Drosophila Neuroblast Asymmetric Cell Division: Recent Advances and Implications for Stem Cell Biology

Cytoarchitecture of mouse and human subventricular zone in developing cerebral neocortex

During cerebral neocortical development, excitatory neurons are generated from radial glial cells in the ventricular zone (VZ) or from secondary progenitor cells in the subventricular zone (SVZ); these neurons then migrate toward the pial surface. We have observed that post-mitotic neurons generated directly in the VZ accumulated just above the VZ with a multipolar morphology, while secondary progenitor cells having a long ascending process left the VZ faster than the post-mitotic neurons. Recent observations of human developing neocortex have revealed the existence of radial glia-like progenitors (oRG cells) in the SVZ. This type of progenitor was first thought to be human specific; however, similar cells have also been found in mouse neocortex, and the morphology of these cells resembled that of some of the secondary progenitor cells that we had previously observed, suggesting the existence of a common architecture for the developing neocortex among mammals. In this review, we discuss the nature of the SVZ and its similarities and differences between humans and mice.

Extracellular matrix and its receptors in Drosophila neural development

Extracellular matrix (ECM) and matrix receptors are intimately involved in most biological processes. The ECM plays fundamental developmental and physiological roles in health and disease, including processes underlying the development, maintenance, and regeneration of the nervous system. To understand the principles of ECM-mediated functions in the nervous system, genetic model organisms like Drosophila provide simple, malleable, and powerful experimental platforms. This article provides an overview of ECM proteins and receptors in Drosophila. It then focuses on their roles during three progressive phases of neural development: (1) neural progenitor proliferation, (2) axonal growth and pathfinding, and (3) synapse formation and function. Each section highlights known ECM and ECM-receptor components and recent studies done in mutant conditions to reveal their in vivo functions, all illustrating the enormous opportunities provided when merging work on the nervous system with systematic research into ECM-related gene functions.

Robust spindle alignment in Drosophila neuroblasts by ultrasensitive activation of pins

Cellular signaling pathways exhibit complex response profiles with features such as thresholds and steep activation (i.e., ultrasensitivity). In a reconstituted mitotic spindle orientation pathway, activation of Drosophila Pins (LGN in mammals) by Gαi is ultrasensitive (apparent Hill coefficient of 3.1), such that Pins recruitment of the microtubule binding protein Mud (NuMA) occurs over a very narrow Gαi concentration range. Ultrasensitivity is required for Pins function in neuroblasts as a nonultrasensitive Pins mutant fails to robustly couple spindle position to cell polarity. Pins contains three Gαi binding GoLoco domains (GLs); Gαi binding to GL3 activates Pins, whereas GLs 1 and 2 shape the response profile. Although cooperative binding is one mechanism for generating ultrasensitivity, we find GLs 1 and 2 act as "decoys" that compete against activation at GL3. Many signaling proteins contain multiple protein interaction domains, and the decoy mechanism may be a common method for generating ultrasensitivity in regulatory pathways.

LGN-dependent orientation of cell divisions in the dermomyotome controls lineage segregation into muscle and dermis

The plane of cell divisions is pivotal for differential fate acquisition. Dermomyotome development provides an excellent system with which to investigate the link between these processes. In the central sheet of the early dermomyotome, single epithelial cells divide with a planar orientation. Here, we report that in the avian embryo, in addition to self-renewing, a subset of progenitors translocates into the myotome where they generate differentiated myocytes. By contrast, in the late epithelium, individual progenitors divide perpendicularly to produce both mitotic myoblasts and dermis. To examine whether spindle orientations influence fate segregation, early planar divisions were randomized and/or shifted to a perpendicular orientation by interfering with LGN function or by overexpressing inscuteable. Clones derived from single transfected cells exhibited an enhanced proportion of mixed dermomyotome/myotome progeny at the expense of `like' daughter cells in either domain. Loss of LGN or Gαi1 function in the late epithelium randomized otherwise perpendicular mitoses and favored muscle development at the expense of dermis. Hence, LGN-dependent early planar divisions are required for the proper allocation of progenitors into either dermomyotome or myotome, whereas late perpendicular divisions are necessary for the normal balance between muscle and dermis production.

NUMB-ing down cancer by more than just a NOTCH

The protein Numb does not live up to its name. This passive-sounding protein is anything but spent. Originally identified as a cell-fate determinant in Drosophila development, Numb received a good deal of attention as an inhibitor of the Notch receptor signaling pathway. It turns out, however, that Numb does a lot more than simply regulate Notch. It has been implicated in a variety of biochemical pathways connected with signaling (it regulates Notch-, Hedgehog- and TP53-activated pathways), endocytosis (it is involved in cargo internalization and recycling), determination of polarity (it interacts with the PAR complex, and regulates adherens and tight junctions), and ubiquitination (it exploits this mechanism to regulate protein function and stability). This complex biochemical network lies at the heart of Numb's involvement in diverse cellular phenotypes, including cell fate developmental decisions, maintenance of stem cell compartments, regulation of cell polarity and adhesion, and migration. Considering its multifaceted role in cellular homeostasis, it is not surprising that Numb has been implicated in cancer as a tumor suppressor. Our major goal here is to explain the cancer-related role of Numb based on our understanding of its role in cell physiology. We will attempt to do this by reviewing the present knowledge of Numb at the biochemical and functional level, and by integrating its apparently heterogeneous functions into a unifying scenario, based on our recently proposed concept of the "endocytic matrix". Finally, we will discuss the role of Numb in the maintenance of the normal stem cell compartment, as a starting point to interpret the tumor suppressor function of Numb in the context of the cancer stem cell hypothesis.

The role of cell cycle in retinal development: cyclin-dependent kinase inhibitors co-ordinate cell-cycle inhibition, cell-fate determination and differentiation in the developing retina

The mature retina is formed through multi-step developmental processes, including eye field specification, optic vesicle evagination, and cell-fate determination. Co-ordination of these developmental events with cell-proliferative activity is essential to achieve formation of proper retinal structure and function. In particular, the molecular and cellular dynamics of the final cell cycle significantly influence the identity that a cell acquires, since cell fate is largely determined at the final cell cycle for the production of postmitotic cells. This review summarizes our current understanding of the cellular mechanisms that underlie the co-ordination of cell-cycle and cell-fate determination, and also describes a molecular role of cyclin-dependent kinase inhibitors (CDKIs) as co-ordinators of cell-cycle arrest, cell-fate determination and differentiation.

