Accessibility of a critical prion protein region involved in strain recognition and its implications for the early detection of prions

Human prion diseases are characterized by the accumulation in the brain of proteinase K (PK)-resistant prion protein designated PrP27-30 detectable by the 3F4 antibody against human PrP109-112. We recently identified a new PK-resistant PrP species, designated PrP^*20, in uninfected human and animal brains. It was preferentially detected with the 1E4 antibody against human PrP 97-108 but not with the anti-PrP 3F4 antibody, although the 3F4 epitope is adjacent to the 1E4 epitope in the PrP^*20 molecule. The present study reveals that removal of the N-terminal amino acids up to residue 91 significantly increases accessibility of the 1E4 antibody to PrP of brains and cultured cells. In contrast to cells expressing wild-type PrP, cells expressing pathogenic mutant PrP accumulate not only PrP^*20 but also a small amount of 3F4-detected PK-resistant PrP27-30. Remarkably, during the course of human prion disease, a transition from an increase in 1E4-detected PrP^*20 to the occurrence of the 3F4-detected PrP27-30 was observed. Our study suggests that an increase in the level of PrP^*20 characterizes the early stages of prion diseases.

Species barriers for chronic wasting disease by in vitro conversion of prion protein

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy that can affect North American cervids (deer, elk, and moose). Using a novel in vitro conversion system based on incubation of prions with normal brain homogenates, we now report that PrP(CWD) of elk can readily induce the conversion of normal cervid PrP (PrP(C)) molecules to a protease-resistant form, but is less efficient in converting the PrP(C) of other species, such as human, bovine, hamster, and mouse. However, when substrate brain homogenates are partially denatured by acidic conditions (pH 3.5), PrP(CWD)-induced conversion can be greatly enhanced in all species. Our results demonstrate that PrP(C) from cervids (including moose) can be efficiently converted to a protease-resistant form by incubation with elk CWD prions, presumably due to sequence and structural similarities between these species. Moreover, partial denaturation of substrate PrP(C) can apparently overcome the structural barriers between more distant species.

An immunological epitope selective for pathological monomer-misfolded SOD1 in ALS

Misfolding of Cu/Zn-superoxide dismutase (SOD1) is emerging as a mechanism underlying motor neuron degeneration in individuals with amyotrophic lateral sclerosis (ALS) who carry a mutant SOD1 gene (SOD1 ALS). Here we describe a structure-guided approach to developing an antibody that specifically recognizes monomer-misfolded forms of SOD1. We raised this antibody to an epitope that is normally buried in the SOD1 native homodimer interface. The SOD1 exposed dimer interface (SEDI) antibody recognizes only those SOD1 conformations in which the native dimer is disrupted or misfolded and thereby exposes the hydrophobic dimer interface. Using the SEDI antibody, we established the presence of monomer-misfolded SOD1 in three ALS mouse models, with G37R, G85R and G93A SOD1 mutations, and in a human individual with an A4V SOD1 mutation. Despite ubiquitous expression, misfolded SOD1 was found primarily within degenerating motor neurons. Misfolded SOD1 appeared before the onset of symptoms and decreased at the end stage of the disease, concomitant with motor neuron loss.

Decreased cell surface prion protein in mouse models of prion disease

Transmissible spongiform encephalopathies are infectious neurodegenerative diseases caused by prions, composed of ordered aggregates of misfolded cellular prion protein. Neural antigen density of prion protein, Thy-1 and glial fibrillary acidic protein was analyzed using flow cytometry of dissociated mouse brain cells after inoculation with mouse-adapted transmissible spongiform encephalopathy agents. Transmissible spongiform encephalopathy gliosis was demonstrated by increased intracellular immunoreactivity for glial fibrillary acidic protein compared with controls. Immunoreactivity for cell surface prion protein was reduced 2.8-3.8-fold compared with control brain cells, whereas surface Thy-1 protein was reduced 1.5-4-fold. Double-staining protocols revealed loss of brain cells highly immunoreactive for prion protein and Thy-1, with a preferential reduction of prion protein, suggesting that prion protein expression, trafficking or consumption may be affected early in disease.

