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Monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptide 1C1 (OATP1C1) are thyroid hormone (TH) transmembrane transporters relevant for the availability of TH in neural cells, crucial for their proper development and function. Mutations in MCT8 or OATP1C1 result in severe disorders with dramatic movement disability related to alterations in basal ganglia motor circuits. Mapping the expression of MCT8/OATP1C1 in those circuits is necessary to explain their involvement in motor control. We studied the distribution of both transporters in the neuronal subpopulations that configure the direct and indirect basal ganglia motor circuits using immunohistochemistry and double/multiple labeling immunofluorescence for TH transporters and neuronal biomarkers. We found their expression in the medium-sized spiny neurons of the striatum (the receptor neurons of the corticostriatal pathway) and in various types of its local microcircuitry interneurons, including the cholinergic. We also demonstrate the presence of both transporters in projection neurons of intrinsic and output nuclei of the basal ganglia, motor thalamus and nucleus basalis of Meynert, suggesting an important role of MCT8/OATP1C1 for modulating the motor system. Our findings suggest that a lack of function of these transporters in the basal ganglia circuits would significantly impact motor system modulation, leading to clinically severe movement impairment.

Monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptide 1C1 (OATP1C1) are thyroid hormones (TH) transmembrane transporters relevant for the availability of TH in neural cells, crucial for their proper development and function. Mutations in MCT8 or OATP1C1 result in severe disorders with dramatic movement disability related to alterations in basal ganglia motor circuits. Mapping the expression of MCT8/OATP1C1 in those circuits is necessary to explain their involvement in motor control. We studied the distribution of both transporters in the neuronal subpopulations that configure the direct and indirect basal ganglia motor circuits using immunohistochemistry and double/multiple labeling immunofluorescence for TH transporters and neuronal biomarkers. We found their expression in the medium-sized spiny neurons of the striatum (the receptor neurons of the corticostriatal pathway), and in various types of its local microcircuitry interneurons, including the cholinergic. We also demonstrate the presence of both transporters in projection neurons of intrinsic and output nuclei of the basal ganglia, motor thalamus and nucleus basalis of Meynert, suggesting an important role of MCT8/OATP1C1 for modulating the motor system. Our findings suggest that a lack of function of these transporters in the basal ganglia circuits would significantly impact motor system modulation, leading to clinically severe movement impairment.

Monocarboxylate transporter 8 (MCT8) and organic anion transporter polypeptide 1C1 (OATP1C1) are thyroid hormone (TH) transmembrane transporters that play an important role in the availability of TH for neural cells, allowing their proper development and function. It is important to define which cortical cellular subpopulations express those transporters to explain why MCT8 and OATP1C1 deficiency in humans leads to dramatic alterations in the motor system. By means of immunohistochemistry and double/multiple labeling immunofluorescence in adult human and monkey motor cortices, we demonstrate the presence of both transporters in long-projection pyramidal neurons and in several types of short-projection GABAergic interneurons in both species, suggesting a critical position of these transporters for modulating the efferent motor system. MCT8 is present at the neurovascular unit, but OATP1C1 is only present in some of the large vessels. Both transporters are expressed in astrocytes. OATP1C1 was unexpectedly found, only in the human motor cortex, inside the Corpora amylacea complexes, aggregates linked to substance evacuation towards the subpial system. On the basis of our findings, we propose an etiopathogenic model that emphasizes these transporters’ role in controlling excitatory/inhibitory motor cortex circuits in order to understand some of the severe motor disturbances observed in TH transporter deficiency syndromes.

Patients lacking functional Monocarboxylate transporter 8 (MCT8), a highly specific thyroid hormone (TH) transporter, present severe psychomotor disabilities. MCT8 deficiency leads to peripheral hyperthyroidism and brain hypothyroidism, the latter due to impaired transport of TH across brain barriers. Available treatments for patients are limited and aim to overcome the limited TH transport across brain barriers. The use of TH analogues such as 3,3',5-triiodothyroacetic acid (TRIAC) that do not require MCT8 to cross the cellular membranes is considered a potential therapy for MCT8 deficiency. Previous studies have shown that systemic administration of TRIAC at therapeutic doses does not increase TRIAC content in the brain, while intracerebroventricular (ICV) administration of therapeutic doses of TRIAC increases TRIAC content in the brain but does not mediate thyromimetic effects. In view of this, we hypothesise that ICV administration of high doses of TRIAC can mediate thyromimetic effects in the brain without worsening the brain hypothyroidism or peripheral hyperthyroidism of patients.400 ng/g of body weight per day of TRIAC were administered ICV to a mouse model of MCT8 deficiency: Mct8-/y and deiodinase 2 (Dio2)-/ - double knockout mice. The effects of this treatment on TH and TRIAC levels/content in blood and tissues were determined by radioimmunoassay and effects on TH-regulated genes were assessed by RT-qPCR in peripheral and central tissues.ICV administration of high doses of TRIAC ameliorated the peripheral hyperthyroidism. In the brain, this treatment did not further aggravate brain hypothyroidism and increased TRIAC content in several brain regions; however, only moderate thyromimetic activity was observed in restricted brain areas.Administration of high doses of TRIAC by ICV delivery at juvenile stages in a mouse model of MCT8 deficiency, is effective in normalising peripheral hyperthyroidism, but exerts minimal thyromimetic activity in the brain.

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.

