Exocytosis of ATP from astrocyte progenitors modulates spontaneous Ca2+ oscillations and cell migration

Aquaporin-4 and transient receptor potential vanilloid 4 balance in early postnatal neurodevelopment

In the adult brain, the water channel aquaporin-4 (AQP4) is expressed in astrocyte endfoot, in supramolecular assemblies, called "Orthogonal Arrays of Particles" (OAPs) together with the transient receptor potential vanilloid 4 (TRPV4), finely regulating the cell volume. The present study aimed at investigating the contribution of AQP4 and TRPV4 to CNS early postnatal development using WT and AQP4 KO brain and retina and neuronal stem cells (NSCs), as an in vitro model of astrocyte differentiation. Western blot analysis showed that, differently from AQP4 and the glial cell markers, TRPV4 was downregulated during CNS development and NSC differentiation. Blue native/SDS-PAGE revealed that AQP4 progressively organized into OAPs throughout the entire differentiation process. Fluorescence quenching assay indicated that the speed of cell volume changes was time-related to NSC differentiation and functional to their migratory ability. Calcium imaging showed that the amplitude of TRPV4 Ca2+ transient is lower, and the dynamics are changed during differentiation and suppressed in AQP4 KO NSCs. Overall, these findings suggest that early postnatal neurodevelopment is subjected to temporally modulated water and Ca2+ dynamics likely to be those sustaining the biochemical and physiological mechanisms responsible for astrocyte differentiation during brain and retinal development.

Ketotifen is a microglial stabilizer by inhibiting secretory vesicle acidification

AIMS

Microglia survey the brain environment by sensing alarm signals to provide the first line of defense against injury or infection after which they acquire an activated phenotype, but they also respond to chemical signals sent from brain mast cells, sentinels of the immune system, when these are degranulated in response to noxious agents. Nevertheless, excessive microglia activation damages the surrounding healthy neural tissue causing progressive loss of neurons and inducing chronic inflammation. Thus, it would be of intense interest the development and application of agents which prevent mast cell mediator release and inhibit the actions of such mediators once released on microglia.

MAIN METHODS

Fluorescence measurements of fura-2 and quinacrine were used to measure intracellular Ca²⁺ signaling and exocytotic vesicle fusion in resting and activated microglia.

KEY FINDINGS

We show that treatment of microglia with a cocktail of mast cell mediators induces microglia activation, phagocytosis, and exocytosis, and reveal by the first-time microglia undergo a phase of vesicular acidification just before the exocytotic fusion occurs. This acidification is an important process for vesicular maturation and contributes with ∼25 % to the content that the vesicle can store and later release by exocytosis. Pre-incubation with ketotifen, a mast cell stabilizer and H1R antagonist completely abolished histamine-mediated calcium signaling and acidification of microglial organelles, and concomitantly reduced the discharge of vesicle contents.

SIGNIFICANCE

These results highlight a key role for vesicle acidification in microglial physiology and provide a potential therapeutic target for diseases related to mast cell and microglia-mediated neuroinflammation.

Connexins Signatures of the Neurovascular Unit and Their Physio-Pathological Functions

Central nervous system (CNS) homeostasis is closely linked to the delicate balance of the microenvironment in which different cellular components of the neurovascular unit (NVU) coexist. Intercellular communication plays a pivotal role in exchanges of signaling molecules and mediators essential for survival functions, as well as in the removal of disturbing elements that can lead to related pathologies. The specific signatures of connexins (Cxs), proteins which form either gap junctions (GJs) or hemichannels (HCs), represent the biological substrate of the pathophysiological balance. Connexin 43 (Cx43) is undoubtedly one of the most important factors in glia–neuro–vascular crosstalk. Herein, Cxs signatures of every NVU component are highlighted and their critical influence on functional processes in healthy and pathological conditions of nervous microenvironment is reviewed.

Transcriptome and metabolome analysis of crGART, a novel cell model of de novo purine synthesis deficiency: Alterations in CD36 expression and activity

In humans, GART [phosphoribosylglycinamide formyltransferase (EC 2.1.2.2) / phosphoribosylglycinamide synthetase (EC 6.3.4.13) / phosphoribosylaminoimidazole synthetase (EC 6.3.3.1)] is a trifunctional protein which catalyzes the second, third, and fifth reactions of the ten step de novo purine synthesis (DNPS) pathway. The second step of DNPS is conversion of phosphoribosylamine (5-PRA) to glycineamide ribonucleotide (GAR). 5-PRA is extremely unstable under physiological conditions and is unlikely to accumulate in the absence of GART activity. Recently, a HeLa cell line null mutant for GART was constructed via CRISPR-Cas9 mutagenesis. This cell line, crGART, is an important cellular model of DNPS inactivation that does not accumulate DNPS pathway intermediates. In the current study, we characterized the crGART versus HeLa transcriptomes in purine-supplemented and purine-depleted growth conditions. We observed multiple transcriptome changes and discuss pathways and ontologies particularly relevant to Alzheimer disease and Down syndrome. We selected the Cluster of Differentiation (CD36) gene for initial analysis based on its elevated expression in crGART versus HeLa as well as its high basal expression, high log2 value, and minimal P-value.

Calcium Oscillation Frequency Is a Potential Functional Complex Physiological Relevance Indicator for a Neuroblastoma-Based 3D Culture Model

In vitro screening for drugs that affect neural function in vivo is still primitive. It primarily relies on single cellular responses from 2D monolayer cultures that have been shown to be exaggerations of the in vivo response. For the 3D model to be physiologically relevant, it should express characteristics that not only differentiate it from 2D but also closely emulate those seen in vivo. These complex physiologically relevant (CPR) outcomes can serve as a standard for determining how close a 3D culture is to its native tissue or which out of a given number of 3D platforms is better suited for a given application. In this study, Fluo-4-based calcium fluorescence imaging was performed followed by automated image data processing to quantify the calcium oscillation frequency of SHSY5Y cells cultured in 2D and 3D formats. It was found that the calcium oscillation frequency is upregulated in traditional 2D cultures while it was comparable to in vivo in spheroid and microporous polymer scaffold-based 3D models, suggesting calcium oscillation frequency as a potential functional CPR indicator for neural cultures.

Astrocyte and Neuronal Pannexin1 Contribute Distinctly to Seizures

ATP- and adenosine-mediated signaling are prominent types of glia–glia and glia–neuron interaction, with an imbalance of ATP/adenosine ratio leading to altered states of excitability, as seen in epileptic seizures. Pannexin1 (Panx1), a member of the gap junction family, is an ATP release channel that is expressed in astrocytes and neurons. Previous studies provided evidence supporting a role for purinergic-mediated signaling via Panx1 channels in seizures; using mice with global deletion of Panx1, it was shown that these channels contribute in maintenance of seizures by releasing ATP. However, nothing is known about the extent to which astrocyte and neuronal Panx1 might differently contribute to seizures. We here show that targeted deletion of Panx1 in astrocytes or neurons has opposing effects on acute seizures induced by kainic acid. The absence of Panx1 in astrocytes potentiates while the absence of Panx1 in neurons attenuates seizure manifestation. Immunohistochemical analysis performed in brains of these mice, revealed that adenosine kinase (ADK), an enzyme that regulates extracellular levels of adenosine, was increased only in seized GFAP-Cre:Panx1^f/f mice. Pretreating mice with the ADK inhibitor, idotubercidin, improved seizure outcome and prevented the increase in ADK immunoreactivity. Together, these data suggest that the worsening of seizures seen in mice lacking astrocyte Panx1 is likely related to low levels of extracellular adenosine due to the increased ADK levels in astrocytes. Our study not only reveals an unexpected link between Panx1 channels and ADK but also highlights the important role played by astrocyte Panx1 channels in controlling neuronal activity.

