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

CaMKIV mediates spine growth deficiency of hippocampal neurons by regulation of EGR3/BDNF signal axis in congenital hypothyroidism

Congenital hypothyroidism (CH) will cause cognitive impairment in the condition of delayed treatment. The hippocampus is one of the most affected tissues by CH, in which the functional structures of hippocampal neurons manifest deficiency due to aberrant expression of effector molecules. The Ca²⁺/Calmodulin-dependent protein kinase, CaMKIV, is downregulated in the hippocampal neurons, influencing the growth of dendritic spines in response to CH. However, the underlying mechanism is not fully elucidated. In the present study, the early growth response factor 3 (EGR3) was regulated by CaMKIV in the hippocampal neurons of CH rat pups, as was analyzed by transcriptome sequencing and in vitro cell experiments. EGR3 localized within hippocampal neurons in CA1, CA3, and dentate gyrus regions. Deficient EGR3 in the primary hippocampal neurons significantly reduced the density of dendritic spines by downregulating the expression of BDNF, and such effects could be rescued by supplementing recombinant BDNF protein. Taken together, CH mediates cognitive impairment of pups through the inactivation of CaMKIV in the hippocampal neurons, which decreases the expression of EGR3 and further reduces the production of BDNF, thereby impairing the growth of dendritic spines. Identifying CaMKIV/EGR3/BDNF pathway in the hippocampal neurons in the context of CH will benefit the drug development of intellectual disability caused by CH.

Congenital hypothyroidism impairs spine growth of dentate granule cells by downregulation of CaMKIV

Congenital hypothyroidism (CH), a common neonatal endocrine disorder, can result in cognitive deficits if delay in diagnose and treatment. Dentate gyrus (DG) is the severely affected subregion of the hippocampus by the CH, where the dentate granule cells (DGCs) reside in. However, how CH impairs the cognitive function via affecting DGCs and the underlying mechanisms are not fully elucidated. In the present study, the CH model of rat pups was successfully established, and the aberrant dendrite growth of the DGCs and the impaired cognitive behaviors were observed in the offspring. Transcriptome analysis of hippocampal tissues following rat CH successfully identified that calcium/calmodulin-dependent protein kinase IV (CaMKIV) was the prominent regulator involved in mediating deficient growth of DGC dendrites. CaMKIV was shown to be dynamically regulated in the DG subregion of the rats following drug-induced CH. Interference of CaMKIV expression in the primary DGCs significantly reduced the spine density of dendrites, while addition of T3 to the primary DGCs isolated from CH pups could facilitate the spine growth of dendrites. Insights into relevant mechanisms revealed that CH-mediated CaMKIV deficiency resulted in the significant decrease of phosphorylated CREB in DGCs, in association with the abnormality of dendrites. Our results have provided a distinct cell type in hippocampus that is affected by CH, which would be beneficial for the treatment of CH-induced cognitive deficiency.

Thyroid disrupting chemicals and developmental neurotoxicity - New tools and approaches to evaluate hormone action

It is well documented that thyroid hormone (TH) action is critical for normal brain development and is mediated by both nuclear and extranuclear pathways. Given this dependence, the impact of environmental endocrine disrupting chemicals that interfere with thyroid signaling is a major concern with direct implications for children's health. However, identifying thyroid disrupting chemicals in vivo is primarily reliant on serum thyroxine (T4) measurements within greater developmental and reproductive toxicity assessments. These studies do not examine known TH-dependent phenotypes in parallel, which complicates chemical evaluation. Additionally, there exist no recommendations regarding what degree of serum T4 dysfunction is adverse, and little consideration is given to quantifying TH action within the developing brain. This review summarizes current testing strategies in rodent models and discusses new approaches for evaluating the developmental neurotoxicity of thyroid disrupting chemicals. This includes assays to identify adverse cellular effects of the brain by both immunohistochemistry and gene expression, which would compliment serum T4 measures. While additional experiments are needed to test the full utility of these approaches, incorporation of these cellular and molecular assays could enhance chemical evaluation in the regulatory arena.

