Ubiquitin conjugate immunoreactivity in the brains of scrapie infected mice

Proteostasis unbalance in prion diseases: Mechanisms of neurodegeneration and therapeutic targets

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are progressive neurodegenerative disorders of the central nervous system that affect humans and animals as sporadic, inherited, and infectious forms. Similarly to Alzheimer's disease and other neurodegenerative disorders, any attempt to reduce TSEs' lethality or increase the life expectancy of affected individuals has been unsuccessful. Typically, the onset of symptoms anticipates the fatal outcome of less than 1 year, although it is believed to be the consequence of a decades-long process of neuronal death. The duration of the symptoms-free period represents by itself a major obstacle to carry out effective neuroprotective therapies. Prions, the infectious entities of TSEs, are composed of a protease-resistant protein named prion protein scrapie (PrP^Sc) from the prototypical TSE form that afflicts ovines. PrP^Sc misfolding from its physiological counterpart, cellular prion protein (PrP^C), is the unifying pathogenic trait of all TSEs. PrP^Sc is resistant to intracellular turnover and undergoes amyloid-like fibrillation passing through the formation of soluble dimers and oligomers, which are likely the effective neurotoxic entities. The failure of PrP^Sc removal is a key pathogenic event that defines TSEs as proteopathies, likewise other neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's disease, characterized by alteration of proteostasis. Under physiological conditions, protein quality control, led by the ubiquitin-proteasome system, and macroautophagy clears cytoplasm from improperly folded, redundant, or aggregation-prone proteins. There is evidence that both of these crucial homeostatic pathways are impaired during the development of TSEs, although it is still unclear whether proteostasis alteration facilitates prion protein misfolding or, rather, PrP^Sc protease resistance hampers cytoplasmic protein quality control. This review is aimed to critically analyze the most recent advancements in the cause-effect correlation between PrP^C misfolding and proteostasis alterations and to discuss the possibility that pharmacological restoring of ubiquitin-proteasomal competence and stimulation of autophagy could reduce the intracellular burden of PrP^Sc and ameliorate the severity of prion-associated neurodegeneration.

Granulovacuolar degeneration: a neurodegenerative change that accompanies tau pathology

Granule-containing vacuoles in the cytoplasm of hippocampal neurons are a neuropathological feature of Alzheimer's disease. Granulovacuolar degeneration (GVD) is not disease-specific and can be observed in other neurodegenerative disorders and even in the brains of non-demented elderly people. However, several studies have reported much higher numbers of neurons undergoing GVD in the hippocampus of Alzheimer's disease cases. Recently, a neuropathological staging system for GVD has facilitated neuropathological assessment. Data obtained by electron microscopy and immunolabeling suggest that GVD inclusions are a special form of autophagic vacuole. GVD frequently occurs together with pathological changes of the microtubule-associated protein tau, but to date, the relationship between the two lesions remains elusive. Originally identified in hematoxylin- and silver-stained sections, immunolabeling has shown that the granules are composed of a variety of proteins, including those related to tau pathology, autophagy, diverse signal transduction pathways, cell stress and apoptosis. Several of these proteins serve as markers of GVD. Most researchers and authors have interpreted the sequestration of proteins into GVD inclusions as either a cellular defense mechanism or one that leads to the impairment of important cellular functions. This review provides a detailed overview of the various aspects of GVD and focuses on the relationship between tau pathology and GVD.

Generation of monoclonal antibody that distinguishes PrPSc from PrPC and neutralizes prion infectivity

To establish PrP(Sc)-specific mAbs, we immunized Prnp(-/-) mice with PrP(Sc) purified from prion-infected mice. Using this approach, we obtained mAb 6H10, which reacted with PrP(Sc) treated with proteinase K, but not with PrP(Sc) pretreated with more than 3 M GdnHCl. In contrast, reactivity of pan-PrP mAbs increased with increasing concentrations of GdnHCl used for pretreatment of PrP(Sc). In histoblot analysis, mAb 6H10 showed a positive reaction on a non-denatured histoblot but reactivity was lower when the histoblot was pretreated by autoclaving. Epitope analysis suggested that the extreme C-terminus of PrP is likely to be part of the epitope for mAb 6H10. MAb 6H10 immunoprecipitated PrP(Sc) from brains of mice, sheep, and cattle infected with prions. Furthermore, pretreatment of purified PrP(Sc) with mAb 6H10 reduced the infectious titer more than 1 log. Taken together, these results suggest that mAb 6H10 recognizes a conformational epitope on PrP(Sc) that is related to prion infectivity.