Calcium Signaling in Glioma Cells: The Role of Nucleotide Receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y₁ and P2Y₁₂ nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Connexins in Astrocyte Migration

Astrocytes have long been considered the supportive cells of the central nervous system, but during the last decades, they have gained much more attention because of their active participation in the modulation of neuronal function. For example, after brain damage, astrocytes become reactive and undergo characteristic morphological and molecular changes, such as hypertrophy and increase in the expression of glial fibrillary acidic protein (GFAP), in a process known as astrogliosis. After severe damage, astrocytes migrate to the lesion site and proliferate, which leads to the formation of a glial scar. At this scar-forming stage, astrocytes secrete many factors, such as extracellular matrix proteins, cytokines, growth factors and chondroitin sulfate proteoglycans, stop migrating, and the process is irreversible. Although reactive gliosis is a normal physiological response that can protect brain cells from further damage, it also has detrimental effects on neuronal survival, by creating a hostile and non-permissive environment for axonal repair. The transformation of astrocytes from reactive to scar-forming astrocytes highlights migration as a relevant regulator of glial scar formation, and further emphasizes the importance of efficient communication between astrocytes in order to orchestrate cell migration. The coordination between astrocytes occurs mainly through Connexin (Cx) channels, in the form of direct cell-cell contact (gap junctions, GJs) or contact between the extracellular matrix and the astrocytes (hemichannels, HCs). Reactive astrocytes increase the expression levels of several proteins involved in astrocyte migration, such as α_vβ₃ Integrin, Syndecan-4 proteoglycan, the purinergic receptor P2X7, Pannexin1, and Cx43 HCs. Evidence has indicated that Cx43 HCs play a role in regulating astrocyte migration through the release of small molecules to the extracellular space, which then activate receptors in the same or adjacent cells to continue the signaling cascades required for astrocyte migration. In this review, we describe the communication of astrocytes through Cxs, the role of Cxs in inflammation and astrocyte migration, and discuss the molecular mechanisms that regulate Cx43 HCs, which may provide a therapeutic window of opportunity to control astrogliosis and the progression of neurodegenerative diseases.

Upregulation of Orai1 and STIM1 expression as well as store-operated Ca2+ entry in ovary carcinoma cells by placental growth factor

Placental growth factor (PlGF) is produced by tumor cells and stimulates tumor growth and metastasis in part by upregulation of hypoxia inducible factor HIF1α. Orchestration of tumor cell proliferation and migration involves oscillations of cytosolic Ca²⁺ activity ([Ca²⁺]_i). The [Ca²⁺]_i oscillations could be accomplished by triggering of intracellular Ca²⁺ release followed by store-operated Ca²⁺-entry (SOCE). Mechanisms accomplishing SOCE include the pore-forming ion channel unit Orai1 and its regulator STIM1. The present study explored whether PlGF influences the expression of Orai1 and STIM1, as well as SOCE and whether this effect impacts on HIF1α expression. To this end, ovary carcinoma cells were cultured for 24 h without and with PlGF (10 ng/ml). Orai1, STIM1 and HIF1α transcript levels were quantified utilizing RT-PCR and Orai1, STIM1 and HIF1α protein levels by Western blotting. [Ca²⁺]_i was estimated from Fura-2-fluorescence and SOCE from increase of [Ca²⁺]_i following Ca²⁺ re-addition after Ca²⁺-store depletion with extracellular Ca²⁺ removal and sarcoendoplasmatic Ca²⁺-ATPase (SERCA) inhibitor thapsigargin (1 μM). As a result, exposure of ovary carcinoma cells to PlGF was followed by a significant increase of Orai1 as well as STIM1 transcript and protein levels. PlGF significantly increased store-operated Ca²⁺-entry following re-addition of extracellular Ca²⁺, an effect virtually abrogated by Orai1 inhibitor MRS1845 (10 μM). PlGF further increased HIF1α transcript and protein levels, an effect again significantly blunted by MRS1845 (10 μM). In conclusion, PlGF upregulates expression of both, Orai1 and STIM1 thus enhancing store-operated Ca²⁺-entry with subsequent upregulation of HIF1α.

Purinergic signaling in the retina: From development to disease

Retinal injuries and diseases are major causes of human disability involving vision impairment by the progressive and permanent loss of retinal neurons. During development, assembly of this tissue entails a successive and overlapping, signal-regulated engagement of complex events that include proliferation of progenitors, neurogenesis, cell death, neurochemical differentiation and synaptogenesis. During retinal damage, several of these events are re-activated with both protective and detrimental consequences. Purines and pyrimidines, along with their metabolites are emerging as important molecules regulating both retinal development and the tissue's responses to damage. The present review provides an overview of the purinergic signaling in the developing and injured retina. Recent findings on the presence of vesicular and channel-mediated ATP release by retinal and retinal pigment epithelial cells, adenosine synthesis and release, expression of receptors and intracellular signaling pathways activated by purinergic signaling in retinal cells are reported. The pathways by which purinergic receptors modulate retinal cell proliferation, migration and death of retinal cells during development and injury are summarized. The contribution of nucleotides to the self-repair of the injured zebrafish retina is also discussed.

Intertwining extracellular nucleotides and their receptors with Ca2+ in determining adult neural stem cell survival, proliferation and final fate

In the central nervous system (CNS), during both brain and spinal cord development, purinergic and pyrimidinergic signalling molecules (ATP, UTP and adenosine) act synergistically with peptidic growth factors in regulating the synchronized proliferation and final specification of multipotent neural stem cells (NSCs) to neurons, astrocytes or oligodendrocytes, the myelin-forming cells. Some NSCs still persist throughout adulthood in both specific 'neurogenic' areas and in brain and spinal cord parenchyma, retaining the potentiality to generate all the three main types of adult CNS cells. Once CNS anatomical structures are defined, purinergic molecules participate in calcium-dependent neuron-to-glia communication and also control the behaviour of adult NSCs. After development, some purinergic mechanisms are silenced, but can be resumed after injury, suggesting a role for purinergic signalling in regeneration and self-repair also via the reactivation of adult NSCs. In this respect, at least three different types of adult NSCs participate in the response of the adult brain and spinal cord to insults: stem-like cells residing in classical neurogenic niches, in particular, in the ventricular-subventricular zone (V-SVZ), parenchymal oligodendrocyte precursor cells (OPCs, also known as NG2-glia) and parenchymal injury-activated astrocytes (reactive astrocytes). Here, we shall review and discuss the purinergic regulation of these three main adult NSCs, with particular focus on how and to what extent modulation of intracellular calcium levels by purinoceptors is mandatory to determine their survival, proliferation and final fate.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

TRPC3-mediated Ca2+ signals as a promising strategy to boost therapeutic angiogenesis in failing hearts: The role of autologous endothelial colony forming cells

