Asymmetric localization of a mammalian numb homolog during mouse cortical neurogenesis

Numb-associated kinase interacts with the phosphotyrosine binding domain of Numb and antagonizes the function of Numb in vivo

During asymmetric cell division, the membrane-associated Numb protein localizes to a crescent in the mitotic progenitor and is segregated predominantly to one of the two daughter cells. We have identified a putative serine/threonine kinase, Numb-associated kinase (Nak), which interacts physically with the phosphotyrosine binding (PTB) domain of Numb. The PTB domains of Shc and insulin receptor substrate bind to an NPXY motif which is not present in the region of Nak that interacts with Numb PTB domain. We found that the Numb PTB domain but not the Shc PTB domain interacts with Nak through a peptide of 11 amino acids, implicating a novel and specific protein-protein interaction. Overexpression of Nak in the sensory organs causes both daughters of a normally asymmetric cell division to adopt the same cell fate, a transformation similar to the loss of numb function phenotype and opposite the cell fate transformation caused by overexpression of Numb. The frequency of cell fate transformation is sensitive to the numb gene dosage, as expected from the physical interaction between Nak and Numb. These findings indicate that Nak may play a role in cell fate determination during asymmetric cell divisions.

Mechanisms of asymmetric cell division during animal development

Recent years have seen the discovery of machineries for asymmetric cell division in a number of different organisms. The Inscuteable protein is a central component of such a machinery in Drosophila. Within dividing Drosophila neural precursor cells, Inscuteable directs both the orientation of the mitotic spindle and the asymmetric segregation of the proteins Numb, Prospero and Miranda into one of the two daughter cells. Numb can act by repressing signalling via the transmembrane receptor Notch, whereas Miranda localizes the transcription factor Prospero which initiates daughter cell specific gene expression. The identification of Numb homologs in other species has suggested that this machinery might be conserved from Drosophila to vertebrates.

Segregation of myogenic lineages in Drosophila requires numb

Terminal divisions of myogenic lineages in the Drosophila embryo generate sibling myoblasts that found larval muscles or form precursors of adult muscles. Alternative fates adopted by sibling myoblasts are associated with distinct patterns of gene expression. Genes expressed in the progenitor cell are maintained in one sibling and repressed in the other. These differences depend on an asymmetric segregation of Numb between sibling cells. In numb mutants, muscle fates associated with repression are duplicated and alternative muscles are lost. If numb is overexpressed the reverse transformation occurs. Numb acts to block Notch-mediated repression of genes expressed in muscle progenitor cells. Thus asymmetric cell divisions are essential determinants of muscle fates during myogenesis in Drosophila

In vitro pattern formation during neurogenesis in neuroectodermal progenitor cells immortalized by p53-deficiency

In vitro neural differentiation was induced in a p53-deficient immortalized neuroectodermal progenitor cell line, NE-4C, by treatment with retinoic acid [K. Schlett and E. Madarász (1997) J. Neurosci. Res. 47, 405-416]. Rearrangement of nestin filaments was an early marker of neuron-formation. The increase in neurofilament protein content was accompanied by a decrease in the expression of nestin filaments in induced precursors. Cells with astroglial features appeared with a delay of 4-5 days compared to the appearence of neurons. Future neurons were sorted out from the substrate-attached population of apparently non-induced cells. The sorting out of future neurons resembled the separation of neural precursors in vivo. The continuous changes in the shape and also in the position of the cells resulted in the formation of characteristic morphological patterns. On the basis of morphological changes, five characteristic stages of in vitro neural differentiation were distinguished. The analysis of the morphological changes revealed that cell-to-cell interactions played an essential role in the cell fate decision made by induced precursors. Our observations indicate that the NE-4C cell line can serve as an in vitro model to investigate some early steps of neurogenesis.

Binding specificity and in vivo targets of the EH domain, a novel protein-protein interaction module

EH is a recently identified protein-protein interaction domain found in the signal transducers Eps15 and Eps15R and several other proteins of yeast nematode. We show that EH domains from Eps15 and Eps15R bind in vitro to peptides containing an asparagine-proline-phenylalanine (NPF) motif. Direct screening of expression libraries with EH domains yielded a number of putative EH interactors, all of which possessed NPF motifs that were shown to be responsible for the interaction. Among these interactors were the human homolog of NUMB, a developmentally reguated gene of Drosophila, and RAB, the cellular cofactor of the HIV REV protein. We demonstrated coimmunoprecipitation of Eps15 with NUMB and RAB. Finally, in vitro binding of NPF-containing peptides to cellular proteins and EST database screening established the existence of a family of EH-containing proteins in mammals. Based on the characteristics of EH-containing and EH-binding proteins, we propose that EH domains are involved in processes connected with the transport and sorting of molecules within the cell.

