floating head and masterblind regulate neuronal patterning in the roof of the forebrain

Bmp activity establishes a gradient of positional information throughout the entire neural plate

Bone morphogenetic proteins (Bmps) are key regulators of dorsoventral (DV) patterning. Within the ectoderm, Bmp activity has been shown to inhibit neural development, promote epidermal differentiation and influence the specification of dorsal neurons and neural crest. In this study, we examine the patterning of neural tissue in mutant zebrafish embryos with compromised Bmp signalling activity. We find that although Bmp activity does not influence anteroposterior (AP) patterning, it does affect DV patterning at all AP levels of the neural plate. Thus, we show that Bmp activity is required for specification of cell fates around the margin of the entire neural plate, including forebrain regions that do not form neural crest. Surprisingly, we find that Bmp activity is also required for patterning neurons at all DV levels of the CNS. In swirl/bmp2b(−) (swr(−)) embryos, laterally positioned sensory neurons are absent whereas more medial interneuron populations are hugely expanded. However, in somitabun(−) (sbn(−)) embryos, which probably retain higher residual Bmp activity, it is the sensory neurons and not the interneurons that are expanded. Conversely, in severely Bmp depleted embryos, both interneurons and sensory neurons are absent and it is the most medial neurons that are expanded. These results are consistent with there being a gradient of Bmp-dependent positional information extending throughout the entire neural and non-neural ectoderm.

Development of a circadian melatonin rhythm in embryonic zebrafish

We investigated the time course of circadian system development in zebrafish and the role of environmental light cycles in this process, using a rhythm in melatonin content of embryos and larvae as a marker of circadian function. When zebrafish were raised in a cycle of 14 h light and 10 h dark at 28.5 degrees C, nocturnal increases in melatonin content were detectable beginning on the second night post-fertilization (PF). When embryos were transferred to constant darkness (DD) at the end of the second light period, a circadian rhythm of melatonin content persisted for at least three cycles. However, when embryos were transferred from light to DD at 14 h PF, no rhythm was detectable in the population. Phase-locked circadian melatonin rhythms were measurable after embryos were exposed to a transition from constant light (LL) to darkness at 26 or 32 h PF, but not at 20 h. These data indicate that a circadian oscillator that regulates melatonin synthesis becomes functional and responsive by light between 20 and 26 h PF. At this stage, pineal photoreceptors have begun to differentiate, but retinal photoreceptors have not, suggesting that the first circadian melatonin rhythms are of pineal origin. The absence of melatonin rhythms in populations of embryos exposed to DD beginning at earlier stages indicates that there is no timed developmental event that sets the circadian clock in the absence of environmental input. Exposure to DD starting at 14 or 20 h PF did not retard overall development as determined by gross morphological staging criteria, and did not prevent later synchronization of melatonin rhythms by light-dark (LD) cycles.

The homeobox gene, Xanf-1, can control both neural differentiation and patterning in the presumptive anterior neurectoderm of the Xenopus laevis embryo

From the onset of neurectoderm differentiation, homeobox genes of the Anf class are expressed within a region corresponding to the presumptive telencephalic and rostral diencephalic primordia. Here we investigate functions of the Xenopus member of Anf, Xanf-1, in the differentiation of the anterior neurectoderm. We demonstrate that ectopic Xanf-1 can expand the neural plate at expense of adjacent non-neural ectoderm. In tadpoles, the expanded regions of the plate developed into abnormal brain outgrowths. At the same time, Xanf-1 can inhibit terminal differentiation of primary neurones. We also show that, during gastrula/neurula stages, the exogenous Xanf-1 can downregulate four transcription regulators, XBF-1, Otx-2, Pax-6 and the endogenous Xanf-1, that are expressed in the anterior neurectoderm. However, during further development, when the exogenous Xanf-1 was presumably degraded, re-activation of XBF-1, Otx-2 and Pax-6 was observed in the abnormal outgrowths developed from blastomeres microinjected with Xanf-1 mRNA. Other effects of the ectopic Xanf-1 include cyclopic phenotype and inhibition of the cement gland, both by Otx-2-dependent and -independent mechanisms. Using fusions of Xanf-1 with the repressor domain of Drosophila engrailed or activator domain of herpes virus VP16 protein, we showed that most of the observed effects of Xanf-1 were probably elicited by its functioning as a transcription repressor. Altogether, our data indicate that the repressor function of Xanf-1 may be necessary for regulation of both neural differentiation and patterning in the presumptive anterior neurectoderm.

