The germinal matrices in the developing dentate gyrus are composed of neuronal progenitors at distinct differentiation stages

Which neurodevelopmental processes continue in humans after birth?

Once we are born, the number and location of nerve cells in most parts of the brain remain unchanged. These types of structural changes are therefore a significant form of flexibility for the neural circuits where they occur. In humans, the postnatal birth of neurons is limited; however, neurons do continue to migrate into some brain regions throughout infancy and even into adolescence. In human infants, multiple migratory pathways deliver interneurons to destinations across the frontal and temporal lobe cortex. Shorter-range migration of excitatory neurons also appears to continue during adolescence, particularly near the amygdala paralaminar nucleus, a region that follows a delayed trajectory of growth from infancy to adulthood. The significance of the timing for when different brain regions recruit new neurons through these methods is unknown; however, both processes of protracted migration and maturation are prominent in humans. Mechanisms like these that reconfigure neuronal circuits are a substrate for critical periods of plasticity and could contribute to distinctive circuit functionality in human brains.

Intraventricular haemorrhage in premature infants: the role of immature neuronal salt and water transport

Intraventricular haemorrhage is a common complication of premature birth. Survivors are often left with cerebral palsy, intellectual disability and/or hydrocephalus. Animal models suggest that brain tissue shrinkage, with subsequent vascular stretch and tear, is an important step in the pathophysiology, but the cause of this shrinkage is unknown. Clinical risk factors for intraventricular haemorrhage are biomarkers of hypoxic-ischaemic stress, which causes mature neurons to swell. However, immature neuronal volume might shift in the opposite direction in these conditions. This is because immature neurons express the chloride, salt and water transporter NKCC1, which subserves regulatory volume increases in non-neural cells, whereas mature neurons express KCC2, which subserves regulatory volume decreases. When hypoxic-ischaemic conditions reduce active ion transport and increase the cytoplasmic membrane permeability, the effects of these transporters are diminished. Consequentially, mature neurons swell (cytotoxic oedema), whereas immature neurons might shrink. After hypoxic-ischaemic stress, in vivo and in vitro multi-photon imaging of perinatal transgenic mice demonstrated shrinkage of viable immature neurons, bulk tissue shrinkage and blood vessel displacement. Neuronal shrinkage was correlated with age-dependent membrane salt and water transporter expression using immunohistochemistry. Shrinkage of immature neurons was prevented by prior genetic or pharmacological inhibition of NKCC1 transport. These findings open new avenues of investigation for the detection of acute brain injury by neuroimaging, in addition to prevention of neuronal shrinkage and the ensuing intraventricular haemorrhage, in premature infants.

Generation of adult hippocampal neural stem cells occurs in the early postnatal dentate gyrus and depends on cyclin D2

Lifelong hippocampal neurogenesis is maintained by a pool of multipotent adult neural stem cells (aNSCs) residing in the subgranular zone of the dentate gyrus (DG). The mechanisms guiding transition of NSCs from the developmental to the adult state remain unclear. We show here, by using nestin-based reporter mice deficient for cyclin D2, that the aNSC pool is established through cyclin D2-dependent proliferation during the first two weeks of life. The absence of cyclin D2 does not affect normal development of the dentate gyrus until birth but prevents postnatal formation of radial glia-like aNSCs. Furthermore, retroviral fate mapping reveals that aNSCs are born on-site from precursors located in the dentate gyrus shortly after birth. Taken together, our data identify the critical time window and the spatial location of the precursor divisions that generate the persistent population of aNSCs and demonstrate the central role of cyclin D2 in this process.

Endogenous neural stem cells characterization using omics approaches: Current knowledge in health and disease

Neural stem cells (NSCs), an invaluable source of neuronal and glial progeny, have been widely interrogated in the last twenty years, mainly to understand their therapeutic potential. Most of the studies were performed with cells derived from pluripotent stem cells of either rodents or humans, and have mainly focused on their potential in regenerative medicine. High-throughput omics technologies, such as transcriptomics, epigenetics, proteomics, and metabolomics, which exploded in the past decade, represent a powerful tool to investigate the molecular mechanisms characterizing the heterogeneity of endogenous NSCs. The transition from bulk studies to single cell approaches brought significant insights by revealing complex system phenotypes, from the molecular to the organism level. Here, we will discuss the current literature that has been greatly enriched in the “omics era”, successfully exploring the nature and function of endogenous NSCs and the process of neurogenesis. Overall, the information obtained from omics studies of endogenous NSCs provides a sharper picture of NSCs function during neurodevelopment in healthy and in perturbed environments.

Fine-tuning of mTOR signaling by the UBE4B-KLHL22 E3 ubiquitin ligase cascade in brain development

Spatiotemporal regulation of the mechanistic target of rapamycin (mTOR) pathway is pivotal for establishment of brain architecture. Dysregulation of mTOR signaling is associated with a variety of neurodevelopmental disorders. Here, we demonstrate that the UBE4B-KLHL22 E3 ubiquitin ligase cascade regulates mTOR activity in neurodevelopment. In a mouse model with UBE4B conditionally deleted in the nervous system, animals display severe growth defects, spontaneous seizures and premature death. Loss of UBE4B in the brains of mutant mice results in depletion of neural precursor cells and impairment of neurogenesis. Mechanistically, UBE4B polyubiquitylates and degrades KLHL22, an E3 ligase previously shown to degrade the GATOR1 component DEPDC5. Deletion of UBE4B causes upregulation of KLHL22 and hyperactivation of mTOR, leading to defective proliferation and differentiation of neural precursor cells. Suppression of KLHL22 expression reverses the elevated activity of mTOR caused by acute local deletion of UBE4B. Prenatal treatment with the mTOR inhibitor rapamycin rescues neurogenesis defects in Ube4b mutant mice. Taken together, these findings demonstrate that UBE4B and KLHL22 are essential for maintenance and differentiation of the precursor pool through fine-tuning of mTOR activity.

