Thioredoxin converts the Syrian hamster (29-231) recombinant prion protein to an insoluble form

Aggregate-prone R120GCRYAB triggers multifaceted modifications of the thioredoxin system

AIMS

The human mutation R120G in the αB-crystallin (CRYAB) causes a multisystemic disease that is characterized by hypertrophic cardiomyopathy and cytoplasmic protein aggregates. In transgenic mice, human R120GCRYAB (hR120GTg) expression in heart sequentially modifies the REDOX status, in part by the activation of the nuclear factor, erythroid derived 2, like 2 (Nrf2). Thioredoxin system (TS) components are NRF2 target genes, so it could be hypothesized that TS was affected in hR120GTg mice.

RESULTS

Transgenic hearts overexpressed thioredoxin 1 (Trx1), which was identified by isotope coded affinity tag-mass spectrometry, among hundreds of peptides displaying an increased reduced/oxidized ratio. Coupled to this higher level of reduced cysteines, the activity of thioredoxin reductase 1 (TrxR1) was augmented by 2.5-fold. Combining mutiple experimental approaches, the enzymatic regulation of TrxR1 by a histone deacetylase 3 (HDAC3)-dependent level of acetylation was confirmed. In vitro and in vivo functional tests established that TrxR1 activity is required to mitigate aggregate development, and this could be mediated by Bcl-2-associated athanogene 3 (BAG3) as a potential TS substrate.

INNOVATION AND CONCLUSIONS

This study uncovers the compartmentalized changes and the involvement of TS in the cardiac stress response elicited by misfolded proteins such as R120GCRYAB. Our work suggests that R120GCRYAB triggers a defensive pathway acting through the newly identified interacting partners HDAC3, TrxR1, and BAG3 to counter aggregate growth. Therefore, those interactors may function as modifier genes contributing to the variable onset and expressivity of such human diseases. Furthermore, our work underscores the potential organismal effects of pharmacological interventions targeting TS and HDAC.

The propagation of hamster-adapted scrapie PrPSc can be enhanced by reduced pyridine nucleotide in vitro

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders caused by an infectious agent termed a prion, which can convert normal cellular prion protein (PrP(C)) into a pathologically misfolded isoform (PrP(Sc)). Taking advantage of protein misfolding cyclic amplification (PMCA), a series of experiments was conducted to investigate the possible influences of pyridine nucleotides on the propagation activities of hamster-adapted scrapie agents 263K and 139A in vitro using normal hamster brain homogenates and recombinant hamster PrP as the substrates. The results showed that PrP(Sc) from both scrapie agent 263K- and 139A-infected brains propagated more efficiently in PMCA with the addition of reduced NADPH, showing an obvious dose-dependent enhancement. Reduced NADH also prompted PrP(Sc) propagation, whereas NADP, NAD and vitamin C failed. Moreover, following incubation with NADPH, recombinant hamster PrP could be efficiently converted into the proteinase K-resistant form when exposed to the trace of PrP(Sc) from infected hamsters. Our data provide evidence that the reduced pyridine nucleotide plays an important role in the propagation of prion and this process seems to target PrP(C) molecules.

Deep membrane insertion of prion protein upon reduction of disulfide bond

The membrane may play a role in the pathogenesis of the prion protein (PrP). Cytoplasmic expression of PrP causes the conversion of PrP to a self-perpetuating PrP(Sc)-like conformation and the interaction of polypeptide chain with the hydrophobic core of the membrane is believed to be closely correlated with neurodegeneration. However, it is still elusive what factors govern the membrane interaction of PrP. Here, we show that PrP penetrates deeply into the membrane when the single disulfide bond is reduced, which results in membrane disruption and leakage. The proteinase K treatment and the fluorescence quenching assays showed that a predicted transmembrane domain of PrP penetrates into the membrane when the disulfide bond was reduced. Therefore, the oxidation state of PrP might be an important factor that influences its neurotoxicity or pathogenesis.

