Pathogenetical significance of porencephalic lesions associated with intracerebral inoculation of sheep with the bovine spongiform encephalopathy (BSE) agent

Altered distribution, aggregation, and protease resistance of cellular prion protein following intracranial inoculation

Prion protein (PrP^C) is a protease-sensitive and soluble cell surface glycoprotein expressed in almost all mammalian cell types. PrP^Sc, a protease-resistant and insoluble form of PrP^C, is the causative agent of prion diseases, fatal and transmissible neurogenerative diseases of mammals. Prion infection is initiated via either ingestion or inoculation of PrP^Sc or when host PrP^C stochastically refolds into PrP^Sc. In either instance, the early events that occur during prion infection remain poorly understood. We have used transgenic mice expressing mouse PrP^C tagged with a unique antibody epitope to monitor the response of host PrP^C to prion inoculation. Following intracranial inoculation of either prion-infected or uninfected brain homogenate, we show that host PrP^C can accumulate both intra-axonally and within the myelin membrane of axons suggesting that it may play a role in axonal loss following brain injury. Moreover, in response to the inoculation host PrP^C exhibits an increased insolubility and protease resistance similar to that of PrP^Sc, even in the absence of infectious prions. Thus, our results raise the possibility that damage to the brain may be one trigger by which PrP^C stochastically refolds into pathogenic PrP^Sc leading to productive prion infection.

Robust Central Nervous System Pathology in Transgenic Mice following Peripheral Injection of α-Synuclein Fibrils

Misfolded α-synuclein (αS) is hypothesized to spread throughout the central nervous system (CNS) by neuronal connectivity leading to widespread pathology. Increasing evidence indicates that it also has the potential to invade the CNS via peripheral nerves in a prion-like manner. On the basis of the effectiveness following peripheral routes of prion administration, we extend our previous studies of CNS neuroinvasion in M83 αS transgenic mice following hind limb muscle (intramuscular [i.m.]) injection of αS fibrils by comparing various peripheral sites of inoculations with different αS protein preparations. Following intravenous injection in the tail veins of homozygous M83 transgenic (M83^+/+) mice, robust αS pathology was observed in the CNS without the development of motor impairments within the time frame examined. Intraperitoneal (i.p.) injections of αS fibrils in hemizygous M83 transgenic (M83^+/-) mice resulted in CNS αS pathology associated with paralysis. Interestingly, injection with soluble, nonaggregated αS resulted in paralysis and pathology in only a subset of mice, whereas soluble Δ71-82 αS, human βS, and keyhole limpet hemocyanin (KLH) control proteins induced no symptoms or pathology. Intraperitoneal injection of αS fibrils also induced CNS αS pathology in another αS transgenic mouse line (M20), albeit less robustly in these mice. In comparison, i.m. injection of αS fibrils was more efficient in inducing CNS αS pathology in M83 mice than i.p. or tail vein injections. Furthermore, i.m. injection of soluble, nonaggregated αS in M83^+/- mice also induced paralysis and CNS αS pathology, although less efficiently. These results further demonstrate the prion-like characteristics of αS and reveal its efficiency to invade the CNS via multiple routes of peripheral administration.

IMPORTANCE

The misfolding and accumulation of α-synuclein (αS) inclusions are found in a number of neurodegenerative disorders and is a hallmark feature of Parkinson's disease (PD) and PD-related diseases. Similar characteristics have been observed between the infectious prion protein and αS, including its ability to spread from the peripheral nervous system and along neuroanatomical tracts within the central nervous system. In this study, we extend our previous results and investigate the efficiency of intravenous (i.v.), intraperitoneal (i.p.), and intramuscular (i.m.) routes of injection of αS fibrils and other protein controls. Our data reveal that injection of αS fibrils via these peripheral routes in αS-overexpressing mice are capable of inducing a robust αS pathology and in some cases cause paralysis. Furthermore, soluble, nonaggregated αS also induced αS pathology, albeit with much less efficiency. These findings further support and extend the idea of αS neuroinvasion from peripheral exposures.

