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From: TSS ()
BSE: How Risky is it to Drink Milk? Prion proteins found in commercial milk In a first-time global breakthrough, a Swiss start-up firm has succeeded in detecting prion proteins in the milk of humans, cows, sheep, and goats. This again raises the question of a "mad cow disease" risk from drinking milk. Tests are underway to verify disease-causing prions in milk. Prions are known to be causes of neurological conditions such as Mad Cow disease (BSE) and Creutzfeldt-Jakob disease in human beings. The causative agent destroys the central nervous system in humans and animals. It is known that prions can also emerge in body fluids such as blood and be transferred by them. In the past it was difficult to estimate the risk of an infection through blood transfusion or drinking milk, since the concentration of prions in body fluids is very low; nor is there a sensitive method to identify prions. Moreover, the incubation time for infection in human beings can take 10 years or longer. Prion proteins in milk from the supermarket From a consumer standpoint, milk and milk products were regarded as safe up till now. But this situation could change, since a team of scientists from the biotechnology firm Alicon AG, headquartered at Schlieren in Canton Zurich, has managed for the first time to detect prion proteins in the milk of human beings, cows, sheep, and goats. Using Alicon's new technology, prion proteins were even found in homogenized and pasteurized milk on supermarket shelves, as reported recently in the international science journal Public Library of Science (PLoS ONE)(1). In the case of the prion proteins detected, it is highly likely that they were of the normal variety posing no danger to health. However, the occurrence of the normal variety could mean that the milk of cows already infected with BSE also contains infectious prion proteins [i.e., prions] of the disease-causing variety. Alicon's head of research, Dr. Ralph Zahn, comments: "So far there has been no scientific basis for assuming that only 'healthy' prion proteins are present in milk and those causing disease were not." Hence, beside blood and urine, milk is another body fluid in which prions causing disease could be present. As an American team of scientists has shown recently, infectious prions even arise in saliva. Further research with infected animals This finding coincides with a study conducted recently in this field by a Swiss prion researcher Adriano Aguzzi, an internationally recognized specialist. Aguzzi's research team at the University of Zurich showed that inflamed udder tissue found in sheep diseased with scrapie (a prion-induced ovine disease) actually contains infectious prions. The Alicon team is now continuing research work on the milk of infected animals. Research findings will show if milk is a potential carrier of BSE. A possible answer to such a threat is introduction of a live BSE test and simultaneous scanning of dairy cows with BSE at an early stage. Alicon is offering the equipment needed for this. At present its team is working on completion of a rapid test to identify prions in milk and milk products. Thumbnail sketch of Alicon AG The biotechnology firm Alicon AG is a successful spin-off from the Swiss Federal Institute of Technology in Zurich. Alicon AG was founded in 2004 and specializes in innovative technologies to identify and filter prions. In the diagnostics sector, Alicon concentrates on development and marketing of the first BSE live test. It seeks to increase bio-security in meat and milk products. Alicon is developing filtration-technology products to remove prions from body fluids such as blood plasma and urine. These will increase bio-security in medications. Alicon technology is patented worldwide and allows the firm to market it exclusively worldwide. (1) Sources Franscini, N, A.E. Gedaily, U. Matthey, S. Franitza, M. Sy, et al. (2006) Prion Protein in Milk. PLoS ONE 1(1): e71. doi:10.1371/journal.pone.0000071 Contact information Dr. Ralph Zahn SOURCE Alicon AG http://www.genengnews.com/news/bnitem.aspx?name=12293532 Subject: Prion Protein in Milk Nicola Franscini,1 Ahmed El Gedaily,1 Ulrich Matthey,1 Susanne Franitza,1 Man-Sun Sy,2 Alexander Bürkle,3 Martin Groschup,4 Ueli Braun,5 and Ralph Zahn1* 1Alicon AG, Schlieren, Switzerland University of Liverpool, United Kingdom * To whom correspondence should be addressed. E-mail: info@alicon.ch Prion protein is highly conserved among mammals [6]. It is primarily synthesized in cells of the central nervous system [7], but is also abundantly expressed in several peripheral tissues [8], [9]. An amino-terminal signal sequence targets prion protein to the endoplasmatic reticulum, where it transits the Golgi and ultimately reaches the external surface of the cell membrane [10]. There it is attached to a carboxy-terminal glycosyl phosphatidylinositol anchor [11]. The mature bovine protein of 217 amino acids contains two consensus acceptor sites for addition of N-linked polysaccharides [12]. Prion proteins (PrPC and PrPSc) have been detected in the cellular fraction of blood [13]–[17], but so far not in milk [18]–[21]. Considering that milk and milk products represent a major component of human nutrition it seems of particular importance to analyze milk for the presence of prion proteins. A first step in this direction is to determine the amount of PrPC in milk of healthy animals. If milk contains a significant amount of PrPC, this could indicate that PrPSc might be present in so far undetectable amounts in milk of TSE infected animals. However, the high concentration of total protein (about 40 mg/ml) and the high amount of lipids (about 35 mg/ml) in the milk make prion protein analysis by common biochemical methods demanding. We have therefore developed an adsorption matrix, Alicon PrioTrap®, which binds with high affinity and specificity to prion proteins PrPC and PrPSc. The exceptional binding properties of Alicon PrioTrap® result from hydrophilic and hydrophobic surface clusters that recognize different prion protein epitopes, allowing quantitative enrichment of extreme low quantity of prion proteins in body fluids and in biological tissues. To confirm specificity of immunochemical detection of PrPC in milk, we compared different anti-PrP monoclonal antibodies, which are directed against non-overlapping epitopes (Figure 2): PrP−mab 8B4 binds to residues 37−44 of mouse PrP [22]; mAB 6H4 targets residues 144−152 [23]; and PrP−mab 8H4 binds to residues 175−185 [24]. The three antibodies recognize the same proteins and thus confirm the presence of PrPC in milk. In control experiments, with non-PrP antibodies, e.g., anti-Tau protein monoclonal antibody (Chemicon International) (Figure 2) and anti-Aβ monoclonal antibody (Calbiochem, Germany; data not shown), none of the PrPC isoforms was detected, thus confirming binding specificity of the anti-PrP monoclonal antibodies. An interesting observation with regard to antibody 8B4 is its “clear” detection profile when compared to 6H4 and 8H4 antibodies. This can be rationalized by 8B4 not recognizing a variety of carboxy-terminal fragments of milk PrPC, which appear as smear in the Western Blot. We further compared the glycoforms of native prion protein in cow milk with those of bovine brain, a tissue where prion protein expression is well characterized. The glycoforms were identified by digestion with PNGase (Figure 3), an enzyme that cuts off oligosaccharides from N-linked glycoproteins, e.g., the two N-linked sugars of PrPC [12]. After partial cleavage with PNGase the upper PrP-isoform in the Western Blot representing diglycolysated PrPC (34 kD) disappears in favour of monoglycosylated (30 kD) and nonglycosylated PrPC (27 kD). In parallel, there seems to be a small shift from the higher molecular weight monoglycosylated form to the lower molecular weight form. A slight downshift of the monoglycosylated PrPC is also observed for brain homogenate after PNGase treatment (Figure 3). The diglycosylated PrPC isoforms differ slightly in molecular mass, indicating that carbohydrate structure of PrPC in milk and brain may not be identical. More stringent reaction conditions result in complete truncation of carbohydrates from PrPC. Most importantly, the apparent molecular masses of nonglycosylated PrPC in milk exactly matches with that of the corresponding PrPC in brain homogenate. Alicon PrioTrap® can also be applied for elimination of prion protein from milk. As shown in Figure 4, after the first treatment of 10 ml milk with Alicon PrioTrap® more than 95% of endogenous PrPC was already removed, and after the second treatment PrPC was completely eliminated. However, the overall protein concentration (measured by bicinchoninic acid assay, Pierce) was constant with about 40 mg/ml before and after PrPC elimination. The protein composition of milk as analyzed by SDS PAGE (Figure 4) was not affected either. Prion protein was also completely removed, when milk