Population dynamics during cell proliferation and neuronogenesis in the developing murine neocortex

A choroid plexus apocrine secretion mechanism shapes CSF proteome and embryonic brain development

We discovered that apocrine secretion by embryonic choroid plexus (ChP) epithelial cells contributes to the cerebrospinal fluid (CSF) proteome and influences brain development in mice. The apocrine response relies on sustained intracellular calcium signaling and calpain-mediated cytoskeletal remodeling. It rapidly alters the embryonic CSF proteome, activating neural progenitors lining the brain's ventricles. Supraphysiological apocrine secretion induced during mouse development by maternal administration of a serotonergic 5HT2C receptor agonist dysregulates offspring cerebral cortical development, alters the fate of CSF-contacting neural progenitors, and ultimately changes adult social behaviors. Critically, exposure to maternal illness or to the psychedelic drug LSD during pregnancy also overactivates the ChP, inducing excessive secretion. Collectively, our findings demonstrate a new mechanism by which maternal exposure to diverse stressors disrupts in utero brain development.

Temporal recording of mammalian development and precancer

Key to understanding many biological phenomena is knowing the temporal ordering of cellular events, which often require continuous direct observations [1, 2]. An alternative solution involves the utilization of irreversible genetic changes, such as naturally occurring mutations, to create indelible markers that enables retrospective temporal ordering [3-8]. Using NSC-seq, a newly designed and validated multi-purpose single-cell CRISPR platform, we developed a molecular clock approach to record the timing of cellular events and clonality in vivo , while incorporating assigned cell state and lineage information. Using this approach, we uncovered precise timing of tissue-specific cell expansion during murine embryonic development and identified new intestinal epithelial progenitor states by their unique genetic histories. NSC-seq analysis of murine adenomas and single-cell multi-omic profiling of human precancers as part of the Human Tumor Atlas Network (HTAN), including 116 scRNA-seq datasets and clonal analysis of 418 human polyps, demonstrated the occurrence of polyancestral initiation in 15-30% of colonic precancers, revealing their origins from multiple normal founders. Thus, our multimodal framework augments existing single-cell analyses and lays the foundation for in vivo multimodal recording, enabling the tracking of lineage and temporal events during development and tumorigenesis.

Neocortex neurogenesis and maturation in the African greater cane rat

BACKGROUND

Neocortex development has been extensively studied in altricial rodents such as mouse and rat. Identification of alternative animal models along the "altricial-precocial" spectrum in order to better model and understand neocortex development is warranted. The Greater cane rat (GCR, Thyronomys swinderianus) is an indigenous precocial African rodent. Although basic aspects of brain development in the GCR have been documented, detailed information on neocortex development including the occurrence and abundance of the distinct types of neural progenitor cells (NPCs) in the GCR are lacking.

METHODS

GCR embryos and fetuses were obtained from timed pregnant dams between gestation days 50-140 and their neocortex was analyzed by immunofluorescence staining using characteristic marker proteins for NPCs, neurons and glia cells. Data were compared with existing data on closely related precocial and altricial species, i.e. guinea pig and dwarf rabbit.

RESULTS

The primary sequence of neuro- and gliogenesis, and neuronal maturation is preserved in the prenatal GCR neocortex. We show that the GCR exhibits a relatively long period of cortical neurogenesis of 70 days. The subventricular zone becomes the major NPC pool during mid-end stages of neurogenesis with Pax6 + NPCs constituting the major basal progenitor subtype in the GCR neocortex. Whereas dendrite formation in the GCR cortical plate appears to initiate immediately after the onset of neurogenesis, major aspects of axon formation and maturation, and astrogenesis do not begin until mid-neurogenesis. Similar to the guinea pig, the GCR neocortex exhibits a high maturation status, containing neurons with well-developed dendrites and myelinated axons and astrocytes at birth, thus providing further evidence for the notion that a great proportion of neocortex growth and maturation in precocial mammals occurs before birth.

CONCLUSIONS

Together, this work has deepened our understanding of neocortex development of the GCR, of the timing and the cellular differences that regulate brain growth and development within the altricial-precocial spectrum and its suitability as a research model for neurodevelopmental studies. The timelines of brain development provided by this study may serve as empirical reference data and foundation in future studies in order to model and better understand neurodevelopment and associated alterations.

Length of the Neurogenic Period-A Key Determinant for the Generation of Upper-Layer Neurons During Neocortex Development and Evolution

The neocortex, a six-layer neuronal brain structure that arose during the evolution of, and is unique to, mammals, is the seat of higher order brain functions responsible for human cognitive abilities. Despite its recent evolutionary origin, it shows a striking variability in size and folding complexity even among closely related mammalian species. In most mammals, cortical neurogenesis occurs prenatally, and its length correlates with the length of gestation. The evolutionary expansion of the neocortex, notably in human, is associated with an increase in the number of neurons, particularly within its upper layers. Various mechanisms have been proposed and investigated to explain the evolutionary enlargement of the human neocortex, focussing in particular on changes pertaining to neural progenitor types and their division modes, driven in part by the emergence of human-specific genes with novel functions. These led to an amplification of the progenitor pool size, which affects the rate and timing of neuron production. In addition, in early theoretical studies, another mechanism of neocortex expansion was proposed—the lengthening of the neurogenic period. A critical role of neurogenic period length in determining neocortical neuron number was subsequently supported by mathematical modeling studies. Recently, we have provided experimental evidence in rodents directly supporting the mechanism of extending neurogenesis to specifically increase the number of upper-layer cortical neurons. Moreover, our study examined the relationship between cortical neurogenesis and gestation, linking the extension of the neurogenic period to the maternal environment. As the exact nature of factors promoting neurogenic period prolongation, as well as the generalization of this mechanism for evolutionary distinct lineages, remain elusive, the directions for future studies are outlined and discussed.

