Activation of Src/kinase/phospholipase C/mitogen-activated protein kinase and induction of neurite expression by ATP, independent of nerve growth factor

Introduction to Purinergic Signalling in the Brain

ATP is a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the brain. There is a widespread presence of both adenosine (P1) and P2 nucleotide receptors in the brain on both neurons and glial cells. Adenosine receptors play a major role in presynaptic neuromodulation, while P2X ionotropic receptors are involved in fast synaptic transmission and synaptic plasticity. P2Y G protein-coupled receptors are largely involved in presynaptic activities, as well as mediating long-term (trophic) signalling in cell proliferation, differentiation and death during development and regeneration. Both P1 and P2 receptors participate in neuron-glial interactions. Purinergic signalling is involved in control of cerebral vascular tone and remodelling and has been implicated in learning and memory, locomotor and feeding behaviour and sleep. There is increasing interest in the involvement of purinergic signalling in the pathophysiology of the CNS, including trauma, ischaemia, epilepsy, neurodegenerative diseases, neuropsychiatric and mood disorders, and cancer, including gliomas.

Role of P2 receptors in normal brain development and in neurodevelopmental psychiatric disorders

The purinergic signaling system, including P2 receptors, plays an important role in various central nervous system (CNS) disorders. Over the last few decades, a substantial amount of accumulated data suggest that most P2 receptor subtypes (P2X1, 2, 3, 4, 6, and 7, and P2Y₁, ₂, ₆, ₁₂, and ₁₃) regulate neuronal/neuroglial developmental processes, such as proliferation, differentiation, migration of neuronal precursors, and neurite outgrowth. However, only a few of these subtypes (P2X2, P2X3, P2X4, P2X7, P2Y₁, and P2Y₂) have been investigated in the context of neurodevelopmental psychiatric disorders. The activation of these potential target receptors and their underlying mechanisms mainly influence the process of neuroinflammation. In particular, P2 receptor-mediated inflammatory cytokine release has been indicated to contribute to the complex mechanisms of a variety of CNS disorders. The released inflammatory cytokines could be utilized as biomarkers for neurodevelopmental and psychiatric disorders to improve the early diagnosis intervention, and prognosis. The population changes in gut microbiota after birth are closely linked to neurodevelopmental/neuropsychiatric disorders in later life; thus, the dynamic expression and function of P2 receptors on gut epithelial cells during disease processes indicate a novel avenue for the evaluation of disease progression and for the discovery of related therapeutic compounds.

High-throughput screen for compounds that modulate neurite growth of human induced pluripotent stem cell-derived neurons

Development of technology platforms to perform compound screens of human induced pluripotent stem cell (hiPSC)-derived neurons with relatively high throughput is essential to realize their potential for drug discovery. Here, we demonstrate the feasibility of high-throughput screening of hiPSC-derived neurons using a high-content, image-based approach focused on neurite growth, a process that is fundamental to formation of neural networks and nerve regeneration. From a collection of 4421 bioactive small molecules, we identified 108 hit compounds, including 37 approved drugs, that target molecules or pathways known to regulate neurite growth, as well as those not previously associated with this process. These data provide evidence that many pathways and targets known to play roles in neurite growth have similar activities in hiPSC-derived neurons that can be identified in an unbiased phenotypic screen. The data also suggest that hiPSC-derived neurons provide a useful system to study the mechanisms of action and off-target activities of the approved drugs identified as hits, leading to a better understanding of their clinical efficacy and toxicity, especially in the context of specific human genetic backgrounds. Finally, the hit set we report constitutes a sublibrary of approved drugs and tool compounds that modulate neurites. This sublibrary will be invaluable for phenotypic analyses and interrogation of hiPSC-based disease models as probes for defining phenotypic differences and cellular vulnerabilities in patient versus control cells, as well as for investigations of the molecular mechanisms underlying human neurite growth in development and maintenance of neuronal networks, and nerve regeneration.

Summary: High-throughput, small molecule screening of hiPSC-derived neurons using a high-content, image-based approach focused on neurite growth identified hit compounds, including approved drugs, which target molecules or pathways known to regulate neurite growth.

