A recent study published in PLOS Pathogens identifies the Leptospira membrane protein L36 (MPL36) as essential for its pathogenesis and, as a result, a potential target for future diagnostics and prophylactics.
Study: MPL36, a major plasminogen (PLG) receptor in pathogenic Leptospira, has an essential role during infection. Image Credit: Kateryna Kon / Shutterstock.com
Background
The human plasminogen (PLG) system provides a proteolytic substrate for pathogen invasiveness and tissue damage, thereby increasing the virulence of various bacterial species. Sequestration of PLG and activation into plasmin (PLA) is important for bacterial survival, as PLA degrades fibrin clots, extracellular matrix (ECM) molecules, and the host's innate immunological proteins to enable bacteria to spread and evade immune responses ultimately.
Leptospirosis, a life-threatening infectious disease, is a zoonotic disease that accounts for over one million infections and about 60,000 deaths each year throughout the world. In severe forms of leptospirosis, patients will often experience severe hemorrhaging and acute kidney injury.
Leptospira enter the host through injured skin or mucous membranes and subsequently travel through viscous environments to ultimately reach the bloodstream. Thereafter, Leptospira participate in cellular adhesion, which is mediated by various surface proteins that interact with ECM components.
Previous studies indicate that many Leptospira proteins act as PLG receptors, some of which include outer membrane proteins (OMPs) like endostatin-like protein A (LenA), as well as Leptospiral immunoglobulin-like proteins A (LigA), LiGB, and LipL32.
To date, there remains a lack of data on the pathogenesis of leptospirosis, which has prevented the development of sensitive diagnostic instruments and efficient prophylactic treatments. The present study reports that MPL36 binds with PLG, promotes adherence to host cells, and degrades fibrinogen by converting bound PLG to PLA.
About the study
The current study utilized Leptospira interrogans serovar Manilae strain L495 and its mutant stains, including Manilae mpl36 mutant complemented strain (Δmpl36+) and Leptospira biflexa serovar Patoc strain Patoc 1. These strains were cultured in Madin-Darby canine kidney (MDCK) cells, following which their growth curves were analyzed using dark-field microscopy, motility, and immunoblot assays.
Recombinant MPL36 (rMPL36) protein was produced using E. coli expression. Intrinsic tryptophan fluorescence spectroscopy and nickel affinity chromatography evaluated the tertiary structure of rMPL36, rMPL36 aa41-305, and rMPL36 aa41-235. The binding capacity of rMPL36 was determined, and the conversion of PLG bound to rMPL36 into active PLA was evaluated.
The binding of rMPL36 to ECM proteins and PLG was determined by staining for fibronectin, laminin, PLG, and bovine serum albumin (BSA). The pathogenicity of Manilae wildtype (WT) and mutant strains was also evaluated in three-week-old male Golden Syrian hamsters.
The tertiary structure of sporulation-related repeat (SPOR) domains from leptospiral species was predicted using AlphaFold and PyMOL software ClustalX to match MPL36 SPOR domain sequences from various Leptospira species. In vivo data was analyzed using Prism 9, Fisher's exact test, and analysis of variance (ANOVA) to ecludate the mechanisms of rMPL36 binding to host components and BSA.
MPL36 is a crucial target for leptopsirosis
The motility assay revealed that, as compared to wild-type cells, mpl36 mutant and complemented strains did not significantly differ in their motility or growth characteristics. Immunofluorescence staining confirmed the subcellular localization of MPL36 in Leptospira as an outer membrane protein in both wild-type and complement cells.
Immunoprecipitation studies revealed that rMPL36 exhibited significant binding to both rLigA and rLigB, whereas no binding was observed between rMPL36 and fibronectin nor laminin.
However, rMPL36 was found to effectively bind to human PLG as compared to other recombinant proteins that were used as positive and negative controls. Within rMPL36, the conserved SPOR domain, which is located at the last 70 amino acids of the protein, is essential for the binding between rMPL36 and PLG.
Hamsters that were infected with the wild-type strain died between eight and nine days following infection with Leptospira. Comparatively, mpl36 mutant hamsters who were infected with 108 leptospires survived without exhibiting any symptoms of leptospirosis, thereby demonstrating the significant role of MPL36 in leptospirosis.
Convalescent sera were obtained from Brazilian individuals with laboratory-confirmed severe leptospirosis. To this end, anti-MPL36 immunoglobulin M (IgM) and IgG were observed in leptospirosis patients’ samples as compared to sera from healthy individuals.
Study takeaways
MPL36 is an outer membrane Leptospira protein that is capable of binding to PLG, degrading fibrinogen, and facilitating host infection. Despite these findings, additional research is needed to better understand leptospiral pathogenesis and advance diagnostic and prevention strategies.
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