Disruption of actin cytoskeleton in cultured rat astrocytes suppresses ATP- and bradykinin-induced [Ca(2+)](i) oscillations by reducing the coupling efficiency between Ca(2+) release, capacitative Ca(2+) entry, and store refilling

Are cytoskeleton changes observed in astrocytes functionally linked to aging?

Astrocytes are active participants in the performance of the Central Nervous System (CNS) in both health and disease. During aging, astrocytes are susceptible to reactive astrogliosis, a molecular state characterized by functional changes in response to pathological situations, and cellular senescence, characterized by loss of cell division, apoptosis resistance, and gain of proinflammatory functions. This results in two different states of astrocytes, which can produce proinflammatory phenotypes with harmful consequences in chronic conditions. Reactive astrocytes and senescent astrocytes share morpho-functional features that are dependent on the organization of the cytoskeleton. However, such changes in the cytoskeleton have yet to receive the necessary attention to explain their role in the alterations of astrocytes that are associated with aging and pathologies. In this review, we summarize all the available findings that connect changes in the cytoskeleton of the astrocytes with aging. In addition, we discuss future avenues that we believe will guide such a novel topic.

Nanotopography and Microconfinement Impact on Primary Hippocampal Astrocyte Morphology, Cytoskeleton and Spontaneous Calcium Wave Signalling

Astrocytes' organisation affects the functioning and the fine morphology of the brain, both in physiological and pathological contexts. Although many aspects of their role have been characterised, their complex functions remain, to a certain extent, unclear with respect to their contribution to brain cell communication. Here, we studied the effects of nanotopography and microconfinement on primary hippocampal rat astrocytes. For this purpose, we fabricated nanostructured zirconia surfaces as homogenous substrates and as micrometric patterns, the latter produced by a combination of an additive nanofabrication and micropatterning technique. These engineered substrates reproduce both nanotopographical features and microscale geometries that astrocytes encounter in their natural environment, such as basement membrane topography, as well as blood vessels and axonal fibre topology. The impact of restrictive adhesion manifests in the modulation of several cellular properties of single cells (morphological and actin cytoskeletal changes) and the network organisation and functioning. Calcium wave signalling was observed only in astrocytes grown in confined geometries, with an activity enhancement in cells forming elongated agglomerates with dimensions typical of blood vessels or axon fibres. Our results suggest that calcium oscillation and wave propagation are closely related to astrocytic morphology and actin cytoskeleton organisation.

The Importance and Control of Low-Grade Inflammation Due to Damage of Cellular Barrier Systems That May Lead to Systemic Inflammation

Systemic low-grade inflammation can be initiated in vivo after traumatic injury or in chronic diseases such as neurodegenerative, metabolic, and autoimmune diseases. Inducers of inflammation trigger production of inflammatory mediators, which alter the functionality of tissues and organs and leads to harmful induction of different barrier systems in the body, where the blood-brain barrier, the blood-retinal barrier, blood-nerve barrier, blood-lymph barrier and the blood-cerebrospinal fluid barrier play major roles. The different barriers are unique but structured in a similar way. They are equipped with sophisticated junctional complexes where different connexins, protein subunits of gap junction channels and hemichannels, constitute important partners. The cells involved in the various barriers are coupled in networks, are excitable but do not express action potentials and may be targets for inflammation leading to changes in several biochemical cellular parameters. During any type of inflammation barrier break-down is observed where any form of injury can start with low-grade inflammation and may lead to systemic inflammation.

Diversity of Evoked Astrocyte Ca2+ Dynamics Quantified through Experimental Measurements and Mathematical Modeling

Astrocytes are a major cell type in the mammalian brain. They are not electrically excitable, but generate prominent Ca²⁺ signals related to a wide variety of critical functions. The mechanisms driving these Ca²⁺ events remain incompletely understood. In this study, we integrate Ca²⁺ imaging, quantitative data analysis, and mechanistic computational modeling to study the spatial and temporal heterogeneity of cortical astrocyte Ca²⁺ transients evoked by focal application of ATP in mouse brain slices. Based on experimental results, we tune a single-compartment mathematical model of IP₃-dependent Ca²⁺ responses in astrocytes and use that model to study response heterogeneity. Using information from the experimental data and the underlying bifurcation structure of our mathematical model, we categorize all astrocyte Ca²⁺ responses into four general types based on their temporal characteristics: Single-Peak, Multi-Peak, Plateau, and Long-Lasting responses. We find that the distribution of experimentally-recorded response types depends on the location within an astrocyte, with somatic responses dominated by Single-Peak (SP) responses and large and small processes generating more Multi-Peak responses. On the other hand, response kinetics differ more between cells and trials than with location within a given astrocyte. We use the computational model to elucidate possible sources of Ca²⁺ response variability: (1) temporal dynamics of IP₃, and (2) relative flux rates through Ca²⁺ channels and pumps. Our model also predicts the effects of blocking Ca²⁺ channels/pumps; for example, blocking store-operated Ca²⁺ (SOC) channels in the model eliminates Plateau and Long-Lasting responses (consistent with previous experimental observations). Finally, we propose that observed differences in response type distributions between astrocyte somas and processes can be attributed to systematic differences in IP₃ rise durations and Ca²⁺ flux rates.

Sildenafil (Viagra(®)) prevents and restores LPS-induced inflammation in astrocytes

Astrocytes are effectively involved in the pathophysiological processes in the central nervous system (CNS) and may contribute to or protect against development of inflammatory and degenerative diseases. Sildenafil is a potent and selective phosphodiesterase-5 (PDE-5) inhibitor, which induces cyclic GMP accumulation. However, the mechanisms of actions on glial cells are not clear. The aim of the present work is to evaluate the role of sildenafil in lipopolysaccharide (LPS)-stimulated astrocytes. The cytoskeleton integrity and Ca(2+) waves were assessed as indicators of inflammatory state. Two main groups were done: (A) one prevention and (B) one restoration. Each of these groups: A1: control; A2: LPS for 24h; A3: sildenafil 1 or 10μM for 4h and then sildenafil 1 or 10μM+LPS for 24h. B1: control; B2: LPS for 24h; B3: LPS for 24h and then LPS+sildenafil 1 or 10μM for 24h. Cytoskeleton integrity was analyzed through GFAP immunolabeling and actin labeling with an Alexa 488-conjugated phalloidin probe. Calcium responses were assessed through a Ca(2+)-sensitive fluorophore Fura-2/AM. The results show that both preventive and restorative treatments with sildenafil (in both concentrations) reduced the Ca(2+) responses in intensity and induced a more organized actin fiber pattern, compared to LPS treated cells. This work demonstrated for the first time that astrocytes are a key part of the sildenafil protective effects in the CNS.

