Asymmetric localization of a mammalian numb homolog during mouse cortical neurogenesis

Cyclin D2 in the basal process of neural progenitors is linked to non-equivalent cell fates

Asymmetric cell division plays an indispensable role during corticogenesis for producing new neurons while maintaining a self-renewing pool of apical progenitors. The cellular and molecular determinants favouring asymmetric division are not completely understood. Here, we identify a novel mechanism for generating cellular asymmetry through the active transportation and local translation of Cyclin D2 mRNA in the basal process. This process is regulated by a unique cis-regulatory sequence found in the 3' untranslated region (3'UTR) of the mRNA. Unequal inheritance of Cyclin D2 protein to the basally positioned daughter cell with the basal process confers renewal of the apical progenitor after asymmetric division. Conversely, depletion of Cyclin D2 in the apically positioned daughter cell results in terminal neuronal differentiation. We demonstrate that Cyclin D2 is also expressed in the developing human cortex within similar domains, thus indicating that its role as a fate determinant is ancient and conserved.

The Notch pathway in hematopoietic stem cells

Hematopoiesis is the process that generates all the cell types of the blood, which are responsible for oxygen transport and immune defense. It has been now more than 50 years from the demonstration that blood cells derive from a common ancestor called Hematopoietic Stem Cell (HSC) McCulloch and Till (1960). Thus, the hematopoietic process relies on the unlimited and distinctive self-renewal ability of HSC, which in the adult mammalian organisms reside in the bone marrow, but their generation occurs during embryonic life. Questions still remain about how HSCs acquire and maintain the features of self-renewal and pluripotency that define stem-cell populations. Notch is a crucial signaling pathway involved in the generation of cell diversity and stem-cell maintenance in different systems. In some cases, Notch prevents differentiation, while in other contexts Notch directly participates in promoting cell differentiation. In the following sections, we will review what is known about the role of Notch in HSC establishment and hematopoietic cell lineage specification.

Golgi during development

The Golgi is essential for processing proteins and sorting them, as well as plasma membrane components, to their final destinations. Not surprisingly, this organelle, a major compartment of the secretory pathway, is an important venue for regulating many aspects of development in both invertebrates and vertebrates. Through its role as a site for protein cleavage and glycosylation as well as through changes in its spatial organization and secretory trafficking, the Golgi exerts highly specific effects on cellular differentiation and morphogenesis by spatially and temporally constraining developmental pathways.

Development and evolution of the human neocortex

The size and surface area of the mammalian brain are thought to be critical determinants of intellectual ability. Recent studies show that development of the gyrated human neocortex involves a lineage of neural stem and transit-amplifying cells that forms the outer subventricular zone (OSVZ), a proliferative region outside the ventricular epithelium. We discuss how proliferation of cells within the OSVZ expands the neocortex by increasing neuron number and modifying the trajectory of migrating neurons. Relating these features to other mammalian species and known molecular regulators of the mouse neocortex suggests how this developmental process could have emerged in evolution.

Dronc caspase exerts a non-apoptotic function to restrain phospho-Numb-induced ectopic neuroblast formation in Drosophila

Drosophila neuroblasts have served as a model to understand how the balance of stem cell self-renewal versus differentiation is achieved. Drosophila Numb protein regulates this process through its preferential segregation into the differentiating daughter cell. How Numb restricts the proliferation and self-renewal potentials of the recipient cell remains enigmatic. Here, we show that phosphorylation at conserved sites regulates the tumor suppressor activity of Numb. Enforced expression of a phospho-mimetic form of Numb (Numb-TS4D) or genetic manipulation that boosts phospho-Numb levels, attenuates endogenous Numb activity and causes ectopic neuroblast formation (ENF). This effect on neuroblast homeostasis occurs only in the type II neuroblast lineage. We identify Dronc caspase as a novel binding partner of Numb, and demonstrate that overexpression of Dronc suppresses the effects of Numb-TS4D in a non-apoptotic and possibly non-catalytic manner. Reduction of Dronc activity facilitates ENF induced by phospho-Numb. Our findings uncover a molecular mechanism that regulates Numb activity and suggest a novel role for Dronc caspase in regulating neural stem cell homeostasis.

Numb links extracellular cues to intracellular polarity machinery to promote chemotaxis

Cell polarization is essential throughout development for proliferation, migration, and differentiation. However, it is not known how extracellular cues correctly orient cell polarity at distinct stages of development. Here, we show that the endocytic adaptor protein Numb, previously characterized for its role in cell proliferation, subsequently plays an important role in cell migration. In neural precursors stimulated with the chemotactic factor BDNF, Numb binds to activated TrkB, the BDNF receptor, and functions both as an endocytic regulator for TrkB and as a scaffold for aPKC (aPKC). Thus, Numb promotes BDNF-dependent aPKC activation. Interestingly, Numb is also a substrate of aPKC. When phosphorylated, Numb exhibits increased efficacy in binding TrkB and in promoting a chemotactic response to BDNF. Therefore, Numb functions in a feed-forward loop to promote chemotaxis of neural precursors, linking BDNF, an extracellular cue, to aPKC, a critical component of the intrinsic polarity machinery.