Zebrafish serotonin N-acetyltransferase-2: marker for development of pineal photoreceptors and circadian clock function

Serotonin N-acetyltransferase (AANAT), the penultimate enzyme in melatonin synthesis, is typically found only at significant levels in the pineal gland and retina. Large changes in the activity of this enzyme drive the circadian rhythm in circulating melatonin seen in all vertebrates. In this study, we examined the utility of using AANAT messenger RNA (mRNA) as a marker to monitor the very early development of pineal photoreceptors and circadian clock function in zebrafish. Zebrafish AANAT-2 (zfAANAT-2) cDNA was isolated and used for in situ hybridization. In the adult, zfAANAT-2 mRNA is expressed exclusively in pineal cells and retinal photoreceptors. Developmental analysis, using whole mount in situ hybridization, indicated that pineal zfAANAT-2 mRNA expression is first detected at 22 h post fertilization. Retinal zfAANAT-2 mRNA was first detected on day 3 post fertilization and appears to be associated with development of the retinal photoreceptors. Time-of-day analysis of 2- to 5-day-old zebrafish larvae indicated that zfAANAT-2 mRNA abundance exhibits a dramatic 24-h rhythm in a 14-h light, 10-h dark cycle, with high levels at night. This rhythm persists in constant darkness, indicating that the zfAANAT-2 mRNA rhythm is driven by a circadian clock at this stage. The techniques described in this report were also used to determine that zfAANAT-2 expression is altered in two well characterized genetic mutants, mindbomb and floating head. The observations described here suggest that zfAANAT-2 mRNA may be a useful marker to study development of the pineal gland and of circadian clock mechanisms in zebrafish.

Molecular mechanisms of holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain in humans. Several distinct human genes for holoprosencephaly have now been identified. They include Sonic hedgehog (SHH), ZIC2, and SIX3. Many additional genes involved in forebrain development are rapidly being cloned and characterized in model vertebrate organisms. These include Patched (Ptc), Smoothened (Smo), cubitus interuptus (ci)/Gli, wingless (wg/Wnt, decapentaplegic (dpp)/BMP, Hedgehog interacting protein (Hip), nodal, Smads, One-eyed pinhead (Oep), and TG-Interacting Factor (TGIF). However, further analysis is needed before their roles in HPE can be established. Here we present an overview of the presently known genes causing human holoprosencephaly and describe candidate genes involved in forebrain development identified in other systems. A model is discussed for how these genes may interact within and between several different signaling pathways to direct the formation of the forebrain.

Zebrafish tbx-c functions during formation of midline structures

Several genes containing the conserved T-box region in invertebrates and vertebrates have been reported recently. Here, we describe three novel members of the T-box gene family in zebrafish. One of these genes, tbx-c, is studied in detail. It is expressed in the axial mesoderm, notably, in the notochordal precursor cells immediately before formation of the notochord and in the chordoneural hinge of the tail bud, after the notochord is formed. In addition, its expression is detected in the ventral forebrain, sensory neurons, fin buds and excretory system. The expression pattern of tbx-c differs from that of the other two related genes, tbx-a and tbx-b. The developmental role of tbx-c has been analysed by overexpression of the full-length tbx-c mRNA and a truncated form of tbx-c mRNA, which encodes the dominant-negative Tbx-c. Overexpression of tbx-c causes expansion of the midline mesoderm and formation of ectopic midline structures at the expense of lateral mesodermal cells. In dominant-negative experiments, the midline mesoderm is reduced with the expansion of lateral mesoderm to the midline. These results suggest that tbx-c plays a role in formation of the midline mesoderm, particularly, the notochord. Moreover, modulation of tbx-c activity alters the development of primary motor neurons. Results of in vitro analysis in zebrafish animal caps suggest that tbx-c acts downstream of early mesodermal inducers (activin and ntl) and reveal an autoregulatory feedback loop between ntl and tbx-c. These data and analysis of midline (ntl−/− and flh−/−) and lateral mesoderm (spt−/−) mutants suggest that tbx-c may function during formation of the notochord.

