Thyroid hormone action: induction of morphological changes and protein secretion in astroglial cell cultures

Effect of stem cells-based therapy on astrogliosis in stroke subjected-mice

This study was planned to continue our previous study to assess effect of combination therapy bone marrow stromal cells (BMSCs) with exercise (EX) and triiodothyronine (T₃) on stroke-induced astrogliosis in mice. Stroke subjected-mice were divided into five monotherapy groups including sham, control, BMSCs, EX and T₃; and three combination therapy groups including BMSCs + EX, BMSCs + T₃ and BMSCs + EX + T₃. Astrogliosis was assessed in ipsilateral hemisphere at day 7 after MCAO. Combination therapy BMSCs with EX and T₃ could significantly decrease stroke-induced astrogliosis. However, monotherapy with BMSCs or EX also improved changes of glial fibrillary acidic protein (GFAP)-positive cells following stroke. Combination therapy BMSCs with EX and T₃ didn't have any added effect on astrogliosis compared to monotherapy with BMSCs or EX. With comparing the present findings with the results of neurobehavioral functioning in our earlier study, it seems that decrease of astrogliosis could be helpful for stroke recovery.

Docosahexaenoic Acid (DHA) Induced Morphological Differentiation of Astrocytes Is Associated with Transcriptional Upregulation and Endocytosis of β2-AR

Docosahexaenoic acid (DHA), an important ω-3 fatty acid, is abundantly present in the central nervous system and is important in every step of brain development. Much of this knowledge has been based on studies of the role of DHA in the function of the neurons, and reports on its effect on the glial cells are few and far between. We have previously reported that DHA facilitates astrocyte differentiation in primary culture. We have further explored the signaling mechanism associated with this event. It was observed that a sustained activation of the extracellular signal-regulated kinase (ERK) appeared to be critical for DHA-induced differentiation of the cultured astrocytes. Prior exposure to different endocytic inhibitors blocked both ERK activation and differentiation of the astrocytes during DHA treatment suggesting that the observed induction of ERK-2 was purely endosomal. Unlike the β₁-adrenergic receptor (β₁-AR) antagonist, atenolol, pre-treatment of the cells with the β₂-adrenergic receptor (β₂-AR) antagonist, ICI-118,551 inhibited the DHA-induced differentiation process, indicating a downstream involvement of β₂-AR in the differentiation process. qRT-PCR and western blot analysis demonstrated a significant induction in the mRNA and protein expression of β₂-AR at 18-24 h of DHA treatment, suggesting that the induction of β₂-AR may be due to transcriptional upregulation. Moreover, DHA caused activation of PKA at 6 h, followed by activation of downstream cAMP response element-binding protein, a known transcription factor for β₂-AR. Altogether, the observations suggest that DHA upregulates β₂-AR in astrocytes, which undergo endocytosis and signals for sustained endosomal ERK activation to drive the differentiation process.

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.

Increased beta(2)-adrenergic receptor activity by thyroid hormone possibly leads to differentiation and maturation of astrocytes in culture

(1) Our earlier studies indicate a downsteam regulatory role of the beta-adrenergic receptor (beta-AR) system in thyroid hormone induced differentiation and maturation of astrocytes. In the present study we have investigated the contributions of the subtypes of beta-AR in the above phenomenon. (2) Primary astrocyte cultures were grown under thyroid hormone deficient as well as under euthyroid conditions. [(125)I]Pindolol ([(125)I]PIN) binding studies showed a gradual increase in the specific binding to beta(2)-AR when observed at 5, 10, 15, and 20 days under both cultural conditions. Thyroid hormone caused an increase in binding of [(125)I]PIN to beta(2)-AR compared to thyroid hormone deficient controls at all ages of astrocyte culture. (3) Saturation studies using [(125)I]PIN in astrocyte membranes prepared from 20-day-old cultures showed a significant increase in the affinity of the receptors (K (D)) in the thyroid hormone treated cells without any change in receptor number (B (max)). (4) beta(2)-AR mRNA levels were measured by real-time PCR during ontogenic development as well as during exposure of 10-day-old hypothyroid cultures to normal levels of thyroid hormone for 2, 6, 12, and 24 h. None of the conditions caused any significant change in the beta(2)-adrenergic receptor mRNA levels when compared with corresponding hypothyroid controls. (5) Over expression of beta(2)-AR cDNA in hypothyroid astrocytes caused morphological transformation in spite of the absence of thyroid hormone in the medium. (6) Taken together, results suggest thyroid hormone causes a selective increase in [(125)I]PIN binding to beta(2)-AR due to increase in receptor affinity, which may lead to maturation of astrocytes.

