Surface translocation and tri-iodothyronine uptake of mutant MCT8 proteins are cell type-dependent

Mutations in the gene encoding the thyroid hormone transporter, monocarboxylate transporter 8 (MCT8), underlie severe mental retardation. We wanted to understand the functional consequences of a series of missense mutations in MCT8 in order to identify therapeutic options for affected patients. We established cell lines stably expressing 12 MCT8 variants in JEG1 and MDCK1 cells. The cell lines were characterized according to MCT8 mRNA and protein expression, tri-iodothyronine (T-3) transport activity, substrate K-M characteristics, surface expression, and responsiveness to T-3 preincubation and chemical chaperones. Functional activities of ins235V and L568P MCT8 mutants depend on the cell type in which they are expressed. These mutants and R271H exhibited considerable transport activity when present at the cell surface as verified by surface biotinylation and kinetic analysis. Most mutants, however, were inactive in T-3 transport even when present at the cell surface (e.g. S194F, A224V, Delta F230, L512P). Preincubation of G558D with T-3 increased T-3 uptake in MDCK1 cells to a small, but significant, extent. Chemical chaperones were ineffective. The finding that the cell type determines surface expression and T-3 transport activities of missense mutants in MCT8 may be important to understand phenotypic variability among carriers of different mutations. In particular, the clinical observation that the severity of derangements of thyroid hormone levels does not correlate with mental impairments of the patients may be based on different residual activity of mutant MCT8 in different cell types.