A soluble form of prion protein in human cerebrospinal fluid: implications for prion-related encephalopathies

Prion protein is secreted in soluble forms in the epididymal fluid and proteolytically processed and transported in seminal plasma

The presence of prion protein in sperm and fluids collected from different parts of the ram genital tract was investigated by immunoblotting with monoclonal antibodies. A slightly immunoreactive 25- to 30-kDa protein was recognized on Western blots of testicular and epididymal sperm extracts. Immunoreactivity increased on ejaculated sperm extracts and 2 other bands at 35 and 43 kDa also reacted. Seminal plasma showed several immunoreactive bands, the main bands being detected at 43 and 35 kDa, whereas less reactive bands were observed at 30, 25, 20, and <14 kDa. All these bands strongly decreased in the seminal plasma after vasectomy, indicating a testicular or an epididymal origin. Testicular fluid showed almost no reactivity, whereas caudal epididymal fluid contained the 2 strong immunoreactive bands at 43 and 35 kDa and in some cases a faint 30-kDa band. The 43-kDa band was also found in the fluid from the proximal caput, whereas the 35-kDa band appeared in the distal caput. Immunoprecipitation of (35)S-labeled proteins secreted in the epididymal fluid indicated that the 43-kDa form was synthesized in caput and caudal regions and the 35-kDa form in the distal caput to the distal corpus. Treatment of caudal fluid and seminal plasma by N-glycosidase resulted in the formation of 3 bands: 1 highly reactive at about 25 kDa, a second less reactive at about 28 kDa, and a third at approximately 20 kDa. The pattern of prion protein distribution in epididymal fluids was found to be similar in scrapie-infected rams to that of healthy rams. Cauda epididymal fluid and seminal plasma from infected animals could not be treated directly with proteinase K, because of the presence of protease inhibitors. However, the prion protein immunoprecipitated from these fluids was completely cleaved by proteinase K, whereas in the same conditions this from an infected sheep brain gave the usual resistant band pattern.

Nonneuronal cellular prion protein

The normal cellular prion protein (PrP(c)) is a membrane sialoglycoprotein of unknown function having the unique property of adopting an abnormal tertiary conformation. The pathological conformer PrP(sc) would be the agent of transmissible spongiform encephalopathies or prion diseases. They include scrapie and bovine spongiform encephalopathy in animals and Creutzfeldt-Jakob disease in humans. The conversion of PrP(c) into PrP(sc) in the brain governs the clinical phenotype of the disease. However, the three-dimensional structure change of PrP(c) can also take place outside the central nervous system, in nonneuronal cells particularly of lymphoid tissue where the agent replicates. In natural infection, PrP(c) in nonneuronal cells of peripheral extracerebral organs may play a key role as the receptor required to enable the entry of the infectious agent into the host. In the present review we have undertaken a first evaluation of compelling data concerning the PrP(c)-expressing cells of nonneuronal origin present in cerebral and extracerebral tissues. The analysis of tissue, cellular, and subcellular localization of PrP(c) may help us better understand the biological function of PrP(c) and provide some information on physiopathological processes underlying prion diseases.

Comparative two-dimensional mapping of prion protein isoforms in human cerebrospinal fluid and central nervous system

The cellular prion protein (PrP(C)) is a glycosylphosphatidylinositol (GPI)-anchored glycoprotein abundant in neurons. Although its precise function is unknown, PrP(C) represents the substrate for the generation of a conformational pathogenic isoform (PrP(Sc)) in human and animal transmissible spongiform encephalopathies, or prion diseases. By applying novel solubilization cocktails, we analyzed normal human brain and cerebrospinal fluid (CSF) PrP(C) by immunoblot of two-dimensional (2-D) gel electrophoresis preparations, using specific antibodies. Here, we show that PrP(C) from brain and CSF is composed of several charge isomers of differently glycosylated isoforms of the full-length PrP(C) and two N-terminally truncated fragments of 20 and 18 kDa. In the CSF, substantial amounts of the highly glycosylated PrP(C) isoforms and of the unglycosylated 18 kDa fragment are detected. Our study, for the first time, provides a detailed 2-D map of human PrP(C) both in brain and CSF, and establishes an innovative and sensitive method that might help in detecting the CSF pathological PrP(Sc) isoform in vivo. It also shows the incredible microheterogeneity of such isoforms (ca. 60 spots!), as revealed in 2-D mapping, as opposed to 3-4 main zones by mono-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE).

