The Forgotten: Identification and Functional Characterization of MHC Class II Molecules H2-Eb2 and RT1-Db2.

In this article, we report the complete coding sequence and to our knowledge, the first functional analysis of two homologous nonclassical MHC class II genes: RT1-Db2 of rat and H2-Eb2 of mouse. They differ in important aspects compared with the classical class II ß1 molecules: their mRNA expression by APCs is much lower, they show minimal polymorphism in the Ag-binding domain, and they lack N-glycosylation and the highly conserved histidine 81. Also, their cytoplasmic region is completely different and longer. To study and compare them with their classical counterparts, we transduced them in different cell lines. These studies show that they can pair with the classical α-chains (RT1-Da and H2-Ea) and are expressed at the cell surface where they can present superantigens. Interestingly, compared with the classical molecules, they have an extraordinary capacity to present the superantigen Yersinia pseudotuberculosis mitogen. Taken together, our findings suggest that the b2 genes, together with the respective α-chain genes, encode for H2-E2 or RT1-D2 molecules, which could function as Ag-presenting molecules for a particular class of Ags, as modulators of Ag presentation like nonclassical nonpolymorphic class II molecules DM and DO do, or even as players outside the immune system.

The LRR receptor Islr2 is required for retinal axon routing at the vertebrate optic chiasm.

BACKGROUND: In the visual system of most binocular vertebrates, the axons of retinal ganglion cells (RGCs) diverge at the diencephalic midline and extend to targets on both ipsi- and contralateral sides of the brain. While a molecular mechanism explaining ipsilateral guidance decisions has been characterized, less is known of how RGC axons cross the midline. RESULTS: Here, we took advantage of the zebrafish, in which all RGC axons project contralaterally at the optic chiasm, to characterize Islr2 as an RGC receptor required for complete retinal axon midline crossing. We used a systematic extracellular protein-protein interaction screening assay to identify two Vasorin paralogs, Vasna and Vasnb, as specific Islr2 ligands. Antibodies against Vasna and Vasnb reveal cellular populations surrounding the retinal axon pathway, suggesting the involvement of these proteins in guidance decisions made by axons of the optic nerve. Specifically, Vasnb marks the membranes of a cellular barricade located anteriorly to the optic chiasm, a structure termed the "glial knot" in higher vertebrates. Loss of function mutations in either vasorin paralog, individually or combined, however, do not exhibit an overt retinal axon projection phenotype, suggesting that additional midline factors, acting either independently or redundantly, compensate for their loss. Analysis of Islr2 knockout mice supports a scenario in which Islr2 controls the coherence of RGC axons through the ventral midline and optic tract. CONCLUSIONS: Although stereotypic guidance of RGC axons at the vertebrate optic chiasm is controlled by multiple, redundant mechanisms, and despite the differences in ventral diencephalic tissue architecture, we identify a novel role for the LRR receptor Islr2 in ensuring proper axon navigation at the optic chiasm of both zebrafish and mouse.

A Floor-Plate Extracellular Protein-Protein Interaction Screen Identifies Draxin as a Secreted Netrin-1 Antagonist.

Floor-plate-derived extracellular signaling molecules, including canonical axon guidance cues of the Netrin family, control neuronal circuit organization. Despite the importance of the floor plate as an essential signaling center in the developing vertebrate central nervous system, no systematic approach to identify binding partners for floor-plate-expressed cell-surface and secreted proteins has been carried out. Here, we used a high-throughput assay to discover extracellular protein-protein interactions, which likely take place in the zebrafish floor-plate microenvironment. The assembled floor-plate network contains 47 interactions including the hitherto-not-reported interaction between Netrin-1 and Draxin. We further characterized this interaction, narrowed down the binding interface, and demonstrated that Draxin competes with Netrin receptors for binding to Netrin-1. Our results suggest that Draxin functions as an extracellular Netrin signaling modulator in vertebrates. A reciprocal gradient of Draxin might shape or sharpen the active Netrin gradient, thereby critically modulating its effect.

Live imaging of endogenous protein dynamics in zebrafish using chromobodies.

