Mitotic spindle rotation and mode of cell division in the developing telencephalon

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, to species diversification. The micromere of the sea urchin embryo may serve as one of those examples: An ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, Activator of G-protein Signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans, leaving a question of what evolutionary modification of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

Mitotic outcomes and errors in fibrous environments

During mitosis, cells round up and utilize the interphase adhesion sites within the fibrous extracellular matrix (ECM) as guidance cues to orient the mitotic spindles. Here, using suspended ECM-mimicking nanofiber networks, we explore mitotic outcomes and error distribution for various interphase cell shapes. Elongated cells attached to single fibers through two focal adhesion clusters (FACs) at their extremities result in perfect spherical mitotic cell bodies that undergo significant 3-dimensional (3D) displacement while being held by retraction fibers (RFs). Increasing the number of parallel fibers increases FACs and retraction fiber-driven stability, leading to reduced 3D cell body movement, metaphase plate rotations, increased interkinetochore distances, and significantly faster division times. Interestingly, interphase kite shapes on a crosshatch pattern of four fibers undergo mitosis resembling single-fiber outcomes due to rounded bodies being primarily held in position by RFs from two perpendicular suspended fibers. We develop a cortex-astral microtubule analytical model to capture the retraction fiber dependence of the metaphase plate rotations. We observe that reduced orientational stability, on single fibers, results in increased monopolar mitotic defects, while multipolar defects become dominant as the number of adhered fibers increases. We use a stochastic Monte Carlo simulation of centrosome, chromosome, and membrane interactions to explain the relationship between the observed propensity of monopolar and multipolar defects and the geometry of RFs. Overall, we establish that while bipolar mitosis is robust in fibrous environments, the nature of division errors in fibrous microenvironments is governed by interphase cell shapes and adhesion geometries.

Rassf7a promotes spinal cord regeneration and controls spindle orientation in neural progenitor cells

Spinal cord injury (SCI) can cause long-lasting disability in mammals due to the lack of axonal regrowth together with the inability to reinitiate spinal neurogenesis at the injury site. Deciphering the mechanisms that regulate the proliferation and differentiation of neural progenitor cells is critical for understanding spinal neurogenesis after injury. Compared with mammals, zebrafish show a remarkable capability of spinal cord regeneration. Here, we show that Rassf7a, a member of the Ras-association domain family, promotes spinal cord regeneration after injury. Zebrafish larvae harboring a rassf7a mutation show spinal cord regeneration and spinal neurogenesis defects. Live imaging shows abnormal asymmetric neurogenic divisions and spindle orientation defects in mutant neural progenitor cells. In line with this, the expression of rassf7a is enriched in neural progenitor cells. Subcellular analysis shows that Rassf7a localizes to the centrosome and is essential for cell cycle progression. Our data indicate a role for Rassf7a in modulating spindle orientation and the proliferation of neural progenitor cells after spinal cord injury.

Progenitor-Based Cell Biological Aspects of Neocortex Development and Evolution

During development, the decision of stem and progenitor cells to switch from proliferation to differentiation is of critical importance for the overall size of an organ. Too early a switch will deplete the stem/progenitor cell pool, and too late a switch will not generate the required differentiated cell types. With a focus on the developing neocortex, a six-layered structure constituting the major part of the cerebral cortex in mammals, we discuss here the cell biological features that are crucial to ensure the appropriate proliferation vs. differentiation decision in the neural progenitor cells. In the last two decades, the neural progenitor cells giving rise to the diverse types of neurons that function in the neocortex have been intensely investigated for their role in cortical expansion and gyrification. In this review, we will first describe these different progenitor types and their diversity. We will then review the various cell biological features associated with the cell fate decisions of these progenitor cells, with emphasis on the role of the radial processes emanating from these progenitor cells. We will also discuss the species-specific differences in these cell biological features that have allowed for the evolutionary expansion of the neocortex in humans. Finally, we will discuss the emerging role of cell cycle parameters in neocortical expansion.

Self-Organization of Tissue Growth by Interfacial Mechanical Interactions in Multilayered Systems

Morphogenesis is a spatially and temporally regulated process involved in various physiological and pathological transformations. In addition to the associated biochemical factors, the physical regulation of morphogenesis has attracted increasing attention. However, the driving force of morphogenesis initiation remains elusive. Here, it is shown that during the growth of multilayered tissues, a morphogenetic process can be self-organized by the progression of compression gradient stemmed from the interfacial mechanical interactions between layers. In tissues with low fluidity, the compression gradient is progressively strengthened during growth and induces stratification by triggering symmetric-to-asymmetric cell division reorientation at the critical tissue size. In tissues with high fluidity, compression gradient is dynamic and induces cell rearrangement leading to 2D in-plane morphogenesis instead of 3D deformation. Morphogenesis can be tuned by manipulating tissue fluidity, cell adhesion forces, and mechanical properties to influence the progression of compression gradient during the development of cultured cell sheets and chicken embryos. Together, the dynamics of compression gradient arising from interfacial mechanical interaction provides a conserved mechanism underlying morphogenesis initiation and size control during tissue growth.

A stochastic model of homeostasis: The roles of noise and nuclear positioning in deciding cell fate

We study a population-based cellular model that starts from a single stem cell that divides stochastically to give rise to either daughter stem cells or differentiated daughter cells. There are three main components in the model: nucleus position, the underlying gene-regulatory network, and stochastic segregation of transcription factors in the daughter cells. The proportion of self-renewal and differentiated cell lines as a function of the nucleus position which in turn decides the plane of cleavage is studied. Both nuclear position and noise play an important role in determining the stem cell genealogies. We have observed both long and short genealogies in model simulation, and these compare well with experimental results from neuroblast and B-cell division. Symmetric divisions are observed in apical nuclei, while asymmetric division occurs when the nucleus is toward the base. In this model, the number of clones decreases over time, although the average clone size increases.

