ABSTRACT: In mammals, the peptide hormone vasopressin controls renal water excretion, largely through its actions in the renal collecting duct to regulate the molecular water channel aquaporin-2 (AQP2). There are two modes of regulation: 1) short-term regulation by membrane trafficking; and 2) long-term regulation involving vasopressin-induced changes in the abundance of the aquaporin-2 protein. Defects in the long-term regulation have been implicated in multiple water balance disorders. With the objetive to identify genes that are transcriptionally regulated in response to vasopressin in cultured mouse collecting duct cells (mpkCCD), we carried out both RNA-seq to measure all transcripts (n=9) and ChIP-seq for RNA polymerase II (Polr2a) binding along the genome (n=3). Observations were made both after 24-hr treatment with the vasopressin analog dDAVP and with vehicle. Protocol:Confluent monolayers mpkCCD cells were washed in ice-cold 1X PBS (Ca+2/Mg+2-free) followed by cross-linking in 1.11% formaldehyde in Covaris Fixing Buffer (5.5 ml) for 5min at room temperature with gentle rocking (PN 520075, Covaris, Woburn, Massachusetts). Cross-linking was terminated by the addition of 0.3 ml of 1X Covaris Quenching Buffer with rocking. The buffer solution was aspirated and 1X ice-cold PBS was added to each plate. Cells were scraped into 15ml conical tubes, spun down (200 x g, 5 min), and washed twice with PBS. After the second wash, cell pellets were resuspended in 10 ml of 1X Covaris Lysis Buffer for 10 min at 4o C with rocking. Nuclei were pelleted by spinning at 1,700 x g for 5 min at 4o C. Nuclear pellets was resuspended in the 1X Covaris Wash Buffer and incubated for 10 mins at 4o C with rocking. Samples were spun at 1,700 x g for 5 min at 4o C. Nuclear pellets were washed twice with the Covaris Non-ionic Shearing Buffer and spun at 1,700 x g for 5 min at 4o C. Nuclei were resuspended in Covaris Non-ionic Shearing Buffer (Maximum of 1-3x 10^7cells per ml of buffer). Shearing. Two 20 ul aliquots of the resuspended pellets were removed prior to shearing to be used as subsequent unsheared controls for immunoblotting and agarose gel electrophoresis. The remaining samples were transferred to AFA tubes (TC12 x 12 mm) and volumes adjusted to 1 ml (V)). Samples were sheared using a Covaris S2 SonoLAB Single system for two minutes (Duty cycle: 5%; Intensity: 4; cycle: 200). Total nuclear protein concentration of each sample in mcg/ml (R) was determined with the BCA Protein Assay Kit (Thermo Scientific, Rockford, IL). To test shearing efficiency, a 20 ul aliquot from each tube was removed after shearing to test the efficacy of shearing. The aliquots, including the unsheared sample, were incubated with 100 ul 1X TE buffer, 10 ul 10% SDS, and RNAse A (10 ug/ul, Cell Signaling Technology, Danvers, Massachusetts) for 30 mins at 37o C. 1 ul proteinase K (20 ug/ul) was added and the samples were incubated overnight at 65o C. The DNA was purified using DNA Clean & Concentrator columns (Catalog #: D4013, Zymo Research, Irvine, CA) and ran on an E-gel EX 2% agarose (Invitrogen, Carlsbad, CA). Target fragment sizes for chromatin immunoprecipitation were in the range of 150-700 bp in length with average fragment size of 300 bp. Testing epitope integrity (immunoblotting). Total protein content was determined with the BCA Protein Assay Kit (Thermo Scientific, Rockford, IL). Samples were then diluted with sample buffer (5X Laemmli buffer) at 20% (v/v) of the entire sample volume. Proteins were resolved by SDS-PAGE (4-20% polyacrylamide gels, Criterion, Bio-Rad, Hercules, CA) and transferred electrophoretically onto nitrocellulose membranes as previously described (Hwang, 2010). Membranes were probed with anti-RNA polymerase II (Catalog #: MMS-126R, Covance). The antibody was used at a 1:500 dilution (anti-RNA polymerase II) (in Odyssey blocking buffer containing 0.1% Tween 20) overnight at 4°C. After 1-h incubation with secondary antibody (Li-Cor Biosciences 680 anti-rabbit immunoglobulin G or 800 anti-mouse immunoglobulin G; Lincoln, NE) at 1:5000 dilution, sites of antibody-antigen reaction were detected using an Odyssey infrared imager (Li-Cor). Chromatin immunoprecipitation. 10 ul of sample equivalent to 2% input was removed prior to IP. Chromatin Immunoprecipitation was performed using 30-90 ug of sheared chromatin in 500 uL of 1X Covaris Non-ionic Shearing Buffer with 10 μl anti-RNA polymerase II antibodies. Samples were incubated overnight at 4°C with rotation, followed by addition of ChIP Grade Protein G Magnet Beads (Catalog #: 9006, Cell Signaling Technology) and incubation for 2 h at 4o C with rotation. The beads were washed a total of four times, including three low salt washes followed by one high salt wash for 5 mins/wash at 4o C (SimpleChIP protocol #9003, Cell Signaling Technology). Immunoprecipitated chromatin was eluted from the beads with 150 ul of 1X ChIP Elution Buffer for 30 mins at 65o C in a thermomixer. To reverse crosslinks, 6 ul of 5M NaCl and 2 ul of 20 ug/ul Proteinase K were added to the chromatin followed by incubation at 65o C overnight. DNA was purified according to SimpleChIP protocol #9003, Cell Signaling Technology. q-PCR. To check the success of the chromatin immunoprecipitation, input and ChIPâ??ed DNA samples were amplified with SYBR Green PCR Master Mix (Applied Biosystems) on a 7900HT Fast Real-Time PCR System. Primer sets corresponding to Hox9a and actin genes were used as positive controls for both histone and RNA polymerase II IPâ??s. (see Supplementary Table X for primer sequences). Library construction, amplification and sequencing. Loaded fixed volume of total eluate. Library construction was performed on a NuGEN instrument using the protocol and reagents outlined in the Ovation SP Ultralow Library Systems kit. Libraries were then enriched via PCR. After amplification, DNA was measured. A fixed amount of the total measured DNA was used for sequencing in an Illumina Hiseq2000 instrument to obtain single read data. High-throughput sequencing and read mapping. Libraries for Illumina sequencing of the samples were prepared using the the protocol and reagents outlined in the Ovation SP Ultralow Library Systems kit , NuGen Mondrian SP+ (NuGEN, San Carlos, CA) . Samples were sequenced using an Illumina HiSeq2000, obtaining 1- by 50-bp reads. We combine RNA-seq and ChIP-seq to identify genes whose mRNA abundances change in tandem with changes in RNA polymerase II binding, as a mean to identify genes that are transcriptionally regulated in response to vasopressin. In addition, the pattern of RNA polymerase II binding along the gene provides important information about regulatory mechanisms. We identify two populations of genes regulated by vasopressin in mpkCCD cells, viz. 1) those with increases in transcript abundance and increases in RNA polymerase II binding throughout the gene body and; 2) genes that show increases in RNA polymerase II binding in the promotor proximal region but do not produce increases in transcript level. The former group is restricted to 35 genes and includes Aqp2. The latter group is made up of the majority of expressed genes and is presumably due to a widespread increase in transcriptional initiation without proportionate release of pausing.