Chromobodies are intracellular nanoprobes that combine the specificity of antibodies with the convenience of live fluorescence imaging in a flexible, DNA-encoded reagent. Here, we present the first application of this technique to an intact living vertebrate organism. We generated zebrafish lines expressing chromobodies that trace the major cytoskeletal component actin and the cell cycle marker PCNA with spatial and temporal specificity. Using these chromobodies, we captured full localization dynamics of the endogenous antigens in different cell types and at different stages of development. For the first time, the chromobody technology enables live imaging of endogenous subcellular structures in an animal, with the remarkable advantage of avoiding target protein overexpression or tagging. In combination with improved chromobody selection systems, we anticipate a rapid adaptation of this technique to new intracellular antigens and model organisms, allowing the faithful description of cellular and molecular processes in their dynamic state.

The Usher gene cadherin 23 is expressed in the zebrafish brain and a subset of retinal amacrine cells.

PURPOSE: To characterize the expression pattern of cadherin 23 (cdh23) in the zebrafish visual system, and to determine whether zebrafish cdh23 mutants have retinal defects similar to those present in the human disease Usher syndrome 1D. METHODS: In situ hybridization and immunohistochemistry were used to characterize cdh23 expression in the zebrafish, and to evaluate cdh23 mutants for retinal degeneration. Visual function was assessed by measurement of the optokinetic response in cdh23 siblings and mutants. RESULTS: We detected cdh23 mRNA expression in multiple nuclei of both the developing and adult central nervous system. In the retina, cdh23 mRNA was expressed in a small subset of amacrine cells, beginning at 70 h postfertilization and continuing through adulthood. No expression was detected in photoreceptors. The cdh23-positive population of amacrine cells was GABAergic. Examination of homozygous larvae expressing two different mutant alleles of cdh23-cdh23(tc317e) or cdh23(tj264a)-revealed no detectable morphological retinal defects or degeneration. In addition, the optokinetic response to moving gratings of varied contrast or spatial frequency was normal in both mutants. CONCLUSIONS: Unlike in other vertebrates, cdh23 is not detectable in zebrafish photoreceptors. Instead, cdh23 is expressed by a small subset of GABAergic amacrine cells. Moreover, larvae with mutations in cdh23 do not exhibit any signs of gross retinal degeneration or dysfunction. The role played by cdh23 in human retinal function is likely performed by either a different gene or an unidentified cdh23 splice variant in the retina that is not affected by the above mutations.

The impact of gene expression regulation on evolution of extracellular signaling pathways.

Extracellular protein interactions are crucial to the development of multicellular organisms because they initiate signaling pathways and enable cellular recognition cues. Despite their importance, extracellular protein interactions are often under-represented in large scale protein interaction data sets because most high throughput assays are not designed to detect low affinity extracellular interactions. Due to the lack of a comprehensive data set, the evolution of extracellular signaling pathways has remained largely a mystery. We investigated this question using a combined data set of physical pairwise interactions between zebrafish extracellular proteins, mainly from the immunoglobulin superfamily and leucine-rich repeat families, and their spatiotemporal expression profiles. We took advantage of known homology between proteins to estimate the relative rates of changes of four parameters after gene duplication, namely extracellular protein interaction, expression pattern, and the divergence of extracellular and intracellular protein sequences. We showed that change in expression profile is a major contributor to the evolution of signaling pathways followed by divergence in intracellular protein sequence, whereas extracellular sequence and interaction profiles were relatively more conserved. Rapidly evolving expression profiles will eventually drive other parameters to diverge more quickly because differentially expressed proteins get exposed to different environments and potential binding partners. This allows homologous extracellular receptors to attain specialized functions and become specific to tissues and/or developmental stages.

Construction of a large extracellular protein interaction network and its resolution by spatiotemporal expression profiling.

Extracellular interactions involving both secreted and membrane-tethered receptor proteins are essential to initiate signaling pathways that orchestrate cellular behaviors within biological systems. Because of the biochemical properties of these proteins and their interactions, identifying novel extracellular interactions remains experimentally challenging. To address this, we have recently developed an assay, AVEXIS (avidity-based extracellular interaction screen) to detect low affinity extracellular interactions on a large scale and have begun to construct interaction networks between zebrafish receptors belonging to the immunoglobulin and leucine-rich repeat protein families to identify novel signaling pathways important for early development. Here, we expanded our zebrafish protein library to include other domain families and many more secreted proteins and performed our largest screen to date totaling 16,544 potential unique interactions. We report 111 interactions of which 96 are novel and include the first documented extracellular ligands for 15 proteins. By including 77 interactions from previous screens, we assembled an expanded network of 188 extracellular interactions between 92 proteins and used it to show that secreted proteins have twice as many interaction partners as membrane-tethered receptors and that the connectivity of the extracellular network behaves as a power law. To try to understand the functional role of these interactions, we determined new expression patterns for 164 genes within our clone library by using whole embryo in situ hybridization at five key stages of zebrafish embryonic development. These expression data were integrated with the binding network to reveal where each interaction was likely to function within the embryo and were used to resolve the static interaction network into dynamic tissue- and stage-specific subnetworks within the developing zebrafish embryo. All these data were organized into a freely accessible on-line database called ARNIE (AVEXIS Receptor Network with Integrated Expression; www.sanger.ac.uk/arnie) and provide a valuable resource of new extracellular signaling interactions for developmental biology.