Current and future molecular diagnostics for prion diseases

It is now widely held that the infectious agents underlying the transmissible spongiform encephalopathies are prions, which are primarily composed of a misfolded, protease-resistant isoform of the host prion protein. Untreatable prion disorders include some human diseases, such as Creutzfeldt-Jakob disease, and diseases of economically important animals, such as bovine spongiform encephalopathy (cattle) and chronic wasting disease (deer and elk). Detection and diagnosis of prion disease (and presymptomatic incubation) is contingent upon developing novel assays, which exploit properties uniquely possessed by this misfolded protein complex, rather than targeting an agent-specific nucleic acid. This review highlights some of the conventional and disruptive technologies developed to respond to this challenge.

Cerebral degeneration predicts survival in amyotrophic lateral sclerosis

OBJECTIVE

To determine the relationship of cerebral degeneration with survival in amyotrophic lateral sclerosis (ALS).

METHODS

Patients with probable or definite ALS underwent magnetic resonance spectroscopic imaging (MRSI) of the brain between July 1996 and May 2002, and were followed prospectively until March 2004. Creatine (Cr), choline (Cho) and the neuronal marker N-acetylaspartate (NAA) were quantified as ratios in the motor cortex.

RESULTS

In 63 patients compared with 18 healthy people, NAA/Cho was reduced by 13% (p<0.001), NAA/Cr was reduced by 5% (p = 0.01) and Cho/Cr was increased by 8% (p = 0.01). NAA/Cho was used for survival analysis, given its larger effect size and superior test accuracy (a sensitivity of 67% and a specificity of 83%). Median survival after MRSI was 24 months. Multivariate analysis showed reduced survival for lower NAA/Cho (hazard ratio (HR) 0.24, 95% confidence interval (CI) 0.08 to 0.72, p = 0.01), older age (HR 1.03, 95% CI 1.00 to 1.06, p = 0.04) and shorter symptom duration (HR 0.96, 95% CI 0.93 to 0.99, p = 0.01). Patients with NAA/Cho <2.11 had a reduced survival of 19.4 v 31.9 months (HR 2.05, 95% CI 1.12 to 4.03, p = 0.02).

CONCLUSIONS

Cerebral degeneration is predictive of reduced survival in ALS.

Progress in prion vaccines and immunotherapies

The transmissible spongiform encephalopathies have presented a challenge to physicians and scientists attempting to develop immunologically-based treatments. Self-tolerance has been one of the major obstacles to successfully raising antibodies against the prion protein (PrP), the host-encoded protein whose misfolded form (PrPSc) is linked to the protein-only infectious agent responsible for these disorders. Recently, it has been shown that antibodies directed against the normal cellular isoform of PrP (PrPC) can reduce or eliminate PrP isoform conversion in both in vitro and in vivo model systems. Similar studies with a PrPSc-specific epitope target are in progress. There is now rational hope that this devastating group of diseases may soon be amenable to immunotherapy and immunoprophylaxis.

Mitochondrial proteomic analysis of a cell line model of familial amyotrophic lateral sclerosis

Mutations in copper-zinc superoxide dismutase (SOD1) have been linked to a subset of familial amytrophic lateral sclerosis (fALS), a fatal neurodegenerative disease characterized by progressive motor neuron death. An increasing amount of evidence supports that mitochondrial dysfunction and apoptosis activation play a critical role in the fALS etiology, but little is known about the mechanisms by which SOD1 mutants cause the mitochondrial dysfunction and apoptosis. In this study, we use proteomic approaches to identify the mitochondrial proteins that are altered in the presence of a fALS-causing mutant G93A-SOD1. A comprehensive characterization of mitochondrial proteins from NSC34 cells, a motor neuron-like cell line, was achieved by two independent proteomic approaches. Four hundred seventy unique proteins were identified in the mitochondrial fraction collectively, 75 of which are newly discovered proteins that previously had only been reported at the cDNA level. Two-dimensional gel electrophoresis was subsequently used to analyze the differences between the mitochondrial proteomes of NSC34 cells expressing wild-type and G93A-SOD1. Nine and 36 protein spots displayed elevated and suppressed abundance respectively in G93A-SOD1-expressing cells. The 45 spots were identified by MS, and they include proteins involved in mitochondrial membrane transport, apoptosis, the respiratory chain, and molecular chaperones. In particular, alterations in the post-translational modifications of voltage-dependent anion channel 2 (VDAC2) were found, and its relevance to regulating mitochondrial membrane permeability and activation of apoptotic pathways is discussed. The potential role of other proteins in the mutant SOD1-mediated fALS is also discussed. This study has produced a short list of mitochondrial proteins that may hold the key to the mechanisms by which SOD1 mutants cause mitochondrial dysfunction and neuronal death. It has laid the foundation for further detailed functional studies to elucidate the role of particular mitochondrial proteins, such as VDAC2, in the pathogenesis of familial ALS.