Notch in the vertebrate nervous system: an old dog with new tricks

The Notch pathway is prominent among those known to regulate neural development in vertebrates. Notch receptor activation can inhibit neurogenesis, maintain neural progenitor character, and in some contexts promote gliogenesis and drive binary fate choices. Recently, a wave of exciting studies has emerged, which has both solidified previously held assertions and expanded our understanding of Notch function during neurogenesis and in the adult brain. These studies have examined pathway regulators and interactions, as well as pathway dynamics, with respect to both gene expression and cell-cell signaling. Here, focusing primarily on vertebrates, we review the current literature on Notch signaling in the nervous system, and highlight numerous recent studies that have generated interesting and unexpected advances.

Numb protects renal proximal tubular cells from puromycin aminonucleoside-induced apoptosis through inhibiting Notch signaling pathway

Numb was originally discovered as an intrinsic cell fate determinant in Drosophila by antagonizing Notch signaling. The present study is to characterize the role of Numb in oxidative stress-induced apoptosis of renal proximal tubular cells. Exposure of NRK52E cells to puromycin aminonucleoside (PA) resulted in caspase 3-dependent apoptosis. Numb expression was downregulated by PA in a time- and dose-dependent manner. Knocking down endogenous Numb by siRNA sensitized NRK52E cells to PA-induced apoptosis, whereas overexpressing Numb protected NRK52E cells from PA-induced apoptosis. Moreover, PA activated Notch signaling in a time- and dose-dependent manner as indicated by increased expression of the intracellular domain of Notch and Hes-1. Notch signaling inhibitor DAPT significantly attenuated Numb siRNA-augmented apoptosis. On the other hand, overexpression of intracellular domain of Notch1 could reverse the protective effect of Numb on PA-induced apoptosis. Taken together, our data demonstrated that, in renal proximal tubular cells, Numb functions as a protective molecule on PA-induced apoptosis through antagonizing Notch signaling activity.

The relevance of symmetric and asymmetric cell divisions to human central nervous system diseases

During development of the embryonic central nervous system (CNS), large numbers of neurons and glia are generated from the neuroepithelium and its progenitor derivatives as a result of symmetric and asymmetric cell divisions. We describe the biology of symmetric and asymmetric cell divisions in the CNS as gleaned from animal models, and discuss the relevance of these processes to human CNS development and disease.

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.

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.

Zebrafish numb and numblike are involved in primitive erythrocyte differentiation

Background

Notch signaling is an evolutionarily conserved regulatory circuitry implicated in cell fate determination in various developmental processes including hematopoietic stem cell self-renewal and differentiation of blood lineages. Known endogenous inhibitors of Notch activity are Numb-Nb and Numblike-Nbl, which play partially redundant functions in specifying and maintaining neuronal differentiation. Nb and Nbl are expressed in most tissues including embryonic and adult hematopoietic tissues in mice and humans, suggesting possible roles for these proteins in hematopoiesis.

Methodology and Principal Findings

We employed zebrafish to investigate the possible functional role of Numb and Numblike during hematopoiesis, as this system allows a detailed analysis even in embryos with severe defects that would be lethal in other organisms. Here we describe that nb/nbl knockdown results in severe reduction or absence of embryonic erythrocytes in zebrafish. Interestingly, nb/nbl knocked-down embryos present severe downregulation of the erythroid transcription factor gata1. This results in erythroblasts which fail to mature and undergo apoptosis. Our results indicate that Notch activity is increased in embryos injected with nb/nbl morpholino, and we show that inhibition of Notch activation can partially rescue the hematopoietic phenotype.

Conclusions and Significance

Our results provide the first in vivo evidence of an involvement of Numb and Numblike in zebrafish erythroid differentiation during primitive hematopoiesis. Furthermore, we found that, at least in part, the nb/nbl morphant phenotype is due to enhanced Notch activation within hematopoietic districts, which in turn results in primitive erythroid differentiation defects.

