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MATERIALS AND
METHODS
1. Embryos handling
In order to obtain Xenopus laevis embryos, each adult female used were injected with 800-1000 units of human chorionic gonadotropin (Gonasi HP 5000 U.I., Amsa) 12-14 hours prior to eggs collection. Ovulated eggs were fertilized with testis homogenate and let to develop in 0.1X MMR. Jelly coats were removed by using the dejelling solution.
2. Embryos microinjection and manipulation
For knock-down experiments, a morpholino antisense oligonucleotide, targeted to the 5’ untranslated region of 5-HT2B mRNA (Mo, Gene Tools, LLC), has been injected together with nuclear-β-galactosidase (n-β-gal) and Green Fluorescent Protein (GFP) in vitro transcribed capped mRNAs (see Results).
X5-HT2B-Mo sequence:
5’-GTGCCAGGGAATGGAGATTGTGTCT-3’
For over-expression experiments, X5-HT2B in vitro transcribe capped mRNA was injected together with nuclear-β-galactosidase (β-gal) and Green Fluorescent Protein (GFP) capped mRNA (see Results).
34 blastomere of two-cells stage embryos. Microinjections were performed using a Drummond "Nanoject" apparatus. Embryos were injected in 0.1X MMR and 3% Ficoll-400 and cultured overnight (O./N.) at 18° C in the same solution. Embryos were subsequently transferred in 0,1X MMR and incubated at Room Temperature (R.T.) until the desired developmental stage. Embryos were staged according to Nieuwkoop and Faber (1967).
When n-β-gal mRNA was used as a lineage tracer in microinjection experiments, β-gal staining was detected by fixing embryos in 1X MEMFA for 30 min. at R.T., washing in 1X PBS pH 7.3 and than incubating them in β-gal staining solution for 20- 30 min. at 37°C. Embryos were than fixed again in 1X MEMFA for 1 hour at R.T. and then gradually dehydrated with ethanol to be processed by whole mount in situ hybridization. The embryos treatment before in situ hybridization on sectioned embryos will be detailed afterward.
2.1. Capped mRNAs
Capped mRNAs were synthesized in vitro from the
template cDNAs using the SP6 mMESSAGE
mMACHINE Kit (Ambion). After DNase digestion to remove template cDNA, transcripts were purified with 0.1 volume of Ammonium Acetate to avoid embryos gastrulation defects, extracted with 1 volume of phenol/chloroform, Precipitate with 2 volume of 100% ethanol, resuspended in water at the 500 ng/µl concentration and stored at -20°C. Concentration and
35 length of transcripts were estimated by agarose gel electrophoresis.
2.1.1. Constructs used for over-expression analysis: linearization and capped mRNA transcription
- n-β-gal (NotI/Sp6), containing the nuclear-β-galactosidase reporter inserted into the pCS2+ vector (Chitnis et al,. 1995).
- pCMTEGFP (NotI/Sp6), containing the Enhanced Green Fluorescent Protein reporter gene inserted into the pCS2+ vector (a gift from D. Gilmour).
2.2. Pharmacological treatments
5-HT2B Selective antagonist (Sigma, cat. R 2533) was used to treat from stage 25 to 37 embryos at a final concentration of 150 μM. Culture medium plus was prepared fresh and changed daily.
3. RNA Extraction, cDNA Synthesis and qPCR
Total RNA was RNA was extracted from 15 embryos head and it was isolated using the TRIZOL Reagent (Invitrogen). To quantify gene expression levels, equal amounts of cDNA were synthesized using ImProm-II™ Reverse Transcription System (Promega) and mixed with GoTaq® qPCR Master Mix (Promega) 1X, primers 0,25 µM e 32 ng di cDNA. GAPDH was amplified as an internal control. All primers were designed to have a Tm of 60 ° C, the following are all sequences. All qPCR was conducted at 50°C for 5 min, and then 45
36 cycles of 95°C for: 95°C for 15s , 60°C for 20 s and 72°C for 40 s. The specificity of the reaction was verified by melt curve analysis. The threshold crossing value was noted for each transcript and normalized to the internal control. The relative quantitation of each mRNA was performed using the comparative “Delta-delta Ct” method. Statistical analysis was performed using the t-test. All experiments has been repeated on biological triplicated.
