Degradation of the Disease-Associated Prion Protein by a Serine Protease from Lichens

Christopher J. Johnson,Richard A. Bessen,Cynthia M. Rodriguez,Steven M. Biro,Tonie E. Rocke,Juan Camilo Duque-Velasquez,James P. Bennett,Jason C. Bartz

doi:10.1371/journal.pone.0019836

Abstract

The disease-associated prion protein (PrPTSE), the probable etiological agent of the transmissible spongiform encephalopathies (TSEs), is resistant to degradation and can persist in the environment. Lichens, mutualistic symbioses containing fungi, algae, bacteria and occasionally cyanobacteria, are ubiquitous in the environment and have evolved unique biological activities allowing their survival in challenging ecological niches. We investigated PrPTSE inactivation by lichens and found acetone extracts of three lichen species (Parmelia sulcata, Cladonia rangiferina and Lobaria pulmonaria) have the ability to degrade prion protein (PrP) from TSE-infected hamsters, mice and deer. Immunoblots measuring PrP levels and protein misfolding cyclic amplification indicated at least two logs of reductions in PrPTSE. Degradative activity was not found in closely related lichen species or in algae or a cyanobacterium that inhabit lichens. Degradation was blocked by Pefabloc SC, a serine protease inhibitor, but not inhibitors of other proteases or enzymes. Additionally, we found that PrP levels in PrPTSE-enriched preps or infected brain homogenates are also reduced following exposure to freshly-collected P. sulcata or an aqueous extract of the lichen. Our findings indicate that these lichen extracts efficiently degrade PrPTSE and suggest that some lichens could have potential to inactivate TSE infectivity on the landscape or be a source for agents to degrade prions. Further work to clone and characterize the protease, assess its effect on TSE infectivity and determine which organism or organisms present in lichens produce or influence the protease activity is warranted.

Highlights

Transmissible spongiform encephalopathies (TSEs) are a group of infectious, neurodegenerative diseases including bovine spongiform encephalopathy, sheep scrapie, cervid chronic wasting disease (CWD) and human Creutzfeldt-Jakob disease (CJD) [1]
We found that following incubation, prion protein (PrP) levels were reduced in brain homogenate (BH) supernatants exposed to P. sulcata, compared to BH incubated with P. squarrosa or in the absence of lichen
We found that extracts of three lichen species have a serine protease activity capable of reducing PrP levels in PrPTSE-enriched preps or infected BH in vitro

Summary

Introduction

Transmissible spongiform encephalopathies (TSEs) are a group of infectious, neurodegenerative diseases including bovine spongiform encephalopathy, sheep scrapie, cervid chronic wasting disease (CWD) and human Creutzfeldt-Jakob disease (CJD) [1]. The resilience of TSE infectivity to inactivation has led to unexpected instances of disease transmission, such as CJD cases caused by contaminated surgical instruments subjected to conventional methods of cleaning and steam sterilization [4]. Scrapie and CWD differ from other TSEs in that epizootics can be maintained by horizontal transmission, as well as being mediated by an environmental reservoir of infectivity [5,6]. Naıve sheep and deer have been infected following habitation in environments contaminated many years prior and, with a lack of evidence for vector-mediated transmission, environmental fomites have been implicated in the spread of these diseases [7,8,9]. Microorganisms, proteases and manganese minerals have all been suggested to have the potential to reduce prion infectivity on the landscape or in engineered systems [12,13,14]

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Conclusion