Reactive Bergmann glia play a central role in spinocerebellar ataxia inflammation via the JNK pathway.

The spinocerebellar ataxias (SCAs) are devastating neurological diseases characterized by progressive cerebellar incoordination. While neurons bear the brunt of the pathology, a growing body of evidence suggests that glial cells are also affected. It has, however, been difficult to understand the role of glia, given the diversity of subtypes, each with their individual contributions to neuronal health. Using human SCA autopsy samples we have discovered that Bergmann glia-the radial glia of the cerebellum, which form intimate functional connections with cerebellar Purkinje neurons-display inflammatory JNK-dependent c-Jun phosphorylation. This phosphorylation defines a signaling pathway not observed in other activated glial populations, providing an opportunity to isolate the role of Bergmann glia in SCA inflammation. Turning to an SCA1 mouse model as a paradigmatic SCA, we demonstrate that inhibiting the JNK pathway reduces Bergmann glia inflammation accompanied by improvements in the SCA1 phenotype both behaviorally and pathologically. These findings demonstrate the causal role for Bergmann glia inflammation in SCA1 and point to a novel therapeutic strategy that could span several ataxic syndromes where Bergmann glia inflammation is a major feature.

The L1 cell adhesion molecule constrains dendritic spine density in pyramidal neurons of the mouse cerebral cortex.

A novel function for the L1 cell adhesion molecule, which binds the actin adaptor protein Ankyrin was identified in constraining dendritic spine density on pyramidal neurons in the mouse neocortex. In an L1-null mouse mutant increased spine density was observed on apical but not basal dendrites of pyramidal neurons in diverse cortical areas (prefrontal cortex layer 2/3, motor cortex layer 5, visual cortex layer 4. The Ankyrin binding motif (FIGQY) in the L1 cytoplasmic domain was critical for spine regulation, as demonstrated by increased spine density and altered spine morphology in the prefrontal cortex of a mouse knock-in mutant (L1YH) harboring a tyrosine (Y) to histidine (H) mutation in the FIGQY motif, which disrupted L1-Ankyrin association. This mutation is a known variant in the human L1 syndrome of intellectual disability. L1 was localized by immunofluorescence staining to spine heads and dendrites of cortical pyramidal neurons. L1 coimmunoprecipitated with Ankyrin B (220 kDa isoform) from lysates of wild type but not L1YH forebrain. This study provides insight into the molecular mechanism of spine regulation and underscores the potential for this adhesion molecule to regulate cognitive and other L1-related functions that are abnormal in the L1 syndrome.

Semaphorin3F Drives Dendritic Spine Pruning Through Rho-GTPase Signaling.

Dendritic spines of cortical pyramidal neurons are initially overproduced then remodeled substantially in the adolescent brain to achieve appropriate excitatory balance in mature circuits. Here we investigated the molecular mechanism of developmental spine pruning by Semaphorin 3F (Sema3F) and its holoreceptor complex, which consists of immunoglobulin-class adhesion molecule NrCAM, Neuropilin-2 (Npn2), and PlexinA3 (PlexA3) signaling subunits. Structure-function studies of the NrCAM-Npn2 interface showed that NrCAM stabilizes binding between Npn2 and PlexA3 necessary for Sema3F-induced spine pruning. Using a mouse neuronal culture system, we identified a dual signaling pathway for Sema3F-induced pruning, which involves activation of Tiam1-Rac1-PAK1-3 -LIMK1/2-Cofilin1 and RhoA-ROCK1/2-Myosin II in dendritic spines. Inhibitors of actin remodeling impaired spine collapse in the cortical neurons. Elucidation of these pathways expands our understanding of critical events that sculpt neuronal networks and may provide insight into how interruptions to these pathways could lead to spine dysgenesis in diseases such as autism, bipolar disorder, and schizophrenia.

Developmental Regulation of Basket Interneuron Synapses and Behavior through NCAM in Mouse Prefrontal Cortex.

