# Peptidoglycan architecture dictates protein interactions, tissue tropism, and arthritis in the Lyme disease spirochete Borrelia burgdorferi

## TL;DR

The structure of peptidoglycan in Borrelia burgdorferi affects protein interactions, tissue targeting, and arthritis in Lyme disease.

## Contribution

Altered peptidoglycan in Borrelia burgdorferi disrupts joint tropism and reduces arthritis, revealing a novel role in disease pathogenesis.

## Key findings

- Changes to peptidoglycan cross-linking and composition do not affect B. burgdorferi in vitro but reduce arthritis in vivo.
- Increased pentapeptide in peptidoglycan sacculi disrupts interactions with the periplasmic protein p83/100, likely decreasing pathogenicity.
- Peptidoglycan architecture is crucial for tissue tropism and arthritis in Lyme disease.

## Abstract

Lyme disease is a vector-borne illness transmitted by infected Ixodes spp. ticks. Dissemination of the Lyme spirochete—Borrelia burgdorferi—from the tick bite site results in a bi-phasic infection; the latter phase can cause severe musculoskeletal disease including arthritis. Lyme arthritis is an inflammatory disorder and maladaptive immune response to B. burgdorferi infection and its cellular products. One such product, which has been implicated as a key mediator of Lyme arthritis, is peptidoglycan. Peptidoglycan (PG) is a near ubiquitous feature of the bacterial cell envelope, but several chemical features make B. burgdorferi PG distinct from other members of the kingdom. We hypothesized the overall chemical composition and structural architecture of the B. burgdorferi cell wall are essential to Lyme disease pathogenesis. To manipulate the PG peptide chemical composition, as well as the native cross-links, we produced an isogenic deletion of a putative PG carboxypeptidase dacA homologue and assessed both the molecular and cellular phenotypes while probing the pathogenicity of our mutant strain. Our combined and comprehensive approach indicates while changes to PG stem peptide and cross-linking have virtually no discernable impact on any B. burgdorferi characteristic in vitro, alterations have significant impacts on tissue tropism and result in a near complete attenuation of Lyme arthritis. PG sacculi containing increased amounts of free and cross-linked pentapeptide surprisingly caused the disassociation of p83/100, an abundant periplasmic protein of unknown function previously implicated in joint tropism, likely contributing to a marked decrease in pathogenicity. These studies strengthen our understanding of the B. burgdorferi cell envelope, its unusual components, and further define bacterial features that mediate infectious arthritis.

Many of the pathogenic mechanisms by which Borrelia burgdorferi causes disease involve cell envelope interactions with host components. To further elucidate the role of peptidoglycan in Lyme disease pathology we assessed a mutant strain with altered proportions of glycopeptides and found a near complete attenuation of arthritis. Cellular and molecular studies indicate this phenotype could be partially explained by disrupting the interactions between peptidoglycan and a previously uncharacterized periplasmic protein known to impact tissue tropism. Our findings suggest that the complex cell wall of B. burgdorferi, and its associated factors, are important contributors to Lyme disease pathogenesis.