Intralineage directional Notch signaling regulates self-renewal and differentiation of asymmetrically dividing radial glia

Asymmetric division of progenitor/stem cells generates both self-renewing and differentiating progeny and is fundamental to development and regeneration. How this process is regulated in the vertebrate brain remains incompletely understood. Here, we use time-lapse imaging to track radial glia progenitor behavior in the developing zebrafish brain. We find that asymmetric division invariably generates a basal self-renewing daughter and an apical differentiating sibling. Gene expression and genetic mosaic analysis further show that the apical daughter is the source of Notch ligand that is essential to maintain higher Notch activity in the basal daughter. Notably, establishment of this intralineage and directional Notch signaling requires the intrinsic polarity regulator Partitioning defective protein-3 (Par-3), which segregates the fate determinant Mind bomb unequally to the apical daughter, thereby restricting the self-renewal potential to the basal daughter. These findings reveal with single-cell resolution how self-renewal and differentiation become precisely segregated within asymmetrically dividing neural progenitor/stem lineages.

Spindle orientation in mammalian cerebral cortical development

In any mitotic cell, the orientation of the mitotic spindle determines the orientation of the cleavage plane and therefore the position of the two daughter cells. When combined with polarization of cellular components, spindle orientation is also a well-conserved means of generating daughter cells with asymmetric cell fates, such as progenitors and differentiated cell types. In the mammalian neocortex, the precise planar spindle orientation observed early during development is vital for symmetric proliferative divisions. During later stages, spindles can be oblique or even vertical but the role of this reorientation is somewhat less clear as asymmetric cell fates can arise independently of spindle orientation during this stage. Although decades of work have identified many key conserved regulators of spindle positioning, its precise role in cell fate determination in the mammalian neocortex has been enigmatic. Recent work focused on mInsc and LGN has now revealed an important role for spindle orientation in determination of specific asymmetric cell fates, namely intermediate progenitors and a new progenitor population, termed outer radial glia. In this way, spindle orientation helps determine the neurogenic outcome of asymmetric progenitor divisions, thereby influencing neuron output and cerebral cortical expansion.

Mitotic spindle orientation can direct cell fate and bias Notch activity in chick neural tube

Live imaging in the chick neural tube reveals that induction of apico-basal divisions in neural progenitor cells results in neuronal differentiation of the apical daughter, while the basal daughter remains a progenitor with high Notch activity.

Inheritance of apical membrane is proposed to maintain vertebrate neural stem cell proliferation. However, evidence for this is contradictory. Using direct clonal analysis and live imaging in chick neural tube, we show that divisions that separate apical and basal components generate an apical daughter, which becomes a neuron, and a basal daughter, which rapidly re-establishes apico-basal polarity and divides again. Using a recently described real-time reporter of Notch activity, we confirm progenitor status and demonstrate that division orientation can influence Notch signalling. In addition, we reveal loss of apical complex proteins on neuronal differentiation onset, suggesting that removal of this inherited complex is part of the neuronal differentiation mechanism. These findings reconcile contradictory data, link asymmetric division to Notch signalling dynamics and identify apical complex loss as a new step towards neuronal differentiation.

Cell polarity proteins and cancer

Cell polarity is essential in many biological processes and required for development as well as maintenance of tissue integrity. Loss of polarity is considered both a hallmark and precondition for human cancer. Three conserved polarity protein complexes regulate different modes of polarity that are conserved throughout numerous cell types and species. These complexes are the Crumbs, Par and Scribble complex. Given the importance of cell polarity for normal tissue homeostasis, aberrant polarity signaling is suggested to contribute to the multistep processes of human cancer. Most human cancers are formed from epithelial cells. Evidence confirming the roles for polarity proteins in different phases of the oncogenic trajectory comes from functional studies using mammalian cells as well as Drosophila and zebrafish models. Furthermore, several reports have revealed aberrant expression and localization of polarity proteins in different human tumors. In this review we will give an overview on the current data available that couple polarity signaling to tumorigenesis, particularly in epithelial cells.

Laminin/β1 integrin signal triggers axon formation by promoting microtubule assembly and stabilization

Axon specification during neuronal polarization is closely associated with increased microtubule stabilization in one of the neurites of unpolarized neuron, but how this increased microtubule stability is achieved is unclear. Here, we show that extracellular matrix (ECM) component laminin promotes neuronal polarization via regulating directional microtubule assembly through β1 integrin (Itgb1). Contact with laminin coated on culture substrate or polystyrene beads was sufficient for axon specification of undifferentiated neurites in cultured hippocampal neurons and cortical slices. Active Itgb1 was found to be concentrated in laminin-contacting neurites. Axon formation was promoted and abolished by enhancing and attenuating Itgb1 signaling, respectively. Interestingly, laminin contact promoted plus-end microtubule assembly in a manner that required Itgb1. Moreover, stabilizing microtubules partially prevented polarization defects caused by Itgb1 downregulation. Finally, genetic ablation of Itgb1 in dorsal telencephalic progenitors caused deficits in axon development of cortical pyramidal neurons. Thus, laminin/Itgb1 signaling plays an instructive role in axon initiation and growth, both in vitro and in vivo, through the regulation of microtubule assembly. This study has established a linkage between an extrinsic factor and intrinsic cytoskeletal dynamics during neuronal polarization.

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.

Neural stem cells maintain their stemness through protein kinase C ζ-mediated inhibition of TRIM32

Several studies over the last couple of years have delivered insights into the mechanisms that drive neuronal differentiation. However, the mechanisms that ensure the maintenance of stemness characteristics in neural stem cells over several rounds of cell divisions are still largely unknown. Here, we provide evidence that the neuronal fate determinant TRIM32 binds to the protein kinase C ζ. Through this interaction, TRIM32 is retained in the cytoplasm. However, during differentiation, this interaction is abrogated and TRIM32 translocates to the nucleus to initiate neuronal differentiation by targeting c-Myc for proteasomal degradation.

