Transcriptomics provides mechanistic indicators of fluoride toxicology on endochondral ossification in the hind limb of Bufo gargarizans

Decrypting the skeletal toxicity of vertebrates caused by environmental pollutants from an evolutionary perspective: From fish to mammals

The rapid development of modern society has led to an increasing severity in the generation of new pollutants and the significant emission of old pollutants, exerting considerable pressure on the ecological environment and posing a serious threat to both biological survival and human health. The skeletal system, as a vital supportive structure and functional unit in organisms, is pivotal in maintaining body shape, safeguarding internal organs, storing minerals, and facilitating blood cell production. Although previous studies have uncovered the toxic effects of pollutants on vertebrate skeletal systems, there is a lack of comprehensive literature reviews in this field. Hence, this paper systematically summarizes the toxic effects and mechanisms of environmental pollutants on the skeletons of vertebrates based on the evolutionary context from fish to mammals. Our findings reveal that current research mainly focuses on fish and mammals, and the identified impact mechanisms mainly involve the regulation of bone signaling pathways, oxidative stress response, endocrine system disorders, and immune system dysfunction. This study aims to provide a comprehensive and systematic understanding of research on skeletal toxicity, while also promoting further research and development in related fields.

Effects of perchlorate and exogenous T4 exposures on body condition and endochondral ossification of Rana chensinensis tadpoles

Perchlorate, as an endocrine-disrupting chemical (EDC), is largely produced and used in the military, fireworks, fertilizers, and other industries and widely exists in water. Although perchlorate is known to destroy the normal function of thyroid hormones (THs) in amphibians and interfere with their growth and development, the impact of TH levels caused by sodium perchlorate (NaClO4) on endochondral ossification and skeletal development is poorly investigated, and the underlying molecular mechanism has not been clarified. The present study aimed to explore the potential effects of NaClO4 and exogenous thyroxine (T4) on the skeletal development of Rana chensinensis tadpoles and elucidate the related molecular mechanisms. Our results showed that histological changes occurred to the femur and tibia-fibula of tadpoles raised in 250 μg/L NaClO4 and 5 μg/L exogenous T4, and the length of their hindlimbs was significantly reduced. In addition, exogenous T4 exposure significantly interfered with the expression of Dio3, TRβ, MMP9, MMP13, and Runx2, inhibiting the endochondral ossification process. Therefore, we provide robust evidence that the changes in TH levels caused by NaClO4 and exogenous T4 will adversely affect the endochondral ossification and skeletal development of R. chensinensis tadpoles.

Copper-mediated shifts in transcriptomic responses of intestines in Bufo gargarizans tadpoles to lead stress

The differential transcriptomic responses of intestines in Bufo gargarizans tadpoles to Pb alone or in the presence of Cu were evaluated. Tadpoles were exposed to 30 μg/L Pb individually and in combination with Cu at 16 or 64 μg/L from Gosner stage (Gs) 26 to Gs 38. After de novo assembly, 105,107 unigenes were generated. Compared to the control group, 7387, 6937, and 11139 differentially expressed genes (DEGs) were identified in the treatment of Pb + Cu0, Pb + Cu16, and Pb + Cu64, respectively. In addition, functional annotation and enrichment analysis of DEGs revealed substantial transcriptional reprogramming of diverse molecular and biological pathways were induced in all heavy metal treatments. The relative expression levels of genes associated with intestinal epithelial barrier and bile acids (BAs) metabolism, such as mucin2, claudin5, ZO-1, Asbt, and Ost-β, were validated by qPCR. This study demonstrated that Pb exposure induced transcriptional responses in tadpoles, and the responses could be modulated by Cu.

