Ire1α Deletion in Adult β Cells Decreases Proinsulin Biosynthesis at the Post-transcriptional Level

To study the function of IRE1α in differentiated β cells and since Ire1α deletion causes embryonic lethality, we analyzed the requirement for IRE1α in β cells by generating and characterizing mice with one floxed Ire1α^Fe allele [39] in combination with either a wild-type (WT) Ire1α⁺ or an Ire1α— null allele [40]. Deletion of the floxed Ire1α^Fe allele is mediated by a Cre recombinase-estrogen receptor fusion protein driven by the rat Ins2 promoter, which is expressed in pancreatic β cells from midgestation (E9–E11.5) and activated by tamoxifen (Tam) administration [41]. This cross yields Ire1α^{Fe/-; Cre+} (herein designated KO) mice at nearly the expected frequency as the Ire1α^{Fe/+; Cre-} WT, Ire1α^{Fe/-; Cre-} (full-body heterozygous at birth; Het-B) and Ire1α^{Fe/+; Cre+} (Tam-induced β cell-specific heterozygous; Het-I) littermate controls. This allows more efficient Ire1α deletion as only one floxed allele requires deletion and provides two different heterozygous genotypes for comparison to the WT and KO groups. Undeleted KO mice exhibit normal blood glycemia; however, glucose intolerance became significant at 2 wk post-Tam injection and peaked at 6 wk (Fig 1A and 1B and S1A Fig). A similar diabetic phenotype was observed upon analysis of Tam-induced deletion of homozygous floxed mice Ire1α^{Fe/Fe; Cre+} (KO) (S1A Fig). Only the KO mice displayed significantly reduced levels of insulin and proinsulin within the serum, pancreas, and islets compared to the heterozygous or WT mice (Fig 1A–1I and S1A–S1C Fig). Peak glucose intolerance for the KO mice occurred 6 wk post-Tam when Ire1α deletion was most efficient (Fig 1A and 1J and S1A Fig); therefore, most islet experiments were conducted at this time, unless otherwise noted. Compared to the developmental deletion models of Ire1α and Xbp1 previously reported [35–37,42], Tam-induced Ire1α deletion in β cells of adult mice caused a significantly greater diabetic phenotype (Fig 1A–1I and S1A–S1C Fig). The decreased basal serum insulin in the KO mice became more pronounced after a fast and refeed, indicating a defect in postprandial insulin secretion (Fig 1C). Consistent with β cell failure, the KO mice also exhibited higher levels of serum proinsulin (S1B Fig). Significantly, although the percent islet mass was slightly reduced in the KO mice, this decrease alone could not account for the diabetic phenotype (Fig 1E). A more accurate measure of total insulin and proinsulin content in pancreas extracts by ELISA showed a significant reduction in both insulin and proinsulin in the KO group, with proinsulin being more reduced than insulin (Fig 1F–1H). To determine whether the reduced pancreatic proinsulin content was accompanied by a defect in proinsulin synthesis, isolated islets were radiolabeled for 30 min under high glucose with [³⁵S]-Cys/Met. Compared to WT and heterozygous islets, proinsulin synthesis was significantly reduced in the islets from KO mice (S1 Data). In addition, infection of WT islets with adenoviruses that express either Cre as control (Ad-Cre) or a dominant-negative IRE1α-K907A mutant (Ad-ΔR) demonstrated only the latter selectively reduced proinsulin synthesis (Fig 1I and S1 Data). Taken together, the slightly reduced islet area coupled with the reduced proinsulin synthesis accounts for the reduced pancreatic insulin and proinsulin contents and abnormal glucose homeostasis in the KO mice (Fig 1A–1I and S1A–S1C Fig). In contrast to the beta cell-specific Xbp1 deletion in which IRE1α RNase hyperactivation occurs [37], the mRNA levels encoding INS1 and INS2 and the proinsulin-processing enzymes—i.e., prohormone convertases (PC1, PC2) and CPE—were not significantly altered upon β cell-specific Ire1α deletion, despite an 88% decrease in the floxed Ire1α mRNA expression and an 81% decrease in Xbp1 mRNA splicing (Figs 1J and 3A). In addition, reduced IRE1α in insulinoma cells was reported to also reduce proinsulin synthesis [35] and is consistent with our findings in islets. Importantly, these results show that adult β cells require functional IRE1α to maintain proinsulin mRNA translation, granule storage, and insulin secretion but not to maintain the expression of Ins1, Ins2, and most other β cell-specific mRNAs (Figs 1J and 3A).

