Pituitary-thyroid function of fetuses of hypothyroid and growth hormone treated hypothyroid rats.

Maternal hypothyroidism induced by surgical thyroidectomy (Tx) of the rat resulted in significantly higher fetal serum levels of thyroid stimulating hormone (TSH) and thyroxine (T4) on day 22 of gestation. Surprisingly, administration of growth hormone (GH) to hypothyroid mothers increased further the fetal serum T4 and TSH. The in vitro uptake of 131I-T4 by erythrocytes was elevated significantly when incubated with serum from fetuses of both hypothyroid and hypothyroid GH-treated mothers. Although the plasma protein levels of hypothyroid mothers and their fetuses are decreased significantly as compared to controls this is not true of hypothyroid GH-treated mothers and their fetuses. The T4 levels of both groups of Tx mothers were significantly below that of controls. However, as in the case of their fetuses, the serum T4 of GH-treated hypothyroid mothers was elevated from that of Tx only animals. It is concluded that the pituitary-thyroid system of fetuses of hypothyroid mothers is activated excessively during late gestation, that considerable T4 can be transported from the fetus to the mother during this period and that these fetuses are in fact born in a hyperthyroid state which is aggravated by maternal treatment with GH.     

Requirement of thyrotropin-releasing hormone for the postnatal functions of pituitary thyrotrophs: ontogeny study of congenital tertiary hypothyroidism in mice

We recently reported that TRH-deficient mice showed characteristic tertiary hypothyroidism. In the present study, we investigated how this tertiary hypothyroidism occurred particularly in pre- and postnatal stages. Immunohistochemical analysis revealed a number of TSH-immunopositive cells in the TRH-/- pituitary on embryonic day 17.5 and at birth. The mutant pituitary at birth in pups born from TRH-deficient dams also showed no apparent morphological changes, indicating no requirement of either maternal or embryonic TRH for the development of pituitary thyrotrophs. In contrast, apparent decreases in number and level of staining of TSH-immunopositive cells were observed after postnatal day 10 in mutant pituitary. Similar decreases were observed in the 8-week-old mutant pituitary, while no apparent changes were observed in other pituitary hormone-producing cells, and prolonged TRH administration completely reversed this effect. Consistent with these morphological results, TRH-/- mice showed normal thyroid hormone levels at birth, but the subsequent postnatal increase was depressed, resulting in hypothyroidism. As expected, TSH content in the TRH-/- pituitary showed a marked reduction to only 40% of that in the wild type. Despite hypothyroidism in the mutant mice, both the pituitary TSHbeta and alpha mRNA levels were lower than those of the wild-type pituitary. These phenotypic changes were specific to the pituitary thyrotrophs. These findings indicated that 1) TRH is essential only for the postnatal maintenance of the normal function of pituitary thyrotrophs, including the normal feedback regulation of the TSH gene by thyroid hormone; 2) neither maternal nor embryonic TRH is required for normal development of the fetal pituitary thyrotroph; and 3) TRH-deficient mice do not exhibit hypothyroidism at birth. Moreover, reflecting its name, TRH has more critical effects on the pituitary thyrotrophs than on other pituitary hormone-producing cells.     

Hypothalamic CRF immunoreactivity in genetically hypothyroid (hyt/hyt) mice

The induction of hypothyroidism in young rats by feeding thiouracil to their mothers during pregnancy has been shown to depress hypothalamic content of bioactive and immunoactive corticotropin-releasing factor (CRF). The present study was done to determine whether genetically hypothyroid young mice (hyt/hyt) born to euthyroid mothers (+/hyt) exhibited a similar depression in hypothalamic CRF immunoreactivity. Young euthyroid and hypothyroid littermate mice were examined by radioimmunoassay for hypothalamic CRF content at 15, 20, 25, or 30 days of age. Mean CRF content was depressed insignificantly (to about 80% of normal) by hypothyroidism, at 15-25 days of age. However, after weaning by the mother, 30-day-old hypothyroid pups demonstrated significantly depressed hypothalamic CRF levels (71%). It is suggested that maternal factors may be assisting in the maintenance of hypothalamic CRF until after weaning. Furthermore, genetic hypothyroidism does not appear to have nearly as marked an influence as thiouracil feeding on hypothalamic CRF levels.     

