Cerebellar astrocytes treated by thyroid hormone modulate neuronal proliferation

Synaptic Plasticity and Cognitive Ability in Experimental Adult-Onset Hypothyroidism

Adult-onset hypothyroidism impairs normal brain function. Research on animal models of hypothyroidism has revealed critical information on how deficiency of thyroid hormones impacts the electrophysiological and molecular functions of the brain, which leads to the well known cognitive impairment in untreated hypothyroid patients. Currently, such information can only be obtained from experiments on animal models of hypothyroidism. This review summarizes important research findings that pertain to understanding the clinical cognitive consequences of hypothyroidism, which will provide a better guiding path for therapy of hypothyroidism. SIGNIFICANCE STATEMENT: Cognitive impairment occurs during adult-onset hypothyroidism in both humans and animal models. Findings from animal studies validate clinical findings showing impaired long-term potentiation, decreased CaMKII, and increased calcineurin. Such findings can only be gleaned from animal experiments to show how hypothyroidism produces clinical symptoms.

Thyroid hormone action in adult neurogliogenic niches: the known and unknown

Over the last decades, thyroid hormones (THs) signaling has been established as a key signaling cue for the proper maintenance of brain functions in adult mammals, including humans. One of the most fascinating roles of THs in the mature mammalian brain is their ability to regulate adult neurogliogenic processes. In this respect, THs control the generation of new neuronal and glial progenitors from neural stem cells (NSCs) as well as their final differentiation and maturation programs. In this review, we summarize current knowledge on the cellular organization of adult rodent neurogliogenic niches encompassing well-established niches in the subventricular zone (SVZ) lining the lateral ventricles, the hippocampal subgranular zone (SGZ), and the hypothalamus, but also less characterized niches in the striatum and the cerebral cortex. We then discuss critical questions regarding how THs availability is regulated in the respective niches in rodents and larger mammals as well as how modulating THs availability in those niches interferes with lineage decision and progression at the molecular, cellular, and functional levels. Based on those alterations, we explore the novel therapeutic avenues aiming at harnessing THs regulatory influences on neurogliogenic output to stimulate repair processes by influencing the generation of either new neurons (i.e. Alzheimer's, Parkinson's diseases), oligodendrocytes (multiple sclerosis) or both (stroke). Finally, we point out future challenges, which will shape research in this exciting field in the upcoming years.

Thyroid hormone biosynthesis and its role in brain development and maintenance

Thyroid hormones are critical modulators in the physiological processes necessary to virtually all tissues, with exceptionally fundamental roles in brain development and maintenance. These hormones regulate essential neurodevelopment events, including neuronal migration, synaptogenesis, and myelination. Additionally, thyroid hormones are crucial for maintaining brain homeostasis and cognitive function in adulthood. This chapter aims to offer a comprehensive understanding of thyroid hormone biosynthesis and its intricate role in brain physiology. Here, we described the mechanisms underlying the biosynthesis of thyroid hormones, their influence on various aspects of brain development and ongoing maintenance, and the proteins in the brain that are responsive to these hormones. This chapter was geared towards broadening our understanding of thyroid hormone action in the brain, shedding light on potential therapeutic targets for neurodevelopmental and neurodegenerative disorders.

Thyroid hormone receptors are required for the melatonin-dependent control of Rfrp gene expression in mice

Mammals adapt to seasons using a neuroendocrine calendar defined by the photoperiodic change in the nighttime melatonin production. Under short photoperiod, melatonin inhibits the pars tuberalis production of TSHβ, which, in turn, acts on tanycytes to regulate the deiodinase 2/3 balance resulting in a finely tuned seasonal control of the intra-hypothalamic thyroid hormone T3. Despite the pivotal role of this T3 signaling for synchronizing reproduction with the seasons, T3 cellular targets remain unknown. One candidate is a population of hypothalamic neurons expressing Rfrp, the gene encoding the RFRP-3 peptide, thought to be integral for modulating rodent's seasonal reproduction. Here we show that nighttime melatonin supplementation in the drinking water of melatonin-deficient C57BL/6J mice mimics photoperiodic variations in the expression of the genes Tshb, Dio2, Dio3, and Rfrp, as observed in melatonin-proficient mammals. Notably, we report that this melatonin regulation of Rfrp expression is no longer observed in mice carrying a global mutation of the T3 receptor, TRα, but is conserved in mice with a selective neuronal mutation of TRα. In line with this observation, we find that TRα is widely expressed in the tanycytes. Altogether, our data demonstrate that the melatonin-driven T3 signal regulates RFRP-3 neurons through non-neuronal, possibly tanycytic, TRα.

Cryopreserved astrocytes maintain biological properties: Support of neuronal survival and differentiation

BACKGROUND

Astrocytes, one of the main glial cell types, play critical roles in the central nervous system (CNS) development and function, including support of neuronal survival and differentiation, blood brain barrier formation, synapse homeostasis and injury response. Cell isolation and culture techniques have been proved to be a powerful tool to study astrocyte physiology and function. Due to financial constraints and rigid biosafety and ethics rules to use animal models, freezing techniques and the creation of cell banks emerged as alternatives to optimize the use of experimental animals. One of the main challenges, however, of these techniques is to guarantee that conserved cells keep their biological properties.

NEW METHOD

In this work, we characterized morphologically and functionally murine secondary astrocyte cultures that have been submitted to freezing/thawing procedures.

RESULTS

Morphological characterization of SAC (secondary astrocyte culture) and SFAC (secondary frozen-astrocyte culture) did not reveal significant differences on astrocyte morphology, confluence time and cell number along culture period. Functionally, SAC and SFAC did not reveal differences in their potential to support neuronal survival, maturation, neuritogenesis and synapse formation.

CONCLUSIONS

Our results suggest that murine astrocytes that are submitted to freezing/thawing procedure maintain morphological and functional characteristics when compared with non-frozen astrocytes. Thus, this methodological approach is a valuable tool for in vitro research and might allow experimental optimization and reduction of animal use.

Titanium dioxide nanoparticles enhanced thyroid endocrine disruption of pentachlorophenol rather than neurobehavioral defects in zebrafish larvae

This study investigated the influences of titanium dioxide nanoparticles (n-TiO₂) on the thyroid endocrine disruption and neurobehavioral defects induced by pentachlorophenol (PCP) in zebrafish (Danio rerio). Embryos (2 h post-fertilization) were exposed to PCP (0, 3, 10, and 30 μg/L) or in combination with n-TiO₂ (0.1 mg/L) until 6 days post-fertilization. The results showed that n-TiO₂ alone did not affect thyroid hormones levels or transcriptions of related genes. Exposure to PCP significantly decreased thyroid hormone thyroxine (T4) content, thyroid stimulating hormone (TSH) level and transcription of thyroglobulin (tg), but significantly increased 3,5,3'-triiodothyronine (T3) level and upregulation of deiodinase 2 (dio2). In comparison, the co-exposure with n-TiO₂ significantly reduced the content of T3 by depressing the potential targets, tg and dio2. For neurotoxicity, the single and co-exposure resulted in similar effects with significant downregulation of neurodevelopment-related genes (ELAV like RNA Binding Protein 3, elavl3; Growth associated protein-43, gap43; α-tubulin) and inhibited locomotor activity. The results indicated that the presence of n-TiO₂ significantly enhanced the PCP-induced thyroid endocrine disruption but not the neurobehavioral defects in zebrafish larvae.

