Apoptosis and its relation to the cell cycle in the developing cerebral cortex

Evaluation of the behavioral, histopathological, and immunohistochemical effects resulting from ventriculosubcutaneous shunt obstruction in kaolin-induced hydrocephalus in rats

PURPOSE

Hydrocephalus is a brain disease prevalent in the pediatric population that presents complex pathophysiology and multiple etiologies. The best treatment is still ventricular shunting. Mechanical obstruction is the most frequent complication, but the resulting pathological effects are still unknown.

OBJECTIVE

Evaluation and comparison of clinical, histopathological, and immunohistochemical aspects in the acute phase of experimental hydrocephalus induced by kaolin, after treatment with adapted shunt, and after shunt obstruction and posterior disobstruction.

METHODS

Wistar rats aged 7 days were used and divided into 4 groups: control group without kaolin injection (n = 6), untreated hydrocephalic group (n = 5), hydrocephalic group treated with ventriculosubcutaneous shunt (DVSC) (n = 7), and hydrocephalic group treated with shunt, posteriorly obstructed and disobstructed (n = 5). The animals were submitted to memory and spatial learning evaluation through the Morris water maze test. The rats were sacrificed at 28 days of age and histological analysis of the brains was performed with luxol fast blue, in addition to immunohistochemical analysis in order to evaluate reactive astrocytosis, inflammation, neuronal labeling, and apoptotic activity.

RESULTS

The group with shunt obstruction had worse performance in memory tests. Reactive astrocytosis was more evident in this group, as was the inflammatory response.

CONCLUSIONS

Obstruction of the shunt results in impaired performance of behavioral tests and causes irreversible histopathological changes when compared to findings in the group with treated hydrocephalus, even after unblocking the system. The developed model is feasible and efficient in simulating the clinical context of shunt dysfunction.

The characteristics of excitatory lineage differentiation and the developmental conservation in Reeler neocortex

The majority of neocortical projection neurons are generated indirectly from radial glial cells (RGCs) mediated by intermediate progenitor cells (IPCs) in mice. IPCs are thought to be a great breakthrough in the evolutionary expansion of the mammalian neocortex. However, the precise ratio of neuron production from IPCs and characteristics of RGC differentiation process are still unclear. Our study revealed that direct neurogenesis was seldom observed and increased slightly at late embryonic stage. Besides, we conducted retrovirus sparse labelling combined carboxyfluorescein diacetate succinimide ester (CFSE) and Tbr2-CreER strain to reconstruct individual lineage tree in situ. The lineage trees simulated the output of RGCs at per round of division in sequence with high temporal, spatial and cellular resolution at P7. We then demonstrated that only 1.90% of neurons emanated from RGCs directly in mouse cerebral neocortex and 79.33% of RGCs contributed to the whole clones through IPCs. The contribution of indirect neurogenesis was underestimated previously because approximately a quarter of IPC-derived neurons underwent apoptosis. Here, we also showed that abundant IPCs from first-generation underwent self-renewing division and generated four neurons ultimately. We confirmed that the intermediate proliferative progenitors expressed higher Cux2 characteristically at early embryonic stage. Finally, we validated that the characteristics of neurogenetic process in lineages and developmental fate of neurons were conserved in Reeler mice. This study contributes to further understanding of neurogenesis in neocortical development.

Microglia complement signaling promotes neuronal elimination and normal brain functional connectivity

Complement signaling is thought to serve as an opsonization signal to promote the phagocytosis of synapses by microglia. However, while its role in synaptic remodeling has been demonstrated in the retino-thalamic system, it remains unclear whether complement signaling mediates synaptic pruning in the brain more generally. Here we found that mice lacking the Complement receptor 3, the major microglia complement receptor, failed to show a deficit in either synaptic pruning or axon elimination in the developing mouse cortex. Instead, mice lacking Complement receptor 3 exhibited a deficit in the perinatal elimination of neurons in the cortex, a deficit that is associated with increased cortical thickness and enhanced functional connectivity in these regions in adulthood. These data demonstrate a role for complement in promoting neuronal elimination in the developing cortex.

SNIP1 and PRC2 coordinate cell fates of neural progenitors during brain development

Stem cell survival versus death is a developmentally programmed process essential for morphogenesis, sizing, and quality control of genome integrity and cell fates. Cell death is pervasive during development, but its programming is little known. Here, we report that Smad nuclear interacting protein 1 (SNIP1) promotes neural progenitor cell survival and neurogenesis and is, therefore, integral to brain development. The SNIP1-depleted brain exhibits dysplasia with robust induction of caspase 9-dependent apoptosis. Mechanistically, SNIP1 regulates target genes that promote cell survival and neurogenesis, and its activities are influenced by TGFβ and NFκB signaling pathways. Further, SNIP1 facilitates the genomic occupancy of Polycomb complex PRC2 and instructs H3K27me3 turnover at target genes. Depletion of PRC2 is sufficient to reduce apoptosis and brain dysplasia and to partially restore genetic programs in the SNIP1-depleted brain in vivo. These findings suggest a loci-specific regulation of PRC2 and H3K27 marks to toggle cell survival and death in the developing brain.

Transcription-associated DNA DSBs activate p53 during hiPSC-based neurogenesis

Neurons are overproduced during cerebral cortical development. Neural progenitor cells (NPCs) divide rapidly and incur frequent DNA double-strand breaks (DSBs) throughout cortical neurogenesis. Although half of the neurons born during neurodevelopment die, many neurons with inaccurate DNA repair survive leading to brain somatic mosaicism. Recurrent DNA DSBs during neurodevelopment are associated with both gene expression level and gene length. We used imaging flow cytometry and a genome-wide DNA DSB capture approach to quantify and map DNA DSBs during human induced pluripotent stem cell (hiPSC)-based neurogenesis. Reduced p53 signaling was brought about by knockdown (p53KD); p53KD led to elevated DNA DSB burden in neurons that was associated with gene expression level but not gene length in neural progenitor cells (NPCs). Furthermore, DNA DSBs incurred from transcriptional, but not replicative, stress lead to p53 activation in neurotypical NPCs. In p53KD NPCs, DNA DSBs accumulate at transcription start sites of genes that are associated with neurological and psychiatric disorders. These findings add to a growing understanding of how neuronal genome dynamics are engaged by high transcriptional or replicative burden during neurodevelopment.

Serotonergic regulation of bipolar cell survival in the developing cerebral cortex

One key factor underlying the functional balance of cortical networks is the ratio of excitatory and inhibitory neurons. The mechanisms controlling the ultimate number of interneurons are beginning to be elucidated, but to what extent similar principles govern the survival of the large diversity of cortical inhibitory cells remains to be investigated. Here, we investigate the mechanisms regulating developmental cell death in neurogliaform cells, bipolar cells, and basket cells, the three main populations of interneurons originating from the caudal ganglionic eminence and the preoptic region. We found that all three subclasses of interneurons undergo activity-dependent programmed cell death. However, while neurogliaform cells and basket cells require glutamatergic transmission to survive, the final number of bipolar cells is instead modulated by serotonergic signaling. Together, our results demonstrate that input-specific modulation of neuronal activity controls the survival of cortical interneurons during the critical period of programmed cell death.

