Analysis of Histone Antibody Specificity with Peptide Microarrays

The goal of this video is to describe an integrated pipeline for the design fabrication and use of peptide microarrays for profiling the specificities of histone antibodies. Antibodies that recognize histones and their diverse post-translational modifications are key reagents in chromatin biochemistry research. We rely on these tools to deepen our understanding of histone modification function.

Recent studies reveal inconsistencies in histone antibody behavior. As antibody choice is the most critical element in epigenetics experiments, vigorous incomprehensive quality control measures are demanded for these reagents. Here, we demonstrate an integrated pipeline we'd developed for designing, fabricating and analyzing peptide microarrays to generate specificity profiles for antibodies that recognize histones and their post-translational modifications.

The pipeline is facilitated by Array Ninja, an open source interactive software application that we developed, which integrates the design and analysis stages of microarray experiments. To begin designing the array slide, under the plan a slide layout heading, select the microarray printer that will be used. Then, type, empty in the load plate field and press enter.

Set the spot diameter to 275 micrometers and the spot spacing to 100 micrometers. Fill in the remaining parameters appropriately for the experiment, ensuring that the layout allows for sufficient replicates. At the resulting layout preview, mouse over each unique peptide feature on the slide and enter a feature identifier in the popup text box.

Once each unique feature has been labeled, click populate. Name the slide layout, and click commit your plate to save the layout and generate a map at the physical locations of each feature in the source plates. Guided by the source plate map, load one to two microliters of each peptide feature into the designated wells of one or more 384 wells, small volume plates, dilute each feature 10 fold with 1x protein microarray printing buffer, supplemented with 1%bovine serum albumin and five micrograms per microliter fluorescin labeled biotin.

Then, centrifuge the plates at 500 x G at room temperature for two minutes. Next, empty the waste container at the microarray printer. Fill the washed solution in humidifier containers with sterile distilled water.

Enter the array slide parameters in the microarray printer software. Set the washing procedure to a one second wash with the single immersion, the post washed to redip the pins five times, and the humidity to 60%Then, insert encoated glass slides into the substrate platinums. Load the platinums into the platinum elevator.

Insert the source plates into the plate holders and load the holders into the source plate elevator. Run the printing process, then, remove the substrate platinums from the instrument. Block the printed slides with 1x protein microarray blocking buffer for 30 minutes while shaking.

Then, wash the slides in room temperature, pH 7.6 phosphate buffered saline for 10 minutes. Repeat the wash with a fresh batch of PBS. Dry the slides by centrifugation for 30 seconds at room temperature.

Next, pre-heat a microarray wax imprinter at 85 degrees celsius for 30 minutes, or until the wax has melted. Then, insert a slide printed side down, into the holder and align the holder with the imprinter mold. Bring the mold into contact with the slide and hold for two seconds, then, quickly remove the slide from the holder and inspect the wax borders to verify that the arrays are properly enclosed.

Store the partitioned slides at four degrees celsius, in a dry dark area. To begin the hybridization procedure, place a partitioned array slide in a plastic dish and cover the slide with hybridization buffer. Equilibrate the slide for 30 minutes at four degree celsius, while shaking at low speed.

Dry the slide by spinning in a microarray slide centrifuge at room temperature. Then, add five microliters of each histone PTM antibody solution to at least two wells of the array and incubate at four degree celsius for one hour. Following incubation, remove the antibody solution and wash the array with cold PBS three times for five minutes each at four degrees celsius.

Next, incubate the array in a fluorescent dye-conjugated secondary antibody solution. For 30 minutes at four degrees celsius, in the absence of light. Wash the slide three times with cold PBS as before.

Dip the array in room temperature 0.1x PBS to remove excess salt and dry the slide by brief centrifugation at room temperature. Image the slide with a microarray scanner at a resolution of at least, 25 microns. To begin the analysis, use image processing software to merge the single channel files from the microarray scanner.

Then, open Array Ninja and select the appropriate microarray printer under the, to quantify images header. Fill in the name of the same slide layout in the load plate dialog box, and press enter. Load the merged image file, and adjust the contrast, brightness and mid point as desired.

Fill in the array scanner resolution, adjust the top and side margins to align slide layout grid with the image. To analyze a set of wells as replicates or perform other selective analysis, fill in the well coordinates in the iPin, jPin, and subA fields and press enter. Click to toggle zoom, to display a magnified image of the area under the cursor.

Then, click spot seek and wait for the process to finish. Repeat the seek function as needed to center the grid circles on the array spots. Reference spots can be selected by pressing the R key and spot showing debris or widely bearing morphology can be deactivated by pressing the A key while hovering over the spot.

Once the background reference spots have been selected, and spots with pore morphology or contamination have been deactivated, click populate, followed by, submit to perform the quantification. A bar graph and table will be displayed in a new page. The binding specificities of two histone PTM antibodies were evaluated using this method.

A histone PTM antibody designed to target trimethylated lysine 9 on histone H3 was also found to bind to three other trimethylated lysine groups. A silylation of neighboring lysine residues was observed to positively influence targeting of lysine 9 on H3.A histone PTM antibody targeting trimethylated lysine 4 on histone H3, was found to interact with singlely methylated and dimethylated lysine 4 on H3 as well. Phosphorelation of the neighboring three anine six, was observed to negatively impact binding at lysine four in this assay.

This pipeline has been used to screen many of the most widely used, commercially available histone antibodies and data generated from these experiments is freely available through the histone antibody specificity database. Variations to this pipeline have been previously described for the analysis of histone readers, writers and erasers, modifying this platform before utility beyond epigenetics research could be easily for any printable library of interest. Several histone peptide microarray platforms are available commercially for users who do not have access to the specialized equipment needed to synthesize peptides and fabricate arrays.