Two novel human NUMB isoforms provide a potential link between development and cancer

We previously identified four functionally distinct human NUMB isoforms. Here, we report the identification of two additional isoforms and propose a link between the expression of these isoforms and cancer. These novel isoforms, NUMB5 and NUMB6, lack exon 10 and are expressed in cells known for polarity and migratory behavior, such as human amniotic fluid cells, glioblastoma and metastatic tumor cells. RT-PCR and luciferase assays demonstrate that NUMB5 and NUMB6 are less antagonistic to NOTCH signaling than other NUMB isoforms. Immunocytochemistry analyses show that NUMB5 and NUMB6 interact and complex with CDC42, vimentin and the CDC42 regulator IQGAP1 (IQ (motif) GTPase activating protein 1). Furthermore, the ectopic expression of NUMB5 and NUMB6 induces the formation of lamellipodia (NUMB5) and filopodia (NUMB6) in a CDC42- and RAC1-dependent manner. These results are complemented by in vitro and in vivo studies, demonstrating that NUMB5 and NUMB6 alter the migratory behavior of cells. Together, these novel isoforms may play a role in further understanding the NUMB function in development and cancer.

Keeping neural progenitor cells on a short leash during Drosophila neurogenesis

The developmental potential of stem cells and progenitor cells must be functionally distinguished to ensure the generation of diverse cell types while maintaining the stem cell pool throughout the lifetime of an organism. In contrast to stem cells, progenitor cells possess restricted developmental potential, allowing them to give rise to only a limited number of post-mitotic progeny. Failure to establish or maintain restricted progenitor cell potential can perturb tissue development and homeostasis, and probably contributes to tumor initiation. Recent studies using the developing fruit fly Drosophila larval brain have provided molecular insight into how the developmental potential is restricted in neural progenitor cells.

Epicardial spindle orientation controls cell entry into the myocardium

During heart morphogenesis, epicardial cells undergo an epithelial-to-mesenchymal transition (EMT) and migrate into the subepicardium. The cellular signals controlling this process are poorly understood. Here, we show that epicardial cells exhibit two distinct mitotic spindle orientations, directed either parallel or perpendicular to the basement membrane. Cells undergoing perpendicular cell division subsequently enter the myocardium. We found that loss of beta-catenin led to a disruption of adherens junctions and a randomization of mitotic spindle orientation. Loss of adherens junctions also disrupted Numb localization within epicardial cells, and disruption of Numb and Numblike expression in the epicardium led to randomized mitotic spindle orientations. Taken together, these data suggest that directed mitotic spindle orientation contributes to epicardial EMT and implicate a junctional complex of beta-catenin and Numb in the regulation of spindle orientation.

Numb regulates meiotic spindle organisation in mouse oocytes

Numb is an adaptor protein that controls the fate of cells in different species through asymmetrical inheritance by sibling cells during division. It has been investigated extensively in mitosis, mostly in neural progenitor cells, but its function in meiosis remains unknown. The present study was designed to investigate the expression, subcellular localisation and functional roles of Numb during mouse oocyte meiotic maturation. Using real-time polymerase chain reaction and western blotting, we found that the expression of Numb increased from the germinal vesicle (GV) to MII stages. Immunofluorescent staining revealed that Numb was mainly concentrated in the GV before meiosis resumption, aggregated in the vicinity of the chromosomes after GV breakdown and then localised to the spindle poles from prometaphase I to MII. Nocodazole treatment resulted in spindle destruction and Numb diffusion into the cytoplasm. However, Numb appeared at the spindle poles again once the spindles had formed when nocodazole-treated oocytes were washed and cultured for spindle recovery. Depletion of Numb by RNA interference resulted in chromosome misalignment, spindle deformation and even doubled spindle formation. Our results suggest that Numb is critical for spindle organisation during mouse oocytes meiosis. The present study provides evidence of a new function for Numb in addition to its action as a cell fate-determining factor.

The endocytic adaptor Numb regulates thymus size by modulating pre-TCR signaling during asymmetric division

Stem cells must proliferate and differentiate to generate the lineages that shape mature organs; understanding these 2 processes and their interaction is one of the central themes in current biomedicine. An intriguing aspect is asymmetric division, by which 2 daughter cells with different fates are generated. Several cell fate determinants participate in asymmetric division, with the endocytic adaptor Numb as the best-known example. Here, we have explored the role of asymmetric division in thymocyte development, visualizing the differential segregation of Numb and pre-TCR in thymic precursors. Analysis of mice where Numb had been inhibited by expressing a dominant negative revealed enhanced pre-T-cell receptor (TCR) signaling and a smaller thymus. Conversely, Numb overexpression resulted in loss of asymmetric division and a larger thymus. The conclusion is that Numb determines the levels of pre-TCR signaling in dividing thymocytes and, ultimately, the size of the pool from which mature T lymphocytes are selected.

