A pancreatic beta-cell-specific homolog of glucose-6-phosphatase emerges as a major target of cell-mediated autoimmunity in diabetes

Transcriptome Analysis of Rheumatoid Arthritis Uncovers Genes Linked to Inflammation-Induced Pain

Autoimmune diseases such as rheumatoid arthritis (RA) can promote states of chronic Inflammation with accompanying tissue destruction and pain. RA can cause inflammatory synovitis in peripheral joints, particularly within the hands and feet, but can also sometimes trigger temporomandibular joint (TMJ) arthralgia. To better understand the effects of ongoing Inflammation-induced pain signaling, dorsal root ganglia (DRGs) were acquired from individuals with RA for transcriptomic study. We conducted RNA sequencing from the L5 DRGs because it contains the soma of the sensory neurons that innervate the affected joints in the foot. DRGs from 5 RA patients were compared with 9 non-arthritic controls. RNA-seq of L5 DRGs identified 128 differentially expressed genes (DEGs) that were dysregulated in the RA subjects as compared to the non-arthritic controls. The DRG resides outside the blood brain barrier and, as such, our initial transcriptome analysis detected signs of an autoimmune disorder including the upregulated expression of immunoglobulins and other immunologically related genes within the DRGs of the RA donors. Additionally, we saw the upregulation in genes implicated in neurogenesis that could promote pain hypersensitivity. overall, our DRG analysis suggests that there are upregulated inflammatory and pain signaling pathways that can contribute to chronic pain in RA.

An Enhancer Within Abcb11 Regulates G6pc2 in C57BL/6 Mouse Pancreatic Islets

G6PC2 is predominantly expressed in pancreatic islet β-cells where it encodes a glucose-6-phosphatase catalytic subunit that modulates the sensitivity of insulin secretion to glucose by opposing the action of glucokinase, thereby regulating fasting blood glucose (FBG). Prior studies have shown that the G6pc2 promoter alone is unable to confer sustained islet-specific gene expression in mice, suggesting the existence of distal enhancers that regulate G6pc2 expression. Using information from both mice and humans and knowledge that single nucleotide polymorphisms (SNPs) both within and near G6PC2 are associated with variations in FBG in humans, we identified several putative enhancers 3' of G6pc2. One region, herein referred to as enhancer I, resides in the 25th intron of Abcb11 and binds multiple islet-enriched transcription factors. CRISPR-mediated deletion of enhancer I in C57BL/6 mice had selective effects on the expression of genes near the G6pc2 locus. In isolated islets, G6pc2 and Spc25 expression were reduced ∼50%, and Gm13613 expression was abolished, whereas Cers6 and nostrin expression were unaffected. This partial reduction in G6pc2 expression enhanced islet insulin secretion at basal glucose concentrations but did not affect FBG or glucose tolerance in vivo, consistent with the absence of a phenotype in G6pc2 heterozygous C57BL/6 mice.

ARTICLE HIGHLIGHTS

Pancreatic islet beta cell-specific deletion of G6pc2 reduces fasting blood glucose

The G6PC1, G6PC2 and G6PC3 genes encode distinct glucose-6-phosphatase catalytic subunit (G6PC) isoforms. In mice, germline deletion of G6pc2 lowers fasting blood glucose (FBG) without affecting fasting plasma insulin (FPI) while, in isolated islets, glucose-6-phosphatase activity and glucose cycling are abolished and glucose-stimulated insulin secretion (GSIS) is enhanced at submaximal but not high glucose. These observations are all consistent with a model in which G6PC2 regulates the sensitivity of GSIS to glucose by opposing the action of glucokinase. G6PC2 is highly expressed in human and mouse islet beta cells however, various studies have shown trace G6PC2 expression in multiple tissues raising the possibility that G6PC2 also affects FBG through non-islet cell actions. Using real-time PCR we show here that expression of G6pc1 and/or G6pc3 are much greater than G6pc2 in peripheral tissues, whereas G6pc2 expression is much higher than G6pc3 in both pancreas and islets with G6pc1 expression not detected. In adult mice, beta cell-specific deletion of G6pc2 was sufficient to reduce FBG without changing FPI. In addition, electronic health record-derived phenotype analyses showed no association between G6PC2 expression and phenotypes clearly unrelated to islet function in humans. Finally, we show that germline G6pc2 deletion enhances glycolysis in mouse islets and that glucose cycling can also be detected in human islets. These observations are all consistent with a mechanism by which G6PC2 action in islets is sufficient to regulate the sensitivity of GSIS to glucose and hence influence FBG without affecting FPI.

MHC Genomics and Disease: Looking Back to Go Forward

Ancestral haplotypes are conserved but extremely polymorphic kilobase sequences, which have been faithfully inherited over at least hundreds of generations in spite of migration and admixture. They carry susceptibility and resistance to diverse diseases, including deficiencies of CYP21 hydroxylase (47.1) and complement components (18.1), as well as numerous autoimmune diseases (8.1). The haplotypes are detected by segregation within ethnic groups rather than by SNPs and GWAS. Susceptibility to some other diseases is carried by specific alleles shared by multiple ancestral haplotypes, e.g., ankylosing spondylitis and narcolepsy. The difference between these two types of association may explain the disappointment with many GWAS. Here we propose a pathway for combining the two different approaches. SNP typing is most useful after the conserved ancestral haplotypes have been defined by other methods.

