Interkinetic nuclear migration: a mysterious process in search of a function

From neural tube to spinal cord: The dynamic journey of the dorsal neuroepithelium

In a developing embryo, formation of tissues and organs is remarkably precise in both time and space. Through cell-cell interactions, neighboring progenitors coordinate their activities, sequentially generating distinct types of cells. At present, we only have limited knowledge, rather than a systematic understanding, of the underlying logic and mechanisms responsible for cell fate transitions. The formation of the dorsal aspect of the spinal cord is an outstanding model to tackle these dynamics, as it first generates the peripheral nervous system and is later responsible for transmitting sensory information from the periphery to the brain and for coordinating local reflexes. This is reflected first by the ontogeny of neural crest cells, progenitors of the peripheral nervous system, followed by formation of the definitive roof plate of the central nervous system and specification of adjacent interneurons, then a transformation of roof plate into dorsal radial glia and ependyma lining the forming central canal. How do these peripheral and central neural branches segregate from common progenitors? How are dorsal radial glia established concomitant with transformation of the neural tube lumen into a central canal? How do the dorsal radial glia influence neighboring cells? This is only a partial list of questions whose clarification requires the implementation of experimental paradigms in which precise control of timing is crucial. Here, we outline some available answers and still open issues, while highlighting the contributions of avian models and their potential to address mechanisms of neural patterning and function.

Decoding Molecular Bases of Rodent Social Hetero-Grooming Behavior Using in Silico Analyses and Bioinformatics Tools

Highly prevalent in laboratory rodents, 'social' hetero-grooming behavior is translationally relevant to modeling a wide range of neuropsychiatric disorders. Here, we comprehensively evaluated known mouse genes linked to aberrant hetero-grooming phenotype and applied bioinformatics tools to construct a network of their established protein-protein interactions (PPI). We next identified several distinct molecular clusters within this network, including neuronal differentiation, cytoskeletal, WNT-signaling and synapsins-associated pathways. Using additional bioinformatics analyses, we further identified 'central' (hub) proteins within these molecular clusters, likely key for mouse hetero-grooming behavior. Overall, a more comprehensive characterization of intricate molecular pathways linked to aberrant rodent grooming may markedly advance our understanding of underlying cellular mechanisms and related neurological disorders, eventually helping discover novel targets for their pharmacological or gene therapy interventions.

Fas2EB112: a tale of two chromosomes

The cell-cell adhesion molecule Fasciclin II (Fas2) has long been studied for its evolutionarily conserved role in axon guidance. It is also expressed in the follicular epithelium, where together with a similar protein, Neuroglian (Nrg), it helps to drive the reintegration of cells born out of the tissue plane. Remarkably, one Fas2 protein null allele, Fas2G0336, demonstrates a mild reintegration phenotype, whereas work with the classic null allele Fas2EB112 showed more severe epithelial disorganization. These observations raise the question of which allele (if either) causes a bona fide loss of Fas2 protein function. The problem is not only relevant to reintegration but fundamentally important to understanding what this protein does and how it works: Fas2EB112 has been used in at least 37 research articles, and Fas2G0336 in at least three. An obvious solution is that one of the two chromosomes carries a modifier that either suppresses (Fas2G0336) or enhances (Fas2EB112) phenotypic severity. We find not only the latter to be the case, but identify the enhancing mutation as Nrg14, also a classic null allele.

SimuCell3D: three-dimensional simulation of tissue mechanics with cell polarization

The three-dimensional (3D) organization of cells determines tissue function and integrity, and changes markedly in development and disease. Cell-based simulations have long been used to define the underlying mechanical principles. However, high computational costs have so far limited simulations to either simplified cell geometries or small tissue patches. Here, we present SimuCell3D, an efficient open-source program to simulate large tissues in three dimensions with subcellular resolution, growth, proliferation, extracellular matrix, fluid cavities, nuclei and non-uniform mechanical properties, as found in polarized epithelia. Spheroids, vesicles, sheets, tubes and other tissue geometries can readily be imported from microscopy images and simulated to infer biomechanical parameters. Doing so, we show that 3D cell shapes in layered and pseudostratified epithelia are largely governed by a competition between surface tension and intercellular adhesion. SimuCell3D enables the large-scale in silico study of 3D tissue organization in development and disease at a great level of detail.

Ascorbic acid and its transporter SVCT2, affect radial glia cells differentiation in postnatal stages

Radial glia (RG) cells generate neurons and glial cells that make up the cerebral cortex. Both in rodents and humans, these stem cells remain for a specific time after birth, named late radial glia (lRG). The knowledge of lRG and molecules that may be involved in their differentiation is based on very limited data. We analyzed whether ascorbic acid (AA) and its transporter SVCT2, are involved in lRG cells differentiation. We demonstrated that lRG cells are highly present between the first and fourth postnatal days. Anatomical characterization of lRG cells, revealed that lRG cells maintained their bipolar morphology and stem-like character. When lRG cells were labeled with adenovirus-eGFP at 1 postnatal day, we detected that some cells display an obvious migratory neuronal phenotype, suggesting that lRG cells continue generating neurons postnatally. Moreover, we demonstrated that SVCT2 was apically polarized in lRG cells. In vitro studies using the transgenic mice SVCT2+/- and SVCT2tg (SVCT2-overexpressing mouse), showed that decreased SVCT2 levels led to accelerated differentiation into astrocytes, whereas both AA treatment and elevated SVCT2 expression maintain the lRG cells in an undifferentiated state. In vivo overexpression of SVCT2 in lRG cells generated cells with a rounded morphology that were migratory and positive for proliferation and neuronal markers. We also examined mediators that can be involved in AA/SVCT2-modulated signaling pathways, determining that GSK3-β through AKT, mTORC2, and PDK1 is active in brains with high levels of SVCT2/AA. Our data provide new insights into the role of AA and SVCT2 in late RG cells.

The nuclei of human adult stem cells can move within the cell and generate cellular protrusions to contact other cells

BACKGROUND

The neuronal transdifferentiation of adult bone marrow cells (BMCs) is still considered an artifact based on an alternative explanation of experimental results supporting this phenomenon obtained over decades. However, recent studies have shown that following neural induction, BMCs enter an intermediate cellular state before adopting neural-like morphologies by active neurite extension and that binucleated BMCs can be formed independent of any cell fusion events. These findings provide evidence to reject the idea that BMC neural transdifferentiation is merely an experimental artifact. Therefore, understanding the intermediate states that cells pass through during transdifferentiation is crucial given their potential application in regenerative medicine and disease modelling.

METHODS

In this study, we examined the functional significance of the variety of morphologies and positioning that cell nuclei of human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can adopt during neural-like differentiation using live-cell nuclear fluorescence labelling, time-lapse microscopy, and confocal microscopy analysis.

