Circadian rhythm of type II 5'deiodinase activity in the rat hypothalamic-pituitary-adrenal axis

The thyroxine inactivating gene, type III deiodinase, suppresses multiple signaling centers in Dictyostelium discoideum

Thyroxine deiodinases, the enzymes that regulate thyroxine metabolism, are essential for vertebrate growth and development. In the genome of Dictyostelium discoideum, a single intronless gene (dio3) encoding type III thyroxine 5' deiodinase is present. The amino acid sequence of D. discoideum Dio3 shares 37% identity with human T4 deiodinase and is a member of the thioredoxin reductase superfamily. dio3 is expressed throughout growth and development and by generating a knockout of dio3, we have examined the role of thyroxine 5' deiodinase in D. discoideum. dio3(-) had multiple defects that affected growth, timing of development, aggregate size, cell streaming, and cell-type differentiation. A prominent phenotype of dio3(-) was the breaking of late aggregates into small signaling centers, each forming a fruiting body of its own. cAMP levels, its relay, photo- and chemo-taxis were also defective in dio3(-). Quantitative RT-PCR analyses suggested that expression levels of genes encoding adenylyl cyclase A (acaA), cAMP-receptor A (carA) and cAMP-phosphodiesterases were reduced. There was a significant reduction in the expression of CadA and CsaA, which are involved in cell-cell adhesion. The dio3(-) slugs had prestalk identity, with pronounced prestalk marker ecmA expression. Thus, Dio3 seems to have roles in mediating cAMP synthesis/relay, cell-cell adhesion and slug patterning. The phenotype of dio3(-) suggests that Dio3 may prevent the formation of multiple signaling centers during D. discoideum development. This is the first report of a gene involved in thyroxine metabolism that is also involved in growth and development in a lower eukaryote.

Cyclic mRNA expression of thyrotropin subunits and deiodinases in red drum, Sciaenops ocellatus

The role of thyrotropin (thyroid-stimulating hormone, TSH) in driving peripheral thyroid function in non-mammalian species is still poorly understood. Thyroxine (T₄), the principal hormone released from the thyroid gland in response to TSH stimulation, circulates with a robust daily rhythm in the teleost fish the red drum. Previous research suggests that the red drum T₄ cycle is circadian in nature, driven by TSH secretion in the early photophase and inhibited by T₄ feedback in the early scotophase. To determine whether TSH is produced in a pattern consistent with feedback inhibition by this T₄ cycle, we used quantitative real time PCR (qPCR) to quantify the daily cycle of expression of the pituitary TSH subunits GSUα, and TSHβ. We found that TSH expression cycled inversely to, and 6-12 h out of phase with, the T₄ cycle, consistent with the hypothesis that TSH secretion drives the T₄ cycle. To examine the potential role of deiodinases in negative feedback regulation of this TSH cycle, we also utilized qPCR to assess the pituitary expression patterns of the TH activating enzyme outer-ring deiodinase (Dio2) and the TH deactivating enzyme inner-ring deiodinase (Dio3). Dio2 was not expressed with an obvious daily cycle, whereas Dio3 expression mirrored the expression of TSH. These results are consistent with circulating T₄ providing the negative feedback signal controlling both TSH production and Dio3 expression in the pituitary, and suggest that TH inactivation by inner ring deiodination is an important component of TSH negative feedback control.

Rhythmic expression of clock genes in the ependymal cell layer of the third ventricle of rodents is independent of melatonin signaling

Reproductive physiology is regulated by the photoperiod in many mammals. Decoding of the photoperiod involves circadian clock mechanisms, although the molecular basis remains unclear. Recent studies have shown that the ependymal cell layer lining the infundibular recess of the third ventricle (EC) is a key structure for the photoperiodic gonadal response. The EC exhibits daylength-dependent changes in the expression of photoperiodic output genes, including the type 2 deiodinase gene (Dio2 ). Here we investigated whether clock genes (Per1 and Bmal1) and the albumin D-binding protein gene (Dbp) are expressed in the EC of Syrian hamsters, and whether their expression differs under long-day and short-day conditions. Expression of all three genes followed a diurnal rhythm; expression of Per1 and Dbp in the EC peaked around lights-off, and expression of Bmal1 peaked in the early light phase. The amplitude of Per1 and Dbp expression was higher in hamsters kept under long-day conditions than in those kept under short-day conditions. Notably, the expression of these genes was not modified by exogenous melatonin within 25 h after injection, whereas Dio2 expression was inhibited 19 h after injection. Targeted melatonin receptor (MT1, MT2, and both MT1 and MT2) disruption in melatonin-proficient C3H mice did not affect the rhythmic expression of Per1 in the EC. These data show the existence of a molecular clock in the rodent EC. In the hamster, this clock responds to long-term changes in the photoperiod, but is independent of acute melatonin signals. In mice, the EC clock is not affected by deletion of melatonin receptors.

