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

Knockout of mafba Causes Inner-Ear Developmental Defects in Zebrafish via the Impairment of Proliferation and Differentiation of Ionocyte Progenitor Cells

Zebrafish is an excellent model for exploring the development of the inner ear. Its inner ear has similar functions to that of humans, specifically in the maintenance of hearing and balance. Mafba is a component of the Maf transcription factor family. It participates in multiple biological processes, but its role in inner-ear development remains poorly understood. In this study, we constructed a mafba knockout (mafba^−/−) zebrafish model using CRISPR/Cas9 technology. The mafba^−/− mutant inner ear displayed severe impairments, such as enlarged otocysts, smaller or absent otoliths, and insensitivity to sound stimulation. The proliferation of p63⁺ epidermal stem cells and dlc⁺ ionocyte progenitors was inhibited in mafba^−/− mutants. Moreover, the results showed that mafba deletion induces the apoptosis of differentiated K⁺-ATPase-rich (NR) cells and H⁺-ATPase-rich (HR) cells. The activation of p53 apoptosis and G0/G1 cell cycle arrest resulted from DNA damage in the inner-ear region, providing a mechanism to account for the inner ear deficiencies. The loss of homeostasis resulting from disorders of ionocyte progenitors resulted in structural defects in the inner ear and, consequently, loss of hearing. In conclusion, the present study elucidated the function of ionic channel homeostasis and inner-ear development using a zebrafish Mafba model and clarified the possible physiological roles.

Zebrafish Minichromosome Maintenance Protein 5 Gene Regulates the Development and Migration of Facial Motor Neurons via Fibroblast Growth Factor Signaling

Minichromosome maintenance protein 5 (MCM5), a member of the microchromosomal maintenance protein family, plays an important role in the initiation and extension of DNA replication. However, its role in neural development in zebrafish remains unclear. Here, we used morpholino (MO) and CRISPR/Cas9 to knock down mcm5 and investigated the developmental features of facial motor neurons (FMNs) in the hindbrain of zebrafish. We found that knockdown of mcm5 using mcm5 MO resulted in a small head, small eyes, and a blurred midbrain-hindbrain boundary, while MO injection of mcm5 led to decrease in FMNs and their migration disorder. However, the mutant of mcm5 only resulted in the migration defect of FMNs rather than quantity change. We further investigated the underlying mechanism of mcm5 in the development of hindbrain using in situ hybridization (ISH) and fgfr1a mRNA co-injected with mcm5 MO. Results from ISH showed that the fibroblast growth factor (FGF) signaling pathway was changed when the MCM5 function was lost, with the decrease in fgfr1a and the increase in fgf8, while that of pea3 had opposite trend. FMN development defects were rescued by fgfr1a mRNA co-injected with mcm5 MO. Our results demonstrated that FGF signaling pathway is required for FMN development in zebrafish. Specifically, mcm5 regulates FMN development during zebrafish growing.

Impact of the Olig Family on Neurodevelopmental Disorders

The Olig genes encode members of the basic helix-loop-helix (bHLH) family of transcription factors. Olig1, Olig2, and Olig3 are expressed in both the developing and mature central nervous system (CNS) and strictly regulate cellular specification and differentiation. Extensive studies have established functional roles of Olig1 and Olig2 in directing neuronal and glial formation during different stages in development. Recently, Olig2 overexpression was implicated in neurodevelopmental disorders down syndrome (DS) and autism spectrum disorder (ASD) but its influence on cognitive and intellectual defects remains unknown. In this review, we summarize the biological functions of the Olig family and how it uniquely promotes cellular diversity in the CNS. This is followed up with a discussion on how abnormal Olig2 expression impacts brain development and function in DS and ASD. Collectively, the studies described here emphasize vital features of the Olig members and their distinctive potential roles in neurodevelopmental disease states.

