Effects of ATP and derivatives on neuropile glial cells of the leech central nervous system

Early evolutionary history (from bacteria to hemichordata) of the omnipresent purinergic signalling: A tribute to Geoff Burnstock inquisitive mind

Purines and pyrimidines are indispensable molecules of life; they are fundamental for genetic code and bioenergetics. From the very early evolution of life purines have acquired the meaning of damage-associated extracellular signaller and purinergic receptors emerged in unicellular organisms. Ancestral purinoceptors are P2X-like ionotropic ligand-gated cationic channels showing 20-40% of homology with vertebrate P2X receptors; genes encoding ancestral P2X receptors have been detected in Protozoa, Algae, Fungi and Sponges; they are also present in some invertebrates, but are absent from the genome of insects, nematodes, and higher plants. Plants nevertheless evolved a sophisticated and widespread purinergic signalling system relying on the idiosyncratic purinoceptor P2K1/DORN1 linked to intracellular Ca²⁺ signalling. The advance of metabotropic purinoceptors starts later in evolution with adenosine receptors preceding the emergence of P2Y nucleotide and P0 adenine receptors. In vertebrates and mammals the purinergic signalling system reaches the summit and operates throughout all tissues and systems without anatomical or functional segregation.

Purinergic modulation of neuronal gap junction circuits in the CNS of the leech

Gap junctions (GJs) are widely distributed in brains across the animal kingdom. To visualize the GJ- coupled networks of two major mechanosensory neurons in the ganglia of medicinal leeches, we injected these cells with the GJ-permeable tracer Neurobiotin. When diffusion time was limited to only 30 min, tracer coupling was highly variable for both cells, suggesting a possible modulation of GJ permeability. In invertebrates the innexins (homologs of vertebrate pannexins) form the GJs. Because extracellular adenosine triphosphate (ATP) modulates pannexin and leech innexin hemichannel permeability and is released by leech glial cells following injury, we tested the effects of bath application of ATP after the injection of Neurobiotin and observed a significant increase in the number of neurons tracer coupled to the sensory neurons. This effect required the elevation of intracellular Ca²⁺ and could be produced by bath application of caffeine. Conversely, scavenging endogenous extracellular ATP with the ATPase apyrase decreased the number of coupled cells. ATP also increased electrical conductance and tracer permeability between the bilateral Retzius neurons. This modulatory effect of ATP on GJ coupling was blocked by siRNA knockdown of a P1-like adenosine receptor. Finally, exposure of leech ganglia to extracellular ATP induced a characteristic low frequency (<0.3 Hz) rhythmic bursting activity that was roughly synchronous among multiple neurons, a behavior that was significantly attenuated by the GJ blocker octanol. These findings highlight the mediation by ATP of a robust physiological mechanism for modifying neuronal circuits by rapidly recruiting neurons into active networks and entraining synchronized bursting activity.

Evolution of Neuroglia

As the nervous system evolved from the diffused to centralised form, the neurones were joined by the appearance of the supportive cells, the neuroglia. Arguably, these non-neuronal cells evolve into a more diversified cell family than the neurones are. The first ancestral neuroglia appeared in flatworms being mesenchymal in origin. In the nematode C. elegans proto-astrocytes/supportive glia of ectodermal origin emerged, albeit the ensheathment of axons by glial cells occurred later in prawns. The multilayered myelin occurred by convergent evolution of oligodendrocytes and Schwann cells in vertebrates above the jawless fishes. Nutritive partitioning of the brain from the rest of the body appeared in insects when the hemolymph-brain barrier, a predecessor of the blood-brain barrier was formed. The defensive cellular mechanism required specialisation of bona fide immune cells, microglia, a process that occurred in the nervous system of leeches, bivalves, snails, insects and above. In ascending phylogeny, new type of glial cells, such as scaffolding radial glia, appeared and as the bran sizes enlarged, the glia to neurone ratio increased. Humans possess some unique glial cells not seen in other animals.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Stratification of astrocytes in healthy and diseased brain

