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

New insights into mechanisms of stem cell daughter fate determination in regenerative tissues

Stem cells can self-renew and differentiate over extended periods of time. Understanding how stem cells acquire their fates is a central question in stem cell biology. Early work in Drosophila germ line and neuroblast showed that fate choice is achieved by strict asymmetric divisions that can generate each time one stem and one differentiated cell. More recent work suggests that during homeostasis, some stem cells can divide symmetrically to generate two differentiated cells or two identical stem cells to compensate for stem cell loss that occurred by direct differentiation or apoptosis. The interplay of all these factors ensures constant tissue regeneration and the maintenance of stem cell pool size. This interplay can be modeled as a population-deterministic dynamics that, at least in some systems, may be described as stochastic behavior. Here, we overview recent progress made on the characterization of stem cell dynamics in regenerative tissues.

The stem cell state

This volume describes the latest findings on transcriptional and translational regulation of stem cells. Both transcriptional activators and repressors have been shown to be crucial for the maintenance of the stem cell state. A key element of stem cell maintenance is repression of differentiation factors or developmental genes - achieved transcriptionally, epigenetically by the Polycomb complex, and post-transcriptionally by RNA-binding proteins and microRNAs. This volume takes two approaches to this topic - (1) illustrating the general principles outlined above through a series of different stem cell examples - embryonic, iPS and adult stem cells, and (2) describing several molecular families that have been shown to have roles in regulation of multiple stem cell populations.

Uncovering the link between malfunctions in Drosophila neuroblast asymmetric cell division and tumorigenesis

Asymmetric cell division is a developmental process utilized by several organisms. On the most basic level, an asymmetric division produces two daughter cells, each possessing a different identity or fate. Drosophila melanogaster progenitor cells, referred to as neuroblasts, undergo asymmetric division to produce a daughter neuroblast and another cell known as a ganglion mother cell (GMC). There are several features of asymmetric division in Drosophila that make it a very complex process, and these aspects will be discussed at length. The cell fate determinants that play a role in specifying daughter cell fate, as well as the mechanisms behind setting up cortical polarity within neuroblasts, have proved to be essential to ensuring that neurogenesis occurs properly. The role that mitotic spindle orientation plays in coordinating asymmetric division, as well as how cell cycle regulators influence asymmetric division machinery, will also be addressed. Most significantly, malfunctions during asymmetric cell division have shown to be causally linked with neoplastic growth and tumor formation. Therefore, it is imperative that the developmental repercussions as a result of asymmetric cell division gone awry be understood.

Cycles and the qualitative evolution of chemical systems

Cycles are abundant in most kinds of networks, especially in biological ones. Here, we investigate their role in the evolution of a chemical reaction system from one self-sustaining composition of molecular species to another and their influence on the stability of these compositions. While it is accepted that, from a topological standpoint, they enhance network robustness, the consequence of cycles to the dynamics are not well understood. In a former study, we developed a necessary criterion for the existence of a fixed point, which is purely based on topological properties of the network. The structures of interest we identified were a generalization of closed autocatalytic sets, called chemical organizations. Here, we show that the existence of these chemical organizations and therefore steady states is linked to the existence of cycles. Importantly, we provide a criterion for a qualitative transition, namely a transition from one self-sustaining set of molecular species to another via the introduction of a cycle. Because results purely based on topology do not yield sufficient conditions for dynamic properties, e.g. stability, other tools must be employed, such as analysis via ordinary differential equations. Hence, we study a special case, namely a particular type of reflexive autocatalytic network. Applications for this can be found in nature, and we give a detailed account of the mitotic spindle assembly and spindle position checkpoints. From our analysis, we conclude that the positive feedback provided by these networks' cycles ensures the existence of a stable positive fixed point. Additionally, we use a genome-scale network model of the Escherichia coli sugar metabolism to illustrate our findings. In summary, our results suggest that the qualitative evolution of chemical systems requires the addition and elimination of cycles.

FACS purification and transcriptome analysis of drosophila neural stem cells reveals a role for Klumpfuss in self-renewal

Drosophila neuroblasts (NBs) have emerged as a model for stem cell biology that is ideal for genetic analysis but is limited by the lack of cell-type-specific gene expression data. Here, we describe a method for isolating large numbers of pure NBs and differentiating neurons that retain both cell-cycle and lineage characteristics. We determine transcriptional profiles by mRNA sequencing and identify 28 predicted NB-specific transcription factors that can be arranged in a network containing hubs for Notch signaling, growth control, and chromatin regulation. Overexpression and RNA interference for these factors identify Klumpfuss as a regulator of self-renewal. We show that loss of Klumpfuss function causes premature differentiation and that overexpression results in the formation of transplantable brain tumors. Our data represent a valuable resource for investigating Drosophila developmental neurobiology, and the described method can be applied to other invertebrate stem cell lineages as well.

klumpfuss distinguishes stem cells from progenitor cells during asymmetric neuroblast division

