Cortical neural precursors inhibit their own differentiation via N-cadherin maintenance of beta-catenin signaling

Cadherin-11 regulates motility in normal cortical neural precursors and glioblastoma

Metastasizing tumor cells undergo a transformation that resembles a process in normal development when non-migratory epithelial cells modulate the expression of cytoskeletal and adhesion proteins to promote cell motility. Here we find a mesenchymal cadherin, Cadherin-11 (CDH11), is increased in cells exiting the ventricular zone (VZ) neuroepithelium during normal cerebral cortical development. When overexpressed in cortical progenitors in vivo, CDH11 causes premature exit from the neuroepithelium and increased cell migration. CDH11 expression is elevated in human brain tumors, correlating with higher tumor grade and decreased patient survival. In glioblastoma, CDH11-expressing tumor cells can be found localized near tumor vasculature. Endothelial cells stimulate TGFβ signaling and CDH11 expression in glioblastoma cells. TGFβ promotes glioblastoma cell motility, and knockdown of CDH11 expression in primary human glioblastoma cells inhibits TGFβ-stimulated migration. Together, these findings show that Cadherin-11 can promote cell migration in neural precursors and glioblastoma cells and suggest that endothelial cells increase tumor aggressiveness by co-opting mechanisms that regulate normal neural development.

LHX2 regulates the neural differentiation of human embryonic stem cells via transcriptional modulation of PAX6 and CER1

The LIM homeobox 2 transcription factor Lhx2 is known to control crucial aspects of neural development in various species. However, its function in human neural development is still elusive. Here, we demonstrate that LHX2 plays a critical role in human neural differentiation, using human embryonic stem cells (hESCs) as a model. In hESC-derived neural progenitors (hESC-NPs), LHX2 was found to be expressed before PAX6, and co-expressed with early neural markers. Conditional ectopic expression of LHX2 promoted neural differentiation, whereas disruption of LHX2 expression in hESCs significantly impaired neural differentiation. Furthermore, we have demonstrated that LHX2 regulates neural differentiation at two levels: first, it promotes expression of PAX6 by binding to its active enhancers, and second, it attenuates BMP and WNT signaling by promoting expression of the BMP and WNT antagonist Cerberus 1 gene (CER1), to inhibit non-neural differentiation. These findings indicate that LHX2 regulates the transcription of downstream intrinsic and extrinsic molecules that are essential for early neural differentiation in human.

Dynamic interactions between intermediate neurogenic progenitors and radial glia in embryonic mouse neocortex: potential role in Dll1-Notch signaling

The mammalian neocortical progenitor cell niche is composed of a diverse repertoire of neuroepithelial cells, radial glia (RG), and intermediate neurogenic progenitors (INPs). Previously, live-cell imaging experiments have proved crucial in identifying these distinct progenitor populations, especially INPs, which amplify neural output by undergoing additional rounds of proliferation before differentiating into new neurons. INPs also provide feedback to the RG pool by serving as a source of Delta-like 1 (Dll1), a key ligand for activating Notch signaling in neighboring cells, a well-known mechanism for maintaining RG identity. While much is known about Dll1-Notch signaling at the molecular level, little is known about how this cell-cell contact dependent feedback is transmitted at the cellular level. To investigate how RG and INPs might interact to convey Notch signals, we used high-resolution live-cell multiphoton microscopy (MPM) to directly observe cellular interactions and dynamics, in conjunction with Notch-pathway specific reporters in the neocortical neural stem cell niche in organotypic brain slices from embryonic mice. We found that INPs and RG interact via dynamic and transient elongate processes, some apparently long-range (extending from the subventricular zone to the ventricular zone), and some short-range (filopodia-like). Gene expression profiling of RG and INPs revealed further progenitor cell diversification, including different subpopulations of Hes1+ and/or Hes5+ RG, and Dll1+ and/or Dll3+ INPs. Thus, the embryonic progenitor niche includes a network of dynamic cell-cell interactions, using different combinations of Notch signaling molecules to maintain and likely diversify progenitor pools.