Endothelial progenitor cells (EPCs) are a sub-population of bone marrow-derived mononuclear cells that are released in circulation to restore damaged endothelium during its physiological turnover or rescue blood perfusion after an ischemic insult. Additionally, they may be mobilized from perivascular niches located within larger arteries' wall in response to hypoxic conditions. For this reason, EPCs have been regarded as an effective tool to promote revascularization and functional recovery of ischemic hearts, but clinical application failed to exploit the full potential of patients-derived cells. Indeed, the frequency and biological activity of EPCs are compromised in aging individuals or in subjects suffering from severe cardiovascular risk factors. Rejuvenating the reparative phenotype of autologous EPCs through a gene transfer approach has, therefore, been put forward as an alternative approach to enhance their therapeutic potential in cardiovascular patients. An increase in intracellular Ca²⁺ concentration constitutes a pivotal signal for the activation of the so-called endothelial colony forming cells (ECFCs), the only known truly endothelial EPC subset. Studies from our group showed that the Ca²⁺ toolkit differs between peripheral blood- and umbilical cord blood (UCB)-derived ECFCs. In the present article, we first discuss how VEGF uses repetitive Ca²⁺ spikes to regulate angiogenesis in ECFCs and outline how VEGF-induced intracellular Ca²⁺ oscillations differ between the two ECFC subtypes. We then hypothesize about the possibility to rejuvenate the biological activity of autologous ECFCs by transfecting the cell with the Ca²⁺ -permeable channel Transient Receptor Potential Canonical 3, which selectively drives the Ca²⁺ response to VEGF in UCB-derived ECFCs.

ATP-induced Ca2+-signalling mechanisms in the regulation of mesenchymal stem cell migration

The ability of cells to migrate to the destined tissues or lesions is crucial for physiological processes from tissue morphogenesis, homeostasis and immune responses, and also for stem cell-based regenerative medicines. Cytosolic Ca²⁺ is a primary second messenger in the control and regulation of a wide range of cell functions including cell migration. Extracellular ATP, together with the cognate receptors on the cell surface, ligand-gated ion channel P2X receptors and a subset of G-protein-coupled P2Y receptors, represents common autocrine and/or paracrine Ca²⁺ signalling mechanisms. The P2X receptor ion channels mediate extracellular Ca²⁺ influx, whereas stimulation of the P2Y receptors triggers intracellular Ca²⁺ release from the endoplasmic reticulum (ER), and activation of both type of receptors thus can elevate the cytosolic Ca²⁺ concentration ([Ca²⁺]_c), albeit with different kinetics and capacity. Reduction in the ER Ca²⁺ level following the P2Y receptor activation can further induce store-operated Ca²⁺ entry as a distinct Ca²⁺ influx pathway that contributes in ATP-induced increase in the [Ca²⁺]_c. Mesenchymal stem cells (MSC) are a group of multipotent stem cells that grow from adult tissues and hold promising applications in tissue engineering and cell-based therapies treating a great and diverse number of diseases. There is increasing evidence to show constitutive or evoked ATP release from stem cells themselves or mature cells in the close vicinity. In this review, we discuss the mechanisms for ATP release and clearance, the receptors and ion channels participating in ATP-induced Ca²⁺ signalling and the roles of such signalling mechanisms in mediating ATP-induced regulation of MSC migration.

SLC10A4 regulates IgE-mediated mast cell degranulation in vitro and mast cell-mediated reactions in vivo

Mast cells act as sensors in innate immunity and as effector cells in adaptive immune reactions. Here we demonstrate that SLC10A4, also referred to as the vesicular aminergic-associated transporter, VAAT, modifies mast cell degranulation. Strikingly, Slc10a4 -/- bone marrow-derived mast cells (BMMCs) had a significant reduction in the release of granule-associated mediators in response to IgE/antigen-mediated activation, whereas the in vitro development of mast cells, the storage of the granule-associated enzyme mouse mast cell protease 6 (mMCP-6), and the release of prostaglandin D2 and IL-6 were normal. Slc10a4-deficient mice had a strongly reduced passive cutaneous anaphylaxis reaction and a less intense itching behaviour in response to the mast cell degranulator 48/80. Live imaging of the IgE/antigen-mediated activation showed decreased degranulation and that ATP was retained to a higher degree in mast cell granules lacking SLC10A4. Furthermore, ATP was reduced by two thirds in Slc10a4 -/- BMMCs supernatants in response to IgE/antigen. We speculate that SLC10A4 affects the amount of granule-associated ATP upon IgE/antigen-induced mast cell activation, which affect the release of granule-associated mast cell mediators. In summary, SLC10A4 acts as a regulator of degranulation in vitro and of mast cell-related reactions in vivo.

Regulation of adult neural progenitor cell functions by purinergic signaling

Extracellular purines are signaling molecules in the neurogenic niches of the brain and spinal cord, where they activate cell surface purinoceptors at embryonic neural stem cells (NSCs) and adult neural progenitor cells (NPCs). Although mRNA and protein are expressed at NSCs/NPCs for almost all subtypes of the nucleotide-sensitive P2X/P2Y, and the nucleoside-sensitive adenosine receptors, only a few of those have acquired functional significance. ATP is sequentially degraded by ecto-nucleotidases to ADP, AMP, and adenosine with agonistic properties for distinct receptor-classes. Nucleotides/nucleosides facilitate or inhibit NSC/NPC proliferation, migration and differentiation. The most ubiquitous effect of all agonists (especially of ATP and ADP) appears to be the facilitation of cell proliferation, usually through P2Y1Rs and sometimes through P2X7Rs. However, usually P2X7R activation causes necrosis/apoptosis of NPCs. Differentiation can be initiated by P2Y2R-activation or P2X7R-blockade. A key element in the transduction mechanism of either receptor is the increase of the intracellular free Ca²⁺ concentration, which may arise due to its release from intracellular storage sites (G protein-coupling; P2Y) or due to its passage through the receptor-channel itself from the extracellular space (ATP-gated ion channel; P2X). Further research is needed to clarify how purinergic signaling controls NSC/NPC fate and how the balance between the quiescent and activated states is established with fine and dynamic regulation. GLIA 2017;65:213-230.

NTPDase2 and purinergic signaling control progenitor cell proliferation in neurogenic niches of the adult mouse brain

Nerve cells are continuously generated from stem cells in the adult mammalian subventricular zone (SVZ) and hippocampal dentate gyrus. We have previously noted that stem/progenitor cells in the SVZ and the subgranular layer (SGL) of the dentate gyrus express high levels of plasma membrane-bound nucleoside triphosphate diphosphohydrolase 2 (NTPDase2), an ectoenzyme that hydrolyzes extracellular nucleoside diphosphates and triphosphates. We inferred that deletion of NTPDase2 would increase local extracellular nucleoside triphosphate concentrations perturbing purinergic signaling and boosting progenitor cell proliferation and neurogenesis. Using newly generated mice globally null for Entpd2, we demonstrate that NTPDase2 is the major ectonucleotidase in these progenitor cell-rich areas. Using BrdU-labeling protocols, we have measured stem cell proliferation and determined long-term survival of cell progeny under basal conditions. Brains of Entpd2 null mice revealed increased progenitor cell proliferation in both the SVZ and the SGL. However, this occurred without noteworthy alterations in long-term progeny survival. The hippocampal stem cell pool and the pool of the intermediate progenitor type-2 cells clearly expanded. However, substantive proportions of these proliferating cells were lost during expansion at around type-3 stage. Cell loss was paralleled by decreases in cAMP response element-binding protein phosphorylation in the doublecortin-positive progenitor cell population and by an increase in labeling for activated caspase-3 levels. We propose that NTPDase2 has functionality in scavenging mitogenic extracellular nucleoside triphosphates in neurogenic niches of the adult brain, thereby acting as a homeostatic regulator of nucleotide-mediated neural progenitor cell proliferation and expansion.