Lengthening Neurogenic Period during Neocortical Development Causes a Hallmark of Neocortex Expansion

A hallmark of the evolutionary expansion of the neocortex is a specific increase in the number of neurons generated for the upper neocortical layers during development. The cause underlying this increase is unknown. Here, we show that lengthening the neurogenic period during neocortical development is sufficient to specifically increase upper-layer neuron generation. Thus, embryos of mouse strains with longer gestation exhibited a longer neurogenic period and generated more upper-layer, but not more deep-layer, neurons than embryos with shorter gestation. Accordingly, long-gestation embryos showed a greater abundance of neurogenic progenitors in the subventricular zone than short-gestation embryos at late stages of cortical neurogenesis. Analysis of a mouse-rat chimeric embryo, developing inside a rat mother, pointed to factors in the rat environment that influenced the upper-layer neuron generation by the mouse progenitors. Exploring a potential maternal source of such factors, short-gestation strain mouse embryos transferred to long-gestation strain mothers exhibited an increase in the length of the neurogenic period and upper-layer neuron generation. The opposite was the case for long-gestation strain mouse embryos transferred to short-gestation strain mothers, indicating a dominant maternal influence on the length of the neurogenic period and hence upper-layer neuron generation. In summary, our study uncovers a hitherto unknown link between embryonic cortical neurogenesis and the maternal gestational environment and provides experimental evidence that lengthening the neurogenic period during neocortical development underlies a key aspect of neocortical expansion.

Timing the spinal cord development with neural progenitor cells losing their proliferative capacity: a theoretical analysis

In the developing neural tube in chicken and mammals, neural stem cells proliferate and differentiate according to a stereotyped spatiotemporal pattern. Several actors have been identified in the control of this process, from tissue-scale morphogens patterning to intrinsic determinants in neural progenitor cells. In a previous study (Bonnet et al. eLife 7, 2018), we have shown that the CDC25B phosphatase promotes the transition from proliferation to differentiation by stimulating neurogenic divisions, suggesting that it acts as a maturating factor for neural progenitors. In this previous study, we set up a mathematical model linking fixed progenitor modes of division to the dynamics of progenitors and differentiated populations. Here, we extend this model over time to propose a complete dynamical picture of this process. We start from the standard paradigm that progenitors are homogeneous and can perform any type of divisions (proliferative division yielding two progenitors, asymmetric neurogenic divisions yielding one progenitor and one neuron, and terminal symmetric divisions yielding two neurons). We calibrate this model using data published by Saade et al. (Cell Reports 4, 2013) about mode of divisions and population dynamics of progenitors/neurons at different developmental stages. Next, we explore the scenarios in which the progenitor population is actually split into two different pools, one of which is composed of cells that have lost the capacity to perform proliferative divisions. The scenario in which asymmetric neurogenic division would induce such a loss of proliferative capacity appears very relevant.

The developing optic tectum: An asymmetrically organized system and the need for a redefinition of the notion of sensitive period

The present article summarizes the main events involved in the isthmic organizer and optic tectum determination and analyses how optic tectum patterning is translated, by the organized operation of several specific cell behaviors, into the terminally differentiated optic tectum. The paper proposes that this assembling of temporally/spatially organized cell behaviors could be incorporated into a wider notion of patterning and that, given the asymmetric organization of the developing optic tectum, the notion of "sensitive period" does not capture the whole complexity of midbrain development and the pathogenesis of congenital disorders. The cell behaviors involved in the optic tectum development are organized in time and space by the isthmic organizer. A comprehensive description of the normal optic tectum development, and also its alterations, should consider both domains. Significantly, the identity of each neuronal cohort depends critically on its "time and place of birth". Both parameters must be considered at once to explain how the structural and functional organization of the optic tectum is elaborated. The notion of "patterning" applies only to the early events of the optic tectum development. Besides, the notion of "sensitive period" considers only a temporal domain and disregards the asymmetric organization of the developing optic tectum. The present paper proposes that these notions might be re-defined: (a) a wider meaning of the term patterning and (b) a replacement of the term "sensitive period" by a more precise concept of "sensitive temporal/spatial window".

From proliferation to target innervation: signaling molecules that direct sympathetic nervous system development

The sympathetic division of the autonomic nervous system includes a variety of cells including neurons, endocrine cells and glial cells. A recent study (Furlan et al. 2017) has revised thinking about the developmental origin of these cells. It now appears that sympathetic neurons and chromaffin cells of the adrenal medulla do not have an immediate common ancestor in the form a "sympathoadrenal cell", as has been long believed. Instead, chromaffin cells arise from Schwann cell precursors. This review integrates the new findings with the expanding body of knowledge on the signalling pathways and transcription factors that regulate the origin of cells of the sympathetic division of the autonomic nervous system.

Endogenous neurogenesis in adult mammals after spinal cord injury

During the whole life cycle of mammals, new neurons are constantly regenerated in the subgranular zone of the dentate gyrus and in the subventricular zone of the lateral ventricles. Thanks to emerging methodologies, great progress has been made in the characterization of spinal cord endogenous neural stem cells (ependymal cells) and identification of their role in adult spinal cord development. As recently evidenced, both the intrinsic and extrinsic molecular mechanisms of ependymal cells control the sequential steps of the adult spinal cord neurogenesis. This review introduces the concept of adult endogenous neurogenesis, the reaction of ependymal cells after adult spinal cord injury (SCI), the heterogeneity and markers of ependymal cells, the factors that regulate ependymal cells, and the niches that impact the activation or differentiation of ependymal cells.

Zbtb20 modulates the sequential generation of neuronal layers in developing cortex

Background

During corticogenesis, genetic programs encoded in progenitor cells at different developmental stages and inherited in postmitotic neurons specify distinct layer and area identities. Transcription factor Zbtb20 has been shown to play a role for hippocampal development but whether it is implicated in mammalian neocortical morphogenesis remains unknown.