Thyroid hormones (TH) are crucial for brain development; their deficiency during neurodevelopment impairs neural cell differentiation and causes irreversible neurological alterations. Understanding TH action, and in particular the mechanisms regulating TH availability in the prenatal human brain is essential to design therapeutic strategies for neurological diseases due to impaired TH signaling during neurodevelopment. We aimed at the identification of cells involved in the regulation of TH availability in the human brain at fetal stages. To this end, we studied the distribution of the TH transporters monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptide 1C1 (OATP1C1), as well as the TH-metabolizing enzymes types 2 and 3 deiodinases (DIO2 and DIO3). Paraffin-embedded human brain sections obtained from necropsies of thirteen fetuses from 14 to 38 gestational weeks were analyzed by immunohistochemistry and in situ hybridization. We found these proteins localized along radial glial cells, in brain barriers, in Cajal-Retzius cells, in migrating fibers of the brainstem and in some neurons and glial cells with particular and complex spatiotemporal patterns. Our findings point to an important role of radial glia in controlling TH delivery and metabolism and suggest two additional novel pathways for TH availability in the prenatal human brain: the outer, and the inner cerebrospinal fluid–brain barriers. Based on our data we propose a model of TH availability for neural cells in the human prenatal brain in which several cell types have the ability to autonomously control the required TH content., The IdiPAZ Biobank is supported by Instituto de Salud Carlos III, Spanish Health Ministry (Retic RD09/0076/00073) and Farmaindustria, through the Cooperation Program in Clinical and Translational Research of the Community of Madrid. We thank Drs. Soledad Bárez-López, José Miguel Cosgaya, Estrella Rausell and Ana Montero-Pedrazuela for the careful reading of the manuscript and their helpful suggestions, and Javier Pérez for his help on the artwork. This work was supported by Grants from the Spanish Plan Nacional de I+D+i (Grant numbers SAF2014-54919-R and SAF2017-86342-R to A.GF and J.B), and the Center for Biomedical Research on Rare Diseases (Ciberer), Instituto de Salud Carlos III, Madrid, Spain and the Sherman Foundation (OTR02211 to A.GF). D.LE is a recipient of a fellowship from “Fellowship Training Program for Advanced Human Capital, BECAS CHILE” from the National Commission for Scientific and Technological Research (CONICYT), Gobierno de Chile. The cost of this publication has been paid in part by FEDER funds.

Thyroid hormones (T3 and T4) are essential for the development and function of the mammalian central nervous system (CNS). For this reason, controlling the availability of thyroid hormones to neural cells is an essential step. This is probably why this is a complex process with several levels of regulation, with the ultimate goal of controlling the appropriate spatial and temporal levels of T3, the nuclear active thyroid hormone, in the CNS. The mechanisms responsible for an appropriate thyroid hormone availability and action during foetal brain development include the expression of thyroid hormone receptors, to which T3 binds to mediate its actions by regulating the expression of T3-target genes; transplacental passage of thyroid hormones to the foetus; expression of transmembrane transporter proteins with the ability to transport thyroid hormones across cell membranes and expression of deiodinase enzymes that can activate (T4 to T3 conversion) or inactivate thyroid hormone action. Lack of thyroid hormone action during brain development can lead to several brain impairments, which functional consequences depend on the timing and the cause for thyroid hormone deficiency.

Hormones are key factors throughout neurodevelopment, and thyroid hormones are essential for a correct maturation of the nervous system. Thyroid hormones’ pleiotropic actions during brain development include the regulation of cell generation and survival, differentiation, migration, growth, and retraction of cellular processes, synaptogenesis, myelination, and establishment of neuronal circuits. Indeed, thyroid hormones act on different progenitor cells, glial cells, and neurons subtypes, at specific regional and temporal patterns. They are key factors involved in the regulation of the expression of target genes, including cell cycle proteins, growth factors, and neurotrophins, cytoskeleton proteins, axon guidance and synaptogenesis-related proteins, myelin proteins, and enzymes related to neurotransmitter metabolism. In the fetus, the maternal thyroid hormone supply is essential for a correct neurodevelopment, until the fetal thyroid gland is functional. Thyroid hormones deficiency during brain development induces mild to devastating impairments on the neurological outcome of the offspring depending on the severity of the deficiency, its duration, and the time window affected.

A euthyroid state in the brain is crucial for its adequate development and function. Impairments in thyroid hormones (THs; T3 or 3,5,3'-triiodothyronine and T4 or thyroxine) levels and availability in brain can lead to neurological alterations and to psychiatric disorders, particularly mood disorders. The thyroid gland synthetizes mainly T4, which is secreted to circulating blood, however, most actions of THs are mediated by T3, the transcriptionally active form. In the brain, intracellular concentrations of T3 are modulated by the activity of type 2 (D2) and type 3 (D3) deiodinases. In the present work, we evaluated learning and memory capabilities and anxiety-like behavior at adult stages in mice lacking D2 (D2KO) and we analyzed the impact of D2-deficiency on TH content and on the expression of T3-dependent genes in the amygdala and the hippocampus. We found that D2KO mice do not present impairments in spatial learning and memory, but they display emotional alterations with increased anxiety-like behavior as well as enhanced auditory-cued fear memory and spontaneous recovery of fear memory following extinction. D2KO mice also presented reduced T3 content in the hippocampus and decreased expression of the T3-dependent gene Dio3 in the amygdala suggesting a hypothyroid status in this structure. We propose that the emotional dysfunctions found in D2KO mice can arise from the reduced T3 content in their brain, which consequently leads to alterations in gene expression with functional consequences. We found a downregulation in the gene encoding for the calcium-binding protein calretinin (Calb2) in the amygdala of D2KO mice that could affect the GABAergic transmission. The current findings in D2KO mice can provide insight into emotional disorders present in humans with DIO2 polymorphisms., This work was supported by institutional grants provided by the Spanish Ministry of Science and Innovation and Economy and Competitiveness (grant numbers BFU2007-62979, BFU2010-16498 and SAF2014-54919-R (AGF); AGL2014-56464-C3-2-R (CV). SBL and DBG were recipients of a fellowship from the Spanish FPI program. The cost of this publication has been paid in part by FEDER funds.