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Fig 1. Tam-induced Ire1α deletion in adult β cells reduces proinsulin synthesis, insulin content, and insulin secretion, without altering insulin mRNA levels.

(A) Blood glucose levels for 16-wk-old male mice following 4 h of fasting with increasing weeks post-Tam. Respectively for 4, 8, and 20 wk post-Tam ([p = 0.042, 0.009, 0.031], [WT^Fe/+ n = 8, KO^{Fe/-; Cre} n = 10]). (B) Glucose tolerance tests (GTTs) performed on 16-wk-old male mice at 6 wk post-Tam and the areas under the curves 6 wk post-Tam. The values for statistical significance in Fig 1A and S1A Fig were calculated from areas under the GTT curves. The data and statistics for the GTTs and all other data except when indicated are within S1 Data. ([WT^Fe/+, Het-I^{Fe/+; Cre}, Het-B^Fe/- and KO^{Fe/-; Cre} n = 8], [p = 1.408 x 10⁻⁷, WT^Fe/+ versus KO^{Fe/-; Cre}]). (C) Serum insulin levels in mice 6 wk post-Tam: fed, 4 h fasted, 30 and 90 min after refeeding ([n = 4, all groups], student’s t test for significance for WT^Fe/+ versus KO^{Fe/-; Cre} [p = 0.044]). (D) Immunofluorescence microscopy of islets co-stained for insulin (red), proinsulin (green), and DAPI (blue); see S3B Fig for additional examples. Scale bar, 100 μm. (E) Percent islet areas were determined on 6-wk post-Tam pancreas by outlining 138, 153, 234, and 297 cross sections from 9, 9, 11, and 14 mice WT^Fe/+, Het-I^{Fe/+; Cre}, and KO^{Fe/-; Cre} groups, respectively. (F and G) Insulin and proinsulin ELISAs of acid ethanol extracts from 6 wk post-Tam mouse pancreas (WT^Fe/+ versus KO^{Fe/-; Cre}; [insulin, p = 0.039, proinsulin, p = 0.031], [n = 5, all groups]). (H) Individual mouse proinsulin/insulin ratios were determined and averaged ([p = 0.009], [n = 5]). (I) Islets were shifted from 4 mM to 25 mM glucose for 30 min in [³⁵S]-Cys/Met in order to determine the synthesis rate during high glucose by antiproinsulin immunoprecipitation IP ([n = 3] for WT^Fe/+, Het-I^{Fe/+; Cre}, and KO^{Fe/-; Cre}), ([n = 6] for +/+ infected with Ad-Cre versus Ad-ΔR (p = 0.019)]. Since limiting amounts of a homemade proinsulin antibody was used for the first five lanes, the Ad experiments used a commercial antibody that produced consistent results. (J) Real-time PCR (quantitative real-time PCR [qRT-PCR]) of total RNA isolated from islets at 6 wk post-Tam ([n = 5], [p = 0.022**, 0.039*, and 0.047*]) for Ire1α deletion-specific, Xbp1 spliced-specific, and Mafa mRNAs, respectively.

https://doi.org/10.1371/journal.pbio.1002277.g001

In addition to the Ad-Cre control used on islets in vitro (Fig 1I), to further ensure the diabetic phenotype did not result from nonspecific expression of the RIP-CreER allele [43] and Ire1α deletion in another tissue in vivo, such as the hypothalamus [44], we measured levels of serum dopamine, which is synthesized in the arcuate nucleus of the hypothalamus. This analysis did not detect a significant difference between the KO and control mice (S1D Fig). In addition, although we detected Cre positive staining in brain sections, there was little difference in growth hormone-releasing hormone (GHRH) expression between the KO mice and the controls (S1E Fig). Finally, the KO islets and the WT islets expressing Ad-ΔR both demonstrated reduced proinsulin synthesis without affecting Ins1 or Ins2 mRNA levels in the KO (Fig 1I, S1 Data and Fig 1J). Therefore, we conclude that Ire1α deletion in β cells reduces proinsulin mRNA translation and is a primary molecular basis for the diabetic phenotype.

