Expression of tubulin, GFAP and of their encoding mRNAs during the proliferation and differentiation of cultured astrocytes

PHOX2B Is Associated with Neuroblastoma Cell Differentiation

Neuroblastoma is a common pediatric malignancy that accounts for ∼15% of tumor-related deaths in children. The tumor is generally believed to originate from neural crest cells during early sympathetic neurogenesis. As the degree of neuroblastoma differentiation has been correlated with clinical outcome, clarifying the molecular mechanisms that drive neuroblastoma progression and differentiation is important for increasing the survival of these patients. In a previous study, the authors identified paired-like homeobox 2b (PHOX2B) as a key mediator of neuroblastoma pathogenesis in a TH-MYCN mouse model. In the present study, they aimed to define whether PHOX2B is also associated with proliferation and differentiation of human neuroblastoma cells. PHOX2B expression in neuroblastoma cells was evaluated by immunoblot analyses, and the effects of PHOX2B on the proliferation of neuroblastoma cells in vitro were determined using clonogenic and sphere formation assays. Xenograft experiments in NOD/SCID mice were used to examine the in vivo response to PHOX2B knockdown. Their data demonstrated that PHOX2B acts as a prognostic marker in neuroblastoma and that retinoic acid-induced neuronal differentiation downregulates PHOX2B expression, thereby suppressing the self-renewal capacity of neuroblastoma cells and inhibiting tumorigenicity. These findings confirmed that PHOX2B is a key regulator of neuroblastoma differentiation and stemness maintenance and indicated that PHOX2B might serve as a potential therapeutic target in neuroblastoma patients.

Glial fibrillary acidic protein is necessary for mature astrocytes to react to beta-amyloid

Upregulation of the glial fibrillary acidic protein (GFAP) in astrocytes is a hallmark of the phenomenon known as reactive gliosis and, yet, the function of GFAP in this process is largely unknown. Our previous studies have shown that mature astrocytes react vigorously to substrate bound beta-amyloid protein (BAP) in a variety of ways (i.e., increased GFAP, enhanced motility, unusual aggregation patterns, inhibitory ECM production). In order to uncover which, if any, of these phenomena are causally related to the function of GFAP, primary cortical astrocytes from transgenic mice lacking GFAP were cultured on BAP substrates at low or high density and at various lengths of time following in vitro maturation. Differences between mutant and control cells became progressively more obvious when cells were matured in vitro for two weeks or longer and especially in cultures that were at high density. Mature control astrocytes show a dramatic response to BAP by aggregating into a meshwork of rope-like structures that completely bridge over the peptide surface. In marked contrast, mature GFAP-null astrocytes initiate the response much more slowly and had a much reduced ability to aggregate tightly. Furthermore, we prepared hippocampal slice cultures from GFAP-/- and GFAP+/+ mice and compared their astrocytic responses to injected BAP. GFAP-/- astrocytes of hippocampal slice cultures failed to form a barrier-like structure around the edge of the BAP deposit as did GFAP+/+ astrocytes. Our data suggest that GFAP may be essential for mature astrocytes to constrain certain types of highly inflammatory lesions in the brain.

Glutamine synthetase (GS) expression is reduced in senile dementia of the Alzheimer type

Glutamine synthetase (GS), a metabolic marker of the mature astrocyte, was investigated in the temporal neocortex of postmortem brain samples of 8 cases, either not demented or affected by senile dementia of the Alzheimer type. A negative correlation between the GS protein level and the density of both classical beta A4 deposits and senile plaques was evidenced. Such a correlation for GS underlies a dysfunction of the astroglial metabolism and particularly of the glutamate and ammonia neutralization. Since GS is sensitive to oxidative lesioning, the changes in GS level that were observed, occurring at the posttranslational stage, might reflect oxidative damage and have severe consequences on the pathological cascade of events.

