BackgroundPrions and amyloid-β (Aβ) oligomers trigger neurodegeneration by hijacking a poorly understood cellular signal mediated by the prion protein (PrP) at the plasma membrane. In early zebrafish embryos, PrP-1-dependent signals control cell-cell adhesion via a tyrosine phosphorylation-dependent mechanism.ResultsHere we report that the Src family kinases (SFKs) Fyn and Yes act downstream of PrP-1 to prevent the endocytosis and degradation of E-cadherin/β-catenin adhesion complexes in vivo. Accordingly, knockdown of PrP-1 or Fyn/Yes cause similar zebrafish gastrulation phenotypes, whereas Fyn/Yes expression rescues the PrP-1 knockdown phenotype. We also show that zebrafish and mouse PrPs positively regulate the activity of Src kinases and that these have an unexpected positive effect on E-cadherin-mediated cell adhesion. Interestingly, while PrP knockdown impairs β-catenin adhesive function, PrP overexpression enhances it, thereby antagonizing its nuclear, wnt-related signaling activity and disturbing embryonic dorsoventral specification. The ability of mouse PrP to influence these events in zebrafish embryos requires its neuroprotective, polybasic N-terminus but not its neurotoxicity-associated central region. Remarkably, human Aβ oligomers up-regulate the PrP-1/SFK/E-cadherin/β-catenin pathway in zebrafish embryonic cells, mimicking a PrP gain-of-function scenario.ConclusionsOur gain- and loss-of-function experiments in zebrafish suggest that PrP and SFKs enhance the cell surface stability of embryonic adherens junctions via the same complex mechanism through which they over-activate neuroreceptors that trigger synaptic damage. The profound impact of this pathway on early zebrafish development makes these embryos an ideal model to study the cellular and molecular events affected by neurotoxic PrP mutations and ligands in vivo. In particular, our finding that human Aβ oligomers activate the zebrafish PrP/SFK/E-cadherin pathway opens the possibility of using fish embryos to rapidly screen for novel therapeutic targets and compounds against prion- and Alzheimer's-related neurodegeneration. Altogether, our data illustrate PrP-dependent signals relevant to embryonic development, neuronal physiology and neurological disease.Electronic supplementary materialThe online version of this article (doi:10.1186/s13024-016-0076-5) contains supplementary material, which is available to authorized users.

BackgroundAs a consequence of gene/genome duplication, the RTN4/Nogo gene has two counterparts in zebrafish: rtn4a and rtn4b. The shared presence of four specific amino acid motifs—M1 to M4—in the N-terminal region of mammalian RTN4, and zebrafish Rtn4b suggests that Rtn4b is the closest homologue of mammalian Nogo-A.ResultsTo explore their combined roles in zebrafish development, we characterized the expression patterns of rtn4a and rtn4b in a comparative manner and performed morpholino-mediated knockdowns. Although both genes were coexpressed in the neural tube and developing brain at early stages, they progressively acquired distinct expression domains such as the spinal cord (rtn4b) and somites (rtn4a). Downregulation of rtn4a and rtn4b caused severe brain abnormalities, with rtn4b knockdown severely affecting the spinal cord and leading to immobility. In addition, the retinotectal projection was severely affected in both morphants, as the retina and optic tectum appeared smaller and only few retinal axons reached the abnormally reduced tectal neuropil. The neuronal defects were more persistent in rtn4b morphants. Moreover, the latter often lacked pectoral fins and lower jaws and had malformed branchial arches. Notably, these defects led to larval death in rtn4b, but not in rtn4a morphants.ConclusionsIn contrast to mammalian Nogo-A, its zebrafish homologues, rtn4a and particularly rtn4b, are essential for embryonic development and patterning of the nervous system.

Background: The conditional Cre/lox system has recently emerged as a valuable tool for studies on both embryonic and adult Zebrafish. Temporal control and site-specific recombination are achieved by using the ligand-inducible CreER T2 and administration of the drug tamoxifen (TAM) or its active metabolite, 4-Hydroxytamoxifen (4-OHT). Results: Here we report the generation of a transgenic Zebrafish line, which expresses an mCherry-tagged variant of CreER T2 under the control of the myelin basic protein a (mbpa) promoter. Our analysis shows that larval and adult expression of the transgene recapitulates the endogenous mbpa expression pattern in oligodendrocytes. Furthermore, combination with a Cre-dependent EGFP reporter results in EGFPexpressing oligodendrocytes in the spinal cord, brain, and optic nerve in TAM-or 4-OHT-treated larvae and 4-month-old fish, but not in untreated controls. Conclusions: The transgenic Zebrafish line Tg(mbpa:mCherry-T2A-CreER T2 ) elicits CreER T2 expression specifically in myelinating glia cells. Cre-inducible targeted recombination of genes in oligodendrocytes will be useful to elucidate cellular and molecular mechanisms of myelination in vivo during development (myelination) and regeneration (remyelination) after injury to the central nervous system (CNS). It will also allow targeted expression and overexpression of genes of interest (transgenes) in oligodendrocytes at defined developmental and adult stages. Developmental Dynamics 246:41-49, 2017. V C 2016 Wiley Periodicals, Inc.

This study explored why lesioned retinal ganglion cell (RGC) axons regenerate successfully in the zebrafish optic nerve despite the presence of Rtn4b, the homologue of the rat neurite growth inhibitor RTN4-A/Nogo-A. Rat Nogo-A and zebrafish Rtn4b possess characteristic motifs (M1-4) in the Nogo-A-specific region, which contains delta20, the most inhibitory region of rat Nogo-A. To determine whether zebrafish M1-4 is inhibitory as rat M1-4 and Nogo-A delta20, proteins were recombinantly expressed and used as substrates for zebrafish single cell RGCs, mouse hippocampal neurons and goldfish, zebrafish and chick retinal explants. When offered as homogenous substrates, neurites of hippocampal neurons and of zebrafish single cell RGCs were inhibited by zebrafish M1-4, rat M1-4, and Nogo-A delta20. Neurite length increased when zebrafish single cell RGCs were treated with receptor-type-specific antagonists and, respectively, with morpholinos (MO) against S1PR2 and S1PR5a-which represent candidate zebrafish Nogo-A receptors. In a stripe assay, however, where M1-4 lanes alternate with polylysine-(Plys)-only lanes, RGC axons from goldfish, zebrafish, and chick retinal explants avoided rat M1-4 but freely crossed zebrafish M1-4 lanes-suggesting that zebrafish M1-4 is growth permissive and less inhibitory than rat M1-4. Moreover, immunostainings and dot blots of optic nerve and myelin showed that expression of Rtn4b is very low in tissue and myelin at 3-5 days after lesion when axons regenerate. Thus, Rtn4b seems to represent no major obstacle for axon regeneration in vivo because it is less inhibitory for RGC axons from retina explants, and because of its low abundance.