Histological evidence of protein aggregation in mutant SOD1 transgenic mice and in amyotrophic lateral sclerosis neural tissues.

The mechanisms leading to neurodegeneration in ALS (amyotrophic lateral sclerosis) are not well understood, but cytosolic protein aggregates appear to be common in sporadic and familial ALS as well as transgenic mouse models expressing mutant Cu/Zn superoxide dismutase (SOD1). In this study, we systematically evaluated the presence of these aggregates in three different mouse models (G93A, G85R, and G37R SOD1) and compared these aggregates to those seen in cases of sporadic and familial ALS. Inclusions and loss of motor neurons were observed in spinal cords of all of these three mutant transgenic lines. Since a copper-mediated toxicity hypothesis has been proposed to explain the cytotoxic gain-of-function of mutant SOD1, we sought to determine the involvement of the copper chaperone for SOD1 (CCS) in the formation of protein aggregates. Although all aggregates contained CCS, SOD1 was not uniformly found in the inclusions. Similarly, CCS-positive skein-like inclusions were rarely seen in ALS neurons. These studies do not provide strong evidence for a causal role of CCS in aggregate formation, but they do suggest that protein aggregation is a common event in all animal models of the disease. Selected proteins, such as the glutamate transporter GLT-1, were not typically observed within the inclusions. Most inclusions were positively stained with antibodies recognizing ubiquitin, proteasome, Hsc70 in transgenic lines, and some Hsc70-positive inclusions were detected in sporadic ALS cases. Overall, these observations suggest that inclusions might be sequestered into ubiquitin-proteasome pathway and some chaperone proteins such as Hsc70 may be involved in formation and/or degradation of these inclusions.

Manganese superoxide dismutase in Saccharomyces cerevisiae acquires its metal co-factor through a pathway involving the Nramp metal transporter, Smf2p.

Eukaryotes express both copper/zinc (SOD1)- and manganese (SOD2)-requiring superoxide dismutase enzymes that guard against oxidative damage. Although SOD1 acquires its copper through a specific copper trafficking pathway, nothing is known regarding the intracellular manganese trafficking pathway for SOD2. We demonstrate here that in Saccharomyces cerevisiae cells delivery of manganese to SOD2 in the mitochondria requires the Nramp metal transporter, Smf2p. SOD2 activity is greatly diminished in smf2Delta mutants, even though the mature SOD2 polypeptide accumulates to normal levels in mitochondria. Treating smf2Delta cells with manganese supplements corrected the SOD2 defect, as did elevating intracellular manganese through mutations in PMR1. Hence, manganese appears to be inaccessible to mitochondrial SOD2 in smf2 mutants. Cells lacking SMF2 also exhibited defects in manganese-dependent steps in protein glycosylation and showed an overall decrease in steady-state levels of accumulated manganese. By comparison, mutations in the cell surface Nramp transporter, Smf1p, had very little impact on manganese accumulation and trafficking. Smf2p resides in intracellular vesicles and shows no evidence of plasma membrane localization, even in an end4 mutant blocked for endocytosis. We propose a model in which Smf2p-containing vesicles play a central role in manganese trafficking to the mitochondria and other cellular sites as well.

Metal transporters that contribute copper to metallochaperones in Saccharomyces cerevisiae.

Copper metallochaperones represent a new family of soluble, low-molecular-weight proteins that function to deliver copper to specific sites within a cell. How the metallochaperones acquire their copper, however, is not known. In this study, we have conducted a survey of known metal ion transporters in bakers' yeast, Saccharomyces cerevisiae, to identify those that contribute copper to pathways involving the metallochaperones Atxlp and Lys7p. The results indicatethat, in addition to the well known Ctr1p and Ctr3p high-affinity copper transporters, the metallochaperones can acquire their copper through pathways involving the relatively non-specific divalent metal ion transporter Fet4p and the putative low-affinitycopper transporter Ctr2p. We have examined the localization of Ctr2p using an epitope tagged version of the protein and find that Ctr2p does not localize to the cell surface but may operate at the level of the vacuole to mobilize intracellular copper. Inaddition to Ctrlp, Ctr2p, Ctr3p and Fet4p, other metal transport systems can act as upstream donors of copper for the metallochaperones when copper availability in the medium is increased. Although the nature of these auxiliary systems is unknown, they do not appear to involve the yeast members of the Nramp family of divalent transporters, or uptake mechanisms that involve endocytosis. Since vastly different metal transporters located at either the cell surface or intracellular sites can all contribute copper to metallochaperones, it is unlikely that the metallochaperones directly interact with the metal transporters to obtain the metal.

