Human T cell cyclophilin18 binds to thiol-specific antioxidant protein Aop1 and stimulates its activity

Targeting Extracellular Cyclophilin A Reduces Neuroinflammation and Extends Survival in a Mouse Model of Amyotrophic Lateral Sclerosis

Neuroinflammation is a major hallmark of amyotrophic lateral sclerosis (ALS), which is currently untreatable. Several anti-inflammatory compounds have been evaluated in patients and in animal models of ALS, but have been proven disappointing in part because effective targets have not yet been identified. Cyclophilin A, also known as peptidylprolyl cis-/trans-isomerase A (PPIA), as a foldase is beneficial intracellularly, but extracellularly has detrimental functions. We found that extracellular PPIA is a mediator of neuroinflammation in ALS. It is a major inducer of matrix metalloproteinase 9 and is selectively toxic for motor neurons. High levels of PPIA were found in the CSF of SOD1^G93A mice and rats and sporadic ALS patients, suggesting that our findings may be relevant for familial and sporadic cases. A specific inhibitor of extracellular PPIA, MM218, given at symptom onset, rescued motor neurons and extended survival in the SOD1^G93A mouse model of familial ALS by 11 d. The treatment resulted in the polarization of glia toward a prohealing phenotype associated with reduced NF-κB activation, proinflammatory markers, endoplasmic reticulum stress, and insoluble phosphorylated TDP-43. Our results indicates that extracellular PPIA is a promising druggable target for ALS and support further studies to develop a therapy to arrest or slow the progression of the disease in patients.SIGNIFICANCE STATEMENT We provide evidence that extracellular cyclophilin A, also known as peptidylprolyl cis-/trans-isomerase A (PPIA), is a mediator of the neuroinflammatory reaction in amyotrophic lateral sclerosis (ALS) and is toxic for motor neurons. Supporting this, a specific extracellular PPIA inhibitor reduced neuroinflammation, rescued motor neurons, and extended survival in the SOD1^G93A mouse model of familial ALS. Our findings suggest selective pharmacological inhibition of extracellular PPIA as a novel therapeutic strategy, not only for SOD1-linked ALS, but possibly also for sporadic ALS. This approach aims to address the neuroinflammatory reaction that is a major hallmark of ALS. However, given the complexity of the disease, a combination of therapeutic approaches may be necessary.

Mitochondrial Thioredoxin System as a Modulator of Cyclophilin D Redox State

The mitochondrial thioredoxin system (NADPH, thioredoxin reductase, thioredoxin) is a major redox regulator. Here we have investigated the redox correlation between this system and the mitochondrial enzyme cyclophilin D. The peptidyl prolyl cis-trans isomerase activity of cyclophilin D was stimulated by the thioredoxin system, while it was decreased by cyclosporin A and the thioredoxin reductase inhibitor auranofin. The redox state of cyclophilin D, thioredoxin 1 and 2 and peroxiredoxin 3 was measured in isolated rat heart mitochondria and in tumor cell lines (CEM-R and HeLa) by redox Western blot analysis upon inhibition of thioredoxin reductase with auranofin, arsenic trioxide, 1-chloro-2,4-dinitrobenzene or after treatment with hydrogen peroxide. A concomitant oxidation of thioredoxin, peroxiredoxin and cyclophilin D was observed, suggesting a redox communication between the thioredoxin system and cyclophilin. This correlation was further confirmed by i) co-immunoprecipitation assay of cyclophilin D with thioredoxin 2 and peroxiredoxin 3, ii) molecular modeling and iii) depleting thioredoxin reductase by siRNA. We conclude that the mitochondrial thioredoxin system controls the redox state of cyclophilin D which, in turn, may act as a regulator of several processes including ROS production and pro-apoptotic factors release.

Fish Peroxiredoxins and Their Role in Immunity

Peroxiredoxins (Prxs) are a family of antioxidant enzymes that protect cells from oxidative damage. In addition, Prxs may act as modulators of inflammation, protect against cell death and tumour progression, and facilitate tissue repair after damage. The most studied roles of Prx1 and Prx2 are immunological. Here we present a review on the effects of some immunostimulant treatments and bacterial, viral, or parasitic infections on the expression of fish Prxs at the gene and/or protein level, and point to their important role in immunity. The Prxs show antioxidant activity as well as a protective effect against infection. Some preliminary data are presented about the role of fish Prx1 and Prx2 in virus resistance although further studies are needed before the role of fish Prx in immunity can be definitively defined.

Hyperoxidized peroxiredoxin 2 interacts with the protein disulfide- isomerase ERp46

Prx (peroxiredoxin) 2 protects cells from deleterious oxidative damage. It catalyses the breakdown of hydroperoxides through a highly reactive cysteine residue and has been linked to chaperone activity that promotes cell survival under conditions of oxidative stress. It may also be involved in redox signalling by binding to other proteins. In the present study we have searched for binding partners of Prx2 in H2O2-treated Jurkat and human umbilical vein endothelial cells and discovered that the hyperoxidized form selectively co-precipitated with the protein disulfide-isomerase ERp46. Mutant analyses revealed that loss of the peroxidative cysteine residue of Prx2 also facilitated complex formation with ERp46, even without H2O2 treatment, whereas the resolving cysteine residue of Prx2 was indispensible for the interaction to occur. The complex involved a stable non-covalent interaction that was disassociated by the reduction of intramolecular disulfides in ERp46, or by disruption of the decameric structure of hyperoxidized Prx2. This is the first example of a protein interaction dependent on the hyperoxidized status of a Prx.

Principles in redox signaling: from chemistry to functional significance

Reactive oxygen and nitrogen species are currently considered not only harmful byproducts of aerobic respiration but also critical mediators of redox signaling. The molecules and the chemical principles sustaining the network of cellular redox regulated processes are described. Special emphasis is placed on hydrogen peroxide (H(2)O(2)), now considered as acting as a second messenger, and on sulfhydryl groups, which are the direct targets of the oxidant signal. Cysteine residues of some proteins, therefore, act as sensors of redox conditions and are oxidized in a reversible reaction. In particular, the formation of sulfenic acid and disulfide, the initial steps of thiol oxidation, are described in detail. The many cell pathways involved in reactive oxygen species formation are reported. Central to redox signaling processes are the glutathione and thioredoxin systems controlling H(2)O(2) levels and, hence, the thiol/disulfide balance. Lastly, some of the most important redox-regulated processes involving specific enzymes and organelles are described. The redox signaling area of research is rapidly expanding, and future work will examine new pathways and clarify their importance in cellular pathophysiology.