Development of subdomains in the medial pallium of Xenopus laevis and Trachemys scripta: Insights into the anamniote-amniote transition

In all vertebrates, the most dorsal region of the telencephalon gives rise to the pallium, which in turn, is formed by at least four evolutionarily conserved histogenetic domains. Particularly in mammals, the medial pallium generates the hippocampal formation. Although this region is structurally different among amniotes, its functions, attributed to spatial memory and social behavior, as well as the specification of the histogenetic domain, appears to be conserved. Thus, the aim of the present study was to analyze this region by comparative analysis of the expression patterns of conserved markers in two vertebrate models: one anamniote, the amphibian Xenopus laevis; and the other amniote, the turtle Trachemys scripta elegans, during development and in adulthood. Our results show that, the histogenetic specification of both models is comparable, despite significant cytoarchitectonic differences, in particular the layered cortical arrangement present in the turtle, not found in anurans. Two subdivisions were observed in the medial pallium of these species: a Prox1 + and another Er81/Lmo4 +, comparable to the dentate gyrus and the mammalian cornu ammonis region, respectively. The expression pattern of additional markers supports this subdivision, which together with its functional involvement in spatial memory tasks, provides evidence supporting the existence of a basic program in the specification and functionality of the medial pallium at the base of tetrapods. These results further suggest that the anatomical differences found in different vertebrates may be due to divergences and adaptations during evolution.

Dentate gyrus morphogenesis is regulated by β-catenin function in hem-derived fimbrial glia

The dentate gyrus, a gateway for input to the hippocampal formation, arises from progenitors in the medial telencephalic neuroepithelium adjacent to the cortical hem. Dentate progenitors navigate a complex migratory path guided by two cell populations that arise from the hem, the fimbrial glia and Cajal-Retzius (CR) cells. As the hem expresses multiple Wnt genes, we examined whether β-catenin, which mediates canonical Wnt signaling and also participates in cell adhesion, is necessary for the development of hem-derived lineages. We report that, in mice, the fimbrial glial scaffold is disorganized and CR cells are mispositioned upon hem-specific disruption of β-catenin. Consequently, the dentate migratory stream is severely affected, and the dentate gyrus fails to form. Using selective Cre drivers, we further determined that β-catenin function is required in the fimbrial glial scaffold, but not in the CR cells, for guiding the dentate migration. Our findings highlight a primary requirement for β-catenin for the organization of the fimbrial scaffold and a secondary role for this factor in dentate gyrus morphogenesis.

Thirty Years' History since the Discovery of Pax6: From Central Nervous System Development to Neurodevelopmental Disorders

Pax6 is a sequence-specific DNA binding transcription factor that positively and negatively regulates transcription and is expressed in multiple cell types in the developing and adult central nervous system (CNS). As indicated by the morphological and functional abnormalities in spontaneous Pax6 mutant rodents, Pax6 plays pivotal roles in various biological processes in the CNS. At the initial stage of CNS development, Pax6 is responsible for brain patterning along the anteroposterior and dorsoventral axes of the telencephalon. Regarding the anteroposterior axis, Pax6 is expressed inversely to Emx2 and Coup-TF1, and Pax6 mutant mice exhibit a rostral shift, resulting in an alteration of the size of certain cortical areas. Pax6 and its downstream genes play important roles in balancing the proliferation and differentiation of neural stem cells. The Pax6 gene was originally identified in mice and humans 30 years ago via genetic analyses of the eye phenotypes. The human PAX6 gene was discovered in patients who suffer from WAGR syndrome (i.e., Wilms tumor, aniridia, genital ridge defects, mental retardation). Mutations of the human PAX6 gene have also been reported to be associated with autism spectrum disorder (ASD) and intellectual disability. Rodents that lack the Pax6 gene exhibit diverse neural phenotypes, which might lead to a better understanding of human pathology and neurodevelopmental disorders. This review describes the expression and function of Pax6 during brain development, and their implications for neuropathology.

Early Life Events and Maturation of the Dentate Gyrus: Implications for Neurons and Glial Cells

The dentate gyrus (DG), an important part of the hippocampus, plays a significant role in learning, memory, and emotional behavior. Factors potentially influencing normal development of neurons and glial cells in the DG during its maturation can exert long-lasting effects on brain functions. Early life stress may modify maturation of the DG and induce lifelong alterations in its structure and functioning, underlying brain pathologies in adults. In this paper, maturation of neurons and glial cells (microglia and astrocytes) and the effects of early life events on maturation processes in the DG have been comprehensively reviewed. Early postnatal interventions affecting the DG eventually result in an altered number of granule neurons in the DG, ectopic location of neurons and changes in adult neurogenesis. Adverse events in early life provoke proinflammatory changes in hippocampal glia at cellular and molecular levels immediately after stress exposure. Later, the cellular changes may disappear, though alterations in gene expression pattern persist. Additional stressful events later in life contribute to manifestation of glial changes and behavioral deficits. Alterations in the maturation of neuronal and glial cells induced by early life stress are interdependent and influence the development of neural nets, thus predisposing the brain to the development of cognitive and psychiatric disorders.