N-cadherin regulates the proliferation and differentiation of ventral midbrain dopaminergic progenitors

Adherens junction (AJ) between dopaminergic (DA) progenitors maintains the structure of ventricular zone and polarity of radial glia cells in the ventral midbrain (vMB) during embryonic development. However, it is unclear how loss of N-cadherin might influence the integrity of the AJ and the process of DA neurogenesis. Here, we used conditional gene targeting approaches to perform the region-specific removal of N-cadherin in the neurogenic niche of DA neurons in the vMB. Removal of N-cadherin in the vMB using Shh-Cre disrupts the AJs of DA progenitors and radial glia processes in the vMB. Surprisingly, loss of N-cadherin in the vMB leads to a significant expansion of DA progenitors, including those expressing Sox2, Ngn2, and Otx2. Cell cycle analyses reveal that the cell cycle exit in the progenitor cells is decreased in the mutants from E11.5 to E12.5. In addition, the efficiency of DA progenitors in differentiating into DA neurons is decreased from E10.5 to E12.5, leading to a marked reduction in the number of DA neurons at E11.5, E12.5, and E17.5. Loss of N-cadherin leads to the diffuse distribution of β-catenin proteins, which are a critical component of AJ and Wnt signaling, from the AJ throughout the entire cytoplasm in neuroepithelial cells, suggesting that canonical Wnt signaling might be activated in the DA progenitors in vMB. Taken together, these results support the notion that N-cadherin regulates the proliferation of DA progenitors and the differentiation of DA neurons through canonical Wnt-β-catenin signaling in the vMB.

AKT activation by N-cadherin regulates beta-catenin signaling and neuronal differentiation during cortical development

BACKGROUND

During cerebral cortical development, neural precursor-precursor interactions in the ventricular zone neurogenic niche coordinate signaling pathways that regulate proliferation and differentiation. Previous studies with shRNA knockdown approaches indicated that N-cadherin adhesion between cortical precursors regulates β-catenin signaling, but the underlying mechanisms remained poorly understood.

RESULTS

Here, with conditional knockout approaches, we find further supporting evidence that N-cadherin maintains β-catenin signaling during cortical development. Using shRNA to N-cadherin and dominant negative N-cadherin overexpression in cell culture, we find that N-cadherin regulates Wnt-stimulated β-catenin signaling in a cell-autonomous fashion. Knockdown or inhibition of N-cadherin with function-blocking antibodies leads to reduced activation of the Wnt co-receptor LRP6. We also find that N-cadherin regulates β-catenin via AKT, as reduction of N-cadherin causes decreased AKT activation and reduced phosphorylation of AKT targets GSK3β and β-catenin. Inhibition of AKT signaling in neural precursors in vivo leads to reduced β-catenin-dependent transcriptional activation, increased migration from the ventricular zone, premature neuronal differentiation, and increased apoptotic cell death.

CONCLUSIONS

These results show that N-cadherin regulates β-catenin signaling through both Wnt and AKT, and suggest a previously unrecognized role for AKT in neuronal differentiation and cell survival during cortical development.

Scratch regulates neuronal migration onset via an epithelial-mesenchymal transition-like mechanism

During neocortical development, the neuroepithelial or neural precursor cells that commit to neuronal fate need to delaminate and start migration toward the pial surface. However, the mechanism that couples neuronal fate commitment to detachment from the neuroepithelium remains largely unknown. Here we show that Scratch1 and Scratch2, members of the Snail superfamily of transcription factors, are expressed upon neuronal fate commitment under the control of proneural genes and promote apical process detachment and radial migration in the developing mouse neocortex. Scratch-induced delamination from the apical surface was mediated by transcriptional repression of the adhesion molecule E-cadherin. These findings suggest that Scratch proteins constitute a molecular link between neuronal fate commitment and the onset of neuronal migration. On the basis of their similarity to proteins involved in the epithelial-mesenchymal transition (EMT), we propose that Scratch proteins mediate the conversion of neuroepithelial cells to migrating neurons or intermediate neuronal progenitors through an EMT-related mechanism.

Adhesion in the stem cell niche: biological roles and regulation

Stem cell self-renewal is tightly controlled by the concerted action of stem cell-intrinsic factors and signals within the niche. Niche signals often function within a short range, allowing cells in the niche to self-renew while their daughters outside the niche differentiate. Thus, in order for stem cells to continuously self-renew, they are often anchored in the niche via adhesion molecules. In addition to niche anchoring, however, recent studies have revealed other important roles for adhesion molecules in the regulation of stem cell function, and it is clear that stem cell-niche adhesion is crucial for stem cell self-renewal and is dynamically regulated. Here, we highlight recent progress in understanding adhesion between stem cells and their niche and how this adhesion is regulated.