Thalamocortical axons control the cytoarchitecture of neocortical layers by area-specific supply of VGF

Neuronal abundance and thickness of each cortical layer are specific to each area, but how this fundamental feature arises during development remains poorly understood. While some of area-specific features are controlled by intrinsic cues such as morphogens and transcription factors, the exact influence and mechanisms of action by cues extrinsic to the cortex, in particular the thalamic axons, have not been fully established. Here, we identify a thalamus-derived factor, VGF, which is indispensable for thalamocortical axons to maintain the proper amount of layer 4 neurons in the mouse sensory cortices. This process is prerequisite for further maturation of the primary somatosensory area, such as barrel field formation instructed by a neuronal activity-dependent mechanism. Our results provide an actual case in which highly site-specific axon projection confers further regional complexity upon the target field through locally secreting signaling molecules from axon terminals.

Global Reprogramming of Apoptosis-Related Genes during Brain Development

To enable long-term survival, mammalian adult neurons exhibit unique apoptosis competence. Questions remain as to whether and how neurons globally reprogram the expression of apoptotic genes during development. We systematically examined the in vivo expression of 1923 apoptosis-related genes and associated histone modifications at eight developmental ages of mouse brains. Most apoptotic genes displayed consistent temporal patterns across the forebrain, midbrain, and hindbrain, suggesting ubiquitous robust developmental reprogramming. Although both anti- and pro-apoptotic genes can be up- or downregulated, half the regulatory events in the classical apoptosis pathway are downregulation of pro-apoptotic genes. Reduced expression in initiator caspases, apoptosome, and pro-apoptotic Bcl-2 family members restrains effector caspase activation and attenuates neuronal apoptosis. The developmental downregulation of apoptotic genes is attributed to decreasing histone-3-lysine-4-trimethylation (H3K4me3) signals at promoters, where histone-3-lysine-27-trimethylation (H3K27me3) rarely changes. By contrast, repressive H3K27me3 marks are lost in the upregulated gene groups, for which developmental H3K4me3 changes are not predictive. Hence, developing brains remove epigenetic H3K4me3 and H3K27me3 marks on different apoptotic gene groups, contributing to their downregulation and upregulation, respectively. As such, neurons drastically alter global apoptotic gene expression during development to transform apoptosis controls. Research into neuronal cell death should consider maturation stages as a biological variable.

The Proapoptotic Gene Bad Regulates Brain Development via p53-Mediated Stress Signals in Zebrafish

Studies have shown that the BH3-only domain Bad regulates brain development via the control of programmed cell death (PCD), but very few studies have addressed its effect on the molecular signaling of brain development in the system. In this work, we examined the novel role of zebrafish Bad in initial programmed cell death for brain morphogenesis through the priming of p53-mediated stress signaling. In a biological function study on the knockdown of Bad by morpholino oligonucleotides, at 24 h post-fertilization (hpf) Bad defects induced abnormal hindbrain development, as determined in a tissue section by means of HE staining which traced the damaged hindbrain. Then, genome-wide approaches for monitoring either the upregulation of apoptotic-related genes (11.8%) or the downregulation of brain development-related genes (29%) at the 24 hpf stage were implemented. The p53/caspase-8-mediated apoptotic death pathway was strongly involved, with the pathway being strongly reversed in a p53 mutant (p53^M214K) line during Bad knockdown. Furthermore, we propose the involvement of a p53-mediated stress signal which is correlated with regulating Bad loss-mediated brain defects. We found that some major genes in brain development, such as crybb1, pva1b5, irx4a, pax7a, and fabp7a, were dramatically restored in the p53^M214K line, and brain development recovered to return movement behavior to normal. Our findings suggest that Bad is required for (PCD) control, exerting a p53 stress signal on caspase-8/tBid-mediated death signaling and brain development-related gene regulation.

Cell-Type-Specific Gene Expression in Developing Mouse Neocortex: Intermediate Progenitors Implicated in Axon Development

Cerebral cortex projection neurons (PNs) are generated from intermediate progenitors (IPs), which are in turn derived from radial glial progenitors (RGPs). To investigate developmental processes in IPs, we profiled IP transcriptomes in embryonic mouse neocortex, using transgenic Tbr2-GFP mice, cell sorting, and microarrays. These data were used in combination with in situ hybridization to ascertain gene sets specific for IPs, RGPs, PNs, interneurons, and other neural and non-neural cell types. RGP-selective transcripts (n = 419) included molecules for Notch receptor signaling, proliferation, neural stem cell identity, apical junctions, necroptosis, hippo pathway, and NF-κB pathway. RGPs also expressed specific genes for critical interactions with meningeal and vascular cells. In contrast, IP-selective genes (n = 136) encoded molecules for activated Delta ligand presentation, epithelial-mesenchymal transition, core planar cell polarity (PCP), axon genesis, and intrinsic excitability. Interestingly, IPs expressed several “dependence receptors” (Unc5d, Dcc, Ntrk3, and Epha4) that induce apoptosis in the absence of ligand, suggesting a competitive mechanism for IPs and new PNs to detect key environmental cues or die. Overall, our results imply a novel role for IPs in the patterning of neuronal polarization, axon differentiation, and intrinsic excitability prior to mitosis. Significantly, IPs highly express Wnt-PCP, netrin, and semaphorin pathway molecules known to regulate axon polarization in other systems. In sum, IPs not only amplify neurogenesis quantitatively, but also molecularly “prime” new PNs for axogenesis, guidance, and excitability.

Creative Destruction: A Basic Computational Model of Cortical Layer Formation

One of the most characteristic properties of many vertebrate neural systems is the layered organization of different cell types. This cytoarchitecture exists in the cortex, the retina, the hippocampus, and many other parts of the central nervous system. The developmental mechanisms of neural layer formation have been subject to substantial experimental efforts. Here, we provide a general computational model for cortical layer formation in 3D physical space. We show that this multiscale, agent-based model, comprising two distinct stages of apoptosis, can account for the wide range of neuronal numbers encountered in different cortical areas and species. Our results demonstrate the phenotypic richness of a basic state diagram structure. Importantly, apoptosis allows for changing the thickness of one layer without automatically affecting other layers. Therefore, apoptosis increases the flexibility for evolutionary change in layer architecture. Notably, slightly changed gene regulatory dynamics recapitulate the characteristic properties observed in neurodevelopmental diseases. Overall, we propose a novel computational model using gene-type rules, exhibiting many characteristics of normal and pathological cortical development.

