Zebrafish mutations in Gli-mediated hedgehog signaling lead to lens transdifferentiation from the adenohypophysis anlage.

It is known that the earliest lens marker delta-crystallin is expressed abundantly in Rathke's pouch of the chicken, suggesting a close relationship between the cell states of the adenohypophysis (pituitary) anlage and the early lens. We show here that the zebrafish midline mutants you-too (yot) and iguana (igu) develop lenses from the adenohypophysis anlage. The early adenohypophysis anlage of normal zebrafish expresses lim3 and six3 but in yot(ty119) mutants the anterior part of the anlage lacks lim3 expression, and instead produces a crystallin-expressing cell population which develops into a large lens structure expressing beta and gamma-crystallins, but is not associated with retina tissues. Among the zebrafish mutants with midline defects, midline lenses were observed in two mutant alleles of yot and an allele of igu, but not in other mutants (syu, con, smh, dtr, uml, spi and lok). Two yot mutant alleles with midline lenses likely encode dominant negative forms of the Gli2 protein which will interfere with transcriptional activation by other Gli proteins. The observation argues that overall inhibition of Shh-Gli signaling leads the adenohypophysis anlage to transdifferentiate into lens.

Comparative synteny cloning of zebrafish you-too: mutations in the Hedgehog target gli2 affect ventral forebrain patterning.

Zebrafish you-too (yot) mutations interfere with Hedgehog (Hh) signaling during embryogenesis. Using a comparative synteny approach, we isolated yot as a zinc finger transcription factor homologous to the Hh target gli2. Two alleles of yot contain nonsense mutations resulting in carboxy-terminally truncated proteins. In addition to causing defects in midline development, muscle differentiation, and retinal axon guidance, yot mutations disrupt anterior pituitary and ventral forebrain differentiation. yot mutations also cause ectopic lens formation in the ventral diencephalon. These findings reveal that truncated zebrafish Gli2 proteins interfere with Hh signaling necessary for differentiation and axon guidance in the ventral forebrain.

Genetic analysis of axon guidance and mapping in the zebrafish.

Systematic genetic screens have been powerful tools in identifying genes responsible for axon guidance in fruitflies and nematodes. This approach has now been extended to the study of axon guidance and the formation of topographic neuronal connections in the vertebrate brain. A systematic genetic screen was used to identify genes responsible for precise axon pathfinding and targeting in the retinotectal system of the zebrafish (Danio rerio). Over 30 genes were found that affect either: (1) retinal axon pathfinding to the contralateral tectal lobe; or (2) the topographic connection between the eye and the tectum. The zebrafish retinotectal mutants represent a new resource for the study of axon guidance in the vertebrate brain.

Mutations affecting development of the midline and general body shape during zebrafish embryogenesis.

Tissues of the dorsal midline of vertebrate embryos, such as notochord and floor plate, have been implicated in inductive interactions that pattern the neural tube and somites. In our screen for embryonic visible mutations in the zebrafish we found 113 mutations in more than 27 genes with altered body shape, often with additional defects in CNS development. We concentrated on a subgroup of mutations in ten genes (the midline-group) that cause defective development of the floor plate. By using floor plate markers, such as the signaling molecule sonic hedgehog, we show that the schmalspur (sur) gene is needed for early floor plate development, similar to one-eyed-pinhead (oep) and the previously described cyclops (cyc) gene. In contrast to oep and cyc, sur embryos show deletions of ventral CNS tissue restricted to the mid- and hindbrain, whereas the forebrain appears largely unaffected. In the underlying mesendodermal tissue of the head, sur is needed only for development of the posterior prechordal plate, whereas oep and cyc are required for both anterior and posterior prechordal plate development. Our analysis of sur mutants suggests that defects within the posterior prechordal plate may cause aberrant development of ventral CNS structures in the mid- and hindbrain. Later development of the floor plate is affected in mutant chameleon, you-too, sonic-you, iguana, detour, schmalhans and monorail embryos; these mutants often show additional defects in tissues that are known to depend on signals from notochord and floor plate. For example, sur, con and yot mutants show reduction of motor neurons; median deletions of brain tissue are seen in sur, con and yot embryos; and cyc, con, yot, igu and dtr mutants often show no or abnormal formation of the optic chiasm. We also find fusions of the ventral neurocranium for all midline mutants tested, which may reveal a hitherto unrecognized function of the midline in influencing differentiation of neural crest cells at their destination. As a working hypothesis, we propose that midline-group genes may act to maintain proper structure and inductive function of zebrafish midline tissues.