The Role of Astrocytes in the Development of the Cerebellum

Astrocytes, initially described as merely support cells, are now known as a heterogeneous population of cells actively involved in a variety of biological functions such as: neuronal migration and differentiation; regulation of cerebral blood flow; metabolic control of extracellular potassium concentration; and modulation of synapse formation and elimination; among others. Cerebellar glial cells have been shown to play a significant role in proliferation, differentiation, migration, and synaptogenesis. However, less evidence is available about the role of neuron-astrocyte interactions during cerebellar development and their impact on diseases of the cerebellum. In this review, we will focus on the mechanisms underlying cellular interactions, specifically neuron-astrocyte interactions, during cerebellar development, function, and disease. We will discuss how cerebellar glia, astrocytes, and Bergmann glia play a fundamental role in several steps of cerebellar development, such as granule cell migration, axonal growth, neuronal differentiation, and synapse formation, and in diseases associated with the cerebellum. We will focus on how astrocytes and thyroid hormones impact cerebellar development. Furthermore, we will provide evidence of how growth factors secreted by glial cells, such as epidermal growth factor and transforming growth factors, control cerebellar organogenesis. Finally, we will argue that glia are a key mediator of cerebellar development and that identification of molecules and pathways involved in neuron-glia interactions may contribute to a better understanding of cerebellar development and associated disorders.

Astrocyte Heterogeneity: Impact to Brain Aging and Disease

Astrocytes, one of the largest glial cell population in the central nervous system (CNS), play a key function in several events of brain development and function, such as synapse formation and function, control of neurotransmitters release and uptake, production of trophic factors and control of neuronal survival. Initially described as a homogenous population, several evidences have pointed that astrocytes are highly heterogeneous, both morphologically and functionally, within the same region, and across different brain regions. Recent findings suggest that the heterogeneity in the expression profile of proteins involved in astrocyte function may predict the selective vulnerability of brain regions to specific diseases, as well as to the age-related cognitive decline. However, the molecular mechanisms underlying these changes, either in aging as well as in brain disease are scarce. Neuroinflammation, a hallmark of several neurodegenerative diseases and aging, is reported to have a dubious impact on glial activation, as these cells release pro- and anti-inflammatory cytokines and chemokines, anti-oxidants, free radicals, and neurotrophic factors. Despite the emerging evidences supporting that reactive astrocytes have a duality in their phenotype, neurotoxic or neuroprotective properties, depending on the age and stimuli, the underlying mechanisms of their activation, cellular interplays and the impact of regional astrocyte heterogeneity are still a matter of discussion. In this review article, we will summarize recent findings on astrocyte heterogeneity and phenotypes, as well as their likely impact for the brain function during aging and neural diseases. We will focus on the molecules and mechanisms triggered by astrocyte to control synapse formation in different brain regions. Finally, we will discuss new evidences on how the modulation of astrocyte phenotype and function could impact the synaptic deficits and glial dysfunction present in aging and pathological states.

Analysis of FK506-mediated functional recovery and neuroprotection in a rat model of spinal cord injury indicates that EGF is modulated in astrocytes

The present study aimed to evaluate whether the application of tacrolimus (FK506) could improve functional recovery in spinal cord injury (SCI) rat models by activating astrocytes, and to further investigate the underlying mechanisms of this action. Male Sprague-Dawley rats (n=56) were used to establish moderate SCI models, which were induced at the T10 spinal segment by dropping a 10-g weight from a height of 25 mm using a New York University Impactor device. The rats were randomly separated into the FK506 or control group (n=28 per group). Rats were treated with FK506 (0.5 mg/kg) or saline intravenously 30 min after sustaining the injury. Functional recovery was evaluated over 42 days following the injury, and epidermal growth factor (EGF) levels were detected. The astrocytes were treated with FK506 in vitro, and the EGF mRNA and protein expression levels were analyzed using reverse transcription-quantitative polymerase chain reaction and ELISA, respectively. DNA microarray analysis was also performed to evaluate the genes in astrocytes. Rats in the FK506 group had improved locomotor functional recovery compared with those of control group. Furthermore, FK506 upregulated EGF expression of astrocytes both in vivo and in vitro. Subsequent to treatment with FK506-conditioned medium (CM), the length of neuronal cells increased 61.06% on the first day, and increased 56.4% on the third day compared with those of C-CM group. Furthermore, addition of anti-EGF neutralizing antibodies could interrupt the promotion of neurite outgrowth by FK506-CM. The present study indicates that astrocytes have an important role as mediators of FK506-improved spinal cord function recovery, and this partially clarifies the role of cell-cell interaction through modulating EGF in this process.

Low omega-6/omega-3 ratio in a maternal protein-deficient diet promotes histone-3 changes in progeny neural cells and favors leukemia inhibitory factor genetranscription

Omega-3 (n-3) fatty acids modulate epigenetic changes critical to genesis and differentiation of neural cells. Conversely, maternal protein-malnutrition can negatively modify these changes. This study investigated whether a low n-6/n-3 ratio in a maternal diet could favor histone-3 (H3) modifications, gene transcription and differentiation in the offspring neural cells even under protein-deficiency. Female rats fed a control (Ct), or 3 types of multideficient diets differing in protein levels or linoleic/alpha-linolenic fatty acid ratios (RBD, RBD-C, RBD-SO) from 30 days prior to mating and during pregnancy. Cerebral cortex tissue and cortical cultures of progeny embryonic neurons and postnatal astrocytes were analyzed. H3K9 acetylation and H3K27 or H3K4 di-methylation levels were assessed by flow cytometry and/or immunocytochemistry. In astrocyte cultures and cortical tissue, the GFAP protein levels were assessed. Glial derived neurotrophic factor (GDNF) and leukemia inhibitory factor (LIF) gene expression were evaluated in the cortical tissue. GFAP levels were similar in astrocytes of Ct, RBD and RBD-C, but 65% lower in RBD-SO group. Higher levels of H3K9Ac were found in the neurons and H3K4Me2 in the astrocytes of the RBD group. No intergroup difference in the cortical GDNF mRNA expression or the H3K27Me2 levels in astrocytes was detected. LIF mRNA levels were higher in the RDB (P=.002) or RBD-C (P=.004) groups than in the control. The findings indicate the importance of dietary n-3 availability for the brain, even under a protein-deficient condition, inducing Histone modifications and increasing LIF gene transcription, involved in neural cell differentiation and reactivity.

Neuroprotective astrocyte-derived insulin/insulin-like growth factor 1 stimulates endocytic processing and extracellular release of neuron-bound Aβ oligomers

Synaptopathy underlying memory deficits in Alzheimer's disease (AD) is increasingly thought to be instigated by toxic oligomers of the amyloid beta peptide (AβOs). Given the long latency and incomplete penetrance of AD dementia with respect to Aβ pathology, we hypothesized that factors present in the CNS may physiologically protect neurons from the deleterious impact of AβOs. Here we employed physically separated neuron-astrocyte cocultures to investigate potential non-cell autonomous neuroprotective factors influencing AβO toxicity. Neurons cultivated in the absence of an astrocyte feeder layer showed abundant AβO binding to dendritic processes and associated synapse deterioration. In contrast, neurons in the presence of astrocytes showed markedly reduced AβO binding and synaptopathy. Results identified the protective factors released by astrocytes as insulin and insulin-like growth factor-1 (IGF1). The protective mechanism involved release of newly bound AβOs into the extracellular medium dependent upon trafficking that was sensitive to exosome pathway inhibitors. Delaying insulin treatment led to AβO binding that was no longer releasable. The neuroprotective potential of astrocytes was itself sensitive to chronic AβO exposure, which reduced insulin/IGF1 expression. Our findings support the idea that physiological protection against synaptotoxic AβOs can be mediated by astrocyte-derived insulin/IGF1, but that this protection itself is vulnerable to AβO buildup.

