MOLECULAR DISSECTION OF BORRELIA BURGDORFERI BB0323 PROTEIN COMPLEX SUPPORTING MICROBIAL BIOLOGY, INFECTIVITY, AND AS A NOVEL THERAPEUTIC TARGET

Abstract

Lyme disease (LD), also known as Lyme borreliosis, is the most common vector-borne disease in the United States, caused by the gram-negative bacteria of the Borrelia burgdorferi sensu lato group. This atypical bacterial group features distinct genomic and antigenic elements, does not possess any classical toxins, and the pathogenesis of LD is primarily due to the immune activity of the host. These multi-organotrophic spirochetes can elicit severe clinical complications in susceptible hosts, including neuroborreliosis, carditis, and arthritis. If diagnosed early, the disease can be treated with a conventional antibiotic regimen; however, persistent, or relapsing symptoms later develop in a subset of patients. Six months to a year after the antibiotic treatment, up to 20% of the patients can experience various subjective symptoms pertaining to pain, cognitive dysfunction, or other neurological complications, collectively termed Post Treatment Lyme Disease Syndrome (PTLDS). The diagnosis, etiology, and treatment of PTLDS remain currently unknown. To better understand microbial pathogenesis, we have characterized a select set of structurally unique spirochete gene products that act as novel virulence determinants and support microbial infection in mammals. The current study focused on the BB0323 protein of B. burgdorferi, a unique and multifunctional virulence determinant undergoing a complex post-translational maturation process. The maturation, stability, and functions of BB0323 require multifaceted protein-protein interaction (PPI) events involving specific B. burgdorferi proteins, such as a protease-chaperone called BbHtrA, and a membrane-associated protein of unknown function annotated as BB0238. In our current study, we have further dissected the biological significances of the protein-protein interaction complex (PPI), either involving BbHtrA: BB0323 and BB0323:BB0238. The latter PPI event was more thoroughly investigated for its role in spirochete biology and infection and as a novel target for therapeutic intervention against B. burgdorferi infection. We identified a cleavage site where BB0323 full-length protein cleaves into N and C termini by BbHtrA. Subsequently, we have introduced point mutations in the recombinant BB0323 (at the cleavage site for BbHtrA- NL residues replaced with AA), as well as generated an isogenic B. burgdorferi isolates (Bbbb0323NL) with the point mutations in native BB0323. Further analyses show that the cleavage site mutated BB0323 protein could not be processed by the recombinant BbHtrA. Notably, despite the inability of BbHtrA to process BB0323 in vitro, within Bbbb0323NL, BB0323 could indeed be processed to some degree, which yields a basal level of mature N-terminal protein. Notably, in these B. burgdorferi cells, at least two other BB0323 polypeptides of lower molecular weight (less than 27 kDa of mature N-term BB0323) were also produced, possibly due to the action of BbHtrA on non-specific sites. However, the Bbbb0323NL mutants were non-infectious in the murine host, demonstrating the importance of precise cleavage of BB0323 full-length protein and optimal production of N-terminal, which needed to form a complex with another PPI partner, BB0238. Overall, these results further underscored the event of BbHtrA and BB0323 interaction for processing the latter protein as an essential prerequisite for spirochete infection in mammals. Our previous studies have shown that BB0323 N-terminal and BB0238 interact and post-translationally stabilize each other. We used an interaction-deficient borrelial mutant, replacing the BB0323 interaction motif in BB238 (termed as bb0238 Delta Interaction Motif, or bb0238∆IM), which despite showing no growth defects in vitro or other abnormalities, is unable to infect mammalian host. We, therefore, explored the possibility of using the BB0323:BB0238 complex as a novel therapeutic target to combat B. burgdorferi infection in mammals. We first examined whether bb0238∆IM mutants (without interaction motifs) can persist in mice for a long term or could be acquired by naïve ticks. The results show that, unlike the wild type or another B. burgdorferi mutant, The bb0238∆IM could not establish the infection in mice and, as a result, could not be acquired by the ticks, suggesting blockade of BB0323:BB0238 interaction by small molecules could be a novel therapeutic approach to combat incidence of LD. An AlphaLisa assay platform was developed in our lab to monitor BB0323-BB0238 PPI on a high-throughput basis using 384-well microtiter plates, which was then miniaturized to 1536 well at the National Center for Advancing Translational Sciences (NCATS) in a collaborative effort. An AlphaLisa quantitative HTS later screened several small molecule libraries available at NCATS, which were further filtered by counter assays, and a selected set of 84 compounds was tested in a secondary, cell-based assay for cell-permeable compounds that impair BB0323-BB0238 interaction with spirochete cells. A B. burgdorferi cell-based assay comprising a dot-blot assay and regrowth assay was developed to examine the PPI inhibitory activities of the molecules inside the cells. We finally selected one of the compounds, Lomibuvir, for the in vivo studies and demonstrated its PPI inhibitory activity in an in vitro experiment. A pharmacokinetic study in mice showed an increase in the level of the compound in plasma and liver over 21 days. Additional in vivo efficacy studies of Lomibuvir to reduce B. burgdorferi infection in mice were performed using vehicle and ceftriaxone as negative and positive controls, respectively. The results showed that the bacterial load in the skin and heart of the mice was significantly lower in the Lomibuvir-treated group, as compared to the vehicle-treated animals; however, the effect was not as dramatically effective as the antibiotic (ceftriaxone) treatment groups. While future medicinal chemistry approaches could be adopted to further enhance the impact of Lomibuvir as an anti-B. burgdorferi agent, to the best of knowledge, is the first proof-of-concept study that highlights the utility of targeting borrelial PPI events as a possible therapeutic target of Lyme disease.