Transmission of prions

Abstract

The "protein only" hypothesis states that the infectious agent causing transmissible spongiform encephalopathies is a conformational isomer of PrP, a host protein predominantly expressed in brain, and is strongly supported by many lines of evidence. Prion diseases are so far unique among conformational diseases in that they are transmissible, not only experimentally but also by natural routes, mainly by ingestion. A striking feature of prions is their extraordinary resistance to conventional sterilization procedures, and their capacity to bind to surfaces of metal and plastic without losing infectivity. This property, first observed in a clinical setting, is now being investigated in experimental settings, both in animals and in cell culture.

Figure 1

Models for the conformational conversion of PrP^C to PrP^Sc. (A) The “refolding” model. The conformational change is kinetically controlled, a high activation energy barrier preventing spontaneous conversion at detectable rates. Interaction with exogenously introduced PrP^Sc causes PrP^C to undergo an induced conformational change to yield PrP^Sc. This reaction could be facilitated by an enzyme or chaperone. In the case of certain mutations in PrP^C, spontaneous conversion to PrP^Sc may occur as a rare event, explaining why familial CJD or GSS arise spontaneously, albeit late in life. Sporadic CJD may come about when an extremely rare event (occurring in about one in a million individuals per year) leads to spontaneous conversion of PrP^C to PrP^Sc. (B) The “seeding” model. PrP^C and PrP^Sc (or a PrP^Sc-like molecule, light) are in equilibrium, with PrP^C strongly favored. PrP^Sc is stabilized only when it adds onto a crystal-like seed or aggregate of PrP^Sc (dark). Seed formation is rare; however, once a seed is present, monomer addition ensues rapidly. To explain exponential conversion rates, aggregates must be continuously fragmented, generating increasing surfaces for accretion.

Figure 2

Possible routes of propagation of ingested prions. After oral uptake, prions may penetrate the intestinal mucosa through M cells and reach Peyer's patches as well as the enteric nervous system. Depending on the host, prions may replicate and accumulate in spleen and lymph nodes. Myeloid dendritic cells are thought to mediate transport within the lymphoreticular system. From the lymphoreticular system and likely from other sites, prions proceed along the peripheral nervous system to finally reach the brain, either directly via the vagus nerve or via the spinal cord, under involvement of the sympathetic nervous system.

Figure 3

Transmission of mouse scrapie prions by stainless steel wire. Stainless steel wires were inserted into the brains of scrapie-infected mice for 5, 30, or 120 min, washed exhaustively, and introduced permanently into brains of indicator mice. Five minutes of contact sufficed for the wire to acquire a maximum load of infectivity, equivalent to the injection of 30 μl of 1% homogenate of the same brain. Data from ref. .

Figure 4

Infection of mouse neuroblastoma cells by plastic-bound prions. Wells of a polystyrene 96-well microtiter plate were exposed to various dilutions of a homogenate of scrapie-prion-infected mouse brain, washed exhaustively, and dried. Ten thousand N2a/Bos2 mouse neuroblastoma cells (77) were cultured in the wells for 3 days, then transferred to 24-well plates and cultured 4 wk, splitting 1:10 twice a week. The cells were then transferred to coverslips and assayed for the presence of PrP^Sc (78). Optimal infectivity resulted when plates were coated with 0.0125% homogenate. High concentrations of brain proteins bound to the plastic appear inhibitory for cell infection (P.-C.K. and C.W., unpublished results).

Figure 5

Neuroblastoma cells are infected by contact with prion-coated stainless steel wires. (A) Stainless steel wires were exposed to scrapie-infected brain homogenates, washed, and placed on a confluent layer of neuroblastoma cells. After periods ranging from 1 to 14 days, the wire, to which a few cells had attached, were placed on a coverslip in a separate well and cultured for further 14 days. Both the cells remaining in the original dish (“remaining cells”) and those derived from the cells clinging to the wire were assayed for PrP^Sc by the cell blot assay (78) and the mouse bioassay. (B Left) The cultures derived from wire-bound cells have been infected, as evidenced by the accumulation of PrP^Sc, whereas residual cells remain uninfected. (Right) The location of cells as revealed by ethidium bromide staining. UN, Blank wire; UB, wire treated with uninfected brain homogenate (M.E. and C.W., unpublished results).