Parvalbumin (PV)-expressing basket interneurons in the prefrontal cortex (PFC) regulate pyramidal cell firing, synchrony, and network oscillations. Yet, it is unclear how their perisomatic inputs to pyramidal neurons are integrated into neural circuitry and adjusted postnatally. Neural cell adhesion molecule NCAM is expressed in a variety of cells in the PFC and cooperates with EphrinA/EphAs to regulate inhibitory synapse density. Here, analysis of a novel parvalbumin (PV)-Cre: NCAM F/F mouse mutant revealed that NCAM functions presynaptically in PV+ basket interneurons to regulate postnatal elimination of perisomatic synapses. Mutant mice exhibited an increased density of PV+ perisomatic puncta in PFC layer 2/3, while live imaging in mutant brain slices revealed fewer puncta that were dynamically eliminated. Furthermore, EphrinA5-induced growth cone collapse in PV+ interneurons in culture depended on NCAM expression. Electrophysiological recording from layer 2/3 pyramidal cells in mutant PFC slices showed a slower rise time of inhibitory synaptic currents. PV-Cre: NCAM F/F mice exhibited impairments in working memory and social behavior that may be impacted by altered PFC circuitry. These findings suggest that the density of perisomatic synapses of PV+ basket interneurons is regulated postnatally by NCAM, likely through EphrinA-dependent elimination, which is important for appropriate PFC network function and behavior.

Detection of DNA Double-Strand Breaks Using Pulsed-Field Gel Electrophoresis.

DNA is one of the most biologically important targets of exogenous and endogenous toxicants as well as carcinogens. Damage to DNA can be of different types (e.g., DNA adducts, DNA protein cross-links, single-strand breaks, oxidized bases, abasic sites, and double-strand breaks (DSBs)). DSBs are considered the most lethal form of DNA damage for eukaryotic cells, and if left unrepaired or misrepaired, can cause cell death, chromosome instability, and cancer. DSBs can arise in the cells through different sources and can be distinguished as endogenous or exogenous DSBs. Exogenous sources can be chemotherapeutic drugs, irradiation, and environmental chemicals. The endogenous causes of DNA DSBs in the cells are mainly reactive oxygen species and faulty repair of oxidative clustered DNA lesions. Qualitative and quantitative analysis of DNA DSBs is of utmost importance to understand physiologically relevant cellular processes as well as to investigate the genotoxic or clastogenic effects of toxicants. Pulsed-field gel electrophoresis (PFGE) is a widely used method for direct quantification of DNA DSBs. In this method, the cells exposed to DSB-inducing agents are embedded in the agarose blocks and lysed. These agarose blocks containing DNA are then run under multiple electric fields which are at 120° angle, to aid in the movement of large DNA strands. It gives a direct and specific measure of DSBs unlike the foci-based assays. This chapter provides a brief overview of the various commonly used approaches to analyze DNA DSBs and describes the theory, advantages and method of PFGE, for use in cells exposed to DNA DSB inducing agents.

Close Homolog of L1 Regulates Dendritic Spine Density in the Mouse Cerebral Cortex Through Semaphorin 3B.

Dendritic spines in the developing mammalian neocortex are initially overproduced and then eliminated during adolescence to achieve appropriate levels of excitation in mature networks. We show here that the L1 family cell adhesion molecule Close Homolog of L1 (CHL1) and secreted repellent ligand Semaphorin 3B (Sema3B) function together to induce dendritic spine pruning in developing cortical pyramidal neurons. Loss of CHL1 in null mutant mice in both genders resulted in increased spine density and a greater proportion of immature spines on apical dendrites in the prefrontal and visual cortex. Electron microscopy showed that excitatory spine synapses with postsynaptic densities were increased in the CHL1-null cortex, and electrophysiological recording in prefrontal slices from mutant mice revealed deficiencies in excitatory synaptic transmission. Mechanistically, Sema3B protein induced elimination of spines on apical dendrites of cortical neurons cultured from wild-type but not CHL1-null embryos. Sema3B was secreted by the cortical neuron cultures, and its levels increased when cells were treated with the GABA antagonist gabazine. In vivo CHL1 was coexpressed with Sema3B in pyramidal neuron subpopulations and formed a complex with Sema3B receptor subunits Neuropilin-2 and PlexinA4. CHL1 and NrCAM, a closely related L1 adhesion molecule, localized primarily to distinct spines and promoted spine elimination to Sema3B or Sema3F, respectively. These results support a new concept in which selective spine elimination is achieved through different secreted semaphorins and L1 family adhesion molecules to sculpt functional neural circuits during postnatal maturation.SIGNIFICANCE STATEMENT Dendritic spines in the mammalian neocortex are initially overproduced and then pruned in adolescent life through unclear mechanisms to sculpt maturing cortical circuits. Here, we show that spine and excitatory synapse density of pyramidal neurons in the developing neocortex is regulated by the L1 adhesion molecule, Close Homolog of L1 (CHL1). CHL1 mediated spine pruning in response to the secreted repellent ligand Semaphorin 3B and associated with receptor subunits Neuropilin-2 and PlexinA4. CHL1 and related L1 adhesion molecule NrCAM localized to distinct spines, and promoted spine elimination to Semaphorin 3B and -3F, respectively. These results support a new concept in which selective elimination of individual spines and nascent synapses can be achieved through the action of distinct secreted semaphorins and L1 adhesion molecules.

