Cell culture and virus propagation. Dengue virus serotype 2 (DENV-2) strain New Guinea C (NGC) was used in our experiments, prepared by propagating in C6/36 cells. These cells were maintained in L-15 medium (Gibco Life Technologies), supplemented with fetal bovine serum and antibiotic antimycotic solution (Himedia Laboratories). Virus containing cell supernatants were harvested upon observation of cytopathic effect or physical changes in the cells, filtered and stored at -70oC till further use. Virus stocks were titered by plaque assays using BHK21 cells.
The STUB1/CHIP gene in HEK293T cells was knocked out with CRISPR-Cas9 plasmid containing CHIP sgRNA as described previously19,31. DNA oligonucleotides were synthesized by using the sgRNA designer tool available in the web interface of Broad Institute, USA from cDNA sequence of CHIP. Source of control sgRNA was firefly (Photinus pyralis) luciferase gene (Supplementary Table S1). Oligonucleotides were mixed with phosphonucleotide kinase enzyme reagent mix and kept for 40 min at 37oC. Reaction was stopped by adding 0.1M NaCl and incubated at 65oC for 20 min. Oligonucleotides were annealed by boiling the mix for 5 min at 100 °C and allowed the mix to bring to room temperature. BbsI digested pSpCas9 (BB)-2A-GFP (pX458) plasmid was mixed with reaction mix for ligation with annealed oligos and transformed in to E. coli to propagate the plasmid. Clone was confirmed by Sanger sequencing and used to transfect HEK-293T cells. The said plasmid has GFP protein to sort the positive cells by sorter (BD FACS Aria-II). Sorted cells were maintained in DMEM containing 10% FBS and antibiotics and gene knockdown was checked by western blot.
Transfections and plasmids. Transfections were performed using linear Polyethyleneimine (MW 25,000; Polysciences Inc., USA) reagents in accordance with the manufacturer’s instructions. Addgene plasmids pCMV5B-HA-Smad2 (# 11734), pCMV5B-Flag-Smad3 (# 11742), pCMV5-Smad7-HA (# 11733), pCMV5B-Flag-Smurf2 wt (# 11746), pCMV5B-Flag-Smurf2 C716A (# 11747) were gifts from Dr. Jeffrey Wrana. Two addgene plasmids; pcDNA Flag-Smad4M (# 14959) and CS2 HA-Smad6 (# 14962) were gifts from Joan Massague. DENV NS1 encoding plasmid (pCDNA 3.1-His-NS1) was kindly provided by Dr. Ronaldo Mohana-Borges, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil. Dr. Chia-Yi Yu, Department of Microbiology, College of Medicine, National Cheng Kung University, Taiwan kindly provided DENV NS5 encoding Plasmid (pCAG‐HA‐NS5).
Western blotting and antibodies. Supernatant of lysed HEK-293T cells were resuspended in SDS sample loading buffer, resolved on 10% SDS-PAGE gel and transferred onto a nitrocellulose membrane (Cat # SCNJ8101XXXX101, MDI, Advanced Microdevices Ltd., India). Membrane was blocked with nonfat milk or BSA and probed with different specific primary antibodies, followed by probing with respective HRP-conjugated secondary Abs (Jackson ImmunoResearch, USA). Membrane was washed and developed using chemiluminescence solutions.
Anti-Smad2 (cat#5339), phospho-SMAD2 (Ser465/Ser467; cat#18338), anti phospho-Smad3 antibody (Ser423/425; cat#9520), anti-Lamin A/C antibody (cat#2032) antibodies were obtained from Cell Signaling Technology; anti-GAPDH antibody (cat#Sc-32233) from Santa Cruz Biotechnology; anti-HA tag antibody (cat #M1001010), anti-His tag antibody (cat#M1001020) from Immunotag/G-Biosciences and anti-FLAG® M2 antibody (cat #F1804), anti-DENV NS1 antibody (cat# SAB2702308) from Sigma-Aldrich.
Cycloheximide chase assay. Degradation kinetics of Smad proteins was studied by cycloheximide chase assay32. Briefly, HEK-293T cells were transfected with respective plasmids for 24 h and then treated with Cycloheximide (cat #239764, Sigma-Aldrich). Cell lysates were prepared at different time points (0,2,4,6,8 and 10 h) and analyzed by immunoblotting.
Activators and inhibitors. Human Transforming Growth Factor-beta 1 (#TC298; Himedia) was used to stimulate Smad signaling. Lysosomal function inhibitor chloroquine (#C6628, Sigma-Aldrich) and MG132 (#C2211, Sigma-Aldrich), a membrane permeable proteasome inhibitor were used to treat HEK-293T cells for 8 hours prior harvesting for immunoblotting.
Preparation of nuclear and cytoplasmic extracts. Nuclear and cytoplasmic fractions were prepared from cells transfected with different plasmids of interest using Nuclear and Cytoplasmic Extraction Kit (G-Biosciences, USA) as per manufacturer's instructions and subjected to immunoblotting. In our experiments, Lamin A/C and GAPDH were used as a nuclear and cytosolic loading control respectively.
Gene expression analysis. Total cellular RNA was extracted from DENV-2 infected or control HEK293T cells using RNeasy Mini Kit (Qiagen) as per the manufacturer’s instructions. The quantity and quality of RNA were determined on a Nanodrop instrument (ThermoFisher Scientific). 500 ng of RNA was subjected to cDNA synthesis by using High Capacity cDNA Reverse Transcription Kit (Applied Biosystems). Quantitative Real time PCR was performed on cDNA samples using PowerUp™ SYBR™ Green Master Mix (Applied Biosystems) on a ABI 7500 Fast Real Time PCR system (Applied Biosystems) as described in manufacturer’s protocol. Set of primers used in this study were described in Supplementary Table 2. Gene expression analysis was performed by ddCt method and fold changes were calculated.
Quantitative real-time PCR for viral replication. Viral RNA was extracted from cell supernatants harvested from DENV infected HEK-293T cells using QIAamp Viral RNA Mini Kit as per manufacturer instructions (Qiagen). Eluted RNA samples were subjected to qRT-PCR with very specific primers and probe of DENV-2 capsid region and TaqMan fast virus one step master mix (Applied Biosystems) as described previously33. Estimation of viral RNA (copies/mL) was carried out by using the standard curve generated from DENV-2 transcripts.
Statistical analysis. Western blot images were analyzed by using imageJ software. All results represented here as mean ± standard error obtained from three independent experiments. For the calculation of fold changes GAPDH was used as endogenous control. Statistical significance was calculated by using student t test and data was considered significant if p < 0.05.