### A. Overview

Extracellular purines (adenosine, ADP, and ATP) and pyrimidines (UDP and UTP) are important signaling molecules that mediate diverse biological effects via cell-surface receptors termed purine receptors. In this review particular emphasis is placed on the discrepancy between the

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Receptors for Purines and Pyrimidines

Inositol phospholipids have long been known to have an important regulatory role in cell physiology. The repertoire of cellular processes known to be directly or indirectly controlled by this class of lipids has now dramatically expanded. Through interactions mediated by their headgroups, which can be reversibly phosphorylated to generate seven species, phosphoinositides play a fundamental part in controlling membrane-cytosol interfaces. These lipids mediate acute responses, but also act as constitutive signals that help define organelle identity. Their functions, besides classical signal transduction at the cell surface, include regulation of membrane traffic, the cytoskeleton, nuclear events and the permeability and transport functions of membranes.

Phosphoinositides in cell regulation and membrane dynamics

http://academic.oup.com/milmed/article-pdf/146/7/506/24126715/milmed-146-7-506.pdf

Goodman and Gilman's The Pharmacological Basis of Therapeutics

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.

/pdf/physiology-and-pathophysiology-of-purinergic-1z94g2hz0f.pdf

Physiology and Pathophysiology of Purinergic Neurotransmission

Inwardly rectifying K+ (Kir) channels allow K+ to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are ...

Inwardly Rectifying Potassium Channels: Their Structure, Function, and Physiological Roles

Extracellular acidification occurs not only in pathological conditions such as inflammation and brain ischemia, but also in normal physiological conditions such as synaptic transmission. Acid-sensing ion channels (ASICs) can detect a broad range of physiological pH changes during pathological and synaptic cellular activities. ASICs are voltage-independent, proton-gated cation channels widely expressed throughout the central and peripheral nervous system. Activation of ASICs is involved in pain perception, synaptic plasticity, learning and memory, fear, ischemic neuronal injury, seizure termination, neuronal degeneration, and mechanosensation. Therefore, ASICs emerge as potential therapeutic targets for manipulating pain and neurological diseases. The activity of these channels can be regulated by many factors such as lactate, Zn(2+), and Phe-Met-Arg-Phe amide (FMRFamide)-like neuropeptides by interacting with the channel's large extracellular loop. ASICs are also modulated by G protein-coupled receptors such as CB1 cannabinoid receptors and 5-HT2. This review focuses on the physiological roles of ASICs and the molecular mechanisms by which these channels are regulated.

Acid-sensing ion channels (ASICs): therapeutic targets for neurological diseases and their regulation

Activity of KCNQ (Kv7) channels requires binding of phosphatidylinositol 4,5-bisphosphate (PIP 2 ) from the plasma membrane. We give evidence that Mg 2+ and polyamines weaken the KCNQ channel–phospholipid interaction. Lowering internal Mg 2+ augmented inward and outward KCNQ currents symmetrically, and raising Mg 2+ reduced currents symmetrically. Polyvalent organic cations added to the pipette solution had similar effects. Their potency sequence followed the number of positive charges: putrescine (+2) 2+ were reversible with sequential whole-cell patching. Internal tetraethylammonium ion (TEA) gave classical voltage-dependent block of the pore with changes of the time course of K + currents. The effect of polyvalent cations was simpler, symmetric, and without changes of current time course. Overexpression of phosphatidylinositol 4-phosphate 5-kinase Iγ to accelerate synthesis of PIP 2 attenuated the sensitivity to polyvalent cations. We suggest that Mg 2+ and other polycations reduce the currents by electrostatic binding to the negative charges of PIP 2 , competitively reducing the amount of free PIP 2 available for interaction with channels. The dose–response curves could be modeled by a competition model that reduces the pool of free PIP 2 . This mechanism is likely to modulate many other PIP 2 -dependent ion channels and cellular processes.

https://rupress.org/jgp/article-pdf/130/3/241/1223794/jgp_200709821.pdf

Electrostatic interaction of internal Mg2+ with membrane PIP2 Seen with KCNQ K+ channels.

