Ursula Schenk

Astrocyte-Derived ATP Induces Vesicle Shedding and IL-1β Release from Microglia

ATP has been indicated as a primary factor in microglial response to brain injury and inflammation. By acting on different purinergic receptors 2, ATP is known to induce chemotaxis and stimulate the release of several cytokines from these cells. The activation of purinergic receptors 2 in microglia can be triggered either by ATP deriving from dying cells, at sites of brain injury or by ATP released from astrocytes, in the absence of cell damage. By the use of a biochemical approach integrated with video microscopy experiments, we investigated the functional consequences triggered in microglia by ATP released from mechanically stimulated astrocytes, in mixed glial cocultures. Astrocyte-derived ATP induced in nearby microglia the formation and the shedding of membrane vesicles. Vesicle formation was inhibited by the ATP-degrading enzyme apyrase or by P2X7R antagonists. Isolation of shed vesicles, followed by IL-1β evaluation by a specific ELISA revealed the presence of the cytokine inside the vesicular organelles and its subsequent efflux into the extracellular medium. IL-1β efflux from shed vesicles was enhanced by ATP stimulation and inhibited by pretreatment with the P2X7 antagonist oxidized ATP, thus indicating a crucial involvement of the pore-forming P2X7R in the release of the cytokine. Our data identify astrocyte-derived ATP as the endogenous factor responsible for microvesicle shedding in microglia and reveal the mechanisms by which astrocyte-derived ATP triggers IL-1β release from these cells.

Purinergic Control of T Cell Activation by ATP Released Through Pannexin-1 Hemichannels

Pannexin hemichannel–mediated release of ATP provides an autocrine, costimulatory signal for T cell activation. ATP Signals T Cells to Activate Sustained influx of extracellular Ca2+ is a critical event in the activation of T cells. One consequence of increased cytosolic Ca2+ concentration is the uptake of Ca2+ by mitochondria, which leads to the synthesis of adenosine triphosphate (ATP). Activation of purinergic receptors upon T cells is known to affect the outcome of stimulation of the T cell receptor (TCR), but how extracellular ATP might affect T cell function in the context of inflammation is unclear. Schenk et al. now show that on TCR triggering, ATP is released from T cells through pannexin hemichannels and functions in an autocrine fashion as a costimulator of T cell activation. Blocking ATP signaling mediated by purinergic P2X receptors on T cells in the context of TCR stimulation led to decreased T cell activation and increased expression of anergy-associated genes. Moreover, administration of a P2X receptor antagonist to mouse models of type 1 diabetes and inflammatory bowel disease substantially inhibited the development of effector T cells and lessened tissue damage compared with that in untreated mice. Together, these data suggest that therapeutic intervention against ATP synthesis and release may be of benefit in the treatment of T cell–mediated inflammatory diseases. T cell receptor (TCR) stimulation results in the influx of Ca2+, which is buffered by mitochondria and promotes adenosine triphosphate (ATP) synthesis. We found that ATP released from activated T cells through pannexin-1 hemichannels activated purinergic P2X receptors (P2XRs) to sustain mitogen-activated protein kinase (MAPK) signaling. P2XR antagonists, such as oxidized ATP (oATP), blunted MAPK activation in stimulated T cells, but did not affect the nuclear translocation of the transcription factor nuclear factor of activated T cells, thus promoting T cell anergy. In vivo administration of oATP blocked the onset of diabetes mediated by anti-islet TCR transgenic T cells and impaired the development of colitogenic T cells in inflammatory bowel disease. Thus, pharmacological inhibition of ATP release and signaling could be beneficial in treating T cell–mediated inflammatory diseases.

