Mitochondrial clearance of calcium facilitated by MICU2 controls insulin secretion

OBJECTIVE

Transport of Ca2+ into pancreatic β cell mitochondria facilitates nutrient-mediated insulin secretion. However, the underlying mechanism is unclear. Recent establishment of the molecular identity of the mitochondrial Ca2+ uniporter (MCU) and associated proteins allows modification of mitochondrial Ca2+ transport in intact cells. We examined the consequences of deficiency of the accessory protein MICU2 in rat and human insulin-secreting cells and mouse islets.

METHODS

siRNA silencing of Micu2 in the INS-1 832/13 and EndoC-βH1 cell lines was performed; Micu2-/- mice were also studied. Insulin secretion and mechanistic analyses utilizing live confocal imaging to assess mitochondrial function and intracellular Ca2+ dynamics were performed.

RESULTS

Silencing of Micu2 abrogated GSIS in the INS-1 832/13 and EndoC-βH1 cells. The Micu2-/- mice also displayed attenuated GSIS. Mitochondrial Ca2+ uptake declined in MICU2-deficient INS-1 832/13 and EndoC-βH1 cells in response to high glucose and high K+. MICU2 silencing in INS-1 832/13 cells, presumably through its effects on mitochondrial Ca2+ uptake, perturbed mitochondrial function illustrated by absent mitochondrial membrane hyperpolarization and lowering of the ATP/ADP ratio in response to elevated glucose. Despite the loss of mitochondrial Ca2+ uptake, cytosolic Ca2+ was lower in siMICU2-treated INS-1 832/13 cells in response to high K+. It was hypothesized that Ca2+ accumulated in the submembrane compartment in MICU2-deficient cells, resulting in desensitization of voltage-dependent Ca2+ channels, lowering total cytosolic Ca2+. Upon high K+ stimulation, MICU2-silenced cells showed higher and prolonged increases in submembrane Ca2+ levels.

CONCLUSIONS

MICU2 plays a critical role in β cell mitochondrial Ca2+ uptake. β cell mitochondria sequestered Ca2+ from the submembrane compartment, preventing desensitization of voltage-dependent Ca2+ channels and facilitating GSIS.

The Vbeta13 T Cell Receptor Monoclonal Antibody Reduces Hyaluronan and CD68+, CD3+, and CD8+ Cell Infiltrations to Delay Diabetes in Congenic BB DRLyp/Lyp Rats

The depleting Vβ13a T cell receptor monoclonal antibody (mAb) 17D5 prevents both induced and spontaneous autoimmune diabetes in BB rats. Here it was tested in congenic DRLyp/Lyp rats, all of which spontaneously developed diabetes. Starting at 40 days of age, rats were injected once weekly with either saline, His42 Vβ16 mAb, or 17D5 mAb and monitored for hyperglycemia. Diabetes occurred in 100% (n = 5/5) of saline-treated rats (median age, 66 days; range 55-73), and in 100% (n = 6/6) of His42-treated rats (median age, 69 days; range 59-69). Diabetes occurred in fewer (n = 8/11, 73%) 17D5-treated rats at a later age (median 76 days, range 60-92). Three (27%) of the 17D5-treated rats were killed at 101-103 days of age without diabetes (17D5 no-diabetes rats). Survival analysis demonstrated that 17D5 mAb delayed diabetes onset. Saline- and His42-treated rats had severely distorted islets with substantial loss of insulin-positive cells. These rats exhibited prominent hyaluronan (HA) staining, with the intra-islet HA+ accumulations measuring 5,000 ± 2,400 µm2 and occupying 36 ± 12% of islet area, and severe (grade 4) insulitis with abundant infiltration by CD68+, CD3+, and CD8+ cells. The 17D5 mAb-treated rats with delayed diabetes onset exhibited less severe insulitis (predominantly grade 3). In contrast, the 17D5 no-diabetes rats had mostly normal islets, with insulin+ cells representing 76 ± 3% of islet cells. In these rats, the islet HA deposits were significantly smaller than in the diabetic rats; the intra-islet HA+ areas were 1,200 ± 300 µm2 and accounted for 8 ± 1% of islet area. Also, islet-associated CD68+ and CD3+ cells occurred less frequently (on average in 60 and 3% of the islets, respectively) than in the diabetes rats (present in >95% of the islets). No CD8+ cells were detected in islets in all 17D5 no-diabetes rats. We conclude that mAb 17D5 delayed diabetes in DRLyp/Lyp rats and markedly reduced expression of HA and concomitant infiltration of CD68+, CD3+, and CD8+ cells. Our findings underscore the importance of refining immune suppression in prevention or intervention clinical trials to use mAb reagents that are directed against specific T cell receptors.

