Netrin signaling breaks the equivalence between two identified zebrafish motoneurons revealing a new role of intermediate targets

Characterization of paralogous uncx transcription factor encoding genes in zebrafish

The paired-type homeodomain transcription factor Uncx is involved in multiple processes of embryogenesis in vertebrates. Reasoning that zebrafish genes uncx4.1 and uncx are orthologs of mouse Uncx, we studied their genomic environment and developmental expression. Evolutionary analyses indicate the zebrafish uncx genes as being paralogs deriving from teleost-specific whole-genome duplication. Whole-mount in situ mRNA hybridization of uncx transcripts in zebrafish embryos reveals novel expression domains, confirms those previously known, and suggests sub-functionalization of paralogs. Using genetic mutants and pharmacological inhibitors, we investigate the role of signaling pathways on the expression of zebrafish uncx genes in developing somites. In identifying putative functional role(s) of zebrafish uncx genes, we hypothesized that they encode transcription factors that coordinate growth and innervation of somitic muscles.

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The Uncx4.1 and Uncx genes derive from the teleost-specific whole-genome duplication.

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Uncx genes are expressed during embryogenesis in unique and overlapping domains.

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Uncx gene expression during somite differentiation is regulated by FGF signaling.

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Synteny and expression profiles correlate Uncx genes with axon guidance.

Knockdown of Laminin gamma-3 (Lamc3) impairs motoneuron guidance in the zebrafish embryo

Background: Previous work in the zebrafish embryo has shown that laminin γ-3 ( lamc3) is enriched in endothelial cells marked by expression of fli1a, but the role of Lamc3 has been unknown. Methods: We use antisense morpholino oligonucleotides, and CRISPR/Cas9 mutagenesis of F0 embryos, to create zebrafish embryos in which lamc3 expression is compromised. Transgenic imaging, immunofluorescence, and in situ hybridisation reveal that Lamc3 loss-of-function affects the development of muscle pioneers, endothelial cells, and motoneurons. Results:  Lamc3 is enriched in endothelial cells during zebrafish development, but it is also expressed by other tissues. Depletion of Lamc3 by use of antisense morpholino oligonucleotides perturbs formation of the parachordal chain and subsequently the thoracic duct, but Lamc3 is not required for sprouting of the cardinal vein. F0 embryos in which lamc3 expression is perturbed by a CRISPR/Cas9 approach also fail to form a parachordal chain, but we were unable to establish a stable lamc3 null line. Lamc3 is dispensable for muscle pioneer specification and for the expression of netrin-1a in these cells. Lamc3 knockdown causes netrin-1a up-regulation in the neural tube and there is increased Netrin-1 protein throughout the trunk of the embryo. Axonal guidance of rostral primary motoneurons is defective in Lamc3 knockdown embryos. Conclusions: We suggest that knockdown of Lamc3 perturbs migration of rostral primary motoneurons at the level of the horizontal myoseptum, indicating that laminin γ3 plays a role in motoneuron guidance.

Lhx3 and Lhx4 suppress Kolmer-Agduhr interneuron characteristics within zebrafish axial motoneurons

A central problem in development is how fates of closely related cells are segregated. Lineally related motoneurons (MNs) and interneurons (INs) express many genes in common yet acquire distinct fates. For example, in mouse and chick Lhx3 plays a pivotal role in the development of both cell classes. Here, we utilize the ability to recognize individual zebrafish neurons to examine the roles of Lhx3 and its paralog Lhx4 in the development of MNs and ventral INs. We show that Lhx3 and Lhx4 are expressed by post-mitotic axial MNs derived from the MN progenitor (pMN) domain, p2 domain progenitors and by several types of INs derived from pMN and p2 domains. In the absence of Lhx3 and Lhx4, early-developing primary MNs (PMNs) adopt a hybrid fate, with morphological and molecular features of both PMNs and pMN-derived Kolmer-Agduhr' (KA') INs. In addition, we show that Lhx3 and Lhx4 distinguish the fates of two pMN-derived INs. Finally, we demonstrate that Lhx3 and Lhx4 are necessary for the formation of late-developing V2a and V2b INs. In conjunction with our previous work, these data reveal that distinct transcription factor families are deployed in post-mitotic MNs to unequivocally assign MN fate and suppress the development of alternative pMN-derived IN fates.

