Cu/Zn superoxide dismutase mutants associated with amyotrophic lateral sclerosis show enhanced formation of aggregates in vitro

Structural and thermodynamic effects of post-translational modifications in mutant and wild type Cu, Zn superoxide dismutase

Aggregation of Cu,Zn superoxide dismutase (SOD1) is implicated in amyotrophic lateral sclerosis. Glutathionylation and phosphorylation of SOD1 is omnipresent in the human body, even in healthy individuals, and has been shown to increase SOD1 dimer dissociation, which is the first step on the pathway toward SOD1 aggregation. We found that post-translational modification of SOD1, especially glutathionylation, promotes dimer dissociation. We discovered an intermediate state in the pathway to dissociation, a conformational change that involves a "loosening" of the β-barrels and a loss or shift of dimer interface interactions. In modified SOD1, this intermediate state is stabilized as compared to unmodified SOD1. The presence of post-translational modifications could explain the environmental factors involved in the speed of disease progression. Because post-translational modifications such as glutathionylation are often induced by oxidative stress, post-translational modification of SOD1 could be a factor in the occurrence of sporadic cases of amyotrophic lateral sclerosis, which represent 90% of all cases of the disease.

Apomyoglobin mutants with single point mutations at val10 can form amyloid structures at permissive temperature

Formation of amyloid-like protein aggregates in human organs and tissues underlies many serious diseases, therefore being in the focus of numerous biochemical, medical, and molecular biological studies. So far, formation of amyloids by globular proteins has been studied mostly under conditions that strongly destabilized their native structure. Here we present our results obtained at permissive temperature by thioflavin T fluorescence, far UV CD, IR spectroscopy, and electron microscopy. We used apomyoglobin and its mutants with Ala or Phe substituted for Val10 that are structurally close to wild type apomyoglobin. It is shown that at permissive temperature the ability of the protein to form amyloids depends on the extent of its structural destabilization, but not on hydrophobicity of the substituting residue. A possible difference between amyloids formed by strongly destabilized proteins and those yielded by proteins with a slightly fluctuating native structure, as well as the stroke and infarction effect on the ability of proteins to form amyloid structures, are discussed.

ALS-causing SOD1 mutations promote production of copper-deficient misfolded species

Point mutations scattered throughout the sequence of Cu,Zn superoxide dismutase (SOD1) cause a subset of amyotrophic lateral sclerosis (ALS) cases. SOD1 is a homodimer in which each subunit binds one copper atom and one zinc atom. Inclusions containing misfolded SOD1 are seen in motor neurons of SOD1-associated ALS cases. The mechanism by which these diverse mutations cause misfolding and converge on the same disease is still not well understood. Previously, we developed several time-resolved techniques to monitor structural changes in SOD1 as it unfolds in guanidine hydrochloride. By measuring the rates of Cu and Zn release using an absorbance-based assay, dimer dissociation through chemical cross-linking, and β-barrel conformation changes by tryptophan fluorescence, we established that wild-type SOD1 unfolds by a branched pathway involving a Zn-deficient monomer as the dominant intermediate of the major pathway, and with various metal-loaded and Cu-deficient dimers populated along the minor pathway. We have now compared the unfolding pathway of wild-type SOD1 with those of A4V, G37R, G85R, G93A, and I113T ALS-associated mutant SOD1. The kinetics of unfolding of the mutants were generally much faster than those of wild type. However, all of the mutants utilize the minority pathway to a greater extent than the wild-type protein, leading to greater populations of Cu-deficient intermediates and decreases in Zn-deficient intermediates relative to the wild-type protein. The greater propensity of the mutants to populate Cu-deficient states potentially implicates these species as a pathogenic form of SOD1 in SOD1-associated ALS and provides a novel target for therapeutic intervention.

Sensitive and molecular size-selective detection of proteins using a chip-based and heteroliganded gold nanoisland by localized surface plasmon resonance spectroscopy

A highly sensitive and molecular size-selective method for the detection of proteins using heteroliganded gold nanoislands and localized surface plasmon resonance (LSPR) is described. Two different heteroligands with different chain lengths (3-mercaptopionicacid and decanethiol) were used in fabricating nanoholes for the size-dependent separation of a protein in comparison with its aggregate. Their ratios on gold nanoisland were optimized for the sensitive detection of superoxide dismutase (SOD1). This protein has been implicated in the pathology of amyotrophic lateral sclerosis (ALS). Upon exposure of the optimized gold nanoisland to a solution of SOD1 and aggregates thereof, changes in the LSPR spectra were observed which are attributed to the size-selective and covalent chemical binding of SOD1 to the nanoholes. With a lower detection limit of 1.0 ng/ml, the method can be used to selectively detect SOD1 in the presence of aggregates at the molecular level.

Size-selective concentration and label-free characterization of protein aggregates using a Raman active nanofluidic device

Trace detection and physicochemical characterization of protein aggregates have a large impact in understanding and diagnosing many diseases, such as ageing-related neurodegeneration and systemic amyloidosis, for which the formation of protein aggregates is one of the pathological hallmarks. Here we demonstrate an innovative label-free method for detecting and characterizing small amounts of early stage protein aggregates using a Raman active nanofluidic device. Sub-micrometre channels formed by a novel elastomeric collapse technique enable the separation and concentration of matured protein aggregates from small protein molecules. The Raman enhancement by gold nanoparticle clusters fixed below a micro/nanofluidic junction allows characterization of intrinsic properties of protein aggregates at concentration levels (∼fM) much lower than can be done with traditional analytical tools. With our device we show for the first time the concentration dependence of protein aggregation over these low concentration ranges. We expect that our method could facilitate definitive diagnosis and possible therapeutics of diseases at early stages.

