Astrocyte stellation in saline media lacking bicarbonate: possible relation to intracellular pH and tyrosine phosphorylation

Glioprotective effects of resveratrol in hypothalamic astrocyte cultures obtained from interferon receptor knockout (IFNα/βR-/-) mice

Astrocytes play essential roles in the central nervous system (CNS), such as the regulation of glutamate metabolism, antioxidant defenses, and inflammatory/immune responses. Moreover, hypothalamic astrocytes seem to be crucial in the modulation of inflammatory processes, including those related to type I interferon signaling. In this regard, the polyphenol resveratrol has emerged as an important glioprotective molecule to regulate astrocyte functions. Therefore, this study aimed to investigate the immunomodulatory and protective effects of resveratrol in hypothalamic astrocyte cultures obtained from mouse depleted of type I interferon receptors (INF-α/β^-/-), a condition that can impair immune and inflammatory functions. Resveratrol upregulated glutamate transporter and glutamine synthetase gene expression, as well as modulated the release of wide range of cytokines and genes involved in the control of inflammatory response, besides the expression of adenosine receptors, which display immunomodulatory functions. Resveratrol also increased genes associated with redox balance, mitochondrial processes, and trophic factors signaling. The putative genes associated with glioprotective effects of resveratrol, including nuclear factor erythroid derived 2 like 2 (Nrf2), heme oxygenase 1 (HO-1), sirtuin 1 (SIRT1), and phosphoinositide 3-kinase (PI3K)/Akt, were further upregulated by resveratrol. Thus, our data show that resveratrol was able to modulate key genes associated with glial functionality and inflammatory response in astrocyte cultures derived from IFNα/βR^-/- mice. These data are in agreement with previous results, reinforcing its glioprotective effects even in hypothalamic astrocytes with altered inflammatory and immune signaling. Finally, this polyphenol can prepare astrocytes to better respond to injuries, including those associated with neuroimmunology defects.

Quantifying Filopodia in Cultured Astrocytes by an Algorithm

Astrocytes in vivo extend thin processes termed peripheral astrocyte processes (PAPs), in particular around synapses where they can mediate glia-neuronal communication. The relation of PAPs to synapses is not based on coincidence, but it is not clear which stimuli and mechanisms lead to their formation and are active during process extension/ retraction in response to neuronal activity. Also, the molecular basis of the extremely fine PAP morphology (often 50 to 100 nm) is not understood. These open questions can be best investigated under in vitro conditions studying glial filopodia. We have previously analyzed filopodial mechanisms (Lavialle et al. PNAS 108:12915) applying an automated method for filopodia morphometry, which is now described in greater detail. The Filopodia Specific Shape Factor (FSSF) developed integrates number and length of filopodia. It quantifies filopodia independent of overall astrocytic shape or size, which can be intricate in itself. The algorithm supplied here permits automated image processing and measurements using ImageJ. Cells have to be sampled in higher numbers to obtain significant results. We validate the FSSF, and characterize the systematic influence of thresholding and camera pixel grid on measurements. We provide exemplary results of substance-induced filopodia dynamics (glutamate, mGluR agonists, EGF), and show that filopodia formation is highly sensitive to medium pH (CO₂) and duration of cell culture. Although the FSSF was developed to study astrocyte filopodia with focus on the perisynaptic glial sheath, we expect that this parameter can also be applied to neuronal growth cones, non-neural cell types, or cell lines.