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.

Thyroid hormone modulates the extracellular matrix organization and expression in cerebellar astrocyte: effects on astrocyte adhesion

The effects of thyroid hormone (T(3)) on extracellular matrix (ECM) expression and organization in cerebellar astrocytes were studied. Control astrocytes exhibit laminin immunostaining distributed in a punctate configuration and fibronectin concentrated in focal points at the cell surface. These cells attach to the substratum by membrane points, as shown by scanning microscopy, possibly by focal points stained to fibronectin. In contrast, after T(3) treatment, laminin assumes a fibrillary pattern and fibronectin becomes organized in filaments homogeneously distributed on the cell surface; the cells acquire a very flat and spread morphology. T(3) treatment also modulates astrocyte adhesion. In addition, increased expression of both laminin and fibronectin was detected by Western blot. These alterations in fibronectin and/or laminin production and organization may be involved in the flat and spread morphology and in altered adhesion. We observed that fibroblast growth factor-2 (FGF(2)) added to cultures had similar effects to those described to T(3). Neutralizing antibodies against FGF(2) reversed T(3) effects on fibronectin and laminin distribution. We also observed that cerebellar neurons co-cultured on T(3)-treated astrocytes had an increase in the number of cells and presented longer neurites. Thus, we propose a novel mechanism of the effect of thyroid hormone on cerebellar development mediated by astrocytes: T(3) may induce astrocyte secretion of growth factors, mainly FGF(2), that autocrinally stimulate astrocyte proliferation, reorganization in ECM proteins, and alterations in cell spreading and adhesion. These effects may indirectly influence neuronal development.

Thyroid hormone actions on neural cells

1. In addition to its role in cellular metabolic activity, thyroid hormone (TH) is critically involved in growth, development, and function of the central nervous system. In the brain, as in other structures, TH is described to exert its major action by the binding of L-3,5,3'-triiodothyronine (T3), considered as the bioactive form of the hormone, to nuclear thyroid hormone receptors (TR) that function as ligand-dependent transcription factors. 2. The transcription of numerous brain genes was indeed shown to be positively or negatively regulated by TH, turning these TR-mediated effects one explanation for the physiological effects of TH. In this context, the knowledge from TR-knockout studies provides some surprising results, since neonatal hypothyroidism is associated to more significant abnormalities than is TR deficiency. Some (nonexclusive) hypotheses include a permissive effect of TH, allowing derepression of unliganded-TR effects and non-TR-mediated effects of the hormone, further emphasizing the importance of a controlled accessibility of neural cells to TH. 3. On the other hand, T3 was demonstrated to directly act not only on neuronal but also on glial cells proliferation and differentiation, contributing to the harmonious development of the brain. Interestingly, in addition to these direct actions on neuronal and glial cells, several lines of evidence, notably developped in our laboratory, point out the role of thyroid hormone in neuronal-glial interactions.

Influence of cell density and thyroid hormone on glial cell development in primary cultures of embryonic rat cerebral hemisphere

The influence of cell density and thyroid hormone (TH) on the development of astrocytes and oligodendrocytes was investigated in primary cultures prepared from rat cerebral hemisphere on embryonic day (E)18. At the beginning of the culture, most of the cells were microtubule-associated protein 2 (MAP2)-positive neurons, whereas O1-positive oligodendrocytes and glial fibrillary acidic protein (GFAP)-positive astrocytes were rarely observed. After the cells were maintained in serum-free defined medium, astrocytes developed at high cell density but rarely at a low one. When leukemia inhibitory factor (LIF) was supplemented in low-density cultures, the levels of GFAP expression markedly increased to almost the same extent as in high-density culture without TH. This suggests that, in low-density cultures, astrocyte progenitors could not differentiate because of insufficient astrocyte-inducing factors. Interestingly, the addition of TH increased GFAP expression levels only at high density. The number of oligodendrocytes increased with TH addition at both cell densities, although the effects were more remarkable at high density. These results suggest that cell density and TH are pivotal factors in the development of both astrocytes and oligodendrocytes. It is also suggested that the effects of TH on glial cell development could be accelerated via cell-cell communications.