The Role of Calmodulin in Tumor Cell Migration, Invasiveness, and Metastasis

Calmodulin (CaM) is the principal Ca²⁺ sensor protein in all eukaryotic cells, that upon binding to target proteins transduces signals encoded by global or subcellular-specific changes of Ca²⁺ concentration within the cell. The Ca²⁺/CaM complex as well as Ca²⁺-free CaM modulate the activity of a vast number of enzymes, channels, signaling, adaptor and structural proteins, and hence the functionality of implicated signaling pathways, which control multiple cellular functions. A basic and important cellular function controlled by CaM in various ways is cell motility. Here we discuss the role of CaM-dependent systems involved in cell migration, tumor cell invasiveness, and metastasis development. Emphasis is given to phosphorylation/dephosphorylation events catalyzed by myosin light-chain kinase, CaM-dependent kinase-II, as well as other CaM-dependent kinases, and the CaM-dependent phosphatase calcineurin. In addition, the role of the CaM-regulated small GTPases Rac1 and Cdc42 (cell division cycle protein 42) as well as CaM-binding adaptor/scaffold proteins such as Grb7 (growth factor receptor bound protein 7), IQGAP (IQ motif containing GTPase activating protein) and AKAP12 (A kinase anchoring protein 12) will be reviewed. CaM-regulated mechanisms in cancer cells responsible for their greater migratory capacity compared to non-malignant cells, invasion of adjacent normal tissues and their systemic dissemination will be discussed, including closely linked processes such as the epithelial–mesenchymal transition and the activation of metalloproteases. This review covers as well the role of CaM in establishing metastatic foci in distant organs. Finally, the use of CaM antagonists and other blocking techniques to downregulate CaM-dependent systems aimed at preventing cancer cell invasiveness and metastasis development will be outlined.

Transport, Metabolism, and Function of Thyroid Hormones in the Developing Mammalian Brain

Ever since the discovery of thyroid hormone deficiency as the primary cause of cretinism in the second half of the 19th century, the crucial role of thyroid hormone (TH) signaling in embryonic brain development has been established. However, the biological understanding of TH function in brain formation is far from complete, despite advances in treating thyroid function deficiency disorders. The pleiotropic nature of TH action makes it difficult to identify and study discrete roles of TH in various aspect of embryogenesis, including neurogenesis and brain maturation. These challenges notwithstanding, enormous progress has been achieved in understanding TH production and its regulation, their conversions and routes of entry into the developing mammalian brain. The endocrine environment has to adjust when an embryo ceases to rely solely on maternal source of hormones as its own thyroid gland develops and starts to produce endogenous TH. A number of mechanisms are in place to secure the proper delivery and action of TH with placenta, blood-brain interface, and choroid plexus as barriers of entry that need to selectively transport and modify these hormones thus controlling their active levels. Additionally, target cells also possess mechanisms to import, modify and bind TH to further fine-tune their action. A complex picture of a tightly regulated network of transport proteins, modifying enzymes, and receptors has emerged from the past studies. TH have been implicated in multiple processes related to brain formation in mammals-neuronal progenitor proliferation, neuronal migration, functional maturation, and survival-with their exact roles changing over developmental time. Given the plethora of effects thyroid hormones exert on various cell types at different developmental periods, the precise spatiotemporal regulation of their action is of crucial importance. In this review we summarize the current knowledge about TH delivery, conversions, and function in the developing mammalian brain. We also discuss their potential role in vertebrate brain evolution and offer future directions for research aimed at elucidating TH signaling in nervous system development.

Thyroid Hormone Disruption in the Fetal and Neonatal Rat: Predictive Hormone Measures and Bioindicators of Hormone Action in the Developing Cortex

Adverse neurodevelopmental consequences remain a primary concern when evaluating the effects of thyroid hormone (TH) disrupting chemicals. Though the developing brain is a known target of TH insufficiency, the relationship between THs in the serum and the central nervous system is not well characterized. To address this issue, dose response experiments were performed in pregnant rats using the goitrogen propylthiouracil (PTU) (dose range 0.1-10 ppm). THs were quantified in the serum and brain of offspring at gestational day 20 (GD20) and postnatal day 14 (PN14), two developmental stages included in OECD and EPA regulatory guideline/guidance studies. From the dose response data, the quantitative relationships between THs in the serum and brain were determined. Next, targeted gene expression analyses were performed in the fetal and neonatal cortex to test the hypothesis that TH action in the developing brain is linked to changes in TH concentrations within the tissue. Results show a significant reduction of T4/T3 in the serum and brain of the GD20 fetus in response to low doses of PTU; interestingly, very few genes were significantly different at any dose tested. In the PN14 pup significant reductions of T4/T3 in the serum and brain were also detected; however, twelve transcriptional targets were identified in the neonatal cortex that correlated well with reduced brain THs. These results show that serum T4 is a good predictor of brain THs, and offer several target genes that could serve as pragmatic readouts of T4/T3 dysfunction within the PN14 cortex.

