Characterization and analysis of the novel CDR gene family in the nematode Caenorhabditis elegans

The mechanism of cellular defense against cadmium toxicity has attracted a great deal of attention. The cdr-1 gene was discovered during a genomic screen for cadmium-responsive genes in C. elegans . Transcription of cdr-1 is significantly induced by cadmium. The CDR-1 protein is expressed exclusively in intestinal cells under cadmium exposure and is targeted to lysosomes. To study the biological function of the CDR-1 protein, CDR-1 expression was inhibited by RNA interference through ingested or injected dsRNA in C. elegans. Strong phenotypes suggest that CDR-1 functions to protect against cadmium toxicity and the biological function of CDR-1 may be in osmoregulation. CDR-1 may function as an ion pump on lysosomal membrane and transport and accumulate cadmium ions into lysosomes, through which it confers resistance to cadmium toxicity. Six homologues of CDR-1 were identified in C. elegans by sequence analysis and subsequently isolated. Sequence comparison among the CDR gene family reveals a high degree of similarity in the lengths of the open reading frames, and the predicted molecular mass, ∼32 kDa. There are high levels of amino acid and nucleotide sequence identities among the homologues. The seven proteins are predicted to be extremely hydrophobic and are all classified as integral membrane proteins. The expression of cdr-1 and cdr-7 can be significantly induced by cadmium. cdr-9 is expressed at a high level in control nematodes, while it is not cadmium-inducible. cdr-5, cdr-6, cdr-8 and cdr-10 are expressed at low levels in both control and cadmium-exposed C. elegans. Therefore, the two homologues, CDR-7 and CDR-9, were selected to study in detail as well as CDR-1. Both cdr-7 and cdr-9 are expressed in all the intestinal cells in control C. elegans. cdr-7 and cdr-9 are expressed in all the post-embryonic developmental stages. In addition, cdr-7 was observed to be transcribed in developing embryos. The biological function of CDR-7 and CDR-9 has been studied with RNA interference. CDR-7 and CDR-9 essentially function in osmoregulation in C. elegans. However, they do not confer resistance to cadmium toxicity. In addition to cadmium, the in vivo effects of arsenic, mercury and zinc on the transcription of cdr-1, cdr-7 and cdr-9 were determined. Besides being strongly induced by cadmium, the transcription of cdr-1 is slightly induced by mercury, but not by arsenic and zinc. cdr-7 is significantly induced by all the metals tested. However, cdr-9 cannot be induced by any of the metals tested. None of CDR-1, CDR-7 and CDR-9 has been observed to confer resistance to mercury toxicity. In intestinal cells, the CDR-7 protein is targeted to lysosomes, as well as CDR-1. (Abstract shortened by UMI.)...