Interactions of the thymic polypeptide hormone thymopoietin with neuronal nicotinic alpha-bungarotoxin binding sites and with muscle-type, but not ganglia-type, nicotinic acetylcholine receptor ligand-gated ion channels

Abstract

Studies were conducted to assess the ability of the thymic polypeptide hormone thymopoietin (TPO) to interact with proto-typical ganglia-type or muscle-type nicotinic acetylcholine receptor ion channels (nAcChoR). Also investigated were interactions of TPO with neuronal nicotinic alpha-bungarotoxin binding sites (nBgtS), which share many features with nAcChoR and may belong to an extended nAcChoR family but do not appear to function as simple ligand-gated ion channels. TPO and alpha-bungarotoxin (Bgt) share the capacity for high affinity (IC50 values in the nanomolar range) interaction with nBgtS, which are expressed as high affinity radioiodinated Bgt binding sites by cells of the SH-SY5Y or IMR-32 human neuroblastomas. TPO and Bgt also share the capacity for high affinity interaction with muscle-type nAcChoR, which are expressed as high affinity binding sites for radioiodinated Bgt or tritium-labeled acetylcholine by cells of the TE671/RD human clone or the BC3H-1 mouse muscle cell line or on membrane preparations from Torpedo electroplax. TPO and Bgt act acutely as high affinity antagonists of muscle-type nAcChoR functional responses, which are measured using an isotopic rubidium ion efflux assay, on TE671/RD or BC3H-1 cells. In contrast, neither TPO nor Bgt are effective, at doses of up to 1 microM, as antagonists of ganglia-type nAcChoR function on SH-SY5Y or IMR-32 cells, nor are they potent as inhibitors of high affinity tritium-labeled acetylcholine binding to sites on putative ganglia-type nAcChoR expressed by SH-SY5Y or IMR-32 cells. These data indicate that some members of the extended nAcChoR family, including nBgtS and functional muscle-type nAcChoR but not ganglia-type nAcChoR, can interact with either Bgt or TPO. The results suggest that TPO may be an endogenous ligand active in both the nervous and immune systems and that some of its actions may be mediated via nBgtS or via functional blockade of muscle-type nAcChoR.