Asymmetric stem cell division balances maintenance of the stem cell pool and generation of diverse cell types by simultaneously allowing one daughter progeny to maintain a stem cell fate and its sibling to acquire a progenitor cell identity. A progenitor cell possesses restricted developmental potential, and defects in the regulation of progenitor cell potential can directly impinge on the maintenance of homeostasis and contribute to tumor initiation. Despite their importance, the molecular mechanisms underlying the precise regulation of restricted developmental potential in progenitor cells remain largely unknown. We used the type II neural stem cell (neuroblast) lineage in Drosophila larval brain as a genetic model system to investigate how an intermediate neural progenitor (INP) cell acquires restricted developmental potential. We identify the transcription factor Klumpfuss (Klu) as distinguishing a type II neuroblast from an INP in larval brains. klu functions to maintain the identity of type II neuroblasts, and klu mutant larval brains show progressive loss of type II neuroblasts due to premature differentiation. Consistently, Klu protein is detected in type II neuroblasts but is undetectable in immature INPs. Misexpression of klu triggers immature INPs to revert to type II neuroblasts. In larval brains lacking brain tumor function or exhibiting constitutively activated Notch signaling, removal of klu function prevents the reversion of immature INPs. These results led us to propose that multiple mechanisms converge to exert precise control of klu and distinguish a progenitor cell from its sibling stem cell during asymmetric neuroblast division.

Neural stem cells in Drosophila: molecular genetic mechanisms underlying normal neural proliferation and abnormal brain tumor formation

Neural stem cells in Drosophila are currently one of the best model systems for understanding stem cell biology during normal development and during abnormal development of stem cell-derived brain tumors. In Drosophila brain development, the proliferative activity of neural stem cells called neuroblasts gives rise to both the optic lobe and the central brain ganglia, and asymmetric cell divisions are key features of this proliferation. The molecular mechanisms that underlie the asymmetric cell divisions by which these neuroblasts self-renew and generate lineages of differentiating progeny have been studied extensively and involve two major protein complexes, the apical complex which maintains polarity and controls spindle orientation and the basal complex which is comprised of cell fate determinants and their adaptors that are segregated into the differentiating daughter cells during mitosis. Recent molecular genetic work has established Drosophila neuroblasts as a model for neural stem cell-derived tumors in which perturbation of key molecular mechanisms that control neuroblast proliferation and the asymmetric segregation of cell fate determinants lead to brain tumor formation. Identification of novel candidate genes that control neuroblast self-renewal and differentiation as well as functional analysis of these genes in normal and tumorigenic conditions in a tissue-specific manner is now possible through genome-wide transgenic RNAi screens. These cellular and molecular findings in Drosophila are likely to provide valuable genetic links for analyzing mammalian neural stem cells and tumor biology.

A big new job for small GTPases

Members of the Ras superfamily of small guanosine triphosphatases (GTPases) function as key nodes within signaling networks in a remarkable range of cellular processes, including cell proliferation, differentiation, growth, cell-cell adhesion and apoptosis. We recently described a novel role for the Ras-like small GTPases Rap1 and Ral in regulating cortical polarity and spindle orientation during asymmetric neuroblast division in Drosophila. The participation of these proteins in promoting cell polarization seems to be a common theme throughout evolution.

Asymmetric segregation and self-renewal of hematopoietic stem and progenitor cells with endocytic Ap2a2

The stem cell–intrinsic model of self-renewal via asymmetric cell division (ACD) posits that fate determinants be partitioned unequally between daughter cells to either activate or suppress the stemness state. ACD is a purported mechanism by which hematopoietic stem cells (HSCs) self-renew, but definitive evidence for this cellular process remains open to conjecture. To address this issue, we chose 73 candidate genes that function within the cell polarity network to identify potential determinants that may concomitantly alter HSC fate while also exhibiting asymmetric segregation at cell division. Initial gene-expression profiles of polarity candidates showed high and differential expression in both HSCs and leukemia stem cells. Altered HSC fate was assessed by our established in vitro to in vivo screen on a subcohort of candidate polarity genes, which revealed 6 novel positive regulators of HSC function: Ap2a2, Gpsm2, Tmod1, Kif3a, Racgap1, and Ccnb1. Interestingly, live-cell videomicroscopy of the endocytic protein AP2A2 shows instances of asymmetric segregation during HSC/progenitor cell cytokinesis. These results contribute further evidence that ACD is functional in HSC self-renewal, suggest a role for Ap2a2 in HSC activity, and provide a unique opportunity to prospectively analyze progeny from HSC asymmetric divisions.

Cortical aPKC kinase activity distinguishes neural stem cells from progenitor cells by ensuring asymmetric segregation of Numb

During asymmetric stem cell division, polarization of the cell cortex targets fate determinants unequally into the sibling daughters, leading to regeneration of a stem cell and production of a progenitor cell with restricted developmental potential. In mitotic neural stem cells (neuroblasts) in fly larval brains, the antagonistic interaction between the polarity proteins Lethal (2) giant larvae (Lgl) and atypical Protein Kinase C (aPKC) ensures self-renewal of a daughter neuroblast and generation of a progenitor cell by regulating asymmetric segregation of fate determinants. In the absence of lgl function, elevated cortical aPKC kinase activity perturbs unequal partitioning of the fate determinants including Numb and induces supernumerary neuroblasts in larval brains. However, whether increased aPKC function triggers formation of excess neuroblasts by inactivating Numb remains controversial. To investigate how increased cortical aPKC function induces formation of excess neuroblasts, we analyzed the fate of cells in neuroblast lineage clones in lgl mutant brains. Surprisingly, our analyses revealed that neuroblasts in lgl mutant brains undergo asymmetric division to produce progenitor cells, which then revert back into neuroblasts. In lgl mutant brains, Numb remained localized in the cortex of mitotic neuroblasts and failed to segregate exclusively into the progenitor cell following completion of asymmetric division. These results led us to propose that elevated aPKC function in the cortex of mitotic neuroblasts reduces the function of Numb in the future progenitor cells. We identified that the acyl-CoA binding domain containing 3 protein (ACBD3) binding region is essential for asymmetric segregation of Numb in mitotic neuroblasts and suppression of the supernumerary neuroblast phenotype induced by increased aPKC function. The ACBD3 binding region of Numb harbors two aPKC phosphorylation sites, serines 48 and 52. Surprisingly, while the phosphorylation status at these two sites directly impinged on asymmetric segregation of Numb in mitotic neuroblasts, both the phosphomimetic and non-phosphorylatable forms of Numb suppressed formation of excess neuroblasts triggered by increased cortical aPKC function. Thus, we propose that precise regulation of cortical aPKC kinase activity distinguishes the sibling cell identity in part by ensuring asymmetric partitioning of Numb into the future progenitor cell where Numb maintains restricted potential independently of regulation by aPKC.

