Cloning and characterization of a complementary DNA for a thyroid hormone-responsive protein in mature rat cerebral tissue

Pathophysiology and Clinical Features of Neuropsychiatric Manifestations of Thyroid Disease

Thyroid hormones (TH) have a cardinal role in the development of the central nervous system during embryogenesis and early infancy. However, the TH-responsive genes in the developing brain cease to respond to TH in adulthood. Nevertheless, thyroid dysfunction in adults is commonly associated with a host of cognitive and psychiatric problems. Cognitive decline, dysphoria, and depression are common manifestations of overt hypothyroidism while hyperthyroidism can cause agitation, acute psychosis, and apathy, especially in older people. Whereas levothyroxine treatment can reverse dementia in the setting of hypothyroidism, the effect of levothyroxine on depressive symptoms in subjects with subclinical hypothyroidism is controversial. The use of supraphysiologic doses of TH to treat depression refractory to antidepressant remains a viable therapeutic tool with the caveat that excessive doses of thyroid hormone to treat depression may have potentially damaging effects on other organ systems. The present communication describes the pathophysiology of neuropsychiatric manifestations of thyroid disease, including changes in neurotransmission, alterations in neuronal or glial cell gene expression, blood-brain barrier dysfunction, increased risk of cerebrovascular disease, and occasionally cerebral inflammatory disease in the context of autoimmune thyroid disease. Elucidating the molecular mechanisms of TH effect on cerebral tissue will help identify novel therapeutic targets for managing people with neuropsychiatric disorders.

Overexpression of the Thyroid Hormone-Responsive (THRSP) Gene in the Striatum Leads to the Development of Inattentive-like Phenotype in Mice

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that affects 8-12% of children globally. Factor analyses have divided ADHD symptoms into two domains: inattention and a combination of hyperactivity and impulsivity. The identification of domain-specific genetic risk variants may help uncover potential genetic mechanisms underlying ADHD. We have previously identified that thyroid hormone-responsive (THRSP) gene expression is upregulated in spontaneously hypertensive rats (SHR/NCrl) and Wistar-Kyoto (WKY/NCrl) rats which exhibited inattention behavior. Thus, we established a line of THRSP overexpressing (OE) mice and assessed their behavior through an array of behavioral tests. The gene and protein overexpression of THRSP in the striatum (STR) was confirmed by quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting. The THRSP OE mice exhibited inattention in the novel-object recognition and Y-maze test, but not hyperactivity in the open-field test and impulsivity in the cliff-avoidance and delay-discounting task. We have also found that expression of dopamine-related genes (dopamine transporter, tyrosine hydroxylase, and dopamine D1 and D2 receptors) in the STR increased. Treatment with methylphenidate (5 mg/kg), the most commonly used medication for ADHD, improved attention and normalized expression levels of dopamine-related genes in THRSP OE mice. Our findings suggest that THRSP plays a role in the inattention phenotype of ADHD and that the THRSP OE mice may be used as an animal model to elucidate the genetic mechanisms of the disorder.

The thyroid hormone responsive protein (THRP) has a critical role in the embryogenesis of Xenopus laevis

The human and mouse homologs of the rat thyroid hormone responsive protein (THRP), c-abl-interacting protein 2 (Abi-2), are critically involved in neurological development. The Abi-2 gene is evolutionarily conserved in vertebrates, and is also found in Xenopus laevis and Drosophila melanogaster. The THRP gene is one of the few genes regulated by thyroid hormone in adult animals. Sequence analysis of the 5'-flanking region of the THRP gene identified a putative thyroid hormone response element (TRE) that is conserved between rat and human. To determine whether or not THRP regulates neural growth and development, THRP was constitutively expressed in transgenic X. laevis. Growth of most animals was halted in early neurulation while the few animals that survived the process developed into grossly malformed tadpoles. In contrast, control animals reached late embryonic stage 25. These observations suggest that THRP over-expression in early development is not compatible with completion of early embryogenesis and that a different strategy needs to be employed to investigate THRP function in this model.