PrP aggregation can be seeded by pre-formed recombinant PrP amyloid fibrils without the replication of infectious prions

Mammalian prions are unusual infectious agents, as they are thought to consist solely of aggregates of misfolded prion protein (PrP). Generation of synthetic prions, composed of recombinant PrP (recPrP) refolded into fibrils, has been utilised to address whether PrP aggregates are, indeed, infectious prions. In several reports, neurological disease similar to transmissible spongiform encephalopathy (TSE) has been described following inoculation and passage of various forms of fibrils in transgenic mice and hamsters. However, in studies described here, we show that inoculation of recPrP fibrils does not cause TSE disease, but, instead, seeds the formation of PrP amyloid plaques in PrP-P101L knock-in transgenic mice (101LL). Importantly, both WT-recPrP fibrils and 101L-recPrP fibrils can seed plaque formation, indicating that the fibrillar conformation, and not the primary sequence of PrP in the inoculum, is important in initiating seeding. No replication of infectious prions or TSE disease was observed following both primary inoculation and subsequent subpassage. These data, therefore, argue against recPrP fibrils being infectious prions and, instead, indicate that these pre-formed seeds are acting to accelerate the formation of PrP amyloid plaques in 101LL Tg mice. In addition, these data reproduce a phenotype which was previously observed in 101LL mice following inoculation with brain extract containing in vivo-generated PrP amyloid fibrils, which has not been shown for other synthetic prion models. These data are reminiscent of the "prion-like" spread of aggregated forms of the beta-amyloid peptide (Aβ), α-synuclein and tau observed following inoculation of transgenic mice with pre-formed seeds of each misfolded protein. Hence, even when the protein is PrP, misfolding and aggregation do not reproduce the full clinicopathological phenotype of disease. The initiation and spread of protein aggregation in transgenic mouse lines following inoculation with pre-formed fibrils may, therefore, more closely resemble a seeded proteinopathy than an infectious TSE disease.

Stability of murine scrapie strain 87V after passage in sheep and comparison with the CH1641 ovine strain

Breed- and prion protein (PRNP) genotype-related disease phenotype variability has been observed in sheep infected with the 87V murine scrapie strain. Therefore, the stability of this strain was tested by inoculating sheep-derived 87V brain material back into VM mice. As some sheep-adapted 87V disease phenotypes were reminiscent of CH1641 scrapie, transgenic mice (Tg338) expressing ovine prion protein (PrP) were inoculated with the same sheep-derived 87V sources and with CH1641. Although at first passage in VM mice the sheep-derived 87V sources showed some divergence from the murine 87V control, all the characteristics of murine 87V infection were recovered at second passage from all sheep sources. These included 100 % attack rates and indistinguishable survival times, lesion profiles, immunohistochemical features of disease-associated PrP accumulation in the brain and PrP biochemical properties. All sheep-derived 87V sources, as well as CH1641, were transmitted to Tg338 mice with identical clinical, pathological, immunohistochemical and biochemical features. While this might potentially indicate that sheep-adapted 87V and CH1641 are the same strain, profound divergences were evident, as murine 87V was unable to infect Tg338 mice but was lethal for VM mice, while the reverse was true for CH1641. These combined data suggest that: (i) murine 87V is stable and retains its properties after passage in sheep; (ii) it can be isolated from sheep showing a CH1641-like or a more conventional scrapie phenotype; and (iii) sheep-adapted 87V scrapie, with conventional or CH1641-like phenotype, is biologically distinct from experimental CH1641 scrapie, despite the fact that they behave identically in a single transgenic mouse line.

Influence of breed and genotype on the onset and distribution of infectivity and disease-associated prion protein in sheep following oral infection with the bovine spongiform encephalopathy agent

The onset and distribution of infectivity and disease-specific prion protein (PrP(d)) accumulation was studied in Romney and Suffolk sheep of the ARQ/ARQ, ARQ/ARR and ARR/ARR prion protein gene (Prnp) genotypes (where A stands for alanine, R for arginine and Q for glutamine at codons 136, 154 and 171 of PrP), following experimental oral infection with cattle-derived bovine spongiform encephalopathy (BSE) agent. Groups of sheep were killed at regular intervals and a wide range of tissues taken for mouse bioassay or immunohistochemistry (IHC), or both. Bioassay results for infectivity were mostly coincident with those of PrP(d) detection by IHC both in terms of tissues and time post infection. Neither PrP(d) nor infectivity was detected in any tissues of BSE-dosed ARQ/ARR or ARR/ARR sheep or of undosed controls. Moreover, four ARQ/ARQ Suffolk sheep, which were methionine (M)/threonine heterozygous at codon 112 of the Prnp gene, did not show any biological or immunohistochemical evidence of infection, while those homozygous for methionine (MARQ/MARQ) did. In MARQ/MARQ sheep of both breeds, initial PrP(d) accumulation was identified in lymphoreticular system (LRS) tissues followed by the central nervous system (CNS) and enteric nervous system (ENS) and finally by the autonomic nervous system and peripheral nervous system and other organs. Detection of infectivity closely mimicked this sequence. No PrP(d) was observed in the ENS prior to its accumulation in the CNS, suggesting that ENS involvement occurred simultaneously to that of, or followed centrifugal spread from, the CNS. The distribution of PrP(d) within the ENS further suggested a progressive spread from the ileal plexus to other ENS segments via neuronal connections of the gut wall. Differences between the two breeds were noted in terms of involvement of LRS and ENS tissues, with Romney sheep showing a more delayed and less consistent PrP(d) accumulation than Suffolk sheep in such tissues. Whether this accounted for the slight delay (∼5 months) in the appearance of clinical signs in Romney sheep is debatable since by the last scheduled kill before animals reached clinical end point, both breeds showed widespread accumulation and similar magnitudes of PrP(d) accumulation in the brain.

