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The IRE1α/XBP1s Pathway Is Essential for the Glucose Response and Protection of β Cells.

Hassler JR, Scheuner DL, Wang S, Han J, Kodali VK, Li P, Nguyen J, George JS, Davis C, Wu SP, Bai Y, Sartor M, Cavalcoli J, Malhi H, Baudouin G, Zhang Y, Yates Iii JR, Itkin-Ansari P, Volkmann N, Kaufman RJ - PLoS Biol. (2015)

Bottom Line: Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions.Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion.The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

View Article: PubMed Central - PubMed

Affiliation: Degenerative Diseases Program, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, California, United States of America; Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan, United States of America.

ABSTRACT
Although glucose uniquely stimulates proinsulin biosynthesis in β cells, surprisingly little is known of the underlying mechanism(s). Here, we demonstrate that glucose activates the unfolded protein response transducer inositol-requiring enzyme 1 alpha (IRE1α) to initiate X-box-binding protein 1 (Xbp1) mRNA splicing in adult primary β cells. Using mRNA sequencing (mRNA-Seq), we show that unconventional Xbp1 mRNA splicing is required to increase and decrease the expression of several hundred mRNAs encoding functions that expand the protein secretory capacity for increased insulin production and protect from oxidative damage, respectively. At 2 wk after tamoxifen-mediated Ire1α deletion, mice develop hyperglycemia and hypoinsulinemia, due to defective β cell function that was exacerbated upon feeding and glucose stimulation. Although previous reports suggest IRE1α degrades insulin mRNAs, Ire1α deletion did not alter insulin mRNA expression either in the presence or absence of glucose stimulation. Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions. In contrast, Ire1α deletion in β cells increased the expression of over 300 mRNAs encoding functions that cause inflammation and oxidative stress, yet only a few of these accumulated during high glucose. Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion. The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

No MeSH data available.


Related in: MedlinePlus

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 KOFe/-; 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 KOFe/-; Cre & WTFe/+ samples; green and red indicate increased and decreased expression. The blue box indicates genes with inverse expression dependent on IRE1α and high glucose.
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pbio.1002277.g002: 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 KOFe/-; 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 KOFe/-; Cre & WTFe/+ samples; green and red indicate increased and decreased expression. The blue box indicates genes with inverse expression dependent on IRE1α and high glucose.

Mentions: One of the most remarkable features of the pancreatic beta cells—those that produce and secrete insulin to regulate glucose homeostasis—is their capacity to increase the synthesis of proinsulin (the insulin precursor) up to 10-fold after glucose stimulation. This dramatic increase in the synthesis of proinsulin is a challenge to the proximal secretory pathway and triggers an adaptive stress response, the unfolded protein response, which is coordinated by the IRE1α enzyme and the X-box-binding protein 1 (XBP1) transcription factor. Deletion of IRE1α specifically from the pancreatic beta cells in adult mice resulted in overt diabetic phenotypes such as high blood glucose. mRNA analysis revealed several hundred genes whose expression was coordinately regulated by glucose and IRE1α and whose functions are important for the beta cell secretory pathway. Furthermore, IRE1α also regulates the expression of mRNAs involved in the production of reactive oxygen species (ROS), and we could show that, in fact, oxidative stress is a primary mechanism that causes beta cell failure upon collapse of the secretory pathway. Finally, in experiments with murine and human islets (the regions of the pancreas where secretory beta cells are located), we observed that while IRE1α does not regulate the expression of the gene encoding insulin, it determines final insulin levels by controlling translation of proinsulin mRNA.


The IRE1α/XBP1s Pathway Is Essential for the Glucose Response and Protection of β Cells.

Hassler JR, Scheuner DL, Wang S, Han J, Kodali VK, Li P, Nguyen J, George JS, Davis C, Wu SP, Bai Y, Sartor M, Cavalcoli J, Malhi H, Baudouin G, Zhang Y, Yates Iii JR, Itkin-Ansari P, Volkmann N, Kaufman RJ - PLoS Biol. (2015)

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 KOFe/-; 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 KOFe/-; Cre & WTFe/+ samples; green and red indicate increased and decreased expression. The blue box indicates genes with inverse expression dependent on IRE1α and high glucose.
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pbio.1002277.g002: 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 KOFe/-; 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 KOFe/-; Cre & WTFe/+ samples; green and red indicate increased and decreased expression. The blue box indicates genes with inverse expression dependent on IRE1α and high glucose.
Mentions: One of the most remarkable features of the pancreatic beta cells—those that produce and secrete insulin to regulate glucose homeostasis—is their capacity to increase the synthesis of proinsulin (the insulin precursor) up to 10-fold after glucose stimulation. This dramatic increase in the synthesis of proinsulin is a challenge to the proximal secretory pathway and triggers an adaptive stress response, the unfolded protein response, which is coordinated by the IRE1α enzyme and the X-box-binding protein 1 (XBP1) transcription factor. Deletion of IRE1α specifically from the pancreatic beta cells in adult mice resulted in overt diabetic phenotypes such as high blood glucose. mRNA analysis revealed several hundred genes whose expression was coordinately regulated by glucose and IRE1α and whose functions are important for the beta cell secretory pathway. Furthermore, IRE1α also regulates the expression of mRNAs involved in the production of reactive oxygen species (ROS), and we could show that, in fact, oxidative stress is a primary mechanism that causes beta cell failure upon collapse of the secretory pathway. Finally, in experiments with murine and human islets (the regions of the pancreas where secretory beta cells are located), we observed that while IRE1α does not regulate the expression of the gene encoding insulin, it determines final insulin levels by controlling translation of proinsulin mRNA.

Bottom Line: Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions.Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion.The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

View Article: PubMed Central - PubMed

Affiliation: Degenerative Diseases Program, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, California, United States of America; Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan, United States of America.

ABSTRACT
Although glucose uniquely stimulates proinsulin biosynthesis in β cells, surprisingly little is known of the underlying mechanism(s). Here, we demonstrate that glucose activates the unfolded protein response transducer inositol-requiring enzyme 1 alpha (IRE1α) to initiate X-box-binding protein 1 (Xbp1) mRNA splicing in adult primary β cells. Using mRNA sequencing (mRNA-Seq), we show that unconventional Xbp1 mRNA splicing is required to increase and decrease the expression of several hundred mRNAs encoding functions that expand the protein secretory capacity for increased insulin production and protect from oxidative damage, respectively. At 2 wk after tamoxifen-mediated Ire1α deletion, mice develop hyperglycemia and hypoinsulinemia, due to defective β cell function that was exacerbated upon feeding and glucose stimulation. Although previous reports suggest IRE1α degrades insulin mRNAs, Ire1α deletion did not alter insulin mRNA expression either in the presence or absence of glucose stimulation. Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions. In contrast, Ire1α deletion in β cells increased the expression of over 300 mRNAs encoding functions that cause inflammation and oxidative stress, yet only a few of these accumulated during high glucose. Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion. The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

No MeSH data available.


Related in: MedlinePlus