Delineation Of The Functional Properties Exhibited By The Zinc-Activated Channel (Zac) And Its High-Frequency Thr(128) Ala Variant (Rs2257020) In Xenopus Oocytes

The signalling characteristics of the Zinc-Activated Channel (ZAC), a member of the Cys-loop receptor (CLR) superfamily, are presently poorly elucidated. The ZACN polymorphism c.454G>A encoding for the Thr(128) Ala variation in ZAC is found in extremely high allele frequencies across ethnicities. In this, the first study of ZAC in Xenopus oocytes by TEVC electrophysiology, ZAC(T)(hr)(128) and ZAC(Ala128 )exhibited largely comparable pharmacological and signalling characteristics, but interestingly the Zn2+- and H+-evoked current amplitudes in ZAC(Ala128) oocytes were dramatically smaller than those in ZAC(Thr128)-oocytes. While the variation thus appeared to impact cell surface expression and/or channel properties of ZAC, the similar expression properties exhibited by ZAC(Thr128) and ZAC(Ala128) in transfected mammalian cells indicated that their distinct functionalities could arise from the latter. In co-expression experiments, wild-type and variant ZAC subunits assembled efficiently into "hetemmeric" complexes in HEK293 cells, while the concomitant presence of ZAC(Ala128) in ZAC(Thr128) :ZAC(Ala128) oocytes did not exert a dominant negative effect on agonist-evoked current amplitudes compared to those in ZAC(Thr128)-oocytes. Finally, the structural determinants of the functional importance of the 1-hydroxyethyl sidechain of Thr(128) appeared to be subtle, as agonist-evoked current amplitudes in ZAC(Ser128)-, ZAC(Val128)- and ZAC(Ile128)-oocytes also were substantially lower than those in ZAC(Thr128)-oocytes. In conclusion, the functional properties exhibited by ZAC in this work substantiate the notion of it being an atypical CLR. While the impact of the Thr(128 )Ala variation on ZAC functionality in oocytes is striking, it remains to be investigated whether and to which extent this translates into an in vivo setting and thus could constitute a source of inter-individual variation in ZAC physiology.