Two pathways of photoproduction of organic hydroperoxides on the donor side of photosystem 2 in subchloroplast membrane fragments

Earlier the catalase-insensitive formation of organic hydroperoxides (via the interaction of organic radicals produced due to redox activity of P₆₈₀^+· (or TyrZ^·) with molecular oxygen) has been found in Mn-depleted PS2 preparations (apo-WOC-PS2) by Khorobrykh et al. (Biochemistry 50:10658-10665, 2011). The present work describes a second pathway of the photoproduction of organic peroxides on the donor side of PS2. It was shown that illumination of CaCl₂-treated PS2 membranes (deprived of the PS2 extrinsic proteins without removal of the Mn-containing water-oxidizing complex) (CaCl₂-PS2) led to the photoproduction of highly lipophilic organic hydroperoxides (LP-OOH) (in amount corresponding to 1.5 LP-OOH per one reaction center of PS2) which significantly increased upon the addition of exogenous electron acceptor potassium ferricyanide (to 4.2 LP-OOH per one reaction center). Addition of catalase (200 U/ml) before illumination inhibited ferricyanide-induced photoproduction of hydroperoxides while no effect was obtained by adding catalase after illumination or by adding inactivated catalase before illumination. The hydroperoxide photoproduction was inhibited by the addition of exogenous electron donor for PS2, diphenylcarbazide or diuron (inhibitor of the electron transfer in PS2). The addition of exogenous hydrogen peroxide to the CaCl₂-PS2 led to the production of highly lipophilic organic hydroperoxides in the dark (3.2 LP-OOH per one reaction center). We suggest that the photoproduction of highly lipophilic organic hydroperoxides in CaCl₂-PS2 preparations occurs via redox activity of H₂O₂ produced on the donor side of PS2.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

Formation of chloroplast protrusions and catalase activity in alpine Ranunculus glacialis under elevated temperature and different CO2/O2 ratios

Chloroplast protrusions (CPs) have frequently been observed in plants, but their significance to plant metabolism remains largely unknown. We investigated in the alpine plant Ranunculus glacialis L. treated under various CO2 concentrations if CP formation is related to photorespiration, specifically focusing on hydrogen peroxide (H2O2) metabolism. Immediately after exposure to different CO2 concentrations, the formation of CPs in leaf mesophyll cells was assessed and correlated to catalase (CAT) and ascorbate peroxidase (APX) activities. Under natural irradiation, the relative proportion of chloroplasts with protrusions (rCP) was highest (58.7 %) after exposure to low CO2 (38 ppm) and was lowest (3.0 %) at high CO2 (10,000 ppm). The same relationship was found for CAT activity, which decreased from 34.7 nkat mg(-1) DW under low CO2 to 18.4 nkat mg(-1) DW under high CO2, while APX activity did not change significantly. When exposed to natural CO2 concentration (380 ppm) in darkness, CP formation was significantly lower (18.2 %) compared to natural solar irradiation (41.3 %). In summary, CP formation and CAT activity are significantly increased under conditions that favour photorespiration, while in darkness or at high CO2 concentration under light, CP formation is significantly lower, providing evidence for an association between CPs and photorespiration.

Chloroplast and oxygen evolution changes in Symbiodinium sp. as a response to latrunculin and butanedione monoxime treatments under various light conditions

The actin cytoskeleton is a dynamic structure that provides an interactive platform for organelles and cellular components. It also serves as track for membranes and vesicles that move via myosin. The actin cytoskeleton of Symbiodinium is a well-organized reticular structure suggestive of multiple membrane interactions, very likely including those of the chloroplast. The Symbiodinium chloroplast membrane network is, in turn, a highly organized structure, suggestive of being under the control of an organizing network. We visualized the chloroplast membranes of cultured Symbiodinium sp. under various light conditions and observed changes dependent on illumination intensity. Since we suspected interaction between these two organelles, and we knew that the Symbiodinium actin cytoskeleton collapses upon treatment with either latrunculin B, an actin microfilament-disrupting agent, or butanedione monoxime, a myosin function inhibitor, we tested the Symbiodinium sp. oxygen evolution in their presence. Upon latrunculin B addition, the oxygen production decreased compared to non-treated cells; however, this was not observed after a 24 h latrunculin treatment. On the contrary, butanedione monoxime treatment caused a non-recoverable dysfunction of the chloroplast causing a severe loss in oxygen production even after long-term exposure. Using electron microscopy, we observed an alteration of the Symbiodinium sp. chloroplast distribution after latrunculin B treatment, with respect to untreated cells. Furthermore, a thorough disorganization of the chloroplast grana was observed after butanedione monoxime treatment. These data suggest that an actomyosin system would be important for chloroplast organization and distribution, and critical for normal photosynthetic function of Symbiodinium sp.

PHOTOSYSTEM II PROTEIN33, a protein conserved in the plastid lineage, is associated with the chloroplast thylakoid membrane and provides stability to photosystem II supercomplexes in Arabidopsis

Photosystem II (PSII) is a multiprotein complex that catalyzes the light-driven water-splitting reactions of oxygenic photosynthesis. Light absorption by PSII leads to the production of excited states and reactive oxygen species that can cause damage to this complex. Here, we describe Arabidopsis (Arabidopsis thaliana) At1g71500, which encodes a previously uncharacterized protein that is a PSII auxiliary core protein and hence is named PHOTOSYSTEM II PROTEIN33 (PSB33). We present evidence that PSB33 functions in the maintenance of PSII-light-harvesting complex II (LHCII) supercomplex organization. PSB33 encodes a protein with a chloroplast transit peptide and one transmembrane segment. In silico analysis of PSB33 revealed a light-harvesting complex-binding motif within the transmembrane segment and a large surface-exposed head domain. Biochemical analysis of PSII complexes further indicates that PSB33 is an integral membrane protein located in the vicinity of LHCII and the PSII CP43 reaction center protein. Phenotypic characterization of mutants lacking PSB33 revealed reduced amounts of PSII-LHCII supercomplexes, very low state transition, and a lower capacity for nonphotochemical quenching, leading to increased photosensitivity in the mutant plants under light stress. Taken together, these results suggest a role for PSB33 in regulating and optimizing photosynthesis in response to changing light levels.

High light-dependent phosphorylation of photosystem II inner antenna CP29 in monocots is STN7 independent and enhances nonphotochemical quenching

Phosphorylation of the photosystem II antenna protein CP29 has been reported to be induced by excess light and further enhanced by low temperature, increasing resistance to these stressing factors. Moreover, high light-induced CP29 phosphorylation was specifically found in monocots, both C3 and C4, which include the large majority of food crops. Recently, knockout collections have become available in rice (Oryza sativa), a model organism for monocots. In this work, we have used reverse genetics coupled to biochemical and physiological analysis to elucidate the molecular basis of high light-induced phosphorylation of CP29 and the mechanisms by which it exerts a photoprotective effect. We found that kinases and phosphatases involved in CP29 phosphorylation are distinct from those reported to act in State 1-State 2 transitions. In addition, we elucidated the photoprotective role of CP29 phosphorylation in reducing singlet oxygen production and enhancing excess energy dissipation. We thus established, in monocots, a mechanistic connection between phosphorylation of CP29 and nonphotochemical quenching, two processes so far considered independent from one another.

Heat-stress and light-stress induce different cellular pathologies in the symbiotic dinoflagellate during coral bleaching

Coral bleaching is a significant contributor to the worldwide degradation of coral reefs and is indicative of the termination of symbiosis between the coral host and its symbiotic algae (dinoflagellate; Symbiodinium sp. complex), usually by expulsion or xenophagy (symbiophagy) of its dinoflagellates. Herein, we provide evidence that during the earliest stages of environmentally induced bleaching, heat stress and light stress generate distinctly different pathomorphological changes in the chloroplasts, while a combined heat- and light-stress exposure induces both pathomorphologies; suggesting that these stressors act on the dinoflagellate by different mechanisms. Within the first 48 hours of a heat stress (32°C) under low-light conditions, heat stress induced decomposition of thylakoid structures before observation of extensive oxidative damage; thus it is the disorganization of the thylakoids that creates the conditions allowing photo-oxidative-stress. Conversely, during the first 48 hours of a light stress (2007 µmoles m⁻² s⁻¹ PAR) at 25°C, condensation or fusion of multiple thylakoid lamellae occurred coincidently with levels of oxidative damage products, implying that photo-oxidative stress causes the structural membrane damage within the chloroplasts. Exposure to combined heat- and light-stresses induced both pathomorphologies, confirming that these stressors acted on the dinoflagellate via different mechanisms. Within 72 hours of exposure to heat and/or light stresses, homeostatic processes (e.g., heat-shock protein and anti-oxidant enzyme response) were evident in the remaining intact dinoflagellates, regardless of the initiating stressor. Understanding the sequence of events during bleaching when triggered by different environmental stressors is important for predicting both severity and consequences of coral bleaching.

