Many organs in vertebrates are left-right asymmetrical located. For example, liver is at the right side and stomach is at the left side in human. Fibroblast growth factor (Fgf) signaling is important for left-right asymmetry. To investigate the roles of Fgfr2 signaling in zebrafish left-right asymmetry, we used splicing blocking morpholinos to specifically block the splicing of fgfr2b and fgfr2c variants, respectively. We found that the relative position of the liver and the pancreas were disrupted in fgfr2c morphants. Furthermore, the left-right asymmetry of the heart became random. Expression pattern of the laterality controlling genes, spaw and pitx2c, also became random in the morphants. Furthermore, lefty1 was not expressed in the posterior notochord, indicating that the molecular midline barrier had been disrupted. It was also not expressed in the brain diencephalon. Kupffer's vesicle (KV) size became smaller in fgfr2c morphants. Furthermore, KV cilia were shorter in fgfr2c morphants. We conclude that the fgfr2c isoform plays an important role in the left-right asymmetry during zebrafish development.
Citation: Liu D-W, Hsu C-H, Tsai S-M, Hsiao C-D, Wang W-P (2011) A Variant of Fibroblast Growth Factor Receptor 2 (Fgfr2) Regulates Left-Right Asymmetry in Zebrafish. PLoS ONE 6(7): e21793. doi:10.1371/journal.pone.0021793
Editor: Hector Escriva, Laboratoire Arago, France
Received: November 24, 2010; Accepted: June 13, 2011; Published: July 1, 2011
Copyright: © 2011 Liu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by grants from National Science Council (NSC96-2311-B-320-002-MY2) and Tzu Chi University (TCMRC-P-99013-01). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
The bodies of most adult animals have left-right symmetry. However, some organs are not symmetrical, including the heart, liver, spleen, stomach, and pancreas . When laterality is disrupted, many defects can result, such as abnormal position of organs, skeletal malformation and failure of neural tube closure . Mechanisms involved in the regulation of laterality for various animal species have been identified. These include motor proteins, ion channel, cytoskeleton, serotonin, cell-cell junction, Ca2+, and cilia . For example, in the mouse, the leftward movement of fluid at the ventral node, called nodal flow, is the critical process for left-right asymmetry . The nodal flow is generated by the clockwise rotation of nodal cilia. This directional flow causes some morphogens to concentrate at left side of the node and leads to left-right polarization , .
Many signaling molecules are involved in left-right asymmetry, including Nodal and Sonic hedgehog (SHH) , , . Signaling of fibroblast growth factors (FGFs) has been shown to regulate Nodal signaling in left-right determination , . The expression of nodal is suppressed by FGF8 in chicken . In contrast, FGF8 can induce nodal expression in mouse . Moreover, FGF signaling triggers secretion of nodal vesicular parcels which carry SHH and retinoic acid . There are 22 FGF ligands in human and mice, and 27 in zebrafish , . Four FGF receptors (FGFRs) have been identified, including FGFR1, FGFR2, FGFR3, and FGFR4 in vertebrates. Furthermore, FGFR1, FGFR2, and FGFR3 can be classified into b and c isoforms by alternative splicing. The binding specificity of FGFs with FGFRs is provided by the diversity of the FGF sequence and alternative splicing of FGFRs , . Among them, FGFR2b can bind with FGF1, FGF3, FGF7, FGF10 and FGF22; FGFR2c can bind with FGF1, FGF2, FGF4, FGF6, FGF9, FGF17 and FGF18.
In zebrafish, Kupffer's vesicle (KV) is equivalent to the mouse node and is important for left-right development . KV is a fluid-filled ciliated organ. Recent results indicate that most cilia are located on the dorsal side and are distributed along the anterior-posterior axis unequally . Interestingly, unlike the leftward fluid flow in mice, fluid flow generated in the KV of zebrafish has a counter-clockwise rotation . Recently, Fgf signaling regulation of laterality has been reported in zebrafish , , . In ace/fgf8 mutant, the asymmetric visceral organs and the proper symmetric craniofacial skeleton are disrupted. Furthermore, the KV morphogenesis is defective in ace/fgf8 mutant fish . Moreover, Fgf signaling can regulate the length of cilia through the Fgf8/Fgf24-Fgfr1 pathway . The downstream effectors of Fgf signaling, Ier2 and Fibp1, are also identified in the process of KV ciliogenesis .
