Use of the chemiluminescent probe lucigenin to monitor the production of the superoxide anion radical in a recombinant Aspergillus niger (B1-D)

Inactivating alternative NADH dehydrogenases: enhancing fungal bioprocesses by improving growth and biomass yield?

Debate still surrounds the physiological roles of the alternative respiratory enzymes found in many fungi and plants. It has been proposed that alternative NADH:ubiquinone oxidoreductases (NADH dehydrogenases) may protect against oxidative stress, conversely, elevated activity of these enzymes has been linked to senescence. Here we show that inhibition of these enzymes in a fungal protein expression system (Aspergillus niger) leads to significantly enhanced specific growth rate, substrate uptake, carbon dioxide evolution, higher protein content, and more efficient use of substrates. These findings are consistent with a protective role of the NADH dehydrogenases against oxidative stress, thus, when electron flow via these enzymes is blocked, flux through the main respiratory pathway rises, leading to enhanced ATP generation. We anticipate that our findings will stimulate further studies in fungal and plant cultures leading to significant improvements in these expression systems, and to deeper insights into the cellular roles of alternative respiration.

Cross-talk between the cellular redox state and the circadian system in Neurospora

The circadian system is composed of a number of feedback loops, and multiple feedback loops in the form of oscillators help to maintain stable rhythms. The filamentous fungus Neurospora crassa exhibits a circadian rhythm during asexual spore formation (conidiation banding) and has a major feedback loop that includes the FREQUENCY (FRQ)/WHITE COLLAR (WC) -1 and -2 oscillator (FWO). A mutation in superoxide dismutase (sod)-1, an antioxidant gene, causes a robust and stable circadian rhythm compared with that of wild-type (Wt). However, the mechanisms underlying the functions of reactive oxygen species (ROS) remain unknown. Here, we show that cellular ROS concentrations change in a circadian manner (ROS oscillation), and the amplitudes of ROS oscillation increase with each cycle and then become steady (ROS homeostasis). The ROS oscillation and homeostasis are produced by the ROS-destroying catalases (CATs) and ROS-generating NADPH oxidase (NOX). cat-1 is also induced by illumination, and it reduces ROS levels. Although ROS oscillation persists in the absence of frq, wc-1 or wc-2, its homeostasis is altered. Furthermore, genetic and biochemical evidence reveals that ROS concentration regulates the transcriptional function of WCC and a higher ROS concentration enhances conidiation banding. These findings suggest that the circadian system engages in cross-talk with the cellular redox state via ROS-regulatory factors.

Modeling the effect of oxygen on the amperometric response of immobilized organoselenium-based S-nitrosothiol sensors

Amperometric detection of S-nitrosothiols (RSNOs) at submicromolar levels in blood samples is of potential importance for monitoring endothelial function and other disease states that involve changes in physiological nitric oxide (NO) production. It is shown here that the elimination of dissolved oxygen from samples is critical when covalently attached diselenocystamine-based amperometric RSNO sensors are used for practical RSNO measurements. The newest generation of RSNO sensors utilizes an amperometric NO gas sensor with a thin organoselenium modified dialysis membrane mounted at the distal sensing tip. Sample RSNOs are catalytically reduced to NO within the dialysis membrane by the immobilized organoselenium species. In the presence of oxygen, the sensitivity of these sensors for measuring low levels of RSNOs (<μM) is greatly reduced. It is demonstrated that the main scavenger of the generated nitric oxide is not the dissolved oxygen but rather superoxide anion radical generated from the reaction of the reduced organoselenium species (the reactive species in the catalytic redox cycle) and dissolved oxygen. Computer simulations of the response of the RSNO sensor using rate constants and diffusion coefficients for the reactions involved, known from the literature or estimated from fitting to the observed amperometric response curves, as well as the specific geometric dimensions of the RSNO sensor, further support that nitric oxide and superoxide anion radical quickly react resulting in near zero sensor sensitivity toward RSNO concentrations in the submicromolar concentration range. Elimination of oxygen from samples helps improve sensor detection limits to ca. 10 nM levels of RSNOs.

The roles of the alternative NADH dehydrogenases during oxidative stress in cultures of the filamentous fungus Aspergillus niger

Despite the importance of filamentous fungi in the biotechnology industry, little is known about their metabolism under the stressful conditions experienced in typical production fermenters. In the present study, oxygen enrichment was used to recreate an industrial batch process, and the effects of the increasing dissolved oxygen tension were studied as regards the cellular metabolism. It was found that elevated dissolved oxygen tension led to an oxidatively stressful environment, as detailed by rapid initial increases in reactive oxygen species (ROS) concentrations and antioxidant enzyme activities. Intracellular protein concentrations also decreased in oxygenated cultures; this appeared to be concomitant with a decrease in the adenosine-5'-triphosphate (ATP) pool in these cultures. Oxygenated cultures showed early senescence and death compared to aerated control cultures. Despite earlier studies proposing various mechanisms for such findings in fungal cultures subjected to oxidative stress, these findings can best be explained by the fact that in such cultures the activity of alternative NADH dehydrogenases was significantly increased, which served to maintain lower ROS concentrations throughout the duration of the process but in doing so also reduced the ability of the organism to create a proton motive force by which to drive ATP synthesis. The findings of the present study help further our understanding of the central roles of these highly conserved enzymes within fungal metabolism under oxidative stress.