Membrane development in purple photosynthetic bacteria in response to alterations in light intensity and oxygen tension

Studies on membrane development in purple bacteria during adaptation to alterations in light intensity and oxygen tension are reviewed. Anoxygenic phototrophic such as the purple α-proteobacterium Rhodobacter sphaeroides have served as simple, dynamic, and experimentally accessible model organisms for studies of the photosynthetic apparatus. A major landmark in photosynthesis research, which dramatically illustrates this point, was provided by the determination of the X-ray structure of the reaction center (RC) in Blastochloris viridis (Deisenhofer and Michel, EMBO J 8:2149-2170, 1989), once it was realized that this represented the general structure for the photosystem II RC present in all oxygenic phototrophs. This seminal advance, together with a considerable body of subsequent research on the light-harvesting (LH) and electron transfer components of the photosynthetic apparatus has provided a firm basis for the current understanding of how phototrophs acclimate to alterations in light intensity and quality. Oxygenic phototrophs adapt to these changes by extensive thylakoid membrane remodeling, which results in a dramatic supramolecular reordering to assure that an appropriate flow of quinone redox species occurs within the membrane bilayer for efficient and rapid electron transfer. Despite the high level of photosynthetic unit organization in Rba. sphaeroides as observed by atomic force microscopy (AFM), fluorescence induction/relaxation measurements have demonstrated that the addition of the peripheral LH2 antenna complex in cells adapting to low-intensity illumination results in a slowing of the rate of electron transfer turnover by the RC of up to an order of magnitude. This is ascribed to constraints in quinone redox species diffusion between the RC and cytochrome bc1 complexes arising from the increased packing density as the intracytoplasmic membrane (ICM) bilayer becomes crowded with LH2 rings. In addition to downshifts in light intensity as a paradigm for membrane development studies in Rba. sphaeroides, the lowering of oxygen tension in chemoheterotropically growing cells results in a gratuitous formation of the ICM by an extensive membrane biogenesis process. These membrane alterations in response to lowered illumination and oxygen levels in purple bacteria are under the control of a number of interrelated two-component regulatory circuits reviewed here, which act at the transcriptional level to regulate the formation of both the pigment and apoprotein components of the LH, RC, and respiratory complexes. We have performed a proteomic examination of the ICM development process in which membrane proteins have been identified that are temporally expressed both during adaptation to low light intensity and ICM formation at low aeration and are spatially localized in both growing and mature ICM regions. For these proteomic analyses, membrane growth initiation sites and mature ICM vesicles were isolated as respective upper-pigmented band (UPB) and chromatophore fractions and subjected to clear native electrophoresis for isolation of bands containing the LH2 and RC-LH1 core complexes. In chromatophores, increasing levels of LH2 polypeptides relative to those of the RC-LH1 complex were observed as ICM membrane development proceeded during light-intensity downshifts, along with a large array of other associated proteins including high spectral counts for the F1FO-ATP synthase subunits and the cytochrome bc1 complex, as well as RSP6124, a protein of unknown function, that was correlated with increasing LH2 spectral counts. In contrast, the UPB was enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane protein assembly factors confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination that give rise to the ICM. The changes in ICM vesicles were correlated to AFM mapping results (Adams and Hunter, Biochim Biophys Acta 1817:1616-1627, 2012), in which the increasing LH2 levels were shown to form densely packed LH2-only domains, representing the light-responsive antenna complement formed under low illumination. The advances described here could never have been envisioned when the author was first introduced in the mid-1960s to the intricacies of the photosynthetic apparatus during a lecture delivered in a graduate Biochemistry course at the University of Illinois by Govindjee, to whom this volume is dedicated on the occasion of his 80th birthday.

In vivo function of Tic22, a protein import component of the intermembrane space of chloroplasts

Preprotein import into chloroplasts depends on macromolecular machineries in the outer and inner chloroplast envelope membrane (TOC and TIC). It was suggested that both machineries are interconnected by components of the intermembrane space (IMS). That is, amongst others, Tic22, of which two closely related isoforms exist in Arabidopsis thaliana, namely atTic22-III and atTic22-IV. We investigated the function of Tic22 in vivo by analyzing T-DNA insertion lines of the corresponding genes. While the T-DNA insertion in the individual genes caused only slight defects, a double mutant of both isoforms showed retarded growth, a pale phenotype under high-light conditions, a reduced import rate, and a reduction in the photosynthetic performance of the plants. The latter is supported by changes in the metabolite content of mutant plants when compared to wild-type. Thus, our results support the notion that Tic22 is directly involved in chloroplast preprotein import and might point to a particular importance of Tic22 in chloroplast biogenesis at times of high import rates.

S1 domain-containing STF modulates plastid transcription and chloroplast biogenesis in Nicotiana benthamiana

• In this study, we examined the biochemical and physiological functions of Nicotiana benthamiana S1 domain-containing Transcription-Stimulating Factor (STF) using virus-induced gene silencing (VIGS), cosuppression, and overexpression strategies. • STF : green fluorescent protein (GFP) fusion protein colocalized with sulfite reductase (SiR), a chloroplast nucleoid-associated protein also present in the stroma. Full-length STF and its S1 domain preferentially bound to RNA, probably in a sequence-nonspecific manner. • STF silencing by VIGS or cosuppression resulted in severe leaf yellowing caused by disrupted chloroplast development. STF deficiency significantly perturbed plastid-encoded multimeric RNA polymerase (PEP)-dependent transcript accumulation. Chloroplast transcription run-on assays revealed that the transcription rate of PEP-dependent plastid genes was reduced in the STF-silenced leaves. Conversely, the exogenously added recombinant STF protein increased the transcription rate, suggesting a direct role of STF in plastid transcription. Etiolated seedlings of STF cosuppression lines showed defects in the light-triggered transition from etioplasts to chloroplasts, accompanied by reduced light-induced expression of plastid-encoded genes. • These results suggest that STF plays a critical role as an auxiliary factor of the PEP transcription complex in the regulation of plastid transcription and chloroplast biogenesis in higher plants.

GUN4-porphyrin complexes bind the ChlH/GUN5 subunit of Mg-Chelatase and promote chlorophyll biosynthesis in Arabidopsis

The GENOMES UNCOUPLED4 (GUN4) protein stimulates chlorophyll biosynthesis by activating Mg-chelatase, the enzyme that commits protoporphyrin IX to chlorophyll biosynthesis. This stimulation depends on GUN4 binding the ChlH subunit of Mg-chelatase and the porphyrin substrate and product of Mg-chelatase. After binding porphyrins, GUN4 associates more stably with chloroplast membranes and was proposed to promote interactions between ChlH and chloroplast membranes-the site of Mg-chelatase activity. GUN4 was also proposed to attenuate the production of reactive oxygen species (ROS) by binding and shielding light-exposed porphyrins from collisions with O₂. To test these proposals, we first engineered Arabidopsis thaliana plants that express only porphyrin binding-deficient forms of GUN4. Using these transgenic plants and particular mutants, we found that the porphyrin binding activity of GUN4 and Mg-chelatase contribute to the accumulation of chlorophyll, GUN4, and Mg-chelatase subunits. Also, we found that the porphyrin binding activity of GUN4 and Mg-chelatase affect the associations of GUN4 and ChlH with chloroplast membranes and have various effects on the expression of ROS-inducible genes. Based on our findings, we conclude that ChlH and GUN4 use distinct mechanisms to associate with chloroplast membranes and that mutant alleles of GUN4 and Mg-chelatase genes cause sensitivity to intense light by a mechanism that is potentially complex.

Nonesterified cholesterol content of lysosomes modulates susceptibility to oxidant-induced permeabilization

Reactive oxygen species (ROS) can induce lysosomal membrane permeabilization (LMP). Photoirradiation of murine hepatoma 1c1c7 cultures preloaded with the photosensitizer NPe6 generates singlet oxygen within acidic organelles and causes LMP and the activation of procaspases. Treatment with the cationic amphiphilic drugs (CADs) U18666A, imipramine, and clozapine stimulated the accumulation of filipin-stainable nonesterified cholesterol/sterols in late endosomes/lysosomes, but not in mitochondria. Concentration-response studies demonstrated an inverse relationship between lysosomal nonesterified cholesterol/sterol contents and susceptibility to NPe6 photoirradiation-induced intracellular membrane oxidation, LMP, and activation of procaspase-9 and -3. Similarly, the kinetics of restoration of NPe6 photoirradiation-induced LMP paralleled the losses of lysosomal cholesterol that occurred upon replating U18666A-treated cultures in CAD-free medium. Consistent with the oxidation of lysosomal cholesterol, filipin staining in U18666A-treated cultures progressively decreased with increasing photoirradiating light dose. U18666A also suppressed the induction of LMP and procaspase activation by exogenously added hydrogen peroxide. However, neither U18666A nor imipramine suppressed the induction of apoptosis by agents that did not directly induce LMP. These studies indicate that lysosomal nonesterified cholesterol/sterol content modulates susceptibility to ROS-induced LMP and possibly does so by being an alternative target for oxidants and lowering the probability of damage to other lysosomal membrane lipids and/or proteins.

