Regulation of neural crest cell fate by the retinoic acid and Pparg signalling pathways

Interference with the retinoic acid signalling pathway inhibits the initiation of teeth and caudal primary scales in the small-spotted catshark Scyliorhinus canicula

The retinoic acid (RA) pathway was shown to be important for tooth development in mammals, and suspected to play a key role in tooth evolution in teleosts. The general modalities of development of tooth and "tooth-like" structures (collectively named odontodes) seem to be conserved among all jawed vertebrates, both with regard to histogenesis and genetic regulation. We investigated the putative function of RA signalling in tooth and scale initiation in a cartilaginous fish, the small-spotted catshark Scyliorhinus canicula. To address this issue, we identified the expression pattern of genes from the RA pathway during both tooth and scale development and performed functional experiments by exposing small-spotted catshark embryos to exogenous RA or an inhibitor of RA synthesis. Our results showed that inhibiting RA synthesis affects tooth but not caudal primary scale development while exposure to exogenous RA inhibited both. We also showed that the reduced number of teeth observed with RA exposure is probably due to a specific inhibition of tooth bud initiation while the observed effects of the RA synthesis inhibitor is related to a general delay in embryonic development that interacts with tooth development. This study provides data complementary to previous studies of bony vertebrates and support an involvement of the RA signalling pathway toolkit in odontode initiation in all jawed vertebrates. However, the modalities of RA signalling may vary depending on the target location along the body, and depending on the species lineage.

Integrated Transcriptome and Microbiota Reveal the Regulatory Effect of 25-Hydroxyvitamin D Supplementation in Antler Growth of Sika Deer

The level of plasma 25-hydroxyvitamin D (25(OH)D) is associated with the growth of the antler, a fast-growing bone organ of Cervidae. However, the benefits of 25(OH)D supplementation on antler growth and the underlying mechanisms remain unclear. Here, the antler growth profile and transcriptome, plasma parameters, rumen bacteria, and metabolites (volatile fatty acids and amino acids) were determined in sika deer in a 25(OH)D supplementation group (25(OH)D, n = 8) and a control group (Ctrl, n = 8). 25(OH)D supplementation significantly increased the antler weight and growth rate. The levels of IGF-1,25(OH)D and 1,25-dihydroxyvitamin D were significantly higher in the 25(OH)D group than in the Ctrl group, while the levels of LDL-C were lower. The levels of valerate and branched-chain amino acids in the rumen fluid were significantly different between the 25(OH)D and Ctrl groups. The bacterial diversity indices were not significantly different between the two groups. However, the relative abundances of the butyrate-producing bacteria (families Lachnospiraceae and Succinivibrionaceae) and the pyruvate metabolism pathway were higher in the 25(OH)D group. The transcriptomic profile of the antler was significantly different between the 25(OH)D and Ctrl groups, with 356 up- and 668 down-regulated differentially expressed genes (DEGs) in the 25(OH)D group. The up-regulated DEGs were enriched in the proteinaceous extracellular matrix and collagen, while the down-regulated DEGs were enriched in the immune system and lipid metabolism pathways. Overall, these results provide novel insights into the effects of 25(OH)D supplementation on the host metabolism, rumen microbiota, and antler transcriptome of sika deer.

Retinoic acid effects on in vitro palatal fusion and WNT signaling

Retinoic acid is the main active vitamin A derivate and a key regulator of embryonic development. Excess of retinoic acid can disturb palate development in mice leading to cleft palate. WNT signaling is one of the main pathways in palate development. We evaluated the effects of retinoic acid on palate fusion and WNT signaling in in vitro explant cultures. Unfused palates from E13.5 mouse embryos were cultured for 4 days with 0.5 μM, 2 μM or without retinoic acid. Apoptosis, proliferation, WNT signaling and bone formation were analyzed by histology and quantitative PCR. Retinoic acid treatment with 0.5 and 2.0 μM reduced palate fusion from 84% (SD 6.8%) in the controls to 56% (SD 26%) and 16% (SD 19%), respectively. Additionally, 2 μM retinoic acid treatment increased Axin2 expression. Retinoic acid also increased the proliferation marker Pcna as well as the number of Ki-67-positive cells in the palate epithelium. At the same time, the WNT inhibitors Dkk1, Dkk3, Wif1 and Sfrp1 were downregulated at least two-fold. Retinoic acid also down-regulated Alpl and Col1a2 gene expression. Alkaline phosphatase (ALP) activity was notably reduced in the osteogenic areas of the retinoic acid- treated palates. Our data suggest that retinoic acid impairs palate fusion and bone formation by upregulation of WNT signaling.

