Immunohistochemical comparison of anti-prion protein (PrP) antibodies in the CNS of mice infected with scrapie

Use of fixatives for immunohistochemistry and their application for detection of retinoic acid synthesizing enzymes in the central nervous system

Retinoic acid (RA) is a lipid signaling molecule that has a crucial role in growth and survival of neurons as well as regulation of neuronal plasticity in the central nervous system. Complete understanding of the distribution of RA is necessary to identify foci of RA signaling. However, RA itself is very difficult to detect by immunohistochemistry as there are few effective antibodies to this lipid and it can be difficult to fix in place in tissue. A set of retinaldehyde dehydrogenases (RALDHs) catalyze the last step of RA synthesis and their distribution can be used as a proxy for RA distribution. This protocol uses the example of RALDH2 expression in the motor neurons of the spinal cord as a demonstration of the approach and describes methods that can improve its effectiveness. Immunodetection is impacted by protein cross linking and protein denaturation as well as antigen/epitope masking by various fixatives. Finding a suitable fixative that preserves morphology and tissue structure by linking cellular component such as proteins and lipids in an indissoluble macromolecular network while keeping functional groups, including antigens, intact is essential. Here, we discuss fixatives in general and also describe a fixation protocol that allows detection of multiple antigens in the same section with a periodate-lysine-paraformaldehyde (PLP) fixative. This keeps tissue structure and antigen well preserved in the adult spinal cord to show RALDH2 expression in motor neurons.

Intraperitoneal Infection of Wild-Type Mice with Synthetically Generated Mammalian Prion

The prion hypothesis postulates that the infectious agent in transmissible spongiform encephalopathies (TSEs) is an unorthodox protein conformation based agent. Recent successes in generating mammalian prions in vitro with bacterially expressed recombinant prion protein provide strong support for the hypothesis. However, whether the pathogenic properties of synthetically generated prion (rec-Prion) recapitulate those of naturally occurring prions remains unresolved. Using end-point titration assay, we showed that the in vitro prepared rec-Prions have infectious titers of around 10⁴ LD₅₀ / μg. In addition, intraperitoneal (i.p.) inoculation of wild-type mice with rec-Prion caused prion disease with an average survival time of 210 – 220 days post inoculation. Detailed pathological analyses revealed that the nature of rec-Prion induced lesions, including spongiform change, disease specific prion protein accumulation (PrP-d) and the PrP-d dissemination amongst lymphoid and peripheral nervous system tissues, the route and mechanisms of neuroinvasion were all typical of classical rodent prions. Our results revealed that, similar to naturally occurring prions, the rec-Prion has a titratable infectivity and is capable of causing prion disease via routes other than direct intra-cerebral challenge. More importantly, our results established that the rec-Prion caused disease is pathogenically and pathologically identical to naturally occurring contagious TSEs, supporting the concept that a conformationally altered protein agent is responsible for the infectivity in TSEs.

The transmissible spongiform encephalopathies (TSEs) are a group of infectious neurodegenerative diseases affecting both humans and animals. The prion hypothesis postulates that prions are protein conformation based infectious agents responsible for TSE infectivity. Prions have been synthetically generated in vitro, but it remains unclear whether the properties of synthetically generated prion are the same as those of TSE agents and whether the disease caused by synthetically generated prion is identical to naturally occurring TSEs. In this study, we demonstrated that similar to the classical TSE agents, the synthetically generated prion has a titratable infectivity and is able to cause prion disease in wild-type mice via routes other than direct intra-cerebral inoculation. More importantly, we showed that the synthetically generated prion induced pathological changes, including the dissemination of disease-specific prion protein accumulation and the route and mechanism of neuroinvasion, were all typical of classical TSEs. These results demonstrate the similarity of synthetically generated prion to the infectious agent in TSEs, providing strong evidence supporting the prion hypothesis.