Inhibition of TNF-α protects in vitro brain barrier from ischaemic damage

Cerebral ischaemia, associated with neuroinflammation and oxidative stress, is known to perturb blood-brain barrier (BBB) integrity and promote brain oedema formation. Using an in vitro model of human BBB composed of brain microvascular endothelial cells and astrocytes, this study examined whether suppression of TNF-α, a potent pro-inflammatory cytokine, might attenuate ischaemia-mediated cerebral barrier damage. Radical decreases in transendothelial electrical resistance and concomitant increases in paracellular flux across co-cultures exposed to increasing periods of oxygen-glucose deprivation alone (0.5-20 h) or followed by 20 h of reperfusion (OGD ± R) confirmed the deleterious effects of ischaemic injury on cerebral barrier integrity and function which concurred with reductions in tight junction protein (claudin-5 and occludin) expressions. OGD ± R elevated TNF-α secretion, NADPH oxidase activity, O2(-) production, actin stress fibre formation, MMP-2/9 activities and apoptosis in both endothelial cells and astrocytes. Increases in MMP-2 activity were confined to its extracellular isoform and treatments with OGD+R in astrocytes where MMP-9 could not be detected at all. Co-exposure of individual cell lines or co-cultures to an anti-TNF-α antibody dramatically diminished the extent of OGD ± R-evoked oxidative stress, morphological changes, apoptosis, MMP-2/9 activities while improving the barrier function through upregulation of tight junction protein expressions. In conclusion, vitiation of the exaggerated release of TNF-α may be an important therapeutic strategy in preserving cerebral integrity and function during and following a cerebral ischaemic attack.

Coupled cell networks are target cells of inflammation, which can spread between different body organs and develop into systemic chronic inflammation

Several organs in the body comprise cells coupled into networks. These cells have in common that they are excitable but do not express action potentials. Furthermore, they are equipped with Ca²⁺ signaling systems, which can be intercellular and/or extracellular. The transport of small molecules between the cells occurs through gap junctions comprising connexin 43. Examples of cells coupled into networks include astrocytes, keratinocytes, chondrocytes, synovial fibroblasts, osteoblasts, connective tissue cells, cardiac and corneal fibroblasts, myofibroblasts, hepatocytes, and different types of glandular cells. These cells are targets for inflammation, which can be initiated after injury or in disease. If the inflammation reaches the CNS, it develops into neuroinflammation and can be of importance in the development of systemic chronic inflammation, which can manifest as pain and result in changes in the expression and structure of cellular components. Biochemical parameters of importance for cellular functions are described in this review.

Actin filament reorganization in astrocyte networks is a key functional step in neuroinflammation resulting in persistent pain: novel findings on network restoration

In recent years, the importance of glial cell activation in the generation and maintenance of long-term pain has been investigated. One novel mechanism underlying long-lasting pain is injury-induced inflammation in the periphery, followed by microglial activation in the dorsal horn of the spinal cord, which results in local neuroinflammation. An increase in neuronal excitability may follow, with intense signaling along the pain tracts to the thalamus and the parietal cortex along with other cortical regions for the identification and recognition of the injury. If the local neuroinflammation develops into a pathological state, then the astrocytes become activated. Previous studies in which lipopolysaccharide (LPS) was used to induce inflammation have shown that in a dysfunctional astrocyte network, the actin cytoskeleton is reorganized from the normally occurring F-actin stress fibers into the more diffusible, disorganized, ring-form globular G-actin. In addition, Ca(2+) signaling systems are altered, Na(+)- and glutamate transporters are downregulated, and pro-inflammatory cytokines, particularly IL-1β, are released in dysfunctional astrocyte networks. In a series of experiments, we have demonstrated that these LPS-induced changes in astrocyte function can be restored by stimulation of Gi/o and inhibition of Gs with a combination of a μ-receptor agonist and ultralow concentrations of a μ-receptor antagonist and by inhibition of cytokine release, particularly IL-1β, by the antiepileptic drug levetiracetam. These findings could be of clinical significance and indicate a novel treatment for long-term pain.

Characterization of adult rat astrocyte cultures

Astrocytes, a major class of glial cells, regulate neurotransmitter systems, synaptic processing, ion homeostasis, antioxidant defenses and energy metabolism. Astrocyte cultures derived from rodent brains have been extensively used to characterize astrocytes' biochemical, pharmacological and morphological properties. The aims of this study were to develop a protocol for routine preparation and to characterize a primary astrocyte culture from the brains of adult (90 days old) Wistar rats. For this we used enzymatic digestion (trypsin and papain) and mechanical dissociation. Medium exchange occurred from 24 h after obtaining a culture and after, twice a week up to reach the confluence (around the 4^th to 5^th week). Under basal conditions, adult astrocytes presented a polygonal to fusiform and flat morphology. Furthermore, approximately 95% the cells were positive for the main glial markers, including GFAP, glutamate transporters, glutamine synthetase and S100B. Moreover, the astrocytes were able to take up glucose and glutamate. Adult astrocytes were also able to respond to acute H₂O₂ exposure, which led to an increase in reactive oxygen species (ROS) levels and a decrease in glutamate uptake. The antioxidant compound resveratrol was able to protect adult astrocytes from oxidative damage. A response of adult astrocytes to an inflammatory stimulus with LPS was also observed. Changes in the actin cytoskeleton were induced in stimulated astrocytes, most likely by a mechanism dependent on MAPK and Rho A signaling pathways. Taken together, these findings indicate that the culture model described in this study exhibits the biochemical and physiological properties of astrocytes and may be useful for elucidating the mechanisms related to the adult brain, exploring changes between neonatal and adult astrocytes, as well as investigating compounds involved in cytotoxicity and cytoprotection.

Modulation of RhoGTPases improves the behavioral phenotype and reverses astrocytic deficits in a mouse model of Rett syndrome

RhoGTPases are crucial molecules in neuronal plasticity and cognition, as confirmed by their role in non-syndromic mental retardation. Activation of brain RhoGTPases by the bacterial cytotoxic necrotizing factor 1 (CNF1) reshapes the actin cytoskeleton and enhances neurotransmission and synaptic plasticity in mouse brains. We evaluated the effects of a single CNF1 intracerebroventricular inoculation in a mouse model of Rett syndrome (RTT), a rare neurodevelopmental disorder and a genetic cause of mental retardation, for which no effective therapy is available. Fully symptomatic MeCP2-308 male mice were evaluated in a battery of tests specifically tailored to detect RTT-related impairments. At the end of behavioral testing, brain sections were immunohistochemically characterized. Magnetic resonance imaging and spectroscopy (MRS) were also applied to assess morphological and metabolic brain changes. The CNF1 administration markedly improved the behavioral phenotype of MeCP2-308 mice. CNF1 also dramatically reversed the evident signs of atrophy in astrocytes of mutant mice and restored wt-like levels of this cell population. A partial rescue of the overexpression of IL-6 cytokine was also observed in RTT brains. CNF1-induced brain metabolic changes detected by MRS analysis involved markers of glial integrity and bioenergetics, and point to improved mitochondria functionality in CNF1-treated mice. These results clearly indicate that modulation of brain RhoGTPases by CNF1 may constitute a totally innovative therapeutic approach for RTT and, possibly, for other disorders associated with mental retardation.