Tachykinin family genes and their receptors are differentially expressed in the hypothyroid ovary and pituitary

Plasma tachykinin levels are known to be altered with sexual acyclicity and loss of reproductive function. Ovulatory dysfunction, as seen in postmenopausal women, is also often encountered in hypothyroid patients. To know the involvement of different tachykinin genes in hypothyroidism-associated reproductive disorders, we performed DD-PCR with the pituitary RNA of control and hypothyroid rats to see the differentially expressed gene profile. Subsequently, we selected a few clones, tachykinin being one of them. Since its expression was up regulated in hypothyroidism as it does in the sexually acyclic females, we wanted to correlate these two phenomena with hypothyroidism associated reproductive disorders. We observed differential expression of tac2 along with other tk genes and their receptors in rat pituitary and ovary, which suggests that hypothyroidism affects the expression of these genes in these tissues. The experiments were repeated in ovarian tissue obtained at surgery from hypothyroid human patients, which showed similar expression pattern of TAC3 (equivalent to rat tac2) and their receptors as in rat ovary. Significant reduction of tac2 expression in reproductively less active rat ovary suggests the association of tac2 with reproductive senescence. Our results suggest that decline in reproductive function in hypothyroidism is associated with altered expression level of tac2 and its receptors. Further investigation in this area could elucidate the possible mechanism of tachykinins' involvement in loss of sexual cyclicity and other reproductive disorders associated with hypothyroidism.

Involvement of Pitx2, a homeodomain transcription factor, in hypothyroidism associated reproductive disorders

Hypothyroid-associated reproductive disorders have now become a striking phenomenon worldwide but the molecular mechanism behind these disorders is not fully known. Pitx2 gene encodes homeodomain transcription factor, which regulates Plod2 gene in brain tissue, transactivates gonadotropin genes in pituitary and plays a substantial role in cell growth and proliferation in different tissues. Pitx2 binds to Plod2 promoter and activates this gene in rat ovary. In this report, we show that Pitx2's expression is markedly reduced in hypothyroid ovary as well as in ovarian granulosa cells, which is recovered with T(3)-supplementation both in vitro and in vivo conditions. Reduced Pitx2 expression could decrease Plod2 expression and hence facilitate ovarian ECM degradation. We have also observed similar pattern of expression of Pitx1 and -3 in hypothyroid and T(3)-supplemented ovaries. The temporal expression of Pitx2 across the estrous cycle shows that it is expressed through all the 4 phases of the cycle and reaches its maximum in the proestrus phase suggesting its possible role in ovulation followed by luteinization. The present study reveals that the reduced Pitx2 expression in hypothyroid ovary could lead to ovarian dysfunction by modulating the Pitx2-Plod2 interaction, further study will be necessary to unravel the complete regulatory mechanism of Pitx2 in ovarian function.

Acute and chronic effects of developmental iron deficiency on mRNA expression patterns in the brain

Because of the multiple biochemical pathways that require iron, iron deficiency can impact brain metabolism in many ways. The goal of this study was to identify a molecular footprint associated with ongoing versus long term consequences of iron deficiency using microarray analysis. Rats were born to iron-deficient mothers, and were analyzed at two different ages: 21 days, while weaning and iron-deficient; and six months, after a five month iron-sufficient recovery period. Overall, the data indicate that ongoing iron deficiency impacts multiple pathways, whereas the long term consequences of iron deficiency on gene expression are more limited. These data suggest that the gene array profiles obtained at postnatal day 21 reflect a brain under development in a metabolically compromised setting that given appropriate intervention is mostly correctable. There are, however, long term consequences to the developmental iron deficiency that could underlie the neurological deficits reported for iron deficiency.

