Inflammatory response and retinal ganglion cell degeneration following intraocular injection of ME7

Age and Environment Influences on Mouse Prion Disease Progression: Behavioral Changes and Morphometry and Stereology of Hippocampal Astrocytes

Because enriched environment (EE) and exercise increase and aging decreases immune response, we hypothesized that environmental enrichment and aging will, respectively, delay and increase prion disease progression. Mice dorsal striatum received bilateral stereotaxic intracerebral injections of normal or ME7 prion infected mouse brain homogenates. After behavior analysis, animals were euthanized and their brains processed for astrocyte GFAP immunolabeling. Our analysis related to the environmental influence are limited to young adult mice, whereas age influence refers to aged mice raised on standard cages. Burrowing activity began to reduce in ME7-SE two weeks before ME7-EE, while no changes were apparent in ME7 aged mice (ME7-A). Object placement recognition was impaired in ME7-SE, NBH-A, and ME7-A but normal in all other groups. Object identity recognition was impaired in ME7-A. Cluster analysis revealed two morphological families of astrocytes in NBH-SE animals, three in NBH-A and ME7-A, and four in NBH-EE, ME7-SE, and ME7-EE. As compared with control groups, astrocytes from DG and CA3 prion-diseased animals show significant numerical and morphological differences and environmental enrichment did not reverse these changes but induced different morphological changes in GFAP+ hippocampal astroglia. We suggest that environmental enrichment and aging delayed hippocampal-dependent behavioral and neuropathological signs of disease progression.

Abnormal Prion Accumulation Associated with Retinal Pathology in Experimentally Inoculated Scrapie-Affected Sheep

The purpose of this study was to characterize the patterns of PrPSc immunoreactivity in the retinae of scrapie-affected sheep and to determine the extent of retinal pathology as indicated by glial fibrillary acidic protein immunoreactivity (GFAP-IR) of Müller glia. Sections from the retina of 13 experimentally inoculated scrapie-affected and 2 negative control sheep were examined with immunohistochemical staining for PrPSc, GFAP, and PrPSc/GFAP double staining. GFAP-IR of Müller glia is suggestive of retinal pathology in the absence of morphologic abnormality detected by light microscopy. Sheep with the least amount of PrPSc in the retina have multifocal punctate aggregates of prion staining in the outer half of the inner plexiform layer and rarely in the outer plexiform layer. In these retinae, GFAP-IR is not localized with prion accumulation, but rather is present in moderate numbers of Müller glia throughout the sections of retina examined. The majority of sheep with retinal accumulation of PrPSc have intense, diffuse PrPSc staining in both plexiform layers, with immunoreactivity in the cytoplasm of multiple ganglion cells and lesser amounts in the optic fiber layer and between nuclei in nuclear layers. This intense PrPSc immunoreactivity is associated with diffuse, intense GFAP-IR that extends from the inner limiting membrane to the outer limiting membrane. This is the first report of a prion disease in a natural host that describes the accumulation of PrPSc in retina associated with retinal pathology in the absence of overt morphologic changes indicative of retinal degeneration.

Neuroglobin and prion cellular localization: investigation of a potential interaction

Neuroglobin (Ngb) and the cellular prion protein (PrP(c)), proteins of unknown function in the nervous system, are known to be expressed in the retina and have been observed in different rat retinal cells. The retina is the site of the highest concentration for Ngb, a heme protein of similar size and conformation to myoglobin. In this study, we demonstrated by immunohistochemical analysis of retinal colocalization of Ngb and PrP(c) in the ganglion cell layer. Considering for these two a common protective role in relation to oxidative stress and a possible transient contact during migration of PrP(c) through the eye or upon neuronal degradation, we undertook in vitro studies of the interaction of the purified proteins. Mixing these two proteins leads to rapid aggregation, even at submicromolar concentrations. As observed with the use of dynamic light scattering, particles comprising both proteins evolve to hundreds of nanometers within several seconds, a first report showing that PrP(c) is able to form aggregates without major structural changes. The main effect would then appear to be a protein-protein interaction specific to the surface charge of the Ngb protein with PrP(c) N-terminal sequence. A dominant parameter is the solvent ionic force, which can significantly modify the final state of aggregation. PrP(c), normally anchored to the cell membrane, is toxic in the cytoplasm, where Ngb is present; this could suggest an Ngb function of scavenging proteins capable of forming deleterious aggregates considering a charge complementarity in the complex.