Regulation of tubulin, Tau and microtubule associated protein 2 expression during mouse brain development

The level of three microtubule proteins, tubulin, Tau and MAP2 and of their encoding mRNA was studied in the mouse brain at an early developmental stage (3 days postnatal) and in adulthood. The level of the mRNA encoding both tubulin and Tau decreased by 85% between these two stages whereas the encoded proteins decreased only by 50% during the same period. Thus, the level of these proteins seems to be regulated both negatively and positively by transcriptional and post translational mechanisms. In vitro transcription assays, performed with nuclei isolated at different postnatal stages, showed that the tubulin and Tau transcripts are produced with some variations during mouse brain development. However these fluctuations are much less important than the drops of the steady state levels of tubulin and Tau mRNA seen in vivo. Thus, the decrease in transcripts levels does not seem to result from reduced transcriptional activities, and can be ascribed to changes in mRNA stability occurring during brain development, i.e. to a post transcriptional mechanism. The situation is even more complex for MAP2: its encoding mRNA level remains constant during development whereas the in vitro transcription activity decreases markedly during the same period. Finally, MAP2 protein level increases during development although its encoding mRNA level remains constant suggesting that this protein is stabilized by a post translational mechanism.

Synthesis of glial fibrillary acidic protein in rat C6 glioma in chemically defined medium: cyclic AMP-dependent transcriptional and translational regulation

Glial fibrillary acidic protein (GFA) expression was induced in rat C6 glioma in chemically defined medium by the addition of N6, O2'-dibutyryl cyclic AMP (dbcAMP). Induction was dependent on the increase in intracellular cyclic AMP (cAMP), which was linearly correlated with added dbcAMP. Contrary to GFA mRNA synthesis, which can be obtained by cAMP-dependent and -independent pathways, translation of mRNA into GFA was observed only above a cellular cAMP concentration of approximately 0.2 fmol/cell. dbcAMP stimulation did not affect the vimentin concentration, which remained at a low level, but changed the cellular morphology from a bipolar to a stellate shape. A similar morphological change was observed after stimulation of C6 with lipopolysaccharide (LPS). However, LPS did not significantly increase the intracellular concentration of cAMP and the LPS-induced mRNA was not translated into GFA. Our results indicate that GFA synthesis is regulated at the mRNA level and at the translational level and that a cAMP-dependent mechanism determines the ultimate synthesis of GFA by a yet unknown mechanism.

Transcriptional regulation of glial fibrillary acidic protein (GFAP)-mRNA expression during postnatal development of mouse brain

During mouse brain maturation, GFAP-mRNA undergoes a two-step developmental expression. It increases between birth and day 15 (period of astrocytic proliferation) and then decreases until day 55 (period of astrocytic morphological differentiation). We have developed an in vitro transcription procedure, as a mean to study the part of transcriptional control in this biphasic expression. After RNA synthesis by endogenous RNA polymerases in nuclei isolated from mouse brain (of 3 to 55 days and 217 days), the relative rates of GFAP-mRNA transcripts were analysed by hybridization with a specific cDNA probe. As early as 3 days after birth, the rate of GFAP-mRNA transcripts was maximal, whereas unexpectedly, it showed a significant decrease in mice of 15 days and stayed low until the 55th day. Therefore, a transcriptional control may take place early in mouse brain postnatal development by increasing the transcriptional rate of the GFAP gene in astrocytes, and during the transition from proliferation to differentiation phase of astrocytes (that occurs at the 15th day after birth) by decreasing this rate. However, posttranscriptional events may also occur to modulate the level of the cytoplasmic GFAP-mRNA. In older mice (217 days), the low rate of GFAP-mRNA transcripts found is not concordant with the high cytoplasmic level generally observed in gliosis of the aging brain. Our data suggest posttranscriptional events at this age.

Melanocortins stimulate proliferation and induce morphological changes in cultured rat astrocytes by distinct transducing mechanisms