Patients lacking the thyroid hormone (TH) transporter MCT8 present abnormal serum levels of TH: low thyroxine and high triiodothyronine. They also have severe neurodevelopmental defects resulting from cerebral hypothyroidism, most likely due to impaired TH transport across the brain barriers. The use of TH analogs, such as triiodothyroacetic acid (TRIAC), that can potentially access the brain in the absence of MCT8 and restore at least a subset of cerebral TH actions could improve the neurological defects in these patients. We hypothesized that direct administration of TRIAC into the brain by intracerebroventricular delivery to mice lacking MCT8 could bypass the restriction at the brain barriers and mediate TH action without causing hypermetabolism. We found that intracerebroventricular administration of therapeutic doses of TRIAC does not increase further plasma triiodothyronine or further decrease plasma thyroxine levels and does not alter TH content in the cerebral cortex. Although TRIAC content increased in the brain, it did not induce TH-mediated actions on selected target genes. Our data suggest that intracerebroventricular delivery of TRIAC has the ability to target the brain in the absence of MCT8 and should be further investigated to address its potential therapeutic use in MCT8 deficiency., This work was funded by the Spanish Plan Nacional de I+D+i (grant number SAF2017-86342-R to AG-F), the Sherman Foundation (OTR02211 to AG-F and SB-L), the Center for Biomedical Research on Rare Diseases (Ciberer to AG-F and CG-M), Instituto de Salud Carlos III, Madrid, Spain. X-HL and SR were supported in part by grant DK 15070 from the National Institutes of Health, USA.

Resumen del póster presentado al 6th Symposium on Biomedical Research: Advances and Perspectives in Molecular Endocrinology "In Homage to Gabriella Morreale", celebrado en el Instituto de Investigaciones Biomédicas Alberto Sols (IIBM-CSIC) el 31 de mayo de 2019., Allan-Herndon-Dudley syndrome is a rare disease caused by mutations in the specific thyroid hormone transporter monocarboxylate transporter 8 (MCT8). Defects in this transporter cause peripheral hyperthyroidism and profound psychomotor alterations in humans. Mice lacking Mct8 were the first murine model used for the study of this disease; they presented peripheral hyperthyroidism but unfortunately no gross neurological abnormalities. This seems to be due to a compensatory mechanism in brain involving the enzyme deiodinase type 2 (Dio2) which converts T4 into T3. Therefore, a new animal model has been proposed: mice lacking both Mct8 transporter and Dio2 enzyme. Here we have analysed the endocrine and neurologic phenotype of these mice at 3 and 6 months of age. Our results show that these animals present peripheral hyperthyroidism and brain hypothyroidism that seems to be permanent and to vary across regions, being the striatum the most sensitive area. Immunohistochemical studies in brain show alterations compatible with thyroid hormone deficiency. We have also found alterations in motor skills evaluations. All these data support the potential of Mct8/Dio2-deficient mice as a new tool to understand the mechanism underlying the pathophysiology of human Mct8 deficiency and to explore new therapeutic strategies., Supported by grants BFU2007-62979 from the Spanish Ministry of Science, Innovation and Universities and from the Sherman Foundation.

In Press., The cold‐ and menthol‐activated ion channel transient receptor potential channel subfamily M member 8 (TRPM8) is the principal detector of environmental cold in mammalian sensory nerve endings. Although it is mainly expressed in a subpopulation of peripheral sensory neurons, it has also been identified in non‐neuronal tissues. Here, we show, by in situ hybridization (ISH) and by the analysis of transgenic reporter expression in two different reporter mouse strains, that TRPM8 is also expressed in the central nervous system. Although it is present at much lower levels than in peripheral sensory neurons, we found cells expressing TRPM8 in restricted areas of the brain, especially in the hypothalamus, septum, thalamic reticular nucleus, certain cortices and other limbic structures, as well as in some specific nuclei in the brainstem. Interestingly, positive fibers were also found traveling through the major limbic tracts, suggesting a role of TRPM8‐expressing central neurons in multiple aspects of thermal regulation, including autonomic and behavioral thermoregulation. Additional ISH experiments in rat brain demonstrated a conserved pattern of expression of this ion channel between rodent species. We confirmed the functional activity of this channel in the mouse brain using electrophysiological patch‐clamp recordings of septal neurons. These results open a new window in TRPM8 physiology, guiding further efforts to understand potential roles of this molecular sensor within the brain., Instituto de Salud Carlos III, Grant/Award Number: PI12/0058; National Institutes of Health, Grant/Award Number: ZIA DE000721‐12; Ministerio de Ciencia e Innovación, Grant/Award Numbers: SAF2009‐11175, SAF2010‐14990‐R, SAF2016‐77233‐R; Ministerio de Economía y Competitividad, Grant/Award Numbers: BES‐2011‐047063, BES‐2017‐080782; Severo Ochoa Programme for Centres of Excellence in R&D, Grant/Award Number: SEV‐2017‐0723 and cofinanced by the European Regional Development Fund; Generalitat Valenciana, Grant/Award Number: GRISOLIA/2008/025

[Background]: Mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) lead to peripheral hyperthyroidism and profound psychomotor alterations in humans. Mice lacking Mct8 present peripheral hyperthyroidism but no gross neurological abnormalities due to brain compensatory mechanisms involving the enzyme deiodinase type 2 (Dio2)., [Methods]: Here we have analyzed the endocrine and neurologic phenotype of mice lacking both Mct8 and Dio2 at three and six months of age. Thyroxine (T4) and 3,5,3′ triiodothyronine (T3) levels/content were measured by specific radioimmunoassays; motor skill performance was evaluated by the footprint, rotarod, four limb hanging wire, and balance beam tests; and brain histological analysis was performed by immunostaining for neurofilament and parvalbumin., [Results]: We have found that this mouse model presents peripheral hyperthyroidism and brain hypothyroidism. Interestingly, the severity of the brain hypothyroidism seems permanent and varies across regions, with the striatum being a particularly affected area. We have also found brain alterations at the histological level compatible with TH deficiency and impaired motor skills., [Conclusions]: These findings indicate the potential of Mct8/Dio2-deficient mice to represent a model for human MCT8 deficiency, to understand the mechanisms underlying its pathophysiology, and ultimately design therapeutic interventions for human patients., This study was supported by grants from the Spanish Plan Nacional de I+D+i (Grant No. SAF2017-86342-R to A.GF), the Sherman Foundation (Grant No. OTR02211 to A.GF and S.BL), and the Center for Biomedical Research on Rare Diseases (Ciberer), Instituto de Salud Carlos III, Madrid, Spain. The cost of this publication has been paid, in part, by FEDER funds.