Thyroid Hormone and Astrocyte Differentiation

Thyroid hormones (THs) have important contributions to the development of the mammalian brain, targeting its actions on both neurons and glial cells. Astrocytes, which constitute about half of the glial cells, characteristically undergo dramatic changes in their morphology during development and such changes become necessary for the proper development of the brain. Interestingly, a large number of studies have suggested that THs play a profound role in such morphological maturation of the astrocytes. This review discusses the present knowledge on the mechanisms by which THs elicit progressive differentiation and maturation of the astrocytes. As a prelude, information on astrocyte morphology during development and its regulations, the role of THs in the various functions of astrocyte shall be dealt with for a thorough understanding of the subject of this review.

Central and peripheral administrations of levothyroxine improved memory performance and amplified brain electrical activity in the rat model of Alzheimer's disease

Alzheimer's disease (AD) is associated with cognitive impairments and a decline in the spontaneous neuronal discharge. In the current study, we evaluated the effect of subcutaneous (S.C.) and intrahippocampal (I.H.) administrations of levothyroxine (LT-4) on the passive avoidance and spatial memory, as well as electrophysiological activity in an animal model of AD. One hundred-sixty male Wistar rats were divided into two main groups. The S.C. group included two Sham and four AD (vehicle or L-T4 25, 50 & 100μg/kg); and the I.H. had consisted of two Sham and two AD (vehicle or L-T4 10μg/kg) subgroups. To make an animal model of AD, amyloid beta (Aβ) plus ibotenic acid (Ibo) were injected I.H. Rats were treated with L-T4 and/or normal saline for ten days. Passive avoidance and spatial memory were evaluated in shuttle box and Morris water maze, respectively. Neuronal single unit recording was assessed from hippocampal dentate gyrus (DG). Results showed that the mean latency time (s) increased significantly (p<0.001) in AD animals and decreased significantly in both S.C. and I.H. L-T4 injected AD animals, compared with the AD group (p<0.001). The percentage of total time that animals spent in goal quarter and the step through latency decreased significantly in AD rats (p<0.001) and increased significantly in both S.C. and I.H. L-T4 injected AD animals in comparison with the AD group (p<0.01, p<0.001). Data showed that the average number of spikes/bin significantly decreased in the AD group (p<0.001). The S.C. and I.H. L-T4 injections in AD rats significantly increased the spike rate in comparison to the AD group (p<0.001). In conclusion, both S.C. and I.H. injections of L-T4 alleviated memory deficits and spontaneous neuronal activity in Aβ-induced AD rats. Also, I.H. microinjection of L-T4 had more beneficial effects on memory and neuronal electrophysiological activity in comparison to S.C. administration.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Enteric Glial Cells: A New Frontier in Neurogastroenterology and Clinical Target for Inflammatory Bowel Diseases

The word "glia" is derived from the Greek word "γλoια," glue of the enteric nervous system, and for many years, enteric glial cells (EGCs) were believed to provide mainly structural support. However, EGCs as astrocytes in the central nervous system may serve a much more vital and active role in the enteric nervous system, and in homeostatic regulation of gastrointestinal functions. The emphasis of this review will be on emerging concepts supported by basic, translational, and/or clinical studies, implicating EGCs in neuron-to-glial (neuroglial) communication, motility, interactions with other cells in the gut microenvironment, infection, and inflammatory bowel diseases. The concept of the "reactive glial phenotype" is explored as it relates to inflammatory bowel diseases, bacterial and viral infections, postoperative ileus, functional gastrointestinal disorders, and motility disorders. The main theme of this review is that EGCs are emerging as a new frontier in neurogastroenterology and a potential therapeutic target. New technological innovations in neuroimaging techniques are facilitating progress in the field, and an update is provided on exciting new translational studies. Gaps in our knowledge are discussed for further research. Restoring normal EGC function may prove to be an efficient strategy to dampen inflammation. Probiotics, palmitoylethanolamide (peroxisome proliferator-activated receptor-α), interleukin-1 antagonists (anakinra), and interventions acting on nitric oxide, receptor for advanced glycation end products, S100B, or purinergic signaling pathways are relevant clinical targets on EGCs with therapeutic potential.

Correlation of manganese with thyroid function in females having hypo- and hyperthyroid disorders

The aim of present study was to compare the level of manganese (Mn) with thyroid functions, thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxin (FT4) of females having hyper- (HPRT) and hypothyroid (HPOT) disorder. For comparative study, females of the same age group (16-30 years) having no thyroid disorders were selected as referents. The serum samples were acid digested prior to analysis by electrothermal atomic absorption spectrometry. The validity and accuracy of the methodology was checked by a certified sample. The resulted data indicated that the mean values of Mn in serum samples of females have hyperthyroidism was significantly higher than referent subjects (p < 0.01), while lower values of Mn was observed in serum samples of hypothyroid patients. The mean values of FT3 and FT4 were found to be lower while TSH higher is in HPRT patients than age-matched healthy control females (p = <0.01). The reverse resulted data of these thyroid hormones was observed in HPOT patients (0.003).

Thyroid hormone and astroglia: endocrine control of the neural environment

Thyroid hormones (THs) play key roles in brain development and function. The lack of THs during childhood is associated with the impairment of several neuronal connections, cognitive deficits and mental disorders. Several lines of evidence point to astrocytes as TH targets and as mediators of TH action in the central nervous system; however, the mechanisms underlying these events are still not completely known. In this review, we focus on advances in our understanding of the effects of THs on astroglial cells and the impact of these effects on neurone-astrocyte interactions. First, we discuss the signalling pathways involved in TH metabolism and the molecular mechanisms underlying TH receptor function. Then, we discuss data related to the effects of THs on astroglial cells, as well as studies regarding the generation of mutant TH receptor transgenic mice that have contributed to our understanding of TH function in brain development. We argue that astrocytes are key mediators of hormone actions on development of the cerebral cortex and cerebellum and that the identification of the molecules and pathways involved in these events might be important for determining the molecular-level basis of the neural deficits associated with endocrine diseases.

The enteric glia: identity and functions

Enteric glial cells were first described at the end of the 19th century, but they attracted more interest from researchers only in the last decades of the 20th. Although, they have a different embryological origin, the enteric GLIA share many characteristics with astrocytes, the main glial cell type of the central nervous system (CNS), such as in their expression of the same markers and in their functions. Here we review the construction of the enteric nervous system (ENS), with a focus on enteric glia, and also the main studies that have revealed the action of enteric glia in different aspects of gastrointestinal tract homeostasis, such as in the intestinal barrier, in communications with neurons, and in their action as progenitor cells. We also discuss recent discoveries about the roles of enteric glia in different disorders that affect the ENS, such as degenerative pathologies including Parkinson's and prion diseases, and in cases of intestinal diseases and injury.

Effects of 3,3',5-triiodothyronine on microglial functions

L-tri-iodothyronine (3, 3', 5-triiodothyronine; T3) is an active form of the thyroid hormone (TH) essential for the development and function of the CNS. Though nongenomic effect of TH, its plasma membrane-bound receptor, and its signaling has been identified, precise function in each cell type of the CNS remained to be investigated. Clearance of cell debris and apoptotic cells by microglia phagocytosis is a critical step for the restoration of damaged neuron-glia networks. Here we report nongenomic effects of T3 on microglial functions. Exposure to T3 increased migration, membrane ruffling and phagocytosis of primary cultured mouse microglia. Injection of T3 together with stab wound attracted more microglia to the lesion site in vivo. Blocking TH transporters and receptors (TRs) or TRα-knock-out (KO) suppressed T3-induced microglial migration and morphological change. The T3-induced microglial migration or membrane ruffling was attenuated by inhibiting Gi /o -protein as well as NO synthase, and subsequent signaling such as phosphoinositide 3-kinase (PI3K), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK). Inhibitors for Na(+) /K(+) -ATPase, reverse mode of Na(+) /Ca(2+) exchanger (NCX), and small-conductance Ca(2+) -dependent K(+) (SK) channel also attenuated microglial migration or phagocytosis. Interestingly, T3-induced microglial migration, but not phagocytosis, was dependent on GABAA and GABAB receptors, though GABA itself did not affect migratory aptitude. Our results demonstrate that T3 modulates multiple functional responses of microglia via multiple complex mechanisms, which may contribute to physiological and/or pathophysiological functions of the CNS.