Regulation of cadherin expression in nervous system development

This review addresses our current understanding of the regulatory mechanisms for classical cadherin expression during development of the vertebrate nervous system. The complexity of the spatial and temporal expression patterns is linked to morphogenic and functional roles in the developing nervous system. While the regulatory networks controlling cadherin expression are not well understood, it is likely that the multiple signaling pathways active in the development of particular domains also regulate the specific cadherins expressed at that time and location. With the growing understanding of the broader roles of cadherins in cell-cell adhesion and non-adhesion processes, it is important to understand both the upstream regulation of cadherin expression and the downstream effects of specific cadherins within their cellular context.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Wnt signaling and forebrain development

Components of the Wnt signaling pathway are expressed in a tightly regulated and spatially specific manner during development of the forebrain, and Wnts are key regulators of regional forebrain identity. Wnt signaling from the cortical hem regulates the expansion and cell-type specification of the adjacent neuroepithelium and, in conjunction with Bmp, Fgf, and Shh signaling, controls dorsal-ventral forebrain patterning. Subsequently, Wnt signaling dynamically regulates the behavior of cortical progenitor cells, initially promoting the expansion of radial glia progenitor cells and later inducing neurogenesis by promoting terminal differentiation of intermediate progenitor cells. A role for Wnt signaling in cell-type specification has also been proposed.

Crucial first steps: the transcriptional control of neuron delamination

A crucial event in the birth of a neuron is the detachment of its apical process from the neuroepithelium. In this issue of Neuron, Rousso et al. (2012) show that repression of N-cadherin by Foxp transcription factors disrupts apical adherens junctions and triggers neurogenesis.

Foxp-mediated suppression of N-cadherin regulates neuroepithelial character and progenitor maintenance in the CNS

Neuroepithelial attachments at adherens junctions are essential for the self-renewal of neural stem and progenitor cells and the polarized organization of the developing central nervous system. The balance between stem cell maintenance and differentiation depends on the precise assembly and disassembly of these adhesive contacts, but the gene regulatory mechanisms orchestrating this process are not known. Here, we demonstrate that two Forkhead transcription factors, Foxp2 and Foxp4, are progressively expressed upon neural differentiation in the spinal cord. Elevated expression of either Foxp represses the expression of a key component of adherens junctions, N-cadherin, and promotes the detachment of differentiating neurons from the neuroepithelium. Conversely, inactivation of Foxp2 and Foxp4 function in both chick and mouse results in a spectrum of neural tube defects associated with neuroepithelial disorganization and enhanced progenitor maintenance. Together, these data reveal a Foxp-based transcriptional mechanism that regulates the integrity and cytoarchitecture of neuroepithelial progenitors.

P-cadherin regulates human hair growth and cycling via canonical Wnt signaling and transforming growth factor-β2

P-cadherin is a key component of epithelial adherens junctions, and it is prominently expressed in the hair follicle (HF) matrix. Loss-of-function mutations in CDH3, which encodes P-cadherin, result in hypotrichosis with juvenile macular dystrophy (HJMD), an autosomal recessive disorder featuring sparse and short hair. Here, we attempted to recapitulate some aspects of HJMD in vitro by transfecting normal, organ-cultured human scalp HFs with lipofectamine and CDH3-specific or scrambled control siRNAs. As in HJMD patients, P-cadherin silencing inhibited hair shaft growth, prematurely induced HF regression (catagen), and inhibited hair matrix keratinocyte proliferation. In situ, membrane β-catenin expression and transcription of the β-catenin target gene, axin2, were significantly reduced, whereas glycogen synthase kinase 3 β (GSK3β) and phospho-β-catenin immunoreactivity were increased. These effects were partially reversed by inhibiting GSK3β. P-cadherin silencing reduced the expression of the anagen-promoting growth factor, IGF-1, whereas that of transforming growth factor β 2 (TGFβ2; catagen promoter) was enhanced. Neutralizing TGFβ antagonized the catagen-promoting effects of P-cadherin silencing. In summary, we introduce human HFs as an attractive preclinical model for studying the functions of P-cadherin in human epithelial biology and pathology. This model demonstrates that cadherins can be successfully knocked down in an intact human organ in vitro, and shows that P-cadherin is needed for anagen maintenance by regulating canonical Wnt signaling and suppressing TGFβ2.