RESULTS

Here, we showed that after neural induction, hBM-MSCs enter an intermediate cellular state in which the nuclei are able to move within the cells, switching shapes and positioning and even generating cellular protrusions as they attempt to contact the cells around them. These findings suggest that changes in nuclear positioning occur because human cell nuclei somehow sense their environment. In addition, we showed the process of direct interactions between cell nuclei, which opens the possibility of a new level of intercellular interaction.

CONCLUSIONS

The present study advances the understanding of the intermediate stage through which hBM-MSCs pass during neural transdifferentiation, which may be crucial to understanding the mechanisms of these cell conversion processes and eventually harness them for use in regenerative medicine. Importantly, our study provides for the first time evidence that the nuclei of hBM-MSC-derived intermediate cells somehow sense their environment, generating cellular protrusions to contact other cells. In summary, human mesenchymal stromal cells could not only help to increase our understanding of the mechanisms underlying cellular plasticity but also facilitate the exact significance of nuclear positioning in cellular function and in tissue physiology.

Fas2EB112: A Tale of Two Chromosomes

The cell-cell adhesion molecule Fasciclin II (Fas2) has long been studied for its evolutionarily-conserved role in axon guidance. It is also expressed in the follicular epithelium, where together with a similar protein, Neuroglian (Nrg), it helps to drive the reintegration of cells born out of the tissue plane. Remarkably, one Fas2 protein null allele, Fas2G0336, demonstrates a mild reintegration phenotype, whereas work with the classic null allele Fas2EB112 showed more severe epithelial disorganization. These observations raise the question of which allele (if either) causes a bona fide loss of Fas2 protein function. The problem is not only relevant to reintegration but fundamentally important to understanding what this protein does and how it works: Fas2EB112 has been used in at least 37 research articles, and Fas2G0336 in at least three. An obvious solution is that one of the two chromosomes carries a modifier that either suppresses (Fas2G0336) or enhances (Fas2EB112) phenotypic severity. We find not only the latter to be the case, but identify the enhancing mutation as Nrg14, also a classic null allele.

MKL/SRF and Bcl6 mutual transcriptional repression safeguards the fate and positioning of neocortical progenitor cells mediated by RhoA

During embryogenesis, multiple intricate and intertwined cellular signaling pathways coordinate cell behavior. Their slightest alterations can have dramatic consequences for the cells and the organs they form. The transcriptional repressor Bcl6 was recently found as important for brain development. However, its regulation and integration with other signals is unknown. Using in vivo functional approaches combined with molecular mechanistic analysis, we identified a reciprocal regulatory loop between B cell lymphoma 6 (Bcl6) and the RhoA-regulated transcriptional complex megakaryoblastic leukemia/serum response factor (MKL/SRF). We show that Bcl6 physically interacts with MKL/SRF, resulting in a down-regulation of the transcriptional activity of both Bcl6 and MKL/SRF. This molecular cross-talk is essential for the control of proliferation, neurogenesis, and spatial positioning of neural progenitors. Overall, our data highlight a regulatory mechanism that controls neuronal production and neocortical development and reveal an MKL/SRF and Bcl6 interaction that may have broader implications in other physiological functions and in diseases.

Directionality of developing skeletal muscles is set by mechanical forces

Formation of oriented myofibrils is a key event in musculoskeletal development. However, the mechanisms that drive myocyte orientation and fusion to control muscle directionality in adults remain enigmatic. Here, we demonstrate that the developing skeleton instructs the directional outgrowth of skeletal muscle and other soft tissues during limb and facial morphogenesis in zebrafish and mouse. Time-lapse live imaging reveals that during early craniofacial development, myoblasts condense into round clusters corresponding to future muscle groups. These clusters undergo oriented stretch and alignment during embryonic growth. Genetic perturbation of cartilage patterning or size disrupts the directionality and number of myofibrils in vivo. Laser ablation of musculoskeletal attachment points reveals tension imposed by cartilage expansion on the forming myofibers. Application of continuous tension using artificial attachment points, or stretchable membrane substrates, is sufficient to drive polarization of myocyte populations in vitro. Overall, this work outlines a biomechanical guidance mechanism that is potentially useful for engineering functional skeletal muscle.

In Vivo Imaging of Radiation-Induced Apoptosis at Single-Cell Resolution in Transgenic Zebrafish Embryos

Among the various types of cell death induced by ionizing radiation, apoptosis is a highly regulated and well-characterized form. Investigating radiation-induced apoptosis in an intact organism offers advantages in capturing the dynamics of apoptosis under preserved physiology, although high resolution imaging remains challenging. Owing to their optical transparency and genetic amenability, zebrafish is an ideal animal model for research into this aspect. In this study, we present a secA5 transgenic zebrafish expressing genetically encoded secreted ANNEXIN V fused with mVenus, a yellow fluorescent protein that enables reporting of radiation-induced apoptosis. Using in vivo imaging approach, we show that after 2 Gy total-body irradiation, apoptosis could be visualized at single-cell resolution in different cell types throughout the embryo. Elevated apoptosis could be imaged and quantified in the neuroepithelium of the embryonic brain, as well as the proliferative zone and parenchyma of the larval brain. In addition, clearance of apoptotic cells by microglia, the professional phagocytes residing in the brain, could be imaged at single-cell resolution in irradiated larvae. These results establish transgenic secA5 zebrafish as a useful and versatile in vivo system for investigating the dynamic process of radiation-induced apoptosis.

Time-lapse mechanical imaging of neural tube closure in live embryo using Brillouin microscopy

Neural tube closure (NTC) is a complex process of embryonic development involving molecular, cellular, and biomechanical mechanisms. While the genetic factors and biochemical signaling have been extensively investigated, the role of tissue biomechanics remains mostly unexplored due to the lack of tools. Here, we developed an optical modality that can conduct time-lapse mechanical imaging of neural plate tissue as the embryo is experiencing neurulation. This technique is based on the combination of a confocal Brillouin microscope and a modified ex ovo culturing of chick embryo with an on-stage incubator. With this technique, for the first time, we captured the mechanical evolution of the neural plate tissue with live embryos. Specifically, we observed the continuous increase in tissue modulus of the neural plate during NTC for ex ovo cultured embryos, which is consistent with the data of in ovo culture as well as previous studies. Beyond that, we found that the increase in tissue modulus was highly correlated with the tissue thickening and bending. We foresee this non-contact and label-free technique opening new opportunities to understand the biomechanical mechanisms in embryonic development.

