Study of a Novel Secreted Effector in Salmonella enterica Serovar Typhimurium That Triggers Cell Death

Menghan Li, Purdue University

Abstract

Salmonella species are Gram-negative intracellular bacterial pathogens that infect a broad range of hosts. In humans, infections by Salmonella enterica serovar Typhimurium and Typhi cause acute gastroenteritis and typhoid fever, respectively. Central to Salmonella virulence is their ability to invade and replicate in nonphagocytic cells. To achieve a successful infection, Salmonella enterica serovars have evolved sophisticated virulence mechanisms to manipulate host cell responses for their own benefit. These include specialized secretion systems capable of injecting "effector" proteins into the host cells to modulate various signaling pathways. Two type 3 secretion systems (T3SS) encoded in Salmonella Pathogenicity Island 1(SPI1) and 2 (SPI2) (T3SS-1 and T3SS-2, respectively) are responsible for the effector delivery. The T3SS uses a needle-like apparatus that penetrates the host cell membrane and delivers effectors directly into the host cell from the bacterial. While the T3SS apparatus are highly conserved, the effector proteins are specialized to target their corresponding host cellular processes, which are necessary for a successful infection. So far more than 60 effectors have been characterized as Salmonella T3SS substrates. Upon contact with the host cells, T3SS-1 (T3SS encoded in SPI-1) is activated leading to the translocation of effectors into host cells to promote bacterial entry, and to aid the formation of the Salmonella Containing Vacuole (SCV). These effectors include, but are not limited to, Salmonella invasion protein (Sip) A, SipC, Salmonella outer protein (Sop) B, SopE and SptP. Once inside, the T3SS-2 steers the maturation of the SCV and facilitates Salmonella survival and replication. Approximately 30 effector proteins are responsible for this process by maintaining the integrity of the SCV, preventing oxidative killing, interfering with the host cytoskeleton and immune signaling. A recent study indicated that SPI-2 T3SS mediates Salmonella exit from infected cells to infect neighboring cells, leading to the formation of new infection foci and facilitates the spreading of the bacterium in the systemic infection(Grant, et al. 2012). Much evidence indicates that there are additional effectors which have not been identified yet. Firstly, effectors are not always encoded in the pathogenicity islands, some are scattered elsewhere on the chromosome. Thus it is not easy to predict where the effectors are. Secondly, unlike the T3SS apparatus, effector proteins are less conserved between species, making them difficult to be identified. Lastly, several T3SS associated phenotypes, including avoidance of NADPH oxidase-dependent killing, delayed macrophage apoptosis and altered subcellular localization of iNOS (inducible nitric oxide synthase), cannot be explained by the known effectors. This means there must be additional effectors with undefined features that are responsible for these phenotypes. In this study, we discovered 13 novel Salmonella effector proteins by detecting of SPI1-dependent translocation. It is demonstrated that one of the effector proteins, YggG, is able to induce apoptotic cell death and facilitate Salmonella infection in mouse. YggG contains a Zn-metalloprotease domain and mutation in this motif abrogates its activity toward Salmonella pathogenesis. Eliciting host cell death during microbial infection is one of the survival strategy of pathogen, and our study discovered YggG as a new regulator of host cell death pathway during Salmonella infection.

Degree

Ph.D.

Advisors

Zhou, Purdue University.

Subject Area

Biology|Physiology

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