Location: Produce Safety and Microbiology Research2020 Annual Report
Transmissible spongiform encephalopathies (TSEs) are animal diseases caused by infectious prion proteins that result in a slow progressive neurodegeneration that is fatal. The observation that prion diseases can be transmitted between animals, including humans, by consumption of contaminated food necessitates strategies to mitigate their occurrence in the food supply. In support of existing public health and food safety measures the USDA conducts TSE surveillance of suspect animals to monitor the incidence of the disease in the livestock population. TSE diagnostic surveillance is dependent on the detection of infectious prions in post mortem brain samples by immunoassay. The limitations of current prion immunoassays necessitate the development of improved prion detection methods that can reliably monitor the: 1) effectiveness of the ruminant feed ban, 2) spontaneous occurrence of disease, and 3) zoonotic transmission of TSE from endemic reservoirs of disease. The objective of this research is to develop immunodiagnostic methodologies that improve the effectiveness of TSE surveillance of livestock. Objective 1: Define methodologies for tissue specific prion sample enrichment to increase immunoassay sensitivity. Subobjective 1.1: Define methods to enrich prions from fresh or frozen tissues. Subobjective 1.2: Define methods to enhance prion detection from aldehyde preserved tissues. Subobjective 1.3: Define methods to enrich prions from decomposed tissues. Objective 2: Generate and validate improved prion monoclonal antibodies to achieve increased selectivity and sensitivity. Subobjective 2.1: Inoculate Prnp(0/0) Balbc/J mice with purified infectious prions and perform hybridoma fusions to generate conformation specific anti-prion monoclonal antibodies. Subobjective 2.2: Characterize the biochemical properties and validate binding specificity of anti-prion monoclonal antibodies. Objective 3: Develop and deploy applied immunoassays for prion detection. Subobjective 3.1: Develop prion immunoassays and evaluate detection sensitivity in agriculturally relevant models. The project will generate transferable technologies useful in the diagnosis of TSEs and the detection of low-level infectious prions in livestock tissues. These technologies will facilitate ante mortem TSE detection tests that will enhance our understanding of TSE disease prevalence in captive and wild animal populations. An effective ante mortem test for prion disease would also be valuable in the diagnosis of the closely related human Creutzfeldt-Jakob disease (CJD) and aid in the discovery of effective therapeutic interventions.
To overcome the obstacle of detecting low-level prions as a result of slow prion propagation following initial infection and allow sampling of non-neuronal tissues for evaluation we will define methodologies for tissue specific prion sample enrichment. These enrichment methods will include the biochemical isolation of prions with lipid rafts from fresh or frozen tissue, the use of chemically mediated antigen retrieval from aldehyde fixed tissue, proteolytic degradation of interfering proteins from decomposing tissues, high molecular weight dialysis to retain large aggregate prion amyloid, and chemical precipitation to concentrate prion enriched samples. The application of these methods will result in an increased yield of prions from target tissues and improve the reliability of prion detection measures. The properties of prion antibodies dictate the sensitivity and selectivity of prion immunoassays used in the determination of disease status. To generate improved prion monoclonal antibodies (mAbs) we will use highly purified prion immunogens, genetically engineered prion-knockout mice, hybridoma technology, and stringent screening methods for the identification of high-affinity anti-prion mAbs. Identified mAbs will be evaluated for prion binding that includes: epitope mapping, affinity measurements, species and strain specificity, and immunoassay application. Rigorous selection criteria will be used to identify high-affinity conformation- dependent anti-prion mAbs for development of enhanced prion immunoassays. Effective and reliable TSE surveillance depends on the sensitive detection of infectious prions by immunoassay. Applied prion tissue enrichment methodologies along with improved anti-prion monoclonal antibodies will be used to develop and optimize immunoassays for prion detection. Construction and deployment of various immunoassay platforms and antibody conjugated reporters (enzymatic, colorimetric, and fluorescent) will address end user needs for sensitive tissue specific prion detection that include: enzyme-linked immunosorbant assay (ELISA), field deployable lateral flow immunoassay (LFIA), Western blotting (WB), and immunohistochemistry.
This report is for a bridging project that began on March 1, 2018 and continues research from project 2030-32000-009-00D. In fiscal year 2020, scientific resources were also put toward addressing Objective 5, "Development of immuno-, bacteriophage-, and mass spectrometry-based methods for rapid detection of foodborne pathogens," in the 2030-42000-050-0D project. Please see that project's report for additional information. Research continued on distinguishing among strains of chronic wasting disease (CWD). CWD is a prion disease spreading throughout the United States, impacting 10 million hunters, and threatening a $38 billion industry. Scientists recently discovered that some CWD strains can infect mice and deer. Scientists from the ARS in Albany, California, in collaboration with their Canadian colleagues, developed a mass spectrometry-based method of distinguishing among CWD strains. They were able to distinguish among CWD strains that infect deer alone or both mice and deer. Such information can be used detect the emergence of new strains of CWD and inform regulatory agencies of their emergence. Research also continued on improving our understanding of prions. ARS scientists used chemical reagents to react with the lysines in prions and then analyzed the results. Prions remained infectious and retained their shape after reacting with the reagents. Researchers can now use chemical reagents to help define the shape of a prion.
1. Selective covalent modification to study prion conformation. Prions are infectious proteins which cause devastating disease and whose infectivity results from a characteristic shape or conformation. Despite intensive research, the structure of a prion remains elusive. ARS scientists in Albany, California, used chemical reagents to react with the lysines in prions and then analyzed the results. Prions remained infectious and retained their shape after reacting with the reagents. This means that the structural information derived from this analysis is relevant to a prion’s structure. These and other reagents can be used to develop new prion-specific diagnostics to help farmers, ranchers, and regulators identify infected animals and help researchers refine their computer models of the prion structure.
Silva, C.J., Erickson-Beltran, M.L., Duque Velásquez, C., Aiken, J.M., McKenzie, D. 2019. A general mass spectrometry-based method of quantitating prion polymorphisms from heterozygous chronic wasting disease-infected cervids. Analytical Chemistry. 92(1):1276-1284. https://doi.org/10.1021/acs.analchem.9b04449.