Prions: The Unfolding Story of a Good Protein Gone Bad

Part II: Neuroinvasion and Replication

By Michele Arduengo, Ph.D.
Promega Corporation

Neuroinvasion 

The prion-only hypothesis states that the infectious agents of TSEs are proteins only—no nucleic acid (1). This hypothesis raises several questions. How does an ingested protein infect the highly protected tissues of the brain—moving from the stomach, crossing the formidable blood-brain barrier into the CNS? How does the infectious protein replicate its aberrant structure in the host organism? How can an aberrant protein be inherited? Work in mouse models has provided clues about how prions invade and damage CNS tissues, but many questions remain.

Route to the CNS

Prions may invade the CNS through several possible routes. Early in infection after oral ingestion, prions accumulate in the Peyer’s patches of the gut (lymphoid tissues), in the intestinal ganglia, and in lymphatic organs. Accumulation in the intestinal ganglia suggests that prions could travel directly from the sympathetic nerves innervating the intestine to the CNS. Studies have shown that a PrP^c-expressing (PrP^c = normal cellular prion) host tissue that is not derived from the bone marrow (non-hematopoetic) is required for neuroinvasion (2), and several authors have suggested that this tissue is the sympathetic nervous system (3). 

Studies have demonstrated that sympathetic innervation in mice determines the rate of neuroinvasion in peripheral prion infections (3,4). When prions are inoculated into the peritoneal cavity of mice (the space in which the internal organs of the gut are located), pathologic prions (PrP^sc) first accumulate in the thoracic segments of the spinal cord that receive the nerves from the area in which the prions were introduced. Another study in hamsters indicates that neuroinvasion from intratongue inoculation is more efficient than invasion from ingestion (59 days v. 101 days), and these authors demonstrate PrP^sc expression in the hypoglossal nerve as soon as 2 days after tongue inoculation (5). These data suggest that PrP^sc  travels from the tongue along the hypoglossal nerve to the brain. Taken together, these observations suggest that PrP^sc might travel “up” sympathetic nerves to the spinal cord and brain.

The molecular nature of this travel along neurons is unknown. Many vesicles and other molecules travel along nerve axons by fast axonal transport, using the microtubule cytoskeleton of the nerve cells as a track for transport. One study has shown that fast axonal transport is not required for neuroinvasion by PrP^sc from peripheral infection in mice (6), so some other mechanism must be involved for PrP^sc transport. Perhaps the PrP^sc structure is replicated by converting normal PrP^c to PrP^sc in a domino fashion along the neurons, although no direct evidence exists to support this hypothesis (4).

Route to the Lymphatic System

In addition to accumulating in the intestinal ganglia, the PrP^sc prions also accumulate in the spleen and the Peyer’s patches of the small intestine. Transport of PrP^sc into the lumen of the gut probably occurs via M cells, and migrating dendritic cells (DCs) are responsible for transporting PrP^sc to the lymphoid tissues (7,8). DCs are hematopoietic cells that belong to the class of immune cells known as Antigen Presenting Cells (APCs). In the spleen, PrP^sc replication takes place in the germinal centers where PrP^sc colonizes the follicular dendritic cells that express PrP^c (FDCs; 9). FDCs are stationery cells that are not of hematopoietic origin. The replication in FDCs is dependent upon lymphotoxin B signaling (inducing differentiation of the FDCs) and B lymphocytes, although it is not necessary for the B lymphocytes to express PrP^c (10). In addition to delivering PrP^sc to the FDCs of the spleen, DCs could also deliver PrP^sc directly to nerves in the skin and liver (tissues in which DCs are closely associated with nerve axon terminals) or perhaps directly to central nervous system tissues (8).