Oxidative stress in industrial fungi

Fungi are amongst the most industrially important microorganisms in current use within the biotechnology industry. Most such fungal cultures are highly aerobic in nature, a character that has been frequently referred to in both reactor design and fungal physiology. The most fundamentally significant outcome of the highly aerobic growth environment in fermenter vessels is the need for the fungal culture to effectively combat in the intracellular environment the negative consequences of high oxygen transfer rates. The use of oxygen as the respiratory substrate is frequently reported to lead to the development of oxidative stress, mainly due to oxygen-derived free radicals, which are collectively termed as reactive oxygen species (ROS). Recently, there has been extensive research on the occurrence, extent, and consequences of oxidative stress in microorganisms, and the underlying mechanisms through which cells prevent and repair the damage caused by ROS. In the present study, we critically review the current understanding of oxidative stress events in industrially relevant fungi. The review first describes the current state of knowledge of ROS concisely, and then the various antioxidant strategies employed by fungal cells to counteract the deleterious effects, together with their implications in fungal bioprocessing are also discussed. Finally, some recommendations for further research are made.

Oxidative Stress in Submerged Cultures of Fungi

It has been known for many years that oxygen (O2) may have toxic effects on aerobically growing microorganisms, mainly due to the threat arising from reactive oxygen species (ROS). In submerged culture industrial fermentation processes, maintenance of adequate levels of O2 (usually measured as dissolved oxygen tension (DOT)) can often be critical to the success of the manufacturing process. In viscous cultures of filamentous cultures, actively respiring, supplying adequate levels of O2 to the cultures by conventional air sparging is difficult and various strategies have been adopted to improve or enhance O2 transfer. However, adoption of those strategies to maintain adequate levels of DOT, that is, to avoid O2 limitation, may expose the fungi to potential oxidative damage caused by enhanced flux through the respiratory system. In the past, there have been numerous studies investigating the effects of DOT on fungal bioprocesses. Generally, in these studies moderately enhanced levels of O2 supply resulted in improvement in growth, product formation and acceptable morphological changes, while the negative impact of higher levels of DOT on morphology and product synthesis were generally assumed to be a consequence of "oxidative stress." However, very little research has actually been focused on investigation of this implicit link, and the mechanisms by which such effects might be mediated within industrial fungal processes. To elucidate this neglected topic, this review first surveys the basic knowledge of the chemistry of ROS, defensive systems in fungi and the effects of DOT on fungal growth, metabolism and morphology. The physiological responses of fungal cells to oxidative stress imposed by artificial and endogenous stressors are then critically reviewed. It is clear that fungi have a range of methods available to minimize the negative impacts of elevated ROS, but also that development of the various defensive systems or responses, can itself have profound consequences upon many process-related parameters. It is also clear that many of the practically convenient and widely used experimental methods of simulating oxidative stress, for example, addition of exogenous menadione or hydrogen peroxide, have effects on fungal cultures quite distinct from the effects of elevated levels of O2, and care must thus be exercised in the interpretation of results from such studies. The review critically evaluates our current understanding of the responses of fungal cultures to elevated O2 levels, and highlights key areas requiring further research to remedy gaps in knowledge.

The band mutation in Neurospora crassa is a dominant allele of ras-1 implicating RAS signaling in circadian output

band, an allele enabling clear visualization of circadianly regulated spore formation (conidial banding), has remained an integral tool in the study of circadian rhythms for 40 years. bd was mapped using single-nucleotide polymorphisms (SNPs), cloned, and determined to be a T79I point mutation in ras-1. Alterations in light-regulated gene expression in the ras-1(bd) mutant suggests that the Neurospora photoreceptor WHITE COLLAR-1 is a target of RAS signaling, and increases in transcription of both wc-1 and fluffy show that regulators of conidiation are elevated in ras-1(bd). Comparison of ras-1(bd) with dominant active and dominant-negative ras-1 mutants and biochemical assays of RAS function indicate that RAS-1(bd) displays a modest enhancement of GDP/GTP exchange and no change in GTPase activity. Because the circadian clock in ras-1(bd) appears to be normal, ras-1(bd) apparently acts to amplify a subtle endogenous clock output signal under standard assay conditions. Reactive oxygen species (ROS), which can affect and be affected by RAS signaling, increase conidiation, suggesting a link between generation of ROS and RAS-1 signaling; surprisingly, however, ROS levels are not elevated in ras-1(bd). The data suggest that interconnected RAS- and ROS-responsive signaling pathways regulate the amplitude of circadian- and light-regulated gene expression in Neurospora.