Reelin sets the pace of neocortical neurogenesis

Migration of neurons during cortical development is often assumed to rely on purely post-proliferative reelin signaling. However, Notch signaling, long known to regulate neural precursor formation and maintenance, is required for the effects of reelin on neuronal migration. Here, we show that reelin gain-of-function causes a higher expression of Notch target genes in radial glia and accelerates the production of both neurons and intermediate progenitor cells. Converse alterations correlate with reelin loss-of-function, consistent with reelin controlling Notch signaling during neurogenesis. Ectopic expression of reelin in isolated clones of progenitors causes a severe reduction in neuronal differentiation. In mosaic cell cultures, reelin-primed progenitor cells respond to wild-type cells by further decreasing neuronal differentiation, consistent with an increased sensitivity to lateral inhibition. These results indicate that reelin and Notch signaling cooperate to set the pace of neocortical neurogenesis, a prerequisite for proper neuronal migration and cortical layering.

The apical polarity determinant Crumbs 2 is a novel regulator of ESC-derived neural progenitors

ESCs undergoing neural differentiation in vitro display an intrinsic heterogeneity with a large subset of the cells forming polarized neural rosettes that maintain the neural progenitor microenvironment. This heterogeneity is not only necessary for normal development but also causes substantial technical challenges for practical applications. Here, we report a novel regulator of early neural progenitors, the apical polarity protein Crb2 (Crumbs homologue 2). Employing monolayer differentiation of mouse ESCs to model neurogenesis in vitro, we find that Crb2 is upregulated with Sox1 and Musashi at the onset of neuroepithelial specification and localizes to the apical side of neural rosettes. Stable Crb2-knockdown (KD) lines die at the onset of neural specification and fail to stabilize several apical polarity proteins. However, these cells are able to proliferate under self-renewing conditions and can be differentiated into mesodermal and endodermal lineages. Conversely, Crb2 overexpression during neural differentiation results in elevated levels of other apical polarity proteins and increases proliferation. Additionally, sustained overexpression of Crb2 reduces terminal differentiation into TuJ1-positive neurons. Furthermore, we demonstrate that Crb2 overexpression under self-renewing conditions increases glycogen synthase kinase (GSK)-3β inhibition, correlating with an increase in clonogenicity. To confirm the importance of GSK-3β inhibition downstream of Crb2, we show that Crb2-KD cells can be forced into neural lineages by blocking GSK-3β function and supplementing Epidermal Growth Factor (EGF) and basic Fibroblast Growth Factor (bFGF). Thus, this is the first demonstration that a member of the Crumbs family is essential for survival and differentiation of ESC-derived neural progenitors.

FGF signaling expands embryonic cortical surface area by regulating Notch-dependent neurogenesis

The processes regulating cortical surface area expansion during development and evolution are unknown. We show that loss of function of all fibroblast growth factor receptors (FgfRs) expressed at the earliest stages of cortical development causes severe deficits in surface area growth by embryonic day 12.5 (E12.5) in the mouse. In FgfR mutants, accelerated production of neurons led to severe loss of radial progenitors and premature termination of neurogenesis. Nevertheless, these mutants showed remarkably little change in cortical layer structure. Birth-dating experiments indicated that a greater proportion of layer fates was generated during early neurogenic stages, revealing that FgfR activity normally slows the temporal progression of cortical layer fates. Electroporation of a dominant-negative FgfR at E11.5 increased cortical neurogenesis in normal mice--an effect that was blocked by simultaneous activation of the Notch pathway. Together with changes in the expression of Notch pathway genes in FgfR mutant embryos, these findings indicate that Notch lies downstream of FgfR signaling in the same pathway regulating cortical neurogenesis and begin to establish a mechanism for regulating cortical surface expansion.

Cell biological regulation of division fate in vertebrate neuroepithelial cells

The developing nervous system derives from neuroepithelial progenitor cells that divide to generate all of the mature neuronal types. For the proper complement of cell types to form, the progenitors must produce postmitotic cells, yet also replenish the progenitor pool. Progenitor divisions can be classified into three general types: symmetric proliferative (producing two progenitors), asymmetric neurogenic (producing one progenitor and one postmitotic cell), and symmetric neurogenic (producing two postmitotic cells). The appropriate ratios for these modes of cell division require intrinsic polarity, which is one of the characteristics that define neuroepithelial progenitor cells. The type of division an individual progenitor undergoes can be influenced by cellular features, or behaviors, which are heterogeneous within the population of progenitors. Here we review three key cellular parameters, asymmetric inheritance, cell cycle kinetics, and interkinetic nuclear migration, and the possible mechanisms for how these features influence progenitor fates.

Cytoarchitecture of mouse and human subventricular zone in developing cerebral neocortex

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

The role of Pax6 in regulating the orientation and mode of cell division of progenitors in the mouse cerebral cortex

Successful brain development requires tight regulation of sequential symmetric and asymmetric cell division. Although Pax6 is known to exert multiple roles in the developing nervous system, its role in the regulation of cell division is unknown. Here, we demonstrate profound alterations in the orientation and mode of cell division in the cerebral cortex of mice deficient in Pax6 function (Pax6(Sey/Sey)) or after acute induced deletion of Pax6. Live imaging revealed an increase in non-vertical cellular cleavage planes, resulting in an increased number of progenitors with unequal inheritance of the apical membrane domain and adherens junctions in the absence of Pax6 function. This phenotype appears to be mediated by the direct Pax6 target Spag5, a microtubule-associated protein, reduced levels of which result in the replication of the Pax6 phenotype of altered cell division orientation. In addition, lack of Pax6 also results in premature delamination of progenitor cells from the apical surface due to an overall decrease in proteins mediating anchoring at the ventricular surface. Moreover, continuous long-term imaging in vitro revealed that Pax6-deficient progenitors generate daughter cells with asymmetric fates at higher frequencies. These data demonstrate a cell-autonomous role for Pax6 in regulating the mode of cell division independently of apicobasal polarity and cell-cell interactions. Taken together, our work reveals several direct effects that the transcription factor Pax6 has on the machinery that mediates the orientation and mode of cell division.

