Strabismus regulates asymmetric cell divisions and cell fate determination in the mouse brain

The planar cell polarity (PCP) pathway organizes the cytoskeleton and polarizes cells within embryonic tissue. We investigate the relationship between PCP signaling and cell fate determination during asymmetric division of neural progenitors (NPs) in mouse embryos. The cortex of Lp/Lp (Loop-tail) mice deficient in the essential PCP mediator Vangl2, homologue of Drosophila melanogaster Strabismus (Stbm), revealed precocious differentiation of neural progenitors into early-born neurons at the expense of late-born neurons and glia. Although Lp/Lp NPs were easily maintained in vitro, they showed premature differentiation and loss of asymmetric distribution of Leu-Gly-Asn–enriched protein (LGN)/partner of inscuteable (Pins), a regulator of mitotic spindle orientation. Furthermore, we observed a decreased frequency in asymmetric distribution of the LGN target nuclear mitotic apparatus protein (NuMa) in Lp/Lp cortical progenitors in vivo. This was accompanied by an increase in the number of vertical cleavage planes typically associated with equal daughter cell identities. These findings suggest that Stbm/Vangl2 functions to maintain cortical progenitors and regulates mitotic spindle orientation during asymmetric divisions in the vertebrate brain.     

Cortical development and asymmetric cell divisions

The development of the mammalian neocortex involves rounds of symmetric and asymmetric cell division of neural progenitors to fulfill needs of both self-renewal of progenitors and production of differe...     

Frizzled regulates localization of cell-fate determinants and mitotic spindle rotation during asymmetric cell division

Cell-fate diversity is generated in part by the unequal segregation of cell-fate determinants during asymmetric cell divisions. In the Drosophila pupa, the pI sense organ precursor cell is polarized along the anterior-posterior axis of the fly and divides asymmetrically to generate a posterior pIIa cell and an anterior pIIb cell. The anterior pIIb cell specifically inherits the determinant Numb and the adaptor protein Partner of Numb (Pon). By labelling both the Pon crescent and the microtubules in living pupae, we show that determinants localize at the anterior cortex before mitotic-spindle formation, and that the spindle forms with random orientation and rotates to line up with the Pon crescent. By imaging living frizzled (fz) mutant pupae we show that Fz regulates the orientation of the polarity axis of pI, the initiation of spindle rotation and the unequal partitioning of determinants. We conclude that Fz participates in establishing the polarity of pI.     

Drosophila Anillin is unequally required during asymmetric cell divisions of the PNS

During Drosophila embryogenesis, timely and orderly asymmetric cell divisions ensure the correct number of each cell type that make up the sensory organs of the larval PNS. We report a role of scraps, Drosophila Anillin, during these divisions. Anillin, a constitutive member of the contractile ring is essential for cytokinesis in Drosophila and vertebrates. During embryogenesis we find that zygotically transcribed scraps is required specifically for the unequal cell divisions, those in which cytokinesis occurs in an “off-centred” manner, of the pIIb and pIIIb neuronal precursor cells, but not the equal cell divisions of the lineage related precursor cells. Complementation and genetic rescue studies demonstrate this effect results from zygotic scraps and leads to polyploidy, ectopic mitosis, and loss of the neuronal precursor daughter cells. The net result of which is the formation of incomplete sense organs and embryonic lethality.     

中枢神经系统发育的不对称细胞分裂机制

无论在无脊椎动物还是脊椎动物中,组成中枢神经系统（CNS）的大多数细胞都是由极性神经祖细胞不对称分裂而来。通过简要综述果蝇（Drosophila melanogaste）成神经母细胞（NB）不对称分裂机制,并与近年来在脊椎动物不对称细胞分裂上取得的研究成果相比较,尝试找出两个系统的相似性和相异性。     

不对称分裂中命运决定子的调节机制

不对称细胞分裂是果蝇等无脊椎动物以及脊椎动物神经发生过程中神经干细胞分化的基本机制.命运决定子的极性定位及其选择性分配,作为不对称细胞分裂中的重要环节,在子细胞命运决定方面发挥至关重要的作用.本文综述了在中枢及外周神经系统发育期间,不对称分裂中调节Numb等命运决定子靶向定位的影响因素及命运决定子的效应机制,并简要探讨命运决定子调节机制的进化保守性.     

Cell cycle and cell-fate determination in Drosophila neural cell lineages

"Normal" development requires a finely tuned equilibrium between cell differentiation and cell proliferation. Important issues in development include whether the cell cycle controls the cell-fate determination and whether cell identity in turn regulates cell-cycle progression. Although, these issues are of general biological relevance, stereotyped Drosophila neural lineages are particularly suited to address these questions and have provided insights into the links between cell-cycle progression and cell-fate specification.     

discordia mutations specifically misorient asymmetric cell divisions during development of the maize leaf epidermis.

