Functional assessment of TprC/D and TprK proteins of syphilis causing spirochete, Treponema pallidum

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SCIENTIFIC ABSTRACT Syphilis has an estimated global prevalence of 36M cases, with more than 11M new infections occurring per year around the world. In the United States alone, the number of cases of infectious syphilis has steadily increased since 2000, indicating that this disease is still a public health concern, particularly considering that it can lead to serious neurological and cardiovascular sequelae. Understanding the biological function of the outer membrane proteins (OMPs) of the syphilis agent, Treponema pallidum subsp. pallidum (T. pallidum), is one of the greatest challenges in the study of syphilis pathogenesis. Progress towards functional characterization of OMPs is hindered by the lack of an independent, pure culture system for T. pallidum, genetic intractability of the pathogen, the need to use of rabbits for bacterial propagation, the paucity of OMPs in T. pallidum outer membrane, and the fragility of this cellular compartment. Despite so many limitations, undertaking the study of T. pallidum OMPs can greatly enhance our understanding of the role of these virulence factors in syphilis pathogenesis and even provide important clues on new approaches to successfully control syphilis, such as by developing a protective vaccine. Twelve of putative OMPs/virulence factors belong to the T. pallidum repeat (Tpr) family, which are a group of highly immunogenic proteins predicted to be homologous to the Treponema denticola (T. denticola) major sheath protein (Msp), a surface virulence factor with porin and adhesin properties. Limited or no experimental evidence is available until now regarding functions of various Tprs. Based on homology to Msp, structural models for the Tprs and our preliminary data, we hypothesize that several Tprs also have dual roles as adhesins and porin transporters. T. pallidum and Borrelia burgdorferi, which causes Lyme disease, are physiologically and structurally related spirochetes. Unlike T. pallidum, B. burgdorferi can be genetically manipulated and transformed to express selected Tprs as if they were constituents of its OM. The validity of the surrogate system to study Tprs is supported by our preliminary data showing that when TprD2 and TprK are expressed on the surface of a non-infectious and poorly adherent B. burgdorferi strain, these proteins enhance adhesion to Glioma and epithelial cells and facilitate amino acids and peptides uptake. Here, we plan to further test our hypothesis and establish function of these important virulence factors by using both our well- tried B. burgdorferi, as well as the T. denticola msp mutant surrogate systems. We will conduct following studies to test our hypothesis: (1) Determine whether T. pallidum TprC, D2, and K proteins mediate attachment to host cells in differential manner by recognizing the specific host receptors/ECM components on each cell type, and (2) Determine the role of T. pallidum TprC, D2, and K proteins as channels involved in nutrient transport. Significance: Our studies will lead to a better understanding of the molecular basis of T. pallidum adhesion to host components, which is a pivotal step in congenital transmission and neuroinvasion during infection and will provide important information to further support the use of these antigens as vaccine candidates in the future.