Transient acidification and subsequent proinflammatory cytokine stimulation of astrocytes induce distinct activation phenotypes

The foot processes of astrocytes cover over 60% of the surface of brain microvascular endothelial cells, regulating tight junction integrity. Retraction of astrocyte foot processes has been postulated to be a key mechanism in pathology. Therefore, movement of an astrocyte in response to a proinflammatory cytokine or even limited retraction of processes would result in leaky junctions between endothelial cells. Astrocytes lie at the gateway to the CNS and are instrumental in controlling leukocyte entry. Cultured astrocytes typically have a polygonal morphology until stimulated. We hypothesized that cultured astrocytes which were induced to stellate would have an activated phenotype compared with polygonal cells. We investigated the activation of astrocytes derived from adult macaques to the cytokine TNF-α under resting and stellated conditions by four parameters: morphology, intermediate filament expression, adhesion, and cytokine secretion. Astrocytes were stellated following transient acidification; resulting in increased expression of GFAP and vimentin. Stellation was accompanied by decreased adhesion that could be recovered with proinflammatory cytokine treatment. Surprisingly, there was decreased secretion of proinflammatory cytokines by stellated astrocytes compared with polygonal cells. These results suggest that astrocytes are capable of multiple phenotypes depending on the stimulus and the order stimuli are applied.

Morphological plasticity of rodent astroglia

In the past two decades, there has been an explosion of research on the role of neuroglial interactions in the control of brain homeostasis in both physiological and pathological conditions. Astrocytes, a subtype of glia in the central nervous system, are dynamic signaling elements that regulate neurogenesis and development of brain circuits, displaying intimate dynamic relationships with neurons, especially at synaptic sites where they functionally integrate the tripartite synapse. When astrocytes are isolated from the brain and maintained in culture, they exhibit a polygonal shape unlike their precursors in vivo. However, cultured astrocytes can be induced to undergo morphological plasticity leading to process formation, either by interaction with neurons or by the influence of pharmacological agents. This review highlights studies on the molecular mechanisms underlying morphological plasticity in astrocyte cultures and intact brain tissue, both in situ and in vivo.

Serofendic acid promotes stellation induced by cAMP and cGMP analogs in cultured cortical astrocytes

We investigated the effect of serofendic acid, a neuroprotective substance derived from fetal calf serum, on the morphological changes in cultured cortical astrocytes. Cultured astrocytes developed a stellate morphology with several processes following exposure to dibutylyl cAMP (dbcAMP), a membrane-permeable cAMP analog; 8-Br-cGMP, a membrane-permeable cGMP analog; or phorbol-12-myristate-13-acetate (PMA), a protein kinase C activator. Serofendic acid significantly accelerated the stellation induced by dbcAMP- and 8-Br-cGMP. In contrast, the PMA-induced stellation was not affected by serofendic acid. Next, we attempted to elucidate the mechanism underlying the dbcAMP-induced stellation and explore the site of action of serofendic acid. Both the stellation induced by dbcAMP and the promotional effect of serofendic acid were partially inhibited by KT5720, a specific protein kinase A (PKA) inhibitor. Furthermore, serofendic acid failed to facilitate the stellation induced by Y-27632, an inhibitor of Rho-associated kinase (ROCK). These results indicate that serofendic acid promotes dbcAMP- and 8-Br-cGMP-induced stellation and the promotional effect on dbcAMP-induced stellation is mediated at least partly by the regulation of PKA activity and not by controlling ROCK activity.

Effect of the atypical neuroleptic risperidone on morphology and S100B secretion in C6 astroglial lineage cells

We investigated the effect of the atypical neuroleptic risperidone on morphology and S100B secretion in C6 glioma cells, considering the putative involvement of astroglial cells in neuropsychiatric disorders. In the presence of high experimental doses of risperidone, C6 cells become stellate, with process-bearing cells and partial retraction of the cell body followed by detachment from the adhesion surface with practically no cell death. These results indicate that risperidone is able to interfere with C6 cell adhesion without toxic effects. RhoA activator LPA prevented the effects of risperidone on cell morphology. From 6 h risperidone induced a statistically significant increment of about 80% in S100B secretion. These data contribute to the proposal that glial cells are targets of risperidone, which could be involved in the therapeutic response of risperidone to improve autism symptoms.

