Cep120 and TACCs control interkinetic nuclear migration and the neural progenitor pool

Platelet activating factor blocks interkinetic nuclear migration in retinal progenitors through an arrest of the cell cycle at the S/G2 transition

Nuclear migration is regulated by the LIS1 protein, which is the regulatory subunit of platelet activating factor (PAF) acetyl-hydrolase, an enzyme complex that inactivates the lipid mediator PAF. Among other functions, PAF modulates cell proliferation, but its effects upon mechanisms of the cell cycle are unknown. Here we show that PAF inhibited interkinetic nuclear migration (IKNM) in retinal proliferating progenitors. The lipid did not, however, affect the velocity of nuclear migration in cells that escaped IKNM blockade. The effect depended on the PAF receptor, Erk and p38 pathways and Chk1. PAF induced no cell death, nor a reduction in nucleotide incorporation, which rules out an intra-S checkpoint. Notwithstanding, the expected increase in cyclin B1 content during G2-phase was prevented in the proliferating cells. We conclude that PAF blocks interkinetic nuclear migration in retinal progenitor cells through an unusual arrest of the cell cycle at the transition from S to G2 phases. These data suggest the operation, in the developing retina, of a checkpoint that monitors the transition from S to G2 phases of the cell cycle.

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.

Kinesin 3 and cytoplasmic dynein mediate interkinetic nuclear migration in neural stem cells

Radial glial progenitor cells exhibit bidirectional cell cycle-dependent nuclear oscillations. The purpose and underlying mechanism of this unusual 'interkinetic nuclear migration' are poorly understood. We investigated the basis for this behavior by live imaging of nuclei, centrosomes and microtubules in embryonic rat brain slices, coupled with the use of RNA interference (RNAi) and the myosin inhibitor blebbistatin. We found that nuclei migrated independent of centrosomes and unidirectionally away from or toward the ventricular surface along microtubules, which were uniformly oriented from the ventricular surface to the pial surface of the brain. RNAi directed against cytoplasmic dynein specifically inhibited nuclear movement toward the apical surface. An RNAi screen of kinesin genes identified Kif1a, a member of the kinesin-3 family, as the motor for basally directed nuclear movement. These observations provide direct evidence that kinesins are involved in nuclear migration and neurogenesis and suggest that a cell cycle-dependent switch between distinct microtubule motors drives interkinetic nuclear migration.

Strategies for analyzing neuronal progenitor development and neuronal migration in the developing cerebral cortex

The emergence of functional neuronal connectivity in the developing cerebral cortex depends on 1) neural progenitor differentiation, which leads to the generation of appropriate number and types of neurons, and 2) neuronal migration, which enables the appropriate positioning of neurons so that the correct patterns of functional synaptic connectivity between neurons can emerge. In this review, we discuss 1) currently available methods to study neural progenitor development and differentiation in the developing cerebral cortex and emerging technologies in this regard, 2) assays to study the migration of descendents of progenitors (i.e., neurons) in vitro and in vivo, and 3) the use of these assays to probe the molecular control of these events in the developing brain and evaluation of gene functions disrupted in human neurodevelopmental disorders.

The centrosome neither persistently leads migration nor determines the site of axonogenesis in migrating neurons in vivo

The position of the centrosome ahead of the nucleus has been considered crucial for coordinating neuronal migration in most developmental situations. The proximity of the centrosome has also been correlated with the site of axonogenesis in certain differentiating neurons. Despite these positive correlations, accumulating experimental findings appear to negate a universal role of the centrosome in determining where an axon forms, or in leading the migration of neurons. To further examine this controversy in an in vivo setting, we have generated cell type-specific multi-cistronic gene expression to monitor subcellular dynamics in the developing zebrafish cerebellum. We show that migration of rhombic lip-derived neurons is characterized by a centrosome that does not persistently lead the nucleus, but which is instead regularly overtaken by the nucleus. In addition, axonogenesis is initiated during the onset of neuronal migration and occurs independently of centrosome proximity. These in vivo data reveal a new temporal orchestration of organelle dynamics and provide important insights into the variation in intracellular processes during vertebrate brain differentiation.

Neural progenitor nuclei IN motion

Interkinetic nuclear migration (INM), the movement of neuroepithelial and radial glial cell nuclei along the apical-basal axis in concert with the cell cycle, underlies the pseudostratification of the ventricular zone (VZ). Recent studies provide insight into the molecular mechanisms of INM and its effects on neural progenitor cell fate determination. Moreover, INM not only has a key role in increasing the VZ progenitor pool, but also may have set the stage for the evolution of subventricular zone progenitors implicated in cortical expansion.

