Acknowledgements
We thank Fan Xia for technical support and Xiong Xiao for the assistance with picture editing. This work was supported by the National Natural Science Foundation of China [81370320, 81570334] (TL Yang), and [81141068, 31571241] (H Huang), Xiangya Eminent Doctor Project #013 (TL Yang), and Zijing Foundation (Q Duan).
Potential conflict of interest
Disclosure forms provided by the authors are available with the full text of this article online at http://dx.doi.org/10.1080/10428194.2016.1219907.