Genoarchitectonic Compartmentalization of the Embryonic Telencephalon: Insights From the Domestic Cat

The telencephalon develops from the alar plate of the secondary prosencephalon and is subdivided into two distinct divisions, the pallium, which derives solely from prosomere hp1, and the subpallium which derives from both hp1 and hp2 prosomeres. In this first systematic analysis of the feline telencephalon genoarchitecture, we apply the prosomeric model to compare the expression of a battery of genes, including Tbr1, Tbr2, Pax6, Mash1, Dlx2, Nkx2-1, Lhx6, Lhx7, Lhx2, and Emx1, the orthologs of which alone or in combination, demarcate molecularly distinct territories in other species. We characterize, within the pallium and the subpallium, domains and subdomains topologically equivalent to those previously described in other vertebrate species and we show that the overall genoarchitectural map of the E26/27 feline brain is highly similar to that of the E13.5/E14 mouse. In addition, using the same approach at the earlier (E22/23 and E24/25) or later (E28/29 and E34/35) stages we further analyze neurogenesis, define the timing and duration of several developmental events, and compare our data with those from similar mouse studies; our results point to a complex pattern of heterochronies and show that, compared with the mouse, developmental events in the feline telencephalon span over extended periods suggesting that cats may provide a useful animal model to study brain patterning in ontogenesis and evolution.

Disruption of postnatal neurogenesis and adult-stage suppression of synaptic plasticity in the hippocampal dentate gyrus after developmental exposure to sterigmatocystin in rats

Exposure to sterigmatocystin (STC) raises concerns on developmental neurological disorders. The present study investigated the effects of maternal oral STC exposure on postnatal hippocampal neurogenesis of offspring in rats. Dams were exposed to STC (1.7, 5.0, and 15.0 ppm in diet) from gestational day 6 until day 21 post-delivery (weaning), and offspring were maintained without STC exposure until adulthood on postnatal day (PND) 77, in accordance with OECD chemical testing guideline Test No. 426. On PND 21, 15.0-ppm STC decreased type-3 neural progenitor cell numbers in the subgranular zone (SGZ) due to suppressed proliferation. Increased γ-H2AX-immunoreactive (+) cell numbers in the SGZ and Ercc1 upregulation and Brip1 downregulation in the dentate gyrus suggested induction of DNA double-strand breaks in SGZ cells. Upregulation of Apex1 and Ogg1 and downregulation of antioxidant genes downstream of NRF2-Keap1 signaling suggested induction of oxidative DNA damage. Increased p21WAF1/CIP1+ SGZ cell numbers and suppressed cholinergic signaling through CHRNB2-containing receptors in GABAergic interneurons suggested potential neurogenesis suppression mechanisms. Multiple mechanisms involving N-methyl-d-aspartate (NMDA) receptor-mediated glutamatergic signaling and various GABAergic interneuron subpopulations, including CHRNA7-expressing somatostatin+ interneurons activated by BDNF-TrkB signaling, may be involved in ameliorating the neurogenesis. Upregulation of Arc, Ptgs2, and genes encoding NMDA receptors and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors suggested synaptic plasticity facilitation. On PND 77, ARC+ granule cells decreased, and Nos2 was upregulated following 15.0 ppm STC exposure, suggesting oxidative stress-mediated synaptic plasticity suppression. Inverse pattern in gene expression changes in vesicular glutamate transporter isoforms, Slc17a7 and Slc17a6, from weaning might also be responsible for the synaptic plasticity suppression. The no-observed-adverse-effect level of maternal oral STC exposure for offspring neurogenesis was determined to be 5.0 ppm, translating to 0.34-0.85 mg/kg body weight/day.

Development of the mammalian cortical hem and its derivatives: the choroid plexus, Cajal-Retzius cells and hippocampus

The dorsal medial region of the developing mammalian telencephalon plays a central role in the patterning of the adjacent brain regions. This review describes the development of this specialized region of the vertebrate brain, called the cortical hem, and the formation of the various cells and structures it gives rise to, including the choroid plexus, Cajal-Retzius cells and the hippocampus. We highlight the ontogenic processes that create these different forebrain derivatives from their shared embryonic origin and discuss the key signalling pathways and molecules that influence the patterning of the cortical hem. These include BMP, Wnt, FGF and Shh signalling pathways acting with Homeobox factors to carve the medial telencephalon into district progenitor regions, which in turn give rise to the choroid plexus, dentate gyrus and hippocampus. We then link the formation of the lateral ventricle choroid plexus with embryonic and postnatal neurogenesis in the hippocampus.

Induction of cellular senescence as a late effect and BDNF-TrkB signaling-mediated ameliorating effect on disruption of hippocampal neurogenesis after developmental exposure to lead acetate in rats

Lead (Pb) exposure causes cognitive deficits in children. The present study investigated the effect of developmental exposure to Pb acetate (PbAc) on postnatal hippocampal neurogenesis. Pregnant rats were administered drinking water containing 0, 2000, or 4000 ppm PbAc from gestational day 6 until day 21 post-delivery (weaning), and offspring were maintained without PbAc exposure until adulthood on postnatal day (PND) 77. There was a dose-related accumulation of Pb in the offspring brain at weaning, while Pb was mainly excreted in adulthood. In the hippocampus, metallothionein I/II immunoreactive ⁽⁺⁾ glia were increased through adulthood as a neuroprotective response to accumulated Pb, accompanied by increased astrocyte and microglia numbers in adulthood, suggesting sustained neural damage. Gene expression changes suggested elevated oxidative stress at weaning and suppression of the antioxidant system in adulthood, as well as continued neuroinflammatory responses. At weaning, granule cell apoptosis was increased and numbers of type-3 neural progenitor cells (NPCs) were decreased. By contrast, type-2a and type-2b NPCs were increased, suggesting suppressed differentiation to type-3 NPCs. In adulthood, there were increased numbers of immature granule cells. In the hilus of the dentate gyrus, somatostatin⁺ interneurons were increased at weaning, while calbindin-D-29K⁺ interneurons were increased throughout adulthood, suggesting a strengthened interneuron regulatory system against the suppressed differentiation at weaning. In the dentate gyrus, Bdnf, Ntrk2, and Chrna7 gene expression were upregulated and numbers of hilar TrkB⁺ interneurons increased at weaning. These findings suggest activation of BDNF-TrkB signaling to increase somatostatin⁺ interneurons and promote cholinergic signaling, thus increasing later production of immature granule cells. In adulthood, Pcna and Apex1 gene expression were downregulated and Chek1 and cyclin-dependent kinase inhibitor expression were upregulated. Furthermore, there was an increase in γ-H2AX⁺ SGZ cells, suggesting induction of cellular senescence of SGZ cells due to Pb genotoxicity.

