Identification of genes mediating thyroid hormone action in the developing mouse cerebellum

Absence of Thyroid Hormone Induced Delayed Dendritic Arborization in Mouse Primary Hippocampal Neurons Through Insufficient Expression of Brain-Derived Neurotrophic Factor

Thyroid hormone (TH) plays important roles in the developing brain. TH deficiency in early life leads to severe developmental impairment in the hippocampus. However, the mechanisms of TH action in the developing hippocampus are still largely unknown. In this study, we generated 3,5,3'-tri-iodo-l-thyronine (T3)-free neuronal supplement, based on the composition of neuronal supplement 21 (NS21), to examine the effect of TH in the developing hippocampus using primary cultured neurons. Effects of TH on neurons were compared between cultures in this T3-free culture medium (-T3 group) and a medium in which T3 was added (+T3 group). Morphometric analysis and RT-qPCR were performed on 7, 10, and 14 days in vitro (DIV). On 10 DIV, a decreased dendrite arborization in -T3 group was observed. Such difference was not observed on 7 and 14 DIV. Brain-derived neurotrophic factor (Bdnf) mRNA levels also decreased significantly in -T3 group on 10 DIV. We then confirmed protein levels of phosphorylated neurotrophic tyrosine kinase type 2 (NTRK2, TRKB), which is a receptor for BDNF, on 10 DIV by immunocytochemistry and Western blot analysis. Phosphorylated NTRK2 levels significantly decreased in -T3 group compared to +T3 group on 10 DIV. Considering the role of BDNF on neurodevelopment, we examined its involvement by adding BDNF on 8 and 9 DIV. Addition of 10 ng/ml BDNF recovered the suppressed dendrite arborization induced by T3 deficiency on 10 DIV. We show that the lack of TH induces a developmental delay in primary hippocampal neurons, likely caused through a decreased Bdnf expression. Thus, BDNF may play a role in TH-regulated dendritogenesis.

Genetic differences in the aryl hydrocarbon receptor and CYP1A2 affect sensitivity to developmental polychlorinated biphenyl exposure in mice: relevance to studies of human neurological disorders

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that remain a human health concern with newly discovered sources of contamination and ongoing bioaccumulation and biomagnification. Children exposed during early brain development are at highest risk of neurological deficits, but highly exposed adults reportedly have an increased risk of Parkinson's disease. Our previous studies found allelic differences in the aryl hydrocarbon receptor and cytochrome P450 1A2 (CYP1A2) affect sensitivity to developmental PCB exposure, resulting in cognitive deficits and motor dysfunction. High-affinity Ahr ^b Cyp1a2(-/-) mice were most sensitive compared with poor-affinity Ahr ^d Cyp1a2(-/-) and wild-type Ahr ^b Cyp1a2(+/+) mice. Our follow-up studies assessed biochemical, histological, and gene expression changes to identify the brain regions and pathways affected. We also measured PCB and metabolite levels in tissues to determine if genotype altered toxicokinetics. We found evidence of AHR-mediated toxicity with reduced thymus and spleen weights and significantly reduced thyroxine at P14 in PCB-exposed pups. In the brain, the greatest changes were seen in the cerebellum where a foliation defect was over-represented in Cyp1a2(-/-) mice. In contrast, we found no difference in tyrosine hydroxylase immunostaining in the striatum. Gene expression patterns varied across the three genotypes, but there was clear evidence of AHR activation. Distribution of parent PCB congeners also varied by genotype with strikingly high levels of PCB 77 in poor-affinity Ahr ^d Cyp1a2(-/-) while Ahr ^b Cyp1a2(+/+) mice effectively sequestered coplanar PCBs in the liver. Together, our data suggest that the AHR pathway plays a role in developmental PCB neurotoxicity, but we found little evidence that developmental exposure is a risk factor for Parkinson's disease.

Isoform-independent and -dependent phosphorylation of microtubule-associated protein tau in mouse brain during postnatal development

