What is the basis of transmissible spongiform encephalopathy induced neurodegeneration and can it be repaired?

Conventional and State-of-the-Art Detection Methods of Bovine Spongiform Encephalopathy (BSE)

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease that belongs to a group of diseases known as transmissible spongiform encephalopathies (TSEs). It is believed that the infectious agent responsible for prion diseases is abnormally folded prion protein (PrP^Sc), which derives from a normal cellular protein (PrP^C), which is a cell surface glycoprotein predominantly expressed in neurons. There are three different types of BSE, the classical BSE (C-type) strain and two atypical strains (H-type and L-type). BSE is primarily a disease of cattle; however, sheep and goats also can be infected with BSE strains and develop a disease clinically and pathogenically indistinguishable from scrapie. Therefore, TSE cases in cattle and small ruminants require discriminatory testing to determine whether the TSE is BSE or scrapie and to discriminate classical BSE from the atypical H- or L-type strains. Many methods have been developed for the detection of BSE and have been reported in numerous studies. Detection of BSE is mainly based on the identification of characteristic lesions or detection of the PrP^Sc in the brain, often by use of their partial proteinase K resistance properties. The objective of this paper was to summarize the currently available methods, highlight their diagnostic performance, and emphasize the advantages and drawbacks of the application of individual tests.

The Role of the Mammalian Prion Protein in the Control of Sleep

Sleep disruption is a prevalent clinical feature in many neurodegenerative disorders, including human prion diseases where it can be the defining dysfunction, as in the case of the "eponymous" fatal familial insomnia, or an early-stage symptom as in certain types of Creutzfeldt-Jakob disease. It is important to establish the role of the cellular prion protein (PrP^C), the key molecule involved in prion pathogenesis, within the sleep-wake system in order to understand fully the mechanisms underlying its contribution to both healthy circadian rhythmicity and sleep dysfunction during disease. Although severe disruption to the circadian rhythm and melatonin release is evident during the pathogenic phases of some prion diseases, untangling whether PrP^C plays a role in circadian rhythmicity, as suggested in mice deficient for PrP^C expression, is challenging given the lack of basic experimental research. We provide a short review of the small amount of direct literature focused on the role of PrP^C in melatonin and circadian rhythm regulation, as well as suggesting mechanisms by which PrP^C might exert influence upon noradrenergic and dopaminergic signaling and melatonin synthesis. Future research in this area should focus upon isolating the points of dysfunction within the retino-pineal pathway and further investigate PrP^C mediation of pinealocyte GPCR activity.

Characterisation of radioiodinated flavonoid derivatives for SPECT imaging of cerebral prion deposits

Prion diseases are fatal neurodegenerative diseases characterised by deposition of amyloid plaques containing abnormal prion protein aggregates (PrP(Sc)). This study aimed to evaluate the potential of radioiodinated flavonoid derivatives for single photon emission computed tomography (SPECT) imaging of PrP(Sc). In vitro binding assays using recombinant mouse PrP (rMoPrP) aggregates revealed that the 4-dimethylamino-substituted styrylchromone derivative (SC-NMe2) had higher in vitro binding affinity (Kd = 24.5 nM) and capacity (Bmax = 36.3 pmol/nmol protein) than three other flavonoid derivatives (flavone, chalcone, and aurone). Fluorescent imaging using brain sections from mouse-adapted bovine spongiform encephalopathy (mBSE)-infected mice demonstrated that SC-NMe2 clearly labelled PrP(Sc)-positive prion deposits in the mice brain. Two methoxy SC derivatives, SC-OMe and SC-(OMe)2, also showed high binding affinity for rMoPrP aggregates with Ki values of 20.8 and 26.6 nM, respectively. In vitro fluorescence and autoradiography experiments demonstrated high accumulation of [(125)I]SC-OMe and [(125)I]SC-(OMe)2 in prion deposit-rich regions of the mBSE-infected mouse brain. SPECT/computed tomography (CT) imaging and ex vivo autoradiography demonstrated that [(123)I]SC-OMe showed consistent brain distribution with the presence of PrP(Sc) deposits in the mBSE-infected mice brain. In conclusion, [(123)I]SC-OMe appears a promising SPECT radioligand for monitoring prion deposit levels in the living brain.