A small molecule accelerates neuronal differentiation in the adult rat

Adult neurogenesis occurs in mammals and provides a mechanism for continuous neural plasticity in the brain. However, little is known about the molecular mechanisms regulating hippocampal neural progenitor cells (NPCs) and whether their fate can be pharmacologically modulated to improve neural plasticity and regeneration. Here, we report the characterization of a small molecule (KHS101) that selectively induces a neuronal differentiation phenotype. Mechanism of action studies revealed a link of KHS101 to cell cycle exit and specific binding to the TACC3 protein, whose knockdown in NPCs recapitulates the KHS101-induced phenotype. Upon systemic administration, KHS101 distributed to the brain and resulted in a significant increase in neuronal differentiation in vivo. Our findings indicate that KHS101 accelerates neuronal differentiation by interaction with TACC3 and may provide a basis for pharmacological intervention directed at endogenous NPCs.

Centrosome motility is essential for initial axon formation in the neocortex

The mechanisms underlying the normal development of neuronal morphology remain a fundamental question in neurobiology. Studies in cultured neurons have suggested that the position of the centrosome and the Golgi may predict the site of axon outgrowth. During neuronal migration in the developing cortex, however, the centrosome and Golgi are oriented toward the cortical plate at a time when axons grow toward the ventricular zone. In the current work, we use in situ live imaging to demonstrate that the centrosome and the accompanying polarized cytoplasm exhibit apical translocation in newborn cortical neurons preceding initial axon outgrowth. Disruption of centrosomal activity or downregulation of the centriolar satellite protein PCM-1 affects axon formation. We further show that downregulation of the centrosomal protein Cep120 impairs microtubule organization, resulting in increased centrosome motility. Decreased centrosome motility resulting from microtubule stabilization causes an aberrant centrosomal localization, leading to misplaced axonal outgrowth. Our results reveal the dynamic nature of the centrosome in developing cortical neurons, and implicate centrosome translocation and microtubule organization during the multipolar stage as important determinants of axon formation.

Control of activating transcription factor 4 (ATF4) persistence by multisite phosphorylation impacts cell cycle progression and neurogenesis

Organogenesis is a highly integrated process with a fundamental requirement for precise cell cycle control. Mechanistically, the cell cycle is composed of transitions and thresholds that are controlled by coordinated post-translational modifications. In this study, we describe a novel mechanism controlling the persistence of the transcription factor ATF4 by multisite phosphorylation. Proline-directed phosphorylation acted additively to regulate multiple aspects of ATF4 degradation. Stabilized ATF4 mutants exhibit decreased β-TrCP degron phosphorylation, β-TrCP interaction, and ubiquitination, as well as elicit early G₁ arrest. Expression of stabilized ATF4 also had significant consequences in the developing neocortex. Mutant ATF4 expressing cells exhibited positioning and differentiation defects that were attributed to early G₁ arrest, suggesting that neurogenesis is sensitive to ATF4 dosage. We propose that precise regulation of the ATF4 dosage impacts cell cycle control and impinges on neurogenesis.

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.

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.

Cdk5rap2 interacts with pericentrin to maintain the neural progenitor pool in the developing neocortex

Primary autosomal-recessive microcephaly (MCPH) and Majewski osteodysplastic primordial dwarfism type II (MOPDII) are both genetic diseases that result in decreased brain size at birth. MCPH is thought to arise from alterations in the size of the neural progenitor pool, but the cause of this defect has not been thoroughly explored. We find that one of the genes associated with MCPH, Cdk5rap2, is highly expressed in the neural progenitor pool and that its loss results in a depletion of apical progenitors and increased cell-cycle exit leading to premature neuronal differentiation. We link Cdk5rap2 function to the pericentriolar material protein pericentrin, loss of function of which is associated with MOPDII. Depletion of pericentrin in neural progenitors phenocopies effects of Cdk5rap2 knockdown and results in decreased recruitment of Cdk5rap2 to the centrosome. Our findings uncover a common mechanism, involving aberrations in the neurogenesis program, that may underlie the development of microcephaly in multiple diseases.