Retinal cell types are differentially affected in sheep with scrapie

Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases characterized microscopically by spongiform lesions (vacuolation) in the neuropil, neuronal loss, and gliosis. Accumulation of the abnormal form of the prion protein (PrP(Sc)) has been demonstrated in the retina of natural and non-natural TSE-affected hosts, with or without evidence of microscopically detectable retinal pathology. This study was conducted to investigate the effect of PrP(Sc) accumulation on retinal neurons in a natural host lacking overt microscopical evidence of retinal degeneration by comparing the distribution of retinal cell type-specific markers in control and scrapie-affected sheep. In retinas with PrP(Sc)-immunoreactivity, there was disruption of the normal immunoreactivity patterns of the alpha isoform of protein kinase C (PKCalpha) and vesicular glutamate transporter 1 (VGLUT1), markers of retinal bipolar cells. Altered immunoreactivity was also observed for microtubule-associated protein 2 (MAP2), a marker of a subset of retinal ganglion cells, and glutamine synthetase (GS), a marker of Müller glia. These results demonstrate alterations of immunoreactivity patterns for proteins associated with specific cell types in retinas with PrP(Sc) accumulation, despite an absence of microscopical evidence of retinal degeneration.

Prion protein expression differences in microglia and astroglia influence scrapie-induced neurodegeneration in the retina and brain of transgenic mice

Activated microglia and astroglia are known to be involved in a variety of neurodegenerative diseases, including prion diseases. In the present experiments, we studied activation of astroglia and microglia after intraocular scrapie infection in transgenic mice expressing prion protein (PrP) in multiple cell types (tg7 mice) or in neurons only (tgNSE mice). In this model, scrapie infection and protease-resistant PrP deposition occurs in the retinas of both strains of mice, but retinal degeneration is observed only in tg7 mice. Our results showed that the retinas of tg7 and tgNSE mice both had astroglial activation with increased chemokine expression during the course of infection. However, only tg7 retinas exhibited strong microglial activation compared to tgNSE retinas, which showed little microglial activation by biochemical or morphological criteria. Therefore, microglial PrP expression might be required for scrapie-induced retinal microglial activation and damage. Furthermore, microglial activation preceded retinal neurodegeneration in tg7 mice, suggesting that activated microglia might contribute to the degenerative process, rather than being a response to the damage. Surprisingly, brain differed from retina in that an altered profile of microglial activation markers was upregulated, and the profiles in the two mouse strains were indistinguishable. Microglial activation in the brain was associated with severe brain vacuolation and neurodegeneration, leading to death. Thus, retinal and brain microglia appeared to differ in their requirements for activation, suggesting that different activation pathways occur in the two tissues.

The toxicity of the PrP106-126 prion peptide on cultured photoreceptors correlates with the prion protein distribution in the mammalian and human retina

In patients affected by Creutzfeldt-Jakob disease and in animals affected by transmissible spongiform encephalopathies, retinal functions are altered, and major spongiform changes are observed in the outer plexiform layer where photoreceptors have their synaptic terminals. In the present study, the prion protein PrP(c) was found to form aggregates in rod photoreceptor terminals from both rat and human retina, whereas no labeling was observed in cone photoreceptors. Discrete staining was also detected in the inner plexiform layer where the prion protein was located at human amacrine cell synapses. In mixed porcine retinal cell cultures, the PrP106-126 prion peptide triggered a 61% rod photoreceptor cell loss by apoptosis as indicated by terminal deoxynucleotidyl transferase dUTP nick-end labeling, whereas cone photoreceptors were not affected. Amacrine cells were also reduced by 47% in contrast to ganglion cells. Although this cell loss was associated with a 5.5-fold increase in microglial cells, the strict correlation between the PrP(c) prion protein expression and the peptide toxicity suggested that this toxicity did not rely on the release of a toxic compound by glial cells. These results provide new insights into the retinal pathophysiology of prion diseases and illustrate advantages of adult retinal cell cultures to investigate prion pathogenic mechanisms.

Neurons and astrocytes respond to prion infection by inducing microglia recruitment

The accumulation and activation of microglial cells at sites of amyloid prion deposits or plaques have been documented extensively. Here, we investigate the in vivo recruitment of microglial cells soon after intraocular injection of scrapie-infected cell homogenate (hgtsc+) using immunohistochemistry on retinal sections. A population of CD11b/CD45-positive microglia was specifically detected within the ganglion and internal plexiform retinal cell layers by 2 d after intravitreal injection of hgtsc+. Whereas no chemotactism properties were ascribed to hgtsc+ alone, a massive migration of microglial cells was observed by incubating primary cultured neurons and astrocytes with hgtsc+ in a time- and concentration-dependent manner. hgtsc+ triggered the recruitment of microglial cells by interacting with both neurons and astrocytes by upregulation of the expression levels of a broad spectrum of neuronal and glial chemokines. We show that, in vitro and in vivo, the microglia migration is at least partly under the control of chemokine receptor-5 (CCR-5) activation, because highly specific CCR-5 antagonist TAK-779 significantly reduced the migration rate of microglia. Activated microglia recruited in the vicinity of prion may, in turn, cause neuronal cell damage by inducing apoptosis. These findings provide insight into the understanding of the cell-cell communication that takes place during the development of prion diseases.

