Imaging and Quantification of P-Smad1/5 in Zebrafish Blastula and Gastrula Embryos

Quantitative Staging of Mid-blastula Zebrafish Embryos by Nuclei Counting

Precise developmental staging of embryos is critical for experimental methods that compare different groups of embryos in developmental biology. During development, gene and protein expression levels change dynamically as cells acquire sequentially more restricted cell fates, from pluri- or multipotent cells to germ layers and to differentiated cells. In particular, zebrafish embryos experience rapid transcriptional changes at the mid-blastula transition, when zygotic transcription is activated in specific domains along the dorsoventral and anteroposterior axes and maternal transcripts are degraded. During these rapid and widespread transcriptional changes, precise staging is needed to accurately compare one group of experimental embryos to another that differ in some experimental variable, such as mutant versus wild-type embryos, or embryos receiving two different experimental treatments. Precise staging ensures that an observed change in gene or protein expression between the experimental groups is due to the experimental variable and not to a difference in developmental stage caused by asynchrony in early development. However, zebrafish embryos exhibit rapid cell divisions and subtle morphological changes during the mid-blastula period, making it difficult to accurately stage embryos using morphology or timing alone. Here, we present a method to quantitatively stage zebrafish embryos based on cell count during the mid-blastula period. Our method can be applied to any zebrafish experiment performed during the mid-blastula period, by counting nuclei in sibling embryos of the experimental ones. After staining with a nuclear marker, nuclei are imaged with a confocal microscope and then quickly identified and counted with standard imaging software. Overall, this quantitative staging method can increase experimental rigor and decrease variability in experiments during the mid-blastula period.

EpicTope: narrating protein sequence features to identify non-disruptive epitope tagging sites

Epitope tagging is an invaluable technique enabling the identification, tracking, and purification of proteins in vivo. We developed a tool, EpicTope, to facilitate this method by identifying amino acid positions suitable for epitope insertion. Our method uses a scoring function that considers multiple protein sequence and structural features to determine locations least disruptive to the protein's function. We validated our approach on the zebrafish Smad5 protein, showing that multiple predicted internally tagged Smad5 proteins rescue zebrafish smad5 mutant embryos, while the N- and C-terminal tagged variants do not, also as predicted. We further show that the internally tagged Smad5 proteins are accessible to antibodies in wholemount zebrafish embryo immunohistochemistry and by western blot. Our work demonstrates that EpicTope is an accessible and effective tool for designing epitope tag insertion sites. EpicTope is available under a GPL-3 license from: https://github.com/FriedbergLab/Epictope.

Formation and characterization of BMP2/GDF5 and BMP4/GDF5 heterodimers

BACKGROUND

Proteins of the TGFβ family, which are largely studied as homodimers, are also known to form heterodimers with biological activity distinct from their component homodimers. For instance, heterodimers of bone morphogenetic proteins, including BMP2/BMP7, BMP2/BMP6, and BMP9/BMP10, among others, have illustrated the importance of these heterodimeric proteins within the context of TGFβ signaling.

RESULTS

In this study, we have determined that mature GDF5 can be combined with mature BMP2 or BMP4 to form BMP2/GDF5 and BMP4/GDF5 heterodimer. Intriguingly, this combination of a BMP2 or BMP4 monomer, which exhibit high affinity to heparan sulfate characteristic to the BMP class, with a GDF5 monomer with low heparan sulfate affinity produces a heterodimer with an intermediate affinity. Using heparin affinity chromatography to purify the heterodimeric proteins, we then determined that both the BMP2/GDF5 and BMP4/GDF5 heterodimers consistently signaled potently across an array of cellular and in vivo systems, while the activities of their homodimeric counterparts were more context dependent. These differences were likely driven by an increase in the combined affinities for the type 1 receptors, Alk3 and Alk6. Furthermore, the X-ray crystal structure of BMP2/GDF5 heterodimer was determined, highlighting the formation of two asymmetric type 1 receptor binding sites that are both unique relative to the homodimers.

CONCLUSIONS

Ultimately, this method of heterodimer production yielded a signaling molecule with unique properties relative to the homodimeric ligands, including high affinity to multiple type 1 and moderate heparan binding affinity.