The endocannabinoid system and retinoic acid signaling combine to influence bone growth

Osteoporosis is an increasing burden on public health as the world-wide population ages and effective therapeutics are severely needed. Two pathways with high potential for osteoporosis treatment are the retinoic acid (RA) and endocannabinoid system (ECS) signaling pathways. We sought to elucidate the roles that these pathways play in bone development and maturation. Here, we use chemical treatments to modulate the RA and ECS pathways at distinct early, intermediate, and late times bone development in zebrafish. We further assessed osteoclast activity later in zebrafish and medaka. Finally, by combining sub-optimal doses of AR and ECS modulators, we show that enhancing RA signaling or reducing the ECS promote bone formation and decrease osteoclast abundance and activity. These data demonstrate that RA signaling and the ECS can be combined as sub-optimal doses to influence bone growth and may be key targets for potential therapeutics.

Dysregulation of lipid metabolism in PLHC-1 and ZFL cells exposed to tributyltin an all-trans retinoic acid

There is increasing awareness that exposure to endocrine disrupters interferes with lipid homeostasis in vertebrates, including fish. Many of these compounds exert their action by binding to nuclear receptors, such as peroxisome proliferator-activated receptors and retinoid X receptor. This work investigates the use of fish liver cells (PLHC-1 and ZFL cells) for the screening of metabolic and lipid disrupters in the aquatic environment by assessing changes in the cell's lipidome after exposure to the model compounds, tributyltin chloride and all-trans retinoic acid. Lipid extracts, analyzed by FIA-ESI (+/-) Orbitrap, evidenced the intracellular accumulation of triglycerides and diglycerides in both cell models after exposure to 100 and 200 nM tributyltin chloride for 24 h. Exposure to 1 μM all-trans retinoic acid led to a significant accumulation of triglycerides in PLHC-1 cells, while few triglycerides were accumulated in ZFL cells. Retinoic acid (cyp26b1, cyp3a65, lrata) and lipid metabolism (fasn, scd, elovl6) related genes were up-regulated by tributyltin chloride and all-trans retinoic acid, while only all-trans retinoic acid down-regulated the expression of dgat1a. The two cell models show sensitivity and responses to tributyltin chloride and all-trans retinoic acid comparable to those previously reported in mammalian cells. These results support the use of fish liver cells as alternative models for the detection of contaminants that act as lipid disrupters in the aquatic environment.

Homotherapy for heteropathy active components and mechanisms of Qiang-Huo-Sheng-Shi decoction for treatment of rheumatoid arthritis and osteoarthritis

Qiang-Huo-Sheng-Shi decoction (QHSSD), a classic traditional Chinese herbal formula, which has been reported to be effective in rheumatoid arthritis (RA) and osteoarthritis (OA). However, the concurrent targeting mechanism of how the aforementioned formula is valid in the two distinct diseases OA and RA, which represents the homotherapy-for-heteropathy principle in traditional Chinese medicine (TCM), have not yet been clarified. In the present study, network pharmacology was adopted to analyze the potential molecular mechanism, and therapeutic effective components of QHSSD on both OA and RA. A total of 153 active ingredients in QHSSD were identified, 142 of which associated with 59 potential targets for the two diseases were identified. By constructing the protein-protein interaction network and the compound-target-disease network, 72 compounds and 10 proteins were obtained as the hub targets of QHSSD against OA and RA. The hub genes of ESR1, PTGS2, PPARG, IL1B, TNF, MMP2, IL6, CYP3A4, MAPK8, and ALB were mainly involved in osteoclast differentiation, the NF-κB and TNF signaling pathways. Moreover, molecular docking results showed that the screened active compounds had a high affinity for the hub genes. This study provides new insight into the molecular mechanisms behind how QHSSD presents homotherapy-for-heteropathy therapeutic efficacy in both OA and RA. For the first time, a two-disease model was linked with a TCM formula using network pharmacology to identify the key active components and understand the common mechanisms of its multi-pathway regulation. This study will inspire more innovative and important studies on the modern research of TCM formulas.