Navigating the Depths and Avoiding the Shallows of Pancreatic Islet Cell Transcriptomes

Islet gene expression has been widely studied to better understand the transcriptional features that define a healthy β-cell. Transcriptomes of FACS-purified α-, β-, and δ-cells using bulk RNA-sequencing have facilitated our understanding of the complex network of cross talk between islet cells and its effects on β-cell function. However, these approaches were by design not intended to resolve heterogeneity between individual cells. Several recent studies used single-cell RNA sequencing (scRNA-Seq) to report considerable heterogeneity within mouse and human β-cells. In this Perspective, we assess how this newfound ability to assess gene expression at single-cell resolution has enhanced our understanding of β-cell heterogeneity. We conduct a comprehensive assessment of several single human β-cell transcriptome data sets and ask if the heterogeneity reported by these studies showed overlap and concurred with previously known examples of β-cell heterogeneity. We also illustrate the impact of the inevitable limitations of working at or below the limit of detection of gene expression at single cell resolution and their consequences for the quality of single-islet cell transcriptome data. Finally, we offer some guidance on when to opt for scRNA-Seq and when bulk sequencing approaches may be better suited.

Virgin Beta Cells Persist throughout Life at a Neogenic Niche within Pancreatic Islets

Postnatal maintenance or regeneration of pancreatic beta cells is considered to occur exclusively via the replication of existing beta cells, but clinically meaningful restoration of human beta cell mass by proliferation has never been achieved. We discovered a population of immature beta cells that is present throughout life and forms from non-beta precursors at a specialized micro-environment or "neogenic niche" at the islet periphery. These cells express insulin, but lack other key beta cell markers, and are transcriptionally immature, incapable of sensing glucose, and unable to support calcium influx. They constitute an intermediate stage in the transdifferentiation of alpha cells to cells that are functionally indistinguishable from conventional beta cells. We thus identified a lifelong source of new beta cells at a specialized site within healthy islets. By comparing co-existing immature and mature beta cells within healthy islets, we stand to learn how to mature insulin-expressing cells into functional beta cells.

Inhibition of peptide aggregation by means of enzymatic phosphorylation

As is the case in numerous natural processes, enzymatic phosphorylation can be used in the laboratory to influence the conformational populations of proteins. In nature, this information is used for signal transduction or energy transfer, but has also been shown to play an important role in many diseases like tauopathies or diabetes. With the goal of determining the effect of phosphorylation on amyloid fibril formation, we designed a model peptide which combines structural characteristics of α-helical coiled-coils and β-sheets in one sequence. This peptide undergoes a conformational transition from soluble structures into insoluble amyloid fibrils over time and under physiological conditions and contains a recognition motif for PKA (cAMP-dependent protein kinase) that enables enzymatic phosphorylation. We have analyzed the pathway of amyloid formation and the influence of enzymatic phosphorylation on the different states along the conformational transition from random-coil to β-sheet-rich oligomers to protofilaments and on to insoluble amyloid fibrils, and we found a remarkable directing effect from β-sheet-rich structures to unfolded structures in the initial growth phase, in which small oligomers and protofilaments prevail if the peptide is phosphorylated.

G6PC2 Modulates the Effects of Dexamethasone on Fasting Blood Glucose and Glucose Tolerance

The glucose-6-phosphatase catalytic subunit 2 (G6PC2) gene encodes an islet-specific glucose-6-phosphatase catalytic subunit. G6PC2 forms a substrate cycle with glucokinase that determines the glucose sensitivity of insulin secretion. Consequently, deletion of G6pc2 lowers fasting blood glucose (FBG) without affecting fasting plasma insulin. Although chronic elevation of FBG is detrimental to health, glucocorticoids induce G6PC2 expression, suggesting that G6PC2 evolved to transiently modulate FBG under conditions of glucocorticoid-related stress. We show, using competition and mutagenesis experiments, that the synthetic glucocorticoid dexamethasone (Dex) induces G6PC2 promoter activity through a mechanism involving displacement of the islet-enriched transcription factor MafA by the glucocorticoid receptor. The induction of G6PC2 promoter activity by Dex is modulated by a single nucleotide polymorphism, previously linked to altered FBG in humans, that affects FOXA2 binding. A 5-day repeated injection paradigm was used to examine the chronic effect of Dex on FBG and glucose tolerance in wild-type (WT) and G6pc2 knockout mice. Acute Dex treatment only induces G6pc2 expression in 129SvEv but not C57BL/6J mice, but this chronic treatment induced G6pc2 expression in both. In 6-hour fasted C57BL/6J WT mice, Dex treatment lowered FBG and improved glucose tolerance, with G6pc2 deletion exacerbating the decrease in FBG and enhancing the improvement in glucose tolerance. In contrast, in 24-hour fasted C57BL/6J WT mice, Dex treatment raised FBG but still improved glucose tolerance, with G6pc2 deletion limiting the increase in FBG and enhancing the improvement in glucose tolerance. These observations demonstrate that G6pc2 modulates the complex effects of Dex on both FBG and glucose tolerance.

Moving on from GWAS: functional studies on the G6PC2 gene implicated in the regulation of fasting blood glucose

Genome-wide association studies (GWAS) have shown that single-nucleotide polymorphisms (SNPs) in G6PC2 are the most important common determinants of variations in fasting blood glucose (FBG) levels. Molecular studies examining the functional impact of these SNPs on G6PC2 gene transcription and splicing suggest that they affect FBG by directly modulating G6PC2 expression. This conclusion is supported by studies on G6pc2 knockout (KO) mice showing that G6pc2 represents a negative regulator of basal glucose-stimulated insulin secretion that acts by hydrolyzing glucose-6-phosphate, thereby reducing glycolytic flux and opposing the action of glucokinase. Suppression of G6PC2 activity might, therefore, represent a novel therapy for lowering FBG and the risk of cardiovascular-associated mortality. GWAS and G6pc2 KO mouse studies also suggest that G6PC2 affects other aspects of beta cell function. The evolutionary benefit conferred by G6PC2 remains unclear, but it is unlikely to be related to its ability to modulate FBG.

Multiple functional polymorphisms in the G6PC2 gene contribute to the association with higher fasting plasma glucose levels

AIMS/HYPOTHESIS

We previously identified the G6PC2 locus as a strong determinant of fasting plasma glucose (FPG) and showed that a common G6PC2 intronic single nucleotide polymorphism (SNP) (rs560887) and two common G6PC2 promoter SNPs (rs573225 and rs13431652) are highly associated with FPG. However, these promoter SNPs have complex effects on G6PC2 fusion gene expression, and our data suggested that only rs13431652 is a potentially causative SNP. Here we examine the effect of rs560887 on G6PC2 pre-mRNA splicing and the contribution of an additional common G6PC2 promoter SNP, rs2232316, to the association signal.