We studied the role of Fgfr2 in liver development (manuscript in preparation). Unexpectedly, we detected that left-right asymmetry of visceral organs was randomized in the different fgfr2 morphants, especially for the fgfr2c. Furthermore, normal heart jogging and looping were disrupted in the morphants. The expression of specific left-sided genes, such as spaw, pitx2c and lefty1, was affected in fgfr2c morphants. The expression of spaw and pitx2c was randomized in left lateral plate mesoderm (LPM) of fgfr2c morphants. However, the expression of lefty1 was absent in most fgfr2c morphants. Furthermore, we found that ciliogenesis was defective in fgfr2c morphants: the cilia length was shorter in fgfr2c morphants. This phenomenon was similar in fgfr1, fgf8, and fgf8/fgf24 morphants. These results suggest that Fgfr2c is important in the regulation of left-right asymmetry.
Visceral Organ Laterality was Affected by fgfr2 Inhibition
Fgf signaling pathways have been proposed to regulate liver specification . However, the critical Fgf receptor(s) that participate in this process have not been fully characterized. We found the expression of earliest marker for developing liver, hematopoietically expressed homeobox gene (hhex), was absented in more than half of fgfr2 morphants (data not shown).
Unexpectedly, the disruption of left-right asymmetry in the fgfr2-ATG morphants was noticed from the expression pattern of foxA3 at 48 hours post fertilization (hpf). In 94.1% (n = 101) of the wild type embryos, the liver bud was located on the left side and the pancreatic bud was located on the right side in zebrafish (Fig. 1A and 1O). In 5.9% of the wild type embryos, abnormal left-right asymmetry was observed, and the relative locations of liver and pancreas were reversed (Fig. 1E and 1O). In fgfr2-ATG morphants, the left-right pattern was affected, and the abnormal percentages increased with dosages (Fig. 1B, C, F, G, and 1O; 8 ng/egg: 14.6% abnormal embryos, n = 41; 16 ng/egg: 30.6% abnormal embryos, n = 49). These results were confirmed by a splice-blocking MO, fgfr2-I4E5 MO (Fig. 1D, 1H, and 1O; 29.6% abnormal embryos, n = 27). In order to verify which fgfr2 variant controlled left-right asymmetry, we designed specific splicing blocking MOs that targeted fgfr2b and fgfr2c, respectively. According the cDNA sequence data (AB094118 and AB083105), isoform-specific exonic regions could be identified. We further confirmed these regions in fgfr2b and fgfr2c were exon8 and exon9, respectively, according to annotated zebrafish Zv9 assembly. In order to specifically inhibit the splicing of fgfr2b and fgfr2c, morpholino target sites were located at E8I8 (fgfr2b) and E9I9 (fgfr2c). The specificity was verified by RT-PCR analysis and sequencing (supplementary Fig. S1). The predicted translated products of fgfr2b and fgfr2c had in frame deletions of 13 and 17 amino acids, respectively. The deleted region of Fgfr2c consists of two critical amino acid residues (I350 and Y352) which form hydrophobic groove to interact with Fgf ligands . We found 29.6% (n = 375) of embryos to be abnormal in fgfr2c morphants (Fig. 1I, 1L, and 1O). However, only 7.4% (n = 27) of embryos were observed to be abnormal in fgfr2b morphants (Fig. 1J, 1M, and 1O). The laterality of visceral organ was not affected in fgfr2c 5-base mismatch (fgfr2c-5 mm) morphants (Fig. 1K, 1N and 1O; 3% abnormal embryos, n = 133). The phenotype in fgfr2c morphants could be rescued with morpholino-resistant fgfr2c mRNA (16.9% abnormal embryos, n = 118). These results indicate that fgfr2c is the major fgfr2 isoform that regulates the left-right pattern of visceral organs.