A fraction of yeast Cu,Zn-superoxide dismutase and its metallochaperone, CCS, localize to the intermembrane space of mitochondria. A physiological role for SOD1 in guarding against mitochondrial oxidative damage.

Cu,Zn-superoxide dismutase (SOD1) is an abundant, largely cytosolic enzyme that scavenges superoxide anions. The biological role of SOD1 is somewhat controversial because superoxide is thought to arise largely from the mitochondria where a second SOD (manganese SOD) already resides. Using bakers' yeast as a model, we demonstrate that Cu,Zn-SOD1 helps protect mitochondria from oxidative damage, as sod1Delta mutants show elevated protein carbonyls in this organelle. In accordance with this connection to mitochondria, a fraction of active SOD1 localizes within the intermembrane space (IMS) of mitochondria together with its copper chaperone, CCS. Neither CCS nor SOD1 contains typical N-terminal presequences for mitochondrial uptake; however, the mitochondrial accumulation of SOD1 is strongly influenced by CCS. When CCS synthesis is repressed, mitochondrial SOD1 is of low abundance, and conversely IMS SOD1 is very high when CCS is largely mitochondrial. The mitochondrial form of SOD1 is indeed protective against oxidative damage because yeast cells enriched for IMS SOD1 exhibit prolonged survival in the stationary phase, an established marker of mitochondrial oxidative stress. Cu,Zn-SOD1 in the mitochondria appears important for reactive oxygen physiology and may have critical implications for SOD1 mutations linked to the fatal neurodegenerative disorder, amyotrophic lateral sclerosis.

Saccharomyces cerevisiae expresses three functionally distinct homologues of the nramp family of metal transporters.

The baker's yeast Saccharomyces cerevisiae expresses three homologues of the Nramp family of metal transporters: Smf1p, Smf2p, and Smf3p, encoded by SMF1, SMF2, and SMF3, respectively. Here we report a comparative analysis of the yeast Smf proteins at the levels of localization, regulation, and function of the corresponding metal transporters. Smf1p and Smf2p function in cellular accumulation of manganese, and the two proteins are coregulated by manganese ions and the BSD2 gene product. Under manganese-replete conditions, Bsd2p facilitates trafficking of Smf1p and Smf2p to the vacuole, where these transport proteins are degraded. However, Smf1p and Smf2p localize to distinct cellular compartments under metal starvation: Smf1p accumulates at the cell surface, while Smf2p is restricted to intracellular vesicles. The third Nramp homologue, Smf3p, is quite distinctive. Smf3p is not regulated by Bsd2p or by manganese ions and is not degraded in the vacuole. Instead, Smf3p is down-regulated by iron through a mechanism that does not involve transcription or protein stability. Smf3p localizes to the vacuolar membrane independently of metal treatment, and yeast cells lacking Smf3p show symptoms of iron starvation. We propose that Smf3p helps to mobilize vacuolar stores of iron.

Copper activation of superoxide dismutase 1 (SOD1) in vivo. Role for protein-protein interactions with the copper chaperone for SOD1.

Insertion of copper into superoxide dismutase 1 (SOD1) in vivo requires the copper chaperone for SOD1 (CCS). CCS encompasses three protein domains: copper binding Domains I and III at the amino and carboxyl termini, and a central Domain II homologous to SOD1. Using a yeast interaction mating system, yeast CCS was seen to physically interact with SOD1, and this interaction required sequences at the predicted dimer interface of CCS Domain II. Interactions with SOD1 also required sequences of Domain III, but not Domain I. Mutations were introduced at the dimer interface of yeast SOD1, and the corresponding mutant failed to interact with CCS. When loaded with copper independent of CCS, this mutant SOD1 exhibited superoxide scavenging activity, but was normally inactive in vivo because CCS failed to recognize the enzyme. Activation of SOD1 by CCS was also examined using an in vivo assay for copper incorporation into SOD1. Yeast CCS was observed to insert copper into a pre-existing pool of apoSOD1 without the need for new SOD1 synthesis or for protein unfolding by the major SSA cytosolic heat shock proteins. Our data are consistent with a model in which prefolded dimers of apoSOD1 serve as substrate for the CCS copper chaperone.

Role of Saccharomyces cerevisiae ISA1 and ISA2 in iron homeostasis.

