Developmental iodine deficiency resulting in hypothyroidism reduces hippocampal ERK1/2 and CREB in lactational and adolescent rats

Hypothyroidism Induces Interleukin-1-Dependent Autophagy Mechanism as a Key Mediator of Hippocampal Neuronal Apoptosis and Cognitive Decline in Postnatal Rats

Thyroid hormone (TH) is essential for brain development, and hypothyroidism induces cognitive deficits in children and young adults. However, the participating mechanisms remain less explored. Here, we examined the molecular mechanism, hypothesizing the involvement of a deregulated autophagy and apoptosis pathway in hippocampal neurons that regulate cognitive functions. Therefore, we used a rat model of developmental hypothyroidism, generated through methimazole treatment from gestation until young adulthood. We detected that methimazole stimulated the autophagy mechanism, characterized by increased LC3B-II, Beclin-1, ATG7, and ATG5-12 conjugate and decreased p-mTOR/mTOR and p-ULK1/ULK1 autophagy regulators in the hippocampus of developing and young adult rats. This methimazole-induced hippocampal autophagy could be inhibited by thyroxine treatment. Subsequently, probing the upstream mediators of autophagy revealed an increased hippocampal neuroinflammation, marked by upregulated interleukin (IL)-1alpha and beta and activated microglial marker, Iba1, promoting neuronal IL-1 receptor-1 expression. Hence, IL-1R-antagonist (IL-1Ra), which reduced hippocampal neuronal IL-1R1, also inhibited the enhanced autophagy in hypothyroid rats. We then linked these events with hypothyroidism-induced apoptosis and loss of hippocampal neurons, where we observed that like thyroxine, IL-1Ra and autophagy inhibitor, 3-methyladenine, reduced the cleaved caspase-3 and TUNEL-stained apoptotic neurons and enhanced Nissl-stained neuronal count in methimazole-treated rats. We further related these molecular results with cognition through Y-maze and passive avoidance tests, demonstrating an IL-1Ra and 3-methyladenine-mediated improvement in learning-memory performances of the hypothyroid rats. Taken together, our study enlightens the critical role of neuroinflammation-dependent autophagy mechanism in TH-regulated hippocampal functions, disrupted in developmental hypothyroidism.

Identification of gene targets of developmental neurotoxicity focusing on DNA hypermethylation involved in irreversible disruption of hippocampal neurogenesis in rats

We have previously found that maternal exposure to 6‐propyl‐2‐thiouracil (PTU), valproic acid (VPA), or glycidol (GLY) has a sustained or late effect on hippocampal neurogenesis at the adult stage in rat offspring. Herein, we searched for genes with hypermethylated promoter region and downregulated transcript level to reveal irreversible markers of developmental neurotoxicity. The hippocampal dentate gyrus of male rat offspring exposed maternally to PTU, VPA, or GLY was subjected to Methyl‐Seq and RNA‐Seq analyses on postnatal day (PND) 21. Among the genes identified, 170 were selected for further validation analysis of gene expression on PND 21 and PND 77 by real‐time reverse transcription‐PCR. PTU and GLY downregulated many genes on PND 21, reflecting diverse effects on neurogenesis. Furthermore, genes showing sustained downregulation were found after PTU or VPA exposure, reflecting a sustained or late effect on neurogenesis by these compounds. In contrast, such genes were not observed with GLY, probably because of the reversible nature of the effects. Among the genes showing sustained downregulation, Creb, Arc, and Hes5 were concurrently downregulated by PTU, suggesting an association with neuronal mismigration, suppressed synaptic plasticity, and reduction in neural stem and progenitor cells. Epha7 and Pvalb were also concurrently downregulated by PTU, suggesting an association with the reduction in late‐stage progenitor cells. VPA induced sustained downregulation of Vgf and Dpysl4, which may be related to the aberrations in synaptic plasticity. The genes showing sustained downregulation may be irreversible markers of developmental neurotoxicity.