A Common Exocytotic Mechanism Mediates Axonal and Dendritic Outgrowth

Outgrowth of the dendrites and the axon is the basis of the establishment of the neuronal shape, and it requires addition of new membrane to both growing processes. It is not yet clear whether one or two exocytotic pathways are responsible for the respective outgrowth of axons and dendrites. We have previously shown that tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) defines a novel network of tubulovesicular structures present both at the leading edge of elongating dendrites and axons of immature hippocampal neurons developing in primary culture and that TI-VAMP is an essential protein for neurite outgrowth in PC12 cells. Here we show that the expression of the N-terminal domain of TI-VAMP inhibits the outgrowth of both dendrites and axons in neurons in primary culture. This effect is more prominent at the earliest stages of the development of neurons in vitro. Expression of the N-terminal domain deleted form of TI-VAMP has the opposite effect. This constitutively active form of TI-VAMP localizes as the endogenous protein, particularly concentrating at the leading edge of growing axons. Our results suggest that a common exocytotic mechanism that relies on TI-VAMP mediates both axonal and dendritic outgrowth in developing neurons.

ATP Inhibits the Generation and Function of Regulatory T Cells Through the Activation of Purinergic P2X Receptors

Purinergic signaling during inflammation converts immunosuppressive CD4+ T cells into proinflammatory ones. T Cells Lose Their Identity Regulatory T cells (Tregs) inhibit the actions of inflammatory T cells during immune responses and prevent autoimmunity. Schenk et al. showed that adenosine triphosphate (ATP) signaling through purinergic receptors on Tregs inhibited their immunosuppressive effects and exacerbated tissue inflammation in mice. Worse still, autocrine ATP signaling made the Tregs lose their identity, through the loss of their signature transcription factor Foxp3, and induced their conversion into proinflammatory, interleukin-17–secreting cells. These data suggest that ATP signaling through purinergic receptors might be an effective therapeutic target to shape immune responses, a suggestion supported by the maintenance of the identity and immunosuppressive function of Tregs through pretreatment with a purinergic receptor antagonist. Extracellular nucleotides are pleiotropic regulators of mammalian cell function. Adenosine triphosphate (ATP) released from CD4+ helper T cells upon stimulation of the T cell receptor (TCR) contributes in an autocrine manner to the activation of mitogen-activated protein kinase (MAPK) signaling through purinergic P2X receptors. Increased expression of p2rx7, which encodes the purinergic receptor P2X7, is part of the transcriptional signature of immunosuppressive CD4+CD25+ regulatory T cells (Tregs). Here, we show that the activation of P2X7 by ATP inhibits the suppressive potential and stability of Tregs. The inflammatory cytokine interleukin-6 (IL-6) increased ATP synthesis and P2X7-mediated signaling in Tregs, which induced their conversion to IL-17–secreting T helper 17 (TH17) effector cells in vivo. Moreover, pharmacological antagonism of P2X receptors promoted the cell-autonomous conversion of naïve CD4+ T cells into Tregs after TCR stimulation. Thus, ATP acts as an autocrine factor that integrates stimuli from the microenvironment and cellular energetics to tune the developmental and immunosuppressive program of the T cell in adaptive immune responses.

Localization and functional relevance of system A neutral amino acid transporters in cultured hippocampal neurons

Glutamine and alanine are important precursors for the synthesis of glutamate. Provided to neurons by neighboring astrocytes, these amino acids are internalized by classical system A amino acid carriers. In particular, System A transporter (SAT1) is a highly efficient glutamine transporter, whereas SAT2 exhibits broad specificity for neutral amino acids with a preference for alanine. We investigated the localization and the functional relevance of SAT1 and SAT2 in primary cultures of hippocampal neurons. Both carriers have been expressed since early developmental stages and are uniformly distributed throughout all neuronal processes. However, whereas SAT1 is present in axonal growth cones and can be detected at later developmental stages at the sites of synaptic contacts, SAT2 does not appear to be significantly expressed in these compartments. The non-metabolizable amino acid analogue α-(methylamino)-isobutyric acid, a competitive inhibitor of system A carriers, significantly reduced miniature excitatory postsynaptic current amplitude in neurons growing on top of astrocytes, being ineffective in pure neuronal cultures. α-(Methylamino)-isobutyric acid did not alter neuronal responsitivity to glutamate, thus excluding a postsynaptic effect. These data indicate that system A carriers are expressed with a different subcellular distribution in hippocampal neurons and play a crucial role in controlling the astrocyte-mediated supply of glutamatergic neurons with neurotransmitter precursors.