Generally, quantitative real-time PCR (qRT-PCR) of β cell-specific mRNAs did not detect a significant difference upon Ire1α deletion, although Mafa (v-maf avian musculoaponeurotic fibrosarcoma oncogene homolog A) mRNA expression was consistently increased within KO islets (Figs 1J and 3A). However, immunofluorescence microscopy demonstrated that the WT islets actually contained higher overall and nuclear MAFA protein than KO islets (S3A Fig), while PDX1 nuclear localization was not affected (S3A Fig). This is consistent with the finding that oxidative stress decreases nuclear localization of MAFA [45,46]. Although the peak of hyperglycemia occurred at 6 wk post-Tam injection, there was no significant increase in terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positivity at this time or at 12 wk post-Tam injection. However, long after Tam-induced deletion (>6 mo), the glucose intolerance slightly improved, suggesting β cell recovery, possibly by expansion of nondeleted β cells and/or adaptation of the deleted cells (S2 Data). Therefore, the mechanism for the diabetic phenotype in the KO mice is not due to excessive β cell death but is most likely due to underlying XBP1s-dependent defects caused by Ire1α deletion in the β cell. Taken together, these results indicate IRE1α is required within the β cell for proinsulin mRNA translation, without significantly affecting insulin mRNA steady-state levels.

Ire1α Deletion in β Cells Causes ER Stress

Islets isolated from KO mice at 6 wk post-Tam injection demonstrated reduced expression of previously described XBP1s target genes Atf6α, Pdia1, Fkbp11, Erdj4, and Wfs1 [34,39,47] and increased expression of ER chaperones Hspa5 (immunoglobulin binding protein [BIP]), Grp94, and Erp72 and proapoptotic Ddit3 (Fig 2A). Immunofluorescence microscopy, qRT-PCR, and mRNA-Seq analyses also demonstrated increased BIP and GRP94 in KO islets (Fig 2A and 2B, S3B, S4A and S5C Figs). In addition, the KO β cells exhibited increased colocalization of the plasma membrane-resident protein GLUT2 with the KDEL-containing ER chaperones BIP and GRP94 (Fig 2B and S3B Fig), indicative of an ER-to-Golgi trafficking defect in KO β cells. Electron microscopy (EM) revealed that KO β cells contain many distended ER/Golgi membranes and/or empty vesicles, a 43% reduction in insulin granules, distended mitochondria, pyknotic nuclei, and multilamellar vesicles suggestive of autophagy (Fig 2C [yellow outlines] and S2A Fig, bottom). These dramatic morphological changes were not observed in Tam-induced Het-I control islets, indicating IRE1α is required to maintain organelle integrity in β cells. Next, we studied the effect of Ire1α deletion on glucose-regulated gene expression, by mRNA-Seq analysis on islets after 72 h incubation in 6 mM or 18 mM glucose to chronically stimulate insulin production and because 18mM glucose was reported to cause insulin mRNA degradation by IRE1α in insulinoma cells [48]. RT-PCR using primers flanking the unconventional intron spliced by IRE1α demonstrated treatment with 18 mM glucose increased Xbp1 mRNA splicing by 56.6% in WT islets compared to undetectable levels in KO islets (Fig 2D). Similarly, qRT-PCR confirmed Ire1α deletion and Xbp1 mRNA splicing were >90% decreased in KO islets (S3A Fig).

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Fig 2. KO islets exhibit ER stress.

(A) qRT-PCR of UPR genes in islets isolated 6 wk post-Tam and incubated in 11 mM glucose 16 h ([n = 5], [p ≤ 0.05]). (B) Immunofluorescence microscopy of pancreas sections stained for KDEL (BIP and GRP94) (green), the plasma membrane protein GLUT2 (red), and nuclei DAPI (blue). Overlap of red/green channels represents defective compartmentalization that was found to be increased in the KO^{Fe/-; Cre} as shown in yellow. Scale bars, 400x = 50 μm, 1,000x = 10 μm, 5,180x = 2 μm and 10,500x = 1 μM. Additional examples are shown in S3B Fig. (C) EM of adult mouse (16 wk old) islets and their β cells from mice 2 wk post-Tam. Scale bars, both panels, 1 μm. Distended mitochondria are outlined with yellow dashes. (D) Conventional PCR flanking the 26 nt intron in Xbp1 mRNA spliced by IRE1α from the islet complementary DNAs (cDNAs) used for mRNA-Seq analysis, 6 mM versus 18 mM glucose. Results representative of n = 5 per genotype. (E) Global heatmap for the ~22,000 mRNAs detected by mRNA-Seq for 18 mM KO^{Fe/-; Cre} & WT^Fe/+ samples; green and red indicate increased and decreased expression. The blue box indicates genes with inverse expression dependent on IRE1α and high glucose.