Imaging extracellular ATP with a genetically-encoded, ratiometric fluorescent sensor

Extracellular adenosine triphosphate (ATP) is a key purinergic signal that mediates cell-to-cell communication both within and between organ systems. We address the need for a robust and minimally invasive approach to measuring extracellular ATP by re-engineering the ATeam ATP sensor to be expressed on the cell surface. Using this approach, we image real-time changes in extracellular ATP levels with a sensor that is fully genetically-encoded and does not require an exogenous substrate. In addition, the sensor is ratiometric to allow for reliable quantitation of extracellular ATP fluxes. Using live-cell microscopy, we characterize sensor performance when expressed on cultured Neuro2A cells, and we measure both stimulated release of ATP and its clearance by ectonucleotidases. Thus, this proof-of-principle demonstrates a first-generation sensor to report extracellular ATP dynamics that may be useful for studying purinergic signaling in living specimens.

Validation of antibodies for neuroanatomical localization of the P2Y11 receptor in macaque brain

Focus on the purinergic receptor P2Y₁₁ has increased following the finding of an association between the sleep disorder narcolepsy and a genetic variant in P2RY11 causing decreased gene expression. Narcolepsy is believed to arise from an autoimmune destruction of the hypothalamic neurons that produce the neuropeptide hypocretin/orexin. It is unknown how a decrease in expression of P2Y₁₁ might contribute to an autoimmune reaction towards the hypocretin neurons and the development of narcolepsy. To advance narcolepsy research it is therefore extremely important to determine the neuroanatomical localization of P2Y₁₁ in the brain with particular emphasis on the hypocretin neurons. In this article we used western blot, staining of blood smears, and flow cytometry to select two antibodies for immunohistochemical staining of macaque monkey brain. Staining was seen in neuron-like structures in cortical and hypothalamic regions. Rats do not have a gene orthologue to the P2Y₁₁ receptor and therefore rat brain was used as negative control tissue. The chromogenic signal observed in macaque monkey brain in neurons was not considered reliable, because the antibodies stained rat brain in a similar distribution pattern. Hence, the neuroanatomical localization of the P2Y₁₁ receptor remains undetermined due to the lack of specific P2Y₁₁ antibodies for brain immunohistochemistry.

A critical look at the function of the P2Y11 receptor

The P2Y11 receptor is a member of the purinergic receptor family. It has been overlooked, somewhat due to the lack of a P2ry11 gene orthologue in the murine genome, which prevents the generation of knockout mice, which have been so helpful for defining the roles of other P2Y receptors. Furthermore, some of the studies reported to date have methodological shortcomings, making it difficult to determine the function of P2Y11 with certainty. In this review, we discuss the lack of a murine "P2Y11-like receptor" and highlight the limitations of the currently available methods used to investigate the P2Y11 receptor. These methods include protein recognition with antibodies that show very little specificity, gene expression studies that completely overlook the existence of a fusion transcript between the adjacent PPAN gene and P2RY11, and agonists/antagonists reported to be specific for the P2Y11 receptor but which have not been tested for activity on numerous other adenosine 5'-triphosphate (ATP)-binding receptors. We suggest a set of criteria for evaluating whether a dataset describes effects mediated by the P2Y11 receptor. Following these criteria, we conclude that the current evidence suggests a role for P2Y11 in immune activation with cell type-specific effects.

Purines in neurite growth and astroglia activation

The mammalian nervous system is a complex, functional network of neurons, consisting of local and long-range connections. Neuronal growth is highly coordinated by a variety of extracellular and intracellular signaling molecules. Purines turned out to be an essential component of these processes. Here, we review the current knowledge about the involvement of purinergic signaling in the regulation of neuronal development. We particularly focus on its role in neuritogenesis: the formation and extension of neurites. In the course of maturation mammals generally lose their ability to regenerate the central nervous system (CNS) e.g. after traumatic brain injury; although, spontaneous regeneration still occurs in the peripheral nervous system (PNS). Thus, it is crucial to translate the knowledge about CNS development and PNS regeneration into novel approaches to enable neurons of the mature CNS to regenerate. In this context we give a general overview of growth-inhibitory and growth-stimulatory factors and mechanisms involved in neurite growth. With regard to neuronal growth, astrocytes are an important cell population. They provide structural and metabolic support to neurons and actively participate in brain signaling. Astrocytes respond to injury with beneficial or detrimental reactions with regard to axonal growth. In this review we present the current knowledge of purines in these glial functions. Moreover, we discuss organotypic brain slice co-cultures as a model which retains neuron-glia interactions, and further presents at once a model for CNS development and regeneration. In summary, the purinergic system is a pivotal factor in neuronal development and in the response to injury. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Long-term treatment with PP2 after spinal cord injury resulted in functional locomotor recovery and increased spared tissue