The role of Rho GTPase proteins in CNS neuronal migration

The architectonics of the mammalian brain arise from a remarkable range of directed cell migrations, which orchestrate the emergence of cortical neuronal layers and pattern brain circuitry. At different stages of cortical histogenesis, specific modes of cell motility are essential to the stepwise formation of cortical architecture. These movements range from interkinetic nuclear movements in the ventricular zone, to migrations of early-born, postmitotic polymorphic cells into the preplate, to the radial migration of precursors of cortical output neurons across the thickening cortical wall, and the vast, tangential migrations of interneurons from the basal forebrain into the emerging cortical layers. In all cases, actomyosin motors act in concert with cell adhesion receptor systems to provide the force and traction needed for forward movement. As key regulators of actin and microtubule cytoskeletons, cell polarity, and adhesion, the Rho GTPases play critical roles in CNS neuronal migration. This review will focus on the different types of migration in the developing neocortex and cerebellar cortex, and the role of the Rho GTPases, their regulators and effectors in these CNS migrations, with particular emphasis on their involvement in radial migration.

The vertebrate retina: a model for neuronal polarization in vivo

The vertebrate retina develops rapidly from a proliferative neuroepithelium into a highly ordered laminated structure, with five distinct neuronal cell types. Like all neurons, these cells need to polarize in appropriate orientations order integrate their neuritic connections efficiently into functional networks. Its relative simplicity, amenability to in vivo imaging and experimental manipulation, as well as the opportunity to study varied cell types within a single tissue, make the retina a powerful model to uncover how neurons polarize in vivo. Here we review the progress that has been made thus far in understanding how the different retinal neurons transform from neuroepithelial cells into mature neurons, and how the orientation of polarization may be specified by a combination of pre-established intrinsic cellular polarity set up within neuroepithelial cells, and extrinsic cues acting upon these differentiating neurons.

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.

Neurogenesis at the brain-cerebrospinal fluid interface

Cerebral cortical progenitor cells can be classified into several different types, and each progenitor type integrates cell-intrinsic and cell-extrinsic cues to regulate neurogenesis. On one hand, cell-intrinsic mechanisms that depend upon appropriate apical-basal polarity are established by adherens junctions and apical complex proteins and are particularly important in progenitors with apical processes contacting the lateral ventricle. The apical protein complexes themselves are concentrated at the ventricular surface, and apical complex proteins regulate mitotic spindle orientation and cell fate. On the other hand, remarkably little is known about how cell-extrinsic cues signal to progenitors and couple with cell-intrinsic mechanisms to instruct neurogenesis. Recent research shows that the cerebrospinal fluid, which contacts apical progenitors at the ventricular surface and bathes the apical complex of these cells, provides growth- and survival-promoting cues for neural progenitor cells in developing and adult brain. This review addresses how the apical-basal polarity of progenitor cells regulates cell fate and allows progenitors to sample diffusible signals distributed by the cerebrospinal fluid. We also review several classes of signaling factors that the cerebrospinal fluid distributes to the developing brain to instruct neurogenesis.

Oblique radial glial divisions in the developing mouse neocortex induce self-renewing progenitors outside the germinal zone that resemble primate outer subventricular zone progenitors

Radial glia cells function as neural stem cells in the developing brain and generate self-renewing and differentiating daughter cells by asymmetric cell divisions. During these divisions, the apical process or basal process of the elongated epithelial structure is asymmetrically partitioned into daughter cells, depending on developmental contexts. However, in mammalian neurogenesis, the relationship between these subcellular structures and self-renewability is largely unknown. We induced oblique cleavages of radial glia cells to split the apical and basal processes into two daughters, and investigated the fate and morphology of the daughters in slice cultures. We observed that the more basal daughter cell that inherits the basal process self-renews outside of the ventricular zone (VZ), while the more apical daughter cell differentiates. These self-renewing progenitors, termed "outer VZ progenitors," retain the basal but not the apical process, as recently reported for the outer subventricular zone (OSVZ) progenitors in primates (Fietz et al., 2010; Hansen et al., 2010); to self-renew, they require clonal Notch signaling between sibling cells. We also found a small endogenous population of outer VZ progenitors in the mouse embryonic neocortex, consistent with a low frequency of oblique radial glia divisions. Our results describe the general role of the basal process in the self-renewal of neural progenitors and implicate the loss of the apical junctions during oblique divisions as a possible mechanism for generating OSVZ progenitors. We propose that mouse outer VZ progenitors, induced by oblique cleavages, provide a model to study both progenitor self-renewal and OSVZ progenitors.

Continuous live imaging of adult neural stem cell division and lineage progression in vitro

Little is known about the intrinsic specification of adult neural stem cells (NSCs) and to what extent they depend on their local niche. To observe adult NSC division and lineage progression independent of their niche, we isolated cells from the adult mouse subependymal zone (SEZ) and cultured them at low density without growth factors. We demonstrate here that SEZ cells in this culture system are primarily neurogenic and that adult NSCs progress through stereotypic lineage trees consisting of asymmetric stem cell divisions, symmetric transit-amplifying divisions and final symmetric neurogenic divisions. Stem cells, identified by their astro/radial glial identity and their slow-dividing nature, were observed to generate asymmetrically and fast-dividing cells that maintained an astro/radial glia identity. These, in turn, gave rise to symmetrically and fast-dividing cells that lost glial hallmarks, but had not yet acquired neuronal features. The number of amplifying divisions was limited to a maximum of five in this system. Moreover, we found that cell growth correlated with the number of subsequent divisions of SEZ cells, with slow-dividing astro/radial glia exhibiting the most substantial growth prior to division. The fact that in the absence both of exogenously supplied growth factors and of signals provided by the local niche neurogenic lineage progression takes place in such stereotypic fashion, suggests that lineage progression is, to a significant degree, cell intrinsic or pre-programmed at the beginning of the lineage.

