Academic journal article Genetics

Mos1 Mutagenesis Reveals a Diversity of Mechanisms Affecting Response of Caenorhabditis Elegans to the Bacterial Pathogen Microbacterium Nematophilum

Academic journal article Genetics

Mos1 Mutagenesis Reveals a Diversity of Mechanisms Affecting Response of Caenorhabditis Elegans to the Bacterial Pathogen Microbacterium Nematophilum

Article excerpt

ABSTRACT

A specific host-pathogen interaction exists between Caenorhabditis elegans and the gram-positive bacterium Microbacterium nematophilum. This bacterium is able to colonize the rectum of susceptible worms and induces a defensive tail-swelling response in the host. Previous mutant screens have identified multiple loci that affect this interaction. Some of these loci correspond to known genes, but many bus genes [those with a bacterially unswollen (Bus) mutant phenotype] have yet to be cloned. We employed Mos1 transposon mutagenesis as a means of more rapidly cloning bus genes and identifying new mutants with altered pathogen response. This approach revealed new infection-related roles for two well-characterized and much-studied genes, egl-8 and tax-4. It also allowed the cloning of a known bus gene, bus-17, which encodes a predicted galactosyltransferase, and of a new bus gene, bus-19, which encodes a novel, albeit ancient, protein. The results illustrate advantages and disadvantages of Mos1 transposon mutagenesis in this system.

ALL animals and plants coexist with potentially lethal pathogens. Whether or not a pathogen infects a particular host, and whether an infection proves lethal, depends on the response of both the pathogen and host to these encounters. Most multicellular organisms rely on only two lines of defense against pathogens: a mechanical defense provided by the epidermis and an innate immune defense provided by germ-line-encoded pathogen recognition and antimicrobial factors and responses. Pathogen success relies on being able to exploit or evade these defenses. For example, carbohydrates on host surfaces are often targets for binding by pathogens (for examples see Johnson 1999). As the outer surfaces ofnematodes such as Caenorhabditis elegans arecomposed mainly of sugar-modified proteins and lipids (for review see BLAXTER and BIRD 1997), it is not surprising to find that the surface of the worm is a site of adherence for pathogens (JANNSON 1994; MENDOZA DE GIVES et al. 1999; HODGKIN et al. 2000; JOSHUA et al. 2003; HOFLICH et al. 2004).

Many studies have shown that C. elegans mounts conserved innate immune responses against invading pathogens. Specifically, evolutionarily distant organisms, such as plants, mammals, and nematodes, employ a p38 MAP kinase cascade or a TGFb pathway in defense against bacterial and fungal pathogens (for reviews see MILLET and EWBANK 2004; SCHULENBURG et al. 2004; GRAVATO-NOBRE and HODGKIN 2005; SIFRI et al. 2005). Further, a nematode DAF-2 insulin-like receptor/IGF signaling pathway contributes to protection against pathogens (GARSIN et al. 2003). This pathway is also implicated in environmental stress response pathways and longevity in C. elegans as well as other organisms (KENYON et al. 1993; TATAR et al. 2003; BABA et al. 2005; GAMI and WOLKOW 2006). These studies have identified conserved host factors used against virulent pathogens with a generalized host range. However, host factors that have coevolved in response to pathogens with a narrow host range may have been missed.

When C. elegans is exposed to Microbacterium nematophilum it becomes sick and develops a swollen tail region, referred to as the deformed anal region (Dar) phenotype. Previous work has shown that this swelling response is mediated through the ERK/MAPK pathway (NICHOLAS and HODGKIN 2004).Worms with mutations in this pathway fail to swell after exposure to M. nematophilum; instead, these mutants experience severe constipation and greatly increased rate of larval arrest, adult sterility, or death. This implicates the ERK/MAPK pathway as a defensive signaling mechanism in the context of this infection.

Previous forward genetic screens have identified many loci that are involved in this host-pathogen interaction. From ethyl methanesulfonate (EMS) and mut-7 transposon screens, >20 loci were found to affect tail swelling in the presence of M. nematophilum, which is a conspicuous and easily scored phenotype (GRAVATO-NOBRE et al. …

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