Uncaria rhynchophylla Upregulates the Expression of MIF and Cyclophilin A in Kainic Acid-Induced Epilepsy Rats: A Proteomic Analysis

Uncaria rhynchophylla (Miq) Jack (UR) is a traditional Chinese herb and is used for the treatment of convulsive disorders, including epilepsy. Our previous study has shown that UR, as well as its major component rhynchophylline (RH), has an anticonvulsive effect and this effect is closely related to its scavenging activities of oxygen free radicals. The purpose of the present study was to investigate the effect of (UR) on the expression of proteins using a proteomics analysis in Sprague-Dawley (SD) rats with kainic acid (KA)-induced epileptic seizures. We profiled the differentially expressed proteins on two-dimensional electrophoresis (2-DE) maps derived from the frontal cortex and hippocampus of rat brain tissue 24 hours after KA-induced epileptic seizures. The results indicated that macrophage migration inhibitory factor (MIF) and cyclophilin A were under expressed in frontal cortex by an average of 0.19- and 0.23-fold, respectively. In the frontal cortex, MIF and cyclophilin A were significantly decreased in the KA group and these decreases were confirmed by the Western blots. However, in the hippocampus, only cyclophilin A was significantly decreased in the KA group. In addition, in real-time quantitative PCR (Q-PCR), MIF and cyclophilin A gene expressions were also significantly under expressed in the frontal cortex, and only the cyclophilin A gene was also significantly under expressed in the hippocampus in the KA group. These under expressions of MIF and cyclophilin A could be overcome by the treatment of UR and RH. In conclusion, the under expressions of MIF and cyclophilin A in the frontal cortex and hippocampus in KA-treated rats, which were overcome by both UR and UH treatment, suggesting that both MIF and cyclophilin A at least partly participate in the anticonvulsive effect of UR.

Novel roles of peroxiredoxins in inflammation, cancer and innate immunity

Peroxiredoxins possess thioredoxin or glutathione peroxidase and chaperone-like activities and thereby protect cells from oxidative insults. Recent studies, however, reveal additional functions of peroxiredoxins in gene expression and inflammation-related biological reactions such as tissue repair, parasite infection and tumor progression. Notably, peroxiredoxin 1, the major mammalian peroxiredoxin family protein, directly interacts with transcription factors such as c-Myc and NF-κB in the nucleus. Additionally, peroxiredoxin 1 is secreted from some cells following stimulation with TGF-β and other cytokines and is thus present in plasma and body fluids. Peroxiredoxin 1 is now recognized as one of the pro-inflammatory factors interacting with toll-like receptor 4, which triggers NF-κB activation and other signaling pathways to evoke inflammatory reactions. Some cancer cells release peroxiredoxin 1 to stimulate toll-like receptor 4-mediated signaling for their progression. Interestingly, peroxiredoxins expressed in protozoa and helminth may modulate host immune responses partly through toll-like receptor 4 for their survival and progression in host. Extracellular peroxiredoxin 1 and peroxiredoxin 2 are known to enhance natural killer cell activity and suppress virus-replication in cells. Peroxiredoxin 1-deficient mice show reduced antioxidant activities but also exhibit restrained tissue inflammatory reactions under some patho-physiological conditions. Novel functions of peroxiredoxins in inflammation, cancer and innate immunity are the focus of this review.

Decarbonylated cyclophilin A Cpr1 protein protects Saccharomyces cerevisiae KNU5377Y when exposed to stress induced by menadione

Cyclophilins are conserved cis-trans peptidyl-prolyl isomerase that are implicated in protein folding and function as molecular chaperones. The accumulation of Cpr1 protein to menadione in Saccharomyces cerevisiae KNU5377Y suggests a possibility that this protein may participate in the mechanism of stress tolerance. Stress response of S. cerevisiae KNU5377Y cpr1Δ mutant strain was investigated in the presence of menadione (MD). The growth ability of the strain was confirmed in an oxidant-supplemented medium, and a relationship was established between diminishing levels of cell rescue enzymes and MD sensitivity. The results demonstrate the significant effect of CPR1 disruption in the cellular growth rate, cell viability and morphology, and redox state in the presence of MD and suggest the possible role of Cpr1p in acquiring sensitivity to MD and its physiological role in cellular stress tolerance. The in vivo importance of Cpr1p for antioxidant-mediated reactive oxygen species (ROS) neutralization and chaperone-mediated protein folding was confirmed by analyzing the expression changes of a variety of cell rescue proteins in a CPR1-disrupted strain. The cpr1Δ to the exogenous MD showed reduced expression level of antioxidant enzymes, molecular chaperones, and metabolic enzymes such as nicotinamide adenine dinucleotide phosphate (NADPH)- or adenosine triphosphate (ATP)-generating systems. More importantly, it was shown that cpr1Δ mutant caused imbalance in the cellular redox homeostasis and increased ROS levels in the cytosol as well as mitochondria and elevated iron concentrations. As a result of excess ROS production, the cpr1Δ mutant provoked an increase in oxidative damage and a reduction in antioxidant activity and free radical scavenger ability. However, there was no difference in the stress responses between the wild-type and the cpr1Δ mutant strains derived from S. cerevisiae BY4741 as a control strain under the same stress. Unlike BY4741, KNU5377Y Cpr1 protein was decarbonylated during MD stress. Decarbonylation of Cpr1 protein in KNU5377Y strain seems to be caused by a rapid and efficient gene expression program via stress response factors Hsf1, Yap1, and Msn2. Hence, the decarbonylated Cpr1 protein may be critical in cellular redox homeostasis and may be a potential chaperone to menadione.

A cyclophilin A CPR1 overexpression enhances stress acquisition in Saccharomyces cerevisiae

Cyclophilins are conserved cis-trans peptidyl-prolyl isomerase that are implicated in protein folding and function as molecular chaperones. We found the expression of cyclophilin A, Cpr1, changes in response to exposure to yeast Saccharomyces cerevisiae to abiotic stress conditions. The effect of Cpr1 overexpression in stress responses was therefore examined. The CPR1 gene was cloned to the yeast expression vector pVTU260 under regulation of an endogenous alcohol dehydrogenase (ADH) promoter. The overexpression of Cpr1 drastically increased cell viability of yeast in the presence of stress inducers, such as cadmium, cobalt, copper, hydrogen peroxide, tert-butyl hydroperoxide (t-BOOH), and sodium dodecyl sulfate (SDS). The Cpr1 expression also enhanced the cell rescue program resulting in a variety of antioxidant enzymes including thioredoxin system (particularly, thioredoxin peroxidase), metabolic enzymes (glucose-6-phosphate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase), and molecular chaperones (Hsp104, Hsp90, Hsp60 and Hsp42). Thus, our study illustrates the importance of Cpr1 as a molecular chaperone that improves cellular stress responses through collaborative relationships with other proteins when yeast cells are exposed to adverse conditions, and it also premises the improvement of yeast strains.

Mitochondrial peroxiredoxin involvement in antioxidant defence and redox signalling

Prxs (peroxiredoxins) are a family of proteins that are extremely effective at scavenging peroxides. The Prxs exhibit a number of intriguing properties that distinguish them from conventional antioxidants, including a susceptibility to inactivation by hyperoxidation in the presence of excess peroxide and the ability to form complex oligomeric structures. These properties, combined with a high cellular abundance and reactivity with hydrogen peroxide, have led to speculation that the Prxs function as redox sensors that transmit signals as part of the cellular response to oxidative stress. Multicellular organisms express several different Prxs that can be categorized by their subcellular distribution. In mammals, Prx 3 and Prx 5 are targeted to the mitochondrial matrix. Mitochondria are a major source of hydrogen peroxide, and this oxidant is implicated in the damage associated with aging and a number of pathologies. Hydrogen peroxide can also act as a second messenger, and is linked with signalling events in mitochondria, including the induction of apoptosis. A simple kinetic competition analysis estimates that Prx 3 will be the target for up to 90% of hydrogen peroxide generated in the matrix. Therefore, mitochondrial Prxs have the potential to play a major role in mitochondrial redox signalling, but the extent of this role and the mechanisms involved are currently unclear.