Cephalopod retinal development shows vertebrate-like mechanisms of neurogenesis

Coleoid cephalopods, including squid, cuttlefish, and octopus, have large and complex nervous systems and high-acuity, camera-type eyes. These traits are comparable only to features that are independently evolved in the vertebrate lineage. The size of animal nervous systems and the diversity of their constituent cell types is a result of the tight regulation of cellular proliferation and differentiation in development. Changes in the process of development during evolution that result in a diversity of neural cell types and variable nervous system size are not well understood. Here, we have pioneered live-imaging techniques and performed functional interrogation to show that the squid Doryteuthis pealeii utilizes mechanisms during retinal neurogenesis that are hallmarks of vertebrate processes. We find that retinal progenitor cells in the squid undergo nuclear migration until they exit the cell cycle. We identify retinal organization corresponding to progenitor, post-mitotic, and differentiated cells. Finally, we find that Notch signaling may regulate both retinal cell cycle and cell fate. Given the convergent evolution of elaborate visual systems in cephalopods and vertebrates, these results reveal common mechanisms that underlie the growth of highly proliferative neurogenic primordia. This work highlights mechanisms that may alter ontogenetic allometry and contribute to the evolution of complexity and growth in animal nervous systems.

Cytoophidia Maintain the Integrity of Drosophila Follicle Epithelium

CTP synthase (CTPS) forms a filamentous structure termed the cytoophidium in all three domains of life. The female reproductive system of Drosophila is an excellent model for studying the physiological function of cytoophidia. Here, we use CTPS^H355A, a point mutation that destroys the cytoophidium-forming ability of CTPS, to explore the in vivo function of cytoophidia. In CTPS^H355A egg chambers, we observe the ingression and increased heterogeneity of follicle cells. In addition, we find that the cytoophidium-forming ability of CTPS, rather than the protein level, is the cause of the defects observed in CTPS^H355A mutants. To sum up, our data indicate that cytoophidia play an important role in maintaining the integrity of follicle epithelium.

Nuclear movement in multinucleated cells

Nuclear movement is crucial for the development of many cell types and organisms. Nuclear movement is highly conserved, indicating its necessity for cellular function and development. In addition to mononucleated cells, there are several examples of cells in which multiple nuclei exist within a shared cytoplasm. These multinucleated cells and syncytia have important functions for development and homeostasis. Here, we review a subset of the developmental contexts in which the regulation of the movement and positioning of multiple nuclei are well understood, including pronuclear migration, the Drosophila syncytial blastoderm, the Caenorhabditis elegans hypodermis, skeletal muscle and filamentous fungi. We apply the principles learned from these models to other systems.

Primary Cilia Influence Progenitor Function during Cortical Development

Corticogenesis is an intricate process controlled temporally and spatially by many intrinsic and extrinsic factors. Alterations during this important process can lead to severe cortical malformations. Apical neuronal progenitors are essential cells able to self-amplify and also generate basal progenitors and/or neurons. Apical radial glia (aRG) are neuronal progenitors with a unique morphology. They have a long basal process acting as a support for neuronal migration to the cortical plate and a short apical process directed towards the ventricle from which protrudes a primary cilium. This antenna-like structure allows aRG to sense cues from the embryonic cerebrospinal fluid (eCSF) helping to maintain cell shape and to influence several key functions of aRG such as proliferation and differentiation. Centrosomes, major microtubule organising centres, are crucial for cilia formation. In this review, we focus on how primary cilia influence aRG function during cortical development and pathologies which may arise due to defects in this structure. Reporting and cataloguing a number of ciliary mutant models, we discuss the importance of primary cilia for aRG function and cortical development.

Functional Cooperation of α-Synuclein and Tau Is Essential for Proper Corticogenesis

Alpha-synuclein (αSyn) and tau are abundant multifunctional neuronal proteins, and their intracellular deposits have been linked to many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Despite the disease relevance, their physiological roles remain elusive, as mice with knock-out of either of these genes do not exhibit overt phenotypes. To reveal functional cooperation, we generated αSyn-/-tau-/- double-knock-out mice and characterized the functional cross talk between these proteins during brain development. Intriguingly, deletion of αSyn and tau reduced Notch signaling and accelerated interkinetic nuclear migration of G2 phase at early embryonic stage. This significantly altered the balance between the proliferative and neurogenic divisions of progenitor cells, resulting in an overproduction of early born neurons and enhanced neurogenesis, by which the brain size was enlarged during the embryonic stage in both sexes. On the other hand, a reduction in the number of neural progenitor cells in the middle stage of corticogenesis diminished subsequent gliogenesis in the αSyn-/-tau-/- cortex. Additionally, the expansion and maturation of macroglial cells (astrocytes and oligodendrocytes) were suppressed in the αSyn-/-tau-/- postnatal brain, which in turn reduced the male αSyn-/-tau-/- brain size and cortical thickness to less than the control values. Our study identifies important functional cooperation of αSyn and tau during corticogenesis.SIGNIFICANCE STATEMENT Correct understanding of the physiological functions of αSyn and tau in CNS is critical to elucidate pathogenesis involved in the etiology of neurodegenerative diseases including Alzheimer's disease and Parkinson's disease. We show here that αSyn and tau are cooperatively involved in brain development via maintenance of progenitor cells. αSyn and tau double-knock-out mice exhibited an overproduction of early born neurons and accelerated neurogenesis at early corticogenesis. Furthermore, loss of αSyn and tau also perturbed gliogenesis at later embryonic stage, as well as the subsequent glial expansion and maturation at postnatal brain. Our findings provide new mechanistic insights and extend therapeutic opportunities for neurodegenerative diseases caused by aberrant αSyn and tau.

Mechanics and functional consequences of nuclear deformations

As the home of cellular genetic information, the nucleus has a critical role in determining cell fate and function in response to various signals and stimuli. In addition to biochemical inputs, the nucleus is constantly exposed to intrinsic and extrinsic mechanical forces that trigger dynamic changes in nuclear structure and morphology. Emerging data suggest that the physical deformation of the nucleus modulates many cellular and nuclear functions. These functions have long been considered to be downstream of cytoplasmic signalling pathways and dictated by gene expression. In this Review, we discuss an emerging perspective on the mechanoregulation of the nucleus that considers the physical connections from chromatin to nuclear lamina and cytoskeletal filaments as a single mechanical unit. We describe key mechanisms of nuclear deformations in time and space and provide a critical review of the structural and functional adaptive responses of the nucleus to deformations. We then consider the contribution of nuclear deformations to the regulation of important cellular functions, including muscle contraction, cell migration and human disease pathogenesis. Collectively, these emerging insights shed new light on the dynamics of nuclear deformations and their roles in cellular mechanobiology.