Spectrosomes and fusomes anchor mitotic spindles during asymmetric germ cell divisions and facilitate the formation of a polarized microtubule array for oocyte specification in Drosophila

In the Drosophila ovary, membrane skeletal proteins such as the adducin-like Hts protein(s), spectrin, and ankyrin are found in the spectrosome, an organelle in germline stem cells (GSC) and their differentiated daughter cells (cystoblasts). These proteins are also components of the fusome, a cytoplasmic structure that spans the cystoblast's progeny that develop to form a germline cyst consisting of 15 nurse cells and an oocyte. Spectrosomes and fusomes are associated with one pole of spindles during mitosis and are implicated in cyst formation and oocyte differentiation. Here we show that the asymmetric behavior of the spectrosome persists throughout the cell cycle of GSC. Eliminating the spectrosome by the htsl mutation leads to randomized spindle orientation, suggesting that the spectrosome anchors the spindle to ensure the asymmetry of GSC division; eliminating the fusome in developing cysts results in defective spindles and randomized spindle orientation as well as asynchronous and reduced cystocyte divisions. These observations suggest that fusomes are required for the proper formation and asymmetric orientation of mitotic spindles. Moreover, they reinforce the notion that fusomes are required for the four synchronous divisions of the cystoblast leading to cyst formation. In htsl cysts which lack fusomes and fail to incorporate a hts gene product(s) into ring canals following cyst formation, polarized microtubule networks do not form, the dynamics of cytoplasmic dynein is disrupted, and oskar and orb RNAs fail to be transported to the future oocyte. These observations support the proposed role of fusomes and ring canals in organizing a polarized microtubule-based transport system for RNA localization that leads to oocyte differentiation.

Miranda is required for the asymmetric localization of Prospero during mitosis in Drosophila

Asymmetric division of Drosophila neuroblasts, sensory organ precursor cells, and cells in the procephalic neurogenic region involves the segregation of Numb and Prospero proteins into one of the two daughter cells. We have isolated a novel gene, miranda, based on the ability of its gene product to interact with the Prospero asymmetric localization domain. miranda expression coincides spatially and temporally with asymmetric cell divisions and asymmetric localization of Prospero. Miranda protein is localized asymmetrically, along with Prospero, to the basal cell membrane during mitosis. Loss of miranda gene function abolishes asymmetric Prospero localization during mitosis. The asymmetric localization of Miranda protein requires inscuteable. Our results suggest that miranda functions downstream of inscuteable and works as an adapter that connects Prospero to the basal cell membrane during asymmetric cell division.

Response diversity and the timing of progenitor cell maturation are regulated by developmental changes in EGFR expression in the cortex

Early cortical progenitor cells of the ventricular zone (VZ) differ from later progenitor cells of the subventricular zone (SVZ) in cell-type generation and their level of epidermal growth factor receptors (EGFRs). To determine whether differences in their behavior are causally related to EGFR number/density, we introduced extra EGFRs into VZ cells with a retrovirus in vivo and in vitro. This results in premature expression of traits characteristic of late SVZ progenitor cells, including migration patterns, differentiation into astrocytes, and proliferation of multipotential cells to form spheres. The choice between proliferation and differentiation depends on ligand concentration and progenitor cell age and may reflect different thresholds of stimulation. The level of EGFRs expressed by progenitor cells in the cortex may therefore contribute to the timing of their maturation and choice of response to pleiotropic environmental signals.

A novel group of pumilio mutations affects the asymmetric division of germline stem cells in the Drosophila ovary

Germline stem cells play a pivotal role in gametogenesis; yet little is known about how they are formed, how they divide to self-renew, and how these processes are genetically controlled. Here we describe the self-renewing asymmetric division of germline stem cells in the Drosophila ovarian germline, as marked by the spectrosome, a cytoplasmic structure rich in membrane skeletal proteins. The ontogeny of the spectrosome marks the lineage of germline stem cells. We identified two new groups of mutations in which the divisional asymmetry is disrupted. The first, which we refer to as ovarette (ovt) mutations, was shown to correspond to a novel class of mutations in the pumilio locus. Since pumilio is known to posttranscriptionally repress the expression of target genes at earlier stages of germ cell development, our results suggest that a similar activity is needed to maintain germ line stem cells. We have also identified a second and novel gene, piwi, whose mutations abolish germline stem cell division.