https://doi.org/10.1371/journal.pbio.1002277.g002

mRNA-Seq Identifies IRE1α- and Glucose-Dependent mRNAs

mRNA-Seq on average detected >22,000 mRNAs in each sample that were visualized by heatmap (Fig 2E). mRNAs having altered expression with p-values ≤ 0.1 or ≤ 0.01 reduced the number to ~4,500 and ~1,700, respectively. Importantly, consistent with our previous results (Fig 1J), the levels of mRNAs encoding INS1 and INS2 were not significantly altered in KO islets as measured by both mRNA-Seq and qRT-PCR (Fig 3A and S4A Fig). In addition, as measured by qRT-PCR, mRNA-Seq also reported that mRNA expression of the XBP1s target genes Wfs1, Atf6α, Edem1, Edem3, and Erdj4 was significantly reduced in KO islets, whereas the mRNA levels encoding BIP, GRP94, and DDIT3/CHOP were increased (Fig 2A, S4A and S5C Figs and S2 Data). Venn diagram analysis of the islet mRNA-Seq data was used to determine mRNAs that were Ire1α dependent, high glucose dependent, or dependent on both. The four-way Venn diagrams identified the expression of 613 decreased and 1,338 increased mRNAs in KO islets, of which 141 and 368 were simultaneously high glucose-dependent, respectively (Fig 3B and 3C and S2 Data). Among the 141 mRNAs that required both IRE1α and high glucose for induction, the most significant gene ontology (GO) terms included ER protein translocation, ER–Golgi protein transport machinery, ribosome and protein biosynthetic components, and the lysosome and glycosidases, as well as other intracellular processes with fewer representative mRNAs of each respective ontological group (Fig 3C, left). Strikingly, 76% of the 141 and 35% of the 368 IRE1α- and high glucose-dependent mRNAs have not yet been functionally characterized or previously associated with the IRE1α pathway (S3 Data). Similarly, only 22 of the 141 mRNAs were previously shown to bind XBP1 within their promoters (S4B Fig, left and S3 Data) [16]. In contrast to the mRNAs reduced upon Ire1α deletion, mRNA-Seq also uncovered 368 inversely regulated mRNAs that were increased in Ire1α-null islets and reduced during high glucose in WT islets (Fig 3B and 3C, right). These mRNAs encode proteins that are known RIDD targets, produce oxidative stress, and are involved in the ECM, plasma membrane, and immune cell signaling, as well as many other uncharacterized transcripts (Figs 3C and 4A, S4C and S5A Figs and S2 Data). The Venn diagram also compared the 368 inversely regulated mRNAs with previously identified XBP1s and RIDD targets (S4B and S4C Fig) [18,19]. These mRNAs increased by high glucose in KO islets but decreased in WT islets upon glucose stimulation could be (1) degraded by IRE1α, (2) repressed by XBP1s, (3) induced by XBP1u, (4) stabilized as a consequence of defective ribosome recruitment to the ER, (5) induced via alternative UPR pathways, (6) induced as a consequence of oxidative stress or inflammation (see below), or (7) derived from alternative cell types. To verify which changes in mRNA levels upon Ire1α deletion correlate with protein levels, isolated WT islets were infected in culture with adenoviruses expressing either β-Galactosidase (Ad-β-Gal) or the IRE1α K907A RNase mutant (Ad-ΔR), incubated in high glucose (18 mM), and analyzed by mass spectrometry after 72 h. Expression of the dominant-negative IRE1α caused a 21% reduction in insulin 1 and 2 peptides (S3 Data). The mass spectrometry analysis identified increases and decreases in proteins correlating with the mRNA changes detected by mRNA-Seq (Fig 3D and S3 Data).

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Fig 3. mRNA sequencing identifies IRE1α- and glucose-dependent mRNAs in islets.

(A) mRNA-Seq data on β cell-specific mRNAs. The results show no significant change to INS1 or INS2 in the KO^{Fe/-; Cre} samples, while MAFA, GCG, and PC5 are increased by deletion ([n = 5], [18 mM KO^{Fe/-; Cre}, p-values ≤ 0.05]). mRNA-Seq expression fold changes were normalized relative to the 6 mM WT^Fe/+ islet context. (B) Four-way Venn diagrams of WT^Fe/+ versus KO^{Fe/-; Cre} islets during 6 mM versus 1 8mM glucose exposure for 72 h. Ire1α-dependent mRNAs are in bold italics, while those also dependent on high glucose are in bold, italicized, and underlined font. At the center, bar graphs representing the Ire1α- and glucose-dependent trends of interest are labeled “Induction” and “Repression.” (C) Combined DAVID (the Database for Annotation, Visualization and Integrated Discovery) and “ConceptGen” GO analysis of Ire1α- and glucose-dependent mRNAs. Categories shown are specifically found in the genotype, while the shared categories have been omitted for simplicity, although no single mRNA was common between the groups. (D) Mass spectrometry of murine islets infected with Ad-IREα-K907A (Ad-ΔR) versus Ad-β-Galactosidase (β-Gal). Proteins with ≥5 unique peptides detected per protein increased or decreased upon infection in triplicate were analyzed for GO using ConceptGen and DAVID web resources (n = 3). The proteins shown (Fig 3D) exhibit the same expression dependence for IRE1α as measured by mRNA-Seq (S2 Data).

https://doi.org/10.1371/journal.pbio.1002277.g003

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Fig 4. KO islets accumulate oxidative stress, inflammation, and fibrosis.

