Mitotic WNT signalling orchestrates neurogenesis in the developing neocortex.

The role of WNT/ß-catenin signalling in mouse neocortex development remains ambiguous. Most studies demonstrate that WNT/ß-catenin regulates progenitor self-renewal but others suggest it can also promote differentiation. Here we explore the role of WNT/STOP signalling, which stabilizes proteins during G2/M by inhibiting glycogen synthase kinase (GSK3)-mediated protein degradation. We show that mice mutant for cyclin Y and cyclin Y-like 1 (Ccny/l1), key regulators of WNT/STOP signalling, display reduced neurogenesis in the developing neocortex. Specifically, basal progenitors, which exhibit delayed cell cycle progression, were drastically decreased. Ccny/l1-deficient apical progenitors show reduced asymmetric division due to an increase in apical-basal astral microtubules. We identify the neurogenic transcription factors Sox4 and Sox11 as direct GSK3 targets that are stabilized by WNT/STOP signalling in basal progenitors during mitosis and that promote neuron generation. Our work reveals that WNT/STOP signalling drives cortical neurogenesis and identifies mitosis as a critical phase for neural progenitor fate.

Expression of human-specific ARHGAP11B in mice leads to neocortex expansion and increased memory flexibility.

Neocortex expansion during human evolution provides a basis for our enhanced cognitive abilities. Yet, which genes implicated in neocortex expansion are actually responsible for higher cognitive abilities is unknown. The expression of human-specific ARHGAP11B in embryonic/foetal mouse, ferret and marmoset neocortex was previously found to promote basal progenitor proliferation, upper-layer neuron generation and neocortex expansion during development, features commonly thought to contribute to increased cognitive abilities. However, a key question is whether this phenotype persists into adulthood and if so, whether cognitive abilities are indeed increased. Here, we generated a transgenic mouse line with physiological ARHGAP11B expression that exhibits increased neocortical size and upper-layer neuron numbers persisting into adulthood. Adult ARHGAP11B-transgenic mice showed altered neurobehaviour, notably increased memory flexibility and a reduced anxiety level. Our data are consistent with the notion that neocortex expansion by ARHGAP11B, a gene implicated in human evolution, underlies some of the altered neurobehavioural features observed in the transgenic mice, such as the increased memory flexibility, a neocortex-associated trait, with implications for the increase in cognitive abilities during human evolution.

Induction of Oxidative Stress and Alteration of Synaptic Gene Expression in Newborn Hippocampal Granule Cells after Developmental Exposure to Aroclor 1254.

INTRODUCTION: Hippocampal newborn neurons integrate into functional circuits where they play an important role in learning and memory. We previously showed that perinatal exposure to Aroclor 1254, a commercial mixture of polychlorinated biphenyls (PCBs) associated with alterations of cognitive function in children, disrupted the normal maturation of excitatory synapses in the dentate gyrus. We hypothesized that hippocampal immature neurons underlie some of the cognitive effects of PCBs. METHODS: We used newly generated neurons to examine the effects of PCBs in mice following maternal exposure. Newborn dentate granule cells were tagged with enhanced green fluorescent protein using a transgenic mouse line. The transcriptome of the newly generated granule cells was assessed using RNA sequencing. RESULTS: Gestational and lactational exposure to 6 mg/kg/day of Aroclor 1254 disrupted the mRNA expression of 1,308 genes in newborn granule cells. Genes involved in mitochondrial functions were highly enriched with 154 genes significantly increased in exposed compared to control mice. The upregulation of genes involved in oxidative phosphorylation was accompanied by signs of endoplasmic reticulum stress and an increase in lipid peroxidation, a marker of oxidative stress, in the subgranular zone of the dentate gyrus but not in mature granule cells in the granular zone. Aroclor 1254 exposure also disrupted the expression of synaptic genes. Using laser-captured subgranular and granular zones, this effect was restricted to the subgranular zone, where newborn neurons are located. CONCLUSION: Our data suggest that gene expression in newborn granule cells is disrupted by Aroclor 1254 and provide clues to the effects of endocrine-disrupting chemicals on the brain.

Response to Comment on "Human TKTL1 implies greater neurogenesis in frontal neocortex of modern humans than Neanderthals".

Herai et al. discuss the known fact that a low percentage of modern humans who lack any overt phenotypes carry the ancestral TKTL1 allele. Our paper demonstrates that the amino acid substitution in TKTL1 increases neural progenitor cells and neurogenesis in the developing brain. It is another question if, and to what extent, this has consequences for the adult brain.

Human TKTL1 implies greater neurogenesis in frontal neocortex of modern humans than Neanderthals.