Isolation and identification of cytoskeleton-associated prolamine mRNA binding proteins from developing rice seeds

The messenger RNA of the rice seed storage protein prolamine is targeted to the endoplasmic reticulum (ER) membranes surrounding prolamine protein bodies via a mechanism, which is dependent upon both RNA sorting signals and the actin cytoskeleton. In this study we have used an RNA bait corresponding to the previously characterized 5'CDS prolamine cis-localization sequence for the capture of RNA binding proteins (RBPs) from cytoskeleton-enriched fractions of developing rice seed. In comparison to a control RNA, the cis-localization RNA bait sequence led to the capture of a much larger number of proteins, 18 of which have been identified by tandem mass spectrometry. Western blots demonstrate that several of the candidate proteins analyzed to date show good to excellent specificity for binding to cis-localization sequences over the control RNA bait. Temporal expression studies showed that steady state protein levels for one RNA binding protein, RBP-A, paralleled prolamine gene expression. Immunoprecipitation studies showed that RBP-A is bound to prolamine and glutelin RNAs in vivo, supporting a direct role in storage protein gene expression. Using confocal immunofluorescence microscopy, RBP-A was found to be distributed to multiple compartments in the cell. In addition to the nucleus, RBP-A co-localizes with microtubules and is associated with cortical ER membranes. Collectively, these results indicate that employing a combination of in vitro binding and in vivo binding and localization studies is a valid strategy for the identification of putative prolamine mRNA binding proteins, such as RBP-A, which play a role in controlling expression of storage protein mRNAs in the cytoplasm.

The variegated mutants lacking chloroplastic FtsHs are defective in D1 degradation and accumulate reactive oxygen species

In the photosynthetic apparatus, a major target of photodamage is the D1 reaction center protein of photosystem II (PSII). Photosynthetic organisms have developed a PSII repair cycle in which photodamaged D1 is selectively degraded. A thylakoid membrane-bound metalloprotease, FtsH, was shown to play a critical role in this process. Here, the effect of FtsHs in D1 degradation was investigated in Arabidopsis (Arabidopsis thaliana) mutants lacking FtsH2 (yellow variegated2 [var2]) or FtsH5 (var1). Because these mutants are characterized by variegated leaves that sometimes complicate biochemical studies, we employed another mutation, fu-gaeri1 (fug1), that suppresses leaf variegation in var1 and var2 to examine D1 degradation. Two-dimensional blue native PAGE showed that var2 has less PSII supercomplex and more PSII intermediate lacking CP43, termed RC47, than the wild type under normal growth light. Moreover, our histochemical and quantitative analyses revealed that chloroplasts in var2 accumulate significant levels of reactive oxygen species, such as superoxide radical and hydrogen peroxide. These results indicate that the lack of FtsH2 leads to impaired D1 degradation at the step of RC47 formation in PSII repair and to photooxidative stress even under nonphotoinhibitory conditions. Our in vivo D1 degradation assays, carried out by nonvariegated var2 fug1 and var1 fug1 leaves, demonstrated that D1 degradation was impaired in different light conditions. Taken together, our results suggest the important role of chloroplastic FtsHs, which was not precisely examined in vivo. Attenuated D1 degradation in the nonvariegated mutants also suggests that leaf variegation seems to be independent of the PSII repair.

Light-induced dissociation of an antenna hetero-oligomer is needed for non-photochemical quenching induction

PsbS plays a major role in activating the photoprotection mechanism known as "non-photochemical quenching," which dissipates chlorophyll excited states exceeding the capacity for photosynthetic electron transport. PsbS activity is known to be triggered by low lumenal pH. However, the molecular mechanism by which this subunit regulates light harvesting efficiency is still unknown. Here we show that PsbS controls the association/dissociation of a five-subunit membrane complex, composed of two monomeric Lhcb proteins (CP29 and CP24) and the trimeric LHCII-M. Dissociation of this supercomplex is indispensable for the onset of non-photochemical fluorescence quenching in high light, strongly suggesting that protein subunits catalyzing the reaction of heat dissipation are buried into the complex and thus not available for interaction with PsbS. Consistently, we showed that knock-out mutants on two subunits participating to the B4C complex were strongly affected in heat dissipation. Direct observation by electron microscopy and image analysis showed that B4C dissociation leads to the redistribution of PSII within grana membranes. We interpreted these results to mean that the dissociation of B4C makes quenching sites, possibly CP29 and CP24, available for the switch to an energy-quenching conformation. These changes are reversible and do not require protein synthesis/degradation, thus allowing for changes in PSII antenna size and adaptation to rapidly changing environmental conditions.

Experimental evidence for ascorbate-dependent electron transport in leaves with inactive oxygen-evolving complexes

Previously, we showed that in barley (Hordeum vulgare) leaves with heat-inactivated oxygen-evolving complexes, photosystem II (PSII) has access to a large pool of alternative electron donors. Based on in vitro data, we proposed that this donor was ascorbate, yet this hypothesis has not been substantiated in vivo. In this paper, with the aid of chlorophyll a fluorescence induced by short (5-ms) light pulses and 820-nm absorbance transient measurements on wild-type and ascorbate-deficient (vtc2-1) mutant leaves of Arabidopsis (Arabidopsis thaliana), we show that in heat-treated leaves the rate of electron donation to PSII as well as the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive electron transport toward photosystem I depend on the ascorbate content of the leaves: upon ascorbate treatment, the donation half-time in the wild type and the mutant decreased from 25 to 22 ms and from 55 to 32 ms, respectively. Thermoluminescence measurements show that Tyr(Z)(+) is involved in the electron transfer from ascorbate to PSII. These data and the similar ascorbate dependencies of the heat-treated and the tris(hydroxymethyl)aminomethane-treated thylakoid membranes, with maximal donation half-times of about 16 ms, show that ascorbate is capable of supporting a sustained electron transport activity in leaves containing inactivated oxygen-evolving complexes. This alternative electron transport appears to be ubiquitous in the plant kingdom and is present in the green alga Chlamydomonas reinhardtii, and its rate depends on the physiological state of the plants and on environmental conditions. Our data suggest that ascorbate, as an alternative PSII electron donor, plays a physiological role in heat-stressed plants.

The role of lutein in the acclimation of higher plant chloroplast membranes to suboptimal conditions

Two mutants of Arabidopsis thaliana deficient in lutein have been investigated with respect to their responses to growth under a range of suboptimal conditions. The first mutant, lut1, was enriched in violaxanthin, antheraxanthin, zeaxanthin and zeinoxanthin compared with the wild-type (WT). In the second mutant, lut2, the lack of lutein was compensated for only by an increase in xanthophyll cycle (XC) carotenoids. Upon transfer of plants grown under optimal conditions to high light (HL), drought or HL + drought, both mutants acclimated during several days to the new conditions to the same extent as the WT. In contrast, transfer to chilling conditions (6 degrees C) for 6 days induced responses that were different between WT and mutants and between the mutants themselves. In contrast to the WT, the lut2 mutant in particular exhibited a large increase in the Chl a/b ratio and the XC pool size, extensive de-epoxidation and an enhanced extent of non-photochemical quenching. It is suggested that although the role of lutein in the structure and organisation of the light-harvesting complexes can be fulfilled by other xanthophylls under excess light conditions at optimal temperatures, this is not the case at low temperature.