Peroxiredoxins in the central nervous system

Oxidative stress is considered one of the causative pathomechanisms of nervous system diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, stroke and excitotoxicity. The basal expression of six different peroxiredoxin (Prx) isozymes show distinct distribution profiles in different brain regions and different cell types. PrxI and VI are expressed in glial cells but not in neurons; while PrxII, III, IV and V are expressed in neurons. Various diseases or models show altered expression levels of these isozymes, such as by upregulation of PrxI, II and VI and downregulation of PrxIII. Thioredoxin (Trx)I mRNA is distributed widely in the rat brain. This distribution pattern may reflect the specific functions of these isozymes. Recently, the neuroprotective roles of Prx III and V against ibotenate-induced-excitotoxicity were reported by two independent groups. Adenovirus transduction of PrxIII eliminated protein nitration and prevented gliosis caused by direct infusion of ibotenate. Systemic administration of recombinant PrxV diminished brain lesions in animals treated with ibotenate. In this chapter, we review the causative mechanisms of oxidative stress in neurodegenerative diseases, as well as describe the basal and disease-induced changes in Prxs/Trxs/Trx reductases expression levels and neuroprotective roles of Trxs and Prxs as demonstrated in overexpression models.

Oxidation of methionine residues in the prion protein by hydrogen peroxide

Reaction of H(2)O(2) with the recombinant SHa(29-231) prion protein resulted in rapid oxidation of multiple methionine residues. Susceptibility to oxidation of individual residues, assessed by mass spectrometry after digestion with CNBr and lysC, was in general a function of solvent exposure. Met 109 and Met 112, situated in the highly flexible amino terminus, and key residues of the toxic peptide PrP (106-126), showed the greatest susceptibility. Met 129, a residue located in a polymorphic position in human PrP and modulating risk of prion disease, was also easily oxidized, as was Met 134. The structural effect of H(2)O(2)-induced methionine oxidation on PrP was studied by CD spectroscopy. As opposed to copper catalyzed oxidation, which results in extensive aggregation of PrP, this reaction led only to a modest increase in beta-sheet structure. The high number of solvent exposed methionine residues in PrP suggests their possible role as protective endogenous antioxidants.

Metal-catalyzed oxidation of alpha-synuclein: helping to define the relationship between oligomers, protofibrils, and filaments

Oxidative stress is implicated in a number of neuro-degenerative diseases and is associated with the selective loss of dopaminergic neurons of the substantia nigra in Parkinson's disease. The role of alpha-synuclein as a potential target of intracellular oxidants has been demonstrated by the identification of posttranslational modifications of synuclein within intracellular aggregates that accumulate in Parkinson's disease brains, as well as the ability of a number of oxidative insults to induce synuclein oligomerization. The relationship between these relatively small soluble oligomers, potentially neurotoxic synuclein protofibrils, and synuclein filaments remains unclear. We have found that metal-catalyzed oxidation of alpha-synuclein inhibited formation of synuclein filaments with a concomitant accumulation of beta sheet-rich oligomers that may represent synuclein protofibrils. Similar results with a number of oxidative and enzymatic treatments suggest that the covalent association of synuclein into higher molecular mass oligomers/protofibrils represents an alternate pathway from filament formation and renders synuclein less prone to proteasomal degradation.

Selective prion protein binding to synaptic components is modulated by oxidative and nitrosative changes induced by copper(II) and peroxynitrite in cholinergic synaptosomes, unveiling a role for calcineurin B and thioredoxin