Disruption of CK2beta in embryonic neural stem cells compromises proliferation and oligodendrogenesis in the mouse telencephalon

Genetic programs that govern neural stem/progenitor cell (NSC) proliferation and differentiation are dependent on extracellular cues and a network of transcription factors, which can be regulated posttranslationally by phosphorylation. However, little is known about the kinase-dependent pathways regulating NSC maintenance and oligodendrocyte development. We used a conditional knockout approach to target the murine regulatory subunit (beta) of protein kinase casein kinase 2 (CK2beta) in embryonic neural progenitors. Loss of CK2beta leads to defects in proliferation and differentiation of embryonic NSCs. We establish CK2beta as a key positive regulator for the development of oligodendrocyte precursor cells (OPCs), both in vivo and in vitro. We show that CK2beta directly interacts with the basic helix-loop-helix (bHLH) transcription factor Olig2, a critical modulator of OPC development, and activates the CK2-dependent phosphorylation of its serine-threonine-rich (STR) domain. Finally, we reveal that the CK2-targeted STR domain is required for the oligodendroglial function of Olig2. These findings suggest that CK2 may control oligodendrogenesis, in part, by regulating the activity of the lineage-specific transcription factor Olig2. Thus, CK2beta appears to play an essential and uncompensated role in central nervous system development.

Gap junctions/hemichannels modulate interkinetic nuclear migration in the forebrain precursors

During mitotic division in the telencephalic proliferative ventricular zone (VZ), the nuclei of the neural precursors move basally away from the ventricular surface for DNA synthesis, and apically return to the surface for mitotic division; a process known as interkinetic migration or "to-and-fro" nuclear translocation. The cell, which remains attached to the ventricular surface, either continues cycling, or exits the cycle and migrates to the subventricular zone or the developing cortical plate. Although gap junctions/hemichannels are known to modulate DNA synthesis via Ca(2+) waves, the role of Ca(+) oscillations and the mechanism of nuclear translocation in the VZ precursors are unclear. Here, we provide evidence that, during apical nuclear migration, VZ precursors display dynamic spontaneous Ca(2+) transients, which depend on functional gap junctions/hemichannels via ATP release and Ca(2+)-mobilizing messenger diffusion. Furthermore, we found that blocking gap junctions/hemichannels or short hairpin RNA-mediated knockdown of Cx43 (connexin 43) retards the apically directed interkinetic nuclear migration accompanied with changes in the nuclear length/width ratio. In addition, we demonstrated that blocking functional gap junctions/hemichannels induces phosphorylation of small GTPase cdc42 in the VZ precursors. The basal phase of interkinetic migration is much slower and appears to be mediated passively by mechanical forces after cell division. Our findings indicate that functional interference with gap junctions/hemichannels during embryonic development may lead to abnormal corticogenesis and dysfunction of the cerebral cortex in adult organisms.

Duplication of the Wolf-Hirschhorn syndrome critical region causes neurodevelopmental delay

Wolf-Hirschhorn Syndrome (WHS) is caused by deletions on chromosome 4p and is clinically well defined. Genotype-phenotype correlations of patients with WHS point to a critical locus to be responsible for the main characteristics of this disorder. Submicroscopic duplications of this region, however, are not known. Here we report a patient with an interstitial 560 kb duplication overlapping this critical locus. The present case shows that not only deletions but also duplications of the Wolf-Hirshhorn critical region cause mental retardation and multiple congenital anomalies. Interestingly, the duplication phenotype overlaps partially with the deletion phenotype. However, his facial phenotype differs from the typical WHS gestalt.

CLERC and centrosomal leucine-rich repeat proteins

The centrosome functions as the microtubule-organizing center and plays a vital role in organizing spindle poles during mitosis. Recently, we identified a centrosomal protein called CLERC (Centrosomal leucine-rich repeat and coiled-coil containing protein) which is a human ortholog of Chlamydomonas Vfl1 protein. The bibliography as well as database searches provided evidence that the human proteome contains at least seven centrosomal leucine-rich repeat proteins including CLERC. CLERC and four other centrosomal leucine-rich repeat proteins contain the SDS22-like leucine-rich repeat motifs, whereas the remaining two proteins contain the RI-like and the cysteine-containing leucine-rich repeat motifs. Individual leucine-rich repeat motifs are highly conserved and present in evolutionarily diverse organisms. Here, we provide an overview of CLERC and other centrosomal leucine-rich repeat proteins, their structures, their evolutionary relationships, and their functional properties.

