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Cadherins are calcium-binding transmembrane glycoproteins that are important mediators of cell-cell association. Here we describe a novel member of this gene family, zebrafish ventral neural cadherin (VN-cad). Multiple VN-cad transcripts are first detectable by Northern blots at 60% epiboly. In the developing neural tube, VN-cad RNA is first found in the neuroectoderm, directly above the notochord, and later was localized to the neural keel. At the 20-somite stage, VN-cad transcripts are confined to the ventral neural tube, otic vesicle, midbrain, and diencephelon. Transcription of VN-cad RNA continues in adult fish. The embryonic pattern of expression is not significantly disrupted in cyclops or no tail mutants, which lack the floor plate and notochord, respectively. Therefore, neither of these structures is absolutely required for VN-cad expression. The localized pattern of VN-cad expression suggests a possible role for this adhesion molecule in the initial formation and subsequent differentiation of the central nervous system.
Electrical synaptic transmission is widespread in the vertebrate CNS and its modulation plays a critical role in altering the properties of coupled neural networks. In order to define further the mechanisms of electrical synaptic plasticity in the vertebrate retina, the electrophysiological characteristics of solitary horizontal cells and horizontal cell pairs from the zebrafish (Brachydanio rerio) were examined by whole-cell patch-clamp recordings from cells in primary cell culture. In solitary cells, the current-voltage relation exhibited inward current at potentials negative to -60 mV, a linear region of high resistance from -50 mV to 0 mV, and outward current positive to +20 mV. The inward current at negative potentials was blocked by substituting Cs+ for K+ in the extracellular medium, while the outward current at positive potentials was blocked by substitution of Cs+ for K+ in the pipette solution. Measurements of gap junctional conductance from electrically coupled cell pairs revealed that zebrafish horizontal cells expressed a mean junctional conductance of considerably smaller magnitude than other teleost retinal horizontal cells. Junctional conductance was found to be voltage dependent, exhibiting time-dependent closure with increasing transjunctional voltage. Voltage sensitivity was symmetrical about 0 mV junctional potential. At +/- 90 mV the ratio of steady state to peak current was approximately 0.5 and the time constant for inactivation of the junctional current was approximately 120 msec. Junctional conductance was also modulated by dopamine and cAMP. Pairs of horizontal cells responded to puff application of dopamine with a two- to threefold reduction in junctional conductance, but there was no discernible effect on extrajunctional conductances. The action of dopamine on coupling was mimicked by application of the dopamine agonist (+/-)-6,7-dihydroxy-2-amino-tetralin (ADTN) and the membrane permeable cAMP analog 8-bromo-cAMP. The selective D1 dopamine receptor antagonist SCH23390 blocked uncoupling by dopamine. These data provide a primary description of the electrophysiological characteristics of solitary horizontal cells and the electrical coupling between pairs of horizontal cells dissociated from the zebrafish retina. They indicate that zebrafish horizontal cells are distinct from the horizontal cells of other teleosts in their coupling characteristics. The results suggest that zebrafish horizontal cells exhibit differences in the regulation of synaptic assembly and maintenance that have important implications for the function of the zebrafish horizontal cell network in vivo.
The history of developmental and genetic analysis in the mouse has made it the model of choice for studying mammalian embryogenesis. Presently lacking is a simple technique for efficiently analyzing early mouse mutant phenotypes in utero. We demonstrate application of a real-time imaging method called ultrasound backscatter microscopy for visualizing mouse early embryonic neural tubes and hearts. This method was used to study live embryos in utero between 9.5 and 11.5 days of embryogenesis, with a spatial resolution close to 50 microns. Ultrasound backscatter microscope images of cultured embryos made it possible to visualize the heart chambers. This noninvasive imaging method was also used for analyzing a neural tube defect. The midhindbrain deletion associated with a null mutation of the Wnt-1 protooncogene was easily recognizable on ultrasound backscatter microscope images of 10.5- and 11.5-day embryos. Computer-generated volumetric renderings of the neural tube cavities were made from three-dimensional image data. This allowed a much clearer definition of the Wnt-1 mutant phenotype. These imaging techniques should be of considerable use in studying mouse development in utero.
