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Loss of Liver Kinase B1 (LKB1) in Beta Cells Enhances Glucose-stimulated Insulin Secretion Despite Profound Mitochondrial Defects.

Swisa A, Granot Z, Tamarina N, Sayers S, Bardeesy N, Philipson L, Hodson DJ, Wikstrom JD, Rutter GA, Leibowitz G, Glaser B, Dor Y - J. Biol. Chem. (2015)

Bottom Line: However, the full spectrum of LKB1 effects and the mechanisms involved in the secretory phenotype remain incompletely understood.Surprisingly, enhanced GSIS is seen despite profound defects in mitochondrial structure and function in LKB1-deficient β cells, expected to greatly diminish insulin secretion via the classic triggering pathway.This study shows that β cells can be manipulated to enhance GSIS to supra-normal levels even in the face of defective mitochondria and without deterioration over months.

View Article: PubMed Central - PubMed

Affiliation: From the Department of Developmental Biology and Cancer Research, The Institute for Medical Research Israel-Canada, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel.

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LKB1 is essential for mitochondrial integrity.A, representative transmission electron micrographs of pancreatic sections showing mitochondria (arrows) in β cells in lox/lox and βLKB mice. Mice were 10 months old, 9 months after tamoxifen injection. B, a section from an islet of an LKB1-deficient mouse showing intact mitochondria in two α-cells. Arrows point to mitochondria. C, quantification of the percentage of β cells with defective mitochondria (swollen mitochondria or lack of cristae) as determined by analysis of EM images. *, p < 0.05. D, Western blot of cytochrome c protein normalized to actin on islets from two wild type mice and two βLKB mice. Mice were 5 months old. E, mitochondrial DNA copy number measured by the ratio between mitochondrial DNA and nuclear DNA from lox/lox and βLKB whole islets (left) or sorted β cells (right) using qPCR. For whole islets, data represent the mean of DNA ratio (cytochrome b/Aprt) from 4–5 mice at 4 and 10 months of age. For sorted β cells, data represent the ratio of DNA (cytochrome b/L1) pooled from 4 mice per genotype at 1 year of age. F, PGC1α and PGC1β mRNA levels quantified by RT-PCR from wild type and βLKB islets normalized to actin. Data represent the mean ± S.E. of 4 mice at age of 4.5 months. ***, p < 0.005.
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Figure 5: LKB1 is essential for mitochondrial integrity.A, representative transmission electron micrographs of pancreatic sections showing mitochondria (arrows) in β cells in lox/lox and βLKB mice. Mice were 10 months old, 9 months after tamoxifen injection. B, a section from an islet of an LKB1-deficient mouse showing intact mitochondria in two α-cells. Arrows point to mitochondria. C, quantification of the percentage of β cells with defective mitochondria (swollen mitochondria or lack of cristae) as determined by analysis of EM images. *, p < 0.05. D, Western blot of cytochrome c protein normalized to actin on islets from two wild type mice and two βLKB mice. Mice were 5 months old. E, mitochondrial DNA copy number measured by the ratio between mitochondrial DNA and nuclear DNA from lox/lox and βLKB whole islets (left) or sorted β cells (right) using qPCR. For whole islets, data represent the mean of DNA ratio (cytochrome b/Aprt) from 4–5 mice at 4 and 10 months of age. For sorted β cells, data represent the ratio of DNA (cytochrome b/L1) pooled from 4 mice per genotype at 1 year of age. F, PGC1α and PGC1β mRNA levels quantified by RT-PCR from wild type and βLKB islets normalized to actin. Data represent the mean ± S.E. of 4 mice at age of 4.5 months. ***, p < 0.005.

Mentions: To understand the basis for the functional defect in mitochondria of βLKB β cells, we examined mitochondrial structure at high resolution using transmission electron microscopy. Islets of control islets presented with a typical pattern of mitochondria, including the fine structure of cristae (Fig. 5A). Islets of βLKB mice, examined 3–10 months after deletion of LKB1, revealed a strikingly different pattern of mitochondria. Mitochondria in mutant β cells were swollen and absent of cristae. The effect was specific to β cells, as adjacent α cells in the same sections had the normal appearance of mitochondria (Fig. 5B). The latter observation also rules out artifacts related to fixation and processing of tissue. The mitochondrial defect was found in 60% of β cells in βLKB islets (n = 6 mice, 15–20 cells examined per mouse) compared with ∼12% of mitochondria that had such appearance in wild type islets (Fig. 5C). In affected β cells, all mitochondria were apparently disrupted. This suggests a cell autonomous, all or none effect, occurring in the LKB1-deleted β cells and sparing cells that have escaped cre-mediated deletion. We have not observed other ultra-structural alterations in βLKB islets beyond the previously reported distended endoplasmic reticulum (42). In particular, we tested whether LKB1-deficient islets had more insulin granules docked at the plasma membrane (up to 200 nm from the membrane) as a potential indication for enhancement in distal release but found no difference between wild type and mutant islets (data not shown).


