Rapid morphological changes in astrocytes are accompanied by redistribution but not by quantitative changes of cytoskeletal proteins

Reactive and Senescent Astroglial Phenotypes as Hallmarks of Brain Pathologies

Astrocytes, as the most abundant glial cells in the central nervous system, are tightly integrated into neural networks and participate in numerous aspects of brain physiology and pathology. They are the main homeostatic cells in the central nervous system, and the loss of astrocyte physiological functions and/or gain of pro-inflammatory functions, due to their reactivation or cellular senescence, can have profound impacts on the surrounding microenvironment with pathological outcomes. Although the importance of astrocytes is generally recognized, and both senescence and reactive astrogliosis have been extensively reviewed independently, there are only a few comparative overviews of these complex processes. In this review, we summarize the latest data regarding astrocyte reactivation and senescence, and outline similarities and differences between these phenotypes from morphological, functional, and molecular points of view. A special focus has been given to neurodegenerative diseases, where these phenotypic alternations of astrocytes are significantly implicated. We also summarize current perspectives regarding new advances in model systems based on astrocytes as well as data pointing to these glial cells as potential therapeutic targets.

Serotonin Plays a Key Role in the Development of Opioid-Induced Hyperalgesia in Mice

Opioid usage for pain therapy is limited by its undesirable clinical effects, including paradoxical hyperalgesia, also known as opioid-induced hyperalgesia (OIH). However, the mechanisms associated with the development and maintenance of OIH remain unclear. Here, we investigated the effect of serotonin inhibition by the 5-HT3 receptor antagonist, ondansetron (OND), as well as serotonin deprivation via its synthesis inhibitor para-chlorophenylalanine, on mouse OIH models, with particular focus on astrocyte activation. Co-administering of OND and morphine, in combination with serotonin depletion, inhibited mechanical hyperalgesia and astrocyte activation in the spinal dorsal horn of mouse OIH models. Although previous studies have suggested that activation of astrocytes in the spinal dorsal horn is essential for the development and maintenance of OIH, herein, treatment with carbenoxolone (CBX), a gap junction inhibitor that suppresses astrocyte activation, did not ameliorate mechanical hyperalgesia in mouse OIH models. These results indicate that serotonin in the spinal dorsal horn, and activation of the 5-HT3 receptor play essential roles in OIH induced by chronic morphine, while astrocyte activation in the spinal dorsal horn serves as a secondary effect of OIH. Our findings further suggest that serotonergic regulation in the spinal dorsal horn may be a therapeutic target of OIH. PERSPECTIVE: The current study revealed that the descending serotonergic pain-facilitatory system in the spinal dorsal horn is crucial in OIH, and that activation of astrocytes is a secondary phenotype of OIH. Our study offers new therapeutic targets for OIH and may help reduce inappropriate opioid use.

Paclitaxel is effective for controlling astrocyte proliferation in vitro: Implications for generating ventral mesencephalic cultures enriched with dopamine neurons

BACKGROUND

Primary embryonic ventral mesencephalic (VM) cultures are a high throughput tool for understanding and manipulating dopamine neurons, to study the mechanisms that trigger their degeneration during Parkinson's disease (PD), and to test new drugs aimed at treating the disease. Unfortunately, primary cell cultures are often quickly overwhelmed by dividing astrocytes which both obscure neuronal cells and distort the cellular composition that exists in vivo.

NEW METHOD

To develop a new in vitro system whereby astrocyte division can be readily controlled while maintaining neuronal integrity, VM cultures were treated with different doses (1.75, 3.5, 7, 14 nM) of the anti-mitotic drug paclitaxel for up to seven days in vitro. The study subsequently sought to determine the importance of astrocytes in dopamine neuron survival when challenged with an exposure to the toxin 6-hydroxydopamine (6-OHDA).

RESULTS

Optical density (O.D.) measures of GFAP expression and counts of β-III tubulin and tyrosine hydroxylase positive neurons reveals that a low dose of 3.5 nM of paclitaxel significantly reduced the density of GFAP + astrocytes in primary VM cultures, while maintaining the viability of neurons and dopamine neurons. Interestingly, a reduction of GFAP + astrocytes within primary VM cultures did not reveal any statistically significant differences in the number of dopamine neurons surviving treatment with 6-OHDA.

CONCLUSIONS

These findings detail a quick and simple method for stabilising astrocyte numbers in primary VM cultures, without affecting the viability of dopamine neurons, and suggest that astrocytes may not enhance the survival of dopamine neurons when challenged with the 6-OHDA toxin.

Bmal1-deficiency affects glial synaptic coverage of the hippocampal mossy fiber synapse and the actin cytoskeleton in astrocytes

Bmal1 is an essential component of the molecular clockwork, which drives circadian rhythms in cell function. In Bmal1-deficient (Bmal1-/-) mice, chronodisruption is associated with cognitive deficits and progressive brain pathology including astrocytosis indicated by increased expression of glial fibrillary acidic protein (GFAP). However, relatively little is known about the impact of Bmal1-deficiency on astrocyte morphology prior to astrocytosis. Therefore, in this study we analysed astrocyte morphology in young (6-8 weeks old) adult Bmal1-/- mice. At this age, overall GFAP immunoreactivity was not increased in Bmal1-deficient mice. At the ultrastructural level, we found a decrease in the volume fraction of the fine astrocytic processes that cover the hippocampal mossy fiber synapse, suggesting an impairment of perisynaptic processes and their contribution to neurotransmission. For further analyses of actin cytoskeleton, which is essential for distal process formation, we used cultured Bmal1-/- astrocytes. Bmal1-/- astrocytes showed an impaired formation of actin stress fibers. Moreover, Bmal1-/- astrocytes showed reduced levels of the actin-binding protein cortactin (CTTN). Cttn promoter region contains an E-Box like element and chromatin immunoprecipitation revealed that Cttn is a potential Bmal1 target gene. In addition, the level of GTP-bound (active) Rho-GTPase (Rho-GTP) was reduced in Bmal1-/- astrocytes. In summary, our data demonstrate that Bmal1-deficiency affects morphology of the fine astrocyte processes prior to strong upregulation of GFAP, presumably because of impaired Cttn expression and reduced Rho-GTP activation. These morphological changes might result in altered synaptic function and, thereby, relate to cognitive deficits in chronodisruption.

A Logic Model of Neuronal-Glial Interaction Suggests Altered Homeostatic Regulation in the Perpetuation of Neuroinflammation

Aberrant inflammatory signaling between neuronal and glial cells can develop into a persistent sickness behavior-related disorders, negatively impacting learning, memory, and neurogenesis. While there is an abundance of literature describing these interactions, there still lacks a comprehensive mathematical model describing the complex feed-forward and feedback mechanisms of neural-glial interaction. Here we compile molecular and cellular signaling information from various studies and reviews in the literature to create a logically-consistent, theoretical model of neural-glial interaction in the brain to explore the role of neuron-glia homeostatic regulation in the perpetuation of neuroinflammation. Logic rules are applied to this connectivity diagram to predict the system's homeostatic behavior. We validate our model predicted homeostatic profiles against RNAseq gene expression profiles in a mouse model of stress primed neuroinflammation. A meta-analysis was used to calculate the significance of similarity between the inflammatory profiles of mice exposed to diisopropyl fluorophostphate (DFP) [with and without prior priming by the glucocorticoid stress hormone corticosterone (CORT)], with the equilibrium states predicted by the model, and to provide estimates of the degree of the neuroinflammatory response. Beyond normal homeostatic regulation, our model predicts an alternate self-perpetuating condition consistent with chronic neuroinflammation. RNAseq gene expression profiles from the cortex of mice exposed to DFP and CORT+DFP align with this predicted state of neuroinflammation, whereas the alignment to CORT alone was negligible. Simulations of putative treatment strategies post-exposure were shown to be theoretically capable of returning the system to a state of typically healthy regulation with broad-acting anti-inflammatory agents showing the highest probability of success. The results support a role for the brain's own homeostatic drive in perpetuating the chronic neuroinflammation associated with exposure to the organophosphate DFP, with and without CORT priming. The deviation of illness profiles from exact model predictions suggests the presence of additional factors or of lasting changes to the brain's regulatory circuitry specific to each exposure.