Gα subunit coordinates with ephrin-B to balance self-renewal and differentiation in neural progenitor cells

Proper development of the mammalian brain requires that neural progenitor cells balance self-renewal and differentiation under precise temporal and spatial regulation, but the underlying mechanisms are not well understood. In this study, we identify Gα subunit as a positive regulator of mammalian neurogenesis, working with the regulator of G protein signaling (RGS)-mediated ephrin-B signaling pathway as two opposing forces to maintain a balance between self-renewal and differentiation in the developing mouse cerebral cortex. Multiple Gα(i) subunits are expressed by cortical neural progenitor cells during the course of cortical neurogenesis. Activation of Gα(i) signaling, through in utero electroporation-mediated expression of wild-type and constitutively active Gα(i) subunits, counteracts the function of ephrin-B in cortical neural progenitors to induce differentiation. Genetic knock-in of an RGS-insensitive G184SGα(i2) causes early cell cycle exit and a reduction of cortical neural progenitor cells and leads to a defect in the production of late born cortical neurons, similar to what is observed in mutant mice with deficiency in ephrin-B reverse signaling pathway. This study reveals a role of Gα subunit in mammalian neurogenesis and uncovers a developmental mechanism, coordinated by the Gα and ephrin-B signaling pathways, for control of the balance between self-renewal and differentiation in neural progenitor cells.

Challenges to the study of asymmetric cell division in corneal and limbal epithelia

Asymmetrical cell division in mammalian corneal epithelia is not widely accepted and it is not well characterized. Although some data led to propose that asymmetrical division occurs along the entire corneal epithelium, evidence from different laboratories implies that asymmetrical cell division in adult individuals could be confined to limbal epithelium, as suggested by the location of the corneal epithelial stem cells and the distribution of some molecular markers involved in regulation of stratification and cell differentiation. Nevertheless, most evidence sustaining the participation of asymmetric mitosis in corneal development and differentiation is merely an inference. Recent results based in cell culture experiments suggest that asymmetric division is part of the differentiation program; in such case, mitotic spindle orientation would be regulated by the structure, composition and active signaling pathways at the stem cell niche. Together, the results support the view that in adults, asymmetric division might be confined to limbus, and hence, the process takes place both in apico-basal oriented cells and in cells in which the mitotic spindle is horizontally oriented. In contrast, during development, asymmetrical divisions would be determined by intrinsic mechanisms involving cell polarity, predominantly occurring in apico-basal oriented cells. Future studies should be oriented to understand the regulation of the asymmetrical/symmetrical division, and the control of cell fate by the niche.

Par3 controls epithelial spindle orientation by aPKC-mediated phosphorylation of apical Pins

BACKGROUND

Formation of epithelial sheets requires that cell division occurs in the plane of the sheet. During mitosis, spindle poles align so the astral microtubules contact the lateral cortex. Confinement of the mammalian Pins protein to the lateral cortex is essential for this process. Defects in signaling through Cdc42 and atypical protein kinase C (aPKC) also cause spindle misorientation. When epithelial cysts are grown in 3D cultures, misorientation creates multiple lumens.

RESULTS

We now show that silencing of the polarity protein Par3 causes spindle misorientation in Madin-Darby canine kidney cell cysts. Silencing of Par3 also disrupts aPKC association with the apical cortex, but expression of an apically tethered aPKC rescues normal lumen formation. During mitosis, Pins is mislocalized to the apical surface in the absence of Par3 or by inhibition of aPKC. Active aPKC increases Pins phosphorylation on Ser401, which recruits 14-3-3 protein. 14-3-3 binding inhibits association of Pins with Gαi, through which Pins attaches to the cortex. A Pins S401A mutant mislocalizes over the cell cortex and causes spindle orientation and lumen defects.

CONCLUSIONS

The Par3 and aPKC polarity proteins ensure correct spindle pole orientation during epithelial cell division by excluding Pins from the apical cortex. Apical aPKC phosphorylates Pins, which results in the recruitment of 14-3-3 and inhibition of binding to Gαi, so the Pins falls off the cortex. In the absence of a functional exclusion mechanism, astral microtubules can associate with Pins over the entire epithelial cortex, resulting in randomized spindle pole orientation.

A Drosophila model for primary coenzyme Q deficiency and dietary rescue in the developing nervous system

Coenzyme Q (CoQ) or ubiquinone is a lipid component of the electron transport chain required for ATP generation in mitochondria. Mutations in CoQ biosynthetic genes are associated with rare but severe infantile multisystemic diseases. CoQ itself is a popular over-the-counter dietary supplement that some clinical and rodent studies suggest might be beneficial for neurodegenerative diseases. Here, we identify mutations in the Drosophila qless gene, which encodes an orthologue of the human PDSS1 prenyl transferase that synthesizes the isoprenoid side chain of CoQ. We show that neurons lacking qless activity upregulate markers of mitochondrial stress and undergo caspase-dependent apoptosis. Surprisingly, even though experimental inhibition of caspase activity did not prevent mitochondrial disruption, it was sufficient to rescue the size of neural progenitor clones. This demonstrates that, within the developing larval CNS, qless activity is required primarily for cell survival rather than for cell growth and proliferation. Full rescue of the qless neural phenotype was achieved by dietary supplementation with CoQ4, CoQ9 or CoQ10, indicating that a side chain as short as four isoprenoid units can provide in vivo activity. Together, these findings show that Drosophila qless provides a useful model for studying the neural effects of CoQ deficiency and dietary supplementation.

Polarity in stem cell division: asymmetric stem cell division in tissue homeostasis

Many adult stem cells divide asymmetrically to balance self-renewal and differentiation, thereby maintaining tissue homeostasis. Asymmetric stem cell divisions depend on asymmetric cell architecture (i.e., cell polarity) within the cell and/or the cellular environment. In particular, as residents of the tissues they sustain, stem cells are inevitably placed in the context of the tissue architecture. Indeed, many stem cells are polarized within their microenvironment, or the stem cell niche, and their asymmetric division relies on their relationship with the microenvironment. Here, we review asymmetric stem cell divisions in the context of the stem cell niche with a focus on Drosophila germ line stem cells, where the nature of niche-dependent asymmetric stem cell division is well characterized.