Altered cleavage plane orientation with increased genomic aneuploidy produced by receptor-mediated lysophosphatidic acid (LPA) signaling in mouse cerebral cortical neural progenitor cells

The brain is composed of cells having distinct genomic DNA sequences that arise post-zygotically, known as somatic genomic mosaicism (SGM). One form of SGM is aneuploidy-the gain and/or loss of chromosomes-which is associated with mitotic spindle defects. The mitotic spindle orientation determines cleavage plane positioning and, therefore, neural progenitor cell (NPC) fate during cerebral cortical development. Here we report receptor-mediated signaling by lysophosphatidic acid (LPA) as a novel extracellular signal that influences cleavage plane orientation and produces alterations in SGM by inducing aneuploidy during murine cortical neurogenesis. LPA is a bioactive lipid whose actions are mediated by six G protein-coupled receptors, LPA₁-LPA₆. RNAscope and qPCR assessment of all six LPA receptor genes, and exogenous LPA exposure in LPA receptor (Lpar)-null mice, revealed involvement of Lpar1 and Lpar2 in the orientation of the mitotic spindle. Lpar1 signaling increased non-vertical cleavage in vivo by disrupting cell-cell adhesion, leading to breakdown of the ependymal cell layer. In addition, genomic alterations were significantly increased after LPA exposure, through production of chromosomal aneuploidy in NPCs. These results identify LPA as a receptor-mediated signal that alters both NPC fate and genomes during cortical neurogenesis, thus representing an extracellular signaling mechanism that can produce stable genomic changes in NPCs and their progeny. Normal LPA signaling in early life could therefore influence both the developing and adult brain, whereas its pathological disruption could contribute to a range of neurological and psychiatric diseases, via long-lasting somatic genomic alterations.

Diversity of activator of G-protein signaling (AGS)-family proteins and their impact on asymmetric cell division across taxa

Asymmetric cell division (ACD) is a cellular process that forms two different cell types through a cell division and is thus critical for the development of all multicellular organisms. Not all but many of the ACD processes are mediated by proper orientation of the mitotic spindle, which segregates the fate determinants asymmetrically into daughter cells. In many cell types, the evolutionarily conserved protein complex of Gαi/AGS-family protein/NuMA-like protein appears to play critical roles in orienting the spindle and/or generating the polarized cortical forces to regulate ACD. Studies in various organisms reveal that this conserved protein complex is slightly modified in each phylum or even within species. In particular, AGS-family proteins appear to be modified with a variable number of motifs in their functional domains across taxa. This apparently creates different molecular interactions and mechanisms of ACD in each developmental program, ultimately contributing to developmental diversity across species. In this review, we discuss how a conserved ACD machinery has been modified in each phylum over the course of evolution with a major focus on the molecular evolution of AGS-family proteins and its impact on ACD regulation.

Cell lineage-dependent chiral actomyosin flows drive cellular rearrangements in early Caenorhabditis elegans development

Proper positioning of cells is essential for many aspects of development. Daughter cell positions can be specified via orienting the cell division axis during cytokinesis. Rotatory actomyosin flows during division have been implied in specifying and reorienting the cell division axis, but how general such reorientation events are, and how they are controlled, remains unclear. We followed the first nine divisions of Caenorhabditis elegans embryo development and demonstrate that chiral counter-rotating flows arise systematically in early AB lineage, but not in early P/EMS lineage cell divisions. Combining our experiments with thin film active chiral fluid theory we identify a mechanism by which chiral counter-rotating actomyosin flows arise in the AB lineage only, and show that they drive lineage-specific spindle skew and cell reorientation events. In conclusion, our work sheds light on the physical processes that underlie chiral morphogenesis in early development.

Acute Lengthening of Progenitor Mitosis Influences Progeny Fate during Cortical Development in vivo

BACKGROUND/AIMS

Prenatal microcephaly is posited to arise from aberrant mitosis of neural progenitors, which disrupts both neuronal production and survival. Although microcephaly has both a genetic and environmental etiology, the mechanisms by which dysregulation of mitosis causes microcephaly are poorly understood. We previously discovered that prolonged mitosis of mouse neural progenitors, either ex vivo or in vitro, directly alters progeny cell fate, -resulting in precocious differentiation and apoptosis. This raises questions as to whether prolonged progenitor mitosis affects cell fate and neurogenesis in vivo, and what are the underlying mechanisms?

METHODS/RESULTS

Towards addressing these knowledge gaps, we developed an in vivo model of mitotic delay. This uses pharmacological inhibition to acutely and reversibly prolong mitosis during cortical development, and fluorescent dyes to label direct progeny. Using this model, we discovered that a causal relationship between mitotic delay of neural progenitors and altered progeny cell fate is evident in vivo. Using transcriptome analyses to investigate the state of delayed cells and their progeny, we uncovered potential molecular mechanisms by which prolonged mitosis induces altered cell fates, including DNA damage and p53 signaling. We then extended our studies to human neural progenitors, demonstrating that lengthened mitosis duration also directly alters neuronal cell fate.

CONCLUSIONS

This study establishes a valuable new experimental paradigm towards understanding mechanisms whereby lengthened mitosis duration may explain some cases of microcephaly.

Inhibition of Autism-Related Crm1 Disrupts Mitosis and Induces Apoptosis of the Cortical Neural Progenitors

De novo microdeletion of chromosome 2p15-16.1 presents clinically recognizable phenotypes that include mental retardation, autism, and microcephaly. Chromosomal maintenance 1 (CRM1) is a gene commonly missing in patients with 2p15-16.1 microdeletion and one of two genes found in the smallest deletion case. In this study, we investigate the role and mechanism of Crm1 in the developing mouse brain by inhibiting the protein or knocking down the gene in vivo. Inhibition of Crm1 reduces the proliferation and increases p53-dependent apoptosis of the cortical neural progenitors, thereby impeding the growth of embryonic cerebral cortex. Live imaging of mitosis in ex vivo embryonic brain slices reveals that inhibition of CRM1 arrests the cortical progenitors at metaphase. The arrested cells eventually slip into a pseudo-G1 phase without chromosome segregation. The mitotic slippage cells are marked by persistent expression of the spindle assembly checkpoint (SAC), repressing of which rescues the cells from apoptosis. Our study reveals that activating the SAC and inducing the mitotic slippage may lead to apoptosis of the cortical neural progenitors. The resulting cell death may well contribute to microcephaly associated with microdeletion of chromosome 2p15-16.1 involving CRM1.

