Two populations of endochondral osteoblasts with differential sensitivity to Hedgehog signalling

Zebrafish sp7:EGFP: a transgenic for studying otic vesicle formation, skeletogenesis, and bone regeneration

We report the expression pattern and construction of a transgenic zebrafish line for a transcription factor involved in otic vesicle formation and skeletogenesis. The zinc finger transcription factor sp7 (formerly called osterix) is reported as a marker of osteoblasts. Using bacterial artificial chromosome (BAC)-mediated transgenesis, we generated a zebrafish transgenic line for studying skeletal development, Tg(sp7:EGFP)b1212. Using a zebrafish BAC, EGFP was introduced downstream of the regulatory regions of sp7 and injected into one cell-stage embryos. In this transgenic line, GFP expression reproduces endogenous sp7 gene expression in the otic placode and vesicle, and in forming skeletal structures. GFP-positive cells were also detected in adult fish, and were found associated with regenerating fin rays post-amputation. This line provides an essential tool for the further study of zebrafish otic vesicle formation and the development and regeneration of the skeleton.

Effects of benzo(a)pyrene on the skeletal development of Sebastiscus marmoratus embryos and the molecular mechanism involved

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants, which have been known to be carcinogenic and teratogenic. However, the skeletal development toxicity of PAHs and the mechanism involved remain unclear. In fishes, the neurocranial and craniofacial skeleton develop as cartilage. The signaling molecules of hedgehog (Hh) family play crucial roles in regulating skeletal development. In the present study, rockfish (Sebastiscus marmoratus) embryos were exposed to benzo(a)pyrene (BaP) for 7 days at environmental levels (0.05, 0.5 and 5nmol/L) which resulted in craniofacial skeleton deformities. BaP exposure reduced the cell proliferation activity in the craniofacial skeleton as detected by quantitative PCR and in situ hybridization. The expression of Sonic hedgehog (Shh), rather than Indian hedgehog (Ihh), was down-regulated in the craniofacial skeleton in the 0.5 and 5nmol/L groups. Consistent with the Shh results, the expression of Ptch1 and Gli2 was decreased by BaP exposure and BMP4 was presented on changes in the 0.5 and 5nmol/L groups. These results suggested that BaP could impair the expression and function of Shh signaling pathway, perturbing the proliferation of chondrocytes and so disturbing craniofacial skeletal development.

Sonic hedgehog (Shh)-Gli signaling controls neural progenitor cell division in the developing tectum in zebrafish

Despite considerable progress, the mechanisms that control neural progenitor differentiation and behavior, as well as their functional integration into adult neural circuitry, are far from being understood. Given the complexity of the mammalian brain, non-mammalian models provide an excellent model to study neurogenesis, including both the cellular composition of the neurogenic microenvironment, and the factors required for precursor growth and maintenance. In particular, we chose to address the question of the control of progenitor proliferation by Sonic hedgehog (Shh) using the zebrafish dorsal mesencephalon, known as the optic tectum (OT), as a model system. Here we show that either inhibiting pharmacologically or eliminating hedgehog (Hh) signaling by using mutants that lack essential components of the Hh pathway reduces neural progenitor cell proliferation affecting neurogenesis in the OT. On the contrary, pharmacological gain-of-function experiments result in significant increase in proliferation. Importantly, Shh-dependent function controls neural progenitor cell behavior as sox2-positive cell populations were lost in the OT in the absence of Hh signaling, as evidenced in slow-muscle-omitted (smu) mutants and with timed cyclopamine inhibition. Expressions of essential components of the Hh pathway reveal for the first time a late dorsal expression in the embryonic OT. Our observations argue strongly for a role of Shh in neural progenitor biology in the OT and provide comparative data to our current understanding of progenitor/stem cell mechanisms that place Shh as a key niche factor in the dorsal brain.

Visualization of Gli activity in craniofacial tissues of hedgehog-pathway reporter transgenic zebrafish

Background

The Hedgehog (Hh)-signaling pathway plays a crucial role in the development and maintenance of multiple vertebrate and invertebrate organ systems. Gli transcription factors are regulated by Hh-signaling and act as downstream effectors of the pathway to activate Hh-target genes. Understanding the requirements for Hh-signaling in organisms can be gained by assessing Gli activity in a spatial and temporal fashion.

Methodology/Principal Findings

We have generated a Gli-dependent (Gli-d) transgenic line, Tg(Gli-d:mCherry), that allows for rapid and simple detection of Hh-responding cell populations in both live and fixed zebrafish. This transgenic line expresses a mCherry reporter under the control of a Gli responsive promoter, which can be followed by using fluorescent microscopy and in situ hybridization. Expression of the mCherry transgene reporter during embryogenesis and early larval development faithfully replicated known expression domains of Hh-signaling in zebrafish, and abrogating Hh-signaling in transgenic fish resulted in the suppression of reporter expression. Moreover, ectopic shh expression in Tg(Glid:mCherry) fish led to increased transgene production. Using this transgenic line we investigated the nature of Hh-pathway response during early craniofacial development and determined that the neural crest skeletal precursors do not directly respond to Hh-signaling prior to 48 hours post fertilization, suggesting that earlier requirements for pathway activation in this population of facial skeleton precursors are indirect.