Prion diseases — close to effective therapy?

The transmissible spongiform encephalopathies could represent a new mode of transmission for infectious diseases--a process more akin to crystallization than to microbial replication. The prion hypothesis proposes that the normal isoform of the prion protein is converted to a disease-specific species by template-directed misfolding. Therapeutic and prophylactic strategies to combat these diseases have emerged from immunological and chemotherapeutic approaches. The lessons learned in treating prion disease will almost certainly have an impact on other diseases that are characterized by the pathological accumulation of misfolded proteins.

Monomeric Cu,Zn-superoxide dismutase is a common misfolding intermediate in the oxidation models of sporadic and familial amyotrophic lateral sclerosis

Proteinacious intracellular aggregates in motor neurons are a key feature of both sporadic and familial amyotrophic lateral sclerosis (ALS). These inclusion bodies are often immunoreactive for Cu,Zn-superoxide dismutase (SOD1) and are implicated in the pathology of ALS. On the basis of this and a similar clinical presentation of symptoms in the familial (fALS) and sporadic forms of ALS, we sought to investigate the possibility that there exists a common disease-related aggregation pathway for fALS-associated mutant SODs and wild type SOD1. We have previously shown that oxidation of fALS-associated mutant SODs produces aggregates that have the same morphological, structural, and tinctorial features as those found in SOD1 inclusion bodies in ALS. Here, we show that oxidative damage of wild type SOD at physiological concentrations ( approximately 40 microm) results in destabilization and aggregation in vitro. Oxidation of either mutant or wild type SOD1 causes the enzyme to dissociate to monomers prior to aggregation. Only small changes in secondary and tertiary structure are associated with monomer formation. These results indicate a common aggregation prone monomeric intermediate for wild type and fALS-associated mutant SODs and provides a link between sporadic and familial ALS.

A prion protein epitope selective for the pathologically misfolded conformation

Conformational conversion of proteins in disease is likely to be accompanied by molecular surface exposure of previously sequestered amino-acid side chains. We found that induction of beta-sheet structures in recombinant prion proteins is associated with increased solvent accessibility of tyrosine. Antibodies directed against the prion protein repeat motif, tyrosine-tyrosine-arginine, recognize the pathological isoform of the prion protein but not the normal cellular isoform, as assessed by immunoprecipitation, plate capture immunoassay and flow cytometry. Antibody binding to the pathological epitope is saturable and specific, and can be created in vitro by partial denaturation of normal brain prion protein. Conformation-selective exposure of Tyr-Tyr-Arg provides a probe for the distribution and structure of pathologically misfolded prion protein, and may lead to new diagnostics and therapeutics for prion diseases.

Intrathecal B-cell clonal expansion, an early sign of humoral immunity, in the cerebrospinal fluid of patients with clinically isolated syndrome suggestive of multiple sclerosis

The development of somatically mutated memory and plasma B cells is a consequence of T cell-dependent antigen-challenged humoral immunity. To investigate the role of B cell-mediated humoral immunity in the initiation and evolution of multiple sclerosis (MS), we analyzed Ig variable heavy chain genes of intrathecal B cells derived from patients with a first clinical manifestation suggestive of MS. Sequences of Ig variable regions showed that B cells in the cerebrospinal fluid from most of these patients were clonally expanded and carried somatic hypermutated variable heavy chain genes. The mutations showed a high replacement-to-silent ratio and were distributed in a way suggesting that these clonally expanded B cells had been positively selected through their antigen receptor. In comparison, intrathecal B-cell clonal expansion often precedes both oligoclonal IgG bands and multiple magnetic resonance imaging lesions. Clinical follow-up study showed that patients with clonally expanded intrathecal B cells had a high rate of conversion to clinically definite MS. The findings provide direct evidence of recruitment of germinal center differentiated B lymphocytes into the central nervous system during the initiation of MS. These results indicate B cell-mediated immune response in the cerebrospinal fluid is an early event of inflammatory reaction in the central nervous system of MS. This procedure also provides a more sensitive method to evaluate the association of humoral immunity in the evolution of MS.