The spinal cord has the ability to regenerate but the microenvironment generated after trauma reduces that capacity. An increase in Src family kinase (SFK) activity has been implicated in neuropathological conditions associated with central nervous system trauma. Therefore, we hypothesized that a decrease in SFK activation by a long-term treatment with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyramidine (PP2), a selective SFK inhibitor, after spinal cord contusion with the New York University (NYU) impactor device would generate a permissive environment that improves axonal sprouting and/or behavioral activity. Results demonstrated that long-term blockade of SFK activation with PP2 increases locomotor activity at 7, 14, 21 and 28 days post-injury in the Basso, Beattie, and Bresnahan open field test, round and square beam crossing tests. In addition, an increase in white matter spared tissue and serotonin fiber density was observed in animals treated with PP2. However, blockade of SFK activity did not change the astrocytic response or infiltration of cells from the immune system at 28 days post-injury. Moreover, a reduced SFK activity with PP2 diminished Ephexin (a guanine nucleotide exchange factor) phosphorylation in the acute phase (4 days post-injury) after trauma. Together, these findings suggest a potential role of SFK in the regulation of spared tissue and/or axonal outgrowth that may result in functional locomotor recovery during the pathophysiology generated after spinal cord injury. Our study also points out that ephexin1 phosphorylation (activation) by SFK action may be involved in the repulsive microenvironment generated after spinal cord injury.

Bcl-xL is necessary for neurite outgrowth in hippocampal neurons

AIMS

B-cell lymphoma-extra large (Bcl-xL) protects survival in dividing cells and developing neurons, but was not known to regulate growth. Growth and synapse formation are indispensable for neuronal survival in development, inextricably linking these processes. We have previously shown that, during synaptic plasticity, Bcl-xL produces changes in synapse number, size, activity, and mitochondrial metabolism. In this study, we determine whether Bcl-xL is required for healthy neurite outgrowth and whether neurite outgrowth is necessary for survival in developing neurons in the presence or absence of stress.

RESULTS

Depletion of endogenous Bcl-xL impairs neurite outgrowth in hippocampal neurons followed by delayed cell death which is dependent on upregulation of death receptor 6 (DR6), a molecule that regulates axonal pruning. Under hypoxic conditions, Bcl-xL-depleted neurons demonstrate increased vulnerability to neuronal process loss and to death compared with hypoxic controls. Endogenous DR6 expression and upregulation during hypoxia are associated with worsened neurite damage; depletion of DR6 partially rescues neuronal process loss, placing DR6 downstream of the effects of Bcl-xL on neuronal process outgrowth and protection. In vivo ischemia produces early increases in DR6, suggesting a role for DR6 in brain injury.

INNOVATION

We suggest that DR6 levels are usually suppressed by Bcl-xL; Bcl-xL depletion leads to upregulation of DR6, failure of neuronal outgrowth in nonstressed cells, and exacerbation of hypoxia-induced neuronal injury.

CONCLUSION

Bcl-xL regulates neuronal outgrowth during development and protects neurites from hypoxic insult, as opposed by DR6. Factors that enhance neurite formation may protect neurons against hypoxic injury or neurodegenerative stimuli.