The budding yeast Saccharomyces cerevisiae contains two homologues of bacterial IscA proteins, designated Isa1p and Isa2p. Bacterial IscA is a product of the isc (iron-sulfur cluster) operon and has been suggested to participate in Fe-S cluster formation or repair. To test the function of yeast Isa1p and Isa2p, single or combinatorial disruptions were introduced in ISA1 and ISA2. The resultant isaDelta mutants were viable but exhibited a dependency on lysine and glutamate for growth and a respiratory deficiency due to an accumulation of mutations in mitochondrial DNA. As with other yeast genes proposed to function in Fe-S cluster assembly, mitochondrial iron concentration was significantly elevated in the isa mutants, and the activities of the Fe-S cluster-containing enzymes aconitase and succinate dehydrogenase were dramatically reduced. An inspection of Isa-like proteins from bacteria to mammals revealed three invariant cysteine residues, which in the case of Isa1p and Isa2p are essential for function and may be involved in iron binding. As predicted, Isa1p is targeted to the mitochondrial matrix. However, Isa2p is present within the intermembrane space of the mitochondria. Our deletion analyses revealed that Isa2p harbors a bipartite N-terminal leader sequence containing a mitochondrial import signal linked to a second sequence that targets Isa2p to the intermembrane space. Both signals are needed for Isa2p function. A model for the nonredundant roles of Isa1p and Isa2p in delivering iron to sites of the Fe-S cluster assembly is discussed.

Copper chaperone for superoxide dismutase is essential to activate mammalian Cu/Zn superoxide dismutase.

Recent studies in Saccharomyces cerevisiae suggest that the delivery of copper to Cu/Zn superoxide dismutase (SOD1) is mediated by a cytosolic protein termed the copper chaperone for superoxide dismutase (CCS). To determine the role of CCS in mammalian copper homeostasis, we generated mice with targeted disruption of CCS alleles (CCS(-/-) mice). Although CCS(-/-) mice are viable and possess normal levels of SOD1 protein, they reveal marked reductions in SOD1 activity when compared with control littermates. Metabolic labeling with (64)Cu demonstrated that the reduction of SOD1 activity in CCS(-/-) mice is the direct result of impaired Cu incorporation into SOD1 and that this effect was specific because no abnormalities were observed in Cu uptake, distribution, or incorporation into other cuproenzymes. Consistent with this loss of SOD1 activity, CCS(-/-) mice showed increased sensitivity to paraquat and reduced female fertility, phenotypes that are characteristic of SOD1-deficient mice. These results demonstrate the essential role of any mammalian copper chaperone and have important implications for the development of novel therapeutic strategies in familial amyotrophic lateral sclerosis.

Saccharomyces cerevisiae ISU1 and ISU2: members of a well-conserved gene family for iron-sulfur cluster assembly.

Recent studies in bacteria and eukaryotes have led to the identification of several new genes implicated in the biogenesis of iron-sulfur (Fe/S) cluster-containing proteins. This report focuses on two genes of bakers yeast Saccharomyces cerevisiae, ISU1 and ISU2, which encode homologues to bacterial IscU and NifU, potential iron-binding or cluster-assembly proteins. As with other yeast genes implicated in Fe/S protein assembly, deletion of either ISU1 or ISU2 results in increased accumulation of iron within the mitochondria, loss of activity of the [4Fe-4S] aconitase enzyme, and suppression of oxidative damage in cells lacking cytosolic copper/zinc superoxide dismutase. Both genes are induced in strains expressing an activated allele of Aft1p, the iron-sensing transcription factor, suggesting that they are regulated by the iron status of the cell. Immunoblotting studies using an antibody directed against Escherichia coli IscU reveal that both Isu1p and Isu2p are localized primarily in the mitochondria and that Isu1p is the predominant form expressed under all growth conditions tested. The possible role of the Isu proteins in the assembly and/or repair of Fe/S clusters is discussed.

A gain of superoxide dismutase (SOD) activity obtained with CCS, the copper metallochaperone for SOD1.

The incorporation of copper ions into the cytosolic superoxide dismutase (SOD1) is accomplished in vivo by the action of the copper metallochaperone CCS (copper chaperone for SOD1). Mammalian CCS is comprised of three distinct protein domains, with a central region exhibiting remarkable homology (approximately 50% identity) to SOD1 itself. Conserved in CCS are all the SOD1 zinc binding ligands and three of four histidine copper binding ligands. In CCS the fourth histidine is replaced by an aspartate (Asp(200)). Despite this conservation of sequence between SOD1 and CCS, CCS exhibited no detectable SOD activity. Surprisingly, however, a single D200H mutation, targeting the fourth potential copper ligand in CCS, granted significant superoxide scavenging activity to this metallochaperone that was readily detected with CCS expressed in yeast. This mutation did not inhibit the metallochaperone capacity of CCS, and in fact, D200H CCS appears to represent a bifunctional SOD that can self-activate itself with copper. The aspartate at CCS position 200 is well conserved among mammalian CCS molecules, and we propose that this residue has evolved to preclude deleterious reactions involving copper bound to CCS.