Centrosomes: The good and the bad for brain development

Centrosomes nucleate and organise the microtubule cytoskeleton in animal cells. These membraneless organelles are key structures for tissue organisation, polarity and growth. Centrosome dysfunction, defined as deviation in centrosome numbers and/or structural integrity, has major impact on brain size and functionality, as compared with other tissues of the organism. In this review, we discuss the contribution of centrosomes to brain growth during development. We discuss in particular the impact of centrosome dysfunction in Drosophila and mammalian neural stem cell division and fitness, which ultimately underlie brain growth defects.

Spindle Orientation-Independent Control of Cell Fate Determination by RGS3 and KIF20A

It was proposed that similar to its role in the invertebrate nervous system, mitotic spindle orientation (or cell cleavage plane orientation) of a dividing neural progenitor cell specifies the fate of daughter cells in the mammalian brain, modulating the production of neurons via symmetric versus asymmetric cell divisions during the course of neurogenesis. Experimental tests of the sufficiency of spindle/cleavage plane orientation in mammalian cell fate determination have yielded conflicting results. On the other hand, the necessity of spindle/cleavage plane orientation in mammalian cell fate determination has not yet been addressed. Here we examined the necessity of spindle/cleavage plane orientation during cortical neurogenesis in mice with loss-of-function of the RGS3-KIF20A interaction axis. We present evidence that while inactivation of RGS3 or KIF20A was linked to a shift in neural progenitor cells from proliferative to differentiative divisions in the developing cortex, these genetic mutations did not lead to anticipated alteration in the orientation of spindle/cleavage plane. Our results indicate that the RGS3-KIF20A axis regulates the balance between proliferation and differentiation in the mammalian cortex employing a mechanism independent of spindle/cleavage plane orientation. These data also caution against using spindle/cleavage plane orientation as the synonym for cell fate determination.

Two-photon microscopic observation of cell-production dynamics in the developing mammalian neocortex in utero

Morphogenesis and organ development should be understood based on a thorough description of cellular dynamics. Recent studies have explored the dynamic behaviors of mammalian neural progenitor cells (NPCs) using slice cultures in which three‐dimensional systems conserve in vivo‐like environments to a considerable degree. However, live observation of NPCs existing truly in vivo, as has long been performed for zebrafish NPCs, has yet to be established in mammals. Here, we performed intravital two‐photon microscopic observation of NPCs in the developing cerebral cortex of H2B‐EGFP or Fucci transgenic mice in utero. Fetuses in the uterine sac were immobilized using several devices and were observed through a window made in the uterine wall and the amniotic membrane while monitoring blood circulation. Clear visibility was obtained to the level of 300 μm from the scalp surface of the fetus, which enabled us to quantitatively assess NPC behaviors, such as division and interkinetic nuclear migration, within a neuroepithelial structure called the ventricular zone at embryonic day (E) 13 and E14. In fetuses undergoing healthy monitoring in utero for 60 min, the frequency of mitoses observed at the apical surface was similar to those observed in slice cultures and in freshly fixed in vivo specimens. Although the rate and duration of successful in utero observations are still limited (33% for ≥10 min and 14% for 60 min), further improvements based on this study will facilitate future understanding of how organogenetic cellular behaviors occur or are pathologically influenced by the systemic maternal condition and/or maternal‐fetal relationships.

NCAM regulates temporal specification of neural progenitor cells via profilin2 during corticogenesis

The development of cerebral cortex requires spatially and temporally orchestrated proliferation, migration, and differentiation of neural progenitor cells (NPCs). The molecular mechanisms underlying cortical development are, however, not fully understood. The neural cell adhesion molecule (NCAM) has been suggested to play a role in corticogenesis. Here we show that NCAM is dynamically expressed in the developing cortex. NCAM expression in NPCs is highest in the neurogenic period and declines during the gliogenic period. In mice bearing an NPC-specific NCAM deletion, proliferation of NPCs is reduced, and production of cortical neurons is delayed, while formation of cortical glia is advanced. Mechanistically, NCAM enhances actin polymerization in NPCs by interacting with actin-associated protein profilin2. NCAM-dependent regulation of NPCs is blocked by mutations in the profilin2 binding site. Thus, NCAM plays an essential role in NPC proliferation and fate decision during cortical development by regulating profilin2-dependent actin polymerization.

Telophase correction refines division orientation in stratified epithelia

During organogenesis, precise control of spindle orientation balances proliferation and differentiation. In the developing murine epidermis, planar and perpendicular divisions yield symmetric and asymmetric fate outcomes, respectively. Classically, division axis specification involves centrosome migration and spindle rotation, events occurring early in mitosis. Here, we identify a novel orientation mechanism which corrects erroneous anaphase orientations during telophase. The directionality of reorientation correlates with the maintenance or loss of basal contact by the apical daughter. While the scaffolding protein LGN is known to determine initial spindle positioning, we show that LGN also functions during telophase to reorient oblique divisions toward perpendicular. The fidelity of telophase correction also relies on the tension-sensitive adherens junction proteins vinculin, α-E-catenin, and afadin. Failure of this corrective mechanism impacts tissue architecture, as persistent oblique divisions induce precocious, sustained differentiation. The division orientation plasticity provided by telophase correction may enable progenitors to adapt to local tissue needs.

A stochastic framework of neurogenesis underlies the assembly of neocortical cytoarchitecture

The cerebral cortex contains multiple areas with distinctive cytoarchitectonic patterns, but the cellular mechanisms underlying the emergence of this diversity remain unclear. Here, we have investigated the neuronal output of individual progenitor cells in the developing mouse neocortex using a combination of methods that together circumvent the biases and limitations of individual approaches. Our experimental results indicate that progenitor cells generate pyramidal cell lineages with a wide range of sizes and laminar configurations. Mathematical modeling indicates that these outcomes are compatible with a stochastic model of cortical neurogenesis in which progenitor cells undergo a series of probabilistic decisions that lead to the specification of very heterogeneous progenies. Our findings support a mechanism for cortical neurogenesis whose flexibility would make it capable to generate the diverse cytoarchitectures that characterize distinct neocortical areas.