Dynamic switching of lateral inhibition spatial patterns

Hes genes are transcriptional repressors activated by Notch. In the developing mouse neural tissue, HES5 expression oscillates in neural progenitors (Manning et al. 2019 Nat. Commun.10, 1–19 (doi:10.1038/s41467-019-10734-8)) and is spatially organized in small clusters of cells with synchronized expression (microclusters). Furthermore, these microclusters are arranged with a spatial periodicity of three–four cells in the dorso-ventral axis and show regular switching between HES5 high/low expression on a longer time scale and larger amplitude than individual temporal oscillators (Biga et al. 2021 Mol. Syst. Biol.17, e9902 (doi:10.15252/msb.20209902)). However, our initial computational modelling of coupled HES5 could not explain these features of the experimental data. In this study, we provide theoretical results that address these issues with biologically pertinent additions. Here, we report that extending Notch signalling to non-neighbouring progenitor cells is sufficient to generate spatial periodicity of the correct size. In addition, introducing a regular perturbation of Notch signalling by the emerging differentiating cells induces a temporal switching in the spatial pattern, which is longer than an individual cell’s periodicity. Thus, with these two new mechanisms, a computational model delivers outputs that closely resemble the complex tissue-level HES5 dynamics. Finally, we predict that such dynamic patterning spreads out differentiation events in space, complementing our previous findings whereby the local synchronization controls the rate of differentiation.

Mechanical constraints to cell-cycle progression in a pseudostratified epithelium

As organs and tissues approach their normal size during development or regeneration, growth slows down, and cell proliferation progressively comes to a halt. Among the various processes suggested to contribute to growth termination,1-10 mechanical feedback, perhaps via adherens junctions, has been suggested to play a role.11-14 However, since adherens junctions are only present in a narrow plane of the subapical region, other structures are likely needed to sense mechanical stresses along the apical-basal (A-B) axis, especially in a thick pseudostratified epithelium. This could be achieved by nuclei, which have been implicated in mechanotransduction in tissue culture.15 In addition, mechanical constraints imposed by nuclear crowding and spatial confinement could affect interkinetic nuclear migration (IKNM),16 which allows G2 nuclei to reach the apical surface, where they normally undergo mitosis.17-25 To explore how mechanical constraints affect IKNM, we devised an individual-based model that treats nuclei as deformable objects constrained by the cell cortex and the presence of other nuclei. The model predicts changes in the proportion of cell-cycle phases during growth, which we validate with the cell-cycle phase reporter FUCCI (Fluorescent Ubiquitination-based Cell Cycle Indicator).26 However, this model does not preclude indefinite growth, leading us to postulate that nuclei must migrate basally to access a putative basal signal required for S phase entry. With this refinement, our updated model accounts for the observed progressive slowing down of growth and explains how pseudostratified epithelia reach a stereotypical thickness upon completion of growth.

Revealing the Impact of Mitochondrial Fitness During Early Neural Development Using Human Brain Organoids

Mitochondrial homeostasis -including function, morphology, and inter-organelle communication- provides guidance to the intrinsic developmental programs of corticogenesis, while also being responsive to environmental and intercellular signals. Two- and three-dimensional platforms have become useful tools to interrogate the capacity of cells to generate neuronal and glia progeny in a background of metabolic dysregulation, but the mechanistic underpinnings underlying the role of mitochondria during human neurogenesis remain unexplored. Here we provide a concise overview of cortical development and the use of pluripotent stem cell models that have contributed to our understanding of mitochondrial and metabolic regulation of early human brain development. We finally discuss the effects of mitochondrial fitness dysregulation seen under stress conditions such as metabolic dysregulation, absence of developmental apoptosis, and hypoxia; and the avenues of research that can be explored with the use of brain organoids.

Mitosis, a springboard for epithelial-mesenchymal transition?

Mitosis is a key process in development and remains critical to ensure homeostasis in adult tissues. Besides its primary role in generating two new cells, cell division involves deep structural and molecular changes that might have additional effects on cell and tissue fate and shape. Specific quantitative and qualitative regulation of mitosis has been observed in multiple morphogenetic events in different embryo models. For instance, during mouse embryo gastrulation, the portion of epithelium that undergoes epithelial to mesenchymal transition, where a static epithelial cell become mesenchymal and motile, has a higher mitotic index and a distinct localization of mitotic rounding, compared to the rest of the tissue. Here we explore the potential mechanisms through which mitosis may favor tissue reorganization in various models. Notably, we discuss the mechanical impact of cell rounding on the cell and its environment, and the modification of tissue physical parameters through changes in cell-cell and cell-matrix adhesion.

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

Repeated nuclear translocations underlie photoreceptor positioning and lamination of the outer nuclear layer in the mammalian retina

In development, almost all stratified neurons must migrate from their birthplace to the appropriate neural layer. Photoreceptors reside in the most apical layer of the retina, near their place of birth. Whether photoreceptors require migratory events for fine-positioning and/or retention within this layer is not well understood. Here, we show that photoreceptor nuclei of the developing mouse retina cyclically exhibit rapid, dynein-1-dependent translocation toward the apical surface, before moving more slowly in the basal direction, likely due to passive displacement by neighboring retinal nuclei. Attenuating dynein 1 function in rod photoreceptors results in their ectopic basal displacement into the outer plexiform layer and inner nuclear layer. Synapse formation is also compromised in these displaced cells. We propose that repeated, apically directed nuclear translocation events are necessary to ensure retention of post-mitotic photoreceptors within the emerging outer nuclear layer during retinogenesis, which is critical for correct neuronal lamination.

Global pattern of interkinetic nuclear migration in tracheoesophageal epithelia of the mouse embryo: Interorgan and intraorgan regional differences

Interkinetic nuclear migration (INM) is an apicobasal (AB) polarity-based regulatory mechanism of proliferation/differentiation in epithelial stem/progenitor cells. We previously documented INM in the endoderm-derived tracheal/esophageal epithelia at embryonic day (E) 11.5 and suggested that INM is involved in the development of both organs. We here investigated interorgan (trachea vs esophagus) and intraorgan regional (ventral vs dorsal) differences in the INM mode in the tracheal and esophageal epithelia of the mouse embryo. We also analyzed convergent extension (CE) and planar cell movement (PCM) in the epithelia based on cell distribution. The pregnant C57BL/6J mice were intraperitoneally injected with 5-ethynyl-2'-deoxyuridine at E11.5 and E12.5 and were sacrificed 1, 4, 6, 8, and 12 hours later to obtain the embryos. The distribution of labeled cell nuclei along the AB axis was chronologically analyzed in the total, ventral, and dorsal sides of the epithelia. The percentage distribution of the nuclei population was represented by histogram and the chronological change was analyzed statistically using multidimensional scaling. The interorgan comparison of the INM mode during E11.5-E12.0, but not E12.5-E13.0, showed a significant difference. During E11.5-E12.0 the trachea, but not the esophagus, showed a significant difference between ventral and dorsal sides. During E12.5-E13.0 neither organ showed regional differences. CE appeared to occur in both organs during E11.5-E12.0 while PCM was unclear in both organs. These findings suggest a difference between the trachea and esophagus, and a regional difference in the trachea, not in the esophagus, in the INM mode, which may be related with the later differential organogenesis/histogenesis of these organs.