The expression pattern of foxA3 in liver (l) and pancreas (p) was shown in wild type, fgfr2-ATG morphants, fgfr2-I4E5 morphants, fgfr2c morphants, fgfr2b and fgfr2c-5 mm morphants (A∼D, I∼K). Abnormal pattern of reverse visceral organs was also observed in these embryos (E∼H, L∼N). All pictures were dorsal view. The bar charts showed the percentage of left-right asymmetry of visceral organs (O).
Heart Laterality was Randomized by fgfr2 Inhibition
Since the laterality of visceral organs was affected in fgfr2c morphants, we wanted to analyze whether the left-right asymmetry of developing heart was also randomized. We examined heart jogging at 30 hpf and looping at 48 hpf using Line 544 (cmlc2:EGFP/β-actin2-mCherry) fish, in which GFP was specifically expressed in heart. Heart jogging occurred after heart-tube formation, and heart looping took place following heart jogging. In 90.2% (n = 41) of the un-injected transgenic line 544 embryos we observed, heart corn migrated toward the left-anterior and formed heart tube (L-jog, Fig. 2A and 2R), and in 92.7% (n = 41) of the un-injected line544 embryos we observed, the atrium was located at the left side of ventricle (D-loop, Fig. 2J and 2R). When laterality was disrupted, the direction of heart jogging became random in 9.8% of the un-injected transgenic embryos (heart corn migrated toward the right-anterior, R-jog; heart corn migrated toward mid line, mid-jog, Fig. 2D, 2G, and 2R), and in 7.3% of the un-injected transgenic embryos, the location of the atrium turned toward the right side of ventricle (L-loop, Fig. 2M and 2R). In fgfr2c morphants, the numbers of embryo with R-jog or mid-jog increased to 49% (n = 51) (Fig. 2E, 2H, and 2R). Furthermore, 60% (n = 45) of fgfr2c morphants exhibited abnormal heart looping (L-loop or no-loop, Fig. 2N, 2P, and 2R). Embryos injected with fgfr2c-5 mm MO were relatively normal in heart jogging (Fig. 2C and 2R; 88.2% L-jog) and heart looping (Fig. 2L and 2R; 86.2% D-loop). According to these results, we conclude that Fgfr2c signaling is required for left-right pattern of heart in zebrafish.
The development of heart was followed using Line 544 (cmlc2:EGFP/β-actin2-mCherry) transgenic fish. Normal direction of heart jogging was toward left side (A∼C, L-jog). Randomization resulted in abnormal patterns of jogging (D∼I, R-jog and mid-jog). Normal heart looping (J∼L, D-loop) and abnormal heart looping (M∼Q, L-loop and no-loop) were detected in un-injected transgenic line 544 embryos, fgfr2c and fgfr2c-5 mm morphants. All pictures were ventral-anterior view. The bar charts showed the percentage of different types of heart jogging and looping (R). A: atrium, V: ventricle. Left-right axis was indicated as labeled.
Expression of lefty1, spaw and pitx2c were Affected by fgfr2c Inhibition
To verify the molecular mechanism of Fgfr2c signaling in left-right asymmetry, we examined the expression of lefty1, spaw and pitx2c. Normally, lefty1 was expressed in the notochord as a molecular midline barrier (Fig. 3A) . Eighty percent (n = 45) of fgfr2c morphants in our experiment showed reduced lefty1 expression in the midline (Fig. 3C). The lefty1 gene was also expressed in the left diencephalon of wild type embryos (Fig. 3B) . However, we found that 97.8% (n = 46) lefty1 was not expressed in this region in fgfr2c morphants (Fig. 3D). We then examined the expression of the left-side genes, spaw and pitx2c. For spaw expression in wild type embryos, we found that 97.4% (n = 77) exhibited left lateral plate mesoderm (LPM) expression; 1.3% exhibited right-side expression; and 1.3% exhibited bilateral expression (Fig. 3M). In the fgfr2c morphants, the expression of spaw was seen in 36.4% (n = 99) in left LPM; 7.1% expressed spaw in right LPM, 12.1% exhibited bilateral expression, and 44.4% showed no expression (Fig. 3E–3H, and 3M). The pitx2c was expressed in wild type embryos (n = 76): left LPM (96.1%); right LPM (1.3%); bilateral LPM (1.3%); and no expression (1.3%) (Fig. 3N). The pitx2c expression was affected in fgfr2c morphants (left: 32.4%, right: 6.7%, bilateral: 0.9% and no expression: 60%; Fig. 3I–3L, and 3N). These results suggest that the abnormal expression of spaw and pitx2c may be due to the defective of molecular midline barrier in fgfr2c morphants.
