Insulinoma-Associated 1 Has a Panneurogenic Role and Promotes the Generation and Expansion of Basal Progenitors in the Developing Mouse Neocortex

Control of Neuronal Development by T-Box Genes in the Brain

T-box transcription factors play key roles in the regulation of developmental processes such as cell differentiation and migration. Mammals have 17 T-box genes, of which several regulate brain development. The Tbr1 subfamily of T-box genes is particularly important in development of the cerebral cortex, olfactory bulbs (OBs), and cerebellum. This subfamily is comprised of Tbr1, Tbr2 (also known as Eomes), and Tbx21. In developing cerebral cortex, Tbr2 and Tbr1 are expressed during successive stages of differentiation in the pyramidal neuron lineage, from Tbr2+ intermediate progenitors to Tbr1+ postmitotic glutamatergic neurons. At each stage, Tbr2 and Tbr1 regulate laminar and regional identity of cortical projection neurons, cell migration, and axon guidance. In the OB, Tbr1 subfamily genes regulate neurogenesis of mitral and tufted cells, and glutamatergic juxtaglomerular interneurons. Tbr2 is also prominent in the development of retinal ganglion cells in nonimage-forming pathways. Other regions that require Tbr2 or Tbr1 in development or adulthood include the cerebellum and adult dentate gyrus. In humans, de novo mutations in TBR1 are important causes of sporadic autism and intellectual disability. Further studies of T-box transcription factors will enhance our understanding of neurodevelopmental disorders and inform approaches to new therapies.

Direct neuronal reprogramming: learning from and for development

The key signalling pathways and transcriptional programmes that instruct neuronal diversity during development have largely been identified. In this Review, we discuss how this knowledge has been used to successfully reprogramme various cell types into an amazing array of distinct types of functional neurons. We further discuss the extent to which direct neuronal reprogramming recapitulates embryonic development, and examine the particular barriers to reprogramming that may exist given a cell's unique developmental history. We conclude with a recently proposed model for cell specification called the ‘Cook Islands’ model, and consider whether it is a fitting model for cell specification based on recent results from the direct reprogramming field.

Intermediate Progenitor Cohorts Differentially Generate Cortical Layers and Require Tbr2 for Timely Acquisition of Neuronal Subtype Identity

Intermediate progenitors (IPs) amplify the production of pyramidal neurons, but their role in selective genesis of cortical layers or neuronal subtypes remains unclear. Using genetic lineage tracing in mice, we find that IPs destined to produce upper cortical layers first appear early in corticogenesis, by embryonic day 11.5. During later corticogenesis, IP laminar fates are progressively limited to upper layers. We examined the role of Tbr2, an IP-specific transcription factor, in laminar fate regulation using Tbr2 conditional mutant mice. Upon Tbr2 inactivation, fewer neurons were produced by immediate differentiation and laminar fates were shifted upward. Genesis of subventricular mitoses was, however, not reduced in the context of a Tbr2-null cortex. Instead, neuronal and laminar differentiation were disrupted and delayed. Our findings indicate that upper-layer genesis depends on IPs from many stages of corticogenesis and that Tbr2 regulates the tempo of laminar fate implementation for all cortical layers.

A Dynamic Unfolded Protein Response Contributes to the Control of Cortical Neurogenesis

The cerebral cortex contains layers of neurons sequentially generated by distinct lineage-related progenitors. At the onset of corticogenesis, the first-born progenitors are apical progenitors (APs), whose asymmetric division gives birth directly to neurons. Later, they switch to indirect neurogenesis by generating intermediate progenitors (IPs), which give rise to projection neurons of all cortical layers. While a direct lineage relationship between APs and IPs has been established, the molecular mechanism that controls their transition remains elusive. Here we show that interfering with codon translation speed triggers ER stress and the unfolded protein response (UPR), further impairing the generation of IPs and leading to microcephaly. Moreover, we demonstrate that a progressive downregulation of UPR in cortical progenitors acts as a physiological signal to amplify IPs and promotes indirect neurogenesis. Thus, our findings reveal a contribution of UPR to cell fate acquisition during mammalian brain development.

Understanding how differentiation is maintained: lessons from the Drosophila brain

The ability to maintain cells in a differentiated state and to prevent them from reprogramming into a multipotent state has recently emerged as a central theme in neural development as well as in oncogenesis. In the developing central nervous system (CNS) of the fruit fly Drosophila, several transcription factors were recently identified to be required in postmitotic cells to maintain differentiation, and in their absence, mature neurons undergo dedifferentiation, giving rise to proliferative neural stem cells and ultimately to tumor growth. In this review, we will highlight the current understanding of dedifferentiation and cell plasticity in the Drosophila CNS.

Relief of hypoxia by angiogenesis promotes neural stem cell differentiation by targeting glycolysis

Blood vessels are part of the stem cell niche in the developing cerebral cortex, but their in vivo role in controlling the expansion and differentiation of neural stem cells (NSCs) in development has not been studied. Here, we report that relief of hypoxia in the developing cerebral cortex by ingrowth of blood vessels temporo-spatially coincided with NSC differentiation. Selective perturbation of brain angiogenesis in vessel-specific Gpr124 null embryos, which prevented the relief from hypoxia, increased NSC expansion at the expense of differentiation. Conversely, exposure to increased oxygen levels rescued NSC differentiation in Gpr124 null embryos and increased it further in WT embryos, suggesting that niche blood vessels regulate NSC differentiation at least in part by providing oxygen. Consistent herewith, hypoxia-inducible factor (HIF)-1α levels controlled the switch of NSC expansion to differentiation. Finally, we provide evidence that high glycolytic activity of NSCs is required to prevent their precocious differentiation in vivo Thus, blood vessel function is required for efficient NSC differentiation in the developing cerebral cortex by providing oxygen and possibly regulating NSC metabolism.

INSM1: A Novel Immunohistochemical and Molecular Marker for Neuroendocrine and Neuroepithelial Neoplasms

OBJECTIVES

Neuroendocrine neoplasms (NENs) are heterogeneous neoplasms, which are sometimes malignant, although predicting metastasis is difficult. INSM1 is a transcription factor expressed transiently in embryonic neuroendocrine (NE) tissue, thought to coordinate termination of cell division with differentiation of NE and neuroepithelial cells. In adult tissues, INSM1 has been identified in multiple tumors of NE or neuroepithelial origin but has not been thoroughly investigated as a potential neoplastic marker.

METHODS

We evaluated INSM1 as a semiquantitative immunohistochemical (IHC) marker for NE and neuroepithelial neoplasms and as a quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) marker for gastrointestinal NENs (GI-NENs).

RESULTS

Using IHC, we found in normal adult tissue that INSM1 expression was highly restricted to nuclei of NE cells and tissues. INSM1 was not detected in any adult nonneoplastic, non-NE tissue. In neoplastic tissue, INSM1 was detectable by IHC in 88.3% of 129 NEN specimens. In contrast, INSM1 was detected by IHC in only one of 27 neoplasms without a neuroepithelial or NE component. Using qRT-PCR, we evaluated INSM1 gene expression in 113 GI-NEN specimens.