Loss of Liver Kinase B1 (LKB1) in Beta Cells Enhances Glucose-stimulated Insulin Secretion Despite Profound Mitochondrial Defects.

Swisa A, Granot Z, Tamarina N, Sayers S, Bardeesy N, Philipson L, Hodson DJ, Wikstrom JD, Rutter GA, Leibowitz G, Glaser B, Dor Y - J. Biol. Chem. (2015)

LKB1 is essential for mitochondrial integrity.A, representative transmission electron micrographs of pancreatic sections showing mitochondria (arrows) in β cells in lox/lox and βLKB mice. Mice were 10 months old, 9 months after tamoxifen injection. B, a section from an islet of an LKB1-deficient mouse showing intact mitochondria in two α-cells. Arrows point to mitochondria. C, quantification of the percentage of β cells with defective mitochondria (swollen mitochondria or lack of cristae) as determined by analysis of EM images. *, p < 0.05. D, Western blot of cytochrome c protein normalized to actin on islets from two wild type mice and two βLKB mice. Mice were 5 months old. E, mitochondrial DNA copy number measured by the ratio between mitochondrial DNA and nuclear DNA from lox/lox and βLKB whole islets (left) or sorted β cells (right) using qPCR. For whole islets, data represent the mean of DNA ratio (cytochrome b/Aprt) from 4–5 mice at 4 and 10 months of age. For sorted β cells, data represent the ratio of DNA (cytochrome b/L1) pooled from 4 mice per genotype at 1 year of age. F, PGC1α and PGC1β mRNA levels quantified by RT-PCR from wild type and βLKB islets normalized to actin. Data represent the mean ± S.E. of 4 mice at age of 4.5 months. ***, p < 0.005.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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Figure 5: LKB1 is essential for mitochondrial integrity.A, representative transmission electron micrographs of pancreatic sections showing mitochondria (arrows) in β cells in lox/lox and βLKB mice. Mice were 10 months old, 9 months after tamoxifen injection. B, a section from an islet of an LKB1-deficient mouse showing intact mitochondria in two α-cells. Arrows point to mitochondria. C, quantification of the percentage of β cells with defective mitochondria (swollen mitochondria or lack of cristae) as determined by analysis of EM images. *, p < 0.05. D, Western blot of cytochrome c protein normalized to actin on islets from two wild type mice and two βLKB mice. Mice were 5 months old. E, mitochondrial DNA copy number measured by the ratio between mitochondrial DNA and nuclear DNA from lox/lox and βLKB whole islets (left) or sorted β cells (right) using qPCR. For whole islets, data represent the mean of DNA ratio (cytochrome b/Aprt) from 4–5 mice at 4 and 10 months of age. For sorted β cells, data represent the ratio of DNA (cytochrome b/L1) pooled from 4 mice per genotype at 1 year of age. F, PGC1α and PGC1β mRNA levels quantified by RT-PCR from wild type and βLKB islets normalized to actin. Data represent the mean ± S.E. of 4 mice at age of 4.5 months. ***, p < 0.005.
Mentions: To understand the basis for the functional defect in mitochondria of βLKB β cells, we examined mitochondrial structure at high resolution using transmission electron microscopy. Islets of control islets presented with a typical pattern of mitochondria, including the fine structure of cristae (Fig. 5A). Islets of βLKB mice, examined 3–10 months after deletion of LKB1, revealed a strikingly different pattern of mitochondria. Mitochondria in mutant β cells were swollen and absent of cristae. The effect was specific to β cells, as adjacent α cells in the same sections had the normal appearance of mitochondria (Fig. 5B). The latter observation also rules out artifacts related to fixation and processing of tissue. The mitochondrial defect was found in 60% of β cells in βLKB islets (n = 6 mice, 15–20 cells examined per mouse) compared with ∼12% of mitochondria that had such appearance in wild type islets (Fig. 5C). In affected β cells, all mitochondria were apparently disrupted. This suggests a cell autonomous, all or none effect, occurring in the LKB1-deleted β cells and sparing cells that have escaped cre-mediated deletion. We have not observed other ultra-structural alterations in βLKB islets beyond the previously reported distended endoplasmic reticulum (42). In particular, we tested whether LKB1-deficient islets had more insulin granules docked at the plasma membrane (up to 200 nm from the membrane) as a potential indication for enhancement in distal release but found no difference between wild type and mutant islets (data not shown).

Bottom Line: However, the full spectrum of LKB1 effects and the mechanisms involved in the secretory phenotype remain incompletely understood.Surprisingly, enhanced GSIS is seen despite profound defects in mitochondrial structure and function in LKB1-deficient β cells, expected to greatly diminish insulin secretion via the classic triggering pathway.This study shows that β cells can be manipulated to enhance GSIS to supra-normal levels even in the face of defective mitochondria and without deterioration over months.

View Article: PubMed Central - PubMed

Affiliation: From the Department of Developmental Biology and Cancer Research, The Institute for Medical Research Israel-Canada, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel.

Show MeSH
Related in: MedlinePlus