Astrocyte transforming growth factor beta 1 promotes inhibitory synapse formation via CaM kinase II signaling

The balance between excitatory and inhibitory synaptic inputs is critical for the control of brain function. Astrocytes play important role in the development and maintenance of neuronal circuitry. Whereas astrocytes-derived molecules involved in excitatory synapses are recognized, molecules and molecular mechanisms underlying astrocyte-induced inhibitory synapses remain unknown. Here, we identified transforming growth factor beta 1 (TGF-β1), derived from human and murine astrocytes, as regulator of inhibitory synapse in vitro and in vivo. Conditioned media derived from human and murine astrocytes induce inhibitory synapse formation in cerebral cortex neurons, an event inhibited by pharmacologic and genetic manipulation of the TGF-β pathway. TGF-β1-induction of inhibitory synapse depends on glutamatergic activity and activation of CaM kinase II, which thus induces localization and cluster formation of the synaptic adhesion protein, Neuroligin 2, in inhibitory postsynaptic terminals. Additionally, intraventricular injection of TGF-β1 enhanced inhibitory synapse number in the cerebral cortex. Our results identify TGF-β1/CaMKII pathway as a novel molecular mechanism underlying astrocyte control of inhibitory synapse formation. We propose here that the balance between excitatory and inhibitory inputs might be provided by astrocyte signals, at least partly achieved via TGF-β1 downstream pathways. Our work contributes to the understanding of the GABAergic synapse formation and may be of relevance to further the current knowledge on the mechanisms underlying the development of various neurological disorders, which commonly involve impairment of inhibitory synapse transmission.

Role of thyroid hormones in different aspects of nervous system regeneration in vertebrates

Spontaneous functional recovery from injury in the adult human nervous system is rare and trying to improve recovery remains a clinical challenge. Nervous system regeneration is a complicated sequence of events involving cell death or survival, cell proliferation, axon extension and remyelination, and finally reinnervation and functional recovery. Successful recovery depends on the cell-specific and time-dependent activation and repression of a wide variety of growth factors and guidance molecules. Thyroid hormones (THs), well known for their regulatory role in neurodevelopment, have recently emerged as important modulators of neuroregeneration. This review focuses on the endogenous changes in the proteins regulating TH availability and action in different cell types of the adult mammalian nervous system during regeneration as well as the impact of TH supplementation on the consecutive steps in this process. It also addresses possible differences in TH involvement between different vertebrate classes, early or late developmental stages and peripheral or central nervous system. The available data show that THs are able to stimulate many signaling pathways necessary for successful neurogeneration. They however also suggest that supplementation with T4 and/or T3 may have beneficial or detrimental influences depending on the dose and more importantly on the specific phase of the regeneration process.

Purkinje cells and Bergmann glia are primary targets of the TRα1 thyroid hormone receptor during mouse cerebellum postnatal development

Thyroid hormone is necessary for normal development of the central nervous system, as shown by the severe mental retardation syndrome affecting hypothyroid patients with low levels of active thyroid hormone. The postnatal defects observed in hypothyroid mouse cerebellum are recapitulated in mice heterozygous for a dominant-negative mutation of Thra, the gene encoding the ubiquitous TRα1 receptor. Using CRE/loxP-mediated conditional expression approach, we found that this mutation primarily alters the differentiation of Purkinje cells and Bergmann glia, two cerebellum-specific cell types. These primary defects indirectly affect cerebellum development in a global manner. Notably, the inward migration and terminal differentiation of granule cell precursors is impaired. Therefore, despite the broad distribution of its receptors, thyroid hormone targets few cell types that exert a predominant role in the network of cellular interactions that govern normal cerebellum maturation.

The effect of low dose lipopolysaccharide on thyroid hormone-regulated actin cytoskeleton modulation and type 2 iodothyronine deiodinase activity in astrocytes

Systemic infection/inflammation can severely interfere with brain development. Lipopolysaccharide (LPS) is a major cell wall component of gram-negative bacteria and commonly used to model the response by infections. Since perinatal exposure to LPS shows neurodevelopmental defects partly similar to those seen in perinatal hypothyroidism, we examined the effect of LPS on thyroxin (T4)-mediated signalings in astrocytes. Initially, C6 rat glioma-derived clonal cells were used, whose biological nature is similar to that of astrocytes. To measure the effects of LPS and T4, actin polymerization and D2 activity assays were carried out. LPS treatment (10 ng/mL) markedly induced actin depolymerization, whereas 10 nM T4 promoted actin polymerization. Furthermore, T4 partly rescued LPS-induced actin depolymerization. LPS treatment (10 ng/mL) increased D2 activity, whereas T4 (10 nM) suppressed this activity. T4 restored LPS-increased D2 activity at 10 nM. LPS-induced actin depolymerization and D2 activity were blocked by p38 MAP kinase inhibitor. Such effects were not seen in T4-mediated changes. Furthermore, similar results were found in the cerebellar primary astrocyte. These results indicate that, although LPS affects T4-regulated cellular events such as actin polymerization and D2 activity, which may induce neurodevelopmental defects similar to those in perinatal hypothyroidism, LPS signaling pathways are independent of T4 signaling pathways.

Avaliação da proteína acídica fibrilar glial como marcador da injúria por isquemia-reperfusão hepática

OBJETIVO: Avaliar a expressão da Proteína Acídica Fibrilar Glial após a injúria por isquemia-reperfusão. MÉTODOS: vinte e quatro ratos foram distribuídos em quatro grupos: Controle, submetidos à anestesia e biópsia hepática; Simulação, injeção de heparina através da veia cava e dissecção do pedículo hepático superior, biópsia após 24 horas; Isquemia 30 minutos, mesmo procedimento do grupo Simulação, acrescido de clampeamento do pedículo hepático superior por 30 minutos; Isquemia 90 minutos, mesmo procedimento do grupo Isquemia 30 minutos, porém com período de clampeamento de 90 minutos. Após 24 horas de observação, os animais foram submetidos à laparotomia e seus fígados avaliados macroscopicamente, microscopicamente, por coloração de Hematoxilina-Eosina (HE) e submetidos à análise da expressão da GFAP por Western Blotting. RESULTADOS: Não se observou diferença no aspecto macroscópico dos fígados entre os diferentes grupos experimentais, tendo todos evidenciado morfologia normal. A análise por HE não evidenciou diferenças significativas, no que diz respeito à morfologia lobular. Por outro lado, nos grupos isquemia, foram encontrados infiltrados neutrofílicos e pequenas áreas de necrose. A expressão de GFAP foi semelhante em todos os grupos, seja qualitativamente quanto quantitativamente. CONCLUSÃO: A expressão da Proteína Acídica Fibrilar Glial não se alterou em nosso modelo de isquemia-reperfusão.

