Micro-respirometry of whole cells and isolated mitochondria

The Biochemical Assessment of Mitochondrial Respiratory Chain Disorders

Mitochondrial respiratory chain (MRC) disorders are a complex group of diseases whose diagnosis requires a multidisciplinary approach in which the biochemical investigations play an important role. Initial investigations include metabolite analysis in both blood and urine and the measurement of lactate, pyruvate and amino acid levels, as well as urine organic acids. Recently, hormone-like cytokines, such as fibroblast growth factor-21 (FGF-21), have also been used as a means of assessing evidence of MRC dysfunction, although work is still required to confirm their diagnostic utility and reliability. The assessment of evidence of oxidative stress may also be an important parameter to consider in the diagnosis of MRC function in view of its association with mitochondrial dysfunction. At present, due to the lack of reliable biomarkers available for assessing evidence of MRC dysfunction, the spectrophotometric determination of MRC enzyme activities in skeletal muscle or tissue from the disease-presenting organ is considered the ‘Gold Standard’ biochemical method to provide evidence of MRC dysfunction. The purpose of this review is to outline a number of biochemical methods that may provide diagnostic evidence of MRC dysfunction in patients.

Glucose uptake as an alternative to oxygen uptake for assessing metabolic rate in Danio rerio larvae

Respirometry, based on oxygen uptake, is commonly employed for measuring metabolic rate. There is a growing need for metabolic rate measurements suitable for developmental studies, particularly in Danio rerio, where many important developmental stages occur at < 4 mm. However, respirometry becomes more challenging as the size of the organism reduces. Additionally, respirometry can be costly and require significant experience and technical knowledge which may prohibit uptake in non-specialist/non-physiology labs. Thus, using equipment routine in most developmental/molecular biology laboratories, we measured glucose uptake in 96-h post fertilisation (hpf) zebrafish larvae and compared it to stop-flow respirometry measures of oxygen uptake to test whether glucose uptake was a suitable alternative measure of metabolic rate. A Passing-Bablok regression revealed that within a 95% limit of agreement, the rate of glucose uptake and the rate of oxygen uptake were equivalent as measures of metabolic rate in 96 hpf Danio rerio larvae. Thus, the methodology we outline here may be a useful alternative or a complementary method for assessing metabolic rate in small organisms.

Uncontrolled Oxygen Levels in Cultures of Retinal Pigment Epithelium: Have We Missed the Obvious?

Retinal pigment epithelium (RPE) is the outermost layer of retina located between the photoreceptor cells and the choroid. This highly-polarized monolayer provides critical support for the functioning of the other parts of the retina, especially photoreceptors. Methods of culturing RPE have been under development since its establishment in 1920s. Despite considering various factors, oxygen (O₂) levels in RPE microenvironments during culture preparation and experimental procedure have been overlooked. O₂ is a crucial parameter in the cultures, and therefore, maintaining RPE cells at O₂ levels different from their native environment (70-90 mm Hg of O₂) could have unintended consequences. Owing to the importance of the topic, lack of sufficient discussion in the literature and to encourage future research, this paper will focus on uncontrolled O₂ level in cultures of RPE cells.

Measuring Mitochondrial Function: From Organelle to Organism

Mitochondrial energy production is crucial for normal daily activities and maintenance of life. Herein, the logic and execution of two main classes of measurements are outlined to delineate mitochondrial function: ATP production and oxygen consumption. Aerobic ATP production is quantified by phosphorus magnetic resonance spectroscopy (³¹PMRS) in vivo in both human subjects and animal models using the same protocols and maintaining the same primary assumptions. Mitochondrial oxygen consumption is quantified by oxygen polarography and applied in isolated mitochondria, cultured cells, and permeabilized fibers derived from human or animal tissue biopsies. Traditionally, mitochondrial functional measures focus on maximal oxidative capacity-a flux rate that is rarely, if ever, observed outside of experimental conditions. Perhaps more physiologically relevant, both measurement classes herein focus on one principal design paradigm; submaximal mitochondrial fluxes generated by graded levels of ADP to map the function for ADP sensitivity. We propose this function defines the bioenergetic role that mitochondria fill within the myoplasm to sense and match ATP demands. Any deficit in this vital role for ATP homeostasis leads to symptoms often seen in cardiovascular and cardiopulmonary diseases, diabetes, and metabolic syndrome.

