Wilson disease-causing mutations in the carboxyl terminus of ATP7B regulates its localization and Golgi exit selectively in the unpolarized cells

Mutational inactivation of the P-type Cu-ATPase ATP7B interferes with its cellular functions to varying extent leading to varied cellular phenotypes. Wilson's disease (WD) primarily affects organs composed of polarized/differentiated epithelial cells. Therefore, phenotypic variability might differ depending on the polarization/differentiation of the cells. The present study investigates the intracellular stability and localization of ATP7B harboring WD mutations in both unpolarized/undifferentiated and polarized/differentiated cell-based models. Green fluorescent protein (GFP)-ATP7B harboring the WD causing mutations, N41S, S653Y, R778Q, G1061E, H1069Q, S1423N, S1426I, and T1434M, are included for investigation. The C-terminal WD mutations (S1423N, S1426I, and T1434M), exhibit distinct localization and Cu(I) responsive anterograde and retrograde trafficking in undifferentiated/unpolarized vs. differentiated/polarized cells. While basal localization of the S1423N mutant gets corrected in the differentiated glia, its Cu(I) responsive anterograde and retrograde trafficking behavior is not identical to the wild-type. But localization and trafficking properties are completely rescued for the S1426I and T1434M mutants in the differentiated cells. Comprehensive meta-analysis on the effect of the reported C-terminal mutations on patient phenotype and cultured cells demonstrate discrete regions having distinct effects. While mutations in the proximal C-terminus affect ATP7B stability, the present study shows that the distal region dictates cell-specific Trans Golgi Network (TGN) localization and exit. The localization and export properties are corrected in the differentiated cells, which is a plausible mechanism for the milder phenotype exhibited by these mutations. It highlights the critical role of the C-terminus in cell-specific TGN retention and exit of ATP7B.