Effects of nitrite exposure on metamorphosis and skeletal development of Bufo gargarizans

Nitrite, as a part of nitrogen cycle, is one of the most common toxic compounds in aquatic ecosystems. Since skeletal development is an essential process during amphibian metamorphosis, exposure of larval amphibians to nitrite might disrupt skeletal development. To evaluate whether nitrite affects skeletal development of amphibian larvae, Bufo gargarizans larvae at Gs26 were exposed to 10, 100, 500 and 1000 μg/L nitrite-nitrogen (NO₂-N) in the present study. The metamorphosis rate, body weight, body length, forelimb length and hindlimb length of B. gargarizans exposed to NO₂-N were decreased. The microscopic structures of thyroid gland were altered under NO₂-N exposure at Gs42. The skeletal lengths of the humerus, femur and fibulare of tadpole at Gs42 were significantly reduced under 100, 500 and 1000 μg/L NO₂-N treatment groups, and the lengths of humerus, tibia-fibula and tibiale of tadpole at Gs46 were significantly reduced under 1000 μg/L NO₂-N treatment groups. In addition, the expression levels of thyroid hormone (TH) and endochondral ossification-related genes of tadpoles at Gs42 and Gs46 were tested by qRT-PCR. Overall, NO₂-N exposure could affect the expressions of these genes and then may influence the activity and function of thyroid gland, further disturbing the amphibian metamorphosis and skeletal development of amphibian larvae.

Inhibition of Metamorphosis, Thyroid Gland, and Skeletal Ossification Induced by Hexavalent Chromium in Bufo gargarizans Larvae

Hexavalent chromium (Cr [VI]) is one of the major detrimental heavy metal pollutants. In the present study, Bufo gargarizans were exposed to 0, 52, 104, 208, and 416 μg/L Cr (VI) from Gosner stage 2 until metamorphosis; and growth, development, and histological characteristics of the thyroid gland and skeletal ossification were examined. The results demonstrated that the survival rate of larvae exposed to Cr (VI) was not different from that measured in animals from the control group. However, high levels of Cr (VI) (104, 208, and 416 μg/L) were associated with significantly delayed growth and development. The suppression of skeletal ossification was observed at high Cr (VI) levels. Besides, histological alterations of the thyroid gland, such as follicular cell hyperplasia, colloid depletion, and peripheral colloid vacuolation, were found in 52 to 416 μg/L Cr (VI) treatments. The results of the present study highlight reductions in growth and development as well as percent metamorphosis and skeletal ossification due to histological alteration of the thyroid gland during exposure to Cr (VI) in B. gargarizans larvae. The present investigation could provide a basis for understanding the detrimental effects of Cr (VI) in amphibian larvae. Environ Toxicol Chem 2021;40:2474-2483. © 2021 SETAC.

Altered temperature affect body condition and endochondral ossification in Bufo gargarizans tadpoles

Bufo gargarizans is one kind of economic animals with higher medicinal value in China. In this study, B. gargarizans (Bufo gargarizans) tadpoles were reared at three different water temperature (15, 22 and 29 °C) from Gosner stages 28-46. We investigated the effects of temperature on growth, development, survival, metamorphic duration, size and skeletal ossification at Gosner stage 40, 42, and 46, as well as thyroid tissue reached metamorphic climax (Gs42). Besides, we examined the transcription levels of endochondral ossification-related genes in hind limb at metamorphic climax (Gs42). Our results showed that the growth and development of tadpoles conform to the temperature-size rule (TSR). While warm temperature resulted in the decrease in body size and hind limb length, and shorten larval period, cold temperature led to increase in body size and hind limb length but prolonged larval period. Histological examinations revealed that warm and cold temperatures caused damage to thyroid tissue. Also, warm and cold temperatures inhibited the degree of ossification with the double staining methodology. Additionally, the real-time PCR results suggested that warm and cold temperatures significantly up-regulated Runx2, VEGF and VEGFR mRNA levels, and down-regulated TRβ, MMP9, MMP13 and Runx3 mRNA levels. The up-regulation of Dio2 level and down-regulation of Dio3 level were observed in warm temperature. TRα mRNA level was significantly increased in warm temperature, but decreased in cold temperature. Collectively, these observations demonstrated that warm and cold temperatures affected endochondral ossification in B. gargarizans tadpoles, which might influence their capacity to terrestrial locomotion.

