Thyroid hormones play vital roles in fetal brain development. Mutations in MCT8, recently recognised as a specific thyroid hormone transporter, define a novel syndrome of severe X-linked psychomotor retardation and thyroid hormone resistance.
N-TERA-2 (NT2) cells (human embryonal cells with characteristics of CNS precursors) were transiently transfected with either WT MCT8 or its L471P, R271H or S448X mutations, described in males affected by severe psychomotor impairment. WT MCT8 showed intense cell surface expression, and co-localized with the plasma membrane marker CD8. R271H MCT8 showed a similar expression pattern. L471P and S448X MCT8 failed to reach the plasma membrane, co-localizing with a marker for the endoplasmic reticulum (ER). Accumulation of L471P MCT8 did not induce ER stress, assessed through C/EBP homologous protein expression in transfected cells.
MTT and tritiated thymidine uptake assays were used to assess the effects of WT or mutated MCT8 on proliferation of NT2 or JEG-3 cells (an MCT8 null cell line), grown in T3 replete or deplete media. Addition of 10nM T3 induced NT2 cell proliferation (8.8% increase, P=0.011), but did not affect proliferation of JEG-3 cells. In contrast, transfection of WT MCT8, with or without T3, dramatically reduced proliferation (33% and 24% reductions for NT2 and JEG-3 proliferation respectively). An analogous effect was seen in cells transfected with either R271H or S448X MCT8. Over-expression of L471P MCT8, however, had no effect on NT-2 proliferation compared to control cells, and induced only a 12% reduction in JEG-3 proliferation. An alternative monocarboxylate transporter, hMCT4, failed to replicate these reductions in proliferation, indicating an MCT8-specific effect. Further, neither WT nor mutated MCT8 augmented activity of malic enzyme thyroid hormone response element in luciferase assays. These results indicate a potential role for MCT8 in addition to T3 transport, mediated through the modulation of cell proliferation in the developing brain.