Mahogunin Ring Finger-1 (MGRN1), a Multifaceted Ubiquitin Ligase: Recent Unraveling of Neurobiological Mechanisms

In healthy cell, inappropriate accumulation of poor or damaged proteins is prevented by cellular quality control system. Autophagy and ubiquitin proteasome system (UPS) provides regular cytoprotection against proteotoxicity induced by abnormal or disruptive proteins. E3 ubiquitin ligases are crucial components in this defense mechanism. Mahogunin Ring Finger-1 (MGRN1), an E3 ubiquitin ligase of the Really Interesting New Gene (RING) finger family, plays a pivotal role in many biological and cellular mechanisms. Previous findings indicate that lack of functions of MGRN1 can cause spongiform neurodegeneration, congenital heart defects, abnormal left-right patterning, and mitochondrial dysfunctions in mice brains. However, the detailed molecular pathomechanism of MGRN1 in cellular functions and diseases is not well known. This article comprehensively represents the molecular nature, characterization, and functions of MGRN1; we also summarize possible beneficiary aspects of this novel E3 ubiquitin ligase. Here, we review recent literature on the role of MGRN1 in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic intervention.

The intricate mechanisms of neurodegeneration in prion diseases

Prion diseases are a group of infectious neurodegenerative diseases with an entirely novel mechanism of transmission, involving a protein-only infectious agent that propagates the disease by transmitting protein conformational changes. The disease results from extensive and progressive brain degeneration. The molecular mechanisms involved in neurodegeneration are not entirely known but involve multiple processes operating simultaneously and synergistically in the brain, including spongiform degeneration, synaptic alterations, brain inflammation, neuronal death and the accumulation of protein aggregates. Here, we review the pathways implicated in prion-induced brain damage and put the pieces together into a possible model of neurodegeneration in prion disorders. A more comprehensive understanding of the molecular basis of brain degeneration is essential to develop a much needed therapy for these devastating diseases.

Molecular pathology of human prion diseases

Prion diseases are fatal neurodegenerative conditions in humans and animals. In this review, we summarize the molecular background of phenotypic variability, relation of prion protein (PrP) to other proteins associated with neurodegenerative diseases, and pathogenesis of neuronal vulnerability. PrP exists in different forms that may be present in both diseased and non-diseased brain, however, abundant disease-associated PrP together with tissue pathology characterizes prion diseases and associates with transmissibility. Prion diseases have different etiological background with distinct pathogenesis and phenotype. Mutations of the prion protein gene are associated with genetic forms. The codon 129 polymorphism in combination with the Western blot pattern of PrP after proteinase K digestion serves as a basis for molecular subtyping of sporadic Creutzfeldt-Jakob disease. Tissue damage may result from several parallel, interacting or subsequent pathways that involve cellular systems associated with synapses, protein processing, oxidative stress, autophagy, and apoptosis.

Prion diseases: from protein to cell pathology

Prion diseases or transmissible spongiform encephalopathies are fatal neurodegenerative conditions in humans and animals that originate spontaneously, genetically or by infection. Conformational change of the normal (cellular) form of prion protein (PrP c) to a pathological, disease-associated form (PrP TSE) is considered central to pathogenesis and formation of the infectious agent or prion. Neuronal damage is central to clinical manifestation of prion diseases but poorly understood. In this review, we analyze the major pathogenetic pathways that lead to tissue pathology in different forms of disease. Neuropathogenesis of prion diseases evolves in complex ways on several front lines, most but not all of which exist also in other neurodegenerative as well as infectious diseases. Whereas intracellular accumulation of PrP forms might significantly impair cell function and lead to cytopathology, mere extracellular deposition of PrP TSE is questionable as a direct cytotoxic factor. Tissue damage may result from several parallel, interacting, or subsequent pathways. Future studies should clarify the trigger(s) and sequence of these processes and whether, and which, one is dominating or decisive.