From Neural Crest to Definitive Roof Plate: The Dynamic Behavior of the Dorsal Neural Tube

Research on the development of the dorsal neural tube is particularly challenging. In this highly dynamic domain, a temporal transition occurs between early neural crest progenitors that undergo an epithelial-to-mesenchymal transition and exit the neural primordium, and the subsequent roof plate, a resident epithelial group of cells that constitutes the dorsal midline of the central nervous system. Among other functions, the roof plate behaves as an organizing center for the generation of dorsal interneurons. Despite extensive knowledge of the formation, emigration and migration of neural crest progenitors, little is known about the mechanisms leading to the end of neural crest production and the transition into a roof plate stage. Are these two mutually dependent or autonomously regulated processes? Is the generation of roof plate and dorsal interneurons induced by neural tube-derived factors throughout both crest and roof plate stages, respectively, or are there differences in signaling properties and responsiveness as a function of time? In this review, we discuss distinctive characteristics of each population and possible mechanisms leading to the shift between the above cell types.

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

Development of the smooth muscle layer in the ileum of mouse embryos

The smooth muscle layer (SML) comprises a significant portion of the intestines and other tubular organs. Whereas epithelial development has recently been extensively studied, SML development has drawn relatively less attention. Previous morphological reports revealed that the inner circular layer (IC) differentiates earlier than the outer longitudinal layer (OL), but detailed development of the SML, including chronological changes in the cell layer number, precise cell orientation, and regional differences in relation to the mesentery, has not been reported. We here observed the development of the SML in the C57BL/6J mouse ileum near the ileocecal junction at embryonic day (E) 13.5, 15.5, and 17.5. By histo-morphometric analyses, in IC, smooth muscle cells (SMCs) were oval-shaped and irregularly arranged in 3-4 layers at E13.5, then adopted an elongated spindle shape and decreased to two cell layers at E15.5 and E17.5. The IC SMC nuclear angle was not vertical, but oriented at 60-80° against the mid-axis of the intestinal lumen. The single SMC layer in OL was observed at E17.5, and the SMC nuclear angle was parallel to the luminal mid-axis. No clear regional difference against the mesentery was observed. Collectively, the findings suggest that development and differentiation of the ileal SML is not simple but regulated in a complex manner and possibly related to the macroscopic organogenesis.

Role of NDE1 in the Development and Evolution of the Gyrified Cortex

An expanded cortex is a hallmark of human neurodevelopment and endows increased cognitive capabilities. Recent work has shown that the cell cycle-related gene NDE1 is essential for proper cortical development. Patients who have mutations in NDE1 exhibit congenital microcephaly as a primary phenotype. At the cellular level, NDE1 is essential for interkinetic nuclear migration and mitosis of radial glial cells, which translates to an indispensable role in neurodevelopment. The nuclear migration function of NDE1 is well conserved across Opisthokonta. In mammals, multiple isoforms containing alternate terminal exons, which influence the functionality of NDE1, have been reported. It has been noted that the pattern of terminal exon usage mirrors patterns of cortical complexity in mammals. To provide context to these findings, here, we provide a comprehensive review of the literature regarding NDE1, its molecular biology and physiological relevance at the cellular and organismal levels. In particular, we outline the potential roles of NDE1 in progenitor cell behavior and explore the spectrum of NDE1 pathogenic variants. Moreover, we assessed the evolutionary conservation of NDE1 and interrogated whether the usage of alternative terminal exons is characteristic of species with gyrencephalic cortices. We found that gyrencephalic species are more likely to express transcripts that use the human-associated terminal exon, whereas lissencephalic species tend to express transcripts that use the mouse-associated terminal exon. Among gyrencephalic species, the human-associated terminal exon was preferentially expressed by those with a high order of gyrification. These findings underscore phylogenetic relationships between the preferential usage of NDE1 terminal exon and high-order gyrification, which provide insight into cortical evolution underlying high-order brain functions.

An Axon-Pathfinding Mechanism Preserves Epithelial Tissue Integrity

Epithelial tissues form the boundaries of organs, where they perform a range of functions, including secretion, absorption, and protection. These tissues are commonly composed of discrete cell layers-sheets of cells that are one-cell thick. In multiple systems examined, epithelial cells round up and move in the apical direction before dividing, likely in response to neighbor-cell crowding [1-6]. Because of this movement, daughter cells may be born displaced from the tissue layer. Reintegration of these displaced cells supports tissue growth and maintains tissue architecture [4]. Two conserved IgCAMs (immunoglobulin superfamily cell adhesion molecules), neuroglian (Nrg) and fasciclin 2 (Fas2), participate in cell reintegration in the Drosophila follicular epithelium [4]. Like their vertebrate orthologs L1CAM and NCAM1/2, respectively, Nrg and Fas2 are cell adhesion molecules primarily studied in the context of nervous system development [7-10]. Consistent with this, we identify another neural IgCAM, Fasciclin 3 (Fas3), as a reintegration factor. Nrg, Fas2, and Fas3 are components of the insect septate junction, the functional equivalent of the vertebrate tight junction, but proliferating follicle cells do not have mature septate junctions, and we find that the septate junction protein neurexin IV does not participate in reintegration [11, 12]. Here, we show that epithelial reintegration works in the same way as IgCAM-mediated axon growth and pathfinding; it relies not only on extracellular adhesion but also mechanical coupling between IgCAMs and the lateral spectrin-based membrane skeleton. Our work indicates that reintegration is mediated by a distinct epithelial adhesion assembly that is compositionally and functionally equivalent to junctions made between axons.

Nuclear crowding and nonlinear diffusion during interkinetic nuclear migration in the zebrafish retina

An important question in early neural development is the origin of stochastic nuclear movement between apical and basal surfaces of neuroepithelia during interkinetic nuclear migration. Tracking of nuclear subpopulations has shown evidence of diffusion - mean squared displacements growing linearly in time - and suggested crowding from cell division at the apical surface drives basalward motion. Yet, this hypothesis has not yet been tested, and the forces involved not quantified. We employ long-term, rapid light-sheet and two-photon imaging of early zebrafish retinogenesis to track entire populations of nuclei within the tissue. The time-varying concentration profiles show clear evidence of crowding as nuclei reach close-packing and are quantitatively described by a nonlinear diffusion model. Considerations of nuclear motion constrained inside the enveloping cell membrane show that concentration-dependent stochastic forces inside cells, compatible in magnitude to those found in cytoskeletal transport, can explain the observed magnitude of the diffusion constant.

