Selective neuronal vulnerability during experimental scrapie infection: insights from an ultrastructural investigation

Selective neuronal vulnerability is involved in cerebellar lesions of Guinea pigs infected with bovine spongiform encephalopathy (BSE) prions: Immunohistochemical and electron microscopic investigations

The cerebellar lesions of bovine spongiform encephalopathy (BSE)-infected guinea pigs were characterized as severe atrophy of the cerebellar cortex associated with the loss of granule cells, decrease in the width of the molecular layer, and intense protease-resistant prion protein (PrP^Sc ) accumulations that are similar to cerebellar lesions in kuru and the VV2 type of sporadic Creutzfeldt-Jakob disease. The aim of this study is to assess the relationships between the distribution and localization of PrP^Sc and synapses expressing neurotransmitter transporters in order to reveal the pathogenesis of the disease. We used cell-type-specific immunohistochemical makers recognizing glutamatergic and γ-aminobutylic acid (GABA)ergic terminals to identify terminals impaired with PrP^Sc accumulations. The distribution of PrP^Sc accumulations and immunoreactivity of synaptic vesicles were studied throughout the neuroanatomical pathways in cerebellar lesions. Time course study demonstrated that PrP^Sc accumulation showed a tendency to spread from granular layer to molecular layer. The immunoreactivity of vesicular glutamate transporter 1 (VGluT1) was localized in axon terminals of cerebellar granule cells, and decreased in association with the severity of PrP^Sc accumulations and loss of granule cells. Immunoreactivities of vesicular glutamate transporter 2 (VGluT2) and vesicular GABA transporter (VGAT) that exist in axon terminals of inferior olivary neurons and GABAergic synapses of Purkinje cells, respectively, were preserved well in these lesions. In brainstem, VGluT1 immunoreactivity decreased selectively in pontine nuclei that are a component of the pontocerebellar pathway, although other neurotransmitter immunoreactivities were preserved well. Our findings suggest that the selective loss of VGluT1-immunoreactive synapses subsequent to PrP^Sc accumulations can contribute to the pathogenesis of cerebellar lesions of BSE-infected guinea pigs.

A brain aggregate model gives new insights into the pathobiology and treatment of prion diseases

Brain aggregates (BrnAggs) derived from fetal mouse brains contain mature neurons and glial cells. We determined that BrnAggs are consistently infected with Rocky Mountain Laboratory scrapie strain prions and produce increasing levels of the pathogenic form of the prion protein (PrP). Their abundant dendrites undergo degeneration shortly after prion infection. Treatment of prion-infected BrnAggs with drugs, such as a γ-secretase inhibitors and quinacrine (Qa), which stop PrP formation and dendritic degeneration, mirrors the results from rodent studies. Because PrP is trafficked into lysosomes by endocytosis and autophagosomes by phagocytosis in neurons of prion strain-infected BrnAggs, we studied the effects of drugs that modulate subcellular trafficking. Rapamycin (Rap), which activates autophagy, markedly increased light-chain 3-II (LC3-II)-positive autophagosomes and cathepsin D-positive lysosomes in BrnAggs but could not eliminate the intracellular PrP within them. Adding Qa to Rap markedly reduced the number of LC3-II-positive autolysosomes. Rap + Qa created a competition between Rap increasing and Qa decreasing LC3-II. Rapamycin + Qa decreased total PrP by 56% compared with that of Qa alone, which reduced PrP by 37% relative to Rap alone. We conclude that the decrease was dominated by the ability of Qa to decrease the formation of PrP. Therefore, BrnAggs provide an efficient in vitro tool for screening drug therapies and studying the complex biology of prions.

A Drosophila model of GSS syndrome suggests defects in active zones are responsible for pathogenesis of GSS syndrome

We have established a Drosophila model of Gerstmann-Sträussler-Scheinker (GSS) syndrome by expressing mouse prion protein (PrP) having leucine substitution at residue 101 (MoPrP(P101L)). Flies expressing MoPrP(P101L), but not wild-type MoPrP (MoPrP(3F4)), showed severe defects in climbing ability and early death. Expressed MoPrP(P101L) in Drosophila was differentially glycosylated, localized at the synaptic terminals and mainly present as deposits in adult brains. We found that behavioral defects and early death of MoPrP(P101L) flies were not due to Caspase 3-dependent programmed cell death signaling. In addition, we found that Type 1 glutamatergic synaptic boutons in larval neuromuscular junctions of MoPrP(P101L) flies showed significantly increased numbers of satellite synaptic boutons. Furthermore, the amount of Bruchpilot and Discs large in MoPrP(P101L) flies was significantly reduced. Brains from scrapie-infected mice showed significantly decreased ELKS, an active zone matrix marker compared with those of age-matched control mice. Thus, altered active zone structures at the molecular level may be involved in the pathogenesis of GSS syndrome in Drosophila and scrapie-infected mice.

