Mutant INS Gene-induced Diabetes Of Youth Research Articles

Distinct states of proinsulin misfolding in MIDY

A precondition for efficient proinsulin export from the endoplasmic reticulum (ER) is that proinsulin meets ER quality control folding requirements, including formation of the Cys(B19)–Cys(A20) “interchain” disulfide bond, facilitating formation of the Cys(B7)–Cys(A7) bridge. The third proinsulin disulfide, Cys(A6)–Cys(A11), is not required for anterograde trafficking, i.e., a “lose-A6/A11” mutant [Cys(A6), Cys(A11) both converted to Ser] is well secreted. Nevertheless, an unpaired Cys(A11) can participate in disulfide mispairings, causing ER retention of proinsulin. Among the many missense mutations causing the syndrome of Mutant INS gene-induced Diabetes of Youth (MIDY), all seem to exhibit perturbed proinsulin disulfide bond formation. Here, we have examined a series of seven MIDY mutants [including G(B8)V, Y(B26)C, L(A16)P, H(B5)D, V(B18)A, R(Cpep + 2)C, E(A4)K], six of which are essentially completely blocked in export from the ER in pancreatic β-cells. Three of these mutants, however, must disrupt the Cys(A6)–Cys(A11) pairing to expose a critical unpaired cysteine thiol perturbation of proinsulin folding and ER export, because when introduced into the proinsulin lose-A6/A11 background, these mutants exhibit native-like disulfide bonding and improved trafficking. This maneuver also ameliorates dominant-negative blockade of export of co-expressed wild-type proinsulin. A growing molecular understanding of proinsulin misfolding may permit allele-specific pharmacological targeting for some MIDY mutants.

An Unusual Form of Type 1 Diabetes With an Elevated Proinsulin Level

Introduction: Diabetes is one of the most prevalent diseases in the world. We recognize the three most common types of diabetes: type1, type2 and gestational diabetes. It is estimated that around 425 million of people worldwide have diabetes and about 90% of those represent type 2 diabetes. The most common types of diabetes are polygenic -- they are caused by a defect in multiple genes. Monogenic diabetes is caused by a mutation in a single gene. We currently have over 10 different types of monogenic diabetes called MODY (Maturity Onset Diabetes of the Young). Sun et al. states that over the past few years, 30 different insulin gene mutations were reported to cause a new syndrome called MIDY (Mutant INS-gene-induced Diabetes of Youth). Most of these mutations lead to proinsulin misfolding in the endoplasmic reticulum. We present a rare case of a young obese female with an elevated proinsulin level and low C-peptide level diagnosed with type 1 diabetes requiring therapy with insulin. Case Description: A 21 year old female with past medical history of chronic diarrhea initially presented with a complaint of dry mouth, dizziness, excessive urination, and thirst. She was found to have hyperglycemia of 203 mg/dL, A1C 8.3, and negative ketones. Patient had a strong family history of diabetes. She had a family history of: father with type 1 diabetes; mother with a past medical history of gestational diabetes who became diabetic postpartum; and three of the patient’s grandparents with a history of diabetes. Patient was started on the oral hypoglycemic agents metformin and glipizide, but she only had partial response to these medications. Because of her strong family history and incomplete response to oral hypoglycemic agents, additional testing was performed. Patient was found to have a low C-peptide level (1.6 ng/mL), elevated proinsulin (72.9 pmol/L), positive GAD antibody (10.3 units/mL) and negative islet cell autoantibody. Patient had a very good response to insulin and subsequently became insulin dependent. She is currently on an insulin pump. Conclusion: Sun et al. reports that proinsulin misfolding causes beta cell failure. Increased misfolding occurs under certain pathological conditions that are currently unknown. We think that there might be some increased proinsulin misfolding abnormality that might be occurring in this patient. There are most likely many epigenetic modifiers that would trigger certain individuals to be more prone to this phenomena of misfolded proinsulin. Future research in diabetes may one day yield antibodies that would specifically recognize misfolded proinsulin in the plasma. Further research is required to elucidate how defective proinsulin folding may lead to beta cell dysfunction and subsequent evolution of diabetes mellitus.