High-throughput identification of transient extracellular protein interactions.

Protein interactions are highly diverse in their biochemical nature, varying in affinity and are often dependent on the surrounding biochemical environment. Given this heterogeneity, it seems unlikely that any one method, and particularly those capable of screening for many protein interactions in parallel, will be able to detect all functionally relevant interactions that occur within a living cell. One major class of interactions that are not detected by current popular high-throughput methods are those that occur in the extracellular environment, especially those made by membrane-embedded receptor proteins. In the present article, we discuss some of our recent research in the development of a scalable assay to identify this class of protein interaction and some of the findings from its application in the construction of extracellular protein interaction networks.

Developmentally regulated impediments to skin reinnervation by injured peripheral sensory axon terminals.

The structural plasticity of neurites in the central nervous system (CNS) diminishes dramatically after initial development, but the peripheral nervous system (PNS) retains substantial plasticity into adulthood. Nevertheless, functional reinnervation by injured peripheral sensory neurons is often incomplete [1-6]. To investigate the developmental control of skin reinnervation, we imaged the regeneration of trigeminal sensory axon terminals in live zebrafish larvae following laser axotomy. When axons were injured during early stages of outgrowth, regenerating and uninjured axons grew into denervated skin and competed with one another for territory. At later stages, after the establishment of peripheral arbor territories, the ability of uninjured neighbors to sprout diminished severely, and although injured axons reinitiated growth, they were repelled by denervated skin. Regenerating axons were repelled specifically by their former territories, suggesting that local inhibitory factors persist in these regions. Antagonizing the function of several members of the Nogo receptor (NgR)/RhoA pathway improved the capacity of injured axons to grow into denervated skin. Thus, as in the CNS, impediments to reinnervation in the PNS arise after initial establishment of axon arbor structure.

A cell surface interaction network of neural leucine-rich repeat receptors.

BACKGROUND: The vast number of precise intercellular connections within vertebrate nervous systems is only partly explained by the comparatively few known extracellular guidance cues. Large families of neural orphan receptor proteins have been identified and are likely to contribute to these recognition processes but due to the technical difficulty in identifying novel extracellular interactions of membrane-embedded proteins, their ligands remain unknown. RESULTS: To identify novel neural recognition signals, we performed a large systematic protein interaction screen using an assay capable of detecting low affinity extracellular protein interactions between the ectodomains of 150 zebrafish receptor proteins containing leucine-rich-repeat and/or immunoglobulin superfamily domains. We screened 7,592 interactions to construct a network of 34 cell surface receptor-ligand pairs that included orphan receptor subfamilies such as the Lrrtms, Lrrns and Elfns but also novel ligands for known receptors such as Robos and Unc5b. A quantitative biochemical analysis of a subnetwork involving the Unc5b and three Flrt receptors revealed a surprising quantitative variation in receptor binding strengths. Paired spatiotemporal gene expression patterns revealed dynamic neural receptor recognition maps within the developing nervous system, providing biological support for the network and revealing likely functions. CONCLUSIONS: This integrated interaction and expression network provides a rich source of novel neural recognition pathways and highlights the importance of quantitative systematic extracellular protein interaction screens to mechanistically explain neural wiring patterns.

Type 3 cystatins; fetuins, kininogen and histidine-rich glycoprotein.