COUP-TF1 Modulates Thyroid Hormone Action in an Embryonic Stem-Cell Model of Cortical Pyramidal Neuronal Differentiation

BACKGROUND

Thyroid hormone is critical for normal brain development and acts in a spatial and temporal specific pattern. Thyroid hormone excess, or deficiency, can lead to irreversible impairment of brain and sensory development. Chicken ovalbumin upstream-transcription factor 1 (COUP-TF1), expressed early in neuronal development, is essential to achieve normal brain structure. Thyroid hormone stimulation of gene expression is inversely correlated with the level of COUP-TF1 expression.

METHODS

An in vitro method of differentiating mouse embryonic stem (mES) cells into cortical neurons was utilized to study the influence of COUP-TF1 on thyroid hormone signaling in brain development. mES cells were cultured and differentiated in specific conditioned media, and a high percentage of nestin-positive progenitor neurons in the first stage, and cortical neurons in the second stage, was obtained with characteristic neuronal firing.

RESULTS

The number of nestin-positive progenitors, as determined by fluorescence-activated cell sorting analysis, was significantly greater with triiodothyronine (T3) treatment compared to control (p < 0.05). T3 enhanced the expression of cortical neuron marker (Tbr1 and Rc3) mRNAs. After COUP-TF1 knockdown, the number of nestin-positive progenitors was reduced compared to control (p < 0.05), but the number increased with T3 treatment. The mRNA of cortical neuronal gene markers was measured after COUP-TF1 knockdown. In the presence of T3, the peak expression of neuron markers Emx1, Tbr1, Camkiv, and Rc3 mRNA was earlier, at day 18 of differentiation, compared to control cells, at day 22. Furthermore, after COUP-TF1 knockdown, T3 induction of Rc3 and Tbr1 mRNA was significantly enhanced compared to cells expressing COUP-TF1.

CONCLUSION

These results indicate that COUP-TF1 plays an important role in modulating the timing and magnitude of T3-stimulated gene expression required for normal corticogenesis.

Thyroid hormone and the brain: Mechanisms of action in development and role in protection and promotion of recovery after brain injury

Thyroid hormone (TH) is essential for normal brain development and may also promote recovery and neuronal regeneration after brain injury. TH acts predominantly through the nuclear receptors, TH receptor alpha (THRA) and beta (THRB). Additional factors that impact TH action in the brain include metabolism, activation of thyroxine (T4) to triiodothyronine (T3) by the enzyme 5'-deiodinase Type 2 (Dio2), inactivation by the enzyme 5-deiodinase Type 3 (Dio3) to reverse T3 (rT3), which occurs in glial cells, and uptake by the Mct8 transporter in neurons. Traumatic brain injury (TBI) is associated with inflammation, metabolic alterations and neural death. In clinical studies, central hypothyroidism, due to hypothalamic and pituitary dysfunction, has been found in some individuals after brain injury. TH has been shown, in animal models, to be protective for the damage incurred from brain injury and may have a role to limit injury and promote recovery. Although clinical trials have not yet been reported, findings from in vitro and in vivo models inform potential treatment strategies utilizing TH for protection and promotion of recovery after brain injury.

Repression of COUP-TFI Improves Bone Marrow-Derived Mesenchymal Stem Cell Differentiation into Insulin-Producing Cells

Identifying molecular mechanisms that regulate insulin expression in bone marrow-derived mesenchymal stem cells (bmMSCs) can provide clues on how to stimulate the differentiation of bmMSCs into insulin-producing cells (IPCs), which can be used as a therapeutic approach against type 1 diabetes (T1D). As repression factors may inhibit differentiation, the efficiency of this process is insufficient for cell transplantation. In this study, we used the mouse insulin 2 (Ins2) promoter sequence and performed a DNA affinity precipitation assay combined with liquid chromatography-mass spectrometry to identify the transcription factor, chicken ovalbumin upstream promoter transcriptional factor I (COUP-TFI). Functionally, bmMSCs were reprogrammed into IPCs via COUP-TFI suppression and MafA overexpression. The differentiated cells expressed higher levels of genes specific for islet endocrine cells, and they released C-peptide and insulin in response to glucose stimulation. Transplantation of IPCs into streptozotocin-induced diabetic mice caused a reduction in hyperglycemia. Mechanistically, COUP-TFI bound to the DR1 (direct repeats with 1 spacer) element in the Ins2 promoter, thereby negatively regulating promoter activity. Taken together, the data provide a novel mechanism by which COUP-TFI acts as a negative regulator in the Ins2 promoter. The differentiation of bmMSCs into IPCs could be improved by knockdown of COUP-TFI, which may provide a novel stem cell-based therapy for T1D.