Similar turnover and shedding of the cellular prion protein in primary lymphoid and neuronal cells

The cellular prion protein (PrP(C)) is essential for pathogenesis and transmission of prion diseases. Although prion replication in the brain is accompanied by neurodegeneration, prions multiply efficiently in the lymphoreticular system without any detectable pathology. We have used pulse-chase metabolic radiolabeling experiments to investigate the turnover and processing of PrP(C) in primary cell cultures derived from lymphoid and nervous tissues. Similar kinetics of PrP(C) degradation were observed in these tissues. This indicates that the differences between these two organs with respect to their capacity to replicate prions is not due to differences in the turnover of PrP(C). Substantial amounts of a soluble form of PrP that lacks the glycolipid anchor appeared in the medium of splenocytes and cerebellar granule cells. Soluble PrP was detected in murine and human serum, suggesting that it might be of physiological relevance.

Absence of protease-resistant prion protein in the cerebrospinal fluid of Creutzfeldt-Jakob disease

Creutzfeldt-Jakob disease (CJD), believed to be caused by a protease-resistant isoform of prion protein (PrP(Sc)), usually manifests itself as a clinically distinctive age-related dementia because of its rapid progression, occasionally accompanied by cerebellar ataxia. Recently, a variant CJD (vCJD) has been described, which has prominent early psychiatric symptoms and an earlier age of death. Although cerebrospinal fluid (CSF) is part of the extracellular fluid of the central nervous system (CNS), the bulk of its proteins are derived from the plasma and there is increasing concern about possible transmission of prion disease by blood. As investigation of CSF has played a significant role in the diagnosis and management of several neurological diseases, it was decided to characterize PrP present in the CSF of CJD individuals. Significant variation was observed in the level of PrP in the CSF from both non-CJD and CJD (including vCJD) patients, and the detected PrP forms are protease-sensitive. Using a conformation-dependent immunoassay, it was further demonstrated that the PrP detected in the CSF from CJD patients was broadly similar in conformation to that found in non-CJD patients. Taken together, the results of this study fail to demonstrate any correlation between the presence of protease-resistant PrP isoform (PrP(Sc)) in the CSF and disease manifestation.

Clinicopathological characterization of prion: a novel marker of activated human hepatic stellate cells

BACKGROUND/AIMS

We recently demonstrated prion as a new marker for hepatic stellate cell activation in rats. Here, we have examined prion expression in normal and diseased human livers.

METHODS

Prion expression was examined at protein level by immunohistochemistry and at mRNA level by in situ hybridization.

RESULTS

While normal livers were negative for prion, all liver specimens but one from patients with chronic liver disease were positively stained. In chronic hepatitis, prion protein expression was found not only in the sinusoidal lining cells within the lobules but also in mesenchymal cells in expanded portal tracts. In alcoholic liver disease, prion-positive cells were found mainly in the areas of alcoholic hepatitis. Immunoelectronmicroscopy revealed that prion-positive cells were activated stellate cells. In situ hybridization demonstrated that the distribution of prion mRNA is similar to that of prion protein. In chronic hepatitis, the number of prion-positive cells correlated with the grade of activity but not with the stage of fibrosis. In alcoholic liver disease, levels of prion protein expression were significantly increased in the presence of alcoholic hepatitis.

CONCLUSION

Prion as a novel marker of activated stellate cells correlates well with disease activity in human chronic liver diseases.

Ultrastructural localization of prion proteins: physiological and pathological implications

The transmissible spongiform encephalopathies (TSE) or prion diseases are fatal neurodegenerative disorders in which the central event is the conversion of a normal host-encoded protein (PrP(c)) into an abnormal isoform (PrP(sc)) which accumulates as amyloid in TSE brain. The two PrP(c) and PrP(sc) prion protein isoforms are membrane sialoglycoproteins synthesized in the central nervous system and various peripheral organ tissues. In this review, we describe the ultrastructural localization of prion proteins in human and animal cerebral and non-cerebral tissues whether or not infected by TSE agents. In addition to the plasma membrane of several cells, PrP(c) was found in association with cytoplasmic organelles of central and nerve-muscle synapses, and secretory granules of epithelial cells. Fibrils of amyloid plaques, synaptic structures, and lysosome-like organelles constitute the subcellular sites harboring PrP(sc). These findings have led to discussions on the physiological role of PrP(c) and the pathological mechanisms underlying prion spongiform encephalopathies.