METHODS

Minigene analyses were used to characterise the effect of rs560887 on G6PC2 pre-mRNA splicing. Fusion gene and gel retardation analyses characterised the effect of rs2232316 on G6PC2 promoter activity and transcription factor binding. The genetic association of rs2232316 with FPG variation was assessed using regression adjusted for age, sex and BMI in 4,220 Europeans with normal FPG.

RESULTS

The rs560887-G allele was shown to enhance G6PC2 pre-mRNA splicing, whereas the rs2232316-A allele enhanced G6PC2 transcription by promoting Foxa2 binding. Genetic analyses provide evidence for association of the rs2232316-A allele with increased FPG (β = 0.04 mmol/l; p = 4.3 × 10(-3)) as part of the same signal as rs560887, rs573225 and rs13431652.

CONCLUSIONS/INTERPRETATION

As with rs13431652, the in situ functional data with rs560887 and rs2232316 are in accord with the putative function of G6PC2 in pancreatic islets, and suggest that all three are potentially causative SNPs that contribute to the association between G6PC2 and FPG.

Enzyme responsive materials: design strategies and future developments

Enzyme responsive materials (ERMs) are a class of stimuli responsive materials with broad application potential in biological settings. This review highlights current and potential future design strategies for ERMs and provides an overview of the present state of the art in the area.

Enzymatic self-assembly of nanostructures for theranostics

Self-assembly of small molecules or macromolecules through non-covalent or covalent bonds to build up supramolecular nanostructures is a prevalent and important process in nature. While most chemists use small molecules to assemble nanostructures with physical or chemical perturbations, nature adopts enzymes to catalyze the reaction to assemble biological, functional nanostructures with high efficiency and specificity. Although enzymatic self-assembly of nanostructures has been remained challenging for chemists, there are still a few examples of using important enzymes to initiate the self-assembly of nanostructures for diagnosis or therapy of certain diseases because down-regulation or overexpression of certain enzymes always associates with abnormalities of tissues/organs or diseases in living body. Herein, we introduce the concept of enzymatic self-assembly and illustrate the design and application of enzyme-catalyzed or -regulated formation of nanostructures for theranostics.

The potential of multimer technologies in type 1 diabetes prediction strategies

Type 1 diabetes is an autoimmune disease which occurs in (human leukocyte antigen) genetically predisposed individuals as a consequence of the organ-specific immune destruction of the insulin-producing β cells in the islets of Langherans within the pancreas. Type 1 diabetes is the result of a breakdown in immune regulation that leads to expansion of autoreactive CD4+ and CD8+ T cells, autoantibody-producing B lymphocytes and activation of the innate immune system. Islet-related autoantibodies revealed themselves to be good predictors of future onset of the disease, although they are not directly pathogenetic; T cells instead play a dominant role in disease initiation and progression. In this review, we first discuss the approaches that several laboratories attempted to measure human islet autoantigen-specific T-cell function in type 1 diabetes. T-cell assays could be used in combination with standardized autoantibody screenings to improve predictive strategies. They could also help to monitor in long-term follow-up the efficacy of tolerogenic immunotherapeutic strategies when established at the onset of the disease, and help to predict the recurrence of disease. Although some recent developments based on enzyme-linked immunosorbent spot and immunoblotting techniques have been able to distinguish with good sensitivity and specificity patients from controls, T-cell results, as revealed by international workshops, were indeed largely inconclusive. Nowadays, novel technologies have been exploited that could contribute to answering the tantalizing question of identifying autoreactive T cells. We particularly focus on and discuss MHC multimer tools and emphasize the advantages they can offer but also their weaknesses when used in combination with other T-cell assays. Copyright © 2011 John Wiley & Sons, Ltd.

Genetic and functional assessment of the role of the rs13431652-A and rs573225-A alleles in the G6PC2 promoter that are strongly associated with elevated fasting glucose levels

OBJECTIVE

Genome-wide association studies have identified a single nucleotide polymorphism (SNP), rs560887, located in a G6PC2 intron that is highly correlated with variations in fasting plasma glucose (FPG). G6PC2 encodes an islet-specific glucose-6-phosphatase catalytic subunit. This study examines the contribution of two G6PC2 promoter SNPs, rs13431652 and rs573225, to the association signal.

RESEARCH DESIGN AND METHODS

We genotyped 9,532 normal FPG participants (FPG <6.1 mmol/l) for three G6PC2 SNPs, rs13431652 (distal promoter), rs573225 (proximal promoter), rs560887 (3rd intron). We used regression analyses adjusted for age, sex, and BMI to assess the association with FPG and haplotype analyses to assess comparative SNP contributions. Fusion gene and gel retardation analyses characterized the effect of rs13431652 and rs573225 on G6PC2 promoter activity and transcription factor binding.

RESULTS

Genetic analyses provide evidence for a strong contribution of the promoter SNPs to FPG variability at the G6PC2 locus (rs13431652: β = 0.075, P = 3.6 × 10(-35); rs573225 β = 0.073 P = 3.6 × 10(-34)), in addition to rs560887 (β = 0.071, P = 1.2 × 10(-31)). The rs13431652-A and rs573225-A alleles promote increased NF-Y and Foxa2 binding, respectively. The rs13431652-A allele is associated with increased FPG and elevated promoter activity, consistent with the function of G6PC2 in pancreatic islets. In contrast, the rs573225-A allele is associated with elevated FPG but reduced promoter activity.

CONCLUSIONS

Genetic and in situ functional data support a potential role for rs13431652, but not rs573225, as a causative SNP linking G6PC2 to variations in FPG, though a causative role for rs573225 in vivo cannot be ruled out.