Thyroid hormone regulated genes in cerebral cortex development

The physiological and developmental effects of thyroid hormones are mainly due to the control of gene expression after interaction of T₃ with the nuclear receptors. To understand the role of thyroid hormones on cerebral cortex development, knowledge of the genes regulated by T₃ during specific stages of development is required. In our laboratory, we previously identified genes regulated by T₃ in primary cerebrocortical cells in culture. By comparing these data with transcriptomics of purified cell types from the developing cortex, the cellular targets of T₃ can be identified. In addition, many of the genes regulated transcriptionally by T₃ have defined roles in cortex development, from which the role of T₃ can be derived. This review analyzes the specific roles of T₃-regulated genes in the different stages of cortex development within the physiological frame of the developmental changes of thyroid hormones and receptor concentrations in the human cerebral cortex during fetal development. These data indicate an increase in the sensitivity to T₃ during the second trimester of fetal development. The main cellular targets of T₃ appear to be the Cajal-Retzius and the subplate neurons. On the other hand, T₃ regulates transcriptionally genes encoding extracellular matrix proteins, involved in cell migration and the control of diverse signaling pathways.

Implications of the thyroid hormone on neuronal development with special emphasis on the calmodulin-kinase IV pathway

Thyroid hormones influence brain development through regulation of gene expression. This is especially true for Ca²⁺-dependent regulation since a major pathway is controlled by the Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) which in turn is induced by the thyroid hormone T₃. In addition, CaMKIV is involved in regulation of alternative splicing of a number of protein isoforms, among them PMCA1a, the neuronal specific isoform of the plasma membrane calcium pump. On the other hand, hypothyroidism or CaMKIV deficiency can have a severe influence on brain development. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

The Ca(2+)/Calmodulin/CaMKK2 Axis: Nature's Metabolic CaMshaft

Calcium (Ca(2+)) is an essential ligand that binds its primary intracellular receptor calmodulin (CaM) to trigger a variety of downstream processes and pathways. Central to the actions of Ca(2+)/CaM is the activation of a highly conserved Ca(2+)/CaM kinase (CaMK) cascade that amplifies Ca(2+) signals through a series of subsequent phosphorylation events. Proper regulation of Ca(2+) flux is necessary for whole-body metabolism and disruption of Ca(2+) homeostasis has been linked to various metabolic diseases. Here we provide a synthesis of recent advances that highlight the roles of the Ca(2+)/CaMK axis in key metabolic tissues. An appreciation of this information is critical to understanding the mechanisms by which Ca(2+)/CaM-dependent signaling contributes to metabolic homeostasis and disease.

The plethora of PMCA isoforms: Alternative splicing and differential expression

In this review the four different genes of the mammalian plasma membrane calcium ATPase (PMCA) and their spliced isoforms are discussed with respect to their tissue distribution, their differences during development and their importance for regulating Ca²⁺ homeostasis under different conditions. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

A novel crosstalk between calcium/calmodulin kinases II and IV regulates cell proliferation in myeloid leukemia cells