Physical and functional interactions of cyclophilin B with neuronal actin and peroxiredoxin-1 are modified by oxidative stress

Presynaptic actin was identified as a new Torpedo cyclophilin B partner captured in pull-down experiments and by coimmunoprecipitation. The cyclophilin B-actin pull-down interaction was insensitive to the blockade of peptidyl cis/trans prolyl isomerase and calcineurin activities and to the latrunculin A- and jasplakinolide-mediated perturbation of F-actin polymerization. Conversely, it was reduced by ATP and stimulated by a low Cu(2+) treatment of synaptosomes and by acrolydan-conjugated cyclophilin B. This Cu(2+)-induced stress, in parallel, stimulates the formation of GSH adducts with cysteines of synaptosomal actin followed by its deglutathionylation and its dimerization in the presence of higher Cu(2+) concentrations. The reversibility of the thiol processing of actin occurred in the same range of Cu(2+) concentrations that mediated a stronger cyclophilin B-actin interaction, suggesting cyclophilin B participation in antioxidant processes. Among 2-Cys-peroxiredoxin isoforms, mainly peroxiredoxin-1 was found in cell bodies and nerve endings. Functionally, both Torpedo and human peroxiredoxin-1 were activated in vitro by Torpedo cyclophilin B. Moreover, cyclophilin B, like thioredoxins, maintained an H(2)O(2)-dependent peroxidase activity of peroxiredoxin-1 in the presence of dithiothreitol. Thus, the monocysteinic Torpedo cyclophilin B is able to sustain peroxiredoxin-1 activity and might be involved in the presynaptic defense against oxidative stress affecting G-actin posttranslational changes and its redox signaling in nerve ending compartments.

Redox characterization of human cyclophilin D: identification of a new mammalian mitochondrial redox sensor?

Mitochondria are metabolically highly active cell organelles that are also implicated in reactive oxygen species production and in cell death regulation. Cyclophilin D, the only human mitochondrial isoform of cyclophilins, plays an essential role in the formation of the mitochondrial permeability transition pore leading to cell necrosis. Recently, it has been shown that redox environment modifies structural and functional properties of some plant cyclophilins. Here, it is shown that oxidation of human cyclophilin D influences the conformation of the enzyme but also its activity. Site-directed mutagenized variants of cyclophilin D allowed the identification of cysteine 203 as an important redox-sensitive residue. Moreover, the redox modulation of cyclophilin D was confirmed in human neuroblastoma SH-SY5Y cells exposed to oxidative stress. Altogether, our results suggest that cyclophilin D may play a role as a redox sensor in mitochondria of mammalian cells transmitting information on the redox environment to target proteins.

Typical 2-Cys peroxiredoxins--modulation by covalent transformations and noncovalent interactions

2-Cys peroxiredoxins are peroxidases devoid of prosthetic groups that mediate in the defence against oxidative stress and the peroxide activation of signaling pathways. This dual capacity relies on the high reactivity of the conserved peroxidatic and resolving cysteines, whose modification embraces not only the usual thiol-disulfide exchange but also higher oxidation states of the sulfur atom. These changes are part of a complex system wherein the cooperation with other post-translational modifications - phosphorylation, acetylation - may function as major regulatory mechanisms of the quaternary structure. More importantly, modern proteomic approaches have identified the oxyacids at cysteine residues as novel protein targets for unsuspected post-translational modifications, such as phosphorylation that yields the unusual sulfi(o)nic-phosphoryl anhydride. In this article, we review the biochemical attributes of 2-Cys peroxiredoxins that, in combination with complementary studies of forward and reverse genetics, have generated stimulating molecular models to explain how this enzyme integrates into cell signaling in vivo.

Peroxiredoxin 2 overexpression protects cortical neuronal cultures from ischemic and oxidative injury but not glutamate excitotoxicity, whereas Cu/Zn superoxide dismutase 1 overexpression protects only against oxidative injury

We previously reported that peroxiredoxin 2 (PRDX2) and Cu/Zn superoxide dismutase 1 (SOD1) proteins are up-regulated in rat primary neuronal cultures following erythropoietin (EPO) preconditioning. In the present study, we have demonstrated that adenovirally mediated overexpression of PRDX2 in cortical neuronal cultures can protect neurons from in vitro ischemia (oxygen-glucose deprivation) and an oxidative insult (cumene hydroperoxide) but not glutamate excitotoxicity. We have also demonstrated that adenovirally mediated overexpression of SOD1 in cortical neuronal cultures protected neurons only against the oxidative insult. Interestingly, we did not detect up-regulation of PRDX2 or SOD1 protein in the rat hippocampus following exposure to either 3 min or 8 min of global cerebral ischemia. Further characterization of PRDX2's neuroprotective mechanisms may aid in the development of a neuroprotective therapy.

AOP-1 interacts with cardiac-specific protein kinase TNNI3K and down-regulates its kinase activity

In the present study, a yeast two-hybrid screening system was used to identify the interaction partners of cardiac troponin I-interacting kinase (TNNI3K) that might serve as regulators or targets, and thus in turn to gain some insights on the roles of TNNI3K. After screening the adult heart cDNA library with a bait construct encoding the ANK motif of TNNI3K, antioxidant protein 1 (AOP-1) was isolated. The interaction between TNNI3K and AOP-1 was confirmed by the in vitro binding assay and coexpression experiments in vivo. The colocalization of TNNI3K and AOP-1 was clarified by confocal immunofluorescence. Moreover, coexpression of AOP-1 inhibited TNNI3K kinase activity in the in vitro kinase assay.

Cyclophilin A participates in the nuclear translocation of apoptosis-inducing factor in neurons after cerebral hypoxia-ischemia

Upon cerebral hypoxia-ischemia (HI), apoptosis-inducing factor (AIF) can move from mitochondria to nuclei, participate in chromatinolysis, and contribute to the execution of cell death. Previous work (Cande, C., N. Vahsen, I. Kouranti, E. Schmitt, E. Daugas, C. Spahr, J. Luban, R.T. Kroemer, F. Giordanetto, C. Garrido, et al. 2004. Oncogene. 23:1514–1521) performed in vitro suggests that AIF must interact with cyclophilin A (CypA) to form a proapoptotic DNA degradation complex. We addressed the question as to whether elimination of CypA may afford neuroprotection in vivo. 9-d-old wild-type (WT), CypA^+/−, or CypA^−/− mice were subjected to unilateral cerebral HI. The infarct volume after HI was reduced by 47% (P = 0.0089) in CypA^−/− mice compared with their WT littermates. Importantly, CypA^−/− neurons failed to manifest the HI-induced nuclear translocation of AIF that was observed in WT neurons. Conversely, CypA accumulated within the nuclei of damaged neurons after HI, and this nuclear translocation of CypA was suppressed in AIF-deficient harlequin mice. Immunoprecipitation of AIF revealed coprecipitation of CypA, but only in injured, ischemic tissue. Surface plasmon resonance revealed direct molecular interactions between recombinant AIF and CypA. These data indicate that the lethal translocation of AIF to the nucleus requires interaction with CypA, suggesting a model in which two proteins that normally reside in separate cytoplasmic compartments acquire novel properties when moving together to the nucleus.