3.1 Primers used for qPCR gene expression analysis XFoxC2:
primer forward: GCCCCTTCTAATGTGTTTGG,
primer reverse: AAGCGCTGAAAACAGCTCAG
XPitx2:
primer forward: TCGGCCTGAAAAACGACATG, primer reverse: TGAGTTTTGCAGCCATCACC
XRaldh3:
primer forward: ATAAGTGGCCGCACTTTTGC, primer reverse: ACAGCCTTGTCAACATCTGC
XFoxC1:
primer forward: TGTGAGACAAACTGCGCTTC, primer reverse: ATGGGCACAGCAATAGGAAC
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XRdh10:
primer forward: CTACCACCCCTGAAACCAGA, primer reverse: AGGAACCGGTACATGCAGAC
XDhrs3a:
primer forward: CAGGCGCAAGAAATCCTAAG, primer reverse: AAAGGCCACGTTACAGGATG
XCyp26A1:
primer forward: AGGTTTGGCTTCATCCCTTT, primer reverse: TTAGCGGGTAGGTTGTCCAC
XGapdh:
primer forward:CTTTGATGCTGATGCTGGAA, primer reverse:GAAGAGGGGTTGACAGGTGA
4. In situ hybridization
4.1. Purification of plasmidic DNA
Plasmidic DNA were prepared according to the alkaline lysis procedure, followed by chromatography over QIAGEN columns. After purification, circular plasmids were digested with appropriate restriction enzymes, in order to linearize templates for in vitro transcriptions. 4.1.1. Constructs used for gene expression analysis: linearization and RNA transcription to generate antisense mRNA probes
38 XDct-pBS: SacI/T7 XRaldh3-pGemTeasy: SpeI/T7 XFoxC1-pCS107: Asp718/SP6 XVax2-pBS: NotI/T7 XPax2-pBSIISK: EcoRI/T3 XTwist-pCR2.1: HindIII/T7
4.2. Synthesis of digoxigenin (DIG) labeled probes Linearized plasmids were used as template to synthesize digoxigenin-11-UTP- labeled (Boehringer Mannheim) antisense mRNA probes employing the RNA polymerases indicated above. In particular, transcription reactions were incubatedin the presence of 10 mM each of rATP, rCTP, rGTP, rUTP/DIG-11-rUTP (3/2) 2 hours at 37°C. After DNase digestion to remove template DNA, the amount and length of transcripts were estimated by agarose gel electrophoresis. Probes were than diluted in hybridization mix at a final concentration of 10 µg/ml and stored at -20°C for several months. 4.3. Whole mount in situ hybridization
Whole-mount in situ hybridization was performed by using standard procedures as described in Harland (1991), except that BM purple (Roche) was used as a substrate for the alkaline phosphatase. After colour development embryos were post- fixed on 1X MEMFA and bleached over a fluorescent light for about 1 hour to remove the pigment. Control experiments were performed with sense probes.
39 For histological examination, whole-mount in situ hybridization processed embryos were embedded in a paraffin solution according to the protocol described in Levin (2004) whit minor modification, and then sectioned at 12 µm thickness using a microtome.
4.4. In situ hybridization on frozen tissue sections Embryos collected for in situ hybridization on cryostat cut tissue sections were fixed 2 hours in 4% paraphormaldeyde in 1X PBS, cryo-protected with 20% sucrose in 1X PBS and subsequently embedded in the optimal cutting temperature (O.C.T.) compound (Sakura Finetek) and stored at -80°C.
In situ hybridization on 12 µm cryostat sectioned embryos was performed as described in Strahle et al., (1994) with minor modifications.
Control experiments were performed with sense probes. 5. Whole mount immunohistochemistry
Muscle specific 12/101 monoclonal antibody (1:10, Developmental Studies Hybridoma Bank), a Nerve specific 3A10 monoclonal antibody (1:100, Developmental Studies Hybridoma Bank) and anti-GFP polyclonal antibody (1:2000, Molecular Probes) were used following a standard immunohistochemistry procedure.
6. Neural crest cells transplantation assay
40 and β-gal capped mRNA as described above. Embryos over-expressing X5-HT2B were starved in MMR 0,1X until stage 15 and selected for the presence of the GFP in the neural plate. NCC transplantation assays were performed as described in Borchers et al., 2000, with minor modifications.
7. Tunel Assay
Tunel staining on whle mount embryos at stage 37 was performed by using the ApopTag Peroxidase Kit (Serologicals), following the manufacturer’s instruction 8. Solutions BLEACHING SOLUTION 2% H2O2 5% Formamide 0.5X SSC 1X PBS DEJELLING SOLUTION DTT (dithiothreitol) 3.2 mM Tris pH 8.8, 0.2 M MEMFA salts 10X MOPS (pH 7.4) 100 mM EGTA 2 mM MgSO4 1 mM Formaldehyde 3.7%
41 MMR (Marc’s Modified Ringer’s) 10X
NaCl 0.1M KCl 2mM MgSO4 1mM CaCl2 2mM HEPES (pH 7.8) 5mM EDTA 0.1mM
PBS (Phosphate Buffer Saline) 10X NaCl 137mM
KCl 2.07mM Na2HPO4 10mM KH2PO4 2mM pH 7.3
β-gal STAINING SOLUTION
K3Fe(CN)6 (Potassium Ferricyanide) 5mM K4Fe(CN)6 (Potassium Ferrocyanide) 5mM MgCl2 2mM
Red-gal substrate 1 mg/ml 1X PBS