The exon junction complex component Magoh controls brain size by regulating neural stem cell division

Brain structure and size requires precise division of neural stem cells (NSCs), which self-renew and generate intermediate neural progenitors (INPs) and neurons. The factors that regulate NSCs remain poorly understood, as do mechanistic explanations of how aberrant NSC division causes reduced brain size as seen in microcephaly. Here we demonstrate that Magoh, a component of the exon junction complex (EJC) that binds RNA, controls mouse cerebral cortical size by regulating NSC division. Magoh haploinsufficiency causes microcephaly due to INP depletion and neuronal apoptosis. Defective mitosis underlies these phenotypes as depletion of EJC components disrupts mitotic spindle orientation and integrity, chromosome number, and genomic stability. In utero rescue experiments revealed that a key function of Magoh is to control levels of the microcephaly-associated protein, LIS1, during neurogenesis. This study uncovers new requirements for the EJC in brain development, NSC maintenance, and mitosis, thus implicating this complex in the pathogenesis of microcephaly.

Numb--from flies to humans

Numb carries the distinction of being the first molecule discovered to influence cell fate by being asymmetrically segregated during cell division. Originally identified from studies in Drosophila, further work has since demonstrated the importance of Numb in mammalian and, in particular, human systems, from diverse fields such as developmental neurobiology to cancer biology and neurodegenerative disease. This review surveys the body of knowledge concerning Numb, and discusses the relevance of Numb to human biology and disease.

NUMBL interacts with TRAF6 and promotes the degradation of TRAF6

Tumor necrosis factor-associated factor 6 (TRAF6) is an essential adaptor protein for IL-1R or TLR-mediated NF-kappaB signaling pathway activation. In previous work we have found NUMBL interacts with TAB2 and negatively regulates NF-kappaB signaling pathway. Here, we report that NUMBL directly binds to TRAF6 in vivo and in vitro. NUMBL down-regulates TRAF6 protein level and shortens its half-life. Furthermore, knockdown of NUMBL significantly increases endogenous TRAF6 protein level in the cultured cortical neurons. In vivo ubiquitination assays indicate that NUMBL promotes the assembly of K48-linked polyubiquitination chains on TRAF6, but has no significant effect on its K63-linked polyubiquitination. Our results collectively reveal that NUMBL interacts with TRAF6 and promotes the degradation of TRAF6 in vivo, leading to the inhibition of NF-kappaB signaling pathway.

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.

The transition from radial glial to intermediate progenitor cell is inhibited by FGF signaling during corticogenesis

During corticogenesis, the balance between the self-renewal of radial glial stem cells and the production of their descendent progenitor cells is essential in generating the correct size and cell composition of the neocortex. How the stem-to-progenitor cell transition is regulated is poorly understood. FGFs are commonly implicated in promoting proliferation of neural precursor cells, but it is unclear how they exert their effects on stem cells, progenitor cells, or both in vivo. Here, three FGF receptor genes are simultaneously deleted during cortical neurogenesis. In these mutants, radial glia are depleted due to an increased transition from an uncommitted state to a more differentiated one, initially causing an increase in progenitors, but ultimately resulting in a smaller cortex. The proliferation rate of progenitors themselves, however, is unchanged. These results indicate that FGFs normally repress the radial glia to progenitor cell transition during corticogenesis.

The multiple functions of Numb

Numb is an evolutionary conserved protein that plays critical roles in cell fate determination. Mammalian Numb displays a higher degree of structural complexity compared to the Drosophila homolog based on the number of encoding genes (Numb and Numb-like) and of alternative spliced isoforms. Accordingly, Numb proteins display a complex pattern of functions such as the control of asymmetric cell division and cell fate choice, endocytosis, cell adhesion, cell migration, ubiquitination of specific substrates and a number of signaling pathways (i.e. Notch, Hedgehog, p53). Recent findings indicate that, besides controlling such physiologic developmental processes, subversion of the above Numb-dependent events plays a critical role in disease (e.g. cancer). We will review here the multiple functions of mNumb and their underlying molecular mechanisms in development and disease.

MARCKS modulates radial progenitor placement, proliferation and organization in the developing cerebral cortex

The radial glial cells serve as neural progenitors and as a migratory guide for newborn neurons in the developing cerebral cortex. These functions require appropriate organization and proliferation of the polarized radial glial scaffold. Here, we demonstrate in mice that the myristoylated alanine-rich C-kinase substrate protein (MARCKS), a prominent cellular substrate for PKC, modulates radial glial placement and expansion. Loss of MARCKS results in ectopic collection of mitotically active radial progenitors away from the ventricular zone (VZ) in the upper cerebral wall. Apical restriction of key polarity complexes [CDC42, beta-catenin (CTNNB1), N-cadherin (CDH2), myosin IIB (MYOIIB), aPKCzeta, LGL, PAR3, pericentrin, PROM1] is lost. Furthermore, the radial glial scaffold in Marcks null cortex is compromised, with discontinuous, non-radial processes apparent throughout the cerebral wall and deformed, bulbous, unbranched end-feet at the basal ends. Further, the density of radial processes within the cerebral cortex is reduced. These deficits in radial glial development culminate in aberrant positioning of neurons and disrupted cortical lamination. Genetic rescue experiments demonstrate, surprisingly, that phosphorylation of MARCKS by PKC is not essential for the role of MARCKS in radial glial cell development. By contrast, the myristoylation domain of MARCKS needed for membrane association is essential for MARCKS function in radial glia. The membrane-associated targeting of MARCKS and the resultant polarized distribution of signaling complexes essential for apicobasal polarity may constitute a critical event in the appropriate placement, proliferation and organization of polarized radial glial scaffold in the developing cerebral cortex.

