Programming of cardiovascular and metabolic functions by thyroid hormone signaling

Abstract

Thyroid Hormones (TH) regulate myriad processes, such as development, metabolism and cardiovascular functions, by controlling gene expression through binding to nuclear thyroid hormone receptors, TRα and TRβ. While patients with mutations in the TRβ gene are well known, patients with TRα mutations have only recently been identified. To determine potential phenotypes of such patients, our lab previously created a mutant mouse line (TRα 1+/m), in which the receptor affinity to TH is reduced. TRα1+/m mice display a wide range of abnormalities including hypermetabolism and mild bradycardia, contrary to what was predicted from receptor-mediated hypothyroidism. The aim of this thesis is to analyze the unexpected metabolic and cardiac phenotype in greater detail. Furthermore, it aims to elucidate the physiological phenotype of offspring from TRα1 mutant mothers.

Hepatic glycogen content serves as a readout of the metabolic status of mammals. In paper I we show that hypermetabolism leads to completely depleted glycogen storage in adult TRα1 mutants. Furthermore we show that the livers of these mice compensate for this loss by upregulating glucose production over glucose degradation, in contrast to what is observed in T3-treated animals, thus raising the possibility that additional TR-mediated regulatory mechanisms are at play. Remarkably, exposure to high maternal thyroid hormone fully restored glycogen levels in the TRα1+/m adult mice, demonstrating that genetic and maternal factors orchestrate the glycogen set point of the embryo.

Studies from our lab showed that cardiomyocytes isolated from the TRα1+/m mice exhibit signs of severe hypothyroidism. Surprisingly, TRα1+/m animals are only slightly bradycardic despite a striking reduction in expression of the pacemaker gene, Hcn2, which in itself can induce a marked reduction of heart rate, paper II. We show that the autonomic regulation of heart rate in TRα1+/m mice fails to switch from sympathetic to parasympathetic tone in response to environmental stimuli such as temperature. In paper III, we find that impaired thyroid hormone signaling during development leads to an irreversible reduction in parvalbumin cell number in the hypothalamus, and that these cells regulate blood pressure and temperature-dependent changes in the heart rate. These data reveal a novel link between thyroid hormones and central regulation of the cardiovascular system.

Recent studies suggest that maternal factors during pregnancy, such as hormones and nutrients, epigenetically affect the development of the embryo, with long-lasting effects on offspring metabolism and behavior. In paper IV we show that TRα1+/m mutant dams produce male wild type offspring that have a metabolically favorable outcome characterized by their decreased body fat mass, increased lean mass and elevated glucose utilization. However, these mice display an increased voluntary wheel-running behavior, reminiscent of animal models of Attention Deficit Hyperactivity Disorder and addictive behavior. We provide evidence that a likely cause of this phenotype is decreased expression of genes involved in regulation of reward circuits and that at least one of these genes is epigenetically regulated. Moreover, we show that the phenotype is transmitted to the second generation, through the paternal line.

In conclusion, we show that proper thyroid hormone signaling is important during development to i) program the glycogen set point of the embryo; ii) regulate development of a novel hypothalamic cell population that controls cardiovascular functions; and iii) epigenetically regulate gene expression in the wild type embryo with long-lasting consequence for metabolism and behavior.