An Adaptive Threshold in Mammalian Neocortical Evolution

Expansion of the neocortex is a hallmark of human evolution. However, it remains an open question what adaptive mechanisms facilitated its expansion. Here we show, using gyrencephaly index (GI) and other physiological and life-history data for 102 mammalian species, that gyrencephaly is an ancestral mammalian trait. We provide evidence that the evolution of a highly folded neocortex, as observed in humans, requires the traversal of a threshold of 10^9 neurons, and that species above and below the threshold exhibit a bimodal distribution of physiological and life-history traits, establishing two phenotypic groups. We identify, using discrete mathematical models, proliferative divisions of progenitors in the basal compartment of the developing neocortex as evolutionarily necessary and sufficient for generating a fourteen-fold increase in daily prenatal neuron production and thus traversal of the neuronal threshold. We demonstrate that length of neurogenic period, rather than any novel progenitor-type, is sufficient to distinguish cortical neuron number between species within the same phenotypic group.Comment: Currently under review; 38 pages, 5 Figures, 13 Supplementary Figures, 2 Table

The involvement of Rcc2 in mammalian neurogenesis

Rcc2 is a Rac guanine nucleotide exchange factor recently identified as a principal signaling component of integrin adhesion complexes that also plays a central role in the completion of mitosis and cytokinesis. Rcc2 mRNA is enriched in a class of neural progenitors in the ventricular zone, short neural precursors. Although Rcc2 mRNA is present at high levels in the ventricular zone during neurogenesis, the impact of Rcc2 on cortical development has not been previously studied. We used two methods to study the role of Rcc2 in vivo. First we isolated a portion of the upstream regulatory region of Rcc2 and used it to express a fluorescent protein. Additionally, we used an shRNA targeting Rcc2 to knockdown expression of Rcc2. We found that the promoter region of Rcc2 labeled cells that were near the board of the ventricular zone and subventricular zone, and tended to be positive for Sox2 but not Tbr2, when compared to the general progenitor population. Progenitors electroporated with Rcc2 shRNA were closer to the ventricular surface than those with functioning Rcc2. Examination of the cell cycle in cells electroporated with Rcc2 shRNA indicated no difference to those with Rcc2. We found that Rcc2 was active during neurogenesis in ventricular zone progenitors. Additionally, our analysis shows that Rcc2 may be involved in the migration of progenitors during neurogenesis. Further works needs to be done to further elucidate the role of Rcc2

Abundant Occurrence of Basal Radial Glia in the Subventricular Zone of Embryonic Neocortex of a Lissencephalic Primate, the Common Marmoset Callithrix jacchus

Subventricular zone (SVZ) progenitors are a hallmark of the developing neocortex. Recent studies described a novel type of SVZ progenitor that retains a basal process at mitosis, sustains expression of radial glial markers, and is capable of self-renewal. These progenitors, referred to here as basal radial glia (bRG), occur at high relative abundance in the SVZ of gyrencephalic primates (human) and nonprimates (ferret) but not lissencephalic rodents (mouse). Here, we analyzed the occurrence of bRG cells in the embryonic neocortex of the common marmoset Callithrix jacchus, a near-lissencephalic primate. bRG cells, expressing Pax6, Sox2 (but not Tbr2), glutamate aspartate transporter, and glial fibrillary acidic protein and retaining a basal process at mitosis, occur at similar relative abundance in the marmoset SVZ as in human and ferret. The proportion of progenitors in M-phase was lower in embryonic marmoset than developing ferret neocortex, raising the possibility of a longer cell cycle. Fitting the gyrification indices of 26 anthropoid species to an evolutionary model suggested that the marmoset evolved from a gyrencephalic ancestor. Our results suggest that a high relative abundance of bRG cells may be necessary, but is not sufficient, for gyrencephaly and that the marmoset's lissencephaly evolved secondarily by changing progenitor parameters other than progenitor typ

Comparative Analysis of the Subventricular Zone in Rat, Ferret and Macaque: Evidence for an Outer Subventricular Zone in Rodents

The mammalian cerebral cortex arises from precursor cells that reside in a proliferative region surrounding the lateral ventricles of the developing brain. Recent work has shown that precursor cells in the subventricular zone (SVZ) provide a major contribution to prenatal cortical neurogenesis, and that the SVZ is significantly thicker in gyrencephalic mammals such as primates than it is in lissencephalic mammals including rodents. Identifying characteristics that are shared by or that distinguish cortical precursor cells across mammalian species will shed light on factors that regulate cortical neurogenesis and may point toward mechanisms that underlie the evolutionary expansion of the neocortex in gyrencephalic mammals. We immunostained sections of the developing cerebral cortex from lissencephalic rats, and from gyrencephalic ferrets and macaques to compare the distribution of precursor cell types in each species. We also performed time-lapse imaging of precursor cells in the developing rat neocortex. We show that the distribution of Pax6+ and Tbr2+ precursor cells is similar in lissencephalic rat and gyrencephalic ferret, and different in the gyrencephalic cortex of macaque. We show that mitotic Pax6+ translocating radial glial cells (tRG) are present in the cerebral cortex of each species during and after neurogenesis, demonstrating that the function of Pax6+ tRG cells is not restricted to neurogenesis. Furthermore, we show that Olig2 expression distinguishes two distinct subtypes of Pax6+ tRG cells. Finally we present a novel method for discriminating the inner and outer SVZ across mammalian species and show that the key cytoarchitectural features and cell types that define the outer SVZ in developing primates are present in the developing rat neocortex. Our data demonstrate that the developing rat cerebral cortex possesses an outer subventricular zone during late stages of cortical neurogenesis and that the developing rodent cortex shares important features with that of primates

