Thyroid hormones are essential for brain maturation, and for brain function throughout life. In adults, thyroid diseases can lead to various clinical manifestations (1, 2). For example, hypothyroidism causes lethargy, hyporeflexia and poor motor coordination. Subclinical hypothyroidism is often associated with memory impairment. Hypothyroidism is also associated to bipolar affective disorders, depression, or loss of cognitive functions, especially in the elderly (3). Hyperthyroidism causes anxiety, irritability, and hyperreflexia. Both, hypothyroidism or hyperthyroidism can lead to mood disorders, dementia, confusion, and personality changes. Most of these disorders are usually reversible with proper treatment, indicating that thyroid hormone alterations of adult onset do not leave permanent structural defects.
The actions of thyroid hormone during development are different, in the sense that they are required to perform certain actions during specific time windows. Thyroid hormone deficiency, even of short duration may lead to irreversible brain damage, the consequences of which depend on the specific timing of onset and duration of thyroid hormone deficiency (4-8).
The rat has been the most widely used animal model in the sudy of thyroid physiology and in the actions of thyroid hormone in brain, and the use of gene knock out and knock in mice is providing new insights. When the results of animal studies are extrapolated to the human, it is important to realize that the timing of development in relation to birth among mammals presents substantial differences, even if the sequence of events is similar (9-11). Most brain growth occurs after birth in man and in rodents, but rats and mice are born with a less developed thyroid axis than humans. As a useful reference “the newborn rat may be compared with a human fetus in the second trimester of pregnancy, and the newborn human baby to a 6-10 day old rat” (12).
EFFECTS OF THYROID HORMONE ON BRAIN DEVELOPMENTAL PROCESSES
Structural Defects Caused by Hypothyroidism
The hypothyroid brain presents many structural defects (for an extensive review see (12):- 1) increases in cell density in the cerebral cortex, due to reduction of the neuropil (13, 14), 2) lower cell numbers in regions with significant postnatal cell acquisitions, for example the olfactory bulb and the granular layers of the hippocampus and cerebellum (14, 15), 3) decreased number of GABAergic interneurons in the cerebellum with accumulation of neuronal precursors (16), and 4) reduced number of interneurons in the cerebral cortex (17). Â Some specific cell types display stunted dendritic and / or axonal growth and maturation, for example cholinergic cells (18, 19), Â Purkinje cells of the cerebellum (Fig 1) (20, 21) and pyramidal cells of cortex layer V (22, 23). Changes of dendritic spine number are also observed in the cortex and hippocampus after adult onset hypothyroidism, and are reversible with thyroxine treatment (24, 25). These morphological changes are a consequence of altered biological processes as follows.
Thyroid hormones are involved in late events of neural development, such as migration and terminal differentiation of neurons and glia. A possible role of thyroid hormones on proliferation and differentiation of neural precursors in the embryonic neurogenic areas has been shown in a few cases in relation to the effects of maternal thyroid hormones (26, 27), and in tadpoles during premetamorphosis (28). Most studies concern the effects of thyroid hormone on neurogenesis in adult animals. Adult neurogenesis occurs in two regions: the subventricular zone (SVZ), and the subgranular zone (SGZ). The SVZ, located below the surface of the lateral ventricles, generates olfactory bulb interneurons in adult rodents. The SGZ is adjacent to the granular layer of the dentate gyrus, and generates granular neurons. Hypothyroidism depresses, and thyroid hormone administration stimulates, neurogenesis in these two areas (29-32). In the SVZ T3 increases differentiation of neuronal precursors (33) and acts through TRa1 to increase the commitment of neural stem cells to migrating neuroblasts. This action correlates with repression of the Sox2 gene (34). The effects of T3 on embryonic and adult neurogenesis appear to be mediated predominantly by TRÎ±1 (26, 28, 34). However, effects of TRÎ²Â have also been shown (35).
Thyroid hormone exerts important influences on cell migration in the cerebral cortex, hippocampus and cerebellum (Fig 1). Among relevant possible mechanisms is the action on the radial glia, one of the first differentiated cells generated in the neuroepithelium. The radial glia extends long processes to the cerebral wall, providing a scaffold that serves for cell migration (36, 37). Later in development, the radial glia differentiates into astrocytes and ependymal cells (38). Radial glia maturation in the fetal rat brain is delayed in the hippocampus of hypothyroid rats (39).
In the cerebral cortex, thyroid hormones are needed for the proper arrangement of the six layer pattern, formed by the timely migration of cells originated in the ventricular neuroepithelium. Deficiency of thyroid hormone during the period of cortical development leads to less defined cortical layers, due to disturbances of cell migration (40-42). Â One mechanism by which thyroid hormones may influence neuronal migration in the cerebral cortex is through the regulation of the expression of the Reln gene (Fig 2).Â The product of this gene, Reelin, is an extracellular matrix protein produced by neurons located in layer I of the cerebral cortex, the Cajal-Retzius cells (43). Thyroid hormone regulates the expression of at least two genes expressed by these cells, Reln, and Ptgds (encoding Prostaglandin D2 synthase) (44, 45). Reelin is essential for the orderly migration and the establishment of neocortical layers. Cajal-Retzius cells have also an important role in the migration of neuronal precursors in the hippocampus and in the establishment of synaptic connections (46).
Neuronal migration in the cerebral cortex is extremely sensitive to thyroid hormones, and even minor deficiencies are associated with migration defects. For example, transient maternal hypothyroidism in pregnant rats at embryonic days 12 to 15 caused significant misplacement of cells in the neocortex and hippocampus of the offspring, when analyzed at 40 days of age, and audiogenic seizures (47). Moderate thyroid hormone deficiency during pregnancy caused neuronal ectopias in the corpus callosum (48).
In the cerebellum thyroid hormones are involved in the late phase of granular cell migration from the external germinal layer to the internal granular layer. This process takes place postnatally in rodents and is completed by P20, when the EGL disappears. Proliferating granular cells migrate through the molecular layer, along the fibers of Bergmann glial cells, when they exit the cell cycle. The main factor for this control is Sonic Hedgehog, produced by the Purkinje cells. It is likely that thyroid hormones are one additional factor controlling cell division. A striking and very characteristic feature of the hypothyroid cerebellum is a delay in the migration of granule cells so that the EGL persists beyond P20 (Fig 1) (49).
Hypothyroidism causes delayed and poor deposition of myelin (50-52) whereas hyperthyroidism accelerates myelination (53). Â After prolonged neonatal hypothyroidism, the number of myelinated axons in adult rats is abnormally low in hypothyroid animals although most of the myelinated axons appear to have a normal thickness of the myelin sheath (fig 3). Thyroid hormones exert important effects on differentiation of oligodendrocytes, the cells that produce myelin. During development, hypothyroidism delays oligodendrocyte differentiation and myelin gene expression, eventually becoming normal even in the absence of thyroid hormone treatment. However, the myelination defect remains in adult animals, although oligodendrocytes are not targets of thyroid hormones in the adult. It is likely that one additional factor might be indirect effects through axonal maturation, which is impaired in hypothyroidism (19, 42). Â The lower diameter of axons in hypothyroid animals would prevent myelination of small axons, with normal myelination of only those axons that reach a critical size (54).