Lesion profiling and subcellular prion localization of cervid chronic wasting disease in domestic cats

Chronic wasting disease (CWD) is an efficiently transmitted, fatal, and progressive prion disease of cervids with an as yet to be fully clarified host range. While outbred domestic cats (Felis catus) have recently been shown to be susceptible to experimental CWD infection, the neuropathologic features of the infection are lacking. Such information is vital to provide diagnostic power in the event of natural interspecies transmission and insights into host and strain interactions in interspecies prion infection. Using light microscopy and immunohistochemistry, we detail the topographic pattern of neural spongiosis (the "lesion profile") and the distribution of misfolded prion protein in the primary and secondary passage of feline CWD (Fel(CWD)). We also evaluated cellular and subcellular associations between misfolded prion protein (PrP(D)) and central nervous system neurons and glial cell populations. From these studies, we (1) describe the novel neuropathologic profile of Fel(CWD), which is distinct from either cervid CWD or feline spongiform encephalopathy (FSE), and (2) provide evidence of serial passage-associated interspecies prion adaptation. In addition, we demonstrate through confocal analysis the successful co-localization of PrP(D) with neurons, astrocytes, microglia, lysosomes, and synaptophysin, which, in part, implicates each of these in the neuropathology of Fel(CWD). In conclusion, this work illustrates the simultaneous role of both host and strain in the development of a unique Fel(CWD) neuropathologic profile and that such a profile can be used to discriminate between Fel(CWD) and FSE.

Mucosal transmission and pathogenesis of chronic wasting disease in ferrets

Chronic wasting disease (CWD) of cervids is almost certainly transmitted by mucosal contact with the causative prion, whether by direct (animal-to-animal) or indirect (environmental) means. Yet the sites and mechanisms of prion entry remain to be further understood. This study sought to extend this understanding by demonstrating that ferrets exposed to CWD via several mucosal routes developed infection, CWD prion protein (PrP(CWD)) amplification in lymphoid tissues, neural invasion and florid transmissible spongiform encephalopathy lesions resembling those in native cervid hosts. The ferrets developed extensive PrP(CWD) accumulation in the nervous system, retina and olfactory epithelium, with lesser deposition in tongue, muscle, salivary gland and the vomeronasal organ. PrP(CWD) accumulation in mucosal sites, including upper respiratory tract epithelium, olfactory epithelium and intestinal Peyer's patches, make the shedding of prions by infected ferrets plausible. It was also observed that regionally targeted exposure of the nasopharyngeal mucosa resulted in an increased attack rate when compared with oral exposure. The latter finding suggests that nasal exposure enhances permissiveness to CWD infection. The ferret model has further potential for investigation of portals for initiation of CWD infection.

Protective role of interferon regulatory factor 3-mediated signaling against prion infection

Abnormal prion protein (PrP(Sc)) generated from the cellular isoform of PrP (PrP(C)) is assumed to be the main or sole component of the pathogen, called prion, of transmissible spongiform encephalopathies (TSE). Because PrP is a host-encoded protein, acquired immune responses are not induced in TSE. Meanwhile, activation of the innate immune system has been suggested to partially block the progression of TSE; however, the mechanism is not well understood. To further elucidate the role of the innate immune system in prion infection, we investigated the function of interferon regulatory factor 3 (IRF3), a key transcription factor of the MyD88-independent type I interferon (IFN) production pathway. We found that IRF3-deficient mice exhibited significantly earlier onset with three murine TSE strains, namely, 22L, FK-1, and murine bovine spongiform encephalopathy (mBSE), following intraperitoneal transmission, than with wild-type controls. Moreover, overexpression of IRF3 attenuated prion infection in the cell culture system, while PrP(Sc) was increased in prion-infected cells treated with small interfering RNAs (siRNAs) against IRF3, suggesting that IRF3 negatively regulates PrP(Sc) formation. Our findings provide new insight into the role of the host innate immune system in the pathogenesis of prion diseases.

