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

Pioneer neurog1 expressing cells ingress into the otic epithelium and instruct neuronal specification

Neural patterning involves regionalised cell specification. Recent studies indicate that cell dynamics play instrumental roles in neural pattern refinement and progression, but the impact of cell behaviour and morphogenesis on neural specification is not understood. Here we combine 4D analysis of cell behaviours with dynamic quantification of proneural expression to uncover the construction of the zebrafish otic neurogenic domain. We identify pioneer cells expressing neurog1 outside the otic epithelium that migrate and ingress into the epithelialising placode to become the first otic neuronal progenitors. Subsequently, neighbouring cells express neurog1 inside the placode, and apical symmetric divisions amplify the specified pool. Interestingly, pioneer cells delaminate shortly after ingression. Ablation experiments reveal that pioneer cells promote neurog1 expression in other otic cells. Finally, ingression relies on the epithelialisation timing controlled by FGF activity. We propose a novel view for otic neurogenesis integrating cell dynamics whereby ingression of pioneer cells instructs neuronal specification

Sox10 contributes to the balance of fate choice in dorsal root ganglion progenitors

The development of functional peripheral ganglia requires a balance of specification of both neuronal and glial components. In the developing dorsal root ganglia (DRGs), these compo- nents form from partially-restricted bipotent neuroglial precursors derived from the neural crest. Work in mouse and chick has identified several factors, including Delta/Notch signal- ing, required for specification of a balance of these components. We have previously shown in zebrafish that the Sry-related HMG domain transcription factor, Sox10, plays an unex- pected, but crucial, role in sensory neuron fate specification in vivo. In the same study we described a novel Sox10 mutant allele, sox10baz1, in which sensory neuron numbers are elevated above those of wild-types. Here we investigate the origin of this neurogenic pheno- type. We demonstrate that the supernumerary neurons are sensory neurons, and that enteric and sympathetic neurons are almost absent just as in classical sox10 null alleles; peripheral glial development is also severely abrogated in a manner similar to other sox10 mutant alleles. Examination of proliferation and apoptosis in the developing DRG reveals very low levels of both processes in wild-type and sox10baz1, excluding changes in the bal- ance of these as an explanation for the overproduction of sensory neurons. Using chemical inhibition of Delta-Notch-Notch signaling we demonstrate that in embryonic zebrafish, as in mouse and chick, lateral inhibition during the phase of trunk DRG development is required to achieve a balance between glial and neuronal numbers. Importantly, however, we show that this mechanism is insufficient to explain quantitative aspects of the baz1 phenotype. The Sox10(baz1) protein shows a single amino acid substitution in the DNA binding HMG domain; structural analysis indicates that this change is likely to result in reduced flexibility in the HMG domain, consistent with sequence-specific modification of Sox10 binding to DNA. Unlike other Sox10 mutant proteins, Sox10(baz1) retains an ability to drive neurogenin1 transcription. We show that overexpression of neurogenin1 is sufficient to produce supernu- merary DRG sensory neurons in a wild-type background, and can rescue the sensory neu- ron phenotype of sox10 morphants in a manner closely resembling the baz1 phenotype. We conclude that an imbalance of neuronal and glial fate specification results from the Sox10 (baz1) protein\u2019s unique ability to drive sensory neuron specification whilst failing to drive glial development. The sox10baz1 phenotype reveals for the first time that a Notch-dependent lat- eral inhibition mechanism is not sufficient to fully explain the balance of neurons and glia in the developing DRGs, and that a second Sox10-dependent mechanism is necessary. Sox10 is thus a key transcription factor in achieving the balance of sensory neuronal and glial fates

Search for enhancers:teleost models in comparative genomic and transgenic analysis of cis regulatory elements

International audienceHomology searches between DNA sequences of evolutionary distant species (phylogenetic footprinting) offer a fast detection method for regulatory sequences. Because of the small size of their genomes, tetraodontid species such as the Japanese pufferfish and green spotted pufferfish have become attractive models for comparative genomics. A disadvantage of the tetraodontid species is, however, that they cannot be bred and manipulated routinely under laboratory conditions, so these species are less attractive for developmental and genetic analysis. In contrast, an increasing arsenal of transgene techniques with the developmental model species zebrafish and medaka are being used for functional analysis of cis regulatory sequences. The main disadvantage is the much larger genome. While comparison between many loci proved the suitability of phylogenetic footprinting using fish and mammalian sequences, fast rate of change in enhancer structure and gene duplication within teleosts may obscure detection of homologies. Here we discuss the contribution and potentials provided by different teleost models for the detection and functional analysis of conserved cis-regulatory elements

Functional heterogeneity in the pineal projection neurons of zebrafish

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A Notch-mediated, temporal asymmetry in BMP pathway activation promotes photoreceptor subtype diversification

International audienceNeural progenitors produce neurons whose identities can vary as a function of the time that specification occurs. Here, we describe the heterochronic specification of two photoreceptor (PhR) subtypes in the zebrafish pineal gland. We find that accelerating PhR specification by impairing Notch signaling favors the early fate at the expense of the later fate. Using in vivo lineage tracing, we show that most pineal PhRs are born from a fate-restricted progenitor. Furthermore, sister cells derived from the division of PhR-restricted progenitors activate the bone morphogenetic protein (BMP) signaling pathway at different times after division, and this heterochrony requires Notch activity. Finally, we demonstrate that PhR identity is established as a function of when the BMP pathway is activated. We propose a novel model in which division of a progenitor with restricted potential generates sister cells with distinct identities via a temporal asymmetry in the activation of a signaling pathway. Author summary A major goal in the field of developmental neurobiology is to identify the mechanisms that underly the diversification of the subtypes of neurons that are needed for the function of the nervous system. When investigating these mechanisms, time is an often-overlooked variable. Here, we show that in the zebrafish pineal gland-a neuroendocrine organ containing mostly photoreceptors (PhRs) and projection neurons-different classes of PhRs appear in a temporal sequence. In this simple system, the decision to adopt a PhR fate is driven by the activation of the bone morphogenetic protein (BMP) signaling pathway. Following the final cell division of a PhR progenitor, the sister cells normally activate the BMP pathway at different times. When Notch signaling activity is abrogated, activation of the BMP pathway occurs earlier and synchronously, which in turn favors the development of early PhR fates at the expense of later fates. We propose a model in which preventing sister cells from activating a signaling pathway in a synchronous fashion after their final division allows diversification of cell fates. PLOS Biology | https://doi.org/10.1371/journal.pbio

Assembly of trigeminal sensory ganglia by chemokine signaling

Sensory neurons with related functions form ganglia, but how these precisely positioned clusters are assembled has been unclear. Here, we use the zebrafish trigeminal sensory ganglion as a model to address this question. We find that some trigeminal sensory neurons are born at the position where the ganglion is assembled, whereas others are born at a distance and have to migrate against opposing morphogenetic movements to reach the site of ganglion assembly. Loss of Cxcr4b-mediated chemokine signaling results in the formation of mispositioned ganglia. Conversely, ectopic sources of the chemokine SDF1a can attract sensory neurons. Transplantation experiments reveal that neuron-neuron interaction and the adhesion molecules E- and N-Cadherin also contribute to ganglion assembly. These results indicate that ganglion formation depends on the interplay of birthplace, chemokine attraction, cell-cell interaction, and cadherin-mediated adhesion