Regulation of iro3 expression in the zebrafish spinal cord

The developmental hourglass model is applicable to the spinal cord based on single-cell transcriptomes and non-conserved cis-regulatory elements

The developmental hourglass model predicts that embryonic morphology is most conserved at the mid‐embryonic stage and diverges at the early and late stages. To date, this model has been verified by examining the anatomical features or gene expression profiles at the whole embryonic level. Here, by data mining approach utilizing multiple genomic and transcriptomic datasets from different species in combination, and by experimental validation, we demonstrate that the hourglass model is also applicable to a reduced element, the spinal cord. In the middle of spinal cord development, dorsoventrally arrayed neuronal progenitor domains are established, which are conserved among vertebrates. By comparing the publicly available single‐cell transcriptome datasets of mice and zebrafish, we found that ventral subpopulations of post‐mitotic spinal neurons display divergent molecular profiles. We also detected the non‐conservation of cis‐regulatory elements located around the progenitor fate determinants, indicating that the cis‐regulatory elements contributing to the progenitor specification are evolvable. These results demonstrate that, despite the conservation of the progenitor domains, the processes before and after the progenitor domain specification diverged. This study will be helpful to understand the molecular basis of the developmental hourglass model.

Tal1, Gata2a, and Gata3 Have Distinct Functions in the Development of V2b and Cerebrospinal Fluid-Contacting KA Spinal Neurons

Vertebrate locomotor circuitry contains distinct classes of ventral spinal cord neurons which each have particular functional properties. While we know some of the genes expressed by each of these cell types, we do not yet know how several of these neurons are specified. Here, we investigate the functions of Tal1, Gata2a, and Gata3 transcription factors in the development of two of these populations of neurons with important roles in locomotor circuitry: V2b neurons and cerebrospinal fluid-contacting Kolmer-Agduhr (KA) neurons (also called CSF-cNs). Our data provide the first demonstration, in any vertebrate, that Tal1 and Gata3 are required for correct development of KA and V2b neurons, respectively. We also uncover differences in the genetic regulation of V2b cell development in zebrafish compared to mouse. In addition, we demonstrate that Sox1a and Sox1b are expressed by KA and V2b neurons in zebrafish, which differs from mouse, where Sox1 is expressed by V2c neurons. KA neurons can be divided into ventral KA″ neurons and more dorsal KA′ neurons. Consistent with previous morpholino experiments, our mutant data suggest that Tal1 and Gata3 are required in KA′ but not KA″ cells, whereas Gata2a is required in KA″ but not KA′ cells, even though both of these cell types co-express all three of these transcription factors. In gata2a mutants, cells in the KA″ region of the spinal cord lose expression of most KA″ genes and there is an increase in the number of cells expressing V3 genes, suggesting that Gata2a is required to specify KA″ and repress V3 fates in cells that normally develop into KA″ neurons. On the other hand, our data suggest that Gata3 and Tal1 are both required for KA′ neurons to differentiate from progenitor cells. In the KA′ region of these mutants, cells no longer express KA′ markers and there is an increase in the number of mitotically-active cells. Finally, our data demonstrate that all three of these transcription factors are required for later stages of V2b neuron differentiation and that Gata2a and Tal1 have different functions in V2b development in zebrafish than in mouse.

Moving the Shh Source over Time: What Impact on Neural Cell Diversification in the Developing Spinal Cord?

A substantial amount of data has highlighted the crucial influence of Shh signalling on the generation of diverse classes of neurons and glial cells throughout the developing central nervous system. A critical step leading to this diversity is the establishment of distinct neural progenitor cell domains during the process of pattern formation. The forming spinal cord, in particular, has served as an excellent model to unravel how progenitor cells respond to Shh to produce the appropriate pattern. In recent years, considerable advances have been made in our understanding of important parameters that control the temporal and spatial interpretation of the morphogen signal at the level of Shh-receiving progenitor cells. Although less studied, the identity and position of Shh source cells also undergo significant changes over time, raising the question of how moving the Shh source contributes to cell diversification in response to the morphogen. Here, we focus on the dynamics of Shh-producing cells and discuss specific roles for these time-variant Shh sources with regard to the temporal events occurring in the receiving field.