Naloxone and ouabain in ultralow concentrations restore Na+/K+-ATPase and cytoskeleton in lipopolysaccharide-treated astrocytes

Astrocytes respond to inflammatory stimuli and may be important modulators of the inflammatory response in the nervous system. This study aimed first to assess how astrocytes in primary culture behave in response to inflammatory stimuli concerning intracellular Ca(2+) responses, expression of Toll-like receptor 4 (TLR4), Na(+)/K(+)-ATPase, actin filament organization, and expression of cytokines. In a cell culture model with lipopolysaccharide (LPS), astrocyte response was assessed first in the acute phase and then after incubation with LPS for 1-48 h. The concentration curve for LPS-stimulated Ca(2+) responses was bell-shaped, and the astrocytes expressed TLR4, which detects LPS and evokes intracellular Ca(2+) transients. After a long incubation with LPS, TLR4 was up-regulated, LPS-evoked Ca(2+) transients were expressed as oscillations, Na(+)/K(+)-ATPase was down-regulated, and the actin filaments were disorganized. Interleukin-1β (IL-1β) release was increased after 24 h in LPS. A second aim was to try to restore the LPS-induced changes in astrocytes with substances that may have dose-dependent anti-inflammatory properties. Naloxone and ouabain were tested separately in ultralow or high concentrations. Both substances evoked intracellular Ca(2+) transients for all of the concentrations from 10(-15) up to 10(-4) M. Neither substance blocked the TLR4-evoked Ca(2+) responses. Naloxone and ouabain prevented the LPS-induced down-regulation of Na(+)/K(+)-ATPase and restored the actin filaments. Ouabain, in addition, reduced the IL-1β release from reactive astrocytes. Notably, ultralow concentrations (10(-12) M) of naloxone and ouabain showed these qualities. Ouabain seems to be more potent in these effects of the two tested substances.

Isoflurane impairs immature astroglia development in vitro: the role of actin cytoskeleton

General anesthetics, either alone or in combination, can be detrimental to the developing mammalian brain and induce extensive apoptotic degeneration of immature neurons when they are administered at the peak of synaptogenesis. Because neuron development and normal functions depend on the integrity of astroglia, we sought to determine whether general anesthesia also causes disturbances in the early development of astroglia. Using isoflurane, an inhaled anesthetic that is highly toxic to immature neurons, we studied primary astroglia cultures, focusing on very early development (Day-In-Vitro 4 treatment). Exposure to 3% isoflurane for 24 hours delayed morphological differentiation and impaired the growth of immature astrocytes. The timing of delayed astroglia maturation and growth coincided with a major disturbance in actin cytoskeleton sculpting that was manifest as impaired actin stress fiber formation and cytoskeletal organization and downregulation of the focal adhesion protein, paxillin. Isoflurane-induced actin cytoskeletal changes were accompanied by a significant decrease in protein levels of the endogenous GTPase RhoA that regulates the phosphorylation of myosin light chain protein, suggesting that isoflurane-induced impairment in glial growth and morphological development is, in part, mediated by the RhoA/myosin light chain protein signaling pathway.

Acute ethanol exposure disrupts actin cytoskeleton and generates reactive oxygen species in c6 cells

Central nervous system dysfunctions are among the most significant effects of exposure to ethanol and the glial cells that play an important role in maintaining neuronal function, are extremely involved with these effects. The actin cytoskeleton plays a crucial role in a wide variety of cellular functions, especially when there is some injury. Therefore the aim of the present study was to analyze the short-term effects of ethanol (50, 100 and 200 mM) on the cytoskeleton of C6 glioma cells. Here we report that acute ethanol exposure profoundly disrupts the actin cytoskeleton in C6 cells decreasing stress fiber formation and downregulating RhoA and vinculin immunocontent. In contrast, microtubule and GFAP networks were not altered. We further demonstrate that anti-oxidants prevent ethanol-induced actin alterations, suggesting that the actions of ethanol on the actin cytoskeleton are related with generation of reactive oxygen species (ROS) in these cells. Our results show that ethanol at concentrations described to be toxic to the central nervous system was able to target the cytoskeleton of C6 cells and this effect could be related with increased ROS generation. Therefore, we propose that the dynamic restructuring of the cytoskeleton of glial cells might contribute to the response to the injury provoked by binge-like ethanol exposure in brain.

Long-term pain, neuroinflammation and glial activation

Nociceptive and neuropathic pain signals are known to result from noxious stimuli, which are converted into electrical impulses within tissue nociceptors. There is a complex equilibrium of pain-signalling and pain-relieving pathways connecting PNS and CNS. Drugs against long-term pain are today directed against increased neuronal excitability, mostly with less success. An injury often starts with acute physiological pain, which becomes inflammatory, nociceptive, or neuropathic, and may be transferred into long-term pain. Recently a low-grade inflammation was identified in the spinal cord and along the pain pathways to thalamus and the parietal cortex. This neuroinflammation is due to activation of glial cells, especially microglia, with production of cytokines and other inflammatory mediators within the CNS. Additionally, substances released to the blood from the injured region influence the blood-brain barrier, and give rise to an increased permeability of the tight junctions of the capillary endothelial cells, leading to passage of blood cells into the CNS. These cells are transformed into reactive microglia. If the inflammation turns into a pathological state the astrocytes will be activated. They are coupled into networks and respond to substances released by the capillary endothelial cells, to cytokines released from microglia, and to neurotransmitters and peptides released from neurons. As the astrocytes occupy a strategic position between the vasculature and synapses, they monitor the neuronal activity and transmitter release. Increased release of glutamate and ATP leads to disturbances in Ca2+ signalling, increased production of cytokines and free radicals, attenuation of the astrocyte glutamate transport capacity, and conformational changes in the astrocytic cytoskeleton, the actin filaments, which can lead to formation and rebuilding of new synapses. New neuronal contacts are established for maintaining and spreading pain sensation with the astrocytic networks as bridges. Thereby the glial cells can maintain the pain sensation even after the original injury has healed, and convert the pain into long-term by altering neuronal excitability. It can even be experienced from other parts of the body. As astrocytes are intimate co-players with neurons in the CNS, more knowledge on astrocyte responses to inflammatory activators may give new insight in our understanding of mechanisms of low-grade inflammation underlying long-term pain states and pain spreading. Novel treatment strategies would be to restore glial cell function and thereby attenuate the neuroinflammation.