(A) mRNA-Seq expression values for 25/368 of the mRNAs identified by Venn analysis (Fig 3C; right panel, underlined) that are reduced by Ire1α because of glucose that accumulates in the KO^{Fe/-; Cre} ([n = 5], [p-values ≤ 0.05]). (B) Oxidized lipid (hydroxyl-octadecadienoic acids, HODEs) from islets of the indicated genotypes infected with Ad-Cre Ad-GFP or no virus control ([n = 2; controls versus n = 3; Ad-Cre], [p = 0.00434]). (C) Antinitrotyrosine immunohistochemistry (IHC) of islets from 8-mo-old WT^Fe/Fe and KO^{Fe/Fe; Cre} mice 15 wk post-Tam with or without BHA diet for 3 wk. (Scale bar, 50 μm) (WT^Fe/Fe [n = 4 with BHA], [n = 5 regular chow]), (KO^{Fe/Fe; Cre} [n = 5 with BHA], [n = 6 regular chow]). (p = 0.00698; WT^Fe/Fe versus WT^Fe/Fe with BHA), (p = 0.04018; WT^Fe/Fe versus KO^{Fe/Fe; Cre}) and (p = 0.04420; KO^{Fe/Fe; Cre} versus KO^{Fe/Fe; Cre} with BHA). (D) Masson’s trichrome stain (blue) for collagens. Results demonstrate increased staining surrounding KO^{Fe/Fe; Cre} islets with haemotoxylin (red) and eosin (black) co-stains. Quantification of percent strong collagen stain is shown below the images. Scale bar, 50 μm. (WT^Fe/Fe [n = 4 with BHA], [n = 5 regular chow]), (KO^{Fe/Fe; Cre} [n = 5 with BHA], [n = 6 without BHA]). Percent strong collagen stain significance for WT^Fe/Fe without BHA versus KO^{Fe/Fe; Cre} without BHA p = 0.01049). (E) 8-mo-old male mice carrying the doubly floxed allele (Ire1α^Fe/Fe) with and without RIP-Cre 12 wk post-Tam had their pre-BHA GTTs taken, and then half were fed the antioxidant BHA supplemented chow diet for 3 wk or not before examining the mice by GTT again. (WT^Fe/Fe [n = 11 with BHA], [n = 12 regular chow], [p = 0.035]), (KO^{Fe/Fe; Cre} [n = 18 with BHA], [n = 16 without BHA], [p = 0.041]). P-values were calculated by one-tailed student’s t test comparison of the areas under the GTT curves for the biological replicates of control group WT^Fe/Fe versus the Tam-induced KO^{Fe/Fe; Cre} group.

https://doi.org/10.1371/journal.pbio.1002277.g004

Ire1α Deletion Causes β Cell Failure Due to Oxidative Stress

The GO terms for those mRNAs that were increased by the absence of IRE1α include ROS-generating enzymes, such as inducible nitric oxide synthase (iNOS/NOS2), and the lysyl-oxidases (LOXs) that are involved in oxidation of collagens (COLs) that were also increased in the ECM (Fig 4A, S4C, S5A–S5C Figs). Consistent with increased oxidative stress, the KO islets also had significantly higher levels of mRNAs encoding glutathione peroxidases 2 and 3 (GPX2 and GPX3) and protein disulfide isomerases 4 and 5 (PDIA4 and PDIA5) (Fig 3D, S5A, S5C and S6A Figs). Therefore, we directly measured oxidative stress in Ire1α-deleted islets. Upon acute Ad-Cre-mediated Ire1α deletion in Ire1α^Fe/Fe isolated islets, lipid peroxides (hydroxyl-octadecadienoic acids [HODEs]) increased 33% compared to Ad-GFP-infected or Ad-Cre-infected Ire1α^+/+ islet control groups (Fig 4B). In addition, increased nitrotyrosine staining was increased in KO pancreas sections after Ire1α deletion by Tam injection in vivo (Fig 4C). Similarly, tenascin C (TNC) mRNA and protein were increased in KO islets as measured by mRNA-Seq and mass spectrometry and then confirmed by immunohistochemistry (S5D Fig). Consistent with the increased collagen mRNA expression, Masson’s trichrome stain identified increased collagen staining surrounding the KO islets (Fig 4D and S5E Fig). Since previous studies suggested that antioxidant treatment reduces ER stress in β cells [9,49–51], we tested whether feeding mice chow supplemented with the antioxidant butylated hydroxyanisole (BHA) could improve β cell function upon Ire1α deletion. Therefore, at 12 wk post-Tam injection, mice were fed control chow or BHA-supplemented chow for 3 wk (12–15 wk post-Tam). Notably, feeding mice with BHA-supplemented chow significantly improved glucose homeostasis in mice with β cell-specific Ire1α deletion (Fig 4E) that was also reflected by decreased trends of nitrotyrosine, collagen, and TNC staining (Fig 4C and 4D, S5D and S5E Fig). Importantly, these findings indicate that Ire1α deletion causes β cell failure, at least in part, due to oxidative stress.