Resveratrol increases glutamate uptake, glutathione content, and S100B secretion in cortical astrocyte cultures

Resveratrol (3,5,4'-trihydroxy-trans-stilbene) is a polyphenol present in grapes and red wine, which has antioxidant properties and a wide range of other biological effects. In this study, we investigated the effect of resveratrol, in a concentration range of 10-250 microM, on primary cortical astrocytes; evaluating cell morphology, parameters of glutamate metabolism such as glutamate uptake, glutamine synthetase activity and glutathione total content, and S100B secretion. Astrocyte cultures were prepared of cerebral cortex from neonate Wistar rats. Morphology was evaluated by phase-contrast microscopy and immunocytochemistry for glial fibrillary acidic protein (GFAP). Glutamate uptake was measured using L-[2,3-3H]glutamate. Glutamine synthetase and content of glutathione were measured by enzymatic colorimetric assays. S100B content was determined by ELISA. Typical polygonal morphology becomes stellated when astrocyte cultures were exposed to 250 microM resveratrol for 24 h. At concentration of 25 microM, resveratrol was able to increase glutamate uptake and glutathione content. Conversely, at 250 microM, resveratrol decreased glutamate uptake. Unexpectedly, resveratrol at this high concentration increased glutamine synthetase activity. Extracellular S100B increased from 50 microM upwards. Our findings reinforce the protective role of this compound in some brain disorders, particularly those involving glutamate toxicity. However, the underlying mechanisms of these changes are not clear at the moment and it is necessary caution with its administration because elevated levels of this compound could contribute to aggravate these conditions.

Regulation of pH in the mammalian central nervous system under normal and pathological conditions: facts and hypotheses

The maintenance of pH homeostasis in the CNS is of key importance for proper execution and regulation of neurotransmission, and deviations from this homeostasis are a crucial factor in the mechanism underlying a spectrum of pathological conditions. The first few sections of the review are devoted to the brain operating under normal conditions. The article commences with an overview of how extrinsic factors modelling the brain at work: neurotransmitters, depolarising stimuli (potassium and voltage changes) and cyclic nucleotides as major signal transducing vehicles affect pH in the CNS. Further, consequences of pH alterations on the major aspects of CNS function and metabolism are outlined. Next, the major cellular events involved in the transport, sequestration, metabolic production and buffering of protons that are common to all the mammalian cells, including the CNS cells. Since CNS function reflects tight interaction between astrocytes and neurons, the pH regulatory events pertinent to either cell type are discussed: overwhelming evidence implicates astrocytes as a key player in pH homeostasis in the brain. The different classes of membrane proteins involved in proton shuttling are listed and their mechanisms of action are given. These include: the Na+/H+ exchanger, different classes of bicarbonate transporters acting in a sodium-dependent- or -independent mode, monocarboxylic acid transporters and the vacuolar-type proton ATPase. A separate section is devoted to carbonic anhydrase, which is represented by multiple isoenzymes capable of pH buffering both in the cell interior and in the extracellular space. Next, impairment of pH regulation and compensatory responses occurring in brain affected by different pathologies: hypoxia/ischemia, epilepsy, hyperammonemic encephalopathies, cerebral tumours and HIV will be described. The review is limited to facts and plausible hypotheses pertaining to phenomena directly involved in pH regulation: changes in pH that accompany metabolic stress but have no distinct implications for the pH regulatory mechanisms are not dealt with. In most cases, the vast body of knowledge derived from in vitro studies remains to be verified in in vivo settings.

Bicarbonate-dependent pH(i) regulation by chondrocytes within the superficial zone of bovine articular cartilage