Decreased stability and increased formation of soluble aggregates by immature superoxide dismutase do not account for disease severity in ALS

Protein aggregation is a hallmark of many diseases, including amyotrophic lateral sclerosis (ALS), where aggregation of Cu/Zn superoxide dismutase (SOD1) is implicated in causing neurodegeneration. Recent studies have suggested that destabilization and aggregation of the most immature form of SOD1, the disulfide-reduced, unmetallated (apo) protein is particularly important in causing ALS. We report herein in depth analyses of the effects of chemically and structurally diverse ALS-associated mutations on the stability and aggregation of reduced apo SOD1. In contrast with previous studies, we find that various reduced apo SOD1 mutants undergo highly reversible thermal denaturation with little aggregation, enabling quantitative thermodynamic stability analyses. In the absence of ALS-associated mutations, reduced apo SOD1 is marginally stable but predominantly folded. Mutations generally result in slight decreases to substantial increases in the fraction of unfolded protein. Calorimetry, ultracentrifugation, and light scattering show that all mutations enhance aggregation propensity, with the effects varying widely, from subtle increases in most cases, to pronounced formation of 40-100 nm soluble aggregates by A4V, a mutation that is associated with particularly short disease duration. Interestingly, although there is a correlation between observed aggregation and stability, there is minimal to no correlation between observed aggregation, predicted aggregation propensity, and disease characteristics. These findings suggest that reduced apo SOD1 does not play a dominant role in modulating disease. Rather, additional and/or multiple forms of SOD1 and additional biophysical and biological factors are needed to account for the toxicity of mutant SOD1 in ALS.

Point mutations in Aβ result in the formation of distinct polymorphic aggregates in the presence of lipid bilayers

A hallmark of Alzheimer's disease (AD) is the rearrangement of the β-amyloid (Aβ) peptide to a non-native conformation that promotes the formation of toxic, nanoscale aggregates. Recent studies have pointed to the role of sample preparation in creating polymorphic fibrillar species. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function on cellular surfaces. There are several mutations clustered around the central hydrophobic core of Aβ near the α-secretase cleavage site (E22G Arctic mutation, E22K Italian mutation, D23N Iowa mutation, and A21G Flemish mutation). These point mutations are associated with hereditary diseases ranging from almost pure cerebral amyloid angiopathy (CAA) to typical Alzheimer's disease pathology with plaques and tangles. We investigated how these point mutations alter Aβ aggregation in the presence of supported lipid membranes comprised of total brain lipid extract. Brain lipid extract bilayers were used as a physiologically relevant model of a neuronal cell surface. Intact lipid bilayers were exposed to predominantly monomeric preparations of Wild Type or different mutant forms of Aβ, and atomic force microscopy was used to monitor aggregate formation and morphology as well as bilayer integrity over a 12 hour period. The goal of this study was to determine how point mutations in Aβ, which alter peptide charge and hydrophobic character, influence interactions between Aβ and the lipid surface. While fibril morphology did not appear to be significantly altered when mutants were prepped similarly and incubated under free solution conditions, aggregation in the lipid membranes resulted in a variety of polymorphic aggregates in a mutation dependent manner. The mutant peptides also had a variable ability to disrupt bilayer integrity.

Human phenotypically distinct TGFBI corneal dystrophies are linked to the stability of the fourth FAS1 domain of TGFBIp

Mutations in the human TGFBI gene encoding TGFBIp have been linked to protein deposits in the cornea leading to visual impairment. The protein consists of an N-terminal Cys-rich EMI domain and four consecutive fasciclin 1 (FAS1) domains. We have compared the stabilities of wild-type (WT) human TGFBIp and six mutants known to produce phenotypically distinct deposits in the cornea. Amino acid substitutions in the first FAS1 (FAS1-1) domain (R124H, R124L, and R124C) did not alter the stability. However, substitutions within the fourth FAS1 (FAS1-4) domain (A546T, R555Q, and R555W) affected the overall stability of intact TGFBIp revealing the following stability ranking R555W>WT>R555Q>A546T. Significantly, the stability ranking of the isolated FAS1-4 domains mirrored the behavior of the intact protein. In addition, it was linked to the aggregation propensity as the least stable mutant (A546T) forms amyloid fibrils while the more stable variants generate non-amyloid amorphous deposits in vivo. Significantly, the data suggested that both an increase and a decrease in the stability of FAS1-4 may unleash a disease mechanism. In contrast, amino acid substitutions in FAS1-1 did not affect the stability of the intact TGFBIp suggesting that molecular the mechanism of disease differs depending on the FAS1 domain carrying the mutation.

Strategies for stabilizing superoxide dismutase (SOD1), the protein destabilized in the most common form of familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a disorder characterized by the death of both upper and lower motor neurons and by 3- to 5-yr median survival postdiagnosis. The only US Food and Drug Administration-approved drug for the treatment of ALS, Riluzole, has at best, moderate effect on patient survival and quality of life; therefore innovative approaches are needed to combat neurodegenerative disease. Some familial forms of ALS (fALS) have been linked to mutations in the Cu/Zn superoxide dismutase (SOD1). The dominant inheritance of mutant SOD1 and lack of symptoms in knockout mice suggest a "gain of toxic function" as opposed to a loss of function. A prevailing hypothesis for the mechanism of the toxicity of fALS-SOD1 variants, or the gain of toxic function, involves dimer destabilization and dissociation as an early step in SOD1 aggregation. Therefore, stabilizing the SOD1 dimer, thus preventing aggregation, is a potential therapeutic strategy. Here, we report a strategy in which we chemically cross-link the SOD1 dimer using two adjacent cysteine residues on each respective monomer (Cys111). Stabilization, measured as an increase in melting temperature, of ∼20 °C and ∼45 °C was observed for two mutants, G93A and G85R, respectively. This stabilization is the largest for SOD1, and to the best of our knowledge, for any disease-related protein. In addition, chemical cross-linking conferred activity upon G85R, an otherwise inactive mutant. These results demonstrate that targeting these cysteine residues is an important new strategy for development of ALS therapies.

Role of partial protein unfolding in alcohol-induced protein aggregation

Proteins aggregate in response to various stresses including changes in solvent conditions. Addition of alcohols has been recently shown to induce aggregation of disease-related as well as nondisease-related proteins. Here we probed the biophysical mechanisms underlying alcohol-induced protein aggregation, in particular the role of partial protein unfolding in aggregation. We have studied aggregation mechanisms due to benzyl alcohol which is used in numerous biochemical and biotechnological applications. We chose cytochrome c as a model protein, for the reason that various optical and structural probes are available to monitor its global and partial unfolding reactions. Benzyl alcohol induced the aggregation of cytochrome c in isothermal conditions and decreased the temperature at which the protein aggregates. However, benzyl alcohol did not perturb the overall native conformation of cytochrome c. Instead, it caused partial unfolding of a local protein region around the methionine residue at position 80. Site-specific optical probes, two-dimensional NMR titrations, and hydrogen exchange all support this conclusion. The protein aggregation temperature varied linearly with the melting temperature of the Met80 region. Stabilizing the Met80 region by heme iron reduction drastically decreased protein aggregation, which confirmed that the local unfolding of this region causes protein aggregation. These results indicate that a possible mechanism by which alcohols induce protein aggregation is through partial rather than complete unfolding of native proteins.