Involvement of nucleotides in glial growth following scratch injury in avian retinal cell monolayer cultures

When retinal cell cultures were mechanically scratched, cell growth over the empty area was observed. Only dividing and migrating, 2 M6-positive glial cells were detected. Incubation of cultures with apyrase (APY), suramin, or Reactive Blue 2 (RB-2), but not MRS 2179, significantly attenuated the growth of glial cells, suggesting that nucleotide receptors other than P2Y1 are involved in the growth of glial cells. UTPγS but not ADPβS antagonized apyrase-induced growth inhibition in scratched cultures, suggesting the participation of UTP-sensitive receptors. No decrease in proliferating cell nuclear antigen (PCNA(+)) cells was observed at the border of the scratch in apyrase-treated cultures, suggesting that glial proliferation was not affected. In apyrase-treated cultures, glial cytoplasm protrusions were smaller and unstable. Actin filaments were less organized and alfa-tubulin-labeled microtubules were mainly parallel to scratch. In contrast to control cultures, very few vinculin-labeled adhesion sites could be noticed in these cultures. Increased Akt and ERK phosphorylation was observed in UTP-treated cultures, effect that was inhibited by SRC inhibitor 1 and PI3K blocker LY294002. These inhibitors and the FAK inhibitor PF573228 also decreased glial growth over the scratch, suggesting participation of SRC, PI3K, and FAK in UTP-induced growth of glial cells in scratched cultures. RB-2 decreased dissociated glial cell attachment to fibronectin-coated dishes and migration through transwell membranes, suggesting that nucleotides regulated adhesion and migration of glial cells. In conclusion, mechanical scratch of retinal cell cultures induces growth of glial cells over the empty area through a mechanism that is dependent on activation of UTP-sensitive receptors, SRC, PI3K, and FAK.

NTPDase2 and the P2Y1 receptor are not required for mammalian eye formation

Eye formation in vertebrates is controlled by a conserved pattern of molecular networks. Homeobox transcription factors are crucially involved in the establishment and maintenance of the retina. A previous study of Massé et al. (Nature, 449: 1058-62, 2007) using morpholino knockdown identified the ectonucleotidase NTPDase2 and the P2Y1 receptor as essential elements for eye formation in embryos of the clawed frog Xenopus laevis. In order to investigate whether a similarly essential mechanism would be active in mammalian eye development, we analyzed mice KO for Entpd2 or P2ry1 as well as double KO for Entpd2/P2ry1. These mice developed normal eyes. In order to identify potential deficits in the molecular identity or in the arrangement of the cellular elements of the retina, we performed an immunohistological analysis using a variety of retinal markers. The analysis of single and double KO mice demonstrated that NTPDase2 and P2Y1 receptors are not required for murine eye formation, as previously shown for eye development in Xenopus laevis.

Identification and staining of distinct populations of secretory organelles in astrocytes

Increasing evidence indicates that astrocytes, the most abundant glial cell type in the brain, respond to an elevation in cytoplasmic calcium concentration ([Ca(2+)]i) by releasing chemical transmitters (also called gliotransmitters) via regulated exocytosis of heterogeneous classes of organelles. By this process, astrocytes exert modulatory influences on neighboring cells and are thought to participate in the control of synaptic circuits and cerebral blood flow. Studying the properties of exocytosis in astrocytes is a challenge, because the cell biological basis of this process is incompletely defined. Astrocytic exocytosis involves multiple populations of secretory vesicles, including synaptic-like microvesicles (SLMVs), dense-core granules (DCGs), and lysosomes. Here we summarize the available information for identifying individual populations of secretory organelles in astrocytes, including DCGs, SLMVs, and lysosomes, and present experimental procedures for specifically staining such populations.

Purinergic and glutamatergic receptors on astroglia

Astroglial cells express many neurotransmitter receptors; the receptors to glutamate and ATP being the most abundant. Here, we provide a concise overview on the expression and main properties of astroglial glutamate receptors (ionotropic receptors represented by AMPA and NMDA subtypes) and metabotropic (mainly mGluR5 and mGluR3 subtypes) and purinoceptors (adenosine receptors of A1, A2A, A2B, and A3 types, ionotropic P2X1/5 and P2X7 subtypes, and metabotropic P2Y purinoceptors). We also discuss the role of these receptors in glial physiology and pathophysiology.

Light-controlled astrocytes promote human mesenchymal stem cells toward neuronal differentiation and improve the neurological deficit in stroke rats

Astrocytes are key components of the central nervous system (CNS) and release factors to support neural stem cell proliferation, differentiation, and migration. Adenosine 5'-triphosphate (ATP) is one of the key factors released upon activation of astrocytes that regulates the neural stem cell's function. However, it is not clear whether ATP derived from the depolarized astrocytes plays a vital role in promoting the neuronal differentiation of mesenchymal stem cells (MSCs) in vitro and in vivo. Herein, for the first time, we co-cultured MSCs with light-stimulated-channelrhodopsin-2 (ChR2)-astrocytes, and observed that the neuronal differentiation of MSCs was enhanced by expressing more neuronal markers, Tuj1 and NeuN. The ChR2-astrocyte-conditioned medium also stimulated MSCs differentiating into neuronal lineage cells by expressing more Tuj1 and Pax6, which was blocked by the P2X receptor antagonist, TNP-ATP. Then we found that light-depolarization of astrocytes significantly increased ATP accumulation in their bathing medium without impairing the cell membrane. We further found that ATP up-regulated the Tuj1, Pax6, FZD8 and β-catenin mRNA levels of MSCs, which could be reversed by application of TNP-ATP. Together these in vitro data provided convergent evidence that ATP from light-depolarized-astrocytes activated the wnt/β-catenin signaling of MSCs through binding to the P2X receptors, and promoted the neuronal differentiation of MSCs. Finally but importantly, our study also demonstrated in stroke rats that light-controlled astrocytes stimulated endogenous ATP release into the ischemic area to influence the transplanted MSCs, resulting in promoting the MSCs towards neuronal differentiation and improvements of neurological deficit.

Neural progenitor cells regulate capillary blood flow in the postnatal subventricular zone

In the postnatal subventricular zone (SVZ), S phase entry of neural progenitor cells (NPCs) correlates with a local increase in blood flow. However, the cellular mechanism controlling this hemodynamic response remains unknown. We show that a subpopulation of SVZ cells, astrocyte-like cells or B-cells, sends projections ensheathing pericytes on SVZ capillaries in young mice. We examined whether calcium increases in pericytes or B-cells led to a vascular response in acute slices using the P2Y(2/4) receptor (P2Y(2/4)R) agonist UTP, electrical stimulation, or transgenic mice expressing exogenous Gq-coupled receptors (MrgA1) in B-cells. UTP increased calcium in pericytes leading to capillary constrictions. Electrical stimulation induced calcium propagation in SVZ cells followed by capillary constrictions involving purinergic receptors. In transgenic mice, selective calcium increases in B-cells induced P2Y(2/4)R-dependent capillary constrictions, suggesting that B-cells release ATP activating purinergic receptors on pericytes. Interestingly, in the presence of a P2Y(2/4)R blocker, dilation was observed. Intraventricular UTP injection transiently decreased blood flow monitored in vivo using laser Doppler flowmetry. Using neonatal electroporation, we expressed MrgA1 in slow cycling radial glia-derived B1 cells, i.e., NPCs. Intraventricular injection of an MrgA1 ligand increased blood flow in the SVZ. Thus, upon intracellular calcium increases B-cells/NPCs release ATP and vasodilating factors that activate purinergic receptors on pericytes triggering a vascular response and blood flow increase in vivo. Considering that NPCs receive signals from other SVZ cells, these findings further suggest that NPCs act as transducers of neurometabolic coupling in the SVZ.