Neurolysosomal pathology in human prosaposin deficiency suggests essential neurotrophic function of prosaposin

A neuropathologic study of three cases of prosaposin (pSap) deficiency (ages at death 27, 89 and 119 days), carried out in the standard autopsy tissues, revealed a neurolysosomal pathology different from that in the non-neuronal cells. Non-neuronal storage is represented by massive lysosomal accumulation of glycosphingolipids (glucosyl-, galactosyl-, lactosyl-, globotriaosylceramides, sulphatide, and ceramide). The lysosomes in the central and peripheral neurons were distended by pleomorphic non-lipid aggregates lacking specific staining and autofluorescence. Lipid storage was borderline in case 1, and at a low level in the other cases. Neurolysosomal storage was associated with massive ubiquitination, which was absent in the non-neuronal cells and which did not display any immunohistochemical aggresomal properties. Confocal microscopy and cross-correlation function analyses revealed a positive correlation between the ubiquitin signal and the late endosomal/lysosomal markers. We suppose that the neuropathology most probably reflects excessive influx of non-lipid material (either in bulk or as individual molecules) into the neurolysosomes. The cortical neurons appeared to be uniquely vulnerable to pSap deficiency. Whereas in case 1 they populated the cortex, in cases 2 and 3 they had been replaced by dense populations of both phagocytic microglia and astrocytes. We suggest that this massive neuronal loss reflects a cortical neuronal survival crisis precipitated by the lack of pSap. The results of our study may extend the knowledge of the neurotrophic function of pSap, which should be considered essential for the survival and maintenance of human cortical neurons.

Application of Ubiquitin Immunohistochemistry to the Diagnosis of Disease

Ubiquitin immunohistochemistry has changed understanding of the pathophysiology of many diseases, particularly chronic neurodegenerative diseases. Protein aggregates (inclusions) containing ubiquitinated proteins occur in neurones and other cell types in the central nervous system in afflicted cells. The inclusions are present in all the neurological illnesses, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, polyglutamine diseases, and rarer forms of neurodegenerative disease. A new cause of cognitive decline in the elderly, "dementia with Lewy bodies," accounting for some 15-30% of cases, was initially discovered and characterized by ubiquitin immunocytochemistry. The optimal methods for carrying out immunohistochemical analyses of paraffin-embedded tissues are described, and examples of all the types of intracellular inclusions detected by ubiquitin immunohistochemistry in the diseases are illustrated. The role of the ubiquitin proteasome system (UPS) in disease progression is being actively researched globally and increasingly, because it is now realized that the UPS controls most pathways in cellular homeostasis. Many of these regulatory mechanisms will be dysfunctional in diseased cells. The goal is to understand fully the role of the UPS in the disorders and then therapeutically intervene in the ubiquitin pathway to treat these incurable diseases.

Neuropathological characterisation of French bovine spongiform encephalopathy cases

Bovine spongiform encephalopathy (BSE) in cattle is a neurodegenerative disease belonging to the transmissible spongiform encephalopathies, a group of diseases including sheep scrapie and human Creutzfeldt-Jakob disease. The pathological characteristics of BSE are vacuolation, mild gliosis, little neuronal degeneration without inflammatory process and abnormal prion protein (PrPsc) accumulation. The aim of this study was to define precisely the neuropathology of BSE in French cases by assessing the distributions of vacuolar lesions and PrPsc within cattle brains. We showed that vacuolation and PrPsc accumulation varied from one structure to the other, and most often coexisted. These distributions were in accordance with British and Portuguese data previously published. Seven types of PrPsc immunolabelling were described based on morphology and localisation. Besides mild gliosis mainly associated with vacuolation, we observed a very slight neuronal apoptosis. In addition, we saw a moderate vimentin labelling colocalised with vacuolation, a discrete ubiquitin staining and no Tau protein staining. This study provides precise histopathological data that will be completed with a quantitative study on more than 100 obex samples of French BSE cases.

