Glucose dependent alterations of mitochondrial ultrastructure and enzyme content in mouse pancreatic islets maintained in tissue culture: a morphometrical and biochemical study

Diabetes reduces β-cell mitochondria and induces distinct morphological abnormalities, which are reproducible by high glucose in vitro with attendant dysfunction

We investigated the impact of a diabetic state with hyperglycemia on morphometry of β cell mitochondria and modifying influence of a K (+) -ATP channel opener and we related in vivo findings with glucose effects in vitro. For in vivo experiments islets from syngeneic rats were transplanted under the kidney capsule to neonatally streptozotocin-diabetic or non-diabetic recipients. Diabetic recipients received vehicle, or tifenazoxide (NN414), intragastrically for 9 weeks. Non-diabetic rats received vehicle. Transplants were excised 7 d after cessation of treatment (wash-out) and prepared for electron microscopy. Morphological parameters were measured from approx. 25,000 mitochondria. Rat islets were cultured in vitro for 2-3 weeks at 27 or 11 (control) mmol/l glucose. Transplants to diabetic rats displayed decreased numbers of mitochondria (-31%, p < 0.05), increased mitochondrial volume and increased mitochondrial outer surface area, p < 0.001. Diabetes increased variability in mitochondrial size with frequent appearance of mega-mitochondria. Tifenazoxide partly normalized diabetes-induced effects, and mega-mitochondria disappeared. Long-term culture of islets at 27 mmol/l glucose reproduced the in vivo morphological abnormalities. High-glucose culture was also associated with reduced ATP and ADP contents, reduced oxygen consumption, reduced signaling by MitoTracker Red and reduction of mitochondrial proteins (complexes I-IV), OPA 1 and glucose-induced insulin release. We conclude that (1) a long-term diabetic state leads to a reduced number of mitochondria and to distinct morphological abnormalities which are replicated by high glucose in vitro; (2) the morphological abnormalities are coupled to dysfunction; (3) K (+) -ATP channel openers may have potential to partly reverse glucose-induced effects.

In vitro and in vivo improvement of islet survival following treatment with nerve growth factor

BACKGROUND

Nerve growth factor (NGF) has been reported to play an important regulatory role in pancreatic beta-cell function. However, the usefulness of NGF in a transplantation setting is unknown.

METHODS

A marginal number of islet cells (260 islet equivalents/recipient) cultured for 24 hr with NGF (500 ng/ml) was syngeneically transplanted under the kidney capsule of streptozotocin-induced diabetic Balb/c mice. Fluorescence microscopy was used to evaluate islet viability. Islet function was evaluated in vitro and in vivo by static assay and glucose tolerance test, respectively.

RESULTS

In vitro, improved viability and survival were found in murine islets cultured in serum-free medium for 96 hr with 500 ng/ml NGF (P<0.05). NGF-treated islets had more insulin secretion than islets cultured without NGF in response to 2.8 mmol/L glucose (P<0.05), and 20 mmol/L glucose conditions. In vivo, 67% of recipients with a submarginal number of islets cultured in NGF attained normoglycemia for more than 120 days, whereas transplanted islets without NGF treatment survived a maximum of 13 days in control mice. At posttransplant day 4, recipients of NGF-cultured islets showed significant improvement of glucose tolerance. On immunohistochemistry, the kidney capsules containing NGF-cultured islets displayed higher insulin content, and more dilated neoplastic microvessels than control renal capsules. The number of apoptotic cells using TUNEL staining decreased by nearly 50% in NGF-cultured islet grafts in comparison to control islet grafts.

CONCLUSIONS

The above data suggest potential advantages of NGF for islet survival following transplantation. This neurotrophic approach may prove beneficial in human islet transplantation.