Autoimmune polyglandular syndromes

The autoimmune polyglandular syndromes-a group of syndromes comprising a combination of endocrine and nonendocrine autoimmune diseases-differ in their component diseases and in the immunologic features of their pathogenesis. One of the three main syndromes, type 1 autoimmune polyglandular syndrome (APS-1), has a unique pathogenic mechanism owing to mutations in the autoimmune regulator (AIRE) gene, which results in the loss of central tolerance-a process by which developing T cells with potential reactivity for self-antigens are eliminated during early differentiation in the thymus. Patients with IPEX (immune dysfunction, polyendocrinopathy, enteropathy, X-linked) syndrome harbor mutations in the forkhead box P3 (FOXP3) gene in regulatory T cells, which leads to severe autoimmunity and immune deficiency. Although both of these disorders are rare, their well-defined mechanisms of disease provide a basis for the understanding of the more common condition, APS-2. In this syndrome, alleles of human leukocyte antigens (HLAs) determine the targeting of specific tissues by autoreactive T cells, which leads to organ-specific autoimmunity as a result of this loss of tolerance. Non-HLA genes also contribute to autoimmunity in APS-2 and, depending on the polymorphism, potentially predispose to a loss of tolerance or influence which organ is specifically targeted. This Review discusses the genetic basis of APS-1, APS-2 and IPEX syndrome, with an emphasis on the mechanisms of autoimmunity and presents currently available therapies to treat their underlying autoimmune disorders.

Immunologic endocrine disorders

Autoimmunity affects multiple glands in the endocrine system. Animal models and human studies highlight the importance of alleles in HLA-like molecules determining tissue-specific targeting that, with the loss of tolerance, leads to organ-specific autoimmunity. Disorders such as type 1A diabetes, Graves disease, Hashimoto thyroiditis, Addison disease, and many others result from autoimmune-mediated tissue destruction. Each of these disorders can be divided into stages beginning with genetic susceptibility, environmental triggers, active autoimmunity, and finally metabolic derangements with overt symptoms of disease. With an increased understanding of the immunogenetics and immunopathogenesis of endocrine autoimmune disorders, immunotherapies are becoming prevalent, especially in patients with type 1A diabetes. Immunotherapies are being used more in multiple subspecialty fields to halt disease progression. Although therapies for autoimmune disorders stop the progress of an immune response, immunomodulatory therapies for cancer and chronic infections can also provoke an unwanted immune response. As a result, there are now iatrogenic autoimmune disorders arising from the treatment of chronic viral infections and malignancies.

Coxsackievirus B4 and type 1 diabetes pathogenesis: contribution of animal models

The role of enteroviruses, in particular type B coxsackieviruses (CV-B), in type 1 diabetes (T1D) pathogenesis is supported by epidemiological, clinical and experimental observations.The investigation of T1D pathogenesis benefits from the contribution of animal models called spontaneously diabetic. Among these animals the non-obese diabetic (NOD) mouse and the bio-breeding diabetes-prone (BBDP) rat present a genetic susceptibility manifested by the expression of an autoimmune diabetes similar to the pathology observed in human beings. Other models whose genetic predisposition is less known are of considerable contribution as well. Numerous major observations relative to several aspects of T1D pathogenesis in the context of CV-B infections, such as susceptibility, diabetogenicity, pancreatotropism, mechanisms of beta cells destruction and others, have been deduced thanks to investigations with animal models. Despite their limits, these models are necessary in improving our knowledge of the role of enteroviruses, like CV-B4, in the pathogenesis of T1D, and the recent advances ensuing from their contribution may have important therapeutic and preventive spin-offs.

Innovative Immune-Based Therapeutic Approaches for the Treatment of Type 1 Diabetes Mellitus

Type 1 diabetes mellitus is an autoimmune disease caused by a culmination of noxious processes of autoimmunity composed of various components of the innate and adaptive immune systems. Current treatment of type 1 diabetes focuses on restraining the endocrine disease without affecting the autoimmune process that underlies it. Prevention of this disease requires immune modulation and early intervention. New therapeutic approaches can be classified on the basis of the immunological arm targeted, that is, T-cell immune modulation (using cytokines, anti-CD3 monoclonal antibodies, and peptide MHC class II dimers), innate immune system modulation (using alpha-galactosylceramide or peptide 277), or specific antigen vaccination (glutamic acid decarboxylase and insulin). Here we review the most promising therapies developed based on these targets and emphasize those that have reached human phase clinical investigation.

Autoimmune polyendocrine syndrome type 1 (APS-1) as a model for understanding autoimmune polyendocrine syndrome type 2 (APS-2)

Autoimmune polyendocrine syndromes type 1 and 2 (APS-1 and APS-2) are diverse in regards to their component diseases and immunologic features of pathogenesis. Animal models and human studies highlight the importance of alleles of HLA (human leukocyte antigen)-like molecules determining tissue specific targeting that with the loss of tolerance leads to organ specific autoimmunity. Knowledge of the syndromes and component diseases allows clinicians to recognize and prevent illness prior to morbidity. With the current understanding of the syndromes, a paradigm for diagnosis, screening and treatment can be established. Once genetically susceptible individuals are identified screening for autoantibodies can be performed. Amongst autoantibody positive individuals, monitoring for physiologic decompensation, with a goal of treating prior to morbidity and in some cases mortality, follows. With continued basic and clinical research, therapies aimed at treating the underlying autoimmunity and disease prevention should become possible.

