Full text loading...
Chapter 20 : Role of Cyclic Di-GMP in Vibrio cholerae Virulence
Category: Microbial Genetics and Molecular Biology
Ebook: Choose a downloadable PDF or ePub file. Chapter is a downloadable PDF file. File must be downloaded within 48 hours of purchase
The bacterial allosteric regulator and secondary messenger cyclic di-GMP (c-di-GMP) plays an integral role in the life cycle of pathogenic Vibrio cholerae by helping to mediate the environment-to-host transition. The regulation of the intracellular concentration of c-di-GMP in V. cholerae is complex, as there is an abundance of genes coding for diguanylate cyclases (DGCs) and phosphodiesterase (PDEs). This chapter focuses on the role of c-di-GMP in the regulation of V. cholerae virulence mechanisms during colonization and dissemination. The two most important virulence factors expressed by V. cholerae are cholera toxin (CT) and the toxin-coregulated pilus (TCP). In vitro expression studies have shown that c-di-GMP has a negative effect on virulence gene expression, specifically CT. Although V. cholerae motility has been well studied, the role of motility in pathogenesis remains unclear. There is an abundance of information regarding physiological behaviors regulated by c-di-GMP, but relatively little is known about the mechanism(s) of this regulation. It is understood that the intracellular concentration of c-di-GMP is modulated by the enzymatic activities of DGCs and PDEs, but it is not known how these changes in concentration signal for the vast alterations in bacterial behavior are observed. Identification of new regulatory pathways will provide new insight into the cdi-GMP regulatory circuit and will help to explain how this secondary messenger molecule is so important to V. cholerae pathogenesis and environmental fitness.
Key Concept Ranking
- Enzyme-Linked Immunosorbent Assay
The ToxR regulon in V. cholerae is repressed by c-di-GMP. Arrows indicate activation of genes or proteins; parallel lines in place of an arrowhead represent repression. The predicted interaction of c-di-GMP with the regulon is shown. High c-di-GMP concentration results in decreased toxT transcription, consequently causing reduced ctxAB expression and CT production. c-di-GMP may also regulate ToxT activity, because tcp gene expression is unaffected by high c-di-GMP. CRP, cAMP receptor protein.
RIVET. RIVET was used to examine the expression of V. cholerae genes during infection of the infant mouse. Transcriptional fusions of tnpR, which encodes a resolvase, to promoters of interest (here, Px) are made. When a promoter is activated in response to undefined signals present in the host, TnpR is produced and targets res sites (indicated by grey rectangles). The res sites flank neo (kanamycin resistance) and sacB (sucrose sensitivity) genes, and recognition by TnpR results in excision, or resolution, of neo-sacB. Thus, loss of kanamycin resistance and sucrose sensitivity represents expression of the gene of interest in a given condition ( 71 ). RIVET was used to show that c-di-GMP concentration is decreased upon infection to allow maximal virulence gene expression ( 15 , 42 ). In addition, a modified RIVET was used to identify genes expressed late during infection, including three GGDEF genes and one GGDEF-EAL hybrid.
c-di-GMP regulates motility gene expression in V. cholerae. Schematic of V. cholerae motility gene hierarchy, represented by the function of genes within each class. Classes are denoted by roman numerals I to IV. Gene regulators or proteins of note are named. Arrows indicate activation of genes, and parallel lines in place of an arrowhead indicate gene repression. Transcriptional profiling studies have shown that c-di-GMP represses the expression of all class III and IV genes and some class II genes, including FliA, which is required for expression of class IV genes. Despite this information, there is no model to account for the effects of c-di-GMP on V. cholerae motility at this time.
The intracellular concentration of c-di-GMP fluctuates throughout the life cycle of V. cholerae. The solid line indicates the relative level of intracellular c-di-GMP, which is predicted to oscillate as V. cholerae shifts from aquatic reservoirs (ex vivo) to the host small intestine (in vivo) and back again; dashed lines demarcate these transition points. c-di-GMP is predicted to be high in the aquatic environment and in the biofilm state. The induction of PDEs upon entry into the host and the negative effect of c-di-GMP, in vitro and in vivo, on virulence gene expression suggest that c-di-GMP must be lowered upon infection. When V. cholerae is disseminated from the host, c-di-GMP must be elevated again to aid survival in the environment. Evidence suggests that elevation of c-di-GMP by V. cholerae begins prior to exiting the host during advanced cholera, possibly in response to changing conditions in the intestine.