Transmembrane anterior posterior transformation 1 regulates BMP signaling and modulates the protein stability of SMAD1/5

The bone morphogenetic protein (BMP) signaling pathway plays pivotal roles in various biological processes during embryogenesis and adult homeostasis. Transmembrane anterior posterior transformation 1 (TAPT1) is an evolutionarily conserved protein involved in murine axial skeletal patterning. Genetic defects in TAPT1 result in complex lethal osteochondrodysplasia. However, the specific cellular activity of TAPT1 is not clear. Herein, we report that TAPT1 inhibits BMP signaling and destabilizes the SMAD1/5 protein by facilitating its interaction with SMURF1 E3 ubiquitin ligase, which leads to SMAD1/5 proteasomal degradation. In addition, we found that the activation of BMP signaling facilitates the redistribution of TAPT1 and promotes its association with SMAD1. TAPT1-deficient murine C2C12 myoblasts or C3H/10T1/2 mesenchymal stem cells exhibit elevated SMAD1/5/9 protein levels, which amplifies BMP activation, in turn leading to a boost in the transdifferentiation or differentiation processing of these distinct TAPT1-deficient cell lines changing into mature osteoblasts. Furthermore, the enhancing effect of TAPT1 deficiency on osteogenic differentiation of C3H/10T1/2 cells was observed in an in vivo ectopic bone formation model. Importantly, a subset of TAPT1 mutations identified in humans with lethal skeletal dysplasia exhibited gain-of-function activity on SMAD1 protein levels. Thus, this finding elucidates the role of TAPT1 in the regulation of SMAD1/5 protein stability for controlling BMP signaling.

Tetrabromobisphenol A perturbs cell fate decisions via BMP signaling in the early embryonic development of zebrafish

Tetrabromobisphenol A (TBBPA) readily accumulates in the egg yolk of aquatic oviparous animals and is transferred to their embryos. Early embryogenesis is vital for organ formation and subsequent development. The developmental toxicity of TBBPA in aquatic animals has been extensively reported. However, few studies have assessed the toxic effects of TBBPA in the early embryonic development. In this work, we found that TBBPA perturbed cell fate decisions along the dorsal-ventral (DV) axis during gastrulation, further disrupting early organogenesis in the entire embryo. TBBPA exposure increased the number of embryonic cells that acquired a ventral cell fate, which formed epidermis, blood and heart tissues. In return, the number of embryonic cells that acquired a dorsal cell fate was greatly decreased, causing the TBBPA-exposed embryos to develop a small brain and small eyes. We revealed that TBBPA elevated the activity gradient of bone morphogenetic protein (BMP) signaling which is responsible for cell fate specification along the DV axis, with up-regulation of BMP ligands (bmp4, bmp7a) and target genes (szl) and promotion signal transduction through phosphorylation of Smad1/5. As the function of BMP signaling in embryogenesis is highly conserved among many vertebrates, these findings highlight the ecological and health risks of TBBPA.

Proteolytic Restriction of Chordin Range Underlies BMP Gradient Formation

A fundamental question in developmental biology is how morphogens, such as bone morphogenetic protein (BMP), form precise signaling gradients to impart positional and functional identity to the cells of the early embryo. We combine rigorous mutant analyses with quantitative immunofluorescence to determine that the proteases Bmp1a and Tolloid spatially restrict the BMP antagonist Chordin in dorsoventral (DV) axial patterning of the early zebrafish gastrula. We show that maternally deposited Bmp1a plays an unexpected and non-redundant role in establishing the BMP signaling gradient, while the Bmp1a/Tolloid antagonist Sizzled is surprisingly dispensable. Combining computational modeling and in vivo analyses with an immobile Chordin construct, we demonstrate that long-range Chordin diffusion is not necessary for BMP gradient formation and DV patterning. Our data do not support a counter-gradient of Chordin and instead favor a Chordin sink, established by Bmp1a and Tolloid, as the primary mechanism that drives BMP gradient formation.

The BMP morphogen generates a precise signaling gradient during axial patterning. In the zebrafish embryo, Tuazon et al. find that proteases Bmp1a/Tolloid are key to this process, preventing the long-range diffusion of the BMP antagonist, Chordin. By regionally restricting Chordin, Bmp1a/Tolloid establish the signaling sink that drives BMP gradient formation.

