The shape of infectious prions

Structural changes were located in the prion protein N-terminus, where a novel reorganization of the beta sheet (in yellow) was observed. In the background, the X-ray diffraction pattern of the crystal composed by the complex prion protein-Nanoboy.

Prions are unique infective agents -- unlike viruses, bacteria, fungi and other parasites, prions do not contain either DNA or RNA. Despite their seemingly simple structure, they can propagate their pathological effects like wildfire, by "infecting" normal proteins. PrPSc (the pathological form of the prion protein) can induce normal prion proteins (PrPC) to acquire the wrong conformation and convert into further disease-causing agents.

"When they are healthy, they look like tiny spheres; when they are malignant, they appear as cubes" stated Giuseppe Legname, principal investigator of the Prion Biology Laboratory at the Scuola Internazionale Superiore di Studi Avanzati (SISSA) in Trieste, when describing prion proteins. Prions are "misfolded" proteins that cause a group of incurable neurodegenerative diseases, including spongiform encephalopathies (for example, mad cow diseases) and Creutzfeldt-Jakob disease. Legname and coworkers have recently published a detailed analysis of the early mechanisms of misfolding. Their research has just been published in the Journal of the American Chemical Society, the most authoritative scientific journal in the field.

"For the first time, our experimental study has investigated the structural elements leading to the disease-causing conversion" explains Legname. "With the help of X-rays, we observed some synthetic prion proteins engineered in our lab by applying a new approach -- we used nanobodies, i.e. small proteins that act as a scaffolding and induce prions to stabilize their structure." Legname and colleagues reported that misfolding originates in a specific part of the protein named "N-terminal." "The prion protein consists of two subunits. The C-terminal has a clearly defined and well-known structure, whereas the unstructured N-terminal is disordered, and still largely unknown. This is the very area where the early prion pathological misfolding occurs" adds Legname. "The looser conformation of the N-terminal likely determines a dynamic structure, which can thus change the protein shape."

"Works like ours are the first, important steps to understand the mechanisms underlying the pathogenic effect of prions" concludes Legname. "Elucidating the misfolding process is essential to the future development of drugs and therapeutic strategies against incurable neurodegenerative diseases."

Source: Sissa Medialab

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Microbiologists discover how gut bacterial resources are hijacked to promote intestinal, foodborne illnesses

Dr. Vanessa Sperandio. Credit: Image courtesy of UT Southwestern Medical Center

UT Southwestern Medical Center microbiologists have identified key bacteria in the gut whose resources are hijacked to spread harmful foodborne E. coli infections and other intestinal illnesses.

Though many E. coli bacteria are harmless and critical to gut health, some E. coli species are harmful and can be spread through contaminated food and water, causing diarrhea and other intestinal illnesses. Among them is enterohemorrhagic E. coli or EHEC, one of the most common foodborne pathogens linked with outbreaks featured in the news, including the multistate outbreaks tied to raw sprouts and ground beef in 2014.

The UT Southwestern team discovered that EHEC uses a common gut bacterium called Bacteroides thetaiotaomicron to worsen EHEC infection. B. thetaiotaomicron is a predominant species in the gut's microbiota, which consists of tens of trillions of microorganisms used to digest food, produce vitamins, and provide a barrier against harmful microorganisms.

"EHEC has learned to how to steal scarce resources that are made by other species in the microbiota for its own survival in the gut," said lead author Dr. Meredith Curtis, Postdoctoral Researcher at UT Southwestern.

The research team found that B. thetaiotaomicron causes changes in the environment that promote EHEC infection, in part by enhancing EHEC colonization, according to the paper, appearing in the journal Cell Host Microbe.

"We usually think of our microbiota as a resistance barrier for pathogen colonization, but some crafty pathogens have learned how to capitalize on this role," said Dr. Vanessa Sperandio, Professor of Microbiology and Biochemistry at UT Southwestern and senior author.

EHEC senses changes in sugar concentrations brought about by B. thetaiotaomicron and uses this information to turn on virulence genes that help the infection colonize the gut, thwart recognition and killing by the host immune system, and obtain enough nutrients to survive. The group observed a similar pattern when mice were infected with their equivalent of EHEC, the gut bacterium Citrobacter rodentium. Mice whose gut microbiota consisted solely of B. thetaiotaomicron were more susceptible to infection than those that had no gut microbiota. Once again, the research group saw that B. thetaiotaomicron caused changes in the environment that promoted C. rodentium infection.

"This study opens up the door to understand how different microbiota composition among hosts may impact the course and outcome of an infection," said Dr. Sperandio, whose lab studies how bacteria recognize the host and how this recognition might be exploited to interfere with bacterial infections. "We are testing the idea that differential gastrointestinal microbiota compositions play an important role in determining why, in an EHEC outbreak, some people only have mild diarrhea, others have bloody diarrhea and some progress to hemolytic uremic syndrome, even though all are infected with the same strain of the pathogen."

The Centers for Diseases Control and Prevention (CDC) estimates that each year roughly 1 in 6 Americans (or 48 million people) gets food poisoning; 128,000 are hospitalized;, and 3,000 die of their food-borne disease. EHEC, which also caused a widespread outbreak in Europe in 2011, can lead to bloody diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome, which in turn can lead to kidney disease and failure. EHEC is among the top five pathogens contributing to domestically acquired foodborne illnesses resulting in hospitalization, according to the CDC. Outbreaks in 2014 were reported in California, Idaho, Massachusetts, Michigan, Missouri, Ohio, Montana, Utah, and Washington.

Source: UT Southwestern Medical Center

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