1801-P: Novel De Novo INS P.T97P Variant Presenting with Severe Neonatal DKA

Objective: A 5 month old, premature male infant with history of hydrops fetalis and hernia repair presented unresponsive in respiratory distress requiring intubation and was found to have pH 6.93, bicarbonate 6 mmol/L, anion gap 23 mmol/L, glucose 669 mg/dL and HbA1c 14.2%. His condition improved with insulin drip, fluid and electrolyte management. Work-up revealed negative autoantibodies and the University of Chicago Neonatal Diabetes Sequencing Panel was performed. Testing revealed a heterozygous sequence variant of unknown significance in the INS gene (c.289A>C), resulting in the amino acid change p.T97P. In silico pathogenicity prediction tools provided contradictory results. Neither parent carried the variant. INS gene mutations have been associated with Mutant INS-gene-induced Diabetes of Youth (MIDY) that includes permanent neonatal diabetes and MODY 10. Expression of mutant proinsulin is thought to lead to misfolded complexes in the endoplasmic reticulum (ER), leading to ER stress and β-cell death. Methods: To determine the effects of this variant on insulin processing and release, we utilized a validated luminescent insulin secretion assay where Gaussia luciferase is inserted into the C-peptide portion of proinsulin. We compared luciferase secretion for wild type and variant INS sequences. Results: The mutation decreased luciferase release ∼10-fold versus wild type. Proteasome inhibition led to partial rescue. However, overall expression of proinsulin-luciferase peptide was unchanged between the wild type and mutant. Therefore, it is likely that the mutation causes disruptions in processes downstream of transcription and translation, such as folding, proteolytic processing and transport. Conclusions: The novel INS p.T97P is the first identified A-chain mutation resulting in MIDY not involving the creation or deletion of cysteine residues. The substitution of the polar threonine with nonpolar proline may be sufficient to disrupt nearby disulfide bonding in the proinsulin folding pathway. Disclosure R. Lal: Consultant; Self; Abbott, Biolinq. H.P. Moeller: None. R. Freeman: None. P. Prahalad: None. J.P. Annes: None. Funding National Institutes of Health (1K12DK122550, T32DK007217, R01DK101530, T32GM113854, P30DK116074)

Role of Proinsulin Self-Association in Mutant INS Gene-Induced Diabetes of Youth.

Abnormal interactions between misfolded mutant and wild-type (WT) proinsulin (PI) in the endoplasmic reticulum (ER) drive the molecular pathogenesis of mutant INS gene-induced diabetes of youth (MIDY). How these abnormal interactions are initiated remains unknown. Normally, PI-WT dimerizes in the ER. Here, we suggest that the normal PI-PI contact surface, involving the B-chain, contributes to dominant-negative effects of misfolded MIDY mutants. Specifically, we find that PI B-chain tyrosine-16 (Tyr-B16), which is a key residue in normal PI dimerization, helps confer dominant-negative behavior of MIDY mutant PI-C(A7)Y. Substitutions of Tyr-B16 with either Ala, Asp, or Pro in PI-C(A7)Y decrease the abnormal interactions between the MIDY mutant and PI-WT, rescuing PI-WT export, limiting ER stress, and increasing insulin production in β-cells and human islets. This study reveals the first evidence indicating that noncovalent PI-PI contact initiates dominant-negative behavior of misfolded PI, pointing to a novel therapeutic target to enhance PI-WT export and increase insulin production.

Misfolded proinsulin impairs processing of precursor of insulin receptor and insulin signaling in β cells.

Insulin resistance in classic insulin-responsive tissues is a hallmark of type 2 diabetes (T2D). However, the pathologic significance of β-cell insulin resistance and the underlying mechanisms contributing to defective insulin signaling in β cells remain largely unknown. Emerging evidence indicates that proinsulin misfolding is not only the molecular basis of mutant INS-gene–induced diabetes of youth (MIDY) but also an important contributor in the development and progression of T2D. However, the molecular basis of β-cell failure caused by misfolded proinsulin is still incompletely understood. Herein, using Akita mice expressing diabetes-causing mutant proinsulin, we found that misfolded proinsulin abnormally interacted with the precursor of insulin receptor (ProIR) in the endoplasmic reticulum (ER), impaired ProIR maturation to insulin receptor (IR), and decreased insulin signaling in β cells. Importantly, using db/db insulin-resistant mice, we found that oversynthesis of proinsulin led to an increased proinsulin misfolding, which resulted in impairments of ProIR processing and insulin signaling in β cells. These results reveal for the first time that misfolded proinsulin can interact with ProIR in the ER, impairing intracellular processing of ProIR and leading to defective insulin signaling that may contribute to β-cell failure in both MIDY and T2D.—Liu, S., Li, X., Yang, J., Zhu, R., Fan, Z., Xu, X., Feng, W., Cui, J., Sun, J., Liu, M. Misfolded proinsulin impairs processing of precursor of insulin receptor and insulin signaling in β cells.