Clinical application of NKT cell biology in type I (autoimmune) diabetes mellitus

Type 1 natural killer T (NKT) cells are a population of CD1d-restricted, regulatory T cells that exhibit various NK cell characteristics and rapidly produce cytokines on stimulation with glycolipid antigen. In type I diabetes (TID), NKT cells are thought to have a tolerogenic function, evidenced by NKT cell numerical and functional deficiencies in the nonobese diabetic (NOD) mouse, which when corrected, can ameliorate disease. The mechanisms by which NKT cells can mediate their immunosuppressive effects in NOD mice are still poorly understood, which makes successful clinical translation of NKT- cell-based therapies challenging. However, new insights into the genetic control of NKT cell deficiencies have provided some understanding of the genes that may control NKT cell number and function, potentially offering a new avenue for assessing TID risk in humans. Here, we review the mechanisms by which NKT cells are thought to prevent TID, discuss the evidence for involvement of NKT cells in the regulation of human TID and examine the genetic control of NKT cell number and function. A greater understanding of these areas will increase the chances of successful clinical manipulation of NKT cells to prevent or treat TID.

Foxa2 and MafA regulate islet-specific glucose-6-phosphatase catalytic subunit-related protein gene expression

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP/G6PC2) is a major autoantigen in both mouse and human type 1 diabetes. IGRP is selectively expressed in islet beta cells and polymorphisms in the IGRP gene have recently been associated with variations in fasting blood glucose levels and cardiovascular-associated mortality in humans. Chromatin immunoprecipitation (ChIP) assays have shown that the IGRP promoter binds the islet-enriched transcription factors Pax-6 and BETA2. We show here, again using ChIP assays, that the IGRP promoter also binds the islet-enriched transcription factors MafA and Foxa2. Single binding sites for these factors were identified in the proximal IGRP promoter, mutation of which resulted in decreased IGRP fusion gene expression in betaTC-3, Hamster insulinoma tumor (HIT), and Min6 cells. ChiP assays have shown that the islet-enriched transcription factor Pdx-1 also binds the IGRP promoter, but mutational analysis of four Pdx-1 binding sites in the proximal IGRP promoter revealed surprisingly little effect of Pdx-1 binding on IGRP fusion gene expression in betaTC-3 cells. In contrast, in both HIT and Min6 cells mutation of these four Pdx-1 binding sites resulted in a approximately 50% reduction in fusion gene expression. These data suggest that the same group of islet-enriched transcription factors, namely Pdx-1, Pax-6, MafA, BETA2, and Foxa2, directly or indirectly regulate expression of the two major autoantigens in type 1 diabetes.

Cellular therapies for type 1 diabetes

Type 1 diabetes mellitus (T1DM) is a disease that results from the selective autoimmune destruction of insulin-producing beta-cells. This disease process lends itself to cellular therapy because of the single cell nature of insulin production. Murine models have provided opportunities for the study of cellular therapies for the treatment of diabetes, including the investigation of islet transplantation, and also the possibility of stem cell therapies and islet regeneration. Studies in islet transplantation have included both allo- and xeno-transplantation and have allowed for the study of new approaches for the reversal of autoimmunity and achieving immune tolerance. Stem cells from hematopoietic sources such as bone marrow and fetal cord blood, as well as from the pancreas, intestine, liver, and spleen promise either new sources of islets or may function as stimulators of islet regeneration. This review will summarize the various cellular interventions investigated as potential treatments of T1DM.

Long-range enhancers are required to maintain expression of the autoantigen islet-specific glucose-6-phosphatase catalytic subunit-related protein in adult mouse islets in vivo

OBJECTIVE

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is selectively expressed in islet beta-cells and is a major autoantigen in both mouse and human type 1 diabetes. This study describes the use of a combination of transgenic and transfection approaches to characterize the gene regions that confer the islet-specific expression of IGRP.

RESEARCH DESIGN AND METHODS

Transgenic mice were generated containing the IGRP promoter sequence from -306, -911, or -3911 to +3 ligated to a LacZ reporter gene. Transgene expression was monitored by 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside staining of pancreatic tissue.

RESULTS

In all the transgenic mice, robust LacZ expression was detected in newborn mouse islets, but expression became mosaic as animals aged, suggesting that additional elements are required for the maintenance of IGRP gene expression. VISTA analyses identified two conserved regions in the distal IGRP promoter and one in the third intron. Transfection experiments demonstrated that all three regions confer enhanced luciferase reporter gene expression in beta TC-3 cells when ligated to a minimal IGRP promoter. A transgene containing all three conserved regions was generated by using a bacterial recombination strategy to insert a LacZ cassette into exon 5 of the IGRP gene. Transgenic mice containing a 15-kbp fragment of the IGRP gene were then generated. This transgene conferred LacZ expression in newborn mouse islets; however, expression was still suppressed as animals aged.

CONCLUSIONS

The data suggest that long-range enhancers 5' or 3' of the IGRP gene are required for the maintenance of IGRP gene expression in adult mice.

Deletion of the gene encoding the ubiquitously expressed glucose-6-phosphatase catalytic subunit-related protein (UGRP)/glucose-6-phosphatase catalytic subunit-beta results in lowered plasma cholesterol and elevated glucagon

In liver, glucose-6-phosphatase catalyzes the hydrolysis of glucose-6-phosphate (G6P) to glucose and inorganic phosphate, the final step in the gluconeogenic and glycogenolytic pathways. Mutations in the glucose-6-phosphatase catalytic subunit (G6Pase) give rise to glycogen storage disease (GSD) type 1a, which is characterized in part by hypoglycemia, growth retardation, hypertriglyceridemia, hypercholesterolemia, and hepatic glycogen accumulation. Recently, a novel G6Pase isoform was identified, designated UGRP/G6Pase-beta. The activity of UGRP relative to G6Pase in vitro is disputed, raising the question as to whether G6P is a physiologically important substrate for this protein. To address this issue we have characterized the phenotype of UGRP knock-out mice. G6P hydrolytic activity was decreased by approximately 50% in homogenates of UGRP(-/-) mouse brain relative to wild type tissue, consistent with the ability of UGRP to hydrolyze G6P. In addition, female, but not male, UGRP(-/-) mice exhibit growth retardation as do G6Pase(-/-) mice and patients with GSD type 1a. However, in contrast to G6Pase(-/-) mice and patients with GSD type 1a, UGRP(-/-) mice exhibit no change in hepatic glycogen content, blood glucose, or triglyceride levels. Although UGRP(-/-) mice are not hypoglycemic, female UGRP(-/-) mice have elevated ( approximately 60%) plasma glucagon and reduced ( approximately 20%) plasma cholesterol. We hypothesize that the hyperglucagonemia prevents hypoglycemia and that the hypocholesterolemia is secondary to the hyperglucagonemia. As such, the phenotype of UGRP(-/-) mice is mild, indicating that G6Pase is the major glucose-6-phosphatase of physiological importance for glucose homeostasis in vivo.