The glial nature of embryonic and adult neural stem cells

Glial cells were long considered end products of neural differentiation, specialized supportive cells with an origin very different from that of neurons. New studies have shown that some glial cells--radial glia (RG) in development and specific subpopulations of astrocytes in adult mammals--function as primary progenitors or neural stem cells (NSCs). This is a fundamental departure from classical views separating neuronal and glial lineages early in development. Direct visualization of the behavior of NSCs and lineage-tracing studies reveal how neuronal lineages emerge. In development and in the adult brain, many neurons and glial cells are not the direct progeny of NSCs, but instead originate from transit amplifying, or intermediate, progenitor cells (IPCs). Within NSCs and IPCs, genetic programs unfold for generating the extraordinary diversity of cell types in the central nervous system. The timing in development and location of NSCs, a property tightly linked to their neuroepithelial origin, appear to be the key determinants of the types of neurons generated. Identification of NSCs and IPCs is critical to understand brain development and adult neurogenesis and to develop new strategies for brain repair.

Mammalian Par3 regulates progenitor cell asymmetric division via notch signaling in the developing neocortex

Asymmetric cell division of radial glial progenitors produces neurons while allowing self-renewal; however, little is known about the mechanism that generates asymmetry in daughter cell fate specification. Here, we found that mammalian partition defective protein 3 (mPar3), a key cell polarity determinant, exhibits dynamic distribution in radial glial progenitors. While it is enriched at the lateral membrane domain in the ventricular endfeet during interphase, mPar3 becomes dispersed and shows asymmetric localization as cell cycle progresses. Either removal or ectopic expression of mPar3 prevents radial glial progenitors from dividing asymmetrically yet generates different outcomes in daughter cell fate specification. Furthermore, the expression level of mPar3 affects Notch signaling, and manipulations of Notch signaling or Numb expression suppress mPar3 regulation of radial glial cell division and daughter cell fate specification. These results reveal a critical molecular pathway underlying asymmetric cell division of radial glial progenitors in the mammalian neocortex.

Neural stem cells in the developing and adult brains

Neural stem cells exist in the mammalian developing and adult nervous system. Recently, tremendous interest in the potential of neural stem cells for the treatment of neurodegenerative diseases and brain injuries has substantially promoted research on neural stem cell self-renewal and differentiation. Multiple cell-intrinsic regulators coordinate with the microenvironment through various signaling pathways to regulate neural stem cell maintenance, self-renewal, and fate determination. This review focuses on essential intracellular regulators that control neural stem cell maintenance and self-renewal in both embryonic brains and adult nervous system. These factors include the orphan nuclear receptor TLX, the high-mobility-group DNA binding protein Sox2, the basic helix-loop-helix transcription factor Hes, the tumor suppressor gene Pten, the membrane-associated protein Numb, and its cytoplasmic homolog Numblike. The aim of this review is to summarize our current understanding of neural stem cell regulation through these important stem cell regulators.

Numb regulates post-endocytic trafficking and degradation of Notch1

Notch is a transmembrane receptor that controls cell fate decisions during development and tissue homeostasis. Both activation and attenuation of the Notch signal are tightly regulated by endocytosis. The adaptor protein Numb acts as an inhibitor of Notch and is known to function within the intracellular trafficking pathways. However, a role for Numb in regulating Notch trafficking has not been defined. Here we show that mammalian Notch1 is constitutively internalized and trafficked to both recycling and late endosomal compartments, and we demonstrate that changes in Numb expression alter the dynamics of Notch1 trafficking. Overexpression of Numb promotes sorting of Notch1 through late endosomes for degradation, whereas depletion of Numb facilitates Notch1 recycling. Numb mutants that do not interact with the ubiquitin-protein isopeptide ligase, Itch, or that lack motifs important for interaction with endocytic proteins fail to promote Notch1 degradation. Our data suggest that Numb inhibits Notch1 activity by regulating post-endocytic sorting events that lead to Notch1 degradation.

Structural basis for self-renewal of neural progenitors in cortical neurogenesis

In mammalian brain development, neuroepithelial cells act as progenitors that produce self-renewing and differentiating cells. Recent technical advances in live imaging and gene manipulation now enable us to investigate how neural progenitors generate the 2 different types of cells with unprecedented accuracy and resolution, shedding new light on the roles of epithelial structure in cell fate decisions and also on the plasticity of neurogenesis.