Connective tissue growth factor (CTGF/CCN2) is negatively regulated during neuron-glioblastoma interaction

Connective-tissue growth factor (CTGF/CCN2) is a matricellular-secreted protein involved in complex processes such as wound healing, angiogenesis, fibrosis and metastasis, in the regulation of cell proliferation, migration and extracellular matrix remodeling. Glioblastoma (GBM) is the major malignant primary brain tumor and its adaptation to the central nervous system microenvironment requires the production and remodeling of the extracellular matrix. Previously, we published an in vitro approach to test if neurons can influence the expression of the GBM extracellular matrix. We demonstrated that neurons remodeled glioma cell laminin. The present study shows that neurons are also able to modulate CTGF expression in GBM. CTGF immnoreactivity and mRNA levels in GBM cells are dramatically decreased when these cells are co-cultured with neonatal neurons. As proof of particular neuron effects, neonatal neurons co-cultured onto GBM cells also inhibit the reporter luciferase activity under control of the CTGF promoter, suggesting inhibition at the transcription level. This inhibition seems to be contact-mediated, since conditioned media from embryonic or neonatal neurons do not affect CTGF expression in GBM cells. Furthermore, the inhibition of CTGF expression in GBM/neuronal co-cultures seems to affect the two main signaling pathways related to CTGF. We observed inhibition of TGFβ luciferase reporter assay; however phopho-SMAD2 levels did not change in these co-cultures. In addition levels of phospho-p44/42 MAPK were decreased in co-cultured GBM cells. Finally, in transwell migration assay, CTGF siRNA transfected GBM cells or GBM cells co-cultured with neurons showed a decrease in the migration rate compared to controls. Previous data regarding laminin and these results demonstrating that CTGF is down-regulated in GBM cells co-cultured with neonatal neurons points out an interesting view in the understanding of the tumor and cerebral microenvironment interactions and could open up new strategies as well as suggest a new target in GBM control.

Hypothyroidism in the adult rat causes incremental changes in brain-derived neurotrophic factor, neuronal and astrocyte apoptosis, gliosis, and deterioration of postsynaptic density

BACKGROUND

Adult hypothyroidism is a highly prevalent condition that impairs processes, such as learning and memory. Even though tetra-iodothyronine (T(4)) treatment can overcome the hypothyroidism in the majority of cases, it cannot fully recover the patient's learning capacity and memory. In this work, we analyzed the cellular and molecular changes in the adult brain occurring with the development of experimental hypothyroidism.

METHODS

Adult male Sprague-Dawley rats were treated with 6-propyl-2-thiouracil (PTU) for 20 days to induce hypothyroidism. Neuronal and astrocyte apoptosis were analyzed in the hippocampus of control and hypothyroid adult rats by confocal microscopy. The content of brain-derived neurotrophic factor (BDNF) was analyzed using enzyme-linked immunosorbent assay (ELISA) and in situ hybridization. The glutamatergic synapse and the postsynaptic density (PSD) were analyzed by electron microscopy. The content of PSD proteins like tyrosine receptor kinase B (TrkB), p75, and N-methyl-D-aspartate receptor (NMDAr) were analyzed by immunoblot.

RESULTS

We observed that the hippocampus of hypothyroid adult rats displayed increased apoptosis levels in neurons and astrocyte and reactive gliosis compared with controls. Moreover, we found that the amount of BDNF mRNA was higher in the hippocampus of hypothyroid rats and the content of TrkB, the receptor for BDNF, was reduced at the PSD of the CA3 region of hypothyroid rats, compared with controls. We also observed that the glutamatergic synapses from the stratum radiatum of CA3 from hypothyroid rats, contained thinner PSDs than control rats. This observation was in agreement with a reduced content of NMDAr subunits at the PSD in hypothyroid animals.

CONCLUSIONS

Our data suggest that adult hypothyroidism affects the hippocampus by a mechanism that alters the composition of PSD, reduces neuronal and astrocyte survival, and alters the content of the signaling neurotrophic factors, such as BDNF.

An in vitro method to study the effects of thyroid hormone-disrupting chemicals on neuronal development

Thyroid hormones (THs) play critical roles for normal cerebellar development. It has been reported that several environmental chemicals may affect cerebellar development through TH system. One such example is the suppression of TH receptor (TR)-mediated transcription by polybrominated diphenyl ethers (PBDEs). To determine the effect of these chemicals on brain development, we established a primary culture system of rat cerebellar Purkinje cells. Using this system, as low as 10(-10)M TH induced Purkinje cell dendritic arborization and such effect was dose-dependent. We examined the effect of decabromodiphenyl ether (BDE209) using this system. Dendritic development of the Purkinje cells was suppressed by 10(-10)M BDE209, that was compatible to the result of the suppression of TR-mediated transcription by using reporter gene assay. These results suggest that TH plays a pivotal role in the development of the Purkinje cell dendrites. Together with in vitro assay system such as reporter gene assay and liquid chemiluminescent DNA-pull down assay, an in vitro protein-DNA binding assay, these assay systems provide us with precise information about environmental chemicals on brain development.

Astrocytes treated by lysophosphatidic acid induce axonal outgrowth of cortical progenitors through extracellular matrix protein and epidermal growth factor signaling pathway

Lysophosphatidic acid (LPA) plays important roles in many biological processes, such as brain development, oncogenesis and immune functions, via its specific receptors. We previously demonstrated that LPA-primed astrocytes induce neuronal commitment of cerebral cortical progenitors (Spohr et al. 2008). In the present study, we analyzed neurite outgrowth induced by LPA-treated astrocytes and the molecular mechanism underlying this event. LPA-primed astrocytes increase neuronal differentiation, arborization and neurite outgrowth of developing cortical neurons. Treatment of astrocytes with epidermal growth factor (EGF) ligands yielded similar results, suggesting that members of the EGF family might mediate LPA-induced neuritogenesis. Furthermore, treatment of astrocytes with LPA or EGF ligands led to an increase in the levels of the extracellular matrix molecule, laminin (LN), thus enhancing astrocyte permissiveness to neurite outgrowth. This event was reversed by pharmacological inhibitors of the MAPK signaling pathway and of the EGF receptor. Our data reveal an important role of astrocytes and EGF receptor ligands pathway as mediators of bioactive lipids action in brain development, and implicate the LN and MAPK pathway in this process.

Thyroid hormone and the neuroglia: both source and target

Thyroid hormone plays a crucial role in the development and function of the nervous system. In order to bind to its nuclear receptor and regulate gene transcription thyroxine needs to be activated in the brain. This activation occurs via conversion of thyroxine to T3, which is catalyzed by the type 2 iodothyronine deiodinase (D2) in glial cells, in astrocytes, and tanycytes in the mediobasal hypothalamus. We discuss how thyroid hormone affects glial cell function followed by an overview on the fine-tuned regulation of T3 generation by D2 in different glial subtypes. Recent evidence on the direct paracrine impact of glial D2 on neuronal gene expression underlines the importance of glial-neuronal interaction in thyroid hormone regulation as a major regulatory pathway in the brain in health and disease.

Enteric glia promote intestinal mucosal healing via activation of focal adhesion kinase and release of proEGF

Wound healing of the gastrointestinal mucosa is essential for the maintenance of gut homeostasis and integrity. Enteric glial cells play a major role in regulating intestinal barrier function, but their role in mucosal barrier repair remains unknown. The impact of conditional ablation of enteric glia on dextran sodium sulfate (DSS)-induced mucosal damage and on healing of diclofenac-induced mucosal ulcerations was evaluated in vivo in GFAP-HSVtk transgenic mice. A mechanically induced model of intestinal wound healing was developed to study glial-induced epithelial restitution. Glial-epithelial signaling mechanisms were analyzed by using pharmacological inhibitors, neutralizing antibodies, and genetically engineered intestinal epithelial cells. Enteric glial cells were shown to be abundant in the gut mucosa, where they associate closely with intestinal epithelial cells as a distinct cell population from myofibroblasts. Conditional ablation of enteric glia worsened mucosal damage after DSS treatment and significantly delayed mucosal wound healing following diclofenac-induced small intestinal enteropathy in transgenic mice. Enteric glial cells enhanced epithelial restitution and cell spreading in vitro. These enhanced repair processes were reproduced by use of glial-conditioned media, and soluble proEGF was identified as a secreted glial mediator leading to consecutive activation of epidermal growth factor receptor and focal adhesion kinase signaling pathways in intestinal epithelial cells. Our study shows that enteric glia represent a functionally important cellular component of the intestinal epithelial barrier microenvironment and that the disruption of this cellular network attenuates the mucosal healing process.