The lefty1 was expressed in the mid line (white arrow head), heart primordium (black arrow head) and diencephalon (white arrow) of wild type (A and B) but not expressed in the corresponding regions of fgfr2c morphants (C and D). Four different expression patterns of spaw and pitx2c in LPM (left, right, bilateral and no-signal) were detected in fgfr2c morphants (black arrow, E∼L). All pictures were dorsal view. The bar charts showed the percentage of different spaw and pitx2c expressing pattern in wild type and morphants (M and N). Left-right axis was indicated as labeled.
The Ciliogenesis in KV was Affected in fgfr2c Morphants
We next analyzed the cilia of KV. We used acetylated tubulin antibody to detect the cilia morphology of KV in 10 somite-stage embryos (Fig. 4A–4F). The ciliogenesis in KV is important for left–right asymmetry , , . Fgfr1, Fgf8, and Fgf24 have been reported to regulate the cilia length in KV and other organs . In our results, the cilia lengths were reduced in fgfr1 morphants (3.1±0.1 µm, 708 cilia, 19 embryos, P<0.0001), fgf8 morphants (3.6±0.1 µm, 752 cilia, 23 embryos for 4 ng/egg, P<0.0001; 3.5±0.2 µm, 426 cilia, 19 embryos for 8 ng/egg, P = 0.0001), and fgf8/fgf24 double morphants (2.8±0.2 µm, 159 cilia, 13 embryos, P<0.0001) compared with wild type embryos (Fig. 4G, 4.2±0.1 µm, 4849 cilia, 97 embryos). The cilia length was also reduced in fgfr2c morphants (Fig. 4G, 3.7±0.1 µm, 627 cilia, 16 embryos, P = 0.0053) compared with wild type embryos. Furthermore, we counted the number of cilia in the KV of various morphants. The cilia number was reduced in fgfr1 morphants (37.3±4.9, P = 0.0245), fgf8 morphants (32.7±3.1, for 4 ng/egg, P = 0.0006; 22.4±2.4, for 8 ng/egg, P<0.0001), and fgf8/fgf24 double morphants (14.5±3.9, P<0.0001) compared with wild type embryos (Fig. 4H, 50.0±2.3). The cilia number was dramatically reduced in fgf8/fgf24 double morphants, and KV disappeared in 45% (n = 20) of fgf8/fgf24 double morphants. However, the cilia number was not significantly changed in statistics for the fgfr2c morphants (Fig. 4H, 39.2±4.9, P = 0.0725). Furthermore, KV size was reduced in fgfr2c morphants compared with wild type embryos (P<0.001, data not shown). Additionally, we found the expression of fgfr2 was dynamic. From 95% to 100% epiboly, expression of fgfr2 could be detected in the marginal YSL (supplementary Fig. S2A∼C). At 5 somite-stage, fgfr2 was expressed in the area near KV (supplementary Fig. S2D and D′). These results indicate that spatial and temporal expression of Fgfr2 could regulate left–right asymmetry by controlling the length of cilia rather than cilia number. In order to investigate the effects of fgfr2c MO specific on dorsal forerunner cells (DFCs), MOs were injected into yolk at 2∼4 hpf . The embryos injected with 4 ng MOs were analyzed by foxA3 and cmlc2 whole mount in situ hybridization (WISH). The laterality of visceral organs was affected in DFCfgfr2c MO morphants (supplementary Fig. S3A, S3B and S3I; 51.8% abnormal embryos, n = 180). The heart looping was also affected in DFCfgfr2c MO (supplementary Fig. S3E, S3F and S3I; 58.5% abnormal embryos, n = 144). In the control DFCfgfr2c-5 mm MO morphants, the laterality was relatively normal (supplementary ; 84.7% for visceral organs, n = 163; 84.4% for heart looping, n = 162). Taken together, the laterality is disrupted when specifically targeting fgfr2c MO in DFCs.