In inflammatory reactive astrocytes co-cultured with brain endothelial cells nicotine-evoked Ca(2+) transients are attenuated due to interleukin-1beta release and rearrangement of actin filaments

The aim of this study was to investigate whether nicotine acetylcholine receptors (nAChRs) are expressed in a more pronounced way in astrocytes co-cultured with microvascular endothelial cells from adult rat brain, compared with monocultured astrocytes, as a sign of a more developed signal transduction system. Also investigated was whether nicotine plays a role in the control of neuroinflammatory reactivity in astrocytes. Ca(2+) imaging experiments were performed using cells loaded with the Ca(2+) indicator Fura-2/AM. Co-cultured astrocytes responded to lower concentrations of nicotine than did monocultured astrocytes, indicating that they are more sensitive to nicotine. Co-cultured astrocytes also expressed a higher selectivity for alpha7nAChR and alpha4/beta2 subunits and evoked higher Ca(2+) transients compared with monocultured astrocytes. The Ca(2+) transients referred to are activators of Ca(2+)-induced Ca(2+) release from intracellular stores, both IP(3) and ryanodine, triggered by influx through receptor channels. The nicotine-induced Ca(2+) transients were attenuated after incubation with the inflammatory mediator lipopolysaccharide (LPS), but were not attenuated after incubation with the pain-transmitting peptides substance P and calcitonin-gene-related peptide, nor with the infection and inflammation stress mediator, leptin. Furthermore, LPS-induced release of interleukin-1beta (IL-1beta) measured by enzyme-linked immunosorbent assay (ELISA) was more pronounced in co-cultured versus monocultured astrocytes. Incubation with both LPS and IL-1beta further attenuated nicotine-induced Ca(2+) response. We also found that LPS and IL-1beta induced rearrangement of the F-actin filaments, as measured with an Alexa488-conjugated phalloidin probe. The rearrangements consisted of increases in ring formations and a more dispersed appearance of the filaments. These results indicate that there is a connection between a dysfunction of nicotine Ca(2+) signaling in inflammatory reactive astrocytes and upregulation of IL-1beta and the rearrangements of actin filaments in the cells.

Latrunculin A depolarizes starfish oocytes

Depolymerization of the actin cytoskeleton may liberate Ca2+ from InsP3-sensitive stores in some cell types, including starfish oocytes, while inhibiting Ca2+ influx in others. However, no information is available on the modulation of membrane potential (V(m)) by actin. The present study was aimed to ascertain whether the widely employed actin depolymerizing drug, latrunculin A (Lat A), affects V(m) in mature oocytes of the starfish Astropecten aranciacus. Lat A induced a membrane depolarization which was mimicked by cytochalasin D, another popular actin disruptor, and prevented by jasplakinolide, a stabilizer of the actin network. Lat A-elicited depolarization consisted in a positive shift in V(m) which reached the threshold of activation of voltage-gated Ca2+ channels (VGCC), thus triggering an action potential. Lat A-promoted depolarization lacked the action potential in Ca2+-free sea water, while it was abolished upon removal of external Na+. Moreover, membrane depolarization was prevented by pre-injection of BAPTA and heparin, but not ryanodine. These data indicate that Lat A induces a membrane depolarization by releasing Ca2+ from InsP3Rs. The Ca2+ signal in turn activates a Ca2+-dependent Na+ entry, which causes the positive shift in V(m) and stimulates the VGCC.

Cytoskeleton as a potential target in the neuropathology of maple syrup urine disease: insight from animal studies

In this short review we provide evidence that the branched-chain keto acids accumulating in the neurometabolic disorder maple syrup urine disease disturb rat cerebral cytoskeleton in a developmentally regulated manner. Alterations of protein phosphorylation leading to brain cytoskeletal misregulation and neural cell death caused by these metabolites are associated with energy deprivation, oxidative stress and excitotoxicity that may ultimately disrupt normal cell function and viability.

Thyroid Hormones Reorganize the Cytoskeleton of Glial Cells Through Gfap Phosphorylation and Rhoa-Dependent Mechanisms

Thyroid hormones (3,5,3'-triiodo-L: -thyronine, T3; 3,5,3',5'-L: -tetraiodothyronine, T4; TH) play crucial roles in the growth and differentiation of the central nervous system. In this study, we investigated the actions of TH on proliferation, viability, cell morphology, in vitro phosphorylation of glial fibrillary acidic protein (GFAP) and actin reorganization in C6 glioma cells. We first observe that long-term exposure to TH stimulates cell proliferation without induce cell death. We also demonstrate that after 3, 6, 12, 18, and 24 h treatment with TH, C6 cells and cortical astrocytes show a process-bearing shape. Furthermore, immunocytochemistry with anti-actin and anti-GFAP antibodies reveals that TH induces reorganization of actin and GFAP cytoskeleton. We also observe an increased in vitro 32P incorporation into GFAP recovered into the high-salt Triton insoluble cytoskeletal fraction after 3 and 24 h exposure to 5 x 10(-8) and 10(-6) M T3, and only after 24 h exposure to 10(-9) M T4. These results show a T3 action on the phosphorylating system associated to GFAP and suggest a T3-independent effect of T4 on this cytoskeletal protein. In addition, C6 cells and astrocytes treated with lysophosphatidic acid, an upstream activator of the RhoA GTPase pathway, totally prevented the morphological alterations induced by TH, indicating that this effect could be mediated by the RhoA signaling pathway. Considering that IF network can be regulated by phosphorylation leading to reorganization of IF filamentous structure and that alterations of the microfilament organization may have important implications in glial functions, the effects of TH on glial cell cytoskeleton could be implicated in essential neural events such as brain development.

F-actin-based Ca signaling-a critical comparison with the current concept of Ca signaling

A short comparative survey on the current idea of Ca signaling and the alternative concept of F-actin-based Ca signaling is given. The two hypotheses differ in one central aspect, the mechanism of Ca storage. The current theory rests on the assumption of Ca-accumulating endoplasmic/sarcoplasmic reticulum-derived vesicles equipped with an ATP-dependent Ca pump and IP3- or ryanodine-sensitive channel-receptors for Ca-release. The alternative hypothesis proceeds from the idea of Ca storage at the high-affinity binding sites of actin filaments. Cellular sites of F-actin-based Ca storage are microvilli and the submembrane cytoskeleton. Several specific features of Ca signaling such as store-channel coupling, quantal Ca release, spiking and oscillations, biphasic and "phasic" uptake kinetics, and Ca-induced Ca release (CICR), which are not adequately described by the current concept, are inherent properties of the F-actin system and its dynamic state of treadmilling.

Propionic acid induces cytoskeletal alterations in cultured astrocytes from rat cerebral cortex

Severe neurological symptoms, cerebral edema, and atrophy are common features of the inherited metabolic disorder propionic acidemia. However, the pathomechanisms involved in the neuropathology of this disease are not well established. In this study, we investigate the effects of propionic acid (PA), a metabolite accumulating in this disorder, on cytoskeletal reorganization, on cell viability, and on the in vitro phosphorylation of glial fibrillary acidic protein (GFAP) and vimentin in cultured astrocytes from cerebral cortex of neonatal rats. We observed that the astrocytes changed their usual polygonal morphology when exposed to 5 mM PA for 72 h, leading to the appearance of fusiform or process-bearing cells, without elicit cell death. We also noticed that after 72 h treatment with 5 mM PA cells showed retracted cytoplasm with bipolar processes containing packed GFAP filaments and disorganized actin stress fibers, as revealed by immunocytochemistry. In addition, the morphological alterations were accompanied by increased in vitro 32P incorporation into GFAP and vimentin recovered into the high-salt Triton-insoluble cytoskeletal fraction. In conclusion, our results indicate that PA lead to cytoskeletal reorganization and to increased in vitro phosphorylation of Triton-insoluble GFAP and vimentin. On the basis of our results we could suppose that Triton-insoluble GFAP and vimentin hyperphosphorylation could be implicated in the reorganization of cellular structure and these findings could be involved in the brain damage characteristic of propionic acidemia patients.