Screening of quorum sensing peptides for biological effects in neuronal cells

Quorum sensing peptides (QSP) are an important class of bacterial peptides which can have an effect on human host cells. These peptides are used by bacteria to communicate with each other. Some QSP are able to cross the blood-brain barrier and reach the brain parenchyma. However, nothing is known about the effects of these peptides in the brain. Therefore, 85 quorum sensing peptides were screened on six different neuronal cell lines using MTT toxicity, neurite differentiation, cytokine production and morphology as biological outcomes. This primary screening resulted in 22 peptides with effects observed on neuronal cell lines, indicating a possible role in the gut-brain axis. Four peptides (Q138, Q143, Q180 and Q212) showed induction of neurite outgrowth while two peptides (Q162 and Q208) inhibited NGF-induced neurite outgrowth in PC12 cells. Eight peptides (Q25, Q135, Q137, Q146, Q151, Q165, Q208 and Q298) induced neurite outgrowth in human SH-SY5Y neuroblastoma cells. Two peptides (Q13 and Q52) were toxic for SH-SY5Y cells and one (Q123) for BV-2 microglia cells based on the MTT assay. Six peptides had an effect on BV-2 microglia, Q180, Q184 and Q191 were able to induce IL-6 expression and Q164, Q192 and Q208 induced NO production. Finally, Q75 and Q147 treated C8D1A astrocytes demonstrated a higher fraction of round cells. Overall, these in vitro screening study results indicate for the first time possible effects of QSP on neuronal cells.

Targeted inhibition of RAGE in substantia nigra of rats blocks 6-OHDA-induced dopaminergic denervation

The receptor for advanced glycation endproducts (RAGE) is a pattern-recognition receptor associated with inflammation in most cell types. RAGE up-regulates the expression of proinflammatory mediators and its own expression via activation of NF-kB. Recent works have proposed a role for RAGE in Parkinson's disease (PD). In this study, we used the multimodal blocker of RAGE FPS-ZM1, which has become available recently, to selectively inhibit RAGE in the substantia nigra (SN) of rats intracranially injected with 6-hydroxydopamine (6-OHDA). FPS-ZM1 (40 μg per rat), injected concomitantly with 6-OHDA (10 μg per rat) into the SN, inhibited the increase in RAGE, activation of ERK1/2, Src and nuclear translocation of NF-kB p65 subunit in the SN. RAGE inhibition blocked glial fibrillary acidic protein and Iba-1 upregulation as well as associated astrocyte and microglia activation. Circulating cytokines in serum and CSF were also decreased by FPS-ZM1 injection. The loss of tyrosine hydroxylase and NeuN-positive neurons was significantly inhibited by RAGE blocking. Finally, FPS-ZM1 attenuated locomotory and exploratory deficits induced by 6-OHDA. Our results demonstrate that RAGE is an essential component in the neuroinflammation and dopaminergic denervation induced by 6-OHDA in the SN. Selective inhibition of RAGE may offer perspectives for therapeutic approaches.

Astrocytes: inexplicable cells in neurodegeneration

Astrocytes are the most explored non-neuronal cells in the brain under neurophysiological and neurodegenerative conditions. Extensive research has been done to understand their specific role during neuropathological conditions but still the existing findings could not conclude their mechanism of action and their specific role in neurodegenerative conditions. This review discusses their physiological and pathological roles, their activation, morphological alterations and their probable use in search of new therapeutic targets for the treatment of neurodegenerative diseases.

Morphological plasticity of astroglia: Understanding synaptic microenvironment

Memory formation in the brain is thought to rely on the remodeling of synaptic connections which eventually results in neural network rewiring. This remodeling is likely to involve ultrathin astroglial protrusions which often occur in the immediate vicinity of excitatory synapses. The phenomenology, cellular mechanisms, and causal relationships of such astroglial restructuring remain, however, poorly understood. This is in large part because monitoring and probing of the underpinning molecular machinery on the scale of nanoscopic astroglial compartments remains a challenge. Here we briefly summarize the current knowledge regarding the cellular organisation of astroglia in the synaptic microenvironment and discuss molecular mechanisms potentially involved in use-dependent astroglial morphogenesis. We also discuss recent observations concerning morphological astroglial plasticity, the respective monitoring methods, and some of the newly emerging techniques that might help with conceptual advances in the area.

Memory Formation Shaped by Astroglia

Astrocytes, the most heterogeneous glial cells in the central nervous system (CNS), execute a multitude of homeostatic functions and contribute to memory formation. Consolidation of synaptic and systemic memory is a prolonged process and hours are required to form long-term memory. In the past, neurons or their parts have been considered to be the exclusive cellular sites of these processes, however, it has now become evident that astrocytes provide an important and essential contribution to memory formation. Astrocytes participate in the morphological remodeling associated with synaptic plasticity, an energy-demanding process that requires mobilization of glycogen, which, in the CNS, is almost exclusively stored in astrocytes. Synaptic remodeling also involves bidirectional astroglial-neuronal communication supported by astroglial receptors and release of gliosignaling molecules. Astroglia exhibit cytoplasmic excitability that engages second messengers, such as Ca²⁺, for phasic, and cyclic adenosine monophosphate (cAMP), for tonic signal coordination with neuronal processes. The detection of signals by astrocytes and the release of gliosignaling molecules, in particular by vesicle-based mechanisms, occurs with a significant delay after stimulation, orders of magnitude longer than that present in stimulus–secretion coupling in neurons. These particular arrangements position astrocytes as integrators ideally tuned to support time-dependent memory formation.

Perisynaptic astroglial processes: dynamic processors of neuronal information

Neuroglial interactions are now recognized as essential to brain functions. Extensive research has sought to understand the modalities of such dialog by focusing on astrocytes, the most abundant glial cell type of the central nervous system. Neuron-astrocyte exchanges occur at multiple levels, at different cellular locations. With regard to information processing, regulations occurring around synapses are of particular interest as synaptic networks are thought to underlie higher brain functions. Astrocytes morphology is tremendously complex in that their processes exceedingly branch out to eventually form multitudinous fine leaflets. The latter extremities have been shown to surround many synapses, forming perisynaptic astrocytic processes, which although recognized as essential to synaptic functioning, are poorly defined elements due to their tiny size. The current review sums up the current knowledge on their molecular and structural properties as well as the functional characteristics making them good candidates for information processing units.

