Low Temperature Mitigates Cardia Bifida in Zebrafish Embryos

<div><p>The coordinated migration of bilateral cardiomyocytes and the formation of the cardiac cone are essential for heart tube formation. We investigated gene regulatory mechanisms involved in myocardial migration, and regulation of the timing of cardiac cone formation in zebrafish embryos. Through screening of zebrafish treated with ethylnitrosourea, we isolated a mutant with a hypomorphic allele of <i>mil</i> (<i>s1pr2</i>)/<i>edg5</i>, called <i>s1pr2^as10</i> (<i>as10</i>). Mutant embryos with this allele expressed less <i>mil</i>/<i>edg5</i> mRNA and exhibited cardia bifida prior to 28 hours post-fertilization. Although the bilateral hearts of the mutants gradually fused together, the resulting formation of two atria and one tightly-packed ventricle failed to support normal blood circulation. Interestingly, cardia bifida of <i>s1pr2^as10</i> embryos could be rescued and normal circulation could be restored by incubating the embryos at low temperature (22.5°C). Rescue was also observed in <i>gata5</i> and <i>bon</i> cardia bifida morphants raised at 22.5°C. The use of DNA microarrays, digital gene expression analyses, loss-of-function, as well as mRNA and protein rescue experiments, revealed that low temperature mitigates cardia bifida by regulating the expression of genes encoding components of the extracellular matrix (<i>fibronectin 1</i>, <i>tenascin-c</i>, <i>tenascin-w</i>). Furthermore, the addition of N-acetyl cysteine (NAC), a reactive oxygen species (ROS) scavenger, significantly decreased the effect of low temperature on mitigating cardia bifida in <i>s1pr2^as10</i> embryos. Our study reveals that temperature coordinates the development of the heart tube and somitogenesis, and that extracellular matrix genes (<i>fibronectin 1</i>, <i>tenascin-c</i> and <i>tenascin-w</i>) are involved.</p></div

Expression of two <i>tenascin</i> genes is affected by temperature.

<p><i>In situ</i> hybridization against <i>tenascin-c</i> (<i>tnc</i>) (A-H) and <i>tenascin-w</i> (<i>tnw</i>) (L-O) in 22-ss wild-type (WT) and <i>s1pr2^as10</i> (<i>as10</i>) mutant embryos raised at 28.5°C or 22.5°C. Expression of <i>tnc</i> increased in the pharyngeal arches (white arrows in B and D) and the cells between the brain and eyes of embryos raised at 22.5°C, but was unaffected in the trunk somite region (E-H). (I) qRT-PCR revealed a trend towards increased expression of <i>tnc</i> mRNA in <i>s1pr2^as10</i> mutant embryos raised at 22.5°C. (J) Knockdown of <i>tnc</i> expression with <i>tnc</i>-MO1 in <i>s1pr2^as10</i> mutants increased the percentage of 26-ss embryos raised at 22.5°C with the Class III cardia bifida phenotype. (K) Injection of <i>s1pr2^as10</i> mutant embryos raised at 28.5°C with human TNC protein partially rescued cardia bifida phenotypes at 24 hpf. (L-O) Decreased <i>tnw</i> expression in all tissues, including scattered epidermal cells in the head (black arrows in M and O) was observed in both 22-ss WT and <i>s1pr2^as10</i> mutants raised at 22.5°C. (P) qRT-PCR was used to confirm that expression of <i>tnw</i> mRNA is significantly reduced in 22-ss WT and <i>s1pr2^as10</i> mutant embryos raised at 22.5°C. (Q) Knockdown of <i>tnw</i> with <i>tnw</i>-MO1 in <i>s1pr2^as10</i> mutant embryos raised at 28.5°C partially rescued cardia bifida phenotypes at 24 hpf. (R) Injection of <i>s1pr2^as10</i> mutant embryos raised at 22.5°C with 100 pg <i>tnw</i> mRNA increased the percentage of 26-ss embryos with the Class III cardia bifida phenotype. Scale bars  = 100 µm. The error bars indicate the standard error. Statistical significance was determined using Student’s <i>t</i>-test. * indicates <i>p</i><0.05, ** indicates <i>p</i><0.01, *** indicates <i>p</i><0.001.</p

Reactive oxygen species (ROS) mediate mitigation of cardia bifida in zebrafish embryos incubated at low temperature.

<p>(A) <i>s1pr2^as10</i> mutant embryos raised at 22.5°C were treated with 50 or 150 µM N-acetyl cysteine (NAC) at the tailbud stage, and were then incubated at 22.5°C. Different degrees of myocardial migration defects were observed at the 26 ss. Class I (single heart tube), Class II (cardiomyocytes in close proximity but not in contact), and Class III (two separate hearts). (B) A proposed model showing how low temperature mitigates cardia bifida in zebrafish embryos. Statistical significance was determined using Student’s <i>t</i>-test. * indicates <i>p</i><0.05.</p

Expression of <i>fibronectin 1</i> is affected by temperature.