Heat-Shock Induces Granule Cell Dispersion and Microgliosis in Hippocampal Slice Cultures

Granule cell dispersion (GCD) has been found in the dentate gyrus (dg) of patients with temporal lobe epilepsy (TLE) and a history of febrile seizures but was also recently observed in pediatric patients that did not suffer from epilepsy. This indicates that GCD might not always be disease related, but instead could reflect normal morphological variation. Thus, distribution of newborn granule cells within the hilar region is part of normal dg development at early stages but could be misinterpreted as pathological GCD. In turn, pathological GCD may be caused, for example, by genetic mutations, such as the reeler mutation. GCD in the reeler mutant goes along with an increased susceptibility to epileptiform activity. Pathological GCD in combination with epilepsy is caused by experimental administration of the glutamate receptor agonist kainic acid in rodents. In consequence, the interpretation of GCD and the role of febrile seizures remain controversial. Here, we asked whether febrile temperatures alone might be sufficient to trigger GCD and used hippocampal slice cultures as in vitro model to analyze the effect of a transient temperature increase on the dg morphology. We found that a heat-shock of 41°C for 6 h was sufficient to induce GCD and degeneration of a fraction of granule cells. Both of these factors, broadening of the granule cell layer (gcl) and increased neuronal cell death within the gcl, contributed to the development of a significantly reduced packaging density of granule cells. In contrast, Reelin expressing Cajal–Retzius (CR) cells in the molecular layer were heat-shock resistant. Thus, their number was not reduced, and we did not detect degenerating CR cells after heat-shock, implying that GCD was not caused by the loss of CR cells. Importantly, the heat-shock-induced deterioration of dg morphology was accompanied by a massive microgliosis, reflecting a robust heat-shock-induced immune response. In contrast, in the study that reported on GCD as a non-specific finding in pediatric patients, no microglia reaction was observed. Thus, our findings underpin the importance of microglia as a marker to distinguish pathological GCD from normal morphological variation.

Dentate gyrus development requires a cortical hem-derived astrocytic scaffold

During embryonic development, radial glial cells give rise to neurons, then to astrocytes following the gliogenic switch. Timely regulation of the switch, operated by several transcription factors, is fundamental for allowing coordinated interactions between neurons and glia. We deleted the gene for one such factor, SOX9, early during mouse brain development and observed a significantly compromised dentate gyrus (DG). We dissected the origin of the defect, targeting embryonic Sox9 deletion to either the DG neuronal progenitor domain or the adjacent cortical hem (CH). We identified in the latter previously uncharacterized ALDH1L1+ astrocytic progenitors, which form a fimbrial-specific glial scaffold necessary for neuronal progenitor migration toward the developing DG. Our results highlight an early crucial role of SOX9 for DG development through regulation of astroglial potential acquisition in the CH. Moreover, we illustrate how formation of a local network, amidst astrocytic and neuronal progenitors originating from adjacent domains, underlays brain morphogenesis.

Neural Stem Cells and Methods for Their Generation From Induced Pluripotent Stem Cells in vitro

Neural stem cells (NSCs) provide promising approaches for investigating embryonic neurogenesis, modeling of the pathogenesis of diseases of the central nervous system, and for designing drug-screening systems. Such cells also have an application in regenerative medicine. The most convenient and acceptable source of NSCs is pluripotent stem cells (embryonic stem cells or induced pluripotent stem cells). However, there are many different protocols for the induction and differentiation of NSCs, and these result in a wide range of neural cell types. This review is intended to summarize the knowledge accumulated, to date, by workers in this field. It should be particularly useful for researchers who are beginning investigations in this area of cell biology.

Decoding the development of the human hippocampus

The hippocampus is an important part of the limbic system in the human brain that has essential roles in spatial navigation and the consolidation of information from short-term memory to long-term memory^1,2. Here we use single-cell RNA sequencing and assay for transposase-accessible chromatin using sequencing (ATAC-seq) analysis to illustrate the cell types, cell linage, molecular features and transcriptional regulation of the developing human hippocampus. Using the transcriptomes of 30,416 cells from the human hippocampus at gestational weeks 16-27, we identify 47 cell subtypes and their developmental trajectories. We also identify the migrating paths and cell lineages of PAX6⁺ and HOPX⁺ hippocampal progenitors, and regional markers of CA1, CA3 and dentate gyrus neurons. Multiomic data have uncovered transcriptional regulatory networks of the dentate gyrus marker PROX1. We also illustrate spatially specific gene expression in the developing human prefrontal cortex and hippocampus. The molecular features of the human hippocampus at gestational weeks 16-20 are similar to those of the mouse at postnatal days 0-5 and reveal gene expression differences between the two species. Transient expression of the primate-specific gene NBPF1 leads to a marked increase in PROX1⁺ cells in the mouse hippocampus. These data provides a blueprint for understanding human hippocampal development and a tool for investigating related diseases.