Chlorophyllase is a rate-limiting enzyme in chlorophyll catabolism and is posttranslationally regulated

Chlorophyll is a central player in harvesting light energy for photosynthesis, yet the rate-limiting steps of chlorophyll catabolism and the regulation of the catabolic enzymes remain unresolved. To study the role and regulation of chlorophyllase (Chlase), the first enzyme of the chlorophyll catabolic pathway, we expressed precursor and mature versions of citrus (Citrus sinensis) Chlase in two heterologous plant systems: (1) squash (Cucurbita pepo) plants using a viral vector expression system; and (2) transiently transformed tobacco (Nicotiana tabacum) protoplasts. Expression of full-length citrus Chlase resulted in limited chlorophyll breakdown in protoplasts and no visible leaf phenotype in whole plants, whereas expression of a Chlase version lacking the N-terminal 21 amino acids (ChlaseDeltaN), which corresponds to the mature protein, led to extensive chlorophyll breakdown in both tobacco protoplasts and squash leaves. ChlaseDeltaN-expressing squash leaves displayed a dramatic chlorotic phenotype in plants grown under low-intensity light, whereas under natural light a lesion-mimic phenotype occurred, which was demonstrated to follow the accumulation of chlorophyllide, a photodynamic chlorophyll breakdown product. Full-length and mature citrus Chlase versions were localized to the chloroplast membrane fraction in expressing tobacco protoplasts, where processing of the N-terminal 21 amino acids appears to occur. Results obtained in both plant systems suggest that Chlase functions as a rate-limiting enzyme in chlorophyll catabolism controlled via posttranslational regulation.

Hsp70 protects against UVB induced apoptosis by preventing release of cathepsins and cytochrome c in human melanocytes

Stress-induced heat shock protein 70 (Hsp70) effectively protects cells against apoptosis, although the anti-apoptotic mechanism is still undefined. Exposure of human melanocytes to heat and subsequent UVB irradiation increased the level of Hsp70 and pre-heating reduced UVB induced apoptosis. Immunofluorescence staining of Hsp70 in combination with staining of lysosomes (Lamp2) or mitochondria (Mitotracker) in pre-heated UVB exposed cells showed co-localization of Hsp70 with both lysosomes and mitochondria in the surviving cell population. Furthermore, UVB induced apoptosis was accompanied by lysosomal and mitochondrial membrane permeabilization, detected as release of cathepsin D and cytochrome c, respectively, which were prevented by heat pre-treatment. In purified fractions of lysosomes and mitochondria, recombinant Hsp70 attached to both lysosomal and mitochondrial membranes. Moreover, in apoptotic cells Bax was translocated from a diffuse cytosolic location into punctate mitochondrial-like structures, which was inhibited by Hsp70 induction. Such inhibition of Bax translocation was abolished by transfection with Hsp70 siRNA. Furthermore, Hsp70 siRNA eliminated the apoptosis preventive effect observed after pre-heating. These findings show Hsp70 to rescue melanocytes from UVB induced apoptosis by preventing release of cathepsins from lysosomes, Bax translocation and cytochrome c release from mitochondria.

[Effect of chronic ionizing irradiation on the structural properties of the apical and mitochondrial membranes of small intestine enterocytes]

The different ways of the radiation-induced effect were revealed in the investigations of chronic ionizing radiation influence in total doses of 0.3, 0.6 and 1.0 Gy (0.0072 Gy/day) on the structural properties of the apical and of the mitochondrial membranes of small intestine enterocytes. The modification of the physical properties of the membrane surface area, the decrease of the structural order of the lipid component and conformational changes of the proteins were shown to be specific for the apical membrane. The disturbance of the dynamic properties and topography of the internal mitochondria membrane was revealed in the investigation of the inductive-resonance energy transfer between the pairs of the fluorophores: tryptophan-pyrene, tryptophan-ANS, pyrene-ANS.

Pro-oxidant effects of extremely low frequency electromagnetic fields in the land snail Helix aspersa

Pro-oxidant effects of extremely low frequency (ELF) 50-Hz magnetic fields were investigated in the land snail Helix aspersa exposed both in short-term laboratory treatments and under field conditions by maintaining the organisms in the proximity of a power line for up to 2 months. Oxidative perturbations were investigated as individual antioxidants (catalase, glutathione reductase, glutathione S-transferases, and total glutathione) and total scavenging capacity toward peroxyl radicals and hydroxyl radicals. Accumulation of lipid peroxidation products, destabilization of lysosomal membranes, and loss of DNA integrity were also evaluated as markers of cell damage. The overall results indicated an oxidative challenge caused by ELF magnetic fields with particularly prompt and sensitive responses for catalase, glutathione reductase, and the overall capability to neutralize peroxyl radicals. Cell injuries occurred to different extents according to duration and intensity of electromagnetic exposure and confirmed complex cause-effect relationships between pro-oxidant factors, efficiency of antioxidant defenses, and the onset of oxidative toxicity. This study highlights the importance of a multimarker approach for detecting a wide panel of biological responses, the necessity of investigating the long-term effects of early oxidative responses, and the role of ELF in enhancing susceptibility to other forms of pathologies or diseases.

Rapamycin-Sensitive Pathway Regulates Mitochondrial Membrane Potential, Autophagy, and Survival in Irradiated MCF-7 Cells

Radiation-induced inhibition of rapamycin-sensitive pathway and its effect on the cellular response to radiation were studied in the human breast cancer cell line MCF-7. Both radiation and rapamycin shared molecular targets and induced similar physiologic responses. Each of these treatments increased immunostaining of mammalian target of rapamycin (mTOR) in the nucleus, and radiation led to decreased phosphorylation of its autophosphorylation site Ser2481. In addition to dephosphorylation of established mTOR downstream effectors 4E-binding protein 1 and p70 ribosomal S6 kinase, both treatments decreased the level of eukaryotic initiation factor 4G. Experiments with the potentiometric dye, JC-1, revealed an oligomycin-dependent increase in mitochondrial membrane potential following radiation or rapamycin treatment, suggesting that both lead to reversal of F0F1ATPase activity. Both radiation and rapamycin induced sequestration of cytoplasmic material in autophagic vacuoles. In both cases, appearance of autophagic vacuoles involved the participation of microtubule-associated protein 1 light chain 3 (LC3). Transient cotransfection of green fluorescent protein-LC3 with either wild-type or dominant-negative mTOR further showed that inactivation of mTOR pathway is sufficient to induce autophagy in these cells. Finally, administration of rapamycin in combination with radiation led to enhanced mitochondria hyperpolarization, p53 phosphorylation, and increased cell death. Taken together, these experiments show that radiation-induced inhibition of rapamycin-sensitive pathway in MCF-7 cells causes changes in mitochondria metabolism, development of autophagy, and an overall decrease in cell survival.

Chloroquine-mediated radiosensitization is due to the destabilization of the lysosomal membrane and subsequent induction of cell death by necrosis

The anti-malarial drug chloroquine (CQ) is also thought to be a potential radiation sensitizer. To gain a better understanding of how the lysomotropic CQ can potentiate the effects of ionizing radiation, we investigated the effects of CQ on lysosomal and mitochondrial membrane stability, the subcellular localization of ceramide, plasma membrane permeability, and the mode of cell death in response to irradiation. We found that CQ accumulated in the lysosomes and thus lysosomal volumes increased. As a result, both the lysosomal and plasma membranes were destabilized. After 7 Gy irradiation, most ceramide was associated with the lysosomes in the cells treated with CQ but not in the CQ-untreated control. The elevated levels of ceramide in the lysosomes of the CQ-treated cells appeared to further destabilize the lysosomal and plasma membranes of the cell. Both CQ-treated and -untreated cells had approximately the same rate of cell death by apoptosis after 7 Gy irradiation (P > 0.05, ns). However, in contrast to the CQ-untreated control, the CQ-treated cells underwent massive cell death by necrosis at 24-48 h after irradiation (P < 0.05). Taken together, our data support the idea that the increase in cytotoxic effects by the combination of CQ and radiation is due to radiation-mediated apoptosis and CQ-mediated necrosis.

Gamma-irradiation induced apoptosis in peritoneal macrophages by oxidative stress. Implications of antioxidants in caspase mitochondrial pathway

The in vivo and in vitro development of apoptosis induced by gamma-irradiation was studied in mouse peritoneal macrophages. The apoptosis index was measured by fluorescence microscopy and DNA electrophoresis. In vivo apoptosis was greatest eight days after 8 Gy total body gamma-irradiation. A DNA ladder electrophoretic pattern was only observed in the gamma-irradiated group. The participation of reactive oxygen species in apoptosis induction was investigated by pretreating mice with the antioxidants superoxide dismutase, catalase, vitamin E or lipopolysaccharide before gamma-irradiation. Measurement of serum lipoperoxides showed oxidative stress in the gamma-irradiated mice and the protection given by the antioxidants. These results were confirmed using in vitro cultures of peritoneal macrophages: gamma-irradiated groups and antioxidant-pretreated gamma-irradiation groups showed results similar to those observed with in vivo irradiation. A loss of mitochondrial membrane potential (delta psi(m)) was also observed by microscopy in the gamma-irradiated cell cultures. Experiments with caspase inhibitors confirmed the participation of caspase 3 and caspase 9.