Thyroid hormone responsive protein (THRP) mediates thyroid hormone-induced cytotoxicity in primary neuronal cultures

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone (TH) in the cerebral cortex of adult rats. To study the effects of THRP on neuronal cell survival, primary neurons cultured from rats at embryonic day 19 were treated with either 10(-7) mol L(-1) 3,5,3'-triiodothyronine (T(3)), or 10(-7) mol L(-1) L: -thyroxine (T(4)). This resulted in decreasing neuronal cell number starting 48 h after treatment. T(3) -related cytotoxicity was also documented by measurement of lactate dehydrogenase release into the medium and by propidium iodide staining. Treatment of cells with 10(-7) mol L(-1) T(3) resulted in a significant increase in THRP mRNA levels as early as 24 h of treatment in a concentration-dependent manner. T(3) treatment did not alter glyceraldehyde 3-phosphate dehydrogenase (G3PDH) mRNA levels. Exogenous expression of THRP by transfecting cells with a THRP expression construct (pSVL-THRP) was associated with a significant increase in cell death as measured by the increased number of propidium iodide staining cells (18.0+/-2.1 cells per field) compared with mock-transfected cells (3.3+/-0.2), P<0.002. To further document THRP-induced cytotoxicity, the cells were either transfected with pSVL (empty vector)+pSV2neo (neomycin resistance vector for cell labeling), pSVL-THRP+pSV2neo, or pSVL-THRP+pc-Abl (cAbl tyrosine kinase expressing vector)+pSV2neo. After 24 h the cells were treated with 500 microg mL(-1) G418 (a congener of neomycin) to eliminate the non-transfected cells. Transfection with pSVL-THRP reduced neuronal survival relative to cells transfected with pSVL (356+/-15.6 compared with 145+/-16.9, P<0.05). Co-transfection of THRP with wild-type c-Abl did not alter the effect of THRP on cell survival. It is concluded that THRP is an important factor in TH-induced neuronal cell death.

Microarray analysis of thyroid hormone-induced changes in mRNA expression in the adult rat brain

To determine which genes in the adult rat brain are regulated by thyroid hormone (TH), we used microarrays to examine the effect of hyperthyroidism on neuron-specific gene expression. Four-month-old male Fisher 344 rats were rendered hyperthyroid by intraperitoneal injection of 3,5,3'-L-triiodothyronine (T3, 15 microg/100 g body weight) for 10 consecutive days. To minimize interindividual variability, pooled cerebral tissue RNA from four-control and five-hyperthyroid rats was hybridized in duplicates to the Affymetrix (Santa Clara, CA) U34N rat neurobiology microarray, which contains probes for 1224 neural-specific genes. Changes in gene expression were considered significant only if they were observed in both pair-wise comparisons as well as by Northern blot analysis. Hyperthyroidism was associated with modest changes in the expression of only 11 genes. The expression of the phosphodiesterase Enpp2, myelin oligodendrocyte glycoprotein (Mog), microtubule-associated protein 2 (MAP2), growth hormone (GH), Ca(2+)/calmodulin-dependent protein kinase beta-subunit (Camk2b), neuron-specific protein PEP-19 (Pcp4), a sodium-dependent neurotransmitter, and the myelin-associated glycoprotein (S-MAG) was significantly increased. Three genes were suppressed by hyperthyroidism, including the activity and neurotransmitter-induced early genes-1 and -7 (ANIA-1 and ANIA-7) and the guanine nucleotide-binding protein one (Gnb1). The present study underscores the paucity of TH responsive genes in adult cerebral tissue.

Effect of thyroid hormone responsive protein (THRP) expression on PC12 cell survival

