Isolation and characterization of Lipomyces starkeyi mutants with greatly increased lipid productivity following UV irradiation

The oleaginous yeast Lipomyces starkeyi is an intriguing lipid producer that can produce triacylglycerol (TAG), a feedstock for biodiesel production. We previously reported that the L. starkeyi mutant E15 with high levels of TAG production compared with the wild-type was efficiently obtained using Percoll density gradient centrifugation. However, considering its use for biodiesel production, it is necessary to further improve the lipid productivity of the mutant. In this study, we aimed to obtain mutants with better lipid productivity than E15, evaluate its lipid productivity, and analyze lipid synthesis-related gene expression in the wild-type and mutant strains. The mutants E15-11, E15-15, and E15-25 exhibiting higher lipid productivity than E15 were efficiently isolated from cells exposed to ultraviolet light using Percoll density gradient centrifugation. They exhibited approximately 4.5-fold higher lipid productivity than the wild-type on day 3. The obtained mutants did not exhibit significantly different fatty acid profiles than the wild-type and E15 mutant strains. E15-11, E15-15, and E15-25 exhibited higher expression of acyl-CoA synthesis- and Kennedy pathway-related genes than the wild-type and E15 mutant strains. Activation of the pentose phosphate pathway, which supplies NADPH, was also observed. These results suggested that the increased expression of acyl-CoA synthesis- and Kennedy pathway-related genes plays a vital role in lipid productivity in the oleaginous yeast L. starkeyi.

Glycogen Metabolism Supports Photosynthesis Start through the Oxidative Pentose Phosphate Pathway in Cyanobacteria

Cyanobacteria experience drastic changes in their carbon metabolism under daily light/dark cycles. During the day, the Calvin-Benson cycle fixes CO₂ and diverts excess carbon into glycogen storage. At night, glycogen is degraded to support cellular respiration. The dark/light transition represents a universal environmental stress for cyanobacteria and other photosynthetic lifeforms. Recent studies revealed the essential genetic background necessary for the fitness of cyanobacteria during diurnal growth. However, the metabolic processes underlying the dark/light transition are not well understood. In this study, we observed that glycogen metabolism supports photosynthesis in the cyanobacterium Synechococcus elongatus PCC 7942 when photosynthesis reactions start upon light exposure. Compared with the wild type, the glycogen mutant ∆glgC showed a reduced photosynthetic efficiency and a slower P700⁺ rereduction rate when photosynthesis starts. Proteomic analyses indicated that glycogen is degraded through the oxidative pentose phosphate (OPP) pathway during the dark/light transition. We confirmed that the OPP pathway is essential for the initiation of photosynthesis and further showed that glycogen degradation through the OPP pathway contributes to the activation of key Calvin-Benson cycle enzymes by modulating NADPH levels. This strategy stimulates photosynthesis in cyanobacteria following dark respiration and stabilizes the Calvin-Benson cycle under fluctuating environmental conditions, thereby offering evolutionary advantages for photosynthetic organisms using the Calvin-Benson cycle for carbon fixation.

Light regulation of coccolithophore host-virus interactions

Viruses that infect photoautotrophs have a fundamental relationship with light, given the need for host resources. We investigated the role of light on Coccolithovirus (EhV) infection of the globally distributed coccolithophore, Emiliania huxleyi. Light was required for EhV adsorption, and viral production was highest when host cultures were maintained in continuous light or at irradiance levels of 150-300 μmol m^-2  s^-1 . During the early stages of infection, photosynthetic electron transport remained high, while RuBisCO expression decreased concomitant with an induction of the pentose phosphate pathway, the primary source of de novo nucleotides. A mathematical model developed and fitted to the laboratory data supported the hypothesis that EhV replication was controlled by a trade-off between host nucleotide recycling and de novo synthesis, and that photoperiod and photon flux could toggle this switch. Laboratory results supported field observations that light was the most robust driver of EhV replication within E. huxleyi populations collected across a 2000 nautical mile transect in the North Atlantic. Collectively, these findings demonstrate that light can drive host-virus interactions through a mechanistic interplay between host metabolic processes, which serve to structure infection and phytoplankton mortality in the upper ocean.

Characterization of multiple SPS knockout mutants reveals redundant functions of the four Arabidopsis sucrose phosphate synthase isoforms in plant viability, and strongly indicates that enhanced respiration and accelerated starch turnover can alleviate the blockage of sucrose biosynthesis