Akt1 interacts with epidermal growth factor receptors and hedgehog signaling to increase stem/transit amplifying cells in the embryonic mouse cortex

A subset of precursors in the embryonic mouse cortex and in neurospheres expresses a higher level of the serine/threonine kinase Akt1 than neighboring precursors. We reported previously that the functional significance of high Akt1 expression was enhanced Akt1 activity, resulting in an increase in survival, proliferation, and self-renewal of multipotent stem/transit amplifying cells. Akt1 can interact with a number of signaling pathways, but the extrinsic factors that are required for specific effects of elevated Akt1 expression have not been identified. In this study we addressed the contributions of signaling via epidermal growth factor (EGF) and hedgehog (Hh) receptors. In EGF receptor-null precursors or following transient inhibition of EGF receptor tyrosine kinase activity, elevating Akt1 by retroviral transduction could still increase survival and proliferation but could not increase self-renewal. We also found that elevated Akt1 expression induced the expression of EGF receptors (EGFRs) in wild-type precursors. Several extrinsic factors, including Shh, can induce EGFR expression by cortical precursors, and we found that elevating Akt1 allowed them to respond to a subthreshold concentration of Shh to induce EGFRs. In precursors that lack the Hh receptor smoothened, however, elevating Akt1 did not increase EGFR expression or self-renewal, though it could still stimulate proliferation. These findings suggest that a subset of precursors in the embryonic cortex that express an elevated level of Akt1 can respond to lower concentrations of Shh than neighboring precursors, resulting in an increase in their expression of EGFRs. Signaling via EGFRs is required for their self-renewal.

SOX2 hypomorphism disrupts development of the prechordal floor and optic cup

Haploinsufficiency for the HMG-box transcription factor SOX2 results in abnormalities of the human ventral forebrain and its derivative structures. These defects include anophthalmia (absence of eye), microphthalmia (small eye) and hypothalamic hamartoma (HH), an overgrowth of the ventral hypothalamus. To determine how Sox2 deficiency affects the morphogenesis of the ventral diencephalon and eye, we generated a Sox2 allelic series (Sox2(IR), Sox2(LP), and Sox2(EGFP)), allowing for the generation of mice that express germline hypomorphic levels (<40%) of SOX2 protein and that faithfully recapitulate SOX2 haploinsufficient human phenotypes. We find that Sox2 hypomorphism significantly disrupts the development of the posterior hypothalamus, resulting in an ectopic protuberance of the prechordal floor, an upregulation of Shh signaling, and abnormal hypothalamic patterning. In the anterior diencephalon, both the optic stalks and optic cups (OC) of Sox2 hypomorphic (Sox2(HYP)) embryos are malformed. Furthermore, Sox2(HYP) eyes exhibit a loss of neural potential and coloboma, a common phenotype in SOX2 haploinsufficient humans that has not been described in a mouse model of SOX2 deficiency. These results establish for the first time that germline Sox2 hypomorphism disrupts the morphogenesis and patterning of the hypothalamus, optic stalk, and the early OC, establishing a model of the development of the abnormalities that are observed in SOX2 haploinsufficient humans.

Cell adhesion and its endocytic regulation in cell migration during neural development and cancer metastasis

Cell migration is a crucial event for tissue organization during development, and its dysregulation leads to several diseases, including cancer. Cells exhibit various types of migration, such as single mesenchymal or amoeboid migration, collective migration and scaffold cell-dependent migration. The migration properties are partly dictated by cell adhesion and its endocytic regulation. While an epithelial-mesenchymal transition (EMT)-mediated mesenchymal cell migration requires the endocytic recycling of integrin-mediated adhesions after the disruption of cell-cell adhesions, an amoeboid migration is not dependent on any adhesions to extracellular matrix (ECM) or neighboring cells. In contrast, a collective migration is mediated by both cell-cell and cell-ECM adhesions, and a scaffold cell-dependent migration is regulated by the endocytosis and recycling of cell-cell adhesion molecules. Although some invasive carcinoma cells exhibit an EMT-mediated mesenchymal or amoeboid migration, other cancer cells are known to maintain cadherin-based cell-cell adhesions and epithelial morphology during metastasis. On the other hand, a scaffold cell-dependent migration is mainly utilized by migrating neurons in normal developing brains. This review will summarize the structures of cell adhesions, including adherens junctions and focal adhesions, and discuss the regulatory mechanisms for the dynamic behavior of cell adhesions by endocytic pathways in cell migration in physiological and pathological conditions, focusing particularly on neural development and cancer metastasis.