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone in the cerebral cortex of adult rats. The biological effects of THRP are currently unknown. Since thyroid hormones (TH) are known to cause cell death in primary neuronal cultures, the effect of exogenous THRP expression on PC12 cell viability was investigated. Co-transfection of the THRP expression plasmid with the selectable marker pSV2neo resulted in a lower number of surviving PC12 cells compared to transfection with pSV2neo and the empty vector, pSVL. Similar results were observed when PC12 cells were transfected with the plasmid pCMV. SPORT beta-gal with and without pSVL-THRP. However, expression of exogenous THRP in the colonic epithelial cell line Caco-2 and the glial cell line U251 had no effect on cell viability. Coexpression of THRP with either the wild-type (WT)-c-Abl or a kinase-defective mutant c-Abl (K290R) did not alter the cell viability changes induced by THRP alone. Under these experimental conditions the predominant form of cell death was necrosis as evidenced by in situ analyses, such as terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) and staining with membrane permeating and non-permeating nuclear dyes, Hoechst 33342 and propidium iodide respectively. In addition cell cycle arrest induced by THRP was demonstrated by reduced (3)H-thymidine incorporation into cellular DNA. The number of PC12 cells treated with 10(-7) M of l-3, 5, 3'-triiodothyronine (T(3)) was significantly reduced after the fourth day of culture. Treatment of the cells with T(3 )resulted in a dose dependent induction of THRP mRNA. It is concluded that: (1). THRP expression induces PC12 cell death; (2). under these experimental conditions the form of cell death is predominantly necrosis although cell cycle arrest may also occur; (3). the effect of THRP on cell viability is not modulated by c-Abl tyrosine kinase; and (4). the effect of T(3 )treatment on PC12 cell survival may be mediated by THRP.

A complex deoxyribonucleic acid response element in the rat Ca(2+)/calmodulin-dependent protein kinase IV gene 5'-flanking region mediates thyroid hormone induction and chicken ovalbumin upstream promoter transcription factor 1 repression

Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is regulated by T(3) in a time- and concentration-dependent manner in the developing rat brain and plays an important role in neuronal-specific gene regulation. T(3) treatment, but not retinoic acid (RA), stimulated endogenous CaMKIV mRNA 5-fold in mouse embryonic stem (ES) cells differentiated into neurons. We localized a region -750 to -700 in the CaMKIV gene 5'-flanking region that conferred T(3) responsiveness and bound thyroid hormone receptor (TR), retinoic acid receptor (RAR), and chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1). T(3) and RA treatment stimulated the CaMKIV hormone response element. Cotransfection of a COUP-TF1 expression vector repressed the T(3) response and augmented the RA response. Mutational analysis identified three half-sites arranged in a direct repeat (AB) and overlapping inverted repeat (BC), required for functional induction and receptor binding. TR and RAR bound predominantly to the BC portion of the element and COUP-TF1 to the AB region, with a close correlation of binding and functional studies. COUP-TF1 binding did not influence TR/retinoid X receptor binding but modestly augmented RAR/retinoid X receptor binding. A single element confers T(3) and COUP-TF1 regulation of CaMKIV expression.

mRNA differential display identification of thyroid hormone-responsive protein (THRP) gene in association with early phase of long-term potentiation

The process of long-term potentiation (LTP) consists of the early induction and late maintenance phases. Few studies have examined the cellular mechanisms underlying these two phases; their respective mRNA expression profiles have not yet been elucidated. Here we used the technique of PCR differential display to identify genes that are differentially expressed between the early and late phases of LTP in vivo. Our results indicated that the cDNA fragment corresponding to one mRNA with preferentially increased expression during the early, but not late, phase of LTP encodes the rat thyroid hormone-responsive protein (THRP) gene. In situ hybridization analysis confirmed the results obtained from the PCR differential display. Prior NMDA receptor blockade with MK801 prevented induction of LTP and decreased THRP mRNA expression in the dentate gyrus, as assayed by quantitative RT-PCR analysis. THRP antisense oligonucleotide treatment before tetanic stimulation also prevented induction of LTP. However, when THRP antisense oligonucleotide was administered after induction of LTP, it did not affect expression and maintenance of LTP. THRP is known to be responsive to thyroid hormone. Our results indicate that direct thyroid hormone (T3) injection into the dentate gyrus produces a long-lasting enhancement of synaptic efficacy of these neurons. T3 injection also markedly increased THRP mRNA expression in the dentate gyrus. Taken together, our results suggest that THRP mRNA expression plays an important role in the early phase, but not the late phase, of LTP and that both THRP and thyroid hormone are involved in synaptic plasticity in hippocampal neurons.