Oxidative stress and iron are implicated in fragmenting vacuoles of Saccharomyces cerevisiae lacking Cu,Zn-superoxide dismutase.

The absence of the antioxidant enzyme Cu,Zn-superoxide dismutase (SOD1) is shown here to cause vacuolar fragmentation in Saccharomyces cerevisiae. Wild-type yeast have 1-3 large vacuoles whereas the sod1Delta yeast have as many as 50 smaller vacuoles. Evidence that this fragmentation is oxygen-mediated includes the findings that aerobically (but not anaerobically) grown sod1Delta yeast exhibit aberrant vacuoles and genetic suppressors of other oxygen-dependent sod1 null phenotypes rescue the vacuole defect. Surprisingly, iron also is implicated in the fragmentation process as iron addition exacerbates the sod1Delta vacuole defect while iron starvation ameliorates it. Because the vacuole is reported to be a site of iron storage and iron reacts avidly with reactive oxygen species to generate toxic side products, we propose that vacuole damage in sod1Delta cells arises from an elevation of iron-mediated oxidation within the vacuole or from elevated pools of "free" iron that may bind nonproductively to vacuolar ligands. Furthermore, additional pleiotropic phenotypes of sod1Delta cells (including increased sensitivity to pH, nutrient deprivation, and metals) may be secondary to vacuolar compromise. Our findings support the hypothesis that oxidative stress alters cellular iron homeostasis which in turn increases oxidative damage. Thus, our findings may have medical relevance as both oxidative stress and alterations in iron homeostasis have been implicated in diverse human disease processes. Our findings suggest that strategies to decrease intracellular iron may significantly reduce oxidatively induced cellular damage.

Multiple protein domains contribute to the action of the copper chaperone for superoxide dismutase.

The copper chaperone for superoxide dismutase (SOD1) inserts the catalytic metal cofactor into SOD1 by an unknown mechanism. We demonstrate here that this process involves the cooperation of three distinct regions of the copper chaperone for SOD1 (CCS): an amino-terminal Domain I homologous to the Atx1p metallochaperone, a central portion (Domain II) homologous to SOD1, and a short carboxyl-terminal peptide unique to CCS molecules (Domain III). These regions fold into distinct polypeptide domains as revealed through proteolysis protection studies. The biological roles of the yeast CCS domains were examined in yeast cells. Surprisingly, Domain I was found to be necessary only under conditions of strict copper limitation. Domain I and Atx1p were not interchangeable in vivo, underscoring the specificity of the corresponding metallochaperones. A putative copper site in Domain II was found to be irrelevant to yeast CCS activity, but SOD1 activation invariably required a CXC in Domain III that binds copper. Copper binding to purified yeast CCS induced allosteric conformational changes in Domain III and also enhanced homodimer formation of the polypeptide. Our results are consistent with a model whereby Domain I recruits cellular copper, Domain II facilitates target recognition, and Domain III, perhaps in concert with Domain I, mediates copper insertion into apo-SOD1.

Variation in the biochemical/biophysical properties of mutant superoxide dismutase 1 enzymes and the rate of disease progression in familial amyotrophic lateral sclerosis kindreds.

Mutations in superoxide dismutase 1 (SOD1) polypeptides cause a form of familial amyotrophic lateral sclerosis (FALS). In different kindreds, harboring different mutations, the duration of illness tends to be similar for a given mutation. For example, patients inheriting a substitution of valine for alanine at position four (A4V) average a 1.5 year life expectancy after the onset of symptoms, whereas patients harboring a substitution of arginine for histidine at position 46 (H46R) average an 18 year life expectancy after disease onset. Here, we examine a number of biochemical and biophysical properties of nine different FALS variants of SOD1 polypeptides, including enzymatic activity (which relates indirectly to the affinity of the enzyme for copper), polypeptide half-life, resistance to proteolytic degradation and solubility, in an effort to determine whether a specific property of these enzymes correlates with clinical progression. We find that although all the mutants tested appear to be soluble, the different mutants show a remarkable degree of variation with respect to activity, polypeptide half-life and resistance to proteolysis. However, these variables do not stratify in a manner that correlates with clinical progression. We conclude that the basis for the different life expectancies of patients in different kindreds of sod1-linked FALS may result from an as yet unidentified property of these mutant enzymes.

