Identification and character of a LIP transgenic zebrafish model
To create a stable LIP transgenic zebrafish line with ubiquitous LIP expression, transgenic zebrafish were established based on the TetOn system for drug-induced overexpression of LIP (Figure 1A). The Tol2-actb2-rtTAM2-TREP-EGFP-P2A-lip was microinjected into one-cell stage fertilized embryos. The transgenic zebrafish were treated with Dox after 4dpf, and high expression of green fluorescent protein appeared in the LIP transgenic zebrafish line (Figure 1B). These larvae were reared into adulthood, and the genomic DNA from their tail fins was isolated for positive screening of the lip gene. The positive rate of the lip gene in F0 fish was 6.33% (Table S2). These positive F0 fish were individually mated with wild-type fish to obtain their F1 offspring. F2 offspring with more stable fluorescence expression were obtained by self-crossing positive F1 fish. As shown in Figure 1C, PCR detection indicated that the lip gene was integrated into a single chromosomal locus. Expression of LIP and GFP was detected by western blot assay, and transgenic zebrafish had both high levels of GFP and LIP expression (Figure 1D), indicating that the lip gene is stably inherited in the progeny of zebrafish. Fluorescence detection of different generations showed that the F4 generation obtained a stable line (Figure 1E, Table S2).
Overexpression of lip gene inhibits embryonic development of zebrafish
For 4-7dpf wild-type and transgenic zebrafish, the survival rate of LIP-overexpressing zebrafish with Dox treatment was lower in the experimental group than in the LIP-overexpressing zebrafish without Dox treatment (Figure 2A). In contrast, the malformation rate of LIP-overexpressing zebrafish with Dox treatment was higher (Figure 2B), indicating that LIP overexpression is lethal or malformation in zebrafish. In addition, no abnormal morphology or pericardial edema was observed in the wild-type (Figure 2C), and severe morphological and developmental abnormalities were observed in transgenic zebrafish treated with Dox, such as spinal curvature, yolk sac edema, pericardium edema, and unhatched larvae (Figure 2C, D). In the phenotypic analysis, edema was more common than in other malformations (Figure 2D). In addition, HE staining revealed that elevated pathology was also evident in LIP-overexpressing larvae in which edema and spinal curvature were increased (Figure 2E). Furthermore, the growth and development of zebrafish showed that LIP overexpression caused significant growth retardation, including body length (Figure 2F, G), eyeball radius (Figure 2H), and yolk sac area (Figure 2I) compared with transgenic zebrafish without Dox treatment and the wild-type. LIP inhibited the development of body length and eyeball radius of 4dpf zebrafish more than 7dpf zebrafish, but transgenic zebrafish at 7dpf had a greater effect on yolk sac absorption than at 4dpf. Collectively, LIP inhibits zebrafish development, including developmental retardation, teratogenicity, and embryonic lethality.
LIP Overexpression causes massive cell death in the heart
To determine the relationship of LIP with pericardial edema, heart rate was recorded in the whole process of Dox treatment. Our results demonstrated that heart rate fluctuations were not significantly different from 48 to 120 hpf in wild-type zebrafish, but heart rate fluctuations in LIP-overexpressing transgenic zebrafish showed a significant increment at 48 hpf, 72 hpf, 96 hpf (Figure 3A). This indicates that LIP overexpression damages the heart function of zebrafish. Then, cell death was detected by acridine orange (AO) in transgenic zebrafish and wild-type zebrafish with Dox or without Dox treatment (Figure 3B), and cell death particle numbers in transgenic zebrafish with Dox treatment significantly increased in whole embryos compared to transgenic zebrafish without Dox treatment (Figure 3C). In addition, the number of apoptotic particles in the hearts of transgenic zebrafish with Dox treatment was obviously upregulated, especially at 48 hpf, and the heart was fully developed (Figure 3D). These data showed that LIP plays an inhibitory role in the growth and development of zebrafish via the control of heart function.
Effect of LIP overexpression on zebrafish heart function by Ferroptosis pathway
To further explore the role of LIP in embryonic development, we used transcriptome analysis, combined with functional validation, to reveal the potential biological functions of LIP from the perspective of functional genomics. Based on the embryonic development process, RNA-seq was performed with transgenic zebrafish at four relatively discrete stages, including myocardial progenitor cells forming a horseshoe-shaped structure (19 hpf, segmentation period), the heart begins to loop (36 hpf, hatching period), heart fully formed (60 hpf, hatching period), and complete most of its morphogenesis (96 hpf, early larvae). Transcriptome sequencing data are standardized data after verification by fragments per kilobase per million (FPKM) (Figure 4A). Following the screening standard, 1129, 614, 819, and 637 DEGs were obtained in comparisons between the wild-type and transgenic groups at different developmental stages, respectively. Among these DEGs, there were 889 upregulated and 240 downregulated genes, 288 upregulated and 327 downregulated genes, 527 upregulated and 292 downregulated genes, and 340 upregulated and 297 downregulated genes, respectively (Figure 4B). A total of 109 of the same DEGs were obtained in all four groups of wild-type and transgenic zebrafish (Figure S1A). To reveal the dynamic change of LIP across different stages, we performed PCA (Figure S1B) and showed a continuous developmental process from lip gene expression in embryos (19 hpf) to the end of embryonic development (96 hpf). Differential gene expression analysis showed that different groups were enriched in functions that corresponded to features of LIP at different stages (Figure S1C).