Genetic dissection of the retinotectal projection.

A systematic search for mutations affecting the retinotectal projection in zebrafish larvae was performed, as part of the large-scale Tubingen screen for homozygous diploid mutants in embryonic development. 2,746 inbred lines (F2 families) from males mutagenized with ethylnitroso urea were screened. In wild-type larvae, developing retinal axons travel along a stereotyped route to the contralateral optic tectum. Here, their terminals form a highly ordered retinotopic map. To detect deviations from this pattern, an axon tracing assay was developed that permits screening of large numbers of mutagenized fish. Two fluorescent tracer dyes (DiI and DiO) were injected at opposite poles of the eyes of day-5 aldehyde-fixed larvae. 12 hours later, retinal axons were labelled over their entire length, and could be observed through the intact skin. The assay procedure (aldehyde fixation, mounting, injection of dyes, microscopic analysis) took about 1 minute per fish. In total, 125,000 individual fish larvae were processed. During the screen, 114 mutations in approx. 35 genes were discovered. For the mutants subjected to complementation testing, the number of alleles per locus ranges from 1 to 15. The mutations affect distinct steps in the retinotectal pathway, from pathfinding between eye and tectum to map formation along the dorsal-ventral and the anterior-posterior axis of the tectum. Mutations that disturb axon pathfinding to the tectum for the most part do not disrupt retinotopic mapping, and vice versa. The majority of the mutants display associated defects in other tissues and die before day 10. These mutants provide new tools for studying the formation of neuronal maps. The results of this screen show that a large-scale genetic approach can be applied to relatively late and circumscribed developmental processes in the vertebrate brain.

Zebrafish mutations affecting retinotectal axon pathfinding.

We have isolated mutants in the zebrafish Danio rerio that have defects in axonal connectivity between the retina and tectum. 5-day-old fish larvae were screened by labeling retinal ganglion cells with DiI and DiO and observing their axonal projections to and on the tectum. 82 mutations, representing 13 complementation groups and 6 single allele loci, were found that have defects in retinal ganglion cell axon pathfinding to the tectum. These pathfinding genes fall into five classes, based on the location of pathfinding errors between eye and tectum. In Class I mutant larvae (belladonna, detour, you-too, iguana, umleitung, blowout) axons grow directly to the ipsilateral tectal lobe after leaving the eye. Class II mutant larvae (chameleon, bashful) have ipsilaterally projecting axons and, in addition, pathfinding mistakes are seen within the eye. In Class III mutant larvae (esrom, tilsit, tofu) fewer axons than normal cross the midline, but some axons do reach the contralateral tectal lobe. Class IV mutant larvae (boxer, dackel, pinscher) have defects in axon sorting after the midline and retinal axons occasionally make further pathfinding errors upon reaching the contralateral tectal lobe. Finally, Class V mutant larvae (bashful, grumpy, sleepy, cyclops, astray) have anterior-posterior axon trajectory defects at or after the midline. The analysis of these mutants supports several conclusions about the mechanisms of retinal axon pathfinding from eye to tectum. A series of sequential cues seems to guide retinal axons to the contralateral tectal lobe. Pre-existing axon tracts seem not to be necessary to guide axons across the midline. The midline itself seems to play a central role in guiding retinal axons. Axons in nearby regions of the brain seem to use different cues to cross the ventral midline. Mutant effects are not all-or-none, as misrouted axons may reach their target, and if they do, they project normally on the tectum. The retinotectal pathfinding mutants reveal important choice points encountered by neuronal growth cones as they navigate between eye and tectum.

Mutations disrupting the ordering and topographic mapping of axons in the retinotectal projection of the zebrafish, Danio rerio.

