Biology subjects

Blader, P.

Publications and source records attributed to Blader, P..

10 recordsLinked to original sources

bioRxiv2024

Local activation of Cxcl12a signaling controls olfactory placode morphogenesis in zebrafish embryos

Morphogenesis and cell-type differentiation are highly coordinated in sensory organs to ensure their function. Morphogenesis of the olfactory epithelium (OE) in zebrafish provides a unique model to study this process as undifferentiated cells aligned around the anterior neural plate mature into clusters of early olfactory neurons across a short time scale. While the Cxcl12a/Cxcr4b signaling pathway drives this process, what constraints on pathway activation apply during morphogenesis are unclear. We developed a mathematical model recapitulating Cxcl12a-mediated OE morphogenesis. Restoring Cxcl12a expression in mutants for the ligand rescues correct morphogenesis both in silico and in vivo. However, where expression of the ligand is restored is crucial for rescue, a point not predicted by our model and suggesting an unexpected level of pathway activation control. Analysis of a Cxcr4b activation reporter supports this idea. We concluded that mosaic and heterochronic Cxcl12a activation along the anteroposterior axis sculpts the olfactory epithelium.

developmental biology↗

bioRxiv2023

Analysis of a shark reveals ancient habenular asymmetries in gnathostomes and points to Wnt regulation as a driving force in their diversification

The origin of left-right asymmetries in the vertebrate habenula remains largely unknown. Using a transcriptomic approach, we show that in a cartilaginous fish, the catshark Scyliorhinus canicula, habenulae exhibit marked asymmetries both in their medial and their lateral component. Comparisons across gnathostomes suggest that asymmetries in the catshark lateral habenulae reflect an ancestral gnathostome trait, independently lost in tetrapods and neopterygians. Analysis of the mechanisms underlying their formation highlights an essential role of Wnt signaling. Wnt activity is submitted to a dynamic, asymmetric regulation during habenula development, with a Nodal dependent left repression at a stage when precursors for lateral habenulae have exited cell cycles. Pharmacological treatments during this time window reveal that Wnt signaling promotes lateral right neuronal identities in the right lateral habenula, while its repression by Nodal in the left one promotes lateral left neuronal identities. Based on comparisons with the zebrafish and the mouse, we propose that habenular asymmetry formation and diversification in gnathostomes involve the same developmental logic, relying on a conserved temporal regulation of neurogenesis, shaping neuronal identities on both sides, and its modification by a dynamic Wnt activity, right-restricted in the ancestral state and prone to variations in time and space during evolution.

developmental biology↗

bioRxiv2023

3D RNA profile of habenulae in the catshark Scyliorhinus canicula: a reference in jawed vertebrates

We report the generation of a spatially resolved RNA profile of the habenulae in a chondrichthyan, the catshark Scyliorhinus canicula. The digital profiles obtained for candidate markers of the broad territories previously characterized in this species reflect their broad characteristics, including left- or right asymmetric distributions. We use spatial autocorrelation and autocorrelation as measures to analyze these data and identify novel territory markers. These analyses lead to a refined molecular characterization of catshark habenulae and lead to extended lists of markers for the major territories previously identified. They confirm the marked asymmetry of the catshark habenulae, both in its lateral and medial components. In addition, they highlight a bilateral lateral to medial organization in radial bands, which may reflect a temporal regulation of neurogenesis. These data lead to a novel reference to explore the lmode of evolution of habenulae and of their asymmetries across gnathostomes.

developmental biology↗

bioRxiv2022

Analysis of photoentrainment in the diurnal zebrafish suggests a profound divergence with nocturnal rodents