Twist3 is required for dedifferentiation during extraocular muscle regeneration in adult zebrafish

Severely damaged adult zebrafish extraocular muscles (EOMs) regenerate through dedifferentiation of residual myocytes involving a muscle-to-mesenchyme transition. Members of the Twist family of basic helix-loop-helix transcription factors (TFs) are key regulators of the epithelial-mesenchymal transition (EMT) and are also involved in craniofacial development in humans and animal models. During zebrafish embryogenesis, twist family members (twist1a, twist1b, twist2, and twist3) function to regulate craniofacial skeletal development. Because of their roles as master regulators of stem cell biology, we hypothesized that twist TFs regulate adult EOM repair and regeneration. In this study, utilizing an adult zebrafish EOM regeneration model, we demonstrate that inhibiting twist3 function using translation-blocking morpholino oligonucleotides (MOs) impairs muscle regeneration by reducing myocyte dedifferentiation and proliferation in the regenerating muscle. This supports our hypothesis that twist TFs are involved in the early steps of dedifferentiation and highlights the importance of twist3 during EOM regeneration.

PPARγ maintains the metabolic heterogeneity and homeostasis of renal tubules

Background

The renal tubules, which have distant metabolic features and functions in different segments, reabsorb >99% of approximately 180 l of water and 25,000 mmol of Na ⁺ daily. Defective metabolism in renal tubules is involved in the pathobiology of kidney diseases. However, the mechanisms underlying the metabolic regulation in renal tubules remain to be defined.

Methods

We quantitatively compared the proteomes of the isolated proximal tubules (PT) and distal tubules (DT) from C57BL/6 mouse using tandem mass tag (TMT) labeling-based quantitative mass spectrometry. Bioinformatics analysis of the differentially expressed proteins revealed the significant differences between PT and DT in metabolism pathway. We also performed in vitro and in vivo assays to investigate the molecular mechanism underlying the distant metabolic features in PT and DT.

Findings

We demonstrate that the renal proximal tubule (PT) has high expression of lipid metabolism enzymes, which is transcriptionally upregulated by abundantly expressed PPARα/γ. In contrast, the renal distal tubule (DT) has elevated glycolytic enzyme expression, which is mediated by highly expressed c-Myc. Importantly, PPARγ transcriptionally enhances the protease iRhom2 expression in PT, which suppresses EGF expression and secretion and subsequent EGFR-dependent glycolytic gene expression and glycolysis. PPARγ inhibition reduces iRhom2 expression and increases EGF and GLUT1 expression in PT in mice, resulting in renal tubule hypertrophy, tubulointerstitial fibrosis and damaged kidney functions, which are rescued by 2-deoxy-d-glucose treatment.

Interpretation

These findings delineate instrumental mechanisms underlying the active lipid metabolism and suppressed glycolysis in PT and active glycolysis in DT and reveal critical roles for PPARs and c-Myc in maintaining renal metabolic homeostasis. FUND: This work was supported by the National Natural Science Foundation of China (grants 81572076 and 81873932; to Q.Z.), the Applied Development Program of the Science and Technology Committee of Chongqing (cstc2014yykfB10003; Q.Z.), the Program of Populace Creativities Workshops of the Science and Technology Committee of Chongqing (Q.Z.), the special demonstration programs for innovation and application of techniques (cstc2018jscx-mszdX0022) from the Science and Technology Committee of Chongqing (Q.Z.).

Nuclear receptor research in zebrafish

Nuclear receptors (NRs) form a superfamily of transcription factors that can be activated by ligands and are involved in a wide range of physiological processes. NRs are well conserved between vertebrate species. The zebrafish, an increasingly popular animal model system, contains a total of 73 NR genes, and orthologues of almost all human NRs are present. In this review article, an overview is presented of NR research in which the zebrafish has been used as a model. Research is described on the three most studied zebrafish NRs: the estrogen receptors (ERs), retinoic acid receptors (RARs) and peroxisome proliferator-activated receptors (PPARs). The studies on these receptors illustrate the versatility of the zebrafish as a model for ecotoxicological, developmental and biomedical research. Although the use of the zebrafish in NR research is still relatively limited, it is expected that in the next decade the full potential of this animal model will be exploited.

Vitamin A Affects Flatfish Development in a Thyroid Hormone Signaling and Metamorphic Stage Dependent Manner

Vitamin A (VA) and retinoid derivatives are known morphogens controlling vertebrate development. Despite the research effort conducted during the last decade, the precise mechanism of how VA induces post-natal bone changes, and particularly those operating through crosstalk with the thyroid hormones (THs) remain to be fully understood. Since effects and mechanisms seem to be dose and time-dependent, flatfish are an interesting study model as they undergo a characteristic process of metamorphosis driven by THs that can be followed by external appearance. Here, we studied the effects of VA imbalance that might determine Senegalese sole (Solea senegalensis) skeletogenetic phenotype through development of thyroid follicles, THs homeostasis and signaling when a dietary VA excess was specifically provided during pre-, pro- or post-metamorphic stages using enriched rotifers and Artemia as carriers. The increased VA content in enriched live prey was associated to a higher VA content in fish at all developmental stages. Dietary VA content clearly affected thyroid follicle development, T3 and T4 immunoreactive staining, skeletogenesis and mineralization in a dose and time-dependent fashion. Gene expression analysis showed that VA levels modified the mRNA abundance of VA- and TH-specific nuclear receptors at specific developmental stages. Present results provide new and key knowledge to better understand how VA and TH pathways interact at tissue, cellular and nuclear level at different developmental periods in Senegalese sole, unveiling how dietary modulation might determine juvenile phenotype and physiology.