The differential regulation of group II(A) and group V low molecular weight phospholipases A(2) in cultured rat astrocytes

In astrocytes, cytokines stimulate the release of secretory phospholipase A(2) (sPLA(2)) activity and group II(A) sPLA(2) expression. This paper reports that two sPLA(2) isoforms, group II(A) and group V, are in fact expressed by astrocytes. Our studies showed that tumor necrosis factor alpha (TNFalpha) enhanced the mRNA of both isoforms, but the time courses of enhancement differed; group V was induced much faster than group II(A). Moreover, TNFalpha stimulated both the NF-kappaB and mitogen-activated protein (MAP) kinase (extracellular signal-regulated kinase, c-Jun NH(2)-terminal kinase, and p38 MAP kinase) signaling pathways in astrocytes. Interestingly, PI 3-kinase activity also was enhanced by TNFalpha, and NF-kappaB pathway was involved in mediating its effect. Specific inhibitors were used to show that both extracellular signal-regulated kinase and p38 MAP kinase may contribute to the effect of TNFalpha and that blocking phosphatidylinositol 3-kinase activity fully reversed the effect of TNFalpha. Furthermore, in astrocytes, TNFalpha-induced release of sPLA(2) activity was partially reversed by thyroid hormone and almost abolished by growth factors. This phenomenon was accompanied by a less marked increase in both group II(A) and group V sPLA(2) mRNA. In the presence of growth factors, the increase in group V mRNA was inhibited early and transiently, in contrast to what was observed with group II(A), which was more persistently inhibited. Although a transcriptional effect of thyroid hormone or growth factors in astrocytes cannot be definitively excluded, both types of factor interfered with sPLA(2) expression in a manner suggesting the existence of regulation of post-transcriptional events.

Thyroid hormone action on astroglial cells from distinct brain regions during development

Astrocytes are target to triiodothyronine (T3) hormone action during rat brain development. In this work, we show that astrocytes from distinct developing brain regions are differently responsive to thyroid hormone. Distinctly from embryonic or newborn cerebral hemisphere and mesencephalic astrocytes, newborn cerebellar and embryonic hippocampal astrocytes do not change their morphology in response of hormone treatment. We also analysed protein synthesis and secretion from these T3-treated astrocytes. The results showed a significant increase in protein synthesis in astrocytes from older brain regions. Maximum effect, however, was observed in cerebral hemisphere astrocytes from newborn rats. The protein secretion effect was also more evident in the cerebral hemisphere as well as in cerebellar astrocytes from newborn rats. In addition, we examined T3 effects on GFAP/vimentin expression by culturing 6-day old cerebellar astrocytes. In this case T3 seems to induce GFAP expression which might be occurring as a first step to astrocyte differentiation.

Role of thyroid hormone in the morphological differentiation and maturation of astrocytes: temporal correlation with synthesis and organization of actin

