A new therapeutic avenue in Clostridioides difficile Infection

Akshita Kumar

Clostridioides difficile is an anaerobic Gram-positive bacteria that is capable of forming spores. Clostridioides difficile infection (CDI) is the most common reason behind hospital-acquired gastrointestinal infections and mortality in industrialised nations, with an estimated 223,900 infections, 12,800 deaths, and a $1 billion healthcare cost in 2017. In recent years, there has also been a rise in the number of community-associated illnesses.
The major virulence factors related to infection are exotoxins TcdA and TcdB. Pathogenic TcdA^–TcdB⁺ bacteria have been obtained in the clinic and TcdB alone can cause the full range of CDI-related illnesses in people. 

TcdB and Receptor recognition

Due to the high complexity of TcdB variation, finding effective therapeutic antibodies, vaccines, and diagnostic tests with sufficient breadth has been challenging.. Another important source of worry is the possibility that TcdB mutants have altered their methods for detecting host receptors for cell entrance. Thus, the identification of novel target receptors is a certain need.
A recent study published in Nature Communications by Peng Chen et al. has unravelled the primary three-dimensional structure of Clostridioides difficile toxin B (TcdB) together with the human receptor chondroitin sulphate proteoglycan 4 (CSPG4).
Previous research had identified CSPG4 as a probable TcdB receptor, but the pathophysiological significance and molecular details remained unclear. The findings of this new study indicate a definite TcdB-CSPG4 binding location, also because of the proven fact that CSPG4-binding residues are largely conserved throughout the bulk of TcdB variations identified to this point.
Many immature progenitor cells, like oligodendrocyte progenitor cells and mesenchymal stem cells, express CSPG4. While its exact function is unknown, it’s been found to stimulate cell proliferation, adhesion, and migration, likewise as facilitate the binding of the many growth factors, including basic fibroblast protein and integrin.
Wnt receptor frizzled proteins (FZDs) are a candidate receptor for TcdB. Many therapeutically relevant TcdB variations, like TcdB2, bind CSPG4 but not FZDs. This is due to residue changes within the FZD-binding region that prevent the CSPG4 from binding to FZDs. 

CSPG4:  A probable novel receptor

The CSPG4-binding residues are highly conserved across most TcdB variants known thus far, and a rationally designed CSPG4-mimicking decoy potently inhibits both TcdB1 and TcdB2. It was further shown that bezlotoxumab disrupts this CSPG4-binding site via an allosteric manner, but its epitopes are prone to escaping mutations in TcdB. These findings demonstrate the critical function of CSPG4 as a major TcdB receptor, as well as methods for creating broad-spectrum CDI treatments.TcdB toxicity in mice is decreased by mutations that preferentially impair CSPG4 binding, and CSPG4-knockout animals had less damage to colonic tissues during C. difficile infections. TcdB1 and CSPG4 structure-guided mutagenesis can verify the binding interface and identify TcdB loss-of-function mutations that preferentially eliminate CSPG4 binding.

The sole FDA-approved therapeutic antibody against TcdB is bezlotoxumab, was found to decrease TcdB binding to CSPG4 in vitro in immunoprecipitation tests in a previous research.

Treatments with broad-spectrum antibiotics are currently the mainstay of therapy for CDI, although they frequently result in illness recurrence. While bezlotoxumab has been shown to lower the recurrence rate of CDI in certain individuals, the results of this and previous trials suggest that it is less effective against specific TcdB types.

The high conservation of the CSPG4-binding site in diverse TcdB isoforms provides a novel therapeutic avenue for combating TcdB. It was found that the recombinant Repeat1-Fc could potently inhibit toxicity of both TcdB1 and TcdB2 in a  cell-based assay, which was superior to bezlotoxumab. In addition, this antibody-like decoy receptor protected mice in vivo from TcdB1 and TcdB2. Since CSPG4 and FZDs act as two independent receptors for TcdB1, bi-specific decoy receptor composed of both Repeat1 and therefore the CRD of FZDs will further improve its potency against TcdB1-like toxins while maintaining its broad protection spectrum

The researchers have envisioned that the CSPG4 decoy could block a good range of target TcdB . As an evolutionarily conserved cellular receptor of TcdB, a CSPG4 decoy molecule would be difficult for TcdB to flee from, since any mutations that disrupt toxin binding to the its natural receptors would likewise be disrupted by the decoy. Taken together,these studies establish the mechanistic and structural foundation for the event of next generation therapeutics for the prevention and treatment of CDI, which is in a position to own broad activities across diverse C. difficile strains.

Also read:Exosomes predicted to carry out protein secretion

Source:

1. Chen, P., Zeng, J., Liu, Z. et al. Structural basis for CSPG4 as a receptor for TcdB and a therapeutic target in Clostridioides difficile infection. Nat Commun 12, 3748 (2021). https://doi.org/10.1038/s41467-021-23878-3
2. University of California, Irvine. (2021, 06 18). Study reveals new therapeutic target for C. difficile infection. Retrieved from phys.org: https://phys.org/news/2021-06-reveals-therapeutic-difficile-infection.html

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About Author: Akshita Kumar is a Lucknow based postgraduate in Biochemistry. An inquisitive science enthusiast with an inclination towards scientific writing aiming to be a scientist.