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.     

High-copy bacterial plasmids diffuse in the nucleoid-free space,replicate stochastically and are randomly partitioned at cell division

Bacterial plasmids play important roles in the metabolism, pathogenesis and bacterial evolution and are highly versatile biotechnological tools. Stable inheritance of plasmids depends on their autonomous replication and efficient partition to daughter cells at cell division. Active partition systems have not been identified for high-copy number plasmids, and it has been generally believed that they are partitioned randomly at cell division. Nevertheless, direct evidence for the cellular location of replicating and nonreplicating plasmids, and the partition mechanism has been lacking. We used as model pJHCMW1, a plasmid isolated from Klebsiella pneumoniae that includes two β-lactamase and two aminoglycoside resistance genes. Here we report that individual ColE1-type plasmid molecules are mobile and tend to be excluded from the nucleoid, mainly localizing at the cell poles but occasionally moving between poles along the long axis of the cell. As a consequence, at the moment of cell division, most plasmid molecules are located at the poles, resulting in efficient random partition to the daughter cells. Complete replication of individual molecules occurred stochastically and independently in the nucleoid-free space throughout the cell cycle, with a constant probability of initiation per plasmid.     

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.     

Closing the GAP: a role for a RhoA GAP in cytokinesis

During cell division, cytokinesis partitions the cytoplasm to form two daughter cells. Recently in Developmental Cell, Minoshima et al. (2003) establish an important connection between three regulatory molecules that coordinate this process, showing that the mitotic kinase aurora B phosphorylates MgcRacGAP to convert it to a GAP for RhoA, a small GTPase that drives cytokinesis.     

Numb与神经发育

不对称性细胞分裂是一个母细胞通过一次分裂,产生两个不同命运的子细胞的分裂方式,是单细胞生物向多细胞生物进化的关键一步。根据现有的证据推论,不称性细胞分裂是在器官发育过程中产生细胞多样化的一种基本方式。Numb是第一个被发现决定多细胞生物不对称细胞分裂的信号蛋白。在果蝇中,Numb通过促进Notch泛素化拮抗Notch信号通路,从而决定子细胞的命运,后来的研究表明Numb是细胞内吞调节蛋白,并用通过内吞参与调节神经细胞的粘附,轴突的生长及细胞迁移等过程;并且发现Numb与肿瘤抑制基因p53、泛素化蛋白HDM2形成三聚体抑制p53的泛素化,从而调节肿瘤的恶性程度。本文系统地分析了Numb发现的历史及后来在脊椎动物中的作用和机制,重点介绍了Numb在神经发育过程中的功能。     

A review of mitosis in the fission yeast Schizosaccharomyces pombe

Mitosis and cell division are the final events of the cell cycle, resulting in the precise segregation of chromosomes into two daughter cells. A highly controlled and accurate segregation of the chromosomes is required to ensure that each daughter cell receives a complete genome and remains viable. The fission yeast, Schizosaccharomyces pombe, is a unicellular eukaryotic organism which is particularly convenient for investigating these problems. It is very amenable to genetic analysis and its predominantly haploid life cycle has allowed the isolation of recessive temperature-sensitive mutants unable to complete the cell cycle. Classical genetic analysis of these mutants has been used to identify over 40 gene functions that are required for cell cycle progress in S. pombe. Many of these genes have now been cloned and sequenced and in some cases the encoded gene product has been identified. This approach, coupling classical and molecular genetics, allows identification of the molecules important in the mitotic processes and provides a means for establishing what functional roles they may play.     

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.     

The P1 plasmid in action: time-lapse photomicroscopy reveals some unexpected aspects of plasmid partition

The prophage of bacteriophage P1 is a low copy number plasmid in Escherichia coli and is segregated to daughter cells by an active partition system. The dynamics of the partition process have now been successfully followed by time-lapse photomicroscopy. The process appears to be fundamentally different from that previously inferred from statistical analysis of fixed cells. A focus containing several plasmid copies is captured at the cell center. Immediately before cell division, the copies eject bi-directionally along the long axis of the cell. Cell division traps one or more plasmid copies in each daughter cell. These copies are free to move, associate, and disassociate. Later, they are captured to the new cell center to re-start the cycle. Studies with mutants suggest that the ability to segregate accurately at a very late stage in the cell cycle is dependent on a novel ability of the plasmid to control cell division. Should segregation be delayed, cell division is also delayed until segregation is successfully completed.     

Proliferation of unilocular fat cells in the primary culture

Mature white fat cells (unilocular fat cells) have generally been considered to be in terminal differentiation and, hence, to have no proliferative ability. A new method, referred to as "ceiling culture," has been devised in our laboratory to culture unilocular fat cells in vitro. Under such culture conditions, the fat cells continue to exhibit specific functions of lipid metabolism and proliferate extensively. Intracytoplasmic lipid droplets did not inhibit division of the cells. There were two modes of proliferation of unilocular fat cells: "loculus-dividing" cell division, in which the single loculus of fat in the dividing cell was broken down into multiple droplets and distributed evenly between the daughter cells, and "loculus-preserving" cell division, in which the loculus in the dividing cell was minimally broken down and inherited with its shape preserved by one of the daughter cells with the other getting only a small number of fine lipid droplets. Such findings suggest that unilocular fat cells in mature fat tissue in vivo are probably capable of proliferation in such modes under some conditions.     