Calcium oscillations in type-1 astrocytes, the effect of a presenilin 1 (PS1) mutation

Stimulation of type-1 astrocytes, by a number of agonists, has been shown to increase cytosolic Ca2+ concentrations in an oscillatory manner. However, it is unknown how these are driven or altered by aging, injury or disease. Therefore, we characterized the signaling properties of rat type-1 astrocytes in monolayer cultures. Ca2+ responses were recorded in astrocytes stimulated with ATP or glutamate. Oscillations were evident in cultures at 3 days in vitro (DIV 3) with a peak percentage (26%) of cells responding by DIV 7. The presence or absence of serum in the culture medium did not influence this percentage. Likewise, long-term culture (DIV 30+) did not increase the oscillating cell numbers. ATP was found to have a more potent effective dose (50 microM) than glutamate (10 mM). Membrane potential was recorded with fluorescent voltage-sensitive dye (membrane potential dye for FLIPR) and was similar regardless of the culture age and intracellular Ca2+ response. This suggests that mechanisms associated with the intracellular release of Ca2+ from endogenous Ca2+ stores, rather than ion fluxes across the plasma membrane, may contribute to the oscillations in the astrocytes. In order to identify a possible pathological significance of this response, we transfected astrocytes with wild-type presenilin (PS1) as well as PS1 harboring a mutation linked to familial Alzheimer's disease (FAD). PS mutation expressing astrocytes oscillated at lower ATP and glutamate concentrations when compared to wild-type PS1 expressing astrocytes. This indicates that PS1 mutation may introduce aberrations in the intracellular Ca2+ mobilization in astrocytes contributing to the accelerated pathogenesis in FAD.

Morphological Alterations and Cell Death Provoked by the Branched-Chain α-Amino Acids Accumulating in Maple Syrup Urine Disease in Astrocytes from Rat Cerebral Cortex

1. Maple syrup urine disease (MSUD) is an inherited metabolic disorder predominantly characterized by neurological dysfunction and cerebral atrophy whose patophysiology is poorly known. 2. We investigated here whether the branched-chain amino acids (BCAA) leucine (Leu), isoleucine (Ile) and valine (Val), which are the biochemical hallmark of this disorder, could alter astrocyte morphology and cytoskeleton reorganization by exposing cultured astrocytes from cerebral cortex of neonatal rats to various concentrations of the amino acids. A change of cell morphology from the usual polygonal to the appearance of fusiform or process-bearing cells was caused by the BCAA. Cell death was also observed when astrocytes were incubated in the presence of BCAA for longer periods. 3. Val-treated astrocytes presented the most dramatic morphological alterations. Immunocytochemistry with anti-actin and anti-GFAP antibodies revealed that all BCAA induced reorganization of actin and GFAP cytoskeleton. In addition, lysophosphatidic acid, an activator of RhoA GTPase pathway, was able to totally prevent the morphological alterations and cytoskeletal reorganization induced by Val, indicating that the RhoA signaling pathway was involved in these effects. 4. Furthermore, creatine attenuated the morphological alterations provoked by the BCAA, the protection being more pronounced for Val, suggesting that impairment of energy homeostasis is partially involved in BCAA cytotoxic action. The data indicate that the BCAA accumulating in MSUD are toxic to astrocyte cells, a fact that may be related to the pathogenesis of the neurological dysfunction of MSUD patients.

Evidence that the branched-chain alpha-keto acids accumulating in maple syrup urine disease induce morphological alterations and death in cultured astrocytes from rat cerebral cortex

Severe neurological symptoms, cerebral edema, and atrophy are common features of the inherited metabolic disorder maple syrup urine disease (MSUD). However, the pathomechanisms involved in the neuropathology of this disease are not well established. In this study, we investigated the effects of the branched-chain keto acids (BCKA) alpha-ketoisocaproic (KIC), alpha-ketoisovaleric (KIV), and alpha-keto-beta-methylvaleric (KMV), which accumulate in MSUD, on astrocyte morphology and cytoskeleton reorganization. Cultured astrocytes from cerebral cortex of neonatal rats were exposed to various concentrations of the BCKA and cell morphology was studied. We observed that these cells changed their usual polygonal morphology when exposed to BCKA, leading to the appearance of fusiform or process-bearing cells. Furthermore, longer exposures to the BCKA elicited cell death at all concentrations studied, attaining massive death at the highest concentrations. Immunocytochemistry with anti-actin or anti-GFAP antibodies revealed that the BCKA induced reorganization of actin and GFAP cytoskeleton. In addition, astrocytes treated with lysophosphatidic acid, an upstream activator of the RhoA GTPase pathway, totally prevented the morphological alterations and cytoskeletal reorganization induced by KIV, indicating that this effect could be mediated by the RhoA signaling pathway. Furthermore, the effects of BCKA on astrocyte morphology were prevented by creatine. In addition, creatine kinase activity was inhibited by KIC and KIV; this inhibition was prevented by creatine, indicating that these keto acids compromise brain energy metabolism. Considering that astroglial cells are critical to brain development and functioning, it is conceivable that alterations of the actin network by BCKA may have important implications in astrocytic function and possibly in the pathogenesis of the neurological dysfunction and brain damage of MSUD patients.

Cleavage of SNAP-25 and VAMP-2 impairs store-operated Ca2+entry in mouse pancreatic acinar cells

We recently reported that store-operated Ca2+entry (SOCE) in nonexcitable cells is likely to be mediated by a reversible interaction between Ca2+channels in the plasma membrane and the endoplasmic reticulum, a mechanism known as “secretion-like coupling.” As for secretion, in this model the actin cytoskeleton plays a key regulatory role. In the present study we have explored the involvement of the secretory proteins synaptosome-associated protein (SNAP-25) and vesicle-associated membrane protein (VAMP) in SOCE in pancreatic acinar cells. Cleavage of SNAP-25 and VAMPs by treatment with botulinum toxin A (BoNT A) and tetanus toxin (TeTx), respectively, effectively inhibited amylase secretion stimulated by the physiological agonist CCK-8. BoNT A significantly reduced Ca2+entry induced by store depletion using thapsigargin or CCK-8. In addition, treatment with BoNT A once SOCE had been activated reduced Ca2+influx, indicating that SNAP-25 is needed for both the activation and maintenance of SOCE in pancreatic acinar cells. VAMP-2 and VAMP-3 are expressed in mouse pancreatic acinar cells. Both proteins associate with the cytoskeleton upon Ca2+store depletion, although only VAMP-2 seems to be sensitive to TeTx. Treatment of pancreatic acinar cells with TeTx reduced the activation of SOCE without affecting its maintenance. These findings support a role for SNAP-25 and VAMP-2 in the activation of SOCE in pancreatic acinar cells and show parallels between this process and secretion in a specialized secretory cell type.