Notch: from neural development to neurological disorders

Notch is an integral membrane protein that functions as receptor for ligands such as jagged and delta that are associated with the surface of neighboring cells. Upon ligand binding, notch is proteolytically cleaved within its transmembrane domain by presenilin-1 (the enzymatic component of the gamma-secretase complex) resulting in the release of a notch intracellular domain which translocates to the nucleus where it regulates gene expression. Notch signaling plays multiple roles in the development of the CNS including regulating neural stem cell (NSC) proliferation, survival, self-renewal and differentiation. Notch is also present in post-mitotic neurons in the adult CNS wherein its activation influences structural and functional plasticity including processes involved in learning and memory. Recent findings suggest that notch signaling in neurons, glia, and NSCs may be involved in pathological changes that occur in disorders such as stroke, Alzheimer's disease and CNS tumors. Studies of animal models suggest the potential of agents that target notch signaling as therapeutic interventions for several different CNS disorders.

The TRIM-NHL protein TRIM32 activates microRNAs and prevents self-renewal in mouse neural progenitors

In the mouse neocortex, neural progenitor cells generate both differentiating neurons and daughter cells that maintain progenitor fate. Here, we show that the TRIM-NHL protein TRIM32 regulates protein degradation and microRNA activity to control the balance between those two daughter cell types. In both horizontally and vertically dividing progenitors, TRIM32 becomes polarized in mitosis and is concentrated in one of the two daughter cells. TRIM32 overexpression induces neuronal differentiation while inhibition of TRIM32 causes both daughter cells to retain progenitor cell fate. TRIM32 ubiquitinates and degrades the transcription factor c-Myc but also binds Argonaute-1 and thereby increases the activity of specific microRNAs. We show that Let-7 is one of the TRIM32 targets and is required and sufficient for neuronal differentiation. TRIM32 is the mouse ortholog of Drosophila Brat and Mei-P26 and might be part of a protein family that regulates the balance between differentiation and proliferation in stem cell lineages.

Timing cell-fate determination during asymmetric cell divisions

From invertebrates to mammals, cell-cycle progression during an asymmetric cell division is accompanied by precisely timed redistribution of cell-fate determinants so that they segregate asymmetrically to enable the two daughter cells to choose different fates. Interestingly, studies on how cell fates are specified in such divisions reveal that the same fate determinants can be reiteratively used to specify a variety of cell types through multiple rounds of cell divisions or to exert seemingly contradictory effects on cell proliferation and differentiation. Here I summarize the molecular mechanisms governing asymmetric cell division and review recent findings pointing to a novel mechanism for coupling intracellular signaling and cell-cycle progression. This mechanism uses changes in the morphology, subcellular distribution, and molecular composition of cellular organelles like the Golgi apparatus and centrosomes, which not only accompany the progression of cell cycle to activate but also temporally constrain the activity of fate determinants during asymmetric cell divisions.

Periventricular notch activation and asymmetric Ngn2 and Tbr2 expression in pair-generated neocortical daughter cells

To understand the cellular and molecular mechanisms regulating cytogenesis within the neocortical ventricular zone, we examined at high resolution the spatiotemporal expression patterns of Ngn2 and Tbr2. Individually DiI-labeled daughter cells were tracked from their birth in slice cultures and immunostained for Ngn2 and Tbr2. Both proteins were initially absent from daughter cells during the first 2 h. Ngn2 expression was then initiated asymmetrically in one of the daughter cells, with a bias towards the apical cell, followed by a similar pattern of expression for Tbr2, which we found to be a direct target of Ngn2. How this asymmetric Ngn2 expression is achieved is unclear, but gamma-secretase inhibition at the birth of daughter cells resulted in premature Ngn2 expression, suggesting that Notch signaling in nascent daughter cells suppresses a Ngn2-Tbr2 cascade. Many of the nascent cells exhibited pin-like morphology with a short ventricular process, suggesting periventricular presentation of Notch ligands to these cells.

Imaging hematopoietic precursor division in real time

Stem cells are thought to balance self-renewal and differentiation through asymmetric and symmetric divisions, but whether such divisions occur during hematopoietic development remains unknown. Using a Notch reporter mouse, in which GFP acts as a sensor for differentiation, we image hematopoietic precursors and show that they undergo both symmetric and asymmetric divisions. In addition we show that the balance between these divisions is not hardwired but responsive to extrinsic and intrinsic cues. Precursors in a prodifferentiation environment preferentially divide asymmetrically, whereas those in a prorenewal environment primarily divide symmetrically. Oncoproteins can also influence division pattern: although BCR-ABL predominantly alters the rate of division and death, NUP98-HOXA9 promotes symmetric division, suggesting that distinct oncogenes subvert different aspects of cellular function. These studies establish a system for tracking division of hematopoietic precursors and show that the balance of symmetric and asymmetric division can be influenced by the microenvironment and subverted by oncogenes.