Tau is a microtubule (MT)-associated protein that regulates MT dynamics in the axons of neurons. Tau binds to MTs via its C-terminal MT-binding repeats. There are two types of tau, those with three (3R) or four (4R) MT-binding repeats; 4R tau has a stronger MT-stabilizing activity than 3R tau. The MT-stabilizing activity of tau is regulated by phosphorylation. Interestingly, both the isoform and phosphorylation change at the time of neuronal circuit formation during postnatal development; highly phosphorylated 3R tau is replaced with 4R tau, which is less phosphorylated. However, it is not known how the transition of the isoforms and phosphorylation are regulated. Here, we addressed this question using developing mouse brains. Detailed analysis of developing brains revealed that the switch from 3R to 4R tau occurred during postnatal day 9 (P9) to P18 under the same time course as the conversion of phosphorylation from high to low. However, hypothyroidism, which is known to delay brain development, delayed the timing of tau dephosphorylation but not the exchange of isoforms, indicating that isoform switching and phosphorylation are not necessarily linked. Furthermore, we confirmed this finding by using mouse brains that expressed a single isoform of human tau. Human tau, either 3R or 4R, reduced phosphorylation levels during development even though the isoform did not change. We also found that 3R tau and 4R tau were phosphorylated differently in vivo even at the same developmental days. These results show for the first time that the phosphorylation and isoform alteration of tau are regulated differently during mouse development.

Impairment of synaptic development in the hippocampus of diabetic Goto-Kakizaki rats

Insulin receptor signaling has been shown to regulate essential aspects of CNS function such as synaptic plasticity and neuronal survival. To elucidate its roles during CNS development in vivo, we examined the synaptic and cognitive development of the spontaneously diabetic Goto-Kakizaki (GK) rats in the present study. GK rats are non-obese models of type 2 diabetes established by selective inbreeding of Wistar rats based on impaired glucose tolerance. Though they start exhibiting only moderate hyperglycemia without changes in plasma insulin levels from 3 weeks postnatally, behavioral alterations in the open-field as well as significant impairments in memory retention compared with Wistar rats were observed at 10 weeks and were worsened at 20 weeks. Alterations in insulin receptor signaling and signs of insulin resistance were detected in the GK rat hippocampus at 3 weeks, as early as in other insulin-responsive peripheral tissues. Significant reduction of an excitatory postsynaptic scaffold protein, PSD95, was found at 5w and later in the hippocampus of GK rats due to the absence of a two-fold developmental increase of this protein observed in Wistar control rats between 3 and 20w. In the GK rat hippocampus, NR2A which is a NMDA receptor subunit selectively anchored to PSD95 was also reduced. In contrast, both NR2B and its anchoring protein, SAP102, showed similar developmental profiles in Wistar and GK rats with expression peaks at 2 and 3w. The results suggest that early alterations in insulin receptor signaling in the GK rat hippocampus may affect cognitive performance by suppressing synaptic maturation.

Reduction in NPY-positive neurons and dysregulation of excitability in young senescence-accelerated mouse prone 8 (SAMP8) hippocampus precede the onset of cognitive impairment

The senescence-accelerated mouse prone 8 (SAMP8) strain is considered a neurodegeneration model showing age-related cognitive deficits with little physical impairment. Young SAMP8 mice, however, exhibit signs of disturbances in development such as marked hyperactivity and reduced anxiety well before the onset of cognitive impairment. As the key enzyme in local regulation of thyroid hormone (TH) signaling, type 2 deiodinase, was significantly reduced in the SAMP8 hippocampus relative to that of the normally aging SAM-resistant 1 (SAMR1), we used these two strains to compare the development of the hippocampal GABAergic system, which is known to be strongly affected by hypothyroidism. Among GABAergic components, neuronal K+ /Cl- co-transporter 2 was down-regulated in SAMP8 transiently at 2 weeks. Although distribution of total GABAergic neurons was similar in both strains, 22-30% reduction was observed in the neuropeptide Y (NPY)-positive subpopulation of GABAergic neurons in SAMP8. Electrophysiological studies on hippocampal slices obtained at 4 weeks revealed that epileptiform activity, induced by high-frequency stimulation, lasted four times longer in SAMP8 compared with SAMR1, indicating a dysregulation of excitability that may be linked to the behavioral abnormalities of young SAMP8 and to neurodegeneration later on in life. Local attenuation of TH signaling may thus impact the normal development of the GABAergic system.

Deciphering direct and indirect influence of thyroid hormone with mouse genetics

T3, the active form of thyroid hormone, binds nuclear receptors that regulate the transcription of a large number of genes in many cell types. Unraveling the direct and indirect effect of this hormonal stimulation, and establishing links between these molecular events and the developmental and physiological functions of the hormone, is a major challenge. New mouse genetics tools, notably those based on Cre/loxP technology, are suitable to perform a multiscale analysis of T3 signaling and achieve this task.

