Competing interests: All authors state no conflict of financial or non-financial competing interest.

Author Contributions: D.K. designed and performed experiments, analyzed data, and prepared the manuscript; D.A.R., K.G. and M.L. provided experimental support; K.S. provided GFP immunohistochemistry and Cre-in situ hybridization staining; D.A.R., K.G. and M.L. provided critical revision of the manuscript for important intellectual content; M.L. administrative and technical support; and H.A. study supervision.

Grant Support: This project is funded by grants AL1174/5-2, AL1174/6-1, AL 1174/8-1 and LE 3222/1-2 from Deutsche Forschungsgemeinschaft. DAR is supported by the German Cancer Aid (70113697).