Long-term exposure to the fluoride blocks the development of chondrocytes in the ducks: The molecular mechanism of fluoride regulating autophagy and apoptosis

Long-term exposure to excessive fluoride causes chronic damage in the body tissues and could lead to skeletal and dental fluorosis. Cartilage damage caused by excessive fluoride intake has gained wide attention, but how fluoride accumulation blocks the development of chondrocytes is still unclear. Here, we report a negative correlation between the length and growth plate width after NaF treatments via apoptosis and autophagy, with shrinkage of cells, nuclear retraction, dissolution of chondrocytes. Whereas, fluoride exposure had no significant effect on the number and distribution of the osteoclasts which were well aligned. More importantly, fluoride exposure induced apoptosis of tibial bone through CytC/Bcl-2/P53 pathways via targeting Caspase3, Caspase9, Bak1, and Bax expressions. Meanwhile, the Beclin1, mTOR, Pakin, Pink, and p62 were elevated in NaF treatment group, which indicated that long-term excessive fluoride triggered the autophagy in the tibial bone and produced the chondrocyte injury. Altogether, fluoride exposure induced the chondrocyte injury by regulating the autophagy and apoptosis in the tibial bone of ducks, which demonstrates that fluoride exposure is a risk factor for cartilage development. These findings revealed the essential role of CytC/Bcl-2/P53 pathways in long-term exposure to fluoride pollution and block the development of chondrocytes in ducks, and CytC/Bcl-2/P53 can be targeted to prevent fluoride induced chondrocyte injury.

Effects of fluoride on PIWI-interacting RNA expression profiling in testis of mice

Excessive fluoride intake can damage testis by breaking the integrity of sperm DNA and changing the expression profiles of testicular mRNAs and microRNAs. However, the effects of fluoride on the expression of PIWI-interacting RNAs (piRNAs) in mouse testes have not been reported. In this study, we determined the effect of fluoride on PIWI-interacting RNA expression profiling in testis of mice, using deep-sequencing technology. Compared to the control, 50 mg/L sodium fluoride (NaF) exposure led to a reduced testicular organ coefficient, semen quality, and testosterone level, and altered the testicular microstructure. Furthermore, NaF exposure also changed the expression of 28 piRNAs that regulate 182 target genes in mouse testes. In mice given water containing 50 mg/L NaF, the following four pathways were enriched and overexpressed: lysosomal, Jak-STAT, chemokine, and ubiquitin-mediated proteolysis. Among the piRNAs affecting the lysosomal pathway, piR-mmu-1277316, piR-mmu-8060747, and piR-mmu-1566415 levels were increased. We also observed increased levels of the following target gene mRNAs in lysosomal pathwa in the 50 mg/L NaF-treated group: Gga2, Ap4e1, Gla, and Ap1s3. These findings are in line with the results of piRNA-sequencing and suggest that piRNAs in the testis could be potential biomarkers for fluoride reproductive toxicity.

Fluoride exposure changed the expression of microRNAs in gills of male zebrafish (Danio rerio)

Fluoride has been found to cause detrimental effects on fish gills. Despite essential roles in various metabolism activities, whether and how miRNAs participate in the course of toxicity caused by fluoride in gills is still unclear. In this study, male zebrafish were exposed to 0, 20, 40 mg/L fluoride for 60 days to study the underlying osmotic regulatory mechanism by determining the influences of fluoride on the miRNAs and regulated genes in gills. mRNAs were isolated from the gills and the expression profiles were analyzed by using Illumina Hiseq 2500 platforms. Expressions of 7 differentially miRNAs and some related-genes in gills were validated by qRT-PCR. The results showed that miRNAs expressions were notably altered by fluoride. A total of 584 and 327 miRNAs were remarkably changed after 20 and 40 mg/L fluoride exposure, of which 322 were increased and 262 were decreased in 20 mg/L fluoride group, whereas 219 were elevated and 108 were reduced in 40 mg/L fluoride group. The differentially expressive miRNAs confirmed by qRT-PCR were consistent with micro-assay data. Cluster of Orthologous Groups of proteins (COG) function classification showed that the target genes of differentially expressive miRNAs are mainly related to signal transduction mechanisms, replication, transcription, inorganic ion transport and metabolism, repair and recombination, and energy formation and transformation. In addition, fluoride disturbed the expressions of target genes involved in the osmoregulation of the gill in the fluoride-exposed zebrafish, such as the increased expressions of OSTF1 and the decreased expressions of Na⁺-K⁺-ATPase, CFTR, and AQP-3, which provides a possibility that miRNAs regulation induced by fluoride has an effects on osmotic regulation, providing new hints to the osmotic regulatory mechanism of the toxicity caused by fluoride in zebrafish, and distinguishes new biomarkers of miRNAs for fluoride toxicity.

