Cell-to-cell heterogeneity in Sox2 and Bra expression guides progenitor motility and destiny.

Although cell-to-cell heterogeneity in gene and protein expression within cell populations has been widely documented, we know little about its biological functions. By studying progenitors of the posterior region of bird embryos, we found that expression levels of transcription factors Sox2 and Bra, respectively involved in neural tube (NT) and mesoderm specification, display a high degree of cell-to-cell heterogeneity. By combining forced expression and downregulation approaches with time-lapse imaging, we demonstrate that Sox2-to-Bra ratio guides progenitor's motility and their ability to stay in or exit the progenitor zone to integrate neural or mesodermal tissues. Indeed, high Bra levels confer high motility that pushes cells to join the paraxial mesoderm, while high levels of Sox2 tend to inhibit cell movement forcing cells to integrate the NT. Mathematical modeling captures the importance of cell motility regulation in this process and further suggests that randomness in Sox2/Bra cell-to-cell distribution favors cell rearrangements and tissue shape conservation

Nodal signalling imposes left-right asymmetry upon neurogenesis in the habenular nuclei

The habenulae are evolutionarily conserved bilateral nuclei in the epithalamus that relay input from the forebrain to the ventral midbrain. In zebrafish, the habenulae display left-right (L/R) asymmetries in gene expression and axonal projections. The elaboration of habenular asymmetries requires the presence of a second asymmetric structure, the parapineal, the laterality of which is biased by unilateral Nodal signalling. Here we show that neurons are present earlier in the left habenula than in the right, but, in contrast to other habenular asymmetry phenotypes, this asymmetry in neurogenesis is not dependent on the parapineal. Embryos in which the L/R asymmetry in Nodal signalling is abolished display symmetric neurogenesis, revealing a requirement for this pathway in asymmetrically biasing neurogenesis. Our results provide evidence of a direct requirement for unilateral Nodal activity in establishing an asymmetry per se, rather than solely in biasing its laterality

Alternative Translation Initiation of Human Fibroblast Growth Factor 2 mRNA Controlled by Its 3′-Untranslated Region Involves a Poly(A) Switch and a Translational Enhancer

International audienceFive fibroblast growth factor 2 (FGF-2) isoforms are synthesized from human FGF-2 mRNA by a process of alternative initiation of translation. The regulation of FGF-2 isoform expression by the mRNA 5823-nucleotide-long 3′-untranslated region containing eight alternative polyadenylation sites was examined. Because previous studies had shown that FGF-2 expression was regulated in primary cells but not in transformed cells, primary human skin fibroblasts were used in this study. Using an approach of cell transfection with synthetic reporter mRNAs, a novel translational enhancer (3′-TE) was identified in the 1370-nucleotide mRNA segment located upstream from the eighth poly(A) site. Deletion mutagenesis showed that the 3′-TE was composed of two domains with additive effects. The 3′-TE exhibited the unique feature of modulating the use of FGF-2 alternative initiation codons, which favored the relative expression of CUG-initiated isoforms. Interestingly, the use of an alternative polydenylation site removing the 3′-TE was detected in skin fibroblasts in response to heat shock and cell density variations. At high cell densities, 3′-TE removal was correlated with a loss of CUG-initiated FGF-2 expression. These data show that the FGF-2 mRNA 3′-untranslated region is able to modulate FGF-2 isoform expression by the coupled processes of translation activation and alternative polyadenylation

Mechano-sensory organ regeneration in adults: the zebrafish lateral line as a model.

