Understanding Unique Aspects of Motility and Chemotaxis in Borrelia burgdorferi

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SUMMARY Lyme disease bacterium Borrelia burgdorferi (Bb) is highly motile and can traverse complex environments inside mammalian and arthropod hosts during its infectious cycle. The central hypothesis of this application is that the motility and chemotaxis of Bb constitute a distinct paradigm and play a pivotal role in the host-vector cycle as well as in the disease process, including invasion, dissemination, tissue tropism, and immune evasion. During the last two funding cycles, we revealed several unique aspects of Bb motility and chemotaxis; however, their underlying molecular mechanisms and precise roles in the disease process remain largely unknown. Building upon our previous findings, this renewal aims to fill this knowledge gap by addressing three key questions: (1) How does Bb control asymmetrical flagellar rotation? Due to its unique cell shape and geometry, Bb must rotate its bipolar periplasmic flagella (PF) asymmetrically in order to run: the anterior PF rotates counterclockwise, and the posterior PF rotates clockwise. Without asymmetrical rotation, the cells become distorted. This is a hallmark feature of spirochete motility; however, its underlying molecular mechanism remains elusive. Aim 1 seeks to unravel this longstanding conundrum by determining the function and structure of FliG1, a noncanonical flagellar motor switch protein, using an integrative approach of genetics, biochemistry, cryo-electron tomography, and crystallography. (2) Has Bb evolved swarming motility to facilitate its invasiveness and virulence? During the enzootic cycle, on several occasions, Bb swims in highly viscous gel-like environments, such as mammalian dermis tissue and the tick-gut basement membrane, which are reminiscent of the environments in which bacteria swarm, a form of movement that allows bacteria to crawl over solid and semi-solid surfaces. It has been speculated that Bb has evolved swarming motility to empower its invasiveness. Aim 2 plans to delineate the underlying mechanism of swarming motility and its role in the pathogenicity of Bb, using a comprehensive approach of genetics, biochemistry, structural biology, and in vivo animal models along with intravital imaging. (3) Does CheA1 control Bb virulence and, if so, how? Bb has evolved unique chemotaxis to accommodate its distinct motility and enzootic cycle, e.g., its genome encodes multiple chemotaxis proteins such as two CheA histidine kinases (HK): CheA1 and CheA2. A longstanding question is why Bb needs multiple chemotaxis proteins. CheA2, but not CheA1, is essential for Bb chemotaxis. The role of CheA1 remains unknown. Interestingly, we recently found that CheA1 is required for Bb hematogenous dissemination in mice and expression of several key virulence factors of Bb. Building upon these results, Aim 3 proposes to elucidate the role and underlying molecular mechanism of CheA1 in Bb pathogenicity, using a multidisciplinary approach of genetics, biochemistry, RNA-seq, and animal models. Achievement of these aims will lead to a better understanding of Bb motility and chemotaxis as well as their precise roles in the pathogenesis of Lyme disease. Moreover, the fundamental knowledge to be gained is highly impactful and likely to aid understanding of these processes in other pathogenic spirochetes as well.