Chronic wasting disease prion trafficking via the autonomic nervous system

Chronic wasting disease (CWD) is a fatal spongiform encephalopathy that is efficiently transmitted among members of the mammalian family Cervidae, including deer, elk, and moose. Typical of prion diseases, CWD is characterized by the conversion of the native protease-sensitive protein PrP(C) to a protease-resistant isoform, denoted PrP(RES). In native species, spread of the disease likely results from the ingestion of prion-containing excreta, including urine, saliva, or feces. Although cervid prion protein-expressing transgenic [Tg(CerPrP)] mice have been shown to be effective surrogates of natural CWD, uncertainties remain regarding the mechanisms by which CWD prions traffic in vivo, including the manner by which CWD prions traffic from the gastrointestinal tract to the central nervous system. We used elk prion protein-expressing transgenic [Tg(CerPrP-E)] mice, infected by three different routes of inoculation, and tissue-based IHC to elucidate that centripetal and centrifugal CWD prion transit pathways involve cells and fibers of the autonomic nervous systems, including the enteric nervous system and central autonomic network. Moreover, we identified CWD PrP(RES) associated with the cell bodies and processes of enteric glial cells within the enteric nervous system of CWD-infected Tg(CerPrP-E) mice. The present findings demonstrate the importance of the peripheral and central autonomic networks in CWD neuroinvasion and neuropathogenesis and suggest that enteroglial cells may facilitate the shedding of prions via the intestinal tract.

Amyloid-specific fluorophores for the rapid, sensitive in situ detection of prion contamination on surgical instruments

Prion diseases or transmissible spongiform encephalopathies (TSEs) are a group of rare, transmissible and fatal neurodegenerative diseases associated with the protein agent (PrP(Sc)). As such, the sensitive and rapid detection of prion PrP(Sc) amyloid on the surface of suspect surgical instruments is of great importance and may even allow remedial action to be taken prior to any further operative intervention and possible iatrogenic transmission. However, conventional PrP(Sc) detection methodologies tend to rely on the inefficient and unreliable removal of suspect material from a surface using swabs or wipes prior to antibody analysis. Here we show how the combination of an advanced light microscope technique, episcopic differential interference contrast/epifluorescence (EDIC/EF) microscopy, and the application of beta-amyloid fluorescent thiazole markers (thioflavin T, thioflavin S) can be used to detect, in situ, submicron (attomole) levels of prion protein amyloid contamination in brain and spleen sections, smears and homogenate on surgical stainless steel surfaces and surgical instruments. This technique, although not specific to an amyloid type, can be used to verify that surgical instruments are substantially free from prion amyloid protein soiling and hence reduce the risk of iatrogenic transmission.

Prion protein is a component of the multimolecular signaling complex involved in T cell activation

In this study we analyzed the interaction of prion protein PrP(C) with components of glycosphingolipid-enriched microdomains in lymphoblastoid T cells. PrP(C) was distributed in small clusters on the plasma membrane, as revealed by immunoelectron microscopy. PrP(C) is present in microdomains, since it coimmunoprecipitates with GM3 and the raft marker GM1. A strict association between PrP(C) and Fyn was revealed by scanning confocal microscopy and coimmunoprecipitation experiments. The phosphorylation protein ZAP-70 was immunoprecipitated by anti-PrP after T cell activation. These results demonstrate that PrP(C) interacts with ZAP-70, suggesting that PrP(C) is a component of the multimolecular signaling complex within microdomains involved in T cell activation.

Microdissection: a method developed to investigate mechanisms involved in transmissible spongiform encephalopathy pathogenesis

Background

The transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases affecting both human and animals. The neuroanatomical changes which occur in the central nervous system (CNS) of TSE infected animals include vacuolation, gliosis, neuronal loss and the deposition of a disease specific protein, PrP^Sc. Experimental murine models of scrapie, a TSE of sheep, have revealed that pathology may be confined to specific brain areas with targeting of particular neuronal subsets depending on route of injection and scrapie isolate.

To assess the biochemical changes which are taking place in these targeted areas it was necessary to develop a reliable sampling procedure (microdissection) which could be used for a variety of tests such as western blotting and magnetic resonance spectroscopy.

Methods

The method described is for the microdissection of murine brains. To assess the usefulness of this dissection technique for producing similar sample types for analysis by various down-stream biochemical techniques, the areas dissected were analysed for PrP^Sc by western blotting and compared to immunocytochemical (ICC) techniques.

Results

Results show that the method generates samples yielding a consistent protein content which can be analysed for PrP^Sc. The areas in which PrP^Sc is found by western blotting compares well with localisation visualised by immunocytochemistry.

Conclusion

The microdisssection method described can be used to generate samples suitable for a range of biochemical techniques. Using these samples a range of assays can be carried out which will help to elucidate the molecular and cellular mechanisms underlying TSE pathogenesis. The method would also be useful for any study requiring the investigation of discrete areas within the murine brain.