Actin filaments play a permissive role in the inhibition of store-operated Ca2+ entry by extracellular ATP in rat brown adipocytes

Stimulation of P2 receptors with micromolar concentration of ATP evokes a transient increase in [Ca2+]i (intracellular free Ca2+ concentration), primarily due to release of Ca2+ from intracellular stores; such stimulation also triggers almost complete suppression of thapsigargin-evoked sustained [Ca2+]i increase mediated through a store-operated Ca2+ entry pathway in rat brown adipocytes. We investigated the role of cytoskeletal actin in the inhibitory effect of the extracellular ATP on store-operated Ca2+ entry, using fura 2 fluorescence for continuous measurement of [Ca2+]i, and using Alexa fluor 488-phalloidin staining of actin. Disassembly of actin networks by cytochalasin D (1 microM) or latrunculin A (3 microM) prevented the inhibitory effect of ATP (10 microM) on the thapsigargin (100 nM)-evoked store-operated Ca2+ entry, without changing the effect of ATP in increasing [Ca2+]i. In normal cells, bath application of ATP induced a transient [Ca2+]i increase, consisting of a rapid increase (the rising phase) and the subsequent decrease (the declining phase) to a lower steady level despite the continued presence of the agonist. Disruption of actin assemblies did not significantly affect the rising phase, but prevented the declining phase. Cells incubated with 10 microM ATP for 4 min demonstrated marked accumulations of actin filaments at the cell periphery, showing protrusions at the cell surface; this actin-assembly process is mediated through P2 receptors. In cells treated with cytochalasin D or latrunculin A, extracellular ATP did not induce actin redistribution. These results suggest that the actin reorganization plays a role in ATP-induced inhibition of store-operated Ca2+ entry in rat brown adipocytes.

Mechanisms of secretion of ATP from cortical astrocytes triggered by uridine triphosphate

The mechanisms involved in autocrine ATP release from cultured astrocytes isolated from the rat cortex were investigated using an online bioluminescence technique. Astrocytes released ATP in response to application of 10 microM uridine triphosphate, which was blocked by the non-specific purinergic receptor antagonist suramin. Intracellular pathways of the uridine triphosphate-stimulated ATP release were seen to involve inositol triphosphate and calcium with the assistance of the Golgi-complex and cytoskeleton as the release was inhibited by phospholipase C antagonist lithium, endoplasmic reticulum calcium-dependent ATPase inhibitor thapsigargin, F-actin interruptor cytochalasin D and Golgi-complex interruptor brefeldin A. The uridine triphosphate-stimulated ATP release was also potently blocked by exocytosis inhibitor botulinum toxin A and anion transporter blockers furosemide and glibenclamide. These results suggest that calcium-dependent exocytosis and transportation via anion transporters are the predominant secretion mechanisms for uridine triphosphate-stimulated ATP release from cortical astrocytes.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

The role of the cytoskeleton in capacitative calcium entry in myenteric glia

Capacitative calcium entry (CCE) is the process by which intracellular calcium is replenished from the external milieu upon depletion of intracellular stores. CCE is thought to participate in chemotaxis, proliferation and cell signalling. A physical interaction between intracellular stores and the plasma membrane is postulated to regulate CCE. We hypothesized that cytoskeletal disruption alters this interaction, inhibiting CCE in enteric glia. Cultured myenteric glia from neonatal guinea-pigs were treated with cytochalasin D (10 micro mol L-1), a microfilament disrupting agent, nocodazole (20 micro mol L-1), a microtubule disrupting agent, or vehicle (dimethyl sulphoxide). Intracellular calcium changes were measured using fura-2 microfluorimetry. To evaluate the rate of cation re-entry, barium was substituted for calcium because barium is not sequestered internally. Cytochalasin D-treated glia had diminished CCE responses (57 +/- 3 nmol L-1) compared with controls (97 +/- 7 nmol L-1) as did nocodazole-treated glia (30 +/- 2 nmol L-1) vs controls (77 +/- 6 nmol L-1). The proportion of cells demonstrating CCE abolition was greater in the cytochalasin (50 +/- 8%) and nocodazole-treated (89 +/- 2%) groups compared with controls (21 +/- 2%, 40 +/- 9%, respectively). Cytochalasin D and nocodazole treatment diminished the rate of cation re-entry based on diminished barium entry in treated vs control cells. From this study, we conclude that disruption of cytoskeletal elements diminishes calcium influx essential to calcium store repletion in myenteric glia.

RhoA and lysophosphatidic acid are involved in the actin cytoskeleton reorganization of astrocytes exposed to ethanol

Astroglial cells play an important role in maintaining neuronal function in the adult and in the developing nervous system. Ethanol exposure induces profound alterations in the astrogliogenesis process, affecting important cell functions, including intracellular protein trafficking. Because the actin cytoskeleton plays a crucial role in intracellular protein transport, the aim of the present study was to analyze the effects of ethanol on actin cytoskeleton organization and the involvement of the RhoA signaling pathway in these effects. We show that RhoA and lysophosphatidic acid (LPA), an upstream activator of RhoA, stimulate the formation of stress fibers and focal adhesion in cortical astrocytes in primary culture. Exposure of cultured astrocytes to different concentrations of ethanol profoundly disorganizes the actin cytoskeleton, leading to the formation of actin rings at the cell periphery and decreasing the content of focal adhesion proteins. Furthermore, LPA treatment or RhoA transfection revert the ethanol-induced actin alterations in astrocytes, whereas transfection with an inactive mutant of RhoA is unable to revert the actin ring organization. In addition, inhibition of endogenous RhoA by C3 exoenzyme effectively blocks ethanol-induced actin ring formation. These results suggest that the effects of alcohol on actin cytoskeleton organization are mediated by the RhoA signaling pathway. Disruptions in actin organization may impair important astrocyte functions, participating in ethanol-induced astroglial and brain damage during development.

Arachidonic acid in astrocytes blocks Ca(2+) oscillations by inhibiting store-operated Ca(2+) entry, and causes delayed Ca(2+) influx

ATP-elicited oscillations of the concentration of free intracellular Ca(2+) ([Ca(2+)](i)) in rat brain astrocytes were abolished by simultaneous arachidonic acid (AA) addition, whereas the tetraenoic analogue 5,8,11,14-eicosatetraynoic acid (ETYA) was ineffective. Inhibition of oscillations is due to suppression by AA of intracellular Ca(2+) store refilling. Short-term application of AA, but not ETYA, blocked Ca(2+) influx, which was evoked by depletion of stores with cyclopiazonic acid (CPA) or thapsigargin (Tg). Addition of AA after ATP blocked ongoing [Ca(2+)](i) oscillations. Prolonged AA application without or with agonist could evoke a delayed [Ca(2+)](i) increase. This AA-induced [Ca(2+)](i) rise developed slowly, reached a plateau after 5 min, could be reversed by addition of bovine serum albumin (BSA), that scavenges AA, and was blocked by 1 microM Gd(3+), indicative for the influx of extracellular Ca(2+). Specificity for AA as active agent was demonstrated by ineffectiveness of C16:0, C18:0, C20:0, C18:2, and ETYA. Moreover, the action of AA was not affected by inhibitors of oxidative metabolism of AA (ibuprofen, MK886, SKF525A). Thus, AA exerted a dual effect on astrocytic [Ca(2+)](i), firstly, a rapid reduction of capacitative Ca(2+) entry thereby suppressing [Ca(2+)](i) oscillations, and secondly inducing a delayed activation of Ca(2+) entry, also sensitive to low Gd(3+) concentration.