Proapoptotic Bad Involved in Brain Development, When Severely Defected, Induces Dramatic Malformation in Zebrafish

The BH3-only molecule Bad regulates cell death via its differential protein phosphorylation, but very few studies address its effect on early embryonic development in vertebrate systems. In this work, we examined the novel role of zebrafish Bad in the initial programmed cell death (PCD) for brain morphogenesis through reducing environmental stress and cell death signaling. Bad was considered to be a material factor that because of the knockdown of Bad by morpholino oligonucleotides, PCD was increased and the reactive oxygen species (ROS) level was enhanced, which correlated to trigger a p53/caspase-8 involving cell death signaling. This Bad knockdown-mediated environmental stress and enhanced cell dying can delay normal cell migration in the formation of the three germ layers, especially the ectoderm, for further brain development. Furthermore, Bad defects involved in three-germ-layers development at 8 hpf were identified by in situ hybridization approach on cyp26, rtla, and Sox17 pattern expression markers. Finally, the Bad knockdown-induced severely defected brain was examined by tissue section from 24 to 48 h postfertilization (hpf), which correlated to induce dramatic malformation in the hindbrain. Our data suggest that the BH3-only molecule Bad regulates brain development via controlling programmed cell death on overcoming environmental stress for reducing secondary cell death signaling, which suggests that correlates to brain developmental and neurological disorders in this model system.

Molecular Profiling Reveals Involvement of ESCO2 in Intermediate Progenitor Cell Maintenance in the Developing Mouse Cortex

Intermediate progenitor cells (IPCs) are neocortical neuronal precursors. Although IPCs play crucial roles in corticogenesis, their molecular features remain largely unknown. In this study, we aimed to characterize the molecular profile of IPCs. We isolated TBR2-positive (+) IPCs and TBR2-negative (-) cell populations in the developing mouse cortex. Comparative genome-wide gene expression analysis of TBR2⁺ IPCs versus TBR2^- cells revealed differences in key factors involved in chromatid segregation, cell-cycle regulation, transcriptional regulation, and cell signaling. Notably, mutation of many IPC genes in human has led to intellectual disability and caused a wide range of cortical malformations, including microcephaly and agenesis of corpus callosum. Loss-of-function experiments in cortex-specific mutants of Esco2, one of the novel IPC genes, demonstrate its critical role in IPC maintenance, and substantiate the identification of a central genetic determinant of IPC biogenesis. Our data provide novel molecular characteristics of IPCs in the developing mouse cortex.

Cell death in the male and female rat medial prefrontal cortex during early postnatal development

Apoptosis, programmed cell death, is a critical component of neurodevelopment occurring in temporal, spatial, and at times, sex-specific, patterns across the cortex during the early postnatal period. During this time, the brain is particularly susceptible to environmental influences that are often used in animal models of neurodevelopmental disorders. In the present study, the timing of peak cell death was assessed by the presence of pyknotic cells in the male and female rat medial prefrontal cortex (mPFC), a cortical region that in humans, is often involved in developmental disorders. One male and one female rat per litter were sacrificed at the following ages: postnatal day (P)2, 4, 6, 8, 10, 12, 14, 16, 18, and 25. The mPFC was Nissl-stained, the densities of pyknotic cells and live neurons were stereologically collected, and the number of pyknotic cells per 100 live neurons, pyknotic cell density, and neuron density were analyzed. Males and females showed a significant peak in the ratio of pyknotic to live neurons on P8, and in females, this elevation persisted through P12. Likewise, the density of pyknotic cells peaked on P8 in both sexes and persisted through P12 in females. The timing of cell death within the rat mPFC will inform study design in experiments that employ early environmental manipulations that might disrupt this process.

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The number of pyknotic cells per live neuron was quantified.

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Postnatal cell death peaked on P8 in the male rat medial prefrontal cortex.

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In females, postnatal cell death peaked from P8 to P12.

Enhanced FGFR3 activity in postmitotic principal neurons during brain development results in cortical dysplasia and axonal tract abnormality

Abnormal levels of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) have been detected in various neurological disorders. The potent impact of FGF-FGFR in multiple embryonic developmental processes makes it challenging to elucidate their roles in postmitotic neurons. Taking an alternative approach to examine the impact of aberrant FGFR function on glutamatergic neurons, we generated a FGFR gain-of-function (GOF) transgenic mouse, which expresses constitutively activated FGFR3 (FGFR3K650E) in postmitotic glutamatergic neurons. We found that GOF disrupts mitosis of radial-glia neural progenitors (RGCs), inside-out radial migration of post-mitotic glutamatergic neurons, and axonal tract projections. In particular, late-born CUX1-positive neurons are widely dispersed throughout the GOF cortex. Such a cortical migration deficit is likely caused, at least in part, by a significant reduction of the radial processes projecting from RGCs. RNA-sequencing analysis of the GOF embryonic cortex reveals significant alterations in several pathways involved in cell cycle regulation and axonal pathfinding. Collectively, our data suggest that FGFR3 GOF in postmitotic neurons not only alters axonal growth of postmitotic neurons but also impairs RGC neurogenesis and radial glia processes.

Novel Insights into the Role of UBE3A in Regulating Apoptosis and Proliferation

The UBE3A gene codes for a protein with two known functions, a ubiquitin E3-ligase which catalyzes ubiquitin binding to substrate proteins and a steroid hormone receptor coactivator. UBE3A is most famous for its critical role in neuronal functioning. Lack of UBE3A protein expression leads to Angelman syndrome (AS), while its overexpression is associated with autism. In spite of extensive research, our understanding of UBE3A roles is still limited. We investigated the cellular and molecular effects of Ube3a deletion in mouse embryonic fibroblasts (MEFs) and Angelman syndrome (AS) mouse model hippocampi. Cell cultures of MEFs exhibited enhanced proliferation together with reduced apoptosis when Ube3a was deleted. These findings were supported by transcriptome and proteome analyses. Furthermore, transcriptome analyses revealed alterations in mitochondria-related genes. Moreover, an analysis of adult AS model mice hippocampi also found alterations in the expression of apoptosis- and proliferation-associated genes. Our findings emphasize the role UBE3A plays in regulating proliferation and apoptosis and sheds light into the possible effects UBE3A has on mitochondrial involvement in governing this balance.

Sex differences and gonadal hormone regulation of brain cardiolipin, a key mitochondrial phospholipid

Cardiolipin (CL) is a phospholipid that is almost exclusively located in the inner mitochondrial membrane of eukaryotic cells. As a result of its unique structure and distribution, CL establishes non-covalent bonds with a long list of proteins involved in ATP production, mitochondria biogenesis, mitophagy and apoptosis. Thus, the amount of CL, as well as its fatty acid composition and location, strongly impacts upon mitochondrial-dependent functions and therefore the metabolic homeostasis of different tissues. The brain is particularly sensitive to mitochondrial dysfunction as a result of its high metabolic demand. Several mitochondrial related-neurodegenerative disorders, as well as physiological ageing, show altered CL metabolism. Furthermore, mice lacking enzymes involved in CL synthesis show cognitive impairments. CL content and metabolism are regulated by gonadal hormones in the developing and adult brain. In neuronal cultures, oestradiol increases CL content, whereas adult ovariectomy decreases CL content and alters CL metabolism in the hippocampal mitochondria. Transient sex differences in brain CL metabolism have been detected during development. At birth, brain CL has a higher proportion of unsaturated fatty acids in the brain of male mice than in the brain of females. In addition, the expression of enzymes involved in CL de novo and recycling synthetic pathways is higher in males. Most of these sex differences are abolished by the neonatal androgenisation of females, suggesting a role for testosterone in the generation of sex differences in brain CL. The regulation of brain CL by gonadal hormones may be linked to their homeostatic and protective actions in neural cells, as well as the manifestation of sex differences in neurodegenerative disorders.