Early postnatal cardiac changes and premature death in transgenic mice overexpressing a mutant form of serum response factor

Serum response factor (SRF) is a key regulator of a number of extracellular signal-regulated genes important for cell growth and differentiation. A form of the SRF gene with a double mutation (dmSRF) was generated. This mutation reduced the binding activity of SRF protein to the serum response element and reduced the capability of SRF to activate the atrial natriuretic factor promoter that contains the serum response element. Cardiac-specific overexpression of dmSRF attenuated the total SRF binding activity and resulted in remarkable morphologic changes in the heart of the transgenic mice. These mice had dilated atrial and ventricular chambers, and their ventricular wall thicknesses were only 1/2 to 1/3 the thickness of that of nontransgenic mice. Also these mice had smaller cardiac myocytes and had less myofibrils in their myocytes relative to nontransgenic mice. Altered gene expression and slight interstitial fibrosis were observed in the myocardium of the transgenic mice. All the transgenic mice died within the first 12 days after birth, because of the early onset of severe, dilated cardiomyopathy. These results indicate that dmSRF overexpression in the heart apparently alters cardiac gene expression and blocks normal postnatal cardiac growth and development.     

Transient hypothyroidism in infants born to mothers with chronic thyroiditis--a nationwide study of twenty-three cases. The Transient Hypothyroidism Study Group

To define the difference in prognosis and the clinical features of transient neonatal hypothyroidism in infants born to mothers with chronic thyroiditis, we conducted a nationwide study of this condition. Sixteen mothers with chronic thyroiditis and twenty-three of their offspring with transient hypothyroidism were registered and reported in this paper. Five (group A) of twenty-two live infants showed physical, mental and/or psychomotor developmental delay (IQ below 80). No significant difference between TSH-binding inhibitor immunoglobulin (TBII) or thyroid-stimulation blocking antibody (TSBAb) activities in groups A and B (normal development) were noted. Moreover, there was no significant difference in thyroid function in the newborn period, ages at the start of thyroid medication or the dose and duration of treatment in the two groups. A striking difference observed between the two groups was the thyroid function of their mothers during pregnancy. In group A, four mothers were hypothyroid during pregnancy, and another mother discontinued thyroid medication in the last trimester and her baby was most delayed at the start thyroid medication. On the other hand, the mothers of only two of seventeen live cases in group B had mild hypothyroidism during pregnancy. There were two sets of siblings whose mother received inadequate treatment during the first pregnancy and adequate treatment during the second pregnancy. The psychomotor, physical and mental developmental delay were observed in their first babies. These findings suggested that maternal thyroid function during pregnancy might be an important factor in the prognosis of infants born to mothers with chronic thyroiditis.     

Cystic fibrosis as a bowel cancer syndrome and the potential role of CK2

Thyroid hormones are major regulators of postnatal brain development. Thyroid hormones act through nuclear receptors to modulate the expression of specific genes in the brain. We have used microarray analysis to identify novel responsive genes in 14-day-old hypothyroid rat brains, and discovered that synaptosomal-associated protein of 25 kDa (SNAP-25) was one of the thyroid hormone-responsive genes. SNAP-25 is a presynaptic plasma membrane protein and an integral component of the vesicle docking and fusion machinery mediating secretion of neurotransmitters and is required for neuritic outgrowth and synaptogenesis. Using microarray analysis we have shown that SNAP-25 was down-regulated in the hypothyroid rat brain compared with the age-matched controls. Real-time RT-PCR and western blotting analysis confirmed that SNAP-25 mRNA and protein levels decreased significantly in the developing hypothyroid rat brain. Our data suggest that in the developing rat brain, SNAP-25 expression is regulated by thyroid hormone, and thyroid hormone deficiency can cause decreased expression of SNAP-25 and this may on some level account for the impaired brain development seen in hypothyroidism.     

Cross-talk between thyroid hormone receptor and liver X receptor regulatory pathways is revealed in a thyroid hormone resistance mouse model

Hypercholesterolemia is found in patients with hypothyroidism and resistance to thyroid hormone. In this study, we examined cholesterol metabolism in a thyroid hormone receptor beta (TR-beta) mutant mouse model of resistance to thyroid hormone. Whereas studies of cholesterol metabolism have been reported in TR-beta knock-out mice, generalized expression of a non-ligand binding TR-beta protein in this knock-in model more fully recapitulates the hypothyroid state, because the hypothyroid effect of TRs is mediated by the unliganded receptor. In the hypothyroid state, a high cholesterol diet increased serum cholesterol levels in wild-type animals (WT) but either did not change or reduced levels in mutant (MUT) mice relative to hypothyroidism alone. 7alpha-Hydroxylase (CYP7A1) is the rate-limiting enzyme in cholesterol metabolism and mRNA levels were undetectable in the hypothyroid state in all animals. triiodothyronine replacement restored CYP7A1 mRNA levels in WT mice but had minimal effect in MUT mice. In contrast, a high cholesterol diet markedly induced CYP7A1 levels in MUT but not WT mice in the hypothyroid state. Elevation of CYP7A1 mRNA levels and reduced hepatic cholesterol content in MUT animals are likely because of cross-talk between TR-beta and liver X receptor alpha (LXR-alpha), which both bind to a direct repeat + 4 (DR+4) element in the CYP7A1 promoter. In transfection studies, WT but not MUT TR-beta antagonized induction of this promoter by LXR-alpha. Electromobility shift analysis revealed that LXR/RXR heterodimers bound to the DR+4 element in the presence of MUT but not WT TR-beta. A mechanism for cross-talk, and potential antagonism, between TR-beta and LXR-alpha is proposed.     