Impaired astrocytic extracellular matrix distribution under congenital hypothyroidism affects neuronal development in vitro

Astrocytes clearly play a role in neuronal development. An indirect mechanism of thyroid hormone (T3) in the regulation of neuronal development mediated by astrocytes has been proposed. T3 alters the production and organization of the extracellular matrix (ECM) proteins and proteoglycans, producing a high-quality substrate for neuronal differentiation. The present study investigated the effect of hypothyroidism on the astrocyte production of fibronectin (FN) and laminin (LN) as well as their involvement in neuronal growth and neuritogenesis. Our results demonstrated that the amount of both FN and LN were significantly reduced in cultures of hypothyroid astrocytes from rat cerebellum compared with normal cells. This effect was accompanied by reduced numbers of neurons and neuritogenesis. Similarly, the proportions of neurons and neurons with neurites were reduced in cultures on ECM prepared from hypothyroid astrocytes in comparison with normal cells. The proportion of both normal and hypothyroid neurons is strongly reduced in astrocyte ECM compared with cocultures on astrocyte monolayers, suggesting that extracellular factors other than ECM proteins are involved in this process. Moreover, treatment of hypothyroid astrocytic cultures with T3 restored the area of both FN and LN immunostaining to normal levels and partially reestablished neuronal survival and neuritogenesis. Taken together, our results demonstrated that hypothyroidism involves impairment of the astrocytic microenvironment and affects the production of ECM proteins. Thus, hypothyroidism is implicated in impaired neuronal development.

Thyroid hormone induces cerebellar neuronal migration and Bergmann glia differentiation through epidermal growth factor/mitogen-activated protein kinase pathway

Cerebellar development in the postnatal period is mainly characterized by an intense cellular proliferation in the external granular layer, followed by migration of granular cells in the molecular layer along the Bergmann glia (BG) fibers. Cerebellar ontogenesis undergoes dramatic modulation by thyroid hormones (THs), although their mechanism of action in this organ is still largely unknown. We previously demonstrated that THs induce astrocytes to secrete epidermal growth factor (EGF), which thus promotes cerebellar neuronal proliferation and extracellular matrix remodeling in vitro. In the present study, we investigated the effect of the TH/EGF pathway on granule neuronal migration. By taking advantage of rat explant and dissociated culture assays, we showed that cerebellar astrocytes treated with TH promote granule cell migration. The addition of neutralizing antibodies against EGF or the pharmacological inhibitor of EGF signaling, bis-tyrphostin, completely inhibited TH-astrocyte-induced migration. Likewise, the addition of EGF itself greatly increased neuronal migration. Treatment of BG-dissociated cultures by EGF dramatically induced an alteration in cell morphology, characterized by an elongation in the glial process. Both neuronal migration and BG elongation were inhibited by the mitogen-activated protein kinase pathway inhibitor PD98059, suggesting that these events might be associated. Together, our results suggest that, by inducing EGF secretion, THs promote neuronal migration through BG elongation. Our data provide new clues to the molecular mechanism of THs in cerebellar development, and may contribute to a better understanding of some neuroendocrine disorders associated with migration deficits.

Effects of the flavonoid casticin from Brazilian Croton betulaster in cerebral cortical progenitors in vitro: direct and indirect action through astrocytes

Neurodegenerative diseases are a major constraint on the social and economic development of many countries. Evidence has suggested that phytochemicals have an impact on brain pathology; however, both their mechanisms of action and their cell targets are incompletely known. Here, we investigated the effects of the flavonoid casticin, extracted from Croton betulaster, a common plant in the state of Bahia in Brazil, on rat cerebral cortex neurons in vitro. Treatment of neural progenitors with 10 microM casticin increased the neuronal population positive for the neuronal marker beta-tubulin III and the neuronal transcriptional factor Tbr2 by approximately 20%. This event was followed by a 50% decrease in neuronal death. Pools of astrocyte (GFAP and S100beta), neural (nestin), and oligodendrocyte (Olig2 and NG2) progenitors were not affected by casticin. Neither neuronal commitment nor proliferation of progenitors was affected by casticin, suggesting a neuroprotective effect of this compound. Culture of neural progenitors on casticin-treated astrocyte monolayers increased the neuronal population by 40%. This effect was reproduced by conditioned medium derived from casticin-treated astrocytes, suggesting the involvement of a soluble factor. ELISA assays of the conditioned medium revealed a 20% increase in interleukin-6 level in response to casticin. In contrast to the direct effect, neuronal death was unaffected, but a 52% decrease in the death of nestin-positive progenitors was observed. Together our data suggest that casticin influences the neuronal population by two mechanisms: 1) directly, by decreasing neuronal death, and 2) indirectly, via astrocytes, by modulating the pool of neuronal progenitors.

Thyroid hormone receptor beta mutation causes severe impairment of cerebellar development

Cerebellar development on the postnatal period is mainly characterized by cellular proliferation in the external granular layer (EGL) followed by migration of granular cells in the molecular layer through the Bergmann glia (BG) fibers in order to form the granular layer in the adult. All these events are drastically affected by thyroid hormones (TH), which actions are mainly mediated by alpha (TRalpha) and beta (TRbeta) nuclear receptor isoforms. Here, we analyzed the effects of a natural human mutation (337T) in the TRbeta locus, which impairs T3 binding to its receptor, on the mouse cerebellum ontogenesis. We report that target inactivation of TRbeta-TH binding leads to a smaller cerebellum area characterized by impaired lamination and foliation. Further, TRbeta mutant mice presented severe deficits in proliferation of granular precursors, arborization of Purkinje cells and organization of BG fibers. Together, our data suggest that the action of TH via TRbeta regulates important events of cerebellar ontogenesis contributing to a better understanding of some neuroendocrine disorders. Further, our data correlate TRbeta with cerebellar foliation, and provide, for the first time, evidence of a receptor-mediated mechanism underlying TH actions on this event.

Thyroid hormone (T3)-induced up-regulation of voltage-activated sodium current in cultured postnatal hippocampal neurons requires secretion of soluble factors from glial cells

We have previously shown that treatment with the thyroid hormone T(3) increases the voltage-gated Na(+)current density (Nav-D) in hippocampal neurons from postnatal rats, leading to accelerated action potential upstrokes and increased firing frequencies. Here we show that the Na(+) current regulation depends on the presence of glial cells, which secrete a heat-instable soluble factor upon stimulation with T(3). The effect of conditioned medium from T(3)-treated glial cells was mimicked by basic fibroblast growth factor (bFGF), known to be released from cerebellar glial cells after T(3) treatment. Neutralization assays of astrocyte-conditioned media with anti-bFGF antibody inhibited the regulation of the Nav-D by T(3). This suggests that the up-regulation of the neuronal sodium current density by T(3) is not a direct effect but involves bFGF release and satellite cells. Thus glial cells can modulate neuronal excitability via secretion of paracrinely acting factors.