PrPSc accumulation in neuronal plasma membranes links Notch-1 activation to dendritic degeneration in prion diseases

Prion diseases are disorders of protein conformation in which PrP^C, the normal cellular conformer, is converted to an abnormal, protease-resistant conformer rPrP^Sc. Approximately 80% of rPrP^Sc accumulates in neuronal plasma membranes where it changes their physical properties and profoundly affects membrane functions. In this review we explain how rPrP^Sc is transported along axons to presynaptic boutons and how we envision the conversion of PrP^C to rPrP^Sc in the postsynaptic membrane. This information is a prerequisite to the second half of this review in which we present evidence that rPrP^Sc accumulation in synaptic regions links Notch-1 signaling with the dendritic degeneration. The hypothesis that the Notch-1 intracellular domain, NICD, is involved in prion disease was tested by treating prion-infected mice with the γ-secretase inhibitor (GSI) LY411575, with quinacrine (Qa), and with the combination of GSI + Qa. Surprisingly, treatment with GSI alone markedly decreased NICD but did not prevent dendritic degeneration. Qa alone produced near normal dendritic trees. The combined GSI + Qa treatment resulted in a richer dendritic tree than in controls. We speculate that treatment with GSI alone inhibited both stimulators and inhibitors of dendritic growth. With the combined GSI + Qa treatment, Qa modulated the effect of GSI perhaps by destabilizing membrane rafts. GSI + Qa decreased PrP^Sc in the neocortex and the hippocampus by 95%, but only by 50% in the thalamus where disease was begun by intrathalamic inoculation of prions. The results of this study indicate that GSI + Qa work synergistically to prevent dendrite degeneration and to block formation of PrP^Sc.

A systems approach to prion disease

Prions cause transmissible neurodegenerative diseases and replicate by conformational conversion of normal benign forms of prion protein (PrP^C) to disease-causing PrP^Sc isoforms. A systems approach to disease postulates that disease arises from perturbation of biological networks in the relevant organ. We tracked global gene expression in the brains of eight distinct mouse strain–prion strain combinations throughout the progression of the disease to capture the effects of prion strain, host genetics, and PrP concentration on disease incubation time. Subtractive analyses exploiting various aspects of prion biology and infection identified a core of 333 differentially expressed genes (DEGs) that appeared central to prion disease. DEGs were mapped into functional pathways and networks reflecting defined neuropathological events and PrP^Sc replication and accumulation, enabling the identification of novel modules and modules that may be involved in genetic effects on incubation time and in prion strain specificity. Our systems analysis provides a comprehensive basis for developing models for prion replication and disease, and suggests some possible therapeutic approaches.

Cellular prion protein electron microscopy: attempts/limits and clues to a synaptic trait. Implications in neurodegeneration process

Prion diseases are caused by an infectious agent constituted by a rogue protein called prion (PrP Sc) of neuronal origin (PrP c) and are exemplified by Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cattle. Considerable efforts have been made to understand the cerebral damage caused by these diseases but a clear comprehensive view cannot be achieved without defining the neurophysiological function of PrP c. This lack of information is in part attributable to our ignorance of the precise localization of PrP c in the brain neuronal cell. One relevant option to explore this aspect is to undertake PrP immunohistochemistry at the electron-microscopy level, knowing that this challenge raises major technical constraints. In describing the attempts and restrictions of the various approaches used, we review here the efforts that have been invested in this particular field of prionology. The common result emerging from these contributions is that the synapse could be the site at which PrP c exerts its critical activity. This location suggests, in the perspective of synaptic regulation, that PrP c can be assigned multiple biological functions and supports the novel concept that prion-like changes are involved in long-term memory formation. The synaptic trait of PrP c and PrP Sc suggests that synapse loss is the key event in neuronal death. Interestingly, synaptic alterations are also considered to be predominant in the pathophysiological mechanism in Alzheimer, Parkinson and Huntington diseases. All these brain disorders, characterized by the formation of a specific amyloid protein of synaptic origin, can be classified under the heading of amyloidogenic synaptopathies.