Clonal dispersion and evidence for asymmetric cell division in ferret cortex

Cell lineage analysis with retroviral libraries suggests that clonal progeny disperse widely in rodent cortex. To determine whether widespread dispersion is a general mammalian plan and to investigate phylogenetic differences in cortical development, we analyzed cell lineage in the ferret, a carnivore and near relative of the cat. The ferret possesses a highly developed, folded cerebral cortex, characteristic of higher mammalian species. Progenitor cells of the ferret cerebral cortex were tagged with an amphotropic retroviral library encoding alkaline phosphatase, and sibling relationships were determined using the polymerase chain reaction. Neuronal clones were single neurons (52%) or large clones (48%; average, 7 neurons) containing neurons and glia in widespread cortical locations. Neuronal clones in the ferret labeled at middle to late neurogenesis (embryonic day 33–35) contained large numbers of neurons and showed little tendency to cluster. The large proportion of single neuron clones, contrasted with the large size of multicell clones, suggests that some progenitors divide asymmetrically, producing a postmitotic neuron and regenerating a multipotential cell.

Differential expression of mammalian Numb, Numblike and Notch1 suggests distinct roles during mouse cortical neurogenesis

During Drosophila neurogenesis, asymmetric cell divisions are achieved by differential segregation of Numb (d-Numb) into one of the daughter cells to cause a bias in the Notch mediated cell-cell interaction. We have isolated a second mammalian gene with significant sequence similarity to d-numb, mouse numblike. When expressed in dividing neural precursors in Drosophila, Numblike is symmetrically distributed in the cytoplasm, unlike endogenous d-Numb or expressed mouse Numb (m-Numb), both of which are asymmetrically localized to one half of the cell membrane. In d-numb loss-of-function mutant embryos, expression of Numblike allows both daughter cells of a neural precursor to adopt the fate of the cell that normally inherits d-Numb. In mice, numblike mRNA is preferentially expressed in adult and embryonic nervous system. In the developing neocortex, Numblike is expressed in postmitotic neurons in the cortical plate, but not in progenitors within the ventricular zone where m-Numb and Notch1 are expressed. We have also found that, in dividing cortical progenitors, Notch1 is distributed around the entire membrane, unlike m-Numb which is asymmetrically localized to the apical membrane. We propose that an interplay between cell-intrinsic mechanisms (executed by m-numb and numblike) and cell-extrinsic mechanisms (mediated by Notch1) may be involved in both progenitor cell proliferation and neuronal differentiation during mammalian cortical neurogenesis.

Cell fate specification and symmetrical/asymmetrical divisions in the developing cerebral cortex

Two different modes of cell division are adopted by progenitor cells to generate the neurons and glia of the cerebral cortex: they either divide symmetrically to generate other progenitors or a pair of postmitotic cells or divide asymmetrically to generate both a progenitor and a postmitotic cell. In this study we used a lineage marker, the BAG retrovirus, in embryonic day 16 rats in combination with bromodeoxyuridine (BrdU) to identify patterns of cell generation in the cerebral cortex, and investigated the relationship between the phenotype of cells and the history of their lineages. The location, phenotype and birth order of clonally related cells were studied in the subsequent 3 weeks. Only pyramidal neurons and/or astrocytes formed discrete clusters in which several generations of family members were present, whereas nonpyramidal neurons were found exclusively in pairs or as single cells. Analysis of BrdU levels in these cells showed that nonpyramidal neurons were originally part of larger clones and were found dispersed in the neocortex because of tangential migration of their progenitors, dispersion of postmitotic cells, or death of clonal relatives. These results suggest that both symmetrical and asymmetrical division can be adopted by progenitor cells to generate cortical neurons and glial cells and that cell extrinsic events contribute to the isolation of nonpyramidal neurons.