The KV and cilia were labeled with antibodies against aPKC (red) and acetylated tubulin (green), respectively, at 10 somite-stage embryos (A∼F). The cilia length was reduced in fgfr1, fgf8 morphants and fgf8/fgf24 double morphants compared to wild type embryos (A∼E, G). The cilia length was also reduced in fgfr2c morphants (F∼G). The cilia number was reduced in fgfr1, fgf8 morphants and fgf8/fgf24 double morphants compared to wild type embryos (A∼E, H). In fgfr2c morphants, the cilia number was not significantly reduced (F, H, 39.2±4.9, P = 0.0725). Various expression patterns of foxj1a were detected in wild type embryos (lateral view, I∼K; dorsal view, I′∼K′) and fgfr2c morphants (lateral view, L∼O; dorsal view, L′∼N′) at 90% epiboly. Error bar, s.e.m. Scale bar: 10 µm.
It's known that foxj1 is important for the motile ciliogenic program . Because the left–right asymmetry is also randomized in foxj1 morphants , we analyzed the expression of foxj1a in 90% epiboly embryos. In wild type embryos, foxj1a was normally expressed in the KV (Fig. 4I and I′, 73%, n = 30). Some embryos (17%) had a scattered expression pattern of foxj1a (Fig. 4J and J′), and some (10%) had a reduced signal (Fig. 4K and K′). In fgfr2c morphants, the number of embryos with normal expression pattern of foxj1a was slightly reduced (Fig. 4L and L′, 63%, n = 30). The morphants with abnormal pattern, including scattered expression patterns, reduced signal, and no signal, were increased to 37% (Fig. 4M∼4O). Consistent to our finding, a recent study also indicated that the expression of foxj1 was downregulated in fgfr1 morphants. Accordingly, cilia length was also reduced in fgfr1 morphants . These results indicate that the reason for the reduced cilia length of fgfr2c morphants was due to affected expression of foxj1a.
In this study, we found that the orientation of asymmetric organs was randomized after knocking down Fgfr2. We further identified fgfr2c as the main fgfr2 variant that regulates the left–right asymmetry. Expression patterns of spaw, pitx2c and lefty1 were abnormal in fgfr2c morphants. These results indicate that the molecular midline barrier was disrupted and further affects the asymmetric expression of spaw and pitx2c. Importantly, the cilia length was reduced in the KV of fgfr2c morphants.
In mice, chickens, and rabbits, left–right asymmetry requires FGF8 , , . In zebrafish, Fgf8 signaling can regulate morphogenesis of the KV. KV is lost in about 30% of ace mutant embryos, and the laterality of visceral organs, including heart and brain, is also disordered . Recently, some evidence has indicated that Fgf signaling could regulate ciliogenesis in the KV to determine left–right asymmetry , , . In the KV of fgfr1 morphants, length of cilia is shorter. This results from the reduction of ciliogenic transcription factors, foxj1 and rfx2, and intraflagellar transport gene ift88 . In ace mutant embryos, the length of cilia is not affected. However, the cilia length is reduced in ace mutant embryos injected with fgf24 MO . These results suggest that fgf8, fgf24, and fgfr1 are important for ciliogenesis. In the other hand, two Fgf8 signaling target genes, ier2 and fibp1, have been identified . The cilia number in KV of ier2 and fibp1 morphants is also reduced. When ier2 and fibp1 mRNA are injected in fgf8 morphants, the cilia number is restored. Therefore, these two genes can mediate Fgf8 signaling in ciliogenesis and are essential for the establishment of laterality. A recent report indicates that Fgf4 signaling is important for left–right asymmetry . The left–right asymmetry of visceral organs and heart are randomized in fgf4 morphants. Furthermore, the expression of lefty1 is absent in the posterior notochord, and the cilia length is reduced despite normal quantities of cilia in fgf4 morphants. In our studies, reducing cilia length rather than cilia number might result in randomizing the left–right asymmetry in fgfr2c morphants. In order to verify which Fgf ligands regulate left–right asymmetry through Fgfr2c, we analyzed the possible synergistic effect of Fgfr2c and the above three mentioned ligands. We examined the heart looping in different low dosage MO combinations, including fgfr2c-fgf4, fgfr2c-fgf8 and fgfr2c-fgf24 (Supplementary Fig. S4). These preliminary results showed Fgfr2c and Fgf ligands did not have obviously synergistic effect except Fgfr2c-Fgf8. So we suggested that Fgfr2c could functionally interact with Fgf8, whereas Fgf4 and Fgf24 were parallel pathways with Fgfr2c for left–right asymmetry. The disruption of laterality in DFCfgf24 MO morphants is not known, whereas left–right asymmetry of visceral organs and the heart was affected in DFCfgfr2c MO but not in DFCfgf4 MO morphants . This observation also supports the independence of Fgfr2c and Fgf4 signaling. .