Isoform pattern of 14-3-3 proteins in the cerebrospinal fluid of patients with Creutzfeldt-Jakob disease

Two-dimensional polyacrylamide gel electrophoresis of CSF has been used in the diagnosis of Creutzfeldt-Jakob disease (CJD). One of the two diagnostic protein spots was identified as isoform(s) of the 14-3-3 family of abundant brain proteins. This has led to the development of one-dimensional 14-3-3 sodium dodecyl sulfate polyacrylamide gel electrophoresis immunoblot, which is currently used to support the diagnosis of CJD. In the present study employing western blot analysis, we have identified the panel of 14-3-3 isoforms that appear in the CSF of 10 patients with CJD compared with 10 patients with other dementias. The results clearly show that the 14-3-3 isoforms beta, gamma, epsilon, and eta are present in the CSF of patients with CJD and can be used to differentiate other dementias. 14-3-3eta also gave a baseline signal in all patients with other dementias, including six patients with Alzheimer's disease. The presence of 14-3-3eta in the CSF of a patient with herpes simplex encephalitis was particularly noteworthy. This study has determined that isoform-specific 14-3-3 antibodies against beta, gamma, and epsilon should be considered for the neurochemical differentiation of CJD from other neurodegenerative diseases.

Cellular biology of prion diseases

Prion diseases are fatal neurodegenerative disorders of humans and animals that are important because of their impact on public health and because they exemplify a novel mechanism of infectivity and biological information transfer. These diseases are caused by conformational conversion of a normal host glycoprotein (PrPC) into an infectious isoform (PrPSc) that is devoid of nucleic acid. This review focuses on the current understanding of prion diseases at the cell biological level. The characteristics of the diseases are introduced, and a brief history and description of the prion hypothesis are given. Information is then presented about the structure, expression, biosynthesis, and possible function of PrPC, as well as its posttranslational processing, cellular localization, and trafficking. The latest findings concerning PrPSc are then discussed, including cell culture systems used to generate this pathogenic isoform, the subcellular distribution of the protein, its membrane attachment, proteolytic processing, and its kinetics and sites of synthesis. Information is also provided on molecular models of the PrPC-->PrPSc conversion reaction and the possible role of cellular chaperones. The review concludes with suggestions of several important avenues for future investigation.

Sulphated glycosaminoglycans prevent the neurotoxicity of a human prion protein fragment

Although a number of features distinguish the disease isoform of the prion protein (PrPSc) from its normal cellular counterpart (PrPC) in the transmissible spongiform encephalopathies (TSEs), the neuropathogenesis of these diseases remains an enigma. The amyloid fibrils formed by fragments of human PrP have, however, been shown to be directly neurotoxic in vitro. We show here that sulphated polysaccharides (heparin, keratan and chondroitin) inhibit the neurotoxicity of these amyloid fibrils and this appears to be mediated via inhibition of the polymerization of the PrP peptide into fibrils. This provides a rationale for the therapeutic effects of sulphated polysaccharides and suggests a rapid in vitro functional screen for TSE therapeutics.

Detection of 14-3-3 protein in the cerebrospinal fluid supports the diagnosis of Creutzfeldt-Jakob disease

The analysis of 14-3-3 protein in cerebrospinal fluid (CSF) was shown to be highly sensitive and specific for the diagnosis of Creutzfeldt-Jakob disease (CJD). However, the predictive value of this test in the clinical diagnosis of, and its relation to, sporadic, genetic, and iatrogenic CJD cases have yet to be established. CSF samples of suspect CJD cases seen in the prospective German surveillance study were tested for the presence of 14-3-3 protein by using a modified western blot (WB) technique. WB detected 14-3-3 protein in 95.4% of definite and 92.8% of probable cases. In two patients classified initially as not having CJD the test was positive, and both were later proved to have definite CJD. The positive predictive value is 94.7% and the negative predictive value is 92.4%. False-positive results in a single CSF analysis were seen in patients with herpes simplex encephalitis, hypoxic brain damage, atypical encephalitis, intracerebral metastases of a bronchial carcinoma, metabolic encephalopathy, and progressive dementia of unknown cause. WB analysis for 14-3-3 protein was positive in only 5 of 10 cases of familial forms of spongiform encephalopathies. CSF analysis for 14-3-3 protein should thus be performed in any case suspect for CJD.