CaMKs link transient increases in intracellular Ca(2+) with biological processes. In myeloid leukemia cells, CaMKII, activated by the bcr-abl oncogene, promotes cell proliferation. Inhibition of CaMKII activity restricts cell proliferation, and correlates with growth arrest and differentiation. The mechanism by which the inhibition of CaMKII results in growth arrest and differentiation in myeloid leukemia cells is still unknown. We report that inhibition of CaMKII activity results in an upregulation of CaMKIV mRNA and protein in leukemia cell lines. Conversely, expression of CaMKIV inhibits autophosphorylation and activation of CaMKII, and elicits G0/G1cell cycle arrest,impairing cell proliferation. Furthermore, U937 cells expressing CaMKIV show elevated levels of Cdk inhibitors p27(kip1) and p16(ink4a) and reduced levels of cyclins A, B1 and D1. These findings were also confirmed in the K562 leukemic cell line. The relationship between CaMKII and CaMKIV is also observed in primary acute myeloid leukemia (AML) cells, and it correlates with their immunophenotypic profile. Indeed, immature MO/M1 AML showed increased CaMKIV expression and decreased pCaMKII, whereas highly differentiated M4/M5 AML showed decreased CaMKIV expression and increased pCaMKII levels. Our data reveal a novel cross-talk between CaMKII and CaMKIV and suggest that CaMKII suppresses the expression of CaMKIV to promote leukemia cell proliferation.

Thyroid hormone regulation of gene expression in primary cerebrocortical cells: role of thyroid hormone receptor subtypes and interactions with retinoic acid and glucocorticoids

The effects of thyroid hormone on brain development and function are largely mediated by the binding of 3,5,3'-triiodo-L-thyronine (T3) to its nuclear receptors (TR) to regulate positively or negatively gene expression. We have analyzed by quantitative polymerase chain reaction the effect of T3 on primary cultured cells from the embryonic mouse cerebral cortex, on the expression of Hr, Klf9, Shh, Dio3, Aldh1a1, and Aldh1a3. In particular we focused on T3 receptor specificity, and on the crosstalk between T3, retinoic acid and dexamethasone. To check for receptor subtype specificity we used cerebrocortical cells derived from wild type mice and from mice deficient in thyroid hormone receptor subtypes. Receptor subtype specificity was found for Dio3 and Aldh1a1, which were induced by T3 only in cells expressing the T3 receptor alpha 1 subtype. Interactions of T3 with retinoic acid signaling through the control of retinoic acid metabolism are likely to be important during development. T3 had opposing influences on retinoic acid synthesizing enzymes, increasing the expression of Aldh1a1, and decreasing Aldh1a3, while increasing the retinoic acid degrading enzyme Cyp26b1. Dexamethasone increased Klf9 and Aldh1a1 expression. The effects of T3 and dexamethasone on Aldh1a1 were highly synergistic, with mRNA increments of up to 20 fold. The results provide new data on thyroid hormone regulation of gene expression and underscore the importance of thyroid hormone interactions with retinoic acid and glucocorticoids during neural development.

Mechanisms of thyroid hormone action

Our understanding of thyroid hormone action has been substantially altered by recent clinical observations of thyroid signaling defects in syndromes of hormone resistance and in a broad range of conditions, including profound mental retardation, obesity, metabolic disorders, and a number of cancers. The mechanism of thyroid hormone action has been informed by these clinical observations as well as by animal models and has influenced the way we view the role of local ligand availability; tissue and cell-specific thyroid hormone transporters, corepressors, and coactivators; thyroid hormone receptor (TR) isoform-specific action; and cross-talk in metabolic regulation and neural development. In some cases, our new understanding has already been translated into therapeutic strategies, especially for treating hyperlipidemia and obesity, and other drugs are in development to treat cardiac disease and cancer and to improve cognitive function.

Ca2+-signaling, alternative splicing and endoplasmic reticulum stress responses

Ca(2+)-signaling, alternative splicing, and stress responses by the endoplasmic reticulum are three important cellular activities which can be strongly interconnected to alter the expression of protein isoforms in a tissue dependent manner or during development depending on the environmental conditions. This integrated network of signaling pathways permits a high degree of versatility and adaptation to metabolic, developmental and stress processes. Defects in its regulation may lead to cellular malfunction.

Thyroid hormone and COUP-TF1 regulate kallikrein-binding protein (KBP) gene expression

