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.     

Asymmetric cell division in the morphogenesis of <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> macrochaetae

Asymmetric cell division (ACD) is the basic process which creates diversity in the cells of multi-cellular 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—bristles (macrochaetae)—of Drosophila are widely used as a model system for studying the genetic control mechanisms of asymmetric division. Bristles 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 (SOP), which differentiates from the wing imaginal disc at the larval stage of the late third age. The basic differentiation and further specialization of the daughter cells of SOP is an asymmetric division process.     

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.     

A dual function of phyllopod in Drosophila external sensory organ development: cell fate specification of sensory organ precursor and its progeny

During Drosophila external sensory organ development, one sensory organ precursor (SOP) arises from a proneural cluster, and undergoes asymmetrical cell divisions to produce an external sensory (es) organ made up of different types of daughter cells. We show that phyllopod (phyl), previously identified to be essential for R7 photoreceptor differentiation, is required in two stages of es organ development: the formation of SOP cells and cell fate specification of SOP progeny. Loss-of-function mutations in phyl result in failure of SOP formation, which leads to missing bristles in adult flies. At a later stage of es organ development, phyl mutations cause the first cell division of the SOP lineage to generate two identical daughters, leading to the fate transformation of neurons and sheath cells to hair cells and socket cells. Conversely, misexpression of phyl promotes ectopic SOP formation, and causes opposite fate transformation in SOP daughter cells. Thus, phyl functions as a genetic switch in specifying the fate of the SOP cells and their progeny. We further show that seven in absentia (sina), another gene required for R7 cell fate differentiation, is also involved in es organ development. Genetic interactions among phyl, sina and tramtrack (ttk) suggest that phyl and sina function in bristle development by antagonizing ttk activity, and ttk acts downstream of phyl. It has been shown previously that Notch (N) mutations induce formation of supernumerary SOP cells, and transformation from hair and socket cells to neurons. We further demonstrate that phyl acts epistatically to N. phyl is expressed specifically in SOP cells and other neural precursors, and its mRNA level is negatively regulated by N signaling. Thus, these analyses demonstrate that phyl acts downstream of N signaling in controlling cell fates in es organ development.     

Where does asymmetry come from? Illustrating principles of polarity and asymmetry establishment in Drosophila neuroblasts

Asymmetric cell division (ACD) is the fundamental process through which one cell divides into two cells with different fates. In animals, it is crucial for the generation of cell-type diversity and for stem cells, which use ACD both to self-renew and produce one differentiating daughter cell. One of the most prominent model systems of ACD, Drosophila neuroblasts, relies on the PAR complex, a conserved set of proteins governing cell polarity in animals. Here, we focus on recent advances in our understanding of the mechanisms that control the orientation of the neuroblast polarity axis, how the PAR complex is positioned, and how its activity may regulate division orientation and cell fate determinant localization and discuss how important findings about the composition polarity complexes in other models may apply to neuroblasts.     

Asymmetric Rab 11 endosomes regulate delta recycling and specify cell fate in the Drosophila nervous system

Drosophila sensory organ precursor (SOP) cells are a well-studied model system for asymmetric cell division. During SOP division, the determinants Numb and Neuralized segregate into the pIIb daughter cell and establish a distinct cell fate by regulating Notch/Delta signaling. Here, we describe a Numb- and Neuralized-independent mechanism that acts redundantly in cell-fate specification. We show that trafficking of the Notch ligand Delta is different in the two daughter cells. In pIIb, Delta passes through the recycling endosome which is marked by Rab 11. In pIIa, however, the recycling endosome does not form because the centrosome fails to recruit Nuclear fallout, a Rab 11 binding partner that is essential for recycling endosome formation. Using a mammalian cell culture system, we demonstrate that recycling endosomes are essential for Delta activity. Our results suggest that cells can regulate signaling pathways and influence their developmental fate by inhibiting the formation of individual endocytic compartments.     

Molecular mechanisms of asymmetric divisions in mammary stem cells

Stem cells have the remarkable ability to undergo proliferative symmetric divisions and self‐renewing asymmetric divisions. Balancing of the two modes of division sustains tissue morphogenesis and homeostasis. Asymmetric divisions of Drosophila neuroblasts (NBs) and sensory organ precursor (SOP) cells served as prototypes to learn what we consider now principles of asymmetric mitoses. They also provide initial evidence supporting the notion that aberrant symmetric divisions of stem cells could correlate with malignancy. However, transferring the molecular knowledge of circuits underlying asymmetry from flies to mammals has proven more challenging than expected. Several experimental approaches have been used to define asymmetry in mammalian systems, based on daughter cell fate, unequal partitioning of determinants and niche contacts, or proliferative potential. In this review, we aim to provide a critical evaluation of the assays used to establish the stem cell mode of division, with a particular focus on the mammary gland system. In this context, we will discuss the genetic alterations that impinge on the modality of stem cell division and their role in breast cancer development.     