Concomitant requirement for Notch and Jak/Stat signaling during neuro-epithelial differentiation in the Drosophila optic lobe

The optic lobe forms a prominent compartment of the Drosophila adult brain that processes visual input from the compound eye. Neurons of the optic lobe are produced during the larval period from two neuroepithelial layers called the outer and inner optic anlage (OOA, IOA). In the early larva, the optic anlagen grow as epithelia by symmetric cell division. Subsequently, neuroepithelial cells (NE) convert into neuroblasts (NB) in a tightly regulated spatio-temporal progression that starts at the edges of the epithelia and gradually move towards its centers. Neuroblasts divide at a much faster pace in an asymmetric mode, producing lineages of neurons that populate the different parts of the optic lobe. In this paper we have reconstructed the complex morphogenesis of the optic lobe during the larval period, and established a role for the Notch and Jak/Stat signaling pathways during the NE-NB conversion. After an early phase of complete overlap in the OOA, signaling activities sort out such that Jak/Stat is active in the lateral OOA which gives rise to the lamina, and Notch remains in the medial cells that form the medulla. During the third instar, a wave front of enhanced Notch activity progressing over the OOA from medial to lateral controls the gradual NE-NB conversion. Neuroepithelial cells at the medial edge of the OOA, shortly prior to becoming neuroblasts, express high levels of Delta, which activates the Notch pathway and thereby maintains the OOA in an epithelial state. Loss of Notch signaling, as well as Jak/Stat signaling, results in a premature NE-NB conversion of the OOA, which in turn has severe effects on optic lobe patterning. Our findings present the Drosophila optic lobe as a useful model to analyze the key signaling mechanisms controlling transitions of progenitor cells from symmetric (growth) to asymmetric (differentiative) divisions.

Asymmetric cell division of T cells upon antigen presentation uses multiple conserved mechanisms

Asymmetric cell division is a potential means by which cell fate choices during an immune response are orchestrated. Defining the molecular mechanisms that underlie asymmetric division of T cells is paramount for determining the role of this process in the generation of effector and memory T cell subsets. In other cell types, asymmetric cell division is regulated by conserved polarity protein complexes that control the localization of cell fate determinants and spindle orientation during division. We have developed a tractable, in vitro model of naive CD8(+) T cells undergoing initial division while attached to dendritic cells during Ag presentation to investigate whether similar mechanisms might regulate asymmetric division of T cells. Using this system, we show that direct interactions with APCs provide the cue for polarization of T cells. Interestingly, the immunological synapse disseminates before division even though the T cells retain contact with the APC. The cue from the APC is translated into polarization of cell fate determinants via the polarity network of the Par3 and Scribble complexes, and orientation of the mitotic spindle during division is orchestrated by the partner of inscuteable/G protein complex. These findings suggest that T cells have selectively adapted a number of evolutionarily conserved mechanisms to generate diversity through asymmetric cell division.

Fragile X protein controls neural stem cell proliferation in the Drosophila brain

Fragile X syndrome (FXS) is the most common form of inherited mental retardation and is caused by the loss of function for Fragile X protein (FMRP), an RNA-binding protein thought to regulate synaptic plasticity by controlling the localization and translation of specific mRNAs. We have recently shown that FMRP is required to control the proliferation of the germline in Drosophila. To determine whether FMRP is also required for proliferation during brain development, we examined the distribution of cell cycle markers in dFmr1 brains compared with wild-type throughout larval development. Our results indicate that the loss of dFmr1 leads to a significant increase in the number of mitotic neuroblasts (NB) and BrdU incorporation in the brain, consistent with the notion that FMRP controls proliferation during neurogenesis. Developmental studies suggest that FMRP also inhibits neuroblast exit from quiescence in early larval brains, as indicated by misexpression of Cyclin E. Live imaging experiments indicate that by the third instar larval stage, the length of the cell cycle is unaffected, although more cells are found in S and G2/M in dFmr1 brains compared with wild-type. To determine the role of FMRP in neuroblast division and differentiation, we used Mosaic Analysis with a Repressible Marker (MARCM) approaches in the developing larval brain and found that single dFmr1 NB generate significantly more neurons than controls. Our results demonstrate that FMRP is required during brain development to control the exit from quiescence and proliferative capacity of NB as well as neuron production, which may provide insights into the autistic component of FXS.

Drosophila adult muscle precursors form a network of interconnected cells and are specified by the rhomboid-triggered EGF pathway

In Drosophila, a population of muscle-committed stem-like cells called adult muscle precursors (AMPs) keeps an undifferentiated and quiescent state during embryonic life. The embryonic AMPs are at the origin of all adult fly muscles and, as we demonstrate here, they express repressors of myogenic differentiation and targets of the Notch pathway known to be involved in muscle cell stemness. By targeting GFP to the AMP cell membranes, we show that AMPs are tightly associated with the peripheral nervous system and with a subset of differentiated muscles. They send long cellular processes running along the peripheral nerves and, by the end of embryogenesis, form a network of interconnected cells. Based on evidence from laser ablation experiments, the main role of these cellular extensions is to maintain correct spatial positioning of AMPs. To gain insights into mechanisms that lead to AMP cell specification, we performed a gain-of-function screen with a special focus on lateral AMPs expressing the homeobox gene ladybird. Our data show that the rhomboid-triggered EGF signalling pathway controls both the specification and the subsequent maintenance of AMP cells. This finding is supported by the identification of EGF-secreting cells in the lateral domain and the EGF-dependent regulatory modules that drive expression of the ladybird gene in lateral AMPs. Taken together, our results reveal an unsuspected capacity of embryonic AMPs to form a cell network, and shed light on the mechanisms governing their specification and maintenance.

[Centrosomes, mitotic spindle and cancer: find the odd one out!]

Centrosomes are essential protagonists during cell division through microtubule nucleation and spindle formation which are key to the harmonious distribution of sister chromatids in the two daughter cells. However, during the past decade, a wealth of new observations has extended their role beyond mitosis, particularly in the asymmetrical partition of cell fate determinants. Remarkably, asymmetric centrosome inheritance per se, through the segregation of differently aged mother -centrioles, seems to regulate the differential behaviour of daughter cells, in part through asynchronous expression of primary cilia, governing the response to environmental signals. It is thus understandable why any quantitative or qualitative dysfunction of centrioles contributes to genomic -instability and thus -tumorigenesis.