Stem cells in the central nervous system

In the vertebrate central nervous system, multipotential cells have been identified in vitro and in vivo. Defined mitogens cause the proliferation of multipotential cells in vitro, the magnitude of which is sufficient to account for the number of cells in the brain. Factors that control the differentiation of fetal stem cells to neurons and glia have been defined in vitro, and multipotential cells with similar signaling logic can be cultured from the adult central nervous system. Transplanting cells to new sites emphasizes that neuroepithelial cells have the potential to integrate into many brain regions. These results focus attention on how information in external stimuli is translated into the number and types of differentiated cells in the brain. The development of therapies for the reconstruction of the diseased or injured brain will be guided by our understanding of the origin and stability of cell type in the central nervous system.

Cell fate determination in Drosophila

A major issue in development is to understand how local heterogeneities are interpreted to determine specific cell fates. The sense organs of Drosophila provide an accessible system for addressing this issue. Most sense organs comprise four types of cells, and their differentiation is the outcome of a complex developmental programme comprising several steps. Recent results illuminate, for several of these steps, the nature of the local heterogeneities and the mechanism used to interpret them in terms of cell fate decisions.

Asymmetric division and polarity of neuroepithelial cells

Neuroepithelial cells, the progenitors to the CNS neurons and glia, undergo both symmetric and asymmetric divisions. Symmetric divisions underlie the proliferation of neuroepithelial cells that predominates early in CNS development. Asymmetric divisions are thought to generate the cell types derived from neuroepithelial cells, such as neurons. Insight into the mechanism of asymmetric division of neuroepithelial cells has come from two lines of research, the study of their epithelial polarity and the analysis of the expression of vertebrate homologues of proteins known to be involved in cell fate determination in Drosophila.

Metaphase spindles rotate in the neuroepithelium of rat cerebral cortex

Time-lapse confocal microscopy has been used to image cells in mitosis at the apical surface of neuroepithelium from the rat cerebral cortex during the period of neurogenesis. Staining with vital chromatin dyes reveals that mitotic spindles that are aligned parallel to the surface of the tissue are highly motile, rotating within the plane of the epithelium throughout metaphase, and come to rest only as anaphase begins. Spindles may make several complete turns, parallel to the epithelium, but only rarely tumble into an orientation perpendicular to the epithelial sheet. Analysis shows that spindles do not rotate randomly; rather, they spend most of their time aligned parallel or antiparallel to the direction in which they will later enter anaphase and undergo cell division. This conclusion is strongly supported by statistical analyses of the data. Stereotyped movements of this kind show that the direction of division is determined early in mitosis. This suggests the existence of intracellular and perhaps intercellular signals that define the polarity of the cell both in the apico-basal direction and within the plane of the epithelium. Such mechanisms may be important for maintaining the structure of the epithelium and cell-cell communication during development and may also provide a mechanism for the precise distribution of cytoplasmic determinants that might influence the fate of the daughter cells at a time when neuronal fate is being determined.

The ins and outs of notch signaling

The Notch gene encodes a cell surface protein that regulates cell fate choices in vertebrates and invertebrates. Given the wide variety of cell types influenced by Notch, it would seem that the signal relayed through Notch activation is not an instructive one per se. Rather, Notch signaling is thought to influence the cell's ability to respond to instructive signals responsible for specific cell fates. Expression and functional studies of Notch support this idea; however, the possibility of additional functions for Notch cannot be excluded. Much of what we know about the Notch signaling pathway comes from studies with Drosophila Notch and the Caenorhabditis elegans Notch-related genes lin-12 and glp-1. With the isolation of multiple vertebrate Notch genes we are beginning to understand and define Notch signaling in vertebrates as well. A number of excellent reviews have been published summarizing the current status of Notch/LIN-12/GLP-1 signaling in Drosophila and C. elegans, as well as recent findings with the vertebrate counterparts. Here I review the structure of the various Notch proteins and their putative ligands, and discuss possible interactions between Notch, its ligands, and other cellular components that affect Notch signal transduction. A role for Notch signaling during normal development and in disease processes is discussed in an accompanying review by T. Gridley (1997, Mol. Cell. Neurosci. 9: 103-108).

Neuroblasts: a model for the asymmetric division of stem cells

The ability of stem cells to self-renew has hung been attributed to an asymmetry in division that generates one daughter cell identical to the mother and another cell committed to differentiation. Recent studies on neuroblasts, a group of neural stem cells responsible for generating various neurons and glial cells in the central nervous system, have revealed exciting mechanisms that underlie self-renewing asymmetric division. Several important localized cell fate determinants have been characterized, and their segregation mechanism has been explored in the context of cytoskeletal organization, cell-cycle type progression, cytokinesis and mitotic orientation. These findings are illuminating in understanding the general mechanism of stem cell division.