Morphological changes and the molecular mechanisms associated with the maturation of astrocytes were studied under normal and thyroid hormone-deficient conditions using long-term (30 days) primary cultures derived from the neonatal rat brain. Immunocytochemical staining of cells with a monoclonal antibody specific to glial fibrillary acidic protein demonstrated for the first time that, similar to their maturation in vivo, astrocytes maintained in normal serum-containing medium can undergo complete maturation involving two distinct stages of morphological differentiation (from radial glia to flat polygonal cells with epithelioid morphology and then to mature process-bearing cells with stellate morphology). Deficiency of thyroid hormone delays the first step and totally blocks the second stage of differentiation in the maturation process. Comparative staining of normal and thyroid hormone-deficient astrocytes with filamentous actin-specific fluorescein isothiocyanate-phalloidin and quantitation of the various forms of intracellular actin using an improved DNase I assay demonstrated that maturation of astroglial cells is associated with characteristic alterations in the level of cytoskeletal and noncytoskeletal filamentous (F) actin. In particular, the maintenance of the epithelioid form of the hypothyroid astrocytes is associated with a progressive increase in the level of cytoskeletal F-actin and a concomitant decline in the level of non-cytoskeletal F-actin. Quantitation of actin mRNA by Northern blot analysis and studies on the rate of actin synthesis at various stages of differentiation showed that the initial transformation into the epithelioid form is associated with an increase in the rate of synthesis of actin and the expression of its mRNA, while the final transformation into the nature process-bearing form is correlated with a decline in these parameters. The results indicates that thyroid hormone plays an obligatory role in promoting the differentiation and maturation of astrocytes, and that during this process the hormone regulates the expression of actin and its intracellular organization in a way conducive to morphological differentiation.

Thyroid hormone and conditioned medium effects on astroglial cells from hypothyroid and normal rat brain: factor secretion, cell differentiation, and proliferation

The effects of triiodothyronine (T3) on cell morphology were examined in cerebral hemisphere and cerebellar astrocyte cultures obtained from normal and hypothyroid neonatal rats. T3-treatment induced morphological changes in astrocytes from cerebral hemispheres. This morphological effect was produced earlier if astrocytes were treated with conditioned medium obtained from cerebral hemisphere astrocyte cultures previously exposed to 50 nM T3. T3 or conditioned medium-treatment produced faster morphological changes in hypothyroid rat cerebral hemisphere astrocyte monolayers. Cerebellar astrocytes from normal brain did not respond to thyroid hormone with morphological changes, but proliferated after T3-treatment. However, hypothyroid cerebellar astrocyte cultures exhibited morphological changes, differently than normal cells. We verified that T3 may induce astrocyte secretion of factor(s) that promotes morphological differentiation in cerebral hemisphere astroglial cultures and stimulates the proliferation of cerebellar astrocytes. Astrocytes obtained from hypothyroid animals were more sensitive to secreted factors than normal cells. These results emphasize the heterogeneity and the importance of glial cells to normal brain development and open new questions about thyroid hormone therapy in hypothyroidism.

Effects of growth factors on phosphatidylinositol-3 kinase in astroglial cells

Growth factors differently regulate astroglial cell differentiation and proliferation. In an effort to understand the early intracellular events promoted by growth factors in astroglial cells, we have determined the effects of insulin-like growth factor I (IGF1), insulin, platelet-derived growth factor (PDGF), epidermal growth factor (EGF) and fibroblast growth factors (FGFs) on phosphatidylinositol-3 kinase (PI(3)-kinase). In astroglial cells cultured in serum-free medium, IGF1, PDGF, and EGF, which stimulate cell proliferation, increased PI(3)-kinase activity immunoprecipitated with anti-phosphotyrosine antibodies as shown by thin layer chromatography and high performance liquid chromatography. FGFa and FGFb, which strongly stimulate proliferation, glutamine synthetase, and deiodinase activities and modify cell morphology, have no effect on PI(3)-kinase activity. Addition of 1 nM PDGF, 10 nM IGF1, or 100 nM EGF to the culture medium rapidly stimulated PI(3)-kinase activity which declined slowly after 2 min. The stimulation of PI(3)-kinase increased with growth factor concentration. The maximum increase in PI(3)-kinase activity occurred with 50 nM IGF1, 1 nM PDGF, or 100 nM EGF. Since insulin was active only at high concentration (1 microM), its effect was probably mediated through IGF1 receptors and not through insulin receptors. IGF1 and PDGF, to a lesser degree, also increased the PI(3)-kinase activity associated with pp60c-src protein. Immunoblots performed with an antibody directed against the p85-subunit of the PI(3)-kinase confirmed that IGF1 increased the number of PI(3)-kinase molecules associated with phosphotyrosine-containing proteins or with c-src protein. Each growth factor affects in a different manner the association of PI(3)-kinase with phosphotyrosine-containing proteins and with pp60c-src and thus probably modulates intracellular signals downstream of PI(3)-kinase in astroglial cells.

