Investigating Rickettsia Interspecies and Host-Specific Lipopolysaccharide Variation

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PROJECT SUMMARY Rickettsiae are Gram-negative obligate intracellular Alphaproteobacteria and metabolic parasites with a wide range of eukaryotic hosts. Across the Rickettsia tree, vector-borne pathogens (i.e., Spotted Fever Group (SFG) and Typhus Group (TG) disease agents) are interspersed with many endosymbionts and other species of unknown pathogenicity. A treasure trove of sequenced genomes allows for robust comparisons to illuminate mechanisms behind vector transmission and pathogenesis. This is crucial for human health, as rising deforestation and urbanization are fueling spikes in rickettsial diseases across the US, with the ever-present chance tetracycline-resistant strains will emerge. Dr. Gillespie uses phylogenomics to identify lineage specific factors that are subsequently characterized for roles in rickettsial pathogenesis. Teaming with Dr. Ernst, an expert on bacterial LPS, has resulted in the very recent discovery that not all Rickettsia lipid A is created equal! Lipid A, the membrane component of LPS that is among the most proinflammatory molecules known, diverges in acyl chain length at a definable point in SFG Rickettsiae evolution − one of the deadliest pathogens (the Rocky Mountain Spotted Fever agent R. rickettsi) adopts a 2’ acyl chain like that found in the highly potent E. coli lipid A! This implies Rickettsiae lipid A interacts variably with the host MD-2/TLR4 receptor. We have also made two other discoveries indicating LPS is variable across Rickettsia species. First, the polysaccharide synthesis operon (pso), which encodes enzymes involved in synthesis of LPS carbohydrate moiety (CaMo), is highly divergent across Rickettsia genomes. Second, genes encoding two enzymes that potential modify LPS with phosphoethanolamine (pEtN by Ept) and phosphorylcholine (ChoP by LicD) have been acquired by lateral gene transfer and are pseudogenized in some non-pathogens that don’t infect vertebrates! These collective data strengthen our hypothesis that LPS is variable across diverse Rickettsiae; further, given the physiological and immunological differences between arthropod and vertebrate cells, we posit that Rickettsia LPS structure changes during shifts between arthropod and vertebrate host environments, similar to other bacteria (i.e. Yersinia pestis and Francisella tularensis), and may be a mechanism for silencing the host innate immune system. In this proposal, two independent (yet complementary) Specific Aims are designed to test these hypotheses. First (AIM 1), we will use FLATn, a rapid method to yield lipid A structures with minimal input sample and no chemical extraction, to determine lipid A acyl chain length variability for ten diverse rickettsiae infecting both arthropod and vertebrate cells. Next (AIM 2), a subset of these species will be used to characterize CaMo structures and gauge gene expression of ps, ept, and licD; furthermore, we will characterize Rickettsia typhi pEtN addition to LPS and mine Rickettsia genomes for additional LPS modification genes. There has been a recent expansion of available genome assemblies for new Rickettsiae on GenBank, so we anticipate making important discoveries regarding LPS biology. Our description of Rickettsia LPS will yield insight on vector transmission dynamics, identify species-specific traits, and set the stage for future assays testing LPS immunostimulatory potential (lipid A) and epitope recognition (CaMo).