The role of inab in axon morphology of an identified zebrafish motoneuron

The ability of an animal to move and to interact with its environment requires that motoneurons correctly innervate specific muscles. Although many genes that regulate motoneuron development have been identified, our understanding of motor axon branching remains incomplete. We used transcriptional expression profiling to identify potential candidate genes involved in development of zebrafish identified motoneurons. Here we focus on inab, an intermediate filament encoding gene dynamically expressed in a subset of motoneurons as well as in an identified interneuron. We show that inab is necessary for proper axon morphology of a specific motoneuron subtype.

Notum homolog plays a novel role in primary motor innervation

To form complex neuronal networks, growth cones use intermediate targets as guideposts on the path to more distant targets. In the developing zebrafish (Danio rerio), the muscle pioneers (MPs) are intermediate targets for primary motor neurons (PMNs) that innervate the trunk musculature. The mechanisms regulating PMN axon guidance at the MPs are not fully understood. We have identified a new member of the Notum family in zebrafish, Notum 2, which is expressed exclusively in the MPs during primary motor innervation. While homologs of Notum, including zebrafish Notum 1a, negatively regulate the Wnt/β-catenin signaling pathway, we discovered a novel function of Notum 2 in regulating motor axon guidance. Knockdown of Notum 2 resulted in a failure of caudal primary (CaP) axons to migrate beyond the MPs, despite the proper specification of the intermediate target. In contrast, mosaic Notum 2 overexpression induced branching of PMN axons. This effect is specific to Notum 2, as overexpression of Notum 1a does not affect PMN axon trajectory. Ectopic expression of Notum 2 by cells contacting the growing CaP axon induced the highest frequency of branching, suggesting that localized Notum 2 expression affects axon behavior. We propose a model where Notum 2 expression at the MPs provides a cue to release CaP motor axons from their intermediate targets, allowing growth cones to proceed to secondary targets in the ventral muscle. This work demonstrates an unexpected role for a Notum homolog in regulating growth cone migration, separate from the well established functions of other Notum homologs in Wnt signaling.

Zebrafish Mnx proteins specify one motoneuron subtype and suppress acquisition of interneuron characteristics

BACKGROUND

Precise matching between motoneuron subtypes and the muscles they innervate is a prerequisite for normal behavior. Motoneuron subtype identity is specified by the combination of transcription factors expressed by the cell during its differentiation. Here we investigate the roles of Mnx family transcription factors in specifying the subtypes of individually identified zebrafish primary motoneurons.

RESULTS

Zebrafish has three Mnx family members. We show that each of them has a distinct and temporally dynamic expression pattern in each primary motoneuron subtype. We also show that two Mnx family members are expressed in identified VeLD interneurons derived from the same progenitor domain that generates primary motoneurons. Surprisingly, we found that Mnx proteins appear unnecessary for differentiation of VeLD interneurons or the CaP motoneuron subtype. Mnx proteins are, however, required for differentiation of the MiP motoneuron subtype. We previously showed that MiPs require two temporally-distinct phases of Islet1 expression for normal development. Here we show that in the absence of Mnx proteins, the later phase of Islet1 expression is initiated but not sustained, and MiPs become hybrids that co-express morphological and molecular features of motoneurons and V2a interneurons. Unexpectedly, these hybrid MiPs often extend CaP-like axons, and some MiPs appear to be entirely transformed to a CaP morphology.

CONCLUSIONS

Our results suggest that Mnx proteins promote MiP subtype identity by suppressing both interneuron development and CaP axon pathfinding. This is, to our knowledge, the first report of transcription factors that act to distinguish CaP and MiP subtype identities. Our results also suggest that MiP motoneurons are more similar to V2 interneurons than are CaP motoneurons.

Electrical activity as a developmental regulator in the formation of spinal cord circuits

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.