Measuring the degree of spatial correlation between histological features in thin sections of brain tissue

Histological features visible in thin sections of brain tissue, such as neuronal perikarya, blood vessels, or pathological lesions may exhibit a degree of spatial association or correlation. In neurodegenerative disorders such as AD, Pick's disease, and CJD, information on whether different types of pathological lesion are spatially correlated may be useful in elucidating disease pathogenesis. In the present article the statistical methods available for studying spatial association in histological sections are reviewed. These include tests of interspecific association between two or more histological features using chi2 contingency tables, measurement of 'complete' and 'absolute' association, and more complex methods that use grids of contiguous samples. In addition, the use of correlation matrices and stepwise multiple regression methods are described. The advantages and limitations of each method are reviewed and possible future developments discussed.

Lymphocyte infiltration in the injured brain: role of proinflammatory cytokines

Studies using mouse axotomised facial motoneuron model show a strong and highly selective entry of CD3+ lymphocytes into the affected nucleus, with a maximum at Day 14, which coincides with the peak of neuronal cell death, microglial phagocytosis, and increased synthesis of interleukin-1 beta (IL1beta), tumour necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma). We explored the possible involvement of these cytokines during the main phase of lymphocyte recruitment into the axotomised facial motor nucleus 7-21 days after nerve cut using mice homozygously deficient for IL1 receptor type 1 (IL1R1-/-), TNF receptor type 1 (TNFR1-/-), type 2 (TNFR2-/-) and type 1 and 2 (TNFR1&2-/-), IFNgamma receptor type 1 (IFNgammaR1-/-), and the appropriate controls for the genetic background. Transgenic deletion of IL1R1 led to a 54% decrease and that of TNFR2 to a 44% reduction in the number of CD3+ T-cells in the axotomised facial motor nucleus, with a similar relative decrease at Day 7, 14, and 21. Deletion of TNFR1 or IFNgammaR1 had no significant effect. Deletion of both TNFR1 and 2 (TNFR1&2-/-) caused a somewhat stronger, 63% decrease than did TNFR2 deletion alone, but this could be due to an almost complete inhibition of neuronal cell death. No mutations seemed to inhibit aggregation of CD3+ T-cells around glial nodules consisting of Ca-ion binding adaptor protein-1 (IBA1)+ phagocytotic microglia and neuronal debris. Altogether, the current data show the importance of IL1R1 and TNFR2 as the key players during the main phase of lymphocyte recruitment to the damaged part of the central nervous system.

Quantitative variations in the pathology of 11 cases of variant Creutzfeldt-Jakob disease (vCJD)

Quantitative variations in the density and distribution of the vacuolation ('spongiform change'), surviving neurons, and prion protein (PrP) deposits were studied in eight brain regions from 11 cases of variant Creutzfeldt-Jakob disease (vCJD). Principal components analysis (PCA) was used to study the similarities and differences between cases and to identify the neuropathological variables which could best account for these variations. Two principal components (PC) were extracted from the data accounting in total for 93.4% of the variance; the majority of the variance (90%) being associated with PC1. Some clustering of the 11 cases in relation to PC1 and PC2 was evident. The densities of the vacuolation in the occipital cortex and the molecular layer of the cerebellum were positively and negatively correlated, respectively, with PC1. No significant variation between cases was associated with PrP deposition. These data suggest that vCJD cases have a consistent neuropathological profile characterised by the presence of vacuolation, neuronal loss and PrP deposition in the form of florid and non-florid deposits. However, there are quantitative variations between cases in the development of the vacuolation especially affecting the occipital cortex and cerebellum.

Tissue plasminogen activator mediates microglial activation via its finger domain through annexin II

Microglia are the immunocompetent cells of the CNS, and their activation is thought to play an important neurotoxic role in many diseases modeled by glutamate-induced excitotoxicity. One molecule whose expression is upregulated after excitotoxic injury is tissue plasminogen activator (tPA), a serine protease with dual roles in the CNS. The catalytic activity of tPA, which converts plasminogen into plasmin, leads to neuronal death during excitotoxicity. Via a nonproteolytic mechanism, tPA also mediates microglial activation. We show here in culture studies that stimulated wild-type neurons and microglia can release the tPA that elicits the activation, and that tPA acts in combination with other factors. We also show that the finger domain of tPA is necessary to trigger the activation and identify annexin II as its probable binding partner-receptor. Together, these findings suggest that tPA released by either neurons or microglia can act as a neural cytokine, signaling through annexin II to activate microglia in settings of disease and injury. Developing methods to inhibit the interaction of tPA with annexin II would offer a new and selective approach to interfere with microglial activation for therapeutic purposes.