Expression of osterix Is Regulated by FGF and Wnt/β-Catenin Signalling during Osteoblast Differentiation

Osteoblast differentiation from mesenchymal cells is regulated by multiple signalling pathways. Here we have analysed the roles of Fibroblast Growth Factor (FGF) and canonical Wingless-type MMTV integration site (Wnt/β-Catenin) signalling pathways on zebrafish osteogenesis. We have used transgenic and chemical interference approaches to manipulate these pathways and have found that both pathways are required for osteoblast differentiation in vivo. Our analysis of bone markers suggests that these pathways act at the same stage of differentiation to initiate expression of the osteoblast master regulatory gene osterix (osx). We use two independent approaches that suggest that osx is a direct target of these pathways. Firstly, we manipulate signalling and show that osx gene expression responds with similar kinetics to that of known transcriptional targets of the FGF and Wnt pathways. Secondly, we have performed ChIP with transcription factors for both pathways and our data suggest that a genomic region in the first intron of osx mediates transcriptional activation. Based upon these data, we propose that FGF and Wnt/β-Catenin pathways act in part by directing transcription of osx to promote osteoblast differentiation at sites of bone formation.

Vitamin A and arachidonic acid altered the skeletal mineralization in Atlantic cod (Gadus morhua) larvae without any interactions on the transcriptional level

The main object of this study was to evaluate the impact of different levels of vitamin A (VA) and arachidonic acid (ARA) in relation to eicosapentaenoic acid (EPA) on mineralization and gene expression in Atlantic cod larvae (Gadus morhua). First-feeding larvae were fed enriched rotifers from start-feeding until 29 days post hatch (dph). Larvae in four tanks were fed one of the following diets: control (EPA/ARA ratio: 15.8, 0.9μg VA g(-1)), control+VA (EPA/ARA ratio: 15.8, 7.8μg VA g(-1)), High ARA (EPA/ARA ratio: 0.9, 1.5μg VA g(-1)) or High ARA+VA (EPA/ARA ratio: 0.9, 12.0μg VA g(-1)). Larvae fed High ARA+VA were shorter at 29dph compared to the other groups and had significantly less mineralized bones when comparing larvae of similar size, showing interaction effects between VA and ARA. Although transcriptomic analysis did not reveal any interaction effects, a higher number of genes were differentially expressed in the high ARA fed larvae compared to control+VA fed larvae. Furthermore, bglap1, bglap2 and col10a1 were all down-regulated in larvae fed High ARA-diets and to a greater extent than larvae fed VA supplemented diet, indicating an additive effect on mineralization. In conclusion, this study showed that the dietary increase in ARA and VA altered the skeletal metabolism during larval development, most likely through signaling pathways specific for each nutrient rather than an interaction. The present study also demonstrates that VA could affect the larval response to ARA, even within the accepted non-toxic/non-deficient range.

Lipid Abundance in Zebrafish Embryos Is Regulated by Complementary Actions of the Endocannabinoid System and Retinoic Acid Pathway

The endocannabinoid system (ECS) and retinoic acid (RA) signaling have been associated with influencing lipid metabolism. We hypothesized that modulation of these pathways could modify lipid abundance in developing vertebrates and that these pathways could have a combinatorial effect on lipid levels. Zebrafish embryos were exposed to chemical treatments altering the activity of the ECS and RA pathway. Embryos were stained with the neutral lipid dye Oil-Red-O (ORO) and underwent whole-mount in situ hybridization (WISH). Mouse 3T3-L1 fibroblasts were differentiated under exposure to RA-modulating chemicals and subsequently stained with ORO and analyzed for gene expression by qRT-PCR. ECS activation and RA exposure increased lipid abundance and the expression of lipoprotein lipase. In addition, RA treatment increased expression of CCAAT/enhancer-binding protein alpha. Both ECS receptors and RA receptor subtypes were separately involved in modulating lipid abundance. Finally, increased ECS or RA activity ameliorated the reduced lipid abundance caused by peroxisome proliferator-activated receptor gamma (PPARγ) inhibition. Therefore, the ECS and RA pathway influence lipid abundance in zebrafish embryos and have an additive effect when treated simultaneously. Furthermore, we demonstrated that these pathways act downstream or independently of PPARγ to influence lipid levels. Our study shows for the first time that the RA and ECS pathways have additive function in lipid abundance during vertebrate development.