Control of chondrocyte pH (pH(i)) determines articular cartilage matrix metabolism. However, the transporters of chondrocytes in situ throughout cartilage zones are unclear, and we tested the hypothesis that chondocytes within the superficial zone (SZ) utilise a HCO(3) (-)-dependent system absent from other zones. Imaging of single BCECF-labelled cells was used to monitor the pH(i) of in situ chondrocytes within the cartilage zones, and also that of cells isolated from the SZ or full depth (FD) explants. Resting pH(i) and intrinsic buffering power (beta(i)) in HEPES-buffered saline was not different between SZ and DZ cells, however the pH(i) of SZ chondrocytes was lower in HCO(3) (-) saline. Ammonium pre-pulse was used to acid-load cells and pH(i) recovery by in situ or isolated SZ chondrocytes shown to be totally dependent on HCO(3) (-). pH(i) recovery rate was significantly (P < 0.05) greater for in situ cells, suggesting that isolation damaged the HCO(3) (-)-dependent system. Recovery of pH(i) by in situ cells was blocked by the anion transport inhibitor DIDS, and partially inhibited by EIPA probably non-specifically. Recovery of pH(i) by acidified MZ or DZ cells or those isolated from FD explants was not affected by HCO(3) (-) (P > 0.05). Na(+)-dependent HCO(3) (-)-(NBC) transporters were identified in SZ chondrocytes by fluorescence immunohistochemistry suggesting that this system might account for the HCO(3) (-)-dependent recovery of pH(i). Bovine articular cartilage chondrocytes possess a HCO(3) (-)-dependent transporter which plays a key role in pH(i) regulation in cells in the SZ, but not in chondrocytes within deeper cartilage zones.

Resveratrol increases glutamate uptake and glutamine synthetase activity in C6 glioma cells

Resveratrol, a phytoalexin found mainly in grapes, is a promising natural product with anti-cancer and cardio-protective activities. Here, we investigated, in C6 glioma cells, the effect of resveratrol on some specific parameters of astrocyte activity (glutamate uptake, glutamine synthetase and secretion of S100B, a neurotrophic cytokine) commonly associated with the protective role of these cells. Cell proliferation was significantly decreased by 8% and 26%, following 24h of treatment with 100 and 250 microM resveratrol. Extracellular S100B increased after 48 h of resveratrol exposure. Short-term resveratrol exposure (from 1 to 100 microM) induced a linear increase in glutamate uptake (up to 50% at 100 microM resveratrol) and in glutamine synthetase activity. Changes in these glial activities can contribute to the protective role of astrocytes in brain injury conditions, reinforcing the putative use of this compound in the therapeutic arsenal against neurodegenerative diseases and ischemic disorders.

Human astrocytes can be induced to differentiate into cells with neuronal phenotype

Several recent studies have proposed that astrocytes may contribute to neurogenesis, not only as a source of trophic substances regulating it, but also as stem cells themselves. In order to better understand these mechanisms, primary astrocyte cultures were established from human fetal brain. After 3-4 weeks in culture, astrocytes (about 95% GFAP+; neurofilament, NF-; neuro-specific enolase, NSE-) were treated with a cocktail of protein kinase activators and FGF-1. After 5 h of treatment, most cells showed morphological changes that increased progressively up to 24-48 h, exhibiting a round cell body with long processes. Immunocytochemistry showed that treatment-induced NF and NSE expression in about 40% of cells. Nestin expression increased after treatment, whereas GFAP immunostaining was not significantly modified. Western blot and RT-PCR confirmed the results. No neuronal electrophysiological properties were observed after treatment, suggesting an incomplete maturation under these experimental conditions. Understanding the regenerative capability and neurogenic potential of astrocytes might be useful in devising therapeutic approaches for a variety of neurological disorders.

Signal transduction mechanisms involved in the proliferation of C6 glioma cells induced by lysophosphatidic acid

We studied pathways involved in the proliferation of rat C6 glioma cells induced by lysophosphatidic acid (LPA), a phospholipid with diverse biological functions. LPA induced a dose-responsive proliferation of C6 cells after 48 h. Proliferation was blocked by inhibitors of the sodium/proton exchanger type 1 (NHE1), Rho-associated kinase, the phosphatidylinositol 3-kinase/Akt pathway (PI3K/Akt), protein kinase C (PKC) and extracellular signal regulated kinase kinase (MEK). Phospho-specific antibodies were used to investigate the pathways involved. LPA induced transient (10 min) phosphorylations of ERK 1/2, Akt and the transcription factor CREB. The LPA-induced phosphorylation of ERK 1/2 and CREB was blocked by inhibition of PI3K, PKC and MEK, but that of Akt was only inhibited by wortmannin, the PI3K inhibitor. Inhibition of Rho kinase or NHE1 did not reduce the LPA-induced phosphorylation of ERK, Akt or CREB. The results were compared with the effects of LPA on transduction pathways in other cell types.