Neuroependymal denudation is in progress in full-term human foetal spina bifida aperta

In human spina bifida aperta (SBA), cerebral pathogenesis [hydrocephalus, Sylvius aqueduct (SA) stenosis and heterotopias] is poorly understood. In animal models, loss of ventricular lining (ependymal denudation) causes SA stenosis and hydrocephalus. We aimed to investigate whether ependymal denudation also takes place in human foetal SBA. Considering that ependymal denudation would be related to alterations in junction proteins, sections through SA of five SBA and six control foetuses (gestational ages ranged between 37 and 40 weeks) were immunostained for markers of ependyma (caveolin 1, βIV-tubulin, S100), junction proteins (N-cadherin, connexin-43, neural cell adhesion molecule (NCAM), blood vessels (Glut-1) and astrocytes [glial fibrillary acidic protein (GFAP)]. In control foetuses, ependymal denudation was absent. In SBA foetuses different stages of ependymal denudation were observed: (i) intact ependyma/neuroepithelium; (ii) imminent ependymal denudation (with abnormal subcellular location of junction proteins); (iii) ependymal denudation (with protrusion of neuropile into SA, formation of rosettes and macrophage invasion); (iv) astroglial reaction. It is suggested that abnormalities in the formation of gap and adherent junctions result in defective ependymal coupling, desynchronized ciliary beating and ependymal denudation, leading to hydrocephalus. The presence of various stages of ependymal denudation within the same full-term SBA foetuses suggests continuation of the process after birth.

Long-term single-cell imaging of mammalian stem cells

Continuous long-term single-cell observation provides insight into the molecular control of cell fate. This is particularly important for rare and heterogeneous populations of cells, such as mammalian stem cells. The current lack of usable off-the-shelf hardware and software for such experiments makes their implementation technically challenging. Here I discuss the need for continuous single-cell quantification to understand molecular cell fate control as well as organizational and technical solutions for long-term imaging and tracking of stem cells.

A lateral belt of cortical LGN and NuMA guides mitotic spindle movements and planar division in neuroepithelial cells

Knockdown or mislocalization of LGN complex components disrupts the stereotypic biphasic spindle movements regulating planar cell division and neuroepithelial structure in chick embryos.

To maintain tissue architecture, epithelial cells divide in a planar fashion, perpendicular to their main polarity axis. As the centrosome resumes an apical localization in interphase, planar spindle orientation is reset at each cell cycle. We used three-dimensional live imaging of GFP-labeled centrosomes to investigate the dynamics of spindle orientation in chick neuroepithelial cells. The mitotic spindle displays stereotypic movements during metaphase, with an active phase of planar orientation and a subsequent phase of planar maintenance before anaphase. We describe the localization of the NuMA and LGN proteins in a belt at the lateral cell cortex during spindle orientation. Finally, we show that the complex formed of LGN, NuMA, and of cortically located Gαi subunits is necessary for spindle movements and regulates the dynamics of spindle orientation. The restricted localization of LGN and NuMA in the lateral belt is instructive for the planar alignment of the mitotic spindle, and required for its planar maintenance.

The transcription factor Pax6 regulates survival of dopaminergic olfactory bulb neurons via crystallin αA

Most neurons in the adult mammalian brain survive for the entire life of an individual. However, it is not known which transcriptional pathways regulate this survival in a healthy brain. Here, we identify a pathway regulating neuronal survival in a highly subtype-specific manner. We show that the transcription factor Pax6 expressed in dopaminergic neurons of the olfactory bulb regulates the survival of these neurons by directly controlling the expression of crystallin αA (CryαA), which blocks apoptosis by inhibition of procaspase-3 activation. Re-expression of CryαA fully rescues survival of Pax6-deficient dopaminergic interneurons in vivo and knockdown of CryαA by shRNA in wild-type mice reduces the number of dopaminergic OB interneurons. Strikingly, Pax6 utilizes different DNA-binding domains for its well-known role in fate specification and this role of regulating the survival of specific neuronal subtypes in the mature, healthy brain.

Radial glia regulate Cajal-Retzius cell positioning in the early embryonic cerebral cortex

The organization of neocortex, along its radial axis, into a six-layered structure is one of the most exquisite features of the brain. Because of their strategic localization in the marginal zone, and their expression of reelin, a signal that controls spatial ordering of cortical layers, Cajal-Retzius (C-R) cells play a crucial role in cortical patterning along this axis. Yet, it remains less well understood how C-R cell targeting itself is regulated. At the onset of corticogenesis when C-R cells first arrive in the cortex via tangential migration, radial glia (RG) are the main cell type present. This suggests that RG may play a role in C-R cell localization. To test this, we used genetic approaches to perturb RG scaffold during early corticogenesis. We found that disrupting RG endfoot adhesion to basal lamina consistently results in C-R cell displacement. These displacements do not appear to result from primary defects in neural progenitor cell proliferation, deficits in the meninges or basement membrane, or cell autonomous defects in C-R cells. Instead, they show close temporal and spatial correlation with RG endfoot retraction. Moreover, ablation of RG via cell cycle blockade similarly results in local displacement of C-R cells. These lines of evidence thus indicate that, during early corticogenesis, RG play a primary role in regulating spatial targeting of C-R cells. Since RG are also neural progenitors as well as neuronal migration scaffolds, these findings suggest that, during nervous system development, neuroepithelial stem cells may not only be responsible for generating a diverse array of neuronal cell types and facilitating their radial migration. They may also, through regulating the placement of guidepost cells, coordinate spatial patterning of the nervous system along its radial axis.