Form follows function: astrocyte morphology and immune dysfunction in SIV neuroAIDS

Cortical function is disrupted in neuroinflammatory disorders, including HIV-associated neurocognitive disorders (HAND). Astrocyte dysfunction includes retraction of foot processes from the blood-brain barrier and decreased removal of neurotransmitters from synaptic clefts. Mechanisms of astrocyte activation, including innate immune function and the fine neuroanatomy of astrocytes, however, remain to be investigated. We quantified the number of glial fibrillary acidic protein (GFAP)-labeled astrocytes per square millimeter and the proportion of astrocytes immunopositive for Toll-like receptor 2 (TLR2) to examine innate immune activation in astrocytes. We also performed detailed morphometric analyses of gray and white matter astrocytes in the frontal and parietal lobes of rhesus macaques infected with simian immunodeficiency virus (SIV), both with and without encephalitis, an established model of AIDS neuropathogenesis. Protoplasmic astrocytes (gray matter) and fibrous astrocytes (deep white matter) were imaged, and morphometric features were analyzed using Neurolucida. Gray matter and white matter astrocytes showed no change in cell body size in animals infected with SIV regardless of encephalitic status. In SIV-infected macaques, both gray and white matter astrocytes had shorter, less ramified processes, resulting in decreased cell arbor compared with controls. SIV-infected macaques with encephalitis showed decreases in arbor length in white matter astrocytes and reduced complexity in gray matter astrocytes compared to controls. These results provide the first evidence that innate immune activation of astrocytes is linked to altered cortical astrocyte morphology in SIV/HIV infection. Here, we demonstrate that astrocyte remodeling is correlated with infection. Perturbed neuron-glia signaling may be a driving factor in the development of HAND.

Dynamics of β-adrenergic/cAMP signaling and morphological changes in cultured astrocytes

The morphology of astrocytes, likely regulated by cAMP, determines the structural association between astrocytes and the synapse, consequently modulating synaptic function. β-Adrenergic receptors (β-AR), which increase cytosolic cAMP concentration ([cAMP]i ), may affect cell morphology. However, the real-time dynamics of β-AR-mediated cAMP signaling in single live astrocytes and its effect on cell morphology have not been studied. We used the fluorescence resonance energy transfer (FRET)-based cAMP biosensor Epac1-camps to study time-dependent changes in [cAMP]i ; morphological changes in primary rat astrocytes were monitored by real-time confocal microscopy. Stimulation of β-AR by adrenaline, noradrenaline, and isoprenaline, a specific agonist of β-AR, rapidly increased [cAMP]i (∼15 s). The FRET signal response, mediated via β-AR, was faster than in the presence of forskolin (twofold) and dibutyryl-cAMP (>35-fold), which directly activate adenylyl cyclase and Epac1-camps, respectively, likely due to slow entry of these agents into the cytosol. Oscillations in [cAMP]i have not been recorded, indicating that cAMP-dependent processes operate in a slow time domain. Most Epac1-camps expressing astrocytes revealed a morphological change upon β-AR activation and attained a stellate morphology within 1 h. The morphological changes exhibited a bell-shaped dependency on [cAMP]i . The 5-10% decrease in cell cross-sectional area and the 30-50% increase in cell perimeter are likely due to withdrawal of the cytoplasm to the perinuclear region and the appearance of protrusions on the surface of astrocytes. Because astrocyte processes ensheath neurons, β-AR/cAMP-mediated morphological changes can modify the geometry of the extracellular space, affecting synaptic, neuronal, and astrocyte functions in health and disease.

Astrocyte-synapse structural plasticity

The function and efficacy of synaptic transmission are determined not only by the composition and activity of pre- and postsynaptic components but also by the environment in which a synapse is embedded. Glial cells constitute an important part of this environment and participate in several aspects of synaptic functions. Among the glial cell family, the roles played by astrocytes at the synaptic level are particularly important, ranging from the trophic support to the fine-tuning of transmission. Astrocytic structures are frequently observed in close association with glutamatergic synapses, providing a morphological entity for bidirectional interactions with synapses. Experimental evidence indicates that astrocytes sense neuronal activity by elevating their intracellular calcium in response to neurotransmitters and may communicate with neurons. The precise role of astrocytes in regulating synaptic properties, function, and plasticity remains however a subject of intense debate and many aspects of their interactions with neurons remain to be investigated. A particularly intriguing aspect is their ability to rapidly restructure their processes and modify their coverage of the synaptic elements. The present review summarizes some of these findings with a particular focus on the mechanisms driving this form of structural plasticity and its possible impact on synaptic structure and function.

Aerosol-induced brucellosis increases TLR-2 expression and increased complexity in the microanatomy of astroglia in rhesus macaques

Brucella melitensis, a bacterial pathogen and agent of epizootic abortion causes multiple pathologies in humans as well as a number of agriculturally important animal species. Clinical human brucellosis manifests as a non-specific, chronic debilitating disease characterized by undulant fever, arthropathies, cardiomyopathies and neurological sequelae. These symptoms can occur acutely for a few weeks or persist for months to years. Within the brain, endothelial and glial cells can be infected leading to downstream activation events including matrix metalloprotease (MMP) and cytokine secretion and Toll-like receptor (TLR) signaling. These events are likely to lead to tissue remodeling, including morphologic changes in neuronal and glial cells, which are linked to neurological complications including depressive behavior, immune activation and memory loss. Our hypothesis was that B. melitensis infection and neurobrucellosis would lead to activation of astrocytes through upregulation of TLR2 and stimulate concurrent changes in the microanatomy. All six animals were infected via inhalation route. TLR2 expression was approximately doubled in white matter astrocytes of infected rhesus macaques. There was also a 50% increase in the number of astrocytes per unit area in subcortical white matter tracts suggesting increased innate immune activation. This coincided with dramatic increases in the length and complexity of the cell arbor of hypertrophic astrocytes in both cortical gray and white matter. Thus, aerosol-induced brucellosis results in dramatically increased innate immune activation of astrocytes in the absence of widespread neuroinflammation.

Amyloid beta(1-40)-induced astrogliosis and the effect of genistein treatment in rat: a three-dimensional confocal morphometric and proteomic study

Astrocytes are highly involved in regulation and homeostasis of the extracellular environment in the healthy brain. In pathological conditions, these cells play a major role in the inflammatory response seen in CNS tissues, which is called reactive astrogliosis and includes hypertrophy and proliferation of astrocytes. Here, we performed 3D confocal microscopy to evaluate the morphological response of reactive astrocytes positive for glial fibrillary acidic protein (GFAP) in rats, to the presence of Aβ(1-40) in the rat brain before and after treatment with genistein. In 50 astrocytes per animal, we measured the volume and surface area for the nucleus, cell body, the entire cell, the tissue covered by single astrocytes and quantified the number and length of branches, the density of the astrocytes and the intensity of GFAP immunoreactivity. Injecting Aβ(1-40) into the brain of rats caused astrogliosis indicated by increased values for all measured parameters. Mass spectrometric analysis of hippocampal tissue in Aβ(1-40)-injected brain showed decreased amounts of tubulins, enolases and myelin basic protein, and increased amounts of dihydropyrimidinase-related protein 2. In Aβ(1-40)-injected rats pretreated with genistein, GFAP intensity was decreased to the sham-operated group level, and Aβ(1-40)-induced astrogliosis was significantly ameliorated.