TGFβ1 regulates the inflammatory response during chronic neurodegeneration

The ME7 model of murine prion disease shows an atypical inflammatory response characterized by morphologically activated microglia and an anti-inflammatory cytokine profile with a marked expression of TGFbeta1. The investigation of the role of TGFbeta1 during a time course disease shows that its expression is correlated with (i) the onset of behavioral abnormalities, (ii) increased activated microglia, (iii) thickening of the basement membrane, and (iv) is associated with increased PrP(sc) deposition. Increasing TGFbeta1 using an adenoviral vector has no significant impact on prion-associated behavioral impairments or on neuropathology. In contrast, inhibition of TGFbeta1 activity using an adenovirus expressing decorin induces severe cerebral inflammation, expression of inducible nitric oxide synthase and acute neuronal death in prion-diseased animals only. These data suggest that TGFbeta1 plays a critical role in the downregulation of microglial responses minimizing brain inflammation and thus avoiding exacerbation of brain damage.

Diagnosis of transmissible spongiform encephalopathies in animals: a review

Transmissible spongiform encephalopathies (TSEs) in animals include, among others, bovine spongiform encephalopathy (BSE), scrapie, chronic wasting disease, and atypical forms of prion diseases. Diagnosis of TSEs is based on identification of characteristic lesions or on detection of the abnormal prion proteins in tissues, often by use of their partial proteinase K resistance property. Correctly sampling of target tissues is of utmost importance as this has a considerable effect on test sensitivity. Most of the rapid or screening tests are based on ELISA or Western immunoblot (WB) analysis, and many are officially approved. Confirmatory testing is normally performed by use of histologic examination, immunohistochemical analysis, certain WB protocols, or detection of prion fibrils by use of electron microscopy (scrapie-associated fibril). The discriminatory methods for diagnostic use are mostly based on WB technology and provide initial identification of the prion strain, particularly for differentiation of BSE from scrapie in small ruminants. Definitive prion strain characterization is performed by use of bioassays, usually in mice. A burgeoning number of transgenic mice have been developed for TSE studies. Development of new tests with higher sensitivity and of more reliable diagnostic applications for live animals tested for food safety reasons is a rapidly developing field. Ultimately, the choice of a test for TSE diagnosis depends on the rationale for the testing.

Disease-related prion protein forms aggresomes in neuronal cells leading to caspase activation and apoptosis

The molecular basis for neuronal death in prion disease is not established, but putative pathogenic roles for both disease-related prion protein (PrP(Sc)) and accumulated cytosolic PrP(C) have been proposed. Here we report that only prion-infected neuronal cells become apoptotic after mild inhibition of the proteasome, and this is strictly dependent upon sustained propagation of PrP(Sc). Whereas cells overexpressing PrP(C) developed cytosolic PrP(C) aggregates, this did not cause cell death. In contrast, only in prion-infected cells, mild proteasome impairment resulted in the formation of large cytosolic perinuclear aggresomes that contained PrP(Sc), heat shock chaperone 70, ubiquitin, proteasome subunits, and vimentin. Similar structures were found in the brains of prion-infected mice. PrP(Sc) aggresome formation was directly associated with activation of caspase 3 and 8, resulting in apoptosis. These data suggest that neuronal propagation of prions invokes a neurotoxic mechanism involving intracellular formation of PrP(Sc) aggresomes. This, in turn, triggers caspase-dependent apoptosis and further implicates proteasome dysfunction in the pathogenesis of prion diseases.

Neuronal cell death in transmissible spongiform encephalopathies (prion diseases) revisited: from apoptosis to autophagy

Neuronal autophagy, like apoptosis, is one of the mechanisms of the programmed cell death (PCD). In this review, we summarize the presence of autophagic vacuoles in experimentally induced scrapie, Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Initially, a part of the neuronal cytoplasm was sequestrated by concentric arrays of double membranes; the enclosed cytoplasm appeared relatively normal except that its density was often increased. Next, electron density of the central area dramatically increased. The membranes then proliferated within the cytoplasm in a labyrinth-like manner and the area sequestrated by these membranes enlarged into a more complex structure consisting of vacuoles, electron-dense areas and areas of normally-looking cytoplasm connected by convoluted membranes. Of note, autophagic vacuoles form not only in neuronal perikarya but also in neurites and synapses. Finally, a large area of the cytoplasm was transformed into a collection of autophagic vacuoles of different sizes. On a basis of ultrastructural studies, we suggest that autophagy plays a major role in transmissible spongiform encephalopathies (TSEs) and may even participate in a formation of spongiform change.