was spiked with PrPSc from mouse Rocky Mountain Laboratory (RML) brain homogenate (data not shown). Thus, Alicon PrioTrap® can be used for enrichment and detection of overall prion protein in milk, but also for complete removal of prions. The presence of PrPC in blood has been documented [14], [15], and is confirmed by our own unpublished observations. To produce one liter of milk, about 400 to 500 liters of blood must pass through the udder of a cow. It is thus possible that the PrPC found in milk derives from blood cells or, alternatively, has been secreted from glandular epithelial cells. Cell types that have been identified in milk from healthy cows are mainly macrophages, and other leucocytes. However, in our assay cells are completely removed by centrifugation. Therefore, the recovered PrPC is not cell associated but most likely binds to other proteins or lipids resulting in stable molecular complexes. The fact that milk contains full-length PrPC, very likely comprising the glycolipid anchor, indicates that prion protein was originally cell-bound and does not represent any of the amino-terminal truncation products of PrPC known to be released from normal cells under physiological conditions [25]. The detection of such a considerable expression of cell membrane bound or derived PrPC in milk constitutes one of the key requirements for the generation of infectious prions in the udder of infected animals. Over the last 10 years, scientific groups, risk assessment agencies, and public health organizations have debated the TSE risk for milk and milk products [26], [27]. Epidemiological and bioassay data so far available have not provided evidence for milk to harbour prion infectivity and infectious prions have as yet not been detected by bioassays in the milk, colostrum or udder of clinical BSE cases in cow [18]–[21]. However, a recent statement of the European Food Safety Authority affirmed that based on a number of observations from research data, there are indications that infectivity in the milk from small ruminants can not be totally excluded [28]. Furthermore, the exclusion of animals with mastitis, an inflammation of the mammal gland, being able to destabilize the blood-milk barrier, is considered a measure able to reduce but not to eliminate the potential contamination risk [28]. The rational of this conclusion is confirmed by a recent study showing that in sheep naturally affected with both scrapie and lymphocyte or lymphofollicular mastitis, PrPSc accumulation was present in lymphoid follicles adjacent to milk ducts [29]. At least in natural sheep scrapie, prion replication can occur following a lymphotropic virus infection in the inflamed mammary gland. This study has not detected PrPSc or prion infectivity in milk itself. However, since under such inflammatory conditions, the total number of immune cells increases in milk of animals, it might be possible that infectious PrPSc is also passing through and reaches the milk. In this context milk from such animals could possibly be responsible for the spread of scrapie from the ewes to their offspring in affected sheep or goat flocks. Moreover, sheep and goat milk could also constitute a TSE exposure risk for mammals (humans) consuming these products. The former Scientific Steering committee of the European Commission and the European Food Safety Authority recommend that research should intensify on the safety of milk of small ruminants with regard to TSE risk. Limited new data are expected to be published in the near future and there is still little research initiated in this area [28]. The Alicon PrioTrap® technology opens a new avenue for studying the biochemical characteristics of prion protein in milk and thus may contribute to offer a feasible approach to perform an appropriate study on the milk safety with regard to TSE risk. Immunochemical PrPC detection We thank Theo Bearth, Roland Ermini, Peter May, Günter Kreissel, and Mathias Schmid for their generous support. We are grateful to Torsten Seuberlich from the NeuroCenter of the University of Bern for kindly providing us brain homogenate from cattle. Competing Interests: The authors have declared that no competing interests exist. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=17183703 TSS http://disc.server.com/discussion.cgi?disc=236650;article=169;title=CJD%20DISCUSSION%20BOARD http://disc.server.com/discussion.cgi?disc=236650;article=170;title=CJD%20DISCUSSION%20BOARD TSS
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