Kallikrein-binding protein (KBP) is a component of the kallikrein-kinin system that mediates vasodilation and inhibits tumor growth by antagonizing vascular endothelial growth factor-mediated angiogenesis. We demonstrate that KBP gene expression is repressed by T(3) and modulated by the orphan nuclear receptor, chicken ovalbumin upstream promoter transcription factor 1 (COUP-TF1). In hypothyroid mice, KBP mRNA expression in the testis was increased 2.1-fold compared with euthyroid mice. We have identified two negative thyroid hormone response elements (nTREs) in the mouse KBP gene, nTRE1 located in the 5' flanking region (-53 to -29) and nTRE2, located in the first intron (104-132). We used functional assays, cofactor knockdown, and chromatin immunoprecipitation assays to characterize nTRE1 and nTRE2 in hepatic (HepG2) and testes (GC-1spg) cell lines. Reporter expression directed by both elements was enhanced with addition of thyroid hormone receptor and repressed with the addition of T(3). COUP-TF1 enhanced basal expression of both elements but blunted unliganded thyroid hormone receptor enhancement and T(3) repression of nTRE1 but not nTRE2. Both nTREs bound nuclear corepressor and binding increased in response to T(3). Nuclear corepressor knockdown resulted in loss of T(3) repression of both nTRE1 and nTRE2. COUP-TF1, which usually represses T(3) induction of positive thyroid hormone response elements, reverses T(3) repression mediated by nTRE1 in the mouse KBP gene. Endogenous KBP expression is repressed by T(3) and two functional nTREs, both of which are required, have been characterized in the KBP gene. COUP-TF1 may be an important factor to modulate expression of genes that are repressed by T(3).

Thyroid hormone regulation of gene expression in the developing rat fetal cerebral cortex: prominent role of the Ca2+/calmodulin-dependent protein kinase IV pathway

Thyroid hormones influence brain development through regulation of gene expression mediated by nuclear receptors. Nuclear receptor concentration increases rapidly in the human fetus during the second trimester, a period of high sensitivity of the brain to thyroid hormones. In the rat, the equivalent period is the last quarter of pregnancy. However, little is known about thyroid hormone action in the fetal brain, and in rodents, most thyroid hormone-regulated genes have been identified during the postnatal period. To identify potential targets of thyroid hormone in the fetal brain, we induced maternal and fetal hypothyroidism by maternal thyroidectomy followed by antithyroid drug (2-mercapto-1-methylimidazole) treatment. Microarray analysis identified differentially expressed genes in the cerebral cortex of hypothyroid fetuses on d 21 after conception. Gene function analysis revealed genes involved in the biogenesis of the cytoskeleton, neuronal migration and growth, and branching of neurites. Twenty percent of the differentially expressed genes were related to each other centered on the Ca(2+) and calmodulin-activated kinase (Camk4) pathway. Camk4 was regulated directly by T(3) in primary cultured neurons from fetal cortex, and the Camk4 protein was also induced by thyroid hormone. No differentially expressed genes were recovered when euthyroid fetuses from hypothyroid mothers were compared with fetuses from normal mothers. Although the results do not rule out a specific contribution from the mother, especially at earlier stages of pregnancy, they indicate that the main regulators of thyroid hormone-dependent, fetal brain gene expression near term are the fetal thyroid hormones.

Repression of Ca2+/calmodulin-dependent protein kinase IV signaling accelerates retinoic acid-induced differentiation of human neuroblastoma cells

Neuroblastoma cells having stem cell-like qualities are widely employed models for the study of neural stem/progenitor cell proliferation and differentiation. We find that human BE(2)C neuroblastoma cells possess a signaling cascade initiated by Ca(2+) influx via voltage-dependent calcium channels and the N-methyl-D-aspartate (NMDA) receptor and culminating in nuclear calmodulin-dependent protein kinase IV (CaMKIV)-mediated phosphorylation and activation of the transcription factors Ca(2+)/cyclic AMP-response element-binding protein (CREB) and ATF1 (activating transcription factor-1). This pathway functions to maintain BE(2)C cells in an undifferentiated, proliferative state. Parallel to this Ca(2+)-dependent pathway is a hormone-responsive program by which retinoic acid (RA) initiates the differentiation of BE(2)C cells toward a neuronal lineage. This is evidenced by RA-dependent induction of the cell cycle inhibitor p21/Cip1 (Cdk-interacting protein 1) and cell cycle arrest, induction of the neuroblastic marker doublecortin and of the neuron-specific intermediate filament protein, peripherin, and by RA-stimulated extension of neuritic processes. During neuronal differentiation there is a complex antagonistic interplay between these two major signaling pathways. RA down-regulates expression of CaMKIV and one of its upstream activators, CaMKK1 (calmodulin-dependent protein kinase kinase 1). This is accompanied by RA-induced suppression of activating phosphorylation of CREB with a time course paralleling that of CaMKIV down-regulation. RA-induced repression of the Ca(2+)/calmodulin-dependent protein kinase kinase/CaMKIV/CREB pathway appears to be involved in regulating the timing of neuronal differentiation, as shown by the effect of RNA interference of CaMKIV to markedly accelerate RA-dependent up-regulation of p21/Cip1 and doublecortin expression and RA-promoted neurite outgrowth. RA-induced repression of the CaMKIV signaling pathway may represent an early event in retinoid-dependent neuronal differentiation.

