Outer surface lipoproteins from the Lyme disease spirochete exploit the molecular switch mechanism of the complement protease C1s

Heterologous Surface Display Reveals Conserved Complement Inhibition and Functional Diversification of Borrelia burgdorferi Elp Proteins

Lyme disease is a tick-borne spirochetosis with diverse clinical manifestations. Genotypic and phenotypic variation among Borrelia burgdorferi strains correlates with variable manifestations of Lyme disease in humans; this diversity is attributed in part to variation in surface-exposed lipoproteins, which are targets of the human antibody response and contribute to tissue adhesion, immune evasion, and other host interactions. Many B. burgdorferi lipoproteins are encoded as multi-copy gene families, such as the OspE/F-like leader peptide (Elp) protein family, which inhibits classical complement activation by binding complement C1s. To characterize Elp allelic variants, we adapted the Pseudomonas syringae ice nucleation protein (INP) system to present B. burgdorferi lipoproteins on the surface of Escherichia coli. Using this system, we identified interactions with classical complement proteins and mapped binding regions, then validated interactions using recombinant proteins and B. burgdorferi surface display. We also discovered a novel potential interaction between Elp proteins and the mammalian basement membrane protein perlecan, thus revealing a bifunctional nature of Elps. Our findings indicate that Elps have undergone functional diversification while maintaining classical complement inhibition mediated by potent and conserved C1s binding and demonstrate that E. coli surface display offers an efficient, cost-effective, and relatively high-throughput approach to characterize B. burgdorferi lipoproteins.

Borrelia burgdorferi lacking all cp32 prophage plasmids retains full infectivity in mice

The causative agent of Lyme disease, Borrelia burgdorferi, contains a unique, segmented genome comprising multiple linear and circular plasmids. To date, the genomes of over 63 sequenced Lyme disease Borrelia carry one or more 32 kbp circular plasmids (cp32) or cp32-like elements. The cp32 plasmids are endogenous prophages and encode, among other elements, a family of surface exposed lipoproteins termed OspEF-related proteins. These lipoproteins are synthesized during mammalian infection and are considered important components of the spirochete's adaptive response to the vertebrate host. Here, we detail the construction and infectivity of the first described B. burgdorferi strain lacking all cp32 plasmids. Despite their universal presence, our findings indicate that B. burgdorferi does not require any cp32 plasmids to complete the experimental mouse-tick-mouse infectious cycle and a total lack of cp32s does not impair spirochete infectivity.

Blocking activation of the C1r zymogen defines a novel mode of complement inhibition

Many hematophagous organisms secrete inhibitors of the coagulation and complement systems as constituents of their salivary fluid. Whereas previous studies on salivary gland extracts from the sandfly Lutzomyia longipalpis identified SALO (salivary anticomplement from L. longipalpis) as a potent inhibitor of the classical complement pathway (CP), its precise mechanism of action has remained elusive. Here, we show that SALO inhibits the CP by binding selectively to the C1r zymogen. Using surface plasmon resonance, we found that SALO expressed by human embryonic kidney 293(T) cells (eSALO-WT) bound with nanomolar affinity to the zymogen of complement protease C1r (pro-C1r), but that it did not bind the enzymatically active form of C1r. To gain insight into the structural basis for CP inhibition by SALO, we solved a 3.3 Å resolution crystal structure of eSALO-WT bound to a recombinant form of C1r that was engineered to remain in a zymogen-like state (zC1r-12SP). eSALO-WT formed extensive interactions with the zymogen activation loop of zC1r-12SP, including groups derived from residues R463 and I464, which compose its scissile peptide bond. Although the interactions with R463 and I464 were mediated by side-chain sulfation of eSALO-WT at position Y51, we found that this modification enhanced the potency of SALO but was not required for its activity. Consistent with our structural observations, subsequent studies showed that eSALO-WT binding to pro-C1r blocked its activation and thereby inhibited the CP in hemolytic assays of complement function. Together, our results define a new mode of inhibiting complement by blocking the farthest upstream enzymatic reaction of the CP.