MMP14 is required for delamination of chick neural crest cells independently of its catalytic activity

Matrix metalloproteinases have a broad spectrum of substrates ranging from extracellular matrix components and adhesion molecules to chemokines and growth factors. Despite being mostly secreted, MMPs have been detected in the cytosol, the mitochondria or the nucleus. Although most of the attention is focused on their role in matrix remodeling, the diversity of their substrates and their complex trafficking open the possibility for non-canonical functions. Yet in vivo examples and experimental demonstration of the physiological relevance of such activities are rare. Here, we have used chick neural crest (NC) cells, a highly migratory stem cell population likened to invasive cancer cells, as a model for physiological epithelial-mesenchymal transition (EMT). We demonstrate that MMP14 is required for NC delamination. Interestingly, this role is independent of its cytoplasmic tail and of its catalytic activity. Our in vivo data indicate that, in addition to being a late pro-invasive factor, MMP14 is also likely to be an early player, owing to its role in EMT.

Disruption in murine Eml1 perturbs retinal lamination during early development

During mammalian development, establishing functional neural networks in stratified tissues of the mammalian central nervous system depends upon the proper migration and positioning of neurons, a process known as lamination. In particular, the pseudostratified neuroepithelia of the retina and cerebrocortical ventricular zones provide a platform for progenitor cell proliferation and migration. Lamination defects in these tissues lead to mispositioned neurons, disrupted neuronal connections, and abnormal function. The molecular mechanisms necessary for proper lamination in these tissues are incompletely understood. Here, we identified a nonsense mutation in the Eml1 gene in a novel murine model, tvrm360, displaying subcortical heterotopia, hydrocephalus and disorganization of retinal architecture. In the retina, Eml1 disruption caused abnormal positioning of photoreceptor cell nuclei early in development. Upon maturation, these ectopic photoreceptors possessed cilia and formed synapses but failed to produce robust outer segments, implying a late defect in photoreceptor differentiation secondary to mislocalization. In addition, abnormal positioning of Müller cell bodies and bipolar cells was evident throughout the inner neuroblastic layer. Basal displacement of mitotic nuclei in the retinal neuroepithelium was observed in tvrm360 mice at postnatal day 0. The abnormal positioning of retinal progenitor cells at birth and ectopic presence of photoreceptors and secondary neurons upon maturation suggest that EML1 functions early in eye development and is crucial for proper retinal lamination during cellular proliferation and development.

Mechanical Regulation of Nuclear Translocation in Migratory Neurons

Neuronal migration is a critical step during the formation of functional neural circuits in the brain. Newborn neurons need to move across long distances from the germinal zone to their individual sites of function; during their migration, they must often squeeze their large, stiff nuclei, against strong mechanical stresses, through narrow spaces in developing brain tissue. Recent studies have clarified how actomyosin and microtubule motors generate mechanical forces in specific subcellular compartments and synergistically drive nuclear translocation in neurons. On the other hand, the mechanical properties of the surrounding tissues also contribute to their function as an adhesive support for cytoskeletal force transmission, while they also serve as a physical barrier to nuclear translocation. In this review, we discuss recent studies on nuclear migration in developing neurons, from both cell and mechanobiological viewpoints.

Interkinetic nuclear movements promote apical expansion in pseudostratified epithelia at the expense of apicobasal elongation

Pseudostratified epithelia (PSE) are a common type of columnar epithelia found in a wealth of embryonic and adult tissues such as ectodermal placodes, the trachea, the ureter, the gut and the neuroepithelium. PSE are characterized by the choreographed displacement of cells’ nuclei along the apicobasal axis according to phases of their cell cycle. Such movements, called interkinetic movements (INM), have been proposed to influence tissue expansion and shape and suggested as culprit in several congenital diseases such as CAKUT (Congenital anomalies of kidney and urinary tract) and esophageal atresia. INM rely on cytoskeleton dynamics just as adhesion, contractility and mitosis do. Therefore, long term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. Here we bypassed this hurdle by generating a 2D agent-based model of a proliferating PSE and compared its output to the growth of the chick neuroepithelium to assess the interplay between INM and these other important cell processes during growth of a PSE. We found that INM directly generates apical expansion and apical nuclear crowding. In addition, our data strongly suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but rather requires a specific elongation program. We then discuss how such program might functionally link INM, tissue growth and differentiation.

Pseudostratified epithelia (PSE) are a common type of epithelia characterized by the choreographed displacement of cells’ nuclei along the apicobasal axis during proliferation. These so-called interkinetic movements (INM) were proposed to influence tissue expansion and suggested as culprit in several congenital diseases. INM rely on cytoskeleton dynamics. Therefore, longer term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. We bypassed this hurdle by generating a mathematical model of PSE and compared it to the growth of an epithelium of reference. Our data show that INM drive expansion of the apical domain of the epithelium and suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but might rather requires a specific elongation program.

Changes in the Distribution of Cell Contacts and Mitotic Cycle Disturbances in Cells of the Allograft of Rat Embryonic Neocortex

Morphological changes in the allograft of rat anterior cerebral vesicle at the early stages after transplantation into the peripheral nerve of an adult rat were studied by immunohistochemical methods. Immunohistochemical reaction to bromodeoxyuridine showed that the delay of mitotic division in neural stem/progenitor cells in the grafts occurred during S/G2 stage. In transplants of rat embryonic neocortex (E13), changes in the cell cycle of neural stem/progenitor cells in 3 h after transplantation into the nerve correlated with abnormal distribution of adherens junctions and interkinetic nuclear migration.

How the extracellular matrix shapes neural development

During development, both cells and tissues must acquire the correct shape to allow their proper function. This is especially relevant in the nervous system, where the shape of individual cell processes, such as the axons and dendrites, and the shape of entire tissues, such as the folding of the neocortex, are highly specialized. While many aspects of neural development have been uncovered, there are still several open questions concerning the mechanisms governing cell and tissue shape. In this review, we discuss the role of the extracellular matrix (ECM) in these processes. In particular, we consider how the ECM regulates cell shape, proliferation, differentiation and migration, and more recent work highlighting a key role of ECM in the morphogenesis of neural tissues.

The spatiotemporal expression pattern of microRNAs in the developing mouse nervous system

MicroRNAs (miRNAs) control various biological processes by inducing translational repression and transcript degradation of the target genes. In mammalian development, knowledge of the timing and expression pattern of each miRNA is important to determine and predict its function in vivo So far, no systematic analyses of the spatiotemporal expression pattern of miRNAs during mammalian neurodevelopment have been performed. Here, we isolated total RNAs from the embryonic dorsal forebrain of mice at different developmental stages and subjected these RNAs to microarray analyses. We selected 279 miRNAs that exhibited high signal intensities or ascending or descending expression dynamics. To ascertain the expression patterns of these miRNAs, we used locked nucleic acid (LNA)-modified miRNA probes in in situ hybridization experiments. Multiple miRNAs exhibited spatially restricted/enriched expression in anatomically distinct regions or in specific neuron subtypes in the embryonic brain and spinal cord, such as in the ventricular area, the striatum (and other basal ganglia), hypothalamus, choroid plexus, and the peripheral nervous system. These findings provide new insights into the expression and function of miRNAs during the development of the nervous system and could be used as a resource to facilitate studies in neurodevelopment.

