Highways for mRNA transport

Two new studies reveal the role of microtubule polarity in the asymmetric localization of mRNAs. In this issue of Cell, Zimyanin et al. (2008) show that the asymmetric localization of oskar mRNA in fruit fly oocytes results from a slight bias in the direction of its transport. Meanwhile, Messitt et al. (2008) reporting in Developmental Cell find a subpopulation of microtubules that is critical for the asymmetric distribution of Vg1 mRNA in frog oocytes.     

Recycling polarity

Three current papers in Cell and in this issue of Developmental Cell highlight the role of the exocyst in recycling of membrane proteins from endosomes to the plasma membrane in asymmetric cell division and polarized epithelial cells.     

Connecting cancer to the asymmetric division of stem cells

Two studies, one in this issue of Cell and the other in Developmental Cell show that the cell-fate determinant Brain Tumor (Brat) suppresses self-renewal in one of the daughter cells that arise from the asymmetric division of a neural stem cell. This work suggests a mechanism by which loss of polarity in stem cells may lead to tumorigenesis.     

Cytoskeleton and Early Development in Fucoid Algae

Cell polarization and asymmetric cell divisions play important roles during development in many multicellular eukaryotes. Fucoid algae have a long history as models for studying early developmental processes, probably because of the ease with which zygotes can be observed and manipulated in the laboratory. This review discusses cell polarization and asymmetric cell divisions in fucoid algal zygotes with an emphasis on the roles played by the cytoskeleton.     

Advocating asymmetry and the POP-1 paradox: noncanonical Wnt signaling in C. elegans

In this issue of Cell, Kidd and colleagues (Kidd et al., 2005) describe their identification of a novel beta-catenin that functions in noncanonical C. elegans Wnt signaling pathways to specify the different fates of daughter cells produced by asymmetric cell division.     

Ran and Rac in mouse eggs: cortical polarity and spindle positioning

Polar body formation in eggs proceeds through two extreme asymmetric divisions, requiring precise coordination between spindle position and the polarized acto-myosin cortex. Two new studies appearing in this issue of Developmental Cell implicate the small GTPases Ran and Rac in cortical polarity of the mouse egg.     

多细胞生物干细胞不对称分裂的内在调节机制研究进展

多细胞生物的发育是从一个受精卵分化成多种类型细胞的过程。细胞多样性形成的基础是不等分裂,不等分裂是干细胞自我更新和自我维持的关键。干细胞不等分裂有细胞内和细胞外两种调节机制。果蝇神经干细胞增殖和分化、植物胚胎发育、表皮气孔形成及根内皮层的分化,是研究不等细胞分裂调节机制最多的发育背景。本综述介绍了果蝇神经干细胞和植物胚胎发育早期、表皮气孔发生及根皮层内皮层中细胞不等分裂内在调节机制的研究进展。     

Orienting the Direction of EGFR Activation

Morphogens are typically distributed symmetrically from their source of production. In this issue of Developmental Cell, Peng et?al. (2012) demonstrate that a bias in the directionality of protrusions emanating from cells secreting the EGFR ligand Spitz leads to asymmetric activation of the pathway.     

The inner compass of spindle positioning and orientation

Polarized cortical cues are known to guide spindle movements to dictate division axis and cleavage site during asymmetric cell division. In a recent issue of Nature Cell Biology, Kiyomitsu and Cheeseman (2012) report two novel spindle-intrinsic signals that regulate spindle orientation and position in symmetrically dividing human cells.     

Cell polarity and asymmetric cell division: the C. elegans early embryo

Cell polarity is crucial for many functions including cell migration, tissue organization and asymmetric cell division. In animal cells, cell polarity is controlled by the highly conserved PAR (PARtitioning defective) proteins. par genes have been identified in Caenorhabditis elegans in screens for maternal lethal mutations that disrupt cytoplasmic partitioning and asymmetric division. Although PAR proteins were identified more than 20 years ago, our understanding on how they regulate polarity and how they are regulated is still incomplete. In this chapter we review our knowledge of the processes of cell polarity establishment and maintenance, and asymmetric cell division in the early C. elegans embryo. We discuss recent findings that highlight new players in cell polarity and/or reveal the molecular details on how PAR proteins regulate polarity processes.     