THYROID HORMONES IN THE BRAIN
The concentrations of T4 and T3 in the brain are controlled by very efficient regulatory mechanisms involving thyroidal secretion, transport to the brain, expression of deiodinases and, in the fetus, transplacental passage of thyroxine (fig 4). T3 equilibrates rapidly between the plasma, liver, or kidney pools, whereas the brain pool equilibrates more slowly. Therefore, when T3 alone is administered as a constant infusion, to thyroidectomized rats, the liver and kidney require lower doses than the brain. However, when T4 is administered, brain T3 is normalized at doses that result in relatively low concentrations in plasma or liver (55, 56). In addition, the brain T3 concentration is maintained within a narrow range under a wide range of T4 dosage, thus avoiding T3 excess.
When T4 is administered to pregnant dams previously treated with MMI in order to block the maternal and fetal thyroids, T3 is found in the fetal brain. In contrast, if T3 is given to the dams, it is not detected in the brain despite crossing the placenta and reaching other tissues such as the liver (57, 58). These results indicate that the T3 found in the fetal brain after T4 administration to the pregnant dams is generated locally, because for some reason circulating T3 in the fetus does not reach the brain.This is not so in postnatal and adult rodents in which brain T3 derives from the blood and from T4 deiodination. In the absence of D2 (see next section) brain T3 is about 50% of normal (59).
Sources of Thyroid Hormone for the Fetus
Before onset of function of its own thyroid gland, the only source of thyroid hormone for the fetus is the maternal thyroid gland. Thyroid hormones are present in the rat embryos as early as 3 days after implantation and in the fetus well before onset of fetal thyroid gland function (60-65). During fetal development the proportion of hormone in the fetus originating in the fetal gland increases, and that of maternal origin decreases, but in the rat at term maternal T4 still accounts for about 17.5% of the fetal extra thyroidal thyroxine pool (66). In humans T4 is already present in the coelomic fluid bathing the yolk sack, as early as the 6th gestational week (67). In the fetal brain, T4 and T3 are present in significant amounts by the 10th week after conception (68). At term, about 30-50% of T4 present in neonates represents the maternal contribution (69).
Expression and Regional Distribution of Deiodinases
The predominant deiodinases expressed in brain are D2 and D3, products of the Dio2 and Dio3 genes respectively (70). Deiodinases are membrane-anchored proteins. D1 and D3 are anchored to the plasma membrane with the catalytic site exposed to the intracellular compartments, whereas D2 is anchored to the endoplasmic reticulum, with the catalytic site exposed to the lumen (70, 71). D3 in the plasma membrane is oriented with its catalytic site exposed to the extracellular fluid (72). In this way it should have easy access to extracellular iodothyronines. There is also evidence that iodothyronines need to be internalized in the cell in order to act as D3 substrates (73). This could be due to the rapid internalization of D3 into endosomes. (72) As described below, Dio2 and Dio3 are expressed in different cells, Dio2 in astrocytes and Dio3 in neurons. Therefore, astrocytes generate the active T3, whereas neurons degrade T4 and T3 to rT3 and T2, respectively. Furthermore, D2 activity is regulated directly by T4, which increases D2 degradation, whereas Dio3 expression is increased by T3. In other cellular contexts the balance between D2 and D3 activities is also regulated by Sonic Hedgehog, which increases D2 degradation and induces Dio3 expression (74, 75). Whether Shh plays similar roles in the developing brain is not known.
Type 2 deiodinase
In Dio2-expressing tissues, such as brain, brown adipose tissue and pituitary, 50% or more of T3 derives from local T4 deiodination (76, 77). In the adult rat brain as much as 80% of nuclear bound T3 is formed locally from T4 (78). D2 activity is detectable in the rat fetal brain, markedly increasing by the end of pregnancy to adult levels (79, 80). At the same time, brain T3 increases by about 18-fold (80). D2 activity increases in hypothyroidism, and is very sensitive to the administration of T4 (81). In iodine deficiency the increased D2 activity tends to maintain T3 concentrations normal despite greatly reduced T4 concentrations in plasma and brain (82). The main regulation of D2 activity occurs at the post translational level involving the actin cytoskeleton (83-85) and the ubiquitin-proteasome pathway (86, 87). In addition, Dio2 expression is also regulated at the mRNA level (88-91).
Strongest Dio2 expression in the brain (Fig 5) occurs in the hypothalamus (92), in specialized glial cells known as tanycytes, which partially line the walls of the third ventricle (91-93). These cells extend long processes to the adjacent hypothalamus and the median eminence (94) ending in capillaries and axon terminals. The tanycytes are involved in the uptake of T4 from the capillaries of the median eminence and basal hypothalamus and/or from the CSF and its conversion to T3. T3 formed in tanycytes would be released back to the CSF and reach other brain regions by diffusion, or it may be delivered to hypothalamic nuclei directly from the tanycytes processes. It is likely that T3 reaches in this way the paraventricular nucleus (PVN) where T3 regulates TRH production and does not express D2 (93). T3 produced by tanycytes could also participate in anterior pituitary regulation after delivery to the portal vessels (95). Activation of the Dio2 gene in the basal hypothalamus is involved in photoperiods and seasonal breeding in birds and mammals (96, 97).
Dio2 is also expressed in astrocytes throughout the brain (90, 93). Specific astrocytic expression of the Dio2 gene has also been found in whole genomic analysis of purified brain cells, revealing that Dio2 is one of the top 50 astrocyte-specific genes (98). Localization of D2 in astrocytes and tanycytes has also been found in the human hypothalamus (99). Expression of D2 in astrocytes contrasts with the localization of the T3 receptor, which is mainly neuronal (100, 101). Therefore, the current thinking is that astrocytes are involved in the uptake of T4 from the brain capillaries, and in T3 generation and further delivery to neurons (see below). This is reminiscent of other forms of coupling between glia and neurons (102) in glucose metabolism, or in the generation of glutamate and GABA. Also in the cochlea, D2 is present in the connective tissue (103) whereas the T3 receptor is expressed in the sensory epithelium and spiral ganglion (103-105).