Transcriptional regulation of cell shape during organ morphogenesis

The emerging field of transcriptional regulation of cell shape changes aims to address the critical question of how gene expression programs produce a change in cell shape. Together with cell growth, division, and death, changes in cell shape are essential for organ morphogenesis. Whereas most studies of cell shape focus on posttranslational events involved in protein organization and distribution, cell shape changes can be genetically programmed. This review highlights the essential role of transcriptional regulation of cell shape during morphogenesis of the heart, lungs, gastrointestinal tract, and kidneys. We emphasize the evolutionary conservation of these processes across different model organisms and discuss perspectives on open questions and research avenues that may provide mechanistic insights toward understanding birth defects.

Radial WNT5A-Guided Post-mitotic Filopodial Pathfinding Is Critical for Midgut Tube Elongation

The early midgut undergoes intensive elongation, but the underlying cellular and molecular mechanisms are unknown. The early midgut epithelium is pseudostratified, and its nuclei travel between apical and basal surfaces in concert with cell cycle. Using 3D confocal imaging and 2D live imaging, we profiled behaviors of individual dividing cells. As nuclei migrate apically for mitosis, cells maintain a basal process (BP), which splits but is inherited by only one daughter. After mitosis, some daughters directly use the inherited BP as a "conduit" to transport the nucleus basally, while >50% of daughters generate a new basal filopodium and use it as a path to return the nucleus. Post-mitotic filopodial "pathfinding" is guided by mesenchymal WNT5A. Without WNT5A, some cells fail to tether basally and undergo apoptosis, leading to a shortened midgut. Thus, these studies reveal previously unrecognized strategies for efficient post-mitotic nuclear trafficking, which is critical for early midgut elongation.

Feedback between tissue packing and neurogenesis in the zebrafish neural tube

Balancing the rate of differentiation and proliferation in developing tissues is essential to produce organs of robust size and composition. Although many molecular regulators have been established, how these connect to physical and geometrical aspects of tissue architecture is poorly understood. Here, using high-resolution timelapse imaging, we find that changes to cell geometry associated with dense tissue packing play a significant role in regulating differentiation rate in the zebrafish neural tube. Specifically, progenitors that are displaced away from the apical surface due to crowding, tend to differentiate in a Notch-dependent manner. Using simulations we show that interplay between progenitor density, cell shape and changes in differentiation rate could naturally result in negative-feedback control on progenitor cell number. Given these results, we suggest a model whereby differentiation rate is regulated by density dependent effects on cell geometry to: (1) correct variability in cell number; and (2) balance the rates of proliferation and differentiation over development to 'fill' the available space.

Ari-1 Regulates Myonuclear Organization Together with Parkin and Is Associated with Aortic Aneurysms

Nuclei are actively positioned and anchored to the cytoskeleton via the LINC (Linker of Nucleoskeleton and Cytoskeleton) complex. We identified mutations in the Parkin-like E3 ubiquitin ligase Ariadne-1 (Ari-1) that affect the localization and distribution of LINC complex members in Drosophila. ari-1 mutants exhibit nuclear clustering and morphology defects in larval muscles. We show that Ari-1 mono-ubiquitinates the core LINC complex member Koi. Surprisingly, we discovered functional redundancy between Parkin and Ari-1: increasing Parkin expression rescues ari-1 mutant phenotypes and vice versa. We further show that rare variants in the human homolog of ari-1 (ARIH1) are associated with thoracic aortic aneurysms and dissections, conditions resulting from smooth muscle cell (SMC) dysfunction. Human ARIH1 rescues fly ari-1 mutant phenotypes, whereas human variants found in patients fail to do so. In addition, SMCs obtained from patients display aberrant nuclear morphology. Hence, ARIH1 is critical in anchoring myonuclei to the cytoskeleton.

Interkinetic nuclear migration in the tracheal and esophageal epithelia of the mouse embryo: Possible implications for tracheo-esophageal anomalies

Interkinetic nuclear migration (INM) is a cell polarity-based phenomenon in which progenitor cell nuclei migrate along the apico-basal axis of the pseudostratified epithelium in synchrony with the cell cycle. INM is suggested to be at least partially cytoskeleton-dependent and to regulate not only the proliferation/differentiation of stem/progenitor cells but also the localized/overall size and shape of organs/tissues. INM occurs in all three of the germ-layer derived epithelia, including the endoderm-derived gut. However, INM has not been documented in the esophagus and respiratory tube arising from the anterior foregut. Esophageal atresia with or without trachea-esophageal fistula (EA/TEF) is a relatively common developmental defect. Transcription factors and signaling molecules have been implicated in EA/TEF, but the etiology of EA/TEF-which has been suggested to involve cell polarity-related mechanisms-remains highly controversial. In the present study, we first examined whether INM exists in the trachea and esophagus of mouse embryos at embryonic day 11.5 (E11.5), just after separation of the two tubes from the anterior foregut. By labeling the DNA-synthesizing stem cell nuclei with 5-ethynyl-2'-deoxyuridine, a nucleotide analogue, and statistically analyzing chronological changes in the distribution pattern of the labeled nuclei by using multidimensional scaling, we showed the existence of INM in both the esophagus and trachea, with differences in the INM magnitude and cycle pattern. We further showed morphological changes from the INM-based pseudostratified single layer to the stratified multilayer in the esophageal epithelium in association with a temporal loss/perturbation of AB polarity, suggesting a possible relation with the pathogenesis of EA/TEF.

Microtubule Motors in Establishment of Epithelial Cell Polarity

Epithelial cells play a key role in insuring physiological homeostasis by acting as a barrier between the outside environment and internal organs. They are also responsible for the vectorial transport of ions and fluid essential to the function of many organs. To accomplish these tasks, epithelial cells must generate an asymmetrically organized plasma membrane comprised of structurally and functionally distinct apical and basolateral membranes. Adherent and occluding junctions, respectively, anchor cells within a layer and prevent lateral diffusion of proteins in the outer leaflet of the plasma membrane and restrict passage of proteins and solutes through intercellular spaces. At a fundamental level, the establishment and maintenance of epithelial polarity requires that signals initiated at cell-substratum and cell-cell adhesions are transmitted appropriately and dynamically to the cytoskeleton, to the membrane-trafficking machinery, and to the regulation of occluding and adherent junctions. Rigorous descriptive and mechanistic studies published over the last 50 years have provided great detail to our understanding of epithelial polarization. Yet still, critical early steps in morphogenesis are not yet fully appreciated. In this review, we discuss how cytoskeletal motor proteins, primarily kinesins, contribute to coordinated modification of microtubule and actin arrays, formation and remodeling of cell adhesions to targeted membrane trafficking, and to initiating the formation and expansion of an apical lumen.