Using a kinase/phosphatase switch to regulate a supramolecular hydrogel and forming the supramolecular hydrogel in vivo

We have designed and synthesized a new hydrogelator Nap-FFGEY (1), which forms a supramolecular hydrogel. A kinase/phosphatase switch is used to control the phosphorylation and dephosphorylation of the hydrogelator and to regulate the formation of supramolecular hydrogels. Adding a kinase to the hydrogel induces a gel-sol phase transition in the presence of adenosine triphosphates (ATP) because the tyrosine residue is converted into tyrosine phosphate by the kinase to give a more hydrophilic molecule of Nap-FFGEY-P(O)(OH)(2) (2); treating the resulting solution with a phosphatase transforms 2 back to 1 and restores the hydrogel. Electron micrographs of the hydrogels indicate that 1 self-assembles into nanofibers. Subcutaneous injection of 2 in mice shows that 80.5 +/- 1.2% of 2 turns into 1 and results in the formation of the supramolecular hydrogel of 1 in vivo. This simple biomimetic approach for regulating the states of supramolecular hydrogels promises a new way to design and construct biomaterials.

Alternative splicing of G6PC2, the gene coding for the islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP), results in differential expression in human thymus and spleen compared with pancreas

AIMS/HYPOTHESIS

Autoimmunity to insulin, glutamic acid decarboxylase and the tyrosine-phosphatase-like protein IA-2 is associated with type 1 diabetes. The production of self-molecules in thymus and secondary lymphoid tissues is critical for self-tolerance; reduced levels may impair tolerance and predispose to autoimmunity, as shown for insulin. Alternative splicing causes differential expression of IA-2 gene (PTPRN) transcripts and IA-2 protein in human thymus and spleen compared with pancreas. IA-2 sequences not present in lymphoid tissues become autoimmune targets in type 1 diabetes. The beta cell molecule islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is an autoantigen in the non-obese diabetic (NOD) mouse, a model of type 1 diabetes. IGRP is a candidate autoantigen in the human disease, but robust assays for IGRP autoantibodies and/or autoreactive T cells are not available. Both full-length and IGRP splice variants encoded by the G6PC2 gene are expressed in the pancreas. In this study we tested the hypothesis that IGRP splice variants could be differentially expressed in thymus and spleen compared with the pancreas.

METHODS

We evaluated the expression of G6PC2 transcripts in matched human thymus, spleen and pancreas specimens by RT-PCR.

RESULTS

Alternative splicing results in differential expression of G6PC2 transcripts in thymus and spleen compared with pancreas. The full-length transcript is expressed in human pancreas but not in thymus or spleen. Five alternative spliced forms are always expressed in pancreas but those lacking exons 2, 3 and 4, alone or in combination, were rarely detected in thymus or spleen.

CONCLUSIONS/INTERPRETATION

Differential tissue expression might favour autoimmune responses to IGRP in humans; target epitopes may be encoded by exons 3 and 4, or at the junctions of the conserved exons in the spliced transcripts. This information may aid in designing synthetic peptides for the identification of IGRP-specific autoreactive T cells in patients with type 1 diabetes.

Immunotherapeutic approaches to prevent, ameliorate, and cure type 1 diabetes

Type 1A diabetes (T1D) is caused by autoimmune islet beta cell destruction precipitated by environmental triggers in genetically predisposed individuals. Islet beta cells produce insulin and are the primary target of this autoimmune disorder. Insulin, glutamic acid decarboxylase, and insulinoma associated-2 autoantibodies (IAA, GAD65, and IA-2) are the autoantibodies that have been associated most clearly with the development of T1D. Despite our current ability to predict T1D using genetic markers and detecting islet autoantibodies, we have yet to find a safe way to prevent the disease. However, there are more than 100 different therapies that prevent T1D in the nonobese diabetic (NOD) mouse model or the BioBreeding (BB) rats. This paper reviews a few select therapeutic approaches that have been or are being evaluated as possibilities for the prevention, amelioration, or cure of T1D.

Insulin as a primary autoantigen for type 1A diabetes

Type 1A diabetes mellitus is caused by specific and progressive autoimmune destruction of the beta cells in the islets of Langerhans whereas the other cell types in the islet (alpha, delta, and PP) are spared. The autoantigens of Type 1A diabetes may be divided into subgroups based on their tissue distributions: Beta-cell-specific antigens like insulin, insulin derivatives, and IGRP (Islet-specific Glucose-6-phosphatase catalytic subunit Related Peptide); neurendocrine antigens such as carboxypeptidase H, insulinoma-associated antigen (IA-2), glutamic acid decarboxylase (GAD65), and carboxypeptidase E; and those expressed ubiquitously like heat shock protein 60 (a putative autoantigen for type 1 diabetes). This review will focus specifically on insulin as a primary autoantigen, an essential target for disease, in type 1A diabetes mellitus. In particular, immunization with insulin peptide B:9-23 can be used to induce insulin autoantibodies and diabetes in animal models or used to prevent diabetes. Genetic manipulation of the insulin 1 and 2 genes reciprocally alters development of diabetes in the NOD mouse, and insulin gene polymorphisms are important determinants of childhood diabetes. We are pursuing the hypothesis that insulin is a primary autoantigen for type 1 diabetes, and thus the pathogenesis of the disease relates to specific recognition of one or more peptides.