This review describes the properties of four structurally related, abundant plasma proteins denoted fetuin-A/alpha-2-Heremans Schmid-glycoprotein (AHSG), fetuin-B (FETUB), kininogen (KNG) and histidine-rich glycoprotein (HRG). These proteins form a subgroup (denoted type 3) within the cystatin superfamily of cysteine protease inhibitors. Apart from KNG, the type 3 proteins appear to lack cystatin activity. AHSG has its major function in regulation of bone mineralization; the physiological role of FETUB is poorly understood. KNG serves dual functions in the assembly of the protein complex initiating the surface-activated blood coagulation cascade and as a precursor for the kinin hormones. The heparin-binding HRG has also been implicated in regulation of coagulation. In addition, several members of the type 3 cystatins have been implicated in tumor growth and shown to regulate endothelial cell function and formation of new blood vessels, angiogenesis. Thus, these proteins may potentially be useful in treatment of diseases characterized by excess angiogenesis such as cancer.

Vesicular glutamate transporter 3 is required for synaptic transmission in zebrafish hair cells.

Hair cells detect sound and movement and transmit this information via specialized ribbon synapses. Here we report that asteroid, a gene identified in an ethylnitrosourea mutagenesis screen of zebrafish larvae for auditory/vestibular mutants, encodes vesicular glutamate transporter 3 (Vglut3). A splice site mutation in exon 2 of vglut3 results in a severe truncation of the predicted protein product and morpholinos directed against the vglut3 ATG start site or the affected splice junction replicate the asteroid phenotype. In situ hybridization shows that vglut3 is exclusively expressed in hair cells of the ear and lateral line organ. A second transporter gene, vglut1, is also expressed in zebrafish hair cells, but the level of vglut1 mRNA is not increased in the absence of Vglut3. Antibodies against Vglut3 label the basal end of hair cells and labeling is not present in asteroid/vglut3 mutants. Based on the localization of Vglut3 in hair cells, we suspected that the lack of vestibulo-ocular and acoustic startle reflexes in asteroid/vglut3 mutants was attributable to a defect in synaptic transmission in hair cells. In support of this notion, action currents in postsynaptic acousticolateralis neurons are absent in asteroid/vglut3 mutants. At the ultrastructural level, mutant asteroid/vglut3 hair cells show a decrease in the number of ribbon-associated synaptic vesicles, indicating a role for Vglut3 in synaptic vesicle biogenesis and/or tethering to the ribbon body. Lack of postsynaptic action currents in the mutants suggests that the remaining hair-cell synaptic vesicles contain insufficient levels of glutamate for generation of action potentials in first-order neurons.

Large-scale screening for novel low-affinity extracellular protein interactions.

Extracellular protein-protein interactions are essential for both intercellular communication and cohesion within multicellular organisms. Approximately a fifth of human genes encode membrane-tethered or secreted proteins, but they are largely absent from recent large-scale protein interaction datasets, making current interaction networks biased and incomplete. This discrepancy is due to the unsuitability of popular high-throughput methods to detect extracellular interactions because of the biochemical intractability of membrane proteins and their interactions. For example, cell surface proteins contain insoluble hydrophobic transmembrane regions, and their extracellular interactions are often highly transient, having half-lives of less than a second. To detect transient extracellular interactions on a large scale, we developed AVEXIS (avidity-based extracellular interaction screen), a high-throughput assay that overcomes these technical issues and can detect very transient interactions (half-lives

Large-scale mapping of mutations affecting zebrafish development.

BACKGROUND: Large-scale mutagenesis screens in the zebrafish employing the mutagen ENU have isolated several hundred mutant loci that represent putative developmental control genes. In order to realize the potential of such screens, systematic genetic mapping of the mutations is necessary. Here we report on a large-scale effort to map the mutations generated in mutagenesis screening at the Max Planck Institute for Developmental Biology by genome scanning with microsatellite markers. RESULTS: We have selected a set of microsatellite markers and developed methods and scoring criteria suitable for efficient, high-throughput genome scanning. We have used these methods to successfully obtain a rough map position for 319 mutant loci from the Tübingen I mutagenesis screen and subsequent screening of the mutant collection. For 277 of these the corresponding gene is not yet identified. Mapping was successful for 80 % of the tested loci. By comparing 21 mutation and gene positions of cloned mutations we have validated the correctness of our linkage group assignments and estimated the standard error of our map positions to be approximately 6 cM. CONCLUSION: By obtaining rough map positions for over 300 zebrafish loci with developmental phenotypes, we have generated a dataset that will be useful not only for cloning of the affected genes, but also to suggest allelism of mutations with similar phenotypes that will be identified in future screens. Furthermore this work validates the usefulness of our methodology for rapid, systematic and inexpensive microsatellite mapping of zebrafish mutations.