Analysis of the cell cycle of neural progenitors in the developing ferret neocortex

Description of the cell cycle features of neural progenitors during late stages of neurogenesis in a gyrencephalic mammal, the ferret

Abundant Occurrence of Basal Radial Glia in the Subventricular Zone of Embryonic Neocortex of a Lissencephalic Primate, the Common Marmoset Callithrix jacchus

Subventricular zone (SVZ) progenitors are a hallmark of the developing neocortex. Recent studies described a novel type of SVZ progenitor that retains a basal process at mitosis, sustains expression of radial glial markers, and is capable of self-renewal. These progenitors, referred to here as basal radial glia (bRG), occur at high relative abundance in the SVZ of gyrencephalic primates (human) and nonprimates (ferret) but not lissencephalic rodents (mouse). Here, we analyzed the occurrence of bRG cells in the embryonic neocortex of the common marmoset Callithrix jacchus, a near-lissencephalic primate. bRG cells, expressing Pax6, Sox2 (but not Tbr2), glutamate aspartate transporter, and glial fibrillary acidic protein and retaining a basal process at mitosis, occur at similar relative abundance in the marmoset SVZ as in human and ferret. The proportion of progenitors in M-phase was lower in embryonic marmoset than developing ferret neocortex, raising the possibility of a longer cell cycle. Fitting the gyrification indices of 26 anthropoid species to an evolutionary model suggested that the marmoset evolved from a gyrencephalic ancestor. Our results suggest that a high relative abundance of bRG cells may be necessary, but is not sufficient, for gyrencephaly and that the marmoset's lissencephaly evolved secondarily by changing progenitor parameters other than progenitor type

Neurodevelopment under the prism of environmental challenges

Prenatal development affects adult health. Exposures to a variety of prenatal environ-mental factors have important effects on fetal development and, in turn, are extensively associated with neurobehavioral, structural and functional phenotypes after birth. Developmental processes are in part promoted by orchestrated levels of glucocorticoids, which are steroid hormones involved in fetal organ maturation. Glucocorticoids also mediate the hormonal stress response of the organism as part of the hypothalamic-pituitary-adrenal axis. During pregnancy levels of glucocorticoids outside of the normal range, either due to maternal pathology including stress-related psychiatric disorders or to antenatal synthetic glucocorticoid treatments, have been associated with altered brain structural and neurobehavioral phenotypes after birth. Interestingly, developmental time-windows seem to interplay with the exposure to influence the direction of post-natal phenotypes. Exposures later in gestation are mainly associated with adverse out-comes while exposures earlier in gestation are additionally associated with potentially beneficial outcomes. While many studies have investigated the effects of glucocorticoids on late developmental time-windows, so far little evidence is available on their effects on early human cortical development and especially during the neurogenic period, which is when neurons are produced. Thus, the potential cellular and molecular underpinnings of the timing dependent divergent effects of glucocorticoids on postnatal phenotypes are not known. To investigate these processes in a complex model of early human neurodevelopment that is reactive to environmental stimuli, I used induced Pluripotent Stem Cells-derived 3-dimensional cerebral organoids and combined them with in vivo mouse neurodevelopment. I found that application of glucocorticoids during neurogenesis increases neurogenic processes that are enriched in species with a gyrified brain, like humans, while are rare in species with a smooth brain, like rodents. These processes contribute to the increased neuronal production and cortical expansion seen in gyrencephalic species. More specifically, at the molecular level this effect is mediated by the glucocorticoid receptor, a transcription factor, which in turn activates ZBTB16 by altering its methylation landscape in specific DNA regulatory elements. Subsequently ZBTB16, a transcription factor itself, increases the expression of PAX6, a key driver of neurogenesis, by activating its promoter. This results in increased numbers of progenitor cells expressing PAX6 and EOMES (a marker of more mature progenitors) in the basal regions of the germinal zones in both organoids and mice. PAX6- and EOMES- positive progenitors are enriched in gyrified species while they are rare in species with smooth brains. The increased numbers of these highly proliferative and neurogenic progenitors lead to an extended neurogenic period and ultimately to increased production of deep layer neurons (BCL11B- positive). Finally, the altered cellular architecture due to glucocorticoids and ZBTB16 potentially mediates beneficial postnatal outcomes as indicated by causal associations with higher educational attainment and increased postnatal cortical thick-ness. This work highlights the importance of early neurodevelopment and specifically of the neurogenic period as a sensitive time-window for glucocorticoid effects. In addition, the molecular and cellular mechanisms as well as the pathways identified could have pro-found implications for our understanding of glucocorticoid effects during early brain development that potentially mediate postnatal outcomes