Expression and function of bradykinin receptors in microglia

Expression of bradykinin (BK) receptors and their cellular function were investigated in microglia. Microglial cells were isolated from mixed cultures of cerebrocortical cells from postnatal day 3 Wistar rats. Reverse transcription-PCR (RT-PCR) showed that rat primary microglia express mRNAs for the type 2 bradykinin (B(2)) receptor subtype but not the type 1 (B(1)) receptor subtype under our experimental condition. However, the expression of B(1) receptor was greatly up-regulated after the treatment of microglia with BK for 24 hours. The expression of B(2) receptor in microglia was further confirmed by immunocytochemistry. Membrane currents were measured using whole-cell recording under voltage-clamp conditions. In 14% of patched cells (12/85 cells), BK (100-200 nM) induced an outward current at the holding potential of -20 mV, with oscillations in 2 cases. The BK-induced outward current was transient and desensitized rapidly. TEA inhibited the BK-induced outward current in a dose-dependent manner. These results suggest that microglia express B(2) receptors and presumably increase the intracellular Ca(2+) concentration via inositol trisphosphate with the subsequent activation of Ca(2+)-dependent K(+) channels. Our data provide the first evidence that microglia express functional BK receptors and support the idea that microglia play an important role in CNS inflammatory responses.

Hypoxic remodeling of Ca2+ stores in type I cortical astrocytes

Prolonged periods of hypoxia are deleterious to higher brain functions and increase the likelihood of developing dementias. Here, we have used fluorimetric techniques to investigate the effects of chronic hypoxia (2.5% O(2), 24 h) on Ca(2+) stores in type I cortical astrocytes, because such stores are crucial to various astrocyte functions, including Ca(2+)-dependent modulation of neuronal activity. Rises of [Ca(2+)](i) evoked by exposure of astrocytes to bradykinin were enhanced following chronic hypoxia, as were transient increases in [Ca(2+)](i) recorded in Ca(2+)-free perfusate. The enhanced responses were due partly to impaired plasmalemmal Na(+)/Ca(2+) exchange following chronic hypoxia. More importantly, chronic hypoxia increased the Ca(2+) content of mitochondria (as determined by exposing cells to mitochondrial inhibitors), such that they were unable to act as Ca(2+) buffers following bradykinin-evoked Ca(2+) release from the endoplasmic reticulum. Hypoxic enhancement of mitochondrial Ca(2+) content was also observed in confocal images of cells loaded with the mitochondrial Ca(2+) indicator, Rhod-2. Confocal imaging of cells loaded with tetramethylrhodamine ethyl ester, an indicator of mitochondrial membrane potential, indicated that mitochondria were hyperpolarized in astrocytes following chronic hypoxia. Our findings indicate that hypoxia disturbs Ca(2+) signaling in type I astrocytes, primarily by causing mitochondrial Ca(2+) overload.

Functional importance of the actin cytoskeleton in contraction of bovine iris sphincter muscle

1. The contractile capacity of smooth muscle cells depends on the cytoskeletal framework of the cell. The aim of this study was to determine the functional importance of both the actin and the tubulin components of the cytoskeleton in contractile responses of the bovine isolated iris sphincter muscle. 2. In each preparation, two contractions to the muscarinic agonist carbachol were obtained. The maximum responses of the first contractions were taken as 100%. The second contractions to carbachol were elicited in the presence of either cytochalasin B (50 and 5 microm), an inhibitor of the actin cytoskeleton, or colchicine (100 microm), an inhibitor of the tubulin cytoskeleton (30 min incubation). 3. Cytochalasin B, at a concentration of 50 microm, significantly decreased the contractions induced by carbachol, with the maximum response reduced to 21.8 +/- 6.6% (n = 12) of the initial maximum. The maximal contractions to carbachol in the presence of colchicine reached 96.2 +/- 7.9% (n = 9) of the initial contraction, which was not significantly different from control second responses to carbachol with neither drug present, which reached 113.3 +/- 7.6% (n = 7). 4. The effect of cytochalasin B was dose-dependent, since at a lower concentration of 5 microm, the drug decreased the maximum contraction to carbachol to 60.3 +/- 8.8% (n = 6). The effect of cytochalasin B was at least partially reversible, since after the use of the higher concentration of 50 microm, contractions to carbachol increased to 62.3 +/- 15.5% (n = 4) of the maximal response, after 1 h repeated washing of the preparations. 5. Cytochalasin D, at a concentration of 50 microm, completely abolished the contractions induced by carbachol (n = 4). 6. These findings suggest that in bovine iris sphincter muscle, contractions to carbachol are highly dependent, from a functional point of view, on actin polymerization, and not, to any important degree, on the polymerization of tubulin.

Calcium influx factor from cytochrome P-450 metabolism and secretion-like coupling mechanisms for capacitative calcium entry in corneal endothelial cells

Notwithstanding extensive efforts, the mechanism of capacitative calcium entry (CCE) remains unclear. Two seemingly opposed theories have been proposed: secretion-like coupling (Patterson, R. L., van Rossum, D. B., and Gill, D. L. (1999) Cell 98, 487-499) and the calcium influx factor (CIF) (Randriamampita, C., and Tsien, R. Y. (1993) Nature 364, 809-814). In the current study, a combinatorial approach was taken to investigate the mechanism of CCE in corneal endothelial cells. Induction of cytochrome P-450s by beta-naphthoflavone (BN) enhanced CCE measured by Sr(2+) entry after store depletion. 5,6-Epoxyeicosatrienoic acid (5,6-EET), a proposed CIF generated by cytochrome P-450s (Rzigalinski, B. A., Willoughby, K. A., Hoffman, S. W., Falck, J. R., and Ellis, E. F. (1999) J. Biol. Chem. 274, 175-182), induced Ca(2+) entry. Both BN-enhanced CCE and the 5,6-EET-induced Ca(2+) entry were inhibited by the CCE blocker 2-aminoethoxydiphenyl borate, indicating a role for cytochrome P-450s in CCE. Treatment with calyculin A (CalyA), which causes condensation of cortical cytoskeleton, inhibited CCE. The actin polymerization inhibitor cytochalasin D partially reversed the inhibition of CCE by CalyA, suggesting a secretion-like coupling mechanism for CCE. However, CalyA could not inhibit CCE in BN-treated cells, and 5,6-EET caused a partial activation of CCE in CalyA-treated cells. These results further support the notion that cytochrome P-450 metabolites may be CIFs. The vesicular transport inhibitor brefeldin A inhibited CCE in both vehicle- and BN-treated cells. Surprisingly, Sr(2+) entry in the absence of store depletion was enhanced in BN-treated cells, which was also inhibited by 2-aminoethoxydiphenyl borate. An integrative model suggests that both CIF from cytochrome P-450 metabolism and secretion-like coupling mechanisms play roles in CCE in corneal endothelial cells.

