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.This study was supported by MCIN/AEI/10.13039/501100011033, Spain (Grant No. SAF2017-86342-R to AG-F); MCIN/AEI/10.13039/501100011033/FEDER “Una manera de hacer Europa”, Spain (Grant No PID2020-113139RB-I00 to AG-F); Consejo Superior de Investigaciones Científicas, Spain (Grant No. 2020AEP044 to AG-F); the Sherman Foundation, Australia (Grant No. OTR02211 to SB-L and AG-F); Asociación Corriendo con el Corazón por Hugo, Spain (Grant No OTR06190 to AG-F), and the BBSRC, United Kingdom (grant number BB/R016879/1 to SB-L). VV-H is recipient of a contract from MCIN/AEI /10.13039/501100011033/FSE “El FSE invierte en tu futuro”, Spain (Grant No PRE2018-086185), MG-Y from the programa de Formación de Profesorado Universitario, Spain (FPU, FPU19/02006) program from the Ministerio de Ciencia, Innovación y Universidades and DL-E was recipient of a fellowship from the “Fellowship Training Program for Advanced Human Capital, Becas Chile” from National Commission for Scientific and Technological Research (CONICYT), Gobierno de Chile

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

Abstract 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

A CRISPR/Cas9-engineered avatar mouse model of monocarboxylate transporter 8 deficiency displays distinct neurological alterations

International audienceInactivating mutations in the specific thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to an X-linked rare disease named MCT8 deficiency or Allan-Herndon-Dudley Syndrome. Patients exhibit a plethora of severe endocrine and neurological alterations, with no effective treatment for the neurological symptoms. An optimal mammalian model is essential to explore the pathological mechanisms and potential therapeutic approaches. Here we have generated by CRISPR/Cas9 an avatar mouse model for MCT8 deficiency with a point mutation found in two MCT8-deficient patients (P253L mice). We have predicted by in silico studies that this mutation alters the substrate binding pocket being the probable cause for impairing thyroid hormone transport. We have characterized the phenotype of MCT8-P253L mice and found endocrine alterations similar to those described in patients and in MCT8-deficient mice. Importantly, we detected brain hypothyroidism, structural and functional neurological alterations resembling the patient's neurological impairments. Thus, the P253L mouse provides a valuable model for studying the pathophysiology of MCT8 deficiency and in the future will allow to test therapeutic alternatives such as in vivo gene therapy and pharmacological chaperone therapy to improve the neurological impairments in MCT8 deficiency

A CRISPR/Cas9-engineered avatar mouse model of monocarboxylate transporter 8 deficiency displays distinct neurological alterations

Inactivating mutations in the specific thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to an X-linked rare disease named MCT8 deficiency or Allan-Herndon-Dudley Syndrome. Patients exhibit a plethora of severe endocrine and neurological alterations, with no effective treatment for the neurological symptoms. An optimal mammalian model is essential to explore the pathological mechanisms and potential therapeutic approaches. Here we have generated by CRISPR/Cas9 an avatar mouse model for MCT8 deficiency with a point mutation found in two MCT8-deficient patients (P253L mice). We have predicted by in silico studies that this mutation alters the substrate binding pocket being the probable cause for impairing thyroid hormone transport. We have characterized the phenotype of MCT8-P253L mice and found endocrine alterations similar to those described in patients and in MCT8-deficient mice. Importantly, we detected brain hypothyroidism, structural and functional neurological alterations resembling the patient's neurological impairments. Thus, the P253L mouse provides a valuable model for studying the pathophysiology of MCT8 deficiency and in the future will allow to test therapeutic alternatives such as in vivo gene therapy and pharmacological chaperone therapy to improve the neurological impairments in MCT8 deficiency.Ministerio de Ciencia e Innovación (MCIN). AEI/10.13039/501100011033 ; AEI/10.13039/501100011033/FEDER; AEI /10.13039/501100011033/FSEConsejo Superior de Investigaciones CientíficasSherman FoundationAsociación Corriendo con el Corazón por HugoBBSRCMinisterio de Ciencia, Innovación y UniversidadesDepto. de Biología CelularFac. de Ciencias BiológicasTRUEpu