Dio2 knock out mice (59) have reduced T3 concentrations in brain, similar to those of hypothyroid mice; however, the mice have minimal neurological impairment. Tests of locomotion, memory, anxiety, etc are normal or reveal minimal impairment. In the earliest studies on Dio2 knock out mice it was realized that expression of some T3 responsive genes, such as Nrgn (RC3/Neurogranin) was not altered in the absence of D2, despite having similar brain T3 concentrations as the hypothyroid mice. The cause for this discrepancy is not known, but the possibility of different roles for the T3 generated locally from T4, and the T3 reaching the neurons directly from the blood, is very intriguing. In fact, we have recently found that in the absence of D2, when all brain T3 is from the circulation, expression of most positively regulated genes in the cerebral cortex is unaffected. In contrast, the negatively regulated genes show an expression level similar to hypothyroid animals. (106, 107). It may be that more T3 is needed intracellularly for gene repression than for gene induction, and that the extra T3 needed is provided by D2 activity. Double Dio1/Dio2 knock out mice also have normal blood and tissue concentrations of T3, similar to those of hypothyroid mice or Dio2 knock outs but Nrgn expression is reduced in the cortex. The double knock outs also have minimal neurological phenotype (108).
D3, the product of the Dio3 gene, inactivates thyroid hormones through inner ring deiodination (109). D3 activity is highest in placenta and in fetal tissues and decreases after birth (79, 110-112). In the human placenta, D3 activity is 200 times higher than that of D2 at all gestational ages (113, 114). Although in cultured astrocytes Dio3 expression can be induced by growth factors (115-117), in vivo it is expressed in neurons (118, 119).
D3 plays an important role in the control of T3 concentration in developing tissues. For example, in metamorphosis D3 is negatively correlated with responsiveness to thyroid hormone of tadpole tissues (120-123). In mammals placental D3 controls the transfer of maternal thyroid hormone to the fetus (124). Dio3 expression is very high in the uterus at the implantation site and in the epithelial cells of the uterine lumen surrounding the fetal cavity (125). In the newborn rat brain discrete and intense expression of D3 occurs in neurons located in areas involved in sexual differentiation of the brain (118, 126), suggesting that these areas need to be protected from a possible interfering action of T3 during critical periods of sexual brain differentiation (127). D3-deficient mice (128, 129) have profound alterations in thyroid hormone economy, with greatly elevated thyroid hormone concentrations during the perinatal period. This hypermetabolic situation progresses into a state of central hypothyroidism, maintained through adulthood. In the brain, the expression of thyroid hormone target genes is elevated during the early postnatal period, and then decreases, in parallel to the thyroid state (130). D3 activation reduces the effects of high doses of T3 on gene expression, and therefore protects the brain from excessive T3 (107). The effect of T3 on the Dio3 gene is selectively mediated through the TRÎ±1 receptor subtype (131).
Astrocyte-neuron coupling in the regulation of thyroid hormone action in the brain.
As indicated above, expression of Dio2 in astrocytes led to the hypothesis that in the brain T4 was converted to T3 in the astrocytes and delivered to neurons in response to local needs (93). This hypothesis was later refined by the realization that D3 was present in the neurons, thereby providing a control of neuronal T3 through degradation. It was later demonstrated in vitro using a co-culture assay that T3 produced in astrocytes could influence neuronal gene expression (132). Proof that this mechanism indeed operates in vivo was recently provided by our laboratory. First, we showed that circulating T3 needs the presence of the Mct8 transporter (see below) in the blood-brain barrier (BBB) to influence neuronal gene expression (133). Absence of Mct8 did not affect the action of T4, presumably because it crosses the BBB through another transporter (see below) and the T3 produced in the astrocytes would compensate the deficit of T3 from the circulation. That this was indeed the case was demonstrated by analyzing cerebral cortex gene expression in mice that lacked Mct8, D2, or both (106). While at least the positively regulated genes were normally expressed in the single Mct8 or D2 deficiency, the absence of both, Mct8 and D2, led to hypothyroid levels of gene expression. This experiment demonstrates the relevance of the two sources of T3, the circulation, and the astrocyte, for thyroid hormone action in the neurons.
Transport of Thyroid Hormones to the Brain
The brain barriers
The passage of substances from blood to brain is restricted by the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (blood-CSF) barrier (134) (Fig 6). The BBB is formed by the endothelial cells of brain capillaries which are apposed by tight junctions, so that substances must leave the circulation and enter the brain parenchyma through transcellular transport. Surrounding the brain capillaries are the astrocytic end-feet. Â It has been proposed that the astrocyte membranes do not cover completely the capillary surface, leaving patches of capillary wall in contact with the insterstitial fluid (135). Â Therefore T4 and T3 transported through the BBB could reach the astrocytes through their end-feet or be delivered directly to the interstitial fluid through the spaces between astrocytes. Other studies however show that the astrocytes cover completely the capillaries, leaving only narrow spaces for free diffusion (136).
The blood-CSF barrier is formed by the epithelial cells lining the ventricular side of the choroid plexus. Administration of labelled T4 or T3 leads to a rapid labelling of the choroid plexus and the appearance of the hormones in the CSF (137). Thyroid hormones are present in the CSF at less than ten times the concentration in serum (138-140), but the free hormone fraction is several fold higher in the CSF due to the low protein concentration. It was suggested that transthyretin (TTR), the major protein of the CSF in many species, plays a role in T4 transport in the choroid plexus (141, 142). However, the T4 transfer rate from plasma to tissue compartments, including the brain, is normal in transthyretin null-mutant mice (143, 144).
The bulk of hormone that reaches most brain structures does it through the capillaries in the parenchyma. The fraction of brain thyroid hormone that is transported through the choroid plexus and the CSF has been estimated to be around 20% (145). Labelled T4 injected directly into the CSF accumulates mainly in the median eminence (146, 147).
Thyroid hormone transporters
The transport of thyroid hormone through the plasma cell membrane is facilitated by several classes of membrane transporters (for recent reviews see (148-153)). The discovery that mutations in one of these transporters, the monocarboxylate transporter 8 (MCT8, SLC16A2 gene) cause a syndrome of severe neurological impairment and thyroid hormone abnormalities, provided a definitive proof for their physiological importance (154, 155) (see later section). The membrane transporters for thyroid hormones belong to several families including the Na+-dependent organic anion transporter (NTCP), the Na+-independent organic anion transporting polypeptides (OATP), the heterodimeric amino acid transporters (HAT) containing the light chains LAT-1 (SLC7A5) and LAT-2 (SLC7A8), and the monocarboxylate anion transporters (MCT) MCT8 and MCT10 (SLC16A10 gene). These transporters have a wide range of tissue distribution, with overlapping patterns of expression in most of them (156).
MCT8 and MCT10 are specific for the transport of iodothyronines, with slightly higher affinity for T3 than for T4. MCT10 have similar kinetic properties than MCT8, and may mediate both, influx and efflux of the hormones. Rodent Mct8 is strongly expressed in the choroid plexus. It is also expressed in the membrane of tanycytes and neurons. In addition, recent data have demonstrated anatomically and functionally that Mct8 expression in the BBB is very important to understand differential T4 and T3 transport through the BBB (133, 157). While MCT8 transports T4 and T3, OATP1C1 (SLCO1C1) transports preferentially T4. This transporter is also expressed in the endothelial cells of the capillaries forming the blood brain barrier, and in the choroid plexus (158). Based on the differential expression of transporters, and the comparative effects of T4 and T3 in the brain of Mct8 knock out mice (106), it can be proposed that in rodents T4 is transported by Oatp1c1 to the astrocytes where it is converted to T3. Circulating T3 crosses the BBB primarily through Mct8, probably reaching the brain interstitial fluid directly. Astrocyte-produced, or blood T3 may be transported to neurons primarily through Mct8 but may also do so through alternative transporters (133). Actually astrocytic-produced T3 reaches the neurons in vivo in the absence of Mct8 (106).