Results

Here, we report that during embyogenesis transcription factor Zbtb20 has a dynamic spatio-temporal expression pattern in mitotic cortical progenitors through which it modulates the sequential generation of cortical neuronal layer identities. Zbtb20 knock out mice exhibited enhanced populations of early born L6-L4 neuronal subtypes and a dramatic reduction of the late born L3/L2 neurons. This defect was due to a temporal misbalance in the production of earlier versus later born neurons, leading to a progressive diminishing of the progenitor pool for the generation of L3-L2 neurons. Zbtb20 implements these temporal effects in part by binding to promoter of the orphan nuclear receptor CoupTF1/Nr2f1. In addition to its effects exerted in cortical progenitors, the postmitotic expression of Zbtb20 in L3/L2 neurons starting at birth may contribute to their proper differentiation and migration.

Conclusions

Our findings reveal Zbtb20 as a novel temporal regulator for the generation of layer-specific neuronal identities.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0242-2) contains supplementary material, which is available to authorized users.

Beyond boundaries--Eph:ephrin signaling in neurogenesis

Eph:ephrin signaling plays an important role in embryonic development as well as tissue homeostasis in the adult. At the cellular level, this transduction pathway is best known for its role in the control of cell adhesion and repulsion, cell migration and morphogenesis. Yet, a number of publications have also implicated Eph:ephrin signaling in the control of adult and embryonic neurogenesis. As is the case for other biological processes, these studies have reported conflicting and sometimes opposite roles for Eph:ephrin signaling in neurogenesis. Herein, we review these studies and we discuss existing mathematical models of stem cell dynamics and neurogenesis that provide a coherent framework and may help reconcile conflicting results.

Modeling local and cross-species neuron number variations in the cerebral cortex as arising from a common mechanism

A massive increase in the number of neurons in the cerebral cortex, driving its size to increase by five orders of magnitude, is a key feature of mammalian evolution. Not only are there systematic variations in cerebral cortical architecture across species, but also across spatial axes within a given cortex. In this article we present a computational model that accounts for both types of variation as arising from the same developmental mechanism. The model employs empirically measured parameters from over a dozen species to demonstrate that changes to the kinetics of neurogenesis (the cell-cycle rate, the progenitor death rate, and the "quit rate," i.e., the ratio of terminal cell divisions) are sufficient to explain the great diversity in the number of cortical neurons across mammals. Moreover, spatiotemporal gradients in those same parameters in the embryonic cortex can account for cortex-wide, graded variations in the mature neural architecture. Consistent with emerging anatomical data in several species, the model predicts (i) a greater complement of neurons per cortical column in the later-developing, posterior regions of intermediate and large cortices, (ii) that the extent of variation across a cortex increases with cortex size, reaching fivefold or greater in primates, and (iii) that when the number of neurons per cortical column increases, whether across species or within a given cortex, it is the later-developing superficial layers of the cortex which accommodate those additional neurons. We posit that these graded features of the cortex have computational and functional significance, and so must be subject to evolutionary selection.

Evo-devo and the primate isocortex: the central organizing role of intrinsic gradients of neurogenesis

Spatial gradients in the initiation and termination of basic processes, such as cytogenesis, cell-type specification and dendritic maturation, are ubiquitous in developing nervous systems. Such gradients can produce a niche adaptation in a particular species. For example, the high density of photoreceptors and neurons in the 'area centralis' of some vertebrate retinas result from the early maturation of its center relative to its periphery. Across species, regularities in allometric scaling of brain regions can derive from conserved spatial gradients: longer neurogenesis in the alar versus the basal plate of the neural tube is associated with relatively greater expansion of alar plate derivatives in larger brains. We describe gradients of neurogenesis within the isocortex and their effects on adult cytoarchitecture within and across species. Longer duration of neurogenesis in the caudal isocortex is associated with increased neuron number and density per column relative to the rostral isocortex. Later-maturing features of single neurons, such as soma size and dendritic spine numbers reflect this gradient. Considering rodents and primates, the longer the duration of isocortical neurogenesis in each species, the greater the rostral-to-caudal difference in neuron number and density per column. Extended developmental duration produces substantial, predictable changes in the architecture of the isocortex in larger brains, and presumably a progressively changed functional organization, the properties of which we do not yet fully understand. Many features of isocortical architecture previously viewed as species- or niche-specific adaptations can now be integrated as the natural outcomes of spatiotemporal gradients that are deployed in larger brains.

Spatial distribution and cellular composition of adult brain proliferative zones in the teleost, Gymnotus omarorum

Proliferation of stem/progenitor cells during development provides for the generation of mature cell types in the CNS. While adult brain proliferation is highly restricted in the mammals, it is widespread in teleosts. The extent of adult neural proliferation in the weakly electric fish, Gymnotus omarorum has not yet been described. To address this, we used double thymidine analog pulse-chase labeling of proliferating cells to identify brain proliferation zones, characterize their cellular composition, and analyze the fate of newborn cells in adult G. omarorum. Short thymidine analog chase periods revealed the ubiquitous distribution of adult brain proliferation, similar to other teleosts, particularly Apteronotus leptorhynchus. Proliferating cells were abundant at the ventricular-subventricular lining of the ventricular-cisternal system, adjacent to the telencephalic subpallium, the diencephalic preoptic region and hypothalamus, and the mesencephalic tectum opticum and torus semicircularis. Extraventricular proliferation zones, located distant from the ventricular-cisternal system surface, were found in all divisions of the rombencephalic cerebellum. We also report a new adult proliferation zone at the caudal-lateral border of the electrosensory lateral line lobe. All proliferation zones showed a heterogeneous cellular composition. The use of short (24 h) and long (30 day) chase periods revealed abundant fast cycling cells (potentially intermediate amplifiers), sparse slow cycling (potentially stem) cells, cells that appear to have entered a quiescent state, and cells that might correspond to migrating newborn neural cells. Their abundance and migration distance differed among proliferation zones: greater numbers and longer range and/or pace of migrating cells were associated with subpallial and cerebellar proliferation zones.