Expression and Function of Neuron-Glia-Related Cell Adhesion Molecule (NrCAM) in the Amygdalar Pathway.

Neuron-Glia related cell adhesion molecule (NrCAM) is a candidate autism risk factor that promotes axon guidance through cytoskeletal linkages in developing brain but its role in limbic circuitry has not been investigated. In situ hybridization (ISH) and immunofluorescence staining showed that NrCAM is expressed in the developing amygdalar pathway of mouse embryos during outgrowth of projections in the stria terminalis, a major limbic tract that interconnects the central amygdala (CeA) with key targets in the bed nucleus of the stria terminalis (BNST). Analysis of fiber tracts in NrCAM mutant mice by Neurofilament protein immunohistochemistry showed pronounced defasciculation and misprojection of fibers in the ST. The defasciculation phenotype may result from impairment in NrCAM homophilic inter-axonal adhesion or axon repulsion from the secreted ligand Semaphorin 3F, which is expressed in limbic areas in proximity to the ST. Behavioral testing indicated that NrCAM null mice were impaired in context-dependent fear conditioning, in accord with altered amygdala-BNST connectivity, but displayed normal cued (tone-shock) conditioning. Results are consistent with the novel finding that NrCAM mediates fasciculation of axon fibers in the ST important for proper amygdalar-BNST circuitry and response to contextual fear conditioning.

Temporal Regulation of Dendritic Spines Through NrCAM-Semaphorin3F Receptor Signaling in Developing Cortical Pyramidal Neurons.

Neuron-glial related cell adhesion molecule NrCAM is a newly identified negative regulator of spine density that genetically interacts with Semaphorin3F (Sema3F), and is implicated in autism spectrum disorders (ASD). To investigate a role for NrCAM in spine pruning during the critical adolescent period when networks are established, we generated novel conditional, inducible NrCAM mutant mice (Nex1Cre-ERT2: NrCAMflox/flox). We demonstrate that NrCAM functions cell autonomously during adolescence in pyramidal neurons to restrict spine density in the visual (V1) and medial frontal cortex (MFC). Guided by molecular modeling, we found that NrCAM promoted clustering of the Sema3F holoreceptor complex by interfacing with Neuropilin-2 (Npn2) and PDZ scaffold protein SAP102. NrCAM-induced receptor clustering stimulated the Rap-GAP activity of PlexinA3 (PlexA3) within the holoreceptor complex, which in turn, inhibited Rap1-GTPase and inactivated adhesive ß1 integrins, essential for Sema3F-induced spine pruning. These results define a developmental function for NrCAM in Sema3F receptor signaling that limits dendritic spine density on cortical pyramidal neurons during adolescence.

Neurocan Inhibits Semaphorin 3F Induced Dendritic Spine Remodeling Through NrCAM in Cortical Neurons.

