Juan Bernal - Academia.edu (original) (raw)

Papers by Juan Bernal

Thyroid : official journal of the American Thyroid Association, 2003

The purpose of this review is to provide an up-to-date report on the molecular and physiologic pr... more The purpose of this review is to provide an up-to-date report on the molecular and physiologic processes involved in the role of thyroid hormone as an epigenetic factor in brain maturation. We summarize the available data on the control of brain gene expression by thyroid hormone, the correlation between gene expression and physiologic effects, and the likely mechanisms of action of thyroid hormone on brain gene expression. In addition we propose a role for unliganded thyroid hormone receptors in the pathogenesis of hypothyroidism. Finally, we review recent data indicating that thyroid hormone receptors have an impact on behavior.

Journal of Clinical Endocrinology & Metabolism, 1988

The total number and saturation of nuclear receptors for T3 were measured in human fetal brain an... more The total number and saturation of nuclear receptors for T3 were measured in human fetal brain and lung from the 9th to the 13th week of fetal life. The concentrations of occupied and unoccupied receptor sites were determined by measuring total binding capacity at 0 and 22 C. At 0 C [125I]T3 was bound mainly to unoccupied sites, whereas at 22 C it was bound to unoccupied sites plus a fraction (70%) of endogenously occupied sites. Saturations of brain and lung receptors were similar (12-27%). From the fractional receptor occupancy and the receptor dissociation constants (34 pmol/L in brain and 56 pmol/L in lung) the concentration of intranuclear free T3 was calculated to be 9 pmol/L in brain and 11 pmol/L in lung. Total and free cytosolic T3 were measured by RIA and equilibrium dialysis. Total T3 was below the limit of detection in lung (90 pmol/L). The concentration of free T3 in brain cytosol was 0.95 pmol/L at 11 weeks and 2.96 pmol/L at 13 weeks, i.e. considerably lower than the nuclear free T3 concentration. These results suggest the presence of a small gradient (3-fold) between nuclear and cytosolic free T3 in both fetal tissues. The data strongly support the idea that thyroid hormones influence human brain development at least from the 9th to the 10th week of gestational age.

Nature Clinical Practice Endocrinology & Metabolism, 2007

Thyroid hormones are important during development of the mammalian brain, acting on migration and... more Thyroid hormones are important during development of the mammalian brain, acting on migration and differentiation of neural cells, synaptogenesis, and myelination. The actions of thyroid hormones are mediated through nuclear thyroid hormone receptors (TRs) and regulation of gene expression. The purpose of this article is to review the role of TRs in brain maturation. In developing humans maternal and fetal thyroid glands provide thyroid hormones to the fetal brain, but the timing of receptor ontogeny agrees with clinical data on the importance of the maternal thyroid gland before midgestation. Several TR isoforms, which are encoded by the THRA and THRB genes, are expressed in the brain, with the most common being TRalpha1. Deletion of TRalpha1 in rodents is not, however, equivalent to hormone deprivation and, paradoxically, even prevents the effects of hypothyroidism. Unliganded receptor activity is, therefore, probably an important factor in causing the harmful effects of hypothyroidism. Accordingly, expression of a mutant receptor with impaired triiodothyronine (T(3)) binding and dominant negative activity affected cerebellar development and motor performance. TRs are also involved in adult brain function. TRalpha1 deletion, or expression of a dominant negative mutant receptor, induces consistent behavioral changes in adult mice, leading to severe anxiety and morphological changes in the hippocampus.

Molecular Brain Research, 1994

Thyroid hormone deficiency has dramatic effects on rat brain maturation. The expression of genes ... more Thyroid hormone deficiency has dramatic effects on rat brain maturation. The expression of genes encoding neurotrophins and the trk family of neurotrophin receptors has been evaluated in several brain regions of normal and of neonatal or adult hypothyroid rats to analyze whether they are subject to thyroid hormone action. We found that hypothyroidism decreased trk mRNA levels in its major site of expression, the striatum, on postnatal days 5 (P5; 45%) and 15 (P15; 25%) and also in adults (35%). In contrast, no differences in trkB or trkC mRNAs levels were observed in any brain region at studied ages. According to previous reports, p75LNGFR mRNA was elevated in hypothyroid cerebellum as compared to age-matched controls on P5 and P15. We have also observed a distinct pattern for neurotrophin genes. The level of NGF mRNA was 20-50% lower in the cortex, hippocampus, and cerebellum of hypothyroid rats on neonatal hypothyroid rats on P15 and also after adult-onset hypothyroidism. Treatment of neonatally-induced hypothyroid rats with a single injection of triiodothyronine led to the recovery of hippocampal but not cortex NGF mRNA levels to that of control animals. On the contrary, no differences in the relatively high expression of the two mRNAs encoding BDNF were observed in any brain area. In contrast to a recent report, we did not find a reduction in brain NT-3 mRNA levels in hypothyroid animals. If any, the effect of thyroid deficiency in the hippocampus and cortex seems to be an early upregulation of NT-3 expression. In summary, our results support a region-specific developmental regulation by thyroid hormone of the expression of particular neurotrophic factors and their receptors in the rat brain. The observed alterations may contribute to the abnormal brain maturation caused by hypothyroidism.