The eye is instrumental for controlling circadian rhythms in mice and human. Here, we address the conservation of this function in the zebrafish, a diurnal vertebrate. Using lakritz (lak) mutant larvae, which lack retinal ganglion cells (RGCs), we show that while a functional eye contributes to masking, it is largely dispensable for the establishment of circadian rhythms of locomotor activity. Furthermore, the eye is dispensable for the induction of a phase delay following a pulse of white light at CT 16 but contributes to the induction of a phase advance upon a pulse of white light at CT21. Melanopsin photopigments are important mediators of photoentrainment, as shown in nocturnal mammals. One of the zebrafish melanopsin genes, opn4xa, is expressed in RGCs but also in photosensitive projection neurons in the pineal gland. Pineal opn4xa+ projection neurons function in a LIGHT ON manner in contrast to other projection neurons which function in a LIGHT OFF mode. We generated an opn4xa mutant in which the pineal LIGHT ON response is impaired. This mutation has no effect on masking and circadian rhythms of locomotor activity, or for the induction of phase shifts, but slightly modifies period length when larvae are subjected to constant light. Finally, analysis of opn4xa;lak double mutant larvae did not reveal redundancy between the function of the eye and opn4xa in the pineal for the control of phase shifts after light pulses. Our results support the idea that the eye is not the sole mediator of light influences on circadian rhythms of locomotor activity and highlight differences in the circadian system and photoentrainment of behaviour between different animal models. Author summaryExperiments performed in mice have established a crucial role for the eye in general and melanopsin expressing cells in particular in the control of circadian rhythms most notably during photoentrainment, by which circadian rhythms adapt to a changing light environment. In marked contrast to this, we show that in zebrafish the eye and photosensitivity dependent on one of the melanopsin genes, opn4xa, which is expressed in both the eye and the pineal gland, are largely dispensable for correct circadian rhythms. These results provide insight that the light sensors orchestrating circadian rhythms of locomotor activity are different between animal models supporting that vertebrates might employ different molecular/cellular circuits for photoentrainment of behaviour depending on their phylogeny and/or temporal niche.

neuroscience↗

bioRxiv2021

Probing small ribosomal subunit RNA helix 45 acetylation across eukaryotic evolution

NAT10 is an essential enzyme that catalyzes the formation of N4-acetylcytidine (ac4C) in eukaryotic transfer RNA (tRNA) and 18S ribosomal RNA (rRNA). Recent studies in human cells suggested that rRNA acetylation is dependent on SNORD13, a non-canonical box C/D small nucleolar RNA (SNORD) predicted to base-pair with 18S rRNA via two antisense elements. However, the selectivity of SNORD13-dependent cytidine acetylation and its relationship to NAT10s essential function in pre-rRNA processing remain to be defined. Here, we used CRISPR-Cas9 genome editing to formally demonstrate that SNORD13 is required for acetylation of a single cytidine residue of human and zebrafish 18S rRNA. In-depth characterization revealed that SNORD13-dependent ac4C is dispensable for yeast or human cell growth, ribosome biogenesis, translation, and the development of multicellular metazoan model organisms. This loss of function analysis inspired a cross-evolutionary survey of the eukaryotic rRNA acetylation machinery that led to the characterization of many novel SNORD13 genes in phylogenetically-distant metazoans and more deeply rooted photosynthetic organisms. This includes an atypical SNORD13-like RNA in D. melanogaster which appears to guide ac4C to 18S rRNA helix 45 despite lacking one of the two rRNA antisense elements. Finally, we discover that C. elegans 18S rRNA is not acetylated despite the presence of an essential NAT10 homolog. Altogether, our findings shed light on the molecular mechanisms underlying SNORD13-mediated rRNA acetylation across the eukaryotic tree of life and raise new questions regarding the biological function and evolutionary persistence of this highly conserved rRNA base modification.

molecular biology↗

bioRxiv2019

Morphogenesis is transcriptionally coupled to neurogenesis during olfactory placode development

Sense organs acquire their distinctive shapes concomitantly with the differentiation of sensory cells and neurons necessary for their function. While our understanding of the mechanisms controlling morphogenesis and neurogenesis in these structures has grown, how these processes are coordinated remains largely unexplored. Neurogenesis in the zebrafish olfactory epithelium requires the bHLH proneural transcription factor Neurogenin1 (Neurog1). To address whether Neurog1 also controls morphogenesis in this system, we analysed the morphogenetic behaviour of early olfactory neural progenitors in neurog1 mutant embryos. Our results indicate that the oriented movements of these progenitors are disrupted in this context. Morphogenesis is similarly affected by mutations in the chemokine receptor gene, cxcr4b, suggesting it is a potential Neurog1 target gene. We find that Neurog1 directly regulates cxcr4b through an E-boxes cluster located just upstream of the cxcr4b transcription start site. Our results suggest that proneural transcription factors, such as Neurog1, directly couple distinct aspects of nervous system development. One Sentence SummaryNeurog1 controls olfactory organ morphogenesis via cxcr4b

developmental biology↗

bioRxiv2019

A novel subtype of pineal projection neurons expressing melanopsin share a common developmental program with classical projection neurons