Glucose Stimulates the IRE1α/XBP1s Pathway to Expand ER Capacity for Proinsulin Synthesis

Although the 141 mRNAs that require IRE1α for glucose induction encoded known functions in protein synthesis and the secretory pathway (Fig 3C, left, and Fig 5A, bottom), approximately half of these mRNAs have never been characterized or associated with the IRE1α/XBP1s pathway (Fig 3D and 3E (left panels), and S3 Data). Significantly, the most prominent GO cluster from the IRE1α-dependent and glucose-inducible group of 141 mRNAs included 12 with functions for SRP recruitment to the ER, translocon components, and the catalytic and structural subunits of the signal peptide cleavage complex (Fig 5A, bottom). These 12 mRNAs were induced up to 2.5-fold by glucose in an IRE1α-dependent manner, whereas their expression was reduced as much as 2.5-fold in the KO islets, i.e., ~4–6-fold difference during glucose stimulation, suggesting a major bottleneck in the signal peptide-dependent proximal secretory pathway of the β cell when compromised by Ire1α deletion.

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Fig 5. IRE1α mediated Xbp1 splicing is necessary for proper signal peptide cleavage of preproinsulin and ribosome distribution.

(A) mRNA-Seq expression values of 24/141 mRNAs identified (Fig 3B–3D) to be both Ire1α- and high glucose-dependent for their induction ([n = 5], [p-values ≤ 0.01]). (B) Autoradiograph from whole cell islet lysates prepared by steady-state (18 h) [³⁵S]-Cys/Met radiolabeling from 12 wk post-Tam mice (n = 3) following peptide gel electrophoresis. (C) Western blotting for proinsulin/preproinsulin, IRE1α, SEC11C, SSR1, and tubulin after peptide gel electrophoresis of lysates prepared 72 h after COS-1 cells were coinfected with adenoviruses expressing WT preproinsulin, the Akita mutant (A), and/or the (G) GFP, (X) XBP1s, or (Δ) the dominant negative IRE1α-RNase mutant K907A representative results shown (n = 4). (D) Ribosomes and their relative position to one another were measured from electron micrographs at a magnification of 25,000x. Total numbers of ribosomes analyzed are shown below the images ([n = 3], [p = 2.2 x 10⁻¹⁵]). (E) Subcellular fractionation and western blot analysis for ribosomal small subunit 9 and tubulin of the Ire1α^Fe/Fe β cell insulinoma line at after 2-h glucose shift from 12 mM to either 4 mM or 36 mM with or without infection by the indicated adenoviruses were blotted representative of (n = 3). The 12 mM condition western blots and the quantified results normalized to tubulin membranous/cytosolic are shown in S6D Fig.