Maintenance of the filamentous actin cytoskeleton is necessary for the activation of store-operated Ca2+ channels, but not other types of plasma-membrane Ca2+ channels, in rat hepatocytes

The roles of the filamentous actin (F-actin) cytoskeleton and the endoplasmic reticulum (ER) in the mechanism by which store-operated Ca(2+) channels (SOCs) and other plasma-membrane Ca(2+) channels are activated in rat hepatocytes in primary culture were investigated using cytochalasin D as a probe. Inhibition of thapsigargin-induced Ca(2+) inflow by cytochalasin D depended on the concentration and time of treatment, with maximum inhibition observed with 0.1 microM cytochalasin D for 3 h. Cytochalasin D (0.1 microM for 3 h) did not inhibit the total amount of Ca(2+) released from the ER in response to thapsigargin but did alter the kinetics of Ca(2+) release. The effects of cytochalasin D (0.1 microM) on vasopressin-induced Ca(2+) inflow were similar to those on thapsigargin-induced Ca(2+) inflow, except that cytochalasin D did inhibit vasopressin-induced release of Ca(2+) from the ER. Cytochalasin D (0.1 microM) inhibited vasopressin-induced Mn(2+) inflow (predominantly through intracellular messenger-activated non-selective cation channels), but the degree of inhibition was less than that of vasopressin-induced Ca(2+) inflow (predominantly through Ca(2+)-selective SOCs). Maitotoxin- and hypotonic shock-induced Ca(2+) inflow were enhanced rather than inhibited by 0.1 microM cytochalasin D. Treatment with 0.1 microM cytochalasin D substantially reduced the amount of F-actin at the cell cortex, whereas 5 microM cytochalasin D increased the total amount of F-actin and caused an irregular distribution of F-actin at the cell cortex. Cytochalasin D (0.1 microM) caused no significant change in the overall arrangement of the ER [monitored using 3',3'-dihexyloxacarbocyanine iodide [DiOC(6)(3)] in fixed cells] but disrupted the fine structure of the smooth ER and reduced the diffusion of DiOC(6)(3) in the ER in live hepatocytes after photobleaching. It is concluded that (i) the concentration of cytochalasin D is a critical factor in the use of this agent as a probe to disrupt the cortical F-actin cytoskeleton in rat hepatocytes, (ii) a reduction in the amount of cortical F-actin inhibits SOCs but not intracellular messenger-activated non-selective cation channels, and (iii) inhibition of the activation of SOCs and reduction in the amount of cortical F-actin is associated with disruption of the organization of the ER.

Signaling and gene expression in the neuron-glia unit during brain function and dysfunction: Holger Hydén in memoriam

Holger Hydén demonstrated almost 40 years ago that learning changes the base composition of nuclear RNA, i.e. induces an alteration in gene expression. An equally revolutionary observation at that time was that a base change occurred in both neurons and glia. From these findings, Holger Hydén concluded that establishment of memory is correlated with protein synthesis, and he demonstrated de novo synthesis of several high-molecular protein species after learning. Moreover, the protein, S-100, which is mainly found in glial cells, was increased during learning, and antibodies towards this protein inhibited memory consolidation. S-100 belongs to a family of Ca(2+)-binding proteins, and Holger Hydén at an early point realized the huge importance of Ca(2+) in brain function. He established that glial cells show more marked and earlier changes in RNA composition in Parkinson's disease than neurons. Holger Hydén also had the vision and courage to suggest that "mental diseases could as well be thought to depend upon a disturbance of processes in glia cells as in the nerve cells", and he showed that antidepressant drugs cause profound changes in glial RNA. The importance of Holger Hydén's findings and visions can only now be fully appreciated. His visionary concepts of the involvement of glia in neurological and mental illness, of learning being associated with changes in gene expression, and of the functional importance of Ca(2+)-binding proteins and Ca(2+) are presently being confirmed and expanded by others. This review briefly summarizes highlights of Holger Hydén's work in these areas, followed by a discussion of recent research, confirming his findings and expanding his visions. This includes strong evidence that glial dysfunction is involved in the development of Parkinson's disease, that drugs effective in mood disorders alter gene expression and exert profound effects on astrocytes, and that neuronal-astrocytic interactions in glutamate signaling, NO synthesis, Ca(2+) signaling, beta-adrenergic activity, second messenger production, protein kinase activities, and transcription factor phosphorylation control the highly programmed events that carry the memory trace through the initial, signal-mediated short-term and intermediate memory stages to protein synthesis-dependent long-term memory.

Recent developments in non-excitable cell calcium entry

Influx of calcium into cells following stimulation of cell surface receptors is a key process controlling cellular activity. However, despite intensive research, there is still no consensus on precisely how calcium entry is controlled in electrically no n-excitable cells. In particular, the regulation of depletion-activated or 'capacitative' calcium entry continues to be a focus of debate. Work published in the last 2 years has lent new impetus to the so-called 'conformational coupling' theory, although evidence for the existence of soluble messengers between the ER and the plasma membrane also continues to appear. In addition, there remains disagreement on whether intra-store [Ca(2+)] has to fall below a threshold before Ca(2+)entry is activated. A further major question is the identity of the putative depletion-operated Ca(2+)channel or channels. Here discussion has largely focussed on whether homologue(s) of the Drosophila TRP ('Transient Receptor Potential') protein is/are the elusive channel, or at least a part of it. Finally, it remains possible that Ca(2+)entry mechanisms other than depletion-activated channels may be important in agonist-evoked Ca(2+)influx. This commentary summarizes recent developments in the field, and highlights both current debates and critical unsolved questions.

Regulation of phosphatidylserine transbilayer redistribution by store-operated Ca2+ entry: role of actin cytoskeleton

The phosphatidylserine transmembrane redistribution at the cell surface is one of the early characteristics of cells undergoing apoptosis and also occurs in cells fulfilling a more specialized function, such as the phosphatidylserine-dependent procoagulant response of platelets after appropriate activation. Although an increase in cytoplasmic Ca2+ is essential to trigger the remodeling of the plasma membrane, little is known about intracellular signals leading to phosphatidylserine externalization. Here, the role of store-operated Ca2+ entry on phosphatidylserine exposure was investigated in human erythroleukemia HEL cells, a pluripotent lineage with megakaryoblastic properties. Ca2+ entry inhibitors (SKF-96365, LaCl(3), and miconazole) inhibited store-operated Ca2+ entry in A23187- or thapsigargin-stimulated cells and reduced the degree of phosphatidylserine externalization concomitantly, providing evidence for a close link between the two processes. In cells pretreated with cytochalasin D, an agent that disrupts the microfilament network of the cytoskeleton, store-operated Ca2+ entry and phosphatidylserine externalization at the cell surface were inhibited. In a context where most of the key actors remain to be identified, these results provide evidence for the implication of both store-operated Ca2+ entry and cytoskeleton architectural organization in the regulation of phosphatidylserine transbilayer migration.