Mycoplasma pneumoniae CARDS toxin exploits host cell endosomal acidic pH and vacuolar ATPase proton pump to execute its biological activities

Abstract

Mycoplasma pneumoniae is the leading cause of bacterial community-acquired pneumonia among hospitalized children in the United States. It is also responsible for a spectrum of other respiratory tract disorders and extrapulmonary manifestations in children and adults. The main virulence factor of M. pneumoniae is a 591 amino acid multifunctional protein called Community Acquired Respiratory Distress Syndrome (CARDS) toxin. The amino terminal region of CARDS toxin (N-CARDS) retains ADP-ribosylating activity and the carboxy region (C-CARDS) contains the receptor binding and vacuolating activities. After internalization, CARDS toxin is transported in a retrograde manner from endosome through the Golgi complex into the endoplasmic reticulum. However, the mechanisms and criteria by which internalized CARDS toxin is transported and activated to execute its cytotoxic effects remain unknown. In this study, we used full-length CARDS toxin and its mutant and truncated derivatives to analyze how pharmacological drugs that alter pH of intracellular vesicles and electrical potential across vesicular membranes affect translocation of CARDS toxin in mammalian cells. Our results indicate that an acidic environment is essential for CARDS toxin retrograde transport to endoplasmic reticulum. Moreover, retrograde transport facilitates toxin clipping and is required to induce vacuole formation. Additionally, toxin-mediated cell vacuolation is strictly dependent on the function of vacuolar type-ATPase.

Community-Acquired Respiratory Distress Syndrome Toxin: Unique Exotoxin for M. pneumoniae

“…Findings showed that the pathogenic effect of CARDS TX is influenced by its structural and functional composition (Kannan and Baseman, 2006;Hardy et al, 2009). Recent studies have shown that V-ATPase plays an important role in CARDS TX-induced vacuole formation (Ramasamy et al, 2021). Existing studies have demonstrated that CARDS TX exerts its biological effects through vacuolar ATPase proton pump and host cell endosomal acidic environment, but the mechanism of its endocytosis is unknown (Ramasamy et al, 2021).…”

“…Recent studies have shown that V-ATPase plays an important role in CARDS TX-induced vacuole formation (Ramasamy et al, 2021). Existing studies have demonstrated that CARDS TX exerts its biological effects through vacuolar ATPase proton pump and host cell endosomal acidic environment, but the mechanism of its endocytosis is unknown (Ramasamy et al, 2021). Further research is needed to determine whether the toxic effects of inhibiting vacuolation on CARDS TX is diminished, and that the repair mechanism of vacuolation is also a good research point.…”

“…Moreover, deacidification of endosomes can significantly reduce the binding of the CARDS TX-Golgi complex, indicating that an acidic endosomal environment is essential for the retrograde transport of CARDS TX to the ER. Meanwhile, endosomal pH affects CARDS TX cleavage, but not binding and/or entry, whereas CARDS TX cleavage is required for tight attachment to the Golgi complex (Ramasamy et al, 2021). Overall, both KELED sequences and acidic endosomal environments play essential roles in retrograde transport and cytopathic effects (Ramasamy et al, 2018(Ramasamy et al, , 2021.…”

“…Meanwhile, endosomal pH affects CARDS TX cleavage, but not binding and/or entry, whereas CARDS TX cleavage is required for tight attachment to the Golgi complex (Ramasamy et al, 2021). Overall, both KELED sequences and acidic endosomal environments play essential roles in retrograde transport and cytopathic effects (Ramasamy et al, 2018(Ramasamy et al, , 2021.…”

“…Notably, V-ATPase (vacuolar ATPase) blockers completely inhibited vacuole formation induced by CARDS TX, whereas endosomal pH had less impact on cell vacuolation. V-ATPase plays an important role in CARDS TXinduced vacuole formation (Ramasamy et al, 2021). Further studies are needed to examine the effect of inhibiting vacuolation on CARDS TX toxicity.…”