Use of Saccharomyces cerevisiae as a whole cell system for aldol condensation in organic medium: Study of the factors affecting the biotransformation

Saccharomyces cerevisiae has been employed as a whole cell catalyst for a number of asymmetric transformations. This work explores the ability of this microorganism to carry out the asymmetric aldol condensation between 4-nitrobenzaldehyde and acetone. For this purpose, lyophilized cells of the FY86 laboratory strain from stationary phase cultures were employed. This reaction shows stereoselectivity, and its progress is affected by the water concentration in the medium, temperature and the growth stage of the yeast culture. Cell lysis experiments indicate that activity responsible for this biotransformation is located in the soluble fraction.     

EGF receptor activation: push comes to shove

A study by Zhang et al. (2006) in this issue of Cell provides compelling evidence that the tyrosine kinase domain of the epidermal growth factor receptor (EGFR) is activated by the formation of an asymmetric dimer, with one kinase domain in the EGF-mediated dimer activating the other through an allosteric mechanism.     

LIS1 and spindle orientation in neuroepithelial cells

Asymmetric stem cell division is thought to require precise orientation of the mitotic spindle. However, a recent study in Cell (Yingling et al., 2008) analyzes the role of LIS1 in the developing mouse brain and shows that spindle orientation is more important during early, symmetric progenitor cell divisions than for later asymmetric divisions.     

Vinegar flies turn to porto for cell division cycle meeting

The Third International Workshop on Drosophila Cell Division Cycle brought together researchers focusing on DNA replication, mitosis, meiosis, cell cycle regulation, checkpoints, asymmetric division, cell and tissue growth, and tumorgenesis. This review describes new findings presented at the meeting that particularly highlight the advantages of the Drosophila systems.     

Microtubule network asymmetry in motile cells: Role of Golgi-derived array

Cell migration requires polarization of the cell into the leading edge and the trailing edge. Microtubules (MTs) are indispensable for polarized cell migration in the majority of cell types. To support cell polarity, MT network has to be functionally and structurally asymmetric. How is this asymmetry achieved? In interphase cells, MTs form a dynamic system radiating from a centrosome-based MT-organizing center (MTOC) to the cell edges. Symmetry of this radial array can be broken according to four general principles. Asymmetry occurs due to differential modulation of MT dynamics, relocation of existing MTs within a cell, adding an asymmetric nucleation site, and/or repositioning of a symmetric nucleation site to one side of a cell. Combinations of these asymmetry regulation principles result in a variety of asymmetric MT networks typical for diverse motile cell types. Importantly, an asymmetric MT array is formed at a non-conventional MT nucleation site, the Golgi. Here, we emphasize the contribution of this array to the asymmetry of MT network.     

A Cell Kinetic and Cytological Study on the Asymmetric Cell Division of Thymic Lymphoblasts of the Embryonic Rat

Cell division of thymus lymphoid cells from embrynonic and young rats was investigated cytologically on cell smears, focusing attention on asymmetric cell division. Some of thymic lymphoblasts displayed features implicating asymmetric cell division. At the telophase of such cells, two immature daughter cells looked dissimilar: one of them was smaller in size and possessed a more condensed nucleus, compared with the counterpart cell. Furthrmore, in most cases the cytoplasm of the smaller daughter cell was stained with Giemsa more deeply. It was suggested that the asymmetry of the nucleus emerges at anaphase and telophase probably due to some polarized situation of the cytoplasm. Asymmetrically-dividing cells were relatively frequently observed during the developmental period when large lymphoblasts actively transform into smaller lymphocytes :16% to 17% of whole dividing cells were under asymmetric cell division on days 16 and 17 of gestation, while less than 5% on day 19 or thereafter. In correlation with this observation, asymmetrically-dividing cells were more frequently observed among large lymphoblasts than among other smaller cell fractions. These results support the view that the asymmetric cell division may play some essential role in the transformation of large lymphoblasts into smaller lymphocytes.     