https://doi.org/10.1371/journal.pbio.1002277.g005

In order to detect signal peptide cleavage in preproinsulin, which occurs very efficiently, special cellular contexts were utilized. Steady-state labeling demonstrated that, compared to control islets, the KO islets accumulated a slower migrating ~12 kDa species corresponding to the size of preproinsulin (Fig 5B and S1 Data). This steady-state labeling also showed decreased amounts of processed intracellular and secreted insulin and increased proinsulin that is likely due to the prolonged high glucose exposure in vitro (Fig 5B). To extend these findings and focus specifically on the preproinsulin-to-proinsulin processing step, signal peptide cleavage was analyzed in COS-1 cells that do not process proinsulin to insulin. Cells were coinfected with Ad-preproinsulin encoding either WT (W) or the Akita misfolded Cys96Tyr mutant (A) [52] and coexpressed with Ad-GFP (G) or Ad-ΔR (Δ). Ad-preproinsulin infection alone or in combination with Ad-β-Gal produced a single 10 kDa species representing proinsulin (Fig 5C, lanes 1–3). In contrast, coinfection of Ad-preproinsulin with Ad-ΔR, the IRE1α RNase-dead mutant incapable of splicing Xbp1 mRNA, caused accumulation of a ~12 kDa insulin immune-reactive species, corresponding to preproinsulin, consistent with a defect in signal peptide cleavage when IRE1α-dependent Xbp1 mRNA splicing is compromised (Fig 5C, lane 4), and this ~12 kDa species accumulated to a greater extent upon expression of the Akita proinsulin mutant (Fig 5C, lane 6). In addition, adenoviral overexpression of XBP1s was sufficient to induce SEC11C and signal sequence receptor 1 (SSR1) protein levels in COS-1 cells (Fig 5C). Because ribosome-membrane recruitment and translocon mRNAs were heavily Ire1α- and high glucose-dependent, we analyzed ribosome distribution in electron micrographs from Het and KO islets at 2 wk post-Tam injection. The KO had significantly more dispersed ribosomes, a hallmark of disordered polysomes, and more total monosome evidence for defect in polysome formation in the absence of IRE1α (Fig 5D) [53,54]. Consistent with this observation, Ad-Cre infection of an immortalized Ire1α^Fe/Fe β cell line increased the monosome/polysome ratio (S6C Fig), further evidence of a translation initiation defect. Finally, we analyzed ribosome recruitment to the ER in the Ire1α^Fe/Fe β cell line at 48 h after infection with Ad-Cre or controls. Cells were shifted from 12 mM glucose to media containing 4 mM, 12 mM, or 36 mM glucose for 2 h and then subjected to subcellular fractionation. Western blotting for the ribosomal small subunit proteins RPS9 and RPSA in cytosolic and membranous fractions from glucose-stimulated floxed Ire1α^Fe/Fe insulinoma cells indicated Ire1α deletion in vitro disrupted glucose-stimulated recruitment of the ribosome to the membranous fraction (Fig 5E and S6D Fig). The increased monosomes detected in islets and the IRE1α-deleted insulinoma line (Fig 5D and S6C Fig) were accompanied by basally increased RPS9 and RPSA protein, but not increased mRNA levels (Fig 5E, S6D and S6E Fig). We next tested whether chemical inhibition of the IRE1α RNase activity in human islets affected INS mRNA levels after 24 h in low versus high glucose (S7A Fig). The chemical inhibition of IRE1α RNase overnight did not cause INS mRNA levels to accumulate but did block glucose-stimulated induction of proinsulin mRNA, supporting the hypothesis that XBP1s is needed for ER expansion to stimulate proinsulin mRNA translation (S7A Fig). The reduced levels of the XBP1s target gene P58^IPK mRNA is consistent with inhibition of XBP1 mRNA splicing (S7A Fig). We then used adenoviral forced expression of XBP1s to induce mRNAs encoding components of the SRP-dependent proximal ER in human islets. Significant expression of GFP was observed at 5 d after infection with Ad-GFP, indicating efficient Ad-mediated expression in human islets (S7B Fig). Interestingly, expression of Ad-Xbp1s, as opposed to its chemical inhibition, had the inverse effect on INS mRNA. Increased XBP1s was sufficient to increase INS mRNA levels, presumably because of increased capacity for mRNA recruitment to the ER and cotranslational translocation (S7A–S7C Fig). We then analyzed the effect of adenoviral expression of IRE1α, its mutants, and XBP1s on proinsulin production in human islets at the protein level. Expression of IRE1α mutants devoid of RNase function (IRE1α- ΔR or ΔCT) greatly reduced proinsulin levels compared to controls (S7D Fig). The results support the conclusion that positive regulation of proinsulin production by IRE1α is conserved between mice and humans.

Ethics Statement

Sanford Burnham Prebys Medical Discovery Institute (SBPMDI) follows the “Guide for the Care and Use of Laboratory Animals: Eighth Edition” standards. The Institute’s Animal Care & Use Program is accredited by AAALAC International, and a Multiple Project Assurance A3053-1 is on file in the OLAW, DHHS. Euthanasia is consistent with the recommendations of the 2013 AVMA Guidelines for the Euthanasia of Animals. All animal care and procedures were conducted according to the protocols and guidelines approved by the University of Michigan Committee on the Use and Care of Animals (UCUCA) and the SBPMDI, as well as by the Institutional Animal Care and Use Committee (IACUC) divisions of the American Association for Laboratory Animal Science (AALAS). IOur AAALAC number is 000710, and our mouse protocol number is 14–036. The human islets were sourced from the Clinical Islet Laboratory at the University of Alberta/Alberta Health Sciences Sanford Burnham Prebys Medical Discovery Institute IRB Code: 100894XX. Islet donors’ written consent was given to Clinical Islet Laboratory at the University of Alberta/Alberta Health Sciences.

