The SSBP3 Co-Regulator Is a Novel Driver of Islet Cell Structure and Function

The activities of transcriptional complexes drive the proper development and function of insulin producing beta-cells, ultimately required for the regulation of glucose homeostasis. Our prior work helped to establish that the LIM-homeodomain transcription factor (TF), Islet-1 (Isl1), directly interacts with the Ldb1 co-regulator in developing and adult beta-cells. We further found that a member of the Single Stranded DNA-Binding Protein (SSBP) co-regulator family, SSBP3, interacts with the Isl1:Ldb1 complex in beta-cells and primary islets to impact critical target genes MafA and Glp1R. Members of the SSBP family of co-regulators stabilize TF complexes in various tissues, ranging from brain to skin, by binding directly to Ldb1 and protecting the factors from ubiquitin-mediated turnover. Because of this, we hypothesized that SSBP3 would have similarly critical roles as Isl1 and Ldb1 for beta-cell development and function in vivo. To assess this, we first developed a novel SSBP3 floxed mouse line, where Cre-mediated recombination is predicted to impart loss of the Ldb1-interacting domain, plus an early termination. We bred this mouse into a Pax6-Cre transgenic line to recombine SSBP3 in the developing pancreatic islet, a model termed SSBP3(islet). We found that SSBP3(islet) neonates become progressively hyperglycemic and both male and female mice are glucose intolerant as early as postnatal day (P) 21. These results are similar to previous Ldb1 and Isl1 knockouts in the embryonic islet, both of which were hyperglycemic by P10. We observed a reduction of the beta-cell maturity marker, MafA, and disruptions in islet cell architecture with an apparent increase in both glucagon(+) alpha-cells and ghrelin(+)epsilon-cells at P10 and P28. In ongoing studies we are generating embryonic day (E)18.5 embryos to determine islet development defects and will conduct chromatin immunoprecipitation (ChIP) experiments to determine the beta-cell and islet genes directly bound by SSBP3 in vivo. These experiments will further elucidate the regulation of islet function by LIM complexes, knowledge that is central not only for our understanding of glucose homeostasis but for the development of novel diabetes therapeutics.