In plant cells, cytokinesis depends on a cytoskeletal structure called a phragmoplast, which directs the formation of a new cell wall between daughter nuclei after mitosis. The orientation of cell division depends on guidance of the phragmoplast during cytokinesis to a cortical site marked throughout prophase by another cytoskeletal structure called a preprophase band. Asymmetrically dividing cells become polarized and form asymmetric preprophase bands prior to mitosis; phragmoplasts are subsequently guided to these asymmetric cortical sites to form daughter cells of different shapes and/or sizes. Here we describe two new recessive mutations, discordia1 (dcd1) and discordia2 (dcd2), which disrupt the spatial regulation of cytokinesis during asymmetric cell divisions. Both mutations disrupt four classes of asymmetric cell divisions during the development of the maize leaf epidermis, without affecting the symmetric divisions through which most epidermal cells arise. The effects of dcd mutations on asymmetric cell division can be mimicked by cytochalasin D treatment, and divisions affected by dcd1 are hypersensitive to the effects of cytochalasin D. Analysis of actin and microtubule organization in these mutants showed no effect of either mutation on cell polarity, or on formation and localization of preprophase bands and spindles. In mutant cells, phragmoplasts in asymmetrically dividing cells are structurally normal and are initiated in the correct location, but often fail to move to the position formerly occupied by the preprophase band. We propose that dcd mutations disrupt an actin-dependent process necessary for the guidance of phragmoplasts during cytokinesis in asymmetrically dividing cells.     

Applications of mRNA injections for analyzing cell lineage and asymmetric cell divisions during segmentation in the leech Helobdella robusta

Synthetic mRNAs can be injected to achieve transient gene expression even for 'non-model' organisms in which genetic approaches are not feasible. Here, we have used this technique to express proteins that can serve as lineage tracers or reporters of cellular events in embryos of the glossiphoniid leech Helobdella robusta (phylum Annelida). As representatives of the proposed super-phylum Lophotrochozoa, glossiphoniid leeches are of interest for developmental and evolutionary comparisons. Their embryos are suitable for microinjection, but no genetic approaches are currently available. We have injected segmentation stem cells (teloblasts) with mRNAs encoding nuclear localized green fluorescent protein (nGFP) and its spectral variants, and have used tandem injections of nGFP mRNA followed by antisense morpholino oligomer (AS MO), to label single blast cell clones. These techniques permit high resolution cell lineage tracing in living embryos. We have applied them to the primary neurogenic (N) lineage, in which alternate segmental founder cells (nf and ns blast cells) contribute distinct sets of progeny to the segmental ganglia. The nf and ns blast cell clones exhibit strikingly different cell division patterns: the increase in cell number within the nf clone is roughly linear, while that in the ns clone is almost exponential. To analyze spindle dynamics in the asymmetric divisions of individual blast cells, we have injected teloblasts with mRNA encoding a tau::GFP fusion protein. Our results show that the asymmetric divisions of n blast cells result from a posterior shift of both the spindle within the cell and the midbody within the mitotic spindle, with differential regulation of these processes between nf and ns.     

Drosophila neuroblast asymmetric divisions: cell cycle regulators, asymmetric protein localization, and tumorigenesis

Over the past decade, many of the key components of the genetic machinery that regulate the asymmetric division of Drosophila melanogaster neural progenitors, neuroblasts, have been identified and their functions elucidated. Studies over the past two years have shown that many of these identified components act to regulate the self-renewal versus differentiation decision and appear to function as tumor suppressors during larval nervous system development. In this paper, we highlight the growing number of molecules that are normally considered to be key regulators of cell cycle events/progression that have recently been shown to impinge on the neuroblast asymmetric division machinery to control asymmetric protein localization and/or the decision to self-renew or differentiate.     