Intramolecular interaction in LGN, an adaptor protein that regulates mitotic spindle orientation

Proper mitotic spindle orientation requires that astral microtubules are connected to the cell cortex by the microtubule-binding protein NuMA, which is recruited from the cytoplasm. Cortical recruitment of NuMA is at least partially mediated via direct binding to the adaptor protein LGN. LGN normally adopts a closed conformation via an intramolecular interaction between its N-terminal NuMA-binding domain and its C-terminal region that contains four GoLoco (GL) motifs, each capable of binding to the membrane-anchored Gα_i subunit of heterotrimeric G protein. Here we show that the intramolecular association with the N-terminal domain in LGN involves GL3, GL4, and a region between GL2 and GL3, whereas GL1 and GL2 do not play a major role. This conformation renders GL1 but not the other GL motifs in a state easily accessible to Gα_i To interact with full-length LGN in a closed state, NuMA requires the presence of Gα_i; both NuMA and Gα_i are essential for cortical recruitment of LGN in mitotic cells. In contrast, mInsc, a protein that competes with NuMA for binding to LGN and regulates mitotic spindle orientation in asymmetric cell division, efficiently binds to full-length LGN without Gα_i and induces its conformational change, enhancing its association with Gα_i In nonpolarized symmetrically dividing HeLa cells, disruption of the LGN-NuMA interaction by ectopic expression of mInsc results in a loss of cortical localization of NuMA during metaphase and anaphase and promotes mitotic spindle misorientation and a delayed anaphase progression. These findings highlight a specific role for LGN-mediated cell cortex recruitment of NuMA.

The Mode of Stem Cell Division Is Dependent on the Differential Interaction of β-Catenin with the Kat3 Coactivators CBP or p300

Normal long-term repopulating somatic stem cells (SSCs) preferentially divide asymmetrically, with one daughter cell remaining in the niche and the other going on to be a transient amplifying cell required for generating new tissue in homeostatic maintenance and repair processes, whereas cancer stem cells (CSCs) favor symmetric divisions. We have previously proposed that differential β-catenin modulation of transcriptional activity via selective interaction with either the Kat3 coactivator CBP or its closely related paralog p300, regulates symmetric versus asymmetric division in SSCs and CSCs. We have previously demonstrated that SSCs that divide asymmetrically per force retain one of the dividing daughter cells in the stem cell niche, even when treated with specific CBP/β-catenin antagonists, whereas CSCs can be removed from their niche via forced stochastic symmetric differentiative divisions. We now demonstrate that loss of p73 in early corticogenesis biases β-catenin Kat3 coactivator usage and enhances β-catenin/CBP transcription at the expense of β-catenin/p300 transcription. Biased β-catenin coactivator usage has dramatic consequences on the mode of division of neural stem cells (NSCs), but not neurogenic progenitors. The observed increase in symmetric divisions due to enhanced β-catenin/CBP interaction and transcription leads to an immediate increase in NSC symmetric differentiative divisions. Moreover, we demonstrate for the first time that the complex phenotype caused by the loss of p73 can be rescued in utero by treatment with the small-molecule-specific CBP/β-catenin antagonist ICG-001. Taken together, our results demonstrate the causal relationship between the choice of β-catenin Kat3 coactivator and the mode of stem cell division.

Laminin β2 Chain Regulates Retinal Progenitor Cell Mitotic Spindle Orientation via Dystroglycan

Vertebrate retinal development follows a pattern during which retinal progenitor cells (RPCs) give rise to all retinal cell types in a highly conserved temporal sequence. RPC proliferation and cell cycle exit are tightly coordinated to ensure proper and timely production of each of the retinal cell types. Extracellular matrix (ECM) plays an important role in eye development, influencing RPC proliferation and differentiation. In this study, we demonstrate that laminins, key ECM components, in the inner limiting membrane, control mitotic spindle orientation by providing environmental cues to the RPCs. In vivo deletion of laminin β2 in mice of both sexes results in a loss RPC basal processes and contact with the ECM, leading to a shift of the mitotic spindle pole orientation toward asymmetric cell divisions. This leads to decreased proliferation and premature RPC pool depletion, resulting in overproduction of rod photoreceptors at the expense of bipolar cells and Müller glia. Moreover, we show that deletion of laminin β2 leads to disruption and mislocalization of its receptors: dystroglycan and β1-integrin. Addition of exogenous β2-containing laminins to laminin β2-deficient retinal explants stabilizes the RPC basal processes and directs their mitotic spindle orientation toward symmetric divisions, leading to increased RPC proliferation, as well as restores proper receptor localization at the retinal surface. Finally, functional blocking of dystroglycan in wild-type retinal explants phenocopies laminin β2 ablation. Our data suggest that dystroglycan-mediated signaling between RPCs and the ECM is of key importance in controlling critical developmental events during retinogenesis.SIGNIFICANCE STATEMENT The mechanisms governing retinogenesis are subject to both intrinsic and extrinsic signaling cues. Although the role of intrinsic signaling has been the subject of many studies, our understanding of the role of the microenvironment in retinal development remains unclear. Using a combination of in vivo and ex vivo approaches, we demonstrate that laminins, key extracellular matrix components, provide signaling cues that control retinal progenitor cell attachment to the basement membrane, mitotic axis, proliferation, and fate adoption. Moreover, we identify, for the first time, dystroglycan as the receptor responsible for directing retinal progenitor cell mitotic spindle orientation. Our data suggest a mechanism where dystroglycan-mediated signaling between the cell and the extracellular matrix controls the proliferative potential of progenitors in the developing CNS.

The Genetics of Primary Microcephaly

Primary microcephaly (MCPH, for "microcephaly primary hereditary") is a disorder of brain development that results in a head circumference more than 3 standard deviations below the mean for age and gender. It has a wide variety of causes, including toxic exposures, in utero infections, and metabolic conditions. While the genetic microcephaly syndromes are relatively rare, studying these syndromes can reveal molecular mechanisms that are critical in the regulation of neural progenitor cells, brain size, and human brain evolution. Many of the causative genes for MCPH encode centrosomal proteins involved in centriole biogenesis. However, other MCPH genes fall under different mechanistic categories, notably DNA replication and repair. Recent gene discoveries and functional studies have implicated novel cellular processes, such as cytokinesis, centromere and kinetochore function, transmembrane or intracellular transport, Wnt signaling, and autophagy, as well as the apical polarity complex. Thus, MCPH genes implicate a wide variety of molecular and cellular mechanisms in the regulation of cerebral cortical size during development.

