Impact of oxidative stress on Acanthamoeba castellanii mitochondrial bioenergetics depends on cell growth stage

Cytological structures and physiological and biochemical characteristics of covered oat (Avena sativa L.) and naked oat (Avena nuda L.) seeds during high-temperature artificial aging

BACKGROUND

Seed aging, a natural and inevitable process occurring during storage. Oats, an annual herb belonging to the Gramineae family and pooideae. In addition to being a healthy food, oats serve as ecological pastures, combating soil salinization and desertification. They also play a role in promoting grassland agriculture and supplementing winter livestock feed. However, the high lipid and fat derivatives contents of oat seeds make them susceptible to deterioration, as fat derivatives are prone to rancidity, affecting oat seed production, storage, development, and germplasm resource utilization. Comparative studies on the effects of aging on physiology and cytological structure in covered and naked oat seeds are limited. Thus, our study aimed to determine the mechanism underlying seed deterioration in artificially aged 'LongYan No. 3' (A. sativa) and 'BaiYan No. 2' (A. nuda) seeds, providing a basis for the physiological evaluation of oat seed aging and serving as a reference for scientifically safe storage and efficient utilization of oats.

RESULTS

In both oat varieties, superoxide dismutase and catalase activities in seeds showed increasing and decreasing trends, respectively. Variance analysis revealed significant differences and interaction in all measured indicators of oat seeds between the two varieties at different aging times. 'LongYan No. 3' seeds, aged for 24-96 h, exhibited a germination rate of < 30%, Conductivity, malondialdehyde, soluble sugar, and soluble protein levels increased more significantly than the 'BaiYan No. 2'. With prolonged aging leading to cell membrane degradation, reactive oxygen species accumulation, disrupted antioxidant enzyme system, evident embryo cell swelling, and disordered cell arrangement, blocking the nutrient supply route. Simultaneously, severely concentrated chromatin in the nucleus, damaged mitochondrial structure, and impaired energy metabolism were noted, resulting in the loss of 'LongYan No. 3' seed vitality and value. Conversely, 'BaiYan No. 2' seeds showed a germination rate of 73.33% after 96 h of aging, consistently higher antioxidant enzyme activity during aging, normal embryonic cell shape, and existence of the endoplasmic reticulum.

CONCLUSIONS

ROS accumulation and antioxidant enzyme system damage in aged oat seeds, nuclear chromatin condensation, mitochondrial structure damage, nucleic acid metabolism and respiration weakened, oat seed vigor decreased. 'LongYan No. 3' seeds were more severely damaged under artificial aging than 'BaiYan No. 2' seeds, highlighting their heightened susceptibility to aging effects.

Alternative Oxidase: From Molecule and Function to Future Inhibitors

In the respiratory chain of the majority of aerobic organisms, the enzyme alternative oxidase (AOX) functions as the terminal oxidase and has important roles in maintaining metabolic and signaling homeostasis in mitochondria. AOX endows the respiratory system with flexibility in the coupling among the carbon metabolism pathway, electron transport chain (ETC) activity, and ATP turnover. AOX allows electrons to bypass the main cytochrome pathway to restrict the generation of reactive oxygen species (ROS). The inhibition of AOX leads to oxidative damage and contributes to the loss of adaptability and viability in some pathogenic organisms. Although AOXs have recently been identified in several organisms, crystal structures and major functions still need to be explored. Recent work on the trypanosome alternative oxidase has provided a crystal structure of an AOX protein, which contributes to the structure-activity relationship of the inhibitors of AOX. Here, we review the current knowledge on the development, structure, and properties of AOXs, as well as their roles and mechanisms in plants, animals, algae, protists, fungi, and bacteria, with a special emphasis on the development of AOX inhibitors, which will improve the understanding of respiratory regulation in many organisms and provide references for subsequent studies of AOX-targeted inhibitors.

Acanthamoeba castellanii Uncoupling Protein: A Complete Sequence, Activity, and Role in Response to Oxidative Stress

Uncoupling proteins (UCPs) are mitochondrial inner membrane transporters that mediate free-fatty-acid-induced, purine-nucleotide-inhibited proton leak into the mitochondrial matrix, thereby uncoupling respiratory substrate oxidation from ATP synthesis. The aim of this study was to provide functional evidence that the putative Acucp gene of the free-living protozoan amoeba, A. castellanii, encodes the mitochondrial protein with uncoupling activity characteristic of UCPs and to investigate its role during oxidative stress. We report the sequencing and cloning of a complete Acucp coding sequence, its phylogenetic analysis, and the heterologous expression of AcUCP in the S. cerevisiae strain InvSc1. Measurements of mitochondrial respiratory activity and membrane potential indicate that the heterologous expression of AcUCP causes AcUCP-mediated uncoupling activity. In addition, in a model of oxidative stress with increased reactive oxygen species levels (superoxide dismutase 1 knockout yeasts), AcUCP expression strongly promotes cell survival and growth. The level of superoxide anion radicals is greatly reduced in the ΔSOD1 strain expressing AcUCP. These results suggest that AcUCP targeted to yeast mitochondria causes uncoupling and may act as an antioxidant system. Phylogenetic analysis shows that the A. castellanii UCP diverges very early from other UCPs, but clearly locates within the UCP subfamily rather than among other mitochondrial anion carrier proteins.