Insulin-secreting cells derived from stem cells: clinical perspectives, hypes and hopes

Diabetes is a degenerative disease that results from the selective destruction of pancreatic beta-cells. These cells are responsible for insulin production and secretion in response to increases in circulating concentrations of nutrients, such as glucose, fatty acids and amino acids. This degenerative disease can be treated by the transplantation of differentiated islets obtained from cadaveric donors, according to a new surgical intervention developed as Edmonton protocol. Compared to the classical double transplant kidney-pancreas, this new protocol presents several advantages, concerning to the nature of the implant, immunosuppressive drug regime and the surgical procedure itself. However, the main problem to face in any islet transplantation program is the scarcity of donor pancreases and the low yield of islets isolated (very often around 50%) from each pancreas. Nevertheless, transplanted patients presented no adverse effects and no progression of diabetic complications. In the search of new cell sources for replacement trials, stem cells from embryonic and adult origins represent a key alternative. In order to become a realistic clinical issue transplantation of insulin-producing cells derived from stem cells, it needs to overcome multiple experimental obstacles. The first one is to develop a protocol that may allow obtaining a pure population of functional insulin-secreting cells as close as possible to the pancreatic beta-cell. The second problem should concern to the transplantation itself, considering issues related to immune rejection, tumour formation, site for implant, implant survival, and biosafety mechanisms. Although transplantation of bioengineered cells is still far in time, experience accumulated in islet transplantation protocols and in experiments with appropriate animal models will give more likely the clues to address this question in the future.

The good turned ugly: immunopathogenic basis for diabetogenic CD8+ T cells in NOD mice

Type 1 diabetes (T1D) in both humans and nonobese diabetic (NOD) mice is a T-cell-mediated autoimmune disease in which the insulin-producing pancreatic islet beta-cells are selectively eliminated. As a result, glucose metabolism cannot be regulated unless exogenous insulin is administered. Both the CD4(+) and the CD8(+) T-cell subsets are required for T1D development. Approximately 20 years ago, an association between certain class II major histocompatibility complex (MHC) alleles and susceptibility to T1D was reported. This finding led to enormous interest in the CD4(+) T cells participating in the development of T1D, while the CD8(+) subset was relatively ignored. However, the isolation of beta-cell-autoreactive CD8(+) T-cell clones from the islets of NOD mice helped to generate interest in the pathogenic role of this subset, as has accumulating evidence that certain class I MHC alleles are additional risk factors for T1D development in humans. Three distinct diabetogenic CD8(+) T-cell populations have now been characterized in NOD mice. Here, we review recent investigations exploring their selection, activation, trafficking, and antigenic specificities. As CD8(+) T cells are suspected contributors to beta-cell demise in humans, continued exploration of these critical areas could very possibly lead to tangible benefits for T1D patients and at-risk individuals.

Immune modulation for prevention of type 1 diabetes mellitus

Prevention of type 1 diabetes mellitus requires early intervention in the autoimmune process directed against beta cells of the pancreatic islets of Langerhans. This autoimmune inflammatory process is thought to be caused by the effect of Th1 cells and their secreted cytokines (e.g. interferon) and to be suppressed by Th2-secreted anti-inflammatory cytokines (e.g. IL-4, IL-10). Various methods aimed specifically at halting or modulating this response have been attempted. An alternative method is the re-induction of tolerance towards the putative self antigen that causes the disease. Proposed antigens such as insulin, glutamic acid decarboxilase (GAD) and the heat shock protein 60 (Hsp60)-derived peptide 277 have been used successfully in murine diabetes models and in initial clinical trials in early diabetes patients. Here, we review the results of these trials.

Identification of CD4+ T Cell-Specific Epitopes of Islet-Specific Glucose-6-Phosphatase Catalytic Subunit-Related Protein: A Novel β Cell Autoantigen in Type 1 Diabetes

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) has been identified as a novel CD8(+) T cell-specific autoantigen in NOD mice. This study was undertaken to identify MHC class II-specific CD4(+) T cell epitopes of IGRP. Peptides named P1, P2, P3, P4, P5, P6, and P7 were synthesized by aligning the IGRP protein amino acid sequence with peptide-binding motifs of the NOD MHC class II (I-A(g7)) molecule. Peptides P1, P2, P3, and P7 were immunogenic and induced both spontaneous and primed responses. IGRP peptides P1-, P2-, P3-, and P7-induced responses were inhibited by the addition of anti-MHC class II (I-A(g7)) Ab, confirming that the response is indeed I-A(g7) restricted. Experiments using purified CD4(+) and CD8(+) T cells from IGRP peptide-primed mice also showed a predominant CD4(+) T cell response with no significant activation of CD8(+) T cells. T cells from P1-, P3-, and P7-primed mice secreted both IFN-gamma and IL-10 cytokines, whereas P2-primed cells secreted only IFN-gamma. Peptides P3 and P7 prevented the development of spontaneous diabetes and delayed adoptive transfer of diabetes. Peptides P1 and P2 delayed the onset of diabetes in both these models. In summary, we have identified two I-A(g7)-restricted CD4(+) T cell epitopes of IGRP that can modulate and prevent the development of diabetes in NOD mice. These results provide the first evidence on the role of IGRP-specific, MHC class II-restricted CD4(+) T cells in disease protection and may help in the development of novel therapies for type 1 diabetes.

Regulatory T-cells in type 1 diabetes

Type 1 diabetes is a T-cell-mediated autoimmune disease, resulting in destruction of the insulin-producing beta cells in the pancreas. Disease progression is thought to involve the action of T-cells, particularly those producing Th1-type cytokines. Given the complexity in understanding the precise etiology of autoimmune diseases, the diversity of autoantigens, and the variability that exists between individual patients, it might be very difficult to eliminate autoaggressive T-cell responses without resorting to generalized means of immunosuppression. However, recent evidence shows that autoimmune processes are composed not only of autoaggressive T-cell responses but also of autoreactive regulatory components. Enhancing regulatory T-cell responses, therefore, has become an area of intense focus as a means of treating autoimmune diseases like type 1 diabetes. This review will concentrate on two different types of regulatory T-cells, the naturally occurring ('professional') CD4+CD25+ T-cells and antigen-induced ('adaptive') CD4+ Th2-like regulatory T-cells.