Nonamyloid aggregates arising from mature copper/zinc superoxide dismutases resemble those observed in amyotrophic lateral sclerosis

Protein aggregation is a hallmark of many diseases, including amyotrophic lateral sclerosis (ALS) where aggregation of copper/zinc superoxide dismutase (SOD1) is implicated in pathogenesis. We report here that fully metallated (holo) SOD1 under physiologically relevant solution conditions can undergo changes in metallation and/or dimerization over time and form aggregates that do not exhibit classical characteristics of amyloid. The relevance of the observed aggregation to disease is demonstrated by structural and tinctorial analyses, including the novel observation of binding of an anti-SOD1 antibody that specifically recognizes aggregates in ALS patients and mice models. ALS-associated SOD1 mutations can promote aggregation but are not essential. The SOD1 aggregation is characterized by a lag phase, which is diminished by self- or cross-seeding and by heterogeneous nucleation. We interpret these findings in terms of an expanded aggregation mechanism consistent with other in vitro and in vivo findings that point to multiple pathways for the formation of toxic aggregates by different forms of SOD1.

Cellular strategies for controlling protein aggregation

The aggregation of misfolded proteins is associated with the perturbation of cellular function, ageing and various human disorders. Mounting evidence suggests that protein aggregation is often part of the cellular response to an imbalanced protein homeostasis rather than an unspecific and uncontrolled dead-end pathway. It is a regulated process in cells from bacteria to humans, leading to the deposition of aggregates at specific sites. The sequestration of misfolded proteins in such a way is protective for cell function as it allows for their efficient solubilization and refolding or degradation by components of the protein quality-control network. The organized aggregation of misfolded proteins might also allow their asymmetric distribution to daughter cells during cell division.

Perturbed amelogenin secondary structure leads to uncontrolled aggregation in amelogenesis imperfecta mutant proteins

Mutations in amelogenin sequence result in defective enamel, and the diverse group of genetically altered conditions is collectively known as amelogenesis imperfecta (AI). Despite numerous studies, the detailed molecular mechanism of defective enamel formation is still unknown. In this study, we have examined the biophysical properties of a recombinant murine amelogenin (rM180) and two point mutations identified from human DNA sequences in two cases of AI (T21I and P41T). At pH 5.8 and 25 °C, wild type (WT) rM180 and mutant P41T existed as monomers, and mutant T21I formed lower order oligomers. CD, dynamic light scattering, and fluorescence studies indicated that rM180 and P41T can be classified as a premolten globule-like subclass protein at 25 °C. Thermal denaturation and refolding monitored by CD ellipticity at 224 nm indicated the presence of a strong hysteresis in mutants compared with WT. Variable temperature tryptophan fluorescence and dynamic light scattering studies showed that WT transformed to a partially folded conformation upon heating and remained stable. The partially folded conformation formed by P41T, however, readily converted into a heterogeneous population of aggregates. T21I existed in an oligomeric state at room temperature and, upon heating, rapidly formed large aggregates over a very narrow temperature range. Thermal denaturation and refolding studies indicated that the mutants are less stable and exhibit poor refolding ability compared with WT rM180. Our results suggest that alterations in self-assembly of amelogenin are a consequence of destabilization of the intrinsic disorder. Therefore, we propose that, like a number of other human diseases, AI appears to be due to the destabilization of the secondary structure as a result of amelogenin mutations.

Lipid molecules induce the cytotoxic aggregation of Cu/Zn superoxide dismutase with structurally disordered regions

Cu/Zn-superoxide dismutase (SOD1) is present in the cytosol, nucleus, peroxisomes and mitochondrial intermembrane space of human cells. More than 114 variants of human SOD1 have been linked to familial amyotrophic lateral sclerosis (ALS), which is also known as Lou Gehrig's disease. Although the ultimate mechanisms underlying SOD1-mediated cytotoxicity are largely unknown, SOD1 aggregates have been strongly implicated as a common feature in ALS. This study examined the mechanism for the formation of SOD1 aggregates in vitro as well as the nature of its cytotoxicity. The aggregation propensity of SOD1 species was investigated using techniques ranging from circular dichroism spectroscopy to fluorescence dye binding methods, as well as electron microscopic imaging. The aggregation of SOD1 appears to be related to its structural instability. The demetallated (apo)-SOD1 and aggregated SOD1 species, with structurally disordered regions, readily undergo aggregation in the presence of lipid molecules, whereas metallated (holo)-SOD1 does not. The majority of aggregated SOD1s that are induced by lipid molecules have an amorphous morphology and exhibit significant cytotoxicity. The lipid binding propensity of SOD1 was found to be closely related to the changes in surface hydrophobicity of the proteins, even at very low levels, which induced further binding and assembly with lipid molecules. These findings suggest that lipid molecules induce SOD1 aggregation under physiological conditions and exert cytotoxicity, and might provide a possible mechanism for the pathogenesis of ALS.

DNA-triggered aggregation of copper, zinc superoxide dismutase in the presence of ascorbate

The oxidative damage hypothesis proposed for the function gain of copper, zinc superoxide dismutase (SOD1) maintains that both mutant and wild-type (WT) SOD1 catalyze reactions with abnormal substrates that damage cellular components critical for viability of the affected cells. However, whether the oxidative damage of SOD1 is involved in the formation of aggregates rich in SOD1 or not remains elusive. Here, we sought to explore the oxidative aggregation of WT SOD1 exposed to environments containing both ascorbate (Asc) and DNA under neutral conditions. The results showed that the WT SOD1 protein was oxidized in the presence of Asc. The oxidation results in the higher affinity of the modified protein for DNA than that of the unmodified protein. The oxidized SOD1 was observed to be more prone to aggregation than the WT SOD1, and the addition of DNA can significantly accelerate the oxidative aggregation. Moreover, a reasonable relationship can be found between the oxidation, increased hydrophobicity, and aggregation of SOD1 in the presence of DNA. The crucial step in aggregation is neutralization of the positive charges on some SOD1 surfaces by DNA binding. This study might be crucial for understanding molecular forces driving the protein aggregation.