The Rap1-Rgl-Ral signaling network regulates neuroblast cortical polarity and spindle orientation

The Rap1–Rgl–Ral signaling network modulates asymmetric Drosophila neuroblast division in cooperation with other intrinsic polarity cues.

A crucial first step in asymmetric cell division is to establish an axis of cell polarity along which the mitotic spindle aligns. Drosophila melanogaster neural stem cells, called neuroblasts (NBs), divide asymmetrically through intrinsic polarity cues, which regulate spindle orientation and cortical polarity. In this paper, we show that the Ras-like small guanosine triphosphatase Rap1 signals through the Ral guanine nucleotide exchange factor Rgl and the PDZ protein Canoe (Cno; AF-6/Afadin in vertebrates) to modulate the NB division axis and its apicobasal cortical polarity. Rap1 is slightly enriched at the apical pole of metaphase/anaphase NBs and was found in a complex with atypical protein kinase C and Par6 in vivo. Loss of function and gain of function of Rap1, Rgl, and Ral proteins disrupt the mitotic axis orientation, the localization of Cno and Mushroom body defect, and the localization of cell fate determinants. We propose that the Rap1–Rgl–Ral signaling network is a novel mechanism that cooperates with other intrinsic polarity cues to modulate asymmetric NB division.

Inscuteable regulates the Pins-Mud spindle orientation pathway

During asymmetric cell division, alignment of the mitotic spindle with the cell polarity axis ensures that the cleavage furrow separates fate determinants into distinct daughter cells. The protein Inscuteable (Insc) is thought to link cell polarity and spindle positioning in diverse systems by binding the polarity protein Bazooka (Baz; aka Par-3) and the spindle orienting protein Partner of Inscuteable (Pins; mPins or LGN in mammals). Here we investigate the mechanism of spindle orientation by the Insc-Pins complex. Previously, we defined two Pins spindle orientation pathways: a complex with Mushroom body defect (Mud; NuMA in mammals) is required for full activity, whereas binding to Discs large (Dlg) is sufficient for partial activity. In the current study, we have examined the role of Inscuteable in mediating downstream Pins-mediated spindle orientation pathways. We find that the Insc-Pins complex requires Gαi for partial activity and that the complex specifically recruits Dlg but not Mud. In vitro competition experiments revealed that Insc and Mud compete for binding to the Pins TPR motifs, while Dlg can form a ternary complex with Insc-Pins. Our results suggest that Insc does not passively couple polarity and spindle orientation but preferentially inhibits the Mud pathway, while allowing the Dlg pathway to remain active. Insc-regulated complex assembly may ensure that the spindle is attached to the cortex (via Dlg) before activation of spindle pulling forces by Dynein/Dynactin (via Mud).

A niche for Drosophila neuroblasts?

Stem cells, which can self-renew and give rise to differentiated daughters, are responsible for the generation of diverse cell types during development and the maintenance of tissue/organ homeostasis in adulthood. Thus, the precise regulation of stem-cell self-renewal and proliferative potential is a key aspect of development. The stem-cell niche confers such control by concentrating localized factors including signaling molecules which favor stem-cell self-renew and regulate stem-cell proliferation in line with developmental programs. In contrast, Drosophila neuroblasts (NBs), often referred to as neural stem cells/progenitors, can undergo asymmetric cell division to self-renew and produce differentiated daughters even in isolation (or in culture). Furthermore, these isolated NBs can also progress through an intrinsically regulated temporal series (of transcription factor expression) to generate diverse cell types in vitro. These data argue that NBs may depend only to a limited extent, if at all, on local environment (a niche) for their maintenance. On the other hand, there is increasing evidence which indicate that the interaction between NBs and their surrounding glia is critical for the control of NB proliferative potential and these glia, in conjunction with systemic regulation, perform the niche function to regulate NB behavior. Thus, these observations emphasize the importance of coordinated local microenvironment (niche activity) and systemic environment (global activity) on the regulation of NB behavior in vivo, and suggest NBs may conform to an alternative stem-cell/progenitor maintenance model.

Shaping microtubules into diverse patterns: molecular connections for setting up both ends

Microtubules serve as rails for intracellular trafficking and their appropriate organization is critical for the generation of cell polarity, which is a foundation of cell differentiation, tissue morphogenesis, ontogenesis and the maintenance of homeostasis. The microtubule array is not just a static railway network; it undergoes repeated collapse and reassembly in diverse patterns during cell morphogenesis. In the last decade much progress has been made toward understanding the molecular mechanisms governing complex microtubule patterning. This review first revisits the basic principle of microtubule dynamics, and then provides an overview of how microtubules are arranged in highly shaped and functional patterns in cells changing their morphology by factors controlling the fate of microtubule ends.