Osteoblast de- and redifferentiation are controlled by a dynamic response to retinoic acid during zebrafish fin regeneration

Zebrafish restore amputated fins by forming tissue-specific blastema cells that coordinately regenerate the lost structures. Fin amputation triggers the synthesis of several diffusible signaling factors that are required for regeneration, raising the question of how cell lineage-specific programs are protected from regenerative crosstalk between neighboring fin tissues. During fin regeneration, osteoblasts revert from a non-cycling, mature state to a cycling, preosteoblastic state to establish a pool of progenitors within the blastema. After several rounds of proliferation, preosteoblasts redifferentiate to produce new bone. Blastema formation and proliferation are driven by the continued synthesis of retinoic acid (RA). Here, we find that osteoblast dedifferentiation and redifferentiation are inhibited by RA signaling, and we uncover how the bone regenerative program is achieved against a background of massive RA synthesis. Stump osteoblasts manage to contribute to the blastema by upregulating expression of the RA-degrading enzyme cyp26b1. Redifferentiation is controlled by a presumptive gradient of RA, in which high RA levels towards the distal tip of the blastema suppress redifferentiation. We show that this might be achieved through a mechanism involving repression of Bmp signaling and promotion of Wnt/β-catenin signaling. In turn, cyp26b1(+) fibroblast-derived blastema cells in the more proximal regenerate serve as a sink to reduce RA levels, thereby allowing differentiation of neighboring preosteoblasts. Our findings reveal a mechanism explaining how the osteoblast regenerative program is protected from adverse crosstalk with neighboring fibroblasts that advances our understanding of the regulation of bone repair by RA.

A ground state of PPARγ activity and expression is required for appropriate neural differentiation of hESCs

BACKGROUND

Several evidences indicate stimulation of peroxisome proliferator activated receptor γ (PPARg), promotes neuronal differentiation. This study was conducted to testify the prominence of PPARγ during neural differentiation of human embryonic stem cells (hESCs).

METHODS

PPARγ expression level was assessed during neural differentiation of hESCs. Meanwhile, the level of endogenous miRNAs, which could be engaged in regulation of PPARγ expression, was measured. Next, natural and synthetic components of PPARγ agonists and antagonist were implemented on neural progenitor formation during neural differentiation of hESCs.

RESULTS

Data showed an increasing wave of PPARγ expression level when human neural progenitors (NPs) were formed upon retinoic acid treatment. Interestingly, there was no significant difference in the amount of PPARγ proteins during the differentiation of hESCs that is inconsistent with what we observed for RNA level. Our results indicated that miRNAs are not involved in the regulation of PPARγ expression, while proteasome-mediated degradation may to some degree be involved in this process. Among numerous treatments, PPARγ inactivation during NPs formation significantly decreased expression of NP markers.

CONCLUSIONS

We conclude that a ground state of PPARγ activity is required for NP formation of hESCs during early neural differentiation. However, high expression and activity of PPARγ could not enhance the required neural differentiation, whereas the PPARγ inactivation could negatively influence NP formation from hESCs by antagonist.

The development and growth of tissues derived from cranial neural crest and primitive mesoderm is dependent on the ligation status of retinoic acid receptor γ: evidence that retinoic acid receptor γ functions to maintain stem/progenitor cells in the absence of retinoic acid

Retinoic acid (RA) signaling is important to normal development. However, the function of the different RA receptors (RARs)--RARα, RARβ, and RARγ--is as yet unclear. We have used wild-type and transgenic zebrafish to examine the role of RARγ. Treatment of zebrafish embryos with an RARγ-specific agonist reduced somite formation and axial length, which was associated with a loss of hoxb13a expression and less-clear alterations in hoxc11a or myoD expression. Treatment with the RARγ agonist also disrupted formation of tissues arising from cranial neural crest, including cranial bones and anterior neural ganglia. There was a loss of Sox 9-immunopositive neural crest stem/progenitor cells in the same anterior regions. Pectoral fin outgrowth was blocked by RARγ agonist treatment. However, there was no loss of Tbx-5-immunopositive lateral plate mesodermal stem/progenitor cells and the block was reversed by agonist washout or by cotreatment with an RARγ antagonist. Regeneration of the caudal fin was also blocked by RARγ agonist treatment, which was associated with a loss of canonical Wnt signaling. This regenerative response was restored by agonist washout or cotreatment with the RARγ antagonist. These findings suggest that RARγ plays an essential role in maintaining stem/progenitor cells during embryonic development and tissue regeneration when the receptor is in its nonligated state.