Astrocyte atrophy and immune dysfunction in self-harming macaques

Background

Self-injurious behavior (SIB) is a complex condition that exhibits a spectrum of abnormal neuropsychological and locomotor behaviors. Mechanisms for neuropathogenesis could include irregular immune activation, host soluble factors, and astrocyte dysfunction.

Methods

We examined the role of astrocytes as modulators of immune function in macaques with SIB. We measured changes in astrocyte morphology and function. Paraffin sections of frontal cortices from rhesus macaques identified with SIB were stained for glial fibrillary acidic protein (GFAP) and Toll-like receptor 2 (TLR2). Morphologic features of astrocytes were determined using computer-assisted camera lucida.

Results

There was atrophy of white matter astrocyte cell bodies, decreased arbor length in both white and gray matter astrocytes, and decreased bifurcations and tips on astrocytes in animals with SIB. This was combined with a five-fold increase in the proportion of astrocytes immunopositive for TLR2.

Conclusions

These results provide direct evidence that SIB induces immune activation of astrocytes concomitant with quantifiably different morphology.

Studying subcellular detail in fixed astrocytes: dissociation of morphologically intact glial cells (DIMIGs)

Studying the distribution of astrocytic antigens is particularly hard when they are localized in their fine, peripheral astrocyte processes (PAPs), since these processes often have a diameter comparable to vesicles and small organelles. The most appropriate technique is immunoelectron microscopy, which is, however, a time-consuming procedure. Even in high resolution light microscopy, antigen localization is difficult to detect due to the small dimensions of these processes, and overlay from antigen in surrounding non-glial cells. Yet, PAPs frequently display antigens related to motility and glia-synaptic interaction. Here, we describe the dissociation of morphologically intact glial cells (DIMIGs), permitting unambiguous antigen localization using epifluorescence microscopy. Astrocytes are dissociated from juvenile (p13-15) mouse cortex by applying papain treatment and cytospin centrifugation to attach the cells to a slide. The cells and their complete processes including the PAPs is thus projected in 2D. The entire procedure takes 2.5-3 h. We show by morphometry that the diameter of DIMIGs, including the PAPs is similar to that of astrocytes in situ. In contrast to cell culture, results derived from this procedure allow for direct conclusions relating to (1) the presence of an antigen in cortical astrocytes, (2) subcellular antigen distribution, in particular when localized in the PAPs. The detailed resolution is shown in an exemplary study of the organization of the astrocytic cytoskeleton components actin, ezrin, tubulin, and GFAP. The distribution of connexin 43 in relation to a single astrocyte's process tree is also investigated.

Expression and activation of STAT3 in the astrocytes of optic nerve in a rat model of transient intraocular hypertension

Lamina cribosa, an astrocyte-rich region, is the origin of axonal degeneration in glaucomatous neuropathy. Astrocytes are particularly activated during optic nerve (ON) degeneration and are likely to contribute to the pathogenesis of glaucomatous optic neuropathy. Signalling mechanisms that regulate different aspects of astrocyte reactiviation in response to intraocular hypertensive injury are not well defined. Signal transducer and activator of transcription protein-3 (STAT3) is a transcription factor that participates in many biological processes and has been implicated as activator of reactive astrogliosis. In this study, we investigated the role of STAT3 in regulating the activation of astrocytes to transient intraocular hypertension in vivo by using a rat ocular hypertension model. ON astrocytes hypertrophy was observed early after intraocular hypertensive stress. Morphological changes in glial fibrillary acidic protein (GFAP) positive cells coupled with axon loss in the optic nerve was detected at day 7 after the injury. Nestin was significantly upregulated in ON astrocytes as early as day 2 post injury and kept elevated through post injury day 7. Phosphorylated STAT3 (pSTAT3) was markedly upregulated in ON astrocytes at post injury day 1, prior to the reactivation of ON astrocytes. These findings indicate that STAT3 signalling is involved in the initiation of astrocyte reactivation in optic nerve injury.

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.

A forward loop between glioma and microglia: glioma-derived extracellular matrix-activated microglia secrete IL-18 to enhance the migration of glioma cells

The mediators and cellular effectors of inflammation are important constituents of the local environment of tumors. In some occasions, oncogenic changes induce an inflammatory microenvironment that promotes the progression of tumors. In gliomas, the presence of microglia may represent tumor-related inflammation and microglia activation, and subsequent inflammatory responses may influence tumor growth and metastasis. Here, we found that C6 glioma--but not primary astrocyte-derived extracellular matrix (ECM) could activate microglia, including primary microglia and BV-2 cell line, and activated microglia-secreted interleukin (IL)-18, a potent inflammatory cytokine of the IL-1 family, to promote C6 migration. In addition, by coating purified ECM components, it was found that secretion of IL-18 by activated microglia was enhanced when microglia encountered with fibronectin and vitronectin. Furthermore, IL-18-induced C6 migration and microfilament disassembly were antagonized by iNOS inhibitor, guanylate cyclase inhibitor, and protein kinase G inhibitor. Taken together, these results indicate that IL-18 secreted by microglia, which was activated by C6 glioma-derived ECM, enhanced migration of C6 glioma through NO/cGMP pathway.

An in vitro trauma model to study rodent and human astrocyte reactivity

Protocols are presented describing a unique in vitro injury model and how to culture and mature mouse, rat, and human astrocytes for its use. This injury model produces widespread injury and astrocyte reactivity that enable quantitative measurements of morphological, biochemical, and functional changes in rodent and human reactive astrocytes. To investigate structural and molecular mechanisms of reactivity in vitro, cultured astrocytes need to be purified and then in vitro "matured" to reach a highly differentiated state. This is achieved by culturing astrocytes on deformable collagen-coated membranes in the presence of adult-derived horse serum (HS), followed by its stepwise withdrawal. These in vitro matured, process-bearing, quiescent astrocytes are then subjected to mechanical stretch injury by an abrupt pressure pulse from a pressure control device that briefly deforms the culture well bottom. This inflicts a measured reproducible, widespread strain that induces reactivity and injury in rodent and human astrocytes. Cross-species comparisons are possible because mouse, rat, and human astrocytes are grown using essentially the same in vitro treatment regimen. Human astrocytes from fetal cerebral cortex are compared to those derived from cortical biopsies of epilepsy patients (ages 1-12 years old), with regard to growth, purity, and differentiation. This opens a unique opportunity for future studies on glial biology, maturation, and pathology of human astrocytes. Prototypical astrocyte proteins including GFAP, S100, aquaporin4, glutamate transporters, and tenascin are expressed in mouse, rat, and human in vitro matured astrocyte. Upon pressure-stretching, rodent and human astrocytes undergo dynamic morphological, gene expression, and protein changes that are characteristic for trauma-induced reactive astrogliosis.