Pathology of SSLOW, a transmissible and fatal synthetic prion protein disorder, and comparison with naturally occurring classical transmissible spongiform encephalopathies

AIMS

Naturally occurring transmissible spongiform encephalopathies (TSEs) accumulate disease-specific forms of prion protein on cell membranes in association with pathognomonic lesions. We wished to determine whether synthetic prion protein disorders recapitulated these and other subcellular TSE-specific changes.

METHODS

SSLOW is a TSE initiated with refolded synthetic prion protein. Five terminally sick hamsters previously intracerebrally inoculated with third passage SSLOW were examined using light and immunogold electron microscopy.

RESULTS

SSLOW-affected hamsters showed widespread abnormal prion protein (PrP(SSLOW) ) and amyloid plaques. PrP(SSLOW) accumulated on plasma lemmas of neurites and glia without pathological changes. PrP(SSLOW) also colocalized with increased coated vesicles and pits, coated spiral membrane invaginations and membrane microfolding. PrP(SSLOW) was additionally observed in lysosomes of microglial cells but not of neurones or astrocytes.

CONCLUSIONS

PrP(SSLOW) is propagated by cell membrane conversion of normal PrP and lethal disease may be linked to the progressive growth of amyloid plaques. Cell membrane changes present in SSLOW are indistinguishable from those of naturally occurring TSEs. However, some lesions found in SSLOW are absent in natural animal TSEs and vice versa. SSLOW may not entirely recapitulate neuropathological features previously described for natural disease. End-stage neuropathology in SSLOW, particularly the nature and distribution of amyloid plaques may be significantly influenced by the early redistribution of seeds within the inoculum and its recirculation following interstitial, perivascular and other drainage pathways. The way in which seeds are distributed and aggregate into plaques in SSLOW has significant overlap with murine APP overexpressing mice challenged with Aβ.

Influence of polymorphisms in the prion protein gene on the pathogenesis and neuropathological phenotype of sheep scrapie after oral infection

The prion protein gene (Prnp) plays a crucial role in the susceptibility of sheep to scrapie in terms of attack rate and/or incubation period. However, the influence of Prnp on the pathogenesis of the disease, specifically the involvement of tissues of the lymphoreticular system (LRS), pathways of neuroinvasion and neuropathological phenotypes, remains controversial. This study reports the onset and progression of disease-associated prion protein (PrP(d)) accumulation in the LRS and nervous tissues of sheep of six different Prnp genotypes infected by oral administration of the same mixed scrapie brain homogenate. Sheep homozygous for glutamine (Q) at codon 171 of PrP, with either valine (V) or alanine (A) at codon 136 (i.e. VRQ/VRQ, VRQ/ARQ and ARQ/ARQ), showed early and consistent PrP(d) accumulation in LRS tissues of the pharynx and gut. In contrast, LRS involvement was minimal, inconsistent and occurred late in the incubation period in sheep heterozygous for arginine (R) at codon 171 (i.e. VRQ/ARR and ARQ/ARR). Despite this difference, all five groups were susceptible to infection and developed clinical disease, albeit with significantly different incubation periods (shortest in VRQ/VRQ and longest in ARQ/ARR sheep). The remaining group of ARR/ARR homozygous sheep did not show evidence of infection at the end of the experiment or at previous predetermined time points. As for LRS tissues, the sites of initial PrP(d) accumulation in the brain were determined immunohistochemically. These were the same in all susceptible sheep (except for ARR/ARR sheep), regardless of their Prnp genotype which, together with an early and consistent accumulation of PrP(d) in circumventricular organs and a late or inconsistent involvement of the enteric and autonomic nervous system and of the spinal cord, suggests neuroinvasion occurring via the blood. The neuropathological phenotype (PrP(d) profile in the central nervous system) of clinically affected sheep was similar in the three V136 carrier groups, but showed some differences in the two A136 homozygous groups, suggesting a codon 136-driven selection of different strains from the mixture contained in the inoculum. ARQ/ARR sheep showed an irregular distribution of brain PrP(d), contrasting with the more widespread distribution of the other four groups. The results indicate that (1) ARQ/ARR sheep are more susceptible to oral scrapie infection than would be predicted from incidence figures in natural disease, (2) amplification and accumulation of PrP(d) in LRS tissues is host genotype dependent, but does not necessarily have a marked effect on the outcome of the infection and (3) the neuropathological phenotype of scrapie is related to the host genotype, but possibly in combination with the infecting source.