Amyloid in Alzheimer's disease and prion-related encephalopathies: studies with synthetic peptides

Deposition of amyloid-beta protein (beta A) in brain parenchyma and vessel walls is a major pathological feature of Alzheimer's disease (AD). In prion-related encephalopathies (PRE), too, an altered form of prion protein (PrPsc) forms amyloid fibrils and accumulates in the brain. In both conditions the amyloid deposition is accompanied by nerve cell loss, whose pathogenesis and molecular basis are not understood. Neuropathological, genetic and biochemical studies indicate a central role of beta A in the AD pathogenesis. Synthetic peptides homologous to beta A and its fragments contribute to investigate the mechanisms of beta A deposit formation and the role played by beta A in AD pathogenesis. The physicochemical studies on the beta-sheet conformation and self-aggregation properties of beta A peptides indicate the conditions and the factors influencing the formation of beta A deposits. The neurotoxic activity of beta A and its fragments support the causal relationship between beta A deposits and the neuropathological events in AD. Numerous studies were performed to clarify the mechanism of neuronal death induced by exposure to beta A peptides. A similar approach has been used to investigate the role of PrPsc in PRE; in these diseases, the association between accumulation of PrPsc and neuropathology is evident and numerous data indicate that PrPsc itself might be the infectious agent responsible for disease transmission. Thus, PrP peptides were used to investigate the pathogenic role of PrPsc in PRE and the conformational change responsible for the conversion PrPc to PrPsc that makes the molecule apparently infectious. In particular, we synthesized a peptide homologous to residues 106-126, an integral part of all abnormal PrP isoforms that accumulate in the brain of subjects' PRE. This peptide is fibrillogenic, has secondary structure largely composed of beta-sheet and proteinase-resistant properties, is neurotoxic and induces astrogliosis. In this review, we summarize and compare the data obtained with beta A and PrP peptides and analyze the significance in terms of amyloidogenic proteins and neurodegeneration.

Vascular variant of prion protein cerebral amyloidosis with tau-positive neurofibrillary tangles: the phenotype of the stop codon 145 mutation in PRNP

Deposition of PrP amyloid in cerebral vessels in conjunction with neurofibrillary lesions is the neuropathologic hallmark of the dementia associated with a stop mutation at codon 145 of PRNP, the gene encoding the prion protein (PrP). In this disorder, the vascular amyloid in tissue sections and the approximately 7.5-kDa fragment extracted from amyloid are labeled by antibodies to epitopes located in the PrP sequence including amino acids 90-147. Amyloid-laden vessels are also labeled by antibodies against the C terminus, suggesting that PrP from the normal allele is involved in the pathologic process. Abundant neurofibrillary lesions are present in the cerebral gray matter. They are composed of paired helical filaments, are labeled with antibodies that recognize multiple phosphorylation sites in tau protein, and are similar to those observed in Alzheimer disease. A PrP cerebral amyloid angiopathy has not been reported in diseases caused by PRNP mutations or in human transmissible spongiform encephalopathies; we propose to name this phenotype PrP cerebral amyloid angiopathy (PrP-CAA).

Mechanisms of prion diseases described as long-lasting disorders of the blood-brain barrier system

Prison diseases can be described as disorders wherein circulating prion molecules of external origin intervene with the normal synthesis of similar molecules in the central nervous system. The hypothesis is that the endogenous prion-like molecules denote the blood-brain barrier discontinuity. In the case of barrier discontinuity, small numbers of hypothetical signal molecules enter circulation and attach to the receptor sites in the neighbouring blood vessels. The specific receptors of the cells in the blood vessels stimulated by endogenous prion-like molecules might initiate the repairing processes of the blood-brain barrier. In prion diseases, prion molecules from external sources are similar to endogenous prion-like molecules or to the hypothetical signal molecules described here. Large numbers of prion molecules enter circulation and initiate repairing processes in large brain areas causing the tissue damage. This damage leads to new barrier discontinuities that would provoke pathological process even when the exogenous prion molecules are no longer present in the circulation.