TNF deficiency causes alterations in the spatial organization of neurogenic zones and alters the number of microglia and neurons in the cerebral cortex

BACKGROUND

Although tumor necrosis factor (TNF) inhibitors are used to treat chronic inflammatory diseases, there is little information about how long-term inhibition of TNF affects the homeostatic functions that TNF maintains in the intact CNS.

MATERIALS AND METHODS

To assess whether developmental TNF deficiency causes alterations in the naïve CNS, we estimated the number of proliferating cells, microglia, and neurons in the developing neocortex of E13.5, P7 and adult TNF knock out (TNF^-/-) mice and wildtype (WT) littermates. We also measured changes in gene and protein expression and monoamine levels in adult WT and TNF^-/- mice. To evaluate long-term effects of TNF inhibitors, we treated healthy adult C57BL/6 mice with either saline, the selective soluble TNF inhibitor XPro1595, or the nonselective TNF inhibitor etanercept. We estimated changes in cell number and protein expression after two months of treatment. We assessed the effects of TNF deficiency on cognition by testing adult WT and TNF^-/- mice and mice treated with saline, XPro1595, or etanercept with specific behavioral tasks.

RESULTS

TNF deficiency decreased the number of proliferating cells and microglia and increased the number of neurons. At the same time, TNF deficiency decreased the expression of WNT signaling-related proteins, specifically Collagen Triple Helix Repeat Containing 1 (CTHRC1) and Frizzled receptor 6 (FZD6). In contrast to XPro1595, long-term inhibition of TNF with etanercept in adult C57BL/6 mice decreased the number of BrdU⁺ cells in the granule cell layer of the dentate gyrus. Etanercept, but not XPro1595, also impaired spatial learning and memory in the Barnes maze memory test.

CONCLUSION

TNF deficiency impacts the organization of neurogenic zones and alters the cell composition in brain. Long-term inhibition of TNF with the nonselective TNF inhibitor etanercept, but not the soluble TNF inhibitor XPro1595, decreases neurogenesis in the adult mouse hippocampus and impairs learning and memory after two months of treatment.

Large-scale death of retinal astrocytes during normal development is non-apoptotic and implemented by microglia

Naturally occurring cell death is a fundamental developmental mechanism for regulating cell numbers and sculpting developing organs. This is particularly true in the nervous system, where large numbers of neurons and oligodendrocytes are eliminated via apoptosis during normal development. Given the profound impact of death upon these two major cell populations, it is surprising that developmental death of another major cell type—the astrocyte—has rarely been studied. It is presently unclear whether astrocytes are subject to significant developmental death, and if so, how it occurs. Here, we address these questions using mouse retinal astrocytes as our model system. We show that the total number of retinal astrocytes declines by over 3-fold during a death period spanning postnatal days 5–14. Surprisingly, these astrocytes do not die by apoptosis, the canonical mechanism underlying the vast majority of developmental cell death. Instead, we find that microglia engulf astrocytes during the death period to promote their developmental removal. Genetic ablation of microglia inhibits astrocyte death, leading to a larger astrocyte population size at the end of the death period. However, astrocyte death is not completely blocked in the absence of microglia, apparently due to the ability of astrocytes to engulf each other. Nevertheless, mice lacking microglia showed significant anatomical changes to the retinal astrocyte network, with functional consequences for the astrocyte-associated vasculature leading to retinal hemorrhage. These results establish a novel modality for naturally occurring cell death and demonstrate its importance for the formation and integrity of the retinal gliovascular network.

A study of the neonatal mouse retina shows that developmental cell death of retinal astrocytes does not occur by apoptosis but is instead mediated by microglia, which kill and engulf astrocytes to effect their developmental removal.

Developmental Cell Death in the Cerebral Cortex

In spite of the high metabolic cost of cellular production, the brain contains only a fraction of the neurons generated during embryonic development. In the rodent cerebral cortex, a first wave of programmed cell death surges at embryonic stages and affects primarily progenitor cells. A second, larger wave unfolds during early postnatal development and ultimately determines the final number of cortical neurons. Programmed cell death in the developing cortex is particularly dependent on neuronal activity and unfolds in a cell-specific manner with precise temporal control. Pyramidal cells and interneurons adjust their numbers in sync, which is likely crucial for the establishment of balanced networks of excitatory and inhibitory neurons. In contrast, several other neuronal populations are almost completely eliminated through apoptosis during the first two weeks of postnatal development, highlighting the importance of programmed cell death in sculpting the mature cerebral cortex.

Low-intensity transcranial magnetic stimulation promotes the survival and maturation of newborn oligodendrocytes in the adult mouse brain

Neuronal activity is a potent extrinsic regulator of oligodendrocyte generation and central nervous system myelination. Clinically, repetitive transcranial magnetic stimulation (rTMS) is delivered to noninvasively modulate neuronal activity; however, the ability of rTMS to facilitate adaptive myelination has not been explored. By performing cre‐lox lineage tracing, to follow the fate of oligodendrocyte progenitor cells in the adult mouse brain, we determined that low intensity rTMS (LI‐rTMS), administered as an intermittent theta burst stimulation, but not as a continuous theta burst or 10 Hz stimulation, increased the number of newborn oligodendrocytes in the adult mouse cortex. LI‐rTMS did not alter oligodendrogenesis per se, but instead increased cell survival and enhanced myelination. These data suggest that LI‐rTMS can be used to noninvasively promote myelin addition to the brain, which has potential implications for the treatment of demyelinating diseases such as multiple sclerosis.

Intermediate progenitors and Tbr2 in cortical development

In developing cerebral cortex, intermediate progenitors (IPs) are transit amplifying cells that specifically express Tbr2 (gene: Eomes), a T-box transcription factor. IPs are derived from radial glia (RG) progenitors, the neural stem cells of developing cortex. In turn, IPs generate glutamatergic projection neurons (PNs) exclusively. IPs are found in ventricular and subventricular zones, where they differentiate as distinct ventricular IP (vIP) and outer IP (oIP) subtypes. Morphologically, IPs have short processes, resembling filopodia or neurites, that transiently contact other cells, most importantly dividing RG cells to mediate Delta-Notch signaling. Also, IPs secrete a chemokine, Cxcl12, which guides interneuron and microglia migrations and promotes thalamocortical axon growth. In mice, IPs produce clones of 1-12 PNs, sometimes spanning multiple layers. After mitosis, IP daughter cells undergo asymmetric cell death in the majority of instances. In mice, Tbr2 is necessary for PN differentiation and subtype specification, and to repress IP-genic transcription factors. Tbr2 directly represses Insm1, an IP-genic transcription factor gene, as well as Pax6, a key activator of Tbr2 transcription. Without Tbr2, abnormal IPs transiently accumulate in elevated numbers. More broadly, Tbr2 regulates the transcriptome by activating or repressing hundreds of direct target genes. Notably, Tbr2 'unlocks' and activates PN-specific genes, such as Tbr1, by recruiting Jmjd3, a histone H3K27me3 demethylase that removes repressive epigenetic marks placed by polycomb repressive complex 2. IPs have played an important role in the evolution and gyrification of mammalian cerebral cortex, and TBR2 is essential for human brain development.