Fully 3D printed fluidic devices with integrated valves and pumps for flow injection analysis

The use of a PolyJet 3D printer to create a microfluidic device that has integrated valves and pumps is described. The process uses liquid support and stacked printing to result in fully printed devices that are ready to use within minutes of fabrication after minimal post-processing. A unique feature of PolyJet printing is the ability to incorporate several different materials of varying properties into one print. In this work, two commercially available materials were used: a rigid-transparent plastic material (VeroClear) was used to define the channel regions and the bulk of the device, while the pumps/valves were printed in a flexible, rubber-like material (Agilus30). The entire process, from initial design to testing takes less than 4 hours to complete. The performance of the valves and pumps were characterized by fluorescence microscopy. A flow injection analysis device that enabled the discrete injections of analyte plugs was created, with on-chip pumps being used to move the fluid streams. The injection process was found to be reproducible and linearly correlated with changes in analyte concentration. The utility was demonstrated with the injection and rapid lysis of fluorescently-labeled endothelial cells. The ability to produce a device with integrated pumps/valves in one process significantly adds to the applicability of 3D printing to create microfluidic devices for analytical measurements.

Fasting and fasting-mimicking treatment activate SIRT1/LXRα and alleviate diabetes-induced systemic and microvascular dysfunction

AIMS/HYPOTHESIS

Homo sapiens evolved under conditions of intermittent food availability and prolonged fasting between meals. Periods of fasting are important for recovery from meal-induced oxidative and metabolic stress, and tissue repair. Constant high energy-density food availability in present-day society contributes to the pathogenesis of chronic diseases, including diabetes and its complications, with intermittent fasting (IF) and energy restriction shown to improve metabolic health. We have previously demonstrated that IF prevents the development of diabetic retinopathy in a mouse model of type 2 diabetes (db/db); however the mechanisms of fasting-induced health benefits and fasting-induced risks for individuals with diabetes remain largely unknown. Sirtuin 1 (SIRT1), a nutrient-sensing deacetylase, is downregulated in diabetes. In this study, the effect of SIRT1 stimulation by IF, fasting-mimicking cell culture conditions (FMC) or pharmacological treatment using SRT1720 was evaluated on systemic and retinal metabolism, systemic and retinal inflammation and vascular and bone marrow damage.

METHODS

The effects of IF were modelled in vivo using db/db mice and in vitro using bovine retinal endothelial cells or rat retinal neuroglial/precursor R28 cell line serum starved for 24 h. mRNA expression was analysed by quantitative PCR (qPCR). SIRT1 activity was measured via histone deacetylase activity assay. NR1H3 (also known as liver X receptor alpha [LXRα]) acetylation was measured via western blot analysis.

RESULTS

IF increased Sirt1 mRNA expression in mouse liver and retina when compared with non-fasted animals. IF also increased SIRT1 activity eightfold in mouse retina while FMC increased SIRT1 activity and expression in retinal endothelial cells when compared with control. Sirt1 expression was also increased twofold in neuronal retina progenitor cells (R28) after FMC treatment. Moreover, FMC led to SIRT1-mediated LXRα deacetylation and subsequent 2.4-fold increase in activity, as measured by increased mRNA expression of the genes encoding ATP-binding cassette transporter (Abca1 and Abcg1). These changes were reduced when retinal endothelial cells expressing a constitutively acetylated LXRα mutant were tested. Increased SIRT1/LXR/ABC-mediated cholesterol export resulted in decreased retinal endothelial cell cholesterol levels. Direct activation of SIRT1 by SRT1720 in db/db mice led to a twofold reduction of diabetes-induced inflammation in the retina and improved diabetes-induced visual function impairment, as measured by electroretinogram and optokinetic response. In the bone marrow, there was prevention of diabetes-induced myeloidosis and decreased inflammatory cytokine expression.

