Developmental thyroid hormone action on pro-opiomelanocortin-expressing cells programs hypothalamic BMPR1A depletion and brown fat activation

Serotonergic and Chemosensory Brain Areas and Sensory Ganglia Expressing Type 3 Deiodinase Mapped With Dio3Cre drivers

Thyroid hormone (triiodothyronine, T3) promotes neurodevelopment but under strict control because unconstrained exposure to T3 impairs brain and sensory functions. Thyroid hormone-inactivating type 3 deiodinase, encoded by Dio3, critically limits T3 signaling and controls diverse neural functions. Accordingly, understanding the cellular basis of T3 action requires identification of Dio3-expressing cell types but this is difficult because of low level, transient expression within the complexity of the nervous system. Here, we derived a knock-in Dio3Cre driver that sensitively labels Dio3-expressing cells in male and female mice. In this anatomical study, we identified Dio3 expression in the immature amygdala and other brain regions associated with emotion and motivation, and in serotonergic raphe nuclei, which influence many behavioral and physiological systems. Notably, expression in circumventricular organs, including the chemosensory subfornical organ and organum vasculosum laminae terminalis, suggested regulation of centers that lack a blood-brain barrier and directly sense signaling factors in the circulation. Expression in trigeminal, dorsal root, cochleovestibular, and other sensory ganglia highlighted contributions to sensory pathways. Although Dio3 expression declines during maturation, a conditional Dio3CreERt2 driver revealed neurons with T3-inducible expression in the adult brain, suggesting ongoing homeostatic functions. These Cre drivers indicate strategically located neuronal groups for control of T3 signaling in behavioral, chemosensory and sensory systems.

An Atlas of Thyroid Hormone Responsive Genes in Adult Mouse Hypothalamus

We present an atlas of genes that respond to thyroid hormone in the adult mouse mediobasal hypothalamus. Based on droplet-based single nuclei RNA-seq method and batch transcriptome analyses, the atlas lists putative target genes of the hormone nuclear receptors in 20 different types of neuronal and glial cells. The transcriptional regulation of these genes varies extensively across neuronal and glial cell types. However, while astrocytes appear to be highly sensitive to thyroid hormone stimulation, differentiated oligodendrocytes are relatively insensitive. This atlas is expected to promote future investigations into the molecular and cellular mechanisms that underlie the numerous functions of thyroid hormone in the hypothalamic circuits.

Thyroid Hormone Clearance in the Paraventricular Nucleus of Male Mice Regulates Lean Mass and Physical Activity

INTRODUCTION

The actions of thyroid hormones (THs) in the central nervous system are relevant to food intake and energy expenditure. TH receptors exhibit high expression in brain areas modulating energy balance, including the arcuate, paraventricular (PVN), supraoptic, and ventromedial (VMH) hypothalamic nuclei.

METHODS

To examine the role of THs in the regulation of energy balance via action in specific hypothalamic nuclei of the adult mouse, we performed experiments of conditional inactivation of DIO3, the enzyme responsible for the clearance of THs, in the lateral hypothalamus (LH), and VMH and PVN hypothalamic nuclei. We accomplished DIO3 genetic inactivation via stereotaxic injection of the AAV-cre vector into adult mice homozygous for a "floxed" Dio3 allele.

RESULTS

Dio3 inactivation in the LH and VMH of males or females did not result in significant changes in body weight 8 weeks after injection. However, inactivation of Dio3 in the PVN resulted in increased body weight (both fat mass and lean mass) and locomotor activity, and decreased hypothalamic Mc4r expression in male, but not female mice. However, PNV-specific Dio3 KO did not cause hyperphagia.

CONCLUSION

These results suggest local TH action influences MC4R signaling and possibly other PVN-associated circuitries, with consequences for body composition and energy balance endpoints, but not for orexigenic pathways. They also support a regulatory role for PVN Dio3 in the central regulation of energy homeostasis in adult life.

