Our understanding of the cellular and molecular mechanisms underlying the adult neural stem cell state remains fragmentary. To provide new models on this issue, we searched for stem cells in the adult brain of the zebrafish. Using BrdU tracing and immunodetection of cell-type-specific markers, we demonstrate that the adult zebrafish telencephalon contains self-renewing progenitors, which show features of adult mammalian neural stem cells but distribute along the entire dorso-ventral extent of the telencephalic ventricular zone. These progenitors give rise to newborn neurons settling close to the ventricular zone within the telencephalon proper. They have no equivalent in mammals and therefore constitute a new model of adult telencephalic neural stem cells. In addition, progenitors from the ventral subpallium generate rapidly dividing progenitors and neuroblasts that reach the olfactory bulb (OB) via a rostral migratory stream and differentiate into GABAergic and TH-positive neurons. These ventral progenitors are comparable to the mammalian neural stem cells of the subependymal zone. Interestingly, dorsal and ventral progenitors in the adult telencephalon express a different combination of transcription factors than their embryonic counterparts. In the case of neurogenin1, this is due to the usage of different enhancer elements. Together, our results highlight the conserved and unique phylogenic and ontogenic features of adult neurogenesis in the zebrafish telencephalon and open the way to the identification of adult neural stem cell characters in cross-species comparative studies.

Anteroposterior patterning of the vertebrate forebrain during gastrulation involves graded Wnt signaling, which segregates anterior fields (telencephalon and eye) from the diencephalon. How the telencephalic and retinal primordia are subsequently subdivided remains largely unknown. We demonstrate that at late gastrulation the Paired-like homeodomain transcription factor Rx3 biases cell specification choices towards the retinal fate within a population of bipotential precursors of the anterior forebrain: direct cell tracing demonstrates that retinal precursors acquire a telencephalic fate in embryos homozygous for the rx3-null allele ckh ne2611 , characterized by an enlarged telencephalon and a lack of eyes. Chimera analyses further indicate that this function of Rx3 is cell autonomous. Transfating of the eye field in the absence of Rx3 function correlates with a substantial posterior expansion of expression of the Wnt antagonist Tlc and the winged-helix transcription factor Foxg1. These results suggest that the process segregating the telencephalic and eye fields is isolated from diencephalic patterning, and is mediated by Rx3.

Transcriptome analysis of a zebrafish mutant that does not respond to amphetamine identifies a network of coordinated gene regulation that may underlie the susceptibility to addiction.

All subdivisions of the adult zebrafish brain maintain niches of constitutive neurogenesis, sustained by quiescent and multipotent progenitor populations. In the telencephalon, the latter potential neural stem cells take the shape of radial glia aligned along the ventricle and are controlled by Notch signalling. With the aim of identifying new markers of this cell type and of comparing the effectors of embryonic and adult neurogenesis, we focused on the family of hairy/enhancer of split [E(spl)] genes. We report the expression of seven hairy/E(spl) (her) genes and the new helt gene in three neurogenic areas of the adult zebrafish brain (telencephalon, hypothalamus, and midbrain) in relation to radial glia, proliferation, and neurogenesis. We show that the expression of most her genes in the adult brain characterizes quiescent radial glia, whereas only few are expressed in progenitor domains engaged in active proliferation or neurogenesis. The low proliferation status of most her-positive progenitors contrasts with the embryonic nervous system, in which her genes are expressed in actively dividing progenitors. Likewise, we demonstrate largely overlapping expression domains of a set of her genes in the adult brain, which is in striking contrast to their distinct embryonic expression profiles. Overall, our data provide a consolidated map of her expression, quiescent glia, proliferation, and neurogenesis in these various subdivisions of the adult brain and suggest distinct regulation and function of Her factors in the embryonic and adult contexts.

The zebrafish adult brain contains numerous neural progenitors and is a good model to approach the general mechanisms of adult neural stem cell maintenance and neurogenesis. Here we use this model to test for a correlation between Fgf signaling and cell proliferation in adult progenitor zones. We report expression of Fgf signals (fgf3,4,8a,8b,17b), receptors (fgfr1-4), and targets (erm, pea3, dusp6, spry1,2,4, and P-ERK) and document that genes of the embryonic fgf8 synexpression group acquire strikingly divergent patterns in the adult brain. We further document the specific expression of fgf3, fgfr1-3, dusp6, and P-ERK in ventricular zones, which contain neural progenitors. In these locations, however, a comparison at the single-cell level of fgfr/P-ERK expression with bromo-deoxy-uridine (BrdU) incorporation and the proliferation marker MCM5 indicates that Fgf signaling is not specifically associated with proliferating progenitors. Rather, it correlates with the ventricular radial glia state, some of which only are progenitors. Together these results stress the importance of Fgf signaling in the adult brain and establish the basis to study its function in zebrafish, in particular in relation to adult neurogenesis.

Background: In Drosophila, mutations in the gene eyes absent (eya) lead to severe defects in eye development. The functions of its mammalian orthologs Eya1-4 are only partially understood and no mouse model exists for Eya3. Therefore, we characterized the phenotype of a new Eya3 knockout mouse mutant.

BackgroundNeurogenesis control and the prevention of premature differentiation in the vertebrate embryo are crucial processes, allowing the formation of late-born cell types and ensuring the correct shape and cytoarchitecture of the brain. Members of the Hairy/Enhancer of Split (Hairy/E(spl)) family of bHLH-Orange transcription factors, such as zebrafish Her3, 5, 9 and 11, are implicated in the local inhibition of neurogenesis to maintain progenitor pools within the early neural plate. To better understand how these factors exert their inhibitory function, we aimed to isolate some of their functional interactors.ResultsWe used a yeast two-hybrid screen with Her5 as bait and recovered a novel zebrafish Hairy/E(spl) factor - Her8a. Using phylogenetic and synteny analyses, we demonstrate that her8a evolved from an ancient duplicate of Hes6 that was recently lost in the mammalian lineage. We show that her8a is expressed across the mid- and anterior hindbrain from the start of segmentation. Through knockdown and misexpression experiments, we demonstrate that Her8a is a negative regulator of neurogenesis and plays an essential role in generating progenitor pools within rhombomeres 2 and 4 - a role resembling that of Her3. Her8a co-purifies with Her3, suggesting that Her8a-Her3 heterodimers may be relevant in this domain of the neural plate, where both proteins are co-expressed. Finally, we demonstrate that her8a expression is independent of Notch signaling at the early neural plate stage but that SoxB factors play a role in its expression, linking patterning information to neurogenesis control. Overall, the regulation and function of Her8a differ strikingly from those of its closest relative in other vertebrates - the Hes6-like proteins.ConclusionsOur results characterize the phylogeny, expression and functional interactions involving a new Her factor, Her8a, and highlight the complex interplay of E(spl) proteins that generates the neurogenesis pattern of the zebrafish early neural plate.

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