Prolonged culture in low glucose induces apoptosis of rat pancreatic beta-cells through induction of c-myc

Prolonged culture in low-glucose concentrations (</=5mM) induces apoptosis in pancreatic beta-cells by a poorly defined mechanism. We now show that, in both purified rat beta-cells and isolated rat islets, culture in the presence of 3 or 5mM (G3-G5) instead of 10mM glucose (G10) induces a large increase in c-myc expression before onset of a caspase-dependent apoptosis. These effects were prevented by addition of leucine and glutamine to G3 and G5, and were mimicked by addition of the mitochondrial poison azide to G10. In contrast, inhibition of Ca(2+) influx and insulin secretion with diazoxide under control conditions did not stimulate islet c-myc expression nor beta-cell apoptosis. In rat beta-cells, adenovirus-mediated c-myc overexpression increased their rate of apoptosis, whereas antisense-c-myc expression reduced low-glucose-induced apoptosis by approximately 50%. In the insulin producing MIN6 cell line, apoptosis induction by either low glucose or an activator of AMP-activated protein kinase (AMPK) was associated with c-myc mRNA and protein upregulation. In conclusion, stimulation of beta-cell apoptosis by prolonged culture at low glucose partly results from early and sustained induction of beta-cell c-myc expression. These effects may be due to sustained restriction in nutrient-derived metabolic signals.

Desensitization of insulin secretion

Desensitization of insulin secretion describes a reversible state of decreased secretory responsiveness of the pancreatic beta-cell, induced by a prolonged exposure to a multitude of stimuli. These include the main physiological stimulator, glucose, but also other nutrients like free fatty acids and practically all pharmacological stimulators acting by depolarization and Ca2+ influx into the beta-cell. Desensitization of insulin secretion appears to be an important step in the manifestation of type 2 diabetes and in the secondary failure of oral antidiabetic treatment. In this commentary, the basic concepts and the controversial issues in the field will be outlined. With regard to glucose-induced desensitization, two fundamentally opposing concepts have emerged. The first is that desensitization is the consequence of functional changes in the beta-cell that impair glucose-recognition. The second is that long-term increased secretory activity leads to a depletion of releasable insulin, often in spite of increased insulin synthesis. The latter concept is more appropriately termed beta-cell exhaustion. The same dichotomy applies to the desensitization evoked by pharmacological stimuli: again the relative contributions of a decreased insulin content versus alterations in signal transduction are in dispute. The action of tolbutamide on beta-cells may be an example of desensitization caused by a lack of releasable insulin since the signaling mechanisms are nearly unchanged, whereas the action of phentolamine, an imidazoline, induces a strong desensitization without reducing insulin content or secretory granules, apparently by abolishing Ca2+ influx. With pharmacological agents it seems that both, alterations in signal transduction and decreased availability of releasable insulin, can contribute to the desensitized state of the beta-cell, the relative contribution being variable depending upon the exact nature of the secretory stimulus.

An ultrastructural change in developing rat cerebral cortex: a morphometrical study

Five age groups, each composed of four animals from each of the following ages, were used to assess age-related ultrastructural changes with development in the neuropil of the III layer of the frontal cortex (area 6) in rats; 1, 2, 3, 5 and 12 weeks old. Random samplings within the neuropil were taken to produce 25 electron micrographs in each rat (totaling 500). The profiles of axon terminals, dendrites and mitochondria in each element in the neuropil of each micrograph were traced. The percentage of their areas for the area of neuropil (relative volume fraction) was examined using the image analyzer system. The size and number of synaptic terminals were counted. The relative volume fractions of both the axon terminals and mitochondria in the terminals for the neuropil were found to have increased in the mature rats. On the other hand, the relative volume fraction of dendrites for the neuropil had been unchanged and the size of the terminals had gradually decreased. The number of terminals had progressively increased with development.