Zebrafish transgenic constructs label specific neurons in Xenopus laevis spinal cord and identify frog V0v spinal neurons

A correctly functioning spinal cord is crucial for locomotion and communication between body and brain but there are fundamental gaps in our knowledge of how spinal neuronal circuitry is established and functions. To understand the genetic program that regulates specification and functions of this circuitry, we need to connect neuronal molecular phenotypes with physiological analyses. Studies using Xenopus laevis tadpoles have increased our understanding of spinal cord neuronal physiology and function, particularly in locomotor circuitry. However, the X. laevis tetraploid genome and long generation time make it difficult to investigate how neurons are specified. The opacity of X. laevis embryos also makes it hard to connect functional classes of neurons and the genes that they express. We demonstrate here that Tol2 transgenic constructs using zebrafish enhancers that drive expression in specific zebrafish spinal neurons label equivalent neurons in X. laevis and that the incorporation of a Gal4:UAS amplification cassette enables cells to be observed in live X. laevis tadpoles. This technique should enable the molecular phenotypes, morphologies and physiologies of distinct X. laevis spinal neurons to be examined together in vivo. We have used an islet1 enhancer to label Rohon-Beard sensory neurons and evx enhancers to identify V0v neurons, for the first time, in X. laevis spinal cord. Our work demonstrates the homology of spinal cord circuitry in zebrafish and X. laevis, suggesting that future work could combine their relative strengths to elucidate a more complete picture of how vertebrate spinal cord neurons are specified, and function to generate behavior. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1007-1020, 2017.

Sox4 regulates choroid fissure closure by limiting Hedgehog signaling during ocular morphogenesis

SoxC transcription factors play critical roles in many developmental processes, including neurogenesis, cardiac formation, and skeletal differentiation. In vitro and in vivo loss-of-function studies have suggested that SoxC genes are required for oculogenesis; however the mechanism was poorly understood. Here, we have explored the function of the SoxC factor Sox4 during zebrafish eye development. We show that sox4a and sox4b are expressed in the forebrain and periocular mesenchyme adjacent to the optic stalk during early eye development. Knockdown of sox4 in zebrafish resulted in coloboma, a structural malformation of the eye that is a significant cause of pediatric visual impairment in humans, in which the choroid fissure fails to close. Sox4 morphants displayed altered proximo-distal patterning of the optic vesicle, including expanded pax2 expression in the optic stalk, as well as ectopic cell proliferation in the retina. We show that the abnormal ocular morphogenesis observed in Sox4-deficient zebrafish is caused by elevated Hedgehog (Hh) signaling, and this is due to increased expression of the Hh pathway ligand Indian Hedgehog b (ihhb). Consistent with these results, coloboma in sox4 morphants could be rescued by pharmacological treatment with the Hh inhibitor cyclopamine, or by co-knockdown of ihhb. Conversely, overexpression of sox4 reduced Hh signaling and ihhb expression, resulting in cyclopia. Finally, we demonstrate that sox4 and sox11 have overlapping, but not completely redundant, functions in regulating ocular morphogenesis. Taken together, our data demonstrate that Sox4 is required to limit the extent of Hh signaling during eye development, and suggest that mutations in SoxC factors could contribute to the development of coloboma.

olig2-Expressing hindbrain cells are required for migrating facial motor neurons

The complicated trajectory of facial motor neuron migration requires coordination of intrinsic signals and cues from the surrounding environment. Migration begins in rhombomere (r) 4 where facial motor neurons are born and proceeds in a caudal direction. Once facial motor neurons reach their target rhombomeres, they migrate laterally and radially from the ventral neural tube. In zebrafish, as facial motor neurons migrate through r5/r6, they pass near cells that express olig2, which encodes a bHLH transcription factor. In this study, we found that olig2 function is required for facial motor neurons to complete their caudal migration into r6 and r7 and form stereotypical clusters. Additionally, embryos that lack mafba function, in which facial motor neurons also fail to complete caudal migration, lack olig2 expression in r5 and r6. Our data raise the possibility that cells expressing olig2 are intermediate targets that help guide facial motor neuron migration.

