Unraveling the mechanism of action of a new type III secretion system effector AexU from Aeromonas hydrophila

We recently characterized a T3SS effector, AexU, from a diarrheal isolate SSU of Aeromonas hydrophila, which exhibited ADP-ribosyltransferase (ADPRT) activity. Here we provided evidence that AexU also possessed GTPase-activating protein (GAP) activity, which was mainly responsible for host cell apoptosis and disruption of actin filaments. Earlier, we showed that the ΔaexU null mutant was attenuated in a mouse model, and we now demonstrated that while the parental A. hydrophila strain could be detected in the lung, liver, and spleen of infected mice, the ΔaexU mutant was rapidly cleared from these organs resulting in increased survivability of animals. Further, AexU prevented phosphorylation of c-Jun, JNK and IκBα and inhibited IL-6 and IL-8 secretion from HeLa cells. Our data indicated that AexU operated by inhibiting NF-κB and inactivating Rho GTPases. Importantly, however, when the ΔaexU null mutant was complemented with the mutated aexU gene devoid of ADPRT and GAP activities, a higher mortality rate in mice with concomitant increase in the production of pro-inflammatory cytokines/chemokines was noted. These data indicated that either such a mutated AexU is a potent inducer of them or that AexU possesses yet another unknown activity that is modulated by ADPRT and GAP activities and results in this aberrant cytokine/chemokine production responsible for increased animal death.