Role of Rho GTPase in astrocyte morphology and migratory response during in vitro wound healing

Small Rho GTPases are key regulators of the cytoskeleton in a great variety of cells. Rho function mediates morphological changes as well as locomotor activity. Using astrocyte cultures established from neonatal mice we investigated the role of Rho in process formation during astrocyte stellation. Using a scratch-wound model, we examined the impact of Rho on a variety of morphological and functional variables such as stellation and migratory activity during wound healing. C3 proteins are widely used to study cellular Rho functions. In addition, C3 derived from Clostridium botulinum (C3bot) is considered selectively to promote neuronal regeneration. Because the latter requires a balanced activity of neurones and glial cells, the effects of C3 protein on glial cells such as astrocytes have to be considered carefully. Low nanomolar concentrations of C3 proteins significantly promoted process outgrowth and increased process branching. Besides enzymatic inactivation of Rho by ADP-ribosylation, changes in protein levels of the various Rho GTPases may also contribute to the observed effects. Furthermore, incubation of scratch-wounded astrocyte cultures with C3bot accelerated wound healing. By inhibiting the Rho downstream effector ROCK with the selective inhibitor Y27632 we were able to demonstrate that the accelerated wound closure resulted from both enhanced polarized process formation and increased migratory activity of astrocytes into the lesion site. These results suggest that Rho negatively regulates astrocytic process growth and migratory responses after injury and that its inactivation by C3bot in nanomolar concentrations promotes astrocyte migration.

Long-term intermittent compressive stimulation improves the composition and mechanical properties of tissue-engineered cartilage

Tissue engineering of articular cartilage is a promising alternative for cartilage repair. However, it has been difficult to develop tissue in vitro that mimicks native cartilage. Cartilaginous tissue formed in vitro does not accumulate enough extracellular matrix, is deficient in collagen, and possesses only a fraction of the mechanical properties of native cartilage. In this study, we investigated whether long-term intermittent compressive stimulation would improve the quality of the generated tissue. Chondrocyte cultures were established on the surface of porous calcium polyphosphate substrates and allowed to form cartilaginous tissue. In vitro-formed tissues were subjected to different stimulation protocols for 1 week. The optimal mechanical stimulation parameters identified in this short-term study were then applied to the cultures for up to 4 weeks. Mechanical stimulation applied at a 5% compressive amplitude at a frequency of 1 Hz for 400 cycles every second day resulted in the greatest increase in collagen synthesis (37 +/- 9% over control) while not significantly affecting proteoglycan synthesis (2 +/- 8% over control). This condition, applied to the chondrocyte cultures for 4 weeks, resulted in a significant increase in the amount of tissue that formed (stimulated, 2.4 +/- 0.2 mg dry wt; unstimulated, 1.61 +/- 0.08 mg dry wt). Stimulated tissues contained approximately 40% more collagen (stimulated, 590 +/- 58 microg; unstimulated, 420 +/- 42 microg), and 30% more proteoglycans (stimulated, 393 +/- 34 microg; unstimulated, 302 +/- 32 microg) as well as displaying a 2- to 3-fold increase in compressive mechanical properties (maximal equilibrium stress: stimulated, 10 +/- 1 kPa; unstimulated, 5 +/- 1 kPa; maximal equilibrium modulus: stimulated, 80 +/- 23 kPa; unstimulated, 24 +/- 6 kPa). The results of this study demonstrate that intermittent mechanical stimulation can increase collagen synthesis and, when applied over a 4-week period, can accelerate extracellular matrix accumulation as well as improve the material properties of the developed tissue. Interestingly, only short periods of mechanical stimulation (6 min every second day) were needed to affect the quality of cartilaginous tissue formed in vitro.