P2X and P2Y receptors—role in the pathophysiology of the nervous system

Purinergic signalling plays a crucial role in proper functioning of the nervous system. Mechanisms depending on extracellular nucleotides and their P2 receptors also underlie a number of nervous system dysfunctions. This review aims to present the role of purinergic signalling, with particular focus devoted to role of P2 family receptors, in epilepsy, depression, neuropathic pain, nervous system neoplasms, such as glioma and neuroblastoma, neurodegenerative diseases like Parkinson’s disease, Alzheimer’s disease and multiple sclerosis. The above-mentioned conditions are associated with changes in expression of extracellular ectonucleotidases, P2X and P2Y receptors in neurons and glial cells, as well as releasing considerable amounts of nucleotides from activated or damaged nervous tissue cells into the extracellular space, which contributes to disturbance in purinergic signalling. The numerous studies indicate a potential possibility of using synthetic agonists/antagonists of P2 receptors in treatment of selected nervous system diseases. This is of particular significance, since numerous available agents reveal a low effectiveness and often produce side effects.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Potentiation of cytotoxicity of paclitaxel in combination with Cl-IB-MECA in human C32 metastatic melanoma cells: A new possible therapeutic strategy for melanoma

Metastatic melanoma monotherapies with drugs such as dacarbazine, cisplatin or paclitaxel (PXT) are associated with significant toxicity and low efficacy rates. These facts reinforce the need for development of novel agents or combinatory strategies. Cl-IB-MECA is a small molecule, orally bioavailable, well tolerated and currently under clinical trials as an anticancer agent. Our aim was to investigate a possible combinatory therapeutic strategy using PXT and Cl-IB-MECA on human C32 melanoma cells and its underlying mechanisms. Cytotoxicity was evaluated using MTT reduction, lactate dehydrogenase leakage and neutral red uptake assays, for different concentrations and combinations of both agents, at 24 and 48 h. Apoptosis was also assessed using fluorescence microscopy and through the evaluation of caspases 8, 9, and 3 activities. We demonstrated, for the first time, that combination of PXT and Cl-IB-MECA significantly increases cytotoxicity for clinically relevant concentrations. This combination seems to act synergistically in disrupting membrane integrity, but also causing lysosomal and mitochondrial dysfunction. When using the lowest PTX concentration (10 ng/mL), co-incubation with CI-IB-MECA (micromolar concentrations) potentiated overall cytotoxic effects and morphological signs of apoptosis. All combinations studied enhanced caspase 8, 9, and 3 activities, suggesting the involvement of both intrinsic and extrinsic apoptotic pathways. The possibility that cytotoxicity elicited by Cl-IB-MECA, alone or in combination with PXT, involves adenosine receptor activation was discarded and results confirmed that oxidative stress is only involved in cytotoxicity after treatment with PXT, alone. Being melanoma a very apoptosis-resistance cancer, this combination seems to hold promise as a new therapeutic strategy for melanoma.

Implications of purinergic receptor-mediated intracellular calcium transients in neural differentiation

Purinergic receptors participate, in almost every cell type, in controlling metabolic activities and many physiological functions including signal transmission, proliferation and differentiation. While most of P2Y receptors induce transient elevations of intracellular calcium concentration by activation of intracellular calcium pools and forward these signals as waves which can also be transmitted into neighboring cells, P2X receptors produce calcium spikes which also include activation of voltage-operating calcium channels. P2Y and P2X receptors induce calcium transients that activate transcription factors responsible for the progress of differentiation through mediators including calmodulin and calcineurin. Expression of P2X2 as well as of P2X7 receptors increases in differentiating neurons and glial cells, respectively. Gene expression silencing assays indicate that these receptors are important for the progress of differentiation and neuronal or glial fate determination. Metabotropic receptors, mostly P2Y1 and P2Y2 subtypes, act on embryonic cells or cells at the neural progenitor stage by inducing proliferation as well as by regulation of neural differentiation through NFAT translocation. The scope of this review is to discuss the roles of purinergic receptor-induced calcium spike and wave activity and its codification in neurodevelopmental and neurodifferentiation processes.