CONCLUSIONS

INSM1 expression was significantly increased in neoplastic vs nonneoplastic tissue. Furthermore, among midgut GI-NENs, neoplasms with known metastases showed significantly higher expression than those that had not yet metastasized.

Insm1 promotes neurogenic proliferation in delaminated otic progenitors

INSM1 is a zinc-finger protein expressed throughout the developing nervous system in late neuronal progenitors and nascent neurons. In the embryonic cortex and olfactory epithelium, Insm1 may promote the transition of progenitors from apical, proliferative, and uncommitted to basal, terminally-dividing and neuron producing. In the otocyst, delaminating and delaminated progenitors express Insm1, whereas apically-dividing progenitors do not. This expression pattern is analogous to that in embryonic olfactory epithelium and cortex (basal/subventricular progenitors). Lineage analysis confirms that auditory and vestibular neurons originate from Insm1-expressing cells. In the absence of Insm1, otic ganglia are smaller, with 40% fewer neurons. Accounting for the decrease in neurons, delaminated progenitors undergo fewer mitoses, but there is no change in apoptosis. We conclude that in the embryonic inner ear, Insm1 promotes proliferation of delaminated neuronal progenitors and hence the production of neurons, a similar function to that in other embryonic neural epithelia. Unexpectedly, we also found that differentiating, but not mature, outer hair cells express Insm1, whereas inner hair cells do not. Insm1 is the earliest known gene expressed in outer versus inner hair cells, demonstrating that nascent outer hair cells initiate a unique differentiation program in the embryo, much earlier than previously believed.

Detection of neuroendocrine tumors using promoter-specific secreted Gaussia luciferase

Accurate detection of neuroendocrine (NE) tumors is critically important for better prognosis and treatment outcomes in patients. To demonstrate the efficacy of using an adenoviral vector for the detection of NE tumors, we have constructed a pair of adenoviral vectors which, in combination, can conditionally replicate and release Gaussia luciferase into the circulation after infecting the NE tumors. The expression of these two vectors is regulated upstream by an INSM1-promoter (insulinoma-associated-1) that is specifically active in NE tumors and developing NE tissues, but silenced in normal adult tissues. In order to retain the tumor-specificity of the INSM1 promoter, we have modified the promoter using the core insulator sequence from the chicken β-globin HS4 insulator and the neuronal restrictive silencing element (NRSE). This modified INSM1-promoter can retain NE tumor specificity in an adenoviral construct while driving a mutated adenovirus E1A gene (∆24E1A), the Metridia, or Gaussia luciferase gene. The in vitro cell line and mouse xenograft human tumor studies revealed the NE specificity of the INSM1-promoter in NE lung cancer, neuroblastoma, medulloblastoma, retinoblastoma, and insulinoma. When we combined the INSM1-promoter driven Gaussia luciferase with ∆24E1A, the co-infected NE tumor secreted higher levels of Gaussia luciferase as compared to the INSM1p-Gaussia virus alone. In a mouse subcutaneous xenograft tumor model, the combination viruses secreted detectable level of Gaussia luciferase after infecting an INSM1-positive NE lung tumor for ≥12 days. Therefore, the INSM1-promoter specific conditional replicating adenovirus represents a sensitive diagnostic tool to aid clinicians in the detection of NE tumors.

Insulinoma-Associated Protein 1 Is a Crucial Regulator of Neuroendocrine Differentiation in Lung Cancer

Insulinoma-associated protein 1 (INSM1) is expressed exclusively in embryonic developing neuroendocrine (NE) tissues. INSM1 gene expression is specific for small-cell lung cancer (SCLC), along with achaete-scute homolog-like 1 (ASCL1) and several NE molecules, such as chromogranin A, synaptophysin, and neural cell adhesion molecule 1. However, the underlying biological role of INSM1 in lung cancer remains largely unknown. We first showed that surgically resected SCLC samples specifically expressed INSM1. Forced expression of the INSM1 gene in adenocarcinoma cell lines (H358 and H1975) induced the expression of ASCL1, brain-2 (BRN2), chromogranin A, synaptophysin, and neural cell adhesion molecule 1; in contrast, knockdown of the INSM1 gene by siRNA in SCLC (H69 and H889) decreased their expression. However, forced/knockdown expression of ASCL1 and BRN2 did not affect INSM1 expression. A chromatin immunoprecipitation study revealed that INSM1 bound to the promoter region of the ASCL1 gene. A xenotransplantation assay using tet-on INSM1 gene-transfected adenocarcinoma cell lines demonstrated that INSM1 induced NE differentiation and growth inhibition. Furthermore, we found that INSM1 was not expressed in non-small-cell lung cancer and some SCLC cell lines expressing Notch1-Hes1. By forced/knockdown expression of Notch1 or Hes1 genes, we revealed that Notch1-Hes1 signaling suppressed INSM1, as well as ASCL1 and BRN2. INSM1, expressed exclusively in SCLC, is a crucial regulator of NE differentiation in SCLCs, and is regulated by the Notch1-Hes1 signaling pathway.

Sustained Pax6 Expression Generates Primate-like Basal Radial Glia in Developing Mouse Neocortex

The evolutionary expansion of the neocortex in mammals has been linked to enlargement of the subventricular zone (SVZ) and increased proliferative capacity of basal progenitors (BPs), notably basal radial glia (bRG). The transcription factor Pax6 is known to be highly expressed in primate, but not mouse, BPs. Here, we demonstrate that sustaining Pax6 expression selectively in BP-genic apical radial glia (aRG) and their BP progeny of embryonic mouse neocortex suffices to induce primate-like progenitor behaviour. Specifically, we conditionally expressed Pax6 by in utero electroporation using a novel, Tis21–CreER^T2 mouse line. This expression altered aRG cleavage plane orientation to promote bRG generation, increased cell-cycle re-entry of BPs, and ultimately increased upper-layer neuron production. Upper-layer neuron production was also increased in double-transgenic mouse embryos with sustained Pax6 expression in the neurogenic lineage. Strikingly, increased BPs existed not only in the SVZ but also in the intermediate zone of the neocortex of these double-transgenic mouse embryos. In mutant mouse embryos lacking functional Pax6, the proportion of bRG among BPs was reduced. Our data identify specific Pax6 effects in BPs and imply that sustaining this Pax6 function in BPs could be a key aspect of SVZ enlargement and, consequently, the evolutionary expansion of the neocortex.

"Humanizing" the expression of the transcription factor Pax6 in cortical progenitors in the developing mouse brain is sufficient to endow these progenitors with a primate-like proliferative capacity.

During development, neural progenitors generate all cells that make up the mammalian brain. Differences in brain size among the various mammalian species are attributed to differences in the abundance and proliferative capacity of a specific class of neural progenitors called basal progenitors. Among these, a specific progenitor type called basal radial glia is thought to have played an important role during evolution in the expansion of the neocortex, the part of the brain associated with higher cognitive functions like conscious thought and language. In the neocortex, the expression of the transcription factor Pax6 in basal progenitors is low in rodents, but high in primates, including humans. In this study, we aimed to mimic the elevated expression pattern of Pax6 seen in humans in basal progenitors of the embryonic mouse neocortex. To this end, we generated a novel, transgenic mouse line that allows sustained expression of the Pax6 gene in basal progenitors. This elevated expression resulted in an increase in the generation of basal radial glia, in the proliferative capacity of basal progenitors, and, ultimately, in the number of neurons produced. Our findings demonstrate that altering the expression of a single transcription factor from a mouse to a human-like pattern suffices to induce a primate-like proliferative behaviour in neural progenitors, which is thought to underlie the evolutionary expansion of the neocortex.

