Dr. Boris Striepen recently joined Penn Vet as a professor in the Department of Pathobiology. Boris studied biology in Bonn and Marburg, Germany and conducted undergrad research on liver flukes in Bonn and cattle trypanosomes in Bobo Dioulasso, Burkina Faso. After earning his Ph.D. with Ralph Schwarz in Marburg, Boris was a postdoc with David Roos at the University of Pennsylvania, after which he joined the faculty of the Center for Tropical & Emerging Global Diseases at the University of Georgia (UGA), where he last served as a Distinguished Research Professor. In addition, Boris has been an instructor at the Marine Biological Laboratory (MBL) Biology of Parasitism summer research course since 2001 and served as course director.

Toxoplasma—a somewhat surprising model organism

When Boris considered graduate projects, his mentor suggested focusing on Toxoplasma gondii, a pathogen that was relatively obscure at the time and was studied by only a tiny research community. Boris had spent time in West-Africa and was much more taken with parasites that caused ‘tropical’ diseases like malaria or sleeping sickness. However, the Toxoplasma project was a collaboration with Jean-François Dubremetz, so Dr. Schwarz was able to entice Boris by cleverly offering to send him to France to learn from the world’s leading Toxoplasma biologists. Toxoplasma infection in humans is very common, but is largely asymptomatic unless the patient is immunosuppressed or infected in utero, in which case it can have devastating consequences. In his dissertation studies, Boris discovered that an important antigen used to differentiate acute infection in pregnant women (which threatens the fetus) from chronic infection (which does not), is not a protein, but actually from a family of glycolipids. He resolved their molecular structures, mapped the epitopes, and delineated the biosynthetic pathways leading to their assembly [1, 2].

Boris’ decision to study Toxoplasma turned out to be timely, as at the time of his thesis defense, the laboratories of Drs. J. Boothroyd, D. Roos, and L. Sibley established genetic manipulation for Toxoplasma. This made Toxoplasma the first genetically tractable apicomplexan parasite and allowed the field to take off. Boris joined Dr. David Roos’s lab to learn how to transfect parasites, and developed the first Toxoplasma expressing fluorescent proteins. He used this model to determine how the parasite sorts and targets its proteins to a variety of critical organelles. In his own laboratory at UGA he built a program that expanded and harnessed the genetic Toxoplasma model to dissect fundamental aspects of parasite biology, addressing such questions as: How are parasites built and assembled? What are the mechanistic nuts and bolts of organelle biogenesis and parasite replication [3, 4] and how does parasite metabolism feed these processes?

Cryptosporidium, an important but challenging problem

The current focus of the Striepen group is Cryptosporidium. This parasite related to Toxoplasma is an important cause of diarrheal disease in the US. The incidence of Cryptosporidium in the US is rising and the Center for Disease Control (CDC) is estimating 750,000 annual cases and implicating the parasite in >50% of waterborne outbreaks. Cryptosporidium is resistant to water chlorination and listed as a category B bioterrorism agent. Globally, Cryptosporidium is the second leading cause of severe diarrheal disease in small children, second only to Rotavirus and an important contributor of child mortality (~10% of under-five mortality is due to diarrhea). There is no fully effective treatment and no vaccine. A main block to such advances has been the poor tractability of the parasite system with significant limitations in culture, animal model and genetics [5].

Molecular genetics to enable drug development and fundamental biology

Following a decade-long effort, the Striepen lab pioneered molecular genetics in Cryptosporidium in 2015 [6] by engineering reporter parasites expressing luciferases and fluorescent proteins. These tools enabled a collaboration with the Novartis Institute for Tropical Diseases to screen a library of anti-malaria compounds for potential activity against Cryptosporidium. Importantly, these studies led to the discovery and development of a PI4K inhibitor that cures cryptosporidiosis in mice and calves [7]. This candidate is now slated for first in man clinical studies. However, while the last three years have seen unprecedented progress in Cryptosporidium drug development, critical knowledge of their mode of action remains scant. As part of the Bill & Melinda Gates Foundation funded Cryptosporidium Drug Accelerator program the Striepen laboratory uses molecular genetics to validate targets, link targets to compounds, and further understanding of parasite metabolism to identify vulnerabilities that could be targeted for drug development.