Melanocyte stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), and several peptides derived from pro-opiomelanocortin, are present in the dorsolateral hypothalamus and arcuate nucleus of several vertebrate species. These peptides affect central nervous system (CNS) functions including behavior, memory, and foetal brain development. In this study we investigated the effects of ACTH1-24, ACTH1-17, ACTH4-10, alpha-MSH, beta-MSH, and a potent analog (Nle4,D-Phe7)-alpha-MSH (melanocortins) on immunocytochemically defined astroglial cells prepared from primary cultures of 1-2-day-old rat brains. A cyclic adenosine 3',5'-monophosphate (cAMP) response to the melanocortins was only detected in astrocytes and not in other cell types in the culture. The extent of the cAMP response was greatest on day 21, the latest time tested. On the other hand, (methyl3H)-thymidine incorporation in astrocytes was significantly stimulated (1.5-2-fold) by melanocortins only in 7 and not in 14 and 21 day cultures. This mitogenic activity of melanocortins was not mimicked by other agents such as forskolin or isoproterenol which efficiently stimulate cAMP production in astrocytes. ACTH1-17 as a melanocortin representative induced significant morphological changes in 7 and 14 day cultures which included rounding of the cell body and process extension. This response, however, resembled that induced by forskolin and hence appears to be cAMP mediated. These findings suggest that astrocytes in the CNS may serve as a target for melanocortins. These peptides appear to affect differentiation and proliferation of these cells during certain developmental periods. While the morphological effects of melanocortins seem to be cAMP mediated, induction of proliferation of the astrocytes by melanocortins appears to involve an alternative signal transduction pathway.

Developmental expression of the glial fibrillary acidic protein (GFAP) gene in the mouse retina

1. In the nervous system, Glial fibrillary acidic protein (GFAP) is a well-known, cell type-specific marker for astrocytes. 2. In the mammalian retina, Muller cells, the major class of retinal glia, do not express GFAP or contain only low amounts of this protein. In retinas with photoreceptor degeneration, however, high levels of GFAP are found. It is possible that GFAP synthesis in these retinas could result from "dedifferentiation" of Muller cells as a consequence of disruption of normal neuron-glia interactions. 3. We have carried out immunocytochemical and in situ hybridization studies to examine whether GFAP or its mRNA is expressed by retinal cells early in embryonic development. 4. Our results show that GFAP-containing cells, which are probably astrocytes, are found only in the ganglion cell and nerve fiber layers and that these cells appear after postnatal day-1 (P-1) and continue to form until P-10. 5. Astrocyte formation starts from the optic disc and moves toward the periphery of the retina at a rate of approximately 160-200 microns per day. 6. An unexpected result from these studies is that GFAP mRNA levels are high in the first week of birth and decline rapidly as the animal develops. 7. Finally, we did not find either GFAP or GFAP mRNA in retinal cells other than astrocytes during normal development.

Tumor necrosis factor-induced proliferation of astrocytes from mature brain is associated with down-regulation of glial fibrillary acidic protein mRNA

Previous results from this laboratory have shown that tumor necrosis factor (TNF) is mitogenic for bovine astrocytes in chemically defined (CD) medium. The maximum mitogenic response was detected with 200 U/ml at 48 h. We have now extended these studies to assess the effect of TNF on message levels for the intermediate filament proteins glial fibrillary acidic protein (GFAP) and vimentin. The results have shown that, whereas TNF had only a slight effect on vimentin mRNA, TNF induced a marked decrease to 4.3 +/- 2.0% of controls in GFAP mRNA which was both time and dose dependent. The lowest effective dose was 50 U/ml and the maximal effective dose was 200 U/ml. Kinetic analysis of this response demonstrated that a marked decrease in GFAP mRNA was present at 12 h and continued to decrease through 72 h. To determine the reversibility of the TNF effect, astrocyte cultures were exposed to 200 U/ml TNF for varying periods of time and then cultured in fresh CD medium. A 1-h pulse with TNF was sufficient to reduce GFAP mRNA levels when measured 24 h later. However, cultures incubated with 200 U/ml TNF for 48 h followed by incubation in CD medium without TNF for 7 days showed that GFAP mRNA levels had returned to 60% of the control values. Nuclear runoff assays showed that the effect of TNF on GFAP mRNA was at the posttranscriptional level. Polyacrylamide gel electrophoretic analysis of astrocyte cytoskeletal proteins demonstrated that GFAP levels were reduced after a 5-day incubation with 200 U/ml TNF whereas protein levels of vimentin and actin were not significantly changed.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of various microtubule-associated protein 2 forms in the developing mouse brain and in cultured neurons and astrocytes