Zebrafish prdm12b acts independently of nkx6.1 repression to promote eng1b expression in the neural tube p1 domain

Background

Functioning of the adult nervous system depends on the establishment of neural circuits during embryogenesis. In vertebrates, neurons that make up motor circuits form in distinct domains along the dorsoventral axis of the neural tube. Each domain is characterized by a unique combination of transcription factors (TFs) that promote a specific fate, while repressing fates of adjacent domains. The prdm12 TF is required for the expression of eng1b and the generation of V1 interneurons in the p1 domain, but the details of its function remain unclear.

Methods

We used CRISPR/Cas9 to generate the first germline mutants for prdm12 and employed this resource, together with classical luciferase reporter assays and co-immunoprecipitation experiments, to study prdm12b function in zebrafish. We also generated germline mutants for bhlhe22 and nkx6.1 to examine how these TFs act with prdm12b to control p1 formation.

Results

We find that prdm12b mutants lack eng1b expression in the p1 domain and also possess an abnormal touch-evoked escape response. Using luciferase reporter assays, we demonstrate that Prdm12b acts as a transcriptional repressor. We also show that the Bhlhe22 TF binds via the Prdm12b zinc finger domain to form a complex. However, bhlhe22 mutants display normal eng1b expression in the p1 domain. While prdm12 has been proposed to promote p1 fates by repressing expression of the nkx6.1 TF, we do not observe an expansion of the nkx6.1 domain upon loss of prdm12b function, nor is eng1b expression restored upon simultaneous loss of prdm12b and nkx6.1.

Conclusions

We conclude that prdm12b germline mutations produce a phenotype that is indistinguishable from that of morpholino-mediated loss of prdm12 function. In terms of prdm12b function, our results indicate that Prdm12b acts as transcriptional repressor and interacts with both EHMT2/G9a and Bhlhe22. However, bhlhe22 function is not required for eng1b expression in vivo, perhaps indicating that other bhlh genes can compensate during embryogenesis. Lastly, we do not find evidence for nkx6.1 and prdm12b acting as a repressive pair in formation of the p1 domain – suggesting that prdm12b is not solely required to repress non-p1 fates, but is specifically needed to promote p1 fates.

Electronic supplementary material

The online version of this article (10.1186/s13064-019-0129-x) contains supplementary material, which is available to authorized users.

Analysis of novel caudal hindbrain genes reveals different regulatory logic for gene expression in rhombomere 4 versus 5/6 in embryonic zebrafish

BACKGROUND

Previous work aimed at understanding the gene regulatory networks (GRNs) governing caudal hindbrain formation identified morphogens such as Retinoic Acid (RA) and Fibroblast growth factors (FGFs), as well as transcription factors like hoxb1b, hoxb1a, hnf1ba, and valentino as being required for rhombomere (r) r4-r6 formation in zebrafish. Considering that the caudal hindbrain is relatively complex - for instance, unique sets of neurons are formed in each rhombomere segment - it is likely that additional essential genes remain to be identified and integrated into the caudal hindbrain GRN.

METHODS

By taking advantage of gene expression data available in the Zebrafish Information Network (ZFIN), we identified 84 uncharacterized genes that are expressed in r4-r6. We selected a representative set of 22 genes and assayed their expression patterns in hoxb1b, hoxb1a, hnf1b, and valentino mutants with the goal of positioning them in the caudal hindbrain GRN. We also investigated the effects of RA and FGF on the expression of this gene set. To examine whether these genes are necessary for r4-r6 development, we analyzed germline mutants for six of the genes (gas6, gbx1, sall4, eglf6, celf2, and greb1l) for defects in hindbrain development.

RESULTS

Our results reveal that r4 gene expression is unaffected by the individual loss of hoxb1b, hoxb1a or RA, but is under the combinatorial regulation of RA together with hoxb1b. In contrast, r5/r6 gene expression is dependent on RA, FGF, hnf1ba and valentino - as individual loss of these factors abolishes r5/r6 gene expression. Our analysis of six mutant lines did not reveal rhombomere or neuronal defects, but transcriptome analysis of one line (gas6 mutant) identified expression changes for genes involved in several developmental processes - suggesting that these genes may have subtle roles in hindbrain development.