Conclusion/Significance

We have determined that early Hh-signaling requirements in craniofacial development are indirect. We further demonstrate the Tg(Gli-d:mCherry) fish are a highly useful tool for studying Hh-signaling dependent processes during embryogenesis and larval stages.

Hedgehog signalling is required for perichondral osteoblast differentiation in zebrafish

In tetrapod long bones, Hedgehog signalling is required for osteoblast differentiation in the perichondrium. In this work we analyse skeletogenesis in zebrafish larvae treated with the Hedgehog signalling inhibitor cyclopamine. We show that cyclopamine treatment leads to the loss of perichondral ossification of two bones in the head. We find that the Hedgehog co-receptors patched1 and patched2 are expressed in regions of the perichondrium that will form bone before the onset of ossification. We also show that cyclopamine treatment strongly reduces the expression of osteoblast markers in the perichondrium and that perichondral ossification is enhanced in patched1 mutant fish. This data suggests a conserved role for Hedgehog signalling in promoting perichondral osteoblast differentiation during vertebrate skeletal development. However, unlike what is seen during long bone development, we did not observe ectopic chondrocytes in the perichondrium when Hedgehog signalling is blocked. This result may point to subtle differences between the development of the skeleton in the skull and limb.

Two families of Xenopus tropicalis skeletal genes display well-conserved expression patterns with mammals in spite of their highly divergent regulatory regions

The origin of bone and cartilage, and their subsequent diversification in specific vertebrate lineages, is intimately linked to the precise transcriptional control of genes involved in matrix mineralization. It is not yet clear, however, to which extent the osteoblasts, osteocytes, and chondrocytes of each of the major vertebrate groups express similar sets of genes. In this study we have focused on the evolution of two independent families of genes that code for extracellular matrix components of the skeleton and that include secreted protein, acidic, cysteine-rich (SPARC), bone sialoprotein (BSP) and dentin matrix protein 1 (DMP1) paralogues, and the osteocalcin (OC) and matrix gla protein (MGP) paralogues. Analyzing developing Xenopus tropicalis skeletal elements, we show that the expression patterns of these genes are well conserved with mammals. The fact that only a few osteoblasts express DMP1, while only some osteocytes express SPARC and BSP, reveals a significant degree of molecular heterogeneity for these two populations of X. tropicalis cells, similarly to what has been described in mouse. Although the cis-regulatory modules (CRM) of the mammalian OC, DMP1, and BSP orthologs have been functionally characterized, we found no evidence of sequence similarity between these regions and the X. tropicalis genome. Furthermore, these regulatory elements evolve rapidly, as they are only poorly conserved between human and rodents. Therefore, the SPARC/DMP1/BSP and the OC/MGP families provide a good paradigm to study how transcriptional output can be maintained in skeletal cells despite extensive sequence divergence of CRM.

Development of an oligo DNA microarray for the European sea bass and its application to expression profiling of jaw deformity

Background

The European sea bass (Dicentrarchus labrax) is a marine fish of great importance for fisheries and aquaculture. Functional genomics offers the possibility to discover the molecular mechanisms underlying productive traits in farmed fish, and a step towards the application of marker assisted selection methods in this species. To this end, we report here on the development of an oligo DNA microarray for D. labrax.

Results

A database consisting of 19,048 unique transcripts was constructed, of which 12,008 (63%) could be annotated by similarity and 4,692 received a GO functional annotation. Two non-overlapping 60mer probes were designed for each unique transcript and in-situ synthesized on glass slides using Agilent SurePrint™ technology. Probe design was positively completed for 19,035 target clusters; the oligo microarray was then applied to profile gene expression in mandibles and whole-heads of fish affected by prognathism, a skeletal malformation that strongly affects sea bass production. Statistical analysis identified 242 transcripts that are significantly down-regulated in deformed individuals compared to normal fish, with a significant enrichment in genes related to nervous system development and functioning. A set of genes spanning a wide dynamic range in gene expression level were selected for quantitative RT-PCR validation. Fold change correlation between microarray and qPCR data was always significant.

Conclusions

The microarray platform developed for the European sea bass has a high level of flexibility, reliability, and reproducibility. Despite the well known limitations in achieving a proper functional annotation in non-model species, sufficient information was obtained to identify biological processes that are significantly enriched among differentially expressed genes. New insights were obtained on putative mechanisms involved on mandibular prognathism, suggesting that bone/nervous system development might play a role in this phenomenon.