Alterations in local thyroid hormone signaling in the hippocampus of the SAMP8 mouse at younger ages: association with delayed myelination and behavioral abnormalities

The senescence-accelerated mouse (SAM) strains were established through selective inbreeding of the AKR/J strain based on phenotypic variations of aging and consist of senescence-prone (SAMP) and senescence-resistant (SAMR) strains. Among them, SAMP8 is considered as a model of neurodegeneration displaying age-associated learning and memory impairment and altered emotional status. Because adult hypothyroidism is one of the common causes of cognitive impairment and various psychiatric disorders, we examined the possible involvement of thyroid hormone (TH) signaling in the pathological aging of SAMP8 using the senescence-resistant SAMR1 as control. Although plasma TH levels were similar in both strains, a significant decrease in type 2 deiodinase (D2) gene expression was observed in the SAMP8 hippocampus from 1 to 8 months of age, which led to a 35–50% reductions at the protein level and 20% reduction of its enzyme activity at 1, 3, and 5 months. D2 is responsible for local conversion of thyroxine into transcriptionally active 3,5,3′-triiodothyronine (T3), so the results suggest a reduction in T3 level in the SAMP8 hippocampus. Attenuation of local TH signaling was confirmed by downregulation of TH-dependent genes and by immunohistochemical demonstration of delayed and reduced accumulation of myelin basic protein, the expression of which is highly dependent on TH. Furthermore, we found that hyperactivity and reduced anxiety were not age-associated but were characteristic of young SAMP8 before they start showing impairments in learning and memory. Early alterations in local TH signaling may thus underlie behavioral abnormalities as well as the pathological aging of SAMP8. © 2012 Wiley Periodicals, Inc.

Genome-wide search reveals the existence of a limited number of thyroid hormone receptor alpha target genes in cerebellar neurons

Thyroid hormone (T3) has a major influence on cerebellum post-natal development. The major phenotypic landmark of exposure to low levels of T3 during development (hypothyroidism) in the cerebellum is the retarded inward migration of the most numerous cell type, granular neurons. In order to identify the direct genetic regulation exerted by T3 on cerebellar neurons and their precursors, we used microarray RNA hybridization to perform a time course analysis of T3 induced gene expression in primary cultures of cerebellar neuronal cell. These experiments suggest that we identified a small set of genes which are directly regulated, both in vivo and in vitro, during cerebellum post-natal development. These modest changes suggest that T3 does not acts directly on granular neurons and mainly indirectly influences the cellular interactions taking place during development.

Thyroid hormone action in cerebellum and cerebral cortex development

Thyroid hormones (TH, including the prohormone thyroxine (T4) and its active deiodinated derivative 3,3′,5-triiodo-L-thyronine (T3)) are important regulators of vertebrates neurodevelopment. Specific transporters and deiodinases are required to ensure T3 access to the developing brain. T3 activates a number of differentiation processes in neuronal and glial cell types by binding to nuclear receptors, acting directly on transcription. Only few T3 target genes are currently known. Deeper investigations are urgently needed, considering that some chemicals present in food are believed to interfere with T3 signaling with putative neurotoxic consequences.

Hypothyroidism impairs chloride homeostasis and onset of inhibitory neurotransmission in developing auditory brainstem and hippocampal neurons

Thyroid hormone (TH) deficiency during perinatal life causes a multitude of functional and morphological deficits in the brain. In rats and mice, TH dependency of neural maturation is particularly evident during the first 1-2 weeks of postnatal development. During the same period, synaptic transmission via the inhibitory transmitters glycine and GABA changes from excitatory depolarizing effects to inhibitory hyperpolarizing ones in most neurons [depolarizing-hyperpolarizing (D/H) shift]. The D/H shift is caused by the activation of the K(+)-Cl(-) co-transporter KCC2 which extrudes Cl(-) from the cytosol, thus generating an inward-directed electrochemical Cl(-) gradient. Here we analyzed whether the D/H shift and, consequently, the onset of inhibitory neurotransmission are influenced by TH. Gramicidin perforated-patch recordings from auditory brainstem neurons of experimentally hypothyroid rats revealed depolarizing glycine effects until postnatal day (P)11, i.e. almost 1 week longer than in control rats, in which the D/H shift occurred at approximately P5-6. Likewise, until P12-13 the equilibrium potential E(Gly) in hypothyroids was more positive than the membrane resting potential. Normal E(Gly) could be restored upon TH substitution in P11-12 hypothyroids. These data demonstrate a disturbed Cl(-) homeostasis following TH deficiency and point to a delayed onset of synaptic inhibition. Interestingly, immunohistochemistry demonstrated an unchanged KCC2 distribution in hypothyroids, implying that TH deficiency did not affect KCC2 gene expression but may have impaired the functional status of KCC2. Hippocampal neurons of hypothyroid P16-17 rats also demonstrated an impaired Cl(-) homeostasis, indicating that TH may have promoted the D/H shift and maturation of synaptic inhibition throughout the brain.