Functional genomics identifies neural stem cell sub-type expression profiles and genes regulating neuroblast homeostasis

The Drosophila larval central brain contains about 10,000 differentiated neurons and 200 scattered neural progenitors (neuroblasts), which can be further subdivided into ~95 type I neuroblasts and eight type II neuroblasts per brain lobe. Only type II neuroblasts generate self-renewing intermediate neural progenitors (INPs), and consequently each contributes more neurons to the brain, including much of the central complex. We characterized six different mutant genotypes that lead to expansion of neuroblast numbers; some preferentially expand type II or type I neuroblasts. Transcriptional profiling of larval brains from these mutant genotypes versus wild-type allowed us to identify small clusters of transcripts enriched in type II or type I neuroblasts, and we validated these clusters by gene expression analysis. Unexpectedly, only a few genes were found to be differentially expressed between type I/II neuroblasts, suggesting that these genes play a large role in establishing the different cell types. We also identified a large group of genes predicted to be expressed in all neuroblasts but not in neurons. We performed a neuroblast-specific, RNAi-based functional screen and identified 84 genes that are required to maintain proper neuroblast numbers; all have conserved mammalian orthologs. These genes are excellent candidates for regulating neural progenitor self-renewal in Drosophila and mammals.

Fragile X mental retardation protein and stem cells

Stem cells, which can self-renew and produce different cell types, are regulated by both extrinsic signals and intrinsic factors. Fragile X syndrome, one of the most common forms of inherited mental retardation, is caused by the functional loss of fragile X mental retardation protein (FMRP). FMRP is a selective RNA-binding protein that forms a messenger ribonucleoprotein (mRNP) complex that associates with polyribosomes. Recently, the role of Fmrp in stem cell biology has been explored in both Drosophila and the mouse. In this chapter, we discuss the role of FMRP in regulating the proliferation and differentiation of stem cells.

Drosophila neuroblasts retain the daughter centrosome

During asymmetric mitosis, both in male Drosophila germline stem cells and in mouse embryo neural progenitors, the mother centrosome is retained by the self-renewed cell; hence suggesting that mother centrosome inheritance might contribute to stemness. We test this hypothesis in Drosophila neuroblasts (NBs) tracing photo converted centrioles and a daughter-centriole-specific marker generated by cloning the Drosophila homologue of human Centrobin. Here we show that upon asymmetric mitosis, the mother centrosome is inherited by the differentiating daughter cell. Our results demonstrate maturation-dependent centrosome fate in Drosophila NBs and that the stemness properties of these cells are not linked to mother centrosome inheritance.

Asymmetric partitioning of centrosomes has been reported in Drosophila neuroblasts, but whether this type of division has implications for stem cell self-renewal is unclear. In this study, the authors show that the asymmetric division of the centrosomes correlates with the asymmetric fate of the cells and that the daughter centrosome is retained by the neuroblast.

Phosphorylation and dephosphorylation events allow for rapid segregation of fate determinants during Drosophila neuroblast asymmetric divisions

Drosophila neuroblasts display remarkable asymmetry throughout mitosis. The most prominent asymmetry is the size difference between daughter cells at cytokinesis. The larger cell retains stem cell identity, i.e., remains a neuroblast while the smaller cell, called a ganglion mother cell (GMC), will generate differentiated neural and glial progeny. Preceding this size difference, several protein complexes localize to opposite sides of the neuroblast cortex (apical and basal in the embryo and, by analogy, referred to as such in larval neuroblasts although their asymmetry no longer correlates with such axis). The plane of division is coordinated with this molecular asymmetry such that apical and basal complexes are unequally partitioned between the two daughter cells: apical complexes are inherited by the self-renewing neuroblast while basal complexes are inherited by the GMC. This unequal segregation has been extensively shown to be functionally significant. Apical complexes contain factors required for cellular selfrenewal and basal complexes contain factors required for the differentiation of the GMC progeny. Curiously, however, some "basal" neuroblast proteins such as the scaffold protein Miranda (Mira) and its associated fate determinant Prospero (Pros), are initially apically localized prior to translocating to the opposite side of the cell cortex by the onset of mitosis. This is because mira mRNA is apically enriched, where it remains throughout the cell cycle, suggesting that Mira protein is translated within the apical environment.1,2 The transition from apical to basal enrichment of Mira and Pros takes place within minutes.2 Here, we summarize the known phosphorylation events and roles during neuroblast asymmetric divisions, as well as very recent work, including our own, identifying the first protein phosphatases implicated in this process. We then discuss models previously proposed, as well as a new model, for apical-to-basal transition of the Mira complex in light of our new results.

Epithelial cell polarity: a major gatekeeper against cancer?

The correct establishment and maintenance of cell polarity are crucial for normal cell physiology and tissue homeostasis. Conversely, loss of cell polarity, tissue disorganisation and excessive cell growth are hallmarks of cancer. In this review, we focus on identifying the stages of tumoural development that are affected by the loss or deregulation of epithelial cell polarity. Asymmetric division has recently emerged as a major regulatory mechanism that controls stem cell numbers and differentiation. Links between cell polarity and asymmetric cell division in the context of cancer will be examined. Apical–basal polarity and cell–cell adhesion are tightly interconnected. Hence, how loss of cell polarity in epithelial cells may promote epithelial mesenchymal transition and metastasis will also be discussed. Altogether, we present the argument that loss of epithelial cell polarity may have an important role in both the initiation of tumourigenesis and in later stages of tumour development, favouring the progression of tumours from benign to malignancy.