The neurochemical nature of PrPc-containing cells in the rat brain

The cellular prion protein (PrP(C)) is a membrane-bound glycoprotein abundantly expressed in neurons and glial cells within the CNS. The scrapie prion protein (PrP(Sc)) is a conformationally altered isoform of PrP(C) that is responsible for prion diseases, also termed transmissible spongiform encephalopathies (TSE), a group of neurodegenerative diseases that affect a wide variety of mammal species, including humans. The presence of the cellular isoform of PrP is necessary for the establishment and further evolution of prion diseases and the physiological conditions where PrP(C) is present seems to modulate the alterations in TSE. In this work, the presence of PrP(C) in GABAergic, glutamatergic, nitrergic, cholinergic, serotoninergic and orexinergic populations of cells within the rat brain is examined. Our observations show that PrP(C) is widely expressed in a subset of neurons that contain markers of inhibitory populations of cells throughout the rat brain. The presence of PrP(C) in other cells types containing important neurotransmitters for the overall brain function is congruent with the imbalances reported for some of them in TSE. Within the cerebral cortex, PrP(C) is scarcely located in a subset of cells expressing the laminin receptor precursor (LRP) to such a low extent that suggests that other LRP-independent mechanisms actively participate during the pathogenic process. Taken together, our data demonstrate that investigation of the chemical partners of PrP(C) within cells gives a rational basis for the interpretation of the histopathological alterations in TSE and might help analyze some pathogenic mechanisms of PrP(Sc).

Assessment of calcium-binding proteins (Parvalbumin and Calbindin D-28K) and perineuronal nets in normal and scrapie-affected adult sheep brains

Scrapie is a prion disease in small ruminants that manifests itself with neurological clinical signs amongst which are ataxia and tremors. These signs can be explained partially by an imbalance in central inhibitory innervation. The study of the brain's inhibitory neuronal GABAergic populations and of their extracellular matrix has been used to define, in part, the pathogenesis of human prion diseases and scrapie models in rodents. The brain's distribution of neuronal GABAergic subpopulations has been monitored carefully using, as markers, antibodies against the calcium binding proteins parvalbumin and calbindin D-28K. The distribution of this perineuronal net marker was evaluated by means of affinity histochemistry with W. floribunda agglutinin. These techniques were performed on the brains of nine scrapie-positive sheep and on four infection-free sheep. These animals had undergone previously a clinical follow-up as well as a lesion profile and an immunohistochemical profile of the scrapie-associated prion protein deposition in the brain. The study of calcium-binding proteins revealed an alteration of the parvalbumin positive GABAergic neuronal subpopulation. In scrapie-positive cases, a reduction in stained neuronal perykaria was observed, along with a marked reduction of neurite labelling. This finding was noticeable in regions such as the neocortex, particularly the motor frontal cortex, and was concomitant with a moderate PrPsc deposition and a mild degree of lesion. No changes were observed in the extracellular matrix study. The results of the present study provide a partial explanation for the mechanisms of scrapie clinical signs due to a disturbance of the parvalbumin-positive inhibitory neuronal population.

Expression of PrP(C) in the rat brain and characterization of a subset of cortical neurons

The cellular prion protein (PrP(C)) is a membrane-bound glycoprotein mainly present in the CNS. The scrapie prion protein (PrP(Sc)) is an isoform of PrP(C), and it is responsible for transmissible spongiform encephalopathies (TSEs), a group of neurodegenerative diseases affecting both humans and animals. The presence of the cellular form is necessary for the establishment and further evolution of prion diseases. Here, we map the regional distribution of PrP(C) in the rat brain and study the chemical nature of these immunopositive neurons. Our observations are congruent with retrograde transport of prions, as shown by the ubiquitous distribution of PrP(C) throughout the rat brain, but especially in the damaged areas that send projections to primarily affected nuclei in fatal familial insomnia. On the other hand, the presence of the cellular isoform in a subset of GABAergic neurons containing calcium-binding proteins suggests that PrP(C) plays a role in the metabolism of calcium. The lack of immunostaining in neurons ensheathed by perineuronal nets indicates that prions do not directly interact with components of these nets. The destruction of these nets is more likely to be the consequence of a factor needed for prions during the early stages of TSEs. This would cause destruction of these nets and death of the surrounded neurons. Our results support the view that destruction of this extracellular matrix is caused by the pathogenic effect of prions and not a primary event in TSEs.