Hippocampal dentate gyrus of rat offspring exposed maternally to PTU, VPA, or GLY was subjected to global methylation analysis on PND 21. Genes downregulated on PND 77 were examined. PTU concurrently downregulated Creb, Arc, and Hes5, suggesting an association with the diverse effects on neurogenesis. PTU also concurrently downregulated Epha7 and Pvalb, suggesting an association with progenitor cell reduction. VPA downregulated Vgf and Dpysl4, suggesting an association with the aberrant synaptic plasticity. In contrast, GLY did not induce sustained downregulation.

Thyroid active agents T3 and PTU differentially affect immune gene transcripts in the head kidney of rainbow trout (Oncorynchus mykiss)

In mammals, numerous reports describe an immunomodulating effect of thyroid-active compounds. In contrast, only few reports have been published on this subject in fish. We previously demonstrated that immune cells of rainbow trout (Oncorhynchus mykiss) possess thyroid hormone receptors (THRs) and that exposure of trout to the thyroid hormone 3,3',5-triiodo-l-thyronine (T3) or the antithyroid drug propylthiouracil (PTU) alters immune cell transcript levels of THR and several immune genes. The present study aims to further characterize the immunomodulating action of thyroid-active compounds in trout immune cells. We report here the use of a custom-designed 60-mer oligo immune-targeted microarray for rainbow trout to analyze the gene expression profiles induced in the head kidney by T3 and PTU. Morphometric analyses of the thyroid showed that PTU exposure increased the size of the epithelial cells, whereas T3 induced no significant effects. Both T3 and PTU had diverse and partly contrasting effects on immune transcript profiles. The strongest differential effects of T3 and PTU on gene expressions were those targeting the Mitogen Associated Protein Kinase (MAPK), NFkB, Natural Killer (NK) and Toll-Like Receptor (TLR) pathways, a number of multipath genes (MPG) such as those encoding pleiotropic transcription factors (atf1, junb, myc), as well as important pro-inflammatory genes (tnfa, tnf6, il1b) and interferon-related genes (ifng, irf10). With these results we show for the first time in a fish species that the in vivo thyroidal status modulates a diversity of immune genes and pathways. This knowledge provides the basis to investigate both mechanisms and consequences of thyroid hormone- and thyroid disruptor-mediated immunomodulation for the immunocompetence of fish.

The Impact of Carrot Enriched in Iodine through Soil Fertilization on Iodine Concentration and Selected Biochemical Parameters in Wistar Rats

Iodine is one of the trace elements which are essential for mammalian life. The major objective of iodine biofortification of plants is to obtain food rich in this trace element, which may increase its consumption by various populations. Additionally, it may reduce the risk of iodine deficiency diseases. In this research for the first time we have assessed the bioavailability of iodine from raw or cooked carrot biofortified with this trace element on iodine concentration in selected tissues and various biochemical parameters as well as mRNA expression of some genes involved in iodine metabolism in Wistar rats. Statistically, a significantly higher iodine level was determined in urine, faeces and selected tissues of rats fed a diet containing biofortified raw carrot as compared to a diet without iodine and a diet containing control cooked carrot. Biofortified raw carrot significantly increased triiodothyronine concentration as compared to animals from other experimental groups. The highest thyroid stimulating hormone level was determined in rats fed control cooked carrots. mRNA expression of selected genes was affected by different dietary treatment in rats’ hearts. Biofortified raw and cooked carrot could be taken into account as a potential source of iodine in daily diets to prevent iodine deficiency in various populations.

Different Degrees of Iodine Deficiency Inhibit Differentiation of Cerebellar Granular Cells in Rat Offspring, via BMP-Smad1/5/8 Signaling

Iodine deficiency (ID) during development results in dysfunction of the central nervous system (CNS) and affects psychomotor and motor function. It is worth noting that maternal mild and marginal ID tends to be the most common reason of preventable neurodevelopmental impairment, via a mechanism that has not been elucidated. Therefore, our aim was to study the effects of developmental mild and marginal ID on the differentiation of cerebellar granule cells (GCs) and investigate the activation of BMP-Smad1/5/8 signaling, which is crucial for the development and differentiation of cerebellum. Three developmental rat models were created by feeding dam rats with a diet deficient in iodine and deionized water supplemented with potassium iodide. Our results showed that different degrees of ID inhibited and delayed the differentiation of cerebellar GCs on postnatal day (PN) 7, PN14, and PN21. Moreover, mild and severe ID reduced the expression of BMP2 and p-Smad1/5/8, and increased the levels of Id2 on PN7, PN14, and PN21. However, marginal ID rarely altered expression of these proteins in the offspring. Our study supports the hypothesis that mild and severe ID during development inhibits the differentiation of cerebellar GCs, which may be ascribed to the down-regulation of BMP-Smad1/5/8 signaling and the overexpression of Id2. Furthermore, it was speculated that maternal marginal ID rarely affected the differentiation of cerebellar GCs in the offspring.