Induction of type III-deiodinase activity in astroglial cells by retinoids

Thyroid hormones and retinoic acid (RA) are important modulators of growth, development, and differentiation. Type III deiodinase (D-III), which catalyzes thyroid hormones degradation in the brain and in cultured astroglial cells, is induced in astroglial cells by multiple pathways, including cAMP, 12.0-tetradecanoylphorbol-13-acetate (TPA), fibroblast growth factors, and thyroid hormones themselves. In the present study, the effects of retinoids on D-III activity were examined in astroglial cells cultures in a chemically defined medium devoid of hormones and growth factors. Incubation of astroglial cells with 5 microM all-trans-RA caused up to 200-fold increase in D-III activity, which reached a plateau after 48 h. The retinoid-induced increase in D-III activity was concentration dependent (0.5 microM all-trans-RA and 9-cis-RA producing half-maximal effect). Retinol was effective at physiological concentrations (1 and 10 microM). The 48 h effects of 5 microM all-trans-RA and 10 nM thyroid hormones on D-III activity were at least additive. Addition of 2 nM acidic fibroblast growth factor or 1 mM 8-bromo-cAMP for the last 8 h of a 48 h incubation with 5 microM all-trans-RA did not alter the induction by all-trans-RA, whereas 0.1 microM TPA in the same conditions produced an additive effect with all-trans-RA. All-trans-RA (5 microM) had little or no effect on type II deiodinase, the enzyme which catalyzes the activation of thyroxine to 3,5,3'-triiodothyronine.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of a tubulin binding protein in rat brain plasma membrane

Studies on the interaction of FITC-tubulin and 125I-tubulin with isolated plasma membrane of neural cells and with primary cultures of neuronal (N) and glial (G) cells of rat brain demonstrate the presence of specific, saturable, high affinity tubulin binding sites in these cells. The positive fluorescence of live unfixed primary cultures of N and G cells following incubation with FITC-tubulin indicate that the tubulin binding sites are located on the outer side of the plasma membrane. Such fluorescence was not observed with FITC-BSA, FITC-conalbumin or freshly dissociated cells from rat tissues or established cell lines. Binding of FITC-tubulin or 125I-tubulin is competed only by tubulin and not by other proteins. Scatchard analysis of the binding of 125I-tubulin to purified plasma membrane indicates very high affinity (Kd = 85 nM) with a Bmax of 7.4 pmol/mg protein. The putative tubulin receptor was partially purified by affinity chromatography on tubulin-sepharose column. Immunoprecipitation of the solubilized tubulin-receptor complex followed by SDS-PAGE analysis and autoradiography, revealed the presence of two components of molecular weights 70 and 45 kDa respectively, presumably representing the two nonidentical subunits of the putative receptor. In conjunction with several recent reports indicating the secretion of high molecular weight proteins from cultured neural cells and the ability of tubulin to modulate adenyl cyclase in synaptic membranes these findings suggest that the binding of exogenous tubulin to sites external to the plasma membrane may be involved in signal transduction.

Cyclic AMP regulation of messenger RNA level of the stimulatory GTP-binding protein Gs alpha. Isoproterenol, forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate increase the level of Gs alpha mRNA in cultured astroglial cells