MCT8 gene mutations produce thyroid hormone (TH) deficiency in the brain, causing severe neuropsychomotor abnormalities not correctable by treatment with TH. This proof-of-concept study examined whether transfer of human MCT8 (hMCT8) cDNA using adeno-associated virus 9 (AAV9) could correct the brain defects of Mct8 knockout mice (Mct8KO).AAV9 vectors delivering long and/or short hMCT8 protein isoforms or an empty vector were injected intravenously (IV) and/or intracerebroventricularly (ICV) into postnatal day 1 Mct8KO and wild type (Wt) mice. Triiodothyronine (T3) was given daily for four days before postnatal day 28, at which time brains were collected after perfusion to assess increase in T3 content and effect on the T3-responsive transcription factor, Hairless.Increased pup mortality was observed after IV injection of the AAV9-long hMCT8 isoform, but not after injection of AAV9-short hMCT8 isoform. Compared to IV, ICV delivery produced more hMCT8 mRNA and protein relative to the viral dose, which was present in various brain regions and localized to the cell membranes. Despite production of abundant hMCT8 mRNA and protein with ICV delivery, only IV delivered AAV9-hMCT8 targeted the choroid plexus and significantly increased brain T3 content and expression of Hairless.These results indicate that MCT8 delivery to brain barriers by IV but not ICV injection is crucial for its proper function. MCT8 has no constitutive activity but acts through an increase in T3 entering the brain tissue. Increasing MCT8 expression in brain cell membranes, including neurons, is insufficient to produce an effect without an increase in brain T3 content. The correct hMCT8 isoform along with an optimized delivery method are critical for an effective gene therapy to provide functional MCT8 in the brain of patients with MCT8 mutations.

Objective: Metainflammation is a chronic low-grade inflammatory state induced by obesity and associated comorbidities, including peripheral insulin resistance. Brown adipose tissue (BAT), a therapeutic target against obesity, is an insulin target tissue sensitive to inflammation. Therefore, it is necessary to find strategies to protect BAT against the effects of inflammation in energy balance. In this study, we explored the impact of moderate sirtuin 1 (SIRT1) overexpression on insulin sensitivity and β-adrenergic responses in BAT and brown adipocytes (BA) under pro-inflammatory conditions. Methods: The effect of inflammation on BAT functionality was studied in obese db/db mice and lean wild-type (WT) mice or mice with moderate overexpression of SIRT1 (SIRT1Tg+) injected with a low dose of bacterial lipopolysaccharide (LPS) to mimic endotoxemia. We also conducted studies on differentiated BA (BA-WT and BA-SIRT1Tg+) exposed to a macrophage-derived pro-inflammatory conditioned medium (CM) to evaluate the protection of SIRT1 overexpression in insulin signaling and glucose uptake, mitochondrial respiration, fatty acid oxidation (FAO), and norepinephrine (NE)-mediated-modulation of uncoupling protein-1 (UCP-1) expression. Results: BAT from the db/db mice was susceptible to metabolic inflammation manifested by the activation of pro-inflammatory signaling cascades, increased pro-inflammatory gene expression, tissue-specific insulin resistance, and reduced UCP-1 expression. Impairment of insulin and noradrenergic responses were also found in the lean WT mice upon LPS injection. In contrast, BAT from the mice with moderate overexpression of SIRT1 (SIRT1Tg+) was protected against LPS-induced activation of pro-inflammatory signaling, insulin resistance, and defective thermogenic-related responses upon cold exposure. Importantly, the decline in triiodothyronine (T3) levels in the circulation and intra-BAT after exposure of the WT mice to LPS and cold was markedly attenuated in the SIRT1Tg+ mice. In vitro BA experiments in the two genotypes revealed that upon differentiation with a T3-enriched medium and subsequent exposure to a macrophage-derived pro-inflammatory CM, only BA-SIRT1Tg+ fully recovered insulin and noradrenergic responses. Conclusions: This study has ascertained the benefit of the moderate overexpression of SIRT1 to confer protection against defective insulin and β-adrenergic responses caused by BAT inflammation. Our results have potential therapeutic value in combinatorial therapies for BAT-specific thyromimetics and SIRT1 activators to combat metainflammation in this tissue., This study was funded by grants RTI2018-094052-B-100 (MICINN/AEI/FEDER, EU), S2017/BMD-3684 (Community of Madrid, Spain), the Ramón Areces Foundation (Spain), and CIBERdem (ISCIII) to A.M.V., grant S2010/BMD-2423 (Community of Madrid, Spain) to M.J.O. and A.M.V., grants SAF2017-83813-C3-1-R (MICINN/AEI/FEDER, EU) and CIBERobn (grant CB06/03/0001, ISCIII, Spain) to L.H. and D.S., grants 2017SGR278 (Government of Catalonia) and 201627-30 (Fundació La Marató of TV3) to D.S., grant RTI2018-095166-B-I00 (MICINN/AEI/FEDER, EU) to F.J.R., and grants SAF2017-86342-R (MICINN/AEI/FEDER, EU) and CIBERer (ISCIII, Spain) to A.G.-F. C.E. is a recipient of an FPU fellowship (Ministry of Universities, Spain) (FPU 15/00251). S.Z. is a recipient of an ANID fellowship from Chile.

Loss of function mutations in the gene encoding the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to severe neurodevelopmental defects in humans associated with a specific thyroid hormone phenotype manifesting high serum 3,5,3’-triiodo-thyronine (T3) and low thyroxine (T4) levels. Patients present a paradoxical state of peripheral hyperthyroidism and brain hypothyroidism, this last one most likely arising from impaired thyroid hormone transport across the brain barriers. The administration of thyroid hormones by delivery pathways that bypass the brain barriers, such as the intranasal delivery route, offers the possibility to improve the neurological defects of MCT8-deficient patients. In this study, the thyroid hormones T4 and T3 were administrated intranasally in different mouse models of MCT8 deficiency. We have found that, under the present formulation, intranasal administration of thyroid hormones does not increase the content of thyroid hormones in the brain and further raises the peripheral thyroid hormone levels. Our data suggests intranasal delivery of thyroid hormones is not a suitable therapeutic strategy for MCT8 deficiency, although alternative formulations could be considered in the future to improve the nose-to-brain transport., This work was supported by the Spanish Ministry of Economy and Competitiveness, grant number SAF2017-86342-R (MINECO/AEI/FEDER, UE) to AG-F, the Sherman Foundation (grant number OTR02211) to AG-F and SB-L, and the BBSRC (grant number BB/R016879/1) to SB-L. CG-M is a recipient of a contract from the Center for Biomedical Research on Rare Diseases (CIBERER), Instituto de Salud Carlos III, Madrid. S. R. was supported by grant DK15079 from the National Institutes of Health, USA