THYROID HORMONE RECEPTORS IN THE BRAIN
In rats the nuclear thyroid hormone receptor protein is present in brain at embryonic day 13.5-14 i.e., several days before onset of thyroid gland function (Fig 7). The receptor increases subsequently and reaches a maximum on postnatal day 6 (159-161). Total brain receptor occupancy by the hormone increases in parallel with plasma and cytosol total and free T3 with a maximum of 50-60% on postnatal day 15 (162). All receptor isoform mRNAs are expressed in the brain, but the predominant TR isoform is TRÎ±1, widely distributed in the CNS from E14 to adulthood (100, 101, 163). From E19 to P0, TRÎ±2 is present in the outer part of the cerebral cortex and hippocampal CA1 field. During the late fetal stage TRÎ±1 becomes expressed in the piriform cortex, superior colliculus, and pyramidal layer of the hippocampus, and in the granular layer of dentate gyrus. In adult rats TRÎ±1 expression is prominent in the cerebral cortex, cerebellum, hippocampus, striatum and olfactory bulb. The pattern of TRÎ²1 expression during development is different than that of TRÎ±1, with restricted low expression during the fetal period, and increasing during the postnatal period and through adulthood. TRÎ²1 mRNA can be detected at E15.5 in the upper tegmental neuroepithelium. Between E17 and E20 only low levels are present in the brain, especially in the pyramidal layer of the hippocampus. On P0 a drastic increase occurs in the accumbens, striatum, and hippocampus. From around P7 TRï¢1 becomes also expressed in the cerebral cortex. The patterns of expression of TRÎ²1 and TRÎ±1 overlap (Fig 8), but in some cells, one of the isoforms is expressed preferentially. For example, in the cerebellum, differentiated granular cells express TRÎ±1 while Purkinje cells express TRÎ²1. Recent studies by VennstrÃ¶m and coworkers (164) have shown that TRÎ±1 is expressed essentially in all postmitotic neurons, with the notable exception of Purkinje cells, and in a few glial cell types.
As with the mRNA, the predominant receptor protein in the brain is TRï¡1, which accounts for about 70-80% of all receptor subtypes. In addition to TRï¡1 and TRï¢1, TRï¢2 isoform, initially considered to be pituitary-specific (165), was later detected by in situ hybridization in several regions of the brain such as the rostral caudate, the hippocampus and the hypothalamus (100). Actually the TRï¢2 protein contributes by about 10 % to the total receptor protein in different tissues, including the brain (166). By immunohistochemistry the TRï¢2 protein was found widely distributed in the brain, in regions where the mRNA cannot be detected, such as layers II-VI of the cerebral cortex, and Purkinje cells of the cerebellum (167). Quantitative studies on the concentrations of TR mRNAs and proteins in different tissues were carried out by Ercan-Fang et al (table 1) (168).
|Table 1: TR isoform protein and mRNA concentrations in rat tissues|
|Protein molecules/cell||mRNA molecules/cell|
|The three TR isoform proteins are present in all tissues. This includes the TRï¢2 isoform, despite that the corresponding mRNA is only detectable in pituitary. Â The concentrations of receptor protein were measured by specific immunoprecipitation of labelled T3 after incubation with tissue nuclear extract. The concentrations of mRNA were measured in northern blots using quantitative procedures. Data from (168).|
In the human brain, the receptor protein is present at low levels in the fetus around the 10th week postconception (68). The T3 receptor mRNAs can be detected during the first trimester (169). Receptor concentration increases 10-fold from the 10th to the 16th-18th weeks postconception (Fig 9). During this time the brain gains in weight and DNA content by about 5 fold, so that the total brain T3 receptor content increases 500 times. This period coincides with that of the active neuroblast proliferation (170). Not only the receptor, but also the ligand T3 is also present in brain from the 10th week of gestation, at enough concentrations to results in about 25% occupancy of receptor (171). At the same time, in other organs such as liver and lung, even with higher receptor concentration, only T4 was present on 18th weeks. This indicates that during the second trimester of pregnancy, the main source of T3 to the fetal brain is local deiodination of T4 by D2. Kester et al (172) showed that T3 increases in the cerebral cortex during the second trimester, in parallel with increased D2 activity. In the cerebellum however a high activity of D3 keeps T3 concentrations low (Fig 10) during the same developmental ages. These data are especially relevant in the context of the pathogenesis of neurological cretinism and for the consequences of maternal hypothyroxinemia on fetal brain development, as discussed below.
The distribution of T3 receptor mRNAs in vivo by in situ hybridization suggests a predominant neuronal expression. However, in neural cell cultures, T3 receptors have been detected in neurons, astrocytes and oligodendrocytes (173, 174). In dorsal root ganglia both sensory neurons and Schwann cells appear to express T3 receptors (175) but whereas neurons express permanently the receptor, glial cells do only transiently (176). Studies in vivo have shown that TR isoforms colocalize with oligodendrocyte markers but not with astrocytic markers (177) and other studies have demonstrated that in primary culture, rat astrocytes do not express T3 receptor, but only the TRï¡2 isoform (178, 179). As mentioned above, recent studies showed expression of the TRï¡1-encoding gene in only a few glial cell types (164).
MECHANISMS OF THYROID HORMONE ACTION IN THE BRAIN
Regulation of Brain Gene Expression by Thyroid Hormone
Most effects of thyroid hormones on developmental processes are carried out through the control of gene expression, but non-genomic actions have also been proposed (180). Many thyroid hormone-dependent genes have been identified mostly during the postnatal period. With few exceptions, the role of thyroid hormone is to accelerate the rate of gene expression changes during development. For example, myelin proteins accumulate rapidly in the brain during the first weeks of life in the rat, in parallel to an increased gene expression and RNA accumulation. The role of thyroid hormone is to control the rate of gene transcription so that accumulation of the gene products occurs at the required rate. In the absence of thyroid hormone, mRNA and protein accumulation slows down. but their final concentrations in the tissue eventually attain a normal value (see an example of this phenomenon in figure 1). Another property of thyroid hormone regulation of gene expression in the brain is the regional specificity, even for genes regulated at the transcriptional level (181). The molecular basis for the time and regional specificity is mostly unknown. Transcription is the result of combinatorial activity of many transcription factors, and one of them is the thyroid hormone receptor, in genes regulated by thyroid hormones at the transcriptional levels. Therefore the action of thyroid hormone will depend upon the specific expression of the receptor and other transcription factors regulating the target gene in specific cells and developmental periods.
