Rapid microtubule bundling and stabilization by the Streptococcus pneumoniae neurotoxin pneumolysin in a cholesterol-dependent, non-lytic and Src-kinase dependent manner inhibits intracellular trafficking

Streptococcus pneumoniae is the most frequent cause of bacterial meningitis, leading to permanent neurological damage in 30% and lethal outcome in 25% of patients. The cholesterol-dependent cytolysin pneumolysin is a major virulence factor of S. pneumoniae. It produces rapid cell lysis at higher concentrations or apoptosis at lower concentrations. Here, we show that sublytic amounts of pneumolysin produce rapid bundling and increased acetylation of microtubules (signs of excessive microtubule stabilization) in various types of cells--neuroblastoma cells, fibroblasts and primary astrocytes. The bundling started perinuclearly and extended peripherally towards the membrane. The effect was not connected to pneumolysin's capacity to mediate calcium influx, macropore formation, apoptosis, or RhoA and Rac1 activation. Cellular cholesterol depletion and neutralization of the toxin by pre-incubation with cholesterol completely inhibited the microtubule phenotype. Pharmacological inhibition of Src-family kinases diminished microtubule bundling, suggesting their involvement in the process. The relevance of microtubule stabilization to meningitis was confirmed in an experimental pneumococcal meningitis animal model, where increased acetylation was observed. Live imaging experiments demonstrated a decrease in organelle motility after toxin challenge in a manner comparable to the microtubule-stabilizing agent taxol, thus proposing a possible pathogenic mechanism that might contribute to the CNS damage in pneumococcal meningitis.

Cholesterol-dependent actin remodeling via RhoA and Rac1 activation by the Streptococcus pneumoniae toxin pneumolysin

The Streptococcus pneumoniae toxin pneumolysin belongs to the group of cholesterol-dependent cytolysins. It produces rapid cell lysis at higher concentrations or apoptosis at lower concentrations. In cell membranes, it forms prepores and pores. Here, we show that sublytic concentrations of pneumolysin produce rapid activation of Rho and Rac GTPases and formation of actin stress fibers, filopodia, and lamellipodia. That Rac1-specific and Rho-associated kinase (ROCK)-specific inhibitors reverted the formation of lamellipodia and stress fibers, respectively, identifies RhoA and Rac1 as key toxin effectors. Live imaging excluded macropore formation (as judged by membrane impermeability toward calcein) but indicated very early membrane depolarization [as judged by bis-(1,3-dibutylbarbituric acid)trimethine oxanol staining], indicative of formation of micropores with ion channel properties. That Rac1-dependent lamellipodia formation was reverted by the voltage-gated calcium channel inhibitor SKF96365 and by toxin exposure in calcium-free medium suggests a role for calcium influx via endogenous calcium channels in the Rac1 activation. Cellular cholesterol depletion by methyl-beta-cyclodextrin or incubation of the toxin with cholesterol before cell treatment eliminated its membrane binding and the subsequent GTPase activation. Thus, that our experiments show small GTPase activation by a cholesterol-dependent cytolysin suggests a membrane cholesterol-dependent activation mechanism.