Melanisation of Teladorsagia circumcincta larvae exposed to sunlight: A role for GTP-cyclohydrolase in nematode survival

Trichostrongylid nematode parasites of livestock inhabit two very different niches during their life-cycle; within the host and free-living in the environment. UV radiation plays a significant role in the survival of free-living, pre-parasitic nematode larvae, with different species exhibiting differing levels of sensitivity. In many eukaryotes, melanisation is a key protective mechanism against UV damage, however there is little information about this process in parasitic nematodes. Caenorhabditis elegans cat-4 mutants, which are deficient in the enzyme guanosine triphosphate-cyclohydrolase I (GTP-CH), have both depleted levels of melanin in their cuticles and an increased sensitivity to anthelmintic drugs. Some parasitic nematodes have very high levels of GTP-CH transcript in their pre-parasitic stages, suggesting an important role for this biopterin synthetic enzyme. Here, we show that the Tci-cat-4 gene, which encodes GTP-CH in Teladorsagia circumcincta, has a role in melanisation and is also capable of rescuing C. elegans cat-4 mutants. In addition, following exposure of T. circumcincta L3s to sunlight, there is a 32% increase in GTP-CH enzyme activity (P = 0.019), and a 21% increase in levels of melanin (P = 0.031) compared with unexposed larvae. These data suggest that one explanation for the high level of GTP-CH present in pre-parasitic stages of trichostrongylid nematodes is to facilitate melanisation in response to UV exposure.

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► UV radiation plays a significant role in the survival of pre-parasitic nematode larvae.
► Some nematodes have high levels of GTP-CH-encoding transcript in pre-parasitic stages.
► UV exposure induces melanisation and GTP-CH activity in Teladorsagia circumcincta larvae.
► Tci-cat-4 (encoding GTP-CH) from T. circumcincta can rescue C. elegans cat-4 mutants.
► Induced GTP-CH activity has a role in protecting T. circumcincta larvae from UV.