[Background]: Loss of function mutations in the thyroid hormone (TH)-specific cell membrane transporter, the monocarboxylate transporter 8 (MCT8), lead to profound psychomotor retardation and abnormal TH serum levels, with low thyroxine (T4) and high triiodothyronine (T3). Several studies point to impaired TH transport across brain barriers as a crucial pathophysiological mechanism resulting in cerebral hypothyroidism. Treatment options for MCT8-deficient patients are limited and are focused on overcoming the brain barriers. The aim of this study was to evaluate the ability of the TH analog sobetirome and its prodrug Sob-AM2 to access the brain and exert thyromimetic actions in the absence of Mct8., [Methods]: Juvenile wild-type (Wt) mice and mice lacking Mct8 and deiodinase type 2 (Mct8/Dio2KO) were treated systemically with daily injections of vehicle, 1 mg of sobetirome/kg body weight/day, or 0.3 mg of SobAM2/kg body weight/day for seven days. Sobetirome content was measured using liquid chromatography–tandem mass spectrometry, and T4 and T3 levels by specific radioimmunoassays. The effect of sobetirome treatment in the expression of T3-dependent genes was measured in the heart, liver, and cerebral cortex by realtime polymerase chain reaction., [Results]: Sob-AM2 treatment in Mct8/Dio2KO animals led to 1.8-fold more sobetirome content in the brain and 2.5-fold less in plasma in comparison to the treatment with the parent drug sobetirome. Both sobetirome and Sob-AM2 treatments in Mct8/Dio2KO mice greatly decreased plasma T4 and T3 levels. Dio1 and Ucp2 gene expression was altered in the liver of Mct8/Dio2KO mice and was not affected by the treatments. In the heart, Hcn2 but not Atp2a2 expression was increased after treatment with the analogs. Interestingly, both sobetirome and Sob-AM2 treatments increased the expression of several T3-dependent genes in the brain such as Hr, Abcd2, Mme, and Flywch2 in Mct8/Dio2KO mice., [Conclusions]: Sobetirome and its amide prodrug Sob-AM2 can access the brain in the absence of Mct8 and exert thyromimetic actions modulating the expression of T3-dependent genes. At the peripheral level, the administration of these TH analogs results in the depletion of circulating T4 and T3. Therefore, sobetirome and Sob-AM2 have the potential to address the cerebral hypothyroidism and the peripheral hyperthyroidism characteristic of MCT8 deficiency., This work was supported by the Spanish Ministry of Economy Industry and Competitiveness (SAF2014-54919-R and SAF2017-86342-R to A.G.F.), the Sherman Foundation (OTR02211 to A.G.F.), and the National Institutes of Health (DK52798 to T.S.S. and 2T32DK007680-21 to M.D.H.), and the National Multiple Sclerosis Society (FG 2023A 1/2 to M.D.H.). The cost of this publication has been paid in part by FEDER funds.

TaqIA is a polymorphismassociated with addictions and dopamine-related traits. It is located in the ankyrin repeat and kinase domain containing 1 gene (ANKK1) nearby the gene for the dopamine D2 receptor (D2R). Since ANKK1 function is unknown, TaqIA-associated traits have been explained only by differences in D2R. Here we report ANKK1 studies in mouse and human brain using quantitative real-time PCR, Western blot, immunohistochemistry, and flow cytometry. ANKK1 mRNA and protein isoforms vary along neurodevelopment in the human and mouse brain. In mouse adult brain ANKK1 is located in astrocytes, nuclei of postmitotic neurons and neural precursors from neurogenic niches. In both embryos and adults, nuclei of neural precursors showsignificant variation of ANKK1 intensity.We demonstrate a correlation between ANKK1 and the cell cycle. Cell synchronization experiments showed a significant increment of ANKK1-kinase in mitotic cells while ANKK1-kinase overexpression affects G1 and M phase that were found to be modulated by ANKK1 alleles and apomorphine treatment. Furthermore, during embryonic neurogenesis ANKK1 was expressed in slow-dividing neuroblasts and rapidly dividing precursors which are mitotic cells. These results suggest a role of ANKK1 during the cell cycle in neural precursors thus providing biological support to brain structure involvement in the TaqIA-associated phenotypes., This work has been supported by the Instituto de Salud Carlos III grants PI11/0731 and PI15/01013 (J.H.), by Ministry of Economy and Competitiveness grants SAF2014-54919-R (A.G.-F.) and SAF2011-25020 (A.P.).