Many of the thyroid hormone-regulated genes identified during the postnatal period in the rat brain are sensitive to the hormone only during a time window that spans the first 2-3 weeks after birth (Fig 11). Most of these genes are not dependent of thyroid hormone in the fetal or in the adult brain. Based on these observations it was thought that the critical period of thyroid hormone sensitivity in the brain is limited to the first 2-3 postnatal weeks in the rat. However, this concept derives from the fact that most searches for thyroid hormone dependent genes in brain have been made during the postnatal period, at the peak of T3 receptor expression and occupancy. Although this period is probably the most sensitive to thyroid hormones in terms of the irreversible consequences of thyroid hormone deprivation, the rat brain is also sensitive, morphologically and functionally, and also in terms of control of gene expression in the fetus and in adult animals. Many genes have been identified regulated by thyroid hormone in brain, both by conventional Molecular Biology approaches, and by microarray analysis (106, 182-188). For clarity only a few selected examples will be described.
The fetal period
Early efforts to demonstrate an effect of thyroid hormones on fetal brain gene expression led to the conclusion that the fetal rat brain at term is not sensitive to thyroid hormones (189). However, regulation of several genes, have been described in cultured fetal neurons (190) or in the brain in vivo (191-193). In the fetal cerebral cortex at the end of gestation thyroid hormone controls the expression of genes involved in biogenesis of the cytoskeleton, neuronal migration and growth, and branching of neurites. Interestingly, a large percentage of the thyroid hormone dependent genes were related to Camk4 signalling pathways (194). Camk4 was regulated directly by T3 in cultured primary neurons, confirming earlier results from other groups (195, 196)
The postnatal period
Most actions of thyroid hormone on brain development have been studied during the postnatal period in the rat (Fig 11). As mentioned above, the first 2-3 weeks of postnatal life is the period of maximal sensitivity of the rat brain to thyroid hormone, in parallel to the postnatal increase in thyroid hormone secretion, and brain D2 activity, and receptor and T3 concentrations. Important developmental events occur during this period: myelination, development of the cerebellum, dentate gyrus, and cochlea, glia cell proliferation, and terminal differentiation of neurons, with synapse formation and axon and dendrite sprouting. The corresponding period in the human is from about the 20th week of gestation to the first 2 years. Practically all these events are influenced by thyroid hormone, and genes involved in these processes whose expression is dependent on the thyroidal status have been identified.
The adult brain
In adult subjects thyroid hormones influence mood and behaviour, and thyroid dysfunction affects neurotransmitter systems (197) often leading to psychiatric disorders (198). High doses of T4 are effective in the treatment of bipolar depression (199, 200). In the adult rat striatum, administration of a large single T3 dose leads up-regulation of 149 genes and down-regulation of 88 genes (182). Physiological doses of T3 given for several days to hypothyroid animals led to up-regulation of 18 genes, and down-regulation of just one gene. Therefore, acute large doses of thyroid hormone causes large changes in gene expression, with more modest changes with lower doses. Some of the regulated genes are related to circadian rythms and to wakefulness, with one of them (Dbp or D-site binding protein) proposed as a candidate gene in bipolar disorders (201), and likely to be regulated directly by TRÎ±1 (188). Many other genes were involved in striatal physiology as components of several signalling pathways (Fig 12).
Genes involved inÂ Myelination
Neonatal hypothyroidism in the rat is associated with decreased myelin content, and reduced axon myelination. Thyroid hormone influences the expression of practically all myelin protein genes. The best characterized are those encoding the structural proteins (proteolipid protein (PLP), myelin basic protein (MBP), and myelin associated glycoprotein (MAG) (202). The period of thyroid hormone sensitivity for these genes in the rat brain extends from about the end of the first postnatal week up to the end of the first month, but the timing of regulation has a strong regional component, in parallel with the wave of myelination (203, 204). Expression of myelin genes becomes normalized with time, even with continuous deprivation of thyroid hormones, but the number of myelinated axons remains lower than normal. The primary action of thyroid hormone on myelination is on oligodendrocyte differentiation (205). Thyroid hormone promotes differentiation of oligodendrocyte precursors in vitro (206). This effect is probably exerted through inhibition of the transcription factor E2F1 (207). The receptor involved in these actions of thyroid hormone is TRÎ±1 (208, 209).
An extensive review on thyroid hormone action on the mitochondria has been published (210). Mitochondria contain truncated forms of TRï¡1 and RXRï¡, and thyroid hormone influences transcriptional activity of liver mitochondria (211, 212). In the brain, the thyroidal status influences mitochondrial morphology and function (213, 214) leading to changes in the expression of nuclear-encoded and mitochondrial-encoded mRNAs and proteins, such as 12S and 16S RNAs, cytochrome c oxidase subunits (214, 215), NADH dehydrogenase subunit 3 (216), and a protein import receptor (44).
Thyroid hormone influences maturation of the radial glia, the path along which radial migration occurs in the cerebral cortex and the hippocampus (39). The mechanisms of thyroid hormone control of cellular migrations have not been defined in detail, but some of the molecules involved are regulated by thyroid hormone. Among them the extracellular protein Reelin, secreted by the Cajal-Retzius cells (46) of cortex and hippocampus, and by granular cells of the cerebellum, is under thyroid hormone control (217). The main function of Reelin is to stop migrating neurons, and it is essential for the inside-out pattern of cerebral cortex development. The protein Disabled1 (Dab1), a component of the Reelin signalling pathway is also under thyroid hormone regulation (46).
Thyroid hormones also regulate negatively the concentrations of other extracellular matrix proteins and adhesion molecules, of multiple functions including not only cell migration, but also neurite outgrowth, growth cone morphology, and axonal guidance and fasciculation. Among them Tenascin C, laminin, L1, and NCAM (181, 218-220). The concentrations of these proteins in neural tissue decrease during the postnatal period and thyroid hormones control the rate of this process.
Genes involved in Neural Cell Differentiation
Thyroid hormone controls the expression of many genes encoding proteins with roles on terminal cell differentiation, such as cell cycle regulators, cytoskeletal proteins, neurotrophins and neurotrophin receptors and extracellular matrix proteins. Among the cell cycle regulators, E2F1, p53, cyclins and cyclin-dependent kinase inhibitors are regulated by thyroid hormone in cell culture (221-223). As already mentioned E2F1 is involved in oligodendrocyte differentiation (207).
Neural cell shape is determined by the cytoskeleton, which consists of microtubules (Tubulin), microfilaments (Actin), and intermediate filaments, specific for neurons (Neurofilaments), glia (Glial Fibrillary Acidic Protein), or maturing cells (Vimentin, Nestin). TubulinsÂ Î±1 andÂ Î±2 are downregulated by thyroid hormone, and tubulinÎ²4 is upregulated (224, 225). Microtubule associated proteins (MAPs) are also under thyroid hormone control at a posttranscriptional level. For example thyroid hormone regulates Map2 protein distribution in the Purkinje cell dendritic tree (226), and conversion of immature forms of Tau protein to mature forms of the TAU protein by alternative splicing of the Tau mRNA (227). The neurofilament genes Nefh, and Nefm are also under thyroid hormone control in the fetal and postnatal cerebral cortex (106, 194).
