Inscuteable-independent apicobasally oriented asymmetric divisions in the Drosophila embryonic CNS

Notch signaling acts before cell division to promote asymmetric cleavage and cell fate of neural precursor cells

Asymmetric cell divisions in the central nervous system generate neurons of diverse fates. In Drosophila melanogaster, the protein Numb localizes asymmetrically to dividing neural precursor cells such that only one daughter cell inherits Numb. Numb inhibits Notch signaling in this daughter cell, resulting in a different cell fate from the Notch-induced fate in the other-Numb-negative-daughter cell. Precursor cells undergo asymmetric cytokinesis generating daughter cells of different sizes. I found that inactivation of Notch in fly embryonic neural precursor cells disrupted the asymmetric positioning of the cleavage furrow and produced daughter cells of the same size and fate. Moreover, inactivation of Notch at different times altered the degree of asymmetric Numb localization, such that earlier inactivation of Notch caused symmetric distribution of Numb and later inactivation produced incomplete asymmetric localization of Numb. The extent of asymmetrically localized Numb positively correlated with the degree of asymmetric cytokinesis and the size disparity in daughter cells. Loss of Numb or expression of constitutively active Notch led to premature specification of the precursor cells into the fate of one of the daughter cells. Thus, in addition to its role in the specification of daughter cell fate after division, Notch controls Numb localization in the precursor cells to determine the size and fate of daughter cells. Numb also inhibits Notch signaling in precursor cells to prevent Notch-induced differentiation of the precursor cell, forming an autoregulatory loop.

Asymmetric cell division and Notch signaling specify dopaminergic neurons in Drosophila

In Drosophila, dopaminergic (DA) neurons can be found from mid embryonic stages of development till adulthood. Despite their functional involvement in learning and memory, not much is known about the developmental as well as molecular mechanisms involved in the events of DA neuronal specification, differentiation and maturation. In this report we demonstrate that most larval DA neurons are generated during embryonic development. Furthermore, we show that loss of function (l-o-f) mutations of genes of the apical complex proteins in the asymmetric cell division (ACD) machinery, such as inscuteable and bazooka result in supernumerary DA neurons, whereas l-o-f mutations of genes of the basal complex proteins such as numb result in loss or reduction of DA neurons. In addition, when Notch signaling is reduced or abolished, additional DA neurons are formed and conversely, when Notch signaling is activated, less DA neurons are generated. Our data demonstrate that both ACD and Notch signaling are crucial mechanisms for DA neuronal specification. We propose a model in which ACD results in differential Notch activation in direct siblings and in this context Notch acts as a repressor for DA neuronal specification in the sibling that receives active Notch signaling. Our study provides the first link of ACD and Notch signaling in the specification of a neurotransmitter phenotype in Drosophila. Given the high degree of conservation between Drosophila and vertebrate systems, this study could be of significance to mechanisms of DA neuronal differentiation not limited to flies.

On the roles of Notch, Delta, kuzbanian, and inscuteable during the development of Drosophila embryonic neuroblast lineages

The generation of cellular diversity in the nervous system involves the mechanism of asymmetric cell division. Besides an array of molecules, including the Par protein cassette, a heterotrimeric G protein signalling complex, Inscuteable plays a major role in controlling asymmetric cell division, which ultimately leads to differential activation of the Notch signalling pathway and correct specification of the two daughter cells. In this context, Notch is required to be active in one sibling and inactive in the other. Here, we investigated the requirement of genes previously known to play key roles in sibling cell fate specification such as members of the Notch signalling pathway, e.g., Notch (N), Delta (Dl), and kuzbanian (kuz) and a crucial regulator of asymmetric cell division, inscuteable (insc) throughout lineage progression of 4 neuroblasts (NB1-1, MP2, NB4-2, and NB7-1). Notch-mediated cell fate specification defects were cell-autonomous and were observed in all neuroblast lineages even in cells born from late ganglion mother cells (GMC) within the lineages. We also show that Dl functions non-autonomously during NB lineage progression and clonal cells do not require Dl from within the clone. This suggests that within a NB lineage Dl is dispensable for sibling cell fate specification. Furthermore, we provide evidence that kuz is involved in sibling cell fate specification in the central nervous system. It is cell-autonomously required in the same postmitotic cells which also depend on Notch function. This indicates that KUZ is required to facilitate a functional Notch signal in the Notch-dependent cell for correct cell fate specification. Finally, we show that three neuroblast lineages (NB1-1, NB4-2, and NB7-1) require insc function for sibling cell fate specification in cells born from early GMCs whereas insc is not required in cells born from later GMCs of the same lineages. Thus, there is differential requirement for insc for cell fate specification depending on the stage of lineage progression of NBs.