Neurocan is a chondroitin sulfate proteoglycan present in perineuronal nets, which are associated with closure of the critical period of synaptic plasticity. During postnatal development of the neocortex dendritic spines on pyramidal neurons are initially overproduced; later they are pruned to achieve an appropriate balance of excitatory to inhibitory synapses. Little is understood about how spine pruning is terminated upon maturation. NrCAM (Neuron-glial related cell adhesion molecule) was found to mediate spine pruning as a subunit of the receptor complex for the repellent ligand Semaphorin 3F (Sema3F). As shown here in the postnatal mouse frontal and visual neocortex, Neurocan was localized at both light and electron microscopic level to the cell surface of cortical pyramidal neurons and was adjacent to neuronal processes and dendritic spines. Sema3F-induced spine elimination was inhibited by Neurocan in cortical neuron cultures. Neurocan also blocked Sema3F-induced morphological retraction in COS-7 cells, which was mediated through NrCAM and other subunits of the Sema3F holoreceptor, Neuropilin-2, and PlexinA3. Cell binding and ELISA assays demonstrated an association of Neurocan with NrCAM. Glycosaminoglycan chain interactions of Neurocan were required for inhibition of Sema3F-induced spine elimination, but the C-terminal sushi domain was dispensable. These results describe a novel mechanism wherein Neurocan inhibits NrCAM/Sema3F-induced spine elimination.

Perineuronal Net Protein Neurocan Inhibits NCAM/EphA3 Repellent Signaling in GABAergic Interneurons.

Perineuronal nets (PNNs) are implicated in closure of critical periods of synaptic plasticity in the brain, but the molecular mechanisms by which PNNs regulate synapse development are obscure. A receptor complex of NCAM and EphA3 mediates postnatal remodeling of inhibitory perisomatic synapses of GABAergic interneurons onto pyramidal cells in the mouse frontal cortex necessary for excitatory/inhibitory balance. Here it is shown that enzymatic removal of PNN glycosaminoglycan chains decreased the density of GABAergic perisomatic synapses in mouse organotypic cortical slice cultures. Neurocan, a key component of PNNs, was expressed in postnatal frontal cortex in apposition to perisomatic synapses of parvalbumin-positive interneurons. Polysialylated NCAM (PSA-NCAM), which is required for ephrin-dependent synapse remodeling, bound less efficiently to neurocan than mature, non-PSA-NCAM. Neurocan bound the non-polysialylated form of NCAM at the EphA3 binding site within the immunoglobulin-2 domain. Neurocan inhibited NCAM/EphA3 association, membrane clustering of NCAM/EphA3 in cortical interneuron axons, EphA3 kinase activation, and ephrin-A5-induced growth cone collapse. These studies delineate a novel mechanism wherein neurocan inhibits NCAM/EphA3 signaling and axonal repulsion, which may terminate postnatal remodeling of interneuron axons to stabilize perisomatic synapses in vivo.

Histone deacetylase inhibition reduces hypothyroidism-induced neurodevelopmental defects in rats.

Thyroid hormone (TH) through its receptor (TRα/ß) influences spatio-temporal regulation of its target gene repertoire during brain development. Though hypothyroidism in WT rodent models of perinatal hypothyroidism severely impairs neurodevelopment, its effect on TRα/ß knockout mice is less severe. An explanation to this paradox is attributed to a possible repressive action of unliganded TRs during development. Since unliganded TRs suppress gene expression through the recruitment of histone deacetylase (HDACs) via co-repressor complexes, we tested whether pharmacological inhibition of HDACs may prevent the effects of hypothyroidism on brain development. Using valproate, an HDAC inhibitor, we show that HDAC inhibition significantly blocks the deleterious effects of hypothyroidism on rat cerebellum, evident by recovery of TH target genes like Bdnf, Pcp2 and Mbp as well as improved dendritic structure of cerebellar Purkinje neurons. Together with this, HDAC inhibition also rescues hypothyroidism-induced motor and cognitive defects. This study therefore provides an insight into the role of HDACs in TH insufficiency during neurodevelopment and their inhibition as a possible therapeutics for treatment.

Neural cell adhesion molecule NrCAM regulates Semaphorin 3F-induced dendritic spine remodeling.