European Journal of Endocrinology, 1997

Journal of Neuroscience Research, 1999

We have characterized an apparently full-length cDNA corresponding to a rat mRNA, SE6C, previousl... more We have characterized an apparently full-length cDNA corresponding to a rat mRNA, SE6C, previously identified by subtractive hybridization as being expressed predominantly in the striatal region of the brain. The SE6C mRNA encodes a 266 amino acid protein with significant similarity to members of the Ras-like GTP-binding protein family; thus, we have chosen the name Rhes, for Ras homolog enriched in striatum. The human homolog was found in a genomic sequence from human chromosome 22q13.1 and shares 95% identity with rat Rhes. Among the family of small G-proteins, Rhes shares 62% identity with Dexras1, a mouse dexamethasone-inducible Ras-like protein. Both Rhes and Dexras1 have substantially longer C-termini than other members of the Ras-like small G-protein family. Divergence between the C-terminal sequences of Rhes and Dexras1 suggests that, although their functions are probably similar, they have unique properties. Bacterially expressed Rhes binds GTP, suggesting that the protein indeed has GTPase functionality. Although Rhes was not induced by dexamethasone, its full expression is dependent upon thyroid hormone availability. Its accumulation is postnatal, consistent with the dependence upon thyroid hormone. It is noteworthy that most striatum-&amp;quot;specific&amp;quot; mRNAs characterized to date encode components of signal transduction cascades.

Journal of Cell Biology, 1993

The c-erbA proto-oncogenes encode nuclear receptors for thyroid hormone (T3), a hormone intimatel... more The c-erbA proto-oncogenes encode nuclear receptors for thyroid hormone (T3), a hormone intimately involved in mammalian brain maturation. To study thyroid hormone receptor (TR) action on neuronal cells in vitro, we expressed the chicken c-erbA/ TRot-1 as well as its oncogenic variant v-erbA in the adrenal medulla progenitor cell line PC12. In the absence of T3, exogenous TRc~-I inhibits NGF-induced neuronal differentiation and represses neuron-specific gene expression. In contrast, TRo~-I allows normal differentiation and neuronal gene expression to occur in the presence of T3. Finally, TRc~-l-expressing cells become NGF-responsive for proliferation when T3 is absent, but NGF-dependent for survival in presence of Cells were treated for 2 h with mitomycin-C (Sigma Chem. Co.) at a final concentration of 20/~g/ml. In pilot experiments, this concentration inhibited thymidine incorporation to <1% of respective controls without producing toxic effects. Monolayers were then washed three times with PBS, incubated for 30 rain with fresh medium, washed again, and then trypsinized and reseeded for differentiation induction. Except for different mitomycin-C concentrations, the same procedure was previously used to demonstrate that differentiation occurs in absence of cell division in myoblasts , macrophages (Beug et al.

Molecular Endocrinology, 1991

To define at the molecular biological level the effects of thyroid hormone on brain development w... more To define at the molecular biological level the effects of thyroid hormone on brain development we have examined cDNA clones of brain mRNAs and identified several whose expression is altered in hypothyroid animals during the neonatal period. Clones were identified with probes prepared by subtractive or differential hybridization, and those corresponding to mRNAs altered in hypothyroidism were further studied by Northern blot analysis. Using RNA prepared from whole brains, no effect of hypothyroidism was found on the expression of the astroglial gene coding for glial fibrillary acidic protein. Among genes of neuronal expression, no significant alterations were found in the steady state levels of mRNAs coding for neuron-specific enolase, microtubule-associated protein-2, Tau, or nerve growth factor. N-CAM mRNA increased slightly in hypothyroid brains. In contrast a 2- to 3-fold decrease was found in the mRNA coding for a novel neuronal gene, RC3. This is the first neuronal gene known to be significantly altered at the mRNA level by thyroid hormone deprivation. The abundance of the mRNAs for the major myelin proteins proteolipid protein, myelin basic protein, and myelin-associated glycoprotein, expressed by oligodendrocytes, were also decreased in hypothyroid brains. Developmental studies on RC3 and myelin-associated glycoprotein expression indicated that the corresponding mRNAs accumulate in the brain of normal rats during the first 15-20 days of neonatal life. A similar accumulation occurred in hypothyroid brains, but at much reduced levels. The results demonstrate that thyroid hormone controls the steady state levels of particular mRNAs during brain development.

Endocrinology, 1985

We have determined the concentration of thyroid hormone receptor binding sites in nuclear extract... more We have determined the concentration of thyroid hormone receptor binding sites in nuclear extracts derived from rat fetal organs throughout gestation and the postnatal period. Before day 14 of gestation nuclear extracts were obtained from whole fetuses. No receptor binding activity could be detected at day 12 of gestational age, and small amounts were detected at day 13 (maximum binding capacity less than 50 fmol/mg DNA). The receptor could be measured in pools of individual organs from day 14 (brain) or from day 16 (heart, liver, and lung) onwards. The order of analog binding affinity at 14 days was triiodothyroacetic acid = T3 greater than T4 greater than rT3, suggesting that at 14 days of fetal age the receptor has the same binding specificity as the receptor from mature tissues. In brain, the concentration of binding sites increased from 77 fmol/mg DNA at 14 days to 210 fmol/mg DNA at 17 days, remaining at this level until birth. Receptor concentration was identical whether the binding assays were performed on purified nuclei or nuclear extracts. There was no effect of maternofetal hypothyroidism on receptor concentration in the brain at 21 days of gestational age. Lung concentrations of receptor also remained constant during the fetal period. During the postnatal period, there was an increase in receptor concentration in brain and lung, with maximum levels at day 6. The pattern of receptor development in heart and liver was different, since its concentration increased progressively throughout the fetal and postnatal periods towards the levels found in adult rat tissues. The results suggest that the appearance of the thyroid hormone receptor coincides with that of the first fetal thyroid gland structures, but that it occurs much before thyroid function is fully established. As far as the receptor is concerned, fetal tissues have the potential to respond to thyroid hormone as early as the 13th day of gestational age.