The interphase microtubule aster is a determinant of asymmetric division orientation in Drosophila neuroblasts

The orientation of stem cell divisions is maintained beyond one cell cycle thanks to microtubule polymerization and apical centrosome positioning.

The mechanisms that maintain the orientation of cortical polarity and asymmetric division unchanged in consecutive mitoses in Drosophila melanogaster neuroblasts (NBs) are unknown. By studying the effect of transient microtubule depolymerization and centrosome mutant conditions, we have found that such orientation memory requires both the centrosome-organized interphase aster and centrosome-independent functions. We have also found that the span of such memory is limited to the last mitosis. Furthermore, the orientation of the NB axis of polarity can be reset to any angle with respect to the surrounding tissue and is, therefore, cell autonomous.

Neural stem cell transcriptional networks highlight genes essential for nervous system development

Neural stem cells must strike a balance between self-renewal and multipotency, and differentiation. Identification of the transcriptional networks regulating stem cell division is an essential step in understanding how this balance is achieved. We have shown that the homeodomain transcription factor, Prospero, acts to repress self-renewal and promote differentiation. Among its targets are three neural stem cell transcription factors, Asense, Deadpan and Snail, of which Asense and Deadpan are repressed by Prospero. Here, we identify the targets of these three factors throughout the genome. We find a large overlap in their target genes, and indeed with the targets of Prospero, with 245 genomic loci bound by all factors. Many of the genes have been implicated in vertebrate stem cell self-renewal, suggesting that this core set of genes is crucial in the switch between self-renewal and differentiation. We also show that multiply bound loci are enriched for genes previously linked to nervous system phenotypes, thereby providing a shortcut to identifying genes important for nervous system development.

Brain tumor stem cells

Primary malignant brain cancer, one of the most deadly diseases, has a high rate of recurrence after treatment. Studies in the past several years have led to the hypothesis that the root of the recurrence may be brain tumor stem cells (BTSCs), stem-like subpopulation of cells that are responsible for propagating the tumor. Current treatments combining surgery and chemoradiotherapy could not eliminate BTSCs because these cells are highly infiltrative and possess several properties that can reduce the damages caused by radiation or anti-cancer drugs. BTSCs are similar to NSCs in molecular marker expression and multi-lineage differentiation potential. Genetic analyses of Drosophila CNS neoplasia, mouse glioma models, and human glioma tissues have revealed a link between increased NSC self-renewal and brain tumorigenesis. Furthermore, data from various rodent models of malignant brain tumors have provided compelling evidence that multipotent NSCs and lineage-restricted neural progenitor cells (NPCs) could be the cell origin of brain tumors. Thus, the first event of brain tumorigenesis might be the occurrence of oncogenic mutations in the stem cell self-renewal pathway in an NSC or NPC. These mutations convert the NSC or NPC to a BTSC, which then initiates and sustains the growth of the tumor. The self-renewal of BTSCs is controlled by several evolutionarily conserved signaling pathways and requires an intact vascular niche. Targeting these pathways and the vascular niche could be a principle in novel brain tumor therapies aimed to eliminate BTSCs.

Lessons from development: A role for asymmetric stem cell division in cancer

Asymmetric stem cell division has emerged as a major regulatory mechanism for physiologic control of stem cell numbers. Reinvigoration of the cancer stem cell theory suggests that tumorigenesis may be regulated by maintaining the balance between asymmetric and symmetric cell division. Therefore, mutations affecting this balance could result in aberrant expansion of stem cells. Although a number of molecules have been implicated in regulation of asymmetric stem cell division, here, we highlight known tumor suppressors with established roles in this process. While a subset of these tumor suppressors were originally defined in developmental contexts, recent investigations reveal they are also lost or mutated in human cancers. Mutations in tumor suppressors involved in asymmetric stem cell division provide mechanisms by which cancer stem cells can hyperproliferate and offer an intriguing new focus for understanding cancer biology. Our discussion of this emerging research area derives insight from a frontier area of basic science and links these discoveries to human tumorigenesis. This highlights an important new focus for understanding the mechanism underlying expansion of cancer stem cells in driving tumorigenesis.

Apical polarity protein PrkCi is necessary for maintenance of spinal cord precursors in zebrafish

During development, neural precursors divide to produce new precursors and cells that differentiate as neurons and glia. In Drosophila, apicobasal polarity and orientation of the mitotic spindle play important roles in specifying the progeny of neural precursors for different fates. We examined orientation of zebrafish spinal cord precursors using time-lapse imaging and tested the function of protein kinase C, iota (PrkCi), a member of the Par complex of proteins necessary for apicobasal polarity in the nervous system. We found that nearly all precursors divide within the plane of the neuroepithelium of wild-type embryos even when they must produce cells that have different fates. In the absence of PrkCi function, neural precursor divisions become oblique during late embryogenesis and excess oligodendrocytes form concomitant with loss of dividing cells. We conclude that PrkCi function and planar divisions are necessary for asymmetric, self-renewing division of spinal cord precursors.

Spindle alignment is achieved without rotation after the first cell cycle in Drosophila embryonic neuroblasts

Spindle alignment along the apicobasal polarity axis is mandatory for proper self-renewing asymmetric division in Drosophila neuroblasts (NBs). In embryonic NBs, spindles have been reported to assemble orthogonally to the polarity axis and later rotate to align with it. In larval NBs, spindles assemble directly aligned with the axis owing to the differential spatiotemporal control of the microtubule organising activity of their centrosomes. We have recorded embryonic NBs that express centrosome and microtubule reporters, from delamination up to the fourth cell cycle, by two-photon confocal microscopy, and have found that the switch between these two spindle orientation modes occurs in the second cell cycle of the NB, the first that follows delamination. Therefore, predetermined spindle orientation is not restricted to larval NBs. On the contrary, it actually applies to all but the first cell cycle of embryonic NBs.