Characterization of Numb expression in astrocytomas

During early neurodevelopment, asymmetric segregation of Numb in mitotic progenitor cells influences the fate of daughter cells, whereby one daughter retains the progenitor phenotype while the other proceeds along a differentiation pathway. Numb has also been reported to function as a tumor suppressor in breast cancers and medulloblastomas. Given its role in maintaining neural progenitor pools in animal models and its reported role as a tumor suppressor, Numb could potentially contribute to astrocytoma oncogenesis. We characterized Numb expression in both human astrocytoma tissue samples and glioblastoma cell lines. We found that Numb is expressed in all grades of astrocytomas, being predominantly cytoplasmic in higher-grade astrocytomas but nuclear in pilocytic astrocytomas. Numb is also present in normal neurons, but not in normal astrocytes. In cultured glioblastoma cells, Numb concentrates in the perinuclear region and process tips. Numb expression in astrocytomas recapitulates that of progenitor cells during neurodevelopment, and suggests a role for Numb in astrocytoma oncogenesis.

Numb mediates the interaction between Wnt and Notch to modulate primitive erythropoietic specification from the hemangioblast

During embryonic development, the establishment of the primitive erythroid lineage in the yolk sac is a temporally and spatially restricted program that defines the onset of hematopoiesis. In this report, we have used the embryonic stem cell differentiation system to investigate the regulation of primitive erythroid development at the level of the hemangioblast. We show that the combination of Wnt signaling with inhibition of the Notch pathway is required for the development of this lineage. Inhibition of Notch signaling at this stage appears to be mediated by the transient expression of Numb in the hemangioblast-derived blast cell colonies. Activation of the Notch pathway was found to inhibit primitive erythropoiesis efficiently through the upregulation of inhibitors of the Wnt pathway. Together, these findings demonstrate that specification of the primitive erythroid lineage is controlled, in part, by the coordinated interaction of the Wnt and Notch pathways, and position Numb as a key mediator of this process.

Single-cell gene profiling defines differential progenitor subclasses in mammalian neurogenesis

Cellular diversity of the brain is largely attributed to the spatial and temporal heterogeneity of progenitor cells. In mammalian cerebral development,it has been difficult to determine how heterogeneous the neural progenitor cells are, owing to dynamic changes in their nuclear position and gene expression. To address this issue, we systematically analyzed the cDNA profiles of a large number of single progenitor cells at the mid-embryonic stage in mouse. By cluster analysis and in situ hybridization, we have identified a set of genes that distinguishes between the apical and basal progenitors. Despite their relatively homogeneous global gene expression profiles, the apical progenitors exhibit highly variable expression patterns of Notch signaling components, raising the possibility that this causes the heterogeneous division patterns of these cells. Furthermore, we successfully captured the nascent state of basal progenitor cells. These cells are generated shortly after birth from the division of the apical progenitors, and show strong expression of the major Notch ligand delta-like 1, which soon fades away as the cells migrate in the ventricular zone. We also demonstrated that attenuation of Notch signals immediately induces differentiation of apical progenitors into nascent basal progenitors. Thus, a Notch-dependent feedback loop is likely to be in operation to maintain both progenitor populations.

Signaling networks during development: the case of asymmetric cell division in the Drosophila nervous system

Remarkable progress in genetics and molecular biology has made possible the sequencing of the genomes from numerous species. In the post-genomic era, technical developments in the fields of proteomics and bioinformatics are poised to further catapult our understanding of protein structure, function and organization into complex signaling networks. One of the greatest challenges in the field now is to unravel the functional signaling networks and their spatio-temporal regulation in living cells. Here, the need for such in vivo system-wide level approach is illustrated in relation to the mechanisms that underlie the biological process of asymmetric cell division. Genomic, post-genomic and live imaging techniques are reviewed in light of the huge impact they are having on this field for the discovering of new proteins and for the in vivo analysis of asymmetric cell division. The proteins, signals and the emerging networking of functional connections that is arising between them during this process in the Drosophila nervous system will be also discussed.

Post-transcriptional regulation of myelin formation

Myelin is a specialized structure of the nervous system that both enhances electrical conductance and protects neurons from degeneration. In the central nervous system, extensively polarized oligodendrocytes form myelin by wrapping cellular processes in a spiral pattern around neuronal axons. Myelin formation requires the oligodendrocyte to regulate gene expression in response to changes in its extracellular environment. Because these changes occur at a distance from the cell body, post-transcriptional control of gene expression allows the cell to fine-tune its response. Here, we review the RNA-binding proteins that control myelin formation in the brain, highlighting the molecular mechanisms by which they control gene expression and drawing parallels from studies in other cell types.