Stereological analysis of normal rabbit pancreatic islets

Stereological methods were applied to obtain morphometric data related to pancreatic islets of Langerhans in the normal rabbit. By light microscopy, it was found that 1 mm3 of pancreatic parenchyma contained 47.7 islets, constituting 2.2% of its volume. Approximately 69% of the islets had diameters less than 80 micrometer; 31% were greater than 80 micrometer. The former group of islets, however, composed only 13% of the volume of the endocrine pancreas and the latter group, 87%. Using electron microscopy, a unit volume of islet tissue was observed to consist of 86% beta cells, 7.7% alpha cells, and 2.2% delta cells. The average beta-cell volume was 1260 micrometer 3 and its cytoplasm consisted of 52.6% matrix, 12.7% rough endoplasmic reticulum, 10.2% secretory granules, 7.8% mitochondria, and 3.3% Golgi apparatus. A typical beta cell contained 662 mitochondria, intermediate (10-nm) filaments whose length totalled 50 mm, and 9,200 secretory granules with a ratio of four mature granules to each immature or "pale" granule. Within alpha and beta cells, three parameters were used for the quantitation of organelles or their component parts: 1) volume; 2) surface; and 3) numerical densities. In the beta cell, the surface densities of rough endoplasmic reticulum and Golgi membranes exceeded, by two- or threefold, their counterparts in alpha cells. Similarly, the number of beta-cell mitochondria exceeded by 30% that of alpha cells; but beta-cell mitochondrial volume was twice that of the alpha cell, as were surface densities of both inner and outer mitochondrial membranes. Volume and surface densities of secretory granules within beta cells were half the values obtained for alpha cells. An alpha cell contained three times the number of granules present in a beta cell.

Long-term effects of glibenclamide on the insulin production, oxidative metabolism and quantitative ultrastructure of mouse pancreatic islets maintained in tissue culture at different glucose concentrations

In order to evaluate long-term effects of sulphonylureas on pancreatic islet structure and function, isolated mouse islets were maintained in tissue culture for one week at various glucose concentrations, and in the absence or presence of glibenclamide. When the islets were cultured at 3.3 or 5.5 mmol/1, but not at 16.7 mmol/1 glucose, it was found that the drug stimulated insulin secretion into the culture medium during the initial 3 days of culture. During the remainder of the culture period no such enhancement of secretion was demonstrated. Insulin release due to glibenclamide apparently resulted in rapid depletion of intracellular insulin stores. The finding of an enlarged B-cell Golgi apparatus in the drug-treated islets was probably associated with granule discharge. The failure of glibenclamide to promote insulin secretion during the whole culture period could reflect the adverse effects of the drug on islet insulin biosynthesis as indicated by short-term experiments performed after culture. Similar experiments showed that the impaired insulin biosynthesis could not be restored by withdrawal of the drug from the culture medium for 3 days. Furthermore, the capacity for insulin release in response to an acute glucose challenge at the end of the culture period, was abolished by culture in the presence of glibenclamide. The drug effects on insulin biosynthesis and intracellular insulin stores, which were most pronounced at 5.5 mmol/1 glucose, possibly resulted from changes in B-cell metabolism as suggested by the diminished islet glucose-oxidation rate. The spatial characteristics of islet mitochondria indicated that these changes might involve an adaptation to substrates other than glucose. In conclusion, our findings suggest that sulphonylureas have an insulinotropic effect, which is however transient. Indeed, it rather seems as if long-term exposure of islet B-cells to sulphonylureas in vitro were accompanied by functional deficiency.

Effect of 1.25-dihydroxycholecalciferol administration on blood glucose and pancreatic islet morphology in mice

1.25-dihydroxycholecalciferol (DHCC) administration to fed and starved mice had no effect on the blood-glucose concentration or the light-microscopic appearance of the endocrine or exocrine pancreas. Electron microscopy, however, disclosed changes which appeared early after the injection and were more marked in starved than in fed animals. The B-cells exhibited mitochondrial hypertrophy, studied both by qualitative and quantitative methods, invagination and vacuolation of mitochondrial membranes, increased occurrence of light secretory granules, multiple rough endoplasmic cisternae, multi-lamellar bodies, and a rather rich Ca2+-containing pyroantimonate precipitation mainly localized to nuclei and mitochondria. A tendency to mitochondrial hypertrophy was observed also in some D-cells. The A-cells were unaffected. The findings indicate that the endocrine pancreas (or at least the B-cells) is affected in some way, directly or indirectly, by DHCC.

Pancreatic islets subjected to different concentrations of glucose in vitro. A study with special regard to mitochondrial changes

Isolated pancreatic islets of mice and gerbils were cultured for 6 days at low (2mM) or high (20mM) concentrations of glucose after which they were studied using qualitative and quantitative electron microscopy, histo- and microchemistry, and X-ray microanalysis. Compared with the islets cultured at high glucose, those subjected to low glucose exhibited enhanced succinate dehydrogenase activity, a decreased content of adenosine triphosphate, and an increased volume of B-cell mitochondria which often were rounded or oval.