Defects in ErbB-dependent establishment of adult melanocyte stem cells reveal independent origins for embryonic and regeneration melanocytes

Adult stem cells are responsible for maintaining and repairing tissues during the life of an organism. Tissue repair in humans, however, is limited compared to the regenerative capabilities of other vertebrates, such as the zebrafish (Danio rerio). An understanding of stem cell mechanisms, such as how they are established, their self-renewal properties, and their recruitment to produce new cells is therefore important for the application of regenerative medicine. We use larval melanocyte regeneration following treatment with the melanocytotoxic drug MoTP to investigate these mechanisms in Melanocyte Stem Cell (MSC) regulation. In this paper, we show that the receptor tyrosine kinase, erbb3b, is required for establishing the adult MSC responsible for regenerating the larval melanocyte population. Both the erbb3b mutant and wild-type fish treated with the ErbB inhibitor, AG1478, develop normal embryonic melanocytes but fail to regenerate melanocytes after MoTP-induced melanocyte ablation. By administering AG1478 at different time points, we show that ErbB signaling is only required for regeneration prior to MoTP treatment and before 48 hours of development, consistent with a role in establishing MSCs. We then show that overexpression of kitla, the Kit ligand, in transgenic larvae leads to recruitment of MSCs, resulting in overproliferation of melanocytes. Furthermore, kitla overexpression can rescue AG1478-blocked regeneration, suggesting that ErbB signaling is required to promote the progression and specification of the MSC from a pre–MSC state. This study provides evidence that ErbB signaling is required for the establishment of adult MSCs during embryonic development. That this requirement is not shared with the embryonic melanocytes suggests that embryonic melanocytes develop directly, without proceeding through the ErbB-dependent MSC. Moreover, the shared requirement of larval melanocyte regeneration and metamorphic melanocytes that develops at the larval-to-adult transition suggests that these post-embryonic melanocytes develop from the same adult MSC population. Lastly, that kitla overexpression can recruit the MSC to develop excess melanocytes raises the possibility that Kit signaling may be involved in MSC recruitment during regeneration.

The promise of regenerative medicine lies in the ability to find or create stem cells that can be manipulated to replace damaged tissues and organs. Such ability requires an understanding of how adult stem cells are established and then later recruited to regenerate different tissues. Here, we study the zebrafish's ability to regenerate melanocytes, a pigment cell shared with humans, to understand these mechanisms. We show that the erbb3b gene is required to establish melanocyte stem cells in the embryo that are responsible for regenerating melanocytes after melanocytes are ablated in the larval zebrafish. Because this adult stem cell is not required for the development of embryonic melanocytes, we conclude that adult melanocyte stem cells develop in parallel to the embryonic tissues that they regulate. We also show that overexpressing kit ligand a will result in over-recruiting these adult stem cells to produce excess melanocytes. Further exploration into the mechanisms by which the zebrafish melanocyte stem cells are maintained and recruited will inform how adult stem cells might be manipulated for medical applications.

Patterns of growth, axonal extension and axonal arborization of neuronal lineages in the developing Drosophila brain

The Drosophila central brain is composed of approximately 100 paired lineages, with most lineages comprising 100-150 neurons. Most lineages have a number of important characteristics in common. Typically, neurons of a lineage stay together as a coherent cluster and project their axons into a coherent bundle visible from late embryo to adult. Neurons born during the embryonic period form the primary axon tracts (PATs) that follow stereotyped pathways in the neuropile. Apoptotic cell death removes an average of 30-40% of primary neurons around the time of hatching. Secondary neurons generated during the larval period form secondary axon tracts (SATs) that typically fasciculate with their corresponding primary axon tract. SATs develop into the long fascicles that interconnect the different compartments of the adult brain. Structurally, we distinguish between three types of lineages: PD lineages, characterized by distinct, spatially separate proximal and distal arborizations; C lineages with arborizations distributed continuously along the entire length of their tract; D lineages that lack proximal arborizations. Arborizations of many lineages, in particular those of the PD type, are restricted to distinct neuropile compartments. We propose that compartments are "scaffolded" by individual lineages, or small groups thereof. Thereby, the relatively small number of primary neurons of each primary lineage set up the compartment map in the late embryo. Compartments grow during the larval period simply by an increase in arbor volume of primary neurons. Arbors of secondary neurons form within or adjacent to the larval compartments, resulting in smaller compartment subdivisions and additional, adult specific compartments.

Blastocyst lineage formation, early embryonic asymmetries and axis patterning in the mouse

The investigation into lineage allocation and early asymmetries in the pre- and peri-implantation mouse embryo is gaining momentum. As we review here, new insights have been gained into the cellular and molecular events that lead to the establishment of the three lineages of the blastocyst, to the determination of the origin and the fates of the visceral endoderm in the peri-implantation mouse embryo, and to the generation of cellular and molecular activities that accompany the emergence of asymmetries in the pre-gastrulation embryo. We also discuss the continuing debate that surrounds the relative impacts of early lineage bias versus the stochastic allocation of cells with respect to the events that pattern the blastocyst and initiate its later asymmetries.

The centrosome and asymmetric cell division

Asymmetric stem cell division is a mechanism widely employed by the cell to maintain tissue homeostasis, resulting in the production of one stem cell and one differentiating cell. However, asymmetric cell division is not limited to stem cells and is widely observed even in unicellular organisms as well as in cells that make up highly complex tissues. In asymmetric cell division, cells must organize their intracellular components along the axis of asymmetry (sometimes in the context of extracellular architecture). Recent studies have described cell asymmetry in many cell types and in many cases such asymmetry involves the centrosome (or spindle pole body in yeast) as the center of cytoskeleton organization. In this review, I summarize recent discoveries in cellular polarity that lead to an asymmetric outcome, with a focus on centrosome function.

Asymmetric stem cell division and pathology: insights from Drosophila stem cell systems

Adult stem cells maintain many tissues and organs throughout the life of an organism by serving as renewable sources of differentiated cells. While stem cells remain in a relatively undifferentiated state, their daughters can commit to differentiation to acquire distinct cell fates. Therefore, a stem cell's choice between self-renewal and commitment to differentiation is of critical importance to the maintenance of functional tissues and organs. Many adult stem cells can divide asymmetrically to produce one self-renewed stem cell and one differentiated daughter, preserving the critical balance between stem cell and differentiated cell populations. Stem cell dysfunction and/or malfunction have been proposed to lead to several human pathologies, including tumourigenesis and tissue degeneration, yet whether a failure of asymmetric division is a primary cause of stem cell-related pathologies remains largely uninvestigated. Here, I discuss the implications of asymmetric stem cell division in pathology.

