Propagation of scrapie in peripheral nerves after footpad infection in normal and neurotoxin exposed hamsters

Transport of Prions in the Peripheral Nervous System: Pathways, Cell Types, and Mechanisms

Prion diseases are transmissible protein misfolding disorders that occur in animals and humans where the endogenous prion protein, PrPC, undergoes a conformational change into self-templating aggregates termed PrPSc. Formation of PrPSc in the central nervous system (CNS) leads to gliosis, spongiosis, and cellular dysfunction that ultimately results in the death of the host. The spread of prions from peripheral inoculation sites to CNS structures occurs through neuroanatomical networks. While it has been established that endogenous PrPC is necessary for prion formation, and that the rate of prion spread is consistent with slow axonal transport, the mechanistic details of PrPSc transport remain elusive. Current research endeavors are primarily focused on the cellular mechanisms of prion transport associated with axons. This includes elucidating specific cell types involved, subcellular machinery, and potential cofactors present during this process.

Distinct patterns of spread of prion infection in brains of mice expressing anchorless or anchored forms of prion protein

Background

In humans and animals, prion protein (PrP) is usually expressed as a glycophosphatidylinositol (GPI)-anchored membrane protein, but anchorless PrP may be pathogenic in humans with certain familial prion diseases. Anchored PrP expressed on neurons mediates spread of prions along axons in the peripheral and central nervous systems. However, the mechanism of prion spread in individuals expressing anchorless PrP is poorly understood. Here we studied prion spread within brain of mice expressing anchorless or anchored PrP.

Results

To create a localized initial point of infection, we microinjected scrapie in a 0.5 microliter volume in the striatum. In this experiment, PrPres and gliosis were first detected in both types of mice at 40 days post-inoculation near the needle track. In mice with anchored PrP, PrPres appeared to spread via neurons to distant connected brain areas by the clinical endpoint at 150 days post-inoculation. This PrPres was rarely associated with blood vessels. In contrast, in mice with anchorless PrP, PrPres spread did not follow neuronal circuitry, but instead followed a novel slower pattern utilizing the drainage system of the brain interstitial fluid (ISF) including perivascular areas adjacent to blood vessels, subependymal areas and spaces between axons in white matter tracts.

Conclusions

In transgenic mice expressing anchorless PrP small amyloid-seeding PrPres aggregates appeared to be transported in the ISF, thus spreading development of cerebral amyloid angiopathy (CAA) throughout the brain. Spread of amyloid seeding by ISF may also occur in multiple human brain diseases involving CAA.

Targeted knock-down of cellular prion protein expression in myelinating Schwann cells does not alter mouse prion pathogenesis

In naturally acquired transmissible spongiform encephalopathies, the pathogenic agents or prions spread from the sites of initial peripheral uptake or replication to the brain where they cause progressive and fatal neurodegeneration. Routing via the peripheral nervous system is considered to be one of the main pathways to the central nervous system. Replication of prions in Schwann cells is viewed as a potentially important mechanism for efficient prion spread along nerves. Here we used a Cre-loxP mouse transgenetic approach to disrupt host-encoded prion protein (PrP(C)) specifically in myelinating Schwann cells. Despite the use of infection routes targeting highly myelinated nerves, there was no alteration in mouse prion pathogenesis, suggesting that conversion-dependent, centripetal spread of prions does not crucially rely on PrP(C) expressed by myelinating Schwann cells.

The Experimental Use of Syrian Hamsters

The Syrian hamster (Mesocricetus auratus) is a widely used experimental animal model. This chapter focuses primarily on the most current research uses of the hamster. More classical uses are covered only as they pertain to these current uses. Hamsters possess unique anatomical and physiological features, which make them desirable research models. Unlike other commonly used laboratory rodents, hamsters possess a cheek pouch, which can be easily everted and examined at both the gross and microscopic level. The hamster's relative size also allows for better visualization of certain biological systems including the respiratory and reproductive systems when compared to the mouse. Further, laboratory hamsters develop a variety of inherited diseases, which display similarities to human conditions. Hamsters possessing some of these inherited traits are commercially available. They are susceptible to a variety of carcinogens and develop tumors that other research animals less commonly develop. Also they are susceptible to the induction of a variety of metabolic disorders through the use of dietary manipulations. The antagonistic nature of hamsters is used to study the effect of treatment on male aggressive and defensive behaviors. Syrian hamsters display several unique characteristics that make them desired models for carcinogenesis studies.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

Crucial role for prion protein membrane anchoring in the neuroinvasion and neural spread of prion infection