Dorsal-zone-specific reduction of sensory neuron density in the olfactory epithelium following long-term exercise or caloric restriction

Exercise (Ex) and caloric restriction (CR) reduce oxidative stress and improve organ function. For instance, voluntary Ex or CR is known to reduce age-related cochlear damage in male C57BL/6J mice. However, the effect of Ex and CR on the olfactory system is unknown. In this study, we confirmed the positive effect of Ex and CR on age-related cochlear damage, but found that Ex and CR affected negatively cell dynamics in the olfactory epithelium (OE) by reducing the number of mature olfactory sensory neurons (OSNs) and increasing the number of proliferative basal cells and apoptotic OSNs in the dorsal zone of the olfactory epithelium (OE), which contains neurons expressing NADPH quinone oxido-reductase 1 (NQO1). In addition, these interventions resulted in lower odor-induced c-fos expression in areas of the olfactory bulb receiving projections from dorsal-zone OSNs than in areas receiving ventral-zone projections. Further, we observed substantial oxidative stress in NQO1-positive cells and apoptotic OSNs in the dorsal zone in Ex and CR animals. These results suggest that, in contrast to their positive effects in other organs, Ex and CR facilitate oxidative stress and negatively impact structure and function in dorsal-zone OSNs, probably in association with NQO1 bioactivation.

Cadherin 8 regulates proliferation of cortical interneuron progenitors

Cortical interneurons are born in the ventral forebrain and migrate tangentially in two streams at the levels of the intermediate zone (IZ) and the pre-plate/marginal zone to the developing cortex where they switch to radial migration before settling in their final positions in the cortical plate. In a previous attempt to identify the molecules that regulate stream specification, we performed transcriptomic analysis of GFP-labelled interneurons taken from the two migratory streams during corticogenesis. A number of cadherins were found to be expressed differentially, with Cadherin-8 (Cdh8) selectively present in the IZ stream. We verified this expression pattern at the mRNA and protein levels on tissue sections and found approximately half of the interneurons of the IZ expressed Cdh8. Furthermore, this cadherin was also detected in the germinal zones of the subpallium, suggesting that it might be involved not only in the migration of interneurons but also in their generation. Quantitative analysis of cortical interneurons in animals lacking the cadherin at E18.5 revealed a significant increase in their numbers. Subsequent functional in vitro experiments showed that blocking Cdh8 function led to increased cell proliferation, with the opposite results observed with over-expression, supporting its role in interneuron generation.

Clonal analysis reveals laminar fate multipotency and daughter cell apoptosis of mouse cortical intermediate progenitors

In developing cerebral cortex, most pyramidal-projection neurons are produced by intermediate progenitors (IPs), derived in turn from radial glial progenitors. Although IPs produce neurons for all cortical layers, it is unknown whether individual IPs produce multiple or single laminar fates, and the potential of IPs for extended proliferation remains uncertain. Previously, we found that, at the population level, early IPs (present during lower-layer neurogenesis) produce lower- and upper-layer neurons, whereas late IPs produce upper-layer neurons only. Here, we employed mosaic analysis with double markers (MADM) in mice to sparsely label early IP clones. Most early IPs produced 1-2 neurons for deep layers only. Less frequently, early IPs produced larger clones (up to 12 neurons) spanning lower and upper layers, or upper layers only. The majority of IP-derived clones (∼66%) were associated with asymmetric cell death after the first division. These data demonstrate that laminar fate is not predetermined, at least in some IPs. Rather, the heterogeneous sizes and laminar fates of early IP clones are correlated with cell division/death/differentiation choices and neuron birthdays, respectively.

Pediatric brain repair from endogenous neural stem cells of the subventricular zone

There is great interest in the regenerative potential of the neural stem cells and progenitors that populate the germinal zones of the immature brain. Studies using animal models of pediatric brain injuries have provided a clearer understanding of the responses of these progenitors to injury. In this review, we have compared and contrasted the responses of the endogenous neural stem cells and progenitors of the subventricular zone in animal models of neonatal cerebral hypoxia-ischemia, neonatal stroke, congenital cardiac disease, and pediatric traumatic brain injury. We have reviewed the dynamic shifts that occur within this germinal zone with injury as well as changes in known signaling molecules that affect these progenitors. Importantly, we have summarized data on the extent to which cell replacement occurs in response to each of these injuries, opportunities available, and obstacles that will need to be overcome to improve neurological outcomes in survivors.

Histology Atlas of the Developing Prenatal and Postnatal Mouse Central Nervous System, with Emphasis on Prenatal Days E7.5 to E18.5

Evaluation of the central nervous system (CNS) in the developing mouse presents unique challenges, given the complexity of ontogenesis, marked structural reorganization over very short distances in 3 dimensions each hour, and numerous developmental events susceptible to genetic and environmental influences. Developmental defects affecting the brain and spinal cord arise frequently both in utero and perinatally as spontaneous events, following teratogen exposure, and as sequelae to induced mutations and thus are a common factor in embryonic and perinatal lethality in many mouse models. Knowledge of normal organ and cellular architecture and differentiation throughout the mouse's life span is crucial to identify and characterize neurodevelopmental lesions. By providing a well-illustrated overview summarizing major events of normal in utero and perinatal mouse CNS development with examples of common developmental abnormalities, this annotated, color atlas can be used to identify normal structure and histology when phenotyping genetically engineered mice and will enhance efforts to describe and interpret brain and spinal cord malformations as causes of mouse embryonic and perinatal lethal phenotypes. The schematics and images in this atlas illustrate major developmental events during gestation from embryonic day (E)7.5 to E18.5 and after birth from postnatal day (P)1 to P21.

Electrical activity controls area-specific expression of neuronal apoptosis in the mouse developing cerebral cortex

Programmed cell death widely but heterogeneously affects the developing brain, causing the loss of up to 50% of neurons in rodents. However, whether this heterogeneity originates from neuronal identity and/or network-dependent processes is unknown. Here, we report that the primary motor cortex (M1) and primary somatosensory cortex (S1), two adjacent but functionally distinct areas, display striking differences in density of apoptotic neurons during the early postnatal period. These differences in rate of apoptosis negatively correlate with region-dependent levels of activity. Disrupting this activity either pharmacologically or by electrical stimulation alters the spatial pattern of apoptosis and sensory deprivation leads to exacerbated amounts of apoptotic neurons in the corresponding functional area of the neocortex. Thus, our data demonstrate that spontaneous and periphery-driven activity patterns are important for the structural and functional maturation of the neocortex by refining the final number of cortical neurons in a region-dependent manner.

Homeostatic interplay between electrical activity and neuronal apoptosis in the developing neocortex

An intriguing feature of nervous system development in most animal species is that the initial number of generated neurons is higher than the number of neurons incorporated into mature circuits. A substantial portion of neurons is indeed eliminated via apoptosis during a short time window - in rodents the first two postnatal weeks. While it is well established that neurotrophic factors play a central role in controlling neuronal survival and apoptosis in the peripheral nervous system (PNS), the situation is less clear in the central nervous system (CNS). In postnatal rodent neocortex, the peak of apoptosis coincides with the occurrence of spontaneous, synchronous activity patterns. In this article, we review recent results that demonstrate the important role of electrical activity for neuronal survival in the neocortex, describe the role of Ca²⁺ and neurotrophic factors in translating electrical activity into pro-survival signals, and finally discuss the clinical impact of the tight relation between electrical activity and neuronal survival versus apoptosis.