[Background]: Monocarboxylate transporter 8 (MCT8) is a thyroid hormone-specific cell membrane transporter. Mutations in the MCT8 gene lead to profound psychomotor retardation and abnormal thyroid hormone serum levels with low thyroxine (T4) and high triiodothyronine (T3). Currently, therapeutic options for patients are limited. Triiodothyroacetic acid (TRIAC) has potential therapeutic value. The aim of this study was to evaluate the effects and efficacy of therapeutic doses of TRIAC on Mct8-deficient mice (Mct8KO)., [Methods]: Wild-type (Wt) and Mct8KO mice were treated with 30 ng TRIAC/g BW/day, given in drinking water, from postnatal day 21 to 30. TRIAC, T4 and T3 levels in plasma, as well as T3 and TRIAC content in the cerebral cortex and striatum were measured by specific radioimmunoassays. The activities of deiodinases 1 and 2 were measured in liver and cortex. The effect of TRIAC treatment in the expression of T3-dependent genes was measured in the heart, cerebral cortex and striatum., [Results]: Plasma TRIAC concentration were the same in Wt and Mct8KO animals after treatment. TRIAC treatment greatly decreased plasma T4 in Wt and Mct8KO mice, and reduced T3 to normal levels in the Mct8KO. Deiodinase 1 activity and gene expression in the liver increased while it did not have any effect on the expression of Serca2a in the heart. TRIAC treatment did not induce the expression of T3-dependent genes in the cerebral cortex or striatum but further decreased expression of Flywch2 in the cortex and Aldh1a1 and Flywch2 in the striatum. Direct measurements of TRIAC and T3 content in the cortex and striatum revealed a decrease in T3 after treatment with no significant increase in the level of endogenous TRIAC., [Conclusions]: Therapeutic doses of TRIAC in Mct8KO mice restored plasma T3 levels but severely decreased T4 levels. TRIAC has a direct effect on deiodinase 1 in the liver and does not have an effect on gene expression in the heart. The increase in the plasma TRIAC levels after treatment is not sufficient to increase TRIAC levels in the brain and to promote the expression of T3-dependent genes in brain cells. Instead, it leads to a state of brain hypothyroidism with reduced T3 content., This work was supported by Grants from the Mehuer Foundation and the Seville's College of Pharmacists, Ramón Areces Foundation (CIVP16A1805), Spanish Ministry of Economy and Competitiveness (SAF2011-25608, SAF2014-54919-R, SAF 2012-32491) and S2010/BMD-2423 from CAM and under the frame of E‑Rare‑2 and the “Centro de Investigación Biomédica en Red de Enfermedades Raras, Instituto de Salud Carlos III” under the frame of the ERA-Net for Research on Rare Diseases. S.B.-L. is recipient of a predoctoral fellowship and contract from the FPI program of the Plan Nacional de I+D+i. The cost of this publication has been paid in part by FEDER funds.

The cellular influx and efflux of thyroid hormones are facilitated by transmembrane protein transporters. Of these transporters, monocarboxylate transporter 8 (MCT8) is the only one specific for the transport of thyroid hormones and some of their derivatives. Mutations in SLC16A2, the gene that encodes MCT8, lead to an X-linked syndrome with severe neurological impairment and altered concentrations of thyroid hormones. Histopathological analysis of brain tissue from patients who have impaired MCT8 function indicates that brain lesions start prenatally, and are most probably the result of cerebral hypothyroidism. A Slc16a2 knockout mouse model has revealed that Mct8 is an important mediator of thyroid hormone transport, especially T 3, through the blood-brain barrier. However, unlike humans with an MCT8 deficiency, these mice do not have neurological impairment. One explanation for this discrepancy could be differences in expression of the T 4 transporter OATP1C1 in the blood-brain barrier; OATP1C1 is more abundant in rodents than in primates and permits the passage of T 4 in the absence of T 3 transport, thus preventing full cerebral hypothyroidism. In this Review, we discuss the relevance of thyroid hormone transporters in health and disease, with a particular focus on the pathophysiology of MCT8 mutations.

et al., [Context]: Mutations in the MCT8 (SLC16A2) gene, encoding a specific thyroid hormone transporter, cause an X-linked disease with profound psychomotor retardation, neurological impairment, and abnormal serumt hyroid hormone levels. The nature of the central nervous system damage is unknown., [Objective]: The objective of the study was to define the neuropathology of the syndrome by analyzing brain tissue sections from MCT8-deficient subjects., [Design]: We analyzed brain sections from a 30th gestational week male fetus and an 11-year-old boy and as controls, brain tissue from a 30th and 28th gestational week male and female fetuses, respectively, and a 10-year-old girl and a 12-year-old boy., [Methods]: Staining with hematoxylin-eosin and immunostaining for myelin basic protein, 70-kDa neurofilament, parvalbumin, calbindin-D28k, and synaptophysin were performed. Thyroid hormone determinations and quantitative PCR for deiodinases were also performed., [Results]: The MCT8-deficient fetus showed a delay in cortical and cerebellar development and myelination, loss of parvalbumin expression, abnormal calbindin-D28k content, impaired axonal maturation, and diminished biochemical differentiation of Purkinje cells. The 11-year-old boy showed altered cerebellar structure, deficient myelination, deficient synaptophysin and parvalbumin expression, and abnormal calbindin-D28k expression. The MCT8-deficient fetal cerebral cortex showed50%reduction of thyroid hormones and increased type 2 deiodinase and decreased type 3 deiodinase mRNAs., [Conclusions]: The following conclusions were reached: 1) brain damage in MCT8 deficiency is diffuse, without evidence of focal lesions,andpresent from fetal stages despite apparent normality at birth; 2) deficient hypomyelination persists up to 11 years of age; and 3) the findings are compatible with the deficient action of thyroid hormones in the developing brain caused by impaired transport to the target neural cells., This study was supported by Grant SAF2011-25608 from the Plan Nacional de IDi and the Center for Research on Rare Diseases (Centro de Investigación Biomédica en Red Enfermedades Raras), Instituto de Salud Carlos III, Madrid, Spain; Grant DK15070 from the National Institutes of Health, Bethesda, Maryland; and a grant from the Sherman Family. D.L.-E. is recipient of a fellowship from the “Fellowship Training Program for Advanced Human Capital, Becas Chile” from the National Commission for Scientific and Technological Research, Gobierno de Chile.

