The germ cell nuclear factor is required for retinoic acid signaling during Xenopus development

Single-cell Transcriptional Landscape of Temporal Neutrophil Response to Burn Wound in Larval Zebrafish

Neutrophils accumulate early in tissue injury. However, the cellular and functional heterogeneity of neutrophils during homeostasis and in response to tissue damage remains unclear. In this study, we use larval zebrafish to understand neutrophil responses to thermal injury. Single-cell transcriptional mapping of myeloid cells during a 3-d time course in burn and control larvae revealed distinct neutrophil subsets and their cell-cell interactions with macrophages across time and conditions. The trajectory formed by three zebrafish neutrophil subsets resembles human neutrophil maturation, with varying transition patterns between conditions. Through ligand-receptor cell-cell interaction analysis, we found that neutrophils communicate more in burns in a pathway and temporal manner. Finally, we identified the correlation between zebrafish myeloid signatures and human burn severity, establishing GPR84+ neutrophils as a potential marker of early innate immune response in burns. This work builds a comparative single-cell transcriptomic framework to identify neutrophil markers of tissue damage using model organisms.

Single-cell transcriptional landscape of temporal neutrophil response to burn wound in larval zebrafish

Neutrophils accumulate early in tissue injury. However, the cellular and functional heterogeneity of neutrophils during homeostasis and in response to tissue damage remains unclear. Here, we use larval zebrafish to understand neutrophil responses to thermal injury. Single-cell transcriptional mapping of myeloid cells during a 3-day time course in burn and control larvae revealed distinct neutrophil subsets and their cell-cell interactions with macrophages across time and conditions. The trajectory formed by three zebrafish neutrophil subsets resembles human neutrophil maturation, with varying transition patterns between conditions. Through ligand-receptor cell-cell interaction analysis, we found neutrophils communicate more in burns in a pathway and temporal manner. Finally, we identified the correlation between zebrafish myeloid signatures and human burn severity, establishing GPR84+ neutrophils as a potential marker of early innate immune response in burns. This work builds the molecular foundation and a comparative single-cell genomic framework to identify neutrophil markers of tissue damage using model organisms.

The expanding roles of Nr6a1 in development and evolution

The Nuclear Receptor (NR) family of transcriptional regulators possess the ability to sense signalling molecules and directly couple that to a transcriptional response. While this large class of proteins are united by sequence and structural homology, individual NR functional output varies greatly depending on their expression, ligand selectivity and DNA binding sequence specificity. Many NRs have remained somewhat enigmatic, with the absence of a defined ligand categorising them as orphan nuclear receptors. One example is Nuclear Receptor subfamily 6 group A member 1 (Nr6a1), an orphan nuclear receptor that has no close evolutionary homologs and thus is alone in subfamily 6. Nonetheless, Nr6a1 has emerged as an important player in the regulation of key pluripotency and developmental genes, as functionally critical for mid-gestational developmental progression and as a possible molecular target for driving evolutionary change in animal body plan. Here, we review the current knowledge on this enigmatic nuclear receptor and how it impacts development and evolution.

Nr6a1 controls Hox expression dynamics and is a master regulator of vertebrate trunk development

The vertebrate main-body axis is laid down during embryonic stages in an anterior-to-posterior (head-to-tail) direction, driven and supplied by posteriorly located progenitors. Whilst posterior expansion and segmentation appears broadly uniform along the axis, there is developmental and evolutionary support for at least two discrete modules controlling processes within different axial regions: a trunk and a tail module. Here, we identify Nuclear receptor subfamily 6 group A member 1 (Nr6a1) as a master regulator of trunk development in the mouse. Specifically, Nr6a1 was found to control vertebral number and segmentation of the trunk region, autonomously from other axial regions. Moreover, Nr6a1 was essential for the timely progression of Hox signatures, and neural versus mesodermal cell fate choice, within axial progenitors. Collectively, Nr6a1 has an axially-restricted role in all major cellular and tissue-level events required for vertebral column formation, supporting the view that changes in Nr6a1 levels may underlie evolutionary changes in axial formulae.