Developmental exposure to diacetoxyscirpenol reversibly disrupts hippocampal neurogenesis by inducing oxidative cellular injury and suppressed differentiation of granule cell lineages in mice

To investigate the developmental exposure effect of diacetoxyscirpenol (DAS) on postnatal hippocampal neurogenesis, pregnant ICR mice were provided a diet containing DAS at 0, 0.6, 2.0, or 6.0 ppm from gestational day 6 to day 21 on weaning after delivery. Offspring were maintained through postnatal day (PND) 77 without DAS exposure. On PND 21, neural stem cells (NSCs) and all subpopulations of proliferating progenitor cells were suggested to decrease in number in the subgranular zone (SGZ) at ≥ 2.0 ppm. At 6.0 ppm, increases of SGZ cells showing TUNEL⁺, metallothionein-I/II⁺, γ-H2AX⁺ or malondialdehyde⁺, and transcript downregulation of Ogg1, Parp1 and Kit without changing the level of double-stranded DNA break-related genes were observed in the dentate gyrus. This suggested induction of oxidative DNA damage of NSCs and early-stage progenitor cells, which led to their apoptosis. Cdkn2a, Rb1 and Trp53 downregulated transcripts, which suggested an increased vulnerability to DNA damage. Hilar PVALB⁺ GABAergic interneurons decreased and Grin2a and Chrna7 were downregulated, which suggested suppression of type-2-progenitor cell differentiation. On PND 77, hilar RELN⁺ interneurons increased at ≥ 2.0 ppm; at 6.0 ppm, RELN-related Itsn1 transcripts were upregulated and ARC⁺ granule cells decreased. Increased RELN signals may ameliorate the response to the decreases of NSCs and ARC-mediated synaptic plasticity. These results suggest that DAS reversibly disrupts hippocampal neurogenesis by inducing oxidative cellular injury and suppressed differentiation of granule cell lineages. The no-observed-adverse-effect level of DAS for offspring neurogenesis was determined to be 0.6 ppm (0.09-0.29 mg/kg body weight/day).

Adult Neural Stem Cells: Born to Last

The generation of new neurons is a lifelong process in many vertebrate species that provides an extra level of plasticity to several brain circuits. Frequently, neurogenesis in the adult brain is considered a continuation of earlier developmental processes as it relies in the persistence of neural stem cells, similar to radial glia, known as radial glia-like cells (RGLs). However, adult RGLs are not just leftovers of progenitors that remain in hidden niches in the brain after development has finished. Rather, they seem to be specified and set aside at specific times and places during embryonic and postnatal development. The adult RGLs present several cellular and molecular properties that differ from those observed in developmental radial glial cells such as an extended cell cycle length, acquisition of a quiescence state, a more restricted multipotency and distinct transcriptomic programs underlying those cellular processes. In this minireview, we will discuss the recent attempts to determine how, when and where are the adult RGLs specified.

Human amniotic epithelial cell transplantation promotes neurogenesis and ameliorates social deficits in BTBR mice

Background

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairments in social interactions and communication and stereotypical patterns of behaviors, interests, or activities. Even with the increased prevalence of ASD, there is no defined standard drug treatment for ASD patients. Currently, stem cells, including human amniotic epithelial cell (hAEC) transplantation, seem to be a promising treatment for ASD, but the effectiveness needs to be verified, and the mechanism has not been clarified.

Methods

We intraventricularly transplanted hAECs into a 2-month-old BTBR T+tf/J (BTBR) mouse model of ASD. Behavior tests were detected 1 month later; hippocampal neurogenesis, neuroprogenitor cell (NPC) pool, and microglia activation were analyzed with immunohistochemistry and immunofluorescence; the levels of pro-inflammatory cytokines, brain-derived neurotrophic factor (BDNF), and TrkB in the hippocampus were determined by real-time PCR or western blotting.

Results

After intraventricular injection of hAECs into adult males, social deficits in BTBR mice were significantly ameliorated. In addition, hAEC transplantation restored the decline of neurogenesis and NPCs in the hippocampus of BTBR mice by expanding the stem cell pool, and the decreased levels of BDNF and TrkB were also rescued in the hippocampus of the hAEC-injected BTBR mice. Meanwhile, the transplantation of hAECs did not induce microglial overactivation or excessive production of pro-inflammatory cytokines in the hippocampus of BTBR mice.

Conclusions

Based on these results, we found that hAEC transplantation ameliorated social deficits and promoted hippocampal neurogenesis in BTBR mice. Our study indicates a promising therapeutic option that could be applied to ASD patients in the future.

Early life stress delays hippocampal development and diminishes the adult stem cell pool in mice

Early life stress predisposes to mental illness and behavioral dysfunction in adulthood, but the mechanisms underlying these persistent effects are poorly understood. Stress throughout life impairs the structure and function of the hippocampus, a brain system undergoing considerable development in early life. The long-term behavioral consequences of early life stress may therefore be due in part to interference with hippocampal development, in particular with assembly of the dentate gyrus (DG) region of the hippocampus. We investigated how early life stress produces long-term alterations in DG structure by examining DG assembly and the generation of a stable adult stem cell pool in routine housing and after stress induced by the limited bedding/nesting paradigm in mice. We found that early life stress leads to a more immature, proliferative DG than would be expected for the animal's age immediately after stress exposure, suggesting that early life stress delays DG development. Adult animals exposed to early life stress exhibited a reduction in the number of DG stem cells, but unchanged neurogenesis suggesting a depletion of the stem cell pool with compensation in the birth and survival of adult-born neurons. These results suggest a developmental mechanism by which early life stress can induce long-term changes in hippocampal function by interfering with DG assembly and ultimately diminishing the adult stem cell pool.