Cellular and molecular bases of axonal pathfinding during embryogenesis of the fish central nervous system

The accessibility of the zebrafish embryo offers unique possibilities to study the mechanisms that guide growing axons in the developing vertebrate central nervous system. This review examines the current understanding of the pathfinding decisions by the growing axons, their substrates, and the recognition molecules that mediate axon-substrate interactions. The detailed analysis of pathfinding at the level of individual axons demonstrates that growing axons chose their paths unerringly. To do so, they rely on cues presented by their environment, in particular by neuroepithelial cells. Our understanding of the molecular bases of axon-substrate interactions is increasing. Members of most classes of recognition molecules have been identified in fish. Experimental evidence for the functions of these molecules in the zebrafish nervous system is accumulating. In the future, this analysis is expected to profit greatly from genetic screens that have recently been initiated.

Pattern formation in zebrafish--fruitful liaisons between embryology and genetics

Vertebrate embryos, despite quite diverse early morphologies, appear to employ similar cellular strategies and conserved biochemical pathways in their development (Eyal-Giladi, 1997). In the past decade, a small tropical teleost, zebrafish (Danio rerio), became an important model system in which to study development (Streisinger et al., 1981). By combining embryology with molecular and classical genetic methods, our understanding of early inductive and morphogenetic events during vertebrate embryogenesis significantly advanced. In zebrafish, dorsal-ventral polarity is established during early cleavage and is dependent on microtubular transport of determinants from the vegetal pole to the blastomeres positioned on top of the yolk cell. The syncytium forming from these marginal blastomeres in the early blastula exhibits dorsal-ventral asymmetry with beta-catenin localized to the nuclei on the presumptive dorsal side of the syncytium. The yolk cell is a source of signals that induce and pattern overlying blastoderm. Therefore, the dorsal yolk syncytial layer is equivalent to the Nieuwkoop center of the amphibian embryo. The embryonic shield, a thickening of the dorsal blastoderm margin, exhibits properties similar to the amphibian Spemann organizer. However, certain inductive and patterning signals from the organizer might be produced before the shield forms or might originate outside of the shield. Similar to the amphibian embryo, the key patterning functions of the fish dorsal organizer (i.e., dorsalization of mesoderm, ectoderm, and coordination of gastrulation movements) are performed by secreted molecules that antagonize the ventralizing activity of the swil (zbmp-2) and zbmp-4 gene products expressed on the ventral side of the embryo. These functions of the dorsal organizer require the activity of the chordino gene (a zebrafish homologue of chordin), bozozok, mercedes and ogon loci.

Patterning of the embryonic forebrain

The vertebrate forebrain is derived from the anterior neural plate, where anteroposterior, dorsoventral and local patterning mechanisms specify regional identify. The recent identification of genetic regulators of these processes has opened the way to elucidating how the major forebrain regions (i.e. cerebral cortex, basal ganglia, thalamus, and hypothalamus) are formed, and how molecular lesions in these processes cause human birth defects.

Neuronal and neuroendocrine expression of lim3, a LIM class homeobox gene, is altered in mutant zebrafish with axial signaling defects

LIM class homeobox genes code for a family of transcriptional regulators that encode important determinants of cell lineage and cell type specificity. The lim3 gene from the zebrafish, Danio rerio, is highly conserved in sequence and expression pattern compared to its homologs in other vertebrates. In this paper we report immunocytochemical analysis of Lim3 protein expression in the pituitary, pineal, hindbrain, and spinal cord of the embryo, revealing an asymmetrical, lateral and late program of pituitary development in zebrafish, distinct from the pattern in other vertebrates. We studied Lim3 expression in no tail, floating head, and cyclops mutant embryos, all of which have midline defects, with special reference to spinal cord differentiation where Lim3 marks mostly motoneurons. cyclops embryos showed essentially normal Lim3 expression in the hindbrain and spinal cord despite the absence of the floor plate, while no tail mutant embryos, which lack a differentiated notochord, displayed an excess of Lim3-expressing cells in a generally normal pattern. In contrast, Lim3-positive cells largely disappeared from the posterior spinal cord in floating head mutants, except in patches that correlated with remnants of apparent floor plate cells. These results support the view that either notochord or floor plate signaling can specify Lim3-positive motoneurons in the spinal cord.