Cryptosporidium has a single host life cycle, with both asexual and sexual processes occurring sequentially in the intestinal epithelium of the same host. Completion of this developmental program is required for continued infection, severe disease, and spread of the pathogen. Despite the fundamental importance of the lifecycle for drug and vaccine development, current understanding of the mechanism is rudimentary. The Striepen laboratory dissects the parasite’s lifecycle at the molecular and cellular level, attempting to answer important questions such as How do gametes develop and find each other within the cells of their host and how could one genetically interrupt the process? Identification of such mutants will not only drive fundamental insight but could also be starting points for attenuation.

Drugs are terrific—a vaccine could be better

Although epidemiological and experimental studies support the likely development of immune-mediated resistance to Cryptosporidium, single exposure does not consistently produce sterilizing immunity and host nutritional status, and host and parasite genetic diversity likely modulate resistance. Moreover, the immunological mechanisms required for control and protection are not well defined, as rodent studies have been challenged by the fact that only severely immune-compromised mice are susceptible to infection with common Cryptosporidium strains. To remedy this situation, the Striepen laboratory derived a natural mouse model for Cryptosporidium infection from parasites isolated from ‘wild’ mice. These parasites are genetically tractable and produce self-limiting infection of the small intestine in immunocompetent mice. Importantly, these mice are protected from reinfection and immunity is transferable. This has opened Cryptosporidium immunology to rigorous experimentation and it is the group’s hope that this may close critical knowledge gaps towards vaccination.

Dr. Striepen’s laboratory is located on the 3rd Floor, Hill Pavilion and his office is found at 317 Hill Pavilion. Research in the Striepen laboratory is supported by the National Institutes of Health (NIH): R01AI112427, R01AI127798, R01AI125362, F32AI124518, and the Bill & Melinda Gates Foundation: OPP1183177 and OPP1161001.

References

1. Striepen, B., Zinecker, C.F., Damm, J.B., Melgers, P.A., Gerwig, G.J., Koolen, M., Vliegenthart, J.F., Dubremetz, J.F., and Schwarz, R.T. (1997). Molecular structure of the "low molecular weight antigen" of Toxoplasma gondii: a glucose alpha 1-4 N-acetylgalactosamine makes free glycosyl- phosphatidylinositols highly immunogenic. J Mol Biol 266, 797-813.
2. Striepen, B., Dubremetz, J.F., and Schwarz, R.T. (1999). Glucosylation of glycosylphosphatidylinositol membrane anchors: identification of uridine diphosphate-glucose as the direct donor for side chain modification in Toxoplasma gondii using carbohydrate analogues. Biochemistry 38, 1478-1487.
3. Francia, M.E. and Striepen, B. (2014). Cell division in apicomplexan parasites. Nat Rev Microbiol 12, 125-136.
4. van Dooren, G.G. and Striepen, B. (2013). The algal past and parasite present of the apicoplast. Annu Rev Microbiol 67, 271-289.
5. Striepen, B. (2013). Parasitic infections: Time to tackle cryptosporidiosis. Nature 503, 189-191.
6. Vinayak, S., Pawlowic, M.C., Sateriale, A., Brooks, C.F., Studstill, C.J., Bar-Peled, Y., Cipriano, M.J., and Striepen, B. (2015). Genetic modification of the diarrhoeal pathogen Cryptosporidium parvum. Nature 523, 477-480.
7. Manjunatha, U.H., Vinayak, S., Zambriski, J.A., Chao, A.T., Sy, T., Noble, C.G., Bonamy, G.M.C., Kondreddi, R.R., Zou, B., Gedeck, P., et al. (2017). A Cryptosporidium PI(4)K inhibitor is a drug candidate for cryptosporidiosis. Nature 546, 376-380.