Transcriptional changes of proteins of the thioredoxin and glutathione systems in Acanthamoeba spp. under oxidative stress - an RNA approach

The thioredoxin (Trx) and the glutathione (GSH) systems represent important antioxidant systems in cells and in particular thioredoxin reductase (TrxR) has been shown to constitute a promising drug target in parasites. For the facultative protozoal pathogen Acanthamoeba, it was demonstrated that a bacterial TrxR as well as a TrxR, characteristic of higher eukaryotes, mammals and humans is expressed on the protein level. However, only bacterial TrxR is strongly induced by oxidative stress in Acanthamoeba castellanii. In this study, the impact of oxidative stress on key enzymes involved in the thioredoxin and the glutathione system of A. castellanii under different culture conditions and of clinical Acanthamoeba isolates was evaluated on the RNA level employing RT-qPCR. Additionally, the effect of auranofin, a thioredoxin reductase inhibitor, already established as a potential drug in other parasites, on target enzymes in A. castellanii was investigated. Oxidative stress induced by hydrogen peroxide led to significant stimulation of bacterial TrxR and thioredoxin, while diamide had a strong impact on all investigated enzymes. Different strains displayed distinct transcriptional responses, rather correlating to sensitivity against the respective stressor than to respective pathogenic potential. Culture conditions appear to have a major effect on transcriptional changes in A. castellanii. Treatment with auranofin led to transcriptional activation of the GSH system, indicating its role as a potential backup for the Trx system. Altogether, our data provide more profound insights into the complex redox system of Acanthamoeba, preparing the ground for further investigations on this topic.

Patterns of virus growth across the diversity of life

Although viruses in their natural habitats add up to less than 10% of the biomass, they contribute more than 90% of the genome sequences [1]. These viral sequences or 'viromes' encode viruses that populate the Earth's oceans [2, 3] and terrestrial environments [4, 5], where their infections impact life across diverse ecological niches and scales [6, 7], including humans [8-10]. Most viruses have yet to be isolated and cultured [11-13], and surprisingly few efforts have explored what analysis of available data might reveal about their nature. Here, we compiled and analyzed seven decades of one-step growth and other data for viruses from six major families, including their infections of archaeal, bacterial and eukaryotic hosts [14-191]. We found that the use of host cell biomass for virus production was highest for archaea at 10%, followed by bacteria at 1% and eukarya at 0.01%, highlighting the degree to which viruses of archaea and bacteria exploit their host cells. For individual host cells, the yield of virus progeny spanned a relatively narrow range (10-1000 infectious particles per cell) compared with the million-fold difference in size between the smallest and largest cells. Furthermore, healthy and infected host cells were remarkably similar in the time they needed to multiply themselves or their virus progeny. Specifically, the doubling time of healthy cells and the delay time for virus release from infected cells were not only correlated (r = 0.71, p < 10-10, n = 101); they also spanned the same range from tens of minutes to about a week. These results have implications for better understanding the growth, spread and persistence of viruses in complex natural habitats that abound with diverse hosts, including humans and their associated microbes.

Effect of oxidative stress on vital indicators of Acanthamoeba castellanii (T4 genotype)

Acanthamoeba has 22 genotypes with the T4 genotype being the main causative agent of amoebic granulomatous encephalitis and keratitis. Because the molecular mechanisms of the immune defenses of neutrophils and macrophages against histoparasites are based on oxidative stress, parasites may rely on their antioxidant systems to preclude immune defenses. Therefore, understanding of the effect of oxidative stress on vital characteristics of Acanthamoeba castellanii (T4 genotype) and the antioxidant defense responses of Acanthamoeba to oxidative status will cast light on immune cell-parasite interactions. Acanthamoeba T4 cells were cultured in RPMI-1640 medium containing different concentrations of hydrogen peroxide (H₂O₂). The survival of Acanthamoeba was evaluated by MTT assay and the IC50 concentration was calculated. The total antioxidant capacity (TAC) of the parasite was determined by the cupric reducing antioxidant capacity (CUPRAC) method. Malondialdehyde (MDA) as a marker of lipid peroxidation, protein carbonyl content as a measure of oxidized protein, total thiol (-SH) groups present on proteins as a major source of cellular antioxidants, and total oxidant status (TOS) were evaluated by colorimetric methods. The reactive oxygen species level increased markedly after induction of oxidative stress by the treatment of Acanthamoeba T4 with H₂O₂. Exposure to H₂O₂ also significantly increased the MDA and protein carbonyl content. The TOS level and total thiol groups also increased in the treated group compared to those in untreated parasites, although the results were not statistically significant. The TAC level was found to be significantly higher in H₂O₂-treated parasites, confirming that the parasite fosters its total antioxidant capacity to overcome oxidative conditions. This study showed that under oxidative stress, the defense reactions of the parasite are in part mediated by increasing its antioxidant activity, which is important for the survival of the parasite.