Crystal structure of the copper chaperone for superoxide dismutase.

Cellular systems for handling transition metal ions have been identified, but little is known about the structure and function of the specific trafficking proteins. The 1.8 A resolution structure of the yeast copper chaperone for superoxide dismutase (yCCS) reveals a protein composed of two domains. The N-terminal domain is very similar to the metallochaperone protein Atx1 and is likely to play a role in copper delivery and/or uptake. The second domain resembles the physiological target of yCCS, superoxide dismutase I (SOD1), in overall fold, but lacks all of the structural elements involved in catalysis. In the crystal, two SOD1-like domains interact to form a dimer. The subunit interface is remarkably similar to that in SOD1, suggesting a structural basis for target recognition by this metallochaperone.

Mutational analysis of Saccharomyces cerevisiae Smf1p, a member of the Nramp family of metal transporters.

We have recently shown that a member of the Nramp family of metal transporters, Saccharomyces cerevisiae Smf1p, is tightly regulated at the level of protein stability and protein sorting. Under metal replete conditions, Smf1p is targeted to the vacuole for degradation in a manner dependent on the S. cerevisiaeBSD2 gene product, but under metal starvation conditions, Smf1p accumulates at the cell surface. Here, we have addressed whether Smf1p activity may be necessary for its regulation by metal ions and Bsd2p. Well conserved residues within transmembrane domain 4 and the transport signature sequence of Smf1p were mutagenized. We identified two mutants, G190A and G424A, which destroyed Smf1p activity as monitored by complementation of a smf1 mutation. Notably, these mutations also abolished control by metal ions and Bsd2p, suggesting that Smf1p metal transport function may be necessary for its regulation. Two additional mutants isolated (Q419A and E423A) exhibited wild-type complementation activity and were properly targeted for vacuolar degradation in a Bsd2-dependent manner. However, these mutants failed to re-distribute to the plasma membrane under conditions of metal starvation. A model is proposed herein describing the probable role of Smf1 protein conformation in directing its movement to the vacuole versus cell surface in response to changes in metal ion availability.

Structure-function analyses of the ATX1 metallochaperone.

Saccharomyces cerevisiae Atx1p represents a member of the family of metallochaperone molecules that escort copper to distinct intracellular targets. Atx1p specifically delivers copper to the Ccc2p copper transporter in the Golgi. Additionally, when overproduced, Atx1p substitutes for superoxide dismutase 1 in preventing oxidative damage; however the mechanistic overlap between these functions is unresolved. The crystal structure of Atx1p has been solved recently. By examining a surface electrostatic potential distribution, multiple conserved lysines are revealed on one face of Atx1p. An additional conserved lysine (Lys65) lies in close proximity to the metal binding site. Through site-directed mutagenesis, residues in the metal binding region including Lys65 were found to be necessary for both copper delivery to Ccc2p and for Atx1p antioxidant activity. Copper trafficking to Ccc2p also relied on the lysine-rich face of Atx1p. Surprisingly however, elimination of these lysines did not inhibit the antioxidant activity of Atx1p. We provide evidence that Atx1p does not suppress oxidative damage by a metallochaperone mechanism but may directly consume superoxide. Purified Cu-Atx1p reacts noncatalytically with superoxide anion in vitro. We conclude that the copper-trafficking and antioxidant functions of Atx1p arise from chemically and structurally distinct attributes of this metallochaperone.

Undetectable intracellular free copper: the requirement of a copper chaperone for superoxide dismutase.

The copper chaperone for the superoxide dismutase (CCS) gene is necessary for expression of an active, copper-bound form of superoxide dismutase (SOD1) in vivo in spite of the high affinity of SOD1 for copper (dissociation constant = 6 fM) and the high intracellular concentrations of both SOD1 (10 microM in yeast) and copper (70 microM in yeast). In vitro studies demonstrated that purified Cu(I)-yCCS protein is sufficient for direct copper activation of apo-ySOD1 but is necessary only when the concentration of free copper ions ([Cu]free) is strictly limited. Moreover, the physiological requirement for yCCS in vivo was readily bypassed by elevated copper concentrations and abrogation of intracellular copper-scavenging systems such as the metallothioneins. This metallochaperone protein activates the target enzyme through direct insertion of the copper cofactor and apparently functions to protect the metal ion from binding to intracellular copper scavengers. These results indicate that intracellular [Cu]free is limited to less than one free copper ion per cell and suggest that a pool of free copper ions is not used in physiological activation of metalloenzymes.