Cell cycle-dependent protein secretion by Saccharomyces cerevisiae.

Synchronized Saccharomyces cerevisiae cell populations were used to examine secretion rates of a heterologous protein as a function of cell cycle position. The synchronization procedure had a profound effect on the type and quality of data obtained. When cell synchrony was induced by cell cycle-arresting drugs, a significant physiological perturbation of cells was observed that obscured representative secretion data. In contrast, synchronization with centrifugal elutriation resulted in synchronized first-generation daughter cells with undetectable perturbation of the physiological state. The synchronized cells did not secrete significant amounts of protein until they reached cell division, suggesting that the secretion process in these cells is strongly cell cycle dependent. However, the maximum secretion rate of the synchronized culture (7-14 molecules/cell/second) was significantly lower than that of an asynchronous culture (29-51 molecules/cell/second). This result indicates that young daughter cells isolated in the synchronization process exhibit different protein secretion behavior than older mother cells that are absent in the synchronized cell population but present in the asynchronous culture.     

Monitoring lymphocyte proliferation in vitro and in vivo with the intracellular fluorescent dye carboxyfluorescein diacetate succinimidyl ester

This protocol outlines the carboxyfluorescein diacetate succinimidyl ester (CFSE) method for following the proliferation of human lymphocytes in vitro and mouse lymphocytes both in vitro and in vivo. The method relies on the ability of CFSE to covalently label long-lived intracellular molecules with the highly fluorescent dye, carboxyfluorescein. Following each cell division, the equal distribution of these fluorescent molecules to progeny cells results in a halving of the fluorescence of daughter cells. The CFSE labeling protocol described, which typically takes <1 h to perform, allows the detection of up to eight cell divisions before CFSE fluorescence is decreased to the background fluorescence of unlabeled cells. Protocols are outlined for labeling large and small numbers of human and mouse lymphocytes, labeling conditions being identified that minimize CFSE toxicity but maximize the number of cell divisions detected. An important feature of the technique is that division-dependent changes in the expression of cell-surface markers and intracellular proteins are easily quantified by flow cytometry.     

Septation and cytokinesis in fungi

Cytokinesis is the ultimate step of a cell cycle resulting in the generation of two progeny. Failure of correct cell division may be lethal for both, mother and daughter cells, and thus such a process must be tightly regulated with other events of the cell cycle. Differing solutions to the same problem have been developed in bacteria and plants while cytokinesis in animal and fungal cells is highly similar and requires a contractile ring containing actomyosin. Cytokinesis in fungi can be viewed as a three-stage process: (i) selection of a division site, (ii) orderly assembly of protein complexes, and finally (iii) dynamic events that lead to a constriction of the contractile ring and septum construction. Elaborate mechanisms known as the Mitotic Exit Network (MEN) and the Septation Initiation Network (SIN) have evolved to link these events, particularly the final steps of cytokinesis, with nuclear division. The purpose of this review was to discuss the latest developments in the fungal field and to describe the central known players required for key steps on the road to cell division. Differences in the cytokinesis of yeast-like fungi that result in complete cell separation in contrast to septation which leads to the compartmentalization of fungal hyphae are highlighted.     

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.     

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.     

Asymmetric cell division from a cell to cells: Shape,length, and location of polarity domain

Asymmetric cell division is one of the most elegant biological systems by which cells create daughter cells with different functions and increase cell diversity. In particular, PAR polarity in the cell membrane plays a critical role in regulating the whole process of asymmetric cell division. Numerous studies have been conducted to determine the underlying mechanism of PAR polarity formation using both experimental and theoretical approaches in the last 10 years. However, they have mostly focused on answering the fundamental question of how this exclusive polarity is established but the precise dynamics of polarity domain have been little notified. In this review, I focused on studies on the shape, length, and location of PAR polarity from a theoretical perspective that may be important for an integrated understanding of the entire process of asymmetric cell division.     

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.     

Stage-dependent density effect in the cell cycle of budding yeast

Yeasts in culture media grow exponentially in early period but eventually stop growing. The saturation of population growth is due to "density effect". The budding yeast, Saccharomyces cerevisiae, is known to exhibit a stage-dependent cell division. Daughter cell, which gives no birth, has longer generation time than mother, because daughter needs maturity time. So far, investigations have been restricted in exponential or non-crowding state; very little is known for the stage dependence of density effect. Here we present a lattice gas model to explore the population dynamics of crowding period. We compare theoretical results with experimental data, and find a stage-dependent density effect. Although small daughter cells can develop to a critical size, the reproduction of large daughter cells suddenly stops when the total density exceeds some critical level. Our results imply the existence of an inhibitor that specifically halts the reproduction of matured daughter cell.