Choline acetyltransferase (ChAT) and choline transport are decreased after nitrosative stress. ChAT activity is altered in scrapie-infected neurons, where oxidative stress develops. Cellular prion protein (PrPc) may play a neuroprotective function in participating in the redox control of neuronal environment and regulation of copper metabolism, a role impaired when PrPc is transformed into PrPSc in prion pathologies. The complex cross-talk between PrPc and cholinergic neurons was analyzed in vitro using peroxynitrite and Cu2+ treatments on nerve endings isolated from Torpedo marmorata, a model of the motoneuron pre-synaptic element. Specific interactions between solubilized synaptic components and recombinant ovine prion protein (PrPrec) could be demonstrated by Biacore technology. Peroxynitrite abolished this interaction in a concentration-dependent way and induced significant alterations of neuronal targets. Interaction was restored by prior addition of peroxynitrite trapping agents. Cu2+ (in the form of CuSO4) treatment of synaptosomes triggered a milder oxidative effect leading to a bell-shaped increase of PrPrec binding to synaptosomal components, counteracted by the natural thiol agents, glutathione and thioredoxin. Copper(II)-induced modifications of thiols in several neuronal proteins. A positive correlation was observed between PrPrec binding and immunoreactive changes for calcineurin B and its partners, suggesting a synergy between calcineurin complex and PrP for copper regulation.

The thioredoxin system?From science to clinic

The thioredoxin system-formed by thioredoxin reductase and its characteristic substrate thioredoxin-is an important constituent of the intracellular redox milieu. Interactions with many different metabolic pathways such as DNA-synthesis, selenium metabolism, and the antioxidative network as well as significant species differences render this system an attractive target for chemotherapeutic approaches in many fields of medicine-ranging from infectious diseases to cancer therapy. In this review we will present and evaluate the preclinical and clinical results available today. Current trends in drug development are emphasized.

Overexpressed protein disulfide isomerase in brains of patients with sporadic Creutzfeldt-Jakob disease

Earlier studies have failed to detect covalent modifications in beta-sheet-rich scrapie isoform prion protein (PrP(Sc)) and have concluded that the conversion of alpha-helix-rich cellular form prion protein (PrP(C)) to PrP(Sc) represents purely conformational transition not involving chemical reactions. However, recent studies have shown that the intradisulfide bond of PrP(C) can play an important role for instability and conformational change to PrP(Sc). Interestingly, we found overexpressed protein disufide isomerase (PDI) in brains of sporadic Creutzfeldt-Jakob disease (sCJD, human prion disease) patients using two dimensional electrophoresis and Western blot analysis but not in other neurodegenerative disorders as Down Syndrome and Alzheimer's disease. However, proteinase K digestion and plasminogen binding assay of brain homogenates incubated with PDI suggest that PDI has no effect on either proteinase resistance or conformational change of PrP. Overexpression of PDI protein in sCJD brain may simply reflect a cellular defense response against the altered prion protein.

Potential involvement of copper and thiol-disulphide interchange in prion proteins' conformational conversion

The prion protein PrP(C) in transmissible spongiform encephalopathy converts to the pathogenic isoform PrP(Sc) containing less alpha-helical structure and a greater beta-pleated sheet content. The stability of PrP(C) protein is partly dependent on the disulphide bond between two alpha-helices designated B and C. Further stability could arise from ligand complexes of Cu(II) ions formed with carboxylic acid side chains in PrP(C). Electron spin resonance (E.S.R.) spectra and atomic absorption measurements have shown for alpha-keratin that the formation of ligands by Cu(II) is 10(2) more rapid than interaction of Cu(II) with ionised thiols X-S(-) which form X-S-Cu(+). X-S(-) destabilises disulphide bonds by thiol-disulphide interchange. When insufficient Cu(II) is present to form ligands with all available sites in PrP(C) then unblocked X-S(-) groups could potentially destabilise the disulphide bonds by thiol-disulphide interchange followed by reformation of the disulphide bond in the beta form of PrP(Sc) and the release of X-S(-) to interact with other PrP(C).

