Cross-talk among epidermal growth factor, Ap(5)A, and nucleotide receptors causing enhanced ATP Ca(2+) signaling involves extracellular kinase activation in cerebellar astrocytes

Geoffrey Burnstock, our friend and magister: the diadenosine polyphosphate connection

Development of science needs the cooperation of many creative brains. Sometimes, ideas on a specific area get suddenly exhausted and then it is the time for a privileged mind to think in a different way and reach the turning point to introduce a new paradigm. This happened to Geoffrey Burnstock, a heterodox thinker and nonconformist scientist that has been the paladin of purinergic signalling since 1972, opening neuroscience to the understanding of organs and tissues functioning and development of a new pharmacology. This review summarizes the contribution of our group to the understanding of the role of the diadenosine polyphosphates, Ap_nA, as signalling molecules, describing their tissue and organ distribution, their transport and storage in secretory vesicles and their release and interaction with purinergic receptors. We also have to acknowledge the friendly and kindly support of Professor Burnstock that showed a great interest in the field from our initial findings and actively stimulated our efforts to establish the extracellular roles and biological significance of these dinucleotides.

P2X7 Nucleotide and EGF Receptors Exert Dual Modulation of the Dual-Specificity Phosphatase 6 (MKP-3) in Granule Neurons and Astrocytes, Contributing to Negative Feedback on ERK Signaling

Extracellular signal-regulated kinases 1 and 2 (ERK1/2) play a central role in the intracellular signaling of P2X7 nucleotide receptors in neurons and glial cells. Fine spatio-temporal tuning of mitogen-activated protein (MAP) kinases is essential to regulate their biological activity. MAP kinase phosphatases (MKPs) are dual specificity protein phosphatases (DUSPs) that dephosphorylate phosphothreonine and phosphotyrosine residues in MAP kinases. This study focuses on how DUSP, DUSP6/MKP3, a phosphatase specific for ERK1/2 is regulated by the P2X7 nucleotide receptor in cerebellar granule neurons and astrocytes. Stimulation with the specific P2X7 agonist, BzATP, or epidermal growth factor (EGF) (positive control for ERK activation) regulates the levels of DUSP6 in a time dependent manner. Both agonists promote a decline in DUSP6 protein, reaching minimal levels after 30 min yet recovering to basal levels after 1 h. The initial loss of protein occurs through proteasomal degradation, as confirmed in experiments with the proteasome inhibitor, MG-132. Studies carried out with Actinomycin D demonstrated that the enhanced transcription of the Dusp6 gene is responsible for recovering the DUSP6 protein levels. Interestingly, ERK1/2 proteins are involved in the biphasic regulation of the protein phosphatase, being required for both the degradation and the recovery phase. We show that direct Ser¹⁹⁷ phosphorylation of DUSP6 by ERK1/2 proteins could be part of the mechanism regulating their cytosolic levels, at least in glial cells. Thus, the ERK1/2 activated by P2X7 receptors exerts positive feedback on these kinase’s own activity, promoting the degradation of one of their major inactivators in the cytosolic compartment, DUSP6, both in granule neurons and astrocytes. This feedback loop seems to function as a common universal mechanism to regulate ERK signaling in neural and non-neural cells.

Prostaglandin E2 Impairs P2Y2/P2Y4 Receptor Signaling in Cerebellar Astrocytes via EP3 Receptors

Prostaglandin E₂ (PGE₂) is an important bioactive lipid that accumulates after tissue damage or inflammation due to the rapid expression of cyclooxygenase 2. PGE₂ activates specific G-protein coupled EP receptors and it mediates pro- or anti-inflammatory actions depending on the cell-context. Nucleotides can also be released in these situations and they even contribute to PGE₂ production. We previously described the selective impairment of P2Y nucleotide signaling by PGE₂ in macrophages and fibroblasts, an effect independent of prostaglandin receptors but that involved protein kinase C (PKC) and protein kinase D (PKD) activation. Considering that macrophages and fibroblasts influence inflammatory responses and tissue remodeling, a similar mechanism involving P2Y signaling could occur in astrocytes in response to neuroinflammation and brain repair. We analyzed here the modulation of cellular responses involving P2Y₂/P2Y₄ receptors by PGE₂ in rat cerebellar astrocytes. We demonstrate that PGE₂ inhibits intracellular calcium responses elicited by UTP in individual cells and that inhibiting this P2Y signaling impairs the astrocyte migration elicited by this nucleotide. Activation of EP3 receptors by PGE₂ not only impairs the calcium responses but also, the extracellular regulated kinases (ERK) and Akt phosphorylation induced by UTP. However, PGE₂ requires epidermal growth factor receptor (EGFR) transactivation in order to dampen P2Y signaling. In addition, these effects of PGE₂ also occur in a pro-inflammatory context, as evident in astrocytes stimulated with bacterial lipopolysaccharide (LPS). While we continue to investigate the intracellular mechanisms responsible for the inhibition of UTP responses, the involvement of novel PKC and PKD in cerebellar astrocytes cannot be excluded, kinases that could promote the internalization of P2Y receptors in fibroblasts.

