An instructive function for Notch in promoting gliogenesis in the zebrafish retina

Molecular Cloning and Functional Expression of zfCx52.6

Gap junction-mediated electrical coupling contributes to synchronous oscillatory activities of neurons, and considerable progress has been made in defining the molecular composition of gap junction channels. In particular, cloning and functional expression of gap junction proteins (connexins (Cx)) from zebrafish retina have shown that this part of the brain possesses a high degree of connexin diversity that may account for differential functional properties of electrical synapses. Here, we report the cloning and functional characterization of a new connexin, designated zebrafish Cx52.6 (zfCx52.6). This connexin shows little similarity to known connexins from fish and higher vertebrates. By combining in situ hybridization with Laser Capture Microdissection and RT-PCR, we found that this novel fish connexin is expressed in horizontal cells in the inner nuclear layer of the retina. Functional expression of zfCx52.6 in neuroblastoma cells and Xenopus oocytes led to functional gap junctional channels and, in single oocytes, induced large non-junctional membrane currents indicative of the formation of hemichannels, which were inhibited in reversible fashion by raising extracellular Ca(2+) concentrations.

Approaches to study neurogenesis in the zebrafish retina

Similar to other vertebrate species, the zebrafish retina is simpler than other regions of the central nervous system (CNS). Relative simplicity, rapid development, and accessibility to genetic analysis make the zebrafish retina an excellent model system for the studies of neurogenesis in the vertebrate CNS. Numerous genetic screens have led to isolation of an impressive collection of mutations affecting the retina and the retinotectal projection in zebrafish. Mutant phenotypes are being studied using a rich variety of markers: antibodies, RNA probes, retrograde and anterograde tracers, as well as transgenic lines. Particularly impressive progress has been made in the characterization of the zebrafish genome. Consequently, positional and candidate cloning of mutant genes are now fairly easy to accomplish in zebrafish. Many mutant genes have, in fact, already been cloned and their analysis has provided important insights into the gene circuitry that regulates retinal neurogenesis. Genetic screens for visual system defects will continue in the future and progressively more sophisticated screening approaches will make it possible to detect a variety of subtle mutant phenotypes in retinal development. The remarkable evolutionary conservation of the vertebrate eye provides the basis for the use of the zebrafish retina as a model of human disorders. Some of the genetic defects of the zebrafish retina indeed resemble human retinopathies. As new techniques are being introduced and improved at a rapid pace, the zebrafish will continue to be an important organism for the studies of the vertebrate visual system.

Neurogenesis and the cell cycle

For a long time, it has been understood that neurogenesis is linked to proliferation and thus to the cell cycle. Recently, the gears that mediate this linkage have become accessible to molecular investigation. This review describes some of the progress that has been made in understanding how the molecular machinery of the cell cycle is used in the processes of size regulation in the brain, histogenesis, neuronal differentiation, and the maintenance of stem cells.

Windows into development: historic, current, and future perspectives on transgenic zebrafish

The recent explosion of transgenic zebrafish lines in the literature demonstrates the value of this model system for detailed in vivo analysis of gene regulation and morphogenetic movements. The optical clarity and rapid early development of zebrafish provides the ability to follow these events as they occur in live, developing embryos. This article will review the development of transgenic technology in zebrafish as well as the current and future uses of transgenic zebrafish to explore the dynamic environment of the developing vertebrate embryo.

Proneural, prosensory, antiglial: the many faces of neurogenins

Previous reports have linked Neurogenin 1 with the general promotion of neurogenesis and gliogenesis. A new study in zebrafish, supported by recent findings in Drosophila, has now challenged this idea by demonstrating that the activity of Neurogenin 1 is limited to activation of development of two classes of sensory neurons.

Analysis of gene function in the zebrafish retina

Mutagenesis screens in zebrafish have uncovered several hundred mutant alleles affecting the development of the retina and established the zebrafish as one of the leading models of vertebrate eye development. In addition to forward genetic mutagenesis approaches, gene function in the zebrafish embryo is being studied using several reverse genetic techniques. Some of these rely on the overexpression of a gene product, others take advantage of antisense oligonucleotides to block function of selected loci. Here we describe these methods in the context of the developing eye.