Neuron-glial related cell adhesion molecule (NrCAM) is a regulator of axon growth and repellent guidance, and has been implicated in autism spectrum disorders. Here a novel postsynaptic role for NrCAM in Semaphorin3F (Sema3F)-induced dendritic spine remodeling was identified in pyramidal neurons of the primary visual cortex (V1). NrCAM localized to dendritic spines of star pyramidal cells in postnatal V1, where it was coexpressed with Sema3F. NrCAM deletion in mice resulted in elevated spine densities on apical dendrites of star pyramidal cells at both postnatal and adult stages, and electron microscopy revealed increased numbers of asymmetric synapses in layer 4 of V1. Whole-cell recordings in cortical slices from NrCAM-null mice revealed increased frequency of mEPSCs in star pyramidal neurons. Recombinant Sema3F-Fc protein induced spine retraction on apical dendrites of wild-type, but not NrCAM-null cortical neurons in culture, while re-expression of NrCAM rescued the spine retraction response. NrCAM formed a complex in brain with Sema3F receptor subunits Neuropilin-2 (Npn-2) and PlexinA3 (PlexA3) through an Npn-2-binding sequence (TARNER) in the extracellular Ig1 domain. A trans heterozygous genetic interaction test demonstrated that Sema3F and NrCAM pathways interacted in vivo to regulate spine density in star pyramidal neurons. These findings reveal NrCAM as a novel postnatal regulator of dendritic spine density in cortical pyramidal neurons, and an integral component of the Sema3F receptor complex. The results implicate NrCAM as a contributor to excitatory/inhibitory balance in neocortical circuits.

Iodine plus n-3 fatty acid supplementation augments rescue of postnatal neuronal abnormalities in iodine-deficient rat cerebellum.

High prevalence of hypothyroxinaemia in iodine-deficient (ID) mothers has serious implications for mental health of the progeny. Independent supplementation of iodine and n-3 fatty acids (FA) markedly improves growth and cognitive performance of school children. Discerning effects of n-3 FA and iodine on the developing cerebellum have not been ascertained. The present study investigates effects of these two micronutrients separately as well as together in an ID rat model. We studied the effects of these micronutrients on progeny of ID dams by instituting the following supplementation diets: (1) low-iodine diet (LID), (2) LID+potassium iodide (KI), (3) LID+n-3 FA and (4) LID+KI+n-3 FA. Pups were investigated for morphological and biochemical parameters at the peak of cerebellar histogenesis on postnatal day (P) 16 and for neurobehavioural as well as motor coordination parameters at P40. Results indicate that n-3 FA alone, without improvement in circulating thyroid hormone (TH), significantly improves functional, morphological and biochemical indices of the developing cerebellum. Further, results show that co-supplementation with iodine and n-3 FA rescues not only the loss of neurotrophic support, but also salvages motor coordination, memory and learning. This additive effect results in significantly improving neurotrophic support and seems to be mediated by parallel significant increase in TH receptor (TR)α and normalisation of TRß, retinoic orphan receptor α and p75 neurotrophin receptor, as well as noteworthy prevention of apoptotic cell death and strengthening of anti-oxidative defence. The overall results indicate important mitigating role that n-3 FA may play in enhancing TH nuclear receptor-mediated signalling in the developing cerebellum.

Calcitriol [1, 25[OH]2 D3] pre- and post-treatment suppresses inflammatory response to influenza A (H1N1) infection in human lung A549 epithelial cells.

PURPOSE: Influenza viruses infect airway epithelial cells, causing respiratory distress. Immune defense is maintained by chemokine/cytokine secretions from airway epithelial cells. While moderate inflammatory response protects from ill effects, hyper-inflammatory response promotes the pathogenesis. High circulating levels of vitamin D are known to mitigate effects of infectious diseases, including respiratory infectious diseases. The question whether and how vitamin D treatment pre-/post-viral exposure modulates inflammatory response is not clear. The present study was undertaken to understand autophagy/apoptosis balance and chemokine/cytokine response to influenza A (H1N1) infection by pre- and post-1, 25-dihydroxyvitamin D3 (1,25[OH]2 D3)[calcitriol] treatment of human lung A549 epithelial cells. METHODS: Influenza A (H1N1) virus was propagated in A549 cell line, titrated using hemagglutination assay, and was used to assess effect of calcitriol. After confirming that 100 nM of calcitriol fails to clear virus, A549 cells were either pre-treated (16 h) with 100 nM or post-treated with 30 nM of 1,25[OH]2 D3 of virus inoculation (1 h). Cells after incubation at 37 °C under 5 % CO2 for 48 h were collected and subjected to RNA and protein extraction. Measurements of viability, influenza M protein, and molecular parameters of cell death and inflammatory response were performed. RESULTS: We report that treatment of these cells with 100/30 nM of 1,25[OH]2 D3 prior to/or post-H1N1 exposure does not affect viral clearance but significantly reduces autophagy and restores increased apoptosis seen on H1N1 infection back to its constitutive level. However, it significantly decreases the levels of H1N1-induced TNF-α (tumor necrosis factor-alpha), IFN-ß (interferon-beta), and IFN-stimulated gene-15 (ISG15). 1,25[OH]2 D3 treatment prior to/or post-H1N1 infection significantly down-regulates IL-8 as well as IL-6 RNA levels. These results demonstrate that calcitriol treatment suppresses the H1N1-induced transcription of the chemokines RANTES and IL-8 in epithelial cells. CONCLUSION: The findings provide support for the initiation of vitamin D supplementation program to VDD populations in reducing the severity of influenza.