Osteogenic differentiation of hypertrophic chondrocytes involves asymmetric cell divisions and apoptosis

We have investigated the early cellular events that take place during the change in lineage commitment from hypertrophic chondrocytes to osteoblast-like cells. We have induced this osteogenic differentiation by cutting through the hypertrophic cartilage of embryonic chick femurs and culturing the explants. Immunocytochemical characterization, [3H]thymidine pulse-chase labeling, in situ nick translation or end labeling of DNA breaks were combined with ultrastructural studies to characterize the changing pattern of differentiation. The first responses to the cutting, seen after 2 d, were upregulation of alkaline phosphatase activity, synthesis of type I collagen and single-stranded DNA breaks, probably indicating a metastable state. Associated with the change from chondrogenic to osteogenic commitment was an asymmetric cell division with diverging fates of the two daughter cells, where one daughter cell remained viable and the other one died. The available evidence suggests that the viable daughter cell then divided and generated osteogenic cells, while the other daughter cell died by apoptosis. The results suggest a new concept of how changes in lineage commitment of differentiated cells may occur. The concepts also reconcile previously opposing views of the fate of the hypertrophic chondrocyte.     

lin-17 mutations of Caenorhabditis elegans disrupt certain asymmetric cell divisions

The identification of a gene necessary for the asymmetry of cell division would be an important first step toward understanding how sister cells come to differ in their developmental fates. The lin-17 gene of the nematode Caenorhabditis elegans is an excellent candidate for being such a gene. lin-17 mutations cause several blast cells that normally generate sister cells of two distinct types to generate instead sister cells of the same type. Moreover, lin-17 mutations cause sister cells to be equal in size as well as equivalent in developmental fate, suggesting that lin-17 acts at or prior to the asymmetric cell division. The lin-17 gene product is involved in asymmetric cell divisions in a variety of tissues, indicating that lin-17 functions in a general mechanism for the establishment of cellular asymmetry in parent cells.     

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.     

The SCARECROW gene's role in asymmetric cell divisions in rice plants

Asymmetric cell division is one of the most important mechanisms in the diversification of cell function and fate. In Arabidopsis, SCARECROW (SCR) is essential for the asymmetric division of the cortex/endodermis progenitor cell in the root. To learn more about how SCR is involved in asymmetric division, we analyzed the rice SCR (OsSCR) expression. In the root tip, OsSCR expression was observed in the endodermal cell layer and downregulated in the daughter cortex cell after asymmetric division, just as with Arabidopsis SCR. In leaf primordia, expression of OsSCR was observed in stomatal and ligule formation. In stomatal development, OsSCR was specifically expressed in the stomatal cell files before formation of guard mother cells (GMCs), and then, its expression was localized in GMCs, when the first asymmetric division occurred to generate the GMCs. Before the second asymmetric division of subsidiary mother cells (SMCs), localized OsSCR expression was observed in SMCs in the area close to the GMCs. Before these asymmetric divisions, the localization of OsSCR mRNA in GMC-forming cells and SMCs was observed in the area of the daughter GMC and subsidiary cells. OsSCR expression was also observed in the initiation area of ligule formation, and its downregulation occurred in the inner L2 cells generated by asymmetric division. Based on these observations, we proposed that OsSCR is involved not only in the asymmetric division of the cortex/endodermis progenitor cell but also during stomata and ligule formation by establishing the polarization of cytoplasm.     

dEHBP1 controls exocytosis and recycling of Delta during asymmetric divisions

Notch signaling governs binary cell fate determination in asymmetrically dividing cells. Through a forward genetic screen we identified the fly homologue of Eps15 homology domain containing protein-binding protein 1 (dEHBP1) as a novel regulator of Notch signaling in asymmetrically dividing cells. dEHBP1 is enriched basally and at the actin-rich interface of pII cells of the external mechanosensory organs, where Notch signaling occurs. Loss of function of dEHBP1 leads to up-regulation of Sanpodo, a regulator of Notch signaling, and aberrant trafficking of the Notch ligand, Delta. Furthermore, Sec15 and Rab11, which have been previously shown to regulate the localization of Delta, physically interact with dEHBP1. We propose that dEHBP1 functions as an adaptor molecule for the exocytosis and recycling of Delta, thereby affecting cell fate decisions in asymmetrically dividing cells.     

Activator of G-protein signaling in asymmetric cell divisions of the sea urchin embryo

An asymmetric fourth cell division in the sea urchin embryo results in formation of daughter cells, macromeres and micromeres, with distinct sizes and fates. Several lines of functional evidence presented here, including pharmacological interference and dominant negative protein expression, indicate that heterotrimeric G protein Gi and its interaction partner, activator of G-protein signaling (AGS), are necessary for this asymmetric cell division. Inhibition of Gi signaling by pertussis toxin interferes with micromere formation and leads to defects in embryogenesis. AGS was isolated in a yeast two-hybrid screen with G alpha i as bait and was expressed in embryos localized to the cell cortex at the time of asymmetric divisions. Introduction of exogenous dominant-negative AGS protein, containing only G-protein regulatory (GPR) domains, selectively prevented the asymmetric division in normal micromere formation. These results support the growing evidence that AGS is a universal regulator of asymmetric cell divisions in embryos.     

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.