Atypical protein kinase C controls sea urchin ciliogenesis

The distribution and function of aPKC are examined during sea urchin ciliogenesis. The kinase concentrates in a ring at the transition zone between the basal body and the elongating axoneme. Inhibition of aPKC results in mislocalization of the kinase and defective ciliogenesis. Thus aPKC controls the growth of motile cilia in invertebrate embryos.

The atypical protein kinase C (aPKC) is part of the conserved aPKC/PAR6/PAR3 protein complex, which regulates many cell polarity events, including the formation of a primary cilium at the apical surface of epithelial cells. Cilia are highly organized, conserved, microtubule-based structures involved in motility, sensory processes, signaling, and cell polarity. We examined the distribution and function of aPKC in the sea urchin embryo, which forms a swimming blastula covered with motile cilia. We found that in the early embryo aPKC is uniformly cortical and becomes excluded from the vegetal pole during unequal cleavages at the 8- to 64-cell stages. During the blastula and gastrula stages the kinase localizes at the base of cilia, forming a ring at the transition zone between the basal body and the elongating axoneme. A dose-dependent and reversible inhibition of aPKC results in mislocalization of the kinase, defective ciliogenesis, and lack of swimming. Thus, as in the primary cilium of differentiated mammalian cells, aPKC controls the growth of motile cilia in invertebrate embryos. We suggest that aPKC might function to phosphorylate kinesin and so activate the transport of intraflagellar vesicles.

DREF is critical for Drosophila bristle development by regulating endoreplication in shaft cells

DREF (DNA replication-related element-binding factor) plays important roles in replication and proliferation in vivo by regulating transcription of various genes. However, due to a lack of appropriate cell biological studies in vivo, roles of DREF during a single cell development are poorly understood. To address this question, we focused our attention on macrochaetes bristle development system. Utilizing cell lineage analysis focusing on a single posterior scutellar (PSC) macrochaete sensory organ precursor (SOP) lineages in combination with GAL4/UAS targeted expression system for DREF double strand RNA, we revealed that DREF plays no apparent role in differentiation process during SOP formation. Rather, DREF regulates the timing of asymmetric cell division but perhaps plays no direct role in differentiation during asymmetric cell division. Most importantly, DREF affected replication and growth in shaft cells and/or socket cells. Further analysis revealed that DREF is necessary but not sufficient for nuclear growth and protein synthesis in shaft cells. Finally, it could be demonstrated that DREF plays a critical role in regulating pcna transcription in endocycling shaft cells. All these results provide evidence that DREF plays critical roles, especially in endoreplication process of bristle development, at least in part by regulating the pcna gene expression.

Transcriptome asymmetry within mouse zygotes but not between early embryonic sister blastomeres

Transcriptome regionalization is an essential polarity determinant among metazoans, directing embryonic axis formation during normal development. Although conservation of this principle in mammals is assumed, recent evidence is conflicting and it is not known whether transcriptome asymmetries exist within unfertilized mammalian eggs or between the respective cleavage products of early embryonic divisions. We here address this by comparing transcriptome profiles of paired single cells and sub-cellular structures obtained microsurgically from mouse oocytes and totipotent embryos. Paired microsurgical spindle and remnant samples from unfertilized metaphase II oocytes possessed distinguishable profiles. Fertilization produces a totipotent 1-cell embryo (zygote) and associated spindle-enriched second polar body whose paired profiles also differed, reflecting spindle transcript enrichment. However, there was no programmed transcriptome asymmetry between sister cells within 2- or 3-cell embryos. Accordingly, there is transcriptome asymmetry within mouse oocytes, but not between the sister blastomeres of early embryos. This work places constraints on pre-patterning in mammals and provides documentation correlating potency changes and transcriptome partitioning at the single-cell level.

On the cutting edge: post-translational modifications in cytokinesis

Cytokinesis represents the final stage in the cell cycle, in which two daughter cells, each with their complement of the duplicated genome, physically separate. At the core of this process sits highly conserved machinery responsible for specifying the plane of division, building a contractile apparatus and ultimately cleaving cells in two. Although the 'parts list' of contributing proteins has been well described, mechanisms by which these parts are spatially and temporally regulated are only beginning to be understood. With advancements in biochemical and proteomic analyses, recent work has uncovered multiple new roles for post-translational modifications in the regulation of cytokinesis. Here, we review these latest findings and interpret our current understanding of cytokinesis in light of relevant modifications.