Lipofection strategy for the study of Xenopus retinal development

The analysis of gene function during retinal development can be addressed by perturbing gene expression either by inhibition or by overexpression in desired regions and at defined stages of development. An in vivo lipofection strategy has been applied for stage-specific and region-specific expression of genes in Xenopus retina. Due to colipofection efficiency, this strategy enables us to study functional interaction of genes by lipofecting multiple expression constructs. This lipofection technique also allows us to transfect morpholino oligonucleotides into retinoblasts to block gene function. We present here various aspects of this technique, including recent improvements and modifications.

The role of notch in promoting glial and neural stem cell fates

The Notch signaling pathway has long been known to influence cell fate in the developing nervous system. However, this pathway has generally been thought to inhibit the specification of certain cell types in favor of others, or to simply maintain a progenitor pool. Recently, this view has been challenged by numerous studies suggesting that Notch may play an instructive role in promoting glial development. This work has inspired a new look at the role of Notch signaling in specifying cell fate. It has also prompted further consideration of the emerging view that in some contexts glia may be multipotent progenitors. This review examines the role of Notch during gliogenesis in both fruit flies and vertebrates, as well as evidence in vertebrates that some glia may be stem cells.

Arteries and veins: making a difference with zebrafish

Arteries and veins are structurally different and have long been functionally defined by the direction of blood flow that they carry. However, a growing body of evidence indicates that the identity of the endothelial cells that line these vessels is determined in the developing embryo, before circulation begins. Recent work on the zebrafish has led to the identification of signals that are responsible for arterial and venous differentiation of endothelial cells, and highlights the unique benefits of this model organism in the study of vascular development.

Disruption of segmental neural crest migration and ephrin expression in delta-1 null mice

Neural crest cells migrate segmentally through the rostral half of each trunk somite due to inhibitory influences of ephrins and other molecules present in the caudal-half of somites. To examine the potential role of Notch/Delta signaling in establishing the segmental distribution of ephrins, we examined neural crest migration and ephrin expression in Delta-1 mutant mice. Using Sox-10 as a marker, we noted that neural crest cells moved through both rostral and caudal halves of the somites in mutants, consistent with the finding that ephrinB2 levels are significantly reduced in the caudal-half somites. Later, mutant embryos had aberrantly fused and/or reduced dorsal root and sympathetic ganglia, with a marked diminution in peripheral glia. These results show that Delta-1 is essential for proper migration and differentiation of neural crest cells. Interestingly, absence of Delta-1 leads to diminution of both neurons and glia in peripheral ganglia, suggesting a general depletion of the ganglion precursor pool in mutant mice.

sonic hedgehog and vascular endothelial growth factor act upstream of the Notch pathway during arterial endothelial differentiation

The appearance of molecular differences between arterial and venous endothelial cells before circulation suggests that genetic factors determine these cell types. We find that vascular endothelial growth factor (vegf) acts downstream of sonic hedgehog (shh) and upstream of the Notch pathway to determine arterial cell fate. Loss of Vegf or Shh results in loss of arterial identity, while exogenous expression of these factors causes ectopic expression of arterial markers. Microinjection of vegf mRNA into embryos lacking Shh activity can rescue arterial differentiation. Finally, activation of the Notch pathway in the absence of Vegf signaling can rescue arterial marker gene expression. These studies reveal a complex signaling cascade responsible for establishing arterial cell fate and suggest differential effects of Vegf on developing endothelial cells.

Notch1 and its ligands Delta-like and Jagged are expressed and active in distinct cell populations in the postnatal mouse brain

Notch signaling plays a pivotal role in the regulation of vertebrate neurogenesis. However, in vitro experiments suggest that Notch1 may also be involved in the regulation of later stages of brain development. We have addressed putative roles in the central nervous system by examining the expression of Notch signaling cascade components in the postnatal mouse brain. In situ mRNA hybridization revealed that Notch1 is associated with cells in the subventricular zone, the dentate gyrus and the rostromigratory stream, all regions of continued neurogenesis in the postnatal brain. In addition, Notch1 is expressed at low levels throughout the cortex and olfactory bulb and shows striking expression in the cerebellar Purkinje cell layer. The Notch ligands, including Delta-like1 and 3 and Jagged1 and Jagged2, show distinct expression patterns in the developing and adult brain overlapping that of Notch1. In addition, the downstream targets of the Notch signaling cascade Hes1, Hes3, Hes5 and the intrinsic Notch regulatory proteins Numb and Numblike also show active signaling in distinct brain regions. Hes5 coincides with the majority of Notch1 expression and can be detected in the cerebral cortex, cerebellum and putative germinal zones. Hes3, on the other hand, shows a restricted expression in cerebellar Purkinje cells. The distribution of Notch1 and its putative ligands suggest distinct roles in specific subsets of cells in the postnatal brain including putative stem cells and differentiated neurons.