Growth, homeostatic regulation and stem cell dynamics in tissues

The regulation of cell growth in animal tissues is a question of critical importance: most tissues contain different types of cells in interconversion and the fraction of each type has to be controlled in a precise way, by mechanisms that remain unclear. Here, we provide a theoretical framework for the homeostasis of stem-cell-containing epithelial tissues using mechanical equations, which describe the size of the tissue and kinetic equations, which describe the interconversions of the cell populations. We show that several features, such as the evolution of stem cell fractions during intestinal development, the shape of a developing intestinal wall, as well as the increase in the proliferative compartment in cancer initiation, can be studied and understood from generic modelling which does not rely on a particular regulatory mechanism. Finally, inspired by recent experiments, we propose a model where cell division rates are regulated by the mechanical stresses in the epithelial sheet. We show that pressure-controlled growth can, in addition to the previous features, also explain with few parameters the formation of stem cell compartments as well as the morphologies observed when a colonic crypt becomes cancerous. We also discuss optimal strategies of wound healing, in connection with experiments on the cornea.

Nucleoside analog labeling of neural stem cells and their progeny

Nucleoside analog pulse labeling is an important technique which can assess the birthdate, cell cycle maintenance, or cycling rates of cells during development. This method has evolved over several decades of use and is now applied to a multitude of tissue subtypes and systems. The methodology in this chapter covers the classic uses for analog pulse labeling as well as their use in conjunction with the newly characterized technique of in utero electroporation (IUE).

Proliferation and cell cycle dynamics in the developing stellate ganglion

Cell proliferation during nervous system development is poorly understood outside the mouse neocortex. We measured cell cycle dynamics in the embryonic mouse sympathetic stellate ganglion, where neuroblasts continue to proliferate following neuronal differentiation. At embryonic day (E) 9.5, when neural crest-derived cells were migrating and coalescing into the ganglion primordium, all cells were cycling, cell cycle length was only 10.6 h, and S-phase comprised over 65% of the cell cycle; these values are similar to those previously reported for embryonic stem cells. At E10.5, Sox10(+) cells lengthened their cell cycle to 38 h and reduced the length of S-phase. As cells started to express the neuronal markers Tuj1 and tyrosine hydroxylase (TH) at E10.5, they exited the cell cycle. At E11.5, when >80% of cells in the ganglion were Tuj1(+)/TH(+) neuroblasts, all cells were again cycling. Neuroblast cell cycle length did not change significantly after E11.5, and 98% of Sox10(-)/TH(+) cells had exited the cell cycle by E18.5. The cell cycle length of Sox10(+)/TH(-) cells increased during late embryonic development, and ∼25% were still cycling at E18.5. Loss of Ret increased neuroblast cell cycle length at E16.5 and decreased the number of neuroblasts at E18.5. A mathematical model generated from our data successfully predicted the relative change in proportions of neuroblasts and non-neuroblasts in wild-type mice. Our results show that, like other neurons, sympathetic neuron differentiation is associated with exit from the cell cycle; sympathetic neurons are unusual in that they then re-enter the cell cycle before later permanently exiting.

Cell cycle and lineage progression of neural progenitors in the ventricular-subventricular zones of adult mice

Proliferating neural stem cells and intermediate progenitors persist in the ventricular-subventricular zone (V-SVZ) of the adult mammalian brain. This extensive germinal layer in the walls of the lateral ventricles is the site of birth of different types of interneurons destined for the olfactory bulb. The cell cycle dynamics of stem cells (B1 cells), intermediate progenitors (C cells), and neuroblasts (A cells) in the V-SVZ and the number of times these cells divide remain unknown. Using whole mounts of the walls of the lateral ventricles of adult mice and three cell cycle analysis methods using thymidine analogs, we determined the proliferation dynamics of B1, C, and A cells in vivo. Achaete-scute complex homolog (Ascl)1(+) C cells were heterogeneous with a cell cycle length (T(C)) of 18-25 h and a long S phase length (T(S)) of 14-17 h. After C cells, Doublecortin(+) A cells were the second-most common dividing cell type in the V-SVZ and had a T(C) of 18 h and T(S) of 9 h. Human glial fibrillary acidic protein (hGFAP)::GFP(+) B1 cells had a surprisingly short Tc of 17-18 h and a T(S) of 4 h. Progenitor population analysis suggests that following the initial division of B1 cells, C cells divide three times and A cells once, possibly twice. These data provide essential information on the dynamics of adult progenitor cell proliferation in the V-SVZ and how large numbers of new neurons continue to be produced in the adult mammalian brain.

Scratch2 modulates neurogenesis and cell migration through antagonism of bHLH proteins in the developing neocortex

Scratch genes (Scrt) are neural-specific zinc-finger transcription factors (TFs) with an unknown function in the developing brain. Here, we show that, in addition to the reported expression of mammalian Scrt2 in postmitotic differentiating and mature neurons in the developing and early postnatal brain, Scrt2 is also localized in subsets of mitotic and neurogenic radial glial (RGP) and intermediate (IP) progenitors, as well as in their descendants-postmitotic IPs and differentiating neurons at the border subventricular/intermediate zone. Conditional activation of transgenic Scrt2 in cortical progenitors in mice promotes neuronal differentiation by favoring the direct mode of neurogenesis of RGPs at the onset of neurogenesis, at the expense of IP generation. Neuronal amplification via indirect IP neurogenesis is thereby extenuated, leading to a mild postnatal reduction of cortical thickness. Forced in vivo overexpression of Scrt2 suppressed the generation of IPs from RGPs and caused a delay in the radial migration of upper layer neurons toward the cortical plate. Mechanistically, our results indicate that Scrt2 negatively regulates the transcriptional activation of the basic helix loop helix TFs Ngn2/NeuroD1 on E-box containing common target genes, including Rnd2, a well-known major effector for migrational defects in developing cortex. Altogether, these findings reveal a modulatory role of Scrt2 protein in cortical neurogenesis and neuronal migration.