Ubiquitin and the molecular pathology of neurodegenerative diseases

Ubiquitin plays a central role in normal cellular function as well as in disease. It is possible to group ubiquitin-immunostained structures into several main groups, the most distinctive being the ubiquitin/intermediate filament/alphaB crystallin family of inclusions that seem to represent a general cellular response to abnormal proteins recently termed the aggresomal response. While ubiquitin immunohistochemistry is a very useful technique for detecting pathological changes and inclusion bodies in the nervous system this alone is not enough to classify inclusions, and a panel of antibodies is recommended to clarify any findings made by screening tissues with anti-ubiquitin. Several mechanistic possibilities now exist to explain the accumulation of ubiquitinated proteins in cells of the nervous system, understanding of which should lead to new therapeutic advances in the group of chronic neurodegenerative diseases.

Intracellular mechanisms of amyloid accumulation and pathogenesis in Alzheimer's disease

Cell-culture studies have revealed some of the fundamental features of the interaction of amyloid Abeta with cells and the mechanism of amyloid accumulation and pathogenesis in vitro. A(beta)1-42, the longer isoform of amyloid that is preferentially concentrated in senile plaque (SP) amyloid deposits in Alzheimer's disease (AD), is resistant to degradation and accumulates as insoluble aggregates in late endosomes or lysosomes. Once these aggregates have nucleated inside the cell, they grow by the addition of aberrantly folded APP and amyloidgenic fragments of APP, that would otherwise be degraded, onto the amyloid lattice in a fashion analogous to prion replication. This accumulation of heterogeneous aggregated APP fragments and Abeta appears to mimic the pathophysiologyof dystrophic neurites, where the same spectrum of components has been identified by immunohistochemistry. In the brain, this residue appears to be released into the extracellular space, possibly by a partially apoptotic mechanism that is restricted to the distal compartments of the neuron. Ultimately, this insoluble residue may be further digested to the protease-resistant A(beta)n-42 core, perhaps by microglia, where it accumulates as senile plaques. Thus, the dystrophic neurites are likely to be the source of the immediate precursors of amyloid in the senile plaques. This is the opposite of the commonly held view that extracellular accumulation of amyloid induces dystrophic neurites. Many of the key pathological events of AD may also be directly related to the intracellular accumulation of this insoluble amyloid. The aggregated, intracellular amyloid induces the production of reactive oxygen species (ROS) and lipid peroxidation products and ultimately results in the leakage of the lysosomal membrane. The breakdown of the lysosomal membrane may be a key pathogenic event, leading to the release of heparan sulfate and lysosomal hydrolases into the cytosol. Together, these observations provide the novel view that amyloid deposits and some of the early events of amyloid pathogenesis initiate randomly within single cells in AD. This pathogenic mechanism can explain some of the more enigmatic features of Alzheimer's pathogenesis, like the focal nature of amyloid plaques, the relationship between amyloid, dystrophic neurites and neurofibrillary-tangle pathology, and the miscompartmentalization of extracellular and cytosolic components observed in AD brain.

Increased levels of oxidative stress markers detected in the brains of mice devoid of prion protein

Although minor abnormalities have been reported in prion protein (PrP) knock-out (Prnp-/-) mice, the normal physiological function of PrP, the causative agent implicated in transmissible spongiform encephalopathies (TSE), remains unresolved. Since there are increasing correlations between oxidative stress and amyloidoses, we decided to investigate whether PrP plays a role in oxidative modulation. We found higher levels of oxidative damage to proteins and lipids in the brain lysates of Prnp-/- as compared to wild-type (WT) mice of the same genetic background. These two indicators, protein oxidation and lipid peroxidation, are hallmarks of cellular oxidative damage. Elevated levels of ubiquitin-protein conjugates were also observed in Prnp-/- mice, a probable consequence of cellular attempts to remove the damaged proteins as indicated by increased proteasome activity. Taken together, these findings are indicative of a role for PrP in oxidative homeostasis in vivo.