Calcium signaling in glioma cells--the role of nucleotide receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present Chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y(1) and P2Y(12) nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Biophysical microenvironment and 3D culture physiological relevance

Force and substrate physical property (pliability) is one of three well established microenvironmental factors (MEFs) that may contribute to the formation of physiologically more relevant constructs (or not) for cell-based high-throughput screening (HTS) in preclinical drug discovery. In 3D cultures, studies of the physiological relevance dependence on material pliability are inconclusive, raising questions regarding the need to design platforms with materials whose pliability lies within the physiological range. To provide more insight into this question, we examine the factors that may underlie the studies inconclusiveness and suggest the elimination of redundant physical cues, where applicable, to better control other MEFs, make it easier to incorporate 3D cultures into state of the art HTS instrumentation, and reduce screening costs per compound.

Extrinsic purinergic regulation of neural stem/progenitor cells: implications for CNS development and repair

There has been tremendous progress in understanding neural stem cell (NSC) biology, with genetic and cell biological methods identifying sequential gene expression and molecular interactions guiding NSC specification into distinct neuronal and glial populations during development. Data has emerged on the possible exploitation of NSC-based strategies to repair adult diseased brain. However, despite increased information on lineage specific transcription factors, cell-cycle regulators and epigenetic factors involved in the fate and plasticity of NSCs, understanding of extracellular cues driving the behavior of embryonic and adult NSCs is still very limited. Knowledge of factors regulating brain development is crucial in understanding the pathogenetic mechanisms of brain dysfunction. Since injury-activated repair mechanisms in adult brain often recapitulate ontogenetic events, the identification of these players will also reveal novel regenerative strategies. Here, we highlight the purinergic system as a key emerging player in the endogenous control of NSCs. Purinergic signalling molecules (ATP, UTP and adenosine) act with growth factors in regulating the synchronized proliferation, migration, differentiation and death of NSCs during brain and spinal cord development. At early stages of development, transient and time-specific release of ATP is critical for initiating eye formation; once anatomical CNS structures are defined, purinergic molecules participate in calcium-dependent neuron-glia communication controlling NSC behaviour. When development is complete, some purinergic mechanisms are silenced, but can be re-activated in adult brain after injury, suggesting a role in regeneration and self-repair. Targeting the purinergic system to develop new strategies for neurodevelopmental disorders and neurodegenerative diseases will be also discussed.

Astrocytes shed large membrane vesicles that contain mitochondria, lipid droplets and ATP

Various cells types, including stem and progenitor cells, can exchange complex information via plasma membrane-derived vesicles, which can carry signals both in their limiting membrane and lumen. Astrocytes, traditionally regarded as mere supportive cells, play previously unrecognized functions in neuronal modulation and are capable of releasing signalling molecules of different functional significance. In the present study, we provide direct evidence that human fetal astrocytes in culture, expressing the same feature as immature and reactive astrocytes, release membrane vesicles larger than the microvesicles described up to now. We found that these large vesicles, ranging from 1-5 to 8 μm in diameter and expressing on their surface β1-integrin proteins, contain mitochondria and lipid droplets together with ATP. We documented vesicle content with fluorescent-specific dyes and with the immunocytochemistry technique we confirmed that mitochondria and lipid droplets were co-localized in the same vesicle. Scanning electron microscopy and transmission electron microscopy confirmed that astrocytes shed from surface membrane vesicles of the same size as the ones detected by fluorescence microscopy. Our results report for the first time that cultured astrocytes, activated by repetitive stimulation of ATP released from neighboring cells, shed from their surface large membrane vesicles containing mitochondria and lipid droplets.

Neuroblast migration and P2Y(1) receptor mediated calcium signalling depend on 9-O-acetyl GD3 ganglioside

Previous studies indicated that a ganglioside 9acGD3 (9-O-acetyl GD3) antibody [the J-Ab (Jones antibody)] reduces GCP (granule cell progenitor) migration in vitro and in vivo. We here investigated, using cerebellar explants of post-natal day (P) 6 mice, the mechanism by which 9acGD3 reduces GCP migration. We found that immunoblockade of the ganglioside with the J-Ab or the lack of GD3 synthase reduced GCP in vitro migration and the frequency of Ca(2+) oscillations. Immunocytochemistry and pharmacological assays indicated that GCPs expressed P2Y(1)Rs (P2Y(1) receptors) and that deletion or blockade of these receptors decreased the migration rate of GCPs and the frequency of Ca(2+) oscillations. The reduction in P2Y(1)-mediated calcium signals seen in Jones-treated and GD3 synthase-null GCPs were paralleled by P2Y(1)R internalization. We conclude that 9acGD3 controls GCP migration by influencing P2Y(1)R cellular distribution and function.

New insights regarding the regulation of chemotaxis by nucleotides, adenosine, and their receptors

The directional movement of cells can be regulated by ATP, certain other nucleotides (e.g., ADP, UTP), and adenosine. Such regulation occurs for cells that are "professional phagocytes" (e.g., neutrophils, macrophages, certain lymphocytes, and microglia) and that undergo directional migration and subsequent phagocytosis. Numerous other cell types (e.g., fibroblasts, endothelial cells, neurons, and keratinocytes) also change motility and migration in response to ATP, other nucleotides, and adenosine. In this article, we review how nucleotides and adenosine modulate chemotaxis and motility and highlight the importance of nucleotide- and adenosine-regulated cell migration in several cell types: neutrophils, microglia, endothelial cells, and cancer cells. We also discuss difficulties in conducting experiments and drawing conclusions regarding the ability of nucleotides and adenosine to modulate the migration of professional and non-professional phagocytes.

Long-range Ca2+ waves transmit brain-damage signals to microglia

Microglia are the resident phagocytes of the brain that are responsible for the clearance of injured neurons, an essential step in subsequent tissue regeneration. How death signals are controlled both in space and time to attract these cells toward the site of injury is a topic of great interest. To this aim, we have used the optically transparent zebrafish larval brain and identified rapidly propagating Ca2+ waves that determine the range of microglial responses to neuronal cell death. We show that while Ca2+-mediated microglial responses require ATP, the spreading of intercellular Ca2+ waves is ATP independent. Finally, we identify glutamate as a potent inducer of Ca2+-transmitted microglial attraction. Thus, this real-time analysis reveals the existence of a mechanism controlling microglial targeted migration to neuronal injuries that is initiated by glutamate and proceeds across the brain in the form of a Ca2+ wave.