Hook3 interacts with PCM1 to regulate pericentriolar material assembly and the timing of neurogenesis

Centrosome functions are important in multiple brain developmental processes. Proper functioning of the centrosome relies on assembly of protein components into the pericentriolar material. This dynamic assembly is mediated by the trafficking of pericentriolar satellites, which are comprised of centrosomal proteins. Here we demonstrate that trafficking of pericentriolar satellites requires the interaction between Hook3 and Pericentriolar Material 1 (PCM1). Hook3, previously shown to link the centrosome and the nucleus in C. elegans, is recruited to pericentriolar satellites through interaction with PCM1, a protein associated with schizophrenia. Disruption of the Hook3-PCM1 interaction in vivo impairs interkinetic nuclear migration, a featured behavior of embryonic neural progenitors. This in turn leads to overproduction of neurons and premature depletion of the neural progenitor pool in the developing neocortex. These results underscore the importance of centrosomal assembly in neurogenesis and provide potential insights into the etiology of brain developmental diseases related to the centrosome dysfunction.

FAK-mediated extracellular signals are essential for interkinetic nuclear migration and planar divisions in the neuroepithelium

During the development of the vertebrate nervous system, mitosis of neural progenitor cells takes place near the lumen, the apical side of the neural tube, through a characteristic movement of nuclei known as interkinetic nuclear migration (INM). Furthermore, during the proliferative period, neural progenitor cells exhibit planar cell divisions to produce equivalent daughter cells. Here, we examine the potential role of extracellular signals in INM and planar divisions using the medaka mutant tacobo (tab). This tab mutant shows pleiotropic phenotypes, including neurogenesis, and positional cloning identified tab as laminin gamma1 (lamc1), providing a unique framework to study the role of extracellular signals in neurogenesis. In tab mutant neural tubes, a number of nuclei exhibit abnormal patterns of migration leading to basally mislocalized mitosis. Furthermore, the orientation of cell division near the apical surface is randomized. Probably because of these defects, neurogenesis is accelerated in the tab neural tube. Detailed analyses demonstrate that extracellular signals mediated by the FAK pathway regulate INM and planar divisions in the neuroepithelium, possibly through interaction with the intracellular dynein-motor system.

Asymmetric centrosome inheritance maintains neural progenitors in the neocortex

Asymmetric divisions of radial glia progenitors produce self-renewing radial glia and differentiating cells simultaneously in the ventricular zone (VZ) of the developing neocortex. Whereas differentiating cells leave the VZ to constitute the future neocortex, renewing radial glia progenitors stay in the VZ for subsequent divisions. The differential behaviour of progenitors and their differentiating progeny is essential for neocortical development; however, the mechanisms that ensure these behavioural differences are unclear. Here we show that asymmetric centrosome inheritance regulates the differential behaviour of renewing progenitors and their differentiating progeny in the embryonic mouse neocortex. Centrosome duplication in dividing radial glia progenitors generates a pair of centrosomes with differently aged mother centrioles. During peak phases of neurogenesis, the centrosome retaining the old mother centriole stays in the VZ and is preferentially inherited by radial glia progenitors, whereas the centrosome containing the new mother centriole mostly leaves the VZ and is largely associated with differentiating cells. Removal of ninein, a mature centriole-specific protein, disrupts the asymmetric segregation and inheritance of the centrosome and causes premature depletion of progenitors from the VZ. These results indicate that preferential inheritance of the centrosome with the mature older mother centriole is required for maintaining radial glia progenitors in the developing mammalian neocortex.

Sun proteins enlighten nuclear movement in development

Regulation of nuclear movement is a critical event in neurogenesis and neuronal migration during brain development. In this issue of Neuron, Zhang et al. identify a role for SUN and the KASH-domain-containing nuclear membrane proteins as the long-sought linker between microtubules and the nucleus during brain development.

SUN1/2 and Syne/Nesprin-1/2 complexes connect centrosome to the nucleus during neurogenesis and neuronal migration in mice

Nuclear movement is critical during neurogenesis and neuronal migration, which are fundamental for mammalian brain development. Although dynein, Lis1, and other cytoplasmic proteins are known for their roles in connecting microtubules to the nucleus during interkinetic nuclear migration (INM) and nucleokinesis, the factors connecting dynein/Lis1 to the nuclear envelope (NE) remain to be determined. We report here that the SUN-domain proteins SUN1 and SUN2 and the KASH-domain proteins Syne-1/Nesprin-1 and Syne-2/Nesprin-2 play critical roles in neurogenesis and neuronal migration in mice. We show that SUN1 and SUN2 redundantly form complexes with Syne-2 to mediate the centrosome-nucleus coupling during both INM and radial neuronal migration in the cerebral cortex. Syne-2 is connected to the centrosome through interactions with both dynein/dynactin and kinesin complexes. Syne-2 mutants also display severe defects in learning and memory. These results fill an important gap in our understanding of the mechanism of nuclear movement during brain development.