In addition to oligodendrocytes and radial glia, other glial cells are also influenced by thyroid hormones: astrocytes (228, 229) cerebellar Golgi epithelial cells (230), and microglia (231). As mentioned above, thyroid hormone influences the in vivo expression of astroglial genes such as those encoding Tenascin C, Laminin, and L1, which also have additional roles in neuronal migration and differentiation, and in axonal fasciculation. In vitro, the effect of T3 on astrocyte differentiation is blocked by ï¢-adrenergic receptor antagonists (232).
Some of the effects of thyroid hormone on differentiation and survival might be mediated through control of neurotrophin expression. Interactions between thyroid hormone and NGF are important for the growth and maintenance of cholinergic neurons in the basal forebrain (18). Changes in NGF, TrkA and p75NTR after hypothyroidism have been described (233). In the cerebellum thyroid hormone also controls the expression of NT-3 in vivo and in cultured cerebellar granule cells and it was suggested that the control of Purkinje cell differentiation by thyroid hormone is mediated through NT-3 produced by granule cells (234).
Genes Involved in Cell Signalling
Proteins directly involved in intracellular signalling are also under direct control of thyroid hormone. Here are cited only a few examples to illustrate the diversity and pleiotropy of thyroid hormone effects on the brain. One of these proteins is RC3/Neurogranin (181), a protein kinase C substrate that binds Calmodulin in the non-phosphorylated state and in the presence of low Ca2+ concentrations (235). Neurogranin (Nrgn) is involved in synaptic plasticity (236) by regulating the free calmodulin available and its distribution within the dendritic spines. Nrgn knock out mice display alterations of spatial memory (237). Nrgn mRNA and protein are under thyroid hormone control in developing rats, mice and goats, and the effect is mediated at the level of transcription through a response element located in the first intron of the gene (238).
Rhes (Ras homolog enriched in striatum), or Rasd2 (239, 240) is striatal-enriched protein of the Ras family, with high homology with Dexras-1 (dexamethasone-inducible Ras protein) or Rasd1. These two proteins define a family within the Ras superfamily of small GTPases. Rhes regulates the AKT pathway, and mediates mTOR and dopamine signalling, and mutant-hungtintin toxicity, among other actions (241-243). The PCP-2 (Purkinje cell-specific protein 2, or L7) contains the G-protein regulatory motif GoLoco, involved in regulation of Gï¡i protein signalling (244). Thyroid hormone also regulates in vivo and in cultured cells the expression of tubby, a gene expressed in hypothalamic nuclei encoding a protein that also acts through G-protein signalling (245, 246). Other genes involved in GPCR signalling are also under thyroid hormone control in the adult striatum (182).
Transcription Factors and Splicing Regulators
Regulation of proteins involved in transcription, and RNA stability and splicing is important for secondary influences in gene regulation. NGFI-A mRNA (Krox-24, Egr-1, Zif-268) is decreased in hypothyroid rats in several areas of the brain (247), and is induced by T3 at the promoter level in vivo (248). T3 regulates the krÃ¼ppel-like factor 9 (Klf9) transcription factor formerly known as BTEB (basic transcription element binding protein), a member of the Sp1 family of transcription factors, in vivo and in cultured neuronal cells (249, 250). Klf9 has been recently shown to mediate the actions of thyroid hormone in the loss of the regenerative capacity of Purkinje cells that occur during postnatal development (251). RORÎ±, a member of the RZR/ROR family of orphan nuclear receptors is thyroid hormone dependent in the cerebellum. Disruption of the RORÎ±Â gene is responsible leads to profound alterations in Purkinje cell growth and differentiation and granule cell migration (staggerer phenotype in mice) so that some of the actions of thyroid hormone on cerebellum could be mediated by this transcription factor (252). Also in the cerebellum, and other brain regions, the expression of the transcription factor Hairless is very sensitive to thyroid hormones (253). Hairless is a corepressor that heterodimerizes with the thyroid hormone receptor (254), so that in theory its induction would presumably tend to buffer other responses to thyroid hormone. Thyroid hormone also influences genes involved in RNA splicing, such as the mammalian homolog of the Drosophila splicing regulator Suppressor-of-white-apricot (SWAP) (255), and Musashi-1. Some of the posttranscriptional effects of thyroid hormone might be mediated through the control of RNA binding proteins controlling RNA stability.
Gene expression in the adult brain
In adult subjects thyroid hormones influence mood and behaviour, and thyroid dysfunction affects neurotransmitter systems  often leading to psychiatric disorders . High doses of T4 are effective in the treatment of bipolar depression [213, 214]. In the adult rat striatum, administration of a large single T3 dose lead up-regulation of 149 genes and down-regulation of 88 genes . Physiological doses of T3 given for several days to hypothyroid animals led to up-regulation of 18 genes, and down-regulation of just one gene. Therefore, acute large doses of thyroid hormone causes large changes in gene expression, with more modest changes with lower doses. Some of the regulated genes are related to circadian rhythms and to wakefulness, with one of them (Dbp or D-site binding protein) proposed as a candidate gene in bipolar disorders . Many other genes were involved in striatal physiology as components of several signalling pathways (Fig 12).
Mechanisms of Gene Regulation by Thyroid Hormone
In some cases thyroid hormone responsive elements (TRE) have been identified in the promoter or intronic regions of thyroid hormone dependent brain genes. Among these, myelin basic protein (256), Â the Purkinje cell specific gene (PCP2) (257), the calmodulin binding and PKC substrate RC3 (238), prostaglandin D2 synthetase (258, 259) the transcription factor Hairless (253), the neuronal cell adhesion molecule (NCAM) (260) and the early response gene NGFI-A (248). Chromatin immunoprecipitation assays have identified many TRÎ²1 binding sites in the developing mouse cerebellum (261) or cerebellar cells (188). Expression of other genes is regulated at the levels of mRNA stability (acetylcholinesterase), protein translation (MAP2) (226) or mRNA splicing (Tau, (227)). Regulation of splicing might be due to a primary action on the transcription of splicing regulators (262).
The major causes of thyroid hormone deficiency during development are iodine deficiency, congenital disorders of the thyroid gland, and maternal and/or fetal hypothyroidism. In addition, the condition known as hypothyroxinemia, defined as low T4 in the presence of normal T3 and TSH, in the pregnant woman is suspected of a possible cause of developmental impairment. Less frequently, but of great importance are mutations of the thyroid hormone transporter MCT8. On the other hand, mutations of thyroid hormone receptors, THRA and THRB also cause variable degree of brain disturbances. The consequences of thyroid hormone deficiency on brain maturation greatly depend on the specific stages of development affected (Fig 13).