Mouse Husbandry

All animal care and procedures were conducted according to the protocols and guidelines approved by the UCUCA and the SBPMDI, as well as by the IACUC division of the AALAS.

qRT-PCR

Mouse and human qRT-PCR primer sequences specific to total levels of mRNAs were obtained from the Harvard Primer Bank: http://pga.mgh.harvard.edu/primerbank/. See online supporting methods for additional details. The primers used to detect the presence and absence of the Ire1α floxed allele are as follows: FWD-cctacaagagtatgtggagc, REV-ggtctctgtgaacaatgttgagag. Spliced-specific Xbp1 primers are as follows: FWD-gagtccgcagcaggtg, REV-gtgtcagagtccatggga. Unspliced-specific Xbp1 primers are as follows: FWD-ctcagactatgtgcacctct (within the 26 nt intron), REV- catgactgggtccaagttgtccag.

Conventional PCR Primers Flanking the 26 nt Intron in XBP1 mRNA

The primer sequences for XBP1 flanking PCR are FWD-ccttgtggttgagaaccagg, REV-gtgtcagagtccatggga amplicon + 211 bp (unspliced) and 185 bp (spliced).

Blood Glucose Measurements and Pancreatic Proinsulin and Insulin Content

Mice were fasted for 4 h, and then fasted blood glucose measurements were recorded. Glucose tolerance tests (GTTs) were performed as previously described by intraperitoneally injecting a glucose solution of 2 g/kg by body weight and recording tail-vein blood measurements over time using a digital glucometer [9]. Insulin and proinsulin content was determined by diluting acid ethanol extracts from the pancreas at 50 mg/ml 1:200 in sample resuspension buffer provided by the ALPCO ELISA kits for insulin (Cat. 80-INSHU-E01.1) and proinsulin (Cat. 80-PINMS-E01)

EM

Freshly dissected pancreas was fixed in Sorenson’s buffer and processed by the University of Michigan Electron Microscopy Core facility. Blind scoring of insulin granules was performed using Cell Profiler software.

Adenoviral Infections

Experiments utilizing adenoviruses were performed in triplicate with graphs representing the average of all experiments except the human islets where the individual’s age is stated within the legends. Mouse islets were infected with adenovirus at 24 h post-isolation, and human islets were typically received 3 d postmortem and after one night in media were infected with adenoviruses. For infection of islets and the immortalized Ire1α^Fe/Fe β cell line, 50 and ten plaque forming units per cell were used, respectively. At 72 h postadenoviral infection, analysis by pulse-chase and measurement for oxidative stress and HODEs was performed as described [9].

mRNA-Seq Transcriptome Sequencing

The Illumina Genome Analyzer II was utilized to analyze 200 nt long, fragmented mRNA converted to cDNA (50 ng/individual) purified from islets of five individual mice per genotype according to Illumina mRNA-Seq kit (Part# 1004898). The islet mRNA-Seq data has been deposited to the SRA Study Accession: http://www.ncbi.nlm.nih.gov/sra/?term=SRP041246, and the bioproject website is http://www.ncbi.nlm.nih.gov/bioproject/242958.

Polysome Profiling

Polysome profiles were obtained byt reating cells with 0.1 mg/mL cycloheximide (CHX) for 10 min at 37°C, washed twice with ice-cold PBS-CHX (phosphate buffered saline containing 0.1 mg/mL CHX), and harvested using polysome lysis buffer (20 mM Tris-HCl pH 7.5, 100 mM NaCl, 10 mM MgCl₂, 0.4% IGEPAL, 50 μg/mL CHX, protease inhibitors, and RNaseIn). Lysates were clarified by centrifugation at 13,000 × g for 10 min at 4°C. Equal amounts of clarified lysates based on the absorption at 260 nm were layered onto 10%–50% sucrose gradient (prepared in 20 mM Tris-HCl pH 7.5, 100 mM NaCl, 10 mM MgCl₂, 50 μg/mL CHX) and centrifuged in an SW41-Ti rotor at 40,000 rpm for 2 h at 4°C. Fractions were collected using a Bio-Rad fraction collector, and the amount of total RNA in each fraction was measured using a NanoDrop spectrophotometer [66].

Subcellular Fractionation

The Pierce kit catalog #78840 was used to isolate subcellular fractions.