Cell-Biological Requirements for the Generation of Dentate Gyrus Granule Neurons

The dentate gyrus (DG) receives highly processed information from the associative cortices functionally integrated in the trisynaptic hippocampal circuit, which contributes to the formation of new episodic memories and the spontaneous exploration of novel environments. Remarkably, the DG is the only brain region currently known to have high rates of neurogenesis in adults (Andersen et al., 1966, 1971). The DG is involved in several neurodegenerative disorders, including clinical dementia, schizophrenia, depression, bipolar disorder and temporal lobe epilepsy. The principal neurons of the DG are the granule cells. DG granule cells generated in culture would be an ideal model to investigate their normal development and the causes of the pathologies in which they are involved and as well as possible therapies. Essential to establish such in vitro models is the precise definition of the most important cell-biological requirements for the differentiation of DG granule cells. This requires a deeper understanding of the precise molecular and functional attributes of the DG granule cells in vivo as well as the DG cells derived in vitro. In this review we outline the neuroanatomical, molecular and cell-biological components of the granule cell differentiation pathway, including some growth- and transcription factors essential for their development. We summarize the functional characteristics of DG granule neurons, including the electrophysiological features of immature and mature granule cells and the axonal pathfinding characteristics of DG neurons. Additionally, we discuss landmark studies on the generation of dorsal telencephalic precursors from pluripotent stem cells (PSCs) as well as DG neuron differentiation in culture. Finally, we provide an outlook and comment critical aspects.

Ablation of proliferating neural stem cells during early life is sufficient to reduce adult hippocampal neurogenesis

Environmental exposures during early life, but not during adolescence or adulthood, lead to persistent reductions in neurogenesis in the adult hippocampal dentate gyrus (DG). The mechanisms by which early life exposures lead to long-term deficits in neurogenesis remain unclear. Here, we investigated whether targeted ablation of dividing neural stem cells during early life is sufficient to produce long-term decreases in DG neurogenesis. Having previously found that the stem cell lineage is resistant to long-term effects of transient ablation of dividing stem cells during adolescence or adulthood (Kirshenbaum, Lieberman, Briner, Leonardo, & Dranovsky, ), we used a similar pharmacogenetic approach to target dividing neural stem cells for elimination during early life periods sensitive to environmental insults. We then assessed the Nestin stem cell lineage in adulthood. We found that the adult neural stem cell reservoir was depleted following ablation during the first postnatal week, when stem cells were highly proliferative, but not during the third postnatal week, when stem cells were more quiescent. Remarkably, ablating proliferating stem cells during either the first or third postnatal week led to reduced adult neurogenesis out of proportion to the changes in the stem cell pool, indicating a disruption of the stem cell function or niche following stem cell ablation in early life. These results highlight the first three postnatal weeks as a series of sensitive periods during which elimination of dividing stem cells leads to lasting alterations in adult DG neurogenesis and stem cell function. These findings contribute to our understanding of the relationship between DG development and adult neurogenesis, as well as suggest a possible mechanism by which early life experiences may lead to lasting deficits in adult hippocampal neurogenesis.

Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults

New neurons continue to be generated in the subgranular zone of the dentate gyrus of the adult mammalian hippocampus. This process has been linked to learning and memory, stress and exercise, and is thought to be altered in neurological disease. In humans, some studies have suggested that hundreds of new neurons are added to the adult dentate gyrus every day, whereas other studies find many fewer putative new neurons. Despite these discrepancies, it is generally believed that the adult human hippocampus continues to generate new neurons. Here we show that a defined population of progenitor cells does not coalesce in the subgranular zone during human fetal or postnatal development. We also find that the number of proliferating progenitors and young neurons in the dentate gyrus declines sharply during the first year of life and only a few isolated young neurons are observed by 7 and 13 years of age. In adult patients with epilepsy and healthy adults (18-77 years; n = 17 post-mortem samples from controls; n = 12 surgical resection samples from patients with epilepsy), young neurons were not detected in the dentate gyrus. In the monkey (Macaca mulatta) hippocampus, proliferation of neurons in the subgranular zone was found in early postnatal life, but this diminished during juvenile development as neurogenesis decreased. We conclude that recruitment of young neurons to the primate hippocampus decreases rapidly during the first years of life, and that neurogenesis in the dentate gyrus does not continue, or is extremely rare, in adult humans. The early decline in hippocampal neurogenesis raises questions about how the function of the dentate gyrus differs between humans and other species in which adult hippocampal neurogenesis is preserved.

Liver X receptor β regulates the development of the dentate gyrus and autistic-like behavior in the mouse

The dentate gyrus (DG) of the hippocampus is a laminated brain region in which neurogenesis begins during early embryonic development and continues until adulthood. Recent studies have implicated that defects in the neurogenesis of the DG seem to be involved in the genesis of autism spectrum disorders (ASD)-like behaviors. Liver X receptor β (LXRβ) has recently emerged as an important transcription factor involved in the development of laminated CNS structures, but little is known about its role in the development of the DG. Here, we show that deletion of the LXRβ in mice causes hypoplasia in the DG, including abnormalities in the formation of progenitor cells and granule cell differentiation. We also found that expression of Notch1, a central mediator of progenitor cell self-renewal, is reduced in LXRβ-null mice. In addition, LXRβ deletion in mice results in autistic-like behaviors, including abnormal social interaction and repetitive behavior. These data reveal a central role for LXRβ in orchestrating the timely differentiation of neural progenitor cells within the DG, thereby providing a likely explanation for its association with the genesis of autism-related behaviors in LXRβ-deficient mice.