Human enteroviral infection impairs autophagy in clonal INS(832/13) cells and human pancreatic islet cells

AIM/HYPOTHESIS

Human enteroviral infections are suggested to be associated with type 1 diabetes. However, the mechanism by which enteroviruses can trigger disease remains unknown. The present study aims to investigate the impact of enterovirus on autophagy, a cellular process that regulates beta cell homeostasis, using the clonal beta cell line INS(832/13) and human islet cells as in vitro models.

METHODS

INS(832/13) cells and human islet cells were infected with a strain of echovirus 16 (E16), originally isolated from the stool of a child who developed type 1 diabetes-associated autoantibodies. Virus production and release was determined by 50% cell culture infectious dose (CCID₅₀) assay and FACS analysis. The occurrence of autophagy, autophagosomes, lysosomes and autolysosomes was detected by western blot, baculoviral-mediated expression of microtubule-associated protein light chain 3 (LC3)II-GFP and LysoTracker Red, and quantified by Cellomics ArrayScan. Autophagy was also monitored with a Cyto-ID detection kit. Nutrient deprivation (low glucose [2.8 mmol/l]), amino acid starvation (Earle's Balanced Salt Solution [EBSS]) and autophagy-modifying agents (rapamycin and chloroquine) were used in control experiments. Insulin secretion and the expression of autophagy-related (Atg) genes and genes involved in autophagosome-lysosome fusion were determined.

RESULTS

E16-infected INS(832/13) cells displayed an accumulation of autophagosomes, compared with non-treated (NT) cells (grown in complete RPMI1640 containing 11.1 mmol/l glucose) (32.1 ± 1.7 vs 21.0 ± 1.2 μm²/cell; p = 0.05). This was accompanied by increased LC3II ratio both in E16-infected cells grown in low glucose (LG) (2.8 mmol/l) (0.42 ± 0.03 vs 0.11 ± 0.04 (arbitrary units [a.u.]); p < 0.0001) and grown in media containing 11.1 mmol/l glucose (0.37 ± 0.016 vs 0.05 ± 0.02 (a.u.); p < 0.0001). Additionally, p62 accumulated in cells after E16 infection when grown in LG (1.23 ± 0.31 vs 0.36 ± 0.12 (a.u.); p = 0.012) and grown in media containing 11.1 mmol/l glucose (1.79 ± 0.39 vs 0.66 ± 0.15 (a.u.); p = 0.0078). mRNA levels of genes involved in autophagosome formation and autophagosome-lysosome fusion remained unchanged in E16-infected cells, except Atg7, which was significantly increased when autophagy was induced by E16 infection, in combination with LG (1.48 ± 0.08-fold; p = 0.02) and at 11.1 mmol/l glucose (1.26 ± 0.2-fold; p = 0.001), compared with NT controls. Moreover, autophagosomes accumulated in E16-infected cells to the same extent as when cells were treated with the lysosomal inhibitor, chloroquine, clearly indicating that autophagosome turnover was blocked. Upon infection, there was an increased viral titre in the cell culture supernatant and a marked reduction in glucose-stimulated insulin secretion (112.9 ± 24.4 vs 209.8 ± 24.4 ng [mg protein]^-1 h^-1; p = 0.006), compared with uninfected controls, but cellular viability remained unaffected. Importantly, and in agreement with the observations for INS(832/13) cells, E16 infection impaired autophagic flux in primary human islet cells (46.5 ± 1.6 vs 34.4 ± 2.1 μm²/cell; p = 0.01).