In nature prion diseases are usually transmitted by extracerebral prion infection, but clinical disease results only after invasion of the central nervous system (CNS). Prion protein (PrP), a host-encoded glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein, is necessary for prion infection and disease. Here, we investigated the role of the anchoring of PrP on prion neuroinvasion by studying various inoculation routes in mice expressing either anchored or anchorless PrP. In control mice with anchored PrP, intracerebral or sciatic nerve inoculation resulted in rapid CNS neuroinvasion and clinical disease (154 to 156 days), and after tongue, ocular, intravenous, or intraperitoneal inoculation, CNS neuroinvasion was only slightly slower (193 to 231 days). In contrast, in anchorless PrP mice, these routes resulted in slow and infrequent CNS neuroinvasion. Only intracerebral inoculation caused brain PrPres, a protease-resistant isoform of PrP, and disease in both types of mice. Thus, anchored PrP was an essential component for the rapid neural spread and CNS neuroinvasion of prion infection.

Spreading of prions from the immune to the peripheral nervous system: a potential implication of dendritic cells

The implication of dendritic cells (DCs) in the peripheral spreading of prions has increased in the last few years. It has been recently described that DCs can transmit prions to primary neurons from the central nervous system. In order to improve the understanding of the earliest steps of prion peripheral neuroinvasion, we studied, using an in vitro model, the effect of exposing primary peripheral neurons to scrapie-infected lymphoid cells. Thanks to this system, there is evidence that bone marrow dendritic cells (BMDCs) are in connection with neurites of peripheral neurons via cytoplasmic extensions. BMDCs are competent to internalize prions independently from the expression of cellular prion protein (PrP(C)) and have the capacity to transmit detergent-insoluble, relatively proteinase K-resistant prion protein (PrP(Sc)) to peripheral neurons after 96 h of coculture. Furthermore, we confirmed the special status of the peripheral nervous system in front of prion diseases. Contrary to central neurons, PrP(Sc) infection does not disturb survival and neurite outgrowth. Our model demonstrates that PrP(Sc)-loaded dendritic cells and peripheral nerve fibers that are included in neuroimmune interfaces can initiate and spread prion neuroinvasion.

Prion propagation in a nerve conduit model containing segments devoid of axons

Prions, the putative causative agents of transmissible spongiform encephalopathies, are neurotropic pathogens that spread to the central nervous system via synaptically linked neural conduits upon peripheral infection. Axons and their transport processes have been suggested as mediators of nerve-associated prion dissemination. However, the exact cellular components and molecular mechanisms underlying neural spread are unknown. This study used an established hamster scrapie model to pursue a novel experimental approach using nerve conduits containing segments devoid of neurites generated by incomplete nerve regeneration following Wallerian degeneration to probe the necessity of axons for the neural propagation of prions. For this purpose, animals were subjected to unilateral sciatic neurectomy 4 weeks before footpad inoculation with scrapie agent. The results showed that the regional nerve is the prime conduit for cerebral neuroinvasion and revealed, as evidenced by the accumulation of pathological prion protein PrP TSE, that prions can proceed along segments of peripheral neural projections without detectable axonal structures.

Relevance of the regional lymph node in scrapie pathogenesis after peripheral infection of hamsters

BACKGROUND

The exact role of the lymphoreticular system in the spread of peripheral prion infections to the central nervous system still needs further elucidation. Against this background, the influence of the regional lymph node (Ln. popliteus) on the pathogenesis of scrapie was monitored in a hamster model of prion infection via the footpad.

METHODS

Surgical lymphadenectomy was carried out at different time points after infection, or prior to inoculation, in order to elucidate the impact of the lymph node on lethal neuroinvasion.

RESULTS

The Ln. popliteus did not show an influence on pathogenesis when a high dose of infectivity was administered. However, it was found to modulate the interval of time until the development of terminal scrapie in a subset of animals lymphadenectomized after low-dose infection. In additon, lymphadenectomy performed four weeks before inoculation prevented cerebral PrP(TSE) deposition and development of disease during the period of observation (314 days) in the majority of hamsters challenged with a very low dose of scrapie agent.

CONCLUSION

Our findings suggest the regional lymph node as a potentially facilitating or even essential factor for invasion of the brain after peripheral challenge with low doses of infectious scrapie agent. The invasive in vivo approach pursued in this study may be applied also to other animal species for further elucidating the involvement of lymphoid tissue in the pathogenesis of experimental and natural TSEs.