We characterized multiple knock-out mutants of the four Arabidopsis sucrose phosphate synthase (SPSA1, SPSA2, SPSB and SPSC) isoforms. Despite their reduced SPS activity, spsa1/spsa2, spsa1/spsb, spsa2/spsb, spsa2/spsc, spsb/spsc, spsa1/spsa2/spsb and spsa2/spsb/spsc mutants displayed wild type (WT) vegetative and reproductive morphology, and showed WT photosynthetic capacity and respiration. In contrast, growth of rosettes, flowers and siliques of the spsa1/spsc and spsa1/spsa2/spsc mutants was reduced compared with WT plants. Furthermore, these plants displayed a high dark respiration phenotype. spsa1/spsb/spsc and spsa1/spsa2/spsb/spsc seeds poorly germinated and produced aberrant and sterile plants. Leaves of all viable sps mutants, except spsa1/spsc and spsa1/spsa2/spsc, accumulated WT levels of nonstructural carbohydrates. spsa1/spsc leaves possessed high levels of metabolic intermediates and activities of enzymes of the glycolytic and tricarboxylic acid cycle pathways, and accumulated high levels of metabolic intermediates of the nocturnal starch-to-sucrose conversion process, even under continuous light conditions. Results presented in this work show that SPS is essential for plant viability, reveal redundant functions of the four SPS isoforms in processes that are important for plant growth and nonstructural carbohydrate metabolism, and strongly indicate that accelerated starch turnover and enhanced respiration can alleviate the blockage of sucrose biosynthesis in spsa1/spsc leaves.

Acute Activation of Oxidative Pentose Phosphate Pathway as First-Line Response to Oxidative Stress in Human Skin Cells

Integrity of human skin is endangered by exposure to UV irradiation and chemical stressors, which can provoke a toxic production of reactive oxygen species (ROS) and oxidative damage. Since oxidation of proteins and metabolites occurs virtually instantaneously, immediate cellular countermeasures are pivotal to mitigate the negative implications of acute oxidative stress. We investigated the short-term metabolic response in human skin fibroblasts and keratinocytes to H2O2 and UV exposure. In time-resolved metabolomics experiments, we observed that within seconds after stress induction, glucose catabolism is routed to the oxidative pentose phosphate pathway (PPP) and nucleotide synthesis independent of previously postulated blocks in glycolysis (i.e., of GAPDH or PKM2). Through ultra-short (13)C labeling experiments, we provide evidence for multiple cycling of carbon backbones in the oxidative PPP, potentially maximizing NADPH reduction. The identified metabolic rerouting in oxidative and non-oxidative PPP has important physiological roles in stabilization of the redox balance and ROS clearance.

Induced pluripotent stem cells from ataxia-telangiectasia recapitulate the cellular phenotype

Pluripotent stem cells can differentiate into every cell type of the human body. Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) therefore provides an opportunity to gain insight into the molecular and cellular basis of disease. Because the cellular DNA damage response poses a barrier to reprogramming, generation of iPSCs from patients with chromosomal instability syndromes has thus far proven to be difficult. Here we demonstrate that fibroblasts from patients with ataxia-telangiectasia (A-T), a disorder characterized by chromosomal instability, progressive neurodegeneration, high risk of cancer, and immunodeficiency, can be reprogrammed to bona fide iPSCs, albeit at a reduced efficiency. A-T iPSCs display defective radiation-induced signaling, radiosensitivity, and cell cycle checkpoint defects. Bioinformatic analysis of gene expression in the A-T iPSCs identifies abnormalities in DNA damage signaling pathways, as well as changes in mitochondrial and pentose phosphate pathways. A-T iPSCs can be differentiated into functional neurons and thus represent a suitable model system to investigate A-T-associated neurodegeneration. Collectively, our data show that iPSCs can be generated from a chromosomal instability syndrome and that these cells can be used to discover early developmental consequences of ATM deficiency, such as altered mitochondrial function, that may be relevant to A-T pathogenesis and amenable to therapeutic intervention.

Changes in the hydrocarbon-synthesizing activity during growth of Botryococcus braunii B70

Botryococcus braunii is a green, colonial microalga that produces large amounts of hydrocarbons. B. braunii B70 was estimated to be B race by the incorporation of radioactivity from l-[methyl(14)C]-methionine into hydrocarbon. The hydrocarbon-synthesizing activity of B70 cells was determined by feeding experiments using (14)C-compounds. NaH(14)CO(3) incorporation rate into the hydrocarbon was high in the early logarithmic growth phase but it declined thereafter. Hydrocarbon-synthesizing activity from [2-(14)C] pyruvate in 15-day cells was 80% of that in 5-day cells. In contrast, hydrocarbon-synthesizing activity from NaH(14)CO(3) and l-[methyl(14)C]-methionine decreased remarkably by 15 days after inoculation. Hence, the allocation of carbon was a regulatory step in hydrocarbon biosynthesis during the early logarithmic growth phase. The high activity of pentose phosphate pathway in the early logarithmic growth was seemed to be the contribution of the supply of NADPH for botryococcene synthesis.

Ultraviolet light-induced changes in the glucose-6-phosphate dehydrogenase activity of porcine corneas

PURPOSE

Corneas have the high activity of the pentose phosphate pathway. To clarify the relevance of the pentose phosphate pathway to their antioxidant defense, we examined the activity of glucose-6-phosphate dehydrogenase (G6PDH), the rate-determining enzyme of the pathway, in corneas exposed to ultraviolet light (UV).

METHODS

Fresh porcine eye globes were exposed to ultraviolet light A (UVA) or ultraviolet light C (UVC), and the tear film-removed eye globes to UVA. After UV exposure, the corneas were dissected from the eye globes and extracted with a saline solution, and the G6PDH activity in the extract was assayed. The G6PDH activity of unilateral opaque corneas, their paired, transparent corneas, and normal corneas, all of which were obtained from fresh, UV-unexposed eye globes, and the L-lactate dehydrogenase (LDH) activity of these opaque and normal corneas also were assayed.