The ubiquitin carboxy-terminal hydrolase-L1 gene S18Y polymorphism does not confer protection against idiopathic Parkinson's disease

The ubiquitin carboxy-terminal hydrolase L1gene (UCH-L1) has been implicated in the aetiology of Parkinson's disease (PD). A rare Ile93Met mutation in UCH-L1 in a German PD sib-pair has been reported. Recently, a S18Y (C54A) polymorphism in exon 3 of UCH-L1 was found to be under-represented in PD patients compared to controls. To test the reproducibility of this negative association, we conducted an allele-association study of the S18Y polymorphism in an Australian case-control sample consisting of 142 PD cases and 142 closely matched control subjects. Genotypes were determined using polymerase chain reaction and RsaI restriction enzyme assay. Analysis revealed no significant difference between PD patients and controls for genotype or allele frequencies of the S18Y polymorphism. The frequency of the S18Y allele in Australian subjects is similar to that reported elsewhere. This study suggests that the S18Y polymorphism in UCH-L1 does not influence the risk for developing PD.

Immunohistochemical studies of the PrP(CJD) deposition in Creutzfeldt-Jakob disease

The PrP(CJD) deposition in eight brains of sporadic Creutzfeldt-Jakob disease (CJD) was examined immunohistochemically using both hydrolytic autoclaving and formic acid pretreatment in order to understand the pathogenesis of CJD. Synaptic-type PrP immunoreactivity was revealed in the gray matter in all cases and had a tendency to be weaker in devastated areas in cases with a longer duration of illness. However, in one particular case with numerous prion plaques, the degeneration was relatively mild while PrP(CJD) immunoreactivity was intense despite the longest duration of illness among the examined cases. Deep layer accentuation of PrP(CJD) immunoreactivity was observed in the cerebral cortices in most cases. This staining pattern, however, disappeared in a burnt-out lesion exhibiting status spongiosus. The granular layer was damaged mostly in the cerebellum of the advanced cases. PrP(CJD) and synaptophysin immunoreactivities decreased as the tissue degeneration progressed. Interestingly, the Purkinje cells had no positivity for PrP(CJD) in all cases, although the neurons in relatively preserved cerebellum showed apparent positivity for synaptophysin. In the Ammon's horn and subiculum the neurons were well preserved despite the marked immunoreactivity for PrP(CJD) in all cases, although some cases demonstrated severe spongiform change. Approximately half of the cases showed intracytoplasmic inclusion body-like immunoreactivity for PrP(CJD) in neurons of the dentate nucleus. These findings suggest that PrP(CJD) deposition may be an event that precedes neuronal degeneration evolving from deeper layers of the cerebral cortex. Although the Ammon's horn and subiculum showed striking PrP(CJD) deposition and spongiform change, neuronal loss did not take place, suggesting that deposited PrP(CJD) itself seems not to be directly harmful to the neurons. Some investigators have assumed that microglia activated by PrP(CJD) plays an important role in neuronal degeneration. Considering this, we speculate that microglia in the Ammon's horn and subiculum may have a unique characteristic of not responding to PrP(CJD).