Optimality in the development of intestinal crypts

Intestinal crypts in mammals are comprised of long-lived stem cells and shorter-lived progenies. These two populations are maintained in specific proportions during adult life. Here, we investigate the design principles governing the dynamics of these proportions during crypt morphogenesis. Using optimal control theory, we show that a proliferation strategy known as a "bang-bang" control minimizes the time to obtain a mature crypt. This strategy consists of a surge of symmetric stem cell divisions, establishing the entire stem cell pool first, followed by a sharp transition to strictly asymmetric stem cell divisions, producing nonstem cells with a delay. We validate these predictions using lineage tracing and single-molecule fluorescence in situ hybridization of intestinal crypts in infant mice, uncovering small crypts that are entirely composed of Lgr5-labeled stem cells, which become a minority as crypts continue to grow. Our approach can be used to uncover similar design principles in other developmental systems.

Pattern, growth, and control

Systems biology seeks not only to discover the machinery of life but to understand how such machinery is used for control, i.e., for regulation that achieves or maintains a desired, useful end. This sort of goal-directed, engineering-centered approach also has deep historical roots in developmental biology. Not surprisingly, developmental biology is currently enjoying an influx of ideas and methods from systems biology. This Review highlights current efforts to elucidate design principles underlying the engineering objectives of robustness, precision, and scaling as they relate to the developmental control of growth and pattern formation. Examples from vertebrate and invertebrate development are used to illustrate general lessons, including the value of integral feedback in achieving set-point control; the usefulness of self-organizing behavior; the importance of recognizing and appropriately handling noise; and the absence of "free lunch." By illuminating such principles, systems biology is helping to create a functional framework within which to make sense of the mechanistic complexity of organismal development.

New production of eosinophils and the corresponding TH1/TH2 balance in the lungs after allergen exposure in BALB/c and C57BL/6 mice

Allergic asthma is associated with eosinophilic inflammation in the airways. Animal models commonly used to elucidate allergic inflammation mechanisms include BALB/c and C57BL/6 mice. Our aim was to evaluate lung eosinophilia and the corresponding Th1/Th2 balance in the two strains after allergen exposure. BALB/c and C57BL/6 mice were subjected to ovalbumin-induced allergic airway inflammation using BrdU to label newly produced cells. The numbers of new eosinophils were evaluated by differential cell count and immunocytochemistry (MBP+BrdU+). Proliferation rate of lung eosinophils was measured by analysis of CD45+CCR3+BrdU+ cells by FACS. Distribution of newly produced eosinophils in the lung and the Th1/Th2 (CD4+T-bet+/CD4+GATA-3+) balance was evaluated by immunohistochemistry. Allergen challenge with ovalbumin induced comparable eosinophilia in bone marrow (BM), blood and lung tissue in both strains of mice compared to phosphate-buffered saline controls, which was confirmed by immunocytochemistry. There was a small increase in the number of lung MBP+BrdU(-) eosinophils in C57BL/6 mice compared to BALB/c mice, which suggests a basal increase in this strain following sensitization. While there was no difference in eosinophilic proliferation in the lung, the distribution of the newly produced eosinophils differs between the two strains. BALB/c mice showed staining primarily around vessels and airways, whereas C57BL/6 mice showed a more even distribution in the lung tissue. No difference in the Th1/Th2 balance was observed between two strains. This study shows that there is a difference in the distribution of eosinophils in the lung between the C57BL/6 and BALB/c mice, but no difference in eosinophil production or Th1/Th2 balance.

Cell lineages and the logic of proliferative control

It is widely accepted that the growth and regeneration of tissues and organs is tightly controlled. Although experimental studies are beginning to reveal molecular mechanisms underlying such control, there is still very little known about the control strategies themselves. Here, we consider how secreted negative feedback factors ("chalones") may be used to control the output of multistage cell lineages, as exemplified by the actions of GDF11 and activin in a self-renewing neural tissue, the mammalian olfactory epithelium (OE). We begin by specifying performance objectives-what, precisely, is being controlled, and to what degree-and go on to calculate how well different types of feedback configurations, feedback sensitivities, and tissue architectures achieve control. Ultimately, we show that many features of the OE-the number of feedback loops, the cellular processes targeted by feedback, even the location of progenitor cells within the tissue-fit with expectations for the best possible control. In so doing, we also show that certain distinctions that are commonly drawn among cells and molecules-such as whether a cell is a stem cell or transit-amplifying cell, or whether a molecule is a growth inhibitor or stimulator-may be the consequences of control, and not a reflection of intrinsic differences in cellular or molecular character.

Navigating neocortical neurogenesis and neuronal specification: a positional information system encoded by neurogenetic gradients

The projection neurons of the neocortex are produced in the pseudostratified ventricular epithelium (PVE) lining the embryonic lateral ventricles. Over a 7 d period in mouse, these neurons arise in an overlapping layer VI-to-II sequence and in an anterolateral to posteromedial gradient [the transverse neurogenetic gradient (TNG)]. At any time in the 7 d neurogenetic interval, a given PVE cell must know what class of precursor cell or neuron to form next. How this information is encoded in the PVE is not known. With comparative experiments in wild-type and double-transgenic mice, overexpressing the cell cycle inhibitor p27(Kip1), we show that a gradient of expression of Lhx2 (inferred from its mRNA levels), a LIM homeodomain transcription factor, together with a gradient in duration of the G1 phase of the cell cycle (T(G1)), are sufficient to specify a positional mapping system that informs the PVE cell what class of neuron to produce next. Lhx2 likely is representative of an entire class of transcription factors expressed along the TNG. This mapping system consisting of a combination of signals from two different sources is a novel perspective on the source of positional information for neuronal specification in the developing CNS.

Nuclear migration during retinal development

In this review we focus on the mechanisms, regulation, and cellular consequences of nuclear migration in the developing retina. In the nervous system, nuclear migration is prominent during both proliferative and post-mitotic phases of development. Interkinetic nuclear migration is the process where the nucleus oscillates from the apical to basal surfaces in proliferative neuroepithelia. Proliferative nuclear movement occurs in step with the cell cycle, with M-phase being confined to the apical surface and G1-, S-, and G2-phases occurring at more basal locations. Later, following cell cycle exit, some neuron precursors migrate by nuclear translocation. In this mode of cellular migration, nuclear movement is the driving force for motility. Following discussion of the key components and important regulators for each of these processes, we present an emerging model where interkinetic nuclear migration functions to distinguish cell fates among retinal neuroepithelia.

