Acknowledgments
This study was supported by the National Natural Science Foundation of China (no. 81473602), the State Key Program for Basic Research of China (973 Program, no. 2012CB518501), the Education Ministry's New Century Excellent Talents Supporting Plan, the Special Fund for the Authors Who Win the 100 Top Doctoral Dissertations Award of China (no. 2014084), and the Sichuan Youth Science & Technology Foundation (no. 15QNJJ0008).

Disclosure
Ke Qiu, Miaomiao Jing, and Ruirui Sun are co-first authors and contributed equally to this paper.

Competing Interests
The authors declare that they have no competing interests.