Syrian hamster prion protein (PrP(C)) is expressed in photoreceptor cells of the adult retina

Prion-induced photoreceptor degeneration begins with misfolded prion protein accumulation in cones at two distinct sites: cilia and ribbon synapses

Accumulation of misfolded host proteins is central to neuropathogenesis of numerous human brain diseases including prion and prion-like diseases. Neurons of retina are also affected by these diseases. Previously, our group and others found that prion-induced retinal damage to photoreceptor cells in mice and humans resembled pathology of human retinitis pigmentosa caused by mutations in retinal proteins. Here, using confocal, epifluorescent and electron microscopy we followed deposition of disease-associated prion protein (PrPSc) and its association with damage to critical retinal structures following intracerebral prion inoculation. The earliest time and place of retinal PrPSc deposition was 67 days post-inoculation (dpi) on the inner segment (IS) of cone photoreceptors. At 104 and 118 dpi, PrPSc was associated with the base of cilia and swollen cone inner segments, suggesting ciliopathy as a pathogenic mechanism. By 118 dpi, PrPSc was deposited in both rods and cones which showed rootlet damage in the IS, and photoreceptor cell death was indicated by thinning of the outer nuclear layer. In the outer plexiform layer (OPL) in uninfected mice, normal host PrP (PrPC) was mainly associated with cone bipolar cell processes, but in infected mice, at 118 dpi, PrPSc was detected on cone and rod bipolar cell dendrites extending into ribbon synapses. Loss of ribbon synapses in cone pedicles and rod spherules in the OPL was observed to precede destruction of most rods and cones over the next 2–3 weeks. However, bipolar cells and horizontal cells were less damaged, indicating high selectivity among neurons for injury by prions. PrPSc deposition in cone and rod inner segments and on the bipolar cell processes participating in ribbon synapses appear to be critical early events leading to damage and death of photoreceptors after prion infection. These mechanisms may also occur in human retinitis pigmentosa and prion-like diseases, such as AD.

Prion protein facilitates retinal iron uptake and is cleaved at the β-site: Implications for retinal iron homeostasis in prion disorders

Prion disease-associated retinal degeneration is attributed to PrP-scrapie (PrP^Sc), a misfolded isoform of prion protein (PrP^C) that accumulates in the neuroretina. However, a lack of temporal and spatial correlation between PrP^Sc and cytotoxicity suggests the contribution of host factors. We report retinal iron dyshomeostasis as one such factor. PrP^C is expressed on the basolateral membrane of retinal-pigment-epithelial (RPE) cells, where it mediates uptake of iron by the neuroretina. Accordingly, the neuroretina of PrP-knock-out mice is iron-deficient. In RPE19 cells, silencing of PrP^C decreases ferritin while over-expression upregulates ferritin and divalent-metal-transporter-1 (DMT-1), indicating PrP^C-mediated iron uptake through DMT-1. Polarization of RPE19 cells results in upregulation of ferritin by ~10-fold and β-cleavage of PrP^C, the latter likely to block further uptake of iron due to cleavage of the ferrireductase domain. A similar β-cleavage of PrP^C is observed in mouse retinal lysates. Scrapie infection causes PrP^Sc accumulation and microglial activation, and surprisingly, upregulation of transferrin despite increased levels of ferritin. Notably, detergent-insoluble ferritin accumulates in RPE cells and correlates temporally with microglial activation, not PrP^Sc accumulation, suggesting that impaired uptake of iron by PrP^Sc combined with inflammation results in retinal iron-dyshomeostasis, a potentially toxic host response contributing to prion disease-associated pathology.

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.

The Cellular Prion Protein Prevents Copper-Induced Inhibition of P2X4Receptors

Although the physiological function of the cellular prion protein (PrP^C) remains unknown, several evidences support the notion of its role in copper homeostasis. PrP^C binds Cu²⁺ through a domain composed by four to five repeats of eight amino acids. Previously, we have shown that the perfusion of this domain prevents and reverses the inhibition by Cu²⁺ of the adenosine triphosphate (ATP)-evoked currents in the P2X₄ receptor subtype, highlighting a modulatory role for PrP^C in synaptic transmission through regulation of Cu²⁺ levels. Here, we study the effect of full-length PrP^C in Cu²⁺ inhibition of P2X₄ receptor when both are coexpressed. PrP^C expression does not significantly change the ATP concentration-response curve in oocytes expressing P2X₄ receptors. However, the presence of PrP^C reduces the inhibition by Cu²⁺ of the ATP-elicited currents in these oocytes, confirming our previous observations with the Cu²⁺ binding domain. Thus, our observations suggest a role for PrP^C in modulating synaptic activity through binding of extracellular Cu²⁺.