BMP heterodimers signal via distinct type I receptor class functions

TGF-β family heterodimeric ligands show increased or exclusive signaling compared to homodimeric ligands in both vertebrate and insect development as well as in therapeutically relevant processes, like osteogenesis. However, the mechanisms that differentiate heterodimer and homodimer signaling remain uncharacterized. We show that BMP antagonists do not account for the exclusive signaling of Bmp2/7 heterodimers in zebrafish development. We found that overexpressed homodimers can signal but surprisingly require two distinct type I receptors, like heterodimers, indicating a required activity of the heteromeric type I receptor complex. We further demonstrate that a canonical type I receptor function has been delegated to only one of these receptors, Acvr1. Our findings should inform both basic and translational research in multiple TGF-β family signaling contexts.

Heterodimeric TGF-β ligands outperform homodimers in a variety of developmental, cell culture, and therapeutic contexts; however, the mechanisms underlying this increased potency remain uncharacterized. Here, we use dorsal–ventral axial patterning of the zebrafish embryo to interrogate the BMP2/7 heterodimer signaling mechanism. We demonstrate that differential interactions with BMP antagonists do not account for the reduced signaling ability of homodimers. Instead, we find that while overexpressed BMP2 homodimers can signal, they require two nonredundant type I receptors, one from the Acvr1 subfamily and one from the Bmpr1 subfamily. This implies that all BMP signaling within the zebrafish gastrula, even BMP2 homodimer signaling, requires Acvr1. This is particularly surprising as BMP2 homodimers do not bind Acvr1 in vitro. Furthermore, we find that the roles of the two type I receptors are subfunctionalized within the heterodimer signaling complex, with the kinase activity of Acvr1 being essential, while that of Bmpr1 is not. These results suggest that the potency of the Bmp2/7 heterodimer arises from the ability to recruit both Acvr1 and Bmpr1 into the same signaling complex.

The BMP signaling gradient is interpreted through concentration thresholds in dorsal-ventral axial patterning

Bone Morphogenetic Protein (BMP) patterns the dorsal–ventral (DV) embryonic axis in all vertebrates, but it is unknown how cells along the DV axis interpret and translate the gradient of BMP signaling into differential gene activation that will give rise to distinct cell fates. To determine the mechanism of BMP morphogen interpretation in the zebrafish gastrula, we identified 57 genes that are directly activated by BMP signaling. By using Seurat analysis of single-cell RNA sequencing (scRNA-seq) data, we found that these genes are expressed in at least 3 distinct DV domains of the embryo. We distinguished between 3 models of BMP signal interpretation in which cells activate distinct gene expression through interpretation of thresholds of (1) the BMP signaling gradient slope; (2) the BMP signal duration; or (3) the level of BMP signal activation. We tested these 3 models using quantitative measurements of phosphorylated Smad5 (pSmad5) and by examining the spatial relationship between BMP signaling and activation of different target genes at single-cell resolution across the embryo. We found that BMP signaling gradient slope or BMP exposure duration did not account for the differential target gene expression domains. Instead, we show that cells respond to 3 distinct levels of BMP signaling activity to activate and position target gene expression. Together, we demonstrate that distinct pSmad5 threshold levels activate spatially distinct target genes to pattern the DV axis.

This study tested three models of how a BMP morphogen gradient is translated into differential gene activation that specifies distinct cell fates, finding that BMP signal concentration thresholds, not gradient shape or signal duration, position three distinct gene activation domains.

Fibrodysplasia ossificans progressiva mutant ACVR1 signals by multiple modalities in the developing zebrafish

Fibrodysplasia ossificans progressiva (FOP) is a rare human genetic disorder characterized by altered skeletal development and extraskeletal ossification. All cases of FOP are caused by activating mutations in the type I BMP/TGFβ cell surface receptor ACVR1, which over-activates signaling through phospho-Smad1/5 (pSmad1/5). To investigate the mechanism by which FOP-ACVR1 enhances pSmad1/5 activation, we used zebrafish embryonic dorsoventral (DV) patterning as an assay for BMP signaling. We determined that the FOP mutants ACVR1-R206H and -G328R do not require their ligand binding domain to over-activate BMP signaling in DV patterning. However, intact ACVR1-R206H has the ability to respond to both Bmp7 and Activin A ligands. Additionally, BMPR1, a type I BMP receptor normally required for BMP-mediated patterning of the embryo, is dispensable for both ligand-independent signaling pathway activation and ligand-responsive signaling hyperactivation by ACVR1-R206H. These results demonstrate that FOP-ACVR1 is not constrained by the same receptor/ligand partner requirements as WT-ACVR1.