A cDNA probe specific to microtubule-associated protein 2 (MAP2) was used to study the expression of the mRNAs encoding the high- and low-molecular-weight MAP2 variants in cultured neurons and astrocytes. The timing and relative abundance of these MAP2 transcripts and of their encoded proteins were also studied in the developing cerebral hemispheres and cerebellum of the mouse. A 9-kb mRNA, known to encode high-molecular-weight MAP2, was expressed in cultured astrocytes, albeit at a lower level than in neurons. The 6-kb transcript, recently shown to encode low-molecular-weight MAP2 (MAP2c), was expressed in neurons and was the predominant MAP2 transcript of the astrocytes. The level of the 9- and 6-kb transcripts decreased at late stages of astroglial and neuronal cell culture. The 9-kb mRNA was detected in the cerebellum and cerebral hemispheres at every developmental stage. Although the levels of this mRNA varied slightly in the cerebral hemispheres, its expression was biphasic in the cerebellum. This might be explained by the differences in timing of development of the various neuronal cell types formed in these two brain areas. The 6-kb transcript was detected only at early developmental stages in the two brain areas. Correlating the temporal expression of the 9-kb mRNA to that of high-molecular-weight MAP2 indicates that the accumulation of this protein is in part regulated at a cytoplasmic level.

DBcAMP effect on the expression of GFAP and of its encoding mRNA in astroglial primary cultures

Short term and chronic dBcAMP effects on the expression of glial fibrillary acidic protein (GFAP) in astroglial primary cultures are investigated. Short (48 h) and long (more than 7 days) treatments with the cAMP derivative induce both cell shape changes and an increase in GFAP immunolabelling. Such effects are only associated with an increase in GFAP and in GFAP-mRNA levels in the long term treatment. These results suggest that the short term effect of dBcAMP induces post-translational modifications of the protein whereas the long term effect is associated with an increase in GFAP mRNA transcription and/or stability.

Effect of thyroid deficiency on glial fibrillary acidic protein (GFAP) and GFAP-mRNA in the cerebellum and hippocampal formation of the developing rat

The concentrations of glial fibrillary acidic protein (GFAP) and its encoding mRNA in the cerebellum and hippocampal formation were assayed during the development of normal and hypothyroid rats. Neonatal hypothyroidism induced a significant reduction in the GFAP concentration in both regions from day 14. The reduction was especially marked on day 35 in the cerebellum (-43%) and the hippocampal formation (-55%). The immunocytochemical study of vimentin showed that the developmental disappearance of this protein from the Bergmann and internal astrocytes is greatly delayed in the cerebellum of the hypothyroid rats. The reduction in GFAP concentration together with the delayed vimentin-GFAP transition could explain how astrocyte morphogenesis is impaired by neonatal thyroid deficiency. The GFAP-mRNA concentration in the hippocampal formation was reduced throughout the development of thyroid-deficient rats, while the GFAP-mRNA concentration in the cerebellum first increased between birth and day 14 to reach a peak well above the normal value (+78%) and decreased thereafter to reach 53% of the normal value by day 35. This transient increase in the cerebellar GFAP-mRNA concentration may be related to the astroglial hyperplasia that occurs in these animals. The difference between the developmental profile of GFAP and its encoding mRNA, especially under pathological conditions, indicates that two distinct mechanisms control the synthesis or stability of the protein and its messenger RNA, as was previously found in the forebrain of the developing normal rat.

GFAP turnover during astroglial proliferation and differentiation

The expression and turnover of the glial fibrillary acidic protein (GFAP) were studied in astroglial primary cultures during postnatal proliferation and maturation. 1. Immunocytochemical studies demonstrated that in immature proliferating astrocytes. GFAP was expressed as a filamentous organized crown around the nucleus whereas in the maturating cells, a GFAP labelled network began to radiate throughout the cytoplasm and GFAP was highly expressed in the astroglial processes. 2. GFAP turnover was studied at 3 periods of culture. The decay of radioactivity from prelabelled GFAP was followed from day 4-12 (immature stage), 11-19 (maturing stage) and 21-29 (morphologically differentiated stage). GFAP displayed a biphasic decay kinetic at each considered period. Two pools of GFAP distinctly appeared. The first one was a fast decaying pool with a half life of 16-18 h and of 5-6 days for the stable one. The unstable pool decreased from 70% to 30% of the total incorporated radioactivity from the proliferating stage to the most mature stage, whereas the stable pool increased proportionally.