Amino acid residues of Leishmania donovani cyclophilin key to interaction with its adenosine kinase: biological implications

Cyclophilins (CyPs), by interacting with a variety of proteins, often modulate their biological activities and thus have been implicated in several cellular functions. However, mechanisms that determine such interactions are poorly understood. We earlier reported that an endoplasmic reticulum (ER)-located cyclophilin (LdCyP) from the purine auxotrophic parasitic protozoan Leishmania donovani reactivated its adenosine kinase (AdK). The AdK-reactivating property of LdCyP was however abolished at high ionic strength but not by nonionic detergents. Modeling of LdCyP, based on its crystal structure solved at 1.97 A resolution, revealed several solvent-exposed hydrophobic and charged residues. Mutagenesis of several of such solvent-exposed residues was performed and their corresponding activities with regard to their (i) AdK reactivation property, (ii) ability to form complex with the enzyme, (iii) capacity to induce red shift in the intrinsic tryptophan fluorescence maxima of AdK, and (iv) efficiency to withdraw the ADP inhibition from the AdK-mediated reaction were compared to the wild-type protein. Results indicated that while the replacement of R147 with either A or D severely impaired all of the above characteristics displayed by the wild-type LdCyP, the effect of mutating K114 and K153 was although relatively less but nevertheless noticeable. Alteration of other exposed hydrophobic and charged residues apparently did not have any discernible effect. Under the condition of cellular stress, the ER-located LdCyP is released into the cytoplasm with concomitant increase both in the specific activity of the cytosol-resident AdK and the uptake of radiolabeled Ado into the cells. These experiments, besides demonstrating the importance of the positive charge, identified R147 as the most crucial residue in the LdCyP-AdK interaction and provide evidence for the stress-induced retrograde translocation of LdCyP from the ER to the cytoplasm. A possible implication of this interaction in the life cycle of the parasite is proposed.

Reversal of ADP-mediated aggregation of adenosine kinase by cyclophilin leads to its reactivation

Cyclophilins have been implicated in several important cellular functions. Our earlier results showed that reactivation of adenosine kinase (AdK) by CyP (LdCyP) from the parasitic protozoa Leishmania donovani is accompanied with disaggregation of the enzyme [Chakraborty, A., et al. (2002) J. Biol. Chem. 277, 47451-47460; Chakraborty, A., et al. (2004) Biochemistry 43, 11862-11872]. However, it remained to be known why the enzyme displayed progressive inhibition during the time-dependent reaction and what LdCyP does to prevent and/or reverse the inhibition. Herein, we demonstrate that one of its reaction products, ADP but not AMP, facilitates the formation of AdK aggregates, leading to its inactivation. Further studies revealed that LdCyP reactivates the enzyme by withdrawing the ADP inhibition. To investigate the molecular mechanism, the intrinsic tryptophan fluorescence and polarization of AdK were monitored in the presence of either LdCyP or ADP and in combination thereof. Whereas in the presence of LdCyP the tryptophan fluorescence emission maxima of AdK exhibited a red shift, ADP had a quenching effect. However, both the red shift and quenching became less noticeable when one (W234) of the two tryptophan residues of AdK was altered, indicating W234 fluorescence is relatively more sensitive to both LdCyP and ADP binding. Kinetic measurements indicated that LdCyP-facilitated reactivation of AdK is accompanied with a concomitant increase in the KD of ADP but not of AMP. Interestingly, addition of myokinase (MK) and pyruvate kinase (PK) along with phosphoenolpyruvate, either singly or in conjunction, to the AdK reaction mixture led to its reactivation. The effect of PK but not of MK could be substituted by CyP and vice versa. Taken together, the results suggest that LdCyP-induced reactivation occurs due to conformational reorientation of AdK in a manner that decreases the affinity of the enzyme for ADP with consequent relief from the ADP-mediated aggregation.

Redox regulation of cyclophilin A by glutathionylation

Using redox proteomics techniques to characterize the thiol status of proteins in human T lymphocytes, we identified cyclophilin A (CypA) as a specifically oxidized protein early after mitogen activation. CypA is an abundantly expressed cytosolic protein, target of the immunosuppressive drug cyclosporin A (CsA), for which a variety of functions has been described. In this study, we could identify CypA as a protein undergoing glutathionylation in vivo. Using MALDI-MS we identified Cys52 and Cys62 as targets of glutathionylation in T lymphocytes, and, using bioinformatic tools, we defined the reasons for the susceptibility of these residues to the modification. In addition, we found by circular dichroism spectroscopy that glutathionylation has an important impact on the secondary structure of CypA. Finally, we suggest that glutathionylation of CypA may have biological implications and that CypA may play a key role in redox regulation of immunity.

Antioxidant-like protein 1 is altered in non-pyramidal cells and expressed in astrocytes in the gerbil hippocampal CA1 region after transient forebrain ischemia

In the present study, we observed chronological changes of antioxidant-like protein 1 (AOP-1) in the gerbil hippocampal CA1 region after 5 min of transient forebrain ischemia using immunohistochemistry and western blot. AOP-1 was significantly altered in the CA1 region after transient ischemia. In the sham-operated group, AOP-1 immunoreactivity was detected in pyramidal and non-pyramidal cells of the CA1 region. At 30 min after ischemic insult, AOP-1 immunoreactivity and protein level was decreased in the CA1 region. At 12 h after ischemic insult, AOP-1 immunoreactivity and protein level was highest in this region. At this time, after ischemia, AOP-1 immunoreactivity in non-pyramidal cells was high compared to the sham-operated group. Based on double immunofluorescence study, AOP-1-immunoreactive neurons were identified as GABAergic, which were stained with GAD or parvalbumin. Thereafter, AOP-1 immunoreactivity and protein levels were decreased time-dependently. From 4 days after ischemic insult, AOP 1 immunoreactivity was generally expressed in astrocytes. Five days after ischemic insult, AOP-1 immunoreactivity and protein level was increased again to 1.4 folds compared to that of the sham-operated group. In brief, AOP-1 immunoreactivity was increased in GABAergic non-pyramidal cells in the hippocampal CA1 region at early time after ischemic insult and was expressed in astrocytes at late time after ischemia. This result suggests that AOP-1 may be important role in homeostasis of GABAergic neurons because these neurons are resistant to ischemic damage.

Isomerase-Independent Chaperone Function of Cyclophilin Ensures Aggregation Prevention of Adenosine Kinase Both in vitro and under in vivo Conditions

Using inactive aggregates of adenosine kinase (AdK) from Leishmania donovani as the model substrate, we recently demonstrated that a cyclophilin (LdCyP) from the same source in an isomerase-independent fashion reactivated the enzyme in vitro by disaggregating its inactive oligomers [Chakraborty et al. (2002) J. Biol. Chem. 277, 47451-47460]. Besides disrupting preformed aggregates, LdCyP also prevents reaggregation of the newly formed active protein that is generated after productive refolding from its urea-denatured state. To investigate possible physiological implications of such phenomena, a unique expression system that simultaneously induces both AdK and LdCyP in naturally AdK-deficient Escherichia coli, was developed. Both in vitro and in vivo experiments revealed that oligomerization is an inherent property of this particular enzyme. In vivo protein cross-linking studies, activity determination analysis and Ado phosphorylation experiments carried out in cells coexpressing both the proteins unequivocally demonstrated that, similar to the phenomena observed in vitro, aggregates of the enzyme formed in vivo are able to interact with both LdCyP and its N-terminal truncated form (N(22-88)DEL LdCyP) in a crowded intracellular environment, resulting in aggregation prevention and reactivation of the enzyme. Our results indicate that the isomerase-independent chaperone function of LdCyP, detected in vitro, participates in vivo as well to keep aggregation-prone proteins in a monomeric state. Furthermore, analogous to yeast/bacterial two-hybrid systems, development of this simple coexpression system may help in the confirmation of interaction of two proteins under simulated in vivo conditions.

