Björn Vennström

Abnormal heart rate and body temperature in mice lacking thyroid hormone receptor alpha 1

Thyroid hormone, acting through several nuclear hormone receptors, plays important roles in thermogenesis, lipogenesis and maturation of the neonatal brain. The receptor specificity for mediating these effects is largely unknown, and to determine this we developed mice lacking the thyroid hormone receptor TRα1. The mice have an average heart rate 20% lower than that of control animals, both under normal conditions and after thyroid hormone stimulation. Electrocardiograms show that the mice also have prolonged QRS‐ and QTend‐durations. The mice have a body temperature 0.5°C lower than normal and exhibit a mild hypothyroidism, whereas their overall behavior and reproduction are normal. The results identify specific and important roles for TRα1 in regulation of tightly controlled physiological functions, such as cardiac pacemaking, ventricular repolarisation and control of body temperature.

Contrasting developmental and tissue-specific expression of alpha and beta thyroid hormone receptor genes.

Thyroid hormones and their receptors (TRs) have critical functions in development. Here we show that a chicken TR beta cDNA clone encodes a receptor with a novel, short N‐terminal domain. In vitro‐expressed TR beta protein bound thyroid hormone with similar affinity as the chicken TR alpha. Comparison of expression of TR alpha and TR beta mRNAs throughout chicken development until 3 weeks post‐hatching revealed ubiquitous expression of TR alpha mRNAs (in 14 different tissues) with some variations in levels, from early embryonic stages. In contast, expression of TR beta mRNA was restricted, occurring notably in brain, eye, lung, yolk sac and kidney, and was subject to striking developmental control, especially in brain where levels increased 30‐fold upon hatching. Levels also sharply increased in late embryonic lung, but were relatively high earlier in embryonic eye and yolk sac. RNase protection analyses detected no obvious mRNAs for alpha and beta TRs with variant C‐termini as demonstrated previously for the rat TR alpha gene. The data suggest a general role for TR alpha and specific developmental functions for TR beta, and that thyroid‐dependent development involves temporal and tissue‐specific expression of the TR beta gene.

Deletion of the thyroid hormone receptor α1 prevents the structural alterations of the cerebellum induced by hypothyroidism

Thyroid hormone (T3) controls critical aspects of cerebellar development, such as migration of postmitotic granule cells and terminal differentiation of Purkinje cells. T3 acts through nuclear receptors (TR) of two types, TRα1 and TRβ, that either repress or activate gene expression. We have analyzed the cerebellar structure of developing mice lacking the TRα1 isoform, which normally accounts for about 80% of T3 receptors in the cerebellum. Contrary to what was expected, granule cell migration and Purkinje cell differentiation were normal in the mutant mice. Even more striking was the fact that when neonatal hypothyroidism was induced, no alterations in cerebellar structure were observed in the mutant mice, whereas the wild-type mice showed delayed granule cell migration and arrested Purkinje cell growth. The results support the idea that repression by the TRα1 aporeceptor, and not the lack of thyroid hormone, is responsible for the hypothyroid phenotype. This conclusion was supported by experiments with the TRβ-selective compound GC-1. Treatment of hypothyroid animals with T3, which binds to TRα1 and TRβ, prevents any defect in cerebellar structure. In contrast, treatment with GC-1, which binds to TRβ but not TRα1, partially corrects Purkinje cell differentiation but has no effect on granule cell migration. Our data indicate that thyroid hormone has a permissive effect on cerebellar granule cell migration through derepression by the TRα1 isoform.

Transforming capacities of avian erythroblastosis virus mutants deleted in the erbA or erbB oncogenes

Mutants of avian erythroblastosis virus (AEV) were constructed by deleting large nucleotide segments in each of the viral oncogenes termed v-erbA and v-erbB. Mutants in erbA (erbA -B +) retained the ability to transform fibroblasts in vitro, and these cells exhibited most of the transformation characteristics that typify wild-type AEV-transformed fibroblasts. In addition, the mutants induced small erythroid colonies upon infection of bone marrow cells in culture. Chickens inoculated with erbA -B + virus or with erbA -B +-transformed cells developed sarcomas or atypical erythroid leukemias. The erythroid cells transformed in vivo or in vitro by the erbA -B + viruses appeared not to be as tightly blocked in differentiation as wild-type transformed cells. In contrast, fibroblasts infected with the erbA +B - mutant resembled normal cells in all transformation parameters tested, and no bone marrow cell transformation was observed with the mutant. The results indicate that the main transforming properties of AEV are encoded in erbB and that its effects are enhanced by erbA.

Selective thyroid hormone receptor-β activation: A strategy for reduction of weight, cholesterol, and lipoprotein (a) with reduced cardiovascular liability

Few treatments for obesity exist and, whereas efficacious therapeutics for hyperlipidemia are available, further improvements are desirable. Thyroid hormone receptors (TRs) regulate both body weight and cholesterol levels. However, thyroid hormones also have deleterious effects, particularly on the heart. The TRβ subtype is involved in cholesterol lowering and possibly elevating metabolic rate, whereas TRα appears to be more important for control of heart rate (HR). In the current studies, we examined the effect of TRβ activation on metabolic rate and HR with either TRα1–/– mice or the selective TRβ agonist KB-141 in mice, rats, and monkeys. 3,5,3′-triiodi-l-thyronine (T3) had a greater effect on increasing HR in WT than in TRα–/– mice (ED15 values of 34 and 469 nmol/kg/day, respectively). T3 increased metabolic rate [whole body oxygen consumption (MVO2)] in both WT and TRα–/– mice, but the effect in the TRα1–/– mice at the highest dose was half that of the WT mice. Thus, stimulation of MVO2 is likely due to both TRα and -β. T3 had equivalent potency for cholesterol reduction in WT and TRα–/– mice. KB-141 increased MVO2 with selectivities of 16.5- and 11.2-fold vs. HR in WT and TRα1–/– mice, respectively. KB-141 also increased MVO2 with a 10-fold selectivity and lowered cholesterol with a 27-fold selectivity vs. HR in rats. In primates, KB-141 caused significant cholesterol, lipoprotein (a), and body-weight reduction (up to 7% after 1 wk) with no effect on HR. TRβ-selective agonists may constitute a previously uncharacterized class of drugs to treat obesity, hypercholesterolemia, and elevated lipoprotein (a).