Asymmetric cell division: recent developments and their implications for tumour biology

The ability of cells to divide asymmetrically is essential for generating diverse cell types during development. The past 10 years have seen tremendous progress in our understanding of this important biological process. We have learned that localized phosphorylation events are responsible for the asymmetric segregation of cell fate determinants in mitosis and that centrosomes and microtubules play important parts in this process. The relevance of asymmetric cell division for stem cell biology has added a new dimension to the field, and exciting connections between asymmetric cell division and tumorigenesis have begun to emerge.

Disturbances in the positioning, proliferation and apoptosis of neural progenitors contribute to subcortical band heterotopia formation

Cortical malformations are commonly associated with intractable epilepsy and other developmental disorders. Our studies utilize the tish rat, a spontaneously occurring genetic model of subcortical band heterotopia (SBH) associated with epilepsy, to evaluate the developmental events underlying SBH formation in the neocortex. Our results demonstrate that Pax6(+) and Tbr2(+) progenitors are mislocalized in tish(+/-) and tish(-/-)- neocortex throughout neurogenesis. In addition, mislocalized tish(-/-) progenitors possess a longer cell cycle than wild type or normally-positioned tish(-/-) progenitors, owing to a lengthened G(2)+M+G(1) time. This mislocalization is not associated with adherens junction breakdown or loss of radial glial polarity in the ventricular zone (VZ), as assessed by immunohistochemistry against phalloidin (to identify F-actin), aPKC-λ and Par3. However, vimentin immunohistochemistry indicates that the radial glial scaffold is disrupted in the region of the tish(-/-) heterotopia. Moreover, lineage tracing experiments using in utero electroporation in tish(-/-) neocortex demonstrate that mislocalized progenitors do not retain contact with the ventricular surface and that ventricular/subventricular zone (SVZ) progenitors produce neurons that migrate into both the heterotopia and cortical plate (CP). Taken together, these findings define a series of developmental errors contributing to SBH formation that differs fundamentally from a primary error in neuronal migration.

FGF signaling gradient maintains symmetrical proliferative divisions of midbrain neuronal progenitors

For the correct development of the central nervous system, the balance between self-renewing and differentiating divisions of the neuronal progenitors must be tightly regulated. To maintain their self-renewing identity, the progenitors need to retain both apical and basal interfaces. However, the identities of fate-determining signals which cells receive via these connections, and the exact mechanism of their action, are poorly understood. The conditional inactivation of Fibroblast growth factor (FGF) receptors 1 and 2 in the embryonic mouse midbrain-hindbrain area results in premature neuronal differentiation. Here, we aim to elucidate the connection between FGF signaling and neuronal progenitor maintenance. Our results reveal that the loss of FGF signaling leads to downregulation of Hes1 and upregulation of Ngn2, Dll1, and p57 in the ventricular zone (VZ) cells, and that this increased neurogenesis occurs cell-autonomously. Yet the cell cycle progression, apico-basal-polarity, cell-cell connections, and the positioning of mitotic spindle in the mutant VZ appear unaltered. Interestingly, FGF8-protein is highly concentrated in the basal lamina. Thus, FGFs may act through basal processes of neuronal progenitors to maintain their progenitor status. Indeed, midbrain neuronal progenitors deprived in vitro of FGFs switched from symmetrical proliferative towards symmetrical neurogenic divisions. We suggest that FGF signaling in the midbrain VZ is cell-autonomously required for the maintenance of symmetrical proliferative divisions via Hes1-mediated repression of neurogenic genes.

Essential role of PDZ-RGS3 in the maintenance of neural progenitor cells

Ephrin-B plays an important role in neural progenitor cells to regulate self-renewal and differentiation. Cellular and embryological evidence suggest this function of ephrin-B is mediated through a PDZ-dependent reverse signaling mechanism. Here, we have genetically investigated the function of PDZ-RGS3, a proposed downstream signaling mediator of ephrin-B function, and found that knockout of PDZ-RGS3 caused early cell cycle exit and precocious differentiation in neural progenitor cells of the developing cerebral cortex, reminiscent of the phenotype observed in ephrin-B1 knockout mice. This resulted in a loss of cortical neural progenitor cells during cortical neurogenesis and led to impairment in the production of late born cortical neurons. These results reveal an essential role of PDZ-RGS3 in maintaining the balance between self-renewal and differentiation of neural progenitor cells and provide genetic evidence linking PDZ-RGS3 to ephrin-B reverse signaling. As ephrin-B molecules are often differentially expressed in different types of neural progenitor/stem cells during development or in adult life, deletion of PDZ-RGS3 can achieve a uniform loss of function of the ephrin-B/regulator of G protein-signaling (RGS) pathway, thereby providing a genetic tool useful for dissecting the mechanisms and functions of the ephrin-B/RGS reverse signaling pathway in neural progenitor/stem cell regulation.

Cortical progenitor expansion, self-renewal and neurogenesis-a polarized perspective

Neural stem and progenitor cells giving rise to neurons in developing mammalian neocortex fall into two principal classes with regard to location of mitosis-apical and basal, and into three principal classes in terms of cell polarity during mitosis-bipolar, monopolar, and nonpolar. Insight has been gained into how inheritance of polarized, apical and basal, cell constituents is related to symmetric versus asymmetric divisions of these progenitors, and how this inheritance is linked to their expansion, self-renewal, and neurogenesis. Retention and inheritance of the basal process emerge as key for self-renewal, notably for the monopolar progenitors of prospective gyrencephalic neocortex that undergo asymmetric mitoses at basal locations. The resulting expansion of the neocortex during evolution is proposed to be associated with an increased cone-shape of radial units.