Interaction between dendritic cells and nerve fibres in lymphoid organs after oral scrapie exposure

In transmissible spongiform encephalopathies (TSEs), the infectious agent, called PrPsc, an abnormal isoform of the cellular prion protein, accumulates and replicates in lymphoid organs before affecting the nervous system. To clarify the cellular requirements for the neuroinvasion of the scrapie agent from the lymphoid organs to the central nervous system, we have studied, by confocal microscopy, the innervations within Peyer's patches, mesenteric lymph nodes and the spleen of mice in physiological conditions and after oral exposure to prion. Contacts between nerve fibres and PrPsc-associated cells, dendritic cells (DCs) and follicular dendritic cells (FDCs), were evaluated in preclinical prion-infected mice. Using a double immunolabelling strategy, we demonstrated the lack of innervation of PrPsc-accumulating cells (FDCs). Contacts between nerve fibers and PrPsc-propagating cells (DCs) were detected in T-cell zones and cell-trafficking areas. This supports, for the first time, the possible implication of dendritic cells in the prion neuroinvasion process.

The spread of prions through the body in naturally acquired transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases that are caused by unconventional pathogens and affect the central nervous system of animals and humans. Several different forms of these diseases result from natural infection (i.e. exposure to transmissible spongiform encephalopathy agents or prions, present in the natural environment of the respective host). This holds true also for scrapie in sheep, bovine spongiform encephalopathy in cattle, chronic wasting disease in elk and deer, or variant Creutzfeldt-Jakob disease in humans, all of which are assumed to originate predominantly from peroral prion infection. This article intends to provide an overview of the current state of knowledge on the spread of scrapie, chronic wasting disease, bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease agents through the body in naturally affected hosts, and in model animals experimentally challenged via the alimentary tract. Special attention is given to the tissue components and spreading pathways involved in the key stages of prion routing through the body, such as intestinal uptake, neuroinvasion of nerves and the central nervous system, and centrifugal spread from the brain and spinal cord to peripheral sites (e.g. sensory ganglia or muscles). The elucidation of the pathways and mechanisms by which prions invade a host and spread through the organism can contribute to efficient infection control strategies and the improvement of transmissible spongiform encephalopathy diagnostics. It may also help to identify prophylactic or therapeutic approaches that would impede naturally acquired transmissible spongiform encephalopathy infections.

Accumulation of pathological prion protein PrPSc in the skin of animals with experimental and natural scrapie

Prion infectivity and its molecular marker, the pathological prion protein PrP^Sc, accumulate in the central nervous system and often also in lymphoid tissue of animals or humans affected by transmissible spongiform encephalopathies. Recently, PrP^Sc was found in tissues previously considered not to be invaded by prions (e.g., skeletal muscles). Here, we address the question of whether prions target the skin and show widespread PrP^Sc deposition in this organ in hamsters perorally or parenterally challenged with scrapie. In hamsters fed with scrapie, PrP^Sc was detected before the onset of symptoms, but the bulk of skin-associated PrP^Sc accumulated in the clinical phase. PrP^Sc was localized in nerve fibres within the skin but not in keratinocytes, and the deposition of PrP^Sc in skin showed no dependence from the route of infection and lymphotropic dissemination. The data indicated a neurally mediated centrifugal spread of prions to the skin. Furthermore, in a follow-up study, we examined sheep naturally infected with scrapie and detected PrP^Sc by Western blotting in skin samples from two out of five animals. Our findings point to the skin as a potential reservoir of prions, which should be further investigated in relation to disease transmission.

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative diseases affecting the central nervous system. According to the prion hypothesis, TSEs are caused by proteinaceous infectious particles (“prions”) that consist essentially of PrP^Sc, an aberrant form of the prion protein with a pathologically altered folding and/or aggregation structure. Scrapie of sheep, chronic wasting disease (CWD) of deer, bovine spongiform encephalopathy (BSE) of cattle, and variant Creutzfeldt-Jakob disease (vCJD) of humans are prominent examples of acquired prion diseases. To further pinpoint the peripheral tissues that could serve as reservoirs of prions in the mammalian body and from which these pathogens could be potentially disseminated into the environment and transmitted to other individuals, we examined the skin of hamsters perorally challenged with scrapie and of naturally infected scrapie sheep for the presence of PrP^Sc. We show that PrP^Sc can accumulate in the skin at late stages of incubation, and that the protein is located primarily in small nerve fibres within this organ. The question of whether the skin may also provide a reservoir for prions in CWD, BSE, or vCJD, and the role of the skin in relation to the natural transmission of scrapie in the field needs further investigation.