Anticancer efficacies of doxorubicin,verapamil and quercetin on FM3A cells under hyperthermic temperature

Hyperthermia (HT) in combination with anticancer drugs (ACDs) had proven to more efficacious in various cancers, although efficacies vary according to chemotherapeutic compounds and cancer types. Presently there are few data that compares anticancer efficacies among ACDs under hyperthermic conditions. Therefore, we selected three commonly used ACDs (quercetin, verapamil and doxorubicin) and compared their antitumor effects when each was treated with 43°C HT exposure. Firstly, FM3A, a murine breast cancer cell line, was treated with each ACD for 1 h followed by 43°C exposure for additional 1 h, and examined the effects of: 1) each drug, 2) 43°C HT exposure, and 3) the combination of each drug and 43°C HT exposure for 1, 6 and 24 h. The determined overall effects on FM3A cells were arrested cell proliferation, clonogenic efficiency and apoptosis. Pre-treatment of FM3A cells to each ACD followed by 43°C HT exposure produced greater antitumor effects including suppressed cell proliferation, reduced clonogenic efficiency and increased apoptotic cell death, compared to ACD treatment or HT exposure alone. Apoptotic cell death occurred in a time-dependent manner. Among the ACDs, antitumor efficacies varied in the order of doxorubicin > verapamil > quercetin. It was concluded that heat exposure during ACD treatment of caner cells may be an important factor to get a better antitumor benefit, even though this benefit may differ from one drug to another.     

Spindle orientation and epidermal morphogenesis

Asymmetric cell divisions (ACDs) result in two unequal daughter cells and are a hallmark of stem cells. ACDs can be achieved either by asymmetric partitioning of proteins and organelles or by asymmetric cell fate acquisition due to the microenvironment in which the daughters are placed. Increasing evidence suggests that in the mammalian epidermis, both of these processes occur. During embryonic epidermal development, changes occur in the orientation of the mitotic spindle in relation to the underlying basement membrane. These changes are guided by conserved molecular machinery that is operative in lower eukaryotes and dictates asymmetric partitioning of proteins during cell divisions. That said, the shift in spindle alignment also determines whether a division will be parallel or perpendicular to the basement membrane, and this in turn provides a differential microenvironment for the resulting daughter cells. Here, we review how oriented divisions of progenitors contribute to the development and stratification of the epidermis.     

Prospero distinguishes sibling cell fate without asymmetric localization in the Drosophila adult external sense organ lineage

The adult external sense organ precursor (SOP) lineage is a model system for studying asymmetric cell division. Adult SOPs divide asymmetrically to produce IIa and IIb daughter cells; IIa generates the external socket (tormogen) and hair (trichogen) cells, while IIb generates the internal neuron and sheath (thecogen) cells. Here we investigate the expression and function of prospero in the adult SOP lineage. Although Prospero is asymmetrically localized in embryonic SOP lineage, this is not observed in the adult SOP lineage: Prospero is first detected in the IIb nucleus and, during IIb division, it is cytoplasmic and inherited by both neuron and sheath cells. Subsequently, Prospero is downregulated in the neuron but maintained in the sheath cell. Loss of prospero function leads to 'double bristle' sense organs (reflecting a IIb-to-IIa transformation) or 'single bristle' sense organs with abnormal neuronal differentiation (reflecting defective IIb development). Conversely, ectopic prospero expression results in duplicate neurons and sheath cells and a complete absence of hair/socket cells (reflecting a IIa-to-IIb transformation). We conclude that (1) despite the absence of asymmetric protein localization, prospero expression is restricted to the IIb cell but not its IIa sibling, (2) prospero promotes IIb cell fate and inhibits IIa cell fate, and (3) prospero is required for proper axon and dendrite morphology of the neuron derived from the IIb cell. Thus, prospero plays a fundamental role in establishing binary IIa/IIb sibling cell fates without being asymmetrically localized during SOP division. Finally, in contrast to previous studies, we find that the IIb cell divides prior to the IIa cell in the SOP lineage.     