The disarrayed mutation results in cell cycle and neurogenesis defects during retinal development in zebrafish

BACKGROUND

The vertebrate retina is derived from proliferative neuroepithelial cells of the optic cup. During retinal development, cell proliferation and the processes of cell cycle exit and neurogenesis are coordinated in neuroepithelial progenitor cells. Previous studies have demonstrated reciprocal influences between the cell cycle and neurogenesis. However the specific mechanisms and exact relationships of cell cycle regulation and neurogenesis in the vertebrate retina remain largely unknown.

RESULTS

We have isolated and characterized a zebrafish mutant, disarrayed (drya64), which exhibits retinal defects in cell cycle regulation and neurogenesis. By 42 hours post fertilization, disarrayed mutants show small eyes and a reduced forebrain. Other aspects of development appear normal. Although retinogenesis is delayed, mutant retinal cells eventually differentiate to all major cell types. Examination of the disarrayed mitotic cycle using BrdU and direct imaging techniques revealed that retinal neuroepithelial cells have an extended cell cycle period and reduced rate of cell cycle exit and neurogenesis, despite the fact that neurogenesis initiates at the appropriate time of development. Genetic mosaic analyses indicate that the cell cycle phenotype of disarrayed is cell-non-autonomous.

CONCLUSION

The disarrayed mutant shows defects in both cell cycle regulation and neurogenesis and provides insights into the coordinated regulation of these processes during retinal development.

Sex differences in adult cell proliferation within the zebrafish (Danio rerio) cerebellum

It has been reported that neurons generated in the adult brain show sex-specific differences in several brain regions of lower vertebrates and mammals. The present study questioned whether cell proliferation and survival in the adult zebrafish (Danio rerio) cerebellum, the most mitotically active area of adult teleost brain, is sexually differentiated. Adult zebrafish were treated with the thymidine analogue 5'-bromo-2'-deoxyuridine (BrdU) and allowed to survive for 24 h (short-term) and for 21 days (long-term). BrdU immunohistochemistry allowed visualization of cells incorporating BrdU at the S phase of mitosis. At short-term survival, male zebrafish had a higher number of labelled cells at proliferation sites of the molecular layer of corpus cerebelli (CCe) and the granular layer of the caudal lobe of the cerebellum (LCa) than did females. In long-term survival, BrdU-positive cells were found at their final destination, but only the granular layer of the medial division of the valvula cerebelli showed sex-specific differences in the number of labelled cells. This higher mitotic activity in male cerebellum might be related to sex-specific motor behaviour observed in male zebrafish. To investigate the role of programmed cell death, the terminal deoxynucleotidyl-mediated dUTP nick-end-labelling (TUNEL) method was applied. The vast majority of apoptotic figures occurred in the granular cell layer of valvula and CCe, only in a few cases within the BrdU-retaining cells. Apoptosis was found specifically at the sites of the final destination of proliferating cells, indicating that the close relation of cell birth and death might represent a possible plasticity mechanism in the adult zebrafish cerebellum.

Computational models of neocortical neuronogenesis and programmed cell death in the developing mouse, monkey, and human

This paper presents a computational model allowing quantitative simulations of acquisition of neocortical neuronal number across mammalian species. When extrapolating scientific findings from rodents to humans, it is particularly pertinent to acknowledge the importance of the accelerated enlargement of the neocortex during human evolution. Neocortex development is marked by discrete stages of neural progenitor cell proliferation and death, neuronal differentiation, and neuronal programmed cell death. We have developed computational models of human and rhesus monkey neocortical neuronal cell acquisition based on experimentally derived parameters of cell cycle length, commitment to cell cycle exit, and cell death. Our model results agree with independent stereological studies estimating neocortical neuron number in adult and developing rhesus monkey and human. Comparisons of our primate models with previously developed rodent models suggest correlations between the lengthening of the duration of the neuronogenesis period and a lengthening of the cellular processes of cell cycle progression and death can account for the vast increase in size of the primate neocortex. Furthermore, when compared with rodents, we predict that cell death may play a larger role in shaping the primate neocortex. Our mathematical models of the development and evolution of the neocortex provide a quantitative, biologically based construct for extrapolation between rodent and humans. These models can assist in focusing future experimental research on the differing mechanisms of rodent versus human neocortical development.

Simulated morphogenesis of developmental folds due to proliferative pressure

We present a simulation that models individual cells as spherical particles that can migrate, interact, divide and differentiate. We simulate the evolution of a progenitor layer of cells that reproduce, leading either to more progenitors or to differentiated daughters. We find that this simplified model produces spontaneous folds whose lengths depend linearly on the ratio of rates of production of progenitors to differentiated daughters. We also find that folds grow approximately exponentially in time, and that larger folds can be placed via patterning events that perturb the positions of selected progenitor cells early in the developmental process.

Individually dosed oral drug administration to socially-living transponder-tagged mice by a water dispenser under RFID control

The sensitivity of behavioral and physiological parameters to even mildly stressful experiences such as drug injections creates a need for alternative methods. We have established a method of stress-free administration of drugs via drinking water that allows multiple, individually specific and exact dosages, even for socially-housed animals. The drug solution is supplied by a dispenser with automated volume control. Animals are PIT microchip-tagged with RFID transponders and identified in realtime at the water port. Computer control permits preprogramming of individual reward quantities so that drug administration is terminated after an individual has collected its daily dose. For our experiments, the substance 5-bromo-2'-deoxyuridine (BrdU) was given as a marker of proliferating cells that we quantified in the hippocampus of adult mice. Experimental groups received BrdU either via intraperitoneal injections or orally via the water dispensers. Immunohistochemical staining of BrdU-positive cells was of the same quality after oral administration as after injection. BrdU-positive cells did not differ statistically in cell numbers. Thus, water dispensers under transponder control allow the individual and stress-free application of drugs even to group-living animals without disturbing their behavior. This is useful where a complex temporal protocol of application is required, and for phenotyping experiments combining behavioral tests with neural, cellular or molecular analyses.