Introduction to bioreactors of shake-flask inocula leads to development of oxidative stress in Aspergillus niger

Inoculation of bioreactors with shake-flask cultures present the organism with an immediate shift from an environment with little O2 to one in which O2 is typically at 100% saturation. The inoculation of such shake-flasks cultures into bioreactors sparged with 1 vvm air or 1 vvm air/O2 mix i.e. 50% O2 enrichment is an oxidatively stressful event, as judged by immediate increases in the intracellular concentrations of superoxide anion radical (O2*-) (from 4,600 to 11,600 RLU mg DCW(-1) and 5,500 to 23,000 RLU mg DCW(-1) respectively) and changes in the activities of the major antioxidant enzymes superoxide dismutase and catalase in all cultures. There are further effects on metabolic indices, particularly decreased nutrient consumption in oxygenated cultures (from 0.16 to 0.12 g starch g DCW h(-1)) and decreased protein production, indicating that inoculation of the bioreactor exerts a global burden on the cellular metabolic networks.

Elevated temperature effects on the oxidant/antioxidant balance in submerged batch cultures of the filamentous fungus Aspergillus niger B1-D

In the present study the relationship between oxidative stress and elevated culture temperature was examined in an industrially relevant fungal culture, Aspergillus niger B1-D. For the first time, both the intracellular levels of the main stressor species (superoxide radical [O(2) (.-)]) and activities of cellular defensive enzymes (superoxide dismutase [SOD], catalase [CAT], and glutathione peroxide [GPx]) were quantified at varying temperature (25, 30, 35, 40 degrees C) to more fully characterize culture response in different growth phases. Elevated culture temperature led to increased O(2) (.-) levels in various culture phases. In the exponential phase this was due to an enhanced generation of O(2) (.-), whereas in stationary phase a decreased dismutation rate may also have contributed. CAT activities generally increased with culture temperature, whereas GPx activity changed little as temperature rose, indicating that GPx played only a minor role in destroying H(2)O(2) in this A. niger. The combination of elevated temperature (35 degrees C) and increased O(2) supply (50% enrichment) led to decreased levels of O(2) (.-) compared to the cultivation at 35 degrees C gassed with air, probably due to enhanced activity of the alternative fungal respiratory pathway. Our findings indicate that while elevated cultivation temperature does clearly induce oxidative stress events, mechanistically, it does so by a rather more complex route than previous studies indicate. Elevated temperature caused a marked disparity in the activities of SOD and CAT, very distinct from the integrated increase in activity of these enzymes in response to oxidative stress.

Chemiluminescence of lucigenin by electrogenerated superoxide ions in aqueous solutions

The chemiluminescence reaction of lucigenin (Luc(2+)c2NO(3) (-), N,N'-dimethyl-9,9'-biacridinium dinitrate) at gold electrodes in dioxygen-saturated alkaline aqueous solutions (pH 10) was investigated in detail by the use of electrochemical emission spectroscopy. We noted that both O(2) and Luc(2+) are reduced on a gold electrode in aqueous solution of pH 10 in almost the same potential region. From this fact, we expected chemiluminescence based on a radical-radical coupling reaction of superoxide ion (O(2).(-)) and one-electron reduced form of Luc(2+) (Luc.(+), a radical cation). Chemiluminescence was actually observed in the potential range where O(2) and Luc(2+) were simultaneously reduced at the electrodes. The effects were examined upon addition of enzymes, i.e. superoxide dismutase (SOD) and catalase, into the solution and the substitution of heavy water (D(2)O) for light water (H(2)O) as a solvent on the chemiluminescence. In the presence of native and active SOD, chemiluminescence was completely absent. On the other hand, chemiluminescence was observed, unchanged in the presence of either denatured and inert SOD or catalase. In addition, the amount of chemiluminescence in D(2)O solution was about three times greater than that in H(2)O solution. These results, together with cyclic voltammetric results, suggest that O(2).(-) participates directly in the chemiluminescence but H(2)O(2) does not, and the chemiluminescence results from the coupling reaction between O(2).(-) and Luc(.+) under the present experimental conditions. These chemically unstable species, O(2).(-) and Luc.(+), are produced during the simultaneous electroreduction of O(2) and Luc(2+). The coupling reaction between those radical species would lead to the formation of a dioxetane-type intermediate and, finally, to chemiluminescence. The chemiluminescence reaction mechanism is discussed.