In research recently published in mBio, researchers from the Abigail Wexner Research Institute (AWRI) at Nationwide Children’s Hospital and Griffith University’s Institute for Glycomics have discovered non-antibiotic (host-targeted) therapies for the effective treatment of Neisseria gonorrhoeae infections by repurposing existing drugs.
Gonorrhoea is the second most commonly reported sexually transmitted infection (STI).
The Centers for Disease Control and Prevention estimates that approximately 1.14 million new gonococcal infections occur in the United States each year.
Gonorrhea is now a major public health threat, with alarming increases in incidence over the past five years (67% in the U.S. and 80% in Australia).
The disease affects both men and women, however, up to 80% of female gonococcal cervicitis is asymptomatic.
When left untreated, it can lead to pelvic inflammatory disease (40% of infected women) that can result in severe long-term health implications such as infertility, adverse pregnancy outcomes and devastating neonatal complications.
The propensity of N. gonorrhoeae to rapidly acquire antibiotic resistance has solidified untreatable gonorrhea as a reality and, in the absence of a vaccine, has fueled interest in developing new antibiotics for gonorrhea treatment.
Traditional antibiotics work by directly targeting (and killing) the bacteria and, as such, are subject to bacterial resistance mechanisms.
Therefore, the development of new antibiotics that similarly target the bacterium will likely only provide a short-term solution to incurable N. gonorrhoeae infections.
“N. gonorrhoeae, the bacterium responsible for gonorrhea infections, is highly variable and exquisitely human-adapted,” said study co-author Michael Jennings, Ph.D., principal research leader and deputy director at Griffith.
“Its treatment evasion strategies have seen resistance develop against every antibacterial ever used to treat it. Our data strongly indicate that two, existing non-antibiotic drugs, methlydopa and carbamazepine, can be repurposed to both prevent and cure gonorrhea infection in women.”
Methlydopa is a prescription medication currently used for the treatment of hypertension in pregnant women, whereas carbamazepine is used for the treatment of epilepsy.
“Carbamazepine and methyldopa work by out-competing the bacteria for binding to a specific molecule (called CR3) on the human cell surface,” said study co-author Jennifer Edwards, Ph.D., principal investigator at AWRI at Nationwide Children’s.
“These drugs block the bacteria from binding, but they also bind in a way that is slightly different from the way the bacteria bind.
“This causes the host cell to act differently than it would when the bacteria bind,” said Dr. Edwards, also an associate professor at The Ohio State University.
“That is, when the drugs bind to the host cell, they prime the cell to kill the bacteria. This type of host-mediated killing would not be expected to be subject to bacterial drug resistance mechanisms. We have not observed any drug resistance over sequential N. gonorrhoeae infections using primary human cell infection models.”
N.gonorrhoeae infects only humans. Experimental infection of women is ethically prohibited. Animal models developed for N. gonorrhoeae research do not have CR3 on the mucosal surface of the female reproductive tract; however, CR3 is required for N. gonorrhoeae to initiate infection in women.
Therefore, the research team, led by Drs. Edwards and Jennings, used primary human cervical epithelial cells to model human cervical infection, which is the main site of infection in women.
Following a single dose of methyldopa or carbamazepine, 100% clearance of N. gonorrhoeae occurred for cervical cells infected with multidrug-resistant (“untreatable”) bacteria.
“Antibiotic resistance has emerged as one of our greatest threats to global health,” said Lauren Bakaletz, Ph.D., principal investigator and director of the Center for Microbial Pathogenesis at AWRI.
“The rising incidence of multi-drug resistance demonstrated by Neisseria gonorrhoeae combined with recent trends of increases in the number of cases of gonorrhea worldwide have created an urgent demand for better treatment strategies, and preferably those that are not reliant on traditional antibiotics.
This new study is novel, highly significant and exceptionally timely.”
Mark von Itzstein, AO, director of the Institute for Glycomics, said the research team’s data suggest a long-term solution to the growing problem of multidrug-resistant Neisseria gonorrhoeae infections.