Molecular components and polarity of radial glial cells during cerebral cortex development

Originating from ectodermal epithelium, radial glial cells (RGCs) retain apico-basolateral polarity and comprise a pseudostratified epithelial layer in the developing cerebral cortex. The apical endfeet of the RGCs faces the fluid-filled ventricles, while the basal processes extend across the entire cortical span towards the pial surface. RGC functions are largely dependent on this polarized structure and the molecular components that define it. In this review, we will dissect existing molecular evidence on RGC polarity establishment and during cerebral cortex development and provide our perspective on the remaining key questions.

Mechanical Forces Program the Orientation of Cell Division during Airway Tube Morphogenesis

Oriented cell division plays a key role in controlling organogenesis. The mechanisms for regulating division orientation at the whole-organ level are only starting to become understood. By combining 3D time-lapse imaging, mouse genetics, and mathematical modeling, we find that global orientation of cell division is the result of a combination of two types of spindles with distinct spindle dynamic behaviors in the developing airway epithelium. Fixed spindles follow the classic long-axis rule and establish their division orientation before metaphase. In contrast, rotating spindles do not strictly follow the long-axis rule and determine their division orientation during metaphase. By using both a cell-based mechanical model and stretching-lung-explant experiments, we showed that mechanical force can function as a regulatory signal in maintaining the stable ratio between fixed spindles and rotating spindles. Our findings demonstrate that mechanical forces, cell geometry, and oriented cell division function together in a highly coordinated manner to ensure normal airway tube morphogenesis.

An interaction between myosin-10 and the cell cycle regulator Wee1 links spindle dynamics to mitotic progression in epithelia

Proper spindle orientation must be achieved before anaphase onset, but whether and how cells link spindle position to anaphase onset is unknown. Sandquist, Larson, et al. identify a novel interaction between the motor protein myosin-10 and the cell cycle regulator wee1 that is proposed to help coordinate preanaphase spindle dynamics and positioning with mitotic exit.

Anaphase in epithelia typically does not ensue until after spindles have achieved a characteristic position and orientation, but how or even if cells link spindle position to anaphase onset is unknown. Here, we show that myosin-10 (Myo10), a motor protein involved in epithelial spindle dynamics, binds to Wee1, a conserved regulator of cyclin-dependent kinase 1 (Cdk1). Wee1 inhibition accelerates progression through metaphase and disrupts normal spindle dynamics, whereas perturbing Myo10 function delays anaphase onset in a Wee1-dependent manner. Moreover, Myo10 perturbation increases Wee1-mediated inhibitory phosphorylation on Cdk1, which, unexpectedly, concentrates at cell–cell junctions. Based on these and other results, we propose a model in which the Myo10–Wee1 interaction coordinates attainment of spindle position and orientation with anaphase onset.

Coupling progenitor and neuronal diversity in the developing neocortex

The adult neocortex is composed of several types of glutamatergic neurons, which are sequentially born from progenitors during development. The extent and nature of progenitor diversity, and how it relates to neuronal diversity, is still poorly understood. In this review, we discuss key features of neocortical progenitors across several species, including their morphological properties, cell cycling behaviour and molecular signatures, and how these features relate to the competence of these cells to generate distinct types of progenies.

Transcriptional regulation of intermediate progenitor cell generation during hippocampal development

During forebrain development, radial glia generate neurons through the production of intermediate progenitor cells (IPCs). The production of IPCs is a central tenet underlying the generation of the appropriate number of cortical neurons, but the transcriptional logic underpinning this process remains poorly defined. Here, we examined IPC production using mice lacking the transcription factor nuclear factor I/X (Nfix). We show that Nfix deficiency delays IPC production and prolongs the neurogenic window, resulting in an increased number of neurons in the postnatal forebrain. Loss of additional Nfi alleles (Nfib) resulted in a severe delay in IPC generation while, conversely, overexpression of NFIX led to precocious IPC generation. Mechanistically, analyses of microarray and ChIP-seq datasets, coupled with the investigation of spindle orientation during radial glial cell division, revealed that NFIX promotes the generation of IPCs via the transcriptional upregulation of inscuteable (Insc). These data thereby provide novel insights into the mechanisms controlling the timely transition of radial glia into IPCs during forebrain development.

Regulation of cytokinesis during corticogenesis: focus on the midbody

Development of the cerebral cortices depends on tight regulation of cell divisions. In this system, stem and progenitor cells undergo symmetric and asymmetric divisions to ultimately produce neurons that establish the layers of the cortex. Cell division culminates with the formation of the midbody, a transient organelle that establishes the site of abscission between nascent daughter cells. During cytokinetic abscission, the final stage of cell division, one daughter cell will inherit the midbody remnant, which can then maintain or expel the remnant, but mechanisms and circumstances influencing this decision are unclear. This review describes the midbody and its constituent proteins, as well as the known consequences of their manipulation during cortical development. The potential functional relevance of midbody mechanisms is discussed.

Genetic and Molecular Approaches to Study Neuronal Migration in the Developing Cerebral Cortex

The migration of neuronal cells in the developing cerebral cortex is essential for proper development of the brain and brain networks. Disturbances in this process, due to genetic abnormalities or exogenous factors, leads to aberrant brain formation, brain network formation, and brain function. In the last decade, there has been extensive research in the field of neuronal migration. In this review, we describe different methods and approaches to assess and study neuronal migration in the developing cerebral cortex. First, we discuss several genetic methods, techniques and genetic models that have been used to study neuronal migration in the developing cortex. Second, we describe several molecular approaches to study aberrant neuronal migration in the cortex which can be used to elucidate the underlying mechanisms of neuronal migration. Finally, we describe model systems to investigate and assess the potential toxicity effect of prenatal exposure to environmental chemicals on proper brain formation and neuronal migration.