The existence of a cAMP-dependent regulation of the expression of the alpha-subunit of the stimulatory guanine nucleotide-binding protein (Gs) in a well characterized astroglial cells culture was established. The culture of astroglial cells for 3-6 h with isoproterenol (10 microM) or forskolin (10 microM) (a cAMP-inducing agent) increased (200-400%) the response of adenylylcyclase to agents which bypass the receptor; GTP, GTP[S] or forskolin. For prolonged exposure times (15 h or more) to isoproterenol or forskolin, the adenylycyclase activity decreased to the value observed in control cells. The same biphasic response of adenylylcyclase to isoproterenol (10 microM) plus GTP (10 microM) occurred in membrane fractions from cells cultured with forskolin, whereas a diminished response to isoproterenol was observed in isoproterenol-treated cells, indicating that the beta-adrenergic receptor was desensitized. To understand the molecular mechanism of these phenomena, we measured the levels of the alpha subunits of the guanine-nucleotide binding protein (Gs and Gi) by Western-blot analysis. The culture of astroglial cells with isoproterenol or forskolin (3-24 h) resulted in a transient increase of both the Gs alpha and the Gi alpha protein levels, while the level of G beta subunits was unaffected. We also identified Gs alpha protein (about 40% of the total cellular protein) in the supernatant fraction of astroglial cells but its level was not modified by the stimulation of cells by forskolin. The level of Gs alpha mRNA measured by Northern-blot analysis was transiently increased (200%) after stimulation of astroglial cells with isoproterenol or forskolin for an incubation period of 6-9 h, then returned to that of control cells for longer period of time. In addition, the Gs alpha mRNA level was threefold increased when cells were cultured for 2-6 h with 8-bromoadenosine 3':5'-cyclic monophosphate (10 microM), a permeant analogue of cAMP. These results indicate that cAMP induces a time-dependent increase of Gs alpha mRNA. The half-life of Gs alpha protein and Gs alpha mRNA were determined. Pulse-chase studies revealed that the decay of Gs alpha protein was clearly biphasic with an early phase (5-6 h) and a slower second phase (20-25 h) but the treatment of cells with forskolin did not accelerate or slow down the turnover of Gs alpha protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyroid hormones influence the astroglial plasticity: changes in the expression of glial fibrillary acidic protein (GFAP) and of its encoding message

Normal development of the brain requires the presence of thyroid hormones. To progress in the understanding of the contribution of astrocytes to brain pathophysiology we investigated the effect of T3, on the astroglial plasticity through the expression of two astroglial proteins: the Glial fibrillary acidic protein (GFAP) and the glutamine synthetase (GS). Western and northern blots were performed using astroglial primary cultures initiated from neocortex and cerebellum of new-born mice. Treatment with T3 caused a decrease of GFAP and of its encoding message level in both areas, suggesting a transcriptional regulation of its expression, whereas it had no apparent effect on GS expression. This reduction in GFAP expression was developmentally regulated; it was significant in proliferating but not in more mature astrocytes. T3 effect on astrocytes was higher in the cerebellum compared to the neocortex, suggesting the presence of astroglial subpopulations differing by their sensitivity to T3. The astroglial specific response to T3, corresponds to a precise, targetted and regulated adaptation of the cell. Factors of the microenvironment may modulate this specific astroglial response in vivo.

Decrease in beta-adrenergic receptors of cerebral astrocytes in hypothyroid rat brain

Studies on the binding of 3H-dihydroalprenolol (3H-DHA) to astrocytes from cerebra of normal and hypothyroid rats show that hypothyroidism results in a decline in the beta-adrenergic receptors. Ontogenic studies indicated that in normal, euthyroid rats, the maximum binding capacity (Bmax) for 3H-DHA progressively increased with age while the affinity (Kd) remained unaltered. In astrocytes prepared from hypothyroid rats, total number of binding sites for 3H-DHA also increased with age, however, at a given age, the number was significantly lower than that for corresponding euthyroid animals while the affinity for 3H-DHA remained unaffected. Correspondingly, primary cultures of astrocytes from normal and hypothyroid brain when maintained in TH-deficient serum, display a similar reduction of 3H-DHA binding. In the case of astrocytes from hypothyroid brain cultured in TH-deficient serum, the decline can be largely restored by supplementing with normal serum. Results suggest that thyroid hormones (TH) directly or indirectly regulates the level of beta-adrenergic receptors in astrocytes from developing rat brain.