Actomyosin is the main driver of interkinetic nuclear migration in the retina

Progenitor cell nuclei in the rapidly expanding epithelium of the embryonic vertebrate central nervous system undergo a process called interkinetic nuclear migration (IKNM). Movements of IKNM are generally believed to involve smooth migration of nuclei from apical to basal and back during the G1 and G2 phases of the cell cycle, respectively. Yet, this has not been formally demonstrated, nor have the molecular mechanisms that drive IKNM been identified. Using time-lapse confocal microscopy to observe nuclear movements in zebrafish retinal neuroepithelial cells, we show that, except for brief apical nuclear translocations preceding mitosis, IKNM is stochastic rather than smooth and directed. We also show that IKNM is driven largely by actomyosin-dependent forces as it still occurs when the microtubule cytoskeleton is compromised but is blocked when MyosinII activity is inhibited.

Myosin II is required for interkinetic nuclear migration of neural progenitors

Interkinetic nuclear migration (INM) is a hallmark of the polarized stem and progenitor cells in the ventricular zone (VZ) of the developing vertebrate CNS. INM is responsible for the pseudostratification of the VZ, a crucial aspect of brain evolution. The nuclear migration toward the apical centrosomes in G2 is thought to be a dynein-microtubule-based process. By contrast, the cytoskeletal machinery involved in the basally directed nuclear translocation away from the centrosome in G1 has been enigmatic. Studying the latter aspect of INM requires manipulation of the cytoskeleton without impairing mitosis and cytokinesis. To this end, we have established a culture system of mouse embryonic telencephalon that reproduces cortical development, and have applied it to explore a role of actomyosin in INM. Using the nonmuscle myosin II inhibitor blebbistatin at a low concentration at which neither cell cycle progression nor cytokinesis is impaired, we show that myosin II is required for the apical-to-basal (ap-->bl), ab-centrosomal INM. Myosin II activity is also necessary for the nuclear translocation during delamination of subventricular zone (SVZ) cells, a second, telencephalon-specific type of neural progenitor. Moreover, the inhibition of ab-centrosomal INM changes the balance between VZ and SVZ progenitor cell fate. Our data suggest a unifying concept in which the actomyosin contraction underlying ab-centrosomal INM sets the stage for the evolutionary increase in VZ pseudostratification and for SVZ progenitor delamination, a key process in cortical expansion.

Relocalization of a microtubule-anchoring protein, ninein, from the centrosome to dendrites during differentiation of mouse neurons

Microtubules in typical cells form radial arrays with their plus-ends pointing toward the cell periphery. In contrast, microtubules in dendrites of neurons are free from centrosomes and have a unique arrangement in which about half have a polarity with a minus-end distal orientation. Mechanisms for generation and maintenance of the microtubule arrangement in dendrites are not well understood. Here, we examined dendritic localization of a centrosomal protein, ninein, which has microtubule-anchoring and stabilizing functions. Immunohistochemical analysis of developing mouse cerebral and cerebellar cortices showed that ninein is localized at the centrosome in undifferentiated neural precursors. In contrast, ninein was barely detected in migrating neurons, such as those in the intermediate layer of the cerebral cortex and the internal granular layer of the cerebellar cortex. High expression was observed in thick dendrite-bearing neurons such as pyramidal neurons of the cerebral cortex and Purkinje neurons in the cerebellar cortex. Ninein was not detected at the centrosome of these cells, but was diffusely present in cell soma and dendrites. In cultured cortical neurons, ninein formed granular structures in soma and dendrites, being not associated with gamma-tubulin. About 60% of these structures showed resistance to detergent and association with microtubules. Our observations suggest that the minus-ends of microtubules may be anchored and stabilized by centrosomal proteins localized in dendrites.