Astrocytes, a subtype of glial cells, come in variety of forms and functions. However, overarching role of these cell is in the homeostasis of the brain, be that regulation of ions, neurotransmitters, metabolism or neuronal synaptic networks. Loss of homeostasis represents the underlying cause of all brain disorders. Thus, astrocytes are likely involved in most if not all of the brain pathologies. We tabulate astroglial homeostatic functions along with pathological condition that arise from dysfunction of these glial cells. Classification of astrocytes is presented with the emphasis on evolutionary trails, morphological appearance and numerical preponderance. We note that, even though astrocytes from a variety of mammalian species share some common features, human astrocytes appear to be the largest and most complex of all astrocytes studied thus far. It is then an imperative to develop humanized models to study the role of astrocytes in brain pathologies, which is perhaps most abundantly clear in the case of glioblastoma multiforme.

Neuroglial ATP release through innexin channels controls microglial cell movement to a nerve injury

Microglia, the immune cells of the central nervous system, are attracted to sites of injury. The injury releases adenosine triphosphate (ATP) into the extracellular space, activating the microglia, but the full mechanism of release is not known. In glial cells, a family of physiologically regulated unpaired gap junction channels called innexons (invertebrates) or pannexons (vertebrates) located in the cell membrane is permeable to ATP. Innexons, but not pannexons, also pair to make gap junctions. Glial calcium waves, triggered by injury or mechanical stimulation, open pannexon/innexon channels and cause the release of ATP. It has been hypothesized that a glial calcium wave that triggers the release of ATP causes rapid microglial migration to distant lesions. In the present study in the leech, in which a single giant glial cell ensheathes each connective, hydrolysis of ATP with 10 U/ml apyrase or block of innexons with 10 µM carbenoxolone (CBX), which decreased injury-induced ATP release, reduced both movement of microglia and their accumulation at lesions. Directed movement and accumulation were restored in CBX by adding ATP, consistent with separate actions of ATP and nitric oxide, which is required for directed movement but does not activate glia. Injection of glia with innexin2 (Hminx2) RNAi inhibited release of carboxyfluorescein dye and microglial migration, whereas injection of innexin1 (Hminx1) RNAi did not when measured 2 days after injection, indicating that glial cells’ ATP release through innexons was required for microglial migration after nerve injury. Focal stimulation either mechanically or with ATP generated a calcium wave in the glial cell; injury caused a large, persistent intracellular calcium response. Neither the calcium wave nor the persistent response required ATP or its release. Thus, in the leech, innexin membrane channels releasing ATP from glia are required for migration and accumulation of microglia after nerve injury.

Excitation of rat spinal ventral horn neurons by purinergic P2X and P2Y receptor activation

ATPgammaS, a nonhydrolyzable ATP analog, was found to dose-dependently generate an inward current at a holding potential of -70 mV (EC(50)=43 microM) in lamina IX neurons of rat spinal cord slices using the whole-cell patch-clamp technique. This inward current had an extrapolated reversal potential of -9 mV and was resistant to the Na(+)-channel blocker tetrodotoxin, glutamate-receptor antagonists or nominally Ca(2+)-free medium. ATP gamma S also increased the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic current (sEPSC); this action was dose-dependent and sensitive to tetrodotoxin. Unlike ATP gamma S, the P2X-receptor agonist, BzATP or alpha,beta-methylene ATP, did not change holding currents, but the current response produced by ATP gamma S disappeared in the presence of the P2-receptor antagonist PPADS. The sEPSC frequency and amplitude increase was observed with alpha,beta-methylene ATP, but not with the P2Y-receptor agonist, 2-methylthio ADP, UTP or UDP. The current response by ATP gamma S was suppressed by the addition of GDP beta S into the patch-pipette solution. As for ATP gamma S, 2-methylthio ADP produced an inward current, while UTP and UDP had no effect on holding currents. The P2Y(1)-receptor antagonist MRS2179 inhibited the ATP gamma S-induced inward current, but did not affect the sEPSC frequency and amplitude increase produced by ATP gamma S. These data indicate that extracellular ATP increases the excitability of lamina IX neurons by membrane depolarization (probably through non-selective cation-channel activation) and spontaneous excitatory transmission enhancement, which may be mediated by P2Y(1) and P2X receptors, respectively. This finding supports the idea that purinergic receptor antagonists provide a therapy for spinal cord injury.