CONCLUSION

We conclude that r4-r6 formation is relatively robust, such that very few genes are absolutely required for this process. However, there are mechanistic differences in r4 versus r5/r6, such that no single factor is required for r4 development while several genes are individually required for r5/r6 formation.

A parental requirement for dual-specificity phosphatase 6 in zebrafish

Background

Signaling cascades, such as the extracellular signal-regulated kinase (ERK) pathway, play vital roles in early vertebrate development. Signals through these pathways are initiated by a growth factor or hormone, are transduced through a kinase cascade, and result in the expression of specific downstream genes that promote cellular proliferation, growth, or differentiation. Tight regulation of these signals is provided by positive or negative modulators at varying levels in the pathway, and is required for proper development and function. Two members of the dual-specificity phosphatase (Dusp) family, dusp6 and dusp2, are believed to be negative regulators of the ERK pathway and are expressed in both embryonic and adult zebrafish, but their specific roles in embryogenesis remain to be fully understood.

Results

Using CRISPR/Cas9 genome editing technology, we generated zebrafish lines harboring germ line deletions in dusp6 and dusp2. We do not detect any overt defects in dusp2 mutants, but we find that approximately 50% of offspring from homozygous dusp6 mutants do not proceed through embryonic development. These embryos are fertilized, but are unable to proceed past the first zygotic mitosis and stall at the 1-cell stage for several hours before dying by 10 h post fertilization. We demonstrate that dusp6 is expressed in gonads of both male and female zebrafish, suggesting that loss of dusp6 causes defects in germ cell production. Notably, the 50% of homozygous dusp6 mutants that complete the first cell division appear to progress through embryogenesis normally and give rise to fertile adults.

Conclusions

The fact that offspring of homozygous dusp6 mutants stall prior to activation of the zygotic genome, suggests that loss of dusp6 affects gametogenesis and/or parentally-directed early development. Further, since only approximately 50% of homozygous dusp6 mutants are affected, we postulate that ERK signaling is tightly regulated and that dusp6 is required to keep ERK signaling within a range that is permissive for proper embryogenesis. Lastly, since dusp6 is expressed throughout zebrafish embryogenesis, but dusp6 mutants do not exhibit defects after the first cell division, it is possible that other regulators of the ERK pathway compensate for loss of dusp6 at later stages.

Electronic supplementary material

The online version of this article (10.1186/s12861-018-0164-6) contains supplementary material, which is available to authorized users.

prdm12b specifies the p1 progenitor domain and reveals a role for V1 interneurons in swim movements

Proper functioning of the vertebrate central nervous system requires the precise positioning of many neuronal cell types. This positioning is established during early embryogenesis when gene regulatory networks pattern the neural tube along its anteroposterior and dorsoventral axes. Dorsoventral patterning of the embryonic neural tube gives rise to multiple progenitor cell domains that go on to differentiate unique classes of neurons and glia. While the genetic program is reasonably well understood for some lineages, such as ventrally derived motor neurons and glia, other lineages are much less characterized. Here we show that prdm12b, a member of the PR domain containing-family of transcriptional regulators, is expressed in the p1 progenitor domain of the zebrafish neural tube in response to Sonic Hedgehog signaling. We find that disruption of prdm12b function leads to dorsal expansion of nkx6.1 expression and loss of p1-derived eng1b-expressing V1 interneurons, while the adjacent p0 and p2 domains are unaffected. We also demonstrate that prdm12b-deficient fish exhibit an abnormal touch-evoked escape response with excessive body contractions and a prolonged response time, as well as an inability to coordinate swimming movements, thereby revealing a functional role for V1 interneurons in locomotor circuits. We conclude that prdm12b is required for V1 interneuron specification and that these neurons control swimming movements in zebrafish.