Effect of thyroid hormone T3 on myosin-Va expression in the central nervous system

Thyroid hormones (THs) are essential for brain development, where they regulate gliogenesis, myelination, cell proliferation and protein synthesis. Hypothyroidism severely affects neuronal growth and establishment of synaptic connections. Triiodothyronine (T3), the biologically active form of TH, has a central function in these activities. So, Myosin-Va (Myo-Va), a molecular motor protein involved in vesicle and RNA transport, is a good candidate as a target for T3 regulation. Here, we analyzed Myo-Va expression in euthyroid and hypothyroid adult rat brains and synaptosomes. We observed a reduction of Myo-Va expression in cultured neural cells from newborn hypothyroid rat brain, while immunocytochemical experiments showed a punctate distribution of this protein in the cytoplasm of cells. Particularly, Myo-Va co-localized with microtubules in neurites, especially in their varicosities. Myo-Va immunostaining was stronger in astrocytes and neurons of controls when compared with hypothyroid brains. In addition, supplementation of astrocyte cultures with T3 led to increased expression of Myo-Va in cells from both euthyroid and hypothyroid animals, suggesting that T3 modulates Myo-Va expression in neural cells both in vivo and in vitro. We have further analyzed Myo-Va expression in U373 cells, a human glioblastoma line, and found the same punctate cytoplasmic protein localization. As in normal neural cells, this expression was also increased by T3, suggesting that the modulatory mechanism exerted by T3 over Myo-Va remains active on astrocyte tumor cells. These data, coupled with the observation that Myo-Va is severely affected in hypothyroidism, support the hypothesis that T3 activity regulates neural motor protein expression, taking Myo-Va as a model. As a consequence, reduced T3 activity could supposedly affect axonal transport and synaptic function, and could therefore explain disturbances seen in the hypothyroid brain.

Thyroid hormones and methylmercury toxicity

Thyroid hormones are essential for cellular metabolism, growth, and development. In particular, an adequate supply of thyroid hormones is critical for fetal neurodevelopment. Thyroid hormone tissue activation and inactivation in brain, liver, and other tissues is controlled by the deiodinases through the removal of iodine atoms. Selenium, an essential element critical for deiodinase activity, is sensitive to mercury and, therefore, when its availability is reduced, brain development might be altered. This review addresses the possibility that high exposures to the organometal, methylmercury (MeHg), may perturb neurodevelopmental processes by selectively affecting thyroid hormone homeostasis and function.

cAMP promotes neurite outgrowth and extension through protein kinase A but independently of Erk activation in cultured rat motoneurons

It is well established that cAMP counteracts myelin inhibition to permit axon regeneration in the central nervous system. On the other hand, the role of cAMP in axonal growth on permissive substrates remains controversial because the evidence available is contradictory. In view that elevation of cAMP represents an attractive therapeutic target to promote nerve regeneration in vivo, we investigated the effect of cAMP on neurite outgrowth and extension in motoneurons. We manipulated cAMP levels pharmacologically in cultured motoneurons and investigated targets downstream of cAMP of neurite outgrowth and extension on a permissive substrate. Reduction of cAMP by the adenylyl cyclase inhibitor SQ22536 inhibited, and elevation of cAMP by forskolin, dibutyryl cAMP, IBMX and rolipram increased outgrowth and extension of neurites. The cAMP-mediated effects occur via activation of protein kinase A (PKA) and were reduced by the inhibitors, H89 and Rp-cAMP. However, cAMP elevation did not lead to Erk activation that is an essential downstream component of neurotrophin signaling. These findings provide evidence for a key role of cAMP in promoting peripheral nerve regeneration after nerve injuries and indicate that this effect is unusual in not being mediated via Erk phosphorylation.

Lysophosphatidic acid receptor-dependent secondary effects via astrocytes promote neuronal differentiation

Lysophosphatidic acid (LPA) is a simple phospholipid derived from cell membranes that has extracellular signaling properties mediated by at least five G protein-coupled receptors referred to as LPA(1)-LPA(5). In the nervous system, receptor-mediated LPA signaling has been demonstrated to influence a range of cellular processes; however, an unaddressed aspect of LPA signaling is its potential to produce specific secondary effects, whereby LPA receptor-expressing cells exposed to, or "primed," by LPA may then act on other cells via distinct, yet LPA-initiated, mechanisms. In the present study, we examined cerebral cortical astrocytes as possible indirect mediators of the effects of LPA on developing cortical neurons. Cultured astrocytes express at least four LPA receptor subtypes, known as LPA(1)-LPA(4). Cerebral cortical astrocytes primed by LPA exposure were found to increase neuronal differentiation of cortical progenitor cells. Treatment of unprimed astrocyte-progenitor cocultures with conditioned medium derived from LPA-primed astrocytes yielded similar results, suggesting the involvement of an astrocyte-derived soluble factor induced by LPA. At least two LPA receptor subtypes are involved in LPA priming, since the priming effect was lost in astrocytes derived from LPA receptor double-null mice (LPA(1)((-/-))/LPA(2)((-/-))). Moreover, the loss of LPA-dependent differentiation in receptor double-null astrocytes could be rescued by retrovirally transduced expression of a single deleted receptor. These data demonstrate that receptor-mediated LPA signaling in astrocytes can induce LPA-dependent, indirect effects on neuronal differentiation.

Guanine derivatives modulate extracellular matrix proteins organization and improve neuron-astrocyte co-culture

Guanine derivatives (GD) have been shown to exert relevant extracellular effects as intercellular messengers, neuromodulators in the central nervous system, and trophic effects on astrocytes and neurons. Astrocytes have been pointed out as the major source of trophic factors in the nervous system, however, several trophic effects of astrocytic-released soluble factors are mediated through modulation of extracellular matrix (ECM) proteins. In this study, we investigated the effects of guanosine-5'-monophosphate (GMP) and guanosine (GUO) on the expression and organization of ECM proteins in cerebellar astrocytes. Moreover, to evaluate the effects of astrocytes pre-treated with GMP or GUO on cerebellar neurons we used a neuron-astrocyte coculture model. GMP or GUO alters laminin and fibronectin organization from a punctate to a fibrillar pattern, however, the expression levels of the ECM proteins were not altered. Guanine derivatives-induced alteration of ECM proteins organization is mediated by activation of mitogen activated protein kinases (MAPK), CA(2+)-calmodulin-dependent protein kinase II (CaMK-II), protein kinase C (PKC), and protein kinase A (PKA) pathways. Furthermore, astrocytes treated with GMP or GUO promoted an increased number of cerebellar neurons in coculture, without altering the neuritogenesis pattern. No proliferation of neurons or astrocytes was observed due to GMP or GUO treatment. Our results show that guanine derivatives promote a reorganization of the ECM proteins produced by astrocytes, which might be responsible for a better interaction with neurons in cocultures.

