Programmed cell death mediated by ced-3 and ced-4 protects Caenorhabditis elegans from Salmonella typhimurium -mediated killing

Abstract

Programmed cell death (PCD) in mammals has been implicated in several disease states including cancer, autoimmune disease, and neurodegenerative disease. In Caenorhabditis elegans, PCD is a normal component of development. We find that Salmonella typhimurium colonization of the C. elegans intestine leads to an increased level of cell death in the worm gonad. S. typhimuriummediated germ-line cell death is not observed in C. elegans ced-3 and ced-4 mutants in which developmentally regulated cell death is blocked, and ced-3 and ced-4 mutants are hypersensitive to S. typhimurium-mediated killing. These results suggest that PCD may be involved in the C. elegans defense response to pathogen attack. S everal human pathogens, including Pseudomonas aeruginosa, Salmonella typhimurium, Serratia marcescens, and Burkholderia pseudomallei, kill the nematode Caenorhabditis elegans when supplied as a food source, and a variety of bacterial virulence factors have been shown to play a role in both nematode and mammalian pathogenesis (1-6). In addition, C. elegans mutants that are either more susceptible or more resistant to bacterial killing can be readily identified (7,8). These facts make C. elegans an attractive host for dissecting the molecular basis of bacterial pathogenesis.An important feature of a variety of mammalian-pathogen interactions is programmed cell death (PCD) of both immune and somatic cells. PCD in mammals has also been implicated in several disease states, including cancer, autoimmune disease, and neurodegenerative disease (9). Because many of the key components of the mammalian cell death machinery were first identified by genetic studies in C. elegans (10, 11), it was of interest to determine the role, if any, of PCD in C. elegans in response to pathogen attack.Mutations in at least three C. elegans genes, ced-3, ced-4, and ced-9, affect the 131 somatic cell deaths that occur during the development of the C. elegans hermaphrodite (12, 13). Loss-offunction mutations in ced-3 or ced-4 or a gain-of-function mutation in the gene ced-9 result in the survival of cells that normally die. CED-3 and CED-4 proapoptotic activity is antagonized by the Bcl-2 family member CED-9 (14). In cells fated to die, EGL-1 binds to and directly inhibits the activity of CED-9 (15).The 131 cells that undergo PCD during the somatic development of C. elegans represent about 12% of the cells in an adult worm. In contrast to somatic cells, germ cells do not have a fixed lineage or population of cells. In normal adult hermaphrodites, over half of all potential oocytes are eliminated by PCD that is mediated by the same core machinery (ced-3, ced-4, and ced-9) responsible for somatic PCD during development (16). Germ cell deaths are more abundant in starving and old worms than in well fed young animals (17), suggesting that PCD in the germ line is regulated by environmental conditions. P. aeruginosa and S. typhimurium kill C. elegans by at least three distinct mechanisms when provided to C. elegans as the sole source of food. At least in t...

Germ Cell Apoptosis and DNA Damage Responses

“…Factors such as the rates of apoptotic corpse-engulfment and germ cell proliferation, the precise age of the worm, and even the temperature have to be considered when reporting the number of germ cell corpses, ideally done as time course experiments. Also the possibility that bacterial contamination might trigger germ cell apoptosis has to be considered (Aballay and Ausubel 2001 ;Anton Gartner, unpublished observation). Methods for detecting germ cell corpses and for labelling them with GFP markers or speci fi c dyes have been described (Gartner et al 2004(Gartner et al , 2008 .…”

“…At present we know most about DNA damage response pathways leading to apoptosis. Finally, stresses such as starvation, heat shock, bacterial infection, or a high glucose diet also trigger excessive germ cell apoptosis (Salinas et al 2006 ;Angelo and Van Gilst 2009 ;Aballay and Ausubel 2001 ;Choi 2011 ) .…”

Caspases: an ancient cellular sword of Damocles

“…A recent study found that infection of C. elegans by the pathogenic bacterium Salmonella typhimurium induced cell death in the animal's germ line, and that these deaths were blocked in ced-3 or ced-4 mutant backgrounds. 37 Interestingly, these mutant animals were also more susceptible to Salmonella-mediated killing, even though the mutants' lifespans were not different from wild-type animals in the absence of infection. 37 It may be that the germline cell deaths observed in this infection model (or perhaps somatic cell deaths that remained undetected in the study) provide a measure of bacterial resistance to C. elegans.…”

“…37 Interestingly, these mutant animals were also more susceptible to Salmonella-mediated killing, even though the mutants' lifespans were not different from wild-type animals in the absence of infection. 37 It may be that the germline cell deaths observed in this infection model (or perhaps somatic cell deaths that remained undetected in the study) provide a measure of bacterial resistance to C. elegans. An alternative but perhaps more intriguing possibility is that ced-3 function might be required for the activation of disease-resistance pathways in the soma, just as certain caspases are in higher metazoans (see below).…”

Physiological and Immunological Regulations in Caenorhabditis elegans Infected with Salmonella enterica serovar Typhi

“…Earlier studies have demonstrated that certain genes were induced against S. Typhimurium infection in Caenorhabditis elegans [6,7]. These studies fail to mimic the essential aspects of immunity induced by S. Typhi [8].…”