Genetic and functional characterization of Caenorhabditis elegans srf-3, a gene involved in regulating surface antigenicity

The widespread emergence of pathogens resistant to the majority of
available antibiotics makes it necessary to find new ways to combat
the corresponding microorganisms. Adaptive immunity is specific to
vertebrates but the mechanisms of innate immunity are ancient and
highly conserved during evolution. Therefore it is reasonable to
use model organisms to study the molecular mechanisms of host
defence and the corresponding mechanisms of pathogenicity.
Microbacterium nematophilum adheres to the rectum of a
Caenorhabditis elegans animal inducing a localized non-lethal
response that causes swelling of the underlying hypodermal tissue.
The aim of this thesis was to gain a first insight into the
molecular mechanisms underlying the resistance of srf-3 animals to
the bacterial pathogens Microbacterium nematophilum and Yersinia
pestis/pseudotuberculosis. M. nematophilum adheres to the cuticle
of wild type animals but fails to adhere to the surface of srf-3
worms. This is a novel type of resistance because pathogens like P.
aeruginosa or Salmonella typhimurium do not adhere to the cuticle
but kill C. elegans by colonization and accumulation in the
intestine. Molecular cloning of srf-3 showed that this gene codes
for a type III transmembrane protein similar to the family of
UDP-galactose transporters. Expression analysis revealed that SRF-3
is expressed in a set of active secretory cells consistent with a
function of this gene in cuticle or surface modification. A
functional characterization of SRF-3 revealed that this protein can
function as a nucleotide sugar transporter. The protein showed
multisubstrate specificity capable of translocating UDP-galactose
and UDP-N-acetylglucosamine in vitro as judged by transport assays
done with Golgi/ER enriched vesicles, as well as in vivo, as shown
by the phenotypic correction of mutants defective in UDP-galactose
or UDP-N-acetylglucosamine transport. The data presented in this
thesis emphasize the importance of glycosylation in regulating the
surface antigenicity of C. elegans. This can help to understand the
process of pathogen adherence, the first step in the establishment
of an infection, as well as how parasitic nematodes modulate the
surface in order to escape the host response.