation in asymmetric division

Asymmetric division is a fundamental mechanism of generating cell diversity during development. One of its hallmarks is asymmetric localization during mitosis of proteins that specify daughter cell fate. Studies in Drosophila show that subcellular localization of many proteins required for asymmetric division of neuronal progenitors correlates with progression through mitosis. Yet, how cell cycle and asymmetric division machineries cooperate remains unclear. Recent data show that (1) key cell cycle regulators are required for asymmetric localization of cell fate determinants and for cell fate determination and (2) molecules that mediate asymmetric division can also act to modulate proliferation potential of progenitor cells.     

Molecular control of cell polarity and asymmetric cell division in Drosophila neuroblasts

In the embryonic central nervous system of the fruit fly Drosophila, most neurons and glial cells are generated by asymmetric division of neural stem cells called neuroblasts. Several genes have been identified that are required for the establishment of neuroblast polarity, for the asymmetric segregation of cell fate determinants and for the proper orientation and geometry of the mitotic spindle. However, little was known about the interactions between these genes and their respective gene products. It has emerged that most of the relevant proteins are assembled into three major protein complexes whose molecular interactions are conserved in evolution.     

Polarization of Drosophila Neuroblasts During Asymmetric Division

During Drosophila development, neuroblasts divide to generate progeny with two different fates. One daughter cell self-renews to maintain the neuroblast pool, whereas the other differentiates to populate the central nervous system. The difference in fate arises from the asymmetric distribution of proteins that specify either self-renewal or differentiation, which is brought about by their polarization into separate apical and basal cortical domains during mitosis. Neuroblast symmetry breaking is regulated by numerous proteins, many of which have only recently been discovered. The atypical protein kinase C (aPKC) is a broad regulator of polarity that localizes to the neuroblast apical cortical region and directs the polarization of the basal domain. Recent work suggests that polarity can be explained in large part by the mechanisms that restrict aPKC activity to the apical domain and those that couple asymmetric aPKC activity to the polarization of downstream factors. Polarized aPKC activity is created by a network of regulatory molecules, including Bazooka/Par-3, Cdc42, and the tumor suppressor Lgl, which represses basal recruitment. Direct phosphorylation by aPKC leads to cortical release of basal domain factors, preventing them from occupying the apical domain. In this framework, neuroblast polarity arises from a complex system that orchestrates robust aPKC polarity, which in turn polarizes substrates by coupling phosphorylation to cortical release.Cells use polarity for remarkably diverse functions. In this article, I discuss a polarity that is harnessed to generate daughter cells with different fates. Using polarity to divide asymmetrically addresses several challenges that complex organisms face. The diversification of cell types and tissues that occurs during the development of complex organisms is one such challenge. Drosophila neuroblasts, the subject of this article, undergo repeated symmetry breaking asymmetric cell divisions (ACDs) to populate the central nervous system. In a similar manner in adult organisms, ACDs are important for adult homeostasis, replenishing cells that are turned over during the course of normal physiology (Betschinger and Knoblich 2004).A fundamental aspect of ACD is the production of daughter cells containing distinct fate determinants. To segregate fate determinants, the cell becomes polarized to form mutually exclusive cortical domains, each with a set of fate determinants appropriate for one of the two daughter cells. The cleavage furrow forms at the interface of the two domains, partitioning the fate determinants into the two daughter cells where they function to either self-renew (to keep the progenitor population) or to differentiate (e.g., by changing the pattern of gene expression). One of the unique features of the symmetry breaking that occurs during ACD, at least as implemented by the neuroblast, is that it is remarkably dynamic, developing early in mitosis and depolarizing following the completion of cytokinesis.Since the discovery of the first polarized components, neuroblasts have been an excellent model system for investigating the mechanisms of cell polarization and have been extensively analyzed. Although aspects of neuroblast polarity remain unclear, a core framework for how polarity is created and maintained is emerging. In this article, I focus on neuroblast polarity as centered around the activity of atypical protein kinase C, which has emerged as a key regulator of the process. In this framework, neuroblast polarity can be explained by events that polarize aPKC and those that couple aPKC activity to the polarization of fate determinants.     