Shh-mediated centrosomal recruitment of PKA promotes symmetric proliferative neuroepithelial cell division

Tight control of the balance between self-expanding symmetric and self-renewing asymmetric neural progenitor divisions is crucial to regulate the number of cells in the developing central nervous system. We recently demonstrated that Sonic hedgehog (Shh) signalling is required for the expansion of motor neuron progenitors by maintaining symmetric divisions. Here we show that activation of Shh/Gli signalling in dividing neuroepithelial cells controls the symmetric recruitment of PKA to the centrosomes that nucleate the mitotic spindle, maintaining symmetric proliferative divisions. Notably, Shh signalling upregulates the expression of pericentrin, which is required to dock PKA to the centrosomes, which in turn exerts a positive feedback onto Shh signalling. Thus, by controlling centrosomal protein assembly, we propose that Shh signalling overcomes the intrinsic asymmetry at the centrosome during neuroepithelial cell division, thereby promoting self-expanding symmetric divisions and the expansion of the progenitor pool.

Automated mitotic spindle tracking suggests a link between spindle dynamics, spindle orientation, and anaphase onset in epithelial cells

Proper spindle positioning at anaphase onset is essential for normal tissue organization and function. Here we develop automated spindle-tracking software and apply it to characterize mitotic spindle dynamics in the Xenopus laevis embryonic epithelium. We find that metaphase spindles first undergo a sustained rotation that brings them on-axis with their final orientation. This sustained rotation is followed by a set of striking stereotyped rotational oscillations that bring the spindle into near contact with the cortex and then move it rapidly away from the cortex. These oscillations begin to subside soon before anaphase onset. Metrics extracted from the automatically tracked spindles indicate that final spindle position is determined largely by cell morphology and that spindles consistently center themselves in the XY-plane before anaphase onset. Finally, analysis of the relationship between spindle oscillations and spindle position relative to the cortex reveals an association between cortical contact and anaphase onset. We conclude that metaphase spindles in epithelia engage in a stereotyped "dance," that this dance culminates in proper spindle positioning and orientation, and that completion of the dance is linked to anaphase onset.

Symmetry Does not Come for Free: Cellular Mechanisms to Achieve a Symmetric Cell Division

During mitosis cells can divide symmetrically to proliferate or asymmetrically to generate tissue diversity. While the mechanisms that ensure asymmetric cell division have been extensively studied, it is often assumed that a symmetric cell division is the default outcome of mitosis. Recent studies, however, imply that the symmetric nature of cell division is actively controlled, as they reveal numerous mechanisms that ensure the formation of equal-sized daughter cells as cells progress through cell division. Here we review our current knowledge of these mechanisms and highlight possible key questions in the field.

Division modes and physical asymmetry in cerebral cortex progenitors

Neural stem cells go through a sequence of timely regulated gene expression and pattern of division mode to generate diverse neurons during brain development. During vertebrate cerebral cortex development, neural stem cells begin with proliferative symmetric divisions, subsequently undergo neurogenic asymmetric divisions, and finally gliogenic divisions. In this review, we explore the relationship between stem cell versus neural fate specification and the division mode. Specifically, we discuss recent findings on the mechanisms of asymmetric divisions, division mode, and developmental progression of neural progenitor identity.

Mutations in genes encoding condensin complex proteins cause microcephaly through decatenation failure at mitosis

Martin et al. report that biallelic mutations in NCAPD2, NCAPH, or NCAPD3, encoding subunits of condensin complexes, cause microcephaly. Frequent anaphase chromatin bridge formation observed in apical neural progenitors during neurogenesis are the consequence of failed sister chromatid disentanglement during chromosome compaction.

Compaction of chromosomes is essential for accurate segregation of the genome during mitosis. In vertebrates, two condensin complexes ensure timely chromosome condensation, sister chromatid disentanglement, and maintenance of mitotic chromosome structure. Here, we report that biallelic mutations in NCAPD2, NCAPH, or NCAPD3, encoding subunits of these complexes, cause microcephaly. In addition, hypomorphic Ncaph2 mice have significantly reduced brain size, with frequent anaphase chromatin bridge formation observed in apical neural progenitors during neurogenesis. Such DNA bridges also arise in condensin-deficient patient cells, where they are the consequence of failed sister chromatid disentanglement during chromosome compaction. This results in chromosome segregation errors, leading to micronucleus formation and increased aneuploidy in daughter cells. These findings establish “condensinopathies” as microcephalic disorders, with decatenation failure as an additional disease mechanism for microcephaly, implicating mitotic chromosome condensation as a key process ensuring mammalian cerebral cortex size.

Building brains in a dish: Prospects for growing cerebral organoids from stem cells

The recent development of organoid techniques, in which embryonic brain-like tissue can be grown from human or mouse stem cells in vitro offers the potential to transform the way in which brain development is studied. In this review, we summarize key aspects of the embryonic development of mammalian forebrains, focussing in particular on the cerebral cortex and highlight significant differences between mouse and primates, including human. We discuss recent work using cerebral organoids that has revealed key similarities and differences between their development and that of the brain in vivo. Finally, we outline the ways in which cerebral organoids can be used in combination with CRISPR/Cas9 genome editing to unravel genetic mechanisms that control embryonic development of the cerebral cortex, how this can help us understand the causes of neurodevelopmental disorders and some of the key challenges which will have to be resolved before organoids can become a mainstream tool to study brain development.

SARA regulates neuronal migration during neocortical development through L1 trafficking

Emerging evidence suggests that endocytic trafficking of adhesion proteins plays a crucial role in neuronal migration during neocortical development. However, molecular insights into these processes remain elusive. Here, we study the early endosomal protein Smad anchor for receptor activation (SARA) in the developing mouse brain. SARA is enriched at the apical endfeet of radial glia of the neocortex. Although SARA knockdown did not lead to detectable neurogenic phenotypes, SARA-suppressed neurons exhibited impaired orientation and migration across the intermediate zone. Mechanistically, we show that SARA knockdown neurons exhibit increased surface expression of the L1 cell adhesion molecule. Neurons ectopically expressing L1 phenocopy the migration and orientation defects caused by SARA knockdown and display increased contact with neighboring neurites. L1 knockdown effectively rescues SARA suppression-induced phenotypes. SARA knockdown neurons eventually overcome their migration defect and enter later into the cortical plate. Nevertheless, these neurons localize at more superficial cortical layers than their control counterparts. These results suggest that SARA regulates the orientation, multipolar-to-bipolar transition and the positioning of cortical neurons via modulating surface L1 expression.