[Background]: Thyroid hormones have a key role in both the developing and adult central nervous system and skeletal muscle. The thyroid gland produces mainly thyroxine (T4) but the intracellular concentrations of 3,5,3′-triiodothyronine (T3; the transcriptionally active hormone) in the central nervous system and skeletal muscle are modulated by the activity of type 2 deiodinase (D2). To date no neurological syndrome has been associated with mutations in the DIO2 gene and previous studies in young and juvenile D2-knockout mice (D2KO) did not find gross neurological alterations, possibly due to compensatory mechanisms. [Aim]: This study aims to analyze the motor phenotype of 3-and-6-month-old D2KO mice to evaluate the role of D2 on the motor system at adult stages in which compensatory mechanisms could have failed. [Results]: Motor abilities were explored by validated tests. In the footprint test, D2KO showed an altered global gait pattern (mice walked slower, with shorter strides and with a hindlimb wider base of support than wild-type mice). No differences were detected in the balance beam test. However, a reduced latency to fall was found in the rotarod, coat-hanger and four limb hanging wire tests indicating impairment on coordination and prehensile reflex and a reduction of muscle strength. In histological analyses of cerebellum and skeletal muscle, D2KO mice did not present gross structural abnormalities. Thyroid hormones levels and deiodinases activities were also determined. In D2KO mice, despite euthyroid T3 and high T4 plasma levels, T3 levels were significantly reduced in cerebral cortex (48% reduction) and skeletal muscle (33% reduction), but not in the cerebellum where other deiodinase (type 1) is expressed. [Conclusions]: The motor alterations observed in D2KO mice indicate an important role for D2 in T3 availability to maintain motor function and muscle strength. Our results suggest a possible implication of D2 in motor disorders., This work was supported by grants from Plan Nacional de I+D: BFU2007-62979 and BFU 2010-16498 and SAF2011-25608 (AGF); SAF2012-32491 (MJO), Community of Madrid: S2010-BMD-2423 (MJO). SBL and DBG were recipients of a fellowship from the FPI program and AMP of a contract all from the Ministry of Science and Innovation and Economy and Competitiveness, Spain.

Cortistatin (CST) is a presumptive neuropeptide that shares 11 of its 14 amino acids with somatostatin (SST). CST and SST are expressed in partially overlapping but distinct populations of cortical interneurons. In the hippocampal formation, most CST-positive cells are also positive for SST. In contrast to SST, administration of CST into the rat brain ventricles reduces locomotor activity and specifically enhances slow wave sleep. Intracerebroventricular injection of CST or SST has been shown to protect against the neurotoxic effects of kainic acid. Here, we show that CST and SST mRNAs respond differently to kainate-induced seizures. Furthermore, comparison of the upstream sequences from the CST and SST precursor genes reveal that they contain binding motifs for different transcriptional regulatory factors. Our data demonstrate that CST and SST, which are often co-expressed in the same neurons, are regulated by different stimuli.

The expression of a high affinity Na⁺- (and Cl⁻) dependent L-proline transporter (PROT) in subpopulations of putative glutamatergic pathways in rat brain raises the possibility of a specific physiological role(s) for this carrier in excitatory neurotransmission (Fremeau, R. T., Jr., Caron, M. G., and Blakely, R. D.(1992) Neuron 8, 915-926). However, the biochemical properties and regional, cellular, and subcellular distribution of the PROT protein have yet to be elucidated. Here, we document the brain-specific expression and neuronal localization of rat PROT mRNA. We also report the first identification and partial biochemical characterization of the mammalian brain PROT protein. An affinity-purified antipeptide antibody was produced that specifically recognized a single 68-kDa PROT protein on immunoblots of rat and human brain tissues. Deglycosylation of rat hippocampal membranes with peptide-N-glycosidase F reduced the apparent molecular mass of the native PROT protein from 68 to 53 kDa, the size of the primary PROT translation product determined by in vitro translation of the rat PROT cDNA in the absence of microsomes. Subcellular fractionation studies demonstrated that the PROT protein was enriched in synaptic plasma membranes but absent from postsynaptic densities. A differential distribution of PROT mRNA and protein was observed in rat striatum, suggesting that the transporter protein is synthesized in neuronal cell bodies in the cortex and exported to axon terminals in the caudate putamen. These findings warrant the consideration of a novel presynaptic regulatory role for this transporter in excitatory synaptic transmission.

10 páginas, 4 figuras, 1 tabla.-- et al., Hypothyroidism is the most common hormonal disease in adults, which is frequently accompanied by learning and memory impairments and emotional disorders. However, the deleterious effects of thyroid hormones deficiency on emotional memory are poorly understood and often underestimated. To evaluate the consequences of hypothyroidism on emotional learning and memory, we have performed a classical Pavlovian fear conditioning paradigm in euthyroid and adult-thyroidectomized Wistar rats. In this experimental model, learning acquisition was not impaired, fear memory was enhanced, memory extinction was delayed and spontaneous recovery of fear memory was exacerbated in hypothyroid rats. The potentiation of emotional memory under hypothyroidism was associated with an increase of corticosterone release after fear conditioning and with higher expression of glucocorticoid and mineralocorticoid receptors in the lateral and basolateral nuclei of the amygdala, nuclei that are critically involved in the circuitry of fear memory. Our results demonstrate for the first time that adult-onset hypothyroidism potentiates fear memory and also increases vulnerability to develop emotional memories. Furthermore, our findings suggest that enhanced corticosterone signaling in the amygdala is involved in the pathophysiological mechanisms of fear memory potentiation. Therefore, we recommend evaluating whether inappropriate regulation of fear in patients with post-traumatic stress and other mental disorders is associated with abnormal levels of thyroid hormones, especially those patients refractory to treatment., This work was supported by grants BFU2007-62979 and BFU 2010-16498 (A.G.-F) and SAF2009-09129 (C.V.) from the Spanish Ministry of Science and Innovation. I.F.-L. was recipient of a fellowship and A.M.-P. of a contract both from the Ministry of Science and Innovation, Spain.

Callosal connections were studied with tracers (horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP)) in normal rats and rats deprived of thyroid hormones with methimazole (Sigma) since embryonic day 14 and thyroidectomized at postnatal day 6. In hypothyroid rats, the auditory areas, in particular the primary auditory area, showed cytoarchitectonic changes including blurred lamination and decrease in the size of layer V pyramidal neurons. In control rats, callosally-projecting neurons were found between layers II and VI with a peak in layer III and upper layer IV. In hypothyroid rats, labelled neurons were found between layers IV and VI with two peaks corresponding to layer IV and upper layer V, and in upper layer VI. Quantitative analysis of radial distribution of callosally-projecting neurons confirmed their shift to infragranular layers in hypothyroid rats. Three-dimensional reconstructions showed a more continuous tangential distribution of callosally-projecting neurons in hypothyroid rats which may be due to the maintenance of a juvenile ‘exuberant’ pattern of projections. These changes in cortical connectivity may be relevant for understanding epilepsy and mental retardation associated with early hypothyroidism in humans and to clarify basic mechanisms of cortical development.

