Efficient Activation of Pathogenic ΔPhe501 Mutation in Monocarboxylate Transporter 8 by Chemical and Pharmacological Chaperones

Monocarboxylate transporter 8 (MCT8) is a thyroid hormone transmembrane transporter expressed in many cell types, including neurons. Mutations that inactivate transport activity of MCT8 cause severe X-linked psychomotor retardation in male patients, a syndrome originally described as the Allan-Herndon-Dudley syndrome. Treatment options currently explored the focus on finding thyroid hormone-like compounds that by pass MCT8 and enter cells through different transporters. Because MCT8 is a multipass transmembrane protein, some pathogenic mutations affect membrane trafficking while potentially retaining some transporter activity. We explore here the effects of chemical and pharmacological chaperones on the expression and transport activity of the MCT8 mutant Delta Phe501. Dimethylsulfoxide, 4-phenylbutyric acid as well as its sodium salt, and the isoflavone genistein increase T-3 uptake into MDCK1 cells stably transfected with mutant MCT8 Delta Phe501. We show that Delta Phe501 represents a temperature-sensitive mutant protein that is stabilized by the proteasome inhibitor MG132. 4-Phenylbutyrate has been used to stabilize Delta Phe508 mutant cystic fibrosis transmembrane conductance regulator protein and is in clinical use in patients with urea cycle defects. Genistein is enriched in soy and available as a nutritional supplement. It is effective in stabilizing MCT8-Delta Phe501 at 100 nM concentration. Expression of the L471P mutant is increased in response to phenylbutyrate, but T-3 uptake activity is not induced, supporting the notion that the chaperone specifically increases membrane expression. Our findings suggest that certain pathogenic MCT8 mutants may be responsive to (co-)treatment with readily available compounds, which increase endogenous protein function.