Mutated otopetrin 1 affects the genesis of otoliths and the localization of Starmaker in zebrafish.

Otoliths in bony fishes and otoconia in mammals are composite crystals consisting of calcium carbonate and proteins. These biominerals are part of the gravity and linear acceleration detection system of the inner ear. Mutations in otopetrin 1 have been shown to result in lack of otoconia in tilted and mergulhador mutant mice. The molecular function of Otopetrin 1, a novel protein that contains ten predicted transmembrane domains, however, has remained elusive. Here we show that a mutation in the orthologous gene in zebrafish is responsible for the complete absence of otoliths in backstroke mutants. We examined the localization of Starmaker, a secreted protein that is highly abundant in otoliths in backstroke mutants. Starmaker protein accumulated within cells of the otic epithelium, indicating a possible defect in secretion. Our data suggest that Otopetrin 1 in zebrafish may be involved in the protein trafficking of components required for formation of biominerals in the ear.

Mutations in cadherin 23 affect tip links in zebrafish sensory hair cells.

Hair cells have highly organized bundles of apical projections, or stereocilia, that are deflected by sound and movement. Displacement of stereocilia stretches linkages at the tips of stereocilia that are thought to gate mechanosensory channels. To identify the molecular machinery that mediates mechanotransduction in hair cells, zebrafish mutants were identified with defects in balance and hearing. In sputnik mutants, stereociliary bundles are splayed to various degrees, with individuals displaying reduced or absent mechanotransduction. Here we show that the defects in sputnik mutants are caused by mutations in cadherin 23 (cdh23). Mutations in Cdh23 also cause deafness and vestibular defects in mice and humans, and the protein is present in hair bundles. We show that zebrafish Cdh23 protein is concentrated near the tips of hair bundles, and that tip links are absent in homozygous sputnik(tc317e) larvae. Moreover, tip links are absent in larvae carrying weak alleles of cdh23 that affect mechanotransduction but not hair bundle integrity. We conclude that Cdh23 is an essential tip link component required for hair-cell mechanotransduction.

The deafness gene dfna5 is crucial for ugdh expression and HA production in the developing ear in zebrafish.

Over 30 genes responsible for human hereditary hearing loss have been identified during the last 10 years. The proteins encoded by these genes play roles in a diverse set of cellular functions ranging from transcriptional regulation to K(+) recycling. In a few cases, the genes are novel and do not give much insight into the cellular or molecular cause for the hearing loss. Among these poorly understood deafness genes is DFNA5. How the truncation of the encoded protein DFNA5 leads to an autosomal dominant form of hearing loss is not clear. In order to understand the biological role of Dfna5, we took a reversegenetic approach in zebrafish. Here we show that morpholino antisense nucleotide knock-down of dfna5 function in zebrafish leads to disorganization of the developing semicircular canals and reduction of pharyngeal cartilage. This phenotype closely resembles previously isolated zebrafish craniofacial mutants including the mutant jekyll. jekyll encodes Ugdh [uridine 5'-diphosphate (UDP)-glucose dehydrogenase], an enzyme that is crucial for production of the extracellular matrix component hyaluronic acid (HA). In dfna5 morphants, expression of ugdh is absent in the developing ear and pharyngeal arches, and HA levels are strongly reduced in the outgrowing protrusions of the developing semicircular canals. Previous studies suggest that HA is essential for differentiating cartilage and directed outgrowth of the epithelial protrusions in the developing ear. We hypothesize that the reduction of HA production leads to uncoordinated outgrowth of the canal columns and impaired facial cartilage differentiation.

Control of crystal size and lattice formation by starmaker in otolith biomineralization.

The stone-like otoliths from the ears of teleost fishes are involved in balance and hearing and consist of calcium carbonate crystallites embedded in a protein framework. We report that a previously unknown gene, starmaker, is required in zebrafish for otolith morphogenesis. Reduction of starmaker activity by injection of modified antisense oligonucleotides causes a change in the crystal lattice structure and thus a change in otolith morphology. The expression pattern of starmaker, along with the presence of the protein on the growing otolith, suggest that the expression levels of starmaker control the shape of the otoliths.