Immunoreactivity to ubiquitin-protein conjugates is present early in the disease process 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.

Toward Therapy of Human Prion Diseases

Three decades after the discovery of prions as the cause of Creutzfeldt-Jakob disease and other transmissible spongiform encephalopathies, we are still nowhere close to finding an effective therapy. Numerous pharmacological interventions have attempted to target various stages of disease progression, yet none has significantly ameliorated the course of disease. We still lack a mechanistic understanding of how the prions damage the brain, and this situation results in a dearth of validated pharmacological targets. In this review, we discuss the attempts to interfere with the replication of prions and to enhance their clearance. We also trace some of the possibilities to identify novel targets that may arise with increasing insights into prion biology.

Lysosomal Quality Control in Prion Diseases

Prion diseases are transmissible, familial or sporadic. The prion protein (PrP), a normal cell surface glycoprotein, is ubiquitously expressed throughout the body. While loss of function of PrP does not elicit apparent phenotypes, generation of misfolded forms of the protein or its aberrant metabolic isoforms has been implicated in a number of neurodegenerative disorders such as scrapie, kuru, Creutzfeldt-Jakob disease, fatal familial insomnia, Gerstmann-Sträussler-Scheinker and bovine spongiform encephalopathy. These diseases are all phenotypically characterised by spongiform vacuolation of the adult brain, hence collectively termed as late-onset spongiform neurodegeneration. Misfolded form of PrP (PrP^Sc) and one of its abnormal metabolic isoforms (the transmembrane ^CtmPrP) are known to be disease-causing agents that lead to progressive loss of structure or function of neurons culminating in neuronal death. The aberrant forms of PrP utilise and manipulate the various intracellular quality control mechanisms during pathogenesis of these diseases. Amongst these, the lysosomal quality control machinery emerges as one of the primary targets exploited by the disease-causing isoforms of PrP. The autophagosomal-lysosomal degradation pathway is adversely affected in multiple ways in prion diseases and may hence be regarded as an important modulator of neurodegeneration. Some of the ESCRT pathway proteins have also been shown to be involved in the manifestation of disease phenotype. This review discusses the significance of the lysosomal quality control pathway in affecting transmissible and familial types of prion diseases.

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

Prion degradation pathways: Potential for therapeutic intervention

Prion diseases are fatal neurodegenerative disorders. Pathology is closely linked to the misfolding of native cellular PrP(C) into the disease-associated form PrP(Sc) that accumulates in the brain as disease progresses. Although treatments have yet to be developed, strategies aimed at stimulating the degradation of PrP(Sc) have shown efficacy in experimental models of prion disease. Here, we describe the cellular pathways that mediate PrP(Sc) degradation and review possible targets for therapeutic intervention. This article is part of a Special Issue entitled 'Neuronal Protein'.

Axonal and transynaptic spread of prions

Natural transmission of prion diseases depends upon the spread of prions from the nervous system to excretory or secretory tissues, but the mechanism of prion transport in axons and into peripheral tissue is unresolved. Here, we examined the temporal and spatial movement of prions from the brain stem along cranial nerves into skeletal muscle as a model of axonal transport and transynaptic spread. The disease-specific isoform of the prion protein, PrP(Sc), was observed in nerve fibers of the tongue approximately 2 weeks prior to PrP(Sc) deposition in skeletal muscle. Initially, PrP(Sc) deposits had a small punctate pattern on the edge of muscle cells that colocalized with synaptophysin, a marker for the neuromuscular junction (NMJ), in >50% of the cells. At later time points PrP(Sc) was widely distributed in muscle cells, but <10% of prion-infected cells exhibited PrP(Sc) deposition at the NMJ, suggesting additional prion replication and dissemination within muscle cells. In contrast to the NMJ, PrP(Sc) was not associated with synaptophysin in nerve fibers but was found to colocalize with LAMP-1 and cathepsin D during early stages of axonal spread. We propose that PrP(Sc)-bound endosomes can lead to membrane recycling in which PrP(Sc) is directed to the synapse, where it either moves across the NMJ into the postsynaptic muscle cell or induces PrP(Sc) formation on muscle cells across the NMJ.

IMPORTANCE

Prion diseases are transmissible and fatal neurodegenerative diseases in which prion dissemination to excretory or secretory tissues is necessary for natural disease transmission. Despite the importance of this pathway, the cellular mechanism of prion transport in axons and into peripheral tissue is unresolved. This study demonstrates anterograde spread of prions within nerve fibers prior to infection of peripheral synapses (i.e., neuromuscular junction) and infection of peripheral tissues (i.e., muscle cells). Within nerve fibers prions were associated with the endosomal-lysosomal pathway prior to entry into muscle cells. Since early prion spread is anterograde and endosome-lysosomal movement within axons is primarily retrograde, these findings suggest that endosome-bound prions may have an alternate fate that directs prions to the peripheral synapse.

