Instrument: NextSeq 550
Strategy: RNA-Seq
Source: TRANSCRIPTOMIC
Selection: cDNA
Layout: SINGLE
Construction protocol: Tissues were snap-frozen in liquid nitrogen immediately after dissection and stored at -80°C till used. For RNA extraction, the tissues were soaked in TRI-reagent (Sigma) and were homogenized by bead beater (Bead Ruptor e24, OMNI), and then proceeded by a standard TRI-reagent based RNA extraction protocol. RNA concentration was determined using NanoDrop™ 2000 Spectrophotometer (Thermo Fisher Scientific). RNA Integrity was validated using 2200 TapeStation (Agilent). cDNA was generated from 1ul of mRNA of each sample. cDNA quantity in each sample was evaluated by qPCR for Actin B gene, and then equivalent amounts of mRNA of each sample were taken for RNAseq library construction. Library construction was performed in a 96-well plate format. First, to open secondary RNA structures and allow annealing of the RT primer, the samples were incubated at 72˚C for 3 min and immediately transferred to 4˚C. Then, RT reaction mix (10 mM DTT, 4 mM dNTP, 2.5 U/µl Superscript III RT enzyme in 50 mM Tris-HCl (pH 8.3), 75 mM KCl, 3 mM MgCl2) was added into each well of the 96-well plate and the reaction was mixed. The 96-well plate was then spun down and moved into a cycler (Eppendorf) for the following incubation: 2 min at 42˚C, 50 min at 50˚C, 5 min at 85˚C. Indexed samples with equivalent amount of cDNA were pooled. The pooled cDNA was converted to double-stranded DNA with a second strand synthesis kit (NEB) in a 20µl reaction, incubating for 2.5 h at 16˚C. The product was purified with 1.4x volumes of SPRI beads, eluted in 8 µl and in-vitro transcribed (with the beads) at 37˚C overnight for linear amplification using the T7 High Yield RNA polymerase IVT kit (NEB). Following IVT, the DNA template was removed with Turbo DNase I (Ambion) 15 min at 37˚C and the amplified RNA (aRNA) purified with 1.2x volumes of SPRI beads. Library preparation for high-throughput sequencing: The aRNA was chemically fragmented into short molecules (median size ~200 nucleotides) by incubating 3 min at 70˚C in Zn2+ RNA fragmentation solution (Ambion) and purified with two volumes of SPRI beads. The aRNA (5 µl) was preincubated 3 min at 70˚C with 1 µl of 100 µM ligation adapter; then, 14 µl of a mix containing 9.5% DMSO, 1 mM ATP, 20% PEG8000 and 1 U/µl T4 ligase in 50 mM Tris HCl pH7.5, 10 mM MgCl2 and 1mM DTT was added. The reaction was incubated at 22˚C for 2 h. The ligated product was reverse transcribed using Affinity Script RT enzyme (Agilent; reaction mix contains Affinity Script RT buffer, 10 mM DTT, 4 mM dNTP, 2.5 U/µl RT enzyme) and a primer complementary to the ligated adapter. The reaction was incubated for 2 min at 42˚C, 45 min at 50˚C and 5 min at 85˚C. The cDNA was purified with 1.5x volumes of SPRI beads. The library was completed and amplified through a nested PCR reaction with 0.5 µM of P5_Rd1 and P7_Rd2 primers and PCR ready mix (Kapa Biosystems). The forward primer contains the Illumina P5-Read1 sequences and the reverse primer contains the P7-Read2 sequences. The amplified pooled library was purified with 0.7x volumes of SPRI beads to remove primer leftovers. Library concentration was measured with a Qubit fluorometer (Life Technologies) and mean molecule size was determined with a 2200 TapeStation instrument (Agilent). MARS-Seq libraries were sequenced using an Illumina HiSeq 1500. 3' RNA-seq for digital gene expression quantitation