N-cadherin specifies first asymmetry in developing neurons

The precise polarization and orientation of developing neurons is essential for the correct wiring of the brain. In pyramidal excitatory neurons, polarization begins with the sprouting of opposite neurites, which later define directed migration and axo-dendritic domains. We here show that endogenous N-cadherin concentrates at one pole of the newborn neuron, from where the first neurite subsequently emerges. Ectopic N-cadherin is sufficient to favour the place of appearance of the first neurite. The Golgi and centrosome move towards this newly formed morphological pole in a second step, which is regulated by PI3K and the actin/microtubule cytoskeleton. Moreover, loss of function experiments in vivo showed that developing neurons with a non-functional N-cadherin misorient their cell axis. These results show that polarization of N-cadherin in the immediate post-mitotic stage is an early and crucial mechanism in neuronal polarity.

Cortical neurogenesis and morphogens: diversity of cues, sources and functions

The cerebral cortex is composed of hundreds of different types of neurons, which underlie its ability to perform highly complex neural processes. How cortical neurons are generated during development constitutes a major challenge in developmental neurosciences with important implications for brain repair and diseases. Cortical neurogenesis is dependent on intrinsic and extrinsic cues, which interplay to generate cortical neurons at the right number, time and place. While the role of intrinsic factors such as proneural and Notch genes has been well established, recent evidence indicate that most classical morphogens, produced by various neural and non-neural sources throughout embryonic development, contribute to the master control and fine tuning of cortical neurogenesis. Here we review some recent advances in the dissection of the molecular logic underlying neurogenesis in the cortex, with special emphasis on the roles of morphogenic cues in this process.

Adherens junctions in mammalian development, homeostasis and disease: lessons from mice

Mice have proven to be a particularly powerful model to study molecular mechanisms of development and disease. The reason for this is the close evolutionary relationship between rodents and humans, similarities in physiological mechanisms in mice and human, and the large number of techniques available to study gene functions in mice. A large number of mice mutations, either germ line, conditional or inducible, have been generated in the past years for adherens junctions components, and the number is still increasing. In this review we will discuss mice models that have contributed to understanding the developmental and physiological role of adherens junctions and their components in mammals and have revealed novel mechanistic aspects of how adherens junctions regulate morphogenesis and tissue homeostasis.

N-cadherin adherens junctions mediate osteogenesis through PI3K signaling

During endochondral ossification, the cartilage is surrounded by a layer of cells that constitute the perichondrium. Communication between osteoblasts in the perichondrium via N-cadherin adherens junctions is essential for endochondral bone growth. We observed that adherens junction molecule N-cadherin and its interacting partners p120, β-catenin and PTEN are expressed by cells present in the perichondrium. To study if N-cadherin mediated adherens junctions play a role in mediating signal transduction events during bone development, we utilized MC3T3E1 preosteoblasts plated at sub confluent (low) and confluent (high) densities to mimic adherens junction formation. When MC3T3E1 cells were plated at high density we observed an increase in phosphorylation of AKTSer473 and its downstream target GSK3Ser9, which coincided with an increase in Osterix, Osteomodulin and Osteoglycin gene expression. Using immunofluorescence, we identified N-cadherin, p120 and β-catenin localized at the membrane of MC3T3E1 cells. Treatment of confluent MC3T3E1 cells with an N-cadherin junction inhibitor-EGTA and a PI3K inhibitor LY294002 resulted in reduction of phosphorylation levels of AKT and GSK3 and expression of Osterix, Osteomodulin and Osteoglycin. Furthermore, utilizing an N-cadherin blocking antibody resulted in reduced AKT signaling and Osterix gene expression, suggesting that osteoblast junction formation is linked to activation of PI3K signaling, which leads to osteoblast differentiation. To further explore the strength of this linkage, we utilized a conditional knockout approach using Dermo1cre to delete β-catenin and PTEN, two important proteins known to be essential for adherens junctions and PI3K signaling, respectively. In the absence of β-catenin, we observed a decrease in adherens junctions and AKT signaling in the perichondrium. PTEN deletion, on the other hand, increased the number of cells expressing N-cadherin in the perichondrium. These observations show that N-cadherin mediated junctions between osteoblasts are needed for osteoblast gene transcription.