Comparing the commonality of the four sets of transcriptomes, we found that LIP not only disturbs lipid metabolism through the PPAR signaling pathway, but also causes cell death, such as the ferroptosis and autophagy pathways. Subsequent Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses suggested a potential impact of LIP protein overexpression on metabolic pathways involving lipid metabolism and cell death (Figure S1, S2). The ferroptosis pathway showed the strongest enrichment, followed by cogent signals that supported the KEGG analysis, including upregulation of ferroptosis marker molecules tf, tfr1a, and acsl4a. We then designed and analyzed the network of molecular interactions of ten major pathways differentially expressed in LIP transgenic zebrafish, which were divided into three groups: mitophagy, lysosome, and ferroptosis. We found that these 10 pathways were related to the ferroptosis pathway (Figure 4C). Damage to mitochondria and lysosomes is a manifestation of ferroptosis, which is a form of cell death caused by lipid metabolism. Furthermore, molecules related to ferroptosis were significantly upregulated in the overexpressed LIP transcriptome (Figure 4D). Therefore, we speculated that LIP triggered ferroptosis during embryonic development in zebrafish. The upregulation of ferroptosis-specific genes was confirmed by qPCR (Figure 4E), which validated the reliability of our transcriptome-wide data.
LIP overexpression in zebrafish larvae causes edema by triggering ferroptosis
Here, we confirmed that LIP overexpression improved the expression of both ACSL4, TP53, and HMGB1 at the protein level by immunoblotting (Figure 5A). In addition, the contents of iron (Figure 5B) and MDA (Figure 5C) in overexpressed LIP zebrafish were higher than those in the control group, while the ratios of GSH and GSH/GSSG (Figure 5D) were lower than those in the control group. The results show that LIP triggers ferroptosis by modulating GSH metabolism, iron metabolism, and lipid peroxidation at multiple levels.
We then quantified the zebrafish phenotype, ranging from P1 = normal phenotype to P4 = very severe edema. More than 60% of LIP-overexpressing zebrafish developed a severe (P3 = 40.33%) or very severe edema phenotype (P4 = 23%, Figure 5E). To investigate whether the edema phenotype was caused by ferroptosis, we used vitamin E as an inhibitor of ferroptosis in transgenic Tg (TRE:EGFP-lip) zebrafish that treat the edema phenotype. In vitamin E-treated zebrafish, a significant increase in survival and edema phenotypes were significantly lower than that in the untreated group, indicating that LIP overexpression in zebrafish larvae causes edema by triggering ferroptosis (Figure 5F, G). To rule out that the observed phenotype was due to VE interacting with LIP, ferric ammonium citrate (FAC) and desferrioxamine (DFO) were used to regulate ferroptosis based on iron levels (Figure 5H). FAC, as a ferroptosis-inducing factor, induces cell death, whereas DFO, as an iron-chelating agent, reduces the iron content in the body. Our results demonstrated that FAC caused edema in zebrafish, while DFO significantly inhibited the edema phenotype caused by FAC and LIP overexpression. Furthermore, we examined cardiomyocyte ultrastructure in vivo and found extensive mitochondrial edema with obvious ferroptosis features (Figure 5I). Therefore, LIP overexpression in zebrafish larvae causes edema by triggering ferroptosis.
Intervention of tfr1a and acsl4a to inhibit ferroptosis induced by LIP overexpression
TFR1 and ACSL4 are markers of ferroptosis and are upregulated in ferroptosis induced by LIP overexpression (18-19). To identify the direct target of LIP, we designed three types of siRNAs for tfr1a and acsl4a. Through qPCR detection of 24 hpf zebrafish embryos, we determined that siRNA tfr1a-799 and siRNA acsl4a-275 worked best (Figure S3C-F), therefore, they were selected for follow-up experiments.
As shown in Figure 6A-E, knocking down two key ferroptosis genes, tfr1a and acsl4a, inhibited the edema phenotype of zebrafish larvae caused by ferroptosis caused by LIP overexpression to a certain extent. Among them, siRNA acsl4a had a significant inhibitory effect (Figure 6F-H), which is characterized by high embryo survival rate, a significant decrease in severe edema phenotype, and a significant decrease in lip transcription level. It should be noted that the mixed injection of the two siRNAs had a more significant inhibitory effect on the edema phenotype. This suggests that ferroptosis caused by LIP overexpression can be inhibited by targeting tfr1a or acsl4a, and acsl4a may be the direct target of LIP. In addition, the mixed injection of two siRNAs also obtained better results, indicating that LIP acts directly on acsl4a and indirectly on tfr1a (Figure 6I-L).
To determine the regulatory effect of ACSL4 on ferroptosis triggered by LIP, ROSI was selected as a small-molecule inhibitor of acsl4a (20). Compared with the control group, the expression level of the acsl4a gene was inhibited in the ROSI group (Figure 7B). The results of qPCR showed that the lip was also significantly downregulated and positively correlated with acsl4a. At the same time, inhibition of acsl4a resulted in the downregulation of lip, indicating that LIP acts directly on acsl4a (Figure 7C). Meanwhile, the abnormal edema phenotype of zebrafish embryos caused by LIP overexpression decreased significantly. The results showed that inhibiting the expression of acsl4a in zebrafish embryos had a protective effect on ferroptosis caused by overexpression of LIP, and could reduce the edema phenotype caused by overexpression of LIP (Figure 7D, E). Using dihydroartemisinin (DHA) as an inducer of ferroptosis, similar experimental results were obtained (Figure S4).