From Pioneer to Repressor: Bimodal foxd3 Activity Dynamically Remodels Neural Crest Regulatory Landscape In Vivo.

The neural crest (NC) is a transient embryonic stem cell-like population characterized by its multipotency and broad developmental potential. Here, we perform NC-specific transcriptional and epigenomic profiling of foxd3-mutant cells in vivo to define the gene regulatory circuits controlling NC specification. Together with global binding analysis obtained by foxd3 biotin-ChIP and single cell profiles of foxd3-expressing premigratory NC, our analysis shows that, during early steps of NC formation, foxd3 acts globally as a pioneer factor to prime the onset of genes regulating NC specification and migration by re-arranging the chromatin landscape, opening cis-regulatory elements and reshuffling nucleosomes. Strikingly, foxd3 then gradually switches from an activator to its well-described role as a transcriptional repressor and potentially uses differential partners for each role. Taken together, these results demonstrate that foxd3 acts bimodally in the neural crest as a switch from "permissive" to "repressive" nucleosome and chromatin organization to maintain multipotency and define cell fates.

Biotagging of Specific Cell Populations in Zebrafish Reveals Gene Regulatory Logic Encoded in the Nuclear Transcriptome.

Interrogation of gene regulatory circuits in complex organisms requires precise tools for the selection of individual cell types and robust methods for biochemical profiling of target proteins. We have developed a versatile, tissue-specific binary in vivo biotinylation system in zebrafish termed biotagging that uses genetically encoded components to biotinylate target proteins, enabling in-depth genome-wide analyses of their molecular interactions. Using tissue-specific drivers and cell-compartment-specific effector lines, we demonstrate the specificity of the biotagging toolkit at the biochemical, cellular, and transcriptional levels. We use biotagging to characterize the in vivo transcriptional landscape of migratory neural crest and myocardial cells in different cellular compartments (ribosomes and nucleus). These analyses reveal a comprehensive network of coding and non-coding RNAs and cis-regulatory modules, demonstrating that tissue-specific identity is embedded in the nuclear transcriptomes. By eliminating background inherent to complex embryonic environments, biotagging allows analyses of molecular interactions at high resolution.

Znf385C mediates a novel p53-dependent transcriptional switch to control timing of facial bone formation.

Jaw formation involves an intricate series of molecular events, whereby a chondrogenic scaffold precedes osteogenesis. The mechanisms coupling timing of cartilage maturation to onset of bone differentiation are poorly understood, particularly for neural crest-derived bones of the head. Here we present a novel zebrafish gene/protein-trap Citrine-fusion line that reveals transient expression of the zinc-finger protein Znf385C in maturing chondrocytes of the jaw. Functional analysis shows that loss of Znf385C disrupts a distinct peak of p21(cip1/waf1) expression in the chondrocytes, as well as causes premature ossification of the zebrafish jaw. We find that Znf385C is expressed as two splice variants which act differentially to activate p21(cip1/waf1) and/or interact with p53 in subcellular compartments. Taken together, the results suggest that Znf385C acts as a developmental switch for p53 function that modulates cell cycle arrest of chondrocytes and regulates timing of jaw cartilage maturation and ossification.

Zebrafish stem/progenitor factor msi2b exhibits two phases of activity mediated by different splice variants.

The Musashi (Msi) family of RNA-binding proteins is important in stem and differentiating cells in many species. Here, we present a zebrafish gene/protein trap line gt(msi2b-citrine)(ct) (57) (a) that expresses a Citrine fusion protein with endogenous Msi2b. Our results reveal two phases of Msi2b expression: ubiquitous expression in progenitor cells in the early embryo and later, tissue-specific expression in differentiating cells in the olfactory organ, pineal gland, and subpopulations of neurons in the central nervous system (CNS). Interestingly, this division between early and late phases is paralleled by differential expression of msi2b alternative splicing products. Whereas the full-length and long variant v3 Msi2b predominate at early stages, the later expression of variants in differentiating tissues appears to be tissue specific. Using the gt(msi2b-citrine)(ct) (57) (a), we characterized tissue-specific expression of Msi2b with cellular resolution in subsets of differentiating cells in the olfactory organ, pineal gland, CNS, and ventral neural tube. By performing transcription activator-like effectors nuclease-mediated biallelic genome editing or morpholino knockdown of Msi2b in zebrafish, our results show that early inactivation of Msi2b results in severe embryonic defects including hypertrophy of the ventricles and shortening of the body, consistent with an important role in cell proliferation and survival. Moreover, specific inactivation of Msi2b full-length indicates that this species is essential for the early role of Msi2b. This line provides a valuable tool both for live imaging of the endogenous Msi2b at subcellular resolution and manipulation of Msi2b-expressing cells.

Laminin ß1a controls distinct steps during the establishment of digestive organ laterality.

Visceral organs, including the liver and pancreas, adopt asymmetric positions to ensure proper function. Yet the molecular and cellular mechanisms controlling organ laterality are not well understood. We identified a mutation affecting zebrafish laminin ß1a (lamb1a) that disrupts left-right asymmetry of the liver and pancreas. In these mutants, the liver spans the midline and the ventral pancreatic bud remains split into bilateral structures. We show that lamb1a regulates asymmetric left-right gene expression in the lateral plate mesoderm (LPM). In particular, lamb1a functions in Kupffer's vesicle (KV), a ciliated organ analogous to the mouse node, to control the length and function of the KV cilia. Later during gut-looping stages, dynamic expression of Lamb1a is required for the bilayered organization and asymmetric migration of the LPM. Loss of Lamb1a function also results in aberrant protrusion of LPM cells into the gut. Collectively, our results provide cellular and molecular mechanisms by which extracellular matrix proteins regulate left-right organ morphogenesis.

A novel FoxD3 gene trap line reveals neural crest precursor movement and a role for FoxD3 in their specification.

Neural crest cells migrate extensively and contribute to diverse derivatives, including the craniofacial skeleton, peripheral neurons and glia, and pigment cells. Although several transgenic lines label neural crest subpopulations, few are suited for studying early events in neural crest development. Here, we present a zebrafish gene/protein trap line gt(foxd3-citrine)(ct110a) that expresses a Citrine fusion protein with FoxD3, a transcription factor expressed in premigratory and migrating neural crest cells. In this novel line, citrine expression exactly parallels endogenous foxd3 expression. High-resolution time-lapse imaging reveals the dynamic phases of precursor and migratory neural crest cell movements from the neural keel stage to times of active cell migration. In addition, Cre-recombination produces a variant line FoxD3-mCherry-pA whose homozygosis generates a FoxD3 mutant. Taking advantage of the endogenously regulated expression of FoxD3-mCherry fusion protein, we directly assess early effects of FoxD3 loss-of-function on specification and morphogenesis of dorsal root ganglia, craniofacial skeleton and melanophores. These novel lines provide new insights and useful new tools for studying specification, migration and differentiation of neural crest cells.