Protective role of Cadherin 13 in interneuron development

Cortical interneurons are generated in the ganglionic eminences and migrate through the ventral and dorsal telencephalon before finding their final positions within the cortical plate. During early stages of migration, these cells are present in two well-defined streams within the developing cortex. In an attempt to identify candidate genes which may play a role in interneuron stream specification, we previously carried out a microarray analysis which identified a number of cadherin receptors that were differentially expressed in these streams, including Cadherin-13 (Cdh13). Expression analysis confirmed Cdh13 to be present in the preplate layer at E13.5 and, later in development, in some cortical interneurons and pyramidal cells. Analysis of Cdh13 knockout mice at E18.5, but not at E15.5, showed a reduction in the number of interneurons and late born pyramidal neurons and a concomitant increase in apoptotic cells in the cortex. These observations were confirmed in dissociated cell cultures using overexpression and short interfering RNAs (siRNAs) constructs and dominant negative inhibitory proteins. Our findings identified a novel protective role for Cdh13 in cortical neuron development.

Patterns of cell death in the perinatal mouse forebrain

The importance of cell death in brain development has long been appreciated, but many basic questions remain, such as what initiates or terminates the cell death period. One obstacle has been the lack of quantitative data defining exactly when cell death occurs. We recently created a "cell death atlas," using the detection of activated caspase-3 (AC3) to quantify apoptosis in the postnatal mouse ventral forebrain and hypothalamus, and found that the highest rates of cell death were seen at the earliest postnatal ages in most regions. Here we have extended these analyses to prenatal ages and additional brain regions. We quantified cell death in 16 forebrain regions across nine perinatal ages from embryonic day (E) 17 to postnatal day (P) 11 and found that cell death peaks just after birth in most regions. We found greater cell death in several regions in offspring delivered vaginally on the day of parturition compared with those of the same postconception age but still in utero at the time of collection. We also found massive cell death in the oriens layer of the hippocampus on P1 and in regions surrounding the anterior crossing of the corpus callosum on E18 as well as the persistence of large numbers of cells in those regions in adult mice lacking the pro-death Bax gene. Together these findings suggest that birth may be an important trigger of neuronal cell death and identify transient cell groups that may undergo wholesale elimination perinatally. J. Comp. Neurol. 525:47-64, 2017. © 2016 Wiley Periodicals, Inc.

Semaphorin3A-neuropilin1 signalling is involved in the generation of cortical interneurons

Cortical interneurons are generated predominantly in the medial ganglionic eminence of the ventral telencephalon and migrate to the cortex during embryonic development. These cells express neuropilin (Nrp1 and Nrp2) receptors which mediate their response to the chemorepulsive class 3 semaphorin (Sema) ligands. We show here that semaphorins Sema3A and Sema3F are expressed in layers adjacent to cortical interneuron migratory streams as well as in the striatum, suggesting they may have a role in guiding these cells throughout their journey. Analysis of Sema3A ^-/- and Sema3F ^-/- mice during corticogenesis showed that absence of Sema3A, but not Sema3F, leads to aberrant migration of cortical interneurons through the striatum. Reduced number of cortical interneurons was found in the cortex of Sema3A ^-/-, Nrp1 ^-/- and Nrp2 ^-/- mice, as well as altered distribution in Sema3F ^-/-, Nrp1 ^-/-, Nrp2 ^-/- animals and especially in neuropilin double mutants. The observed decrease in interneurons in Sema3A ^-/- and Nrp1 ^-/- mice was due to altered proliferative activity of their progenitors highlighted by changes in their mitotic spindle positioning and angle of cleavage plane during cell division. These findings point to a novel role for Sema3A-Nrp1 signalling in progenitor cell dynamics and in the generation of interneurons in the ventral telencephalon.

The Indispensable Roles of Microglia and Astrocytes during Brain Development

Glia are essential for brain functioning during development and in the adult brain. Here, we discuss the various roles of both microglia and astrocytes, and their interactions during brain development. Although both cells are fundamentally different in origin and function, they often affect the same developmental processes such as neuro-/gliogenesis, angiogenesis, axonal outgrowth, synaptogenesis and synaptic pruning. Due to their important instructive roles in these processes, dysfunction of microglia or astrocytes during brain development could contribute to neurodevelopmental disorders and potentially even late-onset neuropathology. A better understanding of the origin, differentiation process and developmental functions of microglia and astrocytes will help to fully appreciate their role both in the developing as well as in the adult brain, in health and disease.

Lithium increases proliferation of hippocampal neural stem/progenitor cells and rescues irradiation-induced cell cycle arrest in vitro

Radiotherapy in children causes debilitating cognitive decline, partly linked to impaired neurogenesis. Irradiation targets primarily cancer cells but also endogenous neural stem/progenitor cells (NSPCs) leading to cell death or cell cycle arrest. Here we evaluated the effects of lithium on proliferation, cell cycle and DNA damage after irradiation of young NSPCs in vitro.

NSPCs were treated with 1 or 3 mM LiCl and we investigated proliferation capacity (neurosphere volume and bromodeoxyuridine (BrdU) incorporation). Using flow cytometry, we analysed apoptosis (annexin V), cell cycle (propidium iodide) and DNA damage (γH2AX) after irradiation (3.5 Gy) of lithium-treated NSPCs.

Lithium increased BrdU incorporation and, dose-dependently, the number of cells in replicative phase as well as neurosphere growth. Irradiation induced cell cycle arrest in G₁ and G₂/M phases. Treatment with 3 mM LiCl was sufficient to increase NSPCs in S phase, boost neurosphere growth and reduce DNA damage. Lithium did not affect the levels of apoptosis, suggesting that it does not rescue NSPCs committed to apoptosis due to accumulated DNA damage.

Lithium is a very promising candidate for protection of the juvenile brain from radiotherapy and for its potential to thereby improve the quality of life for those children who survive their cancer.

A 3D printed microfluidic device for production of functionalized hydrogel microcapsules for culture and differentiation of human Neuronal Stem Cells (hNSC)

We present here a microfluidic device that generates sub-millimetric hollow hydrogel spheres, encapsulating cells and coated internally with a layer of reconstituted extracellular matrix (ECM) of a few microns thick. The spherical capsules, composed of alginate hydrogel, originate from the spontaneous instability of a multi-layered jet formed by co-extrusion using a coaxial flow device. We provide a simple design to manufacture this device using a DLP (digital light processing) 3D printer. Then, we demonstrate how the inner wall of the capsules can be decorated with a continuous ECM layer that is anchored to the alginate gel and mimics the basal membrane of a cellular niche. Finally, we used this approach to encapsulate human Neural Stem Cells (hNSC) derived from human Induced Pluripotent Stem Cells (hIPSC), which were further differentiated into neurons within the capsules with negligible loss of viability. Altogether, we show that these capsules may serve as cell micro-containers compatible with complex cell culture conditions and applications. These developments widen the field of research and biomedical applications of the cell encapsulation technology.