Induced adaptive resistance to oxidative stress in the CNS: a discussion on possible mechanisms and their therapeutic potential

The free radical, nitric oxide (NO), is synthesized by mammalian cells and is utilized for normal cellular functions. High levels of NO are released during disease, injury and inflammation. NO at high concentrations more readily combines with other oxidants to form reactive nitrogenous species (RNS), which can wreak havoc on the cell by damaging a variety of cellular targets, such as DNA and proteins, ultimately leading to apoptosis, mutagenesis or carcinogenesis. Cells have natural resistance mechanisms to nitrooxidative stress that are either defective (as can occur in disease), or overwhelmed (as can occur in injury and inflammation). It has been found recently in the CNS that resistance to normally toxic levels of NO can be induced by nontoxic levels of NO and that this induction is correlated with and dependent upon increased levels and activity of the heme-metabolizing enzyme, heme oxygenase-1 (HO-1). HO1-mediated metabolism of heme groups released from NO-damaged proteins leads to a change in the levels of redox-active iron and a release of carbon monoxide (CO) and bilirubin, all of which have been implicated in cellular resistance to oxidative stress. Perhaps one or more of the products of HO1 heme metabolism is involved in induced adaptive resistance or perhaps a heme-independent mechanism is involved. In fact, a variety of possible mechanisms may be involved in induced resistance to NO in the CNS. Ultimately elucidating these mechanisms will enable us to modulate them for therapeutic potential.

Gabapentin therapy for amyotrophic lateral sclerosis: lack of improvement in neuronal integrity shown by MR spectroscopy

BACKGROUND AND PURPOSE

Proton MR spectroscopy has revealed impaired neuronal integrity in the motor cortex of patients with amyotrophic lateral sclerosis (ALS). We hypothesized that the N-acetylaspartate (NAA)-creatine (Cr) ratios in the motor cortex and adjacent brain could reflect the therapeutic effectiveness of gabapentin (GBP) treatment in ALS.

METHODS

Eight patients with ALS underwent MR spectroscopy before and 26.5 days +/- 8.8 after starting GBP. In 10 patients with ALS who were not treated with GBP, paired spectra were obtained 21.4 days +/- 7.2 apart. Fourteen healthy subjects underwent a single MR spectroscopic examination. The NAA/Cr ratio was measured in the precentral gyrus, the postcentral gyrus, the superior parietal lobule, the supplementary motor area, and the premotor cortex.

RESULTS

The NAA/Cr ratio was decreased in the precentral and postcentral gyri of patients with ALS compared with healthy controls. In those with ALS, the change in the NAA/Cr ratio was not different between treated patients and untreated patients in any of the regions studied.

CONCLUSION

No improvement in neuronal integrity was detected in motor and nonmotor cerebral regions after GBP treatment. This result agrees with that of prior investigations showing the equivocal clinical effectiveness of GBP for ALS and supports the validity of the NAA/Cr ratio as a surrogate of therapeutic effectiveness.

Oxidation-induced misfolding and aggregation of superoxide dismutase and its implications for amyotrophic lateral sclerosis

The presence of intracellular aggregates that contain Cu/Zn superoxide dismutase (SOD1) in spinal cord motor neurons is a pathological hallmark of amyotrophic lateral sclerosis (ALS). Although SOD1 is abundant in all cells, its half-life in motor neurons far exceeds that in any other cell type. On the basis of the premise that the long half-life of the protein increases the potential for oxidative damage, we investigated the effects of oxidation on misfolding/aggregation of SOD1 and ALS-associated SOD1 mutants. Zinc-deficient wild-type SOD1 and SOD1 mutants were extremely prone to form visible aggregates upon oxidation as compared with wild-type holo-protein. Oxidation of select histidine residues that bind metals in the active site mediates SOD1 aggregation. Our results provide a plausible model to explain the accumulation of SOD1 aggregates in motor neurons affected in ALS.