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.     

The secret to life is being different: asymmetric divisions in plant development

Asymmetric cell divisions (ACDs) are used to create organismal form and cellular diversity during plant development. In several embryonic and postembryonic contexts, genes that specify cell fates and networks that provide positional information have been identified. The cellular mechanisms that translate this information into a physically ACD, however, are still obscure. In this review we examine the cell polarization events that precede asymmetric divisions in plants. Using principles derived from studies of other organisms and from postmitotic polarity generation in plants, we endeavor to provide a framework of what is known, what is on the horizon and what is critically needed to develop a rigorous mechanistic understanding of ACDs in plants.     

Flamingo controls the planar polarity of sensory bristles and asymmetric division of sensory organ precursors in Drosophila.

The sensory bristles of the fruit fly Drosophila are organized in a polarized fashion such that bristles on the thorax point posteriorly. These bristles are derived from asymmetric division of sensory organ precursors (SOPs). The Numb protein, which is localized asymmetrically in a cortical crescent in each SOP, segregates into only one of the two daughter cells during cell division, thereby conferring distinct fates to the daughter cells [1] [2]. In neuroblasts, establishment of apical-basal polarity by the protein Inscuteable is crucial for orienting asymmetric division, but this is not the case for division of SOPs [3]. Instead, the Frizzled (Fz) protein mediates a planar polarity signal that controls the anteroposteriorly oriented first division (pl) of SOPs [4]. Here, we report that Flamingo (Fmi), a seven-transmembrane cadherin [5], controls the planar polarity of sensory bristles and the orientation of the SOP pl division. Both the loss of function and overexpression of fmi disrupted bristle polarity. During mitosis of the SOP, the axis of the pl division and the positioning of the Numb crescent were randomized in the absence of Fmi activity. Overexpression of Fmi and Fz caused similar effects. The dependence of proper Fmi localization on Fz activity suggests that Fmi functions downstream of Fz in controlling planar polarity. We also present evidence suggesting that Fz also functions in the Wingless pathway to pattern sensory organs.     

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.     

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。     

Live-cell Imaging of Sensory Organ Precursor Cells in Intact Drosophila Pupae

Since the discovery of Green Fluorescent Protein (GFP), there has been a revolutionary change in the use of live-cell imaging as a tool for understanding fundamental biological mechanisms. Striking progress has been particularly evident in Drosophila, whose extensive toolkit of mutants and transgenic lines provides a convenient model to study evolutionarily-conserved developmental and cell biological mechanisms. We are interested in understanding the mechanisms that control cell fate specification in the adult peripheral nervous system (PNS) in Drosophila. Bristles that cover the head, thorax, abdomen, legs and wings of the adult fly are individual mechanosensory organs, and have been studied as a model system for understanding mechanisms of Notch-dependent cell fate decisions. Sensory organ precursor (SOP) cells of the microchaetes (or small bristles), are distributed throughout the epithelium of the pupal thorax, and are specified during the first 12 hours after the onset of pupariation. After specification, the SOP cells begin to divide, segregating the cell fate determinant Numb to one daughter cell during mitosis. Numb functions as a cell-autonomous inhibitor of the Notch signaling pathway.Here, we show a method to follow protein dynamics in SOP cell and its progeny within the intact pupal thorax using a combination of tissue-specific Gal4 drivers and GFP-tagged fusion proteins ^1,2.This technique has the advantage over fixed tissue or cultured explants because it allows us to follow the entire development of an organ from specification of the neural precursor to growth and terminal differentiation of the organ. We can therefore directly correlate changes in cell behavior to changes in terminal differentiation. Moreover, we can combine the live imaging technique with mosaic analysis with a repressible cell marker (MARCM) system to assess the dynamics of tagged proteins in mitotic SOPs under mutant or wildtype conditions. Using this technique, we and others have revealed novel insights into regulation of asymmetric cell division and the control of Notch signaling activation in SOP cells (examples include references ^{1-6,7 ,8}).     

Asymmetric cell division in plant development

Asymmetric cell division(ACD) is a fundamental process that generates new cell types during development in eukaryotic species.In plant development,post-embryonic organogenesis driven by ACD is universal and more important than in animals,in which organ pattern is preset during embryogenesis.Thus,plant development provides a powerful system to study molecular mechanisms underlying ACD.During the past decade,tremendous progress has been made in our understanding of the key components and mechanism...