SOD1 mutations targeting surface hydrogen bonds promote amyotrophic lateral sclerosis without reducing apo-state stability

In good accord with the protein aggregation hypothesis for neurodegenerative diseases, ALS-associated SOD1 mutations are found to reduce structural stability or net repulsive charge. Moreover there are weak indications that the ALS disease progression rate is correlated with the degree of mutational impact on the apoSOD1 structure. A bottleneck for obtaining more conclusive information about these structure-disease relationships, however, is the large intrinsic variability in patient survival times and insufficient disease statistics for the majority of ALS-provoking mutations. As an alternative test of the structure-disease relationship we focus here on the SOD1 mutations that appear to be outliers in the data set. The results identify several ALS-provoking mutations whose only effect on apoSOD1 is the elimination or introduction of a single charge, i.e. D76V/Y, D101N, and N139D/K. The thermodynamic stability and folding behavior of these mutants are indistinguishable from the wild-type control. Moreover, D101N is an outlier in the plot of stability loss versus patient survival time by having rapid disease progression. Common to the identified mutations is that they truncate conserved salt-links and/or H-bond networks in the functional loops IV or VII. The results show that the local impact of ALS-associated mutations on the SOD1 molecule can sometimes overrun their global effects on apo-state stability and net repulsive charge, and point at the analysis of property outliers as an efficient strategy for mapping out new ALS-provoking features.

Wild-type human SOD1 overexpression does not accelerate motor neuron disease in mice expressing murine Sod1 G86R

Approximately 10% of the cases of amyotrophic lateral sclerosis (ALS) are inherited, with the majority of identified linkages in the gene encoding Cu/Zn superoxide dismutase (SOD1). Recent studies showed that human wild-type SOD1 (SOD1(WT)) overexpression accelerated disease in mice expressing human SOD1 mutants linked to ALS. However, there is a controversy whether the exacerbation mechanism occurs through coaggregation of human SOD1(WT) with SOD1 mutants, stabilization by SOD1(WT) of toxic soluble SOD1 species, or conversion of SOD1(WT) into toxic species through oxidative damage. To further address whether the exacerbation of disease requires misfolding, modifications, and/or interaction of SOD1(WT) with pathogenic forms of SOD1 species, we have studied the effect of human SOD1(WT) overexpression in mice expressing the murine mutant Sod1(G86R). Surprisingly, unlike a previous report with SOD1(G85R) mice, SOD1(WT) overexpression did not affect the life span of Sod1(G86R) mice. Our analysis of spinal cord extracts revealed a lack of heterodimerization or aggregation between human SOD1(WT) and mouse Sod1(G86R) proteins. Moreover, there was no evidence of conversion of SOD1(WT) into misfolded or abnormal SOD1 isoforms based on immunoreactivity with monoclonal antibodies specific to misfolded forms of SOD1 mutants and on analysis of SOD1 isoforms after two-dimensional gel electrophoresis. We conclude that a direct interaction between wild type and mutant forms of SOD1 is required for exacerbation of ALS disease by SOD1(WT) protein.

G93A SOD1 alters cell cycle in a cellular model of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative multifactorial disease characterized, like other diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) or frontotemporal dementia (FTD), by the degeneration of specific neuronal cell populations. Motor neuron loss is distinctive of ALS. However, the causes of onset and progression of motor neuron death are still largely unknown. In about 2% of all cases, mutations in the gene encoding for the Cu/Zn superoxide dismutase (SOD1) are implicated in the disease. Several alterations in the expression or activation of cell cycle proteins have been described in the neurodegenerative diseases and related to cell death. In this work we show that mutant SOD1 can alter cell cycle in a cellular model of ALS. Our findings suggest that modifications in the cell cycle progression could be due to an increased interaction between mutant G93A SOD1 and Bcl-2 through the cyclins regulator p27. As previously described in post mitotic neurons, cell cycle alterations could fatally lead to cell death.

Superoxide dismutase 1 and tgSOD1 mouse spinal cord seed fibrils, suggesting a propagative cell death mechanism in amyotrophic lateral sclerosis

Background

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that specifically affects motor neurons and leads to a progressive and ultimately fatal loss of function, resulting in death typically within 3 to 5 years of diagnosis. The disease starts with a focal centre of weakness, such as one limb, and appears to spread to other parts of the body. Mutations in superoxide dismutase 1 (SOD1) are known to cause disease and it is generally accepted they lead to pathology not by loss of enzymatic activity but by gain of some unknown toxic function(s). Although different mutations lead to varying tendencies of SOD1 to aggregate, we suggest abnormal proteins share a common misfolding pathway that leads to the formation of amyloid fibrils.

Methodology/Principal Findings

Here we demonstrate that misfolding of superoxide dismutase 1 leads to the formation of amyloid fibrils associated with seeding activity, which can accelerate the formation of new fibrils in an autocatalytic cascade. The time limiting event is nucleation to form a stable protein “seed” before a rapid linear polymerisation results in amyloid fibrils analogous to other protein misfolding disorders. This phenomenon was not confined to fibrils of recombinant protein as here we show, for the first time, that spinal cord homogenates obtained from a transgenic mouse model that overexpresses mutant human superoxide dismutase 1 (the TgSOD1^G93A mouse) also contain amyloid seeds that accelerate the formation of new fibrils in both wildtype and mutant SOD1 protein in vitro.

Conclusions/Significance

These findings provide new insights into ALS disease mechanism and in particular a mechanism that could account for the spread of pathology throughout the nervous system. This model of disease spread, which has analogies to other protein misfolding disorders such as prion disease, also suggests it may be possible to design assays for therapeutics that can inhibit fibril propagation and hence, possibly, disease progression.