Drosophila asymmetric division, polarity and cancer

A limited number of adult stem cells (SCs) maintain the homoestasis of different tissues through the lifetime of the individual by generating differentiating daughters and renewing themselves. Errors in the SC division rate or in the fine balance between self-renewal and differentiation might result in tissue overgrowth or depletion, two potentially lethal conditions. A few types of SCs have been identified in Drosophila. These include the SCs of the adult intestine and malpighian tubes, (Micchelli and Perrimon, 2006; Ohlstein and Spradling, 2006; Singh et al., 2007), the prohematocytes that maintain the population of cells involved in the immunoresponse (Lanot et al., 2001; Lemaitre and Hoffmann, 2007), the SC of the follicle epithelia in the ovary (Nystul and Spradling, 2007), germ line SCs (GSCs) of both sexes (Fuller and Spradling, 2007) and neuroblasts (NBs), the fly neural SCs (Yu et al., 2006; Chia et al., 2008; Knoblich, 2008). Drosophila SCs have proved a fruitful model system to unveil some aspects of the molecular logic that sustains SC function. This review focuses on results obtained in the last few years from the study of NBs, particularly from the standpoint of the possible functional connection between asymmetric SC division and cancer.

Role of spindle asymmetry in cellular dynamics

The mitotic spindle is mostly perceived as a symmetric structure. However, in many cell divisions, the two poles of the spindle organize asters with different dynamics, associate with different biomolecules or subcellular domains, and perform different functions. In this chapter, we describe some of the most prominent examples of spindle asymmetry. These are encountered during cell-cycle progression in budding and fission yeast and during asymmetric cell divisions of stem cells and embryos. We analyze the molecular mechanisms that lead to generation of spindle asymmetry and discuss the importance of spindle-pole differentiation for the correct outcome of cell division.

Structural brain mutants: mushroom body defect (mud): a case study

Single-gene mutants of Drosophila have not only increased our understanding of the biochemical processes underlying learning and memory processes, but also established structure-function relationships. The first relevant mutants were identified by Martin Heisenberg nearly 30 years ago in a screen for altered adult brain structure and were used to link the mushroom bodies in the central brain with olfactory learning and memory processes. Because the observed structural defects in the adult are the consequence of deregulated developmental processes, the characterization of these mutants can also provide insight into the genetic programs underlying the establishment, maintenance, and remodeling of functional neuronal circuits. As an example for the value of this approach, we trace the history of mushroom body defect (mud), from the original anatomical description of the mutation to most recent insights of the function of the protein as a regulator of neuronal progenitor cell division.

A conserved nuclear receptor, Tailless, is required for efficient proliferation and prolonged maintenance of mushroom body progenitors in the Drosophila brain

The intrinsic neurons of mushroom bodies (MBs), centers of olfactory learning in the Drosophila brain, are generated by a specific set of neuroblasts (Nbs) that are born in the embryonic stage and exhibit uninterrupted proliferation till the end of the pupal stage. Whereas MB provides a unique model to study proliferation of neural progenitors, the underlying mechanism that controls persistent activity of MB-Nbs is poorly understood. Here we show that Tailless (TLL), a conserved orphan nuclear receptor, is required for optimum proliferation activity and prolonged maintenance of MB-Nbs and ganglion mother cells (GMCs). Mutations of tll progressively impair cell cycle in MB-Nbs and cause premature loss of MB-Nbs in the early pupal stage. TLL is also expressed in MB-GMCs to prevent apoptosis and promote cell cycling. In addition, we show that ectopic expression of tll leads to brain tumors, in which Prospero, a key regulator of progenitor proliferation and differentiation, is suppressed whereas localization of molecular components involved in asymmetric Nb division is unaffected. These results as a whole uncover a distinct regulatory mechanism of self-renewal and differentiation of the MB progenitors that is different from the mechanisms found in other progenitors.

Negative-feedback regulation of proneural proteins controls the timing of neural precursor division

Neurogenesis requires precise control of cell specification and division. In Drosophila, the timing of cell division of the sensory organ precursor (SOP) is under strict temporal control. But how the timing of mitotic entry is determined remains poorly understood. Here, we present evidence that the timing of the G2-M transition is determined by when proneural proteins are degraded from SOPs. This process requires the E3 ubiquitin ligase complex, including the RING protein Sina and the adaptor Phyl. In phyl mutants, proneural proteins accumulate, causing delay or arrest in the G2-M transition. The G2-M defect in phyl mutants is rescued by reducing the ac and sc gene doses. Misexpression of phyl downregulates proneural protein levels in a sina-dependent manner. Phyl directly associates with proneural proteins to act as a bridge between proneural proteins and Sina. As phyl is a direct transcriptional target of Ac and Sc, our data suggest that, in addition to mediating cell cycle arrest, proneural protein initiates a negative-feedback regulation to time the mitotic entry of neural precursors.

Centrosome amplification can initiate tumorigenesis in flies

Centrosome amplification is a common feature of many cancer cells, and it has been previously proposed that centrosome amplification can drive genetic instability and so tumorigenesis. To test this hypothesis, we generated Drosophila lines that have extra centrosomes in approximately 60% of their somatic cells. Many cells with extra centrosomes initially form multipolar spindles, but these spindles ultimately become bipolar. This requires a delay in mitosis that is mediated by the spindle assembly checkpoint (SAC). As a result of this delay, there is no dramatic increase in genetic instability in flies with extra centrosomes, and these flies maintain a stable diploid genome over many generations. The asymmetric division of the larval neural stem cells, however, is compromised in the presence of extra centrosomes, and larval brain cells with extra centrosomes can generate metastatic tumors when transplanted into the abdomens of wild-type hosts. Thus, centrosome amplification can initiate tumorigenesis in flies.

Regulation of glia number in Drosophila by Rap/Fzr, an activator of the anaphase-promoting complex, and Loco, an RGS protein

Glia mediate a vast array of cellular processes and are critical for nervous system development and function. Despite their immense importance in neurobiology, glia remain understudied and the molecular mechanisms that direct their differentiation are poorly understood. Rap/Fzr is the Drosophila homolog of the mammalian Cdh1, a regulatory subunit of the anaphase-promoting complex/cyclosome (APC/C). APC/C is an E3 ubiquitin ligase complex well characterized for its role in cell cycle progression. In this study, we have uncovered a novel cellular role for Rap/Fzr. Loss of rap/fzr function leads to a marked increase in the number of glia in the nervous system of third instar larvae. Conversely, ectopic expression of UAS-rap/fzr, driven by repo-GAL4, results in the drastic reduction of glia. Data from clonal analyses using the MARCM technique show that Rap/Fzr regulates the differentiation of surface glia in the developing larval nervous system. Our genetic and biochemical data further indicate that Rap/Fzr regulates glial differentiation through its interaction with Loco, a regulator of G-protein signaling (RGS) protein and a known effector of glia specification. We propose that Rap/Fzr targets Loco for ubiquitination, thereby regulating glial differentiation in the developing nervous system.