Scrapie-associated PrP accumulation and its inhibition: revisiting the amyloid-glycosaminoglycan connection

An abnormal protease-resistant isoform of the protein PrP accumulates in the brain of hosts with transmissible spongiform encephalopathies (TSEs) and appears to be centrally involved in TSE pathogenesis. Studies with scrapie-infected tissue culture cells have indicated that this abnormal PrP is formed from an apparently normal precursor on the plasma membrane or along an endocytic pathway to the lysosomes. Inhibitors of protease-resistant PrP accumulation might serve as tools for studying the basic mechanism of protease-resistant PrP formation and as potential drugs for TSE therapy. Using scrapie-infected neuroblastoma cells to screen for such compounds in vitro, we found that the amyloid binding dye Congo red and certain sulfated glycans potently inhibited the accumulation of protease-resistant PrP in scrapie-infected cells without apparent effects on the metabolism of the normal isoform. The relative potencies of the sulfated glycans corresponded with their previously determined anti-scrapie activities in vivo, suggesting that the prophylactic effects of sulfated polyanions may be due to inhibition of protease-resistant PrP accumulation. Since protease-resistant PrP amyloid is known to contain sulfated glycosaminoglycans, as do other naturally derived amyloids, we hypothesize that these sulfated inhibitors competitively block binding between PrP and endogenous glycosaminoglycans that is important for its accumulation in a protease-resistant, potentially amyloidogenic state. Drugs which interfere with this (pre)amyloid-glycosaminoglycan interaction may be useful for treating a variety of amyloidoses.

Inhibition of scrapie-associated PrP accumulation. Probing the role of glycosaminoglycans in amyloidogenesis

Accumulation of an abnormal, protease-resistant form of an endogenous protein, PrP, is a characteristic feature of scrapie and related transmissible spongiform encephalopathies. This abnormal isoform is also present in the amyloid plaques that are often observed in these diseases. In mouse neuroblastoma cells persistently infected with scrapie, the abnormal protease-resistant isoform of PrP is derived from an operationally normal protease-sensitive precursor. Conversion of PrP to the protease-resistant state occurs either on the plasma membrane or along an endocytic pathway by an unknown mechanism. Inhibitors of protease-resistant PrP accumulation have been identified, and these include the amyloid-binding dye Congo red and certain sulfated glycans. The similarity of these compounds to sulfated glycosaminoglycans, which are components of all natural amyloids, has led to the hypothesis that the inhibitors act by competitively blocking an interaction between endogenous glycosaminoglycan(s) and PrP that is critical for amyloidogenic PrP accumulation. The proven prophylactic effect of these sulfated glycans in animal models of scrapie suggests that they represent a group of compounds that might interfere with the pathogenic formation of amyloid in a variety of diseases, such as Alzheimer's disease.

Synthetic peptides homologous to prion protein residues 106-147 form amyloid-like fibrils in vitro

Gerstmann-Sträussler-Scheinker disease (GSS) is a prion-related encephalopathy pathologically characterized by massive deposition of prion protein (PrP) amyloid in the central nervous system. The major component of amyloid fibrils isolated from patients of the Indiana kindred of GSS (GSS-Ik) is an 11-kDa fragment of PrP spanning residues 58 to approximately 150. These patients carry a missense mutation of the PRNP gene, causing a Phe-->Ser substitution at codon 198. We investigated fibrillogenesis in vitro by using synthetic peptides homologous to consecutive segments of GSS-Ik amyloid protein (residues 57-64, 89-106, 106-126, and 127-147) as well as peptides from the PrP region with the GSS-Ik mutation (residues 191-205 and 181-205, both wild type and mutant). Peptide PrP-(106-126) formed straight fibrils similar to those extracted from GSS brains, whereas peptide PrP-(127-147) formed twisted fibrils resembling scrapie-associated fibrils isolated from subjects with transmissible spongiform encephalopathies. Congo red staining and x-ray fibril diffraction showed that both straight and twisted fibrils had tinctorial and conformational properties of native amyloid. Conversely, the other peptides did not form amyloid-like fibrils under similar conditions. These findings suggest that the sequence spanning residues 106-147 of PrP is central to amyloid fibril formation in GSS and related encephalopathies.