Exposure of hydrophobic surfaces initiates aggregation of diverse ALS-causing superoxide dismutase-1 mutants

The copper-zinc superoxide dismutase-1 (SOD1) is a highly structured protein and, a priori, one of the least likely proteins to be involved in a misfolding disease. However, more than 140, mostly missense, mutations in the SOD1 gene cause aggregation of the affected protein in familial forms of amyotrophic lateral sclerosis (ALS). The remarkable diversity of the effects of these mutations on SOD1 properties has suggested that they promote aggregation by a variety of mechanisms. Experimental assessment of surface hydrophobicity using a sensitive fluorescent-based assay, revealed that diverse ALS-causing mutations provoke SOD1 aggregation by increasing their propensity to expose hydrophobic surfaces. These findings could not be anticipated from analysis of the amino acid sequence. Our results uncover the biochemical nature of the misfolded aggregation-prone intermediate and reconcile the seemingly diverse effects of ALS-causing mutations into a unifying mechanism. Furthermore, the method we describe here will be useful for investigating and interfering with aggregation of various proteins and thereby provide insight into the molecular mechanisms underlying many neurodegenerative diseases.

Metal-free ALS variants of dimeric human Cu,Zn-superoxide dismutase have enhanced populations of monomeric species

Amino acid replacements at dozens of positions in the dimeric protein human, Cu,Zn superoxide dismutase (SOD1) can cause amyotrophic lateral sclerosis (ALS). Although it has long been hypothesized that these mutations might enhance the populations of marginally-stable aggregation-prone species responsible for cellular toxicity, there has been little quantitative evidence to support this notion. Perturbations of the folding free energy landscapes of metal-free versions of five ALS-inducing variants, A4V, L38V, G93A, L106V and S134N SOD1, were determined with a global analysis of kinetic and thermodynamic folding data for dimeric and stable monomeric versions of these variants. Utilizing this global analysis approach, the perturbations on the global stability in response to mutation can be partitioned between the monomer folding and association steps, and the effects of mutation on the populations of the folded and unfolded monomeric states can be determined. The 2- to 10-fold increase in the population of the folded monomeric state for A4V, L38V and L106V and the 80- to 480-fold increase in the population of the unfolded monomeric states for all but S134N would dramatically increase their propensity for aggregation through high-order nucleation reactions. The wild-type-like populations of these states for the metal-binding region S134N variant suggest that even wild-type SOD1 may also be prone to aggregation in the absence of metals.

Low-level expression of a folding-incompetent protein in Escherichia coli: search for the molecular determinants of protein aggregation in vivo

Aggregation of peptides and proteins into insoluble amyloid fibrils or related intracellular inclusions is the hallmark of many degenerative diseases, including Alzheimer's disease, Parkinson's disease, and various forms of amyloidosis. In spite of the considerable progress carried out in vitro in elucidating the molecular determinants of the conversion of purified and isolated proteins into amyloid fibrils, very little is known on factors governing this process in the complex environment of living organisms. Taking advantage of increasing evidence that bacterial inclusion bodies consist of amyloid-like aggregates, we have expressed in Escherichia coli both wild type and 21 single-point mutants of the N-terminal domain of the E. coli protein HypF. All variants were expressed as folding-incompetent units in a controlled manner, at low and comparable levels. Their solubilities were measured by quantifying the protein amount contained in the soluble and insoluble fractions by Western blot analysis. A significant negative correlation was found between the solubility of the variants in E. coli and their intrinsic propensity to form amyloid fibrils, predicted using an algorithm previously validated experimentally in vitro on a number of unfolded peptides and proteins, and considering hydrophobicity, beta-sheet propensity, and charge as major sequence determinants of the aggregation process. These findings show that the physicochemical parameters previously recognized to govern amyloid formation by fully or partially unfolded proteins are largely applicable in vivo and pave the way for the molecular exploration of a process as complex as protein aggregation in living organisms.

Amyotrophic lateral sclerosis is a non-amyloid disease in which extensive misfolding of SOD1 is unique to the familial form

Amyotrophic lateral sclerosis (ALS) is a conformational disease in which misfolding and aggregation of proteins such as SOD1 (familial ALS) and TDP-43 (sporadic ALS) are central features. The conformations adopted by such proteins within motor neurons in affected patients are not well known. We have developed a novel conformation-specific antibody (USOD) targeted against SOD1 residues 42-48 that specifically recognizes SOD1 in which the beta barrel is unfolded. Use of this antibody, in conjunction with the previously described SEDI antibody that recognizes the SOD1 dimer interface, allows a detailed investigation of the in vivo conformation of SOD1 at the residue-specific level. USOD and SEDI immunohistochemistry of spinal cord sections from ALS cases resulting from SOD1 mutations (A4V and DeltaG27/P28) shows that inclusions within remaining motor neurons contain SOD1 with both an unfolded beta barrel and a disrupted dimer interface. Misfolded SOD1 can also be immunoprecipitated from spinal cord extracts of these cases using USOD. However, in ten cases of sporadic ALS, misfolded SOD1 is not detected by either immunohistochemistry or immunoprecipitation. Using the amyloid-specific dyes, Congo Red and Thioflavin S, we find that SOD1-positive inclusions in familial ALS, as well as TDP-43- and ubiquitin-positive inclusions in sporadic ALS, contain non-amyloid protein deposits. We conclude that SOD1 misfolding is not a feature of sporadic ALS, and that both SOD1-ALS and sporadic ALS, rather than being amyloid diseases, are conformational diseases that involve amorphous aggregation of misfolded protein. This knowledge will provide new insights into subcellular events that cause misfolding, aggregation and toxicity.

Effect of rosmarinic acid in motor dysfunction and life span in a mouse model of familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a late-onset progressive neurodegenerative disease affecting motor neurons. About 2% of patients with the disease are associated with mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1). The purpose of this study is to assess the effect of rosemary extract and its major constituents, rosmarinic acid (RA) and carnosic acid (CA), in human SOD1 G93A transgenic mice, which are well-established mouse models for ALS. The present study demonstrates that intraperitoneal administration of rosemary extract or RA from the presymptomatic stage significantly delayed motor dysfunction in paw grip endurance tests, attenuated the degeneration of motor neurons, and extended the life span of ALS model mice. In addition, RA administration significantly improved the clinical score and suppressed body weight loss compared with a vehicle-treated group. In conclusion, this study provides the first report that rosemary extract and, especially, RA have preventive effects in the mouse model of ALS.

Lack of evidence of monomer/misfolded superoxide dismutase-1 in sporadic amyotrophic lateral sclerosis

OBJECTIVE

In familial amyotrophic lateral sclerosis (fALS) harboring superoxide dismutase (SOD1) mutations (fALS1), SOD1 toxicity has been linked to its propensity to misfold and aggregate. It has recently been proposed that misfolded SOD1 may be causative of all types of ALS, including sporadic cases (sALS). In the present study, we have used a specific antibody to test for the presence of monomer/misfolded SOD1 in sALS.