Selection and validation of endogenous reference genes using a high throughput approach

BACKGROUND

Endogenous reference genes are commonly used to normalize expression levels of other genes with the assumption that the expression of the former is constant in different tissues and in different physiopathological conditions. Whether this assumption is correct it is, however, still matter of debate. In this study, we searched for stably expressed genes in 384 cDNA array hybridization experiments encompassing different tissues and cell lines.

RESULTS

Several genes were identified whose expression was highly stable across all samples studied. The usefulness of 8 genes among them was tested by normalizing the relative gene expression against test genes whose expression pattern was known. The range of accuracy of individual endogenous reference genes was wide whereas consistent information could be obtained when information pooled from different endogenous reference genes was used.

CONCLUSIONS

This study suggests that even when the most stably expressed genes in array experiments are used as endogenous reference, significant variation in test gene expression estimates may occur and the best normalization is achieved when data from several endogenous reference genes are pooled together to minimize minimal but significant variation among samples. We are presently optimizing strategies for the preparation of endogenous reference gene mixtures that could yield information comparable to that of data pooled from individual endogenous reference gene normalizations.

Two Enzymes in One

Although a great deal is known biochemically about peroxiredoxins (Prxs), little is known about their real physiological function. We show here that two cytosolic yeast Prxs, cPrxI and II, which display diversity in structure and apparent molecular weights (MW), can act alternatively as peroxidases and molecular chaperones. The peroxidase function predominates in the lower MW forms, whereas the chaperone function predominates in the higher MW complexes. Oxidative stress and heat shock exposure of yeasts causes the protein structures of cPrxI and II to shift from low MW species to high MW complexes. This triggers a peroxidase-to-chaperone functional switch. These in vivo changes are primarily guided by the active peroxidase site residue, Cys(47), which serves as an efficient "H(2)O(2)-sensor" in the cells. The chaperone function of these proteins enhances yeast resistance to heat shock.

Kinetics and redox-sensitive oligomerisation reveal negative subunit cooperativity in tryparedoxin peroxidase of Trypanosoma brucei brucei

Tryparedoxin peroxidases (TXNPx) are peroxiredoxin-type enzymes that detoxify hydroperoxides in trypanosomatids. Reduction equivalents are provided by trypanothione [T(SH)2] via tryparedoxin (TXN). The T(SH)2-dependent peroxidase system was reconstituted from TXNPx and TXN of T. brucei brucei (TbTXN-Px and TbTXN). TbTXNPx efficiently reduces organic hydroperoxides and is specifically reduced by TbTXN, less efficiently by thioredoxin, but not by glutathione (GSH) or T(SH)2. The kinetic pattern does not comply with a simple rate equation but suggests negative co-operativity of reaction centers. Gel permeation of oxidized TbTXNPx yields peaks corresponding to a decamer and higher aggregates. Electron microscopy shows regular ring structures in the decamer peak. Upon reduction, the rings tend to depolymerise forming open-chain oligomers. Co-oxidation of TbTXNPx with TbTXNC43S yields a dead-end intermediate mimicking the catalytic intermediate. Its size complies with a stoichiometry of one TXN per subunit of TXNPx. Electron microscopy of the intermediate displays pentangular structures that are compatible with a model of a decameric TbTXNPx ring with ten bound TbTXN molecules. The redox-dependent changes in shape and aggregation state, the kinetic pattern and molecular models support the view that, upon oxidation of a reaction center, other subunits adopt a conformation that has lower reactivity with the hydroperoxide.

Advances in our understanding of peroxiredoxin, a multifunctional, mammalian redox protein

Organisms living under aerobic conditions have developed various anti-oxidative mechanisms to protect them from damage by reactive oxygen species (ROS). A novel family of anti-oxidative proteins, designated as peroxiredoxin (Prx), has been identified in the past two decades and currently comprises six members in mammals. They share a common reactive Cys residue in the N-terminal region, and are capable of serving as a peroxidase and involve thioredoxin and/or glutathione as the electron donor. Prx1 to Prx4 have an additional Cys residue in the conserved C-terminal region, and are cross members as judged by the amino acid sequence similarity. Prx5 also contains an additional Cys in its C-terminal region which is less conserved. On the other hand, Prx6 has only one unique Cys. These Prx family members are distributed in the cytosol, mitochondria, peroxisome and plasma, all of which are potential sites of ROS production. In addition to their role as a peroxidase, however, a body of evidence has accumulated to suggest that individual members also serve divergent functions which are associated with various biological processes such as the detoxification of oxidants, cell proliferation, differentiation and gene expression. It would be expected that these functions might not necessarily depend on peroxidase activity and, therefore, it seems likely that the divergence is due to unique molecular characteristics intrinsic to each member. A comparative study of the divergence would lead to a better understanding of the biological significance of the Prx family.

Peroxiredoxins in antioxidant defense and redox regulation

Peroxiredoxins constitute a family of peroxidases that lack prosthetic groups or catalytically active heteroatoms. Instead, their peroxidatic activity is due to a strictly conserved cysteine that is activated within a novel catalytic triad in which the cysteine thiol is coordinated to an arginine and a threonine or serine residue. Donor substrates are thiol compounds which differ between subtypes of peroxiredoxins and species. In pathogenic trypanosomatids that lack heme- or seleno-peroxidases peroxiredoxins have been shown to represent the major devices to detoxify hydroperoxides and an equivalent role may be assumed for other protozoal parasites and many bacterial pathogens. In mammals equipped with more efficient peroxidases the peroxiredoxins appear to be responsible for the redox regulation of diverse metabolic processes. The substantial differences in the cosubstrate requirements of the peroxiredoxins of pathogenic microorganisms and their mammalian host may be exploited to selectively inhibit the antioxidant defense of pathogens. Thereby, the pathogen would be more readily eliminated by the innate immune response of the host's phagocytes.

A Single-domain Cyclophilin from Leishmania donovaniReactivates Soluble Aggregates of Adenosine Kinase by Isomerase-independent Chaperone Function

Disaggregation and reactivation of aggregated proteins by chaperones is well established. However, little is known regarding such kind of function of single-domain small cyclophilins (CyPs). Here we demonstrate that, with increasing concentrations, fully active adenosine kinase (AdK) of Leishmania donovani tends to form soluble aggregates, resulting in inactivation. Using this inactive enzyme as the substrate, it is shown that a CyP from L. donovani (LdCyP) alone can cause complete disaggregation, leading to reactivation of the enzyme. The reactivating ability of LdCyP remains unaffected even in the presence of cyclosporin A and macromolecular crowding agents. The reactivation occurs noncatalytically and is reversible. A truncated LdCyP, devoid of 88 amino acids from the N terminus, is found to be required in near stoichiometric proportion to reactivate AdK, suggesting essentiality of the C-terminal region. Gel filtration and light-scattering experiments together with protein cross-linking studies revealed that both full-length LdCyP and the truncated form directly interact with AdK and convert oligomeric forms of the enzyme to monomeric state. Homology modeling studies suggest that the exposed hydrophobic residues of LdCyP, by interacting with solvent-accessible hydrophobic surface of AdK, pull apart its aggregated inactive oligomers to functional monomers. Clearly, the results are consistent with the interpretation that the higher efficiency of the truncated LdCyP is most likely due to increased exposure of the hydrophobic residues on its surface. These observations, besides establishing L. donovani AdK as one of the model enzymes to study aggregation-disaggregation of proteins, raise the possibility that single-domain small CyPs, under physiological conditions, may regulate the activity of aggregation-prone proteins by ensuring their disaggregation.