Regulated delivery of AMPA receptor subunits to the presynaptic membrane

In recent years, a role for AMPA receptors as modulators of presynaptic functions has emerged. We have investigated the presence of AMPA receptor subunits and the possible dynamic control of their surface exposure at the presynaptic membrane. We demonstrate that the AMPA receptor subunits GluR1 and GluR2 are expressed and organized in functional receptors in axonal growth cones of hippocampal neurons. AMPA receptors are actively internalized upon activation and recruited to the surface upon depolarization. Pretreatment of cultures with botulinum toxin E or tetanus toxin prevents the receptor insertion into the plasma membrane, whereas treatment with α‐latrotoxin enhances the surface exposure of GluR2, both in growth cones of cultured neurons and in brain synaptosomes. Purification of small synaptic vesicles through controlled‐pore glass chromatography, revealed that both GluR2 and GluR1, but not the GluR2 interacting protein GRIP, copurify with synaptic vesicles. These data indicate that, at steady state, a major pool of AMPA receptor subunits reside in synaptic vesicle membranes and can be recruited to the presynaptic membrane as functional receptors in response to depolarization.

Cell-autonomous regulation of hematopoietic stem cell cycling activity by ATP

Extracellular nucleotides regulate many cellular functions through activation of purinergic receptors in the plasma membrane. Here, we show that in hematopoietic stem cell (HSC), ATP is stored in vesicles and released in a calcium-sensitive manner. HSC expresses ATP responsive P2X receptors and in vitro pharmacological P2X antagonism restrained hematopoietic progenitors proliferation, but not myeloid differentiation. In mice suffering from chronic inflammation, HSCs were significantly expanded and their cycling activity was sensitive to treatment with the P2X antagonist periodate-oxidized 2,3-dialdehyde ATP. Our results indicate that ATP acts as an autocrine stimulus in regulating HSCs pool size.

Purinergic P2X7 Receptor Drives T Cell Lineage Choice and Shapes Peripheral γδ Cells

TCR signal strength instructs αβ versus γδ lineage decision in immature T cells. Increased signal strength of γδTCR with respect to pre-TCR results in induction of the γδ differentiation program. Extracellular ATP evokes physiological responses through purinergic P2 receptors expressed in the plasma membrane of virtually all cell types. In peripheral T cells, ATP released upon TCR stimulation enhances MAPK activation through P2X receptors. We investigated whether extracellular ATP and P2X receptors signaling tuned TCR signaling at the αβ/γδ lineage bifurcation checkpoint. We show that P2X7 expression was selectively increased in immature γδ+CD25+ cells. These cells were much more competent to release ATP than pre–TCR-expressing cells following TCR stimulation and Ca2+ influx. Genetic ablation as well as pharmacological antagonism of P2X7 resulted in impaired ERK phosphorylation, reduction of early growth response (Egr) transcripts induction, and diversion of γδTCR-expressing thymocytes toward the αβ lineage fate. The impairment of the ERK-Egr-inhibitor of differentiation 3 (Id3) signaling pathway in γδ cells from p2rx7−/− mice resulted in increased representation of the Id3-independent NK1.1-expressing γδ T cell subset in the periphery. Our results indicate that ATP release and P2X7 signaling upon γδTCR expression in immature thymocytes constitutes an important costimulus in T cell lineage choice through the ERK-Egr-Id3 signaling pathway and contributes to shaping the peripheral γδ T cell compartment.

Regulation of peripheral T cell activation by calreticulin

Porcellini et al. 2006. J. Exp. Med. doi:10.1084/jem.20051519 [OpenUrl][1][Abstract/FREE Full Text][2] [1]: {openurl}?query=rft_id%253Dinfo%253Adoi%252F10.1084%252Fjem.20051519%26rft_id%253Dinfo%253Apmid%252F16492806%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%