Mechanism of action of AexU, a new type III secretion system effector from an emerging human pathogen Aeromonas hydrophila

Our laboratory first reported the complete sequence of the type III secretion system (T3SS) from a diarrheal isolate SSU of A. hydrophila. We identified an effector protein (designated as AexU) of the T3SS, which exhibited ADP-ribosyltransferase (ADPRT) and GTPase-activating protein (GAP) activity. AexU was successfully expressed in the HeLa cell Tet-Off system and I provided evidence that cells expressing and producing the full length AexU showed actin reorganization followed by apoptosis. Earlier, we showed that the ÄaexU null mutant was attenuated in a mouse model, and I now demonstrated that while the parental A. hydrophila strain could be detected in the lung, liver, and spleen of infected mice, the ÄaexU mutant was rapidly cleared from these organs resulting in increased survivability of animals. The GAP activity of AexU was mainly responsible for host cell apoptosis and disruption of actin filaments. Further, AexU prevented phosphorylation of c-Jun, JNK and IêBá and inhibited IL-6 and IL-8 secretion from HeLa cells. Our data indicated that AexU operated by inhibiting NF-êB and inactivating Rho GTPases. Importantly, however, when the ÄaexU 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 proinflammatory 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.