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In the first installment
of this series, we introduced prions, the agents that cause transmissible spongiform encephalopathies (TSEs) in a variety of animals ranging from sheep to deer to humans. We described the structure of the prion agent and discussed the history of several of the fatal neurodegenerative diseases caused by prions. In this second installment, we will explore the mechanisms that prions use to invade the central nervous system (CNS) and replicate their pathologic structure. 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.
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 PrPc-expressing (PrPc = 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
(PrPsc) 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 PrPsc expression in the
hypoglossal nerve as soon as 2 days after tongue inoculation (5). These data
suggest that PrPsc travels from the tongue along the
hypoglossal nerve to the brain. Taken together, these observations suggest that
PrPsc 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
PrPsc from peripheral infection in mice (6), so some other mechanism must be involved for
PrPsc transport. Perhaps the PrPsc structure is replicated by converting normal
PrPc to PrPsc in a domino fashion along the neurons, although no
direct evidence exists to support this hypothesis (4).
In addition to accumulating in the intestinal ganglia, the PrPsc prions also accumulate in the spleen and the Peyer’s patches of the small intestine. Transport of PrPsc into the lumen of the gut probably occurs via M cells, and migrating dendritic cells (DCs) are responsible for transporting PrPsc 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, PrPsc replication takes place in the germinal centers where PrPsc colonizes the follicular dendritic cells that express PrPc (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 PrPc (10). In addition to delivering PrPsc to the FDCs of the spleen, DCs could also deliver PrPsc 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).
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 PrPc 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 PrPsc?
Scientists have proposed two mechanisms to describe the conversion of PrPc to PrPsc (for review see 10). The first model, the refolding model, proposes that introduced PrPsc serves as a template for the refolding of PrPc to the aberrant, protease-resistant, β-sheet form. Presumably the kinetics for spontaneously adopting the PrPsc 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 PrPc and PrPsc. If several PrPsc molecules manage to form a “seed” of three or four ordered PrPsc molecules, they can recruit other PrPsc 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 PrPsc 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 PrPsc were mixed with an excess of PrPc. After cycles in which PrPsc aggregates were sonicated, the sonicated pieces appeared to form seeds for converting more PrPc to PrPsc. It is unclear if this truly represents what happens in vivo.
References
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