Cyclosporin A blocks muscle differentiation by inducing oxidative stress and inhibiting the peptidyl-prolyl-cis-trans isomerase activity of cyclophilin A: cyclophilin A protects myoblasts from cyclosporin A-induced cytotoxicity

Allogenic myoblast transplantation (AMT) is under investigation for treatment of severe genetic myopathies. Data regarding the role of cyclosporine (CsA) and FK-506 in AMT have shown that CsA is less effective than FK-506. For this study, we investigated mechanisms of CsA toxicity during AMT and showed that a high level of reactive oxygen species (ROS) generated by CsA, mediated partly by inhibition of the peptidylprolyl-cis-trans-isomerase (PPIase)-like activity of cyclophilin A (CypA), blocked differentiation and induced apoptosis at an early stage of muscle differentiation. Inhibition of the PPIase-like activity of CypA alone also blocked muscle differentiation. However, CsA toxicity did not depend on the inhibition of calcineurin activity during muscle differentiation. Together, these data suggest that CsA-mediated inhibition of the PPIase-like activity of CypA and the high level of ROS generation contributed to the low efficacy of CsA in AMT. In addition, we showed that a reduction of oxidative stress protected cells from CsA-induced apoptosis, and myoblasts that had survived after preexposure to CsA not only proliferated and differentiated reversibly but also gained resistance to subsequent CsA exposure. Thus, administration of antioxidants or overexpression of CypA either exogenously or endogenously during CsA treatment has the potential to improve the success of this treatment in AMT.

Mitochondrial targeted cyclophilin D protects cells from cell death by peptidyl prolyl isomerization

Cyclophilin D (CyPD) is thought to sensitize opening of the mitochondrial permeability transition pore (mPTP) based on the findings that cyclosporin A (CsA), a pseudo-CyPD substrate, hyperpolarizes the mitochondrial membrane potential (DeltaPsi) and inhibits apoptosis. We provide evidence that contrasts with this model. Using live cell imaging and two photon microscopy, we report that overexpression of CyPD desensitizes HEK293 and rat glioma C6 cells to apoptotic stimuli. By site-directed mutagenesis of CyPD that compromises peptidyl-prolyl cis-trans isomerase (PPIase) activity, we demonstrate that the mechanism involved in this protective effect requires PPIase activity. Furthermore, we show that, under resting conditions, DeltaPsi is hyperpolarized in CyPD wild type-overexpressing cells but not in cells overexpressing mutant forms of CyPD that lack PPIase activity. Finally, in glutathione S-transferase (GST) pull-down assays, we demonstrate that CyPD binding to the adenine nucleotide translocator (ANT), which is considered to be the core component of the mPTP, is not affected by the loss of PPIase activity. Collectively, our data suggest that CyPD should be viewed as a cell survival-signaling molecule and indicate a protective role of CyPD against apoptosis that is mediated by one or more targets other than the ANT.

Cyclophilin-A is involved in excitotoxin-induced caspase activation in rat neuronal B50 cells

Glutamate and the NO donor, nitroprusside, synergistically induced the death of B50 cells from a rat CNS-derived neuroblastoma cell line. With low [nitroprusside] (10 microM) both nitroprusside and glutamate were required. Under these conditions, nuclei became pyknotic and caspases were activated. The activities of caspase-3 and caspase-6 (effector caspases) were higher than those of caspase-8 and caspase-9 (initiator caspases). The activation of all four caspases was inhibited by cyclosporin A, with the order of susceptibility caspase-8=caspase-9=caspase-6>caspase-3. To identify the possible locus of cyclosporin A action, we used an antisense oligodeoxynucleotide to suppress the level of cyclophilin-A to<5% of its control value. Cyclophilin-A suppression largely reproduced the inhibitory effects of cyclosporin A. These results provide the first indication that cyclophilin-A participates in the activation of the caspase cascade in neuronal cells, in particular in the form of cascade elicited by excitotoxic stimuli. It is concluded that neuroprotection by cyclosporin A against excitotoxin-induced apoptosis is, at least partly, due to inhibition of cyclophilin-A.

Peroxiredoxins

Present knowledge on peroxiredoxins is reviewed with special emphasis on catalytic principles, specificities and biological function. Peroxiredoxins are low efficiency peroxidases using thiols as reductants. They appear to be fairly promiscuous with respect to the hydroperoxide substrate; the specificities for the donor substrate vary considerably between the subfamilies, comprising GSH, thioredoxin, tryparedoxin and the analogous CXXC motifs in bacterial AhpF proteins. Peroxiredoxins are definitely responsible for antioxidant defense in bacteria (AhpC), yeast (thioredoxin peroxidase) and trypanosomatids (tryparedoxin peroxidase). They are considered to determine virulence of mycobacteria and trypanosomatids. In higher plants they are involved in balancing hydroperoxide production during photosynthesis. In higher animals peroxiredoxins appear to be involved in the redox-regulation of cellular signaling and differentiation, displaying in part opposite effects.

Mitochondrial intermembrane junctional complexes and their involvement in cell death

Mitochondria establish contact sites between the inner and outer membranes. The contact sites are held together by junctional complexes of the adenine nucleotide translocase (ANT; inner membrane) and the voltage-dependent anion channel (VDAC; outer membrane). The junctional complexes act as multifunctional recruitment centres, binding a range of proteins according to the function to be executed. Some of these, involving kinases and enzymes of lipid transfer, are readily understood as ongoing functions in energy and lipid metabolism. But the roles of other proteins recruited to the junctional complexes are less well defined. Here, we focus on the complexes formed with Bax and with cyclophilin-D, and their possible roles in apoptotic and necrotic cell death. We have isolated both types of complexes using glutathione-S-transferase fusion proteins of Bax and of cyclophilin-D. The VDAC/ANT/cyclophilin-D complex reconstitutes Ca(2+)- and cyclosporin A-sensitive permeability transition pore activity when incorporated into proteoliposomes. The complex forms readily in the absence of factors required for pore opening in isolated mitochondria, suggesting that these factors act on the preexisting complex, rather than drive its assembly, and that the complex is a physiological entity in healthy cells.

Cyclophilin a binds to peroxiredoxins and activates its peroxidase activity

Six distinct peroxiredoxin (Prx) proteins (Prx I-VI) from distinct genes have been identified in mammalian tissues. Prxs are members of a group of peroxidases that have conserved reactive cysteine residue(s) in the active site(s). An immediate physiological electron donor for the peroxidase catalysis for five Prx proteins (Prx I-V) has been identified as thioredoxin (Trx), but that for Prx VI (1-Cys Prx) is still unclear. To identify an immediate electron donor and a binding protein for Prx VI, we performed a Prx VI protein overlay assay. A 20-kDa binding protein was identified by the Prx VI protein overlay assay with flow-through fractions from a High-Q column with rat lung crude extracts. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) and MS-Fit, we identified the 20-kDa Prx VI-binding protein as a cyclophilin A (CyP-A). The binding of recombinant human CyP-A (hCyP-A) to Prx VI was confirmed by using the hCyP-A protein overlay assay and Western immunoblot analysis with hCyP-A-specific antibodies. hCyP-A enhanced the antioxidant activity of Prx VI, as well as the other known mammalian Prx isotypes. hCyP-A supported antioxidant activity of Prx II and Prx VI both against thiol (dithiothreitol)-containing metal-catalyzed oxidation (MCO) systems and ascorbate-containing MCO systems. Prx II was reduced by hCyP-A without help from any other reductant, and the reduction was cyclosporin A-independent. These results strongly suggest that CyP-A not only binds to Prx proteins but also supports its peroxidase activity as an immediate electron donor. In addition, Cys(115) and Cys(161) of hCyP-A were found to be involved in the activation and the reduction of Prx.

