Lyme disease (LD), caused by the spirochete Borrelia burgdorferi (Bb), is the most common arthropod-borne infection in the United States. Bb is maintained in nature via an enzootic cycle that typically involves rodents and Ixodes ticks. To sustain this complex life cycle, Bb must continuously alter its transcriptome, proteome, and metabolome in response to arthropod- and mammalian host-derived signals. Our long term objective has been to characterize the environmental cues, regulatory pathways, and genes critical for these adaptive changes. In our previously funded application, we hypothesized that the alternative sigma factor RpoS acts as a "gatekeeper" that (i) transcribes genes required for tick transmission as well as the establishment of mammalian infection and (ii) represses tick-phase genes during mammalian host-adaptation. Our validation of this hypothesis has yielded powerful new insights into the mechanisms that perpetuate the bacterium in nature and maintain Lyme disease as a global threat to human health. A number of our recent findings relate to the transmission of spirochetes during nymphal feeding, the seminal event for infection of humans as well as mice. Using methodologies developed for tracking and imaging live spirochetes within ticks, we discovered that dissemination of Bb in feeding nymphs involves an initial non-motile phase (termed "adherence-mediated migration") followed by a transition to motility that enables individual organisms to penetrate the midgut. These studies have helped elucidate the range of adaptations, physiological as well as structural, that spirochetes must undergo in order to negotiate the formidable barriers they encounter en route to the mammal. They also have definitively demonstrated that RpoS-regulated genes are required for transmission of Bb (see Progress Report). Our collective results give rise to our primary hypothesis, to be assessed in Aims 1 and 2, that RpoS-upregulated genes (a) promote the biphasic dissemination of spirochetes and (b) collectively comprise a 'Go'signal for tick-to-mammal transmission. Additionally, we and others have identified a global regulatory network controlled by the Hk1/Rrp1 two-component system, which we conceptualize as imparting a tick-adaptive 'Stay'signal via the small nucleotide messenger cyclic di-GMP, a molecule known to promote sessile developmental states in bacteria. Our finding that the Hk1/Rrp1 regulon includes tick-phase genes repressed by RpoS within the mammal, coupled with our live-imaging studies, leads to our secondary hypothesis, to be assessed in Aim 3, that the degree of interplay between RpoS and the Hk1/Rrp1 pathway is a major determinant of spirochete behavior and gene expression during the blood meals. Our studies are based on the premise that improved understanding of the molecular mechanisms whereby Bb is transmitted by the arthropod vector will lead to new strategies for prevention and control of LD.

Public Health Relevance

Borrelia burgdorferi (Bb), the causative agent of Lyme disease, is maintained in nature by an enzootic life cycle that involves an Ixodes tick vector and a mammalian host, typically wild rodents. We have shown that genes transcribed by the alternative sigma factor RpoS are critical for transmission of Bb by ticks as well as infection of mice and, therefore, humans. The experiments in our proposal will define more precisely how and where RpoS-dependent genes function during the transmission process and how they are regulated.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Bacterial Pathogenesis Study Section (BACP)
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Breen, Joseph J
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University of Connecticut
Internal Medicine/Medicine
Schools of Medicine
United States
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Iyer, Radha; Caimano, Melissa J; Luthra, Amit et al. (2015) Stage-specific global alterations in the transcriptomes of Lyme disease spirochetes during tick feeding and following mammalian host adaptation. Mol Microbiol 95:509-38
Caimano, Melissa J; Sivasankaran, Sathesh K; Allard, Anna et al. (2014) A model system for studying the transcriptomic and physiological changes associated with mammalian host-adaptation by Leptospira interrogans serovar Copenhageni. PLoS Pathog 10:e1004004
Miller, Kelly A; Motaleb, Md A; Liu, Jun et al. (2014) Initial characterization of the FlgE hook high molecular weight complex of Borrelia burgdorferi. PLoS One 9:e98338
Kenedy, Melisha R; Luthra, Amit; Anand, Arvind et al. (2014) Structural modeling and physicochemical characterization provide evidence that P66 forms a *-barrel in the Borrelia burgdorferi outer membrane. J Bacteriol 196:859-72
Brandt, Kevin S; Patton, Toni G; Allard, Anna S et al. (2014) Evaluation of the Borrelia burgdorferi BBA64 protein as a protective immunogen in mice. Clin Vaccine Immunol 21:526-33
Harman, Michael; Vig, Dhruv K; Radolf, Justin D et al. (2013) Viscous dynamics of Lyme disease and syphilis spirochetes reveal flagellar torque and drag. Biophys J 105:2273-80
Dunham-Ems, Star M; Caimano, Melissa J; Eggers, Christian H et al. (2012) Borrelia burgdorferi requires the alternative sigma factor RpoS for dissemination within the vector during tick-to-mammal transmission. PLoS Pathog 8:e1002532
Harman, Michael W; Dunham-Ems, Star M; Caimano, Melissa J et al. (2012) The heterogeneous motility of the Lyme disease spirochete in gelatin mimics dissemination through tissue. Proc Natl Acad Sci U S A 109:3059-64
Caimano, Melissa J; Kenedy, Melisha R; Kairu, Toru et al. (2011) The hybrid histidine kinase Hk1 is part of a two-component system that is essential for survival of Borrelia burgdorferi in feeding Ixodes scapularis ticks. Infect Immun 79:3117-30
Eggers, Christian H; Caimano, Melissa J; Malizia, Robert A et al. (2011) The coenzyme A disulphide reductase of Borrelia burgdorferi is important for rapid growth throughout the enzootic cycle and essential for infection of the mammalian host. Mol Microbiol 82:679-97

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