Action of thyroid hormone in brain

Among the most critical actions of thyroid hormone in man and other mammals are those exerted on brain development. Severe hypothyroidism during the neonatal period leads to structural alterations, including hypomyelination and defects of cell migration and differentiation, with long-lasting, irreversible effects on behavior and performance. A complex regulatory mechanism operates in brain involving regulation of the concentration of the active hormone, T3, and the control of gene expression. Most brain T3 is formed locally from its precursor, T4, by the action of type II deiodinase which is expressed in glial cells, tanycytes, and astrocytes. Type III deiodinase (DIII) is also involved in the regulation of T3 concentrations, especially during the embryonic and early post-natal periods. DIII is expressed in neurons and degrades T4 and T3 to inactive metabolites. The action of T3 is mediated through nuclear receptors, which are expressed mainly in neurons. The receptors are ligand-modulated transcription factors, and a number of genes have been identified as regulated by thyroid hormone in brain. The regulated genes encode proteins of myelin, mitochondria, neurotrophins and their receptors, cytoskeleton, transcription factors, splicing regulators, cell matrix proteins, adhesion molecules, and proteins involved in intracellular signaling pathways. The role of thyroid hormone is to accelerate changes of gene expression that take place during development. Surprisingly, null-mutant mice for the T3 receptors show almost no signs of central nervous system involvement, in contrast with the severe effects of hypothyroidism. The resolution of this paradox is essential to understand the role of thyroid hormone and its receptors in brain development and function.

Partial characterization of a cerebral thyroid hormone-responsive protein

The thyroid hormone-responsive protein (THRP) is expressed in rat cerebral tissue and has 83% overall sequence homology with c-Abl interactor protein, Abi-2, which is a substrate for the tyrosine kinase activity of c-Abl. Within the core region of the two proteins, the sequence similarity approaches 99%. To determine whether THRP is a rat homologue of Abi-2 or is a distinct protein with unique properties, the tissue distribution of THRP and Abi-2 mRNA's was examined using a sensitive ribonuclease protection assay and probes specific for THRP and Abi-2, respectively. The THRP mRNA content of cerebral tissue (1340.0 +/- 126.5 arbitrary units) was 2.3-fold higher than Abi-2 mRNA (581.3 +/- 73.7), while the ratio of hepatic content of THRP mRNA (209.0 +/- 49.1) to hepatic Abi-2 mRNA (2923.0 +/- 378.7) was only 0.07 (P < 0.004). Very low levels of Abi-2 mRNA, but not THRP mRNA, were also found in the heart and small intestine. Experiments with PC12 cells transfected with the full-length THRP cDNA and grown in the presence or absence of a tyrosine kinase inhibitor, along with experiments where PC12 cells were cotransfected with the THRP cDNA with or without the wild-type or mutant (tyrosine kinase deficient) c-Abl cDNA, showed that THRP is tyrosine phosphorylated; however, it is not a substrate for c-Abl. These studies demonstrate that THRP and Abi-2 have distinct tissue distribution and distinct biological properties.

Identification of genes enhanced by protein-calorie malnutrition by differential display polymerase chain reaction (expression of fibrinogen B beta chain, B cell translocation gene 1 and thyroid hormone responsive protein genes)

Protein-calorie malnutrition (PCM), as one of global health problems, arises during protein and/or energy deficit due to disease and nutritional inadequacy. Previously, we showed that PCM elicited oxidative stress with activation of the phase II detoxifying gene expression, which was reversed by cysteine supplementation. As part of the attempts to identify the cellular adaptive responses and the associated gene expression during PCM, the current study was initiated to analyze the genes differentially expressed in the rat during PCM. Among 1,916 bands amplified, 85 putative differentially amplified bands were enhanced by PCM in the liver, while the expression of 64 bands was suppressed. Northern and/or reverse transcription-polymerase chain reaction (RT-PCR) analyses revealed that PCM increased the expression of fibrinogen B beta chain, B cell translocation gene I (BTGI) and thyroid hormone responsive protein (THRP) mRNAs. The increase in the hepatic fibrinogen B beta chain mRNA was not prevented by cysteine supplementation, whereas cysteine decreased the enhancement in the rGSTA2 and microsomal epoxide hydrolase mRNA expression. Cysteine was also active in reversing the increase in BTG1 mRNA during PCM. This was supported by the increase in BTG1 mRNA in H4IIE cells exposed to sulfur amino acid-deprived medium. Northern blot analysis revealed that THRP, highly expressed in the brain in a tissue-specific manner, was induced by PCM and that cysteine supplementation abolished the THRP induction. Conversely, the level of hepatic albumin mRNA was markedly decreased by PCM, which was partially restored by cysteine supplementation. Differential display RT-PCR analysis allowed us to identify the genes that are responsive to oxidative stress during PCM and to characterize the differential role of cysteine on the expression of the fibrinogen B beta chain, BTG1 and THRP genes as a homeostatic adaptive response during protein deficiency.