Maternal thyroid hormone deficiency affects the fetal neocorticogenesis by reducing the proliferating pool, rate of neurogenesis and indirect neurogenesis.

Neuronal progenitor cell proliferation and their optimum number are indispensable for neurogenesis, which is determined by cell cycle length and cell cycle quitting rate of the dividing progenitors. These processes are tightly orchestrated by transcription factors like Tbr2, Pax6, and E2f-1. Radial glia and intermediate progenitor cells (IPC) through direct and indirect neurogenesis maintain surface area and neocortical thickness during development. Here we show that fetal neurogenesis is maternal thyroid hormone (MTH) dependent with differential effect on direct and indirect neurogenesis. MTH deficiency (MTHD) impairs direct neurogenesis through initial down-regulation of Pax6 and diminished progenitor pool with recovery even before the onset of fetal thyroid function (FTF). However, persistent decrease in Tbr2 positive IPCs, diminished NeuN positivity in layers I-III of neocortex, and reduced cortical thickness indicate a non-compensatory impairment in indirect neurogenesis. TH deficiency causes disrupted cell cycle kinetics and deranged neurogenesis. It specifically affects indirect neurogenesis governed by intermediate progenitor cells (IPCs). TH replacement in hypothyroid dams partially restored the rate of neurogenesis in the fetal neocortex. Taken together we describe a novel role of maternal TH in promoting IPCs derived neuronal differentiation in developing neo-cortex. We have also shown for the first time that ventricular zone progenitors are TH responsive as they express its receptor, TR alpha-1, transporters (MCT8) and deiodinases. This study highlights the importance of maternal thyroid hormone (TH) even before the start of the fetal thyroid function.

Prenatal iodine deficiency results in structurally and functionally immature lungs in neonatal rats.

Maternal hypothyroidism affects postnatal lung structure. High prevalence of hypothyroxinemia (low T4, normal T3) in iodine-deficient pregnant women and associated risk for neuropsychological development along with high infant/neonatal mortality ascribed to respiratory distress prompted us to study the effects of maternal hypothyroxinemia on postnatal lung development. Female Sprague Dawley rats were given a low-iodine diet (LID) with 1% KClO(4) in drinking water for 10 days, to minimize thyroid hormone differences. Half of these rats were continued on iodine-deficient diet; ID (LID with 0.005% KClO(4)) for 3 mo, whereas the rest were switched to an iodine-sufficient diet; IS [LID + potassium iodide (10 µg iodine/20 g of diet + normal drinking water)]. Pups born to ID mothers were compared with age-matched pups from IS mothers at postnatal days 8 (P8) and 16 (P16) (n = 6-8/group). ID pups had normal circulating T3 but significantly low T4 levels (P < 0.05) and concomitantly approximately sixfold higher thyroid hormone receptor-ß mRNA in alveolar epithelium. Lung histology revealed larger and irregularly shaped alveoli in ID pups relative to controls. Lung function was assessed at P16 using a double-chambered plethysmograph and observed reduced tidal volume, peak inspiratory and expiratory flow, and dynamic lung compliance in ID pups compared with IS pups. Significant lowering of surfactant protein (SP)-B and SP-C mRNA and protein found in ID pups at P16. ID pups had 16-fold lower matrix metalloproteinase-9 mRNA levels in their alveolar epithelium. In addition, mRNA levels of thyroid transcription factor-1 and SP-D were significantly higher (3-fold) compared with IS pups. At P16, significantly lower levels of SP-B and SP-C found in ID pups may be responsible for immature lung development and reduced lung compliance. Our data suggest that maternal hypothyroxinemia may result in the development of immature lungs that, through respiratory distress, could contribute to the observed high infant mortality in ID neonates.