Ubiquitin in homeostasis, development and disease

Ubiquitin is the most phylogenetically conserved protein known. This 8,500 Da polypeptide can be covalently attached to cellular proteins as a posttranslational modification. In most cases, the addition of multiple ubiquitin adducts to a protein targets it for rapid degradation by a multisubunit protease known as the 26S proteasome. While the ubiquitin/26S proteasome pathway is responsible for the degradation of the bulk of cellular proteins during homeostasis, it may also be responsible for the rapid loss of protein during the programmed death of certain cells, such as skeletal muscle during insect metamorphosis. In addition, alterations in the expression and regulation of ubiquitin may play significant roles in pathological disorders. For example, dramatic increases in ubiquitin and ubiquitin-protein conjugates are observed in a wide variety of neurodegenerative disorders, including Alzheimer's disease. Patients suffering from the autoimmune disease systemic lupus erythematosus generate antibodies reacting with ubiquitin and ubiquitinated histones. At present, it is not known whether these changes in ubiquitin expression and regulation initiate pathological changes in these diseases or if they are altered as a consequence of these disorders.

The abnormal isoform of the prion protein accumulates in late-endosome-like organelles in scrapie-infected mouse brain

The prion encephalopathies are characterized by accumulation in the brain of the abnormal form PrPsc of a normal host gene product PrPc. The mechanism and site of formation of PrPsc from PrPc are currently unknown. In this study, ME7 scrapie-infected mouse brain was used to show, both biochemically and by double-labelled immunogold electron microscopy, that proteinase K-resistant PrPsc is enriched in subcellular structures which contain the cation-independent mannose 6-phosphate receptor, ubiquitin-protein conjugates, beta-glucuronidase, and cathepsin B, termed late endosome-like organelles. The glycosylinositol phospholipid membrane-anchored PrPc will enter such compartment for normal degradation and the organelles may therefore act as chambers for the conversion of PrPc into infectious PrPsc in this murine model of scrapie.

Pathology of the transmissible spongiform encephalopathies with special emphasis on ultrastructure

The transmissible spongiform encephalopathies are a group of genetic and infectious disorders which are exemplified by scrapie in animals and Creutzfeldt-Jakob disease in humans. The spongiform encephalopathies are characterized by symmetrical vacuolation of neurons and neuropil. Amyloid plaque formation similar to that found in Alzheimer's disease is conspicuous in many, but not all, of these diseases. The sub-cellular pathology features of the spongiform encephalopathies have been studied by conventional transmission electron microscopy, scanning electron microscopy, freeze fracture, negative staining and most recently by application of immunogold labelling methods. Although these studies have revealed many unusual structures, convincing virus-like particles have not been demonstrated. Considerable data, including important transgenic mouse studies, now suggest that a single cellular protein, designated prion protein, is necessary for infection. Ultrastructural immunogold studies have shown that prion protein is released from the surface of neurons and neurites, diffuses through the extracellular space around infected cells where it accumulates and finally becomes aggregated as amyloid fibrils. It is likely that the accumulation of prion protein within the extracellular space is instrumental in causing nerve cell dysfunction and, ultimately, neurological disease.

Cytokines, prostaglandins and lipocortin-1 are present in the brains of scrapie-infected mice

The presence of cytokines, prostaglandins and lipocortin-1 was investigated in terminally affected mice in two models of scrapie. There was marked induction of glial interleukin-1 beta, tumour necrosis factor alpha, prostaglandin E2, prostaglandin F2 alpha and lipocortin-1 immunoreactivity in those areas of the brain showing the characteristic vacuolation of scrapie. A comparison of these staining patterns with those of GFAP and F4/80 showed that their expression occurred predominantly in astrocytes. It is possible that cytokines play a significant role in the pathogenesis of neurodegeneration in scrapie.

Expression of polyubiquitin and heat-shock protein 70 genes increases in the later stages of disease progression in scrapie-infected mouse brain

We have shown by northern analyses that the expression of the mouse polyubiquitin C gene is increased severalfold in the brains of mice infected with both the ME7 and 87V strains of scrapie. Expression of the polyubiquitin gene does not change significantly, compared with controls, until the later stages of disease progression when there is a 2.5-fold increase in ME7-infected brains and a 1.8-fold increase in 87V-infected brains. The patterns of changes of expression of the polyubiquitin genes in brains infected with the two strains of scrapie resemble those of accumulation of ubiquitin-conjugate-positive structures in the brain that are detected immunohistochemically. A similar increase in the expression of a heat-shock protein 70 gene also occurs.