Dipeptidyl peptidase II in astrocytes of the rat brain. Meningeal cells increase enzymic activity in cultivated astrocytes

Astrocytes grown in media conditioned by meningeal cells (MCM) develop cellular processes and markedly increased protein per cell. One protein component affected is the dipeptidyl peptidase II (DPP II). The increase of DPP II activity is dose- and time-dependent and can also be elicited by the second messenger cAMP. More mature astrocytes express higher levels of DPP II than immature proliferating astrocytes. The rate of proliferation of astrocytes is markedly enhanced by enriched MCM. These observations lead to the assumption that DPP II has a function within the catabolic processes of cellular differentiation. To assess whether the in vitro results may reflect in vivo conditions, we investigated the postnatal development of DPP II in the rat brain. Differentiating astrocytes in vivo are especially found early postnatally and, indeed, during this period high specific activities are found in brain. Depending on the region investigated DPP II activities decrease within the first ten days to one fourth of their P2 level and finally reach at about similar levels in all brain regions. Exceptions are the hypothalamus, where the activity is generally 1.5- to 3-fold higher than elsewhere in brain, and pons and mesencephalon, where the perinatal activity peak is lacking. The bulk activity of DPP II in immature rat brains is attributed to differentiating astrocytes loosing it in later postnatal stages due to a neuronal influence.

Triiodothyronine has diverse and multiple stimulating effects on expression of the major myelin protein genes

If the importance of triiodothyronine (T3) on brain development including myelinogenesis has long been recognized, its mechanism of action at the gene level is still not fully elucidated. We studied the effect of T3 on the expression of myelin protein genes in aggregating brain cell cultures. T3 increases the concentrations of mRNA transcribed from the following four myelin protein genes: myelin basic protein (Mbp), myelin-associated glycoprotein (Mag), proteolipid protein (Plp), and 2',3'-cyclic nucleotide 3'-phosphodiesterase (Cnp). T3 is not only a triggering signal for oligodendrocyte differentiation, but it has continuous stimulatory effects on myelin gene expression. Transcription in isolated nuclei experiments shows that T3 increases Mag and Cnp transcription rates. After inhibiting transcription with actinomycin D, we measured the half-lives of specific mRNAs. Our results show that T3 increases the stability of mRNA for myelin basic protein, and probably proteolipid protein. In vitro translation followed by myelin basic protein-specific immunoprecipitation showed a direct stimulatory effect of T3 on myelin basic protein mRNA translation. Moreover, this stimulation was higher when the mRNA was already stabilized in culture, indicating that stabilization is achieved through mRNA structural modifications. These results demonstrate the diverse and multiple mechanisms of T3 stimulation of myelin protein genes.

Infection by coronavirus JHM of rat neurons and oligodendrocyte-type-2 astrocyte lineage cells during distinct developmental stages

Primary telencephalic cultures derived from neonatal Wistar Furth rats were able to support the growth of coronavirus JHM if a viable neuronal population was maintained. This occurred under serum-free defined, but not serum-supplemented, growth conditions. The importance of neurons in establishing infections in mixed cultures was confirmed by immunocytochemical and electron microscopic studies. Glia, although more abundant than neurons in these cultures, were less frequently infected during the initial 48 h postinoculation. The two glial lineages present in mixed telencephalic cultures were separated into type-1 astrocytes and oligodendrocyte-type-2 astrocyte (O-2A) lineage cells and individually assessed for their ability to support virus growth. Infection could not be established in type-1 astrocytes regardless of the culture conditions employed, consistent with our previous study (S. Beushausen and S. Dales, Virology 141:89-101, 1985). In contrast, infections could be initiated in selected O-2A lineage cells grown in serum-free medium. Virus multiplication was however significantly reduced by preconditioning the medium with mixed telencephalic or enriched type-1 astrocyte cultures, suggesting that intercellular interactions mediated by soluble factor(s) can influence the infectious process in O-2A lineage cells. This presumption was supported by eliciting similar effects with basic fibroblast growth factor and platelet-derived growth factor, two central nervous system cytokines known to control O-2A differentiation. The presence of these cytokines, which synergistically block O-2A cells from differentiating into oligodendrocytes was correlated with specific and reversible resistance to JHM virus (JHMV) infection. These data, combined with our finding that accelerated terminal differentiation of the oligodendrocyte phenotype confers resistance to JHMV (Beushausen and Dales, Virology, 1985), suggest that the permissiveness of O-2A cells for JHMV is restricted to a discrete developmental stage.