“Given the absence of a gonococcal vaccine and the continued emergence of antibiotic-resistant and untreatable strains of Neisseria gonorrhoeae,this scientific finding provides a great deal of hope to millions of people around the globe who are battling this intractable disease,” said von Itzstein.
Neisseria gonorrhoeae (the gonococcus) is an exclusive human pathogen that causes the sexually transmitted infection, gonorrhea. Infections caused by N. gonorrhoeae continue to be a global intractable problem (1).
Asymptomatic cervicitis generates a carrier-like state in up to 80% of N. gonorrhoeae-infected women (2–6) and is the primary cause for the global prevalence of N. gonorrhoeae and its disease sequelae (7).
The absence of a gonococcal vaccine together with the continuing emergence of antibiotic-resistant and untreatable strains indicate that N. gonorrhoeae poses an “urgent” public health threat (8).
N. gonorrhoeae express type IV pili (fimbriae), which are crucial to interactions with epithelial cells (9–11).
These pili are polymers with variable phase and antigenic expression composed of thousands of pilin subunits encoded by the pilE gene (12).
Pilin can be posttranslationally modified with an O-linked monosaccharide, N,N′-diacetylbacillosamine (diNAcBac), or a disaccharide, Gal(α1-3)diNAcBac (Fig. 1) (13–15). Pilin glycosylation results from the activity of pilin glycosyltransferase (pgl) gene products.
For example, PglD is essential for diNAcBac biosynthesis, whereas the glycosyltransferase PglA adds the terminal galactose to Gal(α1-3)diNAcBac (16).
Expression of pglA is phase variable (high frequency, reversible on/off switching of gene expression) (Fig. 1) (17–19). Thereby, the pilin-linked glycan can be either a disaccharide or a monosaccharide depending on pglA expression status (i.e., on or off).
Complement receptor 3 (CR3; also known as integrin αMβ2, CD11b/CD18, and Mac-1) is an innate immune pattern recognition receptor. Expression of CR3 has historically been limited to cells of monocytic lineage; however, CR3 is also expressed on the apical surface of the human cervix (20).
The alpha subunit of CR3, CD11b, contains an approximately 200-amino-acid insertion, known as the I-domain (see Fig. S1 in the supplemental material). The CR3 I-domain is the primary binding site for iC3b and many other protein ligands (21).
Several important human pathogens (e.g., Streptococcus, Mycobacterium, Toxoplasma, and Staphylococci) use CR3 as a mechanism to promote disease (22–31). In this regard, CR3 is critical to N. gonorrhoeae infection of human cervical epithelial cells, both in vivo (20) and ex vivo (20, 32).
Complement receptor 3 I-domain sequence. Complement receptor 3 is a heterodimer composed of an alpha subunit, CD11b, and a beta subunit, CD18. CD11b contains an ∼200-amino-acid insertion domain, or I-domain, and a lectin domain. The human I-domain of CD11b (accession NP_000623.2, amino acids Gly127 to Ala325) is 77% similar to the mouse I-domain (accession NM_001082960.1, amino acids Gly127 to Ala334). *, identical amino acids; •, similar amino acids. The G2 peptide region is boxed in red. Download FIG S1, PDF file, 0.08 MB.Copyright © 2020 Poole et al. – This content is distributed under the terms of the Creative Commons Attribution 4.0 International license.
We previously reported the fundamental discovery that the N. gonorrhoeae-CR3 interaction occurs solely through the I-domain (32) and is mediated by the pilin-linked glycan (33). Infection of primary human cervical epithelial (Pex) cells requires pili with Gal(α1-3)diNAcBac. Bacteria expressing a diNAcBac monosaccharide do not survive Pex cell infection (33).
Given the important role of CR3 in numerous human infections, we sought to define the kinetics and specificity of this novel lectin function for the human CR3 I-domain and apply this information to the development of a novel strategy to prevent and cure gonococcal infections in women.
More information: Jessica Poole et al. Repurposed Drugs That Block the Gonococcus-Complement Receptor 3 Interaction Can Prevent and Cure Gonococcal Infection of Primary Human Cervical Epithelial Cells, mBio (2020). DOI: 10.1128/mBio.03046-19