Light-directed gene delivery by photochemical internalisation

This article reviews a novel technology, named photochemical internalisation (PCI), for light-directed delivery of transgenes. Most gene therapy vectors are taken into the cell by endocytosis and, hence, are located in the endocytic vesicles. Although viral vectors have developed the means to escape from these vesicles, poor endosomal release is one of the major obstacles for non-viral vectors. PCI is a technology that allows liberation of the entrapped vectors carrying a gene in response to illumination. The method is based on chemical compounds (photosensitisers) that localise specifically in the membranes of endocytic vesicles and, following activation by light, induce the rupture of the vesicular membranes. The released transgenes can further be transferred to the nucleus, transcribed and translated. As gene liberation depends on light, enhancement of gene expression is achieved only at illuminated regions. PCI substantially improves gene transfer in vitro not only with non-viral gene vectors, but, surprisingly, also with adenoviruses and adeno-associated viruses. This article will review the background for the PCI technology and its role for gene delivery using both non-viral and viral vectors. Some aspects of the potential of PCI for site-specific gene delivery in therapeutic situations will also be discussed.

Phytosphingosine in combination with ionizing radiation enhances apoptotic cell death in radiation-resistant cancer cells through ROS-dependent and -independent AIF release

The use of chemical modifiers as radiosensitizers in combination with low-dose irradiation may increase the therapeutic effect on cancer by overcoming a high apoptotic threshold. Here, we showed that phytosphingosine treatment in combination with γ-radiation enhanced apoptotic cell death of radiation-resistant human T-cell lymphoma in a caspase-independent manner. Combination treatment induced an increase in intracellular reactive oxygen species (ROS) level, mitochondrial relocalization of B-cell lymphoma-2(Bcl-2)-associated X protein (Bax), poly-adenosine diphosphate (ADP)-ribose polymerase 1 (PARP-1) activation, and nuclear translocation of apoptosis-inducing factor (AIF). siRNA targeting of AIF effectively protected cells from the combination treatment-induced cell death. An antioxidant, N-acetyl-L-cysteine (NAC), inhibited Bax relocalization and AIF translocation but not PARP-1 activation. Moreover, transfection of Bax-siRNA significantly inhibited AIF translocation. Pretreatment of PARP-1 inhibitor, DPQ (3,4-dihydro-5-[4-(1-piperidinyl)-butoxy]-1(2H)-isoquinolinone), or PARP-1-siRNA also partially attenuated AIF translocation, whereas the same treatment did not affect intracellular ROS level and Bax redistribution. Taken together, these results demonstrate that enhancement of cell death of radiation-resistant cancer cells by phytosphingosine treatment in combination with γ-radiation is mediated by nuclear translocation of AIF, which is in turn mediated both by ROS-dependent Bax relocalization and ROS-independent PARP-1 activation. The molecular signaling pathways that we elucidated in this study may provide potential drug targets for radiation sensitization of cancers refractive to radiation therapy. (Blood. 2005;105:1724-1733)

Combined intracellular three-dimensional imaging and selective nanosurgery by a nonlinear microscope

We use near-IR femtosecond laser pulses for a combination of microscopy and nanosurgery on fluorescently labeled structures within living cells. Three-dimensional reconstructions of microtubule structures tagged with green fluorescent protein (GFP) are made during different phases of the cell cycle. Further, the microtubules are dissected using the same laser beam but with a higher laser power than for microscopy. We establish the viability of this technique for the cells of a fission yeast, which is a common model to study the mechanics of cell division. We show that nanosurgery can be performed with submicrometer precision and without visible collateral damage to the cell. The energy is primarily absorbed by the GFP molecules, and not by other native structures in the cell. GFP is particularly suitable for multiphoton excitation, as its excitation wavelength near 900 nm is benign for most cellular structures. The ability to use GFP to label structures for destruction by multiphoton excitation may be a valuable tool in cell biology.

Radiation protection of DNA and membranein vitro by extract ofHemidesmus indicus

Radioprotective effect of H. indicus root extract on lipid peroxidation in rat liver microsomes and plasmid DNA was examined. Hemidesmus indicus (HI) root extract was found to protect microsomal membranes as evident from reduction in lipid peroxidation values. The extract could also protect DNA from radiation induced strand breaks.

Cell Cycle Regulators Modulating Con A Mitogenesis and Apoptosis in Low-Dose Radiation-Exposed Mice

Low doses of ionizing radiation (LDR) are reported to induce transient changes in mouse lymphocytes, such as enhanced response to polyclonal T cell mitogens, increased expression of heat shock proteins, and p53. We evaluated the role of cell cycle proteins and apoptosis in lymphocytes of C57BL/6 mice exposed to 20 cGy fractionated LDR. We found an enhanced cell proliferation in response to the mitogen concanavalin A (con A). The expression of several cell cycle and apoptosis-related intracellular and extracellular proteins was analyzed by flowcytometry following labeling with specific antibodies. An increased response to con A was accompanied by an increase in the expression of proliferating cell nuclear antigen (PCNA) and cyclins D and A. The expression of cyclin B did not change significantly. In 20 cGy-exposed C57BL/6 mice, the caspase activity and apoptosis were reduced in con A-stimulated spleen cells as compared to sham controls. The expression of Fas and Fas ligand was analyzed by labeling with specific antibodies followed by flowcytometry. There was no change in the expression of Fas and Fas ligand. The change in the mitochondrial transmembrane potential was followed by labeling the cells with the dye 5,5',6,6',-tetrachloro-1,1,3,3-tetraethylbenzimidazolcarbocyanine iodide (JC-I) and analyzing by flowcytometry. Mitochondrial stability was increased in spleen cells of LDR-treated mice. These data suggest that LDR induces augmentation of mitogenic response by modulation of expression of cyclins and the mitochondrial membrane potential leading to reduced apoptosis.

Cryptochromes and phytochromes synergistically regulate Arabidopsis root greening under blue light

To increase their fitness, plants sense ambient light conditions and modulate their developmental processes by utilizing multiple photoreceptors such as phytochrome, cryptochrome and phototropin. Even roots, which are normally not exposed to light, express photoreceptors and can respond to light by developing chloroplasts. In the present study, root greening was observed in Arabidopsis thaliana. Seedlings were grown under monochromatic light and chlorophyll levels in the roots were determined. It was found that blue light was far more effective at inducing chloroplast development in Arabidopsis roots than was red light, and this response was under the control of a strong synergistic interaction between phytochromes and cryptochromes. As expected, the cry1 mutant was deficient in this response. Interestingly, the phyAphyB double mutant failed to respond to blue light under these conditions. This strongly suggests that either phytochrome A or phytochrome B, in addition to cryptochrome, was required for this blue light response. It was further demonstrated that the expression of photosynthetic genes was regulated in the same way. Dichromatic irradiation experiments indicated that this interaction depends on the level of phyB P(FR). Analysis of the cop1, det1 and hy5 mutants indicated that the corresponding factors were involved in the response.

[Hormonal regulation of the function of the myocyte intracellular membranes under radiation influence]

It was shown that the experimental hypofunction of thyroid, induced by thyroidectomy and strong gamma-irradiation with a dose of 1 Gy lead to disturbance of function and structure of membranes of sarcoplasmic reticulum of myocites in rats. Introduction in vivo of L-thyroxine rose functional capacity of membranes with insignificaut changes in their lipid bilayer.

Leukemic cell intracellular responses to nanosecond electric fields

Intense, nanosecond (ns) pulsed electric fields (PEFs) are known to affect the intracellular structures of cells. The probability of preferentially inducing subcellular effects increases with decreasing pulse length while effects on the plasma membrane are diminished. This has been demonstrated by applying electrical pulses of 60 and 10 ns duration with electric field intensities of up to 6.5 MV/m to HL-60 cells. Using confocal microscopy, PEF-induced changes in the integrity of the plasma membrane and nucleus were measured by recording fluorescence changes with propidium iodide (PI) and acridine orange (AO), respectively. Results suggest that high voltage, nsPEFs target the nucleus and modify cellular functions while plasma membrane effects are delayed and become smaller as pulse duration is shortened. Cell viability was not affected by these pulses. In spite of the high pulsed electric fields, thermal effects can be neglected because of the ultrashort pulse duration. The results suggest application of this ultrashort pulse technology to modulate nuclear structure and function for potential therapeutic benefit.