RhoA-ROCK-Myosin pathway regulates morphological plasticity of cultured olfactory ensheathing cells

Olfactory ensheathing cells (OECs) are glial cells in the olfactory system with morphological and functional plasticity. Cultured OECs have the flattened and process-bearing shape. Reversible changes have been found between these two morphological phenotypes. However, the molecular mechanism underlying the regulation of their morphological plasticity remains elusive. Using RhoA FRET biosensor, we found that the active RhoA signal mainly distributed in the lamellipodia and/or filopodia of OECs. Local disruption of these active RhoA distributions led to the morphological change from the flattened into process-bearing shape and promoted process outgrowth. Furthermore, RhoA pathway inhibitors, Toxin-B, C3, Y-27632 or over-expression of DN-RhoA blocked serum-induced morphological change of OECs from the process-bearing into flattened shape, whereas the activation of RhoA pathway by lysophosphatidic acid (LPA) promoted the morphological change from the process-bearing into flattened shape. Finally, ROCK-Myosin-F-actin as a downstream of RhoA pathway was involved in morphological plasticity of OECs. Taken together, these results suggest that RhoA-ROCK-Myosin pathway mediates the morphological plasticity of cultured OECs in response to extracellular cues.

Possible mechanisms of trichosanthin-induced apoptosis of tumor cells

Trichosanthin (TCS) is a type I ribosome-inactivating protein that is isolated from the root tubers of the Chinese medicinal herb Trichosanthes kirilowii Maximowicz. TCS has been used as an abortifacient for 1,500 years in China because of its high toxicity on trophoblasts. Over the past 20 years, TCS has been the subject of much research because of its potential antitumor activities. Many reports have revealed that TCS is cytotoxic in a variety of tumor cell lines in vitro and in vivo. Monoclonal antibody-conjugated TCS could enhance its antitumor efficacy; thus, TCS is considered to be a potential biological agent for cancer treatment. TCS is able to inhibit protein synthesis and consequently induce necrosis. Recent studies have demonstrated that TCS does indeed induce apoptosis in several tumor cell lines. Although TCS-induced apoptosis of tumor cell lines is now well known, the underlying mechanisms remain to be elucidated. The purpose of this review was to investigate the effects of TCS and its possible mechanisms of action, based on published literature and the results of our own studies.

Actin cytoskeleton remodeling governs aquaporin-4 localization in astrocytes

Aquaporin-4 (AQP4) is constitutively concentrated in the plasma membrane of the perivascular glial processes, and its expression is altered in certain pathological conditions associated with brain edema or altered glial migration. When astrocytes are grown in culture, they lose their characteristic star-like shape and AQP4 continuous plasma membrane localization observed in vivo. In this study, we differentiated primary astrocyte cultures with cAMP and lovastatin, both able to induce glial stellation through a reorganization of F-actin cytoskeleton, and obtained AQP4 selectively localized on the cell plasma membrane associated with an increase in the plasma membrane water transport level, but only cAMP induced an increase in AQP4 total protein expression. Phosphorylation experiments indicated that AQP4 in astrocytes is neither phosphorylated nor a substrate of PKA. Depolymerization of F-actin cytoskeleton performed by cytochalasin-D suggested that F-actin cytoskeleton plays a primary role for AQP4 plasma membrane localization and during cell adhesion. Finally, AQP4 knockdown does not compromise the ability of astrocytes to stellate in the presence of cAMP, indicating that astrocyte stellation is independent of AQP4.

The cyclic GMP-protein kinase G pathway regulates cytoskeleton dynamics and motility in astrocytes

We have previously demonstrated that inflammatory compounds that increase nitric oxide (NO) synthase expression have a biphasic effect on the level of the NO messenger cGMP in astrocytes. In this work, we demonstrate that NO-dependent cGMP formation is involved in the morphological change induced by lipopolysaccharide (LPS) in cultured rat cerebellar astroglia. In agreement with this, dibutyryl-cGMP, a permeable cGMP analogue, and atrial natriuretic peptide, a ligand for particulate guanylyl cyclase, are both able to induce process elongation and branching in astrocytes resulting from a rapid, reversible and concentration-dependent redistribution of glial fibrillary acidic protein (GFAP) and actin filaments without significant change in protein levels. These effects are also observed in astrocytes co-cultured with neurons. The cytoskeleton rearrangement induced by cGMP is prevented by the specific protein kinase G inhibitor Rp-8Br-PET-cGMPS and involves downstream inhibition of RhoA GTPase since is not observed in cells transfected with constitutively active RhoA. Furthermore, dibutyryl-cGMP prevents RhoA-membrane association, a step necessary for its interaction with effectors. Stimulation of the cGMP-protein kinase G pathway also leads to increased astrocyte migration in an in vitro scratch-wound assay resulting in accelerated wound closure, as seen in reactive gliosis following brain injury. These results indicate that cGMP-mediated pathways may regulate physio-pathologically relevant responses in astroglial cells.

Bacteria and PAMPs activate nuclear factor kappaB and Gro production in a subset of olfactory ensheathing cells and astrocytes but not in Schwann cells

The primary olfactory nerves provide uninterrupted conduits for neurotropic pathogens to access the brain from the nasal cavity, yet infection via this route is uncommon. It is conceivable that olfactory ensheathing cells (OECs), which envelope the olfactory nerves along their entire length, provide a degree of immunological protection against such infections. We hypothesized that cultured OECs would be able to mount a biologically significant response to bacteria and pathogen-associated molecular patterns (PAMPs). The response of OECs to Escherichia coli (E. coli) and various PAMPs was compared to that of Schwann cells (SCs), astrocytes (ACs), and microglia (MG). A subset of OECs displayed nuclear localization of nuclear factor kappaB), an inflammatory transcription factor, after treatment with E. coli (20% +/- 5%), lipopolysacchride (33% +/- 9%), and Poly I:C (25% +/- 5%), but not with peptidoglycan or CpG oligonucleotides. ACs displayed a similar level of activation to these treatments, and in addition responded to peptidoglycan. The activation of OECs and ACs was enhanced by coculture with MG (56% +/- 16% and 85% +/- 13%, respectively). In contrast, SCs did not respond to any treatment or to costimulation by MG. Immunostaining for the chemokine Gro demonstrated a functional response that was consistent with NF kappaB activation. OECs expressed mRNA for Toll-like receptors (TLRs) 2 and 4, but only TLR4 protein was detected by Western blotting and immunohistochemistry. The results demonstrate that OECs possess the cellular machinery that permits them to respond to certain bacterial ligands, and may have an innate immune function in protecting the CNS against infection.

Three-dimensional confocal morphometry - a new approach for studying dynamic changes in cell morphology in brain slices

Pathological states in the central nervous system lead to dramatic changes in the activity of neuroactive substances in the extracellular space, to changes in ionic homeostasis and often to cell swelling. To quantify changes in cell morphology over a certain period of time, we employed a new technique, three-dimensional confocal morphometry. In our experiments, performed on enhanced green fluorescent protein/glial fibrillary acidic protein astrocytes in brain slices in situ and thus preserving the extracellular microenvironment, confocal morphometry revealed that the application of hypotonic solution evoked two types of volume change. In one population of astrocytes, hypotonic stress evoked small cell volume changes followed by a regulatory volume decrease, while in the second population volume changes were significantly larger without subsequent volume regulation. Three-dimensional cell reconstruction revealed that even though the total astrocyte volume increased during hypotonic stress, the morphological changes in various cell compartments and processes were more complex than have been previously shown, including swelling, shrinking and structural rearrangement. Our data show that astrocytes in brain slices in situ during hypotonic stress display complex behaviour. One population of astrocytes is highly capable of cell volume regulation, while the second population is characterized by prominent cell swelling, accompanied by plastic changes in morphology. It is possible to speculate that these two astrocyte populations play different roles during physiological and pathological states.