Differentiation of human induced pluripotent stem cells into brown and white adipocytes: role of Pax3

Identification of molecular mechanisms involved in generation of different types of adipocytes is progressing substantially in mice. However, much less is known regarding characterization of brown (BAP) and white adipocyte progenitors (WAPs) in humans, highlighting the need for an in vitro model of human adipocyte development. Here, we report a procedure to selectively derive BAP and WAPs from human-induced pluripotent stem cells. Molecular characterization of APs of both phenotypes revealed that BMP4, Hox8, Hoxc9, and HoxA5 genes were specifically expressed in WAPs, whereas expression of PRDM16, Dio2, and Pax3 marked BAPs. We focused on Pax3 and we showed that expression of this transcription factor was enriched in human perirenal white adipose tissue samples expressing UCP1 and in human classical brown fat. Finally, functional experiments indicated that Pax3 was a critical player of human AP fate as its ectopic expression led to convert WAPs into brown-like APs. Together, these data support a model in which Pax3 is a new marker of human BAPs and a molecular mediator of their fate. The findings of this study could lead to new anti-obesity therapies based on the recruitment of APs and constitute a platform for investigating in vitro the developmental origins of human white and brown adipocytes.

Possible effects of EXT2 on mesenchymal differentiation--lessons from the zebrafish

Background

Mutations in the EXT genes disrupt polymerisation of heparan sulphates (HS) and lead to the development of osteochondroma, an isolated/sporadic- or a multifocal/hereditary cartilaginous bone tumour. Zebrafish (Danio rerio) is a very powerful animal model which has shown to present the same cartilage phenotype that is commonly seen in mice model and patients with the rare hereditary syndrome, Multiple Osteochondroma (MO).

Methods

Zebrafish dackel (dak) mutant that carries a nonsense mutation in the ext2 gene was used in this study. A panel of molecular, morphological and biochemical analyses was used to assess at what step bone formation is affected and what mechanisms underlie changes in the bone formation in the ext2 mutant.

Results

During bone development in the ext2^-/- zebrafish, chondrocytes fail to undergo terminal differentiation; and pre-osteoblasts do not differentiate toward osteoblasts. This inadequate osteogenesis coincides with increased deposition of lipids/fats along/in the vessels and premature adipocyte differentiation as shown by biochemical and molecular markers. Also, the ext2-null fish have a muscle phenotype, i.e. muscles are shorter and thicker. These changes coexist with misshapen bones. Normal expression of runx2 together with impaired expression of osterix and its master regulator - xbp1 suggest that unfolded protein responses might play a role in MO pathogenesis.

Conclusions

Heparan sulphates are required for terminal differentiation of the cartilaginous template and consecutive formation of a scaffold that is needed for further bone development. HS are also needed for mesenchymal cell differentiation. At least one copy of ext2 is needed to maintain the balance between bone and fat lineages, but homozygous loss of the ext2 function leads to an imbalance between cartilage, bone and fat lineages. Normal expression of runx2 and impaired expression of osterix in the ext2^-/- fish indicate that HS are required by osteoblast precursors for their further differentiation towards osteoblastic lineage. Lower expression of xbp1, a master regulator of osterix, suggests that HS affect the ‘unfolded protein response’, a pathway that is known to control bone formation and lipid metabolism. Our observations in the ext2-null fish might explain the musculoskeletal defects that are often observed in MO patients.

Retinoic acid differentially affects in vitro proliferation, differentiation and mineralization of two fish bone-derived cell lines: different gene expression of nuclear receptors and ECM proteins