Iodine Deficiency Disorders. Endemic Cretinism
Daily adult needs of iodine are of the order of 150, and 250 Âµg during pregnancy and lactation [222, 223]. Iodine deficiency causes a wide spectrum of abnormalities collectively known as iodine deficiency disorders , with high incidence of abortions and stillbirths, increased perinatal and infant mortalities, neonatal goiter and hypothyroidism, and various degrees of psychomotor and mental defects, and cretinism.
Iodine deficiency has been, and still continues to be, endemic in many regions of the planet. The consequence of severe iodine deficiency during development is the syndrome known as cretinism, of which two forms, Neurological and Mixedematous, were described by McCarrison  in the Himalayas and in Papua-New Guinea. In Neurological cretinism the thyroid gland is normal, and there are no signs of hypothyroidism. However, there is severe mental retardation, deaf mutism and a striato-pallidal disorder with spastic diplegia affecting the lower limbs . On the other hand, mixedematous cretins  are hypothyroid, with short stature, poor sexual development, and craniofacial abnormalities. They are also mentally retarded but not as severely as neurological cretins, and signs of neurological involvement are observed only in a minority of cases. In affected regions, usually a combination of the two forms is mor frequently observed.
Until the 80âs of last century, there were great controversies concerning the pathogenesis of each of these syndromes, and whether or not neurological cretinism was really due to thyroid hormone deficiency. Among other confusing reasons was the fact that early postnatal administration of iodine or of thyroid hormone was ineffective. It is clear today that the differences between the two forms of cretinism reflect different a different timing in the action, or lack of action, of thyroid hormones, according to developmental timing of expression of deiodinases, receptors, thyroid secretion, and maternal placental transfer of thyroid hormones. The clinical picture of neurological cretinism suggests damage to the striatum and the cerebral cortex during the second trimester of pregnancy, and it is due to the profound maternal hypothyroxinemia caused by severe iodine deficiency, during the first half of pregnancy. As described before (Fig 9), the T3 receptor is expressed from the second trimester, at a time when most T3 in fetal brain areas, such as the cortex depend on maternal T4, and local D2 activity (Fig 10). In contrast, Mixedematous cretinism is clearly due to failure of the fetal, and infant thyroid gland during the last trimester of pregnancy and the postnatal period. In many cases destruction of the fetal gland is due to a combination of iodine deficiency and dietary goitrogens.
Congenital failure of the thyroid gland occurs in about 1 in 3,000-4,000 newborns (for a review see (269). Neonatal screening and early thyroid hormone treatment have efficiently prevented its consequences. Causes for Congenital hypothyroidism are thyroid dysgenesis including ectopic thyroid gland and agenesia, and inborn errors of thyroid biosynthesis. In a small percentage of thyroid dysgenesis mutations of genes involved in thyroid embryogenesis have been described (270). The great majority of cases are nonetheless due to non Mendelian mechanisms, with the involvement of several genes (271). An isolated family with truncated TSH receptor has also been described (272).
Despite the systematic and early diagnosis and treatment (273) of Congenital hypothyroidism in the prevention of mental retardation some affected individuals may remain with minimal brain damage, indicated by learning disabilities and disturbance of fine motor coordination. The success of thyroid hormone treatment depends on the severity, onset, and duration of hypothyroidism. Children at risk of having neurological sequelae even with early treatment may require a substantially higher replacement dose of T4 than moderate cases (274). It is therefore important to identify these children by the estimation of bone maturation and plasma T4 levels at diagnosis.
Maternal Hypothyroidism and Maternal Hypothyroxinemia
Maternal thyroid hormones cross the placenta (275), and up to 50% of fetal blood T4 at term is of maternal origin (69). Therefore, in congenital hypothyroidism maternal hormones protect the fetal brain until birth, and even children with total thyroid agenesia do not suffer from severe neurological deficits as in neurological cretinism. The important role of maternal hormones is demonstrated in cases of severe maternal and fetal hypothyroidism. For example, in reported cases Pit-1 deficiency (276) or high titers of thyroid stimulation blocking antibodies (277, 278) circulating thyroid hormones are very low in the mother and in the infant. The children suffer from permanent sensorioneural deafness and irreversible neuromotor development resembling neurological cretinism.
Maternal hypothyroidism, in the presence of a normal fetal thyroid, is also harmful for fetal brain development (279, 280). High TSH in mothers during the second trimester of pregnancy is associated with a reduction of 4 points in the children IQ, and impaired attention, language, and visual-motor performance at 7-9 years. (280). Japanese children born from hypothyroid mothers who where treated after the 6th-16th week of gestation showed no signs of altered neurodevelopment (281). The authors argued against a major role of maternal T4 during early pregnancy. It is possible that iodine supply to the fetal thyroid is an important factor, considering the high iodine intake in Japan. At least in the rat, a good functioning of the the fetal thyroid gland is essential for cerebral cortex gene expression (58, 194). The need for antenatal thyroid screening of pregnant women to prevent impaired cognitive development of children born from women with low thyroid hormone levels was examined by Lazarus et al (282) It was found that the cognitive development at 3 years of age of children born from mothers with low thyroid hormone and/or high thyroid-stimulating hormone (TSH), with or without T4 treatment, was not different from that of the control group.
But there is also evidence that not only frank hypothyroidism, but maternal hypothyroxinemia is associated with a higher incidence of neurological alterations in children (283, 284), and increased incidence of Attention Deficit Hyperactivity Disorder (285). While the incidence of hypothyroidism in pregnant women is around 2.5%, hypothyroxinemia is much more prevalent, up to 30% (275, 286, 287), and it is usually due to mild iodine deficiency (288). In these situations, peripheral euthyroidism, and normal TSH levels, are maintained by several compensatory mechanisms, including preferential T3 secretion. For these reasons Iodine supplementation of women considering conception and during pregnancy and lactation is warranted (263). In rats Berbel et al have clearly shown that maternal T4 is needed for proper brain development (289). In pregnant women mild hypothyroxinemia due to iodine deficiency leads to altered neurocognitive performance of the progeny (290) but not if the hypothyroxinemia was corrected with iodine supplements during the first trimester.
Premature babies often present transient hypothyroxinemia lasting several weeks (291, 292), in about 85% of cases. This is due interruption of maternal thyroid hormone supply, and immaturity of the fetal thyroid axis. The issue of whether the hypothyroxinemia of prematurity is âphysiologicalâ, or deserves thyroid hormone treatment has been thoroughly discussed. It is clear that circulating total T4 and FT4 concentrations are lower in preterm neonates than in fetuses of comparable age still in utero (293-295). Therefore, the hypothyroxinemia of prematurity may have clinical consequences (296) with increased risk of neurological impairment (297), risk of cerebral palsy (298) and white matter damage (299). In an experimental model of prematurity, Berbel et al (289) have shown that deprivation of maternal hormones similar to that suffered by premature babies leads to altered development of the neocortex and hippocampus, and that the defects were corrected by T4 treatment of the dams. Despite the importance of this topic, the issue of whether preterm babies should be treated with thyroid hormone is not settled (300-302).