Roles of Hedgehog pathway components and retinoic acid signalling in specifying zebrafish ventral spinal cord neurons

In mouse, Hedgehog (Hh) signalling is required for most ventral spinal neurons to form. Here, we analyse the spinal cord phenotype of zebrafish maternal-zygotic smoothened (MZsmo) mutants that completely lack Hh signalling. We find that most V3 domain cells and motoneurons are lost, whereas medial floorplate still develops normally and V2, V1 and V0v cells form in normal numbers. This phenotype resembles that of mice that lack both Hh signalling and Gli repressor activity. Ventral spinal cord progenitor domain transcription factors are not expressed at 24 hpf in zebrafish MZsmo mutants. However, pMN, p2 and p1 domain markers are expressed at early somitogenesis stages in these mutants. This suggests that Gli repressor activity does not extend into zebrafish ventral spinal cord at these stages, even in the absence of Hh signalling. Consistent with this, ectopic expression of Gli3R represses ventral progenitor domain expression at these early stages and knocking down Gli repressor activity rescues later expression. We investigated whether retinoic acid (RA) signalling specifies ventral spinal neurons in the absence of Hh signalling. The results suggest that RA is required for the correct number of many different spinal neurons to form. This is probably mediated, in part, by an effect on cell proliferation. However, V0v, V1 and V2 cells are still present, even in the absence of both Hh and RA signalling. We demonstrate that Gli1 has a Hh-independent role in specifying most of the remaining motoneurons and V3 domain cells in embryos that lack Hh signalling, but removal of Gli1 activity does not affect more dorsal neurons.

olig1 Expression identifies developing oligodendrocytes in zebrafish and requires hedgehog and notch signaling

Myelin, the isolating sheath around large diameter axons, is formed in the central nervous system (CNS) by oligodendrocytes. We isolated the zebrafish ortholog of olig1, a bHLH transcription factor, and describe the origin and development of oligodendrocytes in the zebrafish brain. Olig1:mem-eGFP transgenic animals demonstrate the highly dynamic nature of oligodendrocyte membrane processes, providing a tool for studying in vivo oligodendrocyte development. Formation of oligodendrocytes and initiation of olig1 expression are under the control of long-range hedgehog and notch signaling while maintenance of olig1 expression only depends on hedgehog. Over-expression of olig1 did not affect myelin formation in the brain and combined over-expression of olig1 and olig2 could not rescue loss of hedgehog signaling, indicating that critical factors other than olig1 and olig2 are necessary. Lastly, knockdown of Olig1 in an Olig2-sensitized background did result in defects in CNS myelination, indicating a functional overlap between Olig1 and Olig2 proteins.

Tcf3 inhibits spinal cord neurogenesis by regulating sox4a expression

The Lef/Tcf factor Tcf3 is expressed throughout the developing vertebrate central nervous system (CNS), but its function and transcriptional targets are uncharacterized. Tcf3 is thought to mediate canonical Wnt signaling, which functions in CNS patterning, proliferation and neurogenesis. In this study, we examine Tcf3 function in the zebrafish spinal cord, and find that this factor does not play a general role in patterning, but is required for the proper expression of Dbx genes in intermediate progenitors. In addition, we show that Tcf3 is required to inhibit premature neurogenesis in spinal progenitors by repressing sox4a, a known mediator of spinal neurogenesis. Both of these functions are mediated by Tcf3 independently of canonical Wnt signaling. Together, our data indicate a novel mechanism for the regulation of neurogenesis by Tcf3-mediated repression.