Drosophila neural stem cells: cell cycle control of self-renewal, differentiation, and termination in brain development

The wealth of neurons that make up the brain are generated through the proliferative activity of neural stem cells during development. This neurogenesis activity involves complex cell cycle control of proliferative self-renewal, differentiation, and termination processes in these cells. Considerable progress has been made in understanding these processes in the neural stem cell-like neuroblasts which generate the brain in the genetic model system Drosophila. Neuroblasts in the developing fly brain generate neurons through repeated series of asymmetrical cell divisions, which balance self-renewal of the neuroblast with generation of differentiated progeny through the segregation of cell fate determinants such as Numb, Prospero, and Brat to the neural progeny. A number of classical cell cycle regulators such as cdc2/CDK1, Polo, Aurora A, and cyclin E are implicated in the control of asymmetric divisions in neuroblasts linking the cell cycle to the asymmetrical division machinery. The cellular and molecular identity of the postmitotic neurons produced by proliferating neuroblasts is influenced by the timing of their exit from the cell cycle through the action of a temporal expression series of transcription factors, which include Hunchback, Kruppel, Pdm, and Castor. This temporal series is also implicated in the control of termination of neuroblast proliferation which is effected by two different cell cycle exit strategies, terminal differentiative division or programmed cell death of the neuroblast. Defects in the asymmetric division machinery which interfere with the termination of proliferation can result in uncontrolled tumorigenic overgrowth. These findings in Drosophila brain development are likely to have general relevance in neural stem cell biology and may apply to cell cycle control in mammalian brain development as well.

A tale of tailless

Drosophila Tailless(Tll) and its vertebrate homologue Tlx are conserved orphan nuclear receptors specifically expressed in the eye and the forebrain. Tll and Tlx act primarily as transcriptional repressors through their interactions with transcriptional corepressors, Atrophin family proteins, and histone-tail/chromatin-modifying factors such as lysine-specific histone demethylase 1 and histone deacetylases. The functional importance of Tll and Tlx is made apparent by the recent discovery that they are expressed in neural stem cells (NSCs) and are required for self-renewal of these cells in both Drosophila and the mouse. This review provides a snapshot of current knowledge about Tll and Tlx and their transcriptional network, which maintains NSCs in developing and adult animals.

Amplicon-dependent CCNE1 expression is critical for clonogenic survival after cisplatin treatment and is correlated with 20q11 gain in ovarian cancer

Genomic amplification of 19q12 occurs in several cancer types including ovarian cancer where it is associated with primary treatment failure. We systematically attenuated expression of genes within the minimally defined 19q12 region in ovarian cell lines using short-interfering RNAs (siRNA) to identify driver oncogene(s) within the amplicon. Knockdown of CCNE1 resulted in G1/S phase arrest, reduced cell viability and apoptosis only in amplification-carrying cells. Although CCNE1 knockdown increased cisplatin resistance in short-term assays, clonogenic survival was inhibited after treatment. Gain of 20q11 was highly correlated with 19q12 amplification and spanned a 2.5 Mb region including TPX2, a centromeric protein required for mitotic spindle function. Expression of TPX2 was highly correlated with gene amplification and with CCNE1 expression in primary tumors. siRNA inhibition of TPX2 reduced cell viability but this effect was not amplicon-dependent. These findings demonstrate that CCNE1 is a key driver in the 19q12 amplicon required for survival and clonogenicity in cells with locus amplification. Co-amplification at 19q12 and 20q11 implies the presence of a cooperative mutational network. These observations have implications for the application of targeted therapies in CCNE1 dependent ovarian cancers.

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.

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.

A cross-disciplinary approach to understanding neural stem cells in development and disease

The Company of Biologists recently launched a new series of workshops aimed at bringing together scientists with different backgrounds to discuss cutting edge research in emerging and cross-disciplinary areas of biology. The first workshop was held at Wilton Park, Sussex, UK, and the chosen theme was 'Neural Stem Cells in Development and Disease', which is indeed a hot topic, not only because of the potential use of neural stem cells in cell replacement therapies to treat neurodegenerative diseases, but also because alterations in their behaviour can, in certain cases, lie at the origin of brain tumours and other diseases.

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.

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.

Hedgehog-dependent down-regulation of the tumor suppressor, vitamin D3 up-regulated protein 1 (VDUP1), precedes lamina development in Drosophila

The tumor suppressor vitamin D(3) up-regulated protein 1 (VDUP1) is expressed throughout the developing and mature Drosophila nervous system, but its regulatory pathways are not well understood. Within the developing Drosophila visual system, down-regulation of VDUP1 in lamina precursor cells (LPCs) coincided with the arrival of retinal axons into the lamina target field, suggesting VDUP1 regulation by an axonally transmitted signal. Hedgehog (Hh) is a signal well known to coordinate LPC proliferation and differentiation in response to retinal axon innervation, and analysis of orthologous dvdup1 promoters identified an evolutionarily conserved binding site for the Hh-dependent transcription factor cubitus interruptus (Ci). Hh-dependent regulation of VDUP1 in the developing lamina was confirmed in Hh loss-of-function backgrounds where VDUP1 expression was maintained in LPCs, inhibiting both cell proliferation and lamina neurogenesis. This putative coupling of VDUP1 to the Hh signaling pathway may provide novel insights into the mechanisms controlling brain growth and development.

Asymmetric cell divisions and asymmetric cell fates

The regulation of self-renewal, cell diversity, and differentiation can occur by modulating symmetric and asymmetric cell divisions. Remarkably, asymmetric cell divisions can arise through multiple processes in which molecules in the cytoplasm and nucleus, as well as template "immortal" DNA strands, can segregate to one daughter cell during cell division. Explaining how these events direct distinct daughter cell fates is a major challenge to understanding how the organism is assembled and maintained for a lifetime. Numerous technical issues that are associated with assessing how distinct cell fates are executed in vivo have resulted in divergent interpretations of experimental findings. This review addresses some of these points and considers different developmental model systems that attempt to investigate how cell fate decisions are determined, as well as the molecules that guide these choices.

