Notch and Fgf signaling during electrosensory versus mechanosensory lateral line organ development in a non-teleost ray-finned fish

Opposing roles for Bmp signalling during the development of electrosensory lateral line organs

The lateral line system enables fishes and aquatic-stage amphibians to detect local water movement via mechanosensory hair cells in neuromasts, and many species to detect weak electric fields via electroreceptors (modified hair cells) in ampullary organs. Both neuromasts and ampullary organs develop from lateral line placodes, but the molecular mechanisms underpinning ampullary organ formation are understudied relative to neuromasts. This is because the ancestral lineages of zebrafish (teleosts) and Xenopus (frogs) independently lost electroreception. We identified Bmp5 as a promising candidate via differential RNA-seq in an electroreceptive ray-finned fish, the Mississippi paddlefish (Polyodon spathula; Modrell et al., 2017, eLife 6: e24197). In an experimentally tractable relative, the sterlet sturgeon (Acipenser ruthenus), we found that Bmp5 and four other Bmp pathway genes are expressed in the developing lateral line, and that Bmp signalling is active. Furthermore, CRISPR/Cas9-mediated mutagenesis targeting Bmp5 in G0-injected sterlet embryos resulted in fewer ampullary organs. Conversely, when Bmp signalling was inhibited by DMH1 treatment shortly before the formation of ampullary organ primordia, supernumerary ampullary organs developed. These data suggest that Bmp5 promotes ampullary organ development, whereas Bmp signalling via another ligand(s) prevents their overproduction. Taken together, this demonstrates opposing roles for Bmp signalling during ampullary organ formation.

Identification of multiple transcription factor genes potentially involved in the development of electrosensory versus mechanosensory lateral line organs

In electroreceptive jawed vertebrates, embryonic lateral line placodes give rise to electrosensory ampullary organs as well as mechanosensory neuromasts. Previous reports of shared gene expression suggest that conserved mechanisms underlie electroreceptor and mechanosensory hair cell development and that electroreceptors evolved as a transcriptionally related "sister cell type" to hair cells. We previously identified only one transcription factor gene, Neurod4, as ampullary organ-restricted in the developing lateral line system of a chondrostean ray-finned fish, the Mississippi paddlefish (Polyodon spathula). The other 16 transcription factor genes we previously validated in paddlefish were expressed in both ampullary organs and neuromasts. Here, we used our published lateral line organ-enriched gene-set (arising from differential bulk RNA-seq in late-larval paddlefish), together with a candidate gene approach, to identify 25 transcription factor genes expressed in the developing lateral line system of a more experimentally tractable chondrostean, the sterlet (Acipenser ruthenus, a small sturgeon), and/or that of paddlefish. Thirteen are expressed in both ampullary organs and neuromasts, consistent with conservation of molecular mechanisms. Seven are electrosensory-restricted on the head (Irx5, Irx3, Insm1, Sp5, Satb2, Mafa and Rorc), and five are the first-reported mechanosensory-restricted transcription factor genes (Foxg1, Sox8, Isl1, Hmx2 and Rorb). However, as previously reported, Sox8 is expressed in ampullary organs as well as neuromasts in a catshark (Scyliorhinus canicula), suggesting the existence of lineage-specific differences between cartilaginous and ray-finned fishes. Overall, our results support the hypothesis that ampullary organs and neuromasts develop via largely conserved transcriptional mechanisms, and identify multiple transcription factors potentially involved in the formation of electrosensory versus mechanosensory lateral line organs.

Identification, Characterization and Functional Analysis of Fibroblast Growth Factors in Black Rockfish (Sebastes schlegelii)

Fibroblast growth factors (FGFs) are short polypeptides that play essential roles in various cellular biological processes, including cell migration, proliferation, and differentiation, as well as tissue regeneration, immune response, and organogenesis. However, studies focusing on the characterization and function of FGF genes in teleost fishes are still limited. In this study, we identified and characterized expression patterns of 24 FGF genes in various tissues of embryonic and adult specimens of the black rockfish (Sebates schlegelii). Nine FGF genes were found to play essential roles in myoblast differentiation, as well as muscle development and recovery in juvelines of S. schlegelii. Moreover, sex-biased expression pattern of multiple FGF genes was recorded in the species' gonads during its development. Among them, expression of the FGF1 gene was recorded in interstitial and sertoli cells of testes, promoting germ-cell proliferation and differentiation. In sum, the obtained results enabled systematic and functional characterization of FGF genes in S. schlegelii, laying a foundation for further studies on FGF genes in other large teleost fishes.