Effects of manganese on thyroid hormone homeostasis: potential links

Manganese (Mn) is an essential trace nutrient that is potentially toxic at high levels of exposure. As a constituent of numerous enzymes and a cofactor, manganese plays an important role in a number of physiologic processes in mammals. The manganese-containing enzyme, manganese superoxide dismutase (Mn-SOD), is the principal antioxidant enzyme which neutralizes the toxic effects of reactive oxygen species. Other manganese-containing enzymes include oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases and glutamine synthetase. Environmental or occupational exposure to high levels of manganese can cause a neuropathy resembling idiopathic Parkinson's disease, commonly referred to as manganism. Manganism and Parkinson's disease are both characterized by motor deficits and damage to nuclei of the basal ganglia, particularly the substantia nigra, with altered dopamine (and its metabolites) contributing to these disorders. Dopamine, a major neurotransmitter plays a crucial role in the modulation of the cognitive function, working memory and/or attention of the prefrontal cortex and the hippocampus. Dopamine is also a known inhibitory modulator of thyroid stimulating hormone (TSH) secretion. The involvement of dopamine and dopaminergic receptors in neurodevelopment, as well as TSH modulation, led us to hypothesize that excessive manganese exposure may lead to adverse neurodevelopmental outcomes due to the disruption of thyroid homeostasis via the loss of dopaminergic control of TSH regulation of thyroid hormones. This disruption may alter thyroid hormone levels, resulting in some of the deficits associated with gestational exposure to manganese. While the effects of manganese in adult populations are relatively well documented, comprehensive data on its neurodevelopmental effects are sparse. Given the importance of this topic, we review the potential participation of thyroid hormone dyshomeostasis in the neurodevelopmental effects of manganese positing the hypotheses that manganese may directly or indirectly affect thyroid function by injuring the thyroid gland or dysregulating dopaminergic modulation of thyroid hormone synthesis.

Disrupting effects of hydroxy-polychlorinated biphenyl (PCB) congeners on neuronal development of cerebellar Purkinje cells: a possible causal factor for developmental brain disorders?

Polychlorinated biphenyls (PCBs) and hydroxy-PCB (OH-PCB) metabolites are widely distributed bioaccumulative environmental chemicals and have similar chemical structures to those of thyroid hormones (THs). Previously, we reported that THs are essential for neuronal development and the low doses of two OH-PCBs, namely, 4-OH-2',3,3',4',5'-pentachlorobiphenyl (4'-OH-PeCB-106) and 4-OH-2',3,3',4',5,5'-hexachlorobiphenyl (4'-OH-HxCB-159), inhibited the TH-dependent dendritic development of Purkinje cells in mouse cerebellar cultures using serum-free defined medium. To determine which type of OH-PCBs affect neuronal development, we further examined several OH-PCBs and other estrogenic chemicals using this simple and sensitive assay system. Two-way ANOVA was used to assess the effects of OH-PCBs and other chemicals on both factors of their concentrations and with/without T4 in the assay of TH-dependent dendritic development of Purkinje cells. Aside from the two OH-PCBs, 4-OH-2',3,4',5,6'-pentachlorobiphenyl (4'-OH-PeCB-121) and bisphenol A significantly inhibited the TH-dependent dendritic development of Purkinje cells, whereas 4-OH-2',3,3',5',6'-pentachlorobiphenyl (4'-OH-PeCB-112), 4-OH-2',3,3',5,5',6'-hexachlorobiphenyl (4'-OH-HxCB-165), 4-OH-2,2',3,4',5,5',6-heptachlorobiphenyl (4-OH-HpCB-187), progesterone and nonylphenol did not induce any inhibition, but significantly promoted the dendritic extension of Purkinje cells in the absence of THs. Other estrogenic chemicals, including beta-estradiol, diethyl stilbestrol and p-octylphenol did not show significant inhibitory or promoting effects. From these results, it is suggested that exposure to OH-PCBs and other environmental chemicals may disrupt normal neuronal development and cause some developmental brain disorders, such as LD, ADHD, and autism.

nAChR-mediated calcium responses and plasticity inDrosophila Kenyon cells

In Drosophila, nicotinic acetylcholine receptors (nAChRs) mediate fast excitatory synaptic transmission in mushroom body Kenyon cells, a neuronal population involved in generation of complex behaviors, including responses to drugs of abuse. To determine whether activation of nAChRs can induce cellular changes that contribute to functional plasticity in these neurons, we examined nicotine-evoked responses in cells cultured from brains of late stage OK107-GAL4 pupae. Kenyon cells can be identified by expression of green fluorescent protein (GFP+). Nicotine activates alpha-bungarotoxin-sensitive nAChRs, causing a rapid increase in intracellular calcium levels in over 95% of the Kenyon cells. The nicotine-evoked calcium increase has a voltage-gated calcium channel (VGCC) dependent component and a VGCC-independent component that involves calcium influx directly through nAChRs. Thapsigargin treatment reduces the nicotine response consistent with amplification by calcium release from intracellular stores. The response to nicotine is experience-dependent: a short conditioning pulse of nicotine causes a transient 50% reduction in the magnitude of the response to a test pulse of nicotine when the interpulse interval is 4 h. This cellular plasticity is dependent on activation of the VGCC-component of the nicotine response and on cAMP-signaling, but not on protein synthesis. These data demonstrate that activation of nAChRs induces a calcium-dependent plasticity in Kenyon cells that could contribute to adult behaviors involving information processing in the mushroom bodies including responses to nicotine.

Toxoplasma gondii prevents neuron degeneration by interferon-gamma-activated microglia in a mechanism involving inhibition of inducible nitric oxide synthase and transforming growth factor-beta1 production by infected microglia

Interferon (IFN)-gamma, the main cytokine responsible for immunological defense against Toxoplasma gondii, is essential in all infected tissues, including the central nervous system. However, IFN-gamma-activated microglia may cause tissue injury through production of toxic metabolites such as nitric oxide (NO), a potent inducer of central nervous system pathologies related to inflammatory neuronal disturbances. Despite potential NO toxicity, neurodegeneration is not commonly found during chronic T. gondii infection. In this study, we describe decreased NO production by IFN-gamma-activated microglial cells infected by T. gondii. This effect involved strong inhibition of iNOS expression in IFN-gamma-activated, infected microglia but not in uninfected neighboring cells. The inhibition of NO production and iNOS expression were parallel with recovery of neurite outgrowth when neurons were co-cultured with T. gondii-infected, IFN-gamma-activated microglia. In the presence of transforming growth factor (TGF)-beta1-neutralizing antibodies, the beneficial effect of the parasite on neurons was abrogated, and NO production reverted to levels similar to IFN-gamma-activated uninfected co-cultures. In addition, we observed Smad-2 nuclear translocation, a hallmark of TGF-beta1 downstream signaling, in infected microglial cultures, emphasizing an autocrine effect restricted to infected cells. Together, these data may explain a neuropreservation pattern observed during immunocompetent host infection that is dependent on T. gondii-triggered TGF-beta1 secretion by infected microglia.

Thyroid hormone-induced morphological differentiation and maturation of astrocytes involves activation of protein kinase A and ERK signalling pathway

Thyroid hormone (TH) has a profound effect on astrocyte differentiation and maturation. Astrocytes cultured under TH-deficient conditions fail to transform from flat polygonal morphology to mature, process-bearing, stellate cells. Supplementation of physiological concentrations of TH initiate gradual transformation of the cells and the process takes approximately 48 h to complete. The signal transduction pathways associated with TH-mediated maturation of astrocytes have been investigated. TH treatment caused an initial activation of protein kinase A (PKA), with a peak activity at 2 h which fell back to basal level there after. Although there was no visible change in morphology of the cells during the observed activation of PKA, it was sufficient to drive the process of transformation to completion, suggesting the involvement of downstream regulators of PKA. PKA inhibitors as well as the MEK inhibitor PD098059 attenuated the TH-induced morphological transformation. Further studies showed that TH treatment resulted in a biphasic response on the cellular phospho-MAP kinase (p-MAPK or p-ERK) level: an initial decline in the p-ERK level followed by an induction at 18-24 h, both of which could be blocked by a PKA inhibitor. Such sustained activation of p-ERK levels by TH at this later stage coincided with initiation of morphological differentiation of the astrocytes and appeared to be critical for the transformation of astrocytes. The nitric oxide synthase (NOS) inhibitor 7-NI inhibited this induction of p-ERK activity. Moreover, the induction was accompanied by a parallel increase in phospho-CREB activity which, however, persisted at the end of the transformation of the astroglial cells.