Neanderthal brains were similar in size to those of modern humans. We sought to investigate potential differences in neurogenesis during neocortex development. Modern human transketolase-like 1 (TKTL1) differs from Neanderthal TKTL1 by a lysine-to-arginine amino acid substitution. Using overexpression in developing mouse and ferret neocortex, knockout in fetal human neocortical tissue, and genome-edited cerebral organoids, we found that the modern human variant, hTKTL1, but not the Neanderthal variant, increases the abundance of basal radial glia (bRG) but not that of intermediate progenitors (bIPs). bRG generate more neocortical neurons than bIPs. The hTKTL1 effect requires the pentose phosphate pathway and fatty acid synthesis. Inhibition of these metabolic pathways reduces bRG abundance in fetal human neocortical tissue. Our data suggest that neocortical neurogenesis in modern humans differs from that in Neanderthals.

Neocortex expansion in development and evolution-from genes to progenitor cell biology.

The evolutionary expansion of the neocortex, the seat of higher cognitive functions in humans, is primarily due to an increased and prolonged proliferation of neural progenitor cells during development. Basal progenitors, and in particular basal radial glial cells, are thought to have a key role in the increased generation of neurons that constitutes a foundation of neocortex expansion. Recent studies have identified primate-specific and human-specific genes and changes in gene expression that promote increased proliferative capacity of cortical progenitors. In many cases, the cell biological basis underlying this increase has been uncovered. Model systems such as mouse, ferret, nonhuman primates, and cerebral organoids have been used to establish the relevance of these genes for neocortex expansion.

Human-Specific ARHGAP11B Acts in Mitochondria to Expand Neocortical Progenitors by Glutaminolysis.

The human-specific gene ARHGAP11B is preferentially expressed in neural progenitors of fetal human neocortex and increases abundance and proliferation of basal progenitors (BPs), which have a key role in neocortex expansion. ARHGAP11B has therefore been implicated in the evolutionary expansion of the human neocortex, but its mode of action has been unknown. Here, we show that ARHGAP11B is imported into mitochondria, where it interacts with the adenine nucleotide translocase (ANT) and inhibits the mitochondrial permeability transition pore (mPTP). BP expansion by ARHGAP11B requires its presence in mitochondria, and pharmacological inhibition of ANT function or mPTP opening mimic BP expansion by ARHGAP11B. Searching for the underlying metabolic basis, we find that BP expansion by ARHGAP11B requires glutaminolysis, the conversion of glutamine to glutamate for the tricarboxylic acid (TCA) cycle. Hence, an ARHGAP11B-induced, mitochondria-based effect on BP metabolism that is a hallmark of highly mitotically active cells appears to underlie its role in neocortex expansion.

Malformations of Human Neocortex in Development - Their Progenitor Cell Basis and Experimental Model Systems.

Malformations of the human neocortex in development constitute a heterogeneous group of complex disorders, resulting in pathologies such as intellectual disability and abnormal neurological/psychiatric conditions such as epilepsy or autism. Advances in genomic sequencing and genetic techniques have allowed major breakthroughs in the field, revealing the molecular basis of several of these malformations. Here, we focus on those malformations of the human neocortex, notably microcephaly, and macrocephaly, where an underlying basis has been established at the level of the neural stem/progenitor cells (NPCs) from which neurons are directly or indirectly derived. Particular emphasis is placed on NPC cell biology and NPC markers. A second focus of this review is on experimental model systems used to dissect the underlying mechanisms of malformations of the human neocortex in development at the cellular and molecular level. The most commonly used model system have been genetically modified mice. However, although basic features of neocortical development are conserved across the various mammalian species, some important differences between mouse and human exist. These pertain to the abundance of specific NPC types and/or their proliferative capacity, as exemplified in the case of basal radial glia. These differences limit the ability of mouse models to fully recapitulate the phenotypes of malformations of the human neocortex. For this reason, additional experimental model systems, notably the ferret, non-human primates and cerebral organoids, have recently emerged as alternatives and shown to be of increasing relevance. It is therefore important to consider the benefits and limitations of each of these model systems for studying malformations of the human neocortex in development.

Evolution and cell-type specificity of human-specific genes preferentially expressed in progenitors of fetal neocortex.

Understanding the molecular basis that underlies the expansion of the neocortex during primate, and notably human, evolution requires the identification of genes that are particularly active in the neural stem and progenitor cells of the developing neocortex. Here, we have used existing transcriptome datasets to carry out a comprehensive screen for protein-coding genes preferentially expressed in progenitors of fetal human neocortex. We show that 15 human-specific genes exhibit such expression, and many of them evolved distinct neural progenitor cell-type expression profiles and levels compared to their ancestral paralogs. Functional studies on one such gene, NOTCH2NL, demonstrate its ability to promote basal progenitor proliferation in mice. An additional 35 human genes with progenitor-enriched expression are shown to have orthologs only in primates. Our study provides a resource of genes that are promising candidates to exert specific, and novel, roles in neocortical development during primate, and notably human, evolution.