Imaging exocytosis of ATP-containing vesicles with TIRF microscopy in lung epithelial A549 cells

Nucleotide release constitutes the first step of the purinergic signaling cascade, but its underlying mechanisms remain incompletely understood. In alveolar A549 cells much of the experimental data is consistent with Ca(2+)-regulated vesicular exocytosis, but definitive evidence for such a release mechanism is missing, and alternative pathways have been proposed. In this study, we examined ATP secretion from A549 cells by total internal reflection fluorescence microscopy to directly visualize ATP-loaded vesicles and their fusion with the plasma membrane. A549 cells were labeled with quinacrine or Bodipy-ATP, fluorescent markers of intracellular ATP storage sites, and time-lapse imaging of vesicles present in the evanescent field was undertaken. Under basal conditions, individual vesicles showed occasional quasi-instantaneous loss of fluorescence, as expected from spontaneous vesicle fusion with the plasma membrane and dispersal of its fluorescent cargo. Hypo-osmotic stress stimulation (osmolality reduction from 316 to 160 mOsm) resulted in a transient, several-fold increment of exocytotic event frequency. Lowering the temperature from 37°C to 20°C dramatically diminished the fraction of vesicles that underwent exocytosis during the 2-min stimulation, from ~40% to ≤1%, respectively. Parallel ATP efflux experiments with luciferase bioluminescence assay revealed that pharmacological interference with vesicular transport (brefeldin, monensin), or disruption of the cytoskeleton (nocodazole, cytochalasin), significantly suppressed ATP release (by up to ~80%), whereas it was completely blocked by N-ethylmaleimide. Collectively, our data demonstrate that regulated exocytosis of ATP-loaded vesicles likely constitutes a major pathway of hypotonic stress-induced ATP secretion from A549 cells.

TI-VAMP/VAMP7 is the SNARE of secretory lysosomes contributing to ATP secretion from astrocytes

BACKGROUND INFORMATION

ATP is the main transmitter stored and released from astrocytes under physiological and pathological conditions. Morphological and functional evidence suggest that besides secretory granules, secretory lysosomes release ATP. However, the molecular mechanisms involved in astrocytic lysosome fusion remain still unknown.

RESULTS

In the present study, we identify tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP, also called VAMP7) as the vesicular SNARE which mediates secretory lysosome exocytosis, contributing to release of both ATP and cathepsin B from glial cells. We also demonstrate that fusion of secretory lysosomes is triggered by slow and locally restricted calcium elevations, distinct from calcium spikes which induce the fusion of glutamate-containing clear vesicles. Downregulation of TI-VAMP/VAMP7 expression inhibited the fusion of ATP-storing vesicles and ATP-mediated calcium wave propagation. TI-VAMP/VAMP7 downregulation also significantly reduced secretion of cathepsin B from glioma.

CONCLUSIONS

Given that sustained ATP release from glia upon injury greatly contributes to secondary brain damage and cathepsin B plays a critical role in glioma dissemination, TI-VAMP silencing can represent a novel strategy to control lysosome fusion in pathological conditions.

Targeting pannexin1 improves seizure outcome

Imbalance of the excitatory neurotransmitter glutamate and of the inhibitory neurotransmitter GABA is one of several causes of seizures. ATP has also been implicated in epilepsy. However, little is known about the mechanisms involved in the release of ATP from cells and the consequences of the altered ATP signaling during seizures. Pannexin1 (Panx1) is found in astrocytes and in neurons at high levels in the embryonic and young postnatal brain, declining in adulthood. Panx1 forms large-conductance voltage sensitive plasma membrane channels permeable to ATP that are also activated by elevated extracellular K(+) and following P2 receptor stimulation. Based on these properties, we hypothesized that Panx1 channels may contribute to seizures by increasing the levels of extracellular ATP. Using pharmacological tools and two transgenic mice deficient for Panx1 we show here that interference with Panx1 ameliorates the outcome and shortens the duration of kainic acid-induced status epilepticus. These data thus indicate that the activation of Panx1 in juvenile mouse hippocampi contributes to neuronal hyperactivity in seizures.

Evidence for the existence of secretory granule (dense-core vesicle)-based inositol 1,4,5-trisphosphate-dependent Ca2+ signaling system in astrocytes

BACKGROUND

The gliotransmitters released from astrocytes are deemed to play key roles in the glial cell-neuron communication for normal function of the brain. The gliotransmitters, such as glutamate, ATP, D-serine, neuropeptide Y, are stored in vesicles of astrocytes and secreted following the inositol 1,4,5-trisphosphate (IP3)-induced intracellular Ca2+ releases. Yet studies on the identity of the IP3-dependent intracellular Ca2+ stores remain virtually unexplored.

PRINCIPAL FINDINGS

We have therefore studied the potential existence of the IP3-sensitive intracellular Ca2+ stores in the cytoplasm of astrocytes using human brain tissue samples in contrast to cultured astrocytes that had primarily been used in the past. It was thus found that secretory granule marker proteins chromogranins and secretogranin II localize in the large dense core vesicles of astrocytes, thereby confirming the large dense core vesicles as bona fide secretory granules. Moreover, consistent with the major IP3-dependent intracellular Ca2+ store role of secretory granules in secretory cells, secretory granules of astrocytes also contained all three (types 1, 2, and 3) IP3R isoforms.

SIGNIFICANCE

Given that the secretory granule marker proteins chromogranins and secretogranin II are high-capacity, low-affinity Ca2+ storage proteins and chromogranins interact with the IP3Rs to activate the IP3R/Ca2+ channels, i.e., increase both the mean open time and the open probability of the channels, these results imply that secretory granules of astrocytes function as the IP3-sensitive intracellular Ca2+ store.

P2X7 receptors mediate ischemic damage to oligodendrocytes

Brain ischemia leading to stroke is a major cause of disability in developed countries. Therapeutic strategies have most commonly focused on protecting neurons from ischemic damage. However, ischemic damage to white matter causes oligodendrocyte death, myelin disruption, and axon dysfunction, and it is partially mediated by glutamate excitotoxicity. We have previously demonstrated that oligodendrocytes express ionotropic purinergic receptors. The objective of this study was to investigate the role of purinergic signaling in white matter ischemia. We show that, in addition to glutamate, enhanced ATP signaling during ischemia is also deleterious to oligodendrocytes and myelin, and impairs white matter function. Thus, ischemic oligodendrocytes in culture display an inward current and cytosolic Ca(2+) overload, which is partially mediated by P2X7 receptors. Indeed, oligodendrocytes release ATP after oxygen and glucose deprivation through the opening of pannexin hemichannels. Consistently, ischemia-induced mitochondrial depolarization as well as oxidative stress culminating in cell death are partially reversed by P2X7 receptor antagonists, by the ATP degrading enzyme apyrase and by blockers of pannexin hemichannels. In turn, ischemic damage in isolated optic nerves, which share the properties of brain white matter, is greatly attenuated by all these drugs. Ultrastructural analysis and electrophysiological recordings demonstrated that P2X7 antagonists prevent ischemic damage to oligodendrocytes and myelin, and improved action potential recovery after ischemia. These data indicate that ATP released during ischemia and the subsequent activation of P2X7 receptor is critical to white matter demise during stroke and point to this receptor type as a therapeutic target to limit tissue damage in cerebrovascular diseases.

NMDA receptors activated by subventricular zone astrocytic glutamate are critical for neuroblast survival prior to entering a synaptic network

Even before integrating into existing circuitry, adult-born neurons express receptors for neurotransmitters, but the intercellular mechanisms and their impact on neurogenesis remain largely unexplored. Here, we show that neuroblasts born in the postnatal subventricular zone (SVZ) acquire NMDA receptors (NMDARs) during their migration to the olfactory bulb. Along their route, neuroblasts are ensheathed by astrocyte-like cells expressing vesicular glutamate release machinery. Increasing calcium in these specialized astrocytes induced NMDAR activity in neuroblasts, and blocking astrocytic vesicular release eliminated spontaneous NMDAR activity. Single-cell knockout of NMDARs using neonatal electroporation resulted in neuroblast apoptosis at the time of NMDAR acquisition. This cumulated in a 40% loss of neuroblasts along their migratory route, demonstrating that NMDAR acquisition is critical for neuroblast survival prior to entering a synaptic network. In addition, our findings suggest an unexpected mechanism wherein SVZ astrocytes use glutamate signaling through NMDARs to control the number of adult-born neurons reaching their final destination.

