James L. Edwards

Diabetes regulates mitochondrial biogenesis and fission in mouse neurons

Aims/hypothesisNormal mitochondrial activity is a critical component of neuronal metabolism and function. Disruption of mitochondrial activity by altered mitochondrial fission and fusion is the root cause of both neurodegenerative disorders and Charcot–Marie–Tooth type 2A inherited neuropathy. This study addressed the role of mitochondrial fission in the pathogenesis of diabetic neuropathy.MethodsMitochondrial biogenesis and fission were assayed in both in vivo and in vitro models of diabetic neuropathy. Gene, protein, mitochondrial DNA and ultrastructural analyses were used to assess mitochondrial biogenesis and fission.ResultsThere was greater mitochondrial biogenesis in dorsal root ganglion neurons from diabetic compared with non-diabetic mice. An essential step in mitochondrial biogenesis is mitochondrial fission, regulated by the mitochondrial fission protein dynamin-related protein 1 (DRP1). Evaluation of diabetic neurons in vivo indicated small, fragmented mitochondria, suggesting increased fission. In vitro studies revealed that short-term hyperglycaemic exposure increased levels of DRP1 protein. The influence of hyperglycaemia-mediated mitochondrial fission on cell viability was evaluated by knockdown of Drp1 (also known as Dnm1l). Knockdown of Drp1 resulted in decreased susceptibility to hyperglycaemic damage.Conclusions/interpretationWe propose that: (1) mitochondria undergo biogenesis in response to hyperglycaemia, but the increased biogenesis is insufficient to accommodate the metabolic load; (2) hyperglycaemia causes an excess of mitochondrial fission, creating small, damaged mitochondria; and (3) reduction of aberrant mitochondrial fission increases neuronal survival and indicates an important role for the fission–fusion equilibrium in the pathogenesis of diabetic neuropathy.

Mechanisms of disease: Mitochondria as new therapeutic targets in diabetic neuropathy

Diabetic neuropathy (DN) is the most common complication of diabetes mellitus, and it imposes a considerable burden on a patients quality of life and the health-care system. Despite the prevalence and severity of DN, there are no effective treatments. Pathogenetic evidence suggests that DN is marked by degeneration of dorsal root ganglion (DRG) neurons in peripheral nerves, and that DRG mitochondria are particularly affected. DRG mitochondria are especially vulnerable because they are the origin of reactive oxygen species production in the hyperglycemic neuron. Accumulating evidence indicates that neuronal mitochondria are subject to damage at the level of their DNA, and their outer and inner membranes, and also via deregulation of mitochondrial fission and fusion proteins that control mitochondrial shape and number. This Review will survey the mechanisms of mitochondrial degeneration in the pathogenesis of DN, highlighting potential mitochondrial sites for therapeutic intervention.

The antioxidant response as a drug target in diabetic neuropathy.

While increasing antioxidant potential is an attractive treatment strategy for diabetic neuropathy, many years of trials using high-dose oral antioxidants have not produced therapeutic results. An increasing understanding of the innate antioxidant response and the pharmacological agents that can regulate this mechanism may open new avenue for drug development. This review describes the current state of antioxidant trials and the potential for targeting the antioxidant response. In combination with antihyperglycemic agents, agents that regulate the antioxidant response may afford superior protection against cellular oxidative injury in diabetes.

Comparative Evaluation of Two Isobaric Labeling Tags, DiART and iTRAQ

Isobaric tags have broad applications in both basic and translational research, as demonstrated by the widely used isobaric tag for relative and absolute quantitation (iTRAQ). Recent results from large-scale quantitative proteomics projects, however, indicate that protein quantification by iTRAQ is often biased in complex biological samples. Here, we report the application of another isobaric tag, deuterium isobaric amine reactive tag (DiART), for quantifying the proteome of Thermoanaerobacter tengcongensis (T. tengcongensis), a thermophilic bacterium first discovered in China. We compared the performance of DiART with iTRAQ from several different aspects, including their fragmentation mechanisms, the number of identified proteins, and the accuracy of quantification. Our results revealed that, as compared with iTRAQ, DiART yielded significantly stronger reporter ions, which did not reduce the number of identifiable peptides, but improved the signal-to-noise ratio (S/N) for quantification. Remarkably, we found that, under identical chromatography and mass spectrometry (MS) conditions, DiART exhibited less reporter ions ratio compression than iTRAQ, probably due to more reporter ions with higher intensities produced by DiART labeling. Taken together, we demonstrate that DiART is a valuable alternative of iTRAQ with enhanced performance for quantitative proteomics.

Subsecond absolute quantitation of amine metabolites using isobaric tags for discovery of pathway activation in mammalian cells.

The absolute quantitation of amine metabolites from mammalian cell samples was achieved by combining amine standards, isobaric tags, and capillary liquid chromatography (LC) tandem mass spectrometry (MS/MS). Our approach allowed 32 specific amines to be analyzed within a single chromatographic run, with the generation of the calibration curve and absolute quantitation of each analyte taking less than 900 ms. Using this strategy, we determined the amine response of human aortic endothelial cells (HAECs) from a glucose challenge. The observed changes of the absolute concentration of these metabolites implied eight enzymatic reactions may change efficiency upon glucose treatment. Five of these reactions have been previously reported as being up-regulated in diabetic conditions. The remaining three reactions were analyzed by measuring the expression of these enzymes, with 66% showing increases. Our data indicate that rapid determination of absolute quantitation is useful in determining novel pathway activation. Furthermore, even though we determined the absolute quantity of 32 metabolites here, the number of analytes that can be measured by this method is limited mainly by commercial availability of amine standards.

Extraction and Quantitation of Ketones and Aldehydes from Mammalian Cells Using Fluorous Tagging and Capillary LC-MS.

The extraction and quantitation of carbonyl metabolites from cell lysate was accomplished using a carbonyl-reactive fluorous tag and capillary liquid chromatography coupled to mass spectrometry (capLC-MS). Selective fluorous tagging for ketones and aldehydes provided a 30-fold increase in sensitivity using electrospray ionization MS. Separation of fluorous tagged carbonyl resulted in good separation of all components, and tandem MS was able to differentiate structural carbonyl isomers. The average limit of detection for carbonyl standards was 37 nM (range 1.5-250 nM), with linearity of R(2) > 0.99. Reproducibility for metabolites in cell lysate averaged 9% RSD. Human aortic endothelial cells (HAECs) were exposed to varying levels of glucose, and their carbonyl metabolite levels were quantified. Significant metabolite changes were seen in glycolysis and the propanoate pathway from a glucose challenge. Using an untargeted approach, 120 carbonyl metabolites were found to change in hyperglycemic HAECs. From this list of compounds, multiple metabolites from the pentose phosphate and tryptophan metabolic pathways were discovered. This system provides excellent sensitivity and quantitation of carbonyl metabolites without the need for isotope standards or labels.