Adenosine triphosphate regulates NADPH oxidase activity leading to hydrogen peroxide production and COX-2/PGE2 expression in A549 cells

Non-small cell lung carcinoma (NSCLC) accounts for most of all lung cancers, which is the leading cause of mortality in human beings. High level of cyclooxygenase-2 (COX-2) is one of the features of NSCLC and related to the low survival rate of NSCLC. However, whether extracellular nucleotides releasing from stressed resident tissues contributes to the expression of COX-2 remains unclear. Here, we showed that stimulation of A549 cells by adenosine 5'-O-(3-thiotriphosphate) (ATPγS) led to an increase in COX-2 gene expression and prostaglandin E(2) (PGE(2)) synthesis, revealed by Western blotting, RT-PCR, promoter assay, and enzyme-linked immunosorbent assay. In addition, ATPγS induced intracellular reactive oxygen species (ROS) generation through the activation of NADPH oxidase. The increase of ROS level resulted in activation of the c-Src/epidermal growth factor receptor (EGFR)/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/nuclear factor (NF)-κB cascade. We also found that activated Akt was translocated into the nucleus and recruited with NF-κB and p300 to form a complex. Thus, activation of p300 modulated the acetylation of histone H4 via the NADPH oxidase/c-Src/EGFR/PI3K/Akt/NF-κB cascade stimulated by ATPγS. Our results are the first to show a novel role of NADPH oxidase-dependent Akt/p65/p300 complex formation that plays a key role in regulating COX-2/PGE(2) expression in ATPγS-treated A549 cells. Taken together, we demonstrated that ATPγS stimulated activation of NADPH oxidase, resulting in generation of ROS, which then activated the downstream c-Src/EGFR/PI3K/Akt/NF-κB/p300 cascade to regulate the expression of COX-2 and synthesis of PGE(2) in A549 cells. Understanding the regulation of COX-2 expression and PGE(2) release by ATPγS on A549 cells may provide potential therapeutic targets of NSCLC.

ATP-mediated Erk1/2 activation stimulates bacterial capture by filopodia, which precedes Shigella invasion of epithelial cells

Shigella, the causative agent of bacillary dysentery in humans, invades epithelial cells, using a type III secretory system (T3SS) to inject bacterial effectors into host cells and remodel the actin cytoskeleton. ATP released through connexin hemichanels on the epithelial membrane stimulates Shigella invasion and dissemination in epithelial cells. Here, we show that prior to contact with the cell body, Shigella is captured by nanometer-thin micropodial extensions (NMEs) at a distance from the cell surface, in a process involving the T3SS tip complex proteins and stimulated by ATP- and connexin-mediated signaling. Upon bacterial contact, NMEs retract, bringing bacteria in contact with the cell body, where invasion occurs. ATP stimulates Erk1/2 activation, which controls actin retrograde flow in NMEs and their retraction. These findings reveal previously unappreciated facets of interaction of an invasive bacterium with host cells and a prominent role for Erk1/2 in the control of filopodial dynamics.

Interaction of purinergic receptors with GPCRs, ion channels, tyrosine kinase and steroid hormone receptors orchestrates cell function

Extracellular purines and pyrimidines have emerged as key regulators of a wide range of physiological and pathophysiological cellular processes acting through P1 and P2 cell surface receptors. Increasing evidence suggests that purinergic receptors can interact with and/or modulate the activity of other classes of receptors and ion channels. This review will focus on the interactions of purinergic receptors with other GPCRs, ion channels, receptor tyrosine kinases, and steroid hormone receptors. Also, the signal transduction pathways regulated by these complexes and their new functional properties are discussed.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

Extracellular ATP stimulates epithelial cell motility through Pyk2-mediated activation of the EGF receptor

Wounding usually causes considerable cell damage, and released ATP promotes migration of nearby epithelium. ATP binds to purinergic receptors on the cell surface and induces transactivation of the EGF receptor through signaling by the Src family kinases (SFKs). Here we tested whether ATP activates these kinases through Pyk2, a member of the focal adhesion kinase family. Pyk2 was rapidly and potently activated by treating corneal epithelial cells with ATP, and physical interaction of Pyk2 with the SFKs was enhanced. Disruption of Pyk2 signaling either by siRNA or by expression of a dominant-negative mutant led to inhibition of ATP-induced activation of the SFKs and the EGF receptor. Inhibiting Pyk2 activity also blocked ATP stimulation of healing of wounds in epithelial cell sheets. These data suggest that ATP stimulates sequential activation of Pyk2, SFKs, and the EGF receptor to induce cell migration.