Maternal Subclinical Hypothyroidism Impairs Neurodevelopment in Rat Offspring by Inhibiting the CREB Signaling Pathway

Thyroid hormone is indispensable for fetal brain development, and maternal thyroid hormone deficiency is thought to result in severe and irreversible brain impairments in learning and memory. Epidemiological and animal studies by our group had shown that maternal subclinical hypothyroidism had significant negative impact on neurodevelopment. But, the underlying mechanisms responsible for these neurological alterations remain unclear. In the present study, we performed thyroidectomy and injected L-T4 daily in Wistar rats to induce maternal subclinical hypothyroidism. Our data indicated that the pups from subclinical group showed prolonged latencies during the learning process in the Morris water maze as compared to the control group. Transcription factor cAMP response element-binding protein (CREB) signaling pathway is closely associated with synaptic plasticity, learning, and memory. Consistent with behavioral results, Western blotting also showed decreased activation of three important upstream modulators of CREB signaling pathway: phospho-mitogen-activated protein kinases (P-ERK1/2), phospho-calcium-dependent-calmodulin kinase IV (P-CaMKIV), phospho-serine/threonine protein kinase AKT(P-AKT), as well as total CREB and phospho-CREB as compared to the control at postnatal day 7 (PND 7) in hippocampus. Our findings suggested that decreased activation of the CREB signaling pathway in pups was related to impairments of cognitive function caused by maternal subclinical hypothyroidism.

The sonic hedgehog-induced type 3 deiodinase facilitates tumorigenesis of basal cell carcinoma by reducing Gli2 inactivation

Thyroid hormone (TH) is an important regulator of growth, development, and metabolism. Most of the active TH T3 is generated by peripheral TH metabolism mediated by the iodothyronine deiodinases. Type 3 deiodinase (D3) inactivates T3 via specific deiodination reactions. It is an oncofetal protein frequently expressed in neoplastic tissues and is a direct target of the sonic hedgehog (Shh) pathway in basal cell carcinomas (BCCs). However, the molecular mechanisms triggered by T3 in BCC are still mostly unrevealed. Here, we demonstrate that D3 action is critical in the proliferation and survival of BCC cells. D3 depletion or T3 treatment induce apoptosis of BCC cells and attenuate Shh signaling. This is achieved through a direct impairment of Gli2 protein stability by T3. T3 induces protein kinase A, which in turn destabilizes Gli2 protein via its C-terminal degron. Finally, in a mouse model of BCC, T3-topical treatment significantly reduces tumor growth. These results demonstrate the existence of a previously unrecognized cross talk between TH and Gli2 oncogene, providing functional and mechanistic evidence of the involvement of TH metabolism in Shh-induced cancer. TH-mediated Gli2 inactivation would be beneficial for therapeutically purposes, because the inhibition of Shh-Gli2 signaling is an attractive target for several anticancer drugs, currently in clinical trials.

Effect of maternal excessive iodine intake on neurodevelopment and cognitive function in rat offspring

Background

Iodine deficiency and iodine excess are both associated with adverse health consequences. Iodine deficiency during pregnancy leads to insufficient maternal thyroid hormone, subsequently causing irreversible adverse effects on the neurological and cognitive functions of the offspring. The results of our previous epidemiological study suggested that mild iodine excess might increase the prevalence of subclinical hypothyroidism. In the present study, female Wistar rats maintained on low-iodine grain were randomly assigned to three groups based on iodated water concentration: low iodine (LI, 1.2 μg/d), normal iodine (NI, 5–6 μg/d), and 3-fold high iodine (3HI, 15–16 μg/d). The present study investigated whether higher-than-normal iodine intake (3HI) by rats from before pregnancy until breastfeeding affects the postnatal (PN) neurodevelopment (PN7 and PN45) of their offspring during particularly sensitive periods in brain development.

