Type 1 diabetes (T1D) is an autoimmune disease characterized by autoreactive T cell-mediated destruction of insulin-producing pancreatic beta-cells. Loss of beta-cells leads to insulin insufficiency and hyperglycemia, with patients eventually requiring lifelong insulin therapy to maintain normal glycemic control. Since T1D has been historically defined as a disease of immune system dysregulation, there has been little focus on the state and response of beta-cells and how they may also contribute to their own demise. Major hurdles to identifying a cure for T1D include a limited understanding of disease etiology and how functional and transcriptional beta-cell heterogeneity may be involved in disease progression. Recent studies indicate that the beta-cell response is not simply a passive aspect of T1D pathogenesis, but rather an interplay between the beta-cell and the immune system actively contributing to disease. Here, we comprehensively review the current literature describing beta-cell vulnerability, heterogeneity, and contributions to pathophysiology of T1D, how these responses are influenced by autoimmunity, and describe pathways that can potentially be exploited to delay T1D.
Edited by Joel M. Gottesfeld Diabetes is characterized by a loss of -cell mass, and a greater understanding of the transcriptional mechanisms governing -cell function is required for future therapies. Previously, we reported that a complex of the Islet-1 (Isl1) transcription factor and the co-regulator single-stranded DNA-binding protein 3 (SSBP3) regulates the genes necessary for -cell function, but few proteins are known to interact with this complex in -cells. To identify additional components, here we performed SSBP3 reverse-cross-linked immunoprecipitation (ReCLIP)-and MSbased experiments with mouse -cell extracts and compared the results with those from our previous Isl1 ReCLIP study. Our analysis identified the E3 ubiquitin ligases ring finger protein 20 (RNF20) and RNF40, factors that in nonpancreatic cells regulate transcription through imparting monoubiquitin marks on histone H2B (H2Bub1), a precursor to histone H3 lysine 4 trimethylation (H3K4me3). We hypothesized that RNF20 and RNF40 regulate similar genes as those regulated by Isl1 and SSBP3 and are important for -cell function. We observed that Rnf20 and Rnf40 depletion reduces -cell H2Bub1 marks and uncovered several target genes, including glucose transporter 2 (Glut2), MAF BZIP transcription factor A (MafA), and uncoupling protein 2 (Ucp2). Strikingly, we also observed that Isl1 and SSBP3 depletion reduces H2Bub1 and H3K4me3 marks, suggesting that they have epigenetic roles. We noted that the RNF complex is required for glucose-stimulated insulin secretion and normal mitochondrial reactive oxygen species levels. These findings indicate that RNF20 and RNF40 regulate -cell gene expression and insulin secretion and establish a link between Isl1 complexes and global cellular epigenetics.
Pancreatic islet cell development is regulated by transcription factors (TFs) that mediate embryonic progenitor differentiation toward mature endocrine cells. Prior studies from our lab and others showed that the islet‐enriched TF, Islet‐1 (Isl1), interacts with the broadly‐expressed transcriptional co‐regulator, Ldb1, to regulate islet cell maturation and postnhyperatal function (by embryonic day (E)18.5). However, Ldb1 is expressed in the developing pancreas prior to Isl1 expression, notably in multipotent progenitor cells (MPCs) marked by Pdx1 and endocrine progenitors (EPs) expressing Neurogenin‐3 (Ngn3). MPCs give rise to the endocrine and exocrine pancreas, while Ngn3+ EPs specify pancreatic islet endocrine cells. We hypothesized that Ldb1 is required for progenitor identity in MPC and EP populations during development to impact islet appearance and function. To test this, we generated a whole‐pancreas Ldb1 knockout, termed Ldb1ΔPanc, and observed severe developmental and postnatal pancreas defects including disorganized progenitor pools, a significant reduction of Ngn3‐expressing EPs, Pdx1HI β‐cells, and early hormone+ cells. Ldb1ΔPanc neonates presented with severe hyperglycemia, hypoinsulinemia, and drastically reduced hormone expression in islets, yet no change in total pancreas mass. This supports the endocrine‐specific actions of Ldb1. Considering this, we also developed an endocrine‐enriched model of Ldb1 loss, termed Ldb1ΔEndo. We observed similar dysglycemia in this model, as well as a loss of islet identity markers. Through in vitro and in vivo chromatin immunoprecipitation experiments, we found that Ldb1 occupies key Pdx1 and Ngn3 promoter domains. Our findings provide insight into novel regulation of endocrine cell differentiation that may be vital toward improving cell‐based diabetes therapies.
Pancreatic islets are comprised of hormone secreting cell types that are vital regulators of glucose metabolism. Specifically, the pancreatic β cell is indispensable for glucose control and its dysfunction is central to diabetes mellitus. β cell development is regulated by transcription factor (TF) cascades, mediating differentiation of progenitors into mature insulin producing β cells. Our prior studies show that TF, Islet1 (Isl1), interacts with a transcriptional co‐regulator, Ldb1, to regulate β cell maturation from embryonic day (E) 18.5 onward. However, Ldb1 is also expressed in early stages of pancreas development, before Isl1 is present (as early as E10.5). The earlier Ldb1+ cell types include Pdx1+ multipotent progenitor cells (MPCs) and endocrine progenitors expressing the TF Neurogenin3 (Ngn3). MPC progeny will populate the entire pancreas (endocrine and exocrine), while Ngn3+ endocrine progenitors (Isl1−) are fated to become islet cells. Our hypothesis is that Ldb1 has Isl1 independent roles in maintaining progenitor identity in these requisite populations during early pancreatic development. To test this we generated a whole pancreas knockout of Ldb1 (Ldb1Δpanc) and observed severe developmental and postnatal phenotypes. At E13.5, Ldb1Δpanc mice exhibit disorganized progenitor pools, suggesting early defects in endocrine identity. At E15.5, Ldb1Δpanc mice had a significant reduction of Ngn3+ progenitors and Pdx1HI immunoreactivity, a mark of presumptive β cells. Ldb1Δpanc mice die by postnatal day 7 (P7) with severe hyperglycemia and hypoinsulinemia due to drastic islet hormone cell reduction. Interestingly, total pancreatic mass remained unchanged in Ldb1Δpanc neonates, suggesting that Ldb1 impacts are islet specific in the pancreas. Considering these observations, we generated a new model of Ldb1 loss specifically in Ngn3+ islet progenitors, termed Ldb1Δendo. We confirmed loss of Ldb1 in endocrine clusters and observed postnatal hyperglycemia, with a reduction of islet cells in neonates, similar to that seen in Ldb1Δpanc mice. Chromatin immunoprecipitation in vivo and in vitro highlights that Ldb1 imparts control on Pdx1 through occupation of the Pdx1 Area I–II regulatory domains, and that Ngn3 control also occurs via direct occupation. We are now further assessing the developmental phenotype, examining markers of proliferation, apoptosis, and altered cell identity through lineage tracing in the Ldb1Δendo model. Concomitantly, our published work revealed that Ldb1 and Isl1 interact in complex with the single stranded DNA binding protein co‐regulator, SSBP3, which helps stabilize this transcriptional complex. To examine the functional relationship between SSBP3, Isl1, and Ldb1 in vivo, we developed a new SSBP3 floxed mouse model in order to generate pancreas wide loss of SSBP3 (SSBP3Δpanc). We observed preliminary reductions in key islet mRNAs along with hyperglycemia in neonates at P1. Our work provides insight into the transcriptional complexes dictating how islet progenitors adopt their cell fate....
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 SSBP3Δislet. We found that SSBP3Δisletneonates 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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.