Acidic pH and detergents enhance in vitro conversion of human brain PrPC to a PrPSc-like form

In the presence of a low concentration of denaturants or detergents, acidic pH triggers a conformational transition of alpha-helices into beta-sheets in recombinant prion protein (PrP), likely mimicking some aspects of the transformation of host-encoded normal cellular PrP (PrP(C)) into its pathogenic isoform (PrP(Sc)). Here we observed the effects of acidic pH and guanidine hydrochloride (GdnHCl) on the physicochemical and structural properties of PrP(C) derived from normal human brain and determined the ability of the acid/GdnHCl-treated PrP to form a proteinase K (PK)-resistant species in the absence and presence of PrP(Sc) template. After treatment with 1.5 m GdnHCl at pH 3.5, PrP(C) from normal brain homogenates was converted into a detergent-insoluble form similar to PrP(Sc). Unlike PrP(Sc), however, the treated brain PrP(C) was protease-sensitive and retained epitope accessibility to monoclonal antibodies 3F4 and 6H4. Brain PrP(C) treated with acidic pH/GdnHCl acquired partial PK resistance upon further treatment with low concentrations of sodium dodecyl sulfate (SDS). Formation of this PrP(Sc)-like isoform was greatly enhanced by incubation with trace quantities of PrP(Sc) from Creutzfeldt-Jakob disease brain. Acid/GdnHCl-treated brain PrP may constitute a "recruitable intermediate" in PrP(Sc) formation. Further structural rearrangement seems essential for this species to acquire PK resistance, which can be promoted by the presence of a PrP(Sc) template.

Transmissible spongiform encephalopathies: vaccine issues

The recent emergence of bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob Disease (vCJD) suggests that transmissible spongiform encephalopathies (TSEs) pose an ongoing threat to human and animal health. To avoid iatrogenic transmission of TSEs in vaccines, strategies must be developed to obviate TSE agent infectivity in cellular substrates, cell culture media components and enzymes, and excipients, and to validate the safety of these components and field vaccines efficiently

All or none fibrillogenesis of a prion peptide

Amyloid proteins and peptides comprise a diverse group of molecules that vary both in size and amino-acid sequence, yet assemble into amyloid fibrils that have a common core structure. Kinetic studies of amyloid fibrillogenesis have revealed that certain amyloid proteins form oligomeric intermediates prior to fibril formation. We have investigated fibril formation with a peptide corresponding to residues 195-213 of the human prion protein. Through a combination of kinetic and equilibrium studies, we have found that the fibrillogenesis of this peptide proceeds as an all-or-none reaction where oligomeric intermediates are not stably populated. This variation in whether oligomeric intermediates are stably populated during fibril formation indicates that amyloid proteins assemble into a common fibrillar structure; however, they do so through different pathways.

Variant Creutzfeldt-Jakob disease: a summary of current scientific knowledge in relation to public health

The prion diseases pose unique scientific, medical, veterinary and regulatory challenges. Here, we summarize current information bearing on the natural history, pathobiology and epidemiology of these disorders and public policy responses to the potential threats to public health posed, particularly, by bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease (vCJD). Six years after the first case reports of vCJD, there is still no clear indication of the magnitude of the primary epidemic, or of the likelihood of lateral transmission of this untreatable disease by iatrogenic means, particularly by blood and blood products. However, the unsettling nature of the available evidence warrants prudence regarding public health policy and regulation, as well as a forward-looking approach to research.

GM2 ganglioside regulates the function of ciliary neurotrophic factor receptor in murine immortalized motor neuron-like cells (NSC-34)