Cadmium induced skeletal underdevelopment, liver cell apoptosis and hepatic energy metabolism disorder in Bufo gargarizans larvae by disrupting thyroid hormone signaling

Cadmium (Cd) is hazardous to human health and it is also highly detrimental to amphibian life. In this study, Bufo gargarizans larvae were exposed to environmentally relevant Cd concentrations of 5, 100 and 200 μg L^-1 from Gosner stage (Gs) 26 to Gs 42 of metamorphic climax about 6 weeks. The results showed thyroid structural injuries and thyroid signaling disruption were induced by high Cd exposure (100 and 200 μg L^-1). Moreover, tadpole skeleton including whole body, vertebrata, forelimb and hindlimb was developmentally delayed by high Cd exposure through downregulating the mRNA expressions of genes involved with skeletal ossification and growth pathway. Moreover, liver histopathological injuries were caused by high Cd exposure featured by hepatocytes malformation, nuclear degeneration and increasing melanomacrophage centers. Meanwhile, liver apoptosis rate showed on the rise in a dose-dependent way and Cd stimulated liver apoptosis by upregulating mRNA expressions of genes related to extrinsic and intrinsic apoptosis pathways. Furthermore, high Cd caused hepatic glucometabolism disorder by decreasing the genetic expressions associated with glycolysis and mitochondrial oxidative phosphorylation. In addition, liver lipid metabolism was disrupted by high Cd exposure through downregulating mRNA levels of genes related to fatty oxidation and upregulating mRNA levels of genes related to fatty acid synthesis. We suggested that Cd did great harm to tadpole health by disturbing thyroid function, skeletal growth, liver cell apoptosis signaling and hepatic energy metabolism pathway.

Involvement of Bmal1 and circadian clock signaling in chondrogenic differentiation of ATDC5 cells by fluoride

Skeletal fluorosis causes growth plate impairment and growth retardation during bone development. However, the mechanism of how fluoride impairs chondrocyte is unclear. To explore the effect of fluoride on chondrocyte differentiation and the regulation of circadian clock signaling pathway during chondrogenesis, we treated ATDC5 cells with fluoride and carried out a series of experiments. 10^-3 M fluoride inhibited cell viability and significantly decreased the expression of Sox9 and Col2a1 (P < 0.05). Fluoride inhibited proteoglycan synthesis and decreased significantly the expression of Aggrecan, Ihh and Col10a1 (P < 0.05). Meanwhile, fluoride significantly inhibited the expression of Bmal1 and disrupted circadian clock signaling pathway (P < 0.05). Furthermore, fluoride disrupted the time-dependent expression of circadian clock molecules and stage-specific differentiation markers. Overexpression of Bmal1 by lentivirus reversed the adverse effects of fluoride on chondrogenesis. These results suggested that fluoride inhibited chondrocyte viability and delayed chondrocyte differentiation. Fluoride delayed chondrogenesis partly via interfering with Bmal1 and circadian clock signaling pathway. Nevertheless, the specific mechanism of circadian clock in fluoride-induced cartilage damage needs to be further studied.