Thyroid hormone transporter mutations
Among the thyroid hormone transporters in the cell membrane, mutations have only been identified in MCT8, a 12 transmembrane protein encoded by SLC16A2, a gene located in chromosome X. They were first described by the groups of Refetoff (154) and De Visser (155) in children. Other cases were subsequently reported (303-305). Affected patients suffer from X-linked mental retardation, with severe developmental delay and neurological damage in addition to altered thyroid hormone levels in blood. Total and free T4 and rT3 are low, whereas total and free T3 are elevated. TSH is either normal or moderately elevated. The neurological syndrome consists of global developmental delay, lack of language development, profound mental deficiency, rotary nistagmus, impaired gaze and hearing, dystonic movements and severe proximal hypotonia with poor head control progressively developing into spastic quadriplegia. Some patients present paroxysmal dyskinesias (306). It was also recognized that the Allan-Herndon-Dudley syndrome (OMIM 300523) is due to MCT8 mutations (307). The neurological syndrome is likely due to defective T3 transport to neural cells during critical periods of fetal development (148, 308, 309), but the lesion responsible for the clinical phenotype is not known. The most likely possibility is hypomyelination . Actually the syndrome may be included within the leukoencephalopaties. About 5% of patients diagnosed of Pelizeaus-Merchbacher disease, a leukoencephalopaty usually due to PLP1 mutations, have instead MCT8 mutations (310).
Mct8-deficient mice (Mct8-/y) (311, 312) have also, as expected, elevated circulating T3 and reduced T4 and rT3. However they have minimal, if any, brain impairment (313), with only modest reductions in the expression of T3 target genes (133). The knock out mice are only partial models of the disease, and have been useful to dissect the mechanisms leading to the unusual pattern of thyroid hormone concentrations in blood (314). Restricted access of T3 to neurons, where Dio3 is expressed, impairs T3 degradation. The increased availability of T3 increases D1 activity in liver causing and increased rT3 degradation and T4 to T3 conversion, thereby reducing rT3 and T4 levels, and further contributing to increase T3. Additionally, thyroid gland secretion is altered because Mct8 is present in the basal membrane of the thyrocytes and plays an important role in thyroid hormone secretion (315, 316). In the brain there is a reduction of T4 supply to the astrocytes which causes an increased D2 activity and increased production of T3 from T4. Therefore, there are two important features of this syndrome: one is the concomitant increase in the activity of the two deiodinases, D1 and D2. The second is the coexistence of âhyperthyroidâ (liver) and âhypothyroidâ (brain) tissues. Even within the brain, the presence of alternative transporters in some cells might perhaps expose these cells to excessive T3, although this remains as a possibility (313).
There is strong evidence that the main restriction to the entry of T3 to neurons is not at the level of the neuronal membranes, but at the BBB (133, 157). Mct8, and other transporters, such as Oatp1c1 (317) are present in the membrane of capillary endothelial cells . In the absence of Mct8, T4 is still transported due to its higher affinity for Oatp1c1, therefore contributing, through T4 to T3 production, to compensate the lack of T3 transport through the BBB. It has been postulated that reason why Mct8 knock out mice do not show neurological impairment in contrast to patients, is due to compensation by a T4 transporter not be present in humans (157, 313). Indeed, the adult and juvenile primate BBB contains MCT8 but little OATP1C1 (318). The OATP1 transporter is detected immunohistochemically in the human fetal brain capillaries (157) but in lower amounts than in rodents.
T3 receptor mutations
The majority of cases (85%) of the classical form of Resistance to Thyroid Hormone (RTH) are due to mutations in the THRB gene (319). RTH patientsï may have learning disabilities, reduced IQ, and increased incidence of Attention Deficit and Hyperactivity Disorder. Thrb knock out mice, or knock in mice expressing receptor mutations are good models of RTH (320). In addition, the phenotypic analysis of these mice has revealed important roles of TRÎ²Â in maturation of somatosensory systems. Thrb mutant mice have deafness and impaired color vision, due to altered maturation of cochlear hair cells, and retinal photoreceptors (321, 322). Mice expressing a mutant TRÎ²1 with strong dominant negative property had cerebellar abnormalities reminiscent of severe hypothyroidism, and neuromotor disability (323).
THRA mutations have recently been described in humans (324, 325). The TRÎ±1 mutant proteins display strong dominant negative activity. The patients suffer from growth retardation, delayed bone development, severe constipation, and mild cognitive deficits, with minimal alterations of circulating thyroid hormones: normal TSH, low T4/T3 ratios and strongly reduced rT3. T4 may be slightly reduced T4 or in the lower limit of normal range, and T3 may be slightly increased or in the upper limit of normal range. These alterations may be caused at least in part by deficient regulation of Dio3 which is a TRÎ±1-regulated gene (131).
Previously, extensive work was aimed at defining the phenotypes of mice with TRÎ±1 inactivation or harboring a mutant TRÎ±1 with the thought that it could help to identify patients (326). In agreement with this, expression of mutant TRÎ±1 with dominant negative activity causes retarded growth (327), low cerebral glucose consumption (328), retarded brain development, and neuromotor impairment in developing animals (329-331), and a profound anxiety in adult animals, ameliorated by T3 treatment (330). The consequences of TRÎ±1 mutations may also be heterogeneous depending on the mutation (332-334).
An important observation was that absence of TRÎ±1 in mice was not equivalent to mutations of the same gene. Despite the fact that most actions of thyroid hormone in brain are mediated by TRÎ±1, absence of this receptor does not lead to a âhypothyroidâ brain. In other terms, receptor deletion is not equivalent to hormone deprivation. This is due to the intrinsic transcriptional activity of the unliganded receptor. In the absence of the hormone, as in hypothyroidism, the unliganded receptor (or aporeceptor) has hormone-independent activity, either repression or activation of transcription, leading to a disturbance of processes which are normally controlled by thyroid hormones (335). Therefore, to some extent it can be said that the hypothyroid phenotype is the consequence of unliganded receptor activity. In the absence of receptor, this activity is suppressed and hypothyroidism is not as detrimental as in its presence (336, 337).
Since unliganded receptors have transcriptional activity, the question is if they have a have a physiological and developmental role in the absence of the hormone (335, 338). The receptors are expressed somewhat before the onset of thyroid gland secretion, and therefore occupancy of receptors during development is dependent as explained above, from the maternal hormones and the activity of deiodinases. D2 activity increases in the human fetal cerebral cortex during the second trimester, and T3 concentration increases in parallel (172). During the same stages, the cerebellum expresses mainly D3 and T3 is concentration is kept very low. These findings suggest that at the same time of development, the liganded and unliganded receptors are involved in development of the cerebral cortex and the cerebellum respectively. Evidence for a role of unliganded receptors have been demonstrated in studies on amphibian metamorposis (339), or in the development of the inner ear in mice (340).
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