Three-dimensional confocal morphometry reveals structural changes in astrocyte morphology in situ

Neuronal activity and many pathological states in the CNS are accompanied by transient astrocytic swelling, which affects excitability, extrasynaptic transmission, and neuron-glia interactions. By using three-dimensional confocal morphometry (3DCM), we quantified the morphometric parameters of astrocytes in intact tissue. In experiments performed in brain cortex slices from transgenic GFAP/EGFP mice, we applied 3DCM to study the dynamic changes in astrocyte morphology during hypotonic stress. Our morphometric analysis showed that the effect of a 10-min application of hypotonic solution (200 mmol/kg) on the swelling of different cell compartments was dependent on the extent of the swelling of the total astrocyte volume. If the swelling of the whole cell, i.e., soma and processes, was less than approximately 10%, there were no differences between the swelling of the soma and the processes. However, if the swelling of the total cell volume was greater than 10%, the swelling of the processes was greater than the swelling of the soma. Analyzing the effect of hypotonic solution on the morphology of these astrocytes revealed that the total cell volume increased; however, certain cell compartments were distinguished in which the volume increased, whereas in other compartments cell volume decreased or apparently did not change, and the structure of some compartments was altered. Our data show that astrocytes in brain slices undergoing hypotonic stress display cell volume regulation as well as transient changes in morphology.

Differentiated astrocytes acquire sensitivity to hydrogen sulfide that is diminished by the transformation into reactive astrocytes

Hydrogen sulfide (H2S) enhances the induction of hippocampal long-term potentiation (LTP) and induces calcium waves in astrocytes. Based on these observations, H2S has been proposed to be a synaptic modulator in the brain. Here we show that differentiated astrocytes acquire sensitivity to H2S that is diminished by their transformation into reactive astrocytes. Although sodium hydrosulfide hydrate (NaHS), a donor of H2S, did not increase the intracellular concentration of Ca2+ in progenitors, exposure of progenitors to leukemia inhibitory factor (LIF), which induces differentiation into glial fibrillary acidic protein (GFAP)-positive astrocytes, greatly increased the sensitivity to NaHS. In contrast, epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), dibutyryl cyclic AMP (db cAMP) and interleukin-1beta (IL-1beta) induced the conversion to reactive astrocytes with diminished sensitivity to NaHS. This suppressive effect of EGF on the sensitivity to NaHS was inhibited by cycloheximide, indicating that de novo protein synthesis was required for the suppression of H2S sensitivity.

CELL CULTURES ON A SUPER WATER-REPELLENT ALKYLKETENE DIMER SURFACE

The fractal alkylketene dimer (AKD) surface is an artificial super water-repellent one with a high contact angle of 174°, therefore, may provide special surface circumstances for studies of biological cells such as cell cultures. The experimental results indicated that the distribution of F-actin in the astrocytes cultured on the fractal AKD-coated dishes showed the stellate shape, while that in the astrocytes cultured on the poly-L-lysine-coated coverslips showed the formation of long alignment. The morphological change of astrocytes is induced by the fractal AKD surface, and the result suggests that astrocyte differentiation is stimulated by the fractal AKD surface.

Potassium currents in primary cultured astrocytes from the rat corpus callosum

The corpus callosum (CC) is the main white matter tract in the brain and is involved in interhemispheric communication. Using the whole-cell voltage-clamp technique, a study was made of K(+)-currents in primary cultured astrocytes from the CC of newborn rats. These cells were positive to glial fibrillary acidic protein after culturing in Dulbecco's Modified Eagle Medium (> 95% of cells) or in serum-free neurobasal medium with G5 supplement (> 99% of cells). Astrocytes cultured in either medium displayed similar voltage-activated ion currents. In 81% of astrocytes, the current had a transient component and a sustained component, which were blocked by 4-aminopyridine and tetraethylammonium, respectively; and both had a reversal potential of -66 mV, indicating that they were carried by K(+) ions. Based on the Ba(2+)-sensitivity and activation kinetics of the K(+)-current, two groups of astrocytes were discerned. One group (55% of cells) displayed a strong Ba(2+) blockade of the K(+)-current whose activation kinetics, time course of decay, and the current-voltage relationship were modified by Ba(2+). This current was greatly blocked (52%) by Ba(2+) in a voltage-dependent way. Another group (45% of cells) presented weak Ba(2+)-blockade, which was only blocked 24% by Ba(2+). The activation kinetics and time course of decay of this current component were unaffected by Ba(2+). These results may help to understand better the roles of voltage-activated K(+)-currents in astrocytes from the rat CC in particular and glial cells in general.

Cholesterol deficiency perturbs actin signaling and glutamate homeostasis in hippocampal astrocytes

This study was undertaken to explore the possibility that cholesterol deficiency may perturb the physiological functions of astrocytes, thus rendering cells vulnerable to the cytotoxicity induced by glutamate (Glu). Cholesterol deprivation induces astrocyte stellation, which is accompanied by disruption of cortical actin, and phosphorylation of extracellular signal-regulated kinase (ERK) in an astrocyte-specific manner. Moreover, cholesterol reduction decreases the activity of glutamine synthetase (GS) while enhancing the capacity of Glu transporter. Using [(3)H]d-aspartate as a tracer, we found a marked efflux of [(3)H]d-aspartate from cholesterol-deficient astrocytes after Glu stimulation. Changes in the actin cytoskeleton, cell morphology, ERK phosphorylation and GS level gradually recovered in astrocytes after the withdrawal of cholesterol depletion. Moreover, withdrawal of cholesterol deprivation attenuated cell loss in cholesterol-deficient astrocytes during Glu exposure. Taken together, our data suggest that, upon Glu exposure, there would be an increase in intracellular Glu as a consequence of enhanced Glu uptake and reduced degradation of Glu by GS in cholesterol-deficient astrocytes. This in turn leads to a concentration gradient favoring Glu release, thereby causing the accumulation of cytotoxic levels of Glu extracellularly. It is thus concluded that the detrimental effect of cholesterol deprivation may, in part, arise from the impairment in Glu homeostasis.

Morphological and functional plasticity of olfactory ensheathing cells

In the primary olfactory pathway, olfactory ensheathing cells (OECs) extend processes to envelop bundles of olfactory axons as they course towards their termination in the olfactory bulb. The expression of growth-promoting adhesion and extracellular matrix molecules by OECs, and their spatially close association with olfactory axons are consistent with OECs being involved in promoting and guiding olfactory axon growth. Because of this, OECs have been employed as a possible tool for inducing axonal regeneration in the injured adult CNS, resulting in significant functional recovery in some animal models and promising outcomes from early clinical applications. However, fundamental aspects of OEC biology remain unclear. This brief review discusses some of the experimental data that have resulted in conflicting views with regard to the identity of OECs. We present here recent findings which support the notion of OECs as a single but malleable phenotype which demonstrate extensive morphological and functional plasticity depending on the environmental stimuli. The review includes a discussion of the normal functional role of OECs in the developing primary olfactory pathway as well as their interaction with regenerating axons and reactive astrocytes in the novel environment of the injured CNS. The use of OECs to induce repair in the injured nervous system reflects the functional plasticity of these cells. Finally, we will explore the possibility that recent microarray data could point to OECs assuming an innate immune function or playing a role in modulating neuroinflammation.