CONCLUSIONS/INTERPRETATION

Enteroviruses disrupt beta cell autophagy by impairing the later stages of the autophagic pathway, without influencing expression of key genes involved in core autophagy machinery. This results in increased viral replication, non-lytic viral spread and accumulation of autophagic structures, all of which may contribute to beta cell demise and type 1 diabetes. Graphical abstract.

Development of renal cell carcinoma in living donor kidney grafts

BACKGROUND

Although development of malignancies after transplantation is well recognized, de novo development of cancer in renal transplants is a rare phenomenon. We describe two cases of de novo development of renal cell carcinoma in two living donor grafts.

MATERIALS AND RESULTS

The recipients were 45 and 4 years, respectively, at transplantation and their fathers were donors. Because of failure to grow, they were both treated with human growth hormone. Over the years a number of cysts developed in the grafts and after 8 and 7 years the echogenecity of some of the cysts changed. Biopsy confirmed the diagnosis renal cell carcinoma 9 and 11 years after transplantation. The grafts were removed and the immunosuppressive therapy discontinued. The two fathers are well with normal function of the native kidney and no signs of cyst formation or cancer.

CONCLUSION

Two cases of de novo development of cancer in living donor kidney transplants are described. Because a stimulatory effect of growth hormone on tumor genesis has been described, this treatment may have been of importance in the tumor development. The findings emphasize the importance of annual ultrasonographic surveillance of renal grafts, especially in the pediatric population.

[Intestinal transplantation. The first Swedish transplantation of the small intestine in an adult patient with pseudoobstruction]

Recent advances, first and foremost the development of new immunosuppressive agents, have markedly improved the outcome of intestinal transplantation, which is a treatment option for patients with serious intestinal diseases who have become dependent on total parenteral nutrition. The first small bowel transplantation in Sweden was performed at Huddinge Hospital in 1997, in the adult patient with intestinal pseudo-obstruction. The article reports the course of this patient and an update of international progress in intestinal transplantation.

Phototoxicity identification by solid phase extraction and photoinduced toxicity to Daphnia magna

The photoinduced toxicity of several environmental pollutants (some Polycyclic Aromatic Hydrocarbons [PAHs]) is a potential threat to aquatic organisms. To identify the cause/s of photoinduced toxicity of a sample, it is not sufficient to simply analyze the content of some known phototoxic compounds; so far too few substances of environmental concern have ever been tested for their photoinduced toxicity. The PAHs as well as other known phototoxic compounds are hydrophobic and are expected to bind to C18 columns. The use of Solid Phase Extraction (SPE) is typically part of the procedure identifying any primary nonpolar toxicant/s, and adding phototoxicity tests to these manipulations would not substantially increase the workload. In this study, therefore, the difference in acute toxicity to Daphnia magna before and after 2 h of UV irradiation was determined for six PAHs. The ratio between EC50 values before and after UV irradiation ranged from 4.6 (for benzo-a-pyrene) to >244 (for 3, 4-benzofluoranthene), demonstrating that the UV enhances the PAH-toxicity. A further characterization technique using binding to Sep-Pak SPE C18 columns and recovery with methanol as an eluting agent was then tested in combination with UV irradiation. The mean recovered UV induced toxicity after binding and elution of the six PAHs was 119% according to the phototoxicity tests made. A linear relationship, between the log10 Kow values for the PAHs and the log10 for the concentration of methanol at peak elution was found. The combined use of C18 column separation and UV activation may, therefore, be used in toxicity identification evaluations (TIE) of organic phototoxic compounds.