Quality Control in the ER: Misfolded Prohormones Get a Checkup.

In Mutant INS-gene-induced Diabetes of Youth (MIDY) syndrome, mutant proinsulin aggregates interfere with the folding of wild-type proinsulin in the endoplasmic reticulum, ultimately decreasing insulin secretion. In this issue of Molecular Cell, Cunningham etal. (2019) identify two mechanisms by which prohormone aggregation is prevented and cleared.

Cells Deploy a Two-Pronged Strategy to Rectify Misfolded Proinsulin Aggregates.

Insulin gene coding sequence mutations are known to cause mutant INS-gene-induced diabetes of youth (MIDY), yet the cellular pathways needed to prevent misfolded proinsulin accumulation remain incompletely understood. Here, we report that Akita mutant proinsulin forms detergent-insoluble aggregates that entrap wild-type (WT) proinsulin in the endoplasmic reticulum (ER), thereby blocking insulin production. Two distinct quality-control mechanisms operate together to combat this insult: the ER luminal chaperone Grp170 prevents proinsulin aggregation, while the ER membrane morphogenic protein reticulon-3 (RTN3) disposes of aggregates via ER-coupled autophagy (ER-phagy). We show that enhanced RTN-dependent clearance of aggregated Akita proinsulin helps to restore ER export of WT proinsulin, which can promote WT insulin production, potentially alleviating MIDY. We also find that RTN3 participates in the clearance of other mutant prohormone aggregates. Together, these results identify a series of substrates of RTN3-mediated ER-phagy, highlighting RTN3 in the disposal of pathogenic prohormone aggregates.

Biosynthesis, structure, and folding of the insulin precursor protein

Insulin synthesis in pancreatic β-cells is initiated as preproinsulin. Prevailing glucose concentrations, which oscillate pre- and postprandially, exert major dynamic variation in preproinsulin biosynthesis. Accompanying upregulated translation of the insulin precursor includes elements of the endoplasmic reticulum (ER) translocation apparatus linked to successful orientation of the signal peptide, translocation and signal peptide cleavage of preproinsulin-all of which are necessary to initiate the pathway of proper proinsulin folding. Evolutionary pressures on the primary structure of proinsulin itself have preserved the efficiency of folding ("foldability"), and remarkably, these evolutionary pressures are distinct from those protecting the ultimate biological activity of insulin. Proinsulin foldability is manifest in the ER, in which the local environment is designed to assist in the overall load of proinsulin folding and to favour its disulphide bond formation (while limiting misfolding), all of which is closely tuned to ER stress response pathways that have complex (beneficial, as well as potentially damaging) effects on pancreatic β-cells. Proinsulin misfolding may occur as a consequence of exuberant proinsulin biosynthetic load in the ER, proinsulin coding sequence mutations, or genetic predispositions that lead to an altered ER folding environment. Proinsulin misfolding is a phenotype that is very much linked to deficient insulin production and diabetes, as is seen in a variety of contexts: rodent models bearing proinsulin-misfolding mutants, human patients with Mutant INS-gene-induced Diabetes of Youth (MIDY), animal models and human patients bearing mutations in critical ER resident proteins, and, quite possibly, in more common variety type 2 diabetes.