GABA(A) Increases Calcium in Subventricular Zone Astrocyte-Like Cells Through L- and T-Type Voltage-Gated Calcium Channels

In the adult neurogenic subventricular zone (SVZ), the behavior of astrocyte-like cells and some of their functions depend on changes in intracellular Ca²⁺ levels and tonic GABA_A receptor activation. However, it is unknown whether, and if so how, GABA_A receptor activity regulates intracellular Ca²⁺ dynamics in SVZ astrocytes. To monitor Ca²⁺ activity selectively in astrocyte-like cells, we used two lines of transgenic mice expressing either GFP fused to a Gq-coupled receptor or DsRed under the human glial fibrillary acidic protein (hGFAP) promoter. GABA_A receptor activation induced Ca²⁺ increases in 40–50% of SVZ astrocytes. GABA_A-induced Ca²⁺ increases were prevented with nifedipine and mibefradil, blockers of L- and T-type voltage-gated calcium channels (VGCC). The L-type Ca²⁺ channel activator BayK 8644 increased the percentage of GABA_A-responding astrocyte-like cells to 75%, suggesting that the majority of SVZ astrocytes express functional VGCCs. SVZ astrocytes also displayed spontaneous Ca²⁺ activity, the frequency of which was regulated by tonic GABA_A receptor activation. These data support a role for ambient GABA in tonically regulating intracellular Ca²⁺ dynamics through GABA_A receptors and VGCC in a subpopulation of astrocyte-like cells in the postnatal SVZ.

The carboxyl-terminal domain of connexin43 is a negative modulator of neuronal differentiation

Connexin43 (Cx43) is widely expressed in embryonic brain, and its expression becomes restricted mainly to astrocytes as the central nervous system matures. Recent studies have indicated that Cx43 plays important, nonchannel, roles during central nervous system development by affecting neuronal cell migration. Here, we evaluated the effects of Cx43 on neuronal differentiation. For that we used an in vitro model of neural cell development (neurospheres) to evaluate, through immunocytochemistry, electrophysiology, and molecular biology, the degree of neuronal maturation from neurospheres derived from wild-type (WT) and Cx43-null mice. Our results indicate that Cx43 is a negative modulator of neuronal differentiation. The percent neurospheres containing differentiated neurons and the number of cells displaying inward currents were significantly higher in Cx43-null than in WT littermate neurospheres. Knockdown of Cx43 with small interfering RNA increased the number of WT neurospheres generating differentiated neurons. Blockade of gap junctional communication with carbenoxolone did not induce neuronal differentiation in WT neurospheres. Transfection of Cx43-null neurospheres with Cx43 mutants revealed that Cx43 carboxyl terminus prevents neuronal maturation. In agreement with these in vitro data, in situ analysis of embryonic day 16 brains revealed increased beta-III-tubulin expression in germinal zones of Cx43-null compared with that of WT littermates. These results indicate that Cx43, and specifically its carboxyl terminus, is crucial for signaling mechanisms preventing premature neuronal differentiation during embryonic brain development.

Functional characterization of intracellular Dictyostelium discoideum P2X receptors

Indicative of cell surface P2X ion channel activation, extracellular ATP evokes a rapid and transient calcium influx in the model eukaryote Dictyostelium discoideum. Five P2X-like proteins (dP2XA-E) are present in this organism. However, their roles in purinergic signaling are unclear, because dP2XA proved to have an intracellular localization on the contractile vacuole where it is thought to be required for osmoregulation. To determine functional properties of the remaining four dP2X-like proteins and to assess their cellular roles, we recorded membrane currents from expressed cloned receptors and generated a quintuple knock-out Dictyostelium strain devoid of dP2X receptors. ATP evoked inward currents at dP2XB and dP2XE receptors but not at dP2XC or dP2XD. beta,gamma-Imido-ATP was more potent than ATP at dP2XB but a weak partial agonist at dP2XE. Currents in dP2XB and dP2XE were strongly inhibited by Na(+) but insensitive to copper and the P2 receptor antagonists pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid and suramin. Unusual for P2X channels, dP2XA and dP2XB were also Cl(-)-permeable. The extracellular purinergic response to ATP persisted in p2xA/B/C/D/E quintuple knock-out Dictyostelium demonstrating that dP2X channels are not responsible. dP2XB, -C, -D, and -E were found to be intracellularly localized to the contractile vacuole with the ligand binding domain facing the lumen. However, quintuple p2xA/B/C/D/E null cells were still capable of regulating cell volume in water demonstrating that, contrary to previous findings, dP2X receptors are not required for osmoregulation. Responses to the calmodulin antagonist calmidazolium, however, were reduced in p2xA/B/C/D/E null cells suggesting that dP2X receptors play a role in intracellular calcium signaling.

Pannexin 1: the molecular substrate of astrocyte "hemichannels"

Purinergic signaling plays distinct and important roles in the CNS, including the transmission of calcium signals between astrocytes. Gap junction hemichannels are among the mechanisms proposed by which astrocytes might release ATP; however, whether the gap junction protein connexin43 (Cx43) forms these "hemichannels" remains controversial. Recently, a new group of proteins, the pannexins, have been shown to form nonselective, high-conductance plasmalemmal channels permeable to ATP, thereby offering an alternative for the hemichannel protein. Here, we provide strong evidence that, in cultured astrocytes, pannexin1 (Panx1) but not Cx43 forms hemichannels. Electrophysiological and fluorescence microscope recordings performed in wild-type and Cx43-null astrocytes did not reveal any differences in hemichannel activity, which was mostly eliminated by treating Cx43-null astrocytes with Panx1-short interfering RNA [Panx1-knockdown (Panx1-KD)]. Moreover, quantification of the amount of ATP released from wild-type, Cx43-null, and Panx1-KD astrocytes indicates that downregulation of Panx1, but not of Cx43, prevented ATP release from these cells.

Point mutation in the mouse P2X7 receptor affects intercellular calcium waves in astrocytes

Purinergic P2 receptors and gap junctions are two groups of proteins involved in the transmission of ICWs (intercellular calcium waves) between astrocytes. The extent to which ICWs spread among these glial cells depends on the amount of ATP released, which can occur through membrane channels, as well as other pathways. Our previous studies have shown that the pore-forming P2X7R (P2X7 receptor) contributes to the amplification of ICW spread by providing sites of ATP release through Panx1 (Pannexin1) channels. To gain insight into the signal transduction events mediating this response we compared the properties of the P2X7R-Panx1 complex in astrocytes from a mouse strain (C57Bl/6) containing a naturally occurring point mutation (P451L) in the C-terminus of the P2X7R to that of non-mutated receptors (Balb/C mice). Electrophysiological, biochemical, pharmacological and fluorescence imaging techniques revealed that the P451L mutation located in the SH3 domain (a Src tyrosine kinase-binding site) of the C-terminus of the P2X7R attenuates Panx1 currents, ATP release and the distance of ICW spread between astrocytes. Similar results were obtained when using the Src tyrosine inhibitor (PP2) and a membrane-permeant peptide spanning the P451L mutation of the P2X7R of the C57Bl6 astrocytes. These results support the participation of a tyrosine kinase of the Src family in the initial steps mediating the opening of Panx1 channels following P2X7R stimulation and in the transmission of calcium signals among astrocytes.