Hypothesis of interference to superinfection between bovine spastic paresis and bovine spongiform encephalopathy; suggestions for experimentation, theoretical and practical interest

Sub-acute transmissible spongiform encephalopathies (TSEs) or prion diseases are diseases of little known etiology. The origin of these diseases would appear to be an abnormal protease-resistant prion protein (PrP(res)) which would be infectious by directly inducing its defective conformation to the normal native protein (PrP(C)). This hypothesis does not account for certain aspects of TSEs, such as interference to superinfection: in laboratory animals, inoculation by means of an attenuated strain with a long incubation period protects against later infection by a very virulent strain with a short incubation period. The hypothesis is put forward that there exists a possibility of interference to superinfection between neurodegenerative diseases of unknown origin, thought to be similar to TSEs, and a later infection by a TSE. The study of this interference between bovine spastic paresis (BSP) and bovine spongiform encephalopathy (BSE) could be used as a model for this hypothesis. BSP is a very rare disease among cattle, of unknown etiology; it is curable, in the very early stages, by using tryptophan and especially lithium, potentiated by copper and manganese. An etiology close to that of TSEs has been suggested on several occasions. If interference could be demonstrated between BSP and BSE, interesting data would be provided concerning the etiology, the pathogenesis and possibly the treatment and prevention of these diseases. Notably, such data could lead to the development of a treatment and a prevention with lithium and amino acids precursors of neuromediators (tryptophan, tyrosine, glutamic acid, etc.), as well as the developing of a vaccine to combat TSEs, especially BSE and scrapie.

Scrapie-infected GT1-1 cells show impaired function of voltage-gated N-type calcium channels (Cav 2.2) which is ameliorated by quinacrine treatment

Prions are transmissible pathogens that cause neurodegenerative diseases, although the mechanisms behind the nervous system dysfunctions are unclear. To study the effects of a prion infection on voltage-gated calcium channels, scrapie-infected gonadotropin-releasing hormone neuronal cells (ScGT1-1) in culture were depolarized by KCl and calcium responses recorded. Lower calcium responses were observed in infected compared to uninfected cells. This effect was still observed when L-type calcium channels were blocked by nimodipine. After inhibition of N-type calcium channels with omega-conotoxin GVIA, there was no difference in calcium responses. The calcium responses after nimodipine treatment became progressively lower during infection, but there was no major loss of the cellular prion protein (PrP(C)) or marked increase in accumulation of the abnormal prion protein (PrP(Sc)) in the cultures. These results indicate that scrapie infection causes a dysfunction of voltage-gated N-type calcium channels, which is exacerbated slowly over time. Quinacrine treatment cleared PrP(Sc) and restored calcium responses in the ScGT1-1 cultures.

Anti-prion antibodies for prophylaxis following prion exposure in mice

Prion disease is characterized by a conformational change of the normal form of the prion protein (PrP(C)) to the scrapie-associated form (PrP(Sc)). Since the emergence of new variant Creutzfeldt-Jakob disease a potentially large human population is at risk for developing prion disease. Currently, no effective treatment or form of post-exposure prophylaxis is available for prion disease. We recently showed that active immunization with recombinant PrP prolongs the incubation period of scrapie. Here we show that anti-PrP antibodies following prion exposure are effective at increasing the incubation period of the infection. Stimulation of the immune system is an important therapeutic target for the prion diseases, as well as for other neurodegenerative illnesses characterized by abnormal protein conformation.

CNS pathogenesis of prion diseases

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases, clinically characterised by cognitive decline, paralleled by severe damage to the central nervous system. Prion diseases have attracted a broad interest because of their unique mechanisms of replication and propagation; however, the underlying pathogenic mechanisms are still highly speculative. In this review, current knowledge about the pathogenesis of prion diseases in the CNS will be highlighted and the most revealing animal models will be discussed, with future perspectives to address immediate questions about the pathogenesis.