Generating asymmetry: with and without self-renewal

At some point during the history of organismal evolution, unicellular, unipotent and mitotically active cells acquired an ability to undergo a special type of cell division called asymmetric division. By this special type of cell division, these cells could divide to generate two different progeny or to self-renew and at the same time generate a progeny that is committed to become a cell different from the mother cell. This type of cell division, which forms the basis for the functioning of totipotent or multipotent stem cells, underlies the fundamental basis for the developmental evolution of organisms. It is not clear if the asymmetric division without self-renewal preceded the asymmetric division with self-renewal. It is reasonable to assume that the asymmetric division without self-renewal preceded the asymmetric division with self-renewal. In this review we explore the genetic regulation of these two types of asymmetric divisions using the Drosophila central nervous system (CNS) as a model system. The results from recent studies argue that for cells to undergo a self-renewing asymmetric division, certain "stem cell" proteins must be maintained or up-regulated, while genes encoding proteins responsible for differentiation must be repressed or down-regulated. As long as a balance between these two classes of proteins is maintained via asymmetric segregation and activation/repression, the progeny that receives stem cell proteins/maintains stem cell competence will have the potential to undergo self-renewing asymmetric division. The other progeny will commit to differentiate. In non-self-renewing asymmetric division, down-regulation of stem cell proteins/competence combined with asymmetric segregation of cell identity specifying factors (either cell-autonomous or a combination of cell autonomous and non-cell autonomous signals) cause the two progeny to assume different differentiated identities. Identification of mutations that confer a stem cell type of division to nonstem cell precursors, or mutations that eliminate asymmetric division, has led the way in elucidating the molecular basis for these divisions. Given that there is a considerable degree of conservation of genes and their function, these studies should provide clear insight into how the self-renewing asymmetric division of stem cells in neural and other lineages is regulated not only in Drosophila but also in vertebrates including humans.

Contrasting mechanisms of stem cell maintenance in Drosophila

Stem cells are self-renewing multipotent cells essential for development or homeostasis of many tissues. Stem cell populations can be found in most multicellular plants and animals. The mechanisms by which these populations are maintained are diverse, utilizing both intrinsic and extrinsic factors to regulate cell division and differentiation. The genetic tools of the fruitfly, Drosophila melanogaster, have permitted detailed characterization of two stem cell populations. In this review, we will examine these contrasting stem cell model systems from Drosophila and their relevance to stem cell populations in other organisms.

Looking back to the embryo: defining transcriptional networks in adult myogenesis

Skeletal muscle has an intrinsic capacity for regeneration following injury or exercise. The presence of adult stem cells in various tissues with myogenic potential provides new opportunities for cell-based therapies to treat muscle disease. Recent studies have shown a conserved transcriptional hierarchy that regulates the myogenic differentiation of both embryonic and adult stem cells. Importantly, the molecules and signalling pathways that induce myogenic determination in the embryo might be manipulated or mimicked to direct the differentiation of adult stem cells either in vivo or ex vivo.

Sticky business: orchestrating cellular signals at adherens junctions

Cohesive sheets of epithelial cells are a fundamental feature of multicellular organisms and are largely a product of the varied functions of adherens junctions. These junctions and their cytoskeletal associations contribute heavily to the distinct shapes, polarity, spatially oriented mitotic spindle planes, and cellular movements of developing tissues. Deciphering the underlying mechanisms that govern these conserved cellular rearrangements is a prerequisite to understanding vertebrate morphogenesis.