Identification of novel retinoic acid target genes

Retinoic acid is required for diverse ontogenic processes and as such identification of the genes and pathways affected by retinoic acid is critical to understanding these pleiotropic effects. The presomitic mesoderm of the E8.5 mouse embryo is composed of undifferentiated cells that are depleted of retinoic acid, yet are competent to respond to the retinoid signal. We have exploited these properties to use this tissue to identify novel retinoic acid-responsive genes, including candidate target genes, by treating E8.5 embryos with retinoic acid and assessing changes in gene expression in the presomitic mesoderm by microarray analysis. This exercise yielded a cohort of genes that were differentially expressed in response to exogenous retinoic acid exposure. Among these were a number of previously characterized retinoic acid targets, validating this approach. In addition, we recovered a number of novel candidate target genes which were confirmed as retinoic acid-responsive by independent analysis. Chromatin immunoprecipitation assays revealed retinoic acid receptor occupancy of the promoters of certain of these genes. We further confirmed direct retinoic acid regulation of the F11r gene, a new RA target, using tissue culture models. Our results reveal a significant number of potential RA targets implicated in embryonic development and offer a novel in vivo system for better understanding of retinoid-dependent transcription.

Germ cell nuclear factor regulates gametogenesis in developing gonads

Expression of germ cell nuclear factor (GCNF; Nr6a1), an orphan member of the nuclear receptor gene family of transcription factors, during gastrulation and neurulation is critical for normal embryogenesis in mice. Gcnf represses the expression of the POU-domain transcription factor Oct4 (Pou5f1) during mouse post-implantation development. Although Gcnf expression is not critical for the embryonic segregation of the germ cell lineage, we found that sexually dimorphic expression of Gcnf in germ cells correlates with the expression of pluripotency-associated genes, such as Oct4, Sox2, and Nanog, as well as the early meiotic marker gene Stra8. To elucidate the role of Gcnf during mouse germ cell differentiation, we generated an ex vivo Gcnf-knockdown model in combination with a regulated CreLox mutation of Gcnf. Lack of Gcnf impairs normal spermatogenesis and oogenesis in vivo, as well as the derivation of germ cells from embryonic stem cells (ESCs) in vitro. Inactivation of the Gcnf gene in vivo leads to loss of repression of Oct4 expression in both male and female gonads.

Revisiting the role of GCNF in embryonic development

GCNF (NR6A1) is essential for embryonic development. GCNF belongs to the nuclear receptor (NR) gene family, it is distantly related to other NRs and is the only member of subfamily 6. As the ligand for GCNF has not been identified, GCNF is designated an orphan nuclear receptor. GCNF has been found to be a transcriptional repressor, through specific binding to DR0 response elements, which is found in the Oct4 proximal promoter for example. GCNF is expressed widely in early mouse embryos, and later in the developing nervous system. GCNF knockout mouse embryos die around E10.5. GCNF is required for the restriction of Oct4 expression to primordial germ cells after gastrulation. GCNF is expressed in ES/EC cells and during their differentiation, and has been reported to be required for pluripotency gene repression during retinoic acid (RA)-induced mES cell differentiation. GCNF can interact with DNA methylation proteins, and is suggested to recruit DNA methylation complexes to repress and silence Oct4 expression. Nuclear receptor regulation in embryonic development is a complex process, as different nuclear receptors have overlapping and distinct functions. In-depth exploration of GCNF function and mechanism of action will help to comprehensively understand the nuclear receptor regulation in embryonic development.

Gonadotropins regulate ovarian germ cell mitosis/meiosis decision in the embryonic chicken

Gonadotropins are required for gametogenesis but in embryonic gonads this mechanism is not well understood. Here we use chicken embryos to investigate the mechanism that gonadotropins regulate the ovarian germ cell mitosis/meiosis decision. Treatment with follicle-stimulating hormone (FSH) delayed germ cell meiosis entry and promoted their proliferation. This action was blocked by an aromatase inhibitor. Treatment with luteinizing hormone (LH) accelerated germ cell meiosis entry and promoted transcription of 3βHSDII to increase progesterone (P4) production. In the cultured ovaries, P4 triggered meiotic initiation in germ cells. MiR181a, which acts to downregulate the NR6A1 transcript to inhibit the meiotic initiation, was upregulated by FSH and downregulated by LH. Collectively, gonadotropins regulate germ cells mitosis and meiotic initiation through steroid hormones and a miR181a-mediated pathway. In particularly, FSH delays germ cell meiosis entry and promotes cell proliferation via estrogen while LH accelerates the meiotic initiation via elevated P4 production.

