A phylogenetic view of the leukocyte ectonucleotidases

The leukocyte ectonucleotidases are a recently defined family included in the last Human Leukocyte Differentiation Antigens Workshop, giving prominence to these membrane proteins whose catalytic activity is expressed outside the cell. Among the most important substrates of the leukocyte ectonucleotidases are extracellular ATP and NAD+ whose transient increases are not immunologically silent but rather perceived as danger signals by the host. Among the host responses to the release of ATP, NAD+ and related small molecules is their breakdown on behalf of a panel of leukocyte ectonucleotidases - CD38, CD39, CD73, CD157, CD203a and CD203c -, whose activities are concatenated to form two nucleotide-catabolizing channels defined as the canonical and non-canonical adenosinergic pathways. Here, after briefly reviewing the structure and function of the proteins involved in these pathwys, we focus on the genes encoding the ectoenzymes of these adenosinergic pathways. The chromosomal localizations of the enzyme-encoding genes yield a first level of information concerning their origins by duplication and modes of regulation. Further information was obtained from phylogenetic analyses that show ectoenzyme orthologs are conserved in major tetrapod species whereas examination of synteny conservation revealed that the chromosomal regions harboring the ADP-ribosyl cyclases on human chromosome 4 and the ENTPDase CD39 on chromosome 10 show striking similarities in gene content consistent with their being paralogous chromosomal regions derived from a vertebrate whole genome duplication. Thus the connections between some of the leukocyte ectoenzymes run deeper than previously imagined.