Microcebus murinus retina: a new model to assess prion-related neurotoxicity in primates

No effective treatment currently exists for prion diseases and therefore the development of experimental non-human primate models of prion neurotoxicity, to better understand the underlying mechanism and to test new treatments relevant to humans, represents an urgent medical need. However, the establishment of such models is challenging due to animal welfare and cost considerations. We describe here the use of Microcebus murinus retina, in primary cultures and in vivo, as a new experimental primate model to rapidly examine the effects in the central nervous system of PrP(106-126), a neurotoxic fragment of the human prion protein. We demonstrate that PrP(106-126) triggered rod photoreceptor cell loss by apoptosis and a change in morphology of microglial cells in mixed neuronal-glial cultures of retinal cells. In addition, 2days after intravitreal injection of PrP(106-126), retinas showed a significant increase in the number of apoptotic nuclei, mainly in the ganglion cell layer.

Amyloid-beta deposits lead to retinal degeneration in a mouse model of Alzheimer disease

PURPOSE

To compare the temporal and spatial expression patterns of amyloid precursor protein (APP), amyloid-beta deposits, inflammatory chemokines, and apoptosis in the retina of a mouse model of Alzheimer disease (AD).

METHODS

Retinas of transgenic mice harboring a mutant presenilin (PS1) and a mutant APP gene were processed for TUNEL and immunohistochemistry with antibodies against APP, amyloid-beta, monocyte chemotactic protein (MCP)-1, and F4/80. Comparisons were made between age groups and between transgenic and wild-type congeners.

RESULTS

The neuroretina demonstrated age-dependent increases in APP in the ganglion cells (RGCs) and in neurons of the inner nuclear layer (INL). Amyloid-beta demonstrated significant age-dependent deposition in the nerve fiber layer (NFL). TUNEL-positive RGC increased in an age-dependent fashion and in transgenic compared with wild-type congeners. Concomitant overexpression of MCP-1 and intense immunoreactivity for F4/80 suggested that RGCs upregulate MCP-1 in response to amyloid-beta. Activated microglia proliferated in response to MCP-1. In the outer retina, retinal pigment epithelium (RPE) demonstrated moderate age-dependent APP immunoreactivity, but nearby drusenlike deposits were not present. Amyloid-beta was observed in the choriocapillaris of the older animals.

CONCLUSIONS

Amyloid-beta deposits accumulate with age in the retina of a transgenic mouse model of AD. The amyloid-beta loads are accompanied by increased immunoreactivity for MCP-1, F4/80, and TUNEL-positive profiles in the RGC layer. The results suggest that amyloid-beta causes neurodegeneration in the retina of the doubly mutant transgenic mouse model of AD.

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.

Anterograde axonal transport of the exogenous cellular isoform of prion protein in the chick visual system

The cellular isoform of endogenous, newly synthesized prion protein (PrPc) can be transported by axons in the anterograde direction. To determine whether a mechanism exists for secreted PrPc to be internalized and then axonally transported, we analyzed internalization and anterograde axonal transport of radiolabeled recombinant PrPc after its intraocular injection in chick embryos. Internalization and axonal transport of exogenous PrPc to the midbrain by retinal ganglion cells (RGCs) is efficient, saturable and likely receptor-mediated. Ultrastructural quantitative localization of radiolabeled PrPc within RGC soma showed significant labeling of vesicular/endosomal compartments and much less labeling present over the Golgi apparatus and lysosomes, which indicates slow degradation of exogenous PrPc in this system. These data show that a mechanism exists to internalize a secreted form of PrPc and then to axonally transport such PrPc in an anterograde direction. This may provide an additional, novel mechanism for prion protein to spread among neurons.