Identical pathogenesis and neuropathological phenotype of scrapie in valine, arginine, glutamine/valine, arginine, glutamine sheep infected experimentally by the oral and conjunctival routes

The pathogenesis of scrapie in sheep after natural or oral exposure to the infectious agent generally involves the early accumulation of disease-associated prion protein (PrP(d)) in the lymphoreticular system (LRS). This phase is followed by neuroinvasion, for which two routes, ascending neural and haematogenous, have been postulated. The present study reports the use of immunohistochemistry to track the tissue progression of PrP(d) deposition in sheep of a single, highly scrapie-susceptible PrP genotype administered by the oral or conjunctival routes. Regardless of the route of infection, the earliest detection of PrP(d) was in gut- and pharynx-associated LRS tissues. Subsequently, the brain became PrP(d) positive simultaneously with other LRS tissues, but before the spinal cord and peripheral nervous tissues of the enteric, parasympathetic and sympathetic systems. The sites of initial PrP(d) accumulation in the brain were the dorsal motor nucleus of the vagus and the hypothalamus and their related circumventricular organs (the area postrema and the median eminence, respectively). These were the same for both routes of infection. Rapid progression to clinical disease was observed in sheep infected orally or conjunctivally, with definite signs of scrapie recorded at around 6 and 8 months after infection, respectively. Longer incubation periods in sheep infected by the conjunctival route were probably due to them receiving a lower dose than those infected orally. Irrespective of the route of infection, clinically affected sheep showed the same pathological phenotype (PrP(d) profile) and PrP(d) distribution throughout the brain. The identical peripheral and central pathogenesis observed in sheep of both groups suggests early dissemination of the infectious agent in the bloodstream and a common neuroinvasion pathway. The late involvement of the enteric and autonomic nervous system supports a haematogenous route of infection to the brain.

Biochemical Characteristics and PrP(Sc) Distribution Pattern in the Brains of Cattle Experimentally Challenged with H-type and L-type Atypical BSE

Besides the classical form of bovine spongiform encephalopathy (BSE) that has been known for almost three decades, two atypical forms designated H-type and L-type BSE have recently been described. While the main diagnostic feature of these forms is the altered biochemical profile of the accumulated PrP^Sc, it was also observed in the initial analysis that L-type BSE displays a distribution pattern of the pathological prion protein (PrP^Sc), which clearly differs from that observed in classical BSE (C-type). Most importantly, the obex region in the brainstem is not the region with the highest PrP^Sc concentrations, but PrP^Sc is spread more evenly throughout the entire brain. A similar distribution pattern has been revealed for H-type BSE by rapid test analysis. Based on these findings, we performed a more detailed Western blot study of the anatomical PrP^Sc distribution pattern and the biochemical characteristics (molecular mass, glycoprofile as well as PK sensitivity) in ten different anatomical locations of the brain from cattle experimentally challenged with H- or L-type BSE, as compared to cattle challenged with C-type BSE. Results of this study revealed distinct differences in the PrP^Sc deposition patterns between all three BSE forms, while the biochemical characteristics remained stable for each BSE type among all analysed brain areas.

