Sel1L-Hrd1 ER-associated degradation maintains β cell identity via TGF-β signaling

beta Cell apoptosis and dedifferentiation are 2 hotly debated mechanisms underlying beta cell loss in type 2 diabetes; however, the molecular drivers underlying such events remain largely unclear. Here, we performed a side-by-side comparison of mice carrying beta cell-specific deletion of ER-associated degradation (ERAD) and autophagy. We reported that, while autophagy was necessary for II cell survival, the highly conserved Sel1L-Hrd1 ERAD protein complex was required for the maintenance of beta cell maturation and identity. Using single-cell RNA-Seq, we demonstrated that Sel1L deficiency was not associated with beta cell loss, but rather loss of beta cell identity. Sel1L-Hrd1ERAD controlled II cell identity via TGF-beta signaling, in part by mediating the degradation of TGF-beta receptor 1. Inhibition of TGF-beta signaling in Sel1L-deficient beta cells augmented the expression of II cell maturation markers and increased the total insulin content. Our data revealed distinct pathogenic effects of 2 major proteolytic pathways in beta cells, providing a framework for therapies targeting distinct mechanisms of protein quality control.