Neurite outgrowth is dependent on the association of c-Src and lipid rafts

Regulation of neurite outgrowth is an important aspect not only for proper development of the nervous system but also for tissue regeneration after nerve injury and the treatment of neuropathological conditions. Here, we report that neurite outgrowth in cortical neuron and neuro 2A (N2A) cell was dependent on intact lipid rafts, as well as the enhanced localization of c-Src in the lipid rafts. Src inhibition or lipid rafts disruption could specifically block c-Src phosphorylation profile, pY416 Src increase and pY529 Src decrease, they also resulted in pY529 Src and c-terminal Src kinase (Csk) partition out of lipid rafts. Thus, we concluded that c-Src signal cascades within the lipid rafts is crucial for efficient neurite outgrowth.

Protein kinase C {delta}-specific activity reporter reveals agonist-evoked nuclear activity controlled by Src family of kinases

Conventional and novel protein kinase C (PKC) isozymes transduce the abundance of signals mediated by phospholipid hydrolysis; however redundancy in regulatory mechanisms confounds dissecting the unique signaling properties of each of the eight isozymes constituting these two subgroups. Previously, we created a genetically encoded reporter (C kinase activity reporter (CKAR)) to visualize the rate, amplitude, and duration of agonist-evoked PKC signaling at specific locations within the cell. Here we designed a reporter, δCKAR, that specifically measures the activation signature of one PKC isozyme, PKC δ, in cells, revealing unique spatial and regulatory properties of this isozyme. Specifically, we show two mechanisms of activation: 1) agonist-stimulated activation at the plasma membrane (the site of most robust PKC δ signaling), Golgi, and mitochondria that is independent of Src and can be triggered by phorbol esters and 2) agonist-stimulated activation in the nucleus that requires Src kinase activation and cannot be triggered by phorbol esters. Translocation studies reveal that the G-protein-coupled receptor agonist UTP induces the translocation of PKC δ into the nucleus by a mechanism that depends on the C2 domain and requires Src kinase activity. However, translocation from the cytosol into the nucleus is not required for the Src-dependent regulation of nuclear activity; a construct of PKC δ prelocalized to the nucleus continues to be activated by UTP by a mechanism dependent on Src kinase activity. These data identify the nucleus as a signaling hub for PKC δ that is driven by receptor-mediated signaling pathways (but not phorbol esters) and differs from signaling at plasma membrane and Golgi in that it is controlled by Src family kinases.

A neuroprotective exercise protocol reduces the adenine nucleotide hydrolysis in hippocampal synaptosomes and serum of rats

Regular and moderate exercise has been considered as an interesting neuroprotective strategy. However, the molecular mechanisms by which physical exercise alters brain function are unclear. Purinergic signaling seems to modulate the pathophysiology of ischemic neuronal damage, since it has been described a neuroprotective activity of adenosine and a dual role of ATP. In the present study, we investigated the effect of daily moderate intensity exercise on ectonucleotidase activities in synaptosomes from hippocampus and the soluble nucleotidases from blood serum of rats. Adult male Wistar rats were assigned to non-exercised (sedentary) group and exercised during 20-min sessions on different programs. The effects of physical activity on hydrolysis of ATP, ADP and AMP were assayed in the synaptosomal fraction obtained from the hippocampus and serum approximately 16 h after the last training session. Our data demonstrated that a neuroprotective exercise protocol, daily 20 min of training in treadmill during 2 weeks, diminished significantly the ADP hydrolysis and there is a trend to reduce the ATP hydrolysis in both hippocampal synaptosomes and blood serum of rats. We suggest that the neuroprotective exercise protocol may modulate nucleotidase activities.