Reactive oxygen species-producing site in radiation-induced apoptosis of human peripheral T cells: involvement of lysosomal membrane destabilization

In our previous studies, we have partly elucidated the mechanism of radiation-induced apoptosis of human peripheral T cells. The exact site of the ROS (reactive oxygen species) formation induced by irradiation has been so far unknown. Therefore, in this study, we investigated the site of ROS formation by utilizing MitoCapture, H2DCFDA (succinimidyl ester of dichlorodihydrofluorescein diacetate), DAPI, and Lysosensor. Our results showed that ROS formation apparently originated in the mitochondria and/or lysosomes instead of in the nuclei of irradiated T cells. Moreover, lysosomal swelling and deformity, possibly revealing lysosomal membrane instability, were observed at 1 h after 5 Gy irradiation of T cells. At 4 h after irradiation of 5 Gy, increase of fluorescence around the lysosomes, possibly revealing lysosomal rupture, was seen. Based on the above results, we concluded the possible existence of a new apoptotic cascade involving early lysosomal membrane destabilization in radiation-induced apoptosis of human peripheral T cells. Therefore, possible involvement of lysosomal protease leakage caused by hydroxyl radical formation in lysosomes (possibly resulting in mitochondrial membrane dysfunction) is considered to play an important role in radiation-induced T cell apoptosis.

Nanoscale spatial induction of ultraviolet photoproducts in cellular DNA by three-photon near-infrared absorption

The high-resolution spatial induction of ultraviolet (UV) photoproducts in mammalian cellular DNA is a goal of many scientists who study UV damage and repair. Here we describe how UV photoproducts can be induced in cellular DNA within nanometre dimensions by near-diffraction-limited 750 nm infrared laser radiation. The use of multiphoton excitation to induce highly localized DNA damage in an individual cell nucleus or mitochondrion will provide much greater resolution for studies of DNA repair dynamics and intracellular localization as well as intracellular signalling processes and cell-cell communication. The technique offers an advantage over the masking method for localized irradiation of cells, as the laser radiation can specifically target a single cell and subnuclear structures such as nucleoli, nuclear membranes or any structure that can be labelled and visualized by a fluorescent tag. It also increases the time resolution with which migration of DNA repair proteins to damage sites can be monitored. We define the characteristics of localized DNA damage induction by near-infrared radiation and suggest how it may be used for new biological investigations.

[Dynamics of structural changes in rat liver after single dose of gamma-irradiation]

Histological changes and alterations in biophysical and biochemical parameters in liver of gamma-irradiate rats have been investigated. The gamma-irradiation of the whole body of rats with a single dose of 1 Gy did not cause any impairments of beam structure of rat liver, but resulted in the lymphocytic infiltrations of portal tracts which were not accompanied by formation of spotty areas of necrosis in adjacent areas of lever parenchyma. gamma-Irradiation stimulated proliferation of the hepatocytes and induced time-dependent mitochondrial structure lesions. Post-irradiation changes in cell cytoplasm appeared as disordering in reticulum-endothelial system, among them enlarging and fragmentation of its cisterns, cytoplasmic vacuolization, enhancement of the number of lysosomes and of the lipid inclusion contents. These facts revealed the mobilization of the additional energy resources for recovery of metabolic processes in rat liver. Post-irradiation increase of the level of the hepatocyte membrane lipid peroxidation products preceded liver morphological alterations. The membrane lipid microviscosity decreased in 1 and 3 days after irradiation. As a result of damages of hepatocyte membrane, the activity of the alanin- and asparagin-aminotransferases in blood serum increased 6 hours after. We can conclude that the whole body single gamma-irradiation with a dose of 1 Gy leads to the reversible but significant damages to the rat liver cell membrane structures. These damages might be the reason of radiation-induced liver morphological alterations.

[Isoflavone genistein-8-c-glycoside prevents the oxidative damages in structure and function of rat liver microsomal membranes]

Bioflavonoids (polyhydroxyphenols) are ubiquitous components of plants, fruits and vegetables; these compounds are efficient scavengers of free oxygen radicals and peroxides. The aim of this study was to investigate the antioxidant and radioprotective effects of genistein-8-C-glicoside (G8CG), an isoflavone, isolated from the flowers of Lipinus luteusl L. G8CG prevents dose-dependently the destruction of the cytochrome P-450 and its conversion to an inactive form cytochrome P-420, inhibits membrane lipid peroxidation and membrane SH-group oxidation in isolated rat liver microsomal membranes under tert-butylhydroperoxide-induced oxidative stress. Single whole-body gamma-irradiation (1 Gy) of rats results in blood plasma and liver microsomal membrane lipid peroxidation, impairments of microsomal membrane structure and function. Rat treatment with G8CG (75 mg/kg) developed the clear protective effect, stabilized membrane structure and improved the parameters of the monooxygenase function. We can conclude that G8CG can be used as antioxidant and radioprotective agent.

Overexpression of the transgene for manganese superoxide dismutase (MnSOD) in 32D cl 3 cells prevents apoptosis induction by TNF-alpha, IL-3 withdrawal, and ionizing radiation

OBJECTIVE

Stabilization of the mitochondria in IL-3-dependent hematopoietic progenitor cell line 32D cl 3 by overexpression of the transgene for manganese superoxide dismutase (MnSOD) prior to ionizing radiation prevents apoptosis. We now demonstrate that overexpression of the MnSOD transgene also protects 32D cl 3 cells from apoptosis caused by exposure to tumor necrosis factor-alpha (TNF-alpha) or withdrawal of interleukin (IL)-3.

MATERIALS AND METHODS

The hematopoietic progenitor cell line, 32D cl 3, and subclones overexpressing the human MnSOD transgene, 1F2 or 2C6, were radiated to 1000 cGy or were exposed to TNF-alpha (0 to 100 etag/mL) or were subjected to IL-3 withdrawal. The cells were then examined at several time points for DNA strand breaks using a comet assay, depolarization of the mitochondrial membrane, activation of caspase-3, PARP cleavage, and apoptosis, and also for changes in cell cycle distribution.

RESULTS

Overexpression of the transgene for MnSOD resulted in increased survival following exposure to radiation, exposure to TNF-alpha, or IL-3 withdrawal. The cell lines overexpressing MnSOD (1F2 or 2C6) displayed decreased radiation-induced, TNF-alpha-induced, or IL-3 withdrawal-induced mitochondrial membrane permeability, caspase-3 and PARP activation, and apoptosis.

CONCLUSIONS

Overexpression of the human MnSOD transgene in 32D cl 3 cells results in stabilization of the mitochondria and reduction in radiation-, TNF-alpha-, or IL-3 withdrawal-induced damage. Thus, MnSOD stabilization of the mitochondrial membrane is relevant to reduction of apoptosis by several classes of oxidative stress inducers.

Chloroplast membrane photostability in chlP transgenic tobacco plants deficient in tocopherols

The phototolerance of three chlP transgenic tobacco (Nicotiana tabacum) lines, affected in geranylgeranyl reductase and, hence, deficient in tocopherols (vitamin E), was estimated by in vivo luminescence and fluorescence measurements and was compared with that of the wild type (WT). Exposure of leaf discs to high light (1 mmol photon m(-2) s(-1)) and low temperature (10 degrees C) led to a rapid inhibition of photosystem II (PSII) photochemistry that showed little dependence on the tocopherol level. PSII photo-inhibition was followed by lipid peroxidation with a time delay of about 4 h, and this phenomenon was exacerbated in the tocopherol-deficient leaves. A linear correlation was observed in these short-term experiments between resistance to photooxidation and tocopherol content. When whole plants were exposed to the same treatment, PSII was severely photo-inhibited in mature leaves of all genotypes. Lipid peroxidation was also observed in all plants, but it occurred much more rapidly in tocopherol-deficient transgenic plants relative to WT plants. The time at which extensive lipid peroxidation occurred was correlated with the tocopherol content of the leaves. The present results show that tocopherols protect thylakoid membranes against photodestruction through lipid peroxidation. However, tocopherol deficiency was compensated in young, developing leaves that were able to photo-acclimate in the long term and did not suffer from photooxidative damage. Soluble antioxidants (glutathione and ascorbate) did not accumulate in photo-acclimated chlP transgenic leaves relative to WT leaves. In contrast, a selective accumulation of xanthophyll cycle pigments was observed in young transgenic leaves, and this could represent a compensatory mechanism for tocopherol deficiency.