Results

After 12 weeks of treatment (before pregnancy), iodine concentrations in urine and thyroid tissue and circulating thyroxine of adult females correlated with iodine intake. Brain-derived neurotrophic factor (BDNF) expression in the hippocampi of pups on PN7 and PN45 was decreased in 3HI group compared to the NI controls (P < 0.05, all) On PN7 and PN45, the BDNF levels of the 3HI pups were 83.5% and 88.8%, respectively, that of the NI pups. In addition, the 3HI group had a higher neuroendocrine-specific protein A (NSP-A) level than the NI controls on PN7 (P < 0.05). NSP-A levels of the 3HI pups were 117.0% that of the NI pups. No significant difference was observed in the expressions of c-Fos or c-Jun in the hippocampal CA1 region of the 3HI group compared to the controls (P > 0.05). Results from the Morris water maze test revealed that pups of the 3HI group had mild learning and spatial memory deficits.

Conclusions

The neurodevelopmental and cognitive deficits of the 3HI pups were mild and temporary, likely related to the changes in hippocampal protein expressions of BDNF and NSP-A.

The effect of folic acid as an antioxidant on the hypothalamic monoamines in experimentally induced hypothyroid rat

Thyroid hormones are recognized as key metabolic hormones that play a critical role in the central nervous system development throughout life. In the present study, we studied the biochemical changes of hypothalamus of hypothyroid rats at post-pubertal stage, and the possible ameliorating effect of folic acid. A total of 50 male albino rats were equally divided into five groups; the first and second groups were the control and folic acid groups, respectively, while the third group was the hypothyroid group in which rats received daily 6-n-propyl-2-thiouracil (PTU) in drinking water for 6 weeks to induce hypothyroidism. The fourth and fifth groups were hypothyroid rats treated with folic acid for 4 weeks during and after receiving PTU, respectively, and were dissected after 6 and 10 weeks, respectively. There was a significant increase in plasma total homocysteine, malondialdehyde (MDA), oxidized glutathione\reduced glutathione and total nitric oxide and hypothalamic MDA, serotonin and norepinephrine in the hypothyroid rats group as compared to the control group. This reflects hyperhomocysteinaemia and oxidative stress associated with hypothyroid state. On the other hand, hypothalamic total nitric oxide and dopamine in the hypothyroid rats group were significantly decreased when compared to the control group. Treatment of hypothyroid rats with folic acid improves the oxidative stress and hypothalamic monoamines. Our results revealed that, folic acid treatment was better if it is administered as an adjuvant after returning to the euthyroid state.

Developmental iodine deficiency and hypothyroidism impair neural development in rat hippocampus: involvement of doublecortin and NCAM-180

BACKGROUND

Developmental iodine deficiency results in inadequate thyroid hormone (TH), which damages the hippocampus. Here, we explored the roles of hippocampal doublecortin and neural cell adhesion molecule (NCAM)-180 in developmental iodine deficiency and hypothyroidism.

METHODS

Two developmental rat models were established with either an iodine-deficient diet, or propylthiouracil (PTU)-adulterated water (5 ppm or 15 ppm) to impair thyroid function, in pregnant rats from gestational day 6 until postnatal day (PN) 28. Silver-stained neurons and protein levels of doublecortin and NCAM-180 in several hippocampal subregions were assessed on PN14, PN21, PN28, and PN42.

RESULTS

The results show that nerve fibers in iodine-deficient and 15 ppm PTU-treated rats were injured on PN28 and PN42. Downregulation of doublecortin and upregulation of NCAM-180 were observed in iodine-deficient and 15 ppm PTU-treated rats from PN14 on. These alterations were irreversible by the restoration of serum TH concentrations on PN42.

CONCLUSION

Developmental iodine deficiency and hypothyroidism impair the expression of doublecortin and NCAM-180, leading to nerve fiber malfunction and thus impairments in hippocampal development.