THE SSBP3 CO-REGULATOR IS REQUIRED FOR GLUCOSE HOMEOSTASIS, PANCREATIC ISLET ARCHITECTURE, AND BETA-CELL IDENTITY
Objective: Transcriptional complex activity drives the development and function of pancreatic islet cells to allow for proper glucose regulation. Our prior work highlighted that the LIM-homeodomain transcription factor (TF), Islet-1 (Isl1), and its interacting co-regulator, Ldb1, are vital effectors of developing and adult ß-cells. We further found that a member of the Single Stranded DNA-Binding Protein (SSBP) co-regulator family, SSBP3, interacts with Isl1 and Ldb1 in ß-cells and primary islets (mouse and human) to impact ß-cell target genes MafA and Glp1R. Members of the SSBP family stabilize TF complexes by binding directly to Ldb1 and protecting the complex from ubiquitin-mediated turnover. In this study, we hypothesized that SSBP3 would have critical roles in pancreatic islet cell development and function in vivo, similar to the Isl1::Ldb1 complex. Methods: We first generated a novel SSBP3 LoxP allele mouse line, where Cre-mediated recombination imparts a predicted early protein termination. We bred this mouse to constitutive Cre lines (Pdx1- and Pax6-driven) to recombine of SSBP3 in the developing pancreas and islet (SSBP3DeltaPanc and SSBP3DeltaIslet), respectively. We assessed glucose tolerance and used immunofluorescence to detect changes in islet cell abundance and markers of ß-cell identity and function. Using an inducible Cre system we also deleted SSBP3 in the adult ß-cell, a model termed SSBP3Deltaß-cell. We measured glucose tolerance as well as glucose-stimulated insulin secretion (GSIS), both in vivo and in isolated islets in vitro. Using islets from control and SSBP3Deltaß-cell we conducted RNA-Seq and compared our results to published datasets for similar ß-cell specific Ldb1 and Isl1 knockouts to identify commonly regulated target genes. Results: SSBP3DeltaPanc and SSBP3DeltaIslet neonates present with hyperglycemia. SSBP3DeltaIslet mice are glucose intolerant by P21 and exhibit a reduction of ß-cell maturity markers MafA, Pdx1, and UCN3. We observe disruptions in islet cell architecture with an increase in glucagon+ a-cells and ghrelin+ e-cells at P10. Inducible loss of ß-cell SSBP3 in SSBP3Deltaß-cell causes hyperglycemia, glucose intolerance, and reduced GSIS. Transcriptomic analysis of  14-week SSBP3Deltaß-cell islets revealed a decrease in ß-cell function gene expression (Ins, MafA, Ucn3), increased stress and dedifferentiation markers (Neurogenin-3, Aldh1a3, Gastrin), and shared differentially expressed genes between SSBP3, Ldb1, and Isl1 in adult ß-cells. Conclusions: SSBP3 drives proper islet development and identity, where its loss causes altered islet-cell abundance and glucose regulatory function. ß-cell SSBP3 is required for GSIS and glucose homeostasis, at least partially through shared regulation of Ldb1 and Isl1 target genes. Overall design: 6 Samples