Bioengineering of the Human Neural Stem Cell Niche: A Regulatory Environment for Cell Fate and Potential Target for Neurotoxicity

Human neural stem/progenitor cells of the developing and adult organisms are surrounded by the microenvironment, so-called neurogenic niche. The developmental processes of stem cells, such as survival, proliferation, differentiation, and fate decisions, are controlled by the mutual interactions between cells and the niche components. Such interactions are tissue specific and determined by the biochemical and biophysical properties of the niche constituencies and the presence of other cell types. This dynamic approach of the stem cell niche, when translated into in vitro settings, requires building up "biomimetic" microenvironments resembling natural conditions, where the stem/progenitor cell is provided with diverse extracellular signals exerted by soluble and structural cues, mimicking those found in vivo. The neural stem cell niche is characterized by a unique composition of soluble components including neurotransmitters and trophic factors as well as insoluble extracellular matrix proteins and proteoglycans. Biotechnological innovations provide tools such as a new generation of tunable biomaterials capable of releasing specific signals in a spatially and temporally controlled manner, thus creating in vitro nature-like conditions and, when combined with stem cell-derived tissue specific progenitors, producing differentiated neuronal tissue structures. In addition, substantial progress has been made on the protocols to obtain stem cell-derived cell aggregates such as neurospheres and self-assembled organoids.In this chapter, we have assessed the application of bioengineered human neural stem cell microenvironments to produce in vitro models of different levels of biological complexity for the efficient control of stem cell fate. Examples of biomaterial-supported two-dimensional and three-dimensional (2D and 3D) complex culture systems that provide artificial neural stem cell niches are discussed in the context of their application for basic research and neurotoxicity testing.

COUP-TFI mitotically regulates production and migration of dentate granule cells and modulates hippocampal Cxcr4 expression

Development of the dentate gyrus (DG), the primary gateway for hippocampal inputs, spans embryonic and postnatal stages, and involves complex morphogenetic events. We have previously identified the nuclear receptor COUP-TFI as a novel transcriptional regulator in the postnatal organization and function of the hippocampus. Here, we dissect its role in DG morphogenesis by inactivating it in either granule cell progenitors or granule neurons. Loss of COUP-TFI function in progenitors leads to decreased granule cell proliferative activity, precocious differentiation and increased apoptosis, resulting in a severe DG growth defect in adult mice. COUP-TFI-deficient cells express high levels of the chemokine receptor Cxcr4 and migrate abnormally, forming heterotopic clusters of differentiated granule cells along their paths. Conversely, high COUP-TFI expression levels downregulate Cxcr4 expression, whereas increased Cxcr4 expression in wild-type hippocampal cells affects cell migration. Finally, loss of COUP-TFI in postmitotic cells leads to only minor and transient abnormalities, and to normal Cxcr4 expression. Together, our results indicate that COUP-TFI is required predominantly in DG progenitors for modulating expression of the Cxcr4 receptor during granule cell neurogenesis and migration.

Diagnostic Significance of Cellular Neuroglial Tissue in Ovarian Immature Teratoma

Background

Immature teratoma (IT) is a tumor containing immature neuroectodermal tissue, primarily in the form of neuroepithelial tubules. However, the diagnosis of tumors containing only cellular neuroglial tissue (CNT) without distinct neuroepithelial tubules is often difficult, since the histological characteristics of immature neuroectodermal tissues remain unclear. Here, we examined the significance of CNT and tried to define immature neuroectodermal tissues by comparing the histological features of neuroglial tissues between mature teratoma (MT) and IT.

Methods

The histological features of neuroglial tissue, including the cellularity, border between the neuroglial and adjacent tissues, cellular composition, mitotic index, Ki-67 proliferation rate, presence or absence of tissue necrosis, vascularity, and endothelial hyperplasia, were compared between 91 MT and 35 IT cases.

Results

CNTs with a cellularity grade of ≥ 2 were observed in 96% of IT cases and 4% of MT cases (p < .001); however, CNT with a cellularity grade of 3 in MT cases was confined to the histologically distinct granular layer of mature cerebellar tissue. Moreover, CNT in IT exhibited significantly higher rates of Ki-67 proliferation, mitoses, and necrosis than those in MT (p < .001). Furthermore, an infiltrative border of neuroglial tissue and glomeruloid endothelial hyperplasia were significantly more frequent in IT cases than in MT cases (p < .001).

Conclusions

Our results suggest that if CNT with a cellularity grade of ≥ 2 is not a component of cerebellar tissue, such cases should be diagnosed as IT containing immature neuroectodermal tissue, particularly if they exhibit an infiltrative border, mitoses, necrosis, and increased Ki-67 proliferation.

Brain size and limits to adult neurogenesis

The walls of the cerebral ventricles in the developing embryo harbor the primary neural stem cells from which most neurons and glia derive. In many vertebrates, neurogenesis continues postnatally and into adulthood in this region. Adult neurogenesis at the ventricle has been most extensively studied in organisms with small brains, such as reptiles, birds, and rodents. In reptiles and birds, these progenitor cells give rise to young neurons that migrate into many regions of the forebrain. Neurogenesis in adult rodents is also relatively widespread along the lateral ventricles, but migration is largely restricted to the rostral migratory stream into the olfactory bulb. Recent work indicates that the wall of the lateral ventricle is highly regionalized, with progenitor cells giving rise to different types of neurons depending on their location. In species with larger brains, young neurons born in these spatially specified domains become dramatically separated from potential final destinations. Here we hypothesize that the increase in size and topographical complexity (e.g., intervening white matter tracts) in larger brains may severely limit the long-term contribution of new neurons born close to, or in, the ventricular wall. We compare the process of adult neuronal birth, migration, and integration across species with different brain sizes, and discuss how early regional specification of progenitor cells may interact with brain size and affect where and when new neurons are added.