beta1 integrin maintains integrity of the embryonic neocortical stem cell niche

During embryogenesis, the neural stem cells (NSC) of the developing cerebral cortex are located in the ventricular zone (VZ) lining the cerebral ventricles. They exhibit apical and basal processes that contact the ventricular surface and the pial basement membrane, respectively. This unique architecture is important for VZ physical integrity and fate determination of NSC daughter cells. In addition, the shorter apical process is critical for interkinetic nuclear migration (INM), which enables VZ cell mitoses at the ventricular surface. Despite their importance, the mechanisms required for NSC adhesion to the ventricle are poorly understood. We have shown previously that one class of candidate adhesion molecules, laminins, are present in the ventricular region and that their integrin receptors are expressed by NSC. However, prior studies only demonstrate a role for their interaction in the attachment of the basal process to the overlying pial basement membrane. Here we use antibody-blocking and genetic experiments to reveal an additional and novel requirement for laminin/integrin interactions in apical process adhesion and NSC regulation. Transient abrogation of integrin binding and signalling using blocking antibodies to specifically target the ventricular region in utero results in abnormal INM and alterations in the orientation of NSC divisions. We found that these defects were also observed in laminin alpha2 deficient mice. More detailed analyses using a multidisciplinary approach to analyse stem cell behaviour by expression of fluorescent transgenes and multiphoton time-lapse imaging revealed that the transient embryonic disruption of laminin/integrin signalling at the VZ surface resulted in apical process detachment from the ventricular surface, dystrophic radial glia fibers, and substantial layering defects in the postnatal neocortex. Collectively, these data reveal novel roles for the laminin/integrin interaction in anchoring embryonic NSCs to the ventricular surface and maintaining the physical integrity of the neocortical niche, with even transient perturbations resulting in long-lasting cortical defects.

Gap-Junction Proteins in Retinal Development: New Roles for the “Nexus”

Gap-junction channels, the cytoplasmic proteins that associate with them, and the transcriptional networks that regulate them are increasingly being viewed as critical communications hubs for cell signaling in health and disease. As a result, the term “nexus,” which was the original structural name for these focal intercellular links, is coming back into use with new proteomic and transcriptomic meanings. The retina is better understood than any other part of the vertebrate central nervous system in respect of its developmental patterning, its diverse neuronal types and circuits, and the emergence of its definitive structure-function correlations. Thus, studies of the junctional and nonjunctional nexus roles of gap-junction proteins in coordinating retinal development should throw useful light on cell signaling in other developing nervous tissues.

Genetic variants associated with cardiac structure and function: a meta-analysis and replication of genome-wide association data

CONTEXT

Echocardiographic measures of left ventricular (LV) structure and function are heritable phenotypes of cardiovascular disease.

OBJECTIVE

To identify common genetic variants associated with cardiac structure and function by conducting a meta-analysis of genome-wide association data in 5 population-based cohort studies (stage 1) with replication (stage 2) in 2 other community-based samples.

DESIGN, SETTING, AND PARTICIPANTS

Within each of 5 community-based cohorts comprising the EchoGen consortium (stage 1; n = 12 612 individuals of European ancestry; 55% women, aged 26-95 years; examinations between 1978-2008), we estimated the association between approximately 2.5 million single-nucleotide polymorphisms (SNPs; imputed to the HapMap CEU panel) and echocardiographic traits. In stage 2, SNPs significantly associated with traits in stage 1 were tested for association in 2 other cohorts (n = 4094 people of European ancestry). Using a prespecified P value threshold of 5 x 10(-7) to indicate genome-wide significance, we performed an inverse variance-weighted fixed-effects meta-analysis of genome-wide association data from each cohort.

MAIN OUTCOME MEASURES

Echocardiographic traits: LV mass, internal dimensions, wall thickness, systolic dysfunction, aortic root, and left atrial size.

RESULTS

In stage 1, 16 genetic loci were associated with 5 echocardiographic traits: 1 each with LV internal dimensions and systolic dysfunction, 3 each with LV mass and wall thickness, and 8 with aortic root size. In stage 2, 5 loci replicated (6q22 locus associated with LV diastolic dimensions, explaining <1% of trait variance; 5q23, 12p12, 12q14, and 17p13 associated with aortic root size, explaining 1%-3% of trait variance).

CONCLUSIONS

We identified 5 genetic loci harboring common variants that were associated with variation in LV diastolic dimensions and aortic root size, but such findings explained a very small proportion of variance. Further studies are required to replicate these findings, identify the causal variants at or near these loci, characterize their functional significance, and determine whether they are related to overt cardiovascular disease.

Rac is involved in the interkinetic nuclear migration of cortical progenitor cells

The small GTPase Rac regulates neuronal behavior, but whether it also functions in neural progenitor cells has not yet been explored. Here we report that Rac contributes to the regulation of nuclear migration in neocortical progenitor cells. Rac1 is expressed by progenitor cells in a unique spatiotemporal pattern. Cross-sectional immunohistochemical examination revealed intense Rac1 immunoreactivity at the ventricular surface. Similar staining patterns were obtained by immunofluorescence for a Rac-activator, Tiam1, and by reactions to detect the GTP-bound (active) form of Rac. En face inspection of the ventricular surface revealed that apical Rac1 localization was most frequent in M-phase cells, and the endfeet of cells in other cell cycle phases also showed apical Rac1 distribution at lower frequencies. To ask whether progenitor cell behavior prior to and during M phase is Rac-dependent, we monitored individual DiI-labeled progenitor cells live in the presence of a Rac inhibitor, NSC23766. We observed significantly retarded adventricular nuclear migration, as well as cytokinesis failures. Similar inhibitory effects were obtained by forced expression of a dominant-negative Rac1. These results suggest that Rac may play a role in interkinetic nuclear migration in the developing mouse brain.