A cDNA has been isolated from human hippocampus that appears to encode a novel Na(+)-dependent, Cl(-)-independent, neutral amino acid transporter. The putative protein, designated SATT, is 529 amino acids long and exhibits significant amino acid sequence identity (39-44%) with mammalian L-glutamate transporters. Expression of SATT cDNA in HeLa cells induced stereospecific uptake of L-serine, L-alanine, and L-threonine that was not inhibited by excess (3 mM) 2-(methylamino)-isobutyric acid, a specific substrate for the System A amino acid transporter. SATT expression in HeLa cells did not induce the transport of radiolabeled L-cysteine, L-glutamate, or related dicarboxylates. Northern blot hybridization revealed high levels of SATT mRNA in human skeletal muscle, pancreas, and brain, intermediate levels in heart, and low levels in liver, placenta, lung, and kidney. SATT transport characteristics are similar to the Na(+)-dependent neutral amino acid transport activity designated System ASC, but important differences are noted. These include: 1) SATT's apparent low expression in ASC-containing tissues such as liver or placenta; 2) the lack of mutual inhibition between serine and cysteine; and 3) the lack of trans-stimulation. SATT may represent one of multiple activities that exhibit System ASC-like transport characteristics in diverse tissues and cell lines.

In the corpus callosum of the cat, the heavy subunit of neurofilaments (NFH) can be demonstrated with the monoclonal antibody NE14, as early as P11, not at P3, and only in a few axons. At P18–19 and more markedly at P29, many more callosal axons have become positive to NE14 and this is similar to what is found in the adult. In contrast, callosal axons become positive to the neurofilament antibody SMI-32 only between P29 and P39 and remain positive in the adult. Treatment with alkaline phosphatase prevents axonal staining with NE14, but results in SMI-32 staining of a few callosal axons as early as P11, but not at P3. Between P11 and P19 the number of axons stained with SMI-32 after alkaline phosphatase treatment increases, in parallel with that of axons stained with NE14. Thus NE14 appears to recognize a phosphorylated form of NFH, while SMI-32 appears to recognize an epitope of NFH which is either masked by phosphate or inaccessible until between P29 and P39, unless the tissue is treated with alkaline phosphatase. These two forms of NFH appear towards the end of the period of massive developmental elimination of callosal axons1. They are also synchronous with changes in the spacing of neurofilaments quantified in a separate ultrastructural study3. These cytoskeletal changes may terminate the juvenile-labile state of callosal axons and allow further axial growth of the axon., Supported by FNS Grant 3.359-0.86 to G.M.I. and by Sandoz Foundation Grant 1988 to G.M.I. and B.M.R.

Nat. Rev. Endocrinol. 11, 406–417 (2015); doi:10.1038/nrendo.2015.66 In the original published article, reference 93 lists the incorrect article. The correct reference is Arjona, F. J. et al. Identification and functional characterization of zebrafish solute carrier Slc16a2 (Mct8) as a thyroid hormone membrane transporter.

10 pages, 3 figures, 7 tables.-- et al., dickkopf (dkk) genes encode a small family of secreted Wnt antagonists, except for dkk3, which is divergent and whose function is poorly understood. Here, we describe the generation and characterization of dkk3 mutant mice. dkk3-deficient mice are viable and fertile. Phenotypic analysis shows no major alterations in organ morphology, physiology, and most clinical chemistry parameters. Since Dkk3 was proposed to function as thyroid hormone binding protein, we have analyzed deiodinase activities, as well as thyroid hormone levels. Mutant mice are euthyroid, and the data do not support a relationship of dkk3 with thyroid hormone metabolism. Altered phenotypes in dkk3 mutant mice were observed in the frequency of NK cells, immunoglobulin M, hemoglobin, and hematocrit levels, as well as lung ventilation. Furthermore, dkk3-deficient mice display hyperactivity., This work was supported by the DFG (SFB 488 A1), grants SAF2001-2243 and FIS RCMN 03/08 (M.-J.O.), BFI2001-2412 and BFU2004-05944 (A.G-F.), NGFN 01GR0430, and 01GR0434, 01GR0458, and 01GR0103 (GMC).

11 pages, 5 figures.-- PMID: 16446739., Hormonal imbalances are involved in many of the age-related pathologies, as neurodegenerative and psychiatric diseases. Specifically, thyroid state alterations in the adult are related to psychological changes and mood disorders as depression. The dentate gyrus of the hippocampal formation undergoes neurogenesis in adult mammals including humans. Recent evidence suggests that depressive disorders and their treatment are tightly related to the number of newly born neurons in the dentate gyrus., We have studied the effect of thyroid hormones (TH) on hippocampal neurogenesis in adult rats in vivo. A short period of adult-onset hypothyroidism impaired normal neurogenesis in the subgranular zone of the dentate gyrus with a 30% reduction in the number of proliferating cells. Hypothyroidism also reduced the number of newborn neuroblasts and immature neurons (doublecortin (DCX) immunopositive cells) which had a severely hypoplastic dendritic arborization. To correlate these changes with hippocampal function, we subjected the rats to the forced swimming and novel object recognition tests. Hypothyroid rats showed normal memory in object recognition, but displayed abnormal behavior in the forced swimming test, indicating a depressive-like disorder. Chronic treatment of hypothyroid rats with TH not only normalized the abnormal behavior but also restored the number of proliferative and DCX-positive cells, and induced growth of their dendritic trees. Therefore, hypothyroidism induced a reversible depressive-like disorder, which correlated to changes in neurogenesis., Our results indicate that TH are essential for adult hippocampal neurogenesis and suggest that mood disorders related to adult-onset hypothyroidism in humans could be due, in part, to impaired neurogenesis., This work was supported by Grants BFI2001-2412 and BFU2004-05944 (A.G.-F), BFI2002-00489 (J.B.), BFI2003-07524 (C.V.) from the Ministry of Science and Technology and FIS, Instituto de Salud Carlos III, Red de Centros RCMN (C03/08). A.M.-P. and I.F.-L. are recipient of a fellowship and A.G.-F. and C.V. of a contract from the Ramón y Cajal Program, all of the Ministry of Science and Technology, Spain.