High-Resolution Mapping of Protein-DNA Interactions in Mouse Stem Cell-Derived Neurons using Chromatin Immunoprecipitation-Exonuclease (ChIP-Exo)

The main advantage of our ChIP-exo method is the improved mapping resolution of protein DNA interactions in the cell compared to the conventional chromatin immunoprecipitation method. To prepare protein G magnetic beads for ChIP, mix the magnetic beads until homogeneous. Then add 25 microliters of the magnetic beads to a two milliliter protein low bind tube.

Add one milliliter of blocking solution to the beads. Mix well by pipetting, then place the tube on a magnetic rack for one minute. Once the supernatant is clear, remove it.

Next, add one milliliter of blocking solution to the beads. Place the tube on a rocking platform at four degrees Celsius and rock for 10 minutes, then briefly spin the tube. Place the tube on the magnetic rack and remove the supernatant.

Add 500 microliters of blocking solution to the magnetic beads. Briefly spin the antibody against Islet-1, then add four micrograms of the antibody to the tube containing the magnetic beads. Place the tube containing magnetic beads and Islet-1 antibody on a rocking platform at four degrees Celsius and rock for six to 24 hours.

To begin the chromatin immunoprecipitation, wash the antibody-coated beads in one milliliter of blocking solution. Place the tube containing the antibody-coated beads on a rocking platform at four degrees Celsius for five minutes. After briefly spinning, place the tube on a magnetic rack and remove the supernatant.

Resuspend the antibody-coated beads in 50 microliters of blocking solution. Add one milliliter of sonicated lysates to the antibody-coated beads, then incubate the sample on a rocking platform at four degrees Celsius overnight. After allowing the sample to incubate overnight, collect liquid from the cap by briefly spinning the tube, then place the tube on a magnetic rack.

And after one minute, remove the supernatant carefully with a pipette. Next, perform a series of washes as follows. Add one milliliter of cold wash buffer containing CPI to the sample.

Mix the sample on a rocking platform at four degrees Celsius for five minutes. Briefly spin the sample tube, then place on a magnetic rack. After one minute, remove the supernatant with a pipette.

For the first enzymatic reaction and repair and DA tailing, add 38 microliters of autoclaved double distilled water to the sample, then add end prep reaction mix and end prep enzyme mix. Mix the contents of the tube by pipetting, then incubate the sample at 20 degrees Celsius for 30 minutes. Next, wash the sample beads with high salt wash buffer, lithium chloride wash buffer, and 10 millimolar Tris hydrogen chloride buffer as described previously.

To perform the index adapter ligation, add 27 microliters of cold 10 millimolar Tris hydrogen chloride buffer to the sample. Then add index adapter, ligation enhancer, and ligase master mix. Incubate the sample at 20 degrees Celsius for 15 minutes.

Again, wash the sample beads with high salt wash buffer, lithium chloride wash buffer, and 10 millimolar Tris hydrogen chloride buffer. Then add 47 microliters of cold 10 millimolar Tris hydrogen chloride buffer to the sample. To repair the nick in the DNA resulting from the missing phosphodiester bond, add 11.1 microliters of the fill-in mix to the sample.

Incubate the sample at 30 degrees Celsius for 20 minutes. After a series of washes, add 50 microliters of cold autoclaved double distilled water to the sample. To digest the ChIP DNA in the five prime to three prime direction, add lambda exonuclease buffer and lambda exonuclease and mix the sample by pipetting.

Incubate the sample at 37 degrees Celsius for 30 minutes. After the digestion is complete, repeat the three washes a final time. To elute ChIP sample from the beads, resuspend the sample in 75 microliters of ChIP elution buffer and incubator at 65 degrees Celsius for 15 minutes at 130 times G.Add 2.5 microliters of 20 milligrams per milliliter proteinase K to the sample.

Vortex the sample briefly, then incubate overnight at 65 degrees Celsius. Briefly spin the sample and place on a magnetic rack. After one minute, transfer the supernatant to a new 1.5 milliliter tube.

Purify and elute the DNA. After purifying the eluted DNA, transfer 16 microliters of the extracted DNA sample to a PCR tube. Add 1.2 microliters of denaturing and primer annealing mix to the sample.

Using the program described in table six of the manuscript, denature and anneal primers to the template DNA. Add three microliters of primer extension mix to the sample and run the PCR program described in table seven of the manuscript. Add 4.1 microliters of DA tailing mix to the sample and run the sample using the PCR program in table eight of the manuscript.

Then add 21.5 microliters of universal adapter ligation mix to the sample and incubate for 15 minutes at 20 degrees Celsius. Add 29 microliters of LM-PCR mix to the sample, then run the sample using the program from table 10 of the manuscript. Following 18 cycles of LM-PCR, ChIP-exo samples from mouse motor neurons were electrophoresed on a 1.5%agarose gel.

The results for ChIP-exo Islet-1 showed amplified DNA libraries around 200 to 400 base pairs indicating that the ChIP-exo libraries were successfully amplified by LM-PCR. The band around 100 base pairs represents artifacts from adapters and PCR primers. The no antibody control demonstrates that non-specific background DNA was digested by the lambda exonuclease treatment.

Using ChIP-exo and ChIP-seq, Islet-1 bound locations were identified. The ChIP-exo signal was highly focused at Islet-1 binding sites detecting multiple clustered Islet-1 transcription factor binding patterns. The ChIP-seq signal displayed broader signals indicating that ChIP-exo had higher mapping resolution than ChIP-seq for Islet-1.

Multiple transcription factors often bind DNA next to each other as a protein-DNA complex. ChIP-exo allows us to study how multiple transcription factors recognize and regulate target DNA.