The transcription factor Nerfin-1 prevents reversion of neurons into neural stem cells

Froldi et al. show that the Drosophila zinc finger transcription factor Nervous fingers 1 (Nerfin-1) locks neurons into differentiation, preventing their reversion into neuroblasts. The loss of Nerfin-1 causes reversion to multipotency and results in tumors in several neural lineages. RNA-seq and ChIP analysis show that Nerfin-1 administers its function by repression of self-renewing genes and activation of differentiation-specific genes.

Cellular dedifferentiation is the regression of a cell from a specialized state to a more multipotent state and is implicated in cancer. However, the transcriptional network that prevents differentiated cells from reacquiring stem cell fate is so far unclear. Neuroblasts (NBs), the Drosophila neural stem cells, are a model for the regulation of stem cell self-renewal and differentiation. Here we show that the Drosophila zinc finger transcription factor Nervous fingers 1 (Nerfin-1) locks neurons into differentiation, preventing their reversion into NBs. Following Prospero-dependent neuronal specification in the ganglion mother cell (GMC), a Nerfin-1-specific transcriptional program maintains differentiation in the post-mitotic neurons. The loss of Nerfin-1 causes reversion to multipotency and results in tumors in several neural lineages. Both the onset and rate of neuronal dedifferentiation in nerfin-1 mutant lineages are dependent on Myc- and target of rapamycin (Tor)-mediated cellular growth. In addition, Nerfin-1 is required for NB differentiation at the end of neurogenesis. RNA sequencing (RNA-seq) and chromatin immunoprecipitation (ChIP) analysis show that Nerfin-1 administers its function by repression of self-renewing-specific and activation of differentiation-specific genes. Our findings support the model of bidirectional interconvertibility between neural stem cells and their post-mitotic progeny and highlight the importance of the Nerfin-1-regulated transcriptional program in neuronal maintenance.

From trans to cis: transcriptional regulatory networks in neocortical development

Transcriptional mechanisms mediated by the binding of transcription factors (TFs) to cis-acting regulatory elements (CREs) in DNA play crucial roles in directing gene expression. While TFs have been extensively studied, less effort has gone towards the identification and functional characterization of CREs and associated epigenetic modulation. However, owing to methodological and analytical advances, more comprehensive studies of regulatory elements and mechanisms are now possible. We summarize recent progress in integrative analyses of these regulatory components in the development of the cerebral neocortex, the part of the brain involved in cognition and complex behavior. These studies are uncovering not only the underlying transcriptional regulatory networks, but also how these networks are altered across species and in neurological and psychiatric disorders.

Deficiency of the chromatin regulator BRPF1 causes abnormal brain development

Epigenetic mechanisms are important in different neurological disorders, and one such mechanism is histone acetylation. The multivalent chromatin regulator BRPF1 (bromodomain- and plant homeodomain-linked (PHD) zinc finger-containing protein 1) recognizes different epigenetic marks and activates three histone acetyltransferases, so it is both a reader and a co-writer of the epigenetic language. The three histone acetyltransferases are MOZ, MORF, and HBO1, which are also known as lysine acetyltransferase 6A (KAT6A), KAT6B, and KAT7, respectively. The MORF gene is mutated in four neurodevelopmental disorders sharing the characteristic of intellectual disability and frequently displaying callosal agenesis. Here, we report that forebrain-specific inactivation of the mouse Brpf1 gene caused early postnatal lethality, neocortical abnormalities, and partial callosal agenesis. With respect to the control, the mutant forebrain contained fewer Tbr2-positive intermediate neuronal progenitors and displayed aberrant neurogenesis. Molecularly, Brpf1 loss led to decreased transcription of multiple genes, such as Robo3 and Otx1, important for neocortical development. Surprisingly, elevated expression of different Hox genes and various other transcription factors, such as Lhx4, Foxa1, Tbx5, and Twist1, was also observed. These results thus identify an important role of Brpf1 in regulating forebrain development and suggest that it acts as both an activator and a silencer of gene expression in vivo.

A current review for biological monitoring of manganese with exposure, susceptibility, and response biomarkers

People can be easily exposed to manganese (Mn), the twelfth most abundant element, through various exposure routes. However, overexposure to Mn causes manganism, a motor syndrome similar to Parkinson disease, via interference of the several neurotransmitter systems, particularly the dopaminergic system in areas. At cellular levels, Mn preferentially accumulates in mitochondria and increases the generation of reactive oxygen species, which changes expression and activity of manganoproteins. Many studies have provided invaluable insights into the causes, effects, and mechanisms of the Mn-induced neurotoxicity. To regulate Mn exposure, many countries have performed biological monitoring of Mn with three major biomarkers: exposure, susceptibility, and response biomarkers. In this study, we review current statuses of Mn exposure via various exposure routes including food, high susceptible population, effects of genetic polymorphisms of metabolic enzymes or transporters (CYP2D6, PARK9, SLC30A10, etc.), alterations of the Mn-responsive proteins (i.e., glutamine synthetase, Mn-SOD, metallothioneins, and divalent metal trnsporter1), and epigenetic changes due to the Mn exposure. To minimize the effects of Mn exposure, further biological monitoring of Mn should be done with more sensitive and selective biomarkers.

Thalamic afferents influence cortical progenitors via ephrin A5-EphA4 interactions

The phenotype of excitatory cerebral cortex neurons is specified at the progenitor level, orchestrated by various intrinsic and extrinsic factors. Here, we provide evidence for a subcortical contribution to cortical progenitor regulation by thalamic axons via ephrin A5-EphA4 interactions. Ephrin A5 is expressed by thalamic axons and represents a high-affinity ligand for EphA4 receptors detected in cortical precursors. Recombinant ephrin A5-Fc protein, as well as ephrin A ligand-expressing, thalamic axons affect the output of cortical progenitor division in vitro. Ephrin A5-deficient mice show an altered division mode of radial glial cells (RGCs) accompanied by increased numbers of intermediate progenitor cells (IPCs) and an elevated neuronal production for the deep cortical layers at E13.5. In turn, at E16.5 the pool of IPCs is diminished, accompanied by reduced rates of generated neurons destined for the upper cortical layers. This correlates with extended infragranular layers at the expense of superficial cortical layers in adult ephrin A5-deficient and EphA4-deficient mice. We suggest that ephrin A5 ligands imported by invading thalamic axons interact with EphA4-expressing RGCs, thereby contributing to the fine-tuning of IPC generation and thus the proper neuronal output for cortical layers.