Differentiating cells mechanically limit progenitor cells’ interkinetic nuclear migration to secure apical cytogenesis

Many proliferative epithelia are pseudostratified due to cell cycle–dependent interkinetic nuclear migration (IKNM, basal during G1 and apical during G2). Although most epithelia, including early embryonic neuroepithelia (≤100 µm thick), undergo IKNM over the entire apicobasal extent, more apicobasally elongated (300 µm) neural progenitor cells (also called “radial glia”) in the mid-embryonic mouse cerebral wall move their nuclei only within its apical (100 µm) compartment, leaving the remaining basal part nucleus-free (fiber-like). How this IKNM range (i.e., the thickness of a pseudostratified “ventricular zone” [VZ]) is determined remains unknown. Here, we report external fencing of IKNM and VZ by differentiating cells. When a tight stack of multipolar cells just basal to VZ was “drilled” via acute neuron-directed expression of diphtheria toxin, IKNM of apicobasally connected progenitor cells continued far basally (200 µm). The unfencing-induced, basally overshot nuclei stay in S phase too long and do not move apically, suggesting that external limitation of IKNM is necessary for progenitors to undergo normal cytogenetic behaviors. Thus, physical collaboration between progenitors and differentiating cells including neurons underlies brain development.

Nuclear movement in fungi

Nuclear movement within a cell occurs in a variety of eukaryotic organisms including yeasts and filamentous fungi. Fungal molecular genetic studies identified the minus-end-directed microtubule motor cytoplasmic dynein as a critical protein for nuclear movement or orientation of the mitotic spindle contained in the nucleus. Studies in the budding yeast first indicated that dynein anchored at the cortex via its anchoring protein Num1 exerts pulling force on an astral microtubule to orient the anaphase spindle across the mother-daughter axis before nuclear division. Prior to anaphase, myosin V interacts with the plus end of an astral microtubule via Kar9-Bim1/EB1 and pulls the plus end along the actin cables to move the nucleus/spindle close to the bud neck. In addition, pushing or pulling forces generated from cortex-linked polymerization or depolymerization of microtubules drive nuclear movements in yeasts and possibly also in filamentous fungi. In filamentous fungi, multiple nuclei within a hyphal segment undergo dynein-dependent back-and-forth movements and their positioning is also influenced by cytoplasmic streaming toward the hyphal tip. In addition, nuclear movement occurs at various stages of fungal development and fungal infection of plant tissues. This review discusses our current understanding on the mechanisms of nuclear movement in fungal organisms, the importance of nuclear positioning and the regulatory strategies that ensure the proper positioning of nucleus/spindle.

Cellular systems for epithelial invagination

Epithelial invagination is a fundamental module of morphogenesis that iteratively occurs to generate the architecture of many parts of a developing organism. By changing the physical properties such as the shape and/or position of a population of cells, invagination drives processes ranging from reconfiguring the entire body axis during gastrulation, to forming the primordia of the eyes, ears and multiple ducts and glands, during organogenesis. The epithelial bending required for invagination is achieved through a variety of mechanisms involving systems of cells. Here we provide an overview of the different mechanisms, some of which can work in combination, and outline the circumstances in which they apply.

This article is part of the themed issue ‘Systems morphodynamics: understanding the development of tissue hardware’.

Neural tube closure: cellular, molecular and biomechanical mechanisms

Neural tube closure has been studied for many decades, across a range of vertebrates, as a paradigm of embryonic morphogenesis. Neurulation is of particular interest in view of the severe congenital malformations - 'neural tube defects' - that result when closure fails. The process of neural tube closure is complex and involves cellular events such as convergent extension, apical constriction and interkinetic nuclear migration, as well as precise molecular control via the non-canonical Wnt/planar cell polarity pathway, Shh/BMP signalling, and the transcription factors Grhl2/3, Pax3, Cdx2 and Zic2. More recently, biomechanical inputs into neural tube morphogenesis have also been identified. Here, we review these cellular, molecular and biomechanical mechanisms involved in neural tube closure, based on studies of various vertebrate species, focusing on the most recent advances in the field.

DNA-PKcs, ATM, and ATR Interplay Maintains Genome Integrity during Neurogenesis

The DNA damage response (DDR) orchestrates a network of cellular processes that integrates cell-cycle control and DNA repair or apoptosis, which serves to maintain genome stability. DNA-PKcs (the catalytic subunit of the DNA-dependent kinase, encoded by PRKDC), ATM (ataxia telangiectasia, mutated), and ATR (ATM and Rad3-related) are related PI3K-like protein kinases and central regulators of the DDR. Defects in these kinases have been linked to neurodegenerative or neurodevelopmental syndromes. In all cases, the key neuroprotective function of these kinases is uncertain. It also remains unclear how interactions between the three DNA damage-responsive kinases coordinate genome stability, particularly in a physiological context. Here, we used a genetic approach to identify the neural function of DNA-PKcs and the interplay between ATM and ATR during neurogenesis. We found that DNA-PKcs loss in the mouse sensitized neuronal progenitors to apoptosis after ionizing radiation because of excessive DNA damage. DNA-PKcs was also required to prevent endogenous DNA damage accumulation throughout the adult brain. In contrast, ATR coordinated the DDR during neurogenesis to direct apoptosis in cycling neural progenitors, whereas ATM regulated apoptosis in both proliferative and noncycling cells. We also found that ATR controls a DNA damage-induced G₂/M checkpoint in cortical progenitors, independent of ATM and DNA-PKcs. These nonoverlapping roles were further confirmed via sustained murine embryonic or cortical development after all three kinases were simultaneously inactivated. Thus, our results illustrate how DNA-PKcs, ATM, and ATR have unique and essential roles during the DDR, collectively ensuring comprehensive genome maintenance in the nervous system.

SIGNIFICANCE STATEMENT

The DNA damage response (DDR) is essential for prevention of a broad spectrum of different human neurologic diseases. However, a detailed understanding of the DDR at a physiological level is lacking. In contrast to many in vitro cellular studies, here we demonstrate independent biological roles for the DDR kinases DNA-PKcs, ATM, and ATR during neurogenesis. We show that DNA-PKcs is central to DNA repair in nonproliferating cells, and restricts DNA damage accumulation, whereas ATR controls damage-induced G₂ checkpoint control and apoptosis in proliferating cells. Conversely, ATM is critical for controlling apoptosis in immature noncycling neural cells after DNA damage. These data demonstrate functionally distinct, but cooperative, roles for each kinase in preserving genome stability in the nervous system.