Ca2+/calmodulin-dependent kinase II signalling cascade mediates P2X7 receptor-dependent inhibition of neuritogenesis in neuroblastoma cells

ATP, via purinergic P2X receptors, acts as a neurotransmitter and modulator in both the central and peripheral nervous systems, and is also involved in many biological processes, including cell proliferation, differentiation and apoptosis. Previously, we have reported that P2X7 receptor inhibition promotes axonal growth and branching in cultured hippocampal neurons. In this article, we demonstrate that the P2X7 receptor negatively regulates neurite formation in mouse Neuro-2a neuroblastoma cells through a Ca2+/calmodulin-dependent kinase II-related mechanism. Using both molecular and immunocytochemical techniques, we characterized the presence of endogenous P2X1, P2X3, P2X4 and P2X7 subunits in these cells. Of these, the P2X7 receptor was the only functional receptor, as its activation induced intracellular calcium increments similar to those observed in primary neuronal cultures, exhibiting pharmacological properties characteristic of homomeric P2X7 receptors. Patch-clamp experiments were also conducted to fully demonstrate that ionotropic P2X7 receptors mediate nonselective cation currents in this cell line. Pharmacological inhibition of the P2X7 receptor and its knockdown by small hairpin RNA interference resulted in increased neuritogenesis in cells cultured in low serum-containing medium, whereas P2X7 overexpression significantly reduced the formation of neurites. Interestingly, P2X7 receptor inhibition also modified the phosphorylation state of focal adhesion kinase, Akt and glycogen synthase kinase 3, protein kinases that participate in the Ca2+/calmodulin-dependent kinase II signalling cascade and that have been related to neuronal differentiation and axonal growth. Taken together, our results provide the first mechanistic insight into P2X7 receptor-triggered signalling pathways that regulate neurite formation in neuroblastoma cells.

Characterization of P2Y receptor subtypes functionally expressed on neonatal rat cardiac myofibroblasts

BACKGROUND AND PURPOSE

Little is known about P2Y receptors in cardiac fibroblasts, which represent the predominant cell type in the heart and differentiate into myofibroblasts under certain conditions. Therefore, we have characterized the phenotype of the cells and the different P2Y receptors at the expression and functional levels in neonatal rat non-cardiomyocytes.

EXPERIMENTAL APPROACH

Non-cardiomyocyte phenotype was determined by confocal microscopy by using discoidin domain receptor 2, alpha-actin and desmin antibodies. P2Y receptor expression was investigated by reverse transcription-polymerase chain reaction and immunocytochemistry, and receptor function by cAMP and inositol phosphate (IP) accumulation induced by adenine or uracil nucleotides in the presence or absence of selective antagonists of P2Y(1) (MRS 2179, 2-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt), P2Y(6) (MRS 2578) and P2Y(11) (NF 157, 8,8'-[carbonylbis[imino-3,1-phenylenecarbonylimino(4-fluoro-3,1-phenylene)carbonylimino]]bis-1,3,5-naphthalene trisulphonic acid hexasodium salt) receptors. G(i/o) and G(q/11) pathways were evaluated by using Pertussis toxin and YM-254890 respectively.

KEY RESULTS

The cells (>95%) were alpha-actin and discoidin domain receptor 2-positive and desmin-negative. P2Y(1), P2Y(2), P2Y(4), P2Y(6) were detected by reverse transcription-polymerase chain reaction and immunocytochemistry, and P2Y(11)-like receptors at protein level. All di- or tri-phosphate nucleotides stimulated IP production in an YM-254890-sensitive manner. AMP, ADPbetaS, ATP and ATPgammaS increased cAMP accumulation, whereas UDP and UTP inhibited cAMP response, which was abolished by Pertussis toxin. MRS 2179 and NF 157 inhibited ADPbetaS-induced IP production. MRS 2578 blocked UDP- and UTP-mediated IP responses.

CONCLUSION AND IMPLICATIONS

P2Y(1)-, P2Y(2)-, P2Y(4)-, P2Y(6)-, P2Y(11)-like receptors were co-expressed and induced function through G(q/11) protein coupling in myofibroblasts. Furthermore, P2Y(2) and P2Y(4) receptor subtypes were also coupled to G(i/o). The G(s) response to adenine nucleotides suggests a possible expression of a new P2Y receptor subtype.