Neurogenesis and asymmetric cell division

The astonishing cellular diversity in the central nervous system (CNS) arises from neural progenitors which can undergo different modes of symmetric and asymmetric divisions to self-renew as well as produce differentiated neuronal and glial progeny. Drosophila CNS neural progenitor cells, neuroblasts, have been utilised as a model to stimulate the understanding of the processes of asymmetric division, generation of neuronal lineages and, more recently, stem cell biology in vertebrates. Here we review some recent developments involving Drosophila and mammalian neural progenitor cells, highlighting some similarities and differences in the mechanisms that regulate their divisions during neurogenesis.

Mechanisms of asymmetric cell division: flies and worms pave the way

Asymmetric cell division is fundamental for generating diversity in multicellular organisms. The mechanisms that govern asymmetric cell division are increasingly well understood, owing notably to studies that were conducted in Drosophila melanogaster and Caenorhabditis elegans. Lessons learned from these two model organisms also apply to cells that divide asymmetrically in other metazoans, such as self-renewing stem cells in mammals.

Distinct behaviors of neural stem and progenitor cells underlie cortical neurogenesis

Neocortical precursor cells undergo symmetric and asymmetric divisions while producing large numbers of diverse cortical cell types. In Drosophila, cleavage plane orientation dictates the inheritance of fate-determinants and the symmetry of newborn daughter cells during neuroblast cell divisions. One model for predicting daughter cell fate in the mammalian neocortex is also based on cleavage plane orientation. Precursor cell divisions with a cleavage plane orientation that is perpendicular with respect to the ventricular surface (vertical) are predicted to be symmetric, while divisions with a cleavage plane orientation that is parallel to the surface (horizontal) are predicted to be asymmetric neurogenic divisions. However, analysis of cleavage plane orientation at the ventricle suggests that the number of predicted neurogenic divisions might be insufficient to produce large amounts of cortical neurons. To understand factors that correlate with the symmetry of cell divisions, we examined rat neocortical precursor cells in situ through real-time imaging, marker analysis, and electrophysiological recordings. We find that cleavage plane orientation is more closely associated with precursor cell type than with daughter cell fate, as commonly thought. Radial glia cells in the VZ primarily divide with a vertical orientation throughout cortical development and undergo symmetric or asymmetric self-renewing divisions depending on the stage of development. In contrast, most intermediate progenitor cells divide in the subventricular zone with a horizontal orientation and produce symmetric daughter cells. We propose a model for predicting daughter cell fate that considers precursor cell type, stage of development, and the planar segregation of fate determinants.

MALS-3 regulates polarity and early neurogenesis in the developing cerebral cortex

Apicobasal polarity plays an important role in regulating asymmetric cell divisions by neural progenitor cells (NPCs) in invertebrates, but the role of polarity in mammalian NPCs is poorly understood. Here, we characterize the function of the PDZ domain protein MALS-3 in the developing cerebral cortex. We find that MALS-3 is localized to the apical domain of NPCs. Mice lacking all three MALS genes fail to localize the polarity proteins PATJ and PALS1 apically in NPCs, whereas the formation and maintenance of adherens junctions appears normal. In the absence of MALS proteins, early NPCs progressed more slowly through the cell cycle, and their daughter cells were more likely to exit the cell cycle and differentiate into neurons. Interestingly, these effects were transient; NPCs recovered normal cell cycle properties during late neurogenesis. Experiments in which MALS-3 was targeted to the entire membrane resulted in a breakdown of apicobasal polarity, loss of adherens junctions, and a slowing of the cell cycle. Our results suggest that MALS-3 plays a role in maintaining apicobasal polarity and is required for normal neurogenesis in the developing cortex.

Timing neurogenesis and differentiation: insights from quantitative clonal analyses of cerebellar granule cells

The cerebellum is an excellent model system to study how developmental programs give rise to exquisite neuronal circuits in the adult brain. Here, we describe our findings regarding granule cell neurogenesis and differentiation using the MADM method (mosaic analysis with double markers) in mice. By following the development of individual granule cell clones, we show that (1) granule cell precursors (GCPs) undergo predominantly symmetric division during postnatal development; (2) clonally related granule cells (GCs) exit the cell cycle within a narrow time window and stack their axons in the molecular layer in chronological order from deep to superficial sublayers; and (3) whereas the average GCP proliferation in the external granular layer is progressively slower as development proceeds, there is a rapid expansion of GCPs shortly before clonally related GCs exit the cell cycle. These properties produce GC clones that are distinct, each having a restricted axonal projection, but that are on average similar in cell number. We discuss possible developmental mechanisms and functional implications of these findings.