Dividing cellular asymmetry: asymmetric cell division and its implications for stem cells and cancer

Cell division is commonly thought to involve the equal distribution of cellular components into the two daughter cells. During many cell divisions, however, proteins, membrane compartments, organelles, or even DNA are asymmetrically distributed between the two daughter cells. Here, we review the various types of asymmetries that have been described in yeast and in animal cells. Asymmetric segregation of protein determinants is particularly relevant for stem cell biology. We summarize the relevance of asymmetric cell divisions in various stem cell systems and discuss why defects in asymmetric cell division can lead to the formation of tumors.

Cbk1 regulation of the RNA-binding protein Ssd1 integrates cell fate with translational control

Spatial control of gene expression, at the level of both transcription and translation, is critical for cellular differentiation [1-4]. In budding yeast, the conserved Ndr/warts kinase Cbk1 localizes to the new daughter cell, where it acts as a cell fate determinant. Cbk1 both induces a daughter-specific transcriptional program and promotes morphogenesis in a less well-defined role [5-8]. Cbk1 is essential in cells expressing functional Ssd1, an RNA-binding protein of unknown function [9-11]. We show here that Cbk1 inhibits Ssd1 in vivo. Loss of this regulation dramatically slows bud expansion, leading to highly aberrant cell wall organization at the site of cell growth. Ssd1 associates with specific mRNAs, a significant number of which encode cell wall remodeling proteins. Translation of these messages is rapidly and specifically suppressed when Cbk1 is inhibited; this suppression requires Ssd1. Transcription of several of these Ssd1-associated mRNAs is also regulated by Cbk1, indicating that the kinase controls both the transcription and translation of daughter-specific mRNAs. This work suggests a novel system by which cells coordinate localized expression of genes involved in processes critical for cell growth and division.

Forced G1-phase reduction alters mode of division, neuron number, and laminar phenotype in the cerebral cortex

The link between cortical precursors G1 duration (TG1) and their mode of division remains a major unresolved issue of potential importance for regulating corticogenesis. Here, we induced a 25% reduction in TG1 in mouse cortical precursors via forced expression of cyclin D1 and cyclin E1. We found that in utero electroporation-mediated gene transfer transfects a cohort of synchronously cycling precursors, necessitating alternative methods of measuring cell-cycle phases to those classical used. TG1 reduction promotes cell-cycle reentry at the expense of differentiation and increases the self-renewal capacities of Pax6 precursors as well as of Tbr2 basal precursors (BPs). A population level analysis reveals sequential and lineage-specific effects, showing that TG1 reduction: (i) promotes Pax6 self-renewing proliferative divisions before promoting divisions wherein Pax6 precursors generate Tbr2 BPs and (ii) promotes self-renewing proliferative divisions of Tbr2 precursors at the expense of neurogenesis, thus leading to an amplification of the BPs pool in the subventricular zone and the dispersed mitotic compartment of the intermediate zone. These results point to the G1 mode of division relationship as an essential control mechanism of corticogenesis. This is further supported by long-term studies showing that TG1 reduction results in cytoarchitectural modifications including supernumerary supragranular neuron production. Modeling confirms that the TG1-induced changes in neuron production and laminar fate are mediated via the changes in the mode of division. These findings also have implications for understanding the mechanisms that have contributed to brain enlargement and complexity during evolution.

Cell cycle independent role of Cyclin E during neural cell fate specification in Drosophila is mediated by its regulation of Prospero function

During development, neural progenitor cells or neuroblasts generate a great intra- and inter-segmental diversity of neuronal and glial cell types in the nervous system. In thoracic segments of the embryonic central nervous system of Drosophila, the neuroblast NB6-4t undergoes an asymmetric first division to generate a neuronal and a glial sublineage, while abdominal NB6-4a divides once symmetrically to generate only 2 glial cells. We had earlier reported a critical function for the G1 cyclin, CyclinE (CycE) in regulating asymmetric cell division in NB6-4t. Here we show that (i) this function of CycE is independent of its role in cell cycle regulation and (ii) the two functions are mediated by distinct domains at the protein level. Results presented here also suggest that CycE inhibits the function of Prospero and facilitates its cortical localization, which is critical for inducing stem cell behaviour, i.e. asymmetric cell division of NB6-4t. Furthermore our data imply that CycE is required for the maintenance of stem cell identity of most other neuroblasts.

Coupling the cell cycle to development

The core machinery that drives the eukaryotic cell cycle has been thoroughly investigated over the course of the past three decades. It is only more recently, however, that light has been shed on the mechanisms by which elements of this core machinery are modulated to alter cell cycle progression during development. It has also become increasingly clear that, conversely, core cell cycle regulators can play a crucial role in developmental processes. Here, focusing on findings from Drosophila melanogaster and Caenorhabditis elegans, we review the importance of modulating the cell cycle during development and discuss how core cell cycle regulators participate in determining cell fates.

PAR-1 phosphorylates Mind bomb to promote vertebrate neurogenesis

Generation of neurons in the vertebrate central nervous system requires a complex transcriptional regulatory network and signaling processes in polarized neuroepithelial progenitor cells. Here we demonstrate that neurogenesis in the Xenopus neural plate in vivo and mammalian neural progenitors in vitro involves intrinsic antagonistic activities of the polarity proteins PAR-1 and aPKC. Furthermore, we show that Mind bomb (Mib), a ubiquitin ligase that promotes Notch ligand trafficking and activity, is a crucial molecular substrate for PAR-1. The phosphorylation of Mib by PAR-1 results in Mib degradation, repression of Notch signaling, and stimulation of neuronal differentiation. These observations suggest a conserved mechanism for neuronal fate determination that might operate during asymmetric divisions of polarized neural progenitor cells.