KV formation is very important for left–right asymmetry. The cellular origin of KV is DFCs which migrate at the leading edge of the blastoderm margin . When DFCs is ablated by laser or the KV morphogenesis is disrupted, the expression pattern of left–right asymmetry genes, including spaw, lefty1 and lefty2, becomes random , . In addition to shorter cilia length, the morphology of KV is changed and its area is reduced significantly in fgfr2c morphants (Fig .4F and data not shown). This phenomenon has not been reported in fgfr1 and fgf4 morphants , . To investigate whether the reduced KV area was due to changes of DFC numbers in fgfr2c morphants, we used casanova (cas) probe to highlight DFCs. Preliminarily we found that the number of cas expressed cells was not reduced in the morphants compared to wild type embryos (33.8±0.8 cells in fgfr2c moprhants, n = 195, and 33.4±0.9 cells in wild type, n = 140; P = 0.7738). Whether the cell size is affected in the morphants needs to be further examined. Notably, we did find the DFC morphology was obviously different in fgfr2c morphants (Supplementary Fig. S5). So, disorganized DFC pattern may cause defects of KV formation. For Fgf related genes in laterality, the ier2 and fibp1 have been indicated to affect KV formation starting at the time of DFCs formation . Since these two genes mediate Fgf8 signaling in left–right asymmetry patterning and Fgf8 and Fgfr2c signaling could have functional interaction, we suggest that Fgf8, Fgfr2c and Ier2/Fibp1 may be the same pathway to regulate DFC patterning. Further examination of fgfr2c MO specifically targets to DFCs also reveals the disruption of laterality of visceral organs and heart (Supplementary Fig. S3). Therefore, we suggest that Fgfr2 may function cell-autonomously in KV to regulate the organization of DFCs during the laterality establishment. However, the detailed mechanism remains unclear. In addition to KV morphogenesis, the expression of lefty1 in midline is also important for the left-side expression of spaw in LPM. In this study, we found that the expression of lefty1 in midline was absent in fgfr2c morphants. The abnormal expression of spaw in the LPM of fgfr2c morphants could be due to the loss of lefty1 in midline. Taken together, we conclude that Fgfr2c signaling controls left–right patterning through regulating the cilia length and controlling the expression of lefty1 to set up a molecular midline barrier. These suggest that Fgfs have multiple roles in left–right patterning.
Materials and Methods
All embryos were handled according to protocols approved by the Institutional Animal Care and Use Committee of Tzu Chi University, Hualien, Taiwan (approval ID: 97062).
The zebrafish (Danio renio) were raised as described in the Zebrafish Book . The AB wild type strain was used for morpholino injection and other experiment. Line 544 (cmlc2:EGFP/β-actin2-mCherry) was generated by Dr. Chung-Der Hsiao.
Tol2 kit was used to rapidly assemble expression vectors by two-fragment gateway recombination cloning. The p5E-β-actin2 5′ entry clone contains 5.3 kb upstream regulatory sequences of β-actin2 gene that sufficient to target transgene ubiquitously express. The pME-mCherry middle entry clone contains mCherry fluorescent reporter gene. The p3E-polyA 3′ entry clone contains late polyA sequence from SV40 virus. Finally, p5E-β-actin2, pME-mCherry and p3E-polyA were assembled together with pDestTol2CG2 by LR reaction to create expression vectors of pDestTol2CG2bactin2-mCherry-pA.