The Munich MIDY Pig Biobank – A unique resource for studying organ crosstalk in diabetes

ObjectiveThe prevalence of diabetes mellitus and associated complications is steadily increasing. As a resource for studying systemic consequences of chronic insulin insufficiency and hyperglycemia, we established a comprehensive biobank of long-term diabetic INSC94Y transgenic pigs, a model of mutant INS gene-induced diabetes of youth (MIDY), and of wild-type (WT) littermates. MethodsFemale MIDY pigs (n = 4) were maintained with suboptimal insulin treatment for 2 years, together with female WT littermates (n = 5). Plasma insulin, C-peptide and glucagon levels were regularly determined using specific immunoassays. In addition, clinical chemical, targeted metabolomics, and lipidomics analyses were performed. At age 2 years, all pigs were euthanized, necropsied, and a broad spectrum of tissues was taken by systematic uniform random sampling procedures. Total beta cell volume was determined by stereological methods. A pilot proteome analysis of pancreas, liver, and kidney cortex was performed by label free proteomics. ResultsMIDY pigs had elevated fasting plasma glucose and fructosamine concentrations, C-peptide levels that decreased with age and were undetectable at 2 years, and an 82% reduced total beta cell volume compared to WT. Plasma glucagon and beta hydroxybutyrate levels of MIDY pigs were chronically elevated, reflecting hallmarks of poorly controlled diabetes in humans. In total, ∼1900 samples of different body fluids (blood, serum, plasma, urine, cerebrospinal fluid, and synovial fluid) as well as ∼17,000 samples from ∼50 different tissues and organs were preserved to facilitate a plethora of morphological and molecular analyses. Principal component analyses of plasma targeted metabolomics and lipidomics data and of proteome profiles from pancreas, liver, and kidney cortex clearly separated MIDY and WT samples. ConclusionsThe broad spectrum of well-defined biosamples in the Munich MIDY Pig Biobank that will be available to the scientific community provides a unique resource for systematic studies of organ crosstalk in diabetes in a multi-organ, multi-omics dimension.

Evaluation of conformational changes in diabetes-associated mutation in insulin a chain: a molecular dynamics study.

Insulin plays a central role in the regulation of metabolism in humans. Mutations in the insulin gene can impair the folding of its precursor protein, proinsulin, and cause permanent neonatal-onset diabetes mellitus known as Mutant INS-gene induced Diabetes of Youth (MIDY) with insulin deficiency. To gain insights into the molecular basis of this diabetes-associated mutation, we perform molecular dynamics simulations in wild-type and mutant (Cys(A7) to Tyr or C(A7)Y) insulin A chain in aqueous solutions. The C(A7)Y mutation is one of the identified mutations that impairs the protein folding by substituting the cysteine residue which is required for the disulfide bond formation. A comparative analysis reveals structural differences between the wild-type and the mutant conformations. The analyzed mutant insulin A chain forms a metastable state with major effects on its N-terminal region. This suggests that MIDY mutant involves formation of a partially folded intermediate with conformational change in N-terminal region in A chain that generates flexible N-terminal domain. This may lead to the abnormal interactions with other proinsulins in the aggregation process.

Prediabetic phenotype in transgenic pigs expressing the mutant insulin C93S

Aims: Mutant INS gene-induced diabetes of youth (MIDY) in humans is caused by missense mutations of the insulin gene. The severity of the disease depends on the type of mutation. We generated transgenic pigs expressing INS C93S to complement the previously established INSC 94 Y transgenic pig model, which develops permanent neonatal diabetes mellitus (Renner et al., Diabetes 62, 1505 – 11, 2013).

Endoplasmic Reticulum Oxidoreductin-1α (Ero1α) Improves Folding and Secretion of Mutant Proinsulin and Limits Mutant Proinsulin-induced Endoplasmic Reticulum Stress