Diversity and pattern in the developing spinal cord

The generation of distinct neuronal cell types in appropriate numbers and at precise positions underlies the assembly of neural circuits that encode animal behavior. Despite the complexity of the vertebrate central nervous system, advances have been made in defining the principles that control the diversification and patterning of its component cells. A combination of molecular genetic, biochemical, and embryological assays has begun to reveal the identity and mechanism of action of molecules that induce and pattern neural tissue and the role of transcription factors in establishing generic and specific neuronal fates. Some of these advances are discussed here, focusing on the spinal cord as a model system for analyzing the molecular control of central nervous system development in vertebrates.

The Drosophila Numb protein inhibits signaling of the Notch receptor during cell-cell interaction in sensory organ lineage

Specification of unequal daughter cell fates in the Drosophila external sense organ lineage requires asymmetric localization of the intrinsic determinant Numb as well as cell-cell interactions mediated by the Delta ligand and Notch receptor. Previous genetic studies indicated that numb acts upstream of Notch, and biochemical studies revealed that Numb can bind Notch. For a functional assay of the action of Numb on Notch signaling, we expressed these proteins in cultured Drosophila cells and used nuclear translocation of Suppressor of Hairless [Su(H)] as a reporter for Notch activity. We found that Numb interfered with the ability of Notch to cause nuclear translocation of Su(H); both the C-terminal half of the phosphotyrosine binding domain and the C terminus of Numb are required to inhibit Notch. Overexpression of Numb during wing development, which is sensitive to Notch dosage, revealed that Numb is also able to inhibit the Notch receptor in vivo. In the external sense organ lineage, the phosphotyrosine binding domain of Numb was found to be essential for the function but not for asymmetric localization of Numb. Our results suggest that Numb determines daughter cell fates in the external sense organ lineage by inhibiting Notch signaling.

Asymmetric cell division and neurogenesis

Several proteins have been identified that are asymmetrically localized in dividing Drosophila and vertebrate neural precursors. Some are involved in establishing cell polarity, whereas others are determinants of cell fate. New insights into how cell diversity is generated during neurogenesis have been obtained and the means for rapid progress in this field now exist.

Control of daughter cell fates during asymmetric division: interaction of Numb and Notch

During development of the Drosophila peripheral nervous system, a sensory organ precursor (SOP) cell undergoes rounds of asymmetric divisions to generate four distinct cells of a sensory organ. Numb, a membrane-associated protein, is asymmetrically segregated into one daughter cell during SOP division and acts as an inherited determinant of cell fate. Here, we show that Notch, a transmembrane receptor mediated cell-cell communication, functions as a binary switch in cell fate specification during asymmetric divisions of the SOP and its daughter cells in embryogenesis. Moreover, numb negatively regulates Notch, probably through direct protein-protein interaction that requires the phosphotyrosine-binding (PTB) domain of Numb and either the RAM23 region or the very C-terminal end of Notch. Notch then positively regulates a transcription factor encoded by tramtrack (ttk). This leads to Ttk expression in the daughter cell that does not inherit Numb. Thus, the inherited determinant Numb bestows a bias in the machinery for cell-cell communication to allow the specification of distinct daughter cell fates.

Intravenous and oral practolol in the acute stages of myocardial infarction

The influence of routine administration of the beta adrenoceptor blocking drug practolol on the outcome of acute myocardial infarction has been studied in 94 patients. The study was restricted to patients under the age of 70 experiencing their first myocardial infarction and in whom there was no contraindication to beta blockade. In the treated group an initial dose intravenous practolol 15 mg was followed by five oral doses of practolol 200 mg at 12 h intervals. A significant reduction in heart rate and systolic blood pressure was apparent in the treated group within 2 h. No difference was detectable in the course of the acute stage of the illness between treated and control patients, apart from a significant reduction in the incidence of atrial fibrillation among those receiving practolol. Patients with inferior infarctions showed a tendency to develop potentially harmful bradycardia and hypotension on receiving practolol which lead to withdrawal of the drug in many cases. At regular review over 7 mth no detectable difference emerged between the treated and control groups in the incidence of cardiac failure, death or reinfarction.