Application of NanoBiT for Monitoring Dimerization of the Null Hong Kong Variant of α-1-Antitrypsin,NHK, in Living Cells

In this study, we investigated expression and dimerization of an ER-associated degradation (ERAD) substrate, a null Hong Kong variant of α-1-antitrypsin (NHK) using immunoblotting assay and a novel NanoLuc complementary reporter system called the NanoBiT (NB) assay. This NB-tagged NHK made it possible to monitor the intra- and extracellular status of NHK in living cells. The values for this NB assay fluctuated in response to distinct pharmacological stimuli and co-transfection of several ERAD-related factors. We then focused on mesencephalic astrocyte-derived neurotrophic factor (MANF), an unclarified ATF6/IRE1-downstream target, and established MANF-deficient Neuro2a (N2a) cells using CRISPR/Cas9 system. MANF-deficient N2a significantly elevated OS-9 protein after tunicamycin treatment; however, no specific differences in intra- and extracellular status of NHK protein were observed between wild-type and MANF-deficient cells. Taken together, intrinsic MANF in N2a cells is not strongly associated with the accumulation and clearance of unfolded proteins within the ER under current condition, but this novel NB assay is a useful approach for characterizing the protein status including ERAD substrates.