Upon chronic up-regulation of proinsulin synthesis, misfolded proinsulin can accumulate in the endoplasmic reticulum (ER) of pancreatic β-cells, promoting ER stress and type 2 diabetes mellitus. In Mutant Ins-gene-induced Diabetes of Youth (MIDY), misfolded mutant proinsulin impairs ER exit of co-expressed wild-type proinsulin, limiting insulin production and leading to eventual β-cell death. In this study we have investigated the hypothesis that increased expression of ER oxidoreductin-1α (Ero1α), despite its established role in the generation of H2O2, might nevertheless be beneficial in limiting proinsulin misfolding and its adverse downstream consequences. Increased Ero1α expression is effective in promoting wild-type proinsulin export from cells co-expressing misfolded mutant proinsulin. In addition, we find that upon increased Ero1α expression, some of the MIDY mutants themselves are directly rescued from ER retention. Secretory rescue of proinsulin-G(B23)V is correlated with improved oxidative folding of mutant proinsulin. Indeed, using three different variants of Ero1α, we find that expression of either wild-type or an Ero1α variant lacking regulatory disulfides can rescue mutant proinsulin-G(B23)V, in parallel with its ability to provide an oxidizing environment in the ER lumen, whereas beneficial effects were less apparent for a redox-inactive form of Ero1. Increased expression of protein disulfide isomerase antagonizes the rescue provided by oxidatively active Ero1. Importantly, ER stress induced by misfolded proinsulin was limited by increased expression of Ero1α, suggesting that enhancing the oxidative folding of proinsulin may be a viable therapeutic strategy in the treatment of type 2 diabetes.

Impaired Cleavage of Preproinsulin Signal Peptide Linked to Autosomal-Dominant Diabetes

Recently, missense mutations upstream of preproinsulin’s signal peptide (SP) cleavage site were reported to cause mutant INS gene-induced diabetes of youth (MIDY). Our objective was to understand the molecular pathogenesis using metabolic labeling and assays of proinsulin export and insulin and C-peptide production to examine the earliest events of insulin biosynthesis, highlighting molecular mechanisms underlying β-cell failure plus a novel strategy that might ameliorate the MIDY syndrome. We find that whereas preproinsulin-A(SP23)S is efficiently cleaved, producing authentic proinsulin and insulin, preproinsulin-A(SP24)D is inefficiently cleaved at an improper site, producing two subpopulations of molecules. Both show impaired oxidative folding and are retained in the endoplasmic reticulum (ER). Preproinsulin-A(SP24)D also blocks ER exit of coexpressed wild-type proinsulin, accounting for its dominant-negative behavior. Upon increased expression of ER–oxidoreductin-1, preproinsulin-A(SP24)D remains blocked but oxidative folding of wild-type proinsulin improves, accelerating its ER export and increasing wild-type insulin production. We conclude that the efficiency of SP cleavage is linked to the oxidation of (pre)proinsulin. In turn, impaired (pre)proinsulin oxidation affects ER export of the mutant as well as that of coexpressed wild-type proinsulin. Improving oxidative folding of wild-type proinsulin may provide a feasible way to rescue insulin production in patients with MIDY.

Interaction of zing with proinsulin

To maintain copious insulin granule stores in the face of ongoing metabolic demand, pancreatic beta cells must produce large quantities of proinsulin, the insulin precursor. Proinsulin biosynthesis can account for up to 30–50% of total cellular protein synthesis of beta cells. This puts pressure on the beta cell secretory pathway, especially the endoplasmic reticulum (ER), where proinsulin undergoes its initial folding, including the formation of three evolutionarily conserved disulfide bonds. In normal beta cells, up to 20% of newly synthesized proinsulin may fail to reach its native conformation, suggesting that proinsulin is a misfolding-prone protein. Misfolded proinsulin molecules can either be refolded to their native structure or degraded through ER associated degradation (ERAD) and autophagy. These degraded molecules decrease proinsulin yield but do not otherwise compromise beta cell function. However, under certain pathological conditions, proinsulin misfolding increases, exceeding the genetically determined threshold of beta cells to handle the misfolded protein load. This results in accumulation of misfolded proinsulin in the ER – a causal factor leading to beta cell failure and diabetes. In patients with Mutant INS-gene induced diabetes of Youth (MIDY), increased proinsulin misfolding due to insulin gene mutations is the primary defect operating as a “first hit” to beta cells. Additionally, increased proinsulin misfolding can be secondary to an unfavorable ER folding environment due to genetic and/or environmental factors. Under these conditions, increased wild-type proinsulin misfolding becomes a “second hit” to the ER and beta cells, aggravating beta cell failure and diabetes. In this article, we describe our current understanding of the normal proinsulin folding pathway in the ER, and then review existing links between proinsulin misfolding, ER dysfunction, and beta cell failure in the development and progression of type 2, type 1, and some monogenic forms of diabetes.