Initiating and growing an axon

The ability of neurons to form a single axon and multiple dendrites underlies the directional flow of information transfer in the central nervous system. Dendrites and axons are molecularly and functionally distinct domains. Dendrites integrate synaptic inputs, triggering the generation of action potentials at the level of the soma. Action potentials then propagate along the axon, which makes presynaptic contacts onto target cells. This article reviews what is known about the cellular and molecular mechanisms underlying the ability of neurons to initiate and extend a single axon during development. Remarkably, neurons can polarize to form a single axon, multiple dendrites, and later establish functional synaptic contacts in reductionist in vitro conditions. This approach became, and remains, the dominant model to study axon initiation and growth and has yielded the identification of many molecules that regulate axon formation in vitro (Dotti et al. 1988). At present, only a few of the genes identified using in vitro approaches have been shown to be required for axon initiation and outgrowth in vivo. In vitro, axon initiation and elongation are largely intrinsic properties of neurons that are established in the absence of relevant extracellular cues. However, the importance of extracellular cues to axon initiation and outgrowth in vivo is emerging as a major theme in neural development (Barnes and Polleux 2009). In this article, we focus our attention on the extracellular cues and signaling pathways required in vivo for axon initiation and axon extension.

Cell adhesion in regulation of asymmetric stem cell division

Adult stem cells inevitably communicate with their cellular neighbors within the tissues they sustain. Indeed, such communication, particularly with components of the stem cell niche, is essential for many aspects of stem cell behavior, including the maintenance of stem cell identity and asymmetric cell division. Cell adhesion mediates this communication by placing stem cells in proximity to the signaling source and by providing a polarity cue that orients stem cells. Here, I review the recent discovery that cell adhesion molecules govern the behavior of stem cells.

Unravelling the development of the visual cortex: implications for plasticity and repair

The visual cortex comprises over 50 areas in the human, each with a specified role and distinct physiology, connectivity and cellular morphology. How these individual areas emerge during development still remains something of a mystery and, although much attention has been paid to the initial stages of the development of the visual cortex, especially its lamination, very little is known about the mechanisms responsible for the arealization and functional organization of this region of the brain. In recent years we have started to discover that it is the interplay of intrinsic (molecular) and extrinsic (afferent connections) cues that are responsible for the maturation of individual areas, and that there is a spatiotemporal sequence in the maturation of the primary visual cortex (striate cortex, V1) and the multiple extrastriate/association areas. Studies in both humans and non-human primates have started to highlight the specific neural underpinnings responsible for the maturation of the visual cortex, and how experience-dependent plasticity and perturbations to the visual system can impact upon its normal development. Furthermore, damage to specific nuclei of the visual cortex, such as the primary visual cortex (V1), is a common occurrence as a result of a stroke, neurotrauma, disease or hypoxia in both neonates and adults alike. However, the consequences of a focal injury differ between the immature and adult brain, with the immature brain demonstrating a higher level of functional resilience. With better techniques for examining specific molecular and connectional changes, we are now starting to uncover the mechanisms responsible for the increased neural plasticity that leads to significant recovery following injury during this early phase of life. Further advances in our understanding of postnatal development/maturation and plasticity observed during early life could offer new strategies to improve outcomes by recapitulating aspects of the developmental program in the adult brain.

ASPP2 regulates epithelial cell polarity through the PAR complex

The PAR complex, consisting of the evolutionarily conserved PAR-3, PAR-6, and aPKC, regulates cell polarity in many cell types, including epithelial cells [1-4]. Consistently, genetic manipulation of its components affects tissue integrity in multiple biological systems [5-9]. However, the regulatory mechanisms of the PAR complex remain obscure. We report here that apoptosis-stimulating protein of p53 (ASPP2 or TP53BP2), which binds to the tumor suppressor p53 and stimulates its proapoptotic function [10-12], positively regulates epithelial cell polarity by associating with the PAR complex. ASPP2 interacts and colocalizes with PAR-3 at apical cell-cell junctions in the polarized epithelial cells. Depletion of ASPP2 in epithelial cells causes defects in cell polarity, such as the formation of tight junctions and the maintenance and development of apical membrane domains. Moreover, depletion of ASPP2 causes a defect in PAR-3 localization, as well as vice versa. Furthermore, disturbance in the interaction between ASPP2 and PAR-3 causes defects in cell polarity. We conclude that ASPP2 regulates epithelial cell polarity in cooperation with PAR-3 to form an active PAR complex. Our results, taken together with the known functions of ASPP2, suggest a close relationship between cell polarity and other cell regulatory mechanisms mediated by ASPP2.

Neural stem cells: the need for a proper orientation

Drosophila neuroblasts and mouse radial glial cells can divide asymmetrically to self-renew while producing differentiating daughter cells that contribute to brain growth. Intense research activity in the past few years has started to unveil some of the processes that govern asymmetric division in these two cell types. Here we discuss the case of centrosome asymmetry and the contribution of spindle orientation and non-spindle-related centrosome functions. Although still fragmentary, the emerging picture suggests that both notable parallelisms and striking differences apply.

Directing astroglia from the cerebral cortex into subtype specific functional neurons

Forced expression of single defined transcription factors can selectively and stably convert cultured astroglia into synapse-forming excitatory and inhibitory neurons.