Cell migration in the normal and pathological postnatal mammalian brain

In the developing brain, cell migration is a crucial process for structural organization, and is therefore highly regulated to allow the correct formation of complex networks, wiring neurons, and glia. In the early postnatal brain, late developmental processes such as the production and migration of astrocyte and oligodendrocyte progenitors still occur. Although the brain is completely formed and structured few weeks after birth, it maintains a degree of plasticity throughout life, including axonal remodeling, synaptogenesis, but also neural cell birth, migration and integration. The subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus are the two main neurogenic niches in the adult brain. Neural stem cells reside in these structures and produce progenitors that migrate toward their ultimate location: the olfactory bulb and granular cell layer of the DG respectively. The aim of this review is to synthesize the increasing information concerning the organization, regulation and function of cell migration in a mature brain. In a normal brain, proteins involved in cell-cell or cell-matrix interactions together with secreted proteins acting as chemoattractant or chemorepellant play key roles in the regulation of neural progenitor cell migration. In addition, recent data suggest that gliomas arise from the transformation of neural stem cells or progenitor cells and that glioma cell infiltration recapitulates key aspects of glial progenitor migration. Thus, we will consider glioma migration in the context of progenitor migration. Finally, many observations show that brain lesions and neurological diseases trigger neural stem/progenitor cell activation and migration toward altered structures. The factors involved in such cell migration/recruitment are just beginning to be understood. Inflammation which has long been considered as thoroughly disastrous for brain repair is now known to produce some positive effects on stem/progenitor cell recruitment via the regulation of growth factor signaling and the secretion of a number of chemoattractant cytokines. This knowledge is crucial for the development of new therapeutic strategies. One of these strategies could consist in increasing the mobilization of endogenous progenitor cells that could replace lost cells and improve functional recovery.

Regulated exocytosis from astrocytes physiological and pathological related aspects

Astrocytes have traditionally been considered ancillary, satellite cells of the nervous system. However, it is a very recent acquisition that glial cells generate signaling loops which are integral to the brain circuitry and participate, interactively with neuronal networks, in the processing of information. Such a conceptual breakthrough makes this field of investigation one of the hottest in neuroscience, as it calls for a revision of past theories of brain function as well as for new strategies of experimental exploration of brain function. Glial cells are electrically not excitable, and it was only the use of optical recording techniques together with calcium sensitive dyes, that allowed the chemical excitability of glial cells to become apparent. Studies using these new techniques have shown for the first time that glial cells are activated by surrounding synaptic activity and translate neuronal signals into their own calcium code. Intracellular calcium concentration([Ca2+]i) elevations in glial cells have then shown to underlie spatial transfer of information in the glial network, accompanied by release of chemical transmitters (gliotransmitters) such as glutamate and back-signaling to neurons. As a consequence, optical imaging techniques applied to cell cultures or intact tissue have become a state-of-the-art technology for studying glial cell signaling. The molecular mechanisms leading to release of "gliotransmitters," especially glutamate, from glia are under debate. Accumulating evidence clearly indicates that astrocytes secrete numerous transmitters by Ca(2+)-dependent exocytosis. This review will discuss the mechanisms underlying the release of chemical transmitters from astrocytes with a particular emphasis to the regulated exocytosis processes.

Fast subplasma membrane Ca2+ transients control exo-endocytosis of synaptic-like microvesicles in astrocytes

Astrocytes are the most abundant glial cell type in the brain. Although not apposite for long-range rapid electrical communication, astrocytes share with neurons the capacity of chemical signaling via Ca(2+)-dependent transmitter exocytosis. Despite this recent finding, little is known about the specific properties of regulated secretion and vesicle recycling in astrocytes. Important differences may exist with the neuronal exocytosis, starting from the fact that stimulus-secretion coupling in astrocytes is voltage independent, mediated by G-protein-coupled receptors and the release of Ca(2+) from internal stores. Elucidating the spatiotemporal properties of astrocytic exo-endocytosis is, therefore, of primary importance for understanding the mode of communication of these cells and their role in brain signaling. We here take advantage of fluorescent tools recently developed for studying recycling of glutamatergic vesicles at synapses (Voglmaier et al., 2006; Balaji and Ryan, 2007); we combine epifluorescence and total internal reflection fluorescence imaging to investigate with unprecedented temporal and spatial resolution, the stimulus-secretion coupling underlying exo-endocytosis of glutamatergic synaptic-like microvesicles (SLMVs) in astrocytes. Our main findings indicate that (1) exo-endocytosis in astrocytes proceeds with a time course on the millisecond time scale (tau(exocytosis) = 0.24 +/- 0.017 s; tau(endocytosis) = 0.26 +/- 0.03 s) and (2) exocytosis is controlled by local Ca(2+) microdomains. We identified submicrometer cytosolic compartments delimited by endoplasmic reticulum tubuli reaching beneath the plasma membrane and containing SLMVs at which fast (time-to-peak, approximately 50 ms) Ca(2+) events occurred in precise spatial-temporal correlation with exocytic fusion events. Overall, the above characteristics of transmitter exocytosis from astrocytes support a role of this process in fast synaptic modulation.

Lack of "hemichannel" activity in insulin-producing cells

Connexins and pannexins have been implicated in the formation of "hemichannels," which may account for the uptake and release of membrane-impermeant molecules in single cells. The in vivo existence of "hemichannels" and their protein composition is still debated. Investigations on these matters are complicated by the lack of adequate negative controls. In search for such essential controls, the authors have investigated transformed (MIN6 line) and primary insulin-producing cells. Here, the authors report that these cells, which express Cx36 and pannexin 1, cannot be shown to display functional "hemichannels," as evaluated by (1) uptake of the membrane-impermeant tracer ethidium bromide, whether in the presence or absence of extracellular Ca(2+), following stimulation of P2X(7) receptors, and after exposure to hypotonic medium; and (2) lack of exocytosis-independent release of endogenous ATP. Moreover, electrophysiological recordings indicated the absence of carbenoxolone-sensitive pannexin 1 currents evoked by membrane potentials above +30 mV. Thus, insulin-producing cells are expected to provide a useful tool in the further characterization of hemichannel composition, properties, and physiological relevance.

Lysosomes are the major vesicular compartment undergoing Ca2+-regulated exocytosis from cortical astrocytes

Although Ca(2+)-dependent exocytosis is considered to be a pathway for gliotransmitter release from astrocytes, the structural and functional bases of this process remain controversial. We studied the relationship between near-membrane Ca(2+) elevations and the dynamics of single astroglial vesicles with styryl (FM) dyes. We show that cultured astrocytes, unlike neurons, spontaneously internalize FM dyes, resulting in the labeling of the entire acidic vesicle population within minutes. Interestingly, metabotropic glutamate receptor activation did not affect the FM labeling. Most FM-stained vesicles expressed sialin, CD63/LAMP3, and VAMP7, three markers for lysosomes and late endosomes. A subset of lysosomes underwent asynchronous exocytosis that required both Ca(2+) mobilization from intracellular stores and Ca(2+) influx across the plasma membrane. Lysosomal fusion occurred within seconds and was complete with no evidence for kiss and run. Our experiments suggest that astroglial Ca(2+)-regulated exocytosis is carried by lysosomes and operates on a timescale orders of magnitude slower than synaptic transmission.