Fluoride impairs ovary development by affecting oogenesis and inducing oxidative stress and apoptosis in female zebrafish (Danio rerio)

Previous studies have shown that waterborne fluoride exposure has adverse effects on the reproductive system of zebrafish. However, the underlying toxic mechanisms were still not clear. In the present study, female zebrafish were exposed to different concentrations of 0.787 (Control), 18.599, 36.832 mg/L of fluoride for 30 d and 60 d, and the effects of different doses of fluoride on ovary development, reproductive hormones, oogenesis, ROS content, antioxidant levels, and the expression of apoptosis-related genes and proteins in the ovaries of female zebrafish were analyzed. The results showed that ovarian weight and GSI were significantly decreased, FSH, LH and VTG levels were significantly reduced, the transcriptional profiles of oogenesis-related genes (tgfβ1, bmp15, gdf9, mprα, mprβ, ptg2β) were remarkably altered, ROS levels was notably increased, the SOD, CAT, GPx activities and GSH content as well as their mRNA expressions were significantly decreased, MDA content was remarkably increased, the expressions of apoptosis-related genes and proteins (caspase3, caspase8, caspase9, Fas-L, Cytochrome C, Bax and Bcl-2) were significantly changed, the ratio of Bax/Bcl-2 protein levels were notably increased. Taken together, this study demonstrated that fluoride exposure significantly affected ovarian development, decreased the reproductive hormones, affected oogenesis, induced oxidative stress, caused apoptosis through both extrinsic and intrinsic pathways in ovary of zebrafish. Indicating that oogenesis, oxidative stress, and apoptosis were responsible for the impairment of ovarian development.

Effects of octylphenol exposure on the lipid metabolism and microbiome of the intestinal tract of Rana chensinensis tadpole by RNAseq and 16s amplicon sequencing

Octylphenol (OP) is a widely distributed endocrine disrupting chemical (EDC), and can be commonly found in various and diverse environmental media. Previous studies have reported that OP exposure could cause many adverse effects on aquatic animals. However, knowledge concerning the impact of OP on lipid metabolism in amphibians was still limited. In our study, Rana chensinensis tadpoles were exposed to different OP concentrations (0, 10^-8, 10^-7 and 10^-6 mol/L) from the Gosner stage (Gs) 25-38. The RNA-seq analysis of tadpole intestines was explored by RNA-seq, and six differentially expressed genes (DEGs) related to the fat digestion and absorption were validated by RT-qPCR. Moreover, we used 16s amplicon sequencing to evaluate effects of OP on intestinal microbiome in tadpoles, further determining the variations of lipid metabolism. Our results revealed that OP exposure influenced gene expression levels related to fat digestion and absorption and led to alteration of structure and composition of intestinal microbiome. At the phylum level, the Firmicutes/Bacteroidetes ratio was gradually decreased in OP exposure groups, which disrupted lipid metabolism. According to the results of intestinal microbial functional prediction, OP exposure interfered with metabolic function and increased risk of disease. These data provide us with powerful resources to assess the effects of OP on lipid metabolism by integrating RNAseq and 16s amplicon sequencing analysis of intestinal tract and intestinal microbiome.

Probing the effects of hexavalent chromium exposure on histology and fatty acid metabolism in liver of Bufo gargarizans tadpoles

Hexavalent chromium is one of the major detrimental heavy metal pollutants. B. gargarizans tadpoles were treated with different concentrations of Cr⁶⁺ (0, 13, 52, 104, 208, and 416 μg Cr⁶⁺ L^-1) from Gs 2 to Gs 42. The effect of Cr⁶⁺ on histopathological alterations and transcript levels of fatty acid metabolism-related genes as well as fatty acids composition and content in liver were examined. Histopathological changes were observed in liver at 52, 104, 208, and 416 μg Cr⁶⁺ L^-1. Moreover, RT-qPCR analyses showed the downregulated mRNA levels of the genes related to fatty acid synthesis (SCD, MECR, TECR and ELOVL1) and fatty acid β-oxidation (ACOT1, PPT1, HADH and ACAA2) at 416 μg Cr⁶⁺ L^-1. However, the mRNA expression of fatty acid β-oxidation-related genes (ECHS1, HADHA and ACAA1) were significantly upregulated at 13, 52, 104, 208 and 416 μg Cr⁶⁺ L^-1. In situ hybridization revealed BSEP was expressed in hepatocyte nucleus and plasma membrane, and HSD17B12 was abundantly expressed in the plasma membrane. The HSD17B12 mRNA levels were significantly upregulated in tadpoles exposed to all Cr⁶⁺ treatment groups, while the BSEP mRNA levels were downregulated at 104, 208 and 416 μg Cr⁶⁺ L^-1 groups compared to control. In addition, an increase in polyunsaturated fatty acids and a decrease in monounsaturated fatty acids were found in 52, 104 and 416 μg Cr⁶⁺ L^-1 groups. Overall, chronic exposure to Cr⁶⁺ may suppress fatty acid synthesis, disturb fatty acid β-oxidation, aggravate disorders of hepatic function and induce hepatic impairment in B. gargarizans tadpoles.