Identification of dopamine responsive mRNAs in glial cells by suppression subtractive hybridization

Recent studies have established that glial cells are important targets of the neurotransmitter dopamine (DA), but the regulatory effects of DA on glial cells have not been extensively studied. In the present study, we have investigated the influence of DA on gene transcription in glial cells. Two-directional (forward and backward) suppression subtraction hybridization (SSH) was performed on astrocytes cultured from rat cerebral tissues in standard media or in culture media treated with DA. PCR-select differential screening was used to further verify the differentially expressed cDNA clones, positive clones were sequenced, and the mRNAs were re-examined on Northern blots. Fourteen sequences were identified of which eleven are homologous to known genes, three are homologous to expressed sequence tags (ESTs). Three novel full-length cDNAs were isolated using the EST fragments as probes to screen a cDNA library constructed from human brain. Analysis of these sequences suggested that complex intracellular signaling pathways, involving crosstalk with growth factor pathways, steroid hormone pathways, and an interferon-regulated 2-5 A pathway, are responsive to DA in astrocytes. The responsive proteins downstream from the signaling pathways were found to fall into at least three groups, including a series of metabolic enzymes, stress proteins, transfer proteins, etc. In addition, several of them have established their relationships with specific neurodegenerative diseases, showing that there is overlap in the pathogenic mechanisms of different diseases. Our results have provided a foundation for better understanding of the molecular basis of glial cell functions in dopaminergic transmission and an approach to find possible medication for the related disorders.

Isolation of hNap1BP which interacts with human Nap1 (NCKAP1) whose expression is down-regulated in Alzheimer's disease

We previously reported the isolation of a novel apoptosis-related gene, human Nap1 (HGMW-approved symbol NCKAP1), the expression of which was strongly down-regulated in sporadic Alzheimer's disease (AD). Human Nap1 proved to be an orthologue of rat Nap1 which binds to the adaptor molecule Nck in signal transduction. In order to further elucidate the function of human Nap1, we performed yeast two-hybrid screening. As a result of screening, we discovered a protein designated hNap1BP (human Nap1 binding protein) which is a member of the tyrosine kinase-binding protein family. In addition, hNap1BP bound to the SH3 domain of c-Abl and Nck. hNap1BP is expressed ubiquitously in various tissues like human Nap1, and intriguingly these genes are co-expressed in hippocampus and cerebral cortex in mouse brain where AD pathological features are strongly evident. Further functional analysis of hNap1BP may clarify its contribution to AD pathology.

Identification of a mammalian homologue of the fungal Tom70 mitochondrial precursor protein import receptor as a thyroid hormone-regulated gene in specific brain regions

Thyroid hormone is an important regulator of mammalian brain maturation. By differential display PCR, we isolated a cDNA clone (S2) that is specifically up-regulated in the striatum of neonatal hypothyroid rats. S2 was identified as KIAA0719, the first human gene distantly homologous to the fungal Tom70, which encodes a member of the translocase mitochondrial outer membrane complex involved in the import of preproteins into the mitochondria. By northern and in situ hybridization studies, KIAA0719 was found to be up-regulated in the striatum, nucleus accumbens, and discrete cortical layers of 15-day-old hypothyroid rats. In contrast, lower expression was found in the olfactory tubercle, whereas no differences were detected in other brain regions. Significantly, treatment of hypothyroid animals with single injections of thyroxine restored the normal levels of KIAA0719 expression. Moreover, treatment of control animals with thyroxine led to a reduced expression, demonstrating a negative hormonal regulation in vivo. Thus, KIAA0719 gene expression is regulated by thyroid hormone in the neonatal rat brain in a region-specific fashion. Given the role of the homologous Tom70 gene, the alteration of KIAA0719 expression may contribute to the changes in mitochondrial morphology and physiology caused by hypothyroidism in the developing rat brain.