Zebrafish V2 cells develop into excitatory CiD and Notch signalling dependent inhibitory VeLD interneurons

The vertebrate spinal cord contains distinct classes of cells that form at precise dorsal-ventral locations and express specific combinations of transcription factors. In amniotes, V2 cells develop in the ventral spinal cord, just dorsal to motoneurons. All V2 cells develop from the same progenitor domain and hence are initially molecularly identical. However, as they start to become post-mitotic and differentiate they subdivide into two intermingled molecularly-distinct subpopulations of cells, V2a and V2b cells. Here we show that the molecular identities of V2a and V2b cells are conserved between zebrafish and amniotes. In zebrafish, these two cell types both develop into interneurons with very similar morphologies, but while V2a cells become excitatory Circumferential Descending (CiD) interneurons, V2b cells become inhibitory Ventral Lateral Descending (VeLD) interneurons. In addition, we demonstrate that Notch signalling is required for V2 cells to develop into V2b cells. In the absence of Notch signalling, all V2b cells develop as V2a cells.

Regulation and function of Dbx genes in the zebrafish spinal cord

Dbx homeodomain proteins are important for spinal cord dorsal/ventral patterning and the production of multiple spinal cord cell types. We have examined the regulation and function of Dbx genes in the zebrafish. We report that Hedgehog signaling is not required for the induction or maintenance of these genes; in the absence of Hedgehog signaling, dbx1a/1b/2 are expanded ventrally with concomitant increases in postmitotic neurons that differentiate from this domain. Also, we find that retinoic acid signaling is not required for the induction of Dbx expression. Furthermore, we are the first to report that knockdown of Dbx1 function causes a dorsal expansion of nkx6.2, which is thought to be the cross-repressive partner of Dbx1 in mouse. Our data confirm that the dbx1a/1b/2 domain in zebrafish spinal cord development behaves similarly to amniotes, while extending knowledge of Dbx1 function in spinal cord patterning.

Neural patterning and CNS functions of Wnt in zebrafish

Wnt signaling plays several important roles in the development of the zebrafish central nervous system (CNS). This chapter outlines both the known and postulated roles of Wnts from the earliest step of neural plate induction to relatively late events such as axon pathfinding and synaptogenesis. The common tools useful for examining Wnt function and nervous system development in zebrafish are first reviewed. Examples are then provided for specific phenotypes resulting from gain and loss of Wnt activity at multiple developmental stages. Finally, specific assays and reagents that can be used to investigate the function of novel Wnt pathway components in CNS development are listed.

Proliferation and patterning are mediated independently in the dorsal spinal cord downstream of canonical Wnt signaling

Canonical Wnt signaling can regulate proliferation and patterning in the developing spinal cord, but the relationship between these functions has remained elusive. It has been difficult to separate the distinct activities of Wnts because localized changes in proliferation could conceivably alter patterning, and gain and loss of function experiments have resulted in both types of defects. To resolve this issue we have investigated canonical Wnt signaling in the zebrafish spinal cord using multiple approaches. We demonstrate that Wnt signaling is required initially for proliferation throughout the entire spinal cord, and later for patterning dorsal progenitor domains. Furthermore, we find that spinal cord patterning is normal in embryos after cell division has been pharmacologically blocked. Finally, we determine the transcriptional mediators of Wnt signaling that are responsible for patterning and proliferation. We show that tcf7 gene knockdown results in dorsal patterning defects without decreasing the mitotic index in dorsal domains. In contrast, tcf3 gene knockdown results in a reduced mitotic index without affecting dorsal patterning. Together, our work demonstrates that proliferation and patterning in the developing spinal cord are separable events that are regulated independently by Wnt signaling.

Cloning of zebrafish nkx6.2 and a comprehensive analysis of the conserved transcriptional response to Hedgehog/Gli signaling in the zebrafish neural tube

Sonic Hedgehog (Shh) signaling helps pattern the vertebrate neural tube, in part by regulating the dorsal/ventral expression of a number of homeodomain containing transcription factors. These Hh responsive genes have been divided into two classes, with Class II genes being activated by Hh signaling and Class I genes being repressed by Hh signaling. While the transcriptional response to varying Hh levels is well defined in chick and mouse, it is only partially described in zebrafish, despite the fact that zebrafish has emerged as a powerful genetic system for the study of neural patterning. To better characterize the Hh response in the zebrafish neural tube, we cloned the zebrafish Class II Hh target genes nkx2.9 and nkx6.2. We then analyzed the expression of a number of Class I and Class II Hh responsive genes in wild type, Hh mutant, and Hh over-expressing zebrafish embryos. We show that expression of Class I and Class II genes is highly conserved in the vertebrate neural tube. Further, ventral-most Class II gene expression was completely lost in all Hh pathway mutants analyzed, indicating high levels of Hh signaling are blocked in all of these mutants. In contrast, more dorsally expressed genes were variably affected in different Hh pathway mutants, indicating mid-levels of Hh signaling are differentially affected. This comprehensive expression study provides an important tool for the characterization of Hh signaling in zebrafish and provides a sensitive assay for determining the degree to which newly identified zebrafish mutants affect Hh signaling.