Prolonged Mitosis of Neural Progenitors Alters Cell Fate in the Developing Brain

Embryonic neocortical development depends on balanced production of progenitors and neurons. Genetic mutations disrupting progenitor mitosis frequently impair neurogenesis; however, the link between altered mitosis and cell fate remains poorly understood. Here we demonstrate that prolonged mitosis of radial glial progenitors directly alters neuronal fate specification and progeny viability. Live imaging of progenitors from a neurogenesis mutant, Magoh(+/-), reveals that mitotic delay significantly correlates with preferential production of neurons instead of progenitors, as well as apoptotic progeny. Independently, two pharmacological approaches reveal a causal relationship between mitotic delay and progeny fate. As mitotic duration increases, progenitors produce substantially more apoptotic progeny or neurons. We show that apoptosis, but not differentiation, is p53 dependent, demonstrating that these are distinct outcomes of mitotic delay. Together our findings reveal that prolonged mitosis is sufficient to alter fates of radial glia progeny and define a new paradigm to understand how mitosis perturbations underlie brain size disorders such as microcephaly.

Brain development is impaired in c-fos -/- mice

c-Fos is a proto-oncogene involved in diverse cellular functions. Its deregulation has been associated to abnormal development and oncogenic progression. c-fos−/− mice are viable but present a reduction in their body weight and brain size. We examined the importance of c-Fos during neocortex development at 13.5, 14.5 and 16.5 days of gestation. At E14.5, neocortex thickness, apoptosis, mitosis and expression of markers along the different stages of Neural Stem Progenitor Cells (NSPCs) differentiation in c-fos−/− and wild-type mice were analyzed. A ∼15% reduction in the neocortex thickness of c-fos−/− embryos was observed which correlates with a decrease in the number of differentiated cells and an increase in apoptosis at the ventricular zone. No difference in mitosis rate was observed, although the mitotic angle was predominantly vertical in c-fos−/− embryos, suggesting a reduced trend of NSPCs to differentiate. At E13.5, changes in differentiation markers start to be apparent and are still clearly observed at E16.5. A tendency of more AP-1/DNA complexes present in nuclear extracts of cerebral cortex from c-fos−/− embryos with no differences in the lipid synthesis activity was found. These results suggest that c-Fos is involved in the normal development of NSPCs by means of its AP-1 activity.

Mapping neurogenesis onset in the optic tectum of Xenopus laevis

Neural progenitor cells have a central role in the development and evolution of the vertebrate brain. During early brain development, neural progenitors first expand their numbers through repeated proliferative divisions and then begin to exhibit neurogenic divisions. The transparent and experimentally accessible optic tectum of Xenopus laevis is an excellent model system for the study of the cell biology of neurogenesis, but the precise spatial and temporal relationship between proliferative and neurogenic progenitors has not been explored in this system. Here we construct a spatial map of proliferative and neurogenic divisions through lineage tracing of individual progenitors and their progeny. We find a clear spatial separation of proliferative and neurogenic progenitors along the anterior-posterior axis of the optic tectum, with proliferative progenitors located more posteriorly and neurogenic progenitors located more anteriorly. Since individual progenitors are repositioned toward more anterior locations as they mature, this spatial separation likely reflects an increasing restriction in the proliferative potential of individual progenitors. We then examined whether the transition from proliferative to neurogenic behavior correlates with cellular properties that have previously been implicated in regulating neurogenesis onset. Our data reveal that the transition from proliferation to neurogenesis is associated with a small change in cleavage plane orientation and a more pronounced change in cell cycle kinetics in a manner reminiscent of observations from mammalian systems. Our findings highlight the potential to use the optic tectum of Xenopus laevis as an accessible system for the study of the cell biology of neurogenesis. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1328-1341, 2016.

The role of adherens junctions in the developing neocortex

The disproportional enlargement of the neocortex through evolution has been instrumental in the success of vertebrates, in particular mammals. The neocortex is a multilayered sheet of neurons generated from a simple proliferative neuroepithelium through a myriad of mechanisms with substantial evolutionary conservation. This developing neuroepithelium is populated by progenitors that can generate additional progenitors as well as post-mitotic neurons. Subtle alterations in the production of progenitors vs. differentiated cells during development can result in dramatic differences in neocortical size. This review article will examine how cadherin adhesion proteins, in particular α-catenin and N-cadherin, function in regulating the neural progenitor microenvironment, cell proliferation, and differentiation in cortical development.

ASPM regulates symmetric stem cell division by tuning Cyclin E ubiquitination

We generate a mouse model for the human microcephaly syndrome by mutating the ASPM locus, and demonstrate a premature exhaustion of the neuronal progenitor pool due to dysfunctional self-renewal processes. Earlier studies have linked ASPM mutant progenitor excessive cell cycle exit to a mitotic orientation defect. Here, we demonstrate a mitotic orientation-independent effect of ASPM on cell cycle duration. We pinpoint the cell fate-determining factor to the length of time spent in early G1 before traversing the restriction point. Characterization of the molecular mechanism reveals an interaction between ASPM and the Cdk2/Cyclin E complex, regulating the Cyclin activity by modulating its ubiquitination, phosphorylation and localization into the nucleus, before the cell is fated to transverse the restriction point. Thus, we reveal a novel function of ASPM in mediating the tightly coordinated Ubiquitin- Cyclin E- Retinoblastoma- E2F bistable-signalling pathway controlling restriction point progression and stem cell maintenance.