Altered proliferative ability of neuronal progenitors in PlexinA1 mutant mice

Cortical interneurons are generated predominantly in the medial ganglionic eminence (MGE) and migrate through the ventral and dorsal telencephalon before taking their final positions within the developing cortical plate. Previously we demonstrated that interneurons from Robo1 knockout (Robo1(-/-)) mice contain reduced levels of neuropilin 1 (Nrp1) and PlexinA1 receptors, rendering them less responsive to the chemorepulsive actions of semaphorin ligands expressed in the striatum and affecting their course of migration (Hernandez-Miranda et al. [2011] J. Neurosci. 31:6174-6187). Earlier studies have highlighted the importance of Nrp1 and Nrp2 in interneuron migration, and here we assess the role of PlexinA1 in this process. We observed significantly fewer cells expressing the interneuron markers Gad67 and Lhx6 in the cortex of PlexinA1(-/-) mice compared with wild-type littermates at E14.5 and E18.5. Although the level of apoptosis was similar in the mutant and control forebrain, proliferation was significantly reduced in the former. Furthermore, progenitor cells in the MGE of PlexinA1(-/-) mice appeared to be poorly anchored to the ventricular surface and showed reduced adhesive properties, which may account for the observed reduction in proliferation. Together our data uncover a novel role for PlexinA1 in forebrain development.

Defining the catalytic activity of nanoceria in the P23H-1 rat, a photoreceptor degeneration model

Purpose

Inorganic catalytic nanoceria or cerium oxide nanoparticles (CeNPs) are bona fide antioxidants that possess regenerative radical scavenging activities in vitro. Previously, we demonstrated that CeNPs had neuroprotective and anti-angiogenic properties in rodent retinal degeneration and neovascularization models. However, the cellular mechanisms and duration of the catalytic activity of CeNPs in preventing photoreceptor cell loss are still unknown. In this study, we sought to answer these questions using the P23H-1 rat, an autosomal dominant retinitis pigmentosa (adRP) model.

Methods

A single dose of either saline or CeNPs was delivered intravitreally into the eyes of P23H-1 rats at 2–3 weeks of age. Retinal functions were examined at 3 to 7 weeks post injection. We quantified retinal proteins by Western blot analyses and counted the number of apoptotic (TUNEL+) profiles in the outer nuclear layer (ONL) of retinal sections. We measured free 8-isoprostanes to quantify lipid peroxidation in retinal tissues.

Results

We observed increased rod and cone cell functions up to three weeks post injection. Apoptotic cells were reduced by 46%, 56%, 21%, and 24% at 3, 7, 14, 21 days, respectively, after CeNPs injection compared to saline. Additionally, reduction of lipid peroxidation in the retinas of CeNPs-treated vs saline-treated animals was detected 14 days post injection.

Conclusions

We validated that CeNPs were effective in delaying loss of photoreceptor cell function in an adRP rat model. This represents the fourth rodent retinal disease model that shows delay in disease progression after a single application of CeNPs. We further demonstrated that CeNPs slowed the rate of photoreceptor cell death. We deduced that the catalytic activity of CeNPs in vivo in this rat model to be undiminished for at least 7 days and then declined over the next 14 days after CeNPs administration.

Insulin and IGF receptor signalling in neural-stem-cell homeostasis

Neural stem cells (NSCs) are found in two regions in the adult brain: the subgranular zone (SGZ) in the hippocampal dentate gyrus and the subventricular zone (SVZ) adjacent to the lateral ventricles. Similarly to other somatic stem cells, adult NSCs are found within specialized niches that are organized to facilitate NSC self-renewal. Alterations in stem-cell homeostasis can contribute to the consequences of neurodegenerative diseases, healthy ageing and tissue repair after damage. Insulin and the insulin-like growth factors (IGFs) function in stem-cell homeostasis across species. Studies in the mammalian central nervous system support essential roles for IGF and/or insulin signalling in NSC self-renewal, neurogenesis, cognition and sensory function through distinct ligand-receptor interactions. IGF-II is of particular interest as a result of its production by the choroid plexus and presence in cerebrospinal fluid (CSF). CSF regulates and supports the development, division and migration of cells in the adult brain and is required for NSC maintenance. In this Review, we discuss emerging data on the functions of IGF-II and IGF and/or insulin receptor signalling in the context of NSC regulation in the SVZ and SGZ. We also propose a model for IGF-II in which the choroid plexus is a major component of the NSC niche.

Regulation of cerebral cortex development by Rho GTPases: insights from in vivo studies

The cerebral cortex is the site of higher human cognitive and motor functions. Histologically, it is organized into six horizontal layers, each containing unique populations of molecularly and functionally distinct excitatory projection neurons and inhibitory interneurons. The stereotyped cellular distribution of cortical neurons is crucial for the formation of functional neural circuits and it is predominantly established during embryonic development. Cortical neuron development is a multiphasic process characterized by sequential steps of neural progenitor proliferation, cell cycle exit, neuroblast migration and neuronal differentiation. This series of events requires an extensive and dynamic remodeling of the cell cytoskeleton at each step of the process. As major regulators of the cytoskeleton, the family of small Rho GTPases has been shown to play essential functions in cerebral cortex development. Here we review in vivo findings that support the contribution of Rho GTPases to cortical projection neuron development and we address their involvement in the etiology of cerebral cortex malformations.

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.

Growth and folding of the mammalian cerebral cortex: from molecules to malformations

The size and extent of folding of the mammalian cerebral cortex are important factors that influence a species' cognitive abilities and sensorimotor skills. Studies in various animal models and in humans have provided insight into the mechanisms that regulate cortical growth and folding. Both protein-coding genes and microRNAs control cortical size, and recent progress in characterizing basal progenitor cells and the genes that regulate their proliferation has contributed to our understanding of cortical folding. Neurological disorders linked to disruptions in cortical growth and folding have been associated with novel neurogenetic mechanisms and aberrant signalling pathways, and these findings have changed concepts of brain evolution and may lead to new medical treatments for certain disorders.

Sonic hedgehog signaling regulates mode of cell division of early cerebral cortex progenitors and increases astrogliogenesis

The morphogen Sonic Hedgehog (SHH) plays a critical role in the development of different tissues. In the central nervous system, SHH is well known to contribute to the patterning of the spinal cord and separation of the brain hemispheres. In addition, it has recently been shown that SHH signaling also contributes to the patterning of the telencephalon and establishment of adult neurogenic niches. In this work, we investigated whether SHH signaling influences the behavior of neural progenitors isolated from the dorsal telencephalon, which generate excitatory neurons and macroglial cells in vitro. We observed that SHH increases proliferation of cortical progenitors and generation of astrocytes, whereas blocking SHH signaling with cyclopamine has opposite effects. In both cases, generation of neurons did not seem to be affected. However, cell survival was broadly affected by blockade of SHH signaling. SHH effects were related to three different cell phenomena: mode of cell division, cell cycle length and cell growth. Together, our data in vitro demonstrate that SHH signaling controls cell behaviors that are important for proliferation of cerebral cortex progenitors, as well as differentiation and survival of neurons and astroglial cells.