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.

Involvement of the S100B in cAMP-induced cytoskeleton remodeling in astrocytes: a study using TRTK-12 in digitonin-permeabilized cells

1. Stellation of astrocytes in culture involves a complex rearrangement of microfilaments, intermediate filaments, and microtubules, which reflects in part the plasticity of these cells observed during development or after injury. 2. An astrocytic calcium-binding protein, S100B, has been implicated in the regulation of plasticity due to its ability to interact with cytoskeletal proteins. 3. We used digitonin-permeabilized astrocytes to introduce TRTK-12, a peptide that binds to the C-terminal of S100B and blocks its interaction with cytoskeletal proteins. 4. TRTK-12 was able to block cAMP-induced astrocyte stellation and this effect was dependent on the concentration of the peptide. These results support the idea that S100B has a modulatory role on astrocyte morphology.

Failure to express GAP-43 leads to disruption of a multipotent precursor and inhibits astrocyte differentiation

The nervous system-specific protein GAP-43 is significantly upregulated in neurons and glia that are differentiating. In P19 EC cells that do not express GAP-43, neurogenesis is inhibited; many immature neurons apoptose and the survivors do not mature morphologically. Here we show that the initial defect is in an early precursor with characteristics of a neural stem cell, which failed to respond normally to retinoic acid (RA). As a consequence, its progeny had altered cell fates: In addition to the neuronal defects previously reported, RC1-labeled radial glia failed to exit the cell cycle, accumulated, and failed to acquire GFAP immunoreactivity. However, leukemia inhibitory factor (LIF) could stimulate GFAP expression suggesting that astrocytes not derived from radial glia are less affected by absence of GAP-43. Differentiation of radial glia-derived astrocytes was also inhibited in glial cultures from GAP-43 (-/-) cerebellum, and in GAP-43 (-/-) telencephalon in vivo, differentiation of astrocytes derived from both radial and nonradial glia lineages were both affected: In the glial wedge, GFAP-labeled radial glia-derived astrocytes were reduced consistent with the interpretation that they may be unable to deflect GAP-43 (-/-) commissural axons toward the midline. At the midline, both radial and nonradial glia-derived astrocytes were also decreased although it fused normally. The results demonstrate that GAP-43 expressed in multipotent precursors is required for appropriate cell fate commitment, and that its absence affects astrocyte as well as neuronal differentiation.

β-Hydroxy-butyrate alters the extracellular content of S100B in astrocyte cultures

Astrocytes have a variety of roles in maintaining neural tissue physiology, including energetic support, uptake and metabolism of glutamate and secretion of neurotrophic factors. Glutamate toxicity has been implicated in neurodegenerative disorders associated with conditions related to energy failure, and to elevation of glutamate extracellular levels in brain. Glucose is the main energetic substrate for brain cells but, in some circumstances, the ketone bodies are used as a supplementary source and have been suggested to be neuroprotective agents against seizure disorders. Here, we investigate some possible biochemical changes in astrocyte cultures induced by beta-hydroxy-butyrate, the predominant blood ketone body. Its effect upon S100B secretion, astrocyte morphology and glutamate uptake was particularly investigated. S100B, a calcium-binding protein expressed and secreted by astrocytes, has neurotrophic activity and a possible role in epileptogenesis. Cell morphology was investigated by phase-contrast microscopy and immunocytochemistry for actin, GFAP and S100B. Our data show that beta-hydroxy-butyrate induces dramatic changes in astrocyte morphology and, independent of this, causes changes in the extracellular content of S100B. We observed an increment in S100B 1 h after beta-hydroxy-butyrate addition and a decrease 24 h later. No changes were observed in glutamate uptake. These astrocytic modifications may be associated with reduced neuronal excitability observed in the ketogenic condition.

Fluid shear in viscous fibronectin gels allows aggregation of fibrous materials for CNS tissue engineering

Fibronectin (Fn) materials prepared from human plasma have been used in various forms as substrates for tissue engineering. Such purposes require that the soluble protein aggregates into insoluble fibrous structures which encourage the attachment and migration of cells. The method of aggregation due to mechanical shear was investigated by applying fluid shear forces directly to a viscous solution of Fn. Structural analysis revealed that mechanical shear resulted in the formation of an orientated fibrous protein material that was less soluble than its non-sheared counterpart. The suitability of this shear aggregated Fn material for CNS repair purposes was assessed in vitro where it supported the growth of fibroblasts, S100 immunoreactive Schwann cells and GFAP immunoreactive astrocytes. Implantation of the shear aggregated Fn material into a rat model of spinal cord injury provided a permissive environment for axonal growth. This was extended using an impermeable coating to improve orientation and straightness of axonal growth.

Lesional Expression of RhoA and RhoB following Traumatic Brain Injury in Humans

Inhibition of the small GTPase Rho or of its downstream target Rho-associated kinase (ROCK) has been shown to promote axon regeneration and to improve functional recovery following traumatic CNS lesions in the adult rat. In order to determine the expression pattern of RhoA and RhoB following human traumatic brain injury (TBI) and to assess whether Rho is a possible target for pharmacological intervention in humans, we investigated expression patterns of RhoA and RhoB in brain specimens from 25 patients who died after closed TBI in comparison to brain tissue derived from four neuropathologically unaffected control patients by immunohistochemistry. A highly significant lesional upregulation of both RhoA and RhoB was observed beginning several hours after the traumatic event and continuing for months after TBI. The cellular sources of both molecules included polymorphonuclear granulocytes, monocytes/macrophages, and reactive astrocytes. Additionally, expression of RhoA was also detected in neuronal cells in some of the cases. From our data, we conclude that inhibition of Rho is a promising mechanism for the development of new pharmacological interventions in human TBI. As the observed upregulation of RhoA and RhoB was still detectable months after TBI, we speculate that even delayed treatment with Rho inhibitors might be a therapeutic option.

Interleukin-1beta induces a reactive astroglial phenotype via deactivation of the Rho GTPase-Rock axis

The cytokine interleukin-1beta (IL-1beta) is critical to the formation of an astrocytic scar after CNS injury, but the mechanisms by which it induces a reactive phenotype remain unresolved. Here, we show that IL-1beta regulates the phenotype of astrocytes via deactivation of the Rho GTPase-Rho kinase (ROCK) pathway, which governs cellular morphology and migration via effects on F-actin and its interactions with focal adhesions, nonmuscle myosin, and microvillar adapter proteins of the ezrin-radixin-moesin (ERM) family. We found that IL-1beta induced cortical reorganization of F-actin and dephosphorylation of focal adhesion kinase, myosin light chain 2, and myosin phosphatase targeting subunit 1 in primary human astrocytes, and that all of these effects were mimicked by Rho-ROCK pathway blockade. We also found that IL-1beta conversely potentiated ERM phosphorylation, and that this effect was mediated via a Rho-ROCK-independent mechanism. Next, we used a rhotekin pulldown assay to confirm directly that IL-1beta deactivates Rho, and further demonstrated that a constitutively active Rho construct rescued astrocytes from developing an IL-1beta-induced reactive phenotype. These data implicate cytokine regulation of the Rho-ROCK pathway in the generation of a reactive astrogliosis, and we suggest that interventions targeted at this level may facilitate manipulation of the glial scar in inflammatory disorders of the human CNS.