Evolution of the mammalian dentate gyrus

The dentate gyrus (DG), a part of the hippocampal formation, has important functions in learning, memory, and adult neurogenesis. Compared with homologous areas in sauropsids (birds and reptiles), the mammalian DG is larger and exhibits qualitatively different phenotypes: 1) folded (C- or V-shaped) granule neuron layer, concave toward the hilus and delimited by a hippocampal fissure; 2) nonperiventricular adult neurogenesis; and 3) prolonged ontogeny, involving extensive abventricular (basal) migration and proliferation of neural stem and progenitor cells (NSPCs). Although gaps remain, available data indicate that these DG traits are present in all orders of mammals, including monotremes and marsupials. The exception is Cetacea (whales, dolphins, and porpoises), in which DG size, convolution, and adult neurogenesis have undergone evolutionary regression. Parsimony suggests that increased growth and convolution of the DG arose in stem mammals concurrently with nonperiventricular adult hippocampal neurogenesis and basal migration of NSPCs during development. These traits could all result from an evolutionary change that enhanced radial migration of NSPCs out of the periventricular zones, possibly by epithelial-mesenchymal transition, to colonize and maintain nonperiventricular proliferative niches. In turn, increased NSPC migration and clonal expansion might be a consequence of growth in the cortical hem (medial patterning center), which produces morphogens such as Wnt3a, generates Cajal-Retzius neurons, and is regulated by Lhx2. Finally, correlations between DG convolution and neocortical gyrification (or capacity for gyrification) suggest that enhanced abventricular migration and proliferation of NSPCs played a transformative role in growth and folding of neocortex as well as archicortex.

Dentate Gyrus Development Requires ERK Activity to Maintain Progenitor Population and MAPK Pathway Feedback Regulation

The ERK/MAPK pathway is an important developmental signaling pathway. Mutations in upstream elements of this pathway result in neuro-cardio-facial cutaneous (NCFC) syndromes, which are typified by impaired neurocognitive abilities that are reliant upon hippocampal function. The role of ERK signaling during hippocampal development has not been examined and may provide critical insight into the cause of hippocampal dysfunction in NCFC syndromes. In this study, we have generated ERK1 and conditional ERK2 compound knock-out mice to determine the role of ERK signaling during development of the hippocampal dentate gyrus. We found that loss of both ERK1 and ERK2 resulted in 60% fewer granule cells and near complete absence of neural progenitor pools in the postnatal dentate gyrus. Loss of ERK1/2 impaired maintenance of neural progenitors as they migrate from the dentate ventricular zone to the dentate gyrus proper, resulting in premature depletion of neural progenitor cells beginning at E16.5, which prevented generation of granule cells later in development. Finally, loss of ERK2 alone does not impair development of the dentate gyrus as animals expressing only ERK1 developed a normal hippocampus. These findings establish that ERK signaling regulates maintenance of progenitor cells required for development of the dentate gyrus.

Ex utero electroporation and organotypic slice culture of mouse hippocampal tissue

Mouse genetics offers a powerful tool determining the role of specific genes during development. Analyzing the resulting phenotypes by immunohistochemical and molecular methods provides information of potential target genes and signaling pathways. To further elucidate specific regulatory mechanisms requires a system allowing the manipulation of only a small number of cells of a specific tissue by either overexpression, ablation or re-introduction of specific genes and follow their fate during development. To achieve this ex utero electroporation of hippocampal structures, especially the dentate gyrus, followed by organotypic slice culture provides such a tool. Using this system to generate mosaic deletions allows determining whether the gene of interest regulates cell-autonomously developmental processes like progenitor cell proliferation or neuronal differentiation. Furthermore it facilitates the rescue of phenotypes by re-introducing the deleted gene or its target genes. In contrast to in utero electroporation the ex utero approach improves the rate of successfully targeting deeper layers of the brain like the dentate gyrus. Overall ex utero electroporation and organotypic slice culture provide a potent tool to study regulatory mechanisms in a semi-native environment mirroring endogenous conditions.

Neurogenesis in the embryonic and adult brain: same regulators, different roles

Neurogenesis persists in adult mammals in specific brain areas, known as neurogenic niches. Adult neurogenesis is highly dynamic and is modulated by multiple physiological stimuli and pathological states. There is a strong interest in understanding how this process is regulated, particularly since active neuronal production has been demonstrated in both the hippocampus and the subventricular zone (SVZ) of adult humans. The molecular mechanisms that control neurogenesis have been extensively studied during embryonic development. Therefore, we have a broad knowledge of the intrinsic factors and extracellular signaling pathways driving proliferation and differentiation of embryonic neural precursors. Many of these factors also play important roles during adult neurogenesis, but essential differences exist in the biological responses of neural precursors in the embryonic and adult contexts. Because adult neural stem cells (NSCs) are normally found in a quiescent state, regulatory pathways can affect adult neurogenesis in ways that have no clear counterpart during embryogenesis. BMP signaling, for instance, regulates NSC behavior both during embryonic and adult neurogenesis. However, this pathway maintains stem cell proliferation in the embryo, while it promotes quiescence to prevent stem cell exhaustion in the adult brain. In this review, we will compare and contrast the functions of transcription factors (TFs) and other regulatory molecules in the embryonic brain and in adult neurogenic regions of the adult brain in the mouse, with a special focus on the hippocampal niche and on the regulation of the balance between quiescence and activation of adult NSCs in this region.

A novel cell migratory zone in the developing hippocampal formation

The hippocampal formation (HF) is a unique structure in the mammalian brain and is subdivided into the dentate gyrus, Ammon's horn, and subiculum by their functions and connectivity in the neuronal circuit. Because behaviors of the neural stem cells, neuronal progenitors, and the differentiating neurons are complex during hippocampal morphogenesis, the differentiation of these subdivisions has not been well understood. In this study, we investigated embryonic and postnatal expression of the proteins Prox1, Math2, and Ctip2, which clearly indicate principal cells of the dentate gyrus (Prox1 positive) and Ammon's horn (Math2 and Ctip2 positive). Expression patterns of Prox1 and Math2 were consistent with previously suggested localization of migratory pathways of the dentate granule cells and hippocampal pyramidal cells. Interestingly, we found intermingling of Prox1-expressing cells and Math2-expressing cells in a cell migratory stream, suggesting previously unknown behaviors of differentiating cells of the HF.