Retinal ganglion cells connect to their target organ, the rectum, in a highly ordered fashion. We performed a large-scale screen for mutations affecting the retinotectal projection of the zebrafish, which resulted in the identification of 114 mutations. 44 of these mutations disturb either the order of RGC axons in the optic nerve and tract, the establishment of a topographic map on the tectum, or the formation of proper termination fields. Mutations in three genes, boxer, dackel and pinscher, disrupt the sorting of axons in the optic tract but do not affect mapping on the tectum. In these mutants, axons from the dorsal retina grow along both the ventral and the dorsal branch of the optic tract. Mutations in two genes, nevermind and who-cares, affect the dorsoventral patterning of the projection. In embryos homozygous for either of these mutations, axons from dorsal retinal ganglion cells terminate ventrally and dorsally in the tectum. In nevermind, the retinotopic order of axons along the optic nerve and tract is changed in a characteristic way as well, while it appears to be unaffected in who-cares. Two mutations in two complementation groups, gnarled and macho, affect the anteroposterior patterning of the projection. In these mutants, nasodorsal axons branch and terminate too soon in the anterior tectum. In 27 mutants belonging to six complementation groups, retinal axons do not form normal termination fields. Some implications for models concerning the formation of topographic projections are discussed.

Lachesin: an immunoglobulin superfamily protein whose expression correlates with neurogenesis in grasshopper embryos.

We describe the developmental expression in grasshopper (Schistocerca americana) and molecular characterization in grasshopper and fruit fly (Drosophila melanogaster) of Lachesin, a novel immunoglobulin superfamily protein. Lachesin is expressed on the surfaces of differentiating neuronal cells from the onset of neurogenesis in both the central and peripheral nervous systems. Lachesin expression begins in some cells of the neurogenic ectoderm immediately after engrailed expression begins in the posterior cells of each future segment. All neurogenic cells express Lachesin early, but only those cells that become neuroblasts continue to express Lachesin. Ectodermal cells in the neurogenic region that adopt non-neuronal fates lose Lachesin at the time that they diverge from a potentially neurogenic pathway. Neuroblasts, ganglion mother cells and neurons all express Lachesin early in their lives, but expression becomes restricted to a subset of neurons as development progresses. Sensory neurons express Lachesin as they delaminate from the body wall ectoderm. Lachesin is also present on growing axons of the CNS and PNS and becomes restricted to a subset of axons later in development. This expression is unique among known insect neurogenic genes and suggests a role for Lachesin in early neuronal differentiation and axon outgrowth. Grasshopper Lachesin is a 38 x 10(3) M(r) protein linked to cell membranes through a glycosyl phosphatidylinositol anchor. We have cloned the Lachesin gene from both grasshopper and fly. The proteins are highly conserved (70% identical) between the two species. Lachesin is similar to Drosophila amalgam, bovine OBCAM and the human poliovirus receptor, putting it into a subgroup of the immunoglobulin superfamily containing one V- and two C2-type immunoglobulin domains. Lachesin is also similar to several other vertebrate immunoglobulin superfamily proteins (TAG-1, F11, L1 and NgCAM) known to function in neurite outgrowth and other cell surface recognition events.

Position-specific expression of the annulin protein during grasshopper embryogenesis.

Annulin, named for its annular expression in developing limb buds, is a approximately 100 kDa membrane-associated protein that is expressed in a complex and changing pattern during grasshopper embryogenesis. Its expression is dynamic along the developing midline and in the mesoderm, transient in neuroepithelial sheath cells around mitotic neuroblasts, and position-specific in circumferential stripes in each limb bud segment. Annulin expression begins along the midline of the embryo at the onset of gastrulation. Mesoderm cells express the protein as they migrate away from the midline as do new cells that come to lie at the midline. During neurogenesis, annulin expression disappears from many midline cells until only a specific subset of midline glial cells expresses high levels of the protein. Starting at the beginning of neurogenesis, sheath cells express annulin in correlation with the mitotic activity of the neuroblasts they surround.

The restricted spatial and temporal expression of a nervous-system-specific antigen involved in axon outgrowth during development of the grasshopper.

To identify molecules important for pathfinding by growing axons, monoclonal antibodies (mAb) have been generated against embryonic grasshopper tissue. One mAb, 2B2, shows labeling exclusively in the nervous system. It recognizes a surface epitope on neuronal growth cones, filopodia and axons in the central nervous system (CNS). Initially, the antigen is expressed on all processes of the CNS; after 70% of embryonic development, localization of the 2B2 mAb is restricted to a small subset of axon tracts within the ganglia. Immunoprecipitation from embryonic membrane extracts with the 2B2 mAb reveals a unique band of 160 x 10(3) Mr. Functional studies with the 2B2 mAb demonstrate that the antigen is important in growth cone-axon interactions during process outgrowth. Growth cones that extend along axonal substrata are either blocked in growth or grow along an aberrant pathway when embryos are cultured in the presence of the 2B2 mAb. However, pioneer neurons that extend processes on non-neuronal substrata grow normally.