Photoregulation of asymmetric cell division followed by rhizoid development in the fern Ceratopteris prothalli

Strap-shaped prothalli of CERATOPTERIS: richardii grown in the dark have an apical meristem, a subapical elongation zone and a basal growth cessation zone [Murata et al. (1997) Plant Cell Physiol. 38: 201]. When the dark-grown prothalli were irradiated with continuous white light, marginal cells of the elongation zone divided asymmetrically, and the resulting smaller cells developed into rhizoids. The asymmetric division was also induced by brief irradiation of red light. The effect of red light was cancelled by subsequent irradiation of far-red light, indicating that the asymmetric division was regulated by phytochrome. Since the response to red light was not observed at 10(1) J m(-2) and saturated at 10(2) J m(-2) and the response is photoreversible by far-red light, the photoresponse was classified as a low-fluence response of phytochrome. Although the asymmetric division was induced by brief irradiation of red light, continuous irradiation of white, blue or red light was necessary to induce rhizoid growth. These results indicate that asymmetric division and subsequent cell growth are independently regulated by light in CERATOPTERIS: prothalli.     

Intestinal stem cell asymmetric division in the Drosophila posterior midgut

Over the past 2 years, our understanding of intestinal stem cells in the Drosophila posterior midgut has advanced greatly. In this review, I will focus on the establishment of these stem cells in their niche during development and the molecular mechanisms that regulate their asymmetric division in adults. J. Cell. Physiol. 224: 581–584, 2010. Published 2010 Wiley‐Liss, Inc.     

Gibberellin-regulated XET is differentially induced by auxin in rice leaf sheath bases during gravitropic bending

The asymmetric distribution of auxin plays a fundamental role in plant gravitropism, yet little is understood about how its lateral distribution stimulates growth. In the present work, the asymmetric distribution not only of auxin, but also that of gibberellins (GAs), was observed in rice leaf sheath bases following gravistimulation. Gravistimulation induced the transient accumulation of greater amounts of both IAA and GA in the lower halves of the leaf sheath bases of rice seedlings. OsGA3ox1, a gene of active GA synthesis, was differentially induced by gravistimulation. Furthermore, 2,3,5-tri-iodobenzoic acid (TIBA), an inhibitor of auxin transport, substantially decreased the asymmetric distribution of IAA and the gradient of OsGA3ox1 expression. Externally applied GA(3) restored the gravitropic curvature of rice leaf sheaths inhibited by either TIBA or by ancymidol, a GA synthesis inhibitor. The expression of XET (encoding xyloglucan endotransglycosylase) was differentially induced in the lower halves of gravistimulated leaf sheath bases and was also up-regulated by exogenous IAA and GA(3). Both ancymidol and TIBA decreased the gradient of XET expression. These data suggest that the asymmetric distribution of auxin effected by gravistimulation induced a gradient of GAs via asymmetric expression of OsGA3ox1 in rice leaf sheath bases, and hence caused the asymmetric expression of XET. Cell wall loosening in the curvature site of the leaf sheath triggered by the expression of XET would contribute to gravitropic growth.     

Hydrophilic and hydrophobic attachment of both globular and asymmetric acetylcholinesterase to frog muscle basal lamina sheaths

We prepared myofiber basal lamina sheaths (BLs) using the in vivo experimental procedure of Sanes et al. (J. Cell Biol.78, 176–198, 1978) on frog cutaneus pectoris muscle. On the 15 days post-operatively, acetylcholinesterase (AChE) is still found concentrated in native BLs and purified BLs preparations and both globular and asymmetric molecular forms coexist (Nicolet et al., J. Cell Biol., 107, 762–768, 1986). We describe here at least two distinct AChE pools, according to their differential solubility in non-ionic detergent and high-salt media. One is detergent-extracted (DE) and the other is detergent-insoluble, high-salt extracted (HSS). In the BLs preparation as well as in control motor end-plate rich regions (MEP-r) of muscle, both globular and asymmetric forms of AChE are found as DE and HSS variants. These observations suggest that all AChE forms are present in the extracellular muscle basal lamina and are bound through not only hydrophilic but also hydrophobic bonds, to probably distinct structural domains of the muscle basal lamina.