Maternal thyroid hormone before the onset of fetal thyroid function regulates reelin and downstream signaling cascade affecting neocortical neuronal migration.

Though aberrant neuronal migration in response to maternal thyroid hormone (TH) deficiency before the onset of fetal thyroid function (embryonic day [E] 17.5) in rat cerebral cortex has been described, molecular events mediating morphogenic actions have remained elusive. To investigate the effect of maternal TH deficiency on neocortical development, rat dams were maintained on methimazole from gestational day 6 until sacrifice. Decreased number and length of radial glia, loss of neuronal bipolarity, and impaired neuronal migration were correctible with early (E13-15) TH replacement. Reelin downregulation under hypothyroidism is neither due to enhanced apoptosis in Cajal-Retzius cells nor mediated through brain-derived neurotrophic factor-tyrosine receptor kinase B alterations. Results based on gel shift and chromatin immunoprecipitation assays show the transcriptional control of reelin by TH through the presence of intronic TH response element. Furthermore, hypothyroidism significantly increased TH receptor α1 with decreased reelin, apolipoprotein E receptor 2, very low-density lipoprotein receptor expression, and activation of cytosolic adapter protein disabled 1 that compromised the reelin signaling. Integrins (α(v) and ß1) are significantly decreased without alteration of α3 indicating intact neuroglial recognition but disrupted adhesion and glial end-feet attachment. Results provide mechanistic basis of essentiality of adequate maternal TH levels to ensue proper fetal neocortical cytoarchitecture and importance of early thyroxine replacement.

Effect of hypothyroxinemia on thyroid hormone responsiveness and action during rat postnatal neocortical development.

Neurological deficits due to maternal and neonatal hypothyroxinemia under mild-moderate iodine deficiency are a major preventable health problem worldwide. The present study assesses the impact of hypothyroxinemia on postnatal neocortical development and also compares it to the known effects of severe hypothyroidism. Our results strongly suggest that even within elevated circulating triiodothyronine (T3) levels, hypothyroxinemia significantly impairs thyroid hormone responsiveness in developing rat neocortex. The significant compensatory alteration in deiodinase levels with unaltered monocarboxylate transporter 8 (MCT8) and thyroid hormone receptors (TRs), although found to be similar in hypothyroxinemic and hypothyroid condition, is more pronounced under later condition. The resultant downregulation of nuclear myelin binding protein (MBP) and mitochondrial transcripts Cytochrome oxidase III (Cox III) as well as significantly enhanced mitochondrial localization of Bax and reduced Bcl-2 and Bcl-xL accompanied by enhanced release of Cytochrome c and Smac with activation of caspase-3 indicates pronounced apoptosis leading to compromised cellular survival. The similarities of this responsiveness albeit with difference in degree under hypothyroidism and hypothyroxinemic state with adequate availability of T3 are suggestive of an independent role of thyroxine in neocortex development. Taken together, this study brings forth the neurophysiological aspects of hypothyroxinemia and underscores the importance of adequate iodine nutrition along with mandatory thyroxin monitoring during pregnancy and after birth.

Evidence of a bigenomic regulation of mitochondrial gene expression by thyroid hormone during rat brain development.

Hypothyroidism during early mammalian brain development is associated with decreased expression of various mitochondrial encoded genes along with evidence for mitochondrial dysfunction. However, in-spite of the similarities between neurological disorders caused by perinatal hypothyroidism and those caused by various genetic mitochondrial defects we still do not know as to how thyroid hormone (TH) regulates mitochondrial transcription during development and whether this regulation by TH is nuclear mediated or through mitochondrial TH receptors? We here in rat cerebellum show that hypothyroidism causes reduction in expression of nuclear encoded genes controlling mitochondrial biogenesis like PGC-1alpha, NRF-1alpha and Tfam. Also, we for the first time demonstrate a mitochondrial localization of thyroid hormone receptor (mTR) isoform in developing brain capable of binding a TH response element (DR2) present in D-loop region of mitochondrial DNA. These results thus indicate an integrated nuclear-mitochondrial cross talk in regulation of mitochondrial transcription by TH during brain development.