Thyroid Hormone Receptors Function in GABAergic Neurons During Development and in Adults

The nuclear receptors of thyroid hormone exert a broad influence on brain development and then on adult brain physiology. However, the cell-autonomous function of the receptors is combined with their indirect influence on cellular interactions. Mouse genetics allows one to distinguish between these 2 modes of action. It revealed that 1 of the main cell-autonomous functions of these receptors is to promote the maturation of GABAergic neurons. This review presents our current understanding of the action of thyroid hormone on this class of neurons, which are the main inhibitory neurons in most brain areas.

Mice lacking DIO3 exhibit sex-specific alterations in circadian patterns of corticosterone and gene expression in metabolic tissues

Disruption of circadian rhythms is associated with neurological, endocrine and metabolic pathologies. We have recently shown that mice lacking functional type 3 deiodinase (DIO3), the enzyme that clears thyroid hormones, exhibit a phase shift in locomotor activity, suggesting altered circadian rhythm. To better understand the physiological and molecular basis of this phenotype, we used Dio3+/+ and Dio3-/- mice of both sexes at different zeitgeber times (ZTs) and analyzed corticosterone and thyroxine (T4) levels, hypothalamic, hepatic, and adipose tissue expression of clock genes, as well as genes involved in the thyroid hormone action or physiology of liver and adipose tissues. Wild type mice exhibited sexually dimorphic circadian patterns of genes controlling thyroid hormone action, including Dio3. Dio3-/- mice exhibited altered hypothalamic expression of several clock genes at ZT12, but did not disrupt the overall circadian profile. Expression of clock genes in peripheral tissues was not disrupted by Dio3 deficiency. However, Dio3 loss in liver and adipose tissues disrupted circadian profiles of genes that determine tissue thyroid hormone action and physiology. We also observed circadian-specific changes in serum T4 and corticosterone as a result of DIO3 deficiency. The circadian alterations manifested sexual dimorphism. Most notable, the time curve of serum corticosterone was flattened in Dio3-/- females. We conclude that Dio3 exhibits circadian variations, influencing the circadian rhythmicity of thyroid hormone action and physiology in liver and adipose tissues in a sex-specific manner. Circadian disruptions in tissue physiology may then contribute to the metabolic phenotypes of DIO3-deficient mice.

Epigenetic developmental programming and intergenerational effects of thyroid hormones

Mounting evidence is showing that altered signaling through the nuclear hormone receptor superfamily can cause abnormal, long-term epigenetic changes which translate into pathological modifications and susceptibility to disease. These effects seem to be more prominent if the exposure occurs early in life, when transcriptomic profiles are rapidly changing. At this time, the coordination of the complex coordinated processes of cell proliferation and differentiation that characterize mammalian development. Such exposures may also alter the epigenetic information of the germ line, potentially leading to developmental changes and abnormal outcomes in subsequent generations. Thyroid hormone (TH) signaling is mediated by specific nuclear receptors, which have the ability to markedly change chromatin structure and gene transcription, and can also regulate other determinants of epigenetic marks. TH exhibits pleiotropic effects in mammals, and during development, its action is regulated in a highly dynamic manner to suit the rapidly evolving needs of multiple tissues. Their molecular mechanisms of action, timely developmental regulation and broad biological effects place THs in a central position to play a role in the developmental epigenetic programming of adult pathophysiology and, through effects on the germ line, in inter- and trans-generational epigenetic phenomena. These areas of epigenetic research are in their infancy, and studies regarding THs are limited. In the context of their characteristics as epigenetic modifiers and their finely tuned developmental action, here we review some of the observations underscoring the role that altered TH action may play in the developmental programming of adult traits and in the phenotypes of subsequent generations via germ line transmission of altered epigenetic information. Considering the relatively high prevalence of thyroid disease and the ability of some environmental chemicals to disrupt TH action, the epigenetic effects of abnormal levels of TH action may be important contributors to the non-genetic etiology of human disease.