All morpholinos (MOs) were purchased from GeneTools. The sequences of MO used were as follows: fgfr1 MO, 5′-GCAGCAGCGTGGTCTTCATTATCAT-3′ ; fgfr2-ATG MO, 5′-CAGAAGCCACCCTCGGGCGAACATC-3′; fgfr2-I4E5 MO 5′-GTCGAACCTGGAACGGGAAAGCGTA-3′ ; fgfr2b MO, 5′-CGCTCCTGCTTTTTTACCTGGTATG-3′; fgfr2c MO, 5′-AAGCAGTGGAAGGTGAGTTTATACC-3′; 5-base mismatch for fgfr2c MO, 5′-AAcCAcTGcAAGGTcAcTTTATACC-3′; fgf4 MO, 5′- TTTCATACTCACAGATCCGTAAAGC-3′ ; fgf8 MO, 5′-GAGTCTCATGTTTATAGCCTCAGTA-3′ ; and fgf24 MO, 5′-AGGAGACTCCCGTACCGTACTTGCC-3′ . The MOs were injected into the cell at one cell stage. To target MOs to DFCs, MOs were injected into yolk at 2∼4 hpf. To analyze splicing defects after fgfr2b and fgfr2c MO injection, reverse transcription was carried out using the ImProm-II kit (Promega). The sequences of PCR primers for fgfr2b and fgfr2c were: 5′-GCTCGGGCATAAACAGCTCGG-3′ (fgfr2b-forward), 5′-CGGCAGGTGTGAACACTACGG-3′ (fgfr2c-forward), and 5′-CTCCGGCGAGTGGTGATTCTG-3′ (fgfr2-reverse). The coding region of fgfr2c was amplified from a fgfr2c plasmid  and subcloned into pCS2+ vector. The primer sequences were: 5′-ACTATCGATATGTTCGCCCGAGGGTGG-3′ (forward), 5′-TGGCTCGAGTCATGTTTTTATGCCGCC-3′ (reverse). Transposase RNA was synthesized in vitro by using pCS-transposase plasmid (kindly provided by Dr. Koichi Kawakami) as a template. Capped mRNA was prepared with the mMESSAGE mMACHINE kit (Ambion). To perform rescue experiment, fgfr2c mRNA (50 pg/egg) was coinjected with fgfr2c MO (2 ng/egg).
Creation of cmlc2:EGFP/beta-actin2-mCherry transgenic zebrafish
For generation of transgenic zebrafish, we mixed expression constructs of pDestTol2CG2bactin2-mCherry-pA (50 ng/µl) with in vitro transcribed transposases mRNA (50 ng/µl) and injected about 1–3 nl DNA solution into the animal pole of one-cell stage embryos. The injected embryos were raised to adulthood and the putative founders were screened according to the green fluorescent signals in the heart of their F1 progenies. We totally identified 10 independent lines out of 89 crosses and used the most robust expression line Tg(cmcl2:EGFP; bactin2:mCherry)cy1 for the following experiments.
Whole mount in situ hybridization
The following in situ probes were used: cas , cmlc2 , fgfr2 , foxA3 , foxj1a , lefty1 and spaw (both were provided by Dr. Karuna Sampath, The National University of Singapore), and pitx2c . The DIG-labeled probes were generated by in vitro transcription using a DIG RNA labeling kit (Roche). For whole mount in situ hybridization, DIG-labeled probes were used to hybridize the embryos overnight at 65 or 70°C and then washed with high stringency condition. The embryos were treated with blocking buffer (Roche) and incubated with AP-counjugated anti-DIG antibody overnight at 4°C (1∶8000, Roche). Excess antibody was washed and the embryos were colored with NBT/BCIP.