Nutrition during brain activation: does cell-to-cell lactate shuttling contribute significantly to sweet and sour food for thought?

Functional activation of astrocytic metabolism is believed, according to one hypothesis, to be closely linked to excitatory neurotransmission and to provide lactate as fuel for oxidative metabolism in neighboring neurons. However, review of emerging evidence suggests that the energetic demands of activated astrocytes are higher and more complex than recognized and much of the lactate presumably produced by astrocytes is not locally oxidized during activation. In vivo activation studies in normal subjects reveal that the rise in consumption of blood-borne glucose usually exceeds that of oxygen, especially in retina compared to brain. When the contribution of glycogen, the brain's major energy reserve located in astrocytes, is taken into account the magnitude of the carbohydrate-oxygen utilization mismatch increases further because the magnitude of glycogenolysis greatly exceeds the incremental increase in utilization of blood-borne glucose. Failure of local oxygen consumption to equal that of glucose plus glycogen in vivo is strong evidence against stoichiometric transfer of lactate from astrocytes to neighboring neurons for oxidation. Thus, astrocytes, not nearby neurons, use the glycogen for energy during physiological activation in normal brain. These findings plus apparent compartmentation of metabolism of glycogen and blood-borne glucose during activation lead to our working hypothesis that activated astrocytes have high energy demands in their fine perisynaptic processes (filopodia) that might be met by glycogenolysis and glycolysis coupled to rapid lactate clearance. Tissue culture studies do not consistently support the lactate shuttle hypothesis because key elements of the model, glutamate-induced increases in glucose utilization and lactate release, are not observed in many astrocyte preparations, suggesting differences in their oxidative capacities that have not been included in the model. In vivo nutritional interactions between working neurons and astrocytes are not as simple as implied by "sweet (glucose-glycogen) and sour (lactate) food for thought."

The influence of the extracellular matrix on the morphology and intracellular pH of cultured astrocytes exposed to media lacking bicarbonate

In previous work we showed that the polygonal shape of hippocampal astrocytes cultured on poly-L-lysine changes to a stellate morphology with loss of actinomyosin stress fibers on exchanging the culture medium for saline buffered with HEPES [Brain Res 946 (2002)12]. By contrast, in bicarbonate-buffered saline containing Ca(2+) astrocytes remained polygonal and continued to express stress fibers. Evidence suggests that stellation induced by saline buffered with HEPES is related to intracellular acidification due to the absence of bicarbonate. Here we studied the influence of the matrix used in preparing astrocyte cultures. Stellation in HEPES-saline occurred on a matrix of fibronectin, but not on matrices of collagen I or IV provided Ca(2+) was present. Laminin partially prevented stellation in HEPES-saline. Further, the intracellular acidification induced by HEPES-saline observed in astrocytes cultured on polylysine was abolished in cells cultured on collagens and was attentuated on a matrix of laminin. Two observations suggested the involvement of integrins and focal adhesions. (1) Treatment of cultures on collagens with a blocking antibody to the beta1 integrin subunit abolished protection against HEPES-induced stellation. (2) Compared with polylysine, astrocytes cultured on collagens expressed increased contents of phosphotyrosine proteins, focal adhesion proteins vinculin and paxillin, the beta1 integrin subunit and increased numbers of focal adhesions labelled with anti-vinculin. The observation that astrocytes cultured on collagen I or IV, in contrast to polylysine, express stress fibers and a constant intracellular pH in the absence of buffering by bicarbonate may be related to the fact that in the intact brain astrocytic processes (or end-feet) encounter and bind to collagen IV and laminin in the basement membrane of the endothelial cells which surround the cerebral capillaries. It is also possible that astrocytes retain this capacity from early development when fibrous matrix proteins are present.