We used the phosphatase substrate 2-(5'-chloro-2'-phosphoryloxyphenyl)-6- chloro-4-[3H]-quinazolinone, with standard alkaline phosphatase-mediated immunohistochemical techniques, to visualize a number of antibodies that bind to adult zebrafish retinal tissue. This compound, known as the ELF (enzyme-labeled-fluorescence) phosphatase substrate, produces a precipitate that fluoresces at approximately 500-580 nm (bright yellow-green). We show that the precipitated product from the ELF phosphatase substrate has a number of characteristics that make it superior to fluorescein-labeled secondary reagents. The staining produced with the ELF substrate is much more photostable than that produced by fluorescein-labeled secondary reagents, thus allowing time to examine, focus, and photograph the ELF-labeled tissue under high magnification. Moreover, the ELF precipitate exhibits a Stokes shift of greater than 100 nm, a characteristic that has enabled us to overcome the problem of distinguishing signal from background in this autofluorescent tissue. In addition, we show that the ELF product's large Stokes shift makes the ELF substrate ideal for multicolor applications.
One of the first intercellular signalling events in the vertebrate embryo leads to mesoderm formation and axis determination. In the mouse, a gene encoding a new member of the TGF-beta superfamily, nodal, is disrupted in a mutant deficient in mesoderm formation (Zhou et al., 1993, Nature 361, 543). nodal mRNA is found in prestreak mouse embryos, consistent with a role in the development of the dorsal axis. To examine the biological activities of nodal, we have studied the action of this factor in eliciting axis determination in the zebrafish, Danio rerio. Injection of nodal mRNA into zebrafish embryos caused the formation of ectopic axes that included notochord and somites. Axis duplication was preceded by the generation of an apparent ectopic shield (organizer equivalent) in nodal-injected embryos, as indicated by the appearance of a region over-expressing gsc and lim1; isolation and expression in the shield of the lim1 gene is reported here. These results suggest a role for a nodal-like factor in pattern formation in zebrafish.
The ELF alkaline phosphate substrate can be used to fluorescently label a wide variety of biological targets. This substrate yields a bright, photostable yellow-green fluorescent precipitate at the site of enzymatic activity. ELF labeling can be as much as 40 times as bright and hundreds of times as photostable as labeling with conventional fluorophores and yields signals capable of very fine submicroscopic resolution. Signal development is also extremely rapid, making the signal amplification technology well suited for applications such as RNA in situ hybridization.
1. Transmission at electrical synapses is modulated by a variety of physiological signals, and this modulation is a potentially general mechanism for regulating signal integration in neural circuits and networks. In the outer plexiform layer of the retina, modulation of horizontal-cell electrical coupling by dopamine alters the extent of spatial integration in the horizontal-cell network. By analyzing the activity of individual gap-junction channels in low-conductance electrical synapses of zebrafish retinal horizontal cells, we have defined the properties of these synaptic ion channels and characterized the functional changes in them during modulation of horizontal-cell electrical synapses. 2. Zebrafish horizontal-cell gap-junction channels have a unitary conductance of 50-60 pS and exhibit open times of several tens of milliseconds. The kinetic process of channel closure is best described by the sum of two rate constants. 3. Dopamine, and its agonist, (+/-)-6,7-dihydroxy-2-amino-tetralin (ADTN), modulates electrical synaptic transmission between horizontal cells predominantly by affecting channel-gating kinetics. These agents reduced the open probability of gap-junction channels two- to threefold by reducing both the duration and frequency of channel openings. Both time constants for channel open duration were reduced, whereas the duration of shut periods was increased. Similar changes in open-time kinetics were observed in power spectra of higher conductance gap junctions. 4. These results provide a description of rapid electrical synaptic modulation at the single channel level. The description should be useful in understanding the mechanisms of plasticity at these synapses throughout the vertebrate central nervous system.
An antibody was used to detect antigens in zebrafish that appear to be homologous to the frog homeodomain-containing protein XlHbox 1. These antigens show a restricted expression in the anteroposterior axis and an anteroposterior gradient in the pectoral fin bud, consistent with the distribution of XlHbox 1 protein in frog and mouse embryos. In the somitic mesoderm, a sharp anterior limit of expression coincides exactly with the boundary between somites 4 and 5, and the protein level fades out posteriorly. A similar, graded expression of the antigen is seen within the series of Rohon-Beard sensory neurons of the CNS. We also immunostained the mutant spt-1 ('spadetail'), in which the trunk mesoderm is greatly depleted and disorganized in the region of XlHbox 1 expression. The defects stem from misdirected cell movements during gastrulation, but nervertheless, newly recruited cells that partially refill the trunk mesoderm express the antigen within the normal span of the anteroposterior axis. This finding suggests that the mutation does not delete positional information required for activation of the XlHbox 1 gene.