Designed the experiments: QL, XN, and ZP. Conceived and supervised the study: XN, QW, ZP, and LN. Performed the experiments: QL, YZ, and HS. Data analyzed by: QL, ZP, KP, and BJ. Wrote the manuscript: QL, ZP, and DZ. Proofread the manuscript: DZ and HW. All authors read and approved the final manuscript.