CONCLUSIONS/INTERPRETATION

Taken together, activation of SIRT1 signalling by IF or through pharmacological activation represents an effective therapeutic strategy that provides a mechanistic link between the advantageous effects associated with fasting regimens and prevention of microvascular and bone marrow dysfunction in diabetes.

Assessment of the Maximal Activity of Complex IV in the Inner Mitochondrial Membrane by Tandem Electrochemistry and Respirometry

Assessment of activities of mitochondrial electron transport enzymes is important for understanding mechanisms of metabolic diseases, but structural organization of mitochondria and low sample availability pose distinctive challenges for in situ functional studies. We report the development of a tandem microfluidic respirometer that simultaneously tracks both the reduction of mediators on the electrode and the ensuing reduction of O2 by complex IV in the inner mitochondrial membrane. The response time of O2 consumption to multiple alternating potential steps is of approximately 10 s for a 150 μm-thick sample. Steady O2 depletion shows good quantitative correlation with the supplied electric charge, Pearson's r = 0.994. Reduction of mediators on biocompatible gold electrodes modified with carbon ink or fumed silica can compete with the oxidation of mediators by mitochondria, yielding an overall respiratory activity comparable to that upon chemical reduction by ascorbate. The dependence of O2 consumption on mediator and mitochondrial suspension concentrations shows that mass transport between the electrode and mitochondria does not limit biological activity of the latter. The mediated electrochemical approach is validated by the radiometric measurements of simulated changes in the intrinsic mitochondrial activity upon partial inhibition of complex IV by NaN3. This approach enables the development of O2-independent, biomimetic electrochemical assays narrowly targeting components of the electron transport chains in their native environments.

Mitochondrial Ceramide Effects on the Retinal Pigment Epithelium in Diabetes

Mitochondrial damage in the cells comprising inner (retinal endothelial cells) and outer (retinal pigment epithelium (RPE)) blood-retinal barriers (BRB) is known to precede the initial BRB breakdown and further histopathological abnormalities in diabetic retinopathy (DR). We previously demonstrated that activation of acid sphingomyelinase (ASM) is an important early event in the pathogenesis of DR, and recent studies have demonstrated that there is an intricate connection between ceramide and mitochondrial function. This study aimed to determine the role of ASM-dependent mitochondrial ceramide accumulation in diabetes-induced RPE cell damage. Mitochondria isolated from streptozotocin (STZ)-induced diabetic rat retinas (7 weeks duration) showed a 1.64 ± 0.29-fold increase in the ceramide-to-sphingomyelin ratio compared to controls. Conversely, the ceramide-to-sphingomyelin ratio was decreased in the mitochondria isolated from ASM-knockout mouse retinas compared to wild-type littermates, confirming the role of ASM in mitochondrial ceramide production. Cellular ceramide was elevated 2.67 ± 1.07-fold in RPE cells derived from diabetic donors compared to control donors, and these changes correlated with increased gene expression of IL-1β, IL-6, and ASM. Treatment of RPE cells derived from control donors with high glucose resulted in elevated ASM, vascular endothelial growth factor (VEGF), and intercellular adhesion molecule 1 (ICAM-1) mRNA. RPE from diabetic donors showed fragmented mitochondria and a 2.68 ± 0.66-fold decreased respiratory control ratio (RCR). Treatment of immortalized cell in vision research (ARPE-19) cells with high glucose resulted in a 25% ± 1.6% decrease in citrate synthase activity at 72 h. Inhibition of ASM with desipramine (15 μM, 1 h daily) abolished the decreases in metabolic functional parameters. Our results are consistent with diabetes-induced increase in mitochondrial ceramide through an ASM-dependent pathway leading to impaired mitochondrial function in the RPE cells of the retina.