v-erbA oncogene activation entails the loss of hormone-dependent regulator activity of c-erbA

The v-erbA oncogene, one of the two oncogenes of the avian erythroblastosis virus, efficiently blocks erythroid differentiation and suppresses erythrocyte-specific gene transcription. Here we show that the overexpressed thyroid hormone receptor c-erbA effectively modulates erythroid differentiation and erythrocyte-specific gene expression in a T3-dependent fashion, when introduced into erythroid cells via a retrovirus. In contrast, the endogenous thyroid hormone receptor does not detectably affect erythroid differentiation. The analysis of a series of chimeric v-/c-erbA proteins suggests that the v-erbA oncoprotein has lost one type of thyroid hormone receptor function (regulating erythrocyte gene transcription in response to T3), but constitutively displays another function: it represses transcription in the absence of T3. The region responsible for the loss of hormone-dependent regulator activity of v-erbA has been mapped to the very C-terminus of c-erbA, encompassing a cluster of highly conserved amino acid residues with the potential to form an amphipathic alpha-helix.

Retardation of post-natal development caused by a negatively acting thyroid hormone receptor α1

Most patients with the syndrome resistance to thyroid hormone (RTH) express a mutant thyroid hormone receptor β (TRβ) with transdominant negative transcriptional effects. Since no patient with a mutant TRα has been identified, we introduced a point mutation into the mouse thyroid hormone receptor (TRα1) locus originally found in the TRβ gene, that reduces ligand binding 10‐fold. Heterozygous 2‐ to 3‐week‐ old mice exhibit a severe retardation of post‐natal development and growth, but only a minor reduction in serum thyroxine levels. Homozygous mice died before 3 weeks of age. Adult heterozygotes overcome most of these defects except for cardiac function abnormalities, suggesting that other factors compensate for the receptor defect. However, the additional deletion of the TRβ gene in this mouse strain caused a 10‐fold increase in serum thyroxine, restored hormonal regulation of target genes for TRs, and rescued the growth retardation. The data demonstrate a novel array of effects mediated by a dominant negative TRα1, and may provide important clues for identification of a potentially unrecognized human disorder and its treatment.

Distinct functions for thyroid hormone receptors alpha and beta in brain development indicated by differential expression of receptor genes.

Thyroid hormones are essential for correct brain development, and since vertebrates express two thyroid hormone receptor genes (TR alpha and beta), we investigated TR gene expression during chick brain ontogenesis. In situ hybridization analyses showed that TR alpha mRNA was widely expressed from early embryonic stages, whereas TR beta was sharply induced after embryonic day 19 (E19), coinciding with the known hormone‐sensitive period. Differential expression of TR mRNAs was striking in the cerebellum: TR beta mRNA was induced in white matter and granule cells after the migratory phase, suggesting a main TR beta function in late, hormone‐dependent glial and neuronal maturation. In contrast, TR alpha mRNA was expressed in the earlier proliferating and migrating granule cells, and in the more mature granular and Purkinje cell layers after hatching, indicating a role for TR alpha in both immature and mature neural cells. Surprisingly, both TR genes were expressed in early cerebellar outgrowth at E9, before known hormone requirements, with TR beta mRNA restricted to the ventricular epithelium of the metencephalon and TR alpha expressed in migrating cells and the early granular layer. The results implicate TRs with distinct functions in the early embryonic brain as well as in the late phase of hormone requirement.

A new species of virus-coded low molecular weight RNA from cells infected with adenovirus type 2

A virus-coded low molecular weight RNA (5.2S), which migrates slightly faster on polyacrylamide gels than the well characterized adenovirus-specific 5.5S RNA, has been isolated from cells infected with adenovirus type 2. Hybridization-competition experiments and RNA fingerprints indicate that the two virus-associated (VA) RNAs differ in their primary structures. The gene for 5.2S RNA is located to the right of the gene for 5.5S RNA, on the I strand of a DNA segment which extends between positions 30.3 and 32.2 on the map of adenovirus type 2 DNA. Both 5.5S and 5.2S RNA can be detected early after infection and also in the presence of cytosine-arabinoside or cycloheximide. After the onset of viral DNA replication, the synthesis of 5.2S RNA levels off, whereas 5.5S RNA is synthesized in increasing amounts. Both 5.2S and 5.5S RNAs are synthesized in isolated nuclei by an enzyme which resembles RNA polymerase III in its sensitivity to alpha-amanitin. In isolated nuclei, both RNA species are labeled with beta-32P-labeled GTP, which suggests that they are initiated at separate promotor sites.

International Union of Pharmacology. LIX. The Pharmacology and Classification of the Nuclear Receptor Superfamily: Thyroid Hormone Receptors

The initial identification of thyroid hormone receptors (TRs[1][1]) was based on binding studies ([Oppenheimer et al., 1972][2]). The TR main ligand is 3,5,3′-triiodo-l-thyronine (T3). T3 production primarily results from deiodination of thyroxine (T4), which is secreted by the thyroid gland. Most