The Crystal Structure of the Michaelis-Menten Complex of C1 Esterase Inhibitor and C1s Reveals Novel Insights into Complement Regulation

The ancient arm of innate immunity known as the complement system is a blood proteolytic cascade involving dozens of membrane-bound and solution-phase components. Although many of these components serve as regulatory molecules to facilitate controlled activation of the cascade, C1 esterase inhibitor (C1-INH) is the sole canonical complement regulator belonging to a superfamily of covalent inhibitors known as serine protease inhibitors (SERPINs). In addition to its namesake role in complement regulation, C1-INH also regulates proteases of the coagulation, fibrinolysis, and contact pathways. Despite this, the structural basis for C1-INH recognition of its target proteases has remained elusive. In this study, we present the crystal structure of the Michaelis-Menten (M-M) complex of the catalytic domain of complement component C1s and the SERPIN domain of C1-INH at a limiting resolution of 3.94 Å. Analysis of the structure revealed that nearly half of the protein/protein interface is formed by residues outside of the C1-INH reactive center loop. The contribution of these residues to the affinity of the M-M complex was validated by site-directed mutagenesis using surface plasmon resonance. Parallel analysis confirmed that C1-INH-interfacing residues on C1s surface loops distal from the active site also drive affinity of the M-M complex. Detailed structural comparisons revealed differences in substrate recognition by C1s compared with C1-INH recognition and highlight the importance of exosite interactions across broader SERPIN/protease systems. Collectively, this study improves our understanding of how C1-INH regulates the classical pathway of complement, and it sheds new light on how SERPINs recognize their cognate protease targets.

DnaA modulates the gene expression and morphology of the Lyme disease spirochete

All bacteria encode a multifunctional DNA-binding protein, DnaA, which initiates chromosomal replication. Despite having the most complex, segmented bacterial genome, little is known about Borrelia burgdorferi DnaA and its role in maintaining the spirochete's physiology. In this work we utilized inducible CRISPR-interference and overexpression to modulate cellular levels of DnaA to better understand this essential protein. Dysregulation of DnaA, either up or down, increased or decreased cell lengths, respectively, while also significantly slowing replication rates. Using fluorescent microscopy, we found the DnaA CRISPRi mutants had increased numbers of chromosomes with irregular spacing patterns. DnaA-depleted spirochetes also exhibited a significant defect in helical morphology. RNA-seq of the conditional mutants showed significant changes in the levels of transcripts involved with flagellar synthesis, elongation, cell division, virulence, and other functions. These findings demonstrate that the DnaA plays a commanding role in maintaining borrelial growth dynamics and protein expression, which are essential for the survival of the Lyme disease spirochete.

The molecular determinants of classical pathway complement inhibition by OspEF-related proteins of Borrelia burgdorferi

The complement system serves as the first line of defense against invading pathogens by promoting opsonophagocytosis and bacteriolysis. Antibody-dependent activation of complement occurs through the classical pathway and relies on the activity of initiating complement proteases of the C1 complex, C1r and C1s. The causative agent of Lyme disease, Borrelia burgdorferi, expresses two paralogous outer surface lipoproteins of the OspEF-related protein family, ElpB and ElpQ, that act as specific inhibitors of classical pathway activation. We have previously shown that ElpB and ElpQ bind directly to C1r and C1s with high affinity and specifically inhibit C2 and C4 cleavage by C1s. To further understand how these novel protease inhibitors function, we carried out a series of hydrogen-deuterium exchange mass spectrometry (HDX-MS) experiments using ElpQ and full-length activated C1s as a model of Elp-protease interaction. Comparison of HDX-MS profiles between unbound ElpQ and the ElpQ/C1s complex revealed a putative C1s-binding site on ElpQ. HDX-MS-guided, site-directed ElpQ mutants were generated and tested for direct binding to C1r and C1s using surface plasmon resonance. Several residues within the C-terminal region of ElpQ were identified as important for protease binding, including a single conserved tyrosine residue that was required for ElpQ- and ElpB-mediated complement inhibition. Collectively, our study identifies key molecular determinants for classical pathway protease recognition by Elp proteins. This investigation improves our understanding of the unique complement inhibitory mechanism employed by Elp proteins which serve as part of a sophisticated complement evasion system present in Lyme disease spirochetes.

Erp and Rev Adhesins of the Lyme Disease Spirochete's Ubiquitous cp32 Prophages Assist the Bacterium during Vertebrate Infection

Almost all spirochetes in the genus Borrelia (sensu lato) naturally contain multiple variants of closely related prophages. In the Lyme disease borreliae, these prophages are maintained as circular episomes that are called circular plasmid 32 kb (cp32s). The cp32s of Lyme agents are particularly unique in that they encode two distinct families of lipoproteins, namely, Erp and Rev, that are expressed on the bacterial outer surface during infection of vertebrate hosts. All identified functions of those outer surface proteins involve interactions between the spirochetes and host molecules, as follows: Erp proteins bind plasmin(ogen), laminin, glycosaminoglycans, and/or components of complement and Rev proteins bind fibronectin. Thus, cp32 prophages provide their bacterial hosts with surface proteins that can enhance infection processes, thereby facilitating their own survival. Horizontal transfer via bacteriophage particles increases the spread of beneficial alleles and creates diversity among Erp and Rev proteins.