Increased immunohistochemical labelling for prion protein occurs in diverse neurological disorders of sheep: relevance for normal cellular PrP function

The classical prion diseases (e.g. scrapie of sheep and goats and bovine spongiform encephalopathy of cattle) are characterized by the accumulation of abnormal forms of the prion protein (PrP), usually recognized by their relative resistance to proteolysis compared with the physiological cellular forms of PrP. However, novel prion diseases have been detected in sheep, cattle and man, in which the abnormal PrP has less resistance to proteolysis than identified previously. These more subtle differences between abnormal and normal forms of PrP can be problematic in routine diagnostic tests and raise questions in respect of the range of PrP disorders. Abnormal accumulations of PrP in atypical and classical prion diseases can be recognized by immunohistochemistry. To determine whether altered PrP expression or trafficking might occur in nosological entities not previously connected with prion disease, the brains of sheep affected with diverse neurological conditions were examined for evidence of altered PrP labelling. Such altered immunolabelling was detected in association with either basic lesions or specific diseases. Some reactive glial cells and degenerate neurons found in several different recognized disorders and non-specific inflammatory processes were associated with abnormal PrP labelling, which was absent from brains of healthy, age-matched sheep. The results agree with previous indications that normal PrP function may be linked with the oxidative stress response, but the data also suggest that PrP functions are more extensive than simple protective responses against stress insults.

Neuroinvasion in prion diseases: the roles of ascending neural infection and blood dissemination

Prion disorders are infectious, neurodegenerative diseases that affect humans and animals. Susceptibility to some prion diseases such as kuru or the new variant of Creutzfeldt-Jakob disease in humans and scrapie in sheep and goats is influenced by polymorphisms of the coding region of the prion protein gene, while other prion disorders such as fatal familial insomnia, familial Creutzfeldt-Jakob disease, or Gerstmann-Straussler-Scheinker disease in humans have an underlying inherited genetic basis. Several prion strains have been demonstrated experimentally in rodents and sheep. The progression and pathogenesis of disease is influenced by both genetic differences in the prion protein and prion strain. Some prion diseases only affect the central nervous system whereas others involve the peripheral organs prior to neuroinvasion. Many experiments undertaken in different species and using different prion strains have postulated common pathways of neuroinvasion. It is suggested that prions access the autonomic nerves innervating peripheral organs and tissues to finally reach the central nervous system. We review here published data supporting this view and additional data suggesting that neuroinvasion may concurrently or independently involve the blood vascular system.

Pathogenesis of natural goat scrapie: modulation by host PRNP genotype and effect of co-existent conditions

After detection of a high prevalence of scrapie in a large dairy goat herd, 72 infected animals were examined by immunohistochemistry with prion protein (PrP) antibody Bar224 to study the pathogenesis of the infection. Tissues examined included the brain and thoracic spinal cord (TSC), a wide selection of lymphoreticular system (LRS) tissues, the distal ileum and its enteric nervous system (ENS), and other organs, including the mammary gland. The whole open reading frame of the PRNP gene was sequenced and antibodies to caprine arthritis-encephalitis virus (CAEV) infection were determined. Unexpectedly, accumulation of disease-associated PrP (PrP^d) in the brain was more frequent in methionine carriers at codon 142 (24/32, 75.0%) than amongst isoleucine homozygotes (14/40, 35.0%). The latter, however, showed significantly greater amounts of brain PrP^d than the former (average scores of 9.3 and 3.0, respectively). A significant proportion of the 38 goats that were positive in brain were negative in the ENS (44.7%) or in the TSC (39.5%). These results, together with the early and consistent involvement of the circumventricular organs and the hypothalamus, point towards a significant contribution of the haematogenous route in the process of neuroinvasion. Chronic enteritis was observed in 98 of the 200 goats examined, with no association with either scrapie infection or presence of PrP^d in the gut. Lymphoproliferative interstitial mastitis was observed in 13/31 CAEV-positive and scrapie-infected goats; PrP^d in the mammary gland was detected in five of those 13 goats, suggesting a possible contribution of CAEV infection in scrapie transmission via milk.

Characterization of strains of ovine transmissible spongiform encephalopathy with a short PrPd profiling method

Scrapie is the transmissible spongiform encephalopathy (TSE) that naturally affects sheep and goats; these species are also susceptible to experimental infection with the bovine spongiform encephalopathy (BSE) agent. Discrimination between different strains of sheep scrapie and ovine BSE has been achieved by descriptive and quantitative profiling of deposits of the disease-associated prion protein (PrPd) in different areas of the brain, but this process is time-consuming and difficult to standardize between laboratories. The present paper describes an alternative PrPd profiling method that is less demanding and addresses these difficulties. It is based on the scoring of similar 14 PrPd types in 11 precisely defined areas of the telencephalon. When applied to 48 archived cases of experimental sheep BSE, SSBP/1, CH1641 and natural scrapie, it gave comparable results to the original profiling method, previously conducted on the same brains, and allowed differentiation between the different infectious sources. This new 'short PrPd profiling' method has the advantages of being less time-consuming and easier to standardize, so that it can be readily adopted by different laboratories to provide comparable results.