Fetal and maternal thyroid hormones

It is well known that insufficient production of thyroid hormones during the fetal and neonatal period of development may result in permanent brain damage unless treatment with thyroid hormone is instituted very soon after birth. But congenital hypothyroidism is not the only situation in which brain damage may be related to insufficient thyroid function. Cretinism is the most severe manifestation of iodine deficiency disorders found in areas where iodine intake is greatly reduced. Some of the manifestations of cretinism suggest that the insult to the developing brain starts earlier than in the case of congenital hypothyroidism. Hypothyroxinemia of mothers with adequate iodine intake may also leave permanent, though less severe, mental retardation. For these reasons the possible role of maternal transfer of thyroid hormones during early fetal development have been reinvestigated, using the rat to obtain various experimental models. It has been shown that thyroid hormones are found in embryonic tissues before onset of fetal thyroid function and that thyroidectomy of the mother results in delayed development of the concepta. The concentrations of T4 and T3 in embryonic tissues from thyroidectomized dams were undetectable before the onset of fetal thyroid function, and still reduced in some tissues near term, despite the onset of fetal thyroid function. Treatment of control and thyroidectomized dams with methyl-mercaptoimidazole to block fetal thyroid function reduced thyroid hormone concentrations in fetal tissues near term, but this decrease could be partially avoided by infusion of physiological doses of thyroxine to the mothers. Iodine deficiency of the mothers resulted in thyroid hormone deficiency of the developing embryo, which was very marked until term in all tissues including the brain. The results strongly support a role of maternal thyroid hormones in fetal thyroid hormone economy both before and after the onset of the fetal thyroid function, at least in the rat. They also support a role of the hypothyroxinemia of iodine-deficient mothers in initiating the brain damage of the endemic cretin, a damage which would not be corrected once the fetal thyroid becomes active, as iodine-deficiency of the fetus would impair adequate production of hormones by its own thyroid, and maternal transfer would continue to be low.     

Iodothyronine-5'-deiodinase activity in progenies of hypothyroid rats

Iodothyronine-5'-deiodinase activity (I-5'DA) was measured in the progenies of control rats, hypothyroid (Tx) rats, and hypothyroid treated with ovine GH (Tx + GH) during gestation. The enzyme was measured in cerebral cortex and cerebellum at 22 days gestation and at 5, 10, 30 and 60 days postpartum. In addition, the pituitary I-5'-DA was assessed in the postnatal animals. The experiments were undertaken because the tissues of the progenies of rats that were hypothyroid during pregnancy appeared in many ways to resemble those of hypothyroid animals, even at ages when serum thyroxine (T4) and triiodothyronine (T3) levels were normal. It was found that the progenies of Tx mothers had low liver 5'-deiodinase activities. This is a likely cause of the low serum T3 levels with normal T4 levels seen in these progenies in the neonatal period. Cerebral and cerebellar 5'-deiodinase activities were low in these progenies during the thyroid hormone-dependent perinated period of brain development. The progenies of GH-treated Tx dams had higher enzyme activities than the progenies of untreated Tx dams. These pups from GH-treated Tx mothers have been shown previously to have significantly less neurological impairment than the progenies of untreated Tx mothers. As most of the brain intracellular T3 is produced in situ, a functional thyroid deficiency could result from such a 5'-deiodinase deficiency. As the deiodinase deficiency was still seen in the progenies of Tx mothers at 60 days of age, such a deficiency could explain why, even though serum T4 and T3 levels were normal, brain metabolism was in many ways characteristic of hypothyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Airway injury in lung disease pathophysiology: selective depletion of airway stem and progenitor cell pools potentiates lung inflammation and alveolar dysfunction