Beta-catenin asymmetry is regulated by PLA1 and retrograde traffic in C. elegans stem cell divisions

Asymmetric division is an important property of stem cells. In Caenorhabditis elegans, the Wnt/beta-catenin asymmetry pathway determines the polarity of most asymmetric divisions. The Wnt signalling components such as beta-catenin localize asymmetrically to the cortex of mother cells to produce two distinct daughter cells. However, the molecular mechanism to polarize them remains to be elucidated. Here, we demonstrate that intracellular phospholipase A(1) (PLA(1)), a poorly characterized lipid-metabolizing enzyme, controls the subcellular localizations of beta-catenin in the terminal asymmetric divisions of epithelial stem cells (seam cells). In mutants of ipla-1, a single C. elegans PLA(1) gene, cortical beta-catenin is delocalized and the asymmetry of cell-fate specification is disrupted in the asymmetric divisions. ipla-1 mutant phenotypes are rescued by expression of ipla-1 in seam cells in a catalytic activity-dependent manner. Furthermore, our genetic screen utilizing ipla-1 mutants reveals that reduction of endosome-to-Golgi retrograde transport in seam cells restores normal subcellular localization of beta-catenin to ipla-1 mutants. We propose that membrane trafficking regulated by ipla-1 provides a mechanism to control the cortical asymmetry of beta-catenin.

Temporal transcription factors and their targets schedule the end of neural proliferation in Drosophila

The timing mechanisms responsible for terminating cell proliferation toward the end of development remain unclear. In the Drosophila CNS, individual progenitors called neuroblasts are known to express a series of transcription factors endowing daughter neurons with different temporal identities. Here we show that Castor and Seven-Up, members of this temporal series, regulate key events in many different neuroblast lineages during late neurogenesis. First, they schedule a switch in the cell size and identity of neurons involving the targets Chinmo and Broad Complex. Second, they regulate the time at which neuroblasts undergo Prospero-dependent cell-cycle exit or Reaper/Hid/Grim-dependent apoptosis. Both types of progenitor termination require the combined action of a late phase of the temporal series and indirect feedforward via Castor targets such as Grainyhead and Dichaete. These studies identify the timing mechanism ending CNS proliferation and reveal how aging progenitors transduce bursts of transcription factors into long-lasting changes in cell proliferation and cell identity.

Divide and conquer: how asymmetric division shapes cell fate in the hematopoietic system

A fundamental mechanism by which cells can give rise to daughters with different fates is via asymmetric division. During asymmetric division, a mother cell generates daughter cells that go on to adopt different fates because of differential segregation of cell fate determinants. Although originally characterized in invertebrates, asymmetric division has recently been shown to regulate cell fate decisions in the mammalian hematopoietic system, playing crucial roles in stem cell renewal, lymphocyte activation, and leukemogenesis. These discoveries have opened new doors to understanding how regulation of division pattern contributes to the normal development and function of the immune system as well as how its dysregulation can lead to cancer.

Regulation of neural progenitor cell state by ephrin-B

Maintaining a balance between self-renewal and differentiation in neural progenitor cells during development is important to ensure that correct numbers of neural cells are generated. We report that the ephrin-B–PDZ-RGS3 signaling pathway functions to regulate this balance in the developing mammalian cerebral cortex. During cortical neurogenesis, expression of ephrin-B1 and PDZ-RGS3 is specifically seen in progenitor cells and is turned off at the onset of neuronal differentiation. Persistent expression of ephrin-B1 and PDZ-RGS3 prevents differentiation of neural progenitor cells. Blocking RGS-mediated ephrin-B1 signaling in progenitor cells through RNA interference or expression of dominant-negative mutants results in differentiation. Genetic knockout of ephrin-B1 causes early cell cycle exit and leads to a concomitant loss of neural progenitor cells. Our results indicate that ephrin-B function is critical for the maintenance of the neural progenitor cell state and that this role of ephrin-B is mediated by PDZ-RGS3, likely via interacting with the noncanonical G protein signaling pathway, which is essential in neural progenitor asymmetrical cell division.

Hyaluronan-CD44 pathway regulates orientation of mitotic spindle in normal epithelial cells

Orientation of mitotic spindle and cell division axis can impact normal physiological processes, including epithelial tissue branching and neuron generation by asymmetric cell division. Microtubule dynamics and its interaction with cortical proteins regulate the orientation of mitotic spindle axis. However, the nature of extracellular signals that control proper orientation of mitotic spindle axis is largely unclear. Here, we show that signals from two distinct surface contact, "bi-surface-contact," sites are required for the orientation of mitotic spindle axis in normal epithelial cells. We identified apical and basal surface-membrane as required bi-surface-contact sites. We showed that high molecular weight (HMW) hyaluronan (HA)-CD44 signaling from the apical surface-membrane regulated the orientation of mitotic spindle axis to align parallel to the basal extracellular matrix (ECM). The same effect was achieved by fibronectin-integrin alphavbeta6 signaling from the basal surface-membrane or by inhibition of ROCK activity. On the contrary, HMW HA-CD44 signaling from the basal surface-membrane regulated the orientation of mitotic spindle axis to align oblique-perpendicular to the basal ECM. We also found that microtubule dynamics is required for HMW HA-CD44 mediated regulation of mitotic spindle orientation. Our findings thus provide a novel mechanism for the regulation of mitotic spindle orientation.

Miranda cargo-binding domain forms an elongated coiled-coil homodimer in solution: implications for asymmetric cell division in Drosophila

Miranda is a multidomain adaptor protein involved in neuroblast asymmetric division in Drosophila melanogaster. The central domain of Miranda is necessary for cargo binding of the neural transcription factor Prospero, the Prospero-mRNA carrier Staufen, and the tumor suppressor Brat. Here, we report the first solution structure of Miranda central "cargo-binding" domain (residues 460-660) using small-angle X-ray scattering. Ab initio modeling of the scattering data yields an elongated "rod-like" molecule with a maximum linear dimension (D(max)) of approximately 22 nm. Moreover, circular dichroism and cross-linking experiments indicate that the cargo-binding domain is predominantly helical and forms a parallel coiled-coil homodimer in solution. Based on the results, we modeled the full-length Miranda protein as a double-headed, double-tailed homodimer with a long central coiled-coil region. We discuss the cargo-binding capacity of the central domain and propose a structure-based mechanism for cargo release and timely degradation of Miranda in developing neuroblasts.