N-cadherin regulates radial glial fiber-dependent migration of cortical locomoting neurons

During cerebral cortical development, post-mitotic neurons exhibit a multi-step migration. The locomotion mode covers most of the neuronal migration path. Although for many decades, locomoting neurons have been known to migrate along radial glial fibers, how the cortical locomoting neurons attach to and migrate along radial glial fibers was largely unknown. We recently reported that N-cadherin is required for cortical neuronal migration in vivo. Knockdown or dominant negative inhibition of N-cadherin results in severe neuronal migration defects. Furthermore, suppression of Rab5-dependent endocytosis increases cell surface levels of N-cadherin and perturbs neuronal migration. We showed here that N-cadherin overexpression, which would mimic Rab5 suppression, weakly suppressed neuronal migration, suggesting that excess N-cadherin also disturbs neuronal migration. Interestingly, however, N-cadherin knockdown and overexpression in neurons resulted in different morphologies. While N-cadherin-overexpressing cells closely attached to the radial glial fibers similar to control or Rab5-knockdown cells, N-cadherin knockdown weakened the attachment as the average distance between the soma and radial glial fibers was significantly increased. Taken together, these findings suggest that N-cadherin controls the neuronal attachment to radial glial fibers and that N-cadherin-mediated adhesion complexes are reconstituted through Rab GTPases-dependent endocytic pathways to maintain the proper surface N-cadherin level and to promote neuronal migration.

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.

Transcriptome analysis of neural progenitor cells by a genetic dual reporter strategy

Global analysis of stem/progenitor cells promises new insight into mechanisms that govern self-renewal and cellular potential, an unresolved question of stem/progenitor cell biology. Despite rapid advance of genome-wide profiling methods, the difficulty in cell purification remains a major challenge for global analysis of somatic stem/progenitor cells. Genetic tagging with a reporter provides a powerful tool for identification and isolation of a specific mature cell type; however, for stem/progenitor cells, reporter retention by progeny may be a concern for impurity. Here, we describe a genetic system combining a progenitor cell specific label with a second tag for marking differentiation. We present evidence that differential labeling of neural progenitor cells and their progeny enables prospective purification of these two cell types, whereas isolation based on a single marker compromises the purity of the intended progenitor population. Comparative expression profiling between the purified progenitors and progeny documents a neural progenitor cell transcriptome and uncovers an important role of Tyro3/Axl/Mer receptor tyrosine kinases in the maintenance of neural progenitor cells. This study establishes a general strategy for isolation of somatic stem/progenitor cells and provides a transcriptome database of neural progenitor cells useful for identification of causal factors of neural progenitor cell state, global dissection of epigenetic control of cellular potential, as well as for developing biomarkers or targets of brain cancer stem/initiating cells for therapeutic interventions.

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.

Deriving excitatory neurons of the neocortex from pluripotent stem cells

The human cerebral cortex is an immensely complex structure that subserves critical functions that can be disrupted in developmental and degenerative disorders. Recent innovations in cellular reprogramming and differentiation techniques have provided new ways to study the cellular components of the cerebral cortex. Here, we discuss approaches to generate specific subtypes of excitatory cortical neurons from pluripotent stem cells. We review spatial and temporal aspects of cortical neuron specification that can guide efforts to produce excitatory neuron subtypes with increased resolution. Finally, we discuss distinguishing features of human cortical development and their translational ramifications for cortical stem cell technologies.

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.