Insm1 promotes endocrine cell differentiation by modulating the expression of a network of genes that includes Neurog3 and Ripply3

Insulinoma associated 1 (Insm1) plays an important role in regulating the development of cells in the central and peripheral nervous systems, olfactory epithelium and endocrine pancreas. To better define the role of Insm1 in pancreatic endocrine cell development we generated mice with an Insm1^GFPCre reporter allele and used them to study Insm1-expressing and null populations. Endocrine progenitor cells lacking Insm1 were less differentiated and exhibited broad defects in hormone production, cell proliferation and cell migration. Embryos lacking Insm1 contained greater amounts of a non-coding Neurog3 mRNA splice variant and had fewer Neurog3/Insm1 co-expressing progenitor cells, suggesting that Insm1 positively regulates Neurog3. Moreover, endocrine progenitor cells that express either high or low levels of Pdx1, and thus may be biased towards the formation of specific cell lineages, exhibited cell type-specific differences in the genes regulated by Insm1. Analysis of the function of Ripply3, an Insm1-regulated gene enriched in the Pdx1-high cell population, revealed that it negatively regulates the proliferation of early endocrine cells. Taken together, these findings indicate that in developing pancreatic endocrine cells Insm1 promotes the transition from a ductal progenitor to a committed endocrine cell by repressing a progenitor cell program and activating genes essential for RNA splicing, cell migration, controlled cellular proliferation, vasculogenesis, extracellular matrix and hormone secretion.

Control of cerebral size and thickness

The mammalian neocortex is a sheet of cells covering the cerebrum that provides the structural basis for the perception of sensory inputs, motor output responses, cognitive function, and mental capacity of primates. Recent discoveries promote the concept that increased cortical surface size and thickness in phylogenetically advanced species is a result of an increased generation of neurons, a process that underlies higher cognitive and intellectual performance in higher primates and humans. Here, we review some of the advances in the field, focusing on the diversity of neocortical progenitors in different species and the cellular mechanisms of neurogenesis. We discuss recent views on intrinsic and extrinsic molecular determinants, including the role of epigenetic chromatin modifiers and microRNA, in the control of neuronal output in developing cortex and in the establishment of normal cortical architecture.

Progenitor genealogy in the developing cerebral cortex

The mammalian cerebral cortex is characterized by a complex histological organization that reflects the spatio-temporal stratifications of related stem and neural progenitor cells, which are responsible for the generation of distinct glial and neuronal subtypes during development. Some work has been done to shed light on the existing filiations between these progenitors as well as their respective contribution to cortical neurogenesis. The aim of the present review is to summarize the current views of progenitor hierarchy and relationship in the developing cortex and to further discuss future research directions that would help us to understand the molecular and cellular regulating mechanisms involved in cerebral corticogenesis.

Developmental stage-dependent transcriptional regulatory pathways control neuroblast lineage progression

Neuroblasts generate neurons with different functions by asymmetric cell division, cell cycle exit and differentiation. The underlying transcriptional regulatory pathways remain elusive. Here, we performed genetic screens in C. elegans and identified three evolutionarily conserved transcription factors (TFs) essential for Q neuroblast lineage progression. Through live cell imaging and genetic analysis, we showed that the storkhead TF HAM-1 regulates spindle positioning and myosin polarization during asymmetric cell division and that the PAR-1-like kinase PIG-1 is a transcriptional regulatory target of HAM-1. The TEAD TF EGL-44, in a physical association with the zinc-finger TF EGL-46, instructs cell cycle exit after the terminal division. Finally, the Sox domain TF EGL-13 is necessary and sufficient to establish the correct neuronal fate. Genetic analysis further demonstrated that HAM-1, EGL-44/EGL-46 and EGL-13 form three transcriptional regulatory pathways. We have thus identified TFs that function at distinct developmental stages to ensure appropriate neuroblast lineage progression and suggest that their vertebrate homologs might similarly regulate neural development.

Insulinoma-associated 1a (Insm1a) is required for photoreceptor differentiation in the zebrafish retina

The zinc-finger transcription factor insulinoma-associated 1 (Insm1, previously IA-1) is expressed in the developing nervous and neuroendocrine systems, and is required for cell type specific differentiation. Expression of Insm1 is largely absent in the adult, although it is present in neurogenic regions of the adult brain and zebrafish retina. While expression of Insm1 has also been observed in the embryonic retina of numerous vertebrate species, its function during retinal development has remained unexplored. Here, we demonstrate that in the developing zebrafish retina, insm1a is required for photoreceptor differentiation. Insm1a-deficient embryos were microphthalmic and displayed defects in rod and cone photoreceptor differentiation. Rod photoreceptor cells were more sensitive to loss of insm1a expression than were cone photoreceptor cells. Additionally, we provide evidence that insm1a regulates cell cycle progression of retinoblasts, and functions upstream of the bHLH transcription factors ath5/atoh7 and neurod, and the photoreceptor specification genes crx and nr2e3. Finally, we show that insm1a is negatively regulated by Notch-Delta signaling. Taken together, our data demonstrate that Insm1 influences neuronal subtype differentiation during retinal development.

Trnp1 regulates expansion and folding of the mammalian cerebral cortex by control of radial glial fate

Evolution of the mammalian brain encompassed a remarkable increase in size of the cerebral cortex, which includes tangential and radial expansion. However, the mechanisms underlying these key features are still largely unknown. Here, we identified the DNA-associated protein Trnp1 as a regulator of cerebral cortex expansion in both of these dimensions. Gain- and loss-of-function experiments in the mouse cerebral cortex in vivo demonstrate that high Trnp1 levels promote neural stem cell self-renewal and tangential expansion. In contrast, lower levels promote radial expansion, with a potent increase of the number of intermediate progenitors and basal radial glial cells leading to folding of the otherwise smooth murine cerebral cortex. Remarkably, TRNP1 expression levels exhibit regional differences in the cerebral cortex of human fetuses, anticipating radial or tangential expansion. Thus, the dynamic regulation of Trnp1 is critical to control tangential and radial expansion of the cerebral cortex in mammals.

Spatial distribution of prominin-1 (CD133)-positive cells within germinative zones of the vertebrate brain

BACKGROUND

In mammals, embryonic neural progenitors as well as adult neural stem cells can be prospectively isolated based on the cell surface expression of prominin-1 (CD133), a plasma membrane glycoprotein. In contrast, characterization of neural progenitors in non-mammalian vertebrates endowed with significant constitutive neurogenesis and inherent self-repair ability is hampered by the lack of suitable cell surface markers. Here, we have investigated whether prominin-1-orthologues of the major non-mammalian vertebrate model organisms show any degree of conservation as for their association with neurogenic geminative zones within the central nervous system (CNS) as they do in mammals or associated with activated neural progenitors during provoked neurogenesis in the regenerating CNS.

METHODS

We have recently identified prominin-1 orthologues from zebrafish, axolotl and chicken. The spatial distribution of prominin-1-positive cells--in comparison to those of mice--was mapped in the intact brain in these organisms by non-radioactive in situ hybridization combined with detection of proliferating neural progenitors, marked either by proliferating cell nuclear antigen or 5-bromo-deoxyuridine. Furthermore, distribution of prominin-1 transcripts was investigated in the regenerating spinal cord of injured axolotl.

