Her5 acts as a prepattern factor that blocks neurogenin1 and coe2 expression upstream of Notch to inhibit neurogenesis at the midbrain-hindbrain boundary

Cortical development: the art of generating cell diversity

The fascinating question of how the enormous diversity of neuronal and glial cells in the cerebral cortex is generated during development was recently discussed at a meeting on cortical development and stem cells in Greece. What emerged from this meeting is an equally fascinating answer, namely that precursor diversity at rather early stages of development anticipates later cell type diversity.

Identification of target genes for the Xenopus Hes-related protein XHR1, a prepattern factor specifying the midbrain-hindbrain boundary

The midbrain-hindbrain boundary (MHB) acts as a local organizer in the development of the CNS in vertebrates. Previously, we identified an MHB-specific bHLH-WRPW transcriptional repressor gene, Xenopus Hes-related 1 (XHR1), which is initially expressed in the presumptive MHB (pre-MHB) region at the early gastrula stage. To better understand the gene cascades involved in MHB formation, we investigated the genes downstream from XHR1 by differential screening using a Xenopus cDNA macroarray and a dexamethasone (DEX)-inducible, dominant-negative transcriptional activator construct of XHR1 (XHR1-VP16-GR). Among the newly identified candidate target genes of XHR1 were Enhancer of split-related genes (ESR1, ESR3/7, and ESR9) and Xenopus laevis cleavage 2 (XLCL2). XHR1-VP16-GR induced the expression of the ESR genes and XLCL2 as well as Xdelta1, Xngnr1, and XHR1 itself in the presence of DEX even after pretreatment with the protein synthesis inhibitor, cycloheximide. This suggests that these genes are direct targets of XHR1. XHR1-knockdown experiments with antisense morpholino oligos and ectopic expression of wild-type XHR1 revealed that XHR1 is necessary and sufficient to repress ESR genes in the pre-MHB region. Misexpression of the ESR genes in the pre-MHB region repressed the MHB marker gene, Pax2, suggesting that the repression of the ESR genes by XHR1 is at least partly required for the early development of the pre-MHB. Our data also show that XHR1 is not activated by Notch signaling, differing from ESR genes. Taken together, we propose a model in which XHR1 defines the pre-MHB region as a prepattern gene by repressing those possible direct target genes.

The REF-1 Family of bHLH Transcription Factors Pattern C. elegans Embryos through Notch-Dependent and Notch-Independent Pathways

Much of the patterning of early C. elegans embryos involves a series of Notch interactions that occur in rapid succession and have distinct outcomes; however, none of the targets for these interactions have been identified. We show that the REF-1 family of bHLH transcription factors is a major target of Notch signaling in all these interactions and that most examples of Notch-mediated transcriptional repression can be attributed to REF-1 activities. The REF-1 family is expressed and has similar functions in both Notch-dependent and Notch-independent pathways, and this dual mode of deployment is used repeatedly to pattern the embryo. REF-1 proteins are unusual in that they contain two different bHLH domains and lack the distinguishing characteristics of Hairy/Enhancer of Split (HES) bHLH proteins that are Notch targets in other systems. Our results show that the highly divergent REF-1 proteins are nonetheless HES-like bHLH effectors of Notch signaling.

Patterning of proneuronal and inter-proneuronal domains by hairy- and enhancer of split-related genes in zebrafish neuroectoderm

In teleosts and amphibians, the proneuronal domains, which give rise to primary-motor, primary-inter and Rohon-Beard (RB) neurons, are established at the beginning of neurogenesis as three longitudinal stripes along the anteroposterior axis in the dorsal ectoderm. The proneuronal domains are prefigured by the expression of basic helix-loop-helix (bHLH) proneural genes,and separated by domains (inter-proneuronal domains) that do not express the proneural genes. Little is known about how the formation of these domains is spatially regulated. We have found that the zebrafish hairy- and enhancer of split-related (Her) genes her3 and her9are expressed in the inter-proneuronal domains, and are required for their formation. her3 and her9 expression was not regulated by Notch signaling, but rather controlled by positional cues, in which Bmp signaling is involved. Inhibition of Her3 or Her9 by antisense morpholino oligonucleotides led to ectopic expression of the proneural genes in part of the inter-proneuronal domains. Combined inhibition of Her3 and Her9 induced ubiquitous expression of proneural and neuronal genes in the neural plate, and abolished the formation of the inter-proneuronal domains. Furthermore,inhibition of Her3/Her9 and Notch signaling led to ubiquitous and homogeneous expression of proneural and neuronal genes in the neural plate, revealing that Her3/Her9 and Notch signaling have distinct roles in neurogenesis. These data indicate that her3 and her9 function as prepattern genes that link the positional dorsoventral polarity information in the posterior neuroectoderm to the spatial regulation of neurogenesis.

Inhibition of neurogenesis at the zebrafish midbrain-hindbrain boundary by the combined and dose-dependent activity of a new hairy/E(spl) gene pair

The intervening zone (IZ) is a pool of progenitor cells located at the midbrain-hindbrain boundary (MHB) and important for MHB maintenance, midbrain-hindbrain growth and the generation of midbrain-hindbrain neurons. Recently, we implicated the Hairy/E(spl) transcription factor Her5 in the formation of the medial (most basal) part of the IZ (MIZ) in zebrafish; the molecular bases for lateral IZ (LIZ) formation, however, remain unknown. We now demonstrate that her5 is physically linked to a new family member, him, displaying an identical MHB expression pattern. Using single and double knockdowns of him and her5, as well as a him+her5 deletion mutant background (b404), we demonstrate that Him and Her5 are equally necessary for MIZ formation, and that they act redundantly in LIZ formation in vivo. We show that these processes do not involve cross-regulation between Him and Her5 expression or activities, although Him and Her5 can heterodimerize with high affinity. Increasing the function of one factor when the other is depleted further shows that Him and Her5 are functionally interchangeable. Together, our results demonstrate that patterning and neurogenesis are integrated by the her5-him gene pair to maintain a progenitor pool at the embryonic MHB. We propose a molecular mechanism for this process where the global 'Him+Her5' activity inhibits ngn1 expression in a dose-dependent manner and through different sensitivity thresholds along the medio-lateral axis of the neural plate.

Expression of the zebrafish Iroquois genes during early nervous system formation and patterning

Iroquois genes are involved in many patterning processes during development. In particular, they act as prepattern genes to control proneural gene expression both in Drosophila and in vertebrates. In this paper, we have analyzed the expression during embryogenesis of the 11 zebrafish Iroquois genes, with special interest for nervous system formation and patterning. During the first 2 days of development, Iroquois genes are expressed in distinct domains in the neuroepithelium, as well as in groups of neuronal progenitors and neurons. They are also expressed at different stages of placodal development. These expression patterns are similar to the patterns of the murine irx genes and also show features specific to teleosts. For the zebrafish Iroquois gene family, we find both specific patterns and patterns conserved within a cluster, between paralogues, or in most genes of the family. Overall, these expression data suggest functions for the Iroquois family of transcription factors in neural and placodal patterning, neurogenesis, and neuronal specification.