Early post-hatching sex differences in cell proliferation and survival in the quail telencephalic ventricular zone and intermediate medial mesopallium

Previous studies indicated that avian telencephalic areas related to learned behavior, such as song perception and production, are sexually dimorphic. Our study focused on the eventual occurrence of dimorphism in the intermediate medial mesopallium, an area associated with learning in non-singing birds. During early post-hatching life (days 1 and 5) cell proliferation and survival of newborn cells were studied by means of 5-bromo-2-deoxy-uridine immunocytochemistry. Programmed cell death (apoptosis) was investigated at post-hatching day 10. The ventricular zone, intermediate medial part of mesopallium and lateral septal area was analyzed using stereological methods for cell counts. Short-term experiments revealed significantly higher numbers of newborn cells in male ventricular zone of mesopallium compared to female at post-hatching day 1. Long-term survival until post-hatching day 20 showed significantly higher numbers of labeled cells in the male compared to female intermediate medial part of mesopallium, which is the final destination of migrating cells born in the overlying ventricular zone. The vast majority of these early post-hatching newborn cells residing in the intermediate medial part of mesopallium expressed a neuronal phenotype. In addition to neurogenesis, higher numbers of apoptotic figures were found in the male intermediate medial part of mesopallium at post-hatching day 10, suggesting that cell death plays a role in the control of telencephalic regional cell density in males. Our findings indicate that sex-specific mechanisms possibly stimulate increased cell genesis and survival, as well as the counteracting event of increased apoptotic cell death that characterized the male intermediate medial part of mesopallium.

Transforming growth factor beta 1 promotes cell cycle exit through the cyclin-dependent kinase inhibitor p21 in the developing cerebral cortex

During cortical neurogenesis, cell proliferation and cell cycle exit are carefully regulated to ensure that the appropriate numbers of cells are produced. The antiproliferative agent transforming growth factor beta1 (TGFbeta1) and its receptors are endogenously expressed in proliferative zones of the developing cerebral cortex, thus implicating the growth factor in cell cycle regulation. The present study tested the hypothesis that TGFbeta1 promotes cell cycle exit in the cortical ventricular zone (VZ) through modulation of cell cycle protein expression, in particular cyclin D1 and the cyclin-dependent kinase inhibitors p27 and p21. Although it did not affect the length of the cell cycle, TGFbeta1 decreased the fraction of VZ-cycling cells by 21% and increased the number of VZ cells exiting the cell cycle a commensurate 24%. TGFbeta1 selectively increased the expression of p21 in the VZ. In addition, high p21 expression levels were observed in VZ cells as they exited the cell cycle, and TGFbeta1 increased the number p21-positive cells exiting the cell cycle. Collectively, these data show the following: (1) TGFbeta1 promotes cell cycle exit, (2) p21 upregulation is correlated with cell cycle exit, and (3) TGFbeta1-induced cell cycle exit is mediated by p21.

Ethanol disrupts cell cycle regulation in developing rat cortex interaction with transforming growth factor beta1

Ethanol is a potent teratogenic agent that disrupts several aspects of neuronogenesis, including the proliferation rate of cortical precursors. With regard to corticogenesis, possible targets of ethanol toxicity include soluble factors, like transforming growth factor beta1 (TGFbeta1), that regulate cortical growth and cell cycle proteins that control the kinetics of the cell cycle. The effect of ethanol on normal cell proliferation and TGFbeta1-regulated cell proliferation in the developing cortex was assessed using an organotypic slice culture model. Ethanol elongated the cell cycle, possibly through a decrease in the expression of G1 cell cycle protein cyclin D1. Further, ethanol exposure antagonized the anti-proliferative action of TGFbeta1 and blocked TGFbeta1-dependent increases in cell cycle inhibitor p21. Collectively, this evidence suggests that disruption of appropriate cell cycle protein expression and inhibition of TGFbeta1 activity are potential mechanisms underlying the effect of ethanol on cortical development.

BM88 is an early marker of proliferating precursor cells that will differentiate into the neuronal lineage

Progression of progenitor cells towards neuronal differentiation is tightly linked with cell cycle control and the switch from proliferative to neuron-generating divisions. We have previously shown that the neuronal protein BM88 drives neuroblastoma cells towards exit from the cell cycle and differentiation into a neuronal phenotype in vitro. Here, we explored the role of BM88 during neuronal birth, cell cycle exit and the initiation of differentiation in vivo. By double- and triple-labelling with the S-phase marker BrdU or the late G2 and M-phase marker cyclin B1, antibodies to BM88 and markers of the neuronal or glial cell lineages, we demonstrate that in the rodent forebrain, BM88 is expressed in multipotential progenitor cells before terminal mitosis and in their neuronal progeny during the neurogenic interval, as well as in the adult. Further, we defined at E16 a cohort of proliferative progenitors that exit S phase in synchrony, and by following their fate for 24 h we show that BM88 is associated with the dynamics of neuron-generating divisions. Expression of BM88 was also evident in cycling cortical radial glial cells, which constitute the main neurogenic population in the cerebral cortex. In agreement, BM88 expression was markedly reduced and restricted to a smaller percentage of cells in the cerebral cortex of the Small eye mutant mice, which lack functional Pax6 and exhibit severe neurogenesis defects. Our data show an interesting correlation between BM88 expression and the progression of progenitor cells towards neuronal differentiation during the neurogenic interval.