The proximal islet-specific glucose-6-phosphatase catalytic subunit-related protein autoantigen promoter is sufficient to initiate but not maintain transgene expression in mouse islets in vivo

We have previously reported the discovery of an islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) that is predominantly expressed in islet beta-cells. IGRP has recently been identified as a major autoantigen in a mouse model of type 1 diabetes. The analysis of IGRP-chloramphenicol acetyltransferase (CAT) fusion gene expression in transiently transfected islet-derived hamster insulinoma tumor and betaTC-3 cells revealed that the promoter region located between -306 and +3 confers high-level reporter gene expression. To determine whether this same promoter region is sufficient to confer islet beta-cell-specific gene expression in vivo, it was ligated to a beta-galactosidase reporter gene, and transgenic mice expressing the resulting fusion gene were generated. In two independent founder lines, this -306 to +3 promoter region was sufficient to drive beta-galactosidase expression in newborn mouse islets, predominantly in beta-cells, which was initiated during the expected time in development, around embryonic day 12.5. However, unlike the endogenous IGRP gene, beta-galactosidase expression was also detected in the cerebellum. Moreover, beta-galactosidase expression was almost completely absent in adult mouse islets, suggesting that cis-acting elements elsewhere in the IGRP gene are required for determining appropriate IGRP tissue-specific expression and for the maintenance of IGRP gene expression in adult mice.

A comprehensive guide to antibody and T-cell responses in type 1 diabetes

Type 1 diabetes (T1D) is an organ-specific autoimmune disease in which the insulin-producing beta cells in the pancreatic islets are selectively eliminated. T cells specific for beta-cell antigens are the mediators of this precise cellular destruction. However, antibodies to beta-cell proteins are also generated and may be used for predicting disease in at-risk populations. Over the past two decades, numerous beta-cell proteins and lipids have been implicated as autoantigens in patients or in non-obese diabetic (NOD) mice, a well-studied animal model of T1D. Here, we present a review of these antigens, accompanied by their T-cell epitopes, where known, and a discussion of our current understanding of why particular self-proteins become disease-inciting antigens. Although two dozen beta-cell antigens have been identified to date, few of these have been confirmed to be recognized by pathogenic T cells early in the disease process. Further identification and characterization of initiating beta-cell antigens targeted by pathogenic T cells should be a priority for future studies.

Differential regulation of islet-specific glucose-6-phosphatase catalytic subunit-related protein gene transcription by Pax-6 and Pdx-1

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is selectively expressed in islet beta cells and is a major autoantigen in a mouse model of type I diabetes. The analysis of IGRP-chloramphenicol acetyltransferase (CAT) fusion gene expression through transient transfection of islet-derived betaTC-3 cells revealed that a promoter region, located between -273 and -254, is essential for high IGRP-CAT fusion gene expression. The sequence of this promoter region does not match that for any known islet-enriched transcription factor. However, data derived from gel retardation assays, a modified ligation-mediated polymerase chain reaction in situ footprinting technique and a SDS-polyacrylamide separation/renaturation procedure led to the hypothesis that this protein might be Pax-6, a conclusion that was confirmed by gel supershift assays. Additional experiments revealed a second non-consensus Pax-6 binding site in the -306/-274 IGRP promoter region. Pax-6 binding to these elements is unusual in that it appears to require both its homeo and paired domains. Interestingly, loss of Pax-6 binding to the -273/ -246 element is compensated by Pax-6 binding to the -306/-274 element and vice versa. Gel retardation assays revealed that another islet-enriched transcription factor, namely Pdx-1, binds four non-consensus elements in the IGRP promoter. However, mutation of these elements has little effect on IGRP fusion gene expression. Although chromatin immunoprecipitation assays show that both Pax-6 and Pdx-1 bind to the IGRP promoter within intact cells, in contrast to the critical role of these factors in beta cell-specific insulin gene expression, IGRP gene transcription appears to require Pax-6 but not Pdx-1.

Identification of a beta-cell-specific HLA class I restricted epitope in type 1 diabetes

Type 1 diabetes is an autoimmune disease in which pancreatic beta-cells are destroyed by cytotoxic T-cells that recognize peptide epitopes presented by HLA class I molecules. The identification of human beta-cell epitopes may significantly improve the prospects for immunodiagnosis and immunotherapy in type 1 diabetes. Using algorithms to predict nonameric beta-cell peptides that would bind to the common HLA allele, HLA-A*0201, we identified a potential epitope from the leader sequence of islet amyloid polypeptide (human islet amyloid polypeptide [IAPP] precursor protein [preproIAPP] 5-13: KLQVFLIVL). Peripheral blood mononuclear cells (PBMCs) were isolated from 18 HLA-A*0201 patients with type 1 diabetes (9 with recent-onset [<180 days; range, 1-120 days] and 9 with long-standing diabetes [>180 days; range, 183-3,273 days]) and 9 healthy, nondiabetic control subjects. PBMCs were screened for peptide recognition using interferon-gamma enzyme-linked immunospot (ELISpot) assays. Of the nine patients with recent-onset type 1 diabetes, six had ELISpot responses to preproIAPP 5-13 that were >3 SDs above the mean of the nondiabetic control subjects (P = 0.002). In contrast, no patients with type 1 diabetes for >180 days had a response above this threshold. In summary, preproIAPP 5-13 is a novel HLA class I epitope recognized by a significant proportion of cytotoxic T-cells from HLA-A*0201 patients with recent-onset type 1 diabetes and may prove to be a useful tool for the prediction and/or prevention of this disease.