Molecular mechanisms of projection neuron production and maturation in the developing cerebral cortex

The cerebral cortex is a brain structure unique to mammals and highly adapted to process complex information. Through multiple developmental steps, the cerebral cortex is assembled as a huge diversity of neurons comprising a complex laminar structure, and with both local and long-distance connectivity within the nervous system. Key processes must take place during its construction, including: (i) regulation of the correct number of neurons produced by progenitor cells, (ii) temporal and spatial generation of neuronal diversity, and (iii) control of neuron migration and laminar positioning as well as terminal differentiation within the mature cortex. Here, we seek to highlight recent cellular and molecular findings underlying these sequential steps of neurogenesis, cell fate specification and migration during cortical development, with particular emphasis on cortical projection neurons.

A nuclear-envelope bridge positions nuclei and moves chromosomes

Positioning the nucleus is essential for the formation of polarized cells, pronuclear migration, cell division, cell migration and the organization of specialized syncytia such as mammalian skeletal muscles. Proteins that are required for nuclear positioning also function during chromosome movement and pairing in meiosis. Defects in these processes lead to human diseases including laminopathies. To properly position the nucleus or move chromosomes within the nucleus, the cell must specify the outer surface of the nucleus and transfer forces across both membranes of the nuclear envelope. KASH proteins are specifically recruited to the outer nuclear membrane by SUN proteins, which reside in the inner nuclear membrane. KASH and SUN proteins physically interact in the perinuclear space, forming a bridge across the two membranes of the nuclear envelope. The divergent N-terminal domains of KASH proteins extend from the surface of the nucleus into the cytoplasm and interact with the cytoskeleton, whereas the N-termini of SUN proteins extend into the nucleoplasm to interact with the lamina or chromatin. The bridge of SUN and KASH across the nuclear envelope functions to transfer forces that are generated in the cytoplasm into the nucleoplasm during nuclear migration, nuclear anchorage, centrosome attachment, intermediate-filament association and telomere clustering.

The adenomatous polyposis coli protein is an essential regulator of radial glial polarity and construction of the cerebral cortex

Radial glia are highly polarized cells that serve as neuronal progenitors and as scaffolds for neuronal migration during construction of the cerebral cortex. How radial glial cells establish and maintain their morphological polarity is unknown. Using conditional gene targeting in mice, we demonstrate that adenomatous polyposis coli (APC) serves an essential function in the maintenance of polarized radial glial scaffold during brain development. In the absence of APC, radial glial cells lose their polarity and responsiveness to the extracellular polarity maintenance cues, such as neuregulin-1. Elimination of APC further leads to marked instability of the radial glial microtubule cytoskeleton. The resultant changes in radial glial function and loss of APC in radial glial progeny lead to defective generation and migration of cortical neurons, severely disrupted cortical layer formation, and aberrant axonal tract development. Thus, APC is an essential regulator of radial glial polarity and is critical for the construction of cerebral cortex in mammals.

Establishment of axon-dendrite polarity in developing neurons

Neurons are among the most highly polarized cell types in the body, and the polarization of axon and dendrites underlies the ability of neurons to integrate and transmit information in the brain. Significant progress has been made in the identification of the cellular and molecular mechanisms underlying the establishment of neuronal polarity using primarily in vitro approaches such as dissociated culture of rodent hippocampal and cortical neurons. This model has led to the predominant view suggesting that neuronal polarization is specified largely by stochastic, asymmetric activation of intracellular signaling pathways. Recent evidence shows that extracellular cues can play an instructive role during neuronal polarization in vitro and in vivo. In this review, we synthesize the recent data supporting an integrative model whereby extracellular cues orchestrate the intracellular signaling underlying the initial break of neuronal symmetry leading to axon-dendrite polarization.