Identification of early events that contribute to the establishment of chronic lung disease has been complicated by the variable involvement of the airway and alveolar compartments in the complex physiology of end-stage disease. In particular, the impact of airway injury on alveolar integrity and function has not been addressed and would be facilitated by development of animal models of lung disease that specifically target a single cell type within the airway epithelium. We have previously demonstrated that ganciclovir treatment of CCtk transgenic mice, which express the herpes simplex thymidine kinase gene under regulation of the mouse Clara cell secretory protein (CCSP) promoter, results in elimination of the airway progenitor and stem cell pools and a consequent failure of airway regeneration that is associated with rapid morbidity and mortality. In this study, we used the CCtk model to test the hypothesis that selective airway injury initiates profound lung dysfunction through mechanisms that compromise alveolar integrity. Results demonstrate that elimination of the CCSP-expressing cell population results in secondary alveolar inflammation, edema, and depletion of the alveolar type II cell population. On the basis of these data we conclude that selective airway injury can serve as the inciting injury in diseases characterized by severely compromised alveolar function.     

Identification of genes mediating thyroid hormone action in the developing mouse cerebellum

Despite the indispensable role thyroid hormone (TH) plays in brain development, only a small number of genes have been identified to be directly regulated by TH and its precise mechanism of action remains largely unknown, partly because most of the previous studies have been carried out at postnatal day 15 or later. In the present study, we screened for TH-responsive genes in the developing mouse cerebellum at postnatal day 4 when morphological alterations because of TH status are not apparent. Among the new candidate genes selected by comparing gene expression profiles of experimentally hypothyroid, hypothyroid with postnatal thyroxine replacement, and control animals using oligoDNA microarrays, six genes were confirmed by real-time PCR to be positively ( orc1l, galr3, sort1, nlgn3, cdk5r2 , and zfp367 ) regulated by TH. Among these, sort1 , cdk5r2, and zfp367 were up-regulated already at 1 h after a single injection of thyroxine to the hypothyroid or control animal, suggesting them to be possible primary targets of the hormone. Cell proliferation and apoptosis examined by BrdU incorporation and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling assay revealed that hypothyroidism by itself did not enhance apoptosis at this stage, but rather increased cell survival, possibly through regulation of these newly identified genes.     

Human insulin-like growth factor-IA expression in transgenic mice promotes adenomatous hyperplasia but not pulmonary fibrosis

Insulin-like growth factor-I (IGF-I) has been implicated in postnatal alveolar development, pulmonary fibrosis, and non-small cell lung cancer. To further investigate the role of IGF-I, we created a line of transgenic mice in which alveolar type II epithelial cells express human IGF-IA under the control of the surfactant protein C promoter. We determined the effect of pulmonary overexpression of human IGF-IA on 1) pulmonary inflammation and fibrosis in response to intratracheal instillation of bleomycin, 2) premalignant pulmonary adenomatous hyperplasia, and 3) adenoma formation. Transgenic expression of human IGF-IA had no effect on baseline gross lung pathology, cellularity of bronchoalveolar lavage, or total lung collagen content. In addition, there were no significant differences between transgenic mice and nontransgenic littermate controls in the development of pulmonary inflammation or pulmonary fibrosis in response to intratracheal bleomycin instillation. However, pulmonary expression of human IGF-IA in older mice (>12 mo) significantly increased the incidence of premalignant adenomatous hyperplastic lesions compared with littermate controls without affecting adenoma formation. These findings suggest that increased expression of human IGF-IA in alveolar air spaces does not affect the development of pulmonary fibrosis but promotes premalignant changes in the alveolar epithelium.     

The c-myc proto-oncogene regulates cardiac development in transgenic mice.

During the maturation of the cardiac myocyte, a transition occurs from hyperplastic to hypertrophic growth. The factors that control this transition in the developing heart are unknown. Proto-oncogenes such as c-myc have been implicated in the regulation of cellular proliferation and differentiation, and in the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc can influence myocyte proliferation or differentiation, we examined the in vivo effect of increasing c-myc expression during embryogenesis and of preventing the decrease in c-myc mRNA expression that normally occurs during cardiac development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. In these transgenic mice, increased c-myc mRNA expression was found to be associated with both atrial and ventricular enlargement. This increase in cardiac mass was secondary to myocyte hyperplasia, with the transgenic hearts containing more than twice as many myocytes as did nontransgenic hearts. The results suggest that in the transgenic animals there is additional hyperplastic growth during fetal development. However, this additional proliferative growth is not reflected in abnormal myocyte maturation, as assessed by the expression of the cardiac and skeletal isoforms of alpha-actin. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth and suggest a regulatory role for this proto-oncogene in cardiac myogenesis.