Ubiquitination as a probe for neurodegeneration in the brain in schizophrenia: the prefrontal cortex

Abnormalities in brain structure and brain function have been described in schizophrenia. It is not yet known whether these are caused by an abnormality of brain development, some form of birth injury, or a neurodegenerative process. Using immunocytochemical methods and a marker for neurodegeneration (ubiquitin), we examined an area of prefrontal cortex from elderly schizophrenic and control subjects for the presence of ubiquitin-positive degeneration products. There was no statistical difference in the degree of ubiquitination between the control and the patient samples. The findings provide no evidence to support a neurodegenerative process.

Ubiquitin immunocytochemistry in human spongiform encephalopathies

The distribution of ubiquitin was studied by immunocytochemistry in eight cases of human spongiform encephalopathy and compared with the findings in seven age- and sex-matched cases of Alzheimer's disease and six non-demented control cases. The results were also compared with the immunocytochemical distribution of prion protein and the lysosomal aspartic protease cathepsin D. In the human spongiform encephalopathies, ubiquitin immunoreactivity was found in a punctate distribution at the periphery of prion protein amyloid plaques and in a finely granular pattern in the neuropil around and within areas of spongiform change. Cortical nerve cells contained scanty ubiquitinated dot-like inclusions, and occasional microglia around the areas of spongiform change also gave a positive staining reaction for ubiquitin, as did multiple irregular thread-like structures in the neuropil and white matter. The ubiquitin-containing structures at the plaque periphery in human spongiform encephalopathies resemble the neuritic processes at the periphery of the senile plaque in Alzheimer's disease. The granular positivity for ubiquitin associated with areas of spongiform change closely resembles the pattern of immunostaining seen with the antibodies to the prion protein and cathepsin D, consistent with the reported accumulation of ubiquitinated proteins and prion protein in lysosomes in the murine scrapie model. Further studies are required to investigate the role of lysosomes in this group of disorders, and to study the localization of other cell stress proteins and prion protein in spongiform encephalopathies.

Ubiquitin in neurodegenerative diseases

Immunochemical staining to detect ubiquitin has become an essential technique in evaluating neurodegenerative processes. Age related staining is seen in myelin, in nerve processes in lysosome-related dense bodies, and in corpora amylacea. There is a constant association between filamentous inclusions and the presence of ubiquitin. Intermediate filaments associated with ubiquitin, alpha B crystallin and enzymes of the ubiquitin pathway are the basis of Lewy bodies and Rosenthal fibres, as well as related bodies outside the nervous system. Neurofibrillary tangles in diverse diseases are associated with ubiquitin as are several other tau containing inclusions in both neurones and glia. Inclusions in motor neurones and non-motor cortex characterizing amyotrophic lateral sclerosis (ALS) and certain related forms of frontal lobe dementia can only be readily detected by anti-ubiquitin. Anti-ubiquitin also identifies both filamentous and lysosomal structures in neuronal processes as well as in some swollen neurones. Involvement of ubiquitin-containing elements of the lysosomal system appears important in pathogenesis of prion encephalopathies. Despite great advances in understanding cell biology of the ubiquitin pathway there are as yet few insights into the precise role played by ubiquitin in neuronal disease.

Immunoreactivity to ubiquitin-protein conjugates is present early in the disease process in the brains of scrapie-infected mice