Expression patterns of antioxidant proteins in brains of patients with sporadic Creutzfeldt-Jacob disease

Using two-dimensional gel electrophoresis (2-DE) and Western blot analysis, we were able to identify and quantify six antioxidant proteins, peroxiredoxin (Prx) I, Prx II, Prx III, 1-Cys Prx, putative peroxisomal antioxidant enzyme (PLP), and mitochondrial Mn superoxide dismutase (Mn-SOD) in two individual brain regions, cerebellum and frontal cortex of patients with sporadic Creutzfeldt-Jacob (sCJD). Among six antioxidant proteins, 1-Cys Prx showed significant increase (P > 0.05) in sCJD frontal cortex whereas Prx I was decreased (P > 0.01). In cerebellum, levels of all antioxidant proteins studied were comparable to those of controls. Our findings provide evidence for the link between aberrant expression of antioxidant proteins, 1-Cys Prx and Prx I and CJD neuropathogenesis and we discuss the neuropathological meaning of these dysregulated antioxidant proteins in sCJD brain.

Isolation and characterization of a polymerized prion protein

A polymerized form of recombinant mouse prion protein (mPrP) domain 23-231 [mPrP-(23-231)], designated mPrP-z, was generated at acidic pH (pH 2-5) in the presence of selected concentrations of denaturant (2 M guanidinium chloride or 5 M urea). This isoform of mPrP is stable in acidic solution after removal of denaturant. It can be isolated and purified using reversed-phase HPLC or size-exclusion HPLC. mPrP-z bears structural properties that partially resemble those of scrapie prion. Unlike the native mPrP-(23-231) (mPrP-N), mPrP-z exhibits a high content of beta-sheet structure, as shown by CD spectroscopy, and exists as an oligomer with an approximate molecular mass of 340000 Da, as measured by light scattering. However, similarly to mPrP-N, mPrP-z contains the intact disulphide bond and is sensitive to digestion by proteinase K.

The role of dimerization in prion replication

The central theme in prion diseases is the conformational transition of a cellular protein from a physiologic to a pathologic (so-called scrapie) state. Currently, two alternative models exist for the mechanism of this autocatalytic process; in the template assistance model the prion is assumed to be a monomer of the scrapie conformer, whereas in the nucleated polymerization model it is thought to be an amyloid rod. A recent variation on the latter assumes disulfide reshuffling as the mechanism of polymerization. The existence of stable dimers, let alone their mechanistic role, is not taken into account in either of these models. In this paper we review evidence supporting that the dimerization of either the normal or the scrapie state, or both, has a decisive role in prion replication. The contribution of redox changes, i.e., the temporary opening and possible rearrangement of the intramolecular disulfide bridge is also considered. We present a model including these features largely ignored so far and show that it adheres satisfactorily to the observed phenomenology of prion replication.

Copper-catalyzed oxidation of the recombinant SHa(29-231) prion protein

Metal-catalyzed oxidation may result in structural damage to proteins and has been implicated in aging and disease, including neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis. The selective modification of specific amino acid residues with high metal ion affinity leads to subtle structural changes that are not easy to detect but may have dramatic consequences on physical and functional properties of the oxidized protein molecules. PrP contains a histidine-rich octarepeat domain that binds copper. Because copper-binding histidine residues are particularly prone to metal-catalyzed oxidation, we investigated the effect of this reaction on the recombinant prion protein SHaPrP(29-231). Using Cu2+/ascorbate, we oxidized SHaPrP(29-231) in vitro. Oxidation was demonstrated by liquid chromatography/mass spectrometry, which showed the appearance of protein species of higher mass, including increases in multiples of 16, characteristic of oxygen incorporation. Digestion studies using Lys C indicate that the 29-101 region, which includes the histidine-containing octarepeats, is particularly affected by oxidation. Oxidation was time- and copper concentration-dependent and was evident with copper concentrations as low as 1 microM. Concomitant with oxidation, SHaPrP(29-231) suffered aggregation and precipitation, which was nearly complete after 15 min, when the prion protein was incubated at 37 degrees C with a 6-fold molar excess of Cu2+. These findings indicate that PrP, a copper-binding protein, may be particularly susceptible to metal-catalyzed oxidation and that oxidation triggers an extensive structural transition leading to aggregation.