LAUSR

Background Patients with inactive thyroid hormone (TH) transporter MCT8 display intellectual disability due to an insufficient TH transport and action in the CNS. As a therapeutic strategy, application of Triac (3, 5, 3’-triiodothyroacetic acid) and Ditpa (3, 5 -diiodo-thyropropionic acid) have been proposed as both thyromimetic compounds are not dependent on MCT8 for cellular entry. Here, we tested and directly compared the thyromimetic actions of Triac versus Ditpa in Mct8/Oatp1c1 double knockout mice (Dko), a mouse model for human MCT8 deficiency. Methods Newborn Dko mice were daily injected during the first three postnatal weeks with either Triac (50 ng/g or 400 ng/g) or Ditpa (400 ng/g or 4000 ng/g) and compared with Wt and Dko mice receiving saline injections. A second cohort of Dko mice was daily injected with Triac (400 ng/g) only between postnatal week 3 and 6. Thyromimetic effects in the CNS and peripheral tissues were monitored at different postnatal time points by immunofluorescence stainings for neural marker proteins, in situ hybridization and quantitative real time PCR. Locomotor performance was assessed in rotarod and hanging wire test. Acute brain slices of Triac treated Dko mice and their respective controls were used for electrophysiological recordings. Results Only Dko mice injected with Triac (400 ng/g) during the first three postnatal weeks showed normalized myelination, differentiation of cortical GABAergic interneurons as well as locomotor performance. Electrophysiological recordings revealed an increased frequencies of cortical spontaneous miniature inhibitory postsynaptic currents in Dko mice and a normalization of this parameter in Triac treated Dko mice. In comparison, treatment of Dko mice with Ditpa at 4000 ng/g during the first three postnatal weeks resulted in normal myelination and cerebellar development but was less effective in restoring neuronal parameters and locomotor function. Finally, Triac was more potent than Ditpa in suppressing Trh and Tshb expression, respectively, and exerts stronger thyromimetic effects in liver and kidneys. Conclusions In newborn Dko deficient mice, Triac is highly effective and more efficient than Ditpa in promoting CNS maturation and function. Yet, Triac treatment needs to be initiated directly after birth to achieve the most beneficial effects.