Insights on the role of thyroid hormone transport in neurosensory organs and implication for the Allan-Herndon-Dudley syndrome

Thyroid hormones play an important role during the development and functioning of the different sensory systems. In order to exert their actions, thyroid hormones need to access their target cells through transmembrane transporter proteins, among which the monocarboxylate transporter 8 (MCT8) stands out for its pathophysiological relevance. Mutations in the gene encoding for MCT8 lead to the Allan-Herndon-Dudley syndrome (AHDS), a rare disease characterised by severe neuromotor and cognitive impairments. The impact of MCT8 deficiency in the neurosensory capacity of AHDS patients is less clear, with only a few patients displaying visual and auditory impairments. In this review we aim to gather data from different animal models regarding thyroid hormone transport and action in the different neurosensory systems that could aid to identify potential neurosensorial alterations in MCT8-deficient patients.

Neurovascular unit disruption and blood-brain barrier leakage in MCT8 deficiency

BACKGROUND

The monocarboxylate transporter 8 (MCT8) plays a vital role in maintaining brain thyroid hormone homeostasis. This transmembrane transporter is expressed at the brain barriers, as the blood-brain barrier (BBB), and in neural cells, being the sole known thyroid hormone-specific transporter to date. Inactivating mutations in the MCT8 gene (SLC16A2) cause the Allan-Herndon-Dudley Syndrome (AHDS) or MCT8 deficiency, a rare X-linked disease characterized by delayed neurodevelopment and severe psychomotor disorders. The underlying pathophysiological mechanisms of AHDS remain unclear, and no effective treatments are available for the neurological symptoms of the disease.

METHODS

Neurovascular unit ultrastructure was studied by means of transmission electron microscopy. BBB permeability and integrity were evaluated by immunohistochemistry, non-permeable dye infiltration assays and histological staining techniques. Brain blood-vessel density was evaluated by immunofluorescence and magnetic resonance angiography. Finally, angiogenic-related factors expression was evaluated by qRT-PCR. The studies were carried out both in an MCT8 deficient subject and Mct8/Dio2KO mice, an AHDS murine model, and their respective controls.

RESULTS

Ultrastructural analysis of the BBB of Mct8/Dio2KO mice revealed significant alterations in neurovascular unit integrity and increased transcytotic flux. We also found functional alterations in the BBB permeability, as shown by an increased presence of peripheral IgG, Sodium Fluorescein and Evans Blue, along with increased brain microhemorrhages. We also observed alterations in the angiogenic process, with reduced blood vessel density in adult mice brain and altered expression of angiogenesis-related factors during brain development. Similarly, AHDS human brain samples showed increased BBB permeability to IgG and decreased blood vessel density.

CONCLUSIONS

These findings identify for the first time neurovascular alterations in the MCT8-deficient brain, including a disruption of the integrity of the BBB and alterations in the neurovascular unit ultrastructure as a new pathophysiological mechanism for AHDS. These results open a new field for potential therapeutic targets for the neurological symptoms of these patients and unveils magnetic resonance angiography as a new non-invasive in vivo technique for evaluating the progression of the disease.

Maternal Administration of the CNS-Selective Sobetirome Prodrug Sob-AM2 Exerts Thyromimetic Effects in Murine MCT8-Deficient Fetuses

Background: Monocarboxylate transporter 8 (MCT8) deficiency is a rare X-linked disease where patients exhibit peripheral hyperthyroidism and cerebral hypothyroidism, which results in severe neurological impairments. These brain defects arise from a lack of thyroid hormones (TH) during critical stages of human brain development. Treatment options for MCT8-deficient patients are limited and none have been able to prevent or ameliorate effectively the neurological impairments. This study explored the effects of the TH agonist sobetirome and its CNS-selective amide prodrug, Sob-AM2, in the treatment of pregnant dams carrying fetuses lacking Mct8 and deiodinase type 2 (Mct8/Dio2 KO), as a murine model for MCT8 deficiency. Methods: Pregnant dams carrying Mct8/Dio2 KO fetuses were treated with 1 mg of sobetirome/kg body weight/day, or 0.3 mg of Sob-AM2/kg body weight/day for 7 days, starting at embryonic day 12.5 (E12.5). As controls, pregnant dams carrying wild-type and pregnant dams carrying Mct8/Dio2 KO fetuses were treated with daily subcutaneous injections of vehicle. Dams TH levels were measured by enzyme-linked immunosorbent assay (ELISA). Samples were extracted at E18.5 and the effect of treatments on the expression of triiodothyronine (T3)-dependent genes was measured in the placenta, fetal liver, and fetal cerebral cortex by real-time polymerase chain reaction. Results: Maternal sobetirome treatment led to spontaneous abortions. Sob-AM2 treatment, however, was able to cross the placental as well as the brain barriers and exert thyromimetic effects in Mct8/Dio2 KO fetal tissues. Sob-AM2 treatment did not affect the expression of the T3-target genes analyzed in the placenta, but it mediated thyromimetic effects in the fetal liver by increasing the expression of Dio1 and Dio3 genes. Interestingly, Sob-AM2 treatment increased the expression of several T3-dependent genes in the brain such as Hr, Shh, Dio3, Kcnj10, Klf9, and Faah in Mct8/Dio2 KO fetuses. Conclusions: Maternal administration of Sob-AM2 can cross the placental barrier and access the fetal tissues, including the brain, in the absence of MCT8, to exert thyromimetic actions by modulating the expression of T3-dependent genes. Therefore, Sob-AM2 has the potential to address the cerebral hypothyroidism characteristic of MCT8 deficiency from fetal stages and to prevent neurodevelopmental alterations in the MCT8-deficient fetal brain.

Deficient thyroid hormone transport to the brain leads to impairments in axonal caliber and oligodendroglial development

Mutations in the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to profound brain alterations, including myelination impairments, in humans. We aimed to further explore the pathophysiological mechanisms underlying the MCT8 deficiency-associated myelination impairments to unravel new biomarkers and therapeutic targets. We have performed brain histological analysis on an MCT8-deficient subject and histological, ultrastructural, and magnetic resonance imaging (MRI) analysis in the brain of a mouse model of the syndrome, lacking MCT8 and enzyme deiodinase type 2 (DIO2, Mct8/Dio2 KO). We have found that the MCT8-deficient subject presents severely reduced myelin lipid and protein staining and increased proportion of small-caliber myelinated axons in detriment of large-caliber ones. Mct8/Dio2 KO mice present myelination impairments and abnormal oligodendroglial development. We conclude that the greater proportion of small-caliber axons and impairments in the oligodendroglia lineage progression arise as potential mechanisms underlying the permanent myelination defects in MCT8-deficiency. Moreover, we present the Mct8/Dio2 KO mouse model, and MRI as a non-invasive biomarker, as highly valuable tools for preclinical studies involving MCT8 deficiency. These findings contribute to the understanding of the pathological mechanisms in MCT8 deficiency and suggest new biomarkers and therapeutic targets to consider therapeutic options for the neurological defects in patients.