RESULTS

Remarkably, a conserved association of prominin-1 with germinative zones of the CNS was uncovered as manifested in a significant co-localization with cell proliferation markers during normal constitutive neurogenesis in all species investigated. Moreover, an enhanced expression of prominin-1 became evident associated with provoked, compensatory neurogenesis during the epimorphic regeneration of the axolotl spinal cord. Interestingly, significant prominin-1-expressing cell populations were also detected at distinct extraventricular (parenchymal) locations in the CNS of all vertebrate species being suggestive of further, non-neurogenic neural function(s).

CONCLUSION/INTERPRETATION

Collectively, our work provides the first data set describing a comparative analysis of prominin-1-positive progenitor cells across species establishing a framework for further functional characterization in the context of regeneration.

FoxP2 regulates neurogenesis during embryonic cortical development

The transcription factor FoxP2 has been associated with the development of human speech but the underlying cellular function of FoxP2 is still unclear. Here we provide evidence that FoxP2 regulates genesis of some intermediate progenitors and neurons in the mammalian cortex, one of the key centers for human speech. Specifically, knockdown of FoxP2 in embryonic cortical precursors inhibits neurogenesis, at least in part by inhibiting the transition from radial glial precursors to neurogenic intermediate progenitors. Moreover, overexpression of human, but not mouse, FoxP2 enhances the genesis of intermediate progenitors and neurons. In contrast, expression of a human FoxP2 mutant that causes vocalization deficits decreases neurogenesis, suggesting that in the murine system human FoxP2 acts as a gain-of-function protein, while a human FoxP2 mutant acts as a dominant-inhibitory protein. These results support the idea that FoxP2 regulates the transition from neural precursors to transit-amplifying progenitors and ultimately neurons, and shed light upon the molecular changes that might contribute to evolution of the mammalian cortex.

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

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

Snail/Gfi-1 (SNAG) family zinc finger proteins in transcription regulation, chromatin dynamics, cell signaling, development, and disease

The Snail/Gfi-1 (SNAG) family of zinc finger proteins is a group of transcriptional repressors that have been intensively studied in mammals. SNAG family members are similarly structured with an N-terminal SNAG repression domain and a C-terminal zinc finger DNA binding domain, however, the spectrum of target genes they regulate and the ranges of biological functions they govern vary widely between them. They play active roles in transcriptional regulation, formation of repressive chromatin structure, cellular signaling and developmental processes. They can also result in disease states due to deregulation. We have performed a thorough investigation of the relevant literature and present a comprehensive mini-review. Based on the available information, we also propose a mechanism by which SNAG family members may function.

Adult periventricular neural stem cells: outstanding progress and outstanding issues

Twenty years have past since the existence of neural stem cells (NSCs) within the walls of the adult lateral ventricles was discovered. During this period of time, great strides have been made in every facet of our understanding of this adult periventricular NSC population. In this review, some of the fields' major advancements regarding the nature and function of adult periventricular NSCs are examined. We bring attention to issues related to NSC identity, potential, and the role of Notch signaling in regulating quiescence and activation that warrant further investigation. Progress in the understanding of human adult NSCs will aid in the development of tools required to advance therapies not only for brain repair after injury or disease but may also lead to novel therapeutics for brain tumors.

Primary cilia and forebrain development

With a microtubule-based axoneme supporting its plasma membrane-ensheathed projection from the basal body of almost all cell types in the human body, and present in only one copy per cell, the primary cilium can be considered an organelle sui generis. Although it was first observed and recorded in histological studies from the late 19th century, the tiny structure was essentially forgotten for many decades. In the past ten years, however, scientists have turned their eyes once again upon primary cilia and realized that they are very important for the development of almost all organs in the mammalian body, especially those dependent upon the signaling from members Hedgehog family, such as Indian and Sonic hedgehog. In this review, we outline the roles that primary cilia play in forebrain development, not just in the crucial transduction of Sonic hedgehog signaling, but also new results showing that cilia are important for cell cycle progression in proliferating neural precursors. We will focus upon cerebral cortex development but will also discuss the importance of cilia for the embryonic hippocampus, olfactory bulb, and diencephalon.

Slit/Robo signaling modulates the proliferation of central nervous system progenitors

Neurogenesis relies on a delicate balance between progenitor maintenance and neuronal production. Progenitors divide symmetrically to increase the pool of dividing cells. Subsequently, they divide asymmetrically to self-renew and produce new neurons or, in some brain regions, intermediate progenitor cells (IPCs). Here we report that central nervous system progenitors express Robo1 and Robo2, receptors for Slit proteins that regulate axon guidance, and that absence of these receptors or their ligands leads to loss of ventricular mitoses. Conversely, production of IPCs is enhanced in Robo1/2 and Slit1/2 mutants, suggesting that Slit/Robo signaling modulates the transition between primary and intermediate progenitors. Unexpectedly, these defects do not lead to transient overproduction of neurons, probably because supernumerary IPCs fail to detach from the ventricular lining and cycle very slowly. At the molecular level, the role of Slit/Robo in progenitor cells involves transcriptional activation of the Notch effector Hes1. These findings demonstrate that Robo signaling modulates progenitor cell dynamics in the developing brain.

Molecular control of neurogenesis: a view from the mammalian cerebral cortex

The mammalian nervous system is the most complex organ of any living organism. How this complexity is generated during neural development is just beginning to be elucidated. This article discusses the signaling, transcriptional, and epigenetic mechanisms that are involved in neural development. The first part focuses on molecules that control neuronal numbers through regulation of the timing of onset of neurogenesis, the timing of the neuronal-to-glial switch, and the rate of progenitor proliferation. The second part focuses on molecules that control neuronal diversity by generating spatially or temporally distinct populations of neuronal progenitors. Most of the studies discussed in this article are focused on the developing mammalian cerebral cortex, because this is one of the main model systems for neural developmental studies and many of the mechanisms identified in this tissue also operate elsewhere in the developing brain and spinal cord.

Insm1a-mediated gene repression is essential for the formation and differentiation of Müller glia-derived progenitors in the injured retina

In zebrafish, retinal injury stimulates Müller glia (MG) reprograming; allowing them to generate multipotent progenitors that regenerate damaged cells and restore vision. Recent studies suggest transcriptional repression may underlie these events. To identify these repressors, we compared the transcriptomes of MG and MG-derived progenitors and identified insm1a, a transcriptional repressor exhibiting a biphasic pattern of expression that is essential for retina regeneration. Insm1a was found to suppress ascl1a and its own expression and link injury-dependent ascl1a induction with dickkopf (dkk) suppression, which is necessary for MG dedifferentiation. We also found that Insm1a was responsible for sculpting the zone of injury-responsive MG by suppressing hb-egf_a expression. Finally, we provide evidence that Insm1a stimulates progenitor cell cycle exit by suppressing a genetic program driving progenitor proliferation. Our studies identify Insm1a as a key regulator of retina regeneration and provide a mechanistic understanding of how it contributes to multiple phases of this process.