Spatial correlations between the vacuolation, prion protein deposits, and surviving neurons in the cerebral cortex in sporadic Creutzfeldt-Jakob disease

In the cerebral cortex of cases of sporadic Creutzfeldt-Jakob disease (sCJD), the vacuolation (spongiform change) and PrP deposits are aggregated into clusters which are regularly distributed parallel to the pia mater. The objective of the present study was to determine the spatial relationships between the clusters of the vacuoles and PrP deposits and between the pathological changes and variations in the density of surviving neurons. In areas with low densities of pathological change, clusters of vacuoles were spatially correlated with the surviving neurons and not with the PrP deposits. By contrast, in more significantly affected areas, clusters of vacuoles were spatially correlated with those of the PrP deposits and not with the surviving neurons. In addition, areas with a high density of vacuoles and a low density of PrP deposits exhibited no spatial correlations between the variables. These data suggest that the spatial relationships between the vacuolation, PrP deposits and surviving neurons in sCJD depend on the density of lesions present. Differences in the pattern of correlation may reflect the developmental stage of the pathology in particular cortical areas.

Spatial pattern of prion protein deposits in patients with sporadic Creutzfeldt-Jakob disease

The spatial pattern of the prion protein (PrP) deposits was studied in the cerebral cortex and cerebellum in 10 patients with sporadic Creutzfeldt-Jakob disease (CJD). In all patients the PrP deposits were aggregated into clusters and, in 90% of cortical areas and in 50% of cerebellar sections, the clusters exhibited a regular periodicity parallel to the tissue boundary; a spatial pattern also exhibited by beta-amyloid (Abeta) deposits in Alzheimer's disease (AD). In the cerebral cortex, the incidence of regular clustering of the PrP deposits was similar in the upper and lower cortical laminae. The sizes of the PrP clusters in the upper and lower cortex were uncorrelated. No significant differences in mean cluster size of the PrP deposits were observed between brain regions. The size, location and distribution of the PrP deposit clusters suggest that PrP deposition occurs in relation to specific anatomical pathways and supports the hypothesis that prion pathology spreads through the brain via such pathways. In addition, the data suggest that there are similarities in the pathogenesis of extracellular protein deposits in prion disease and in AD.

The spatial patterns of prion protein deposits in Creutzfeldt-Jakob disease: comparison with beta-amyloid deposits in Alzheimer's disease

Similar pathological processes may be involved in the deposition of extracellular proteins in the brains of patients with Creutzfeldt-Jakob disease (CJD) and Alzheimer's disease (AD). Hence, this study compared the spatial patterns of prion protein (PrP) deposits in the cerebral cortex and hippocampus in cases of sporadic CJD with those of beta-amyloid (Abeta) deposits in sporadic AD. PrP and Abeta deposits were aggregated into clusters and, in 90% of brain areas in CJD and 57% in AD, the clusters were regularly distributed parallel to the tissue boundary. In a significant proportion of cortical analyses, the mean diameter of the clusters of PrP and Abeta deposits were similar to those of the cells of origin of the cortico-cortical pathways. Abeta deposits in AD were distributed more frequently in larger-sized clusters than PrP deposits in CJD. In addition, in the hippocampus and dentate gyrus, clustering of Abeta deposits was observed in AD but PrP deposits were rare in these regions in CJD. The size, location and distribution of the extracellular protein deposits within the cortex of both disorders was consistent with the degeneration of the cortico-cortical pathways. Furthermore, spread of the pathology along these pathways may be a pathogenic feature common to CJD and AD.

The spatial pattern of the vacuolation in patients with sporadic Creutzfeldt-Jakob disease

1600 microm in diameter and, in the majority of tissue sections, the vacuole clusters were distributed with regular periodicity parallel to the tissue boundary. The size of the vacuole clusters was positively correlated with patient age in the lower laminae of the occipital cortex and the inferior temporal gyrus (ITG) and negatively correlated with age in the hippocampus. In addition, the size of the vacuole clusters was positively correlated with disease duration in the upper laminae of the ITG. The size and distribution of the vacuole clusters suggests that the vacuolation in CJD reflects the degeneration of specific brain pathways and supports the hypothesis that prion pathology may spread through the brain along well defined anatomical pathways.