Overexpression of p27 Kip 1, probability of cell cycle exit, and laminar destination of neocortical neurons

Neocortical projection neurons arise from a pseudostratified ventricular epithelium (PVE) from embryonic day 11 (E11) to E17 in mice. The sequence of neuron origin is systematically related to mechanisms that specify neuronal class properties including laminar fate destination. Thus, the neurons to be assembled into the deeper layers are the earliest generated, while those to be assembled into superficial layers are the later generated neurons. The sequence of neuron origin also correlates with the probability of cell cycle exit (Q) and the duration of G1-phase of the cell cycle (T(G1)) in the PVE. Both Q and T(G1) increase as neuronogenesis proceeds. We test the hypothesis that mechanisms regulating specification of neuronal laminar destination, Q and T(G1) are coordinately regulated. We find that overexpression of p27(Kip1) in the PVE from E12 to E14 increases Q but not T(G1) and that the increased Q is associated with a commensurate increase in the proportion of exiting cells that is directed to superficial layers. We conclude that mechanisms that govern specification of neocortical neuronal laminar destination are coordinately regulated with mechanisms that regulate Q and are independent of mechanisms regulatory to cell cycle duration. Moreover, they operate prior to postproliferative mechanisms necessary to neocortical laminar assembly.

The role of cell death during neocortical neurogenesis and synaptogenesis: implications from a computational model for the rat and mouse

We are quantitatively evaluating the acquisition of neocortical neurons through key stages of development including neurogenesis, migration, and synaptogenesis. Here we expand upon a previous computational model describing neocortical neurogenesis in the rat and mouse [Dev. Neurosci. 24 (2002) 467], to include the period of synaptogenesis (P0-P14) when programmed cell death (PCD) is known to play a major role in shaping the neocortex. We also quantitatively evaluate differing hypotheses on the role of cell death during neurogenesis. This new model construct allows prediction of acquisition of adult neuronal number in the rat and mouse neocortex from the beginning of neurogenesis through synaptogenesis. The mathematical model output is validated by independently derived stereologically determined neuron number estimates in the adult rat and mouse. Simulations suggest cell death during synaptogenesis reduces the neocortical neuronal population by 20-30%, while cell death of progenitor cells and newly formed neurons during neurogenesis may reduce output by as much as 24%. However, higher death rates during neurogenesis as suggested by some research would deplete the progenitor population, not allowing for the vast expansion that is the hallmark of the mammalian neocortex. Furthermore, our simulations suggest the clearance time of dying neurons labeled by TUNEL or pyknosis is relatively short, between 1 and 4 h, corroborating experimental research. This novel mathematical model for adult neocortical neuronal acquisition allows for in silico analysis of normal and perturbed states of neocortical development as well as interspecies and evolutionary analyses of neocortical development.

Late granule cell genesis in quail cerebellum

Proliferation of avian cerebellar neurons, including granule cells, is thought to be completed during embryonic life, and aspects of cell addition in cerebellar lobules in posthatching life are unknown. The present study tested the hypothesis that cell genesis in late embryonic and posthatching stages of quail cerebellum occurs in parallel with the performance of motor programs. After exposure to bromodeoxyuridine, short (20 hours) and long survival time points were selected to investigate survival and migration of labeled cells. Quantitative analysis of the lobular distribution of labeled cells was performed with the stereological disector method. External granular layer (EGL) proliferation did not cease after hatching, indicating that there is an extended posthatching period, lasting until P20, when cells can be added into the internal granular layer, modifying the cerebellar circuitry and function. Indeed, long survival experiments suggested that EGL-labeled cells migrated into the internal granular layer and survived for a prolonged time, although many of the progenitor cells remained in the EGL for days. Double-labeling experiments revealed that most of the late-generated granule cells were NeuN positive, but only few expressed nitric oxide synthase. In addition to granule cells, the white matter and a glutamic acid decarboxylase (GAD)-positive cell population in the molecular layer around Purkinje somata showed bromodeoxyuridine labeling. Although all lobules showed significant posthatching proliferation, an anteroposterior gradient was evident. The index of granule cell production and survival supports a spatiotemporal pattern, in correlation with the functional division of cerebellum into anterior and posterior domains.

Taste bud cell dynamics during normal and sodium-restricted development

Taste bud volume increases over the postnatal period to match the number of neurons providing innervation. To clarify age-related changes in fungiform taste bud volume, the current study investigated developmental changes in taste bud cell number, proliferation rate, and life span. Taste bud growth can largely be accounted for by addition of cytokeratin-19-positive taste bud cells. Examination of taste bud cell kinetics with 3H-thymidine autoradiography revealed that cell life span and turnover periods were not altered during normal development but that cells were produced more rapidly in young rats, a prominent modification that could lead to increased taste bud size. By comparison, dietary sodium restriction instituted during pre- and postnatal development results in small taste buds at adulthood as a result of fewer cytokeratin-19-positive cells. The dietary manipulation also had profound influences on taste bud growth kinetics, including an increased latency for cells to enter the taste bud and longer life span and turnover periods. These studies provide fundamental, new information about taste bud development under normal conditions and after environmental manipulations that impact nerve/target matching.

Asymmetric production of surface-dividing and non-surface-dividing cortical progenitor cells

Mature neocortical layers all derive from the cortical plate (CP), a transient zone in the dorsal telencephalon into which young neurons are continuously delivered. To understand cytogenetic and histogenetic events that trigger the emergence of the CP, we have used a slice culture technique. Most divisions at the ventricular surface generated paired cycling daughters (P/P divisions) and the majority of the P/P divisions were asymmetric in daughter cell behavior; they frequently sent one daughter cell to a non-surface (NS) position, the subventricular zone (SVZ), within a single cell-cycle length while keeping the other mitotic daughter for division at the surface. The NS-dividing cells were mostly Hu+ and their daughters were also Hu+, suggesting their commitment to the neuronal lineage and supply of early neurons at a position much closer to their destiny than from the ventricular surface. The release of a cycling daughter cell to SVZ was achieved by collapse of the ventricular process of the cell, followed by its NS division. Neurogenin2 (Ngn2) was immunohistochemically detected in a certain cycling population during G1 phase and was further restricted during G2-M phases to the SVZ-directed population. Its retroviral introduction converted surface divisions to NS divisions. The asymmetric P/P division may therefore contribute to efficient neuron/progenitor segregation required for CP initiation through cell cycle-dependent and lineage-restricted expression of Ngn2.