An oblique view on the role of spindle orientation in vertebrate neurogenesis

Neurogenesis is a dynamic process that produces a diverse number of glial and neural cell types from a limited number of neural stem cells throughout development and into adulthood. After an initial period of amplification through symmetric division, neural stem cells rely on asymmetric modes of division to self-renew while producing more committed progeny. Understanding the molecular mechanisms regulating the choice between symmetric and asymmetric modes of division is essential to understand human brain development and pathologies, and to explain the increasing cortical complexity observed in evolution. A popular model states the existence of a causal relationship between the orientation of the axis of division of stem cells and the fate of their progeny in many different tissues, but the validity of the model in neural stem cells is not clear. In this review, we briefly present the diversity of neural stem cells and intermediate progenitors in the developing central nervous system. We then draw a historic overview of the assumed causal relationship between spindle orientation and fate determination. We show how this prompted a search for regulators of spindle orientation, and present the current state of knowledge on the mechanism. Finally, we review data on the effect of defective spindle orientation and try to integrate conflicting observations by presenting alternative mechanisms that may regulate the choice between symmetric and asymmetric outcomes.

Asymmetric segregation of the double-stranded RNA binding protein Staufen2 during mammalian neural stem cell divisions promotes lineage progression

Asymmetric cell divisions are a fundamental feature of neural development, and misregulation can lead to brain abnormalities or tumor formation. During an asymmetric cell division, molecular determinants are segregated preferentially into one daughter cell to specify its fate. An important goal is to identify the asymmetric determinants in neural progenitor cells, which could be tumor suppressors or inducers of specific neural fates. Here, we show that the double-stranded RNA-binding protein Stau2 is distributed asymmetrically during progenitor divisions in the developing mouse cortex, preferentially segregating into the Tbr2(+) neuroblast daughter, taking with it a subset of RNAs. Knockdown of Stau2 stimulates differentiation and overexpression produces periventricular neuronal masses, demonstrating its functional importance for normal cortical development. We immunoprecipitated Stau2 to examine its cargo mRNAs, and found enrichment for known asymmetric and basal cell determinants, such as Trim32, and identified candidates, including a subset involved in primary cilium function.

GSK3 temporally regulates neurogenin 2 proneural activity in the neocortex

The neocortex is comprised of six neuronal layers that are generated in a defined temporal sequence. While extrinsic and intrinsic cues are known to regulate the sequential production of neocortical neurons, how these factors interact and function in a coordinated manner is poorly understood. The proneural gene Neurog2 is expressed in progenitors throughout corticogenesis, but is only required to specify early-born, deep-layer neuronal identities. Here, we examined how neuronal differentiation in general and Neurog2 function in particular are temporally controlled during murine neocortical development. We found that Neurog2 proneural activity declines in late corticogenesis, correlating with its phosphorylation by GSK3 kinase. Accordingly, GSK3 activity, which is negatively regulated by canonical Wnt signaling, increases over developmental time, while Wnt signaling correspondingly decreases. When ectopically activated, GSK3 inhibits Neurog2-mediated transcription in cultured cells and Neurog2 proneural activities in vivo. Conversely, a reduction in GSK3 activity promotes the precocious differentiation of later stage cortical progenitors without influencing laminar fate specification. Mechanistically, we show that GSK3 suppresses Neurog2 activity by influencing its choice of dimerization partner, promoting heterodimeric interactions with E47 (Tcfe2a), as opposed to Neurog2-Neurog2 homodimer formation, which occurs when GSK3 activity levels are low. At the functional level, Neurog2-E47 heterodimers have a reduced ability to transactivate neuronal differentiation genes compared with Neurog2-Neurog2 homodimers, both in vitro and in vivo. We thus conclude that the temporal regulation of Neurog2-E47 heterodimerization by GSK3 is a central component of the neuronal differentiation "clock" that coordinates the timing and tempo of neocortical neurogenesis in mouse.

Crucial first steps: the transcriptional control of neuron delamination

A crucial event in the birth of a neuron is the detachment of its apical process from the neuroepithelium. In this issue of Neuron, Rousso et al. (2012) show that repression of N-cadherin by Foxp transcription factors disrupts apical adherens junctions and triggers neurogenesis.

Regulation of archicortical arealization by the transcription factor Zbtb20

The molecular mechanisms of regionalization of the medial pallium (MP), the anlage of the hippocampus, and transitional (cingulate and retrosplenial) cortices are largely unknown. Previous analyses have outlined an important role of the transcription factor (TF) Zbtb20 for hippocampal CA1 field specification (Nielsen et al. (2007) Development 134:1133-1140; Nielsen et al. (2010) Cereb Cortex 20:1904-1914; Xie et al. (2010) Proc Natl Acad Sci USA 107:6510-6515). Here, we present novel data showing that Zbtb20 exhibits a ventral(high)-to-dorsal(low) gradient of expression in MP progenitors as well as an expression in postmitotic cells at the transitional cortex/neocortex border. Our detailed pattern analysis revealed that in Zbtb20 loss-of-function the molecular borders between neocortical, transitional, and hippocampal fields are progressively shifted ventrally, leading to an ectopic positioning of all dorsal fields into the neighboring ventrally located areas. Thus, in addition to its known importance for the specification of the hippocampal CA1 sector, the graded expression of TF Zbtb20 in ventricular zone of MP appears to translate early positional information for establishment of all developing MP fields. Our data also suggest that the signaling factor Wnt3a is a putative molecular partner of TF Zbtb20 in this patterning process.

Kdm2b promotes induced pluripotent stem cell generation by facilitating gene activation early in reprogramming

Transcription-factor-directed reprogramming from somatic cells to induced pluripotent stem cells (iPSCs) is by nature an epigenetic process of cell fate change. Previous studies have demonstrated that this inefficient process can be facilitated by the inclusion of additional factors. To gain insight into the reprogramming mechanism, we aimed to identify epigenetic enzymes capable of promoting iPSC generation. Here we show that Kdm2b, a histone H3 Lys 36 dimethyl (H3K36me2)-specific demethylase, has the capacity to promote iPSC generation. This capacity depends on its demethylase and DNA-binding activities, but is largely independent of its role in antagonizing senescence. Kdm2b functions at the beginning of the reprogramming process and enhances activation of early responsive genes in reprogramming. Kdm2b contributes to gene activation by binding to and demethylating the gene promoters. Our studies not only identify an important epigenetic factor for iPSC generation, but also reveal the molecular mechanism underlying how Kdm2b contributes to reprogramming.

Transcriptional control of differentiation and neurogenesis in autonomic ganglia

Autonomic neuron development is controlled by a network of transcription factors, which is induced by bone morphogenetic protein signalling in neural crest progenitor cells. This network intersects with a transcriptional program in migratory neural crest cells that pre-specifies autonomic neuron precursor cells. Recent findings demonstrate that the transcription factors acting in the initial specification and differentiation of sympathetic neurons are also important for the proliferation of progenitors and immature neurons during neurogenesis. Elimination of Phox2b, Hand2 and Gata3 in differentiated neurons affects the expression of subtype-specific and/or generic neuronal properties or neuron survival. Taken together, transcription factors previously shown to act in initial neuron specification and differentiation display a much broader spectrum of functions, including control of neurogenesis and the maintenance of subtype characteristics and survival of mature neurons.