In this report, we present a study of regeneration of the lateral line, a collection of mechano-sensory organ, in the adult zebrafish caudal fin. As all neuromasts are innervated by axon fibers, neuronal regeneration is a key issue in the regenerating process. We first show that support cells from the last neuromast adjacent to the amputation plane divide and migrate to colonize the blastema in order to reform the missing part of the lateral line. We then show that nerve re-growth takes place later than neuromast progenitor cell migration. We also provide evidence that new growth cones form at the amputation plane and subsequently follow the migrating placode-like structure to re-innervate regenerated neuromasts as they differentiate. Altogether, our observations indicate that caudal lateral line regeneration is not a mere recapitulation of the ontogenic process

No evidence for mate copying in Danio rerio

International audienceThe zebrafish Danio rerio is an important model organism, but little is known about its mating preferences and how these are influenced by personality traits like boldness. In this study, we tested two strains of zebrafish and addressed whether females used social information to build a mating preference, a behavior called mate copying, and whether this social learning was affected by boldness. Thus, we provided positive social information for small males to test whether female zebrafish changed their mate preference after observing a pair of a small and a large male with a demonstrator female next to the small one. After that, we tested the observer female in a test maze to evaluate boldness. We found no significant evidence for mate copying as females did not change their preference for the small male after witnessing the large male alone and the small male interacting with another female and chose consistently larger males in a control without opportunity to copy. Whether the female was defined as shy or bold had no effect on mate copying. We conclude that mate copying is probably inexistent or only relatively weak in this species

The importance of population heterogeneities in detecting social learning as the foundation of animal cultural transmission

National audienceHigh levels of within-population behavioural variation can have drastic demographic consequences, thus changing the evolutionary fate of populations. A major source of within-population heterogeneity is personality. Nonetheless, it is still relatively rarely accounted for in social learning studies that constitute the most basic process of cultural transmission. Here, we performed in female mosquitofish (Gambusia holbrooki) a social learning experiment in the context of mate choice, a situation called mate copying (MC), and for which there is strong evidence that it can lead to the emergence of persistent traditions of preferring a given male phenotype. When accounting for the global tendency of females to prefer larger males but ignoring differences in personality, we detected no evidence for MC. However, when accounting for the bold–shy dichotomy, we found that bold females did not show any evidence for MC, while shy females showed significant amounts of MC. This illustrates how the presence of variation in personality can hamper our capacity to detect MC. We conclude that MC may be more widespread than we thought because many studies ignored the presence of within-population heterogeneities

Habenular Neurogenesis in Zebrafish Is Regulated by a Hedgehog, Pax6 Proneural Gene Cascade

<div><p>The habenulae are highly conserved nuclei in the dorsal diencephalon that connect the forebrain to the midbrain and hindbrain. These nuclei have been implicated in a broad variety of behaviours in humans, primates, rodents and zebrafish. Despite this, the molecular mechanisms that control the genesis and differentiation of neural progenitors in the habenulae remain relatively unknown. We have previously shown that, in zebrafish, the timing of habenular neurogenesis is left-right asymmetric and that in the absence of Nodal signalling this asymmetry is lost. Here, we show that habenular neurogenesis requires the homeobox transcription factor Pax6a and the redundant action of two proneural bHLH factors, Neurog1 and Neurod4. We present evidence that Hedgehog signalling is required for the expression of <i>pax6a</i>, which is in turn necessary for the expression of <i>neurog1</i> and <i>neurod4</i>. Finally, we demonstrate by pharmacological inhibition that Hedgehog signalling is required continuously during habenular neurogenesis and by cell transplantation experiments that pathway activation is required cell autonomously. Our data sheds light on the mechanism underlying habenular development that may provide insights into how Nodal signalling imposes asymmetry on the timing of habenular neurogenesis.</p></div

Expression of <i>pax6a</i> is unaffected in <i>neurog1</i>^{<i>hi1059</i>}<i>;neurod4</i> morphant embryos.