Neuroendocrine disruption without direct endocrine mode of action: Polychloro-biphenyls (PCBs) and bisphenol A (BPA) as case studies.

Endocrine disruption is commonly thought to be restricted to a direct endocrine mode of action i.e. the perturbation of the activation of a given type of hormonal receptor by its natural ligand. Consistent with the WHO definition of an endocrine disrupter, a key issue is the "altered function(s) of the endocrine system". Such altered functions can result from different chemical interactions, beyond agonistic or antagonistic effect at a given receptor. Based on neuroendocrine disruption by polychlorinated biphenyls and bisphenol A, this paper proposes different mechanistic paradigms that can result in adverse health effects. They are a consequence of altered endocrine function(s) secondary to chemical interaction with different steps in the physiological regulatory processes, thus accounting for a possibly indirect endocrine mode of action.

Current Changes in Pubertal Timing: Revised Vision in Relation with Environmental Factors Including Endocrine Disruptors.

The aim of this chapter is to revise some common views on changes in pubertal timing. This revision is based on recent epidemiological findings on the clinical indicators of pubertal timing and data on environmental factor effects and underlying mechanisms. A current advancement in timing of female puberty is usually emphasized. It appears, however, that timing is also changing in males. Moreover, the changes are towards earliness for initial pubertal stages and towards lateness for final stages in both sexes. Such observations indicate the complexity of environmental influences on pubertal timing. The mechanisms of changes in pubertal timing may involve both the central neuroendocrine control and peripheral effects at tissues targeted by gonadal steroids. While sufficient energy availability is a clue to the mechanism of pubertal development, changes in the control of both energy balance and reproduction may vary under the influence of common determinants such as endocrine-disrupting chemicals (EDCs). These effects can take place right before puberty as well as much earlier, during fetal and neonatal life. Finally, environmental factors can interact with genetic factors in determining changes in pubertal timing. Therefore, the variance in pubertal timing is no longer to be considered under absolutely separate control by environmental and genetic determinants. Some recommendations are provided for evaluation of EDC impact in the management of pubertal disorders and for possible reduction of EDC exposure along the precautionary principle.

Contribution of the Endocrine Perspective in the Evaluation of Endocrine Disrupting Chemical Effects: The Case Study of Pubertal Timing.

Debate makes science progress. In the field of endocrine disruption, endocrinology has brought up findings that substantiate a specific perspective on the definition of endocrine disrupting chemicals (EDCs), the role of the endocrine system and the endpoints of hormone and EDC actions among other issues. This paper aims at discussing the relevance of the endocrine perspective with regard to EDC effects on pubertal timing. Puberty involves particular sensitivity to environmental conditions. Reports about the advancing onset of puberty in several countries have led to the hypothesis that the increasing burden of EDCs could be an explanation. In fact, pubertal timing currently shows complex changes since advancement of some manifestations of puberty (e.g. breast development) and no change or delay of others (e.g. menarche, pubic hair development) can be observed. In a human setting with exposure to low doses of tenths or hundreds of chemicals since prenatal life, causation is most difficult to demonstrate and justifies a translational approach using animal models. Studies in rodents indicate an exquisite sensitivity of neuroendocrine endpoints to EDCs. Altogether, the data from both human and animal studies support the importance of concepts derived from endocrinology in the evaluation of EDC effects on puberty.

Alteration of rat fetal cerebral cortex development after prenatal exposure to polychlorinated biphenyls.

Polychlorinated biphenyls (PCBs) are environmental contaminants that persist in environment and human tissues. Perinatal exposure to these endocrine disruptors causes cognitive deficits and learning disabilities in children. These effects may involve their ability to interfere with thyroid hormone (TH) action. We tested the hypothesis that developmental exposure to PCBs can concomitantly alter TH levels and TH-regulated events during cerebral cortex development: progenitor proliferation, cell cycle exit and neuron migration. Pregnant rats exposed to the commercial PCB mixture Aroclor 1254 ended gestation with reduced total and free serum thyroxine levels. Exposure to Aroclor 1254 increased cell cycle exit of the neuronal progenitors and delayed radial neuronal migration in the fetal cortex. Progenitor cell proliferation, cell death and differentiation rate were not altered by prenatal exposure to PCBs. Given that PCBs remain ubiquitous, though diminishing, contaminants in human systems, it is important that we further understand their deleterious effects in the brain.