The neurodegeneration sequence in prion diseases: evidence from functional, morphological and ultrastructural studies of the GABAergic system

Loss of the GABAergic system of neurons has been reported to be the first detectable neuropathological change in prion diseases, which features the accumulation of an aberrant isoform of the prion protein (PrP(Sc)). To determine the timing of GABAergic system dysfunction and degeneration and its relationship to PrP(Sc) accumulation during the course of prion disease in Syrian hamsters, we applied 3 approaches: i) quantifying GABA-immunopositive neurons and their processes by light and electron microscopy to test for selective loss; ii) measuring evoked [3H]-GABA release from synaptosomes to test for functional abnormalities; and iii) determining the kinetics of PrP(Sc) accumulation in subcellular fractions to correlate it with GABAergic dysfunction. At the terminal stages of disease, we found a significant increase in the number of GABA-positive and -negative presynaptic boutons with abnormally aggregated synaptic vesicles. At the same stage, we also found an equal degree of GABA-immunopositive and -immunonegative presynaptic bouton loss. In contrast, GABA-positive neocortical cell bodies increased, based on stereologic estimates in the terminal stage of scrapie. In the context of these abnormalities, evoked release of [3H]-GABA from cortical and thalamic synaptosomes was significantly decreased, which correlated well with the accumulation of PrP(Sc) in synaptosomes and cell membrane fractions.

Prion protein (PrPc) immunocytochemistry and expression of the green fluorescent protein reporter gene under control of the bovine PrP gene promoter in the mouse brain

Expression of the cellular prion protein (PrP(c)) by host cells is required for prion replication and neuroinvasion in transmissible spongiform encephalopathies. As a consequence, identification of the cell types expressing PrP(c) is necessary to determine the target cells involved in the cerebral propagation of prion diseases. To identify the cells expressing PrP(c) in the mouse brain, the immunocytochemical localization of PrP(c) was investigated at the cellular and ultrastructural levels in several brain regions. In addition, we analyzed the expression pattern of a green fluorescent protein reporter gene under the control of regulatory sequences of the bovine prion protein gene in the brain of transgenic mice. By using a preembedding immunogold technique, neuronal PrP(c) was observed mainly bound to the cell surface and presynaptic sites. Dictyosomes and recycling organelles in most of the major neuron types also exhibited PrP(c) antigen. In the olfactory bulb, neocortex, putamen, hippocampus, thalamus, and cerebellum, the distribution pattern of both green fluorescent protein and PrP(c) immunoreactivity suggested that the transgenic regulatory sequences of the bovine PrP gene were sufficient to promote expression of the reporter gene in neurons that express immunodetectable endogenous PrP(c). Transgenic mice expressing PrP-GFP may thus provide attractive murine models for analyzing the transcriptional activity of the Prnp gene during prion infections as well as the anatomopathological kinetics of prion diseases.

Cytosolic prion protein in neurons

Localizing the cellular prion protein (PrPC) in the brain is necessary for understanding the pathogenesis of prion diseases. However, the precise ultrastructural localization of PrPC still remains enigmatic. We performed the first quantitative study of the ultrastructural localization of PrPC in the mouse hippocampus using high-resolution cryoimmunogold electron microscopy. PrPC follows the standard biosynthetic trafficking pathway with a preferential localization in late endosomal compartments and on the plasma membrane of neurons and neuronal processes. PrPC is found with the same frequency within the synaptic specialization and perisynaptically, but is almost completely excluded from synaptic vesicles. Unexpectedly, PrP is also found in the cytosol in subpopulations of neurons in the hippocampus, neocortex, and thalamus but not the cerebellum. Cytosolic PrP may have altered susceptibility to aggregation, suggesting that these neurons might play a significant role in the pathogenesis of prion diseases, in particular those mammals harboring mutant PrP genes.

Changes in calcium currents and GABAergic spontaneous activity in cultured rat hippocampal neurons after a neurotropic influenza A virus infection

In order to study mechanisms by which a neurotropic strain of influenza A virus (A/WSN/33) may affect neuronal function or cause nerve cell death, hippocampal cultures from embryonic rats were infected with this virus. Approximately 70% of the neurons in the infected cultures became immunopositive for viral antigens and showed reduced voltage-dependent Ca(2+) currents in whole-cell patch clamp recordings, but no changes in other membrane properties or in cytosolic Ca(2+) concentration were seen. These immunopositive neurons underwent apoptosis 3-4 days after infection. Ca(2+) channel inhibitors had no significant effect on neuronal survival. The immunonegative population of neurons survived, but displayed increased frequency of miniature inhibitory postsynaptic currents of gamma-amino-butyric acid origin compared with controls. The frequency of alpha-amino-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) receptor-mediated miniature excitatory postsynaptic currents was not altered. Viral nucleoproteins, overexpressed using the Semliki Forest virus system, were localized to the dendritic spines as shown by double immunolabeling with actinin, but did not by themselves cause neuronal death or changes in synaptic transmission as measured by AMPA-mediated excitatory postsynaptic currents. Our results show that an influenza A virus infection can cause selective neurophysiological changes in hippocampal neurons and that these can persist even after the viral antigens have been cleared.