Amidation and structure relaxation abolish the neurotoxicity of the prion peptide PrP106-126 in vivo and in vitro

One of the major pathological hallmarks of transmissible spongiform encephalopathies (TSEs) is the accumulation of a pathogenic (scrapie) isoform (PrP(Sc)) of the cellular prion protein (PrP(C)) primarily in the central nervous system. The synthetic prion peptide PrP106-126 shares many characteristics with PrP(Sc) in that it shows PrP(C)-dependent neurotoxicity both in vivo and in vitro. Moreover, PrP106-126 in vitro neurotoxicity has been closely associated with the ability to form fibrils. Here, we studied the in vivo neurotoxicity of molecular variants of PrP106-126 toward retinal neurons using electroretinographic recordings in mice after intraocular injections of the peptides. We found that amidation and structure relaxation of PrP106-126 significantly reduced the neurotoxicity in vivo. This was also found in vitro in primary neuronal cultures from mouse and rat brain. Thioflavin T binding studies showed that amidation and structure relaxation significantly reduced the ability of PrP106-126 to attain fibrillar structures in physiological salt solutions. This study hence supports the assumption that the neurotoxic potential of PrP106-126 is closely related to its ability to attain secondary structure.

The Octapeptide Repeats in Mammalian Prion Protein Constitute a pH-dependent Folding and Aggregation Site

Structural studies of mammalian prion protein at pH values between 4.5 and 5.5 established that the N-terminal 100 residue domain is flexibly disordered. Here, we show that at pH values between 6.5 and 7.8, i.e. the pH at the cell membrane, the octapeptide repeats in recombinant human prion protein hPrP(23-230) encompassing the highly conserved amino acid sequence PHGGGWGQ are structured. The nuclear magnetic resonance solution structure of the octapeptide repeats at pH 6.2 reveals a new structural motif that causes a reversible pH-dependent PrP oligomerization. Within the aggregation motif the segments HGGGW and GWGQ adopt a loop conformation and a beta-turn-like structure, respectively. Comparison with the crystal structure of HGGGW-Cu(2+) indicates that the binding of copper ions induces a conformational transition that presumably modulates PrP aggregation. The knowledge that the cellular prion protein is immobilized on the cell surface along with our results suggests a functional role of aggregation in endocytosis or homophilic cell adhesion.

Influence of pH on NMR structure and stability of the human prion protein globular domain

The NMR structure of the globular domain of the human prion protein (hPrP) with residues 121-230 at pH 7.0 shows the same global fold as the previously published structure determined at pH 4.5. It contains three alpha-helices, comprising residues 144-156, 174-194, and 200-228, and a short anti-parallel beta-sheet, comprising residues 128-131 and 161-164. There are slight, strictly localized, conformational changes at neutral pH when compared with acidic solution conditions: helix alpha1 is elongated at the C-terminal end with residues 153-156 forming a 310-helix, and the population of helical structure in the C-terminal two turns of helix alpha 2 is increased. The protonation of His155 and His187 presumably contributes to these structural changes. Thermal unfolding monitored by far UV CD indicates that hPrP-(121-230) is significantly more stable at neutral pH. Measurements of amide proton protection factors map local differences in protein stability within residues 154-157 at the C-terminal end of helix alpha 1 and residues 161-164 of beta-strand 2. These two segments appear to form a separate domain that at acidic pH has a larger tendency to unfold than the overall protein structure. This domain could provide a "starting point" for pH-induced unfolding and thus may be implicated in endosomic PrPC to PrPSc conformational transition resulting in transmissible spongiform encephalopathies.

Ultrastructural changes in the optic nerves of rodents with experimental Creutzfeldt-Jakob Disease (CJD), Gerstmann-Sträussler-Scheinker disease (GSS) or scrapie