<p>(A-H) <i>In situ</i> hybridization against <i>fibronectin 1</i> (<i>fn1</i>) in 22-ss wild-type (WT) (A, B, E, F) and <i>s1pr2^as10</i> (<i>as10</i>) mutant (C, D, G, H) embryos raised at 28.5 or 22.5°C. Expression of <i>fn1</i> mRNA increased at the midline region (white arrows in B and D) and bilateral LPM (black arrows in B and D) of both WT and <i>s1pr2^as10</i> mutant embryos raised at 22.5°C, as compared to those raised at 28.5°C (A and C). Expression of <i>fn1</i> at the tail bud or yolk regions was not affected (E-H). (I) qRT-PCR revealed a trend towards increased expression of <i>fn1</i> mRNA in <i>s1pr2^as10</i> mutant embryos raised at 22.5°C. The error bars indicate the standard error. (J) Knockdown of <i>fn1</i> expression in <i>s1pr2^as10</i> mutants increased the percentage of 26-ss embryos raised at 22.5°C with the Class III cardia bifida phenotype. (K) Injection of human fibronectin protein into <i>s1pr2^as10</i> mutant embryos raised at 28.5°C partially rescued the cardia bifida phenotypes at 24 hpf. (K’) The red dye indicates the protein injection region at 14–16 hpf. Scale bars  = 100 µm. Statistical significance was determined using Student’s <i>t</i>-test. * indicates <i>p</i><0.05, *** indicates <i>p</i><0.001.</p

Gene ontology (GO) and pathway analyses for differentially expressed genes identified by DNA microarray analysis.

<p>GO and pathway analyses were conducted on different groups of genes using two strategies. (A) Genes with a greater than 1.3-fold change in expression between embryos raised at 28.5 and 22.5°C were used in strategy 1. GO analysis was performed using the zebrafish database. (B) Genes with a greater than 1.5-fold change in expression between embryos raised at 28.5 and 22.5°C were used in strategy 2. GO analysis was performed using the human database. The GO and pathway analyses were performed on genes from both Groups I and II or on genes from Groups III, IV and V. Box and Whisker plots for each group indicate the median (line within the box), 25^th and 75^th percentiles (top and bottom of the box), and 10^th and 90^th percentiles (Whiskers/error bars). The P-value from GO and pathway analyses are smaller than 0.1.</p

The phenotype of the <i>s1pr2^as10</i> mutant.

<p>(A-H) Lateral views of wild-type (WT) (A, C, E and G) and <i>s1pr2^as10</i> mutants (B, D, F and H). Pericardial edema (B’), small eyes, and tail blisters (B’) were observed in 72-hpf <i>s1pr2^as10</i> mutant embryos (B). Two separate hearts (white arrows, D) were detected at 24 hpf, whereas a fused heart (white arrow, F) was observed at 48 hpf in <i>s1pr2^as10</i> mutant embryos. Paraffin sectioning with haematoxylin and eosin staining revealed the presence of two atria and one ventricle in <i>s1pr2^as10</i> mutant embryos at 72 hpf (H). (I-M) Ventral views of wild-type (WT) (I), <i>s1pr2^as10</i> (J), <i>mil</i> (K), and the intercross mutant progeny of <i>s1pr2^as10</i> and <i>mil</i> mutants (L and M) at 24 hpf are shown. The arrows indicate the positions of the hearts. (N) Cardiomyocytes were labeled via <i>cmlc2</i> staining. Two representative <i>s1pr2^as10</i> mutant embryos are shown. Both contacting and separated cardiomyocytes were detected in the <i>s1pr2^as10</i> mutant embryos from the 22 ss to 28 hpf. (O) Percentages of WT and <i>s1pr2^as10</i> mutants containing contacting myocardia at different developmental stages. Scale bar  = 100 µm. The error bars indicate the standard error.</p

Low temperature shifts the timing of formation of the cardiac cone.

<p>(A) Lateral views of 22-ss wild-type (WT) and <i>s1pr2^as10</i> mutant embryos raised at 28.5 or 22.5°C. (B) Time-lapse analyses of the medial migration of bilateral cardiomyocytes during the ∼17–30-ss in <i>Tg</i>(<i>cmlc2:EGFP, cmlc2:H2AFZmCherry</i>)<i>^cy13</i> transgenic fish (WT) or <i>s1pr2^as10</i> mutant embryos raised at 28.5 or 22.5°C. Five WT or <i>s1pr2^as10</i> mutant embryos were analyzed for each stage. The images outlined in red indicate the stage at which cardiac cone formation took place under each condition. (C) <i>In situ</i> hybridization of 19-ss wild-type (WT) or 26-ss <i>s1pr2^as10</i> mutant embryos raised at 28.5°C or 22.5°C with a <i>cmlc2</i> RNA probe. The number of embryos displaying <i>cmlc2</i> staining/the total number of embryos analyzed is shown for each panel. (D) Different degrees of myocardial migration defects were observed in <i>gata5</i> and <i>bon</i> morphants at 24 hpf. Class I (single heart tube), Class II (cardiomyocytes in close proximity but not in contact), and Class III (two separate hearts) defects are shown. (E) Percentages of each class of myocardial migration defects in 10 ng <i>gata5</i>-MO or <i>bon</i>-MO-injected embryos raised at 28.5 or 22.5°C. Scale bar  = 100 µm. Statistical significance was determined using Student’s <i>t</i>-test. * indicates <i>p</i><0.05.</p