Drug-Induced Apoptosis: Mechanism by which Alcohol and Many Other Drugs Can Disrupt Brain Development

Maternal ingestion of alcohol during pregnancy can cause a disability syndrome termed Fetal Alcohol Spectrum Disorder (FASD), which may include craniofacial malformations, structural pathology in the brain, and a variety of long-term neuropsychiatric disturbances. There is compelling evidence that exposure to alcohol during early embryogenesis (4th week of gestation) can cause excessive death of cell populations that are essential for normal development of the face and brain. While this can explain craniofacial malformations and certain structural brain anomalies that sometimes accompany FASD, in many cases these features are absent, and the FASD syndrome manifests primarily as neurobehavioral disorders. It is not clear from the literature how alcohol causes these latter manifestations. In this review we will describe a growing body of evidence documenting that alcohol triggers widespread apoptotic death of neurons and oligodendroglia (OLs) in the developing brain when administered to animals, including non-human primates, during a period equivalent to the human third trimester of gestation. This cell death reaction is associated with brain changes, including overall or regional reductions in brain mass, and long-term neurobehavioral disturbances. We will also review evidence that many drugs used in pediatric and obstetric medicine, including general anesthetics (GAs) and anti-epileptics (AEDs), mimic alcohol in triggering widespread apoptotic death of neurons and OLs in the third trimester-equivalent animal brain, and that human children exposed to GAs during early infancy, or to AEDs during the third trimester of gestation, have a significantly increased incidence of FASD-like neurobehavioral disturbances. These findings provide evidence that exposure of the developing human brain to GAs in early infancy, or to alcohol or AEDs in late gestation, can cause FASD-like neurodevelopmental disability syndromes. We propose that the mechanism by which alcohol, GAs and AEDs produce neurobehavioral deficit syndromes is by triggering apoptotic death and deletion of neurons and OLs (or their precursors) from the developing brain. Therefore, there is a need for research aimed at deciphering mechanisms by which these agents trip the apoptosis trigger, the ultimate goal being to learn how to prevent these agents from causing neurodevelopmental disabilities.

Cell death atlas of the postnatal mouse ventral forebrain and hypothalamus: effects of age and sex

Naturally occurring cell death is essential to the development of the mammalian nervous system. Although the importance of developmental cell death has been appreciated for decades, there is no comprehensive account of cell death across brain areas in the mouse. Moreover, several regional sex differences in cell death have been described for the ventral forebrain and hypothalamus, but it is not known how widespread the phenomenon is. We used immunohistochemical detection of activated caspase-3 to identify dying cells in the brains of male and female mice from postnatal day (P) 1 to P11. Cell death density, total number of dying cells, and regional volume were determined in 16 regions of the hypothalamus and ventral forebrain (the anterior hypothalamus, arcuate nucleus, anteroventral periventricular nucleus, medial preoptic nucleus, paraventricular nucleus, suprachiasmatic nucleus, and ventromedial nucleus of the hypothalamus; the basolateral, central, and medial amygdala; the lateral and principal nuclei of the bed nuclei of the stria terminalis; the caudate-putamen; the globus pallidus; the lateral septum; and the islands of Calleja). All regions showed a significant effect of age on cell death. The timing of peak cell death varied between P1 to P7, and the average rate of cell death varied tenfold among regions. Several significant sex differences in cell death and/or regional volume were detected. These data address large gaps in the developmental literature and suggest interesting region-specific differences in the prevalence and timing of cell death in the hypothalamus and ventral forebrain.

Differences in amygdala cell proliferation between adolescent and young adult rats

Adolescence is characterized by changes in both behavior and neural organization. During this period, the amygdala, a structure that mediates social and emotional behaviors, is changing in terms of neural and glia density. We examined cell proliferation within the amygdala of adolescent (post natal day (PND) 31) and adult (PND 70) male Sprague-Dawley rats using BrdU (bromodeoxyuridine) to label dividing cells. BrdU-labeled cells were distributed throughout the amygdala, often found in fibers surrounding major nuclei. Using two independent cell counting strategies under light and confocal microcopy, respectively, we found significantly more labeled cells in the amygdala in adolescent compared to adult animals (239.3 ± 87.18 vs. 44.75 ± 13.68; n=4/group; p<.05). BrdU/doublecortin (DCX) positive cells constitute approximately 30% of all dividing cells in the amygdala in both adolescents and adults. These data suggest that compared to young adulthood, adolescence is a relatively active period of cell proliferation in the amygdala. Moreover, the normal decline in dividing cells with age does not preferentially affect cells co-containing DCX-immunoreactivity.

Fractionated irradiation-induced altered spatio-temporal cell distribution in the rat forebrain

Ionizing radiation as one of the strongest cytogenetic factors can induce significant injury to the adult brain. In the present study, adult male Wistar rats were exposed to whole-body irradiation with fractionated doses of gamma rays (a total dose of 3Gy). Seven, 14 and 21 days after irradiation the cell types located in the neurogenic anterior subventricular zone (SVZa) were labeled using immunohistochemistry for SVZa-derived young neurons and astrocytes. Cell counting was performed in four anatomical parts along the pathway known as the rostral migratory stream (RMS) represented by the SVZa, vertical arm, elbow and horizontal arm of the RMS. A considerable increase was seen in the number of neuroblasts in the SVZa, vertical arm and elbow on day 7 after irradiation. Until days 14 and 21 there was a marked decline in the density of young neurons, mostly in the horizontal arm of the RMS. In contrast, the number of astrocytes gradually increased in the caudal parts of the RMS until day 14 after irradiation. Strong enhancement was replaced by a steep decline within the RMS up to 21 days after treatment. Our results showed that the radiation response of proliferating cells originating from the SVZa may play a contributory role in the development of more adverse late radiation-induced effects.

Early neural cell death is an extensive, dynamic process in the embryonic chick and mouse retina

Orchestrated proliferation, differentiation, and cell death contribute to the generation of the complex cytoarchitecture of the central nervous system, including that of the neuroretina. However, few studies have comprehensively compared the spatiotemporal patterns of these 3 processes, or their relative magnitudes. We performed a parallel study in embryonic chick and mouse retinas, focusing on the period during which the first neurons, the retinal ganglion cells (RGCs), are generated. The combination of in vivo BrdU incorporation, immunolabeling of retinal whole mounts for BrdU and for the neuronal markers Islet1/2 and β III-tubulin, and TUNEL allowed for precise cell scoring and determination the spatiotemporal patterns of cell proliferation, differentiation, and death. As predicted, proliferation preceded differentiation. Cell death and differentiation overlapped to a considerable extent, although the magnitude of cell death exceeded that of neuronal differentiation. Precise quantification of the population of recently born RGCs, identified by BrdU and β III-tubulin double labeling, combined with cell death inhibition using a pan-caspase inhibitor, revealed that apoptosis decreased this population by half shortly after birth. Taken together, our findings provide important insight into the relevance of cell death in neurogenesis.