Abrin triggers cell death by inactivating a thiol-specific antioxidant protein

Abrin A-chain (ABRA) inhibits protein synthesis by its N-glycosidase activity as well as induces apoptosis, but the molecular mechanism of ABRA-induced cell death has been obscure. Using an ABRA mutant that lacks N-glycosidase activity as bait in a yeast two-hybrid system, a 30-kDa antioxidant protein-1 (AOP-1) was found to be an ABRA(E164Q)-interacting protein. The interaction was further confirmed in vitro by a glutathione S-transferase pull-down assay. The colocalization of endogenous AOP-1 and exogenous ABR proteins in the cell was demonstrated by confocal immunofluorescence. We also demonstrated that ABRA attenuates AOP-1 antioxidant activity in a dose-dependent manner and the intracellular level of reactive oxygen species (ROS) increases in ABR-treated cells. Moreover, ROS scavengers N-acetylcysteine and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl delayed programmed cell death. This indicates that ROS are important mediators of ABR-induced apoptosis. When ectopically expressed, AOP-1 blocked the release of cytochrome c and prevented apoptosis in ABR-treated cells. These findings suggest that the binding of ABRA to AOP-1 promotes apoptosis by inhibiting the mitochondrial antioxidant protein AOP-1, resulting in the increase of intracellular ROS and the release of cytochrome c from the mitochondria to the cytosol, which activates caspase-9 and caspase-3.

Comparison of the decameric structure of peroxiredoxin-II by transmission electron microscopy and X-ray crystallography

The decameric human erythrocyte protein torin is identical to the thiol-specific antioxidant protein-II (TSA-II), also termed peroxiredoxin-II (Prx-II). Single particle analysis from electron micrographs of Prx-II molecules homogeneously orientated across holes in the presence of a thin film of ammonium molybdate and trehalose has facilitated the production of a >/=20 A 3-D reconstruction by angular reconstitution that emphasises the D5 symmetry of the ring-like decamer. The X-ray structure for Prx-II was fitted into the transmission electron microscopic reconstruction by molecular replacement. The surface-rendered transmission electron microscopy (TEM) reconstruction correlates well with the solvent-excluded surface of the X-ray structure of the Prx-II molecule. This provides confirmation that transmission electron microscopy of negatively stained specimens, despite limited resolution, has the potential to reveal a valid representation of surface features of protein molecules. 2-D crystallisation of the Prx-II protein on mica as part of a TEM study resulted in the formation of a p2 crystal form with parallel linear arrays of stacked rings. This latter 2-D form correlates well with that observed from the 2.7 A X-ray structure of Prx-II solved from a new orthorhombic 3-D crystal form.

Regulation of macrophage migration inhibitory factor and thiol-specific antioxidant protein PAG by direct interaction

Macrophage migration inhibitory factor (MIF) is an important mediator that plays a central role in the control of the host immune and inflammatory response. To investigate the molecular mechanism of MIF action, we have used the yeast two-hybrid system and identified PAG, a thiol-specific antioxidant protein, as an interacting partner of MIF. Association of MIF with PAG was found in 293T cells transiently expressing MIF and PAG. The use of PAG mutants (C52S, C71S, and C173S) revealed that this association was significantly affected by C173S, but not C52S and C71S, indicating that a disulfide involving Cys(173) of PAG is responsible for the formation of MIF-PAG complex. In addition, the interaction was highly dependent on the reducing conditions such as dithiothreitol or beta-mercaptoethanol but not in the presence of H2O2. Analysis of the activities of the interacting proteins showed that the D-dopachrome tautomerase activity of MIF was decreased in a dose-dependent manner by coexpression of wild-type PAG, C52S, and C71S, whereas C173S was almost ineffective, suggesting that the direct interaction may be involved in the control of D-dopachrome tautomerase activity of MIF. Moreover, MIF has been shown to bind to PAG and it also inhibits the antioxidant activity of PAG.

Cyclophilin A regulates HIV-1 infectivity, as demonstrated by gene targeting in human T cells

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein binds most members of the cyclophilin family of peptidyl-prolyl isomerases. Of 15 known human cyclophilins, cyclophilin A (CypA) has been the focus of investigation because it was detected in HIV-1 virions. To determine whether CypA promotes HIV-1 replication, we deleted the gene encoding CypA (PPIA) in human CD4(+) T cells by homologous recombination. HIV-1 replication in PPIA(-/-) cells was decreased and not inhibited further by cyclosporin or gag mutations that disrupt Gag's interaction with cyclophilins, indicating that no other cyclophilin family members promote HIV-1 replication. The defective replication phenotype was specific for wild-type HIV-1 since HIV-2/SIV isolates, as well as HIV-1 bearing a gag mutation that confers cyclosporin resistance, replicated the same in PPIA(+/+) and PPIA(-/-) cells. Stable re-expression of CypA in PPIA(-/-) cells restored HIV-1 replication to an extent that correlated with steady-state levels of CypA. Finally, virions from PPIA(-/-) cells possessed no obvious biochemical abnormalities but were less infectious than virions from wild-type cells. These data formally demonstrate that CypA regulates the infectivity of HIV-1 virions.

Peroxiredoxin is ubiquitously expressed in rat skin: isotype-specific expression in the epidermis and hair follicle

Peroxiredoxins are a family of peroxidases that are ubiquitously and abundantly expressed in mammalian tissues; however, comparatively less is known about their expression in the skin. In this study, we examined the expression of three isotypes of peroxiredoxins (I-III) in rat skin. Western blot analyses showed strong expression of peroxiredoxins I-III in the epidermis and dermis of intact skin. Additionally, they were expressed in cultured rat keratinocytes and fibroblasts. Confocal image analyses revealed that peroxiredoxin II was present in the cytoplasm as a diffuse, reticulated pattern. In immunohistochemical staining of rat skin, peroxiredoxin expression was mainly localized to the epidermis, hair follicles, and sebaceous glands. In the epidermis, peroxiredoxins I and II were expressed in all layers with a gradient of increasing expression to the granular layer. In contrast, peroxiredoxin III was expressed in all layers with a gradient of expression decreasing to the granular layer. In the hair follicle, peroxiredoxins I-III were mainly expressed in the outer root sheath, except peroxiredoxin II, which was strongly expressed in the inner root sheath. In situ hybridization showed that mRNA expression was commensurate with the level of protein. Ultraviolet B radiation increased peroxiredoxin II expression in rat skin within 15 min after irradiation. From this study we conclude that peroxiredoxin isoforms are ubiquitously expressed in rat skin, and expression of at least peroxiredoxin II can be regulated by ultraviolet irradiation as a peroxidase in the skin. J Invest Dermatol 115:1108-1114 2000

Isolation, characterization and targeted disruption of mouse ppia: cyclophilin A is not essential for mammalian cell viability