Astroglia from the postnatal cerebral cortex can be reprogrammed in vitro to generate neurons following forced expression of neurogenic transcription factors, thus opening new avenues towards a potential use of endogenous astroglia for brain repair. However, in previous attempts astroglia-derived neurons failed to establish functional synapses, a severe limitation towards functional neurogenesis. It remained therefore also unknown whether neurons derived from reprogrammed astroglia could be directed towards distinct neuronal subtype identities by selective expression of distinct neurogenic fate determinants. Here we show that strong and persistent expression of neurogenic fate determinants driven by silencing-resistant retroviral vectors instructs astroglia from the postnatal cortex in vitro to mature into fully functional, synapse-forming neurons. Importantly, the neurotransmitter fate choice of astroglia-derived neurons can be controlled by selective expression of distinct neurogenic transcription factors: forced expression of the dorsal telencephalic fate determinant neurogenin-2 (Neurog2) directs cortical astroglia to generate synapse-forming glutamatergic neurons; in contrast, the ventral telencephalic fate determinant Dlx2 induces a GABAergic identity, although the overall efficiency of Dlx2-mediated neuronal reprogramming is much lower compared to Neurog2, suggesting that cortical astroglia possess a higher competence to respond to the dorsal telencephalic fate determinant. Interestingly, however, reprogramming of astroglia towards the generation of GABAergic neurons was greatly facilitated when the astroglial cells were first expanded as neurosphere cells prior to transduction with Dlx2. Importantly, this approach of expansion under neurosphere conditions and subsequent reprogramming with distinct neurogenic transcription factors can also be extended to reactive astroglia isolated from the adult injured cerebral cortex, allowing for the selective generation of glutamatergic or GABAergic neurons. These data provide evidence that cortical astroglia can undergo a conversion across cell lineages by forced expression of a single neurogenic transcription factor, stably generating fully differentiated neurons. Moreover, neuronal reprogramming of astroglia is not restricted to postnatal stages but can also be achieved from terminally differentiated astroglia of the adult cerebral cortex following injury-induced reactivation.

The brain consists of two major cell types: neurons, which transmit information, and glial cells, which support and protect neurons. Interestingly, evidence suggests that some glial cells, including astroglia, can be directly converted into neurons by specific proteins, a transformation that may aid in the functional repair of damaged brain tissue. However, in order for the repaired brain areas to function properly, it is important that astroglia be directed into appropriate neuronal subclasses. In this study, we show that non-neurogenic astroglia from the cerebral cortex can be reprogrammed in vitro using just a single transcription factor to yield fully functional excitatory or inhibitory neurons. We achieved this result through forced expression of the same transcription factors that instruct the genesis of these distinct neuronal subtypes during embryonic forebrain development. Moreover we demonstrate that reactive astroglia isolated from the adult cortex after local injury can be reprogrammed into synapse-forming excitatory or inhibitory neurons following a similar strategy. Our findings provide evidence that endogenous glial cells may prove a promising strategy for replacing neurons that have degenerated due to trauma or disease.

Neurons derive from the more apical daughter in asymmetric divisions in the zebrafish neural tube

In the developing CNS, asymmetric cell division is critical for maintaining the balanced production of differentiating neurons while renewing the population of neural progenitors. In invertebrates, this process depends on asymmetric inheritance of fate determinants during progenitor divisions. A similar mechanism is widely believed to underlie asymmetrically fated divisions in vertebrates, but compelling evidence for this is missing. We used live imaging of individual progenitors in the intact zebrafish embryo CNS to test this hypothesis. We found that asymmetric inheritance of a subcellular domain is strongly correlated with asymmetric daughter fates and our results reveal an unexpected feature of this process. The daughter cell destined to become a neuron was derived from the more apical of the two daughters, whereas the more basal daughter inherited the basal process and replenished the apical progenitor pool.

The apical complex couples cell fate and cell survival to cerebral cortical development

Cortical development depends upon tightly controlled cell fate and cell survival decisions that generate a functional neuronal population, but the coordination of these two processes is poorly understood. Here we show that conditional removal of a key apical complex protein, Pals1, causes premature withdrawal from the cell cycle, inducing excessive generation of early-born postmitotic neurons followed by surprisingly massive and rapid cell death, leading to the abrogation of virtually the entire cortical structure. Pals1 loss shows exquisite dosage sensitivity, so that heterozygote mutants show an intermediate phenotype on cell fate and cell death. Loss of Pals1 blocks essential cell survival signals, including the mammalian target of rapamycin (mTOR) pathway, while mTORC1 activation partially rescues Pals1 deficiency. These data highlight unexpected roles of the apical complex protein Pals1 in cell survival through interactions with mTOR signaling.

OSVZ progenitors of human and ferret neocortex are epithelial-like and expand by integrin signaling

A major cause of the cerebral cortex expansion that occurred during evolution is the increase in subventricular zone (SVZ) progenitors. We found that progenitors in the outer SVZ (OSVZ) of developing human neocortex retain features of radial glia, in contrast to rodent SVZ progenitors, which have limited proliferation potential. Although delaminating from apical adherens junctions, OSVZ progenitors maintained a basal process contacting the basal lamina, a canonical epithelial property. OSVZ progenitor divisions resulted in asymmetric inheritance of their basal process. Notably, OSVZ progenitors are also found in the ferret, a gyrencephalic nonprimate. Functional disruption of integrins, expressed on the basal process of ferret OSVZ progenitors, markedly decreased the OSVZ progenitor population size, but not that of other, process-lacking SVZ progenitors, in slice cultures of ferret neocortex. Our findings suggest that maintenance of this epithelial property allows integrin-mediated, repeated asymmetric divisions of OSVZ progenitors, providing a basis for neocortical expansion.

Mutations in N-cadherin and a Stardust homolog, Nagie oko, affect cell-cycle exit in zebrafish retina

It has been reported that the loss of apicobasal cell polarity and the disruption of adherens junctions induce hyperplasia in the mouse developing brain. However, it is not fully understood whether hyperplasia is caused by an enhanced cell proliferation, an inhibited neurogenesis, or both. In this study, we found that the ratio of the number of proliferating progenitor cells to the total number of retinal cells increases in the neurogenic stages in zebrafish n-cadherin (ncad) and nagie oko (nok) mutants, in which the apicobasal cell polarity and adherens junctions in the retinal epithelium are disrupted. The cell-cycle progression was not altered in the ncad and nok mutants. Rather, the ratio of the number of cells undergoing neurogenic cell division to the total number of cells undergoing mitosis decreased in the ncad and nok mutant retinas, suggesting that the switching from proliferative cell division to neurogenic cell division was compromised in these mutant retinas. These findings suggest that the inhibition of neurogenesis is a primary defect that causes hyperplasia in the ncad and nok mutant retinas. The Hedgehog-protein kinase A signaling pathway and the Notch signaling pathway regulate retinal neurogenesis in zebrafish. We found that both signaling pathways are involved in the generation of neurogenic defects in the ncad and nok mutant retinas. Taken together, these findings suggest that apicobasal cell polarity and epithelial integrity are essential for retinal neurogenesis in zebrafish.