Alternative oxidase inhibitors as antiparasitic agents against scuticociliatosis

Philasterides dicentrarchi causes a severe disease in turbot, and at present there are no drugs available to treat infected fish. We have previously demonstrated that, in addition to the classical respiratory pathway, P. dicentrarchi possesses an alternative mitochondrial respiratory pathway that is cyanide-insensitive and salicylhydroxamic acid (SHAM)-sensitive. In this study, we found that during the initial phase of growth in normoxia, ciliate respiration is sensitive to the natural polyphenol resveratrol (RESV) and to Antimycin A (AMA). However, under hypoxic conditions, the parasite utilizes AMA-insensitive respiration, which is completely inhibited by RESV and by the antioxidant propyl gallate (PG), an alternative oxidase (AOX) inhibitor. PG caused significantly dose-dependent inhibition of the in vitro growth of the parasite under normoxia and hypoxia and an over-expression of heat shock proteins of the Hsp70 subfamily. RESV and PG may affect the protective role of the AOX against mitochondrial oxidative stress, leading to an impaired mitochondrial membrane potential and mitochondrial dysfunction, which the parasite attempts to neutralize by increasing the expression of Hsp70. In view of the antiparasitic effects induced by AOX inhibitors and the absence of AOX in their host, this enzyme constitutes a potential target for the development of new drugs against scuticociliatosis.

Encystment in Acanthamoeba castellanii: a review

Differentiation of Acanthamoeba castellanii trophozoites involves massive turnover of cellular components and remodelling of organelle structure and function so as to produce a cryptobiotic cell, resistant to desiccation, heat, freezing, and chemical treatments. This review presents a summary of a decade of research on the most studied aspects of the biochemistry of this process, with emphasis on problems of biocide and drug resistances, putative new targets, molecular and cell biology of the process of encystment, and the characteristics of the encysted state. As well as the intrinsic pathogenicity of the organism towards the cornea, and the ability of related species to invade the human brain, its propensity for harbouring and transmitting pathogenic bacteria and viruses is considerable and leads to increasing concerns. The long-term survival and resistance of cysts to drugs and biocides adds another layer of complexity to the problem of their elimination.

Evidence of an alternative oxidase pathway for mitochondrial respiration in the scuticociliate Philasterides dicentrarchi

The presence of an alternative oxidase (AOX) in the mitochondria of the scuticociliate P. dicentrarchi was investigated. The mitochondrial oxygen consumption was measured in the presence of KCN, an inhibitor of cytochrome pathway (CP) respiration and salicylhydroxamic acid (SHAM), a specific inhibitor of alternative pathway (AP) respiration. AOX expression was monitored by western blotting with an AOX polyclonal antibody. The results showed that P. dicentrarchi possesses a branched mitochondrial electron transport chain with both cyanide-sensitive and -insensitive oxygen consumption. Mitochondrial respiration was partially inhibited by cyanide and completely inhibited by the combination of cyanide and SHAM, which is direct evidence for the existence of an AP in this ciliate. SHAM significantly inhibited in vitro growth of trophozoites both under normoxic and hypoxic conditions. AOX is a 42kD monomeric protein inducible by hypoxic conditions in experimental infections and by CP inhibitors such as cyanide and antimycin A, or by AP inhibitors such as SHAM. CP respiration was greatly stimulated during the exponential growth phase, while AP respiration increased during the stationary phase, in which AOX expression is induced. As the host does not possess AOX, and because during infection P. dicentrarchi respires via AP, it may be possible to develop inhibitors targeting the AP as a novel anti-scuticociliate therapy.

Functional expression and characterization of an iron-containing superoxide dismutase of Acanthamoeba castellanii

Acanthamoeba spp. are free-living amoebae, but opportunistic infections of some strains of the organisms cause severe diseases such as acanthamoebic keratitis, pneumonitis, and granulomatous amoebic encephalitis in human. In this study, we identified a gene encoding iron superoxide dismutase of Acanthamoeba castellanii (AcFe-SOD) and characterized biochemical and functional properties of the recombinant enzyme. Multiple sequence alignment of the deduced amino acid sequence of AcFe-SOD with those of previously reported iron-containing SODs (Fe-SODs) from other protozoan parasites showed that AcFe-SOD shared common metal-binding residues and motifs that are conserved in Fe-SODs. The genomic length of the AcFe-SOD gene was 926 bp consisting of five exons interrupted by four introns. The recombinant AcFe-SOD showed similar biochemical characteristics with its native enzyme and shared typical biochemical properties with other characterized Fe-SODs, including molecular structure, broad pH optimum, and sensitivity to hydrogen peroxide. Immunolocalization analysis revealed that the enzyme localized in the cytosol of the trophozoites. Activity and expression level of the enzyme were significantly increased under oxidative stressed conditions. These results collectively suggest that AcFe-SOD may play essential roles in the survival of the parasite not only by protecting itself from endogenous oxidative stress but also by detoxifying oxidative killing of the parasite by host immune effector cells.