Brains from mice infected with either the 87V or the ME7 strains of mouse-passaged sheep scrapie were taken at stages during the disease process and immunostained to show the localization of ubiquitin-protein conjugates. In both models, conjugates were seen as fine, dot-like structures; as coarser, granular lesions within or adjacent to neurones; and in areas surrounding plaques. The dot-like structures were visible at 28 days post-ME7 infection and at 55 days in 87V-infected mice. In both models, the extent of immunoreactive changes increased as the disease progressed and terminal infection was as described earlier by us (Lowe et al., J. Pathol 1990; 162: 61-66). The patterns of development of these features were distinctive in two ways: progression from region to region was observable and the density of the pathological lesions grew exponentially as the clinical symptoms appeared. The earliest pathological dot-like structures corresponded temporally with the earliest detection of PrPSC by Western blotting, and immunogold electron microscopic investigation of the dot-like lesions indicated that they were the multi-vesicular, lysosome-related, dense bodies that we have described previously in terminal disease (Laszlo et al., J Pathol 1992; 166: 333-341). Until now, ubiquitin-protein conjugates were seen mainly in inclusion bodies associated with the terminal stages of a range of human degenerative diseases. This study establishes that ubiquitin-protein conjugates accumulate in lysosome-related bodies very early and appear to be intimately related to the pathological processes in the animal disorders that we have studied.

Protein processing in lysosomes: the new therapeutic target in neurodegenerative disease

A little recognised feature of neurons is their large complement of lysosomes. Studies of the accumulation of the abnormal isoform of the prion protein (PrPSC) in the prion encephalopathies and the formation of beta/A4 protein from its precursor in Alzheimer's disease suggest that generation of these key proteins takes place in lysosome-related organelles. The release of hydrolytic enzymes from lysosomes may be a primary cause of neuronal damage. Although molecular genetic approaches have identified protein mutations central to the main neurodegenerative disease, cell biological observations are now beginning to unravel the intracellular pathways involved in the molecular pathogenesis of neurodegeneration: as a result, it is now appropriate to consider therapeutic manipulation of the lysosomal system as an approach to treatment.

Lysosomes as key organelles in the pathogenesis of prion encephalopathies

The causation, structural origin, and mechanism of formation of spongiform lesions in transmissible encephalopathies are unknown. We have used immunogold electron microscopy to locate ubiquitin conjugates, hsp 70, and beta-glucuronidase (markers of the lysosomal compartment) and prion protein (PrP) in both control and scrapie-infected mouse brain. In scrapie-infected brain, lysosomes and lysosome-related structures (multivesicular and tubulovesicular dense bodies) are present in abnormally high numbers in neuronal cell processes. These structures contain PrP, together with the lysosomal markers ubiquitin conjugates, hsp 70, and beta-glucuronidase, which could also be identified spilling from tubulovesicular dense bodies into areas of early rarefaction in neuronal processes; we suggest that these areas of rarefaction are the precursor lesions of spongiform change. We advance the hypothesis that spongiform change is brought about by cytoskeletal disruption in neuronal processes caused by liberation of hydrolytic enzymes from lysosomes overloaded with the abnormal isoform of PrP (PrPsc). We suggest that the lysosomal system is probably acting as the bioreactor for processing of normal PrP to the abnormal isoform. The continuous production of increasing quantities of abnormal PrPsc in lysosome-related bodies will eventually cause disruption of the lysosomal membrane with destruction of the neuronal cytoskeleton and the initiation of vacuolation. Later, death of the cell will be associated with release of the PrPsc isoform into the extracellular environment. Repeated rounds of phagocytosis, lysosomal biogenesis of PrPsc, lysosomal membrane rupture, hydrolytic enzyme release, and neuronal lysis will lead to an exponential increase in cell damage and cell death.(ABSTRACT TRUNCATED AT 250 WORDS)

Ubiquitin-protein conjugates and alpha B crystallin are selectively present in cells undergoing major cytomorphological reorganisation in early chicken embryos

Ubiquitin-protein conjugates and alpha B crystallin are detected immunohistochemically in cells undergoing extensive morphological reorganisation in early chicken embryos. Cytoplasmic ubiquitinated proteins and alpha B crystallin are coordinately found in cells of the lens, notochord and myotome. The antigens appear in the myotome cells precisely at the point at which the cells begin to migrate from the dorsomedial lip of the dermamyotome. The findings indicate that ubiquitin and alpha B crystallin may have a coordinate role in the extensive architectural remodeling which occurs in these developing tissues in the early chick embryo. Some form of functional association between protein ubiquitination and alpha B crystallin in cells may explain why alpha B crystallin is found with ubiquitin-protein deposits in some neurodegenerative diseases.