Near-IR irradiation of the S2 state of the water oxidizing complex of photosystem II at liquid helium temperatures produces the metalloradical intermediate attributed to S1Y(Z*)

Near-IR (NIR) excitation at liquid He temperatures of photosystem II (PSII) membranes from the cyanobacterium Synechococcus vulcanus or from spinach poised in the S2 state results in the production of a g = 2.035 EPR resonance, reminiscent of metalloradical signals. The signal is smaller in the spinach preparations, but it is significantly enhanced by the addition of exogenous quinones. Ethanol (2-3%, v/v) eliminates the ability to trap the signal. The g = 2.035 signal is identical to the one recently obtained by Nugent et al. by visible-light illumination of the S1 state, and preferably assigned to S1Y(Z*) [Nugent, J. H. A., Muhiuddin, I. P., and Evans, M. C. W. (2002) Biochemistry 41, 4117-4126]. The production of the g = 2.035 signal by liquid He temperature NIR excitation of the S2 state is paralleled by a significant reduction (typically 40-45% in S. vulcanus) of the S2 state multiline signal. This is in part due to the conversion of the Mn cluster to higher spin states, an effect documented by Boussac et al. [Boussac, A., Un, S., Horner, O., and Rutherford, A. W. (1998) Biochemistry 37, 4001-4007], and in part due to the conversion to the g = 2.035 configuration. Following the decay of the g = 2.035 signal at liquid helium temperatures (decay halftimes in the time range of a few to tens of minutes depending on the preparation), annealing at elevated temperatures (-80 degrees C) results in only partial restoration of the S2 state multiline signal. The full size of the signal can be restored by visible-light illumination at -80 degrees C, implying that during the near-IR excitation and subsequent storage at liquid helium temperatures recombination with Q(A-) (and therefore decay of the S2 state to the S1 state) occurred in a fraction of centers. In support of this conclusion, the g = 2.035 signal remains stable for several hours (at 11 K) in centers poised in the S2...Q(A) configuration before the NIR excitation. The extended stability of the signal under these conditions has allowed the measurement of the microwave power saturation and the temperature dependence in the temperature range of 3.8-11 K. The signal intensity follows Curie law temperature dependence, which suggests that it arises from a ground spin state, or a very low-lying excited spin state. The P1/2 (microwave power at half-saturation) value is 1.7 mW at 3.8 K and increases to 96 mW at 11 K. The large width of the g = 2.035 signal and its relatively fast relaxation support the assignment to a radical species in the proximity of the Mn cluster. The whole phenomenology of the g = 2.035 signal production is analogous to the effects of NIR excitation on the S3 state [Ioannidis, N., Nugent, J. H. A., and Petrouleas, V. (2002) Biochemistry 41, 9589-9600] producing an S2'Y(Z*) intermediate. In the present case, the intermediate is assigned to S1Y(Z*). The NIR-induced increase in the oxidative capability of the Mn cluster is discussed in relation to the photochemical properties of a Mn(III) ion that exists in both S2 and S3 states. The EPR properties of the S1Y(Z*) intermediate cannot be reconciled easily with our current understanding of the magnetic properties of the S1 state. It is suggested that oxidation of tyr Z alters the magnetic properties of the Mn cluster via exchange of a proton.

Light-harvesting ionic dendrimer porphyrins as new photosensitizers for photodynamic therapy

Photodynamic therapy (PDT) is a promising therapeutic modality for treatment of solid tumors. In this study, third-generation aryl ether dendrimer porphyrins (DPs) with either 32 quaternary ammonium groups (32(+)DPZn) or 32 carboxylic groups (32(-)DPZn) were evaluated as a novel, supramolecular class of photosensitizers for PDT. DPs showed a different cell-association profile depending on the positive or negative charge on the periphery, and both DPs eventually localized in membrane-limited organelles. In contrast, protoporphyrin IX (PIX), which is a hydrophobic and relatively low molecular weight photosensitizer used as a control in this study, diffused through the cytoplasm except the nucleus. Confocal fluorescent imaging using organelle-specific dyes indicated that PIX induced severe photodamage to disrupt membranes and intracellular organelles, including the plasma membrane, mitochondrion, and lysosome. On the other hand, cells treated with DPs kept the characteristic fluorescent pattern of such organelles even after photoirradiation. However, notably 32(+)DPZn achieved remarkably higher (1)O(2)-induced cytotoxicity against LLC cells than PIX. Furthermore, both dendrimer porphyrins had far lower dark toxicity as compared with PIX, demonstrating their highly selective photosensitizing effect in combination with a reduced systemic toxicity.

Protection of DNA and microsomal membranes in vitro by Glycyrrhiza glabra L. against gamma irradiation

The radioprotective effect of the root extract of Glycyrrhiza glabra L on lipid peroxidation in rat liver microsomes and plasmid pBR322 DNA was investigated. The extract was found to protect microsomal membranes, as evident from reduction in lipid peroxidation, and could also protect plasmid DNA from radiation-induced strand breaks.

MEK/ERK pathway protects ionizing radiation-induced loss of mitochondrial membrane potential and cell death in lymphocytic leukemia cells

MEK/ERK-mediated signals have recently been found to inhibit Fas-mediated cell death through inhibition of caspase-8 activity. It remains unknown whether MEK/ERK-mediated signals affect ionizing radiation (IR)-induced cell death. Here we demonstrate that MEK/ERK-mediated signals selectively inhibit IR-induced loss of mitochondrial membrane potential (DeltaPsi(m)) and subsequent cell death. In Jurkat cells, TPA strongly activated ERK and inhibited the IR-induced caspase-8/Bid cleavage and the loss of DeltaPsi(m). The inhibitory effect of TPA was mostly abrogated by pretreatment of a specific MEK inhibitor PD98059, indicating that the effect depends upon MEK/ERK-mediated signals. Moreover, BAF-B03 transfectants expressing IL-2 receptor (IL-2R) beta(c) chain lacking the acidic region, which is responsible for MEK/ERK-mediated signals, revealed higher sensitivity to IR than the transfectants expressing wild-type IL-2R. Interestingly, the signals could neither protect the DeltaPsi(m) loss nor cell death in UV-irradiated cells. These data imply that the anti-apoptotic effect of MEK/ERK-mediated signals appears to selectively inhibit the IR-induced cell death through protection of the DeltaPsi(m) loss. Our data enlighten an anti-apoptotic function of MEK/ERK pathway against IR-induced apoptosis, thereby implying its contribution to radioresistance.

Human fibroblast alterations induced by low power laser irradiation at the single cell level using confocal microscopy

Low power laser irradiation is regarded to have a significant role in triggering cellular proliferation and in treating diseases of diverse etiologies. The present work contributes to the understanding of the mechanisms of action by studying low power laser effects in human fibroblasts. Confocal laser scanning microscopy is used for irradiation and observation of the same area of interest allowing the imaging of laser effects at the single cell level and in real time. Coverslip cultures were placed in a small incubation chamber for in vivo microscopic observation. Laser stimulation of the cells was performed using the 647 nm line of the confocal laser through the objective lens of the microscope. Mitochondrial membrane potential (delta psi(m)), intracellular pH, calcium alterations and generation of reactive oxygen species (ROS) were monitored using specific fluorescent vital probes. The induced effects were quantified using digital image processing techniques. After laser irradiation, a gradual alkalinization of the cytosolic pH and an increase in mitochondrial membrane potential were observed. Recurrent spikes of intracellular calcium concentration were also triggered by laser. Reactive oxygen species were generated as a result of biostimulation. No such effects were monitored in microscopic fields other than the irradiated ones.

PUVA-induced apoptosis involves mitochondrial dysfunction caused by the opening of the permeability transition pore

The mechanism of cell death was investigated in Jurkat cells exposed to the combination of psoralen and UVA irradiation (PUVA). Apoptosis was by far prevailing over necrosis and involved mitochondrial dysfunction. The collapse of mitochondrial membrane potential, appears to be caused by the opening of the mitochondrial permeability transition pore since its inhibitor, cyclosporin A, prevented mitochondrial dysfunction and largely attenuated apoptosis. Apoptosis also occurred in cells treated with the photoproducts generated by irradiating psoralen in vitro with an oxygen-dependent process. Thus, the involvement of reactive oxygen species in the onset of PUVA-induced apoptosis appears mostly related to psoralen photooxidation.