The zebrafish pineal organ is a photoreceptive structure containing two main neuronal populations (photoreceptors and projections neurons). Here we describe a new pineal cell type that harbors both characteristics of projection neurons and photoreceptors. Indeed, a subpopulation of projection neurons expresses the melanopsin gene opn4xa suggesting photoreceptive properties. This population of hybrid cell fates, share a similar behaviour regarding dependency for BMP and Notch signalling pathways with classical non-photosensitive projection neurons (PNs) suggesting they are closer to the PN population. We describe two distinct types of activity within PNs: an achromatic LIGHT OFF activity in opn4xa- PNs and a LIGHT ON activity elicited by green and blue light in opn4xa+ PNs. Altogether the discovery and characterization of opn4xa+ PNs suggest a previously unanticipated heterogeneity in the projection neuron population.

neuroscience↗

bioRxiv2019

Sox1a mediates the ability of the parapineal to impart habenular left-right asymmetry

Left-right asymmetries in the zebrafish habenular nuclei are dependent upon the formation of the parapineal, a unilateral group of neurons that arise from the medially positioned pineal complex. In this study, we show that both the left and right habenula are competent to adopt left-type molecular character and efferent connectivity upon the presence of only a few parapineal cells. This ability to impart left-sided character is lost in parapineal cells lacking Sox1a function, despite the normal specification of the parapineal itself. Precisely timed laser ablation experiments demonstrate that the parapineal influences neurogenesis in the left habenula at early developmental stages as well as neurotransmitter phenotype and efferent connectivity during subsequent stages of habenular differentiation. These studies reveal a tight coordination between the formation of the unilateral parapineal nucleus and emergence of asymmetric habenulae, ensuring that appropriate lateralised character is propagated within left and right-sided circuitry.

developmental biology↗

bioRxiv2019

Notch signaling restricts FGF pathway activation in parapineal cells to promote their collective migration

Coordinated migration of cell collectives is important during embryonic development and relies on cells integrating multiple mechanical and chemical cues. Recently, we described that focal activation of the FGF pathway promotes the migration of the parapineal in the zebrafish epithalamus. How FGF activity is restricted to leading cells in this system is, however, unclear. Here, we address the role of Notch signaling in modulating FGF activity within the parapineal. While Notch loss-of-function results in an increased number of parapineal cells activating the FGF pathway, global activation of Notch signaling decreases it; both contexts result in defects in parapineal migration and specification. Decreasing or increasing FGF signaling in a Notch loss-of-function context respectively rescues or aggravates parapineal migration defects without affecting parapineal cells specification. We propose that Notch signaling controls the migration of the parapineal through its capacity to restrict FGF pathway activation to a few leading cells.

cell biology↗

bioRxiv2017

Cell-type heterogeneity in the zebrafish olfactory placode is generated from progenitors within preplacodal ectoderm

Vertebrate olfactory placodes consists of a variety of neuronal populations, which are thought to have distinct embryonic origins. In the zebrafish, while ciliated sensory neurons arise from preplacodal ectoderm (PPE), previous lineage tracing studies suggest that both Gonadotropin releasing hormone 3 (Gnrh3) and microvillous sensory neurons derive from cranial neural crest (CNC). We find that the expression of Islet1/2 is restricted to Gnrh3 neurons associated with the olfactory placode. Unexpectedly, however, we find no change in Islet1/2+ cell numbers in sox10 mutant embryos, calling into question their CNC origin. Lineage reconstruction based on backtracking in time-lapse confocal datasets, and confirmed by photoconversion experiments, reveals that Gnrh3 neurons derive from the anterior/medial PPE. Similarly, all of the microvillous sensory neurons we have traced arise from preplacodal progenitors. Our results suggest that rather than originating from separate ectodermal populations, cell-type heterogeneity is generated from overlapping pools of progenitors within the preplacodal ectoderm.

developmental biology↗