The latent membrane protein-1 in Epstein-Barr virus-transformed lymphoblastoid cells is found with ubiquitin-protein conjugates and heat-shock protein 70 in lysosomes oriented around the microtubule organizing centre

Immunofluorescence studies on Epstein-Barr virus (EBV)-transformed lymphoblastoid cells have previously shown that the latent membrane transforming protein (LMP-1) is found in patch-like inclusions which also immunostain for vimentin. We now show that EBV transformation causes a major reorganization of intermediate filaments, microtubules, mitochondria, and lysosomal elements, which generally become oriented around the microtubule organizing centre. Immunogold electron microscopy shows that LMP-1 is primarily concentrated in secondary lysosomes together with ubiquitin-protein conjugates and heat-shock protein 70. Intermediate filament inclusion formation with the above characteristics may be a general response triggered by other membrane glycoproteins; as seen, for example, in major human neurodegenerative diseases such as diffuse Lewy body disease.

Ubiquitin in health and disease

Studies in recent years have shown that ubiquitin has increasingly important functions in eukaryotic cells; roles which were previously not suspected in healthy and diseased cells. The interplay between molecular pathological and molecular cell biological findings has indicated that ubiquitin may be pivotal in the cell stress response in chronic degenerative and viral diseases. Furthermore, the studies have led to the notion that ubiquitination may not only serve as a signal for nonlysosomal protein degradation but may be a unifying covalent protein modification for the major intracellular protein catabolic systems; these can act to identify proteins for cytosolic proteinases or direct intact and fragmented proteins into the lysosome system for breakdown to amino acids. This unifying role could explain why ubiquitin is restricted to eukaryotic cells, which possess extensive endomembrane systems in addition to a nuclear envelope. Protein ubiquitination is a feature of most filamentous inclusions and certain other intracellular conglomerates that are found in some degenerative and viral diseases. The detection of ubiquitin-protein conjugates is not of great diagnostic importance in these diseases. Protein ubiquitination is not only essential for the normal physiological turnover of proteins but appears to have been adapted as part of an intracellular surveillance system that can be activated by altered, damaged, or foreign proteins and organelles. The purpose of this system is to isolate and eliminate these noxious structures from the cell: as a cytoprotective mechanism this appears to have evolved in the cell akin perhaps to an 'intracellular immune system'. Other heat shock proteins such as hsp 70 may be involved in this process. It is apparent that ubiquitin has a role in embryonic development. Protein ubiquitination is presumably involved in the reorganisation of cytoplasm that accompanies cell differentiation. Ubiquitin is also necessary for the gross intracellular degradative processes which are consequent upon programmed cell death. Cell elimination is of key importance for a number of developmental morphogenetic changes. An understanding of the molecular details of these processes will no doubt provide further insights into the wide ranging roles of ubiquitin in the life process. As it says in the book 'Ubiquitin'; there is no doubt that ubiquitin is a 'lucky' protein. It is lucky in many ways: lucky for scientific progress, lucky for biomedical scientists and lucky for life! If you have not already done so, why don't you get lucky and look for a role for ubiquitin in your experimental system. As Avram Hershko has said "there is plenty to go round"!

The scrapie fibril protein and its cellular isoform

Proteins need help to fold and attain their functional conformation (Ellis and Hemmingsen 1989), and mechanisms have evolved to prevent the accumulation of misfolded protein aggregates within cells (Pelham 1988). These mechanisms fail to prevent the formation of protease-resistant, misfolded forms of PrP (ScPrP) during the development of scrapie and other transmissible spongiform encephalopathies, and ScPrP is a biochemical marker of these diseases. Much is now known about the structure and expression of the PrP gene, but the physiological function of the PrP protein and the mechanism by which the TDE pathogen replicates and specifically interferes with PrP metabolism remain a mystery--a mystery which will entertain prion-ophiliacs for some time yet.