Intermediate progenitors are increased by lengthening of the cell cycle through calcium signaling and p53 expression in human neural progenitors

During development, neurons can be generated directly from a multipotent progenitor or indirectly through an intermediate progenitor (IP). This last mode of division amplifies the progeny of neurons. The mechanisms governing the generation and behavior of IPs are not well understood. In this work, we demonstrate that the lengthening of the cell cycle enhances the generation of neurons in a human neural progenitor cell system in vitro and also the generation and expansion of IPs. These IPs are insulinoma-associated 1 (Insm1)(+)/BTG family member 2 (Btg2)(-), which suggests an increase in a self-amplifying IP population. Later the cultures express neurogenin 2 (Ngn2) and become neurogenic. The signaling responsible for this cell cycle modulation is investigated. It is found that the release of calcium from the endoplasmic reticulum to the cytosol in response to B cell lymphoma-extra large overexpression or ATP addition lengths the cell cycle and increases the number of IPs and, in turn, the final neuron outcome. Moreover, data suggest that the p53-p21 pathway is responsible for the changes in cell cycle. In agreement with this, increased p53 levels are necessary for a calcium-induced increase in neurons. Our findings contribute to understand how calcium signaling can modulate cell cycle length during neurogenesis.

ATP13A2 (PARK9) polymorphisms influence the neurotoxic effects of manganese

INTRODUCTION

A higher prevalence of individuals affected by Parkinsonism was found in Valcamonica, Italy. This may be related to ferro-alloy smelters in the area, releasing manganese (Mn) in the air, soil and water for about a century. There exists individual susceptibility for Mn neurotoxicity.

AIM

To analyse how polymorphism in genes regulating Mn metabolism and toxicity can modify neurophysiological effects of Mn exposure.

MATERIALS AND METHODS

Elderly (N=255) and adolescents (N=311) from Northern Italy were examined for neuromotor and olfactory functions. Exposure to Mn was assessed in blood and urine by atomic absorption spectroscopy and in soil by a portable instrument based on X-Ray fluorescence technology. Polymorphisms in the Parkinson-related gene ATPase type 13A2 (ATP13A2, also called PARK9: rs3738815, rs2076602, rs4920608, rs2871776 and rs2076600), and in the secretory pathway Ca(2+)/Mn(2+) ATPase isoform 1 gene (SPCA1: rs218498, rs3773814 and rs2669858) were analysed by TaqMan probes.

RESULTS

For both adolescents and elderly, negative correlations between Mn in soil and motor coordination (R(s)=-0.20, p<0.001; R(s)=-0.13, p=0.05, respectively) were demonstrated. Also among adolescents, negative correlations were seen between Mn in soil with odor identification (R(s)=-0.17, p<0.01). No associations were seen for Mn in blood or urine. ATP13A2 polymorphisms rs4920608 and rs2871776 significantly modified the effects of Mn exposure on impaired motor coordination in elderly (p for interaction=0.029, p=0.041, respectively), also after adjustments for age and gender. The rs2871776 altered a binding site for transcription factor insulinoma-associated 1.

CONCLUSIONS

ATP13A2 variation may be a risk marker for neurotoxic effects of Mn in humans.

Basolateral rather than apical primary cilia on neuroepithelial cells committed to delamination

Delamination of neural progenitors from the apical adherens junction belt of the neuroepithelium is a hallmark of cerebral cortex development and evolution. Specific cell biological processes preceding this delamination are largely unknown. Here, we identify a novel, pre-delamination state of neuroepithelial cells in mouse embryonic neocortex. Specifically, in a subpopulation of neuroepithelial cells that, like all others, exhibit apical-basal polarity and apical adherens junctions, the re-establishing of the primary cilium after mitosis occurs at the basolateral rather than the apical plasma membrane. Neuroepithelial cells carrying basolateral primary cilia appear at the onset of cortical neurogenesis, increase in abundance with its progression, selectively express the basal (intermediate) progenitor marker Tbr2, and eventually delaminate from the apical adherens junction belt to become basal progenitors, translocating their nucleus from the ventricular to the subventricular zone. Overexpression of insulinoma-associated 1, a transcription factor known to promote the generation of basal progenitors, increases the proportion of basolateral cilia. Basolateral cilia in cells delaminating from the apical adherens junction belt are preferentially found near spot-like adherens junctions, suggesting that the latter provide positional cues to basolateral ciliogenesis. We conclude that re-establishing a basolateral primary cilium constitutes the first known cell biological feature preceding neural progenitor delamination.

A new approach to manipulate the fate of single neural stem cells in tissue

A challenge in the field of neural stem cell biology is the mechanistic dissection of single stem cell behavior in tissue. Although such behavior can be tracked by sophisticated imaging techniques, current methods of genetic manipulation do not allow researchers to change the level of a defined gene product on a truly acute time scale and are limited to very few genes at a time. To overcome these limitations, we established microinjection of neuroepithelial/radial glial cells (apical progenitors) in organotypic slice culture of embryonic mouse brain. Microinjected apical progenitors showed cell cycle parameters that were indistinguishable to apical progenitors in utero, underwent self-renewing divisions and generated neurons. Microinjection of single genes, recombinant proteins or complex mixtures of RNA was found to elicit acute and defined changes in apical progenitor behavior and progeny fate. Thus, apical progenitor microinjection provides a new approach to acutely manipulating single neural stem and progenitor cells in tissue.

Basal progenitor cells in the embryonic mouse thalamus - their molecular characterization and the role of neurogenins and Pax6

Background

The size and cell number of each brain region are influenced by the organization and behavior of neural progenitor cells during embryonic development. Recent studies on developing neocortex have revealed the presence of neural progenitor cells that divide away from the ventricular surface and undergo symmetric divisions to generate either two neurons or two progenitor cells. These 'basal' progenitor cells form the subventricular zone and are responsible for generating the majority of neocortical neurons. However, not much has been studied on similar types of progenitor cells in other brain regions.

Results

We have identified and characterized basal progenitor cells in the embryonic mouse thalamus. The progenitor domain that generates all of the cortex-projecting thalamic nuclei contained a remarkably high proportion of basally dividing cells. Fewer basal progenitor cells were found in other progenitor domains that generate non-cortex projecting nuclei. By using intracellular domain of Notch1 (NICD) as a marker for radial glial cells, we found that basally dividing cells extended outside the lateral limit of radial glial cells, indicating that, similar to the neocortex and ventral telencephalon, the thalamus has a distinct subventricular zone. Neocortical and thalamic basal progenitor cells shared expression of some molecular markers, including Insm1, Neurog1, Neurog2 and NeuroD1. Additionally, basal progenitor cells in each region also expressed exclusive markers, such as Tbr2 in the neocortex and Olig2 and Olig3 in the thalamus. In Neurog1/Neurog2 double mutant mice, the number of basally dividing progenitor cells in the thalamus was significantly reduced, which demonstrates the roles of neurogenins in the generation and/or maintenance of basal progenitor cells. In Pax6 mutant mice, the part of the thalamus that showed reduced Neurog1/2 expression also had reduced basal mitosis.