Cellular and sub-cellular pathology of animal prion diseases: relationship between morphological changes, accumulation of abnormal prion protein and clinical disease

The transmissible spongiform encephalopathies (TSEs) or prion diseases of animals are characterised by CNS spongiform change, gliosis and the accumulation of disease-associated forms of prion protein (PrP(d)). Particularly in ruminant prion diseases, a wide range of morphological types of PrP(d) depositions are found in association with neurons and glia. When light microscopic patterns of PrP(d) accumulations are correlated with sub-cellular structure, intracellular PrP(d) co-localises with lysosomes while non-intracellular PrP(d) accumulation co-localises with cell membranes and the extracellular space. Intracellular lysosomal PrP(d) is N-terminally truncated, but the site at which the PrP(d) molecule is cleaved depends on strain and cell type. Different PrP(d) cleavage sites are found for different cells infected with the same agent indicating that not all PrP(d) conformers code for different prion strains. Non-intracellular PrP(d) is full-length and is mainly found on plasma-lemmas of neuronal perikarya and dendrites and glia where it may be associated with scrapie-specific membrane pathology. These membrane changes appear to involve a redirection of the predominant axonal trafficking of normal cellular PrP and an altered endocytosis of PrP(d). PrP(d) is poorly excised from membranes, probably due to increased stabilisation on the membrane of PrP(d) complexed with other membrane ligands. PrP(d) on plasma-lemmas may also be transferred to other cells or released to the extracellular space. It is widely assumed that PrP(d) accumulations cause neurodegenerative changes that lead to clinical disease. However, when different animal prion diseases are considered, neurological deficits do not correlate well with any morphological type of PrP(d) accumulation or perturbation of PrP(d) trafficking. Non-PrP(d)-associated neurodegenerative changes in TSEs include vacuolation, tubulovesicular bodies and terminal axonal degeneration. The last of these correlates well with early neurological disease in mice, but such changes are absent from large animal prion disease. Thus, the proximate cause of clinical disease in animal prion disease is uncertain, but may not involve PrP(d).

Prions and the proteasome

Prion diseases are fatal neurodegenerative disorders that include Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in animals. They are unique in terms of their biology because they are caused by the conformational re-arrangement of a normal host-encoded prion protein, PrPC, to an abnormal infectious isoform, PrPSc. Currently the precise mechanism behind prion-mediated neurodegeneration remains unclear. It is hypothesised than an unknown toxic gain of function of PrPSc, or an intermediate oligomeric form, underlies neuronal death. Increasing evidence suggests a role for the ubiquitin proteasome system (UPS) in prion disease. Both wild-type PrPC and disease-associated PrP isoforms accumulate in cells after proteasome inhibition leading to increased cell death, and abnormal beta-sheet-rich PrP isoforms have been shown to inhibit the catalytic activity of the proteasome. Here we review potential interactions between prions and the proteasome outlining how the UPS may be implicated in prion-mediated neurodegeneration.

Disease-associated prion protein oligomers inhibit the 26S proteasome

The mechanism of cell death in prion disease is unknown but is associated with the production of a misfolded conformer of the prion protein. We report that disease-associated prion protein specifically inhibits the proteolytic beta subunits of the 26S proteasome. Using reporter substrates, fluorogenic peptides, and an activity probe for the beta subunits, this inhibitory effect was demonstrated in pure 26S proteasome and three different cell lines. By challenge with recombinant prion and other amyloidogenic proteins, we demonstrate that only the prion protein in a nonnative beta sheet conformation inhibits the 26S proteasome at stoichiometric concentrations. Preincubation with an antibody specific for aggregation intermediates abrogates this inhibition, consistent with an oligomeric species mediating this effect. We also present evidence for a direct relationship between prion neuropathology and impairment of the ubiquitin-proteasome system (UPS) in prion-infected UPS-reporter mice. Together, these data suggest a mechanism for intracellular neurotoxicity mediated by oligomers of misfolded prion protein.

Molecular analysis of bovine spongiform encephalopathy infection by cDNA arrays

Here, the first cDNA array analysis of differential gene expression in bovine spongiform encephalopathy (BSE) is reported, using a spotted cDNA array platform representing nearly 17 000 mouse genes. Array analysis identified 296 gene candidates for differential expression in brain tissue from VM mice in late-stage infection with the 301V strain of BSE, compared with brain tissue from normal, age-matched VM mice. Real-time PCR confirmed differential expression of 25 of 31 genes analysed. Some of the genes identified by array analysis as being expressed differentially are associated with ubiquitin/proteasome function, lysosomal function, molecular chaperoning of protein folding or apoptosis. Other genes are involved in calcium ion binding/homeostasis, zinc ion binding/homeostasis or regulation of transcription. Principal-component analysis shows that the global gene-expression profiles of the BSE-infected samples have gene-expression signatures that are markedly different from, and completely non-overlapping with, those obtained from the normal controls.