Thyroid hormone and cerebellar development

Thyroid hormone (TH) plays a key role in mammalian brain development. The developing brain is sensitive to both TH deficiency and excess. Brain development in the absence of TH results in motor skill deficiencies and reduced intellectual development. These functional abnormalities can be attributed to maldevelopment of specific cell types and regions of the brain including the cerebellum. TH functions at the molecular level by regulating gene transcription. Therefore, understanding how TH regulates cerebellar development requires identification of TH-regulated gene targets and the cells expressing these genes. Additionally, the process of TH-dependent regulation of gene expression is tightly controlled by mechanisms including regulation of TH transport, TH metabolism, toxicologic inhibition of TH signaling, and control of the nuclear TH response apparatus. This review will describe the functional, cellular, and molecular effects of TH deficit in the developing cerebellum and emphasize the most recent findings regarding TH action in this important brain region.

The influence of calcium signaling on the regulation of alternative splicing

In this review the influence of calcium signaling on the regulation of alternative splicing is discussed with respect to its influence on cell- and developmental-specific expression of different isoforms of the plasma membrane calcium pump (PMCA). In a second part the possibility is discussed that due to the interaction of the calcium-binding protein ALG-2 with a spliceosomal regulator of alternative splicing, RBM22, Ca2+-signaling may thus influence its regulatory property.

Thyroid hormone-dependent gene expression in differentiated embryonic stem cells and embryonal carcinoma cells: identification of novel thyroid hormone target genes by deoxyribonucleic acid microarray analysis

T3 is required for normal early development, but relatively few T3-responsive target genes have been identified. In general, in vitro stem cell differentiation techniques stimulate a wide range of developmental programs, including thyroid hormone receptor (TR) pathways. We developed several in vitro stem cell models to more specifically identify TR-mediated gene expression in early development. We found that embryonic carcinoma (EC) cells have reduced T3 nuclear binding capacity and only modestly express the known T3 target genes, neurogranin (RC3) and Ca2+/calmodulin-dependent protein kinase IV (CaMKIV), in response to T3. Full T3 induction in transient transfection of EC cells was restored with cotransfection of a TR expression vector. We, therefore, performed gene expression profiles in wild-type embryonic stem (ES) cells compared with expression in cells with deficient (EC) or mutant TR (TRalpha P398H mutant ES cells), to identify T3 target genes. T3 stimulation of wild-type ES cells altered mRNA expression of 610 known genes (26% of those studied), although only approximately 60 genes (1%) met criteria for direct T3 stimulation based on the magnitude of induction and requirement for the presence of TR. We selected five candidate T3 target genes, neurexophilin 2, spermatid perinuclear RNA-binding protein (SPNR), kallikrein-binding protein (KBP), prostate-specific membrane antigen (PSMA), and synaptotagmin II, for more detailed study. T3 responsiveness of these genes was evaluated in both in vitro endogenous gene expression and in vivo mouse model systems. These genes identified in a novel stem cell system, including those induced and repressed in response to T3, may mediate thyroid hormone actions in early development.

Control of thyroid hormone action in the developing rat brain

Thyroid hormones play important roles in brain development. The physiologic function of thyroid hormones in the developing brain is to provide a timing signal that leads to the induction of differentiation and maturation programs during precise stages of development. Inappropriate initiation of these timing events leads to asynchrony in developmental processes and a deleterious outcome. The developing brain is protected from premature thyroid hormone signaling through a variety of measures. Firstly, local brain levels of both thyroxine and triiodothyronine are controlled by ontogenically regulated patterns of production and metabolism. Secondly, developmentally regulated expression of nuclear proteins involved with the nuclear TH response apparatus control the temporal response of brain genes to thyroid hormone. Finally, developmental regulation of TH action modulating transcription factor expression also controls TH action in the developing brain. Together these molecular mechanisms cooperatively act to temporally control TH action during brain development. A description of these controlling mechanisms is the subject of this review.