Cyclophilins (CyPs) are a family of proteins found in organisms ranging from prokaryotes to humans. These molecules exhibit peptidyl-prolyl isomerase activity in vitro, suggesting that they influence the conformation of proteins in cells. CyPs also bind with varying affinities to the immunosuppressive drug cyclosporin A (CsA), a compound used clinically to prevent allograft rejection. The founding member of the family, cyclophilin A (CyPA), is an abundant, ubiquitously expressed protein of unknown function that binds with nanomolar affinity to CsA. Here, we describe the isolation and characterization of mouse Ppia (mPpia), the gene encoding CyPA. Ppia was isolated using a PCR screen that distinguishes the expressed gene from multiple pseudogenes present in the mouse genome. mPpia consists of 5 exons and 4 introns spanning roughly 4.5 kb and maps to chromosome 11 near the centromere. Sequence analysis of a 369-bp fragment from the proximal promoter region of mPpia revealed the presence of a TATA box and sites recognized by several transcriptional regulators, including Sp1, AP-2, GATA factors, c-Myb, and NF-IL-6. This region is sufficient to drive high-level reporter gene expression in transfected cells. Both copies of Ppia were disrupted in murine embryonic stem (ES) cells via gene targeting. Ppia(-/-) ES cells grow normally and differentiate into hematopoeitic precursor cells in vitro, indicating that CyPA is not essential for mammalian cell viability.

Enzymes that catalyse the restructuring of proteins

The large enzyme families of protein disulfide isomerases and peptidyl prolyl cis/trans isomerases have been shown to assist polypeptide restructuring. Various folding states of polypeptides may serve as substrates of the catalysed reaction. Our understanding of the cellular function of these enzymes is increasing as a result of the availability of more specific inhibitors, the discovery of natural substrates and the use of genetically modified organisms. Further highlights of these studies include insights into the three-dimensional structures of enzyme-ligand complexes, as well as into the mechanism of slow folding phases on the atomic level.

Oxygen stress increases prolyl cis/trans isomerase activity and expression of cyclophilin 18 in rabbit blastocysts

The peptidyl-prolyl cis/trans isomerase (PPIase) activity and the expression of cyclophilins were studied in 6-day-old rabbit preimplantation embryos cultured under physiological and increased oxygen concentrations of 5% and 20% O(2), respectively. The PPIase activity was completely inhibited by cyclosporin A (CsA). The inhibitor of FK506-binding proteins, rapamycin, had no effect on the PPIase activity, indicating that the PPIase activity in rabbit blastocysts originates from cyclophilins. Using CsA affinity chromatography, only one cyclophilin with a molecular mass of about 17.8 kDa was separated. The cDNA of rabbit cyclophilin was cloned and sequenced. Analysis of the 682-base pair cDNA revealed an open reading frame coding for a polypeptide of 164 amino acid residues with a molecular weight of 17.83 kDa. Homologies of 90% and 96% for the cDNA and amino acid sequence, respectively, to the human CyP18 were found, suggesting that the novel rabbit cyclophilin is a member of the CyP18 family (rabCyP18). The transcription level of rabCyP18 mRNA was 8.3 +/- 0.6 pg in 100 ng total RNA in noncultured blastocysts. In vitro culture with moderate oxygen stress (20% O(2)) resulted in a 1.5-fold increase in rabCyP18 transcription and an increased PPIase activity compared to that of blastocysts cultured with 5% O(2). Increase in transcription rate and PPIase activity by oxygen stress suggests an involvement of CyP18 in oxygen defense in rabbit preimplantation embryos.

Biochemical and structural characterization of a divergent loop cyclophilin from Caenorhabditis elegans

Cyclophilin 3 (CYP-3) is one of the most abundantly expressed cyclophilin isoforms in the free living nematode Caenorhabditis elegans. The detailed post-embryonic expression pattern of the cyp-3 transcript is unusual, peaking during early larval development. The spatial expression pattern was examined via reporter gene analysis demonstrating that the cyp-3 transcript is exclusively expressed in the single anterior excretory cell. Recombinant cyclophilin 3 has been purified, crystallized and solved to a resolution of 1.8 A. The peptidyl-prolyl isomerase activity of CYP-3 has been characterized against the substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide, and gives a k(cat)/K(m) value of 2.4 x 10(6) M(-1) s(-1). The immunosuppressive drug cyclosporin A binds and inhibits CYP-3 with an IC(50) value of 16 nM, comparable with the range of values found for human cyclophilin A. The x-ray structure shows that the overall fold and active site geometry is similar to other cyclophilin structures. There are however a number of distinctive features, and we use this structure and amino acid sequence alignment analysis to identify a subgroup of "divergent-loop cyclophilins". This subgroup has a number of uniquely conserved features: an additional loop between residues 48 and 54 (KSGKPLH); two cysteine residues (Cys(40) and Cys(168)) that are in close proximity but remain in the unoxidized form, and two other conserved residues, His(54) and Glu(83). We suggest that these features are functionally important for the role played by this class of cyclophilins during cellular responses to stress caused by changes in the redox environment or by up-regulation of cellular activity. This study represents a detailed biological, biochemical, and structural characterization of a single cyclophilin isoform in the model organism Caenorhabditis elegans.

Glutathione and trypanothione in parasitic hydroperoxide metabolism

Thiol-dependent hydroperoxide metabolism in parasites is reviewed in respect to potential therapeutic strategies. The hydroperoxide metabolism of Crithidia fasciculata has been characterized to comprise a cascade of three enzymes, trypanothione reductase, tryparedoxin, and tryparedoxin peroxidase, plus two supportive enzymes to synthesize the redox mediator trypanothione from glutathione and spermidine. The essentiality of the system in respect to parasite vitality and virulence has been verified by genetic approaches. The system appears to be common to all genera of the Kinetoplastida. The terminal peroxidase of the system belongs to the protein family of peroxiredoxins which is also represented in Entamoeba and a variety of metazoan parasites. Plasmodial hydroperoxide metabolism displays similarities to the mammalian system in comprising glutathione biosynthesis, glutathione reductase, and at least one glutathione peroxidase homolog having the active site selenocysteine replaced by cysteine. Nothing precise is known about the antioxidant defence systems of Giardia, Toxoplasma, and Trichomonas species. Also, the role of ovothiols and mycothiols reportedly present in several parasites remains to be established. Scrutinizing known enzymes of parasitic antioxidant defence for suitability as drug targets leaves only those of the trypanosomatid system as directly or indirectly validated. By generally accepted criteria of target selection and feasibility considerations tryparedoxin and tryparedoxin peroxidase can at present be rated as the most appealing target structures for the development of antiparasitic drugs.

Housekeeping genes as internal standards: use and limits

Quantitative studies are commonly realised in the biomedical research to compare RNA expression in different experimental or clinical conditions. These quantifications are performed through their comparison to the expression of the housekeeping gene transcripts like glyceraldehyde-3-phosphate dehydrogenase (G3PDH), albumin, actins, tubulins, cyclophilin, hypoxantine phsophoribosyltransferase (HRPT), L32. 28S, and 18S rRNAs are also used as internal standards. In this paper, it is recalled that the commonly used internal standards can quantitatively vary in response to various factors. Possible variations are illustrated using three experimental examples. Preferred types of internal standards are then proposed for each of these samples and thereafter the general procedure concerning the choice of an internal standard and the way to manage its uses are discussed.