The application of magnets directs the orientation of neurite outgrowth in cultured human neuronal cells

Electric and magnetic fields have been known to influence cellular behavior. In the present study, we hypothesized that the application of static magnetic fields to neurons will cause neurites to grow in a specific direction. In cultured human neuronal SH-SY5Y cells or PC12 cells, neurite outgrowth was induced by forskolin, retinoic acid, or nerve growth factor (NGF). We applied static magnetic fields to the neurons and analyzed the direction and morphology of newly formed neuronal processes. In the presence of the magnetic field, neurites grew in a direction perpendicular to the direction of the magnetic field, as revealed by the higher orientation index of neurites grown under the magnetic field compared to that of the neurites grown in the absence of the magnetic field. The neurites parallel to the magnetic field appeared to be dystrophic, beaded or thickened, suggesting that they would hinder further elongation processes. The co-localized areas of microtubules and actin filaments were arranged into the vertical axis to the magnetic field, while the levels of neurofilament and synaptotagmin were not altered. Our results suggest that the application of magnetic field can be used to modulate the orientation and direction of neurite formation in cultured human neuronal cells.

Extracellular ATP and cancer: an overview with special reference to P2 purinergic receptors

Purinergic signal transduction mechanisms have been appreciated as a complex intercellular signalling network that plays an important regulatory role in both short- and long-term processes in practically every living cell. One of the most intriguing aspects of the field is the participation of ATP and other purine nucleotides in the determination of cell fate and the way they direct cells towards proliferation, differentiation or apoptosis, thereby possibly taking part in promoting or preventing malignant transformation. In this review, following a very brief introduction to the historical aspects of purinergic signalling and a concise overview of the structure of and signal transduction pathways coupled to P2 purinergic receptors, the current theories concerning the possible ways how extracellular ATP can alter the function of tumour cells and the effectiveness of anticancer therapies are discussed, including pharmacological, nutritional, vasoactive and 'anti-antioxidant' actions of the nucleotide. The effects of ATP on animals inoculated with human tumours and on patients with cancer are looked over next, and then an overview of the literature regarding the expression and presumed functions of P2 purinoceptors on tumour cells in vitro is presented, sorted out according to the relevant special clinical fields. The article is closed by reviewing the latest developments in the diagnostic use of P2 purinergic receptors as tumour markers and prognostic factors, while discussing some of the difficulties and pitfalls of the therapeutic use of ATP analogues.

Purinergic agonists stimulate lens Na-K-ATPase-mediated transport via a Src tyrosine kinase-dependent pathway

The Na-K-ATPase is vital for maintenance of lens transparency. Past studies using intact lens suggested the involvement of tyrosine kinases in short-term regulation of Na-K-ATPase. Furthermore, in vitro phosphorylation of a lens epithelial membrane preparation by Src family kinases (SFKs), a family of nonreceptor tyrosine kinases, resulted in modification of Na-K-ATPase activity. Here, the effect of purinergic agonists, ATP and UTP, on Na-K-ATPase function and SFK activation was examined in the rabbit lens. Na-K-ATPase function was examined using two different approaches, measurement of ouabain-sensitive potassium ((86)Rb) uptake by the intact lens, and Na-K-ATPase activity in lens epithelial homogenates. ATP and UTP caused a significant increase in ouabain-sensitive potassium ((86)Rb) uptake. Na-K-ATPase activity was increased in the epithelium of lenses pretreated with ATP. Lenses treated with ATP or UTP displayed activation of SFKs as evidenced by increased Western blot band density of active SFK (phosphorylated at the active loop Y416) and decreased band density of inactive SFKs (phosphorylated at the COOH terminal). A single PY416-Src immunoreactive band at approximately 60 kDa was observed, suggesting not all Src family members are activated. Immunoprecipitation studies showed that band density of active Src, and to a lesser extent active Fyn, was significantly increased, while active Yes did not change. Preincubation of the lenses with SFK inhibitor PP2 abolished the ATP-induced increase in ouabain-sensitive potassium ((86)Rb) uptake. The results suggest selective activation of Src and/or Fyn is part of a signaling mechanism initiated by purinergic agonists that increases Na-K-ATPase-mediated transport in the organ-cultured lens.

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.