Fluoride regulates the expression of extracellular matrix HSPG and related signaling pathways FGFR3 and Ihh/PTHrP feedback loop during endochondral ossification

Skeletal fluorosis causes growth plate impairment and growth retardation during bone development. Longitudinal bone development is accomplished by endochondral ossification in growth plate. However, the mechanism of fluoride impairs growth plate is unclear. To explore the effect of fluoride on various glycosaminoglycans (GAGs) and related signaling pathways in growth plate during endochondral ossification, SD rats and ATDC5 cells were treated with fluoride and carried out a series of experiments. We found that the expression of heparan sulfate (HS), a kind of GAGs in extracellular matrix, was significantly increased in the growth plate of fluoride-treated rats compared with control rats. Furthermore, the expression of HS synthetic enzyme exostosin 1 (EXT1) and glypican 6 (GPC6), a core protein of HS proteoglycan (HSPG), were significantly increased in fluoride-treated ATDC5 cells compared with control cells (P < 0.05). The expression of related molecules including fibroblast growth factor receptor-3 (FGFR3), signal transducer and activator of transcription 1 (STAT1) and parathyroid hormone-related protein (PTHrP) were significantly increased in the fluoride-treated groups compared with control groups (P < 0.05), and there was significantly decreased in the expression of Indian hedgehog (Ihh) in fluoride-treated groups compared with control groups (P < 0.05). Our data suggested that fluoride increased the content of HSPG in extracellular matrix by promoting the expression of EXT1 and GPC6. Fluoride also activated FGFR3 signaling pathway, inhibited Ihh/PTHrP feedback loop and inhibited endochondral ossification. Nevertheless, the regulation of fluoride on HSPG and related pathways FGFR3 and Ihh/PTHrP feedback loop during endochondral ossification needs to be further studied.

Effects of fluoride on the histology, lipid metabolism, and bile acid secretion in liver of Bufo gargarizans larvae

In our study, Bufo gargarizans (B. gargarizans) larvae were exposed to control, 0.5, 5, 10 and 50 mg/L of NaF from Gs 26 to 42. At Gs 42, we evaluated the changes of liver histology and the mRNA levels of target genes in liver. In addition, we also examined the composition and content of fatty acids. Histological analysis revealed that fluoride caused liver injury, such as the increase of number of melanomacrophage centres, atrophy of nucleus, dilation of bile canaliculus, and decrease of quantity, degradation and deposition of lipid droplets. The results of RT-qPCR indicated that exposure to 5, 10 and 50 mg/L of NaF significantly decreased the transcript levels of genes related to fatty acid synthesis (FASN, FAE, MECR, KAR and TECR) in liver. Besides, mRNA expression of genes involved in fatty acid β-oxidation (ECHS1, HADHA, SCP2, CPT2, ACAA1 and ACAA2) and oxidative stress (SOD, GPx, MICU1 and HSP90) was significantly downregulated in 0.5, 5, 10 and 50 mg/L of NaF treatment groups. Also, in the relative expression of genes associated with synthesis and secretion of bile acid, BSEP significantly increased at 0.5, 5 and 50 mg/L of NaF while HSD3B7 significantly reduced in 0.5, 5, 10 and 50 mg/L of NaF. Finally, the fatty acid extraction and GC-MS analysis showed that the content of saturated fatty acids (SFAs) was decreased and the content of polyunsaturated fatty acids (PUFAs) was increased in all fluoride treatment groups. Taken together, the present results indicated that fluoride-induced the histological alterations of liver might be linked to the disorder of lipid metabolism, oxidative damage.