For overexpression of human Otx2, the hsp-Otx2 line generated by Leuzinger et al. was used [Leuzinger et al.: Equivalence of the fly orthodenticle gene and the human OTX genes in embryonic brain development of Drosophila. Development 1998; 125:1703-1710.]. All fly stocks were kept on standard cornmeal/yeast/agar medium at 25°C. Embryos were collected overnight for 12 hours on grape juice plates, further kept for 4 hours at 25°C and then subjected to a 37°C heat shock for 25 min, followed by a recovery period of 25 min at 25°C before RNA isolation. Therefore, at the time of RNA isolation these embryos were at embryonic stages 10-17 [Leemans et al.: Quantitative transcript imaging in normal and heat-shocked Drosophila embryos by using high-density oligonucleotide arrays. Proc Natl Acad Sci U S A 2000; 97:12138-12143.]. Embryos younger than embryonic stage 10 were not used, since heat shock in these earlier stages results in lethality [Walter set al.: Heat shock causes the collapse of the intermediate filament cytoskeleton in Drosophila embryos. Dev Genet 1990; 11:270-279.]
Total RNA was isolated from 200 mg of embryonic tissue, using guanidinium isothiocyanate in combination with acidic phenol (pH 4.0) (fast RNA tube green kit from BIO101) in a fast prep homogenizer FP120 (BIO 101). After precipitation, the RNA was dissolved in DEPC-treated water (Ambion) and spectrophotometrically quantified using a GeneQuant RNA/DNA calculator (Pharmacia Biotech). cDNA was synthesized upon total RNA as a template, using the SuperScript Choice System for cDNA synthesis (Gibco/BRL) with a T7-(T)24 DNA primer. This primer (5'-GCCAGTGAATTGTAATACGACTCACTATAGGGAGGCGG-(T)24VN-3') was purified by PAGE. For first-strand cDNA synthesis, a typical 40 ul reaction contained 25 ug RNA, 200 pmol T7-(T)24 primer, 500 M of each dNTP and 800 units reverse transcriptase (AMV Superscript II). The reaction was incubated for 1 h at 42°C. Second-strand cDNA synthesis was carried out at 18°C for 2 h in a total volume of 340 ul, using 20 units Escherichia coli DNA ligase, 80 units E. coli DNA polymerase I and 4 units RNase H in the presence of 250 M of each dNTP. After second-strand cDNA synthesis, 0.5 ul RNase A (100 mg/ml)(Qiagen) was added and the samples were incubated at 37°C for 30 min. Thereafter, 7.5 ul proteinase K (10 mg/ml) (Sigma) was added and the samples were further incubated at 37°C for another 30 min. After cDNA synthesis was completed, samples were phenol-chloroform extracted, using Phase Lock Gel (5 Prime-3 Prime) and ethanol precipitated. Biotinylated antisense cRNA was synthesized from the dsDNA template, using T7 RNA polymerase (MEGAscript T7 Kit: Ambion.). A 20 l reaction volume contained between 0.3-1.5 g cDNA, 7.5 mM of both ATP and GTP, 5.6 mM of both UTP and CTP and 1.8 mM of both biotinylated Bio-16-UTP and Bio-11-CTP (ENZO diagnostics) and 2 ul 10x T7 enzyme mix. The reaction was incubated at 37°C for 8 h. Thereafter, the unincorporated NTPs were removed by running the sample over an RNeasy spin column (Qiagen). Samples were precipitated, taken up in 20 ul DEPC-treated water and spectrophotometrically quantified. Thereafter, 40 ug of the biotinylated antisense cRNA was fragmented by heating the sample to 95°C for 35 min in a volume of 25 ul, containing 40 mM tris-acetate (pH 8.1), 100 mM potassium acetate, 30 mM magnesium acetate. After the fragmentation, the samples were placed on ice. Gene Chips were pre-hybridized with 220 ul hybridization buffer (1x MES (pH 6.7), 1 M NaCl, 0.01% Triton, 0.5 g/l acetylated BSA, 0.5 g/l sonicated herring sperm DNA) for 15 min at 45°C on a rotisserie (Heidolph) at 60 rpm. Hybridization was done in a final volume of 220 ul hybridization buffer, containing 40 g fragmented biotinylated cRNA. The samples were heated to 95°C for 5 min and briefly spun down. Hybridizations were carried out for 16 h at 45°C with mixing on a rotisserie at 60 rpm. After hybridization, the arrays were briefly rinsed with 6x SSPE-T (0.9 M NaCl, 0.06 M NaH2PO4, 6 mM EDTA, 0.01% Triton) and washed on a Fluidics station (Affymetrix). Hybridized arrays were stained with 220 ul detection solution (1x MES buffer, containing 2.5 l streptavidin-R phycoerythrin conjugate (1 mg/ml) (Molecular Probes)) and 2.0 mg/ml acetylated BSA (Sigma) at 40°C for 15 min and washed again.
The data was normalized against the mean of the total sums of Avg Diff values across all 16 arrays used in this study.