[Mechanisms of asymmetric cell division: from model organisms to tumorigenesis]

Asymmetric cell division is the process by which a single cell gives rise to two different daughter cells. This process is important to generate cell diversity during the development of multicellular organisms, as well as for stem cell self-renewal in adults. Current knowledge on so-called cancer stem cells suggests that a loss of asymmetry during their division could lead to overproliferation and favour tumorigenesis, highlighting the importance of deciphering the mechanisms governing asymmetric cell division. Two mechanisms can lead to an asymmetric cell division: asymmetry can either be governed by proximity to a given cellular environment (or niche), in which case the mechanism is referred to as extrinsic, or the mother cell polarizes itself without external intervention, in which case the mechanism is referred to as intrinsic. In the last 20 years, our understanding of intrinsic mechanisms leading to asymmetric cell division has progressed, largely after studies carried out in model organisms such as the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. These models allowed the identification of molecular complexes used by nearly all the cells that divide asymmetrically, including human cells. Here we review the main intrinsic mechanisms of asymmetric cell division as described in model organisms and discuss their relevance towards mammalian tumorigenesis.

Emx1-lineage progenitors differentially contribute to neural diversity in the striatum and amygdala

In the developing mammalian basal telencephalon, neural progenitors from the subpallium generate the majority of inhibitory medium spiny neurons (MSNs) in the striatum, while both pallial- and subpallial-derived progenitors contribute to excitatory and inhibitory neuronal diversity in the amygdala. Using a combination of approaches, including genetic fate mapping, cell birth dating, cell migration assays, and electrophysiology, we find that cells derived from the Emx1 lineage contribute to two distinct neuronal populations in the mature basal forebrain: inhibitory MSNs in the striatum and functionally distinct subclasses of excitatory neurons in the amygdala. Our cell birth-dating studies reveal that these two populations are born at different times during early neurogenesis, with the amygdala population born before the MSNs. In the striatum, Emx1-lineage neurons represent a unique subpopulation of MSNs: they are disproportionately localized to the dorsal striatum, are found in dopamine receiving, reelin-positive patches, and are born throughout striatal neurogenesis. In addition, our data suggest that a subpopulation of these Emx1-lineage cells originate in the pallium and subsequently migrate to the developing striatum and amygdala. Our intersectional fate-mapping analysis further reveals that Emx1-lineage cells that coexpress Dlx exclusively generate MSNs but do not contribute to the excitatory neurons in the amygdala. Thus, both the timing of neurogenesis and differential combinatorial gene expression appear to be key determinants of striatal versus amygdala fate decisions of Emx1-lineage cells.

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.

Retinoic acid from the meninges regulates cortical neuron generation

Extrinsic signals controlling generation of neocortical neurons during embryonic life have been difficult to identify. In this study we demonstrate that the dorsal forebrain meninges communicate with the adjacent radial glial endfeet and influence cortical development. We took advantage of Foxc1 mutant mice with defects in forebrain meningeal formation. Foxc1 dosage and loss of meninges correlated with a dramatic reduction in both neuron and intermediate progenitor production and elongation of the neuroepithelium. Several types of experiments demonstrate that retinoic acid (RA) is the key component of this secreted activity. In addition, Rdh10- and Raldh2-expressing cells in the dorsal meninges were either reduced or absent in the Foxc1 mutants, and Rdh10 mutants had a cortical phenotype similar to the Foxc1 null mutants. Lastly, in utero RA treatment rescued the cortical phenotype in Foxc1 mutants. These results establish RA as a potent, meningeal-derived cue required for successful corticogenesis.

The apicobasal polarity kinase aPKC functions as a nuclear determinant and regulates cell proliferation and fate during Xenopus primary neurogenesis

During neurogenesis in Xenopus, apicobasally polarised superficial and non-polar deep cells take up different fates: deep cells become primary neurons while superficial cells stay as progenitors. It is not known whether the proteins that affect cell polarity also affect cell fate and how membrane polarity information may be transmitted to the nucleus. Here, we examine the role of the polarity components, apically enriched aPKC and basolateral Lgl2, in primary neurogenesis. We report that a membrane-tethered form of aPKC (aPKC-CAAX) suppresses primary neurogenesis and promotes cell proliferation. Unexpectedly, both endogenous aPKC and aPKC-CAAX show some nuclear localisation. A constitutively active aPKC fused to a nuclear localisation signal has the same phenotypic effect as aPKC-CAAX in that it suppresses neurogenesis and enhances proliferation. Conversely, inhibiting endogenous aPKC with a dominant-negative form that is restricted to the nucleus enhances primary neurogenesis. These observations suggest that aPKC has a function in the nucleus that is important for cell fate specification during primary neurogenesis. In a complementary experiment, overexpressing basolateral Lgl2 causes depolarisation and internalisation of superficial cells, which form ectopic neurons when supplemented with a proneural factor. These findings suggest that both aPKC and Lgl2 affect cell fate, but that aPKC is a nuclear determinant itself that might shuttle from the membrane to the nucleus to control cell proliferation and fate; loss of epithelial cell polarity by Lgl2 overexpression changes the position of the cells and is permissive for a change in cell fate.

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.