Doreen Braun, Ulrich Schweizer

Abstract: 

Monocarboxylate transporter 8 (MCT8) is a thyroid hormone transmembrane transporter expressed in many cell types, including neurons. Mutations which 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 focus on finding thyroid hormone-like compounds which bypass MCT8 and enter cells through different transporters. SinceMCT8is 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 expression and transport activity of the MCT8 mutant Phe501. Dimethylsulfoxide, 4-phenylbutyric acid as well as its sodium salt, and the isoflavone genistein increase T3 uptake into MDCK1 cells stably transfected with mutant MCT8- Phe501. We show that Phe501 represents a temperature sensitive mutant protein which is stabilized by the proteasome inhibitor MG132. 4-phenylbutyrate has been used to stabilizePhe508 mutant CFTR 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- Phe501 at 100 nM concentration. Expression of the L471P mutant is increased in response to phenylbutyrate, but T3-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.