Reelin, Rap1 and N-cadherin orient the migration of multipolar neurons in the developing neocortex

Projection neurons migrate from the ventricular zone to the neocortical plate during mouse brain development. Their overall movement is radial, but they become multipolar and move non-radially in the intermediate zone. Here we show that Reelin, the Rap1 GTPase, and N-cadherin (NCad) are important for multipolar neurons to polarize their migration towards the cortical plate. Inhibition and rescue experiments indicate that Reelin regulates migration through Rap1 and Akt, and that Rap1-regulated GTPases, RalA/B, Rac1 and Cdc42, are also involved. We find that Rap1 regulates plasma membrane localization of N-cadherin, and N-cadherin rescues radial polarization when Rap1 is inhibited. Curiously, inhibition of Rap1 or N-cadherin has little effect on glia-dependent locomotion. We propose a multi-step mechanism in which Reelin activates Rap1, Rap1 up-regulates N-cadherin, and N-cadherin is needed to orient cell migration.

Reelin regulates cadherin function via Dab1/Rap1 to control neuronal migration and lamination in the neocortex

Neuronal migration is critical for establishing neocortical cell layers and migration defects can cause neurological and psychiatric diseases. Recent studies show that radially migrating neocortical neurons use glia-dependent and glia-independent modes of migration, but the signaling pathways that control different migration modes and the transitions between them are poorly defined. Here, we show that Dab1, an essential component of the reelin pathway, is required in radially migrating neurons for glia-independent somal translocation, but not for glia-guided locomotion. During migration, Dab1 acts in translocating neurons to stabilize their leading processes in a Rap1-dependent manner. Rap1, in turn, controls cadherin function to regulate somal translocation. Furthermore, cell-autonomous neuronal deficits in somal translocation are sufficient to cause severe neocortical lamination defects. Thus, we define the cellular mechanism of reelin function during radial migration, elucidate the molecular pathway downstream of Dab1 during somal translocation, and establish the importance of glia-independent motility in neocortical development.

DISC1-dependent switch from progenitor proliferation to migration in the developing cortex

Regulatory mechanisms governing the sequence from progenitor cell proliferation to neuronal migration during corticogenesis are poorly understood1–10. Here we report that phosphorylation of DISC1, a major susceptibility factor for several mental disorders, acts as a molecular switch from maintaining proliferation of mitotic progenitor cells to activating migration of postmitotic neurons. Unphosphorylated DISC1 regulates canonical Wnt signaling via an interaction with GSK3β, whereas specific phosphorylation at Serine 710 (S710) triggers the recruitment of Bardet-Biedl-Syndrome (BBS) proteins to the centrosome. In support of this model, loss of BBS1 leads to defects in migration, but not proliferation, while DISC1 knockdown leads to deficits in both. A phospho-dead mutant can only rescue proliferation, while a phospho-mimic mutant rescues exclusively migration defects. These data highlight a dual role for DISC1 in corticogenesis and suggest that phosphorylation of this protein at S710 activates a key developmental switch.

Loss of desmocollin-2 confers a tumorigenic phenotype to colonic epithelial cells through activation of Akt/β-catenin signaling

This study provides evidence that decreased expression of the desmosomal cadherin desmocollin-2 enhances intestinal epithelial cell proliferation and promotes tumor formation via an Akt/β-catenin pathway.

Desmocollin-2 (Dsc2) and desmoglein-2 (Dsg2) are transmembrane cell adhesion proteins of desmosomes. Reduced expression of Dsc2 has been reported in colorectal carcinomas, suggesting that Dsc2 may play a role in the development and/or progression of colorectal cancer. However, no studies have examined the mechanistic contribution of Dsc2 deficiency to tumorigenesis. Here we report that loss of Dsc2 promotes cell proliferation and enables tumor growth in vivo through the activation of Akt/β-catenin signaling. Inhibition of Akt prevented the increase in β-catenin–dependent transcription and proliferation following Dsc2 knockdown and attenuated the in vivo growth of Dsc2-deficient cells. Taken together, our results provide evidence that loss of Dsc2 contributes to the growth of colorectal cancer cells and highlight a novel mechanism by which the desmosomal cadherins regulate β-catenin signaling.