Transcriptome profiles of sturgeon lateral line electroreceptor and mechanoreceptor during regeneration

Background

The electrosensory ampullary organs (AOs) and mechanosensory neuromasts (NMs) found in sturgeon and some other non-neopterygian fish or amphibians are both originated from lateral line placodes. However, these two sensory organs have characteristic morphological and physiological differences. The molecular mechanisms for the specification of AOs and NMs are not clearly understood.

Results

We sequenced the transcriptome for neomycin treated sturgeon AOs and NMs in the early regeneration stages, and de novo assembled a sturgeon transcriptome. By comparing the gene expression differences among untreated AOs, NMs and general epithelia (EPs), we located some specific genes for these two sensory organs. In sturgeon lateral line, the voltage-gated calcium channels and voltage-gated potassium channels were predominant calcium and potassium channel subtypes, respectively. And by correlating gene expression with the regeneration process, we predicated several candidate key transcriptional regulation related genes might be involved in AOs and NMs regeneration.

Conclusions

Genes with specific expression in the two lateral line sensory organs suggests their important roles in mechanoreceptor and electroreceptor formation. The candidate transcriptional regulation related genes may be important for mechano- and electro- receptor specification, in a “dosage-related” manner. These results suggested the molecular basis for specification of these two sensory organs in sturgeon.

Hes5.9 Coordinate FGF and Notch Signaling to Modulate Gastrulation via Regulating Cell Fate Specification and Cell Migration in Xenopus tropicalis

Gastrulation drives the establishment of three germ layers and embryonic axes during frog embryonic development. Mesodermal cell fate specification and morphogenetic movements are vital factors coordinating gastrulation, which are regulated by numerous signaling pathways, such as the Wnt (Wingless/Integrated), Notch, and FGF (Fibroblast growth factor) pathways. However, the coordination of the Notch and FGF signaling pathways during gastrulation remains unclear. We identified a novel helix–loop–helix DNA binding domain gene (Hes5.9), which was regulated by the FGF and Notch signaling pathways during gastrulation. Furthermore, gain- and loss-of-function of Hes5.9 led to defective cell migration and disturbed the expression patterns of mesodermal and endodermal marker genes, thus interfering with gastrulation. Collectively, these results suggest that Hes5.9 plays a crucial role in cell fate decisions and cell migration during gastrulation, which is modulated by the FGF and Notch signaling pathways.

Functional genetic analysis in a jawless vertebrate, the sea lamprey: insights into the developmental evolution of early vertebrates

Lampreys and hagfishes are the only surviving relicts of an ancient but ecologically dominant group of jawless fishes that evolved in the seas of the Cambrian era over half a billion years ago. Because of their phylogenetic position as the sister group to all other vertebrates (jawed vertebrates), comparisons of embryonic development between jawless and jawed vertebrates offers researchers in the field of evolutionary developmental biology the unique opportunity to address fundamental questions related to the nature of our earliest vertebrate ancestors. Here, we describe how genetic analysis of embryogenesis in the sea lamprey (Petromyzon marinus) has provided insight into the origin and evolution of developmental-genetic programs in vertebrates. We focus on recent work involving CRISPR/Cas9-mediated genome editing to study gene regulatory mechanisms involved in the development and evolution of neural crest cells and new cell types in the vertebrate nervous system, and transient transgenic assays that have been instrumental in dissecting the evolution of cis-regulatory control of gene expression in vertebrates. Finally, we discuss the broad potential for these functional genomic tools to address previously unanswerable questions related to the evolution of genomic regulatory mechanisms as well as issues related to invasive sea lamprey population control.