Targeted germ line disruptions reveal general and species-specific roles for paralog group 1 hox genes in zebrafish

Background

The developing vertebrate hindbrain is transiently segmented into rhombomeres by a process requiring Hox activity. Hox genes control specification of rhombomere fates, as well as the stereotypic differentiation of rhombomere-specific neuronal populations. Accordingly, germ line disruption of the paralog group 1 (PG1) Hox genes Hoxa1 and Hoxb1 causes defects in hindbrain segmentation and neuron formation in mice. However, antisense-mediated interference with zebrafish hoxb1a and hoxb1b (analogous to murine Hoxb1 and Hoxa1, respectively) produces phenotypes that are qualitatively and quantitatively distinct from those observed in the mouse. This suggests that PG1 Hox genes may have species-specific functions, or that anti-sense mediated interference may not completely inactivate Hox function in zebrafish.

Results

Using zinc finger and TALEN technologies, we disrupted hoxb1a and hoxb1b in the zebrafish germ line to establish mutant lines for each gene. We find that zebrafish hoxb1a germ line mutants have a more severe phenotype than reported for Hoxb1a antisense treatment. This phenotype is similar to that observed in Hoxb1 knock out mice, suggesting that Hoxb1/hoxb1a have the same function in both species. Zebrafish hoxb1b germ line mutants also have a more severe phenotype than reported for hoxb1b antisense treatment (e.g. in the effect on Mauthner neuron differentiation), but this phenotype differs from that observed in Hoxa1 knock out mice (e.g. in the specification of rhombomere 5 (r5) and r6), suggesting that Hoxa1/hoxb1b have species-specific activities. We also demonstrate that Hoxb1b regulates nucleosome organization at the hoxb1a promoter and that retinoic acid acts independently of hoxb1b to activate hoxb1a expression.

Conclusions

We generated several novel germ line mutants for zebrafish hoxb1a and hoxb1b. Our analyses indicate that Hoxb1 and hoxb1a have comparable functions in zebrafish and mouse, suggesting a conserved function for these genes. In contrast, while Hoxa1 and hoxb1b share functions in the formation of r3 and r4, they differ with regards to r5 and r6, where Hoxa1 appears to control formation of r5, but not r6, in the mouse, whereas hoxb1b regulates formation of r6, but not r5, in zebrafish. Lastly, our data reveal independent regulation of hoxb1a expression by retinoic acid and Hoxb1b in zebrafish.

prdm12b specifies the p1 progenitor domain and reveals a role for V1 interneurons in swim movements

Proper functioning of the vertebrate central nervous system requires the precise positioning of many neuronal cell types. This positioning is established during early embryogenesis when gene regulatory networks pattern the neural tube along its anteroposterior and dorsoventral axes. Dorsoventral patterning of the embryonic neural tube gives rise to multiple progenitor cell domains that go on to differentiate unique classes of neurons and glia. While the genetic program is reasonably well understood for some lineages, such as ventrally derived motor neurons and glia, other lineages are much less characterized. Here we show that prdm12b, a member of the PR domain containing-family of transcriptional regulators, is expressed in the p1 progenitor domain of the zebrafish neural tube in response to Sonic Hedgehog signaling. We find that disruption of prdm12b function leads to dorsal expansion of nkx6.1 expression and loss of p1-derived eng1b-expressing V1 interneurons, while the adjacent p0 and p2 domains are unaffected. We also demonstrate that prdm12b-deficient fish exhibit an abnormal touch-evoked escape response with excessive body contractions and a prolonged response time, as well as an inability to coordinate swimming movements, thereby revealing a functional role for V1 interneurons in locomotor circuits. We conclude that prdm12b is required for V1 interneuron specification and that these neurons control swimming movements in zebrafish.