Cdc42 and Gsk3 modulate the dynamics of radial glial growth, inter-radial glial interactions and polarity in the developing cerebral cortex

Polarized radial glia are crucial to the formation of the cerebral cortex. They serve as neural progenitors and as guides for neuronal placement in the developing cerebral cortex. The maintenance of polarized morphology is essential for radial glial functions, but the extent to which the polarized radial glial scaffold is static or dynamic during corticogenesis remains an open question. The developmental dynamics of radial glial morphology, inter-radial glial interactions during corticogenesis, and the role of the cell polarity complexes in these activities remain undefined. Here, using real-time imaging of cohorts of mouse radial glia cells, we show that the radial glial scaffold, upon which the cortex is constructed, is highly dynamic. Radial glial cells within the scaffold constantly interact with one another. These interactions are mediated by growth cone-like endfeet and filopodia-like protrusions. Polarized expression of the cell polarity regulator Cdc42 in radial glia regulates glial endfeet activities and inter-radial glial interactions. Furthermore, appropriate regulation of Gsk3 activity is required to maintain the overall polarity of the radial glia scaffold. These findings reveal dynamism and interactions among radial glia that appear to be crucial contributors to the formation of the cerebral cortex. Related cell polarity determinants (Cdc42, Gsk3) differentially influence radial glial activities within the evolving radial glia scaffold to coordinate the formation of cerebral cortex.

Future perspectives: therapeutic targeting of notch signalling may become a strategy in patients receiving stem cell transplantation for hematologic malignancies

The human Notch system consists of 5 ligands and 4 membrane receptors with promiscuous ligand binding, and Notch-initiated signalling interacts with a wide range of other intracellular pathways. The receptor signalling seems important for regulation of normal and malignant hematopoiesis, development of the cellular immune system, and regulation of immune responses. Several Notch-targeting agents are now being developed, including natural receptor ligands, agonistic and antagonistic antibodies, and inhibitors of intracellular Notch-initiated signalling. Some of these agents are in clinical trials, and several therapeutic strategies seem possible in stem cell recipients: (i) agonists may be used for stem cell expansion and possibly to enhance posttransplant lymphoid reconstitution; (ii) receptor-specific agonists or antagonists can be used for immunomodulation; (iii) Notch targeting may have direct anticancer effects. Although the effects of therapeutic targeting are difficult to predict due to promiscuous ligand binding, targeting of this system may represent an opportunity to achieve combined effects with earlier posttransplant reconstitution, immunomodulation, or direct anticancer effects.

Rab GTPases-dependent endocytic pathways regulate neuronal migration and maturation through N-cadherin trafficking

Although membrane trafficking pathways are involved in basic cellular functions, the evolutionally expanded number of their related family proteins suggests additional roles for membrane trafficking in higher organisms. Here, we show that several Rab-dependent trafficking pathways differentially participate in neuronal migration, an essential step for the formation of the mammalian-specific six-layered brain structure. In vivo electroporation-mediated suppression of Rab5 or dynamin to block endocytosis caused a severe neuronal migration defect in mouse cerebral cortex. Among many downstream endocytic pathways, suppression of Rab11-dependent recycling pathways exhibited a similar migration disorder, whereas inhibition of Rab7-dependent lysosomal degradation pathways affected only the final phase of neuronal migration and dendrite morphology. Inhibition of Rab5 or Rab11 perturbed the trafficking of N-cadherin, whose suppression also disturbed neuronal migration. Taken together, our findings reveal physiological roles of endocytic pathways, each of which has specific functions in distinct steps of neuronal migration and maturation during mammalian brain formation.

Beta-catenin signaling negatively regulates intermediate progenitor population numbers in the developing cortex

Intermediate progenitor cells constitute a second proliferative cell type in the developing mammalian cerebral cortex. Little is known about the factors that govern the production of intermediate progenitors. Although persistent expression of stabilized β-catenin was found to delay the maturation of radial glial progenitors into intermediate progenitors, the relationship between β-catenin signaling and intermediate progenitors remains poorly understood. Using a transgenic reporter mouse for Axin2, a direct target of Wnt/β-catenin signaling, we observed that β-catenin signaling is decreased in intermediate progenitor cells relative to radial glial progenitors. Conditional deletion of β-catenin from mouse cortical neural progenitors increased intermediate progenitor numbers, while conditional expression of stabilized β-catenin reduced the intermediate progenitor population. Together, these findings provide evidence that β-catenin signaling in radial progenitors negatively regulates intermediate progenitor cell number during cortical development.