The cell biology of neural stem and progenitor cells and its significance for their proliferation versus differentiation during mammalian brain development

The switch of neural stem and progenitor cells from proliferation to differentiation during development is a crucial determinant of brain size. This switch is intimately linked to the architecture of the two principal classes of neural stem and progenitor cells, the apical (neuroepithelial, radial glial) and basal (intermediate) progenitors, which in turn is crucial for their symmetric versus asymmetric divisions. Focusing on the developing rodent neocortex, we discuss here recent advances in understanding the cell biology of apical and basal progenitors, place key regulatory molecules into subcellular context, and highlight their roles in the control of proliferation versus differentiation.

Putting a notch in our understanding of nuclear migration

The nuclei of progenitor cells in developing neural epithelia change their position during the cell cycle through a process called interkinetic nuclear migration. Del Bene et al. (2008) report that defects in the machinery controlling this process lead to altered exposure to Notch signals and systemic effects on neurogenesis in the retina.

Neural stem cells: balancing self-renewal with differentiation

Stem cells are captivating because they have the potential to make multiple cell types yet maintain their undifferentiated state. Recent studies of Drosophila and mammalian neural stem cells have shed light on how stem cells regulate self-renewal versus differentiation and have revealed the proteins, processes and pathways that all converge to regulate neural progenitor self-renewal. If we can better understand how stem cells balance self-renewal versus differentiation, we will significantly advance our knowledge of embryogenesis, cancer biology and brain evolution, as well as the use of stem cells for therapeutic purposes.

Cell-cycle-specific nestin expression coordinates with morphological changes in embryonic cortical neural progenitors

During brain development, neural progenitor cells extend across the thickening brain wall and undergo mitosis. To understand how these two completely different cellular events are coordinated, we focused on the transcription pattern of the nestin gene (Nes), which encodes an intermediate filament protein strongly expressed in neural progenitor cells. To visualize nestin expression in vivo, we generated transgenic mice that expressed a destabilized fluorescent protein under the control of Nes second intronic enhancer (E/nestin:dVenus). During the neurogenic stage, when the brain wall thickens markedly, we found that nestin was regulated in a cell-cycle-dependent manner. Time-lapse imaging showed that nestin gene expression was upregulated during G1-S phase, when the neural progenitor cells elongate their fibers. However, nestin expression dramatically declined in G2-M phase, when progenitor cells round up to undergo mitosis. The cell-cycle-dependent phosphorylation of an upstream regulator class III POU transcription factor (Pou3f2 or Brn2) reduced its binding activity to the nestin core enhancer element and was therefore responsible for the decreased Nes transcription in G2-M phase. Collectively, these findings demonstrate precisely orchestrated gene regulation that correlates with the 3D morphological changes in neural progenitor cells in vivo.

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

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

A mechanism for inside-out lamination in the neocortex

We outline a unified model for inside-out layering of the neocortex, hinging on a new interpretation for the effects of Reelin on neuronal migrations. The effects of Reelin on cortical structure have been analyzed in great detail, but it has been unclear how individual migrating cells respond to Reelin. In our opinion, many published results might be explained if Reelin acts on neurons when their leading processes reach the marginal zone. Reelin then stimulates two parallel events: detachment from radial glia and translocation of the cell soma to the top of the developing cortical plate. This 'detach and go' model explains many aspects of inside-out lamination, defects in the Reeler mutant and results of recent genetic and in utero experiments.

The ups and downs of neural progenitors: Cep120 and TACCs control interkinetic nuclear migration

The nuclei of dividing neural progenitors undergo a cell-cycle-dependent change in position along the apico-basal axis known as interkinetic nuclear migration (INM). The functional relationship between INM and the mode of division of neural progenitors remains elusive, in part because its regulation at the molecular level is poorly understood. In this issue of Neuron, Xie et al. identify two centrosomal proteins (Cep120 and TACCs) regulating the INM of cortical neural progenitors.

Development of three-dimensional architecture of the neuroepithelium: role of pseudostratification and cellular 'community'

This review discusses the development of the neuroepithelium (NE) and its derivative ventricular zone (VZ), from which the central nervous system (CNS) is formed. First, the histological features of the NE and VZ are summarized, highlighting the phenomenon of pseudostratification, which is achieved by polarization and interkinetic nuclear migration (INM) of neural progenitor cells. Next, our current understanding of the cellular and molecular mechanisms and biological significance of INM and pseudostratification are outlined. The recent three-dimensional time-lapse observations revealing heterogeneity in cell lineages within the NE and VZ are also described, focusing on the neuronal lineage. Finally, the necessity of comprehensive studies on cell-cell interactions in the NE/VZ is discussed, as well as the importance of electrophysiological and biomechanical approaches. In particular, we suggest that a systems biology approach to the NE/VZ as a cellular 'community' may be fruitful.