2-Part Invasion

One model for neuroinvasion proposes a 2-part invasion process. First, migrating DCs transport prions to lymphoid organs where they colonize organs, such as the spleen and replicate. Second, they would invade the CNS from the peripheral sympathetic nerves that innervate the lymphatic organs. The questions that need to be answered in support of this model include: How are prions transported along SNS nerves, since fast axonal transport has been ruled out? What cells are required for prion replication in the lymphatic reticular system? Which cells must express PrP^c as a requirement for invasion? How are the prions transferred from the cells in the lymphatic organs to the sympathetic nerve endings? What is the mechanism of replication of PrP^sc?

Replication

Scientists have proposed two mechanisms to describe the conversion of PrP^c to PrP^sc (for review see 10). The first model, the refolding model, proposes that introduced PrP^sc serves as a template for the refolding of PrP^c to the aberrant, protease-resistant, β-sheet form. Presumably the kinetics for spontaneously adopting the PrP^sc structure are unfavorable and only the presence of the aberrantly folded protein can induce conversion. The second model is the “seeding” or “nucleation” model. In this model, an equilibrium exists between PrP^c and PrP^sc. If several PrP^sc molecules manage to form a “seed” of three or four ordered PrP^sc molecules, they can recruit other PrP^sc molecules and aggregate to form prion amyloid deposits. (Amyloid proteins are proteins that are birefringent upon Congo red staining; these proteins are now described as proteins that attain their energy minimum as highly ordered aggregates.) These seeds can fragment, forming more nuclei that in turn can recruit more PrP^sc and form additional aggregates. Prion replication has been achieved in vitro (12) in a manner conceptually similar to PCR amplification of DNA. Homogenates from hamster containing barely detectable PrP^sc were mixed with an excess of PrP^c. After cycles in which PrP^sc aggregates were sonicated, the sonicated pieces appeared to form seeds for converting more PrP^c to PrP^sc. It is unclear if this truly represents what happens in vivo. 

Read the other installments in this series.

Part I  Introduction to Prion Diseases and Biology  
Part II Neuroinvasion and Replication (this issue)
Part III Research: Transmission and Diagnostics

References  

1. Prusiner, S.B. (1982) Novel proteinaceous infectious particles cause Scrapie. Science 216, 136–44.
2. Blättler, T. et al. (1997) PrP-expressing tissue required for transfer of scrapie infectivity from spleen to brain. Nature 389, 69–73.
3. Glatzel, M. et al. (2001) Sympathetic innervation of lymphoreticular organs is rate limiting for prion neuroinvasion. J. Neurosci. 31, 25–44.
4. Glatzel, M. and Aguzzi, A. (2000) PrP^c expression in the peripheral nervous system is a determinant of prion neuroinvasion. J. Gen. Virol. 81, 2813–21.
5. Bartz, J.C. et al. (2003) Rapid prion neuroinvasion following tongue infection. J. Virol. 77, 583–91.
6. Kunzi, V. et al. (2002) Unhampered prion neuroinvasion despite impaired fast axonal transport in transgenic mice overexpressing four-repeat Tau. J. Neurosci. 22, 7471–77.
7. Huang, F-P. et al. (2002) Migrating intestinal dendritic cells transport PrPsc from the gut. J. Gen. Virol. 83, 267–71.
8. Luhr, K.M. et al. (2002) Processing and degradation of exogenous prion protein by CD11c+ myeloid dendritic cells in vitro. J. Virol. 76, 12259–64.
9. Hill, A.F. et al. (1997) Diagnosis of new variant Creutzfeldt-Jakob disease by tonsil biopsy. Lancet 349, 99–100.
10. Nicotera, P. (2001) A route for prion neuroinvasion. Neuron 31, 345–8.
11. Aguzzi, A., Montrasio, F. and Kaeser, P.S. (2001) Prions: Health scare and biological challenge. Nat. Rev. 2, 118–26.
12. Saborio, G., Bruno, P. and Soto, C. (2001) Sensitive detection of pathological prion protein by cyclic amplification of protein misfolding. Nature 411, 810–13.

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