The Nucleotide Excision Repair Pathway Protects Borrelia burgdorferi from Nitrosative Stress in Ixodes scapularis Ticks

Abstract

The Lyme disease spirochete Borrelia burgdorferi encounters a wide range of environmental conditions as it cycles between ticks of the genus Ixodes and its various mammalian hosts. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are potent antimicrobial molecules generated during the innate immune response to infection, however, it is unclear whether ROS and RNS pose a significant challenge to B. burgdorferi in vivo. In this study, we screened a library of B. burgdorferi strains with mutations in DNA repair genes for increased susceptibility to ROS or RNS in vitro. Strains with mutations in the methyl-directed mismatch repair gene mutS1 are hypersensitive to killing by ROS, while strains lacking the nucleotide excision repair (NER) gene uvrB show increased susceptibility to both ROS and RNS. Therefore, mutS1-deficient and uvrB-deficient strains were compared for their ability to complete their infectious cycle in Swiss Webster mice and I. scapularis ticks to help identify sites of oxidative and nitrosative stresses encountered by B. burgdorferi in vivo. Both mutS1 and uvrB were dispensable for infection of mice, while uvrB promoted the survival of spirochetes in I. scapularis ticks. The decreased survival of uvrB-deficient B. burgdorferi was associated with the generation of RNS in I. scapularis midguts and salivary glands during feeding. Collectively, these data suggest that B. burgdorferi must withstand cytotoxic levels of RNS produced during infection of I. scapularis ticks.

The relapsing fever spirochete Borrelia turicatae persists in the highly oxidative environment of its soft‐bodied tick vector

“…Previous studies examining the hard‐bodied tick I. scapularis have implicated dual oxidase ( duox ) and nitric oxide synthase ( nos ) as the primary sources of ROS and RNS production in the tick midgut during infection (Bourret et al, ; Yang, Smith, Williams, & Pal, ). Our RNAseq analyses indicate that two putative duox genes ( duox1 and duox2 ) and a single nos gene are expressed in O. turicata salivary glands (Table ).…”

“…Previous work indicated NOS is expressed in both the salivary glands and midguts of the hard‐bodied tick I. scapularis by immunofluorescence using a universal NOS antibody (Bourret et al, ). In contrast to I. scapularis , NOS expression appears to be highest in the midguts of O. turicata , whereas the salivary glands show very little expression of NOS (Figure ).…”

“…In contrast, the pH of Ornithodoros saliva is unknown. Previously, we showed that Ixodes scapularis , the hard‐bodied tick vector of the Lyme disease (LD) spirochete Borrelia burgdorferi , produced RNS in both its salivary glands and midguts (Bourret et al, ). We have found ROS are generated in these tissues as well (data not shown).…”

“…We have found ROS are generated in these tissues as well (data not shown). ROS and RNS appear to be substantial environmental challenges for B. burgdorferi during both blood meal acquisition and during starvation in I. scapularis nymphs, as ROS‐sensitive and RNS‐sensitive B. burgdorferi strains harbouring mutations in antioxidant defence genes show poor survival compared with their wild‐type counterparts (Bourret et al, ; Eggers et al, ; Li et al, ). To the best of our knowledge, the production of ROS and RNS in Ornithodoros ticks has not been reported; however, transcriptomic and proteomic studies have identified a variety of oxidant metabolism and antioxidant defence genes expressed in soft‐bodied ticks that transmit RF Borrelia (Francischetti, Mans, et al, ; Francischetti, Meng, et al, ; Landulfo et al, ; Oleaga, Obolo‐Mvoulouga, Manzano‐Roman, & Perez‐Sanchez, ; Oleaga, Obolo‐Mvoulouga, Manzano‐Roman, & Perez‐Sanchez, ; Oleaga, Obolo‐Mvoulouga, Manzano‐Roman, & Perez‐Sanchez, ).…”

A high-throughput genetic screen identifies previously uncharacterized Borrelia burgdorferi genes important for resistance against reactive oxygen and nitrogen species

“…burgdorferi must sense and respond. ROS and RNS are detectable in the salivary glands and midgut of Ixodes ticks following a bloodmeal [9] and are also likely present in the vertebrate host at the tick bite site due to the recruitment of inflammatory cells to this site [10, 11]. …”