We previously reported that ciliary neurotrophic factor (CNTF) increased the serum-free cell survival of immortalized motor neuron-like cells (NSC-34), and addition of the exogenous ganglioside GalNAc beta4(Neu5Ac alpha3)Gal beta4GlcCer (GM2) facilitated cell survival together with CNTF. Moreover beta 1,4 N-acetylgalactosaminyltransferase (GM2 synthase) activity increased in NSC-34 cells cultured with CNTF. We now have examined whether CNTF-induced cell survival is associated with the collaboration between GM2 and the CNTF receptor (CNTF-R). Despite the presence of CNTF (50 ng/ml), anti-CNTF-R antibody caused cell death and prevented the up-regulation of GM2 synthase expression. The addition of GM2 (1 to 20 microM) abrogated the anti-CNTF-R antibody effect which shortened cell survival and blocked GM2 synthase activation. Use of [125I]CNTF showed the specificity of CNTF binding in NSC-34 cells in situ. GM2 produced a 5-fold increase in the CNTF binding affinity per cell but did not change the binding site number. The study by metabolic labeling with [1-(14)C]N-acetyl-D-galactosamine ([14C]GalNAc) showed that biosynthesized GM2 was involved in the immunoprecipitation of CNTF-R. These findings indicate that up-regulated GM2 synthesis induces functional conversion of CNTF-R to the activated state, in which it has affinity for CNTF. We conclude that GM2 is a bio-regulating molecule of CNTF-R in motor neurons.

The failure of Daudi cells to express the cellular prion protein is caused by a lack of glycosyl-phosphatidylinositol anchor formation

The cellular prion protein (PrPc) is a glycosyl-phosphatidylinositol (GPI)-linked cell surface protein, which is expressed at high density on nervous tissues and at lower levels on most other solid-organ tissues. It is also expressed on peripheral blood mononuclear cells (PBMC) of all lineages. In lymphocytes, its level of expression is dependent upon the state of cell activation, and polyclonal anti-PrP antisera partially block lectin-induced T-cell activation, suggesting a functional role of the protein in this process. Using the monoclonal antibody (mAb) 3F4 we examined PrPc surface immunoreactivity on leukaemic cell lines of T- and B-cell origin, and unexpectedly observed a complete lack of PrPc cell-surface expression in Daudi cells, while all other cell lines displayed discernible reactivity. We demonstrated the intracellular presence of PrP-specific mRNA and PrP protein. The lack of surface PrPc is unrelated to the well-known defect of beta2-microglobulin (beta2m) expression in Daudi cells as other beta2m-deficient cells, such as the melanoma cell line F0-1 and spleen cells from beta2m gene-deleted mice, were not deficient in cell-surface PrPc. Daudi cells failed to bind antibodies directed against all GPI-linked cell surface proteins. In somatic hybridization experiments using murine spleen cells as partners, we observed de novo expression of human PrPc, CD55 and CD59, thus demonstrating in Daudi cells the availability of these gene products for GPI linkage and cell-surface expression.

Serum insulin-like growth factor-I (IGF-I) does not correlate positively with isometric strength, fatigue, and quality of life in post-polio syndrome

OBJECTIVES AND BACKGROUND

To determine if serum insulin-like growth factor-I (IGF-I) levels are associated with strength, body mass index (BMI), fatigue, or quality of life in post-poliomyelitis syndrome (PPS). PPS is likely due to a distal disintegration of enlarged post-polio motor units as a result of terminal axonal sprouting. Age-related decline in growth hormone and IGF-I (which support terminal axonal sprouts) is proposed as a contributing factor.

METHODS

As part of the North American Post-Poliomyelitis Pyridostigmine Study (NAPPS), baseline data on maximum voluntary isometric contraction (MVIC), BMI, subjective fatigue (fatigue severity scale, Hare fatigue symptom scale), health-related quality of life (short form health survey-36; SF-36), and serum IGF-I levels were gathered on 112 PPS patients. Pearson correlation coefficients were calculated to evaluate the association between serum IGF-I and MVIC in 12 muscles, BMI, two fatigue scales, and SF-36 scale scores.

RESULTS

There is a significant inverse correlation of IGF-I levels with MVIC in left ankle dorsiflexors (r=-0.30, P<0.01), and left and right knee extensors (r=-0.22, -0.25, P=<0.01, 0.01), but no significant correlations in other muscles. When men and women were evaluated separately, inverse correlations of IGF-I levels with MVIC were found only in men. IGF-I correlated inversely with BMI (r=-0.32, P=0006) and age (r=-0.32, P=0.0005). IGF-I did not correlate with the fatigue or SF-36 scales.

CONCLUSIONS

In this exploratory study, we found that contrary to our expectations, IGF-I did not correlate positively with strength. IGF-I correlated negatively with strength in several lower extremity muscles, BMI, and age. IGF-I is likely not an important factor in the pathogenesis of fatigue and in determining quality of life in PPS, but its role on strength should be studied further.