METHODS

Sections from lumbar spinal cords of 5 fALS1 cases, 13 sALS cases, and 1 non-SOD1 fALS case were labeled immunocytochemically using SOD1-exposed-dimer-interface (SEDI) antibody, which we have previously validated as being specific for pathological monomer/misfolded forms of SOD1.

RESULTS

Monomer/misfolded SOD1 was detected with SEDI antibody in all 5 of the fALS1 cases, localizing predominantly to hyaline conglomerate inclusions, a specific pathological feature of fALS1. In contrast, monomer/misfolded SOD1 was not detected in any of the 13 sALS cases or in the non-SOD1 fALS cases. These results were confirmed by immunoprecipitation.

INTERPRETATION

Although SEDI antibody does not necessarily label all misfolded forms of SOD1, these findings show a distinct difference between fALS1 and sALS, and do not support that monomer/misfolded SOD1 is a common disease entity linking all types of ALS. This is important to our understanding of ALS disease pathogenesis and to considerations of the applicability of using therapeutics that target misfolded SOD1 to non-SOD1-related cases. Ann Neurol 2009;66:75-80.

Amyloidogenic regions and interaction surfaces overlap in globular proteins related to conformational diseases

Protein aggregation underlies a wide range of human disorders. The polypeptides involved in these pathologies might be intrinsically unstructured or display a defined 3D-structure. Little is known about how globular proteins aggregate into toxic assemblies under physiological conditions, where they display an initially folded conformation. Protein aggregation is, however, always initiated by the establishment of anomalous protein-protein interactions. Therefore, in the present work, we have explored the extent to which protein interaction surfaces and aggregation-prone regions overlap in globular proteins associated with conformational diseases. Computational analysis of the native complexes formed by these proteins shows that aggregation-prone regions do frequently overlap with protein interfaces. The spatial coincidence of interaction sites and aggregating regions suggests that the formation of functional complexes and the aggregation of their individual subunits might compete in the cell. Accordingly, single mutations affecting complex interface or stability usually result in the formation of toxic aggregates. It is suggested that the stabilization of existing interfaces in multimeric proteins or the formation of new complexes in monomeric polypeptides might become effective strategies to prevent disease-linked aggregation of globular proteins.

The aggregation of proteins in tissues is associated with the pathogenesis of more than 40 human diseases. The polypeptides underlying disorders such as Alzheimer's and Parkinson's are devoid of any regular structure, whereas the polypeptides causing familial amyotrophic lateral sclerosis or nonneuropathic systemic amyloidosis correspond to globular proteins. Little is known about the mechanism by which globular proteins under physiological conditions aggregate from their initially folded and soluble conformations. Interestingly, several of these pathogenic proteins display quaternary structure or are bound to other proteins in their physiological context. In the present work, we show that protein-protein interaction surfaces and regions with high aggregation propensity significantly overlap in these polypeptides. This suggests that the formation of native complexes and self-aggregation reactions probably compete in the cell, explaining why point mutations affecting the interface or the stability of the protein complex lead in many cases to the formation of toxic aggregates. This study proposes general strategies to fight against diseases associated with the deposition of globular polypeptides.

Metal deficiency increases aberrant hydrophobicity of mutant superoxide dismutases that cause amyotrophic lateral sclerosis

The mechanisms by which mutant variants of Cu/Zn-superoxide dismutase (SOD1) cause familial amyotrophic lateral sclerosis are not clearly understood. Evidence to date suggests that altered conformations of amyotrophic lateral sclerosis mutant SOD1s trigger perturbations of cellular homeostasis that ultimately cause motor neuron degeneration. In this study we correlated the metal contents and disulfide bond status of purified wild-type (WT) and mutant SOD1 proteins to changes in electrophoretic mobility and surface hydrophobicity as detected by 1-anilinonaphthalene-8-sulfonic acid (ANS) fluorescence. As-isolated WT and mutant SOD1s were copper-deficient and exhibited mobilities that correlated with their expected negative charge. However, upon disulfide reduction and demetallation at physiological pH, both WT and mutant SOD1s underwent a conformational change that produced a slower mobility indicative of partial unfolding. Furthermore, although ANS did not bind appreciably to the WT holoenzyme, incubation of metal-deficient WT or mutant SOD1s with ANS increased the ANS fluorescence and shifted its peak toward shorter wavelengths. This increased interaction with ANS was greater for the mutant SOD1s and could be reversed by the addition of metal ions, especially Cu(2+), even for SOD1 variants incapable of forming the disulfide bond. Overall, our findings support the notion that misfolding associated with metal deficiency may facilitate aberrant interactions of SOD1 with itself or with other cellular constituents and may thereby contribute to neuronal toxicity.

A role for copper in the toxicity of zinc-deficient superoxide dismutase to motor neurons in amyotrophic lateral sclerosis

In the 16 years since mutations to copper, zinc superoxide dismutase (SOD1) were first linked to familial amyotrophic lateral sclerosis (ALS), a multitude of apparently contradictory results have prevented any general consensus to emerge about the mechanism of toxicity. A decade ago, we showed that the loss of zinc from SOD1 results in the remaining copper in SOD1 to become extremely toxic to motor neurons in culture by a mechanism requiring nitric oxide. The loss of zinc causes SOD1 to become more accessible, more redox reactive, and a better catalyst of tyrosine nitration. Although SOD1 mutant proteins have a modestly reduced affinity for zinc, wild-type SOD1 can be induced to lose zinc by dialysis at slightly acidic pH. Our zinc-deficient hypothesis offers a compelling explanation for how mutant SOD1s have an increased propensity to become selectively toxic to motor neurons and also explains how wild-type SOD1 can be toxic in nonfamilial ALS patients. One critical prediction is that a therapeutic agent directed at zinc-deficient mutant SOD1 could be even more effective in treating sporadic ALS patients. Although transgenic mice experiments have yielded contradictory evidence to the zinc-deficient hypothesis, we will review more recent studies that support a role for copper in ALS. A more careful examination of the role of copper and zinc binding to SOD1 may help counter the growing disillusion in the ALS field about understanding the pathological role of SOD1.