Drosophila abelson interacting protein (dAbi) is a positive regulator of abelson tyrosine kinase activity

Human and mouse Abelson interacting proteins (Abi) are SH3-domain containing proteins that bind to the proline-rich motifs of the Abelson protein tyrosine kinase. We report a new member of this gene family, a Drosophila Abi (dAbi) that is a substrate for Abl kinase and that co-immunoprecipitates with Abl if the Abi SH3 domain is intact. We have identified a new function for both dAbi and human Abi-2 (hAbi-2). Both proteins activate the kinase activity of Abl as assayed by phosphorylation of the Drosophila Enabled (Ena) protein. Removal of the dAbi SH3 domain eliminates dAbi's activation of Abl kinase activity. dAbi is an unstable protein in cells and is present at low steady state levels but its protein level is increased coincident with phosphorylation by Abl kinase. Expression of the antisense strand of dAbi reduces dAbi protein levels and abolishes activation of Abl kinase activity. Modulation of Abi protein levels may be an important mechanism for regulating the level of Abl kinase activity in the cell.

Identification of the mammalian homolog of the splicing regulator Suppressor-of-white-apricot as a thyroid hormone regulated gene

Mammalian brain development is controlled by thyroid hormone through the regulation of target genes. In this study, we describe for the first time that a splicing regulator gene is under thyroid hormone control in the rat brain during the critical period of neuronal differentiation. By differential display, we have identified the mammalian homolog of the Drosophila splicing regulator Suppressor-of-white-apricot (SWAP) as a thyroid hormone-regulated gene in an immortal line of rat neuroblasts, E18 cells. Using Northern blotting and in situ hybridization, we found that expression of SWAP is under thyroid control in the developing rat brain. SWAP gene expression is highest during the first 10 days of life (P0-P10), preferentially in cerebral cortex, cerebellum, subventricular epithelium, piriform cortex, hippocampus, amygdala, and caudate putamen. At later stages (P15-P30) SWAP expression decreases, being detectable only in the cerebellum, hippocampus, and layers II/III of cerebral and piriform cortexes. We found that hypothyroidism causes an abnormal high level of SWAP RNA expression at P5-P15 throughout the brain except the cerebellum. Significantly, thyroid hormone treatment in vivo of hypothyroid animals led to a normalization of SWAP RNA expression. Furthermore, similar hormone treatment caused a decrease in SWAP expression in control rats. By modulating the expression of SWAP and perhaps other splicing regulators thyroid hormone may exert wide regulatory effects on multiple genes. The regulation of SWAP gene defines a novel mechanism of action of thyroid hormone which can be important for its effects in the developing brain.

Age-related changes in thyroid hormone responsive protein (THRP) expression in cerebral tissue of rats

To determine if aging in rats is associated with insensitivity of cerebral tissue to thyroid hormones (TH), the expression of a TH responsive protein or (THRP) in cerebral tissue was studied in male Fischer rats at 4, 12 and 24 months of age during euthyroid, hypothyroid and hyperthyroid states. The basal levels of THRP mRNA was significantly increased in 24-month-old and in 12-month-old rats while THRP mass measured by Western blots was decreased compared to 4-month-old rats. Compared to euthyroid rats, hyperthyroidism in 4-month-old rats was associated with 5.1-fold increase in THRP mRNA and 3.7-fold increase in protein content while in hyperthyroid aged rats, the increase of THRP mRNA was only 1.6-fold and the increase in the protein was 2.4-fold. Hypothyroidism did not significantly alter THRP or its mRNA in either young or aged rats. It is concluded that aging in rats is associated with reduced cerebral tissue responsiveness to thyroid hormones.