Spatiotemporal characteristics of calcium dynamics in astrocytes

Although Ca(i)(2+) waves in networks of astrocytes in vivo are well documented, propagation in vivo is much more complex than in culture, and there is no consensus concerning the dominant roles of intercellular and extracellular messengers [inositol 1,4,5-trisphosphate (IP(3)) and adenosine-5'-triphosphate (ATP)] that mediate Ca(i)(2+) waves. Moreover, to date only simplified models that take very little account of the geometrical struture of the networks have been studied. Our aim in this paper is to develop a mathematical model based on realistic cellular morphology and network connectivity, and a computational framework for simulating the model, in order to address these issues. In the model, Ca(i) (2+) wave propagation through a network of astrocytes is driven by IP(3) diffusion between cells and ATP transport in the extracellular space. Numerical simulations of the model show that different kinetic and geometric assumptions give rise to differences in Ca(i)(2+) wave propagation patterns, as characterized by the velocity, propagation distance, time delay in propagation from one cell to another, and the evolution of Ca(2+) response patterns. The temporal Ca(i)(2+) response patterns in cells are different from one cell to another, and the Ca(i)(2+) response patterns evolve from one type to another as a Ca(i)(2+) wave propagates. In addition, the spatial patterns of Ca(i)(2+) wave propagation depend on whether IP(3), ATP, or both are mediating messengers. Finally, two different geometries that reflect the in vivo and in vitro configuration of astrocytic networks also yield distinct intracellular and extracellular kinetic patterns. The simulation results as well as the linear stability analysis of the model lead to the conclusion that Ca(i)(2+) waves in astrocyte networks are probably mediated by both intercellular IP(3) transport and nonregenerative (only the glutamate-stimulated cell releases ATP) or partially regenerative extracellular ATP signaling.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Effect of diadenosine polyphosphates in achondroplasic chondrocytes: inhibitory effect of Ap4A on FGF9 induced MAPK cascade

Achondroplasia is characterised by a mutation in the gene that encodes for the FGF receptor type 3 (FGFR3), producing a hyperactivation of this receptor and a subsequent increase in MAPK activity. We have tested the ability of nucleotides to decrease the activation of MAPK in chondrocytes with achondroplasic FGFR3 receptor. Diadenosine tetraphosphate, Ap(4)A, reduced the phosphorylation of pERK1/2 triggered by FGF9 (38% reduction). Ap(4)A diminished the expression of achondroplasic FGFR3 receptor (65% reduction), stimulating FGFR3 receptor degradation. The action of Ap(4)A seems to be mediated by a dinucleotide receptor rather than by any other ATP receptor.

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.

Dinucleoside polyphosphates and their interaction with other nucleotide signaling pathways

Dinucleoside polyphosphates or Ap(n)A are a family of dinucleotides formed by two adenosines joined by a variable number of phosphates. Ap(4)A, Ap(5)A, and Ap(6)A are stored together with other neurotransmitters into secretory vesicles and are co-released to the extracellular medium upon stimulation. These compounds can interact extracellularly with some ATP receptors, both metabotropic (P2Y) and ionotropic (P2X). However, specific receptors for these substances, other than ATP receptors, have been described in presynaptic terminals form rat midbrain. These specific dinucleotide receptors are of ionotropic nature and their activation induces calcium entry into the terminals and the subsequent neurotransmitter release. Calcium signals that cannot be attributable to the interaction of Ap(n)A with ATP receptors have also been described in cerebellar synaptosomes and granule cell neurons in culture, where Ap(5)A induces CaMKII activation. In addition, cerebellar astrocytes express a specific Ap(5)A receptor coupled to ERK activation. Ap(5)A engaged to MAPK cascade by a mechanism that was insensitive to pertussis toxin and required the involvement of src and ras proteins. Diadenosine polyphosphates, acting on their specific receptors and/or ATP receptors, can also interact with other neurotransmitter systems. This broad range of actions and interactions open a promising perspective for some relevant physiological roles for the dinucleotides. However, the physiological significance of these compounds in the CNS is still to be determined.

P2 receptors and neuronal injury

Extracellular adenosine 5'-triphosphate (ATP) was proposed to be an activity-dependent signaling molecule that regulates glia-glia and glia-neuron communications. ATP is a neurotransmitter of its own right and, in addition, a cotransmitter of other classical transmitters such as glutamate or GABA. The effects of ATP are mediated by two receptor families belonging either to the P2X (ligand-gated cationic channels) or P2Y (G protein-coupled receptors) types. P2X receptors are responsible for rapid synaptic responses, whereas P2Y receptors mediate slow synaptic responses and other types of purinergic signaling involved in neuronal damage/regeneration. ATP may act at pre- and postsynaptic sites and therefore, it may participate in the phenomena of long-term potentiation and long-term depression of excitatory synaptic transmission. The release of ATP into the extracellular space, e.g., by exocytosis, membrane transporters, and connexin hemichannels, is a widespread physiological process. However, ATP may also leave cells through their plasma membrane damaged by inflammation, ischemia, and mechanical injury. Functional responses to the activation of multiple P2 receptors were found in neurons and glial cells under normal and pathophysiological conditions. P2 receptor-activation could either be a cause or a consequence of neuronal cell death/glial activation and may be related to detrimental and/or beneficial effects. The present review aims at demonstrating that purinergic mechanisms correlate with the etiopathology of brain insults, especially because of the massive extracellular release of ATP, adenosine, and other neurotransmitters after brain injury. We will focus in this review on the most important P2 receptor-mediated neurodegenerative and neuroprotective processes and their beneficial modulation by possible therapeutic manipulations.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.