This report describes the ultrastructural changes in the optic nerves of (1) hamsters infected with the Echigo-1 strain of Creutzfeldt-Jakob disease (CJD), (2) hamsters infected with the 263K or 22C-H strain of scrapie, and (3) mice infected with the Fujisaki strain of Gerstmann-Sträussler-Scheinker disease (GSS). Vacuolation of myelinated fibres was present in the myelin sheaths, with splitting of myelin lamellae. These vacuoles contained typical secondary vacuoles and curled membrane fragments. Myelinated fibre vacuolation was also accompanied by an exuberant cellular reaction consisting of macrophages containing numerous mitochondria, abundant rough endoplasmic reticulum, and secondary lysosomes filled with digested myelin debris and other electron-dense material. Within macrophages, myelin fragments undergoing active digestion, lyre-like bodies and paracrystalline inclusions were frequently noted. Astrocytes and their processes were prominent; glial filaments and many mitochondria were readily detected. Proliferation of inner mesaxons was observed. Cross-sectional profiles of innumerable myelinated fibres contained membranous organelles continuous with the inner lamellae of the oligodendroglial cells. The proliferations of inner mesaxons formed whorls and loops, and intrusion of the membranous tongue of the inner mesaxon into the axoplasm was occasionally observed; dystrophic neurites were relatively numerous. In mice infected with the Fujisaki strain of GSS, fibres had undergone demyelination with stripping of the myelin lamellae, while others showed vesicular myelin degeneration.

Temporally controlled site-specific mutagenesis in the germ cell lineage of the mouse testis

We have obtained a PrP-Cre-ER(T) transgenic mouse line (28.8) that selectively expresses in testis the tamoxifen-inducible Cre-ER(T) recombinase under the control of a mouse Prion protein (PrP) promoter-containing genomic fragment. Cre-ER(T) is expressed in spermatogonia and spermatocytes, but not in Sertoli and Leydig cells. We also established reporter PrP-L-EGFP-L transgenic mice harboring a LoxP-flanked enhanced green fluorescent protein (EGFP) Cre reporter cassette under the control of the same PrP promoter-containing genomic fragment that exhibits prominent EGFP expression in brain and testis. Using the PrP-L-EGFP-L as well as other Cre-reporter mice, we demonstrate that tamoxifen administration efficiently and selectively induces Cre-mediated recombination in the germ cell lineage. The established PrP-Cre-ER(T) line should provide a valuable tool for studying functions of germ cell-expressed genes involved in spermatogenesis.

Temporally controlled targeted somatic mutagenesis in the mouse brain

To develop spatio-temporally controlled somatic mutagenesis in the adult mouse nervous system, we established transgenic mice expressing the tamoxifen-inducible Cre-ERT recombinase under the control of the mouse prion protein (PrP) promoter. Cre-ERT was expressed in most regions of the brain and in the retina of one transgenic line, whereas its expression was mostly restricted to the hippocampus and the cerebellum in another line. As tamoxifen efficiently induced Cre-mediated recombination in the various neuronal cell types expressing Cre-ERT in the brain of adult mice, the PrP-Cre-ERT lines should be valuable tools to study the functions of genes involved in neurodegenerative diseases or regeneration, and in complex processes such as behaviour, learning and memory. Some limitations of presently available reporter lines for Cre-mediated recombination in adult mouse CNS are discussed.

Function of PrP(C) as a copper-binding protein at the synapse

The prion protein (PrP(C)) shows cooperative copper binding of the N-terminal octarepeat (PHGGGWGO) x4. In brain homogenates, PrP(C) is found in highest concentration in synaptosomal fractions. Mice devoid of PrP(C) (Prnp0/0 mice) show synaptosomal copper concentrations diminished by 50% as compared to normal mice. PrP(C) in the synaptic cleft may serve as a copper buffer. Alternatively it may play a role in the re-uptake of copper into the presynapse or may be of structural importance for the N-terminus and thus may influence binding of PrP(C) to other proteins.

In vivo cytotoxicity of the prion protein fragment 106-126

Transmissible spongiform encephalopathies are fatal neurological diseases characterized by astroglyosis, neuronal loss, and by the accumulation of the abnormal isoform of the prion protein. The amyloid prion protein fragment 106-126 (P106-126) has been shown to be toxic in cultured hippocampal neurons (). Here, we show that P106-126 is also cytotoxic in vivo. Taking advantage of the fact that retina is an integral part of the central nervous system, the toxic effect of the peptide was investigated by direct intravitreous injection. Aged solutions of P106-126 induced apoptotic-mediated retinal cell death and irreversibly altered the electrical activity of the retina. Neither apoptosis nor electroretinogram damages were observed with freshly diluted P106-126, suggesting that the toxicity is linked to the aggregation state of the peptide. The retina provides a convenient in vivo system to look for potential inhibitors of cytotoxicity associated with spongiform encephalopathies.