Activation of phospholipase C (PLC) through G-protein-coupled receptors produces a large number of second messengers and regulates many physiological processes. Many membrane proteins including ion channels require the phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) to function. Activation of PLC can shut down their activity if it depletes the PIP2 pool strongly. Such a mechanism accounts for the muscarinic suppression of current in KCNQ channels. We describe a variety of methods used to show that these channels require PIP2 and that current in the channels is suppressed when receptor-activated PLC depletes PIP2. The methods include observing translocation of lipid-sensitive protein domains, overexpression of enzymes of phosphoinositide metabolism, engineering these enzymes to move to the plasma membrane in response to a chemical signal, and direct chemical analysis of phospholipids. These approaches are general and can be used to test for PIP2 requirements of other membrane proteins.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.2007.132647

Regulation of KCNQ channels by manipulation of phosphoinositides.

The β-subunits of voltage-gated Ca2+ (CaV) channels regulate the functional expression and several biophysical properties of high-voltage–activated CaV channels. We find that CaV β-subunits also determine channel regulation by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). When CaV1.3, -2.1, or -2.2 channels are cotransfected with the β3-subunit, a cytosolic protein, they can be inhibited by activating a voltage-sensitive lipid phosphatase to deplete PIP2. When these channels are coexpressed with a β2a-subunit, a palmitoylated peripheral membrane protein, the inhibition is much smaller. PIP2 sensitivity could be increased by disabling the two palmitoylation sites in the β2a-subunit. To further test effects of membrane targeting of CaV β-subunits on PIP2 regulation, the N terminus of Lyn was ligated onto the cytosolic β3-subunit to confer lipidation. This chimera, like the CaV β2a-subunit, displayed plasma membrane localization, slowed the inactivation of CaV2.2 channels, and increased the current density. In addition, the Lyn-β3 subunit significantly decreased CaV channel inhibition by PIP2 depletion. Evidently lipidation and membrane anchoring of CaV β-subunits compete with the PIP2 regulation of high-voltage–activated CaV channels. Compared with expression with CaV β3-subunits alone, inhibition of CaV2.2 channels by PIP2 depletion could be significantly attenuated when β2a was coexpressed with β3. Our data suggest that the CaV currents in neurons would be regulated by membrane PIP2 to a degree that depends on their endogenous β-subunit combinations.

https://www.pnas.org/content/pnas/109/8/3161.full.pdf

Membrane-localized β-subunits alter the PIP2 regulation of high-voltage activated Ca2+ channels

1. The adenosine receptor in mouse pinealocytes was identified and characterized using pharmacological and physiological approaches. 2. Expression of the two adenosine receptor subtypes A2B and A3 was detected in mouse pineal glands and PGT-beta cells by polymerase chain reaction and nucleotide sequencing. 3. Adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) evoked cyclic AMP generation but the A2)-selective agonist 2-(4-(2-carboxyethyl)phenylethylamino)adenosine-5'-N-ethylcarboxamideadenosine (CGS 21680) and the A1-specific agonists R-N(6)-(2-phenylisopropyl)adenosine (R-PIA) and N(6)-cyclopentyladenosine (CPA) had little effect on intracellular cyclic AMP levels. The A2B receptor selective antagonists alloxazine and enprofylline completely blocked NECA-mediated cyclic AMP accumulation. 4. Treatment of cells with the A3-selective agonist N(6)-(3-iodobenzyl)-5'-(N-methylcarbamoyl)adenosine (IB-MECA) inhibited the elevation of the cyclic AMP level induced by NECA or isoproterenol in a concentration-dependent manner with maximal inhibition of 40 - 50%. These responses were blocked by the specific A3 adenosine receptor antagonist MRS 1191. Pretreatment of the cells with pertussis toxin attenuated the IB-MECA-induced responses, suggesting that this effect occurred via the pertussis toxin-sensitive inhibitory G proteins. 5. IB-MECA also caused a concentration-dependent elevation in [Ca(2+)]i and IP3 content. Both the responses induced by IB-MECA were attenuated by treatment with U73122 or phorbol 12-myristate 13-acetate. 6. These data suggest the presence of both A2B and A3 adenosine receptors in mouse pineal tumour cells and that the A2B receptor is positively coupled to adenylyl cyclase whereas the A3 receptor is negatively coupled to adenylyl cyclase and also coupled to phospholipase C.

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1038/sj.bjp.0704218

Byung-Chang Suh

Papers

Acid-sensing ion channels (ASICs): therapeutic targets for neurological diseases and their regulation

Electrostatic interaction of internal Mg2+ with membrane PIP2 Seen with KCNQ K+ channels.

Regulation of KCNQ channels by manipulation of phosphoinositides.

Membrane-localized β-subunits alter the PIP2 regulation of high-voltage activated Ca2+ channels

Pharmacological characterization of adenosine receptors in PGT-β mouse pineal gland tumour cells