Aggregation of copper-zinc superoxide dismutase in familial and sporadic ALS

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal neurodegenerative disease characterized by the selective death of motor neurons. While the most common form of ALS is sporadic and has no known cause, a small subset of cases is familial because of underlying genetic mutations. The best-studies example of familial ALS is that caused by mutations in the protein copper-zinc superoxide dismutase. The formation of SOD1-rich inclusions in the spinal cord is an early and prominent feature of SOD1-linked familial ALS in human patients and animal models of this disease. These inclusions have been shown to consist of SOD1-rich fibrils, suggesting that the conversion of soluble SOD1 into amyloid fibrils may play an important role in the etiology of familial ALS. SOD1 is also present in inclusions found in spinal cords of sporadic ALS patients, allowing speculations to arise regarding a possible involvement of SOD1 in the sporadic form of this disease. We here review the recent research on the significance, causes, and mechanisms of SOD1 fibril formation from a biophysical perspective.

Direct magnetic resonance evidence for peroxymonocarbonate involvement in the cu,zn-superoxide dismutase peroxidase catalytic cycle

Cu,Zn-superoxide dismutase (SOD1) is a copper- and zinc-dependent enzyme. The main function of SOD1 is believed to be the scavenging and detoxification of superoxide radicals. Nevertheless, the last 30 years have seen a rapid accumulation of evidence indicating that SOD1 may also act as a peroxidase, an alternative function that was implicated in the onset and progression of familial amyotrophic lateral sclerosis. Although SOD1 peroxidase activity and its dependence on carbon dioxide have been well described, the molecular basis of the SOD1 peroxidase cycle remains obscure, because none of the proposed catalytic intermediates have so far been identified. In view of recent observations, we hypothesized that the SOD1 peroxidase cycle relies on two steps: 1) reduction of SOD-Cu(II) by hydrogen peroxide followed by 2) oxidation of SOD-Cu(I) by peroxymonocarbonate, the product of the spontaneous reaction of bicarbonate with hydrogen peroxide, to produce SOD-Cu(II) and carbonate radical anion. This hypothesis has been investigated through electron paramagnetic resonance and nuclear magnetic resonance to provide direct evidence for a peroxycarbonate-driven, SOD1-catalyzed carbonate radical production. The results gathered herein indicate that peroxymonocarbonate (HOOCO(2)(-)) is a key intermediate in the SOD1 peroxidase cycle and identify this species as the precursor of carbonate radical anions.

Sensitive and colorimetric detection of the structural evolution of superoxide dismutase with gold nanoparticles

The detection and characterization of protein aggregates are critical in terms of advanced diagnostic applications and investigations of protein stability. A variety of analytical methods (e.g., circular dichroism, size exclusion chromatography, and fluorescence microscopy) have been used in this regard, but they are limited in the trace detection of the structural evolution of protein aggregation. Here we report the gold nanoparticle (AuNP)-based highly sensitive and colorimetric detection of the temporal evolution of superoxide dismutase (SOD1) aggregates implicated in the pathology of amyotrophic lateral sclerosis (ALS). For the temporal discrimination of SOD1 aggregation, AuNPs were conjugated with SOD1 monomers (SOD1-AuNPs). Upon exposure of the probes (SOD1-AuNPs) with SOD1 aggregates, significant changes in both surface plasmon resonance spectra and concomitant colors were observed which are attributed to the formation of probe aggregates of variable sizes onto the SOD1 aggregates.

Amyloid formation by globular proteins under native conditions

The conversion of proteins from their soluble states into well-organized fibrillar aggregates is associated with a wide range of pathological conditions, including neurodegenerative diseases and systemic amyloidoses. In this review, we discuss the mechanism of aggregation of globular proteins under conditions in which they are initially folded. Although a conformational change of the native state is generally necessary to initiate aggregation, we show that a transition across the major energy barrier for unfolding is not essential and that aggregation may well be initiated from locally unfolded states that become accessible, for example, via thermal fluctuations occurring under physiological conditions. We review recent evidence on this topic and discuss its significance for understanding the onset and potential inhibition of protein aggregation in the context of diseases.

Polymorphism of the SOD1-DNA aggregation species can be modulated by DNA

Proteinaceous aggregates rich in copper, zinc superoxide dismutase (SOD1) have been found in both in vivo and in vitro models. We have shown that double-stranded DNA that acts as a template accelerates the in vitro formation of wild-type SOD1 aggregates. Here, we examined the polymorphism of templated-SOD1 aggregates generated in vitro upon association with DNA under different conditions. Electron microscopy imaging indicates that this polymorphism is capable of being manipulated by the shapes, structures, and doses of the DNAs tested. The nanometer- and micrometer-scale aggregates formed under acidic conditions and under neutral conditions containing ascorbate fall into three classes: aggregate monomers, oligomeric aggregates, and macroaggregates. The aggregate monomers observed at given DNA doses exhibit a polymorphism that is markedly corresponded to the coiled shapes of linear DNA and structures of plasmid DNA. On the other hand, the regularly branched structures observed under both atomic force microscopy and optical microscope indicate that the DNAs tested are simultaneously condensed into a nanoparticle with a specific morphology during SOD1 aggregation, revealing that SOD1 aggregation and DNA condensation are two concurrent phenomena. The results might provide the basis of therapeutic approaches to suppress the formation of toxic protein oligomers or aggregates by screening the toxicity of the protein aggregates with various sizes and morphologies.

Protein aggregation and protein instability govern familial amyotrophic lateral sclerosis patient survival

The nature of the "toxic gain of function" that results from amyotrophic lateral sclerosis (ALS)-, Parkinson-, and Alzheimer-related mutations is a matter of debate. As a result no adequate model of any neurodegenerative disease etiology exists. We demonstrate that two synergistic properties, namely, increased protein aggregation propensity (increased likelihood that an unfolded protein will aggregate) and decreased protein stability (increased likelihood that a protein will unfold), are central to ALS etiology. Taken together these properties account for 69% of the variability in mutant Cu/Zn-superoxide-dismutase-linked familial ALS patient survival times. Aggregation is a concentration-dependent process, and spinal cord motor neurons have higher concentrations of Cu/Zn-superoxide dismutase than the surrounding cells. Protein aggregation therefore is expected to contribute to the selective vulnerability of motor neurons in familial ALS.

