No abstract

During zebrafish development, the left-right (LR) asymmetric signals are first established around the Kupffer vesicle (KV), a ciliated organ generating directional fluid flow. Then, LR asymmetry is conveyed and stabilized in the lateral plate mesoderm. Although numerous molecules and signaling pathways are involved in controlling LR asymmetry, mechanistic difference and concordance between different organs during LR patterning are poorly understood. Here we show that RA signaling regulates laterality decisions at two stages in zebrafish. Before the 2-somite stage (2So), inhibition of RA signaling leads to randomized visceral laterality through bilateral expression of nodal/spaw in the lateral plate mesoderm, which is mediated by increases in cilia length and defective directional fluid flow in KV. Fgf8 is required for the regulation of cilia length by RA signaling. Blockage of RA signaling before 2So also leads to mild defects of heart laterality, which become much more severe through perturbation of cardiac bmp4 asymmetry when RA signaling is blocked after 2So. At this stage, visceral laterality and the left-sided Nodal remain unaffected. These findings suggest that RA signaling controls visceral laterality through the leftsided Nodal signal before 2So, and regulates heart laterality through cardiac bmp4 mainly after 2So, first identifying sequential control and concordance of visceral and heart laterality.

As one of the model organisms of Parkinson’s disease (PD) research, the zebrafish has its advantages, such as the 87% homology with human genome and transparent embryos which make it possible to observe the development of dopaminergic neurons in real time. However, there is no midbrain dopaminergic system in zebrafish when compared with mammals, and the location and projection of the dopaminergic neurons are seldom reported. In this study, Vmat2:GFP transgenic zebrafish was used to observe the development and distribution of dopaminergic neurons in real time. We found that diencephalons (DC) 2 and DC4 neuronal populations were detected at 24 h post fertilization (hpf). All DC neuronal populations as well as those in locus coeruleus (LC), raphe nuclei (Ra) and telencephalon were detected at 48 hpf. Axons were detected at 72 hpf. At 96 hpf, all the neuronal populations were detected. For the first time we reported axons from the posterior tubercle (PT) of ventral DC projected to subpallium in vivo. However, when compared with results from whole mount tyrosine hydroxylase (TH) immunofluorescence staining in wild type (WT) zebrafish, we found that DC2 and DC4 neuronal populations were mainly dopaminergic, while DC1, DC3, DC5 and DC6 might not. Neurons in pretectum (Pr) and telencephalon were mainly dopaminergic, while neurons in LC and Ra might be noradrenergic. Our study makes some corrections and modifications on the development, localization and distribution of zebrafish dopaminergic neurons, and provides some experimental evidences for the construction of the zebrafish PD model.

The cell cycle needs to strictly coordinate with developmental processes to ensure correct generation of the body plan and different tissues. However, the molecular mechanism underlying the coordination remains largely unknown. In this study, we investigate how the cell cycle coordinates gastrulation cell movements in zebrafish. We present a system to modulate the cell cycle in early zebrafish embryos by manipulating the geminin-Cdt1 balance. Alterations of the cell cycle change the apoptotic level during gastrulation, which correlates with the nuclear level of antiapoptotic nuclear factor κB (NF-κB). NF-κB associates with the Snail1a promoter region on the chromatin and directly activates Snail1a, an important factor controlling cell delamination, which is the initial step of mesendodermal cell movements during gastrulation. In effect, the cell cycle coordinates the delamination of mesendodermal cells through the transcription of Snail1a. Our results suggest a molecular mechanism by which NF-κB and Snail1a coordinate the cell cycle through gastrulation.

In many organisms, transcriptional and post-transcriptional regulation of components of pathways or processes has been reported. However, to date, there are few reports of translational co-regulation of multiple components of a developmental signaling pathway. Here, we show that an RNA element which we previously identified as a dorsal localization element (DLE) in the 3′UTR of zebrafish nodal-related1/squint (ndr1/sqt) ligand mRNA, is shared by the related ligand nodal-related2/cyclops (ndr2/cyc) and the nodal inhibitors, lefty1 (lft1) and lefty2 mRNAs. We investigated the activity of the DLEs through functional assays in live zebrafish embryos. The lft1 DLE localizes fluorescently labeled RNA similarly to the ndr1/sqt DLE. Similar to the ndr1/sqt 3′UTR, the lft1 and lft2 3′UTRs are bound by the RNA-binding protein (RBP) and translational repressor, Y-box binding protein 1 (Ybx1), whereas deletions in the DLE abolish binding to Ybx1. Analysis of zebrafish ybx1 mutants shows that Ybx1 represses lefty1 translation in embryos. CRISPR/Cas9-mediated inactivation of human YBX1 also results in human NODAL translational de-repression, suggesting broader conservation of the DLE RNA element/Ybx1 RBP module in regulation of Nodal signaling. Our findings demonstrate translational co-regulation of components of a signaling pathway by an RNA element conserved in both sequence and structure and an RBP, revealing a ‘translational regulon’.