Alexis U. Oguh1, Matthew W. Haemmerle1, Sabyasachi Sen1, Andrea V. Rozo1, Shristi Shrestha2, Jean-Philippe Cartailler2, Hossein Fazelinia3, Hua Ding3, Sam Preza1,4, Juxiang Yang1, Xiaodun Yang1, Lori Sussel5, Juan R. Alvarez-Dominguez1,4, Nicolai Doliba6, Lynn A. Spruce3, Rafael Arrojo e Drigo2 and Doris A. Stoffers1 1Institute for Diabetes, Obesity, and Metabolism, Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19146, USA; 2Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232, USA; 3Proteomics Core Facility, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19146, USA; 4Department of Cell and Developmental Biology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19146, USA; 5Department of Pediatrics and Cell and Developmental Biology, Barbara Davis Center for Diabetes, University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045, USA; 6Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19146, USA Corresponding author: stofferspennmedicine.upenn.edu Abstract

The Cullin-3 E3 ligase adaptor protein SPOP targets proteins for ubiquitination and proteasomal degradation. We previously established the β-cell transcription factor (TF) and human diabetes gene PDX1 as an SPOP substrate, suggesting a functional role for SPOP in the β cell. Here, we generated a β-cell-specific Spop deletion mouse strain (SpopβKO) and found that Spop is necessary to prevent aberrant basal insulin secretion and for maintaining glucose-stimulated insulin secretion through impacts on glycolysis and glucose-stimulated calcium flux. Integration of proteomic, TF-regulatory gene network, and biochemical analyses identified XBP1 as a functionally important SPOP substrate in pancreatic β cells. Furthermore, loss of SPOP strengthened the IRE1α–XBP1 axis of unfolded protein response (UPR) signaling. ER stress promoted proteasomal degradation of SPOP, supporting a model whereby SPOP fine-tunes XBP1 activation during the UPR. These results position SPOP as a regulator of β-cell function and proper UPR activation.

Received June 10, 2024. Accepted November 7, 2024.