Embryos were fixed overnight in 4% paraformaldehyde at 4°C. Fixed embryos were washed with PBST (containing 0.3% TritonX-100) and treated with 10 mM Tris, 1 mM EDTA, 0.05% Tween20 for 5 minutes in 95°C. The embryos were subsequently blocked in PBST containing 4% BSA for one hour. Embryos were incubated in mouse anti-acetylated tubulin (Sigma T-6793, 1∶200) and rabbit anti-aPKC (Santa Cruz sc-216, 1∶100) at 4°C for overnight. After washed with PBST, embryos were incubated in goat anti-mouse Alexa Fluor 488 (Molecular Probes A-11029, 1∶200) and goat anti-rabbit Alexa Fluor 647 (Molecular Probes A-21245, 1∶200) at 4°C for overnight. After washed with PBST, embryos were mounted in SlowFade Gold antifade reagent with DAPI (Molecular Probes S-36938). Embryos were imaged using a LEICA TCS SP2 AOBS confocal microscope. Ciliary length and number were measured using Leica Confocal Software. KV size was analyzed by ImageJ using arbitrary unit. Two-tailed Student's t-test was used for analyzing on cilia length and number.
The effects of fgfr2b and fgfr2c specific MOs. (A and B): Blue arrows were primer sites for RT-PCR to detect the splicing products. Red thick lines were MO target sites. Injection of fgfr2b (4 ng per embryo) or fgfr2c (1, 2 or 4 ng per embryo) MOs caused partial deletion of exon 8 (b) and exon 9 (c), respectively, that had been confirmed by sequencing. The original splice donor sites were blocked and cryptic splice donor sites in exon8 and exon9 were activated (indicated by bottom red lines) by the corresponding MOs. The partial cDNA sequences were shown (exon7, 8, 10 and 11 for fgfr2b and exon 7, 9 10 and 11 for fgfr2c). Underline indicated the primer sequence. The deleted regions were highlighted.
Expression pattern of fgfr2. The expression of fgfr2 was detected in marginal YSL (A∼C, arrow, 95%∼100% epiboly) and in the area near KV (arrowhead in D′, 5 somite-stage). Boxed area shown in panel D is enlarged in panel D′.
The effects of fgfr2c MO specific on DFCs. The normal expression pattern of foxA3 in liver (l) and pancreas (p) was shown in DFCfgfr2c MO and DFCfgfr2c-5 mm MO morphants (A and C). Abnormal pattern of visceral organs was also observed in these embryos (B and D). The development of heart was examined using cmlc2 probe (E∼H). Normal (E and G) and abnormal heart looping (F and H) can be observed in both morphants. The bar charts showed the percentage of embryos with different expression distribution of foxA3 or cmlc2 in both morphants (I). Panel A to D were dorsal view and panel E to H were ventral-anterior view. A: atrium, V: ventricle.
The percentage of abnormal heart looping in fgfr2c and fgf ligand morphants. In order to test the synergistic effect of Fgfr2c and Fgf ligands (Fgf4, Fgf8 and Fgf24), different combinations of low dosage fgfr2c MO and fgf MOs were injected into Line 544. Double morphants of fgfr2c and fgf4 (2, 1 or 0.5 ng/embryo for fgfr2c MO; 34, 22.5 or 12 ng/embryo for fgf4 MO) did not have synergistic effect on the abnormal heart looping, including L-loop and no loop pattern (A). Co-injection with fgfr2c MO (0.5 ng/embryo) and fgf24 MO (1.25 ng/embryo) also did not greatly increase the abnormal percentage (B). In contrast to above results, a synergistic effect was detected in fgfr2c-fgf8 double morphants (0.5 ng/embryo for fgfr2c MO and 1 ng/embryo for fgf8 MO; C).
The cas expression pattern in fgfr2c morphants. Embryos at 90% epiboly were stained with cas probe for labeling DFCs. The morphology of normal DFC cluster in wild type was shown in panel A (79%, n = 140). The mild and severe disorganization of DFC pattern could also be detected. However, the percentages of abnormal pattern were increased in fgfr2c morphants (B and C, 56.9%, n = 195).
We thank Dr. W. Driever for foxA3 probe, Dr. S. P. Hwang for cas and cmlc2 probes, Dr. M. Rebagliati for pitx2c probe, Dr. S. Roy for foxj1a probe, Dr. K. Sampath for lefty1 and spaw probes, and Dr. K. Yamasu for fgfr2 probe.
Conceived and designed the experiments: D-WL C-HH W-PW. Performed the experiments: D-WL C-HH. Analyzed the data: D-WL W-PW. Contributed reagents/materials/analysis tools: C-DH. Wrote the paper: D-WL S-MT W-PW.
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