Comparison of neurolin (ALCAM) and neurolin-like cell adhesion molecule (NLCAM) expression in zebrafish

Many immunoglobulin (Ig)-superfamily cell adhesion molecules influence skeletal muscle formation. In Drosophila, dumbfounded (duf/kirre), irreC, sticks and stones and hibris encode related Ig-family proteins expressed in subsets of neurons and muscle precursor cells. The family mediates cell migration, axon guidance and fusion of myoblasts. Despite the importance of these genes in invertebrate myogenesis, no obvious functional parallels are known in vertebrate myogenesis. Here we investigate the gene expression pattern and phylogenetic and protein-structural relationships of the duf-related molecules neurolin and neurolin-like cell adhesion molecule (NLCAM), members of the activated leukocyte cell adhesion molecule (ALCAM) sub-family of Ig-molecules. These proteins are among the closest to Duf/Kirre by sequence. During zebrafish development, neurolin is expressed in subsets of somite and muscle cells, heart and numerous sites of neuronal maturation. The new ALCAM-family member, NLCAM, appears to have arisen by duplication of neurolin/ALCAM. NLCAM is expressed widely during gastrulation, particularly in the nascent neural plate, but later becomes predominantly expressed in sites of muscle and nerve maturation and in the fin fold. The expression of each gene is often in groups of cells in similar parts of the embryo; for example, in the region of Rohon Beard neurons, trigeminal ganglion and fusing fast and migrating slow muscle fibres. However, expression can also be distinct and dynamic; for example, muscle pioneer fibres express neurolin but not NLCAM at high level. Both molecules are expressed in subsets of muscle precursors at times prior to fusion.

How do genes regulate simple behaviours? Understanding how different neurons in the vertebrate spinal cord are genetically specified

Understanding how the vertebrate central nervous system develops and functions is a major goal of a large body of biological research. This research is driven both by intellectual curiosity about this amazing organ that coordinates our conscious and unconscious bodily processes, perceptions and actions and by the practical desire to develop effective treatments for people with spinal cord injuries or neurological diseases. In recent years, we have learnt an impressive amount about how the nerve cells that communicate with muscles, motoneurons, are made in a developing embryo and this knowledge has enabled researchers to grow motoneurons from stem cells. Building on the success of these studies, researchers have now started to unravel how most of the other nerve cells in the spinal cord are made and function. This review will describe what we currently know about spinal cord nerve cell development, concentrating on the largest category of nerve cells, which are called interneurons. I will then discuss how we can build and expand upon this knowledge base to elucidate the complete genetic programme that determines how different spinal cord nerve cells are made and connected up into neural circuits with particular functions.

Expression of the zebrafish Iroquois genes during early nervous system formation and patterning

Iroquois genes are involved in many patterning processes during development. In particular, they act as prepattern genes to control proneural gene expression both in Drosophila and in vertebrates. In this paper, we have analyzed the expression during embryogenesis of the 11 zebrafish Iroquois genes, with special interest for nervous system formation and patterning. During the first 2 days of development, Iroquois genes are expressed in distinct domains in the neuroepithelium, as well as in groups of neuronal progenitors and neurons. They are also expressed at different stages of placodal development. These expression patterns are similar to the patterns of the murine irx genes and also show features specific to teleosts. For the zebrafish Iroquois gene family, we find both specific patterns and patterns conserved within a cluster, between paralogues, or in most genes of the family. Overall, these expression data suggest functions for the Iroquois family of transcription factors in neural and placodal patterning, neurogenesis, and neuronal specification.