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.

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.

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.

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.

Expression of protein gene product 9.5, a neuronal ubiquitin C-terminal hydrolase, and its developing change in sertoli cells of mouse testis

Protein gene product 9.5 (PGP9.5), originally isolated as a neuron-specific protein, belongs to a family of ubiquitin carboxyl-terminal hydrolases that play important roles in the nonlysosomal proteolytic pathway. Antibodies against PGP9.5 have been used for immunohistochemical detection of neural elements, although some non-neuronal cells are also immunoreactive for PGP9.5. In the present study, developing testes of the mouse were immunostained after autoclave pretreatment of sections. In the testes of days 8 and 16, PGP9.5 was only localized on the spermatogonia, whereas on day 30 and in adults it appeared not only on spermatogonia, but also on Sertoli cells. In the testis of the male sterile W/W(v) mutant, very little, but strong, immunoreactivity was detected at some Sertoli cells, which were phagocytizing Sertoli cell aggregations that had fallen from the basal membrane. Additionally, it was confirmed that the nucleotide sequence of PGP9.5 in mice was highly conserved, like that in other mammals. These results suggest that PGP9.5 is a useful marker for activated Sertoli cells, playing an important role in degradation of abnormal proteins.

Immunocytochemical appearance of cytokines, prostaglandin E2 and lipocortin-1 in the CNS during the incubation period of murine scrapie correlates with progressive PrP accumulations

The appearance of immunoreactive interleukin (IL)-1beta, IL-6 and tumour necrosis factor (TNF)-alpha, prostaglandin (PG) E2 and lipocortin-1 in the central nervous system was investigated during the development of lesions in a 301V/VM murine scrapie model. Focal PrP(Sc) deposition was present after 30 days of the 115-120 day incubation period; this immunoreactivity increased in intensity and distribution thereafter. Staining for IL-1beta and TNF alpha in perivascular macrophages, and PGE2 immunoreactivity in astrocytes, was detected in those areas showing PrP(Sc) deposition from 60 days. Increased GFAP and F4/80 immunoreactivity, indicating activation of astrocytes and microglia, was also evident in these areas from 60 days. Glial cytokine and lipocortin immunoreactivity was detected after 90 days, in the absence of clinical signs. The disease-induced cytokine, PG and lipocortin immunoreactivity occurred only in those brain areas showing PrP(Sc) deposition, glial activation and, in later stages, vacuolation. These findings support the concept that PrP(Sc) deposition induces glial cytokine production. These glial cytokines may contribute to the development of the pathological lesions in scrapie.

Endosome-lysosomes, ubiquitin and neurodegeneration

Before the advent of ubiquitin immunochemistry and immunogold electron microscopy, there was no known intracellular molecular commonality between neurodegenerative diseases. The application of antibodies which primarily detect ubiquitin protein conjugates has shown that all of the human and animal idiopathic and transmissible chronic neurodegenerative diseases, (including Alzheimer's disease (AD), Lewy body disease (LBD), amyotrophic lateral sclerosis (ALS), Creutzfeldt-Jakob disease (CJD) and scrapie) are related by some form of intraneuronal inclusion which contains ubiquitin protein conjugates. In addition, disorders such as Alzheimer's disease, CJD and sheep scrapie, are characterised by deposits of amyloid, arising through incomplete breakdown of membrane proteins which may be associated with cytoskeletal reorganisation. Although our knowledge about these diseases is increasing, they remain largely untreatable. Recently, attention has focused on the mechanisms of production of different types of amyloid and the likely involvement within cells of the endosome-lysosome system, organelles which are immuno-positive for ubiquitin protein conjugates. These organelles may be 'bioreactor' sites for the unfolding and partial degradation of membrane proteins to generate the amyloid materials or their precursors which subsequently become expelled from the cell, or are released from dead cells, and accumulate as pathological entities. Such common features of the disease processes give new direction to therapeutic intervention.

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.

Endosome-lysosomes and neurodegeneration

A number of the major human and animal neurodegenerative diseases, such as Alzheimer's disease and sheep scrapie, are characterised by deposits of amyloid, arising through incomplete breakdown of membrane proteins. Although our knowledge concerning these diseases is increasing, they remain largely untreatable. Recently, attention has focussed on the mechanisms of production of different types of amyloid and the likely involvement within cells of acid compartments called endosome-lysosomes. These organelles may be 'bioreactor' sites for the unfolding and partial degradation of membrane proteins to generate the amyloid materials. These subsequently become expelled from the cell, or are released from dead cells, and accumulate as pathological entities. Common features of the disease processes give new direction to therapeutic intervention.

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.

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.