Conclusions

Our current study establishes the existence of a unique and significant population of basal progenitor cells in the thalamus and their dependence on neurogenins and Pax6. These progenitor cells may have important roles in enhancing the generation of neurons within the thalamus and may also be critical for generating neuronal diversity in this complex brain region.

A novel function of the proneural factor Ascl1 in progenitor proliferation identified by genome-wide characterization of its targets

Proneural genes such as Ascl1 are known to promote cell cycle exit and neuronal differentiation when expressed in neural progenitor cells. The mechanisms by which proneural genes activate neurogenesis--and, in particular, the genes that they regulate--however, are mostly unknown. We performed a genome-wide characterization of the transcriptional targets of Ascl1 in the embryonic brain and in neural stem cell cultures by location analysis and expression profiling of embryos overexpressing or mutant for Ascl1. The wide range of molecular and cellular functions represented among these targets suggests that Ascl1 directly controls the specification of neural progenitors as well as the later steps of neuronal differentiation and neurite outgrowth. Surprisingly, Ascl1 also regulates the expression of a large number of genes involved in cell cycle progression, including canonical cell cycle regulators and oncogenic transcription factors. Mutational analysis in the embryonic brain and manipulation of Ascl1 activity in neural stem cell cultures revealed that Ascl1 is indeed required for normal proliferation of neural progenitors. This study identified a novel and unexpected activity of the proneural gene Ascl1, and revealed a direct molecular link between the phase of expansion of neural progenitors and the subsequent phases of cell cycle exit and neuronal differentiation.

Herpes simplex virus induces the marked up-regulation of the zinc finger transcriptional factor INSM1, which modulates the expression and localization of the immediate early protein ICP0

Background

Herpes simplex viruses (HSVs) rapidly shut off macromolecular synthesis in host cells. In contrast, global microarray analyses have shown that HSV infection markedly up-regulates a number of host cell genes that may play important roles in HSV-host cell interactions. To understand the regulatory mechanisms involved, we initiated studies focusing on the zinc finger transcription factor insulinoma-associated 1 (INSM1), a host cell protein markedly up-regulated by HSV infection.

Results

INSM1 gene expression in HSV-1-infected normal human epidermal keratinocytes increased at least 400-fold 9 h after infection; INSM1 promoter activity was also markedly stimulated. Expression and subcellular localization of the immediate early HSV protein ICP0 was affected by INSM1 expression, and chromatin immunoprecipitation (ChIP) assays revealed binding of INSM1 to the ICP0 promoter. Moreover, the role of INSM1 in HSV-1 infection was further clarified by inhibition of HSV-1 replication by INSM1-specific siRNA.

Conclusions

The results suggest that INSM1 up-regulation plays a positive role in HSV-1 replication, probably by binding to the ICP0 promoter.

Microarray analysis of XOPS-mCFP zebrafish retina identifies genes associated with rod photoreceptor degeneration and regeneration

PURPOSE

XOPS-mCFP transgenic zebrafish experience a continual cycle of rod photoreceptor development and degeneration throughout life, making them a useful model for investigating the molecular determinants of rod photoreceptor regeneration. The purpose of this study was to compare the gene expression profiles of wild-type and XOPS-mCFP retinas and identify genes that may contribute to the regeneration of the rods.

METHODS

Adult wild-type and XOPS-mCFP retinal mRNA was subjected to microarray analysis. Pathway analysis was used to identify biologically relevant processes that were significantly represented in the dataset. Expression changes were verified by RT-PCR. Selected genes were further examined during retinal development and in adult retinas by in situ hybridization and immunohistochemistry and in a transgenic fluorescent reporter line.

RESULTS

More than 600 genes displayed significant expression changes in XOPS-mCFP retinas compared with expression in wild-type controls. Many of the downregulated genes were associated with phototransduction, whereas upregulated genes were associated with several biological functions, including cell cycle, DNA replication and repair, and cell development and death. RT-PCR analysis of a subset of these genes confirmed the microarray

RESULTS

Three transcription factors (sox11b, insm1a, and c-myb), displaying increased expression in XOPS-mCFP retinas, were also expressed throughout retinal development and in the persistently neurogenic ciliary marginal zone.

CONCLUSIONS

This study identified numerous gene expression changes in response to rod degeneration in zebrafish and further suggests a role for the transcriptional regulators sox11b, insm1a, and c-myb in both retinal development and rod photoreceptor regeneration.

Genes expressed in Atoh1 neuronal lineages arising from the r1/isthmus rhombic lip

During embryogenesis, the rhombic lip of the fourth ventricle is the germinal origin of a diverse collection of neuronal populations that ultimately reside in the brainstem and cerebellum. Rhombic lip neurogenesis requires the bHLH transcription factor Atoh1 (Math1), and commences shortly after neural tube closure (E9.5). Within the rhombomere 1-isthmus region, the rhombic lip first produces brainstem and deep cerebellar neurons (E9.5-E12), followed by granule cell precursors after E12. While Atoh1 function is essential for all of these populations to be specified, the downstream genetic programs that confer specific properties to early and late born Atoh1 lineages are not well characterized. We have performed a comparative microarray analysis of gene expression within early and later born cohorts of Atoh1 expressing neural precursors purified from E14.5 embryos using a transgenic labeling strategy. We identify novel transcription factors, cell surface molecules, and cell cycle regulators within each pool of Atoh1 lineages that likely contribute to their distinct developmental trajectories and cell fates. In particular, our analysis reveals new insights into the genetic programs that regulate the specification and proliferation of granule cell precursors, the putative cell of origin for the majority of medulloblastomas.

Distinct and conserved prominin-1/CD133-positive retinal cell populations identified across species

Besides being a marker of various somatic stem cells in mammals, prominin-1 (CD133) plays a role in maintaining the photoreceptor integrity since mutations in the PROM1 gene are linked with retinal degeneration. In spite of that, little information is available regarding its distribution in eyes of non-mammalian vertebrates endowed with high regenerative abilities. To address this subject, prominin-1 cognates were isolated from axolotl, zebrafish and chicken, and their retinal compartmentalization was investigated and compared to that of their mammalian orthologue. Interestingly, prominin-1 transcripts--except for the axolotl--were not strictly restricted to the outer nuclear layer (i.e., photoreceptor cells), but they also marked distinct subdivisions of the inner nuclear layer (INL). In zebrafish, where the prominin-1 gene is duplicated (i.e., prominin-1a and prominin-1b), a differential expression was noted for both paralogues within the INL being localized either to its vitreal or scleral subdivision, respectively. Interestingly, expression of prominin-1a within the former domain coincided with Pax-6-positive cells that are known to act as progenitors upon injury-induced retino-neurogenesis. A similar, but minute population of prominin-1-positive cells located at the vitreal side of the INL was also detected in developing and adult mice. In chicken, however, prominin-1-positive cells appeared to be aligned along the scleral side of the INL reminiscent of zebrafish prominin-1b. Taken together our data indicate that in addition to conserved expression of prominin-1 in photoreceptors, significant prominin-1-expressing non-photoreceptor retinal cell populations are present in the vertebrate eye that might represent potential sources of stem/progenitor cells for regenerative therapies.