A quantitative analysis of the kinetics of Gal4 activator and effector gene expression in the zebrafish

Using a temperature-inducible hsp70:Gal4 activator and UAS:myc-notch1a-intra as effector, we determined quantitatively the kinetics of expression of both transgenes and analysed the effects of varying their expressivity on several phenotypic traits in the developing zebrafish. hsp70:Gal4 is transcribed within 15 min after temperature-mediated induction, but Gal4 RNA decays rapidly. The Gal4 protein was found to be quite stable, as functional Gal4, which was detectable 1.5 h after heat shock (HS), persisted for at least 13 h. myc-notch1a-intra RNA is expressed approximately 1.5 h after HS, but unlike the Gal4 RNA, it was found to be very stable; it continues to accumulate during the succeeding 17 h after HS. Fully penetrant phenotypic effects are obtained after a relatively long activator induction with a 30-min HS.

Targeted gene expression in transgenic Xenopus using the binary Gal4-UAS system

The transgenic technique in Xenopus allows one to misexpress genes in a temporally and spatially controlled manner. However, this system suffers from two experimental limitations. First, the restriction enzyme-mediated integration procedure relies on chromosomal damage, resulting in a percentage of embryos failing to develop normally. Second, every transgenic embryo has unique sites of integration and unique transgene copy number, resulting in variable transgene expression levels and variable phenotypes. For these reasons, we have adapted the Gal4-UAS method for targeted gene expression to Xenopus. This technique relies on the generation of transgenic lines that carry "activator" or "effector" constructs. Activator lines express the yeast transcription factor, Gal4, under the control of a desired promoter, whereas effector lines contain DNA-binding motifs for Gal4-(UAS) linked to the gene of interest. We show that on intercrossing of these lines, the effector gene is transcribed in the temporal and spatial manner of the activator's promoter. Furthermore, we use the Gal4-UAS system to misexpress Xvent-2, a transcriptional target of bone morphogenetic protein 4 (BMP4) signaling during early embryogenesis. Embryos inheriting both the Gal4 activator and Xvent-2 effector transgenes display a consistent microcephalic phenotype. Finally, we exploit this system to characterize the neural and mesodermal defects obtained from early misexpression of Xvent-2. These results emphasize the potential of this system for the controlled analyses of gene function in Xenopus.

Pattern formation in the zebrafish retina

During the past 15 years, the zebrafish has become established as a genetic model organism to study vertebrate development. It is particularly well suited for the analysis of the retina, and several genetic screens have yielded a large number of mutants affecting retinal development. Most of these mutants still await thorough analysis and molecular characterization, but work on a handful of genes has already generated interesting results that shed some light on patterning mechanisms employed in the vertebrate retina.

The art and design of genetic screens: zebrafish

Inventive genetic screens in zebrafish are revealing new genetic pathways that control vertebrate development, disease and behaviour. By exploiting the versatility of zebrafish, biological processes that had been previously obscured can be visualized and many of the responsible genes can be isolated. Coupled with gene knockdown and overexpression technologies, and small-molecule-induced phenotypes, genetic screens in zebrafish provide a powerful system by which to dissect vertebrate gene function and gene networks.

Neuronal potential and lineage determination by neural stem cells

How do neural stem cells ensure that they give rise to the right number and type of neurons at the right time? Over the past year several regulatory mechanisms have been identified, including promotion of neurogenesis by proneural bHLH genes, instruction of gliogenesis by Notch, and cell-intrinsic changes in the neurogenic capacity of stem cells in culture and in vivo.

Lineage diversity in the Drosophila nervous system

The detailed descriptions of cellular lineages in the Drosophila nervous system have provided the foundations for an in-depth genetic analysis of the mechanisms that regulate fate decisions at every cell cycle.