Age-related modifications of the morphological organization of pituicytes are associated with alteration of the GABAergic and dopaminergic innervation afferent to the neurohypophysial lobe

Ageing is known to induce a marked activation of astrocytes within various regions of the central nervous system. To date, the age-related factors responsible for these modifications are unknown. The neural lobe of the hypophysis (NL) is a particular brain region which does not contain neurons but does contain specialized astrocytes, called pituicytes, and numerous terminals of afferent axons, including (i) peptidergic neurohypophysial axons which terminate on the NL blood vessels, and (ii) axons containing both gamma amino-butyric acid (GABA) and dopamine (DA) which form contacts with pituicytes. Because evidence has recently been provided that GABA signalling mediates the morphological organization of astrocytes, the present study was designed to determine whether modifications of pituicytes during ageing were associated with modifications of the GABAergic axons innervating the NL. We show here that, in adult rats, GABA/DA axons form preferential synaptic-like contacts with pituicytes which express both GABAA and D2 dopamine receptors. We then show that, during ageing, pituicytes undergo dramatic modifications of their morphology, correlatively with marked modifications of the GABA/DA fibres innervating the NL. Lastly, in vitro experiments indicate that modifications of the morphology of pituicytes similar to those observed during ageing were obtained by incubating isolated NL of adult rats with a GABAA receptor agonist and/or a D2 dopamine receptor antagonist, whereas inverse modifications were observed when NL of aged rats were incubated with a GABAA receptor antagonist and a D2 dopamine receptor agonist. Taken together, these data suggest that the age-related morphological changes of pituicytes result from the alteration of the GABA/DAergic innervation of the NL.

Neighborly interactions of metabolically-activated astrocytes in vivo

Metabolic responses of brain cells to a stimulus are governed, in part, by their enzymatic specialization and interrelationships with neighboring cells, and local shifts in functional metabolism during brain activation are likely to be influenced by the neurotransmitter system, subcellular compartmentation, and anatomical structure. Selected examples of functional activation illustrate the complexity of metabolic interactions in working brain and of interpretation of changes in brain lactate levels. The major focus of this article is the disproportionately higher metabolism of glucose compared to oxygen in normoxic brain, a phenomenon that occurs during activation in humans and animals. The glucose utilized in excess of oxygen is not fully explained by accumulation of glucose, lactate, or glycogen in brain or by lactate efflux from brain to blood. Thus, any lactate derived from the excess glucose could not have been stoichiometrically exported to and metabolized by neighboring neurons because oxygen consumption would have otherwise increased and matched that of glucose. Metabolic labeling of tricarboxylic acid cycle-derived amino acids increased during brief sensory stimulation, reflecting a rise in oxidative metabolism. Brain glycogen is mainly in astrocytes, and its level falls throughout the stimulus and early post-activation interval. Glycogenolysis cannot be accounted for by lactate accumulation or oxidation; there must be rapid product clearance. Glycogen restoration is slow and diversion of glucose from oxidative pathways for its re-synthesis could reduce the global O(2)/glucose uptake ratio; astrocytes could downshift this ratio for up to an hour after 5 min stimulus. Morphological studies of astrocytes reveal a paucity of cytoplasm and organelles in the fine processes that surround synapses and form gap junction connections with neighboring astrocytes. Specialized regions of astrocytes, e.g. their endfeet and thin peripheral lamellae, are likely to have compartmentalized metabolic activities. Anatomical constraints imposed upon the fine processes might require preferential utilization of glycolysis to satisfy their energy demands, but rapid lactate clearance would then be essential, since its accumulation would inhibit glycolysis. Gap junctional connections between neighboring astrocytes provide a mechanism for rapid metabolite spreading via the astrocytic syncytium and elimination of by-products. Local structure-function relationships need to be incorporated into experimental models of neuron-astrocyte and astrocyte-astrocyte interactions in working brain.

Regulation of CNS glial phenotypes in N20.1 cells

The N20.1 oligodendroglial cell line, immortalized with SV40 T antigen, simultaneously expresses oligodendroglial markers and glial fibrillary acidic protein (GFAP), an astroglial marker. This study examines the plasticity of N20.1 cells with regard to GFAP expression, and its relationship to expression of SV40 T antigen, p53, and a novel nuclear antigen detected by the A007 monoclonal antibody. Marked changes occur in GFAP levels and cell morphology when N20.1 cells are switched from the permissive temperature (34 degrees C) to the non-permissive temperature (39 degrees C), and with cyclic AMP elevation at 39 degrees C. At 34 degrees C, levels of GFAP are high; when cells are switched to 39 degrees C, GFAP levels decrease significantly, then increase slightly when forskolin is added. At both temperatures, the cells display feathery GFAP immunostaining. When forskolin is added at 39 degrees C, however, cells display bright fibrous GFAP staining in elongated processes. The changes in GFAP were compared to changes in T antigen and p53. As expected, the decrease in T antigen at 39 degrees C was accompanied by movement of p53 from the nucleus to cytoplasm. Total p53 levels did not change, however, and forskolin did not alter the respective distribution or levels of p53 at either temperature. At both temperatures, the cell bodies and processes show internal expression of sulfatide, as demonstrated with the O4, Sulph I, and A007 antibodies. We show, for the first time, abundant nuclear immunoreactivity with the A007 monoclonal antibody in the N20.1 cells. This nuclear reactivity is seen at 34 degrees C, but not at 39 degrees C, similar to p53, and is not detected with the other sulfatide antibodies. Double-label immunostaining shows that the nuclear A007 immunoreactivity is co-localized in nuclear structures with T antigen and p53 at 34 degrees C, but is not found in every nucleus containing these antigens. We conclude that regulation of GFAP expression and morphology in N20.1 cells is dependent on a combination of T antigen expression and level of cAMP and may be related to regulation of p53 and the A007 nuclear antigen.

Morphological plasticity of olfactory ensheathing cells is regulated by cAMP and endothelin-1

Olfactory ensheathing cells (ECs) are a promising tool for the repair of injury in the adult central nervous system. However, important aspects of the cell biology of ECs remain unclear, such as whether ECs exist as a single population or as two subpopulations with Schwann cell-like and astrocyte-like characteristics. The morphologies of these subpopulations are used as defining characteristics, yet ECs are known to be morphologically plastic. To elucidate this apparent inconsistency, we investigated the morphological plasticity of ECs in culture. We defined purified ECs as immunopositive for both p75 neurotrophin receptor and glial fibrillary acidic protein. In MEM (D)-valine modification + 10% dialyzed fetal calf serum, 87%-90% of ECs displayed a flat morphology. In three different serum-free media (N2 medium, neurobasal medium + B27 supplement, and DMEM/F-12 medium + G5 supplement), 78%-84% of ECs displayed process-bearing morphology. Ensheathing cells switched reversibly between these morphologies within a day of the serum conditions being changed. Exposure to 1 nM endothelin-1 in serum-free medium prevented the switch from flat to process-bearing morphology, while 1 mM dibutyryl cAMP accelerated this change. The effects of both agents were completely reversible and similar to that reported for astrocytes. Both flat and process-bearing ECs were immunopositive for brain-derived neurotrophic factor, nerve growth factor, neurotrophin-4, and TrkB but not TrkA. Together, these results suggest that ECs exist as a single morphologically plastic population.