The threat of instability: neurodegeneration predicted by protein destabilization and aggregation propensity

A new method based on protein stability and aggregation propensity reveals a strong correlation between the properties of mutant Cu/Zn-superoxide dismutases associated with amyotrophic lateral sclerosis and patient survival.

Initiation and elongation in fibrillation of ALS-linked superoxide dismutase

Familial amyotrophic lateral sclerosis (fALS) caused by mutations in copper-zinc superoxide dismutase (SOD1) is characterized by the presence of SOD1-rich inclusions in spinal cords. Similar inclusions observed in fALS transgenic mice have a fibrillar appearance suggestive of amyloid structure. Metal-free apo-SOD1 is a relatively stable protein and has been shown to form amyloid fibers in vitro only when it has been subjected to severely destabilizing conditions, such as low pH or reduction of its disulfide bonds. Here, by contrast, we show that a small amount of disulfide-reduced apo-SOD1 can rapidly initiate fibrillation of this exceptionally stable and highly structured protein under mild, physiologically accessible conditions, thus providing an unusual demonstration of a specific, physiologically relevant form of a protein acting as an initiating agent for the fibrillation of another form of the same protein. We also show that, once initiated, elongation can proceed via recruitment of either apo- or partially metallated disulfide-intact SOD1 and that the presence of copper, but not zinc, ions inhibits fibrillation. Our findings provide a rare glimpse into the specific changes in a protein that can lead to nucleation and into the ability of amyloid nuclei to recruit diverse forms of the same protein into fibrils.

SOD1-associated ALS: a promising system for elucidating the origin of protein-misfolding disease

Amyotropic lateral sclerosis (ALS) is a neurodegenerative disease linked to misfolding and aggregation of the homodimeric enzyme superoxide dismutase (SOD1). In contrast to the precursors of other neurodegenerative diseases, SOD1 is a soluble and simple-to-study protein with immunoglobulin-like structure. Also, there are more than 120 ALS-provoking SOD1 mutations at the disposal for detailed elucidation of the disease-triggering factors at molecular level. In this article, we review recent progress in the characterization of the folding and assembly pathway of the SOD1 dimer and how this is affected by ALS-provoking mutations. Despite the diverse nature of these mutations, the results offer so far a surprising simplicity. The ALS-provoking mutations decrease either protein stability or net repulsive charge: the classical hallmarks for a disease mechanism triggered by association of non-native protein. In addition, the mutant data identifies immature SOD1 monomers as the species from which the cytotoxic pathway emerges, and point at compromised folding cooperativity as a key disease determinant. The relative ease by which these data can be obtained makes SOD1 a promising model for elucidating also the origin of other neurodegenerative diseases where the precursor proteins are structurally more elusive.

A drosophila model for amyotrophic lateral sclerosis reveals motor neuron damage by human SOD1

Amyotrophic lateral sclerosis (ALS) is a motor neuron disease that leads to loss of motor function and early death. About 5% of cases are inherited, with the majority of identified linkages in the gene encoding copper, zinc-superoxide dismutase (SOD1). Strong evidence indicates that the SOD1 mutations confer dominant toxicity on the protein. To provide new insight into mechanisms of ALS, we have generated and characterized a model for familial ALS in Drosophila with transgenic expression of human SOD1. Expression of wild type or disease-linked (A4V, G85R) mutants of human SOD1 selectively in motor neurons induced progressive climbing deficits. These effects were accompanied by defective neural circuit electrophysiology, focal accumulation of human SOD1 protein in motor neurons, and a stress response in surrounding glia. However, toxicity was not associated with oligomerization of SOD1 and did not lead to neuronal loss. These studies uncover cell-autonomous injury by SOD1 to motor neurons in vivo, as well as non-autonomous effects on glia, and provide the foundation for new insight into injury and protection of motor neurons in ALS.

Conformational stability and folding mechanisms of dimeric proteins

The folding of multisubunit proteins is of tremendous biological significance since the large majority of proteins exist as protein-protein complexes. Extensive experimental and computational studies have provided fundamental insights into the principles of folding of small monomeric proteins. Recently, important advances have been made in extending folding studies to multisubunit proteins, in particular homodimeric proteins. This review summarizes the equilibrium and kinetic theory and models underlying the quantitative analysis of dimeric protein folding using chemical denaturation, as well as the experimental results that have been obtained. Although various principles identified for monomer folding also apply to the folding of dimeric proteins, the effects of subunit association can manifest in complex ways, and are frequently overlooked. Changes in molecularity typically give rise to very different overall folding behaviour than is observed for monomeric proteins. The results obtained for dimers have provided key insights pertinent to understanding biological assembly and regulation of multisubunit proteins. These advances have set the stage for future advances in folding involving protein-protein interactions for natural multisubunit proteins and unnatural assemblies involved in disease.

SOD1 and amyotrophic lateral sclerosis: mutations and oligomerization

There are about 100 single point mutations of copper, zinc superoxide dismutase 1 (SOD1) which are reported (http://alsod.iop.kcl.ac.uk/Als/index.aspx) to be related to the familial form (fALS) of amyotrophic lateral sclerosis (ALS). These mutations are spread all over the protein. It is well documented that fALS produces protein aggregates in the motor neurons of fALS patients, which have been found to be associated to mitochondria. We selected eleven SOD1 mutants, most of them reported as pathological, and characterized them investigating their propensity to aggregation using different techniques, from circular dichroism spectra to ThT-binding fluorescence, size-exclusion chromatography and light scattering spectroscopy. We show here that these eleven SOD1 mutants, only when they are in the metal-free form, undergo the same general mechanism of oligomerization as found for the WT metal-free protein. The rates of oligomerization are different but eventually they give rise to the same type of soluble oligomeric species. These oligomers are formed through oxidation of the two free cysteines of SOD1 (6 and 111) and stabilized by hydrogen bonds, between beta strands, thus forming amyloid-like structures. SOD1 enters the mitochondria as demetallated and mitochondria are loci where oxidative stress may easily occur. The soluble oligomeric species, formed by the apo form of both WT SOD1 and its mutants through an oxidative process, might represent the precursor toxic species, whose existence would also suggest a common mechanism for ALS and fALS. The mechanism here proposed for SOD1 mutant oligomerization is absolutely general and it provides a common unique picture for the behaviors of the many SOD1 mutants, of different nature and distributed all over the protein.