Asymmetric localization of the adaptor protein Miranda in neuroblasts is achieved by diffusion and sequential interaction of Myosin II and VI

The adaptor protein Miranda plays a pivotal role in the asymmetric cell division of neuroblasts by asymmetrically segregating key differentiation factors. Miranda localization requires Myosin VI and Myosin II. The apical-then-basal localization pattern of Miranda detected in fixed tissue, and the localization defects in embryos lacking Myosin VI, suggest that Miranda is transported to the basal pole as a Myosin VI cargo. However, the mode and temporal sequence of Miranda localization have not been characterized in live embryos. Furthermore, it is unknown whether Miranda and PON, a second adaptor protein required for asymmetric protein localization, are both regulated by Myosin II. By combining immunofluorescence studies with time-lapse confocal microscopy, we show that Miranda protein forms an apical crescent at interphase, but is ubiquitously localized at prophase in a Myosin-II-dependent manner. FRAP analysis revealed that Miranda protein reaches the basal cortex by passive diffusion throughout the cell, rather than by long-range Myosin VI-directed transport. Myosin VI acts downstream of Myosin II in the same pathway to deliver diffusing Miranda to the basal cortex. PON localization occurs mainly along the cortex and requires Myosin II but not Myosin VI, suggesting that distinct mechanisms are employed to localize different adaptor proteins during asymmetric cell division.

Drosophila optic lobe neuroblasts triggered by a wave of proneural gene expression that is negatively regulated by JAK/STAT

Neural stem cells called neuroblasts (NBs) generate a variety of neuronal and glial cells in the central nervous system of the Drosophila embryo. These NBs, few in number, are selected from a field of neuroepithelial (NE) cells. In the optic lobe of the third instar larva, all NE cells of the outer optic anlage (OOA) develop into either NBs that generate the medulla neurons or lamina neuron precursors of the adult visual system. The number of lamina and medulla neurons must be precisely regulated because photoreceptor neurons project their axons directly to corresponding lamina or medulla neurons. Here, we show that expression of the proneural protein Lethal of scute [L(1)sc] signals the transition of NE cells to NBs in the OOA. L(1)sc expression is transient, progressing in a synchronized and ordered ;proneural wave' that sweeps toward more lateral NEs. l(1)sc expression is sufficient to induce NBs and is necessary for timely onset of NB differentiation. Thus, proneural wave precedes and induces transition of NE cells to NBs. Unpaired (Upd), the ligand for the JAK/STAT signaling pathway, is expressed in the most lateral NE cells. JAK/STAT signaling negatively regulates proneural wave progression and controls the number of NBs in the optic lobe. Our findings suggest that NBs might be balanced with the number of lamina neurons by JAK/STAT regulation of proneural wave progression, thereby providing the developmental basis for the formation of a precise topographic map in the visual center.

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.

Molecular and cell biology of brain tumor stem cells: lessons from neural progenitor/stem cells

✓ The results of studies conducted in the past several years have suggested that malignant brain tumors may harbor a small fraction of tumor-initiating cells that are likely to cause tumor recurrence. These cells are known as brain tumor stem cells (BTSCs) because of their multilineage potential and their ability to self-renew in vitro and to recapitulate original tumors in vivo. The understanding of BTSCs has been greatly advanced by knowledge of neural progenitor/stem cells (NPSCs), which are multipotent and self-renewing precursor cells for neurons and glia. In this article, the authors summarize evidence that genetic mutations that deregulate asymmetric cell division by affecting cell polarity, spindle orientation, or cell fate determinants may result in the conversion of NPSCs to BTSCs. In addition, they review evidence that BTSCs and normal NPSCs may reside in similar vascularized microenvironments, where similar evolutionarily conserved signaling pathways control their proliferation. Finally, they discuss preliminary evidence that mechanisms of BTSC-associated infiltrativeness may be similar to those underlying the migration of NPSCs and neurons.

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.

Amplification of neural stem cell proliferation by intermediate progenitor cells in Drosophila brain development

Background

In the mammalian brain, neural stem cells divide asymmetrically and often amplify the number of progeny they generate via symmetrically dividing intermediate progenitors. Here we investigate whether specific neural stem cell-like neuroblasts in the brain of Drosophila might also amplify neuronal proliferation by generating symmetrically dividing intermediate progenitors.

Results

Cell lineage-tracing and genetic marker analysis show that remarkably large neuroblast lineages exist in the dorsomedial larval brain of Drosophila. These lineages are generated by brain neuroblasts that divide asymmetrically to self renew but, unlike other brain neuroblasts, do not segregate the differentiating cell fate determinant Prospero to their smaller daughter cells. These daughter cells continue to express neuroblast-specific molecular markers and divide repeatedly to produce neural progeny, demonstrating that they are proliferating intermediate progenitors. The proliferative divisions of these intermediate progenitors have novel cellular and molecular features; they are morphologically symmetrical, but molecularly asymmetrical in that key differentiating cell fate determinants are segregated into only one of the two daughter cells.

Conclusion

Our findings provide cellular and molecular evidence for a new mode of neurogenesis in the larval brain of Drosophila that involves the amplification of neuroblast proliferation through intermediate progenitors. This type of neurogenesis bears remarkable similarities to neurogenesis in the mammalian brain, where neural stem cells as primary progenitors amplify the number of progeny they generate through generation of secondary progenitors. This suggests that key aspects of neural stem cell biology might be conserved in brain development of insects and mammals.

The PAR proteins: fundamental players in animal cell polarization

The par genes were discovered in genetic screens for regulators of cytoplasmic partitioning in the early embryo of C. elegans, and encode six different proteins required for asymmetric cell division by the worm zygote. Some of the PAR proteins are localized asymmetrically and form physical complexes with one another. Strikingly, the PAR proteins have been found to regulate cell polarization in many different contexts in diverse animals, suggesting they form part of an ancient and fundamental mechanism for cell polarization. Although the picture of how the PAR proteins function remains incomplete, cell biology and biochemistry are beginning to explain how PAR proteins polarize cells.