Blocking of electron donation by Mn(II) to Y(Z*) following incubation of Mn-depleted photosystem II membranes with Fe(II) in the light

The donation of electrons by Mn(II) and Fe(II) to Y(Z*) through the high-affinity (HA(Z)) site in Mn-depleted photosystem II (PSII) membranes has been studied by flash-probe fluorescence yield measurements. Mn(II) and Fe(II) donate electrons to Y(Z*) with about the same efficiency, saturating this reaction at the same concentration (ca. 5 microM). However, following a short incubation of the membranes with 5 microM Fe(II), but not with Mn(II) in room light, added Mn(II) or Fe(II) can no longer be photooxidized by Y(Z)(*). This blocking effect is caused by specifically bound, photooxidized Fe [> or =Fe(III)] and is accompanied by a delay in the fluorescence yield decay kinetics attributed to the slowing down of the charge recombination rate between Q(a-) and Y(Z*). Exogenously added Fe(III), on the other hand, does not donate electrons to Y(Z*), does not block the donation of electrons by added Mn(II) and Fe(II), and does not change the kinetics of the decay of the fluorescence yield. These results demonstrate that the light-dependent oxidation of Fe(II) by Y(Z*) creates an Fe species that binds at the HA(Z) site and causes the blocking effect. The pH dependence of Mn(II) electron donation to Y(Z*) via the HA(Z) site and of the Fe-blocking effect is different. These results, together with sequence homologies between the C-terminal ends of the D1 and D2 polypeptides of the PSII reaction center and several diiron-oxo enzymes, suggest the involvement of two or perhaps more (to an upper limit of four to five) bound iron cations per reaction center of PSII in the blocking effect. Similarities in the interaction of Fe(II) and Mn(II) with the HA(Z) Mn site of PSII during the initial steps of the photoactivation process are discussed. The Fe-blocking effect was also used to investigate the relationship between the HA(Z) Mn site and the HA sites on PSII for diphenylcarbazide (DPC) and NH2OH oxidation. Blocking of the HA(Z) site with specifically bound Fe leads to the total inhibition of electron donation to Y(Z*) by DPC. Since DPC and Mn(II) donation to PSII is noncompetitive [Preston, C., and Seibert, M. (1991) Biochemistry 30, 9615-9624], the Fe bound to the HA(Z) site can also block the DPC donation site. On the other hand, electron donation by NH2OH to PSII still occurs in Fe-blocked membranes. Since hydroxylamine does not reduce the Fe [> or =Fe(III)] specifically bound to the HA(Z) site, NH2OH must donate to Y(Z*) through its own site or directly to P680+.

Antiapoptotic effects induced by different wavelengths of ultraviolet light

Cells receive signals for survival as well as for death, and the balance between the two ultimately determines the fate of the cells. UV-triggered apoptotic signaling has been well documented, whereas UV-induced survival effects have received little attention. We have reported previously that UVB irradiation prevented apoptosis, which is partly dependent on activation of the phosphatidylinositol 3-kinase (PI3-kinase)-Akt pathway (Ibuki Y. and Goto, R. [2000] Biochem. Biophys. Res. Commun. 279, 872-878). In this study, antiapoptotic effects and survival signals of UV with different wavelength ranges, UVA, UVB and UVC, were examined. NIH3T3 cells showed apoptotic cell death by detachment from the extracellular matrix under serum-free conditions, which was prevented by all wavelengths. However, the effect of UVA was less than those of UVB and UVC, as determined by metabolism of fluoresceine diacetate and the appearance of chromatin-condensed cells. Furthermore, the effects of three wavelengths of UV on the apoptotic pathway upstream of the nuclear signals were examined. Reduction of mitochondrial transmembrane potential (delta psi) and activation of caspase-9 and -3 were suppressed by all three wavelengths of UV, showing wavelength-dependent effects as mentioned previously. Shorter wavelengths showed stronger inhibitory effects on caspase-8 activity. The P13-kinase inhibitor wortmannin partially inhibited the UVB- and UVC-induced suppression of apoptosis but not the inhibitory effect of UVA. Furthermore, normal delta psi maintained by UVA was not changed in the presence of wortmannin, but those by UVB and UVC were reduced. Akt was clearly phosphorylated by all three wavelengths. The phosphorylation by UVB and UVC was completely inhibited by addition of wortmannin, but that by UVA was not, in agreement with the results of survival and of delta psi. These results suggested the existence of two different survival pathways leading to suppression of apoptosis, one for UVA that is independent of the PI3-kinase-Akt pathway and the other for UVB and UVC that is dependent on this pathway.

Manganese superoxide dismutase (SOD2) inhibits radiation-induced apoptosis by stabilization of the mitochondrial membrane

To define the molecular pathways involved in radiation-induced apoptosis and the role of the mitochondria, 32D cl 3 hematopoietic cells and subclones overexpressing either the human manganese superoxide dismutase (SOD2) transgene (1F2 and 2C6) or BCL2L1 (also known as Bcl-xl) transgene (32D-Bcl-xl) were compared for their response to radiation at the subcellular level, comparing nuclear to mitochondrial localized pathways. All cell lines showed complete detectable DNA repair by 30 min after irradiation, and clearly delayed migration of BAX and active stress-activated protein (SAP) kinases MAPK1 (also known as p38) and MAPK8 (also known as JNK1) to the mitochondria at 3 h. Radioresistant clonal lines 1F2, 2C6 and 32D-Bcl-xl showed significant decreases in mitochondrial membrane permeability, cytochrome C release, caspase 3 and poly(adenosine diphosphate-ribose) polymerase (PARP) activation at 6-12 h, and in apoptosis at 24 h. Since the nuclear-to-cytoplasm events preceding the release of cytochrome C were similar in all cell lines, and increased expression of either the SOD2 or the BCL2L1 transgene provided radiation protection, we conclude that events at the level of the mitochondria are critically involved in radiation-induced apoptosis.

Photochemical internalisation: a novel drug delivery system

The present report reviews a number of recently published papers on a novel technology for the cytosolic delivery of macromolecules named photochemical internalisation (PCI). PCI is based upon the light activation of a drug (a photosensitizer) specifically located in the membrane of endocytic vesicles. Light which is absorbed by the photosensitizer induces the formation of reactive oxygen species, of which singlet oxygen ((1)O(2)) is the predominant form. Singlet oxygen oxidizes biomolecules in the membranes of endosomes and lysosomes, resulting in a subsequent release of the contents of these compartments into the cytosol. Photosensitizers have a higher affinity for tumour tissues than for most normal tissues and are used in photodynamic therapy of various types of cancers. We have taken advantage of the PCI strategy to enhance the delivery of a variety of macromolecules, including ribosome-inactivating toxins, an immunotoxin, horse radish peroxidase, a ras peptide, RNA, oligonucleotides and protein encoding DNA, to the cytosol. Normally, a major intracellular barrier to the application of therapeutically interesting peptides and proteins or the application of DNA and RNA in gene therapy is the degradation of the macromolecules in the endocytic vesicles after uptake by endocytosis. Therefore, a photochemically induced rupture of endocytic vesicles and the subsequent cytosolic release of the macromolecules aids these molecules in escaping attack by the lysosomal hydrolases, thereby maintaining their biological activity. Thus, PCI represents a novel principle for the cytosolic delivery of biologically active macromolecules which overcomes the pivotal intracellular barrier of endosomes and lysosomes. In addition to being utilised as a new site-specific cancer therapy method, PCI can also be applied as a research tool for macromolecule delivery both in vitro and in vivo.

Enhancement of lysosomal osmotic sensitivity induced by the photooxidation of membrane thiol groups

The osmotic lysis of photodamaged lysosomes is a critical event for killing tumor cells. How the photodamage increases lysosomal osmotic sensitivity is still unclear. In this work, the effect of the photooxidation of membrane thiol groups on the lysosomal osmotic sensitivity was studied by measuring the thiol groups with 5,5'-dithiobis(2-nitrobenzoic acid) and examining the lysosomal beta-hexosaminidase latency loss in a hypotonic sucrose medium. The results show that methylene blue-mediated photooxidation of lysosomes decreased their membrane thiol groups and produced cross-linkage of membrane proteins (molecular weight ranging from 75000 to 125000), which was visualized by sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Simultaneously, the lysosomal osmotic sensitivity increased. These photoinduced alterations of the lysosomes could be recovered by reducing the oxidized thiol groups with dithiothreitol. It indicates that the photooxidation of membrane thiol groups can increase the lysosomal osmotic sensitivity and therefore provides a new explanation for the photoinduced lysosomal lysis.

Early events in the apoptotic cascade initiated in cells treated with medium from the progeny of irradiated cells

Genomic instability and the bystander effect have recently been linked experimentally. It has previously been shown that medium from irradiated cells can induce early events in the apoptotic cascade, such as mobilisation of intracellular calcium, loss of mitochondrial membrane potential and an increase in reactive oxygen species, in unhit cells. The aim of this study was to determine if medium from the progeny of irradiated cells could also initiate apoptosis in unhit cells. Human keratinocytes were irradiated (0.5 and 5 Gy) and medium was harvested up to the 7th passage post-irradiation and transferred to unirradiated keratinocytes. Intracellular calcium levels, mitochondrial membrane potential and the level of reactive oxygen species were all monitored for a period of 24 h following medium transfer. Rapid calcium fluxes (within 30 s), loss of mitochondrial membrane potential and increases in reactive oxygen species (from 6 h after medium transfer) were observed. There was no significant difference between medium generated by cells irradiated at the different doses. The data suggest that initiating events in the apoptotic cascade were induced in unhit cells by a signal produced by irradiated cells and that this signal can still be produced in the progeny of irradiated cells.