MicroRNA-mediated posttranscriptional regulation is required for maintaining undifferentiated properties of blastoderm and primordial germ cells in chickens

MicroRNAs (miRNAs) play a critical role in determining the differentiation fate of pluripotent stem cells and germ cells in mammals. However, the mechanism(s) of miRNA-mediated posttranscriptional regulation with regard to lineage specification and differentiation in chick development require further investigation. Therefore, we conducted miRNA expression profiling to explore specific miRNA signatures in undifferentiated blastoderm and primordial germ cells (PGCs). We identified seven miRNAs that are highly expressed in blastoderm and 10 that are highly expressed in PGCs. In this study, miR-302a and miR-456 for blastoderm and miR-181a* for PGCs were analyzed further for their target transcripts and regulatory pathways. Both miR-302a and miR-456 bound directly to the sex-determining region Y box 11 transcript and could act as posttranscriptional coregulators to maintain the undifferentiated state of the chicken blastoderm through the suppression of somatic gene expression and differentiation. Moreover, miR-181a* showed a bifunctional role in PGCs by binding to two different transcripts. miR-181a* inhibited the somatic differentiation of PGCs by silencing homeobox A1 expression. Additionally, miR-181a* prevented PGCs from entering meiosis through the repression of the nuclear receptor subfamily 6, group A, member 1 transcript. Collectively, our data demonstrate that in chickens miRNAs intrinsically regulate the differentiation fate of blastoderms and PGCs and that the specific timing of germ cell meiosis is controlled through miRNA expression.

Disruption of the somitic molecular clock causes abnormal vertebral segmentation

Somites are the precursors of the vertebral column. They segment from the presomitic mesoderm (PSM) that is caudally located and newly generated from the tailbud. Somites form in synchrony on either side of the embryonic midline in a reiterative manner. A molecular clock that operates in the PSM drives this reiterative process. Genetic manipulation in mouse, chick and zebrafish has revealed that the molecular clock controls the activity of the Notch and WNT signaling pathways in the PSM. Disruption of the molecular clock impacts on somite formation causing abnormal vertebral segmentation (AVS). A number of dysmorphic syndromes manifest AVS defects. Interaction between developmental biologists and clinicians has lead to groundbreaking research in this area with the identification that spondylocostal dysostosis (SCD) is caused by mutation in Delta-like 3 (DLL3), Mesoderm posterior 2 (MESP2), and Lunatic fringe (LFNG); three genes that are components of the Notch signaling pathway. This review describes our current understanding of the somitic molecular clock and highlights how key findings in developmental biology can impact on clinical practice.

The novel gene asb11: a regulator of the size of the neural progenitor compartment

From a differential display designed to isolate genes that are down-regulated upon differentiation of the central nervous system in Danio rerio embryos, we isolated d-asb11 (ankyrin repeat and suppressor of cytokine signaling box–containing protein 11). Knockdown of the d-Asb11 protein altered the expression of neural precursor genes sox2 and sox3 and resulted in an initial relative increase in proneural cell numbers. This was reflected by neurogenin1 expansion followed by premature neuronal differentiation, as demonstrated by HuC labeling and resulting in reduced size of the definitive neuronal compartment. Forced misexpression of d-asb11 was capable of ectopically inducing sox2 while it diminished or entirely abolished neurogenesis. Overexpression of d-Asb11 in both a pluripotent and a neural-committed progenitor cell line resulted in the stimulus-induced inhibition of terminal neuronal differentiation and enhanced proliferation. We conclude that d-Asb11 is a novel regulator of the neuronal progenitor compartment size by maintaining the neural precursors in the proliferating undifferentiated state possibly through the control of SoxB1 transcription factors.

Loss of orphan nuclear receptor GCNF function disrupts forebrain development and the establishment of the isthmic organizer

The isthmic organizer, which is located at the midbrain-hindbrain boundary, is important for midbrain development. The mechanism by which the development of the organizer is initiated and maintained is not well understood. Inactivation of the gene encoding the orphan nuclear receptor, GCNF, diminishes the expression of secreted signaling molecules, Fgf8 and Wnt1, the paired box genes Pax2/5, En1/2, and homeodomain transcription factor Gbx2; all of which are essential for isthmic organizer function. In addition, full neuronal differentiation is not observed in the midbrain region of GCNF-/- embryos. Increased cell death may contribute to the loss of midbrain structure in GCNF-/- embryos. These results indicate that GCNF is required for establishment of the isthmic organizer, thereby regulating the midbrain development.

