This work was supported by European Molecular Biology Organization and California Institute for Regenerative Medicine (CIRM) postdoctoral fellowships to MG; predoctoral fellowships from the National Science Foundation and the University of California San Francisco (UCSF) Mentorship and Research Assistantship Program to CC; a predoctoral fellowship from the Faculty of Science and Technology in Portugal to AGM; and grants from the UCSF Institute for Regeneration Medicine, UCSF School of Medicine Research Evaluation and Allocation Committee, the National Institute of Diabetes and Digestive and Kidney Diseases Diabetes Endocrinology Research Center, and CIRM to MRS. CC and HL acknowledge partial support from National Institutes of Health grant GM070808 and the Packard Fellowship in Science and Engineering to HL.