ATP and NO dually control migration of microglia to nerve lesions

Microglia migrate rapidly to lesions in the central nervous system (CNS), presumably in response to chemoattractants including ATP released directly or indirectly by the injury. Previous work on the leech has shown that nitric oxide (NO), generated at the lesion, is both a stop signal for microglia at the lesion and crucial for their directed migration from hundreds of micrometers away within the nerve cord, perhaps mediated by a soluble guanylate cyclase (sGC). In this study, application of 100 microM ATP caused maximal movement of microglia in leech nerve cords. The nucleotides ADP, UTP, and the nonhydrolyzable ATP analog AMP-PNP (adenyl-5'-yl imidodiphosphate) also caused movement, whereas AMP, cAMP, and adenosine were without effect. Both movement in ATP and migration after injury were slowed by 50 microM reactive blue 2 (RB2), an antagonist of purinergic receptors, without influencing the direction of movement. This contrasted with the effect of the NO scavenger cPTIO (2-(4-carboxyphenyl)-4,4,5,5-teramethylimidazoline-oxyl-3-oxide), which misdirected movement when applied at 1 mM. The cPTIO reduced cGMP immunoreactivity without changing the immunoreactivity of eNOS (endothelial nitric oxide synthase), which accompanies increased NOS activity after nerve cord injury, consistent with involvement of sGC. Moreover, the sGC-specific inhibitor LY83583 applied at 50 microM had a similar effect, in agreement with previous results with methylene blue. Taken together, the experiments support the hypothesis that ATP released directly or indirectly by injury activates microglia to move, whereas NO that activates sGC directs migration of microglia to CNS lesions.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

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.

Calcium signaling in invertebrate glial cells

Calcium signaling studies in invertebrate glial cells have been performed mainly in the nervous systems of the medicinal leech (Hirudo medicinalis) and the sphinx moth Manduca sexta. The main advantages of studing glial cells in invertebrate nervous systems are the large size of invertebrate glial cells and their easy accessibility for optical and electrophysiological recordings. Glial cells in both insects and annelids express voltage-gated calcium channels and, in the case of leech glial cells, calcium-permeable neurotransmitter receptors, which allow calcium influx as one major source for cytosolic calcium transients. Calcium release from intracellular stores can be induced by metabotropic receptor activation in leech glial cells, but appears to play a minor role in calcium signaling. In glial cells of the antennal lobe of Manduca, voltage-gated calcium signaling changes during postembryonic development and is essential for the migration of the glial cells, a key step in axon guidance and in stabilization of the glomerular structures that are characteristic of primary olfactory centers.

Developmental expression of metabotropic P2Y(1) and P2Y(2) receptors in freshly isolated astrocytes from rat hippocampus

There are at least three subtypes of cloned metabotropic P2 receptors linked to intracellular Ca(2+) rises in rat brain cells, namely, P2Y(1), P2Y(2) and P2Y(4). In this study we explore the subtypes of the metabotropic P2 receptors seen in freshly isolated astrocytes (FIAs) from P8-P25 rats. We found by single cell RT-PCR that in process-bearing FIAs from hippocampi of P8-P12 rats, 31% of the glial fibrillary acidic protein (GFAP) mRNA (+) cells expressed P2Y(1) mRNA while only 5% of the cells tested expressed P2Y(2) mRNA. The expression of P2Y(1) receptor mRNA was not changed in FIAs from the hippocampi of P18-P25 rats, but 38% of the GFAP mRNA (+) cells in the P18-P25 age group then showed P2Y(2) mRNA. We also studied whether the mRNA was expressing functional receptor protein by measuring Ca(2+) responses to specific agonists for P2Y(1) and P2Y(2). We found that similar proportions of GFAP mRNA (+) FIAs responded to ATP or UTP as showed mRNAs for P2Y (1) and P2Y(2,) respectively. Total tissue RNA from P9 and P24 rat hippocampus showed a 2.8-fold increase in P2Y(2) mRNA levels from P9 to P24 with a decrease in P2Y(1) mRNA. Thus, this study shows a marked up-regulation of mRNA for P2Y(2) from 9 to 24 days in rat hippocampus, and some of this increase is likely due to the protoplasmic astrocytes which is being translated into functional receptor protein in these cells.