ShAR1alpha and ShAR1beta: novel putative nicotinic acetylcholine receptor subunits from the platyhelminth blood fluke Schistosoma.

The cDNAs for two novel neuronal-type nicotinic acetylcholine receptor (nAChR) subunits have been cloned and characterised from the parasitic trematode blood fluke Schistosoma haematobium. One of these encodes a putative nAChR alpha-subunit named ShAR1alpha, whilst the second encodes a potential non-alpha subunit, ShAR1beta. These ShARs possess the key structural features common to all nAChRs, but they are unusual in that they have very large cytoplasmic domains spanning M3 and M4. Overall, the ShAR1alpha and ShAR1beta proteins share 37% identity and 53% similarity, but excluding the residues of the M3-M4 domain this rises to 52% identity and 71% similarity. Sequence comparisons with other nAChR polypeptides indicate that both ShARs are most similar to the invertebrate alpha7-like subunits identified in insects and nematodes, and to the vertebrate subunits alpha7 and alpha8. Outside of the M3-M4 domain, 45% and 40%, respectively, of the ShAR1alpha and ShAR1beta residues are conserved in the ACR-16 subunit from Caenorhabditis elegans. Phylogenetic analysis suggests that the ShARs share a common lineage with members of the ACR-16 group as well as alpha7 and alpha8. Immunolocalisation studies revealed distinct and non-overlapping patterns of distribution for ShAR1alpha and ShAR1beta within the parasite. ShAR1beta was localised within the musculature and on discrete cell bodies within the connective parenchyma. In contrast, ShAR1alpha was localised exclusively to the surface membranes, suggesting it may contribute to the regulatory nAChR we have characterised previously. In Xenopus oocyte expression studies, ShAR1alpha did not form functional channels on its own or in combination with ShAR1beta or the chick beta2 subunit. Furthermore, a chimera in which the M3-M4 domain of ShAR1alpha was replaced with that of chick alpha7 was also non-functional. We discuss our findings in the context of the proposed role for surface nAChRs in the regulation of glucose uptake in the parasite, and the potential exploitation of these receptors as targets for cholinergic schistosomicides.