RESULTS

The G6PDH activity of corneas increased with UVA exposure, and decreased with long-term UVC exposure, although it increased with short-term UVC exposure. A UV-blocking contact lens screened corneas from the UVA-induced increase. Removal of the tear film enhanced the UVC-induced decrease. The G6PDH activity of unilateral opaque corneas was lower than that of paired, transparent corneas or normal corneas, all of which were obtained from fresh, UV-unexposed eye globes. The LDH activity of the opaque corneas was much higher than that of the normal corneas.

CONCLUSION

Exposure of corneas to UVA or a small dose of UVC enhances the G6PDH activity, i.e., the pentose phosphate pathway. This activity enhancement may play an important role in corneal antioxidant defense against UV-induced oxidative stress. However, exposure of corneas to large doses of UVC appears to damage the pathway.

Synergistic effect of UVB radiation and age on HMPS enzymes in rat lens homogenate

The behaviour of rat lenticular enzymes, glucose-6-phosphate dehydrogenase (G6PD, EC: 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGD, EC: 1.1.1.44) as a function of age and UVB irradiation (in vitro) was investigated by irradiating the lens homogenate from 3- and 12-month-old rats at 300 nm (100 microW cm-2). In the 3-month-old group the specific activities of G6PD and 6PGD were reduced by 26% and 42%, respectively, after 24 h of irradiation, whereas in the 12-month-old group the decrease was 38% and 49% respectively, which suggests that the susceptibility of HMPS enzymes to UVB damage is higher in older lenses. The decrease in specific activity was associated with a change in apparent K(m) and Vmax (marginal in 3 months and significant in 12 months) of these enzymes due to UVB irradiation. UVB irradiation also decreased the levels of NADPH and NADPH/NADP ratio. These changes, altered activities of G6PD and 6PGD and altered levels of NADPH, may in turn have a bearing on lens transparency.

Effects of ultraviolet A and B irradiation on lipid peroxidation and activity of the antioxidant enzymes in keratinocytes in culture

Human keratinocytes (NCTC 2544) in culture were exposed to various combinations of ultraviolet A (UVA) and UVB irradiation and at 0.5 h postirradiation the level of lipid peroxidation and activities of antioxidant enzymes were measured. The results suggest that UV irradiation is capable of inducing lipid peroxidation reactions, as parameters of which the amount of thiobarbituric acid-reactive material and the number of conjugated diene double bonds were measured. Both UVA and UVB irradiation were also found to affect the activities of antioxidant enzymes. Following UVB irradiation the activity of superoxide dismutase (Cu/Zn form) was decreased, and combination of increasing doses of UVA irradiation to a given dose of UVB irradiation decreased the activity of both catalase and superoxide dismutase. In summary, this study suggests that both UVA and UVB irradiation are capable of inducing lipid peroxidation reactions and an impairment of the enzymic antioxidant system in human keratinocytes in culture.

Changes of hexose monophosphate pathway and methemoglobin reductase enzyme activity after radiation in guinea pigs

HMP pathway activity changes occurring after exposure to ionizing radiation (LD50 dose) have been investigated. The study was carried out on 18 experimental guinea pigs subjected to 5 successive exposures of 150 rads 3 or 4 days apart. The control animals were sham radiated but were otherwise treated identically as those of the experimental groups. Blood samples were taken by cardiac puncture before radiation and 30 min after each exposure of 150 rads. The red cells were re-suspended in their own plasma and HMP pathway activity was measured in the suspension. The pathway activity showed a consistent but minor reduction in the experimental group, which became statistically significant after the total dose of 750 rads (P less than 0.020). In a separate study the changes induced by ionizing radiation in the erythrocyte enzyme NADH-methemoglobin reductase were measured using the same experimental protocol. The enzyme activity in the red cells of the experimental group varied between 34.90 +/- 2.17 to 161.95 +/- 5.34 I.U./ml erythrocyte pack. Its activity declined toward the initial value after reaching the peak by the 12th day of ionizing radiation with 600 rads (P less than 0.001).

The radiosensitivity of rat thymocytes

gamma-Irradiation in vitro apparently blocked a plasma-membrane associated, superoxide-producing, NADPH oxidase in rat thymocytes. Differential centrifugation of the mixed thymocytes indicated the smaller lymphocytes (approx. 6 microns diameter) to be the radiosensitive population. The oxidase system co-isolated in part with thymus nuclei and could be solubilized by detergent treatment [Bellavite, Jones, Cross, Papini & Rossi (1984) Biochem. J. 223, 639-648]. Endogenous NADPH was the rate-limiting component for superoxide formation in vitro. The level of NADPH was lowered by gamma-irradiation, an effect mimicked by GSSG in the presence of 50 microM-ZnCl2 to inhibit GSSG reductase. These findings are suggested as the metabolic basis for interphase death of small lymphocytes exposed to ionizing radiation.