Retinoic acid (RA), the main active metabolite of vitamin A, regulates vertebrate morphogenesis through signaling pathways not yet fully understood. Such process involves the specific activation of retinoic acid and retinoid X receptors (RARs and RXRs), which are nuclear receptors of the steroid/thyroid hormone receptor superfamily. Teleost fish are suitable models to study vertebrate development, such as skeletogenesis. Cell systems capable of in vitro mineralization have been developed for several fish species and may provide new insights into the specific cellular and molecular events related to vitamin A activity in bone, complementary to in vivo studies. This work aims at investigating the in vitro effects of RA (0.5 and 12.5 μM) on proliferation, differentiation and extracellular matrix (ECM) mineralization of two gilthead seabream bone-derived cell lines (VSa13 and VSa16), and at identifying molecular targets of its action through gene expression analysis. RA induced phenotypic changes and cellular proliferation was inhibited in both cell lines in a cell type-dependent manner (36-59% in VSa13 and 17-46% in VSa16 cells). While RA stimulated mineral deposition in VSa13 cell cultures (50-62% stimulation), it inhibited the mineralization of extracellular matrix in VSa16 cells (11-57% inhibition). Expression of hormone receptor genes (rars and rxrs), and extracellular matrix-related genes such as matrix and bone Gla proteins (mgp and bglap), osteopontin (spp1) and type I collagen (col1a1) were differentially regulated upon exposure to RA in proliferating, differentiating and mineralizing cultures of VSa13 and VSa16 cells. Altogether, our results show: (i) RA affects proliferative and mineralogenic activities in two fish skeletal cell types and (ii) that during phenotype transitions, specific RA nuclear receptors and bone-related genes are differentially expressed in a cell type-dependent manner.

Nuclear receptors in bone physiology and diseases

During the last decade, our view on the skeleton as a mere solid physical support structure has been transformed, as bone emerged as a dynamic, constantly remodeling tissue with systemic regulatory functions including those of an endocrine organ. Reflecting this remarkable functional complexity, distinct classes of humoral and intracellular regulatory factors have been shown to control vital processes in the bone. Among these regulators, nuclear receptors (NRs) play fundamental roles in bone development, growth, and maintenance. NRs are DNA-binding transcription factors that act as intracellular transducers of the respective ligand signaling pathways through modulation of expression of specific sets of cognate target genes. Aberrant NR signaling caused by receptor or ligand deficiency may profoundly affect bone health and compromise skeletal functions. Ligand dependency of NR action underlies a major strategy of therapeutic intervention to correct aberrant NR signaling, and significant efforts have been made to design novel synthetic NR ligands with enhanced beneficial properties and reduced potential negative side effects. As an example, estrogen deficiency causes bone loss and leads to development of osteoporosis, the most prevalent skeletal disorder in postmenopausal women. Since administration of natural estrogens for the treatment of osteoporosis often associates with undesirable side effects, several synthetic estrogen receptor ligands have been developed with higher therapeutic efficacy and specificity. This review presents current progress in our understanding of the roles of various nuclear receptor-mediated signaling pathways in bone physiology and disease, and in development of advanced NR ligands for treatment of common skeletal disorders.

RIPPLY3 is a retinoic acid-inducible repressor required for setting the borders of the pre-placodal ectoderm

Retinoic acid signaling is a major component of the neural posteriorizing process in vertebrate development. Here, we identify a new role for the retinoic acid receptor (RAR) in the anterior of the embryo, where RAR regulates Fgf8 expression and formation of the pre-placodal ectoderm (PPE). RARα2 signaling induces key pre-placodal genes and establishes the posterolateral borders of the PPE. RAR signaling upregulates two important genes, Tbx1 and Ripply3, during early PPE development. In the absence of RIPPLY3, TBX1 is required for the expression of Fgf8 and hence, PPE formation. In the presence of RIPPLY3, TBX1 acts as a transcriptional repressor, and functions to restrict the positional expression of Fgf8, a key regulator of PPE gene expression. These results establish a novel role for RAR as a regulator of spatial patterning of the PPE through Tbx1 and RIPPLY3. Moreover, we demonstrate that Ripply3, acting downstream of RAR signaling, is a key player in establishing boundaries in the PPE.

Pannexin 3 is a novel target for Runx2, expressed by osteoblasts and mature growth plate chondrocytes

Pannexins are a class of chordate channel proteins identified by their homology to insect gap junction proteins. The pannexin family consists of three members, Panx1, Panx2, and Panx3, and the role each of these proteins plays in cellular processes is still under investigation. Previous reports of Panx3 expression indicate enrichment in skeletal tissues, so we have further investigated this distribution by surveying the developing mouse embryo with immunofluorescence. High levels of Panx3 were detected in intramembranous craniofacial flat bones, as well as long bones of the appendicular and axial skeleton. This distribution is the result of expression in both osteoblasts and hypertrophic chondrocytes. Furthermore, the Panx3 promoter contains putative binding sites for transcription factors involved in bone formation, and we show that the sequence between bases -275 and -283 is responsive to Runx2 activation. Taken together, our data suggests that Panx3 may serve an important role in bone development, and is a novel target for Runx2-dependent signaling.