Temporal dynamics of type 2 deiodinase expression after melatonin injections in Syrian hamsters

In many species living in temperate zones, reproduction is controlled by the photoperiod. Recent findings have clarified that type 2 iodothyronine deiodinase (Dio2) plays a significant role in the photoperiodic response of gonads in the mediobasal hypothalamus, converting the prohormone T(4) into bioactive T(3). In mammals, Dio2 expression is suppressed by long-term melatonin injections, although the signal transduction pathways that link the melatonin signal to Dio2 expression are unknown. As a first step to approach the problem, we have here investigated the temporal dynamics of the melatonin effect on Dio2 expression using male Syrian hamsters. Dio2 mRNA levels were found to show diurnal rhythms under long-day conditions in an area adjacent to the tuberoinfundibular sulcus and in the ependymal cell layer lining the ventrobasal walls of the third ventricle. Daily sc melatonin injections given in the late afternoon under long-day condition suppressed the Dio2 mRNA levels already at the first day after the onset of the treatment in the ependymal cell layer lining the ventrobasal walls of the third ventricle, and 1 d later in an area adjacent to the tuberoinfundibular sulcus. These suppressive effects were sustained for at least 2 d after a single injection. Furthermore, we examined the temporal changes of the Dio2 expression after the onset of the treatment, showing that the suppression did not occur until midday of the next day. These data suggest that melatonin is involved in the signal transduction mechanisms controlling the photoperiodic response of gonads by acting on Dio2 expression rather rapidly through indirect pathways.

Daily variations in type II iodothyronine deiodinase activity in the rat brain as controlled by the biological clock

Type II deiodinase (D2) plays a key role in regulating thyroid hormone-dependent processes in, among others, the central nervous system (CNS) by accelerating the intracellular conversion of T4 into active T3. Just like the well-known daily rhythm of the hormones of the hypothalamo-pituitary-thyroid axis, D2 activity also appears to show daily variations. However, the mechanisms involved in generating these daily variations, especially in the CNS, are not known. Therefore, we decided to investigate the role the master biological clock, located in the hypothalamus, plays with respect to D2 activity in the rat CNS as well as the role of one of its main hormonal outputs, i.e. plasma corticosterone. D2 activity showed a significant daily rhythm in the pineal and pituitary gland as well as hypothalamic and cortical brain tissue, albeit with a different timing of its acrophase in the different tissues. Ablation of the biological clock abolished the daily variations of D2 activity in all four tissues studied. The main effect of the knockout of the suprachiasmatic nuclei (SCN) was a reduction of nocturnal peak levels in D2 activity. Moreover, contrary to previous observations in SCN-intact animals, in SCN-lesioned animals, the decreased levels of D2 activity are accompanied by decreased plasma levels of the thyroid hormones, suggesting that the SCN separately stimulates D2 activity as well as the hypothalamo-pituitary-thyroid axis.

Halometabolites and cellular dehalogenase systems: an evolutionary perspective

We review the role of iodothyronine deiodinases (IDs) in the evolution of vertebrate thyroidal systems within the larger context of biological metabolism of halogens. Since the beginning of life, the ubiquity of organohalogens in the biosphere has provided a major selective pressure for the evolution and conservation of cellular mechanisms specialized in halogen metabolism. Among naturally available halogens, iodine emerged as a critical component of unique developmental and metabolic messengers. Metabolism of iodinated compounds occurs in the three major domains of life, and invertebrate deuterostomes possess several biochemical traits and molecular homologs of vertebrate thyroidal systems, including ancestral homologs of IDs identified in urochordates. The finely tuned cellular regulation of iodometabolite uptake and disposal is a remarkable event in evolution and might have been decisive for the explosive diversification of ontogenetic strategies in vertebrates.

Possible role of brain thyroid hormones in the effects of bright light on mood and behavior

Considerable evidence suggests that light may significantly affect the metabolism of cerebral thyroid hormones. Changes in the brain thyroid economy, independent of peripheral changes in thyroid status, may produce considerable behavioral effects. Exposure to bright light affects the mood and behavior of healthy people and individuals with psychiatric disorders. The author suggests that the effects of bright light on mood and behavior may be partly mediated by light-induced changes in the metabolism of brain thyroid hormones.