Global analysis of RAR-responsive genes in the Xenopus neurula using cDNA microarrays

Retinoid signaling is important for patterning the vertebrate hindbrain and midaxial regions. We recently showed that signaling through retinoic acid receptors (RARs) is essential for anteroposterior patterning along the entire body axis. To further investigate the mechanisms through which RARs act, we used microarray analysis to investigate the effects of modulating RAR activity on target gene expression. We identified 334 up-regulated genes (92% of which were validated), including known RA-responsive genes, known genes that have never been proposed as RA targets and many hypothetical and unidentified genes (n = 166). Sixty-seven validated down-regulated genes were identified, including known RA-responsive genes and anterior marker genes. The expression patterns of selected up-regulated genes (n = 45) were examined at neurula stages using whole-mount in situ hybridization. We found that most of these genes were expressed in the neural tube and many were expressed in anterior tissues such as neural crest, brain, eye anlagen, and cement gland. Some were expressed in tissues such as notochord, somites, pronephros, and blood islands, where retinoic acid (RA) plays established roles in organogenesis. Members of this set of newly identified RAR target genes are likely to play important roles in neural patterning and organogenesis under the control of RAR signaling pathways, and their further characterization will expand our understanding of RA signaling during development.

The expression level of the orphan nuclear receptor GCNF (germ cell nuclear factor) is critical for neuronal differentiation

The germ cell nuclear factor (GCNF) is essential for normal embryonic development and gametogenesis. To test the prediction that GCNF is additionally required for neuronal differentiation, we used the mouse embryonal carcinoma cell line PCC7-Mz1, which represents an advantageous model to study neuronal cells from the stage of fate choice until the acquirement of functional competence. We generated stable transfectants that express gcnf sense or antisense RNA under the control of a tetracycline-regulated promoter. After retinoic acid-induced withdrawal from the cell cycle, sense clones developed a neuron network with changed properties, and the time course of neuron maturation was shortened. Consistent with these data, differentiation of neuronal precursor cells was impaired in antisense cultures. This involved a delay in 1) the down-regulation of nestin, a marker for undifferentiated neuroepithelial cells and stem cells of the central nervous system, and 2) up-regulation of the somatodendritic protein microtubule-associated protein 2 and the synaptic vesicle protein synaptophysin. Neuronal cells in the antisense cultures acquired functional competence, although with a significant delay. Our data propose that the level of GCNF is critical for differentiation and maturation of neuronal precursor cells.

The varied roles of nuclear receptors during vertebrate embryonic development

Nuclear receptors comprise a superfamily of sequence-specific transcription factors whose members have diverse roles during development. This review will summarize the developmental roles of selected members of the nuclear receptor superfamily.

The function of Xenopus germ cell nuclear factor (xGCNF) in morphogenetic movements during neurulation

The germ cell nuclear factor (GCNF, NR6A1) is a nuclear orphan receptor first described in the mouse testis and subsequently identified as an essential transcription factor in vertebrate embryogenesis. Here, we analyze the phenotype of Xenopus embryos after depletion of embryonic GCNF (xEmGCNF) protein using a specific morpholino antisense oligonucleotide. Morphological defects after xEmGCNF knockdown became obvious from neurulation onward. Among the abnormalities observed, defective formation of the neural tube and a short and curved main body axis were the most remarkable traits. Histological analysis, lineage tracing of injected blastomeres, and Keller sandwich explants revealed that xEmGCNF function is required for different patterns of cell intercalation during neurulation and consequently for the sequence of morphogenetic movements leading to formation of the neural tube. Further characterization of the phenotype at the molecular level showed an abnormal distribution of the extracellular matrix protein fibronectin and a reduction in the expression level of the integrin subunits alpha5 and alpha6, the limiting components of the laminin and fibronectin receptors, respectively. We propose integrin-mediated cell-matrix interaction as a process that requires xEmGCNF function and provides, in concert with cadherins-mediated cell-cell interactions, a molecular basis for morphogenetic cell movements during neurulation.