Asymmetric cell division of thoracic neuroblast 6-4 to bifurcate glial and neuronal lineage in Drosophila

In the development of the Drosophila central nervous system, some of the neuroblasts designated as neuroglioblasts generate both glia and neurons. Little is known about how neuroglioblasts produce these different cell types. NB6-4 in the thoracic segment (NB6-4T) is a neuroglioblast, although the corresponding cell in the abdominal segment (NB6-4A) produces only glia. Here, we describe the cell divisions in the NB6-4T lineage, following changes in cell number and cell arrangement. We also examined successive changes in the expression of glial cells missing (gcm) mRNA and protein, activity of which is known to direct glial fate from the neuronal default state. The first cell division of NB6-4T occurred in the medial-lateral orientation, and was found to bifurcate the glial and neuronal lineage. After division, the medial daughter cell expressed GCM protein to produce three glial cells, while the lateral daughter cell with no GCM expression produced ganglion mother cells, secondary precursors of neurons. Although gcm mRNA was present evenly in the cytoplasm of NB6-4T before the first cell division, it became detected asymmetrically in the cell during mitosis and eventually only in the medial daughter cell. In contrast, NB6-4A showed a symmetrical distribution of gcm mRNA and GCM protein through division. Our observations suggest that mechanisms regulating gcm mRNA expression and its translation play an important role in glial and neuronal lineage bifurcation that results from asymmetric cell division.     

The PDZ protein Canoe regulates the asymmetric division of Drosophila neuroblasts and muscle progenitors

Asymmetric cell division is a conserved mechanism to generate cellular diversity during animal development and a key process in cancer and stem cell biology. Despite the increasing number of proteins characterized, the complex network of proteins interactions established during asymmetric cell division is still poorly understood. This suggests that additional components must be contributing to orchestrate all the events underlying this tightly modulated process. The PDZ protein Canoe (Cno) and its mammalian counterparts AF-6 and Afadin are critical to regulate intracellular signaling and to organize cell junctions throughout development. Here, we show that Cno functions as a new effector of the apical proteins Inscuteable (Insc)-Partner of Inscuteable (Pins)-Galphai during the asymmetric division of Drosophila neuroblasts (NBs). Cno localizes apically in metaphase NBs and coimmunoprecipitates with Pins in vivo. Furthermore, Cno functionally interacts with the apical proteins Insc, Galphai, and Mushroom body defect (Mud) to generate correct neuronal lineages. Failures in muscle and heart lineages are also detected in cno mutant embryos. Our results strongly support a new function for Cno regulating key processes during asymmetric NB division: the localization of cell-fate determinants, the orientation of the mitotic spindle, and the generation of unequal-sized daughter cells.     

The orientation of cell division influences cell-fate choice in the developing mammalian retina

Asymmetric segregation of cell-fate determinants during cell division plays an important part in generating cell diversity in invertebrates. We showed previously that cells in the neonatal rat retina divide at various orientations and that some dividing cells asymmetrically distribute the cell-fate determinant Numb to the two daughter cells. Here, we test the possibility that such asymmetric divisions contribute to retinal cell diversification. We have used long-term videomicroscopy of green-fluorescent-protein (GFP)-labeled retinal explants from neonatal rats to visualize the plane of cell division and follow the differentiation of the daughter cells. We found that cells that divided with a horizontal mitotic spindle, where both daughter cells should inherit Numb, tended to produce daughters that became the same cell type, whereas cells that divided with a vertical mitotic spindle, where only one daughter cell should inherit Numb, tended to produce daughters that became different. Moreover, overexpression of Numb in the dividing cells promoted the development of photoreceptor cells at the expense of interneurons and Müller glial cells. These findings indicate that the plane of cell division influences cell-fate choice in the neonatal rat retina and support the hypothesis that the asymmetric segregation of Numb normally influences some of these choices.     

Upregulation of Mitimere and Nubbin acts through cyclin E to confer self-renewing asymmetric division potential to neural precursor cells

In the Drosophila CNS, neuroblasts undergo self-renewing asymmetric divisions, whereas their progeny, ganglion mother cells (GMCs), divide asymmetrically to generate terminal postmitotic neurons. It is not known whether GMCs have the potential to undergo self-renewing asymmetric divisions. It is also not known how precursor cells undergo self-renewing asymmetric divisions. Here, we report that maintaining high levels of Mitimere or Nubbin, two POU proteins, in a GMC causes it to undergo self-renewing asymmetric divisions. These asymmetric divisions are due to upregulation of Cyclin E in late GMC and its unequal distribution between two daughter cells. GMCs in an embryo overexpressing Cyclin E, or in an embryo mutant for archipelago, also undergo self-renewing asymmetric divisions. Although the GMC self-renewal is independent of inscuteable and numb, the fate of the differentiating daughter is inscuteable and numb-dependent. Our results reveal that regulation of Cyclin E levels, and asymmetric distribution of Cyclin E and other determinants, confer self-renewing asymmetric division potential to precursor cells, and thus define a pathway that regulates such divisions. These results add to our understanding of maintenance and loss of pluripotential stem cell identity.