<p>Confocal sections (A,B) or 10μm maximum projections (A’,B’) of the head of wild type (A; n = 6/6) or <i>neurog1</i>^{<i>hi1059</i>};<i>neurod4</i> Mo injected embryos (B; n = 10/10) at 36 hpf after a whole-mount <i>in situ</i> hybridization against <i>pax6a</i> (red) and an immunostaining against HuC/D protein (green); cell nuclei staining (in grey) makes visible brain structures (A,B). As previously described, the neuronal marker HuC/D is expressed in the epithalamus, both in epiphyseal projection neurons (ep) and in habenular neurons (Hb, white brackets) located on either sides of the epiphysis. The expression of HuC/D is abrogated or strongly reduced in the habenular domain of <i>neurog1</i>^{<i>hi1059</i>};<i>neurod4</i> morphant embryos, while it is still detected in the telencephalon (Tel), in the epiphysis and in neurons of the tectum (Tc) (B,B’). On the contrary, no change is seen in the expression of <i>pax6a</i> in the same region, suggesting that the expression of <i>neurog1</i> and <i>neurod4</i> does not regulate <i>pax6a</i> during habenular neurogenesis. Embryos are viewed dorsally with anterior up. The expression of HuC/D and <i>pax6a</i> in <i>neurog1</i>^{<i>hi1059</i>} mutants (n = 5/5) and <i>neurod4</i> Mo injected embryos (n = 11/11) was similar to that observed in the non-injected controls (data not shown).</p

Hedgehog signalling is required continuously from 16hpf for correct habenular development.

<p>Whole-mount <i>in situ</i> hybridization against <i>cxcr4b</i> (A-D’) or <i>brn3a</i> (E-H’) showing the epithalamus at 36 or 48 hpf, respectively, in control treated (A-D, E-H) or cyclopamine treated embryos from various stages (A’-D’, E’-H’). Cyclopamine treatment from 16 hpf completely abolishes the expression of <i>cxcr4b</i> (n = 19/21) and <i>brn3a</i> (n = 22/22) in habenular progenitors and neurons respectively. Treatment from 18 hpf results in either a similar abrogation of <i>cxcr4b</i> and <i>brn3a</i> expression (respectively n = 6/11 and n = 6/8) or in a strong decrease (n = 5/11 and n = 2/8, representative pictures in B’ and F’). The expression of <i>cxcr4b</i> and <i>brn3a</i> recovers if embryos are treated at progressively later stages. Although their expression level is significantly reduced, <i>cxcr4b</i> and <i>brn3a</i> are expressed in the habenulae of about half of the embryos treated with cyclopamine from 20 hpf (respectively C’, n = 20/37 and G’, n = 12/22) and in most of the embryos treated from 24 hpf (D’, n = 37/42 and H’, n = 28/34); in the remaining embryos, the expression was not detected (C’, n = 17/37, G’, n = 10/22, D’, n = 5/42 and H’, n = 6/34). For ethanol treated control embryos, the following numbers of embryos were examined: n = 19 (A), n = 3 (B), n = 10 (C), n = 9 (D), n = 19 (E), n = 5 (F), n = 8 (G), n = 7 (H). Embryos are viewed dorsally with anterior up.</p

Left/right asymmetric collective migration of parapineal cells is mediated by focal FGF signaling activity in leading cells

International audienceThe ability of cells to collectively interpret surrounding environmental signals underpins their capacity to coordinate their migrationin various contexts, including embryonic development and cancer metastasis. One tractable model for studying collectivemigration is the parapineal, a left-sided group of neurons that arises from bilaterally positioned precursors that undergo acollective migration to the left side of the brain. In zebrafish, the migration of these cells requires Fgf8 and, in this study, we resolvehow FGF signaling correlates with—and impacts the migratory dynamics of—the parapineal cell collective. The temporal and spatialdynamics of an FGF reporter transgene reveal that FGF signaling is activated in only few parapineal cells usually located at theleading edge of the parapineal during its migration. Overexpressing a constitutively active Fgf receptor compromises parapinealmigration in wild-type embryos, while it partially restores both parapineal migration and mosaic expression of the FGF reportertransgene in fgf8−/− mutant embryos. Focal activation of FGF signaling in few parapineal cells is sufficient to promote the migrationof the whole parapineal collective. Finally, we show that asymmetric Nodal signaling contributes to the restriction and leftwards biasof FGF pathway activation. Our data indicate that the first overt morphological asymmetry in the zebrafish brain is promoted byFGF pathway activation in cells that lead the collective migration of the parapineal to the left. This study shows that cell-state differencesin FGF signaling in front versus rear cells is required to promote migration in a model of FGF-dependent collective migration