Craniosynostosis and multiple skeletal anomalies in humans and zebrafish result from a defect in the localized degradation of retinoic acid

Excess exogenous retinoic acid (RA) has been well documented to have teratogenic effects in the limb and craniofacial skeleton. Malformations that have been observed in this context include craniosynostosis, a common developmental defect of the skull that occurs in 1 in 2500 individuals and results from premature fusion of the cranial sutures. Despite these observations, a physiological role for RA during suture formation has not been demonstrated. Here, we present evidence that genetically based alterations in RA signaling interfere with human development. We have identified human null and hypomorphic mutations in the gene encoding the RA-degrading enzyme CYP26B1 that lead to skeletal and craniofacial anomalies, including fusions of long bones, calvarial bone hypoplasia, and craniosynostosis. Analyses of murine embryos exposed to a chemical inhibitor of Cyp26 enzymes and zebrafish lines with mutations in cyp26b1 suggest that the endochondral bone fusions are due to unrestricted chondrogenesis at the presumptive sites of joint formation within cartilaginous templates, whereas craniosynostosis is induced by a defect in osteoblastic differentiation. Ultrastructural analysis, in situ expression studies, and in vitro quantitative RT-PCR experiments of cellular markers of osseous differentiation indicate that the most likely cause for these phenomena is aberrant osteoblast-osteocyte transitioning. This work reveals a physiological role for RA in partitioning skeletal elements and in the maintenance of cranial suture patency.

Retinoic acid induces two osteocalcin isoforms and inhibits markers of osteoclast activity in Atlantic cod (Gadus morhua) ex vivo cultured craniofacial tissues

Nutritional status including vitamin A could explain some of the developmental deformities observed in cultivated teleosts, including Atlantic cod (Gadus morhua). In the present study we aimed to investigate the transcriptional effect of retinoic acid (RA) on bone related genes using Atlantic cod craniofacial explants tissue cultures. Two different osteoblast specific osteocalcin/bone gla protein isoforms were discovered in cod. Transcription of both isoforms was up-regulated following RA treatment of 65 dph cod lower jaw explants. In contrast, transcripts coding for genes related to bone resorption and osteoclast activity, matrix metalloproteinase 9 and cathepsin K were down-regulated following RA treatment. This could be linked to the decreased transcriptional ratio between receptor activator of nuclear factor kappa-B ligand rankl and osteoprotegerin observed in the same tissue samples. RA treatment of juvenile explants had no effect on runt-related transcription factor 2 and osterix mRNA levels. However, osterix was significantly down-regulated in 25 dph cod head explants following RA treatment. In situ hybridizations revealed differential spatial distribution of the two isoforms and the predominant expression of cathepsin K in bone surrounding tissues. The present study indicates that RA causes a shift in the balance between osteoclast activity and osteoblast activity in favor of the latter.

White adipose tissue development in zebrafish is regulated by both developmental time and fish size

Adipocytes are heterogeneous. Whether their differences are attributed to anatomical location or to different developmental origins is unknown. We investigated whether development of different white adipose tissue (WAT) depots in zebrafish occurs simultaneously or whether adipogenesis is influenced by the metabolic demands of growing fish. Like mammals, zebrafish adipocyte morphology is distinctive and adipocytes express cell-specific markers. All adults contain WAT in pancreatic, subcutaneous, visceral, esophageal, mandibular, cranial, and tail-fin depots. Unlike most zebrafish organs that form during embryogenesis, WAT was not found in embryos or young larvae. Instead, WAT was first identified in the pancreas on 12 days postfertilization (dpf), and then in visceral, subcutaneous, and cranial stores in older fish. All 30 dpf fish exceeding 10.6 mm standard length contained the adult repertoire of WAT depots. Pancreatic, esophageal, and subcutaneous WAT appearance correlated with size, not age, as found for other features appearing during postembryonic zebrafish development.

In vivo analysis of white adipose tissue in zebrafish

White adipose tissue (WAT) is the major site of energy storage in bony vertebrates, and also serves central roles in the endocrine regulation of energy balance. The cellular and molecular mechanisms underlying WAT development and physiology are not well understood. This is due in part to difficulties associated with imaging adipose tissues in mammalian model systems, especially during early life stages. The zebrafish (Danio rerio) has recently emerged as a new model system for adipose tissue research, in which WAT can be imaged in a transparent living vertebrate at all life stages. Here we present detailed methods for labeling adipocytes in live zebrafish using fluorescent lipophilic dyes, and for in vivo microscopy of zebrafish WAT.