Genetic mechanisms regulating stem cell self-renewal and differentiation in the central nervous system of Drosophila

Recent studies using the Drosophila central nervous system as a model have identified key molecules and mechanisms underlying stem cell self-renewal and differentiation. These studies suggest that proteins like Aurora-A, atypical protein kinase C, Prospero and Brain tumor act as key regulators in a tightly coordinated interplay between mitotic spindle orientation and asymmetric protein localization. These data also provide initial evidence that both processes are coupled to cell cycle progression and growth control, thereby regulating a binary switch between proliferative stem self-renewal and differentiative progenitor cell specification. Considering the evolutionary conservation of some of the mechanisms and molecules involved, these data provide a rationale and genetic model for understanding stem cell self-renewal and differentiation in general. The new data gained in Drosophila may therefore lead to conceptual advancements in understanding the aetiology and treatment of human neurological disorders such as brain tumor formation and neurodegenerative diseases.

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.

Jak-STAT regulation of male germline stem cell establishment during Drosophila embryogenesis

Germline stem cells (GSCs) in Drosophila are descendants of primordial germ cells (PGCs) specified during embryogenesis. The precise timing of GSC establishment in the testis has not been determined, nor is it known whether mechanisms that control GSC maintenance in the adult are involved in GSC establishment. Here, we determine that PGCs in the developing male gonad first become GSCs at the embryo to larval transition. This coincides with formation of the embryonic hub; the critical signaling center that regulates adult GSC behavior within the stem cell microenvironment (niche). We find that the Jak-STAT signaling pathway is activated in a subset of PGCs that associate with the newly-formed embryonic hub. These PGCs express GSC markers and function like GSCs, while PGCs that do not associate with the hub begin to differentiate. In the absence of Jak-STAT activation, PGCs adjacent to the hub fail to exhibit the characteristics of GSCs, while ectopic activation of the Jak-STAT pathway prevents differentiation. These findings show that stem cell formation is closely linked to development of the stem cell niche, and suggest that Jak-STAT signaling is required for initial establishment of the GSC population in developing testes.

Self-renewal of stem cells

Asymmetric division is one of the most fundamental characteristics of adult stem cells , which ensures one daughter cell maintains stem cell status and the other daughter cell becomes committed to differentiation. New data emerged recently that allow us to conclude that asymmetric division has another important aspect: it enables self-maintenance of stem cells.

CDKB1;1 forms a functional complex with CYCA2;3 to suppress endocycle onset

The mitosis-to-endocycle transition requires the controlled inactivation of M phase-associated cyclin-dependent kinase (CDK) activity. Previously, the B-type CDKB1;1 was identified as an important negative regulator of endocycle onset. Here, we demonstrate that CDKB1;1 copurifies and associates with the A2-type cyclin CYCA2;3. Coexpression of CYCA2;3 with CDKB1;1 triggered ectopic cell divisions and inhibited endoreduplication. Moreover, the enhanced endoreduplication phenotype observed after overexpression of a dominant-negative allele of CDKB1;1 could be partially complemented by CYCA2;3 co-overexpression, illustrating that both subunits unite in vivo to form a functional complex. CYCA2;3 protein stability was found to be controlled by CCS52A1, an activator of the anaphase-promoting complex. We conclude that CCS52A1 participates in endocycle onset by down-regulating CDKB1;1 activity through the destruction of CYCA2;3.

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.

Molecular properties of CD133+ glioblastoma stem cells derived from treatment-refractory recurrent brain tumors

Glioblastoma multiforme (GBM) remains refractory to conventional therapy. CD133+ GBM cells have been recently isolated and characterized as chemo-/radio-resistant tumor-initiating cells and are hypothesized to be responsible for post-treatment recurrence. In order to explore the molecular properties of tumorigenic CD133+ GBM cells that resist treatment, we isolated CD133+ GBM cells from tumors that are recurrent and have previously received chemo-/radio-therapy. We found that the purified CD133+ GBM cells sorted from the CD133+ GBM spheres express SOX2 and CD44 and are capable of clonal self-renewal and dividing to produce fast-growing CD133− progeny, which form the major cell population within GBM spheres. Intracranial injection of purified CD133+, not CD133− GBM daughter cells, can lead to the development of YKL-40+ infiltrating tumors that display hypervascularity and pseudopalisading necrosis-like features in mouse brain. The molecular profile of purified CD133+ GBM cells revealed characteristics of neuroectoderm-like cells, expressing both radial glial and neural crest cell developmental genes, and portraying a slow-growing, non-differentiated, polarized/migratory, astrogliogenic, and chondrogenic phenotype. These data suggest that at least a subset of treated and recurrent GBM tumors may be seeded by CD133+ GBM cells with neural and mesenchymal properties. The data also imply that CD133+ GBM cells may be clinically indolent/quiescent prior to undergoing proliferative cell division (PCD) to produce CD133− GBM effector progeny. Identifying intrinsic and extrinsic cues, which promote CD133+ GBM cell self-renewal and PCD to support ongoing tumor regeneration may highlight novel therapeutic strategies to greatly diminish the recurrence rate of GBM.