The influence of GAP-43 on orientation of cell division through G proteins

Recent studies have shown that GAP-43 is highly expressed in horizontally dividing neural progenitor cells, and G protein complex are required for proper mitotic-spindle orientation of those progenitors in the mammalian developing cortex. In order to verify the hypothesis that GAP-43 may influence the orientation of cell division through interacting with G proteins during neurogenesis, the GAP-43 RNA from adult C57 mouse was cloned into the pEGFP-N1 vector, which was then transfected into Madin-Darby Canine Kidney (MDCK) cells cultured in a three-dimensional (3D) cell culture system. The interaction of GAP-43 with Gαi was detected by co-immunoprecipitation (co-IP), while cystogenesis of 3D morphogenesis of MDCK cells and expression of GAP-43 and Gαi were determined by immunofluorescence and Western blotting. The results showed are as follows: After being transfected by pEGFP-N1-GAP-43, GAP-43 was localized on the cell membrane and co-localized with Gαi, and this dramatically induced a defective cystogenesis in 3D morphogenesis of MDCK cells. The functional interaction between GAP-43 and Gαi proteins was proven by the co-IP assay. It can be considered from the results that the GAP-43 is involved in the orientation of cell division by interacting with Gαi and this should be an important mechanism for neurogenesis in the mammalian brain.

Physical biology of human brain development

Neurodevelopment is a complex, dynamic process that involves a precisely orchestrated sequence of genetic, environmental, biochemical, and physical events. Developmental biology and genetics have shaped our understanding of the molecular and cellular mechanisms during neurodevelopment. Recent studies suggest that physical forces play a central role in translating these cellular mechanisms into the complex surface morphology of the human brain. However, the precise impact of neuronal differentiation, migration, and connection on the physical forces during cortical folding remains unknown. Here we review the cellular mechanisms of neurodevelopment with a view toward surface morphogenesis, pattern selection, and evolution of shape. We revisit cortical folding as the instability problem of constrained differential growth in a multi-layered system. To identify the contributing factors of differential growth, we map out the timeline of neurodevelopment in humans and highlight the cellular events associated with extreme radial and tangential expansion. We demonstrate how computational modeling of differential growth can bridge the scales-from phenomena on the cellular level toward form and function on the organ level-to make quantitative, personalized predictions. Physics-based models can quantify cortical stresses, identify critical folding conditions, rationalize pattern selection, and predict gyral wavelengths and gyrification indices. We illustrate that physical forces can explain cortical malformations as emergent properties of developmental disorders. Combining biology and physics holds promise to advance our understanding of human brain development and enable early diagnostics of cortical malformations with the ultimate goal to improve treatment of neurodevelopmental disorders including epilepsy, autism spectrum disorders, and schizophrenia.

Asymmetric cell division of granule neuron progenitors in the external granule layer of the mouse cerebellum

The plane of division of granule neuron progenitors (GNPs) was analysed with respect to the pial surface in P0 to P14 cerebellum and the results showed that there was a significant bias towards the plane of cell division being parallel to pial surface across this developmental window. In addition, the distribution of β-Catenin in anaphase cells was analysed, which showed that there was a significant asymmetry in the distribution of β-Catenin in dividing GNPs. Further, inhibition of Sonic Hedgehog (Shh) signalling had an effect on plane of cell division. Asymmetric distribution of β-Catenin was shown to occur towards the source of a localized extracellular cue.

Regulation of cerebral cortical neurogenesis by the Pax6 transcription factor

Understanding brain development remains a major challenge at the heart of understanding what makes us human. The neocortex, in evolutionary terms the newest part of the cerebral cortex, is the seat of higher cognitive functions. Its normal development requires the production, positioning, and appropriate interconnection of very large numbers of both excitatory and inhibitory neurons. Pax6 is one of a relatively small group of transcription factors that exert high-level control of cortical development, and whose mutation or deletion from developing embryos causes major brain defects and a wide range of neurodevelopmental disorders. Pax6 is very highly conserved between primate and non-primate species, is expressed in a gradient throughout the developing cortex and is essential for normal corticogenesis. Our understanding of Pax6’s functions and the cellular processes that it regulates during mammalian cortical development has significantly advanced in the last decade, owing to the combined application of genetic and biochemical analyses. Here, we review the functional importance of Pax6 in regulating cortical progenitor proliferation, neurogenesis, and formation of cortical layers and highlight important differences between rodents and primates. We also review the pathological effects of PAX6 mutations in human neurodevelopmental disorders. We discuss some aspects of Pax6’s molecular actions including its own complex transcriptional regulation, the distinct molecular functions of its splice variants and some of Pax6’s known direct targets which mediate its actions during cortical development.

Cerebrospinal fluid-derived Semaphorin3B orients neuroepithelial cell divisions in the apicobasal axis

The spatial orientation of cell divisions is fundamental for tissue architecture and homeostasis. Here we analysed neuroepithelial progenitors in the developing mouse spinal cord to determine whether extracellular signals orient the mitotic spindle. We report that Semaphorin3B (Sema3B) released from the floor plate and the nascent choroid plexus in the cerebrospinal fluid (CSF) controls progenitor division orientation. Delivery of exogenous Sema3B to neural progenitors after neural tube opening in living embryos promotes planar orientation of their division. Preventing progenitor access to cues present in the CSF by genetically engineered canal obstruction affects the proportion of planar and oblique divisions. Sema3B knockout phenocopies the loss of progenitor access to the CSF. Sema3B binds to the apical surface of mitotic progenitors and exerts its effect via Neuropilin receptors, GSK3 activation and subsequent inhibition of the microtubule stabilizer CRMP2. Thus, extrinsic control mediated by the Semaphorin signalling orients progenitor divisions in neurogenic zones.

Cortical neurogenesis in the absence of centrioles

Neuronal production in the mammalian cortex depends on extensive mitoses of radial glial progenitors (RGPs) residing in the ventricular zone (VZ). Here, we examine the function of centrioles in RGPs during cortical neurogenesis in mice by conditional removal of SAS-4, a protein required for centriole biogenesis. SAS-4 deletion leads to a progressive loss of centrioles, accompanied by RGP detachment from the VZ. Delocalized RGPs do not become outer subventricular zone RGPs (oRGs). While remaining proliferative, ectopic RGPs, as well as those in the VZ with a centrosomal deficit exhibit prolonged mitosis, p53 up-regulation and apoptosis, resulting in neuronal loss and microcephaly. Simultaneous removal of p53 fully rescues RGP death and microcephaly, but not RGP delocalization and randomized mitotic spindle orientation. Our findings define centriolar functions in anchoring RGPs in the VZ and ensuring their efficient mitoses, and also reveal the remarkable adaptability of RGPs in the developing cortex.