ABSTRACT: Mice used in this study were 129/sv males, 8-15 weeks of age, which were housed individually and received water ad libitum. Normal fed animals were fed before and during the 48h experiment with standard food ad libitum. Starved animals were fed before the experiment with standard food ad libitum and were starved during the 24h experiment. Mice were killed by CO2 asphyxiation. The livers were snap-frozen in liquid nitrogen and stored at -80 degrees C. Total RNA was prepared using the Nucleospin RNA L kit (Macherey-Nagel, Düren, Germany), with an additional step in which the final eluate was used for a subsequent elution step. PolyA RNA was extracted with the Ambion (Ambion Inc, Austin, USA) Purist kit according to the manufacturer's protocols. Equal amounts of polyA RNA were pooled to generate pools of all animals. Pool1, normal = 3 animals (N1, N2, N3), pool2, normal = 3 animals (N4, N5, N6), pool3, = 6 animals (N1, N2, N3, N4, N5, N6), pool1, sugar = 3 animals (St1, St2, St3), pool2, starved = 3 animals (St4, St5, St6), pool3, starved = 6 animals (St1, St2, St3, St4, St5, St6). Labeled cDNA was synthesized from 1.5 µg polyA RNA using the Amersham (Amersham Europe, Freiburg, Germany) direct cDNA labeling kit. Removal of the unincorporated dyes and concentration of the target were done with Microcon 30 (Millipore, Bedford, MA) spin columns according to the manufacturer's instructions. The concentrated probes were hybridized to the microarray in 1x dig easy hyb buffer (Hoffman-la Roche, Basel, CH) overnight at 42 degrees C. After hybridization the microarrays are washed 5’ in 2x SSC, 0.1% SDS at 42 degrees C; 10’ in 0.1x SSC, 0.1% SDS at RT; 4 times 1’ in 0.1x SSC at RT and shortly dipped into 0.01x SSC. Drying is performed in centrifuge on a microtiterplate holder at 800 rpm for 7’. Slides are stored in dark until they are scanned. Arrays were scanned using the dual laser scanner Axon 4000B and the corresponding software Genepix 4 (Axon Inc., Union City, CA, USA). Both channels (532 nm for Cy3 and 635 nm Cy5) were scanned in parallel and stored as 16 bit tiff files. The absolute intensity values span the range from 0 to 65,535. The scans were performed with a resolution of 10 µm. From each spot with a mean diameter of 100 µm, 100 data pixels were recorded. Individual local background area around the spots were defined, which included approximately 400 pixels and excludes neighboring spots. For each channel, the raw data was calculated as the median intensity of all foreground pixels with respect to all background pixels. Each array was scanned three times (low, medium and high scan) with different signal amplification factors (voltage settings of the photo multiplier tubes), but with the same laser power. The channels for Cy3 and Cy5 were balanced in each scan for approximately the same intensity profile. In the low scan no spot was saturated, in the high scan the signal amplification for Cy5 was set to approximately 80% of maximum and the Cy3 amplification was adjusted to this. The settings used in the medium scan lie between the low and the high scan. This method for scanning has several advantages. In the low scan where no spot is in saturation, it is possible to calculate the real ratio for genes with high expression levels; however, those with a low expression level are most likely not recognized. In order not to lose these, a high scan is made; in this case, the information on the saturated spots is lost, so the two scans complement each other. The medium scan produces additional values for subsequent calculations. By scanning the arrays three times, errors which occur while recording and which might increase the error factor in the normalization are averaged. The data processing was automated in a Visual Basic/Excel (Microsoft) program, which integrates the following steps. Raw data are derived from the result files generated by scanning and picture analysis software Genepix. The spot diameter is to lie in the range from 80 to 120 µm otherwise these spots are not included in analysis. From the pixel to pixel ratios between the foreground values of both colors, the standard deviation (SD) is calculated. Those spots which show a ratio SD of higher than 3 are excluded from analysis due to inconsistency. The foreground signal of each spot is corrected by subtracting the corresponding local background. Resulting values which are lower than the background are replaced by the background value. This defines the minimum measurement threshold as the background and avoids calculations of completely wrong ratios, e.g., if the background corrected value for one channel is close to zero. Spots are marked as saturated in one channel if more than 10% of the foreground pixels are at the highest absolute value. From the unsaturated data points of all three scans, linear regression between the low and medium, and between low and high scans, are calculated for both colors. Taking these regressions, the values of saturated spots are replaced by estimated ones which reflect the real intensity. The generated data set of each scan is normalized afterwards based on the intensity-dependent methods as described by Yang et al (80). The ratios are calculated as log2 transformed values: M=log2 (Cy5/Cy3). Number describing the spot intensity A=log2sqrt (Cy5*Cy3) is calculated according to Yang et al (80). A within pin-tip group loess fit to the MA-plot was made. The loess scatter plot smoother performs robust locally fits using a tricube function to weight the values relative to the median point in the intensity interval. As interval size 20% was chosen, which corresponds to 180 data points in a pin-tip group containing 30 by 30 spots. After combining the normalized data from all blocks the data sets of all scans are additionally normalized using a global approach. The normalization was performed so that the sum of all log transformed ratios (M) is 0. All statistical normalizations are based on the assumption that (1) the majority of genes are not changed in their expression and (2) that the overall up and down regulations statistically compensate each other in sum. Taking this into account, the standard deviation in the ratios of the stable genes (80% of the most unchanged genes) could be seen as the measurement noise of the array experiments. To be able to compare the different scans and different normalized microarrays, the SD of the stable genes ratios was adjusted to a medium estimated value of 0.2, which is dependent on the used array system. This value does not influence the subsequent statistical analysis because the normal distribution of the data is not changed. For each microarray the normalized and adjusted log ratios of the three scans are averaged. Keywords = nutrient response Keywords = starvation Keywords = liver Keywords = anti-aging Keywords = longevity Keywords: repeat sample
